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Autonomic Regulation of Cellular Immune Function
Denise L. Bellinger, Dianne Lorton
PII: S1566-0702(14)00008-3DOI: doi: 10.1016/j.autneu.2014.01.006Reference: AUTNEU 1625
To appear in: Autonomic Neuroscience: Basic and Clinical
Received date: 12 December 2013Accepted date: 17 January 2014
Please cite this article as: Bellinger, Denise L., Lorton, Dianne, Autonomic Regulationof Cellular Immune Function, Autonomic Neuroscience: Basic and Clinical (2014), doi:10.1016/j.autneu.2014.01.006
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Autonomic Regulation of Cellular Immune Function
Denise L. Bellingera, Dianne Lortonb
Department of Human Anatomy and Pathology
aDepartment of Human Anatomy and Pathology, Loma Linda University, School of Medicine,
Loma Linda, California, 92350, USA;
bCollege of Arts and Sciences, Kent State University and the Kent Summa Initiative for Clinical
and Translational Research, Summa Health System, Akron, OH 44304,USA.
*Corresponding Author and Contact Information:
Denise L. Bellinger, Ph.D.
Departments of Pathology and Human Anatomy
Loma Linda University School of Medicine
11021 Campus Street, AH 325
Loma Linda, CA 92350
Tel.: 909-558-7069
Fax: 909-558-0432
Email: [email protected]
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Abstract
The nervous system and the immune system (IS) are two integrative systems that work
together to detect threats and provide host defense, and to maintain/restore homeostasis.
Cross-talk between the nervous system and the IS is vital for health and well-being. One of the
major neural pathways responsible for regulating host defense against injury and foreign
antigens and pathogens is the sympathetic nervous system (SNS). Stimulation of adrenergic
receptors (ARs) on immune cells regulates immune cell development, survival, and proliferative
capacity, circulation and trafficking of immune cells for immune surveillance and for recruitment,
and directs the cell surface expression of molecules and cytokine production important for cell-
to-cell interactions and “positioning” immune cells for a coordinated immune response (IR).
Finally, AR activation of effector immune cells regulates the activational state of immune cells
and modulates the functional capacity. This review focuses on our current understanding of the
role of the SNS in regulating the IS important for host defense and immune homeostasis. SNS
regulation of IS functioning is a critical link to the development and exacerbation of chronic
immune-mediated diseases. There are many diverse mechanisms that need to be further
unraveled in order to develop sound treatment strategies that act on neural-immune interaction
to resolve or prevent chronic inflammatory diseases, and to improve health and quality of life.
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Keywords: sympathetic nervous system; innate and adaptive immunity; bone marrow; spleen,thymus;
lymph nodes; mucosal associated lymphoid tissue; hematopoeis.
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Introduction
Altered immune function can be achieved using classical behavioral conditioning
paradigms whereby an immune stimulus is repeatedly paired with a non-immune type of
stimulus (associative training), and subsequently results in the non-immune stimulus producing
a similar effect on immune function as the immune stimulus (reviewed in Cohen et al., 1994;
Riether et al., 2008). Learning and memory are required for behavioral conditioning, indicating
mediation of conditioning effects via the central nervous system (CNS). Moreover, behavioral
conditioning can alter inflammation and disease outcome (Bovbjerg et al., 1987; Ader and
Cohen, 1982; Giang et al., 1996) indicating an important role for CNS-to-immune system (IS)
regulation in health and illness. Although, all neuroendocrine hormones can affect immunity,
there are two major pathways that mediate CNS-to-IS signaling, the sympathetic nervous
system (SNS) and the hypothalamic-pituitary-adrenocortical (HPA) axis. Both pathways are
regulated centrally by limbic and autonomic circuits known to mediate the effects of stressor on
body functions. Here, we review the central autonomic circuits that influence SNS-to-IS
signaling, and how they affect immune function. A large body of research indicate that
sympathetic activation either pharmacologically or through physical or psychosocial stressors
can significantly alter immune function (Kemeny and Schedlowski, 2007), with consequences
for mental and physical health. Lesioning or stimulating or blocking the activity of, specific
autonomic/limbic brain regions/nuclei known to regulate autonomic/neuroendocrine outflow also
alters measures of immune function (reviewed Felten et al., 1991). Finally, experimental
manipulation of the autonomic outflow from the CNS to immune organs, can affect such
functions as delayed-type sensitivity reactions, natural killer (NK) cytotoxicity, cytokine
secretion, lymphocyte proliferation, and antibody production (reviewed in Bellinger et al., 2008b;
Elenkov et al., 2001). The effects of these manipulations are generally reproducible, dose-
dependent when adrenergic ligands are used and reversible by blocking the sympathetic
manipulative actions of the treatment. It is clear from these findings that the ANS provides
“hard-wiring” to and from the CNS to lymphoid tissues and SNS.
The autonomic nervous system (ANS) plays a critical role in physiological regulation under
basal conditions and in response to acute and chronic stressors. The ANS, particularly the SNS
regulates the homeostatic and host defense functions of the IS. Foreign substances termed
antigens or tissue damage are perceived by the brain as stressors, and therefore activate the
SNS, which, in turn, regulates host defense mechanisms. Psychological and physical stressor,
via activation of the HPA and SNS modulate homeostasis of the IS, can induce inflammation,
and affect the IS’s ability to heal wounds and fight infections. In this paper, we review the
regulation of the IS by the SNS.
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Both arms of the ANS, the PaSNS and SNS are classically “two-neuron chain circuits”
consisting of a pre- to postganglionic neuron connection. However, central autonomic circuits
between the forebrain, limbic system, and certain hypothalamic regions/nuclei and brainstem
nuclei make up the brain-ANS-immune axis. Postganglionic sympathetic neurons from specific
sympathetic ganglia innervate immune organs and tissues (Fig. 1); however, evidence is
lacking for parasympathetic innervation of immune organs (discussed in Bellinger et al., 2013).
Vagal afferents, as well as other visceral afferents, convey immune signals to regions of the
brain that regulate sympathetic outflow to immune organs (reviewed in Bellinger et al., 2013).
Additionally, preganglionic neurons supply the adrenal medulla, which releases the
catecholamines, norepinephrine and epinephrine into the circulation. Therefore the SNS can
have both localized and systemic effects on immunity. These pathways provide the anatomical
substrate for SNS-immune cross-talk. Afferent visceral fibers, including those in the vagus
nerves, are an important route for conveying information concerning tissue damage or antigen
exposure and subsequent immune activation to the central nervous system (CNS). Additionally,
as a slower route of communication circulating cytokines, particularly proinflammatory cytokines
can interact with the brain at circumventricular organs (CVOs), which lack a blood-brain barrier
or can be actively transported into certain regions of the brain (reviewed in Dantzer, 2001).
Immune activation also induces the production of proinflammatory cytokines centrally (Rothwell,
1991; Watkins and Maier, 2000; Neumann, 2001; Felderhoff-Mueser et al., 2005).
“Figure 1 here”
While multiple neuromediators are released from sympathetic nerves, norepinephrine is
the major neurotransmitter that regulates cells of the IS, and most research has focused on
noradrenergic regulation. Cells of the IS respond to noradrenergic signaling via the cell surface
expression of α- and β-adrenergic receptors (ARs) (Fig. 1). Research is beginning to unravel
the mechanisms through which the intracellular signaling pathways used by α- and β-ARs
communicate with the signaling pathways used to regulate immune functions.
Here, we discuss our current understanding of the brain-SNS-immune axis, sympathetic
regulation of immune cell development and immune function. An overview of recent findings
indicating that activation of β2-AR signaling pathways suppresses T-helper (Th)1 responses and
cellular immunity, and drives Th2 responses important for humoral immunity, and adrenergic
regulation of more recently characterized Th17- and Treg-cells are presented. Under chronic
conditions where the IS fails to eliminate the threat and/or to restore immune homeostasis, the
SNS can promote inflammation and boost cellular immunity, the goal of which is presumed to
localize the inflammatory response and provide protection systemically from the detrimental
effects of inflammation. SNS regulation of immune cell development in the bone marrow (BM)
and thymus is complex and is not well understood. Most limited is our understanding of the
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effects of sympathetic outflow on mucosal immunity, and effects on effectors cells of the innate
IS. Research reviewed here underscores the need for better understanding central mechanism
that influence sympathetic outflow, and α1- and β-AR-mediated regulation of immune function in
order to develop early-intervention therapeutics aimed at recovery of sympathetic-immune
dysregulatory mechanisms under certain physiological conditions and disease states.
Central Regulation of Autonomic Outflow to Immune Organs
Central descending sympathetic pathways responsible for regulating sympathetic nerve
activity (SNA) in lymphoid organs (LOs) and tissues are best defined for the spleen. The spleen
is innervated by neurons in the superior mesenteric –celiac ganglion (SMCG), and subsequently
sympathetic preganglionic neuron (SPNs) (Bellinger et al., 1993; Wan et al., 1993). Supraspinal
sympathetic circuits regulate resting tonic and bursting patterns of SNA in the spleen, as well
as, the relationship between splenic SNA and other target organs that must work together to
achieve an integrative sympathetic response during homeostasis and to acute or chronic stress
(Kenney et al., 2003a,b; Osborn et al., 1987; Taylor and Schramm, 1987; Loewy, 1990;
Dampney, 1994; Sun, 1995). Although limited, available data about central sympathetic
regulatory circuits derive from transsynaptic retrograde tracing, electrophysiological recording
combined with brain lesions and/or microinjection of pharmacological agents into discrete brain
regions, and brain lesioning studies. Brain lesioning and stimulation studies are consistent with
the transsynaptic tracing findings. Cano and colleagues (2000, 2001) have mapped the central
location/density of pseudorabies virus (PRV) across time after its injection into the rat spleen.
They report that postganglionic SMCG neurons innervating the spleen receive input from
intermediolateral cell column SPNs from T3 to T12 (Cano et al., 2001), consistent with region-
specific SPN-evoked splenic SNA activity (Taylor and Weaver, 1992).
Between 72 and 110 h post-inoculation there is progressively increased PRV-positive
neurons in central autonomic regulatory nuclei/regions. Their findings are similar to PRV tracing
studies in other sympathetically-targeted organs/tissues (i.e., adrenal medulla, pineal gland,
heart, urinary bladder, rat tail skin, and adipose tissue), supports a basic hierarchy of central
autonomic regions involved in regulating sympathetic outflow, but does not delineate the organ-
specific microcircuitry that regulate sympathetic outflow (Janig and McLachlan, 2013). For
example, the parvocellular region of the paraventricular nucleus (PVN), A5 region, ventral,
medial and lateral medulla, and caudal raphe project to SPNs (Strack et al., 1989a,b; Jansen et
al., 1993; Schramm et al., 1993). Nuclei labeled after longer intervals post-PRV treatment agree
with reports of indirect projects to the SPNs to influence sympathetic activity (Loewy, 1990).
Cano and colleagues corroborate the absence of efferent parasympathetic innervation of the
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spleen (Bellinger et al., 1993; Wan et al., 1993). Labeled neurons in the A7, NTS (medial part)
and C3 cell group are unique (Cano et al., 2001) compared with other transneural tracing
studies (Strack et al., 1989a,b; Jansen et al., 1993; Schramm et al., 1993). Importantly, their
findings generally agree with the general view of the CNS circuitry that regulates autonomic
function and the stress response (Loewy, 1990; Saper, 1995).
As for the exceptions, the lateral parabrachial nucleus and autonomic-regulating thalamic
nuclei are not infected in this study. Secondly, Barrington’s nucleus (BN), subcoeruleus and EW
labeled after PRV injection (Cano et al., 2000, 2001) are generally not included in central
sympathetic circuitry. The BN and EW regulate parasympathetic outflow, but more complex
broader roles have been suggested (Jansen et al., 1997; Farkas et al., 1998; Cano et al., 2000;
Pardini et al., 1989), such as integrating supraspinal inputs for coordinating central sympathetic
and parasympathetic activity. The BN receives inputs from diverse brain regions and connects
with systems mediating stress responses (Chen and Herbert, 1995; Imaki et al., 1991, 1993;
Palkovits et al., 1997; discussed in Sved et al., 2002). Studies that examine autonomic circuits
to the spleen and other visceral organs using different strategies (i.e., brain lesioning,
electrophysiology, and CRH microinjection) are consistent with these anatomical findings.
Paraventricular Nucleus (PVN). As a major SNS regulator, the PVN integrates
information from the brainstem, forebrain, neuroendocrine and limbic regions (Swanson and
Sawchenko, 1983; Nunn et al., 2011), providing a neural substrate for generating complex
output patterns to sympathetic nerves supplying visceral organs, like the spleen. Its projections
to the RVLM, raphe nuclei, parabrachial nucleus, BN, and intermediolateral cell column
(Swanson and Sawchenko, 1983; Cano et al., 2001) form the anatomical substrate for
influencing sympathetic outflow (Swanson and Sawchenko, 1983; Luitten et al., 1985; Krukoff et
al., 1994; Badoer, 2001). Moreover, functional studies support PVN regulation of sympathetic
outflow to the spleen. Microinjecting glutamate into the PVN (Katafuchi et al., 1993a,b)
increases splenic nerve activity, and electrolytic lesions of PVN reduces splenocyte mitogenic
responsiveness (Brooks et al., 1982). Similarly, hypothalamic lesions that include the PVN
attenuate stress-mediated suppression of splenocyte proliferation (Pezzone et al., 1994).
Multiple neurotransmitters/neuromodulators and their receptors interact in the PVN in
complex ways, including -aminobutyric acid (GABA) via GABAA receptors, nitric oxide,
glutamate and angiotensin II (Kenney et al., 2001b, 2003a,b; Herman et al., 2000).
Electrophysiological studies after neurotransmitter microinjection into the PVN has increased
our understanding of its regulation of splenic sympathetic outflow. In conscious or anesthetized
rats, glutamate or other excitatory amino acids injected into the PVN generally increases plasma
catecholamine content (Martin and Haywood, 1992) and SNA in spleen (Katafuchi et al.,
1993a,b), brown fat, and other visceral organs (Kannan et al., 1989; Katafuchi et al., 1988;
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Deering and Coote, 2000; Lu et al., 1991; Yoshimatsu et al., 1993). However, amino acids
transmitters can suppress renal SNA activity (Katafuchi et al., 1988; Deering and Coote, 2000;
Lu et al., 1991) or exert mixed effects in cardiac sympathetic nerves (Deering & Coote, 2000).
Directionally opposing responses in SNA in many targets are reported for many experimental
manipulations in the PVN (Morrison, 2001). Additionally, selectivity in frequency-domain
characteristics of sympathetic nerve discharge bursts in different targets occurs after various
manipulating in the PVN (Kenney, 1994; Claassen et al., 1998; Kenney et al., 1999). Kenney et
al. (2003a,b) report that the PVN can elicit multiple response profiles in different target organs.
For example, D,L-homocysteic acid (excitatory amino acid) microinjection induces non-uniform
discharge patterns in sympathetic nerves from different targets (including the spleen), whereas
bicuculline (GABAA blocker) administration promotes uniform frequency characteristics in SNA.
Similarly, central IL-1 can differentially regulate tonic splenic and renal SNA in intact rats (Lu et
al., 2003). These findings suggest anatomically-distinct functional circuits in the PVN regulate
SNS outflow in an organ-specific manner.
Medial Preoptic Area (MPO). Bilaterally lesioning the MPO or microinjecting glutamate
suppresses splenic SNA (Katafuchi et al., 1993a), whereas interferon (IFN)-α microinjection
augments splenic SNA, blockable by prior splenic denervation (Take et al., 1993). Electrolytic
lesioning also increases splenic macrophage activity (Roszman et al., 1982) and splenocytes
number (Cross et al., 1982), but reduces splenic NK activity (Cross et al., 1984) and mitogen-
induce lymphocyte proliferation (Brooks et al., 1982; Roszman et al., 1982). The posterior and
lateral hypothalamus also may be involved in regulating splenic immune activity. Cano et al.
(2001) consistently found PRV-infected neurons in the lateral hypothalamus at early survival
times, suggesting lateral hypothalamic innervation of splenic SPNs or of spinal interneurons that
innervate these SPNs. Electrically stimulating the posterior or lateral hypothalamus suppresses
(Weber and Pert, 1990) or increases (Wenner et al., 1996) splenic NK activity, respectively.
Locus Coeruleus (LC). Cano et al. (2001) consistently found PRV-infected neurons in the
LC (ventral region) and lateral hypothalamus at survival times at the second earliest time point,
suggesting sparse LC innervation of splenic SPNs or of spinal interneurons that innervate these
SPNs. Another report (Sawchenko and Swanson, 1982) showing the LC ventral region
projecting to the spinal cord is consistent with this view. The LC responds to a variety, but not all
types, of stressors. The LC can modulate splenic immune function. Ablating brainstem
noradrenergic neurons, including the LC, by i.p. treatment with N-(2-chloroethyl)-N-ethyl-2-
bromobenzylamine (DSP-4) suppresses splenic immune functions, like LPS-induced splenocyte
proinflammatory cytokine production and the percentage of NK-cells (Engler et al., 2010). This
finding supports the importance of central noradrenergic tone in maintaining normal immune
function. The LC also participates in enhancing behaviorally conditioned recall-mediated splenic
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neutrophil activity (Chao et al., 2005) and the induction phase of a graft-vs.-host reaction in rats
(Furukawa et al., 2004). Finally, LC stimulation by local CRH administration suppresses in vitro
splenic mitogen-induced proliferation and antibody response to T-cell receptor antigens
(Rassnick et al., 1994; Caroleo et al., 1993). Therefore, data supports the LC involvement in
CNS circuitry regulating stress-induced immunosuppression in the spleen.
Edinger-Westphal Nucleus (EW). The EW projects to the periaqueductal gray and some
other brainstem areas, where sympathetic premotor neurons reside (Luppi et al., 1988; Klooster
et al., 1993), indirectly supporting EW regulation of the SNS. EW is a major central source of
urocortin (Vaughan et al., 1995). In the EW, urocortin activation of CRH receptors alters the
stress response (Weninger et al., 2000) and intracerebroventricular (icv) administration of
urocortin induces a sympathetically-driven immunosuppression in the spleen (Okamoto et al.,
1998). The stress-responsive LC and PVN project to the EW, consistent with a role regulatory
role for the EW in autonomic functioning during stress (Weninger et al., 2000).
Dorsal Vagal Complex (DVC). Cano et al. (2001) show anatomical linkage between the
NTS/AP and splenic innervation, since post-PRV inoculation of the spleen, the NTS and the C3
cell group were infected at intermediate intervals, followed by PRV-infected neurons in the AP
and dorsal motor nucleus of the vagus (DMV) boundaries later on. Peripheral nerves convey
immune signals to the NTS, a component of the DVC (Watkins et al., 1995). Similarly, immune
stimuli activate the brain at CVOs like the AP, which express interleukin (IL)-1 receptor-1
(Ericsson et al., 1995) and projects to the NTS (Cunningham et al., 1994), which in turn projects
to PVN and RVLM (Sawchenko and Swanson, 1981). Intravenous injection of
lipopolysaccharide (LPS) (Elmquist and Saper, 1996) or IL-1 (Ericsson et al., 1994) induces Fos
expression in the NTS and AP (with higher doses needed for expression in the latter). Similarly,
intraperitoneal treatment with IL-1 increases c-fos mRNA in the rat NTS and AP (Brady et al.,
1994). However, AP lesions do not alter IL-1-induced Fos expression in NTS or PVN (Ericsson
et al., 1997). Still IL-1β (285 ng/kg, intravenous) in an anesthetized rat increases splenic SNA,
an effect abrogated by high cervical spinal or midbrain transection, indicating that an intact
forebrain is necessary for splenic (and lumbar) sympathoexcitatory responses to IL-1β (Kenney
et al., 2002). In senescent rats, the responsiveness of sympathetic circuits to either hypo- or
hyperthermia are attenuated (Kenney and Fels, 2002; Kenney and Musch, 2004; Helwig et al.,
2006). IL-1β, with or without mild hypothermia or applied in different sequences, produces non-
uniform changes in SNA and SNA-coupling between renal and IBAT nerves or the splenic and
lumbar nerves (Kenney et al., 2002), supporting functional plasticity in the central sympathetic
circuits through complex relationships between immune mediators and central sympathetic
circuits. Collectively, these studies support both the NTS and AP as part of the circuitry involved
in sympathetic control of splenic immune activity, and the transduction of peripheral immune
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signals conveyed by cytokines like IL-1. Injection of IL-1 icv, but not IL-6, enhances splenic
norepinephrine turnover, as well as other organs (e.g., lung, pancreas, but not kidney, liver or
heart) (Terao et al., 1994), suggesting that central IL-1 can act on specific sympathetic circuit to
effect SNA to some organs.
Rostral Ventrolateral Medulla (RVLM). The RVLM is a key CNS region for maintaining
basal SNA and mediating sympathetic nerve responses evoked from supraspinal sites. The
RVLM receives somatic and visceral inputs, including the splenic nerve, suggesting the RVLM is
an integrative center of allostatic responses to internal and external environmental changes
(Ermirio et al., 1993). Sympathetic input is not topographically organized in the RVLM, but the
RVLM still regulates sympathetic excitatory or inhibitory drive (Beluli and Weaver, 1991b).
RVLM neurons non-uniformly regulate the resting discharge in splenic and renal sympathetic
nerves (Hayes and Weaver, 1990; Beluli and Weaver 1991a,b) under the influence of GABA.
GABAA receptor activation suppresses basal and discharge rates in splenic and other visceral
nerves (e.g., cardiac and renal) as well as bursts after activation by a variety of stimuli (e.g.,
hyperthermia, icv IL-1 or IL-6) (Hosking et al., 2009). Applying excitatory amino acids (i.e.,
glycine) or GABAA receptor agonists into the RVLM has a greater effect on renal, gastric,
hepatic, adrenal or lumbar than splenic or intestinal SNA (Meckler and Weaver, 1985, 1988a,b;
Qu et al., 1988; Stein & Weaver, 1988; Yardley et al., 1989; Stein et al., 1989; Beluli and
Weaver 1991; Hayes and Weaver, 1990), effects reducible by spinal cord transection in the
renal, but not the splenic nerve. Similarly, amino acid-activated pontine reticular neurons
tonically and actively drive splenic sympathetic nerves by distinctly different populations of
reticular neurons (Hayes and Weaver, 1992).
The identity of central sites of autonomic regulation provides potential targets for future
pharmacological and physiological studies that increase our understanding of brain-immune
interactions under normal and pathological conditions. Anatomical and functional differences
exist between sympathetic nerves supplying the spleen and non-immune organs. The electrical
activity of both the splenic and renal nerves relate to cardiovascular functions, but the number of
fibers showing cardiac-related firing pattern and the magnitude of the responses of the nerve
activity to the changes in arterial blood pressure are less in the splenic nerve than in the renal
nerve (Meckler and Weaver, 1988a,b). Additionally, the SPNs that regulate the splenic SNA
distribute more uniformly in the lateral spinal gray horn than those activating the renal nerve
(Taylor and Weaver, 1992). Finally, the splenic sympathetic nerve elicits a greater response,
both in terms of latency and frequency, to systemic administration of LPS than the renal nerve
(MacNeil et al., 1996, 1997). Moreover, functional studies like those described for the RVLM
suggest that tonic influences from supraspinal sympathetic regulatory sites distributed in a non-
uniform pattern and differentially affect the outflow sympathetic nerves under basal and
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allostatic load conditions. SNA to the kidney is more dependent upon excitatory drive from the
RVLM than SNA directed to the spleen and intestine. They also suggest discrete circuits that
regulate the outflow to specific sympathetic nerves with complex mechanisms for cross-talk
between these sympathetic circuits at multiple levels of the neuroaxis in order to coordinate an
appropriate stress-mediated sympathetic response across target organs.
Evidence for Parasympathetic Innervation of Lymphoid Organs is Lacking
No creditable neuroanatomical data are available to indicate parasympathetic efferent
nerves in immune organs (Nance et al., 1987; Nance and Sanders, 2007). Definitive markers of
parasympathetic fibers and their terminals have not been demonstrated in any immune organs
(Schäfer et al., 1998; Nance and Sanders, 2007). However, a role for afferent fibers in the
vagus, glossopharyngeal, and other sensory nerves in immune regulation has been
demonstrated (Antonica et al., 1991; Maier et al., 1998; Rosas-Ballina and Tracey, 2009), which
indirectly mediates immune functions after the information these nerves convey to the CNS.
Tracy and colleagues (Rosas-Ballina and Tracey, 2009; Rosas-Ballina et al., 2008; Tracey,
2007) has coined the term “the cholinergic anti-inflammatory reflex” in reference to the pathway
mediating IRs resulting from efferent vagal stimulation. This term, however, is a misnomer on
two accounts. First, they have not determined whether the pathway that mediates the vagal
response meets the criteria for a reflex. Anatomically, the cellular target, and afferent and
efferent limbs of this so-called reflex have not been defined. Similarly, the specific stimulus
initiating the response and the immunological response that result from stimulation also has not
been defined. Secondly, since the major neurotransmitter of postganglionic parasympathetic
neurons is acetylcholine, the term “cholinergic” infers that parasympathetic system functions as
the efferent limb, which is not consistent with neuroanatomical findings. Therefore, the proposal
by Tracey and colleagues that the efferent vagus nerves plays an important role in regulating
local and systemic inflammation by blocking splenic macrophage TNF-α production is clearly an
over interpretation of their data and not supported by the existing literature. They report that
electrically stimulating the cut efferent vagus nerve inhibits endotoxin-induced sepsis, TNF-α
production, and localized inflammation in a dermal air pouch, all dependent on the activation of
cholinergic α7 nicotinic receptors (Wang et al., 2003). No studies have been carried out by this
group to rule out the possibility that their effects could be mediated by the SNS, even though
Nance, Greenberg and colleagues (Brown et al., 1991; Vriend et al., 1993) and others (Yoon et
al., 2006) have reported similar suppression of local and systemic inflammation and splenic
macrophages TNF-α production that are sympathetically-mediated.
More recently, Tracey (2009) has acknowledged the absence of efferent vagal innervation
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of the spleen, proposing instead that the vagal efferents innervate prevertebral sympathetic
postganglionic neurons that distribute to the spleen. However, carefully performed transsynaptic
(Cano et al., 2001) and anterograde (Nance and Burns, 1989) studies are not consistent with
this hypothesis, as they failed to demonstrate a vagal-sympathetic-spleen connection.
Moreover, a recent systematic electrophysiological study (Bratton et al., 2012) has further
substantiated that direct vagal-splenic nerve connection do not exist in rats. Stimulating the
intact vagus nerve in which sensory fibers are present produces an electrophysiological
response in the splenic nerve characterized by inhibition and then excitation. Crushing the
central branch of the vagus nerve (comprised of afferent fibers) ameliorates this response
indicating that the link between the splenic nerve and the vagal stimulation occurs via vagal
afferent input to the CNS, consistent with the lack of efferent vagal innervation of the spleen.
Splanchnic nerve stimulation immediately increases the firing rate in postganglionic cell bodies
that specifically target the spleen, but vagal stimulation has no effect on their firing rates
(Bratton et al., 2012). Finally, they find no contacts between vagal efferent terminals and splenic
sympathetic motor neurons after combined retrograde and anterograde tracing from the spleen
and DMV, respectively (Bratton et al., 2012), consistent with their electrophysiological findings
and previous neuroanatomical studies (Nance and Burns, 1989; Cano et al., 2001). Vida et al.
(2011a) show that vagally-mediated splenic macrophage TNF-α production after LPS treatment
remains intact in α7 knockout mice even after cutting the efferent end of the vagus nerve,
discounting the requirement for α7 nicotinic receptor involvement or efferent vagal motor
neurons. Altered immune regulation reported in α7 knockout mice may reflect defects in the
SNS documented in these mice (i.e., diminished norepinephrine release and adrenergic ligand
sensitivity of target organs), whereas the PaSNS remains normal (Franceschini et al., 2000).
Moreover, they report that direct stimulation of the splenic sympathetic nerve in α7 knockout
mice dramatically suppresses LPS-induced TNF-α to a similar extent as that produced by
afferent vagal stimulation, an effect that is not present in β2-AR knockout mice (Vida et al.,
2011b). Finally, β-AR agonists protect mice from septic shock, long after vagal stimulation and
nicotinic agonists failed to do so (Vita et al., 2011b). Collectively, these findings underscore the
importance of vagal afferents and splenic sympathetic efferents in the regulation of immune
function. Since afferent sensory nerves in other body regions can active splenic SNA to
suppress innate immunity (Sato, 1997), the afferent vagal response is not unique, but rather
shares this function with many other afferent nerves.
Anti-inflammatory effects mediated via efferent vagal stimulation require the spleen, the
splenic nerve and β2-ARs (Vida et al., 2011a, Vida et al., 2011b). Since vagal efferent fibers do
not synapse on splenic postganglionic sympathetic neurons vagal efferent-mediated anti-
inflammatory effects must also be mediated by an indirect pathway with a non-neural
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intermediary. At this time, the non-neural intermediary and the mechanism for splenic nerve
discharges remain unknown. In another animal model system, non-neural intermediary release
of hormones from the gut is mediated by the efferent vagus nerve (Uvnäs-Moberg et al., 1992).
As an example, skeletal (electrical) or cutaneous (thermal or vibratory) stimulation increases gut
release of cholecystokinin (CCK) and gastrin into the circulation via a vagal cholinergically-
mediated mechanism that can be blocked by co-stimulation of the vagal and splanchnic nerves
(the adrenal medullary, gut and splenic sympathetic nerves are present in the splanchnic nerve)
(Uvnäs-Moberg et al., 1992). The vagus nerve-mediated CCK production suppresses
inflammatory cytokine release due to a high-fat meal (Luyer et al., 2005), during endotoxic
shock (Ling et al., 2001) and in the spleen post-LPS treatment (Meng et al., 2002) – all
comparable to that mediated by efferent vagal stimulation in the studies by Tracey group.
Circulating CCK activates intact vagal afferent fibers (Lubbers et al., 2010), as well as, central
autonomic regulatory sites known to be activated by immune stimuli, like LPS (Verbalis et al.,
1991). Additionally, churning of the gut induced by chronic efferent vagus stimulation would
activate alimentary tract and abdominal visceral sensory afferent fibers as well. Sensory
feedback would, in turn, activate descending sympathetic central and peripheral neural
pathways producing β2-AR-mediated anti-inflammatory actions, including blockade of LPS-
induced splenic TNF-α production, anti-septic shock, and TNF-α production in the inflamed air
pouch model (Aoki et al., 2005; Nance et al., 2005; Kwon et al., 2003). Thus, efferent vagal
stimulation can elicit the release of many signaling molecules from gut mucosa and/or gut IS
that activate central sympathetic pathways via the afferent vagus nerves or circulation in the
blood may explain these findings, but remains to be determined. It is clear that the anti-
inflammatory response that results from vagal stimulation is NOT mediated by a direct neural
link with the SNS or the spleen (see Martelli, McKinley & McAllen 2014, Cervi, Lukewich and
Lomax 2014 in this issue of Autonomic Neuroscience).
Sympathetic Nerves Innervate Lymphoid Organs (LOs) and Tissues
Norepinephrine, the major neurotransmitter in sympathetic nerves, fulfills the criteria for
neurotransmission with immunocytes in primary and secondary LOs (SLOs) (reviewed in
Madden et al., 1995), confirmed by a large literature using a variety of in vivo, ex vivo, and in
vitro approaches. In primary and SLOs, noradrenergic sympathetic nerves innervate the
vasculature and parenchymal fields of lymphocytes and associated cells in all mammalian
species examined. These organs include bone marrow, thymus, spleen, lymph nodes (LNs),
and mucosa-associated lymphoid tissues (MALT) – gut (GALT: the appendix, sacculus
rotundus, and Peyer’s patch) and bronchus (BALT: tonsils).
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Noradrenergic varicosities closely appose many immune cell types – lymphocytes,
macrophages, dendritic cells (DCs), eosinophils, mast cells (MCs), thymocytes, thymic epithelial
cells, and BM hematopoietic stem cells (HSCs) and others. In fact, electron microscopic studies
in the splenic white pulp have demonstrated that sympathetic nerve terminals in direct
apposition to T-cells and adjacent to both DCs and B-cells, with the neuroimmune junction being
approximately 6 nm wide (Felten et al., 1985; Felten and Olschowka, 1987) in contrast to a
typical CNS synapse that is approximately 20 nm wide or the 205 nm wide junctions in other
SNS-effector target. In this paper, we refer to these non-synaptic junctions as neuroeffector
junctions, meaning that they are sites where sympathetic motor neurons release
neurotransmitter, predominantly norepinephrine, to affect non-neuronal immune target cells.
Characteristic of these junctions are varicose nerves that release transmitters ‘en passage’ to
act on transmitter-specific receptors expressed on target immune cells with varying distances
from where the transmitter is released. Finally, there are no post-junctional membrane
specializations, but the target cells express abundant receptors for norepinephrine and for other
neurotransmitters released by sympathetic nerves.
The presence of “classical” or the atypical close contact to cells is probably not
prerequisite for neurotransmission in lymphoid tissue. Since there are no physical barriers,
norepinephrine released from noradrenergic nerve terminals diffuses through the parenchyma
establishing a high-to-low gradient from the site of release. This type of release and subsequent
diffuse to target cells is called paracrine release (Lever et al., 1965; Su and Bevan, 1970). This
route of signaling is particularly likely in the spleen where high concentrations of norepinephrine
reach the venous drainage (Brown, 1964). Noradrenergic innervation is regional and specific,
generally distributing to T-cell and plasma cell (PC) compartments, but avoiding nodular regions
and zones where B-cells develop or mature. Neuropeptides, like neuropeptide Y, colocalize
with norepinephrine in sympathetic nerve terminals, can be differentially released based on the
firing patterns and activity of the nerve, and can modulate the functional effects of released
norepinephrine (Felten et al., 1985; Bellinger et al., 2013). Paracrine release suggests that the
juxtaposition of the sympathetic terminal to immunocytes in geographically specific patterns may
be considerably important functionally.
After its release, norepinephrine binds to α- and/or β-ARs on the cell surface of
immunocytes, including antigen-presenting cells (macrophages, DCs), T- and B-cells, HSCs,
thymocytes, granulocytes, and other cells of the IS (Felten et al., 1985; Ricci et al., 1999;
Bellinger et al., 2013). The effects of catecholamines on immune measures may be determined
in part by the available concentration of norepinephrine in an immune cells milieu, for example
excitatory at low and inhibitory at higher norepinephrine, the concentration, the AR subtype,
density, activational state, and the type of G protein coupled to the AR. Generally, β2-ARs are
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predominantly expressed in cells of the IS, and in physiological concentrations their activation
limits the magnitude of acute and chronic inflammation, promotes Th2-driven antibody response
and suppresses cellular IRs (e.g., cytotoxic T-cell, NK-cell, neutrophil, and macrophage activity).
Sympathetic nerve function can modulate both the initiation and effector phases of an IR
(summarized later). Differential effects on Th1-driven antibody responses have been reported
depending on Th1-cell activational state, and their mechanism of activation. Chemical
sympathectomy by selective neurotoxins can have variable effects on antibody response,
depending on where nerve loss occurs after birth (neonatal) or in adult rodents. The drive
towards a Th2 antibody response results from a shift in the balance of cytokine production
towards a Th2 phenotype – that is an increase in IL-4 and IL-10, and a decrease in production
of IFN-γ and IL-12. Catecholamines inhibit TNF-α and IL-1, major proinflammatory cytokines
produced by antigen-presenting cells, and in concert with IL-12 play a central pathogenic role in
Th1-driven autoimmune reactions. In contrast to β2-AR mediated effects, α-agonist treatment is
generally proinflammatory, promoting an increase in TNF-α, IL-1, and a decrease in IL-10 and
IL-6. Therefore, the balance in the activation of α- vs. β-ARs may explain differential effects of
sympathetic activation on immune functions, and mechanistically, a way to either ramp up or
resolve an IR.
Bone Marrow (BM). The BM is the main site of hematopoietic stem cells (HSCs) from
which all blood cells derive. The BM also is integral for inflammation, infection or injury by
increasing production and release of platelets, leukocytes and HSCs, and promoting the acute
phase response. The SNS regulates hematopoietic cell function via direct AR-mediated effects
on hematopoietic progenitor stem cells (HPSCs) and HSCs and indirect actions on stromal and
endosteal preosteoblastic cells (Muthu et al., 2007). Intercellular interactions occur within HSC
niches innervated by sympathetic nerves with the vasculature and BM cells as neuroeffector
targets.
The SNS has important roles in regulating (1) HSC hibernation, self-renewal rate and fate;
(2) HSC pool size; (3) blood cell differentiation and functional changes; and (4) HSC and mature
blood cell ingress/egress (reviewed in Bellinger et al., 2008a). BM cells differentially express
specific α- and β-AR subclasses. Beta2-AR stimulation promotes hematopoiesis for all types of
blood cells (Fig. 2), and enhances the acute phase response after infection or tissue damage,
including neutrophil mobilization. Beta2-AR blockade (β2B) decreases the number of proliferating
HSCs. Chronic SNS activation from systemic infection or traumatic injury reduces HSC/HPSC
numbers and consequently causes anemia (Elhassan et al., 2011; Fonseca et al., 2005),
promoting multiple organ system failure. The mechanisms mediating this response vary
depending on the type of injury/infection (Baranski et al., 2011). Beta2-AR-mediated anti-
proliferative effects are evident in neutrophil, monocyte, and erythrocyte precursors (Dresch et
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al., 1981; Maestroni et al., 1998).
“Figure 2 here”
Alpha-AR activation also regulates hematopoiesis (Fig. 2). In normal mice, or after
syngeneic BM transplantation, α-AR stimulation greatly suppresses granulocyte-monocyte
colony-forming units and platelet production, but increases lymphopoiesis (Maestroni, 1995;
Maestroni and Conti, 1994a,b), effects mediated in part by transforming growth factor-β (TGF-β)
(Maestroni, 1995). Prazosin, an α1-AR antagonist, reduces thymocyte and splenic T- and B-cells
numbers after BM transplantation without affecting the relative proportions of thymocyte subsets
(Maestroni and Conti, 1994a,b). In α1B-AR knockout mice, hematopoietic recovery after
irradiation is impaired; pre-B-cell and erythrocyte regeneration is faster than in wildtype mice
(Maestroni, 2000a). Sympathetic activation can rescue hematopoiesis from sublethal X-ray
irradiation or chemotherapeutic drugs, increasing survival in mice (Dresch et al., 1984; Togni
and Maestroni, 1996). Surviving mice have greater leukocyte and platelets counts and
granulocyte/monocyte colony-forming units in the BM. These effects are mediated by α1- (Togni
and Maestroni, 1996) and β2- (Dresch et al., 1984) ARs. Lipopolysaccharide (LPS)-induced
sympathetic activation induces CD14 expression in macrophages, necessary for efficient LPS
signaling via toll-like receptors (TLRs) (Ebong et al., 2001; Wright et al., 1990) and subsequently
tumor necrosis factor (TNF)-α production (Muthu et al., 2005).
The SNS controls the circadian rhythm of hematopoiesis (Fig. 3). BM norepinephrine and
dopamine concentrations and their metabolites display diurnal rhythms (Maestroni et al., 1998)
with higher SNS activity at night, when cell division and HSC release into the blood is greatest.
Selective sympathetic denervation of the tibia ameliorates circadian progenitor egress pattern
without affecting cell viability (Méndez-Ferrer et al., 2008). Although the mechanisms for
circadian HSC/HPSC egress are not entirely understood, the SNS inhibits nocturnal CXCL12
up-regulation, reducing binds to CXCR4, which inhibit circadian HSC egress (Fig. 3) (Méndez-
Ferrer et al., 2008; Katayama et al., 2006). Stromal cell β3-ARs and osteoblast β2-ARs
participate in this response by exerting opposing actions on CXCL12 expression during the day
and night, respectively (Fig. 3) (Méndez-Ferrer et al., 2008). Beta-AR activation on osteoclast
also stimulates matrix metalloproteinase-9 release, which degrades adhesion molecules that
retain HSCs/HPSCs in the BM, and receptor activator of nuclear factor kappa-B ligand to reduce
CXCL12 expression (Kollet et al., 2006).
“Figure 3 here”
Granulocyte colony-stimulating factor (G-CSF) is required for CXCL12 and CXCR4-
mediated progenitor mobilization (Petit et al., 2002; Tesio et al., 2011). Physical/emotional
distress or injury/infection increases SNA, which induces G-CSF-mediated HSC mobilization,
reduces CXCL12 expression, and triggers specific protease-mediated degradation of adhesion
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molecules (Katayama et al., 2006; Dar et al., 2011). Disrupting sympathetic neurotransmission
in mice (i.e., chemical sympathectomy or β-adrenoceptor block [βB]) markedly impairs G-CSF-
induced HSC mobilization. Additionally, TGF-β release from Schwann cells that surround
sympathetic nerves also retains HSC via TGF-β type II receptor/Smad signal transduction
(Yamazaki et al., 2011). Collectively, these findings have clinical implications. Augmenting SNA
may be beneficial in potentiating G-CSF-induced HSC mobilization for transplantation, attracting
HSCs to HSC niches after transplantation, and developing myeloablative-protective strategies
for patients receiving chemo- and/or radiotherapies. Sympathetically-driven HSC
expansion/mobilization may improve health outcomes for wound healing and preventing anemia
during sepsis and certain cancer treatments. The SNS-mediated-hemopoietic regulation may
provide new target for re-establish homeostatic maintenance of hematopoiesis under a number
of conditions where, βB-induced suppression of HPSC mobilization improves survival, like
traumatic brain injury, burns, noncardiac surgery and trauma – conditions which strongly
activates the SNS (Beiermeister et al., 2010).
Thymus. Sympathetically-mediated effects on thymocytes and thymic non-lymphoid cells
(i.e., thymic epithelial, smooth muscle, and mast cells) occur predominantly by catecholamine
binding predominantly to β2- and α1-ARs (Loveland et al., 1981; Marchetti et al., 1994;
Kavelaars, 2002; Pešić et al., 2009). Additionally, catecholamines can translocate to the nucleus
where they influence transcription processes of steroid receptors and nuclear factor kappa-light-
chain-enhancer of activated B-cells (NF-B) to modulate apoptosis in thymocytes (Bergquist et
al., 1997). Sympathetic nerves are “positioned” in the thymic cortex for regulating key
mechanisms responsible for the (1) ingress of HPSC; (2) thymocyte proliferation and
maturation; (3) T-cell receptor (TCR) gene rearrangement; and (4) positive and negative
selection (reviewed in Leposavić et al., 2006; Bellinger et al., 2008a). Sympathetic nerves
distribute to vascular and cellular compartments in the thymus including thymocytes, with
particularly high density of fibers at the corticomedullary junction (Fig. 4A). Additionally,
noradrenergic nerves course along the thymic vasculature, where they may regulate HPSC
ingress and thymocyte egress. These nerves also closely appose MCs, macrophages,
eosinophils, fibroblast and other accessory cells (Novotny et al., 1990; Vizi et al., 1995). The
functional role of the SNS for these cell populations in the thymus is not clear. It is presumed
that norepinephrine ligation of α- and/or β-ARs expressed on multiple cell types in the thymus
participates in regulating T-cell development, with the AR subclasses exerting differential
influences on T-cell development. “Figure 4A-C here”
Remodeling of the thymus and its nerve supply occur under certain physiological
conditions (i.e., stress, immune activation, infection, pregnancy, and aging) causing thymic
involution largely due to increased activation of the HPA axis and the SNS (Leposavić and
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Perisić, 2008; Jondal et al., 2004). Age-related decline in the secretion of hormones that
promote growth is also associated with thymic involution (Patel and Taub, 2009). With thymic
involution, the density of noradrenergic nerves increase as thymic volume shrinks without
altering the normal nerve distribution, nerve volume, or norepinephrine content (Bellinger et al.,
1988; ThyagaRajan et al., 2011). Normal or increased SNA in a smaller thymic volume will
increase norepinephrine availability for target cells with consequences for AR regulation in
target cells. Similarly, antigen challenge causes greater thymocyte apoptosis and/or egress to
SLOs, which consequently increases nerve density (Novotny and Hsu, 1993). Maintained
sympathetic nerve integrity indicates that neuroprotective support is not compromised under all
of these conditions. Interestingly, sympathetic innervation of the rat thymus declines
concomitant with reduce thymic cortical mass as pregnancy proceeds, but recovers to pre-
pregnancy conditions at postpartum (Kendall et al., 1994).
The SNS is not required for development of normal T cells, and most studies support a
suppressive role of the SNS in this process. Mice deficient in dopamine-β-hydroxylase (DBH),
which converts dopamine to norepinephrine, have no intrinsic developmental or functional
defects in immunity (Alaniz et al., 1999). Moreover, sympathectomy permits thymocyte
development in fetal nude mice, and in fetal nonlymphoid thymic rudiments transplanted into a
sympathetically denervated eye (Singh, 1985a,b). Still, housing DBH-deficient mice in normal
vivarium conditions compared with a pathogen-free environment greatly increases mortality.
The increased mortality is associated with marked thymic involution, reduced frequency of
immature CD4+CD8+ (double positive, DP) thymocytes (Alaniz et al., 1999), a cell population
most affected by stress and rapidly turns over (Kroemer, 1995). After primary infection with
Listeria monocytogenes, DBH―/― mice housed under pathogen-free conditions have greater
liver and splenic bacterial load and higher mortality 5-8 days post-challenge than control mice
(Alaniz et al., 1999). Re-challenge with bacteria 3 weeks later leads to higher bacterial burden in
DBH―/― mice, without affecting mortality. Innate immunity limits bacterial growth the first 3 days
post-infection. Then, antigen-specific, T-cell-mediated immunity eliminates active infection and
protects against secondary infection, in part by producing cytokines, like IFN-γ and TNF-α.
Therefore, resistance to bacterial infection is reduced in DBH―/― mice, suggesting defective T-
cell responses to infection. Consistently, ex vivo proliferation and cytokine production is reduced
in anti-CD3-stimulated splenocytes from mice with primary and secondary infections.
Suppressed Th1-specific immune responses in DBH―/― mice exposed to Mycobacterium
tuberculosis or TNP-keyhole limpet hemocyanin are consistent with these findings; further
supporting impaired T-cell responses.
The SNS has a regulatory role in thymic seeding of CD4―CD8― (double negative, DN)
HPSCs (Singh, 1985b), and maintaining thymic cellularity by modulating the balance between
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apoptosis, proliferation and egress (Singh 1985a,b; Alaniz et al., 1999; Leposavić et al., 2006,
2010). Support for sympathetic modulation of thymocyte development is mostly from ex vivo
studies and flow cytometric staining for cell surface markers of T-cell differentiation after
manipulations that prevent AR ligand binding, either by in vivo treatment with α- or β-blockers or
chemical or surgical sympathectomy in rodents (Leposavić et al., 2000, 2006; Madden and
Felten, 2001; Rauski et al., 2003; Pešić et al., 2007). The α1-blocker used for some of these
studies also has central effects on sympathetic outflow, so future work needs to verify a1-
mediated effects on thymocyte development.
Before discussing SNS effects further, a brief review of thymocyte development is
warranted. After entering the thymic medulla, BM-derived HPSCs migrate to the outer cortex
seeding the thymus with thymocytes that subsequently migrate toward the medulla as they
mature. Most die by apoptosis, but those that survive acquire TCRαβ, CD4, CD8 and other cell
surface markers; these processes are tightly regulated by the local cortical milieu. The TCRαβ
allows T-cells to recognize short peptides presented by antigen-presenting cells (APCs),
whereas CD4 and CD8 define T-cells as Th- or cytotoxic T (Tc)-cells, respectively, and serve
distinct functions in these T-cell populations. In the cortex, the β chain of the TCR is rearranged
by gene recombination allowing for β chain selection (Fig. 5A). The TCR must retain structural
properties that permit cell surface expression with pre-TCRα or the thymocyte is eliminated by
apoptosis. TCRβ chain rearrangement produces a T-cell repertoire that recognizes an
impressively diverse array of peptides, which is critical for host defense.
“Figure 5A-B here”
After β chain rearrangement/selection, thymocytes increase CD4 and CD8 co-expression
(double-positive cells, DP) (Fig. 5A), and then undergo TCRα chain rearrangement at or near the
corticomedullary junction. CD4 and CD8 are co-receptors needed for TCR binding to major
histocompatibility complex (MHC) in the next major developmental stage, positive selection. DP
TCRαβ+ thymocytes must highly express TCRs that are capable of binding to MHC to pass
positive selection and survive. Next, surviving cells mature into CD8+ or CD4+ (single-positive
(SP) thymocytes by down-regulating both co-receptors and then up-regulating the appropriate
co-receptors for the MHC class that bind its TCR (i.e., CD8 or CD4 for MHC class I or II,
respectively). Finally, autoreactive thymocytes (those with high affinity TCR binding to self-
peptides presented by MHC I or II) are eliminated in the medulla by negative selection (Fig. 5A).
Medullary epithelial cells express multiple peripheral self-antigens, aiding in negative selection.
At this stage, some thymocytes, usually those with intermediate affinity for self-peptides are
selected to become Treg-cells (thymocytes expressing CD25+RT6.1+ with CD4 or CD8). The
SNS affects thymocyte develop and differentiation in part by modulating the expression of these
cell surface proteins on thymocytes.
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Figure 5A illustrates the effects of α- and β-AR stimulation (upper and lower panels,
respectively) on thymocyte differentiation. In the cortex, α-ARs exert their greatest influence on
TCRβ rearrangement, increasing the frequency of DP TCRαβ cells without affecting DN TCR―
cell frequency (Fig. 5A, upper panel, cortex) (Rauski, et al., 2003; Pešić et al., 2009;
Leposavić et al., 2010). In the medulla, α-AR activation increases CD8+ thymocyte and reduces
CD4+ and Treg thymocyte frequencies (Fig. 5A, upper panel, medulla). Irrespective of
thymocyte phenotype, α-AR activation generally suppresses thymocyte proliferation and
reduces thymocyte yield (Plećaš-Solarović et al., 2005; Pešić et al., 2009; Leposavić et al.,
2010). Interestingly, α1-AR expression is reduced with thymocyte maturation, supporting early
effects on thymocyte development. In contrast, β2-AR stimulation (Fig. 5A; lower panel)
negatively influences positive and negative selection (Leposavić et al., 2006; Pešić et al., 2007;
Madden and Felten, 2001; Pešić et al., 2007). Activation of the β2-AR-cAMP-PKA pathway (Fig.
5B) suppresses the expression of thymus leukemia and Thy-1, antigens that are associated
with thymocyte differentiation (Wajeman-Chao et al., 1998; LaJevic et al., 2010). This inhibitory
effect is mediated by reduced Thy-1 mRNA stability (LaJevic et al., 2010). The down-regulation
of Thy-1 is involved in TCR-dependent selection (Wajeman-Chao et al., 1998; Hueber et al.,
1997; Killeen, 1997). βB can raise Thy-1 expression in DP TCRαβlow/high thymocytes
approximately 2-fold (Leposavić et al., 2006; Pešić et al., 2007), consistent with norepinephrine
or cAMP reducing Thy-1 mRNA in S49 murine thymoma cells (Wajeman-Chao et al., 1998;
LaJevic et al., 2010;). Thymocytes from Thy-1―/― mice exhibit greater TCR signaling,
exaggerated negative selection, and lower de novo production of mature SP T-cells (Hueber et
al., 1997). Finally, some β-AR-mediated effects on thymocyte development may be indirect as
β-AR-ligand binding in LPS-activated thymic epithelial cells increases their cytokine production
(i.e., IL-6 and TNF-α production) (von Patay et al., 1998). Since these cytokines, particularly IL-
6, are important regulators of thymocyte development, this may be an important indirect route
for “fine tuning” CD4+ and CD8+ T-cell maturation under homeostatic conditions and with
immune activation.
Collectively, these data indicate that α- and β-ARs generally induce tonic inhibition of
thymocyte development through differential mechanisms (Plećaš-Solarović et al., 2004, 2005;
Leposavić et al., 2006; Pešić et al., 2009). Activation of β2-AR reduces thymocyte proliferation,
the efficiency of positive selection, and negative selection in part by their influence on Thy-1
expression. Importantly, α1B-ARs stimulation enhances DP TCRαβlow/high frequency and skews
thymocyte lineage commitment towards greater CD8+ and lower CD4+ thymocyte frequency.
Secondary LOs (SLO) and Tissues. All SLOs (spleen (Fig. 4B), LNs (Fig. 4C)) and
MALT) receive sympathetic innervation (Stevens-Felten and Bellinger, 1997; Bellinger et al.,
1992b; Felten et al., 1984). Sympathetic neural relationships with specific types of cellular
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compartments are similar across LOs and tissues. In these immune sites, noradrenergic nerves
travel with the local vasculature and associated connective tissue. From these neurovascular
plexuses, noradrenergic nerves enter into the surrounding lymphoid parenchyma where they
form neuroeffector junctions with cells of the IS. Species/strain differences exist in sympathetic
innervation of SLOs, and innervation can be affected by changes in certain physiological or
pathological conditions. For example, in the spleen, the extent of parenchymal nerves in the red
pulp, and the density of noradrenergic nerves in the white pulp can change under certain
physiological and pathological conditions. With advancing age, noradrenergic nerves in the
white pulp are lost (Bellinger et al., 1987), and increased in the red pulp in autoimmune arthritis
(Lorton et al., 2005, 2009). Similar to the spleen, nerve density in T-cells zones is influenced by
aging (Bellinger et al., 1992a; Bellinger et al., 1992b), LN location (Li and Novotny, 2001),
available neurotrophic support (Carlson et al., 1995), the activational state of the IS (Novotny et
al., 1994), and by diseases, particularly autoimmune disorders (del Rey et al., 2006; Lorton et
al., 1997, 2005, 2009).
GALT and BALT are innervated by sympathetic nerves (Felten, et al., 1981; Felten et al.,
1985; Straub et al., 2008; Elenkov et al., 2000; Bellinger et al., 2008a; Nohr and Weihe, 1991).
Like SLOs, sympathetic nerves extend from vascular beds in the gut and bronchial walls into
lymphoid compartments that make up GALT and BALT, respectively. In GALT, these nerves
enter into T-cell-dependent zones, course among enterochromaffin cells, and interdomally-
located plasma cells. Sympathetic nerves also supply the appendix, Peyer’s patches and
lymphoid tissue in the distal ileal “sac” that transitions into the colon (Felten et al., 1981; Felten
et al., 1985). In both GALT and BALT, sympathetic nerves avoid the germinal center of lymph
nodules, where B-cells reside. In BALT, noradrenergic nerves predominate around the lymphoid
epithelium associated with smooth muscle and the vasculature, and commonly extend into the
periphery of the follicle (Nohr and Weihe, 1991; Russo et al., 2009; Ueyama et al., 1990;
Sirot'áková et al., 2002). Innervation of the tonsil is similar to BALT (Felten et al., 1985).
Noradrenergic nerves reside near MCs and ED1+-macrophages, supporting their regulation of
these cell types. Generally, noradrenergic nerves are assumed to be sympathetic, but their
origin is not known; however, in the palatine tonsils they arise from cranial cervical and
cervicothoracic ganglia.
Tertiary Lymphoid Tissues (TLT)
The development of tertiary lymphoid tissues (TLTs) has recently been reviewed by Neyt
and colleagues (2012). With chronic inflammation, TLT or TLOs can be induced in inflamed
tissue or organs. TLT has a similar organization and structural features to lymphoid follicles in
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SLOs, including postcapillary high endothelial venules (Neyt et al., 2012), which are specialized
by cuboidal endothelial cells that permit circulating lymphocyte to move into the lymph nodes for
immune surveillance. Infiltrated cells organize into structured lesions or distinct T- and B-cell-
rich aggregates associated with follicular reticular cells or follicular dendritic cells (FDCs),
respectively (Link et al., 2011; Fletcher et al., 2010). TLO induction can occur by LT-inducer
(LTi) cells, Th-cells, or Th17-cells. In both SLO and TLO genesis, retinoic acid signals LTi or
immune cells via the retinoic-acid receptor-related orphan receptor γT (ROR)γT (Eberl et al.,
2004; Moyron-Quiroz et al., 2004; Rangel-Moreno et al., 2011; Lochner et al., 2011), which is
critical for their interaction with stromal LT organizer (LTo) cells and lymphotoxin α1β2 signaling
via lymphotoxin-β receptors. This interaction induces LTo cells to express several adhesion
molecules (vascular endothelial growth factor-1, intercellular adhesion molecule-1 (ICAM-1),
mucosal addressin cell adhesion molecule (MADCAM1) and homeostatic CCL19, CCL21, and
CXCL13. CCL19 and CCL21 acting via CCR7 attract and organize T-cells and DCs, whereas
CXCL13 attracts and organize B cells in a FDC network, so these chemokines drive the
infiltration of lymphocytes (Rangel-Moreno et al., 2007; Wengner et al., 2007; Moyron-Quiroz et
al., 2004; McDonald et al., 2005). An increase in lymphangiogenic growth factors, vascular
endothelial growth factor (VEGF)-C, VEGF-D, fibroblast growth factor-2 and hepatocyte growth
factor induce the formation of lymphatic vessels (Vondenhoff et al., 2009; Peduto et al., 2009).
LTo cells develop into FDCs and follicular reticular cells, which provide the framework on which
T- and B-cells migrate and interact with each other. Fibroblasts production of CXCL13 attracts
LTi cells causing their aggregation. Signals derived from the neurons are proposed to provide
the initial signal for LN development (van de Pavert et al., 2009; Veiga-Fernandes et al., 2007),
and possibly TLT (Rangel-Moreno et al., 2007; Wengner et al., 2007; Moyron-Quiroz et al.,
2004; McDonald et al., 2005). Specifically, both SLO and TLO appear to be controlled by
retinoic acid-mediated expression of CXCL13 and peripheral nerves may be the source of
retinoic acid (RA). Neurons contain an enzyme, retinaldehyde dehydrogenase 2, necessary for
retinoic acid synthesis, and Schwann cells produce RA. RA production increases with peripheral
nerve injury, which occurs with chronic inflammation. Despite these findings no studies have
examined the distribution of sympathetic nerves in TLTs or the role of the SNS in their
development. However, since much of the BALT and GALT are formed after birth in response to
external triggers, many investigators consider them to be at least in part TLT (Pabst and
Tschernig, 2010). Thus, the sympathetic nerve supply to BALT described above supports the
likelihood that TLT receives sympathetic nerve supply with a compartmentation similar to that
described for SLOs.
SNS Regulation of Resident and Infiltrating Inflammatory Innate Immune Cells
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Generally, cells of the innate IS express both α- and β-ARs. Alpha-AR expression of in
monocytes/macrophages may be induced after homing to tissues, since it is reported that
peripheral blood mononuclear cells (PBMCs) are α-AR-negative (Casale and Kaliner, 1984).
Dibutyryl cAMP up-regulates α1B-ARs in PBMCs by activating protein kinase A (PKA) (Rouppe
van der Voort et al., 1999, 2000b). Similarly, TNF-α or IL-1β can induce α1A-AR expression in
PBMCs (Heijnen et al., 2002). Catecholamines primarily increase circulating NK-cell and
granulocyte numbers in response to stressors, leaving T- and B-cell numbers remain relatively
unaffected (Ottaway and Husband, 1992, 1994; Benschop et al., 1996).
Stimulation of α1-AR exert effector cell in innate immune cells that are usually directionally
opposite to that seen with β2-AR stimulation, the former augmenting and the latter suppressing
effector functions. The effects of α- or β-AR stimulation on targeted immune cells are
summarized below and in Figure 6. Under homeostatic conditions, β-AR-mediated signals
usually predominates; however, when homeostatic conditions cannot be maintained an adaptive
or maladaptive response can occur whereby there is a shift towards a predominant α-AR
response. Under these conditions, β-AR may uncouple to Gs-cAMP-PKA pathway and can
promiscuously switch coupling to other G-protein signaling cascades (Daaka et al., 1997)
The roles of β2-AR in regulating resident and infiltrating inflammatory cells have been
largely studied for their role in the pathophysiology of respiratory diseases, particularly chronic
and exercise-induced asthma, for which β2-AR drugs are used therapeutically. Beta2-AR
agonists, both short (~ 4 h, albuterol) and, particularly, long acting agonists (8 and more than 20
h, formoterol and salmeterol, respectively), exert both direct and indirect inhibitory effects on
inflammatory cell activation, release of pro-inflammatory mediators, and cell survival, expansion
and recruitment. In many cases, inflammatory cell responsiveness/sensitivity to β2-agonists is
determined by β2-AR density and efficiency of coupling to Gs proteins.
“Figure 6 here”
Granulocytes. Granulocytes or polymorphonuclear leukocytes are white blood cells with
varying nuclear shapes that are loaded with secretory granules, including neutrophils,
eosinophils and basophils. Basophils have many similarities with MCs, which reside in tissues,
and will be discussed below with MCs. Cell components of innate immunity, neutrophils provide
first line host defense against invading bacteria by phagocytosis, release of cytotoxic mediators
and products that activate monocytes/macrophages promote phagocytosis and the formation of
oxidative free radicals for intracellular killing.
Granulocytes mobilize rapidly to sites of injury or infection. The limited available studies
indicates an inhibitory role of β2-ARs stimulation on neutrophil interactions with endothelial cells
(ECs) (i.e., rolling, adhesion, trans-endothelial migration) (Sato, 2004) and neutrophil function
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(Fig. 6). Activation of β2-ARs (in the nanomolar range for catecholamines) suppresses
neutrophil chemotaxis to number of chemotactic factors (i.e., C5a, leukotriene B4, and formyl-
methionyl-leucyl-phenylalanine) (Harvath et al., 1991), and suppresses phagocytosis (at higher
catecholamine concentrations ~10-5 M) (Zurier et al., 1974), lysosomal enzymes release (Zurier
et al., 1974), and respiratory bursts that is linked to degranulation (Nielson, 1987; Weiss et al.,
1996; Barnett et al., 1997). Catecholamines lower the rate of superoxide formation and shorten
the time of release (Gibson-Berry et al., 1993). Similar effects of β-agonists on eosinophils are
reported (Koyama et al., 1999). Intracellular cAMP (cAMPi) regulation of neutrophil function is
complex, involving cAMP extrusion and rate of cAMP metabolism, and its compartmentalization,
timing and magnitude of rise in concentration (Harvath et al., 1991). Data is limited for α1-AR-
mediated actions in neutrophils. Alpha-AR agonists induce granulocytes release (particularly
neutrophils) into the circulation from the marginal pool, lungs and BM (Fig. 6) (Benschop et al.,
1996). Pre-treatment with an α-AR antagonist or reserpine to deplete catecholamines reduces
LPS-induced neutrophilia, suggesting an α1-AR-mediated effect (Altenburg et al., 1997), which
may be either direct and/or indirect.
Salbutamol (β2-AR agonists) inhibits chemotactic responses of human eosinophils (Fig. 6),
without affect their synthesis of platelet-activating factor or leukotriene C4 after activation (Tool
et al., 1996). Consistent with this study, β-AR stimulation reduces chemotaxis and arylsufatase
release in guinea pig eosinophil after ingestion of Candida albicans (C. albicans) and marked it
for degradation, a process called opsonization (Masuyama & Ishikawa 1985). Arylsulfatase is
an enzyme that breaks down leukotrienes, a family of eicosanoid inflammatory mediators,
supporting an anti-inflammatory role for β2-AR activation. In contrast, α-agonist treatment
suppresses opsonized C. albicans phagocytosis and O2‒ production (Fig. 6). These findings
suggest that specific subtypes of ARs differentially regulate eosinophil functions.
MCs. MCs and basophils are major players in inflammation, particularly allergy and
anaphylaxis, but also serve protective functions against pathogens and in wound healing. MCs
and basophils are loaded with granules that contain histamine- and heparin, the main mediators
of allergic and anaphylactic responses, as well as other bioactive mediators that prevent blood
coagulation and promote inflammation. MC and basophils degranulation occurs after IgE that
has bound antigen binds to their cell surface.
Mast cells and basophils express β2-ARs, and their activation may be effective in treating
allergen-induced late-phase reaction or inhibit immediate-phase reaction (Yamaguchi et al.,
1995). Procaterol, a selective β2-agonist dose-dependently suppresses IL-8 and complement-
induced basophil migration, with 10-7 M reducing migration by these factors by 30%. In vitro
studies demonstrate that β2-agonists can completely inhibit histamine release from activated
MCs (formoterol>salmeterol>isoproterenol>albuterol) (Fig. 6) (Butchers et al., 1991), as well as
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other inflammatory mediators. Similarly, salmeterol, a long-acting β2-agonist, prevents IgE-
mediated histamine release from human basophils (Kleine-Tebbe et al., 1994), although β2-ARs
are not as effective in blocking this response as it is in MCs (Morita and Miyamoto, 1987;
Chazan et al., 1991). Agonist inhibition of MC mediator release is prevented by propranolol, a β-
AR antagonist, supporting β2-AR-mediated agonist effects (Johnson, 2002). Important for
clinical use, β2-agonists completely inhibit MC degranulation in the same concentration range as
that for relaxing respiratory smooth muscle (Butchers et al., 1991). Less consistent results
regarding β2-AR regulation of MC histamine release is reported from in vivo studies (Howarth et
al., 1982; Boulet et al., 1997; Proud et al., 1998). This is largely because MCs are more
susceptible to β2-AR desensitization (histamine>prostaglandin D2 (PGD2) or leukotrienes) than
bronchial smooth muscle (Chong et al., 1995, 2003; Chong and Peachell, 1999; Tsuji et al.,
2004); mediator-dependent differences in desensitization reflect differential β-AR-Gs-protein
uncoupling (Tsuji et al., 2004; McGraw and Liggett, 1997).
Beta2-AR-mediated MC degranulation occurs by the cAMP-PKA pathway, and to a less
extent β2-AR-mediated closing of Ca2+-activated K+ channels (iKCa1) (Duffy et al., 2005). No
mRNA or radioligand bindings data exists for α-ARs on MCs, and functional studies for α-ARs in
MCs are scarce. In a murine MC line, phentolamine, an α1-antagonist augments histamine
release that is blockable by phentolamine preincubation, suggesting α-AR down-regulation (Fig.
6) (Moroni et al., 1977). Additionally, prazosin (α1-antagonist) given during heart ischemia-
reperfusion injury prevents the decrease in MC peroxidase induced by norepinephrine-
preconditioning in a rat model that affords cardioprotection (Parikh and Singh, 1999).
Natural Killer (NK)-Cells. NK-cells are cytotoxic lymphocytes critical for both innate and
adaptive immunity. As part of the innate response, they are first responders to attack virally-
infected, stressed (apoptotic), and tumor cells. In adaptive immunity, NK-cells play a role in
developing antigen-specific immunological memory important for a rapid response to a
secondary infection with the same antigen. NK-cells are among the most responsive to the
suppressive effect of stress and catecholamines (Fig. 6) (Elenkov et al., 2000; Irwin, 1994).
Catecholamines have bimodal effects on NK-cells that are time-dependent relative to cell
activation. Infusion of norepinephrine or epinephrine, acute psychological stress or physical
exercise transiently increases circulating NK-cells by 400-600% (Benschop et al., 1996;
Schedlowski et al., 1996). NK-cells are mobilized predominantly from the spleen, but also the
marginating pool that binds to blood vessels (see Fig. 7) (Schedlowski et al., 1996). Acutely,
catecholamines reduce NK-EC interactions, an effect mediated by β-AR on NK-cells, and
independent of soluble ICAM-1, soluble E-selectin, or soluble vascular cell adhesion molecule-1
expression, but dependent on Ca2+/Mg2+ (Benschop et al., 1997), although dynamic exercise
can cause β-AR-mediated shedding of ICAM-1 (Rehman et al., 1997).
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“Figure 7 here”
In contrast, chronic stress or one-week treatment with β2-AR agonists suppresses
circulating NK-cells (as well as Tc-cells). Catecholamines or β2-agonists via β2-ARs potently
inhibit NK-cell activity (Shakhar and Ben-Eliyahu, 1998), Beta2-AR stimulation also act indirectly
on NK-cell function by inhibiting IL-12, IFN-γ or IFN-α production, cytokines essential for NK-cell
activity (Helstrand and Hermodsson, 1989; Elenkov et al., 1996; Hilbert et al., 2013). In mice,
two-hour restraint stress reduces NK-cell numbers in the lungs and blood, but not the spleen,
effects reversed by βB (Kanemi et al., 2005).
Directly stimulating the rat splenic nerve reduces NK activity, as well as Tc-cells and
activated macrophages, effects inhibited by βB (Katafuchi et al., 1993). The reduced NK activity
results from suppressed production of perforin, granzyme B and IFN- (Dokur et al., 2004).
Central administration of proinflammatory cytokines (i.e., IFN-, IFN-α, or IL-1β) or LPS
stimulate splenic SNA to suppress NK activity (Sundar et al., 1989; Katafuchi et al., 1993).
Catecholamines, β-AR ligands, and psychosocial stressors, like social disruption (SDR), inhibit
the effector functions of NK-cells against NK-sensitive tumor cells (Hellstrand and Hermodsson,
1989; Shakhar and Ben-Eliyahu, 1998; Whalen and Bankhurst, 1990). In C57Bl/6 mice, SDR
without immune stimulation changes cell surface expression on lung and splenic NK-cells
(reduced NKG2a and Ly49a, and increased CD16 and CD69) (Tarr et al., 2012) support SDR-
induced NK-cell activation. Moreover, greater CD107a expression on, and increased killing
activity and IFN-γ secretion by IgG/IL-2-costimuated splenic NK-cells suggests that SDR primes
NK-cells. SDR-induced priming of NK-cells is prevented by βB, supporting SNS-mediated
protection against non-specific cell-mediated and anti-viral/tumor immunity. NK-cells also
express A1- and A2-AR mRNAs; while less studied, their stimulation increases NK-cell
cytotoxicity by signaling via PLC- and PKA-mediated pathways, respectively (Xiao et al., 2010).
Antigen-Presenting Cells (APCs). Dendritic cells (DCs), macrophages, and B-cells can
present antigens to T-cells. DCs and macrophages phagocytize pathogens and cellular debris,
secrete immune cytokine/chemokines to orchestrate an appropriate IR, and initiate an adaptive
response by antigens presentation to T-cells via “immunological synapses” (Fig. 8. The
“immunological synapse occurs via MHC-antigen complex binding to T-cell receptor (TCR)
(signal 1) (Fig. 8A) combined with binding of CD28 on antigen-specific T-cell clones to B7
(CD80 or CD86) expressed on macrophages (or DCs) (signal 2) (Fig. 8B). Both signals are
required for T-cell activation. Antigen presentation in the absence of signal 2 (Fig. 8C) results in
T-cell anergy or apoptosis, or cytotoxic T-lymphocyte antigen 4 or (CTLA-4; also called CD154)
binding with CD40 “turns-off” the response. B cells also present antigen bound to cell surface
immunoglobulins via an “immunological synapse”, but without prior antigens processing
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(discussed below). Two subtypes of DCs have been described based on differences in the
secretion of cytokines. The predominant myeloid DCs express TLR2 and TLR4 and secrete IL-
12, whereas plasmacytoid DCs express TLR7 and TLR9 and secrete IFN-α, suggesting
differential functions in their IR to bacteria, and viruses or stressed cells, respectively. B-cells
can also present antigen to T-cells , which will be discussed below with the other lymphocyte
populations. Here, we review the available data for sympathetic regulation of
monocytes/macrophages and DCs via both α- and β-ARs.
““Figure 8 here”
Monocyte/Macrophages. Beta2-AR stimulation suppresses LPS-stimulated PBMN
expression of surface markers involved in regulating Th-cell differentiation and Th-cell balance,
including I-CAM1, CD40, and CD14 (Kuroki et al., 2004). Similarly, a1-AR antagonists can
induce IL-18 production, ICAM and CD40 in human monocytes (Takahashi et al., 2005).
Catecholamines and β-agonists also suppress LPS-induced TNF- production in monocytes,
macrophages and astrocytes (Fig. 6) (Hetier et al., 1991; Severn et al., 1992; van der Poll et al.,
1994; Nakamura et al., 1998). Similarly, IgE-activated MC TNF- secretion is blocked by β-
agonists (Bissonnette and Befus, 1997), whereas IL-10 production is enhanced (van der Poll
and Lowry, 1997). Since TNF- stimulates IL-1β production, catecholamines indirectly
suppress IL-1β (Koff et al., 1986). Catecholamines strongly suppress IL-12 production in
monocytes, LPS-activated macrophages, and DCs via β2-AR stimulation (Elenkov et al., 1996;
Panina-Bordignon et al., 1997; Haskó et al., 1998). IL-12 inhibition is critically involved in the
differentiation of CD4+ Th-cells (Panina-Bordignon et al., 1997) (see section on Th-cells below).
Catecholamines and β2-agonists also up-regulate IL-6 secretion by thymic epithelial-cells, brown
adipocytes, astrocytes and microglia (von Patay et al., 1998; Burýsek and Houstek, 1997;
Maimone et al., 1993; Norris and Benveniste, 1993). All of these effects are mediated via β2-AR
activation of the cAMP-PKA signaling pathway (Elenkov et al., 1996; van der Poll et al., 1996;
Suberville et al., 1996; Siegmund et al., 1998), except in brown adipocytes where cAMP-PKA
signaling is activated via β3-ARs.
Catecholamines suppress IFN-γ-mediated macrophage killing of melanoma cells or herpes
simplex virus-infected cells (Koff and Dunegan, 1985; 1986). Conversely, catecholamines
augment the anti-mycobacterial activity, an effect mediated via α2-ARs (Miles et al., 1996).
Stimulation by an α-agonist also increases phagocytosis by murine peritoneal macrophages in
vitro (Javierre et al., 1975). Similarly, α2-AR-stimulated peritoneal macrophages induce TNF-α
secretion in mice (Spengler et al., 1990, 1994; Szelényi et al., 2000). However, another study
reports that treating human-derived monocytes with epinephrine inhibits Mycobacterium avium
killing (Bermudez et al., 1990). With β2-AR stimulation or increased cAMPi, macrophage TNF-α,
IL-1, and IL-10 production is suppressed (Koff et al., 1986; Chou et al., 1996; Suberville et al.,
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1996; Haskó et al., 1998; Németh et al., 1997). Several mechanism account for discrepancies in
catecholamines effects on macrophages. Beta2-ARs inhibit, and α1-ARs potentiate, macrophage
effector functions. Beta2-ARs on naïve monocytes, down-regulate with continued stimulation
and/or coupling to Gi protein signaling, all of which mitigates β-AR-mediated inhibition.
Moreover, monocytes acquire α-ARs after entry from the blood into tissues, and expression
increases with activation, and causes an increase in α1B- and α1D-AR, and extracellular signal-
regulated kinase (ERK)2 expression (Herman et al., 1990; Rouppe van der Voort et al., 1999;
2000a,b, Kavelaars, 2002).
Dendritic Cells (DC). DCs express both α- and β-ARs that mediate SNS effects on DC
functions and their interactions with T-cells, which are critical for shaping the adaptive response
after antigen challenge (Maestroni, 2005, 2006; Manni and Maestroni, 2008; Kim and Jones,
2010; Yanagawa et al. 2010; Manni et al., 2011; Hilbert et al., 2013) (Fig. 6). Seiffert et al.
(2002) report that catecholamines inhibit the capacity of epidermal DCs to present antigen in
vitro. Consistent with this finding, intradermally injection of a catecholamine inhibits contact
hypersensitivity responses induced by epicutaneously administered haptens. Although
sympathetic nerves do not innervate the epidermis, there is no barrier to prevent norepinephrine
diffusion into the epidermis. Immature (iDCs), but not mature DCs (mDCs), express α1B-AR
mRNA, which contributes to their differential responses to adrenergic ligands. Available studies
generally use contact sensitivity models for in vivo studies, and rodent BM-derived DCs
(BMDCs) or human peripheral blood cells stimulated to drive DC development/maturation and
enriched for DCs. Collectively, these studies support a role for β2-ARs in DCs driving Th2- and
Th17- and suppressing Th1-type IRs by cytokine production. Beta-AR-cAMP signaling in CD40-
stimulated DCs suppresses IL-12 secretion, leading to suppressed Th1- and augmented Th2-
cell differentiation (Panina-Bordignon et al., 1997). Similarly, Goyarts et al. (2008) reports that
β2-agonists inhibit TNF-α, IL-6, IL-12p40, and IL-23 production in human DCs stimulated with
LPS. In TLR9- and TLR4-activated human plasmacytoid DCs, β2-agonists inhibit the secretion
of IFN-α and TNF secretion, respectively (Hilbert et al., 2013). Short-term exposure of LPS- or
keyhole limpet hemocyanin-activated BMDCs to norepinephrine at the start of culture attenuates
IL-12 and elevates IL-10 production, a response dependent on α2- and β-ARs and CD40 ligation
(Maestroni, 2002). Treating mice with norepinephrine-exposed DCs polarizes the IR toward a
Th2 profile. Consistent with these in vitro studies, Maestroni (2005, 2006) find a similar cytokine
response profile from skin DCs in a mouse contact sensitivity model. Similarly, β2-AR
stimulation reduces LPS-induced TNF-α, IL-1, and IL-6 secretion and reduced inflammation in
an allergy-challenge mouse model (Hu et al., 2012). Beta2-AR-mediated effects reduced
signaling through NF-B, MAPK and ERK pathways. Therefore, both in vitro and in vivo studies
consistently report β2-AR-cAMP signaling induced a Th2-type response bias by DCs activated
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by a number of different immune stimuli.
Beta2-AR agonists can also drive TLR2 and nucleotide-binding oligomerization domain-
containing protein 2 (NOD2)-co-activated BMDCs towards a Th17-type IR based on ex vivo
cytokine profiles (high IL-6 and IL-23 with IL-12 inhibition) and increased Th17/Th1-cell ratio
(Manni et al., 2011). Similarly, LPS-stimulated murine BMDCs express greater IL-33 levels in
the presence of catecholamines or cAMP, an effect reversed by β2B or a PKA inhibitor
(Yanagawa et al., 2011). Epinephrine pretreatment of BMDCs co-cultured with LPS- or
CD3/CD28-activated splenic CD4+ T-cells enhances DC MHCII, CD80, and CD86 expression,
and promotes a cytokine profile that drives Th2/Th17-type IRs in the presence of a pathogenic
stimulus (Kim and Jones, 2010) (Fig. 6). These findings may explain stress-associated diseases
that are mediated by dysregulated Th2- or Th17-type IRs.
Exposing mature DCs to a β2-agonist also reduces cross-presentation of protein antigens
while retaining exogenous peptide presentation capability, an effect mediated through Gαi/0-
protein (Hervé et al., 2013). Antigen uptake and costimulatory molecule expression are not
altered, but phagosomal Ag degradation is impaired. Cross-talk occurs between TLR4 and β2-
AR transduction pathways at the NF-κB level. Collectively, these findings support that in vivo β2-
AR agonist treatment in mice inhibits Ag protein cross-presentation to CD8+ T-cells, but
preserves their exogenous MHC class I peptide presentation capability.
Using a DC-based cancer vaccine in the murine E.G7-ovalbumin (OVA) model (Botta and
Maestroni, 2008), β2-AR expression at the site of DC inoculation can influence anti-tumor
efficacy by increasing local innate cytokine responses in tumor-bearing mice. In tumor-bearing
mice, adoptive transfer of immature OVA-loaded DCs after skin preconditioning increases the
antitumor response and diminishes tumor growth, but transfer of mature DCs treated the same
way has an opposite effect. Response differences were attributed to differences in DC migration
to draining LNs and subsequent recruitment of endogenous DCs that regulated the OVA-
specific Tc-lymphocyte response via a distinct cytokine profile and reduced indoleamine 2,3-
dioxygenase in the draining LNs.
In the presence of β1/β2B, intradermal injection of a TLR2, but not a TLR4, agonist and a
soluble protein shifts the recall memory response toward a Th1-type response (Manni and
Maestroni, 2008), mediated via increased local expression of CXCR3 ligands, IFN-β, IFN-γ, IL-
12 and IL-23 during the innate immune phase. More antigen-positive plasmacytoid DCs
migrated to the draining LNs. This is the first report of a β-AR-mediated drive in Th1 IRs, and
suggests a role for the SNS in Th1-sustained inflammatory diseases, such as psoriasis and
rheumatoid arthritis.
Stimulating α-ARs on DCs affects antigen detection/processing and DCs maturation and
migration (Fig. 6) (Maestroni, 2000b; Yanagawa et al., 2010). After norepinephrine treatment
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antigen, (OVA) capture, uptake and endocytosis by DCs increases, an effect blockable by an
α2-AR antagonist, and mediated by PI3K-ERK1/2 signaling (Yanagawa et al., 2010). These
findings support a role for SNS in immune enhancement early after antigen exposure.
Norepinephrine acting on α1-ARs enhances spontaneous skin DC emigration Systemic α2B-AR
mobilizes skin DCs, but local α1B-ARs inhibit DC migration (Maestroni, 2000b). Local α1B-AR at
the time of sensitization inhibits contact hypersensitivity on day 6 post-immunization.
Norepinephrine promotes the migration of immature BMDCs, but not mature CD40-activated
BMDCs via α1-AR stimulation, and is chemotactic for immature DC (Maestroni, 2000a,b),
monocytic cells and MC (Straub et al., 2000), likely contributing to the close proximity of these
cell populations to sympathetic nerves. Therefore, α-ARs direct DC migration from inflammatory
sites to regional LNs.
β2-AR, Lymphocytes and Adaptive Immunity
Lymphocytes are white blood cells that include large granular NK T-cells and smaller T-
and B-cells that mediate the adaptive IR. These immune cells are defined by specific cell
surface markers. NK T-cells lyse virally-infected and tumor cells. Similarly, Tc-cells kill virally-
infected cells, tumor cells and allografts. T-helper (Th)-cells regulate other immune cells through
the release of growth factors and cytokines. B-cells differentiate into antibody-secreting plasma
cells. T- and B-cell clones are activated by specific antigens, and after migration into sites where
antigens are present, become effector T- and plasma cells. The differentiation,
trafficking/migration, and effector functions of all of these lymphocyte subpopulations are
regulated by sympathetic nerve release of norepinephrine acting on ARs.
Lymphocyte Trafficking. The mechanisms that modulate catecholamine-regulated
lymphocyte distribution are not well established. The SNS innervates vascular smooth muscle to
regulate regional blood flow, and deliver lymphocytes to tissues (Benschop et al., 1996).
Lymphocytes traffic to and from tissues by passing through postcapillary venules. The SNS also
regulates the flow of lymph through tissues, which affects lymphocyte output. For example, α1-
agonists infused into the afferent lymphatic vessels of PLNs from sheep increase the
lymphocyte output of efferent lymph (Ottaway and Husband, 1992, 1994), indicating egress of
lymphocytes from the PLNs. Lymphocyte subsets differ in their density and activity of β-AR
expression which may influence catecholamine-mediated lymphocyte distribution. Lymphocytes
expressing more β-AR and/or generating more β-AR-stimulated cAMP (Tc-cells) are more
readily mobilized by catecholamine administration than those with fewer receptors and/or
weaker second messenger signaling (i.e., B-cells, Th2-cells). Like NK-cells, β2-AR-induced
mobilization of lymphocyte subsets from the marginal pool occurs via the activation of immune
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cells, not the ECs, and results from the break in adhesion between CD11a expressed on
leukocytes and CXC3R1 expressed on the EC (Fig. 7) (Dimitrov et al., 2010). In healthy human
subjects, infusion of isoproterenol mobilizes memory/activated CD8+CD29high T-cells 30 min
later, but not CD8+ CD45RA+ CD62L+ naïve T-cells (Mills et al., 2000). These effects are
prevented by βB.
Beta2-AR Expression and Th0-Cell Differentiation. Upon appropriate activation, naïve
or Th-0-cells can differentiate into a number of different types of Th-cells, based on the up-
regulation of specific intracellular transcription factors and cell surface markers (i.e., up-
regulation of the transcription factors T-bet, GATA-3, RORt, and FoxP3 in Th1-, Th2-, Th17-,
and Treg-cells, respectively; Fig. 9). Naïve T-cells, and CD4+ Th1- and Treg-cells predominantly
express β2-AR (Fig. 9A-D), while Th2-cells do not express β2-ARs (Fig. 9B) (Sanders et al.,
1997, 2001; Guereschi et al., 2013). Beta2-AR expression in other CD4+ Th subsets, i.e., Th3-
or Th17-cells, remains to be demonstrated, but functional studies support their expression in
Th17-cells (Ebbinghaus et al., 2012; Kim and Jones, 2010).
“Figure 9 here”
Figure 9 summarizes the effects of AR stimulation in naïve Th-cells (Th0-cells) on
differentiation, their dependence on the local presence of specific cytokines (black above and
below horizontal arrows) from other cellular sources. For Th1-cell differentiation, β 2-AR
stimulation induces greater IFN- secreted from activated naïve CD4+ T-cells cultured in the
presence of IL-12 (Fig. 9A) (Swanson et al., 2001; Sanders, 2012). Greater IL-12
concentrations drive greater IFN- secretion per Th1-cell and cell surface IL-12R expression,
and induce expression of the transcription factor, T-bet. In contrast, adding a β2-agonist to
activated effector Th1-cells causes suppression of IFN- secretion per cell. Beta2-AR effects on
Th1-cell differentiation are time dependent, such that β2-AR stimulation before, at the time of, or
after T-cell activation, causes no change, an increase, or a decrease, respectively, (black
vertical arrows, Fig. 9A) in IFN- production than with T-cells activated in the absence of a β2-
AR agonist (Ramer-Quinn et al., 1997, 2000; Sanders, 2012).
In the presence of IL-4 and IL-10, β2-AR stimulation on naïve T-cells drives Th2-cell
differentiation, increases IL-4 secretion per cell, and induces nuclear expression of the
transcription factor, GATA-3 (Fig. 9B). However, when activated naïve T-cells are cultured with
low or high concentrations of IL-4 and a β2-agonist, Th2-cells that develop produce greater or
low-to-normal IL-4 levels, respectively. However, when effector Th2-cells are activated with a
β2-agonist, IL-4 production does not differ from Th2-cells cultured without the β2-agonist. These
findings are consistent with the lack of β2-AR expression in Th2-cells; whereas other agents that
raise cAMP augment IL-4 production. These studies demonstrate the complexity of SNS-
immune cell interactions and indicate that a better understanding of the intracellular
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mechanisms responsible for these differences is needed before they can be translated to the
clinic in any meaningful way. Disease processes, particularly chronic inflammatory diseases,
can change β2-AR expression, intracellular signaling and/or functioning in naïve or effector
CD4+ T-cells, which contributes to altered cellular function and the development and/or disease
progression. An in depth understanding of disease-mediated changes in β2-AR expression and
functioning over the disease course is also necessary to provide rational neural-immune
approaches for restoring sympathetic regulation and for disease prevention.
Th1- and Th2-cells. In vivo studies indicate added complexities compared with in vitro
studies. Despite no β2-AR expression on Th2-cells, systemically applied catecholamines drive
Th2-type humoral IRs (Fig. 9B). Beta2-AR-driven humoral immunity is indirectly orchestrated by
APC TLR activation after they bind to intracellular microbial or viral components (Fig. 9B). TLR
activation induces DC maturation (APC in Fig. 9B) and their production of TNF- and IL-12
particularly with α1-AR co-stimulation, which drive Th0-cell differentiation towards cell-mediated
immunity (Th1-mediated) in the context of APC co-stimulation (Fig. 9A). NE binding to β2-ARs
in Th0- and differentiating Th0-cells increases IFN- secretion of per cell and together with APC
cytokine production (augmented by 1-AR) strongly drives Th0-cell differentiation towards T-
bet+Th1-effector-cells. Once Th0 cells are fully differentiated to Th1 effector cells, β2-AR
agonists either have no effect, or suppress IFN-, depending on pathogen clearance (right side,
Fig. 9A). Thus, β2-ARs exert bimodal effects on IFN- secretion, depending of the time of
stimulation relative to challenge, pathogen clearance and the state of Th-cell differentiation
(Ramer-Quinn et al., 1997, 2000). Therefore, β2-AR signaling via cAMP-PKA suppresses IFN-
production by Th1-effector cells (Saxena et al., 1999), and provides an important negative
feedback mechanism for restoring immune homeostasis after pathogen clearance. The
importance of feedback inhibition in cell-mediated autoimmunity is underscored by our recent
findings that this inhibitory feedback mechanism is not functional in lymph nodes that drain the
arthritic hind limbs in adjuvant-induced arthritis (Lorton et al., 2013). Administration of a β2-
agonist increases T-cell-derived IFN- production by altering the phosphorylation of β2-ARs-
which shifts β2-AR signaling from a cAMP-PKA to an ERK1/2 pathway. Reducing SNS tone
(unpublished findings) or co-treatment with an -and β2-AR antagonist and agonist,
respectively, restores this feedback inhibition and ameliorates the disease (Lubahn et al., 2004).
On the other hand, APC presentation of parasitic components or allergens (Fig. 9B)
suppresses DC TNF-α and IL-12 production particularly with β2-AR co-stimulation, which drives
IL-4 secretion by differentiating GATA-3+Th2 cells, and IL-6 and 10 production by the APC. This
cytokine profile suppresses Th1 differentiation and promotes Th2 differentiation and a Th2
cytokine profile (i.e., IL-4, IL-5, and antigen-depending on the antigen IL-13) (Fig. 9B; red lines
between Fig. 9A and B). APC production of TNF- and IL-12 is suppressed and IL-10 and IL-
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1β-induced IL-6 production enhanced (Koff et al., 1986; Hetier et al., 1991; Elenkov et al., 1995;
Nakamura et al., 1998; Severn et al., 1992; van der Poll and Lowry, 1997). IL-12, the main
driver of Th1 responses, reduces T-cell secretion of IFN- and increases IL-4 synthesis (Elenkov
et al., 1996; Blotta et al., 1997; Haskó et al., 1998). Additionally, β2-AR stimulation of Th0-cells
inhibits IFN- production to suppress Th1-cell and indirectly drive Th2-cell differentiation, without
actually affecting IL-4 production by Th2-cells (Borger et al., 1998; Sanders et al., 1997). In this
manner, β2-AR activation drives Th2-cell differentiation, Th2 cytokine production and humoral
(antibody) responses (Maimone et al., 1993; Norris and Benveniste, 1993; Elenkov et al., 1996;
Elenkov and Chrousos, 1999; van der Poll et al., 1996). Negative, cytokine-mediated counter-
regulation between Th1- and Th2-cell differentiation (red lines between Fig. 9A and B) in the
appropriate context ensures that the appropriate Th-cell response to a specific antigen
challenge is mounted.
Th17- and Treg-cells. Information regarding SNS regulation of other Th-cell subsets,
such as Treg- and Th17-cells, is limited, in part, because these are relatively newly-discovered
T-cell subsets. Th17-cells are a subset of Th-cells that secrete the proinflammatory cytokines,
IL-17, IL-21, IL-22, and IL-23 that perpetuate inflammation and autoimmune diseases, including
multiple sclerosis, psoriasis, autoimmune uveitis, and rheumatoid arthritis. In Th0 cells, β2-AR
stimulation also promotes Th17-cell differentiation and secretion of IL-17A and IL-23 (Fig. 9C)
(Kim and Jones, 2010). The differentiation of Th17-cells requires expression of cytokines, IL-6
and TGF-β. Kim and Jones (2010) provide data to support a β2-AR-mediated bias towards
Th17-cell activation, since β2-AR stimulation enhanced TGF-β in the presence of IL-6, which
promotes IL-17A production during Th0-cell differentiation (Fig. 9C). In contrast, β2-AR agonists
suppress the production of IL-17A in effector Th17 cells (Fig. 9C), suggesting that after
pathogen clearance, the SNS suppress Th17 effector-cells, restoring homeostasis.
Treg-cells are essential for sustaining self-tolerance and preventing the development of
autoimmune diseases, since some thymocytes with specificity to self-antigens can escape
deletion in the thymus. Elimination of self-antigens as part of normal tissue repair occurs by
processing and presentation to Th0 cells by APCs (i.e., immature DCs (iDC)) without inducing
an immune response. Presentation of a self-antigen to Th0-cells induces retinoic acids, which
drives their differentiation to Treg-cells (Fig. 9D). In this context, stimulation of β2-ARs on Th0-
cells increases TGF-β production, which facilitates retinoic acid-mediated Th0-cell differentiation
to FoxP3+Treg-cells (Fig. 9D) (Guereschi et al., 2013). Through secretion of TGF-β and IL-10,
and by direct interaction, Treg cells suppress Th0 differentiation into effector Th-cells to
maintain self-tolerance. Beta2-AR stimulation enhances Treg-cell suppression by inhibiting IL-2
and IL-2R expression in activated Th0 cells after self-antigen presentation (red lines, Fig. 9D),
since IL-2 and its receptor binding is required for proliferation of Th0-cells (referred to as clonal
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expansion). Also, β-agonists promote the production of TGF-β and IL-10, however the cell types
that produce these cytokines are not clear. In vivo manipulations of sympathetic functioning in
several studies support these in vitro findings. Sympathectomy increases Treg-cells numbers in
spleen and LNs, effects dependent on TGF-β and retinoic acid. Moreover, sympathectomy
inhibits the induction of experimental autoimmune encephalitis (Bhowmick et al., 2009).
Consistently, β2-AR activation augments OVA-induced oral tolerance in a rodent model of RA
(Cobelens et al. 2002). Therefore, both in vivo and in vitro studies support that noradrenergic
nerves promote Treg-cell differentiation and functions, however the mechanisms are unclear.
One possibility is that Treg cells require cAMP for their effector function to prevent
development of autoreactive T cells, however, they do not make cAMP. Treg cells obtain cAMP
from Th0 cells via gap junctions. Activation of β2-AR would increase cAMP in these cells,
providing a greater source of cAMP for Treg cells. Additionally, β2-AR agonists and c-AMP-
elevating agents increase Tc-lymphocyte antigen-4 (CTLA-4) expression in Treg-cells, an
important mediator of cell contact-dependent Treg-cell suppression (not shown in Fig. 9D)
(Guereschi et al., 2013; Vendetti et al., 2002). These β2-AR-mediated effects are PKA-
dependent and inhibited by β2B. Additionally, the SNS may play a role in the trafficking of Treg-
cells, as acute psychological stress reduces circulating CD4+Foxp3+ Treg- and Treg-cells
expressing CTLA-4, and up-regulates β1-AR expression on Treg-cells (Freier et al., 2010).
Together, these findings suggest that the SNS plays a role in tolerance and autoimmunity via its
regulation of Treg- and Th17-cell differentiation, and is consistent with severe stressors
preceding autoimmune onset and sympathetic dysregulation in many chronic inflammatory
autoimmune disease, including rheumatoid arthritis, multiple sclerosis, and lupus
erythematosus.
Cytotoxic T (Tc)-Cells. Available data for SNS effects on Tc-cells, while sparse, generally
support catecholamine-enhanced Tc-cell functions when they are present during initial Tc-cell
activation (Livnat et al., 1987), but subsequently inhibit effector functions (reviewed in Elenkov
et al., 2000). Inhibition of Tc-cell effector functions occurs in part by down-regulating TNF-α
production (Kalinichenko et al., 1999) and delaying the generation of effector Tc-cells (Livnat et
al., 1987). Cook-Mills et al. (1995) have shown that β-agonists inhibit the generation of anti-
MOPC-315 tumor cytotoxicity by murine splenic Tc-cells ex vivo. Similarly, β-AR agonist or
increased cAMPi suppresses the development of murine memory Tc activity against an
influenza virus (Bender et al., 1993). Catecholamines or cAMP analogs available at the time of
sensitization, augments the murine Tc-mediated cytotoxicity in a mixed lymphocyte reaction
(Hatfield et al., 1986).
B-Cells. B-cells are part of the adaptive arm of immunity, responsible for making
antibodies against antigens (i.e., humoral immunity) after they undergo differentiation into
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plasma cells. B-cells recognize antigens via the presence of B-cell receptors (BCR), a cell-
bound antibody on their cell surface. Th-cells support their maturation and function by cytokine
release and upon antigen-specific activation involving the TCR and MHC II presentation by an
antigen-presenting B-cell through an “immunological synapse” (Fig. 10). Most antigens are T-
cell-dependent and therefore require both BCR-antigen binding (1. in Fig. 10) and the co-
stimulation by antigen-specific Th-cells (2. in Fig. 10) for maximal antibody production.
Therefore, β-AR-mediated changes (green arrows in T-cell; Fig. 10) in Th-cell functions
described above can indirectly (black arrows; Fig. 10) affect B-cell function, whereas β-AR
expressed on B-cells can directly affect antibody production (green arrows in B-cell in Fig.
10). Conversely, catecholamines that bind to β2-ARs on B-cells regulate their function (green
arrows in B-cell in Fig. 10), providing another indirect route for the SNS to reciprocally
influence T-cell effector functions (blue arrow in Fig. 10), like cytokine secretion.
“Figure 10 here”
Norepinephrine depletion in severe combined immunodeficiency (SCID) mice before
reconstitution with B-cells and antigen-specific Th2-cells reduces serum antibody and
suppresses the formation of splenic germinal centers (Kohm and Sanders, 1999). These in vivo
effects of catecholamines are, at least in part, due to norepinephrine regulation of CD86
expression, a costimulatory molecule required for germinal centers development (Borriello et al.,
1997). B-cells stimulated with norepinephrine stabilize CD86 mRNA, elevate CD86 transcription,
and increase CD86 surface expression after BCR stimulation (Kohm et al., 2002). Elevated
CD86 expression drives the costimulation of Th2-cells and subsequently CD86-mediated signal
transduction in B-cells to augment IL-4-dependent production of IgG1 and IgE (Fig 11)
(Kasprowicz et al., 2000). Mice that received antigen-specific B-cells from norepinephrine-
depleted hosts and administered CD40L and IL-4 to activate B-cells, have increased serum
IgG1, an effect that is further elevated by anti-CD86 or terbutaline (β2-agonist) treatment (Kohm
et al., 2002). Dual activation of CD86 and β2-AR on B-cells has an additive effect on augmenting
IgG1 production, with the β2-agonist effects mediated via CREB and OCA-B (Fig. 11A). Data
support that CD86 engagement of a B-cell induces a signal in the B-cell that directly affects
IgG1 production per cell, independent of its effect on Th2-cells and subsequent IgG1 switching
(Kasprowicz et al., 2000; Podojil and Sanders, 2003). Elevated IgG1 per B-cell results from
greater 3’IgH enhancer activity and rates of mRNA for IgG1 (Podojil et al., 2004; Podojil and
Sanders, 2003). Additionally, Pongratz and coworkers (2006) reports that in the presence of IL-
4, stimulation of β2-AR in activated B-cells increases IgE expression, an effect that is cAMP-,
PKA- and p38MAPK-dependent; but independent of CREB (Fig. 11B). A β2-AR-mediated
increase in p38MAPK potentiates the B-cell activational rise in p38MAPK and STAT6 the
signals an increase in IgE production and secretion.
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“Figure 11A-B here”
Catecholamines have indirect effects on B-cells by modulating T-cell cytokine production
(Figs. 9, 10). While IL-4-mediated Th2-cell differentiation is not affected by catecholamines,
their effect on IFN-γ secretion by Th1-cells indirectly suppresses IL-4 production by Th2-cells
and high IL-10 production by Th2-cells can inhibit Th-cell functions (Fig. 9), which can impact
IgG1 production. Exposing Th1-cell clones or newly generated effector cells to norepinephrine
before they are activated by B-cells also reduces T-cell production of IFN-γ (Sanders, et al.,
1997), and greater IFN-γ by naïve T-cells exposed to norepinephrine augments the B-cell
production of IgG2a (Swanson, et al., 2000). Collectively, norepinephrine augments or
suppresses B-cell function indirectly by altering Th1-cytokine production and this effect is
dependent on when T-cells are exposed to norepinephrine, and their state of activation. Cyclic-
AMP-elevating agents like catecholamines can up-regulate B7 expression on B-cells (Watts et
al., 1993), which binds to the CD28 a T-cell costimulatory molecule that is part of the
“immunological synapse”, indicating increased T-cell-B-cell interaction.
The activation of B-cells requires that a critical threshold of cAMP concentration being
reached for antibody production (Pollok et al., 1991). Elevated cAMPi can sensitize resting B-
cells to immune complex-induced hyporesponsiveness (Stein and Phipps, 1991). Beta2-AR
stimulation-induced cAMP may be contributory to regulation of antibody production under these
conditions; however, future studies are needed to determine whether this is a mechanism
through which norepinephrine affects B-cell function.
Additionally, catecholamines can directly affect B-cell function via binding to ARs
expressed on their cell surface, which are predominantly β2-ARs. Both in vitro and in vivo
studies indicate that catecholamines directly affect B-cell function, and play a critical role in
determining antibody production. Like Th-cells, their effects depend on the activational state of
B-cells. Kasprowicz et al. (2000) find that incubating B-cells with catecholamines during antigen
processing or within 12 h of Th2-cell co-culture potentiates IgG1 and IgE production. However,
when B- and Th2-cell co-cultures are exposed to norepinephrine 12 h after starting the culture,
only IgG2a is elevated. Lastly, adding norepinephrine to plasma cells reduces the amount of
antibody produced (Melmon et al., 1974). These studies demonstrate the ability of
catecholamines to influence B-cell antibody production in an activation-dependent manner.
Summary
The SNS is a major integrative and regulatory pathway between the CNS and the IS. Our
understanding of the central circuits that regulate sympathetic outflow to immune organs is
limited, and in large part based on studies after immune activation with LPS, some
electrophysiological studies, and a couple of studies mapping neuronal transsynaptic infection
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with PRV after its injection into the spleen by a single research group. It is well documented that
sympathetic noradrenergic nerves innervate primary and SLOs and that norepinephrine meets
the criteria for neurotransmission, at least in the spleen. Our understanding of the interactions
between these two systems has grown considerably since the criteria for sympathetic
neurotransmission with immune cells as effector cells were first demonstrated. However, this
field is still in its infancy, and many questions remain unanswered. It is clear that the SNS
regulates all aspects of innate and adaptive immune responses, but mechanisms are still being
unraveled. Most studies use in vitro or ex vivo methods in rodent models in the presence or
absence of adrenergic agonist/antagonists or other strategies to manipulate the sympathetic
neurotransmission in selected lymphoid organs. While these studies certainly contributed
valuable information, they may not mimic in vivo functions, particularly, the in vitro studies.
Mixed results can result from differences in species or cell lines used for study, and other
factors such as timing, source and activational state of immune cells, psychological stress, age
and gender. Additionally, sympathetic regulation of blood flow, cellular infiltration and stromal
cell functions cannot be easily mimicked in vitro, and may account for disparate findings
between in vivo, ex vivo and in vitro findings. Moreover, available data supports an intimate
interaction between catecholamine signaling and another key immune regulator, the HPA
pathway suggesting cooperative homeostatic regulation of the IS; however further research is
needed to mechanistically understand this three-way interaction. In this paper, we have focused
on direct sympathetic-to-immune cell interactions via NE released from nerve terminals.
However, sympathetic nerves release other neurotransmitters/neuromodulators, such as
neuropeptide Y, adenosine, and ATP, that can influence immune function, and interact with
noradrenergic signaling to facilitate or dampen the immune response. Some studies suggest
that these mediators are differential released from sympathetic nerves depending on firing
activity, but the role of these mediators either alone or in combination with norepinephrine in
neural-immune regulation remains undefined.
Noradrenergic nerves innervate stromal and immune cells. Alpha2-ARs expressed
presynaptically negatively regulate norepinephrine release. Although, these circuits are often
termed “hardwired”, neural plasticity is a prominent feature of neural-immune interactions
allowing for the SNS to adapt to changes in its microenvironment. Immune cells respond to
norepinephrine preferentially through signaling of β2-ARs, and with lower affinity to α1-ARs, but
α1-ARs under certain conditions play important roles in sympathetic-immune regulation. The
intracellular signaling pathways activated by AR stimulation interact at multiple sites with known
signaling pathways used by immune cells to carry out their cellular functions. It is clear from
existing data that catecholamines regulate hematopoiesis for all types of blood cells and
thymocyte development, the major functions of primary LOs. The SNS also can regulate the
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circadian proliferation of BM cells, and the egress/ingress of HSCs and blood cells in the BM. A
rise in circulating epinephrine in response to an acute stressor mobilizes leukocytes from the
BM, but most of the effects of the SNS on BM are mediated by sympathetic nerves in primary,
as well as, SLOs. Sympathetic nerves in the thymus influence key steps in thymocyte
development via AR-mediated regulation of cell surface molecules, like Thy-1, and cytokine
release from stromal cells and thymocytes. Still, only a few research groups are investigating
SNS-immune cell development in primary LOs and remain understudied areas in the field of
psychoneuroimmunology, particularly sympathetic regulation of supporting stromal cells in the
BM and thymus that are critical for normal hematopoietic and thymocyte development.
With regard to adaptive immunity, the SNS clearly regulates the differentiation of Th cells,
particularly Th1- and Th2-cells. The SNS, via β2-ARs, promote differentiation Th0 cells to Th2,
and inhibit differentiation of Th1 cells. Much less is known regarding the influence of
sympathetic nerves on the differentiation of the more recently discovered Th17- and Treg-cells.
However, recent studies indicate these cells express β2-AR, and that the SNS regulates Th0 cell
differentiation toward Th17- and Treg-cells. Future studies are needed to understand the
mechanisms for SNS regulation of these T-cell subsets. Currently, Th17- and Treg-cells are
intensively studied by immunologists due to their importance in autoimmunity and cancer –
diseases linked to SNS dysfunction and where clear relationships between psychosocial stress
and disease outcome are reported.
Sympathetic regulation of mucosal immunity also remains extremely limited (see
McGovern & Mazzone, Sharkey & Savidge in this issue of Autonomic Neuroscience), but a few
studies support current views held for other SLOs, i.e., that the SNS generally suppresses Th1
and drives Th2-type cytokine production and antibody responses, and is anti-inflammatory. It is
apparent from this review that the interactions between these two systems are highly complex,
and that unraveling the mechanisms responsible for SNS-immune interplay is still in its infancy.
The anti-inflammatory properties mediated via β2-ARs have been utilized in a limited way in the
clinical setting, for example in the treatment of asthma, but the research reviewed here
underscore the necessity of future research to better understand the mechanisms for
sympathetic-immune crosstalk under normal and pathological conditions before they can be
translated to treatments that prevent or successfully treat chronic inflammatory diseases.
However, there are exceptions to the anti-inflammatory effects of the SNS under
conditions of chronic inflammation and in autoimmunity, as we have found LOs-dependent
sympathetic response in rats with adjuvant-induced arthritis (Lorton et al., 2013). In DLNs, β2-
AR agonists drive Th1 responses promoting inflammation, while in the spleen they push Th2 or
have no effect. The catecholamine-mediated Th2 drive and Th1 suppression in the spleen can
have beneficial or have detrimental consequence under certain conditions. The Th2-drive may
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be protective, particularly systemically, by preventing organ damage and anemia, from high
circulating Th1 proinflammatory cytokines and damaging products produced by the cells of
innate immunity. A chronic Th2 drive may increase susceptibility to infections that are most
efficiently combated by Th1-type response, increasing the risk for infectious complications.
Moreover chronically high SNA increases metabolism, which promote wasting, and taxes the
cardiovascular, renal, and hepatic systems. Defects in the sympathetic-immune interplay, and/or
an abnormal activity of the SNS in either direction, have been reported in many chronic
inflammatory diseases, and likely contributory to their pathophysiology.
The actions of the SNS can be pro- or anti-inflammatory, depending on many factors,
including presence and nature of the antigen, the state of the local microenvironment, where the
antigen is introduced, the activation/differentiation state of cells of the IS, activational state of
the stress pathways, and the G-coupled protein receptor (GCPR) composition, density on
immune cells, the G-proteins to which they are coupled and how they are phosphorylated. The
nature of the SNS as a homeostatic regulatory system is consistent with a proinflammatory role
at the onset of challenge or injury that facilitates the rapid clearing of microbes and/or injured
tissue and promotes healing. During the effector phase, the role of the SNS assumes an anti-
inflammatory function and suppresses immune effector cells, with the goal of “turning off” the IS
and restoring homeostasis to the fullest extent possible. Therefore, sympathetic-mediated
responses are time-dependent relative to the antigen-presentation. An in depth understanding
of disease-mediated changes in β2-AR expression and functioning under chronic inflammatory
conditions is imperative to provide rational neural-immune based therapies for the prevention or
treatment of chronic inflammatory diseases.
Sympathetic modulation of innate immunity has been much less studied than adaptive
immunity. The role of 1-ARs is and their cross-talk with β2-AR in innate immunity, particularly,
DCs, is poorly understood. We know that the intracellular pathways activated by AR stimulation
can influence the activation of immune cells by modifying cell surface molecules critical for
antigen detection and processing/presentation in APCs, and interface with intracellular
pathways used by TLRs, TCRs and BCRs at multiple sites. Diversity in the responses by β-ARs
may result from the presence of multiple isoforms of ACs and PKA, and from promiscuity in
receptor coupling to either Gs or Gi, or uncoupling from ACs to other signaling pathways. Lipid
rafts play an important role in AC-mediated actions. The recent discovery of biased β2-AR
agonists that preferentially signal via Gs or Gi will be useful tools to decipher how this
differential signaling impacts immune responses. Similarly, there are two G-proteins to which α-
ARs can bind and Gq coupling activates three intracellular pathways, each of them interfacing
with signaling pathways that mediate immune cell responses. Collectively, our current
understanding of how and where sympathetically-activated intracellular pathways in immune
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cells converge on to those activated with immune stimuli is sparse. Additional biomolecular
research is needed to fully understand how catecholamine-mediated changes in the functional
state of specific immune cells populations that interact via “immunological synapses” across time
and under well-defined microenvironments. With this knowledge we can begin to make
reasonable predictions about local and systemic outcomes in vivo under normal and disease
conditions. Only then will it be possible to design new diagnostic pharmaceuticals tools that are
both safe and efficacious, and are truly tailored for the prevention or successfully treatment of
the underlying causes of inflammatory diseases, rather than the treatment of their symptoms
and/or retarding their progression.
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Figure 1. Overview of SNS-Immune System Cross-Talk. The brain responds to sensory information from the periphery, including circulating cytokines (particularly, interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)-α) and sensory neurons that project to the tissues (lower left corner). Central nervous system (CNS) responses to sensory information are transmitted through many CNS pathways, thus alter the firing of preganglionic neurons to regulate SNS tone to sympathetic target organs, including lymphoid organs. Preganglionic sympathetic neurons in the intermediolateral cell column of the thoracolumbar region of the spinal cord (green circles) send their axons to postganglionic sympathetic neurons (green circles) located in sympathetic ganglia and to chromaffin cells in the adrenal medulla. Postganglionic neurons send their axons to lymphoid organs and tissues where they signal cells of the immune system and smooth muscle cells of the vasculature by releasing norepinephrine. Immune cells express predominantly β2-ARs (orange); however, some immune cells do express α-AR subtypes (pink). Preganglionic sympathetic nerves supply adrenal medullary chromaffin cells. These cells release predominantly epinephrine (~80%) and to a lesser extent norepinephrine (~20%) into the circulation (i.e., hormonal release) to interact with ARs expressed on immune cells, subsequently activating signaling pathways that alters their cellular functions. Through these connections, the SNS regulates immune system homeostasis, and under conditions of stress or immune activation. Figure 2. SNS Regulation of Hematopoiesis. Schematic illustrating sites where hematopoiesis of specific type of blood cells is either positively or negatively influenced by catecholamines or adrenergic receptor (AR) agonists. Catecholamines activating via α- or β-ARs promote lymphopoiesis, and β-agonists promote erythropoiesis (upper and lower left side). The β2-AR-mediated drive in erythropoiesis requires the presence of erythropoietin. In contrast, α- or β-agonists suppress in vivo granulocyte/monocyte production (lower left side). Catecholamines also promote the proliferation of pluripotent hematopoietic stem cells (HSCs) via β-AR-mediated mechanisms (upper left side). Additionally, both β3- and β2-ARs expressed
on bone marrow stromal and bone cells, respectively, regulate circadian (~) proliferation of
HSCs and trafficking into the circulation, and subsequently back into the bone marrow (upper right corner). Sympathetic regulation of bone marrow functions may be exploited clinically to improve bone marrow transplantation, or prevent anemia that can result under conditions of
chronic stress and/or inflammation. 1
Byron, 1971; 2
Maestroni et al., 1992, Maestroni & Conti,
1994a,b; 3
Maestroni et al., 1998, Mendez-Ferrier et al., 2010; 4
Mladenovic & Adamson, 1984. Figure 3. Sympathetic Regulation of Circadian HSC Proliferation and HSC Egress from BM. Photic cues are conveyed from the eye to the suprachiasmatic nucleus (SCN) through the retinal-hypothalamic tract (RHT), the central pacemaker in the brain (upper right side). Clock gene expression (like Bmal) in neurons of the suprachiasmatic nucleus (SCN), orchestrates the β3-AR-mediated circadian proliferation and release of hematopoietic stem cells (HSCs)/hematopoietic progenitor stem cell (HPSCs) (purple cells in large box). These signals are transmitted to the bone marrow (BM) via the sympathetic nervous system (SNS, green),
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causing the circadian (~) release of norepinephrine into the BM microenvironment. During the
rest period (day for mice; night for humans indicated by open or closed eye on the right side of the diagram, respectively), the rise in sympathetic activity in the bone marrow is driven by increased G-CSF release in the BM (upper large box). NE binding to β2-ARs on endosteal osteoblasts increase chemokine (C-X-C motif) 12 (CXCL12) secretion (also known as stromal cell-derived growth factor (SDF-1) (lower left of large box). CXCL12 subsequently binds to CXC receptor type 4 (CXCR4; see small box inset) on quiescent HSCs/HPSCs that are “tethered’ to the BM by the adhesion molecules, very late antigen 4 (VLA4) and vascular cell cellular adhesion molecules (VCAM-1)). CXCL12-CXCR4 binding increases metalloprotease 9 (MMP9) by stromal cells, which degrade the adhesion molecules and releases HSC for mobilization into the bloodstream. Clinically, G-CSF is used to increase recovery from depleted white blood cell counts. NE also binds to β3-adrenergic receptor (AR) on mesenchymal stromal cells (MSCs) (left side of large box), which reduces CXCL12 expression (lower left of large box). Beta2-ARs upregulate CXCL12 expression, whereas β3-ARs down-regulate CXCL12 expression, which permits or inhibits HSC/HPSC egress, respectively. As a result of β2- AR stimulation, protein kinase A phosphorylates specificity protein 1 (Sp1), which enhances its DNA binding activity and subsequently transcription of target genes that down-regulate CXCL12 expression (not shown). Conversely, circadian stimulation of β3-AR-Gi/o protein signaling pathway reduces cAMP production and consequently Sp1 phosphorylation, which increases Cxcl12 and CXCL12 expression and prevents HSC and HPSC release from BM. Based on findings from Méndez-Ferrer et al. 2008. Figure 4. HF Staining in Primary and Secondary Lymphoid Organs from Rodents. Fluorescence histochemical (HF) staining demonstrates noradrenergic innervation of the thymus (A), spleen (B), and lymph node (C). A. Thymic noradrenergic nerves derive from the superior cervical and stellate ganglia, and form neurovascular plexuses from which nerves extend into the surrounding parenchyma, particularly in the cortex (CTX). Sympathetic nerves form neuroeffector junctions with thymocytes and cells in the stroma throughout the cortex, so they are “positioned” for regulating key mechanisms responsible for ingress of HSPC and their migration from inner to outer cortex, thymocyte proliferation and maturation, T-cell receptor gene rearrangement, and positive and negative selection. Additionally, noradrenergic nerves closely appose immune cells that associate with the vasculature and connective tissue (i.e., capsule and septae) of the thymus, including perivascular mast cells, macrophages, eosinophils, fibroblast and other accessory cells. In comparison to the cortex, sympathetic nerves sparsely supply the medulla (M), and mainly associate with the vasculature, where hematopoietic progenitor stem cells ingress and mature T-cells egress. The close relationship between medullary vasculature and sympathetic nerves suggest a role for noradrenergic nerves in the ingress of hematopoietic stem cells, and egress of mature thymocytes. In man, but not rodents, noradrenergic nerves closely associate with medullary epithelial cells that form Hassall’s corpuscles; the significance of this finding is not clear. YAC, yellow autofluorescent cells. B. Noradrenergic fibers enter the spleen around the splenic artery, and travel with the vasculature in plexuses (not shown). Noradrenergic nerves continue into the spleen along the trabeculae in trabecular plexuses. Fibers continue mainly along the central artery (ca) and its branches. Noradrenergic nerves extend from these plexuses into the surrounding white pulp among lymphocytes in the periarteriolar lymphatic sheath (pals), but avoid the nodular areas containing naïve B cells and the parenchyma of the red pulp. Noradrenergic nerves are closely associated with CD4+ and CD8+ T-cells. In the marginal (mz) and the parafollicular zones that surround the pals, noradrenergic nerves course among B-cells and macrophages. Close associations between noradrenergic nerves and follicular dendritic cells, or stromal cells (i.e., reticuloendothelial cells and fibroblasts) have not been described, but are likely to occur given
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the overlapping distribution of these nerves and these cells. C. Noradrenergic innervation of a popliteal lymph node (PLN) showing fluorescent noradrenergic nerves traveling along a blood vessel (BV), in the paracortical regions (PC). Some fibers (arrowhead) branch from the vascular plexuses into the surrounding parenchyma and end among lymphocytes and other cells in this region (arrowheads). In the medulla (M) of LNs, sympathetic nerves reside adjacent to the medullary sinuses among T-cells, reticuloendothelial cells, macrophages and plasma cells of the medullary cords, and in the cortex, noradrenergic nerves reside among T-cells and autofluorescent cells, presumed to be macrophages. Sympathetic nerves provide only minimal innervation of B-lymphocyte-rich follicles and germinal centers. Figure 5A. SNS and Thymocyte Development. Schematic illustration of the proposed α- and β-adrenoceptor-mediated modulation of thymocyte development (upper and lower panel, respectively). Also in the upper pale yellow panel, the gray arrows represent the regions in the thymus where the following developmental changes occur in immature thymocytes: 1. β selection and rearrangement in the outer cortex; 2. Positive selection mid-cortex to the corticomedullary junction (CMJ); and 3. Negative selection in the CMJ and medulla. In the lower panel the cortex, corticomedullary junction (CMJ) and medulla are indicated. The major developmental stages in thymocytes are shown from left to right. Catecholamines have very little effect on the early developmental stages, i.e., double negative (DN) T-cell receptor (TCR)― thymocytes (right side of figure) either by α- or β-adrenergic receptors (ARs) stimulation (upper or lower panels, respectively). Stimulation of α-ARs increases (+) the frequency of double-positive (DP, CD4+CD8+) T-cell receptor (TCR)low thymocytes without affecting the frequency of double-negative (DN, CD4‒CD8‒ ) TCR‒ thymocytes. Activation of α-ARs also increases the frequency of single-positive (SP) CD8+ and reduces (‒ ) the frequency of CD4+- and CD4+CD25+RT6.1+ Treg-thymocytes (left side of upper panel). In contrast, stimulation of β-ARs (lower panel) negatively (‒ ) regulates both positive and negative selection, reducing the frequency of DP TCRhigh thymocytes and SP CD8+TCRhigh-, CD4+TCRhigh-, and CD8+CD25+RT6.1+-Treg-thymocytes. B. Cross-talk between the β-AR-Gs-cAMP-PKA and the p38 MAPK pathways occurs in the thymus. Although the β-AR signaling with the MAPK pathway in thymocytes has not been clearly determined, norepinephrine is proposed to binds to β-ARs expressed on thymocytes to activate the canonical Gs protein-AC-cAMP—PKA pathway. Cyclic AMP signals PKA then phosphorylates p38, which phosphorylates activating transcription factor-2 (ATF-2). ATF-2 travels to the nucleus to upregulate Fas ligand (FasL) mRNA, which increases cell death. This is a proposed mechanism by which the SNS promotes reduced cellularity and thymocyte numbers and increased apoptosis of thymocytes. Signaling is terminated by the degradation of cAMP to 5′-AMP by phosphodiesterase (PDE). Adapted from LaJevic et al. (2011). Figure 6. Summary of β- and α-AR-Mediated Effects on Cells of Innate Immunity. Sympathetic nerves (center) release norepinephrine as the major neurotransmitter (pale green circles), which binds to β- (left side, orange) or α-adrenergic receptors (ARs) (right side, pink) expressed on cells of the innate immune system to positively (dark green text) or negatively (red text) affect specific cellular functions of (from top to bottom) eosinophils, neutrophils, mast cells/basophils, natural killer (NK) cells, and macrophages/dendritic cells (MAC/DC). EC, endothelial cells; FMLP, formyl-methionyl-leucyl-phenylalanine; Ag, antigen; IFN-γ; LPS,
lipopolysaccharide; TNF-α, tumor necrosis factor-α; IL, interleukin; MIP-1α, macrophage inflammatory protein-1α; I-CAM, intercellular adhesion molecule; Figure 7. Circulating and Marginating Pools of Effector Cells and Release into the Circulation. A. The circulating pool of peripheral blood leukocytes consist of lymphocytes, polymorphonuclear (PMN) cells, granulocytes (top of 7A), and basophils (not shown). Five
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cytotoxic effector leukocyte subsets make up the marginal pool: CD8+ effector T-cells, DN (CD4─/CD8─
) TCRγ/δ or gamma/delta T-cells, CD3+CD56+ NKT-like cells, CD16+CD56dim
cytotoxic NK-cells, and CD14dimCD16+ proinflammatory monocytes (bottom of 7A). These subsets are attached to the basement membrane of capillary and venule endothelial cells by adhesion molecules, CX3CR1, and CD11a expressed on immune cells. B. Catecholamines induce the mobilization of lymphocytes subsets from the marginal pool by activating β2-ARs expressed on immune cells, not on the endothelial cells (EC), resulting in a breakdown in adhesion between CD11a expressed on leukocytes and CXC3R1 expressed on EC (4th cell from the left). Activation of β2-ARs stimulates the release of proteases (scissors). Beta-AR agonists mobilize memory/activated CD8+CD29high T-cells within 30 min, but not CD8+ CD45RA+ CD62L+ naïve T-cells, an effect blocked by β-AR antagonists. Epinephrine rapidly attenuates endothelium attachment via CD11a and CX3CR1, permitting demargination and release into the circulation, to rapidly mobilize protective host defense against invading pathogens. Based on findings from Dimitrov et al. (2010). Figure 8. Effects of CD28 and CTLA4 (CD152) Recognition in the “Immunological Synapse on APC Functions. A. Recognition of the T-cell receptor (TCR) with a specific major histocompatibility complex (MHC)–peptide (Ag) complex (signal 1) in the absence of CD28 binding to B7.1 or B7.2 (collectively B7; CD80 or CD86, respectively) causes T-cell anergy or apoptosis. B. Simultaneous recognition of the TCR with a specific MHC-peptide complex and CD28-B7 (signal 2) induces T-cell proliferation (clonal expansion) and differentiation in to mature, functional effector T-cells. C. The upregulation and subsequent cell surface expression of cytotoxic T-lymphocyte antigen 4 (CTLA-4) and its binding to CD40 in the presence of TCR recognition of a specific MHC-peptide complex prevents T cell activation and clonal expansion. Figure 9. Summary of the β-AR-Mediated Effects on Th-Cell Differentiation. In secondary lymphoid organs, antigen-presenting cells (APCs; i.e., dendritic cells, macrophages or B-cells) drive the differentiation of T-helper (Th)0-cells toward specific subtypes of Th effector-cells, depending on the type of antigen, the cytokine milieu and adrenergic receptor (AR) activation. This figure illustrates the influence of AR stimulation at the time of antigen presentation and Th0-cell activation (left side of figure) or after Th0-cell differentiation into effector Th-cells (right side of figure). A-B. Cytokines regulate Th1- and Th-2-cell drive. IL-12 from APCs and IFN-γ
from Th0 cells drive Th1-cell and inhibit Th-2 cell differentiation, whereas tumor necrosis factor-α (TNF-α) and IL-10 from APCs and IL-4 from Th0 cells drive Th2-cell and inhibit Th1-cell differentiation (red lines). A. APC presentation of microbial/viral antigens to Th0-cells induces a
cell-mediated response by promoting APC production of TNF- and IL-12. These cytokines
promote IFN- secretion by Th0-cells. Activation of α1-ARs increases APC TNF-α, while activation of Th0 cell β2-ARs enhances IFN-γ production to drive differentiation towards a Th1-cell phenotype and promote cell-mediated immunity. Also, by increasing IFN-γ production by
Th0-cell, β2-AR activation indirectly inhibits Th2-cell differentiation by reducing IL-4. Th1-cells are characterized by expression of the transcription factor T-bet and cell surface expression of IL-12 receptor (IL-12R). Activation of Th0 β2-ARs results in greater IFN-γ production per Th1-cell. In contrast, β2-AR stimulation of effectorTh1-cells (pink) reduces IFN-γ production by the differentiated Th1-cells. By decreasing IFN-γ in Th1-cells, the SNS inhibits cell-mediated immunity and acts as a negative feedback mechanism to restore homeostasis. B. APC presentation of parasitic or allergenic antigens in the presence of APC produced IL-10, Th0-cell released IL-4 (green circles) drives Th0-cell differentiation towards a Th2-cell phenotype (GATA-3 and IL-4R expression) and an increase in IL-4 production by the differentiating Th-cells. Activation of β2-ARs increases the production of APC IL-10, to promote humoral immunity. Beta2-AR stimulation of effector Th1-cells (right side of figure) indirectly promotes cell-mediated immunity, by reducing IFN-γ production, which would inhibit IL-4 production. C. In the context of
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elevated IL-6 and norepinephrine, APC presentation of self-antigens to Th0-cells can drive Th-cell differentiation towards a Th17-cell phenotype (RORγt and IL-23R expression), characterized
by the secretion of transforming growth factor-β (TGF-β), IL-17A, and IL-23. However, β2-AR stimulation of effector Th17-cells (right side of figure) suppresses IL-17A production. D. In the context of elevated retinoic acid and TGF-β, Th0-cells differentiation into Treg-cells. The SNS promotes Th0- to Treg-cell differentiation and β-agonists promote TGF-β and IL-10 production, but the cell type(s) that produce(s) these cytokines is/are not clear. Treg-cells are characterized by increased FoxP3 expression and production of TGF-β and IL-10. FoxP3, forkhead box P3; RORγt, retinoic acid-related orphan receptor γt. Figure 10. Direct and indirect effects of β-adrenergic receptor (AR) stimulation on APC and T-cell functions are illustrated. Simultaneous TCR recognition of a specific MHC-Ag complex and B7 by CD28 induces gene transcription (gray arrows) in both the T-cell and the antigen-presenting cell (APC - dendritic cells (DCs), macrophages or B-cells), which directs specific cellular responses necessary to coordinate an appropriate immune response. Activation of β-ARs via norepinephrine (green circles) binding directly effects gene transcription in the APC (on right) or the T-cell (on left), and the subsequent changes in gene transcription (green arrows) which influences their cellular responses. Additionally, β-AR-mediated changes in the cellular response of the APC can indirectly affect T-cell response (blue arrows). Similarly, β-AR-mediated changes in the cellular response of the activated T-cell can indirectly affect APC response (black arrows). BCR, B-cell receptor. Figure 11. Sympathetic Influence on Antibody Production. A. Schematic showing the molecular mechanism proposed by Sanders et al. (2012) of β2-adrenergic receptor (β2-AR) convergence with the CD86 signaling pathway in an activated B-cell. CD86 ligation increases NF-κB by removing IκB-α-mediated inhibition, and increasing the expression OCA-B, a co-activator protein (a) (left side of figure). CD86 also activates Lyn kinase, CD19, and Akt (not shown). At the time of CD86 ligation, β2-AR stimulation (b) activates adenylate cyclase (AC), which increases cAMP production and consequently activates PKA, which in turn phosphorylates CREB and increases OCA-B, a transcription factor (c). More OCA-B-Oct-2 complexes (d) translocate to the nucleus where they bind to the 3′-IgH-enhancer. OCA-B/Oct-2 binding augments the rate of IgG1 transcription (e), synthesis and levels secreted by the B-cell (f). Beta2-AR stimulation does not affect the number of naïve B-cell that undergo IgM-to-IgG1 switching. B. Schematic demonstrating the molecular mechanisms through which β2-AR (R) signaling modulates the production of IgE in an activated B-cell. STAT6 is essential for IL-4-induced IgE transcription. Upon B-cell activation (a), simultaneous stimulation of β2-ARs (b) activates the cAMP-PKA signal transduction pathway (c). Phosphorylation of PKA activates hematopoietic protein tyrosine phosphatase (HePTP), which increases the available pool of p38MAPK (d) that can be activated by CD40 ligation. Increased p38MAPK phosphorylation along with STAT6 (e) causes an increase in CD23 synthesis and its cleavage from the cell surface (not shown), leading to greater soluble CD23 (sCD23). STAT6/phosphorylated p38MAPK-mediated shedding of CD23 creates a microenvironment that promotes IgE gene transcription (f) and synthesis (g) per B-cell. Blue double-line, cell membrane; blue double, dashed-line, nuclear membrane. L, ligand (norepinephrine); IgE, immunoglobulin E; Gs, Gs protein; AC, adenylate cyclase, PKA, protein kinase A; p38-mitogen-activated protein kinase; STAT6, signal transducer and activator of transcription 6. Both panels are adapted from Sanders et al., 2012.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5A
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Figure 5B
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11A
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Figure 11B
