Prostaglandins, Leukotrienes and Essential Fatty Acids 73 (2005) 17–30 Adipose tissue and the immune system Caroline M. Pond Department of Biological Sciences, The Open University, Milton Keynes MK7 6AA, UK Abstract Adipocytes anatomically associated with lymph nodes (and omental milky spots) have many special properties including fatty acid composition and the control of lipolysis that equip them to interact locally with lymphoid cells. Lymph node lymphocytes and tissue dendritic cells acquire their fatty acids from the contiguous adipocytes. Lymph node-derived dendritic cells suppress lipolysis in perinodal adipocytes but those that permeate the adipose tissue stimulate lipolysis, especially after minor, local immune stimulation. Inflammation alters the composition of fatty acids incorporated into dendritic cells, and that of node-containing adipose tissue, counteracting the effects of dietary lipids. Thus these specialised adipocytes partially emancipate the immune system from fluctuations in the abundance and composition of dietary lipids. Prolonged, low-level immune stimulation induces the local formation of more adipocytes, especially adjacent to the inflamed lymph node. This mechanism may contribute to hypertrophy of the mesentery and omentum in chronic inflammatory diseases such as HIV-infection, and in smokers. Paracrine interactions between adipose and lymphoid tissues are enhanced by diets rich in n-6 fatty acids and attentuated by fish oils. The latter improve immune function and body conformation in animals and people. The partitioning of adipose tissue in many depots, some specialised for local, paracrine interactions with other tissues, is a fundamental feature of mammals. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction Research into the functional association between adipose tissue and the immune system began in the early 1990s, when adipsin secreted from adipocytes was shown to be identical to complement factor D produced in the immune system [1–3]. Since then, many more protein secretions and/or cytokine receptors have been described [4]. In most cases, cytokines such as tumor necrosis factor-a (TNFa) and many interleukins were isolated first from the immune system and later found to be secreted by and/or taken up by adipocytes, but others, notably leptin [5], were identified first in adipose tissue and later shown to modulate immune function. The possibility of using leptin or other appetite regulators as anti-obesity drugs has prompted intensive study of the metabolic interactions between the control of energy storage and immune function [6,7]. Unfortu- nately much of the data come from mice, especially transgenics [8], and human biopsy samples. Mice and young rats are so small and lean that only the largest depots contain enough adipose tissue for most biochem- ical analyses. The choice is narrowed to the perirenal or gonadal (epididymal or parametrial) depots, both of which incorporate no lymphoid structures, and some- times the inguinal, which contains a few nodes at the dorsal end. To avoid collateral damage, biopsy sites from humans and larger animals are always chosen for their surgical accessibility, which in practice means remoteness from lymph nodes and vessels, rather than for known site-specific properties. Consequently, infor- mation about the contributions of depots that incorpo- rate lymphoid structures and those that are ‘pure’ adipose tissue is fragmentary, and does not readily suggest a coherent hypothesis. Adipocytes in the omental, mesenteric and other depots that incorporate lymphoid structures synthesize ARTICLE IN PRESS www.elsevier.com/locate/plefa 0952-3278/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.plefa.2005.04.005 Tel.: +44 1908 655077; fax: +44 1908 654167. E-mail address: [email protected].
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Prostaglandins, Leukotrienes and Essential Fatty Acids 73 (2005) 17–30
www.elsevier.com/locate/plefa
Adipose tissue and the immune system
Caroline M. Pond�
Department of Biological Sciences, The Open University, Milton Keynes MK7 6AA, UK
Abstract
Adipocytes anatomically associated with lymph nodes (and omental milky spots) have many special properties including fatty
acid composition and the control of lipolysis that equip them to interact locally with lymphoid cells. Lymph node lymphocytes and
tissue dendritic cells acquire their fatty acids from the contiguous adipocytes. Lymph node-derived dendritic cells suppress lipolysis
in perinodal adipocytes but those that permeate the adipose tissue stimulate lipolysis, especially after minor, local immune
stimulation. Inflammation alters the composition of fatty acids incorporated into dendritic cells, and that of node-containing
adipose tissue, counteracting the effects of dietary lipids. Thus these specialised adipocytes partially emancipate the immune system
from fluctuations in the abundance and composition of dietary lipids.
Prolonged, low-level immune stimulation induces the local formation of more adipocytes, especially adjacent to the inflamed
lymph node. This mechanism may contribute to hypertrophy of the mesentery and omentum in chronic inflammatory diseases such
as HIV-infection, and in smokers. Paracrine interactions between adipose and lymphoid tissues are enhanced by diets rich in n-6
fatty acids and attentuated by fish oils. The latter improve immune function and body conformation in animals and people. The
partitioning of adipose tissue in many depots, some specialised for local, paracrine interactions with other tissues, is a fundamental
feature of mammals.
r 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Research into the functional association betweenadipose tissue and the immune system began in theearly 1990s, when adipsin secreted from adipocytes wasshown to be identical to complement factor D producedin the immune system [1–3]. Since then, many moreprotein secretions and/or cytokine receptors have beendescribed [4]. In most cases, cytokines such as tumornecrosis factor-a (TNFa) and many interleukins wereisolated first from the immune system and later found tobe secreted by and/or taken up by adipocytes, butothers, notably leptin [5], were identified first in adiposetissue and later shown to modulate immune function.The possibility of using leptin or other appetite
regulators as anti-obesity drugs has prompted intensivestudy of the metabolic interactions between the control
e front matter r 2005 Elsevier Ltd. All rights reserved.
of energy storage and immune function [6,7]. Unfortu-nately much of the data come from mice, especiallytransgenics [8], and human biopsy samples. Mice andyoung rats are so small and lean that only the largestdepots contain enough adipose tissue for most biochem-ical analyses. The choice is narrowed to the perirenal orgonadal (epididymal or parametrial) depots, both ofwhich incorporate no lymphoid structures, and some-times the inguinal, which contains a few nodes at thedorsal end. To avoid collateral damage, biopsy sitesfrom humans and larger animals are always chosen fortheir surgical accessibility, which in practice meansremoteness from lymph nodes and vessels, rather thanfor known site-specific properties. Consequently, infor-mation about the contributions of depots that incorpo-rate lymphoid structures and those that are ‘pure’adipose tissue is fragmentary, and does not readilysuggest a coherent hypothesis.Adipocytes in the omental, mesenteric and other
depots that incorporate lymphoid structures synthesize
ARTICLE IN PRESSC.M. Pond / Prostaglandins, Leukotrienes and Essential Fatty Acids 73 (2005) 17–3018
much less leptin per unit mass than depots with little orno lymphoid tissue [9–12]. As well as adipocytes, manyother cell types are now known to secrete and/orrespond to leptin [13] Bone marrow adipocytes producea fair amount of leptin, at least in vitro [14], but so dothe osteoblasts and chondrocytes themselves [15]. Leptinseems to be important for osteogenesis [16,17]. No netuptake of leptin was measured in the human splanchnicor pulmonary regions, in spite of the presence of manylymph nodes and other lymphoid tissues in the spleen,omentum, mesentery, gut and lungs, although the legs,with relatively fewer lymph nodes, are net leptinexporters [18]. These observations are sufficient to showthat conclusions that assume that the properties of large‘pure adipose tissue’ depots or cell lines in vitro arerepresentative of all adipose tissue can be misleading.Much of the confusion and paradoxical findings aboutthe relationship between adipose stores and immunefunction can be clarified by taking account of site-specific properties of adipose tissue, and paracrineinteractions between adipocytes and adjacent lymphoidcells.
2. Anatomical relations between adipose and lymphoid
tissues
All experimental immunologists are aware that lymphnodes are embedded in adipose tissue and some mentionremoving the tissue as the first stage of an investigation.The parallel development of adipose and lymphoidtissues in foetal and neonatal mammals was pointed outmore than 50 years ago [19]. Nonetheless, descriptions,whole mounts, sections, and cartoons of lymph nodes intextbooks and articles, not excepting the most recent[20], almost always appear without reference to theadipose tissue. Experimental studies of lymph node andvessel formation and repair also ignore the associatedadipose tissue [21–23].Histological examination of the outer capsule of
lymph nodes reveals a fairly thin, loose layer ofcollagenous material, with numerous very fine lymphvessels that branch from the main vessel and enter thenode over almost its entire surface [24]. Such tiny vesselsare permeable to large molecules and certain small cells[25]. Fine, branched vessels would convey fluids muchmore slowly than a single, wider vessel, but thearrangement does increase the area of vessels passingthrough the adipose tissue immediately surrounding thenode, where they may take up lipolytic products releasedby adjacent adipocytes into the extracellular space.Other sites where lymphoid tissues are in similarintimate contact with adipocytes include the omentum[26,27] and bone marrow [14,28].Adipocytes in lymphoid tissue-containing depots are
generally smaller than those of nodeless depots [29,30].
For experimental convenience, adipocytes found withinabout 2mm of a lymph node are designated perinodal,those approximately 5mm from a node as ‘middle’ andthose beyond 10mm as ‘remote from node’, thoughthese categories are not defined by any naturalboundaries that can be seen in vivo. The perinodaladipocytes are always the smallest (Fig. 1), withprogressively larger cells forming a continuous gradientfrom the node. However, this site-specific difference isonly relative (i.e. the absolute size of all adipocytes isgreater in larger or fatter animals [31]), and cannot beseen with the naked eye in fresh tissue. Very precisesampling using the nodes as ‘landmarks’ is needed toobtain consistency like Fig. 1. As shown by the large SEsin Fig. 1(A), some variation in the absolute size ofhomologous adipocytes is unavoidable even at constantbody composition, because people [32] and animals [33]differ in whether their adipose tissue consists of manysmall adipocytes, or fewer, larger adipocytes. Thus thevariance can be much reduced by expressing the volumesof the popliteal adipocytes as a fraction of those of thelarge perirenal depot for each rat, as in Fig. 1(B).As the data in Fig. 1 show, even very mild immune
stimuli that persist over weeks can reduce the mean sizeof adipocytes, especially in node-containing depots. Themass of the depot does not change, because moremature adipocytes appear in response to chronicinflammation (Fig. 2) [34]. Cytokines are probablyamong the mediators of this process. Perinodal adipo-cytes respond more strongly to a wide range of cytokinesas well as to norepinephrine [35]. Cytokine receptors onthe adipocyte surfaces can be stained to visualiseperinodal adipose tissue in situ [36].Dendritic cells are antigen-presenting cells that
permeate most non-neural tissues [37]; so few could becollected from the large, commonly studied depots bystimulation with chemokines such as C6Kine (CCL21)that DCs were believed to be absent from adipose tissue[38]. However, this method does yield substantialnumbers from adipose tissue associated with lymphoidstructures [39] especially mesenteric perinodal samplesand the milky spot-rich region of the omentum. Theseintra-abdominal lymphoid structures are continuouslyactive in euthermic mammals that are feeding regularly[27,40], while the popliteal lymph node is quiescentunless the foot and lower leg that it drains are inflamed[41].As might be expected, the numbers of dendritic cells
increase following chronic, mild immune stimulation,but this response is consistent only in the poplitealdepots (where the immune stimuli were applied), and inthe mesenteric perinodal and milky spot-rich region ofthe omentum. However, the effects of dietary lipids onyields of dendritic cells are as strong as those of the lowdose of lipolysaccharide used [39]. Dietary lipidsmodulate the abundance of dendritic cells in lymphoid
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Perirenal (x 10)
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Fig. 2. Haemocytometer counts of numbers of adipocytes in the
popliteal adipose depot that surrounded lymph nodes that were locally
stimulated by subcutaneous injection of 10 mg or 20 mg lipopolysac-
charide, or sterile saline, 3 times/week for 6 weeks (red bars), the other
(unstimulated) popliteal depot (blue bars) and the perirenal depot
(green bars). N ¼ 3� 18 large male rats. y Significantly different from
saline control, Po0:05; * stimulated and unstimulated popliteal depotsdifferent at Po0:05.
tissue-containing adipose tissue both adjacent to andremote from stimulated lymph nodes, with and withoutimmune stimulation. Fish oil (rich in n-3 fatty acids)
limits detectable responses in tissues adjacent to the siteof immune stimulation and in perinodal adipose tissuearound other lymph nodes. Diets enriched with sun-flower oil (with a low ratio of n-3/n-6 fatty acids)support more dendritic cells in adipose depots asso-ciated with lymphoid tissues, and increase the involve-ment of tissues anatomically distant from the site ofimmune stimulation in mild inflammatory responses.These effects may be among the ways in which dietarylipids modulate the immune response.
3. Paracrine traffic in fatty acids
Although small (each less than 3% of the totaladipose mass of an adult rat), the popliteal depot hasmany advantages for research into paracrine interac-tions between adipose and lymphoid tissues: its largelymph node that drains the lower leg and foot is in aconstant anatomical position, and is easily stimulated byminimally invasive procedures [34]. Lipids (mostlytriacylglycerols) extracted from popliteal perinodaladipose tissue (about 2mm from the node) containproportionately more polyunsaturated fatty acids, andfewer saturates, than those further from nodes or innodeless depots (Fig. 3A). This pattern is found inall node-containing depots with minor quantitativedifferences (Fig. 3B). Changes to the lipid compositionof the animals’ diet does not obliterate the pattern,
Fig. 3. Means7SE of the proportions of saturated FAs, monoenoic FAs, linoleic acid (18:2n-6) and a-linolenic acid (18:3n-3) extracted from the
triacylglycerols in samples of adipose tissue from: (A) Six sites in the popliteal depot (samples 1 and 2 were from as near as possible to the node on the
distal and proximal sides; 3 and 4 from the middle of the depot near where the sciatic nerve runs through it towards the gastrocnemius muscle,
respectively about 4mm and 6mm anterior to the node; sample 5 was from as far as possible from the node going towards the anterior, behind the
knee joint; sample 6 was from as far as possible from the node going dorsally). (B) Perinodal (Pn, within 2–3mm) or remote (Re, more than 10mm
from a lymph node) adipose tissue from 4 superficial node-containing depots and the mesentery, plus regions of the omentum with many or few milky
spots, and two samples (near and remote from a visible knot of blood vessels) from the nodeless perirenal. N ¼ 17 adult guinea-pigs. Asterisks refer
to differences between the composition of sample 1 and others from the same depot, assessed using Student’s t-test: *** significantly different at
Po0:001; ** significantly different at Po0:01; * significantly different at Po0:05.
although it changes the absolute abundance of indivi-dual fatty acids: after several weeks on a diet enrichedwith beef tallow (that consists almost entirely of
triacylglycerols containing saturated and monounsatu-rated fatty acids), perinodal adipose tissue samples fromeight different depots still contain ‘more than their
share’ of polyunsaturated fatty acids, and fewersaturates [42].Fig. 4 shows similar patterns of site-specific differ-
ences in fatty acid composition of the lipids extractedfrom the dendritic cells that permeate adipose tissue
associated with lymphoid structures [43]. In each case,the perinodal (or milky spot-rich) samples have morepolyunsaturated fatty acids than remote from nodesamples, but there were minor differences betweendepots: the ‘reciprocal’ fatty acid was oleic acid
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Unsaturation Index FAs in DCs (mostly phospholipids)
+ 20% fish oil+ 20% sunflower oilPlain chow
r = 0.99
Rats' diet for 6 weeks
r = 0.99 r = 0.89
UI
adip
ocyt
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mos
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Fig. 5. The correlations (r) between mean unsaturation indices (UI) of
fatty acids extracted from dendritic cells (DCs) and those of adipocytes
isolated from the corresponding sample of adipose tissue from
unstimulated rats (mesenteric perinodal and remote from nodes,
omental with many milky spots and with few milky spots, and
popliteal remote). Rats fed on unmodified chow (black type),
r ¼ 0:992; rats fed on chow + 20% sunflower oil for 6 weeks (green
type), r ¼ 0:989; rats fed on chow + 20% fish oil for 6 weeks (blue
type), r ¼ 0:891. N ¼ 6 sets of homologous samples from 3 similarly
treated cage-mate rats for each dietary group. The standard errors of
(C18:1) for dendritic cells from both the popliteal depot(Fig. 4A) and the omentum (Fig. 4B), but in themesentery (Fig. 4B), this role was taken by the twosaturated fatty acids.Arachidonic acid and docosahexaenoic acid are
particularly important in lymphoid cells because, aswell as being incorporated into membrane phospholi-pids, these polyunsaturated fatty acids are specificprecursors for prostaglandins and leukotrienes, short-lived messenger molecules that act in an autocrine orparacrine mode. The best known such lipid signalmolecules are eicosanoids, derived from arachidonicacid (20:4n-6) [44], but some, and the more recentlydescribed resolvins, are docosanoids, metabolites ofdocosahexaenoic acid (22:6n-3) and perhaps other long-chain polyunsaturated fatty acids of the n-3 family [45]The selective accumulations of arachidonic acid anddocosahexaenoic acid in perinodal dendritic cells(Fig. 4) [43] and of their precursors linoleic acid(18:2n-6) and a-linolenic acid (18:3n-3) in perinodaladipose tissue (Fig. 3) [42] suggest that these sitessynthesize and secrete more prostaglandins and leuko-trienes than similar tissues further from lymphoidstructures.The composition of fatty acids from lipids in dendritic
cells, summarized as unsaturation index in Fig. 5,correlates with that of the adipocytes to which theywere adjacent in vivo [43]. As always, diet is a majorinfluence, but the correlation between the compositionsof adipose and lymphoid cells remains. The simplest
explanation for this similarity is that the dendritic cellstake up their lipids from the contiguous adipocytes,rather than from the blood or lymph, as was previouslyassumed. Other lymphoid cells in lymph nodes alsoobtain many, perhaps all, of the fatty acids incorporatedinto lymphocytes newly formed in response to animmune stimulus from the adjacent adipose tissue [46].Some anecdotal evidence suggests paracrine interac-tions between adipose and lymphoid tissues alsooccur in humans: diurnal changes in the compositionof lymph in the human leg during bedrest or lightexercise are consistent with the conclusion that glycerolcomes mainly from adjacent adipocytes, while lipo-proteins, albumen and other metabolites are fromremote sources [47]Following mild immune stimulation, the proportions
of n-6 and n-3 polyunsaturated fatty acids become moreequal (ratios of n-6/n-3 polyunsaturated fatty acidsdecrease) and the proportions of very long-chain fattyacids increases, both changes consistent with increasingthe availability of eicosanoid and docosanoid signalmolecules [43].Like other lymphoid cells, dendritic cells have
hitherto been characterised by their numerous and verydiverse surface proteins. The demonstration of site-specific differences in fatty acid composition adds afurther layer of dendritic cell diversity. Phospholipidsconsisting of different fatty acids may determine howand where their surface proteins are manifest. Sincedendritic cells also communicate with each other andwith other lymphoid cells (especially T-lymphocytes) bymeans of lipid-derived molecules [48], availability of thepolyunsaturated fatty acid precursors could directlymodulate immune responses.The adipocytes associated with lymph nodes appar-
ently assist these changes in fatty acids in contiguousdendritic cells by adjusting the fatty acid composition oftheir triacylglycerols [43]. Ratios of n-6/n-3 polyunsatu-rated fatty acids in perinodal adipocytes convergefollowing chronic immune stimulation (i.e. values fromrats fed on sunflower oil decrease, those from fish oil-fedrats increase), partially obliterating the changes intro-duced by diet, without changing UIs. These effects wereobserved only in the locally stimulated depot, in thiscase the popliteal. Similar data from samples fromomental, mesenteric and perirenal adipocytes did notdiffer significantly. The mechanisms behind these effectsremain to be explored; one possible contributor isselective lipolysis that has been demonstrated inadipocytes from large depots that do not incorporatelymphoid structures [49]. Selective lipolysis seems to belargely non-enzymatic and therefore not completelyspecific to particular fatty acids, except in the case ofhighly unsaturated polyunsaturated fatty acids [50].Of the depots studied in Figs. 3 and 5, the popliteal is
by far the easiest to stimulate locally in vivo, and has the
ARTICLE IN PRESSC.M. Pond / Prostaglandins, Leukotrienes and Essential Fatty Acids 73 (2005) 17–30 23
additional advantages that each of two homologousdepots incorporate one large lymph node (and in a fewspecies including rats and pigs, a second small node). Tostudy the response of perinodal adipocytes to a minor,local, transient immune stimulus, adult guinea-pigs weregiven a single subcutaneous injection of 10 mg lipopoly-saccharide (less than 1% of the dose needed to generatefever in guinea-pigs) to the hind leg (the region drainedby the popliteal depot) during the evening and night,and killed 0–24 h later [51]. The immune stimulusproduces detectable changes in lipolysis from perinodaladipocytes isolated from the adjacent popliteal depotwithin an hour, and the response increases for thefollowing 6–9 h before waning. The peak rate in theremote-from-node samples is lower, and delayed relativeto perinodal. Parallel measurements from perirenaladipocytes (that are not associated with lymphoidstructures in vivo) showed no changes in glycerol releaseover the 24-h period. This experiment shows thatlipolysis from perinodal adipocytes and to a lesserextent, from those elsewhere in node-containing depotscan be stimulated via very mild, transient activation ofthe enclosed lymph node. The control of lipolysis fromlymph nodes may be mediated by cytokine receptors,whose appearance follows a similar time course inresponse to a local immune stimulus [36].Repeated stimulation and simultaneous measurements
of adipocyte lipolysis in depots other than that adjacentto the stimulated node shows that the response ofperinodal adipocytes spreads from locally stimulated tounstimulated depots [52]. Three hours after a singleinjection of LPS, lipolysis is raised only in the localperinodal adipocytes, but if a priming dose is given 12hearlier, lipolysis is higher in all the local poplitealadipocytes, and in perinodal adipocytes of the poplitealdepot in the other leg. If the priming dose is given 24hearlier, lipolysis is high throughout both popliteal depots,except the remote-from-node sample of the unstimulatedleg. Further stimulation within 24 h produces a similarpattern of responses. As well as acting on the remotepopliteal adipocytes, the mesenteric adipocytes alsorespond to subcutaneous injection of 20mg LPS intothe hind leg, but in a slightly different way. Althoughperinodal adipocytes increase lipolysis within 12h of aremote immune stimulus, the middle and remote sampleshardly change, even with repeated stimulation [52].The responses of adipocytes both adjacent to and
remote from the locally activated lymph node are muchinfluenced by diet, as shown in Fig. 6 [53]. Supplement-ing the animals’ diet with 10% sunflower oil increasesthe maximum LPS-stimulated lipolysis in perinodaladipocytes, and the rates attained in adipocytes else-where in the contiguous depot (Fig. 6(A)) and others(Fig. 6(B)). Feeding fish oil (which has a lower ratio n-6/n-3 polyunsaturated fatty acids) has the opposite effect[53]. The manifestation of receptors for tumour necrosis
factor-a on perinodal adipocytes following local injec-tion of LPS is also delayed after several weeks of dietslow in polyunsaturated fatty acids [54]. The mechanismsbehind these effects remain to be investigated, but theyindicate that the involvement of adipose tissue in minor,transient immune responses is much influenced by quitesmall changes in dietary lipids.The data in Fig. 7 suggest that itinerant dendritic cells
are among the possible mediators of these effects [55].Dendritic cells extracted from the adipose tissue stimu-late lipolysis, while those from an adjacent lymph nodeinhibit the process. Simulation of mild, chronic inflam-mation with lipolysaccharide amplifies both effects,which are strongest in perinodal and milky spot-richsamples and minimal in the remote from node adipocytes[55]. Switching from anti-lipolytic to lipolytic secretionsseems to be among the transformations that dendriticcells undergo as they migrate between the lymph nodesand the adjacent adipose tissue, and thus should beconsidered as part of the maturation process [56].All these effects are strongest in the adipocytes closest
to the lymphoid structures, and decline with anatomicaldistance from them. An example is shown in Fig. 8 forwhich the popliteal lymph nodes of rats were locallystimulated with 20 mg lipopolysaccharide three times aweek for the final 2 weeks of life [55]. More of the b-adrenoreceptor ligand [3H]-CGP 12177, and of the a-adrenoreceptor ligand [3H]-RX821002, binds to mem-branes from perinodal adipocytes than to the otherpopliteal samples in the absence of local immunestimulation. LPS-stimulation in vivo increases bindingof [3H]-CGP 12177 to the membranes of all poplitealadipocytes, with the greatest rise occurring in the middlesample, so that a clear gradient of binding fromperinodal to remote-from node appears. Binding of[3H]-CGP 12177 is unaltered after pre-incubation forfour hours with the dendritic cell activator, C6kine, forall the popliteal samples. But pre-incubation with thedendritic cell inhibitor, CEC, reduces CGP-binding withand without immune stimulation in the poplitealperinodal and middle samples, and abolishes the effectsof LPS in popliteal remote-from-node (Fig. 8) and in allintra-abdominal samples except the milky spot-richregion of the omentum [55].Although perinodal adipocytes respond strongly to
signals emanating from dendritic cells and lymph nodes,and to cytokines in vitro, they contribute less to whole-body lipid supplies than nodeless ‘general purpose’depots such as epididymal and perirenal [35].These experiments demonstrate that perinodal adipo-
cytes selectively take up and store dietary lipids (andperhaps other precursors) that are released in responseto local, paracrine signals from adjacent lymphoidcells that take them up. The depots that incorporatelymphoid tissue, especially the perinodal regions, arespecialised to support the growth and metabolism of
ARTICLE IN PRESS
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remote (10 mm from node)different from left adjacent bar
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10 -7 10 -50
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Fig. 6. The effects of dietary lipids on the spread of activation of lipolysis in adipocytes (A) within the locally stimulated popliteal depot and (B) from
the locally stimulated to the unstimulated popliteal depot. Adult guinea-pigs were fed for 6 weeks on plain chow (black/grey) or chow + 10% beef
tallow (red bars), chow + 10% sunflower oil (green bars) or chow + 10% fish oil (blue bars), then injected with 20 mg lipopolysaccharide every dayfor the final 4 days. Measurements of lipolysis from homologous samples of perinodal (about 2mm from node), middle (about 5mm from node) and
remote (more than 10mm from node) collagenase-isolated adipocytes are shown to the same scale for (A) the locally stimulated and (B) the other,
adjacent leukocytes. The concept of local, paracrineexchange of signals and nutrients between adjacentadipose and lymphoid tissues provides an explanationfor why adipose tissue in mammals is partitioned into a
few large and many small depots, most of the latterassociated with lymphoid structures [57].Lipid reserves that are dedicated to supplying the
immune system may be essential to combining fever with
ARTICLE IN PRESS
3.5
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Perinodal Middle Remote Perinodal Remote Milky spots Few MS Perirenal
+ dendritic cells from a dipose tissue
+ dendritic cells from local 1 ymph node
No dendritic cells
Popliteal Mesenteric Omental
+ LPSUnstim.
Fig. 7. Lipolysis over 3 h (expressed as mmol glycerol released/hour/100 adipocytes) in the presence of 10�5M norepinephrine in collagenase-isolated
adipocytes from 3 popliteal, 2 mesenteric, and 2 omental sites defined by their anatomical relations to lymphoid structures, and perirenal; black/white
bars: control, without dendritic cells; red/pink bars: with about 2000 dendritic cells that originated from the same adipose tissue as the adipocytes;
blue bars: with about 2000 dendritic cells collected from the adjacent lymph nodes (or the concentrated milky-spot region of the omentum); paler
bars: from unstimulated rats; darker bars from rats injected with 20 mg lipopolysaccharide (LPS) in both hind legs 3 times a week for the final 2 weeks.N ¼ 24 adult male rats for all bars. Differences between LPS-stimulated and unstimulated by 2-way ANOVA: * significant at Po0:05; ** significantat Po0:01; *** significant at Po0:001. Unmarked ¼ not significant (P40:05). Differences between control without dendritic cells by Student’s t-test:
y significant at Po0:05; yy significant at Po0:01; yyy significant at Po0:001. All these values are significant at Po0:001 for all pairs of samples inboth perinodal and omental milky spot categories, so for clarity, the yyy are omitted.
10
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Popliteal adipocyte membranes
+ LPSUnstim.
After pre-incubation with CEC (1 ng/ml)
After pre-incubation with C6 Kine (1 ng/ml)Control ([ 3H]-CGP 12177 alone)
Effect of LPS stimulation significantat: *** P<0.001; Effect of pre- incubation significantat: ††† P<0.001.
Fig. 8. Binding to membranes (expressed as disintegrations per min/mg protein) prepared from adipocytes from the three popliteal sites of the b-adrenoreceptor ligand [3H]-CGP 12177 alone (stippled bars), after incubation for 4 h at 25 1C with 1 ng/ml C6Kine (blue diagonal stripes) or with DC
inhibitor chloroethylclonidine (CEC, 1 ng/ml) (red diagonal stripes). Pale bars: from unstimulated rats; dark bars from rats injected with 20 mg LPS inboth hind legs 3 times a week for the final 2 weeks. N ¼ 17 for all bars. Differences between lipopolysaccharide-stimulated and unstimulated by 2-
way ANOVA: ** significant at Po0:01; *** significant at Po0:001. Unmarked ¼ not significant (P40:05). Differences between control with [3H]-
CGP 12177 alone, and after incubation with DC agonists by Student’s t-test: yyy significant at Po0:001.
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immune responses in defense against pathogens and toenabling both processes to be combined with anorexia.Many of the polyunsaturated fatty acids selectivelyaccumulated in triacylglycerols of perinodal adipocytes(see Fig. 3) and in dendritic cell lipids (Figs. 4 and 5) aredietary essential, and can be scarce, especially duringperiods of high metabolic demand (e.g. pregnancy,lactation) and reduced food intake (e.g. while disabledby injury or disease). Local control of lipolysis by theimmune system manages supplies of these essential fattyacids efficiently, and minimizes competition with othertissues (e.g. pyrogenic tissues such as muscles and liver)for circulating lipids.By releasing fatty acids only to lymphoid cells and
only when and where they are required, the perinodaladipose tissue partially emancipates immune functionfrom dependence on the quantity and lipid compositionof food. The incorporation of fatty acids into complexlipids does not have the equivalents of mRNA, tRNAand ribosomes that ensure the correct precursors areassembled into proteins, so can be less specific. Thefunctional consequences of different combinations offatty acids in structural lipids are not as well understoodas those arising from the substitution of different aminoacids in proteins. Possibilities include the requirementsof appropriate fatty acids for the formation of lipidrafts that are believed to have a major role in cellsignalling in lymphoid cells [58,59] and caveolae, whichare numerous in adipocyte membranes and essentialfor their responses to insulin [60], and probablyother signal molecules. Non-esterified fatty acids canact as regulators of surface receptors [61], and genetranscription [62].Paracrine interactions may also account for some
features of the anatomy of lymph vessels and nodesdescribed above. The branching of lymph vessels nearnodes would slow the passage of lymph and bring agreater surface area of vessels into contact withadipocytes, thus facilitating the exchange of signalsand metabolites.These site-specific differences and local, paracrine
interactions should be considered in interpreting datafrom the emerging field of lipidomics [63,64], inassessing the effects of dietary and blood-borne lipidson the properties of lymphoid cells, as well as for thetreatment and prevention of obesity [65,66]. Thecomparative biology and evolutionary implications ofthe functional associations between adipose and lym-phoid tissues are reviewed elsewhere [57].
4. Adipose tissue growth in inflammation, obesity and
starvation
Control of the proliferation of pre-adipocytes and oftheir maturation into adipocytes has long been studied
because of its implications for obesity in humans anddomestic livestock, but the emphasis has been on dietand energy balance [67] and angiogenic factors [68,69].More recently, the increasing prevalence of HIV-associated lipodystrophy and similar human disordershas directed attention to the possibility that inflamma-tory cytokines and other immune-derived factors alsohave a role in regulating adipogenesis [70,71]. Majorsystemic immune responses induce anorexia, andeventually cachexia, which deplete the adipose tissue,but chronic mild inflammation induces the formation ofmore adipocytes (Fig. 2) [34]. This effect may underliethe local growth of adipose tissue associated with long-standing lymphatic disorders in humans [72].In most types of cells, expansion usually triggers cell
division, resulting in an increase in tissue mass [73].Replete general-purpose adipocytes seem to be partiallyemancipated from these controls: in naturally obeseanimals, they can become very large for weeks,apparently without triggering the formation of excessivenumbers of adipocytes. An example is Svalbard reindeerthat live as far north as 801N and fatten rapidly duringthe brief summer and autumn: total adipocyte comple-ment is, as always, a bit variable, but no correlationbetween it and age can be found [74]. This property ofconstancy of cell complement may not be true ofadipocytes in lymphoid tissue-containing depots.Chronic inflammation stimulates an increase in theirprotein content rather than their lipid content, and moreadipocytes form without changes in total fatness (seeFigs. 5 and 6). In this respect, these adipocytes behavemore like typical mammalian cells and in so doing, maypartially abdicate their role as metabolic sinks for takingup excess lipid [75]. The capacity of a virus-inducedexcess of leptin to stimulate fat oxidation in whiteadipocytes [76] has only been demonstrated in theepididymal, a nodeless general purpose depot.The formation of additional adipocytes following
prolonged, chronic inflammation (Figs. 1 and 2) may bebehind reports of an association between markers ofviral infection and enlargement of certain adiposedepots in domesticated birds, captive primates andpeople [77]. The fact that several different pathogenshave been identified as causes of obesity [78] points tothe immune responses, rather than direct action of theorganisms themselves, as the main pathological mechan-ism. Such ‘obesity’ may in fact be the slow, selectivehypertrophy of adipose depots that enclose lymphoidtissue [39]. The effects are most obvious in (though notnecessarily confined to) the mesentery and omentum,large adipose depots that incorporate much lymphoidtissue and respond to remote as well as to localinflammation [35,39,42,53,55].Unpublished recent data (Sadler et al.) indicate that
local hypertrophy of adipose tissue associated withlymph nodes reverses very slowly over 3 months after
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the end of experimental inflammation. Increases ininfiltrating dendritic cells and in adipocyte apoptosiswere found, as expected, in the adipose tissue around theinflamed lymph node, and also in the perinodalmesenteric and milky spot-rich parts of the omental.This finding is another case in which adipocytes in theseintra-abdominal depots have well developed propertiesassociated with paracrine interactions with lymphoidcells and actively participate in immune responses thatin other respects are confined to remote part of thebody.The specialised lymphoid tissue-containing depots
remain relatively constant in mass in wild animals,including those that naturally become obese for part ofthe year [74,79,80]. In this respect, the anatomy ofnatural obesity differs strikingly from that of captiveprimates [81,82] and humans [83–85], in which the intra-abdominal depots, especially the mesentery and omen-tum often enlarge greatly. Hypertrophy of the mesenteryand omentum eventually leads to swelling of theabdomen and to high waist/hip ratios in human [86].Thick waists are common among people of averagebody mass who smoke heavily, which continuallyexposes them to toxins and irritants in tobacco [84], orwho are frequently exposed to a wide variety of parasitesand pathogens [87]. The link between hypertrophy ofintra-abdominal adipose tissue and susceptibility to typeII diabetes is long established [88]. More recently,chronic inflammation has been implicated in theprogress of long-standing obesity to insulin resistance[89], and diabetes [90]. Obesity is accompanied bychanges in blood cytokines, at least some of whichmay come from ‘non-fat cells’ found in human adiposetissue [91].Famines, wars and other tragedies have many times
demonstrated in humans the association between under-nutrition and increased susceptibility to pathogens andparasites, especially those that invade through the gutand skin, presumably due to immune inadequacies [92].But there are some paradoxes. Observations from wildanimals that naturally undergo large changes in bodycomposition suggest that ‘stress’ rather than weight lossper se is more important for impaired immune function.Although maintaining the immune system is energeti-cally expensive [93], many wild animals manage toremain healthy and breed normally while very lean, aswell as while very obese [31]. Studies of human athletes[94] suggest that endocrine and paracrine changescaused by ‘stress’, rather than weight loss per se, impairimmune function. Under some circumstances, notablyprolonged anorexia nervosa, immune function remainssurprisingly efficient in spite of massive reduction inadipose tissue mass [95], less fever in response toinfection [96] and altered plasma cytokines [97].Endocrine regulators of appetite, such as leptin [6,7]
and ghrelin [98] modulators of adipocyte metabolism
and energy utilization such as adenosine [99] alsoregulate the immune system. These findings supportthe concept of ‘coupling the metabolic axis to theimmune system’. The problem with this approach is thatit creates the impression that competition between theimmune system and other tissues for energy and othermetabolic resources is limiting, which is incompatiblewith some universal and very familiar aspects of normalresponses to infection.The ‘competition for resources’ model fails to explain
why fever generated by endogenous thermogenesisuniversally accompanies major immune responses tobacteria in mammals, and many cytokines associatedwith both reactions to infection promote anorexia [100]:nutrient intake stops just as energy expenditure increasesabruptly, hardly an efficient arrangement if the immunesystem and pyrogenic tissues were normally in directcompetition for fuels and other resources. Fever,systemic immune responses combined with anorexiacause small mammals (including human infants) to loseweight, and are thus regarded as deleterious. However,when adult mice were experimentally infected withListeria and fed forcibly or ad lib. over the followingdays, weight loss (due to the combination of anorexiaand high-energy expenditure) correlated positively withsurvival [101]. Overriding anorexia by force-feedingseems to accelerate the progress of the pathogens andhasten death, the opposite of what would be expected ifdepletion of lipid and protein reserves suppressedimmune function.A notable feature of naturally lean mammals is the
retention of a small amount of perinodal adipose tissuearound major lymph nodes [57,102]. As long as localinteractions between adipose and lymphoid tissues areunimpaired, the immune system can probably functionover a wide range of body compositions. Obviouscachexia with extensive depletion of muscle proteinseems to set in at about the same time as this perinodaladipose tissue disappears. Thus it is possible thatdeficiencies in perinodal adipose tissue and its capacityto support immune function, rather than reduction inwhole body energy supplies per se, are the mechanism bywhich nutritional ‘stress’ impairs immune function.
5. Conclusions
Adipose tissue around lymph nodes and in theomentum is specialised and the tissues function together.Adipocytes associated with lymphoid structures selec-tively accumulate and store certain fatty acids, especiallythose that are essential precursors for eicosanoids anddocosanoids, and release them in response to locallipolytic signals. Local provisioning of lymphoid tissuespartially emancipates immune function from changes inthe quantity and composition of food. Paracrine control
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of lipolysis by lymphoid cells reduces competition withother tissues for energy stores, thus enabling fever andother energetically expensive defences against pathogensto take place simultaneously with immune responses,and unrelated functions such as lactation and exercise.Animal experiments demonstrate that chronic inflam-mation can lead to hypertrophy of adjacent and remoteperinodal adipose tissue, particularly the omentum andmesentery. Paracrine interactions are difficult to detectin humans, as they have little or no blood manifestation,but they may be important for some forms of humanobesity and for changes in adipose tissue distributionassociated with chronic inflammation.
Acknowledgements
I thank The Leverhulme Trust, Bristol-Myers Squibb(USA), The Open University Trustees’ fund in 2002 andThe North West London Hospital Trust (St Mark’sHospital, Northwick Park) for financial support
