j ourna l ho me page: www.elsev ier .com/ locate /ysmim
eview
therosclerosis – A matter of unresolved inflammation
oana Violaa,∗, Oliver Soehnleina,b,c,∗
Institute for Cardiovascular Prevention (IPEK), LMU Munich, GermanyDepartment of Pathology, Academic Medical Center (AMC), Amsterdam, The NetherlandsGerman Centre for Cardiovascular Research (DZHK), Munich Heart Alliance, Munich, Germany
r t i c l e i n f o
rticle history:eceived 27 January 2015eceived in revised form 19 March 2015ccepted 27 March 2015
eywords:therosclerosis
a b s t r a c t
Atherosclerosis is commonly looked upon as a chronic inflammatory disease of the arterial wall arisingfrom an unbalanced lipid metabolism and a maladaptive inflammatory response. However, atherosclero-sis is not merely an inflammation of the vessel wall. In fact, the cardinal signs of unstable atheroscleroticlesions are primarily characteristics of failed resolution of a chronic inflammation. In contrast to acuteinflammatory events which are typically self-limiting, atherosclerosis is an unresolved inflammatorycondition, lacking the switch from the pro-inflammatory to the pro-resolving phase, the latter charac-
nflammation resolutionacrophage polarization
fferocytosis
terized by termination of inflammatory cell recruitment, removal of inflammatory cells from the siteof inflammation by apoptosis and dead cell clearance, reprogramming of macrophages toward an anti-inflammatory, regenerative phenotype, and finally egress of effector cells and tissue regeneration. Herewe present an overview on mechanisms of failed resolution contributing to atheroprogression and delivera summary of novel therapeutic strategies to restore resolution in inflamed arteries.
. Atherosclerosis – continued inflammation and failedesolution
Atherosclerosis is a complex, progressive disorder affectingarge and medium-sized arteries. Therapeutically, major concernsrise from the silent progression of this worldwide malady, oftenith no clinical evidence until occurrence of ischemic damageue to thrombosis or severe stenosis. Intrinsically arterioscle-otic vascular disease is an inflammatory condition characterizedy aberrant lipid metabolism and a maladaptive inflammatoryesponse. Classically, arterial inflammation is triggered by an insulto the endothelium, often at arterial branch points or at areasxperiencing disturbed flow, ultimately leading to endothelial cellctivation and recruitment of inflammatory cells to the vesselall. At the site of endothelial activation structural alterations,
n particular the exposure of proteoglycans, facilitate the reten-ion of low-density lipoprotein (LDL) particles in the intima [1,2],here they are susceptible to oxidative modification by reactive
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
xygen species (ROS) and enzymes released from inflammatoryells. As macrophages progressively take up modified lipo-roteins they give rise to foam cells. The continuous intracellular
accumulation of lipids (including cholesterol, oxysterols and otherfatty acids) induces endoplasmic reticulum stress triggering foamcell apoptosis [3]. However, aberrations in foam cells have beendescribed, such as the deficiency of pro-apoptotic factors (e.g. Baxand p53), that prevent cell apoptosis contributing to atheroscle-rosis progression [4,5]. In advanced atherosclerosis the sources ofapoptotic cells sturdily overwhelm the efferocytic program. Suchdefective efferocytosis allows apoptotic cells to undergo secondarynecrosis, thereby feeding the necrotic core and a constant flow ofpro-inflammatory mediators that override existing pro-resolutionsignals. This highly inflamed and necrotic core is central to theatherosclerotic plaque – vulnerable to structural disruption and animmediate precursor of acute cardiovascular clinical events.
In contrast to acute inflammatory events which are typi-cally self-limiting, atherosclerosis is an unresolved inflammatorycondition, lacking the switch from pro-inflammatory to anti-inflammatory mediators that characterizes the resolution phase.The resolution phase of inflammation embraces termination ofinflammatory cell recruitment, removal of inflammatory cells fromthe site of inflammation by apoptosis and dead cell clearance,reprogramming of macrophages toward an anti-inflammatory,regenerative phenotype, and finally egress of effector cells andtissue regeneration [6]. Understanding the different aspects of
s – A matter of unresolved inflammation, Semin Immunol (2015),
failed resolution in atherosclerosis provides the opportunity toidentify alternative therapeutic targets, theoretically with minimalside-effects. Thus, this review will concentrate on mechanisms offailed resolution in atherosclerosis, specifically on (1) continued
eukocyte accumulation (as result of continued recruitment, pro-iferation, and failed egress), (2) unbalanced M1/M2 macrophageolarization, and (3) impaired efferocytosis. Finally, we will outlineossible therapeutic ideas, many of which stem from preclinicaltudies of acute inflammatory models.
. Leukocytes gradually accumulate in atheroscleroticesions
Monocyte-derived cells are the most abundant leukocyte sub-et in the atherosclerotic plaque. The pertinent role of monocytes intherosclerosis was clearly evidenced when depletion of these cellsrom the circulation was shown to drastically reduce plaque forma-ion [7,8]. However, depletion of monocytes at later atherosclerotictages did not have any effect on the accumulation of macrophagesithin the lesion and also not in plaque composition or necrotic
ore formation [9], underscoring the importance of continuedecruitment of these cells to the initiation of the disease. Consistent
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
ith this notion, continued monocyte recruitment is a hallmarkuring atherosclerosis progression and regression of atheroscle-otic lesions is primarily driven by halted recruitment of monocytes10]. Besides monocytes, also the presence of neutrophils in the
ig. 1. Mechanisms of failed resolution in atherosclerosis. (a) Perpetuated leukocyte recresolution of inflammation. In the vessel wall other mechanisms take place that aid to purvival, failed egress of abundant monocyte-derived cells, and M1 favored polarization. Mxpression, whereupon survival and tumor suppressor genes are up and down-regulated,
ells are cleared) is hampered, and later on, when plaque rupture occurs, platelet aggrego occur, aggravating the already ongoing leukocyte recruitment. (b) Under hypercholesteukocyte recruitment via platelet chemokine (CCL5) deposition on the endothelium andlatelet derived P-selectin also plays a major role, mediating the delivery of pro-inflammathe environment characterized by low availability of nitric oxide (NOS) and bountiful reaecreted by endothelial cells as well as macrophages. Contrarily, pro-resolution mediatelease of cytokines and leukocyte recruitment. Also pentraxin 3 (PTX3) is known as anonocyte-derived cells via formyl peptide receptor 2 (FPR2). (c) Under inflammatory co
redominate, fueling a M1 macrophage polarization. M1 macrophages express high levelsRF5. Compared to M2 macrophages, M1 macrophages also possess a higher capacity to pravor an M2 phenotype, where high PPAR� and STAT6 activity result in increased capacitncluding resolution of inflammation. Apoptotic cells release “find me” and “eat me” signysophosphatidylcholine 8LPC), CX3CL1 or the nucleotides adenosine or uridine-5′-triphore responsible for the engulfment process. At later stages of atherosclerosis this action isecoys (oxLDL-derived LPC) that deviate efferocytic cells from their apoptotic targets bsuch as lactadherin or Gas6) as well as receptors (Mer tyrosine kinase, MERTK, or CD36elate to shedding of receptors directly enrolled in efferocytosis or decreased availabilityhrombospondin-1 (THBS1), which can bind to apoptotic cells and increase their clearancehenotype of macrophages and creating an imbalanced M1/M2 ratio that delays resolutio
PRESSmunology xxx (2015) xxx–xxx
atherosclerotic plaque has been reported, and a causal contribu-tion of neutrophils during various stages of atherosclerosis hasbeen established [11,12]. Continued leukocyte accumulation in thelesion site feeds an inflammatory milieu and prevents a turnovertoward resolution of inflammation.
Several processes contribute to the progressive accumulationof leukocytes in the atherosclerotic plaque, the most obvious,and supra mentioned, being leukocyte recruitment [7,10,13,14].However macrophage survival [15] and proliferation [16,17] inthe plaque as well as the limited ability of leukocytes to leaveatherosclerotic lesions [18] represent important processes criti-cally controlling the number of macrophages within atheroscleroticlesions (Fig. 1a).
2.1. Mechanisms of continued leukocyte recruitment
The classical cascade of leukocyte recruitment includes leuko-cyte rolling, activation, arrest and migration. Capture and
s – A matter of unresolved inflammation, Semin Immunol (2015),
rolling are mediated by selectins and the P-selectin glycopro-tein ligand-1 (PSGL1) receptor, whereas leukocyte arrest is ledby chemokine-activated integrins mainly lymphocyte function-associated antigene 1, LFA1, and very late antigene 4, VLA4,
uitment is one of the hallmarks of atherosclerosis, and it contributes to adjourningutting off resolution, namely: proliferation of resident macrophages, macrophageacrophages partial escape of apoptotic mechanisms is supported by aberrant gene
respectively. At later stages of disease, efferocytosis (the process by which apoptoticates are frequent and the close contact with oxLDL from the plaque is more likelyerolemia, increased LDL and oxLDL activate platelets upon contact contributing to
the formation of aggregates (neutrophil–platelet or platelet–platelet aggregates).ory molecules to the endothelium as well as to circulating monocytes. Furthermore,ctive oxygen species (ROS) favors the synthesis of leukotriene B4 (LTB4) and CCL2
ors, such as lipoxin A4 (LXA4), resolving D1 (RvD1) or Protectin D1 (PD) stop the endogenous inhibitor of engagement of neutrophils, whereas Annexin A1 acts onnditions, stimuli such as tumor necrosis factor (TNF) and interferon gamma (INF�)
of CD86 receptor as well as MHC II, and act via the transcription factors STAT1 andocess lipids. In a pro-resolution environment, cytokines such as IL-4, IL-13 and IL-10y of dead cell clearance. (d) Clearance of dead cells is required for many processes,als in order to be cleared. “Find me” signals such as sphingosine 1 phosphate (S1P),sphate (ATP and UTP) encourage macrophages migration, whereas “eat me” signals
hampered. Possible contributions are attributed to lysophosphatidylcholine (LPC)y keeping the cell receptors occupied, or to low expression of bridging molecules
for example) directly enrolled in the phagocytic process. Alternative explanations of bridging molecules or other similar mediators, such as complement 1q (C1q) or. Overall, the lesion milieu is mainly pro-inflammatory favoring an M1 polarizationn of inflammation.
ctivated by chemokines via inside-out signal. Therefore any stim-lus that, directly or indirectly, feeds this process ultimately results
n leukocyte recruitment. In this context, it is important to considerdiosyncratic markers of the atherosclerotic condition such as lowioavailability of nitric oxide (NO), high levels of reactive oxygenpecies (ROS), and low density lipoprotein (LDL), as these are con-tant factors throughout the disease and represent a continuous,ersistent stimulus (Fig. 1b).
Indeed, LDL is an important mediator to take into consider-tion, as in both its oxidized (oxLDL) and non-modified formt has been shown to activate platelets, hereby having an indi-ect effect on arterial leukocyte recruitment [19,20] and foamell formation [21,22]. Platelets have been strongly implicated ineukocyte recruitment in atherosclerosis [23,24]. Their action is
ultifaceted: platelets can deliver pro-inflammatory factors toeukocytes upon up-regulation of P-selectin [23,25], induce leuko-yte recruitment via the CX3CL1–CX3CR1 axis [26] or by chemokineCL5 deposition on the endothelium [7,11,20], and enhance neu-rophil transmigration in response to oxLDL [27,28]. In circulation,xidized phospholipids exist as part of lipoproteins [29], but it ist later stages of the disease, upon endothelial damage or plaqueupture, that the likelihood that adhering platelets are put in closeontact with oxLDL increases [30]. At these later stages plateletsan also infiltrate the plaque via leakage or rupture of newly formedicrovessels contributing to lipid accumulation and necrotic core
ormation [31–33]. Indeed, when the fibrous cap is vulnerable,hrombosis as well as neoangiogenesis occur [34], and provide newites for leukocytes to enter the lesion. Interestingly, clinical andxperimental evidences have demonstrated that thrombi are pre-erred areas to recruit circulating leukocytes [35,36] – a processlso mediated by platelets [36,37].
In atherosclerosis the availability of NO (nitric oxide) is lowhile ROS (reactive oxygen species) are known to be abun-ant. Interestingly, the synthesis of leukotrienes, driven by the-lipoxigenase enzyme, depends on ROS and it is inhibited byO [38]. Leukotrienes are inflammatory lipid mediators derived
rom the arachidonic acid (AA), which mediate leukocyte recruit-ent as well as survival [39–42]. Mechanistically leukotriene
4 (LTB4) in particular potentiates atherosclerosis mainly byncreasing the expression of fatty acid translocase/CD36 and of
onocyte chemoattractant protein-1 (MCP-1/CCL2) inducing aositive feedback loop to recruit leukocytes [43–45]. The outputf the leukotriene synthetic pathway is regulated by several fac-ors, for instance the amount of free arachidonic acid releasedrom cell-membrane phospholipids and the availability of NO andeactive oxygen intermediates, that modulate the activity of 5-ipoxygenase. These facts call attention to the driving force ofTB4 synthesis in atherosclerosis, especially as 5-lipoxigenase haslso been shown to correlate with the severity of the atheroscle-otic condition [46–48]. It may be possible that the number ofpoptotic cells and the inflammatory environment support theigh availability of AA by phospholipase A2, and that the lowioavailability of NO, together with the increased ROS, fuel the highctivity, and potentiate the expression of 5-lipoxigenase. Simulta-eously, the continued inflammatory signals induce the expressionf LTB4 receptors [49], amplifying this lipid mediator’s responses.he excessive production of leukotrienes in relation to prores-lution lipid mediators, such as lipoxin A4 (LXA4), resolvin D1RvD1) or protectin 1 (PD1), shifts the balance toward a proin-ammatory milieu. Accordingly, two case control studies [50,51] onuman variants of 12–15 lipoxygenase (12/15-LO), the key enzyme
nvolved in LXA4 biosynthesis, support a protective role for the
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
nzyme against coronary artery disease. 12/15-LO overexpressionn murine models of atherosclerosis underscores these findings,nd it further suggests that a deficiency in the enzyme’s avail-bility predisposes the chronic inflammation [52]. Mechanistically,
PRESSmunology xxx (2015) xxx–xxx 3
LXA4 leads to downregulation of CCL5 in murine macrophages, andtogether with RvD1 and PD1 (two other downstream products of12/15LO), suppresses a vast array of proinflammatory cytokinesand stimulates efferocytosis [52].
Continued leukocyte recruitment might also be caused bymalfunctioning of restraining or homeostatic mechanisms. Morespecifically, a number of endogenous inhibitors of leukocyterecruitment have been described [53], including pentraxin 3 (PTX-3) [54], developmental endothelial locus-1 (del-1) [55], galectin-1[56,57], and growth differentiation factor 15 (GDF-15) [58]. It istherefore reasonable to assume that an inhibition or decreasedavailability of these factors can contribute to continued leukocyterecruitment. In atherosclerosis however, up to date only PTX-3has been shown to be protective, whereas the effects of GDF-15are controversial and those of galectin-1 and del-1 remain unad-dressed. More specifically, PTX-3 deficiency results in increasedatherosclerosis accompanied by increased bone marrow monocy-tosis and macrophage accumulation in the lesion [59]. In addition,Apoe−/−PTX3−/− mice have an increment in expression of adhe-sion molecules, cytokines, and chemokines in the vascular wall,suggesting a modulator function of PTX-3 in vascular-associatedinflammatory responses. The atheroprotective effect of PTX-3 andits increased expression found in atherosclerotic samples [60] thussuggest an effort to restrict excessive leukocyte recruitment andmaintain a certain homeostasis. Whether the continued leuko-cyte engagement is due to insufficient increase in the endogenousinhibitor or a signal overriding owing to massive pro-inflammatorymediators, is unclear but likely a combination of both. On the otherhand GDF-15 deficiency studies in atherosclerosis are not conclu-sive: although lesser accumulation of macrophages in the lesion hasbeen reported [61], it seems to vary depending on the mouse strainand the duration of the western diet [62,63]. Also its effects on theatherosclerotic lesion are contradictory [62,64]. Recently, annexinA1 was added to this pool of endogenous breaks during atheroscle-rosis [65]. Annexin A1 acts as a proresolving ligand, via formylpeptide receptor 2 (FPR2), and abolishes chemokine-mediated acti-vation inhibiting leukocyte recruitment, hereby resulting in smallerearly lesions.
2.2. Leukocyte survival and proliferation
Macrophage accumulation observed during the developmentof atherosclerosis has been perceived for some time as a conse-quence of continued leukocyte recruitment. Nevertheless in thelast two decades numerous reports have consistently shown cellproliferation within the plaque, including of macrophage origin,suggesting a contribution of this action to the chronic inflammation[66]. Moreover, several approaches targeting cell cycle regulatorssupport a role for proliferation in atherosclerosis [67–69]. How-ever, the relative importance of macrophage proliferation to thismalady was only recently addressed. The study combining contin-uous bromodeoxyuridine (BrdU) delivery and parabiosis revealedthat the macrophage turnover in the lesions is rapid, thus signifi-cantly contributing to accumulation [16]. Indeed, macrophages, inparticular resident, can be self-maintained locally [70] and respondto inflammation with proliferation [71], possibly involving type1 scavenger receptor class A, SR-A, in the case of atherosclero-sis [16]. In the context of resolution, the answer to what specificsignals put an end to the proliferation stimulus, so as the fullcharacterization of these macrophages plasticity, is naturally ofinterest. Once addressed, these questions could potentially provideclinicians with new therapeutic targets to possibly push the envi-
s – A matter of unresolved inflammation, Semin Immunol (2015),
ronment toward the resolution phase.In addition to cell proliferation, sustained cell survival in par-
ticular of macrophage and at early stages of atherosclerosis whenefferocytosis is functional – accelerates disease progression [72].
urvival genes, such as Toso or cIAP2, cellular inhibitor of apo-tosis protein 2 [73], are upregulated during atherosclerosis whileew tumor-supressor genes (i.e. p27Kip1) are silenced, leading tooam cell aberration and survival [67]. This process continuouslyostpones resolution, as clearance of apoptotic cells induces theelease of anti-inflammatory mediators. Recently MafB, a tran-cription factor that induced myelomonocytic differentiation, wasdded to the list of apoptosis inhibitors in the context of atheroscle-osis. MafB expression is dominant in plaque foam cells, wheret inhibits apoptosis via de expression of apoptosis inhibitor of
acrophages (AIM) mediated by the liver X receptor/retinoid Xeceptor (LXR/RXR) axis [74]. Lipopolysacharide binding proteinLPB), another target gene of LXR, alike MafB is macrophage-specificnd promotes cell survival and atherogenesis [75]. In addition,lso chemokines and non-coding RNAs have been reported to pro-ote cell survival and to contribute to this chronic disease. The
bsence of CX3CR1 or its ligand results in decreased atheroscle-otic lesion, in an Apoe KO murine model. The introduction of Bcl2ene (involved in cell survival) restores the wild type mouse phe-otype, and the addition of CX3CL1 to cultured human monocyteseverts the induced cell death process, confirming the enrolment ofX3CR1–CX3CL1 axis in cell survival [15]. The long intergenic non-oding RNA (Linc)RNA-p21 is a direct transcriptional target of p53.t also functions as a component of the p53 pathway by interacting
ith p53 repressive complex to downregulate several other p53argets, it suppresses translation by associating with target mRNAs,nd it was recently found downregulated in the atheroscleroticlaque of Apoe−/− murine models [69]. LincRNA-p21 was found toegulate p53 pathway by interacting with mouse double minute 2MDM2) resulting in decreased cell proliferation and increased apo-tosis. Interestingly, the expression of LincRNA-p21 was found toorrelate with coronary heart disease, as its expression in patientsas found to be significantly lower as compared to control donors.aturally, these findings open doors to new possible therapeutic
argets to revert or slow down the development of atherosclerosisiming at increasing macrophage and foam cell apoptosis at earlytages, and fuelling efferocytosis as a proresolution process.
.3. Failed macrophage egress is a characteristic oftheroprogression
In acute inflammatory responses the return to tissue homeo-tasis and functionality is characterized by egress of infiltratedeukocytes. Early work has postulated that after performing theirentral tasks in resolution, macrophages emigrate to the drainingymph node, where they may play an important role in the pre-entation of antigens from the inflamed site [76]. Herein, matrixetalloproteinase (MMP)-mediated shedding of �2 integrin seems
mportant for macrophage egress from the injured tissue [77,78].lthough the importance of macrophage egress during inflamma-
ion resolution was recently challenged [79] it is still believedhat in acute inflammation macrophages disappear from the sitef inflammation by egress. In contrast, a failure in macrophagegress results in their accumulation and may potentially lead tohronic inflammatory diseases, such as atherosclerosis [80]. In fact,arly work has shown that lesional macrophages have the ability togress from atherosclerotic plaques. Herein, studies in atheroscle-otic pigs [81] and in monkeys [82], in which electron micrographshowed images compatible with macrophage foam cells exitingetween endothelial cells and the arterial lumen, support the con-ept of macrophage egress. The availability of atherosclerotic miceoupled with methods to follow cell trafficking, which are rel-
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
tively convenient in mice, led to a direct demonstration afterortic transplantation that during disease progression, emigrationas low, whereas placement of plaques into a regression environ-ent readily demonstrated macrophage exit [80,83]. The increase
PRESSmunology xxx (2015) xxx–xxx
of macrophage egress during plaque regression fits well withthe concept of atherosclerosis being an example of failed resolu-tion. Mechanistically, macrophages are retained in atheroscleroticlesions due to the presence of factors that boycott macrophageegress. One recently identified factor is netrin-1, which inhibitsthe chemotactic responses of macrophages to several chemokines[84]. Other factors promoting the retention of macrophages includeenhanced expression of adhesion molecules. These are more highlyexpressed in macrophages in progressing versus regressing plaques[85] and may thus contribute to macrophage enrichment duringatheroprogression.
Upon inflammation circulating monocytes infiltrate the dam-aged tissue, where they differentiate into macrophages, whosephenotype is heavily shaped by the surrounding milieu: growthfactors (macrophage colony stimulating factor, M-CSF, versus gran-ulocyte macrophage CSF, GM-CSF), cytokines (interferon gamma,INF�, IL-4 and IL-13 or IL-10), chemokines (CXCL4) [86], and otherplaque components such as oxidized proteins [87]. Macrophageheterogeneity is overwhelming with several subpopulationsdescribed in the literature [88], typically well characterized in vitrobut to a lesser extent in vivo. However, the initial and simplisticview of classically (pro-inflammatory phenotype) and alterna-tively (anti-inflammatory phenotype) polarized macrophages, alsoreferred to as type M1 and M2, respectively (reflecting the Th1and Th2 nomenclature in T cells), is frequently employed. Owingto the complexity of macrophage heterogeneity and the differ-ence between murine and human markers, this simplistic viewis preferred to assess the macrophages’ phenotype throughoutthe different stages of inflammation. Evidence implies that animbalance in macrophage polarization sustains the inflammatoryenvironment: plaque macrophages express arginase-1 (Arg1, amarker for M2 phenotype) at early stages of atherosclerosis, whenefferocytosis is fully functional, whereas at later stages of disease,when the inflammation is chronically perpetuated, the expres-sion of arginase-2 (typical for M1) predominates [89]. Accordingly,induction of disease regression shows a switch of macrophagemarkers from pro-inflammatory (monocyte chemotactic protein-1 and tumor necrosis factor) to anti-inflammatory (Arg1, mannosereceptor, MR, and CD163) [90]. However, translating these find-ings to human pathophysiology is not straightforward, as in vitropolarized human macrophages do not express Arg1 as well asother markers well-described for murine macrophage subtypes[91,92]. Nevertheless, in epicardial adipose tissue of patients withcoronary artery disease (CAD) the M1/M2 ratio is changed com-pared to non-CAD patients, with the M1/M2 macrophage ratiopositively correlating to the disease severity [93]. Recently, inan attempt to address the M1/M2 ratio in hypercholesterolemicpatients, peripheral blood samples were analyzed for circulatingCD68+CCR2+ and CX3CR1+CD206+/CD163+, which rather than rep-resenting the traditional monocyte subsets aimed at reflectinghuman macrophage polarization (M1 and M2, respectively), whichtakes place after tissue infiltration [94]. Despite this study’s limi-tations, including the much debated assumption that M1 and M2macrophages derive from classical and nonclassical monocytes,respectively, it clearly showed an increase in the CD68+CCR2+ pop-ulation in hypercholesterolemic samples as compared to healthycontrols. Moreover, the (CD68+CCR2+)/(CX3CR1+CD206+/CD163+)ratio (referred in the study as M1/M2 ratio) was increasedin patients with plaques compared to those without. Impor-
s – A matter of unresolved inflammation, Semin Immunol (2015),
tantly, traditional gating to identify monocyte subsets showed nodifferences in the distribution of classical, intermediate and non-classical monocytes, supporting the study’s correlation between(CD68+CCR2+)/(CX3CR1+CD206+/CD163+) and M1/M2 ratio.
Hence, according to this current hypothesis classically activatedacrophages feed inflammation and plaque vulnerability, whereas
n alternative phenotype is likely to support plaque stability orven regression (Fig. 1c). In agreement, human atheroscleroticlaque M2 macrophages, identified as CD68+/MR+, poorly handleholesterol and have strong phagocytic properties, attributed toigh PPAR� and low LXR� activities [95]. The role of CD68+/MR−
nd CD68+/MR+ macrophages is further corroborated by their local-zation in the lesion: the first cell markers colocalize with rupturerone plaque areas and lipid core, much unlike the second [95,96].evertheless, an inconsistent M1/M2 markers expression profilef lesional foam cells is observed [96], once more underscoring thentricacy in characterization of macrophages subtype in vivo. Thembivalent expression spectrum by foam cells, combining few M1nd M2 markers, is likely to stem from the myriad of stimuli thatopulate the lesion as well as the ensuing microenvironments.
In conclusion, in atherosclerosis the shift of macrophage phe-otype from M1 to M2 appears to be put on hold, and shapinglaque macrophages toward an alternatively activated phenotypeay facilitate plaque stability or even regression, via resolution
f inflammation. Cytokines such as M-CSF, and interleukins 4, 13nd 10 (IL-4, IL-13, IL-10) activate specific transcription factors inacrophages (i.e., signal transducer and activator of transcription
and 6, STATs 3 and 6, interferon regulatory factor 3, IRF3, PPARnd LXR�) that result in the expression of proresolution mediatorsuch as Arg1 (in mice), tumor growth factor � (TGF �) and IL-10,ereby inducing an anti-inflammatory phenotype [97]. Therefore,odulation of such transcription factors can be beneficial to induce
Clearance of apoptotic cells by phagocytes, involving a processermed efferocytosis, impedes the accumulation of necrotic corpsesnd triggers phagocyte re-programming toward anti-inflammatoryhenotypes hereby boosting inflammation resolution [6]. Duringtherosclerosis, effective efferocytosis is hampered resulting in theccumulation of apoptotic cells [98,99] and the delay of resolutionf inflammation [100] (Fig. 1d). Apoptotic cells produce a plethoraf “find-me” and “eat-me” signals that mediate phagocyte attrac-ion, interaction, and finally engulfment [101,102]. Consequently,n insufficient production of attraction and recognition moleculesnd/or an altered interaction and phagocytosis may explainefective efferocytosis during atherosclerosis. Several “find-me”ignals released by apoptotic cells have been identified includingysophosphatidylcholine (LPC), fractalkine (CX3CL1), sphingosine--phosphate (S1P) and the nucleotides adenosine-5′-triphosphatend uridine-5′-triphosphate [101,102]. During cell apoptosis, aaspase-3 dependent activation of the phospholipase A2 (PLA2)esults in the production and release of LPC triggering phagocyteigration and engulfment [103]. During hypercholesterolemia,
xLDL [98] or LPC generated by lipoprotein associated-PLA2 (Lp-LA2)-dependent oxLDLs hydrolization [104] reduces apoptoticell clearance. By competing with the same receptors, enhancedevels of oxLDL-derived LPC may impair apoptotic cell elimination.n addition to inhibiting efferocytosis, other pro-inflammatoryctions are associated with LPC [105] and may explain the positiveorrelation of both LPC and Lp-PLA2 with symptomatic plaques,acrophage content and inflammatory cytokine production [105].n the other hand, S1P acts as “find-me” and “eat-me” signal that
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
as been associated with an anti-inflammatory and atheroprotec-ive function, since administration of a synthetic homolog induceslaque stability by reducing plaque inflammation and necroticore formation [106]. Of the various receptors mediating responses
PRESSmunology xxx (2015) xxx–xxx 5
to S1P, specific activation of S1P receptor 1 was shown to reduceplaque inflammation [107].
Once phagocytes are adjacent to the apoptotic cell, “eat-me”molecules expressed in the apoptotic membranes enable theirrecognition [101,102]. Leukocyte-borne pentraxin 3 is exposedupon neutrophil apoptosis promoting clearance by macrophages[108]. Interestingly, pentraxin 3 accumulates in the lesion of agingmice and genetic depletion of pentraxin 3 results in enhancedplaque size characterized by a hyperinflammatory phenotype [59].Other molecules such as trombospondin-1 or complement C1q bindto apoptotic cells acting as “eat-me” signals and its depletion resultsin enhanced plaque vulnerability due to defective apoptotic cellclearance [109,110]. Phosphatidylserine (PS) is the most commoneat-me signals. The soluble molecule MFG-E8 (lactadherin) acts asa bridge molecule linking PS on apoptotic cells with vitronectinreceptor on macrophages. Genetic deletion of MFG-E8 results inaccumulation of apoptotic debris and promotes advanced plaques[111,112]. Interestingly, the authors found a decrease of MFG-E8expression in advanced atherosclerotic lesion so that impairmentin bridging molecule expression or accelerated turn-over due tofacilitated degradation may provide an explanation for failed effe-rocytosis in atherosclerosis.
On the other hand, several phagocytic receptors are implicatedin apoptotic cell clearance and its modulation has relevant effectson apoptotic corps accumulation during advanced atherosclero-sis. The Mer tyrosine kinase (MERTK) receptor which binds themolecule Gas6 is crucially implicated in lesional efferocytosis sinceits depletion induces apoptotic cell accumulation and expansionof necrotic core and plaque sizes [113,114]. While mechanismsof defective efferocytosis at late stage atherosclerosis despite theabundance of lesional macrophages are not entirely clear, shed-ding of receptors involved in dead cell clearance may provide oneexplanation. Shedding of engulfment receptors not only depletesphagocytic capacity of the host cell but also generates a potentialcompetitive inhibitor that can block efferocytosis on neighbor-ing efferocytes. Notable examples of soluble receptors involved indead cell clearance include CD36, lectin-like oxidized low-densitylipoprotein receptor-1, low-density lipoprotein receptor-relatedprotein-1, and MERTK. Evidence for soluble MERTK is found ina variety of chronic inflammatory disorders, including systemiclupus erythematosus (SLE), rheumatoid arthritis, and cardiovascu-lar diseases. Interestingly, the metalloproteinase ADAM17 inducesMERTK receptor shedding [115] releasing a soluble form whichinhibits efferocytosis [116]. In addition, ADAM17 may shed CD36,a scavenger receptor not just involved in phagocytosis of LDL butalso in thrombospondin-mediated efferocytosis, and thus reduceapoptotic cell clearance [117]. Finally, the enzyme transglutamin-ase 2 (TG2), which is involved in MFG-E8-dependent apoptotic cellengulfment, is also implicated in lesional efferocytosis. Bone mar-row transplant of TG2-deficient cells in Ldlr−/− mice resulted inenhanced plaque size, and necrotic core with reduced efferocytosiscapacity [118].
With the importance of bridging molecules and their respectivereceptors in dead cell clearance in atherosclerotic lesions, recentstudies have aimed at identifying transcriptional regulators forthese molecules. Genetic variation at the chromosome 9p21 risklocus promotes cardiovascular disease. A recent study has revealedthat loss of Cdkn2b, one candidate gene at this locus, resultsin development of advanced atherosclerotic lesions with largenecrotic cores [119]. Furthermore, human carriers of the 9p21risk allele had reduced expression of CDKN2B in atheroscleroticplaques, which was associated with impaired expression of cal-
s – A matter of unresolved inflammation, Semin Immunol (2015),
reticulin, a ligand required for activation of engulfment receptorson phagocytic cells [120]. Another recent study sheds light onERK5 as master regulator of molecules involved in efferocytosis[121]. ERK5 is a member of the mitogen-activated protein kinase
amily activated by redox and hyperosmotic stresses, growthactors, and pathways activated by certain G-protein-coupledeceptors. Lack of ERK5 reduces expression of bridging moleculesnd receptors involved in efferocytosis on macrophages. As aonsequence, atherosclerotic lesions in these mice are larger andharacterized by increased necrotic core areas and traits of plaqueestabilization [121]. Finally, it has been established that advancedtherosclerotic lesions are hypoxic. Recently, it was shown thatypoxia accelerates atherosclerosis by repressing the expressionf efferocytosis receptors MERTK and CD36 [122]. Reversal ofesional hypoxia by breathing hyperoxic carbogen could preventecrotic core expansion by enhancing efferocytosis.
. Therapeutic implications
Development of specific plaque stabilization therapies is aajor goal of preventive cardiology to avoid or reduce the inci-
ence of future coronary events in high risk patients. Randomizedlinical trials have demonstrated the beneficial effect of statins,nti-platelet, or anti-hypertensive compounds in diminishing ACSrevalence, and accordingly, current clinical guidelines recom-end their administration in patients with atherosclerotic vascular
isease. The primary aim of these strategies is to deplete thenitiating stimulus for atherosclerosis, i.e. arterial hypertension,latelet activation, and hypercholesterolemia. Despite the successf lipid lowering statins the residual burden of cardiovascularisease in developed countries remains immense so that a largeart of research during the last 20 years has focused on develop-ent of anti-inflammatory strategies. Such included the inhibition
f myeloid cell recruitment by use of antagonist to chemokineeceptors or cell adhesion molecules, blockade of matrix-degradingnzymes, or neutralization of inflammatory cytokines [123–126].owever, many of these approaches were unsuccessful at pre-linical or clinical stages. Reasons for these failures include thetriking redundancy of inflammatory mediators, rendering inter-erence with just one molecule insufficient, prominent off-targetffects due to cross-reactivity with receptors of similar struc-ure, discrepancies between animal models and human diseases,nd the importance of the targeted molecule in host defensend consequently compromised immune responses. Atheroscle-osis, however, is not merely an inflammation of the vessel wall.n fact, the cardinal signs of unstable atherosclerotic lesions arerimarily characteristics of failed resolution of a chronic inflam-ation [127], such as perpetuated inflammatory cell recruitment,
nsuccessful removal of inflammatory cells from the site of inflam-ation, and failed reprogramming of macrophages toward an
nti-inflammatory, regenerative phenotype [6]. A change in thenderstanding of the pathophysiology of atherosclerosis from ahronic inflammatory disorder toward a pathophysiology primar-ly originating from failed resolution of inflammation also requires
shift in emphasis from inhibitory therapy to replacement ther-py, i.e. from antagonism to agonism. Anti-inflammatory treatments commonly based on neutralizing or inhibiting a molecule or aathway. With the redundancy of signaling pathways involved in
nflammatory leukocyte recruitment and activation, neutralizationf just one molecule may often not be satisfactory. Resolution-nducing receptor agonists on the other hand may overcome thisedundancy by overruling a pathway common to several inflamma-ory molecules, e.g. chemokine-driven integrin activation. Anotherdvantage of resolution-inducing agonists is their superior bene-t:risk ratio. Treatment of atherosclerosis is naturally a chronic
ntervention and drugs interfering with homeostatic, regulatory
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
rocesses such as antagonists to chemokine receptors or adhe-ion molecules also impact on leukocyte trafficking during steadytate. In addition, the widespread expression of these molecu-ar target structures also affects non-immune cells. In contrast,
PRESSmunology xxx (2015) xxx–xxx
resolution-inducing receptors such as FPR2 are predominantlyexpressed on myeloid cells. In addition, induction of resolutionis only operational during inflammation and therapeutic target-ing does hence not influence homeostatic processes. Interestingly,resolution-inducing pathways are also protective in models ofsevere polymicrobial sepsis [128,129], thus further supporting thelow risk profile of proresolution strategies. Nevertheless, withatherosclerosis manifesting primarily in elderly people likely suf-fering from other comorbidities, safety of resolution-inductionwill require tissue-specific treatment. In recent years, severalnanomedicine-based strategies have emerged for delivery of pep-tides and nucleic acids have emerged [130,131] for use in animalmodels of atherosclerosis and further optimization is required fortesting in humans.
In a recent study, Fredman et al. employ the FPR2 agonis-tic peptide Ac2-26 to boycott inflammatory processes and toinduce a dominant program of resolution [132]. To ensure depo-sition of Ac2-26 in advanced atherosclerotic lesions the peptideis packed in collagen IV targeting nanoparticles [133] thus likelyreducing systemic side effects. Atherosclerotic lesions of micereceiving Ac2-26 containing nanoparticles are characterized bylower macrophage numbers, reduced proteolytic activity, loweroxidative stress, smaller necrotic core sizes, and higher amounts ofanti-inflammatory IL10 levels [132]. The protective effect of Ac2-26 was abolished in mice lacking formyl peptide receptor (FPR)2 (and FPR3) thus indicating that this receptor may be of spe-cial importance during therapeutic induction of arterial resolution.Although the study provides only little mechanistic insights, pre-vious work may explain how Ac2-26 functions in the context ofplaque progression. Herein, Annexin A1 was identified as a bridg-ing molecule opsonizing apoptotic cells to mediate efferocytosispossibly explaining smaller necrotic core sizes [134]. In addition,Annexin A1 induces a favorable macrophage M2a phenotype [135]which is characterized by the release of the cytokines IL10 and TGF�and an enhanced capacity to clear dead cells. Finally, a recent studyrevealed that Annexin A1 and Ac2-26 overrule chemokine-evokedintegrin activation and thus inhibit arterial recruitment of myeloidcells [65]. Interestingly Ac2-26 had the ability to counteract integrinactivation exerted by various chemokines so that delivery of Ac2-26may be superior in inhibition of arterial leukocyte recruitment ascompared to blocking individual chemokine receptors. As a conse-quence, repetitive administration of Ac2-26 during early stages ofatherosclerosis led to reduction in atherosclerotic lesion sizes andlower lesional macrophage numbers. Chemerin-derived peptideshave similar activities as Ac2-26 and hence it is interesting to spec-ulate about the potential protective effect of chemerin fragments.In fact, while chemerin primarily attracts antigen-presenting cells,C-terminal peptides released from chemerin by cysteine proteaseshave opposite effects – they block myeloid cell tissue infiltrationand the release of pro-inflammatory mediators from classicallyactivated macrophages [136]. In addition, chemerin-derived pep-tides promote clearance of dead cells by macrophages [137,138].
Among the resolution-inducing agents, resolving lipid media-tors like LXA4, resolvin E1 and D1, and Protectins exert multipleproresolving effects, including inhibition of neutrophil tissue infil-tration, induction of neutrophil apoptosis and efferocytosis andstimulation of tissue repair [139]. In animal models of inflam-mation, resolvin E1 is protective in a model of colitis as shownby decreased neutrophil tissue infiltration, pro-inflammatory geneexpression, and improved survival [140]. Similarly, resolvin E1reduces leukocyte infiltration in a mouse model of asthma andultimately improves lung function [141]. LXA4 is equally potent in
s – A matter of unresolved inflammation, Semin Immunol (2015),
inducing resolution, however due to rapid degradation in the bloodstream, various delivery strategies have evolved, including the useof stable forms such as fluorinated analogs [142]. Nanoparticles canbe enriched with aspirin-triggered resolvin D1 or a LXA4 analog to
ccelerate resolution in experimental peritonitis [143]. However,nly little is known about resolving lipid mediators in the context oftherosclerosis. Dietary administration of omega-3 fatty acids, thusncreasing the production of resolving lipid mediators, has beenhown to reverse defective efferocytosis in murine atheroscleroticesions [144]. In line, dietary omega-3 supplementation attenu-tes atherogenesis [145,146] and high-dose eicosapentaenoic acidEPA) causes lesion regression [147]. Genetically mediated increasef the n − 3/n − 6 ratio in mice also yields less atherosclerosis [148].oreover, overexpression of 12/15-lipoxygenase, a key enzyme in
he production of resolving lipid mediators, limited atherosclerosisroviding support for a role of endogenous biosynthesis of resolving
ipid mediators and for the possible targetability of these media-ors [52]. Aspirin, on the other hand, although triggering resolving
ediators it is only unconditionally recommended for secondaryardiovascular disease prevention (CVD), i.e. for patients with priorardiovascular events, and controversially, not for primary preven-ion (patients with no cardiovascular events). Such status is relatedo the uncertainty of a precise risk/benefit ratio, as the statisticallyignificant reduction in the risk of a first myocardial infarction (inon-diabetic patients) is accompanied by a substantial increase inisk of both gastrointestinal bleeding and hemorrhagic stroke [149].owever, Halvorsen and colleagues, after revising several random-
zed controlled trials recommend aspirin therapy in primary CVDrevention according to patients baseline risk [149].
With the dominance of M1 macrophages in advancedtherosclerosis and the fact that M2 macrophages accumulate inodels of plaque regression, induction of M2 polarization may
ave a favorable outcome [83,90,150]. From a functional pointf view, the properties of M2 macrophages align well with theacroscopic changes observed in regressing plaques the loss of
nflammatory cells and the remodeling of tissue to a morphol-gy associated with less risk of rupture, such as a reduction in theecrotic core. That the M1/M2 state might undergo manipulation
n human plaques as a preventive or therapeutic approach to ACSas suggested by one clinical study, in which patients who receivedigh-density lipoprotein infusions before peripheral atherectomieshowed a tendency for decreased inflammatory mediators in thelaques relative to those excised from the control group [151].xperimental studies support this notion. Two recent studies intherosclerotic mice in which the M2 state was induced and main-ained by injection of either IL-13 (a strong polarizer in vitro, whichn addition to IL-4 is secreted by TH2 lymphocytes) or helminthntigens showed reduced atherosclerosis progression and plaquenflammation [152]. Also inhibition of transcription factors typi-al for M1 (i.e. IRF5) is able to reprogram macrophage phenotype153]. M2 macrophages are also characterized by enhanced abilityo clear dead cells. In this context, several nuclear receptors suchs PPAR and LXR are linked to efferocytosis activity and its acti-ation through specific agonists has been shown to be effectiven experimental inflammatory models, including atherosclerosis88]. However, their association to several adverse side effectsas limited their use [154]. Interestingly, the novel LXR agonist,211945, has been recently tested in a rabbit model of atherosclero-is thereby inducing atherosclerosis regression without secondarynwanted effects [155].
. Perspectives
Since early studies by Rudolf Virchow atherosclerosis is lookedpon as a lipid-driven continuous inflammation of the arterialessel wall. However, experimental evidence from models of
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
theroprogression and atheroregression has consistently shownhat the pathophysiology of atherosclerosis has many character-stics of failed inflammation. While much of this discussion maye academic, its value becomes important when designing specific
PRESSmunology xxx (2015) xxx–xxx 7
interference strategies. In fact, while anti-inflammatory treatmentstrategies primarily rely on inhibitory strategies, resolution-inducing strategies make use of agonist delivery approaches. Suchparadigm shift provides opportunities for novel therapeutic manip-ulation in the future.
Disclosures
None.
Acknowledgements
The study was supported by the DFG (SO876/6-1, SFB914 TP B08,SFB1123 TP A06 and TP B05), the Else Kröner Fresenius Stiftung, theNWO (VIDI project 91712303), the LMU Excellence and the FöFoLeProgram of the LMU Munich.
References
[1] J. Boren, K. Olin, I. Lee, A. Chait, T.N. Wight, T.L. Innerarity, Identification of theprincipal proteoglycan-binding site in LDL. A single-point mutation in apo-B100 severely affects proteoglycan interaction without affecting LDL receptorbinding, J. Clin. Investig. 101 (1998) 2658–2664.
[2] G.P. Kwon, J.L. Schroeder, M.J. Amar, A.T. Remaley, R.S. Balaban, Contributionof macromolecular structure to the retention of low-density lipoprotein atarterial branch points, Circulation 117 (2008) 2919–2927.
[3] R. Salvayre, N. Auge, H. Benoist, A. Negre-Salvayre, Oxidized low-densitylipoprotein-induced apoptosis, Biochim. Biophys. Acta 1585 (2002) 213–221.
[4] J. Liu, D.P. Thewke, Y.R. Su, M.F. Linton, S. Fazio, M.S. Sinensky, Reducedmacrophage apoptosis is associated with accelerated atherosclerosis in low-density lipoprotein receptor-null mice, Arterioscler. Thromb. Vasc. Biol. 25(2005) 174–179.
[5] B.J. van Vlijmen, G. Gerritsen, A.L. Franken, L.S. Boesten, M.M. Kockx, M.J. Gij-bels, et al., Macrophage p53 deficiency leads to enhanced atherosclerosis inAPOE*3-Leiden transgenic mice, Circ. Res. 88 (2001) 780–786.
[6] A. Ortega-Gomez, M. Perretti, O. Soehnlein, Resolution of inflammation: anintegrated view, EMBO Mol. Med. 5 (2013) 661–674.
[7] O. Soehnlein, M. Drechsler, Y. Doring, D. Lievens, H. Hartwig, K. Kemmerich,et al., Distinct functions of chemokine receptor axes in the atherogenic mobi-lization and recruitment of classical monocytes, EMBO Mol. Med. 5 (2013)471–481.
[8] R. Ylitalo, O. Oksala, S. Yla-Herttuala, P. Ylitalo, Effects of clodronate(dichloromethylene bisphosphonate) on the development of experimentalatherosclerosis in rabbits, J. Lab. Clin. Med. 123 (1994) 769–776.
[9] V. Stoneman, D. Braganza, N. Figg, J. Mercer, R. Lang, M. Goddard, et al., Mono-cyte/macrophage suppression in CD11b diphtheria toxin receptor transgenicmice differentially affects atherogenesis and established plaques, Circ. Res.100 (2007) 884–893.
[10] S. Potteaux, E.L. Gautier, S.B. Hutchison, N. van Rooijen, D.J. Rader, M.J. Thomas,et al., Suppressed monocyte recruitment drives macrophage removal fromatherosclerotic plaques of Apoe−/− mice during disease regression, J. Clin.Investig. 121 (2011) 2025–2036.
[11] M. Drechsler, R.T. Megens, M. van Zandvoort, C. Weber, O. Soehnlein,Hyperlipidemia-triggered neutrophilia promotes early atherosclerosis, Cir-culation 122 (2010) 1837–1845.
[12] O. Soehnlein, Multiple roles for neutrophils in atherosclerosis, Circ. Res. 110(2012) 875–888.
[13] F.K. Swirski, M.J. Pittet, M.F. Kircher, E. Aikawa, F.A. Jaffer, P. Libby, et al., Mono-cyte accumulation in mouse atherogenesis is progressive and proportional toextent of disease, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 10340–10345.
[14] F. Tacke, D. Alvarez, T.J. Kaplan, C. Jakubzick, R. Spanbroek, J. Llodra, et al.,Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumu-late within atherosclerotic plaques, J. Clin. Investig. 117 (2007) 185–194.
[15] L. Landsman, L. Bar-On, A. Zernecke, K.W. Kim, R. Krauthgamer, E. Shagdar-suren, et al., CX3CR1 is required for monocyte homeostasis and atherogenesisby promoting cell survival, Blood 113 (2009) 963–972.
[16] C.S. Robbins, I. Hilgendorf, G.F. Weber, I. Theurl, Y. Iwamoto, J.L. Figueiredo,et al., Local proliferation dominates lesional macrophage accumulation inatherosclerosis, Nat. Med. 19 (2013) 1166–1172.
[17] S.N. Zhu, M. Chen, J. Jongstra-Bilen, M.I. Cybulsky, GM-CSF regulates intimalcell proliferation in nascent atherosclerotic lesions, J. Exp. Med. 206 (2009)2141–2149.
[18] G.J. Randolph, Emigration of monocyte-derived cells to lymph nodes dur-ing resolution of inflammation and its failure in atherosclerosis, Curr. Opin.
s – A matter of unresolved inflammation, Semin Immunol (2015),
Lipidol. 19 (2008) 462–468.[19] A. Assinger, Y. Wang, L.M. Butler, G.K. Hansson, Z.Q. Yan, C. Soderberg-Naucler,
et al., Apolipoprotein B100 danger-associated signal 1 (ApoBDS-1) triggersplatelet activation and boosts platelet-leukocyte proinflammatory responses,Thromb. Haemost. 112 (2014) 332–341.
[20] P. von Hundelshausen, K.S. Weber, Y. Huo, A.E. Proudfoot, P.J. Nelson, K. Ley,et al., RANTES deposition by platelets triggers monocyte arrest on inflamedand atherosclerotic endothelium, Circulation 103 (2001) 1772–1777.
[21] S. Badrnya, W.C. Schrottmaier, J.B. Kral, K.C. Yaiw, I. Volf, G. Schabbauer,et al., Platelets mediate oxidized low-density lipoprotein-induced monocyteextravasation and foam cell formation, Arterioscler. Thromb. Vasc. Biol. 34(2014) 571–580.
[22] D. Siegel-Axel, K. Daub, P. Seizer, S. Lindemann, M. Gawaz, Platelet lipopro-tein interplay: trigger of foam cell formation and driver of atherosclerosis,Cardiovasc. Res. 78 (2008) 8–17.
[23] Y. Huo, A. Schober, S.B. Forlow, D.F. Smith, M.C. Hyman, S. Jung, et al., Cir-culating activated platelets exacerbate atherosclerosis in mice deficient inapolipoprotein E, Nat. Med. 9 (2003) 61–67.
[24] S. Massberg, K. Brand, S. Gruner, S. Page, E. Muller, I. Muller, et al., A criticalrole of platelet adhesion in the initiation of atherosclerotic lesion formation,J. Exp. Med. 196 (2002) 887–896.
[26] O. Postea, E.M. Vasina, S. Cauwenberghs, D. Projahn, E.A. Liehn, D. Lievens,et al., Contribution of platelet CX(3)CR1 to platelet-monocyte complex forma-tion and vascular recruitment during hyperlipidemia, Arterioscler. Thromb.Vasc. Biol. 32 (2012) 1186–1193.
[27] S. Badrnya, L.M. Butler, C. Soderberg-Naucler, I. Volf, A. Assinger, Plateletsdirectly enhance neutrophil transmigration in response to oxidised low-density lipoprotein, Thromb. Haemost. 108 (2012) 719–729.
[29] S. Tsimikas, E.S. Brilakis, E.R. Miller, J.P. McConnell, R.J. Lennon, K.S. Kornman,et al., Oxidized phospholipids, Lp(a) lipoprotein, and coronary artery disease,N. Engl. J. Med. 353 (2005) 46–57.
[30] A. Ravandi, G. Leibundgut, M.Y. Hung, M. Patel, P.M. Hutchins, R.C. Murphy,et al., Release and capture of bioactive oxidized phospholipids and oxidizedcholesteryl esters during percutaneous coronary and peripheral arterial inter-ventions in humans, J. Am. Coll. Cardiol. 63 (2014) 1961–1971.
[31] V. Jeney, G. Balla, J. Balla, Red blood cell, hemoglobin and heme in the pro-gression of atherosclerosis, Front. Physiol. 5 (2014) 379.
[32] M.M. Kockx, K.M. Cromheeke, M.W. Knaapen, J.M. Bosmans, G.R. De Meyer,A.G. Herman, et al., Phagocytosis and macrophage activation associated withhemorrhagic microvessels in human atherosclerosis, Arterioscler. Thromb.Vasc. Biol. 23 (2003) 440–446.
[33] F.D. Kolodgie, H.K. Gold, A.P. Burke, D.R. Fowler, H.S. Kruth, D.K. Weber, et al.,Intraplaque hemorrhage and progression of coronary atheroma, N. Engl. J.Med. 349 (2003) 2316–2325.
[34] A.J. Lusis, Atherosclerosis, Nature 407 (2000) 233–241.[35] I.A. Hagberg, H.E. Roald, T. Lyberg, Adhesion of leukocytes to growing arterial
thrombi, Thromb. Haemost. 80 (1998) 852–858.[36] D. Kirchhofer, M.A. Riederer, H.R. Baumgartner, Specific accumulation of cir-
culating monocytes and polymorphonuclear leukocytes on platelet thrombiin a vascular injury model, Blood 89 (1997) 1270–1278.
[37] M. Ghasemzadeh, Z.S. Kaplan, I. Alwis, S.M. Schoenwaelder, K.J. Ashworth, E.Westein, et al., The CXCR1/2 ligand NAP-2 promotes directed intravascularleukocyte migration through platelet thrombi, Blood 121 (2013) 4555–4566.
[38] M.J. Coffey, S.M. Phare, M. Peters-Golden, Prolonged exposure to lipopolysac-charide inhibits macrophage 5-lipoxygenase metabolism via induction ofnitric oxide synthesis, J. Immunol. 165 (2000) 3592–3598.
[39] E. Lee, T. Lindo, N. Jackson, L. Meng-Choong, P. Reynolds, A. Hill, et al., Reversalof human neutrophil survival by leukotriene B(4) receptor blockade and 5-lipoxygenase and 5-lipoxygenase activating protein inhibitors, Am. J. Respir.Crit. Care Med. 160 (1999) 2079–2085.
[40] V.L. Ott, J.C. Cambier, J. Kappler, P. Marrack, B.J. Swanson, Mast cell-dependentmigration of effector CD8+ T cells through production of leukotriene B4, Nat.Immunol. 4 (2003) 974–981.
[41] B. Samuelsson, Leukotrienes: mediators of immediate hypersensitivity reac-tions and inflammation, Science 220 (1983) 568–575.
[42] T. Yokomizo, T. Izumi, K. Chang, Y. Takuwa, T. Shimizu, A G-protein-coupledreceptor for leukotriene B4 that mediates chemotaxis, Nature 387 (1997)620–624.
[43] A. Matsukawa, C.M. Hogaboam, N.W. Lukacs, P.M. Lincoln, R.M. Strieter, S.L.Kunkel, Endogenous monocyte chemoattractant protein-1 (MCP-1) protectsmice in a model of acute septic peritonitis: cross-talk between MCP-1 andleukotriene B4, J. Immunol. 163 (1999) 6148–6154.
[44] K. Subbarao, V.R. Jala, S. Mathis, J. Suttles, W. Zacharias, J. Ahamed, et al., Roleof leukotriene B4 receptors in the development of atherosclerosis: potentialmechanisms, Arterioscler. Thromb. Vasc. Biol. 24 (2004) 369–375.
[45] L. Huang, A. Zhao, F. Wong, J.M. Ayala, M. Struthers, F. Ujjainwalla, et al.,Leukotriene B4 strongly increases monocyte chemoattractant protein-1 inhuman monocytes, Arterioscler. Thromb. Vasc. Biol. 24 (2004) 1783–1788.
[46] M. Mehrabian, H. Allayee, J. Wong, W. Shi, X.P. Wang, Z. Shaposhnik, et al.,Identification of 5-lipoxygenase as a major gene contributing to atheroscle-rosis susceptibility in mice, Circ. Res. 91 (2002) 120–126.
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
[47] H. Qiu, A. Gabrielsen, H.E. Agardh, M. Wan, A. Wetterholm, C.H. Wong,et al., Expression of 5-lipoxygenase and leukotriene A4 hydrolase in humanatherosclerotic lesions correlates with symptoms of plaque instability, Proc.Natl. Acad. Sci. U.S.A. 103 (2006) 8161–8166.
PRESSmunology xxx (2015) xxx–xxx
[48] R. Spanbroek, R. Grabner, K. Lotzer, M. Hildner, A. Urbach, K. Ruhling, et al.,Expanding expression of the 5-lipoxygenase pathway within the arterialwall during human atherogenesis, Proc. Natl. Acad. Sci. U.S.A. 100 (2003)1238–1243.
[49] A. Toda, T. Yokomizo, K. Masuda, A. Nakao, T. Izumi, T. Shimizu, Cloningand characterization of rat leukotriene B(4) receptor, Biochem. Biophys. Res.Commun. 262 (1999) 806–812.
[50] T.L. Assimes, J.W. Knowles, J.R. Priest, A. Basu, A. Borchert, K.A. Volcik, et al.,A near null variant of 12/15-LOX encoded by a novel SNP in ALOX15 and therisk of coronary artery disease, Atherosclerosis 198 (2008) 136–144.
[51] J. Wittwer, M. Bayer, A. Mosandl, J. Muntwyler, M. Hersberger, The c.-292C>Tpromoter polymorphism increases reticulocyte-type 15-lipoxygenase-1activity and could be atheroprotective, Clin. Chem. Lab. Med.: CCLM/FESCC45 (2007) 487–492.
[52] A.J. Merched, K. Ko, K.H. Gotlinger, C.N. Serhan, L. Chan, Atherosclerosis: evi-dence for impairment of resolution of vascular inflammation governed byspecific lipid mediators, FASEB J.: Off. Publ. Feder. Am. Soc. Exp. Biol. 22 (2008)3595–3606.
[53] G. Hajishengallis, T. Chavakis, Endogenous modulators of inflammatory cellrecruitment, Trends Immunol. 34 (2013) 1–6.
[54] L. Deban, R.C. Russo, M. Sironi, F. Moalli, M. Scanziani, V. Zambelli, et al., Reg-ulation of leukocyte recruitment by the long pentraxin PTX3, Nat. Immunol.11 (2010) 328–334.
[55] E.Y. Choi, E. Chavakis, M.A. Czabanka, H.F. Langer, L. Fraemohs, M.Economopoulou, et al., Del-1, an endogenous leukocyte-endothelial adhe-sion inhibitor, limits inflammatory cell recruitment, Science 322 (2008)1101–1104.
[56] D. Cooper, L.V. Norling, M. Perretti, Novel insights into the inhibitory effectsof Galectin-1 on neutrophil recruitment under flow, J. Leukoc. Biol. 83 (2008)1459–1466.
[57] M. La, T.V. Cao, G. Cerchiaro, K. Chilton, J. Hirabayashi, K. Kasai, et al., Anovel biological activity for galectin-1: inhibition of leukocyte-endothelialcell interactions in experimental inflammation, Am. J. Pathol. 163 (2003)1505–1515.
[58] T. Kempf, A. Zarbock, C. Widera, S. Butz, A. Stadtmann, J. Rossaint, et al., GDF-15 is an inhibitor of leukocyte integrin activation required for survival aftermyocardial infarction in mice, Nat. Med. 17 (2011) 581–588.
[59] G.D. Norata, P. Marchesi, V.K. Pulakazhi Venu, F. Pasqualini, A. Anselmo, F.Moalli, et al., Deficiency of the long pentraxin PTX3 promotes vascular inflam-mation and atherosclerosis, Circulation 120 (2009) 699–708.
[60] A. Savchenko, M. Imamura, R. Ohashi, S. Jiang, T. Kawasaki, G. Hasegawa, et al.,Expression of pentraxin 3 (PTX3) in human atherosclerotic lesions, J. Pathol.215 (2008) 48–55.
[61] M.R. Preusch, M. Baeuerle, C. Albrecht, E. Blessing, M. Bischof, H.A. Katus,et al., GDF-15 protects from macrophage accumulation in a mousemodel ofadvanced atherosclerosis, Eur. J. Med. Res. 18 (2013) 19.
[62] G.A. Bonaterra, S. Zugel, J. Thogersen, S.A. Walter, U. Haberkorn, J. Strelau,et al., Growth differentiation factor-15 deficiency inhibits atherosclerosis pro-gression by regulating interleukin-6-dependent inflammatory response tovascular injury, J. Am. Heart Assoc. 1 (2012) e002550.
[63] S.C. de Jager, B. Bermudez, I. Bot, R.R. Koenen, M. Bot, A. Kavelaars, et al.,Growth differentiation factor 15 deficiency protects against atherosclero-sis by attenuating CCR2-mediated macrophage chemotaxis, J. Exp. Med. 208(2011) 217–225.
[64] H. Johnen, T. Kuffner, D.A. Brown, B.J. Wu, R. Stocker, S.N. Breit, Increasedexpression of the TGF-b superfamily cytokine MIC-1/GDF15 protectsApoE(−/−) mice from the development of atherosclerosis, Cardiovasc.Pathol.: Off. J. Soc. Cardiovasc. Pathol. 21 (2012) 499–505.
[65] M. Drechsler, R.J. de Jong, J. Rossaint, J. Viola, G. Leoni, J.M. Wang, et al., AnnexinA1 counteracts chemokine-induced arterial myeloid cell recruitment, Circ.Res. 116 (2015) 827–835.
[66] M.E. Rosenfeld, Macrophage proliferation in atherosclerosis: an historical per-spective, Arterioscler. Thromb. Vasc. Biol. 34 (2014) e21–e22.
[67] J.J. Fuster, P. Fernandez, H. Gonzalez-Navarro, C. Silvestre, Y.N. Nabah, V.Andres, Control of cell proliferation in atherosclerosis: insights from animalmodels and human studies, Cardiovasc. Res. 86 (2010) 254–264.
[68] V.I. Sayin, O.M. Khan, L.E. Pehlivanoglu, A. Staffas, M.X. Ibrahim, A. Asplund,et al., Loss of one copy of Zfp148 reduces lesional macrophage proliferationand atherosclerosis in mice by activating p53, Circ. Res. 115 (2014) 781–789.
[69] G. Wu, J. Cai, Y. Han, J. Chen, Z.P. Huang, C. Chen, et al., LincRNA-p21 regulatesneointima formation, vascular smooth muscle cell proliferation, apopto-sis, and atherosclerosis by enhancing p53 activity, Circulation 130 (2014)1452–1465.
[70] D. Hashimoto, A. Chow, C. Noizat, P. Teo, M.B. Beasley, M. Leboeuf, et al.,Tissue-resident macrophages self-maintain locally throughout adult lifewith minimal contribution from circulating monocytes, Immunity 38 (2013)792–804.
[71] S.J. Jenkins, D. Ruckerl, P.C. Cook, L.H. Jones, F.D. Finkelman, N. van Rooijen,et al., Local macrophage proliferation, rather than recruitment from the blood,is a signature of TH2 inflammation, Science 332 (2011) 1284–1288.
[72] E.L. Gautier, T. Huby, J.L. Witztum, B. Ouzilleau, E.R. Miller, F. Saint-Charles,
s – A matter of unresolved inflammation, Semin Immunol (2015),
et al., Macrophage apoptosis exerts divergent effects on atherogenesis as afunction of lesion stage, Circulation 119 (2009) 1795–1804.
[73] L. Sleiman, R. Beanlands, M. Hasu, M. Thabet, A. Norgaard, Y.X. Chen, et al.,Loss of cellular inhibitor of apoptosis protein 2 reduces atherosclerosis in
atherogenic apoE−/− C57BL/6 mice on high-fat diet, J. Am. Heart Assoc. 2(2013) e000259.
[74] M. Hamada, M. Nakamura, M.T. Tran, T. Moriguchi, C. Hong, T. Ohsumi, et al.,MafB promotes atherosclerosis by inhibiting foam-cell apoptosis, Nat. Com-mun. 5 (2014) 3147.
[75] T. Sallam, A. Ito, X. Rong, J. Kim, C. van Stijn, B.T. Chamberlain, et al., Themacrophage LBP gene is an LXR target that promotes macrophage survivaland atherosclerosis, J. Lipid Res. 55 (2014) 1120–1130.
[76] G.J. Bellingan, H. Caldwell, S.E. Howie, I. Dransfield, C. Haslett, In vivo fate ofthe inflammatory macrophage during the resolution of inflammation: inflam-matory macrophages do not die locally, but emigrate to the draining lymphnodes, J. Immunol. 157 (1996) 2577–2585.
[77] C. Cao, D.A. Lawrence, D.K. Strickland, L. Zhang, A specific role of integrinMac-1 in accelerated macrophage efflux to the lymphatics, Blood 106 (2005)3234–3241.
[78] I.G. Gomez, J. Tang, C.L. Wilson, W. Yan, J.W. Heinecke, J.M. Harlan,et al., Metalloproteinase-mediated Shedding of Integrin beta2 promotesmacrophage efflux from inflammatory sites, J. Biol. Chem. 287 (2012)4581–4589.
[79] E.L. Gautier, S. Ivanov, P. Lesnik, G.J. Randolph, Local apoptosis mediates clear-ance of macrophages from resolving inflammation in mice, Blood 122 (2013)2714–2722.
[80] J. Llodra, V. Angeli, J. Liu, E. Trogan, E.A. Fisher, G.J. Randolph, Emigrationof monocyte-derived cells from atherosclerotic lesions characterizes regres-sive, but not progressive, plaques, Proc. Natl. Acad. Sci. U.S.A. 101 (2004)11779–11784.
[81] R.G. Gerrity, The role of the monocyte in atherogenesis: II. Migration of foamcells from atherosclerotic lesions, Am. J. Pathol. 103 (1981) 191–200.
[82] A. Faggiotto, R. Ross, L. Harker, Studies of hypercholesterolemia in the non-human primate, I. Changes that lead to fatty streak formation, Arteriosclerosis4 (1984) 323–340.
[83] J.E. Feig, J.X. Rong, R. Shamir, M. Sanson, Y. Vengrenyuk, J. Liu, et al., HDLpromotes rapid atherosclerosis regression in mice and alters inflammatoryproperties of plaque monocyte-derived cells, Proc. Natl. Acad. Sci. U.S.A. 108(2011) 7166–7171.
[84] J.M. van Gils, M.C. Derby, L.R. Fernandes, B. Ramkhelawon, T.D. Ray, K.J. Rayner,et al., The neuroimmune guidance cue netrin-1 promotes atherosclerosis byinhibiting the emigration of macrophages from plaques, Nat. Immunol. 13(2012) 136–143.
[85] J.E. Feig, Y. Vengrenyuk, V. Reiser, C. Wu, A. Statnikov, C.F. Aliferis, et al.,Regression of atherosclerosis is characterized by broad changes in the plaquemacrophage transcriptome, PLoS ONE 7 (2012) e39790.
[86] C.A. Gleissner, I. Shaked, K.M. Little, K. Ley, CXC chemokine ligand 4 inducesa unique transcriptome in monocyte-derived macrophages, J. Immunol. 184(2010) 4810–4818.
[87] A. Kadl, A.K. Meher, P.R. Sharma, M.Y. Lee, A.C. Doran, S.R. Johnstone, et al.,Identification of a novel macrophage phenotype that develops in response toatherogenic phospholipids via Nrf2, Circ. Res. 107 (2010) 737–746.
[88] G. Chinetti-Gbaguidi, S. Colin, B. Staels, Macrophage subsets in atherosclero-sis, Nat. Rev. Cardiol. 12 (2015) 10–17.
[89] J. Khallou-Laschet, A. Varthaman, G. Fornasa, C. Compain, A.T. Gaston, M.Clement, et al., Macrophage plasticity in experimental atherosclerosis, PLoSONE 5 (2010) e8852.
[90] J.E. Feig, S. Parathath, J.X. Rong, S.L. Mick, Y. Vengrenyuk, L. Grauer, et al.,Reversal of hyperlipidemia with a genetic switch favorably affects the contentand inflammatory state of macrophages in atherosclerotic plaques, Circula-tion 123 (2011) 989–998.
[91] P.J. Murray, T.A. Wynn, Protective and pathogenic functions of macrophagesubsets, Nat. Rev. Immunol. 11 (2011) 723–737.
[92] P.J. Murray, T.A. Wynn, Obstacles and opportunities for understandingmacrophage polarization, J. Leukoc. Biol. 89 (2011) 557–563.
[93] Y. Hirata, M. Tabata, H. Kurobe, T. Motoki, M. Akaike, C. Nishio, et al., Coro-nary atherosclerosis is associated with macrophage polarization in epicardialadipose tissue, J. Am. Coll. Cardiol. 58 (2011) 248–255.
[94] G.P. Fadini, F. Simoni, R. Cappellari, N. Vitturi, S. Galasso, S. Vigili de Kreutzen-berg, et al., Pro-inflammatory monocyte-macrophage polarization imbalancein human hypercholesterolemia and atherosclerosis, Atherosclerosis 237(2014) 805–808.
[95] G. Chinetti-Gbaguidi, M. Baron, M.A. Bouhlel, J. Vanhoutte, C. Copin, Y. Sebti,et al., Human atherosclerotic plaque alternative macrophages display lowcholesterol handling but high phagocytosis because of distinct activities ofthe PPARgamma and LXRalpha pathways, Circ. Res. 108 (2011) 985–995.
[96] J.L. Stoger, M.J. Gijbels, S. van der Velden, M. Manca, C.M. van der Loos, E.A.Biessen, et al., Distribution of macrophage polarization markers in humanatherosclerosis, Atherosclerosis 225 (2012) 461–468.
[97] T. Lawrence, G. Natoli, Transcriptional regulation of macrophage polarization:enabling diversity with identity, Nat. Rev. Immunol. 11 (2011) 750–761.
[98] D.M. Schrijvers, G.R. De Meyer, M.M. Kockx, A.G. Herman, W. Martinet, Phago-cytosis of apoptotic cells by macrophages is impaired in atherosclerosis,Arterioscler. Thromb. Vasc. Biol. 25 (2005) 1256–1261.
[99] C. Silvestre-Roig, M.P. de Winther, C. Weber, M.J. Daemen, E. Lutgens, O.
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
[102] K.S. Ravichandran, Beginnings of a good apoptotic meal: the find-me and eat-me signaling pathways, Immunity 35 (2011) 445–455.
[103] K. Lauber, E. Bohn, S.M. Krober, Y.J. Xiao, S.G. Blumenthal, R.K. Lindemann,et al., Apoptotic cells induce migration of phagocytes via caspase-3-mediatedrelease of a lipid attraction signal, Cell 113 (2003) 717–730.
[104] T. Aprahamian, I. Rifkin, R. Bonegio, B. Hugel, J.M. Freyssinet, K. Sato, et al.,Impaired clearance of apoptotic cells promotes synergy between atherogen-esis and autoimmune disease, J. Exp. Med. 199 (2004) 1121–1131.
[105] I. Goncalves, A. Edsfeldt, N.Y. Ko, H. Grufman, K. Berg, H. Bjorkbacka, et al., Evi-dence supporting a key role of Lp-PLA2-generated lysophosphatidylcholinein human atherosclerotic plaque inflammation, Arterioscler. Thromb. Vasc.Biol. 32 (2012) 1505–1512.
[106] J.R. Nofer, M. Bot, M. Brodde, P.J. Taylor, P. Salm, V. Brinkmann, et al.,FTY720, a synthetic sphingosine 1 phosphate analogue, inhibits develop-ment of atherosclerosis in low-density lipoprotein receptor-deficient mice,Circulation 115 (2007) 501–508.
[107] F. Poti, F. Gualtieri, S. Sacchi, G. Weissen-Plenz, G. Varga, M. Brodde, et al.,KRP-203, sphingosine 1-phosphate receptor type 1 agonist, amelioratesatherosclerosis in LDL-R−/− mice, Arterioscler. Thromb. Vasc. Biol. 33 (2013)1505–1512.
[108] S. Jaillon, P. Jeannin, Y. Hamon, I. Fremaux, A. Doni, B. Bottazzi, et al., Endoge-nous PTX3 translocates at the membrane of late apoptotic human neutrophilsand is involved in their engulfment by macrophages, Cell Death Differ. 16(2009) 465–474.
[109] M.J. Lewis, T.H. Malik, M.R. Ehrenstein, J.J. Boyle, M. Botto, D.O. Haskard,Immunoglobulin M is required for protection against atherosclerosis in low-density lipoprotein receptor-deficient mice, Circulation 120 (2009) 417–426.
[110] R. Moura, M. Tjwa, P. Vandervoort, S. Van Kerckhoven, P. Holvoet, M.F.Hoylaerts, Thrombospondin-1 deficiency accelerates atherosclerotic plaquematuration in ApoE−/− mice, Circ. Res. 103 (2008) 1181–1189.
[111] H. Ait-Oufella, K. Kinugawa, J. Zoll, T. Simon, J. Boddaert, S. Heeneman, et al.,Lactadherin deficiency leads to apoptotic cell accumulation and acceleratedatherosclerosis in mice, Circulation 115 (2007) 2168–2177.
[112] N. Deroide, X. Li, D. Lerouet, E. Van Vre, L. Baker, J. Harrison, et al., MFGE8inhibits inflammasome-induced IL-1beta production and limits postischemiccerebral injury, J. Clin. Investig. 123 (2013) 1176–1181.
[113] H. Ait-Oufella, V. Pouresmail, T. Simon, O. Blanc-Brude, K. Kinugawa, R. Mer-val, et al., Defective mer receptor tyrosine kinase signaling in bone marrowcells promotes apoptotic cell accumulation and accelerates atherosclerosis,Arterioscler. Thromb. Vasc. Biol. 28 (2008) 1429–1431.
[114] E. Thorp, D. Cui, D.M. Schrijvers, G. Kuriakose, I. Tabas, Mertk receptormutation reduces efferocytosis efficiency and promotes apoptotic cell accu-mulation and plaque necrosis in atherosclerotic lesions of apoe−/− mice,Arterioscler. Thromb. Vasc. Biol. 28 (2008) 1421–1428.
[115] E. Thorp, T. Vaisar, M. Subramanian, L. Mautner, C. Blobel, I. Tabas, Shed-ding of the Mer tyrosine kinase receptor is mediated by ADAM17 proteinthrough a pathway involving reactive oxygen species, protein kinase Cdelta,and p38 mitogen-activated protein kinase (MAPK), J. Biol. Chem. 286 (2011)33335–33344.
[116] S. Sather, K.D. Kenyon, J.B. Lefkowitz, X. Liang, B.C. Varnum, P.M. Henson, et al.,A soluble form of the Mer receptor tyrosine kinase inhibits macrophage clear-ance of apoptotic cells and platelet aggregation, Blood 109 (2007) 1026–1033.
[117] W.S. Driscoll, T. Vaisar, J. Tang, C.L. Wilson, E.W. Raines, Macrophage ADAM17deficiency augments CD36-dependent apoptotic cell uptake and the linkedanti-inflammatory phenotype, Circ. Res. 113 (2013) 52–61.
[118] W.A. Boisvert, D.M. Rose, A. Boullier, O. Quehenberger, A. Sydlaske, K.A. John-son, et al., Leukocyte transglutaminase 2 expression limits atheroscleroticlesion size, Arterioscler. Thromb. Vasc. Biol. 26 (2006) 563–569.
[119] Y. Kojima, K. Downing, R. Kundu, C. Miller, F. Dewey, H. Lancero, et al., Cyclin-dependent kinase inhibitor 2B regulates efferocytosis and atherosclerosis, J.Clin. Investig. 124 (2014) 1083–1097.
[120] S.J. Gardai, K.A. McPhillips, S.C. Frasch, W.J. Janssen, A. Starefeldt, J.E. Murphy-Ullrich, et al., Cell-surface calreticulin initiates clearance of viable or apoptoticcells through trans-activation of LRP on the phagocyte, Cell 123 (2005)321–334.
[121] K.S. Heo, H.J. Cushman, M. Akaike, C.H. Woo, X. Wang, X. Qiu, et al., ERK5 acti-vation in macrophages promotes efferocytosis and inhibits atherosclerosis,Circulation 130 (2014) 180–191.
[122] E. Marsch, T.L. Theelen, J.A. Demandt, M. Jeurissen, M. van Gink, R. Verjans,et al., Reversal of hypoxia in murine atherosclerosis prevents necrotic coreexpansion by enhancing efferocytosis, Arterioscler. Thromb. Vasc. Biol. 34(2014) 2545–2553.
[123] I.F. Charo, R. Taub, Anti-inflammatory therapeutics for the treatment ofatherosclerosis, Nat. Rev. Drug Discov. 10 (2011) 365–376.
[124] R. Horuk, Chemokine receptor antagonists: overcoming developmental hur-dles, Nat. Rev. Drug Discov. 8 (2009) 23–33.
[125] R.R. Koenen, C. Weber, Chemokines: established and novel targets inatherosclerosis, EMBO Mol. Med. 3 (2011) 713–725.
s – A matter of unresolved inflammation, Semin Immunol (2015),
[126] P. Libby, P.M. Ridker, G.K. Hansson, Progress and challenges in translating thebiology of atherosclerosis, Nature 473 (2011) 317–325.
[127] P. Libby, I. Tabas, G. Fredman, E.A. Fisher, Inflammation and its resolution asdeterminants of acute coronary syndromes, Circ. Res. 114 (2014) 1867–1879.
[128] T. Gobbetti, S.M. Coldewey, J. Chen, S. McArthur, P. le Faouder, N. Cenac, et al.,Nonredundant protective properties of FPR2/ALX in polymicrobial murinesepsis, Proc. Natl. Acad. Sci. U.S.A. 111 (2014) 18685–18690.
[129] M. Spite, L.V. Norling, L. Summers, R. Yang, D. Cooper, N.A. Petasis, et al.,Resolvin D2 is a potent regulator of leukocytes and controls microbial sepsis,Nature 461 (2009) 1287–1291.
[130] W.J. Mulder, F.A. Jaffer, Z.A. Fayad, M. Nahrendorf, Imaging and nanomedicinein inflammatory atherosclerosis, Sci. Transl. Med. 6 (2014) 239sr1.
[131] M. Schiener, M. Hossann, J.R. Viola, A. Ortega-Gomez, C. Weber, K. Lauber,et al., Nanomedicine-based strategies for treatment of atherosclerosis, TrendsMol. Med. 20 (2014) 271–281.
[132] G. Fredman, N. Kamaly, S. Spolitu, D. Ghorpade, G. Kuriakose, J. Milton,M. Perretti, O. Farokhad, I. Tabas, Targeted nanoparticles containing thepro-resolving peptide Ac2-26 protect against advanced atherosclerosis, Sci.Transl. Med. 7 (2015) 275ra20.
[133] N. Kamaly, G. Fredman, M. Subramanian, S. Gadde, A. Pesic, L. Cheung,et al., Development and in vivo efficacy of targeted polymeric inflammation-resolving nanoparticles, Proc. Natl. Acad. Sci. U.S.A. 110 (2013) 6506–6511.
[134] S. Arur, U.E. Uche, K. Rezaul, M. Fong, V. Scranton, A.E. Cowan, et al., AnnexinI is an endogenous ligand that mediates apoptotic cell engulfment, Dev. Cell4 (2003) 587–598.
[135] Y. Li, L. Cai, H. Wang, P. Wu, W. Gu, Y. Chen, et al., Pleiotropic regulation ofmacrophage polarization and tumorigenesis by formyl peptide receptor-2,Oncogene 30 (2011) 3887–3899.
[136] J.L. Cash, R. Hart, A. Russ, J.P. Dixon, W.H. Colledge, J. Doran, et al., Syntheticchemerin-derived peptides suppress inflammation through ChemR23, J. Exp.Med. 205 (2008) 767–775.
[137] J.L. Cash, S. Bena, S.E. Headland, S. McArthur, V. Brancaleone, M. Per-retti, Chemerin15 inhibits neutrophil-mediated vascular inflammation andmyocardial ischemia–reperfusion injury through ChemR23, EMBO Rep. 14(2013) 999–1007.
[138] J.L. Cash, A.R. Christian, D.R. Greaves, Chemerin peptides promote phago-cytosis in a ChemR23- and Syk-dependent manner, J. Immunol. 184 (2010)5315–5324.
[139] C.N. Serhan, Pro-resolving lipid mediators are leads for resolution physiology,Nature 510 (2014) 92–101.
[140] M. Arita, M. Yoshida, S. Hong, E. Tjonahen, J.N. Glickman, N.A. Petasis, et al.,Resolvin E1, an endogenous lipid mediator derived from omega-3 eicosapen-taenoic acid, protects against 2,4,6-trinitrobenzene sulfonic acid-inducedcolitis, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 7671–7676.
[141] O. Haworth, M. Cernadas, R. Yang, C.N. Serhan, B.D. Levy, Resolvin E1 regulates
Please cite this article in press as: J. Viola, O. Soehnlein, Atherosclerosihttp://dx.doi.org/10.1016/j.smim.2015.03.013
interleukin 23, interferon-gamma and lipoxin A4 to promote the resolutionof allergic airway inflammation, Nat. Immunol. 9 (2008) 873–879.
[142] C.B. Clish, J.A. O’Brien, K. Gronert, G.L. Stahl, N.A. Petasis, C.N. Serhan, Localand systemic delivery of a stable aspirin-triggered lipoxin prevents neutrophilrecruitment in vivo, Proc. Natl. Acad. Sci. U.S.A. 96 (1999) 8247–8252.
PRESSmunology xxx (2015) xxx–xxx
[143] L.V. Norling, M. Spite, R. Yang, R.J. Flower, M. Perretti, C.N. Serhan,Cutting edge: humanized nano-proresolving medicines mimic inflammation-resolution and enhance wound healing, J. Immunol. 186 (2011) 5543–5547.
[144] S. Li, Y. Sun, C.P. Liang, E.B. Thorp, S. Han, A.W. Jehle, et al., Defective phago-cytosis of apoptotic cells by macrophages in atherosclerotic lesions of ob/obmice and reversal by a fish oil diet, Circ. Res. 105 (2009) 1072–1082.
[145] J.M. Brown, S. Chung, J.K. Sawyer, C. Degirolamo, H.M. Alger, T.M. Nguyen,et al., Combined therapy of dietary fish oil and stearoyl-CoA desaturase 1inhibition prevents the metabolic syndrome and atherosclerosis, Arterioscler.Thromb. Vasc. Biol. 30 (2010) 24–30.
[146] C. Degirolamo, K.L. Kelley, M.D. Wilson, L.L. Rudel, Dietary n-3 LCPUFA fromfish oil but not alpha-linolenic acid-derived LCPUFA confers atheroprotectionin mice, J. Lipid Res. 51 (2010) 1897–1905.
[147] K. Nakajima, T. Yamashita, T. Kita, M. Takeda, N. Sasaki, K. Kasahara,et al., Orally administered eicosapentaenoic acid induces rapid regres-sion of atherosclerosis via modulating the phenotype of dendritic cells inLDL receptor-deficient mice, Arterioscler. Thromb. Vasc. Biol. 31 (2011)1963–1972.
[148] J.B. Wan, L.L. Huang, R. Rong, R. Tan, J. Wang, J.X. Kang, Endogenouslydecreasing tissue n-6/n-3 fatty acid ratio reduces atherosclerotic lesions inapolipoprotein E-deficient mice by inhibiting systemic and vascular inflam-mation, Arterioscler. Thromb. Vasc. Biol. 30 (2010) 2487–2494.
[149] S. Halvorsen, F. Andreotti, J.M. ten Berg, M. Cattaneo, S. Coccheri, R. Marchioli,et al., Aspirin therapy in primary cardiovascular disease prevention: a positionpaper of the European Society of Cardiology working group on thrombosis, J.Am. Coll. Cardiol. 64 (2014) 319–327.
[150] K.J. Rayner, F.J. Sheedy, C.C. Esau, F.N. Hussain, R.E. Temel, S. Parathath, et al.,Antagonism of miR-33 in mice promotes reverse cholesterol transport andregression of atherosclerosis, J. Clin. Investig. 121 (2011) 2921–2931.
[151] J.A. Shaw, A. Bobik, A. Murphy, P. Kanellakis, P. Blombery, N. Mukhame-dova, et al., Infusion of reconstituted high-density lipoprotein leads to acutechanges in human atherosclerotic plaque, Circ. Res. 103 (2008) 1084–1091.
[152] L. Cardilo-Reis, S. Gruber, S.M. Schreier, M. Drechsler, N. Papac-Milicevic, C.Weber, et al., Interleukin-13 protects from atherosclerosis and modulatesplaque composition by skewing the macrophage phenotype, EMBO Mol. Med.4 (2012) 1072–1086.
[153] G. Courties, T. Heidt, M. Sebas, Y. Iwamoto, D. Jeon, J. Truelove, et al., In vivosilencing of the transcription factor IRF5 reprograms the macrophage pheno-type and improves infarct healing, J. Am. Coll. Cardiol. 63 (2014) 1556–1566.
[154] S.E. Nissen, K. Wolski, Effect of rosiglitazone on the risk of myocardial infarc-tion and death from cardiovascular causes, N. Engl. J. Med. 356 (2007)
s – A matter of unresolved inflammation, Semin Immunol (2015),
2457–2471.[155] E. Vucic, C. Calcagno, S.D. Dickson, J.H. Rudd, K. Hayashi, J. Bucerius, et al.,
Regression of inflammation in atherosclerosis by the LXR agonist R211945: anoninvasive assessment and comparison with atorvastatin, JACC Cardiovasc.Imaging 5 (2012) 819–828.
