Chemokines in Atherosclerosis

Genome-wide association identifies OBFC1 as a locus involved in human leukocyte telomere biology

Telomeres are engaged in a host of cellular functions, and their length is regulated by multiple genes. Telomere shortening, in the course of somatic cell replication, ultimately leads to replicative senescence. In humans, rare mutations in genes that regulate telomere length have been identified in monogenic diseases such as dyskeratosis congenita and idiopathic pulmonary fibrosis, which are associated with shortened leukocyte telomere length (LTL) and increased risk for aplastic anemia. Shortened LTL is observed in a host of aging-related complex genetic diseases and is associated with diminished survival in the elderly. We report results of a genome-wide association study of LTL in a consortium of four observational studies (n = 3,417 participants with LTL and genome-wide genotyping). SNPs in the regions of the oligonucleotide/oligosaccharide-binding folds containing one gene (OBFC1; rs4387287; P = 3.9 x 10(-9)) and chemokine (C-X-C motif) receptor 4 gene (CXCR4; rs4452212; P = 2.9 x 10(-8)) were associated with LTL at a genome-wide significance level (P < 5 x 10(-8)). We attempted replication of the top SNPs at these loci through de novo genotyping of 1,893 additional individuals and in silico lookup in another observational study (n = 2,876), and we confirmed the association findings for OBFC1 but not CXCR4. In addition, we confirmed the telomerase RNA component (TERC) as a gene associated with LTL (P = 1.1 x 10(-5)). The identification of OBFC1 through genome-wide association as a locus for interindividual variation in LTL in the general population advances the understanding of telomere biology in humans and may provide insights into aging-related disorders linked to altered LTL dynamics.

Disruption of SM22 promotes inflammation after artery injury via nuclear factor kappaB activation

RATIONALE

SM22 (or transgelin), an actin-binding protein abundant in vascular smooth muscle cells (VSMCs), is downregulated in atherosclerosis, aneurysm and various cancers. Abolishing SM22 in apolipoprotein E knockout mice accelerates atherogenesis. However, it is unclear whether SM22 disruption independently promotes arterial inflammation.

OBJECTIVE

To investigate whether SM22 disruption directly promotes inflammation on arterial injury and to characterize the underlying mechanisms.

METHODS AND RESULTS

Using carotid denudation as an artery injury model, we showed that Sm22 knockout (Sm22(-/-)) mice developed enhanced inflammatory responses with higher induction of proinflammatory genes, including Vcam1, Icam1, Cx3cl1, Ccl2, and Ptgs2. Higher expression of these genes was confirmed in primary Sm22(-/-) VSMCs and in PAC1 cells after Sm22 knockdown, whereas SM22 recapitulation in primary Sm22(-/-) VSMCs decreased their expression. NFKB2 was prominently activated in both injured carotids of Sm22(-/-) mice and in PAC1 cells after Sm22 knockdown and may mediate upregulation of these proinflammatory genes. As a NF-kappaB activator, reactive oxygen species (ROS) increased in primary Sm22(-/-) VSMCs and in PAC1 cells after Sm22 knockdown. ROS scavengers blocked NF-kappaB activation and induction of proinflammatory genes. Furthermore, Sm22 knockdown increased Sod2 expression and activated p47phox, reflecting contributions of mitochondria and NADPH oxidase to the augmented ROS production; this may result from actin and microtubule cytoskeletal remodeling.

CONCLUSIONS

Our findings show that SM22 downregulation can induce proinflammatory VSMCs through activation of ROS-mediated NF-kappaB pathways. This study provides initial evidence linking VSMC cytoskeleton remodeling with arterial inflammation.

Association of the SDF1-3'A polymorphism with susceptibility to myocardial infarction in Chinese Han population

SDF-1 has been demonstrated to be involved in the pathophysiology of atherosclerosis. This study was aimed to investigate whether the SDF1-3'A polymorphism (rs1801157) is associated to myocardial infarction (MI) in a sample of Chinese Han population. A total of 560 patients with MI and 532 controls were enrolled in the study. The SDF1-3'A polymorphism was determined by polymerase chain reaction -restriction fragment length polymorphism (PCR-RFLP) analysis. A significant difference in genotype distribution and allele frequency was observed between patients and controls (P = 0.003 and P = 0.001, respectively). The A allele carriers had a significantly reduced MI risk compared with the GG homozygotes (OR, 0.69; 95% CI, 0.52-0.92; adjusted P = 0.007) in a logistic regression model after controlling conventional risk factors. The present study showed a significant association between the SDF1-3'A polymorphism and MI in Chinese Han population.

Expression of cytokine genes in the aorta is altered by the deficiency in MCP-1: effect of a high-fat, high-cholesterol diet

BACKGROUND

Monocyte chemoattractant protein-1 (MCP-1) facilitates the recruitment of monocytes/macrophages into vascular intima, and it is probably involved in the regulation of other signaling pathways relevant to the pathogenesis of arteriosclerosis and metabolic disturbances. However, chemokines are redundant. Consequently, the protective effect of MCP-1 deficiency may be mediated by changes in other cytokine signals.

METHODS AND RESULTS

Changes in the pattern of gene expression in the aorta were evaluated in LDLr(-/-) and MCP-1(-/-) LDLr(-/-) mice fed either chow or Western-style diet. Functional analyses were used to characterize the pathways affected and to identify biological processes in which MCP-1 may play an additional role. Some data also suggest that MCP-5 may act as a surrogate for MCP-1 deletion. Arteriosclerosis lesion and plaque composition are associated with enrichment in the cytokine-cytokine receptor interaction pathway.

CONCLUSIONS

There is a complex network of interactions linking MCP-1 and other cytokines. The lack of MCP-1 limits the aortic response to atherogenic stimuli, but does not completely protect against neointima formation. Activation of alternative inflammatory pathways in the vascular wall in response to MCP-1 deficiency should be considered to fully understand the actual role of this chemokine.

Therapeutic targeting of chemokine interactions in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial wall that is characterized by a disturbed equilibrium of immune responses and lipid accumulation, leading to the development of plaques. The atherogenic influx of mononuclear cells is orchestrated by chemokines and their receptors. Studies using gene-deficient mice and antagonists based on peptides and small molecules have generated insight into targeting chemokine-receptor axes for treating atherosclerosis, which might complement lipid-lowering strategies and risk factor modulation. Combined inhibition of multiple chemokine axes could interfere with the contributions of chemokines to disease progression at specific cells, stages or sites. In addition, the recently characterized heterophilic interactions of chemokines might present a novel target for the treatment and prevention of inflammatory diseases such as atherosclerosis.

Mouse aorta smooth muscle cells differentiate into lymphoid tissue organizer-like cells on combined tumor necrosis factor receptor-1/lymphotoxin beta-receptor NF-kappaB signaling

OBJECTIVE

Mouse aorta smooth muscle cells (SMC) express tumor necrosis factor receptor superfamily member 1A (TNFR-1) and lymphotoxin beta-receptor (LTbetaR). Circumstantial evidence has linked the SMC LTbetaR to tertiary lymphoid organogenesis in hyperlipidemic mice. Here, we explored TNFR-1 and LTbetaR signaling in cultured SMC.

METHODS AND RESULTS

TNFR-1 signaling activated the classical RelA NF-kappaB pathway, whereas LTbetaR signaling activated the classical RelA and alternative RelB NF-kappaB pathways, and both signaling pathways synergized to enhance p100 inhibitor processing to the p52 subunit of NF-kappaB. Microarrays showed that simultaneous TNFR-1/LTbetaR activation resulted in elevated mRNA encoding leukocyte homeostatic chemokines CCL2, CCL5, CXCL1, and CX3CL1. Importantly, SMC acquired features of lymphoid tissue organizers, which control tertiary lymphoid organogenesis in autoimmune diseases through hyperinduction of CCL7, CCL9, CXCL13, CCL19, CXCL16, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1. TNFR-1/LTbetaR cross-talk resulted in augmented secretion of lymphorganogenic chemokine proteins. Supernatants of TNFR-1/LTbetaR-activated SMC markedly supported migration of splenic T cells, B cells, and macrophages/dendritic cells. Experiments with ltbr(-/-) SMC indicated that LTbetaR-RelB activation was obligatory to generate the lymphoid tissue organizer phenotype.

CONCLUSIONS

SMC may participate in the formation of tertiary lymphoid tissue in atherosclerosis by upregulation of lymphorganogenic chemokines involved in T-lymphocyte, B-lymphocyte, and macrophage/dendritic cell attraction.

Stem/Progenitor Cells, Atherosclerosis and Cardiovascular Regeneration

Regenerative cell based therapy has potential to become effective adjuvant treatment for patients with atherosclerotic disease. Although data from animal studies support this notion, clinical studies undertaken in patients with acute and chronic coronary artery disease do not conclusively demonstrate benefits of such therapy. There are many questions on the stem cell translational roadmap. The basic mechanisms of stem cell-dependent tissue regeneration are not well understood. There is a debate regarding characterization of specific cell types conferring therapeutic effects. In particular, the role of endothelial progenitor cells as a specific reparative cell subtype is questioned, and the role of myeloid cell linage in fostering of vasculo- and angiogenesis is being increasingly appreciated. Intense discussions surround the place of stem/progenitor cells in atherosclerosis progression, plaque destabilization and vessel remodeling. This paper summarizes the current knowledge on the regenerative stem/progenitor cell definitions, mechanisms of stem cell trafficking, homing and their involvement in atherosclerosis progression.

Stem/Progenitor cells, atherosclerosis and cardiovascular regeneration

Regenerative cell based therapy has potential to become effective adjuvant treatment for patients with atherosclerotic disease. Although data from animal studies support this notion, clinical studies undertaken in patients with acute and chronic coronary artery disease do not conclusively demonstrate benefits of such therapy. There are many questions on the stem cell translational roadmap. The basic mechanisms of stem cell-dependent tissue regeneration are not well understood. There is a debate regarding characterization of specific cell types conferring therapeutic effects. In particular, the role of endothelial progenitor cells as a specific reparative cell subtype is questioned, and the role of myeloid cell linage in fostering of vasculo- and angiogenesis is being increasingly appreciated. Intense discussions surround the place of stem/progenitor cells in atherosclerosis progression, plaque destabilization and vessel remodeling. This paper summarizes the current knowledge on the regenerative stem/progenitor cell definitions, mechanisms of stem cell trafficking, homing and their involvement in atherosclerosis progression.

Epigenetics in atherosclerosis and inflammation

Atherosclerosis is a multifactorial disease with a severe burden on western society. Recent insights into the pathogenesis of atherosclerosis underscore the importance of chronic inflammation in both the initiation and progression of vascular remodelling. Expression of immunoregulatory molecules by vascular wall components within the atherosclerotic lesions is accordingly thought to contribute to the ongoing inflammatory process. Besides gene regulatory proteins (transcription factors), epigenetic mechanisms also play an essential and fundamental role in the transcriptional control of gene expression. These epigenetic mechanisms change the accessibility of chromatin by DNA methylation and histone modifications. Epigenetic modulators are thus critically involved in the regulation of vascular, immune and tissue-specific gene expression within the atherosclerotic lesion. Importantly, epigenetic processes are reversible and may provide an excellent therapeutic target. The concept of epigenetic regulation is gradually being recognized as an important factor in the pathogenesis of atherosclerosis. Recent research provides an essential link between inflammation and reprogramming of the epigenome. In this review we therefore discuss the basis of epigenetic regulation – and the contribution thereof in the regulation of inflammatory processes in general and during atherosclerosis in particular. Moreover we highlight potential therapeutic interventions based on epigenetic mechanisms.

Monocytes in atherosclerosis: subsets and functions

Chronic inflammation drives atherosclerosis, the leading cause of cardiovascular disease. Over the past two decades, data have emerged showing that immune cells are involved in the pathogenesis of atherosclerotic plaques. The accumulation and continued recruitment of leukocytes are associated with the development of 'vulnerable' plaques. These plaques are prone to rupture, leading to thrombosis, myocardial infarction or stroke, all of which are frequent causes of death. Plaque macrophages account for the majority of leukocytes in plaques, and are believed to differentiate from monocytes recruited from circulating blood. However, monocytes represent a heterogenous circulating population of cells. Experiments are needed to address whether monocyte recruitment to plaques and effector functions, such as the formation of foam cells, the production of nitric oxide and reactive oxygen species, and proteolysis are critical for the development and rupture of plaques, and thus for the pathophysiology of atherosclerosis, as well as elucidate the precise mechanisms involved.

P-selectin glycoprotein ligand-1 plays a crucial role in the selective recruitment of leukocytes into the atherosclerotic arterial wall

Leukocyte recruitment to the arterial vessel wall is the first step in the development of atherosclerotic lesions. Leukocyte homing in this event proceeds through a well-defined adhesion cascade, which includes tethering, rolling, adhesion, and transmigration. Selectins, including the P-, E-, and L-selectins, and their ligands mediate the initial tethering and rolling. Interactions between selectins and their ligands serve as a braking system to decelerate fast-flowing leukocytes from the central blood stream and enable them to adhere to and transmigrate underneath the activated endothelium. The best characterized ligand for selectins is P-selectin glycoprotein ligand-1, an extended homodimeric mucin on leukocytes that binds to all three selectins. Recent studies show that differential expression or glycosylation of P-selectin glycoprotein ligand-1 in different leukocytes mediates selective recruitment of different subsets of monocytes or lymphocytes to atherosclerotic arteries.

CCL5 promotes macrophage recruitment and survival in human adipose tissue

OBJECTIVE

To examine the role of adipose-produced chemokine, chemokine ligand (CCL) 5, on the recruitment and survival of macrophages in human white adipose tissue (WAT).

METHODS AND RESULTS

CCL5 levels measured by enzyme immunoassay in serum and by real-time polymerase chain reaction in WAT were higher in obese compared to lean subjects. CCL5, but not CCL2, secretion was higher in visceral compared to subcutaneous WAT. CCL5 mRNA expression was positively correlated with the inflammatory macrophage markers as CD11b, tumor necrosis factor-alpha, and IL-6 in visceral WAT (n=24 obese subjects), and was higher in macrophages than other WAT cells. We found that CCL5 triggered adhesion and transmigration of blood monocytes to/through endothelial cells of human WAT. Whereas in obese WAT apoptotic macrophages were located around necrotic adipocytes, we demonstrated that CCL5, but not CCL2, protected macrophages from free cholesterol-induced apoptosis via activation of the Akt/Erk pathways.

CONCLUSIONS

CCL5 could participate in the inflammation of obese WAT by recruiting blood monocytes and exerting antiapoptotic properties on WAT macrophages. This specific role of CCL5 on macrophage survival with maintenance of their lipid scavenging function should be taken into account for future therapeutic strategies in obesity-related diseases.

The chemokine network in relation to infarct size and left ventricular remodeling following acute myocardial infarction

Increased circulating chemokines have been reported during acute myocardial infarction and might give prognostic information about future ischemic events. However, data on the chemokine network in relation to infarct size and measures of left ventricular remodeling after successful percutaneous coronary intervention (PCI) are lacking. A total of 42 patients with first-time ST-segment elevation acute myocardial infarction with a single occluded vessel were recruited, and cardiac magnetic resonance was used for serial assessment (2, 7, and 60 days) of infarct size and left ventricular remodeling. The chemokines were analyzed before and after PCI. After PCI, high levels of CCL4, CXCL16, CXCL10, and, in particular, CXCL8 within the first week after PCI correlated positively with the degree of myocardial damage, as reflected by correlations with the maximum troponin T levels and infarct size after 2 months, as assessed by cardiac magnetic resonance, and with impaired myocardial function after 2 months as assessed by cardiac magnetic resonance and neurohormonal methods. In contrast, the plasma levels of CCL3 and CXCL7 during the first week correlated negatively with myocardial dysfunction after 2 months. In conclusion, our findings suggest a role for chemokines in both adaptive and maladaptive responses after myocardial infarction and might support a role for CCL4, CXCL16, CXCL10, and, in particular, CXCL8 in postmyocardial infarction reperfusion and remodeling.

Role of polymorphonuclear neutrophils in atherosclerosis: current state and future perspectives

Contrary to the long-standing and widely accepted belief that polymorphonuclear neutrophils (PMN) are of marginal relevance in atherosclerosis, evidence revealing a previously unappreciated role of PMN in the process of atherosclerosis is being accumulating. Systemic inflammation involving activated PMN is clearly associated with unstable conditions of coronary artery disease and an increased number of circulating neutrophils is a well-known risk indicator of future cardiovascular outcomes. Furthermore, PMN are activated in a number of clinical conditions associated with high risk of developing atherosclerosis and are detectable into culprit lesions of patients with coronary artery disease. At present, pharmacological interventions aimed at blocking neutrophil emigration from the blood into the arterial wall and/or inhibiting neutrophil-mediated inflammatory functions are not an option for treating atherosclerosis. Nevertheless, several lines of evidence suggest that part of the atheroprotective effects of statins as well as HDL and HDL apolipoproteins may be related to their ability to modulate neutrophilic inflammation in the arterial wall. These hypotheses are not definitely established and warrant for further study. This Review describes the evidence suggesting that PMN may have a causative role in atherogenesis and atheroprogression and discusses the potential importance of modulating neutrophilic inflammation as part of a novel, improved strategy for preventing and treating atherosclerosis.

Macrophage migration inhibitory factor: a noncanonical chemokine important in atherosclerosis

In the recent years, atherogenesis has increasingly been linked to inflammatory processes in the injured vessel wall. Recruitment and arrest of monocytes, T cells, and neutrophils via the concerted actions of multiple chemokines and their chemokine receptors have been the subject of intense research and are being appreciated as key events underlying atherosclerotic lesion formation and progression. The evolutionary conserved cytokine macrophage migration inhibitory factor (MIF) exhibits prominent proinflammatory and proatherogenic functions, and the latest findings on its chemotactic and chemokine-like properties imply MIF as a crucial drug target for the treatment of inflammatory diseases. In this review, the role of MIF in atherosclerosis and injury-induced neointima formation is discussed. We place an emphasis on its proinflammatory and chemokine-like functions in the context of underlying extra- and intracellular signaling mechanisms. These findings clearly distinguish MIF from other cytokines in atherosclerosis and justify the intensive search for inhibitors targeting MIF in the treatment of inflammatory diseases, including advanced atherosclerosis.

A functional heteromeric MIF receptor formed by CD74 and CXCR4

MIF is a chemokine-like inflammatory mediator that triggers leukocyte recruitment by binding to CXCR2 and CXCR4. MIF also interacts with CD74/invariant chain, a single-pass membrane-receptor. We identified complexes between CD74 and CXCR2 with a role in leukocyte recruitment. It is unknown whether CD74 also binds to CXCR4. We demonstrate that CD74/CXCR4 complexes formed when CD74 was expressed with CXCR4 in HEK293 cells. Expression of CD74-variants lacking an ER-retention signal showed CD74/CXCR4 complexes at the cell surface. Importantly, endogenous CD74/CXCR4 complexes were isolated by co-immunoprecipitation from monocytes. Finally, MIF-stimulated CD74-dependent AKT activation was blocked by anti-CXCR4 and anti-CD74 antibodies and AMD3100, whereas CXCL12-stimulated AKT activation was not reduced by anti-CD74. Thus, CD74 forms functional complexes with CXCR4 that mediate MIF-specific signaling.

G2A and LPC: regulatory functions in immunity

The G2A receptor was originally identified by virtue of its transcriptional induction in murine B lymphoid cells in response to oncogenic transformation and treatment with various DNA-damaging agents. While preliminary characterization of cellular responses to G2A overexpression in fibroblastic cell lines suggested that this receptor may negatively regulate cell growth under conditions of proliferative and genotoxic stress, subsequent studies driven by the discovery of lysophosphatidylcholine (LPC) as a regulator of G2A signaling in immunoregulatory cells point to an important role for this receptor in innate and adaptive immunity.

Neutrophil granule proteins tune monocytic cell function

Polymorphonuclear leukocytes (PMNs) release the contents of granules during their migration to inflammatory sites. On liberation from the first leukocyte to enter injured tissue, the granule proteins play a central role in the early inflammatory response. In particular, mononuclear phagocytes interact intimately with PMNs and their secretion products. PMN granule proteins enhance the adhesion of monocytes to the endothelium and stimulate subsequent extravasation of inflammatory monocytes. At the site of inflammation, PMN granule proteins activate macrophages to produce and release cytokines and to phagocytose IgG-opsonized bacteria. Furthermore, by direct cell-cell contacts, PMNs activate monocyte-derived dendritic cells, thereby enhancing antigen presentation. Efforts in this field might lead to the development of drugs for specific modulation of innate immune functions.

Mechanisms underlying neutrophil-mediated monocyte recruitment

Extravasation of polymorphonuclear leukocytes (PMNs) to the site of inflammation precedes a second wave of emigrating monocytes. That these events are causally connected has been established a long time ago. However, we are now just beginning to understand the molecular mechanisms underlying this cellular switch, which has become even more complex considering the emergence of monocyte subsets, which are affected differently by signals generated from PMNs. PMN granule proteins induce adhesion as well as emigration of inflammatory monocytes to the site of inflammation involving beta(2)-integrins and formyl-peptide receptors. Furthermore, modification of the chemokine network by PMNs and their granule proteins creates a milieu favoring extravasation of inflammatory monocytes. Finally, emigrated PMNs rapidly undergo apoptosis, leading to the discharge of lysophosphatidylcholine, which attracts monocytes via G2A receptors. The net effect of these mechanisms is the accumulation of inflammatory monocytes, thus promoting proinflammatory events, such as release of inflammation-sustaining cytokines and reactive oxygen species. As targeting PMNs without causing serious side effects seems futile, it may be more promising to aim at interfering with subsequent PMN-driven proinflammatory events.

Adaptive immunity and atherosclerosis

Atherosclerosis involves the formation of inflammatory arterial lesions and is one of the most common causes of death globally. It has been evident for more than 20 years that adaptive immunity and T cells in particular regulate the magnitude of the atherogenic pro-inflammatory response. T cells also influence the stability of the atherosclerotic lesion and thus the propensity for thrombus formation and the clinical outcome of disease. This review summarizes our current understanding of T cells in atherogenesis, including which antigens they recognize, the role of T cell costimulation/coinhibition, and their secretion of pro- and anti-inflammatory mediators. Furthermore, we outline future areas of research and potential clinical intervention strategies.

Absence of p55 TNF receptor reduces atherosclerosis, but has no major effect on angiotensin II induced aneurysms in LDL receptor deficient mice

Background

The aim of the current study was to investigate the role of p55 TNF Receptor (p55 TNFR), the main signaling receptor for the pro-inflammatory cytokine tumor necrosis factor (TNF), in the development of two vascular disorders: atherosclerosis and angiotensin (Ang) II-induced abdominal aortic aneurysms (AAA).

Methodology/Principal Findings

p55 TNFR deficient mice were crossed to an LDL receptor deficient background and were induced for the development of either atherosclerosis or AngII-induced AAA, and compared to littermate controls, wild-type for p55 TNFR expression. p55 TNFR deficient mice developed 43% smaller atherosclerotic lesions in the aortic sinuses compared to controls. Moreover, expression of CD68, a macrophage specific marker, exhibited a 50% reduction in the aortic arches. Decreased atherosclerosis correlated with a strong down-regulation in the expression of adhesion molecules, such as VCAM-1 and ICAM-1, by p55 TNFR deficient endothelium. In addition, expression levels of the pro-inflammatory cytokines and chemokines TNF, IL-6, MCP-1 and RANTES were significantly reduced in aortas of p55 TNFR deficient mice. In contrast, in the AngII-induced model of AAA, p55 TNFR deficiency correlated with a slight trend towards increased aneurismal lethality, but the incidence of aortic rupture due to a dissecting aneurysm, and the expansion of the suprarenal aorta were not significantly different compared to controls.

Conclusion/Significance

We found that p55 TNFR expression promotes atherosclerosis, among other mechanisms, by enhancing expression of endothelial adhesion molecules, while it seems to have no major role in the development of AngII-induced AAA.

Inflammatory mechanisms in atherosclerosis

Atherosclerosis, the cause of myocardial infarction, stroke and ischemic gangrene, is an inflammatory disease. When LDL accumulates in the intima, it activates the endothelium to express leukocyte adhesion molecules and chemokines that promote recruitment of monocytes and T cells. Monocyte-derived macrophages upregulate pattern recognition receptors, including scavenger receptors that mediate uptake of modified LDL, and Toll-like receptors, which transmit activating signals leading to release of cytokines, proteases, and vasoactive molecules. T cells in lesions recognize local antigens and mount Th1 responses with secretion of pro-inflammatory cytokines, thus contributing to local inflammation and growth of the plaque. Intensified inflammatory activation may lead to local proteolysis, plaque rupture, and thrombus formation, triggering ischemia and infarction. Inflammatory markers are already used to monitor the disease process and anti-inflammatory therapy may be useful to control disease activity.

Atherogenic lipids induce high-density lipoprotein uptake and cholesterol efflux in human macrophages by up-regulating transmembrane chemokine CXCL16 without engaging CXCL16-dependent cell adhesion

Atherosclerosis is a complex pathologic process in which chemokine-mediated leukocyte accumulation in arterial walls is thought to be an important mechanism of pathogenesis. An interesting exception to this paradigm is the chemokine CXCL16, also known as the scavenger receptor for phosphatidylserine and oxidized low density lipoprotein, which is highly expressed in mouse and human atherosclerotic lesions, yet appears to be atheroprotective. In this study, we address potential mechanisms responsible for this activity. Consistent with its presence in atherosclerotic plaque, we found that atherogenic lipids up-regulated CXCL16 in primary human monocyte-derived macrophages. However, the same lipids down-regulated the CXCL16-targeted protease ADAM10, resulting in preferential expression of CXCL16 as the transmembrane form, not the shed form. Although transmembrane CXCL16 is known to mediate cell-cell adhesion by binding its receptor CXCR6, and atherogenic lipids are known to stimulate macrophage adhesion to coronary artery smooth muscle cells, we found that heterotypic adhesion of these cell types occurred in a CXCL16-independent manner. Instead we found that in macrophages, CXCL16 promoted internalization of both oxidized low density lipoprotein and high density lipoprotein, as well as release of cholesterol. Moreover, CXCL16 deficiency in macrophages interfered with oxidized low density lipoprotein-induced up-regulation of atheroprotective genes: adenosine triphosphate-binding cassette transporter A1 and G1 as well as apolipoprotein E. Thus, our findings support the hypothesis that CXCL16 mediates atheroprotection through its scavenger role in macrophages and not by cell-cell adhesion.

Importance of junctional adhesion molecule-C for neointimal hyperplasia and monocyte recruitment in atherosclerosis-prone mice-brief report

OBJECTIVE

Although junctional adhesion molecule (JAM)-C has been implicated in the control of inflammatory leukocyte recruitment, its role in neointima formation after arterial injury has not been elucidated.

METHODS AND RESULTS

In apolipoprotein E-deficient (Apoe(-/-)) mice fed an atherogenic diet, antibody blockade of JAM-C significantly reduced neointimal hyperplasia after wire injury of carotid arteries without altering medial area and decreased neointimal macrophage but not smooth muscle cell (SMC) content. An increased expression of JAM-C was detected in colocalization with luminal SMCs 1 day after injury and neointimal SMCs after 3 weeks. Blocking JAM-C inhibited monocytic cell arrest and leukocyte adhesion to carotid arteries perfused ex vivo and in vivo. Furthermore, monocyte adhesion to activated coronary artery SMCs under flow conditions in vitro was diminished by blocking JAM-C.

CONCLUSIONS

Our data provide the first evidence for a crucial role of JAM-C in accelerated lesion formation and leukocyte recruitment in atherosclerosis-prone mice.

Immune system and atherosclerotic disease: heterogeneity of leukocyte subsets participating in the pathogenesis of atherosclerosis

Atherosclerosis is an inflammatory disease in which a systemic inflammatory reaction is combined with an accumulation of immune cells, such as monocytes/macrophages, dendritic cells (DCs), and numerous lymphocytes, in atherosclerotic plaques. The immune system, comprising innate immunity and adaptive immunity, has been implicated in all stages of atherosclerosis, from initiation through progression and in atherothrombotic complications. It is clear that different subpopulations of leukocytes are involved in the pathogenesis of atherosclerosis and plaque instability. Recent studies have also demonstrated that each heterogeneity of immune-associated cells contributes to the atherogenic and atheroprotective axis. This review highlights recent advances in research and explores the role of the complex heterogeneity of leukocyte subsets, especially monocytes/macrophages (inflammatory monocytes, resident monocytes, M1, and M2), DCs (myeloid DCs, plasmacytoid DCs, pre DCs, conventional DCs, inflammatory DCs), and CD4(+) cells (T-helper 1, T-helper 2, regulatory T, and T-helper 17 cells), in the initiation and development of atherosclerotic disease and its complications.

Genetic diversity of CX3CR1 gene and coronary artery disease: new insights through a meta-analysis

A significant portion of current medical research is devoted to the pursuit of genetic markers that can be used to identify disease or predict susceptibility to disease. In such a quest many investigators hypothesized that genetic variations that alter signalling pathways involved in atherosclerosis affect susceptibility to coronary artery disease (CAD). Fractalkine (FKN) is a small cytokine involved in monocyte chemotaxis and activation. Two single nucleotide polymorphisms, V249I and T280M, have been identified in the receptor coding sequence of FKN. The polymorphisms alter ligand-receptor affinity and are believed to influence an individual's susceptibility to atherosclerosis. Several investigators have tested the latter hypothesis with inconsistent results. In order to clarify the effect of the two polymorphisms on susceptibility to CAD we performed a meta-analysis, using pooled data retrieved from seven case-control studies. In total, 2000 CAD patients and 2841 subjects without evidence of cardiovascular disease were included in the meta-analysis. The 280M allele was associated with a reduced risk for CAD in the heterozygous state. Consequently, this effect was attributed to the only 280M-containing haplotype: I(249)M(280). The latter haplotype was found to be significantly more frequent in the control population's gene pool. Although we do not believe that the retrieved odds ratios render the T280M polymorphism a candidate genetic marker for clinical applications, we do believe that the above genotype-phenotype interaction is indicative of the strong associations between FKN-induced pathways and CAD.

Myeloid cells in atherosclerosis: initiators and decision shapers

Chronic inflammation is the underlying pathophysiological mechanism of atherosclerosis. Prominent suspects being involved in atherosclerosis are lymphocytes, platelets, and endothelial cells. However, recent advances suggest a potent role for myeloid leukocytes, specifically monocyte subsets, polymorphonuclear leukocytes, and mast cells. These three cell types are not just rapidly recruited or already reside in the vascular wall but also initiate and perpetuate core mechanisms in plaque formation and destabilization. Dendritic cell subsets as well as endothelial and smooth muscle progenitor cells may further emerge as important regulators of atheroprogression. To stimulate further investigations about the contribution of these myeloid cells, we highlight the current mechanistic understanding by which these cells tune atherosclerosis.

Disrupting functional interactions between platelet chemokines inhibits atherosclerosis in hyperlipidemic mice

Atherosclerosis is characterized by chronic inflammation of the arterial wall due to chemokine-driven mononuclear cell recruitment. Activated platelets can synergize with chemokines to exacerbate atherogenesis; for example, by deposition of the chemokines platelet factor-4 (PF4, also known as CXCL4) and RANTES (CCL5), triggering monocyte arrest on inflamed endothelium. Homo-oligomerization is required for the recruitment functions of CCL5, and chemokine heteromerization has more recently emerged as an additional regulatory mechanism, as evidenced by a mutual modulation of CXCL8 and CXCL4 activities and by enhanced monocyte arrest resulting from CCL5-CXCL4 interactions. The CCL5 antagonist Met-RANTES reduces diet-induced atherosclerosis; however, CCL5 antagonism may not be therapeutically feasible, as suggested by studies using Ccl5-deficient mice which imply that direct CCL5 blockade would severely compromise systemic immune responses, delay macrophage-mediated viral clearance and impair normal T cell functions. Here we determined structural features of CCL5-CXCL4 heteromers and designed stable peptide inhibitors that specifically disrupt proinflammatory CCL5-CXCL4 interactions, thereby attenuating monocyte recruitment and reducing atherosclerosis without the aforementioned side effects. These results establish the in vivo relevance of chemokine heteromers and show the potential of targeting heteromer formation to achieve therapeutic effects.

The multifaceted contributions of leukocyte subsets to atherosclerosis: lessons from mouse models

Chronic inflammation drives the development of atherosclerosis, and details regarding the involvement of different leukocyte subpopulations in the pathology of this disease have recently emerged. This Review highlights the surprising contribution of granulocyte subsets and mast cells to early atherogenesis and subsequent plaque instability, and describes the complex, double-edged role of monocyte, macrophage and dendritic-cell subsets through crosstalk with T cells and vascular progenitor cells. Improved understanding of the selective contributions of specific cell types to atherogenesis will pave the way for new targeted approaches to therapy.