Coronary artery disease (CAD) arising from atherosclerosis is a leading cause of death and morbidity worldwide. The underlying pathogenesis involves an imbalanced lipid metabolism and a maladaptive immune response entailing a chronic inflammation of the arterial wall. The disturbed equilibrium of lipid accumulation, immune responses and their clearance is shaped by leukocyte trafficking and homeostasis governed by chemokines and their receptors. New pro- and anti-inflammatory pathways linking lipid and inflammation biology have been discovered, and genetic profiling studies have unveiled variations involved in human CAD. The growing understanding of the inflammatory processes and mediators has uncovered an intriguing diversity of targetable mechanisms that can be exploited to complement lipid-lowering therapies. Here we aim to systematically survey recently identified molecular mechanisms, translational developments and clinical strategies for targeting lipid-related inflammation in atherosclerosis and CAD.

The cytokine macrophage migration inhibitory factor (MIF) plays a critical role in inflammatory diseases and atherogenesis. We identify the chemokine receptors CXCR2 and CXCR4 as functional receptors for MIF. MIF triggered G(alphai)- and integrin-dependent arrest and chemotaxis of monocytes and T cells, rapid integrin activation and calcium influx through CXCR2 or CXCR4. MIF competed with cognate ligands for CXCR4 and CXCR2 binding, and directly bound to CXCR2. CXCR2 and CD74 formed a receptor complex, and monocyte arrest elicited by MIF in inflamed or atherosclerotic arteries involved both CXCR2 and CD74. In vivo, Mif deficiency impaired monocyte adhesion to the arterial wall in atherosclerosis-prone mice, and MIF-induced leukocyte recruitment required Il8rb (which encodes Cxcr2). Blockade of Mif but not of canonical ligands of Cxcr2 or Cxcr4 in mice with advanced atherosclerosis led to plaque regression and reduced monocyte and T-cell content in plaques. By activating both CXCR2 and CXCR4, MIF displays chemokine-like functions and acts as a major regulator of inflammatory cell recruitment and atherogenesis. Targeting MIF in individuals with manifest atherosclerosis can potentially be used to treat this condition.

Apoptosis is a pivotal process in embryogenesis and postnatal cell homeostasis and involves the shedding of membranous microvesicles termed apoptotic bodies. In response to tissue damage, the CXC chemokine CXCL12 and its receptor CXCR4 counteract apoptosis and recruit progenitor cells. Here, we show that endothelial cell-derived apoptotic bodies are generated during atherosclerosis and convey paracrine alarm signals to recipient vascular cells that trigger the production of CXCL12. CXCL12 production was mediated by microRNA-126 (miR-126), which was enriched in apoptotic bodies and repressed the function of regulator of G protein (heterotrimeric guanosine triphosphate-binding protein) signaling 16, an inhibitor of G protein-coupled receptor (GPCR) signaling. This enabled CXCR4, a GPCR, to trigger an autoregulatory feedback loop that increased the production of CXCL12. Administration of apoptotic bodies or miR-126 limited atherosclerosis, promoted the incorporation of Sca-1+ progenitor cells, and conferred features of plaque stability on different mouse models of atherosclerosis. This study highlights functions of microRNAs in health and disease that may extend to the recruitment of progenitor cells during other forms of tissue repair or homeostasis.

We studied whether circulating activated platelets and platelet-leukocyte aggregates cause the development of atherosclerotic lesions in apolipoprotein-E-deficient (Apoe(-/-)) mice. Circulating activated platelets bound to leukocytes, preferentially monocytes, to form platelet-monocyte/leukocyte aggregates. Activated platelets and platelet-leukocyte aggregates interacted with atherosclerotic lesions. The interactions of activated platelets with monocytes and atherosclerotic arteries led to delivery of the platelet-derived chemokines CCL5 (regulated on activation, normal T cell expressed and secreted, RANTES) and CXCL4 (platelet factor 4) to the monocyte surface and endothelium of atherosclerotic arteries. The presence of activated platelets promoted leukocyte binding of vascular cell adhesion molecule-1 (VCAM-1) and increased their adhesiveness to inflamed or atherosclerotic endothelium. Injection of activated wild-type, but not P-selectin-deficient, platelets increased monocyte arrest on the surface of atherosclerotic lesions and the size of atherosclerotic lesions in Apoe(-/-) mice. Our results indicate that circulating activated platelets and platelet-leukocyte/monocyte aggregates promote formation of atherosclerotic lesions. This role of activated platelets in atherosclerosis is attributed to platelet P-selectin-mediated delivery of platelet-derived proinflammatory factors to monocytes/leukocytes and the vessel wall.

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SUMMARY Unique among leukocytes, neutrophils follow daily cycles of release from and migration back into the bone marrow, where they are eliminated. Because removal of dying cells generates homeostatic signals, we explored whether neutrophil elimination triggers circadian events in the steady-state. Here, we report that the homeostatic clearance of neutrophils provides cues that modulate the physiology of the bone marrow. We identify a population of CD62LLO CXCR4HI neutrophils that have “aged” in the circulation and are eliminated at the end of the resting period in mice. Aged neutrophils infiltrate the bone marrow and promote reductions in the size and function of the hematopoietic niche. Modulation of the niche depends on macrophages and activation of cholesterol-sensing nuclear receptors, and is essential for the rhythmic egress of hematopoietic progenitors into the circulation. Our results unveil a process that synchronizes immune and hematopoietic rhythms, and expand the ascribed functions of neutrophils beyond inflammation.

Background-Inflammation and activation of immune cells are key mechanisms in the development of atherosclerosis. Previous data indicate important roles for monocytes and T lymphocytes in lesion formation, whereas the contribution of neutrophils remains to be firmly established. Here, we investigate the effect of hypercholesterolemia on peripheral neutrophil counts, neutrophil recruitment to atherosclerotic lesions, and the importance of neutrophils in atherosclerotic lesion formation in Apoe Ϫ/Ϫ mice. Methods and Results-Hypercholesterolemia induces neutrophilia, which was attributable to enhanced granulopoiesis and enhanced mobilization from the bone marrow. The degree of hypercholesterolemia-induced neutrophilia was positively correlated with the extent of early atherosclerotic lesion formation. In turn, neutropenic mice display reduced plaque sizes at early but not late stages of atherosclerotic lesion formation. Flow cytometry of enzymatically digested aortas further shows altered cellular plaque composition in neutropenic mice with reduced numbers of inflammatory monocytes and macrophages. Aortic neutrophil infiltration peaks 4 weeks after the start of a high-fat diet and decreases afterward. The recruitment of neutrophils to large arteries was found to depend on CCR1, CCR2, CCR5, and CXCR2, which contrasts to peripheral venous recruitment, which requires CCR2 and CXCR2 only. The involvement of CCR1 and CCR5 corresponded to the endothelial deposition of the platelet-derived chemokine CCL5 in arteries but not in veins. Conclusions-Our data provide evidence that hypercholesterolemia-induced neutrophilia is multifactorial and that neutrophils infiltrate arteries primarily during early stages of atherosclerosis. Collectively, these data suggest an important role of neutrophils in the initiation of atherosclerosis. (Circulation. 2010;122:1837-1845.)

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.