Human immunodeficiency virus-type 1 (HIV-1) entry requires fusion cofactors on the CD4+ target cell. Fusin, a heterotrimeric GTP-binding protein (G protein)-coupled receptor, serves as a cofactor for T cell line-tropic isolates. The chemokines RANTES, MIP-1alpha, and MIP-1beta, which suppress infection by macrophage-tropic isolates, selectively inhibited cell fusion mediated by the corresponding envelope glycoproteins (Envs). Recombinant CC CKR5, a G protein-coupled receptor for these chemokines, rendered CD4-expressing nonhuman cells fusion-competent preferentially with macrophage-tropic Envs. CC CKR5 messenger RNA was detected selectively in cell types susceptible to macrophage-tropic isolates. CC CKR5 is thus a fusion cofactor for macrophage-tropic HIV-1 strains.

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The role of retinal microglial cells (MCs) in age-related macular degeneration (AMD) is unclear. Here we demonstrated that all retinal MCs express CX3C chemokine receptor 1 (CX3CR1) and that homozygosity for the CX3CR1 M280 allele, which is associated with impaired cell migration, increases the risk of AMD. In humans with AMD, MCs accumulated in the subretinal space at sites of retinal degeneration and choroidal neovascularization (CNV). In CX3CR1-deficient mice, MCs accumulated subretinally with age and albino background and after laser impact preceding retinal degeneration. Raising the albino mice in the dark prevented both events. The appearance of lipid-bloated subretinal MCs was drusen-like on funduscopy of senescent mice, and CX3CR1-dependent MC accumulation was associated with an exacerbation of experimental CNV. These results show that CX3CR1-dependent accumulation of subretinal MCs evokes cardinal features of AMD. These findings reveal what we believe to be a novel pathogenic process with important implications for the development of new therapies for AMD.

Background-Monocytes are critical mediators of atherogenesis. Deletion of individual chemokines or chemokine receptors leads to significant but only partial inhibition of lesion development, whereas deficiency in other signals such as CXCL16 or CCR1 accelerates atherosclerosis. Evidence that particular chemokine pathways may cooperate to promote monocyte accumulation into inflamed tissues, particularly atherosclerotic arteries, is still lacking. Methods and Results-Here, we show that chemokine-mediated signals critically determine the frequency of monocytes in the blood and bone marrow under both noninflammatory and atherosclerotic conditions. Particularly, CCL2-, CX3CR1-, and CCR5-dependent signals differentially alter CD11b ϩ Ly6G Ϫ 7/4 hi (also known as Ly6C hi ) and CD11b ϩ Ly6G Ϫ 7/4 lo (Ly6C lo ) monocytosis. Combined inhibition of CCL2, CX3CR1, and CCR5 in hypercholesterolemic, atherosclerosis-susceptible apolipoprotein E-deficient mice leads to abrogation of bone marrow monocytosis and to additive reduction in circulating monocytes despite persistent hypercholesterolemia. These effects are associated with a marked and additive 90% reduction in atherosclerosis. Interestingly, lesion size highly correlates with the number of circulating monocytes, particularly the CD11b ϩ Ly6G Ϫ 7/4 lo subset. Conclusions-CCL2, CX3CR1, and CCR5 play independent and additive roles in atherogenesis. Signals mediated through these pathways critically determine the frequency of circulating monocyte subsets and thereby account for almost all macrophage accumulation into atherosclerotic arteries. (Circulation. 2008;117:1649-1657.)

Complicated abdominal aortic aneurysm (AAA) is a major cause of mortality in elderly men. Ang II-dependent TGF-β activity promotes aortic aneurysm progression in experimental Marfan syndrome. However, the role of TGF-β in experimental models of AAA has not been comprehensively assessed. Here, we show that systemic neutralization of TGF-β activity breaks the resistance of normocholesterolemic C57BL/6 mice to Ang II-induced AAA formation and markedly increases their susceptibility to the disease. These aneurysms displayed a large spectrum of complications on echography, including fissuration, double channel formation, and rupture, leading to death from aneurysm complications. The disease was refractory to inhibition of IFN-γ, IL-4, IL-6, or TNF-α signaling. Genetic deletion of T and B cells or inhibition of the CX3CR1 pathway resulted in partial protection. Interestingly, neutralization of TGF-β activity enhanced monocyte invasiveness, and monocyte depletion markedly inhibited aneurysm progression and complications. Finally, TGF-β neutralization increased MMP-12 activity, and MMP-12 deficiency prevented aneurysm rupture. These results clearly identify a critical role for TGF-β in the taming of the innate immune response and the preservation of vessel integrity in C57BL/6 mice, which contrasts with its reported pathogenic role in Marfan syndrome.

Atrophic age-related macular degeneration (AMD) is associated with the subretinal accumulation of mononuclear phagocytes (MPs). Their role in promoting or inhibiting retinal degeneration is unknown. We here show that atrophic AMD is associated with increased intraocular CCL2 levels and subretinal CCR2+ inflammatory monocyte infiltration in patients. Using age- and light-induced subretinal inflammation and photoreceptor degeneration in Cx3cr1 knockout mice, we show that subretinal Cx3cr1 deficient MPs overexpress CCL2 and that both the genetic deletion of CCL2 or CCR2 and the pharmacological inhibition of CCR2 prevent inflammatory monocyte recruitment, MP accumulation and photoreceptor degeneration in vivo. Our study shows that contrary to CCR2 and CCL2, CX3CR1 is constitutively expressed in the retina where it represses the expression of CCL2 and the recruitment of neurotoxic inflammatory CCR2+ monocytes. CCL2/CCR2 inhibition might represent a powerful tool for controlling inflammation and neurodegeneration in AMD.

Background-Fractalkine (CX3CL1), a CX3C chemokine, is expressed in the vessel wall and mediates the firm adhesion and chemotaxis of leukocytes expressing its receptor, CX3CR1. A polymorphism in the CX3CR1 gene is associated with low CX3CR1 expression and reduced risk of acute coronary disease in humans. Methods and Results-We generated CX3CR1-deficient mice (CX3CR1 Ϫ/Ϫ ) by targeted gene disruption and crossed them with the proatherogenic apolipoprotein E-deficient mice (apoE Ϫ/Ϫ ). Here we show that the extent of lipid-stained lesions in the thoracic aorta was reduced by 59% in CX3CR1/apoE double knockout mice compared with their CX3CR1 ϩ/ϩ / apoE Ϫ/Ϫ littermates. The development of atherosclerosis in the aortic sinus was also markedly altered in the double knockout mice, with 50% reduction in macrophage accumulation. Although lesions of CX3CR1 Ϫ/Ϫ mice were smaller in size, they retained a substantial accumulation of smooth muscle cells and collagen, features consistent with a stable plaque phenotype. Finally, CX3CR1 ϩ/Ϫ /apoE Ϫ/Ϫ mice showed the same reduction in atherosclerosis as the CX3CR1 Ϫ/Ϫ / apoE Ϫ/Ϫ mice. Conclusions-The CX3CR1-CX3CL1 pathway seems to play a direct and critical role in monocyte recruitment and atherosclerotic lesion development in a mouse model of human atherosclerosis.

Cells undergoing apoptosis are efficiently located and engulfed by phagocytes. The mechanisms by which macrophages, the professional scavenging phagocytes of apoptotic cells, are attracted to sites of apoptosis are poorly defined. Here we show that CX3CL1/fractalkine, a chemokine and intercellular adhesion molecule, is released rapidly from apoptotic lymphocytes, via caspase-and Bcl-2-regulated mechanisms, to attract macrophages. Effective chemotaxis of macrophages to apoptotic lymphocytes is dependent on macrophage fractalkine receptor, CX3CR1. CX3CR1 deficiency caused diminished recruitment of macrophages to germinal centers of lymphoid follicles, sites of high-rate B-cell apoptosis. These results provide the first demonstration of chemokine/chemokine-receptor activity in the navigation of macrophages toward apoptotic cells and identify a mechanism by which macrophage infiltration of tissues containing apoptotic lymphocytes is achieved. (Blood. 2008;112:5026-5036) IntroductionWhen apoptosis occurs at high rates in mammalian tissues, apoptotic cells are almost invariably encountered in situ in association with macrophages. 1 These professional scavengers are attracted to the dying cells and engage in their safe, nonphlogistic disposal by phagocytosis. Examples of this innate immune response to dying cells are readily apparent during normal organogenesis, in normal adult tissues, such as the germinal centers of lymphoid follicles, in inflammatory responses, and in pathologic conditions including tumors. The efficient clearance of apoptotic cells by phagocytes is a homeostatic mechanism that militates against histotoxic, proinflammatory, or immunogenic effects that may result from persistence of apoptotic cells. [1][2][3] In recent years, much progress has been made in improving our understanding of the molecular mechanisms underlying the interactions between apoptotic cells and macrophages and the immunologic implications of those interactions. [1][2][3][4][5][6] Before the tethering/ engulfment phases of macrophage-mediated apoptotic-cell clearance, phagocytes are required to navigate effectively to sites of apoptosis. Active release of chemoattractant ("find-me") signals from apoptotic cells at an early stage after engagement of the cell-death program would be predicted to underpin this process, but knowledge of the molecules involved is currently limited. Lysophosphatidylcholine (LPC) is released from apoptotic cells and functions in soluble form as a chemoattractant for mononuclear phagocytes. 7 Significantly, no chemokine family members have previously been implicated in this chemotactic process. Here we show that the chemokine and adhesion molecule CX3CL1, 8,9 also known as neurotactin or fractalkine (FKN), together with its cognate receptor CX3CR1, 10,11 plays an active role in the chemotaxis of macrophages to apoptotic cells. FKN is a type I transmembrane protein, the extracellular portion of which comprises the chemokine domain attached to a mucin stalk. Well known for its roles in inflammatory processe...