Muscle injury triggers inflammation in which infiltrating mononuclear phagocytes are crucial for tissue regeneration. The interaction of the CCL2/CCR2 and CX3CL1/CX3CR1 chemokine axis that guides ...phagocyte infiltration is incompletely understood. Here, we show that CX3CR1 deficiency promotes muscle repair and rescues Ccl2(-/-) mice from impaired muscle regeneration as a result of altered macrophage function, not infiltration. Transcriptomic analysis of muscle mononuclear phagocytes reveals that Apolipoprotein E (ApoE) is upregulated in mice with efficient regeneration. ApoE treatment enhances phagocytosis by mononuclear phagocytes in vitro, and restores phagocytic activity and muscle regeneration in Ccl2(-/-) mice. Because CX3CR1 deficiency may compensate for defective CCL2-dependant monocyte recruitment by modulating ApoE-dependent macrophage phagocytic activity, targeting CX3CR1 expressed by macrophages might be a powerful therapeutic approach to improve muscle regeneration.
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.
The eyes of patients with atrophic AMD feature high CCL2 and CCR2+ monocytes. This is modeled in Cx3cr1 KO mice in which Ccl2 and Ccr2 deletion, CCR2 inhibition and monocyte depletion diminished subretinal inflammation and photoreceptor degeneration.
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.
We generated CX3CR1-deficient mice (CX3CR1(-/-)) by targeted gene disruption and crossed them with the proatherogenic apolipoprotein E-deficient mice (apoE(-/-)). 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(+/+)/apoE(-/-) 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(-/-) 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(+/-)/apoE(-/-) mice showed the same reduction in atherosclerosis as the CX3CR1(-/-)/apoE(-/-) mice.
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.
Visceral leishmaniasis (VL) is a systemic disease with multifaceted clinical manifestations, including neurological signs, however, the involvement of the nervous system during VL is underestimated. ...Accordingly, we investigated both brain infection and inflammation in a mouse model of VL. Using bioluminescent Leishmania donovani and real-time 2D-3D imaging tools, we strikingly detected live parasites in the brain, where we observed a compartmentalized dual-phased inflammation pattern: an early phase during the first two weeks post-infection, with the prompt arrival of neutrophils and Ly6C
macrophages in an environment presenting a variety of pro-inflammatory mediators (IFN-γ, IL-1β, CXCL-10/CXCR-3, CCL-7/CCR-2), but with an intense anti-inflammatory response, led by IL-10; and a re-inflammation phase three months later, extremely pro-inflammatory, with novel upregulation of mediators, including IL-1β, TNF-α and MMP-9. These new data give support and corroborate previous studies connecting human and canine VL with neuroinflammation and blood-brain barrier disruption, and conclusively place the brain among the organs affected by this parasite. Altogether, our results provide convincing evidences that Leishmania donovani indeed infects and inflames the brain.
Alterations of the chemokine receptor CX3CR1 gene were associated with a reduced risk of myocardial infarction in human and limited atherosclerosis in mice. In this study, we addressed whether CX3CR1 ...antagonists are potential therapeutic tools to limit acute and chronic inflammatory processes in atherosclerosis.
Treatment with F1, an amino terminus-modified CX3CR1 ligand endowed with CX3CR1 antagonist activity, reduced the extent of atherosclerotic lesions in both Apoe(-/-) and Ldlr(-/-) proatherogenic mouse models. Macrophage accumulation in the aortic sinus was reduced in F1-treated Apoe(-/-) mice but the macrophage density of the lesions was similar in F1-treated and control mice. Both in vitro and in vivo F1 treatment reduced CX3CR1-dependent inflammatory monocyte adhesion, potentially limiting their recruitment. In addition, F1-treated Apoe(-/-) mice displayed reduced numbers of blood inflammatory monocytes, whereas resident monocyte numbers remained unchanged. Both in vitro and in vivo F1 treatment reduced CX3CR1-dependent inflammatory monocyte survival. Finally, F1 treatment of Apoe(-/-) mice with advanced atherosclerosis led to smaller lesions than untreated mice but without reverting to the initial phenotype.
The CX3CR1 antagonist F1 is a potent inhibitor of the progression of atherosclerotic lesions by means of its selective impact on inflammatory monocyte functions. Controlling monocyte trafficking and survival may be an alternative or complementary therapy to lipid-lowering drugs classically used in the treatment of 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.
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.
Physiologically, the retinal pigment epithelium (RPE) expresses immunosuppressive signals such as FAS ligand (FASL), which prevents the accumulation of leukocytes in the subretinal space. Age‐related ...macular degeneration (AMD) is associated with a breakdown of the subretinal immunosuppressive environment and chronic accumulation of mononuclear phagocytes (MPs). We show that subretinal MPs in AMD patients accumulate on the RPE and express high levels of APOE. MPs of Cx3cr1−/− mice that develop MP accumulation on the RPE, photoreceptor degeneration, and increased choroidal neovascularization similarly express high levels of APOE. ApoE deletion in Cx3cr1−/− mice prevents pathogenic age‐ and stress‐induced subretinal MP accumulation. We demonstrate that increased APOE levels induce IL‐6 in MPs via the activation of the TLR2‐CD14‐dependent innate immunity receptor cluster. IL‐6 in turn represses RPE FasL expression and prolongs subretinal MP survival. This mechanism may account, in part, for the MP accumulation observed in Cx3cr1−/− mice. Our results underline the inflammatory role of APOE in sterile inflammation in the immunosuppressive subretinal space. They provide rationale for the implication of IL‐6 in AMD and open avenues toward therapies inhibiting pathogenic chronic inflammation in late AMD.
Synopsis
In age‐related macular degeneration, subretinal mononuclear phagocytes (MP) express high APOE levels, which prolongs their survival and accumulation by inducing IL‐6, which in turn promotes chronic subretinal inflammation.
Pathogenic subretinal mononuclear phagocytes, observed in age‐related macular degeneration in human donor tissue and in the Cx3cr1−/− mouse model of subretinal inflammation, express high levels of APOE.
High levels of APOE activate the TLR2‐CD14‐dependent innate immunity receptor cluster and induce IL‐6 in mononuclear phagocytes.
IL‐6 inhibits FASL expression in the retinal pigment epithelium and increases subretinal mononuclear phagocyte survival.
ApoE deletion and pharmacological IL‐6 inhibition prevents pathogenic age‐ and stress‐induced subretinal MP accumulation in Cx3cr1−/−mice.
In age‐related macular degeneration, subretinal mononuclear phagocytes (MP) express high APOE levels, which prolongs their survival and accumulation by inducing IL‐6, which in turn promotes chronic subretinal inflammation.
The cells of the mononuclear phagocyte system are essential for the correct healing of adult skin wounds, but their specific functions remain ill-defined. The absence of granulation tissue ...immediately after skin injury makes it challenging to study the role of mononuclear phagocytes at the initiation of this inflammatory stage. To study their recruitment and migratory behavior within the wound bed, we developed a new model for real-time in vivo imaging of the wound, using transgenic mice that express green and cyan fluorescent proteins and specifically target monocytes. Within hours after the scalp injury, monocytes invaded the wound bed. The complete abrogation of this infiltration in monocyte-deficient CCR2(-/-) mice argues for the involvement of classical monocytes in this process. Monocyte infiltration unexpectedly occurred as early as neutrophil recruitment did and resulted from active release from the bloodstream toward the matrix through microhemorrhages rather than transendothelial migration. Monocytes randomly scouted around the wound bed, progressively slowed down, and stopped. Our approach identified and characterized a rapid and earlier than expected wave of monocyte infiltration and provides a novel framework for investigating the role of these cells during early stages of wound healing.
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