Chemokines modulate inflammatory responses that are prerequisites for organ fibrosis upon liver injury. Monocyte-derived hepatic macrophages are critical for the development, maintenance, and resolution of hepatic fibrosis. The specific role of monocyte-associated chemokine (C-X3-C motif) receptor 1 (CX₃CR1) and its cognate ligand fractalkine [chemokine (C-X3-C motif) ligand 1)] in liver inflammation and fibrosis is currently unknown. We examined 169 patients with chronic liver diseases and 84 healthy controls; we found that CX₃CL1 is significantly up-regulated in the circulation upon disease progression, whereas CX₃CR1 is down-regulated intrahepatically in patients with advanced liver fibrosis or cirrhosis. To analyze the functional relevance of this pathway, two models of experimental liver fibrosis were applied to wild-type (WT) and CX₃CR1-deficient mice. Fractalkine expression was induced upon liver injury in mice, primarily in hepatocytes and hepatic stellate cells. CX₃CR1(-/-) animals developed greater hepatic fibrosis than WT animals with carbon tetrachloride-induced and bile duct ligation-induced fibrosis. CX₃CR1(-/-) mice displayed significantly increased numbers of monocyte-derived macrophages within the injured liver. Chimeric animals that underwent bone marrow transplantation revealed that CX₃CR1 restricts hepatic fibrosis progression and monocyte accumulation through mechanisms exerted by infiltrating immune cells. In the absence of CX₃CR1, intrahepatic monocytes develop preferentially into proinflammatory tumor necrosis factor-producing and inducible nitric oxide synthase-producing macrophages. CX₃CR1 represents an essential survival signal for hepatic monocyte-derived macrophages by activating antiapoptotic bcl2 expression. Monocytes/macrophages lacking CX₃CR1 undergo increased cell death after liver injury, which then perpetuates inflammation, promotes prolonged inflammatory monocyte infiltration into the liver, and results in enhanced liver fibrosis.
Conclusion: CX₃CR1 limits liver fibrosis in vivo by controlling the differentiation and survival of intrahepatic monocytes. The opposing regulation of CX₃CR1 and fractalkine in patients suggests that pharmacological augmentation of this pathway may represent a possible therapeutic antifibrotic strategy.