Chemokine receptors CCR2 and CX3CR1 regulate skin fibrosis in the mouse model of cytokine-induced systemic sclerosis.

BACKGROUND Skin fibrotic disorders such as systemic sclerosis (SSc) are characterized by an excessive accumulation of extracellular matrix (ECM), and develop under the influence of certain cytokines. We previously established a mouse model of skin fibrosis induced by exogenous application of cytokines. We have revealed that both the number of macrophages and the levels of macrophage chemoattractant protein-1 (MCP-1) mRNA positively correlate with the extent of skin fibrosis. Macrophages can be divided into two subsets, the first expressing CCR2, and the second expressing CX3CR1. OBJECTIVE To elucidate the role of skin infiltrating macrophages based on CCR2 and CX3CR1 in this cytokine-induced murine fibrosis model. METHODS We examined the amounts of collagen deposited in granulation tissues, the numbers of macrophages and the levels of several mRNA in wild type (WT) mice, CCR2(-/-) mice, and CX3CR1(-/-) mice during injections of transforming growth factor-β (TGF-β) followed by injections of connective tissue growth factor (CTGF). RESULTS TGF-β injection increased the expressions of MCP-1, fractalkine, CCR2 and CX3CR1 mRNA in WT mice. The overproduction of collagen induced by TGF-β was significantly reduced by CCR2 deficiency, while collagen contents induced by CTGF were restored to wild-type levels. In contrast, overproduction of collagen in CX3CR1-deficient mice decreased nearly 50% by both TGF-β and CTGF stimulations. CONCLUSION The involvement of CCR2/MCP-1 interaction (CCR2-dependent loop) was during the TGF-β phase. In contrast, the fractalkine/CX3CR1 interaction contributes to the initiation of fibrosis by TGF-β and its maintenance by CTGF. Collectively, two subsets of macrophages both cooperatively and independently play important roles in the development of fibrosis.