CX3CL1-CX3CR1 interaction mediates macrophage-mesothelial cross talk and promotes peritoneal fibrosis.

Peritoneal dialysis (PD) is limited by chronic fibrotic remodeling of the peritoneal wall, a transforming growth factor-ß (TGF-ß)-mediated process. The fractalkine (CX3CL1) receptor CX3CR1 is expressed on macrophages and monocytes, where it is a marker of TGFß expression. Detection of its ligand CX3CL1 on the peritoneal mesothelium led us to hypothesize a pathophysiologic role of CX3CL1-CX3CR1 interaction in peritoneal fibrosis. We found that CX3CL1 was expressed on peritoneal mesothelial cells from PD patients and in a murine PD model. CX3CR1, mostly expressed on macrophages in the peritoneal wall, promoted fibrosis induced by chronic dialysate exposure in the mouse model. Our data suggest a positive feedback loop whereby direct interaction with CX3CR1-expressing macrophages promotes mesothelial expression of CX3CL1 and TGFß expression. In turn, TGFß upregulates CX3CR1 in murine and human monocytic cells. Upstream, macrophage cytokines including interleukin-1ß (IL-1ß) promote mesothelial CX3CR1 and TGFß expression, providing a starting point for CX3CL1-CX3CR1 interaction. IL-1ß expression was enhanced by exposure to dialysate both in vitro and in the mouse models. Our data suggest that macrophage-mesothelial cell crosstalk through CX3CR1-CX3CL1 interaction enhances mesothelial TGFß production, promoting peritoneal fibrosis in response to dialysate exposure. This interaction could be a novel therapeutic target in PD-associated chronic peritoneal fibrosis.