Protective roles of the fractalkine/CX3CL1-CX3CR1 interactions in alkali-induced corneal neovascularization through enhanced antiangiogenic factor expression

Macrophages accumulate during the course of corneal neovascularization, but its mechanisms and roles still remain elusive. To address these points, we herein examined corneal neovascularization after alkali injury in mice deficient in fractalkine receptor/CX3CR1, which is normally expressed by macrophages. After alkali injury, the mRNA expression of CX3CR1 was augmented along with accumulation of F4/80-positive macrophages and Gr-1-positive neutrophils in the corneas. Compared with wild-type mice, CX3CR1-deficient mice exhibited enhanced corneal neovascularization 2 wk after injury, as evidenced by enlarged CD31-positive areas. Concomitantly, the accumulation of F4/80-positive macrophages, but not Gr-1-positive neutrophils, was markedly attenuated in CX3CR1-deficient mice compared with wild-type mice. The intraocular mRNA expression of vascular endothelial growth factor (VEGF) was enhanced to similar extents in wild-type and CX3CR1-deifient mice after the injury. However, the mRNA expression of antiangiogenic factors, thrombospondin (TSP) 1, TSP-2, and a disintegrin and metalloprotease with thrombospondin (ADAMTS) 1, was enhanced to a greater extent in wild-type than CX3CR1-deificient mice. A double-color immunofluorescence analysis demonstrated that F4/80-positive cells also expressed CX3CR1 and ADAMTS-1 and that TSP-1 and ADAMTS-1 were detected in CX3CR1-positive cells. CX3CL1 enhanced TSP-1 and ADAMTS-1, but not VEGF, expression by peritoneal macrophages. Moreover, topical application of CX3CL1 inhibited corneal neovascularization at 2 wk, along with enhanced intraocular expression of TSP-1 and ADAMTS-1 but not VEGF. Thus, these observations indicate that accumulation of CX3CR1-positive macrophages intraocularly can dampen alkali-induced corneal neovascularization by producing antiangiogenic factors such as TSP-1 and ADAMTS-1 and suggest the potential therapeutic efficacy of using CX3CL1 against alkali-induced corneal neovascularization.