Monocyte/macrophages promote vasculogenesis in choroidal neovascularization in mice by stimulating SDF-1 expression in RPE cells

Monocyte-macrophages play important roles in choroidal neovascularization (CNV); however, the mechanism is unclear. This study investigated the effects of monocyte depletion on laser-induced CNV in mice, especially the involvement of bone marrow-derived cells (BMCs) and underlying molecular mechanisms. Clodronate-liposomes (lip) were used to deplete monocytes and their effect on retinal pigmental epithelium (RPE) cells, endothelial cells, and BMCs was analyzed. Green fluorescent protein (GFP)-chimeric mice were developed by transplanting bone marrow cells from GFP transgenic mice to C57BL/6 J mice. CNV was induced by laser photocoagulation. Chimeric mice were intravenously treated with clodronate-lip, PBS-lip or PBS, 1 day before and after lasering. Histopathological and choroidal flatmount analysis were performed to measure CNV severity and BMCs recruitment. BMCs expression of endothelial cell marker CD31 and vascular smooth muscle cell marker alpha-SMA in CNV were detected by immunofluorescence. Expression of stromal cell-derived factor-1 (SDF-1) protein in vivo was detected by immunofluorescence as well as ELISA assay. SDF-1 was also examined by RT-PCR and ELISA in a human monocytes-RPE cells co-culturing system. No valid evidence for the toxicity of clodronate-lip was found. Depletion led to significant inhibition of CNV and BMCs recruitment into laser spots on days 3 and 14, reduced BMC expression of CD31 and alpha-SMA on day 14, and decreased expression of SDF-1 in vivo on day 3. SDF-1 was mostly within and around the RPE cells in the laser lesion. SDF-1 was dramatically up-regulated in RPE cells after co-culturing with monocytes. Monocytes may promote experimental CNV, especially BMC contribution in mice, by promoting SDF-1 production in RPE cells.