Retinal pigment epithelial (RPE) cells convert bone marrow derived macrophages into myeloid suppressor cells with novel phenotype

Abstract

AbstractPurpose We have shown previously that macrophages/microglia accumulate in the subretinal space and express CD68 and Arginase‐1 in the aging eye. We hypothesize that Retinal Pigment Epithelial (RPE) cells may play an important role in regulating macrophage/microglial phenotype and function.Methods Bone marrow derived macrophages (BMDMs) and RPE were cultured from C57BL/6J mice and the phenotype was confirmed by CD11b and F4/80 (for BMDMs) and RPE65 and cytokeratin (for RPE cells) stainings. BMDMs were co‐cultured with RPE cells for different times. Macrophages were then isolated for phenotypic and functional assays.Results Co‐culture of BMDMs with RPE cells resulted in a time‐dependent down‐regulation of MHC‐II and the generation of CD11b+F4/80+Ly6G+ myeloid‐derived suppressor cells (MDSC). The MDSCs expressed high levels of IL‐6, IL‐1β, Arginase‐1, and complement inhibitor C1INH, but lower levels of IL‐12p40 and TNF‐a compared to naïve BMDMs. The expression levels of iNOS, TGF‐β and Ym1 did not change compared to naive BMDMs. Furthermore, MDSCs had reduced phagocytic activity and lower ability to stimulate T cell activation and proliferation. When RPE cells were pre‐treated with oxidized photoreceptor outer segments, the expression of IL‐1β and IL‐6 in BMDMs was increased and the expression of Arginase‐1 was decreased.Conclusion Our results suggest that healthy RPE cells can convert BMDMs into myeloid‐derived suppressor cells under in vitro culture conditions. RPE‐induced myeloid‐derived suppressor cells are CD11b+F4/80+Ly6G+MHC‐IIlowIL‐6+IL‐1b+Arg‐1+. This ability of RPE cells is reduced when suffering from oxidative insults.