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.