Role of ocular pigment epithelial cells in regional ocular immunity

Purpose To whether soluble factors by retinal pigment epithelial cells (RPE) promote the generation of T regulatory cells in vitro. Methods Primary cultured RPE cells were established from normal C57BL/6 mice. T cells were co‐cultured with RPE, x‐irradiated, and used as regulators (RPE Treg cells). Target bystander T cells were established from normal splenic T cells with anti‐CD3 antibodies. T‐cell activation was assessed for proliferation by [3H]–thymidine incorporation. Expression of cytotoxic T lymphocyte antigen‐2α (CTLA‐2α) and cathepsin L on RPE and T cells was evaluated with oligonucleotide microarray, RT‐PCR, immune staining, western blots and flow cytometry. Recombinant mouse CTLA‐2α and anti‐mouse CTLA‐2α abs were used for the assay. For induction of experimental autoimmune uveitis (EAU), mice were immunized with interphotoreceptor retinoid‐binding protein peptide emulsified in complete Freund’s adjuvant. Results RPE converted CD4+ T cells into Treg cells by producing and secreting CTLA‐2α, a cathepsin L inhibitor. CTLA‐2α secreted by RPE cells selectively inhibited cathepsin L in the T cells and the cathepsin L‐lacking T cells exhibited Treg phenotype, i.e. expression of Foxp3 and production of transforming growth factor beta (TGFβ). CTLA‐2α enhanced their production of active forms of TGFβ. In addition, CD4+ T cells from EAU‐induced cathepsin L knockout (KO) donors contained high population of Foxp3+ T cells and EAU in cathepsin L KO mice was significantly less than those in wild type mice. Furthermore, treatment with recombinant CTLA‐2α significantly suppressed EAU. Conclusion These results indicate that immunosuppressive factors derived from RPE participate in the establishment of immune regulation in the posterior segment of the eye.