PURPOSE. To determine whether RPE cells can suppress a novel T helper subset, the Th22 cells, via costimulatory interactions. METHODS. Primary RPE cells were established from normal C57BL/6 mice. The target CD4 þ Th22 cells from spleen cells in wild-type control or knockout donors were used. T cell activation was assessed by examining BrdU incorporation (proliferation) and cytokine production. Expression of costimulatory molecules on RPE cells and expression of costimulatory receptors on target

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... Th22 cells were evaluated by flow cytometry. Neutralizing antibodies were used to abolish the suppression function. In addition, human RPE cells and target Th22 cells induced from human CD4 þ cells were also used in similar experiments. RESULTS. Cultured RPE cells significantly suppressed activation of target Th22 cells (e.g., T cell proliferation and IL-22 production). Moreover, human RPE cells suppressed Th22 cell lines and T cell clones established from active uveitis patients. Although cultured RPE cells expressed various costimulatory molecules including programmed cell death 1 ligand 1 (PD-L1), only PD-L1 on the RPE cells was actually delivered to the target Th22 cells. Th22 cells greatly express programmed cell death 1 (PD-1), and RPE cells failed to suppress IL-22 expression by target Th22 cells from PD-1 knockout donors. In addition, if neutralizing antibodies for PD-L1 were cocultured with RPE cells, Th22 suppression was impaired. CONCLUSIONS. RPE cells express PD-L1 costimulatory molecules and suppress bystander Th22type PD-1 þ bystander T cells through negative costimulatory interactions.