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(+) 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 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(+) 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 Th22-type PD-1(+) bystander T cells through negative costimulatory interactions.
Keywords: T cells; costimulatory molecules; cytokine.