Retinal pigment epithelial cells activate uveitogenic T cells when they express high levels of MHC class II molecules, but inhibit T cell activation when they express restricted levels.

The retinal pigment epithelium (RPE) plays an important role in maintaining the privileged status of the eye. In this study, we examined the outcome of the in vitro interaction between uveitogenic T cells and non-activated or activated RPE cells. An interphotoreceptor retinoid-binding protein (IRBP)-reactive T cell line was established and RPE cell monolayers prepared. The expression of MHC class II molecules on RPE cells activated by graded doses of IFN-gamma was determined by flow cytometry. Proliferation and cytokine production of IRBP-reactive T cells were measured after exposure to IFN-gamma-activated RPE cells. We found that RPE cells expressed various levels of MHC class II molecules depending on the concentration of IFN-gamma used. IRBP-specific T cells could be stimulated or inhibited by exposure to RPE cells. The dual effect of RPE cells might be regulated via different levels of MHC class II molecules on the RPE cells. It suggests that controlling the immune activities of uveitogeneic T cells by varying the levels of MHC class II expression on the RPE might be one of the regulatory mechanisms by which the RPE maintains immune privilege in the eye.