Expression of indoleamine 2,3-dioxygenase in human corneal cells as a local immunosuppressive factor

PURPOSE. To identify the localization of indoleamine 2,3-dioxygenase (IDO) in human corneal cells and to evaluate its functional ability as a local immunosuppressive factor. METHODS. The expression profile of IDO was identified in primary cultures of human corneal cells (fibroblasts, epithelial cells, endothelial cells) by RT-PCR and Western blot analysis. The immunosuppressive function of IDO was assessed by culturing human CD4(+) T cells with conditioned medium derived from the three types of human corneal cells, and changes in proliferation and apoptosis were determined. IDO expression and its apoptotic effects on CD4(+) T cells were also investigated after IFN-gamma treatment. RESULTS. Among the three types of human corneal cells, IDO mRNA and protein expression were observed in human corneal fibroblasts and epithelial cells, with higher levels in the human corneal fibroblasts. Human CD4(+) T cells cultivated in conditioned medium derived from human corneal fibroblasts showed decreased cell proliferation and increased apoptosis. IFN-gamma treatment significantly induced IDO expression and showed apoptotic effects on immune cells. CONCLUSIONS. These results suggest that human corneal fibroblasts are relatively immunoresistant and that the IDO expression can act as one of the factors for the maintenance of immune privilege in the cornea.