Iris pigment epithelial (IPE) cells from the anterior segment in the eye are able to suppress activation of bystander responder T cells in vitro. The cultured IPE cells fully suppress proliferation and cytokine production by responder T cells via direct cell-to-cell contact. We have now investigated whether primary cultured human iris pigment epithelial (h-IPE) cells that were established from fresh iris tissues can also inhibit the activation of T cells in vitro. We found that cultured h-IPE cells significantly inhibited T cell proliferation and the IFN-gamma production by the target T cells from both the allogeneic and autogeneic peripheral blood mononuclear cells (PBMCs). The h-IPE cells also inhibited the activation of CD4(+) T cells from patients with active uveitis. The suppression by h-IPE occurred in a completely contact-dependent manner. The h-IPE constitutively expressed transforming growth factor beta (TGFbeta) and the receptors, and the T cells exposed to h-IPE greatly expressed Smad transcripts. In addition, TGFbeta2-siRNA transfected h-IPE failed to inhibit activation of responder T cells. Similarly, h-IPE cells in the presence of anti-TGFbeta neutralizing antibodies or recombinant TGFbeta receptor blocking proteins failed to inhibit the T-cell activation. In conclusion, cultured human iris pigment epithelium fully inhibits T cell activation in vitro. Our data support the hypothesis that the ocular resident cells play a critical role in immunosuppression in the eye.