Human fetal retinal pigment epithelium suppresses the activation of CD4+ and CD8+ T-cells

Background: The suppressive effect of human fetal retinal pigment epithelium (HFRPE) on the activation of human CD4(+) and CD8(+) T-cells was evaluated in vitro. Methods: Pure populations of CD4(+) and CD8(+) T-cells were isolated from human peripheral blood-derived buffy coats by negative immunomagnetic selection. The purity of the cells was examined by flow cytometry using anti-CD3-FITC, anti-CD4-FITC, anti-CD8-PE, and anti-CD20-PE mAbs. HFRPE cells were isolated from fetal eyes and pure cultures were obtained. The effect of normal or IFN-gamma-activated HFRPE cells at early (P3) or late (P6) passages on the activation of CD4(+) and CD8(+) T-cells was assessed in two different T-cell activation assays. In both activation models anti-CD3 mAb (OKT3) provided the antigen-specific signal. The secondary signal for the activation of CD4(+) and CD8(+) T-cells was provided with anti-CD18 mAb (TS1/18) and anti-CD28 mAb (9.3) in the first and the second assay respectively, Crosslinking of these soluble mAbs was performed with sheep-anti-mouse IgG-coated latex beads. The T-cell activation was determined by cell proliferation measured by [H-3]thymidine incorporation. In each activation assay T-cells were incubated with HFRPE cells in a ratio of T-cells to HFRPE of 1:1 or 1:4. Results: CD4(+) and CD8(+) T-cells were activated by cross-linking CD3 and CD18 in the first assay (CD3/CD18) and CD3 and CD28 in the second assay (CD3/CD28). In both assays HFRPE inhibited the activation of CD4(+) and CD8(+) T-cells. IFN-gamma-activated HFRPE cells totally suppressed the T-cell activation at a 1:1 ratio. This suppressive effect was weaker at lower cell ratios. Some donor variation was observed in the inhibition at the lower cell ratios, especially for the inhibition of CD8(+) T-cell activation with anti-CD3/CD18. The passaging of HFRPE cells did not alter their suppressive effect on CD4(+) and CD8(+) T-cells. Conclusions: HFRPE cells suppressed the activation of both CD4+ and CD8(+) T-cells in vitro. These findings suggest that RPE-induced immune suppression may play a significant role in maintaining immune privilege in the subretinal space.