Functional characterization of OX40 expressed on human CD8+ T cells

In the present study, we investigated the expression of OX40 on human CD8+ T cells with regard to expression induction, costimulatory function and possible involvement in cytotoxicity. Human CD8+ T cells were purified from peripheral blood mononuclear cell (PBMC) of healthy donors and cocultured with allogeneic monocyte-derived dendritic cells. Flow cytometric analysis showed that expression of OX40 was induced on CD8+ T cells within 1 day and increased to the maximum levels on day 3. An addition of anti-OX40 ligand (OX40L) mAb suppressed CD25 expression, proliferation and IFN-γ production of CD8+ T cells, suggesting that OX40 functions as a costimulatory molecule not only for CD4+ T cells but also for CD8+ T cells. In parallel, coculture of pre-activated CD8+ T cells with OX40L-transfected murine epithelial cells (MMCE-OX40L) resulted in an increase in CD25 expression, proliferation and IFN-γ producing cells, compared with that with the mock control (MMCE-mock). Finally, non-specific cytotoxic activity of preactivated CD8+ T cells was examined using OKT3 hybridoma as target cells after coculture with these transfectants. Coculture with MMCE-OX40L induced slightly higher cytotoxicity of CD8+ T cells than that with MMCE-mock. These results indicate that OX40 is induced transiently on CD8+ T cells upon activation and its signals contribute to both clonal expansion and functional reinforcement.