Vascular endothelial cells provide T cells with costimulatory signals via the OX40/gp34 system.

We investigated whether gp34, the ligand of OX40, expressed on EC is involved in costimulation of T cells. Normal CD4+ T cells were stimulated with anti-CD3-coated beads, phytohemagglutinin (PHA), or concanavalin A (Con A) in the presence or absence of irradiated human umbilical vein endothelial cells (HUVEC). Stimulation of T cells with each of these mitogens results in significant T-cell proliferation only when HUVEC were present, and this proliferation was inhibited markedly by anti-OX40 or anti-gp34 monoclonal antibody (mAb). T cells cultured with HUVEC produced more interleukin (IL)-2 than those cultured without HUVEC. The addition of anti-IL-2R alpha chain and anti-IL-2R beta chain mAbs abolished the costimulatory effects of HUVEC. Thus, the augmentation of T-cell proliferation appears to be attributable to increased IL-2 production. These results suggest that gp34 expressed on HUVEC plays a role in potentiation of T-cell immune response by providing OX40+ T cells with costimulatory signals.