Ox-40 ligand: a potent costimulatory molecule for sustaining primary CD4 T cell responses.

Ox-40 and Ox-40 ligand (Ox-40L) are thought to be involved in T cell-APC interactions. However, their exact role in T cell responses is undefined. Using fibroblast transfectants expressing Ox-40L and/or B7-1, and CD4 cells from TCR transgenic mice, we investigated the effect of Ox-40 signaling on primary responses to the Ag pigeon cytochrome c. Ox-40 expression on naive CD4 cells peaked 2 to 3 days after activation, and was lost by 4 to 5 days. APCs with Ox-40L promoted partial activation of naive T cells with some IL-2 secretion, but were unable to enhance proliferation, unlike those with B7-1. APCs coexpressing Ox-40L with B7-1 induced large quantities of IL-2 and promoted proliferative responses that persisted for several days. Effector cells taken 5 days after naive T cell activation reexpressed Ox-40 within 4 h and responded strongly to APCs expressing Ox-40L, whereas B7-1 had little effect. Synergy was also seen between Ox-40L and B7-1, with primarily IL-2 being elevated, although IL-4 and IL-5 were also up-regulated. The most striking action was on effector T cell proliferation, which continued at high levels for up to 4 days, with little proliferation evident at this time in the absence of Ox-40 signals. These data suggest that Ox-40/Ox-40L interactions act after initial activation events to prolong clonal expansion and enhance effector cytokine secretion, and may be involved in promoting long-lived primary CD4 responses.