CD28/B7 Interactions Deliver a Unique Signal to Naive T Cells That Regulates Cell Survival but Not Early Proliferation

CD28/B7 ligation provides costimulatory signals important for the development of T cell responses. In the present study, we examined whether CD28/B7 interactions have a specialized role in the regulation of cell cycle progression and sustained T cell proliferative responses in naive T cell populations using TCR transgenic mice. CD28-mediated signaling was shown to be uniquely capable of regulating cell survival compared with TCR-mediated signaling. Increasing the strength of the TCR-mediated signal 1 increased early proliferative responses, but had no effect on sustained cell survival. In contrast, CD28 ligation, signal 2, was not required for early proliferative responses, but dramatically influenced long term T cell survival. The increased cell survival after CD28 ligation was not due to increased IL-2 production, but was linked to up-regulation of Bcl-x_L. The Bcl-x_L protein could not be induced following increased TCR cross-linking in the absence of CD28 signaling. In addition, survival of T cells from Bcl-x_L transgenic mice was not inhibited by blocking CD28 ligation, suggesting that CD28-induced T cell survival is regulated by Bcl-x_L expression. Together, these results suggest that the unique role of CD28 signaling is not to costimulate the initial activation of naive T cells, but is, in fact, to sustain the late proliferative response and enhance long term cell survival.