CD40 ligand (CD154) triggers a short-term CD4(+) T cell activation response that results in secretion of immunomodulatory cytokines and apoptosis

Signals generated through CD28-B7 and CD40 Ligand (CD40L)-CD40 interactions have been shown to be crucial for the induction of long-term allograft sur vivability. We have recently demonstrated that humanized anti-CD40L (hu5C8) prevents rejection of mismatched renal allografts in primates. To investig ate potential mechanisms of CD40L-induced allograft acceptance, we coimmobi lized hu5C8 with suboptimal amounts of anti-CD3 to stimulate CD4(+) T cells . We now report that anti-CD3/CD40L costimulation results in CD28-independe nt activation and subsequent deletion of resting T cells. Coligation of CD3 and CD40L increased expression of CD69, CD25, and CD54 on CD4(+) T cells. We also round that costimulation with anti-CD3/CD40L resulted in enhanced p roduction of interleukin (IL)-10, interferon gamma, and turner necrosis fac tor alpha but not IL-2 or IL-6. Interestingly, alter several days, anti-CD3 /CD40L-mediated activation was followed by apoptosis in a significant popul ation of cells. Consistent with that observation, anti-CD3/CD40L did not en hance the antiapoptotic proteins Bcl-2 and Bcl-xL. Further, the addition of CD28 at 24 h failed to rescue those cells induced to dir after costimulati on with anti-CD3/CD40L. Together, these data suggest that the graft-sparing effect of hu5C8 in vivo may result in part from early and direct effects o n CD4(+) T cells, including a vigorous induction of immunomodulatory cytoki nes and/or apoptosis of allograft-specific T cells.