CD40 ligand-dependent T cell activation: requirement of B7-CD28 signaling through CD40.

The role of CD40 ligand (CD40L) in the primary activation of T cells is not clear. The cellular and humoral immune responses to adenoviral vectors in a murine model of liver-directed gene transfer were studied to define the mechanisms responsible for CD40L-dependent T cell priming. CD40L-deficient mice did not develop effective cytotoxic T cells to transduced hepatocytes, and T cell-dependent B cell responses were absent. Full reconstitution of cellular and humoral immunity was achieved in CD40L-deficient mice by administration of an activating antibody to CD40 that increased expression of B7.2 on spleen cells. Wild-type mice could be made nonresponsive to vector by administration of antibodies to B7. Thus, CD40L-dependent activation of T cells occurs through signaling of CD40 in the antigen-presenting cell to enhance requisite costimulatory pathways that include B7.