CD40 ligand is required for protective cell-mediated immunity to Leishmania major.

The CD40-CD40 ligand (CD40L) signaling process is a pivotal component of multiple immunoregulatory pathways. Although the role that CD40L plays in humoral immune responses is fairly well defined, its function(s) in cell-mediated responses in vivo has not been established. We investigated this issue by assessing the course of Leishmania major infection in CD40L knockout (CD40LKO) mice that were generated on a resistant background. In response to parasite challenge, CD40LKO mice developed ulcerating cutaneous lesions and failed to mount a vigorous Th1-like response. The impaired Th1-like response appears to be related to a defect in the ability of CD40LKO T cells to induce the production of IL-12 from macrophages. Treatment with exogenous IL-12 prevented disease progression in CD40LKO mice, and administration of recombinant CD40L provided partial protection against infection. Thus, a protective cell-mediated immune response to L. major appears to be dependent upon CD40L-induced IL-12 secretion by antigen-presenting cells.