IL-12p70 production by Leishmania major-harboring human dendritic cells isa CD40/CD40 ligand-dependent process

Leishmaniasis, a vector-borne parasitic disease, is transmitted during a sa ndfly blood meal as the parasite is delivered into the dermis, The parasite displays a unique immune evasion mechanism: prevention of IL-12 production within its host cell, the macrophage (i.e., where it differentiates and mu ltiplies). Given the close proximity of skin dendritic tells (DC) to the si te of parasite delivery, their critical role in initiating immune responses and the self-healing nature of Leishmania major (Lm) infection, we examine d the interaction between myeloid-derived human DC and Lm metacyclic promas tigotes (infectious-stage parasites) to model the early "natural" events of infection. We found that DC can take up Lm and, after this internalization , undergo changes in surface phenotype suggesting "maturation". Despite the intracellular location of the parasite and resultant up-regulation of cost imulatory and class II molecules, there was no detectable cytokine release by these Lm-harboring DC. However, using intracellular staining and flow cy tometry to analyze cytokine production at the single-cell level, we found t hat Lm-harburing DC, but not monocytes, produce large amounts of IL-12p70 i n a CD40 ligand (CD40L)-dependent manner. Finally, DC generated from mononu clear cells from patients with cutaneous leishmaniasis (Lm), once loaded wi th live metacyclic promastigotes, were found to reactivate autologous prime d T lymphocytes and induce a CD40L-dependent IFN-gamma response. Our result s link the required CD40/CD40L interactions for healing with DC-derived IL- 12p70 production and provide a mechanism to explain the genesis of a protec tive T cell-mediated response in the face of local immune evasion within th e macrophage at the site of Leishmania delivery.