In its vertebrate host, Leishmania major encounters cells that express Toll-Like Receptors (TLRs) The importance of the MyD88 adaptor molecule for the development of protective immunity against L. major has been documented. Some TLR (2, 3, 4, and 9) have been reported to be the target of some PAMPs from Leishmania. Using genetically resistant C57Bl/6 mice deficient for either TLR2, 4 or 9, we showed that only TLR9-deficient (TLR 9-/-) mice are more susceptible to infection with L. major. TLR9-/- mice resolved their lesions and controlled parasites growth with much lower efficiency than wild type C57Bl/6 mice. The absence of TLR9 also transiently inhibited the development of curative Th1 response. In an attempt to analyze the possible basis for such aberrant response in TLR9-/- mice, we have studied the importance of TLR9 for activation of dendritic cells (DCs) by L. major. Our results showed that bone marrow derived DCs as well as DCs freshly isolated from the spleen of C57Bl/6 mice can be activated by either heat-killed or live L. major in vitro. In sharp contrast, L. major failed to activate DCs from TLR9-/- mice. Interestingly, DNA purified from L. major was capable of activating DCs in a strictly TLR9-dependent manner. Similarly purified vertebrate DNA had no effect, suggesting that this TLR9-dependent response of DCs was restricted to L. major DNA. The importance of TLR9 signaling in the capacity of DCs to recruit and activate T cells is currently assessed in vitro. We will assess the capacity of the different subtypes of DCs (alone or in combination) to activate specific T cells. To this end we will use easily traceable T cells, i.e. T cells from mice transgenic for a TCR specific for OVA. The specific activation by L. major DNA will also be investigated.