Toll-like Receptor-9 (TLR-9) Signaling Is Crucial for Inducing Protective Immunity following Immunization with Genetically Modified Live Attenuated Parasites.

No human vaccine is available for visceral leishmaniasis (VL). Live attenuated centrin gene-deleted () parasite vaccine has been shown to induce robust innate immunity and provide protection in animal models. Toll-like receptors (TLRs) are expressed in innate immune cells and are essential for the early stages of infection. Among TLRs, TLR-9 signaling has been reported to induce host protection during infection. Importantly, TLR-9 ligands have been used as immune enhancers for non-live vaccination strategies against leishmaniasis. However, the function of TLR-9 in the generation of a protective immune response in live attenuated vaccines remains unknown. In this study, we investigated the function of TLR-9 during infection and found that it increased the expression of TLR-9 on DCs and macrophages from ear-draining lymph nodes and spleen. The increase in TLR-9 expression resulted in changes in downstream signaling in DCs mediated through signaling protein myeloid differentiation primary response 88 (MyD88), resulting in activation and nuclear translocation of nuclear factor-κB (NF-κB). This process resulted in an increase in the DC's proinflammatory response, activation, and DC-mediated CD4T cell proliferation. Further, immunization in TLR-9 mice resulted in a significant loss of protective immunity. Thus, vaccine naturally activates the TLR-9 signaling pathway to elicit protective immunity against virulent challenge.