The essential role of Toll-like receptors (TLR) in innate immune responses to bacterial pathogens is increasingly recognized, but very little is known about the role of TLRs in host defense against infections with eukaryotic pathogens. For the present study, we investigated whether TLRs contribute to the innate and acquired immune response to infection with the intracellular protozoan parasite Leishmania major. Our results show that TLR4 contributes to the control of parasite growth in both phases of the immune response. We also addressed the mechanism that results in killing or growth of the intracellular parasites. Control of parasite replication correlates with the early induction of inducible nitric oxide synthase in TLR4-competent mice, whereas increased parasite survival in host cells from TLR4-deficient mice correlates with a higher activity of arginase, an enzyme known to promote parasite growth. This is the first study showing that TLR4 contributes to the effective control of Leishmania infection in vivo.

We studied the expression of a subset of chemokines, including RANTES/CCL5, MIP-1␣/CCL3, IP-10/ CXCL10, and MCP-1/CCL2, in Toll-like receptor (TLR)-competent and -deficient mice after infection with Leishmania major. Chemokine expression at the site of infection (the footpad), in the draining lymph nodes and in the spleens of infected animals was determined by using two different methods of analysis. The results indicate that L. major infection causes overall upregulation of RANTES/CCL5, MIP-1␣/CCL3, IP-10/CXCL10, and MCP-1/CCL2 in the footpads and lymph nodes, while expression of these chemokines is constitutive in the spleens of TLR4-competent mice (C57BL/10ScSn) and TLR4-deficient mice (C57BL10/ScN). Different patterns of expression were detected depending on the time postinfection, but there was little variation in the expression of these four chemokines in the presence or absence of TLR4.

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