MyD88 is essential for clearance of Leishmania major: possible role for lipophosphoglycan and Toll-like receptor 2 signaling

Leishmania major is an obligate intracellular eukaryotic pathogen of mononuclear phagocytes. Invasive promastigotes gain entry into target cells by receptor-mediated phagocytosis, transform into non-motile amastigotes and establish in the phagolysosome. Glycosylphosphatidylinositol-anchored lipophosphoglycan (LPG) is a virulence factor and a major parasite molecule involved in this process. We observed that mice lacking the Toll-like receptor (TLR) pathway adaptor protein MyD88 were more susceptible to infection with L. major than wild-type C57BL/6 mice, demonstrating a central role for this innate immune recognition pathway in control of infection, and suggesting that L. major possesses a ligand for TLR. We sought to identify parasite molecules capable of activating the protective Toll pathway, and found that purified Leishmania LPG, but not other surface glycolipids, activate innate immune signaling pathways via TLR2. Activation of cytokine synthesis by LPG required the presence of the lipid anchor and a functional MyD88 adaptor protein. LPG also induced the expression of negative regulatory pathways mediated by members of thesuppressors of cytokine signaling family SOCS-1 and SOCS-3. Thus, the Toll pathway is required for resistance to L. major and LPG is a defined TLR agonist from this important human pathogen.