NKT cells play an important role in autoimmune diseases, tumor surveillance, and infectious diseases, providing in most cases protection against infection. NKT cells are reactive to CD1d presented glycolipid antigens. They can modulate immune responses by promoting the secretion of type 1, type 2, or immune regulatory cytokines. Pathogen-derived signals to dendritic cells mediated via Toll like Receptors (TLR) can be modulated by activated invariant Natural Killer T (iNKT) cells. The terminal β-(1–4)-galactose residues of glycans can modulate host responsiveness in a T helper type-1 direction via IFN-γ and TLRs. We have attempted to develop a defined immunotherapeutic, based on the cooperative action of a TLR ligand and iNKT cell using a mouse model of visceral leishmaniasis. We evaluated the anti- Leishmania immune responses and the protective efficacy of the β-(1–4)-galactose terminal NKT cell ligand glycosphingophospholipid (GSPL) antigen of L. donovani parasites. Our results suggest that TLR4 can function as an upstream sensor for GSPL and provoke intracellular inflammatory signaling necessary for parasite killing. Treatment with GSPL was able to induce a strong effective T cell response that contributed to effective control of acute parasite burden and led to undetectable parasite persistence in the infected animals. These studies for the first time demonstrate the interactions between a TLR ligand and iNKT cell activation in visceral leishmaniasis immunotherapeutic.

NKT cells respond to presentation of specific glycolipids with release of both Th1‐ and Th2‐type cytokines. Leishmania donovani (LD)‐infected splenic macrophages (sMϕ(I)) and bone marrow‐derived dendritic cells (BMDC(I)) failed to activate NKT cells in response to α‐galactosyl ceramide (α‐GalCer). The defective antigen presentation could be corrected by treating the cells with the immunostimulating glycosphingophospholipid (GSPL) of LD parasites. In vitro pulsing of BMDC(I) or sMϕ(I) with GSPL, caused the activation of the Vα14+ CD1d1‐specific NKT cell hybridoma DN32.D3. Localization of MHC II and CD1d molecules to membrane lipid rafts has been suggested to play an important role in antigen presentation. Confocal analysis clearly demonstrated that LD infection changed the pattern of CD1d distribution to the non‐lipid raft regions and this change could be reversed by GSPL treatment. Isoelectric focusing gel shift assay indicated that GSPL binds to CD1d. GSPL‐treated but not untreated BMDC(I) formed immune synapses with NKT cells and this was associated with calcium mobilization. In conclusion, GSPL treatment was associated with modification of BMDC(I)/sMϕ(I) lipid raft structure, which is a site for immune regulation.

Infection with antimony-resistant Leishmania donovani (SbRLD) induces aggressive pathology in the mammalian hosts as compared with ones with antimony-sensitive L. donovani (SbSLD) infection. SbRLD, but not SbSLD, interacts with TLR2/TLR6 to induce IL-10 by exploiting p50/c-Rel subunits of NF-κB in infected macrophages (Mϕs). Most of the TLRs exploit the universal adaptor protein MyD88 to activate NF-κB. We now show that infection of Mϕs from MyD88−/− mice with SbRLD gave rise to significantly higher intracellular parasite number coupled with elevated IL-10/IL-12 ratio in the culture supernatant as compared with infection in wild type (WT) Mϕs. Τhese attributes were not seen with SbSLD in similar experiments. Further, SbRLD infection upregulated miR-466i, which binds with 3′-untranslated region, leading to the downregulation of MyD88. Infection of MyD88−/− Mϕ or IL-12−/− Mϕ with SbRLD induced IL-10 surge at 4 h, whereas the same in WT Mϕ started from 12 h. Thus, absence of IL-12 in MyD88−/− mice favored early binding of NF-κB subunits to the IL-10 promoter, resulting in IL-10 surge. Infection of MyD88−/− mice with SbRLD showed significantly higher organ parasites coupled with ill-defined and immature hepatic granulomas, whereas in WT mice there were less organ parasites and the granulomas were well defined. From the survival kinetics it was observed that SbRLD-infected MyD88−/− mice died by 60 d postinfection, whereas the WT mice continued to survive. Our results demonstrate that SbRLD has evolved a unique strategy to evade host antileishmanial immune repertoire by manipulating host MyD88 to its advantage.