Nucleic acid sensing activates the innate cytosolic surveillance pathway and promotes parasite survival in visceral leishmaniasis

Microbial pattern recognition critically contributes to innate response, both at extracellular and intracellular cytosolic surveillance pathway (CSP) interface. However, the role of pattern recognition by host innate receptors in CSP is poorly understood in Leishmania donovani infection. Here, we have demonstrated that cytosolic targeting of L.donovani DNA (Ld-DNA) inhibits macrophage responsiveness to IFNɣ, through decreased MHC-II expression and lowered pSTAT1 (Y701) levels, involving host three-prime repair exonuclease-1 (TREX-1). The Ld-DNA potently induced type-1 IFNs, i.e. significant over-production of IFNβ through activation of the IRF pathway. Interestingly, knockdown of TRIF or MyD88 expression in macrophages had no effect on cytosolic Ld-DNA transfection-mediated IFN-β production, indicating involvement of a TLR independent pathway. Contrastingly, Ld-DNA failed to induce IFNβ in both TBK-1 and IRF3KO knockout macrophages. Although IFNβ was not induced by Ld-DNA in STING- knockout macrophages, STING alone was not enough for the induction. Evidently, besides STING, Ld-DNA recognition for induction of IFNβ critically required cytosolic cyclic GMP-AMP synthase (cGAS). Furthermore, the cGAS dependent targeting of Ld-DNA induced IFNβ over-production that contributed to antimony resistance in L.donovani infection. We provide the first evidence that enhanced cytosolic sensing of Ld-DNA in infection by antimony resistant (SBR-LD), but not antimony sensitive L.donovani strains (SBS-LD), was critically regulated by host MDRs, multi drug resistant associated protein 1 (MRP 1) and permeability glycoprotein (P-gp) in macrophages. Collectively, our results disclose Ld-DNA as a vital pathogen associated molecular pattern (PAMP) driving host Type-I IFN responses and antimony resistance. The findings may help in future development of policies for novel anti-leishmanial therapeutics.