Combination of Mycobacterium indicus pranii and heat-induced promastigotes cures drug-resistant Leishmania infection: Critical role of IL-6 producing classical dendritic cells.

The major issues in available therapeutic modalities against leishmaniasis are cost, toxicity, and the emergence of drug-resistance. The aim of this work was to develop a successful therapeutic adjuvant against drug-resistant Leishmania donovani infection in combination with Mycobacterium indicus pranii (MIP) and heat-induced promastigotes (HIP). One-month post-infected Balb/c mice were administered, s.c., with MIP (10(8) cells) and HIP (100 mug) for five days. Spleens were harvested for flow cytometric and reverse transcriptase PCR analysis. The anti-leishmanial effect of the combination strategy was associated with induction of disease resolving Th1 and Th17 response with simultaneous downregulation of CD4(+)CD25(+)Foxp3(+) (nTreg) cells and CD4(+)CD25(-)Foxp3(-) (Tr1 cells) in the spleen. The significant expansion of CD4(+) TCM (CD4(+)CD44(hi)CD11a(hi)CD62L(hi)) was also an interesting outcome of this therapeutic strategy in context to long-term protection of hosts against secondary infection. TLR2 was found instrumental in this anti-parasitic therapy. Induced IL-6 production from expanded CD11c(+)CD8alpha(+) (cDC1) and CD11c(+)CD11b(+) (cDC2) dendritic cells, and essentially not from the CD11b(+)Ly6c(+) inflammatory monocytes (iMOs), was found critical in the protective expansion of Th17 as evidenced by the in vivo IL-6 neutralization assay. It also promoted the hematopoietic conversion towards DC progenitors (pre-DC) from its immediate precursors CDP in the bone marrow. This novel combinational strategy demonstrated that expansion of Th17 by IL-6, released from CD11c(+) classical DCs is crucial along with conventional Th1 response to control drug-resistant infection.