MicroRNA-21 Deficiency Promotes the Early Th1 Immune Response and Resistance toward Visceral Leishmaniasis

MicroRNA-21 (miR-21) inhibits IL-12 expression and impairs the Th1 response necessary for control of Leishmania infection. Recent studies have shown that Leishmania infection induces miR-21 expression in dendritic cells and macrophages, and inhibition of miR-21 restores IL-12 expression. Because miR-21 is known to be expressed due to inflammatory stimuli in a wide range of hematopoietic cells, we investigated the role of miR-21 in regulating immune responses during visceral leishmaniasis (VL) caused by Leishmania donovani infection. We found that miR-21 expression was significantly elevated in dendritic cells, macrophages, inflammatory monocytes, polymorphonuclear neutrophils, and in the spleen and liver tissues after L. donovani infection, concomitant with an increased expression of disease exacerbating IL-6 and STAT3. Bone marrow dendritic cells from miR-21 knockout (miR-21KO) mice showed increased IL-12 production and decreased production of IL-10. On L. donovani infection, miR-21KO mice exhibited significantly greater numbers of IFN-γ- and TNF-α-producing CD4⁺ and CD8⁺ T cells in their organs that was associated with increased production of Th1-associated IFN-γ, TNF-α, and NO from the splenocytes. Finally, miR-21KO mice displayed significantly more developing and mature hepatic granulomas leading to reduction in organ parasitic loads compared with wild type counterparts. Similar results were noted in L. donovani-infected wild type mice after transient miR-21 depletion. These observations indicate that miR-21 plays a critical role in pathogenesis of VL by suppressing IL-12- and Th1-associated IFN-γ and also inducing disease-promoting induction of the IL-6 and STAT-3 signaling pathway. miR-21 could therefore be used as a potential target for developing host-directed treatment for VL.