CD4+ T helper 1 (Th1) cells producing
interferon gamma (IFN-γ) are critical for the resolution of
visceral leishmaniasis (VL).
MicroRNA 155 (miR155) promotes CD4+ Th1 responses and IFN-γ production by targeting suppressor of
cytokine signaling-1 (SOCS1) and Src homology-2 domain-containing
inositol 5-phosphatase 1 (SHIP-1) and therefore could play a role in the resolution of VL. To determine the role of miR155 in VL, we monitored the course of Leishmania donovani
infection in miR155 knockout (miR155KO) and wild-type (WT) C57BL/6 mice. miR155KO mice displayed significantly higher liver and spleen parasite loads than WT controls and showed impaired hepatic
granuloma formation. However, parasite growth eventually declined in miR155KO mice, suggesting the induction of a compensatory miR155-independent antileishmanial pathway. Leishmania
antigen-stimulated splenocytes from miR155KO mice produced significantly lower levels of Th1-associated IFN-γ than controls. Interestingly, at later time points, levels of Th2-associated
interleukin-4 (IL-4) and
IL-10 were also lower in miR155KO splenocyte supernatants than in WT mice. On the other hand, miR155KO mice displayed significantly higher levels of IFN-γ, iNOS, and TNF-α gene transcripts in their livers than WT mice, indicating that distinct organ-specific
antiparasitic mechanisms were involved in control of L. donovani
infection in miR155KO mice. Throughout the course of
infection, organs of miR155KO mice showed significantly more PDL1-expressing Ly6Chi inflammatory monocytes than WT mice. Conversely, blockade of Ly6Chi inflammatory monocyte recruitment in miR155KO mice significantly reduced parasitic loads, indicating that these cells contributed to
disease susceptibility. In conclusion, we found that miR155 contributes to the control of L. donovani but is not essential for
infection resolution.