The relationship established between Leishmania infantum and the vertebrate host can lead to a self-healing infection or to the manifestation of visceral leishmaniasis, a chronic systemic infection associated with high rates of mortality. We hypothesized that regulatory cytokines, such as interleukin-27 (IL-27), play a role in susceptibility to L. infantum infection. IL-27 is a heterodimeric cytokine composed of IL-27p28 and EBi3 subunits which, when combined, bind to IL-27R, leading to STAT-1 and -3 activation, playing a role in the regulation of the immune response. We observed in this work that IL-27 regulates the Th1/Th17 profiles in a mouse model of visceral leishmaniasis (VL) caused by L. infantum We showed here that the pathogen recognition by endosomal Toll-like receptors triggers a type I interferon (IFN) response, which acts through the type I IFN receptor and interferon regulatory factor 1 to induce IL-27 production by macrophages. Furthermore, IL-27 plays a major regulatory role in vivo, because Ebi3(-/-) mice can efficiently control parasite replication despite reduced levels of IFN-γ compared to wild-type mice. On the other hand, the absence of Ebi3 leads to exacerbated IL-17A production in the infected organs as well as in a coculture system, suggesting a direct regulatory action of IL-27 during L. infantum infection. As a consequence of exacerbated IL-17A in Ebi3(-/-) mice, a greater neutrophil influx was observed in the target organs, playing a role in parasite control. Thus, this work unveiled the molecular steps of IL-27 production after L. infantum infection and demonstrated its regulatory role in the IL-17A-neutrophil axis.
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