Toll-like receptor 4 promotes control of Leishmania infantum infection through inducement of leishmanicidal activity in host macrophages: role of mitogen activated kinases.

Establishment of Leishmania infection inside macrophages requires deactivation of various signaling pathways that are dispensable for effective immune responses against the parasite. In the present study, we provide evidence that Leishmania infantum promastigotes attachment on the surface of peritoneal macrophages, internalization and transformation to amastigotes abrogated the activation of extracellular signal-regulated protein kinases (ERK) 1/2, p38 mitogen activated protein kinases (MAPK) and c-Jun NH2-terminal kinases (JNK) and the production of pro-inflammatory cytokines IL-12 and TNFalpha. Subsequent macrophage stimulation with lipopolysaccharide (LPS) during the first hours of exposure to parasite or infection resulted in restoration of MAPK phosphorylation. However, LPS-mediated MAPK activation required parasite internalization (uptake) since cytochalasin-D pretreated macrophages did not responded to LPS stimulation. IL-12, TNFalpha, and NO production was positively regulated with MAPK phosphorylation in contrast to nuclear factor-kappaB (NF-kB) which was MAPK independent. Specifically, inhibition of MAPK activation with specific inhibitors revealed that IL-12 production required p38 MAPK activation, whereas TNFalpha and NO production required all three MAPK. The restoration of NO production resulted in decrease of infection rates. Hence, these results suggest that in contrast to phagocytosis of L. infantum promastigotes, establishment of infection does not desensitize macrophages to subsequent stimulation with LPS, resulting in parasite elimination through MAPK and NF-κB activation and partial restoration of IL-12, TNFalpha and NO synthesis.