Mannose receptor (MR) and Toll-like receptor 2 (TLR2) influence phagosome maturation during Leishmania infection.

Leishmania enter macrophages through receptor-mediated phagocytosis and survive the harsh environment of a phagolysosome. Here, we investigated the interaction between mannose receptor (MR), Toll-like receptor 2 (TLR2), and Leishmania, and the subsequent impact on phagosome maturation. Leishmania parasites are able to delay phagosome maturation, not reaching full maturation until 5 hours post-engulfment. Here, maturation of Leishmania major- and Leishmania donovani-containing phagosomes proceeded as expected in the WT macrophages becoming LAMP1 positive by 6 hours. Interestingly, MR-/- macrophages become LAMP1 positive by ~2 hours and ~4 hours post-infection Leishmania-containing phagosomes lost LAMP1 expression and gained the early marker EEA1. LAMP1 expression was again observed by 6 hours. Leishmania LPG was essential for the delay in both WT and MR-/- macrophages but was not essential for the early maturation (2 hours) observed in MR-/- macrophages. Serum opsonization of Leishmania prior to infection induced identical phagosome maturation patterns in WT and MR-/- macrophages. In the absence of MyD88 or TLR2 on macrophages, Leishmania phagosomes matured significantly faster, becoming LAMP1 positive by ~1-2 hours. These studies add to the knowledge that phagosome maturation is influenced by multiple receptor-ligand interactions and signalling pathways.

Leishmania major inhibits IL-12 in macrophages by signalling through CR3 (CD11b/CD18) and down-regulation of ETS-mediated transcription.

Leishmania major is an aetiological agent of cutaneous leishmaniasis. The parasite primarily infects immune sentinel cells, specifically macrophages and dendritic cells, in the mammalian host. Infection is receptor mediated and is known to involve parasite binding to cell surface protein complement receptor 3 (CR3, Mac-1, CD11b/CD18). Engagement of CR3 by various ligands inhibits production of interleukin-12 (IL-12), the cytokine that drives antileishmanial T helper 1-type immune responses. Likewise, L. major infection inhibits IL-12 production and activation of host macrophages. Our data indicate that in the absence of CR3, L. major-infected bone marrow-derived macrophages produce more IL-12 and nitric oxide compared with WT cells upon lipopolysaccharide (LPS) stimulation. We therefore investigated multiple signalling pathways by which L. major may inhibit IL-12 transcription through CR3 ligation. We demonstrate that L. major infection does not elicit significant NFκB p65, MAPK, IRF-1 or IRF-8 activation in WT or CD11b-deficient macrophages. Furthermore, infection neither inhibits LPS-induced MAPK or NFκB activation nor blocks IFN-γ-activated IRF-1 and IRF-8. ETS-mediated transcription, however, is inhibited by L. major infection independently of CR3. Our data indicate that L. major-mediated inhibition of IL-12 occurs through CR3 engagement; however, the mechanism of inhibition is independent of NFκB, MAPK, IRF and ETS.