No abstract

Leishmania mexicana is the causal agent of cutaneous leishmaniasis in Mexico. Dendritic cells (DC) are one of the host cells of Leishmania parasites. Intracellular microorganisms inhibit host cell apoptosis as a strategy to ensure their survival in infected cells. We have previously shown that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDC), but the mechanisms underlying the inhibition of apoptosis of DC by Leishmania have not been established. MAP kinases and PI3K participate in the process of apoptosis and are modulated by different species of Leishmania. As shown in this study, the infection of moDC with L. mexicana amastigotes diminished significantly the phosphorylation of the MAP kinases p38 and JNK. The inhibition of both kinases diminished significantly DNA fragmentation in moDC stimulated with camptothecin. On the other hand, L. mexicana amastigotes were able to activate the anti-apoptotic pathways PI3K and AKT. Our results indicate that L. mexicana amastigotes have the capacity to diminish MAP kinases activation and activate PI3K and AKT, which is probably one of the strategies employed by L. mexicana amastigotes to inhibit apoptosis in the infected moDC.

NKT cells have been associated with protection against Leishmania donovani, yet their role in infections with Leishmania mexicana has not been addressed, nor has the activation pathway been defined after stimulation with Leishmania mexicana lipophosphoglycan (LPG). We analyzed the activation of NKT cells and their cytokine production in response to Leishmania mexicana LPG. Additionally we compared NKT-cell numbers and cytokine profile in lymph nodes of skin lesions induced by Leishmania mexicana in BALB/c and C57BL/6 mice. We show that LPG activates NKT cells primarily through the indirect pathway, initiating with TLR2 stimulation of dendritic cells (DC), thereby enhancing TLR2, MHC II, and CD86 expressions and IL-12p70 production. This leads to IFN-γ production by NKT cells. C57BL/6 mice showed enhanced DC activation, which correlated with augmented IFN-γ production by NKT cells. Additionally, infected C57BL/6 mice showed elevated percentages of NKT cells with higher IFN-γ and IL-4 production in lymph nodes. We conclude that the response of NKT cells towards Leishmania mexicana LPG initiates with the indirect activation, after binding of LPG to TLR2 in DC. This indirect activation pathway enables NKT cells to produce IFN-γ during the innate phase of Leishmania infection, the magnitude of which differs between mouse strains.

Leishmania species are dimorphic protozoan parasites that live and replicate in the gut of sand flies as promastigotes or in mammalian hosts as amastigotes. Different immune cells, including DCs, and receptors differ in their involvement in phagocytosis of promastigotes and amastigotes and in recognition of different Leishmania species. In the case of L. mexicana, differences in phagocytosis of promastigotes and amastigotes by DCs and participation of C-type lectin receptors (CLRs) have not been established. In the present study, flow cytometry and confocal microscopy were used to investigate the phagocytosis by monocyte-derived dendritic cells (moDCs) of L. mexicana promastigotes and amastigotes in the presence or absence of immune serum during various periods of time. Blocking antibodies against mannose receptors and DC-SIGN were used to explore the participation of these receptors in the phagocytosis of L. mexicana by moDC. The major differences in interactions of L. mexicana promastigotes and amastigotes with moDC were found to occur within the first 3 hr, during which phagocytosis of promastigotes predominated as compared with opsonization of promastigotes and amastigotes. However, after 6 hr of incubation, opsonized promastigotes were preferentially phagocytosed as compared with unopsonized promastigotes and amastigotes and after 24 hr of incubation there were no differences in the phagocytosis of promastigotes and amastigotes. Finally, after 3 hr incubation, DC-SIGN was involved in the phagocytosis of promastigotes, but not of amastigotes.