Regulation of the expression of nitric oxide synthase by Leishmania mexicana amastigotes in murine dendritic cells

In mammalian hosts, Leishmania parasites are obligatory intracellular organisms that invade macrophages (Mϕ) and dendritic cells (DC). In Mϕ, the production of nitric oxide (NO) catalyzed by the inducible nitric oxide synthase (iNOS) has been implicated as a major defense against Leishmania infection. The modulation of this microbicidal mechanism by different species of Leishmania has been well studied in Mϕ. Although DC are permissive for infection with Leishmania both in vivo and in vitro, the effect of this parasite in the expression of iNOS and NO production in these cells has not been established. To address this issue, we analyzed the regulation of iNOS by Leishmania mexicana amastigotes in murine bone marrow-derived dendritic cells (BMDC) stimulated with LPS and IFN-γ. We show that the infection of BMDC with amastigotes down regulated NO production and diminished iNOS protein levels in cells stimulated with LPS alone or in combination with IFN-γ. The reduction in iNOS protein levels and NO production did not correlate with a decrease in iNOS mRNA expression, suggesting that the parasite affects post-transcriptional events of NO synthesis. Although amastigotes were able to reduce NO production in BMDC, the interference with this cytotoxic mechanism was not sufficient to permit the survival of L. mexicana. At 48 h post-infection, BMDC stimulated with LPS + IFN-γ were able to eliminate the parasites. These results are the first to identify the regulation of iNOS by L. mexicana amastigotes in DC.