Murine epidermal Langerhans cells do not express inducible nitric oxide synthase.

In Leishmania-infected macrophages (M phi), the formation of reactive nitrogen intermediates by the inducible isoform of nitric oxide synthase (iNOS) is critical for the killing of the intracellular parasites. We have recently shown that, in addition to M phi, epidermal Langerhans cells (LC) can phagocytose Leishmania major, but they do not allow parasite replication. Therefore, we analyzed whether LC and M phi display the same leishmanicidal effector mechanism. Unlike M phi, stimulation of unselected epidermal cells with interferon-gamma/lipopolysaccharide did not lead to the release of nitric oxide (NO), and inhibition of NO production had no effect on the rate of infection of LC. iNOS mRNA was clearly detectable in M phi as well as unselected epidermal cells (the majority of which consists of keratinocytes) after stimulation with different cytokines. In contrast, pure LC obtained by single-cell picking from cytokine-activated or L. major-infected epidermal cells did not express iNOS mRNA. Addition of the NO donor S-nitroso-N-acetylpenicillamine to already-infected LC did not alter their rate of infection, indicating that LC do not utilize exogenous NO for the control of intracellular Leishmania. These results suggest that in the L. major-infected skin, activated M phi and keratinocytes, but not LC have the ability to express iNOS activity. Therefore, an as yet unidentified, NO-independent mechanism appears to be responsible for the control of parasite replication in LC.