In the Leishmania major mouse model of cutaneous leishmaniasis inducible nitric oxide synthase (iNOS) is crucial for the killing of the parasite in the skin and draining lymph node. However, the effector mechanism operating against L. major in the spleen is unknown. As reactive oxygen intermediates might play a role, we analyzed macrophages and mice lacking the gp91phox subunit of the phagocyte NADPH oxidase (phox) for their ability to combat an infection with L. major. Macrophages from wild-type and gp91phox -/-mice had an equal capacity to kill L. major after activation by cytokines. Unlike iNOS, the activity of phox was dispensable for the resolution of the acute skin lesions and exerted only a limited effect on the containment of the parasites in the draining lymph node, but was essential for the clearance of L. major in the spleen. During the chronic phase of infection, parasites persisted at high levels in gp91phox -/-mice, and cutaneous lesions re-emerged in approximately 60% of these mice. gp91phox deficiency did not impair the expression of iNOS or the production of TNF and IFN-+ . These results demonstrate that iNOS and phox are both required for the control of L. major in vivo and display unexpected organ-and stage-specific anti-leishmanial effects.

IL‐12 is indispensable for the control of many intracellular pathogens, but the components of the signaling pathway that are essential for its function in vivo are incompletely understood. Here, we investigated in the Leishmania major mouse model whether Tyk2 kinase is required for the generation of a protective immune response. Unlike C57BL/6 controls, Tyk2–/–mice developed severe skin lesions after infection that frequently ulcerated, but ultimately healed. NK cell cytotoxicity was absent in infected Tyk2–/– mice, even after IL‐12 pretreatment, which correlated with a STAT4 activation defect. IFN‐α / β, which was still able to activate STAT1 in Tyk2–/– NK cells, reconstituted their cytotoxic activity, but not their IL‐12responsiveness. The IL‐12‐induced production of IFN‐γ by NK cells and CD8+ T cells was strongly suppressed in Tyk2–/– mice at day 1 of infection, but partly regained during the late phase of infection. Tyk2–/– CD4+ T cells developed into Th1 cells (although in a delayed fashion) and infected Tyk2–/– mice expressed normals levels of inducible NO synthase. Thus, Tyk2 is required for the IL‐12 response of NK cells and CD8+ T cells in L. major‐infected mice, but not for the generation of Th1 cells and the ultimate control of the disease.