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Leishmania major promastigotes were found to avoid activation of mouse bone marrow-derived macrophages (BMM0) in vitro for production of cytokines that are typically induced during infection with other intracellular pathogens. Coexposure of BMM0 to the parasite and other microbial stimuli resulted in complete inhibition of interleukin (IL) 12 (p40) mRNA induction and IL-12 release. In contrast, mRNA and protein levels for IL-1(alpha), IL-1(beta), tumor necrosis factor (TNF) alpha, and inducible NO synthase (iNOS) were only partially reduced, and signals for IL-10 and monocyte chemoattractant protein (MCP-1/JE) were enhanced. The parasite could provide a detectable trigger for TNF-alpha and iNOS in BMM0 primed with interferon (IFN) gamma, but still failed to induce IL-12. Thus IL-12 induction is selectively impaired after infection, whereas activation pathways for other monokine responses remain relatively …