Evidence suggests a protective immune response against infection with Mtb is derived mainly from IFN-γ-producing Th1 cells that activate infected macrophages, since CD4-deficient, IFN-γ- and inducible nitric oxide synthase (iNOS)-KO mice are highly susceptible to Mtb infection compared to wild-type strains (9–12). Here, NOS2 is linked to infection.