Interferon-γ production and host protective response against Mycobacterium tuberculosis in mice lacking both IL-12p40 and IL-18

Interferon (IFN)-γ plays an essential role in host defense against infection with Mycobacterium tuberculosis, and its synthesis is critically regulated by interleukin (IL)-12, IL-18 and the recently identified IL-23. The present study was designed to determine the roles of these cytokines in IFN-γ-mediated host defenses against M. tuberculosis. For this purpose, we compared host protective responses in IL-12p40 and IL-18 double-knockout (DKO) mice (which lacked both IL-12/IL-18 and also IL-23) and IFN-γ gene-disrupted (GKO) mice. DKO mice were more resistant to the infection than GKO mice, as indicated by their extended survival and reduced live colony numbers in spleen, liver and lung. IFN-γ was detected by ELISA in liver and lung homogenates, but not in spleen and serum, and in all organs by RT-PCR in DKO mice at comparable or reduced levels to those in wild-type mice. IFN-γ production was reduced by depletion of CD4+ T cells, but not of natural killer (NK), NKT, γδT and dendritic cells. Neutralization of IFN-γ or TNF-α by specific monoclonal antibodies (mAbs) significantly shortened the survival time of the infected DKO mice. Furthermore, anti-TNF-α mAb partially attenuated IFN-γ synthesis in the liver of these mice. Finally, the expression level of inducible nitric oxide synthase (iNOS) mRNA in the spleen, liver and lung was considerable in DKO mice but only marginal or undetected in GKO mice. Our results indicate the presence of IL-12-, IL-18- and IL-23-independent host protective responses against mycobacterial infection mediated by IFN-γ, which was secreted from helper T cells.