The proinflammatory cytokine-induced IRG1 protein associates with mitochondria.

Interferon-gamma (IFN-gamma) and tumor necrosis factor (TNF) are essential cytokines for successful clearance of microbial infections. Activation of macrophages by synergistic effects of these cytokines leads to induction of antimicrobial effector systems like reactive oxygen and reactive nitrogen intermediates. Strikingly, IFN-gammaR(-/-) and TNFRp55(-/-) mice are considerably more susceptible to infections than inducible nitric oxide synthase(-/-) and p47phox(-/-) mice. Thus we applied transcriptome-profiling studies to identify genes synergistically upregulated by IFN-gamma and TNF in macrophages which are potentially involved in the defense against intracellular pathogens. From a total of 234 regulated genes we found 35 genes that were upregulated by combined effects of IFN-gamma and TNF and were at least 2-fold induced. The majority of these genes are involved in signal transduction and transcriptional regulation. However, we found several genes were poorly characterized with regard to immunological functions. As a prototypic TNF- and IFN-gamma-coregulated gene we characterized the expression and the subcellular localization of immunoresponsive gene 1 (IRG1) in murine macrophages. IRG1 is highly upregulated in murine ANA-1 macrophages by several proinflammatory cytokines and Toll-like receptor (TLR) agonists, as well as in spleen and lung of Listeria monocytogenes or Toxoplasma gondii infected mice, respectively. Furthermore, this study identifies 35 genes that constitute the IFN-gamma/TNF-triggered effector program in innate immunity.