Arginine is a crucial amino acid that serves to modulate the cellular immune response during infection. Arginine is also a common substrate for both inducible nitric oxide synthase (iNOS) and arginase. The generation of nitric oxide from arginine is responsible for efficient immune response and cytotoxicity of host cells to kill the invading pathogens. On the other hand, the conversion of arginine to ornithine and urea via the arginase pathway can support the growth of bacterial and parasitic pathogens. The competition between iNOS and arginase for arginine can thus contribute to the outcome of several parasitic and bacterial infections. There are two isoforms of vertebrate arginase, both of which catalyze the conversion of arginine to ornithine and urea, but they differ with regard to tissue distribution and subcellular localization. In the case of infection with Mycobacterium, Leishmania, Trypanosoma, Helicobacter, Schistosoma, and Salmonella spp., arginase isoforms have been shown to modulate the pathology of infection by various means. Despite the existence of a considerable body of evidence about mammalian arginine metabolism and its role in immunology, the critical choice to divert the host arginine pool by pathogenic organisms as a survival strategy is still a mystery in infection biology.

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Activation of macrophages by interferon gamma (IFN-c) and the subsequent production of nitric oxide (NO) are critical for the host defence against Salmonella enterica serovar Typhimurium infection. We report here the inhibition of IFN-c-induced NO production in RAW264.7 macrophages infected with wild-type Salmonella. This phenomenon was shown to be dependent on the nirC gene, which encodes a potential nitrite transporter. We observed a higher NO output from IFN-c-treated macrophages infected with a nirC mutant of Salmonella. The nirC mutant also showed significantly decreased intracellular proliferation in a NO-dependent manner in activated RAW264.7 macrophages and in liver, spleen and secondary lymph nodes of mice, which was restored by complementing the gene in trans. Under acidified nitrite stress, a twofold more pronounced NO-mediated repression of SPI2 was observed in the nirC knockout strain compared to the wild-type. This enhanced SPI2 repression in the nirC knockout led to a higher level of STAT-1 phosphorylation and inducible nitric oxide synthase (iNOS) expression than seen with the wild-type strain. In iNOS knockout mice, the organ load of the nirC knockout strain was similar to that of the wild-type strain, indicating that the mutant is exclusively sensitive to the host nitrosative stress. Taken together, these results reveal that intracellular Salmonella evade killing in activated macrophages by downregulating IFN-c-induced NO production, and they highlight the critical role of nirC as a virulence gene.

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