Inducible Nitric Oxide Is Essential for Host Control of Persistent but Not Acute Infection with the Intracellular Pathogen Toxoplasma gondii

Abstract

The induction by IFN-γ of reactive nitrogen intermediates has been postulated as a major mechanism of host resistance to intracellular pathogens. To formally test this hypothesis in vivo, the course of Toxoplasma gondii infection was assessed in nitric oxide synthase (iNOS)−/− mice. As expected, macrophages from these animals displayed defective microbicidal activity against the parasite in vitro. Nevertheless, in contrast to IFN-γ−/− or IL-12 p40−/− animals, iNOSdeficient mice survived acute infection and controlled parasite growth at the site of inoculation. This early resistance was ablated by neutralization of IFN-γ or IL-12 in vivo and markedly diminished by depletion of neutrophils, demonstrating the existence of previously unappreciated NO independent mechanisms operating against the parasite during early infection. By 3-4 wk post infection, however, iNOS knockout mice did succumb to T. gondii. At that stage parasite expansion and pathology were evident in the central nervous system but not the periphery suggesting that the protective role of nitric oxide against this intracellular infection is tissue specific rather than systemic.

Rapid Recruitment of Neutrophils Containing Prestored IL-12 During Microbial Infection

“…Notably, experiments in cytokine gene and cytokine receptor knockout animals demonstrated that parasite-triggered IL-12 release occurred independently of endogenous IFN-␥ and did not require signaling through the TNF p55 receptor. We and others also found that C57BL/6 mice succumb acutely to infection when depleted of their granulocytes, confirming the importance of these cells during in vivo infection (6,20,21).…”

“…Our previous work and that of others established that neutrophils are necessary to survive acute infection with the low virulence strain ME49 (6,20,21). To ascertain whether their impact may be explained at least in part by an immunoregulatory function, we measured serum IL-12 levels in neutrophil-depleted mice from the time of infection until they became clinically ill, which occurred 8 days p.i.…”

Lack of IL-15 results in the suboptimal priming of CD4+T cell response against an intracellular parasite

“…Mutant animals lacking IL-12 (17) or normal mice treated with anti IL-12 antibody (6) exhibit severe reduction in IFN-␥ response against the parasite and are highly susceptible to infection. Similarly, studies from our laboratory have shown that exogenous IL-7 administration can enhance the IFN-␥-producing cytotoxic T lymphocyte levels in infected animals (8).…”

Cutting Edge: IL-12 Is Required for the Maintenance of IFN-γ Production in T Cells Mediating Chronic Resistance to the Intracellular Pathogen,Toxoplasma gondii

“…For experimental infection, each mouse received 20 cysts of the ME49 strain of T. gondii given i.p. (15).…”

“…Sort-purified T cells (1 ϫ 10 6 /ml) were cultured with irradiated IL-12-deficient splenocytes (1 ϫ 10 7 /ml) and stimulated with IL-12, STAg, or a combination as described above. Culture supernatants were collected at 48 h poststimulation, and the levels of IFN-␥ were measured using a previously described two-site ELISA assay (15).…”