Interferon-gamma suppresses the growth of Toxoplasma gondii in human fibroblasts through starvation for tryptophan

The effect of human recombinant interferon-γ (IFN-γ) on Toxoplasma gondii in cultured human fibroblasts is predominantly parasitostatic. This effect is dependent upon the induction in the host cell of a potent indoleamine 2,3-dioxygenase that converts tryptophan to N-formylkynurenine. This product is, in turn, degraded to kynurenine by a formamidase. Within 24 h of treatment with IFN-γ most of the tryptophan originally present in the medium is converted to these products together with some minor metabolites. When added to the medium of infected cultures at concentrations equimolar to the tryptophan content, neither N-formylkynurenine nor kynurenine suppresses the growth of T. gondii, although at higher concentrations they are effective. The medium of uninfected cultures treated with IFN-γ for 24 h has no effect on the growth of T. gondii, when transferred to fresh cultures provided that the residual IFN-γ is first removed by ultrafiltration or neutralized with a specific monoclonal antibody. Thus minor metabolites produced from tryptophan in response to IFN-γ and excreted into the medium are not parasitostatic. When cultures treated with IFN-γ for 24 h are incubated with medium that contains [³H]tryptophan, the radioactive amino acid is converted to N-formylkynurenine and kynurenine as rapidly as it enters the cell. This degradation not only results in a very low intracellular concentration of tryptophan but also produces intracellular concentrations of tryptophan metabolites that are significantly higher than the tryptophan concentration in control cells. However, it is unlikely that either metabolite reaches intracellular concentrations that are sufficient to suppress the growth of the parasite. The parasitostatic effect of IFN-γ is most likely to result from the starvation of T. gondii for tryptophan.