IFN-γ-Dependent Nitric Oxide Production Is Not Linked to Resistance in Experimental African Trypanosomiasis

Resistance to African trypanosomes is dependent on B cell and Th1 cell responses to the variant surface glycoprotein (VSG). While B cell responses to VSG control levels of parasitemia, the cytokine responses of Th1 cells to VSG appear to be linked to the control of parasites in extravascular tissues. We have recently shown that IFN-γ knockout (IFN-γ KO) mice are highly susceptible to infection and have reduced levels of macrophage activation compared to the wild-type C57BL/6 (WT) parent strain, even though parasitemias were controlled by VSG-specific antibody responses in both strains. In the present work, we examine the role of IFN-γ in the induction of nitric oxide (NO) production and host resistance and in the development of suppressor macrophage activity in mice infected withTrypanosoma brucei rhodesiense.In contrast to WT mice, susceptible IFN-γ KO mice did not produce NO during infection and did not develop suppressor macrophage activity, suggesting that NO might be linked to resistance but that suppressor cell activity was not associated with resistance or susceptibility to trypanosome infection. To further examine the consequence of inducible NO production in infection, we monitored survival, parasitemia, and Th cell cytokine production in iNOS KO mice. While survival times and parasitemia of iNOS KO mice did not differ significantly from WT mice, VSG-specific Th1 cells from iNOS KO mice produced higher levels of IFN-γ and IL-2 than cells from WT mice. Together, these results show for the first time that inducible NO production is not the central defect associated with susceptibility of IFN-γ KO mice to African trypanosomes, that IFNγ-induced factors other than iNOS may be important for resistance to the trypanosomes, and that suppressor macrophage activity is not linked to either the resistance or the susceptibility phenotypes.