ROLES OF GAMMA-INTERFERON AND OTHER CYTOKINES IN SUPPRESSION OF THE SPLEEN-CELL PROLIFERATIVE RESPONSE TO CONCANAVALIN-A AND TOXOPLASMA ANTIGEN DURING ACUTE TOXOPLASMOSIS

Suppressed splenocyte proliferation in response to mitogen and toxoplasma lysate antigen (TLA) is observed in mice acutely infected with Toxoplasma gondii. Recently, we reported that N-G-monomethyl-L-arginine (NMMA), an inhibitor of reactive nitrogen intermediate (RNI) production, partially restored proliferative responses of splenocytes from infected mice. In the present study we have examined the effect of NMMA on production of cytokines by splenocytes from mice acutely infected with T. gondii and assessed the role of gamma interferon (IFN-gamma) and interleukin-10 (IL-10) in the RNI-mediated suppression. Stimulation with concanavalin A (ConA) or TLA of splenocytes from CBA/Ca mice infected for 7 days resulted in increased production of IFN-gamma, IL-4 and IL-10 but reduced levels of IL-2 when compared with cultures of splenocytes from uninfected mice. Whereas addition of NMMA did not alter levels of cytokines produced by splenocytes from uninfected mice, splenocytes from infected mice stimulated with ConA produced significantly higher levels of IL-10 and reduced levels of IL-2 and IL-4. Addition of anti-IFN-gamma monoclonal antibodies to cultures of spleen cells from mice infected for 7 or 14 days remarkably decreased the levels of nitrite and resulted in a 47- and 4-fold increase in proliferation induced by stimulation with ConA or TLA, respectively. Anti-IL-10 did not reduce levels of nitrite produced in culture but did result in a fourfold increase in the proliferative response of splenocytes from mice infected for 14 days. In vivo administration of anti-IFN-gamma or anti-IL-10 monoclonal antibodies to infected mice partially restored ex vivo spleen cell proliferative responses by approximately 40 and 15%, respectively. Our data indicate that IFN-gamma is important in inducing the RNI-mediated immunosuppression, which, in turn, affects production of cytokines by splenocytes. Our data also demonstrate that IL-IO is involved in the suppression observed but that this activity is independent of RNI.