Suppression of human lymphocyte responses by Trypanosoma cruzi

Virtually nothing is known about the basis for the immunosuppression associated with human T. cruzi infection. We have used an in vitro system to explore this effect. Incubation of human peripheral blood mononuclear cells (PBMC) with blood forms of T. cruzi abrogated their responses to suboptimal, optimal and supraoptimal doses of Con A, PHA or PWM, whether or not monocytes were depleted. Killed parasites were not suppressive. Maximal suppression (74%) occurred when the parasites were present during the entire culture period (96 hr), although significant suppression (33%) was seen when the organisms were added 24, 48 or 72 hr after initiation, suggesting that the early stages of lymphocyte activation had been impaired and that a second generation of cells was also affected. The 4-day supernatant medium of a T. cruzi suspension supported PBMC responses to Con A as well as medium incubated alone, indicating that suppression did not result from parasite removal of essential nutrients. Furthermore, 96 hr after mitogenic stimulation, the proportions of viable PBMC in cultures containing or lacking the parasites were comparable. Although T. cruzi binds Con A and PHA, this absorption was not the cause of reduced responsiveness since optimal concentrations of Con A and PHA remained in solution under our conditions. Levels of IL-2 in PHA-stimulated PBMC cultures were markedly reduced in the presence of T. cruzi. However, exogenous IL-2 failed to restore lymphocyte responsiveness. T. cruzi neither absorbed nor inactivated IL-2. Thus, the noted suppression appeared to involve at least deficient production and utilization of IL-2.