Experimental Chagas' disease: kinetics of lymphocyte responses and immunological control of the transition from acute to chronic Trypanosoma cruzi infection.

The responses of spleen cells from mice infected with Trypanosoma cruzi to stimulation with T (concanavalin A and phytohemagglutinin) or B (lipopolysaccharide) cell-specific mitogens were monitored during the acute, transition, and chronic states of the disease. A marked reduction in the responses of infected mouse cells with respect to those of uninfected animals was observed during the acute stage, regardless of whether or not the infective dose was lethal. Reduced or absent responses were recorded with suboptimal, optimal, and supraoptimal concentrations of the mitogens. Normal levels of responsiveness to concanavalin A, phytohemagglutinin, and lipopolysaccharide were observed during the chronic stage of the disease. The trend of return to normal responses was initiated around day 40 after infection with 25 parasites. At this time, a marked decline in parasitemia levels, cessation of mortality, and disappearance of visible signs of disease began to be observed defining the transition stage that precedes establishment of chronicity. T cell levels of the spleen were markedly reduced during the acute period and returned during the chronic phase. Instead, absolute levels of B cells were significantly increased during the acute period but also normalize in the chronic phase. Immunosuppression of chronically infected mice with cyclophosphamide led to a temporary return to acute infection-type conditions, even in animals with undetectable levels of parasitemia before treatment. These results suggest that reduced T cell responses during acute experimental Chagas' disease might in part to be due to depletion of the T cell compartment. Decreased B cell responses in the presence of significant numbers of B lymphocytes implies a suppressive phenomenon, B cell alteration, or a combination of both possibilities. Recrudescence of the disease after immunosuppression with cyclophosphamide suggests that immunological mechanisms play an important role, not only in the gain of control over T. cruzi infected by the host but also in the maintenance of the chronic status.