Trypanosoma cruzi induces suppression of DNA synthesis and inhibits expression of interleukin-2 receptors by stimulated human B lymphocytes

Abstract

Trypanosoma cruzi, the causative agent of Chagas' disease, suppresses immune responses during the acute phase and has been shown to induce multiple cellular alterations in activated human T lymphocytes. However, no information is available regarding the effects of this parasite on human B cells. Using an in vitro culture system, in which purified T. cruzi are co-cultured with either peripheral blood mononuclear cells (PBMC) or B-cell-enriched preparations (BCE), we studied whether the organism can induce alterations in DNA synthesis after stimulation with Pansorbin (PS). This response was markedly reduced by the parasite at both suboptimal and optimal PS concentrations, and the extent of the inhibition was augmented as the parasite concentration was increased. Maximal reduction in DNA synthesis was observed when the trypanosomes were incorporated into the cultures at 0 time (i.e. together with PS); the effect was of a much lesser magnitude and undetectable when the parasites were added at 24 and 48 hr, respectively. These results imply that T. cruzi affects a relatively early event during B-cell stimulation. This inference was confirmed by the finding that the proportion of PS-stimulated B cells expressing interleukin-2 (IL-2) receptors was significantly reduced when the parasite was present in the culture. Addition of recombinant human IL-2 did not restore B-cell responsiveness to normal levels. Suppressed B-cell responses were also observed when T. cruzi was separated from the PBMC or the BCE by a cell-impermeable filter, indicating that a soluble factor(s) released by the organism mediated the effect. Accordingly, supernatants of T. cruzi suspensions were found to be suppressive. These results demonstrate for the first time that T. cruzi can affect human B-cell responses and that the mechanism involves inhibition of IL-2 receptor expression.