Naturally occurring CD4+Foxp3+ regulatory T cells are an essential, IL-10-independent part of the immunoregulatory network in Schistosoma mansoni egg-induced inflammation.

In acute and chronic schistosomiasis, survival of the host requires a carefully balanced immune response against highly immunogenic parasite eggs. We characterized the phenotype, distribution, and functional role of CD4(+)Foxp3(+) naturally occurring regulatory T cells (naTregs) in schistosome egg-induced inflammation. In adoptive transfer experiments and by intracellular staining for Foxp3, we demonstrate significant frequencies of naTregs in hepatic granulomas and draining lymphoid tissues of mice infected with the trematode Schistosoma mansoni. Strikingly, egg-induced inflammation does not change the normal ratio between naTregs and effector CD4(+) T cells at the inflammatory site or in lymphoid organs in acute or chronic disease. However, increasing frequencies of CD103-expressing cells in the naTreg compartment indicate a change in phenotype for naTregs with disease progression. Because CD103 was described recently as an activation marker for naTregs, we speculate that naTregs in chronic schistosomiasis are potentially more suppressive. Furthermore, we found that most naTregs do not contribute to egg-induced IL-4 and IL-10 production. Importantly, depletion of CD25(+) naTregs strongly enhances the frequency of IL-4-producing effector T cells in acute egg-induced inflammation. It does not change clonal expansion of activated CD4(+) T cells. This regulation of egg-induced cytokine production does not require the presence of IL-10. These data demonstrate that naTregs limit egg-induced effector-cytokine production in our model. Our results identify naTregs as an important, IL-10-independent part of the regulatory network in schistosome egg-induced inflammation.