CD4⁺ (and not CD25⁺) T cells are the predominant interleukin-10-producing cells in the circulation of filaria-infected patients

Background. Interleukin (IL)-10 plays an important role in down-regulating the immune response to filarial parasites. The goal of this study was to characterize the predominant cellular source of IL-10 in human filarial infections. Methods. Multicolor flow cytometry was used to determine the frequencies of IL-10 production from various lymphocyte populations in the circulation of 23 patients with filarial infections and 8 uninfected control subjects. Results. The frequencies of cells spontaneously producing IL-10 was significantly greater in filaria-infected patients than in uninfected control subjects (geometric mean, 93 vs. 18 IL-10 -producing cells/100,000 peripheral blood mononuclear cells; P = .03). Most IL-10-producing cells in filaria-infected patients were T cells, with CD4⁺ and CD8⁺ cells accounting for 48% and 27%, respectively, of all IL-10-producing cells; CD19⁺ B cells, CD14⁺ monocytes, and CD56⁺ NK cells accounted for 10%, 8%, and 7%, respectively. Surprisingly, only 12% of the IL-10-producing CD3⁺CD4⁺ cells were CD25⁺. Seventy-seven percent of IL-10-producing CD4⁺ T cells stained negatively for both IL-4 and interferon (IFN)-γ, 22% were positive for IL-4, and <1% were positive for IFN-γ. Conclusions. These experiments demonstrate that the most frequent producers of IL-10 in human filarial infections are CD4⁺ T cells, many of which are skewed toward a type 2 phenotype and most of which are not CD25⁺.