Suppressive activity of IgM memory and transitional B cells depends on IL-10 secretion. (A) Suppressive effect of B-cell subsets on proliferation of CFSE-labeled CD4⁺ T cells in the presence or absence of IL-10 blockade. Flow cytometry histograms show CD4⁺ T cells cultured alone, with switched memory B cells, naïve B cells, IgM memory B cells, and transitional B cells in the presence or absence of IL-10 mAb. (B) Comparison of inhibitory effects by different B-cell subsets on proliferating CD4⁺ T cells, with or without IL-10 blockade. Bars indicate medians, and whiskers indicate the upper range for 3 healthy donors. *P < .05 vs positive control or IL-10R blockade by nonparametric ANOVA. (C) The addition of IL-10 to cocultures of CD4⁺T cells and sorted naïve and switched memory B cells has a minimal suppressive effect. Bars represent medians, and whiskers indicate the upper ranges for 3 healthy individuals.