Direct cell-cell contact contributes to the T cell–suppressive activity of IL-10⁺ IgM memory and transitional B-cell subsets. (A) Representative flow cytometry histograms showing proliferation of CD4⁺ T cells cultured either in separate transwell chambers or directly with CD19⁺ B-cell subsets: CD4⁺ T cells cultured alone or with switched memory B cells, naïve B cells, IgM memory B cells, or transitional B cells at a ratio of 1:1. (B) Impact of direct contact with CD4⁺ T cells in suppressive activity of IL-10⁺ B-cell subsets. (C) IgM memory and transitional B cell–mediated suppression of CD4⁺ T cells depends on both IL-10 and cell-cell contact. In both panels, the data shown are medians and upper ranges from 3 independent experiments with 5 samples per group. *P < .05 by nonparametric ANOVA; ns, not significant.