IL-10 production is enriched in the CD24^hiCD27⁺ and the CD24⁻CD27^hiCD38^hi plasmablast B-cell pools in healthy patients and in AHSCT recipients with or without cGVHD. Whole PBMCs were cultured for 66 hours with CpG-B (3 μg/mL, left panel) followed by restimulation with PMA + ionomycin in the presence of brefeldin A the last 5 hours of culture, fixed, permeabilized, and intracellular IL-10 was measured in live CD19⁺ cells by flow cytometry. (A) Histograms of the expression of IgD, CD27, and CD20 by CD19⁺CD24⁻CD38^hi plasmablasts (red) and CD19⁺CD24⁺CD38⁻ cells (blue) vs control isotype (orange) in CD19⁺ cells from a representative AHSCT recipient. (B) Determination of the cell surface phenotype of human IL-10⁺ B cells. Representative histograms of the expression of CD27, CD24, CD38, CD138, CD5, IgD, IgM, CD20, CD43, CD70, in IL-10⁺ B cells (red) vs IL-10⁻ B cells (blue) and control isotype (orange) in a healthy donor. (C) Dot plots of CD19⁺IL-10⁺ cells in B-cell subsets defined by the expression of CD24 and CD27 (top panel), CD27 and CD38 (middle), CD24 and CD38 (bottom panel) in a representative allogeneic transplant recipient with no cGVHD. Dot plots are gated on live CD19⁺ cells. (D) IL-10⁺ cells in proportion to each B-cell subset defined by the expression of CD24 and CD27 (top panel), CD27 and CD38 (middle), CD24 and CD38 (bottom panel) in healthy donors (n = 14), patients with no cGVHD (n = 9), patients with active cGVHD (n = 6), and cGVHD in remission (n = 8). Dot plots are gated on live CD19⁺ cells. PB, plasmablast; TR, transitional B cell.