Decreased number and impaired ability of BM Bregs to secrete inhibitory cytokines in patients with ITP. (A) Representative density plots of BM and PB CD19⁺CD24^hiCD38^hi Bregs in patients with ITP and HCs. (B, C) The percentages of BM (median [range], 5.50% [0.03%-16.77%] vs 18.67% [4.58%-36.72%]; P < .001) and PB (median [range], 0.31% [0.02%-2.27%] vs 3.05% [0.77%-15.19%]; P < .001) Bregs in CD19⁺ cells were significantly lower in patients with ITP compared with their respective counterparts in HCs. (D, E) Both patients with ITP and HCs had more Bregs in BM B cells than in PB B cells (patients with ITP: median [range], 5.50% [0.03%-16.77%] vs 0.31% [0.02%-2.27%], P < .001; HCs: median [range], 18.67% [4.58%-36.72%] vs 3.05% [0.77%-15.19%], P = .001). (F, G) Quantification of IL-10⁺ or TGF-β⁺ B cells in BM CD19⁺ cells from patients with ITP and HCs, respectively. (H, I) BM B cells from patients with ITP produced less IL-10 (2.35 ± 0.51% vs 7.15 ± 0.86%; P < .001) and TGF-β (median [range], 1.32% [0.39%-4.11%] vs 4.10% [1.34%-5.91%]; P = .028) compared with HCs after stimulation with CPG-ODN. *P < .05 , **P < .01; ***P < .001. APC, allophycocyanin; CPG-ODN, CPG oligonucleotide; FITC, fluorescein isothiocyanate; PE, phycoerythrin; SSC, side scatter.