B cells from cGVHD patients show deficient IL-10 production. PBMCs from cGVHD (n = 11) or no-cGVHD (n = 11) patients and controls (n = 12) were incubated with L cells alone for 48 hours. PMA, ionomycin, and BFA were added for the last 6 hours of culture, and the cells were subsequently stained for the surface expression of CD19, CD3, and PI, and then intracellularly for IL-10. (A) Representative flow cytometry plots of IL-10 staining of gated CD19⁺ B cells from patients with or without cGVHD. Frequencies of IL10⁺ CD19⁺ B cells were assessed relative to the isotype controls. (B) cGVHD patients had lower frequencies and absolute numbers of IL-10⁺ B cells after stimulation with L cells and were assessed in relation to either the no-cGVHD patients or healthy controls (HC). The absolute number of CD19⁺ B-cell subsets was calculated by multiplying their frequencies as determined by flow cytometry by the absolute lymphocyte number (cells/µL) obtained from a diagnostic complete blood count (CBC) panel performed on the same day. CBC numbers were available for absolute CD19⁺IL-10⁺ B-cell calculation for 5 healthy controls. All values are reported as medians and interquartile ranges indicated by brackets. **P = .001 for cGVHD vs HC or no GVHD by nonparametric ANOVA.