Treatment with abatacept (CTLA4 immunoglobulin) restores FO B-cell maturation coincident with decreased CD40L levels on effector T cells in CTLA4 deficiency. (A-B) Flow cytometric analysis of peripheral blood B-cell populations in patients with CTLA4 deficiency before (n = 11) and after abatacept (n = 8) therapy (6 patients captured at paired intervals before/after abatacept therapy) and healthy controls (n = 16). Representative contour plots and histograms from a healthy control (left) and the same patient with CTLA4 deficiency (middle and right) before/after abatacept therapy (homozygous CTLA4 3′ UTR mutation carrier in panel A, and heterozygous CTLA4 c.410C>T, p.P137L mutation carrier in panel B) shown at 10% and 5% of events, respectively. Quantitation of B-cell subset frequency as percent IgD⁺CD27⁻ B cells in the peripheral blood. Symbols represent unique individuals; bars represent means (± SD) of all data; dotted lines represent paired pretreatment to posttreatment time points. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001, or as listed by analysis of variance (ANOVA) for multiple comparisons or by Wilcoxon matched-pairs signed-rank test, as indicated. (C) scRNA-seq of total IgD⁺CD27⁻ B cells (from heterozygous CTLA4 c.410C>T, p.P137L mutation carrier) before and after abatacept therapy. UMAP projection of cells is shown by cell type clustering and state of abatacept treatment. Fraction representation of each clustered scRNA-seq cell type. (D) Flow cytometric analysis of peripheral blood T-cell populations in patients with CTLA4 deficiency before (n = 8) and after abatacept (n = 6) therapy (4 patients captured at paired intervals before/after abatacept therapy) and healthy controls (n = 10). Representative overlaid histograms from the same patient with CTLA4 deficiency before/after abatacept therapy (heterozygous CTLA4 c.173G>C p.C58S mutation carrier). Quantitation of total cellular CD40L by intracellular staining in CD4⁺ T-cell subsets. Symbols represent unique individuals; bars represent means (± SD) of all data; dotted lines represent paired before to after treated unique patients. ∗P < .05 or as listed by ANOVA for multiple comparisons or by Wilcoxon matched-pairs signed-rank test, as indicated. (E) Correlation between FO B-cell frequency as percent IgD⁺CD27⁻ B cells in the peripheral blood (data from panel A; x-axis) and total cellular CD40L levels as MFI in TEMRA CD4⁺ T cells (data from panel D; y-axis). Data are from untreated patients with CTLA4 deficiency, treated patients with CTLA4 deficiency, and healthy controls, as indicated. Simple linear regression with correlation coefficient (R²), P value, and 95% CI shown. ns, not significant.