NFAT1-deficient patient has an accumulation of hyperproliferative naïve B cells. (A-C) scRNA-seq of PBMCs from the patient (II-1), heterozygous controls (n = 3), and healthy control (n = 2) stimulated with P/I for 4 hours. (A) Uniform manifold approximation and projection (UMAP) visualization of stimulated cell subsets. Cell labels indicated on right. (B) Volcano plot showing DE in naïve B cells between patient and 5 healthy controls. Dark colors, significance; dim colors, nominal significance but not after adjusted P value; green, not significant. (C) Violin plots of significantly differentially expressed genes of interest in naïve B cells comparing patient and healthy controls. (D) Expression of NFAT1 in patient and control naïve B cells. Mean fluorescence intensities (MFIs) are indicated. (E) Frequency of IgM⁺⁺CD38⁺⁺ transitional B cells in the patient (II-1) and heterozygous (n = 4) and healthy controls (n = 8). (F) Quantification of E. (G) Frequency of IgD⁺CD27⁻ naïve (marked as “N”) and IgD⁻CD27⁺ memory (marked as “M”) B cells in the patient and one representative control. Schematic of quadrants that correspond to each cell population and frequency shown at top right. (H-I) Quantification of G. (J) Frequency of CD27⁺CD38⁺ plasmablasts in mature CD19⁺ B cells. (K) IgD and IgM expression in each individual. Mean fluorescence intensities are indicated. (L) Frequency of Ki67⁺ naïve B cells in the patient and a representative control. (M) Quantification of L. (N) Frequency of TNF-α⁺ naïve B cells in the patient and a representative control. (O) Quantification of N. (P-Q) Isolated naïve B cells from patient and heterozygous (n = 1) and healthy controls (n = 6) expanded for 6 days. (P) Frequency of class-switched memory B cells and (Q) plasmablasts before and after expansion. ∗P < .05, ∗∗P < .01, one-way analysis of variance and Tukey’s post hoc test. NS, not significant. Green circles, healthy control; blue circle, II-2; blue upright triangle, I-2; blue inverted triangle, I-1; blue square, II-3; red diamond, II-1.