Accumulation of Fas-deficient cells among switched memory B cells. (A) Proportion of Fas-deficient B cells (FAS^{c.3G>T/c.3G>T}) among ex vivo FACS-sorted naïve or switched memory single B cells of P2, by single-cell PCR and sequencing of the specific FAS mutation. The numbers above the bars indicate number of sequenced single B cells. (B) Percent of plasmablasts in the CD19⁺ gate in blood of HD (n = 45), P1, and P2—3 independent measurements. (C) Percent of plasmablasts expressing Fas, measured in blood of a HD (n = 44) and P2 (3 independent measurements). Mean value indicated as a line. (D) Surface Fas expression on B cells of P2 and HD before and after 24-hour stimulation with CD40L, representative of 3 independent experiments. (E) Proportion of Fas-deficient (FAS^wt/c.652G>A), EBV-immortalized, single B-cell clones derived from FACS-sorted naïve or switched memory B cells of P1. The numbers above the bars indicate number of sequenced clones. (F) Fas-deficient and Fas-competent B cells show a similar proliferation profile. Proliferation by CFSE dilution of magnetically isolated B cells from P2 in response to stimulation with CD40L+IL-21 for 4 and 6 days. ***P < .001 (Fisher’s exact test).