Elevated transitional B cell output in patients without increased cell survival or replication in the peripheral blood. (A) Flow cytometric identification of CD10⁺CD21⁺ transitional B cells (top) in patient versus control (Ctrl) PBMCs, including subsets defined by CD24 and CD38 expression (bottom). Gates delineate T1, T2/T3, mature naive, and memory B cells. Gates were adjusted for P4 based on separate matched controls, analyzed on a different day. (B) Ex vivo survival assay for purified naive B cells from patients and controls, with cell death quantitated daily using Annexin V and propidium iodide staining. Data are representative of four independent experiments. (C) Comparison of in vivo replication history by KREC analysis (displayed as number of cell divisions) using sorted transitional and mature naive B cells from controls and patients. The control shown is distinct from that shown in A. (D) CD10 and Ki-67 staining of tonsil sections from control, P2, and P3, with positive staining restricted to GCs. Representative photographs are shown. Bars: (top) 500 µm; (bottom) 200 µm.