CD21^−/lo naive B cells do not proliferate after BCR and CD40 cotriggering and are prone to cell death. (A) Total naive B cells from healthy donors (HD), and RA and CVID patients containing both CD21^−/lo and CD21⁺ B cells were labeled with CFSE and stimulated with F(ab′2) anti-IgM and/or recombinant human CD40L. Dot plots show CD21 expression according to CFSE dilution after 3 days. CFSE dilution in CD21⁺ but not in CD21^−/lo B cells stimulated with F(ab′2) anti-IgM and recombinant human CD40L reveals that some CD21⁺ B cells underwent up to 2 divisions, whereas CD21^−/lo B cells did not divide. (B) Increased frequency of annexin V⁺ propidium iodide–negative (PI⁻) early apoptotic, and annexin V⁺ PI⁺ late apoptotic and dead cells in freshly isolated CD21^−/lo naive B cells from RA and CVID patients compared with their CD21⁺ B-cell counterparts. (C) BCR, CD40, and/or TLR9 triggering do not prevent the apoptosis and cell death of CD21^−/lo naive B cells. Total naive B cells containing both CD21^−/lo and CD21⁺ B cells from RA01 or CVID321 patients were plated, and annexin V and PI staining was assessed after 12 hours. BCR, CD40, and TLR9 stimulation prevents late apoptosis and cell death in CD21⁺ but not in CD21^−/lo B cells.