ABCs accumulate with age and can be generated from FO B cells on TLR stimulation in vitro. (A) Percentages of B-cell subsets in untreated young, aged, and autoreconstituted aged mice. The latter group of mice was sublethally irradiated (500R) on day 0 and allowed to reconstitute for 12 weeks. Gating strategy is the same as that described for Figure 1A. Similar results were observed in 3 independent experiments. (B) Cell-cycle analysis by DAPI for splenic ABC, FO, and BM pro-/pre- B cells from aged mice. (C) FO B cells from aged mice were sorted and CFSE-labeled, then cultured with stimuli shown for 3 days. Dead cells were excluded by DAPI staining. CD21/35 and CD23 expression on live cells is shown; CFSE dilution indicates cell division. CD19⁺ splenic B cells were analyzed on the same day as a reference for CD21/35 and CD23 expression (right-most plot).