Impaired function of ABCs in vitro. (A) Flow cytometrically sorted splenic CD21/35⁻CD23⁻ (ABC) and FO B cells from aged or young mice were labeled with CFSE and then cultured with stimuli shown for 72 hours. Dead cells were stained by TO-PRO-3; CFSE dilution indicates proliferation. (B) Representative CFSE histograms for the same cultures shown in panel A show proliferation of ABCs (bold line) and FO (filled gray) B cells from aged mice. Similar results were found in 5 independent experiments.