ABCs respond to TLR ligands in vitro. (A) Flow cytometrically sorted splenic ABCs from aged mice were labeled with CFSE then cultured with stimuli shown for 72 hours. Dead cells were stained by TO-PRO-3; CFSE dilution indicates proliferation. Similar results were found in 2 independent experiments. (B) CFSE histograms for the same cultures shown in panel A show proliferation of cells with (bold lines) or without (gray filled histograms) anti-μ. (C) TLR7 and TLR9 expression by qPCR in FO, MZ, and ABCs from young or aged mice. Fold change is relative to the level of each receptor observed in young FO (1.0 = the average level in FO B cells from young mice).