Peripheral B cells from old mice suppress B-cell lymphopoiesis. Splenic B cells from young and old mice were adoptively transferred to young hCD20Tg mice that were treated (and confirmed by blood stain) for B-cell depletion. Bone marrow and spleen from recipient mice were quantified for B-cell lymphopoiesis 28 days after depletion. (A) Schematic diagram showing details of the kinetics of the experiment. (B-C) Analysis of BM cells for the indicated mice. For analysis, viable lymphocytes were defined by forward scatter and light side scatter, and gates were set to analyze pro-B (pro) and pre-B (pre) cells (B220⁺CD93⁺IgM^–) and immature B cells (B220⁺CD93⁺IgM⁺). Shown are representative plots for a single mouse from each group; arrows indicate populations (B). Absolute cell numbers (C). Graph depicts mean from 4 mice in each group ± SE. (D-E) Analysis of spleen cells for the indicated mice. Gates were set to quantify newly generated host B cells as B220⁺hCD20⁺. Shown are representative plots for individual mice from each group (D) and absolute cell numbers (E). Graph depicts means from 4 mice in each group ± SE. (F-I) IL-7–driven BM cultures to grow B cells in vitro were prepared from young (F-G) or old (H-I) mice. In these experiments, the fetal calf serum (FCS) in culture media was replaced with 1% fresh mouse serum from the indicated mice. After 5 days, cells were harvested, counted, and stained for surface markers and analyzed for pro-B and pre-B (B220⁺/IgM^–) and immature (B220+/IgM+) B cells. Shown are representative results from a single experiment (F,H) and absolute B-cell counts (G,I) in the cultures. Graphs depict means from 5 experiments ± SE. dep, depleted.