Ablation of BAFF-R induces chronic B-cell deficiency and prevents aging of B lymphopoiesis in the BM of old mice. Young (4 months) and old (20 months) C57Bl6 BAFF-R^FL/FL/Mx-cre and young and old (age-matched) control C57Bl6 BAFF-R^FL/+/Mx-cre mice received intraperitoneal injections of 3 doses of poly I:C (400 μg/mouse each time) to ablate the loxP-flanked BAFF-R allele. Before treatment, old mice were prescreened to have less than 5% AA4.1⁺ B cells in peripheral blood as an indication for an “old-like” B-cell phenotype. After 90 days, BM was analyzed for B lymphopoiesis. (A) BM cells were stained and analyzed by fluorescence-activated cell sorter (FACS) with a lymphocyte gate as defined by light scatter. The IgM versus AA4.1 plots were gated for CD19⁺/B220⁺ cells, and the B220 versus CD43 plots were gated on IgM⁻ cells. (B) Absolute numbers of CD19⁺/B220⁺ cells in the BM of the indicated mice. The B-cell numbers were calculated based on the total number of nucleated cells purified from 2 femurs and 2 tibias (n = 3 mice in each group). (C) Quantification of B-cell subsets in the BM. The relative proportion of each B-cell subset was determined in the B220-gated population. Shown are representative results of young (n = 3) and old (n = 3) control BAFF-R^FL/+/Mx-cre mice and 2 different Old BAFF-R^FL/FL/Mx-cre mice (from n = 3).