B-cell depletion expands populations of BM hematopoietic progenitor cells in old mice. Old (20 months) hCD20 Tg Balb/c mice (prescreened to have < 5% AA4.1⁺ B cells in peripheral blood as an indication for an “old-like” B-cell phenotype) were treated for B-cell depletion by intraperitoneal injection of mouse anti–human CD20 monoclonal antibodies (clone 2H7). BM cells were analyzed by flow cytometry 70 days later for the frequencies of Lin⁻/c-Kit⁺/IL7R⁺ cells (CLPs) and Lin⁻/c-Kit⁺/CD34⁺/Flt3⁺ cells (MPPs) and compared with old (age-matched) and young (4 months old) hCD20 Balb/c controls. (A) Representative FACS profiles of the indicated mice showing CLPs. (B) BM frequencies of CLPs from the young, old, and old-depleted mice (n = 4 in each group). (C) Representative FACS profiles of the indicated mice showing MPPs. (D) BM frequencies of MPPs from the young, old, and old-depleted mice (n = 4 in each group). Statistically different differences are specified.