B cell precursors are decreased in senescent BALB c mice, but retain normal mitotic activity in vivo and in vitro

The numbers of phenotypic (sIg^- Ly5[220]⁺) and functional B cell precursors were significantly reduced in the bone marrow of senescent (22-24 months old) BALB c mice when compared to their young (2-4 months old) cohorts. Little alteration in the numbers of B cell precursors occurred during the first 12 months of life in this strain. In contrast, an accelerated loss of B cell precursors between 15 and 18 months of age was observed. In particular, the levels of small Ly5(220)⁺ B cell precursors were decreased with advanced age, although a decline in numbers of large sIg^- Ly5(220)⁺ B cell precursors was also evident. The percentages of large sIg^- Ly5(220)⁺ B cell precursors in (S + G2 M) stages of cell cycle were similar (e.g., 60-80%) in aged and young BALB c mice. Importantly, Ly5(220)⁺ pre-B cells from both young and aged BALB c mice, either present in vivo or derived from Ly5(220)^- cells in vitro, were capable of proliferation in response to rIL-7. These observations suggest that the aging process results in a progressive decline in the numbers of pre-B cells; however, this apparently is not due to failure of B lineage precursor cells to respond to growth mediators either in vivo or in vitro.