Apoptosis during B lymphopoiesis in mouse bone marrow.

To evaluate the magnitude of cell death and the critical stages at which it occurs during B lymphopoiesis in mouse bone marrow (BM), we have examined the kinetics of apoptosis at defined stages of B cell differentiation. FACS-sorted B220+ BM cells exhibited a low incidence of morphologically apoptotic cells by electron microscopy. In freshly prepared BM suspensions, the incidence of hypodiploid cells detected by multiparameter flow cytometry was greater among large dividing B220+ surface IgM- (sIgM-) precursor B cells and sIgM(low) immature B lymphocytes than among terminal deoxynucleotidyl transferase+ (TdT+) pro-B cells, small nondividing B220+ sIgM- precursors, and surface IgD+ mature B lymphocytes. During short-term culture, apoptotic cells, identified by both DNA content and in situ DNA strand break labeling, increased linearly with time without macrophage ingestion, providing an assay for the rate of entry into apoptosis. B220+ B lineage cells accumulated in apoptosis more rapidly than cells of other lineages. The apoptotic rate was greater among B220+ sIgM- precursor cells than sIgM+ B cells, and was highest among B220+ mu- pro-B cells. Coculture with stromal cells reduced the apoptotic rate of B220+ sIgM- precursors to a greater extent than that of sIgM+ B lymphocytes. The results lead to estimates of the actual number of B lineage cells undergoing apoptosis per unit time in successive differentiation compartments. The findings indicate that, although influenced by local microenvironmental factors, apoptotic cell death occurs most markedly at two developmental stages associated with Ig heavy chain gene rearrangement and Ag receptor expression, respectively.