In rat B lymphocyte genesis sixty percent is lost from the bone marrow at the transition of nondividing pre‐B cell to sIgM+ B lymphocyte, the stage of Ig light chain gene expression

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Abstract
The cycling B precursor cells in rat bone marrow (BM) that carry the B220 antigen and no surface Ig daily produce 780 million new cells. The pool of recirculating B lymphocytes in the rat, however, renew at a rate of only about 40 million cells/day. To analyze at which stages in B lymphocyte genesis the cell loss occurs, we identified post‐mitotic cells in the rat BM B lineage, and determined their renewal rates. We used 5‐bromo‐deoxyuridine (BrdUrd) to label DNA‐synthesizing cells, identifying incorporated BrdUrd with the mouse monoclonal antibody BU‐1. B lineage cell subsets were identified by the markers HIS24 antigen (rat B220), terminal deoxynucleotidyl transferase (TdT), Ig μ heavy chain, and complete Ig. By use of double and triple immunocytology, we determined the extent of BrdUrd incorporation in the various B lineage compartments [HIS24+TdT−Ig−, TdT+, cytoplasmic μ chain (cμ)+ surface (s) IgM− pre‐B, sIgM+ B]. Both sIgM+ B lymphocytes and all B precursors with cell diameters < 11–12 μm were virtually devoid of DNA synthesis, as indicated by S‐phase indices below 2%. In contrast, S‐phase indices of large B precursors ranged between 43%–66%.We established the renewal rates of nondividing BM B lineage cells by placing osmotic minipumps containing BrdUrd subcutaneously in the flank of rats. The nondividing BM B lineage cells all renewed rapidly at rates between 2.4% and 5.6%/h, representing average half‐lives of 29 to 12 h. In absolute numbers, the renewal/day/whole body BM was 165 × 106 for sIgM+ B lymphocytes, 422 × 106 for small cμ+ sIgM− pre‐B cells, 89 × 106 for small TdT+ cells and 35 × 106 for small HIS24+TdT−Ig− cells. Assuming that recirculating B lymphocytes in the periphery are the descendants of BM sIgM+ B lymphocytes, which in their turn are the progeny of small pre‐B cells, the renewal data indicate the following. Of the 165 million potentially available BM B lymphocytes, only 40 million cells become incorporated in the pool of recirculating B lymphocytes, representing a loss of 75%. BM B lymphocytes, in turn, use only (165/422 × 100% =) 40% of the potential output from their immediate precursors. The 60% loss that occurs here may reflect the extent of aberrant Ig light chain gene rearrangement in normal B lymphocyte genesis. In the earlier stages in B lymphocyte genesis, many of the 780 × 106 new cells produced by proliferating precursors apparently do not reach the small pre‐B compartment (which takes up only 422 × 106 new cells/day). The overall loss amounts to a total of 95%, with [(780‐165)/780 × 100% =] 80% in the BM and [(165‐40)/780 × 100% =] 15% in the periphery. We discuss mechanisms potentially underlying the loss at the various stages in B lymphocyte development.