Population dynamics of B lymphocytes and their precursors: demonstration of high turnover in the central and peripheral lymphoid organs.

The population dynamics of B lymphocytes and their precursors was followed in normal, adult mice, after killing dividing cells with high doses of hydroxyurea (HU), by immunofluorescence techniques identifying mature B cells (slgM+), lymphoid cells that show perinuclear staining with rhodamine-conjugated anti-p antibodies (slgM+), and lg-negative precursors that express membrane structures cross-reactive with “public” antibody idiotypes (Id+, slgM+). In addition, functional reactivities were analyzed by determining in limiting dilution the frequency of mitogen-reactive B cells competent to be activated to clonal proliferation and IgM secretion by 2 different mitogens. HU treatment results in a sharp decrease in the total number of B cells in all organs, and it depletes, in an average of 5 experiments, 80 to 90% of the starting population by 24 to 48 hr in bone marrow (BM), 60 to 80% by 24 to 48 hr in the thoracic duct (TD), and 50 to 60% by 48 to 72 hr in the spleen (SP). Among all BM cells with markers of the B lymphocyte lineage, the greatest depletion was observed in the Id+, IgM− set, and the least affected were IgM+ cells. Both these and mature IgM+ cells, although reduced in absolute numbers, actually showed increased frequencies in BM after treatment, whereas the frequencies of IgM+ cells dropped in TD and remained fairly constant in SP. Lipopolysaccharide- (LPS) and lipoprotein- (LP) reactive B cells were also drastically reduced, to a range of 20 to 30% of control numbers by 48 to 72 hr in BM and TD, and to 50 to 60% by day 4 or 5 in SP. The relative frequencies of mitogen-reactive cells, however, were unchanged or slightly enhanced in HU-treated animals, suggesting that this B lymphocyte population has an average lifetime that is similar to or only slightly longer than that of the majority to B cells present in these organs. These results demonstrate that the majority of peripheral B lymphocytes or their immediate precursors in normal mice have recently divided. Since, on the other hand, not more than 1% of such B cells are large (cycling) blasts, this conclusion suggests that the large numbers of B cells continuously generated in BM are in fact exported to the periphery.