EFFECT OF A BCL-2 TRANSGENE ON PRODUCTION AND LOCALIZATION OF PRECURSOR B-CELLS IN MOUSE BONE-MARROW

Many B cell precursors die while differentiating in mouse bone marrow. To ascertain the mechanisms involved in this process, populations of B lineage cells and their tissue localization were analyzed in bone ma rrow of transgenic mice overexpressing the apoptosis inhibitor, Bcl-2. Immunofluorescence labeling and mit otic arrest were used to quantita te the number and proliferative activity of u(-) pro-B cells (terminal deoxynucleotidyl transferase [TdT](+)B220(-), TdT(+)220(+), and TdT(- )B220(+)); pre-B cells (c mu(+)); and B cells (s mu(+)). Mature B cell s (IgM(+)IgD(+)) were increased 16- to 20-fold. In addition, immature B lymphocytes (IgM(+)IgD(-/low)), representing newly formed cells, wer e increased three- to sixfold, whereas pre-B cells and late pro-B cell s were increased 30 to 60% in production rate. Earlier pro-B cells exp ressing TdT were unaffected. In spleen, both mature and immature B cel ls were greatly increased, but cells of precursor phenotype were few a nd TdT(+) cells were absent. The in vivo location of B cells was exami ned by autoradiography using light and electron microscopy after intra venous injection of I-125-labeled antibodies. B lineage cells (B220(+) ) were increased throughout bone marrow, often within dilated venous s inusoids, particularly in subosteal regions. Many intravascular and pe risinusoidal cells were IgD(high) mature B lymphocytes. In contrast, m any other IgM(+) and IgD(low) immature B lymphocytes clustered extrava scularly around the central venous sinus. Plasma cells with distended endoplasmic reticulum were numerous. These findings provide evidence t hat, in addition to expanding the recirculating pool of B cells enteri ng bone marrow from the blood stream, high levels of Bcl-2 can inhibit some of the apoptosis occurring during B cell differentiation, thereb y expanding populations of B lymphopoietic precursor cells within the bone marrow parenchyma.