Influences on the lifespan of B cell subpopulations defined by different phenotypes

Abstract

The turnover of mature and immature B cells defined by a range of cell surface markers was investigated by feeding normal or bcl ‐2‐transgenic (bcl‐2‐Tg) mice 5'‐bromo‐2‐deoxyuridine (BrdUrd) for up to 6 weeks. In peripheral lymphoid tissue, B cells accumulated BrdUrd with a 50% labeling time of 4.3 weeks and a pattern of uptake indicative of the presence of both long‐lived and short‐lived cells. These two kinetic populations could be resolved into immature B220^lo/heat‐stable antigen (HSA)^hi cells which labeled rapidly, and B220^hiHSA^lo cells which were uniformly long‐lived with a half‐life of about 6 weeks. During loading and pulse‐chase experiments, BrdUrd uptake by cells within the mature B220^hiHSA^lo population clearly followed an exponential kinetic pattern, suggesting that their loss was governed by stochastic processes. Using other surface markers, the long‐lived population could also be defined by high expression of IgD, representing cells in the follicular mantle zone of the spleen, and by the phenotype IgM^hiIgD^loHSA^lo which most likely represented marginal zone memory B cells. CD23 expression on B cells did not differentiate well between long and short‐lived cells. Only about half of newly labeled B cells appearing in the spleen progressed to the long‐lived compartment, a proportion which was not altered significantly in bcl ‐2‐Tg mice. The most likely explanation was that a combination of both positive and negative selection was operating at this site which was mediated by pathways not regulated by bcl‐2. On the other hand, overexpression of bcl‐2 did result in a two‐to threefold increase in the rate of appearance of newly labeled B cells in the spleen, consistent with a possible role for this protein during early selection events within the bone marrow. Selection processes appeared to be very active in young mice during the shaping of the B cell repertoire, since B cells from 6‐week‐old non‐Ig mice displayed a rapid rate of turnover irrespective of their surface phenotype, and a significant population of long‐lived cells did not become evident until the mice had reached about 12 weeks of age.