Regulation of growth and proliferation in B cell subpopulations.

The activation of B lymphocyte subpopulations by anti-immunoglobulin and by LPS has been examined. All resting B cells were stimulated to grow larger (i.e. to go from G0 phase to mid G1 phase of the cell cycle) by the continuous presence of anti-mu antibodies. A subpopulation oif these B cells, 30-50% in normal mouse strains, entered S phase in response to large doses of anti-mu. This subpopulation, probably Lyb5+, was completely absent in mice with the xid-determined immune defect. Another, apparently distinct subpopulation, comprising about 25% of the cells, and probably present in xid mice, was sensitive to a proliferative signal delivered by LPS, if the cells had first been cultured for 24 h in the presence of a dose of anti-mu that was sufficient to cause cell enlargement. The fraction of B cells responding to LPS in this way was significantly larger than the fraction responding to LPS alone, suggesting that anti-mu is superior to LPS at inducing the G0 to G1 transition. Based on these results we propose a model of the control of B cell growth and proliferation. Anti-Ig antibodies, or epitopes on conventional antigens, combine with and cross-link B cell receptors, causing the cells to enter G1 and to develop sensitivity to late G1 stimuli, which determine whether they will then enter S phase. These stimuli are provided either by a high dose of anti-mu or by LPS. These agents may work directly or may stimulate other cells to produce B cell Growth Factor (BCGF) and/or related regulatory molecules which may be the actual late G1 stimuli. Distinct B cell types are sensitive to distinct mechanisms for control of proliferation. A new monoclonal antibody, 14G8, which recognizes only a fraction of B cells (30% in normal mice and about 65% in xid mice), was used to separate B cell subpopulations based on the presence or absence of the cell surface antigen recognized by this antibody. The results suggest that 14G8 expression is negatively correlated with Lyb5 expression, although not absolutely. Indeed 14G8+ B cells respond quite well to anti-mu (32% the cells enter S phase). Since Lyb5- B cells are believed not to proliferate in response to anti-mu, this would suggest that a sizeable fraction of the 14G8+ B cells are also Lyb5+. The 14G8+ and 14G8- B cell subpopulations were found to be functionally distinct in that the former responded very well to LPS, whereas the latter responded very poorly. Models of B cell development based on expression of these membrane antigens are presented.