Expression of a bcl-2 transgene reduces proliferation and slows turnover of developing B lymphocytes in vivo.

B lymphocyte differentiation proceeds through a series of alternating stages of proliferative expansion interspersed with noncycling stationary phases during which cells undergo either positive selection or apoptotic cell death. The molecular control of cell cycle progression and that of apoptosis appear to be interconnected. Overexpression of Bcl-2 in lymphocytes or fibroblasts antagonizes apoptosis and delays their transition from the quiescent state into the cell cycle. We have undertaken a systematic analysis of the impact of bcl-2 transgene expression on cell cycle distribution and turnover rate of developing B lymphocytes in normal mice and in mutant animals in which B cell differentiation is arrested at the pro-B/pre-BI or the pre-BII stage. These experiments revealed that overexpression of Bcl-2 reduces proliferation and slows turnover of B cells at all stages of development. This demonstrates that Bcl-2 can retard transition of B cells between the quiescent and the cycling state regardless of the mitogenic stimulus and the differentiation stage. The implications of these results for the normal control of B lymphopoiesis and for lymphomagenesis are discussed.