B cell precursors in senescent mice exhibit decreased recruitment into proliferative compartments and altered expression of Bcl-2 family members.

In vitro, aged pro-B cells generally exhibit limited expansion in response to IL-7 when compared to young pro-B cells. CFSE-labeling in vitro indicated that aged mice have a lower frequency of pro-B cells which are capable of undergoing extensive proliferation upon stimulation with exogenous IL-7. Protein levels of the survival molecule Bcl-x(L) were consistently reduced in IL-7 expanded aged pro-B cells. Levels of both Bcl-2 and Baxalpha proteins were variable in aged B cell precursors. The expansion of aged pro-B cells in response to IL-7 in vitro correlated inversely with the ratio of the pro-apoptotic protein Baxalpha to the survival protein Bcl-2. Expansion of aged pro-B cells in vitro is likely dictated by both recruitment of pro-B cells into proliferative compartments and their survival; in aged B cell precursors, the latter is favored by low Baxalpha to Bcl-2 protein ratios.