Figure 1. Based on our results we hypothesize that homeostatic pressures regulate the cellular composition of stem and B lineage cell compartments in aging. We suggest that these homeostatic pressures are set by the long lived B cells accumulating in the periphery with aging to alter the stem cell compartment and to suppress B lymphopoiesis. Changing the homeostatic equilibrium by depletion of B cells in old mice, increases frequencies of lymphoid-committed stem cells to reactivate B lymphopoiesis and to increase B cell output from the BM to the periphery. The nature of these cross-talk homeostasis mechanisms is yet to be defined, but our results suggest that they are sensed by the stem cell compartments.