Independent homeostatic regulation of B cell compartments.

In the present study we used mice with a developmental arrest of B cell production to study the ability of a limited number of normal B cell precursors to populate peripheral B cell pools. In chimeras reconstituted with mixtures of bone marrow (BM) cells from normal and B cell-deficient donors, we show that the rate of BM B cell production is a constant function of the number of BM pre-B cells and is not modified by the peripheral B cell pool size, i.e. there is no feedback regulation of the central pre-B cell compartment by the number of mature B cells. We also show that the physiological number of peripheral B cells requires a minimum continuous input of newly formed cells, but is not determined by the number of B cell precursors. Chimeras with a threefold reduced rate of BM B cell production have normal numbers of peripheral B cells. Parabiosis between normal and B cell-deficient mice showed that the BM B cell production of one mouse suffices to replenish the B cell pool of three mice. Finally, we show that the compartment of activated IgM-secreting B cells is homeostatically autonomous since the number of cells it comprises is regulated independently of the size of the mature B cell pool. The results presented here support a model of the immune system in which the size of the different B cell compartments, i.e. pre-B, resting B and IgM-secreting, is autonomously regulated.