In Vitro Tolerance Induction of Bone Marrow Cells: A Marker for B Cell Maturation

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Abstract
B cell subpopulations have been defined by various morphologic criteria and differences in functional capacity. In a recent report from this laboratory, the in vitro splenic focus system was utilized to define a subpopulation of neonatal splenic B cells on the basis of their susceptibility to tolerance induction. Since this susceptibility to tolerance induction progressively decreases with age, tolerance susceptibility in this system has been attributed to the immaturity of the cells which constitute developing B cell clones. In the present study, the splenic focus technique was utilized to characterize the maturational state(s) of neonatal and adult bone marrow B cells as assessed by the susceptibility of these cells to tolerance induction, and to compare these responses to the responses of splenic B cells. The results demonstrate that the majority of neonatal bone marrow B cells reach maturity, by the criterion of tolerance susceptibility, later than splenic B cells. Furthermore, in contrast to the spleen, a portion (25%) of bone marrow B cells remain susceptible to tolerance induction even in the adult. Thus, the generative pool of B cells appears to shift from the spleen to the bone marrow after the first few days of neonatal life and resides primarily in the bone marrow thereafter. Finally, the susceptibility of B cells to tolerance induction may serve as a marker for the functional maturation of any given B cell population, and consequently may help to define distinct B cell subpopulations within the B cell pool.