Lethally irradiated mice reconstituted with adult thymus cells and neonatal or fetal liver cells produce an anti-2,4-dinitrophenyl or anti-bovine gamma globulin response of restricted heterogeneity of affinity in comparison with the response of mice reconstituted with B cells from adult donors. In addition, mice reconstituted with day 15 fetal B cells and adult thymus cells produce relatively few indirect plaque-forming cells (PFC). It was found that B cells acquire the capacity to produce a heterogeneous response, of predominantly indirect PFC within 7 days of transfer only when thymus cells are transferred along with the B cells. B cells from fetal or neonatal donors transferred without young adult thymus cells develop the capacity to generate indirect PFC within 13 days after transfer to adult recipients, but continue to produce a response of restricted heterogeneity of affinity for up to 28 days after transfer. Thus, it has been shown that cells present in the thymus facilitate, or are necessary for the functional maturation of B lymphocytes. Furthermore, the data suggest that maturation of the B-cell population to produce a heterogeneous response is controlled independently of its maturation to be capable of producing indirect PFC.

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