The maturation of B-lymphocyte function during fetal development was studied in vivo and in an in vitro organ culture system. The results indicated that the progenitors for 2,4-dinitrophenol (DNP)-specific B cells are present as early as 14 d of gestation in liver and possibly as early as 15 d in spleen. In addition, it was found that the organ culture system supports the development of B lymphocytes as measured by an increase in both the percentage of surface immunoglobulin-positive cells and the frequency of clonable DNP-specific B cells after culturing. The majority of anti-DNP-secreting clones resulting from the antigenic stimulation of fetal B cells produced only the IgM isotype, and the ability to secrete the IgG isotypes increased as a function of gestational age. Because fetal DNP precursors from spleens and livers that had been incubated in organ culture resulted in a greater proportion of clones secreting IgG compared with age-matched uncultured controls, it was concluded that the maturation with regard to the ability to secrete IgG can occur in vitro. In studies relating to the ontogenetic development of tolerance susceptibility, it was found that up to one-half of the DNP-specific B-cell precursors from livers and spleens less than 18 or 19 d of gestation were resistant to tolerogen treatment for 24 h as if in a pretolerant phase. However, if tolerogen were present for 3--5 d during organ culture there was near total elimination of potential DNP clones. This finding suggested that the 24-h induction period was insufficient for affecting the DNP-specific precursors in livers and spleens from the earlier gestational ages, and that a proportion of precursors could subsequently form DNP clones in the splenic focus assay after the removal of tolerogen.

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