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Growth and maturation requirements of human B cell precursors.

The present study describes our efforts to induce the proliferation of human B cell precursors (BCP). Committed BCP (CD10+, sIgM-) isolated from fetal bone marrow (18-25 weeks) were induced to proliferate at low levels in the presence of IL7. IL3 potentiated this effect of IL7 on BCP, while IL4 partially inhibited this proliferation. However, neither of these cytokines allowed the emergence of mature B cells. The growth of BCP was strongly potentiated by the presence of an adherent fibroblastic bone marrow stromal layer devoid of cells of hematopoietic origin. Addition of IL7 to such cocultures further increased BCP proliferation. BCP were shown to proliferate as stroma-adherent and non adherent cells. Total cell numbers expanded during 3 weeks, as much as 8 fold in the presence of IL7 when compared with input BCP numbers. Finally, BCP remained sIgM- in stroma dependent cultures, and only a subpopulation of cells became CD20+ in the presence of IL7. Our present study demonstrates the feasibility of human BCP expansion in vitro. However, the signals required for the transition of BCP to mature B cells remain to be determined.