Interleukin 7 independent development of human B cells

Mammalian hematopoietic stem cell (HSC) commitment and differentiation into lymphoid lineage cells proceed through a series of developmentally restricted progenitor compartments. A complete understanding of this process, and how it differs from HSC commitment and differentiation into cells of the myeloid/erythroid lineages, requires the development of model systems that support HSC commitment to the lymphoid lineages. We now describe a human bone marrow stromal cell culture that preferentially supports commitment and differentiation of human HSC to CD19⁺ B-lineage cells. Fluorescence activated cell sorter-purified CD34⁺⁺/lineage^- cells were isolated from fetal bone marrow and cultured on human fetal bone marrow stromal cells in serum-free conditions containing no exogenous cytokines. Over a period of 3 weeks, CD34⁺⁺/lineage^- cells underwent commitment, differentiation, and expansion into the B lineage. Progressive changes included: loss or CD34, acquisition of and graded increases in the level of cell surface CD19, and appearance of immature B cells expressing μ/κ or μ/λ cell surface Ig receptors. The tempo and phenotype of B-cell development was not influenced by the addition of IL-7 (10 ng/ml), or by the addition of goat anti-IL-7 neutralizing antibody. These results indicate a profound difference between mouse and human in the requirement for IL-7 in normal B-cell development, and provide an experimental system to identify and characterize human bone marrow stromal cell-derived molecules crucial for human B lymphopoiesis.