Regulation of human B lymphopoiesis by the transforming growth factor-β superfamily in a newly established coculture system using human mesenchymal stem cells as a supportive microenvironment

Objectives To characterize and evaluate the validity of a novel coculture system for studying human B-lymphocyte developmental biology. Materials and Methods We developed a long-term culture system to produce B lymphocytes from human CD34+ cells purified from umbilical cord blood using human mesenchymal stem cells (hMSC) as stroma. We evaluated the effects of several low molecular weight inhibitors, recombinant proteins, and neutralizing antibodies (Abs) as potential regulators of B-lymphocyte development. Results Our cocultures of 2000 CD34+ cells in the presence of stem cell factor and Flt3-ligand produced 1–5 × 105 CD10+ cells after 4 weeks of culture. Surface IgM+ immature B cells began to appear after 4 weeks. We evaluated the negative-regulatory effects of the transforming growth factor (TGF)-β superfamily on human B lymphopoiesis, and found that adding an anti–activin A antibody enhanced generation of CD10+ cells two- to three-fold. As well, the proportion of CD10+ cells in the generated cells increased markedly, indicating that activin A downregulated B lymphopoiesis more efficiently than myelopoiesis. Addition of TGF-β1 suppressed B-lymphocyte production by 20% to 30%, while addition of an anti–bone morphogenetic protein (BMP)-4 antibody or recombinant BMP-4 had no effect. Therefore, the strength of ability to suppress human B lymphopoiesis seemed to be activin A > TGF-β1 > BMP-4. None of these three factors influenced the emergence of IgM+ cells. Conclusions hMSC coculture supported human B lymphopoiesis. Activin A selectively suppressed B lymphocyte production.