B-cell–promoting culture of UCB- and hESC-derived hematopoietic progenitors. (A) CD34⁺ UCB cells (○, ▵, □) proliferate in B-cell promoting culture but CD34⁺ hESC-derived cells (●, ▴, ■) do not. Hematopoietic progenitors were seeded into coculture on MS-5 stromal cells in RPMI 1640 containing 10% FBS, 1% P/S, and 10 ng/mL each of SCF and G-CSF (Reg: ○,●), or additionally supplemented with 10 ng/mL IL-7 (Reg + IL-7: ▵, ▴), or with 10 ng/mL IL-7 and Flt3L (Reg + IL-7 + Flt3L: □, ■). Hematopoietic cells were harvested after 13, 20, or 35 days and live cells counted to determine proliferation. (B) CD34⁺ UCB cells form B cells in MS-5 culture but CD34⁺ hESCs do not. Cells cultured for 35 days on MS-5 cells plus SCF and G-CSF were collected and analyzed for expression of human CD45 and CD19 by flow cytometry. Similar phenotypic results were seen with the other cytokine conditions in panel A.