The effect of cytokines on lymphoid lineage differentiation in vitro. (A) Addition of SCF, IL-3, and IL-15 promotes increased NK-cell differentiation. CD34⁺ hESC/S17 (left panels) and CD34⁺ UCB (right panels) cells were cocultured with OP9-DL1 stromal cells for 30 days. Cultures were supplemented with exogenous IL-7 and Flt3L (T-cell conditions) or with IL-7, Flt3L, SCF, IL-3, and IL-15 (NK-cell conditions). Cells were harvested and analyzed by flow cytometry for expression of CD4, CD8, and CD56. (B) To determine other sources of cytokines that could stimulate hESC-derived cells, the indicated mouse stromal cell lines were analyzed by RT-PCR for expression of the cytokines Scf, Flt3L, IL-7, and β-actin.