Derivation of NK cells from hESC capable of direct cytolysis of diverse human tumor cell lines. (A) Flow cytometric identification of CD56⁺CD45⁺ NK cells derived from hESC and UCB progenitor cells cultured in NK-cell conditions for 35 days. Histograms demonstrate that CD56⁺CD45⁺-gated NK cells express a repertoire of receptors important for regulating NK-cell activity, including CD16, C-type lectin-like receptors (CD94 and NKG2D), natural cytotoxicity receptors (NKp46), and killer-cell Ig-like receptors (CD158). (B) In vitro direct cytolysis was evaluated by standard 4-hour ⁵¹Cr-release assay after 35 days in NK-cell culture. NK cell–mediated killing was assessed by incubating effector cells with tumor targets from K562 (n = 7), MCF7 (n = 3), NTERA2 (n = 3), PC3 (n = 3), and U87 (n = 3) at indicated effector-to-target cell ratios. hESC-derived NK cells (■) demonstrate a significantly higher cytolytic activity against K562, MCF7, and PC3 tumor cells compared with UCB-derived NK cells (▲). Mean ± SEM is shown. *P < .05.