hESC-derived NK cells demonstrate clearance of established K562 tumors xenografted in mice with higher efficacy compared with NK cells generated from UCB. NOD/SCID mice were inoculated with luc⁺ K562 tumor cells, and cells were allowed to engraft for 3 days before animals were given 1 systemic (intravenous) infusion of NK cells. All hESC- and UCB-derived NK cells were injected after 30 to 35 days of culture. Mice were monitored by bioluminescent imaging at days 0, 4, 7, 14, and 21. In addition, some mice demonstrating tumor regression were monitored long-term (up to 8 weeks) for tumor recurrence. (A) Representative in vivo bioluminescent images of animals at the day of tumor inoculation and 21 days after tumor inoculation. Non-NK cell–treated control animals did receive the IL-2/IL-15 cytokine regimen and typically developed large tumors as indicated by luciferase expression. Mice treated with UCB-derived NK (UCB-NK) cells typically display slower tumor progression. All mice treated with hESC-derived NK (hESC-NK) cells displayed a complete clearance of tumor cells. (B) Analysis of luciferase activity (photons/second) of individual mice and the mean activity in each treatment group. Luciferase activity was analyzed from the site of tumor inoculation in control (n = 20, ■), UCB-NK treated (n = 13, []), and hESC-NK–treated (n = 13, ●) mice. Error bars indicated SEM. *P < .01 for hESC-NK vs UCB-NK and control. (C) CD56⁺ NK cells were magnetically sorted and injected intravenously into tumor-bearing mice. Mice treated with either sorted CD56⁺ hESC-NK cells (n = 3, —) or sorted CD56⁺ UCB-NK cells (n = 2, ) demonstrate tumor regression similar to mice treated with unsorted hESC- or UCB-NK cells.