IPS-MSCs impair NK cytotoxicity. (A) NK cells were tested in a 4-h ⁵¹Cr-release assay against K562 cells at different effector/target ratios. NK cells were cultured in medium alone (resting) or were supplemented with IL-2 (250 U/mL) (■), BM-MSCs (♦), H9-MSCs (●), or IPS-MSCs (▴; NK/MSC ratio, 4:1) before the assays. (B) The bar graphs represent the percentage release of ⁵¹Cr at 30:1 and 15:1 NK/K562 ratios. Results are representative of 5 independent experiments and are shown as the means ± SD of triplicate samples. Statistically significant differences between NK cells cultured with or with MSCs are indicated by asterisks. ***P < .001; **P < .01 compared with IL-2–activated NK cells alone.