Generalized impairment in NK signaling in patients with MDS. (A) Using K562, 721.221, and MDS1 cells as the target, antibody blocking experiments were performed using PBMCs from healthy donors at a 50:1 effector-target (E/T) ratio in the presence of medium alone or 5 μg/mL of the following antibodies: isotype control antibody, anti-NKG2D, anti-NKp30, anti-NKp46, anti-NKG2D plus anti-NKp30, anti-NKG2D plus anti-NKp46, and anti-NKp30 plus anti-NKp46. (B) Direct cytotoxicity of K562, MDS1, and 721.221 tumor cells in 5-hour ⁵¹Cr-release assays using PBMCs at a 50:1 E/T ratio from MDS group A patients with low K562 lysis (less than 13%) (n = 8), MDS group B patients with normal lysis of K562 (13% or more) (n = 11), and healthy control donors (n = 12). Horizontal lines represent the mean of that group. (C) Antibody blocking experiments were performed using PBMCs from MDS patients at a 50:1 E/T ratio. Effector cells were incubated in the absence of blocking antibodies (medium, black bars) or in the presence of 5 μg/mL isotype control antibody (hatched bars) and anti-NKG2D antibody (gray bars). NKL and NK92 cells were used at a 10:1 E/T ratio. The graphic representation of the percent specific lysis at a 50:1 effector-target (E/T) ratio is shown. The graphs represent the average of triplicate samples; SD is indicated by the error bars, and asterisks indicate statistical significance at P ≤ .01 as determined by a Wilcoxon rank sum test.