Inhibition of NK cell-mediated killing with mAbs to ULBP on K562 cells. (A) Monoclonal antibody to ULBP2 dose-dependently inhibited the killing of GPI⁺ K562 cells alone. (B) Killing of GPI⁺ cells was inhibited with 25 μg/mL mAbs to ULBPs 1 and 2 but none of mAb to ULBP3. The mixture of mAbs to ULBPs 1 and 2 completely abolished the difference in the killing between GPI⁺ and GPI^- cells. Each value represents the mean (± SD) of triplicate assays (E/T ratio, 16:1). □ indicates GPI⁺ K562 cells; ▪, GPI^- K562 cells; and Control, isotype-matched control Ig. ^*P < .01 compared with antibody-free group.