NK cell–mediated cell lysis of primary MM cells is enhanced through ULBP2-BB4. (A-D) Primary malignant plasma cell samples from 3 patients (A-C) or peripheral blood lymphocytes from a healthy donor (D) were used as target cells for an europium release assay with purified NK cells in different effector-target ratios in the presence of ULBP2-BB4 or BB4. (E) The killing of malignant plasma cells of patient no. 3 was determined (effector-target ratio, 10:1) with or without blocking of the NKG2D receptor using an α-NKG2D antibody. Note that the NK cells used in panels A-E are derived from independent donors. Lysis rates were measured in triplicate, and the means with standard deviation are indicated.