ULBP-BB4–dependent NK cell stimulation in the autologous setting. (A) Primary malignant plasma cell samples from were incubated with IL-6 (10 ng/mL) and used for an europium release assay with PBLs from the same patient in different effector-target ratios in the presence of BB4 or ULBP2-BB4. Lysis values were measured in triplicates, and the means ± SD are indicated. (B) Europium release assay using MM cells and PBLs from another patient with 2 independent labeled target cell samples. The absolute counts of the spontanous release (SR) and the release of cells in the presence of BB4 or ULBP2-BB4 are indicated as well as the percentage lysis of the maximum release (100 × [experimental release - spontanous release]/[maximum release - spontanous release]). The spontanous release of labeled primary tumor cells did not exceed 25% of the maximum release.