Knock-down of surface β₂M/MHC class I abrogates β₂M-specific mAb– but not dexamethasone-induced apoptosis in myeloma cells. (A) Levels (mean fluorescence intensity) of surface β₂M, HLA-ABC and CD38 on untreated myeloma cells and on cells treated with mock transfection or transfected with control (ctr) siRNA or β₂M-specific siRNA. Shown are the results of the MM.1S myeloma cell line. Similar results were obtained from ARP-1. Analysis was performed 72 hours after transfection. (B) Apoptosis of myeloma cells (ARP-1 and MM.1S) transfected with control (ctr) siRNA or β₂M-siRNA in cultures with β₂M-specific mAb D1 (50 μg/mL) or dexamethasone (10 μM) in the presence or absence of IL-6 (10 ng/mL). In these experiments, cells were transfected with 400 nM siRNA, and 72 hours later, washed and incubated with β₂M-specific mAbs or dexamethasone for another 48 hours. Apoptosis was detected by Annexin-V–binding assay. Results of 4 experiments performed are shown. Similar results were obtained from other myeloma cell lines and anti-β₂M mAb E6. Error bars indicate SD.