Effects of IL-6 and IGF-I on β₂M-specific mAb– or dexamethasone-induced apoptosis in myeloma cells. (A) Apoptosis of ARP-1 and MM.1S myeloma cells in 48-hour cultures with β₂M-specific mAb D1 (50 μg/mL) or dexamethasone (10 μM) in the presence or absence of IL-6 (10 ng/mL) or IGF-I (50 ng/mL). Similar results were obtained with other myeloma cell lines. (B) Apoptosis of 2 IL-6–dependent cell lines XG1 and ANBL-6 in 24-hour cultures with β₂M-specific mAb D1 (50 μg/mL) or dexamethasone (10 μM) in the presence or absence of IL-6 (10 ng/mL). Flow cytometry analysis from (C) a representative patient with myeloma and (D) pooled data from all patients showing apoptosis of primary myeloma cells in 24-hour cultures with β₂M-specific mAb D1 (50 μg/mL) or dexamethasone (10 μM) in the presence or absence of IL-6 (10 ng/mL) or IGF-I (50 ng/mL). Similar results were obtained with primary myeloma cells with anti-β₂M mAb E6. Apoptotic cells were determined by Annexin-V–binding assay. Results of 3 experiments are shown. Error bars indicate SD. *P < .05; **P < .01.