Immature DCs phagocytose apoptotic MM bodies. (A) U266 cells were irradiated and stained with annexin-V–FITC and PI after 0, 2, 4, 6, 8, and 12 hours incubation at 37°C. Early apoptotic cells were defined as annexin-V–FITC⁺ and PI^– using flow cytometry. Results are representative of experiments with 3 MM cell lines. (B) U266 cells labeled red with Vybrant Dil Cell-Labeling Solution were incubated 4 hours at 37°C after 30-Gy irradiation to allow apoptosis to occur and then were cocultured with immature DCs stained green with Vybrant DiO Cell-Labeling Solution at a ratio of 1:1 for 0, 2, 4, 6, and 8 hours at 37°C or 4°C. Cells were analyzed by flow cytometry and double-positive cells indicate uptake of apoptotic cells by immature DCs. Culturing at 4°C blocked phagocytosis of apoptotic bodies by immature DCs.