Effect of PS-341 plus TRAIL on purging tumor cells from the bone marrow. C1498 cells (10⁴ cells/mL) were added to bone marrow cells (5 × 10⁶ cells/mL), and these mixtures were incubated for 4 hours with PS-341 (20 nM) followed by a further 16 hours in the presence or absence of TRAIL (1000 ng/mL). (A) Dilutions of the cells from the various incubations were then put in media, and total tumor cell numbers (± SEM) were estimated at various time points using MTS staining as described in “Materials and methods.” ○ indicate C1498 cells alone; •, C1498 + bone marrow; ▵, C1498 + bone marrow + TRAIL (1 μg/mL); ▴, C1498 + bone marrow + PS-341 (20 nM); and ▪, C1498 + bone marrow + PS-341 (20 nM) + TRAIL (1 μg/mL). (B) Cells from the various incubations were transferred to irradiated C57BL/6 recipient mice as described in “Materials and methods.” □ indicate C1498 + bone marrow; ▵, C1498 + bone marrow + PS-341 (20 nM); ▿, C1498 + bone marrow + TRAIL (1 μg/mL); and ○, C1498 + bone marrow + PS-341 (20 nM) + TRAIL (1 μg/mL). Mice were continuously monitored, and survival was followed for 100 days after the bone marrow transplantations. **P < .001 comparing survival of mice receiving PS-341—treated bone marrow—tumor cell mixtures with controls; *P < .001 comparing PS-341 plus TRAIL—treated mixtures with PS-341 alone using the log-rank test. (C) At 95 days after bone marrow transfer, the percent of CD45.1⁺ donor-derived lymphoid cells in the spleen was determined by FACS analysis in (▪) mice receiving bone marrow but no tumor cells, (▴) surviving mice that had received bone marrow—tumor cell mixtures treated with PS-341 alone, and (•) surviving mice that had received bone marrow—tumor cell mixtures treated with PS-341 plus TRAIL. Values from individual mice from 3 independent experiments are shown with the bar on the right indicating the mean value.