Macrophages protect myeloma cells from apoptosis. (A) Dot plots showing apoptotic myeloma (MM.1S) cells in cultures or cocultures with nMφs or TCCM-treated Mφs (tMφ) in the presence or absence of dexamethasone (Dex) or melphalan (Mel). Numbers inside dot plots indicate the percentages of live cells. (B) Apoptotic myeloma cells (percentage of melphalan control, which is 100% of apoptotic cells) in cocultures with either nMφs or tMφs in the presence of melphalan. Four commonly used myeloma cell lines, ARP-1, MM.1S, RPMI-8226, and U266, were tested. (C) Percentages of apoptotic myeloma cells in cocultures without or with tMφs either in direct contact (MM + tMφ) or separated by Transwell inserts (MM + tMφ, Transwell) in the presence of melphalan. Culture of myeloma cells in medium and coculture of myeloma cells with tMφs served as controls. (D) Effects of anti–ICAM-1 antibody on blocking tMφ-mediated myeloma apoptosis protection. Shown is the percentage of apoptotic myeloma ARP-1 cells pretreated with 10 μg/mL anti–ICAM-1 antibody or mouse IgG, in (co)cultures with tMφs in the presence of melphalan. Culture of myeloma cells in medium and coculture of myeloma cells with tMφs served as controls. Similar results were obtained with other myeloma cell lines. (E) Percentages of spontaneous apoptotic primary myeloma cells in culture medium only (primary MM) or in cocultures with tMφs (primary MM + tMφ) at 24 hours and 48 hours after isolation of the myeloma cells. Representative results from experiments with primary myeloma cells from one patient of 4 examined are shown. (F) Percentage of apoptotic myeloma ARP-1 cells in coculture with Mφs generated from MM patients in the presence of melphalan. Representative results from experiments with Mφs from 1 patient of 3 examined using this and other cell lines are shown. *P < .05; **P < .01.