BMSCs and BMSC-sup reduce CD38 expression on MM cells. (A) RPMI 8226 and MOLP-8 cells were cocultured with or without 2 different bone marrow stromal cells (BMSC #1 or #2) for 5 days. After incubation, CD38 expression was measured by flow cytometry, and relative CD38 MFI was calculated in comparison with MFI of control. (B) CD138⁺ primary MM cells from 15 MM patients were cocultured with or without autologous BMMCs for 5 days. After incubation, CD38 expression on 7-AAD⁻ and CD138⁺ MM cells was measured by flow cytometry, and relative CD38 MFI was calculated in comparison with MFI of MM cells cultured without BMMCs. (C) RPMI 8226, MOLP-8, H929, and MM.1S cells were cultured with control culture medium (control, n = 3 for RPMI 8226, and n = 2 for the other cell lines) or BMSC-sup from different MM patients (n = 17 for RPMI 8226, and n = 6 for the other cell lines) for 72 hours. CD38 expression on 7-AAD⁻ cells was measured by flow cytometry, and relative CD38 MFI was calculated in comparison with MFI of control. (D) Representative histograms of CD38 expression of panel C. (E) Relative CD38 MFIs were measured after MM cells were cultured with control medium or 2 different BMSC-sup (BMSC-sup #1 or #2) for 24 to 168 hours. (F) Histograms of CD38 expression in RPMI 8226 and MOLP-8 cells cultured with control medium, BMSC-sup #1 or #2 for 168 hours. Data are shown as mean plus or minus standard error of the mean. *P < .05; **P < .01; ***P < .001.