Effect of human myeloma cells on CFU-F formation in human BM cultures. Human BM mononuclear cells were cocultured in the appropriate medium with (A) human myeloma cell lines (HMCLs; RPMI-8226, OPM2, XG-6, XG-1, U266, and JJN3) and (B) BM MM cells obtained from 4 different patients with MM (MM1, MM2, MM3, MM4), or with BM B lymphocytes from healthy donors. (C) In some experiments, HMCLs were incubated in the presence or absence of a transwell insert. CFU-F formation was evaluated after 14 days as described in “Patients, materials, and methods” and quantified. Graphs represent the mean number of CFU-Fs/well ± SD of 6 independent experiments performed in triplicate. (D) Alkaline phosphatase staining was performed in BM cocultures at higher magnification (images were obtained on a Nikon Eclipse TE 300 microscope at 10×/18 using a DS-U1 digital sight and a 4×/0.12 objective lens. (Nikon Instruments). (original magnification, × 5). C indicates control; ^*P < .01; ^**P < .001.