Decorin reduces the stimulatory effects of MSCs and osteoclasts (OC) on myeloma cell growth. (A,B) Myeloma cells from 6 patients were cocultured for approximately 5 days with MSCs infected with lentivirus expressing empty vector (EV) or decorin (DCN). At the end of each experiment, (A) myeloma cell viability was determined and (B) MTT assay was performed with the use of cells from 4 of those patients. Decorin-expressing MSCs demonstrated reductions in myeloma cell viability and growth. (C) Myeloma cells were cultured alone (n = 5) or cocultured with osteoclasts (n = 9) in the presence or absence of decorin (10 μg/mL). Osteoclasts supported long-term growth of myeloma cells (P < .005, MM vs MM + OC), and decorin inhibited this effect in 6 of 9 experiments (P < .007, MM + OC vs MM + OC + DCN).