Survival and proliferation of primary CD138⁺ MM cells in culture. (A) Primary CD138⁺ MM cells were cultured for 72 hours in RPMI with either 10% FCS or 20% plasma pooled from MM patients (CM). Viable CD138⁺ cells after 72 hours, expressed as a percentage of the initial viable CD138⁺ population (mean ± SEM, n = 38; 6 of whom had circulating plasma cells). (B) CD138⁺ cells were cultured in RPMI alone (0%), RPMI/10% FCS, or CM for 72 hours. Viability was determined using annexin V/PI staining and FACS analysis, viable fraction was annexin V–/PI-negative (n = 14, 1 with circulating plasma cells). (C) Four representative cases from panel B are shown (percentage indicates viable fraction). (D) Absolute number of CD138⁺ cells in S/G₂M (proliferative fraction) at the time of isolation (0 hours) and after 72 hours culture in either 10% FCS or CM. Data are presented as mean ± SEM (n = 34, 6 with circulating plasma cells).