Inhibitory effect of B-B4–DM1 on proliferation of CD138⁺ and CD138^- cells adherent to BMSCs. (A) Ocy-My5 or SUDHL-4 cells (2 × 10⁴) were seeded on 70% to 80% confluent BMSCs for 24 hours. Cell proliferation was measured by [³H]thymidine ([³H]-TdR) incorporation following 72 hours of treatment with 10 nM B-B4–DM1. Values represent the mean [³H]-TdR incorporation (cpm) of triplicate cultures. Error bars indicate SD. (B) Patient MM cells were cultured on BMSC layers and exposed for 72 hours to the 10 nM B-B4–DM1 immunoconjugate. CD56 and CD138 expression was evaluated by flow cytometry. We have previously established that B-B4–DM1, even at a concentration as high as 240 nM, did not affect binding of PE-labeled anti-CD138 antibody (Syndecan-1 DL-101; Santa Cruz Biotechnology, Santa Cruz, CA) to CD138-expressing cells. Figure is representative of 2 experiments.