IM-induced alterations in OP9 cells promote the interaction between OP9 cells and leukemic cells. (A) Microscopic visualization of cocultured OP9 cells and mCherry-labeled leukemic cells treated with vehicle (IM−) or IM for 4 days (top: phase contrast; bottom: mCherry fluorescence). (B) Quantification of leukemic cell clusters in (A). (C) Microscopic visualization of leukemic cell clusters formed within 2 hours after seeding of mouse BCR-ABL⁺ ALL cells onto OP9 cells pretreated with the vehicle (IM−) or IM for 4 days (top: phase contrast; bottom: mCherry fluorescence). (D) Quantification of leukemic cell clusters in (C). (E) Microscopic visualization of leukemic cell clusters formed within 2 hours after seeding of IRLCs onto OP9 cells pretreated with the vehicle (IM−) or IM for 4 days. (F) Quantification of leukemic cell clusters in (E). (G) Microscopic visualization of leukemic cell clusters formed within 2 hours after seeding of leukemic cells onto OP9 cells pretreated with a vehicle (control), dasatinib, sunitinib, or erlotinib for 4 days. Note that, like IM, dasatinib and sunitinib are TKIs, whereas erlotinib is an epidermal growth factor receptor inhibitor. (H) Quantification of leukemic cell clusters in (G). (I) Heat map showing the top 100 differentially expressed genes in OP9 cells treated with a vehicle (−) or IM for 32, 64, or 96 hours. Scale bars, 50 µm. Data are shown as the mean ± SEM. *P < .05 as determined by Student t test.