MSCs protect CML CD34⁺CD38⁻ and CD34⁺CD38⁺ cells from TKI treatment. Primary CML and normal (NL) progenitor (CD34⁺) cells were stained with CFSE. CFSE⁺ primitive cells (Lin⁻CD34⁺CD38⁻) and committed cells (Lin⁻CD34⁺CD38⁺) were sorted by flow cytometry and cultured for 96 hours with or without MSCs and were treated with IM (5 μm), nilotinib (Nil; 5 μm), or dasatinib (Das; 0.15 μm) or were left untreated. The percentages of apoptotic primitive cells (A) and committed cells (B) were assessed on the basis of Annexin V⁺ labeling. A proliferation index was calculated based on reduction in CFSE levels for CML and NL CD34⁺CD38⁻ cells (C) and CD34⁺CD38⁺ cells (D) treated with IM, nilotinib, or dasatinib compared with controls (Ctrls) with and without MSCs. (E) Representative flow cytometry plots and (F) graph showing apoptosis (Annexin V⁺ cells) within undivided (CFSE bright) and dividing (CFSE dim) CML CD34⁺CD38⁻ cells treated with IM with and without MSCs. (G) Percentages of undivided (CFSE bright) CML CD34⁺CD38⁻ and CD34⁺CD38⁺ cells after culture with or without IM treatment and with or without MSCs. (H) CML CD34⁺CD38⁻ and CD34⁺CD38⁺ cells cultured for 96 hours with or without MSCs with or without treatment with IM (5 µm), nilotinib (5 µm), or dasatinib (0.15 µm) were plated in methylcellulose progenitor assays, and colony-forming capacity (CFC) frequencies were determined. (I) Representative flow cytometry plot and (J) graph showing CD34⁺ expression after culture of CML CD34⁺ cells with or without IM treatment and with or without MSCs. ns, not significant. n = 3. *P < .05.