Analysis of the sensitivity of normal and CML Lin⁺CD34⁺, Lin⁻CD34⁺, and Lin⁻CD34⁻ cells to IM treatment in vitro. Assessment of the clonogenic activity of normal and CML Lin⁻CD34⁺ (A) and Lin⁻CD34⁻ (B) cells upon treatment with increasing concentrations (0.1μM, 1μM, and 10μM) of IM for 48 hours. The results are expressed as mean values ± SD from 4 independent experiments. The clonogenic efficiency of normal CD34⁺ and CD34⁻ cells was 2.85% ± 0.7% and 0.27% ± 0.02%, respectively. The clonogenic efficiency of CML CD34⁺ and CD34⁻ cells was 4.3% ± 1.3% and 0.68% ± 0.24%, respectively. Also in this set of experiments, the clonogenic efficiency of CML progenitor/stem cells was significantly higher (P < .05) than that of normal counterparts; *P < .05 in CML versus normal cells. Quantitative analysis of BCR-ABL transcript in CML Lin⁻CD34⁺ and Lin⁻CD34⁻ upon treatment with increasing concentrations of IM for 48 hours (C). The results are expressed as mean values ± SD from 3 independent experiments. The number of cells analyzed for BCR-ABL transcript was always > 10⁵. The range of ABL copies for CD34⁺ and CD34⁻ cells was 1737-25 899 and 1000-38 868, respectively. Samples with less than 1000 ABL copies were not considered evaluable and were not analyzed; *P < .05 in untreated versus IM-treated cells. Results of flow cytometric detection of CrkL phosphorylation levels in normal (n = 5) and CML (n = 5) Lin⁺CD34⁺, Lin⁻CD34⁺ and Lin⁻CD34⁻ cells (D). CrkL phosphorylation levels were normalized with respect to untreated CML Lin⁺CD34⁺ samples, set as MIF = 1. The results are expressed as mean values ± SD; *P < .05 in CML versus normal cells. (E) CML untreated control Lin⁺CD34⁺ cells PCrkL MIF levels were similarly set to 1 to compare CML subpopulations (n = 5 experiments) before and after the treatment with IM 5μM for 16 hours. The results are expressed as mean values ± SD; *P < .05 in untreated versus IM-treated cells. MIF indicates mean fluorescence intensity; IM, imatinib mesylate.