Elastase gives a growth advantage to CML cells in cocultures of normal and CML CD34⁺ cells. Mixtures of 4 × 10⁴ normal (HLA-A2⁺) and CML (HLA-A2^-) CD34⁺ cells/mL were cultured in 1 mL serum-free medium in the presence of 10 ng/mL G-CSF, GM-CSF, and SCF with different concentrations of elastase. After 8 days of culture, nucleated cells were counted, harvested, and analyzed for BCR/ABL or HLA-A2 expression. CML and normal cells cultured separately under the same conditions were used as controls. (A) Percentage and (B) absolute number of BCR/ABL-positive and -negative cells in the presence of 0, 1, or 5 μg/mL elastase (▪, BCR/ABL^- cells; □, BCR/ABL⁺ cells). Data are representative of 2 independent experiments. (C) Percentage and (D) absolute number of HLA-A2-positive (normal) and -negative (CML) cells in the presence of increasing doses of elastase (▪, HLA-A2⁺ cells; □, HLA-A2^-). Data represent mean ± SEM of triplicates. CML (HLA-A2^-) and normal (HLA-A2⁺) CD34⁺ cells were sorted by size and granularity and cultured in the same conditions mentioned. After 8 days of culture, nucleated cells were counted, harvested, and analyzed for HLA-A2 expression by FACS. (E) Percentage and (F) absolute number of HLA-A2⁺ (normal) and HLA-A2^- (CML) cells in the presence of 0, 1, or 5 μg/mL elastase (▪, HLA-A2⁺ cells; □; HLA-A2^-). Data represent mean ± SEM of triplicates) and are representative of 2 independent experiments.