Hypoxia enhances the colony forming capacity of committed CML progenitors. (A) Schematic illustrating the experimental procedure used to assay the effect of hypoxia and imatinib on progenitor cells. CML progenitors were exposed to imatinib with or without hypoxia in serum-free media supplemented with growth factors for 96 hours. Cells were then harvested, washed, and assayed for CFCs, LTC-ICs, engraftment potential, and other assays, as described in the text. (B) Representative images taken at 14 days of a colony-forming assay, after a 96-hour treatment period. (C) CML (n = 6 and 5) cells obtained from peripheral blood or BM and normal cord blood cells (n = 3) (unselected or CD34⁺ selected) were treated under 21% or 0.5% O₂ for 96 hours. The cells were washed and plated in drug-free methylcellulose for 2 weeks. The numbers of CFCs are expressed as percentages of the untreated group (0 µM imatinib) incubated at 21% O₂ and are indicated on the top of each column. For experiments performed with CP CD34⁺ cells, the CFC frequency per 1000 CD34⁺ cells is 185.5 ± 29.9 (supplemental Table 1). Error bars show standard of the mean. *P < .05 and **P < .01 denote Student t test statistical significance over 21% O₂ without imatinib controls.