Hypoxia impacts T-ALL cell regrowth activity in vitro and in vivo. (A-B) Growth of T-ALL after hypoxia cultures have been stopped. (A) Leukemic cell numbers of T-ALL precultured in low and high O2 levels after replating 7 days at 21% of O2 with MS5-DL1 stromal cells. (B) Same except the secondary cultures were maintained for 28 days. Every experiment was done in triplicate. Shown are mean ± SEM of cultures with each T-ALL. (C) Engraftment levels of T-ALL after cultures in low and high O2 levels. (i) Representative flow cytometry analysis 8 weeks after transplantation of mouse #1 BM injected with T-ALL #2 precultured in normoxia. (ii) Kinetic of leukemia development in BM after transplantation of 500 cells of T-ALL #2 precultured in normoxia or hypoxia. Shown are percent of leukemic (human CD45+CD7+) cells. (D) Survival curves of mice transplanted with 500 leukemic cells isolated from cultures in normoxia or in hypoxia (4-5 mice per condition). Statistics were determined with the Friedman test and the log-rank (Mantel-Cox) test for mice survival: **P < .01, ***P < .001.