Hypoxia enhances T-ALL chemoresistance. (A-B) Effect of vincristine on T-ALL resistance in high and low O₂. (A) Numbers of live cells recovered after treatment during 72 hours with 10 nM (+vincristine) or without (Ø) vincristine. Mean ± SEM of triplicate cultures are represented. Shown are mean ± SEM of cultures with T-ALL #1, T-ALL #, T-ALL #3, T-ALL #4, and T-ALL #5, n = 9 experiments. (B) Same result is presented in percentage of live cells after treatment (+vincristine in panel A) compared with nontreated (Ø in panel A) cells. Every experiment was done in triplicate. Every dot is the mean of those triplicates with T-ALL #1, n = 3exp; T-ALL #2, n = 5exp; T-ALL #3, n = 3exp; T-ALL #4, n = 2exp; and T-ALL #5, n = 1exp. (C) Leukemic cell production after 7 days in normoxia without treatment, following T-ALL treatment with vincristine in normoxia or hypoxia. Every experiment was done in triplicate. Shown are mean ± SEM of cultures with each T-ALL. (D) Propagating activity of T-ALL treated ex vivo with vincristine in high or low O₂ levels. Shown is a kinetic analysis (4, 6, and 8 weeks) of leukemia development in BM after transplantation of 500 leukemic cells of T-ALL #2 recovered from normoxic or hypoxic culture with vincristine treatment, represented in percent of leukemic cells. (i) Representative mouse. (ii) Analysis of 5 mice/group from T-ALL #2. (E) Survival of mice transplanted with 500 cells precultured in normoxia or in hypoxia with vincristine (+Vinc). T-ALL #1, T-ALL #2, and T-ALL #3, 4-5 mice per condition. Statistics were determined using the Friedman test and the log-rank (Mantel-Cox) test for mice survival: *P < .05, **P < .01, ***P < .001. exp, experiment.