In vitro sensitivity of APL cells to chemotherapy. (A) Decreased apoptosis of APL^luc cells following culture with stromal cells. APL^luc cells (10³ cells/well) were cultured in 96-well black-walled tissue culture plates in the presence or absence of the murine bone marrow stromal cell line M2-10B4. After 48 hours, APL^luc cells were incubated for an additional 48 hours in medium containing 40 ng/mL cytarabine (Ara-C), 105 ng/mL Ara-C, 12 ng/mL daunorubicin (DNR), 40 ng/mL DNR, or vehicle alone (control). APL^luc cell survival was assessed by bioluminescent imaging and results are presented as a percent of the no stroma control. Each bar represents the mean plus or minus SD of 2 separate experiments, for which each sample was assayed in quadruplicate. (B) Decreased apoptosis of APL cells following culture in M2-10B4–conditioned medium. Cell-free culture supernatant was obtained from 3-day cultures of M2-10B4 stromal cells. APL cells (10³ cells/well) were incubated for 48 hours in unconditioned medium (no stroma) or M2-10B4–conditioned medium (+stroma) containing 40 ng/mL Ara-C, 105 ng/mL Ara-C, 12 ng/mL DNR, 40 ng/mL DNR, or vehicle alone (control). APL cell death was assessed by flow cytometry using annexin V–PE and 7-amino-actinomycin D. Each bar represents the mean ± SD of 2 separate experiments, for which each sample was assayed in triplicate. *P < .05; **P < .01; and ***P < .001.