Inhibition of DOX-induced apoptosis by CD40L L cells in lymphoma cell lines. Lymphoma cells were incubated with DOX for 24 hours (0.5 or 1 μg/mL) or 30 hours (0.1 μg/mL) with or without irradiated (75 Gy) L cells expressing CD40L or CDw32, then washed, and cultured without DOX, but otherwise in the same conditions during 24 additional hours and tested for apoptosis using the TUNEL assay as indicated in Materials and Methods. (A and B) Inhibition of DOX-induced apoptosis of the BL36 cell line by CD40L. The intensity of fluorescence is proportional to the number of fluorescein-labeled DNA strand breaks within lymphoma cells. The threshold level of fluorescence intensity beyond which cells were considered to be in apoptosis was 10¹; 5% of lymphoma cells not treated with DOX were above this level. BL36 cells were cultured 48 hours without DOX (negative control) (5% ± 0.4% of apoptotic cells) or with 0.5 μg/mL of DOX during the first 24 hours (65% ± 1% of apoptotic cells) with irradiated CD40L L cells (40% ± 1% of apoptotic cells) (A) or CDw32 L cells (65% ± 1% of apoptotic cells) (B). (C) Inhibition of DOX-induced apoptosis by CD40L L cells in lymphoma cell lines. Cell lines were exposed to various concentrations of DOX during the first 24 hours: 0 μg/mL (1), 0.1 μg/mL (2) and (3), 0.5 μg/mL (4) and (5), 1 μg/mL (6) and (7) and cocultured in the presence of irradiated L cells expressing CD40L (3), (5), and (7). The SD of the percentages is under 2% in all conditions. This experiment is representative of seven different experiments.