Activity of APO2L/TRAIL on 4 Hodgkin disease cell lines. (A) Western blot analysis of the TRAIL receptor expression in HD cell lines. The TRAIL death receptors R1 and R2 were expressed in all cell lines (R1 was weakly expressed in KM-H2). The decoy receptor R4 but not R3 was also expressed in all 4 HD cell lines. (B) A representative experiment demonstrating APO2L/TRAIL–induced apoptosis in HD cell lines. Cells were incubated with 1 μg/mL TRAIL for 48 hours, and cell viability was determined by MTS assay. ⋄ indicates RPMI; ○, TRAIL. (C) Summary data showing TRAIL activity in HD cell lines. Data for each cell line and normal peripheral blood lymphocytes represent a mean (± SEM) of at least 3 independent experiments performed in triplicate. The viable cell number was determined by MTS assay after 48 hours of incubation. TRAIL was most effective in the HD-MYZ and HD-LM2 cell lines. (D) In the 2 sensitive cell lines, APO2L/TRAIL activated caspases 8, 10, 9, 7, and 3 and cleaved BID and PARP. APO2L/TRAIL also inhibited the phosphorylation of ERK without significant effect on the total amount of ERK in these 2 cell lines.