TRAIL-Rs expression and sensitivity to TRAIL in AML. (A) Flow cytometric analysis of TRAIL-Rs expression in primary AML cells. Cells were stained with specific mAbs anti–TRAIL-R1, anti–TRAIL-R2, anti–TRAIL-R3, and anti–TRAIL-R4, as described in “Methods.” Specific fluorescence histograms (▩) are superimposed to negative controls (isotype-matched irrelevant mAbs; empty histograms). One representative experiment is shown out of 6. (B) Quantification of TRAIL-Rs expression on the surface of cells from 6 patients with AML. Absolute numbers of surface antigens expressed per cell (MESF). (C) Apoptosis of primary AML cells treated with increasing doses of TRAIL for 48 hours. Jurkat cells were used as control (empty histogram; 100 ng/mL TRAIL for 48 hours). Primary AML cells are resistant to TRAIL-induced apoptosis. Data from 6 independent experiments are expressed as means plus or minus SD.