The sTRAIL present in supernatants of IFNα-treated monocytes enhances apoptosis in human leukemic cell lines. Apoptosis rate of J32, MEG-01, and KU-812 leukemic cell lines was assessed by annexin-V–FLUOS staining after a 2-day culture in the presence of conditioned medium prepared from resting (□) and IFNα-stimulated (▪) monocytes. Cell incubation was carried out in the absence or the presence of a combination of 100 ng/mL TRAIL-R1/Fc and TRAIL-R2/Fc chimeras. The experiment depicted is representative of 3.