Flow cytometry analysis of the surface expression of TRAIL-Receptors and TRAIL ligand in NK and CTL cells. (A) IL-2–stimulated NK cells. (B) IL-15–stimulated NK cells. (C) CD8⁺ T cells, at different times of activation. (D) Surface expression of TRAIL-Rs by the NK-like cell line NK 3.3. The cell line was routinely cultured in RPMI 1640 + 15% FCS + 100 U/mL rIL-2. (E) Expression of surface TRAIL-ligand (TRAIL-L) by activated (day 10) NK and CD8⁺ T cells. Empty histograms indicate isotype-matched irrelevant antibody, negative controls.