FTY720 induces apoptotic cell death in leukemic NK cells in a dose-dependent manner. (A) Human NKL cell line was treated with 5, 10, and 15μM FTY720 or DMSO for 24 hours, then cells were assayed for apoptosis by flow cytometry. (B) Rat RNK-16 cell line was treated with 2.5, 5, and 10μM FTY720 or DMSO for 24 hours then cells were assayed for apoptosis by flow cytometry. (C) PBMCs from one acute NK-cell leukemia patient (no. 1) and 3 chronic NK-cell leukemia patients (nos. 2, 3, 4) were treated with 2.5, 5, and 10μM FTY720 or DMSO for 18 hours, then cells were assayed for apoptosis by flow cytometry. *P < .05, **P < .005, and ***P < .0005 indicate significant differences of FTY720-treated cells versus DMSO-treated cells (Student t test).