FTY720 inhibits CysLT secretion by mast cells. (A-B) RBL-2H3 mast cells were sensitized overnight with anti–DNP IgE (1 μg/mL), treated without or with FTY720 (1 μM, ▪) for 2 hours, and then stimulated with Ag (10 ng/mL) for 15 minutes. Secreted CysLT was measured by ELISA, and data were expressed as means ± SEM (n = 4). (B) Sensitized RBL-2H3 cells were pretreated without or with 1 μM S1P, FTY720, or FTY720-P for the indicated times and then stimulated with Ag for 15 minutes. CysLT secretion was measured, and results were expressed as percent inhibition of Ag-induced release ± SD. (C) Sensitized LAD2 mast cells were treated without or with FTY720 (1 μM, ▪) for 2 hours, and then stimulated with Ag (10 ng/mL) for 15 minutes. Secreted CysLT was measured by ELISA. (D) FTY720 has no effect on TNF-α formation. RBL-2H3 mast cells were sensitized overnight with anti–DNP IgE, treated without or with FTY720 (1 μM, ▪) for 2 hours, and then stimulated without or with Ag (10 ng/mL) for an additional 16 hours. TNF-α secretion was determined by ELISA. Data are means ± SD of quadruplicate determinations. Similar results were obtained in 3 independent experiments.