Mice were sensitized by an i.p. injection of OVA/alum on days 0 and 7. On days 19–21, mice were subjected to OVA aerosol challenges. Thirty minutes before each aerosol challenge, mice received an i.t. injection of vehicle, 0.25 μg FTY720, or S1P 10^–6 M. (A and B) BAL fluid was analyzed by flow cytometry. (C and D) Mediastinal LN cell suspensions were restimulated in vitro for 4 days with OVA and assayed for cytokine production using ELISA assay. (E–H) H&E staining of lung sections of mice treated or not with FTY720 and S1P. Data in E and F are from the same experiments as in A and B, respectively. (I) FTY720 inhibits the induction of BHR. Airway obstruction to increasing doses of inhaled methacholine was measured as increase in Penh values (% increase from saline) 24 hours after the last Ag exposure. Labels indicate immunization/treatment/challenge; n = 6–8 mice per group. Data are shown as mean ± SEM. (J) Sensitized BALB/c mice were first subjected to OVA aerosol challenges on days 17–19. Subsequently, mice were subjected to an additional series of 3 daily OVA or PBS aerosol challenges on days 20–22. Thirty minutes before each of these aerosol challenges, mice received an i.t. injection of control vehicle or 0.25 μg FTY720. Twenty-four hours after the last OVA or PBS exposure, BAL was performed. Labels indicate sensitization/first series of aerosol challenges/treatment/second series of aerosol challenges. Data are shown as mean ± SEM. *P < 0.05; **P < 0.01; ***P < 0.001, representative of 3 independent experiments.