FTY720-P, but not FTY720 causes lymphopenia in Sphk2 knockout mice. (A) Lymphocyte counts in whole blood before and 24 hours after oral (left) or intravenous (right) treatment with 1 mg/kg FTY720. (B) FTY720 and FTY720-P plasma levels of the same individuals as in panel A at 24 hours. (C) Lymphocyte counts in whole blood before and after intravenous treatment with 1 mg/kg FTY720-P. (D) FTY720 and FTY720-P plasma levels 6 and 24 hours after dosage of FTY720-P. Results are expressed as the mean ± SD, n = 6. Statistical significance was calculated using Student t test (^**P < .01; ^***P < .001; n.s., not significant) in the case of panels A and C comparing lymphocyte counts before treatment (0 hours) to FTY720- or FTY720-P-treated groups, or for panels B and D comparing Sphk-null mice versus WT mice.