FTY720 enhances CXCR4-dependent transmigration of Lin^- cells through bone marrow endothelium in vitro. (A) In an in vitro assay of transendothelial migration across bone marrow endothelium,⁵  FTY720 did not affect spontaneous migration of Lin^- cells isolated from peripheral blood, whereas SDF-1-induced migration (MS-5 conditioned medium) was enhanced by FTY720 in a dose-dependent manner after preincubation of cells for 3 hours with the indicated concentrations of FTY720. Data represent the mean ± SEM from 3 independent experiments. Spontaneous migration: ⋄; SDF-1-induced migration: ♦. (B). The triggering effect of FTY720 on SDF-1-driven transmigration (MS-5 conditioned medium; CM) could be completely blocked by preincubation of Lin^- cells with an anti-CXCR4 mAb or pertussis toxin (PTX), indicating that CXCR4 is the only receptor involved in this system. Data represent the mean ± SEM from 5 independent experiments. ^*P < .05 versus CM.