FTY modulates effector T-cell migration to lymphoid organs but does not uniformly trap effectors in LNs or prevent effector migration to GVHD target parenchymal organs. (A-I) Lethally irradiated BALB/c mice received B6 wild-type BM and 2 × 10⁶ B6 GFP⁺ purified CD25-depleted T cells. Mice were administered sterile water or FTY by oral gavage daily from day −1 to day of imaging. A total of 3 mice per group per time point was imaged. Data were reproduced in a second experiment in the same strain combination and in a third experiment in a different strain combination; findings were similar. A representative image is shown of the indicated organ on day 4 and day 7. See “Materials and methods” for imaging details.