FTY administered only prior to transplantation inhibits GVHD. (A) Lethally irradiated B6 mice were given BALB/c BM and splenocytes (15 × 10⁶) and administered sterile water or FTY by oral gavage at the indicated schedule. Survival is indicated (Control and FTY: day −10 to −2, pool of 2 experiments, n = 16 per group; FTY day −10 to day 28 and FTY day −1 to day 28 in 1 experiment, n = 8 per group; P < .004, control vs any FTY group; P > .13 for any FTY schedule comparison). (B-D) Lethally irradiated B6 mice were given BALB/c BM and CFSE-stained splenocytes (50 × 10⁶). FTY was administered daily from day −10 to day −3 (FTY pre) or day −1 to day 4 (FTY post) by oral gavage. Controls received sterile water. Spleens were harvested on days 3 and 5, and CFSE divisions were evaluated on gated donor (H2^d) CD4⁺ and CD8⁺ cells. (B) A representative donor CD4⁺ CFSE histogram from each group for both days is shown. CFSE-stained splenocytes placed in culture served as control to illustrate no divisions. (C,D) Shown is average CD4 (C) and CD8 (D) responder frequency plus or minus 1 SD calculated from CFSE data according to reference in “Materials and methods.” (n = 3 per group per time point; *P ≤ .014 FTY pre or post vs Control; ^P < .05 FTY pre vs FTY post). Day-3 data were reproduced in a second experiment.