FTY and T_reg cells are additive for GVHD inhibition. (A) Lethally irradiated B6 mice were given BALB/c BM and splenocytes (15 × 10⁶). Cohorts received 1 infusion of BALB/c L Sel⁺ ex vivo activated and expanded T_reg cells (3.5 × 10⁶) on day 0. FTY or water was administered daily from day −1 to day 28. Shown is survival (pool of 2 experiments, n = 16 per group; P < .001, control vs any treatment group; P < .03, T_reg cells or FTY vs FTY + T_reg cells). (B-E) Lethally irradiated B6 mice were given BALB/c BM and CFSE-stained splenocytes (50 × 10⁶). Cohorts received 1 infusion of BALB/c L Sel⁺ ex vivo activated and expanded T_reg cells (10 × 10⁶) on day 0. Sterile water or FTY was administered daily from day −1 to day 4 by oral gavage. Spleens were harvested on day 5, and CFSE divisions were evaluated on gated donor (H2^d) CD4⁺ and CD8⁺ cells. Shown is average CD4⁺ and CD8⁺ responder frequency and proliferative capacity as indicated plus or minus 1 SD calculated from CFSE data according to “Materials and methods” (n = 4 per group per time point; *P ≤ .018 vs control; ^P ≤ .037, FTY vs T_reg cells or FTY + T_reg cells).