Interaction of donor T cells and host antigen-presenting cells (APCs) in secondary lymphoid organs is critical for inducing graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). Activation-induced cell death (AICD) is a chief mechanism of clonal deletion that is responsible for the rapid contraction of alloreactive donor T cells following an initial massive expansion. We hypothesized that sequestration of donor T cells in lymph nodes (LNs) would lead to prolonged interaction of donor T cells and host APCs in LNs and cause AICD of alloreactive donor T cells after allogeneic BMT. To test this hypothesis, recipient mice were given FTY720, which inhibits migration of T cells from LNs, after BMT. Lethally irradiated B6 (H-2b) mice were injected with 10 x 106 purified T cells and 5 x 106 bone marrow cells harvested from congeneic B6.Ly-5a (H-2b, CD45.1+) donors. FTY720 was administered by daily oral gavage at a dose of 0.3mg/kg from day 0. Six days after congeneic BMT, numbers of donor T cells in mesenteric LNs were greater in FTY720-treated mice as expected. By contrast, when B6D2F1 (H-2b/d) recipients were transplanted with 4 x 106 purified T cells and 5 x 106 bone marrow cells from allogeneic B6. Ly-5a donors, numbers of donor T cells in mesenteric LNs were greater in FTY720-treated mice than controls 4 days after allogeneic BMT but were subsequently less in FTY720-treated mice than controls (Figure). Five days after allogeneic BMT, frequencies of apoptotic donor T cells assessed by AnnexinV positivity were significantly greater in FTY720-treated mice (33±3 vs 24±5%, P<.05) than those in controls. Serum level of IFN-γ (7.4±1.1 ng/ml vs 12.4±1.6 ng/ml, P<.05), an index of systemic T cell responses, was significantly reduced in FTY720-treated mice on day7. These results suggest that administration of FTY720 enhances contraction of donor T cells by inducing apoptosis of donor T cells after allogeneic BMT. This FTY720-induced reduction of serum levels of IFN-γ was similarly observed when Fas-deficient B6-lpr/lpr mice were used as donors, indicating Fas-independent AICD in this phenomenon. Administration of FTY720 until day10 after allogeneic BMT improved survivals of recipients (67% vs 0% at day 21, P<.05). When reduced number of T cells (1 x 106) were infused, administration of FTY720 until day10 improved clinical GVHD scores (6.0±0.5 vs 4.1±0.3 at day14, P<.05) and survivals (100% vs 50% at day 30, P<.05) of recipients. Next, 1 x 106 donor T cells isolated from FTY720-treated and control-treated recipients 6 days after allogeneic BMT were adoptively transferred to lethally irradiated B6D2F1 mice together with 2 x 106 bone marrow cells from syngeneic B6D2F1 donors. T cells from FTY720-treated mice caused significantly decreased GVH reaction compared to T cells from control-treated mice (p<.01), suggesting the reduced alloreactivity of T cells from FTY720-treated mice. These findings suggest that FTY720 could modulate GVHD by the induction of AICD of donor T cells in LNs as well as by the inhibition of migration of donor T cells to target tissue.