Regulatory T cells (T reg s) that constitutively express FOXP3 are instrumental to the maintenance of tolerance and may suppress graft-versus-host disease (GVHD) in humans. To determine whether regulatory T cells in allogeneic stem cell transplants (SCTs) ameliorate GVHD after transplantation, we quantitated the coexpression of FOXP3 on CD4 ؉ T cells in 32 donor SCTs infused into HLA-matched siblings and examined GVHD incidence in respective recipients. High CD4 ؉ FOXP3 ؉ Tcell count in the donor was associated with a reduced risk of GVHD. We monitored T reg s during immune reconstitution in 21 patients with leukemia undergoing a T-cell-depleted allogeneic SCT. Early after SCT, there was a significant expansion in the CD4 ؉ FOXP3 ؉ T-cell compartment. A low CD4 ؉ FOXP3 ؉ Tcell count early after SCT (day 30) was associated with an increased risk of GVHD, and the ratio of CD4 ؉ FOXP3 ؉ T cells to CD4 ؉ CD25 ؉ FOXP3 ؊ T cells was significantly reduced in patients with GVHD, suggesting diminished control of effector T cells. Our findings suggest that graft T reg content may predict for risk of GVHD after SCT. Determining the T reg levels in the donor and manipulating T reg s early after transplantation may provide a new approach to

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