Donor-type CD4⁺CD25⁺ Regulatory T Cells Suppress Lethal Acute Graft-Versus-Host Disease after Allogeneic Bone Marrow Transplantation

Acute graft-versus-host disease (aGVHD) is still a major obstacle in clinical allogeneic bone marrow (BM) transplantation. CD4⁺CD25⁺ regulatory T (T_reg) cells have recently been shown to suppress proliferative responses of CD4⁺CD25⁻ T cells to alloantigenic stimulation in vitro and are required for ex vivo tolerization of donor T cells, which results in their reduced potential to induce aGVHD. Here we show that CD4⁺CD25⁺ T cells isolated from the spleen or BM of donor C57BL/6 (H-2^b) mice that have not been tolerized are still potent inhibitors of the alloresponse in vitro and of lethal aGVHD induced by C57BL/6 CD4⁺CD25⁻ T cells in irradiated BALB/c (H-2^d) hosts in vivo. The addition of the CD4⁺CD25⁺ T_reg cells at a 1:1 ratio with responder/inducer CD4⁺CD25⁻ T cells resulted in a >90% inhibition of the mixed leukocyte reaction and marked protection from lethal GVHD. This protective effect depended in part on the ability of the transferred CD4⁺CD25⁺ T cells to secrete interleukin 10 and occurred if the T_reg cells were of donor, but not host, origin. Our results demonstrate that the balance of donor-type CD4⁺CD25⁺ T_reg and conventional CD4⁺CD25⁻ T cells can determine the outcome of aGVHD.