Donor-specific CD8⁺Foxp3⁺ T cells protect skin allografts and facilitate induction of conventional CD4⁺Foxp3 ⁺ regulatory T cells

CD4⁺ regulatory T cells play a critical role in tolerance induction in transplantation. CD8⁺ suppressor T cells have also been shown to control alloimmune responses in preclinical and clinical models. However, the exact nature of the CD8⁺ suppressor T cells, their induction and mechanism of function in allogeneic transplantation remain elusive. In this study, we show that functionally suppressive, alloantigen-specific CD8⁺Foxp3⁺ T cells can be induced and significantly expanded by stimulating naïve CD8⁺ T cells with donor dendritic cells in the presence of IL-2, TGF-β1 and retinoic acid. These CD8⁺Foxp3⁺ T cells express enhanced levels of CTLA-4, CCR4 and CD103, inhibit the up-regulation of costimulatory molecules on dendritic cells, and suppress CD4 and CD8 T cell proliferation and cytokine production in a donor-specific and contact-dependent manner. Importantly, upon adoptive transfer, the induced CD8⁺Foxp3⁺ T cells protect full MHC-mismatched skin allografts. In vivo, the CD8⁺Foxp3 ⁺ T cells preferentially traffic to the graft draining lymph node where they induce conventional CD4⁺Foxp3⁺ T cells and concurrently suppress effector T cell expansion. We conclude that donor-specific CD8⁺Foxp3⁺ suppressor T cells can be induced and exploited as an effective form of cell therapy for graft protection in transplantation. Alloantigen-specific CD8⁺Foxp3⁺ regulatory T cells exert their suppressive function by inducing the down-regulation of costimulatory molecules on CD11c⁺CD8α ⁺ dendritic cells and facilitating the conversion of naïve CD4⁺ T cells into CD4⁺CD25⁺Foxp3⁺ cells.