Regulatory T cells and human myeloid dendritic cells promote tolerance via programmed death ligand-1

Human regulatory T cells inhibit graft-versus-host disease that can occur after tissue transplantation, in part through expression of programmed death ligand 1 and modulation of antigen-presenting cells.
Immunotherapy using regulatory T cells (Treg) has been proposed, yet cellular and molecular mechanisms of human Tregs remain incompletely characterized. Here, we demonstrate that human Tregs promote the generation of myeloid dendritic cells (DC) with reduced capacity to stimulate effector T cell responses. In a model of xenogeneic graft-versus-host disease (GVHD), allogeneic human DC conditioned with Tregs suppressed human T cell activation and completely abrogated posttransplant lethality. Tregs induced programmed death ligand-1 (PD-L1) expression on Treg-conditioned DC; subsequently, Treg-conditioned DC induced PD-L1 expression in vivo on effector T cells. PD-L1 blockade reversed Treg-conditioned DC function in vitro and in vivo, thereby demonstrating that human Tregs can promote immune suppression via DC modulation through PD-L1 up-regulation. This identification of a human Treg downstream cellular effector (DC) and molecular mechanism (PD-L1) will facilitate the rational design of clinical trials to modulate alloreactivity.
Author Summary Graft-versus-host disease (GVHD) is the most serious complication of bone marrow transplants between individuals (so-called allogenic transplants). The class of suppressor immune cells called regulatory T cells (Tregs) inhibit GVHD by dampening the effects of donor immune cells in the grafted tissue. The cellular and molecular mechanisms involved in this process have not been fully characterized, particularly for human cells. In this study, we report that human Tregs, which we generated from precursor cells ex vivo, express high levels of a cell surface protein called PD-L1 (programmed death ligand-1) that is known to mediate immune suppression. Coculture of these Tregs with allogeneic antigen-presenting cells (APCs), which are known to initiate GVHD, increased, in turn, the amount of PD-L1 on the APCs. The Treg-conditioned APCs were then less able than unconditioned APCs to provoke GVHD in a mouse model of the condition, preventing the death of the animals after transplantation. We found that an antibody against PD-L1 blocked the immunosuppressive effects of Tregs or Treg-conditioned APCs, indicating that this protein is an important part of the molecular mechanism. These findings are potentially important for attempts to modulate immune responses in disease by transplanting T cells into patients.