Abstract
The ability of the immune system to differentiate self from nonself is critical in determining the immune response to antigens expressed on transplanted tissue. Even with conventional immunosuppression, acceptance of the allograft is an active process often determined by the presence of regulatory T cells (Tregs). Tregs classically are CD4(+) cells that constitutively express high levels of the IL-2 receptor α chain CD25, along with the transcription factor Foxp3. The use of Tregs in the field of solid organ transplantation is related specifically to the objective of achieving tolerance, with the goal of reducing or eliminating immunosuppressive drugs as well as maintaining tissue repair and managing acute rejection. A key issue in clinical use of Tregs is how to effectively expand the number of Tregs, either through increasing numbers of endogenous Tregs or by the direct infusion of exogenously expanded Tregs. In order to realize the benefits of Treg therapy in solid organ transplantation, a number of outstanding challenges need to be overcome, including assuring an effective expansion of Tregs, improving long-term Treg stability and reduction of risk-related to off-target, nonspecific, immunosuppressive effects related specially to cancer.