T regulatory cells as an immunotherapy for transplantation

Advances in immunosuppressive therapies have made tissue and organ transplantation a common procedure in clinical medicine. However, true donor and recipient tolerance is not regularly achieved and almost all transplant recipients continue to require immunosuppressants throughout life, which is associated with side effects of the drugs. The identification and characterisation of regulatory T cells (Tregs) has recently opened up exciting opportunities for new ways of adoptive immunotherapy in transplantation. CD4+CD25+ Tregs of thymic origin have been shown to be key regulators of unseasoned immune responses in mice and in humans, preventing graft-versus-host disease and organ graft rejection in the transplantation setting. Although these cells can be found in the peripheral blood of healthy individuals, their isolation to a satisfying degree of purity is time-consuming and ineffective. Therefore, a variety of different methods to expand or induce regulatory T cells ex vivo have been advocated. Antigen-specific activation of Tregs is a prerequisite for their optimal function, making the design of new strategies to create and expand antigen-specific Tregs highly desirable. This review will focus on recent advances achieved in the field of transplantation tolerance using naturally occurring Tregs (CD4+CD25+), as well as other Tregs, and will discuss future applications of these cells in immunotherapy.