Regulatory T-cell dysfunction and its implication for cell therapy

Regulatory T cells (Tregs) are a subtype of CD4(+) T cells that can mediate immune tolerance by a multitude of immunomodulatory mechanisms. Treg-based adoptive immunotherapy is currently being tested in multiple phases I and II clinical trials in transplantation and autoimmune diseases. We have learned from the work done on conventional T cells that distinct mechanistic states can define their dysfunctions, such as exhaustion, senescence, and anergy. All three can negatively impact the therapeutic effectiveness of T-cell-based therapies. However, whether Tregs are susceptible to such dysfunctional states is not well studied, and results are sometimes found to be controversial. In addition, Treg instability and loss of FOXP3 expression is another Treg-specific dysfunction that can decreasein their suppressive potential. A better understanding of Treg biology and pathological states will be needed to compare and interpret the results of the different clinical and preclinical trials. We will review herein Tregs' mechanisms of action, describe different T-cell dysfunction subtypes and how and if they apply to Tregs (exhaustion, senescence, anergy, and instability), and finally how this knowledge should be taken into consideration when designing and interpreting Treg adoptive immunotherapy trials. Recent advances in Treg-based immunotherapy, as an immune tolerance approach for transplantation and autoimmune diseases, exhibited the need for a better understanding of Treg biology. Classical mechanisms of dysfunction in T cells such as exhaustion, senescence, and anergy are poorly understood in Tregs. Previous works in auto- and alloimmune diseases, as well as in cell-engineering, have shown susceptibility of Tregs to such complications, furthering the importance of this knowledge for future design and interpretation of Treg adoptive immunotherapy trials.