Regulatory T Cells: Promises and Challenges

Soon after their discovery in 1972, regulatory T cells (Tregs) appeared as nature’s gift to induce tolerance in auto- and alloimmune settings. We review here the barriers to the use of these bona fide cells and the strategies implanted to overcome them. Tregs are extremely rare and heterogenous, rendering them difficult to isolate and expand in vitro. Furthermore, their adoptive transfer was hurdled by their phenotypic instability and lack of intrinsic survival signals. Most studies in the field have focused on identifying distinctive Tregs markers for their pure isolation. This has also led to the discovery of different subtypes within this regulatory population, such as TR1 cells and CD8 + Tregs. In parallel, a myriad of techniques has been implanted for the targeted delivery of IL-2 as well as the bioengineering of Tregs with antigen-specific chimeric antigen receptors. Despite the drawbacks of the use of Tregs, strategies based on targeted delivery of IL-2 and hijacking conventional CD4 cells into suppressive cells are emerging as a promising tool in the field of transplantation. However, more studies are still required, as the long-term safety and stability of these bioengineered cells remain under investigation.