Regulatory T cells in stem cell transplantation: Main characters or walk-on actors?

Almost 10 years have passed since the first experiments specifically addressing the role of regulatory T cells (Treg) in stem cell transplantation (SCT). It is now time to try to dissect what is their actual impact in such a therapeutic context. In the present review we will initially remind the preliminary experiments exploring the Treg behavior in murine models of SCT, then moving to the first studies evaluating their influence on graft versus host disease and graft versus leukemia in humans. After focusing on some interesting aspects of Treg mechanisms of action after SCT, we will evaluate their activity after donor lymphocyte infusion as well as after reduced intensity conditioning and autologous SCT. We will conclude by briefly discussing possible therapeutic applications of Treg and highlighting biological and clinical issues which deserve to be further assessed in the next future.