My laboratory has had a longstanding interest in the biology of graft-versus-host disease (GVHD) for ~30 years with studies into the basic biology, translational clinical approaches, and clinical trials. We identified ex vivo tolerization approaches to prevent GVHD using costimulatory pathway blockade, showed that natural regulatory T cells (nTregs) were required for tolerance induction by costimulatory blockade, and tex vivo expanded murine nTregs could prevent GVHD lethality. We were able to isolate and expand human nTregs, demonstrate their capacity to prevent xenogeneic GVHD, and translate these findings into a first-in-human study. We developed novel methods to expand human nTregs to massive numbers, being tested in an ongoing clinical trial and to produce and expand inducible Tregs (iTregs), for a first-in-human study. We also has provided the first proof-of-principle in rodent models that lead to studies in patients of the following strategies to prevent GVHD or promote alloengraftment including the drugs FK506 (Tacrolimus), and rapamycin (Sirolimus). We are pursuing approaches to improve Treg potency in vivo.