Acute graft-versus-host disease (aGVHD) is a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). T regulatory (Tregs) cells are a subset of CD4+ T cells that co-express high levels of the IL-2Ra chain (CD25) and the transcription factor, FoxP3. Adoptive transfer of natural Tregs (nTregs, CD4+CD25brFoxP3+) that arise in the thymus have already been shown to prevent aGVHD and autoimmunity in mice but high ratios of nTreg:donor T cells are required. Clinical application of nTregs has been hampered by low frequency and low proliferative potential of nTregs derived from peripheral blood (PB). In the periphery, a population of non-Treg CD4+ T-cells can be induced to acquire a Treg phenotype and function - these cells are referred to as inducible Tregs (iTregs). Our group was the first to show that human iTregs could be reliably activated in the presence of IL-2, rapamycin and TGFss, leading to a population with stable Foxp3 expression and potent suppressive of aGVHD in a xenogeneic GVHD murine model. Furthermore, we have recently developed a manufacturing procedure using GMP reagents for cell purification, activation and expansion. Importantly, we have already completed scale-up studies and have demonstrated that we can reproducibly culture >109 iTregs from a PB buffy coat and ~240x109 cells from a full apheresis PB product within 2 weeks of culture initiation. With these expansion rates, we expect to achieve an iTreg:donor T cell ratio up to 5:1, remarkably in excess of the targeted minimum of 1:1 ratio for aGVHD prevention, as shown in preclinical murine models. However, it is possible that higher ratios or multiple doses of iTreg may be required in patients with active disease. With the ultimate goal of manufacturing a clinical grade 'off the shelf' 3rd party iTreg product for aGVHD prophylaxis and treatment, we will embark on a series of 'first-in-human' clinical trials to methodically evaluate the safety and potential efficacy of iTregs. Here, we will establish the safety and kinetics of HLA matched, partially matched, and unmatched iTregs for aGVHD prevention. We will perform a series of phase I trials with an extension phase to determine the MTD, safety profile and potential efficacy of fresh and subsequently cryopreserved iTregs. We will first establish the safety of fresh HLA matched iTregs in recipients of HLA matched sibling donor HSCT, followed by a follow-up study using fresh (and later cryopreserved) HLA mismatched iTregs obtained from a haploidentical relative for use in recipients of HLA-matched, unrelated donor HSCT. The successful completion of these first two clinical trials and the subsequent testing of cryopreserved iTregs, will justify the penultimate trial testing banked 'off-the-shelf' iTregs from an HLA mismatched donor. In parallel to the clinical trials, we will optimize the culture conditions for iTreg expanson required to permit multi-dose infusions and establishment of an off-the-shelf product. At the completion of these studies, we will have developed a bank of iTregs for immediate 'off-the-shelf' use for the prevention and treatment of aGVHD. PUBLIC HEALTH RELEVANCE: One of the major life threatening complications after bone marrow transplantation is acute graft-versus-host disease (GVHD) caused by donor white blood cells called lymphocyte cells which attacks the patient's body. Inducible Tregs (iTregs) are a type of white blood cells normally found in small amounts in the blood and studies in mice have shown that iTregs given during transplantation can prevent GVHD. We plan to test whether iTregs can prevent and treat GVHD in humans.