A common procedure used to try and cure patients who have a cancer of the immune system is to eliminate the patient's immune system and replace it with someone else's. This procedure is known as stem cell transplantation, or SCT. Across the world, over 25,000 people with cancer receive this therapy every year. Unfortunately the success of SCT is limited by a life-threatening complication known as graft-versus-host-disease (GVHD). GVHD occurs when the new immune cells from the donor of the transplant attack the body of the transplanted patient. There is evidence that specialized cells known as regulatory T cells, or Tregs, can control GVHD. We want to develop ways to use human Tregs as a therapy for SCT patients and to study how they stop GVHD. The aims will ultimately lead to improved outcomes for patients who receive a stem cell transplant in an attempt to cure their cancer.