Graft-vs.-Host Disease (GVHD) is one of the major limitations of allogeneic hematopoietic stem cell transplants (HSCT). This complication is major cause of morbidity and mortality and is thought to be initiated by activation of donor T-cells through recognition of "foreign" cells resident in the transplant recipient. Acute GVHD is associated with damage to the liver, skin, gastrointestinal tract and mucosa. Moderate to severe GVHD Grades II-IV occurs in 30-50% of matched related HSCTs and 50-70% of unrelated donor recipients. Severe GHVD requires intense immunosuppression involving steroids and additional agents to get it under control, and patients may develop severe infections as a result of such immunosuppression. An agent or cell therapy that could reduce the incidence and/or severity of GVHD without increasing relapse or infectious risk in HSCT patients would provide substantial benefits.