Approximately 6300 patients receive allogeneic hematopoietic stem cell transplants in the United States each year (IBMTR, 2003). Nearly 50% (approximately 3,150) of these patients develop acute GVHD (Goker et al). A fraction of these patients (approximately 870) will progress to the severe stages of the disease, Grades III-IV. It is estimated that nearly 82% of those patients with severe acute GVHD will be steroid refractory (Przepiorka et al., 1995) and of these, only 50% of steroid-refractory patients wll respond to secondary and tertiary treatments (Greinix et al., 2000). Thus, roughly 350 patients each year face tremendous odds against survival. In addition, most patients who initially responded to secondary and tertiary treatments have a high risk of dying within the first year (Remberger et al., 2001; Anasetti et al., 1994). Development of new therapeutic agents and strategies to rescue patients with steroid refractory, acute GVHD would provide a significant benefit in an area of unmet medical need.