The study plans to treat at least 60 pediatric subjects, male and female, between the ages of 2 months and 17 years inclusive with acute Graft versus Host Disease (aGVHD) following allogeneic hematopoietic stem cell transplant (HSCT) that has failed to respond to treatment with systemic corticosteroid therapy. Subjects may have Grades C and D aGVHD involving the skin, liver and/or gastrointestinal (GI) tract or Grade B aGVHD involving the liver and/or GI tract, with or without concomitant skin disease.