For the treatment of pediatric patients who have failed to respond to steroid treatment for acute graft-versus-host disease (GVHD). Failing steroid treatment for acute GVHD is defined as any Grade B-D (IBMTR grading) of acute GVHD that is not improving after at least 3 days of methylprednisolone (≥ 1 mg/kg/day) or equivalent.