In-vivo Regulatory T Cell Enhancement With Cyclophosphamide and Sirolimus

Inclusion Criteria: * Patients must have a documented clinical diagnosis of grade II-IV acute graft-versus- host disease defined as graft versus host disease (GVHD) occurring within the first 100 days of transplantation * Patients must be steroid-refractory defines as progression after 3 days of corticosteroid therapy or no response after 5 days of corticosteroid therapy. * Progression is defined as up-grading * No response is defined as no down-grading * Progression after 3 days requires patients to have received at least 2 mg/mg/day for a total of 6 mg/kg of methylprednisolone or its equivalent. * No response after 5 days requires patient to have received at least 2 mg/kg/d for a total of 10 mg/kg of methylprednisolone or its equivalent. * Patients with exacerbation of GVHD during steroid taper will require re-treatment with 2mg/kg/d of corticosteroids and will need to meet the criteria * Age 18-70 * Patients must have received an allogeneic hematopoietic stem cell transplant within 100 days of study enrollment. * Serum creatinine \< 2 mg/dL Exclusion Criteria: * Patients cannot have active central nervous system (CNS) disease. * Patients must not have received cyclophosphamide for GVHD prophylaxis * Patients must not have pneumonia requiring oxygen supplementation * Unable or unwilling to sign informed consent.