Siplizumab for Sickle Cell Disease Transplant

The purpose of this study is to find out whether siplizumab is safe and effective for patients with SCD undergoing an allogeneic transplant and to prevent development of Graft versus Host Disease (GVHD) and graft failure. The main goals of this study are : * To determine if acute GVHD occurs and how severe the acute GVHD is in subjects receiving the study drug * To determine if graft failure occurs in subjects receiving the study drugs In this study, participants will receive 5 infusions of the study drug, siplizumab, while getting a stem cell transplant for SCD. Before siplizumab infusion, participants will be given medications to reduce the risks of allergic reaction to the drug.

Phase I/II label single arm safety assessment study of Siplizumab-based conditioning. It is designed to determine the safety and feasibility of hematopoietic stem cell transplantation for patients with advanced sickle cell disease using in vivo T cell depletion with Siplizumab. It is a three-stage design with a target sample size of at most 18 patients. At the first stage, a total of 6 patients will be recruited, the study will be stopped, if 2 or more of patients experience any of the following events: graft failure/Grade 3-4 acute GVHD/death in the first 100 days. If not, the study will proceed to the second stage and recruit additional 6 patients. The study will be stopped if 4 or more patients experience an event as specified above among the 12 patients. If not, the study will proceed to the third stage and recruit additional 6 patients, the study will be stopped any time if 6 or more patients experience an event among the 18 patients. The study will be stopped at any time if a patient death occurs.