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.