Haploidentical Donor Hematopoietic Cell Transplant for Sickle Cell Disease

The purpose of this study it to evaluate a reduced toxicity conditioning regimen for haploidentical donor HCT followed by a GVHD prophylaxis regimen comprising of post-transplant cyclophosphamide, sirolimus and abatacept with the goal to improve the GVHD-free rejection-free survival (GRFS) to greater than 90% after haploidentical donor HCT in children and young adults with SCD. Primary Objective: \- To assess the GVHD-free and rejection free survival (GRFS) after haploidentical donor HCT in children and young adults with SCD. Secondary Objectives: * Assess the overall survival (OS) and disease-free survival (DFS) after haploidentical donor HCT for SCD. * Estimate incidence and severity of acute and chronic GVHD after haploidentical donor HCT for SCD. * Assess the neutrophil and platelet engraftment kinetics after haploidentical donor HCT for SCD.