Reduced Intensity Transplantation for Severe Sickle Cell Disease

This study is being done to test a transplant method that may have fewer side effects (or less toxic, less harmful) than conventional high dose chemotherapy conditioning-based transplants for children and young adults with Sickle Cell Disease (SCD). Patients less than or equal to 25 years old with SCD who would likely benefit from allogeneic hematopoietic cell transplantation (HCT) will be included in this study. Patients with a suitable HLA matched sibling donor (MSD) will be enrolled on the MSD arm while patients without an eligible MSD who have a suitable haploidentical (HAPLO) donor available will be enrolled on the HAPLO arm of the study. Primary Objective To assess the donor T-cell chimerism at 1-year post transplant in each respective arm (MSD, HAPLO) of the trial. Secondary Objectives * Assess the overall survival and 1-year, 2-year and 3-year post-transplant graft versus host disease (GVHD)-free SCD-free survival. * Estimate the primary and secondary graft rejection rate at 1-year, 2-year and 3-year post- transplant. * Estimate the incidence and severity of acute and chronic (GVHD). * Estimate the incidence of SCD recurrence after transplant * Assess the neutrophil and platelet recovery kinetics post-transplant. Exploratory Objectives * Record immune reconstitution parameters, including chimerism analysis, quantitative lymphocyte subsets, T cell receptor excision circle (TREC) analysis, V-beta spectratyping, and lymphocyte phenotype and function. * Conduct longitudinal examination of impact of HCT on patient health-related quality of life (HRQL) and adjustment, and parental adjustment. * Examine impact of HCT on patient cognitive and academic function. * Determine factors that influenced the decision to undergo HCT, explore perceptions of the HCT experience, and assess decisional satisfaction/regret. * Develop and evaluate an objective/quantitative imaging biomarker to assess organ (liver and heart) function/disease status and changes following HCT. *