Haploidentical HCT for Severe Aplastic Anemia

This study is a prospective, single center phase II clinical trial in which patients with Severe Aplastic Anemia (SAA) ) will receive a haploidentical transplantation. The purpose of this study is to learn more about newer methods of transplanting blood forming cells donated by a family member that is not fully matched to the patient. This includes studying the effects of the chemotherapy, radiation, the transplanted cell product and additional white blood cell (lymphocyte) infusions on the patient's body, disease and overall survival. The primary objective is to assess the rate of engraftment at 30 days and overall survival (OS) and event free survival (EFS) at 1 year post-hematopoietic cell transplantation (HCT). Primary Objectives * To estimate the rate of engraftment at 30 days after TCR αβ+ T-cell-depleted graft infusion in patients receiving a single dose of post graft infusion cyclophosphamide. * To estimate the overall survival and event free survival at 1-year post transplantation. Secondary Objectives * To calculate the incidence of acute and chronic GVHD after HCT. * To calculate the rate of secondary graft rejection at 1-year post transplantation * To calculate the cumulative incidence of viral reactivation (CMV, EBV and adenovirus). * To describe the immune reconstitution after TCR αβ+ T-cell-depleted graft infusion at 1 month, 3 months, 6 months, 9 months, and 1 year. Exploratory Objectives * To longitudinally assess the phenotype and epigenetic profile of T-cells in SAA patients receiving HCT for SAA. * To assess the phenotype and epigenetic profile of T-cells in DLI administered to SAA patients post HCT. * To longitudinally assess CD8 T cell differentiation status in SAA patients using an epigenetic atlas of human CD8 T cell differentiation. * To examine the effector functions and proliferative capacity of CD8 T cells isolated from SAA patients before and after DLI. * Quantify donor derived Treg cells at different time points in patients received HCT. * Determine Treg activation status at different stages after HCT. * Are specific features of the DLI product associated with particular immune repertoire profiles post-transplant? * How does the diversity and functional profile of the DLI product alter the response to pathogens in the recipient? * Do baseline features of the recipient's innate and adaptive immune cells correlate with post-transplant immune repertoires and response profiles?

Inclusion Criteria for Transplant Recipient 1. Age less than or equal to 21 years at time of enrollment. 2. Confirmed diagnosis of SAA or a single lineage cytopenia (a) SAA or single lineage cytopenia will be defined as follows: * i. Bone marrow cellularity \< 25% or hypocellular marrow for age, AND * ii. One or more of the following (in peripheral blood): (i) Neutrophils \< 0.5 x10\^9/L (ii) Platelets \< 20 x10\^9/L, or platelet transfusion dependence (iii) Hemoglobin \ 40% or shortening fraction ≥ 25%. 2. Creatinine clearance (CrCl) or glomerular filtration rate (GFR) ≥ 50 ml/ min/1.73m2. 3. Forced vital capacity (FVC) ≥ 50% of predicted value; or pulse oximetry 4. ≥ 92% on room air if patient is unable to perform pulmonary function testing. 5. Karnofsky or Lansky (age-dependent) performance score ≥ 50. 6. Bilirubin ≤ 3 times the upper limit of normal for age. 7. Alanine aminotransferase (ALT) or Aspartate aminotransferase (AST) ≤ 5 times the upper limit of normal for age. 8. Females and males of childbearing potential must agree to practice 2 effective methods of contraception at the same time or agree to abstinence until after the last dose of chemotherapy has been administered Exclusion Criteria for Transplant Recipient: 1. Diagnosis of Fanconi anemia. Fanconi anemia must be excluded by diepoxybutane (DEB) or equivalent testing. 2. Known clinical or genetic diagnosis of dyskeratosis congenita 3. Clonal cytogenetic abnormalities consistent with pre-myelodysplastic syndrome (pre- MDS) or MDS on marrow examination (e.g. Monosomy 7). 4. Diagnosis of myelodysplastic syndrome (MDS). 5. Presence of anti-donor HLA antibodies (positive anti-donor HLA antibody is defined as a positive cross-match test of any titer by complement- dependent cytotoxicity or flow cytometric testing or the presence of anti- donor HLA antibody to the high expression loci HLA-A, B, C, DRB1, or DPB1 with mean fluorescence intensity (MFI) \> 1000 by solid phase immunoassay). 6. Prior allogeneic hematopoietic cell transplant. 7. Prior solid organ transplant. 8. Known life-threatening reaction (i.e., anaphylaxis) to ATG that would prohibit use for the patient. 9. Uncontrolled bacterial, viral, or fungal infection at the time of enrollment. Uncontrolled is defined as progression or no clinical improvement on appropriate medical treatment. 10. Female patients who are pregnant (per institutional practice) or breast- feeding. 11. Prior malignancies except resected basal cell carcinoma or treated cervical carcinoma in situ. Cancer treated with curative intent \> 5 years previously will be allowed. Cancer treated with curative intent ≤ 5 years previously will not be allowed unless approved by the PI. 12. Alemtuzumab or ATG within 2 weeks of enrollment. Inclusion Criteria for Haploidentical Donor 1. At least single haplotype matched (≥ 3 of 6) family member. 2. At least 18 years of age. 3. HIV negative. 4. Not pregnant as confirmed by negative serum or urine pregnancy test within 14 days prior to enrollment (if female). 5. Not breast feeding. 6. Related donors must be ruled out for telomere disease by appropriate clinical and diagnostic measures (for example, clinical evaluation, telomere length testing, genetic testing, and/or bone marrow examination). 7. The HAPLO donor and/or legal guardian must be able to sign informed consent documents. 8. The potential HAPLO donor must be willing and able to donate PBSCs.