Haploidentical HCT for Severe Aplastic Anemia

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 \<8g/dL, or red blood cell transfusion dependence 3. Does not have a suitable HLA-matched sibling donor (MSD) or volunteer 10/10 HLA-matched unrelated donor (MUD) available in the necessary time for progenitor cell donation. 4. Failed at least one trial of immunosuppressive therapy (IST) by being refractory (persistence of severe cytopenias and fulfillment of SAA disease criteria at least 3 months after initial IST) or having relapsed (initial improvement of cytopenias after first-line IST but then a later return to fulfillment of SAA disease criteria when IST is decreased or ceased). IST could have included ATG based regimens, calcineurin inhibitors and/or other higher dose therapy directed at the treatment of primary SAA. Patients with very severe aplastic anemia who are likely not to benefit from IST do not need to have failed a trial of IST and can proceed directly to HCT if they meet the rest of the criteria. 5. Has a suitable single haplotype matched (≥ 3 of 6) family member donor. 6. Patient and/or legal guardian must sign informed consent for HCT. 7. Adequate organ function defined as: 1. Left ventricular ejection fraction \> 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.