A Study of Treatment of Inflammation Before Stem Cell Transplant in People With a Primary Immune Regulatory Disorder (PIRD) and/or an Autoinflammatory Condition

Inclusion Criteria: Patients receiving first allo-HCT for the following immunologic conditions: Primary Immune Regulatory Disorder with or without a genetic lesion as defined by the Primary Immune Deficiency Treatment Consortium (PIDTC) Patients with autoinflammatory disorders evidenced by cytokine and inflammation assays with at least 1.5x ULN of measured cytokines and/or an elevated ferritin or ESR > 2 ULN For inclusion on the emapalumab group, the lesion must be isolated to the IFNγ pathway (or mediators thereof) with an elevated CXCL9 >1.5 ULN (or already controlled on a JAK inhibitor, provided that CXCL9 levels were elevated prior to initiation of the JAK inhibitor). Inclusion on the Fludarabine/dexamethasone group requires inflammation (as defined above) other than an isolated IFNγ/CXCL9 defect Able to tolerate cytoreduction (based on adequate organ function as described below) Patients of any age can enroll so long as they meet other inclusion criteria: Adequate organ function is required, defined as follows: Hepatic: Serum bilirubin ≤ 2 mg/dL, unless benign congenital hyperbilirubinemia. Patients with hyperbilirubinemia related to paroxysmal nocturnal hemoglobinuria or other hemolytic disorders related to their PIRD diagnosis are eligible. Hepatic: AST, ALT, and alkaline phosphatase < 2.5 times the upper limit of normal unless thought to be disease-related. Investigator will need to perform clinically indicated evaluations to assess if disease related or intrinsic liver disease. Additional testing may be done if clinically indicated, after the pre-transplant immune prophase and prior to start of conditioning as this will provide additional data to confirm disease related versus intrinsic liver dysfunction. Renal: serum creatinine <1.5x normal for age. If serum creatinine is outside the normal range, then CrCl > 50 mL/min/1.73m2 (calculated or estimated) or GFR (mL/min/1.72m2) >30% of predicted normal for age. Normal GFR by Age Cardiac: LVEF ≥ 50% by MUGA or resting echocardiogram. Pulmonary: Pulmonary function testing (FEV1 and corrected DLCO) ≥ 50% predicted (pediatric patients unable to complete PFTs will need oxygen saturation as recorded by pulse oximetry of ≥92% on room air). Adequate performance status: Age ≥ 16 years: ECOG ≤ 1 or Karnofsky 70% Age < 16 years: Lansky 70% Each patient must be willing to participate as a research subject and must sign an informed consent form or legal guardian with assent as appropriate. Exclusion Criteria: Uncontrolled infection at the time of enrollment. Patients who have undergone previous allo-HCT. Patient seropositivity for HIV I/II and/or HTLV I/II. Females who are pregnant or breastfeeding. Patients unwilling to use contraception during the study period. Patient or parent or guardian unable to give informed consent or unable to comply with the treatment protocol including research tests. Donor Inclusion Criteria: Related Donors: 8/8 or 7/8 HLA matched at A, B, C, and DRB1 loci, as tested by DNA analysis. Haploidentical donors at A, B, C and DRB1 loci, as tested by DNA analysis Unrelated Donors: o 8/8 or 7/8 matched at A, B, C, and DRB1 loci, as tested by DNA analysis. Able to provide informed consent for the donation process per institutional standards. Meet standard criteria for donor collection (e.g. National Marrow Donor Program Guidelines or collecting center guidelines as approved by treating physician).