IFN-γ to Treat Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS) That Has Relapsed After Allogeneic Hematopoietic Stem Cell Transplantation

Inclusion Criteria:

* Recipients of allogeneic stem cell transplantation for acute myeloid leukemia or myelodysplastic syndrome from a human leukocyte antigen (HLA) matched donor
* Relapsed of primary disease with 5% to 20% of blasts in the bone marrow by flow cytometry in the bone marrow with an clear leukemia-associated immunophenotype (If the patient received therapy to treat the relapse, he or she must have 5-20% residual blasts prior to enrollment on this study)
* Performance status KPS score \>60% (ECOG 0-2)
* No increases in systemic immunosuppression in the prior four weeks other than to maintain therapeutic levels
* No systemic corticosteroid with a dose higher than 0.5mg/kg/day prednisone or equivalent
* No history of grade IV acute GVHD
* No new systemic immunosuppressive medications in the prior two weeks initiated due to GVHD
* Willingness to have bone marrow and peripheral blood collected as per the study protocol
* Must be able to give informed consent
* Age 18 or older

Exclusion Criteria:

* Contraindication to receive IFN-γ including known hypersensitivity to interferon-gamma, E. coli derived products or any component of the product
* Subjects with a positive pregnancy test or who are breastfeeding
* For men or women of childing bearing potential (age \< 50 without hysterectomy or oophorectomy or documented menopause), unwilling to use effective contraception for the duration of the study.
* Primary engraftment failure
* Active cardiac arrhythmias not controlled by medical management or current NYHA class II or higher congestive heart failure
* Active ischemic heart disease not well controlled with medications
* A seizure disorder not well controlled by medications
* Estimated GFR \<30 mL/min
* AST/SGOT or ALT/SPOT \> 5 x ULN
* Total bilirubin \> 3 x ULN
* Chemotherapy (other than hypomethylating and/or venetoclax therapy) within the prior 4 weeks
* Body surface area at or less than 1.5 m2, or greater than 2.5 m2 so as to minimize the variation in IFN-γ exposure based on differences in BSA.
* Patients less than 18 years old.
* Pregnant or breastfeeding patients.