Nivolumab for High-Risk MDS/AML Patients After Allogeneic Stem Cell Transplant With Post-Transplant Cyclophosphamide

Inclusion Criteria for initial enrollment: * Written informed consent form (ICF), according to local guidelines, signed by the patient or by a legal guardian prior to the performance of any study-related screening procedures (i.e., prior to conditioning). * Male and female patients between ≥18 and ˂66 years-of-age * Patients with high-risk AML defined as: AML with ≥5% bone marrow blast burden prior to allogeneic SCT who failed ≥2 lines of cytoreductive anti-leukemic therapy * Patients with high risk MDS defined as: MDS with ≥10% bone marrow blasts prior to allogeneic SCT despite 1 line of prior cytoreductive anti-leukemic treatment with chemotherapy or hypomethylating agent * Patients will receive a MAC MUD or MAC haploidentical SCT followed by PTCy as treatment for AML or MDS. * A 10/10 HLA-match is required for patients that will receive a MUD SCT. * A 5/10 HLA-match or greater is required for patients that will receive a haploidentical SCT. * Patients must be able to swallow and retain oral medication. * Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0, 1, or Inclusion Criteria after SCT, prior to start of nivolumab: * Patients who have achieved a CR post-allogeneic SCT * Patients who have persistent disease post-allogeneic SCT * Greater than 50% PB donor T cell chimerism * Adequate renal function defined as serum creatinine ≤2.0 mg/dL or creatinine clearance ≥40 mL/min measured or calculated by Cockcroft-Gault equation. * Females of childbearing potential must have a negative serum or urine pregnancy test result within 72 hours prior to the first dose of nivolumab and must agree to follow instructions for method(s) of contraception, using two forms of acceptable contraception, including one barrier method, for the duration of treatment with nivolumab and for 7 months following their last dose of the study drug. Females of non-childbearing potential are those who are postmenopausal greater than 1 year or who have had a bilateral tubal ligation or hysterectomy. * Male patients with female partners of childbearing potential are required to use contraceptive methods, during their participation in the study and for 7 months following the last dose of study drug. Male patients must also refrain from donating sperm during their participation in the study and for 7 months following the last dose of study drug. * Ability to understand the nature of this study and to comply with study and follow-up procedures. Exclusion Criteria for initial enrollment: * Prior allogeneic SCT * Pregnant or lactating women * Evidence of active uncontrolled bacterial, fungal, parasitic, or viral infection. Infections are considered controlled if appropriate therapy has been instituted and, at the time of screening, no signs of active infection progression are present. This is assessed by the site clinicians including consulting physicians from infectious disease regarding adequacy of therapy. These infections include, but are not limited to: * Known human immunodeficiency virus (HIV) infection * Active tuberculosis infection * History of autoimmune pneumonitis within the last 5 years * Prior diagnosis of an inflammatory bowel disease (e.g., Crohn's disease, ulcerative colitis) * History of severe hypersensitivity reactions to monoclonal antibodies * Psychological, familial, sociological, or geographical conditions that do not permit compliance with the protocol. Exclusion Criteria after SCT, prior to start of nivolumab: * Current evidence of any grade of active aGVHD at Day 40-60 at the time of study enrollment * Prior history of Grade II or higher aGVHD (Appendix E) * Prior or concurrent treatment with DLI * Use of a study drug ≤21 days or 5 half-lives (whichever is shorter) prior to the first dose of nivolumab, except antimicrobial drugs. For study drugs for which 5 half-lives is ≤21 days, a minimum of 10 days between termination of the study drug and administration of nivolumab is required, except antimicrobial drugs. * Evidence of active uncontrolled bacterial, fungal, parasitic, or viral infection. Infections are considered controlled if appropriate therapy has been instituted and, at the time of screening, no signs of infection progression are present. * Active autoimmune disease that has persisted or recurred after allogeneic SCT, except vitiligo or resolved childhood asthma/atopy. * Inadequate organ function: * Clinically significant or uncontrolled cardiac disease, including unstable angina, acute myocardial infarction within 6 months of enrollment, New York Heart Association Class III or IV congestive heart failure (Appendix B), circulatory collapse requiring vasopressor or inotropic support, or arrhythmia that requires therapy. If the patient does not meet the minimum criteria and/or receive cardiotoxic agents, cardiology consultation and clearance is recommended. * Significant respiratory disease that requires mechanical ventilation support or a resting O2 saturation \<90% by pulse oximetry. FEV1/FVC/DLCO \<50%. If the patient does not meet the minimum criteria, pulmonary consultation and clearance is recommended. * Serum creatinine ≥2.0 mg/dL. If the patient does not meet the minimum criteria, nephrology consultation and clearance is recommended. * Serum bilirubin \>2.5 mg/dL (except for hemolysis or Gilbert's syndrome) and transaminases ≥3 upper limit of normal (ULN). If the patient does not meet minimum criteria, hepatology consultation and clearance should be considered. If warranted, a liver biopsy should be performed prior to transplant. * Any corticosteroid therapy for indications other than GVHD at doses \>1 mg/kg/day methylprednisolone or equivalent within 7 days of start of nivolumab. * Any corticosteroid therapy for prior GVHD including topicals, budesonide and beclomethasone PO at any dose within 7 days of start of nivolumab.