This study aims to evaluate the efficacy and safety of PD-1 inhibitor, Azacitidine, and low-dose DLI in AML relapse After allogeneic hematopoietic stem cell transplantation

Inclusion Criteria:

1. Patients with a diagnosis of AML relapse after allogeneic hematopoietic stem cell transplantation.
2. Adequate organ function.
3. Be able to understand and sign informed consent.
4. Age 18 to 60 years old.
5. Serum pregnancy test for females of childbearing potential that is negative within one week prior to initiation of first dose of treatment. Female patients of childbearing potential and sexually active males must agree to use a highly effective method of contraception throughout the study and for at least 90 days after the last dose of assigned treatment.
6. ECOG performance status ≤ 1.
7. Known HLA-matched donor without contraindications to donate.
8. Life expectancy > 3 months.

Exclusion Criteria:

1. Diagnosis of anther malignant disease.
2. Suspected or proven acute or chronic GVHD.
3. Proven central nervous system leukemia.
4. Prior treatment with anti-PD-1, anti-PD-L1, or DLI.
5. HLA loss positive.
6. Known active viral infection with known human immunodeficiency virus (HIV) or viral hepatitis type B (HBV) or C (HCV) or Corona Virus Disease 2019(COVID-19);
7. Uncontrolled systemic fungal, bacterial, or viral infection.
8. Known or suspected hypersensitivity to PD-1 inhibitor or azacytidine.
9. Participation in another clinical study within 3 months.
10. Any serious medical or psychiatric illness that could, in the investigator's opinion, potentially interfere with the completion of study procedures.