First-Line Jaktinib for Acute Graft-Versus-Host Disease (aGVHD)

Inclusion Criteria: 1. Voluntarily provide signed informed consent and be ≥18 years of age at the time of consent. 2. Recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT) using bone marrow, peripheral blood stem cells, or umbilical cord blood. 3. Have received systemic glucocorticoid therapy for no more than 2 days prior to enrollment. 4. Demonstrate clear myeloid and platelet engraftment: absolute neutrophil count (ANC) \> 1.0 × 10⁹/L and platelet count \> 50 × 10⁹/L (permitted use of growth factors or transfusion support). 5. Clinical diagnosis of grade II-IV acute GVHD (aGVHD) per the MAGIC (Mount Sinai Acute GVHD International Consortium) criteria (Appendix 1). 6. ECOG performance status of 0-2. 7. Life expectancy \> 4 weeks. 8. Able to swallow tablets. 9. Willing and able to comply with study procedures and follow-up. Exclusion Criteria: 1. History of ≥2 allo-HSCT procedures. 2. Development of aGVHD following unplanned donor lymphocyte infusion (DLI) for relapse of underlying malignancy. Note: Planned DLI as part of the transplant protocol is permitted. 3. Concurrent treatment with another JAK inhibitor. Note: Patients who previously discontinued JAK inhibitors due to toxicity (not refractory aGVHD) are eligible. 4. Active bleeding. 5. Diagnosed or suspected chronic GVHD. 6. Uncontrolled active infection, defined as sepsis-induced hemodynamic instability or progressive symptoms/signs/imaging findings attributable to infection. Asymptomatic or persistent fever alone is not exclusionary. 7. unresolved toxicity or complications from allo-HSCT (excluding aGVHD). 8. Clinically significant abnormalities that may compromise safety, including: a) Uncontrolled diabetes (fasting glucose \>13.9 mmol/L); b) Hypertension unresponsive to ≥2 agents (systolic BP ≥160 mmHg or diastolic BP ≥100 mmHg); c) Peripheral neuropathy ≥Grade 2 (per NCI CTCAE v5.0). 9. Within 6 months prior to screening: NYHA Class III/IV heart failure, unstable angina, myocardial infarction, cerebrovascular accident, or pulmonary embolism. 10. Arrhythmia requiring treatment or QTcB interval \>480 ms at screening. 11. Severe renal impairment (serum creatinine \>1.5 × ULN) at screening. 12. Pre-transplant history of gastrointestinal ulcers, gastrectomy, or intestinal resection that may impair drug absorption. 13. Major surgery within 4 weeks prior to screening without full recovery. 14. Cholestatic disorders or hepatic sinusoidal obstruction syndrome (SOS/VOD) at screening (defined as persistent hyperbilirubinemia and organ dysfunction unrelated to GVHD). 15. Active uncontrolled viral infections at screening: HBV: HBsAg⁺ with detectable HBV-DNA, or detectable HBV-DNA regardless of HBsAg status; HCV: Anti-HCV antibody⁺ with detectable HCV-RNA. 16. History of active tuberculosis within 6 months prior to screening. 17. Epilepsy or current use of psychotropic/sedative medications. 18. Pregnancy, lactation, or intention to conceive; male patients unwilling to use condoms during treatment and for 2 days after the last dose. 19. Other active malignancies (excluding the transplanted hematologic malignancy) within 5 years. 20. Current use of anticoagulants or antiplatelet agents (except low molecular weight heparin). 21. Any condition that, in the investigator's judgment, may compromise patient safety or protocol compliance. 22. Known hypersensitivity to jaktinib, its analogs, or excipients. 23. Participation in another interventional clinical trial within 4 weeks prior to screening. 24. Investigator determination of unsuitability for the study.