RecruitingDrug trialFirst-Line Jaktinib for Acute Graft-Versus-Host Disease (aGVHD)This study aims to evaluate the optimal dose (Recommended Phase 2 Dose, RP2D), preliminary safety, and efficacy of gecacitinib (also known as jaktinib) in combination with glucocorticoids as first-line therapy for patients with grade II-IV acute graft-versus-host disease (aGVHD) following allogeneic hematopoietic stem cell transplantation (allo-HSCT).