Safety and Efficacy of Early Second Infusion of Axi-cel Based on ctDNA for R/R Large B - Cell Lymphoma

The goal of this clinical trial is to evaluate the efficacy and safety of early secondary infusion of CD19 CAR T-cell therapy in adults with relapsed/refractory diffuse large B-cell lymphoma (DLBCL), guided by ctDNA monitoring. The main questions it aims to answer are: 1. Efficacy: Does early secondary CAR-T infusion improve the 3-month complete remission (CR) rate and long-term survival outcomes (e.g., 1-year PFS, OS)? 2. Safety: What are the adverse events associated with secondary CAR-T infusion, such as cytokine release syndrome (CRS), immune effector cell-associated neurotoxicity (ICANS), and infections? This is a single-arm, single-center, prospective study. All participants will receive: * Leukapheresis to collect T cells for CAR-T manufacturing. * Preconditioning chemotherapy (fludarabine and cyclophosphamide) to prepare the body for CAR-T infusion. * Two CD19 CAR-T infusions: The first infusion (2×10⁶ cells/kg) followed by a second infusion (same dose) if ctDNA remains positive when PET/CT shows CR or PET/CT shows PR within 60 days post-first infusion. Participants will undergo: * Frequent hospital monitoring for ≥14 days post-infusion to manage potential toxicities. * Regular follow-ups (e.g., blood tests, ctDNA analysis, PET/CT scans) at scheduled intervals up to 12 months. * Continuous safety assessments, including CRS grading, neurological evaluations, and infection monitoring.