Exploratory Study of Anti-BCMA-CD19 CAR-T Cell Therapy in Relapsed or Refractory IgG4-Related Disease

The goal of this clinical trial is to test the safety and potential benefit of a new immune cell therapy called anti-BCMA-CD19 CAR-T cells in adults (18-75 years) with IgG4-related disease (IgG4-RD) that has come back or not improved after standard treatments such as glucocorticoids or rituximab. The main questions this study aims to answer are: * What medical problems (side effects) occur after receiving anti-BCMA-CD19 CAR-T cell therapy? * Does anti-BCMA-CD19 CAR-T cell therapy improve IgG4-RD disease activity scores at 12 weeks and 26 weeks? Participants will: * Have their own blood immune cells collected by a procedure called leukapheresis * Receive short-term chemotherapy to prepare the immune system * Receive one intravenous infusion of anti-BCMA-CD19 CAR-T cells * Return for regular clinic visits over 26 weeks for safety checks, blood tests, and imaging * May be followed for up to one year in total