Allogeneic UCB-derived CAR-T for SLE

* The main questions it aims to answer are: What CAR-T-related adverse events (AEs) occur within 3 months after the UCAR-T cell infusion? Which dose level is the optimal biological dose (OBD)? What are the changes of disease activity status, proportion of patients achieving DORIS remission, percentage of participants achieving maintenance of drug-free DORIS remission, proportion of patients achieving SRI-4 remission, percentage of participants achieving maintenance of LLDAS? * Participants will may receive lymphodepletion chemotherapy (fludarabine plus cyclophosphamide) if clinically needed. If lymphodepletion chemotherapy is administered, rest for 2 days on Day -2 and Day -1. Receive UCAR-T cells infusion on Day 0. Then be hospitalized for at least 7 days post-infusion for close safety monitoring and remain within 2 hours of the treatment facility for at least 28 days. Visit the clinic at Day 14, Day 28, month 3, month 6, month 9, month 12, month 18 and month 24 after UCAR-T cells infusion.