Adults 18–75 with relapsed/refractory MM or plasma cell leukemia, progressive and measurable (serum/urine M‑protein, sFLC ≥100 mg/L with abnormal ratio, or ≥2% circulating PCs), ECOG 0–1, survival >12 wks, adequate organ function, consent, effective contraception. Exclude: pregnancy/lactation, HIV/syphilis/CMV‑IgM/EBV, active HBV/HCV, infections, unresolved AEs, prior BCMA CAR‑T, allo‑SCT, recent auto‑SCT/therapy/steroids/vaccines, severe allergy, LVEF <50%, O2‑dependent, autoimmune disease, other malignancy, CNS mets, recent major surgery, noncompliance.