Clinical Study of the Safety and Efficacy of BCMA CAR-NK

Inclusion Criteria: * Age 18 years or above, no gender preference. * Patients who have received at least 3 prior lines of treatment for multiple myeloma and have failed at least proteasome inhibitor and immunomodulator therapy; Each line has at least 1 complete treatment cycle unless the best remission status for that treatment is documented as progressive disease (PD) (as per the IMWG efficacy evaluation criteria published in 2016, Appendix 4); PD must be documented during or within 12 months after receiving the last treatment. * Presence of measurable lesions at screening, which are defined as any of the following situations: Serum M protein≥1 g/dL (≥10 g/L) Urinary M protein≥200 mg/24 hours Serum free light chain (FLC): abnormal serum FLC ratio (\<0.26 or \>1.65) and involved FLC≥10 mg/dL (100 mg/L) * ECOG score (Appendix 1): 0\~1. * Expected survival≥3 months. * The following test values within 7 days prior to lymphocyte clearance meet the following criteria: Hematology Absolute lymphocyte count:≥0.5×109/L\[Granulocyte colony-stimulating factor (G-CSF) is allowed, but subjects should not have received this supportive therapy within 7 days prior to laboratory test during the screening period\] Absolute neutrophil count:≥1.0×10\^9 /L\[Granulocyte colony-stimulating factor (G-CSF) is allowed, but subjects should not have received this supportive therapy within 7 days prior to laboratory test during the screening period\]. Platelets:Subjects platelet count ≥50 x 10\^9/L (subjects must not receive transfusion support within 7 days prior to laboratory test during the screening period) Hemoglobin:≥8.0 g/dL (recombinant human erythropoietin is allowed) \[subjects have not received a red blood cell (RBC) transfusion within 7 days prior to laboratory test during the screening period\]. Liver Total bilirubin (serum) :Total bilirubin (serum) ≤1.5 × ULN AST and ALT:≤3× ULN * Peripheral venous pathway meets the requirements of intravenous drip. * Subjects agree to use reliable methods for contraception from the time of signing the informed consent till 1 year after the transfusion. * Voluntary participation in the clinical trial and signing of the informed consent form. Exclusion Criteria: * Subjects who have had a severe anaphylactic reaction. * Subjects who received the following anti-MM therapy within a specific time prior to lymphocyte clearance. Small molecule targeted therapy within 2 weeks or 5 half-lives (whichever is longer). Large-molecule drug within 4 weeks or 2 half-lives (whichever is longer). Cytotoxic drugs, modern Chinese medicine preparations with antitumor effects within 2 weeks. Immunomodulators therapy within 1 week. * Subjects who have received a live or attenuated vaccine within 4 weeks prior to lymphocyte clearance. * Subjects who have received the following therapy within 7 days prior to lymphocyte clearance, or that requires long-term treatment during the study period according to the investigators: Systemic steroid therapy (except for inhaled one or topical use). Immunosuppressive therapy. Treatment of graft-versus-host response. * Subjects presenting with incomplete recovery or stabilization to grade 1 (NCI-CTCAE v5.0) of toxicity (including peripheral neuropathy) caused by prior treatments. * Cardiac disease: episode of myocardial infarction≤6 months prior to lymphocyte clearance; episode of unstable angina, severe arrhythmia as judged by the investigators, or coronary artery bypass graft≤3 months prior to lymphocyte clearance. * Women who are pregnant or breastfeeding. * Subjects who, in the opinion of the investigators, have any clinical or laboratory test abnormalities or other reasons that make them ineligible to participate in this clinical study.