RecruitingPhase 1Biological interventionDC/MM Fusion Vaccine With BCMA CAR-T in R/R MMThis study is to evaluate the safety and effectiveness of dendritic cell DC/MM fusion vaccine in combination with standard of care B-cell maturation antigen (BCMA) CAR-T cell therapy in participants with relapsed/refractory multiple myeloma. The names of the study drugs involved in this study are: * DC/MM fusion vaccine (a type of personalized cancer vaccine) * Granulocyte-macrophage colony-stimulating factor (GM-CSF) (a type of growth factor or hormone)