Adverse events

Patients complete apheresis and proceed to lymphodepleting chemotherapy with cyclophosphamide 300mg/m\^2 and fludarabine 30mg/m\^2 for 3 consecutive days followed by the infusion of BCMA CAR T-cells at a target dose of 450 x10\^6 cells. Maintenance lenalidomide, starting at 5mg a day for 21 days of a 28-day cycle will be initiated at a minimum of at least 30 days, but no later than 180 days after the CAR T-cell infusion and will continue until the patient reaches 12 months post CAR T-cell infusion and continue free of progression. Lenalidomide: Maintenance lenalidomide, starting at 5 mg a day for 21 days of a 28-day cycle, will be initiated at a minimum of at least 30 days, but no later than 180 days after the CAR T-cell infusion and will continue until the participant reaches 12 months post CAR T-cell infusion and continues free of progression bb2121: Participants receive infusion of BCMA CAR T-cells at a target dose of 450 x 10\^6 cells. Cyclophosphamide: 300 mg/m\^2/day for 3 consecutive days prior to the CAR T infusion Fludarabine: 30 mg/m\^2/day or 3 consecutive days prior to the CAR T infusion leukapheresis: Placement of central line catheter and leukapheresis