JCARH125

DLT is defined as adverse events (AEs) that occur within 21 days following JCARH125 infusion and meet any of the following criteria: * Treatment-emergent Grade ≥3 allergic reactions related to JCARH125; * Treatment-emergent Grade 3 seizures, regardless of attribution, that do not resolve to Grade ≤2 within 3 days in participants who have no evidence of central nervous system (CNS) involvement of Multiple Myeloma or other CNS pathology; * Treatment-emergent autoimmune toxicity Grade ≥3, regardless of attribution (excluding B-cell aplasia); * Treatment-emergent Grade 3 CRS that does not resolve to Grade ≤2 within 72 hours; * Any other treatment-emergent Grade 3 AE related to JCARH125 that does not resolve to Grade ≤2 within 7 days; * Any treatment-emergent Grade 4 AE related to JCARH125 that does not resolve to Grade ≤2 within 7 days; * Treatment-emergent Grade 4 Cytokine Release Syndrome of any duration; * Any treatment-emergent Grade 5 toxicity not due to the underlying malignancy

TEAE is defined as an AE that starts any time from initiation of JCARH125 administration through and including 90 days following the JCARH125 infusion graded using the NCI Common Terminology Criteria for Adverse Events (CTCAE), version 4.03. Any AE occurring after the initiation of another anticancer treatment will not be considered a TEAE. Grade 3=Severe; Grade 4=Life-threatening; and Grade 5=death.

Clinically significant laboratory abnormalities are assessed by investigator and are reported as treatment-emergent adverse event (TEAE). TEAE is defined as an AE that starts any time from initiation of JCARH125 administration through and including 90 days following the JCARH125 infusion graded by Common Terminology Criteria for Adverse Events (CTCAE), version 4.03. Any AE occurring after the initiation of another anticancer treatment is not considered a TEAE. Grade 3=Severe; Grade 4=Life-threatening.

Number of participants receiving prophylactic anakinra with grade ≥ 2 CRS relative to the number of participants treated at the recommended Phase 2 dose (RP2D) in the Phase 1 dose escalation portion of the trial with grade ≥ 2 CRS. CRS grade is defined by the most severe symptom (excluding fever). Grade 2=moderate; Grade 3=severe; Grade 4=life-threatening

Time to first onset of Grade ≥2 CRS in participants receiving prophylactic anakinra relative to onset of Grade ≥ 2 CRS in participants treated at the RP2D(s) in the Phase 1 dose escalation portion of the trial. Time to onset is calculated from the latest JCARH125 infusion prior to the first onset of Grade \>= 2 CRS. CRS grade is defined by the most severe symptom (excluding fever). Grade 2=moderate; Grade 3=severe; Grade 4=life-threatening

The number of participants receiving prophylactic anakinra with no CRS occurring on study days 1, 2, or 3. CRS grade is defined by the most severe symptom (excluding fever). Grade 2=moderate; Grade 3=severe; Grade 4=life-threatening

ORR is defined as stringent complete response (sCR), complete response (CR), very good partial response (VGPR), or partial response (PR), according to IMWG criteria. Participants without any reported disease response assessments will be considered non-responders. sCR=complete response plus normal free light chain ratio and absence of clonal cells in bone marrow biopsy by immunohistochemistry; CR=negative immunofixation of serum and urine and disappearance of any soft tissue plasmacytomas and \< 5% plasma cells in bone marrow aspirates. When the only method to measure disease is by serum FLC levels, CR can be defined as a normal FLC ratio of 0.26 to 1.65; VGPR=serum and urine M-protein detectable by immunofixation but not on electrophoresis or ≥ 90% reduction in serum M-protein level plus urine M-protein level \< 100 mg/24 h; PR=≥ 50% reduction of serum M protein plus reduction in 24-hour urinary M-protein by ≥ 90% or to \< 200 mg/24 h