BCMA-Targeted CAR T-cell Therapy Plus Radiotherapy for the Treatment of Refractory Myeloma Reveals Potential Synergy

We present a case of a patient with multiply relapsed, refractory myeloma whose clinical course showed evidence of a synergistic abscopal-like response to chimeric antigen receptor (CAR) T-cell therapy and localized radiotherapy (XRT). Shortly after receiving B-cell maturation antigen (BCMA)-targeted CAR T-cell therapy, the patient required urgent high-dose steroids and XRT for spinal cord compression. Despite the steroids, the patient had a durable systemic response that could not be attributed to XRT alone. Post-XRT findings included a second wave of fever and increased CRP and IL6, beginning 21 days after CAR T cells, which is late for cytokine-release syndrome from CAR T-cell therapy alone on this trial. Given this response, which resembled cytokine-release syndrome, immediately following XRT, we investigated changes in the patient's T-cell receptor (TCR) repertoire over 10 serial time points. Comparing T-cell diversity via Morisita's overlap indices ( ), we discovered that, although the diversity was initially stable after CAR T-cell therapy compared with baseline ( = 0.89-0.97, baseline vs. 4 time points after CAR T cells), T-cell diversity changed after the conclusion of XRT, with >30% newly expanded TCRs ( = 0.56-0.69, baseline vs. 4 time points after XRT). These findings suggest potential synergy between radiation and CAR T-cell therapies resulting in an abscopal-like response.