Uncommon biphasic CAR-T expansion induces hemophagocytic lymphohistiocytosis-like syndrome and fatal multiple infections following BCMA CAR-T cell therapy: a case report.

B-cell maturation antigen(BCMA)-directed chimeric antigen receptor (CAR)-T-cell therapy has significantly improved the treatment of relapsed or refractory multiple myeloma (MM). Nevertheless, the uncommon phenomenon of biphasic CAR-T cell expansion in vivo and its related severe toxicities have not been methodically described and studied. Herein, we report a case of patients with MM who experienced two CAR-T cell expansion peaks and subsequently developed multiple severe toxicities following BCMA CAR-T cell infusion. The first expansion peak occurred on Day 7, accompanied by grade 3 cytokine release syndrome. The second peak occurred on Day 28, associated with severe immune effector cell-associated hematotoxicity (ICAHT), immune effector-cell associated hemophagocytic lymphohistiocytosis-like syndrome (IEC-HS), and polymicrobial infections. Both ICAHT and IEC-HS were refractory to our standard treatments; however, human umbilical cord mesenchymal stem cell infusion exhibited some efficacy in improving cytopenia. Despite the administration of a broad-spectrum anti-infective regimen, cytomegalovirus viremia remained uncontrollable, resulting in the development of central nervous system infection, neurological symptoms, and ultimately death. Additionally, we also employed high-dimensional 33-color spectral flow cytometry to describe the dynamic changes in immune cell components and functions between the two expansion peaks. Collectively, this case provides novel insights into the biphasic CAR-T expansion and related immune effector cell-associated toxicities.