Massive CAR T-cell expansion coincides with toxicity. (A) Time course of adverse events following CAR T-cell infusion. The change in color of the MNTs indicates the timing of cyclophosphamide. (B) Therapeutic strategies deployed for CAR T-cell–related toxicity. (C) Absolute lymphocyte count (ALC) and CAR T-cell count (cells/μL blood) are plotted on the left y-axis, and vector copy number (VCN) per cell is plotted on the right y-axis. Absolute CAR T-cell number decreases following cyclophosphamide therapy, but CAR T cells remain the predominant population of circulating lymphocytes. (D) M protein is plotted on the left y-axis, and the difference between involved and uninvolved serum-free light chains is plotted on the right y-axis. Both biomarkers decrease rapidly following CAR T-cell infusion and remain low for the duration of follow-up, consistent with an ongoing stringent complete response. (E) Immunophenotyping demonstrates CAR T-cell expansion, gated on CD3⁺, viable lymphocytes. (F) Magnetic resonance imaging (MRI) of the brain showed that the caudate, striatum, and other structures comprising the basal ganglia were normal. A prior comparison MRI of the brain was not available. (G-H) Brain (18)F-fluorodeoxyglucose–positron emission tomography images showed normal metabolic activity in the basal ganglia and occipital lobes (G) and mild bilateral hypometabolism in the frontal lobes, anterior cingulate gyri, and, to a lesser degree, parietal lobes (H). APC, allophycocyanin; DIC, disseminated intravascular coagulation; FLC, free light chains; PE, phycoerythrin; sBCMA, soluble BCMA; UTI, urinary tract infection.