Higher peak of IL-15 is associated with a higher CD19 CAR T cell AUC_0-90 in humans. The CAR transgene (FlapEF1⍺ copies per μg of DNA) is shown as a polynomial regression line using the locally estimated scatterplot smoothing method (bold lines) for 3 distinct tertiles of serum IL-15 peak concentration. The shaded areas represent the 95% confidence interval of the locally estimated scatterplot smoothing estimates. The dashed line indicates the limit of quantitation of the CAR transgene quantitative polymerase chain reaction assay.