CARhi T cells arise from transient CAR transcript upregulation. a Genomic copy number and mRNA expression of CARhi and CARlo populations as determined by quantitative PCR relative to the housekeeping gene β-actin. The lowest expression level or copy number is set to 1. b EGFRt expression level of sorted cells maintained in culture with exogenous IL-2 and IL-15 and no antigen stimulation. c EGFRt and CAR surface expression on CD8+ CAR-T cells as measured by Erbitux and Protein-L staining, respectively, over 4 days of co-incubation with or without CD19+ K562 target cells. Values in b are averages of quadruplicates with error bars indicating ± 1 s.d.; all other plots show average values of triplicates with error bars indicating ± 1 s.d.