The authors would like to thank the staff in the Product Development and Correlative Sciences laboratory for helpful discussions and analytical support, staff in Clinical Cell and Vaccine Production Facility for manufacturing and analytical support, and staff in the Stem Cell and Xenograft Facility for animal support (University of Pennsylvania, Philadelphia, PA). The authors would like to thank E. Sotillo and A. Thomas-Tikhonenko (Children’s Hospital of Philadelphia, PA) for providing the CD19 CRISPR–Cas9 knock-out NALM-6 cells and B. Jena and L. Cooper (MD Anderson Cancer Center, Houston, TX) for providing the Alexa-Fluor-647-conjugated anti-idiotype antibody. The chimeric antigen receptor used in this study was obtained under a Material transfer agreement (MTA) from Campana and Imai at St. Jude Children’s Research Hospital and was subsequently modified by cloning into a lentiviral vector and expressed with a eukaryotic promoter. This work was supported by grants from the University of Pennsylvania–Novartis Alliance (principal investigator (PI), C.H.J), National Institutes of Health (NIH) 5R01CA120409 (PI, C.H.J), the EMD–Serono Cancer Immunotherapy Clinical Fellowship by the Society for Immunotherapy of Cancer (SITC) (PI, M.R.), the Bristol–Myers Squibb Oncology Fellowship in Clinical Cancer Research by the American Association for Cancer Research (AACR) (PI, M.R.), the Gabrielle’s Angel Foundation (PI, M.R.; PI, D.M.B.; and PI, J.A.F.), the SIES–AIL fellowship by the Italian Society for Experimental Hematology and the Italian Leukemia Association (PI, M.R.), the ASH-Scholar Award (P.I., M.R.), NIH NCI 1K99CA212302-01A1 (PI, M.R.), NIH NCI P01CA214278-01 (PI, C.H.J.) the St. Baldrick’s Foundation Scholar Award (PI, D.M.B.) and NCI T32CA009140 (J.A.F).