CD19 and acute lymphoblastic leukemia: The binding of murine scFv to the CD19 epitope may trigger the HLA-restricted T-cell-mediated immunoregulatory response, leading to the diminished persistence of CD19 CAR T-cells in vivo and even ALL relapse (8), while humanized scFv can reduce the antigenicity of CAR T-cells and enhance its persistence in vivo to improve its efficacy.