CD19 and acute lymphoblastic leukemia: Therefore, the mechanism of CD19 antigen down-regulation after CAR T therapy as the unique cause of leukemic relapse is not fully established: on the one hand, the induction by CAR T cells of CD19 down-regulation in ALL has not been described in vitro and, on the other hand, UPENN data seem to demonstrate that tisagenlecleucel is equally effective in vitro and in vivo against CD19dim cells, so that further research in this direction is necessary to be able to affirm that down-modulation of the target antigen is a mechanism involved in CD19-negative relapse after CAR T therapy.