IL-15 produced by the engineered T cells signals through IL-2Rβ (CD122) on the CAR T cells to boost their activation, proliferation, and persistence; the CAR T cells then kill CD19/CD22/BCMA-positive tumor cells via CAR/CD3ζ activation and perforin/granzyme-mediated lysis. IL-2Rβ-expressing cells are not the cytotoxic targets.