Although CD19 CAR NK cells efficaciously killed the two CD19-positive B-cell precursor ALL cell lines REH and BV-173 in overnight cultures in vitro, our in vivo experiments clearly demonstrated that support of the CD19 CAR NK cells by IL15 co-expression is an absolute prerequisite for the control of the ALL blasts in vivo. While the co-expression of soluble IL15 was required to increase survival of the animals, as also reported by others (17, 67), the group of animals that received CAR NK cells, where IL15 is tethered to the IL15 receptor α-chain, lived slightly longer. Here, CD19 is linked to acute lymphoblastic leukemia.