CD33 and acute myeloid leukemia: Importantly, the sustained serial killing capacity of CD33-CAR-NK cells was also observed when the immune cells were re-challenged with AML cells supplied with fresh medium, indicating that the tumor cell death was not mediated by secondary mechanisms (for example increased levels of IFN-γ or TNF-α) but by sustained CAR-directed cytotoxic activity (Fig. 1H).