Cytotoxicity of NK cells from NOD/SCID mice. (A) NK cells were harvested after 28-day culture of NOD/SCID BM (n = 2, ▧) or spleen (n = 3, ▦) cells in the presence of IL-15, FL, and SCF. For comparison, NK cells were purified from human PB mononuclear cells (n = 2, □). Their cytotoxic activity was tested against the NK-sensitive K562 target cell line for 4 hours with an enzymatic LDH release assay. (B) NOD/SCID mice were injected subcutaneously with K562 cells (1 × 10⁷) alone (▪) or 1 day later by NK cells (2-5 × 10⁶) from human PB (□) or generated in vitro from NOD/SCID BM (▧). NOD/SCID mice that had undergone transplantation with CB CD34⁺ cells were injected with human IL-15 and FL for 7 days and were inoculated with K562 cells 1 day later (▦). Tumor size is expressed as mean ± SEM areas of 2 to 4 animals per treatment group. Significant differences compared with the group without NK cells are marked (*P < .05; **P < .005).