NK cell cytotoxicity is suppressed in tumor-bearing mice. Seven-week-old BALB/c mice were injected subcutaneously with TS/A tumor cells (3 × 10⁵) or PBS as a control. Four weeks after tumor challenge, the mice were anesthetized and injected intravenously with YAC-1–Luc cells (1 × 10⁶). After injection of D-luciferin the mice were imaged at hours 0, 2, 4, and 6 (A), and the total photon count per minute (photons per minute) calculated (5 animals) using Living Image software. The efficiency of NK cell killing of injected YAC-1–Luc cells was determined by measuring the numbers of photons collected at the imaging time points divided by at 0 hours collected (B). *P < .05; **P < .001. On completion of the imaging studies, NK cells (DX5⁺) were isolated from the spleen, and then stimulated with recombinant IL-2 (100 U/mL) for 5 days. After the incubation period, the NK cells were added to YAC-1–Luc or TS/A-Luc cells at varying effector-target (E/T) ratios (10:1, 20:1, and 40:1) as indicated in panels C and D. The cytotoxicity of NK cells to YAC-1–Luc (C) or TS/A-Luc (D) was determined using an NK cell cytotoxic assay as described in “Cytotoxicity assay.” The data represent the mean ± SEM from 5 mice from each group.