Gr-1⁺CD11b⁺F4/80⁺ cells are eliminated by NK cells in vitro and in vivo. (A) IL-2–activated NK cells were cultured with Gr-1⁺ cells isolated from PBLs of tumor-bearing mice at the indicated effector-target ratios for 12 hours. Percentages of Gr-1⁺CD11b⁺F4/80⁺ cells among Gr-1⁺ were determined by flow cytometry. Data are presented as mean plus or minus SD of triplicate cultures. One representative experiment of 3 independent experiments is shown. (B) NK cells were isolated from naive C57BL/6 mice. NK cells (4-6 × 10⁶) were injected intravenously into C57BL/6-Ly5.1 tumor-bearing mice (n = 9) at day 18 or 19 after tumor cell inoculation. Percentages of Gr-1⁺CD11b⁺F4/80⁺ and Gr-1⁺CD11b⁺F4/80⁻ cells among total PBLs were determined by flow cytometry 4 hours before and 5 hours after transfer. Injected NK cells were excluded by gating on Ly5.2-negative cells. In control experiments, mice were mock-treated as described in “Methods.” *P < .05. NS indicates not statistically significant.