NK cells cocultured with Gr-1⁺CD11b⁺F4/80⁺ cells produce large amounts of IFN-γ. (A-D) Purified Gr-1⁺CD11b⁺F4/80⁺ or Gr-1⁺CD11b⁺F4/80⁻ cells from tumor-bearing mice were cocultured with NK cells at the indicated ratios. Supernatants were harvested after 24 hours and IFN-γ amounts were determined by ELISA. (A) NK cells from naive mice were cultured alone or in the presence of Gr-1⁺CD11b⁺F4/80⁺ cells (left panel) or Gr-1⁺CD11b⁺F4/80⁻ (right panel) in the absence of IL-12. (B) NK cells were cultured in IL-12 in the absence or in the presence of Gr-1⁺CD11b⁺F4/80⁺ (left panel) or Gr-1⁺CD11b⁺F4/80⁻ (right panel) cells at the indicated ratios. (C) Tumor-infiltrating Gr-1⁺CD11b⁺F4/80⁺ cells were purified and cultured with NK cells in the presence of IL-12. (D) NK cells isolated from C57BL/6 wt or IFN-γ^−/− mice were cultured in IL-12 in the absence of presence of Gr-1⁺CD11b⁺F4/80⁺ cells. Data depict mean plus or minus SD of triplicate cultures and represent one of 2 (A,C,D) or one of 5 (B) independent experiments. Asterisks indicate statistical significance (P < .01) determined by Student t test. ND indicates not detectable.