Gr-1⁺CD11b⁺F4/80⁺ cells from RMA-S tumor-bearing mice do not affect NK cell proliferation and cytotoxic activity. (A) NK cells isolated from naive mice were cultured with 400 U/mL IL-2 for 48 hours in the absence or presence of Gr-1⁺CD11b⁺F4/80⁺ cells from tumor-bearing mice at the indicated ratios. Proliferation was determined by ³H-thymidine incorporation assay. (B) Purified Gr-1⁺CD11b⁺F4/80⁺ or Gr-1⁺CD11b⁺F4/80⁻ cells from tumor-bearing mice were cultured with NK cells from naive mice for 4 hours and used as effector cells against YAC-1 targets at the indicated ratios. Specific killing was determined by a standard 4-hour ⁵¹Cr-release assay. Data represent mean plus or minus SD of triplicate cultures. (A,B) One representative experiment of 2 independent experiments is shown.