Tumor-derived IL-6 mediates tumorigenic effects of IL-17. (A and B) IL-6 secretion in the tumor microenvironment correlates with IL-17 levels. (A) Single-cell suspensions prepared from B16 tumors harvested from the indicated mice were cultured in vitro overnight; supernatants were then collected and assayed for IL-6 levels, combining four mice from two independent experiments (WT vs. IL-17^−/−, P = 0.0026; IFN-γ^−/− vs. IFN-γ^−/−IL-17^−/−, P = 0.005). Data are means ± SEM. (B) Single-cell suspensions prepared from MB49 tumors were enriched for CD45. Both CD45⁺ (immune cells) and CD45⁻ (tumor cells) were cultured overnight before supernatants were collected for ELISA (n = 4 mice representing three independent experiments; IFN-γ^−/− vs. IFN-γ^−/−IL-17^−/−, P = 0.03). Data are means ± SEM. (C) 10⁵ B16 tumor cells were injected subcutaneously into IFN-γ^−/− and IFN-γ^−/−IL-17^−/− mice. 6 d after tumor implantation, tumor-bearing IFN-γ^−/− mice were treated with IL-6–neutralizing antibodies or control rat IgG every other day. Data are means ± SEM (n = 7 mice from two independent experiments). (D) Western blot analyses of phospho-Stat3, total Stat3, Bcl-x_L, and β-actin levels from protein extracts prepared from B16 tumors harvested from the indicated mice (representative of two independent experiments).