Reciprocal relationship between IFN-γ and IL-17 production by tumor-infiltrating T lymphocytes. Single-cell suspensions were prepared from B16 or MB49 tumors harvested 20 d after tumor implantation. (A) IL-17 expression in CD4⁺ T cells from B16 (top) or MB49 (bottom) tumors; flow cytometric patterns shown (one tumor per group) are representative of three independent experiments. Percentages are shown. (B) IL-17 secretion by intratumoral CD4⁺ T cells sorted from B16 tumors grown in WT, IL-17^−/−, IFN-γ^−/−, or IFN-γ^−/−IL-17^−/− mice was assessed by ELISA after a 24-h culture, combining results from three independent experiments of three to four pooled tumor samples (P = 0.001). Data represent means ± SEM. (C) Percentages of CD4⁺ and CD8⁺ T cells within the total intratumoral leukocyte population harvested on days 20 (top row) and 14 (second from top row). (second from bottom row) IFN-γ expression in CD8⁺ T cells derived from tumors of WT or IL-17^−/− mice. (bottom row) IFN-γ expression by tumor-infiltrating CD4⁺ T cells (representative of three independent experiments). Percentages are shown. (D) Increased secretion of IFN-γ by sorted tumor-infiltrating CD8⁺ (top) and CD4⁺ (bottom) T cells, combining results from three independent experiments of pooled tumor samples (P = 0.002). Data represent means ± SEM. TIL, tumor-infiltrating T lymphocytes.