IL-17 down-regulation of Th1 differentiation required host DCs. (A-C) Sorted CD4⁺ T cells (2 × 10⁵) from WT or IL-17^−/− donors were stimulated with plate-bound anti-CD3 and anti-CD28 as well as IL-12 (10 ng/mL) in culture for 72 hours. The percentage of IFN-γ⁺ or IL-17⁺ cells among cultured donor CD4⁺ T cells were determined by intracellular staining, and the IFN-γ concentration in culture supernatant was measured by ELISA. One representative of 4 replicated intracellular staining experiments, the mean (± SE) percentage of IFN-γ⁺ cells among donor CD4⁺ T cells, and the mean (± SE) of the IFN-γ concentration in the culture supernatants are shown. (D-F) Sorted donor CD4⁺ T cells (2 × 10⁵) from WT or IL-17^−/− donors were cocultured with enriched DCs from irradiated host BALB/c mice for 96 hours. The percentage of IFN-γ⁺ or IL-17⁺ cells among cultured donor CD4⁺ T cells were determined by intracellular staining, and the IFN-γ concentration in culture supernatant was measured by ELISA. One representative of 4 replicated intracellular staining experiment, the mean (± SE) of the percentage of IFN-γ⁺ cells among donor CD4⁺ T cells, and the mean (± SE) of the IFN-γ concentration in the culture supernatants are shown. (G-I) Sorted naive CD44^loCD4⁺ T cells from WT or IL-17^−/− donors were cocultured with enriched DCs from irradiated host BALB/c mice for 5 days. The percentage of IFN-γ⁺ or IL-17⁺ cells among cultured donor CD4⁺ T cells were determined by intracellular staining, and the IFN-γ concentration in culture supernatant was measured by ELISA. Dead cells were excluded by fixable aqua dead cell stain kit. One representative of 3 replicated experiments, the mean (± SE) percentage of IFN-γ⁺ cells among donor CD4⁺ T cells, and the mean (± SE) of the IFN-γ concentration in the culture supernatants of triplicates are shown. (J) IFN-γ concentration in the culture supernatants of naive donor WT and IL-17^−/− CD4⁺ T cells and host DCs with or without the presence of neutralizing anti–IL-12 Antibody. (K) IFN-γ concentration in the culture supernatants of IL-17^−/− donor CD4⁺ T cells and host DCs with or without addition of IL-17 or IL-17 plus IL-12.