Effect of IL-18 secreted from DC1s on Th1 response. IL-18 secreted from DC1 does not drive a Th1 response. (A-B) Th1 (IFN-γ) and Th2 (IL-4) cytokine release from polarized Th cells. As outlined in Figure 1, allogeneic naive Th lymphocytes were cocultured with DC1s (either immature [DC1 w/o CD40L] or matured by a further 24-hour culture with CD40L-expressing P3 × TBA7 cells [DC1 + CD40L]) or DC2s as indicated for 6 days. During this period, neutralizing antibodies (anti–IL-12, anti–IL-18, and isotype-matched control immunoglobulin) were present as indicated. On day 6, Th cells were harvested, counted, and restimulated for 24 hours with anti-CD3 and anti-CD28. IFN-γ and IL-4 released into the supernatant were detected by ELISA. Neutralization of IL-12 resulted in a substantial decrease of IFN-γ release, whereas anti–IL-18 mAb had no significant effect. Th cells obtained through coculture with DC2 secreted substantially more IL-4 than those from DC1 cocultures. One of 2 experiments is shown. (C) Th phenotype determined by intracellular FACS. Th cells generated through coculture of naive allogeneic Th cells with DC1, as described, were restimulated for 5 hours, and intracellular accumulation of IFN-γ and IL-4 was analyzed. The percentages of positive cells are given in the respective quadrants. Anti–IL-12 mAb during coculture significantly decreased the number of IFN-γ⁺IL-4^– cells. One experiment of 2 is shown.