Allogeneic proliferation of naive Th cells. DC2s were obtained by culture of pre-DC2s with IL-3 (DC2) or IL-3 plus IL-18 (IL-18–DC2). After washing, DC2s and IL-18–DC2s were either immediately cocultured with allogeneic naive Th cells for 3 days (w/o CD40L) or, alternatively, matured by addition of CD40L-expressing irradiated P3 × TBA7 cells (+ CD40L), followed after 24 hours by addition of Th cells. Proliferation of Th cells was determined on day 3 of Th/DC coculture by addition of ³H-thymidine for 19 hours (n = 4). Error bars indicate SEM.