Induction of P1A-specific CTLs by immunization with peptide-pulsed DCs activated by CD40 triggering. Mice were injected s.c. in the 4 footpads on days 0 and 14 with 3 x 10⁵ P1A-loaded DCs previously cocultured with CD40L-transfected 3T3 cells (CD40L-activated DC + peptide P1A) or with untransfected 3T3 cells (DC + peptide P1A). The spleen cells from injected mice were harvested on day 21 and stimulated in vitro with P815 tumor cells. Six days later, these cultures were submitted to a second short-term stimulation (4 days) with P815 tumor cells and subsequently tested on ⁵¹Cr-labeled P815, L1210 tumor cells transfected or not with the P1A gene and YAC-1 tumor cells in a three hours and half chromium release assay. Each curve gives the CTL response of spleen cells from individually injected mice. Data are from one representative experiment of 4 performed. Lytic activities were also expressed in lytic units (LU) per 10⁶ effector cells, where one LU is defined as the number of CTL required to lyse 50% of 10⁴ target cells. Each result is the mean of the LU calculated for every mouse of a group.