Specificity of CTL lines raised against three HLA-A2-binding peptides by immunization with the HCV-core DNA vaccine. Spleen cells from mice immunized 6 mo previously with the HCV-core DNA vaccine were stimulated in vitro with the indicated peptides at 10 μM. After three to four weekly in vitro stimulations, the effector CTL were tested for lysis of 3,000 C1R-AAD target cells/well in the presence of the indicated concentrations of each peptide, at an effector-to-target cell ratio of 100:1. Lysis on targets in the absence of peptide was <5%, as shown on the left axis. Furthermore, each line killed targets only with the homologous peptide, and lysis on targets with each of the other two respective peptides was ≤5% (data not shown). Similar results were obtained 2 mo and 14 mo after immunization. At all three time points, no specific CTL were detected after similar peptide stimulation of spleen cells from mice immunized with a control DNA vaccine plasmid (<5% lysis).