The DNA minigene vaccine pHI-Db induces specific CTL killing of FLK-1⁺ endothelial cells but not of FLK-1^- tumor cells. (A) Surface expression of FLK-1 by endothelial cell line MS1 and murine prostate carcinoma cell line RM9. Gray dotted lines indicate phycoerythrin (PE)–conjugated isotype control antibody; black solid lines, PE-conjugated anti–FLK-1. Groups of immunized C57BL/6J mice (n = 4) were killed 2 weeks after the last immunization and isolated splenocytes were stimulated with irradiated MS1 cells for 5 days. Thereafter, cytotoxicity assays were performed with MS1 (B) or RM9 (C) serving as target cells. ♦, pCMV control group;□, pHI group; ▴, pHI-Db group. Experiments were repeated 3 times with similar results. ^*P < 0.001 compared with pCMV or pHI control groups. E/T indicates the ratio of effector and target cells.