Successful immunogene therapy using colon cancer cells (colon 26) transfected with plasmid vector containing mature interleukin-18 cDNA and the Igκ leader sequence

IL-18 is a novel cytokine that induces interferon (IFN) -γ secretion and plays an important role in antitumor immunity. In the present study, we constructed plasmid vectors encoding the murine mature IL- 18 cDNA linked with the Igκ leader sequence and the pro-IL-18 cDNA to estimate the efficacy of the mature IL-18 vector and to evaluate IL-18-producing tumor cells as a tumor vaccine. Colon 26 cells were transfected with the abovementioned vectors or with vector alone (mock). Reverse transcription-polymerase chain reaction analysis showed increased expression of murine IL-18 cDNA in both mature IL-18 and pro-IL-18 transfectants in comparison to that in mock transfected cells. The ability of the culture supernatants of mature IL-18 transfectants to induce IFN-γ secretion was extremely high (40-140 pg/10⁶ cells) in comparison to that of pro-IL-18 transfectants (4-18 pg/10⁶ cells). When injected into syngeneic BALB/c mice, the growth of mature IL-18 transfectants, but not pro-IL-18 transfectants, was significantly less than that in mock transfected cells (P+ and anti-CD8⁺ antibodies. These data suggest that the rejection of mature IL-18/colon 26 cells was mediated through T-cell activation. Gene therapy using mature IL-18 transfectants containing a plasmid vector and the Igκ leader sequence may be a useful tumor vaccine.