Objective: To evaluate the efficacy of antitumour vaccines comprising irradiated allogeneic or autologous whole cells expressing cytosine deaminase (CD) which are first killed ex vivo by prodrug activation using 5-fluorocytosine (5-FC), as the immunogenicity of tumour cells used as irradiated vaccines depends both on antigen expression and on the mode of their death.
Materials and methods: The PA3 rat prostate cell line and MATLyLu, an androgen-insensitive subline, were grown and transfected with CD (designated PCD and MCD). In vitro drug-sensitivity was assessed in the cell lines using a viability assay, and the mode of cell death quantified by assessing apoptosis. Bax and bcl-2 expression were assessed by Western blot analysis. For in vivo experiments, male 8-10-week-old Lobund-Wistar rats were vaccinated (using vehicle in control groups) with 5 x 10(6) cells, all cells being irradiated before injection, to give groups with PA3, PCD, PCD killed with 5-FC, MatLyLu, MCD, and MCD killed with 5-FC. After 7 days all animals were given a subcutaneous tumour challenge of PA3 cells, and tumour volume measured subsequently. Immune responses were assessed in splenocytes.
Results: The efficiency of cell kill varied between the cell lines assessed, but cell death was by induced apoptosis. Single doses of vaccine were most effective in the allogeneic setting, causing significantly slower growth of syngeneic tumour challenge (P < 0.01), and 25% better survival at 50 days (P < 0.02) than irradiated untransfected cells. This was consistent with the greater proliferative response after allogeneic than autologous vaccination.
Conclusion: The immunogenicity of irradiated tumour cells is enhanced when they are killed ex-vivo using suicide-gene therapy. This approach would be clinically applicable in terms of ease of vaccine production, safety, storage and avoidance of potential toxicities of in vivo gene transfer.