The Role of Immunosuppression in the Efficacy of Cancer Gene Therapy Using Adenovirus Transfer of the Herpes Simplex Thymidine Kinase Gene

OBJECTIVE: To determine whether the immune system limits or improves the therapeutic efficacy of an adenovirus vector expressing the herpes simplex thymidine kinase (HSVtk) gene in a subcutaneous tumor model. BACKGROUND DATA: Enhanced immune reactions against tumors may be therapeutically useful. However, recent studies with adenoviral vectors show that immune responses limit the efficacy and persistence of gene expression. The effect of the immune response on cancer gene therapy with HSVtk gene delivery by an adenovirus vector followed by treatment with ganciclovir is unclear. METHODS: After adenoviral transduction of a Fischer rat syngeneic mesothelioma cell line with the HSVtk gene in vitro, subcutaneous flank tumors were established. The ability of the HSVtk/ganciclovir system to inhibit tumor growth was compared among normal Fischer rats, immunodeficient nude rats, and Fischer rats immunosuppressed with cyclosporin. RESULTS: HSVtk/ganciclovir therapy was more effective in nude rats and immunosuppressed Fischer rats than in immunocompetent Fischer rats. CONCLUSION: These results indicate that the immune response against adenovirally transduced cells limits the efficacy of the HSVtk/ganciclovir system and that immunosuppression appears to be a useful adjunct. These findings have important implications for clinical trials using currently available adenovirus vectors as well as for future vector design.