Helper Virus-Free Herpes Simplex Virus Type 1 Amplicon Vectors for Granulocyte-Macrophage Colony-Stimulating Factor-Enhanced Vaccination Therapy for Experimental Glioma

Subcutaneous vaccination therapy with glioma cells, which are retrovirally transduced to secrete granulocytemacrophage colony-stimulating factor (GM-CSF), has previously proven effective in C57BL/6 mice harboring intracerebral GL261 gliomas. However, clinical ex vivo gene therapy for human gliomas would be difficult, as transgene delivery via retroviral vectors occurs only in dividing cells andex vivo glioma cells have a low growth fraction. To circumvent this problem, a helper virus-free herpes simplex virus type 1 (HSV-1) amplicon vector was used. When primary cultures of human glioblastoma cells were infected with HSV-1 amplicon vectors at an MOI of 1, more than 90% of both dividing and nondividing cells were transduced. When cells were infected with an amplicon vector, HSVGM, bearing the GM-CSF cDNA in the presence of Polybrene, GM-CSF secretion into the medium during the first 24 hr after infection was 1026 ng/10⁶ cells, whereas mock-infected cells did not secrete detectable GM-CSF. Subcutaneous vaccination of C57BL/6 mice with 5 X 10⁵ irradiated HSVGM-transduced GL261 cells 7 days prior to intracerebral implantation of 10⁶ wild-type GL261 cells yielded 60% long-term survivors (> 80 days), similar to the 50% long-term survivors obtained by vaccination with retrovirally GM-CSF-transduced GL261 cells. In contrast, animals vaccinated with the same number of nontranduced GL261 cells or with GL261 cells infected with helper virus-free packaged HSV1 amplicon vectors carrying no transgene showed only 10% long-term survivors. In conclusion, helper virusfree HSV-1 amplicon vectors appear to be effective for cytokine-enhanced vaccination therapy of glioma, with the advantages that both dividing and nondividing tumor cells can be infected, no viral proteins are expressed, and these vectors are safe and compatible with clinical use.