Herpes simplex virus thymidine kinase gene therapy in experimental rat BT4C glioma model: Effect of the percentage of thymidine kinase-positive glioma cells on treatment effect, survival time, and tissue reactions

Herpes simplex virus thymidine kinase (HSV-tk) gene transfer and ganciclovi r (GCV) administration have been suggested for the treatment of malignant g liomas. To understand tissue responses and possible ways to improve the tre atment effect, we studied tumor growth, tissue reactions, and survival time after HSV-tk/GCV treatment in a syngeneic BT4C rat glioma model by mixing various ratios of stably transfected HSV-tk-expressing BT4C-tk glioma cells with wild-type BT4C glioma cells (percentage of BT4C-tk cells: 0%, 1%, 10% , 30%, 50%, and 100%), followed by injection into BDIX rat brains (n = 79). With the exception of some animals with end-stage tumors, very little astr oglia or microglia reactivity was detected in the wild-type tumors as analy zed by immunocytochemistry using glial fibrillary acid protein (GFAP)-, vim entin-,human histocompatibility leukocyte antigen-DR-, OX-42-, and CD68-spe cific monoclonal antibodies. After 14 days of GCV treatment, tumors induced with greater than or equal to 10% BT4C-tk cells showed a significant reduc tion in tumor size (P < .05) and prolonged survival time (P < .01). Astrogl iosis, as indicated by a strong GFAP and vimentin immunoreactivity, was see n in the tumor scar area. GFAP and vimentin reactivity was already present after the GCV treatment in tumors induced with 1% BT4C-tk cells. Much less human histocompatibility leukocyte antigen-DR-positive microglia was seen i n the treated animals, indicating low microglia reactivity and immunoactiva tion against the tumor. However, GCV-treated tumors were positive for apopt osis, indicating that apoptosis is an important mechanism for cell death in the BT4C-tk glioma model. Our results suggest that greater than or equal t o 10% transfection efficiency is required for a successful reduction in BT4 C glioma tumor size with HSV-tk/GCV treatment in vivo. Tissue reactions aft er 14 days of GCV treatment are characterized by astrogliosis and apoptosis , whereas microglia response and immunoactivation of the brain cells appear to play a minor role. Stimulation of the microglia response by gene transf er or other means might improve the efficacy of the HSV-tk/GCV treatment in vivo.