To explore the potential for molecular
immunotherapies in the treatment of
malignant gliomas, we evaluated the efficacy of subcutaneous
tumor cell
vaccines in the treatment of intracranial 9L
tumors, using 9L
gliosarcoma cell lines stably transduced with the murine
interleukin-4 cDNA (9L-IL4), the herpes simplex virus-
thymidine kinase cDNA (9L-Tk) or both (9L-IL4-Tk). The expression of multiple genes from a single transcript was achieved by incorporating internal ribosomal entry site (IRES) cassettes in the retroviral constructs. Subcutaneous immunization of rats with nonirradiated 9L-IL4 cells or 9L-IL4-Tk cells followed by treatment with
ganciclovir (GCV) completely protected the animals from a subsequent intracranial challenge with wild-type 9L cells. In contrast, only 50% of animals immunized with 9L-Tk cells and 0% of 9L-neo immunized animals rejected the same challenge with wild-type 9L. More importantly, treatment of established (day 3) intracranial 9L
tumors with genetically engineered
tumor cells resulted in long-term survival (> 100 days) for 25-43% of 9L-IL4-Tk immunized animals and for 27% of nonirradiated 9L-IL4 immunized animals. In striking contrast, no 9L-Tk, 9L-neo or irradiated 9L-IL4 immunized animals survived for more than 33 days. As a marker of a cellular immune response, splenocytes from nonirradiated 9L-IL4, 9L-Tk or 9L-IL4-Tk immunized animals produced
interferon-gamma (IFN-gamma) in greater amounts than those from 9L-neo immunized or Hank's balanced
salts solution (HBSS) treated animals when stimulated with wild-type 9L in vitro. Our findings support the use of
tumor cell
vaccines expressing the
IL-4 and HSVtk genes for the treatment of
malignant gliomas.