Object. Glioblastoma multiforme (GBM) is a malignant tumor of the central nervous system that directly suppresses immunological defenses in vitro and in vivo. The authors used the peripheral delivery of continuously infused granulocyte—macrophage colony-stimulating factor (GM-CSF) in the presence of irradiated tumor antigens as a tumor-specific stimulant to dendritic cells to initiate an immune response to GBM in rats.
Methods. The 9L gliosarcoma tumors were established in the flanks of syngeneic Fischer 344 rats. Osmotic minipumps implanted in the animals' contralateral flanks continuously delivered recombinant GM-CSF (0, 0.1, 1, or 10 ng/day) for 28 days. Irradiated gliosarcoma cells were intermittently injected at the site of the GM-CSF infusion. Animals in the saline control group (0 ng/day GM-CSF) died on Day 59 with average tumor volumes greater than 30,000 mm3. This control group was significantly different from the GM-CSF—treated animals, which all survived with average tumor volumes that peaked on Day 23 and later regressed completely. Tumor growth as well as peak tumor volumes (5833 ± 2284 mm3, 3294 ± 1632 mm3, and 1979 ± 1142 mm3 for 0.1, 1, and 10 ng/day GM-CSF, respectively) in the different treatment groups reflected a significant dose-response relationship with the GM-CSF concentrations. All animals treated with GM-CSF and irradiated cells were resistant to additional challenges of peripheral and intracerebral gliosarcoma, even when they were inoculated 8 months after initial immunotherapy. The colocalization of GM-CSF and inactivated tumor antigens was required to stimulate immunoprotection. To test the efficacy of a peripherally administered immunological therapy on intracerebral brain tumors the authors transplanted 106 gliosarcoma cells into the striatum of treated and control animals. Subcutaneous pumps that released GM-CSF (10 ng/day) and irradiated gliosarcoma cells were placed in the treated animals. The control animals all died within 31 days after intracerebral tumor implantation. In contrast, 40% of the animals receiving GM-CSF—irradiated cell vaccinations survived beyond 300 days. These long-term survivors showed no evidence of gliosarcoma at the injection site on evaluation by magnetic resonance imaging.
Conclusions. These results suggest that the continuous localized delivery of subcutaneous GM-CSF in conjunction with inactivated tumor antigens can initiate a systemic response that leads to the regression of distant peripheral and intracerebral tumors. The success of this treatment illustrates the feasibility of tumor-specific peripheral immunological stimulation after tumor resection to prevent the recurrence of malignant brain tumors.
