INDUCTION OF ANTITUMOR IMMUNITY BY INTRACEREBRALLY IMPLANTED RAT C6 GLIOMA-CELLS GENETICALLY-ENGINEERED TO SECRETE CYTOKINES

To test whether cytokine gene therapy can be applied to an immunologic ally privileged site, such as the brain, we investigated antitumor imm unity in the brain induced by cytokine-secreting glioma cells. Three c ytokine genes, interleukin-2 (IL-2), interluekin-4 (IL-4), and granulo cyte-macrophage colony-stimulating factor (GM-CSF) were transduced int o a rat C6 glioma cell line via a retroviral vector, S2. Rats intracer ebrally (IC) implanted with the C6 cells genetically engineered to sec rete the cytokines, especially GM-CSF, manifested significantly higher survival rates than those with C6 cells or with C6 cells bearing the control vector (p < 0.002). In vivo, C6 tumors bearing the cytokine ge nes grew more slowly than wild-type tumors at any time point, and even tually diminished within 6 weeks after tumor cell implantation. Histop athological and immunohistochemical studies revealed that different cy tokines induced diverse immune reactions. In the IL-2 group, CD4(+) an d CD8(+) T cells dominated from day 3 to week 4, but disappeared at we ek 6. Some granulocytes were noted between weeks 2 and 4. In the IL-4 group, eosinophils were noted from day 3 to week 4, and CD4(+) and CD8 (+) T cells, as well as macrophages at week 2. At week 6, only residua l levels of macrophages and CD8(+) T cells remained. In the GM-CSF gro up, granulocytes appeared as early as day 1 post-IC tumor implantation , and macrophages at day 2. CD4(+) and CD8(+) T cells were found from day 3 to week 4. At week 6, only residual CD4(+) T cells and macrophag es remained. Long-lasting antitumor immunity was confirmed in all grou ps by rechallenging surviving rats with wild-type C6 cells in the brai n 100 days after implanting cytokine gene-bearing C6 cells. In vivo de pletion of GM-CSF by anti-GM-CSF antibody further confirmed that the i mmune reaction induced by GM-CSF-secreting tumor cells were mainly fro m the action of GM-CSF, rather than the immunogenicity of C6 cells.