Protection against experimental cerebral metastases of murine melanoma B16 by active immunization

Melanoma patients often develop brain metastases despite effective systemic immunotherapy against melanoma. We have attempted to establish a mouse model to develop strategies to combat this problem. Immunization of C57BL/6 (H-2b) mice with a combination of the syngeneic G3.12/BM2 melanoma (a B16 subclone) and the allogeneic Cloudman-S91 melanoma was effective in preventing the growth of 10,000 viable, s.c. injected G3.12 cells in 93% of the mice. Irradiated whole tumor cells pretreated with gamma-interferon for 2 days were most effective. A nonspecific adjuvant (DETOX) was injected routinely together with the tumor cells. Active immunization with 2 different doses of irradiated melanoma cells (1 x 10(5) or 2.5 x 10(6) cells/injection x 5 injections) protected against intracerebral challenge with 200 live G3.12 cells in 69% of the mice. This challenge caused the death of all control mice within 30 days. T-cell-mediated, tumor-specific cytotoxicity against G3.12 melanoma was demonstrated in the spleen of immunized mice. Histological observations in the brain, 80 days after tumor challenge, indicated complete eradication of the melanoma, but although CD4+ and CD8+ T-cells and macrophages were present, their number was low. Gliosis was present in both immunized and control animals. Thus, in this murine melanoma model syngeneic mice were protected from death by s.c. and intracerebrally inoculated tumor cells if pretreated with a sufficient number of irradiated syngeneic and allogeneic melanoma cells and an immunological adjuvant. Whether this regimen can treat established tumors of the brain, alone or in combination, is uncertain. Yet its success suggests that the "blood-brain barrier" impeding immunity to tumors may not be absolute.