Therapeutic vaccination against malignant gliomas based on allorecognition and syngeneic tumour antigens: proof of principle in two strains of rat

In the present study we investigated whether allogeneic glioma cells can be utilized to evoke prophylactic or therapeutic immune-mediated elimination of syngeneic glioma in two rat strains. Fisher 344 and Sprague–Dawley (SD) rats were injected with two syngeneic glioma cell lines, 9L and C6, respectively, resulting in progressive tumor growth. 9L is syngeneic to the Fisher 344 and allogeneic to the SD rats, while C6 cells are syngeneic to SD rats and allogeneic to Fisher 344 rats. Both rat strains were subcutaneously injected with their respective allogeneic tumor cells, which proved unable to grow progressively. The allogeneic cells were either rejected immediately in SD rats or within 25 days in Fisher rats, after limited tumor outgrowth. Both rat strains were subsequently challenged with their respective syngeneic glioma tumor cells and once more 10 days later with a fivefold higher dose. SD rats, even after reinjection with five times the original dosage of C6 cells, remained tumor free for at least 360 days. Similarly, Fisher rats, after initially rejecting allogeneic tumors, failed to develop syngeneic tumors. To determine anti-tumor immunity against established glioma tumors under more demanding therapeutic conditions, rats were first injected subcutaneously with their respective syngeneic tumor and vaccinated once or repeatedly (at 5-day intervals) with a mixture of the allogeneic or xenogeneic cells, with or without a lysate from the same syngeneic tumor, which served as a therapeutic vaccine preparations. The control group received either no treatment or syngeneic instead of allogeneic cells. In both strains of rats, we demonstrated that the therapeutically vaccinated groups were able to significantly reduce tumor growth, while complete rejection of tumors was noted in the SD rats. Immunization with syngeneic tumor cells alone failed to evoke anti-tumor immunity. We conclude that therapeutic immunization with a combination of allogeneic cells and syngeneic lysates induces rejection of malignant gliomas and offers a protective effect against challenge with syngeneic tumor cells. This immunization approach may prove useful as a post-surgery adjuvant therapy in future cancer treatment protocols, or even as a stand-alone therapeutic tumor vaccination