IMMUNIZATION WITH MUTAGEN-TREATED (TUM-) CELLS CAUSES REJECTION OF NONIMMUNOGENIC RAT GLIOMA ISOGRAFTS

The ethyl-N-nitrosourea-induced rat glioma N32 was treated with the mutagenic compound N-methyl-N'-nitro-N-nitrosoguanidine and the surviving cells cloned by limiting dilution. Out of 20 clones tested 8 did not produce tumors subcutaneously even after challenge doses 3 log units above the minimal tumor dose for N32. All of 5 clones grew in a retarded manner intracerebrally but produced tumors in some animals. Preimmunizations with three of the rejected clones (tum-) gave protection against subcutaneous and intracerebral isografts of the unmutated N32. This effect could be enhanced if the cells used for immunizations were pretreated with interferon gamma (IFNgamma) for 48 h. If immunizations were started subsequent to challenge, only immunization with one of two tested tum- clones pretreated with IFNgamma induced significant rejection against intracerebral N32 isografts. Both N32 and its tum- clones were MHC class I positive and MHC class II negative. IFNgamma treatment enhanced the MHC class I expression with 20%-90% on the tum- clones and with 40% on N32. MHC class II expression could be induced on N32 cells after 7 days of IFNgamma treatment but not on any of the tum- clones tested. We conclude that the enhancing effect of IFNgamma treatment on tumor isograft rejection may depend on up-regulation of MHC class I but not of MHC class II. This investigation demonstrates that it is possible to induce rejection of weakly immunogenic intracerebral brain tumors by immunization with selected highly immunogenic tumor cell mutants. In conjunction with relevant cytokines, the cross-protective effect of these tum- variants might be further enhanced and serve as a model for immunotherapy against malignant human brain tumors.