Glioblastoma is the most common primary malignant brain tumor. Treatment of glioblastoma consists of surgical resection and concurrent treatment with chemotherapy and radiation. Given the recent success of immunotherapy in other tumors, there is great interest in immunotherapy for glioblastoma.
A large number of glioblastomas carry a mutation in the isocitrate dehydrogenase (IDH) gene. This mutation results in the production of the oncometabolite 2-hydroxyglutarate, which causes a genome-wide decrease in gene expression. Our first line of defense against tumor cells is the innate immune system, which is responsible for distinguishing between normal “self” and cancerous “non-self” cells. A major constituent of this system is natural killer (NK) cells, which play a role in discriminating between self and non-self through the recognition of NKG2D ligands on tumor cells. In our work published in the journal Neuro-Oncology, we demonstrated that IDH mutant glioblastomas escape NK cell recognition by suppressing the expression of NKG2D ligands. We hypothesized that we can make IDH mutant glioblastomas visible to the immune system by reversing the suppressive effects of IDH mutation.
In this study, we explored the efficacy of a demethylating agent, Decitabine (DAC), to restore expression of NKG2D ligands, which would allow recognition by NK cells and lead to tumor cell death. We implanted laboratory animals with IDH-wildtype and IDH-mutant tumor cells and treated the animals with Decitabine every seven days once the tumors were established. IDH-mutant tumor growth was significantly inhibited with treatment. To explore the specific role of NK cells in tumor cell growth inhibition, the IDH-wildtype and IDH-tumor mice were treated with Decitabine along with an antibody directed against NK cells (anti-NK1.1 IgG). The effect of Decitabine on tumor growth inhibition in IDH mutant mice was negated when treated with anti-NK1.1 IgG, emphasizing the role of NK cells in the inhibition of tumor growth. In order to confirm that the NK cells were able to infiltrate the tumors, immunohistochemistry was performed to analyze tissue sections of IDH mutant tumors. IDH mutant tumors were found to have increased NK cell concentrations when treated with Decitabine compared to non-treated IDH mutant tumors as well as IDH wildtype tumors. In addition to increased NK cell infiltration, the micro-environment of the treated IDH mutant tumors showed an increase in other immune cell types, including macrophages and dendritic cells. There was also a decrease in monocyte and myeloid-derived suppressor cells, consistent with an overall change of the tumor micro-environment to a more immunologically active one.
Our bench-top research has identified a new mechanism of how IDH mutant glioblastomas escape the immune system. Furthermore, we performed a proof-of-principle study using animal models revealing that we can reverse IDH mutant immune escape by reversing the genomic effects of the IDH mutation using Decitabine. Overall, our research has led to the development of a new approach to immunotherapy for IDH mutant glioblastoma.