Inhibition of D-2HG leads to upregulation of a proinflammatory gene signature in a novel HLA-A2/HLA-DR1 transgenic mouse model of IDH1R132H-expressing glioma

Background: Long-term prognosis of WHO grade II, isocitrate dehydrogenase (IDH)-mutated low-grade glioma (LGG) is poor due to high risks of recurrence and malignant transformation into high-grade glioma. Immunotherapy strategies are attractive given the relatively intact immune system of patients with LGG and the slow tumor growth rate. However, accumulation of the oncometabolite D-2-hydroxyglutarate (D-2HG) in IDH-mutated gliomas leads to suppression of inflammatory pathways in the tumor microenvironment, thereby contributing to the 'cold' tumor phenotype. Inhibiting D-2HG production presents an opportunity to generate a robust antitumor response following tumor antigen vaccination and immune checkpoint blockade.

Methods: An IDH1^R132H glioma model was created in syngeneic HLA-A2/HLA-DR1-transgenic mice, allowing us to evaluate the vaccination with the human leukocyte antigens (HLA)-DR1-restricted, IDH1^R132H mutation-derived neoepitope. The effects of an orally available inhibitor of mutant IDH1 and IDH2, AG-881, were evaluated as monotherapy and in combination with the IDH1^R132H peptide vaccination or anti-PD-1 immune checkpoint blockade.

Results: The HLA-A2/HLA-DR1-syngeneic IDH1^R132H cell line expressed the IDH1 mutant protein and formed D-2HG producing orthotopic gliomas in vivo. Treatment of tumor-bearing mice with AG-881 resulted in a reduction of D-2HG levels in IDH1^R132H glioma cells (10 fold) and tumor-associated myeloid cells, which demonstrated high levels of intracellular D-2HG in the IDH1^R132H gliomas. AG-881 monotherapy suppressed the progression of IDH1^R132H gliomas in a CD4⁺ and CD8⁺ cell-dependent manner, enhanced proinflammatory IFNγ-related gene expression, and increased the number of CD4⁺ tumor-infiltrating T-cells. Prophylactic vaccination with the HLA-DR1-restricted IDH1^R132H peptide or tumor-associated HLA-A2-restricted peptides did not enhance survival of tumor-bearing animals; however, vaccination with both HLA-A2-IDH1^R132H and DR1-IDH1^R132H peptides in combination with the IDH inhibitor significantly prolonged survival. Finally, tumor-bearing mice treated with both AG-881 and a PD-1 blocking antibody demonstrated improved survival when compared with either treatment alone.

Conclusion: The development of effective IDH1^R132H-targeting vaccine may be enhanced by integration with HLA class I-restricted cytotoxic T cell epitopes and AG-881. Our HLA-A2/HLA-DR1-syngeneic IDH1^R132H glioma model should allow us to evaluate key translational questions related to the development of novel strategies for patients with IDH-mutant glioma.