Approaches to address the immunogenicity and immunosuppressive features of Isocitrate Dehydrogenase 1 mutated gliomas

The discovery of exploitable tumor-specific targets is central to the development of clinically relevant immunotherapeutic strategies for cancer. Mutations in isocitrate dehydrogenase 1 (IDH1), frequent in diffuse gliomas is one such target. The mutation IDH1R132H (mIDH1) is immunogenic, presented on human MHC-II molecules and induces CD4 T-cell responses in MHC-humanized A2DR1 tumor models as well as IDH1-mutated glioma patients when vaccinated with mIDH1-specific peptide vaccine. mIDH1, however plays the role of a double-edged knife. Its immunogenicity is compensated by the strong immunosuppressive environment orchestrated by the production of oncometabolite 2-HG that is imported by T-cells and leads to a suppression of their activation and proliferation. Inhibition of mIDH1 using small molecule inhibitors (IDH1i) has shown benefit in pre-clinical studies as therapy and has been associated with disease control in early clinical trials. Preclinical studies have also shown the ability of IDH1i to alleviate immunosuppression in the tumor microenvironment (TME). The impact of this inhibition on tumor infiltrating T-cells, however, has not been sufficiently evaluated. The first part of this work, therefore, aimed at deciphering the effect of IDH1i on tumor-infiltrating T-cell activity and fate in the tumor microenvironment using single-cell RNA and VDJ sequencing of tumor infiltrating immune cells. The lack of suitable murine glioma models where mIDH1 is presented in its native immunogenic capacity on human MHC-II molecules prompted the development of a novel syngeneic mIDH1 glioma model in MHC-humanized A2DR1 mice. Single-cell transcriptomic and T-cell receptor analysis of tumor infiltrating immune cells in IDH1i treated mice revealed an accumulation of infiltrating T-cells potentiated by IDH1i with an increased abundance of CD4 T-cells with a tumor reactive phenotype and a reduction in frequency regulatory T-cells as well as restoration of functional intercellular T-cell communication. Combination treatment of IDH1i and ICB provided a synergistic therapeutic benefit for mIDH1 A2DR1 gliomas. These findings suggest that reduction of 2-HG levels is necessary for enabling a functional anti-tumor immune response which is then exploitable by immune checkpoint blockade and warrants for clinical trials testing the efficacy of IDH1 inhibitors in combination with adjuvant immunotherapies such as vaccines or immune checkpoint inhibitors in patients with mIDH1 gliomas. The second part of this work focused on developing a setup for the identification and validation of mIDH1-reactive T-cell receptors (TCR) from IDH1RH-specific vaccinated A2DR1 mice and from the resected lesion of a glioma patient part of the NOA16 mIDH1 peptide vaccine trial. The latter revealed a unique transcriptional signature of mIDH1 reactive CD4 T-cells in the tumor microenvironment characterized by CXCL13 expression. The proof of principle identification of mIDH1 reactive TCRs demonstrates the feasibility of exploiting immune responses against CD4-restricted neo-epitopes as a first step in developing an adoptive TCR-transgenic T-cell therapy for glioma patient