IMMU-26. TARGETING CHK2-YBX1/3 INTERACTION TO OVERCOME IMMUNOSUPPRESSION IN GLIOMAS

Abstract Whereas the contribution of tumor microenvironment to the profound immune suppression of glioblastoma (GBM) is clear, tumor-cell intrinsic mechanisms that regulate resistance to CD8 T cells are less studied. We performed an in vivo CRISPR screen to identify glioma intrinsic kinases that contribute to evasion of tumor cells from CD8 T cells. The screen revealed a DNA damage response gene-checkpoint kinase 2 (Chek2) to be the most important kinase contributing to escape from CD8 T-cell recognition. Genetic depletion or pharmacological inhibition of Chek2 with blood-brain-barrier permeable drugs that are in the clinical trials, in combination with PD-1/PD-L1 blockade, led to survival benefit in preclinical glioma models. Analysis of human GBMs demonstrated that Chek2 expression is inversely associated with antigen presentation and T-cell activation. However, the mechanism underlying the immunosuppressive function of Chek2 in gliomas is unknown. Here, we identified that glioma tumor cell intrinsic Chek2 interacts with Y-Box Binding Protein 1 and 3 (Ybx1&3) in ex vivo gliomas using immunoprecipitation followed by mass spectrometry. Independently, YBX3 was also identified as one of the most significantly phosphorylated protein upon Chek2 knockout using phosphoproteomics analysis. The YBX1&3 are DNA/RNA binding proteins implicated in transcriptional regulation of immune-modulatory genes. In silico docking using Alphafold and PIPER tool in Schrodinger revealed that the YBX1 cold-shock domain interacted near the ATP/ADP binding site in CHK2. This interface was also part of the CHK2-CHK2 dimer interface. Consistent with our model, disruption of CHK2 dimerization using prexasertib led to increased colocalization of CHK2-YBX1 in human GBM patient derived xenograft cells. Further, targeting of CHK2-YBX1 interaction with YBX1 inhibitor SU056 reversed the repression of IFNγ response genes in glioma cells. Together this study unveils a kinase independent (non-catalytic) function of CHK2, providing insight into the non-canonical immunosuppressive role of CHK2 in gliomas.