Clinically relevant concurrent BRAF and MEK inhibition alters differentiation states and sensitizes BRAF V600E-mutated high-grade gliomas to immune checkpoint blockade

BRAF and MEK inhibitor combinations have shown promising results against several types of BRAF V600E mutated cancers. Patients with high-grade BRAF V600E mutated gliomas frequently experience therapy failure with concurrent BRAF V600E and MEK inhibition (BRAFi+MEKi). A step toward overcoming therapy resistance includes improving the understanding how these inhibitors affect tumor cells and (immune) microenvironment. In novel syngeneic murine models and patient-derived cell lines of BRAF V600E-mutated high-grade astrocytomas we analyzed effects of BRAF V600E expression and BRAF V600E inhibitor Dabrafenib and MEK inhibitor Trametinib. BRAFi+MEKi promoted glial differentiation as determined by immunostaining and RNA expression profiling. In addition, increased interferon alpha and gamma signatures and pro-inflammatory cytokines were detected in response to BRAFi+MEKi. Quantitative rtPCR validated upregulation of known downstream targets of interferon gamma, the major histocompatibility genes, and programmed death (PD-1) receptor signaling. Cytometry analyses showed heightened frequency of tumor-infiltrating T cells positive for PD-1 and tumor cells co-expressing programmed death ligand-1 (PD-L1). Combining BRAFi+MEKi with dual immune checkpoint inhibition by anti-PD-L1 and anti-CTLA-4 treatment decreased T cell deactivation and resulted in a T cell-dependent survival benefit of mice with orthotopic BRAF V600E-mutated high-grade glioma. These data showed that clinically relevant BRAFi+MEKi sensitizes BRAF V600E-mutated gliomas to the anti-tumor activity of concurrent dual immune checkpoint blockade. Immunofluorescence staining identified an active T cell infiltrate in human MAPK pathway altered gliomas. Collectively, our data suggest that an improved therapeutic benefit could be derived from combining BRAFi+MEKi with immune checkpoint inhibitors in patients with BRAF V600E high-grade gliomas.   ### Competing Interest Statement The authors have declared no competing interest.