Dual MAPK Inhibition Triggers Pro-inflammatory Signals and Sensitizes BRAF V600E Glioma to T Cell-Mediated Checkpoint Therapy.

BRAF ^V600E pediatric low-grade gliomas frequently transform into high-grade gliomas (HGG) and poorly respond to chemotherapy, resulting in high mortality. Although combined BRAF and MEK inhibition (BRAFi+MEKi) outperforms chemotherapy, ∼70% of BRAF ^V600E HGG patients are therapy resistant and undergo unbridled tumor progression. BRAF ^V600E glioma have an immune-rich microenvironment suggesting that they could be responsive to immunotherapy but effects of BRAFi+MEKi on anti-tumor immunity are unclear. Using patient tumor tissue before and after BRAFi+MEKi, two novel syngeneic murine models of BRAF ^V600E HGG, and patient-derived cell lines, we examined the effects of clinically relevant BRAFi+MEKi with dabrafenib and trametinib on tumor growth, cell states, and tumor-infiltrating T cells. We find that BRAFi+MEKi treatment: i) upregulated programmed cell death protein-1 (PD-1) signaling genes and PD-1 ligand (PD-L1) protein expression in murine BRAF ^V600E HGG by stimulating IFNγ and IL-27, ii) attenuated T cell activity by IL-23, IL-27 and IL-32 production, which can promote the expansion of regulatory T cells, and iii) induced glial differentiation linked to a therapy-resistant PD-L1+ compartment through Galectin-3 secretion by tumor cells. Murine BRAF ^V600E HGG shrinkage by BRAFi+MEKi is associated with the upregulation of interferon-gamma response genes, MHC class I/II expression, and antigen presentation and processing programs, indicative of increased anti-tumor immunity. Combined BRAFi+MEKi with therapeutic antibodies inhibiting the PD-1 and cytotoxic T-lymphocyte associated protein 4 (CTLA-4) immune checkpoints re-activate T cells and provide a survival benefit over single therapy in a T cell-dependent manner. The quadruple treatment overcame BRAFi+MEKi resistance by invigorating T cell-mediated anti-tumor immunity in murine BRAF ^V600E HGG. PD-L1 expression was elevated in human BRAF-mutant versus BRAF-wildtype glioblastoma clinical specimen, complementing experimental findings and suggesting translational relevance for patient care.