DDDR-82. Inflammation-driven Resistance to MAPK inhibition in Pediatric Gliomas: Mechanisms and Therapeutic Strategies

Abstract Targeted inhibition of MAPK pathway with BRAF and MEK inhibitors (BRAFi/MEKi) shows initial efficacy in pediatric low- and high-grade gliomas (pL/HGGs), but durable responses are rare due to rapid resistance. Clinical benefit is further limited by tumor rebound upon treatment cessation. Our recent work demonstrates that immune checkpoint inhibitors – specifically anti-PD-L1/PD-1 and anti-CTLA-4 antibodies – extend survival in murine models, but only when combined with BRAFi/MEKi. However, the anticipated toxicity of the quadruple combination is a clinical challenge. We identified the beta-galactosidase-binding Galectin-3 as a secreted factor induced by BRAFi/MEKi in patient-derived high-grade glioma and glioblastoma cells [Xing, Panovska et al., Cell Reports Medicine, In Press]. While Galectin-3 appears to promote immune evasion by upregulating PD-L1 and enhancing tumor cell plasticity, its potential to synergize with pro-inflammatory cytokines and its therapeutic value in the context of BRAFi/MEKi treatment remain untested. To better understand the mechanisms of resistance to BRAFi/MEKi we combined single-cell transcriptomics, multiplexed nELISA, phospho-kinome activity mapping (HT-KAM), and a unique toolkit – before and after BRAFi/MEKi treated patient tissue, three syngeneic BRAF V600E-mutant mouse models and patient-derived pL/HGG cell lines. In addition to previously characterized resistance mediators such as mTOR, PLK and AKT, we identified a broader, immunosuppressive, pro-inflammatory resistance program regulated by interferons and interleukins, which activate the JAK/STAT pathway and upregulate proto-oncogene kinase activity. CellChat analysis predicted that after BRAFi/MEKi treatment of mouse models these candidates remodel the tumor microenvironment, dampen T cell function and promote glial cell state transitions towards astrocytic and oligodendrocytic profiles, similar to normal glial differentiation mediated by AKT/mTOR activation. Currently, we are exploring in vitro therapeutic restoration of T cell function through inhibition of these immunosuppressive, pro-inflammatory resistance programs. Our data support a novel therapeutic rationale to intercept both cell-intrinsic and inflammation-mediated resistance mechanisms in MAPK-driven gliomas.