BIOM-69. TP53-DRIVEN IMMUNOSUPPRESSIVE MICROENVIRONMENT UNDERLIES ANTI-PD1 RESISTANCE IN GLIOBLASTOMA

Abstract Glioblastoma (GBM) is a highly aggressive brain tumor with limited treatment options. Although immune checkpoint inhibitors (ICIs) have shown success in other malignancies, phase III clinical trials in GBM have not demonstrated significant survival benefits, with only a minority of patients experiencing durable responses. To better understand the determinants of ICI response, we analyzed a retrospective cohort of 89 malignant glioma patients, including 56 treated with anti-PD1 therapy. Patients were stratified by the timing of ICI administration (upfront vs. recurrent), and clinical, genomic, and transcriptomic data were integrated using bulk and single-cell RNA sequencing.Overall, ICI treatment did not improve survival compared to standard care. Age at diagnosis was the only clinical factor significantly associated with prognosis. Among molecular features, tumor mutational burden (TMB) correlated with outcomes, while predicted neoantigen burden did not. Notably, TP53 mutations were significantly associated with poor survival specifically in ICI-treated patients, but not in non-ICI cohorts, suggesting an immunotherapy-specific resistance mechanism.Functional analyses showed that TP53 mutations increased TMB without enhancing tumor immunogenicity. Instead, TP53-mutant GBMs exhibited a distinct tumor microenvironment enriched with tumor-associated neutrophils (TANs). Transcriptomic analysis revealed upregulation of neutrophil-recruiting chemokines in non-responders with TP53 mutations. This TAN-related gene signature predicted poor prognosis in the ICI-treated cohort and strongly correlated with neutrophil clusters in single-cell datasets.These findings suggest that TP53 mutations contribute to immunotherapy resistance in GBM not through intrinsic tumor properties but by shaping an immunosuppressive microenvironment. This highlights a novel resistance mechanism and suggests that targeting TANs may offer a therapeutic strategy to overcome ICI failure in TP53-mutant GBMs. S