CTIM-36. IN-SITU VACCINATION OF BIOPSY-ONLY GBM TUMORS BY TUMOR TREATING FIELDS PLUS ANTI-PD-1 IMMUNOTHERAPY RESULTS IN ROBUST ANTIGEN-SPECIFIC T CELL SELECTION AND EXPANSION, HIGH RESPONSE RATE, AND SIGNIFICANTLY EXTENDED SURVIVAL

Abstract We have recently described a complete in-situ immunizing protocol against GBM using Tumor Treating Fields (TTFields) to activate the DNA sensor-dependent inflammasomes and immunogenic cell death. Here, we report successful in-situ vaccination of patients with newly diagnosed GBM, especially in those with bulky, biopsy-only tumors as compared to those with maximally resected disease, using TTFields together with pembrolizumab, an anti-PD-1 immunotherapy. Patients with biopsy-only tumors displayed a marked improvement in both progression-free survival (27.2m vs 9.6m; HR 0.37, 95%CI 0.16-0.85; P=0.014), overall survival (31.6m vs 18.8m; HR 0.4, 95%CI 0.17-0.92; P=0.023), and response rates (66.6% vs 25%) when compared to those with maximally resected tumors. Using a combination of bulk RNA-seq of enriched T cells and single-cell RNA-seq and TCR clonotyping of tumors and PBMC, we tracked clonal evolution of T cells and confirmed that TTFields’ adaptive immune activation followed a type 1 interferon trajectory (T1IFN) specifically through the DC compartment in patients with biopsy-only tumor. From DC, the T1IFN pathway engaged the adaptive immune system, highly correlated with T cell activation (Pearson r=0.81; P=0.000027), and strongly predicted for extended survival (HR 0.3; P=0.034). Importantly, we discovered the adaptive replacement of expanded T cell clones induced by TTFields and enhanced by pembrolizumab predicted for prolonged survival (P=0.031), leading to sustained T cell activation (Pearson r=0.72; P=0.0018) and robust generation of central memory CD4+ and CD8+ T cells, especially in responders with biopsy-only tumors. In recurrent tumors, the PD-1/PD-L1 axis was suppressed, while concurrent upregulation of alternative immune checkpoints was observed in the periphery and TME. This was coupled with ineffective compensatory T cell activation, suggesting potential mechanisms of resistance. These findings highlight the potent in-situ vaccination effects of TTFields and support its co-application with immune checkpoint inhibitors as a promising strategy for GBM, especially in patients with substantial residual tumor.