Abstract A004: Tertiary lymphoid structures drive early response to combined anti-angiogenic and immune checkpoint therapy in recurrent ovarian cancer: A biomarker discovery study

Abstract Background: High-grade serous ovarian cancer (HGSOC) is the deadliest gynecological malignancy, with limited treatment options once platinum resistance occurs. Preclinical studies suggest that VEGF and PGE2 blockade can enhance T-cell infiltration by disrupting the tumor endothelial barrier, potentially synergizing with PD-L1 blockade. The EORTC-1508GCG phase II trial evaluated atezolizumab (anti–PD-L1), bevacizumab (anti–VEGF), and low-dose aspirin in platinum-resistant ovarian cancer (PROC). While the trial did not meet its primary endpoint, translational analyses revealed a subgroup with high tumor-infiltrating lymphocytes (TILs) who derived clinical benefit, prompting further investigation into immune correlates of response. Methods: We analyzed longitudinal tumor and blood samples from patients treated on the EORTC-1508GCG trial using immunohistochemistry (IHC), multiplex immunofluorescence (mIF), TCR sequencing, and spatial transcriptomics (Visium CytAssist, 10x Genomics). Paired biopsies were collected at baseline and before cycle 3. Response was evaluated by RECIST v1. 1 at 6 months. In total, 116 tumors were analyzed by mIF, 54 by spatial transcriptomics, and 26 blood samples by TCRseq. Results: Although progression-free survival at 6 months was insufficient across treatment arms, the combination of atezolizumab and bevacizumab showed numerical improvement and prolonged time to first subsequent therapy (TFST). mIF revealed increased CD8+ TILs in atezolizumab-containing arms post-treatment, and higher baseline TILs were associated with prolonged TFST. Responders (Rs) exhibited elevated CD8+ TILs, CD11c+ myeloid cells, and CD19+ B cells, alongside enriched tertiary lymphoid structures (TLS), which were sustained during treatment. Spatial analysis highlighted active immune cell crosstalk and MECA+ high endothelial venules in Rs, suggesting TLS-mediated TIL recruitment and activation. In contrast, non-responders (NRs) had higher CD105+ microvessel density, indicative of an immunosuppressive tumor microenvironment. TCR sequencing showed greater TCR richness and lower clonality in Rs. Conclusion: This is the first randomized trial to explore the combination of PD-L1 and VEGF blockade with aspirin in PROC. While not meeting efficacy endpoints, translational analyses revealed that a subgroup of patients with pre-existing immune activation and TLS enrichment may benefit from therapy. Ongoing spatial transcriptomic profiling aims to identify molecular programs driving TIL activation, TLS formation, and resistance. These findings support the development of tissue-based biomarkers to better stratify patients and guide future immunotherapy strategies in PROC. Citation Format: Eleonora Ghisoni, Nicolas Rayroux, Denarda Dangaj Laniti. Tertiary lymphoid structures drive early response to combined anti-angiogenic and immune checkpoint therapy in recurrent ovarian cancer: A biomarker discovery study abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl): Abstract nr A004.