Abstract 2208: <i>Phocaeicola vulgatus</i> mediates immunotherapy resistance in hepatocellular carcinoma via reducing indoleacetic acid production

Abstract Background: Gut microbiota has been widely reported to influence anti-PD-1 efficacy in melanoma and lung cancer, in both clinical specimens and animal models. However, the role of gut microbiota in hepatocellular carcinoma (HCC) remains less explored, with limited understanding of the underlying mechanisms. Method: Advanced HCC patients treated with anti-PD-1 with/without anti-CTLA-4 were prospectively enrolled, and categorized into responders and non-responders groups based on their radiological response after 6 months of immunotherapy according to modified Response Evaluation Criteria for Solid Tumors (mRECIST) guideline. The fecal metagenomics and plasma metabolome data were integrated to reveal the profiles of gut microbiota. Meanwhile, fecal microbiota transplantation (FMT), single strain and metabolite intervention animal models, as well as in vitro CD8+T cell culture were employed to study its mechanism of modulating immunotherapy efficacy in HCC. Results: In the metagenomic analysis of 65 patients, the responders and non-responders exhibited distinct gut microbiota profiles, which were replicated in mice via FMT. Lefse analysis revealed a higher abundance of Phocaeicola vulgatus (P. vulgatus) in non-responders compared to responders. Moreover, patients with higher P. vulgatus had a mean progression free survival (PFS) of 11.1 months and overall survival (OS) of 22.9 months, which were significantly shorter than those with lower P. vulgatus (PFS: 30.8 months, p = 0.047; OS: 63.2 months, p = 0.037). Notably, P. vulgatus enriched in non-responders restrained anti-PD-1 efficacy in syngeneic and orthotopic tumor models and inhibited the CD8+T cell cytotoxicity, evidenced by decreased IFN-γ+ and GzmB+ populations. Mechanistically, P. vulgatus reduced gut microbiota-derived indoleacetic acid (IAA), weakening CD8+ T cell effector functions and impairing immune response. Supplementation with IAA restored CD8+ T cell cytotoxicity and counteracted the immune-suppressive effects of P. vulgatus. Conclusion: P. vulgatus impaired anti-PD-1 efficacy in HCC via reducing IAA production, which in turn diminishes the effector functions of CD8+ T cells. This study establishes a causal link between P. vulgatus and anti-PD-1 resistance in HCC, highlighting gut microbiota-derived IAA as a potential therapeutic target. Citation Format: Caining Zhao, Shanshan Li, Thomas Yau, Wenqi Chen, Ren Ji, Xinyuan Guan, Fengming Spring Kong. Phocaeicola vulgatus mediates immunotherapy resistance in hepatocellular carcinoma via reducing indoleacetic acid production [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2208.