S100A9 Promotes Resistance to Anti-PD-1 Immunotherapy in Hepatocellular Carcinoma by Degrading PARP1 and Activating the STAT3/PD-L1 Pathway

Background Immune checkpoint inhibitors (ICIs), such as anti-programmed cell death protein-1 (PD-1) immunotherapy, have emerged as promising treatments for advanced hepatocellular carcinoma (HCC), significantly improving clinical outcomes. However, resistance to ICIs remains a major challenge, and the underlying mechanisms of this resistance are not yet fully understood. This study aimed to investigate the role of S100 calcium-binding protein A9 (S100A9) in mediating resistance to anti-PD-1 therapy. Approach and Results: We conducted RNA sequencing (RNA-seq) on tumor samples from anti-PD-1 responders and non-responders in HCC patients. Differential expression analysis identified S100A9 as a potential driver gene of resistance to anti-PD-1 therapy. Subcutaneous tumor models and an orthotopic HCC model established via hydrodynamic transfection were utilized to evaluate the impact of S100A9 on the efficacy of PD-1 therapy. Our findings revealed that S100A9 promotes resistance to anti-PD-1 therapy in HCC. Mechanistically, S100A9 directly interacted with PARP1 and induced its degradation via the ubiquitin-proteasome pathway. This process increased STAT3 phosphorylation at Tyr705, thereby enhancing PD-L1 transcription. Notably, treatment with the S100A9 inhibitor Tasquinimod significantly improved the efficacy of anti-PD-1 therapy in HCC. Conclusions Our study reveals that S100A9 facilitates immune evasion in HCC by enhancing PARP1 ubiquitination, STAT3 phosphorylation, and PD-L1 expression. Furthermore, combining S100A9 inhibitors with anti-PD-1 antibodies markedly enhances the therapeutic efficacy of ICIs in HCC. These findings highlight S100A9 as a potential therapeutic target for overcoming resistance to immunotherapy in HCC.