Circular RNA circE7 Encoded by Human Papillomavirus Facilitates Immune Evasion in Head and Neck Squamous Cell Carcinoma through Epigenetically Downregulating Galectin-9

Abstract Immune evasion represents a crucial milestone in the progression of cancer and serves as the theoretical foundation for tumor immunotherapy. Currently, immune checkpoint inhibitors such as anti-PD-1/PD-L1 have shown promising therapeutic efficacy in clinical settings for various tumors. However, a significant proportion of tumor patients fail to benefit from this class of immune checkpoint inhibitors, implying potential involvement of other immune checkpoint molecules in tumor immune escape. In this study, we unveiled a negative association between Human Papillomavirus (HPV)-encoded circular RNA circE7 and the presence of infiltrating CD8+ T cells in head and neck squamous cell carcinoma (HNSCC). Both in vitro and in vivo experiments demonstrated that circE7 suppressed the function and activity of T cells by downregulating the transcription of LGALS9, which encodes the galectin-9 protein. The unexpected molecular mechanism involved the binding of circE7 to acetyl-CoA carboxylase 1 (ACC1), enhancing its dephosphorylation, and thereby activation of ACC1, which resulted in a reduction in the acetylation level of H3K27 in the promoter region of LGALS9 gene and subsequent downregulation of galectin-9 expression. Notably, galectin-9 interacts with immune checkpoint molecules TIM-3 and PD-1 present on T cell surface, triggering T cells to secrete cytotoxic cytokines and hindering T cell apoptosis. In essence, HPV-mediated downregulation of galectin-9 expression through circE7 impairs T cell function and activity, ultimately promoting immune evasion in HNSCC. Importantly, we confirmed that a combination of monoclonal antibodies simultaneously targeting TIM-3 and PD-1 significantly improved the immunotherapeutic efficacy in HNSCC both in vitro and in animal models. Our study unveils a novel mechanism through which HPV promotes immune evasion in HNSCC via circE7-mediated epigenetic modification, offering a new therapeutic strategy for enhanced HNSCC immunotherapy.