HBO1 Suppresses Anti-Tumor Immunity of CD8⁺ T Cells in Thyroid Cancer by Recruiting BRD4 to Regulate VTCN1 Expression

Thyroid cancer is the most common endocrine malignancy. Although surgery remains the primary treatment, advanced disease frequently recurs or metastasizes, and subsequent therapies yield suboptimal outcomes. Immunotherapy holds promise, yet the underlying immune-evasion mechanisms remain incompletely understood. HBO1, a multifunctional acyltransferase, plays a significant role in tumor immune regulation. Our previous research revealed that HBO1 is highly expressed in thyroid cancer tissues, promotes malignant progression, and inhibits CD8⁺ T cell function. To further elucidate the role and mechanism of HBO1 in thyroid cancer immune regulation, we conducted the following investigations. Firstly, using an in vitro co-culture model analyzed by flow cytometry and ELISA assays, we found that HBO1 overexpression induces CD8⁺ T cell apoptosis and significantly suppresses their cytotoxic function and secretion of effector molecules. Secondly, leveraging bioinformatic analysis of CUT&Tag datasets combined with in vitro cellular experiments, we discovered that HBO1 activates the immune checkpoint VTCN1 via H3K14 acetylation, thereby promoting CD8⁺ T cell exhaustion. Subsequently, employing RNA-seq analysis, ChIP-qPCR, dual-luciferase reporter assays, co-immunoprecipitation (Co-IP), and in vivo experiments, we delineated the specific mechanism: HBO1 recruits the transcription factor BRD4 through H3K14 acetylation to activate VTCN1 transcription, ultimately mediating anti-tumor immune suppression in thyroid cancer. Finally, through IP-MS and Co-IP experiments, we identified the E3 ligase TRIM21 as an upstream regulator of HBO1. TRIM21 modulates HBO1's acetyltransferase function by promoting its ubiquitination-mediated degradation, consequently influencing HBO1's role in thyroid cancer immune escape. In summary, this study demonstrates that HBO1 recruits BRD4 to regulate VTCN1 expression in thyroid cancer, thereby mediating the suppression of CD8⁺ T cell anti-tumor immunity. Targeting TRIM21 shows promise for reversing HBO1's effects in thyroid cancer. Our experimental results will provide novel experimental evidence for advancing immunotherapy in thyroid cancer.