Dissecting the role of SEPHS1 in shaping an immunosuppressive microenvironment to promote tumor progression.

BACKGROUND: Cancer immunotherapy has revolutionized the treatment landscape for multiple malignancies, particularly melanoma. However, therapeutic resistance remains common, highlighting the need to identify novel regulators of antitumor immunity. Selenium is an essential micronutrient that modulates redox homeostasis and immune function through its incorporation into selenoproteins. Yet, the immunological roles of selenium metabolism-related enzymes, especially Selenophosphate Synthetase 1 (SEPHS1), remain poorly defined. METHODS: We performed a comprehensive pan-cancer analysis using TCGA, CCLE, CPTAC, and cBioPortal datasets to evaluate the expression patterns and clinical relevance of selenium metabolism-related genes. A SELENOAMINO Score (SAS) was established to quantify pathway activity and explore its association with prognosis and tumor immune features. CRISPR-Cas9 functional screening data were integrated to identify selenium metabolism genes linked to immunotherapy response. SEPHS1 was further investigated in melanoma through in vitro and in vivo experiments, including gene knockdown, T cell co-culture, flow cytometry, and transcriptomic profiling. RESULTS: Selenium metabolism-related genes exhibited heterogeneous expression and prognostic associations across cancers. SAS correlated with immune infiltration and clinical outcomes, suggesting an immunoregulatory role of selenium metabolism. SEPHS1 was frequently overexpressed and associated with poor prognosis, driven by promoter hypomethylation and copy number amplification. In melanoma, high SEPHS1 expression was linked to reduced CD8⁺ T cell infiltration and activation of immunosuppressive pathways. Knockdown of SEPHS1 enhanced CD8⁺ T cell recruitment and effector function, upregulated CXCL9/10, and significantly improved the therapeutic efficacy of anti-PD-1 blockade. CONCLUSIONS: SEPHS1 promotes immune evasion in melanoma by suppressing chemokines and limiting CD8⁺ T cell infiltration. Targeting SEPHS1 restores immune activity and potentiates immune checkpoint blockade, suggesting a novel immunometabolic strategy to enhance cancer immunotherapy.