Chen Feng, Ce Li, Dapeng Lei
Background: Head and neck squamous cell carcinoma (HNSC) is a highly heterogeneous malignancy with poor prognosis and frequent recurrence. Beyond tumor-intrinsic alterations, the immune microenvironment plays a decisive role in tumor initiation and progression. However, the causal contribution of systemic plasma proteins to immune regulation and HNSC susceptibility remains poorly defined. Methods: We conducted a multi-sample Mendelian randomization (MR) study integrating large-scale plasma proteomics, immune cell phenotypes, and HNSC. Mediation analyses were performed to identify immune cell phenotypes that potentially mediate protein-HNSC associations. The findings were further supported by immune infiltration analyses, molecular docking and molecular dynamics simulations and validation using clinical HNSC specimens, including single-cell RNA sequencing of collected samples, scTenifoldKnk virtual knockout modeling and immunofluorescence staining/histological assessment of HNSC tissues. Results: monocytes) as the only immune trait causally linked to both PFDN2 and cancer risk. Analysis using multiple deconvolution algorithms demonstrated a consistent negative correlation between PFDN2 expression and monocyte infiltration. Single-cell RNA sequencing revealed predominant PFDN2 expression in epithelial tumor cells, whereas FCGR1A expression was restricted to monocytes. Virtual knockout of PFDN2 selectively activated monocyte-associated inflammatory programs. Molecular docking and dynamics simulations supported a stable protein-protein interaction between PFDN2 and CD64. Tissue analyses further confirmed PFDN2 downregulation and CD64 upregulation in HNSC, correlating with advanced tumor grade and stage. Conclusions: Our findings establish PFDN2 as a protective plasma protein that restrains HNSC progression by suppressing CD64 on monocyte-mediated inflammatory immune microenvironments, highlighting the PFDN2-CD64 axis as a potential prognostic biomarker and therapeutic target.