P06.12 PCIF1 correlates with immunosuppressive tumor microenvironment and predicts response to immune checkpoint blockade therapy in hepatocellular carcinoma

Background Hepatocellular carcinoma (HCC) remains a global health burden with high morbidity and mortality. The response rate to immunotherapy in HCC patients remains limited to 15-30%, underscoring the clinical urgency for reliable predictive biomarkers to guide treatment decisions. While Phosphorylated CTD Interaction Factor 1 (PCIF1), a unique m6Am methyltransferase, has been implicated in tumorigenesis, its functional role in regulating immune landscape and predicting immune checkpoint blockade (ICB) efficacy in HCC remains unclear.Methods TCGA database analysis was performed to explore PCIF1 expression and its correlation with immune cell infiltration and Tumor Immune Dysfunction and Exclusion (TIDE) scores. A total of 245 patients with advanced HCCs were enrolled, among whom 170 underwent immunohistochemical analysis of PCIF1 expression in tumor tissues to assess its prognostic significance. Multiplex IHC (mIHC) staining for PCIF1, CD4, CD8, and FOXP3 was conducted on another 75 paired HCC tissues (tumor vs. adjacent non-tumor) to assess PCIF1 levels in ICB responders. In the Hepa 1-6 subcutaneous model, Pcif1-knockout and control tumor-bearing mice received anti-PD-1 injections to evaluate therapeutic efficacy.Results Compared to adjacent non-tumor tissues, PCIF1 is significantly upregulated in HCC and independently predicts poor patient survival for both TCGA and 245 HCC patients. Elevated PCIF1 in HCC from TCGA correlated with reduced CD8 + T cell infiltration, increased Tregs, and higher TIDE scores (indicating ICB resistance). mIHC categorized the 75 paired HCC cases tissues into three clusters, with the highest PCIF1 cluster (C2) showing the lowest CD8+ T cells and highest Treg cells density, establishing an immune-suppressive microenvironment. Clinically, HCC patients with low PCIF1-expression exhibited superior response to ICB and prolonged progression-free survival (responders vs. non-responders, media PFS: 10.0 months vs. 6.0 months, HR = 0.36, P < 0.01) and overall survival (responders vs. non-responders, media OS: 40.0 months vs. 22.3 months, HR = 0.56, P < 0.001). In subcutaneous mouse model, Pcif1-knockout tumors exhibited 95.2% growth inhibition after anti-PD-1 therapy, with significantly lower final tumor weight (0.039 ± 0.012g vs. 0.830 ± 0.329g in controls, P < 0.05).Conclusions PCIF1 modulates HCC’s immune landscape by regulating CD8 + T cell and Treg infiltration. It serves as a predictive biomarker for ICB response, with higher expression linked to poor prognosis and ICB resistance. Targeting PCIF1 enhances anti-PD-1 therapy efficacy, providing a potential therapeutic strategy for HCC.