Hepatitis B Virus-triggered PTEN/β-Catenin/c-Myc Signaling Enhances PD-L1 Expression To Promote Immune Evasion

Hepatitis B virus(HBV),a small hepatotropic DNA virus,is a major cause of chronic liver disease and can lead to viral hepatitis,cirrhosis,and hepatocellular carcinoma(HCC).An estimated 250 million people around the world are chronically infected with HBV.Like other pathogens,HBV exploits multiple strategies to evade host immune surveillance or interfere with immune signaling pathways and induce immunosuppression.However,it remains largely unknown as to how HBV infection elevates PD-L1 expression in hepatocytes.Programmed cell death 1(PD-1)/PD-1 ligand 1(PD-L1)signaling plays a critical role in regulating T cell homeostasis.PD-1 is an immune checkpoint receptor that is upregulated on activated T cells for the induction of immune tolerance.Tumor cells frequently overexpress the ligand for PD-1,programmed death ligand 1(PD-L1),thus facilitating their escape from immune surveillance.During the past few years,antiPDL1/PD-L1 therapy has taken center stage in immunotherapies for human cancers.Meanwhile,a number of investigations have focused on checkpoint molecules expressed by HBV-infected hepatocytes.By blocking PD-1 in peripheral blood mononuclear cells from CHB patients,antiviral function of effector T cells is significantly enhanced.Despite the advances in understanding the increased antiviral function of HBV-specific T cells by blocking the interaction of PD-1 with PD-L1,the precise mechanism by which HBV infection induces PD-L1 expression in hepatocytes is not fully understood.In this study,we have identified a novel functional role and regulatory mechanism of tensin homolog deleted on chromosome 10(PTEN)/β-catenin/c-Myc signaling in HBVtriggered PD-L1 expression.We confirmed the downregulation of PTEN and upregulation of β-catenin/PD-L1 in HepG2.2.15 cells compared to HepG2 cells,or transfected with pHBV1.3 or pUC18.We observed significant decrease in the expression of phosphatase and tensin homolog deleted on chromosome 10(PTEN),and increase in β-catenin/PD-L1 expression in liver tissues from mice subjected to pHBV1.3 hydrodynamic injection.Mechanistically,in the co-culture system of hepatocytes and Jurkat T cells,PTEN negatively regulated β-catenin/c-Myc signaling and PD-L1 expression in HBV-expressing hepatoma cells,which in turn augmented PD-1 expression,lowered IL-2 secretion,and induced T cell apoptosis.However,disruption of β-catenin inhibited PTEN-mediated PD-L1 expression in hepatocytes transfected with siRNA targeting β-catenin,which was accompanied by decreased PD-1 expression,and increased IL-2 production in T cells.Luciferase reporter assays revealed that c-Myc stimulated transcriptional activity of PD-L1 gene in HBVexpressing hepatoma cells.In addition,HBV X protein(HBx)and HBV polymerase(HBp)contributed to PTEN downregulation and β-catenin/PD-L1 upregulation by overexpression of HBV proteins or transfection with siRNA.Strikingly,PTEN overexpression in hepatocytes inhibited β-catenin/PD-L1 signaling,decreased the expression of HBsAg,HBeAg and HBcAg,and promoted HBV clearance in a mouse model of HBV infection.Using a mouse model of HBV infection and co-culture experiments with HBVexpressing hepatoma cells and Jurkat T cells in vitro,our findings provide insights into the mechanisms by which HBV-triggered PTEN/β-catenin/c-Myc signaling via HBx and HBp enhances PD-L1 expression,leading to inhibition of T cell response,and promotes HBV immune evasion,which may serve as a potential target for immunotherapy of CHB.