B-Cell Lymphoma 6 (BCL6) Is a Host Restriction Factor That Can Suppress HBV Gene Expression and Modulate Immune Responses.

Yeng-Jey Yang,Shih-Hui Chen,Cheng-Hsuan Wu,Yue-Ting Hsieh,Lih-Hwa Hwang,Chun-Ta Lin

doi:10.3389/fmicb.2018.03253

Abstract

Hepatitis B virus (HBV) infection causes acute and chronic liver inflammation. Recent studies have demonstrated that some viral antigens can suppress host innate and adaptive immunity, and thus lead to HBV liver persistency. However, the cellular factors that can help host cells to clear HBV during acute infection remain largely unknown. Here, we used HBV-cleared and HBV-persistent mouse models to seek for cellular factors that might participate in HBV clearance. HBV replicon DNA was delivered into the mouse liver by hydrodynamic injection. RNA-Seq analysis was conducted to identify immune-related genes that were differentially expressed in HBV-persistent and HBV-cleared mouse models. A cellular factor, B cell lymphoma 6 (BCL6), was found to be significantly upregulated in the liver of HBV-cleared mice upon HBV clearance. Co-expression of BCL6 and a persistent HBV clone rendered the clone largely cleared, implicating an important role of BCL6 in controlling HBV clearance. Mechanistic studies demonstrated that BCL6 functioned as a repressor, binding to and suppressing the activities of the four HBV promoters. Correspondingly, BCL6 expression significantly reduced the levels of HBV viral RNA, DNA, and proteins. BCL6 expression could be stimulated by inflammatory cytokines such as TNF-α; the BCL6 in turn synergized TNF-α signaling to produce large amounts of CXCL9 and CXCL10, leading to increased infiltrating immune cells and elevated cytokine levels in the liver. Thus, positive feedback loops on BCL6 expression and immune responses could be produced. Together, our results demonstrate that BCL6 is a novel host restriction factor that exerts both anti-HBV and immunomodulatory activities. Induction of BCL6 in the liver may ultimately assist host immune responses to clear HBV.

Highlights

Hepatitis B virus (HBV) infection may cause severe diseases in the liver such as hepatitis, liver cirrhosis, and hepatocellular carcinoma (Xu et al, 2014)
We have identified a cellular factor, B cell lymphoma 6 (BCL6), whose levels were significantly upregulated in the B6.2S-injected mouse liver during HBV clearance
Taking advantage of the HBV persistent model established in the inbred FVB/N mice and the minimal sequence variation between the B6.2 and B6.2S clones, reducing the undesired heterogeneity of immune responses, FIGURE 8 | TNF-α induces B-cell lymphoma 6 (Bcl6) gene expression in the hepatocytes and Bcl6 overexpression upregulates the TNF-α levels in vivo. (A) The RNA levels of Tnf-α, Il-6, Il-12, Cxcl9, and Cxcl10 were examined by Reverse Transcription-Quantitative PCR (RT-qPCR) in B6.2- and B6.2S-injected mouse liver isolated during the HBV clearance stage. (B) AML12 cells were treated with or without TNF-α (100 ng/ml) for the indicated time periods and Bcl6 RNA was analyzed by RT-qPCR

Summary

Introduction

Hepatitis B virus (HBV) infection may cause severe diseases in the liver such as hepatitis, liver cirrhosis, and hepatocellular carcinoma (Xu et al, 2014). According to an estimation by the World Health Organization (WHO), ∼257 million people are chronically infected with HBV and ∼1 million patients die from HBV-associated diseases each year (Lozano et al, 2012; Ott et al, 2012). Public health policy and a preventive vaccine have provided effective blockage of HBV transmission, there is no effective therapy to cure chronic HBV infection. The major difficulties for curing chronic HBV infection are the exhaustion and depletion of HBV-specific immune responses (see review in Ferrari, 2015) and the persistence of covalently closed circular DNA (cccDNA) in hepatocyte nuclei (Moraleda et al, 1997; Dandri et al, 2000)

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Conclusion