Hepatitis B virus X protein suppresses virus-triggered IRF3 activation and IFN-beta induction by disrupting the VISA-associated complex

A MAVS/TICAM-1-independent interferon-inducing pathway contributes to regulation of hepatitis B virus replication in the mouse hydrodynamic injection model

Toll-like receptors (TLRs) and cytoplasmic RNA sensors have been reported to be involved in the regulation of hepatitis B virus (HBV) replication, but remain controversial due to the lack of a natural infectious model. Our current study sets out to characterize aspects of the role of the innate immune system in eliminating HBV using hydrodynamic-based injection of HBV replicative plasmid and knockout mice deficient in specific pathways of the innate system. The evidence indicated that viral replication was not affected by MAVS or TICAM-1 knockout, but absence of interferon regulatory factor 3 (IRF-3) and IRF-7 transcription factors, as well as the interferon (IFN) receptor, had an adverse effect on the inhibition of HBV replication, demonstrating the dispensability of MAVS and TICAM-1 pathways in the early innate response against HBV. Myd88(-/-) mice did not have a significant increase in the initial viremia, but substantial viral antigen persisted in the mice sera, a response similar to Rag2(-/-) mice, suggesting that the MyD88-dependent pathway participated in evoking an adaptive immune response against the clearance of intrahepatic HBV. Taken together, we show that the RNA-sensing pathways do not participate in the regulation of HBV replication in a mouse model; meanwhile MyD88 is implicated in the HBV clearance.

Recognition of human oncogenic viruses by host pattern-recognition receptors

Human oncogenic viruses include Epstein-Barr virus, hepatitis B virus, hepatitis C virus, human papilloma virus, human T-cell lymphotropic virus, Kaposi's associated sarcoma virus, and Merkel cell polyomavirus. It would be expected that during virus-host interaction, the immune system would recognize these pathogens and eliminate them. However, through evolution, these viruses have developed a number of strategies to avoid such an outcome and successfully establish chronic infections. The persistent nature of the infection caused by these viruses is associated with their oncogenic potential. In this article, we will review the latest information on the interaction between oncogenic viruses and the innate immune system of the host. In particular, we will summarize the available knowledge on the recognition by host pattern-recognition receptors of pathogen-associated molecular patterns present in the incoming viral particle or generated during the virus' life cycle. We will also review the data on the recognition of cell-derived danger associated molecular patterns generated during the virus infection that may impact the outcome of the host-pathogen interaction and the development cancer.

Innate immune responses in hepatitis B virus (HBV) infection

Hepatitis B virus (HBV) infection has a low rate of chronicity compared to HCV infection, but chronic liver inflammation can evolve to life threatening complications. Experimental data from HBV infected chimpanzees and HBV transgenic mice have indicated that cytotoxic T cells are the main cell type responsible for inhibition of viral replication, but also for hepatocyte lysis during chronic HBV infection. Their lower activation and impaired function in later stages of infection was suggested as a possible mechanism that allowed for low levels of viral replication. The lack of an interferon response in these models also indicated the importance of adaptive immunity in clearing the infection. Increased knowledge of the signalling pathways and pathogen associated molecular patterns that govern activation of innate immunity in the early stages of viral infections in general has led to a re-evaluation of the innate immune system in HBV infection. Numerous studies have shown that HBV employs active strategies to evade innate immune responses and induce immunosuppression. Some of the immune components targeted by HBV include dendritic cells, natural killer cells, T regulatory cells and signalling pathways of the interferon response. This review will present the current understanding of innate immunity in HBV infection and of the challenges associated with clearing of the HBV infection.

Suppression of interferon-mediated antiviral immunity by hepatitis B virus: an overview of research progress

Interferon (IFN)-α is an indispensable drug for hepatitis B treatment in clinical settings. However, hepatitis B virus (HBV) can attenuate IFN-mediated antiviral responses to avoid being inhibited or cleared. Much progress has been made in exploring how the IFN-induced anti-HBV effect is inhibited. This review examines and summarizes new advances regarding the molecular mechanism underlying the HBV-induced suppression of type I IFN-mediated antiviral immunity.

Melanoma differentiation-associated gene 5 senses hepatitis B virus and activates innate immune signaling to suppress virus replication

Retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) belong to the RIG-I-like receptors family of pattern recognition receptors. Both RIG-I and MDA5 have been shown to recognize various viral RNAs, but whether they mediate hepatitis B virus (HBV) infection remains unclear. In this study, we demonstrated that the expression of MDA5, but not RIG-I, was increased in Huh7 cells transfected with the HBV replicative plasmid and in the livers of mice hydrodynamically injected with the HBV replicative plasmid. To further determine the effect of RIG-I-like receptors on HBV replication, we cotransfected the HBV replicative plasmid with RIG-I or MDA5 expression plasmid into Huh7 cells and found that MDA5, but not RIG-I at a similar protein level, significantly inhibited HBV replication. Knockdown of endogenous MDA5, but not RIG-I, in Huh7 cells transfected with the HBV replicative plasmid significantly increased HBV replication. Of particular interest, we found that MDA5, but not RIG-I, was able to associate with HBV-specific nucleic acids, suggesting that MDA5 may sense HBV. Finally, we performed in vivo experiments by hydrodynamic injection of the HBV replicative plasmid into wild-type, MDA5⁻/⁻, MDA5⁺/⁻, or RIG-I⁺/⁻ mice, and found that MDA5⁻/⁻ and MDA5⁺/⁻ mice, but not RIG-I⁺/⁻ mice, exhibited an increase of HBV replication as compared with wild-type mice. Collectively, our in vitro and in vivo studies both support a critical role for MDA5 in the innate immune response against HBV infection.

Reversal of hepatitis B virus-induced systemic immune tolerance by intrinsic innate immune stimulation

Systemic immune tolerance induced by chronic hepatitis B virus (HBV) infection is a significant question, but the mechanism of which remains unclear. In this mini-review, we summarize the impaired innate and adaptive immune responses involved in immune tolerance in chronic HBV infection. Furthermore, we delineate a novel dual functional small RNA to inhibit HBV replication and stimulate innate immunity against HBV, which proposed a promising immunotherapeutic intervention to interrupt HBV-induced immunotolerance. A mouse model of HBV persistence was established and used to observe the immune tolerant to HBV vaccination, the cell-intrinsic immune tolerance of which might be reversed by chemically synthesized dual functional small RNA (3p-hepatitis B Virus X gene [HBx]-small interfering RNA) in vitro experiments and by biologically constructed dual functional vector (single-stranded RNA-HBx- short hairpin RNA) in vivo experiment using HBV-carrier mice.

MicroRNA-548 down-regulates host antiviral response via direct targeting of IFN-λ1

Interferon (IFN)-mediated pathways are a crucial part of the cellular response against viral infection. Type III IFNs, which include IFN-λ1, 2 and 3, mediate antiviral responses similar to Type I IFNs via a distinct receptor complex. IFN-λ1 is more effective than the other two members. Transcription of IFN-λ1 requires activation of IRF3/7 and nuclear factor-kappa B (NF-κB), similar to the transcriptional mechanism of Type I IFNs. Using reporter assays, we discovered that viral infection induced both IFN-λ1 promoter activity and that of the 3'-untranslated region (UTR), indicating that IFN-λ1 expression is also regulated at the post-transcriptional level. After analysis with microRNA (miRNA) prediction programs and 3'UTR targeting site assays, the miRNA-548 family, including miR-548b-5p, miR-548c-5p, miR-548i, miR-548j, and miR-548n, was identified to target the 3'UTR of IFN-λ1. Further study demonstrated that miRNA-548 mimics down-regulated the expression of IFN-λ1. In contrast, their inhibitors, the complementary RNAs, enhanced the expression of IFN-λ1 and IFN-stimulated genes. Furthermore, miRNA-548 mimics promoted infection by enterovirus-71 (EV71) and vesicular stomatitis virus (VSV), whereas their inhibitors significantly suppressed the replication of EV71 and VSV. Endogenous miRNA-548 levels were suppressed during viral infection. In conclusion, our results suggest that miRNA-548 regulates host antiviral response via direct targeting of IFN-λ1, which may offer a potential candidate for antiviral therapy.

Hepatitis B virus: from immunobiology to immunotherapy

Owing to the major limitations of current antiviral therapies in HBV (hepatitis B virus) infection, there is a strong need for novel therapeutic approaches to this major health burden. Stimulation of the host's innate and adaptive immune responses in a way that results in the resolution of viral infection is a promising approach. A better understanding of the virus-host interaction in acute and chronic HBV infection revealed several possible novel targets for antiviral immunotherapy. In the present review, we will discuss the current state of the art in HBV immunology and illustrate how control of infection could be achieved by immunotherapeutic interventions.

The role of innate immunity in HBV infection

Hepatitis B virus (HBV) infection is one of the main causes of chronic liver diseases. Whether HBV infection is cleared or persists is determined by both viral factors and host immune responses. It becomes clear that innate immunity is of importance in protecting the host from HBV infection and persistence. However, HBV develops strategies to suppress the antiviral immune responses. A combined therapeutic strategy with both viral suppression and enhancement of antiviral immune responses is needed for effective long-term clearance and cure for chronic HBV infection. We and others confirmed that bifunctional siRNAs with both gene silencing and innate immune activation properties are beneficial for inhibition of HBV and represent a potential approach for treatment of viral infection. Understanding the nature of liver innate immunity and their roles in chronic HBV progression and HBV clearance may aid in the design of novel therapeutic strategies for chronic HBV infection.

Hepatitis B virus regulatory HBx protein binding to DDB1 is required but is not sufficient for maximal HBV replication

Robust hepatitis B virus (HBV) replication is stimulated by the regulatory HBx protein. HBx binds the cellular protein DDB1; however, the importance of this interaction for HBV replication remains unknown. We tested whether HBx binding to DDB1 was required for HBV replication using a plasmid based replication assay in HepG2 cells. Three DDB1 binding-deficient HBx point mutants (HBx(69), HBx(90/91), HBx(R96E)) failed to restore wildtype levels of replication from an HBx-deficient plasmid, which established the importance of the HBx-DDB1 interaction for maximal HBV replication. Analysis of overlapping HBx truncation mutants revealed that both the HBx-DDB1 binding domain and the carboxyl region are required for maximal HBV replication both in vitro and in vivo, suggesting the HBx-DDB1 interaction recruits regulatory functions critical for replication. Finally we demonstrate that HBx localizes to the Cul4A-DDB1 complex, and discuss the possible implications for models of HBV replication.

Involvement of activation of PKR in HBx-siRNA-mediated innate immune effects on HBV inhibition

RNA interference (RNAi) of virus-specific genes offers the possibility of developing a new anti-hepatitis B virus (anti-HBV) therapy. Recent studies have revealed that siRNAs can induce an innate immune response in vitro and in vivo. Here, HBVx (HBx) mRNA expression and HBV replication were significantly inhibited, followed by the enhancement of expression of type I interferons (IFNs), IFN-stimulated genes (ISG15 and ISG56) and proinflammatory cytokines after HepG2.2.15 cells were transfected with chemically synthesized HBx-siRNAs. Transfection with HBx-siRNAs also significantly increased expression of dsRNA-dependent protein kinase R (PKR) in HepG2.2.15 cells, followed by activation of downstream signaling events such as eukaryotic initiation factor 2α (eIF2-α). In PKR-over-expressing HepG2.2.15 cells, HBx-siRNAs exerted more potent inhibitory effects on HBV replication and greater production of type I IFNs. By contrast, the inhibitory effect of HBx-siRNAs on HBV replication was attenuated when PKR was inhibited or silenced, demonstrating that HBx-siRNAs greatly promoted PKR activation, leading to the higher production of type I IFN. Therefore, we concluded that PKR is involved in the innate immune effects mediated by HBx-siRNAs and further contributes to HBV inhibition. The bifunctional siRNAs with both gene silencing and innate immune activation properties may represent a new potential strategy for treatment of HBV.

Reversal of hepatitis B virus-induced immune tolerance by an immunostimulatory 3p-HBx-siRNAs in a retinoic acid inducible gene I-dependent manner

It is extensively accepted that hepatitis B virus (HBV) escapes from innate immunity by inhibiting type I interferon (IFN) production, but efficient intervention to reverse the immune tolerance is still not achieved. Here, we report that 5'-end triphosphate hepatitis B virus X gene (HBx)-RNAs (3p-HBx-short interfering [si]RNAs) exerted significantly stronger inhibitory effects on HBV replication than regular HBx-siRNAs in stably HBV-expressing hepatoplastoma HepG2.2.15 cells through extremely higher expression of type I IFNs, IFN-induced genes and proinflammatory cytokines, and retinoic acid inducible gene I (RIG-I) activation. Also, 3p-HBx-siRNA were more efficient to stimulate type I IFN response than HBx sequence-unrelated 3p-scramble-siRNA in HepG2.2.15 cells, indicating that a stronger immune-stimulating effect may partly result from the reversal of immune tolerance through decreasing HBV load. In RIG-I-overexpressed HepG2.2.15 cells, 3p-HBx-siRNAs exerted stronger inhibitory effects on HBV replication with greater production of type I IFNs; on the contrary, in RIG-I-silenced HepG2.2.15 cells or after blockade of IFN receptor by monoclonal antibody, inhibitory effect of 3p-HBx-siRNAs on HBV replication was largely attenuated, indicating that immunostimulatory function of 3p-HBx-siRNAs was RIG-I and type I IFN dependent. Moreover, in HBV-carrier mice, 3p-HBx-siRNA more strongly inhibited HBV replication and promoted IFN production than HBx-siRNA in primary HBV(+) hepatocytes and, therefore, significantly decreased serum hepatitis B surface antigen and increased serum IFN-β.

CONCLUSION

3p-HBx-siRNAs may not only directly inhibit HBV replication, but also stimulate innate immunity against HBV, which are both beneficial for the inversion of HBV-induced immune tolerance.

Mitochondrion: an emerging platform critical for host antiviral signaling

INTRODUCTION

Toll-like receptors (TLRs) and cytoplasmic retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are the two major receptor systems for detecting RNA viruses. RLRs play essential roles within the cytosol of various cell types. The signaling pathways converge at the mitochondrial antiviral signalling protein (MAVS) on the outer membrane of mitochondria. Recent research has surprisingly shown that many mitochondrial intrinsic factors play novel functions in RLR signaling, conferring a new perspective of therapeutic drug design to inhibit viral infection.

AREAS COVERED

This review covers the literature of the past 6 years and summarizes the components, regulation and mechanisms of RLR signaling, highlighting the function and regulation of mitochondrial proteins, such as MAVS and translocase of outer membrane (Tom)70 in this process. The authors attempt to delineate the complicated cross-talk among subcellular organelles in the context of intracellular antiviral signaling.

EXPERT OPINION

It is imperative to elucidate the specific roles and mechanisms of post-translational modifications in MAVS signaling. The sub-cellular dynamics of regulatory proteins should be correlated with their multiple functions. Animal models are needed to further probe the integration of mitochondria with innate immunity evolutionarily; they will be instrumental for identifying novel antiviral targets and, ultimately, for developing specific clinical drugs.

Mechanism of inhibiting type I interferon induction by hepatitis B virus X protein

Hepatitis B virus (HBV) is regarded as a stealth virus, invading and replicating efficiently in human liver undetected by host innate antiviral immunity. Here, we show that type I interferon (IFN) induction but not its downstream signaling is blocked by HBV replication in HepG2.2.15 cells. This effect may be partially due to HBV X protein (HBx), which impairs IFNβ promoter activation by both Sendai virus (SeV) and components implicated in signaling by viral sensors. As a deubiquitinating enzyme (DUB), HBx cleaves Lys63-linked polyubiquitin chains from many proteins except TANK-binding kinase 1 (TBK1). It binds and deconjugates retinoic acid-inducible gene I (RIG I) and TNF receptor-associated factor 3 (TRAF3), causing their dissociation from the downstream adaptor CARDIF or TBK1 kinase. In addition to RIG I and TRAF3, HBx also interacts with CARDIF, TRIF, NEMO, TBK1, inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase epsilon (IKKi) and interferon regulatory factor 3 (IRF3). Our data indicate that multiple points of signaling pathways can be targeted by HBx to negatively regulate production of type I IFN.

Hepatitis B virus regulatory HBx protein binds to adaptor protein IPS-1 and inhibits the activation of beta interferon

Hepatitis B virus (HBV) encodes the regulatory HBx protein, which is required for virus replication, although its specific role(s) in the replication cycle remains under investigation. An immunoprecipitation/mass spectrometry approach was used to identify four novel HBx binding proteins from the cytoplasmic fraction of HBx transgenic mouse livers. One of these HBx binding partners is beta interferon promoter stimulator 1 (IPS-1), an adaptor protein that plays a critical role in mediating retinoic acid-inducible gene I (RIG-I) signaling, which leads to the activation of beta interferon (IFN-β). The HBx-IPS-1 protein interaction was confirmed in plasmid-transfected HepG2 cells by reciprocal coimmunoprecipitation and Western blotting. We hypothesized that HBx might alter IPS-1 function since proteins of hepatitis C virus and hepatitis A virus similarly bind IPS-1 and target it for inactivation. The effect of HBx on IPS-1-mediated IFN-β signaling was tested in transfected 293T and HepG2 cells, and we show that HBx inhibits double-stranded DNA (dsDNA)-mediated IFN-β activation in a dose-dependent manner when expressed either alone or within the context of HBV replication. However, HBx does not inhibit poly(I:C)-activated IFN-β signaling. These results demonstrate that HBx interferes with the RIG-I pathway of innate immunity. Hepatitis B virus now joins hepatitis C virus and hepatitis A virus in targeting the same innate immune response pathway, presumably as a shared strategy to benefit replication of these viruses in the liver.