Re-Designed Recombinant Hepatitis B Virus Vectors Enable Efficient Delivery of Versatile Cargo Genes to Hepatocytes with Improved Safety

Establishment and Characterization of a New Cell Culture System for Hepatitis B Virus Replication and Infection

Hepatitis B virus (HBV) is a primary cause of chronic liver diseases in humans. HBV infection exhibits strict host and tissue tropism. HBV core promoter (Cp) drives transcription of pregenomic RNA (pgRNA) and plays a key role in the viral life cycle. Hepatocyte nuclear factor 4α (HNF4α) acts as a major transcriptional factor that stimulates Cp. In this work, we reported that BEL7404 ​cell line displayed a high efficiency of DNA transfection and high levels of HBV antigen expression after transfection of HBV replicons without prominent viral replication. The introduction of exogenous HNF4α and human sodium taurocholate cotransporting polypeptide (hNTCP) expression into BEL7404 made it permissive for HBV replication and susceptible to HBV infection. BEL7404-derived cell lines with induced HBV permissiveness and susceptibility were constructed by stable co-transfection of hNTCP and Tet-inducible HNF4α followed by limiting dilution cloning. HBV replication in such cells was sensitive to inhibition by nucleotide analog tenofovir, while the infection was inhibited by HBV entry inhibitors. This cell culture system provides a new and additional tool for the study of HBV replication and infection as well as the characterization of antiviral agents.

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BEL7404 ​cells are characterized by a high transfection efficiency, but do not support canonical HBV replication.

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BEL7404 ​cells lack endogenous HNF4α expression, and exogenous HNF4α rescues canonical HBV replication.

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BEL7404 ​cells with stable hNTCP and inducible HNF4α expression support HBV infection and inducible replication.

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BEL7404-derived cell lines supporting HBV infection retain high transfection efficiencies and allow testing of antivirals.

Determination of infectious hepatitis B virus particles by an end-point dilution assay identifies a novel class of inhibitors

The quantification of infectious virus particles is fundamental to perform in vitro virology studies. To determine the number of hepatitis B virus (HBV) genome-containing particles in vitro, the genome equivalents (GEq) are measured using quantitative PCR (qPCR). However, in addition to infectious virions, HBV DNA-containing, non-infectious HBV particles are also produced in vitro, which can lead to an over-estimation of the number of infectious HBV particles when analyzed by qPCR. Here, we establish an end-point dilution assay that can precisely determine the number of infectious HBV particles. The cell-based HBV infection assay uses a 384-well plate format and enables the calculation of the 50% tissue culture infective dose (TCID₅₀) in a semi-automated manner. Cell culture-derived HBV (HBVcc), produced by either stable HBV-replicating cells (HepAD38) or HBV-infected HepG2-NTCP cells, as well as patient-derived HBV sera were serially diluted and used to infect naïve target cells. Applying the end-point dilution assay, we infected HepG2-NTCP cells with PEG precipitated HBV derived from HepAD38-and HepG2-NTCPsec+ cell supernatants, calculated the TCID₅₀/mL, converted to plaque-forming units (PFUs), and generated the specific infectivity (ratio of PFU/GEq). As a result, a TCID₅₀/mL of 7.22 × 10⁶ and 2.16 × 10⁶, and the specific infectivity of 1/13,816 and 1/8798 were calculated for HepAD38 and HepG2-NTCPsec+ cell supernatants, respectively. The specific infectivity further increased by approximately 2-fold after removal of non-infectious "naked" particles by immunoprecipitation. Purification of HepAD38 cell supernatants by heparin columns increased the TCID₅₀/mL and specific infectivity by 18- and 15-fold, respectively. Interestingly, non-purified patient-derived HBV sera from two individuals had a specific infectivity of 1/88 and 1/3609. After converting TCID₅₀ to multiplicity of infection (MOI) values, we inoculated HepG2-NTCP cells with HBVcc based on GEq or MOI values and demonstrated that MOI-based infection leads to more reproducible infection rates. Furthermore, the assay was validated using serially diluted lamivudine, an HBV replication inhibitor, inhibiting HBV DNA secretion and infectious viral progeny by approx. 56- and 470-fold, respectively. Interestingly, we identified dexmedetomidine (DMM), an alpha-2 adrenergic agonist, inhibiting the secretion of infectious viral progeny by approx. 6-fold, without interfering in the secretion of HBV DNA. Taken together, we developed an assay that is suitable for the standard quantification of infectious HBV particles. We identified DMM as a novel inhibitor that exclusively interferes with the secretion of infectious HBV particles without affecting the secretion of HBV genomes. This end-point dilution assay enables the precise determination of the number of infectious HBV particles, assessment of the specific infectivity and stability of HBV particles, and identification of novel classes of HBV inhibitors.

Transfected DNA is targeted by STING-mediated restriction

DNA transfection is routinely used for delivering expression of gene of interest to target cells. Transfected DNA has been known to activate cellular DNA sensor(s) and innate immune responses, but the effects of such responses on transfected DNA are not fully understood. STING (stimulator of interferon genes) is an important adaptor protein in cellular innate immune response to various DNA and RNA stimuli and upon activation induces significant type I interferon responses. In this work, we characterized the effects of STING on gene expression driven by transfected double-stranded DNA. We observed that gene expression from transfected DNA was repressed in the presence of overexpressed STING, but increased if endogenous STING was knocked down through RNA interference. Endogenous chromosomal genes and chromosome-integrated exogenous genes were not affected by such STING-mediated restriction, which did not depend on DNA circularity or linearity, promoter used, or bacterial sequences in transfected DNA. Mechanistically, STING-mediated repression of transfected DNA correlates with reduced mRNA levels, and partially involves the induction of interferon β production by STING. Collectively, these data indicate that episomal double-stranded DNA is targeted by STING-mediated cell defense.

Construction of a replication-competent hepatitis B virus vector carrying secreted luciferase transgene and establishment of new hepatitis B virus replication and expression cell lines

BACKGROUND

Previously, we have successfully constructed replication-competent hepatitis B virus (HBV) vectors by uncoupling the P open reading frame (ORF) from the preC/C ORF to carefully design the transgene insertion site to overcome the compact organization of the HBV genome and maintain HBV replication competence. Consequently, the replication-competent HBV vectors carrying foreign genes, including pCH-BsdR, carrying blasticidin resistance gene (399 bp), and pCH-hrGFP, carrying humanized renilla green fluorescent protein gene (720 bp), were successfully obtained. However, the replication efficiency of the former is higher but it is tedious to use, while that of the latter is poor and cannot be quantified. Hence, we need to search for a new reporter gene that is convenient and quantifiable for further research.

AIM

To establish a helpful tool for intracellular HBV replication and anti-viral drugs screening studies.

METHODS

We utilized the replication-competent HBV viral vectors constructed by our laboratory, combined with the secreted luciferase reporter gene, to construct replication-competent HBV vectors expressing the reporter gene secretory Nanoluc Luciferase (SecNluc). HepG2.TA2-7 cells were transfected with this vector to obtain cell lines with stably secreted HBV particles carrying secNluc reporter gene.

RESULTS

The replication-competent HBV vector carrying the SecNluc reporter gene pCH-sNLuc could produce all major viral RNAs and a full set of envelope proteins and achieve high-level secreted luciferase expression. HBV replication intermediates could be produced from this vector. Via transfection with pTRE-sNLuc and selection by hygromycin, we obtained isolated cell clones, named HBV-NLuc-35 cells, which could secrete secNLuc recombinant viruses, and were sensitive to existing anti-HBV drugs. Using differentiated HepaRG cells, it was verified that recombinant HBV possessed infectivity.

CONCLUSION

Our research demonstrated that a replication-competent HBV vector carrying a secreted luciferase transgene possesses replication and expression ability, and the established HBV replication and expression cell lines could stably secrete viral particles carrying secNluc reporter gene. More importantly, the cell line and the secreted recombinant viral particles could be used to trace HBV replication or infection.

The HBx-CTTN interaction promotes cell proliferation and migration of hepatocellular carcinoma via CREB1

Hepatitis B virus-encoded X protein (HBx) acts as a tumor promoter during hepatocellular carcinoma (HCC) development, probably by regulating the expression of host proteins through protein–protein interaction. A proteomics approach was used to identify HBx-interacting proteins involved in HBx-induced hepatocarcinogenesis. We validated the proteomics findings by co-immunoprecipitation and confocal microscopy. We performed cell proliferation, migration assays and cell cycle analyses in HCC cells. Finally, we confirmed the clinical significance of our findings in samples from patients. We found that cortactin (CTTN) is a novel HBx-interacting protein, and HBx regulates the expression of CTTN in the HCC cell lines MHCC-LM3 and HepG2. Mechanistically, by upregulating the expression of cAMP response element-binding protein (CREB1) and its downstream targets, such as cyclin D1 and MMP-9, the effects of the HBx-CTTN interaction on the enhancement of cellular proliferation and migration were maintained by inhibiting cell cycle arrest. In addition, we demonstrated that the levels of CTTN and CREB1 were closely correlated in clinical samples from HBV-infected patients with HCC. Overall, our data suggests that HBx contributes to cell migration and proliferation of HCC cells by interacting with CTTN and regulating the expression of CTTN and CREB1. Therefore, the HBx/CTTN/CREB1 axis is a potential novel therapeutic target in HCC.

Establishment of an in vitro reporter system for screening HBx-targeting molecules

Chronic hepatitis B virus (HBV) infection remains a global public health problem. HBV-encoded X protein (HBx) is a multifunctional regulator that is required to initiate and maintain productive HBV infection, and is involved in HBV-related hepatocellular carcinoma (HCC). Inhibitors that interfere with the functions of HBx could be useful not only for the inhibition of HBV replication but also for the prevention or treatment of HBV-related HCC. To screen molecules that target HBx on a large scale remains a technical challenge due to a lack of sensitive and high-throughput system. In this work, we established an in vitro bioluminescent reporter system for screening HBx-targeting molecules. The system is based on a secretory fusion protein that combines HBx and NanoLuc (HBx-Nluc). The measured activity of NanoLuc in the culture supernatant of HBx-Nluc-expressing cells directly reflects the level of secreted HBx-Nluc. HBx protein-targeting intracellular anti-HBx single-chain variable fragment and RNA-targeting shRNA significantly reduced the secreted NanoLuc activity in HBx-Nluc-expressing cells. This system is simple and sensitive, and compatible with continuous non-disruptive screening, suggesting its potential usefulness for high-throughput screening and evaluating HBx-targeting molecules.

Luteolin-7-O-Glucoside Present in Lettuce Extracts Inhibits Hepatitis B Surface Antigen Production and Viral Replication by Human Hepatoma Cells in Vitro

Hepatitis B virus (HBV) infection is endemic in Asia and chronic hepatitis B (CHB) is a major public health issue worldwide. Current treatment strategies for CHB are not satisfactory as they induce a low rate of hepatitis B surface antigen (HBsAg) loss. Extracts were prepared from lettuce hydroponically cultivated in solutions containing glycine or nitrate as nitrogen sources. The lettuce extracts exerted potent anti-HBV effects in HepG2 cell lines in vitro, including significant HBsAg inhibition, HBV replication and transcription inhibition, without exerting cytotoxic effects. When used in combination interferon-alpha 2b (IFNα-2b) or lamivudine (3TC), the lettuce extracts synergistically inhibited HBsAg expression and HBV replication. By using differential metabolomics analysis, Luteolin-7-O-glucoside was identified and confirmed as a functional component of the lettuce extracts and exhibited similar anti-HBV activity as the lettuce extracts in vitro. The inhibition rate on HBsAg was up to 77.4%. Moreover, both the lettuce extracts and luteolin-7-O-glucoside functioned as organic antioxidants and, significantly attenuated HBV-induced intracellular reactive oxygen species (ROS) accumulation. Luteolin-7-O-glucoside also normalized ROS-induced mitochondrial membrane potential damage, which suggests luteolin-7-O-glucoside inhibits HBsAg and HBV replication via a mechanism involving the mitochondria. Our findings suggest luteolin-7-O-glucoside may have potential value for clinical application in CHB and may enhance HBsAg and HBV clearance when used as a combination therapy.

The current status and future directions of hepatitis B antiviral drug discovery

INTRODUCTION

The current standard care of chronic hepatitis B fails to induce a durable off-drug control of hepatitis B virus (HBV) replication in the majority of treated patients. The primary reasons are its inability to eliminate the covalently closed circular (ccc) DNA, the nuclear form of HBV genome, and restoration of the dysfunctional host antiviral immune response against the virus. Accordingly, discovery and development of therapeutics to completely stop HBV replication, eliminate or functionally inactivate cccDNA as well as activate a functional antiviral immune response against HBV are the primary efforts for finding a cure for chronic hepatitis B. Area covered: Herein, the authors highlight the current efforts of HBV drug discovery and offer their opinions for the future directions of this research. Expert opinion: The authors believe that through a consecutive or overlapping three-stage antiviral and immunotherapy program to: (i) completely stop HBV replication and cccDNA amplification; (ii) reduce viral antigen load and induce HBV surface antigen (HBsAg) seroclearance through eradication or inactivation of cccDNA and RNA interference-mediated degradation of viral mRNA and (iii) activate a functional antiviral immune response against HBV through therapeutic immunization or immunotherapy, a functional cure of chronic HBV infection can be achieved in the majority of chronic HBV carriers.