Limited disassembly of cytoplasmic hepatitis B virus nucleocapsids restricts viral infection in murine hepatic cells

Mechanisms underlying delayed loss of HBeAg and HBV DNA following HBsAg seroclearance in PEG-IFNα treated patients of chronic hepatitis B

BACKGROUND & AIMS

A notable proportion of CHB patients undergoing PEG-IFNα based therapy experience lagged serum HBeAg and/or HBV DNA disappearance in patients achieving HBsAg loss. In this study, we explored the molecular mechanisms behind this clinical phenomenon, offering novel insights into the sustainability of chronic HBV infection.

METHODS

Two independent clinical cohorts were enrolled to validate this phenomenon. Then comprehensive analysis was performed using public datasets, coupled with a series of molecular biology experiments.

RESULTS

Approximately 17-20% CHB patients underwent PEG-IFNα based therapy experienced seroclearance of HBsAg, while serum HBeAg and/or HBV DNA remained positive. These patients are more prone to serum HBsAg reappearance compared to those achieving complete virological response. Analysis of public datasets revealed that compared to the PC/BCP, the SP1/SP2 promoter displayed more pronounced inhibitory epigenetic modifications in HBeAg-negative patients and SP1/SP2 in-frame mutation peaked in immune active patients. In vitro experiments demonstrated that introduced SP1/SP2 inactive mutations would enhance PC/BCP transcriptional activity by a mechanism known as adjacent transcriptional interference. Furthermore, the deletion of L-HBsAg facilitated intracellular cccDNA replenishment.

CONCLUSION

This study elucidates that under IFNα treatment and low viral load, transcriptional suppression of SP1/SP2 promoters through mutations and/or epigenetic changes would favour the maintenance of sustain chronic HBV infection, via enhancing the transcription activity of BCP to promote cccDNA replenishment.

IMPACT AND IMPLICATIONS

In clinical practice with IFNα antiviral treatment for CHB patients, a "paradoxical" phenomenon is observed where serum HBsAg disappears while HBV DNA or/and HBeAg remains at low positive levels, with delayed disappearance. Our study confirms this clinical phenomenon using two independent clinical cohorts and explores the potential mechanisms behind the persistence of chronic HBV infection under IFNα treatment and low viral load. Transcriptional suppression of SP1/SP2 promoters through mutations and/or epigenetic changes supports the maintenance of chronic HBV infection by enhancing the transcriptional activity of the BCP, which in turn promotes cccDNA replenishment.

HighlightsApproximately 20% of patients with CHB who have just achieved HBsAg loss under IFNα treatment show positive serum HBV DNA and/or HBeAg.During disease progression, in frame indel mutations accumulate in the HBV genome's SP1 and SP2 promoters, with epigenetic modifications contributing to their suppression.In frame indel mutations in the HBV genome's SP1 and SP2 promoters inhibit the transcription of HBV S mRNA and promote the transcription of 3.5 kb HBV RNA.The loss of L-HBs and envelop proteins leads to an increase in intracellular cccDNA, promoting the maintenance of chronic infection.

First detection of HBV nucleic acid in the lung tissue of a patient with spontaneous recovery from HBV infection by tNGS: a case report

BACKGROUND

After recovery from hepatitis B virus (HBV) infection, the virus is likely to reactivate once the body is in an immunosuppressed state. Currently, routine liver biopsies are difficult to obtain, and there are few reports on HBV nucleic acid detection in biopsies of other human organs and tissues. Targeted Next-Generation Sequencing (tNGS) can precisely focus on specific gene regions. Through high-throughput sequencing techniques, it can efficiently obtain a large amount of target nucleic acid sequence information and is widely applied in fields such as disease gene detection and tumor molecular diagnosis. With the development of tNGS, a new path has been opened for HBV nucleic acid detection in different organs, tissues and body fluids, offering hope to break through the current predicament.

CASE PRESENTATION

This case is about a 78-year-old male patient. He was first hospitalized for pulmonary infection. During this hospitalization, tNGS of the bronchoalveolar lavage fluid sample detected a small amount of hepatitis B virus nucleic acid sequences. Two months later, he was re - hospitalized due to cough and expectoration. TNGS of his transthoracic lung fine needle aspiration sample showed a high number of hepatitis B virus nucleic acid sequences. The serological test results during hospitalization indicated that the patient was in a state of spontaneous recovery from hepatitis B virus infection, with HBsAg negative, Anti - HBs positive, Anti - HBc positive, and HBV nucleic acid negative. Fluorescent quantitative PCR detected HBV nucleic acid in the bronchoalveolar lavage fluid sample and the transthoracic lung fine needle aspiration sample at different times.

CONCLUSION

This case study is the first to use tNGS to precisely analyze the bronchoalveolar lavage fluid sample and the transthoracic lung fine needle aspiration sample from different sampling periods. It accurately identifies HBV nucleic acid replication in the lungs of patients with spontaneous HBV - infection recovery, offering new ideas and evidence for HBV research in extra - hepatic tissues.

Identification of NTCP animal orthologs supporting hepatitis B virus binding and infection

Hepatitis B virus (HBV) infection is a significant global health threat, resulting in more than 800,000 deaths annually. Since HBV naturally infects only humans and chimpanzees, the development and evaluation of new therapies for chronic HBV infection are hindered by the lack of suitable animal models. Human sodium-taurocholate cotransporting polypeptide (NTCP) is a critical factor for HBV binding and entry, exhibiting species-specific differences in the amino acid sequences. This study investigated NTCP orthologs from various species to determine their capability to support HBV binding and infection. We demonstrate that nonhuman NTCP orthologs from woodchuck, ferret, aardvark, horse, rabbit, whale, big brown bat, cat, and rhinoceros support HBV binding and cellular entry, thereby rendering HepG2 cells susceptible to HBV infection upon expression. NTCP orthologs from hamster, goat, and cow support HBV binding but require specific amino acid exchanges to facilitate HBV infection. We show that replacement of the functional region, amino acids (aa) 84-87, in hamster NTCP with the human counterpart allows infection of HepG2 cells expressing the chimeric NTCP variant. Furthermore, we demonstrate that aa 82 in goat and cow NTCP, close to this functional region, needs to be modified to support HBV infection. This study could help identify previously unknown HBV reservoirs and may facilitate the establishment of new animal models.IMPORTANCEThe bona fide HBV entry receptor NTCP provides a natural barrier for cross-species transmission. We identified species-specific NTCP orthologues from woodchuck, ferret, aardvark, horse, rabbit, whale, big brown bat, cat, and rhinoceros that support HBV infection. This may reveal potential HBV reservoirs and facilitate the development of new HBV animal models.

Hepatitis B virus hijacks MRE11-RAD50-NBS1 complex to form its minichromosome

Chronic hepatitis B virus (HBV) infection can significantly increase the incidence of cirrhosis and liver cancer, and there is no curative treatment. The persistence of HBV covalently closed circular DNA (cccDNA) is the major obstacle of antiviral treatments. cccDNA is formed through repairing viral partially double-stranded relaxed circular DNA (rcDNA) by varies host factors. However, the detailed mechanisms are not well characterized. To dissect the biogenesis of cccDNA, we took advantage of an in vitro rcDNA repair system to precipitate host factors interacting with rcDNA and identified co-precipitated proteins by mass spectrometry. Results revealed the MRE11-RAD50-NBS1 (MRN) complex as a potential factor. Transiently or stably knockdown of MRE11, RAD50 or NBS1 in hepatocytes before HBV infection significantly decreased viral markers, including cccDNA, while reconstitution reversed the effect. Chromatin immunoprecipitation assay further validated the interaction of MRN complex and HBV DNA. However, MRN knockdown after HBV infection showed no effect on viral replication, which indicated that MRN complex inhibited the formation of cccDNA without affecting its stability or transcriptional activity. Interestingly, Mirin, a MRN complex inhibitor which can inhibit the exonuclease activity of MRE11 and MRN-dependent activation of ATM, but not ATM kinase inhibitor KU55933, could decrease cccDNA level. Likewise, the MRE11 endonuclease activity inhibitor PFM01 treatment decreased cccDNA. MRE11 nuclease assays indicated that rcDNA is a substrate of MRE11. Furthermore, the inhibition of ATR-CHK1 pathway, which is known to be involved in cccDNA formation, impaired the effect of MRN complex on cccDNA. Similarly, inhibition of MRE11 endonuclease activity mitigated the effect of ATR-CHK1 pathway on cccDNA. These findings indicate that MRN complex cooperates with ATR-CHK1 pathway to regulate the formation of HBV cccDNA. In summary, we identified host factors, specifically the MRN complex, regulating cccDNA formation during HBV infection. These findings provide insights into how HBV hijacks host enzymes to establish chronic infection and reveal new therapeutic opportunities.

Hepatitis B virus entry, assembly, and egress

SUMMARYHepatitis B virus (HBV) is an important human pathogen that chronically infects approximately 250 million people in the world, resulting in ~1 million deaths annually. This virus is a hepatotropic virus and can cause severe liver diseases including cirrhosis and hepatocellular carcinoma. The entry of HBV into hepatocytes is initiated by the interaction of its envelope proteins with its receptors. This is followed by the delivery of the viral nucleocapsid to the nucleus for the release of its genomic DNA and the transcription of viral RNAs. The assembly of the viral capsid particles may then take place in the nucleus or the cytoplasm and may involve cellular membranes. This is followed by the egress of the virus from infected cells. In recent years, significant research progresses had been made toward understanding the entry, the assembly, and the egress of HBV particles. In this review, we discuss the molecular pathways of these processes and compare them with those used by hepatitis delta virus and hepatitis C virus , two other hepatotropic viruses that are also enveloped. The understanding of these processes will help us to understand how HBV replicates and causes diseases, which will help to improve the treatments for HBV patients.

SARS-CoV-2 nsp13 suppresses hepatitis B virus replication by targeting cccDNA transcription

SARS-CoV-2 nonstructural protein 13 (nsp13) has been shown to selectively suppress the transcription of episomal DNA while sparing chromosomal DNA. Hepatitis B Virus (HBV) harbors covalently closed circular DNA (cccDNA), a form of viral episomal DNA found within infected hepatocyte nuclei. The persistence of cccDNA is the major cause of chronic HBV infection. In this study, we investigated the impact of SARS-CoV-2 nsp13 on HBV replication, particularly in the context of cccDNA. Our findings demonstrate that nsp13 effectively hinders HBV replication by suppressing the transcription of HBV cccDNA, both in vitro and in vivo. Additionally, we observed that SARS-CoV-2 nsp13 binds to HBV cccDNA and its NTPase and helicase activities contribute significantly to inhibiting HBV replication. Furthermore, our screening identified the interaction between nsp13 and structural maintenance of chromosomes 4, opening new avenues for future mechanistic inquiries. This study presents the evidence suggesting the potential utilization of SARS-CoV-2 nsp13 as a strategy to impede HBV replication by specifically targeting cccDNA. These findings provide a proof of concept for exploring nsp13 as a prospective approach in combating HBV infection.

IMPORTANCE

To effectively combat hepatitis B virus (HBV), it is imperative to develop potent antiviral medications targeting covalently closed circular DNA (cccDNA). Our investigation aimed to assess the impact of SARS-CoV-2 nsp13 on HBV replication across diverse HBV models, confirming its ability to significantly reduce several HBV replication markers. Additionally, our identification of the interaction between nsp13 and SMC4 opens the door for further mechanistic exploration. This marks a paradigm shift in our approach to HBV antiviral therapy, introducing an entirely novel perspective. Our findings propose a novel strategy for developing anti-HBV drugs that specifically target HBV cccDNA.

The feasibility of establishing a hamster model for HBV infection: in vitro evidence

Chronic hepatitis B virus (HBV) infection remains a significant public health burden with no cure currently available. The research to cure HBV has long been hampered by the lack of immunocompetent small animal models capable of supporting HBV infection. Here, we set out to explore the feasibility of the golden Syrian hamster as an immunocompetent small rodent model for HBV infection. We first started with in vitro assessments of the HBV replication cycle in primary hamster hepatocytes (PHaHs) by adenoviral HBV (Ad-HBV) transduction. Our results demonstrated that PHaHs support HBV reverse transcription and subsequent cccDNA formation via the intracellular recycling pathway. Next, with luciferase reporter assays, we confirmed that PHaHs support the activities of all HBV major promoters. Then, we transduced PHaHs with an adenoviral vector expressing HBV receptor human Na+/taurocholate cotransporting polypeptide NTCP (Ad-huNTCP), followed by HBV inoculation. While the untransduced PHaHs did not support HBV infection, Ad-huNTCP-transduced PHaHs supported de novo cccDNA formation, viral mRNA transcription, and expression of viral antigens. We then humanized the amino acid (aa) residues of hamster NTCP (haNTCP) critical for HBV entry, aa84-87 and aa157-165, and transfected HepG2 cells with constructs expressing wild-type haNTCP and humanized-haNTCP, H84R/P87N and H84R/P87N/G157K/M160V/M165L, respectively, followed by HBV inoculation. The results showed that the humanization of H84R/P87N alone was sufficient to support HBV infection at a level comparable to that supported by huNTCP. Taken together, the above in vitro evidence supports the future direction of humanizing haNTCP for HBV infection in vivo.IMPORTANCEOne of the biggest challenges in developing an HBV cure is the lack of immunocompetent animal models susceptible to HBV infection. Developing such models in mice has been unsuccessful due to the absence of a functional HBV receptor, human NTCP (huNTCP), and the defect in supporting viral cccDNA formation. In search of alternative models, we report herein multiple lines of in vitro evidence for developing a golden Syrian hamster model for HBV infection. We demonstrate that the primary hamster hepatocytes (PHaHs) support HBV replication, transcription, and cccDNA formation, and PHaHs are susceptible to de novo HBV infection in the presence of huNTCP. Furthermore, expressing hamster NTCP with two humanized residues critical for HBV entry renders HepG2 cells permissive to HBV infection. Thus, our work lays a solid foundation for establishing a gene-edited hamster model that expresses humanized NTCP for HBV infection in vivo.

Major open questions in the hepatitis B and D field - Proceedings of the inaugural International emerging hepatitis B and hepatitis D researchers workshop

The early and mid-career researchers (EMCRs) of scientific communities represent the forefront of research and the future direction in which a field takes. The opinions of this key demographic are not commonly aggregated to audit fields and precisely demonstrate where challenges lie for the future. To address this, we initiated the inaugural International Emerging Researchers Workshop for the global Hepatitis B and Hepatitis D scientific community (75 individuals). The cohort was split into small discussion groups and the significant problems, challenges, and future directions were assessed. Here, we summarise the outcome of these discussions and outline the future directions suggested by the EMCR community. We show an effective approach to gauging and accumulating the ideas of EMCRs and provide a succinct summary of the significant gaps remaining in the Hepatitis B and Hepatitis D field.

The nuclear matrix protein HNRNPU restricts hepatitis B virus transcription by promoting OAS3-based activation of host innate immunity

Heterogeneous nuclear protein U (HNRNPU) plays a pivotal role in innate immunity by facilitating chromatin opening to activate immune genes during host defense against viral infection. However, the mechanism by which HNRNPU is involved in Hepatitis B virus (HBV) transcription regulation through mediating antiviral immunity remains unknown. Our study revealed a significant decrease in HNRNPU levels during HBV transcription, which depends on HBx-DDB1-mediated degradation. Overexpression of HNRNPU suppressed HBV transcription, while its knockdown effectively promoted viral transcription, indicating HNRNPU as a novel host restriction factor for HBV transcription. Mechanistically, HNRNPU inhibits HBV transcription by activating innate immunity through primarily the positive regulation of the interferon-stimulating factor 2'-5'-oligoadenylate synthetase 3, which mediates an ribonuclease L-dependent mechanism to enhance innate immune responses. This study offers new insights into the host immune regulation of HBV transcription and proposes potential targets for therapeutic intervention against HBV infection.

Progress of Infection and Replication Systems of Hepatitis B Virus

Despite the long-standing availability of effective prophylaxis, chronic hepatitis B virus (HBV) infection remains a formidable public health threat. Antiviral treatments can limit viral propagation, but prolonged therapy is necessary to control HBV replication. Robust in vitro models of HBV infection are indispensable prerequisites for elucidating viral pathogenesis, delineating virus-host interplay and developing novel therapeutic, preventative countermeasures. Buoyed by advances in molecular techniques and tissue culture systems, investigators have engineered numerous in vitro models of the HBV life cycle. However, all current platforms harbor limitations in the recapitulation of natural infection. In this article, we comprehensively review the HBV life cycle, provide an overview of existing in vitro HBV infection and replication systems, and succinctly present the benefits and caveats in each model with the primary objective of constructing refined experimental models that closely mimic native viral infection and offering robust support for the ambitious "elimination of hepatitis by 2030" initiative.

Establishment of a hydrodynamic delivery system in ducks

Chronic hepatitis B virus (HBV) poses a significant global health challenge as it can lead to acute or chronic liver disease and hepatocellular carcinoma (HCC). To establish a safety experimental model, a homolog of HBV-duck HBV (DHBV) is often used for HBV research. Hydrodynamic-based gene delivery (HGD) is an efficient method to introduce exogenous genes into the liver, making it suitable for basic research. In this study, a duck HGD system was first constructed by injecting the reporter plasmid pLIVE-SEAP via the ankle vein. The highest expression of SEAP occurred when ducks were injected with 5 µg/mL plasmid pLIVE-SEAP in 10% bodyweight volume of physiological saline for 6 s. To verify the distribution and expression of exogenous genes in multiple tissues, the relative level of foreign gene DNA and β-galactosidase staining of LacZ were evaluated, which showed the plasmids and their products were located mainly in the liver. Additionally, β-galactosidase staining and fluorescence imaging indicated the delivered exogenous genes could be expressed in a short time. Further, the application of the duck HGD model on DHBV treatment was investigated by transferring representative anti-HBV genes IFNα and IFNγ into DHBV-infected ducks. Delivery of plasmids expressing IFNα and IFNγ inhibited DHBV infection and we established a novel efficient HGD method in ducks, which could be useful for drug screening of new genes, mRNAs and proteins for anti-HBV treatment.

Hepatitis B virus RNAs co-opt ELAVL1 for stabilization and CRM1-dependent nuclear export

Hepatitis B virus (HBV) chronically infects 296 million people worldwide, posing a major global health threat. Export of HBV RNAs from the nucleus to the cytoplasm is indispensable for viral protein translation and genome replication, however the mechanisms regulating this critical process remain largely elusive. Here, we identify a key host factor embryonic lethal, abnormal vision, Drosophila-like 1 (ELAVL1) that binds HBV RNAs and controls their nuclear export. Using an unbiased quantitative proteomics screen, we demonstrate direct binding of ELAVL1 to the HBV pregenomic RNA (pgRNA). ELAVL1 knockdown inhibits HBV RNAs posttranscriptional regulation and suppresses viral replication. Further mechanistic studies reveal ELAVL1 recruits the nuclear export receptor CRM1 through ANP32A and ANP32B to transport HBV RNAs to the cytoplasm via specific AU-rich elements, which can be targeted by a compound CMLD-2. Moreover, ELAVL1 protects HBV RNAs from DIS3+RRP6+ RNA exosome mediated nuclear RNA degradation. Notably, we find HBV core protein is dispensable for HBV RNA-CRM1 interaction and nuclear export. Our results unveil ELAVL1 as a crucial host factor that regulates HBV RNAs stability and trafficking. By orchestrating viral RNA nuclear export, ELAVL1 is indispensable for the HBV life cycle. Our study highlights a virus-host interaction that may be exploited as a new therapeutic target against chronic hepatitis B.

Epigenetic addition of m5C to HBV transcripts promotes viral replication and evasion of innate antiviral responses

Eukaryotic five-methylcytosine (m5C) is an important regulator of viral RNA splicing, stability, and translation. However, its role in HBV replication remains largely unknown. In this study, functional m5C sites are identified in hepatitis B virus (HBV) mRNA. The m5C modification at nt 1291 is not only indispensable for Aly/REF export factor (ALYREF) recognition to promote viral mRNA export and HBx translation but also for the inhibition of RIG-I binding to suppress interferon-β (IFN-β) production. Moreover, NOP2/Sun RNA methyltransferase 2 (NSUN2) catalyzes the addition of m5C to HBV mRNA and is transcriptionally downregulated by the viral protein HBx, which suppresses the binding of EGR1 to the NSUN2 promoter. Additionally, NSUN2 expression correlates with m5C modification of type I IFN mRNA in host cells, thus, positively regulating IFN expression. Hence, the delicate regulation of NSUN2 expression induces m5C modification of HBV mRNA while decreasing the levels of m5C in host IFN mRNA, making it a vital component of the HBV life cycle. These findings provide new molecular insights into the mechanism of HBV-mediated IFN inhibition and may inform the development of new IFN-α based therapies.

The Cre/loxP-Based Recombinant HBV cccDNA System In Vitro and In Vivo

Covalently closed circular DNA (cccDNA) exists as a stable episomal minichromosome in the nucleus of hepatocytes and is responsible for hepatitis B virus (HBV) persistence. We recently reported a technique involving recombinant cccDNA (rcccDNA) of HBV by site-specific DNA recombination. A floxed monomeric HBV genome was engineered into a precursor plasmid (prcccDNA) which was excised via Cre/loxP-mediated DNA recombination to form a 3.3-kb rcccDNA bearing a loxP-chimeric intron. The foreign sequence was efficiently removed during RNA splicing, rendering a functionally seamless insertion. We characterized rcccDNA formation, effective viral transcription, and replication induced by rcccDNA both in vitro and in vivo. Furthermore, we closely simulated chronic hepatitis by using a replication-defective recombinant adenoviral vector to deliver rcccDNA to the transgenic mice expressing Cre recombinase, which led to prominent HBV persistence. Here, we describe a detailed protocol about how to construct and evaluate Cre/loxP-based recombinant HBV cccDNA system both in vitro and in vivo.

Nucleolin binds to and regulates transcription of hepatitis B virus covalently closed circular DNA minichromosome

Hepatitis B virus (HBV) remains a major public health threat with nearly 300 million people chronically infected worldwide who are at a high risk of developing hepatocellular carcinoma. Current therapies are effective in suppressing HBV replication but rarely lead to cure. Current therapies do not affect the HBV covalently closed circular DNA (cccDNA), which serves as the template for viral transcription and replication and is highly stable in infected cells to ensure viral persistence. In this study, we aim to identify and elucidate the functional role of cccDNA-associated host factors using affinity purification and protein mass spectrometry in HBV-infected cells. Nucleolin was identified as a key cccDNA-binding protein and shown to play an important role in HBV cccDNA transcription, likely via epigenetic regulation. Targeting nucleolin to silence cccDNA transcription in infected hepatocytes may be a promising therapeutic strategy for a functional cure of HBV.

G-quadruplex in hepatitis B virus pregenomic RNA promotes its translation

Hepatitis B virus (HBV) is a hepatotropic DNA virus that has a very compact genome. Due to this genomic density, several distinct mechanisms are used to facilitate the viral life cycle. Recently, accumulating evidence show that G-quadruplex (G4) in different viruses play essential regulatory roles in key steps of the viral life cycle. Although G4 structures in the HBV genome have been reported, their function in HBV replication remains elusive. In this study, we treated an HBV replication-competent cell line and HBV-infected cells with the G4 structure stabilizer pyridostatin (PDS) and evaluated different HBV replication markers to better understand the role played by the G4. In both models, we found PDS had no effect on viral precore RNA (pcRNA) or pre-genomic RNA (pgRNA), but treatment did increase HBeAg/HBc ELISA reads and intracellular levels of viral core/capsid protein (HBc) in a dose-dependent manner, suggesting post-transcriptional regulation. To further dissect the mechanism of G4 involvement, we used in vitro-synthesized HBV pcRNA and pgRNA. Interestingly, we found PDS treatment only enhanced HBc expression from pgRNA but not HBeAg expression from pcRNA. Our bioinformatic analysis and CD spectroscopy revealed that pgRNA harbors a conserved G4 structure. Finally, we introduced point mutations in pgRNA to disrupt its G4 structure and observed the resulting mutant failed to respond to PDS treatment and decreased HBc level in in vitro translation assay. Taken together, our data demonstrate that HBV pgRNA contains a G4 structure that plays a vital role in the regulation of viral mRNA translation.

Hepatitis B virus hijacks TSG101 to facilitate egress via multiple vesicle bodies

Hepatitis B virus (HBV) chronically infects 296 million individuals and there is no cure. As an important step of viral life cycle, the mechanisms of HBV egress remain poorly elucidated. With proteomic approach to identify capsid protein (HBc) associated host factors and siRNA screen, we uncovered tumor susceptibility gene 101 (TSG101). Knockdown of TSG101 in HBV-producing cells, HBV-infected cells and HBV transgenic mice suppressed HBV release. Co-immunoprecipitation and site mutagenesis revealed that VFND motif in TSG101 and Lys-96 ubiquitination in HBc were essential for TSG101-HBc interaction. In vitro ubiquitination experiment demonstrated that UbcH6 and NEDD4 were potential E2 ubiquitin-conjugating enzyme and E3 ligase that catalyzed HBc ubiquitination, respectively. PPAY motif in HBc and Cys-867 in NEDD4 were required for HBc ubiquitination, TSG101-HBc interaction and HBV egress. Transmission electron microscopy confirmed that TSG101 or NEDD4 knockdown reduces HBV particles count in multivesicular bodies (MVBs). Our work indicates that TSG101 recognition for NEDD4 ubiquitylated HBc is critical for MVBs mediated HBV egress.