A hydrophobic domain in the large envelope protein is essential for fusion of duck hepatitis B virus at the late endosome

Both middle and large envelope proteins can mediate neutralization of hepatitis B virus infectivity by anti-preS2 antibodies: escape by naturally occurring preS2 deletions

Hepatitis B virus (HBV) expresses co-terminal large (L), middle (M), and small (S) envelope proteins containing preS1/preS2/S, preS2/S, and S domain alone, respectively. S and preS1 domains mediate sequential virion attachment to heparan sulfate proteoglycans and sodium taurocholate cotransporting polypeptide (NTCP), respectively, which can be blocked by anti-S and anti-preS1 antibodies. How anti-preS2 antibodies neutralize HBV infectivity remains enigmatic. The late stage of chronic HBV infection often selects for mutated preS2 translation initiation codon to prevent M protein expression, or in-frame preS2 deletions to shorten both L and M proteins. When introduced to infectious clone of genotype C or D, both M-minus mutations and most 5' preS2 deletions sustained virion production. Such mutant progeny viral particles were infectious in NTCP-reconstituted HepG2 cells. Neutralization experiments were performed on the genotype D clone. Although remaining susceptible to anti-preS1 and anti-S neutralizing antibodies, M-minus mutants were only partially neutralized by two anti-preS2 antibodies tested while preS2 deletion mutants were resistant. By infection experiments using viral particles with lost versus increased M protein expression, or a neutralization escaping preS2 deletion only present on L or M protein, we found that both full-length L and M proteins contributed to virus neutralization by the two anti-preS2 antibodies. Thus, immune escape could be a driving force for the selection of M-minus mutations, and especially preS2 deletions. The fact that both L and M proteins could mediate neutralization by anti-preS2 antibodies may shed light on the underlying molecular mechanism.IMPORTANCEThe large (L), middle (M), and small (S) envelope proteins of hepatitis B virus (HBV) contain preS1/preS2/S, preS2/S, and S domain alone, respectively. The discovery of heparan sulfate proteoglycans and sodium taurocholate cotransporting polypeptide (NTCP) as the low- and high-affinity HBV receptors could explain neutralizing potential of anti-S and anti-preS1 antibodies, respectively, but how anti-preS2 neutralizing antibodies work remains enigmatic. In this study, we found two M-minus mutants in the context of genotype D partially escaped two anti-preS2 neutralizing antibodies in NTCP-reconstituted HepG2 cells, while several naturally occurring preS2 deletion mutants escaped both antibodies. By point mutations to eliminate or enhance M protein expression, and by introducing preS2 deletion selectively to L or M protein, we found binding of anti-preS2 antibodies to both L and M proteins contributed to neutralization of wild-type HBV infectivity. Our finding may shed light on the possible mechanism(s) whereby anti-preS2 antibodies neutralize HBV infectivity.

Cellular Factors Involved in the Hepatitis D Virus Life Cycle

Hepatitis D virus (HDV) is a defective RNA virus with a negative-strand RNA genome encompassing less than 1700 nucleotides. The HDV genome encodes only for one protein, the hepatitis delta antigen (HDAg), which exists in two forms acting as nucleoproteins. HDV depends on the envelope proteins of the hepatitis B virus as a helper virus for packaging its ribonucleoprotein complex (RNP). HDV is considered the causative agent for the most severe form of viral hepatitis leading to liver fibrosis/cirrhosis and hepatocellular carcinoma. Many steps of the life cycle of HDV are still enigmatic. This review gives an overview of the complete life cycle of HDV and identifies gaps in knowledge. The focus is on the description of cellular factors being involved in the life cycle of HDV and the deregulation of cellular pathways by HDV with respect to their relevance for viral replication, morphogenesis and HDV-associated pathogenesis. Moreover, recent progress in antiviral strategies targeting cellular structures is summarized in this article.

A fusion peptide in preS1 and the human protein disulfide isomerase ERp57 are involved in hepatitis B virus membrane fusion process

Cell entry of enveloped viruses relies on the fusion between the viral and plasma or endosomal membranes, through a mechanism that is triggered by a cellular signal. Here we used a combination of computational and experimental approaches to unravel the main determinants of hepatitis B virus (HBV) membrane fusion process. We discovered that ERp57 is a host factor critically involved in triggering HBV fusion and infection. Then, through modeling approaches, we uncovered a putative allosteric cross-strand disulfide (CSD) bond in the HBV S glycoprotein and we demonstrate that its stabilization could prevent membrane fusion. Finally, we identified and characterized a potential fusion peptide in the preS1 domain of the HBV L glycoprotein. These results underscore a membrane fusion mechanism that could be triggered by ERp57, allowing a thiol/disulfide exchange reaction to occur and regulate isomerization of a critical CSD, which ultimately leads to the exposition of the fusion peptide.

Hepatitis B virus pre-S region: Clinical implications and applications

Hepatitis B virus (HBV) infection is a major threat to global public health, which can result in many acute and chronic liver diseases. HBV, a member of the family Hepadnaviridae, is a small enveloped DNA virus containing a circular genome of 3.2 kb. Located upstream of the S-open-reading frame of the HBV genome is the pre-S region, which is vital to the viral life cycle. The pre-S region has high variability and many mutations in the pre-S region are associated with several liver diseases, such as fulminant hepatitis (FH), liver cirrhosis (LC), and hepatocellular carcinoma (HCC). In addition, the pre-S region has been applied in the development of several pre-S-based materials and systems to prevent or treat HBV infection. In conclusion, the pre-S region plays an essential role in the occurrence, diagnosis, and treatment of HBV-related liver diseases, which may provide a novel perspective for the study of HBV infection and relevant diseases.

Intracellular Trafficking of HBV Particles

The human hepatitis B virus (HBV), that is causative for more than 240 million cases of chronic liver inflammation (hepatitis), is an enveloped virus with a partially double-stranded DNA genome. After virion uptake by receptor-mediated endocytosis, the viral nucleocapsid is transported towards the nuclear pore complex. In the nuclear basket, the nucleocapsid disassembles. The viral genome that is covalently linked to the viral polymerase, which harbors a bipartite NLS, is imported into the nucleus. Here, the partially double-stranded DNA genome is converted in a minichromosome-like structure, the covalently closed circular DNA (cccDNA). The DNA virus HBV replicates via a pregenomic RNA (pgRNA)-intermediate that is reverse transcribed into DNA. HBV-infected cells release apart from the infectious viral parrticle two forms of non-infectious subviral particles (spheres and filaments), which are assembled by the surface proteins but lack any capsid and nucleic acid. In addition, naked capsids are released by HBV replicating cells. Infectious viral particles and filaments are released via multivesicular bodies; spheres are secreted by the classic constitutive secretory pathway. The release of naked capsids is still not fully understood, autophagosomal processes are discussed. This review describes intracellular trafficking pathways involved in virus entry, morphogenesis and release of (sub)viral particles.

The Hepatitis B Virus Envelope Proteins: Molecular Gymnastics Throughout the Viral Life Cycle

New hepatitis B virions released from infected hepatocytes are the result of an intricate maturation process that starts with the formation of the nucleocapsid providing a confined space where the viral DNA genome is synthesized via reverse transcription. Virion assembly is finalized by the enclosure of the icosahedral nucleocapsid within a heterogeneous envelope. The latter contains integral membrane proteins of three sizes, collectively known as hepatitis B surface antigen, and adopts multiple conformations in the course of the viral life cycle. The nucleocapsid conformation depends on the reverse transcription status of the genome, which in turn controls nucleocapsid interaction with the envelope proteins for virus exit. In addition, after secretion the virions undergo a distinct maturation step during which a topological switch of the large envelope protein confers infectivity. Here we review molecular determinants for envelopment and models that postulate molecular signals encoded in the capsid scaffold conducive or adverse to the recruitment of envelope proteins.

Hepatitis B Virus Entry into Cells

Hepatitis B virus (HBV), an enveloped partially double-stranded DNA virus, is a widespread human pathogen responsible for more than 250 million chronic infections worldwide. Current therapeutic strategies cannot eradicate HBV due to the persistence of the viral genome in a special DNA structure (covalently closed circular DNA, cccDNA). The identification of sodium taurocholate co-transporting polypeptide (NTCP) as an entry receptor for both HBV and its satellite virus hepatitis delta virus (HDV) has led to great advances in our understanding of the life cycle of HBV, including the early steps of infection in particular. However, the mechanisms of HBV internalization and the host factors involved in this uptake remain unclear. Improvements in our understanding of HBV entry would facilitate the design of new therapeutic approaches targeting this stage and preventing the de novo infection of naïve hepatocytes. In this review, we provide an overview of current knowledge about the process of HBV internalization into cells.

Bioprocess optimization for purification of chimeric VLP displaying BVDV E2 antigens produced in yeast Hansenula polymorpha

Chimeric virus-like particles (VLP) are known as promising tools in the development of safe and effective subunit vaccines. Recently, a technology platform to produce VLP based on the small surface protein (dS) of the duck hepatitis B virus was established. In this study, chimeric VLP were investigated displaying the 195 N-terminal amino acids derived from the glycoprotein E2 of the bovine viral diarrhea virus (BVDV) on their surface. Isolation of the VLP from methylotrophic yeast Hansenula polymorpha was allowed upon co-expression of wild-type dS and a fusion protein composed of the BVDV-derived antigen N-terminally fused to the dS. It was shown the VLP could be purified by a process adapted from the production of a recombinant hepatitis B VLP vaccine. However, the process essentially depended on costly ultracentrifugation which is critical for low cost production. In novel process variants, this step was avoided after modification of the initial batch capture step, the introduction of a precipitation step and adjusting the ion exchange chromatography. The product yield could be improved by almost factor 8 to 93 ± 12 mg VLP protein per 100 g dry cell weight while keeping similar product purity and antigenicity. This allows scalable and cost efficient VLP production.

Hepatitis C virus p7 mediates membrane-to-membrane adhesion

Viroporin p7 of the hepatitis C virus (HCV) acts as an ion channel for pH equilibration to stabilize HCV particles; most studies of p7 have focused on this role. However, pH equilibration by p7 via its ion channel activity does not fully explain the importance of p7 in HCV particle production. Indeed, several researchers have suggested p7 to have an unidentified ion channel-independent function. Here, we show that p7 has a novel role as a lipid raft adhesion factor, which is independent of its ion channel activity. We found that p7 targets not only the liquid-disordered (Ld) phase, but also the negatively-charged liquid-ordered (Lo) phase that can be represented as a lipid raft. p7 clusters at the phase boundary of the neutral Ld phase and the negatively-charged Lo phase. Interestingly, p7 targeting the Lo phase facilitates membrane-to-membrane adhesion, and this activity is not inhibited by p7 ion channel inhibitors. Our results demonstrated that HCV p7 has dual roles as a viroporin and as a lipid raft adhesion factor. This ion channel-independent function of p7 might be an attractive target for development of anti-HCV compounds.

Visualization of hepatitis B virus entry - novel tools and approaches to directly follow virus entry into hepatocytes

Hepatitis B virus (HBV) is a widespread human pathogen, responsible for chronic infections of ca. 240 million people worldwide. Until recently, the entry pathway of HBV into hepatocytes was only partially understood. The identification of human sodium taurocholate cotransporting polypeptide (NTCP) as a bona fide receptor of HBV has provided us with new tools to investigate this pathway in more details. Combined with advances in virus visualization techniques, approaches to directly visualize HBV cell attachment, NTCP interaction, virion internalization and intracellular transport are now becoming feasible. This review summarizes our current understanding of how HBV specifically enters hepatocytes, and describes possible visualization strategies applicable for a deeper understanding of the underlying cell biological processes.

Strategies to inhibit entry of HBV and HDV into hepatocytes

Although there has been much research into the pathogenesis and treatment of hepatitis B virus (HBV) and hepatitis D virus (HDV) infections, we still do not completely understand how these pathogens enter hepatocytes. This is because in vitro infection studies have only been performed in primary human hepatocytes. Development of a polarizable, HBV-susceptible human hepatoma cell line and studies of primary hepatocytes from Tupaia belangeri have provided important insights into the viral and cellular factors involved in virus binding and infection. The large envelope (L) protein on the surface of HBV and HDV particles has many different functions and is required for virus entry. The L protein mediates attachment of virions to heparan sulfate proteoglycans on the surface of hepatocytes. The myristoylated N-terminal preS1 domain of the L protein subsequently binds to the sodium taurocholate cotransporting polypeptide (NTCP, encoded by SLC10A1), the recently identified bona fide receptor for HBV and HDV. The receptor functions of NTCP and virus entry are blocked, in vitro and in vivo, by Myrcludex B, a synthetic N-acylated preS1 lipopeptide. Currently, the only agents available to treat chronic HBV infection target the viral polymerase, and no selective therapies are available for HDV infection. It is therefore important to study the therapeutic potential of virus entry inhibitors, especially when combined with strategies to induce immune-mediated killing of infected hepatocytes.

Initiation of duck hepatitis B virus infection requires cleavage by a furin-like protease

The entry mechanism of hepatitis B virus (HBV) has not been defined, and this impedes development of antiviral therapies aimed at an early step in the viral life cycle. HBV infection has both host and tissue specificities. For the related duck hepatitis B virus (DHBV), duck carboxypeptidase D (DCPD) has been proposed as the species-specific docking receptor, while glycine decarboxylase (DGD) may serve as a tissue-specific cofactor or secondary receptor. DGD binds to several truncated versions of the viral large envelope protein but not to the full-length protein, suggesting a need for proteolytic cleavage of the envelope protein by a furin-like proprotein convertase. In the present study, we found that transfected DCPD could confer DHBV binding to non-duck cell lines but that this was followed by rapid virus release from cells. Coexpression of furin led to DCPD cleavage and increased virus retention. Treatment of DHBV particles with endosome prepared from duck liver led to cleavage of the large envelope protein, and such viral preparation could generate a small amount of covalently closed circular DNA in LMH cells, a chicken hepatoma cell line resistant to DHBV infection. A furin inhibitor composed of decanoyl-RVKR-chloromethylketone blocked endosomal cleavage of the large envelope protein in vitro and suppressed DHBV infection of primary duck hepatocytes in vivo. These findings suggest that furin or a furin-like proprotein convertase facilitates DHBV infection by cleaving both the docking receptor and the viral large envelope protein.

Hepatitis B virus requires intact caveolin-1 function for productive infection in HepaRG cells

Investigation of the entry pathways of hepatitis B virus (HBV), a member of the family Hepadnaviridae, has been hampered by the lack of versatile in vitro infectivity models. Most concepts of hepadnaviral infection come from the more robust duck HBV system; however, whether the two viruses use the same mechanisms to invade target cells is still a matter of controversy. In this study, we investigate the role of an important plasma membrane component, caveolin-1 (Cav-1), in HBV infection. Caveolins are the main structural components of caveolae, plasma membrane microdomains enriched in cholesterol and sphingolipids, which are involved in the endocytosis of numerous ligands and complex signaling pathways within the cell. We used the HepaRG cell line permissive for HBV infection to stably express dominant-negative Cav-1 and dynamin-2, a GTPase involved in vesicle formation at the plasma membrane and other organelles. The endocytic properties of the newly established cell lines, designated HepaRG(Cav-1), HepaRG(Cav-1Delta1-81), HepaRG(Dyn-2), and HepaRG(Dyn-2K44A), were validated using specific markers for different entry routes. The cells maintained their properties during cell culture, supported differentiation, and were permissive for HBV infection. The levels of both HBV transcripts and antigens were significantly decreased in cells expressing the mutant proteins, while viral replication was not directly affected. Chemical inhibitors that specifically inhibit clathrin-mediated endocytosis had no effect on HBV infection. We concluded that HBV requires a Cav-1-mediated entry pathway to initiate productive infection in HepaRG cells.

The first transmembrane domain of the hepatitis B virus large envelope protein is crucial for infectivity

The early steps of the hepatitis B virus (HBV) life cycle are still poorly understood. Indeed, neither the virus receptor at the cell surface nor the mechanism by which nucleocapsids are delivered to the cytosol of infected cells has been identified. Extensive mutagenesis studies in pre-S1, pre-S2, and most of the S domain of envelope proteins revealed the presence of two regions essential for HBV infectivity: the 77 first residues of the pre-S1 domain and a conformational motif in the antigenic loop of the S domain. In addition, at the N-terminal extremity of the S domain, a putative fusion peptide, partially overlapping the first transmembrane (TM1) domain and preceded by a PEST sequence likely containing several proteolytic cleavage sites, was identified. Since no mutational analysis of these two motifs potentially implicated in the fusion process was performed, we decided to investigate the ability of viruses bearing contiguous deletions or substitutions in the putative fusion peptide and PEST sequence to infect HepaRG cells. By introducing the mutations either in the L and M proteins or in the S protein, we demonstrated the following: (i) that in the TM1 domain of the L protein, three hydrophobic clusters of four residues were necessary for infectivity; (ii) that the same clusters were critical for S protein expression; and, finally, (iii) that the PEST sequence was dispensable for both assembly and infection processes.

HBV life cycle: entry and morphogenesis

Hepatitis B virus (HBV) is a major cause of liver disease. HBV primarily infects hepatocytes by a still poorly understood mechanism. After an endocytotic process, the nucleocapsids are released into the cytoplasm and the relaxed circular rcDNA genome is transported towards the nucleus where it is converted into covalently closed circular cccDNA. Replication of the viral genome occurs via an RNA pregenome (pgRNA) that binds to HBV polymerase (P). P initiates pgRNA encapsidation and reverse transcription inside the capsid. Matured, rcDNA containing nucleocapsids can re-deliver the RC-DNA to the nucleus, or be secreted via interaction with the envelope proteins as progeny virions.

Enveloped viral fusion: insights into the fusion of hepatitis B viruses

Viral fusion, the mechanism by which viruses gain entry into the host cell, is a key step in the replication cycle and an important new target in antiviral therapy and vaccine strategies owing to the conservation of the envelope domains involved and their resistance to immune pressure. The fusion domains of HIV-1 have been studied intensively resulting in the potent antiviral agent T20 and the identification of broadly neutralizing antibody epitopes for vaccine development. Another chronic disease-causing virus, HBV, requires the identification of new antiviral agents to deal with the disease burden of 350 million chronically-infected individuals worldwide, 20% of whom will develop liver cancer. The aim of this review is to bring together basic knowledge on the envelope signatures, mechanisms and strategies for the study of viral fusion and how that knowledge has been applied to the study of hepadnaviral fusion.

Duck hepatitis B virus requires cholesterol for endosomal escape during virus entry

The identity and functionality of biological membranes are determined by cooperative interaction between their lipid and protein constituents. Cholesterol is an important structural lipid that modulates fluidity of biological membranes favoring the formation of detergent-resistant microdomains. In the present study, we evaluated the functional role of cholesterol and lipid rafts for entry of hepatitis B viruses into hepatocytes. We show that the duck hepatitis B virus (DHBV) attaches predominantly to detergent-soluble domains on the plasma membrane. Cholesterol depletion from host membranes and thus disruption of rafts does not affect DHBV infection. In contrast, depletion of cholesterol from the envelope of both DHBV and human HBV strongly reduces virus infectivity. Cholesterol depletion increases the density of viral particles and leads to changes in the ultrastructural appearance of the virus envelope. However, the dual topology of the viral envelope protein L is not significantly impaired. Infectivity and density of viral particles are partially restored upon cholesterol replenishment. Binding and entry of cholesterol-deficient DHBV into hepatocytes are not significantly impaired, in contrast to their release from endosomes. We therefore conclude that viral but not host cholesterol is required for endosomal escape of DHBV.

New insights into hepatitis B and hepatitis delta virus entry

Notwithstanding the medical importance of the HBV infection, our understanding of how this pathogen enters hepatocytes is incomplete. This reflects a long-lasting dependence of in vitro infection studies solely on primary human hepatocytes, which are difficult to obtain and maintain in a susceptible state. The establishment of a polarizable HBV-susceptible human hepatoma cell line (HepaRG) and the utilization of Tupaia belangeri hepatocytes (PTHs) resolved this issue. Since then, important insight into viral and cellular determinants participating in HBV binding and infection have been achieved. We now know that the large viral surface protein (L) plays a pivotal role in HBV entry. It mediates diverse functions, commencing binding of virions to heparan sulfate proteoglycans at the hepatocytes surface as a prerequisite for entry. Subsequently, (a) highly specific event(s) involving the myristoylated N-terminal preS1 subdomain of L, as well as the cytosolic and antigenic loops of the S-domain, initiates a series of less well understood steps, resulting in a pH independent, reduction-sensitive fusion of the viral membrane with a cellular membrane. One of these steps is highly sensitive to synthetic N-acylated preS1 lipopeptides and can be blocked in vitro and in vivo at picomolar concentrations. This opens novel therapeutic options addressing virus entry. Future approaches aiming at the elucidation of HBV hepatotropism, the identification of (a) specific receptor(s), the clarification of the endocytic entry pathway and imaging of fluorescently-labeled virions will allow us to decipher more precisely the HBV entry pathway in the near future. Furthermore, clinical efficacy studies with HBV–preS-derived lipopeptides will tell us whether entry inhibition is a passable way to defend acute and chronic HBV and hepatitis delta virus infections.

Host-range and pathogenicity of hepatitis B viruses

Hepatitis B viruses are small enveloped DNA viruses referred to as Hepadnaviridae that cause transient or persistent (chronic) infections of the liver. This family is divided into two genera, orthohepadnavirus and avihepadnavirus, which infect mammals or birds as natural hosts, respectively. They possess a narrow host range determined by the initial steps of viral attachment and entry. Hepatitis B virus is the focus of biomedical research owing to its medical significance. Approximately 2 billion people have serological evidence of hepatitis B, and of these approximately 350 million people have chronic infections (World Health Organisation, Fact Sheet WHO/204, October 2000). Depending on viral and host factors, the outcomes of infection with hepatitis B virus vary between acute hepatitis, mild or severe chronic hepatitis or cirrhosis. Chronic infections are associated with an increased risk for the development of hepatocellular carcinoma.

The translocation motif of hepatitis B virus envelope proteins is dispensable for infectivity

The early events of hepatitis B virus (HBV) infection remain unclear. In 2006, Stoeckl et al. proposed a new entry mechanism involving a translocation motif (TLM) present in the pre-S2 domain of envelope proteins (L. Stoeckl, A. Funk, A. Kopitzki, B. Brandenburg, S. Oess, H. Will, H. Sirma, and E. Hildt, Proc. Natl. Acad. Sci. USA 103:6730-6734, 2006). After receptor binding and internalization into the endosomal compartment, this motif would allow the translocation of HBV particles through the endosomal membrane into the cytosol. In this study we have used two different mutated viruses containing a truncated TLM and showed their ability to infect human hepatocytes in primary culture, thus demonstrating the dispensability of the TLM for HBV infectivity.

Infectivity determinants of the hepatitis B virus pre-S domain are confined to the N-terminal 75 amino acid residues

The N-terminal pre-S domain of the large hepatitis B virus (HBV) envelope protein plays a pivotal role at the initial step of the viral entry pathway. In the present study, the entire pre-S domain was mapped for infectivity determinants, following a reverse-genetics approach and using in vitro infection assays with hepatitis delta virus (HDV) or HBV particles. The results demonstrate that lesions created within the N-terminal 75 amino acids of the pre-S region abrogate infectivity, whereas mutations between amino acids 76 and 113, overlapping the matrix domain, had no effect. In contrast to the results of a recent study (L. Stoeckl, A. Funk, A. Kopitzki, B. Brandenburg, S. Oess, H. Will, H. Sirma, and E. Hildt, Proc. Natl. Acad. Sci. 103:6730-6734, 2006), the deletion of a cell membrane translocation motif (TLM) located between amino acids 148 and 161 at the C terminus of pre-S2 did not interfere with the infectivity of the resulting HDV or HBV mutants. Furthermore, a series of large deletions overlapping the pre-S2 domain were compatible with infectivity, although the efficiency of infection was reduced when the deletions extended to the pre-S1 domain. Overall, the results demonstrate that the activity of the pre-S domain at viral entry solely depends on the integrity of its first 75 amino acids and thus excludes any function of the matrix domain or TLM.

Entry of duck hepatitis B virus into primary duck liver and kidney cells after discovery of a fusogenic region within the large surface protein

Hepatitis B viruses exhibit a narrow host range specificity that is believed to be mediated by a domain of the large surface protein, designated L. For duck hepatitis B virus, it has been shown that the pre-S domain of L binds to carboxypeptidase D, a cellular receptor present in many species on a wide variety of cell types. Nonetheless, only hepatocytes become infected. It has remained vague which viral features determine host range specificity and organotropicity. By using chymotrypsin to treat duck hepatitis B virus, we addressed the question of whether a putative fusogenic region within the amino-terminal end of the small surface protein may participate in viral entry and possibly constitute one of the determinants of the host range of the virus. Addition of the enzyme to virions resulted in increased infectivity. Remarkably, even remnants of enzyme-treated subviral particles proved to be inhibitory to infection. A noninfectious deletion mutant devoid of the binding region for carboxypeptidase D could be rendered infectious for primary duck hepatocytes by treatment with chymotrypsin. Although because of the protease treatment mutant and wild-type viruses may have become infectious in an unspecific and receptor-independent manner, their host range specificity was not affected, as shown by the inability of the virus to replicate in different hepatoma cell lines, as well as primary chicken hepatocytes. Instead, the organotropicity of the virus could be reduced, which was demonstrated by infection of primary duck kidney cells.

Two potentially important elements of the hepatitis B virus large envelope protein are dispensable for the infectivity of hepatitis delta virus

Previous studies have attempted to clarify the roles of the pre-S1 and pre-S2 domains of the large envelope protein of hepatitis B virus (HBV) in attachment and entry into susceptible cells. Difficulties arise in that these domains contain regions involved in the nucleocapsid assembly of HBV and overlapping with the coding regions of the viral polymerase and RNA sequences required for reverse transcription. Such difficulties can be circumvented with hepatitis delta virus (HDV), which needs the HBV large envelope protein only for infectivity. Thus, mutated HBV envelope proteins were examined for their effects on HDV infectivity. Changing the C-terminal region of pre-S1 critical for HBV assembly allowed the envelopment of HDV and had no effect on infectivity in primary human hepatocytes. Similarly, a deletion of the 12 amino acids of a putative translocation motif (TLM) in pre-S2 had no effect. Thus, these two regions are not necessary for HDV infectivity and, by inference, are not needed for HBV attachment and entry into susceptible cells.

Avian hepatitis B viruses: Molecular and cellular biology, phylogenesis, and host tropism

The human hepatitis B virus (HBV) and the duck hepatitis B virus (DHBV) share several fundamental features. Both viruses have a partially double-stranded DNA genome that is replicated via a RNA intermediate and the coding open reading frames (ORFs) overlap extensively. In addition, the genomic and structural organization, as well as replication and biological characteristics, are very similar in both viruses. Most of the key features of hepadnaviral infection were first discovered in the DHBV model system and subsequently confirmed for HBV. There are, however, several differences between human HBV and DHBV. This review will focus on the molecular and cellular biology, evolution, and host adaptation of the avian hepatitis B viruses with particular emphasis on DHBV as a model system.

Viral and cellular determinants involved in hepadnaviral entry

Hepadnaviridae is a family of hepatotropic DNA viruses that is divided into the genera orthohepadnavirus of mammals and avihepadnavirus of birds. All members of this family can cause acute and chronic hepatic infection, which in the case of human hepatitis B virus (HBV) constitutes a major global health problem. Although our knowledge about the molecular biology of these highly liver-specific viruses has profoundly increased in the last two decades, the mechanisms of attachment and productive entrance into the differentiated host hepatocytes are still enigmatic. The difficulties in studying hepadnaviral entry were primarily caused by the lack of easily accessible in vitro infection systems. Thus, for more than twenty years, differentiated primary hepatocytes from the respective species were the only in vitro models for both orthohepadnaviruses (e.g. HBV) and avihepadnaviruses (e.g. duck hepatitis B virus [DHBV]). Two important discoveries have been made recently regarding HBV: (1) primary hepatocytes from tree-shrews; i.e., Tupaia belangeri, can be substituted for primary human hepatocytes, and (2) a human hepatoma cell line (HepaRG) was established that gains susceptibility for HBV infection upon induction of differentiation in vitro. A number of potential HBV receptor candidates have been described in the past, but none of them have been confirmed to function as a receptor. For DHBV and probably all other avian hepadnaviruses, carboxypeptidase D (CPD) has been shown to be indispensable for infection, although the exact role of this molecule is still under debate. While still restricted to the use of primary duck hepatocytes (PDH), investigations performed with DHBV provided important general concepts on the first steps of hepadnaviral infection. However, with emerging data obtained from the new HBV infection systems, the hope that DHBV utilizes the same mechanism as HBV only partially held true. Nevertheless, both HBV and DHBV in vitro infection systems will help to: (1) functionally dissect the hepadnaviral entry pathways, (2) perform reverse genetics (e.g. test the fitness of escape mutants), (3) titrate and map neutralizing antibodies, (4) improve current vaccines to combat acute and chronic infections of hepatitis B, and (5) develop entry inhibitors for future clinical applications.