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

Heterologous replacement of the supposed host determining region of avihepadnaviruses: high in vivo infectivity despite low infectivity for hepatocytes

Hepadnaviruses, including hepatitis B virus (HBV), a highly relevant human pathogen, are small enveloped DNA viruses that replicate via reverse transcription. All hepadnaviruses display a narrow tissue and host tropism. For HBV, this restricts efficient experimental in vivo infection to chimpanzees. While the cellular factors mediating infection are largely unknown, the large viral envelope protein (L) plays a pivotal role for infectivity. Furthermore, certain segments of the PreS domain of L from duck HBV (DHBV) enhanced infectivity for cultured duck hepatocytes of pseudotyped heron HBV (HHBV), a virus unable to infect ducks in vivo. This implied a crucial role for the PreS sequence from amino acid 22 to 90 in the duck tropism of DHBV. Reasoning that reciprocal replacements would reduce infectivity for ducks, we generated spreading-competent chimeric DHBVs with L proteins in which segments 22–90 (Du-He4) or its subsegments 22–37 and 37–90 (Du-He2, Du-He3) are derived from HHBV. Infectivity for duck hepatocytes of Du-He4 and Du-He3, though not Du-He2, was indeed clearly reduced compared to wild-type DHBV. Surprisingly, however, in ducks even Du-He4 caused high-titered, persistent, horizontally and vertically transmissable infections, with kinetics of viral spread similar to those of DHBV when inoculated at doses of 10⁸ viral genome equivalents (vge) per animal. Low-dose infections down to 300 vge per duck did not reveal a significant reduction in specific infectivity of the chimera. Hence, sequence alterations in PreS that limited infectivity in vitro did not do so in vivo. These data reveal a much more complex correlation between PreS sequence and host specificity than might have been anticipated; more generally, they question the value of cultured hepatocytes for reliably predicting in vivo infectivity of avian and, by inference, mammalian hepadnaviruses, with potential implications for the risk assessment of vaccine and drug resistant HBV variants.

Hepatitis B virus (HBV) associated liver disease is a leading cause of death worldwide. Host range restrictions limit experimental HBV infections largely to chimpanzees or isolated human hepatocytes. A narrow host range is shared by the animal hepadnaviruses, e.g. from ducks (DHBV) and herons (HHBV); HHBV does not infect ducks though it can establish a low-level infection in cultured duck hepatocytes. Host tropism is thought to be mediated by the PreS domain of the large viral envelope protein, because certain duck virus PreS segments introduced into the envelope of spreading-incompetent HHBV pseudotypes enhanced infectivity for duck hepatocytes. Expecting that reciprocal exchanges in DHBV would negatively impact duck tropism, we generated chimeric DHBVs in which the PreS regions in question are derived from HHBV and which are autonomously spreading-competent; this allowed us to directly compare their infectivity for duck hepatocytes and ducks. Surprisingly, even the chimera with the largest portion of HHBV sequence was as infectious for ducks as authentic DHBV; in vitro infectivity, however, was substantially reduced. These unexpected differences question the value of cultured hepatocytes to reliably predict in vivo infectivity of avihepadnaviruses and, by inference, also that of vaccine escape and therapy resistant HBV variants.

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.

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.

Hepadnaviruses have a narrow host range — do they?

Host range describes the range of species that a virus can infect to productively propagate itself. Productive infection requires compatibility between virus and host molecules. Thus host range may be restricted by lack of appropriate permissivity factors;alternatively, hosts may actively counteract infection using restriction factors. Incompatibility between virus and host can manifest on the level of individual cells,of tissues or organs,and of the entire organism. All hepatitis B viruses are hepatotropic,but individual viruses infect the livers of only selected mammalian (orthohepadnaviruses) and avian (avihepadnaviruses) hosts. Hence a narrow host range is thought to be a salient feature of hepadnaviruses. Here we briefly review general mechanisms of host range restriction,and summarise older as well as recent data pertaining to hepadnaviral host range. Clearly,the term species-specific is inadequate for many hepadnaviruses because they can infect different species from one genus,and even species from different genera. For a few others,only a single species,or genus,has been identified that supports efficient infection;however,this could as well relate to the restricted number of experimentally addressable test species. Together with the uncertainty about quantitative phylogenetic relationships between species,still largely based on morphological rather than molecular criteria,this leaves the term narrow open to interpretation. Finally,few if any of the host molecules enabling productive infection by a hepadnavirus have unambiguously been identified,the role of restriction factors has not yet been assessed,and even on the virus side the so-called host determining regions in the PreS domains of the large envelope proteins appear to be relevant only under specialised experimental conditions. Hence this important aspect of hepadnavirus biology is still far from being understood.

Hepatitis B virus infection initiates with a large surface protein-dependent binding to heparan sulfate proteoglycans

Contrary to many other viruses, the initial steps of the hepatitis B virus (HBV) infection, including attachment to hepatocytes, specific receptor interactions, and membrane fusion, are unsolved. Using HepaRG cells as an in vitro cell culture system, we here report that HBV entry into hepatocytes depends on the interaction with the glycosaminoglycan (GAG) side chains of cell-surface-associated heparan sulfate proteoglycans. Binding to GAGs requires the integrity of the pre-S domain as a part of the large (L-) viral envelope protein. HBV infection was abrogated by incubation of virions with heparin, but not the structurally related GAGs chondroitin sulfate A, B, and C. Infection was also abolished by suramin, a known inhibitor of duck hepatitis B virus infection or highly sulfated dextran sulfate. Polycationic substances such as poly-L-lysine, polybrene, and protamine also prevented infection, however, by addressing cellular components. Enzymatic removal of defined acidic carbohydrate structures from the cell surface using heparinase I/III or the obstruction of GAG synthesis by sodium chlorate inhibited HBV infection of HepaRG cells and, moreover, led to a reduction of HBV cell surface binding sites. The biochemical analysis showed selective binding of L-protein-enriched viral particles (virions or filaments) to heparin. GAG-dependent binding of HBV was improved by polyethylene glycol, a substance that specifically enhances HBV infection.

CONCLUSION

HBV infection requires the initial attachment to the carbohydrate side chains of hepatocyte-associated heparan sulfate proteoglycans as attachment receptors. This interaction initializes the multistep entry process of HBV and cannot be bypassed by alternative routes.

Entry of hepatitis delta virus requires the conserved cysteine residues of the hepatitis B virus envelope protein antigenic loop and is blocked by inhibitors of thiol-disulfide exchange

Hepatitis delta virus (HDV) particles are coated with the envelope proteins (large, middle, and small) of the hepatitis B virus (HBV). The large protein bears an infectivity determinant in its pre-S1 domain, whereas a second determinant has been proposed to map to the cysteine-rich antigenic loop (AGL) within the S domain of all three envelope proteins (G. Abou Jaoudé and C. Sureau, J. Virol. 79:10460-10466, 2006). In this study, the AGL cysteines were substituted by serine or alanine, and the mutants were evaluated for their function at viral entry using HDV particles and susceptible HepaRG cells. Mutations of cysteines 121 to 149 were tolerant of the production of HDV virions. The mutations altered the structure and antigenicity of the conserved "a" determinant of the AGL, as measured by conformation-sensitive antibodies, and they created a block to infectivity. Substitution of Cys-90 or Cys-221, located outside of the AGL, had no impact on the "a" determinant or viral entry. Furthermore, infectivity was maintained when the AGL CxxC motif at position 121 to 124 was modified by single-amino-acid deletion or insertion, suggesting that cysteines 121 and 124 are not catalyzers of thiol/disulfide exchange. However, membrane-impermeable inhibitors of thiol/disulfide isomerazation demonstrated a dose-dependent inhibition of infection in an in vitro assay when applied to the virus prior to inoculation or during the virus-cell interaction period. Overall, the results demonstrate the essential role of the AGL cysteines at viral entry, and they establish a correlation between the cysteine disulfide network, the conformation of the "a" determinant, and infectivity.

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.