Fine mapping of neutralization epitopes on duck hepatitis B virus (DHBV) pre-S protein using monoclonal antibodies and overlapping peptides

Potent inhibition of late stages of hepadnavirus replication by a modified cell penetrating peptide

Cationic cell-penetrating peptides (CPPs) and their lipid domain-conjugates (CatLip) are agents for the delivery of (uncharged) biologically active molecules into the cell. Using infection and transfection assays we surprisingly discovered that CatLip peptides were able to inhibit replication of Duck Hepatitis B Virus (DHBV), a reference model for human HBV. Amongst twelve CatLip peptides we identified Deca-(Arg)₈ having a particularly potent antiviral activity, leading to a drastic inhibition of viral particle secretion without detectable toxicity. Inhibition of virion secretion was correlated with a dose-dependent increase in intracellular viral DNA. Deca-(Arg)₈ peptide did neither interfere with DHBV entry, nor with formation of mature nucleocapsids nor with their travelling to the nucleus. Instead, Deca-(Arg)₈ caused envelope protein accumulation in large clusters as revealed by confocal laser scanning microscopy indicating severe structural changes of preS/S. Sucrose gradient analysis of supernatants from Deca-(Arg)₈-treated cells showed unaffected naked viral nucleocapsids release, which was concomitant with a complete arrest of virion and surface protein-containing subviral particle secretion. This is the first report showing that a CPP is able to drastically block hepadnaviral release from infected cells by altering late stages of viral morphogenesis via interference with enveloped particle formation, without affecting naked nucleocapsid egress, thus giving a view inside the mode of inhibition. Deca-(Arg)₈ may be a useful tool for elucidating the hepadnaviral secretory pathway, which is not yet fully understood. Moreover we provide the first evidence that a modified CPP displays a novel antiviral mechanism targeting another step of viral life cycle compared to what has been so far described for other enveloped viruses.

In vivo electroporation improves therapeutic potency of a DNA vaccine targeting hepadnaviral proteins

This preclinical study investigated the therapeutic efficacy of electroporation (EP)-based delivery of plasmid DNA (pDNA) encoding viral proteins (envelope, core) and IFN-γ in the duck model of chronic hepatitis B virus (DHBV) infection. Importantly, only DNA EP-therapy resulted in a significant decrease in mean viremia titers and in intrahepatic covalently closed circular DNA (cccDNA) levels in chronic DHBV-carrier animals, compared with standard needle pDNA injection (SI). In addition, DNA EP-therapy stimulated in all virus-carriers a humoral response to DHBV preS protein, recognizing a broader range of major antigenic regions, including neutralizing epitopes, compared with SI. DNA EP-therapy led also to significant higher intrahepatic IFN-γ RNA levels in DHBV-carriers compared to other groups, in the absence of adverse effects. We provide the first evidence on DNA EP-therapy benefit in terms of hepadnaviral infection clearance and break of immune tolerance in virus-carriers, supporting its clinical application for chronic hepatitis B.

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.

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.

Enzymatic treatment of duck hepatitis B virus: topology of the surface proteins for virions and noninfectious subviral particles

The large surface antigen L of duck hepatitis B virus exhibits a mixed topology with the preS domains of the protein alternatively exposed to the particles' interior or exterior. After separating virions from subviral particles (SVPs), we compared their L topologies and showed that both particle types exhibit the same amount of L with the following differences: 1--preS of intact virions was enzymatically digested with chymotrypsin, whereas in SVPs only half of preS was accessible, 2--phosphorylation of L at S118 was completely removed by phosphatase treatment only in virions, 3--iodine-125 labeling disclosed a higher ratio of exposed preS to S domains in virions compared to SVPs. These data point towards different surface architectures of virions and SVPs. Because the preS domain acts in binding to a cellular receptor of hepatocytes, our findings implicate the exclusion of SVPs as competitors for the receptor binding and entry of virions.

Identification of T-cell epitopes associated with immunity within the surface protein of duck hepatitis B virus

Duck hepatitis is a convenient model of hepatitis B virus (HBV) infection, but the lack of immunological reagents hampers investigation of pathogenesis and vaccine development. The aim of this study was to define T-cell epitopes in the surface peptide recognized by vaccinated immune birds. Blastogenesis assays were used to test the proliferative response of spleen mononuclear cells to synthetic peptides spanning the pre-S/S region in 22 naïve and 13 immunized and challenged immune ducks. Roughly > or = 50% of the immune ducks responded to five immunodominant peptides eliciting a statistically greater proliferative response than in naïve birds. Fewer ducks responded to an additional six peptides. No statistically significant difference could be shown for the response to 11 peptides between the immune ducks and the naïve ducks. There was no clustering of the immunodominant peptides which were located throughout the surface antigen at sites of major swings in hydrophobicity. A number of peptides which induce lymphoblastogenesis in vaccinated immune ducks have been identified. Their role in spontaneous recovery from duck hepatitis B infection merits investigation.

Single B or T-cell epitope-based DNA vaccine using modified vector induces specific immune response against hepadnavirus

Epitope-based DNA vaccine is an effective and powerful approach against a variety of pathogens or tumors. In present study, we reconstructed a vector that could effectively express short B and T-cell epitope of duck/hepatitis B virus, and investigated the role of the epitope-based DNA vaccination. The pUC19 was modified by inserting the compact transient framework (CTF), including HCMV IE1 promoter, enhancer, Kozak sequence, dual stop codon and 3' terminal bovine growth hormone terminal signal and so on. This modified vector was designated pEC(K) and supposed to effectively express short peptide. A well-defined single B-cell and T-cell epitope encoding gene of duck/hepatitis B virus has been synthesized as candidate epitope and cloned into pEC(K) plasmid, respectively. Transfection of the recombinant DNA into C(2)C(12) cell showed that modified plasmid could effectively express both the single B-cell and T-cell short epitope in the culture supernatant as confirmed by dot immunoblot assay (DIA). The recombinant single B and T-cell epitope-based DNA vaccine was administrated to C57BL/6 mice and could greatly induce specific humoral and CTL response. In addition, the specific antibody against B epitope could specifically bind to the DHBV particles. This report demonstrated that single epitope-based DNA vaccine using modified plasmid vector pEC(K) could induce effective specific immune responses and could be of great use for DNA vaccines.

Glycine decarboxylase mediates a postbinding step in duck hepatitis B virus infection

Envelope protein precursors of many viruses are processed by a basic endopeptidase to generate two molecules, one for receptor binding and the other for membrane fusion. Such a cleavage event has not been demonstrated for the hepatitis B virus family. Two binding partners for duck hepatitis B virus (DHBV) pre-S envelope protein have been identified. Duck carboxypeptidase D (DCPD) interacts with the full-length pre-S protein and is the DHBV docking receptor, while duck glycine decarboxylase (DGD) has the potential to bind several deletion constructs of the pre-S protein in vitro. Interestingly, DGD but not DCPD expression was diminished following prolonged culture of primary duck hepatocytes (PDH), which impaired productive DHBV infection. Introduction of exogenous DGD promoted formation of protein-free viral genome, suggesting restoration of several early events in viral life cycle. Conversely, blocking DGD expression in fresh PDH by antisense RNA abolished DHBV infection. Moreover, addition of DGD antibodies soon after virus binding reduced endogenous DGD protein levels and impaired production of covalently closed circular DNA, the template for DHBV gene expression and genome replication. Our findings implicate this second pre-S binding protein as a critical cellular factor for productive DHBV infection. We hypothesize that DCPD, a molecule cycling between the cell surface and the trans-Golgi network, targets DHBV particles to the secretary pathway for proteolytic cleavage of viral envelope protein. DGD represents the functional equivalent of other virus receptors in its interaction with processed viral particles.

Identification of antigenic regions of duck hepatitis B virus core protein with antibodies elicited by DNA immunization and chronic infection

The induction of humoral response in ducks by DNA-based immunization against duck hepatitis B virus (DHBV) core protein (DHBc) was investigated. In addition, the amino acid specificity of the induced response was compared by using peptide scanning to that elicited either by protein immunization or during chronic DHBV infection. Immunization of ducks with a plasmid expressing DHBc protein led to the induction of a long-lasting antibody response able to specifically recognize viral protein in chronically infected duck livers. Peptide scanning analysis of anti-DHBc response induced during chronic DHBV infection allowed us to identify six major antigenic regions (AR1 to AR6). The reactivity spectrum of duck sera elicited by protein immunization appeared narrower and was restricted to only four of these antigenic regions in spite of higher anti-DHBc antibody titers. Interestingly, anti-DHBc antibodies induced by DNA-based immunization recognized five of six antigenic regions, and the epitope pattern was broader and more closely related to that observed in chronic viral infections. To gain more insight into the location of antigenic regions, we built a three-dimensional (3-D) model of DHBc protein based on human and duck core sequence alignment data and the HBc 3-D crystal structure. The results suggest that two identified antigenic regions (AR2, amino acids [aa] (64)T-P(84), and AR5, aa (183)A-R(210)) are located at positions on the protein surface equivalent to those of the two HBc major epitopes. Moreover, we identified another antigenic region (AR3, aa (99)I-I(112)) that was recognized by all sera from chronically infected, DNA- or protein-immunized ducks within the large 45-aa insertion in DHBc protein, suggesting that this region, which lacks HBc, is externally exposed.

Antiviral effect of adefovir in combination with a DNA vaccine in the duck hepatitis B virus infection model

BACKGROUND/AIMS

Combination of antiviral drugs with immunotherapeutic approaches may be a promising approach for the treatment of chronic hepatitis B. We used the duck HBV (DHBV) infection model to evaluate the efficacy of the combination of adefovir with DNA-immunization by comparison with the respective monotherapies.

METHODS

Pekin ducks chronically infected with DHBV received adefovir treatment alone or in association with intramuscular immunization with a plasmid (pCI-preS/S) expressing the DHBV large envelope protein. Ducks immunized with pCI-preS/S plasmid alone and two control groups receiving empty plasmid injections or no treatment were followed in parallel.

RESULTS

All animals treated with adefovir showed a marked drop in viremia titers during drug administration, followed by a rebound of viral replication after drug withdrawal. Eight weeks after the third DNA boost, the median of viremia within the duck group receiving the combination therapy tended to be lower compared to that of the other groups. In addition, our results suggest a trend to an additive effect of adefovir and DNA vaccine since a 51% decrease in DHBV DNA was observed in autopsy liver samples from combination therapy group, whereas pCI-preS/S or adefovir monotherapies decreased intrahepatic viral DNA by 38 and 14%, respectively. This effect was sustained since it was observed 12 weeks after the end of therapy.

CONCLUSIONS

Our results suggest that combination of adefovir with DNA-vaccine may be able to induce a sustained antiviral effect in vivo.

Antibody responses of cattle with respiratory coronavirus infections during pathogenesis of shipping fever pneumonia are lower with antigens of enteric strains than with those of a respiratory strain

The serum antibody responses of cattle with respiratory coronavirus infections during the pathogenesis of shipping fever pneumonia were analyzed with different bovine coronavirus antigens, including those from a wild-type respiratory bovine coronavirus (RBCV) strain (97TXSF-Lu 15-2) directly isolated from lung tissue from a fatally infected bovine, a wild-type enteropathogenic bovine coronavirus (EBCV) strain (Ly 138-3), and the highly cell culture-adapted, enteric prototype strain (EBCV L9-81). Infectivity-neutralizing (IN) and hemagglutinin-inhibiting (HAI) activities were tested. Sequential serum samples, collected during the onset of the respiratory coronavirus infection and at weekly intervals for 5 weeks thereafter, had significantly higher IN and HAI titers for antigens of RBCV strain 97TXSF-Lu15-2 than for the wild-type and the highly cell culture-adapted EBCV strains, with P values ranging from <0.0001 to 0.0483. The IN and HAI antibody responses against the two EBCV strains did not differ significantly, but the lowest titers were detected with EBCV strain L9-81.

Characterization of the antiviral effect of 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine in the duck hepatitis B virus infection model

A novel L-nucleoside analog of deoxycytidine, 2',3'-dideoxy-2', 3'-didehydro-beta-L-5-fluorocytidine (beta-L-Fd4C), was recently shown to strongly inhibit hepatitis B virus (HBV) replication in the 2.2.15 cell line. Therefore, its antiviral activity was evaluated in the duck HBV (DHBV) infection model. Using a cell-free system for the expression of the DHBV polymerase, beta-L-Fd4C-TP exhibited a concentration-dependent inhibition of dCTP incorporation into viral minus-strand DNA with a 50% inhibitory concentration of 0.2 microM which was lower than that of other tested deoxycytidine analogs, i.e. , lamivudine-TP, ddC-TP, and beta-L-FddC-TP. Further analysis showed that beta-L-Fd4C-TP is likely to be a competitive inhibitor of dCTP incorporation and to cause premature DNA chain termination. In primary duck hepatocyte cultures infected in vitro, beta-L-Fd4C administration exhibited a long-lasting inhibitory effect on viral DNA synthesis but could not clear viral covalently closed circular DNA (CCC DNA). Results of short-term antiviral treatment in experimentally infected ducklings showed that beta-L-Fd4C exhibited the most potent antiviral effect, followed by beta-L-FddC, lamivudine, and ddC. Longer administration of beta-L-Fd4C induced a sustained suppression of viremia (>95% of controls) and of viral DNA synthesis within the liver. However, the persistence of trace amounts of viral CCC DNA detected only by PCR was associated with a recurrence of viral replication after drug withdrawal. In parallel, beta-L-Fd4C treatment suppressed viral antigen expression within the liver and decreased intrahepatic inflammation and was not associated with any sign of toxicity. Our data, therefore, demonstrate that in the duck model of HBV infection, beta-L-Fd4C is a potent inhibitor of DHBV reverse transcriptase activity in vitro and suppresses viral replication in the liver in vivo.

Carboxypeptidase D is an avian hepatitis B virus receptor

The receptor molecules for human and animal hepatitis B viruses have not been defined. Previous studies have described a 170 to 180 kDa molecule (p170 or gp180) that binds in vitro to the pre-S domain of the large envelope protein of duck hepatitis B virus (DHBV); cDNA cloning revealed the binding protein to be duck carboxypeptidase D (DCPD). In the present study, the DCPD cDNA was transfected into several nonpermissive human-, monkey-, and avian species-derived cell lines. Cells transfected with a plasmid encoding the full-length DCPD protein bound DHBV particles, whereas cells expressing truncated versions of DCPD protein that fail to bind the pre-S protein did not. The DHBV binding to DCPD-reconstituted cells was blocked by a monoclonal antibody that neutralizes DHBV infection of primary duck hepatocytes (PDH) and also by a pre-S peptide previously shown to inhibit DHBV infection of PDH. In addition to promoting virus binding, DCPD expression was associated with internalization of viral particles. The entry process was prevented by incubation of reconstituted cells with DHBV at 4 degrees C and by the addition of energy-depleting agents known to block DHBV entry into PDH. These results demonstrated that DCPD is a DHBV receptor. However, the lack of complete viral replication in DCPD-reconstituted cells suggested that additional factors are required for postentry events in immortalized cell lines.

Identification and expression of glycine decarboxylase (p120) as a duck hepatitis B virus pre-S envelope-binding protein

A 120-kilodalton protein (p120) was identified in the duck liver that binds to several truncated versions of duck hepatitis B virus (DHBV) pre-S envelope protein, suggesting p120 may serve as a DHBV co-receptor. The amino acid sequences of tryptic peptides from purified p120 were found to be the duck p protein of the glycine decarboxylase complex (DGD). DGD cDNA cloning revealed extensive protein conservation with the chicken homologue except for several insertions in the N-terminal leader sequence. The DGD cDNA contained no in-frame AUG codon at the predicted initiation site of the open reading frame, and site-directed mutagenesis experiments established an AUU codon as the translational initiator. The DGD protein expressed in rabbit reticulocyte lysates bound truncated DHBV pre-S protein identical to that of p120 derived from duck liver confirming DGD as p120. Moreover, transfection studies in liver- and kidney-derived cells revealed both cell surface and cytoplasmic expression of the protein. Cloning of the glycine decarboxylase cDNA will permit a direct test of whether it functions as a cell surface co-receptor or as a co-factor in the DHBV replication cycles.

Residues critical for duck hepatitis B virus neutralization are involved in host cell interaction

To date, no detailed analysis of the neutralization properties of duck hepatitis B virus (DHBV) has been reported, and it is not clear whether any of the known neutralization epitopes correspond to the viral receptor binding site or to sequences involved in the cell entry pathway. We demonstrate here that antibodies directed against two overlapping peptides (amino acids 83 to 97 and 93 to 107), covering the sequences of most DHBV pre-S neutralizing epitopes, both inhibit virus binding to primary duck hepatocytes and neutralize virus infectivity. An extensive mutagenesis of the motif 88WTP90, which is the shortest sequence of the epitope recognized by the virus-neutralizing monoclonal antibody (MAb) 900 was performed in order to define the amino acids involved in these interactions. Single point mutations within this epitope affected neither virus replication nor infectivity but abolished virus neutralization by MAb 900 completely. Interestingly, mutants with two and three consecutive residue replacements (SIP and SIH) within this epitope retained replication competence but were no longer infectious. The loss of infectivity of SIH and SIP mutant particles was associated with significantly reduced binding to primary duck hepatocytes and could be rescued by trans complementation with wild-type pre-S protein. Taken together, these results indicate that each amino acid of the DHBV pre-S sequence 88WTP90 is critical for recognition by the neutralizing MAb 900 and that replacement of the first two or all three residues strongly reduces virus interaction with hepatocytes and abrogates infectivity. These data imply that the motif 88WTP90 contains key residues which are critical for interaction with both the neutralizing MAb and the host cell.

Enhanced duck hepatitis B virus gene expression following aflatoxin B1 exposure

Epidemiological studies have suggested synergistic interactions between chronic hepatitis B virus (HBV) infection and aflatoxin B1 (AFB1) exposure in the etiology of hepatocellular carcinoma (HCC), although the molecular mechanisms of their interactions are still not understood. The aim of this study was to use the Pekin duck model to investigate the impact of AFB1 exposure on duck hepatitis B virus (DHBV) replication during the early stages of virus-carcinogen interactions. Six-week-old chronic DHBV-carrier or uninfected ducks were exposed to AFB1 for 5 weeks or treated with dimethylsulfoxide (DMSO) as a control. Animals were observed for 6 to 13 weeks after AFB1 treatment to study the influence of AFB1 exposure on DHBV replication and liver pathologies. Histological analysis showed more marked changes in the livers of AFB1-treated ducks, and these were enhanced by DHBV infection. A significant increase in serum and liver DHBV DNA level was observed in AFB1-treated ducks as compared with DMSO-treated controls. In addition, viral RNAs, in particular the pregenomic RNA that is the template of viral replication, and intrahepatic DHBV DNA replicative intermediates, were significantly increased by AFB1 treatment. Moreover, an overexpression and accumulation of DHBV large envelope (L) protein was observed in the hepatocytes of AFB1-exposed animals. The in vitro study has further confirmed an increase in intracellular viral DNA and in virus release in AFB1-treated primary duck hepatocytes. Taken together, our results indicate that AFB1 exposure leads to an increase in virus gene expression associated with intrahepatic accumulation of DHBV L protein and enhanced liver pathology.

Protective and therapeutic effect of DNA-based immunization against hepadnavirus large envelope protein

BACKGROUND & AIMS

Studies in the murine model suggest that injection of DNA encoding hepatitis B virus structural proteins is promising for the induction of a specific immune response. We used the duck hepatitis B virus (DHBV) model to study the protective and therapeutic effects of naked DNA immunization against hepadnaviral large envelope protein.

METHODS

A pCI-preS/S plasmid expressing the DHBV large protein was used for intramuscular immunization of ducks. The humoral response was tested by enzyme-linked immunosorbent assay, immunoblotting, neutralization, and in vivo protection tests. For DNA therapy, DHBV-carrier ducks received four injections of this plasmid. Viremia was monitored for 10 months; thereafter, liver biopsies were performed.

RESULTS

Immunization with pCI-preS/S plasmid induced a specific, long-lasting, neutralizing, and highly protective anti-preS humoral response in uninfected animals. After pCI-preS/S treatment, a significant and sustained decrease in serum and liver DHBV DNA was observed for carrier ducks compared with the controls.

CONCLUSIONS

DNA immunization against DHBV large protein results in a potent and protective anti-preS response in the duck model. The results of long-term follow-up of DNA-treated chronically infected ducks are promising and show the usefulness of this model for the study of genetic immunization in chronic hepatitis B therapy.

Phosphorylation of DHBV pre-S: identification of the major site of phosphorylation and effects of mutations on the virus life cycle

Four potential serine/threonine phosphorylation sites [(S/T)-P motif], designated P1-P4, on the pre-S protein of duck hepatitis B virus (DHBV) have been mutated. Mutants include single (P2, P3, P4) and double amino acid substitutions (P1 + P2, P3 + P4) and one with all four sites mutated (4P). Serine at position 118 (P3) was identified as the major site of phosphorylation by Western blotting and radioimmunoprecipitation after in vitro cell labeling with [35S]methionine or [33P]orthophosphate. Mutant virions generated by transfection of LMH cells were infectious both in vitro in duck hepatocyte primary cultures and in vivo in Pekin ducks. Intracellular relaxed circular (RC) and covalently closed circular (ccc) DNA syntheses were not affected by the P3 mutation or even the quadruple mutant. Extracellular virus production was slightly increased when the P3 site was mutated. CsCl gradient centrifugation showed no clear difference between mutant and wild-type virus with respect to the ratios of enveloped virus and nucleocapsid particles in hepatocyte culture supernatants. Trypsin or V8 protease digestion with or without NP-40 indicated that phosphorylation of the pre-S domain is not involved in determining the transmembrane topology of DHBV large protein. This phenotypic analysis indicates that DHBV pre-S phosphorylation has no apparent effect on DHBV replication and formation of mature viral particles in duck hepatocyte primary culture and does not affect infectivity in ducklings.

Enhancement of hepatitis B virus infection by noninfectious subviral particles

The biological function of the huge excess of subviral particles over virions in hepatitis B virus infections is unknown. Using the duck hepatitis B virus as a model, we unexpectedly found that subviral particles strongly enhance intracellular viral replication and gene expression. This effect is dependent on the multiplicity of infection, the ratio of virions over subviral particles, and the time point of addition of subviral particles. Most importantly, we show that the pre-S protein of the subviral particles triggers enhancement and requires the presence of the binding regions for putative cell-encoded virus receptor proteins. These data suggest that enhancement is due either to the recently described transactivation function of the pre-S protein or to signalling pathways which become activated upon binding of subviral particles to cellular receptors. The findings are of clinical importance, since they imply that infectivity of sera containing hepadnaviruses depends not only on the amount of infectious virions but also decisively on the number of particles devoid of nucleic acids. A similarly dramatic enhancing effect of noninfectious particles in other virus infections is well conceivable.

Protective efficacy of DNA vaccines against duck hepatitis B virus infection

The efficacy of DNA vaccines encoding the duck hepatitis B virus (DHBV) pre-S/S and S proteins were tested in Pekin ducks. Plasmid pcDNA I/Amp DNA containing the DHBV pre-S/S or S genes was injected intramuscularly three times, at 3-week intervals. All pre-S/S and S-vaccinated ducks developed total anti-DHBs and specific anti-S antibodies with similar titers reaching 1/10,000 to 1/50,000 and 1/2,500 to 1/4,000, respectively, after the third vaccination. However, following virus challenge, significant differences in the rate of virus removal from the bloodstream and the presence of virus replication in the liver were found between the groups. In three of four S-vaccinated ducks, 90% of the inoculum was removed between <5 and 15 min postchallenge (p.c.) and no virus replication was detected in the liver at 4 days p.c. In contrast, in all four pre-S/S-vaccinated ducks, 90% of the inoculum was removed between 60 and 90 min p.c. and DHBsAg was detected in 10 to 40% of hepatocytes. Anti-S serum abolished virus infectivity when preincubated with DHBV before inoculation into 1-day-old ducklings and primary duck hepatocyte cultures, while anti-pre-S/S serum showed very limited capacity to neutralize virus infectivity in these two systems. Thus, although both DNA vaccines induced high titers of anti-DHBs antibodies, anti-S antibodies induced by the S-DNA construct were highly effective in neutralizing virus infectivity while similar levels of anti-S induced by the pre-S/S-DNA construct conferred only very limited protection. This phenomenon requires further clarification, particularly in light of the development of newer HBV vaccines containing pre-S proteins and a possible discrepancy between anti-HBs titers and protective efficacy.

In vivo selection of duck hepatitis B virus pre-S variants which escape from neutralization

To better understand the role of specific residues within the duck hepatitis B virus (DHBV) pre-S protein in neutralization and infectivity, we have selected and identified pre-S variants which escape neutralization. A highly neutralizing monoclonal antibody (Mab 900) which recognizes an epitope 83IPQPQWTP90 localized previously on the DHBV pre-S protein, within a region suspected to mediate the virus interaction with hepatocytes, was used as immune pressure. After only two in vivo neutralization rounds with Mab 900, five different pre-S mutant genomes were identified, which harbored point mutations affecting only proline residues located at position 90 within this epitope (83IPQPQWTP90) and/or at a distance at position 5. We have shown that a single (P5L) or double proline (P5L + P90H) substitution affect neither virus replication capacity nor in vivo infectivity. However, the P5 mutation reduces mutant recognition by Mab 900 twofold, while the substitution of both prolines 5 and 90 almost completely abolishes mutant P5L + P90H reactivity with this Mab and leads to a decrease of neutralization. Therefore we describe here an experimental system which allows rapid in vivo selection and identification of DHBV pre-S variants and provide evidence that residues within and at a distance from the neutralization epitope are important in DHBV neutralization but do not affect its replication capacity and infectivity.

Characterization of a 120-Kilodalton pre-S-binding protein as a candidate duck hepatitis B virus receptor

Infection by human and animal hepadnaviruses displays remarkable host and tissue tropism. The infection cycle probably initiates with binding of the pre-S domain of viral envelope protein to surface receptors present on the hepatocyte. Three types of neutralizing monoclonal antibodies against duck hepatitis B virus (DHBV) have their binding sites clustered within residues 83 to 107 of the pre-S protein, suggesting that this region may constitute a major receptor binding site. A 170- or 180-kDa duck protein (p170 or gp180) which binds DHBV particles through this part of the pre-S sequence has been identified recently. Although the p170 binding protein is host (duck) specific, its distribution is not restricted to DHBV-infectible tissues. Using the pre-S protein fused to glutathione S-transferase and immobilized on Sepharose beads, we have now identified an additional binding protein with a size of 120 kDa (p120). p120 expression is restricted to the liver, kidney, and pancreas, the three major organs of DHBV replication. While optimal p170 binding requires an intact pre-S protein, binding to p120 occurs much more efficiently with a few N- or C-terminally truncated forms. The p120 binding site was mapped to residues 98 to 102 of the pre-S region, which overlaps with a cluster of known virus-neutralizing epitopes. Site-directed mutagenesis revealed residues 100 to 102 (Phe-Arg-Arg) as the critical p120 contact site; nonconservative substitution in any of the three positions abolished p120 binding. Double mutations at positions 100 to 102 markedly reduced DHBV infectivity in cell culture. Short pre-S peptides covering the clustered neutralizing epitopes (also p170 and p120 binding sites) reduced DHBV infectivity in primary duck hepatocyte cultures. Thus, p120 represents a candidate component of the DHBV receptor complex.

Design of synthetic peptides for oral vaccination

The rational design of effective oral vaccines based on synthetic peptides is a very ambitious undertaking, and involves the solution of huge problems related to protection of the antigens against degradation in the alimentary tract, efficient uptake of the antigens by the relevant cells, and efficient induction of long lasting systemic immunity, local immunity, or both. This paper summarises the steps, necessary to develop such synthetic oral vaccines. These steps involve: (1) the definition of B-cell epitopes; (2) the definition of T-cell epitopes; (3) definition of the carrier or backbone molecule; (4) definition of an immunomodulating element; (5) definition of an adjuvant element; and (6) definition of a targeting element. Good progress is being made with respect to the first three steps, the other steps still provide major challenges, notably the definition of targeting elements. Nevertheless, the first synthetic oral vaccines may become reality in the near future, depending on the speed by which new technology in the area of molecular recognition will develop, i.e. the appropriate chemistry, organic chemistry, molecular modelling, resolution of the molecular interaction of key molecules in microbiology and immunology.

Interaction between duck hepatitis B virus and a 170-kilodalton cellular protein is mediated through a neutralizing epitope of the pre-S region and occurs during viral infection

Identification of cell surface viral binding proteins is important for understanding viral attachment and internalization. We have fused the pre-S domain of the duck hepatitis B virus (DHBV) large envelope protein to glutathione S-transferase and demonstrated a 170-kDa binding protein (p170) in [35S]methionine-labeled duck hepatocyte lysates. This glycoprotein was found abundantly in all extrahepatic tissues infectible with DHBV and in some noninfectible tissues, though it is not secreted into the blood. The interaction of pre-S fusion protein with p170 was competitively inhibited by wild-type DHBV in a dose-dependent manner. In addition, infection of hepatocytes with DHBV blocked the binding of pre-S fusion protein to p170, which suggests a biological role for p170 during natural infection. The p170 binding site was mapped to a conserved sequence of 16 amino acid residues (positions 87 to 102) by using 24 pre-S deletion mutants; this binding domain coincides with a major virus-neutralizing antibody epitope. Furthermore, site-directed mutagenesis revealed that an arginine residue at position 97 is critical for p170 binding. p170 was purified by a combination of ion-exchange and affinity chromatographies, and four peptide sequences were obtained. Two peptides showed significant similarities to human and animal carboxypeptides H, M, and N. Taken together, these results raise the possibility that the p170 binding protein is important during the replication cycle of DHBV.

Epitope mapping: synthetic approaches to the understanding of molecular recognition in the immune system

Progress in the field of immunochemistry is rapidly increasing due to very efficient methods of epitope mapping. Experimental results on the allele-specific sequence motifs of MHC-binding peptides allow the exact forecast of T-cell epitopes and, in combination with B-cell prediction methods and synthetic adjuvant systems, fully synthetic vaccines may be constructed. Methods of multiple peptide synthesis are of particular use for such constructs and for the fine mapping of monoclonal antibodies or sera of patients. Peptide libraries, containing hundred thousands of different oligopeptides are made available for novel screening procedures. These techniques and their applications in various fields are summarized and discussed with respect to efficiency and productivity.

Duck hepatitis B virus (DHBV) as a model for understanding hepadnavirus neutralization

The role of the immune response to the human hepatitis B virus (HBV) envelope proteins in neutralization of viral infectivity has been well documented. The similarity between HBV, prototype member of the hepadnavirus family, and the closely related duck hepatitis B virus (DHBV) has allowed, use of the latter as a convenient model for the study of molecular mechanisms of HBV replication and neutralization. In this brief review, we will examine the HBV and DHBV envelope proteins and their role as targets for virus neutralization.