Epitope-specific antibody response to the surface antigen of duck hepatitis B virus in infected ducks

Spread of hepatitis B viruses in vitro requires extracellular progeny and may be codetermined by polarized egress

Viruses can spread by different mechanisms: via intracellular particles through cell junctions to neighboring cells or via secreted virions to adjacent or remote cells. The observation of clusters of hepadnavirus-infected cells both in vivo and in primary hepatocytes neither proves the first mechanism nor excludes the second. In order to test which mechanism, if not both, is used by hepatitis B viruses in order to spread, we used primary duck hepatocytes and duck hepatitis B virus (DHBV) as an infection model. If extracellular progeny virus alone determines spreading, neutralizing antisera or drugs blocking virus binding to hepatocytes should abolish secondary infection. In order to test this, we used DHBV envelope-specific neutralizing antisera, as well as suramin, a known inhibitor of infection. Both reagents strongly reduced hepatocellular attachment of viral particles and almost completely abolished primary infection, whereas an ongoing intracellular infection was not affected as long as no progeny virus was released. In contrast, incubation of infected primary hepatocytes with these reagents during release of progeny virus completely prevented secondary infection. Moreover, the combination of electron and immunofluorescence microscopy analyses revealed the residence of viral particles in cytoplasmic vesicles preferentially located near the basolateral membrane of infected hepatocytes. Taken together, these data strongly suggest that hepatitis B viruses mainly spread by secreted, extracellular progeny and point to polarized egress of viral particles into intercellular compartments, which restricts their diffusion and favors transmission of virus to adjacent cells.

Development of a sensitive PCR inhibition method to demonstrate HBV nucleic acid inactivation

BACKGROUND

The evaluation of pathogen reduction technologies with relevant viruses currently contaminating the blood supply is limited by the availability of high-titer virus inocula and sensitive in vitro or in vivo infectivity assays. Because HBV infectivity can only be assessed by in vivo studies with chimpanzees, a sensitive PCR inhibition assay was developed to measure PEN110 inactivation of HBV.

STUDY DESIGN AND METHODS

PCR amplification of 1.1 kb of HBV genome was optimized to determine DNA damage introduced by treatment with PEN110 in RBCs. Inactivation of duck HBV (DHBV) in RBCs, with measurement of the in vitro infectivity, was performed to validate the PCR assay.

RESULTS

The PCR was highly specific and sensitive for amplification of the HBV genome and used to demonstrate a reduction of at least 7.2 and 8.1 log geq per mL within the first 18 hours of PEN110 treatment. PEN110 inactivation of DHBV was also achieved within the first 18 hours with a reduction factor of at least 5.0 log tissue culture infectious dose 50 percent per mL, suggesting that PCR inhibition is an alternative to infectivity assays.

CONCLUSION

This study establishes PCR inhibition as a reasonable approach to assess the efficiency of PEN110 inactivation of human pathogens with human plasma donations that have been found to contain high titers of relevant agents during different stages of infection.

Comparison of the kinetics of the specific cellular immune response to duck hepatitis B virus in infected and immune ducks

The kinetics of the cell mediated immune response by ducks acutely and chronically infected with, or immune to infection by duck hepatitis B virus (DHBV) was determined. This was measured by an antigen specific blastogenesis assay to duck hepatitis B surface antigen (DHBsAg) and duck hepatitis B core antigen (DHBcAg) using peripheral blood mononuclear cells (PBMC). The three outcomes of acute infection by DHBV were either clearance from both serum and liver, clearance from serum but not liver, or the development of persistent viraemia. Acutely infected ducks that failed to clear the infection also failed to develop a significant cellular immune response to both antigens. Ducks with chronic infection acquired as neonates or as the result of the failure to clear acute infection had an increasing cellular immune response over time. Two groups of immune ducks were examined. These were either ducks that had become immune following infection or that had been vaccinated. Both groups of ducks demonstrated significant cellular responses following challenge with DHBV irrespective of the level of their responses before challenge. However, there was a reduction in the response of their PBMC over a 4-week-period postchallenge. The range of cellular immune responses to DHBV antigens observed in this study has a number of counterparts in hepatitis B infection of humans. Coupled with the defined clinical outcomes that can be established in the duck/DHBV model, further study of the cellular immune response to DHBV is warranted.

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.

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.

DHBV manipulation and prediction of the outcome of infection

BACKGROUND/AIMS

The immunological response of ducks to acute infection with duck hepatitis B virus (DHBV) has not been fully characterised. In this study the relationship between viral dose and the outcome of infection in immune competent 26-day-old ducks was examined.

METHODS

Indirect ELISA assays were developed to detect the presence of antibody to DHB surface antigen and DHB core antigen. A DHBV serum pool was titrated in 1-day-old and 26-day-old ducklings.

RESULTS

The ID50 dose of the ducks injected at 26 days of age was found to be 1000 times that of the ducks injected on day of hatch. The antibody responses and serum DHBV DNA were followed in eight ducks inoculated with DHBV positive serum when 26 days of gene and in three ducks infected with DHBV on day of hatch. The three ducks infected on day of hatch were viraemic by day 7 and remained highly viraemic throughout the experimental period. In the older ducks, inoculation with 1000ID50 resulted in the development of chronic carriage, while inoculation with either 100 or 10ID50 doses resulted in acute infection with or without viraemia. These ducks were able to clear the infection from their circulation, but only 50% cleared DHBV from the liver within the experimental period. All infected ducks developed anti-core activity. Only non-viraemic ducks developed anti-surface activity.

CONCLUSION

DHBV infection can be established in immune competent adolescent ducks, with variable disease outcomes comparable to HBV infection in humans.

The pre-S domain of the large viral envelope protein determines host range in avian hepatitis B viruses

In addition to their well-recognized hepatotropism, all hepatitis B viruses (HBVs) display marked species specificity, growing poorly or not at all in species other than those closely related to their natural hosts. We have examined the molecular basis for this narrow host range, using duck HBV (DHBV) and heron HBV (HHBV) as a model system. HHBV virions will not infect ducks in vivo and infect cultured duck hepatocytes extremely inefficiently in vitro. Mutant HHBV genomes lacking all viral envelope proteins (HHBV env-) can be complemented in trans with DHBV envelope proteins; the resulting pseudotyped virions can efficiently infect duck hepatocytes. Further complementation analysis reveals that of the two viral surface proteins (L and S), it is the L protein that determines host range. Pseudotyping of HHBV env- with DHBV/HHBV chimeric envelope proteins reveals that replacement of as few as 69 amino acids of the pre-S domain of the HHBV L protein by their DHBV counterparts is sufficient to permit infection of duck hepatocytes. These studies indicate that the species-specificity of hepadnaviral infection is determined at the level of virus entry and is governed by the pre-S domain of the viral L protein.

A cell surface protein that binds avian hepatitis B virus particles

We have identified a 180-kDa cellular glycoprotein (gp180) that binds with high affinity to duck hepatitis B virus (DHBV) particles. The protein was detected by coprecipitating labeled duck hepatocyte proteins with virions or recombinant DHBV envelope proteins, using nonneutralizing monoclonal antibodies to the virion envelope. Binding of gp180 requires only the pre-S region of the viral large envelope protein, since recombinant fusion proteins bearing only this region efficiently coprecipitate gp180. The DHBV-gp180 interaction is blocked by two independent neutralizing monoclonal antibodies. The protein is found on both internal and surface membranes of the cell, and the species distribution of gp180 binding activity mirrors the known host range of DHBV infection. Functional gp180 is expressed in a wide variety of tissues in susceptible ducks.

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.

Naturally occurring point mutation in the C terminus of the polymerase gene prevents duck hepatitis B virus RNA packaging

A duck hepatitis B virus (DHBV) genome cloned from a domestic duck from the People's Republic of China has been sequenced and exhibits no variation in sequences known to be important in viral replication or generation of gene products. Intrahepatic transfection of a dimer of this viral genome into ducklings did not result in viremia or any sign of virus infection, indicating that the genome was defective. Functional analysis of this mutant genome, performed by transfecting the DNA into a chicken hepatoma cell line capable of replicating wild-type virus, indicated that viral RNA is not encapsidated. However, virus core protein is made and can assemble into particles in the absence of encapsidation of viral nucleic acid. Using genetic approaches, it was determined that a change of cysteine to tyrosine in position 711 in the polymerase (P) gene C terminus led to this RNA-packaging defect. By site-directed mutagenesis, it was found that while substitution of Cys-711 with tryptophan also abolished packaging, substitution with methionine did not affect packaging or viral replication. Therefore, Cys-711, which is conserved in all published sequences of DHBV, may not be involved in a disulfide bridge structure essential to viral RNA packaging or replication. Our results, showing that a missense mutation in the region of the DHBV polymerase protein thought to be primarily the RNase H domain results in packaging deficiency, support the previous findings that multiple regions of the complex hepadnaviral polymerase protein may be required for viral RNA packaging.

Peptide mapping of neutralizing and nonneutralizing epitopes of duck hepatitis B virus pre-S polypeptide

Antibodies to the envelope proteins of duck hepatitis B virus neutralize viral infection in vitro. Using a library of murine monoclonal antibodies (Mabs) against the envelope proteins, we previously identified four neutralizing and two non-neutralizing epitopes on the pre-S region of the large envelope proteins. In this study we report the localization of all but one of these epitopes at the amino acid level. All but 28 nucleotides of the pre-S and S genes were cloned in pUC vectors and expressed in Escherichia coli. All Mabs in this study reacted with the expressed gene products in Western blots. Deletion mutants of the pre-S region were generated and their expressed products tested on Western blots for reactivity with the Mabs. Of the three epitopes involved in neutralization, the epitope found to be immunodominant in convalescent ducks was localized to nine amino acids of the middle portion of the pre-S gene product, while a second epitope was mapped to nine amino acids upstream of the immunodominant epitope and the third epitope to seven amino acids adjacent to the S gene. One of the two non-neutralizing epitopes was located between the two groups of neutralizing epitopes while the other mapped to the same region as one of the neutralizing epitopes. Our data indicate that several regions of the pre-S polypeptide may play a role in neutralization of hepadnaviruses.