Characterization of recombinant polioviruses expressing regions of rotavirus VP4, hepatitis B surface antigen, and herpes simplex virus type 2 glycoprotein D

Recombinant polioviruses expressing antigens from rotavirus, herpes simplex virus type 2, and hepatitis B virus were generated. Fusion of the heterologous polypeptides to the amino terminus of the poliovirus polyprotein did not prevent myristylation of VP0, suggesting a novel mechanism of myristylation for these recombinant viruses. The effects of the parental genetic background, different foreign sequences, and different insert sizes on growth characteristics were compared. Both the size and the nature of the heterologous sequence appeared to be factors influencing the growth and stability of recombinant polioviruses. All of the recombinants showed a temperature-sensitive phenotype, regardless of the genetic background (attenuated or wild type) from which they were derived. Preliminary studies with transgenic mice carrying the poliovirus receptor gene are discussed.

Active compounds from Saussurea lappa Clarks that suppress hepatitis B virus surface antigen gene expression in human hepatoma cells

We have examined the antiviral activity of the crude extract prepared from the root of Saussurea lappa Clarks, a Chinese medicinal herb which is widely used for many illnesses including cancer. Two active components, costunolide and dehydrocostus lactone, were identified which show strong suppressive effect on the expression of the hepatitis B surface antigen (HBsAg) in human hepatoma Hep3B cells, but have little effect on the viability of the cells. Both costunolide and dehydrocostus lactone suppress the HBsAg production by Hep3B cells in a dose-dependent manner with IC50s of 1.0 and 2.0 microM, respectively. Northern blotting analysis shows that the suppression of HBsAg gene expression by both costunolide and dehydrocostus lactone were mainly at the mRNA level. Furthermore, the suppressive effect of costunolide and dehydrocostus lactone on HBsAg and hepatitis B e antigen (HBeAg), a marker for hepatitis B viral genome replication in human liver cells, was also observed in another human hepatoma cell line HepA2 which was derived from HepG2 cells by transfecting a tandemly repeat hepatitis B virus (HBV) DNA. Similarly, the mRNA of HBsAg in HepA2 cells was also suppressed by these two compounds. Our findings suggest that costunolide and dehydrocostus lactone may have potential to develop as specific anti-HBV drugs in the future.

Tumor necrosis factor receptors in patients with chronic hepatitis B virus infection

BACKGROUND/AIMS

Patients with chronic hepatitis B infection have elevated plasma tumor necrosis factor (TNF) alpha levels. Two TNF-alpha receptors have been identified, each responsible for distinct TNF-alpha activities. The aim of this study was to evaluate the biological function of the elevated TNF-alpha in chronic hepatitis B virus infection by examining the two TNF signaling pathways in the evolution of hepatitis B-related liver injury.

METHODS

The hepatic expression of the two TNF receptors and the corresponding serum levels of the soluble forms of both TNF receptors were determined and correlated with hepatic inflammation and virus replication in 98 chronic hepatitis B surface antigen carriers. Forty hepatitis B e antigen-positive patients were also studied prospectively, while on interferon alfa treatment, to examine the TNF receptor response during viral clearance.

RESULTS

In chronic hepatitis B virus infection, the hepatic expression and serum levels of TNF receptors, in particular 75-kilodalton TNF receptor subtype (TNF-R p75), are significantly enhanced in association with hepatic inflammation and hepatocytolysis but not with hepatitis B virus replication. During interferon alfa treatment, a significant increase of soluble TNF-R p75 always precedes the hepatitis B e antigen antibody against hepatitis B e antigen seroconversion in responders to treatment.

CONCLUSIONS

In chronic active hepatitis B infection, there is an up-regulation of the TNF receptor system, preferentially the TNF-R p75 signaling pathway, which suggests that the TNF-alpha/TNF receptor system has an important role in the pathogenesis of liver damage and viral clearance.

Hepatitis B virus: specific binding and internalization of small HBsAg by human hepatocytes

Previously, we identified human liver endonexin II (EII) present on human hepatocyte plasma membrane as a specific hepatitis B surface antigen (HBsAg) binding protein. We also showed the spontaneous development of anti-idiotypic (anti-HBsAg) antibodies in rabbits immunized with EII and in chicken immunized with the F(ab')2 fragment of rabbit anti-EII IgG. These findings suggest the existence of a receptor-ligand relationship between EII and HBsAg. In the present study, we demonstrate that small HBsAg conjugated to 10 nm colloidal gold also binds specifically to human hepatocytes. Invagination of the coated pit region at the HBsAg binding sites on the human hepatocyte plasma membrane results in the internalization of the HBsAg-gold particles. The binding and consequently the internalization of HBsAg is inhibited by anti-EII or anti-idiotypic (anti-HBsAg) antibodies. These findings indicate that EII is directly involved in the binding and uptake of hepatitis B envelope proteins.

Characterization of the hepatitis B virus large surface antigen promoter Sp1 binding site

A binding site for the transcription factor Sp1 was identified in the hepatitis B virus (HBV) large surface antigen promoter between nucleotides -49 and -29 by DNase I footprinting analysis with purified recombinant Sp1 protein. Gel retardation analysis using Huh7 nuclear extracts demonstrated that formation of complexes between this sequence element and DNA-binding proteins was specifically inhibited by the HBV major surface antigen and nucleocapsid promoter Sp1 binding sites, as well as by the Sp1 consensus recognition sequence. In addition, gel supershift analysis showed that this sequence element bound factor(s) present in Huh7, HeLa S3, and HepG2.1 cell nuclear extracts which were completely supershifted by the Sp1 antibody and appeared to be the same or similar to the factor(s) which bound the consensus Sp1 recognition sequence. The function of the large surface antigen promoter Sp1 recognition sequence was examined by transient transfection analysis in Drosophila melanogaster Schneider line-2 (SL2) cells. In the context of a minimal promoter element, this Sp1 site was able to mediate transcriptional transactivation by exogenously expressed Sp1. These results suggest that the HBV large surface antigen promoter contains a functional Sp1 binding site which may be involved in the coordinate regulation of HBV transcription by the ubiquitous transcription factor Sp1.

Selection of peptide inhibitors of interactions involved in complex protein assemblies: association of the core and surface antigens of hepatitis B virus

As an example for studies of contacts involved in complex biological systems, peptide ligands that bind to the core antigen of hepatitis B virus (HBcAg) have been selected from a random hexapeptide library displayed on filamentous phage. Affinity-purified phage bearing aa sequence LLGRMK, or some related sequences, bound full-length or truncated HBcAg but did not bind denatured HBcAg. The long (L), but not the short (S), hepatitis B virus envelope polypeptide, when synthesized in an in vitro system, bound firmly to HBcAg, indicating that interaction between HBcAg and the pre-S region of the L polypeptide is critical for virus morphogenesis. This interaction was inhibited by peptide ALLGRMKG, suggesting that this and related small molecules may inhibit viral assembly.

Phenotypic mixing of rodent but not avian hepadnavirus surface proteins into human hepatitis B virus particles

The virus family Hepadnaviridae comprises two genera: orthohepadnaviruses isolated from humans (hepatitis B virus [HBV]) and rodents (e.g., woodchuck hepatitis virus [WHV]) and avihepadnaviruses isolated from birds (e.g., duck hepatitis B virus [DHBV]). They carry in their envelopes two (DHBV) or three (HBV and WHV) coterminal proteins referred to as small (S), middle (M), or large (L) surface protein. These proteins are also secreted from infected cells as subviral particles consisting of surface protein and lipid (e.g., 20-nm hepatitis B surface antigen for HBV). To investigate the assembly of these proteins, we asked whether surface proteins from different hepadnaviruses are able to mix phenotypically with each other. By coexpression and coimmunoprecipitation with species-specific antibodies, we could show the formation of mixed subviral particles and disulfide-linked heterodimers between the WHV S and HBV M proteins whereas the DHBV and HBV surface proteins did not coassemble. Complementation of HBV genomes defective in expressing the S or L protein and therefore incompetent to form virions was possible with the closely related WHV S protein or a WHV pre-S-HBV S chimera, respectively, but not with the less related DHBV S or L protein or with a DHBV L-HBV S chimera. The results suggest that the assembly of HBV subviral particles and virion envelopes requires relatively precise molecular interactions of their surface proteins, which are not conserved between the two hepadnavirus genera. This contrasts with the ability of, e.g., rhabdoviruses or retroviruses, to incorporate envelope proteins even from unrelated viruses.

Phospholipid interactions of the putative fusion peptide of hepatitis B virus surface antigen S protein

One of the first steps in the infective cycle of an enveloped virus consists of the fusion of the viral and cellular membranes. This process is usually achieved as a result of membrane destabilization brought about by a viral fusion peptide located at the amino terminus of one of the viral envelope glycoproteins. Previous sequence similarity studies by Rodríguez-Crespo et al. (Journal of General Virology 75, 637-639, 1994) have shown that a hydrophobic stretch in the amino-terminal sequence of the S protein of hepatitis B virus shares several characteristics with fusion peptides of retroviruses and paramyxoviruses. A 16 residue peptide with this sequence was synthesized and its interaction with liposomes characterized. This peptide was able to mediate vesicle aggregation, lipid mixing and liposome leakage in a pH dependent manner at concentrations ranging from 3.5 to 52.0 microM. These effects were specific for negatively charged phospholipid vesicles. The peptide was also able to haemolyse erythrocytes. This study supports the notion that the sequence might be important in the initial infective steps of this virus, interacting with the target membranes and bringing about their subsequent destabilization.

Fibronectin of human liver sinusoids binds hepatitis B virus: identification by an anti-idiotypic antibody bearing the internal image of the pre-S2 domain

Anti-idiotypic antibodies (anti-Ids) have been successfully used to characterize and isolate receptors of several cell ligands. To prepare an immunological probe for identification of cellular components interacting with the hepatitis B virus (HBV), polyclonal antisera against a panel of five HBV-specific monoclonal antibodies (MAbs) were produced in syngeneic BALB/c mice. MAbs to HBV used for immunization (Ab1) recognized biologically important and potentially neutralizing epitopes, located in the pre-S1, pre-S2, or S region-encoded domains of HBV proteins. All the anti-Ids (Ab2) were specific to idiotopes of the homologous Ab1 and inhibited their interaction with the corresponding viral epitopes, suggesting that they recognized unique determinants on the paratope of each immunizing Ab1. Therefore, all five generated polyclonal anti-Ids were of the Ab2 beta type and could represent internal images of viral epitopes. Ab2 raised against the pre-S2 region-specific MAb F124 bound to the extracellular matrix fibronectin of human liver sinusoids. Immunohistochemical studies demonstrated the attachment of viral and recombinant (S, M) hepatitis B surface antigen particles with the pre-S2 region-encoded epitopes to the fibronectin of human liver sinusoids. In contrast, recombinant (S, L*) hepatitis B surface antigen particles, in which the epitope recognized by F124 MAb was not expressed, did not show any binding capacity. These findings suggest that human liver fibronectin may bind HBV in vivo by the pre-S2 region-encoded epitopes in a species-restricted manner. Furthermore, binding of the circulating virus to liver sinusoids could facilitate its subsequent uptake by hepatocytes.

Cloning and production of functional active recombinant hepatitis B virus surface antigen binding protein

Endonexin II present on the surface of human hepatocytes has recently been identified as a hepatitis B virus surface antigen (HBsAg) binding protein. A full-length cDNA clone encoding human endonexin II was isolated from a human liver cDNA library and was placed under the control of the polyhedrin promoter of Autographa californica nuclear polyhedrosis virus (AcNPV). Infection of Spodoptera frugiperda cells with recombinant virus resulted in the production of high amounts of recombinant protein. This protein has the same molecular weight and iso-electric point as native human endonexin II. It can be easily purified by methods analogous to those described for the native protein. Moreover, the recombinant product binds very efficiently to hepatitis B surface proteins (HBsAg) in a similar fashion as native human endonexin II.

The topology of the S protein in the yeast-derived hepatitis B surface antigen particles

Hepatitis B surface antigen particles are highly immunogenic and have been shown to provide a suitable support for the presentation of foreign epitopes. More information about the topology of their constitutive protein, the S (small envelope) protein, is a prerequisite to any rational attempt to replace region of this protein with foreign epitopes without modifying the assembly of the particle. The topology of the S protein within the lipid membrane was investigated here by combining extensive proteolysis of the external protein domains with proteinase K and (FTIR-ATR). The proteolytic hydrolysis of the S protein and the identification of the digestion products allowed characterization of the membrane-protected regions of the protein. FTIR spectra of the digested hepatitis B particles revealed that the peptides associated with the particles are rich in alpha-helix structure. The kinetic of 2H/H exchange provided evidence that a large fraction of the native S protein is poorly accessible to the aqueous medium.

[Comparative study of the expression of IGF-2 and HBxAg in human hepatocellular carcinoma and liver cirrhosis]

60 Cases of hepatocellular carcinoma (HCC) and 47 cases of liver cirrhosis (LC) were examined with immunocytochemistry methods using antibodies against IGF-2 and HBxAg on paraffin embedded sections. 32 HCC and 37 LC were found to be positive to HBxAg, in which the positive rates of IGF-2 were 100% (32/32) and 94.6% (35/37) respectively. 28 HCC and 10 LC were found to be HBxAg negative, IGF-2 was positive in 23 HCC (82.1%) and 6 LC (60%). The positive expression rates of IGF-2 in the HBxAg positive tissues were significantly higher than those in the HBxAg negative tissues (P < 0.05). There were three types of IGF-2 distribution in HCC and LC: (1) perinucleus; (2) diffuse in cytoplasm; (3) in nucleus. Small polygonal liver cells (SPLC) were found in the liver tissue surrounding the tumor or cirrhosis, the SPLC were positive to both IGF-2 and HBxAg. The positive rates of IGF-2 in SPLC were 86.4% (38/44) in HBxAg positive group and 40.5% (15/37) in the HBxAg negative group. The above findings suggest that IGF-2 plays an important role in abnormal proliferation of HCC and SPLC. The relation of IGF-2 and HBxAg and the nature of SPLC were also discussed.

The putative cell receptors for hepatitis B virus (HBV), annexin V, and apolipoprotein H, bind to lipid components of HBV

Annexin V and apolipoprotein H, reported to bind hepatitis B surface antigen (HBsAg) and presumed to react specifically with the HBsAg S protein and to play an important role in initiation of infection by hepatitis B virus, did not bind to delipidated HBsAg (dlHBsAg). Binding activity was restored by adding lipids to dlHBsAg. These results are consistent with the established affinity of annexin V and apolipoprotein H for lipids.

C-erb-B-2 oncoprotein and epidermal growth factor receptor in human hepatocellular carcinoma: an immunohistochemical study

The aim of this study was to evaluate the immunohistochemical expression of epidermal growth factor (EGFR) and c-erb-B-2 oncoprotein in a series of 71 hepatocellular carcinomas as well as in the adjacent hepatic tissue and to assess any correlation with HBsAg expression. The total of the 71 hepatocellular carcinomas (HCCs) was classified into 17 low grade and 54 high grade cases with adjacent non-neoplastic liver parenchyma, observed in 14 and 28 cases respectively. Coexisting cirrhosis or fibrosis was noticed in the adjacent non-neoplastic parenchyma in 12 cases of low grade and 22 cases of high grade HCC. The immunohistochemical avidin-biotin-peroxidase complex (ABC) method was performed on formalin-fixed paraffin sections for the detection of EGFR, c-erb-B-2 oncoprotein and HBsAg using monoclonal antibodies. The expression of c-erb-B-2 was observed in 29.5% (21/71) of the HCCs showing no statistically significant correlation with histological grade. The c-erb-B-2 was also detected in the adjacent non-neoplastic parenchyma in 7/14 low grade HCCs, and in 9/28 high grade HCCs. No statistically significant differences in c-erb-B-2 oncoprotein expression were observed between the HCCs and the adjacent non-neoplastic parenchyma. In addition, HBsAg was detected in 10/42 examined cases of HCC with adjacent non-neoplastic parenchyma, while only 4 cases of HCCs were simultaneously positive for c-erb-B-2 and HBsAg. EGFR was detected in only 3/71 cases of HCC, while the antigen was not detected at all in the adjacent non neoplastic parenchyma. HBsAg expression was not observed in any of the EGFR-positive HCCs. Our results suggest that both c-erb-B-2-oncoprotein and EGFR do not seem to be predominantly involved in the transformation of hepatocytes to the malignant phenotype.

Hepatitis delta virus attaches to human hepatocytes via human liver endonexin II, a specific HBsAg binding protein

Recently we identified human liver endonexin II (EII) as a specific hepatitis B surface antigen (HBsAg) binding protein. To investigate whether EII is also able to interact with the HBsAg envelope of the hepatitis delta virus (H delta V), immunoprecipitation experiments were performed. H delta V particles could be co-precipitated by polyclonal rabbit anti-EII, but not by rabbit anti-glutathiontransferase (GST pi) antibodies from an H delta V-enriched fraction containing EII or GST pi. These findings suggest that H delta V particles were co-precipitated by anti-EII as a consequence of the binding between HBsAg present in the H delta V envelope and EII. Furthermore, binding of H delta V particles to human hepatocytes could be inhibited by incubation of the liver cells with rabbit anti-EII IgG or the H delta V particles with anti-idiotypic (anti-HBsAg) antibodies, developed spontaneously in rabbits immunized with EII. These findings support the assumption that small HBsAg present in the H delta V envelope is important for the attachment to the hepatocytes and that EII plays an important role in this process.

A carboxy-terminal portion of the preS1 domain of hepatitis B virus (HBV) occasioned retention in endoplasmic reticulum of HBV envelope proteins expressed by recombinant vaccinia viruses

The large envelope glycoprotein (L protein) of Hepatitis B virus (HBV) contains the preS1 domain, which is responsible for retention of the protein in the endoplasmic reticulum. To identify sequences of the preS1 domain involved in this phenomenon we constructed vaccinia virus-HBV recombinants containing the gene for L protein in which the preS1 coding sequence had been partially deleted. The retention of L protein in the endoplasmic reticulum was found to be mediated by a sequence contained within a region of 35 amino acids of the preS1 C-terminus, and not exclusively by amino acid sequences of the N-terminus of the preS1 domain as proposed by Kuroki et al. (Mol. Cell. Biol. 9, 4459-4466, 1989). Our finding could be explained by a specifically VV promoter sequence leading to exclusive synthesis of L or deleted (delta)L proteins, respectively. The ability of the coexpressed HBV S protein to facilitate export of the delta L proteins was demonstrated by coinfection experiments.

Expression of hepatitis B surface and core antigens and transforming growth factor-alpha in "oval cells" of the liver in patients with hepatocellular carcinoma

Recent studies have identified epithelial cell populations in human livers that are similar to the "oval cells" and "transitional cells" seen in rat livers during the early stages of chemical carcinogenesis. It has been suggested that these cells might be precursors of hepatocytes and theoretically could be involved in hepatocarcinogenesis. The hepatitis B virus (HBV) also is believed to play a role in the etiology of hepatocellular carcinoma (HCC). Therefore, a study was conducted in nontumorous livers adjacent to HCCs obtained from 26 patients from China to determine whether HBV antigens could be identified in oval cells and transitional cells using an immunohistochemical technique. Hepatitis B surface antigen (HBsAg) was detected in the nontumorous livers of 22/26 (85%) patients. HBsAg was detected in oval cells in 18/26 (69%), in transitional cells in 21/26 (81%), and in mature hepatocytes in 22/26 (85%), but not in bile duct or ductule cells. Transforming growth factor-alpha (TGF-alpha) was expressed in oval cells, transitional cells, and bile duct cells in 24/26 (92%) patients, an in mature hepatocytes in 25/26 (96%). Coexpression of HBsAg and TGF-alpha was identified in the same cells in populations of oval cells and transitional cells of selected patients. Because of the possibility that oval cells could be a source of evolving HCC, these findings suggest that expression of TGF-alpha associated with HBV infection of oval cells could be a mechanism of human hepatocarcinogenesis. Thus, oval cells could be a site (or one of the sites) where HBV participates in the development of HCC.

Adsorption and penetration of hepatitis B virus in a nonpermissive cell line

A cell monolayer radioimmunoassay was established to detect hepatitis B virus (HBV)-binding activity. HBV was shown to bind to HepG2 and HuH7 cells, but not to LTK- and HeLa cells. The binding activity was inhibited by peptide 21-47 from the large hepatitis B surface antigen (L-HBsAg) region, but not by other peptides from the L- and middle-HBsAg. A monoclonal antibody to L-HBsAg (MA18/7) and a polyclonal antibody to HBsAg (anti-HBs) also inhibited the binding. However, only the Fab fragment of MA18/7 showed blocking activity, suggesting that these two antibodies may neutralize virus by different mechanisms. Excess HBV was able to saturate the binding activity of HepG2 cells. The virus was also shown to be internalized; virus DNA was detected in the cytoplasmic fraction 1-2 hr postadsorption and was associated with the nuclear fraction 2 hr later. Furthermore, immunoprecipitation experiments showed that the virus DNA remained encapsidated 48 hr after internalization. Nuclear fractionation experiments showed that the encapsidated HBV DNA remained associated with the nuclear membrane. Trypsinization of intact nuclei resulted in disassociation of the nucleocapsid from the nuclear membranes, suggesting that the nuclear membrane presents a barrier to intact HBV virions or nucleocapsids. This may explain why HepG2 cells are refractile to infection although permissive for HBV replication after transfection of viral DNA.

Soluble transferrin mediates targeting of hepatitis B envelope antigen to transferrin receptor and its presentation by activated T cells

In a previous study, we identified that transferrin receptor (TfR) is the receptor utilized by hepatitis B virus (HBV) to enter T cells. We demonstrated that hepatitis B envelope antigen (HBenvAg) is taken up by activated T cells via TfR, processed in endosomal compartments, and presented on class II molecules to specific CD4+ T cell clones. Herein, we report that binding to soluble ferric Tf by HBenvAg is needed in TfR-mediated endocytosis. Accordingly, presentation of HBenvAg by activated T cells is not observed in serum-free medium and is restored by addition of soluble Tf. Moreover, we provide evidence that pre-S2 and S regions of HBenvAg contain the critical residues for the interaction with soluble Tf. Our data not only explain HBV entry into a variety of host activated cells, but may also help in developing strategies to alter the course of chronic HBV infection.

Grafting of a hepatitis B S-preS(2) T-cell epitope on lysozyme enhances the immunogenicity of lysozyme in responder mice primed with the T-cell epitope

Subunit immunogens composed of well-defined T- and B-cell epitopes might represent a valuable approach to design vaccines. The reduction of the size of the T-cell epitope is clearly in the line of this strategy. In this study we evaluated the capacity of a hepatitis B S-preS(2) surface antigen-derived T-cell epitope (i.e., S2b) to enhance the humoral immune response towards lysozyme when covalently linked to this antigen. We hereby anticipated that new problems, related to processing of a subunit immunogen, may emerge when grafting minimalized T-cell epitopes on protein antigens. Indeed, insertion of a T-cell epitope containing peptide (i.e., S2b) in a new protein context does not warrant a correct processing of the T-cell epitope. To avoid such potential processing problems an acid labile linker between T-cell and B-cell epitopes was devised in order to provide a processing-independent cleavage site. Using a T-cell hybridoma specific for the S2b T-cell epitope the S2bC-lysozyme conjugate was found to be presented by functional antigen-presenting cells. However, fixed APC did not present the conjugate in vitro indicating that processing is required for the release and presentation of S2b. The ability of the conjugate to generate an enhanced immune response was investigated in vivo. In S2b-primed mice the S2bC-lysozyme conjugate was found to elicit a faster and higher anti-lysozyme humoral response, as compared to uncoupled mixtures of lysozyme and S2b.(ABSTRACT TRUNCATED AT 250 WORDS)

Post-translational alterations in transmembrane topology of the hepatitis B virus large envelope protein

The preS domain at the N-terminus of the large envelope protein (LHBs) of the hepatitis B virus is involved in (i) envelopment of viral nucleocapsids and (ii) binding to the host cell. While the first function suggests a cytosolic location of the preS domain during virion assembly, the function as an attachment site requires its translocation across the lipid bilayer and final exposure on the virion surface. We compared the transmembrane topology of newly synthesized LHBs in the endoplasmic reticulum (ER) membrane with its topology in the envelope of secreted virions. Protease sensitivity and the absence of glycosylation suggest that the entire preS domain of newly synthesized LHBs remains at the cytosolic side of ER vesicles. However, virions secreted from transfected cell cultures or isolated from the blood of persistent virus carriers expose antibody binding sites and proteolytic cleavage sites of the preS domain at their surface in approximately half of the LHBs molecules. Thus, preS domains appear to be transported across the viral lipid barrier by a novel post-translational translocation mechanism to fulfil a dual function in virion assembly and attachment to the host cell.

Expression of hepatitis delta virus RNA deletions: cis and trans requirements for self-cleavage, ligation, and RNA packaging

The hepatitis delta virus (HDV) genome is a circular, single-stranded, rod-shaped, 1.7-kb RNA that replicates via a rolling-circle mechanism. Viral ribozymes function to cleave replication intermediates which are then ligated to generate the circular product. HDV expresses two forms of a single protein, the small and large delta antigens (delta Ag-S and delta Ag-L), which associate with viral RNA in a ribonucleoprotein (RNP) structure. While delta Ag-S is required for RNA replication, delta Ag-L inhibits this process but promotes the assembly of the RNP into mature virions. In this study, we have expressed full-length and deleted HDV RNA inside cells to determine the minimal RNA sequences required for self-cleavage, ligation, RNP packaging, and virion assembly and to assess the role of either delta antigen in each of these processes. We report the following findings. (i) The cleavage and ligation reactions did not require either delta antigen and were not inhibited in their presence. (ii) delta Ag-L, in the absence of delta Ag-S, formed an RNP with HDV RNA which could be assembled into secreted virus-like particles. (iii) Full-length HDV RNAs were stabilized in the presence of either delta antigen and accumulated to much higher levels than in their absence. (iv) As few as 348 nucleotides of HDV RNA were competent for circle formation, RNP assembly, and incorporation into virus-like particles. (v) An HDV RNA incapable of folding into the rod-like structure was not packaged by delta Ag-L.

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.

Hepatitis B virus surface antigen binds to apolipoprotein H

We have previously demonstrated that a plasma membrane-enriched fraction isolated from human liver is capable of binding recombinant hepatitis B surface antigen (rHBsAg) (P. Pontisso, M. A. Petit, M. Bankowski, and M. E. Peeples, J. Virol. 63:1981-1988, 1989). In this study we have separated the plasma membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used a ligand-blotting technique to identify a 46-kDa rHBsAg-binding protein. This protein could be removed from the membranes with a weakly acidic buffer, implying that it is peripherally bound. Examination of human serum revealed that the 46-kDa binding protein is a serum protein. Isolation of plasma lipoproteins revealed that the binding protein is in part associated with chylomicrons and high-density lipoproteins, both of which are targeted to the hepatocyte during the normal course of lipid metabolism. The binding protein was identified as apolipoprotein H (apo H), also known as beta 2-glycoprotein I, on the basis of copurification of the rHBsAg-binding activity with the apo H protein and the ability of cDNA-expressed apo H to bind rHBsAg. Serum-derived HBsAg also binds to apo H, indicating that binding is not unique to rHBsAg. Binding is saturable, requires only the small S protein of rHBsAg, and is inhibited by excess rHBsAg, antibodies to HBsAg, and antibodies to apo H. The binding activity of apo H is destroyed upon reduction, indicating that 1 or more of its 22 disulfide bonds are required for interaction with rHBsAg. The possibility that an interaction between hepatitis B virus particles and lipoprotein particles may facilitate entry of the virus into hepatocytes is discussed.

In vitro binding properties of the hepatitis delta antigens to the hepatitis B virus envelope proteins: potential significance for the formation of delta particles

To investigate the possible existence of (a) reactive binding site(s) on the hepatitis B surface antigen (HBsAg) for the hepatitis delta antigen (delta Ag) in the hepatitis delta virus (HDV), we performed binding studies using recombinant (rec)Small, recMiddle, recLarge HBsAg and recombinant small (S) and large (L) hepatitis delta antigen (recS delta Ag, recL delta Ag). Rec delta Ag was immobilized onto microtiter plates and incubated with recSmall, recMiddle and recLarge HBsAg. Of the three HBsAg proteins only the recMiddle HBsAg was found to bind to recS delta Ag. This binding was inhibited by the addition of synthetic PreS2 peptide but not by small HBsAg, indicating that the S delta Ag exhibits a PreS2 binding site. RecL delta Ag bound to all three forms of HBsAg. The binding of the HBsAg to recL delta Ag was saturable and could be blocked with an excess of HBsAg, but not with BSA. The region of the additional 19 amino acids of the L delta Ag is therefore responsible for the creation of the small HBsAg binding site on the L delta Ag. We therefore suggest that all HBsAg proteins but particularly the small HBsAg in the HDV coat seem to be involved in the interaction with the HDV core particle and that the PreS2 region of the middle HBsAg plays a crucial role in binding to small delta Ag during HDV particle formation, probably to increase the stability of the HDV particle.