Polypeptides coded for by the region pre-S and gene S of hepatitis B virus DNA with the receptor for polymerized human serum albumin: expression on hepatitis B particles produced in the HBeAg or anti-HBe phase of hepatitis B virus infection

There are four polypeptides coded for by the region Pre-S and gene S on DNA of hepatitis B virus that carry the receptor for polymerized human serum albumin (poly-HSA), i.e., P31 and P39, as well as their glycosylated counterparts P35 and P43. With the use of monoclonal antibodies directed to Pre-S(1) sequence and Pre-S(2) sequence (bearing the receptor for poly-HSA), the content of these polypeptides, as well as their expression on the surface, was determined for hepatitis B particles of various categories. P39 and P43, carrying both Pre-S(1) and Pre-S(2) sequences, were contained abundantly in Dane and tubular particles, and to a much lesser extent in small spherical particles, all of which were purified from plasma containing hepatitis B e antigen (HBeAg). P31 and P35, carrying Pre-S(2) but not Pre-S(1) sequence, were contained comparably in these three categories of hepatitis B particles. In remarkable contrast, small spherical particles derived from plasma containing antibody to HBeAg were very low in the content of any Pre-S polypeptides. P31 and P39 showed higher activities for poly-HSA receptor than their glycosylated versions. When Dane particles were digested with trypsin, the poly-HSA receptor was deprived in parallel with the loss of antigenicity for Pre-S(2) sequence. The antigenicity for Pre-S(1) sequence was much less affected, and that for the product of gene S was virtually unchanged by the digestion.     

Characteristics of pre-S2 region of hepatitis B virus

The nucleotide sequence of our cloned HBV DNA (subtype adw) has been determined. When the 165-nucleotide sequence of the pre-S2 region was compared with 7 other published sequences (subtypes adw, adr, ayw, and adyw), we found 38 nucleotide substitutions among different subtypes and 4 (adr) or 6 (adw and ayw) substitutions within the same subtype. Analysis of the predicted amino acid sequence from the known nucleotide sequences indicates that: there are 20 amino acid substitutions, the longest conserved amino acid sequence is located between amino acids 23 to 34, and the 54th amino acid is identical within the same subtype but varies in different subtypes.     

In vitro and in vivo synthesis of the hepatitis B virus surface antigen and of the receptor for polymerized human serum albumin from recombinant human adenoviruses.

We have developed an adenovirus vector to express foreign proteins under the control of the adenovirus E1a promoter. Two recombinant plasmids, harbouring either the S gene or the pre-S2 region and the S gene of hepatitis B virus under the control of the E1a promoter, were used to construct two recombinant adenoviruses. These two viruses direct the synthesis of hepatitis B virus surface antigen (HBsAg) particles during the time course of an infectious cycle. When the pre-S2 region is present in the constructed virus, the synthesis of particles carrying the receptor for polymerized human serum albumin (pHSA) is observed. Moreover, the inoculation of rabbits with this latter purified recombinant adenovirus elicits the production of antibodies that react with both HBsAg and pHSA receptor.     

Duck hepatitis B virus can tolerate insertion, deletion, and partial frameshift mutation in the distal pre-S region.

In-frame and frameshift mutations were introduced into the pre-S region (1,212 base pairs) of duck hepatitis B virus. The in-frame mutants retained the inserted 12 nucleotides, while the frameshift mutants either reverted to wild type or exhibited a 10-nucleotide compensatory deletion downstream of the original mutation site. Thus, although duck hepatitis B virus has a compact and highly economical genome organization, it can replicate despite alterations of up to 9 amino acid codons in the pre-S and P open reading frames.     

Identification of polymerized-albumin receptor domain in the pre-S2 region of hepatitis B virus surface antigen M protein.

The pre-S2-coding region in the hepatitis B virus surface antigen M (P31; pre-S2 + S) protein gene was modified to identify a polymerized-albumin receptor (PAR) domain by deleting restriction fragments or performing site-directed mutagenesis. The modified M protein genes (M-P31x; x = d, e, f, h and i) were cloned into the yeast generalized-expression vector pGLD 906-1 and expressed in Saccharomyces cerevisiae under the control of yeast glyceraldehyde-3-phosphate dehydrogenase gene promoter. The PAR activities of these gene products suggested that the PAR domain is located in the hydrophilic and highly conserved domain in the pre-S2 region (around Leu12 approximately Tyr21). Antibodies specific for a pre-S2 peptide (Phe8 approximately Pro34, subtype adr), which covers the PAR domain, were purified from sera of rabbits immunized with yeast-derived M protein particles having a natural PAR domain. Immune electron microscopy showed that the purified antibodies could aggregate HBV particles. Therefore, it was speculated that the PAR domain overlapped with the dominant virus-neutralizing and virus-protecting epitopes.     

Mapping of the hepatitis B virus pre-S1 domain involved in receptor recognition

Hepatitis B virus (HBV) and woolly monkey hepatitis B virus (WMHBV) are primate hepadnaviruses that display restricted tissue and host tropisms. Hepatitis D virus (HDV) particles pseudotyped with HBV and WMHBV envelopes (HBV-HDV and WM-HDV) preferentially infect human and spider monkey hepatocytes, respectively, thereby confirming host range bias in vitro. The analysis of chimeric HBV and WMHBV large (L) envelope proteins suggests that the pre-S1 domain may comprise two regions that affect infectivity: one within the amino-terminal 40 amino acids of pre-S1 and one downstream of this region. In the present study, we further characterized the role of the amino terminus of pre-S1 in infectivity by examining the ability of synthetic peptides to competitively block HDV infection of primary human and spider monkey hepatocytes. A synthetic peptide representing the first 45 residues of the pre-S1 domain of the HBV L protein blocked infectivity of HBV-HDV and WM-HDV, with a requirement for myristylation of the amino terminal residue. Competition studies with truncated peptides suggested that pre-S1 residues 5 to 20 represent the minimal domain for inhibition of HDV infection and, thus, presumably represent the residues involved in virus-host receptor interaction. Recombinant pre-S1 proteins expressed in insect cells blocked infection with HBV-HDV and WM-HDV at a concentration of 1 nanomolar. The ability of short pre-S1 peptides to efficiently inhibit HDV infection suggests that they represent suitable ligands for identification of the HBV receptor and that a pre-S1 mimetic may represent a rational therapy for the treatment of HBV infection.     

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.     

Analysis of the pre-S2 N- and O-linked glycans of the M surface protein from human hepatitis B virus

The surface antigen of hepatitis B virus comprises a nested set of small (S), middle (M), and large (L) proteins, all of which are partially glycosylated in their S domains. The pre-S2 domain, present only in M and L proteins, is further N-glycosylated at Asn-4 exclusively in the M protein. Since the pre-S2 N-glycan appears to play a crucial role in the secretion of viral particles, the M protein may be considered as a potential target for antiviral therapy. For characterization of the pre-S2 glycosylation, pre-S2 (glyco)peptides were released from native, patient-derived hepatitis B virus subviral particles by tryptic digestion, separated from remaining particles, purified by reversed-phase high performance liquid chromatography, and identified by amino acid and N-terminal sequence analysis as well as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Pre-S2 N-glycans were characterized by anion exchange chromatography, methylation analysis, and on target sequential exoglycosidase digestions in combination with MALDI-TOF-MS, demonstrating the presence of partially sialylated diantennary complex-type oligosaccharides. In addition, the pre-S2 domain of M protein, but not that of L protein, was found to be partially O-glycosylated by a Gal(beta1-3)GalNAcalpha-, Neu5Ac(alpha2-3)Gal(beta1-3)GalNAcalpha-, or GalNAcalpha-residue. The respective O-glycosylation site was assigned to Thr-37 by digestion with carboxypeptidases in combination with MALDI-TOF-MS and by quadrupole time-of-flight electrospray mass spectrometry. Analytical data further revealed that about 90% of M protein is N-terminally acetylated.     

Studies of albumin binding to rat liver plasma membranes. Implications for the albumin receptor hypothesis

To characterize a previously proposed hepatocyte albumin receptor, we examined the binding of native and defatted 125I-labeled rat albumin to rat liver plasma membranes. After incubation for 30 min, binding was determined from the distribution of radioactivity between membrane pellet and supernatant following initial centrifugation (15000 X g for 15 min), and after repeated cycles of washing with buffer and re-centrifugation. 125I-labeled albumin recovered in the initial membrane pellet averaged only 4% of that incubated. Moreover, this albumin was only loosely associated with the membrane, as indicated by recovery in the pellet of under 0.5% of the counts after three washes. Binding of 125I-labeled albumin to the plasma membranes was no greater than to erythrocyte ghosts, was not inhibited by excess unlabeled albumin, and was not decreased by heat denaturation of the membranes, all suggestive of a lack of specific binding. Failure to observe albumin binding to the membranes was not due to a rapid dissociation rate or 'off-time', as incubations in the presence of sufficient ultraviolet light to promote covalent binding of ligands to receptors did not increase 125I counts bound to the membrane. Finally, affinity chromatography over albumin/agarose gel of solubilized membrane proteins provided no evidence of a membrane protein with a high affinity for albumin. These studies, therefore, do not support the hypothesis that liver cell plasma membranes contain a specific albumin receptor.     

Fermentation study for the production of hepatitis B virus pre-S2 antigen by the methylotrophic yeastHansenula polymorpha

Summary Various physico-chemical parameters have been studied in order to improve the production of hepatitis B virus pre-S2 antigen (middle surface antigen) by the methylotrophic yeastHansenula polymorpha. Antigen production was done in two steps: first, production of cells on glycerol (Phase 1), followed by induction of antigen expression with methanol (Phase 2). Dense cultures ofH. polymorpha, equivalent to 35–40 g/l (dry weight), were readily obtained in small fermenters using minimal medium containing glycerol as carbon source. Antigen expression in this minimal medium, after induction with methanol, was however low and never exceeded 1.6 mg/l of culture. Antigen production was greatly enhanced by adding complex organic nitrogen sources along with methanol at induction time; yeast extract was the best of all the sources tested. In shake flasks, antigen production was proportional to yeast extract concentration up to 7% (w/v) yeast extract. it became clear that the nutritional conditions for good antigen expression were different from those for good biomass production. The effects of yeast extract were reproduced in small fermenters: antigen levels reached 8–9 mg/l in medium containing 6% (w/v) yeast extract during induction with methanol. The mechanisms of yeast extract's effects are still unknown but are probably nutritional. The recombinantH. polymorpha strain produced both periplasmic and intracellular antigen. The periplasmic antigen was shown to be present as 20–22-nm particles and was therefore immunogenic. Immunoblotting indicated that part of the pre-S2 antigen was present as a 24-kDa degradation product. These studies have led to a 140-fold increase in volumetric productivity of antigen and to a 4.6-fold increase in specific production.Part of these results have been presented at the Deuxième Congrès de la Société Française de Microbiologie, Strasbourg, France, September 1989.     

The N-terminal (pre-S2) domain of a hepatitis B virus surface glycoprotein is translocated across membranes by downstream signal sequences.

The coding region for the hepatitis B virus surface antigens contains three in-phase ATG codons which direct the synthesis of three related polypeptides. The 24-kilodalton major surface (or S) glycoprotein is initiated at the most distal ATG and is a transmembrane protein whose translocation across the bilayer is mediated by at least two uncleaved signal sequences. The product of the next upstream ATG is the 31-kilodalton pre-S2 protein, which contains 55 additional amino acids attached to the N terminus of the S protein. This pre-S2-specific domain is translocated into the endoplasmic reticulum. Using a coupled in vitro translation-translocation system, we showed that (i) the pre-S2 domain itself lacks functional signal sequence activity, (ii) its translocation across the endoplasmic reticulum membrane is mediated by downstream signals within the S domain, and (iii) the N-terminal signal sequence of the S protein can translocate upstream protein domains in the absence of other signals. The hepatitis B virus pre-S2 protein is an example of a natural protein which displays upstream domain translocation, a phenomenon whose existence was originally inferred from the behavior of synthetic fusion proteins in vitro.     

Characterization of two differently glycosylated molecular species of yeast-derived hepatitis B vaccine carrying the pre-S2 region.

Modified hepatitis B virus surface antigen M protein particles (HBsAg M-P31c) produced in yeast is mainly composed of two differently glycosylated proteins, GP37 and GP34. GP37 has an N-linked sugar chain and O-linked sugar chains; and GP34 has an N-linked sugar chain bound to the peptide backbone P31. Although M-P31c vaccine elicits both anti-S and anti-pre-S2 antibodies, whether there are any differences between GP37 and GP34 in the ability to elicit these antibodies is still unknown. To clarify this issue, we prepared particles which were composed solely of GP37 or GP34 by affinity chromatography, using polymerized human serum albumin as a ligand and digestion with alpha-mannosidase. We also prepared particles composed solely of P31 by successive digestion with alpha-manosidase and endo-beta-N-acetylglycosaminidase H. The vaccines derived from these three kinds of particles elicited both anti-S and anti-pre-S2 antibodies in mice to the same extent as the original M-P31c vaccine. These results suggest that the N- and O-linked sugar chains of M-P31c component proteins produced in the host yeast cells have no effect on the ability to elicit anti-S and anti-pre-S2 antibodies and that there are no differences with respect to antibody response in mice between the two major components of M-P31c, GP37 and GP34.     

An 80-kilodalton protein that binds to the pre-S1 domain of hepatitis B virus

It has been suggested that hepatitis B virus (HBV) binds to a receptor on the plasma membrane of human hepatocytes via the pre-S1 domain of the large envelope protein as an initial step in HBV infection. However, the nature of the receptor remains controversial. In an attempt to identify a cell surface receptor for HBV, purified recombinant fusion protein of the pre-S1 domain of HBV with glutathione S-transferase (GST), expressed in Escherichia coli, was used as a ligand. The surface of human hepatocytes or HepG2 cells was biotinylated, and the cell lysate (precleared lysate) which did not bind to GST and glutathione-Sepharose beads was used as a source of receptor molecules. The precleared lysate of the biotinylated cells was incubated with the GST-pre-S1 fusion protein, and the bound proteins were visualized by Western blotting and enhanced chemiluminescence. An approximately 80-kDa protein (p80) was shown to bind specifically to the pre-S1 domain of the fusion protein. The receptor binding assay using serially or internally deleted segments of pre-S1 showed that amino acid residues 12 to 20 and 82 to 90 are essential for the binding of pre-S1 to p80. p80 also bound specifically to the pre-S1 of native HBV particles. Analysis of the tissue and species specificity of p80 expression in several available human primary cultures and cell lines of different tissue origin showed that p80 expression is not restricted to human hepatocytes. Taken together the results suggest that p80 may be a component of the viral entry machinery.     

Stability of murine, chimeric and humanized antibodies against pre-S2 surface antigen of hepatitis B virus

We have constructed a humanized antibody with specificity for the pre-S2 surface antigen of hepatitis B virus (HBV) by grafting the complementarity determining regions (CDRs) of parental murine monoclonal antibody (mAb) into human anti-Sm antibody framework regions. The humanized antibody has a substitution at position 94 in a framework region of the heavy chain variable region, and exhibits the same antigen binding affinity as the parental murine monoclonal and chimeric antibodies. In order to assess the stability of these antibodies, thermal inactivation of the parental, chimeric and humanized antibodies was analyzed. Fifty percent inactivation of the chimeric and humanized antibodies was observed at 63.7 degrees C and 68.7 degrees C, respectively, compared to 55.0 degrees C for murine antibody. The humanized antibody also exhibited increased stability against denaturant. Guanidine-induced unfolding monitored by the changes in fluorescence intensity at 360 nm showed that midpoints of the transition of the chimeric and humanized antibodies were 2.47 M and 2.56 M, respectively, whereas that of the murine antibody was 1.36 M.     

Study of the antigenic structure of hepatitis B virus proteins. I. Synthesis of pre-S fragments of hepatitis B virus proteins and their immunogenicity

Fragments of hepatitis B virus envelope proteins corresponding to the parts of the pre-S domain were synthesised and immobilized on the carriers with low own immunogenicity. The highest stimulation of the antibody production was observed for the antigens immobilized on microspherical carriers or gelatin modified by H-Gly-Tyr-OH. Among peptides used for immunization, pre-S fragment 134-144, conjugated with microspherical carrier, proved to be the most active.     

Interaction between various polymerized human albumins and hepatitis B surface antigen.

A variety of albumin polymers were prepared and tested for binding with hepatitis B surface antigen (HBsAg): synthetic polymers cross-linked by either glutaraldehyde or carbodiimide; heat-aggregated polymers made by heating albumin solutions at 60 degrees C for 10 h with or without albumin stabilizer; and polymers isolated from fresh or long-stored commercial therapeutic albumin solutions. A sensitive solid-phase, competitive-inhibition radioimmunoassay, which can detect as little as 10 ng of glutaraldehyde-cross-linked human albumin polymer (PHALB-G), was developed and used to measure binding. The binding of PHALB-G with HBsAg was 150- to 1,000-fold greater than that of any other albumin polymer. Glutaraldehyde-cross-linked bovine albumin polymer showed no binding. Albumin monomer and dimer fractions produced by glutaraldehyde treatment exhibited some binding, albeit much weaker than PHALB-G. As measured by a direct-binding assay with solid-phase PHALB-G, the attachment of HBsAg particles from sera positive for antibody to the e antigen was less efficient than that from sera positive for e antigen, even when all sera were tested at equal HBsAg concentrations. In protein blot experiments with radiolabeled albumin preparations, PHALB-G bound almost exclusively to HBsAg polypeptide P31 and showed no binding with the major polypeptides P23 and P26. None of the other radiolabeled albumin polymers was reactive. These results indicate that the interaction between PHALB-G and HBsAg is not due to polymerization of albumin per se, but rather is unique and site specific.     

Intracellular transport of formaldehyde-treated serum albumin in liver endothelial cells after uptake via scavenger receptors.

Endocytosis of formaldehyde-treated serum albumin (FSA) mediated by the scavenger receptor was studied in rat liver endothelial cells. Suspended cells had about 8000 receptors/cell, whereas cultured cells had about 19,000 receptors/cell. Kd was 10(-8) M in both systems. Cell-surface scavenger receptors were found exclusively in coated pits by electron microscopy, by using ligand labelled with colloidal gold. Cell-surface-bound FSA could be released by decreasing the pH to 6.0; it was therefore possible to assess the rate of internalization of surface-bound ligand. This rate was very high: t1/2 for internalization of ligand prebound at 4 degrees C was 24 s. The endocytic rate constant at 37 degrees C, Ke, measured as described by Wiley & Cunningham [(1982) J. Biol. Chem. 257, 4222-4229], was 2.44 min-1, corresponding to t1/2 = 12 s. Uptake of FSA at 37 degrees C after destruction of one cell-surface pool of receptors by Pronase was decreased to 60%. This finding is compatible with a relatively large intracellular pool of receptors. The intracellular handling of 125I-tyramine-cellobiose-labelled FSA (125I-TC-FSA) was studied by subcellular fractionation in sucrose gradients, Nycodenz gradients or by differential centrifugation. The density distributions of degraded and undegraded 125I-TC-FSA after fractionation of isolated non-parenchymal cells and whole liver were similar, when studied in Nycodenz and sucrose gradients, suggesting that the subcellular distribution of the ligand was not influenced by the huge excess of non-endothelial material in a whole liver homogenate. Fractionation in sucrose gradients showed that the ligand was sequentially associated with organelles banding at 1.14, 1.17 and 1.21 g/ml. At 9-12 min after intravenous injection the ligand was in a degradative compartment, as indicated by the accumulation of acid-soluble radioactivity at 1.21 g/ml. A rapid transfer of ligand to the lysosomes was also indicated by the finding that a substantial proportion of the ligand could be degraded by incubating mitochondrial fractions prepared 12 min after intravenous injection of the ligand. The results indicate that FSA is very rapidly internalized and transferred through an endosomal compartment to the lysosomes. The endosomes are gradually converted into lysosomes between 9 and 12 min after injection of FSA. The rate-limiting step in the intracellular handling of 125I-TC-FSA is the degradation in the lysosomes.     

Production of infectious hepatitis delta virus in vitro and neutralization with antibodies directed against hepatitis B virus pre-S antigens.

Hepatitis delta virus (HDV) particles were produced in Huh7 human hepatoma cells by transfection with cloned hepatitis B virus (HBV) DNA and HDV cDNA. The particles were characterized by their buoyant density, the presence of encapsidated viral RNA, and their ability to infect primary cultures of chimpanzee hepatocytes. Successful infection was evidenced by the appearance of increasing amounts of intracellular HDV RNA after exposure to particles. Infection was prevented when particles were incubated with antibodies directed against synthetic peptides specific for epitopes of the pre-S1 or pre-S2 domains of the HBV envelope proteins before exposure to hepatocytes. These data demonstrate that HDV particles produced in vitro are infectious and indicate (i) that infectious particles are coated with HBV envelope proteins that contain the pre-S1 and pre-S2 regions, (ii) that epitopes of the pre-S1 and pre-S2 domains of HBV envelope proteins are exposed at the surface of HDV particles, and (iii) that antibodies directed against those epitopes have neutralizing activity against HDV.