Human and monoclonal antibodies to hepatitis B core antigen recognise a single immunodominant epitope

Sizing up large protein complexes by electrospray ionisation-based electrophoretic mobility and native mass spectrometry: morphology selective binding of Fabs to hepatitis B virus capsids

The capsid of hepatitis B virus (HBV) is a major viral antigen and important diagnostic indicator. HBV capsids have prominent protrusions ('spikes') on their surface and are unique in having either T = 3 or T = 4 icosahedral symmetry. Mouse monoclonal and also human polyclonal antibodies bind either near the spike apices (historically the 'α-determinant') or in the 'floor' regions between them (the 'β-determinant'). Native mass spectrometry (MS) and gas-phase electrophoretic mobility molecular analysis (GEMMA) were used to monitor the titration of HBV capsids with the antigen-binding domain (Fab) of mAb 3120, which has long defined the β-determinant. Both methods readily distinguished Fab binding to the two capsid morphologies and could provide accurate masses and dimensions for these large immune complexes, which range up to ~8 MDa. As such, native MS and GEMMA provide valuable alternatives to a more time-consuming cryo-electron microscopy analysis for preliminary characterisation of virus-antibody complexes.

Epitope-distal effects accompany the binding of two distinct antibodies to hepatitis B virus capsids

Infection of humans by hepatitis B virus (HBV) induces the copious production of antibodies directed against the capsid protein (Cp). A large variety of anticapsid antibodies have been identified that differ in their epitopes. These data, and the status of the capsid as a major clinical antigen, motivate studies to achieve a more detailed understanding of their interactions. In this study, we focused on the Fab fragments of two monoclonal antibodies, E1 and 3120. E1 has been shown to bind to the side of outward-protruding spikes whereas 3120 binds to the "floor" region of the capsid, between spikes. We used hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) to investigate the effects on HBV capsids of binding these antibodies. Conventionally, capsids loaded with saturating amounts of Fabs would be too massive to be readily amenable to HDX-MS. However, by focusing on the Cp protein, we were able to acquire deuterium uptake profiles covering the entire 149-residue sequence and reveal, in localized detail, changes in H/D exchange rates accompanying antibody binding. We find increased protection of the known E1 and 3120 epitopes on the capsid upon binding and show that regions distant from the epitopes are also affected. In particular, the α2a helix (residues 24-34) and the mobile C-terminus (residues 141-149) become substantially less solvent-exposed. Our data indicate that even at substoichiometric antibody binding an overall increase in the rigidity of the capsid is elicited, as well as a general dampening of its breathing motions.

Antigenic switching of hepatitis B virus by alternative dimerization of the capsid protein

Chronic hepatitis B virus (HBV) infection afflicts millions worldwide with cirrhosis and liver cancer. HBV e-antigen (HBeAg), a clinical marker for disease severity, is a nonparticulate variant of the protein (core antigen, HBcAg) that forms the building-blocks of capsids. HBeAg is not required for virion production, but is implicated in establishing immune tolerance and chronic infection. Here, we report the crystal structure of HBeAg, which clarifies how the short N-terminal propeptide of HBeAg induces a radically altered mode of dimerization relative to HBcAg (∼140° rotation), locked into place through formation of intramolecular disulfide bridges. This structural switch precludes capsid assembly and engenders a distinct antigenic repertoire, explaining why the two antigens are cross-reactive at the T cell level (through sequence identity) but not at the B cell level (through conformation). The structure offers insight into how HBeAg may establish immune tolerance for HBcAg while evading its robust immunogenicity.

Molecular basis for the high degree of antigenic cross-reactivity between hepatitis B virus capsids (HBcAg) and dimeric capsid-related protein (HBeAg): insights into the enigmatic nature of the e-antigen

The hepatitis B virus core gene codes for two closely related antigens: a 21-kDa protein that forms dimers that assemble as multimegadalton capsids, and a 17-kDa protein that also forms dimers but that do not assemble. The proteins, respectively referred to as core antigen (HBcAg) and e-antigen (HBeAg), share a sequence of 149 residues but have different amino- and carboxy-termini. Their structural and serological relationship has long been unclear. With insights gained from recent structural studies on immune complexes of the capsids, the relationship was reassessed using recombinant forms of the antigens and a panel of monoclonal antibodies (mAbs) commonly believed to discriminate between core and e-antigen. Surface plasmon resonance (SPR) was used to measure the affinities, in contrast to previous studies that used more error-prone and less sensitive plate-type assays. Four of the six mAbs did not discriminate between core and e-antigen, nor did they discriminate between e-antigen and dimers of dissociated core antigen capsids. One mAb (3120) was specific for assembled capsids and one (e6) was specific for unassembled dimers. Epitope valency of the e-antigen was also studied, using a sandwich SPR assay where e-antigen was captured with one mAb and probed with a second. The e-antigen is often considered to be a monomeric protein on the basis of monovalent reactivity with antibody pairs specific for either an alpha or beta epitope (in a prior nomenclature for e-antigen specificity). This model, however, is incorrect, because recombinant e-antigen is a stable dimer and its apparent monovalency is due to steric blockage. This was proven by the formation of a 2:1 Fab e6-e-antigen complex. These results suggest new approaches for the isolation of the authentic e-antigen, its biological assay, and its stabilization as an immune complex for structural studies.

Non-canonical binding of an antibody resembling a naïve B cell receptor immunoglobulin to hepatitis B virus capsids

The hepatitis B virus capsid (core antigen) is able to bind to and activate naïve B cells and these become efficient primary antigen-presenting cells for the priming of T cells. We have investigated this interaction by using cryo-electron microscopy, three-dimensional image reconstruction, and molecular modeling to visualize capsids decorated with Fab fragments of a receptor immunoglobulin, and surface plasmon resonance to measure the binding affinity. By both criteria, the mode of binding differs from those of the six monoclonal anti-core antigen antibodies previously characterized. The Fab interacts with two sites approximately 30 A apart. One interaction is canonical, whereby the CDR loops engage the tip of one of the 25 A spikes that protrude from the capsid surface. The second interaction is non-canonical; in it, the Fab framework contacts the tip of an adjacent spike. The binding affinity of this Fab for capsids, K(D) approximately 4 x 10(-7) M, is relatively low for an antibody-antigen interaction, but is approximately 150-fold lower still ( approximately 2.5 x 10(-5) M) for unassembled capsid protein dimers. The latter observation indicates that both of the observed interactions are required to achieve stable binding of capsids by this receptor immunoglobulin. Considerations of conserved sequence motifs in other such molecules suggest that other naïve B cells may interact with HBV capsids in much the same way.

Monoclonal antibody against non-dominant epitopes of HBV e Ag was raised by antigen–antibody co-immunization

Detection of hepatitis B e antigen (HBeAg) in the sera of individuals infected with hepatitis B virus (HBV) can indicate both a high infectivity of the disease and a poor prognosis of disease treatment. Most of monoclonal antibodies raised against HBV e proteins interact with immuno-dominant epitopes, such as HBeAg-beta. In order to raise antibodies against non-dominant epitopes of HBV e protein, in this study, mice were immunized with both recombinant HBeAg (rHBeAg) and an anti-HBeAg antibody (EWB) recognizing a dominant antigenic epitope of HBeAg (HBeAg-beta epitope). With this strategy, we successfully selected two monoclonal antibodies, S-29-3 and S-72-3. Both S-29-3 and S-72-3 bind to recombinant HBeAg with a high affinity. The epitope mapping assay determined that the S-73-2 recognizes the N-terminal of HBeAg (1-118 aa) and the S-29-3 recognizes the C-terminal of HBeAg (91-149 aa). Further experiment showed that these two antibodies could be formed a pair-Abs that is used in detecting native HBeAg from the sera of HBV patients. The conclusion is that the developed method is useful to raise mAb against non-dominant epitopes in given Ag.

Diversity of core antigen epitopes of hepatitis B virus

Core antigen (cAg), the viral capsid, is one of the three major clinical antigens of hepatitis B virus. cAg has been described as presenting either one or two conformational epitopes involving the "immunodominant loop." We have investigated cAg antigenicity by cryo-electron microscopy at approximately 11-A resolution of capsids labeled with monoclonal Fabs, combined with molecular modeling, and describe here two conformational epitopes. Both Fabs bind to the dimeric external spikes, and each epitope has contributions from the loops on both subunits, explaining their discontinuous nature: however, their binding aspects and epitopes differ markedly. To date, four cAg epitopes have been characterized: all are distinct. Although only two regions of the capsid surface are accessible to antibodies, local clustering of the limited number of exposed peptide loops generates a potentially extensive set of discontinuous epitopes. This diversity has not been evident from competition experiments because of steric interference effects. These observations suggest an explanation for the distinction between cAg and e-antigen (an unassembled form of capsid protein) and an approach to immunodiagnosis, exploiting the diversity of cAg epitopes.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

Characterization of humoral and CD4+ cellular responses after genetic immunization with retroviral vectors expressing different forms of the hepatitis B virus core and e antigens

The humoral and CD4+ cellular immune responses in mice following genetic immunization with three retroviral vectors encoding different forms of hepatitis B virus core antigen (HBcAg) and e antigen (HBeAg) were analyzed. The retroviral vectors induced expression of intracellular HBcAg (HBc[3A4]), secreted HBeAg (HBe[5A2]), or an intracellular HBcAg-neomycin phosphoryltransferase fusion protein (HBc-NEO[6A3]). Specific antibody levels and immunoglobulin G isotype restriction were highly dependent on both the host major histocompatibility complex and the transferred gene. Humoral and CD4+ cellular HBcAg and/or HBeAg (HBc/eAg)-specific immune responses following retroviral vector immunization were of a lower magnitude but followed the same characteristics compared with those after immunization with HBc/eAg in adjuvant. Two factors influenced the humoral responses. First, in vivo depletion of CD8+ cells in HBc-NEO[6A3]-immunized H-2k mice abrogated both HBcAg-specific antibodies and in vitro-detectable cytotoxic T lymphocytes. Second, priming of H-2b mice with an HBc/eAg-derived T-helper (Th) peptide in adjuvant prior to retroviral vector immunization greatly enhanced the HBc/eAg-specific humoral responses to all three vectors, suggesting that insufficient HBc/eAg-specific CD4+ Th-cell priming limits the humoral responses. In conclusion, direct injection of retroviral vectors seems to be effective in priming HBc/eAg-specific CD8+ but comparatively inefficient in priming CD4+ Th cells and subsequently specific antibodies. However, the limited HBc/eAg-specific CD4+ cell priming can effectively be circumvented by prior administration of a recombinant or synthetic form of HBc/eAg in adjuvant.

A monoclonal inhibition enzyme immunoassay for detection of antibodies against hepatitis B core antigen: confirmation of an immunodominant epitope

Monoclonal antibodies (mAbs) were raised against hepatitis B virus core produced by a recombinant clone of Escherichia coli (rHBc). The three mAbs recognized rHBc by Western blot, suggesting that they reacted with non-conformational epitopes. Competition experiments between mAbs and human anti-HBc sera confirmed the existence of an immunodominant HBc epitope within the viral antigen. A monoclonal competition enzyme immunoassay using an IgM mAb conjugated to biotin and streptavidin-peroxidase as the detection system yielded 99% sensitivity and 100% specificity, when compared to other commercial assays.

Immune recognition of linear antigenic regions within the hepatitis B pre-C and C-gene translation products using synthetic peptides

The antibody recognition of linear regions within the amino acid (aa) sequence of hepatitis B (HB) core antigen (HBcAg), e antigen (HBeAg), and pre-C region was investigated in 46 patients infected with hepatitis B virus (HBV), and one immunized rabbit. Peptide analogues were synthesized to cover the complete product of the C-gene, including the pre-C region using various synthetic methods. Two carriers of hepatitis B surface antigen (HBsAg) with anti-HBe, recognized pin-bound decapeptides covering amino acid (aa) 76-83 of HBc/eAg, and the most essential residues were found to be Asp78, Pro79, Arg82, and Asp83. Pre-C peptides were recognized by IgG1 or IgG3 in sera from two out of ten cases with acute HB, in four out of twelve sera from HBeAg-positive carriers of HBsAg, and in two out of twelve sera from anti-HBe-positive carriers of HBsAg. Two sera from the cases of acute HB showed strong reactivity of the IgG3 isotype with HBc/eAg peptides 61-85. Five of the sera from HBeAg-positive carriers of HBsAg were weakly reactive with peptides 41-60, 61-85, 121-140, and/or 141-160. Eight of the sera from anti-HBe-positive carriers of HBsAg recognized aa 121-140 of HBc/e with IgG1, IgG3, and/or IgG4 isotypes. IgG from one immunized rabbit recognized peptides 1-20, 61-85, and 71-90, and the T-cells recognized peptides 1-20 and 71-90. Thus, human and rabbit antibodies recognize linear antigenic regions within the pre-C, and within regions 1-20, 41-60, 61-85, 121-140, and 141-160 of HBcAg.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of minor and major antigenic regions within the hepatitis B virus nucleocapsid

Hepatitis B core antibodies (anti-HBc) appear very early during the course of the hepatitis B virus infection and often persist years after viral clearance. In order to characterize the immunodominant domain of the HBcAg, the human immune response against the HBV nucleocapsid (HBcAg) was analyzed by using 14 synthetic peptides. Anti-HBc antibodies were detected by an indirect enzyme-linked immunosorbent assay (ELISA) with HBc peptides. Results suggest that the anti-HBc response is heterogeneous and directed against the whole primary structure of the HBc protein. Results also indicate that the epitopes recognized by anti-HBc antibodies can vary with the stages of the disease. In most sera from patients with serological evidence of acute HBV infection, anti-HBc antibodies recognized all the HBc peptides; conversely, after the acute phase, anti-HBc antibodies recognized predominantly epitopes located within the central region of the HBc protein from residue 74 to 123. Our results suggest that the HBV core protein is made up of two antigenic regions: a major one expressing a family of immunodominant epitopes from residue 74 to 123, whereas the minor encompasses the rest of the protein. The concept of the conformational nature of the unique HBcAg determinant is discussed, suggesting numerous families of linear epitopes.

Epitopes recognized by antibodies to denatured core protein of hepatitis B virus

Particulate and denatured core protein as well as e-antigen (HBe) of hepatitis B virus (HBV) differ in part immunologically but this has not been studied in sufficient detail. Therefore, in this study the B-cell immune response to native and denatured HBV core protein which both can exhibit HBe-specific epitopes was examined using a panel of mouse MABs and rabbit polyclonal antibodies to native and denatured core protein and polyclonal anti-HBe/anti-HBc antibodies from sera of infected patients. Epitope mapping was performed using a set of partially overlapping synthetic HBc peptides, carboxy-terminally truncated HBc proteins and various HBc fusion proteins. A major immunogenic region between amino acids 134-140 and two less immunogenic regions, one spanning amino acids 2-10 and one with three partially overlapping epitopes between amino acid positions 138 and 154, were defined by mouse MABs. Polyclonal rabbit antibodies to denatured HBc, woodchuck and ground squirrel hepatitis core proteins (WHc and GSHc) recognized similar epitopes but in addition occasionally region 61-85, and the latter was also recognized on particulate HBc. Two antigenic regions (amino acid positions 2-10 and 138-145) were found to be exposed on HBe from human serum, and were recognized by mouse anti-HBe but not by anti-HBc antibodies from sera of infected patients. This study demonstrates a more complex pattern of HBc and HBe epitopes than detected previously and provides tools to study conformational changes which may take place during HBc/HBe processing, transport and core particle assembly.

Molecular basis of the diversity of hepatitis B virus core-gene products

All hepatitis B viruses examined to date code for at least two different core-gene products which are referred to as the c- and the e-protein. In the case of the human hepatitis B virus, they are known as the HBcAg and the HBeAg. Although these proteins share most of their primary amino acid sequence, they exhibit quite distinct properties. The e-protein is located in the cytoplasm and the nucleus of infected cells and very efficiently assembles into nucleocapsids. By contrast, the e-protein does not form particles. It enters the secretory pathway and is actively secreted by the cells. Here we describe the biosynthetic pathways by which the c- and e-proteins are expressed and summarize recent data from our laboratory showing that the antigenic and biophysical properties which distinguish the HBeAg from the HBcAg are primarily due to the 10 amino acid long portion of the HBeAg leader sequence that remains attached to the HBeAg after cleavage.

A cysteine and a hydrophobic sequence in the noncleaved portion of the pre-C leader peptide determine the biophysical properties of the secretory core protein (HBe protein) of human hepatitis B virus

The molecular basis of the biophysical and antigenic differences between the cellular core protein (HBc protein) and the secreted core protein (HBe protein) of human hepatitis B virus was examined. The data show that the properties which distinguish the HBe protein from the HBc protein are due mostly to the 10-amino-acid portion of the HBe leader sequence which remains attached to the HBe protein after cleavage. A cysteine located within this region determines the quaternary structure and the antigenicity of the HBe protein. If this cysteine is lacking, the HBe protein, which is predominantly a monomer with only HBe antigenicity, is expressed as a disulfide-linked homodimer showing both HBe and HBc antigenicity. However, dimerization of the HBe protein was found to be neither sufficient nor required for particle formation. In fact, aggregation of the HBe protein was found to be inhibited by the strongly hydrophobic tripeptide Trp-Leu-Trp, which is also located in the noncleaved portion of the signal sequence. If this tripeptide was converted into either Asp-Asn-Asn or Ala-Asp-Leu, the HBe protein assembled into particles, independent of the presence of the cysteine.

Immunodominant regions within the hepatitis C virus core and putative matrix proteins

The complete amino acid sequences of hepatitis C virus (HCV) core (residues 1 to 115) and putative matrix (residues 116 to 190) proteins were synthesized as 18-residue-long peptides with an 8-amino-acid overlap. The peptides were assayed with 50 human serum samples with antibodies to HCV (anti-HCV) and 46 serum samples without anti-HCV, as determined by several commercial assays. Immunodominant regions were defined within residues 1 to 18, 11 to 28, 21 to 38, 51 to 68, and 101 to 118. The peptides that covered these regions were recognized by 40 of 50 (80%), 42 of 50 (84%), 36 of 50 (72%), 34 of 48 (68%), and 36 of 48 (72%) of the anti-HCV positive serum samples, respectively. Two anti-HCV negative serum samples were each repeatedly reactive with one peptide, but both were found to be negative by confirmatory anti-HCV assays. Four serum samples that were confirmed to be positive for anti-HCV in commercial assays did not recognize any of the peptides that cover the HCV core-matrix regions. Ninety-two percent of anti-HCV-positive serum samples reacted with a combination of peptides covering residues 1 to 18 and 11 to 28. Testing of peptides that contain the reported genotypic variations of the HCV core within the regions at residues 1 to 18, 51 to 68, and 101 to 118 showed that a change from Thr-110 to Asn-110 decreased the reactivities of eight serum samples. In conclusion, we found that human antibodies to the HCV core-matrix protein(s) are mainly directed to linear determinants and can easily be reproduced by using short synthetic peptides. We also found that such antibodies develop in more than 90% of HCV-infected people.

Immune response to a single peptide containing an immunodominant region of hepatitis C virus core protein: the isotypes and the recognition site

We have used one single peptide covering the 17 N-terminal amino acids of the hepatitis C virus (HCV) core protein (c) to analyse the human immune response against B-cell epitope(s) within this region. The sequence MSTNPKPQRKTKRNTNR was obtained from two sequenced HCV genomes, and the peptide was synthesized by a newly developed method. The peptide was assayed with 144 human sera which had all been assayed for antibodies to HCV (anti-HCV) using commercial assays. Forty-nine sera were found to be positive for anti-HCV using these assays; 40 of these were found to be positive with our anti-HCV IgG peptide assay. The class (IgM, IgG) and subclass (IgA1, IgG1-4) specific reactions were determined using the polyclonal and monoclonal anti-HCV peptide enzyme immunoassays. Isotypes of mainly IgG1 and IgG3, but also IgG4, IgM and IgG2, gave specific reactions with this region. Using omission peptide analogues of the region 1-18, the sequence RKTKRNTN within residues 9-16 was common to 34 out of 37 sera of which the IgG antibody binding site could be mapped. It is unusual for a single peptide assay to have such high sensitivity since B cell epitopes within a protein are often discontinuous. It seems that at least 80% of HCV infected individuals develop antibodies of various isotypes to the antigenic site RKTKRNTN, located in the N-terminal portion of the HCV core. Thus, the immune response to this peptide should be further investigated with regard to the reactive Ig isotypes developing during HCV infection.

The position of heterologous epitopes inserted in hepatitis B virus core particles determines their immunogenicity

The nucleocapsid (HBcAg) of the hepatitis B virus (HBV) has been suggested as a carrier moiety for vaccine purposes. We investigated the influence of the position of the inserted epitope within hybrid HBcAg particles on antigenicity and immunogenicity. For this purpose, genes coding for neutralizing epitopes of the pre-S region of the HBV envelope proteins were inserted at the amino terminus, the amino terminus through a precore linker sequence, the truncated carboxy terminus, or an internal site of HBcAg by genetic engineering and were expressed in Escherichia coli. All purified hybrid HBc/pre-S polyproteins were particulate. Amino- and carboxy-terminal-modified hybrid HBc particles retained HBcAg antigenicity and immunogenicity. In contrast, insertion of a pre-S(1) sequence between HBcAg residues 75 and 83 abrogated recognition of HBcAg by 5 of 6 anti-HBc monoclonal antibodies and diminished recognition by human polyclonal anti-HBc. Predictably, HBcAg-specific immunogenicity was also reduced. With respect to the inserted epitopes, a pre-S(1) epitope linked to the amino terminus of HBcAg was not surface accessible and not immunogenic. A pre-S(1) epitope fused to the amino terminus through a precore linker sequence was surface accessible and highly immunogenic. A carboxy-terminal-fused pre-S(2) sequence was also surface accessible but weakly immunogenic. Insertion of a pre-S(1) epitope at the internal site resulted in the most efficient anti-pre-S(1) antibody response. Furthermore, immunization with hybrid HBc/pre-S particles exclusively primed T-helper cells specific for HBcAg and not the inserted epitope. These results indicate that the position of the inserted B-cell epitope within HBcAg is critical to its immunogenicity.

The quaternary structure, antigenicity, and aggregational behavior of the secretory core protein of human hepatitis B virus are determined by its signal sequence

Human hepatitis B virus encodes a secretory core protein, referred to as the HBe protein, whose secretion is mediated by the pre-C signal sequence. Here we examined whether this sequence is important only for translocation of the HBe precursor (the precore protein) or whether it also contributes to the structural and biophysical properties of the mature HBe protein. When a truncated hepatitis B virus precore protein, lacking the basic C-terminal domain which is cleaved from the wild-type protein during its conversion into HBe, was expressed in human hepatoma cells, only a small amount of HBe-like protein was produced. This protein was slightly smaller than the wild-type HBe protein, suggesting that C-terminal cleavage of the precore protein does not occur at the suggested site. When the authentic signal sequence of the precore protein (the pre-C sequence) was replaced by the unrelated signal sequence of an influenza virus hemagglutinin, not only the full-length but also the C-terminally truncated protein was expressed and secreted with high efficiency. Western blot (immunoblot) analyses with nonreducing gels and conformation-specific monoclonal antibodies revealed that the HBe protein secreted under control of the pre-C signal sequence was a monomer with HBe antigenicity, whereas the HBe-like protein secreted under control of the hemagglutinin signal sequence was a disulfide-bridge-linked dimer with both HBe and HBc antigenicity. Electron microscopic examination of gradient-purified particulate core gene products showed that HBe protein secreted under control of the hemagglutinin signal sequence forms core particles, whereas HBe protein secreted under control of the pre-C sequence does not. Thus, the pre-C sequence not only mediates the secretion but also determines the structural and aggregational properties of the HBe protein.

Rapid "tea-bag" peptide synthesis using 9-fluorenylmethoxycarbonyl (Fmoc) protected amino acids applied for antigenic mapping of viral proteins

The role of individual amino acids in binding human and macaque antibodies were determined in the human immunodeficiency virus type 1 (HIV-1) gp41, residues 594-613, and for human antibodies in the hepatitis B (HB) virus core/e antigens (HBc/eAg), residues 121-140. Decapeptides with 9 amino acids (aa) overlap were synthesised using a rapid method for simultaneous multiple peptide synthesis with 9-fluorenylmethoxycarbonyl (Fmoc) protection for the alpha-amino group of the aas. One coupling cycle including washing steps was performed within 60-90 min. The crude products were analysed by reversed-phase HPLC and PD-mass spectrometry. With the 11 decapeptides covering residues 594-613 of HIV-1 gp41, the sequences SGKLI at aa 599-603 was found to be the main recognition site for 19 human anti-HIV positive sera. Two macaques repeatedly immunized with a peptide covering aa 594-613 of gp41, preferentially recognised the sequence CTTAVPW at residues 604-610 after 1-2 months of immunisation. One macaque also recognised the sequence CSGKLI, with sera sampled greater than 10 months after start of immunisation. Out of 9 human sera from patients with chronic HB, and reactive to a peptide covering residues 121-140 of HBc/eAg, 8 were found to recognise the sequence TPPA at residues 128-131, with an individual variation within residues 125-133 in regard to N- and C-terminal ends of the recognised antigenic site. Thus, human recognition of this antigenic site overlaps the reported T- and the B-cell recognition site found in mice. We believe that this simple and rapid approach to obtain large numbers of immunologically active peptides can be useful for most laboratories interested in the immunological characterisation of proteins.

Human and murine B-cells recognize the HBeAg/beta (or HBe2) epitope as a linear determinant

The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), deduced from the genome of the HBV ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to the beta (or HBe2) epitope of hepatitis B e antigen (HBe/b mAbs; 57/8, 78/3, 141/158 and 141/207). Cross-competition between the mAbs with a mAb to the HBe/alpha epitope (or HBe1) and an anti-HBc mAb showed that all the HBe/b mAbs specifically inhibited human anti-HBe/b binding. Screening the HBc/e peptides showed that all anti-HBe/b mAbs recognized a peptide covering the residues 126-135. Three of the mAbs, 78/3, 141/152 and 141/207, had a less restricted reactivity than the other two, suggesting the recognition of the HBe/b as a discontinuous determinant. Fine mapping of the region aa 126-135 was performed by synthesizing decapeptides with nine overlapping aas, covering residues aa 121-140. All mAbs, except 78/3, reacted with the linear sequence TPPAYR, at residues 128-133. An additional set of peptides was synthesized, where the six aas within the epitope 128-133 were substituted in turn by the other 19 possible aas. By this approach, the essential aas for mAb 57/8 were found to be the sequence of PPA at residues 129-131, and for mAb 141/158 the sequence PP-Y, at residues 129, 130 and 132, respectively. Human recognition of the linear HBe/b epitope was investigated by using a peptide covering residues 121-140 (p 33). Thirty-one sera from chronic carriers of HBsAg, of which seven were positive for HBeAg and the remaining 24 for anti-HBe, were investigated. Of the sera with HBeAg, two had low levels of anti/-HBe/b in the p 33 assay. Out of the sera with anti-HBe, eight were positive in the p 33 EIA. Thus, murine monoclonals and human sera may recognize the HBe/b epitope as a linear determinant residing around aa 130.

Characterisation of a linear binding site for a monoclonal antibody to hepatitis B core antigen

The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to HBcAg (35/312, 37/275, and 7/275). All the mAbs specifically inhibited human anti-HBc by cross competition in assays for anti-HBc and anti-HBe. The mAb 35/312 recognised a peptide covering residues 76-85 of the HBcAg sequence. The other two mAbs did not react specifically with any linear peptide, suggesting discontinuous epitopes for these mAbs. The linear sequence EDPASR at residues 77-82 was found to constitute the epitope for mAb 35/312 when fine mapping the binding site. The most essential aas for mAb 35/312 were found to be the DP at residues 79-80, when peptides were synthesized where the aas at 77-83, were substituted by the other 19 aas. Since the mAb 35/312 inhibits the binding of human anti-HBc positive sera, which are known to recognise an SDS labile epitope, the sequence 77-82 might be a part of a larger discontinuous epitope. Alternatively the mAb 35/312 blocks the binding of human anti-HBc by steric hindrance.

The hepatitis B virus

Of all the hepatotropic viruses, HBV is associated with the greatest worldwide morbidity and mortality. This is because of the ease of transmission and the potential for progression to a chronic infective carrier state, with the complications of cirrhosis and hepatocellular carcinoma. The use of PCR has shown that some of the earlier concepts concerning the interpretation of serological data were inaccurate. Many patients with anti-HBe and anti-HBs have viral DNA detectable by PCR, and some hepatocellular carcinoma patients have detectable HBV DNA in their livers in the absence of all serological markers of HBV disease. The clearance of HBV infected cells from the liver is dependent on the interplay between the interferon system and the cellular limb of the host immune response. The importance of the nucleocapsid proteins as targets for sensitized cytotoxic T cells has been established for chronic HBV infection. The importance of pre-S sequences as inducers and targets of the virus-neutralizing humoral immune response is becoming established, but their precise role must await the development of in vitro models of hepadnavirus infection and a greater understanding of the mechanisms of viral uptake. The epidemiology and clinical course of the disease can be modified by immunization, immune stimulation and antiviral chemotherapy. For the developing world, a programme of immunization at birth would be the most effective way of eliminating this disease, but at present the cost is prohibitive. For the developed world, immunization is realistic for the at-risk population, and anti-viral and immunostimulatory therapy available for those already infected. In adult acquired chronic HBV infection alpha-interferon produces HBe antigen clearance in 40-60% of cases and is followed by resolution of the hepatic inflammation. Results in neonatally acquired infection are less impressive and prednisolone priming followed by interferon may be needed. The presence of a mutation in the pre-core region of some virus isolates has recently been described. Hepatocytes infected with this virus cannot produce HBe antigen and the course of the liver disease is fairly rapid. Whether this mutant causes liver damage in the same way as the wild virus or is directly cytopathic remains unclear, and its relationship to fulminant hepatitis is under investigation.

Comparison of class and subclass distribution of antibodies to the hepatitis B core and B e antigens in chronic hepatitis B

The IgG subclasses IgM and IgA1 of antibodies to hepatitis B core antigen (anti-HBc) and hepatitis B e antigen (anti-HBe) were assayed in sera from 82 patients with chronic hepatitis B utilising class/subclass-specific enzyme immunoassays (EIA). The solid-phase was either recombinant hepatitis B core antigen (rHBcAg) or rHBcAg converted to HBeAg by addition of 0.1% SDS with remaining HBcAg antigenicity blocked with monoclonal anti-HBc. Anti-HBc IgG1 was detected in 81 sera at a geometrical mean titre (GMT) of 296,110 x divided by 2.9. Anti-HBc IgG2 was not detected in any of the sera, and anti-HBc IgG3 and IgG4 were detected in 50 and 37 sera, respectively. Anti-HBc IgM and IgA1 were both significantly correlated to the presence of HBV DNA. The predominant antibody to HBeAg was found to be IgG1, being detected in 45 sera with a GMT of 1,035 x divided by 3.3. Anti-HBe IgG2 was not detected in any serum, while anti-HBe IgG3 and IgG4 were found in 8 and 23 sera, respectively. Anti-HBe IgG1, IgG3, and IgG4 were mainly detected in sera positive for anti-HBe in RIA (Abbott). No patient was found positive for anti-HBe IgA1 or IgM. Thus, in contrast to HBcAg, HBeAg does not trigger a persistent IgM and IgA1 response in chronic hepatitis B. The levels of anti-HBe IgG1 and IgG3 were much lower than the levels of anti-HBc IgG1 and IgG3. The presence of anti-HBe IgG4 was significantly correlated to that of anti-HBc IgG4.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoglobulin isotypes of anti-HBc and anti-HBe and hepatitis B virus (HBV) DNA elimination in acute hepatitis B

Antibodies to hepatitis B core antigen (anti-HBc) and e antigen (anti-HBe) were assayed in 46 sera from ten patients with acute hepatitis B utilizing immunoglobulin class- and subclass-specific enzyme immunoassays (EIAs). The sera were sampled 1 to 512 days after onset of hepatic symptoms. Four patients cleared HBsAg rapidly, within 24 days, and six patients cleared HBsAg slowly, within 27-74 days after the onset of symptoms. In three of the patients with rapid clearance of HBsAg, hepatitis B virus (HBV) DNA was not detected in sera tested during the first week after onset. The fourth patient was not tested until 12 days after onset and was then found to be negative for HBV DNA. In four of the patients with slow clearance of HBsAg, HBV DNA was present during the first week of illness. In the other two patients, HBV DNA was not detected in the first serum, 11 and 17 days after the onset of illness. Anti-HBc IgM and IgA1 were detected in all patients, with maximum titers shortly after onset. Anti-HBc IgG1 was present in all sera tested. Anti-HBc IgG2 was not detected in any of the sera. Anti-HBc IgG3 and IgG4 were detected in all patient sera, with IgG3 paralleling IgG1, and IgG4 mainly in sera long after onset. Anti-HBe IgG1, IgG3, and IgG4 were detected in three, two, and two patients, respectively. Anti-HBe IgG2, IgM, IgA1, or IgA2 was not found in any patient. The time required for maximum titer of anti-HBc IgG1 was shorter in the patients with rapid clearance of HBsAg.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of antibody to hepatitis B core antigen (anti-HBc) using a direct (antiglobulin) format and development of a confirmatory assay for anti-HBc

A direct (antiglobulin) solid-phase enzyme immunoassay for the detection of antibody to hepatitis B core antigen (anti-HBc) is described. The assay utilizes recombinant hepatitis B core antigen as the solid-phase 'capture' reagent and a mixture of monoclonal antibodies specific for human IgG and IgM conjugated to horseradish peroxidase as the 'detector' reagent. The direct assay demonstrated excellent sensitivity and specificity when compared with a commercially available competitive enzyme immunoassay. The direct assay format lends itself to a confirmatory assay for anti-HBc by addition of monoclonal anti-HBc to the reaction mixture. Feasibility of the confirmatory assay for anti-HBc was demonstrated using specimens reactive for anti-HBc as documented by both the direct and competitive assays.

The Leeuwenhoek lecture, 1985. A molecular biologist's view of viral hepatitis

Three forms of viral hepatitis can be distinguished serologically. Hepatitis A virus is a picornavirus, which is being studied increasingly after its propagation in cell cultures. The B virus (HBV) is the prototype of a family now termed hepadna viruses and is by far the best understood. The third, by exclusion, is non-A non-B, about which little else is known. Molecular cloning methods enable copies of viral genes to be propagated and analysed quite readily and provide the means for isolation and expression of individual genes in microbial and animal cells. Determination of the nucleotide sequences of HBV DNA revealed its genetic organization and so guided studies of the mechanism of gene expression both in infected animals and cultures of transformed cells. Replication of the viral genome has also been studied in natural infections, particularly with duck HBV, but also with the human virus. Expression of HBV genes in microbial cells is valuable as a source of antigens for diagnostic reagents and vaccine preparations, but has also been of consequence for the identification of viral gene products not previously recognized and which are of considerable current interest. The methods and materials now available provide additional opportunities for inquiring into the course of viral infection, replication of the virus and, for HBV, the possible role in the development of hepatomas of integration of the viral genome into the host chromosome.

Expression and characterization of hepatitis B virus precore-core antigen in E. coli

The hepatitis B virus core antigen, including the precore sequence (HBcAg-p25), was expressed at very high levels in bacteria. Three expression vectors were constructed in which the synthesis of HBcAg-p25 was controlled by the tac promoter, and the number of nucleotides between the bacterial ribosome binding site and the precore initiation codon was varied in order to maximize HBcAg-p25 synthesis. The relative amount of HBcAg-p25 polypeptide expressed by the different vectors was estimated by SDS-polyacrylamide gel electrophoresis and immunoblot. HBcAg-p25 was associated with an insoluble fraction of bacterial extracts and required ionic detergents for solubilization. Comparison by ELISA of the immunoreactivity of HBcAg with and without the precore sequence suggested that human anti-HBcAg IgG preferentially recognizes HBcAg lacking the precore sequence.