Hepatitis B virus infection in microcarrier-attached immortalized human hepatocytes cultured in molecularporous membrane bags: a model for long-term episomal replication of HBV

Studies on the pathobiology of chronic (long-term) hepatitis B virus (HBV) infection and in vitro drug testing have been hampered by the lack of appropriate systems for culturing susceptible cells chronically infected with HBV. Most of the in vitro studies of HBV replication have been performed with HBV genome-transduced cell lines. In this system, viral production is mainly the result of chromosomal replication. In an in vitro infection system, owing to medium refreshment (which leads to the removal of infectious particles necessary for the perpetuation of infection) and to trypsinization for cell passages, it is difficult, if not impossible, to maintain chronic HBV infection, despite the use of susceptible cells. To circumvent these unfavourable factors for chronic HBV infection in vitro, we cultured microcarrier-attached immortalized human hepatocytes, infected with HBV, in molecularporous (MW 12,000-14,000) membrane (dialysis) bags for a duration of 2 months. HBV covalently-closed-circular (ccc) DNA, HBV precore/core and X mRNAs were detected in the cells cultured in this system following infection until the end of the experiment (day 58), while in classical culture conditions (monolayer), markers of HBV replication were also detected. Production of hepatitis B surface antigen (HBsAg) and HBV DNA was detected and their levels in culture medium (collected at the end of experiments from the molecularporous membrane bags) were increased 2.86- and 3.28-fold respectively. Using Southern blot analysis, HBV replicative intermediates could also be demonstrated throughout the experiments. However, integrated HBV DNA was not present. In contrast, HBV ccc DNA, HBV precore/core and X mRNAs, and replicative intermediates were not demonstrable in FTO 2B rat hepatoma cells infected in the same manner in parallel experiments. This in vitro infection system, using susceptible, immortalized human hepatocytes, therefore provides a new tool for studying the long-term effect of HBV infection, mainly involving episomal replication in hepatocytes, and for drug testing.

Hepatitis B virus replication in human HepG2 cells mediated by hepatitis B virus recombinant baculovirus

A novel transient mechanism for studying hepatitis B virus (HBV) gene expression and replication using recombinant HBV baculovirus to deliver the HBV genome to HepG2 cells was generated. In HBV baculovirus infected HepG2 cells, HBV transcripts, and intracellular and secreted HBV antigens are produced; replication occurs as evidenced by the presence of high levels of intracellular replicative intermediates and protected HBV DNA in the medium. Density-gradient analysis of extracellular HBV DNA indicated that the DNA was contained predominantly in enveloped HBV virions. Covalently closed circular (CCC) DNA is present indicating that, in this system, HBV core particles are capable of delivering newly synthesized HBV genomes back into the nuclei of infected cells. HBV gene expression is driven exclusively from endogenous promoters. Levels of HBV gene expression and replication can be achieved in HBV baculovirus-infected HepG2 cells which far exceed levels found in HepG2 2.2.15 cells. HBV baculovirus infection of HepG2 cells lends itself readily to experimental manipulation as follows: 1) HBV expression can be initiated any time relative to seeding of HepG2 cells; 2) levels of HBV replication can be regulated over a wide range simply by changing the baculovirus multiplicity of infection; 3) HBV replication is readily detectable by one day post infection with HBV baculovirus and persists at least through day eleven post infection; and (4) the transient nature of the infection can be extended and/or enhanced by superinfecting the cultures. We conclude that infection of HepG2 cells by HBV recombinant baculovirus represents a simple to use and highly flexible system for studying the effects of antivirals and/or cytokines on HBV production and for understanding HBV replication and pathogenesis at the molecular level.

Different vaccine strategies used to protect against Theileria annulata

SPAG-1, a sporozoite surface antigen of T. annulata, has previously been shown to elicit partial protection when used, as an hepatitis B core antigen fusion, to immunize cattle. The objective of this study was to try and improve the protective capacity of this antigen by enlisting different vaccine strategies. Cattle were immunized with SPAG-1, as a fusion protein with a His6 tag, either incorporated into ISCOMs, with or without the merozoite antigens TAMS 1-1 and 1-2, or with RWL as adjuvant three times at monthly intervals. Another group of cattle were immunized with p67, the T. parva sporozoite antigen, in RWL to assess whether any cross-protection could be induced. The animals were then challenged with an estimated LD50 of T. annulata sporozoites, and their ability to resist the infection was investigated. Serum responses and T-cell proliferative responses were analyzed throughout the trial. Post-challenge analyses included lymph node biopsies and blood smears to check for the presence of parasites, routine hematological parameters, and observation for clinical manifestations of the disease. The results of this trial will be discussed.

Efficient conditional transgene expression in hepatitis C virus cDNA transgenic mice mediated by the Cre/loxP system

Conditional gene expression has greatly facilitated the examination of the functions of particular gene products. Using the Cre/loxP system, we developed efficient conditional transgene activation of hepatitis C virus (HCV) cDNA (nucleotides 294-3435) in transgenic mice. Efficient recombination was observed in transgenic mouse liver upon intravenous administration of adenovirus that expresses Cre DNA recombinase. After transgene activation, most hepatocytes were stained with anti-core polyclonal antibody, and 21-, 37-, and 64-kDa proteins were detected by Western blot analysis in liver lysates using anti-core, E1, and E2 monoclonal antibodies, respectively. Serum core protein was detected in transgenic mice 7 days after transgene activation with concurrent increases in serum alanine aminotransferase levels. Subsequently, an anti-core antibody response was detected 14 days after infection. Furthermore, a CD4 and CD8 positive cell depletion assay normalized both the serum alanine aminotransferase increases and pathological changes in the liver. These results suggest that HCV proteins are not directly cytopathic and that the host immune response plays a pivotal role in HCV infection. Thus, this HCV cDNA transgenic mouse provides a powerful tool with which to investigate the immune responses and pathogenesis of HCV infection.

Differential regulation of the pre-C and pregenomic promoters of human hepatitis B virus by members of the nuclear receptor superfamily

Synthesis of the pre-C and pregenomic RNAs of human hepatitis B virus (HBV) is directed by two overlapping yet separate promoters (X. Yu and J. E. Mertz, J. Virol. 70:8719-8726, 1996). Previously, we reported the identification of a binding site for the nuclear receptor hepatocyte nuclear factor 4 (HNF4) spanning the TATA box-like sequence of the pre-C promoter. This HNF4-binding site consists of an imperfect direct repeat of the consensus half-site sequence 5'-AGGTCA-3' separated by one nucleotide; i.e., it is a DR1 hormone response element (HRE). We show here that other receptors, including chicken ovalbumin upstream promoter transcription factor 1 (COUP-TF1), human testicular receptor 2 (TR2), and peroxisome proliferator-activated receptors (PPARs) as heterodimers with retinoid X receptors (RXRs), can also specifically bind this DR1 HRE. Synthesis of the pre-C and pregenomic RNAs was affected both in transfected hepatoma cells and in a cell-free transcription system by the binding of factors to this DR1 HRE. Interestingly, whereas some members of the hormone receptor superfamily differentially repressed synthesis of the pre-C RNA (e.g., HNF4 and TR2) or activated synthesis of the pregenomic RNA (e.g., PPARgamma-RXRalpha), other members (e.g., COUP-TF1) coordinately repressed synthesis of both the pre-C and pregenomic RNAs. Thus, HBV likely regulates its expression and replication in part via this DR1 HRE. These findings indicate that appropriate ligands to nuclear receptors may be useful in the treatment of HBV infection.

[Transgenic plants as medicine production systems]

Transgenic plants are emerging as an important system for the expression of many recombinant proteins, especially those intended for therapeutic purpose. The production of foreign proteins in plants has several advantages. In terms of required equipment and cost, mass production in plants is far easier to achieve than techniques involving animal cells. Successful production of several proteins in plants, including human serum albumin, haemoglobin, monoclonal antibodies, viral antigens (vaccines), enkephalin, and trichosanthin, has been reported. Particularly, the demonstration that vaccine antigens can be produced in plants in their native, immunogenic forms opens exciting possibilities for the "bio-farming" of vaccines. If the antigens are orally active, food-based "edible vaccines" could allow economical production. In this review, I will discuss the progress that has been made by several groups in what is now an expanding area of medicine research that utilizes transgenic plants.

Mosaic hepatitis B virus core particles allow insertion of extended foreign protein segments

Because of its particular immunological properties, the core protein of hepatitis B virus (HBcAg) has become one of the favoured 'virus-like particles' for use as a carrier of foreign epitopes. A new strategy to construct core particles presenting extended foreign protein segments was established based on the introduction of a linker containing a translational stop codon between sequences encoding a C-terminally truncated HBcAg (HBcAg delta) and a foreign protein sequence. Expression in an Escherichia coli suppressor strain allowed the simultaneous synthesis of both HBcAg delta and a read-through fusion protein containing a part of the hantavirus nucleocapsid protein. After purification, the presence of core-like mosaic particles with HBc and hantavirus antigenicity was demonstrated by electron microscopy and immunological tests. This strategy of partial stop codon suppression should improve the use of HBcAg as a carrier of foreign epitopes by allowing insertion of long foreign sequences into particle-forming proteins. The resulting mosaic particles should be of general interest for further vaccine developments.

Simple method for efficient production of hepatitis B virus core antigen in Escherichia coli

To obtain good antigenicity and high purity of the hepatitis B virus core antigen (HBcAg) in large quantities without using the fused protein technique employed in recombinant DNA technology, a protein molecule with the same primary sequence as that of wild-type HBcAg (subtype adr) was directly expressed in Escherichia coli JM109 (DE3) using pGd1 expression vector. Purification of the expressed HBcAg yielded high-quality protein by means of simple purification steps, such as sonication, ammonium sulphate precipitation and heat treatment, before final purification by conventional ultra-centrifugation. The HBcAg preparation thus obtained contains small round particles similar in appearance to the HBcAg particles from the HBV-infected human liver tissue.

Reduced antigen production by hepatitis B virus harbouring nucleotide deletions in the overlapping X gene and precore-core promoter

Hepatitis B virus (HBV) genomes with deletions in the precore-core (preC-C) promoter have been detected in HBV infections without serological markers. To address whether the mutations are responsible for the reduced production of virus antigenes, either an 8 bp (8d, position 1763 to 1770) or a 20 bp (20d, 1753 to 1772) deletion was created in a wild-type (wt) HBV clone. Both mutations cause premature termination of the overlapping X ORF. When introduced into HepG2 cells, both mutants produced reduced amounts of HBsAg, HBcAg and HBeAg, but released the same or more virion-associated DNA compared with the wt. A co-transfection of the 20d mutant with a small amount of intact X gene resulted in a 3-fold increase of HBcAg production compared to transfection with either the 20d or wt alone. When the promoter region was cloned into CAT plasmids, the 8d preC promoter showed weak activity and its initiation site was shifted 6 to 10 bp downstream. The preC promoter activity of 20d was not detectable by CAT ELISA and 5' RACE. The levels of C transcripts of both mutants were higher than that of the wt, and their start sites were not altered. Therefore, the deletions cause the reduction of HBsAg, HBcAg and HBeAg although the mutant viruses can still replicate in cultured cells. The reduction of HBeAg is due to both the reduced preC promoter activity and the defect in HBx. The reduction of HBcAg is due to the disrupted X gene, despite augmented C promoter activity.

Relative functional affinity of specific anti-core IgG in different categories of hepatitis B virus infection

While resolution of hepatitis B virus (HBV) infection occurs in most cases, a carrier state can exist in which the HBV surface antigen (HBsAg) persists. Some carriers are also positive for the HBV "e" antigen (HBeAg), indicative of high viral replication. Others are HBV "e" antibody (anti-HBe)-positive carriers in whom there appears to be a fall in the level of viral replication with the appearance of antibodies against the "e" antigen. The former group of carrier is considered to be at a higher risk of transmitting HBV infection than the latter. In order that a carrier state may occur, some degree of tolerance to the infectious agent must exist. A study of the rate of increase of specific antibody avidity following infection provides a means of assessing the maturity of the immune response to an infectious agent. Since antibodies specific for the HBV core antigen (HBcAg) are produced in almost all cases of HBV infection and the HBeAg and HBcAg share a large number of amino acids and some B- and T-cell epitopes, the increase in the avidity of antibodies against the HBV core antigen (anti-HBc) in cases of acute, resolving HBV infection and in HBV carriers has, therefore, been studied. An increase in the avidity of specific antibody, similar to that seen in other viral infections, was observed following acute, resolving infection. However, low avidity antibody persisted longer in carriers who remained positive for HBeAg, whereas in cases where there were antibodies specific for HBeAg, the anti-HBc antibody was of high avidity. Analysis of sequential sera from carriers who seroconverted from HBeAg-positive to anti-HBe-positive showed that an increase in anti-core avidity could predate seroconversion from HBeAg-positive to anti-HBe-positive status. Thus, anti-HBc avidity studies may be of diagnostic and prognostic significance.

Mutated epitopes of hepatitis B surface antigen fused to the core antigen of the virus induce antibodies that react with the native surface antigen

Fusion of peptide epitopes to the core antigen (HBcAg) of hepatitis B virus (HBV) enhances their immunogenicity, both quantitatively and qualitatively. In a number of vaccine-induced mutants of HBV, glycine145 of the surface antigen S polypeptide (HBsAg) has been replaced by arginine, resulting in loss of cross-reactivity with antibodies to normal (wild-type) HBsAg. HBcAg fusion proteins carrying the immunodominant epitope of HBsAg, in which glycine145 was replaced by arginine, glutamic acid, or lysine, were produced in Escherichia coli and formed particles that displayed HBc antigenicity and immunogenicity similar to that of HBcAg itself. The fusion proteins also elicited T-cell proliferative responsiveness to HBcAg and HBsAg. Fusions carrying either wild-type or mutated epitopes of HBsAG showed HBs antigenicity in immunoblot analysis and antigen-capture immunoradiometric assay, but both mutant and wild-type derivatives induced antibodies that cross-reacted with wild-type HBsAG. The results emphasise the potential for HBcAg fusion proteins in vaccines by broadening the antibody response in a way that could confer protection against both wild-type and variant form of HBV.

Immunization with hybrid hepatitis B virus core particles carrying circumsporozoite antigen epitopes protects mice against Plasmodium yoelii challenge

The hepatitis B virus nucleocapsid antigen (HBcAg) was investigated as a carrier moiety for circumsporozoite protein (CS) repeat B cell epitopes of the rodent malaria agent Plasmodium yoelii. A vector expressing a hybrid gene coding for the dominant CS repeat epitope (QGPGAP)4 was constructed and transformed into avirulent Salmonella typhimurium. The resulting hybrid HBcAg-CS polyproteins were purified from recombinant Salmonella typhimurium. They purified as particles and displayed HBc as well as P. yoelii CS antigenicity. To investigate immunogenicity and protective efficacy, BALB/c mice were immunized with the hybrid HBcAg-CS particles. Immunization resulted in high titered antinative CS serum IgG antibody litres. BALB/c mice immunized with hybrid HBcAgCS particles were between 90-100% protected against subsequent P. yoelli challenge. Protective immunity persisted for a minimum of three months. These data confirm the previous suggestion (Schödel et al., 1994), that hybrid HBcAg particles could become a useful component of future human malaria vaccines.

Enhanced core protein production by hepatitis B virus bearing a mutation in the precore region

An HBV DNA tandem dimer bearing GTG instead of the precore initiation codon (p2WPC-), that bearing an amber mutation at precore codon 28 (p2WPCTer) and that of a wild-type were introduced into HepG2 cells. p2WPC- produced no HBeAg, the same amount of HBsAg as the wild-type, and 2-4 times as much HBcAg and progeny virus DNA. p2WPCTer produced no HBeAg, the same amount of HBsAg, 2 times as much HBcAg and a slightly increased amount of progeny virus. The amounts of p21c peptide in both mutants determined by immunoblotting correlated well with the ELISA titers. These results suggest that these precore single point mutations are responsible for the enhanced core peptide production.

Expression of a human cytomegalovirus gp58 antigenic domain fused to the hepatitis B virus nucleocapsid protein

Hepatitis B virus core antigen (HBcAg) has been used as a carrier for expression and presentation of a variety of heterologous viral epitopes in particulate form. The aim of this study was to produce hybrid antigens comprising HBcAg and an immunogenic epitope of human cytomegalovirus (HCMV). A direct comparison was made of amino and carboxyl terminal fusions in order to investigate the influence of position of the foreign epitope on hybrid core particle formation, antigenicity and immunogenicity. HCMV DNA encoding a neutralising epitope of the surface glycoprotein gp58 was either inserted at the amino terminus or fused to the truncated carboxyl terminus of HBcAg and expressed in Escherichia coli. The carboxyl terminal fusion (HBc3-144-HCMV) was expressed at high levels and assembled into core like particles resembling native HBcAg. Protein with a similar fusion at the amino terminus (HCMV-HBc1-183) could not be purified or characterised immunologically, although it formed core like particles. HBc3-144-HCMV displayed HBc antigenicity but HCMV antigenicity could not be detected by radioimmunoassay or western blotting using anti-HCMV monoclonal antibody 7-17 or an anti-HCMV human polyclonal antiserum. Following immunisation of rabbits with HBc3-144-HCMV, a high titre of anti-HBc specific antibody was produced along with lower titres of HCMV/gp58 specific antibody.

Hepatitis B virus core-preS2 particles expressed by recombinant vaccinia virus

Vaccinia virus (VV) recombinants expressing hepatitis B virus (HBV) surface (HBsAg) or core (HBcAg) antigens (Kunke et al., Virology 195, 132 - 139 (1993)] have been shown to raise specific antibodies in mice, nevertheless the levels of antibodies reactive with the preS2 and S antigens were low. In an attempt to enhance the immunogenicity of HBsAg-preS2, a fused C-preS2 gene was constructed. The fusion protein was expressed in E. coli and displayed both HBcAg and preS2 antigen as demonstrated by enzyme-linked immunosorbent assay (ELISA). The same gene was then expressed using recombinant VV and chimerical particles whose size and density were similar to those of native HBV core particles produced in CV-1 cells infected with recombinant VV. Unlike HBcAg, preS2 antigen could not be detected on these particles by ELISA but was revealed by immunoblot analysis only. The immunogenicity of the recombinant VV was evaluated in mice. Antibodies to HBcAg and VV antigen but not to preS2 antigen were found in sera of animals inoculated with 10(7) PFU of the recombinant VV. Presumably, HBcAg-preS2 particles produced in E. coli and in eukaryotic cells have a different conformation, and the presence of preS2 antigen on the surface of chimerical particle might be necessary for a pronounced antibody response.

Effects of a naturally occurring mutation in the hepatitis B virus basal core promoter on precore gene expression and viral replication

The basal core promoter (BCP) of hepatitis B virus (HBV) controls the transcription of both the precore RNA and the core RNA. The precore RNA codes for the secreted e antigen, while the core RNA codes for the major core protein and the DNA polymerase and also is the pregenomic RNA. The double mutation of nucleotides 1762 and 1764 in the BCP from A and G to T and A, respectively, is frequently observed in HBV sequences isolated from chronic patients. Several papers have reported conflicting results regarding whether this double mutation is important for e antigen expression. In order to address this issue, we have introduced this double mutation into the HBV genome and studied its effects on HBV gene expression and replication. Our results indicate that the mutated BCP can no longer bind a liver-enriched transcription factor(s) and that the transcription of only precore RNA and, consequently, the expression of e antigen were reduced. The reduction of precore gene expression was accompanied by an increase in progeny virus production. This increase was found to occur at or immediately prior to the encapsidation of the pregenomic RNA. Thus, the results of our in vitro study resolve the discrepancy of previous clinical observations and indicate that this double mutation suppresses but does not abolish the e antigen phenotype. The implications of these findings in the pathogenesis of HBV are discussed.

Design of immunogens as components of a new generation of molecular vaccines

Three new approaches to design effective immunogens are considered. At first, we derived an expression vector from bacteriophage M13 allowing the exposure of short peptides on the virion surface. EIA demonstrates that antibodies against a recombinant phage carrying the antigenic determinant of the HIV-1 gag protein reacted with the 17-kDa core protein of the virus and also with its polyprotein precursor p55 in immunoblotting. In another approach, we chose the hepatitis B core antigen (HBcAg) particle as a vehicle for the presentation of foreign antigenic determinants to the immune system. Chimerical particles of HBcAg containing epitope of the VEE virus were obtained. A vector system for insertion of foreign antigenic determinants and production of both hybrid and wild HBcAg proteins were also obtained. The third approach relies on construction of immunogens from different T- and B-cell epitopes of the HIV-1. We suggested to construct HIV-1 vaccines in a form of the TBI (T- and B-cell epitopes containing Immunogen) with a predetermined tertiary structure, namely, a four-alpha-helix bundle. The gene of the TBI protein consisting of nine HIV-1 epitopes was synthesized and expressed in Escherichia coli cells. Mice immunized with TBI showed humoral and cellular immune responses to HIV-1. Anti-TBI antibodies displayed HIV-1 neutralizing activity. These new approaches offer promise in the development of new effective vaccines.

Antisense oligonucleotides are effective inhibitors of hepatitis B virus replication in vitro

Antisense oligonucleotides are currently being used in numerous laboratories as potential anticancer and antiviral agents. The unique replication cycle of hepatitis B virus (HBV) contains several different steps which are potentially amenable to modulation by these molecules. We have examined the ability of 56 different single-stranded, oligodeoxyribonucleotides (14-23 nucleotides in length), which target several HBV-specific functions, to inhibit HBV replication in the human hepatoblastoma cell line, 2.2.15. None of the oligonucleotides examined were toxic at concentrations up to 500 microM. Oligonucleotides directed against the HBV surface antigen (HBsAg) gene (S gene), the preS1 open reading frame, and the HBV core antigen (HBcAg) gene (C gene) were effective at depressing virus production, while molecules targeting the HBV e antigen (HBeAg) open reading frame and the HBV polymerase (POL) gene were ineffective. Oligonucleotides directed against the HBV encapsidation signal/structure (epsilon) comprised some of the most effective antiviral molecules against HBV. None of 5 oligonucleotides complementary (i.e., 'sense' orientation) to the antisense oligonucleotides targeting HBsAg, HBcAg, HBeAg, preS1 and POL had any measurable effect on HBV production. The relative effectiveness of oligonucleotides targeting the S and C genes on HBV replication was highly correlated with an effect on HBsAg or HBcAg levels, respectively. None of the antisense oligonucleotides examined affected either the levels or the sizes of HBV-specific RNA transcripts. Since antisense oligonucleotides can exert biologic effects on HBV in 2.2.15 cell cultures in a sequence-specific manner which are consistent with predicted modes of action, such molecules may have practical applications in the therapy of chronic HBV infection.

In vivo activity of the hepatitis B virus core promoter: tissue specificity and temporal regulation

The contribution of the hepatitis B virus enhancers I and II in the regulation of the activity of the core and the X promoters was assessed in transgenic mice. Surprisingly, despite the presence of heterologous promoters linked 5' of the X gene, the transgene expression is mostly due to core promoter (Cp) activity present in the X coding sequence. Moreover, the restriction of Cp activity to hepatic tissue required the combined action of both enhancers I and II, whereas the proximity of these two enhancers was insufficient to confer tissue specificity on Xp activity. Furthermore, the liver-specific activity of the Cp was developmentally regulated in an enhancer I-independent manner.

Deletion of integrated hepatitis B virus genome and cellular flanking sequences in hepatocellular carcinoma cells in BALB/c mice

We have previously reported the establishment of well-differentiated BALB/c mouse liver (ML) cell lines. Transfection of these cell lines with hepatitis B virus (HBV) DNA led to the expression of HBV-specific antigens and integration of HBV sequences in the cellular genome. Two cloned HBV-transfected ML cell lines, ML-2(HBV) and ML-3(HBV), expressed viral antigens and were highly tumorigenic in nude mice. However, the tumorigenicity of the two cell lines was significantly reduced in BALB/c mice. Southern blot analyses showed that the integrated HBV sequences were retained in tumors growing in nude mice but deleted in tumors growing in BALB/c mice. Furthermore, the deletion of HBV DNA was accompanied by deletion of chromosomal sequences flanking the HBV integration sites. In ML-2(HBV) cells, a significant reduction in chromosomal number was also observed. These results suggest that the immune response of BALB/c mice selected against hepatocellular carcinoma (HCC) cells expressing viral antigens and led to the proliferation of cells with deleted HBV sequences and concomitant chromosome aberrations. By using this mechanism, HCC cells escape the immune surveillance and gain the advantage of cell growth.

Hepatitis core antigen produced in Escherichia coli: subunit composition, conformational analysis, and in vitro capsid assembly

The production and biochemical and physiocochemical analysis are described of recombinant-produced hepatitis B virus core antigen (HBcAg capsid) and the corresponding particle produced by a deletion mutant missing the C-terminal 39 residues (HBeAg). Conditions for producing HBeAg from HBcAg capsids by in vitro proteolysis are also described. The morphology and masses of these capsids were determined by scanning transmission electron microscopy. Both HBcAg and HBeAg capsids comprise two size classes that correspond to icosahedral lattices with triangulation numbers (T) of 3 and 4, containing 180 and 240 subunits per capsid, respectively. This dimorphism was confirmed by sedimentation equilibrium and sedimentation velocity measurements on a Beckman Optima XL-A analytical ultracentrifuge. More than 60% of HBcAg capsids were T = 4, whereas only 15-20% of HBeAg capsids were of this size class: the remainder, in each case, were T = 3. Circular dichroism and Raman spectroscopy were used to determine the overall secondary structures of HBcAg and HBeAg capsids. Both have high alpha-helical contents, implying that this capsid protein does not conform to the canonical beta-barrel motif seen for all plant and animal icosahedral viral capsids solved to date. We suggest that the C-terminal domain of HBcAg has a random coil conformation. In vitro dissociation of HBeAg capsids under relatively mild conditions yielded stable dimers. The reassociation of HBeAg dimers into capsids appears to be driven by hydrophobic processes at neutral pH. Capsid assembly is accompanied by little change in subunit conformation as judged by circular dichroism and fluorescence spectroscopy. The thermal stability of HBcAg capsids was compared calorimetrically with that of in vitro assembled HBeAg capsids. Both have melting temperatures > 90 degrees C, implying that the C-terminal region makes little difference to the thermal stability of HBcAg: nevertheless, we discuss its possible role in facilitating disassembly and the release of viral nucleic acid.

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.

Assembly and antigenicity of hepatitis B virus core particles

Recent studies in Xenopus oocytes and other systems have led to an understanding of the HBV capsid, or core particle, assembly process. Nascent HBV core polypeptides rapidly dimerize. Accumulation of free dimers to a signature concentration (approximately 0.8 microM) then triggers a highly cooperative capsid assembly reaction. This dimer-to-capsid transition is accompanied by a switch from HBe to HBc antigenicity and appears to be nucleated by interaction between core protein and RNA: deletion of a protamine-like RNA binding domain at the C-terminus of the core protein markedly increases the concentration of dimers needed to drive capsid assembly. The simple assembly pathway seen for HBV capsids mirrors that of R17 bacteriophage.

Hybrid hepatitis B virus core-pre-S proteins synthesized in avirulent Salmonella typhimurium and Salmonella typhi for oral vaccination

Avirulent salmonellae expressing foreign genes are attractive for use as oral vaccine carriers. To facilitate the stable expression of heterologous genes without conferring antibiotic resistance, a deletion of the asdA1 gene was introduced into Salmonella typhimurium and S. typhi delta cya delta crp mutant vaccine strains. An asd-complementing plasmid expressing hybrid hepatitis B virus nucleocapsid-pre-S (HBcAg-pre-S) particles was constructed. These hybrid HBcAg-pre-S particle genes were stably expressed in S. typhimurium and S. typhi delta cya delta crp mutant vaccine strains in this balanced, lethal host-vector combination. A single oral immunization of BALB/c mice with a recombinant S. typhimurium delta cya delta crp mutant synthesizing hybrid HBcAg-pre-S elicited potentially virus-neutralizing anti-pre-S serum immunoglobulin G antibodies. In addition, serum immunoglobulin G recognizing S. typhimurium lipopolysaccharide was induced. Distribution in tissue after oral immunization was analyzed in one plasmid-strain combination. The recombinant S. typhimurium colonized the gut-associated lymphoid tissue and the spleen and persisted for over 4 weeks, retaining the HBcAg-pre-S expression plasmid. An isogenic virulence plasmid-cured S. typhimurium delta cya delta crp strain expressing the same HBcAg-pre-S gene had reduced immunogenicity for the carried antigen after oral immunization.

Comparison of three different recombinant hepatitis B virus core particles expressed in Escherichia coli

The properties of three different recombinant hepatitis B virus core proteins expressed in Escherichia coli were compared: an N-terminal fusion protein, a C-terminally truncated protein and a sequence-authentic protein. All three proteins assembled into capsid-like particles with typical HBc-antigenicity, sedimentation behavior and distinctive electron microscopical images. Apart from this, however, variant HBc proteins displayed properties different from sequence-authentic HBc protein p21.4. Unlike p21.4, the particles of the N-terminal fusion protein p22.2 were sensitive to proteolytic attack by trypsin at variable sites within its arginine-rich C-terminus but not in its extended N-terminus. We therefore conclude that the C-terminal region is located on the surface of the p22.2 particle. These particles also showed increased HBe-antigenicity, as did the C-terminally truncated core particles p17.6, and to an even greater extent p18* particles which were derived from p22.2 by tryptic digestion. This might be interpreted as evidence for an--albeit minor--structural change. All variant core particles were less stable and contained less RNA. Electron microscopic indication for DNA binding of C-terminal deleted p17.6 particles was obtained using an aqueous spreading technique.

Immune reactions against hepatitis B viral antigens lead to the rejection of hepatocellular carcinoma in BALB/c mice

Human hepatitis B virus (HBV) is closely associated with hepatocellular carcinoma. However, the mechanism of carcinogenesis and the immune responses to HBV infection and hepatocellular carcinoma are not clearly understood. Recently, we established BALB/c mouse liver (ML) cell lines and demonstrated that transfection of ML cell lines with HBV dimer DNA resulted in the expression of HBV antigens (1). The HBV-transfected ML cells and the parental ML cells showed similar tumorigenicity in nude mice. However, the HBV-transfected cells had much lower tumorigenicity in BALB/c mice. Similar results were also obtained in two cloned ML cell lines, ML-1.1 and ML-1.2, transfected with plasmid DNA containing HBs, HBc, or HBx gene. Furthermore, adoptive transfer of spleen cells from BALB/c mice immunized with HBsAg- or HBcAg-expressing ML-1.1 cells caused regression of tumor cells expressing the corresponding antigens in nude mice. In addition, transfer of spleen cells from BALB/c mice immunized with purified HBsAg or HBcAg also caused tumor regression. These results demonstrate that HBsAg and HBcAg can induce immunity which leads to the rejection of hepatocellular carcinoma in vivo.

Chimeric hepatitis B virus core particles with parts or copies of the hepatitis C virus core protein

Either parts or multiple copies of the core gene of hepatitis C virus (HCV) were fused to the 3' terminus of the hepatitis B virus (HBV) core gene with 34 codons removed. As many as four copies of HCV core protein (720 amino acids) were fused to the carboxy terminus of truncated HBV core protein (149 amino acids) without preventing the assembly of HBV core particles. Chimeric core particles were sandwiched between monoclonal antibody to HBV core and that to HCV core, thereby indicating that antigenic determinants of both HBV and HCV cores were accessible on them. Proteolytic digestion deprived chimeric core particles of the antigenicity for the HCV core without affecting that of the HBV core, confirming the surface exposure of HCV core determinants. The density of HCV core determinants on chimeric core particles increased as copies of fused HCV core protein were increased. Hybrid core particles with multiple HCV core determinants would be instrumental as an antigen probe for detecting class-specific antibodies to the HCV core in patients with acute and chronic hepatitis C and for simultaneous detection of antibodies to HBV core and those to HCV core in donated blood.

Stability governs the apparent expression of "particulate" hepatitis B e antigen by mutant hepatitis B virus core particles

The p21.5 capsid or core protein of hepatitis B virus carries two distinct classes of epitopes. Core (HBc) epitopes are found exclusively on the surface of the 28-nm viral icosahedral capsids or core particles, while HBe epitopes are normally expressed only by subparticulate forms of the core protein. Recent studies have suggested that a "particulate" form of HBe is expressed on the surface of capsid particles assembled from p17, a truncated core protein that lacks the carboxy-terminal protamine-like region of p21.5 and hence the ability to bind and encapsidate RNA. In this report we have used epitope-specific ELISAs in conjunction with capsids assembled from a series of carboxy-terminally truncated core proteins to address the mechanistic basis for particulate HBe. Specifically, we sought to test the idea that particulate HBe expression might be linked to the loss of RNA binding. However, our results strongly suggest that expression of HBe by mutant core particles is a result of their intrinsic instability which increases sharply when RNA binding is lost. We show that core particles assembled from mutant core proteins lacking Cys residues also express HBe, again because of capsid instability. We report mild conditions that can induce the dissociation of the mutant capsids and discuss our findings in terms of the factors that control capsid stability.

Hybrid hepatitis B virus nucleocapsid bearing an immunodominant region from hepatitis B virus surface antigen

A hepatitis B core antigen (HBcAg) gene bearing the 39-amino-acid-long domain A of hepatitis B surface antigen (HBsAg) within the HBcAg immunodominant loop has been constructed and expressed in Escherichia coli. Chimeric capsids demonstrated HBs but not HBc antigenicity and elicited in mice B-cell and T-cell responses against native HBcAg and HBsAg.

Hepatitis B virus differentially suppresses myelopoiesis and displays tropism for immature hematopoietic cells

The hematopoietic cell lines HL-60 and THP-1 were challenged with hepatitis B virus (HBV) in vitro to study interactions between the virus and host cell. Exposure to HBV suppressed the ability of HL-60 cells to differentiate into granulocytes after treatment with retinoic acid (RA) or dimethyl sulfoxide (DMSO), and RA-induced activation of the monocytic cell line THP-1 was also suppressed. Terminal differentiation of both cell lines by phorbol 12-myristate 13-acetate (PMA) was not affected by HBV. The suppressive effect on RA- or DMSO-induced differentiation was unique to HBV, since cell exposure to human cytomegalovirus, another virus that inhibits hematopoiesis, failed to block cellular differentiation. At 5 days postinfection, extracellular viral DNA was detected in immature but not in differentiated cultures and higher levels of core antigen (HBcAg) and surface antigen (HBsAg) were seen in undifferentiated cells than in RA- or PMA-treated cells. In addition, release of HBsAg into the medium was 2 to 12 times greater in untreated cultures than for RA- or PMA-treated cells. Thus, HBV suppresses hematopoiesis by blocking the maturational development of progenitors and selectively infects immature myeloid cells compared with mature end-stage cells.

Carboxy-terminal truncations of the HBV core protein affect capsid formation and the apparent size of encapsidated HBV RNA

Deletion mutations were introduced into the hepatitis B virus (HBV) capsid (core) gene to determine the effect on capsid formation, pregenome encapsidation, reverse transcription, and second-strand DNA synthesis. Carboxy-truncated HBV core proteins were expressed in insect cells using recombinant baculoviruses and were tested for capsid forming ability. Sucrose gradient sedimentation analysis revealed that core proteins missing 39 carboxy terminal amino acids produced capsids while removal of an additional 9 amino acids prevented capsid formation. Truncated core proteins co-expressed in the human hepatoma cell line Huh7 were assayed for their ability to complement in trans an HBV genomic plasmid containing a defective core gene. Mutants lacking 7 and 12 carboxy terminal residues complemented the defective core gene of the HBV plasmid as assayed by synthesis of HBV DNA via reverse transcription of the encapsidated RNA pregenome, although the mutant lacking 12 residues was partially defective in completing second-strand DNA synthesis. Capsids formed using a core deletion mutant missing 20 carboxy terminal residues contained HBV RNA but contained little if any HBV DNA. However, the largest encapsidated RNA species was only 1.7 kb, about half the size of the 3.5-kb RNA found in wild-type HBV capsids. Hybridization analysis revealed that the shorter RNA lacked sequences corresponding to the 3' half of the pregenomic RNA. Implications of these findings on HBV packaging are discussed.

High-level expression of hepatitis B viral antigens in fibrosing cholestatic hepatitis

The expression of hepatitis B viral antigens was quantified in liver tissue from four transplant recipients with fibrosing cholestatic hepatitis (FCH) and compared with five other transplant recipients who did not develop this syndrome and 30 patients with chronic hepatitis B virus (HBV) infection. As measured by radioimmunoassays, the liver tissue from patients with FCH had significantly greater amounts of both hepatitis B surface antigen (HBsAg) and nucleocapsid antigens than to transplant patients without this syndrome (P less than 0.01) or patients with chronic HBV infection (P less than 0.001). Intrahepatic expression of pre-S1/pre-S2 in FCH was also extensive with a distribution parallel to that of HBsAg. High-level expression of intrahepatic HBsAg and hepatitis B core antigen in the explanted liver was associated with subsequent development of FCH in the liver graft, suggesting that viral/host factors may also be important. This pattern of intrahepatic hepatitis viral antigen expression, by analogy with Chisari's transgenic mice model and Roingeard's HBV-transfected HepG2 cell model, may be the cause of direct hepatocytopathic injury in this condition.

Production of hepatitis B virion-like particles in yeast

We constructed a yeast strain that simultaneously expresses four genes encoding the major S, middle S, large S hepatitis B viral envelope proteins and the core protein under the control of the yeast glyceraldehyde-3-phosphate dehydrogenase promoter and terminator. The lysate from this cell line, examined by immunological, physicochemical methods and electron microscopy, was found to contain spherical particles with a diameter of about 40 nm and a density of 1.25 g/ml. These particles reacted with anti-envelope antibodies, but not with anti-core antibodies. However, core antigenicity appeared upon treatment with 3% Nonidet P-40 that eliminates an outer envelope. These observations suggest production of a virion-like complex structure, or at least its DNA-less analog, consisting of core particle enveloped by antibody-reactive envelope. Such a structure was made only when all the four gene products were synthesized in a yeast cell. This system may be useful for the study of virus structure and assembly, and for improved vaccine development.

Co-expression of hepatitis B virus antigens by a non-defective adenovirus vaccine vector

Adenovirus type 7 vaccine strain was engineered to express foreign antigens from both the E3 early promoter in the E3 region and the major late promoter inserted between the E4 region and the right inverted terminal repeat. This multiple expression vector was used to express hepatitis B core antigen (HBcAg), hepatitis B e antigen (HBeAg), and hepatitis B surface antigen (HBsAg). The gene inserted in the E3 region was derived from the core gene of the hepatitis B virus genome. When the precore region was present, an immunoreactive group of proteins with molecular weights ranging from 15,000 to 19,000 was secreted into the media. Velocity sedimentation centrifugation of media and lysates from cells infected with recombinants containing the core gene with the precore region resulted in peaks of HBeAg at the top of the gradient where authentic HBeAg should be found. In addition to the core gene in the E3 region, the surface antigen gene of hepatitis B virus was inserted behind the major late promoter in the E4 region resulting in an adeno-hepatitis recombinant virus capable of expressing both the core gene and the HBsAg cells. Cells infected with the adeno-hepatitis recombinants could also be stained with peroxidase-conjugates after reacting to antibody against HBcAg. Inoculation of dogs with the recombinant viruses which contained the core gene, with and without the precore sequence, resulted in a significant antibody response to HBcAg/HBeAg. The dogs also produced a significant antibody response to HBsAg as well as neutralizing antibody to adenovirus.

Expression in yeast of amino-terminal peptide fusions to hepatitis B core antigen and their immunological properties

Hepatitis B core protein (HBcAg) is a potent antigen that gives both a T-cell-dependent and a T-cell-independent antibody response. It has been shown that a foreign epitope can be fused to the amino terminus of HBcAg without affecting particle integrity, and that the resulting chimaeric cores retain the immunogenicity of the foreign epitope. Here we describe the efficient expression in yeast of two different chimaeric cores, carrying epitopes of Foot and Mouth Disease Virus (FMDV) or human chorionic gonadotrophin (hCG), which are candidates for FMD and contraceptive vaccines, respectively. These cores could not be produced in E. coli in soluble form but were expressed to high levels in yeast. We constructed a yeast expression vector that allows rapid production of different chimaeric cores by cloning in cassettes encoding foreign epitopes. Both FMDV and hCG-cores were shown to present the epitopes at the surface of the particles. The FMDV-cores produced in yeast were efficient inducers of neutralising antibodies in guinea-pigs after one low dose.

Synthesis of hepatitis B virus core antigen polypeptide in E. coli using pKK223-3 plasmid, a vector for expression, with tac promoter

A hybrid plasmid was constructed by insertion of the HBc gene encoding HBcAg into the pKK223-3 plasmid at the SmaI cleavage site in the correct direction just downstream from the tac promoter and upstream from the rrnB terminator. The recombinant plasmid carrying the HBc gene was introduced into E. coli and cloned. HBcAg was synthesized in E. coli by using the expression plasmid under the regulation of the tac promoter and rrnB terminator. The tac promoter, derived from sequences of trp and lac UV5 promoters, has identical sequences in two domains (-35 and -10 regions) with optimal distance, and the Shine-Dalgarno sequence, which enables protein synthesis to start at the ATG of the adjacent HBc gene. The nucleotide sequence of the HBc gene and its predicted amino acid sequence were almost identical to those previously reported. Purified HBcAg has a molecular weight of 21,500. This polypeptide gave a positive reaction with anti-HBcAg and anti-HBe antibodies, and was assembled into spherical particles 37 nm in diameter. The recombinant plasmid, carrying the HBc gene between the tac promoter (trp-lac hybrid promoter) and the rrnB terminator in expression plasmid pKK223-3, was useful for efficient expression of the HBc gene and production of HBcAg particles in E. coli.

Expression mechanism of the hepatitis B virus (HBV) C gene and biosynthesis of HBe antigen

The C gene of the hepatitis B virus, which contains two in-phase initiation codons delimiting the pre-C sequence and the C region, directs the synthesis of the major protein of the capsid (HBcAg) and of a precore protein which upon processing results in the secretion of the HBeAg. We used an adenovirus-based vector to study in the human 293 cell line the C gene products, the intermediates of the precore protein processing and the kind of protease involved in this processing. The synthesis of the 21-kDa HBcAg polypeptide was dependent on the deletion of the pre-C sequence suggesting that a pre-C mRNA is not used for the synthesis of the major capsid protein. With the construct containing the complete C gene, two proteins of 25 and 22 kDa were detected intracellularly, corresponding to the unprocessed and partially processed precore protein, respectively. In addition, a 15-kDa protein (HBeAg) was secreted in the culture medium. Using pepstatin, an inhibitor specific for aspartyl proteinases, reduction of HBeAg secretion and accumulation of the 22-kDa processing intermediate were observed, suggesting the involvement of an aspartyl proteinase in the conversion of the 22-kDa protein into HBeAg.

Replication and gene expression of hepatitis B virus in a transgenic mouse that contains the complete viral genome

We have sought to address the problem of the host and tissue specificity of the hepatitis B virus (HBV) by using transgenic mice obtained after injection of head-to-tail dimers of the HBV genome. Viral DNA replication and protein synthesis were obtained in one of nine transgenic mice containing integrated HBV DNA. The RNAs encoding the HBV surface antigen and the core antigen were synthesized in the liver, the kidney, and the heart. In these organs, DNA replicative intermediates similar to those found during normal infection were associated with corelike structures. Large amounts of core polypeptides and capsids were detected in the nuclei in the absence of any pathological effect. These results show that the different steps of HBV multiplication can take place in nonliver nonhuman cells once the problem of entry into the host cell is overcome. In the absence of a small laboratory animal infectable by HBV, such transgenic mice should be helpful for the study of many aspects of viral multiplication.

Hepatitis B virus core antigen: synthesis in Escherichia coli and application in diagnosis

Fragments of hepatitis B virus DNA cloned in plasmid pBR322 carrying the gene for the viral core antigen have been placed under the control of the lac promoter of Escherichia coli. Several of the new recombinants direct higher levels of synthesis of the antigen, but the degree of enhancement varies with the different structures of the plasmids and hence the mRNAs produced. The antigen in crude bacterial lysates is a satisfactory diagnostic reagent for antibodies to the core antigen in serum samples.

Expression of cloned hepatitis B virus DNA in human cell cultures

DNA was isolated from the ayw subtype of hepatitis B virus (HBV) that had been incubated in vitro with all four deoxynucleoside triphosphates in order to complete the circular viral genome by means of the endogenous DNA polymerase. The purified viral DNA was cleaved with EcoRI restriction endonuclease, inserted into the EcoRI site of plasmid pBR322, and cloned in Escherichia coli chi 1776. DNA from a clone, pHBV-1, that contained a 3200-base-pair insert of HBV DNA was cleaved with EcoRI and incubated with phage T4 ligase under conditions favoring intramolecular ligation. HeLa cell cultures exposed to this DNA showed marked cytopathic changes, accompanied by production of hepatitis B core and surface antigens, 11-14 days after subculture. Electron microscopic examination of anti-hepatitis B surface antigen immunoprecipitates from culture media of these cells revealed both 42-nm particles with central cores and 20-nm round particles. Although neither intact circular nor EcoRI-cleaved linear pHBV-1 DNAs evoked these effects in HeLa cells, both cytopathic changes and intranuclear hepatitis B core antigen were detected in HeLa cells infected with Dane particles.