Expression of processed envelope protein of hepatitis C virus in mammalian and insect cells

Kinetic and structural analyses of hepatitis C virus polyprotein processing

Recombinant vaccinia viruses were used to study the processing of hepatitis C virus (HCV) nonstructural polyprotein precursor. HCV-specific proteins and cleavage products were identified by size and by immunoprecipitation with region-specific antisera. A polyprotein beginning with 20 amino acids derived from the carboxy terminus of NS2 and ending with the NS5B stop codon (amino acids 1007 to 3011) was cleaved at the NS3/4A, NS4A/4B, NS4B/5A, and NS5A/5B sites, whereas a polyprotein in which the putative active site serine residue was replaced by an alanine remained unprocessed, demonstrating that the NS3-encoded serine-type proteinase is essential for cleavage at these sites. Processing of the NS3'-5B polyprotein was complex and occurred rapidly. Discrete polypeptide species corresponding to various processing intermediates were detected. With the exception of NS4AB-5A/NS5A, no clear precursor-product relationships were detected. Using double infection of cells with vaccinia virus recombinants expressing either a proteolytically inactive NS3'-5B polyprotein or an active NS3 proteinase, we found that cleavage at the NS4A/4B, NS4B/5A, and NS5A/5B sites could be mediated in trans. Absence of trans cleavage at the NS3/4A junction together with the finding that processing at this site was insensitive to dilution of the enzyme suggested that cleavage at this site is an intramolecular reaction. The trans-cleavage assay was also used to show that (i) the first 211 amino acids of NS3 were sufficient for processing at all trans sites and (ii) small deletions from the amino terminus of NS3 selectively affected cleavage at the NS4B/5A site, whereas more extensive deletions also decreased processing efficiencies at the other sites. Using a series of amino-terminally truncated substrate polyproteins in the trans-cleavage assay, we found that NS4A is essential for cleavage at the NS4B/5A site and that processing at this site could be restored by NS4A provided in cis (i.e., together with the substrate) or in trans (i.e., together with the proteinase). These results suggest that in addition to the NS3 proteinase, NS4A sequences play an important role in HCV polyprotein processing.

Peptide immunogen mimicry of putative E1 glycoprotein-specific epitopes in hepatitis C virus

Hepatitis C virus (HCV) accounts for most cases of acute and chronic non-A and non-B hepatitis with serious consequences that may lead to hepatocellular carcinoma. The putative envelope glycoproteins (E1 and E2) of HCV probably play a role in the pathophysiology of the virus. In order to map the immunodominant domains of the E1 glycoprotein, two epitopes from amino acid residues 210 to 223 (P1) and 315 to 327 (P2) were predicted from the HCV sequence. Immunization of mice with the synthetic peptides conjugated to bovine serum albumin induced an antibody response, and the antisera immunoprecipitated the E1 glycoprotein (approximately 33 kDa) of HCV expressed by recombinant vaccinia virus. A panel of HCV-infected human sera was also tested with the synthetic peptides by enzyme-linked immunosorbent assay for epitope-specific responses. Of 38 infected serum samples, 35 (92.1%) demonstrated a spectrum of reactivity to the P2 peptide. On the other hand, only 17 of 38 (44.7%) serum samples were reactive to the P1 peptide. Strains of HCV exhibit a striking genomic diversity. The predicted P1 epitope showed localization in the sequence-variable region, and the P2 epitope localized in a highly conserved domain. Results from this study suggest that the E1 glycoprotein of HCV contains at least two potential antigenic epitopes. Synthetic peptides corresponding to these epitopes and antisera to these peptides may serve as the monospecific immunological reagents to further determine the role of E1 glycoprotein in HCV infection.

A study of primary- and re-infection with hepatitis C virus in blood transfusion recipients

A nested polymerase chain reaction was used to assess viraemia in blood transfusion recipients with no serological evidence of hepatitis C virus (HCV) infection (naive recipients) and in recipients with prior or existing HCV infection (infected recipients), who were transfused with HCV-positive blood. In 10 hepatitis cases in naive recipients, defined as primary infection, nine showed clinical hepatitis, and one was sub-clinical; the time between transfusion and elevation of alanine aminotransferase (ALT) levels was 15-60 days (37.9 +/- 13.9). All 10 naive recipients showed abnormal ALT, and 10/10 and 7/10 were persistently positive for anti-HCV and HCV-RNA, respectively, for more than 1 year. Similarly, in five cases in previously infected recipients, defined as re-infection, 4/5 showed clinical hepatitis, the time to elevation of ALT was 30-46 days (34.8 +/- 6.4), and 5/5 and 3/5 were persistently positive for anti-HCV and HCV-RNA, respectively, for more than 1 year. All five infected recipients showed abnormal ALT. In conclusion, there was no significant difference (P = 0.05) in the frequency of the markers of infection resulting from primary or re-infection with HCV, suggesting that primary infection fails to induce a protective immune response.

Characterization of hepatitis C virus envelope glycoprotein complexes expressed by recombinant vaccinia viruses

We constructed recombinant vaccinia virus vectors for expression of the structural region of hepatitis C virus (HCV). Infection of mammalian cells with a vector (vv/HCV1-906) encoding C-E1-E2-NS2 generated major protein species of 22 kDa (C), 33 to 35 kDa (E1), and 70 to 72 kDa (E2), as observed previously with other mammalian expression systems. The bulk of the E1 and E2 expressed by vv/HCV1-906 was found integrated into endoplasmic reticulum membranes as core-glycosylated species, suggesting that these E1 and E2 species represent intracellular forms of the HCV envelope proteins. HCV E1 and E2 formed E1-E2 complexes which were precipitated by either anti-E1 or anti-E2 serum and which sedimented at approximately 15 S on glycerol density gradients. No evidence of intermolecular disulfide bonding between E1 and E2 was detected. E1 and E2 were copurified to approximately 90% purity by mild detergent extraction followed by chromatography on Galanthus nivalus lectin-agarose and DEAE-Fractogel. Immunization of chimpanzees with purified E1-E2 generated high titers of anti-E1 and anti-E2 antibodies. Further studies, to be reported separately, demonstrated that purified E1-E2 complexes were recognized at high frequency by HCV+ human sera (D. Y. Chien, Q.-L. Choo, R. Ralston, R. Spaete, M. Tong, M. Houghton, and G. Kuo, Lancet, in press) and generated protective immunity in chimpanzees (Q.-L. Choo, G. Kuo, R. Ralston, A. Weiner, D. Chien, G. Van Nest, J. Han, K. Berger, K. Thudium, J. Kansopon, J. McFarland, A. Tabrizi, K. Ching, B. Mass, L. B. Cummins, E. Muchmore, and M. Houghton, submitted for publication), suggesting that these purified HCV envelope proteins display native HCV epitopes.

Suppression of hepatitis B virus expression and replication by hepatitis C virus core protein in HuH-7 cells

Hepatitis B and C viruses (HBV and HCV, respectively) are associated with acute and chronic liver diseases and hepatocellular carcinoma. To elucidate the molecular status of superinfection with these two hepatitis viruses, we cotransfected the full-length or truncated version of HCV structural genes (core and envelope 1) together with the cloned HBV DNA into a human hepatoma cell line (HuH-7). Expression of HBV-specific major transcripts (3.5 and 2.1 kb), as well as HBV antigens (hepatitis B surface antigen and hepatitis B e and core antigens), was reduced about two- to fourfold by the presence of the HCV structural genes. In addition, the secretion of HBV viral particles, including the viral nucleocapsid and mature virion, was drastically suppressed about 20-fold. Analysis of the intracellular HBV core protein-associated nucleic acid indicated that the encapsidated HBV pregenomic RNA was similarly reduced about 14-fold. Deletion analysis of the HCV structural genes demonstrated that the core gene alone or the fragment containing the core protein's N-terminal 122 amino acid residues conferred the same level of suppressive activity as the full-length structural genes. By indirect immunofluorescence, we found that the core protein of HCV was located in the cytoplasm of transfected HuH-7 cells at day 3 posttransfection and was targeted to the nucleus at day 6. Thus, the kinetics of the suppressive effect exerted by HCV constructs matched the timing of core protein entrance into the nucleus. Our results substantiate the clinical finding that HBV markers are suppressed by superinfection with HCV and further imply that this inhibitory effect may occur in the processes of transcription and encapsidation of HBV pregenomic RNA and may be mediated by the core protein of HCV. The deduced amino acid sequence of the HCV core protein has revealed that it is a basic protein which contains a putative DNA-binding motif (SPRG), as well as triplicate nuclear localization signals and several putative protein kinase A and C recognition sites. These characteristics imply that the HCV core protein can also function as a gene-regulatory protein.

Structural proteins of hepatitis C virus

Despite the lack of an in vitro replication system, genes coding for viral proteins of hepatitis C virus have been identified. The putative nucleocapsid (p22) and envelope (gp35 and gp58) proteins have been expressed in cells by different vectors under various foreign promoters. Detection of antibodies to these proteins in patient sera suggests that they are indeed synthesized during the course of viral replication.

Superinfection of heterologous hepatitis C virus in a patient with chronic type C hepatitis

A Taiwanese woman who had chronic infection of type II hepatitis C virus was superinfected by type III hepatitis C virus after blood transfusion. The subtypes of the hepatitis C virus were determined by direct sequencing of the envelope region of the viral genome in serial serum samples before and after transfusion. The original virus in the patient had a 95.6% homology to the Taiwanese isolate (a type II virus) by comparing nucleotide sequences of the envelope region. After transfusion, markedly elevated serum aminotransferase activities were noted and the virus sequenced showed only a 55.2% homology to the Taiwanese isolate but had a 88.9% homology to a Japanese isolate (a type III virus). After recovery from the acute episode, the newly introduced type III virus became undetectable and type II virus predominated again but with significant genetic variation in the follow-up samples as compared with the original type II virus. It was concluded that superinfection of hepatitis C virus indeed occurs in humans, and this should be taken into consideration in the pathogenesis of reactivation of chronic type C hepatitis.

Characterization of the hepatitis C virus-encoded serine proteinase: determination of proteinase-dependent polyprotein cleavage sites

Processing of the hepatitis C virus (HCV) H strain polyprotein yields at least nine distinct cleavage products: NH2-C-E1-E2-NS2-NS3-NS4A-NS4B-NS5A-NS5B-CO OH. As described in this report, site-directed mutagenesis and transient expression analyses were used to study the role of a putative serine proteinase domain, located in the N-terminal one-third of the NS3 protein, in proteolytic processing of HCV polyproteins. All four cleavages which occur C terminal to the proteinase domain (3/4A, 4A/4B, 4B/5A, and 5A/5B) were abolished by substitution of alanine for either of two predicted residues (His-1083 and Ser-1165) in the proteinase catalytic triad. However, such substitutions have no observable effect on cleavages in the structural region or at the 2/3 site. Deletion analyses suggest that the structural and NS2 regions of the polyprotein are not required for the HCV NS3 proteinase activity. NS3 proteinase-dependent cleavage sites were localized by N-terminal sequence analysis of NS4A, NS4B, NS5A, and NS5B. Sequence comparison of the residues flanking these cleavage sites for all sequenced HCV strains reveals conserved residues which may play a role in determining HCV NS3 proteinase substrate specificity. These features include an acidic residue (Asp or Glu) at the P6 position, a Cys or Thr residue at the P1 position, and a Ser or Ala residue at the P1' position.

Expression and identification of hepatitis C virus polyprotein cleavage products

Hepatitis C virus (HCV) is the major cause of transfusion-acquired non-A, non-B hepatitis. HCV is an enveloped positive-sense RNA virus which has been classified as a new genus in the flavivirus family. Like the other two genera in this family, the flaviviruses and the pestiviruses, HCV polypeptides appear to be produced by translation of a long open reading frame and subsequent proteolytic processing of this polyprotein. In this study, a cDNA clone encompassing the long open reading frame of the HCV H strain (3,011 amino acid residues) has been assembled and sequenced. This clone and various truncated derivatives were used in vaccinia virus transient-expression assays to map HCV-encoded polypeptides and to study HCV polyprotein processing. HCV polyproteins and cleavage products were identified by using convalescent human sera and a panel of region-specific polyclonal rabbit antisera. Similar results were obtained for several mammalian cell lines examined, including the human HepG2 hepatoma line. The data indicate that at least nine polypeptides are produced by cleavage of the HCV H strain polyprotein. Putative structural proteins, located in the N-terminal one-fourth of the polyprotein, include the capsid protein C (21 kDa) followed by two possible virion envelope proteins, E1 (31 kDa) and E2 (70 kDa), which are heavily modified by N-linked glycosylation. The remainder of the polyprotein probably encodes nonstructural proteins including NS2 (23 kDa), NS3 (70 kDa), NS4A (8 kDa), NS4B (27 kDa), NS5A (58 kDa), and NS5B (68 kDa). An 82- to 88-kDa glycoprotein which reacted with both E2 and NS2-specific HCV antisera was also identified (called E2-NS2). Preliminary results suggest that a fraction of E1 is associated with E2 and E2-NS2 via disulfide linkages.

Antigenic regions within the hepatitis C virus envelope 1 and non-structural proteins: identification of an IgG3-restricted recognition site with the envelope 1 protein

Antibody binding to antigenic regions of hepatitis C virus (HCV) envelope 1 (E1; residues 183-380, E2/non-structural (NS) 1 (residues 380-437), NS1 (residues 643-690), and NS4 (1684-1751) proteins were assayed for 50 sera with antibodies to HCV (anti-HCV) and for 46 sera without anti-HCV. Thirty-four peptides, 18 residues long with an eight-amino acid overlap within each HCV region, were synthesized and tested with all 96 sera. Within the E region 183-380, the major binding site was located to residues 203-220, and was recognized by eight sera. Within the E2/NS1 region 380-437, the peptide covering residues 410-427 was recognized by two sera, and within the NS1 region 643-690, peptides covering residues 663-690 were recognized by four sera. Within the NS4 region 1684-1751, 27 sera were reactive to one or more of the NS4 peptides, and 21 out of these were reactive with peptide 1694-1711. One part of the major binding site could be located to residues 1701-1704, with the sequence Leu-Tyr-Arg-Glu. The IgG1, IgG3 and IgG4 subclasses were reactive with the five antigenic regions of HCV core, residues 1-18, 11-28, 21-38, 51-68 and 101-118. Reactivity to the major envelope site consisted almost exclusively of IgG3, and reactivity to the major site of NS4 consisted only of IgG1. Thus, a non-restricted IgG response to linear HCV-encoded binding sites was found to the core protein, whereas IgG subclass-restricted linear binding sites were found within the E1 protein, and within the NS4 protein.

Hepatitis C: looking at a virus that hasn't been seen

Hepatitis C (HCV) is the first virus to be discovered by molecular cloning without direct use of biological or biophysical methods. HCV was first recognised in 1974 as non-A, non-B (NANB) hepatitis resulting from blood transfusions. It took almost 15 years to identify it successfully--by detecting a clone in a library of cDNA prepared from the nucleic acids extracted from plasma known to be infectious for chimpanzees. The clone was derived from a positive-sense RNA of about 10,000 nucleotides, with a long open reading frame encoding for a polyprotein of about 3000 amino acids. The structure of the RNA and the encoded polyprotein had properties similar to known flaviviruses and pestiviruses. Functions of viral proteins produced by proteolytic cleavage of the polyprotein are estimated by analogy with known viruses of similar genomic organisation. Each of the HCV proteins has been produced in recombinant organisms and used as an antigen in immunoassays to investigate serological responses during the course of infection. Seroconversions to both structural and non-structural antigens are observed during the course of disease but typical diagnostic serological patterns have not yet evolved. Immunoassays for HCV antibodies reacting with recombinant antigens are used widely for screening blood donations and for studying the epidemiology of HCV infection. Comparisons of the nucleic acid sequences from different isolates of HCV have shown considerable variability throughout the genome. The importance of this genomic heterogeneity will be a challenging problem for the future.