CD81-dependent binding of hepatitis C virus E1E2 heterodimers

Hepatitis C virus (HCV) is the leading cause of chronic liver disease worldwide. HCV is also the major cause of mixed cryoglobulinemia, a B-lymphocyte proliferative disorder. Direct experimentation with native viral proteins is not feasible. Truncated versions of recombinant E2 envelope proteins, used as surrogates for viral particles, were shown to bind specifically to human CD81. However, truncated E2 may not fully mimic the surface of HCV virions because the virus encodes two envelope glycoproteins that associate with each other as E1E2 heterodimers. Here we show that E1E2 complexes efficiently bind to CD81 whereas truncated E2 is a weak binder, suggesting that truncated E2 is probably not the best tool with which to study cellular interactions. To gain better insight into virus-cell interactions, we developed a method by which to isolate E1E2 complexes that are properly folded. We demonstrate that purified E1E2 heterodimers bind to cells in a CD81-dependent manner. Furthermore, engagement of B cells by purified E1E2 heterodimers results in their aggregation and in protein tyrosine phosphorylation, a hallmark of B-cell activation. These studies provide a possible clue to the etiology of HCV-associated B-cell lymphoproliferative diseases. They also delineate a method by which to isolate biologically functional E1E2 complexes for the study of virus-host cell interaction in other cell types.     

Novel AAV-based genetic vaccines encoding truncated dengue virus envelope proteins elicit humoral immune responses in mice

The envelope protein of dengue virus is involved in host cell attachment for entry and induction of protective immunity. Current efforts are focused on producing a tetravalent vaccine by mixing four monovalent vaccine components. In this work, we developed a genetic vaccine based on a novel adeno-associated viral (AAV) vector expressing the carboxy-terminal truncated envelope protein (79E) of dengue virus. The expression of the recombinant 79E protein in HEK 293 cells was confirmed by Western blot. Vectors packaged with novel AAV capsids (AAV2/8 or AAV2/rh32.33) were injected into C57BL/6 mice intramuscularly. Dengue virus antigen was produced in the mice and induced long-lasting antibody responses against the dengue virus still detectable 20 weeks after immunization. AAV2/8 vaccine induced higher anti-dengue virus antibody levels than AAV2/rh32.33 vaccine or AAV plasmid. Furthermore, the anti-dengue antibodies could neutralize homogeneous dengue virus. These results demonstrated that the AAV vaccines possessed appropriate immunogenicity and could be used for the development of an effective dengue vaccine.     

Mapping of a region of dengue virus type-2 glycoprotein required for binding by a neutralizing monoclonal antibody.

Envelope glycoprotein E of flaviviruses is exposed at the surface of the virion, and is responsible for eliciting a neutralizing antibody (Ab) response, as well as protective immunity in the host. In this report, we describe a method for the fine mapping of a linear sequence of the E protein of dengue virus type-2 (DEN-2), recognized by a type-specific and neutralizing monoclonal Ab (mAb), 3H5. First, an Escherichia coli expression vector containing a heat-inducible lambda pL promoter was used to synthesize several truncated, and near-full length E polypeptides. Reactivities of these polypeptides with polyclonal mouse hyperimmune sera, as well as the 3H5 mAb revealed the location of the 3H5-binding site to be within a region of 166 amino acids (aa) between aa 255 and 422. For fine mapping, a series of targeted deletions were made inframe within this region using the polymerase chain reaction (PCR). The hydrophilicity pattern of this region was used as a guide to systematically delete the regions encoding the various groups of surface aa residues within the context of a near-full-length E polypeptide by using PCR. The 3H5-binding site was thus precisely mapped to a region encoding 12 aa (between aa 386 and 397). A synthetic peptide containing this sequence was able to bind to the 3H5 mAb specifically, as shown by enzyme-linked immunosorbent assay. In addition, we show that rabbit Abs raised against the synthetic peptide of 12 aa were able to bind to the authentic E protein, and to neutralize DEN-2 virus in a plaque reduction assay.     

Dengue virus-specific cross-reactive CD8+ human cytotoxic T lymphocytes

Stimulation with live dengue virus of peripheral blood mononuclear cells from a dengue virus type 4-immune donor generated virus-specific, serotype-cross-reactive, CD8+, class I-restricted cytotoxic T lymphocytes (CTL) capable of lysing dengue virus-infected cells and cells pulsed with dengue virus antigens of all four serotypes. These CTL lysed autologous fibroblasts infected with vaccinia virus-dengue virus recombinant viruses containing the E gene or several nonstructural dengue virus type 4 genes. These results demonstrate that both dengue virus structural and nonstructural proteins are targets for the cytotoxic T-cell-mediated immune response to dengue virus and suggest that serotype-cross-reactive CD8+ CTL may be important mediators of viral clearance and of virus-induced immunopathology during secondary dengue virus infections.     

Variable surface epitopes in the crystal structure of dengue virus type 3 envelope glycoprotein

Dengue virus is an emerging global health threat. The major envelope glycoprotein, E, mediates viral attachment and entry by membrane fusion. Antibodies that bind but fail to neutralize noncognate serotypes enhance infection. We have determined the crystal structure of a soluble fragment of the envelope glycoprotein E from dengue virus type 3. The structure closely resembles those of E proteins from dengue type 2 and tick-borne encephalitis viruses. Serotype-specific neutralization escape mutants in dengue virus E proteins are all located on a surface of domain III, which has been implicated in receptor binding. While antibodies against epitopes in domain I are nonneutralizing in dengue virus, there are neutralizing antibodies that recognize serotype-conserved epitopes in domain II. The mechanism of neutralization for these antibodies is probably inhibition of membrane fusion. Our structure shows that neighboring glycans on the viral surface are spaced widely enough (at least 32 A) that they can interact with multiple carbohydrate recognition domains on oligomeric lectins such as DC-SIGN, ensuring maximum affinity for these putative receptors.     

Mice immunized with recombinant vaccinia virus expressing dengue 4 virus structural proteins with or without nonstructural protein NS1 are protected against fatal dengue virus encephalitis.

We have constructed vaccinia virus recombinants expressing dengue virus proteins from cloned DNA for use in experimental immunoprophylaxis. A recombinant virus containing a 4.0-kilobase DNA sequence that codes for three structural proteins, capsid (C), premembrane (pre-M), and envelope (E), and for nonstructural proteins NS1 and NS2a produced authentic pre-M, E, and NS1 in infected CV-1 cells. Mice immunized with this recombinant were protected against an intracerebral injection of 100 50% lethal doses of dengue 4 virus. A recombinant containing only genes C, pre-M, and E also induced solid resistance to challenge. Deletion of the putative C-terminal hydrophobic anchor of the E glycoprotein did not result in secretion of E from recombinant-virus-infected cells. Recombinants expressing only the E protein preceded by its own predicted N-terminal hydrophobic signal or by the signal of influenza A virus hemagglutinin or by the N-terminal 71 amino acids of the G glycoprotein of respiratory syncytial virus produced glycosylated E protein products of expected molecular sizes. These vaccinia virus recombinants also protected mice.     

Structural Changes in Dengue Virus When Exposed to a Temperature of 37°C

Previous binding studies of antibodies that recognized a partially or fully hidden epitope suggest that insect cell-derived dengue virus undergoes structural changes at an elevated temperature. This was confirmed by our cryo-electron microscopy images of dengue virus incubated at 37°C, where viruses change their surface from smooth to rough. Here we present the cryo-electron microscopy structures of dengue virus at 37°C. Image analysis showed four classes of particles. The three-dimensional (3D) map of one of these classes, representing half of the imaged virus population, shows that the E protein shell has expanded and there is a hole at the 3-fold vertices. Fitting E protein structures into the map suggests that all of the interdimeric and some intradimeric E protein interactions are weakened. The accessibility of some previously found cryptic epitopes on this class of particles is discussed.     

Expression of dengue virus structural proteins and nonstructural protein NS1 by a recombinant vaccinia virus.

A recombinant vaccinia virus containing cloned DNA sequences coding for the three structural proteins and nonstructural proteins NS1 and NS2a of dengue type 4 virus was constructed. Infection of CV-1 cells with this recombinant virus produced dengue virus structural proteins as well as the nonstructural protein NS1. These proteins were precipitated by specific antisera and exhibited the same molecular size and glycosylation patterns as authentic dengue virus proteins. Infection of cotton rats with the recombinant virus induced NS1 antibodies in 1 of 11 animals. However, an immune response to the PreM and E glycoproteins was not detected. A reduced level of gene expression was probably the reason for the limited serologic response to these dengue virus antigens.     

West Nile virus discriminates between DC-SIGN and DC-SIGNR for cellular attachment and infection

The C-type lectins DC-SIGN and DC-SIGNR bind mannose-rich glycans with high affinity. In vitro, cells expressing these attachment factors efficiently capture, and are infected by, a diverse array of appropriately glycosylated pathogens, including dengue virus. In this study, we investigated whether these lectins could enhance cellular infection by West Nile virus (WNV), a mosquito-borne flavivirus related to dengue virus. We discovered that DC-SIGNR promoted WNV infection much more efficiently than did DC-SIGN, particularly when the virus was grown in human cell types. The presence of a single N-linked glycosylation site on either the prM or E glycoprotein of WNV was sufficient to allow DC-SIGNR-mediated infection, demonstrating that uncleaved prM protein present on a flavivirus virion can influence viral tropism under certain circumstances. Preferential utilization of DC-SIGNR was a specific property conferred by the WNV envelope glycoproteins. Chimeras between DC-SIGN and DC-SIGNR demonstrated that the ability of DC-SIGNR to promote WNV infection maps to its carbohydrate recognition domain. WNV virions and subviral particles bound to DC-SIGNR with much greater affinity than DC-SIGN. We believe this is the first report of a pathogen interacting more efficiently with DC-SIGNR than with DC-SIGN. Our results should lead to the discovery of new mechanisms by which these well-studied lectins discriminate among ligands.     

A poxvirus protein with a RING finger motif binds zinc and localizes in virus factories.

Shope fibroma virus (SFV) is a Leporipoxvirus closely related to the highly virulent myxoma virus. The DNA sequence of the BamHI N fragment of the SFV DNA genome was determined, and the single complete open reading frame (N1R) was characterized. The protein encoded by the N1R gene was found to contain a C3HC4 RING finger motif at the C terminus. This C3HC4 motif is the hallmark of a growing family of proteins, many of which are involved in regulation of gene expression, DNA repair, or DNA recombination. Complete homologs of the SFV N1R gene were also detected in variola virus, myxoma virus, and vaccinia virus strain IHD-W. In contrast, the gene is completely absent from vaccinia virus strain Copenhagen, and in vaccinia virus strain WR, the open reading frame is truncated prior to the zinc binding domain because of an 11-bp deletion, thus producing a frameshift and premature stop codon. Recombinant N1R protein from SFV was expressed in Escherichia coli and shown to bind zinc in a specific manner. Using fluorescence microscopy to visualize a peptide epitope tag (derived from ICP27 of herpes simplex virus) fused to the N terminus of the poxvirus proteins, we observed that the N1R protein of SFV and its homologs in myxoma virus and vaccinia virus IHD-W were localized primarily to the virus factories in the cytoplasm of infected cells and, to a lesser degree, the host cell nucleus. The truncated protein of vaccinia virus strain WR failed to localize in this manner but instead was observed throughout the cytoplasm.     

A27L Protein Mediates Vaccinia Virus Interaction with Cell Surface Heparan Sulfate

Vaccinia virus has a wide host range and infects mammalian cells of many different species. This suggests that the cell surface receptors for vaccinia virus are ubiquitously expressed and highly conserved. Alternatively, different receptors are used for vaccinia virus infection of different cell types. Here we report that vaccinia virus binds to heparan sulfate, a glycosaminoglycan (GAG) side chain of cell surface proteoglycans, during virus infection. Soluble heparin specifically inhibits vaccinia virus binding to cells, whereas other GAGs such as condroitin sulfate or dermantan sulfate have no effect. Heparin also blocks infections by cowpox virus, rabbitpox virus, myxoma virus, and Shope fibroma virus, suggesting that cell surface heparan sulfate could be a general mediator of the entry of poxviruses. The biochemical nature of the heparin-blocking effect was investigated. Heparin analogs that have acetyl groups instead of sulfate groups also abolish the inhibitory effect, suggesting that the negative charges on GAGs are important for virus infection. Furthermore, BSC40 cells treated with sodium chlorate to produce undersulfated GAGs are more refractory to vaccinia virus infection. Taken together, the data support the notion that cell surface heparan sulfate is important for vaccinia virus infection. Using heparin-Sepharose beads, we showed that vaccinia virus virions bind to heparin in vitro. In addition, we demonstrated that the recombinant A27L gene product binds to the heparin beads in vitro. This recombinant protein was further shown to bind to cells, and such interaction could be specifically inhibited by soluble heparin. All the data together indicated that A27L protein could be an attachment protein that mediates vaccinia virus binding to cell surface heparan sulfate during viral infection.     

Truncated amplification: a method for high-fidelity template-driven nucleic acid amplification

The error rate of conventional PCR is problematic when amplifying from single cells or amplifying segments for protein functional analysis by in vitro translation. We describe truncated amplification, a method for high-fidelity amplification in which DNA polymerase errors are not propagated efficiently and original DNA templates exert greater influence on the amplification process. Truncated amplification utilizes pairs of oligonucleotides and thermal cycling, but it differs from PCR. Truncated amplification amplifies non-exponentially with one or two chimeric oligonucleotides and produces truncated terminal products that are no more than three rounds of replication from the original template. Exon 6 of the p53 gene was utilized as a model system to demonstrate proof of principle. Chimeric oligonucleotides containing three 3'-->5' reversed-deoxynucleotides or 2'-OMe-ribonucleotides at 6-8 nucleotides from the 3 'terminus retained sequence specificity and primer extension activity. With PfuTurbo but not with Taq or Vent (exo-) DNA polymerases, the modified nucleotides completely truncated the DNA polymerase elongation. The resulting truncated terminal products are not templates for further amplification because of the short length of the 3' complementary region. Truncated amplific ation can amplify quadratically or geometrically depending on whether two or one chimeric oligonucleotides are used. Truncated amplification is a promising approach when template-driven amplification is desired to increase thefrequency of error-free products.     

Crystal structure of west nile virus envelope glycoprotein reveals viral surface epitopes

West Nile virus, a member of the Flavivirus genus, causes fever that can progress to life-threatening encephalitis. The major envelope glycoprotein, E, of these viruses mediates viral attachment and entry by membrane fusion. We have determined the crystal structure of a soluble fragment of West Nile virus E. The structure adopts the same overall fold as that of the E proteins from dengue and tick-borne encephalitis viruses. The conformation of domain II is different from that in other prefusion E structures, however, and resembles the conformation of domain II in postfusion E structures. The epitopes of neutralizing West Nile virus-specific antibodies map to a region of domain III that is exposed on the viral surface and has been implicated in receptor binding. In contrast, we show that certain recombinant therapeutic antibodies, which cross-neutralize West Nile and dengue viruses, bind a peptide from domain I that is exposed only during the membrane fusion transition. By revealing the details of the molecular landscape of the West Nile virus surface, our structure will assist the design of antiviral vaccines and therapeutics.     

Cleavage of dengue virus NS1-NS2A requires an octapeptide sequence at the C terminus of NS1.

The length of amino acid sequence at the NS1-NS2A juncture of dengue virus that is required for specific cleavage effected by the cis-acting function of NS2A was identified by deletion analysis. Recombinant DNA sequences of NS1-NS2A, each containing a deletion in NS1 followed by a sequence of 3 to 20 amino acids at the C terminus of NS1 preceding the cleavage site, were constructed and expressed with vaccinia virus as a vector. The NS1 product of recombinant vaccinia virus-infected cells was immunoprecipitated and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The occurrence of cleavage between NS1 and NS2A was indicated by the appearance of shortened NS1. Failure to cleave this site yielded a large NS1-NS2A fusion protein. This analysis indicated that a minimum length of eight amino acids at the NS1 C terminus preceding the NS1-NS2A juncture is required for cleavage to take place. Comparison of this eight-amino-acid sequence of the NS1 C terminus of dengue type 4 virus with the analogous sequences of 12 other flaviviruses suggests that the consensus cleavage site sequence is as follows: (table; see text)     

Immunization of mice with recombinant vaccinia virus expressing authentic dengue virus nonstructural protein NS1 protects against lethal dengue virus encephalitis.

The protective immunity conferred by a set of recombinant vaccinia viruses containing the entire coding sequence of dengue virus type 4 nonstructural glycoprotein NS1 plus various flanking sequences was evaluated by using a mouse encephalitis model. Mice immunized with recombinant vNS1-NS2a, which expresses authentic NS1, were solidly protected against intracerebral dengue virus challenge. However, mice immunized with recombinants vNS1-15%NS2a and vRSVG/NS1-15%NS2a, which express aberrant forms of NS1, were only partially protected (63 to 67% survival rate). Serologic analysis showed that mice immunized with vNS1-NS2a developed high titers of antibodies to NS1 as measured by radioimmunoprecipitation, enzyme-linked immunosorbent assay, and complement-mediated cytolytic assays. In addition, a pool of sera from these animals was protective in a passive transfer experiment. Lower titers of NS1-specific antibodies were detected in sera of animals immunized with vNS1-15%NS2a or vRSVG/NS1-15%NS2a by all three assays. These data support the view that protection against dengue virus infection in mice may be mediated at least in part by NS1-specific antibodies through a mechanism of complement-mediated lysis of infected cells. Additionally, immunization with two recombinant viruses expressing authentic NS1 of dengue virus type 2 conferred partial protection (30-50%) against dengue virus type 2 challenge.     

Yellow fever virus/dengue-2 virus and yellow fever virus/dengue-4 virus chimeras: biological characterization,immunogenicity, and protection against dengue encephalitis in the mouse model

Two yellow fever virus (YFV)/dengue virus chimeras which encode the prM and E proteins of either dengue virus serotype 2 (dengue-2 virus) or dengue-4 virus within the genome of the YFV 17D strain (YF5.2iv infectious clone) were constructed and characterized for their properties in cell culture and as experimental vaccines in mice. The prM and E proteins appeared to be properly processed and glycosylated, and in plaque reduction neutralization tests and other assays of antigenic specificity, the E proteins exhibited profiles which resembled those of the homologous dengue virus serotypes. Both chimeric viruses replicated in cell lines of vertebrate and mosquito origin to levels comparable to those of homologous dengue viruses but less efficiently than the YF5.2iv parent. YFV/dengue-4 virus, but not YFV/dengue-2 virus, was neurovirulent for 3-week-old mice by intracerebral inoculation; however, both viruses were attenuated when administered by the intraperitoneal route in mice of that age. Single-dose inoculation of either chimeric virus at a dose of 10(5) PFU by the intraperitoneal route induced detectable levels of neutralizing antibodies against the homologous dengue virus strains. Mice which had been immunized in this manner were fully protected from challenge with homologous neurovirulent dengue viruses by intracerebral inoculation compared to unimmunized mice. Protection was associated with significant increases in geometric mean titers of neutralizing antibody compared to those for unimmunized mice. These data indicate that YFV/dengue virus chimeras elicit antibodies which represent protective memory responses in the mouse model of dengue encephalitis. The levels of neurovirulence and immunogenicity of the chimeric viruses in mice correlate with the degree of adaptation of the dengue virus strain to mice. This study supports ongoing investigations concerning the use of this technology for development of a live attenuated viral vaccine against dengue viruses.     

Fusion of intra- and extracellular forms of vaccinia virus with the cell membrane.

The membrane fusion activities of the isolated single-envelope intracellular form of vaccinia virus (INV) and the double-envelope extracellular (EEV) form were studied by using a lipid-mixing assay based on the dilution of a fluorescent probe. Fluorescently labeled INV and EEV from both the IHD-J and WR strains of vaccinia virus fused with HeLa cells at neutral pH, suggesting that fusion occurs with the plasma membrane during virus entry. EEV fused more efficiently and with faster kinetics than INV: approximately 50% of bound EEV particles fused over the course of 1 h, compared with only 25% of the INV particles. Fusion of INV and EEV was strongly temperature dependent, being decreased by 50% at 34 degrees C and by 90% at 28 degrees C. A monoclonal antibody to a 14-kilodalton envelope protein of INV that has been implicated in the fusion reaction (J. F. Rodriguez, E. Paez, and M. Esteban, J. Virol. 61:395-404, 1987) completely suppressed the initial rate of fusion of INV but had no effect on the fusion activity of EEV, suggesting that vaccinia virus encodes two or more membrane fusion proteins. Finally, cells infected with the WR strain of vaccinia virus formed syncytia when briefly incubated at pH 6.4 or below, indicating that an acid-activated viral fusion protein is expressed on the cell surface. However, WR INV and EEV did not display increased fusion activity at acid pH, suggesting that the acid-dependent fusion factor is not incorporated into virions or that its activity there is masked.     

Recombinant dengue virus type 2 envelope/hepatitis B surface antigen hybrid protein expressed in Pichia pastoris can function as a bivalent immunogen

A truncated version of the dengue virus type 2 envelope protein (Den2E) encoding the first 395 amino acid (aa) residues, and Den2E fused in-frame with the full-length 226-aa hepatitis B surface antigen (Den2E-HBsAg) protein were expressed in the methylotrophic yeast, Pichia pastoris. Both the recombinant proteins showed evidence of the capacity to form high molecular weight aggregates. Electron microscopic analysis of the purified proteins showed that while Den2E displayed an amorphous morphology, Den2E-HBsAg existed as well-structured virus-like particles (VLPs). Using immuno-gold electron microscopy, these VLPs were demonstrated to contain both components of the Den2E-HBsAg hybrid protein. Seroanalysis showed that the hybrid VLPs could function in vivo as bivalent immunogens, which could elicit immune responses directed against both components of the hybrid protein, as evidenced by ELISA, immunoprecipitation and immunofluorescence data.     

A recombinant vaccinia virus containing the papilloma E2 protein promotes tumor regression by stimulating macrophage antibody-dependent cytotoxicity

Human papillomavirus infection is associated with cervical cancer. The E6 and E7 papillomavirus proteins are normally required for the maintenance of the malignant phenotype. Expression of these proteins in infected cells is negatively regulated by the binding of the papilloma E2 protein to the long terminal control region of the papilloma virus genome. The E2 protein can also promote cell arrest and apoptosis in HeLa cells. Therefore, it is clear that this protein has the potential of inhibiting the malignant phenotype. Because, anticancer vaccines based in vaccinia viruses have recently been shown to be an effective way to treat and to eradicate tumors, a recombinant vaccinia virus expressing the E2 gene of bovine papilloma virus (Modified Vaccinia Ankara, MVA E2) was created, to explore further the antitumor potential of the E2 protein. A series of rabbits, containing the VX2 transplantable papilloma carcinoma, were treated with MVA E2. An impressive tumor regression, up to a complete disappearance of tumor, was observed in most animals (80%). In contrast, very little or no regression was detected if the normal vaccinia virus was used. Lymphocytes isolated from MVA E2-treated rabbits did not show cytotoxic activity against tumor cells. However, in these animals a humoral immune response against tumor cells was observed. These antitumor antibodies were capable of activating macrophages to destroy tumor cells efficiently. These data indicate that injecting the MVA E2 recombinant vaccinia virus directly into the tumor results in a robust and long-lasting tumor regression. Data also suggest that antitumor antibodies are responsible, at least in part, for eliminating tumors by activating macrophage antibody-dependent cytotoxicity. Received: 23 November 1999 / Accepted: 12 April 2000