Interaction of classical swine fever virus with membrane-associated heparan sulfate: role for virus replication in vivo and virulence

Viral and cellular determinants of the hepatitis C virus envelope-heparan sulfate interaction

Cellular binding and entry of hepatitis C virus (HCV) are the first steps of viral infection and represent a major target for antiviral antibodies and novel therapeutic strategies. We have recently demonstrated that heparan sulfate (HS) plays a key role in the binding of HCV envelope glycoprotein E2 to target cells (Barth et al., J. Biol. Chem. 278:41003-41012, 2003). In this study, we characterized the HCV-HS interaction and analyzed its inhibition by antiviral host immune responses. Using recombinant envelope glycoproteins, virus-like particles, and HCV pseudoparticles as model systems for the early steps of viral infection, we mapped viral and cellular determinants of HCV-HS interaction. HCV-HS binding required a specific HS structure that included N-sulfo groups and a minimum of 10 to 14 saccharide subunits. HCV envelope binding to HS was mediated by four viral epitopes overlapping the E2 hypervariable region 1 and E2-CD81 binding domains. In functional studies using HCV pseudoparticles, we demonstrate that HCV binding and entry are specifically inhibited by highly sulfated HS. Finally, HCV-HS binding was markedly inhibited by antiviral antibodies derived from HCV-infected individuals. In conclusion, our results demonstrate that binding of the viral envelope to a specific HS configuration represents an important step for the initiation of viral infection and is a target of antiviral host immune responses in vivo. Mapping of viral and cellular determinants of HCV-HS interaction sets the stage for the development of novel HS-based antiviral strategies targeting viral attachment and entry.

Virulent variants emerging in mice infected with the apathogenic prototype strain of the parvovirus minute virus of mice exhibit a capsid with low avidity for a primary receptor

The mechanisms involved in the emergence of virulent mammalian viruses were investigated in the adult immunodeficient SCID mouse infected by the attenuated prototype strain of the parvovirus Minute Virus of Mice (MVMp). Cloned MVMp intravenously inoculated in mice consistently evolved during weeks of subclinical infection to variants showing altered plaque phenotypes. All the isolated large-plaque variants spread systemically from the oronasal cavity and replicated in major organs (brain, kidney, liver), in sharp contrast to the absolute inability of the MVMp and small-plaque variants to productively invade SCID organs by this natural route of infection. The virulent variants retained the MVMp capacity to infect mouse fibroblasts, consistent with the lack of genetic changes across the 220-to-335 amino acid sequence of VP2, a capsid domain containing main determinants of MVM tropism. However, the capsid of the virulent variants shared a lower affinity than the wild type for a primary receptor used in the cytotoxic infection. The capsid gene of a virulent variant engineered in the MVMp background endowed the recombinant virus with a large-plaque phenotype, lower affinity for the receptor, and productive invasiveness by the oronasal route in SCID mice, eventually leading to 100% mortality. In the analysis of virulence in mice, both MVMp and the recombinant virus similarly gained the bloodstream 1 to 2 days postoronasal inoculation and remained infectious when adsorbed to blood cells in vitro. However, the wild-type MVMp was cleared from circulation a few days afterwards, in contrast to the viremia of the recombinant virus, which was sustained for life. Significantly, attachment to an abundant receptor of primary mouse kidney epithelial cells by both viruses could be quantitatively competed by wild-type MVMp capsids, indicating that virulence is not due to an extended receptor usage in target tissues. We conclude that the selection of capsid-receptor interactions of low affinity, which favors systemic infection, is a major evolutionary process in the adaptation of parvoviruses to new hosts and in the cause of disease.

Mutation of E1 glycoprotein of classical swine fever virus affects viral virulence in swine

Transposon linker insertion mutagenesis of a full-length infectious clone (IC) (pBIC) of the pathogenic classical swine fever virus (CSFV) strain Brescia was used to identify genetic determinants of CSFV virulence and host range. Here, we characterize a virus mutant, RB-C22v, possessing a 19-residue insertion at the carboxyl terminus of E1 glycoprotein. Although RB-C22v exhibited normal growth characteristics in primary porcine macrophage cell cultures, the major target cell of CSFV in vivo, it was markedly attenuated in swine. All RB-C22v-infected pigs survived infection remaining clinically normal in contrast to the 100% mortality observed for BICv-infected animals. Comparative pathogenesis studies demonstrated a delay in RB-C22v spread to, and decreased replication in the tonsils, a 10(2) to 10(7) log10 reduction in virus titers in lymphoid tissues and blood, and an overall delay in generalization of infection relative to BICv. Notably, RB-C22v-infected animals were protected from clinical disease when challenged with pathogenic BICv at 3, 5, 7, and 21 days post-RB-C22v inoculation. Viremia, viral replication in tissues, and oronasal shedding were reduced in animals challenged at 7 and 21 DPI. Notably BICv-specific RNA was not detected in tonsils of challenged animals. These results indicate that a carboxyl-terminal domain of E1 glycoprotein affects virulence of CSFV in swine, and they demonstrate that mutation of this domain provides the basis for a rationally designed and efficacious live-attenuated CSF vaccine.

The carboxy-terminal sequence of the pestivirus glycoprotein E(rns) represents an unusual type of membrane anchor

The E(rns) protein is a structural glycoprotein of pestiviruses that lacks a typical membrane anchor sequence and is known to be secreted from the infected cell. However, major amounts of the protein are retained within the cell and attached to the virion by a so far unknown mechanism. Transient-expression studies with cDNA constructs showed that in a steady-state situation, 16% of the protein is found in the supernatant of the transfected cells while 84% appears as intracellular protein. We show here that E(rns) represents a membrane-bound protein. Membrane binding occurs via the carboxy-terminal region of E(rns). By fusion of this sequence to the carboxy terminus of green fluorescent protein (GFP), the subcellular localization of the reporter protein switched from cytosolic to membrane bound. A core sequence of 11 amino acids necessary for membrane binding was elicited in truncation experiments with GFP constructs. However, this peptide is not sufficient to confer membrane anchoring but needs either upstream or downstream accessory sequences. Analyses with different extraction procedures showed that E(rns) is neither easily stripped from the membrane, like a peripheral membrane protein, nor as tightly membrane bound as a transmembrane protein.

Dimerization of glycoprotein E(rns) of classical swine fever virus is not essential for viral replication and infection

The pestivirus glycoprotein E(rns), a ribonuclease, is expressed on the surface of virions and in infected cells as a disulfide-linked homodimer. E(rns) is involved in the infection process and its RNase activity is probably involved in viral replication and pathogenesis. The most C-terminal cysteine residue forms an intermolecular disulfide bond with another E(rns) monomer, resulting in an E(rns) dimer. To study the function of dimerisation of E(rns) for viral replication, the cysteine residue at amino acid position 438 was mutated into a serine residue. The mutated C438S gene was cloned into a vector containing an infectious cDNA copy of the CSFV C-strain genome. Using reverse genetics, a mutant virus was generated that only expressed monomeric E(rns), confirming that Cys 438 is essential for homo-dimerization. Characterization of this mutant virus and of a baculovirus-expressed C438S mutant protein indicated that the loss of the dimeric state of E(rns) reduced the affinity of binding of virions and E(rns) to heparan sulphate (HS), the receptor for E(rns) on the cell surface of SK6 cells. This suggests that interaction of virus-bound E(rns) homodimers with membrane associated HS may be a joined action of the two HS-binding domains (one in each monomer) present in the homodimer.

Hepatitis C virus and hepatitis B virus bind to heparin: purification of largely IgG-free virions from infected plasma by heparin chromatography

Binding to heparin of hepatitis C virus (HCV) and hepatitis B virus (HBV) from chronic carriers was investigated. Eighty per cent of HCV RNA from an agammaglobulinaemic patient (IgG-free virus) was retained on immobilized heparin and eluted with > or =0.4 M NaCl, in contrast to approximately 20 % from immunocompetent chronic carriers (with < or =8 % IgG-free virus). Increased binding to heparin of the HCV fraction that was not retained by a protein G column suggested that antibodies complexed to the virions partially inhibited the interaction. A higher proportion (15-80 %) of HBV from chronic carriers bound to heparin and eluted with > or =0.4 M NaCl. After washing of the heparin columns with 0.3 M NaCl, <1 % of total plasma proteins co-eluted with HCV or HBV. By this one-step heparin chromatography, without ultracentrifugation, IgG-free HCV and IgG-free HBV were preferentially purified from human plasma by 1000-fold and greater than 500-fold, respectively. Following assessment with an anti-E2 envelope protein antibody, the amount of immunoprecipitated HCV particles after heparin purification was similar to that in the original plasma, suggesting that undamaged virions were purified. This was further supported by heparin-purified HCV binding to lymphocyte cell lines in a dose-dependent manner. Intact HBV particles were detected by electron microscopy. It was concluded that HCV and HBV from chronically infected patients bind to heparin, the closest homologue of liver heparan sulfate, and that heparin chromatography is an efficient and gentle method for purifying these viruses from human plasma. In the absence of cell-culture systems or alternative robust purification methods, heparin chromatography may help greatly in binding and infectivity studies.

The E2 glycoprotein of classical swine fever virus is a virulence determinant in swine

To identify genetic determinants of classical swine fever virus (CSFV) virulence and host range, chimeras of the highly pathogenic Brescia strain and the attenuated vaccine strain CS were constructed and evaluated for viral virulence in swine. Upon initial screening, only chimeras 138.8v and 337.14v, the only chimeras containing the E2 glycoprotein of CS, were attenuated in swine despite exhibiting unaltered growth characteristics in primary porcine macrophage cell cultures. Additional viral chimeras were constructed to confirm the role of E2 in virulence. Chimeric virus 319.1v, which contained only the CS E2 glycoprotein in the Brescia background, was markedly attenuated in pigs, exhibiting significantly decreased virus replication in tonsils, a transient viremia, limited generalization of infection, and decreased virus shedding. Chimeras encoding all Brescia structural proteins in a CS genetic background remained attenuated, indicating that additional mutations outside the structural region are important for CS vaccine virus attenuation. These results demonstrate that CS E2 alone is sufficient for attenuating Brescia, indicating a significant role for the CSFV E2 glycoprotein in swine virulence.

Characterization of classical swine fever virus entry by using pseudotyped viruses: E1 and E2 are sufficient to mediate viral entry

Classical swine fever virus (CSFV) is the causative agent of classical swine fever. Its envelope comprises glycoproteins E(rns), E1, and E2. In this study, we showed that the unmodified CSFV glycoproteins could incorporate into the HIV core to generate an infectious CSFV pseudotyped virus. The infection was specific to several porcine cell lines, and could be neutralized by anti-E2 monoclonal antibodies (mAbs) completely and by anti-E(rns) mAbs partially, indicating that this pseudotyped virus can mimic the early infection steps of parental CSFV. To investigate the specific role of each envelope protein involved in viral entry, a series of pseudotyped viruses were generated bearing CSFV glycoproteins in various combinations. It was found that specific infectivity was also achieved with non-E(rns) pseudotyped virus carrying E1 and E2 glycoproteins. This indicated that E1 and E2 are sufficient to mediate CSFV entry, and E(rns) is not indispensable in this process.

Antibody responses of pigs to defined Erns fragments after infection with classical swine fever virus

Antibody responses of pigs to defined Erns fragments, after classical swine fever virus (CSFV) infection, were studied by using an enzyme-linked immunosorbent assay (ELISA). Selection of various E(rns) fragments was based on an immunodominant Erns region encompassing three overlapping antigenic regions, amino acids 65 to 145 (Erns(aa)65-145) (AR1), 84 to 160 (Erns(aa)84-160) (AR2), and 109 to 220 (Erns(aa)109-220) (AR3), identified earlier by our group (M. Lin, E. Trottier, J. Pasick, and M. Sabara, J. Biochem., in press). Defined Erns fragments, including AR1, AR2, AR3, Erns(aa)65-160 (AR12), Erns(aa)84-220 (AR23), Erns(aa)65-220 (AR123), Erns(aa)109-145 (the consensus region defined by the three overlapping regions), and Erns(aa)109-160 (a fragment 15 amino acids larger than the consensus region), were expressed in Escherichia coli, purified by nickel chelate affinity chromatography, and used to measure antibody responses in 20 sera serially collected from pigs experimentally infected with CSFV. Based on the optimum cutoffs determined by receiver operating characteristic analysis after testing 238 negative field sera from Canadian sources, all the Erns fragments were capable of distinguishing positive from negative antibody responses with sensitivities ranging between 75 and 90% and specificities ranging between 83.2 and 100%. Detection of antibody responses to refolded Erns(aa)109-145 and Erns(aa)109-160 by ELISA (this study) but not by Western blots (Lin et al., in press) indicated that the epitopes within the consensus region are conformational. When cutoff values were raised to give a specificity of 100%, four Erns fragments (AR2, AR23, Erns(aa)109-145, and Erns(aa)109-160) offered much higher sensitivities (75 to 90%) than those obtained with other fragments (20 to 65%). Erns(aa)109-145 and Erns(aa)109-160 were capable of detecting antibody responses in infected pigs as early as 7 days postinfection. Demonstration of antibody responses to either one of the four fragments can thus be an alternative to use of the full-length protein in ELISA for serological diagnosis of CSFV infection. An advantage of such a test would be its utilization for serological survey in a classical swine fever-free country (e.g., Canada) in biocontainment level 2 laboratories.

Determinants of virulence of classical swine fever virus strain Brescia

Two related classical swine fever virus (CSFV) strain Brescia clones were isolated from blood samples from an infected pig. Virus C1.1.1 is a cell-adapted avirulent variant, whereas CoBrB is a virulent variant. Sequence analysis revealed 29 nucleic acid mutations in C1.1.1, resulting in 9 amino acid substitutions compared to the sequence of CoBrB (476)R. Using reverse genetics, parts of the genomes of these viruses, which contain differences that lead to amino acid changes, were exchanged. Animal experiments with chimeric viruses derived from C1.1.1 and CoBrB (476)R showed that a combination of amino acid changes in the structural and nonstructural regions reduced the virulence of CSFV in pigs. Moreover, the presence of a Leu at position 710 in structural envelope protein E2 seemed to be an important factor in the virulence of the virus. Changing the Leu at position 710 in the CoBrB (476)S variant into a His residue did not affect virulence. However, the (710)His in the C1.1.1/CoBrB virus, together with adaptive mutations (276)R, (476)R, and (477)I in E(rns), resulted in reduced virulence in pigs. These results indicated that mutations in E(rns) and E2 alone do not determine virulence in pigs. The results of in vitro experiments suggested that a high affinity for heparan sulfate of C1.1.1 E(rns) may reduce the spread of the C1.1.1/CoBrB virus in pigs and together with the altered surface structure of E2 caused by the (710)L-->H mutation may result in a less efficient infection of specific target cells in pigs. Both these features contributed to the attenuation of the C1.1.1/CoBrB virus in vivo.

Heparan sulfate-independent infection attenuates high-neurovirulence GDVII virus-induced encephalitis

The high-neurovirulence Theiler's murine encephalomyelitis virus (TMEV) strain GDVII uses heparan sulfate (HS) as a coreceptor to enter target cells. We report here that GDVII virus adapted to growth in HS-deficient cells exhibited two amino acid substitutions (R3126L and N1051S) in the capsid and no longer used HS as a coreceptor. Infectious-virus yields in CHO cells were 25-fold higher for the adapted virus than for the parental GDVII virus, and the neurovirulence of the adapted virus in intracerebrally inoculated mice was substantially attenuated. The adapted virus showed altered cell tropism in the central nervous systems of mice, shifting from cerebral and brainstem neurons to spinal cord anterior horn cells; thus, severe poliomyelitis, but not acute encephalitis, was observed in infected mice. These data indicate that the use of HS as a coreceptor by GDVII virus facilitates cell entry and plays an important role in cell tropism and neurovirulence in vivo.

An avirulent chimeric Pestivirus with altered cell tropism protects pigs against lethal infection with classical swine fever virus

A chimeric Pestivirus was constructed using an infectious cDNA clone of bovine viral diarrhea virus (BVDV) [J. Virol. 70 (1996) 8606]. After deletion of the envelope protein E2-encoding region, the respective sequence of classical swine fever virus (CSFV) strain Alfort 187 was inserted in-frame resulting in plasmid pA/CP7_E2alf. After transfection of in vitro-transcribed CP7_E2alf RNA, autonomous replication of chimeric RNA in bovine and porcine cell cultures was observed. Efficient growth of chimeric CP7_E2alf virus, however, could only be demonstrated on porcine cells, and in contrast to the parental BVDV strain CP7, CP7_E2alf only inefficiently infected and propagated in bovine cells. The virulence, immunogenicity, and "marker vaccine" properties of the generated chimeric CP7_E2alf virus were determined in an animal experiment using 27 pigs. After intramuscular inoculation of 1 x 10(7) TCID(50), CP7_E2alf proved to be completely avirulent, and neither viremia nor virus transmission to contact animals was observed; however, CSFV-specific neutralizing antibodies were detected from day 11 after inoculation. In addition, sera from all animals reacted positive in an E2-specific CSFV-antibody ELISA, but were negative for CSFV-E(RNS)-specific antibodies as determined with a CSFV marker ELISA. After challenge infection with highly virulent CSFV strain Eystrup, pigs immunized with CP7_E2alf were fully protected against clinical signs of CSFV infection, viremia, and shedding of challenge virus, and almost all animals scored positive in a CSFV marker ELISA. From our results, we conclude that chimeric CP7_E2alf may not only serve as a tool for a better understanding of Pestivirus attachment, entry, and assembly, but also represents an innocuous and efficacious modified live CSFV "marker vaccine".

Attenuation of Sindbis virus variants incorporating uncleaved PE2 glycoprotein is correlated with attachment to cell-surface heparan sulfate

Sindbis virus virions incorporating uncleaved precursor envelope protein PE2 bind efficiently to cell-surface heparan sulfate (HS) because the furin cleavage site (a consensus HS-binding domain) is retained in the mature virus particle. However, they are essentially nonviable. Resuscitating mutations selected in the E3 or E2 protein preserve the PE2 noncleaving phenotype and HS binding, but facilitate fusion, and thereby restore wild-type infectivity on cultured cells. Here, we have demonstrated that the resuscitated PE2 noncleaving virus was almost avirulent in vivo, but mutated during the infection. Mutants had increased virulence and cleavage of PE2, with reduced HS binding capacity. We hypothesize that HS binding leads to sequestration of PE2 noncleaving virus particles and suppression of serum viremia, thereby selecting for evolution of the virus into a PE2-cleaving, low HS-binding phenotype.

Heparan sulphate mediates swine vesicular disease virus attachment to the host cell

Heparan sulphate (HS) has been found to serve as receptor for initial cell binding of numerous viruses. Different glycosaminoglycans (GAGs), including heparin and HS, were analysed for their ability to bind swine vesicular disease virus (SVDV), a picornavirus with close homology to human coxsackie B5 virus. Binding of SVDV was established by heparin-affinity chromatography. In addition, infection of IB-RS-2 epithelial porcine cells was inhibited by treating the virus with soluble HS, heparin, and chondroitin sulphate B (CS-B), as well as by enzymic digestion of cell surface GAGs. Analysis of the infection course showed that SVDV uses cellular HS for its binding to the cell surface and that this interaction occurs during attachment of the virus, prior to its internalization into the cell. Sequence analysis of SVDV variants selected for their lack of sensitivity to heparin inhibition in vitro led to the identification of two residues (A2135V and I1266K) potentially involved in heparin/HS interaction. The location of these residues in a three-dimensional model shows that they are clustered in a well-exposed region of the capsid, providing a physical mechanism that could account for the heparin-binding phenotype.

Establishment and characterisation of two cDNA-derived strains of classical swine fever virus, one highly virulent and one avirulent

The virulence of classical swine fever virus (CSFV) strains including established laboratory strains as well as field isolates ranges from avirulent to highly virulent. Here, we describe the construction and characterisation of two cDNA-derived CSFV strains, each corresponding to one of these extremes. The recombinant virus vEy-37 caused acute disease indistinguishable from that provoked by infection with the highly virulent parent strain Eystrup. In contrast, vRiems-3, a molecular clone of the CSFV vaccine strain Riems, was avirulent and induced protective immunity in pigs. After repeated passage of vEy-37 in porcine kidney SK-6 cells adaptive mutations in the E(rns) gene were observed. The respective reconstructed mutant virus grew to titres that were almost 4log units higher when compared to vEy-37. The mutation in the E(rns) gene had only a minor effect on the virulence of the virus. The complete genomic sequences of the two CSFV strains, Eystrup and Riems, have been deposited in GenBank (accession number AF326963 for CSFV Eystrup, AY259122 for CSFV Riems/IVI).

CD46 is a cellular receptor for bovine viral diarrhea virus

Various monoclonal antibodies (MAbs) that recognize cell surface proteins on bovine cells were previously shown to efficiently block infection with bovine viral diarrhea virus (BVDV) (C. Schelp, I. Greiser-Wilke, G. Wolf, M. Beer, V. Moennig, and B. Liess, Arch. Virol. 140:1997-2009, 1995). With one of these MAbs, a 50- to 58-kDa protein was purified from calf thymus by immunoaffinity chromatography. Microchemical analysis of two internal peptides revealed significant sequence homology to porcine and human CD46. The cDNA of bovine CD46 (CD46(bov)) was cloned and further characterized. Heterologously expressed CD46(bov) was detected by the MAb used for purification. A putative function of CD46(bov) as a BVDV receptor was studied with respect to virus binding and susceptibility of nonpermissive cells. While the expression of CD46(bov) correlated well with the binding of [(3)H]uridine-labeled BVDV, the susceptibility of cells nonpermissive for BVDV was not observed. However, the expression of CD46(bov) resulted in a significant increase in the susceptibility of porcine cells to BVDV. These results provide strong evidence that CD46(bov) serves as a cellular receptor for BVDV.

Manipulation of cell surface macromolecules by flaviviruses

Cell surface macromolecules play a crucial role in the biology and pathobiology of flaviviruses, both as receptors for virus entry and as signaling molecules for cell–cell interactions in the processes of vascular permeability and inflammation. This review examines the cell tropism and pathogenesis of flaviviruses from the standpoint of cell surface molecules, which have been implicated as receptors in both virus–cell as well as cell–cell interactions. The emerging picture is one that encompasses extensive regulation and interplay among the invading virus, viral immune complexes, Fc receptors, major histocompatibility complex antigens, and adhesion molecules.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

Role for bovine viral diarrhea virus Erns glycoprotein in the control of activation of beta interferon by double-stranded RNA

Production of alpha/beta interferon in response to viral double-stranded RNA (dsRNA) produced during viral replication is a first line of defense against viral infections. Here we demonstrate that the Erns glycoprotein of the pestivirus bovine viral diarrhea virus can act as an inhibitor of dsRNA-induced responses of cells. This effect is seen whether Erns is constitutively expressed in cells or exogenously added to the culture medium. The Erns effect is specific to dsRNA since activation of NF-kappaB in cells infected with Semliki Forest virus or treated with tumor necrosis factor alpha was not affected. We also show that Erns contains a dsRNA-binding activity, and its RNase is active against dsRNA at a low pH. Both the dsRNA binding and RNase activities are required for the inhibition of dsRNA signaling, and we discuss here a model to account for these observations.

Cellular binding of hepatitis C virus envelope glycoprotein E2 requires cell surface heparan sulfate

The conservation of positively charged residues in the N terminus of the hepatitis C virus (HCV) envelope glycoprotein E2 suggests an interaction of the viral envelope with cell surface glycosaminoglycans. Using recombinant envelope glycoprotein E2 and virus-like particles as ligands for cellular binding, we demonstrate that cell surface heparan sulfate proteoglycans (HSPG) play an important role in mediating HCV envelope-target cell interaction. Heparin and liver-derived highly sulfated heparan sulfate but not other soluble glycosaminoglycans inhibited cellular binding and entry of virus-like particles in a dose-dependent manner. Degradation of cell surface heparan sulfate by pretreatment with heparinases resulted in a marked reduction of viral envelope protein binding. Surface plasmon resonance analysis demonstrated a high affinity interaction (KD 5.2 x 10-9 m) of E2 with heparin, a structural homologue of highly sulfated heparan sulfate. Deletion of E2 hypervariable region-1 reduced E2-heparin interaction suggesting that positively charged residues in the N-terminal E2 region play an important role in mediating E2-HSPG binding. In conclusion, our results demonstrate for the first time that cellular binding of HCV envelope requires E2-HSPG interaction. Docking of E2 to cellular HSPG may be the initial step in the interaction between HCV and the cell surface resulting in receptor-mediated entry and initiation of infection.

The pestivirus Erns glycoprotein interacts with E2 in both infected cells and mature virions

E(rns) is a pestivirus envelope glycoprotein indispensable for virus attachment and infection of target cells. Unlike the other two envelope proteins E1 and E2, E(rns) lacks a transmembrane domain and a vast quantity is secreted into the medium of infected cells. The protein is also present in fractions of pure pestivirus virions, raising the important and intriguing question regarding the mechanism of its attachment to the pestivirus envelope. In this study a direct interaction between E(rns) and E2 glycoproteins was demonstrated in both pestivirus-infected cells and mature virions. By co- and sequential immunoprecipitation we showed that an E(rns)-E2 heterodimer is assembled very early after translation of the viral polyprotein and before its processing is completed. Our results suggest that E(rns) is attached to the pestivirus envelope via a direct interaction with E2 and explain the role of E(rns) in the initial virus-target cell interaction.

Determination of the sequence of the complete open reading frame and the 5'NTR of the Paderborn isolate of classical swine fever virus

The classical swine fever (CSF) epidemic in the Netherlands in 1997-1998 lasted 14 months, during which 429 infected and 1300 at risk herds were culled, at an estimated economical cost of 2 billion US dollars. Despite the overwhelming scale of the epizootic, the CSF virus (CSFV) strain causing the outbreak has remained largely uncharacterized. The Dutch epizootic is epidemiologically linked to a small CSF outbreak in 1997, in Paderborn in Germany. E2 and partial 5' NTR sequencing has shown that the index Paderborn isolate, and several Dutch isolates taken during the 1997-1998 epizootic, are virtually identical, confirming that the Paderborn isolate triggered the Dutch outbreak, and furthermore showing that this single isolate was stable throughout the whole Dutch outbreak (the above reviewed in [C. Terpstra, A. J. de Smit, Veterinary Microbiol. 77 (2000) 3-15]). We determined the nucleotide sequence of the 5' NTR (by 5' RACE) and the complete open reading frame of the Paderborn isolate (GenBank AY072924). Our sequence was identical to previously published partial 5'NTR and E2 sequences for the index Paderborn 1997 and Dutch 1997 (Venhorst) isolates, confirming the identity of the virus we sequenced. Phylogenetic analysis based on the complete open reading frame showed that Paderborn is genetically very different from common European laboratory reference strains. Neutralization studies showed that Paderborn is also antigenically very different from common laboratory strains such as Alfort 187. Paderborn is the only recent European CSFV field isolate for which a complete sequence is available, and given Paderborns genetic and antigenic uniqueness, the Paderborn sequence may have practical use for diagnostic and vaccine antigen development.

Evaluation of genetically engineered derivatives of a Chinese strain of foot-and-mouth disease virus reveals a novel cell-binding site which functions in cell culture and in animals

Adaptation of field isolates of foot-and-mouth disease virus (FMDV) to grow in cells in culture can result in changes in viral properties that include acquisition of the ability to bind to cell surface heparan sulfate (HS). After 13 passages on BHK cells to produce a vaccine, a Cathay topotype isolate of FMDV serotype O from China (O/CHA/90) extended its cell culture host range and bound to heparin-Sepharose, although it did not require cell surface HS as a receptor molecule. To understand these phenomena, we constructed chimeric viruses by using a type A(12) infectious cDNA and the capsid protein-coding regions of O/CHA/90 and its cell culture-adapted derivative (vac-O/CHA/90). Using a set of viruses derived from these chimeras by exchanging portions of the capsid-coding regions, we discovered that a group of amino acid residues that surround the fivefold axis of the icosahedral virion determine host range in cell culture and influence pathogenicity in pigs. These residues included aromatic amino acids at positions 108 and 174 and positively charged residues at positions 83 and 172 in protein 1D. To test if these residues participated in non-integrin-dependent cell binding, the integrin-binding RGD sequence in protein 1D was changed to KGE in two different chimeras. Evaluation of these KGE viruses indicated that growth in cell culture was not dependent on HS. One of these viruses was tested in pigs, where it produced a mild disease and maintained its KGE sequence. These results are discussed in terms of receptor utilization and pathogenesis of this important pathogen.