Classical swine fever virus: independent induction of protective immunity by two structural glycoproteins

Strategies for differentiating infection in vaccinated animals (DIVA) for foot-and-mouth disease, classical swine fever and avian influenza

The prophylactic use of vaccines against exotic viral infections in production animals is undertaken exclusively in regions where the disease concerned is endemic. In such areas, the infection pressure is very high and so, to assure optimal protection, the most efficient vaccines are used. However, in areas considered to be free from these diseases and in which there is the possibility of only limited outbreaks, the use of Differentiation of Infected from Vaccinated Animals (DIVA) or marker vaccines allows for vaccination while still retaining the possibility of serological surveillance for the presence of infection. This literature review describes the current knowledge on the use of DIVA diagnostic strategies for three important transboundary animal diseases: foot-and-mouth disease in cloven-hoofed animals, classical swine fever in pigs and avian influenza in poultry.

A new type of signal peptidase cleavage site identified in an RNA virus polyprotein

Pestiviruses, a group of enveloped positive strand RNA viruses belonging to the family Flaviviridae, express their genes via a polyprotein that is subsequently processed by proteases. The structural protein region contains typical signal peptidase cleavage sites. Only the site at the C terminus of the glycoprotein E(rns) is different because it does not contain a hydrophobic transmembrane region but an amphipathic helix functioning as the E(rns) membrane anchor. Despite the absence of a hydrophobic region, the site between the C terminus of E(rns) and E1, the protein located downstream in the polyprotein, is cleaved by signal peptidase, as demonstrated by mutagenesis and inhibitor studies. Thus, E(rns)E1 is processed at a novel type of signal peptidase cleavage site showing a different membrane topology. Prevention of glycosylation or introduction of mutations into the C-terminal region of E(rns) severely impairs processing, presumably by preventing proper membrane interaction or disturbing a conformation critical for the protein to be accepted as a substrate by signal peptidase.

Interferon-gamma induction correlates with protection by DNA vaccine expressing E2 glycoprotein against classical swine fever virus infection in domestic pigs

Classical swine fever (CSF) is a highly contagious viral infection affecting domestic and wild pigs. For classical swine fever virus (CSFV), immunization with plasmids expressing different versions of glycoprotein E2 has proven an effective way to induce protection. Previously, we have also shown that immunization with DNA vaccine expressing glycoprotein E2 (DNA-E2) induced specific T helper cell responses in the absence of neutralizing antibodies. However, the role of T cell responses in protection against CSFV is largely unknown. Here we have extended these studies to deeply characterize the role of T cell responses by a DNA-E2 and their correlation with protection against CSFV infection. Thus, pigs vaccinated with the DNA vaccine induced a strong cellular immune response, characterized by the specific induction IFN-gamma expressing T cells after vaccination without any detectable levels of CSFV neutralizing antibodies. Constant levels of CSFV-specific IFN-gamma producing cells observed from the beginning of the infection until 7 days after challenge in vaccinated animals might contribute to early control of CSFV replication, at least until neutralizing antibodies are developed. Severe clinical signs of the disease, including high titers of viremia, pyrexia and virus spread to different organs, were recorded in the non-vaccinated challenged animals, in comparison to the vaccinated animals where only one animal showed mild clinical signs and a short peak of viremia. Lack of complete protection in this animal correlated with a delay on the induction of neutralizing antibodies, detectable only from day 11 post-CSFV challenge. Conversely, the rest of the pigs within the group developed neutralizing antibodies as early as at day two post-challenge, correlating with sterile protection. Finally, an inverse correlation seemed to exist between early induction of IFN-alpha and the protection observed, while IL-10 seemed to be differentially regulated in vaccinated and non-vaccinated animals. Our results support the relevance of the induction of a strong T cellular response to confer a solid protection upon DNA vaccination against CSFV. Further experiments are needed to be done in order to clarify the key cytokines playing a role in CSFV-protection and to obtain emergency vaccines capable to confer robust and fast protection.

Yeast-expressed classical swine fever virus glycoprotein E2 induces a protective immune response

Classical swine fever (CSF) is an economically important swine disease worldwide. The glycoprotein E2 of classical swine fever virus (CSFV) is a viral antigen that can induce a protective immune response against CSF. A recombinant E2 protein was constructed using the yeast Pichia pastoris expression system and evaluated for its vaccine efficacy. The yeast-expressed E2 (yE2) was shown to have N-linked glycosylation and to form homodimer molecules. Four 6-week-old specified-pathogen-free (SPF) piglets were intramuscularly immunized with yE2 twice at 3-week intervals. All yE2-vaccinated pigs could mount an anamnestic response after booster vaccination with neutralizing antibody titers ranging from 1:96 to 1:768. Neutralizing antibody titers at 10 weeks post booster vaccination ranged from 1:16 to 1:64. At this time, the pigs were subjected to challenge infection with a dose of 1x10(5)TCID(50) (50% tissue culture infective dose) virulent CSFV strain. At 1 week post challenge infection, all of the yE2-immunized pigs were alive and without symptoms or signs of CSF. Neutralizing antibody titers at this time ranged from 1:4,800 to 1:12,800 and even to 1:51,200 one week later. In contrast, the control pigs continuously exhibited signs of CSF and had to be euthanized because of severe clinical symptoms at 6 days post challenge infection. All of the yE2-vaccinated pigs were E(rns) antibody negative and had seroconverted against E(rns) by post challenge day 11, suggesting that yE2 is a potential DIVA (differentiating infected from vaccinated animals) vaccine. The yeast-expressed E2 protein retains correct immunogenicity and is able to induce a protective immune response against CSFV infection.

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

Mutation of cysteine 171 of pestivirus E rns RNase prevents homodimer formation and leads to attenuation of classical swine fever virus

Pestiviruses represent important pathogens of farm animals that have evolved unique strategies and functions to stay within their host populations. E(rns), a structural glycoprotein of pestiviruses, exhibits RNase activity and represents a virulence factor of the viruses. E(rns) forms disulfide linked homodimers that are found in virions and virus-infected cells. Mutation or deletion of cysteine 171, the residue engaged in intermolecular disulfide bond formation, results in loss of dimerization as tested in coprecipitation and native protein gel electrophoresis analyses. Nevertheless, stable virus mutants with changes affecting cysteine codon 171 could be recovered in tissue culture. These mutants grew almost as well as the parental viruses and exhibited an RNase-positive phenotype. E(rns) dimerization-negative mutants of classical swine fever virus were found to be attenuated in pigs even though the virus clearly replicated and induced a significant neutralizing antibody response in the animals.

Mucosal or systemic administration of rE2 glycoprotein antigen loaded PLGA microspheres

We have evaluated the ability of recombinant E2 antigen, as a surfactant free formulation of poly (D,L-lactide-co-glycolide) (PLGA) microspheres, to elicit a systemic immune response after administration by mucosal routes (oral and nasal) in comparison to intramuscular route. The sequence encoding a truncated E2 glycoprotein of the classical swine fever virus (CSFV) was expressed in insect cells following infection with recombinant baculovirus, as a His-tagged recombinant antigen. The recombinant E2 glycoprotein (rE2) antigen was co-encapsulated with rabbit serum albumin (RSA) as a protein stabilizer. rE2/RSA loaded PLGA microspheres, with a mean diameter of 4 microm were obtained by a water in oil in water solvent extraction method (w/o/w). Rabbits were immunized with 10 microg of rE2 formulated in PLGA microspheres administrated by three different routes (oral, nasal and intramuscular). After 60 days, each rabbit in all three groups was challenge with 5 microg of rE2 glycoprotein solution by intradermal administration. Blood samples were collected weekly for 90 days and specific rE2 antigen antibodies measured. This work showed that rE2 antigen loaded microspheres was able to initiate an immune response. The intradermal challenge after nasal and oral administration had a clear boost effect on the systemic immune response. Moreover, the response after nasal administration was more intense and less variable than oral route. In conclusion, these data demonstrate a high potential of rE2 loaded PLGA microspheres for their use as a mucosal subunit vaccine.

Baculovirus surface display of E(rns) envelope glycoprotein of classical swine fever virus

Classical swine fever virus (CSFV) causes significant losses in the pig industry in many countries. E(rns) is an envelope glycoprotein of CSFV which is known to induce virus-neutralizing antibodies and protective immunity in the natural host. In this study, one recombinant baculoviruses BacSC-E(rns) expressing histidine-tagged E(rns) with the transmembrane domain (TM) and cytoplasmic domain (CTD) derived from baculovirus envelope protein gp64 was constructed and its immunizing efficacy was evaluated in a mouse model. After infection, E(rns) was expressed and anchored on the plasma membrane of Sf-9 cells, as demonstrated by Western-blot and confocal microscopy. Immunogold electron microscopy demonstrated that the E(rns) glycoprotein was successfully displayed on the baculoviral envelope. Vaccine tests in animals showed that BacSC-E(rns) elicited significantly higher E(rns) antibody titers in the immunized mouse models than the control group. This demonstrates that the BacSC-E(rns) vaccine can be used potentially against CSFV infections. This is the first report demonstrating the potential of E(rns)-pseudotyped baculovirus as a CSFV vaccine.

Characterization of antibody responses against a neutralizing epitope on the glycoprotein E2 of classical swine fever virus

The sequence TAVSPTTLR is a conserved and linear neutralizing epitope on the glycoprotein E2 of classical swine fever virus. In this study, TAVSPTTLR-directed antibodies, induced either by virions or by an epitope-focused immunogen, were characterized. The results revealed that despite the same epitope specificity, the antibodies induced by different immunogens varied significantly both in the neutralizing test and in binding inhibition assays. This suggests that the protective immunity induced by this epitope is due to more than simply the epitope specificity and that this epitope might need essential contributions from its flanking context to induce functional epitope-specific antibodies.

A common neutralizing epitope on envelope glycoprotein E2 of different pestiviruses: Implications for improvement of vaccines and diagnostics for classical swine fever (CSF)?

The Pestivirus genus within the family of Flaviviridae consists of at least three species; classical swine fever virus (CSFV) found in swine and wild boar, bovine viral diarrhoea virus type 1 and type 2 (BVDV-I and BVDV-II) mainly isolated from cattle and border disease virus (BDV) preferably replicating in ovine species. Many features demonstrate differences between CSFV and other pestiviruses, BVDV-I, BVDV-II and BDV, here defined as nonCSFV, whereas other features show similarities between all different species of pestiviruses. Focussing on the major envelope glycoprotein E2, the immunodominant protein of pestiviruses, CSFV seems to be a more distinct species within the Pestivirus genus. Here we confirm on one hand the more separated grouping of CSFV by isolation of monoclonal antibodies (MAbs) raised against E2 of BVDV-I and BVDV-II. None of these MAbs recognize E2 of CSFV strains. On the other hand, only one MAb, MAb 912, was isolated against E2 of BDV. MAb 912 binds to E2 of CSFV strains and partly neutralizes CSFV. The epitope of MAb 912 is mapped in antigenic domain B of CSFV-E2. This common epitope of CSFV strains and nonCSFV strains could have implications for development of DIVA vaccines and serological diagnostics for CSF.

The pestivirus glycoprotein Erns is anchored in plane in the membrane via an amphipathic helix

E(rns) is a structural glycoprotein of pestiviruses found to be attached to the virion and to membranes within infected cells via its COOH terminus, although it lacks a hydrophobic anchor sequence. The COOH-terminal sequence was hypothesized to fold into an amphipathic alpha-helix. Alanine insertion scanning revealed that the ability of the E(rns) COOH terminus to bind membranes is considerably reduced by the insertion of a single amino acid at a wide variety of positions. Mutations decreasing the hydrophobicity of the apolar face of the putative helix led to reduction of membrane association. Proteinase K protection assays showed that E(rns) translated in vitro in the presence of microsomal membranes was protected, whereas a mutant with an artificial transmembrane region and a short cytosolic tag was shortened by the protease treatment. A tag fused to the COOH terminus of wild type E(rns) was not accessible for antibodies within digitonin-permeabilized cells, but the variant with the tag located downstream of the artificial transmembrane region was detected under the same conditions. These results are in accordance with the model that the COOH-terminal membrane anchor of E(rns) represents an amphipathic helix embedded in plane into the membrane. The integrity of the membrane anchor was found to be important for recovery of infectious virus.

Recent advances in the development of recombinant vaccines against classical swine fever virus: cellular responses also play a role in protection

Classical swine fever virus (CSFV) is the causative agent of one of the most devastating porcine haemorrhagic viral diseases, classical swine fever (CSF). CSFV mainly infects endothelial cells and macrophages and at the same time promotes bystander apoptosis of the surrounding T cells, causing strong immune suppression and high mortality rates. Most animals experience acute infection, during which they either die or survive by producing neutralising antibodies to the virus. However, in a few cases, the impaired immune system cannot control viral progression, leading to chronic infection. Efficient live attenuated vaccines against CSFV exist and are routinely used only in endemic countries. The ability of these vaccines to replicate in the host, even at very low rates, makes it extremely difficult to distinguish vaccinated from infected animals, favouring a restricted policy regarding vaccination against CSFV in non-endemic countries. There is a clear need for efficient and safer marker vaccines to assist in the control of future CSF outbreaks. In this review article, some of the most recent advances in the field of recombinant vaccines against CSFV are presented and the nature of the protective immune responses they induce is discussed.

Mutations in the carboxyl terminal region of E2 glycoprotein of classical swine fever virus are responsible for viral attenuation in swine

We have previously reported [Risatti, G.R., Borca, M.V., Kutish, G.F., Lu, Z., Holinka, L.G., French, R.A., Tulman, E.R., Rock, D.L. 2005a. The E2 glycoprotein of classical swine fever virus is a virulence determinant in swine. J. Virol. 79, 3787-3796] that chimeric virus 319.1v containing the E2 glycoprotein gene from Classical Swine Fever Virus (CSFV) vaccine strain CS with the genetic background of highly virulent CSFV strain Brescia (BICv) was markedly attenuated in pigs. To identify the amino acids mediating 319.1v attenuation a series of chimeric viruses containing CS E2 residues in the context of the Brescia strain were constructed. Chimera 357v, containing CS E2 residues 691 to 881 of CSFV polyprotein was virulent, while chimera 358v, containing CS E2 residues 882 to 1064, differing in thirteen amino acids from BICv, was attenuated in swine. Single or double substitutions of those amino acids in BICv E2 to CS E2 residues did not affect virulence. Groups of amino acids were then substituted in BICv E2 to CS E2 residues. Mutant 32v, with six substitutions between residues 975 and 1059, and mutant 33v, with six substitutions between 955 and 994, induced disease indistinguishable from BICv. Mutant 31v, with seven substitutions between residues 882 and 958, induced a delayed onset of lethal disease. Amino acids abrogating BICv virulence were then determined by progressively introducing six CS residues into 31v. Mutant 39v, containing nine residue substitutions, was virulent. Mutant 40v, containing ten residue substitutions, induced mild disease. Mutant 42v, containing twelve substitutions, and mutant 43v, with an amino acid composition identical to 358v, were attenuated in swine indicating that all substitutions were necessary for attenuation of the highly virulent strain Brescia. Importantly, 358v protected swine from challenge with virulent BICv at 3 and 28 days post-infection.

Bovine viral diarrhea virus: prevention of persistent fetal infection by a combination of two mutations affecting Erns RNase and Npro protease

Different genetically engineered mutants of bovine viral diarrhea virus (BVDV) were analyzed for the ability to establish infection in the fetuses of pregnant heifers. The virus mutants exhibited either a deletion of the overwhelming part of the genomic region coding for the N-terminal protease N(pro), a deletion of codon 349, which abrogates the RNase activity of the structural glycoprotein E(rns), or a combination of both mutations. Two months after infection of pregnant cattle with wild-type virus or either of the single mutants, the majority of the fetuses contained virus or were aborted or found dead in the uterus. In contrast, the double mutant was not recovered from fetal tissues after a similar challenge, and no dead fetuses were found. This result was verified with a nonrelated BVDV containing similar mutations. After intrauterine challenge with wild-type virus, mutated viruses, and cytopathogenic BVDV, all viruses could be detected in fetal tissue after 5, 7, and 14 days. Type 1 interferon (IFN) could be detected in fetal serum after challenge, except with wild-type noncytopathogenic BVDV. On days 7 and 14 after challenge, the largest quantities of IFN in fetal serum were induced by the N(pro) and RNase-negative double mutant virus. The longer duration of fetal infection with the double mutant resulted in abortion. Therefore, for the first time, we have demonstrated the essential role of both N(pro) and E(rns) RNase in blocking interferon induction and establishing persistent infection by a pestivirus in the natural host.

Novel marker vaccines against classical swine fever

Classical swine fever (CSF) is one of the most devastating epizootic diseases of pigs worldwide. For eradication and control purposes, CSF vaccination is an important tool, and efficacious and safe attenuated vaccines have been available for many decades (for example, the C-strain vaccines). In addition to administering them parenterally, live attenuated vaccines are also administered orally for the control and eradication of CSF in wild boar populations. However, antibodies against live attenuated vaccines do not allow to differentiate infected from vaccinated animals (DIVA principle) and the mechanism responsible for attenuation is not known. Only a few years ago the first DIVA vaccines based on baculovirus-expressed E2 glycoprotein have been put on the market [Hulst MM, Westra DF, Wensvoort G, Moormann RJ. Glycoprotein E1 of hog cholera virus expressed in insect cells protects swine from hog cholera. J Virol 1993;67(9):5435-42]. However, these subunit E2 marker vaccines are less efficient and more than one parenteral application is necessary. Furthermore, oral vaccination is not possible. Taking these disadvantages into account, the development of novel CSF vaccines has been focussed on five different strategies, mainly based on genetically engineered constructs: (1) immunogenic CSFV peptides, (2) DNA vaccines, (3) viral vectors expressing CSFV proteins, (4) chimeric pestiviruses, and (5) trans-complemented deleted CSFV genomes (replicons).

Marker vaccine strategies and candidate CSFV marker vaccines

Classical swine fever (CSF) is an economically important highly contagious disease of swine worldwide. Classical swine fever virus (CSFV) is its etiological agent, and the only natural hosts are domestic pigs and wild boars. Although field CSFV strains vary in the virulence, they all result in serious losses in pig industry. Highly virulent field strains generally cause acute disease and high mortality; moderately virulent field strains raise subacute or chronic infections; postnatal infection by low virulent field strains produces subclinical infection and mortality in the new-born piglets. CSFV can cross the placental barrier, and this transplacental transmission usually results in mortality of fetuses and birth of congenitally infected pigs with a late-onset disease and death. Two main strategies to control CSF epidemic are systematic prophylactic vaccination with live attenuated vaccines (such as C-strain) and non-vaccination stamping-out policy. But neither of them is satisfying enough. Marker vaccine and companion serological diagnostic test is thought to be a promising strategy for future control and eradication of CSF. During the past 15 years, various candidate marker vaccines were constructed and evaluated in the animal experiments, including recombinant chimeric vaccines, recombinant deletion vaccines, DNA vaccines, subunit vaccines and peptide vaccines. Among them, two subunit vaccines entered the large scale marker vaccine trial of EU in 1999. Although they failed to fulfil all the demands of the Scientific Veterinary Committee, they successfully induced solid immunity against CSFV in the vaccinated pigs. It can be expected that new potent marker vaccines might be commercially available and used in systematic prophylactic vaccination campaign or emergency vaccination in the next 15 years. Here, we summarized current strategies and candidate CSFV marker vaccines. These strategies and methods are also helpful for the development of new-generation vaccines against other diseases.

N-linked glycosylation status of classical swine fever virus strain Brescia E2 glycoprotein influences virulence in swine

E2 is one of the three envelope glycoproteins of classical swine fever virus (CSFV). Previous studies indicate that E2 is involved in several functions, including virus attachment and entry to target cells, production of antibodies, induction of protective immune response in swine, and virulence. Here, we have investigated the role of E2 glycosylation of the highly virulent CSFV strain Brescia in infection of the natural host. Seven putative glycosylation sites in E2 were modified by site-directed mutagenesis of a CSFV Brescia infectious clone (BICv). A panel of virus mutants was obtained and used to investigate whether the removal of putative glycosylation sites in the E2 glycoprotein would affect viral virulence/pathogenesis in swine. We observed that rescue of viable virus was completely impaired by removal of all putative glycosylation sites in E2 but restored when mutation N185A reverted to wild-type asparagine produced viable virus that was attenuated in swine. Single mutations of each of the E2 glycosylation sites showed that amino acid N116 (N1v virus) was responsible for BICv attenuation. N1v efficiently protected swine from challenge with virulent BICv at 3 and 28 days postinfection, suggesting that glycosylation of E2 could be modified for development of classical swine fever live attenuated vaccines.

Identification of a novel virulence determinant within the E2 structural glycoprotein of classical swine fever virus

Classical swine fever virus (CSFV) E2 glycoprotein contains a discrete epitope (TAVSPTTLR, residues 829-837 of CSFV polyprotein) recognized by monoclonal antibody (mAb) WH303, used to differentiate CSFV from related ruminant pestiviruses, Bovine Viral Diarrhea Virus (BVDV) and Border Disease Virus (BDV), that infect swine without causing disease. Progressive mutations were introduced into mAb WH303 epitope in CSFV virulent strain Brescia (BICv) to obtain the homologous amino acid sequence of BVDV strain NADL E2 (TSFNMDTLA). In vitro growth of mutants T1v (TSFSPTTLR), T2v (TSFNPTTLR), T3v (TSFNMTTLR) was similar to parental BICv, while mutants T4v (TSFNMDTLR) and T5v (TSFNMDTLA) exhibited a 10-fold decrease in virus yield and reduced plaque size. In vivo, T1v, T2v or T3v induced lethal disease, T4v induced mild and transient disease and T5v induced mild clinical signs. Protection against BICv challenge was observed at 3 and 21 days post-T5v infection. These results indicate that E2 residues TAVSPTTLR play a significant role in CSFV virulence.

Quadruple antigenic epitope peptide producing immune protection against classical swine fever virus

Research on epitope-based vaccines is a current focus in the development of new vaccines against classical swine fever virus (CSFV). The present study aimed to engineer a quadruple antigenic epitope peptide of the CSFV immunogen E2 glycoprotein by splice overlap extension (SOE) PCR, expressed in E. coli fused with glutathione S-transferase (GST), and named rGST-4E. Enzyme-linked immunosorbent assay (ELISA) and Western blot analysis showed that purified rGST-4E had an excellent immunoreactivity with swine anti-CSFV serum and rabbit anti-E2 serum. Animal vaccination trials showed that the rGST-4E was more immunogenic than mono-epitope peptide and was able to produce effective immune protection in rabbits against challenge with hog cholera lapinized virus, and in pigs against challenge with virulent CSFV. These data show that the recombinant repeated epitope peptide could be considered a potential epitope-based vaccine for prevention of the disease.

Candidate peptide-vaccines induced immunity against CSFV and identified sequential neutralizing determinants in antigenic domain A of glycoprotein E2

Antigenic domain A is a highly conserved unit on envelope protein E2 of classical swine fever virus (CSFV). It was found that mutant E2 containing only unit A, with the unit BC deleted, provided immunized pigs with complete protection against the lethal challenge. In this study, six overlapping peptides (A1-A6) covering this unit were synthesized and conjugated to bovine serum albumin (BSA). Two candidate multi-peptide-vaccines (MPVs) using aluminum adjuvant successfully induced potent immunity against CSFV in pigs. Although both candidate MPVs failed to provide complete protection, they showed better protective activity than that induced by C-strain. Subsequently, neutralizing epitopes in unit A were identified using a panel of peptide-vaccines (PVs). Six candidate peptide-vaccines (PV-An, n=1-6) were separately given to six groups of pigs. Among these candidates, PV-A2 and PV-A6 exhibited the most potent protective activity, while the other four showed weaker or almost no effects. Moreover, the polyclonal antibodies induced by PV-A2 and PV-A6 were capable of neutralizing C-strain virus at the dilution 1:16 in vitro. Thus, two principal sequential neutralizing determinants covered by peptide A2 (aa792-814) and A6 (aa844-865) were demonstrated to exist in the antigenic domain A, and can be recruited in developing new effective "marker vaccine" against CSFV.

Secreted expression of the classical swine fever virus glycoprotein Erns in yeast and application to a sandwich blocking ELISA

E(rns) is an envelope glycoprotein of classical swine fever virus (CSFV) with RNase activity. The purpose of this study was to produce an active E(rns) for further applications using the yeast secreted expression system. The E(rns) gene was cloned into the expression vector pGAPZalphaC which was introduced into Pichia pastoris. Expression of E(rns) protein in culture supernatant was confirmed by Western blot analysis using both the monoclonal antibody against CSFV E(rns) and CSFV-positive swine serum. The yeast-expressed E(rns) (yE(rns)) was shown to have N-linked glycosylation and to form homodimer of 74 kDa molecules. All monomer, homodimer, and deglycosylated forms of yE(rns) demonstrated intrinsic ribonuclease activity and a clear preference for uridine-rich sequence. A direct sandwich blocking enzyme-linked immunosorbent assay (ELISA) based on the yE(rns) was developed with a high sensitivity and specificity. The yE(rns) which possesses enzymatic activity and retains antigenicity may provide a useful material for developing a diagnostic kit.

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.

Characterization of epitopes for neutralizing monoclonal antibodies to classical swine fever virus E2 and Erns using phage-displayed random peptide library

Infection of cells with classical swine fever virus (CSFV) is mediated by the interaction of envelope glycoproteins E2 and Erns with receptor molecules on the cell surface. These proteins are also the major antigens for eliciting neutralizing antibodies and conferring protective immunity. Here we report the identification of multiple neutralizing epitopes on these proteins by screening a phage-displayed random peptide library with CSFV-specific neutralizing monoclonal antibodies. Two different E2-specific neutralizing mAbs (a18 and 24/10) were found to bind to a common motif SPTxL, which is similar to the sequence SPTTL of the E2 protein (aa 289-293), indicating that this is likely to be an immunodominant epitope. Similarly, an immunodominant epitope corresponding to the sequence DKN of Erns (aa 117-119) was identified for two independent Erns-specific neutralizing antibodies, b4-22 and 24/16, respectively. Another binding motif, CxNNxTC, was identified for mAb 24/16, but not for b4-22. Sequencing analysis of the genes coding for the light chain of these mAbs was conducted to ensure that all mAbs were derived from different hybridomas, rather than from different subclones of a common parent line. Inhibition studies using immunofluorescent antibody assay and virus neutralization test demonstrated that the mimotope peptides truly mimicked the antibody binding determinants on the viral proteins. The detailed mapping data for these neutralizing epitopes will be useful for development of improved diagnostic tests and perhaps a peptide-based vaccine for this important swine disease.

Oronasal vaccination with classical swine fever virus (CSFV) replicon particles with either partial or complete deletion of the E2 gene induces partial protection against lethal challenge with highly virulent CSFV

A cDNA clone of the classical swine fever virus (CSFV) strain Alfort/187 [Ruggli N, Tratschin JD, Mittelholzer C, Hofmann MA. Nucleotide sequence of classical swine fever virus strain Alfort/187 and transcription of infectious RNA from stably cloned full-length cDNA. J Virol 1996;70(6):3478-87] was used to construct two E2 deletion mutants lacking either the complete E2 gene or, alternatively, a stretch of 204 nucleotides encoding 68 amino acids located in the C-terminal region of the E2 glycoprotein. The respective in vitro synthesized mutant RNAs replicated in SK-6 cells but no infectious virus was generated. Both replicons could be packaged into virus particles in SK-6 cells constitutively expressing E2 of CSFV. For the resulting CSF virus replicon particles (CSF-VRP) A187-E2del373 and A187-E2del68 titers of 10(6) and 10(7) TCID(50)/ml, respectively, were obtained. Oronasal vaccination with 10(7) TCID(50) of either of the two CSF-VRP protected pigs against a challenge with a lethal dose of CSFV strain Eystrup. In contrast, after intradermal vaccination VRP A187-E2del68 but not VRP A187-E2del373 lacking the complete E2 gene induced a protective immune response. We conclude that E2-complemented CSF-VRP have the potential to be used as live-attenuated non-transmissible oral vaccines for pigs. In addition, our data suggest that E2 of CSFV is dispensable for the induction of mucosal but not of parenteral immunity.

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.

Vaccinology of classical swine fever: from lab to field

There are two types of classical swine fever vaccines available: the classical live and the recently developed E2 subunit vaccines. The live Chinese strain vaccine is the most widely used. After a single vaccination, it confers solid immunity within a few days that appears to persist lifelong. The E2 subunit vaccine induces immunity from approximately 10-14 days after a single vaccination. The immunity may persist for more than a year, but is then not complete. The Chinese strain vaccine may establish a strong herd immunity 1-2 weeks earlier than the E2 vaccine. The ability of the Chinese vaccine strain to prevent congenital infection has not been reported, but the E2 subunit vaccine does not induce complete protection against congenital infection. Immunological mechanisms that underlie the protective immunity are still to be elucidated. Both types of vaccine are considered to be safe. A great advantage of the E2 subunit vaccine is that it allows differentiation of infected pigs from vaccinated pigs and is referred to as a DIVA vaccine. However, the companion diagnostic E(rns) ELISA to actually make that differentiation should be improved. Many approaches to develop novel vaccines have been described, but none of these is likely to result in a new DIVA vaccine reaching the market in the next 5-10 years. Countries where classical swine fever is endemic can best control the infection by systematic vaccination campaigns, accompanied by the normal diagnostic procedures and control measures. Oral vaccination of wild boar may contribute to lowering the incidence of classical swine fever, and consequently diminishing the threat of virus introduction into domestic pigs. Free countries should not vaccinate and should be highly alert to rapidly diagnose any new outbreak. Once a new introduction of classical swine fever virus in dense pig areas has been confirmed, an emergency vaccination programme should be immediately instituted, for maximum benefit. The question is whether the time is ripe to seriously consider global eradication of classical swine fever virus.

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.

Proportions and phenotypic expression of peripheral blood leucocytes in pigs vaccinated with an attenuated C strain and a subunit E2 vaccine against classical swine fever

The influence of an attenuated classical swine fever virus C strain vaccine and a subunit E2 vaccine against classical swine fever on the peripheral blood leucocyte proportion and phenotypic expression in 12-week-old pigs was studied. The C strain was amplified in minipig kidney cell culture and final product contained 10(4 +/- 0.15) TCID50/ml, while the subunit vaccine contained 32 microg per dose of gp E2. Haematological findings showed that the vaccines did not cause leucopenia or lymphocytopenia and the number of neutrophils and eosinophils during the observation period was within physiological range. The results of the proportion of CD4a+, CD5a+, CD8a+, wCD21+, CD45RA+, CD45RC+ , non-T non-B, SWC3a+ and CD11b+ cells were gained by single-colour flow cytometry. At the end of the trial a significantly increase of percentage of CD4+, CD5a+, CD8+, wCD21+ cells has been found in pigs that received the subunit vaccine and the percentage of CD4+, CD5a+, CD8+, CD45RA+ and CD45RC+ cells was higher in pigs that received the attenuated vaccine. Twenty-eight days after vaccination the percentage of CD4+, CD45RA+ and CD45RC+ was significantly higher in pigs vaccinated with the C strain than in pigs vaccinated with the subunit vaccine. In contrary, the percentage of the wCD21- cells was higher in pigs that received the subunit vaccine. Statistically higher values of SWC3a+ and lower values of CD11b+ cells was observed in pigs that received the attenuated vaccine than in pigs vaccinated with the subunit vaccine. Taken altogether, our results showed that the subunit vaccine produced a better stimulation of B cells and CD11b+ monocytes/macrophages /granulocytes/NK cells, whereas the attenuated vaccine induced a higher response of Th cells, naive/memory cells and macrophages/neutrophils. Thus, both vaccines were able to influence the porcine immune system, by activating different subsets of the immune effector/accessory cells.

Comparison of antibody values in sera of pigs vaccinated with a subunit or an attenuated vaccine against classical swine fever

Ten pigs, aged 85 days, were vaccinated with a subunit vaccine containing 32 microg of classical swine fever virus glycoprotein E2 (gp E2) (group 1), and a further 10 pigs were vaccinated with a C strain vaccine (10(4+/-0.15) TCID50/ml), produced by amplification in minipig kidney (MPK) cell culture (group 2). Nine non-vaccinated pigs served as a control group (group 3). Serum samples were collected before (day 0) and at 4, 10, 21 and 28 days after vaccination and were analysed by two commercially available enzyme immunoassays and by a neutralizing peroxidase-linked assay (NPLA). At the same times, peripheral blood was taken for determining the total leukocyte count and the body temperature was taken daily. Antibodies were not detected in serum samples collected before vaccination (day 0), and no side-effects that could be connected with vaccination were observed during the trial. Ten days after vaccination 6/10 pigs vaccinated with the subunit vaccine were seropositive. On days 21 and 28, the ratios of serologically positive to vaccinated pigs were 9/10 and 10/10, respectively. Four of the ten pigs that were vaccinated with the C strain vaccine were positive on day 21 and 9/10 on day 28. However, the results of the NPLA showed that only 4/10 pigs had an antibody titre > 1:32 at the end of the trial in both the vaccinated groups, even though the subunit vaccine initiated an earlier and higher level of neutralizing antibodies than the vaccine produced from the C strain. Challenge was performed 28 days after vaccination on four randomly selected pigs from both vaccinated groups. The pigs survived the challenge without showing any clinical signs of classical swine fever (CSF), while two nonvaccinated control pigs died on the 10th and 12th days after infection.

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.

Candidate peptide vaccine induced protection against classical swine fever virus

Former investigations demonstrated that the envelope glycoprotein E2 could protect pigs from classical swine fever virus (CSFV). Based on these findings, we prepared synthetic peptide vaccine using E2 N-terminal antigenic units B/C and hoped to induce protective activity against lethal challenge of virulent CSFV strain Shimen. Five overlapped peptides sequence-covering amino acids 693-777 on E2 of Shimen were synthesized and then conjugated with bovine serum albumin (BSA), respectively. In the vaccination course, the candidate peptide vaccines in combination (multi-peptide vaccine (MPV)) were applied for immunization of pigs (n=10) and induced strong antibody response against CSFV. It is subsequently demonstrated that this peptide vaccine could provide immunized pigs complete protection against lethal CSFV challenge as C-strain does, while all non-immunized pigs in negative control group manifested obvious typical symptoms and died during the second and third weeks after viral challenge. In order to confirm the neutralizing activity of the polyclonal antibodies induced by MPV, neutralization assay were carried out on rabbits. The live C-strain alone could ordinarily induce typical fever on rabbits. The typical fever of rabbits induced by the live C-strain could be inhibited by pre-incubation with the anti-sera (dilution 1:4 and 1:16) induced by MPV, but not inhibited by pre-incubation with the same anti-sera from which the antibodies against five peptides were removed by peptide-specific affinity chromatography, which indicates that these peptide-specific antibodies in the anti-sera induced by MPV provided protective activity against CSFV. Our finding provides a new way to develop marker vaccine against CSFV.

A structural model of pestivirus E(rns) based on disulfide bond connectivity and homology modeling reveals an extremely rare vicinal disulfide

E(rns) is a pestivirus envelope glycoprotein and is the only known viral surface protein with RNase activity. E(rns) is a disulfide-linked homodimer of 100 kDa; it is found on the surface of pestivirus-infected cells and is secreted into the medium. In this study, the disulfide arrangement of the nine cysteines present in the mature dimer was established by analysis of the proteolytically cleaved protein. Fragments were obtained after digestion with multiple proteolytic enzymes and subsequently analyzed by liquid chromatography-electrospray ionization mass spectrometry. The analysis demonstrates which cysteine is involved in dimerization and reveals an extremely rare vicinal disulfide bridge of unknown function. With the assistance of the disulfide arrangement, a three-dimensional model was built by homology modeling based on the alignment with members of the Rh/T2/S RNase family. Compared to these other RNase family members, E(rns) shows an N-terminal truncation, a large insertion of a cystine-rich region, and a C-terminal extension responsible for membrane translocation. The homology to mammalian RNase 6 supports a possible role of E(rns) in B-cell depletion.

Role of disulfide bond formation in the folding and assembly of the envelope glycoproteins of a pestivirus

Bovine viral diarrhea virus (BVDV) is a pestivirus member of the Flaviviridae family, closely related to, and used as a surrogate model for the hepatitis C virus. Its envelope contains the E1 and E2 glycoproteins, disulfide linked into homo- and heterodimers. In this study, we investigate the role of disulfide bond formation in the folding, assembly, and stability of BVDV glycoproteins. We provide molecular evidence that intact disulfide bonds are critical for the acquirement of a stable conformation of E2 monomers. Forcing the E2 glycoproteins to adopt a reduced conformation either co- or post-translationally before assembly into dimers, determines their misfolding and degradation by proteasome. In contrast, dimerization of E2 glycoproteins results in a conformation resistant to reducing agents and degradation. Furthermore, inhibition of the ER-alpha-mannosidase activity leads to impairment of misfolded E2 degradation, demonstrating the involvement of this enzyme in targeting viral proteins towards proteasomal degradation.

Recovery of virulent and RNase-negative attenuated type 2 bovine viral diarrhea viruses from infectious cDNA clones

Cloned cDNA derived from the genome of the virulent type 2 bovine viral diarrhea virus (BVDV) strain NY'93/C was sequenced and served for establishment of the infectious cDNA clone pKANE40A. Virus recovered from pKANE40A exhibited growth characteristics similar to those of wild-type BVDV NY'93/C and proved to be clinically indistinguishable from the wild-type virus in animal experiments. A virus mutant in which the RNase residing in the viral glycoprotein E(rns) was inactivated, revealed an attenuated phenotype. The plasmid pKANE40A represents the first infectious cDNA clone established for a type 2 BVDV and offers a variety of new approaches to analyze the mechanisms of BVDV-induced disease in cattle.

Identification of T-cell epitopes in the structural and non-structural proteins of classical swine fever virus

To identify new T-cell epitopes of classical swine fever virus (CSFV), 573 overlapping, synthetic pentadecapeptides spanning 82% of the CSFV (strain Glentorf) genome sequence were synthesized and screened. In proliferation assays, 26 peptides distributed throughout the CSFV viral protein sequences were able to induce specific T-cell responses in PBMCs from a CSFV-Glentorf-infected d/d haplotype pig. Of these 26 peptides, 18 were also recognized by PBMCs from a CSFV-Alfort/187-infected d/d haplotype pig. In further experiments, it could be shown that peptide 290 (KHKVRNEVMVHWFDD), which corresponds to amino acid residues 1446-1460 of the CSFV non-structural protein NS2-3 could induce interferon-gamma secretion after secondary in vitro restimulation. The major histocompatibility complex (MHC) restriction for stimulation of T-cells by this pentadecapeptide was identified as being mainly MHC class II and partially MHC class I. In cytolytic assays, CSFV-specific cytotoxic T-lymphocytes (CTLs) were able to lyse peptide 290-loaded target cells. These findings indicate the existence of a CSFV-specific helper T-cell epitope and a CTL epitope in this peptide.

Experimental non-transmissible marker vaccines for classical swine fever (CSF) by trans-complementation of E(rns) or E2 of CSFV

Three mutants with deletions in the E2 gene of the infectious DNA copy of the classical swine fever virus (CSFV) strain-C were constructed: one missing the B/C domain of CSFV-E2 between amino acids (aa) 693 and 746, one missing the A domain between aa 800 and 864, and one missing the complete E2 between aa 689 and 1062. All three CSFV-E2 deletion mutants were unable to generate viable virus, indicating that each of the antigenic domains of E2 is essential for viability of CSFV. To rescue the CSFV-E2 deletion mutants SK6 cell lines constitutively expressing glycoprotein E2 of CSFV were generated. The rescued viruses infected and replicated in SK6 cells as demonstrated by expression of viral proteins, but this primary infection did not result in reproduction of infectious virus. Thus, these E2 complemented viruses are considered non-transmissible. In previous experiments, we showed that simultaneous injection of E(rns) complemented virus (Flc23) via intradermal (ID), intramuscular (IM) or intranasal (IN) routes conferred protection to pigs against a lethal challenge with CSFV [J. Virol. 74 (2000) 2973]. Here, we evaluate different routes of application (ID, IM or IN) with E(rns) complemented virus Flc23 in order to find the best route for complemented CSFVs. Intradermal injection with Flc23 protected pigs against a lethal CSFV challenge, whereas intramuscular injection induced partial protection, and intranasal injection did not mediate a protective immune response in pigs at all. We used the intradermal route of vaccination to test the E2 complemented viruses. Vaccination of pigs via the intradermal route with the E2 complemented CSFVs also resulted in the induction of antibodies and in (partial) protection against CSFV challenge. Pigs vaccinated with E2 complemented virus Flc4 (deletion B/C domain) survived a lethal CSFV challenge, whereas partial protection was induced in pigs vaccinated with Flc47 (deletion E2) or Flc48 (deletion A domain) E2 complemented viruses. Serological data demonstrate that these E2 complemented mutant viruses are, in combination with well known diagnostic tests based on E2, potential marker vaccines for CSF.

Translocation activity of C-terminal domain of pestivirus Erns and ribotoxin L3 loop

The pestivirus envelope glycoprotein E(rns) has RNase activity and therefore was suspected to enter cells to cleave RNA. The protein contains an RNase domain with a C-terminal extension, which shows homology with a membrane-active peptide. The modular architecture and the C-terminal homology suggested that the C terminus could be responsible for the presumed translocation. Peptides corresponding to the C-terminal domain of E(rns) and also the homologous L3 loop of ribotoxin II were indeed able to translocate across the eukaryotic cell membrane and were targeted to the nucleoli. The entire E(rns) protein was also able to translocate into the cell. Furthermore, other labeled proteins and even active enzymes could be transported inside the cell when they were attached to the C-terminal E(rns) peptide. Translocation was energy-independent and not mediated by a protein receptor. The peptides showed no specificity for cell type or species.

Classical swine fever (CSF) marker vaccine. Trial I. Challenge studies in weaner pigs

Two commercial marker vaccines against classical swine fever virus (CSFV) and companion diagnostic tests were examined in 160 conventional pigs. To test the vaccines in a "worst case scenario", group of 10 weaners were vaccinated using a single dose of an E2 (gp55) based vaccine at days -21, -14, -10 or -7, and subsequently challenged at day 0. The challenge virus was CSFV 277, originating from a recent outbreak of classical swine fever (CSF) in Germany. In all groups, only 5 out of 10 pigs were challenged; the remaining 5 pigs served as vaccinated contact controls. Also, three control groups, each consisting of 10 non-vaccinated pigs, were challenged in parallel to the vaccinated animals. CSFV could be isolated from all non-vaccinated pigs. Among these pigs 40% displayed a chronic course of the infection (virus positive for more than 10 days). Pigs vaccinated 21 or 14 days before challenge displayed no clinical signs of CSFV after challenge. However, they were still able to replicate CSFV when challenged, as measured by reisolation of CSFV from leukocytes of the directly challenged pigs. CSFV could be isolated from the leucocytes of 25% of the pigs vaccinated 21 days before challenge and 50% of the pigs vaccinated 14 days before challenge. Chronic infection was not observed, but transmission to one vaccinated contact pig occurred. From all pigs vaccinated 10 or 7 days before challenge, CSFV could be reisolated. We observed a chronic course of infection in 5% of pigs vaccinated 10 days before challenge and in 30% of pigs vaccinated 7 days before challenge. The mortality rate was 20% in the pigs vaccinated 10 days before challenge, and varied between 20 and 80% in pigs vaccinated 7 days prior to challenge. The contact animals had lower mortality (0-20%) than directly challenged pigs, probably mirroring the delayed time point of infection. There was thus some protection against clinical illness by both marker vaccines, but not a solid protection against infection and virus shedding. The efficacy of the vaccine was best if used 3 weeks before challenge and a clear correlation between time interval from vaccination to challenge and the level of virus shedding was observed. Each vaccine had its own accompanying discriminatory ELISA, but 18% of the virus positive pigs never seroconverted in these tests.

An E2 sub-unit marker vaccine does not prevent horizontal or vertical transmission of classical swine fever virus

An experimental infection with classical swine fever (CSF) virus in E2 sub-unit marker vaccine vaccinated gilts was conducted in order to evaluate the effect of vaccination on virus transmission and course of the disease. Therefore, clinical signs as well as horizontal and vertical virus transmission were monitored in two inoculated, non-vaccinated and 10 vaccinated conventional gilts, housed in individual sow boxes. Within 10 days post-inoculation, all vaccinated gilts became infected. Depending on the definition of the infectious period, two different estimates of R0 were calculated (R0=14.8 and 3.3), both significantly larger than 1 (P<0.01). In three out of the eight vaccinated pregnant gilts vertical virus transmission occurred, resulting in infected offspring. Based on the results of this experiment, it can be concluded that double vaccination with an E2 sub-unit marker vaccine only protects pregnant gilts from the clinical course of the disease but does not prevent horizontal nor vertical spread of the CSF virus.

Construction of recombinant swinepox viruses and expression of the classical swine fever virus E2 protein

To explore the swinepox virus (SPV) as a potential live vector for immunization, a vector was developed for the construction of a recombinant SPV carrying foreign genes. In this system, a foreign gene placed under the strong vaccinia virus promoter P(11) can be inserted into the viral thymidine kinase (TK) gene, and the recombinant virus can be isolated in a non-selective medium by the co-expression of E. coli lacZ gene. Compared with the wild type virus, the TK(-)recombinant SPV showed a modest level of attenuation in porcine cells while more attenuation was observed in monkey or human cells. Using this system, a recombinant virus expressing the E2 glycoprotein of classical swine fever virus (CSFV) was produced. Engineered with the gX signal sequence of the pseudorabies virus, and transmembrane domain of E2, the E2 protein was expressed as a dimeric form in the cytoplasm of the infected cells.