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

Development of a dual immunochromatographic test strip to detect E2 and Erns antibodies against classical swine fever

Background

It is essential to consider a practical antibody test to successfully implement marker vaccines and validate vaccination efficacy against classical swine fever virus (CSFV). The test should include a serological antibody assay, combined with a tool for differentiating infected from vaccinated animals (DIVA). The immunochromatographic test strip (ICS) has been exclusively designed for detecting CSFV E2 antibodies while lacking in detecting Erns antibodies, which can be employed and satisfy DIVA strategy. This study developed a novel ICS for detecting CSFV E2/Erns dual-antibody. The effectiveness of ICS in evaluating the DIVA capability of two novel chimeric pestivirus vaccine candidates was assessed.

Methods

Recombinant E2 or Erns protein was transiently expressed in the plant benthamiana using Agrobacterium tumefaciens. ICS was subsequently assembled, and goat anti-rabbit IgG and recombinant CSFV E2 or Erns protein were plated onto the nitrocellulose membrane as control and test lines, respectively. The sensitivity and specificity of ICS were evaluated using sera with different neutralizing antibody titers or positive for antibodies against CSFV and other pestiviruses. The coincidence rates for detecting E2 and Erns antibodies between ICS and commercial enzyme-linked immunosorbent assay (ELISA) kits were also computed. ICS performance for DIVA capability was evaluated using sera from pigs vaccinated with conventional vaccine or chimeric vaccine candidates.

Results

E2 and Erns proteins were successfully expressed in N. benthamiana-produced recombinant proteins. ICS demonstrated high sensitivity in identifying CSFV E2 and Erns antibodies, even at the low neutralizing antibody titers. No cross-reactivity with antibodies from other pestiviruses was confirmed using ICS. There were high agreement rates of 93.0 and 96.5% between ICS and two commercial ELISA kits for E2 antibody testing. ICS also achieved strong coincidence rates of 92.9 and 89.3% with two ELISA kits for Erns antibody detection. ICS confirmed the absence of CSFV Erns-specific antibodies in sera from pigs vaccinated with chimeric vaccine candidates.

Conclusion

E2 and Erns proteins derived from the plant showed great potential and can be used to engineer a CSFV E2/Erns dual-antibody ICS. The ICS was also highly sensitive and specific for detecting CSFV E2 and Erns antibodies. Significantly, ICS can fulfill the DIVA concept by incorporating chimeric vaccine candidates.

Development and application of an indirect ELISA for detection of antibodies against emerging atypical porcine pestivirus

BACKGROUND

Atypical porcine pestivirus (APPV) is a newly discovered swine pestivirus, which can cause congenital tremor and high mortality in newborn piglets and subclinical infection in adult pigs, leading to significant impacts on the pig industry. Currently, there is no approved serological method to assess APPV infection status in pig farms.

METHODS

In this study, the envelope glycoprotein E2 of APPV was highly expressed in suspension HEK293 cells, and further an indirect enzyme-linked immunosorbent assay based on the recombinant E2 protein (E2-iELISA) was developed and evaluated.

RESULTS

The reaction parameters of the E2-iELISA were optimized, and the cutoff value was determined to be 0.2 by analyzing S/P values of 165 negative sera against APPV that were confirmed by virus neutralization test (VNT). Specificity test showed that the method had no cross-reaction with other common swine viruses. The E2-iELISA was evaluated using a panel of swine sera, and showed high sensitivity (113/120, 94.2%) and specificity (65/70, 92.9%), and the agreement rate with VNT was 93.7% (178/190). Subsequently, the E2-iELISA was utilized to investigate the seroprevalence of APPV in pig herds of China. When detecting 1368 pig serum samples collected from nine provinces in China, the overall seroprevalence of APPV was 73.9% (1011/1368).

CONCLUSION

Our findings suggest that the E2-iELISA is specific and sensitive, and could be a valuable tool for serological surveillance of APPV infection in pigs.

Efficacy evaluation of a bivalent subunit vaccine against classical swine fever virus and porcine circovirus type 2

Classical swine fever virus (CSFV) and porcine circovirus type 2 (PCV2) are two of the most devastating and economically significant pathogens affecting pig populations worldwide. Administration of a combination of vaccines against swine pathogens has been demonstrated to be as efficacious as the administration of single vaccines. In this study, we developed and tested a novel bivalent subunit vaccine against CSFV and PCV2. The safety and efficacy of this vaccine were demonstrated in mice and specific pathogen-free (SPF) piglets. In addition to investigating the serological responses after immunization, challenge studies with both viruses were also conducted. The results showed that this CSFV/PCV2 bivalent vaccine elicited a high level of neutralizing antibodies against both viruses and provided protection in challenge studies. In conclusion, the CSFV/PCV2 bivalent vaccine is safe and effective against CSFV or PCV2 challenge.

Performance of a Differentiation of Infected from Vaccinated Animals (DIVA) Classical Swine Fever Virus (CSFV) Serum and Oral Fluid Erns Antibody AlphaLISA Assay

Classical swine fever virus (CSFV) is an OIE-listed disease that requires effective surveillance tools for its detection and control. The aim of this study was to develop and evaluate the diagnostic performance of a novel CSFV Erns IgG AlphaLISA for both serum and oral fluid specimens that would likewise be compatible with the use of CSFV E2 DIVA vaccines. Test performance was evaluated using a panel of well-characterized serum (n = 760) and individual (n = 528) or pen-based (n = 30) oral fluid samples from four groups of animals: (1) negative controls (n = 60 pigs); (2) inoculated with ALD strain wild-type CSFV (n = 30 pigs); (3) vaccinated with LOM strain live CSFV vaccine (n = 30 pigs); and (4) vaccinated with live CSFV marker vaccine on commercial farms (n = 120 pigs). At a cutoff of S/P ≥ 0.7, the aggregate estimated diagnostic sensitivities and specificities of the assay were, respectively, 97.4% (95% CI 95.9%, 98.3%) and 100% for serum and 95.4% (95% CI 92.9%, 97.0%) and 100% for oral fluid. The Erns IgG antibody AlphaLISA combined DIVA capability with solid diagnostic performance, rapid turnaround, ease of use, and compatibility with both serum and oral fluid specimens.

The regulation of cell homeostasis and antiviral innate immunity by autophagy during classical swine fever virus infection

CSFV (classical swine fever virus) is currently endemic in developing countries in Asia and has recently re-emerged in Japan. Under the pressure of natural selection pressure, CSFV keeps evolving to maintain its ecological niche in nature. CSFV has evolved mechanisms that induce immune depression, but its pathogenic mechanism is still unclear. In this study, using transcriptomics and metabolomics methods, we found that CSFV infection alters innate host immunity by activating the interferon pathway, inhibiting host inflammation, apoptosis, and remodelling host metabolism in porcine alveolar macrophages. Moreover, we revealed that autophagy could alter innate immunity and metabolism induced by CSFV infection. Enhanced autophagy further inhibited CSFV-induced RIG-I-IRF3 signal transduction axis and JAK-STAT signalling pathway and blocked type I interferon production while reducing autophagy inhibition of the NF-κB signalling pathway and apoptosis in CSFV infection cells. Furthermore, the level of CSFV infection-induced glycolysis and the content of lactate and pyruvate, as well as 3-phosphoglyceraldehyde, a derivative of glycolysis converted to serine, was altered by autophagy. We also found that silencing HK2 (hexokinase 2), the rate-limiting enzyme of glycolytic metabolism, could induce autophagy but reduce the interferon signalling pathway, NF-κB signalling pathway, and inhibition of apoptosis induced by CSFV infection. In addition, inhibited cellular autophagy by silencing ATG5 or using 3-Methyladenine, could backfill the inhibitory effect of silencing HK2 on the cellular interferon signalling pathway, NF-κB signalling pathway, and apoptosis.

Identification of two novel T cell epitopes on the E2 protein of classical swine fever virus C-strain

C-strain, also known as the HCLV strain, is a well-known live attenuated vaccine against classical swine fever (CSF), a devastating disease caused by classical swine fever virus (CSFV). Vaccination with C-strain induces a rapid onset of protection, which is associated with virus-specific gamma interferon (IFN-γ)-secreting CD8+ T cell responses. The E2 protein of CSFV is a major protective antigen. However, the T cell epitopes on the E2 protein remain largely unknown. In this study, eight overlapping nonapeptides of the E2 protein were predicted and synthesized to screen for potential T cell epitopes on the CSFV C-strain E2 protein. Molecular docking was performed on the candidate epitopes with the swine leukocyte antigen-1*0401. The analysis obtained two highly conserved T cell epitopes, 90STEEMGDDF98 and 331ATDRHSDYF339, which were further identified by enzyme-linked immunospot assay. Interestingly, the mutants deleting or substituting the epitopes are nonviable. Further analysis demonstrated that 90STEEMGDDF98 is crucial for the E2 homodimerization, while CSFV infection is significantly inhibited by the 331ATDRHSDYF339 peptide treatment. The two novel T cell epitopes can be used to design new vaccines that are able to provide rapid-onset protection.

Generation and Efficacy of Two Chimeric Viruses Derived from GPE- Vaccine Strain as Classical Swine Fever Vaccine Candidates

A previous study proved that vGPE^- mainly maintains the properties of classical swine fever (CSF) virus, which is comparable to the GPE^- vaccine seed and is a potentially valuable backbone for developing a CSF marker vaccine. Chimeric viruses were constructed based on an infectious cDNA clone derived from the live attenuated GPE^- vaccine strain as novel CSF vaccine candidates that potentially meet the concept of differentiating infected from vaccinated animals (DIVA) by substituting the glycoprotein E^rns of the GPE^- vaccine strain with the corresponding region of non-CSF pestiviruses, either pronghorn antelope pestivirus (PAPeV) or Phocoena pestivirus (PhoPeV). High viral growth and genetic stability after serial passages of the chimeric viruses, namely vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns, were confirmed in vitro. In vivo investigation revealed that two chimeric viruses had comparable immunogenicity and safety profiles to the vGPE^- vaccine strain. Vaccination at a dose of 10^4.0 TCID₅₀ with either vGPE^-/PAPeV E^rns or vGPE^-/PhoPeV E^rns conferred complete protection for pigs against the CSF virus challenge in the early stage of immunization. In conclusion, the characteristics of vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns affirmed their properties, as the vGPE^- vaccine strain, positioning them as ideal candidates for future development of a CSF marker vaccine.

The recombinant Erns and truncated E2-based indirect enzyme-linked immunosorbent assays to distinguishably test specific antibodies against classical swine fever virus and bovine viral diarrhea virus

Background

Classical swine fever (CSF) virus is the causative agent of an economically important, highly contagious disease of pigs. CSFV is genetically and serologically related to bovine viral diarrhea virus (BVDV). BVDV infection in pigs can mimic CSF clinical signs, which cause difficulty in differentiation. Serological test for detection of virus specific antibodies is a valuable tool for diagnosis and surveillance of CSFV and BVDV infections in animals. The aim of this study was to develop the CSFV Erns and BVDV tE2 -based ELISAs to distinguishably test specific antibodies against CSFV and BVDV.

Methods

The CSFV Erns and truncated E2 (tE2, residues 690–865) of BVDV were expressed in E. coli and purified by Ni–NTA affinity chromatography, respectively. Employing Erns or tE2 protein as diagnostic antigen, indirect ELISAs were developed to distinguishably test specific antibodies against CSFV and BVDV. The specificity and sensitivity of ELISAs were evaluated using a panel of virus specific sera of pigs, immunized rabbits and immunized mice. A total 150 clinical serum samples from farm pigs were measured by the developed ELISAs and compared with virus neutralizing test (VNT).

Results

Indirect ELISA was established based on recombinant CSFV Erns or BVDV tE2 protein, respectively. No serological cross-reaction between antibodies against CSFV and BVDV was observed in sera of immunized rabbits, immunized mice or farm pigs by detections of the Erns and tE2 -based ELISAs. Compared to VNT, the CSFV Erns -based ELISA displayed a high sensitivity (93.3%), specificity (92.0%) and agreement rate (92.7%), and the sensitivity, specificity and agreement rate of BVDV tE2 -based ELISA was 92.3%, 95.2% and 94.7%, respectively.

Conclusion

The newly developed ELISAs are highly specific and sensitive and would be valuable tools for serological diagnosis for CSFV and BVDV infections.

Characterization of monoclonal antibodies that specifically differentiate field isolates from vaccine strains of classical swine fever virus

Classical swine fever virus (CSFV) is a major animal pathogen threatening the global pork industry. To date, numerous anti-CSFV monoclonal antibodies (mAbs) and their recognizing epitopes have been reported. However, few mAbs were systematically characterized for the capacity to differentiate field CSFV isolates from CSF vaccine strains, and the molecular basis associated with antigenic differences between vaccines and field isolates is still largely unknown. In the present study, recombinant CSFV structural glycoproteins E2 of both virulent and vaccine strains and Erns of vaccine strain were expressed using eukaryotic cells and murine mAbs generated against E2 and Erns. After serial screening and cloning of the hybridomas, the viral spectra of mAbs were respectively determined by indirect fluorescent antibody assay (IFA) using 108 CSFVs, followed by Western blot analysis using expressed glycoproteins of all CSFV sub-genotypes including vaccine strains. The antigenic structures recognized by these mAbs were characterized by epitope mapping using truncated, chimeric, and site-directed mutated E2 and Erns proteins. We have identified two vaccine-specific, one field isolate-specific, and two universal CSFV-specific mAbs and five novel conformational epitopes with critical amino acid (aa) motifs that are associated with these five mAbs: 213EPD215, 271RXGP274, and 37LXLNDG42 on E2 and 38CKGVP42, W81, and D100/V107 on Erns. Particularly, E213 of E2 is field isolate-specific, while N40 of E2 and D100/V107 of Erns are vaccine strain-specific. Results from our study further indicate that N40D of E2 mutation in field strains was likely produced under positive selection associated with long-term mass vaccination, leading to CSFV evasion of host immune response. Taking together, this study provides new insights into the antigenic structure of CSFV E2 and Erns and the differentiating mAbs will contribute to the development of a diagnostic strategy to differentiate C-strain vaccination from natural infection (DIVA) of CSFV in terms of elimination of CSF in China.

A Novel Competitive ELISA for Specifically Measuring and Differentiating Immune Responses to Classical Swine Fever C-Strain Vaccine in Pigs

Classical swine fever can be controlled effectively by vaccination with C-strain vaccine. In this study, we developed a novel competitive enzyme-linked immunosorbent assay (cELISA) based on a C-strain E^rns specific monoclonal antibody (mAb 1504), aiming to serologically measure immune responses to C-strain vaccine in pigs, and finally to make the C-strain become a DIVA-compatible vaccine. The cELISA system was established based on the strategy that mAb 1504 will compete with the C-strain induced antibodies in the pig serum to bind the C-strain E^rns protein. The cELISA was optimized and was further evaluated by testing different categories of pig sera. It can efficiently differentiate C-strain immunized from wild-type CSFV-infected pigs and lacks cross-reaction with other common swine viruses and viruses in genus Pestivirus such as Bovine viral diarrhea virus (BVDV). The C-strain antibody can be tested in pigs 7-14 days post vaccination with this cELISA. The sensitivity and specificity of the established cELISA were 100% (95% confidence interval: 95.60 to 100%) and 100% (95% confidence interval: 98.30 to 100%), respectively. This novel cELISA is a reliable tool for specifically measuring and differentiating immune responses to C-strain vaccine in pigs. By combining with the wild-type CSFV-specific infection tests, it can make the C-strain have DIVA capability.

Dendrimer nanoplatforms for veterinary medicine applications: A concise overview

Within the nanoparticle (NP) space, dendrimers are becoming increasingly important in the field of nanomedicine, not only to treat human diseases, but also in veterinary medicine, which represents a new therapeutic approach. Major applications include using dendrimers to tackle highly contagious foot-and-mouth disease virus (FMDV) and swine fever virus (SFV) in pigs, FMDV in cattle, hypothermic circulatory arrest (HCA) in dogs, rabies, and H9N2 avian influenza virus in chickens. As we review here, intramuscular (im) subcutaneous (sc), intravenous (iv), and intraperitoneal (ip) routes of administration can be used for the successful application of dendrimers in animals.

A Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Protects Pigs against PCV2b Challenge and Induces Serum Neutralizing Antibody Response against CSFV

Porcine circovirus type 2 (PCV2) is endemic worldwide. PCV2 causes immunosuppressive infection. Co-infection of pigs with other swine viruses, such as pseudorabies virus (PRV) and classical swine fever virus (CSFV), have fatal outcomes, causing the swine industry significant economic losses in many if not all pig-producing countries. Currently available inactivated/modified-live/vectored vaccines against PCV2/CSFV/PRV have safety and efficacy limitations. To address these shortcomings, we have constructed a triple gene (thymidine kinase, glycoprotein E [gE], and gG)-deleted (PRVtmv) vaccine vector expressing chimeric PCV2b-capsid, CSFV-E2, and chimeric E^rns-fused with bovine granulocytic monocyte-colony stimulating factor (E^rns-GM-CSF), designated as PRVtmv+, a trivalent vaccine. Here we compared this vaccine’s immunogenicity and protective efficacy in pigs against wild-type PCV2b challenge with that of the inactivated Zoetis Fostera Gold PCV commercial vaccine. The live PRVtmv+ prototype trivalent subunit vaccine is safe and highly attenuated in pigs. Based on PCV2b-specific neutralizing antibody titers, viremia, viral load in lymphoid tissues, fecal-virus shedding, and leukocyte/lymphocyte count, the PRVtmv+ yielded better protection for vaccinated pigs than the commercial vaccine after the PCV2b challenge. Additionally, the PRVtmv+ vaccinated pigs generated low to moderate levels of CSFV-specific neutralizing antibodies.

Purification of Classical Swine Fever Virus E2 Subunit Vaccines Based on High Affinity Peptide Ligand

BACKGROUND

The purification of expressed proteins is the most critical part of subunit-- vaccine production. Protein-purification methods such as affinity chromatography and ion exchange still have the shortcomings of being time consuming and complicated. With the rapid development of computational molecular-simulation technology, structure-based peptide-ligand design has become feasible. Objection: We aimed to apply molecular docking for a peptide ligand designed for classical swine fever virus (CSFV) E2 purification.

METHODS

Computational-derived peptides were synthesized, and the in vitro binding interaction with E2 was investigated. The effects of purification on E2 were also evaluated.

RESULTS

The best peptide recognizing E2 was P₆, which had a sequence of KKFYWRYWEH. Based on kinetic surface plasmon resonance (SPR) analysis, the apparent affinity constant of P6 was found to be 148 nM. Importantly, P₆ showed suitable binding affinity and specificity for E2 purification from transgenic rice seeds. Evaluation of immune antibodies in mice showed that the antibody- blocking rate on day 42 after inoculation reached 86.18% and 90.68%.

CONCLUSION

The computational-designed peptide in this study has high sensitivity and selectivity and is thus useful for the purification of CSFV E2. The novel method of design provided a broad platform and powerful tool for protein-peptide screening, as well as new insights into CSFV vaccine design.

Research Progress and Challenges in Vaccine Development against Classical Swine Fever Virus

Classical swine fever (CSF), caused by CSF virus (CSFV), is one of the most devastating viral epizootic diseases of swine in many countries. To control the disease, highly efficacious and safe live attenuated vaccines have been used for decades. However, the main drawback of these conventional vaccines is the lack of differentiability of infected from vaccinated animals (DIVA concept). Advances in biotechnology and our detailed knowledge of multiple basic science disciplines have facilitated the development of effective and safer DIVA vaccines to control CSF. To date, two types of DIVA vaccines have been developed commercially, including the subunit vaccines based on CSFV envelope glycoprotein E2 and chimeric pestivirus vaccines based on infectious cDNA clones of CSFV or bovine viral diarrhea virus (BVDV). Although inoculation of these vaccines successfully induces solid immunity against CSFV, none of them could ideally meet all demands regarding to safety, efficacy, DIVA potential, and marketability. Due to the limitations of the available choices, researchers are still striving towards the development of more advanced DIVA vaccines against CSF. This review summarizes the present status of candidate CSFV vaccines that have been developed. The strategies and approaches revealed here may also be helpful for the development of new-generation vaccines against other diseases.

Charged Residues in the Membrane Anchor of the Pestiviral Erns Protein Are Important for Processing and Secretion of Erns and Recovery of Infectious Viruses

The pestivirus envelope protein E^rns is anchored in membranes via a long amphipathic helix. Despite the unusual membrane topology of the E^rns membrane anchor, it is cleaved from the following glycoprotein E1 by cellular signal peptidase. This was proposed to be enabled by a salt bridge-stabilized hairpin structure (so-called charge zipper) formed by conserved charged residues in the membrane anchor. We show here that the exchange of one or several of these charged residues reduces processing at the E^rns carboxy-terminus to a variable extend, but reciprocal mutations restoring the possibility to form salt bridges did not necessarily restore processing efficiency. When introduced into an E^rns-only expression construct, these mutations enhanced the naturally occurring E^rns secretion significantly, but again to varying extents that did not correlate with the number of possible salt bridges. Equivalent effects on both processing and secretion were also observed when the proteins were expressed in avian cells, which points at phylogenetic conservation of the underlying principles. In the viral genome, some of the mutations prevented recovery of infectious viruses or immediately (pseudo)reverted, while others were stable and neutral with regard to virus growth.

Generation and immunogenicity analysis of recombinant classical swine fever virus glycoprotein E2 and Erns expressed in baculovirus expression system

Classical swine fever (CSF) caused by the classical swine fever virus (CSFV) is a highly contagious swine disease resulting in large economical losses worldwide. The viral envelope glycoprotein E2 and E^rns are major targets for eliciting antibodies against CSFV in infected animals. In this report, the glycoprotein E2 and E^rns were expressed using the baculovirus system and their protective immunity in rabbits were tested. Twenty CSFV seronegative rabbits were randomly divided into five groups. Each rabbit was intramuscularly immunized with CSFV-E2, CSFV-E^rns, or their combination (CSFV-E2 + E^rns). Besides, a commercial CSFV vaccine (C-strain) and PBS were used as positive or negative controls, respectively. Four weeks after the second immunization, all the rabbits were challenged with 100 RID₅₀ of CSFV C-strain. High levels of CSFV E2-specific antibody, neutralizing antibody and cellular immune responses to CSFV were elicited in the rabbits inoculated with C-strain, CSFV-E2, and CSFV-E2 + E^rns. And the rabbits inoculated with the three vaccines received complete protection against CSFV C-strain. However, no neutralizing antibody was detected in the E^rns vaccinated rabbits and the rabbits exhibited fever typical of CSFV, suggesting the E^rns alone is not able to induce a protective immune response. Taken together, while the E^rns could not confer protection against CSFV, E2 and E2 + E^rns could not only elicit humoral and cell-mediated immune responses but also confer complete protection against CSFV C-strain in rabbits.

Broad neutralization of CSFV with novel monoclonal antibodies in vivo

Classical swine fever is a highly contagious disease in China. Although vaccination against Classical swine fever virus (CSFV) has been widely carried out in China, CSFV cases still emerge in an endless stream. Therefore, it is necessary to take new antiviral measures to eliminate CSFV. Glycoprotein E2 of CSFV is the major vaccine candidate that confers protective immunity. Thus, in this study, a batch of neutralizing monoclonal antibodies (mAbs) against E2, as alternative antiviral strategies, were produced. Among them, mAbs 6D10, 8D8 and 3C12 presented neutralizing reactivity against CSFV in a dose-dependent manner. Based on truncated overlapping fragments of E2 and mutants, three linear neutralizing epitopes were identified highly conserved in various CSFV strains. Epitopes ⁸YRYAIS¹³ and ²⁵⁴HECLIG²⁵⁹ were reported for the first time. All the three epitopes are involved in virus internalization and attachment as shown in pre- or post-attachment neutralization. Recombinant polypeptides carrying epitopes successfully inhibit virus infection in PK-15 cells, indicating epitopes were located in receptor-binding domain (RBD). Further, both prophylactic and therapeutic functions of neutralizing antibody were evaluated in rabbits upon CSFV challenge, confirming the efficacy in vivo. These findings provide alternative antiviral strategies against CSFV and deepen the understanding in E2 function during virus entry.

Downstream Sequences Control the Processing of the Pestivirus Erns-E1 Precursor

Like other enveloped viruses, pestiviruses employ cellular proteases for processing of their structural proteins. While typical signal peptidase cleavage motifs are present at the carboxy terminus of the signal sequence preceding E^rns and the E1/E2 and E2/P7 sites, the E^rns-E1 precursor is cleaved by signal peptidase at a highly unusual structure, in which the transmembrane sequence upstream of the cleavage site is replaced by an amphipathic helix. As shown before, the integrity of the amphipathic helix is crucial for efficient processing. The data presented here demonstrate that the E1 sequence downstream of this cleavage site is also important for the cleavage. Carboxy-terminal truncation of the E1 moiety as well as internal deletions in E1 reduced the cleavage efficiency to less than 30% of the wild-type (wt) level. Moreover, the C-terminal truncation by more than 30 amino acids resulted in strong secretion of the uncleaved fusion proteins. The reduced processing and increased secretion were even observed when 10 to 5 amino-terminal residues of E1 were left, whereas extensions by 1 or 3 E1 residues resulted in reduced processing but no significantly increased secretion. In contrast to the E1 sequences, a 10-amino-acid c-myc tag fused to the E^rns C terminus had only marginal effect on secretion but was also not processed efficiently. Mutation of the von Heijne sequence upstream of E2 not only blocked the cleavage between E1 and E2 but also prevented the processing between E^rns and E2. Thus, processing at the E^rns-E1 site is a highly regulated process.IMPORTANCE Cellular signal peptidase (SPase) cleavage represents an important step in maturation of viral envelope proteins. Fine tuning of this system allows for establishment of concerted folding and processing processes in different enveloped viruses. We report here on SPase processing of the E^rns-E1-E2 glycoprotein precursor of pestiviruses. E^rns-E1 cleavage is delayed and only executed efficiently when the complete E1 sequence is present. C-terminal truncation of the E^rns-E1 precursor impairs processing and leads to significant secretion of the protein. The latter is not detected when internal deletions preserving the E1 carboxy terminus are introduced, but also these constructs show impaired processing. Moreover, E^rns-E1 is only processed after cleavage at the E1/E2 site. Thus, processing of the pestiviral glycoprotein precursor by SPase is done in an ordered way and depends on the integrity of the proteins for efficient cleavage. The functional importance of this processing scheme is discussed in the paper.

Classical swine fever virus: the past, present and future

Classical swine fever (CSF) is among the most relevant viral epizootic diseases of swine. Due to its severe economic impact, CSF is notifiable to the world organisation for animal health. Strict control policies, including systematic stamping out of infected herds with and without vaccination, have permitted regional virus eradication. Nevertheless, CSF virus (CSFV) persists in certain areas of the world and has re-emerged regularly. This review summarizes the basic established knowledge in the field and provides a comprehensive and updated overview of the recent advances in fundamental CSFV research, diagnostics and vaccine development. It covers the latest discoveries on the genetic diversity of pestiviruses, with implications for taxonomy, the progress in understanding disease pathogenesis, immunity against acute and persistent infections, and the recent findings in virus-host interactions and virulence determinants. We also review the progress and pitfalls in the improvement of diagnostic tools and the challenges in the development of modern and efficacious marker vaccines compatible with serological tests for disease surveillance. Finally, we highlight the gaps that require research efforts in the future.

A classical swine fever virus E2 fusion protein produced in plants elicits a neutralizing humoral immune response in mice and pigs

Classical swine fever (CSF) is one of the most important viral diseases of swine worldwide. Although live or attenuated virus vaccines have been used to control CSFV, it is difficult to distinguish vaccinated pigs from infected pigs; this leads to restrictions on import and export. Subunit vaccines based on the CSFV E2 glycoprotein have been developed using baculovirus or insect cell systems, but some weaknesses remain. Here, we describe production of an E2 recombinant protein using a Nicotiana benthamiana plant expression system. To do this, we took advantage of the ability of the swine Fc domain to increase solubility and stability of the fusion protein and to strengthen immune responses in target animals. N. benthamiana expressed high amounts of pFc2-fused E2 proteins, which were isolated and purified by affinity chromatography to yield a high pure recombinant protein in a cost-effective manner. Native-polyacrylamide gel electrophoresis and size exclusion chromatography confirmed that the pmE2:pFc2 fusion exists as a multimer rather than as a dimer. Injection of recombinant pmE2 protein into mice or piglets generated anti-pmE2 antibodies with efficient neutralizing activity against CSFV. These results suggest that a purified recombinant E2 protein produced in N. benthamiana generates high titers of neutralizing antibodies in vivo; as such, the protein could be developed as a subunit vaccine against CSFV.

Efficient mucosal vaccination of a novel classical swine fever virus E2-Fc fusion protein mediated by neonatal Fc receptor

Classical swine fever (CSF) remains one of the most important highly contagious and fatal viral disease of swine with high morbidity and mortality. CSF is caused by classical swine fever virus (CSFV), a small, enveloped RNA virus of the genus Pestivirus. The aim of this study was to construct the a novel CSFV Fc-fusion recombinant protein and evaluate the efficacy as a vaccine against CSFV. Here, we obtained a novel subunit vaccine expressing CSFV E2 recombinant fusion protein in CHO-S cells. Functional analysis revealed that CSFV Fc-fusion recombinant protein (CSFV-E2-Fc) could bind to FcγRI on antigen-presenting cells (APCs) and significantly increase IgA levels in serum and feces, inducing stronger mucosal immune response in swine. Additionally, CSFV-E2-Fc immunization enhanced CSFV-specific T cell immune response with a Th1-like pattern of cytokine secretion, remarkably stimulated the Th1-biased cellular immune response and humoral immune response. Further, the protective effects of CSFV-E2-Fc subunit vaccines were confirmed. The data suggest that CSFV E2-Fc recombinant fusion protein may be a promising candidate subunit vaccine to elicit immune response and protect against CSFV.

Identification of E2 with improved secretion and immunogenicity against CSFV in piglets

BACKGROUND

Outbreaks of Classical swine fever virus (CSFV) cause significant economic losses in the swine industry. Vaccination is the major method to prevent and control the disease. As live attenuated vaccines fail to elicit differentiable immunity between infected and vaccinated animals, subunit vaccine was considered as an alternative candidate to prevent and eradicate CSFV. Subunit vaccines present advantages in DIVA immunogenicity and safety. The technology was limited due to the low yield and the high cost with multiple and large doses. The native E2 signal peptide has not been well defined before. Here, the aim of this study is to develop a cost-effective and efficacious E2 vaccine candidate against CSFV with signal peptide and E2 sequence selection.

RESULTS

A novel CSFV E2 sequence (E2ZJ) was identified from an epidemic strain of Zhejiang for outstanding secretion in baculovirus and enhanced immunogenicity. E2 secretion induced with the selected signal peptide, SPZJ (SP23), increase at least 50% as compared to any other signal peptides tested. Besides, unique antigenic features were identified in E2ZJ. As indicated with immunized sera in IFA against CSFV infection, E2ZJ elicited CSFV antibodies at the earlier stage than other E2 types tested in mice. Moreover, higher level of neutralizing and CSFV antibodies against CSFV with E2ZJ was detected than other E2s with the same dosage at 28 dpi. Further, E2ZJ successfully elicited neutralizing immunity in piglets. A single dose of 5 μg of E2ZJ was sufficient to induce protective antibodies against CSFV in piglets and provided 100% protection against lethal virus challenge.

CONCLUSIONS

Our studies provide evidence that E2ZJ guided by a novel E2 signal peptide (SPZJ) was efficiently secreted and presented significantly improved immunogenicity than conventional E2 vaccines. Moreover, a single dose of 5 μg E2ZJ is efficacious against CSFV in piglets.

Selection and Characterization of CSFV-Specific Single-Domain Antibodies and Their Application along with Immunomagnetic Nanobeads and Quantum Dots

Outbreak of classical swine fever (CSF) results in high mortality and thus causes severe economic losses in the swine industry. Single-domain antibody (sdAb) is the smallest antigen-binding molecule derived from camelid heavy-chain antibodies and has the potential to be used as a molecular probe for detection of CSF virus (CSFV). In this study, two sdAb fragments against the E2 antigen of CSFV were obtained, expressed in vitro. The functional characteristics analysis indicated that the recombinant sdAbE2-1 and sdAbE2-2 have excellent binding activity, specificity, and high affinity with equilibrium constant value of 3.34 × 10⁻⁷ and 1.35 × 10⁻⁸ M to E2 protein. Then, sdAbE2s were conjugated with quantum dots (QD)/AF488 to synthesize two molecular probes for imaging CSFV distribution in cells. The sdAbE2-1 was also labeled with carboxyl-magnetic beads to construct immunomagnetic nanobeads (IMNBs) able to capture CSFV virions and recombinant E2 protein. QD/AF455-sdAbE2s probes colocalised with CSFV virions in swine testis cells, and IMNBs were used as a detection template and proved to bind specifically with CSFV virions and E2 protein. The selected sdAb fragments and sdAb-based molecular probes may be used for the rapid identification of CSFV during field outbreaks and for research on CSFV and host interactions.

Serum Lipidomics Analysis of Classical Swine Fever Virus Infection in Piglets and Emerging Role of Free Fatty Acids in Virus Replication in vitro

Lipids metabolism plays a significant role in cellular responses to virus pathogens. However, the impact of lipids metabolism in CSFV infection is not yet confirmed. In the present study, for the fist time, we performed serum lipidomics analysis of piglets infected with CSFV based on ultra-high performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS), and identified 167 differentially expressed lipid metabolites. Interestingly, free fatty acids (FFAs) accumulated significantly in these metabolites, accompanied by an increase in sphingolipids and a decrease in glycerolipids and glycerophospholipids, suggesting that CSFV infection markedly changed the serum lipid metabolism of piglets. FFAs are the principal constituents of many complex lipids and are essential substrates for energy metabolism. Based on this, we focused on whether FFAs play a prominent role in CSFV infection. We found that CSFV infection induced FFAs accumulation in vivo and in vitro, which is due to increased fatty acid biosynthesis. Meanwhile, we discovered that alteration of cellular FFAs accumulation by a mixture of FFAs or inhibitors of fatty acid biosynthesis affects progeny virus production in vitro. Furthermore, in the absence of glucose or glutamine, CSFV still has replication capacity, which is significantly reduced with the addition of fatty acid beta oxidation inhibitors, suggesting that the process of FFAs enter the mitochondria for beta oxidation to produce ATP is necessary for virus replication. Finally, we demonstrated CSFV induced FFAs accumulation results in impaired type I IFN signaling-mediated antiviral responses by down-regulating RIG-I-like receptors (RLRs) signaling molecules, which may represent a mechanism of CSFV replication. Taken together, these findings provide the first data on lipid metabolites during CSFV infection and reveal a new view that CSFV infection requires FFAs to enhance viral replication.

Recombinant pseudorabies virus expressing E2 of classical swine fever virus (CSFV) protects against both virulent pseudorabies virus and CSFV

Both classical swine fever (CSF) and pseudorabies are highly contagious, economically significant diseases of swine in China. Although vaccination with the C-strain against classical swine fever virus (CSFV) is widely carried out and severe outbreaks of CSF seldom occur in China, CSF is sporadic in many pig herds and novel sub-subgenotypes of CSFV endlessly emerge. Thus, new measures are needed to eradicate CSFV from Chinese farms. The emergence of a pseudorabies virus (PRV) variant also posed a new challenge for the control of swine pseudorabies. Here, the recombinant PRV strain JS-2012-ΔgE/gI-E2 expressing E2 protein of CSFV was developed by inserting the E2 expression cassette into the intergenic region between the gG and gD genes of the gE/gI-deletion PRV variant strain JS-2012-ΔgE/gI. The recombinant virus was stable when passaged in vitro. A single vaccination of JS-2012-ΔgE/gI-E2 via intramuscular injection fully protected against lethal challenges of PRV and CSFV. Vaccination of piglets with the recombinant JS-2012-ΔgE/gI-E2 in the presence of high levels of maternally derived antibodies (Abs) to PRV can provide partial protection against lethal challenge of CSFV. Vaccination of the recombinant PRV JS-2012-ΔgE/gI-E2 strain did not induce the production of Abs to the gE protein of PRV or to the CSFV proteins other than E2. Thus, JS-2012-ΔgE/gI-E2 appears to be a promising recombinant marker vaccine candidate against PRV and CSFV for the control and eradication of the PRV variant and CSFV.

Apoptosis, Autophagy, and Pyroptosis: Immune Escape Strategies for Persistent Infection and Pathogenesis of Classical Swine Fever Virus

Classical swine fever (CSF) is a severe acute infectious disease that results from classical swine fever virus (CSFV) infection, which leads to serious economic losses in the porcine industry worldwide. In recent years, numerous studies related to the immune escape mechanism of the persistent infection and pathogenesis of CSFV have been performed. Remarkably, several independent groups have reported that apoptosis, autophagy, and pyroptosis play a significant role in the occurrence and development of CSF, as well as in the immunological process. Apoptosis, autophagy, and pyroptosis are the fundamental biological processes that maintain normal homeostatic and metabolic function in eukaryotic organisms. In general, these three cellular biological processes are always understood as an immune defense response initiated by the organism after perceiving a pathogen infection. Nevertheless, several viruses, including CSFV and other common pathogens such as hepatitis C and influenza A, have evolved strategies for infection and replication using these three cellular biological process mechanisms. In this review, we summarize the known roles of apoptosis, autophagy, and pyroptosis in CSFV infection and how viruses manipulate these three cellular biological processes to evade the immune response.

Evaluation of surface glycoproteins of classical swine fever virus as immunogens and reagents for serological diagnosis of infections in pigs: a recombinant Newcastle disease virus approach

Classical swine fever (CSF) is an important viral disease of domestic pigs and wild boar. The structural proteins E2 and E^rns of classical swine fever virus (CSFV), which participate in the attachment of the virion to the host cell surface and its subsequent entry, are immunogenic. The E2 and E^rns proteins are used for diagnosis and the development of vaccines against CSFV infection in swine. Newcastle disease virus (NDV) has been successfully used as a viral vector to express heterologous proteins. In the present study, the E2 and E^rns proteins of CSFV were expressed in cell culture as well as embryonated chicken eggs, using recombinant NDV (rNDV). Rescued rNDV expressing the E2 and E^rns proteins induced the production of CSFV-neutralizing antibodies upon intranasal vaccination of pigs. Serum samples from vaccinated animals were found to neutralize both homologous and heterologous CSFV strains. Furthermore, rNDV expressing the E2 and E^rns proteins of CSFV was used to develop an indirect ELISA, which was used to measure the the antibody titers of randomly collected serum samples. The results suggested that the ELISA based on rNDV-expressed E2 and E^rns proteins could be used to screen for CSFV infections. This study shows that rNDV-based expression of CSFV antigens is potentially applicable for development of vaccines and diagnostic tests for CSFV infection. This approach could be an economically favorable alternative to the existing vaccine and diagnostics for CSFV in pigs.

Characterization of the Humoral Immune Response Induced after Infection with Atypical Porcine Pestivirus (APPV)

Atypical porcine pestivirus (APPV) is a widely distributed pathogen causing congenital tremor (CT) in piglets. So far, no data are available regarding the humoral immune response against APPV. In this study, piglets and their sows from an affected herd were tested longitudinally for viral genome and antibodies. APPV genome was detected in the majority of the piglets (14/15) from CT affected litters. Transient infection of gilts was observed. Kinetics of E^rns- and E2-specific antibodies and their neutralizing capacity were determined by recently (E^rns) and newly (E2) developed antibody ELISAs and virus neutralization assays. Putative maternally derived antibodies (MDA) were detected in most piglets, but displayed only low to moderate neutralizing capacity (ND₅₀ ≤ 112). Horizontal APPV transmission occurred when uninfected and infected piglets were mingled on the flat deck. Horizontally infected piglets were clinically inapparent and showed only transient viremia with subsequently consistently high E2 antibody levels. For piglets from CT affected litters, significantly lower neutralizing antibody titers were observed. Results indicate that E2 represents the main target of neutralizing antibodies. Characterization of the humoral immune response against APPV will help to provide valuable serological diagnosis, to understand the epidemiology of this novel pathogen, and to implement tailored prevention strategies.

Bead-Based Multiplex Assay for the Simultaneous Detection of Antibodies to African Swine Fever Virus and Classical Swine Fever Virus

African swine fever (ASF) and Classical swine fever (CSF) are both highly contagious diseases of domestic pigs and wild boar. In the last years, several cases of both diseases have been reported in the Caucasus, Russian Federation and Eastern Europe. Thus, the probability of encountering these two viruses in the same area is increasing. Since differentiation by clinical or post-mortem examination is not possible, laboratory tools for differential diagnosis are required. In the present work, we have developed a triplex bead-based assay using some of the most immunogenic antigens of each virus, for the simultaneous detection of antibodies; i.e. the VP72 and VP30 of ASF virus (ASFV) and the E2 protein of CSF virus (CSFV). The assay was firstly set up and optimized using well characterized reference serum samples specific for each pathogen. Then, a panel of 352 sera from experimentally infected animals with either ASFV or CSFV were analyzed in the multiplex assay. A collection of 253 field negative sera was also included in the study. The results of the multiplex analysis were compared to those obtained by two commercially available ELISAs for detection of antibodies against ASFV or CSFV, and considered in this study as the reference techniques. The data obtained showed values of 97.3% sensitivity and 98.3% specificity for detection of antibodies to ASFV and 95.7% of sensitivity and 99.8% specificity for detection of antibodies to CSFV. This multiplex assay allows the simultaneous and differential detection of antibodies against ASFV and CSFV, providing a valuable tool for surveillance studies. Moreover, this method is rather versatile, offering the possibility of increasing the panel of antigens from other swine diseases that could be of interest for a differential diagnosis along with ASF and CSF.

Positive selection pressure on E2 protein of classical swine fever virus drives variations in virulence, pathogenesis and antigenicity: Implication for epidemiological surveillance in endemic areas

Classical swine fever (CSF), caused by CSF virus (CSFV), is considered one of the most important infectious diseases with devasting consequences for the pig industry. Recent reports describe the emergence of new CSFV strains resulting from the action of positive selection pressure, due mainly to the bottleneck effect generated by ineffective vaccination. Even though a decrease in the genetic diversity of the positively selected CSFV strains has been observed by several research groups, there is little information about the effect of this selective force on the virulence degree, antigenicity and pathogenicity of this type of strains. Hence, the aim of the current study was to determine the effect of the positive selection pressure on these three parameters of CSFV strains, emerged as result of the bottleneck effects induced by improper vaccination in a CSF-endemic area. Moreover, the effect of the positively selected strains on the epidemiological surveillance system was assessed. By the combination of in vitro, in vivo and immunoinformatic approaches, we revealed that the action of the positive selection pressure induces a decrease in virulence and alteration in pathogenicity and antigenicity. However, we also noted that the evolutionary process of CSFV, especially in segregated microenvironments, could contribute to the gain-fitness event, restoring the highly virulent pattern of the circulating strains. Besides, we denoted that the presence of low virulent strains selected by bottleneck effect after inefficient vaccination can lead to a relevant challenge for the epidemiological surveillance of CSF, contributing to under-reports of the disease, favouring the perpetuation of the virus in the field. In this study, B-cell and CTL epitopes on the E2 3D-structure model were also identified. Thus, the current study provides novel and significant insights into variation in virulence, pathogenesis and antigenicity experienced by CSFV strains after the positive selection pressure effect.

Classical Swine Fever in China-An Update Minireview

Classical swine fever (CSF) remains one of the most economically important viral diseases of domestic pigs and wild boar worldwide. The causative agent is CSF virus, it is highly contagious, with high morbidity and mortality rates; as such, it is an OIE-listed disease. Owing to a nationwide policy of vaccinations of pigs, CSF is well-controlled in China, with large-scale outbreaks rarely seen. Sporadic outbreaks are however still reported every year. In order to cope with future crises and to eradicate CSF, China should strengthen and support biosecurity measures such as the timely reporting of suspected disease, technologies for reliable diagnoses, culling infected herds, and tracing possible contacts, as well as continued vaccination and support of research into drug and genetic therapies. This mini-review summarizes the epidemiology of and control strategies for CSF in China.

Lentiviral-mediated delivery of classical swine fever virus Erns gene into porcine kidney-15 cells for production of recombinant ELISA diagnostic antigen

Classical swine fever virus (CSFV), a member of the Pestivirus genus within the Flaviviridae family causes contagious fatal disease in swine. Antibodies against E2, Erns and NS3 proteins of virus can be detected in infected animals. Development of an ELISA coating antigen to improve the sensitivity of detecting Erns-specific antibodies in pig sera is always desirable for diagnosis as well as for differentiation of infected from vaccinated animals. In present study, a lentivirus-based gene delivery system was used to develop a stable PK-15 cell line expressing Erns (PK-Erns) for production of diagnostic antigen. The Lenti-Erns virus was purified from the supernatant of co-transfected 293LTV cells and used to transduce PK-15 cells. The homogenous PK-Erns cell line was produced by single cell cloning by monitoring eGFP expression. The Erns gene in the genomic DNA and RNA transcripts in total RNA isolated from PK-Erns cells were detected by PCR and RT-PCR, respectively. Expression of 45 kDa Erns glycoprotein was detected in western blot using CSFV-specific hyperimmune sera. The use of PK-Erns cell lysate as antigen in serial dilution and single dilution ELISAs with known positive and negative pig sera was investigated. The PK-Erns ELISA revealed sensitivity equivalent to commercial HerdChek ELISA kit. The sensitivity, specificity and accuracy of the PK-Erns ELISA was 95%, 100% and 96.66%, respectively compared to ELISA using purified CSFV as coating antigen. When field pig sera (n = 69) were tested in PK-Erns ELISA, a significant correlation between the titers from serial dilution and single dilution ELISA was observed. This indicated that PK-Erns cell line can serve as continuous source of ELISA diagnostic antigen for detection of CSFV-specific antibodies in pig sera.

Plant-made E2 glycoprotein single-dose vaccine protects pigs against classical swine fever

Classical Swine Fever Virus (CSFV) causes classical swine fever, a highly contagious hemorrhagic fever affecting both feral and domesticated pigs. Outbreaks of CSF in Europe, Asia, Africa and South America had significant adverse impacts on animal health, food security and the pig industry. The disease is generally contained by prevention of exposure through import restrictions (e.g. banning import of live pigs and pork products), localized vaccination programmes and culling of infected or at-risk animals, often at very high cost. Current CSFV-modified live virus vaccines are protective, but do not allow differentiation of infected from vaccinated animals (DIVA), a critical aspect of disease surveillance programmes. Alternatively, first-generation subunit vaccines using the viral protein E2 allow for use of DIVA diagnostic tests, but are slow to induce a protective response, provide limited prevention of vertical transmission and may fail to block viral shedding. CSFV E2 subunit vaccines from a baculovirus/insect cell system have been developed for several vaccination campaigns in Europe and Asia. However, this expression system is considered expensive for a veterinary vaccine and is not ideal for wide-spread deployment. To address the issues of scalability, cost of production and immunogenicity, we have employed an Agrobacterium-mediated transient expression platform in Nicotiana benthamiana and formulated the purified antigen in novel oil-in-water emulsion adjuvants. We report the manufacturing of adjuvanted, plant-made CSFV E2 subunit vaccine. The vaccine provided complete protection in challenged pigs, even after single-dose vaccination, which was accompanied by strong virus neutralization antibody responses.

Development of Recombinant Protein-Based Vaccine Against Classical Swine Fever Virus in Pigs Using Transgenic Nicotiana benthamiana

Classical swine fever virus (CSFV) is highly contagious, and fatal to infected pigs. Vaccines against CSFV have been developed from attenuated or modified live viruses. These vaccines are effective for immunization of animals, but they are associated with problems such as the accidental spreading of viruses to animals in the field, and with barriers to trade following vaccination. Here, we report the generation of transgenic Nicotiana benthamiana plants for large-scale, cost-effective production of E2 fusion protein for use as a recombinant vaccine against CSFV in pigs. Transgenic N. benthamiana plants harboring an intergenic, single-copy insertion of a chimeric gene encoding E2 fusion protein had high levels of transgene expression. For large-scale production of E2 fusion protein from leaf tissues, we developed a protein-purification protocol consisting of cellulose-binding domain (CBD)-cellulose-based affinity purification and size-exclusion gel-filtration chromatography. E2 fusion proteins showed high immunogenicity in piglets and provided protection against CSFV challenge. The CBD in the E2 fusion protein was also highly immunogenic. These results suggest that plant-produced recombinant E2 fusion proteins can be developed into cost-effective vaccines against CSFV, with the CBD as a marker antigen to differentiate between vaccination and natural infection.

Toward the development of a one-dose classical swine fever subunit vaccine: antigen titration, immunity onset, and duration of immunity

Highly contagious classical swine fever (CSF) remains a major trade and health problem in the pig industry, resulting in large economic losses worldwide. In CSF-endemic countries, attenuated CSF virus (CSFV) vaccines have been routinely used to control the disease. However, eradication of CSFV in a geographical area would require permanent reduction to zero presence of the virus. It is therefore of paramount importance to develop a safe, potent, and non-infectious CSF vaccine. We have previously reported on a cost-effective CSF E2 subunit vaccine, KNB-E2, which can protect against CSF symptoms in a single dose containing 75 µg of recombinant CSFV glycoprotein E2. In this study, we report on a series of animal studies undertaken to elucidate further the efficacy of KNB-E2. We found that pigs vaccinated with a single KNB-E2 dose containing 25 µg of recombinant CSFV glycoprotein E2 were protected from clinical symptoms of CSF. In addition, KNB-E2-mediated reduction of CSF symptoms was observed at two weeks post-vaccination and the vaccinated pigs continued to exhibit reduced CSF clinical signs when virus challenged at two months and four months post-vaccination. These results suggest that KNB-E2 effectively reduces CSF clinical signs, indicating the potential of this vaccine for safely minimizing CSF-related losses.

Detection of classical swine fever virus (CSFV) E2 and Erns antibody (IgG, IgA) in oral fluid specimens from inoculated (ALD strain) or vaccinated (LOM strain) pigs

The objective of this study was to describe oral fluid and serum antibody (IgG, IgA) responses against classical swine fever virus (CSFV) E2 and E^rns proteins in pigs (n = 60) inoculated with a moderately virulent field strain (ALD, n = 30) or a modified live virus vaccine strain (LOM, n = 30). Oral fluid (n = 1391) and serum (n = 591) samples were collected from individually-housed pigs between day post inoculation (DPI) -14 to 28. Testing revealed the synchronous appearance of E2- and E^rns-specific IgG and IgA antibodies in serum and oral fluids over time, with E2 and E^rns IgG ELISAs providing better diagnostic performance than the IgA ELISAs. Overall the data suggest the feasibility of large-scale, cost-effective screening of populations for CSFV using oral fluid samples. Given the historic issues of cross-reactivity among pestiviruses, future research should focus on the development of CSFV-specific testing platforms for the detection of E2 and/or E^rns IgG in oral fluid, ideally to be used in combination with DIVA vaccines.

The E2 glycoprotein is necessary but not sufficient for the adaptation of classical swine fever virus lapinized vaccine C-strain to the rabbit

Classical swine fever virus (CSFV) C-strain was developed through hundreds of passages of a highly virulent CSFV in rabbits. To investigate the molecular basis for the adaptation of C-strain to the rabbit (ACR), a panel of chimeric viruses with the exchange of glycoproteins E^rns, E1, and/or E2 between C-strain and the highly virulent Shimen strain and a number of mutant viruses with different amino acid substitutions in E2 protein were generated and evaluated in rabbits. Our results demonstrate that Shimen-based chimeras expressing E^rns-E1-E2, E^rns-E2 or E1-E2 but not E^rns-E1, E^rns, E1, or E2 of C-strain can replicate in rabbits, indicating that E2 in combination with either E^rns or E1 confers the ACR. Notably, E2 and the amino acids P108 and T109 in Domain I of E2 are critical in ACR. Collectively, our data indicate that E2 is crucial in mediating the ACR, which requires synergistic contribution of E^rns or E1.

Genetic evolution of classical swine fever virus under immune environments conditioned by genotype 1-based modified live virus vaccine

Modified live vaccines (MLVs) based on genotype 1 strains, particularly C-strain, have been used to prevent and control classical swine fever virus (CSFV) worldwide. Nevertheless, a shift in the predominant CSFV strains circulating in the field from genotype 1 or 3 to genotype 2 is seen. Genotype 2 is genetically distant from the vaccine strains and was recently reported during outbreaks after vaccine failure; this has raised concerns that vaccination has influenced viral evolution. In Korea in 2016, there was an unexpected CSF outbreak in a MLV-vaccinated commercial pig herd. The causative CSFV strain was genetically distinct from previously isolated Korean strains but similar to recent Chinese strains exhibiting enhanced capacity to escape neutralization; this suggests the need for global cooperative research on the evolution of CSFV. We analysed global E2 sequences, using bioinformatics tools, revealing the evolutionary pathways of CSFV. Classical swine fever virus genotypes 1 and 2 experienced different degrees and patterns of evolutionary growth. Whereas genotype 1 stayed relatively conserved over time, the genetic diversity of genotype 2 has progressively expanded, with few fluctuations. It was determined that genotype 2 evolved under lower immune pressures and at a higher evolutionary rate than genotype 1. Further, several selected codons, under diversifying selection in genotype 1 but under purifying selection in genotype 2, correspond to antigenic determinants, which could lead to evasion of vaccine-induced immunity. Our findings provide evidence that evolutionary changes in CSFV are the result of the disproportionate usage of the CSF MLVs in endemic areas; this underscores the need to develop mitigation strategies to minimize the substantial risk associated with the emergence of vaccine-escaping mutants.

African and classical swine fever: similarities, differences and epidemiological consequences

For the global pig industry, classical (CSF) and African swine fever (ASF) outbreaks are a constantly feared threat. Except for Sardinia, ASF was eradicated in Europe in the late 1990s, which led to a research focus on CSF because this disease continued to be present. However, ASF remerged in eastern Europe in 2007 and the interest in the disease, its control and epidemiology increased tremendously. The similar names and the same susceptible species suggest a similarity of the two viral diseases, a related biological behaviour and, correspondingly, similar epidemiological features. However, there are several essential differences between both diseases, which need to be considered for the design of control or preventive measures. In the present review, we aimed to collate differences and similarities of the two diseases that impact epidemiology and thus the necessary control actions. Our objective was to discuss critically, if and to which extent the current knowledge can be transferred from one disease to the other and where new findings should lead to a critical review of measures relating to the prevention, control and surveillance of ASF and CSF. Another intention was to identify research gaps, which need to be closed to increase the chances of a successful eradication of ASF and therefore for a decrease of the economic threat for pig holdings and the international trade.

Corrigendum to "Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows"

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and supports the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Expression and purification of classical swine fever virus E2 protein from Sf9 cells using a modified vector

OBJECTIVE

To develop a simple method for efficient expression of classical swine fever virus (CSFV) E2 protein.

RESULTS

The pFastBac HT B vector (pFastHTB-M1) was modified by adding a melittin signal peptide sequence. The E2 gene fragment without the transmembrane region was cloned into pFastHTB-M1. The modified vector has clear advantage over the original one, as evidenced by the purified recombinant E2 protein that was detected significantly by SDS-PAGE.

CONCLUSIONS

The modified vector has the potential for large-scale production and easy purification of the CSFV E2 protein or other proteins of interests.

Epidemiology, diagnosis and control of classical swine fever: Recent developments and future challenges

Classical swine fever (CSF) represents a major health and trade problem for the pig industry. In endemic countries or those with a wild boar reservoir, CSF remains a priority for Veterinary Services. Surveillance as well as stamping out and/or vaccination are the principle tools of prevention and control, depending on the context. In the past decades, marker vaccines and accompanying diagnostic tests allowing the discrimination of infected from vaccinated animals have been developed. In the European Union, an E2 subunit and a chimeric live vaccine have been licensed and are available for the use in future disease outbreak scenarios. The implementation of commonly accepted and globally harmonized concepts could pave the way to replace the ethically questionable stamping out policy by a vaccination-to-live strategy and thereby avoid culling of a large number of healthy animals and save food resources. Although a number of vaccines and diagnostic tests are available worldwide, technological advancement in both domains is desirable. This work provides a summary of an analysis undertaken by the DISCONTOOLS group of experts on CSF. Details of the analysis can be downloaded from the web site at http://www.discontools.eu/.

Complex Virus-Host Interactions Involved in the Regulation of Classical Swine Fever Virus Replication: A Minireview

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is one of the most devastating epizootic diseases of pigs in many countries. Viruses are small intracellular parasites and thus rely on the cellular factors for replication. Fundamental aspects of CSFV-host interactions have been well described, such as factors contributing to viral attachment, modulation of genomic replication and translation, antagonism of innate immunity, and inhibition of cell apoptosis. However, those host factors that participate in the viral entry, assembly, and release largely remain to be elucidated. In this review, we summarize recent progress in the virus-host interactions involved in the life cycle of CSFV and analyze the potential mechanisms of viral entry, assembly, and release. We conclude with future perspectives and highlight areas that require further understanding.

Efficacy of E2 glycoprotein fused to porcine CD154 as a novel chimeric subunit vaccine to prevent classical swine fever virus vertical transmission in pregnant sows

Here we evaluated the effect of double vaccination with a novel subunit marker vaccine candidate based in the CSFV E2 glycoprotein fused to the porcine CD154 to prevent CSFV vertical transmission. A lentivirus-based gene delivery system was used to obtain a stable recombinant HEK 293 cell line for the expression of E2 fused to porcine CD154 molecule. Six pregnant sows were distributed in two groups and at 64days of gestation animals numbered 1-4 (group 1) were vaccinated via intramuscular inoculation with 50μg of E2-CD154 subunit vaccine. Animals from group 2 (numbered 5 and 6, control animals) were injected with PBS. Seventeen days later sows from group 1 were boosted with the same vaccine dose. Twenty-seven days after the first immunization, the sows were challenged with a virulent CSFV Margarita strain and clinical signs were registered. Samples were collected during the experiment and at necropsy to evaluate immune response and virological protection. Between 14 and 18days after challenge, the sows were euthanized, the foetuses were obtained and samples of sera and tissues were collected. E2-CD154 vaccinated animals remained clinically healthy until the end of the study; also, no adverse reaction was shown after vaccination. An effective boost effect in the neutralizing antibody response after the second immunization and viral challenge was observed and support the virological protection detected in these animals after vaccination. Protection against CSFV vertical transmission was found in the 100% of serums samples from foetus of vaccinated sows. Only two out of 208 samples (0.96%) were positive with Ct value about 36 corresponding to one tonsil and one thymus, which may be non-infective viral particles. Besides, its DIVA potential and protection from vertical transmission, the novel CSFV E2 bound to CD154 subunit vaccine, is a promising alternative to the live-attenuated vaccine for developing countries.

Classical Swine Fever-An Updated Review

Classical swine fever (CSF) remains one of the most important transboundary viral diseases of swine worldwide. The causative agent is CSF virus, a small, enveloped RNA virus of the genus Pestivirus. Based on partial sequences, three genotypes can be distinguished that do not, however, directly correlate with virulence. Depending on both virus and host factors, a wide range of clinical syndromes can be observed and thus, laboratory confirmation is mandatory. To this means, both direct and indirect methods are utilized with an increasing degree of commercialization. Both infections in domestic pigs and wild boar are of great relevance; and wild boars are a reservoir host transmitting the virus sporadically also to pig farms. Control strategies for epidemic outbreaks in free countries are mainly based on classical intervention measures; i.e., quarantine and strict culling of affected herds. In these countries, vaccination is only an emergency option. However, live vaccines are used for controlling the disease in endemically infected regions in Asia, Eastern Europe, the Americas, and some African countries. Here, we will provide a concise, updated review on virus properties, clinical signs and pathology, epidemiology, pathogenesis and immune responses, diagnosis and vaccination possibilities.

The double-antigen ELISA concept for early detection of Erns -specific classical swine fever virus antibodies and application as an accompanying test for differentiation of infected from marker vaccinated animals

Emergency vaccination with live marker vaccines represents a promising control strategy for future classical swine fever (CSF) outbreaks, and the first live marker vaccine is available in Europe. Successful implementation is dependent on a reliable accompanying diagnostic assay that allows differentiation of infected from vaccinated animals (DIVA). As induction of a protective immune response relies on virus-neutralizing antibodies against E2 protein of CSF virus (CSFV), the most promising DIVA strategy is based on detection of E^rns -specific antibodies in infected swine. The aim of this study was to develop and to evaluate a novel E^rns -specific prototype ELISA (pigtype CSFV E^rns Ab), which may be used for CSF diagnosis including application as an accompanying discriminatory test for CSFV marker vaccines. The concept of a double-antigen ELISA was shown to be a solid strategy to detect E^rns -specific antibodies against CSFV isolates of different genotypes (sensitivity: 93.5%; specificity: 99.7%). Furthermore, detection of early seroconversion is advantageous compared with a frequently used CSFV E2 antibody ELISA. Clear differences in reactivity between sera taken from infected animals and animals vaccinated with various marker vaccines were observed. In combination with the marker vaccine CP7_E2alf, the novel ELISA represents a sensitivity of 90.2% and a specificity of 93.8%. However, cross-reactivity with antibodies against ruminant pestiviruses was observed. Interestingly, the majority of samples tested false-positive in other E^rns -based antibody ELISAs were identified correctly by the novel prototype E^rns ELISA and vice versa. In conclusion, the pigtype CSFV E^rns Ab ELISA can contribute to an improvement in routine CSFV antibody screening, particularly for analysis of sera taken at an early time point after infection and is applicable as a DIVA assay. An additional E^rns antibody assay is recommended for identification of false-positive results in a pig herd immunized with the licensed CP7_E2alf marker vaccine.

Classical swine fever vaccines-State-of-the-art

Due to its impact on animal health and pig industry, classical swine fever (CSF) is still one of the most important viral diseases of pigs. To control the disease, safe and highly efficacious live attenuated vaccines exist for decades. These vaccines have usually outstanding efficacy and safety but lack differentiability of infected from vaccinated animals (DIVA or marker strategy). In contrast, the first generation of E2 subunit marker vaccines shows constraints in efficacy, application, and production. To overcome these limitations, new generations of marker vaccines are developed. A wide range of approaches have been tried including recombinant vaccines, recombinant inactivated vaccines or subunit vaccines, vector vaccines, and DNA/RNA vaccines. During the last years, especially attenuated deletion vaccines or chimeric constructs have shown potential. At present, especially two new constructs have been intensively tested, the adenovirus-delivered, Semliki Forest virus replicon-vectored marker vaccine candidate "rAdV-SFV-E2" and the pestivirus chimera "CP7_E2alf". The later was recently licensed by the European Medicines Agency. Under field conditions, all marker vaccines have to be accompanied by a potent test system. Particularly this point shows still weaknesses and it is important to embed vaccination in a well-established vaccination strategy and a suitable diagnostic workflow. In summary, conventional vaccines are a standard in terms of efficacy. However, only vaccines with DIVA will allow improved eradication strategies e.g. also under emergency vaccination conditions in free regions. To answer this demand, new generations of marker vaccines have been developed and add now to the tool box of CSF control.