Evaluation of a multiplex real-time RT-PCR for quantitative and differential detection of wild-type viruses and C-strain vaccine of Classical swine fever virus

Isolation and Molecular Characterization of Atypical Porcine Pestivirus Emerging in China

Atypical porcine pestivirus (APPV) is a recently discovered and very divergent species of the genus Pestivirus within the family Flaviviridae, which causes congenital tremor (CT) in newborn piglets. In this study, an APPV epidemiological investigation was conducted by studying 975 swine samples (562 tissue and 413 serum samples) collected from different parts of China from 2017 to 2021. The results revealed that the overall positive rate of the APPV genome was 7.08% (69/975), among which 50.7% (35/69) of the samples tested positive for one or more other common swine viruses, especially porcine circovirus type 2 (PCV2) with a coinfection rate of 36.2% (25/69). Subsequently, a novel APPV strain, named China/HLJ491/2017, was isolated in porcine kidney (PK)-15 cells for the first time from a weaned piglet that was infected with both APPV and PCV2. The new APPV isolate was confirmed by RT-PCR, sequencing, immunofluorescence assay, and transmission electron microscopy. After clearing PCV2, a pure APPV strain was obtained and further stably propagated in PK-15 cells for more than 30 passages. Full genome sequencing and phylogenetic analysis showed that the China/HLJ491/2017 strain was classified as genotype 2, sharing 80.8 to 97.6% of its nucleotide identity with previously published APPV strains. In conclusion, this study enhanced our knowledge of this new pestivirus and the successful isolation of the APPV strain provides critical material for the investigation of the biological and pathogenic properties of this emerging virus, as well as the development of vaccines and diagnostic reagents.

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.

Development of a quadruple PCR-based gene microarray for detection of vaccine and wild-type classical swine fever virus, African swine fever virus and atypical porcine pestivirus

BACKGROUND

Classical swine fever (CSF), African swine fever (ASF), and atypical porcine pestivirus (APPV) are acute, virulent, and contagious viral diseases currently hampering the pig industry in China, which result in mummification or stillbirths in piglets and mortality in pigs. Diagnostic assays for the differentiation of infection and vaccination of CSFV, in addition to the detection of ASFV and APPV, are urgently required for better prevention, control, and elimination of these viral diseases in China.

METHODS

A quadruple PCR-based gene microarray assay was developed in this study to simultaneously detect wild-type and vaccine CSFV strains, ASFV and APPV according to their conserved regions. Forty-two laboratory-confirmed samples, including positive samples of 10 other swine viral diseases, were tested using this assay to confirm its high specificity.

RESULTS

This assay's limit of detections (LODs) for the wild-type and vaccine CSFV were 6.98 and 6.92 copies/µL. LODs for ASFV and APPV were 2.56 × 10 and 1.80 × 10 copies/µL, respectively. When compared with standard RT-PCR or qPCR for CSFV (GB/T 26875-2018), ASFV (MARR issue No.172), or APPV (CN108611442A) using 219 clinical samples, the coincidence was 100%. The results showed that this assay with high sensitivity could specifically distinguish ASFV, APPV, and CSFV, including CSFV infection and immunization.

CONCLUSION

This assay provides a practical, simple, economic, and reliable test for the rapid detection and accurate diagnosis of the three viruses and may have good prospects for application in an epidemiological investigation, prevention, and control and elimination of these three diseases.

Differentiation of Classical Swine Fever Virus Virulent and Vaccine Strains by CRISPR/Cas13a

As a notifiable terrestrial and aquatic animal disease listed by World Organisation for Animal Health (formerly the Office International des Epizooties [OIE]), classical swine fever (CSF) has caused great economic losses to the swine industry worldwide during recent decades. Differentiation of infected and vaccinated animals (DIVA) is urgent for eradication of CSF. In this study, a diagnostic platform based on CRISPR/Cas13a was established with the ability to differentiate between classical swine fever virus (CSFV) virulent and vaccine strains. In combination with reverse transcription recombinase-aided amplification (RT-RAA), the detection limit for CSFV synthetic RNA templates reached 3.0 × 102 copies/μL. In addition, with boiling and chemical reduction, heating unextracted diagnostic samples to obliterate nucleases (HUDSON) treatment was introduced to inactivate nucleases and release viral genome, achieving robust pretreatment of tested sample before CRISPR/Cas13a detection without the need to extract viral nucleic acids. HUDSON-RT-RAA-CRISPR/Cas13a can directly detect cell cultures of virulent Shimen strain and vaccine hog cholera lapinized virus (HCLV) strain, with the detection limit of 3.5 × 102 copies/μL and 1.8 × 102 copies/μL, respectively, which was equally sensitive to nested PCR (nPCR) and 100 times more sensitive than antigen enzyme-linked immunosorbent assay (ELISA). Meanwhile, HUDSON-RT-RAA-CRISPR/Cas13a showed no cross-reactivity with bovine viral diarrhea virus (BVDV), atypical porcine pestivirus (APPV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine epidemic diarrhea virus (PEDV), African swine fever virus (ASFV), pseudorabies virus (PRV), and porcine circovirus 2 (PCV2), exhibiting good specificity. At last, a total of 50 pig spleen samples with suspected clinical signs were also assayed with HUDSON-RT-RAA-CRISPR/Cas13a, nPCR, and antigen ELISA in parallel. HUDSON-RT-RAA-CRISPR/Cas13a showed 100.0% with nPCR and 82.0% coincident rate with antigen ELISA, respectively. IMPORTANCE Classical swine fever (CSF) is a World Organisation for Animal Health (formerly the Office International des Epizooties [OIE]) notifiable terrestrial and aquatic animal disease, causing great economic losses to the swine industry worldwide during the past decades. Due to the use of the most effective and safe attenuated live vaccine for CSF prevention, differentiation of infected and vaccinated pigs is vital work, as well as a bottleneck for eradication of CSF. Methods with the ability to precisely differentiate classical swine fever virus (CSFV) virulent strains from vaccine strain hog cholera lapinized virus (HCLV) are urgently needed. Combining the high sensitivity of isothermal recombinase-aided amplification (RAA) with the accurate molecular sensing ability of Cas13a, we presented a novel method for CSFV detection without the need to extract viral nucleic acids, which showed great advantage to traditional detection methods for precise differentiation of CSFV virulent strains and vaccine strain, providing a novel powerful tool for CSF eradication.

The Unique Glycosylation at Position 986 on the E2 Glycoprotein of Classical Swine Fever Virus Is Responsible for Viral Attenuation and Protection against Lethal Challenge

Classical swine fever (CSF) is an economically important disease of pigs caused by classical swine fever virus (CSFV). The live attenuated vaccine C-strain (also called HCLV strain) against CSF was produced by multiple passages of a highly virulent strain in rabbits. However, the molecular determinants for its attenuation and protection remain unclear. In this study, we identified a unique glycosylation at position 986 (⁹⁸⁶NYT⁹⁸⁸) on the E2 glycoprotein Domain IV of C-strain but not (⁹⁸⁶NYA⁹⁸⁸) the highly virulent CSFV Shimen strain. We evaluated the infectivity, virulence, and protective efficacy of the C-strain-based mutant rHCLV-T988A lacking the glycosylation and Shimen strain mutant rShimen-A988T acquiring an additional glycosylation at position 986. rShimen-A988T showed a significantly decreased viral replication ability in SK6 cells, while rHCLV-T988A exhibited a growth kinetics indistinguishable from that of C-strain. Removal of the C-strain glycosylation site does not affect viral replication in rabbits and the attenuated phenotype in pigs. However, rShimen-A988T was attenuated and protected the pigs from a lethal challenge at 14 days postinoculation. In contrast, the rHCLV-T988A-inoculated pigs showed transient fever, a few clinical signs, and pathological changes in the spleens upon challenge with the Shimen strain. Mechanistic investigations revealed that the unique glycosylation at position 986 influences viral spreading, alters the formation of E2 homodimers, and leads to increased production of neutralizing antibodies. Collectively, our data for the first time demonstrate that the unique glycosylation at position 986 on the E2 glycoprotein is responsible for viral attenuation and protection.

IMPORTANCE Viral glycoproteins involve in infectivity, virulence, and host immune responses. Deglycosylation on the E^rns, E1, or E2 glycoprotein of highly virulent classical swine fever virus (CSFV) attenuated viral virulence in pigs, indicating that the glycosylation contributes to the pathogenicity of the highly virulent strain. However, the effects of the glycosylation on the C-strain E2 glycoprotein on viral infectivity in cells, viral attenuation, and protection in pigs have not been elucidated. This study demonstrates the unique glycosylation at position 986 on the C-strain E2 glycoprotein. C-strain mutant removing the glycosylation at the site provides only partial protection against CSFV challenge. Remarkably, the addition of the glycan to E2 of the highly virulent Shimen strain attenuates the viral virulence and confers complete protection against the lethal challenge in pigs. Our findings provide a new insight into the contribution of the glycosylation to the virus attenuation and protection.

Advances and insights in the diagnosis of viral infections

Viral infections are the most common among diseases that globally require around 60 percent of medical care. However, in the heat of the pandemic, there was a lack of medical equipment and inpatient facilities to provide all patients with viral infections. The detection of viral infections is possible in three general ways such as (i) direct virus detection, which is performed immediately 1-3 days after the infection, (ii) determination of antibodies against some virus proteins mainly observed during/after virus incubation period, (iii) detection of virus-induced disease when specific tissue changes in the organism. This review surveys some global pandemics from 1889 to 2020, virus types, which induced these pandemics, and symptoms of some viral diseases. Non-analytical methods such as radiology and microscopy also are overviewed. This review overlooks molecular analysis methods such as nucleic acid amplification, antibody-antigen complex determination, CRISPR-Cas system-based viral genome determination methods. Methods widely used in the certificated diagnostic laboratory for SARS-CoV-2, Influenza A, B, C, HIV, and other viruses during a viral pandemic are outlined. A comprehensive overview of molecular analytical methods has shown that the assay's sensitivity, accuracy, and suitability for virus detection depends on the choice of the number of regions in the viral open reading frame (ORF) genome sequence and the validity of the selected analytical method.

P108 and T109 on E2 Glycoprotein Domain I Are Critical for the Adaptation of Classical Swine Fever Virus to Rabbits but Not for Virulence in Pigs

The classical swine fever virus (CSFV) live attenuated vaccine C-strain is adaptive to rabbits and attenuated in pigs, in contrast with the highly virulent CSFV Shimen strain. Previously, we demonstrated that P108 and T109 on the E2 glycoprotein (E2^P108-T109) in domain I (E2^DomainI) rather than R132, S133, and D191 in domain II (E2^DomainII) determine C-strain's adaptation to rabbits (ATR) (Y. Li, L. Xie, L. Zhang, X. Wang, C. Li, et al., Virology 519:197-206, 2018). However, it remains elusive whether these critical amino acids affect the ATR of the Shimen strain and virulence in pigs. In this study, three chimeric viruses harboring E2^P108-T109, E2^DomainI, or E2^DomainII of C-strain based on the non-rabbit-adaptive Shimen mutant vSM-HCLVE^rns carrying the E^rns glycoprotein of C-strain were generated and evaluated. We found that E2^P108-T109 or E2^DomainI but not E2^DomainII of C-strain renders vSM-HCLVE^rns adaptive to rabbits, suggesting that E2^P108-T109 in combination with the E^rns glycoprotein (E2^P108-T109-E^rns) confers ATR on the Shimen strain, creating new rabbit-adaptive CSFVs. Mechanistically, E2^P108-T109-E^rns of C-strain mediates viral entry during infection in rabbit spleen lymphocytes, which are target cells of C-strain. Notably, pig experiments showed that E2^P108-T109-E^rns of C-strain does not affect virulence compared with the Shimen strain. Conversely, the substitution of E2^DomainII and E^rns of C-strain attenuates the Shimen strain in pigs, indicating that the molecular basis of the CSFV ATR and that of virulence in pigs do not overlap. Our findings provide new insights into the mechanism of adaptation of CSFV to rabbits and the molecular basis of CSFV adaptation and attenuation.IMPORTANCE Historically, live attenuated vaccines produced by blind passage usually undergo adaptation in cell cultures or nonsusceptible hosts and attenuation in natural hosts, with a classical example being the classical swine fever virus (CSFV) lapinized vaccine C-strain, which was developed by hundreds of passages in rabbits. However, the mechanism of viral adaptation to nonsusceptible hosts and the molecular basis for viral adaptation and attenuation remain largely unknown. In this study, we demonstrated that P108 and T109 on the E2 glycoprotein together with the E^rns glycoprotein of the rabbit-adaptive C-strain confer adaptation to rabbits on the highly virulent CSFV Shimen strain by affecting viral entry during infection but do not attenuate the Shimen strain in pigs. Our results provide vital information on the different molecular bases of CSFV adaptation to rabbits and attenuation in pigs.

Recent Advances in the Diagnosis of Classical Swine Fever and Future Perspectives

Classical swine fever (CSF) is a highly contagious viral disease of pigs, including wild boar. It is regarded as one of the major problems in the pig industry as it is still endemic in many regions of the world and has the potential to cause devastating epidemics, particularly in countries free of the disease. Rapid and reliable diagnosis is of utmost importance in the control of CSF. Since clinical presentations of CSF are highly variable and may be confused with other viral diseases in pigs, laboratory diagnosis is indispensable for an unambiguous diagnosis. On an international level, well-established diagnostic tests of CSF such as virus isolation, fluorescent antibody test (FAT), antigen capture antibody enzyme-linked immunosorbent assay (ELISA), reverse-transcription polymerase chain reaction (RT-PCR), virus neutralization test (VNT), and antibody ELISA have been described in detail in the OIE Terrestrial Manual. However, improved CSF diagnostic methods or alternatives based on modern technologies have been developed in recent years. This review thus presents recent advances in the diagnosis of CSF and future perspectives.

Development of a marker vaccine candidate against classical swine fever based on the live attenuated vaccine C-strain

Classical swine fever (CSF) is a highly contagious and economically damaging disease. Classical swine fever virus (CSFV) lapinized vaccine C-strain against CSF worldwide lacks the capacity for the serological differentiation between infected and vaccinated animals (DIVA). To develop a marker C-strain complying with the DIVA principle, we generated and evaluated mutants rHCLV-E2F117A, rHCLV-E2G119A, and rHCLV-E2P122A, which harbor the single amino acid mutation at ¹¹⁷F, ¹¹⁹G or ¹²²P of the monoclonal antibody HQ06-recognized epitope on the E2 glycoprotein in rabbits and pigs. Viral intravenous administration demonstrated that all the mutants retain the phenotype of C-strain in rabbits, including fever response induction and replication in the spleen. Notably, the HQ06-recognized epitope did not react with the antibodies induced by rHCLV-E2P122A in rabbits, in contrast with C-strain and other two mutants. Intramuscular administration of rHCLV-E2P122A in pigs induced anti-CSFV neutralizing antibodies but not antibodies against the HQ06-recognized epitope at 28 days post-inoculation. Collectively, our data demonstrate that rHCLV-E2P122A is a promising marker vaccine candidate against CSF.

Simultaneous identification of 6 pathogens causing porcine reproductive failure by using multiplex ligation-dependent probe amplification

We developed a multiplex ligation-dependent probe amplification (MLPA) assay for the simultaneous detection of 6 clinically relevant viral pathogens causing porcine reproductive failure, that is porcine reproductive and respiratory syndrome virus (PRRSV), Japanese encephalitis virus (JEV), classical swine fever virus (CSFV), porcine circovirus type 2 (PCV2), pseudorabies virus (PRV) and porcine parvovirus (PPV). The limits of detection for the assay varied among the 6 target organisms from 1 to 8 copies per MLPA assay. The MLPA assay was evaluated with 346 heparinized porcine umbilical cord blood specimens, and the results of the assay were compared to those of real-time PCR. The MLPA assay showed specificities and sensitivities of 99.2% and 100%, respectively, for PRRSV; 100% and 100%, respectively, for CSFV, PCV2, PRV and PPV. No sample was found to be positive for JEV by either the MLPA assay or the real-time PCR. In conclusion, the MLPA assay has comparable clinical sensitivity to that of real-time PCR assay and provides a useful tool for fast screening porcine reproductive failure-associated viruses.

A neutralizing monoclonal antibody-based competitive ELISA for classical swine fever C-strain post-vaccination monitoring

Background

Virus neutralization test (VNT) is widely used for serological survey of classical swine fever (CSF) and efficacy evaluation of CSF vaccines. However, VNT is a time consuming procedure that requires cell culture and live virus manipulation. C-strain CSF vaccine is the most frequently used vaccine for CSF control and prevention. In this study, we presented a neutralizing monoclonal antibody (mAb) based competitive enzyme-linked immunosorbent assay (cELISA) with the emphasis on the replacement of VNT for C-strain post–vaccination monitoring.

Results

One monoclonal antibody (6B211) which has potent neutralizing activity against C-strain was generated. A novel cELISA was established and optimized based on the strategy that 6B211 can compete with C-strain induced neutralizing antibodies in pig serum to bind capture antigen C-strain E2. By testing C-strain VNT negative pig sera (n = 445) and C-strain VNT positive pig sera (n = 70), the 6B211 based cELISA showed 100% sensitivity (95% confidence interval: 94.87 to 100%) and 100% specificity (95% confidence interval: 100 to 100%). The C-strain antibody can be tested in pigs as early as 7 days post vaccination with the cELISA. By testing pig sera (n = 139) in parallel, the cELISA showed excellent agreement (Kappa = 0.957) with VNT. The inhibition rate of serum samples in the cELISA is highly correlated with their titers in VNT (r² = 0.903, p < 0.001). In addition, intra- and inter-assays of the cELISA exhibited acceptable repeatability with low coefficient of variations (CVs).

Conclusions

This novel cELISA demonstrated excellent agreement and high level correlation with VNT. It is a reliable tool for sero-monitoring of C-strain vaccination campaign because it is a rapid, simple, safe and cost effective assay that can be used to monitor vaccination-induced immune response at the population level.

Porcine RING Finger Protein 114 Inhibits Classical Swine Fever Virus Replication via K27-Linked Polyubiquitination of Viral NS4B

Porcine RING finger protein 114 (pRNF114) is a member of the RING domain E3 ligases. In this study, it was shown that pRNF114 is a potential anti-CSFV factor and the anti-CSFV effect of pRNF114 depends on its E3 ligase activity. Notably, pRNF114 targets and catalyzes the K27-linked polyubiquitination of the NS4B protein and then promotes proteasome-dependent degradation of NS4B, inhibiting the replication of CSFV. To our knowledge, pRNF114 is the first E3 ligase to be identified as being involved in anti-CSFV activity, and targeting NS4B could be a crucial route for antiviral development.

In the host, many RING domain E3 ligases have been reported to inhibit viral replication through various mechanisms. In a previous screen, we found that porcine RING finger protein 114 (pRNF114), a RING domain E3 ubiquitin ligase, inhibits classical swine fever virus (CSFV) replication. This study aimed to clarify the underlying antiviral mechanism of pRNF114 against CSFV. Upon CSFV infection, pRNF114 mRNA was upregulated both in vitro and in vivo. CSFV replication was significantly suppressed in PK-pRNF114 cells stably expressing pRNF114 by the lentivirus-delivered system, whereas CSFV growth was enhanced in PK-15 cells with RNF114 knockout by the CRISPR/Cas9 system. The RING domain of pRNF114, which has E3 ubiquitin ligase activity, is crucial for its antiviral activity. Mechanistically, pRNF114 interacted with the CSFV NS4B protein through their C-terminal domains, which led to the K27-linked polyubiquitination and degradation of NS4B through a proteasome-dependent pathway. Collectively, these findings indicate that pRNF114 as a critical regulator of CSFV replication and uncover a mechanism by which pRNF114 employs its E3 ubiquitin ligase activity to inhibit CSFV replication.

IMPORTANCE Porcine RING finger protein 114 (pRNF114) is a member of the RING domain E3 ligases. In this study, it was shown that pRNF114 is a potential anti-CSFV factor and the anti-CSFV effect of pRNF114 depends on its E3 ligase activity. Notably, pRNF114 targets and catalyzes the K27-linked polyubiquitination of the NS4B protein and then promotes proteasome-dependent degradation of NS4B, inhibiting the replication of CSFV. To our knowledge, pRNF114 is the first E3 ligase to be identified as being involved in anti-CSFV activity, and targeting NS4B could be a crucial route for antiviral development.

Engagement of cellular cholesterol in the life cycle of classical swine fever virus: its potential as an antiviral target

Classical swine fever virus (CSFV), the etiological agent of classical swine fever in pigs, is a member of the Pestivirus genus within the Flaviviridae family. It has been proposed that CSFV infection is significantly inhibited by methyl-β-cyclodextrin (MβCD) treatment. However, the exact engagement of cellular cholesterol in the life cycle of CSFV remains unclear. Here, we demonstrated that pretreatment of PK-15 cells with MβCD significantly decreased the cellular cholesterol level and resulted in the inhibition of CSFV infection, while replenishment of exogenous cholesterol in MβCD-treated cells recovered the cellular cholesterol level and restored the viral infection. Moreover, we found that depletion of cholesterol acted on the early stage of CSFV infection and blocked its internalization into the host cells. Furthermore, we showed that 25-hydroxycholesterol, a regulator of cellular cholesterol biosynthesis, exhibited a potent anti-CSFV activity by reducing cellular cholesterol level. Taken together, our findings highlight the engagement of cholesterol in the life cycle of CSFV and its potential use as an antiviral target.

Comprehensive evaluation of the host responses to infection with differentially virulent classical swine fever virus strains in pigs

Classical swine fever virus (CSFV) infection causes most variable clinical syndromes from chronic or latent infection to acute death, and it is generally acknowledged that the course of disease is affected by both virus and host factors. To compare host immune responses to differentially virulent CSFV strains in pigs, fifteen 8-week-old specific-pathogen-free pigs were randomly divided into four groups and inoculated with the CSFV Shimen strain (a highly virulent strain), the HLJZZ2014 strain (a moderately virulent strains), C-strain (an avirulent strain), and DMEM (mock control), respectively. Infection with the Shimen or HLJZZ2014 strain resulted in fever, clinical signs and histopathological lesions, which were not observed in the C-strain-inoculated pigs, though low viral genome copies were detected in the peripheral blood and tissue samples. The data showed that the virulence of the strains affected the outcome of duration and intensity of the disease rather than the tissue tropism of the virus. Furthermore, leukopenia, lymphocytopenia, differentiation of T-cells, and the secretion of cytokines associated with inflammation or apoptosis such as interferon alpha (IFN-α), tumor necrosis factor alpha (TNF-α), interleukin 2 (IL-2), IL-4, IL-6, and IL-10 were induced by the virulent CSFV infection, the differences reflected in onset and extent of the regulation. Taken together, our results revealed that the major differences among the three strains resided in the kinetics of host response to the infection: severe and immediate with the highly virulent strain, while progressive and delayed with the moderately virulent one. This comparative study will help to dissect the pathogenesis of CSFV.

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.

A Review of Classical Swine Fever Virus and Routes of Introduction into the United States and the Potential for Virus Establishment

Classical swine fever (CSF) is caused by CSF virus (CSFV) which can be the source of substantial morbidity and mortality events in affected swine. The disease can take one of several forms (acute, chronic, or prenatal) and depending on the virulence of the inoculating strain may result in a lethal infection irrespective of the form acquired. Because of the disease-free status of the United States and the high cost of a viral incursion, a summary of US vulnerabilities for viral introduction and persistence is provided. The legal importation of live animals as well as animal products, byproducts, and animal feed serve as a potential route of viral introduction. Current import regulations are described as are mitigation strategies that are commonly utilized to prevent pathogens, including CSFV, from entering the US. The illegal movement of suids and their products as well as an event of bioterrorism are both feasible routes of viral introduction but are difficult to restrict or regulate. Ultimately, recommendations are made for data that would be useful in the event of a viral incursion. Population and density mapping for feral swine across the United States would be valuable in the event of a viral introduction or spillover; density data could further contribute to understanding the risk of infection in domestic swine. Additionally, ecological and behavioral studies, including those that evaluate the effects of anthropogenic food sources that support feral swine densities far above the carrying capacity would provide invaluable insight to our understanding of how human interventions affect feral swine populations. Further analyses to determine the sampling strategies necessary to detect low levels of antibody prevalence in feral swine would also be valuable.

Secreted Expression of the Cap Gene of Porcine Circovirus Type 2 in Classical Swine Fever Virus C-Strain: Potential of C-Strain Used as a Vaccine Vector

Bivalent vaccines based on live attenuated viruses expressing a heterologous protein are an attractive strategy to address co-infections with various pathogens in the field. Considering the excellent efficacy and safety of the lapinized live attenuated vaccine C-strain (HCLV strain) of classical swine fever virus (CSFV), we proposed that C-strain has the potential as a viral vector for developing bivalent vaccines. To this end, we generated three recombinant viruses based on C-strain, one expressing the capsid (Cap) gene of porcine circovirus type 2 (PCV2) with the nuclear localization signal (NLS) (rHCLV-2ACap), and the other two expressing the PCV2 Cap gene without the NLS yet containing the signal peptide of the prolactin gene (rHCLV-pspCap) or that of the ubiquitin-specific peptidase gene (rHCLV-uspCap). All the recombinant viruses exhibited phenotypes similar to those of the parental virus and produced high-level anti-CSFV neutralizing antibodies (NAbs) in rabbits. Interestingly, rHCLV-uspCap and rHCLV-pspCap, but not rHCLV-2ACap, elicited detectable anti-Cap and -PCV2 NAbs in rabbits. Taken together, our data demonstrate that C-strain can be used as a viral vector to develop bivalent vaccines.

Simultaneous detection of multiple pathogens by multiplex PCR coupled with DNA biochip hybridization

Traditional serological enzyme-linked immunosorbent assay (ELISA) is routinely used to monitor pathogens during quarantine in most animal facilities to prevent possible infection. However, the ELISA platform is a single-target assay, and screening all targeted pathogens is time-consuming and laborious. In this study, to increase sensitivity and to reduce diagnosis time for high-throughput processes, multiplex PCR and DNA biochip techniques were combined to develop a multi-pathogen diagnostic method for use instead of routine ELISA. Eight primer sets were designed for multiplex PCR to detect genes from seven targeted bacterial and viral pathogens. DNA-DNA hybridization was conducted on a biochip following the multiple PCR analysis. Using this method, a total of 24 clinical samples were tested, and the result showed that not only single infection but also co-infection by multi-pathogens can be detected. In conclusion, multiplex PCR coupled with a DNA biochip is an efficient method for detecting multi-pathogens in a reaction. This platform is a useful tool for quarantine services and disease prevention in animal facilities.

Classical swine fever virus Shimen infection increases p53 signaling to promote cell cycle arrest in porcine alveolar macrophages

Classical swine fever virus (CSFV) replicates in macrophages and causes persistent infection. Despite its role in disastrous economic losses in swine industries, the molecular mechanisms underlying its pathogenesis are poorly understood. The virus evades the neutralizing immune response, subverting the immune system to ensure its own survival and persistence. Our genome-wide analysis of porcine alveolar macrophage transcriptional responses to CSFV Shimen infection using the Solexa/Illumina digital gene expression system revealed that p53 pathway components and cell cycle molecules were differentially regulated during infection compared to controls. Further, we investigated the molecular changes in macrophages infected with CSFV Shimen, focusing on the genes involved in the p53 pathway. CSFV Shimen infection led to phosphorylation and accumulation of p53 in a time-dependent manner. Furthermore, CSFV Shimen infection upregulated cyclin-dependent kinase inhibitor 1A (p21) mRNA and protein. In addition, CSFV Shimen infection induced cell cycle arrest at the G1 phase, as well as downregulation of cyclin E1 and cyclin-dependent kinase 2 (CDK2). The expression of genes in the p53 pathway did not change significantly after p53 knockdown by pifithrin-α during CSFV Shimen infection. Our data suggest that CSFV Shimen infection increases expression of host p53 and p21, and inhibits expression of cyclin E1 and CDK2, leading to cell cycle arrest at the G1 phase. CSFV may utilize this strategy to subvert the innate immune response and proliferate in host cells.

Interferon-Inducible Oligoadenylate Synthetase-Like Protein Acts as an Antiviral Effector against Classical Swine Fever Virus via the MDA5-Mediated Type I Interferon-Signaling Pathway

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), which poses a serious threat to the global pig industry. Interferons (IFNs) and IFN-stimulated genes (ISGs) play a key role in host antiviral defense. We have previously screened the porcine 2′-5′-oligoadenylate synthetase-like protein (pOASL) as a potential anti-CSFV ISG using a reporter CSFV. This study aimed to clarify the underlying antiviral mechanism of pOASL against CSFV. We confirmed that CSFV replication was significantly suppressed in lentivirus-delivered, pOASL-overexpressing PK-15 cells, whereas silencing the expression of endogenous pOASL by small interfering RNAs markedly enhanced CSFV growth. In addition, the transcriptional level of pOASL was upregulated both in vitro and in vivo upon CSFV infection. Interestingly, the anti-CSFV effects of pOASL are independent of the canonical RNase L pathway but depend on the activation of the type I IFN response. Glutathione S-transferase pulldown and coimmunoprecipitation assays revealed that pOASL interacts with MDA5, a double-stranded RNA sensor, and further enhances MDA5-mediated type I IFN signaling. Moreover, we showed that pOASL exerts anti-CSFV effects in an MDA5-dependent manner. In conclusion, pOASL suppresses CSFV replication via the MDA5-mediated type I IFN-signaling pathway.

IMPORTANCE The host innate immune response plays an important role in mounting the initial resistance to viral infection. Here, we identify the porcine 2′-5′-oligoadenylate synthetase-like protein (pOASL) as an interferon (IFN)-stimulated gene (ISG) against classical swine fever virus (CSFV). We demonstrate that the anti-CSFV effects of pOASL depend on the activation of type I IFN response. In addition, we show that pOASL, as an MDA5-interacting protein, is a coactivator of MDA5-mediated IFN induction to exert anti-CSFV actions. This work will be beneficial to the development of novel anti-CSFV strategies by targeting pOASL.

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.

Estimating the scale of adverse animal welfare consequences of movement restriction and mitigation strategies in a classical swine fever outbreak

Background

The study aim was to quantify the impact of movement restriction on the well-being of pigs and the associated mitigation responses during a classical swine fever (CSF) outbreak. We developed a stochastic risk assessment model and incorporated Indiana swine industry statistics to estimate the timing and number of swine premises that would encounter overcrowding or feed interruption resulting from movement restriction. Our model also quantified the amount of on-farm euthanasia and movement of pigs to slaughter plants required to alleviate those conditions. We simulated various single-site (i.e., an outbreak initiated from one location) and multiple-site (i.e., an outbreak initiated from more than one location) outbreak scenarios in Indiana to estimate outputs.

Results

The study estimated that 14% of the swine premises in Indiana would encounter overcrowding or feed interruption due to movement restriction implemented during a CSF outbreak. The number of premises that would experience animal welfare conditions was about 2.5 fold of the number of infected premises. On-farm euthanasia needed to be performed on 33% of those swine premises to alleviate adverse animal welfare conditions, and more than 90% of on-farm euthanasia had to be carried out within 2 weeks after the implementation of movement restriction. Conversely, movement of pigs to slaughter plants could alleviate 67% of adverse animal welfare conditions due to movement restriction, and only less than 1% of movement of pigs to slaughter plants had to be initiated in the first 2 weeks of movement restrictions. The risk of secondary outbreaks due to movement of pigs from movement restriction areas to slaughter plants was low and only seven pigs from each shipment needed to be tested for CSF infection to prevent a secondary outbreak.

Conclusions

We found that the scale of adverse animal welfare consequences of movement restriction during a CSF outbreak in Indiana was substantial, and controlled movement of pigs to slaughter plants was an efficient and low-risk alternative mitigation response to on-farm euthanasia. The output estimates generated from this study provide empirical evidence for decision makers to properly incorporate required resources for mitigating adverse animal welfare conditions in CSF outbreak management strategic planning.

A TaqMan real time probe assay for specific detection and quantification of lapinized vaccine strain of classical swine fever virus

Classical swine fever (CSF) is a highly contagious viral disease having a tremendous socio-economic impact. In CSF endemic countries such as India, control strategies depend heavily on the use of a live attenuated vaccine. The persistence of vaccine virus in tissue/blood of vaccinated animals may interfere with the diagnosis when RT-PCR assay is used for CSF diagnosis. A TaqMan based real-time RT-PCR assay was developed with primers and probe targeting the NS5B region of the lapinized CSFV (vaccine strain) used in India for detection and quantification of lapinized CSFV. The real-time TaqMan RT-PCR assay presented in the current study provided a rapid, specific and sensitive diagnostic tool for the detection of LCSFV and for quantification of virus load in various organs. This real time RT-PCR assay can differentiate the infected from vaccinated animals and quantification assay can be used for post-vaccination studies.

Efficacy evaluation of the C-strain-based vaccines against the subgenotype 2.1d classical swine fever virus emerging in China

Classical swine fever (CSF) is a devastating infectious disease of pigs caused by classical swine fever virus (CSFV). The disease has been controlled following extensive vaccination with the lapinized attenuated vaccine C-strain for decades in China. However, frequent CSF outbreaks occurred recently in a large number of C-strain-vaccinated pig farms in China and a new subgenotype 2.1d of CSFV has been reported to be responsible for the outbreaks. Here we analyzed the molecular variations and antigenic differences among the C-strain-based commercial vaccines of different origins from different manufacturers in China, and reevaluated the vaccines against the emerging subgenotype 2.1d strain of CSFV. The results showed that the C-strain adapted to the continuous ST cell line (C_ST) contain a unique M290K variation on the E2 protein, compared to those of primary BT cells (C_BT) or rabbit origin (C_RT) and the traditional C-strain sequences available in the GenBank database. Serum neutralization test revealed the antigenic differences between C_ST and C_BT or C_RT. Notably, the neutralizing titers of porcine anti-C-strain sera against the CSFV isolate of subgenotype 2.1d were significantly lower than those against C-strain or Shimen strain. The C-strain-vaccinated, subgenotype 2.1d HLJZZ2014 strain-challenged pigs did not show any clinical signs and all survived. However, these pigs displayed mild pathological and histological lesions, and the CSFV viral RNA was detected in the various tissue and blood samples. Taken together, the C-strain-based vaccines of different origins showed molecular variations and antigenic differences, and could provide clinical but not pathological and virological protection against the subgenotype 2.1d CSFV emerging in China. Further investigation is needed to comprehensively assess the efficacy of C-strain of different doses against the subgenotype 2.1d CSFV.

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.

Mitogen-Activated Protein Kinase Kinase 2, a Novel E2-Interacting Protein, Promotes the Growth of Classical Swine Fever Virus via Attenuation of the JAK-STAT Signaling Pathway

The mitogen-activated protein kinase kinase/extracellular regulated kinase (MEK1/2/ERK1/2) cascade is involved in the replication of several members of the Flaviviridae family, including hepatitis C virus and dengue virus. The effects of the cascade on the replication of classical swine fever virus (CSFV), a fatal pestivirus of pigs, remain unknown. In this study, MEK2 was identified as a novel binding partner of the E2 protein of CSFV using yeast two-hybrid screening. The E2-MEK2 interaction was confirmed by glutathione S-transferase pulldown, coimmunoprecipitation, and laser confocal microscopy assays. The C termini of E2 (amino acids [aa] 890 to 1053) and MEK2 (aa 266 to 400) were mapped to be crucial for the interaction. Overexpression of MEK2 significantly promoted the replication of CSFV, whereas knockdown of MEK2 by lentivirus-mediated small hairpin RNAs dramatically inhibited CSFV replication. In addition, CSFV infection induced a biphasic activation of ERK1/2, the downstream signaling molecules of MEK2. Furthermore, the replication of CSFV was markedly inhibited in PK-15 cells treated with U0126, a specific inhibitor for MEK1/2/ERK1/2, whereas MEK2 did not affect CSFV replication after blocking the interferon-induced Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway by ruxolitinib, a JAK-STAT-specific inhibitor. Taken together, our results indicate that MEK2 positively regulates the replication of CSFV through inhibiting the JAK-STAT signaling pathway.

IMPORTANCE Mitogen-activated protein kinase kinase 2 (MEK2) is a kinase that operates immediately upstream of extracellular regulated kinase 1/2 (ERK1/2) and links to Raf and ERK via phosphorylation. Currently, little is known about the role of MEK2 in the replication of classical swine fever virus (CSFV), a devastating porcine pestivirus. Here, we investigated the roles of MEK2 and the MEK2/ERK1/2 cascade in the growth of CSFV for the first time. We show that MEK2 positively regulates CSFV replication. Notably, we demonstrate that MEK2 promotes CSFV replication through inhibiting the interferon-induced JAK-STAT signaling pathway, a key antiviral pathway involved in innate immunity. Our work reveals a novel role of MEK2 in CSFV infection and sheds light on the molecular basis by which pestiviruses interact with the host cell.

Development of an updated PCR assay for detection of African swine fever virus

Due to the current unavailability of vaccines or treatments for African swine fever (ASF), which is caused by African swine fever virus (ASFV), rapid and reliable detection of the virus is essential for timely implementation of emergency control measures and differentiation of ASF from other swine diseases with similar clinical presentations. Here, an improved PCR assay was developed and evaluated for sensitive and universal detection of ASFV. Primers specific for ASFV were designed based on the highly conserved region of the vp72 gene sequences of all ASFV strains available in GenBank, and the PCR assay was established and compared with two OIE-validated PCR tests. The analytic detection limit of the PCR assay was 60 DNA copies per reaction. No amplification signal was observed for several other porcine viruses. The novel PCR assay was more sensitive than two OIE-validated PCR assays when testing 14 strains of ASFV representing four genotypes (I, V, VIII and IX) from diverse geographical areas. A total of 62 clinical swine blood samples collected from Uganda were examined by the novel PCR, giving a high agreement (59/62) with a superior sensitive universal probe library-based real-time PCR. Eight out of 62 samples tested positive, and three samples with higher Ct values (39.15, 38.39 and 37.41) in the real-time PCR were negative for ASFV in the novel PCR. In contrast, one (with a Ct value of 29.75 by the real-time PCR) and two (with Ct values of 29.75 and 33.12) ASFV-positive samples were not identified by the two OIE-validated PCR assays, respectively. Taken together, these data show that the novel PCR assay is specific, sensitive, and applicable for molecular diagnosis and surveillance of ASF.

Isolation and Characterization of a Moderately Virulent Classical Swine Fever Virus Emerging in China

Classical swine fever (CSF) is a devastating infectious disease of pigs caused by classical swine fever virus (CSFV). In China, CSF has been under control owing to extensive vaccination with the lapinized attenuated vaccine (C-strain) since 1950s, despite sporadic or endemic in many regions. However, recently, CSF outbreaks occurred in a large number of swine herds in China. Here, we isolated 15 CSFV strains from diverse C-strain-vaccinated pig farms in China and characterized the genetic variations and antigenicity of the new isolates. The new strains showed unique variations in the E2 protein and were clustered to the subgenotype 2.1d of CSFV recently emerging in China in the phylogenetic tree. Cross-neutralization test showed that the neutralizing titres of porcine anti-C-strain sera against the new isolates were substantially lower than those against both the highly virulent Shimen strain and the subgenotype 2.1b strains that were isolated in China in 2006 and 2009, respectively. In addition, experimental animal infection showed that the HLJZZ2014 strain-infected pigs displayed lower mortality and less severe clinical signs and pathological changes compared with the Shimen strain-infected pigs. The HLJZZ2014 strain was defined to be moderately virulent based on a previously established assessment system for CSFV virulence evaluation, and the virus shedding and the viral load in various tissues of the CSFV HLJZZ2014 strain-infected pigs were significantly lower than those of the Shimen strain-infected pigs. Taken together, the subgenotype 2.1d isolate of CSFV is a moderately virulent strain with molecular variations and antigenic alterations.

Pre-Clinical Evaluation of a Real-Time PCR Assay on a Portable Instrument as a Possible Field Diagnostic Tool: Experiences from the Testing of Clinical Samples for African and Classical Swine Fever Viruses

African swine fever (ASF) and classical swine fever (CSF) are two highly infectious transboundary animal diseases (TADs) that are serious threats to the pig industry worldwide, including in China, the world's largest pork producer. In this study, a duplex real-time PCR assay was developed for the rapid detection and differentiation of African swine fever virus (ASFV) and classical swine fever virus (CSFV). The assay was performed on a portable, battery-powered PCR thermocycler with a low sample throughput (termed as 'T-COR4 assay'). The feasibility and reliability of the T-COR4 assay as a possible field method was investigated by testing clinical samples collected in China. When evaluated with reference materials or samples from experimental infections, the assay performed in a reliable manner, producing results comparable to those obtained from stationary PCR platforms. Of 59 clinical samples, 41 had results identical to a two-step CSFV real-time PCR assay. No ASFV was detected in these samples. The T-COR4 assay was technically easy to perform and produced results within 3 h, including sample preparation. In combination with a simple sample preparation method, the T-COR4 assay provides a new tool for the field diagnosis and differentiation of ASF and CSF, which could be of particular value in remote areas.

Enhanced protective immunity of the chimeric vector-based vaccine rAdV-SFV-E2 against classical swine fever in pigs by a Salmonella bacterial ghost adjuvant

Classical swine fever (CSF) is a highly contagious swine disease caused by classical swine fever virus (CSFV). Previously, we demonstrated that rAdV-SFV-E2, an adenovirus-delivered, Semliki Forest virus replicon-vectored marker vaccine against CSF, is able to protect pigs against lethal CSFV challenge. From an economical point of view, it will be beneficial to reduce the minimum effective dose of the vaccine. This study was designed to test the adjuvant effects of Salmonella enteritidis-derived bacterial ghosts (BG) to enhance the protective immunity of rAdV-SFV-E2 in pigs. Groups of 5-week-old pigs (n = 4) were immunized intramuscularly twice with 10⁵ median tissue culture infective doses (TCID₅₀) rAdV-SFV-E2 combined with 10¹⁰ colony forming units (CFU) BG, 10⁶ or 10⁵ TCID₅₀ rAdV-SFV-E2 alone or 10¹⁰ CFU BG alone at an interval of 3 weeks, and challenged with the highly virulent CSFV Shimen strain at 1 week post-booster immunization. The results show that the pigs inoculated with 10⁵ TCID₅₀ rAdV-SFV-E2 plus BG or 10⁶ TCID₅₀ rAdV-SFV-E2 alone were completely protected from lethal CSFV challenge, in contrast with the pigs vaccinated with 10⁵ TCID₅₀ rAdV-SFV-E2 or BG alone, which displayed partial or no protection following virulent challenge. The data indicate that BG are a promising adjuvant to enhance the efficacy of rAdV-SFV-E2 and possibly other vaccines.

Hypervariable antigenic region 1 of classical swine fever virus E2 protein impacts antibody neutralization

Envelope glycoprotein E2 of classical swine fever virus (CSFV) is the major antigen that induces neutralizing antibodies and confers protection against CSFV infection. There are three hypervariable antigenic regions (HAR1, HAR2 and HAR3) of E2 that are different between the group 1 vaccine C-strain and group 2 clinical isolates. This study was aimed to characterize the antigenic epitope region recognized by monoclonal antibody 4F4 (mAb-4F4) that is present in the group 2 field isolate HZ1-08, but not in the C-strain, and examine its impact on neutralization titers when antisera from different recombinant viruses were cross-examined. Indirect ELISA with C-strain E2-based chimeric proteins carrying the three HAR regions showed that the mAb-4F4 bound to HAR1 from HZ1-08 E2, but not to HAR2 or HAR3, indicating that the specific epitope is located in the HAR1 region. Of the 6 major residues differences between C-strain and field isolates, Glu713 in the HAR1 region of strain HZ1-08 is critical for mAb-4F4 binding either at the recombinant protein level or using intact recombinant viruses carrying single mutations. C-strain-based recombinant viruses carrying the most antigenic part of E2 or HAR1 from strain HZ1-08 remained non-pathogenic to pigs and induced good antibody responses. By cross-neutralization assay, we observed that the anti-C-strain serum lost most of its neutralization capacity to RecC-HZ-E2 and QZ-14 (subgroup 2.1d field isolate in 2014), and vice versa. More importantly, the RecC-HAR1 virus remained competent in neutralizing ReC-HZ-E2 and QZ-14 strains without compromising the neutralization capability to the recombinant C-strain. Thus, we propose that chimeric C-strain carrying the HAR1 region of field isolates is a good vaccine candidate for classical swine fever.

Piglets with maternally derived antibodies from sows immunized with rAdV-SFV-E2 were completely protected against lethal CSFV challenge

Classical swine fever (CSF) is an economically important infectious disease of pigs caused by Classical swine fever virus (CSFV). To facilitate the eradication of CSF in endemic areas, a marker vaccine enabling differentiation of infected from vaccinated animals (DIVA) is urgently needed. Previously, we have demonstrated that the DIVA vaccine rAdV-SFV-E2, an adenovirus-vectored Semliki Forest virus replicon expressing the E2 glycoprotein of CSFV, induces complete protection from lethal CSFV challenge. The aim of this study was to investigate whether maternally derived antibodies (MDAs) from sows immunized with rAdV-SFV-E2 can effectively protect piglets against lethal CSFV challenge. Three groups of five-week-old piglets (n=4), with or without MDAs, were challenged with the highly virulent CSFV Shimen strain. Clinical signs, CSFV-specific antibodies, viremia and pathological and histopathological changes were monitored. The results showed that the piglets with MDAs from the sow immunized with rAdV-SFV-E2 were protected clinically, virologically and pathologically, while the piglets with undetectable MDAs from the rAdV-SFV-E2-immunized sow were partially protected (2/4 survival), in contrast with the piglets from the non-vaccinated sow, which displayed CSF-typical clinical signs, viremia, deaths (4/4) and pathological/histopathological lesions. These results indicate that MDAs from the sow immunized with rAdV-SFV-E2 are able to confer full passive immunity to newborn piglets.

Safety and immunogenicity of a gE/gI/TK gene-deleted pseudorabies virus variant expressing the E2 protein of classical swine fever virus in pigs

Classical swine fever (CSF) and pseudorabies (PR) are both major infectious diseases of pigs, causing enormous economic losses to the swine industry in many countries. A marker vaccine that enables differentiation of infected from vaccinated animals (DIVA) is highly desirable for control and eradication of these two diseases in endemic areas. Since late 2011, PR outbreaks have been frequently reported in many Bartha-K61-vaccinated pig farms in China. It has been demonstrated that a pseudorabies virus (PRV) variant with altered antigenicity and increased pathogenicity was responsible for the outbreaks. Previously, we showed that rPRVTJ-delgE/gI/TK, a gE/gI/TK-deleted PRV variant, was safe for susceptible animals and provided a complete protection against lethal PRV variant challenge, indicating that rPRVTJ-delgE/gI/TK can be used as an attractive vaccine vector. To develop a safe bivalent vaccine against CSF and PR, we generated a recombinant virus rPRVTJ-delgE/gI/TK-E2 expressing the E2 protein of classical swine fever virus (CSFV) based on rPRVTJ-delgE/gI/TK and evaluated its safety and immunogenicity in pigs. The results indicated that pigs (n=5) immunized with rPRVTJ-delgE/gI/TK-E2 of different doses did not exhibit clinical signs or viral shedding following immunization, the immunized pigs produced anti-PRV or anti-CSFV neutralizing antibodies and the pigs immunized with 10(6) or 10(5) TCID50 rPRVTJ-delgE/gI/TK-E2 were completely protected against the lethal challenge with either CSFV Shimen strain or variant PRV TJ strain. These findings suggest that rPRVTJ-delgE/gI/TK-E2 is a promising bivalent DIVA vaccine candidate against CSFV and PRV coinfections.

Guanylate-Binding Protein 1, an Interferon-Induced GTPase, Exerts an Antiviral Activity against Classical Swine Fever Virus Depending on Its GTPase Activity

Many viruses trigger the type I interferon (IFN) pathway upon infection, resulting in the transcription of hundreds of interferon-stimulated genes (ISGs), which define the antiviral state of the host. Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), a highly contagious viral disease endangering the pig industry in many countries. However, anti-CSFV ISGs are poorly documented. Here we screened 20 ISGs that are commonly induced by type I IFNs against CSFV in lentivirus-delivered cell lines, resulting in the identification of guanylate-binding protein 1 (GBP1) as a potent anti-CSFV ISG. We observed that overexpression of GBP1, an IFN-induced GTPase, remarkably suppressed CSFV replication, whereas knockdown of endogenous GBP1 expression by small interfering RNAs significantly promoted CSFV growth. Furthermore, we demonstrated that GBP1 acted mainly on the early phase of CSFV replication and inhibited the translation efficiency of the internal ribosome entry site of CSFV. In addition, we found that GBP1 was upregulated at the transcriptional level in CSFV-infected PK-15 cells and in various organs of CSFV-infected pigs. Coimmunoprecipitation and glutathione S-transferase (GST) pulldown assays revealed that GBP1 interacted with the NS5A protein of CSFV, and this interaction was mapped in the N-terminal globular GTPase domain of GBP1. Interestingly, the K51 of GBP1, which is crucial for its GTPase activity, was essential for the inhibition of CSFV replication. We showed further that the NS5A-GBP1 interaction inhibited GTPase activity, which was critical for its antiviral effect. Taking our findings together, GBP1 is an anti-CSFV ISG whose action depends on its GTPase activity.

IMPORTANCE Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), an economically important viral disease affecting the pig industry in many countries. To date, only a few host restriction factors against CSFV, including interferon-stimulated genes (ISGs), have been characterized. Using a minilibrary of porcine ISGs, we identify porcine guanylate-binding protein 1 (GBP1) as a potent antiviral ISG against CSFV. We further show that the anti-CSFV action of GBP1 depends on its GTPase activity. The K51 of GBP1, critical for its GTPase activity, is essential for the antiviral action of GBP1 against CSFV replication, and the binding of the NS5A protein to GBP1 antagonizes the GTPase activity and thus the antiviral effect. This study will facilitate the development of anti-CSFV therapeutic agents by targeting host factors and may provide a new strategy for the control of CSF.

RNA interference screening of interferon-stimulated genes with antiviral activities against classical swine fever virus using a reporter virus

Classical swine fever (CSF) caused by classical swine fever virus (CSFV) is a highly contagious and often fatal disease of pigs, which leads to significant economic losses in many countries. Viral infection can induce the production of interferons (IFNs), giving rise to the transcription of hundreds of IFN-stimulated genes (ISGs) to exert antiviral effects. Although numerous ISGs have been identified to possess antiviral activities against different viruses, rare anti-CSFV ISGs have been reported to date. In this study, to screen anti-CSFV ISGs, twenty-one ISGs reported previously were individually knocked down using small interfering RNAs (siRNAs) followed by infection with a reporter CSFV expressing Renilla luciferase (Rluc). As a result, four novel anti-CSFV ISGs were identified, including natural-resistance-associated macrophage protein 1 (NRAMP1), cytosolic 5'-nucleotidase III A (NT5C3A), chemokine C-X-C motif ligand 10 (CXCL10), and 2'-5'-oligoadenylate synthetase 1 (OAS1), which were further verified to exhibit antiviral activities against wild-type CSFV. We conclude that the reporter virus is a useful tool for efficient screening anti-CSFV ISGs.

Co-expression of the C-terminal domain of Yersinia enterocolitica invasin enhances the efficacy of classical swine-fever-vectored vaccine based on human adenovirus

The use of adenovirus vector-based vaccines is a promising approach for generating antigen-specific immune responses. Improving vaccine potency is necessary in other approaches to address their inadequate protection for the majority of infectious diseases. This study is the first to reconstruct a recombinant replication-defective human adenovirus co-expressing E2 and invasin C-terminal (InvC) glycoproteins (rAd-E2-InvC). rAd-E2-InvC with 2 x 10(6) TCID50 was intramuscularly administered two times to CSFV-free pigs at 14 day intervals. No adverse clinical reactions were observed in any of the pigs after the vaccination. The CSFV E2-specific antibody titer was significantly higher in the rAd-E2-InvC group than that in the rAdV-E2 group as measured by NPLA and blocking ELISA. Pigs immunized with rAd-E2-InvC were completely protected against lethal challenge. Neither CSFV RNA nor pathological changes were detected in the tissues after CSFV challenge. These results demonstrate that rAd-E2-InvC could be an alternative to the existing CSF vaccine. Moreover, InvC that acts as an adjuvant could enhance the immunogenicity of rAdV-E2 and induce high CSFV E2-specific antibody titer and protection level.

Generation and evaluation of a chimeric classical swine fever virus expressing a visible marker gene

Classical swine fever virus (CSFV) is a noncytopathogenic virus, and the incorporation of an enhanced green fluorescent protein (EGFP) tag into the viral genome provides a means of direct monitoring of viral infection without immunostaining. It is well established that the 3' untranslated region (3'-UTR) of the CSFV plays an important role in viral RNA replication. Although CSFV carrying a reporter gene and chimeric CSFV have been generated and evaluated, a chimeric CSFV with a visible marker has not yet been reported. Here, we generated and evaluated a chimeric virus containing the EGFP tag and the 3'-UTR from vaccine strain HCLV (C-strain) in the genetic background of the highly virulent CSFV Shimen strain. The chimeric marker CSFV was fluorescent and had an approximately 100-fold lower viral titer, lower replication level of viral genome, and weaker fluorescence intensity than the recombinant CSFV with only the EGFP tag or the parental virus. Furthermore, the marker chimera was avirulent and displayed no viremia in inoculated pigs, which were completely protected from lethal CSFV challenge as early as 15 days post-inoculation. The chimeric marker virus was visible in vitro and attenuated in vitro and in vivo, which suggests that CSFV can be engineered to produce attenuated variants with a visible marker to facilitate in vitro studies of CSFV infection and replication and to develop of novel vaccines against CSF.

Generation and Efficacy Evaluation of a Recombinant Pseudorabies Virus Variant Expressing the E2 Protein of Classical Swine Fever Virus in Pigs

Classical swine fever (CSF) is an economically important infectious disease of pigs caused by classical swine fever virus (CSFV). Pseudorabies (PR), which is caused by pseudorabies virus (PRV), is another important infectious disease of pigs and other animals. Coinfections of pigs with PRV and CSFV occur occasionally in the field. The modified live vaccine Bartha-K61 strain has played an important role in the control of PR in many countries, including China. Since late 2011, however, increasing PR outbreaks caused by an emerging PRV variant have been reported in Bartha-K61-vaccinated swine populations on many farms in China. Previously, we generated a gE/gI-deleted PRV (rPRVTJ-delgE) based on this PRV variant, which was shown to be safe and can provide rapid and complete protection against lethal challenge with the PRV variant in pigs. Here, we generated a new recombinant PRV variant expressing the E2 gene of CSFV (rPRVTJ-delgE/gI-E2) and evaluated its immunogenicity and efficacy in pigs. The results showed that rPRVTJ-delgE/gI-E2 was safe for pigs, induced detectable anti-PRV and anti-CSFV neutralizing antibodies, and provided complete protection against the lethal challenge with either the PRV TJ strain or the CSFV Shimen strain. The data indicate that rPRVTJ-delgE/gI-E2 is a promising candidate bivalent vaccine against PRV and CSFV coinfections.

eEF1A Interacts with the NS5A Protein and Inhibits the Growth of Classical Swine Fever Virus

The NS5A protein of classical swine fever virus (CSFV) is involved in the RNA synthesis and viral replication. However, the NS5A-interacting cellular proteins engaged in the CSFV replication are poorly defined. Using yeast two-hybrid screen, the eukaryotic elongation factor 1A (eEF1A) was identified to be an NS5A-binding partner. The NS5A-eEF1A interaction was confirmed by coimmunoprecipitation, glutathione S-transferase (GST) pulldown and laser confocal microscopy assays. The domain I of eEF1A was shown to be critical for the NS5A-eEF1A interaction. Overexpression of eEF1A suppressed the CSFV growth markedly, and conversely, knockdown of eEF1A enhanced the CSFV replication significantly. Furthermore, eEF1A, as well as NS5A, was found to reduce the translation efficiency of the internal ribosome entry site (IRES) of CSFV in a dose-dependent manner, as demonstrated by luciferase reporter assay. Streptavidin pulldown assay revealed that eEF1A could bind to the CSFV IRES. Collectively, our results suggest that eEF1A interacts with NS5A and negatively regulates the growth of CSFV.

Thioredoxin 2 Is a Novel E2-Interacting Protein That Inhibits the Replication of Classical Swine Fever Virus

The E2 protein of classical swine fever virus (CSFV) is an envelope glycoprotein that is involved in virus attachment and entry. To date, the E2-interacting cellular proteins and their involvement in viral replication have been poorly documented. In this study, thioredoxin 2 (Trx2) was identified to be a novel E2-interacting partner using yeast two-hybrid screening from a porcine macrophage cDNA library. Trx2 is a mitochondrion-associated protein that participates in diverse cellular events. The Trx2-E2 interaction was further confirmed by glutathione S-transferase (GST) pulldown, in situ proximity ligation, and laser confocal assays. The thioredoxin domain of Trx2 and the asparagine at position 37 (N37) in the E2 protein were shown to be critical for the interaction. Silencing of the Trx2 expression in PK-15 cells by small interfering RNAs significantly promotes CSFV replication, and conversely, overexpression of Trx2 markedly inhibits viral replication of the wild-type (wt) CSFV and to a greater extent that of the CSFV N37D mutant, which is defective in binding Trx2. The wt CSFV but not the CSFV N37D mutant was shown to reduce the Trx2 protein expression in PK-15 cells. Furthermore, we demonstrated that Trx2 increases nuclear factor kappa B (NF-κB) promoter activity by promoting the nuclear translocation of the p65 subunit of NF-κB. Notably, activation of the NF-κB signaling pathway induced by tumor necrosis factor alpha (TNF-α) significantly inhibits CSFV replication in PK-15 cells, whereas blocking the NF-κB activation in Trx2-overexpressing cells no longer suppresses CSFV replication. Taken together, our findings reveal that Trx2 inhibits CSFV replication via the NF-κB signaling pathway.

IMPORTANCE

Thioredoxin 2 (Trx2) is a mitochondrion-associated protein that participates in diverse cellular events, such as antioxidative and antiapoptotic processes and the modulation of transcription factors. However, little is known about the involvement of Trx2 in viral replication. Here, we investigated, for the first time, the role of Trx2 in the replication of classical swine fever virus (CSFV), a devastating pestivirus of pigs. By knockdown and overexpression, we showed that Trx2 negatively regulates CSFV replication. Notably, we demonstrated that Trx2 inhibits CSFV replication by promoting the nuclear translocation of the p65 subunit of NF-κB, a key regulator of the host's innate immunity and inflammatory response. Our findings reveal a novel role of Trx2 in the host's antiviral response and provide new insights into the complex mechanisms by which CSFV interacts with the host cell.

Differential detection of classical swine fever virus challenge strains in C-strain vaccinated pigs

Background

Control of classical swine fever (CSF) by vaccination ideally requires that field strain infection can be detected irrespective of the vaccination status of the herd. To inform on the usefulness of molecular tests compatible with genetic Differentiation of Infected from Vaccinated Animals (DIVA) principles when using live-attenuated vaccines, tonsil homogenates from a vaccination-challenge experiment were analyzed using a differential real-time qRT-PCR for the C-strain vaccine or real-time qRT-PCR assays developed to specifically detect the challenge strains used.

Results

In animals with high or moderate levels of blood viraemia, which were not, or not fully, protected by vaccination, challenge virus RNA was readily detected in tonsil homogenates. In three out of the seven vaccinated animals that had high or moderate viraemia, the vaccine strain RNA also could be detected but at lower levels. Lower but varying levels of challenge and/or vaccine virus RNA were detected in tonsil homogenate samples from animals with no or low-level viraemia, and in groups solely consisting of such animals, no transmission of infection to naïve in-contact animals occurred. In one group of animals that were vaccinated 3 days prior to challenge, viraemia levels varied from high to absent and transmission of challenge virus to naïve in-contact animals occurred. The DIVA assay revealed challenge virus in all tonsil homogenates from this group, even in those animals that did not have viraemia and were protected from clinical disease by vaccination. Such animals, particularly in a low biosecurity/informal farm setting, could constitute a risk for disease control in the field.

Conclusions

Genetic DIVA testing is useful for detecting the presence of field virus infection especially in non-viraemic animals without overt clinical signs but which are incompletely protected by vaccination. Such tests could particularly be useful to inform decisions prior to and during cessation of a control strategy that employs vaccination.

Development of a TaqMan-based real-time reverse transcription polymerase chain reaction assay for the detection of encephalomyocarditis virus

Encephalomyocarditis virus (EMCV) is one of the major zoonosis pathogens and can cause acute myocarditis in young pigs or reproductive failure in sows. In this study, a TaqMan-based real-time reverse transcription polymerase chain reaction (RT-PCR) assay targeting 3D gene of EMCV was developed and their sensitivities and specificities were investigated. The results indicated that the standard curve had a wide dynamic range (10(1)-10(6) copies/μL) with a linear correlation (R(2)) of 0.996 between the cycle threshold (Ct) value and template concentration. The real-time RT-PCR assay is highly sensitive and able to detect 1.4×10(2) copies/μL of EMCV RNA, as no cross-reaction was observed with other viruses. These data suggested that the real-time RT-PCR assay developed in this study will be suitable for future surveillance and specific diagnosis of EMCV-infection.

Specific ligands for classical swine fever virus screened from landscape phage display library

Classical swine fever (CSF) is a devastating infectious disease caused by classical swine fever virus (CSFV). The screening of CSFV-specific ligands is of great significance for diagnosis and treatment of CSF. Affinity selection from random peptide libraries is an efficient approach to discover ligands with high stability and specificity. Here, we screened phage ligands for the CSFV E2 protein from f8/8 landscape phage display library by biopanning and obtained four phage clones specific for the E2 protein of CSFV. Viral blocking assays indicated that the phage clone displaying the octapeptide sequence DRATSSNA remarkably inhibited the CSFV replication in PK-15 cells at a titer of 10(10) transduction units, as evidenced by significantly decreased viral RNA copies and viral titers. The phage-displayed E2-binding peptides have the potential to be developed as antivirals for CSF.

Classical swine fever in China: a minireview

Classical swine fever (CSF), caused by Classical swine fever virus (CSFV), is an OIE-listed, highly contagious, often fatal disease of swine worldwide. Currently, the disease is controlled by prophylactic vaccination in China and many other countries using the modified live vaccines derived from C-strain, which was developed in China in the mid-1950s. This minireview summarizes the epidemiology, diagnostic assays, control and challenges of CSF in China. Though CSF is essentially under control, complete eradication of CSF in China remains a challenging task and needs long-term, joint efforts of stakeholders.