Development of single dilution immunoassay to detect E2 protein specific classical swine fever virus antibody

Development of classical swine fever virus E2-protein based indirect ELISA for detection of antibodies against the virus in pigs

Classical swine fever (CSF) is an economically important and highly contagious disease of pigs caused by CSF virus, genus Pestivirus. Serological diagnosis of the disease is highly valuable for surveillance and thereby containment of spread of the disease. In this study, we have demonstrated the development of CSFV envelope glycoprotein E2-based indirect ELISA (E2-iELISA) for the detection of CSFV specific antibodies. The full-length E2 protein was expressed in E. coli and the purified protein was used as a coating antigen in indirect ELISA for detecting CSFV specific antibodies in pigs. A panel of 506 pig sera samples was used to validate the ELISA and the results were highly comparable to the results obtained with the commercial antibody detection kit (PrioCHECK CSFV Ab kit). The in-house E2-iELISA demonstrated high diagnostic sensitivity (95.4%) and specificity (95.5%), highlighting its potential application for sero-surveillance or monitoring of the disease in the swine population.

A Novel Blocking Enzyme-Linked Immunosorbent Assay Based on a Biotinylated Nanobody for the Rapid and Sensitive Clinical Detection of Classical Swine Fever Virus Antibodies

Monoclonal and polyclonal antibodies are mostly used for the development of traditional enzyme-linked immunosorbent assays (ELISAs), but the use of certain conventional antibodies may be limited by their low yield, the difficulty of their isolation, and their high cost. Heavy-chain antibodies derived from camelids with naturally missing light chains can overcome these deficiencies and are an excellent alternative to conventional antibodies. In this study, a nanobody (Nb)-AviTag fusion protein was constructed, and the feasibility of its use as a high-sensitivity probe in a blocking ELISA (bELISA) for classical swine fever virus (CSFV) was investigated. The CSFV E2 recombinant protein expressed by the CHO expression system exhibited good reactogenicity and immunogenicity and induced the production of high CSFV antibody levels in rabbits. Three different clones of Nbs were successfully isolated using a phage display system in alpaca, and an Nb1-AviTag fusion protein was successfully expressed using an Escherichia coli expression system. The purified Nb1-AviTag fusion protein was then biotinylated in vitro to obtain Nb1-biotin. A novel bELISA was developed for the detection of CSFV antibodies in clinical serum using Nb1-biotin as a probe. The cutoff value of bELISA was 32.18%, the sensitivity of bELISA was higher than that of the bELISA kit with IDEXX antibody, and the coincidence rate was 94.7%. A rapid, low-cost, highly sensitive and highly specific CSFV E2 antibody-based bELISA method was successfully established and can be used for the serological evaluation of CSFV E2 subunit vaccines and the ELISA-based diagnosis of CSFV infection. IMPORTANCE Currently, the epidemic situation of classical swine fever (CSF) is sporadic, and cases of atypical swine fever are on the rise in China. Therefore, it is necessary to accurately eliminate suspected cases by using highly sensitive and specific diagnostic techniques. In our study, a rapid, low-cost, highly sensitivity, highly reliable and reproducible, and highly specific classical swine fever virus (CSFV) E2 antibody-based blocking ELISA method was successfully established by using the phage display system and the Nb1-AviTag fusion expression platform. It provides a new technique for serological evaluation of CSFV vaccines and ELISA-based diagnosis of CSFV infection.

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.

A chemiluminescence immunoassay for rapid detection of classical swine fever virus E2 antibodies in pig serum samples

The high performance of chemiluminescence immunoassays (CLIAs) in diagnosis has been gradually recognized in recent years, but their application in the diagnosis of classical swine fever (CSF) has not been reported. Here, a recombinant E2 (rE2) protein and a peroxidase-conjugated monoclonal antibody (MAb G5) were used to develop a competition-based chemiluminescence immunoassay (cCLIA) for rapid and accurate detection of E2-specific antibodies in pig serum. To evaluate the feasibility of cCLIA in the diagnosis of CSF, we developed a competition-based enzyme-linked immunosorbent assay (cELISA) as a control. Under the optimum test conditions, cCLIA showed a higher signal-to-noise ratio than that of the control cELISA. The best signal-to-noise ratios of cCLIA and cELISA were 70 and 17, respectively. Then, the diagnostic performance of the two assays was compared by examining a panel of pig serum samples (n = 285) with a confirmed status, and cCLIA showed higher diagnostic sensitivity (Dn) and diagnostic specificity (Dp) values than those of cELISA. The Dn and Dp of cCLIA were 97.49% and 96.08%, respectively, and those of cELISA were 93.97% and 94.12%, respectively. Furthermore, cCLIA can provide results within 20 min, whereas the control cELISA requires at least 1 hr. According to these findings, the newly developed cCLIA has potential application in the diagnosis of CSF and offers an alternative approach for efficient and rapid detection of E2-specific antibodies.

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.

Evaluation of Japanese encephalitis virus E and NS1 proteins immunogenicity using a recombinant Newcastle disease virus in mice

Japanese encephalitis (JE) is the most important cause of acute encephalitis syndrome (AES). Japanese encephalitis virus (JEV), the prototype member of the JE serocomplex, belongs to the genus Flavivirus. The immunogenic proteins envelope (E) and non-structural protein 1 (NS1) of JEV are widely explored for the development of vaccines and diagnostics against JEV. However, there are underlying concerns such as the risk of reversion of live-attenuated vaccines to high virulence, the incomplete inactivation of pathogens in inactivated vaccines and partial vaccine coverage. Newcastle disease virus (NDV) is an efficient viral vaccine vector to express several human and animal immunogenic proteins. In the present study, we have developed a recombinant NDV (rNDV), individually expressing the E and NS1 proteins of JEV (rNDV-Ejev and rNDV-NS1jev). The recovered rNDV-Ejev and rNDV-NS1jev were characterized in 9-day-old SPF embryonated chicken eggs and in cell culture. The vaccination of rNDV-Ejev and rNDV-NS1jev showed effective immunity against JEV upon intranasal immunization in BALB/c mice. The rNDVs vaccination produced effective neutralization antibody titers against both NDV and JEV. The cytokine profiling of the vaccinated mice showed an effective Th1 and Th2 mediated immune response. The study also provided an insight that E, when used in combination with NS1 could reduce the efficacy of only E based immunization in mice. Our results suggested rNDV-Ejev to be a promising live viral vectored vaccine against JEV. This study implies an alternative and economical strategy for the development of a recombinant vaccine against JEV.

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.

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.

Glycoprotein D peptide-based diagnostic approach for the detection of avian infectious laryngotracheitis antibodies

Infectious laryngotracheitis (ILT) is a highly contagious respiratory disease of chickens, pheasants, and peafowl. It is caused by the alpha herpesvirus, infectious laryngotracheitis virus (ILTV). Glycoprotein D (gD) of ILTV is immunogenic and helps in its binding to the susceptible host cell receptor. In the present study, a recombinant gD protein was expressed in a prokaryotic system to develop a single serum dilution ELISA. In addition, two immunogenic peptides, corresponding to regions 77-89 and 317-328, were identified in gD protein. The peptides were synthesized using solid-phase peptide synthesis, purified using reversed-phase HPLC, and characterized using mass spectrometry. The peptides displayed a good titre and were found to be promising antigens to coat the ELISA plate to detect the ILTV antibodies in the serum sample. The developed ELISA showed 96.9% sensitivity, 87.5% specificity, and 95.3% accuracy as compared to OIE referenced standard indirect ILTV ELISA (whole viral coated). The assay may not differentiate vaccinated from infected birds when the flocks are administered with live attenuated vaccines. However, the assay could be useful to detect the disease condition in birds vaccinated with recombinant vaccine expressing glycoproteins other than gD. The developed ILTV single serum dilution ELISA could be an alternative to the existing diagnostics for the detection of ILTV antibodies.

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.

A magnetoelastic biosensor based on E2 glycoprotein for wireless detection of classical swine fever virus E2 antibody

A wireless magnetoelastic (ME) biosensor immobilized with E2 glycoprotein was first developed to detect classical swine fever virus (CSFV) E2 antibody. The detection principle is that a sandwich complex of CSFV E2 - rabbit anti-CSFV E2 antibody - alkaline phosphatase (AP) conjugated goat anti-rabbit IgG formed on the ME sensor surface, with biocatalytic precipitation used to amplify the mass change of antigen-antibody specific binding reaction, induces a significant change in resonance frequency of the biosensor. Due to its magnetostrictive feature, the resonance vibrations and resonance frequency can be actuated and wirelessly monitored through magnetic fields. The experimental results show that resonance frequency shift increases with the augmentation of the CSFV E2 antibody concentration. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and fluorescence microscopy analysis proved that the modification and detection process were successful. The biosensor shows a linear response to the logarithm of CSFV E2 antibody concentrations ranging from 5 ng/mL to 10 μg/mL, with a detection limit (LOD) of 2.466 ng/mL and the sensitivity of 56.2 Hz/μg·mL-1. The study provides a low-cost yet highly-sensitive and wireless method for selective detection of CSFV E2 antibody.