Development of an indirect ELISA, blocking ELISA, fluorescent microsphere immunoassay and fluorescent focus neutralization assay for serologic evaluation of exposure to North American strains of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus: a review of detection, inhibition of host gene expression and evasion of host innate immune

As one of the most important swine enteropathogenic coronavirus, porcine epidemic diarrhea virus (PEDV) is the causative agent of an acute and devastating enteric disease that causes lethal watery diarrhea in suckling piglets. Recent progress in studying PEDV has revealed many intriguing findings on its prevalence and genetic evolution, rapid diagnosis, suppression of host gene expression, and suppression of the host innate immune system. Due to the continuous mutation of the PEDV genome, viral evasions from innate immune defenses and mixed infection with other coronaviruses, the spread of the virus is becoming wider and faster, making it even more necessary to prevent the infections caused by wild-type PEDV variants. It has also been reported that PEDV nsp1 is an essential virulence determinant and is critical for inhibiting host gene expression by structural and biochemical analyses. The inhibition of host protein synthesis employed by PEDV nsp1 may contribute to the regulation of host cell proliferation and immune evasion-related biological functions. In this review, we critically evaluate the recent studies on these aspects of PEDV and assess prospects in understanding the function of PEDV proteins in regulating host innate immune response and viral virulence.

Development and Clinical Application of a Molecular Assay for Four Common Porcine Enteroviruses

Porcine epidemic diarrhea virus (PEDV), porcine transmissible gastroenteritis virus (TGEV), porcine deltacoronavirus (PDCoV), and porcine rotavirus-A (PoRVA) are the four main pathogens that cause viral diarrhea in pigs, and they often occur in mixed infections, which are difficult to distinguish only according to clinical symptoms. Here, we developed a multiplex TaqMan-probe-based real-time RT-PCR method for the simultaneous detection of PEDV, TGEV, PDCoV, and PoRVA for the first time. The specific primers and probes were designed for the M protein gene of PEDV, N protein gene of TGEV, N protein gene of PDCoV, and VP7 protein gene of PoRVA, and corresponding recombinant plasmids were constructed. The method showed extreme specificity, high sensitivity, and excellent repeatability; the limit of detection (LOD) can reach as low as 2.18 × 102 copies/μL in multiplex real-time RT-PCR assay. A total of 97 clinical samples were used to compare the results of the conventional reverse transcription PCR (RT-PCR) and this multiplex real-time RT-PCR for PEDV, TGEV, PDCoV, and PoRVA detection, and the results were 100% consistent. Subsequently, five randomly selected clinical samples that tested positive were sent for DNA sequencing verification, and the sequencing results showed consistency with the detection results of the conventional RT-PCR and our developed method in this study. In summary, this study developed a multiplex real-time RT-PCR method for simultaneous detection of PEDV, TGEV, PDCoV, and PoRVA, and the results of this study can provide technical means for the differential diagnosis and epidemiological investigation of these four porcine viral diarrheic diseases.

Production of virus-like particles of porcine circovirus 2 in baculovirus expression system and its application for antibody detection

BACKGROUND

Porcine circovirus 2 (PCV-2) is one of the pathogens that leads to a growing and persistent threat in pigs. Thus, the development of serological detection methods for PCV-2 is of great necessity for clinical diagnosis as well as epidemiological investigations. This study aimed to establish an indirect enzyme-linked immunosorbent assay (ELISA) to examine antibodies against PCV-2 based on virus-like particles (VLPs).

RESULTS

Recombinant PCV-2 Cap protein was expressed in the baculovirus-insect cells system and PCV-2 VLPs were observed over transmission electron microscopy (TEM). The PCV-2 VLPs were shown to have good immunogenicity in mice and stimulated a high level of PCV-2 antibody titers. Using PCV-2 VLPs as coating antigen, the indirect ELISA can detect PCV-2 antibodies in animals with diagnostic sensitivity and specificity of 98.33% and 93.33% compared to immunofluorescence assay (IFA), respectively. The intra- and inter-assay coefficient variations (CVs) were < 10% in a batch, and < 15% in different batches, indicating good repeatability. There was no cross-reaction of this ELISA with antibodies against other porcine viruses. A total of 170 serum samples collected from different pig farms in China were tested for PCV-2 antibodies, and 151 (88.8%) samples were PCV-2 antibody positive.

CONCLUSION

Our findings suggest that this ELISA was rapid, specific, and reproducible and can be used for large-scale serological investigations of PCV-2 antibodies in pigs.

Development of an Indirect Enzyme-Linked Immunosorbent Assay Based on the Yeast-Expressed CO-26K-Equivalent Epitope-Containing Antigen for Detection of Serum Antibodies against Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea (PED) is a severe contagious intestinal disease caused by the porcine epidemic diarrhea virus (PEDV), which leads to high mortality in piglets. In this study, by analyzing a total of 53 full-length spike genes and COE domain regions of PEDVs, the conserved COE fragment of the spike protein from the dominant strain SC1402 was chosen as the target protein and expressed successfully in Pichia pastoris (P. pastoris). Furthermore, an indirect enzyme-linked immunosorbent assay (iELISA) based on the recombinant COE protein was developed for the detection of anti-PEDV antibodies in pig sera. The results showed that under the optimized conditions, the cut-off value of COE-based indirect ELISA (COE-iELISA) was determined to be 0.12. Taking the serum neutralization test as standard, the relative sensitivity of the COE-iELISA was 94.4% and specificity 92.6%. Meanwhile, no cross-reactivity to other porcine pathogens was noted with this assay. The intra-assay and inter-assay coefficients of variation were less than 7%. Moreover, 164 vaccinated serum samples test showed that overall agreement between COE-iELISA and the actual diagnosis result was up to 99.4%. More importantly, the developed iELISA exhibited a 95.08% agreement rate with the commercial ELISA kit (Kappa value = 0.88), which suggested that the expressed COE protein was an effective antigen in serologic tests and the established COE-iELISA is reliable for monitoring PEDV infection in pigs or vaccine effectiveness.

Comparison of Two Diagnostic Assays for the Detection of Serum Neutralizing Antibody to Porcine Epidemic Diarrhea Virus

Lactogenic immunity is important for the protection of piglets against many pathogens including porcine epidemic diarrhea virus. Circulating neutralizing antibodies levels in sow sera may help determine if a detectable immune response could confer protection to piglets. Neutralizing antibodies can be detected through various diagnostic assays. This study evaluated the diagnostic characteristics of two neutralizing antibody assays for porcine epidemic diarrhea virus neutralizing antibodies in serum of challenged gilts. Four treatment groups, control, non-vaccinated, vaccinated prior to challenge, and vaccinated following challenge, were comprised of 20 gilts. Serum sample were collected from each gilt prior to and following challenge with porcine epidemic diarrhea virus. Samples were evaluated for the presence of neutralizing antibodies via a fluorescent focus neutralization assay and a high-throughput neutralization assay. Diagnostic sensitivity and specificity for the fluorescent focus neutralization and high-throughput neutralization assays for this study were optimized at a cutoff of a dilution of 80 and 80% fluorescent reduction respectively and demonstrated moderate agreement based off the kappa statistic. The focus fluorescent neutralization and high-throughput neutralization assays can be used to monitor the status of neutralizing antibodies within animals or a population of animals. The high-throughput assay has advantages over the focus fluorescent assay in that it has a higher specificity at the indicated cut-off and the nature of the results allows for more discrimination between individual results.

Combined in vitro and cell-based selection display method producing specific binders against IL-9 receptor in high yields

We combined cell-free ribosome display and cell-based yeast display selection to build specific protein binders to the extracellular domain of the human interleukin 9 receptor alpha (IL-9Rα). The target, IL-9Rα, is the receptor involved in the signalling pathway of IL-9, a pro-inflammatory cytokine medically important for its involvement in respiratory diseases. The successive use of modified protocols of ribosome and yeast displays allowed us to combine their strengths-the virtually infinite selection power of ribosome display and the production of (mostly) properly folded and soluble proteins in yeast display. The described experimental protocol is optimized to produce binders highly specific to the target, including selectivity to common proteins such as BSA, and proteins potentially competing for the binder such as receptors of other cytokines. The binders were trained from DNA libraries of two protein scaffolds called 57aBi and 57bBi developed in our laboratory. We show that the described unconventional combination of ribosome and yeast displays is effective in developing selective small protein binders to the medically relevant molecular target.

Current State of Molecular and Serological Methods for Detection of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV), a member of the Coronaviridae family, is the etiological agent of an acute and devastating enteric disease that causes moderate-to-high mortality in suckling piglets. The accurate and early detection of PEDV infection is essential for the prevention and control of the spread of the disease. Many molecular assays have been developed for the detection of PEDV, including reverse-transcription polymerase chain reaction (RT-PCR), real-time RT-PCR (qRT-PCR) and loop-mediated isothermal amplification assays. Additionally, several serological methods have been developed and are widely used for the detection of antibodies against PEDV. Some of them, such as the immunochromatography assay, can generate results very quickly and in field conditions. Molecular assays detect viral RNA in clinical samples rapidly, and with high sensitivity and specificity. Serological assays can determine prior immune exposure to PEDV, can be used to monitor the efficacy of vaccination strategies and may help to predict the duration of immunity in piglets. However, they are less sensitive than nucleic acid-based detection methods. Sanger and next-generation sequencing (NGS) allow the analysis of PEDV cDNA or RNA sequences, and thus, provide highly specific results. Furthermore, NGS based on nonspecific DNA cleavage in clustered regularly interspaced short palindromic repeats (CRISPR)–Cas systems promise major advances in the diagnosis of PEDV infection. The objective of this paper was to summarize the current serological and molecular PEDV assays, highlight their diagnostic performance and emphasize the advantages and drawbacks of the application of individual tests.

Development of an indirect ELISA to detect PEDV specific IgA antibody based on a PEDV epidemic strain

Background

Porcine epidemic diarrhea (PED), a swine epidemic disease caused by porcine epidemic diarrhea virus (PEDV), is characterized by severe watery diarrhea, vomiting, dehydration and high mortality in piglets, and has caused serious economic losses to the global porcine industry. The level of PEDV IgA antibody is a key marker to assess the extent of passive immunity of the resistance against PEDV infection. However, current commercial structure proteins-based kits for detection of PEDV antibody are not affordable, and those kits require complicated antigen preparation procedures, which cannot meet the scope of economic benefits of many large-scale pig companies in China. Therefore, there is an urgent need to develop an accurate, simple, and economical method for IgA detection in clinical samples. In this study, an indirect ELISA (i-ELISA) method was developed based on a purified PEDV epidemic strain (NH-TA2020).

Results

The results show that optimal working dilution ratios of PEDV antigen and HRP anti-swine IgA are at 1: 1000 and 1:15000 respectively. The sensitivity of this method is high with the maximum dilution of samples up to 1:160, and coefficients of variation (CV) of both the intra assays and inter assays were no more than 15%. In addition, the relative sensitivities of the i-ELISA were above 90% compared with values from commercial kits in both serum and oral fluid samples.

Conclusions

Our results suggested that the i-ELISA developed in this study was an accurate, simple, and economical method for PEDV-IgA detection in clinical samples.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03419-w.

Development and Primary Application of an Indirect ELISA Based on Rep Protein to Analyze Antibodies against Porcine Cocirvirus-like Virus (PCLV)

Porcine circovirus-like virus (PCLV) is a member of circovirus that contains a single-strand DNA genome, which may be one of the pathogens that causes diarrheal symptoms in pigs. The Rep protein encoded by the genome of PCLV may be responsible for viral genome replication. The development of serological detection methods for PCLV is of great necessity for clinical diagnosis, as well as epidemiological investigations. Therefore, this study attempted to build an indirect enzyme-linked immunosorbent assay (ELISA) to examine antibodies against PCLV based on the His-tagged recombinant Rep protein. Full-length PCLV Rep protein was induced and expressed in E. coli and was purified as an antigen to establish an ELISA detection kit. The purified Rep protein was used to inject into mice to produce specific antibodies. There was no cross-reaction of Rep-based ELISA with antisera against other porcine viruses. The intra-assay and inter-assay coefficient variations (CVs) were 0.644–8.211% and 0.859–7.246%, respectively, indicating good repeatability. The non-cross-reaction with TGEV, PRRSV and PCV2 testing showed high sensitivity and high specificity for this ELISA assay. A total of 1593 serum samples collected from different pig farms in Jiangxi Province were tested for anti-PCLV Rep antibodies, and 284 (17.83%) of the 1593 samples were Rep antibody positive. Altogether, the indirect ELISA detection tool developed in this study could be applied to examine serum of PCLV antibodies with good repeatability, high sensitivity and high specificity. In addition, field sample detection results suggested that the PCLV antibody has a low prevalence in pig populations in Jiangxi Province of China.

Development of a Blocking ELISA Kit for Detection of ASFV Antibody Based on a Monoclonal Antibody against Full Length p72

BACKGROUND

African swine fever virus (ASFV) is the etiologic agent of African swine fever (ASF), a disease of highly contagious and significant threat to pork production. At present, the sensitive detection methods are the keys to the disease control.

OBJECTIVE

Full length p72 is produced by eukaryotic system and its monoclonal antibody (mAb) 34C10 is subsequently recovered. A blocking ELISA kit for detection of ASFV antibody is developed based on p72 trimers and 34C10.

METHODS

Full length p72 is expressed and is used as immunogen to prepare a panel of monoclonal antibodies. The mAb 34C10 is verified by immunofluorescent and tested by ELISAs with positive serums. The constant affinity of 34C10 is then confirmed. A blocking ELISA kit is further developed and is compared with two commercial kits.

RESULTS

The mAb 34C10 is specifically bound to p72 protein, and it exhibits blocking affection to positive serum. IFA experiment shows that 34C10 could bind to p72 expressed by baculoviruses and the binding affinity of 34C10 is found to be as high as 1.85 × 1011 L/mol. The blocking ELISA kit shows high coincidence with a commercial ELISA kit. The sensitivity between these two kits is 97.6% (95%, CI: 90.65-99.58) and the specificity between them is 100% (95%, CI: 98.34-100).

CONCLUSION

The blocking ELISA developed in this study may have great potential for diagnosis of ASF. The structure of the antigen p72 is found to be a key factor for the performance of the kit.

HIGHLIGHTS

For the first time, the eukaryotic expressed full-length p72 protein is utilized to recover the monoclonal antibody and it is coated as antigen during the development of the blocking ELISA kit. This study sheds new light on the development of the blocking ELISA kits, especially for the development of diagnostic kit for the contagious virus with bio-safety problems.

Development and characterization of recombinant ASFV CD2v protein nanoparticle-induced monoclonal antibody

African swine fever (ASF) caused by African swine fever virus (ASFV) is becoming a serious threat to the swine industry worldwide. CD2v is a key pathogenic factor of ASFV and the protective antigen with low immunogenicity, whereas viral protein-based nanoparticles have advantages of precise assembly and high immunogenicity. In this study, the CD2v protein fused with Norovirus (NoV) P particle assembled into nanoparticle for improved immunogenicity. Then, CD2v protein nanoparticle and monomer CD2v protein were expressed in HEK293F cells. The former induced higher levels of antibodies, and thus highly potent monoclonal antibodies (mAbs) were generated and characterized. The highest antibody titration of mAb 10A3 reached 1:2048000, and mAb 2E9 had the highest inhibition percent of 84% when competed with ASFV positive serum. Meanwhile, all mAbs reacted specifically with the denatured CD2v protein, and the linear epitope with the location of amino acids 28th to 51st of CD2v extracellular domain sequence was identified. In summary, this study produced a highly immunogenic CD2v protein and generated high-titer mAbs, the precise location of linear epitope on the CD2v was further determined. These findings may provide a powerful help for etiology and serological detection of ASFV.

Detection of porcine parainfluenza virus type-1 antibody in swine serum using whole-virus ELISA, indirect fluorescence antibody and virus neutralizing assays

BACKGROUND

Porcine parainfluenza virus 1 (PPIV-1) is a respiratory virus in the family Paramyxoviridae and genus Respirovirus. It is closely related to bovine parainfluenza virus 3, human parainfluenza virus 1, and Sendai virus. Recent reports suggest PPIV-1 is widespread in swine herds in the United States and abroad. However, seroprevalence studies and the ability to evaluate cross neutralization between heterologous strains is not possible without validated antibody assays. This study describes the development of an indirect fluorescence antibody (IFA) assay, a whole virus enzyme-linked immunosorbent assay (wv-ELISA) and a serum virus neutralization (SVN) assay for the detection of PPIV-1 antibodies using 521 serum samples collected from three longitudinal studies and two different challenge strains in swine.

RESULTS

The area under the curve (AUC) of the wv-ELISA (95% CI, 0.93-0.98) was significantly higher (p = 0.03) compared to the IFA (95% CI, 0.90-0.96). However, no significant difference was observed between the IFA and wv-ELISA when compared to the SVN (95% CI, 0.92-0.97). All three assays demonstrated relatively uniform results at a 99% true negative rate, with only 11 disagreements observed between the IFA, wv-ELISA and SVN.

CONCLUSIONS

All three serology assays detected PPIV-1 antibody in swine serum of known status that was collected from experimental studies. The SVN detected seroconversion earlier compared to the IFA and the wv-ELISA. Both the wv-ELISA and the SVN had similar diagnostic performance, while the IFA was not as sensitive as the wv-ELISA. All three assays are considered valid for routine diagnostic use. These assays will be important for future studies to screen seronegative swine for research, determine PPIV-1 seroprevalence, and to evaluate vaccine efficacy against PPIV-1 under experimental and field conditions.

Development and Application of a Reverse-Transcription Recombinase-Aided Amplification Assay for Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV) is a coronavirus currently widespread worldwide in the swine industry. Since PEDV was discovered in China in 1984, it has caused huge economic losses in the swine industry. PEDV can infect pigs of all ages, but piglets have the highest infection with a death rate as high as 100%, and the clinical symptoms are watery diarrhea, vomiting, and dehydration. At present, there is not any report on PEDV detection by RT-RAA. In this study, we developed an isothermal amplification technology by using reverse-transcription recombinase-aided amplification assay (RT-RAA) combined with portable instruments to achieve a molecular diagnosis of PEDV in clinical samples from China. By designing a pair of RT-RAA primers and probes based on the PEDV N gene, this method breaks the limitations of existing detection methods. The assay time was within 30 min at 41 °C and can detect as few as 10 copies of PEDV DNA molecules per reaction. Sixty-two clinical tissue samples were detected by RT-qPCR and RT-RAA. The positive and negative rates for the two methods were 24.19% and 75.81%, respectively. Specificity assay showed that the RT-RAA had specifically detected PEDV and was not reactive for porcine parvovirus (PPV), transmissible gastroenteritis virus (TGEV), porcine circovirus type 2 (PCV2), porcine pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), swine flu virus (SIV), or porcine Japanese encephalitis virus (JEV). The results suggested that RT-RAA had a strong specificity and high detection sensitivity when combined with a portable instrument to complete the detection under a constant temperature of 30 min, which are more suitable for preventing and controlling PEDV onsite in China.

Porcine Enteric Coronavirus Infections in Wild Boar in Poland - a Pilot Study

Introduction

Porcine epidemic diarrhoea virus (PEDV) of the Coronaviridae family causes significant economic losses in the pig industry worldwide. Wild boars contribute to the transmission of different viral, bacterial and parasitic infections to livestock animals and humans. However, their role in the maintenance and transmission of PEDV has not been established.

Material and Methods

In this study, blood and faecal samples from 157 wild boars were collected from 14 provinces of Poland during the 2017–2018 hunting season. RNA was extracted from the faecal homogenate supernatant and subjected to quantitative RT-PCR (RT-qPCR), while clotted blood samples were used for detection of antibodies against PEDV by ELISA.

Results

Five blood samples (3.2%) were seropositive in ELISA, while none of the faecal samples were found positive using RT-qPCR assays.

Conclusion

The results of this analysis indicate the need for additional studies incorporating a larger number of samples and preferably comparing different serological methods, to confirm whether wild boars in Poland act as PEDV reservoirs.

Development of an Indirect ELISA Based on Spike Protein to Detect Antibodies against Feline Coronavirus

Feline coronavirus (FCoV) is a pathogenic virus commonly found in cats that causes a benign enteric illness and fatal systemic disease, feline infectious peritonitis. The development of serological diagnostic tools for FCoV is helpful for clinical diagnosis and epidemiological investigation. Therefore, this study aimed to develop an indirect enzyme-linked immunosorbent assay (iELISA) to detect antibodies against FCoV using histidine-tagged recombinant spike protein. FCoV S protein (1127–1400 aa) was expressed and used as an antigen to establish an ELISA. Mice and rabbits immunized with the protein produced antibodies that were recognized and bound to the protein. The intra-assay coefficient of variation (CV) was 1.15–5.04% and the inter-assay CV was 4.28–15.13%, suggesting an acceptable repeatability. iELISA did not cross-react with antisera against other feline viruses. The receiver operating characteristic curve analysis revealed an 86.7% sensitivity and 93.3% specificity for iELISA. Serum samples (n = 107) were tested for anti-FCoV antibodies, and 70.09% of samples were positive for antibodies against FCoV. The iELISA developed in our study can be used to measure serum FCoV antibodies due to its acceptable repeatability, sensitivity, and specificity. Additionally, field sample analysis data demonstrated that FCoV is highly prevalent in cat populations in Fujian province, China.

Evolution, Interspecies Transmission, and Zoonotic Significance of Animal Coronaviruses

Coronaviruses are single-stranded RNA viruses that affect humans and a wide variety of animal species, including livestock, wild animals, birds, and pets. These viruses have an affinity for different tissues, such as those of the respiratory and gastrointestinal tract of most mammals and birds and the hepatic and nervous tissues of rodents and porcine. As coronaviruses target different host cell receptors and show divergence in the sequences and motifs of their structural and accessory proteins, they are classified into groups, which may explain the evolutionary relationship between them. The interspecies transmission, zoonotic potential, and ability to mutate at a higher rate and emerge into variants of concern highlight their importance in the medical and veterinary fields. The contribution of various factors that result in their evolution will provide better insight and may help to understand the complexity of coronaviruses in the face of pandemics. In this review, important aspects of coronaviruses infecting livestock, birds, and pets, in particular, their structure and genome organization having a bearing on evolutionary and zoonotic outcomes, have been discussed.

Seroprevalence of SARS-CoV-2 antibodies among rural healthcare workers

The objective of this longitudinal cohort study was to determine the seroprevalence of antibodies to severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2) in healthcare workers employed at healthcare settings in three rural counties in eastern South Dakota and western Minnesota from May 13, 2020, through December 22, 2020. Three blood draws were performed at five clinical sites and tested for the presence of antibodies against the SARS‐CoV‐2. Serum samples were tested for the presence of antibodies using a fluorescent microsphere immunoassay (FMIA), neutralization of SARS‐CoV‐2 spike‐pseudotyped particles (SARS‐CoV‐2pp) assay, and serum virus neutralization (SVN) assay. The seroprevalence was determined to be 1/336 (0.29%) for samples collected from 5/13/20 to 7/13/20, 5/260 (1.92%) for samples collected from 8/13/20 to 9/25/20, and 35/235 (14.89%) for samples collected from 10/16/20 to 12/22/20. Eight of the 35 (22.8%) seropositive individuals identified in the final draw did not report a previous diagnosis with COVID‐19. There was a high correlation (>90%) between the FMIA and virus neutralization assays. Each clinical site's seroprevalence was higher than the cumulative incidence for the general public in the respective county as reported by state public health agencies. As of December 2020, there was a high percentage (85%) of seronegative individuals in the study population.

COVID-19 Diagnostic Strategies Part II: Protein-Based Technologies

After the initiation of the current outbreak, humans' lives have been profoundly impacted by COVID-19. During the first months, no rapid and reliable detecting tool was readily available to sufficiently respond to the requirement of massive testing. In this situation, when the development of an effective vaccine requires at least a few months, it is crucial to be prepared by developing and commercializing affordable, accurate, rapid and adaptable biosensors not only to fight Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) but also to be armed to avoid the pandemic in the earliest stages in the future. The COVID-19 diagnostic tools are categorized into two main groups of Nucleic Acid (NA)-based and protein-based tests. To date, nucleic acid-based detection has been announced as the gold-standard strategy for coronavirus detection; however, protein-based tests are promising alternatives for rapid and large-scale screening of susceptible groups. In this review, we discuss the current protein-based biosensing tools, the research advances and the potential protein-detecting strategies for COVID-19 detection. This narrative review aims to highlight the importance of the diagnostic tests, encourage the academic research groups and the companies to eliminate the shortcomings of the current techniques and step forward to mass-producing reliable point-of-care (POC) and point-of-need (PON) adaptable diagnostic tools for large-scale screening in the future outbreaks.

PEDV Infection Generates Conformation-Specific Antibodies That Can Be Effectively Detected by a Cell-Based ELISA

Porcine epidemic diarrhea virus (PEDV) is a coronavirus that causes serious and highly contagious enteric disease in swine worldwide. In this study, we constructed a recombinant baculovirus (S-Bac) expressing full-length spike protein of the virulent epidemic genotype 2b (G2b) PEDV strain for serological studies of infected pigs. We found that most spike-specific antibodies produced upon PEDV infection in pigs are conformation-specific and they could be detected on S-Bac-infected insect cells by immunofluorescent assay, but they were insensitive to Western blot analysis, the typical method for antiserum analysis. These results indicated that spike conformation is crucial for serum recognition. Since it is difficult to purify trimeric spike membrane protein for conventional enzyme-linked immunosorbent assay (ELISA), we used S-Bac to generate a novel cell-based ELISA for convenient PEDV detection. We analyzed 100 pig serum samples, and our cell-based ELISA exhibited a sensitivity of 100%, a specificity of 97%, and almost perfect agreement [Cohen’s kappa coefficient value (κ) = 0.98] with immunocytochemical staining results. Our cell-based ELISA rapidly presented antigen for proper detection of conformation-specific antibodies, making PEDV detection more convenient, and it will be useful for detecting many viral diseases in the future.

Assessment of three commercial ELISAs for the detection of antibodies against Porcine epidemic diarrhea virus at different stages of the immune response

Three commercial ELISAs -two based on spike (E1 and E3) and one on nucleocapsid protein (E2)-were used to analyze the development and persistence of antibodies against Porcine epidemic diarrhea virus (PEDV). Seventy-five four-week-old PEDV-negative piglets were inoculated orally with a European G1b PEDV (INOC) and fourteen were kept as controls (CTRL). After the inoculation, E3 detected positive animals as soon as 7 days post inoculation (dpi), while the earliest detection with E1 and E2 was at 14 dpi. All samples were positive at 21 and 28 dpi using E1 and E3, respectively, while E2 failed to detect 23.3 % of the inoculated pigs at any time point. The percentages of positive samples were different through the study: E1 and E3 > E2 from 14 to 56 dpi; and E3 > E1 > E2 from 56 to 154 dpi (P < 0.05). Five months after the inoculation, E3 still detected 92.0 % (IC95 % = 85.1-98.8 %) of pigs as positive, while E1 and E2 detected only 27.0 % (IC95 % = 16.0-37.9 %) and 0%, respectively. The sensitivity for E2 never exceeded 0.62. Specificity was 1 for all ELISAs. These different outcomes could be related to the ELISA strategies (indirect versus competition), the antigens used, the cut-off, or to other intrinsic factors of each test. The observed differences could be of importance when assessing whether older animals, such as fatteners or gilts, had previously been in contact with PEDV.

Piglet immunization with a spike subunit vaccine enhances disease by porcine epidemic diarrhea virus

Immunization with an insect cell lysate/baculovirus mixture containing recombinant porcine epidemic diarrhea virus (PEDV) spike protein induced high levels of neutralizing antibodies in both mice and piglets. However, immunization of piglets with this vaccine resulted in enhancement of disease symptoms and virus replication in vaccine recipients exposed to PEDV challenge. Thus, these observations demonstrate a previously unrecognized challenge of PEDV vaccine research, which has important implications for coronavirus vaccine development.

Novel Method for Isolation of Porcine Epidemic Diarrhea Virus with the Use of Suspension Vero Cells and Immunogenicity Analysis

In February and December of 2019, two large-scale outbreaks of diarrhea were observed in the same swine farm with 3,000 sows in Shanghai, China. We successfully isolated two porcine epidemic diarrhea virus (PEDV) isolates (strains shxx1902 and shxx1912 in February and December, respectively) from clinical samples in this farm using suspension Vero cells. A third PEDV strain (SH1302) tested positive in another farm of Shanghai, China, in 2013 and was also isolated using suspension Vero cells. The three isolates were better adapted to growth in adherent Vero cells through serial passages in the suspension Vero cells. The three isolated strains were detected positive by an immunofluorescence assay (IFA) and observed through electron microscopy. Phylogenetic analysis of the complete genomic sequence demonstrated that shxx1902 (the 5th passage) and shxx1912 (the 5th passage) clustered with a new GII subgroup (GII-c), which consisted of SINDEL strains from America (e.g., OH851), and their S gene belonged to GII-a. Both strains(the 35th passage) have incurred dramatic changes in their genomes compared with the 5th passage. The 5th and 35th passages of SH1302 belonged to the GI-b genotype. The anti-N protein antibody titer of the strain shxx1902 was elevated to the same level as the vaccine strain (CV777) in mice. The use of the suspension Vero cells to isolate and propagate PEDV provides an effective approach for studies of the epidemiological characteristics and vaccine development of this virus.

New Diagnostic Assays for Differential Diagnosis Between the Two Distinct Lineages of Bovine Influenza D Viruses and Human Influenza C Viruses

Influenza D virus (IDV), a novel orthomyxovirus, is currently emerging in cattle worldwide. It shares >50% sequence similarity with the human influenza C virus (HICV). Two clades of IDV are currently co-circulating in cattle herds in the U.S. New assays specific for each lineage are needed for accurate surveillance. Also, differential diagnosis between zoonotic human influenza C virus and the two clades of IDV are important to assess the zoonotic potential of IDV. We developed an enzyme-linked immunosorbent assay (ELISA) based on two different epitopes HEF and NP and four peptides, and fluorescent focus neutralization assay to differentiate between IDV bovine and swine clades. Calf sera were obtained, and bovine samples underwent surveillance. Our results highlight the importance of position 215 with 212 in determining the heterogeneity between the two lineages. We needed IFA and FFN for tissue culture-based analysis and a BSL2 facility for analyzing virus interactions. Unfortunately, these are not available in many veterinary centers. Hence, our second aim was to develop an iELISA using specific epitopes to detect two lineages of IDVs simultaneously. Epitope-iELISA accurately detects neutralizing and non-neutralizing antibodies against the IDV in non-BSL2 laboratories and veterinary clinics and is cost-effective and sensitive. To differentiate between IDVs and HICVs, whole antigen blocking, polypeptides, and single-peptide ELISAs were developed. A panel of ferret sera against both viruses was used. Results suggested that both IDV and ICV had a common ancestor, and IDV poses a zoonotic risk to individuals with prior or current exposure to cattle. IDV peptides IANAGVK (286-292 aa), KTDSGR (423-428 aa), and RTLTPAT (448-455 aa) could differentiate between the two viruses, whereas peptide AESSVNPGAKPQV (203-215 aa) detected the presence of IDV in human sera but could not deny that it could be ICV, because the only two conserved influenza C peptides shared 52% sequence similarity with IDV and cross-reacted with IDV. However, blocking ELISAs differentiated between the two viruses. Diagnostic tools and assays to differentiate between ICV and IDV are required for serological and epidemiological analysis to clarify the complexity and evolution and eliminate misdiagnosis between ICV and IDV in human samples.

Porcine Reproductive and Respiratory Syndrome Virus Promotes SLA-DR-Mediated Antigen Presentation of Nonstructural Proteins To Evoke a Nonneutralizing Antibody Response In Vivo

The humoral immune response against porcine reproductive and respiratory syndrome virus (PRRSV) infection is characterized by a rapid induction of nonneutralizing antibodies (non-NAbs) against nonstructural proteins (NSPs). Here, we systematically investigated the potential mechanism for the induction of PRRSV NSP-specific non-NAbs. Our data suggested that PRRSV NSP-specific antibodies appeared within 10 days after PRRSV infection in vivo In the in vitro model, functional upregulation of swine leukocyte antigen (SLA)-DR was observed in bone marrow-derived dendritic cells (BMDCs) and porcine alveolar macrophages (PAMs), whereas remarkable inhibition at the mRNA level was observed after infection by both PRRSV-1 and PRRSV-2 isolates. Notably, the inconsistency in SLA-DR expression between the mRNA and protein levels resulted from deubiquitination of SLA-DR via the ovarian tumor (OTU) domain of PRRSV NSP2, which inhibited ubiquitin-mediated degradation. Moreover, mass spectrometry-based immunopeptidome analysis identified immunopeptides originating from multiple PRRSV NSPs within SLA-DR of PRRSV-infected BMDCs. Meanwhile, these PRRSV NSP-derived immunopeptides could be specifically recognized by serum from PRRSV-infected piglets. Notably, certain NSP-derived immunopeptides characterized in vitro could be identified from PAMs or hilar lymph nodes from PRRSV-infected piglets. More importantly, an in vitro neutralizing assay indicated that serum antibodies against NSP immunopeptides were unable to neutralize PRRSV in vitro Conversely, certain structural protein (SP)-derived immunopeptides were identified and could be recognize by pig hyperimmune serum against PRRSV, which further indicates that the NSP-derived antibody response is nonprotective in vivo In conclusion, our data suggested that PRRSV infection interferes with major histocompatibility complex class II (MHC-II) molecule-mediated antigen presentation in antigen-presenting cells (APCs) via promoting SLA-DR expression to present immunopeptides from PRRSV NSPs, which contributes to the induction of non-NAbs in vivo IMPORTANCE PRRSV has haunted the swine industry for over 30 years since its emergence. Besides the limited efficacy of PRRSV modified live vaccines (MLVs) against heterogeneous PRRSV isolates, rapid induction of nonneutralizing antibodies (non-NAbs) against PRRSV NSPs after MLV immunization or wild-strain infection is one of the reasons why development of an effective vaccine has been hampered. By using in vitro-generated BMDCs as models to understand the antigen presentation process of PRRSV, we obtained data indicating that PRRSV infection of BMDCs promotes functional SLA-DR upregulation to present PRRSV NSP-derived immunopeptides for evoking a non-NAb response in vivo Our work not only uncovered a novel mechanism for interference in host antigen presentation by PRRSV but also revealed a novel insight for understanding the rapid production of nonneutralizing antibodies against PRRSV NSPs, which may have benefit for developing an effective vaccine against PRRSV in the future.

Interferon Inducing Porcine Reproductive and Respiratory Syndrome Virus Vaccine Candidate Protected Piglets from HP-PRRSV Challenge and Evoke a Higher Level of Neutralizing Antibodies Response

Although widespread administration of attenuated porcine reproductive and respiratory syndrome virus (PRRSV) vaccines has been implemented since they first became commercially available two decades ago, PRRSV infection prevalence in swine herds remains high. The limited success of PRRSV vaccines is partly due to the well-established fact that a given vaccine strain confers only partial or no protection against heterologous strains. In our past work, A2MC2-P90, a novel PRRSV vaccine candidate that induced a type I IFNs response in vitro, conferred complete protection against challenge with genetically heterologous PRRSV strains. Here we assessed the ability of the PRRSV vaccine candidate A2MC2-P90 to protect piglets against the HP-PRRSV challenge and compared its efficacy to that of a licensed HP-PRRSV-specific vaccine (TJM-F92) assessed in parallel. A2MC2-P90 provided vaccinated piglets with 100% protection from a lethal challenge with extremely virulent HP-PRRSV-XJA1, while 100% mortality was observed for unvaccinated piglets by day 21 post-challenge. Notably, comparison of partial sequence (GP5) of XJA1 to A2MC2-P90 suggested there was only 88.7% homology. When comparing post-HP-PRRSV challenge responses between piglets administered A2AMC2-P90 versus those immunized with licensed vaccine TJM-F92, A2MC2-P90-vaccinated piglets rapidly developed a stronger protective humoral immune response, as evidenced by much higher titers of neutralizing antibodies, more rapid clearance of viremia and less nasal virus shedding. In conclusion, our data suggest that this novel vaccine candidate A2MC2-P90 has improved protection spectrum against heterologous HP-PRRSV strains.

Inactivating Three Interferon Antagonists Attenuates Pathogenesis of an Enteric Coronavirus

Coronaviruses (CoVs) have repeatedly emerged from wildlife hosts and infected humans and livestock animals to cause epidemics with significant morbidity and mortality. CoVs infect various organs, including respiratory and enteric systems, as exemplified by newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The constellation of viral factors that contribute to developing enteric disease remains elusive. Here, we investigated CoV interferon antagonists for their contribution to enteric pathogenesis. Using an infectious clone of an enteric CoV, porcine epidemic diarrhea virus (icPEDV), we generated viruses with inactive versions of interferon antagonist nonstructural protein 1 (nsp1), nsp15, and nsp16 individually or combined into one virus designated icPEDV-mut4. Interferon-responsive PK1 cells were infected with these viruses and produced higher levels of interferon responses than were seen with wild-type icPEDV infection. icPEDV-mut4 elicited robust interferon responses and was severely impaired for replication in PK1 cells. To evaluate viral pathogenesis, piglets were infected with either icPEDV or icPEDV-mut4. While the icPEDV-infected piglets exhibited clinical disease, the icPEDV-mut4-infected piglets showed no clinical symptoms and exhibited normal intestinal pathology at day 2 postinfection. icPEDV-mut4 replicated in the intestinal tract, as revealed by detection of viral RNA in fecal swabs, with sequence analysis documenting genetic stability of the input strain. Importantly, icPEDV-mut4 infection elicited IgG and neutralizing antibody responses to PEDV. These results identify nsp1, nsp15, and nsp16 as virulence factors that contribute to the development of PEDV-induced diarrhea in swine. Inactivation of these CoV interferon antagonists is a rational approach for generating candidate vaccines to prevent disease and spread of enteric CoVs, including SARS-CoV-2.IMPORTANCE Emerging coronaviruses, including SARS-CoV-2 and porcine CoVs, can infect enterocytes, cause diarrhea, and be shed in the feces. New approaches are needed to understand enteric pathogenesis and to develop vaccines and therapeutics to prevent the spread of these viruses. Here, we exploited a reverse genetic system for an enteric CoV, porcine epidemic diarrhea virus (PEDV), and outline an approach of genetically inactivating highly conserved viral factors known to limit the host innate immune response to infection. Our report reveals that generating PEDV with inactive versions of three viral interferon antagonists, nonstructural proteins 1, 15, and 16, results in a highly attenuated virus that does not cause diarrhea in animals and elicits a neutralizing antibody response in virus-infected animals. This strategy may be useful for generating live attenuated vaccine candidates that prevent disease and fecal spread of enteric CoVs, including SARS-CoV-2.

Virus-like particle vaccine with B-cell epitope from porcine epidemic diarrhea virus (PEDV) incorporated into hepatitis B virus core capsid provides clinical alleviation against PEDV in neonatal piglets through lactogenic immunity

Porcine epidemic diarrhea virus (PEDV) has had a negative economic impact on the global swine industry for decades since its first emergence in the 1970s in Europe. In 2013, PEDV emerged for the first time in the United States, causing immense economic losses to the swine industry. Efforts to protect U.S. swine herds from PEDV infection and limit PEDV transmission through vaccination had only limited success so far. Following the previous success in our virus-like particle (VLP) based vaccine in mouse model, in this study we determined the immunogenicity and protective efficacy of a VLP-based vaccine containing B-cell epitope ⁷⁴⁸YSNIGVCK⁷⁵⁵ from the spike protein of PEDV incorporated into the hepatitis B virus core capsid (HBcAg), in a comprehensive pregnant gilt vaccination and piglet challenge model. The results showed that the vaccine was able to induce significantly higher virus neutralization response in gilt milk, and provide alleviation of clinical signs for piglets experimentally infected with PEDV. Piglets from pregnant gilt that was vaccinated with the VLP vaccine had faster recovery from the clinical disease, less small intestinal lesions, and higher survival rate at 10 days post-challenge (DPC).

Characterization of the Humoral Immune Response to Porcine Epidemic Diarrhea Virus Infection under Experimental and Field Conditions Using an AlphaLISA Platform

Coronavirus infections are a continuous threat raised time and again. With the recent emergence of novel virulent strains, these viruses can have a large impact on human and animal health. Porcine epidemic diarrhea (PED) is considered to be a reemerging pig disease caused by the enteropathogenic alphacoronavirus PED virus (PEDV). In the absence of effective vaccines, infection prevention and control through diagnostic testing and quarantine are critical. Early detection and differential diagnosis of PEDV infections increase the chance of successful control of the disease. Therefore, there is a continuous need for development of reduced assay-step protocols, no-wash, high-throughput immunoassays. This study described the characterization of the humoral immune response against PEDV under experimental and field conditions using a rapid, sensitive, luminescent proximity homogenous assay (AlphaLISA). PEDV IgG and IgA antibodies were developed toward the beginning of the second week of infection. PEDV IgG antibodies were detected for at least 16 weeks post-exposure. Remarkably, the serum IgA levels remained high and relatively stable throughout the study, lasting longer than the serum IgG response. Overall, AlphaLISA allows the detection and characterization of pathogen-specific antibodies with new speed, sensitivity, and simplicity of use. Particularly, the bridge assay constitutes a rapid diagnostic that substantially improves upon the "time to result" metric of currently available immunoassays.

EuNPs-mAb fluorescent probe based immunochromatographic strip for rapid and sensitive detection of porcine epidemic diarrhea virus

Porcine epidemic diarrhea (PED), induced by porcine epidemic diarrhea virus (PEDV) causes acute diarrhea, vomiting, dehydration and high mortality in neonatal piglets, resulting in significant economic losses in the pig industries. In this study, an immunochromatographic assay (ICA) based on a EuNPs-mAb fluorescent probe was developed and optimized for rapid detection of PEDV. The limit of detection (LOD) of the ICA was 0.218 μg/mL (2.725 × 10³ TCID₅₀/mL) and its linear detection range was 0.03125-8 μg/mL (3.91 × 10²-10⁵ TCID₅₀/mL). The ICA was also validated for the detection of PEDV in swine stool samples. 60 swine stool samples from southern China were analyzed by the ICA and RT-PCR, and the results showed that the coincidence rate of the ICA to RT-PCR was 86.67%, which was significantly higher than that of AuNPs based ICA. The ICA is sensitive and specific and can achieve on-site rapid detection of swine stool samples. Therefore, the ICA has a great potential for PED diagnosis and prevention.

Porcine Coronaviruses

Transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhoea virus (PEDV), and porcine deltacoronavirus (PDCoV) are enteropathogenic coronaviruses (CoVs) of swine. TGEV appearance in 1946 preceded identification of PEDV (1971) and PDCoV (2009) that are considered as emerging CoVs. A spike deletion mutant of TGEV associated with respiratory tract infection in piglets appeared in 1984 in pigs in Belgium and was designated porcine respiratory coronavirus (PRCV). PRCV is considered non-pathogenic because the infection is very mild or subclinical. Since PRCV emergence and rapid spread, most pigs have become immune to both PRCV and TGEV, which has significantly reduced the clinical and economic importance of TGEV. In contrast, PDCoV and PEDV are currently expanding their geographic distribution, and there are reports on the circulation of TGEV-PEDV recombinants that cause a disease clinically indistinguishable from that associated with the parent viruses. TGEV, PEDV and PDCoV cause acute gastroenteritis in pigs (most severe in neonatal piglets) and matches in their clinical signs and pathogenesis. Necrosis of the infected intestinal epithelial cells causes villous atrophy and malabsorptive diarrhoea. Profuse diarrhoea frequently combined with vomiting results in dehydration, which can lead to the death of piglets. Strong immune responses following natural infection protect against subsequent homologous challenge; however, these viruses display no cross-protection. Adoption of advance biosecurity measures and effective vaccines control and prevent the occurrence of diseases due to these porcine-associated CoVs. Recombination and reversion to virulence are the risks associated with generally highly effective attenuated vaccines necessitating further research on alternative vaccines to ensure their safe application in the field.

Detection of porcine epidemic diarrhea virus-neutralizing antibody using high-throughput imaging cytometry

Porcine epidemic diarrhea virus (PEDV) is an emerging porcine coronavirus that causes a tremendous economic burden on the swine industry. The assessment of PEDV-neutralizing antibody levels provides a valuable tool to assess and predict herd immunity. We evaluated the performance of a PEDV imaging cytometry-based high-throughput neutralization test (HTNT) and compared the HTNT to a fluorescent focus neutralization (FFN) assay using serum samples from pigs of known PEDV infection status (n = 159). Estimates of diagnostic sensitivity and specificity for HTNT and FFN assays derived from receiver-operator characteristic (ROC) curve analyses showed that both PEDV FFN and HTNT provided excellent diagnostic performance. However, in the laboratory, imaging cytometry provided an objective and semi-automated approach that removed human subjectivity from the testing process and reduced the read-time of a 96-well plate to < 4 min. In addition, imaging cytometry facilitated the rapid collection and long-term storage of test images and data for further evaluation or client consultation. For PEDV and other pathogens, imaging cytometry could provide distinct advantages over classic virus neutralization or FFN assays for the detection and quantitation of neutralizing antibody.

Identification of the immunodominant neutralizing regions in the spike glycoprotein of porcine deltacoronavirus

Porcine deltacoronavirus (PDCoV), is an emerging enteropathogenic coronavirus in pigs, that poses a novel threat to swine husbandry worldwide. Crucial to halting PDCoV transmission and infection is the development of effective therapies and vaccines. The spike (S) protein of coronavirus is the major target of host neutralizing antibodies, however the immunodominant neutralizing region in the S protein of PDCoV has not been defined. Here, three truncations of the PDCoV S protein were generated, the N-terminal domain of the S1 subunit (NTD, amino acids (aa) 50-286), the C-terminal domain of the S1 subunit (CTD, aa 278-616), and S2 subunit (aa 601-1087). The proteins were expressed using an E. coli expression system. Polyclonal antisera against the three recombinant proteins were produced in rabbits and mice. All three antisera were able to inhibit PDCoV infection in vitro, as determined by virus neutralization assay, fluorescent focus neutralization assay, and plaque-reduction neutralization. The CTD-specific antisera had the most potent PDCoV-neutralizing effect, indicating that the CTD region may contain the major neutralizing epitope(s) in the PDCoV S protein. Based on these findings, CTD may be a promising target for development of an effective vaccine against PDCoV infection in pigs.

Development and comparison of enzyme-linked immunosorbent assays based on recombinant trimeric full-length and truncated spike proteins for detecting antibodies against porcine epidemic diarrhea virus

BACKGROUND

Since 2010, outbreaks of genotype 2 (G2) porcine epidemic diarrhea virus (PEDV) have caused high mortality in neonatal piglets and have had devastating impacts on the swine industry in many countries. A reliable serological assay for evaluating the PEDV-specific humoral and mucosal immune response is important for disease survey, monitoring the efficacy of immunization, and designing strategies for the prevention and control of PED. Two PEDV spike (S) glycoprotein-based indirect enzyme-linked immunosorbent assays (ELISAs) were developed using G2b PEDV-Pintung 52 (PEDV-PT) trimeric full-length S and truncated S^1-501 proteins derived from the human embryonic kidney (HEK)-293 cell expression system. The truncated S^1-501 protein was selected from a superior expressed stable cell line. The sensitivity and specificity of these two ELISAs were compared to immunostaining of G2b PEDV-PT infected cells and to a commercial nucleocapsid (N)-based indirect ELISA kit using a panel of PEDV negative and hyperimmune sera.

RESULTS

The commercial N-based ELISA exhibited a sensitivity of 37%, a specificity of 100%, and a fair agreement (kappa = 0.37) with the immunostaining result. In comparison, the full-length S-based ELISA showed a sensitivity of 97.8%, a specificity of 94%, and an almost perfect agreement (kappa = 0.90) with the immunostaining result. Interestingly, the S^1-501-based ELISA had even higher sensitivity of 98.9% and specificity of 99.1%, and an almost perfect agreement (kappa = 0.97) with the immunostaining result. A fair agreement (kappa< 0.4) was seen between the commercial N-based ELISA and either of our S-based ELISAs. However, the results of the full-length S-based ELISA shared an almost perfect agreement (kappa = 0.92) with that of S^1-501-based ELISA.

CONCLUSIONS

Both full-length S-based and S^1-501-based ELISAs exhibit high sensitivity and high specificity for detecting antibodies against PEDVs. Considering the high protein yield and cost-effectiveness, the S^1-501-based ELISA could be used as a reliable, sensitive, specific, and economic serological test for PEDV.

Development of indirect enzyme-linked immunosorbent assay for detection of porcine epidemic diarrhea virus specific antibodies (IgG) in serum of naturally infected pigs

BACKGROUND

Porcine epidemic diarrhea virus (PEDV) infection is a highly contagious infectious disease causing watery diarrhea, vomiting, dehydration and high mortality rate in newborn piglets. PEDV infection can cause high economic losses in pig industry. In Japan, a PEDV outbreak occurred with high mortality from 2013 to 2015. Even though until now, PEDV infection occurs sporadically. For the control and monitoring of PEDV infection, not only symptomatic pigs, but also asymptomatic pigs should be identified. The objective of this study is to develop and optimize novel indirect ELISA as a simple, rapid, sensitive and specific method for the detection of anti-PEDV antibodies and evaluate the efficacy of the assay as a diagnostic method for PED.

RESULTS

One hundred sixty-two serum samples, consisting of 81 neutralization test (NT) positive and 81 NT negative sera, were applied to the assay. Indirect ELISA test based on whole virus antigen (NK94P6 strain) derived from Vero cell culture was evaluated by receiver operating characteristic (ROC) analysis with neutralization test (NT) as a reference method, and cut-off value was determined as 0.320 with sensitivity and specificity of 92.6 and 90.1%, respectively. The area under curve (AUC) was 0.949, indicating excellent accuracy of indirect ELISA test. There was significant positive correlation between indirect ELISA and neutralization test (R = 0.815, P < 0.05). Furthermore, the kappa statics showed the excellent agreement between these two tests (kappa value = 0.815). In addition, the sensitivity and specificity of preserved plates with different periods (1 day, 2 weeks, 1, 2, 3, 4, 5 and 6 months) after drying antigen coated plates were 100% and 80-100%, respectively.

CONCLUSIONS

The developed indirect ELISA test in our study would be useful as a reliable test for serological survey and disease control of PEDV infection, and our pre-antigen coated ELISA plates can be preserved at 4 °C until at least 6 months.

A Minimally Replicative Vaccine Protects Vaccinated Piglets Against Challenge With the Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV), is an economically important enteric coronavirus, with over a 90% mortality rate in neonatal piglets. The virus emerged in the US in 2013, resulting in severe production losses. Effective vaccine development against PEDV is a challenge. Inactivated vaccines are of questionable efficacy. Attenuated vaccines, while more effective, require a relatively long lead development time, are associated with safety concerns and are also unable to prevent new field outbreaks. To combine the safety and efficacy advantages of inactivated and attenuated PEDV vaccines, respectively, in this study, we tested the hypothesis that subjecting PEDV virions to heat treatment at 44°C for 10 min to reversibly unfold structural proteins, followed by exposure to RNAse to fragment the genome, would result in a vaccine preparation with intact viral structure/antigenicity but highly diminished replicative abilities. We expected the vaccine to be both safe and effective in a piglet challenge model. Following the heat and RNAse treatment, PEDV virions had an intact electron microscopic ultrastructure and were amplified only in the 3rd passage in Vero cells, indicating that diminished replication was achieved in vitro. Strong PEDV spike-protein specific and virus neutralizing antibody responses were elicited in vaccinated piglets. Upon challenge, all vaccinated pigs were protected against fecal viral shedding and intestinal pathology, while the unvaccinated controls were not. The vaccine virus was not detected in the fecal matter of vaccinated pigs prior to challenge; nor did they develop intestinal lesions. Thus, the described approach has significant promise in improving current approaches for PEDV immunization.

Specific recombinant proteins of porcine epidemic diarrhea virus are immunogenic, revealing their potential use as diagnostic markers

Given the highly contagious and acute nature of porcine epidemic diarrhea (PED), especially in piglets, there is an urgent need for the development of rapid and sensitive diagnostic assays. The diagnostic potentials of specific porcine epidemic diarrhea virus (PEDV) accessory and nonstructural proteins, if any, have not yet been investigated. In order to determine and compare which of the viral proteins may be useful as diagnostic antigens, whole virus (WV) particles and a panel of structural and nonstructural PEDV proteins [spike subunit 1 (S1), the C-terminal part of ORF3 (ORF3C), envelope (E), nonstructural protein 1 (Nsp1), Nsp2, Ac (acidic domain of Nsp3), and ADRP (ADP-ribose-1-monophosphatase domain of Nsp3), expressed individually in bacterial and/or mammalian cells] were tested for reactivity with sera from PEDV-infected pigs by ELISA and/or western blot analysis. According to western blots, serum antibody interactions with the S1 protein were relatively more sensitive and specific than ORF3C, E and Ac. Furthermore, a total of 851 serum samples from diarrheal pigs of different ages were analyzed by ELISA, with most showing immune-reactivity towards the WV, S1, ORF3C, and E proteins. The earliest IgG antibody response was observed in the one-week-old piglets, with similar antibody ontogeny and patterns of seroconversion for S1, ORF3C, E, and WV antigens. In addition, the pattern of neutralizing antibody was more similar to that of IgA in weaning piglets after PEDV infection. Collectively, these data provide more reliable information on the host immune response to different viral proteins, which will be useful for development of novel serological assays and for design of vaccines that better stimulate protective immunity.

Coronavirus Endoribonuclease Activity in Porcine Epidemic Diarrhea Virus Suppresses Type I and Type III Interferon Responses

Coronaviruses (CoVs) can emerge from an animal reservoir into a naive host species to cause pandemic respiratory or gastrointestinal diseases with significant mortality in humans or domestic animals. Porcine epidemic diarrhea virus (PEDV), an alphacoronavirus (alpha-CoV), infects gut epithelial cells and macrophages, inducing diarrhea and resulting in high mortality in piglets. How PEDV suppresses the innate immune response was unknown. We found that mutating a viral endoribonuclease, EndoU, results in a virus that activates both the type I interferon response and the type III interferon response in macrophages and epithelial cells. This activation of interferon resulted in limited viral replication in epithelial cell cultures and was associated with reduced virus shedding and mortality in piglets. This study reveals a role for EndoU activity as a virulence factor in PEDV infection and provides an approach for generating live-attenuated vaccine candidates for emerging coronaviruses.

Identifying viral antagonists of innate immunity and determining if they contribute to pathogenesis are critical for developing effective strategies to control emerging viruses. Previously, we reported that an endoribonuclease (EndoU) encoded by murine coronavirus plays a pivotal role in evasion of host innate immune defenses in macrophages. Here, we asked if the EndoU activity of porcine epidemic diarrhea coronavirus (PEDV), which causes acute diarrhea in swine, plays a role in antagonizing the innate response in porcine epithelial cells and macrophages, the sites of viral replication. We constructed an infectious clone of PEDV-Colorado strain (icPEDV-wt) and an EndoU-mutant PEDV (icPEDV-EnUmt) by changing the codon for a catalytic histidine residue of EndoU to alanine (His226Ala). We found that both icPEDV-wt and icPEDV-EnUmt propagated efficiently in interferon (IFN)-deficient Vero cells. In contrast, the propagation of icPEDV-EnUmt was impaired in porcine epithelial cells (LLC-PK1), where we detected an early and robust transcriptional activation of type I and type III IFNs. Infection of piglets with the parental Colorado strain, icPEDV-wt, or icPEDV-EnUmt revealed that all viruses replicated in the gut and induced diarrhea; however, there was reduced viral shedding and mortality in the icPEDV-EnUmt-infected animals. These results demonstrate that EndoU activity is not required for PEDV replication in immortalized, IFN-deficient Vero cells, but is important for suppressing the IFN response in epithelial cells and macrophages, which facilitates replication, shedding, and pathogenesis in vivo. We conclude that PEDV EndoU activity is a key virulence factor that suppresses both type I and type III IFN responses.

IMPORTANCE Coronaviruses (CoVs) can emerge from an animal reservoir into a naive host species to cause pandemic respiratory or gastrointestinal diseases with significant mortality in humans or domestic animals. Porcine epidemic diarrhea virus (PEDV), an alphacoronavirus (alpha-CoV), infects gut epithelial cells and macrophages, inducing diarrhea and resulting in high mortality in piglets. How PEDV suppresses the innate immune response was unknown. We found that mutating a viral endoribonuclease, EndoU, results in a virus that activates both the type I interferon response and the type III interferon response in macrophages and epithelial cells. This activation of interferon resulted in limited viral replication in epithelial cell cultures and was associated with reduced virus shedding and mortality in piglets. This study reveals a role for EndoU activity as a virulence factor in PEDV infection and provides an approach for generating live-attenuated vaccine candidates for emerging coronaviruses.

Development and application of an indirect enzyme-linked immunosorbent assay using recombinant S1 for serological testing of porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus (PEDV) causes severe infectious diseases in all ages of swine and leads to serious economic losses. Serologic tests are widely accepted and used to detect anti-PEDV antibodies that could indicate PEDV infection or vaccination. In this study, PEDV recombinant S1 protein (rS1) was expressed with the Bac-to-Bac system and purified by nickel-affinity chromatography. An indirect enzyme-linked immunosorbent assay based on rS1 (rS1-ELISA) was then developed and optimized by checkerboard assays with serial dilutions of antigen and serum. Serum samples from 453 domestic pigs and 42 vaccinated pigs were analyzed by the indirect fluorescent antibody (IFA) test and rS1-ELISA. Taking IFA as a gold standard, rS1-ELISA produced a high sensitivity (90.7%) and specificity (94.6%) by a receiver operating characteristic (ROC) curve. In addition, ROC analysis also revealed that rS1-ELISA was consistent with IFA (area under the curve 0.9583 ± 0.0082). This rS1-ELISA was then applied to antibody detection in inactivated PEDV vaccinated pigs. The antibody could be detected 2-4 weeks after the first inoculation. These results indicated that the rS1-ELISA established in this study provides a promising and reliable tool for serologic detection of anti-PEDV IgG antibodies in infected or vaccinated pigs.

Two critical N-terminal epitopes of the nucleocapsid protein contribute to the cross-reactivity between porcine epidemic diarrhea virus and porcine transmissible gastroenteritis virus

Both porcine epidemic diarrhoea virus (PEDV) and porcine transmissible gastroenteritis virus (TGEV), which cause high mortality in piglets and produce similar clinical symptoms and histopathological morphology, belong to the genus Alphacoronavirus. Serological diagnosis plays an important role in distinguishing pathogen species. Together with the spike (S) protein, the nucleocapsid (N) protein is one of the immunodominant regions among coronaviruses. In this study, two-way antigenic cross-reactivity between the N proteins of PEDV and TGEV was observed by indirect immunofluorescence assay (IFA) and Western blot analysis. Furthermore, the PEDV N protein harbouring truncations of amino acids (aa) 1 to 170 or aa 125 to 301 was demonstrated to cross-react with the anti-TGEV N polyclonal antibody (PAb), whereas the truncation-expressing aa 302 to 401 resulted in a specific reaction with the anti-PEDV N PAb but not with the anti-TGEV N PAb. Mutants of the PEDV N protein were generated based on sequence alignment and structural analysis; we then confirmed that the N-terminal residues 58-RWRMRRGERIE-68 and 78-LGTGPHAD-85 contributed to the cross-reactivity. All the results provide vital clues for the development of precise diagnostic assays for porcine coronaviruses.

A convenient colorimetric assay for the quantification of porcine epidemic diarrhea virus and neutralizing antibodies

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PEDV is an economically important, enteric coronavirus of swine.

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Visual reading of 96 well plates for virus neutralization (V/N) or TCID₅₀ assays is time consuming.

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The newly developed colorimetric PEDV V/N and TCID₅₀ assays are reliable and convenient.

Neonatal enteritis caused by the porcine epidemic diarrhea virus (PEDV) is an important cause of high mortality and economic losses to the swine industry. Virus neutralization (V/N) assays are commonly requested in diagnostic laboratories for the assessment of protective antibodies. However, the visual assessment of viral cytopathic effects by operators to determine antibody titers or for viral quantification is a tedious, subjective and time-consuming process, especially when high volume testing is involved. To improve the ease of testing, a colorimetric virus neutralization and TCID₅₀ assays were developed and validated in this study using (3-(4,5-dimethylthiazol-2-yl) Tr-2,5-diphenyltetrazolium- bromide) (MTT), a colorimetric agent which measures cell viability. The respective conventional assays were used as the gold standards. An OD cut off value of ≤0.53, selected by receiver operating characteristics analysis, could distinguish between wells with and without CPE accurately. Performance and reproducibility parameters of the colorimetric assays were comparable to the conventional assays. The described methods can reduce testing time in diagnostic laboratories, while significantly improving current protocols.

The effect of chemical clarification of oral fluids on porcine epidemic diarrhea virus antibody responses

Routine testing of breeding herd oral fluid (OF) samples for porcine epidemic diarrhea virus (PEDV) IgG and/or IgA is used to track levels of PEDV immunity over time. However, OFs contain particles of feed, feces, and inorganic material that detract from the quality of the sample. We clarified swine OF samples using lyophilized chitosan-based formulas (A-C) tested by PEDV IgG and IgA ELISAs. To evaluate both the immediate and residual effects of treatment on antibody detection, samples were tested immediately post-treatment, then stored at 4°C and retested at 2, 4, and 6 days post-treatment (DPT). Formulations were shown to effectively clarify samples. Statistical analysis comparing treated to untreated OF samples at 0 DPT found that neither chitosan nor Tween 20 affected the OF ELISA IgA and IgG sample-to-positive (S/P) ratio results ( p > 0.05). Furthermore, pairwise comparisons of 0 DPT to 2, 4, and 6 DPT results detected no significant differences ( p > 0.05) in IgA and IgG S/P ratios (i.e., treated OF samples were stable over time). Therefore, chitosan efficiently clarified OF specimens without affecting the results of the PEDV IgG and IgA antibody ELISAs.

Epitope selection and their placement for increased virus neutralization in a novel vaccination strategy for porcine epidemic diarrhea virus utilizing the Hepatitis B virus core antigen

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Hepatitis B core antigen (HBcAg) protein with PEDV epitopes can assemble into virus like particles.

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Epitope placement within HBcAg can affect the immunogenicity of a vaccine.

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The YSNIGVCK antigen from PEDV has a strong correlation with virus neutralization.

Porcine epidemic diarrhea virus (PEDV) is a member of the Alphacoronaviridae genus within the Coronaviridae family. It is the causative agent of porcine epidemic diarrhea, a disease that can have mortality rates as high as 100% in suckling piglets. PEDV causes severe economic loss, and has been in existence for decades. A panzootic starting in 2010 renewed interest in the development of a universal vaccine toward PEDV. This report details several design changes made to a Hepatitis B virus core antigen (HBcAg)-based recombinant vaccine strategy, and their effect in vivo. Initially, several multi-antigen vaccine candidates were able to elicit antibodies specific to three out of four B-cell epitopes inserted into the chimeric proteins. However, a lack of virus neutralization led to a redesign of the vaccines. The focus of the newly redesigned vaccines was to elicit a strong immune response to the YSNIGVCK amino acid motif from PEDV. Genetically modified new vaccine candidates were able to elicit a strong antibody (Ab) response to the YSNIGVCK epitope, which correlated with an increased ability to neutralize the CO strain of PEDV. Additionally, the location of the inserted PEDV epitopes within the vector protein was shown to affect the immune recognition toward the native HBcAg during vaccination.

Passive immunity to porcine epidemic diarrhea virus following immunization of pregnant gilts with a recombinant orf virus vector expressing the spike protein

Passive immunity is critical for protection of neonatal piglets against porcine epidemic diarrhea virus (PEDV). Here, we investigated the immunogenicity of an orf virus (ORFV) vector expressing the full-length spike (S) protein of PEDV (ORFV-PEDV-S) in pregnant gilts and its ability to confer passive immunity and protection in piglets. Three doses of ORFV-PEDV-S were given to two groups of PEDV-negative pregnant gilts, with the last dose being administered two weeks prior to farrowing. One of the two groups immunized with the ORFV-PEDV-S recombinant virus was also exposed to live PEDV orally on day 31 post-immunization (pi). Antibody responses were assessed in serum, colostrum and milk of immunized gilts, and passive transfer of antibodies was evaluated in piglet sera. The protective efficacy of ORFV-PEDV-S was evaluated after challenge of the piglets with PEDV. PEDV-specific IgG, IgA and neutralizing antibody (NA) responses were detected in ORFV-PEDV-S-immunized and ORFV-PEDV-S-immunized/PEDV-exposed gilts. PEDV NA, IgG and IgA were detected in the serum of piglets born to immunized gilts, demonstrating the transfer of antibodies through colostrum and milk. Piglets born to immunized gilts showed reduced morbidity and a marked reduction in mortality after PEDV challenge in comparison to control piglets. Piglets born to gilts that received ORFV-PEDV-S and were exposed to live PEDV showed stronger NA responses and lower clinical scores when compared to piglets born to gilts immunized with ORFV-PEDV-S alone. These results demonstrate the potential of ORFV as a vaccine delivery platform capable of eliciting passive immunity against PEDV.

Development and application of an indirect enzyme-linked immunosorbent assay based on recombinant capsid protein for the detection of mink circovirus infection

BACKGROUND

Mink circovirus (MiCV) is a newly discovered pathogen associated with mink diarrhea. The prevalence and economic importance of this virus remain poorly understood, and no specific serological assay has been developed for the diagnosis of MiCV infection.

RESULTS

In this study, a recombinant capsid protein antigen expressed in Escherichia coli was utilized to establish an indirect enzyme-linked immunosorbent assay (iELISA). Results revealed that the assay had no cross-reactivity with other related pathogens, and the respective sensitivity and specificity of the proposed iELISA were 92.31% and 91.67% compared with those obtained of Western blot on 138 serum samples from minks. The correlation coefficient between iELISA and Western blot was 0.838 (p > 0.05). iELISA was applied to detect MiCV antibodies in 683 clinical serum samples from different farms from the major mink industry province in China, and 21 of 24 farms with 163 of 683 (23.87%) individuals were tested positive for MiCV antibodies. The positive rates of each of the 21 flocks ranged from 2.33% to 73.68%.

CONCLUSIONS

These results indicated that iELISA was a sensitive and specific method suitable for the large-scale detection of MiCV infections in mink. This study provided an effective method for the serological diagnosis and positive rate investigation of MiCV infection.

Vaccination of sows with a dendritic cell-targeted porcine epidemic diarrhea virus S1 protein-based candidate vaccine reduced viral shedding but exacerbated gross pathological lesions in suckling neonatal piglets

Porcine epidemic diarrhea virus (PEDV) poses a serious threat to swine worldwide as evidenced by its recent introduction into the USA and the devastating economic impact it caused to the USA swine industry. Commercial vaccines against PEDV are available but their efficacies are inadequate. Therefore, vaccines with improved efficacy are needed to effectively control PEDV infections. We previously determined the immunogenicity of a novel dendritic cell (DC)-targeted PEDV S1 protein-based subunit vaccine in weaned piglets in which the PEDV antigen was targeted to DCs through a porcine Langerin-specific antibody. In this study, we evaluated the protective efficacy of this DC-targeting vaccine by immunizing sows at 5 and 2 weeks prior to farrowing and by challenging the 5-day-old piglets with PEDV. The results showed that immunization of sow with DC-targeted PEDV vaccine did not eliminate faecal virus shedding in piglets but significantly reduced faecal viral RNA levels in the early days after virus challenge. The vaccine also reduced the amount of PEDV antigen in intestinal tissues presented with intestinal villi regrowth. However, the DC-targeted vaccine neither mitigated PEDV clinical signs nor affected viral RNA loads in intestinal tissues of piglets. In the vaccinated sow, DC-targeted PEDV vaccine enhanced T helper 1-like cluster of differentiation (CD)4 T cell responses and induced IgG but not IgA-specific immune responses. The suckling piglets in the DC-targeted vaccine group showed increased gross pathological lesions in the small intestine. Results in this study provide insights into the effects of sow cellular immune responses to PEDV infection in suckling piglets.

Investigation of three outbreaks of Porcine Epidemic Diarrhea in Germany in 2016 demonstrates age dependent differences in the development of humoral immune response

Porcine epidemic diarrhea (PED) has reemerged in Europe since 2014. Characterized by a rapid onset of diarrhea in pigs of all ages, morbidity can reach up to 100% whereas mortality is variable. The virus strains involved in the recent European outbreaks all cluster together with US strains (S INDEL) that lead to less severe clinical signs. In this study, fattening pigs and suckling piglets (n = 105) on farms with no prior PED history were monitored after an acute outbreak of the disease, caused by an S INDEL strain of PED virus (PEDV). For diagnostic investigations in the affected farms, real time RT-PCR was performed to detect PEDV RNA in individually taken fecal samples, and two commercial ELISA kits, both based on the N protein of PEDV, were used to detect IgG in serum samples of pigs experiencing acute signs of the disease. PEDV RNA could be detected in fecal samples up to 14 days after initial sampling. Comparing both ELISAs by Cohens Kappa showed substantial agreement (κ = 0,771). Antibodies were detectable in all fattening pigs (100%) within 10 days after the occurrence of first clinical signs and remained detectable for about two months at least in 20.6% (farm 1) and 45.7% (farm 2) of the animals, respectively. In contrast, only 18 of 34 (52.9%) suckling piglets seroconverted. Although, PEDV RNA was found in fecal samples of all piglets, 13 piglets did not demonstrate antibodies at any sampling day. PCR to detect PEDV RNA in fecal samples seems to be a reliable diagnostic tool during and after the acute outbreak. In the present study, IgG ELISA kits proved to be a feasible diagnostic tool, but age dependent differences in detection rate and persistence of antibodies need to be considered.

Complete Genome Sequence of Porcine Deltacoronavirus Strain CH/JXJGS01/2016, Isolated in Jiangxi Province, China, 2016

The complete genome sequence of a variant of porcine deltacoronavirus, isolated from a diarrheal piglet and designated CH/JXJGS01/2016, was sequenced and analyzed. Phylogenetic analysis demonstrated that CH/JXJGS01/2016 shares the highest nucleotide and amino acid identities with the Chinese strain NH (GenBank accession number KU981059).

The S2 glycoprotein subunit of porcine epidemic diarrhea virus contains immunodominant neutralizing epitopes

The porcine epidemic diarrhea virus (PEDV) spike (S) protein is the major target of neutralizing antibodies against PEDV. Here immunodominant neutralizing epitopes of PEDV were identified using a panel of S-specific monoclonal antibodies (mAbs). Ten of eleven S-specific mAbs successfully neutralized PEDV infectivity in vitro. Notably, epitope mapping by peptide ELISAs revealed that nine of these mAbs recognized linear neutralizing epitopes located in the N-terminus of the S2 glycoprotein subunit (amino acids [aa] 744-759, 747-774 and/or 756-771). Additionally, one mAb recognized a neutralizing epitope located in the C-terminus of S2 (aa 1371-1377), while only one neutralizing mAb reacted against a region of the S1 glycoprotein subunit (aa 499-600). Notably, mAbs that recognized epitopes within the S2 subunit presented the highest neutralizing activity against PEDV. Together these results indicate that the S2 glycoprotein subunit contains major antigenic determinants and, perhaps, the immunodominant neutralizing epitopes of PEDV.

Porcine Epidemic Diarrhea

Porcine epidemic diarrhea (PED) first appeared in England and Belgium in the 1970s. The etiological agent of the disease is porcine epidemic diarrhea virus (PEDV), which belongs to Coronaviridae. The disease has spread globally and became an endemic disease in many Asian and European countries causing transient diarrhea in postweaning pigs with low mortalities for several decades. Since late 2010, field outbreaks of PED, which reemerged in China, spread to Asian and some European countries and emerged in North America; all led to enormous economic losses in porcine industry. New variants of PEDV exhibit not only significant genetic variations as compared to historic PEDV strains but also more virulent causing severe vomiting and watery yellowish diarrhea in suckling piglets under 1 week of age. Factors underlying the potential pathogenesis of the recent PEDV outbreaks include the mutation of the virus, the lacking of maternal antibodies for the protection of piglets, and the slower turnover rate of enterocytes (5–7 days) of the neonatal piglets as compared to postweaning pigs (2–3 days). The emerging and reemerging of the new variants of PEDV highlight the importance of reviewing the etiology, pathogenesis, diagnosis, and epidemiology of the disease.