Novel P22-monoclonal antibody based blocking ELISA for the detection of african swine fever virus antibodies in serum

Identification of two novel B cell epitopes on E184L protein of African swine fever virus using monoclonal antibodies

African swine fever virus (ASFV) is a large double-stranded DNA virus with a complex structural architecture and encodes more than 150 proteins, where many are with unknown functions. E184L has been reported as one of the immunogenic ASFV proteins that may contribute to ASFV pathogenesis and immune evasion. However, the antigenic epitopes of E184L are not yet characterized. In this study, recombinant E184L protein was expressed in prokaryotic expression system and four monoclonal antibodies (mAbs), designated as 1A10, 2D2, 3H6, and 4C10 were generated. All four mAbs reacted specifically with ASFV infected cells. To identify the epitopes of the mAbs, a series of overlapped peptides of E184L were designed and expressed as maltose binding fusion proteins. Accordingly, the expressed fusion proteins were probed with each E184L mAb separately by using Western blot. Following a fine mapping, the minimal linear epitope recognized by mAb 1A10 was identified as 119IQRQGFL125, and mAbs 2D2, 3H6, and 4C10 recognized a region located between 153DPTEFF158. Alignment of amino acids of E184L revealed that the two linear epitopes are highly conserved among different ASFV isolates. Furthermore, the potential application of the two epitopes in ASFV diagnosis was assessed through epitope-based ELISA using 24 ASFV positive and 18 negative pig serum and the method were able to distinguish positive and negative samples, indicating the two epitopes are dominant antigenic sites. To our knowledge, this is the first study to characterize the B cell epitopes of the antigenic E184L protein of ASFV, offering valuable tools for future research, as well as laying a foundation for serological diagnosis and epitope-based marker vaccine development.

A highly efficient blocking ELISA based on p72 monoclonal antibody for the detection of African swine fever virus antibodies and identification of its linear B cell epitope

Due to the absence of effective vaccine and treatment, African swine fever virus (ASFV) control is entirely dependent on accurate and early diagnosis, along with culling of infected pigs. The B646L/p72 is the major capsid protein of ASFV and is an important target for developing a diagnostic assays and vaccines. Herein, we generated a monoclonal antibody (mAb) (designated as 2F11) against the trimeric p72 protein, and a blocking ELISA (bELISA) was established for the detection of both genotype I and II ASFV antibodies. To evaluate the performance of the diagnostic test, a total of 506 porcine serum samples were tested. The average value of percent of inhibition (PI) of 133 negative pig serum was 8.4 % with standard deviation (SD) 6.5 %. Accordingly, the cut-off value of the newly established method was set at 28 % (mean + 3SD). Similarly, a receiver operating characteristic (ROC) was applied to determine the cut off value and the p72-bELISA exhibited a sensitivity of 100 % and a specificity of 99.33 % when the detection threshold was set at 28 %. The bELISA was also able to specifically recognize anti-ASFV sera without cross-reacting with other positive serums for other major swine pathogens. Moreover, by designing a series of overlapped p72 truncated proteins, the linear B cell epitope recognized by 2F11 mAb was defined to be 283NSHNIQ288. Amino acid sequence comparison revealed that the amino acid sequence 283NSHNIQ288 is highly conserved between different ASFV isolates. Our findings indicate that the newly established mAb based blocking ELISA may have a great potential in improving the detection of ASFV antibodies and provides solid foundation for further studies.

Highly lethal genotype I and II recombinant African swine fever viruses detected in pigs

African swine fever virus (ASFV) poses a great threat to the global pig industry and food security. Currently, 24 ASFV genotypes have been reported but it is unclear whether recombination of different genotype viruses occurs in nature. In this study, we detect three recombinants of genotype I and II ASFVs in pigs in China. These recombinants are genetically similar and classified as genotype I according to their B646L gene, yet 10 discrete fragments accounting for over 56% of their genomes are derived from genotype II virus. Animal studies with one of the recombinant viruses indicate high lethality and transmissibility in pigs, and deletion of the virulence-related genes MGF_505/360 and EP402R derived from virulent genotype II virus highly attenuates its virulence. The live attenuated vaccine derived from genotype II ASFV is not protective against challenge of the recombinant virus. These naturally occurring recombinants of genotype I and II ASFVs have the potential to pose a challenge to the global pig industry.

Dynamics of Serological and Mucosal Antibody Responses against African Swine Fever Viruses in Experimentally Infected Pigs

African swine fever virus (ASFV) is a lethal swine pathogen, and there is no effective vaccine or treatment available for ASFV infection. Recently, the occurrence of ASFV genotype I and genotype II natural mutants that manifest as subacute, longer-incubation, or persistent infections poses threats to preventing ASFV infection. The dynamics of antibody responses to ASFV are still completely unrevealed, especially the secretion of mucosal antibodies in oral fluid. Here, a systematic analysis was performed of serological and mucosal antibody secretion against 6 ASFV antigens after direct or indirect infection with four different ASFV strains or genotypes, namely, the field virulent genotype II isolate ASFV HLJ/18, the artificially attenuated genotype II strain HLJ/18-7GD, the naturally attenuated genotype II isolate HLJ/HRB1/20, and genotype I isolate SD/DY-I/21. Severe clinical signs of HLJ/18 infection were observed in pigs from 4 days postinoculation. However, no clinical signs were observed in HLJ/18-7GD-infected pigs. The contact pigs cohoused with the pigs intramuscularly infected with the isolate SD/DY-I/21 or HLJ/HRB1/20 only showed chronic clinical signs. Interestingly, the oral fluid sIgA responses to all the selected antigens were significantly stronger and earlier than the serum IgG responses in both HLJ/18- and HLJ/18-7GD-challenged pigs. Although significant fluctuations and individual differences appeared in oral swab sIgA responses in the contact transmission group, they were earlier than the corresponding serological IgG responses. Moreover, according to the comparative analysis of the three infection groups, P54 was proposed to be a dominant target for serological IgG diagnosis, while P30, CD2v, P54, P22, and P10 were more advantageous as mucosal sIgA diagnosis targets. These results highlight the important role of mucosal antibodies in the early diagnosis of ASFV infection and can provide references to screen appropriate targets for ASFV detection.

Novel Epitopes Mapping of African Swine Fever Virus CP312R Protein Using Monoclonal Antibodies

African Swine Fever (ASF) is a highly contagious and lethal pig disease and poses a huge threat to the pig industry worldwide. ASF virus (ASFV) encodes more than 150 different proteins, but the biological properties of most viral proteins are still unknown. ASFV CP312R protein has been proven to be one of the most immunogenic proteins during ASFV infection in pigs; however, its specific epitopes have yet to be identified. In this study, we verified the immunogenicity of CP312R protein in the sera from attenuated ASFV-inoculated pigs. We generated seven anti-ASFV CP312R mouse monoclonal antibodies (mAbs) from mice immunized with recombinant CP312R protein (rCP312R). All seven mAbs are the IgG2b-Kappa isotype and specifically interacted with the CP312R protein expressed in various cells that were infected by ASFVs or transfected with plasmid CP312R. The epitope mapping was performed by using these characterized mAbs and the peptide scanning (Pepscan) method followed by Western blot. As a result, two antigenic determinant regions were identified: two of the seven mAbs recognized the ¹²²KNEQGEEIYP¹³¹ amino acids, and the remaining five mAbs recognized the ⁷⁸DEEVIRMNAE⁸⁷ amino acids of the CP312R protein. These antigenic determinants of CP312R are conserved in different ASFV strains of seven genotypes. By using the characterized mAb, confocal microscopy observation revealed that the CP312R was mainly localized in the cytoplasm and, to some extent, in nuclei and on the nuclear membrane of infected host cells. In summary, our results benefit our understanding on the antigenic regions of ASFV CP312R and help to develop better serological diagnosis of ASF and vaccine research.

Novel p22 and p30 dual-proteins combination based indirect ELISA for detecting antibodies against African swine fever virus

Introduction

African swine fever virus (ASFV) infection is one of the most complex and fatal hemorrhagic viral diseases, causing a devastating loss to the swine industry. Since no effective vaccine is available, prevention and control of ASFV heavily depends on early diagnostic detection.

Methods

In this study, a novel indirect ELISA was established for detecting antibodies against ASFV using dual-proteins, p22 and p30. Recombinants p22 and p30 were expressed and purified from E.coli vector system by recombined plasmids pET-KP177R and pET-CP204L. p22 and p30 were mixed as antigens for developing the indirect ELISA.

Results

Through optimizing coating concentrations of p30 and p22, coating ratio (p30: p22 = 1:3), and serum dilution (as 1:600), the established ELISA performed higher specificity, sensitivity, and repeatability against ASFV-positive serum. Furthermore, 184 clinical serum samples from suspected diseased pigs were verified the established ELISA in clinical diagnosis. The results showed that compared with two commercial ELISA kits, the established ELISA possessed higher sensitivity and almost uniform coincidence rate.

Conclusion

The novel indirect ELISA based on dual-proteins p30 and p22 performed a valuable role in diagnostic detection of ASFV, providing a broad insight into serological diagnostic methods of ASFV.

Development of an ELISA Method to Differentiate Animals Infected with Wild-Type African Swine Fever Viruses and Attenuated HLJ/18-7GD Vaccine Candidate

African swine fever (ASF) is a highly contagious hemorrhagic disease of pigs, posing a significant threat to the world pig industry. Several researchers are investigating the possibilities for developing a safe and efficient vaccine against ASF. In this regard, significant progress has been made and some gene-deleted ASFVs are reported as potential live attenuated vaccines. A seven-gene-deleted live attenuated vaccine candidate HLJ/18-7GD (among which CD2v is included) has been developed in our laboratory and reported to be safe and protective, and it is expected to be commercialized in the near future. There is an urgent need for developing a diagnostic method that can clearly discriminate between wild-type-ASFV-infected and vaccinated animals (DIVA). In the present study, a dual indirect ELISA based on p54 and CD2v proteins was successfully established to specifically distinguish serum antibodies from pigs infected with wild-type ASFV or possessing vaccine immunization. To evaluate the performance of the assay, a total of 433 serum samples from four groups of pigs experimentally infected with the wild-type HLJ/18 ASFV, immunized with the HLJ/18-7GD vaccine candidate, infected with the new lower virulent variant, and specific-pathogen-free pigs were used. Our results showed that the positive rate of immunized serum was 96.54% (p54) and 2.83% (CD2v), and the positive rate of the infection by wild-type virus was 100% (p54) and 97.8% (CD2v). Similarly, the positive rate to infection by the new low-virulent ASFV variant in China was 100% (p54) and 0% (CD2v), indicating the technique was also able to distinguish antibodies from wild-type and the new low-virulent ASFV variant in China. Moreover, no cross-reaction was observed in immune sera from other swine pathogens, such as CSFV, PEDV, PRRSV, HP-PRRSV, PCV2, and PrV. Overall, the developed dual indirect ELISA exhibited high diagnostic sensitivity, specificity, and repeatability and will provide a new approach to differentiate serum antibodies between wild virulent and CD2v-unexpressed ASFV infection, which will play a great role in serological diagnosis and epidemiological monitoring of ASF in the future.