High level expression of the major antigenic African swine fever virus proteins p54 and p30 in baculovirus and their potential use as diagnostic reagents

Modification of African classical swine fever p30 protein with magnetic nanoparticles and establishment of a novel rapid detection method

African swine fever has caused huge losses to the global pig industry. In the absence of effective vaccines, reliable detection methods are crucial. The p30 protein of ASFV is often used as a target for detection due to its high antigenicity in the early stage of virus replication. In this study, we modified the p30 protein using magnetic nanoparticles to increase its specific surface area and suspension in solution. Based on this modified p30 protein, we optimized the experimental steps to determine the best reaction conditions and then established a double probe competitive ELISA method. The established detection method can produce results within 45 min, with repeatability between and within batches <15 %, and it does not react with other pathogens, which demonstrates high specificity. In conclusion, we modified the ASFV p30 protein with magnetic nanoparticles and established a rapid, ultra-sensitive, and convenient detection method for ASF based on the modified p30. This also provides a new reference for the rapid detection of other veterinary diseases.

Development and Validation of an Indirect and Blocking ELISA for the Serological Diagnosis of African Swine Fever

African swine fever (ASF) is an economically devastating viral disease of pigs caused by the ASF virus (ASFV). The rapid global spread of ASF has increased the demand for ASF diagnostics to be readily available and accessible. No commercial ASF enzyme-linked immunosorbent assay (ELISA) kits are manufactured and licensed in North America. Here, we report the development of two serological diagnostic assays, a blocking ELISA (bELISA) based on ASFV glycoprotein p54 and an indirect ELISA (iELISA) based on ASFV glycoproteins p54 and p72. The assays showed high sensitivity and specificity and detected anti-ASFV antibodies in serum samples from experimentally infected animals as early as 8 days post-infection. The two assays were produced commercially (AsurDx™ bELISA and iELISA) and subjected to extensive validation. Based on data from a set of characterized reference sera, the prototype commercial assays, while maintaining 100.00% specificity, showed 97.67% (AsurDx™ bELISA) and 83.72% (AsurDx™ iELISA) sensitivity. Both prototype assays detected anti-ASFV antibodies in serum samples collected from pigs experimentally infected with multiple ASFV strains and field samples collected from sick, recovering, and vaccinated animals. The two commercially available assays can be used in routine ASF diagnostics, serological surveys, and for evaluating serological responses to ASF vaccine candidates.

Monoclonal antibody development for early detection of ASFV I73R protein: Identification of a linear antigenic epitope

African swine fever virus (ASFV), which was first identified in northern China in 2018, causes high mortality in pigs. Since the I73R protein in ASFV is abundantly expressed during the early phase of virus replication, it can be used as a target protein for early diagnosis. In this study, the I73R protein of ASFV was expressed, and we successfully prepared a novel monoclonal antibody (mAb), 8G11D7, that recognizes this protein. Through both indirect immunofluorescence and Western blotting assays, we demonstrated that 8G11D7 can detect ASFV strains. By evaluating the binding of the antibody to a series of I73R-truncated peptides, the definitive epitope recognized by the monoclonal antibody 8G11D7 was determined to be 58 DKTNTIYPP 66. Bioinformatic analysis revealed that the antigenic epitope had a high antigenic index and conservatism. This study contributes to a deeper understanding of ASFV protein structure and function, helping establish ASFV-specific detection method.

Evaluation of early African swine fever virus detection using CP204L gene encoding the p30 protein using quantitative polymerase chain reaction

Background and Aim

The African swine fever virus (ASFV), spanning 170-193 kb, contains over 200 proteins, including p72 and p30, which play crucial roles in the virus's entry and expression. This study investigated the capability of detecting ASFV early through the analysis of genes B646L and CP204L, encoding p72 and p30 antigen proteins, by employing ASFV, diagnosis, immunohistochemistry (IHC), quantitative polymerase chain reaction (qPCR), and IHC techniques.

Materials and Methods

Samples were taken from both experimentally and field-infected pigs to evaluate the effectiveness of qPCR and IHC in detecting ASFV. Twenty-two infected pigs were necropsied at 3-, 5-, 7-, and 9-day post-infection to obtain the first set of samples, collecting anticoagulated blood and tissues each time. The thymus, spleen, and lymph nodes were processed by fixing in 10% formalin, paraffin-blocking, and undergoing IHC staining. Forty anticoagulated blood samples were collected from clinically infected sows at a pig farm for the second batch of samples. Based on the lowest Ct values, three blood samples were diluted fivefold for qPCR DNA testing, and their tissues were used for both qPCR and IHC analyses.

Results

At 1-day post-infection, p30-qPCR identified more ASFV-positive pigs and measured lower Ct values compared to p72-qPCR. At later time points, both methods showed similar levels of detection. ASFV was detected earlier and with lower Ct values in lymphoid tissues using p30-qPCR compared to p72-qPCR, particularly in the spleen and lymph nodes. In a field outbreak study, p30-qPCR demonstrated superior sensitivity and lower Ct values when detecting ASFV in blood samples compared to p72-qPCR.

Conclusion

The early detection of the CP204L gene encoding p30 and its corresponding antigenic protein in ASFV diagnosis compared to the gene encoding p72 suggests that CP204L and p30 are promising candidates for the development of more effective antigen and antibody testing methods.

Overview of Modern Commercial Kits for Laboratory Diagnosis of African Swine Fever and Swine Influenza A Viruses

Rapid and early detection of infectious diseases in pigs is important, especially for the implementation of control measures in suspected cases of African swine fever (ASF), as an effective and safe vaccine is not yet available in most of the affected countries. Additionally, analysis for swine influenza is of significance due to its high morbidity rate (up to 100%) despite a lower mortality rate compared to ASF. The wide distribution of swine influenza A virus (SwIAV) across various countries, the emergence of constantly new recombinant strains, and the danger of human infection underscore the need for rapid and accurate diagnosis. Several diagnostic approaches and commercial methods should be applied depending on the scenario, type of sample and the objective of the studies being implemented. At the early diagnosis of an outbreak, virus genome detection using a variety of PCR assays proves to be the most sensitive and specific technique. As the disease evolves, serology gains diagnostic value, as specific antibodies appear later in the course of the disease (after 7-10 days post-infection (DPI) for ASF and between 10-21 DPI for SwIAV). The ongoing development of commercial kits with enhanced sensitivity and specificity is evident. This review aims to analyse recent advances and current commercial kits utilised for the diagnosis of ASF and SwIAV.

A Robust Quadruple Protein-Based Indirect ELISA for Detection of Antibodies to African Swine Fever Virus in Pigs

African swine fever (ASF) emerged in domestic pigs and wild boars in China in 2018 and rapidly spread to neighboring Asian countries. Currently, no effective vaccine or diagnostic tests are available to prevent its spread. We developed a robust quadruple recombinant-protein-based indirect enzyme-linked immunosorbent assay (QrP-iELISA) using four antigenic proteins (CD2v, CAP80, p54, and p22) to detect ASF virus (ASFV) antibodies and compared it with a commercial kit (IDvet) using ASFV-positive and -negative serum samples. The maximum positive/negative value was 24.033 at a single antigen concentration of 0.25 μg/mL and quadruple ASFV antigen combination of 1 μg/mL at a 1:100 serum dilution. Among 70 ASFV-positive samples, 65, 67, 65, 70, 70, and 14 were positive above the cut-offs of 0.121, 0.121, 0.183, 0.065, 0.201, and 0.122, for CD2v, CAP80, p54, p22-iELISA, QrP-iELISA, and IDvet, respectively, with sensitivities of 92.9%, 95.7%, 92.9%, 100%, 100%, and 20%, respectively, all with 100% specificity. The antibody responses in QrP-iELISA and IDvet were similar in pigs infected with ASFV I. QrP-iELISA was more sensitive than IDvet for early antibody detection in pigs infected with ASFV II. These data provide a foundation for developing advanced ASF antibody detection kits critical for ASF surveillance and control.

Computational analysis of immunogenic epitopes in the p30 and p54 proteins of African swine fever virus

African swine fever (ASF) is a highly infectious viral disease of pigs, which causes acute fatal haemorrhage and is a severe concern to the global pork industry. The present study followed computational approaches to identify B- and T-cell epitopes for the p30 and p54 proteins of the African swine fever virus (ASFV) by interacting with the swine leukocyte antigen (SLA) alleles. The amino acid sequences of p30 and p54 were analysed for variability and relative solvent accessibility, and their three-dimensional structures were predicted and validated. Molecular dynamics simulation was performed to study the structural and dynamic properties of the protein. Six and five linear B-cell epitopes have been predicted for p30 and p54, respectively. Four and three discontinuous B-cell epitopes have been predicted for p30 and p54, respectively. Further, the top five T-cell epitopes for SLA-1, 2, and 3 have been listed for both proteins. These results can help us to understand the immunodominant regions in the p30 and p54 proteins of ASFV and potentially assist in designing peptide-based diagnostics and vaccines. Also, the identified T-cell epitopes may be considered for peptide-based vaccine design against ASFV.Communicated by Ramaswamy H. Sarma.

Establishment of a Dual-Antigen Indirect ELISA Based on p30 and pB602L to Detect Antibodies against African Swine Fever Virus

African swine fever (ASF) is an acute, virulent, and highly fatal infectious disease caused by the African swine fever virus (ASFV). There is no effective vaccine or diagnostic method to prevent and control this disease currently, which highlights the significance of ASF early detection. In this study, we chose an early antigen and a late-expressed antigen to co-detect the target antibody, which not only helps in early detection but also improves accuracy and sensitivity. CP204L and B602L were successfully expressed as soluble proteins in an Escherichia coli vector system. By optimizing various conditions, a dual-antigen indirect ELISA for ASFV antibodies was established. The assay was non-cross-reactive with antibodies against the porcine reproductive and respiratory syndrome virus, classical swine fever virus, porcine circovirus type 2, and pseudorabies virus. The maximum serum dilution for detection of ASFV-positive sera was 1:1600. The intra-batch reproducibility coefficient of variation was <5% and the inter-batch reproducibility coefficient of variation was <10%. Compared with commercial kits, the dual-antigen indirect ELISA had good detection performance. In conclusion, we established a detection method with low cost, streamlined production process, and fewer instruments. It provides a new method for the serological diagnosis of ASF.

A new B cell epitope of pC129R protein of African swine fever virus identified by monoclonal antibodies

African swine fever virus (ASFV) is a most important pathogen which causes huge damage in swine production in the world. pC129R protein is one of the most abundant ASFV proteins in infected Vero cells and WSL-HP cells, which consequently could be a target for ASF detection and surveillance. In this study, 5-6-week-old female BALB/c mice were immunized with rpC129R protein expressed by a prokaryotic system. And three hybridomas, 1B1, 1B4 and 4H4, steadily secreted anti-pC129R monoclonal antibodies were screened by an indirect enzyme linked immunosorbent assay (ELISA). Among them, 1B4 and 4H4 had IgG2a isotype with Kappa light chain, while 1B1 had IgG1 isotype with Kappa light chain. Western blot and indirect immunofluorescence assay showed that three monoclonal antibodies (mAbs) specifically reacted with ASFV. Epitope mapping was performed with truncated polypeptides. And a new B cell epitope, 18KHYVLIPK25 was identified by the mAbs, which was highly conserved in most genotypes of ASFV. These findings not only provide a monoclonal antibody tool for further study of the function of C129R, but also lay the foundation for serological diagnosis and vaccine development.

Screening and identification of the dominant antigens of the African swine fever virus

African swine fever is a highly lethal contagious disease of pigs for which there is no vaccine. Its causative agent African swine fever virus (ASFV) is a highly complex enveloped DNA virus encoding more than 150 open reading frames. The antigenicity of ASFV is still unclear at present. In this study, 35 proteins of ASFV were expressed by Escherichia coli, and ELISA was developed for the detection of antibodies against these proteins. p30, p54, and p22 were presented as the major antigens of ASFV, positively reacting with all five clinical ASFV-positive pig sera, and 10 pig sera experimentally infected by ASFV. Five proteins (pB475L, pC129R, pE199L, pE184L, and pK145R) reacted well with ASFV-positive sera. The p30 induced a rapid and strong antibody immune response during ASFV infection. These results will promote the development of subunit vaccines and serum diagnostic methods against ASFV.

A chemiluminescent magnetic microparticle immunoassay for the detection of antibody against African swine fever virus

The p30 protein is abundantly expressed in the early stage of African swine fever virus (ASFV) infection. Thus, it is an ideal antigen candidate for serodiagnosis with the use of an immunoassay. In this study, a chemiluminescent magnetic microparticle immunoassay (CMIA) was developed for the detection of antibodies (Abs) against ASFV p30 protein in porcine serum. Purified p30 protein was coupled to magnetic beads, and the experimental conditions including concentration, temperature, incubation time, dilution ratio, buffers, and other relevant variables were evaluated and optimized. To evaluate the performance of the assay, a total of 178 pig serum samples (117 negative and 61 positive samples) were tested. According to receiver operator characteristic curve analysis, the cut-off value of the CMIA was 104,315 (area under the curve, 0.998; Youden's index, 0.974; 95% confidence interval: 99.45 to 100%). Sensitivity results showed that the dilution ratio of p30 Abs in ASFV-positive sera detected by the CMIA is much higher when compared to commercial blocking ELISA kit. Specificity testing showed that no cross-reactivity was observed with sera positive for other porcine disease viruses. The intraassay coefficient of variation (CV) was < 5%, and the interassay CV was < 10%. The p30-magnetic beads could be stored at 4 °C for more than 15 months without loss of activity. The kappa coefficient between CMIA and INGENASA blocking ELISA kit was 0.946, showing strong agreement. In conclusion, our method showed superiority with high sensitivity, specificity, reproducibility, and stability and potentialized its application in the development of a diagnostic kit for the detection of ASF in clinical samples. KEY POINTS: • ASFV tag-free p30 was successfully purified. • High sensitivity, specificity, relatively simple, and time-saving to detect antibody against ASFV were developed. • The development of CMIA will help the clinical diagnosis of ASFV and will be useful for large-scale serological test.

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.

One-Step Rapid and Sensitive ASFV p30 Antibody Detection via Nanoplasmonic Biosensors

African swine fever (ASF) is one of the most serious transnational swine diseases in the world. The case fatality rate of susceptible pigs is up to 100%. Currently, no commercial vaccine is available, so the prevention and control of ASF mainly relies on early diagnosis and culling of infected pigs. As the ASF virus continues to evolve, develop, and diversify, nucleic acid testing becomes less efficient. Here, we developed a method for the rapid and direct optical measurement of African swine fever virus (ASFV) antibody in vitro. This one-step procedure requires nearly no sample preparation and involves p30 protein-specific label-free integration into standard 96-well plates. Using a nanoplasmonic biosensor with extraordinary optical transmission (EOT) effect, one-step sample addition, ASFV antibody was detected within 20 min. The positive antibody showed a satisfactory sensitivity and linear relationship in the dilution ratio of 1:100-1:16000. It was used for the detection of clinical serum samples with a coincidence rate of 96.6%. The measurement results can be automatically analyzed and displayed on a conventional microplate meter computer and connected device. Our detection method can be widely applied in point-of-care testing (POCT) of ASFV antibody in pig farms. IMPORTANCE African swine fever (ASF) is a serious transnational disease caused by the African swine fever virus (ASFV), which is highly contagious in wild boars and domestic pigs. There is currently no available vaccine for ASF; therefore, development efforts are a key priority as ASFV continues to evolve and diversify. The ASF antibody rapid detection platform comprising the nanoplasmonic biosensor with extraordinary optical transmission effect can greatly reduce the detection time and improve detection flux while maintaining detection sensitivity and specificity. The one-step sample addition can effectively avoid cross contamination of samples in the detection process. The detection method provides a solution for the rapid and accurate real-time monitoring of ASF in pig farms.

Development of an indirect ELISA against African swine fever virus using two recombinant antigens, partial p22 and p30

African swine fever (ASF) is a highly fatal viral disease affecting pigs. It is caused by the ASF virus (ASFV), and causes serious economic losses to the swine industry worldwide, including in Korea. Commercially available enzyme-linked immunosorbent assay (ELISA) kits for detecting anti-ASFV antibodies are used for the diagnosis and surveillance of ASF. In this study, an ELISA was developed to detect anti-ASFV antibodies using two recombinant proteins, p22 and p30, from genotype II ASFV. Recombinant transmembrane domain-deleted p22 (p22∆TM) and p30 were expressed in E.coli vector system pET32a and mixed for use as antigens in indirect ELISA. The p22∆TM/p30-based indirect ELISA was validated using 31 sera from genotype I ASFV-infected pigs and 1133 sera from uninfected pigs. Area under the curve of this test was 0.999 [95 % concentration interval 0.992-1.000], and sensitivity and specificity were 93.5 % and 99.8 %, respectively. The between run coefficient of variation for internal quality control serum was 6.61 %. In the seroconversion analysis, the p22∆TM/p30-based indirect ELISA showed equal or better ability to detect antibodies in pigs experimentally challenged with ASFV p72 genotypes I and II (p < 0.05). In conclusion, the p22∆TM/p30-based indirect ELISA is a reliable diagnostic method for detecting anti-ASFV antibodies.

Development of a highly sensitive Gaussia luciferase immunoprecipitation assay for the detection of antibodies against African swine fever virus

In recent years, African swine fever (ASF) has caused a devastating blow to the swine industry globally. Since no effective vaccine is available, strict biosafety measures and rapid diagnosis are the most effective strategies for ASF control. ASFV p30 is one of the most antigenic viral proteins that have been widely used in the field for serological diagnosis of ASF infection. In this study, we developed a luciferase immunoprecipitation system (LIPS) assay for the detection of ASFV antibodies in pig serum using Gaussia luciferase (GLuc)-tagged p30 as a diagnostic antigen. The optimal GLuc-p30 input of 10⁷ luminance units (LU) and optimal serum dilution factor of 1/100 were set to achieve the highest P/N ratio. Based on 87 ASFV-positive and negative pig sera, the cutoff value of the S/N ratio could be set between 2.298 and 30.59 to achieve 100% sensitivity and 100% specificity. Moreover, the diagnostic sensitivity of this LIPS is comparable to that of a commercial enzyme-linked immunosorbent assay (ELISA) and the specificity of LIPS is even superior to the tested ELISA. In conclusion, we have established a LIPS assay for ASFV antibody detection, which could be a potential method for ASFV diagnosis in laboratories and farms.

K205R specific nanobody-horseradish peroxidase fusions as reagents of competitive ELISA to detect African swine fever virus serum antibodies

Background

African swine fever virus (ASFV) is a highly contagious hemorrhagic disease and often lethal, which has significant economic consequences for the swine industry. Due to lacking of commercial vaccine, the prevention and control of ASF largely depend on early large-scale detection and screening. So far, the commercial ELISA kits have a long operation time and are expensive, making it difficult to achieve large-scale clinical applications. Nanobodies are single-domain antibodies produced by camelid animals, and have unique advantages such as smaller molecular weight, easy genetic engineering modification and low-costing of mass production, thus exhibiting good application prospects.

Results

The present study developed a new method for detection of ASFV specific antibodies using nanobody-horseradish peroxidase (Nb-HRP) fusion proteins as probe. By using camel immunization, phage library construction and phage display technology, five nanobodies against K205R protein were screened. Then, Nb-HRP fusion proteins were produced using genetic modification technology. Based on the Nb-HRP fusion protein as specific antibodies against K205R protein, a new type of cELISA was established to detect ASFV antibodies in pig serum. The cut-off value of the cELISA was 34.8%, and its sensitivity, specificity, and reproducibility were good. Furthermore, the developed cELISA exhibited 99.3% agreement rate with the commercial available ELISA kit (kappa value = 0.98).

Conclusions

The developed cELISA method has the advantages of simple operation, rapid and low-costing, and can be used for monitoring of ASFV infection in pigs, thus providing a new method for the prevention and control of ASF.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03423-0.

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.

HRP-conjugated-nanobody-based cELISA for rapid and sensitive clinical detection of ASFV antibodies

Abstract

African swine fever (ASF), which is caused by the ASF virus (ASFV), is a highly contagious hemorrhagic disease that causes high mortality to domestic porcine and wild boars and brings huge economic losses to world swine industry. Due to the lack of an effective vaccine, the control of ASF must depend on early, efficient, and cost-effective detection and strict control and elimination strategies. Traditional serological testing methods are generally associated with high testing costs, complex operations, and high technical requirements. As a promising alternative diagnostic tool to traditional antibodies, nanobodies (Nb) have the advantages of simpler and faster generation, good stability and solubility, and high affinity and specificity, although the system is dependent on the immunization of Bactrian camels to obtain the specific VHH library of the target protein. The application of Nbs in the detection of ASFV antibodies has not yet been reported yet. Using a phage display technology, one Nb against the ASFV p54 protein that exhibited high specificity and affinity, Nb8, was successfully screened. A HEK293T cell line stably expressing Nb8-horseradish peroxidase (HRP) fusion protein was established using the lentiviral expression system. Following the optimization of the reaction conditions, the Nb8-HRP fusion protein was successfully used to establish a competitive enzyme-linked immunosorbent assay (cELISA) to detect ASFV-specific antibodies in pig serum, for the first time. There was no cross-reaction with healthy pig serum, porcine pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), porcine epidemic diarrhea virus (PEDV), and classical swine fever virus (CSFV) positive sera. The optimal cut-off value for the cELISA by ROC analysis was 52.5%. A total of 209 serum samples were tested using the developed cELISA and a commercial ELISA kit. The results showed that the relative specificity of the cELISA was 98.97%, and the relative sensitivity of the cELISA was 93.3%, with the percent agreement between the two ELISA methods being 98.56%. In conclusion, a specific, sensitive, and repeatable cELISA was successfully developed based on the Nb8 as a probe, providing a promising method for the detection of anti-ASFV antibodies in clinical pig serum.

Key points

• We successfully screened a specific, high affinity nanobody against ASFV p54 protein.

• We establish a method for continuous and stable expression of Nb-HRP fusion protein using a lentiviral packaging system.

• We establish a nanobody cELISA detection method that can monitor an ASF infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11981-4.

Viral Vector Vaccines Against ASF: Problems and Prospectives

African swine fever (ASF) is a highly contagious viral disease affecting pigs, with mortality rates a primary focus as they can reach up to 100%. The widespread and colossal economic losses from ASF have impacts on the development of animal husbandry practices in most countries within Africa, Asia, and Europe. Currently, a variety of approaches toward the development of vaccines against ASF are being employed. A promising new concept centered around more economical and time-consuming vaccine production is based on the use of viral vectors to deliver selected immunogens. This review discusses the results obtained from testing various viral vectors as carriers of targeted ASF virus genes. The safety and prospects of viral vectors, the possibilities around modulating cellular and humoral immune responses by choosing genes expressing immunodominant antigens, and the degree of protection in experimental animals from infection with a lethal dose of virulent ASF virus strains have been shown and discussed.

Preparation and epitope mapping of monoclonal antibodies against African swine fever virus P30 protein

African swine fever virus (ASFV) causes acute, febrile, and highly contagious diseases in swine. Early diagnosis is critically important for African swine fever (ASF) prevention and control in the absence of an effective vaccine. P30 is one of the most immunogenic proteins that are produced during the early stage of an ASFV infection. This makes P30 a good serological target for ASF detection and surveillance. In this study, two P30-reactive monoclonal antibodies (mAbs), 2H2 and 5E8, were generated from mice immunized with recombinant P30 protein (rP30). Epitope mapping was performed with overlapping polypeptides, alanine mutants, and synthetic peptides. The mapping results revealed that 2H2 recognized a region located in the N-terminal, 16-48 aa. In contrast, 5E8 recognized a linear epitope in the C-terminal, 122-128 aa. Further analysis indicated that the epitope recognized by 2H2 was highly conserved in genotypes I and II, while the 5E8 epitope was conserved in most genotypes and the Ser to Pro change at position 128 in genotypes IV, V, and VI did not affect recognition. Overall, the results of this study provide valuable information on the antigenic regions of ASFV P30 and lay the foundation for the serological diagnosis of ASF and vaccine research. KEY POINTS: • Two specific and reactive mAbs were prepared and their epitopes were identified. • 2H2 recognized a novel epitope highly conserved in genotypes I and II. • 5E8 recognized a seven-amino acid linear epitope highly conserved in most genotypes.

Development of an indirect ELISA to specifically detect antibodies against African swine fever virus: bioinformatics approaches

BACKGROUND

African swine fever (ASF), characterized by acute, severe, and fast-spreading, is a highly lethal swine infectious disease caused by the African swine fever virus (ASFV), which has caused substantial economic losses to the pig industry worldwide in the past 100 years.

METHODS

This study started with bioinformatics methods and verified the epitope fusion protein method's reliability that does not rely on traditional epitope identification. Meanwhile, it will also express and purify the constructed genes through prokaryotic expression and establish antibody detection methods.

RESULTS

The results indicated that the protein had good reactivity and did not cross-react with other swine diseases. The receiver-operating characteristic analysis was performed to verify the determination. The area under the receiver-operating characteristic curve was 0.9991 (95% confidence interval 0.9973 to 1.001).

CONCLUSIONS

It was proved that the recombinant protein is feasible as a diagnostic antigen to distinguish ASFV and provides a new idea for ASFV antibody detection.

Development of Diagnostic Tests Provides Technical Support for the Control of African Swine Fever

African swine fever is a highly contagious global disease caused by the African swine fever virus. Since African swine fever (ASF) was introduced to Georgia in 2007, it has spread to many Eurasian countries at an extremely fast speed. It has recently spread to China and other major pig-producing countries in southeast Asia, threatening global pork production and food security. As there is no available vaccine at present, prevention and control must be carried out based on early detection and strict biosecurity measures. Early detection should be based on the rapid identification of the disease on the spot, followed by laboratory diagnosis, which is essential for disease control. In this review, we introduced the prevalence, transmission routes, eradication control strategies, and diagnostic methods of ASF. We reviewed the various methods of diagnosing ASF, focusing on their technical characteristics and clinical test results. Finally, we give some prospects for improving the diagnosis strategy in the future.

Characterization of Anti-p54 Monoclonal Antibodies and Their Potential Use for African Swine Fever Virus Diagnosis

African swine fever (ASF) is a highly lethal hemorrhagic viral disease of domestic pigs caused by African swine fever virus (ASFV). Although a good advance has been made to understand the virus, a safe and effective vaccine against ASFV is still lacking and its eradication solely depends on its early and accurate diagnosis. Thus, improving the available diagnostic assays and adding some validated techniques are useful for a range of serological investigations. The aim of this study was to produce and characterize p54 monoclonal antibodies with an ultimate goal of developing a monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for ASFV antibody detection. Five monoclonal antibodies against p54 protein expressed in Escherichia coli was generated and their characterizations were investigated. Furthermore, a competitive enzyme-linked immunosorbent assay (cELISA) based on a monoclonal antibody designated as 2A7 was developed. To evaluate the performance of the assay, a total of 365 pig serum samples (178 negative and 187 positive samples) were tested and a receiver-operating characteristic (ROC) analysis was applied to determine the cut-off value. Based on the ROC analysis, the area under the curve (AUC) was 0.982 (95% confidence interval: 96.9% to 99.4%), besides a sensitivity of 92.5% and a specificity of 98.9% was achieved when the percent inhibition of 20% was selected as a threshold. Moreover, the result showed an excellent agreement when compared to other commercially available blocking ELISA (kappa value = 0.912) and showed no reaction to other swine pathogens. Overall, the newly developed cELISA method offers a promising approach for a rapid and convenient ASFV serodiagnosis, which could be used as alternative to other serological assays for screening possible ASFV infection.

African Swine Fever Virus: An Emerging DNA Arbovirus

African swine fever virus (ASFV) is the sole member of the family Asfarviridae, and the only known DNA arbovirus. Since its identification in Kenya in 1921, ASFV has remained endemic in Africa, maintained in a sylvatic cycle between Ornithodoros soft ticks and warthogs (Phacochoerus africanus) which do not develop clinical disease with ASFV infection. However, ASFV causes a devastating and economically significant disease of domestic (Sus scrofa domesticus) and feral (Sus scrofa ferus) swine. There is no ASFV vaccine available, and current control measures consist of strict animal quarantine and culling procedures. The virus is highly stable and easily spreads by infected swine, contaminated pork products and fomites, or via transmission by the Ornithodoros vector. Competent Ornithodoros argasid soft tick vectors are known to exist not only in Africa, but also in parts of Europe and the Americas. Once ASFV is established in the argasid soft tick vector, eradication can be difficult due to the long lifespan of Ornithodoros ticks and their proclivity to inhabit the burrows of warthogs or pens and shelters of domestic pigs. Establishment of endemic ASFV infections in wild boar populations further complicates the control of ASF. Between the late 1950s and early 1980s, ASFV emerged in Europe, Russia and South America, but was mostly eradicated by the mid-1990s. In 2007, a highly virulent genotype II ASFV strain emerged in the Caucasus region and subsequently spread into the Russian Federation and Europe, where it has continued to circulate and spread. Most recently, ASFV emerged in China and has now spread to several neighboring countries in Southeast Asia. The high morbidity and mortality associated with ASFV, the lack of an efficacious vaccine, and the complex makeup of the ASFV virion and genome as well as its lifecycle, make this pathogen a serious threat to the global swine industry and national economies. Topics covered by this review include factors important for ASFV infection, replication, maintenance, and transmission, with attention to the role of the argasid tick vector and the sylvatic transmission cycle, current and future control strategies for ASF, and knowledge gaps regarding the virus itself, its vector and host species.

Epitope mapping of African swine fever virus (ASFV) structural protein, p54

In the absence of a vaccine for African swine fever virus (ASFV), diagnostic tools are critical for early detection and implementation of control measures. Along with other immunogenic proteins, p54 is a good serological target for conducting ASF detection and surveillance. In this study, a panel of 12 mouse monoclonal antibodies (mAbs) was prepared against a baculovirus-expressed p54(60-178) polypeptide. Further screening showed that five mAbs were positive for reactivity against ASFV-infected cells and recombinant p54 proteins. Mapping studies using five polypeptides and 12 oligopeptides, showed that mAb #154-1 recognized a conserved polypeptide sequence, p54(65-75), and was placed into Group 1. Mabs #143-1 and #7 recognized a region covered by p54(93-113) and were placed into Group 2. Group 3 consisted of mAbs #101 and #117, which recognized p54(118-127). Sera from pigs infected with the low virulent OURT 88/3 strain recognized the same p54 region covered by the Group 3 mAbs. When tested in a neutralization format, only mAb #143-1 showed neutralization activity above background. Together, the results identify important antigenic and immunogenic regions located on p54, which provide new tools for improving ASFV diagnostics.

Development and characterization of monoclonal antibodies against p30 protein of African swine fever virus

Among the structural proteins that compose the virion of African swine fever virus (ASFV), p30 is one of the most immunogenic proteins and is produced during early stage of ASFV infection. These two characteristics make p30 a good target for diagnostic assays to detect ASFV infection. In this study, we describe a panel of newly generated p30-specific monoclonal antibodies (mAbs). The reactivity of these mAbs was confirmed by immunoprecipitation and Western blot analysis in Vero cells infected with alphavirus replicon particles that express p30 (RP-p30). Furthermore, this panel of mAbs recognized ASFV strains BA71 V (Genotype I) and Georgia/2007 (Genotype II) in immunofluorescence assays on virus-infected Vero cells and swine macrophages, respectively. These mAbs also detected p30 expression by immunohistochemistry in tissue samples from ASFV-infected pigs. Epitope mapping revealed that a selected mAb from the panel recognized a linear epitope within the 32-amino acid region, 61-93. In contrast, two of the mAbs recognize the C-terminal region of the protein, which is highly hydrophilic, enriched in glutamic acid residues, and predicted to contain an intrinsically disordered protein region (IDPR). This panel of mAbs and mAb-based diagnostic assays potentially represent valuable tools for ASFV detection, surveillance and disease control.

Evaluation of an African swine fever (ASF) vaccine strategy incorporating priming with an alphavirus-expressed antigen followed by boosting with attenuated ASF virus

In this study, an alphavirus vector platform was used to deliver replicon particles (RPs) expressing African swine fever virus (ASFV) antigens to swine. Alphavirus RPs expressing ASFV p30 (RP-30), p54 (RP-54) or pHA-72 (RP-sHA-p72) antigens were constructed and tested for expression in Vero cells and for immunogenicity in pigs. RP-30 showed the highest expression in Vero cells and was the most immunogenic in pigs, followed by RP-54 and RP-sHA-p72. Pigs primed with two doses of the RP-30 construct were then boosted with a naturally attenuated ASFV isolate, OURT88/3. Mapping of p30 identified an immunodominant region within the amino acid residues 111-130. However, the principal effect of the prime-boost was enhanced recognition of an epitope covered by the peptide sequence 61-110. The results suggest that a strategy incorporating priming with a vector-expressed antigen followed by boosting with an attenuated live virus may broaden the recognition of ASFV epitopes.

Roles of African Swine Fever Virus Structural Proteins in Viral Infection

African swine fever virus (ASFV) is a large, double-stranded DNA virus and the sole member of the Asfarviridae family. ASFV infects domestic pigs, wild boars, warthogs, and bush pigs, as well as soft ticks (Ornithodoros erraticus), which likely act as a vector. The major target is swine monocyte-macrophage cells. The virus can cause high fever, haemorrhagic lesions, cyanosis, anorexia, and even fatalities in domestic pigs. Currently, there is no vaccine and effective disease control strategies against its spread are culling infected pigs and maintaining high biosecurity standards. African swine fever (ASF) spread to Europe from Africa in the middle of the 20^th century, and later also to South America and the Caribbean. Since then, ASF has spread more widely and thus is still a great challenge for swine breeding. The genome of ASFV ranges in length from about 170 to 193 kbp depending on the isolate and contains between 150 and 167 open reading frames (ORFs). The ASFV genome encodes 150 to 200 proteins, around 50 of them structural. The roles of virus structural proteins in viral infection have been described. These proteins, such as pp220, pp62, p72, p54, p30, and CD2v, serve as the major component of virus particles and have roles in attachment, entry, and replication. All studies on ASFV proteins lay a good foundation upon which to clarify the infection mechanism and develop vaccines and diagnosis methods. In this paper, the roles of ASFV structural proteins in viral infection are reviewed.

BA71ΔCD2: a New Recombinant Live Attenuated African Swine Fever Virus with Cross-Protective Capabilities

African swine fever is a highly contagious viral disease of mandatory declaration to the World Organization for Animal Health (OIE). The lack of available vaccines makes its control difficult; thus, African swine fever virus (ASFV) represents a major threat to the swine industry. Inactivated vaccines do not confer solid protection against ASFV. Conversely, live attenuated viruses (LAV), either naturally isolated or obtained by genetic manipulation, have demonstrated reliable protection against homologous ASFV strains, although little or no protection has been demonstrated against heterologous viruses. Safety concerns are a major issue for the use of ASFV attenuated vaccine candidates and have hampered their implementation in the field so far. While trying to develop safer and efficient ASFV vaccines, we found that the deletion of the viral CD2v (EP402R) gene highly attenuated the virulent BA71 strain in vivo. Inoculation of pigs with the deletion mutant virus BA71ΔCD2 conferred protection not only against lethal challenge with the parental BA71 but also against the heterologous E75 (both genotype I strains). The protection induced was dose dependent, and the cross-protection observed in vivo correlated with the ability of BA71ΔCD2 to induce specific CD8⁺ T cells capable of recognizing both BA71 and E75 viruses in vitro. Interestingly, 100% of the pigs immunized with BA71ΔCD2 also survived lethal challenge with Georgia 2007/1, the genotype II strain of ASFV currently circulating in continental Europe. These results open new avenues to design ASFV cross-protective vaccines, essential to fight ASFV in areas where the virus is endemic and where multiple viruses are circulating.

IMPORTANCE African swine fever virus (ASFV) remains enzootic in most countries of Sub-Saharan Africa, today representing a major threat for the development of their swine industry. The uncontrolled presence of ASFV has favored its periodic exportation to other countries, the last event being in Georgia in 2007. Since then, ASFV has spread toward neighboring countries, reaching the European Union's east border in 2014. The lack of available vaccines against ASFV makes its control difficult; so far, only live attenuated viruses have demonstrated solid protection against homologous experimental challenges, but they have failed at inducing solid cross-protective immunity against heterologous viruses. Here we describe a new LAV candidate with unique cross-protective abilities: BA71ΔCD2. Inoculation of BA71ΔCD2 protected pigs not only against experimental challenge with BA71, the virulent parental strain, but also against heterologous viruses, including Georgia 2007/1, the genotype II strain of ASFV currently circulating in Eastern Europe.

Reemergence of African Swine Fever in Zimbabwe, 2015

Zimbabwe is the only country in southern Africa with no reported African swine fever (ASF) outbreaks during 1993–2014. However, the 2015 discovery of genotype II ASF virus in Zimbabwe indicates the reemergence of ASF in this country and suggests that this viral genotype may be spreading through eastern and southern Africa.

Detection of African Swine Fever Virus Antibodies in Serum and Oral Fluid Specimens Using a Recombinant Protein 30 (p30) Dual Matrix Indirect ELISA

In the absence of effective vaccine(s), control of African swine fever caused by African swine fever virus (ASFV) must be based on early, efficient, cost-effective detection and strict control and elimination strategies. For this purpose, we developed an indirect ELISA capable of detecting ASFV antibodies in either serum or oral fluid specimens. The recombinant protein used in the ELISA was selected by comparing the early serum antibody response of ASFV-infected pigs (NHV-p68 isolate) to three major recombinant polypeptides (p30, p54, p72) using a multiplex fluorescent microbead-based immunoassay (FMIA). Non-hazardous (non-infectious) antibody-positive serum for use as plate positive controls and for the calculation of sample-to-positive (S:P) ratios was produced by inoculating pigs with a replicon particle (RP) vaccine expressing the ASFV p30 gene. The optimized ELISA detected anti-p30 antibodies in serum and/or oral fluid samples from pigs inoculated with ASFV under experimental conditions beginning 8 to 12 days post inoculation. Tests on serum (n = 200) and oral fluid (n = 200) field samples from an ASFV-free population demonstrated that the assay was highly diagnostically specific. The convenience and diagnostic utility of oral fluid sampling combined with the flexibility to test either serum or oral fluid on the same platform suggests that this assay will be highly useful under the conditions for which OIE recommends ASFV antibody surveillance, i.e., in ASFV-endemic areas and for the detection of infections with ASFV isolates of low virulence.

African swine fever virus serodiagnosis: a general review with a focus on the analyses of African serum samples

African swine fever (ASF) is an infectious disease that causes heavy mortality in domestic pigs. At present there is no vaccine against ASF, and eradication in countries where the disease is endemic is based only on competent diagnosis programs and the sacrifice of infected animals. Due to the presence of natural attenuated strains, certain infection conditions may result in reduced mortality. In these situations, the disease can be diagnosed by detection of specific antibodies. The use of classical and validated diagnosis assays, such as ELISA and Indirect Immunofluorescence or Immunoblotting, allowed the eradication of ASF in the Iberian Peninsula in the 1990s. However, given that conventional tests include the use of antigens obtained from ASF virus (ASFV)-infected cells, they have several disadvantages, such as difficulties to achieve standardization and also the risks associated with the manipulation of live virus. Such drawbacks have led to the development of alternative and more robust systems for the production of ASFV antigens for use in anti-ASFV antibody detection systems. In the present review, we provide an update on current knowledge about antigen targets for ASFV serodiagnosis, the significant progress made in recombinant antigen production, and the refinement of ASF serological diagnostic assays. Moreover, we describe the accuracy of an ELISA developed for the serodiagnosis of ASFV in Africa. This assay is based on a novel p30 recombinant protein (p30r) obtained from an Eastern African viral isolate (Morara strain), which shares 100% amino acid sequence identity with the Georgia virus isolate. That study included the analyses of 587 field sera collected from domestic pigs and warthogs in Senegal (West Africa), the Democratic Republic of Congo (Central Africa), Mozambique (South-East Africa), and South Africa. The results revealed that the novel p30r-based ELISA allows the accurate detection of antibodies against ASFV, independently of the geographical origin of the sera.

DNA vaccination partially protects against African swine fever virus lethal challenge in the absence of antibodies

The lack of available vaccines against African swine fever virus (ASFV) means that the evaluation of new immunization strategies is required. Here we show that fusion of the extracellular domain of the ASFV Hemagglutinin (sHA) to p54 and p30, two immunodominant structural viral antigens, exponentially improved both the humoral and the cellular responses induced in pigs after DNA immunization. However, immunization with the resulting plasmid (pCMV-sHAPQ) did not confer protection against lethal challenge with the virulent E75 ASFV-strain. Due to the fact that CD8⁺ T-cell responses are emerging as key components for ASFV protection, we designed a new plasmid construct, pCMV-UbsHAPQ, encoding the three viral determinants above mentioned (sHA, p54 and p30) fused to ubiquitin, aiming to improve Class I antigen presentation and to enhance the CTL responses induced. As expected, immunization with pCMV-UbsHAPQ induced specific T-cell responses in the absence of antibodies and, more important, protected a proportion of immunized-pigs from lethal challenge with ASFV. In contrast with control pigs, survivor animals showed a peak of CD8⁺ T-cells at day 3 post-infection, coinciding with the absence of viremia at this time point. Finally, an in silico prediction of CTL peptides has allowed the identification of two SLA I-restricted 9-mer peptides within the hemagglutinin of the virus, capable of in vitro stimulating the specific secretion of IFNγ when using PBMCs from survivor pigs. Our results confirm the relevance of T-cell responses in protection against ASF and open new expectations for the future development of more efficient recombinant vaccines against this disease.

Recombinant protein-based viral disease diagnostics in veterinary medicine

Identification of pathogens or antibody response to pathogens in human and animals modulates the treatment strategies for naive population and subsequent infections. Diseases can be controlled and even eradicated based on the epidemiology and effective prophylaxis, which often depends on development of efficient diagnostics. In addition, combating newly emerging diseases in human as well as animal healthcare is challenging and is dependent on developing safe and efficient diagnostics. Detection of antibodies directed against specific antigens has been the method of choice for documenting prior infection. Other than zoonosis, development of inexpensive vaccines and diagnostics is a unique problem in animal healthcare. The advent of recombinant DNA technology and its application in the biotechnology industry has revolutionized animal healthcare. The use of recombinant DNA technology in animal disease diagnosis has improved the rapidity, specificity and sensitivity of various diagnostic assays. This is because of the absence of host cellular proteins in the recombinant derived antigen preparations that dramatically decrease the rate of false-positive reactions. Various recombinant products are used for disease diagnosis in veterinary medicine and this article discusses recombinant-based viral disease diagnostics currently used for detection of pathogens in livestock and poultry.

Recombinant antigen targets for serodiagnosis of African swine fever

African swine fever (ASF) is an infectious and economically important disease of domestic pigs. There is no vaccine, and so reliable diagnosis is essential for control strategies. The performance of four recombinant ASF virus (ASFV) protein (pK205R, pB602L, p104R, and p54)-based enzyme-linked immunosorbent assays (ELISAs) was evaluated with European porcine field sera that had been established by Office International des Epizooties (OIE)-approved tests to be ASFV negative (n = 119) and ASFV positive (n = 80). The kappa values showed that there was almost perfect agreement between the results of the "gold standard" test (immunoblotting) and the results obtained by the p54-specific ELISA (kappa = 0.95; 95% confidence interval [CI], 0.90 to 0.99) and the pK205R-specific ELISA or the pB602L-specific ELISA (kappa = 0.92; 95% CI, 0.86 to 0.97). For the pA104R-specific ELISA, there was substantial to almost perfect agreement (kappa = 0.81; 95% CI, 0.72 to 0.89). Similar results were observed by the OIE-approved ELISA (kappa = 0.89; 95% CI, 0.82 to 0.95). Importantly, antibodies against these proteins were detectable early after infection of domestic pigs. Preliminary testing of 9 positive and 17 negative serum samples from pigs from West Africa showed identical results by the recombinant protein-based ELISA and the OIE-approved tests. In contrast, there was a high degree of specificity but a surprisingly a low level of sensitivity with 7 positive and 342 negative serum samples from pigs from East Africa. With poorly preserved sera, only the p104R-specific ELISA showed a significant reduction in sensitivity compared to that of the OIE-approved ELISA. Finally, these recombinant proteins also detected antibodies in the sera of the majority of infected warthogs. Thus, recombinant ASFV proteins p54, pB602L, and pK205R provide sensitive and specific targets for the detection of antibodies in European and West African domestic pigs and warthogs.

African swine fever virus protein p30 interaction with heterogeneous nuclear ribonucleoprotein K (hnRNP-K) during infection

Heterogeneous nuclear ribonucleoprotein K (hnRNP-K) was identified as interacting cellular protein with the abundant immediate early protein p30 from African swine fever virus (ASFV) in a macrophage cDNA library screening. The interacting regions of hnRNP-K with p30 were established within residues 35-197, which represent KH1 and KH2 domains responsible for RNA binding. Colocalization of hnRNP-K and p30 was observed mainly in the nucleus, but not in the cytoplasm of infected cells and infection modified hnRNP-K subcellular distribution and decreased the incorporation of 5-fluorouridine into nascent RNA. Since similar effects were observed in cells transiently expressing p30, this interaction provides new insights into p30 function and could represent a possible additional mechanism by which ASFV downregulates host cell mRNA translation.

Systematic analysis of longitudinal serological responses of pigs infected experimentally with African swine fever virus

The protective immune response to African swine fever virus (ASFV) includes both cellular and serological components. In this study, the role of antibodies in the pathogenicity and diagnosis of African swine fever (ASF) was explored. Accordingly, total and Ig isotype antibody responses against the 12 viral proteins previously demonstrated to be the main targets of serological immunity were evaluated in longitudinally collected sera from pigs infected experimentally with the non-pathogenic ASFV/NH/P68 isolate. Strong total IgG antibody responses were observed against viral proteins E183L/p54, K205R/'unassigned', A104R/histone-like and B602L/'unassigned'; therefore, IgM, IgG1 and IgG2 responses to these proteins were also determined. One protein stimulating IgM (K205R) may have practical potential for the detection of recently infected animals. There was a clear trend towards an IgG1 response to all of the proteins. This may reflect a dominant Th2-controlled immune response. In order to identify possible correlations between these serological responses and the pathogenesis of ASF, total IgG responses to the 12 recombinant proteins were compared in asymptomatic and chronically infected animals. For the proteins NP419L/DNA ligase, CP312R, B646L/p73, K196R/thymidine kinase and K205R, the antibody titres were significantly higher in animals developing lesions. One exception was the antibody response to the A104R/histone-like protein, which was higher in asymptomatic than in chronically infected pigs, suggesting that antibodies against this protein might be an indicator of an effective immune response or that this response is somehow involved in protection.

Development of a low-cost, insect larvae-derived recombinant subunit vaccine against RHDV

Vaccine antigens against rabbit hemorrhagic disease virus (RHDV) are currently derived from inactivated RHDV obtained from livers of experimentally infected rabbits. Several RHDV-derived recombinant immunogens have been reported. However, their application in vaccines has been restricted due to their high production costs. In this paper, we describe the development of an inexpensive, safe, stable vaccine antigen for RHDV. A baculovirus expressing a recombinant RHDV capsid protein (VP60r) was used to infect Trichoplusia ni insect larvae. It reached an expression efficiency of 12.5% of total soluble protein, i.e. approximately 2 mg of VP60r per larva. Preservation of the antigenicity and immunogenicity of the VP60r was confirmed by immunological and immunization experiments. Lyophilized crude larvae extracts, containing VP60r, were stable, at room temperature, for at least 800 days. In all cases, rabbits immunized with a single dose of VP60r by the intramuscular route were protected against RHDV challenge. Doses used were as low as 2 microg of VP60r in the presence of adjuvant or 100 microg without one. Orally administered VP60r in the absence of an adjuvant gave no protection. The potential costs of an RHDV vaccine made using this technology would be reduced considerably compared with producing the same protein in insect cells maintained by fermentation. In conclusion, the larva expression system may provide a broad-based strategy for production of recombinant subunit antigens (insectigens) for human or animal medicines, especially when production costs restrain their use.

Optimization and validation of recombinant serological tests for African Swine Fever diagnosis based on detection of the p30 protein produced in Trichoplusia ni larvae

We describe the validation of an enzyme-linked immunosorbent assay (ELISA) and confirmatory immunoblotting assays based on a recombinant p30 protein (p30r) produced in insect larvae using a baculovirus vector. Such validation included the following: (i) the scaling up and standardization of p30r production and the associated immunoassays, (ii) a broad immunological analysis using a large number of samples (a total of 672) from Spain and different African locations, and (iii) the detection of the ASF virus (ASFV)-antibody responses at different times after experimental infection. Yields of p30r reached up to 15% of the total protein recovered from the infected larvae at 3 days postinfection. Serological analysis of samples collected in Spain revealed that the p30r-based ELISA presented similar sensitivity to and higher specificity than the conventional Office International des Epizooties-approved ASFV ELISA. Moreover, the p30r ELISA was more sensitive than the conventional ELISA test in detecting ASFV-specific antibodies in experimentally infected animals at early times postinfection. Both the recombinant and conventional ELISAs presented variable rates of sensitivity and specificity with African samples, apparently related to their geographical origin. Comparative analyses performed on the sequences, predicted structures, and antigenicities of p30 proteins from different Spanish and African isolates suggested that variability among isolates might correlate with changes in antigenicity, thus affecting detection by the p30r ELISA. Our estimations indicate that more than 40,000 ELISA determinations and 2,000 confirmatory immunoblotting tests can be performed with the p30r protein obtained from a single infected larva, making this a feasible and inexpensive strategy for production of serological tests with application in developing countries.

Antigenic properties and diagnostic potential of African swine fever virus protein pp62 expressed in insect cells

African swine fever (ASF) is an infectious and economically important disease of domestic pigs. The absence of vaccine renders the diagnostic test the only tool that can be used for the control of new outbreaks of the disease. At present, the enzyme-linked immunosorbent assay (ELISA) test is the most useful method for large-scale ASF serological studies, although false positives have been detected, mainly on poorly preserved sera. In order to improve the current diagnostic test available for ASF, we have studied the antigenic properties of the ASF virus polyprotein pp62 and its suitability for use in a novel ELISA. Two well-known antigenic proteins of ASF virus, p32 and p54, were also included in this study. These proteins were expressed in the baculovirus expression system and used as antigens in ASF serological tests. Our results indicate that the use of these recombinant proteins as antigens in the ELISAs improves the sensitivity and specificity obtained with the conventional diagnosis test used to detect antibodies against ASF virus. Furthermore, the use of polyprotein pp62 in an ELISA for testing poorly preserved sera allows performance of the diagnosis of ASF without the need to confirm the results by the immunoblot test. These features make pp62 one of the most interesting viral proteins to be used for serological ASF diagnosis.

Serodiagnosis of African swine fever using the recombinant protein p30 expressed in insect larvae

African swine fever (ASF) has a substantial economic impact in many African developing countries and its eradication is based only on an efficient diagnosis program because of the absence of an available vaccine. Previous data suggested the convenience of using the highly antigenic virus protein p30 as ELISA antigen for serological diagnosis of this disease. A simple and efficient method is described for producing the recombinant protein p30 from ASF virus in Trichoplusia ni larvae (cabbage looper) in order to facilitate the large-scale production of this recombinant protein in the absence of fermentation procedures. A baculovirus encoding the virus protein p30 was used to infect insect larvae, showing that recombinant protein production had a sharp optimal peak with a time of occurrence dependent on the initial virus dose inoculated to the larvae. Crude lysates of infected larvae were used without further purification as coating antigen in ELISA to analyse a limited number of sera from natural or experimentally ASF virus infected pigs. Remarkably, the recombinant protein obtained from a single infected larva was sufficient for serological diagnosis of at least 3750 serum samples. Recombinant p30 obtained by this procedure was also used in a confirmatory immunoblotting, reacting with all positive sera tested previously by ELISA. In conclusion, production of the recombinant ASF virus protein p30 in larvae should be applicable to large-scale production of diagnostic reagents for this disease in developing countries, eliminating the need for specialised facilities for tissue culture.

Functionality and cell anchorage dependence of the African swine fever virus gene A179L, a viral bcl-2 homolog, in insect cells

The African swine fever virus gene A179L has been shown to be a functional member of the ced9/bcl-2 family of apoptosis inhibitors in mammalian cell lines. In this work we have expressed the A179L gene product (p21) under the control of the baculovirus polyhedrin promoter using a baculovirus system. Expression of the A179L gene neither altered the baculovirus replication phenotype nor delayed the shutoff of cellular protein synthesis, but it extended the survival of the infected insect cells to very late times postinfection. The increase in cell survival rates correlated with a marked apoptosis reduction after baculovirus infection. Interestingly, prevention of apoptosis was observed when recombinant baculovirus infections were carried out in monolayer cell cultures but not when cells were infected in suspension, suggesting a cell anchorage dependence for p21 function in insect cells. Cell survival was enhanced under optimal conditions of cell attachment and cell-to-cell contact as provided by extracellular matrix components or poly-D-lysine. Since it was observed that cytoskeleton organization varied depending on culture conditions of insect cells (grown in monolayer versus grown in suspension), these results suggested that A179L might regulate apoptosis in insect cells only when the cytoskeletal support of intracellular signaling is maintained upon cell adhesion. Thus, cell shape and cytoskeleton status might allow variations in intracellular transduction of signals related to cell survival in virus-infected cells.

Detection of African swine fever virus in infected pig tissues by immunocytochemistry and in sity hybridisation

The techniques for determining cellular sites of establishment and persistence of African swine fever virus (ASFV) were established in susceptible domestic pigs and the resistant African reservoir hosts, the warthog and bushpig. Detection, both in vitro and in vivo, was achieved by in situ hybridisation and immunocytochemistry, focusing principally on specific probes for vp73, a major capsid protein. Hybridisation of radio-labelled probes for DNA and RNA was relatively insensitive and time consuming whereas hybridisation of non-radioactive DNA probes was quicker and more sensitive. Detection of vp73 protein by immunocytochemistry was as sensitive as non-radioactive DNA hybridisation, offering in addition improved speed, ease and morphology. Both non-radioactive DNA hybridisation and immunocytochemistry were therefore used to detect ASFV DNA and protein in a range of porcine cells infected in vitro with different ASFV isolates. Malta and Malawi isolates infected both alveolar and bone marrow macrophages, but infected negligible numbers of endothelial (< 1%) and kidney cells (IBRS2 cells). Attenuated Uganda isolate, however, infected a high percentage of endothelial cells and IBRS2 cells as well as alveolar and bone marrow macrophages. When used to investigate the cell tropism of ASFV in tissues from pigs infected with the highly virulent Malawi isolate of ASFV, both techniques identified virus principally in cells of the mononuclear phagocytic system. In the lung, double staining revealed that pulmonary intravascular macrophages, but not alveolar macrophages, were infected.

The African swine fever virus proteins p54 and p30 are involved in two distinct steps of virus attachment and both contribute to the antibody-mediated protective immune response

The nature of the initial interactions of African swine fever (ASF) virus with target cells is only partially known, and to date only the ASF virus protein p12 has been identified as a viral attachment protein. More recently, antibodies to viral proteins p54 and p30 have been shown to neutralize the virus, inhibiting virus binding and internalization, respectively. Therefore, we investigated the role of these proteins in the receptor-mediated ASF virus endocytosis in swine macrophages, the natural host cells. Proteins p54 and p30, released from ASF virus particles after treatment of virions with a nonionic detergent, bound to virus-sensitive alveolar pig macrophages. Binding of these proteins was found to be specifically inhibited by neutralizing antibodies obtained from a convalescent pig or from pigs immunized with recombinant p54 or p30 proteins. The baculovirus-expressed proteins p54 and p30 retained the same biological properties as the viral proteins, since they also bound specifically to these cells, and their binding was equally inhibited by neutralizing antibodies. Binding of 35S-labeled recombinant p54 and p30 proteins to macrophages was specifically competed by an excess of unlabeled p54 and p30, respectively. However, cross-binding inhibition was not observed, suggesting the existence of two different saturable binding sites for these proteins in the susceptible cells. In addition, protein p54 blocked the specific binding of virus particles to the macrophage, while protein p30 blocked virus internalization. Both proteins independently prevented virus infection and in a dose-dependent manner, suggesting that binding interactions mediated by both proteins are necessary to give rise to a productive infection. The relevance of blockade of virus-cell interactions mediated by p54 and p30 in the protective immune response against ASF virus was then investigated. Immunization of pigs with either recombinant p54 or p30 proteins induced neutralizing antibodies which, as expected, inhibited virus attachment or internalization, respectively. However, immunized pigs were not protected against lethal infection and the disease course was not modified in these animals. In contrast, immunization with a combination of p54 and p30 proteins simultaneously stimulated both virus neutralizing mechanisms and modified drastically the disease course, rendering a variable degree of protection ranging from a delay in the onset of the disease to complete protection against virus infection. In conclusion, the above results strongly suggest that proteins p54 and p30 mediate specific interactions between ASF virus and cellular receptors and that simultaneous interference with these two interactions has a complementary effect in antibody-mediated protection.