Neutralizing antibodies to African swine fever virus proteins p30, p54, and p72 are not sufficient for antibody-mediated protection

The antibodies against the A137R protein drive antibody-dependent enhancement of African swine fever virus infection in porcine alveolar macrophages

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF), a highly contagious disease that can kill up to 100% of domestic pigs and wild boars. It has been shown that the pigs inoculated with some ASF vaccine candidates display more severe clinical signs and die earlier than do pigs not immunized. We hypothesize that antibody-dependent enhancement (ADE) of ASFV infection may be caused by the presence of some unidentified antibodies. In this study, we found that the ASFV-encoded structural protein A137R (pA137R) can be recognized by the anti-ASFV positive sera, indicating that the anti-pA137R antibodies are induced in the ASFV-infected pigs. Interestingly, our results demonstrated that the anti-pA137R antibodies produced in rabbits or pigs enhanced viral replication of different ASFV strains in primary porcine alveolar macrophages (PAMs), the target cells of ASFV. Mechanistic investigations revealed that anti-pA137R antibodies were able to promote the attachment of ASFV to PAMs and two types of Fc gamma receptors (FcγRs), FcγRII and FcγRIII, mediated the ADE of ASFV infection. Taken together, anti-pA137R antibodies are able to drive ASFV ADE in PAMs. These findings shed new light on the roles of anti-ASFV antibodies and have implications for the pathophysiology of the disease and the development of ASF vaccines.

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.

Comprehensive Characterization of the Genetic Landscape of African Swine Fever Virus: Insights into Infection Dynamics, Immunomodulation, Virulence and Genes with Unknown Function

African Swine Fever (ASF) is a lethal contagious hemorrhagic viral disease affecting the swine population. The causative agent is African Swine Fever Virus (ASFV). There is no treatment or commercial vaccine available at present. This virus poses a significant threat to the global swine industry and economy, with 100% mortality rate in acute cases. ASFV transmission occurs through both direct and indirect contact, with control measures limited to early detection, isolation, and culling of infected pigs. ASFV exhibits a complex genomic structure and encodes for more than 50 structural and 100 non-structural proteins and has 150 to 167 open reading frames (ORFs). While many of the proteins are non-essential for viral replication, they play crucial roles in mediating with the host to ensure longevity and transmission of virus in the host. The dynamic nature of ASFV research necessitates constant updates, with ongoing exploration of various genes and their functions, vaccine development, and other ASF-related domains. This comprehensive review aims to elucidate the structural and functional roles of both newly discovered and previously recorded genes involved in distinct stages of ASFV infection and immunomodulation. Additionally, the review discusses the virulence genes and genes with unknown functions, and proposes future interventions.

Defining correlates of protection for mammalian livestock vaccines against high-priority viral diseases

Enhancing livestock biosecurity is critical to safeguard the livelihoods of farmers, global and local economies, and food security. Vaccination is fundamental to the control and prevention of exotic and endemic high-priority infectious livestock diseases. Successful implementation of vaccination in a biosecurity plan is underpinned by a strong understanding of correlates of protection-those elements of the immune response that can reliably predict the level of protection from viral challenge. While correlates of protection have been successfully characterized for many human viral vaccines, for many high-priority livestock viral diseases, including African swine fever and foot and mouth disease, they remain largely uncharacterized. Current literature provides insights into potential correlates of protection that should be assessed during vaccine development for these high-priority mammalian livestock viral diseases. Establishment of correlates of protection for biosecurity purposes enables immune surveillance, rationale for vaccine development, and successful implementation of livestock vaccines as part of a biosecurity strategy.

Thoughts on the research of African swine fever live-attenuated vaccines

African swine fever (ASF) is a contagious and fatal disease caused by the African swine fever virus (ASFV), which can infect pigs of all breeds and ages. Most infected pigs have poor prognosis, leading to substantial economic losses for the global pig industry. Therefore, it is imperative to develop a safe and efficient commercial vaccine against ASF. The development of ASF vaccine can be traced back to 1960. However, because of its large genome, numerous encoded proteins, and complex virus particle structure, currently, no effective commercial vaccine is available. Several strategies have been applied in vaccine design, some of which are potential candidates for vaccine development. This review provides a comprehensive analysis on the safety and effectiveness, suboptimal immunization effects at high doses, absence of standardized evaluation criteria, notable variations among strains of the same genotype, and the substantial impact of animal health on the protective efficacy against viral challenge. All the information will be helpful to the ASF vaccine development.

Advancing vaccine development: Evaluation of a mannose-modified lipid nanoparticle-based candidate for African swine fever p30 mRNA vaccine eliciting robust immune response in mice

The African swine fever virus (ASFV) continues to pose significant economic and pandemic risks. Consequently, discovering new, efficient vaccines is crucial. Messenger RNA (mRNA) vaccines have emerged as promising candidates, providing minimal risk of insertional mutagenesis, high safety profiles, effectiveness, rapid scalability in production, and cost-effectiveness. In this study, we have developed an ASF p30 mRNA vaccine candidate (mRNA/Man-LNP) employing mannose-modified lipid nanoparticles (LNPs). The mRNA/Man-LNP exhibited effective antigen presentation and facilitated dendritic cells (DCs) maturation. Notably, it elicited strong IgG titers and activated CD4+ and CD8+ T-cells in immunized mice, all while adhering to stringent biosafety standards. This investigation demonstrates that mRNA/Man-LNP can trigger both humoral and cellular immune responses, suggesting its potential as a potent and promising vaccine candidate for controlling African swine fever (ASF).

Identification of an Immunodominant B-Cell Epitope in African Swine Fever Virus p30 Protein and Evidence of p30 Antibody-Mediated Antibody Dependent Cellular Cytotoxicity

African Swine Fever Virus (ASFV) is a large dsDNA virus that encodes at least 150 proteins. The complexity of ASFV and lack of knowledge of effector immune functions and protective antigens have hindered the development of safe and effective ASF vaccines. In this study, we constructed four Orf virus recombinant vectors expressing individual ASFV genes B602L, -CP204L, E184L, and -I73R (ORFVΔ121-ASFV-B602L, -CP204L, -E184L, and -I73R). All recombinant viruses expressed the heterologous ASFV proteins in vitro. We then evaluated the immunogenicity of the recombinants by immunizing four-week-old piglets. In two independent animal studies, we observed high antibody titers against ASFV p30, encoded by CP204L gene. Using Pepscan ELISA, we identified a linear B-cell epitope of 12 amino acids in length (Peptide 15) located in an exposed loop region of p30 as an immunodominant ASFV epitope. Additionally, antibodies elicited against ASFV p30 presented antibody-dependent cellular cytotoxicity (ADCC) activity. These results underscore the role of p30 on antibody responses elicited against ASFV and highlight an important functional epitope that contributes to p30-specific antibody responses.

Analysis of the Immunogenicity of African Swine Fever F317L Protein and Screening of T Cell Epitopes

The African swine fever virus (ASFV) encodes numerous proteins characterized by complex immune escape mechanisms. At present, the structure and function of these proteins, including the F317L protein, have yet to be fully elucidated. In this study, we examined the immunogenicity of the F317L protein. Mice were subcutaneously immunized with the F317L protein using initial and subsequent booster doses, and, at the 28th day post-treatment, we assessed the humoral and cellular immune responses of mice. The F317L protein stimulated production of specific antibodies and activated humoral immune responses. In addition, F317L stimulated the production of large amounts of IFN-γ by splenic lymphocytes, thereby activating cellular immune responses. Using informatics technology, we predicted and synthesized 29 F317L protein T cell epitopes, which were screened using IFN-γ ELISpot. Among these, the F25 (246SRRSLVNPWT255) peptide was identified as having a stronger stimulatory effect than the full-length protein. Collectively, our findings revealed that the ASFV F317L protein can stimulate both strong humoral and cellular immunity in mice, and that the F25 (246SRRSLVNPWT255) peptide may be a potential active T cell epitope. These findings will provide a reference for further in-depth studies of the F317L protein and screening of antigenic epitopes.

Deleting the C84L Gene from the Virulent African Swine Fever Virus SY18 Does Not Affect Its Replication in Porcine Primary Macrophages but Reduces Its Virulence in Swine

(1) Background: African swine fever (ASF) is a highly contagious disease that causes high pig mortality. Due to the absence of vaccines, prevention and control are relatively challenging. The pathogenic African swine fever virus (ASFV) has a complex structure and encodes over 160 proteins, many of which still need to be studied and verified for their functions. In this study, we identified one of the unknown functional genes, C84L. (2) Methods: A gene deficient strain was obtained through homologous recombination and several rounds of purification, and its replication characteristics and virulence were studied through in vitro and in vivo experiments, respectively. (3) Results: Deleting this gene from the wild-type virulent strain SY18 did not affect its replication in porcine primary macrophages but reduced its virulence in pigs. In animal experiments, we injected pigs with a 102 TCID50, 105 TCID50 deletion virus, and a 102 TCID50 wild-type strain SY18 intramuscularly. The control group pigs reached the humane endpoint on the ninth day (0/5) and were euthanized. Two pigs in the 102 TCID50(2/5) deletion virus group survived on the twenty-first day, and one in the 105 TCID50(1/5) deletion virus group survived. On the twenty-first day, the surviving pigs were euthanized, which was the end of the experiment. The necropsies of the survival group and control groups' necropsies showed that the surviving pigs' liver, spleen, lungs, kidneys, and submaxillary lymph nodes did not show significant lesions associated with the ASFV. ASFV-specific antibodies were first detected on the seventh day after immunization; (4) Conclusions: This is the first study to complete the replication and virulence functional exploration of the C84L gene of SY18. In this study, C84L gene was preliminarily found not a necessary gene for replication, gene deletion strain SY18ΔC84L has similar growth characteristics to SY18 in porcine primary alveolar macrophages. The C84L gene affects the virulence of the SY18 strain.

Immunoinformatics-guided approach for designing a pan-proteome multi-epitope subunit vaccine against African swine fever virus

Despite being identified over a hundred years ago, there is still no commercially available vaccine for the highly contagious and deadly African swine fever virus (ASFV). This study used immunoinformatics for the rapid and inexpensive designing of a safe and effective multi-epitope subunit vaccine for ASFV. A total of 18,858 proteins from 100 well-annotated ASFV proteomes were screened using various computational tools to identify potential epitopes, or peptides capable of triggering an immune response in swine. Proteins from genotypes I and II were prioritized for their involvement in the recent global ASFV outbreaks. The screened epitopes exhibited promising qualities that positioned them as effective components of the ASFV vaccine. They demonstrated antigenicity, immunogenicity, and cytokine-inducing properties indicating their ability to induce potent immune responses. They have strong binding affinities to multiple swine allele receptors suggesting a high likelihood of yielding more amplified responses. Moreover, they were non-allergenic and non-toxic, a crucial prerequisite for ensuring safety and minimizing any potential adverse effects when the vaccine is processed within the host. Integrated with an immunogenic 50S ribosomal protein adjuvant and linkers, the epitopes formed a 364-amino acid multi-epitope subunit vaccine. The ASFV vaccine construct exhibited notable immunogenicity in immune simulation and molecular docking analyses, and stable profiles in secondary and tertiary structure assessments. Moreover, this study designed an optimized codon for efficient translation of the ASFV vaccine construct into the Escherichia coli K-12 expression system using the pET28a(+) vector. Overall, both sequence and structural evaluations suggested the potential of the ASFV vaccine construct as a candidate for controlling and eradicating outbreaks caused by the pathogen.

A vesicular stomatitis virus-based African swine fever vaccine prototype effectively induced robust immune responses in mice following a single-dose immunization

Introduction

African swine fever (ASF) is a highly contagious hemorrhagic fever disease in pigs caused by African swine fever virus (ASFV). It is very difficult to control and prevent ASF outbreaks due to the absence of safe and effective vaccines.

Methods

In order to develop a safe and effective ASF vaccine for the control and prevention of ASF, two ASFV recombinant vesicular stomatitis virus (VSV) live vector vaccine prototypes, containing the gene of p72, and a chimera of p30 and p54, were developed based on the replication-competent VSV, and named VSV-p72 and VSV-p35. The immune potency of VSV-p72 or VSV-p35 alone and in combination was evaluated in BALB/c mice via intramuscular and intranasal vaccination.

Results

The results indicated that whether administered alone or in combination, the two vaccine prototypes showed acceptable safety in mice and, more importantly, induced high-level specific antibodies against p72, p30, and p54 of ASFV and a strong cellular immune response 28 days after vaccination. The sera from mice vaccinated with the vaccine prototypes significantly inhibited ASFV from infecting porcine alveolar macrophages (PAMs) in vitro. Most notably, the immunized sera from a mixture of VSV-p35 and VSV-p72 inhibited ASFV from infecting PAMs, with an inhibition rate of up to 78.58%.

Conclusion

Overall, our findings suggest that ASFV recombinant VSV live vector vaccine prototypes may become a promising candidate vaccine for the control and prevention of ASF.

ASFV epitope mapping by high density peptides microarrays

African swine fever (ASF) is an acute, highly contagious and deadly infectious disease. It is a threat to animal health with major potential economic and societal impact. Despite decades of ASF vaccine research, still some gaps in knowledge are hindering the development of a functional vaccine. Worth mentioning are gaps in understanding the mechanism of ASF infection and immunity, as well as the fact that - in case of this disease - virus proteins, so-called protective antigens, responsible for inducing protective immune responses in pigs are not identified yet. In this paper we elaborate on a methodology to identify protective antigens based on epitope mapping by microarray technology. High density peptide microarrays, combined with fluorescence scanning, have been used to analyze the interaction of peptide sequences of African swine fever virus (ASFV) proteins with antibodies present in inactivated serum from infected and healthy animals. The study evidenced ASFV proteins already under the radar for vaccine development, such as p54, and identified specific sequences in those proteins that may become the focus for future vaccine candidates. Such methodology is amenable to automation and high-throughput and may help developing better targeting for next generation vaccines.

Identification of the p34 Protein of African Swine Fever Virus as a Novel Viral Antigen with Protection Potential

African swine fever (ASF) is a highly contagious disease caused by African swine fever virus (ASFV), affecting domestic and wild boars. The polyprotein pp220 of ASFV is responsible for producing the major structural proteins p150, p37, p14, p34, and p5 via proteolytic processing. The p34 protein is the main component of the ASFV core shell. However, the immunologic properties of the p34 protein in vitro and in vivo remain unclear. The results showed that the recombinant p34 protein expressed in prokaryotes and eukaryotes could react with convalescent swine sera to ASFV, suggesting that p34 is an immunogenic protein. Significantly, anti-p34 antibodies were found to inhibit the replication of ASFV in target cells. Furthermore, rabbits immunized with the recombinant C-strain of classical swine fever virus containing p34 produced both anti-p34 humoral and cellular immune responses. In addition, the p34 protein could induce a cell-mediated immune response, and a T-cell epitope on the p34 protein was identified using immunoinformatics and enzyme-linked immunospot (ELIspot) assay. Our study demonstrates that the p34 protein is a novel antigen of ASFV with protective potential.

Efficient delivery of the immunodominant genes of African swine fever virus by adeno-associated virus serotype 2

Background and Aim

Adeno-associated virus serotype 2 (AAV2) represents a promising basis for developing a virus-vector vaccine against African swine fever (ASF). This study aimed to create genetic constructs based on AAV2 to deliver the immunodominant genes of ASF virus (ASFV) and to evaluate their functionality in vitro. The efficiency and specificity of transgene expression, as well as their non-toxicity in cells of target animals, were evaluated.

Materials and Methods

Bioinformatics analysis methods were used to identify the immunodominant genes of ASFV. The target genes B646L, E183L, CP204L, and CP530R were identified and subsequently cloned into the pAAV-MCS vector. Assembly of recombinant AAV2 (rAAV2) was performed by cotransfection of AAV293 cells with the following plasmids: pAAV-MCS with the gene of interest, envelope, and packaging. Quantitative polymerase chain reaction was used to determine the AAV2 titer. The functionality of the constructs was evaluated in HEK293 and SPEV cells by determining the presence of mature proteins in the cell lysate and the expression levels of messenger RNA. The specificity of the target proteins in cell lysates was confirmed by Western blotting.

Results

The proposed AAV2 assembly protocol makes it possible to achieve a concentration of mature viral particles of at least 280 billion/mL of virus-containing material. The rAAV2 could effectively transduce host SPEV cells. The expression of both cistrons was detectable during the transduction of cells; therefore, the combined expression of immunogens in the cells of target animals should be possible using this method.

Conclusion

This study demonstrated the potential of using genetic constructs based on AAV2 for the delivery of ASFV genes in vitro.

Designing a multi-epitope candidate vaccine by employing immunoinformatics approaches to control African swine fever spread

The African swine fever virus has been circulating for decades and is highly infectious, often fatal to farmed and wild pigs. There is currently no approved vaccine or treatment for the disease, making prevention even more difficult. Therefore, vaccine development is necessary and urgent to limit the consequences of ASF and ensure the food chain and sustainability of the swine industry. This research study was conducted to design a multi-epitope vaccine for controlling veterinary diseases caused by the African swine fever virus. We employed the immunoinformatics approaches to reveal 37 epitopes from different viral proteins of ASFV. These epitopes were linked to adjuvants and linkers to form a full-fledged immunogenic vaccine construct. The tertiary structure of the final vaccine was predicted using a deep-learning approach. The molecular docking and molecular dynamics predicted stable interactions between the vaccine and immune receptor TLR5 of Sus scrofa (Pig). The MD simulation studies reflect that the calculated parameters like RMSD, RMSF, number of hydrogen bonds, and finally, the buried interface surface area for the complex remained stable throughout the simulation time. This analysis suggests the stability of interface interactions between the TLR5 and the multi-epitope vaccine construct. Further, the physiochemical analysis demonstrated that our designed vaccine construct was expected to have high stability and prolonged half-life time in mammalian cells. Traditional vaccine design experiments require significant time and financial input from the development stage to the final product. Studies like this can assist in accelerating vaccine development while minimizing the cost.Communicated by Ramaswamy H. Sarma.

Advanced Strategies for Developing Vaccines and Diagnostic Tools for African Swine Fever

African swine fever (ASF) is one of the most lethal infectious diseases affecting domestic pigs and wild boars of all ages. Over a span of 100 years, ASF has continued to spread over continents and adversely affects the global pig industry. To date, no vaccine or treatment has been approved. The complex genome structure and diverse variants facilitate the immune evasion of the ASF virus (ASFV). Recently, advanced technologies have been used to design various potential vaccine candidates and effective diagnostic tools. This review updates vaccine platforms that are currently being used worldwide, with a focus on genetically modified live attenuated vaccines, including an understanding of their potential efficacy and limitations of safety and stability. Furthermore, advanced ASFV detection technologies are presented that discuss and incorporate the challenges that remain to be addressed for conventional detection methods. We also highlight a nano-bio-based system that enhances sensitivity and specificity. A combination of prophylactic vaccines and point-of-care diagnostics can help effectively control the spread of ASFV.

Bridging the Gap: Can COVID-19 Research Help Combat African Swine Fever?

African swine fever (ASF) is a highly contagious and economically devastating disease affecting domestic pigs and wild boar, caused by African swine fever virus (ASFV). Despite being harmless to humans, ASF poses significant challenges to the swine industry, due to sudden losses and trade restrictions. The ongoing COVID-19 pandemic has spurred an unparalleled global research effort, yielding remarkable advancements across scientific disciplines. In this review, we explore the potential technological spillover from COVID-19 research into ASF. Specifically, we assess the applicability of the diagnostic tools, vaccine development strategies, and biosecurity measures developed for COVID-19 for combating ASF. Additionally, we discuss the lessons learned from the pandemic in terms of surveillance systems and their implications for managing ASF. By bridging the gap between COVID-19 and ASF research, we highlight the potential for interdisciplinary collaboration and technological spillovers in the battle against ASF.

Mucosal and cellular immune responses elicited by nasal and intramuscular inoculation with ASFV candidate immunogens

African swine fever (ASF) is an infectious disease caused by African swine fever virus (ASFV) that is highly contagious and has an extremely high mortality rate (infected by virulent strains) among domestic and wild pigs, causing huge economic losses to the pig industry globally. In this study, SDS-PAGE gel bands hybridized with ASFV whole virus protein combined with ASFV-convalescent and ASFV-positive pig serum were identified by mass spectrometry. Six antigens were detected by positive serum reaction bands, and eight antigens were detected in ASFV-convalescent serum. In combination with previous literature reports and proteins corresponding to MHC-II presenting peptides screened from ASFV-positive pig urine conducted in our lab, seven candidate antigens, including KP177R (p22), K78R (p10), CP204L (p30), E183L (p54), B602L (B602L), EP402R-N (CD2V-N) and F317L (F317L), were selected. Subunit-Group 1 was prepared by mixing above-mentioned seven ASFV recombinant proteins with MONTANIDETM1313 VG N mucosal adjuvant and immunizing pigs intranasally and intramuscularly. Subunit-Group 2 was prepared by mixing four ASFV recombinant proteins (p22, p54, CD2V-N1, B602L) with Montanide ISA 51 VG adjuvant and immunizing pigs by intramuscular injection. Anticoagulated whole blood, serum, and oral fluid were collected during immunization for flow cytometry, serum IgG as well as secretory sIgA antibody secretion, and cytokine expression testing to conduct a comprehensive immunogenicity assessment. Both immunogen groups can effectively stimulate the host to produce ideal humoral, mucosal, and cellular immune responses, providing a theoretical basis for subsequent functional studies, such as immunogens challenge protection and elucidation of the pathogenic mechanism of ASFV.

Epidemiological impacts of attenuated African swine fever virus circulating in wild boar populations

African swine fever virus (ASFV) genotype II has been present in wild boar in the European Union since 2014. Control measures have reduced the incidence of the ASF, but highly virulent as well as attenuated ASFV strains continue to circulate. We present the intraherd epidemiological parameters of low and highly virulent ASFV in wild boar from experimental data, and for the first time, evaluate the impact of attenuated strain circulation through unique deterministic compartmental model simulations under various potential scenarios and hypotheses. Using an estimated PCR infectious threshold of TPCR = 36.4, we obtained several transmission parameters, like an Rx (experimental intraherd R0) value of 4.5. We also introduce two novel epidemiological parameters: infectious power and resistance power, which indicate the ability of animals to transmit the infection and the reduction in infectiousness after successive exposures to varying virulence strains, respectively. The presence of ASFV attenuated strains results in 4-17% of animals either remaining in a carrier state or becoming susceptible again when exposed to highly virulent ASFV for more than two years. The timing between exposures to viruses of different virulence also influences the percentage of animals that die or remain susceptible. The findings of this study can be utilized in epidemiological modelling and provide insight into important risk situations that should be considered for surveillance and future potential ASF vaccination strategies in wild boar.

Immunization of pigs with replication-incompetent adenovirus-vectored African swine fever virus multi-antigens induced humoral immune responses but no protection following contact challenge

Introduction

African swine fever virus (ASFV) is a pathogen of great economic importance given that continues to threaten the pork industry worldwide, but there is no safe vaccine or treatment available. Development of a vaccine is feasible as immunization of pigs with some live attenuated ASFV vaccine candidates can confer protection, but safety concerns and virus scalability are challenges that must to be addressed. Identification of protective ASFV antigens is needed to inform the development of efficacious subunit vaccines.

Methods

In this study, replication-incompetent adenovirus-vectored multicistronic ASFV antigen expression constructs that covered nearly 100% of the ASFV proteome were generated and validated using ASFV convalescent serum. Swine were immunized with a cocktail of the expression constructs, designated Ad5-ASFV, alone or formulated with either Montanide ISA-201™ (ASFV-ISA-201) or BioMize^® adjuvant (ASFV-BioMize).

Results

These constructs primed strong B cell responses as judged by anti-pp62-specific IgG responses. Notably, the Ad5-ASFV and the Ad5-ASFV ISA-201, but not the Ad5-ASFV BioMize^®, immunogens primed significantly (p < 0.0001) higher anti-pp62-specific IgG responses compared with Ad5-Luciferase formulated with Montanide ISA-201™ adjuvant (Luc-ISA-201). The anti-pp62-specific IgG responses underwent significant (p < 0.0001) recall in all the vaccinees after boosting and the induced antibodies strongly recognized ASFV (Georgia 2007/1)-infected primary swine cells. However, following challenge by contact spreaders, only one pig nearly immunized with the Ad5-ASFV cocktail survived. The survivor had no typical clinical symptoms, but had viral loads and lesions consistent with chronic ASF.

Discussion

Besides the limited sample size used, the outcome suggests that in vivo antigen expression, but not the antigen content, might be the limitation of this immunization approach as the replication-incompetent adenovirus does not amplify in vivo to effectively prime and expand protective immunity or directly mimic the gene transcription mechanisms of attenuated ASFV. Addressing the in vivo antigen delivery limitations may yield promising outcomes.

Evaluation of humoral and cellular immune responses induced by a cocktail of recombinant African swine fever virus antigens fused with OprI in domestic pigs

BACKGROUND

African swine fever (ASF) is a highly fatal disease in domestic pigs caused by ASF virus (ASFV), for which there is currently no commercial vaccine available. The genome of ASFV encodes more than 150 proteins, some of which have been included in subunit vaccines but only induce limited protection against ASFV challenge.

METHODS

To enhance immune responses induced by ASFV proteins, we expressed and purified three fusion proteins with each consisting of bacterial lipoprotein OprI, 2 different ASFV proteins/epitopes and a universal CD4⁺ T cell epitope, namely OprI-p30-modified p54-TT, OprI-p72 epitopes-truncated pE248R-TT, and OprI-truncated CD2v-truncated pEP153R-TT. The immunostimulatory activity of these recombinant proteins was first assessed on dendritic cells. Then, humoral and cellular immunity induced by these three OprI-fused proteins cocktail formulated with ISA206 adjuvant (O-Ags-T formulation) were assessed in pigs.

RESULTS

The OprI-fused proteins activated dendritic cells with elevated secretion of proinflammatory cytokines. Furthermore, the O-Ags-T formulation elicited a high level of antigen-specific IgG responses and interferon-γ-secreting CD4⁺ and CD8⁺ T cells after stimulation in vitro. Importantly, the sera and peripheral blood mononuclear cells from pigs vaccinated with the O-Ags-T formulation respectively reduced ASFV infection in vitro by 82.8% and 92.6%.

CONCLUSIONS

Our results suggest that the OprI-fused proteins cocktail formulated with ISA206 adjuvant induces robust ASFV-specific humoral and cellular immune responses in pigs. Our study provides valuable information for the further development of subunit vaccines against ASF.

Molecular Signatures in Swine Innate and Adaptive Immune Responses to African Swine Fever Virus Antigens p30/p54/CD2v Expressed Using a Highly Efficient Semliki Forest Virus Replicon System

African swine fever virus (ASFV) causes a devastating viral hemorrhagic disease in domestic pigs and Eurasian wild boars, posing a foremost threat to the swine industry and pig farming. The development of an effective vaccine is urgently needed, but has been hampered by the lack of an in-depth, mechanistic understanding of the host immune response to ASFV infection and the induction of protective immunity. In this study, we report that immunization of pigs with Semliki Forest Virus (SFV) replicon-based vaccine candidates expressing ASFV p30, p54, and CD2v, as well as their ubiquitin-fused derivatives, elicits T cell differentiation and expansion, promoting specific T cell and humoral immunity. Due to significant variations in the individual non-inbred pigs in response to the vaccination, a personalized analysis was conducted. Using integrated analysis of differentially expressed genes (DEGs), Venn, KEGG and WGCNA, Toll-like receptor, C-type lectin receptor, IL17 receptor, NOD-like receptor and nucleic acid sensor-mediated signaling pathways were demonstrated to be positively correlated to the antigen-stimulated antibody production and inversely correlated to the IFN-γ secreting cell counts in peripheral blood mononuclear cells (PBMCs). An up-regulation of CIQA, CIQB, CIQC, C4BPA, SOSC3, S100A8 and S100A9, and down-regulation of CTLA4, CXCL2, CXCL8, FOS, RGS1, EGR1 and SNAI1 are general in the innate immune response post-the second boost. This study reveals that pattern recognition receptors TLR4, DHX58/DDX58 and ZBP1, and chemokines CXCL2, CXCL8 and CXCL10 may play important roles in regulating this vaccination-stimulated adaptive immune response.

New perspective on African swine fever: a bibliometrics study and visualization analysis

Introduction

African swine fever (ASF) is a contagious viral disease that can have devastating effects on domestic pigs and wild boars. Over the past decade, there has been a new wave of this ancient disease spreading around the world, prompting many scholars to dedicate themselves to researching this disease. This research aims to use bibliometric methods to organize, analyze and summarize the scientific publications on ASF that have been amassed in the past two decades.

Methods

This paper used VOSviewer, CiteSpace, and a bibliometric online analysis platform to conduct performance analysis and visualization studies on 1,885 academic papers about ASF in the Web of Science from January 2003 to December 2022.

Results

The amount of literature published on ASF has increased exponentially in recent years, and the development trend of related research is good. A group of representative scholars have appeared in this research field, and some cooperative networks have been formed. Transboundary and Emerging Diseases is the journal with the most publications in this field, while Virus Research is the journal with the most citation per article. High-productivity countries are led by China in terms of the number of articles published followed by the United States and Spain. In regard to the average number of citations, the scholars in the UK are in the lead. The institution with the most articles was the Chinese Academy of Agricultural Sciences. The analysis of high-frequency keywords showed that the pathogens and epidemiology of ASF were the research hotspots in this field, and the research content was closely related to molecular biology and immunology. The burst keywords "transmission", "identification", "virulence", "replication", and "gene" reflects the research frontier. In addition, by collating and analyzing highly cited journals and highly co-cited references, we explored the knowledge structure and theoretical basis of this field.

Discussion

This is the first bibliometric analysis report on ASF research, which highlights the key characteristics of ASF research and presents the research status and evolution trend in this field from a new perspective. It provides a valuable reference for further research.

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.

Comparison of Genotype II African Swine Fever Virus Strain SY18 Challenge Models

African swine fever (ASF) is a viral haemorrhagic disease found in domestic and wild boars caused by the African swine fever virus (ASFV). A highly virulent strain was used to evaluate the efficacy of newly developed vaccine candidates. The ASFV strain SY18 was isolated from the first ASF case in China and is virulent in pigs of all ages. To evaluate the pathogenesis of ASFV SY18 following intraoral (IO) and intranasal (IN) infections, a challenge trial was conducted in landrace pigs, with intramuscular (IM) injection as a control. The results showed that the incubation period of IN administration with 40–1000 50 % tissue culture infective dose (TCID50) was 5–8 days, which was not significantly different from that of IM inoculation with 200 TCID50. A significantly longer incubation period, 11–15 days, was observed in IO administration with 40–5000 TCID50. Clinical features were similar among all infected animals. Symptoms, including high fever (≥40.5 °C), anorexia, depression, and recumbency, were observed. No significant differences were detected in the duration of viral shedding during fever. There was no significant difference in disease outcome, and all animals succumbed to death. This trial showed that IN and IO infections could be used for the efficacy evaluation of an ASF vaccine. The IO infection model, similar to that of natural infection, is highly recommended, especially for the primary screening of candidate vaccine strains or vaccines with relatively weak immune efficacy, such as live vector vaccines and subunit vaccines.

Orally administered recombinant Lactobacillus expressing African swine fever virus antigens that induced immunity responses

African swine fever (ASF) is a highly contagious, acute, febrile disease caused by the African swine fever virus (ASFV), with morbidity and mortality rates approaching 100% in domestic and wild swine, resulting in massive economic losses to the pig industry worldwide. This study aimed to express the p30, p54, and p72 proteins encoded by ASFV in vitro using the Lactobacillus lactis (L. lactis) expression system. Here, six new functional recombinant L. lactis were constructed, and the expression of the p30 protein, p54 protein, p72 protein, p30-LTB (heat-labile enterotoxin B, LTB) fusion protein, p54-LTB fusion protein, and the p72-LTB fusion protein was successfully detected by Western blot analysis. Following oral immunization of rabbits with recombinant L. lactis, serum IgG, intestinal mucosal sIgA, cytokines (IL-4 and INF-γ), and splenocyte viability were higher than in the control group via ELISA. Notably, without the LTB adjuvant group, humoral and Th1 cellular immunity were promoted, whereas, with the LTB adjuvant group, local mucosal immunity, humoral immunity, and Th2 cellular immunity were promoted, providing new insights into the design and development of an ASFV subunit vaccine.

Vaccines for African swine fever: an update

African swine fever (ASF) is a fatal infectious disease of swine caused by the African swine fever virus (ASFV). Currently, the disease is listed as a legally notifiable disease that must be reported to the World Organization for Animal Health (WOAH). The economic losses to the global pig industry have been insurmountable since the outbreak of ASF. Control and eradication of ASF are very critical during the current pandemic. Vaccination is the optimal strategy to prevent and control the ASF epidemic, but since inactivated ASFV vaccines have poor immune protection and there aren't enough cell lines for efficient in vitro ASFV replication, an ASF vaccine with high immunoprotective potential still remains to be explored. Knowledge of the course of disease evolution, the way of virus transmission, and the breakthrough point of vaccine design will facilitate the development of an ASF vaccine. In this review, the paper aims to highlight the recent advances and breakthroughs in the epidemic and transmission of ASF, virus mutation, and the development of vaccines in recent years, focusing on future directions and trends.

Indirect ELISA Using Multi-Antigenic Dominants of p30, p54 and p72 Recombinant Proteins to Detect Antibodies against African Swine Fever Virus in Pigs

African swine fever (ASF) caused by ASF virus (ASFV) is a fatal disease in pigs and results in great economic losses. Due to the lack of available vaccines and treatments, serological diagnosis of ASF plays a key role in the surveillance program, but due to the lack of knowledge and the complexity of the ASFV genome, the candidate target viral proteins are still being researched. False negativity is still a big obstacle during the diagnostic process. In this study, the high antigenic viral proteins p30, p54 and p72 were screened to find the antigenic dominant domains and the tandem His-p30-54-72 was derived. An indirect enzyme-linked immunosorbent assay (iELISA) coated with His-p30-54-72 was developed with a cut-off value of 0.371. A total of 192 clinical samples were detected by His-p30-54-72-coated indirect ELISA (iELISA) and commercial ASFV antibody kits. The results showed that the positive rate of His-p30-54-72-coated iELISA was increased by 4.7% and 14.6% compared with a single viral protein-based commercial ASFV antibody kits. These results provide a platform for future ASFV clinical diagnosis and vaccine immune effect evaluation.

African Swine Fever Modified Live Vaccine Candidates: Transitioning from Discovery to Product Development through Harmonized Standards and Guidelines

The recent centennial anniversary of R.E. Montgomery's seminal published description of "a form of swine fever" disease transmitted from wild African pigs to European domestic pigs is a call to action to accelerate African Swine Fever (ASF) vaccine research and development. ASF modified live virus (MLV) first-generation gene deleted vaccine candidates currently offer the most promise to meet international and national guidelines and regulatory requirements for veterinary product licensure and market authorization. A major, rate-limiting impediment to the acceleration of current as well as future vaccine candidates into regulatory development is the absence of internationally harmonized standards for assessing vaccine purity, potency, safety, and efficacy. This review summarizes the asymmetrical landscape of peer-reviewed published literature on ASF MLV vaccine approaches and lead candidates, primarily studied to date in the research laboratory in proof-of-concept or early feasibility clinical safety and efficacy studies. Initial recommendations are offered toward eventual consensus of international harmonized guidelines and standards for ASF MLV vaccine purity, potency, safety, and efficacy. To help ensure the successful regulatory development and approval of ASF MLV first generation vaccines by national regulatory associated government agencies, the World Organisation for Animal Health (WOAH) establishment and publication of harmonized international guidelines is paramount.

Cross-protection against African swine fever virus upon intranasal vaccination is associated with an adaptive-innate immune crosstalk

African swine fever virus (ASFV) is causing a worldwide pandemic affecting the porcine industry and leading to important global economic consequences. The virus causes a highly lethal hemorrhagic disease in wild boars and domestic pigs. Lack of effective vaccines hampers the control of virus spread, thus increasing the pressure on the scientific community for urgent solutions. However, knowledge on the immune components associated with protection is very limited. Here we characterized the in vitro recall response induced by immune cells from pigs intranasally vaccinated with the BA71ΔCD2 deletion mutant virus. Vaccination conferred dose-dependent cross-protection associated with both ASFV-specific antibodies and IFNγ-secreting cells. Importantly, bulk and single-cell transcriptomics of blood and lymph node cells from vaccinated pigs revealed a positive feedback from adaptive to innate immunity. Indeed, activation of Th1 and cytotoxic T cells was concomitant with a rapid IFNγ-dependent triggering of an inflammatory response characterized by TNF-producing macrophages, as well as CXCL10-expressing lymphocytes and cross-presenting dendritic cells. Altogether, this study provides a detailed phenotypic characterization of the immune cell subsets involved in cross-protection against ASFV, and highlights key functional immune mechanisms to be considered for the development of an effective ASF vaccine.

African swine fever (ASF) pandemic is currently the number one threat for the porcine industry worldwide. Lack of treatments hampers its control, and the insufficient knowledge regarding the immune effector mechanisms required for protection hinders rational vaccine design. Here we present the first comprehensive study characterizing the complex cellular immune response involved in cross-protection against ASF. We show that, upon in vitro reactivation, cells from immune pigs induce a Th1-biased recall response that in turn enhances the antiviral innate response. Our results suggest that this positive feedback regulation of innate immunity plays a key role in the early control of ASF virus infection. Altogether, this work represents a step forward in the understanding of ASF immunology and provide critical immune components that should be considered to more rationally design future ASF vaccines.

The Presence of Virus Neutralizing Antibodies Is Highly Associated with Protection against Virulent Challenge in Domestic Pigs Immunized with ASFV live Attenuated Vaccine Candidates

African swine fever virus (ASFV) is currently producing a pandemic affecting a large area of Eurasia, and more recently, the Dominican Republic in the Western Hemisphere. ASFV is a large and structurally complex virus with a large dsDNA genome encoding for more than 150 genes. Live attenuated virus strains can induce protection in domestic swine against disease produced by homologous virulent parental viruses. The roles of the different immune mechanisms induced by the attenuated strains in protection still need to be understood. In particular, the role of ASFV neutralizing antibody in protection still is an important controversial issue to be elucidated. Here we present the development of a novel methodology to detect virus neutralizing antibodies based on the reduction of virus infectivity in a Vero cell adapted ASFV strain. The described method was used to assess levels of virus neutralizing antibodies in domestic swine inoculated with live attenuated ASFV. Results demonstrated a high association between the presence of virus neutralizing antibodies and protection in 84 animals immunized with the recombinant vaccine candidates ASFV-G-Δ9GL/ΔUK or ASFV-G-ΔI177L. To our knowledge, this is the first report demonstrating an association between virus neutralizing antibodies and protection against virulent challenge in such a large number of experimental individuals.

Inguinal lymph node sample collected by minimally invasive sampler helps to accurately diagnose ASF in dead pigs without necropsy

African swine fever (ASF) is a highly contagious hemorrhagic and transboundary animal disease, and it threatens global food security. A full necropsy to harvest the sample matrices for diagnosis in the farm may lead to contamination of the premises and directly threaten to the herds. In the present study, we compared the ASFV loads of the common samples that can be collected without necropsy. The unmatched nasal, throat, rectal samples were randomly taken using cotton swabs, and inguinal lymph node samples were collected by the minimally invasive samplers from the dead pigs of an ASF field outbreak farm. The ASFV loads of the samples were detected by qPCR and the results suggested that the overall ASFV nucleic acids levels of inguinal lymph node samples were higher than the swabs. What's more, sets of matched nasal swabs, rectal swabs, throat swabs, inguinal lymph nodes, serums, spleens and lungs samples were collected from 15 dead ASFV naturally infected pigs. Similarly, the results showed that inguinal lymph node samples, together with serum, spleen and lungs samples, contained more ASFV nucleic acids than the swabs. Our findings demonstrated that the inguinal lymph node collected by minimally invasive sampler is an ideal tissue for diagnosing ASFV infection in dead pigs without necropsy.

African Swine Fever Vaccinology: The Biological Challenges from Immunological Perspectives

African swine fever virus (ASFV), a nucleocytoplasmic large DNA virus (NCLDV), causes African swine fever (ASF), an acute hemorrhagic disease with mortality rates up to 100% in domestic pigs. ASF is currently epidemic or endemic in many countries and threatening the global swine industry. Extensive ASF vaccine research has been conducted since the 1920s. Like inactivated viruses of other NCLDVs, such as vaccinia virus, inactivated ASFV vaccine candidates did not induce protective immunity. However, inactivated lumpy skin disease virus (poxvirus) vaccines are protective in cattle. Unlike some experimental poxvirus subunit vaccines that induced protection, ASF subunit vaccine candidates implemented with various platforms containing several ASFV structural genes or proteins failed to protect pigs effectively. Only some live attenuated viruses (LAVs) are able to protect pigs with high degrees of efficacy. There are currently several LAV ASF vaccine candidates. Only one commercial LAV vaccine is approved for use in Vietnam. LAVs, as ASF vaccines, have not yet been widely tested. Reports thus far show that the onset and duration of protection induced by the LAVs are late and short, respectively, compared to LAV vaccines for other diseases. In this review, the biological challenges in the development of ASF vaccines, especially subunit platforms, are discussed from immunological perspectives based on several unusual ASFV characteristics shared with HIV and poxviruses. These characteristics, including multiple distinct infectious virions, extremely high glycosylation and low antigen surface density of envelope proteins, immune evasion, and possible apoptotic mimicry, could pose enormous challenges to the development of ASF vaccines, especially subunit platforms designed to induce humoral immunity.

Host Responses to Live-Attenuated ASFV (HLJ/18-7GD)

African swine fever (ASF) is a highly contagious and fatal disease caused by the African swine fever virus. Recently, the multigene family and CD2v gene-deleted ASF vaccine candidate HLJ/18-7GD was found to be safe and effective in laboratory and clinical trials. However, the immune-protective mechanisms underlying the effects of HLJ/18-7GD remain unclear. We assessed samples from pigs immunized with a single dose of 10⁶ TCID₅₀ HLJ/18-7GD. We found that pigs immunized with HLJ/18-7GD showed high levels of specific antibodies. T lymphocyte subsets (helper T cells (Th); cytotoxic T lymphocytes (CTL); double-positive T cells (DP-T cells)) were temporarily increased in peripheral blood mononuclear cells (PBMCs) after HLJ/18-7GD immunization. Once the HLJ/18-7GD-immunized pigs had been challenged with virulent HLJ/18, the percentage of Th, CTL, and DP-T cells increased significantly. PBMCs extracted from the pigs induced higher levels of CD8⁺ T cells after infection with the HLJ/18 strain in vitro. The levels of GM-CSF, IFN-γ, and TNF-α were upregulated at 7 days post-inoculation; this finding was contrary to the results obtained after HLJ/18 or HLJ/18ΔCD2v infection. The immune protection from HLJ/18-7GD resulted from many synergies, which could provide a theoretical basis for HLJ/18-7GD as a safe and effective ASF vaccine.

Examination of immunogenic properties of recombinant antigens based on p22 protein from African swine fever virus

Introduction

The single member of the Asfarviridae family is African swine fever virus (ASFV). This double-stranded DNA virus infects wild and farmed swine and loses the pig industry large sums of money. An inner envelope, capsid, and outer envelope are parts of the ASFV particle containing structural proteins playing different roles in the process of infection or host immune defence evasion. When expressed by the baculovirus system, the p22 protein from the inner envelope was found to induce partial protection against a virulent virus strain. This study aimed to express a part of this protein in a different system and evaluate its immunogenicity.

Material and Methods

We designed two proteins, the extracellular (C terminal) part of the p22 protein (p22Ct) and its fusion with the heat-labile enterotoxin B subunit from Escherichia coli (LTB-p22Ct), which is supposed to be a potent enhancer of the immune response. Both proteins were produced in the E. coli expression system and subsequently used for mice immunisation to analyse their safety and immunogenicity.

Results

The protein fused with LTB did not show the expected adjuvant properties and did not prove safe, because abscess formation was observed after immunisation. In contrast, immunisation with the p22Ct protein alone induced a higher antibody titre but caused no adverse symptoms.

Conclusion

These results show the high potential of the p22Ct region as an immunogenic protein for ASFV serological detection purposes.

African Swine Fever Virus: A Review

African swine fever (ASF) is a viral disease with a high fatality rate in both domestic pigs and wild boars. ASF has greatly challenged pig-raising countries and also negatively impacted regional and national trade of pork products. To date, ASF has spread throughout Africa, Europe, and Asia. The development of safe and effective ASF vaccines is urgently required for the control of ASF outbreaks. The ASF virus (ASFV), the causative agent of ASF, has a large genome and a complex structure. The functions of nearly half of its viral genes still remain to be explored. Knowledge on the structure and function of ASFV proteins, the mechanism underlying ASFV infection and immunity, and the identification of major immunogenicity genes will contribute to the development of an ASF vaccine. In this context, this paper reviews the available knowledge on the structure, replication, protein function, virulence genes, immune evasion, inactivation, vaccines, control, and diagnosis of ASFV.

Analysis of gene expression in monocytes of immunized pigs after infection with homologous or heterologous African swine fever virus

African swine fever is a deadly disease of pigs caused by the large DNA virus (ASFV). Despite intensive research, little is known about the molecular mechanisms of ASFV pathogenesis. Transcriptome analysis of host and viral genes in infected macrophages revealed changes in expression of genes involved in various biological processes, including immune response, inflammatory response and apoptosis. To understand the mechanisms of virus pathogenesis, we used transcriptome analysis to identify the differences in gene expression between peripheral blood monocytes (PBMCs) isolated from pigs immunized with attenuated Congo ASFV strain (KK262), and then infected in vitro with virulent homologous Congo strain (K49) or heterologous Mozambique strain (M78). We found that overexpression of IFN-γ was detected only in cells infected with M78, although the expression of interferon-stimulated genes was increased in both types of cells. In addition, up-regulation of pro-inflammatory cytokines and chemokines was found in PBMCs infected with the heterologous strain M78, in contrast to the cells infected with K49. These data may indicate the beginning of an early immune response in cells infected with a heterologous, but not homologous strain. Transcriptome analysis revealed down-regulation of genes involved in endocytosis and phagocytosis in cells infected with the K49 strain, but not in PBMCs infected with M78. On the contrary, we detected activation of endoplasmic reticulum stress response genes in cells infected with a homologous strain, but not in cells infected with a heterologous strain. This study is the first attempt to determine the differences in the response to ASF infection between homologous and heterologous strains at the cellular level. Our results showed that not only genes of the immune response, but also genes involved in endocytosis and cellular stress response may be important for the formation of cross-protective immunity. This data may be useful for vaccine development or testing of candidate vaccines.

African Swine Fever Control and Prevention: An Update on Vaccine Development

African swine fever (ASF) is a lethal and highly contagious viral disease of domestic and wild pigs, listed as a notifiable disease reported to the World Organization for Animal Health (OIE). Despite its limited host range and absent zoonotic potential, the socio-economic and environmental impact of ASF is very high, representing a serious threat to the global swine industry and the many stakeholders involved. Currently, only control and eradication measures based mainly on early detection and strict stamping-out policies are available, however, the rapid spread of the disease in new countries, and in new regions in countries already affected, show these strategies to be lacking. In this review, we discuss approaches to ASF vaccinology, with emphasis on the advances made over the last decade, including the development of virulence-associated gene deleted strains such as the very promising ASFV-G-ΔI177L/ΔLVR, that replicates efficiently in a stable porcine epithelial cell line, and the cross-protecting BA71ΔCD2 capable of stably growing in the commercial COS-1 cell line, or the naturally attenuated Lv17/WB/Rie1 which shows solid protection in wild boar. We also consider the key constraints involved in the scale-up and commercialization of promising live attenuated and virus-vectored vaccine candidates, namely cross-protection, safety, lack of suitable animal models, compatibility with wildlife immunization, availability of established and licensed cell lines, and differentiating infected from vaccinated animals (DIVA) strategy.

Impact of human behavior on the spread of African swine fever virus: what every veterinarian should know

African swine fever virus was first identified and characterized in Africa in the early 1900s, but it has spread exponentially in Europe, Asia, and the Caribbean since 2018. While it is a disease that exclusively affects swine, thus posing no infectious risk to human health, the virus's resiliency and human behavior have facilitated the rapid global dissemination of the virus over the past 4 years. In this Currents in One Health, we will review its epidemiology, viral characteristics, host range, and current prevention strategies; the current perspective on what a response would look like and who would be affected; and if the virus was ever found in the US. Due to the fact that the virus affects all breeds of Sus scrofa, including those used for food and companionship, it is vital for all veterinarians to work together to keep the virus out of the US. It is only through the collaborative efforts of multiple disciplines working locally, nationally, and globally that we can contain the spread of this virus.

[Problems of specific prevention of African swine fever]

This review presents the current state of the problem of development and application of the specific prevention of African swine fever (ASF) with a brief description of its etiology and pathogenesis. The unique nature of the ASF virus (ASFV) determines some limitations and the complexity of solving the problem of vaccine development. Such situation stimulated the development of highly specific diagnostic methods for rapid and accurate detection of the ASFV. In this regard, results of studies, including our own, concerning the comparative analysis of the genome of vaccine and virulent strains of the ASFV, as well as immunodiagnostic approaches to determine causes of high virulence and low protective activity of the ASFV, are briefly presented. Special attention is given to the issue related to the development of safe and effective vaccines against ASF. In this context disadvantages and possible advantages of live attenuated (LAV) and recombinant (RV) vaccines are considered in details. Results of recent studies on the assessment of the immunogenicity of genetically modified vaccines (GMV) which developed in various laboratories around the world are presented. The obtained data indicate that ASF vaccination is currently the most promising measure to stop the spread of this disease in our country and in the world, however, previous experience with ASF vaccination has revealed some problems in its development and application. The significant contribution of foreign researchers to the study of the basics of virulence of this pathogen and the study of its genes functions are noted. The possible further expansion of ASF in Europe and Asia in bordering Russia territories, as well as the established fact of the persistence of ASFV in wild boar population indicate a constant threat of its re-introduction into our country. In conclusion, the importance of developing a safe effective vaccine against ASF and the assessing of the possible risks of creating the artificial sources of the infection in nature as a result of its use is emphasized.

Structural Design and Assessing of Recombinantly Expressed African Swine Fever Virus p72 Trimer in Saccharomyces cerevisiae

In an effort to control the outbreak of the African Swine Fever Virus (ASFV), there is an urgent need to develop an effective method to prevent the pandemic, including vaccines and diagnostic methods. The major capsid protein of ASFV p72 (B646L), which forms a trimer with each monomer adopting a double jelly roll fold, is the main component of the virus particle and major antigen of ASFV. Thus, the p72 protein may be considered an antigen candidate for vaccine and diagnostic development. However, the development of ASFV p72 trimer for the industry application, including veterinary usage, faces unavoidable challenges: firstly, the low cost of the antigen production is required in vaccine and diagnostic application; and, secondly, whether produced antigen folds in its native conformation. Here, based on the information provided by the atomic structure of p72, we have successfully performed rational mutagenesis on p72 trimers and expressed it in Saccharomyces cerevisiae with high yields. The cryo-EM structure of recombinant expressed p72 trimer is determined at 4.18 Å in resolution. The correlation coefficient between this structure and the ASFV virus structure is 0.77, suggesting a highly similar fold of this trimer with the native protein on the virus particle.

Serum Neutralizing and Enhancing Effects on African Swine Fever Virus Infectivity in Adherent Pig PBMC

African swine fever virus (ASFV) causes hemorrhagic fever with mortality rates of up to 100% in domestic pigs. Currently, there are no commercial vaccines for the disease. Only some live-attenuated viruses have been able to protect pigs from ASFV infection. The immune mechanisms involved in the protection are unclear. Immune sera can neutralize ASFV but incompletely. The mechanisms involved are not fully understood. Currently, there is no standardized protocol for ASFV neutralization assays. In this study, a flow cytometry-based ASFV neutralization assay was developed and tested in pig adherent PBMC using a virulent ASFV containing a fluorescent protein gene as a substrate for neutralization. As with previous studies, the percentage of infected macrophages was approximately five time higher than that of infected monocytes, and nearly all infected cells displayed no staining with anti-CD16 antibodies. Sera from naïve pigs and pigs immunized with a live-attenuated ASFV and fully protected against parental virus were used in the assay. The sera displayed incomplete neutralization with MOI-dependent neutralizing efficacies. Extracellular, but not intracellular, virions suspended in naïve serum were more infectious than those in the culture medium, as reported for some enveloped viruses, suggesting a novel mechanism of ASFV infection in macrophages. Both the intracellular and extracellular virions could not be completely neutralized.

Adenovirus-Vectored African Swine Fever Virus pp220 Induces Robust Antibody, IFN-γ, and CTL Responses in Pigs

African Swine Fever Virus (ASFV) poses a serious threat to the pork industry worldwide; however, there is no safe vaccine or treatment available. The development of an efficacious subunit vaccine will require the identification of protective antigens. The ASFV pp220 polyprotein is essential for virus structural integrity. This polyprotein is processed to generate p5, p34, p14, p37, and p150 individual proteins. Immunization of pigs with a cocktail of adenoviruses expressing the proteins induced significant IgG, IFN-γ-secreting cells, and cytotoxic T lymphocyte responses. Four predicted SLA-I binding nonamer peptides, namely p34161−169, p37859−867, p1501363−1371, and p1501463−1471, recalled strong IFN-γ+ PBMC and splenocyte responses. Notably, peptide p34161−169 was recognized by PBMCs isolated from 7/10 pigs and by splenocytes isolated from 8/10 pigs. Peptides p37859−867 and p1501363−1371 stimulated recall IFN-γ+ responses in PBMCs and splenocytes isolated from 8/10 pigs, whereas peptide p1501463−1471 recalled responses in PBMCs and splenocytes isolated from 7/10 to 9/10 pigs, respectively. The results demonstrate that the pp220 polyprotein contains multiple epitopes that induce robust immune responses in pigs. Importantly, these epitopes are 100% conserved among different ASFV genotypes and were predicted to bind multiple SLA-I alleles. The outcomes suggest that pp220 is a promising candidate for inclusion in a prototype subunit vaccine.

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.

ASF -survivors’ sera do not inhibit African swine fever virus replication in vitro

Introduction

African swine fever virus (ASFV) causes one of the most dangerous diseases of pigs and wild boar – African swine fever (ASF). Since its second introduction into Europe (in 2007), the disease has been spreading consistently, and now ASF-free European countries are at risk. Complex interactions between the host’s immune system and the virus have long prevented the development of a safe vaccine against ASF. This study analysed the possibility of neutralisation of the ASFV in vitro by sera collected from ASF-survivor animals.

Material and Methods

Two pig and three wild boar serum samples were collected from previously selected potential ASF survivors. All sera presented high antibody titres (>5 log₁₀/mL). Primary alveolar macrophages were cultured in growth medium containing 10% and 20% concentrations of selected sera and infected with a haemadsorbing ASFV strain (Pol18_28298_O111, genotype II). The progress of infection was investigated under a light microscope by observing the cytopathic effect (CPE) and the haemadsorption phenomenon. Growth kinetics were investigated using a real-time PCR assay.

Results

Haemadsorption inhibition was detected in the presence of almost all selected sera; however, the inhibition of virus replication in vitro was excluded. In all samples, a CPE and decreasing quantification cycle values of the viral DNA were found.

Conclusion

Anti-ASFV antibodies alone are not able to inhibit virus replication. Interactions between the humoral and cellular immune response which effectively combat the disease are implicated in an ASF-survivor’s organism.

Identification of Linear B Cell Epitopes on CD2V Protein of African Swine Fever Virus by Monoclonal Antibodies

The CD2-like (CD2V) protein is a crucial antigen of African swine fever virus (ASFV). CD2V interacts with the cellular AP-1 protein, participates in intracellular transport of virus, and induces neutralizing antibodies to partly protect swine from virus attack. In this study, a specific CD2V dimeric protein was designed to enhance antigenicity and immunogenicity, expressed in a Bac-to-Bac baculovirus expression vector system and purified by Ni-affinity chromatography. After animal immunization, five monoclonal antibodies (mAbs) (7E12, 22B3, 18A3, 13G11, and 43C2) against CD2V were developed. The variable regions of heavy chains and light chains of the mAbs were sequenced to prove that the five mAbs differed from one another. The mAbs of CD2V could combine with ASFV by immunoperoxidase monolayer assay (IPMA). B cell epitopes of CD2V were screened using the five mAbs by indirect enzyme-linked immunosorbent assay (ELISA) and Dot-ELISA. Therefore, three B cell epitopes (¹⁴⁷FVKYT¹⁵¹, ¹⁵⁷EYNWN¹⁶¹, and ¹⁹⁵SSNY¹⁹⁸) were identified. This is the first time that mAbs of the ASFV CD2V protein have been developed and the sequencing of heavy chains and light chains of mAbs has been completed. Linear B cell epitopes, which were core targets of immunoprotection of the CD2V protein, were identified by mAbs for the first time. This study provides efficient epitopes for the development of ASFV subunit vaccines.

IMPORTANCE The ASFV CD2V protein is a crucial antigen on the outer envelopes of virus particles. A modified ASFV CD2V dimeric protein was expressed in the Bac-to-Bac baculovirus expression vector system. Five monoclonal antibodies (mAbs) against CD2V were developed, sequenced, and applied to identify CD2V protein B cell epitopes. Three B cell epitopes, ¹⁴⁷FVKYT¹⁵¹, ¹⁵⁷EYNWN¹⁶¹, and ¹⁹⁵SSNY¹⁹⁸, were identified. This is the first time CD2V mAbs have been developed, the sequencing of heavy chains and light chains of CD2V mAbs have been completed, and CD2V B cell epitopes have been identified by using scanning peptide method and bioinformatics methods.

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.

Immune Escape Mechanism and Vaccine Research Progress of African Swine Fever Virus

African swine fever virus (ASFV) is the causative agent of the epidemic of African swine fever (ASF), with virulent strains having a mortality rate of up to 100% and presenting devastating impacts on animal farming. Since ASF was first reported in China in 2018, ASFV still exists and poses a potential threat to the current pig industry. Low-virulence and genotype I strains of ASFV have been reported in China, and the prevention and control of ASF is more complicated. Insufficient understanding of the interaction of ASFV with the host immune system hinders vaccine development. Physical barriers, nonspecific immune response and acquired immunity are the three barriers of the host against infection. To escape the innate immune response, ASFV invades monocytes/macrophages and dendritic cells, thereby inhibiting IFN expression, regulating cytokine expression and the body’s inflammatory response process. Meanwhile, in order to evade the adaptive immune response, ASFV inhibits antigen presentation, induces the production of non-neutralizing antibodies, and inhibits apoptosis. Recently, significant advances have been achieved in vaccine development around the world. Live attenuated vaccines (LAVs) based on artificially deleting specific virulence genes can achieve 100% homologous protection and partial heterologous protection. The key of subunit vaccines is identifying the combination of antigens that can effectively provide protection and selecting carriers that can effectively deliver the antigens. In this review, we introduce the epidemic trend of ASF and the impact on the pig industry, analyze the interaction mechanism between ASFV and the body’s immune system, and compare the current status of potential vaccines in order to provide a reference for the development of effective ASF vaccines.