African swine fever virus (ASFV) protection mediated by NH/P68 and NH/P68 recombinant live-attenuated viruses

The attenuated African swine fever vaccine HLJ/18-7GD provides protection against emerging prevalent genotype II variants in China

Genetic changes have occurred in the genomes of prevalent African swine fever viruses (ASFVs) in the field in China, which may change their antigenic properties and result in immune escape. There is usually poor cross-protection between heterogonous isolates, and, therefore, it is important to test the cross-protection of the live attenuated ASFV vaccines against current prevalent heterogonous isolates. In this study, we evaluated the protective efficacy of the ASFV vaccine candidate HLJ/18-7GD against emerging isolates. HLJ/18-7GD provided protection against a highly virulent variant and a lower lethal isolate, both derived from genotype II Georgia07-like ASFV and isolated in 2020. HLJ/18-7GD vaccination prevented pigs from developing ASF-specific clinical signs and death, decreased viral shedding via the oral and rectal routes, and suppressed viral replication after challenges. However, HLJ/18-7GD vaccination did not provide solid cross-protection against genotype I NH/P68-like ASFV challenge in pigs. HLJ/18-7GD vaccination thus shows great promise as an alternative strategy for preventing and controlling genotype II ASFVs, but vaccines providing cross-protection against different ASFV genotypes may be needed in China.

Cryo-EM structure of DNA polymerase of African swine fever virus

African swine fever virus (ASFV) is one of the most important causative agents of animal diseases and can cause highly fatal diseases in swine. ASFV DNA polymerase (DNAPol) is responsible for genome replication and highly conserved in all viral genotypes showing an ideal target for drug development. Here, we systematically determined the structures of ASFV DNAPol in apo, replicating and editing states. Structural analysis revealed that ASFV DNAPol had a classical right-handed structure and showed the highest similarity to the structure of human polymerase delta. Intriguingly, ASFV DNAPol has a much longer fingers subdomain, and the thumb and palm subdomain form a unique interaction that has never been seen. Mutagenesis work revealed that the loss of this unique interaction decreased the enzymatic activity. We also found that the β-hairpin of ASFV DNAPol is located below the template strand in the editing state, which is different from the editing structures of other known B family DNAPols with the β-hairpin above the template strand. It suggests that B family DNAPols have evolved two ways to facilitate the dsDNA unwinding during the transition from replicating into editing state. These findings figured out the working mechanism of ASFV DNAPol and will provide a critical structural basis for the development of antiviral drugs.

African Swine Fever Virus Immunosuppression and Virulence-Related Gene

African swine fever virus (ASFV), a highly contagious pathogen characterized by a complex structure and a variety of immunosuppression proteins, causes hemorrhagic, acute, and aggressive infectious disease that severely injures the pork products and industry. However, there is no effective vaccine or treatment. The main reasons are not only the complex mechanisms that lead to immunosuppression but also the unknown functions of various proteins. This review summarizes the interaction between ASFV and the host immune system, along with the involvement of virulence-related genes and proteins, as well as the corresponding molecular mechanism of immunosuppression of ASFV, encompassing pathways such as cGAS-STING, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), Janus Kinase (JAK) and JAK Signal Transducers and Activators of Transcription (STAT), apoptosis, and other modulation. The aim is to summarize the dynamic process during ASFV infection and entry into the host cell, provide a rational insight into development of a vaccine, and provide a better clear knowledge of how ASFV impacts the host.

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.

A Comprehensive Review of Our Understanding and Challenges of Viral Vaccines against Swine Pathogens

Pig farming has become a strategically significant and economically important industry across the globe. It is also a potentially vulnerable sector due to challenges posed by transboundary diseases in which viral infections are at the forefront. Among the porcine viral diseases, African swine fever, classical swine fever, foot and mouth disease, porcine reproductive and respiratory syndrome, pseudorabies, swine influenza, and transmissible gastroenteritis are some of the diseases that cause substantial economic losses in the pig industry. It is a well-established fact that vaccination is undoubtedly the most effective strategy to control viral infections in animals. From the period of Jenner and Pasteur to the recent new-generation technology era, the development of vaccines has contributed significantly to reducing the burden of viral infections on animals and humans. Inactivated and modified live viral vaccines provide partial protection against key pathogens. However, there is a need to improve these vaccines to address emerging infections more comprehensively and ensure their safety. The recent reports on new-generation vaccines against swine viruses like DNA, viral-vector-based replicon, chimeric, peptide, plant-made, virus-like particle, and nanoparticle-based vaccines are very encouraging. The current review gathers comprehensive information on the available vaccines and the future perspectives on porcine viral vaccines.

Administration Routes and Doses of the Attenuated African Swine Fever Virus Strain PSA-1NH Influence Cross-Protection of Pigs against Heterologous Challenge

African swine fever (ASF) is a lethal hemorrhagic disease of Suidae, i.e., domestic pigs and wild boars, caused by African swine fever virus (ASFV). The development of cross-protective vaccines against ASF is imperative for effective disease control, particularly in regions where ASF is endemic, potentially featuring multiple circulating ASFV isolates. The investigation of non-hemadsorbing naturally attenuated isolates and laboratory recombinant strains with a deletion in the EP402R gene has attracted interest. Our study aimed to assess the impacts of various administration routes and doses of the naturally attenuated ASFV-PSA-1NH (immunotype IV, genotype I) isolate on the manifestation of clinical signs of ASF and the level of protection against the heterologous ASFV-Stavropol 01/08 strain (seroimmunotype VIII, genotype II). The results demonstrated that the intranasal administration of a low dose of ASFV-PSA-1NH to pigs minimized the clinical signs of ASF and established a high level of protection against the heterologous strain ASFV-Stavropol 01/08. Despite the challenges in standardizing the dosage for intranasal administration, this approach appears as a viable alternative in ASF vaccination.

Deletion of MGF505-2R Gene Activates the cGAS-STING Pathway Leading to Attenuation and Protection against Virulent African Swine Fever Virus

African swine fever virus (ASFV) is the etiological agent causing African swine fever (ASF), affecting domestic pigs and wild boar, which is currently the biggest animal epidemic in the world and a major threat to the swine sector. At present, some safety concerns about using LAVs against ASFV still exist despite a commercial vaccine licensed in Vietnam. Therefore, the efforts to identify virulence factors and their mechanisms, as well as to generate new vaccine prototypes, are of major interest. In this work, we have identified the MGF505-2R gene product as an inhibitor of the cGAS/STING pathway, specifically through its interaction with STING protein, controlling IFN-β production. In addition, immunization of a recombinant virus lacking this gene, Arm/07-ΔMGF505-2R, resulted in complete attenuation, demonstrating its involvement in ASFV virulence. Finally, immunization with Arm/07-ΔMGF505-2R induced the generation of antibodies and proved to be partially protective against virulent ASFV strains. These results identify MGF505-2R, as well as its mechanism of action, as a gene contributing to understanding the molecular mechanisms of ASFV virulence, which will be of great value in the design of future vaccine prototypes.

Recent progress and major gaps in the vaccine development for African swine fever

The swine industry across the globe is recently facing a devastating situation imparted by a highly contagious and deadly viral disease, African swine fever. The disease is caused by a DNA virus, the African swine fever virus (ASFV) of the genus Asfivirus. ASFV affects both wild boars and domestic pigs resulting in an acute form of hemorrhagic fever. Since the first report in 1921, the disease remains endemic in some of the African countries. However, the recent occurrence of ASF outbreaks in Asia led to a fresh and formidable challenge to the global swine production industry. Culling of the infected animals along with the implementation of strict sanitary measures remains the only options to control this devastating disease. Efforts to develop an effective and safe vaccine against ASF began as early as in the mid-1960s. Different approaches have been employed for the development of effective ASF vaccines including inactivated vaccines, subunit vaccines, DNA vaccines, virus-vectored vaccines, and live attenuated vaccines (LAVs). Inactivated vaccines are a non-feasible strategy against ASF due to their inability to generate a complete cellular immune response. However genetically engineered vaccines, such as subunit vaccines, DNA vaccines, and virus vector vaccines, represent tailored approaches with minimal adverse effects and enhanced safety profiles. As per the available data, gene deleted LAVs appear to be the most potential vaccine candidates. Currently, a gene deleted LAV (ASFV-G-∆I177L), developed in Vietnam, stands as the sole commercially available vaccine against ASF. The major barrier to the goal of developing an effective vaccine is the critical gaps in the knowledge of ASFV biology and the immune response induced by ASFV infection. The precise contribution of various hosts, vectors, and environmental factors in the virus transmission must also be investigated in depth to unravel the disease epidemiology. In this review, we mainly focus on the recent progress in vaccine development against ASF and the major gaps associated with it.

Identification of L11L and L7L as virulence-related genes in the African swine fever virus genome

Introduction

African swine fever (ASF) is an infectious disease that causes considerable economic losses in pig farming. The agent of this disease, African swine fever virus (ASFV), is a double-stranded DNA virus with a capsid membrane and a genome that is 170-194 kb in length encoding over 150 proteins. In recent years, several live attenuated strains of ASFV have been studied as vaccine candidates, including the SY18ΔL7-11. This strain features deletion of L7L, L8L, L9R, L10L and L11L genes and was found to exhibit significantly reduced pathogenicity in pigs, suggesting that these five genes play key roles in virulence.

Methods

Here, we constructed and evaluated the virulence of ASFV mutations with SY18ΔL7, SY18ΔL8, SY18ΔL9, SY18ΔL10, and SY18ΔL11L.

Results

Our findings did not reveal any significant differences in replication efficiency between the single-gene deletion strains and the parental strains. Pigs inoculated with SY18ΔL8L, SY18ΔL9R and SY18ΔL10L exhibited clinical signs similar to those inoculated with the parental strains. Survival rate of pigs inoculated with 103.0TCID50 of SY18ΔL7L was 25%, while all pigs inoculated with 103.0TCID50 of SY18ΔL11L survived, and 50% inoculated with 106.0TCID50 SY18ΔL11L survived.

Discussion

The results indicate that L8L, L9R and L10L do not affect ASFV SY18 virulence, while the L7L and L11L are associated with virulence.

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.

The Production of Recombinant African Swine Fever Virus Lv17/WB/Rie1 Strains and Their In Vitro and In Vivo Characterizations

Lv17/WB/Rie1-Δ24 was produced via illegitimate recombination mediated by low-dilution serial passage in the Cos7 cell line and isolated on PAM cell culture. The virus contains a huge ~26.4 Kb deletion in the left end of its genome. Lv17/WB/Rie1-ΔCD-ΔGL was generated via homologous recombination, crossing two ASFV strains (Lv17/WB/Rie1-ΔCD and Lv17/WB/Rie1-ΔGL containing eGFP and mCherry markers) during PAM co-infection. The presence of unique parental markers in the Lv17/WB/Rie1-ΔCD-ΔGL genome indicates at least two recombination events during the crossing, suggesting that homologous recombination is a relatively frequent event in the ASFV genome during replication in PAM. Pigs infected with Lv17/WB/Rie1-Δ24 and Lv17/WB/Rie1/ΔCD-ΔGL strains have shown mild clinical signs despite that ASFV could not be detected in their sera until a challenge infection with the Armenia/07 ASFV strain. The two viruses were not able to induce protective immunity in pigs against a virulent Armenia/07 challenge.

Thermostable T Cell Multiepitope Nanoparticle Antigens Inducing Potent Immune Responses against the Swine Fever Virus

African swine fever (ASF) is caused by the African swine fever virus (ASFV) and is a highly contagious, acute, febrile disease that has high morbidity and mortality rates in domestic and wild swine. However, a safe and effective vaccine against ASF remains unavailable as single antigens fail to provide sufficient protection. Therefore, a combination of multiple antigens with an efficient delivery system might be an alternative strategy. Herein, a de novo-designed antigen with multiple T-cell epitopes (TEPs) of ASFV was conjugated for surface display on self-assembled nanoparticles (NPs) of Aquifex aeolicus lumazine synthase (AaLS) and Quasibacillus thermotolerans encapsulin (QT) through the SpyCatcher/SpyTag system to construct nanovaccines (TEP-Spy-NPs). TEP-Spy-NPs exhibited significantly more thermal, storage, and freeze-thaw stability in comparison to TEP monomers. TEP-Spy-NPs were highly immunogenic and induced strong polyclonal antibody responses in mice and pigs. The specific antibody titers against the TEP of the TEP-Spy-AaLS and TEP-Spy-QT groups were significantly higher than those of the TEP monomer immune group after the second booster immunization. The antibody titer against TEP of the TEP-Spy-QT group was approximately twice that of the TEP-Spy-AaLS group in mice. ELISpot analysis demonstrated that more IFN-γ- and IL-2-secreting splenic lymphocytes were produced by TEP-Spy-AaLS- and TEP-Spy-QT-immunized mice than by TEP monomer-immunized mice. TEP-Spy-NPs elicited stronger cellular immunity and in vivo immunity in immunized pigs than did TEP monomers. Thus, the TEP nanovaccine successfully induced strong humoral and cellular immune responses in mice and pigs, and TEP-Spy-NPs have the potential as candidate vaccines for ASFV.

Signal peptide and N-glycosylation of N-terminal-CD2v determine the hemadsorption of African swine fever virus

IMPORTANCE

African swine fever virus (ASFV) is the cause of the current major animal epidemic worldwide. This disease affects domestic pigs and wild boars, has spread since 2007 through Russia, Eastern Europe, and more recently to Western European countries, and since 2018 emerged in China, from where it spread throughout Southeast Asia. Recently, outbreaks have appeared in the Caribbean, threatening the Americas. It is estimated that more than 900,000 animals have died directly or indirectly from ASFV since 2021 alone. One of the features of ASFV infection is hemoadsorption (HAD), which has been linked to virulence, although the molecular and pathological basis of this hypothesis remains largely unknown. In this study, we have analyzed and identified the key players responsible of HAD, contributing to the identification of new determinants of ASFV virulence, the understanding of ASFV pathogenesis, and the rational development of new vaccines.

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.

Capsid-Specific Antibody Responses of Domestic Pigs Immunized with Low-Virulent African Swine Fever Virus

African swine fever (ASF) is a lethal disease in pigs that has grave socio-economic implications worldwide. For the development of vaccines against the African swine fever virus (ASFV), immunogenic antigens that generate protective immune responses need to be identified. There are over 150 viral proteins-many of which are uncharacterized-and humoral immunity to ASFV has not been closely examined. To profile antigen-specific antibody responses, we developed luciferase-linked antibody capture assays (LACAs) for a panel of ASFV capsid proteins and screened sera from inbred and outbred animals that were previously immunized with low-virulent ASFV before challenge with virulent ASFV. Antibodies to B646L/p72, D117L/p17, M1249L, and E120R/p14.5 were detected in this study; however, we were unable to detect B438L-specific antibodies. Anti-B646L/p72 and B602L antibodies were associated with recovery from disease after challenges with genotype I OUR T88/1 but not genotype II Georgia 2007/1. Antibody responses against M1249L and E120R/p14.5 were observed in animals with reduced clinical signs and viremia. Here, we present LACAs as a tool for the targeted profiling of antigen-specific antibody responses to inform vaccine development.

Low transmission risk of African swine fever virus between wild boar infected by an attenuated isolate and susceptible domestic pigs

African swine fever (ASF) is a lethal infectious disease that affects domestic and wild pigs. This complex virus has already affected five continents and more than 70 countries and is considered to be the main threat to the global swine industry. The disease can potentially be transmitted directly through contact with infectious animals, or indirectly by means of contaminated feed or environments. Nevertheless, the knowledge regarding the transmission patterns of different ASF virus isolates at the wildlife-livestock interface is still limited. We have, therefore, assessed the potential transmission of an attenuated ASF virus isolate between infectious wild boar and directly exposed domestic pig. We registered 3,369 interspecific interactions between animals, which were brief and mostly initiated by wild boar. The major patterns observed during the study were head-to-head contact owing to sniffing, thus suggesting a high probability of pathogen transmission. However, only one of the five domestic pigs had a short period of viremia and became serologically positive for ASF virus antibodies. It was additionally discovered that the wild boar did not transmit the virulent virus isolate to the domestic pigs, which suggests that the presence of attenuated ASF virus isolates in affected areas may control the spreading of other more virulent isolates. These outcomes may help make decisions related to large-scale targeted management actions against ASF in field conditions.

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.

Deletion of the CD2 Gene in the Virulent ASFV Congo Strain Affects Viremia in Domestic Swine, but Not the Virulence

African swine fever (ASF) is an infectious disease that causes the most significant losses to the pig industry. One of the effective methods for combating this disease could be the development of vaccines. To date, experimental vaccines based on the use of live attenuated strains of the ASF virus (ASFV) obtained by the deletion of viral genes responsible for virulence are the most effective. Deletion of the EP402R gene encoding a CD2-like protein led to the attenuation of various strains of the ASFV, although the degree of attenuation varies among different isolates. Here we have shown that the deletion of the EP402R gene from the genome of a high-virulent Congo isolate did not change either the virulence of the virus or its ability to replicate in the swine macrophage cell cultures in vitro. However, in vivo, animals infected with ΔCongo-v_CD2v had a delay in the onset of the disease and viremia compared to animals infected with the parental strain. Thus, deletion of the CD2 gene in different isolates of the ASFV has a different effect on the virulence of the virus, depending on its genetic background.

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

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

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.

Involvement of the MGF 110-11L Gene in the African Swine Fever Replication and Virulence

African swine fever (ASF) is a highly lethal hemorrhagic viral disease that causes extensive economic and animal welfare losses in the Eurasian pig (Sus scrofa) population. To date, no effective and safe vaccines have been marketed against ASF. A starting point for vaccine development is using naturally occurring attenuated strains as a vaccine base. Here, we aimed to remove the multigene family (MGF) 110 gene of unknown function from the Lv17/WB/Rie1 genome to improve the usability of the virus as a live-attenuated vaccine, reducing unwanted side effects. The MGF 110-11L gene was deleted using the CRISPR/Cas9 method, and the safety and efficacy of the virus were tested in pigs after isolation. The vaccine candidates administered at high doses showed reduced pathogenicity compared to the parental strain and induced immunity in vaccinated animals, although several mild clinical signs were observed. Although Lv17/WB/Rie1/d110-11L cannot be used as a vaccine in its current form, it was encouraging that the undesirable side effects of Lv17/WB/Rie1 at high doses can be reduced by additional mutations without a significant reduction in its protective capacity.

African swine fever virus I73R is a critical virulence-related gene: A potential target for attenuation

African swine fever virus (ASFV) is a large, double-stranded DNA virus that causes a fatal disease in pigs, posing a threat to the global pig industry. Whereas some ASFV proteins have been found to play important roles in ASFV-host interaction, the functional roles of many proteins are still largely unknown. In this study, we identified I73R, an early viral gene in the replication cycle of ASFV, as a key virulence factor. Our findings demonstrate that pI73R suppresses the host innate immune response by broadly inhibiting the synthesis of host proteins, including antiviral proteins. Crystallization and structural characterization results suggest that pI73R is a nucleic-acid-binding protein containing a Zα domain. It localizes in the nucleus and inhibits host protein synthesis by suppressing the nuclear export of cellular messenger RNA (mRNAs). While pI73R promotes viral replication, the deletion of the gene showed that it is a nonessential gene for virus replication. In vivo safety and immunogenicity evaluation results demonstrate that the deletion mutant ASFV-GZΔI73R is completely nonpathogenic and provides effective protection to pigs against wild-type ASFV. These results reveal I73R as a virulence-related gene critical for ASFV pathogenesis and suggest that it is a potential target for virus attenuation. Accordingly, the deletion mutant ASFV-GZΔI73R can be a potent live-attenuated vaccine candidate.

ASFV proteins presented at the surface of T7 phages induce strong antibody responses in mice

African swine fever virus (ASFV) infection causes substantial economic losses to the swine industry worldwide, and there are still no safe and effective vaccines or therapeutics available. The granulated virus antigen improves the antigen present process and elicits high antibody reaction than the subunit antigen. In this study, the SpyTag peptide-p10 fusion protein was altered and displayed on the surface of the T7 phage to construct an engineered phage (T7-ST). At the same time, ASFV antigen-Spycatcher C-terminal-fused protein (antigen-SC) was expressed and purified by an E. coli prokaryotic expression system. Five virus-like particles (VLPs) displaying the main ASFV antigenic proteins P30, P54, P72, CD2v, and K145R were reconstructed by the isopeptide bond between SpyTag and antigen-SC proteins. The stability of five ASFV VLPs in high temperature and extreme pH conditions was evaluated by transmission electron microscopy (TEM) and plaque analysis. All ASFV VLPs induced a high titer antigen-specific antibody response in mice. Our results showed that the granulated antigen displaying ASFV protein on the surface of the T7 phage provides a robust potential vaccine and diagnostic tool to address the challenge of the ASFV pandemic.

Inactivation Performance of Pseudorabies Virus as African Swine Fever Virus Surrogate by Four Commercialized Disinfectants

This study was based on similar physicochemical characteristics of pseudorabies virus (PRV) and African swine fever virus (ASFV). A cellular model for evaluation of disinfectants was established with PRV as an alternative marker strain. In the present study, we evaluated the disinfection performance of commonly used commercialized disinfectants on PRV to provide a reference for the selection of good ASFV disinfectants. In addition, the disinfection (anti-virus) performances for four disinfectants were investigated based on the minimum effective concentration, onset time, action time, and operating temperature. Our results demonstrated that glutaraldehyde decamethylammonium bromide solution, peracetic acid solution, sodium dichloroisocyanurate, and povidone-iodine solution effectively inactivated PRV at concentrations 0.1, 0.5, 0.5, and 2.5 g/L on different time points 30, 5, 10, and 10 min, respectively. Specifically, peracetic acid exhibits optimized overall performance. Glutaraldehyde decamethylammonium bromide is cost effective but requires a long action time and the disinfectant activity is severely affected by low temperatures. Furthermore, povidone-iodine rapidly inactivates the virus and is not affected by environmental temperature, but its application is limited by a poor dilution ratio such as for local disinfection of the skin. This study provides a reference for the selection of disinfectants for ASFV.

I329L: A Dual Action Viral Antagonist of TLR Activation Encoded by the African Swine Fever Virus (ASFV)

The African Swine Fever Virus (ASFV) is an economically important, large DNA virus which causes a highly contagious and frequently fatal disease in domestic pigs. Due to the acute nature of the infection and the complexity of the protective porcine anti-ASFV response, there is no accepted vaccine in use. As resistance to ASFV is known to correlate with a robust IFN response, the virus is predicted to have evolved strategies to inhibit innate immunity by modulating the IFN response. The deletion of virus host evasion gene(s) inhibiting IFN is a logical solution to develop an attenuated virus vaccine. One such candidate, the ASFV ORF I329L gene, is highly conserved in pathogenic and non-pathogenic virus isolates and in this study we confirm and extend the conclusion that it has evolved for the inhibition of innate immunity initiated through Toll-like receptors (TLRs). Specifically, the ASFV I329L extracellular (ECD) and intracellular (ICD) domains inhibit TLR signalling by two entirely different mechanisms. Bioinformatics modelling suggests that the ECD inhibits several TLR signalling pathways through a short sequence homologous to the conserved TLR dimerization domain, here termed the putative dimerization domain (PDD). Remarkably, both full length and PDD constructs of I329L were demonstrated to inhibit activation, not only of TLR3, but also TLR4, TLR5, TLR8 and TLR9. Additionally, the demonstration of a weak association of I329L with TLR3 is consistent with the formation of a non-signalling I329L-TLR3 heterodimer, perhaps mediated through the PDD of I329L. Finally, the ICD associates with TRIF, thereby impacting on both TLR3 and TLR4 signalling. Thus, I329L offers potential as a general inhibitor of TLR responses and is a rational candidate for construction and testing of an I329L deletion mutant 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.

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.

Recombinant African Swine Fever Virus Arm/07/CBM/c2 Lacking CD2v and A238L Is Attenuated and Protects Pigs against Virulent Korean Paju Strain

African swine fever (ASF) is an obligated declaration swine disease, provoking farm isolation measures and the closing of affected country boarders. ASF virus (ASFV) is currently the cause of a pandemic across China and Eurasia. By the end of 2019, ASF was detected in nine EU Member States: Bulgaria, Romania, Slovakia, Estonia, Hungary, Latvia, Lithuania, Poland and Belgium. The affected area of the EU extended progressively, moving mostly in a southwestern direction (EFSA). Inactivated and/or subunit vaccines have proven to fail since certain virus replication is needed for protection. LAVs are thus the most realistic option, which must be safe, effective and industrially scalable. We here generated a vaccine prototype from the Arm/07/CBM/c2 genotype II strain, in which we have deleted the EP402R (CD2v) and A238L genes by CRISPR/Cas9 in COS-1 cells, without detectable further genetic changes. The successful immunization of pigs has proven this vaccine to be safe and fully protective against the circulating Korean Paju genotype II strain, opening the possibility of a new vaccine on the market in the near future.

Review of the Pig-Adapted African Swine Fever Viruses in and Outside Africa

The region in eastern, central and southern Africa (ECSA) where African swine fever (ASF) originated in a sylvatic cycle is home to all the p72 genotypes of ASF virus identified so far. While 20 of the 24 genotypes have been isolated from outbreaks in domestic pigs in the region, only five of the genotypes (I, II, VIII, IX, X) have an extended field presence associated with domestic pigs. Of the genotypes that appear to be strongly adapted to domestic pigs, two have spread beyond the African continent and have been the focus of efforts to develop vaccines against ASF. Most of the experimental ASF vaccines described do not protect against a wider spectrum of viruses and may be less useful in the event of incursions of different strains or where multiple genotypes co-exist. The other three pig-adapted strains that are currently restricted to the ECSA region might spread, and priority should be given to understanding not only the genetic and antigenic characteristics of these viruses but also their history. We review historic and current knowledge of the distribution of these five virus genotypes, and note that as was the case for genotype II, some pig-associated viruses have the propensity for geographical range expansion. These features are valuable for prioritizing vaccine-development efforts to ensure a swift response to virus escape. However, whilst ASF vaccines are critical for high-production systems, global food security relies on parallel efforts to improve biosecurity and pig production in Africa and on continued ASFV surveillance and characterisation in the ECSA region.

Deletion of the CD2v Gene from the Genome of ASFV-Kenya-IX-1033 Partially Reduces Virulence and Induces Protection in Pigs

Infection of pigs with the African swine fever virus (ASFV) leads to a devastating hemorrhagic disease with a high mortality of up to 100%. In this study, a CD2v gene deletion was introduced to a genotype IX virus from East Africa, ASFV-Kenya-IX-1033 (ASFV-Kenya-IX-1033-∆CD2v), to investigate whether this deletion led to reduced virulence in domestic pigs and to see if inoculation with this LA-ASFV could induce protective immunity against parental virus challenge. All pigs inoculated with ASFV-Kenya-IX-1033-ΔCD2v survived inoculation but presented with fever, reduced appetite and lethargy. ASFV genomic copies were detected in only one animal at one time point. Seven out of eight animals survived subsequent challenge with the pathogenic parental strain (87.5%) but had mild to moderate clinical symptoms and had a gross pathology compatible with chronic ASFV infection. All mock-immunised animals developed acute ASF upon challenge with ASFV-Kenya-IX-1033 and were euthanised upon meeting the humane endpoint criteria. ASFV genome copy numbers after challenge were similar in the two groups. ASFV-Kenya-IX-1033-∆CD2v is therefore a useful tool to investigate the development of immunity to ASFV genotype IX, but safety concerns preclude its use as a candidate vaccine without further attenuation.

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.

Non-Invasive Sampling in the Aspect of African Swine Fever Detection-A Risk to Accurate Diagnosis

African swine fever remains one of the most economically important and dangerous diseases of the Suidae family. Until now, neither a safe vaccine nor a treatment against ASF has been available, which is why prevention of the disease involves biosecurity measures and early recognition based on accurate diagnosis. Nowadays, different strategies for ASF detection are discussed to reduce both animal suffering and the costs of ASF surveillance. This article aims to indicate the risk, with regard to non-invasive sampling, for the detection of ASFV. In this study, we analyzed data from three independent animal trials, in the framework of the detection of positive samples in different matrices (blood, sera, oral and rectal swabs) collected from nineteen domestic pigs infected with similar doses but under different scenarios, including different ASFV strains or routes of infection. Genetic material of ASFV was found in all matrices, but detection occurred earlier in the blood samples than in the oral and the rectal swabs. Furthermore, analyses revealed that at relevant sampling timepoints, PCR-positive blood samples were detected more frequently and reached higher percentages (up to 100% during fever) than oral and rectal swabs. Moreover, mean Ct values in blood samples collected from animals infected with virulent strains were significantly lower than in oral and rectal swabs, ensuring a higher probability of ASFV detection. High Ct values and occasional shedding in all tested matrices, in the cases of animals infected by an attenuated ASFV-strain, showed that blood sampling may be necessary to confirm the presence of anti-ASFV antibodies in sera. This study showed that during veterinary surveillance, blood sampling (for both PCR and serological analyses) is essential for the accurate diagnosis of ASF and provides the highest probability of detection of the disease.

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.

The attenuated ASFV strains MK-200 and FK-32/135 as possible models for investigation of protective immunity by ASFV infection

African swine fever (ASF) is an infectious disease of domestic and wild pigs of all breeds and ages, with the acute form of the disease being characterized by high fever, hemorrhages in the reticuloendothelial system and a high mortality rate. Registered safe and efficacious ASF vaccines are not available. The development of experimental ASF vaccines, particularly live attenuated, have considerably intensified in the last years. There is much variability in experimental approaches undertaken by laboratories attempting to develop first generation vaccines, rendering it difficult to interpret and make comparisons across trials. ASF virus (ASFV) genotyping does not fully correlate with available cross-protection data and may be of limited value in predicting cross-protective vaccine efficacy. Recently, ASFV strains were assigned to a respective nine groups by seroimmunotype (from I to IX): in vivo the grouping is based on results of cross protection of pigs survived after their infection with a virulent strain (bioassay), while in vitro this grouping is based on hemadsorption inhibition assay (HADIA) data. Here we demonstrate the antigenic and protective properties of two attenuated ASFV strains MK200 and FK-32/135. Pronounced differences in the HADIA and in immunological test in animals allow us to consider them and the corresponding reference virulent strains of the ASFV of Mozambique-78 (seroimmunotype III, genotype V) and France-32 (seroimmunotype IV, genotype I) as useful models for studying the mechanisms of protective immunity and evaluation of the candidate vaccines.

Vaccination With a Gamma Irradiation-Inactivated African Swine Fever Virus Is Safe But Does Not Protect Against a Challenge

African swine fever (ASF) is among the most devastating viral diseases of pigs and wild boar worldwide. In recent years, the disease has spread alarmingly. Despite intensive research activities, a commercialized vaccine is still not available, and efficacious live attenuated vaccine candidates raise safety concerns. From a safety perspective, inactivated preparations would be most favourable. However, both historical and more recent trials with chemical inactivation did not show an appreciable protective effect. Under the assumption that the integrity of viral particles could enhance presentation of antigens, we used gamma irradiation for inactivation. To this means, gamma irradiated ASFV “Estonia 2014” was adjuvanted with either Polygen™ or Montanide™ ISA 201 VG, respectively. Subsequently, five weaner pigs per preparation were immunized twice with a three-week interval. Six weeks after the first immunization, all animals were challenged with the highly virulent ASFV strain “Armenia 2008”. Although ASFV p72-specific IgG antibodies were detectable in all vaccinated animals prior challenge, no protection could be observed. All animals developed an acute lethal course of ASF and had to be euthanized at a moderate humane endpoint within six days. Indeed, the vaccinated pigs showed even higher clinical scores and a higher inner body temperature than the control group. However, significantly lower viral loads were detectable in spleen and liver of immunized animals at the time point of euthanasia. This phenomenon suggests an immune mediated disease enhancement that needs further investigation.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

HIGHLIGHTS

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

Cell Lines for the Development of African Swine Fever Virus Vaccine Candidates: An Update

African swine fever virus (ASFV) is the etiological agent of a highly lethal disease in both domestic and wild pigs. The virus has rapidly spread worldwide and has no available licensed vaccine. An obstacle to the construction of a safe and efficient vaccine is the lack of a suitable cell line for ASFV isolation and propagation. Macrophages are the main targets for ASFV, and they have been widely used to study virus-host interactions; nevertheless, obtaining these cells is time-consuming and expensive, and they are not ethically suitable for the production of large-scale vaccines. To overcome these issues, different virulent field isolates have been adapted on monkey or human continuous cells lines; however, several culture passages often lead to significant genetic modifications and the loss of immunogenicity of the adapted strain. Thus, several groups have attempted to establish a porcine cell line able to sustain ASFV growth. Preliminary data suggested that some porcine continuous cell lines might be an alternative to primary macrophages for ASFV research and for large-scale vaccine production, although further studies are still needed. In this review, we summarize the research to investigate the most suitable cell line for ASFV isolation and propagation.

Recombinant ASF Live Attenuated Virus Strains as Experimental Vaccine Candidates

African swine fever (ASF) is causing a pandemic affecting swine in a large geographical area of the Eastern Hemisphere, from Central Europe to East and Southeast Asia, and recently in the Americas, the Dominican Republic and Haiti. The etiological agent, ASF virus (ASFV), infects both domestic and wild swine and produces a variety of clinical presentations depending on the virus strain and the genetics of the pigs infected. No commercial vaccines are currently available, although experimental recombinant live attenuated vaccine candidates have been shown to be efficacious in protecting animals against disease when challenged with homologous virulent strains. This review attempts to systematically provide an overview of all the live attenuated strains that have been shown to be experimental vaccine candidates. Moreover, it aims to analyze the development of these vaccine candidates, obtained by deleting specific genes or group of genes, and their efficacy in preventing virus infection and clinical disease after being challenged with virulent isolates. This report summarizes all the experimental vaccine strains that have shown promise against the contemporary pandemic strain of African swine fever.

An Assessment of Diagnostic Assays and Sample Types in the Detection of an Attenuated Genotype 5 African Swine Fever Virus in European Pigs over a 3-Month Period

African swine fever virus causes hemorrhagic disease in swine. Attenuated strains are reported in Africa, Europe, and Asia. Few studies on the diagnostic detection of attenuated ASF viruses are available. Two groups of pigs were inoculated with an attenuated ASFV. Group 2 was also vaccinated with an attenuated porcine reproductive and respiratory syndrome virus vaccine. Commercially available ELISA, as well as extraction and qPCR assays, were used to detect antibodies in serum and oral fluids (OF) and nucleic acid in buccal swabs, tonsillar scrapings, OF, and blood samples collected over 93 days, respectively. After 12 dpi, serum (88.9% to 90.9%) in Group 1 was significantly better for antibody detection than OF (0.7% to 68.4%). Group 1′s overall qPCR detection was highest in blood (48.7%) and OF (44.2%), with the highest detection in blood (85.2%) from 8 to 21 days post inoculation (dpi) and in OF (83.3%) from 1 to 7 dpi. Group 2′s results were not significantly different from Group 1, but detection rates were lower overall. Early detection of attenuated ASFV variants requires active surveillance in apparently healthy animals and is only reliable at the herd level. Likewise, antibody testing will be needed to prove freedom from disease.

Regulation of antiviral immune response by African swine fever virus (ASFV)

African swine fever (ASF) is a highly contagious and acute hemorrhagic viral disease with a high mortality approaching 100% in domestic pigs. ASF is an endemic in countries in sub-Saharan Africa. Now, it has been spreading to many countries, especially in Asia and Europe. Due to the fact that there is no commercial vaccine available for ASF to provide sustainable prevention, the disease has spread rapidly worldwide and caused great economic losses in swine industry. The knowledge gap of ASF virus (ASFV) pathogenesis and immune evasion is the main factor to limit the development of safe and effective ASF vaccines. Here, we will summarize the molecular mechanisms of how ASFV interferes with the host innate and adaptive immune responses. An in-depth understanding of ASFV immune evasion strategies will provide us with rational design of ASF vaccines.

Transcriptome Profiling Reveals Features of Immune Response and Metabolism of Acutely Infected, Dead and Asymptomatic Infection of African Swine Fever Virus in Pigs

African swine fever virus (ASFV) infection can result in lethal disease in pigs. ASFV encodes 150-167 proteins, of which only approximately 50 encoded viral structure proteins are functionally known. ASFV also encodes some nonstructural proteins that are involved in the regulation of viral transcription, viral replication and evasion from host defense. However, the understanding of the molecular correlates of the severity of these infections is still limited. The purpose of this study was to compare host and viral gene expression differences and perform functional analysis in acutely infected, dead and cohabiting asymptomatic pigs infected with ASFV by using RNA-Seq technique; healthy pigs were used as controls. A total of 3,760 and 2,874 upregulated genes and 4,176 and 2,899 downregulated genes were found in healthy pigs vs. acutely infected, dead pigs or asymptomatic pigs, respectively. Additionally, 941 upregulated genes and 956 downregulated genes were identified in asymptomatic vs. acutely infected, dead pigs. Different alternative splicing (AS) events were also analyzed, as were gene chromosome locations, and protein-protein interaction (PPI) network prediction analysis was performed for significantly differentially expressed genes (DEGs). In addition, 30 DEGs were validated by RT-qPCR, and the results were consistent with the RNA-Seq results. We further analyzed the interaction between ASFV and its host at the molecular level and predicted the mechanisms responsible for asymptomatic pigs based on the selected DEGs. Interestingly, we found that some viral genes in cohabiting asymptomatic pigs might integrate into host genes (DP96R, I73R and L83L) or remain in the tissues of cohabiting asymptomatic pigs. In conclusion, the data obtained in the present study provide new evidence for further elucidating ASFV-host interactions and the ASFV infection mechanism and will facilitate the implementation of integrated strategies for controlling ASF spread.

Analyses of the Impact of Immunosuppressive Cytokines on Porcine Macrophage Responses and Susceptibility to Infection to African Swine Fever Viruses

African swine fever viruses (ASFV), currently a serious threat to the global pig industry, primarily target porcine macrophages. Macrophages are characterized by their remarkable plasticity, being able to modify their phenotype and functions in response to diverse stimuli. Since IL-10 and TGF-β polarize macrophages toward an anti-inflammatory phenotype, we analyzed their impact on porcine monocyte-derived macrophages’ (moMΦ) susceptibility to infection and their responses to two genotype I ASFV strains, virulent 26544/OG10 and attenuated NH/P68. At a low multiplicity of infection (MOI), NH/P68, but not 26544/OG10, presented a higher ability to infect moM(IL-10) compared to moMΦ and moM(TGF-β), but no differences were appreciated at a higher MOI. Both strains replicated efficiently in all moMΦ subsets, with no differences at later times post-infection. Both strains downregulated CD14 and CD16 expression on moMΦ, irrespective of the activation status. ASFV’s modulation of CD163 and MHC II DR expression and cytokine responses to NH/P68 or 26544/OG10 ASFV were not affected by either IL-10 or TGF-β pre-treatment. Our results revealed little impact of these anti-inflammatory cytokines on moMΦ interaction with ASFV, which likely reflects the ability of the virus to effectively modulate macrophage responses.

Performance Characteristics of Real-Time PCRs for African Swine Fever Virus Genome Detection—Comparison of Twelve Kits to an OIE-Recommended Method

African swine fever (ASF) is a major threat to pig production, and real-time PCR (qPCR) protocols are an integral part of ASF laboratory diagnosis. With the pandemic spread of ASF, commercial kits have risen on the market. In Germany, the kits have to go through an approval process and thus, general validation can be assumed. However, they have never been compared to each other. In this study, 12 commercial PCR kits were compared to an OIE-recommended method. Samples representing different matrices, genome loads, and genotypes were included in a panel that was tested under diagnostic conditions. The comparison included user-friendliness, internal controls, and the time required. All qPCRs were able to detect ASFV genome in different matrices across all genotypes and disease courses. With one exception, there were no significant differences when comparing the overall mean. The overall specificity was 100% (95% CI 87.66–100), and the sensitivity was between 95% and 100% (95% CI 91.11–100). As can be expected, variability concerned samples with low genome load. To conclude, all tests were fit for purpose. The test system can therefore be chosen based on compatibility and prioritization of the internal control system.

Development of a one-step multiplex qRT–PCR assay for the detection of African swine fever virus, classical swine fever virus and atypical porcine pestivirus

Background

African swine fever virus (ASFV), classical swine fever virus (CSFV) and atypical porcine pestivirus (APPV) have caused great economic losses to the swine industry in China. Since coinfections of ASFV, CSFV and APPV occur in certain pig herds, it is necessary to accurately and differentially detect these pathogens in field-collected samples. In this study, a one-step multiplex real-time quantitative reverse transcription-polymerase chain reaction (multiplex qRT–PCR) was developed for the simultaneous and differential detection of ASFV, CSFV and APPV.

Results

The one-step multiplex qRT–PCR presented here was able to simultaneously detect ASFV, CSFV and APPV but could not amplify other viruses, including porcine circovirus type 2 (PCV2), pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV), foot-and-mouth disease virus (FMDV), porcine parvovirus (PPV), porcine epidemic diarrhoea virus (PEDV), transmissible gastroenteritis virus (TGEV), porcine rotavirus (PRoV), porcine deltacoronavirus (PDCoV), border disease virus (BDV), bovine viral diarrhoea virus type 1 (BVDV-1), BVDV-2, etc. The limit of detection (LOD) of the assay was 2.52 × 10¹ copies/μL for ASFV, CSFV and APPV. A repeatability test using standard recombinant plasmids showed that the intra- and interassay coefficients of variation (CVs) were less than 2%. An assay of 509 clinical samples collected in Guangxi Province, southern China, from October 2018 to December 2020 showed that the positive rates of ASFV, CSFV and APPV were 45.58, 12.57 and 3.54%, respectively, while the coinfection rates of ASFV and CSFV, ASFV and APPV, CSFV and APPV were 4.91, 1.38, 0.98%, respectively. Phylogenetic analysis based on the nucleotide sequences of the partial ASFV p72 gene showed that all ASFV strains from Guangxi Province belonged to genotypes I and II.

Conclusion

A one-step multiplex qRT–PCR with high specificity, sensitivity and repeatability was successfully developed for the simultaneous and differential detection of ASFV, CSFV and APPV.

Role of African Swine Fever Virus Proteins EP153R and EP402R in Reducing Viral Persistence in Blood and Virulence in Pigs Infected with BeninΔDP148R

The limited knowledge on the role of many of the approximately 170 proteins encoded by African swine fever virus restricts progress toward vaccine development. Previously, the DP148R gene was deleted from the genome of genotype I virulent Benin 97/1 isolate. This virus, BeninΔDP148R, induced transient moderate clinical signs after immunization and high levels of protection against challenge. However, the BeninΔDP148R virus and genome persisted in blood over a prolonged period. In the current study, deletion of either EP402R or EP153R genes individually or in combination from BeninΔDP148R genome was shown not to reduce virus replication in macrophages in vitro. However, deletion of EP402R dramatically reduced the period of infectious virus persistence in blood in immunized pigs from 28 to 14 days and virus genome from 59 to 14 days while maintaining high levels of protection against challenge. The additional deletion of EP153R (BeninΔDP148RΔEP153RΔEP402R) further attenuated the virus, and no viremia or clinical signs were observed postimmunization. This was associated with decreased protection and detection of moderate levels of challenge virus in blood. Interestingly, the deletion of EP153R alone from BeninΔDP148R did not result in further virus attenuation and did not reduce the period of virus persistence in blood. These results show that EP402R and EP153R have a synergistic role in reducing clinical signs and levels of virus in blood. IMPORTANCE African swine fever virus (ASFV) causes a disease of domestic pigs and wild boar which results in death of almost all infected animals. The disease has a high economic impact, and no vaccine is available. We investigated the role of two ASFV proteins, called EP402R and EP153R, in determining the levels and length of time virus persists in blood from infected pigs. EP402R causes ASFV particles and infected cells to bind to red blood cells. Deletion of the EP402R gene dramatically reduced virus persistence in blood but did not reduce the level of virus. Deletion of the EP153R gene alone did not reduce the period or level of virus persistence in blood. However, deleting both EP153R and EP402R resulted in undetectable levels of virus in blood and no clinical signs showing that the proteins act synergistically. Importantly, the infected pigs were protected following infection with the wild-type virus that kills pigs.

Safety of African Swine Fever Vaccine Candidate Lv17/WB/Rie1 in Wild Boar: Overdose and Repeated Doses

African swine fever (ASF) is a highly lethal infectious disease that affects domestic pigs and wild boar. Outbreaks of ASF have grown considerably in the last decade causing important economic consequences for the swine industry. Its control is hampered by the lack of an effective treatment or vaccine. In Europe, the wild boar is a key wild reservoir for ASF. The results of the oral vaccination trial of wild boar with Lv17/WB/Rie1 are hope for this problem. However, this vaccine candidate has certain safety concerns, since it is a naturally attenuated vaccine. Therefore, the current study aims to evaluate the safety of this vaccine candidate in terms of overdose (high dose) and repeated doses (revaccination) in wild boar. Low-dose orally vaccinated animals developed only a slight transient fever after vaccination and revaccination. This was also the case for most of the high-dose vaccinated wild boar, except for one of them which succumbed after revaccination. Although this fatality was related to hierarchical fights between animals, we consider that further studies are required for clarification. Considering these new results and the current epidemiological situation of ASF in wild boar, this vaccine prototype is a promising tool for the control of the disease in these wild populations, although further studies are needed.

The A179L Gene of African Swine Fever Virus Suppresses Virus-Induced Apoptosis but Enhances Necroptosis

A179L, a non-structural protein of African swine fever virus (ASFV), is capable of suppressing apoptosis by binding the BH3 domain of the pro-apoptotic Bcl-2 family proteins via a conserved ligand binding groove. Our present study aims to determine if A179L affects necroptosis, the second form of programmed cell death induced by DNA and RNA viruses. Here we report that A179L enhanced TNF-α or TSZ (TNF-α, Smac, and Z-Vad)-induced receptor-interacting protein kinase (RIPK1), RIPK3, and mixed lineage kinase domain like peudokinase (MLKL) phosphorylation in L929 cells, a murine fibrosarcoma cell line. Sytox green staining revealed that A179L significantly increased the number of necroptotic cells in TSZ-treated L929 cells. Using human herpes simplex virus 1 (HSV-1) to model DNA virus-induced cell death, we found that A179L blocked the HSV-1-induced cleavage of poly (ADP-ribose) polymerase (PARP), caspase 8, and caspase 3 and decreased the number of apoptotic cells in HSV-1-infected IPEC-DQ cells, a porcine intestinal epithelial cell line. In contrast, A179L transfection of IPEC-DQ cells enhanced HSV-1-induced RIPK1, RIPK3, and MLKL phosphorylation and increased the number of necroptotic cells. Consistently, A179L also suppressed apoptosis but enhanced the necroptosis induced by two RNA viruses, Sendai virus (SeV) and influenza virus (IAV). Our study uncovers a previously unrecognized role of A179L in regulating cell death and suggests that A179L re-directs its anti-apoptotic activity to necroptosis.

Evaluation of Cellular Immunity with ASFV Infection by Swine Leukocyte Antigen (SLA)-Peptide Tetramers

African swine fever virus (ASFV) causes acute hemorrhagic fever in domestic pigs and wild boars, resulting in incalculable economic losses to the pig industry. As the mechanism of viral infection is not clear, protective antigens have not been discovered or identified. In this study, we determined that the p30, pp62, p72, and CD2v proteins were all involved in the T cell immune response of live pigs infected with ASFV, among which p72 and pp62 proteins were the strongest. Panoramic scanning was performed on T cell epitopes of the p72 protein, and three high-frequency positive epitopes were selected to construct a swine leukocyte antigen (SLA)-tetramer, and ASFV-specific T cells were detected. Subsequently, the specific T cell and humoral immune responses of ASFV-infected pigs and surviving pigs were compared. The results demonstrate that the specific T cellular immunity responses gradually increased during the infection and were higher than that in the surviving pigs in the late stages of infection. The same trend was observed in specific humoral immune responses, which were highest in surviving pigs. In general, our study provides key information for the exploration of ASFV-specific immune responses and the development of an ASFV vaccine.