An African swine fever virus virulence-associated gene NL-S with similarity to the herpes simplex virus ICP34.5 gene

Inhibition of STING-mediated type I IFN signaling by African swine fever virus DP71L

African swine fever virus (ASFV) is nucleocytoplasmic large DNA arbovirus and encodes many proteins involved in the interaction with host molecules to evade antiviral immune responses. Especially, evasion strategies of type I interferon (IFN-I)-mediated immune responses are crucial for early ASFV replication. However, there is still a lack of information regarding the immune evasion mechanism of ASFV proteins. Here, we demonstrated that ASFV DP71L suppresses STING-mediated antiviral responses. The conserved phosphatase 1 (PP1) motif of DP71L specifically interact with the C-terminal tail (CTT) of STING and in particular, amino acids P371, L374, and R375 of STING were important for interaction with DP71L. Consequently, this interaction disrupted the binding between STING and TANK-binding kinase 1 (TBK1), thereby inhibiting downstream signaling including phosphorylation of TBK1, STING and IRF3 for antiviral signaling. DP71L significantly interfered with viral DNA induced interferon production and IFN-mediated downstream signaling in vitro. Consistently, knockdown of DP71L enhanced antiviral gene expression in ASFV-infected cells. Taken together, these results highlight the important role of DP71L with respect to inhibition of interferon responses and provide guidance for a better understanding of ASFV pathogenesis and the development of live attenuated ASFV vaccines.

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.

Attenuated African swine fever viruses and the live vaccine candidates: a comprehensive review

The African swine fever virus (ASFV) is spreading worldwide and causing huge economic losses to the global pig industry. The ASFV genome is 170-193 kb in length, contains approximately 150 open reading frames, and encodes more than 200 proteins, most of which have unknown functions. Owing to the unique viral structure, replication strategy, large number of genes of unknown function, and complicated pathogenesis, vaccine development research is challenging. Several naturally attenuated ASFV isolates have been extensively investigated and many genetically manipulated, gene-deleted, and cell-adapted ASFVs have been reported. Currently, live attenuated viruses prepared from weakly virulent strains are an efficient method to provide effective protection in vaccinated pigs; however, these have seldom been widely approved for vaccine use, except in Vietnam. Herein, we summarize the attenuated isolates or vaccine candidates for live vaccines derived from different sources, including naturally mutated, attenuated, cell-adapted, and genetically modified recombinant ASFVs. This will help to understand the gene function and immunogenicity of attenuated live ASFV, as well as the shortcomings of these viruses as vaccine candidates, and provide clues to prepare live, efficient, and safe vaccines for African swine fever.IMPORTANCEOutbreaks of African swine fever (ASF) have caused devastating losses to the global pig industry. Pigs immunized with ASFV attenuated virus can resist the lethal challenge of a strongly virulent virus. Here, we summarize the virulence of naturally mutated, cell-adapted, and genetically recombinant ASFV for pigs, and the protective effect after facing an attack challenge. We also analyze the advantages and disadvantages of ASFV attenuated viruses as vaccine candidates to provide clues for the preparation of efficient and safe live African swine fever vaccines.

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.

Function investigation of p11.5 in ASFV infection

Virus replication relies on complex interactions between viral proteins. In the case of African swine fever virus (ASFV), only a few such interactions have been identified so far. In this study, we demonstrate that ASFV protein p72 interacts with p11.5 using co-immunoprecipitation and liquid chromatography-mass spectrometry (LC-MS). It was found that protein p72 interacts specifically with p11.5 ​at sites amino acids (aa) 1-216 of p72 and aa 1-68 of p11.5. To assess the importance of p11.5 in ASFV infection, we developed a recombinant virus (ASFVGZΔA137R) by deleting the A137R gene from the ASFVGZ genome. Compared with ASFVGZ, the infectious progeny virus titers of ASFVGZΔA137R were reduced by approximately 1.0 logs. In addition, we demonstrated that the growth defect was partially attributable to a higher genome copies-to-infectious virus titer ratios produced in ASFVGZΔA137R-infected MA104 ​cells than in those infected with ASFVGZ. This finding suggests that MA104 ​cells infected with ASFVGZΔA137R may generate larger quantities of noninfectious particles. Importantly, we found that p11.5 did not affect virus-cell binding or endocytosis. Collectively, we show for the first time the interaction between ASFV p72 and p11.5. Our results effectively provide the relevant information of the p11.5 protein. These results extend our understanding of complex interactions between viral proteins, paving the way for further studies of the potential mechanisms and pathogenesis of ASFV infection.

Recent progress on gene-deleted live-attenuated African swine fever virus vaccines

African Swine Fever (ASF) is a highly lethal viral disease in swine, with mortality rates approaching 100%. The disease has spread to many swine-producing countries, leading to significant economic losses and adversely impacting global food security. Extensive efforts have been directed toward developing effective ASF vaccines. Among the vaccinology approaches tested to date, live-attenuated virus (LAV) vaccines produced by rational deleting virulence genes from virulent African Swine Fever Virus (ASFV) strains have demonstrated promising safety and efficacy in experimental and field conditions. Many gene-deleted LAV vaccine candidates have been generated in recent years. The virulence genes targeted for deletion from the genome of virulent ASFV strains can be categorized into four groups: Genes implicated in viral genome replication and transcription, genes from the multigene family located at both 5' and 3' termini, genes participating in mediating hemadsorption and putative cellular attachment factors, and novel genes with no known functions. Some promising LAV vaccine candidates are generated by deleting a single viral virulence gene, whereas others are generated by simultaneously deleting multiple genes. This article summarizes the recent progress in developing and characterizing gene-deleted LAV vaccine candidates.

Evaluation of an I177L gene-based five-gene-deleted African swine fever virus as a live attenuated vaccine in pigs

African swine fever (ASF) is a highly contagious disease of domestic and wild pigs caused by the African swine fever virus (ASFV). The current research on ASF vaccines focuses on the development of naturally attenuated, isolated, or genetically engineered live viruses that have been demonstrated to produce reliable immunity. As a result, a genetically engineered virus containing five genes deletion was synthesized based on ASFV Chinese strain GZ201801, named ASFV-GZΔI177LΔCD2vΔMGF. The five-gene-deleted ASFV was safe and fully attenuated in pigs and provides reliable protection against the parental ASFV strain challenge. This indicates that the five-gene-deleted ASFV is a potential candidate for a live attenuated vaccine that could control the spread of ASFV.

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.

SY18ΔL60L: a new recombinant live attenuated African swine fever virus with protection against homologous challenge

Introduction

African swine fever (ASF) is an acute and highly contagious disease and its pathogen, the African swine fever virus (ASFV), threatens the global pig industry. At present, management of ASF epidemic mainly relies on biological prevention and control methods. Moreover, due to the large genome of ASFV, only half of its genes have been characterized in terms of function.

Methods

Here, we evaluated a previously uncharacterized viral gene, L60L. To assess the function of this gene, we constructed a deletion strain (SY18ΔL60L) by knocking out the L60L gene of the SY18 strain. To evaluate the growth characteristics and safety of the SY18ΔL60L, experiments were conducted on primary macrophages and pigs, respectively.

Results

The results revealed that the growth trend of the recombinant strain was slower than that of the parent strain in vitro. Additionally, 3/5 (60%) pigs intramuscularly immunized with a 105 50% tissue culture infectious dose (TCID50) of SY18ΔL60L survived the 21-day observation period. The surviving pigs were able to protect against the homologous lethal strain SY18 and survive. Importantly, there were no obvious clinical symptoms or viremia.

Discussion

These results suggest that L60L could serve as a virulence- and replication-related gene. Moreover, the SY18ΔL60L strain represents a new recombinant live-attenuated ASFV that can be employed in the development of additional candidate vaccine strains and in the elucidation of the mechanisms associated with ASF infection.

Deletion of the H240R Gene in African Swine Fever Virus Partially Reduces Virus Virulence in Swine

African swine fever (ASF) is a highly contagious disease that affects wild and domestic swine. Currently, the disease is present as a pandemic affecting pork production in Eurasia and the Caribbean region. The etiological agent of ASF is a large, highly complex structural virus (ASFV) harboring a double-stranded genome encoding for more than 160 proteins whose functions, in most cases, have not been experimentally characterized. We show here that deletion of the ASFV gene H240R from the genome of the highly virulent ASFV-Georgia2010 (ASFV-G) isolate partially decreases virus virulence when experimentally inoculated in domestic swine. ASFV-G-∆H240R, a recombinant virus harboring the deletion of the H240R gene, was produced to evaluate the function of the gene in the development of disease in pigs. While all animals intramuscularly inoculated with 10² HAD₅₀ of ASFV-G developed a fatal form of the disease, forty percent of pigs receiving a similar dose of ASFV-G-∆H240R survived the infection, remaining healthy during the 28-day observational period, and the remaining sixty percent developed a protracted but fatal form of the disease compared to that induced by ASFV-G. Additionally, all animals inoculated with ASFV-G-∆H240R presented protracted viremias with reduced virus titers when compared with those found in animals inoculated with ASFV-G. Animals surviving infection with ASFV-G-∆H240R developed a strong virus-specific antibody response and were protected against the challenge of the virulent parental ASFV-G.

Deletions of MGF110-9L and MGF360-9L from African swine fever virus are highly attenuated in swine and confer protection against homologous challenge

African swine fever, caused by a large icosahedral DNA virus (African swine fever virus, ASFV), is a highly contagious disease in domestic and feral swine, thus posing a significant economic threat to the global swine industry. Currently, there are no effective vaccines or the available methods to control ASFV infection. Attenuated live viruses with deleted virulence factors are considered to be the most promising vaccine candidates; however, the mechanism by which these attenuated viruses confer protection is unclear. Here, we used the Chinese ASFV CN/GS/2018 as a backbone and used homologous recombination to generate a virus in which MGF110-9L and MGF360-9L, two genes antagonize host innate antiviral immune response, were deleted (ASFV-ΔMGF110/360-9L). This genetically modified virus was highly attenuated in pigs and provided effective protection of pigs against parental ASFV challenge. Importantly, we found ASFV-ΔMGF110/360-9L infection induced higher expression of Toll-like receptor 2 (TLR2) mRNA compared with parental ASFV as determined by RNA-Seq and RT-PCR analysis. Further immunoblotting results showed that parental ASFV and ASFV-ΔMGF110/360-9L infection inhibited Pam3CSK4-triggered activating phosphorylation of proinflammatory transcription factor NF-κB subunit p65 and phosphorylation of NF-κB inhibitor IκBα levels, although NF-κB activation was higher in ASFV-ΔMGF110/360-9L-infected cells compared with parental ASFV-infected cells. Additionally, we show overexpression of TLR2 inhibited ASFV replication and the expression of ASFV p72 protein, whereas knockdown of TLR2 had the opposite effect. Our findings suggest that the attenuated virulence of ASFV-ΔMGF110/360-9L might be mediated by increased NF-κB and TLR2 signaling.

Deletion of DP148R, DP71L, and DP96R Attenuates African Swine Fever Virus, and the Mutant Strain Confers Complete Protection against Homologous Challenges in Pigs

The African swine fever virus (ASFV) has caused a devastating pandemic in domestic and wild swine, causing economic losses to the global swine industry. Recombinant live attenuated vaccines are an attractive option for ASFV treatment. However, safe and effective vaccines against ASFV are still scarce, and more high-quality experimental vaccine strains need to be developed. In this study, we revealed that deletion of the ASFV genes DP148R, DP71L, and DP96R from the highly virulent isolate ASFV CN/GS/2018 (ASFV-GS) substantially attenuated virulence in swine. Pigs infected with 104 50% hemadsorbing doses of the virus with these gene deletions remained healthy during the 19-day observation period. No ASFV infection was detected in contact pigs under the experimental conditions. Importantly, the inoculated pigs were protected against homologous challenges. Additionally, RNA sequence analysis showed that deletion of these viral genes induced significant upregulation of the host histone H3.1 gene (H3.1) and downregulation of the ASFV MGF110-7L gene. Knocking down the expression of H3.1 resulted in high levels of ASFV replication in primary porcine macrophages in vitro. These findings indicate that the deletion mutant virus ASFV-GS-Δ18R/NL/UK is a novel potential live attenuated vaccine candidate and one of the few experimental vaccine strains reported to induce full protection against the highly virulent ASFV-GS virus strain. IMPORTANCE Ongoing outbreaks of African swine fever (ASF) have considerably damaged the pig industry in affected countries. Thus, a safe and effective vaccine is important to control African swine fever spread. Here, an ASFV strain with three gene deletions was developed by knocking out the viral genes DP148R (MGF360-18R), NL (DP71L), and UK (DP96R). The results showed that the recombinant virus was completely attenuated in pigs and provided strong protection against parental virus challenge. Additionally, no viral genomes were detected in the sera of pigs housed with animals infected with the deletion mutant. Furthermore, transcriptome sequencing (RNA-seq) analysis revealed significant upregulation of histone H3.1 in virus-infected macrophage cultures and downregulation of the ASFV MGF110-7L gene after viral DP148R, UK, and NL deletion. Our study provides a valuable live attenuated vaccine candidate and potential gene targets for developing strategies for anti-ASFV treatment.

Attenuated African swine fever virus through serial passaging of viruses in cell culture: a brief review on the knowledge gathered during 60 years of research

African swine fever virus (ASFV) is a highly pathogenic double-stranded DNA virus. It affects various breeds of pigs, causing serious economic losses and health threats because of its rapid spread and high pathogenicity and infectivity. This situation is not helped by the lack of a validated vaccine or effective therapies. Since the 1960s, different strains of ASFV have been subjected to serial passage in a variety of cell lines. The attenuated ASFV strains obtained through serial passage are not only candidates for ASF vaccine research, but also are useful to study the molecular genetic characteristics and pathogenic mechanism of the virus. This review summarizes related studies on the attenuated strains of ASFV acquired through cell passage over the last 60 years, with the aim of providing inspiration for the rational design of vaccines in future.

A Spontaneously Occurring African Swine Fever Virus with 11 Gene Deletions Partially Protects Pigs Challenged with the Parental Strain

African swine fever (ASF) is an infectious Suidae disease caused by the ASF virus (ASFV). Adaptation to less susceptible, non-target host cells is one of the most common techniques used to attenuate virulent viruses. However, this may induce many mutations and large-scale rearrangements in the viral genome, resulting in immunostimulatory potential loss of the virus in vivo. This study continuously maintained the virulent ASFV strain, Armenia2007 (Arm07), to establish an attenuated ASFV strain with minimum genetic alteration in a susceptible host cell line, immortalized porcine kidney macrophage (IPKM). A mutant strain was successfully isolated via repeated plaque purification in combination with next-generation sequencing analysis. The isolated strain, Arm07ΔMGF, which was obtained from a viral fluid at a passage level of 20, lacked 11 genes in total in the MGF300 and MGF360 regions and showed marked reduction in virulence against pigs. Moreover, all the pigs survived the challenge with the parental strain when pigs were immunized twice with 10⁵ TCID₅₀ of Arm07ΔMGF, although viremia and fever were not completely prevented after the challenge infection. These findings suggest that this naturally attenuated, spontaneously occurring ASFV strain may provide a novel platform for ASF vaccine development.

Co-Deletion of A238L and EP402R Genes from a Genotype IX African Swine Fever Virus Results in Partial Attenuation and Protection in Swine

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF), resulting in up to 100% mortality in pigs. Although endemic in most sub-Saharan African countries, where all known ASFV genotypes have been reported, the disease has caused pandemics of significant economic impact in Eurasia, and no vaccines or therapeutics are available to date. In endeavors to develop live-attenuated vaccines against ASF, deletions of several of the ~170 ASFV genes have shown contrasting results depending on the genotype of the investigated ASFV. Here, we report the in vivo outcome of a single deletion of the A238L (5EL) gene and double deletions of A238L (5EL) and EP402R (CD2v) genes from the genome of a highly virulent genotype IX ASFV isolate. Domestic pigs were intramuscularly inoculated with (i) ASFV-Ke-ΔA238L to assess the safety of A238L deletion and (ii) ASFV-Ke-ΔEP402RΔA238L to investigate protection against challenge with the virulent wildtype ASFV-Ke virus. While A238L (5EL) gene deletion did not yield complete attenuation, co-deletion of A238L (5EL) and EP402R (CD2v) improved the safety profile of the single deletions, eliciting both humoral and cellular immune responses and conferred partial protection against challenge with the virulent wildtype ASFV-Ke virus.

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.

Deletion of African Swine Fever Virus Histone-like Protein, A104R from the Georgia Isolate Drastically Reduces Virus Virulence in Domestic Pigs

African swine fever virus (ASFV) is the etiological agent of a frequently lethal disease, ASF, affecting domestic and wild swine. Currently, ASF is causing a pandemic affecting pig production in Eurasia. There are no vaccines available, and therefore control of the disease is based on culling infected animals. We report here that deletion of the ASFV gene A104R, a virus histone-like protein, from the genome of the highly virulent ASFV-Georgia2010 (ASFV-G) strain induces a clear decrease in virus virulence when experimentally inoculated in domestic swine. A recombinant virus lacking the A104R gene, ASFV-G-∆A104R, was developed to assess the role of the A104R gene in disease production in swine. Domestic pigs were intramuscularly inoculated with 10² HAD₅₀ of ASFV-G-∆A104R, and compared with animals that received a similar dose of virulent ASFV-G. While all ASFV-G inoculated animals developed a fatal form of the disease, animals receiving ASFV-G-∆A104R survived the challenge, remaining healthy during the 28-day observational period, with the exception of only one showing a protracted but fatal form of the disease. ASFV-G-∆A104R surviving animals presented protracted viremias with reduced virus titers when compared with those found in animals inoculated with ASFV-G, and all of them developed a strong virus-specific antibody response. This is the first report demonstrating that the A104R gene is involved in ASFV virulence in domestic swine, suggesting that A104R deletion may be used to increase the safety profile of currently experimental vaccines.

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.

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.

Genome Plasticity of African Swine Fever Virus: Implications for Diagnostics and Live-Attenuated Vaccines

African swine fever (ASF) is a highly contagious transboundary viral hemorrhagic disease of domestic and wild pigs presenting a significant threat to the global swine industry. Following its introduction in Caucasus, Georgia, in 2007, the genome of the genotype II of African swine fever virus (ASFV) strain Georgia-07 and its derivatives accumulated significant mutations, resulting in the emergence of genetic variants within short epidemiological timescales as it spreads and infects different hosts in diverse ecosystems, causing outbreaks in Europe, South Asia, South East Asia and the Caribbean. This suggests that ASFV, with a comparatively large and complex DNA genome, is susceptible to genetic mutations including deletions and that although the virus is environmentally stable, it is genetically unstable. This has implications for the development of vaccines and diagnostic tests for disease detection and surveillance. Analysis of the ASFV genome revealed recombination hotspots, which in double-stranded DNA (dsDNA) viruses represent key drivers of genetic diversity. The ability of pox virus, a dsDNA virus with a genome complexity similar to ASFV, regaining virulence following the deletion of a virulence gene via gene amplification, coupled with the recent emergence and spread of live-attenuated ASFV vaccine strains causing disease and death in pigs in China, raise legitimate concerns around the use of live-attenuated ASFV vaccines in non-endemic regions to control the potential introduction. Further research into the risk of using live-attenuated ASFV in non-endemic regions is highly needed.

Isolation of Porcine Bone Marrow Cells and Generation of Recombinant African Swine Fever Viruses

Genetic manipulation of ASFV has been increasingly used not only for the development of live attenuated vaccines but also as an indispensable tool to further our understanding of the virus-host interactions. Here we present methods for isolation of porcine bone marrow cells and purification of recombinant ASFV using both chromogenic and fluorescent reporters. We also describe in detail a newly developed method to purify genetically modified ASFV using fluorescence-activated cell sorting (FACS).

African Swine Fever Virus Bearing an I226R Gene Deletion Elicits Robust Immunity in Pigs to African Swine Fever

African swine fever (ASF) is a severe hemorrhagic infectious disease in pigs caused by African swine fever virus (ASFV), leading to devastating economic losses in epidemic regions. Its control currently depends on thorough culling and clearance of the diseased and surrounding suspected pigs. An ASF vaccine has been extensively explored for years worldwide, especially in hog-intensive areas where it is highly desired, but it is still unavailable for numerous reasons. Here, we report another ASF vaccine candidate, named SY18ΔI226R, bearing a deletion of the I226R gene with a replacement of an enhanced green fluorescent protein (eGFP) expression cassette at the right end of the viral genome. This deletion results in the complete loss of virulence of SY18 as the gene-deleted strain does not cause any clinical symptoms in all pigs inoculated with a dosage of either 10^4.0 or 10^7.0 50% tissue culture infective doses (TCID₅₀). Apparent viremia with a gradual decline was monitored, while virus shedding was detected only occasionally in oral or anal swabs. ASFV-specific antibody appeared at 9 days postinoculation. After intramuscular challenge with its parental strain ASFV SY18 at 21 days postinoculation, all the challenged pigs survived, without obvious febrile or abnormal clinical signs. No viral DNA could be detected upon the dissection of any tissue when viremia disappeared. These results indicated that SY18ΔI226R is safe in swine and elicits robust immunity to virulent ASFV infection.

IMPORTANCE Outbreaks of African swine fever have resulted in devastating losses to the swine industry worldwide, but there is currently no commercial vaccine available. Although several vaccine candidates have been reported, none has been approved for use for several reasons, especially ones concerning biosafety. Here, we identified a new undescribed functional gene, I226R. When deleted from the ASFV genome, the virus completely loses its virulence in swine. Importantly, pigs infected with this gene-deleted virus were resistant to infection by intramuscular challenge with 10^2.5 or 10^4.0 TCID₅₀ of its virulent parental virus. Furthermore, the nucleic acid of the gene-deleted virus and its virulent parental virus was rarely detected from oral or anal swabs. Viruses could not be detected in any tissues after necropsy when viremia became negative, indicating that robust immunity was achieved. Therefore, SY18ΔI226R is a novel, ideal, and efficacious vaccine candidate for genotype II ASF.

Deletion of the A137R Gene from the Pandemic Strain of African Swine Fever Virus Attenuates the Strain and Offers Protection against the Virulent Pandemic Virus

African swine fever virus (ASFV) is causing a devastating pandemic in domestic and wild swine within an extended geographical area from Central Europe to East Asia, resulting in economic losses for the regional swine industry. There are no commercial vaccines; therefore, disease control relies on identification and culling of infected animals. We report here that the deletion of the ASFV gene A137R from the highly virulent ASFV-Georgia2010 (ASFV-G) isolate induces a significant attenuation of virus virulence in swine. A recombinant virus lacking the A137R gene, ASFV-G-ΔA137R, was developed to assess the role of this gene in ASFV virulence in domestic swine. Animals inoculated intramuscularly with 10² 50% hemadsorption doses (HAD₅₀) of ASFV-G-ΔA137R remained clinically healthy during the 28-day observational period. All animals inoculated with ASFV-G-ΔA137R had medium to high viremia titers and developed a strong virus-specific antibody response. Importantly, all ASFV-G-ΔA137R-inoculated animals were protected when challenged with the virulent parental strain ASFV-G. No evidence of replication of challenge virus was observed in the ASFV-G-ΔA137R-inoculated animals. Therefore, ASFV-G-ΔA137R is a novel potential live attenuated vaccine candidate and one of the few experimental vaccine strains reported to induce protection against the highly virulent ASFV Georgia virus that is the cause of the current Eurasian pandemic. IMPORTANCE No commercial vaccine is available to prevent African swine fever. The ASF pandemic caused by ASFV Georgia2007 strain (ASFV-G) is seriously affecting pork production in a contiguous area from Central Europe to East Asia. Here we report the rational development of a potential live attenuated vaccine strain by deleting a virus-specific gene, A137R, from the genome of ASFV-G. The resulting virus presented a completely attenuated phenotype and, importantly, animals infected with this genetically modified virus were protected from developing ASF after challenge with the virulent parental virus. ASFV-G-ΔA137R confers protection even at low doses (10² HAD₅₀), demonstrating its potential as a vaccine candidate. Therefore, ASFV-G-ΔA137R is a novel experimental ASF vaccine protecting pigs from the epidemiologically relevant ASFV Georgia isolate.

Rapid CRISPR/Cas9 Editing of Genotype IX African Swine Fever Virus Circulating in Eastern and Central Africa

African swine fever virus (ASFV) is the etiological agent of a contagious and fatal disease of domestic pigs that has significant economic consequences for the global swine industry. Due to the lack of effective treatment and vaccines against African swine fever, there is an urgent need to leverage cutting-edge technologies and cost-effective approaches for generating and purifying recombinant virus to fast-track the development of live-attenuated ASFV vaccines. Here, we describe the use of the CRISPR/Cas9 gene editing and a cost-effective cloning system to produce recombinant ASFVs. Combining these approaches, we developed a recombinant virus lacking the non-essential gene A238L (5EL) in the highly virulent genotype IX ASFV (ASFV-Kenya-IX-1033) genome in less than 2 months as opposed to the standard homologous recombination with conventional purification techniques which takes up to 6 months on average. Our approach could therefore be a method of choice for less resourced laboratories in developing nations.

African swine fever virus protein MGF-505-7R promotes virulence and pathogenesis by inhibiting JAK1- and JAK2-mediated signaling

African swine fever virus (ASFV) is a large DNA virus that is highly contagious and pathogenic in domestic pigs with a mortality rate up to 100%. However, how ASFV suppresses JAK-STAT1 signaling to evade the immune response remains unclear. In this study, we found that the ASFV-encoded protein MGF-505-7R inhibited proinflammatory IFN-γ-mediated JAK-STAT1 signaling. Mechanistically, MGF-505-7R was found to interact with JAK1 and JAK2 and mediate their degradation. Further study indicated that MGF-505-7R promoted degradation of JAK1 and JAK2 by upregulating the E3 ubiquitin ligase RNF125 expression and inhibiting expression of Hes5, respectively. Consistently, MGF-505-7R-deficient ASFV induced high levels of IRF1 expression and displayed compromised replication both in primary porcine alveolar macrophages and pigs compared with wild-type ASFV. Furthermore, MGF-505-7R deficiency attenuated the virulence of the ASFV and pathogenesis of ASF in pigs. These findings suggest that the JAK-STAT1 axis mediates the innate immune response to the ASFV and that MGF-505-7R plays a critical role in the virulence of the ASFV and pathogenesis of ASF by antagonizing this axis. Thus, we conclude that deletion of MGF-505-7R may serve as a strategy to develop attenuated vaccines against the ASFV.

Molecular Pathogenesis and Immune Evasion of Vesicular Stomatitis New Jersey Virus Inferred from Genes Expression Changes in Infected Porcine Macrophages

The molecular mechanisms associated with the pathogenesis of vesicular stomatitis virus (VSV) in livestock remain poorly understood. Several studies have highlighted the relevant role of macrophages in controlling the systemic dissemination of VSV during infection in different animal models, including mice, cattle, and pigs. To gain more insight into the molecular mechanisms used by VSV to impair the immune response in macrophages, we used microarrays to determine the transcriptomic changes produced by VSV infection in primary cultures of porcine macrophages. The results indicated that VSV infection induced the massive expression of multiple anorexic, pyrogenic, proinflammatory, and immunosuppressive genes. Overall, the interferon (IFN) response appeared to be suppressed, leading to the absence of stimulation of interferon-stimulated genes (ISG). Interestingly, VSV infection promoted the expression of several genes known to downregulate the expression of IFNβ. This represents an alternate mechanism for VSV control of the IFN response, beyond the recognized mechanisms mediated by the matrix protein. Although there was no significant differential gene expression in macrophages infected with a highly virulent epidemic strain compared to a less virulent endemic strain, the endemic strain consistently induced higher expression of all upregulated cytokines and chemokines. Collectively, this study provides novel insights into VSV molecular pathogenesis and immune evasion that warrant further investigation.

Development and in vivo evaluation of MGF100-1R deletion mutant in an African swine fever virus Chinese strain

African swine fever virus (ASFV) is a large nucleoplasmic DNA virus, in which the genome is around 170-198 kilobases (kb). More than 50 % genes have unknown functions. Here, MGF100-1R gene is chosen to study the primary function and sublocalization. The gene was located at the left variable region of the ASFV genome that belongs to MGF100 families. It located at the cytoplasm without cytotoxic activities. However, it related to induce the transcriptional levels of pro-inflammatory cytokines. A deletion mutant of MGF100-1R gene was constructed based on ASFV Chinese strain GZ201801. The recombinant deletion mutant (ASFV△MGF100-1R) was demonstrated in vitro that the gene is non-essential for virus replication with a similar replication kinetics in bone marrow-derived macrophages (BMDMs) cell cultures when compared to parental virus. In vivo evaluation, ASFV△MGF100-1R was inoculated intramuscularly and led to a similar pathogenesis that caused by the parental ASFV GZ201801, confirming that deletion of MGF100-1R gene from the ASFV genome does not impact virulence.

Autophagy impairment by African swine fever virus

African swine fever is a devastating disease of domestic swine and wild boar caused by a large double-stranded DNA virus that encodes for more than 150 open reading frames. There is no licensed vaccine for the disease and the most promising current candidates are modified live viruses that have been attenuated by deletion of virulence factors. Like many viruses African swine fever virus significantly alters the host cell machinery to benefit its replication and viral genes that modify host pathways represent promising targets for development of gene deleted vaccines. Autophagy is an important cellular pathway that is involved in cellular homeostasis, innate and adaptive immunity and therefore is manipulated by a number of different viruses. Autophagy is regulated by a complex protein cascade and here we show that African swine fever virus can block formation of autophagosomes, a critical functional step of the autophagy pathway through at least two different mechanisms. Interestingly this does not require the A179L gene that has been shown to interact with Beclin-1, an important autophagy regulator.

African Swine Fever Virus CD2v Protein Induces β-Interferon Expression and Apoptosis in Swine Peripheral Blood Mononuclear Cells

African swine fever (ASF) is a hemorrhagic disease of swine characterized by massive lymphocyte depletion in lymphoid tissues due to the apoptosis of B and T cells, a process likely triggered by factors released or secreted by infected macrophages. ASFV CD2v (EP402R) has been implicated in viral virulence and immunomodulation in vitro; however, its actual function(s) remains unknown. We found that CD2v expression in swine PK15 cells induces NF-κB-dependent IFN-β and ISGs transcription and an antiviral state. Similar results were observed for CD2v protein treated swine PBMCs and macrophages, the major ASFV target cell. Notably, treatment of swine PBMCs and macrophages with CD2v protein induced apoptosis. Immunoprecipitation and colocalization studies revealed that CD2v interacts with CD58, the natural host CD2 ligand. Additionally, CD58 knockdown in cells or treatment of cells with an NF-κB inhibitor significantly reduced CD2v-mediated NF-κB activation and IFN-β induction. Further, antibodies directed against CD2v inhibited CD2v-induced NF-κB activation and IFN-β transcription in cells. Overall, results indicate that ASFV CD2v activates NF-κB, which induces IFN signaling and apoptosis in swine lymphocytes/macrophages. We propose that CD2v released from infected macrophages may be a significant factor in lymphocyte apoptosis observed in lymphoid tissue during ASFV infection in pigs.

Thoughts on African Swine Fever Vaccines

African swine fever (ASF) is an acute viral hemorrhagic disease of domestic swine with mortality rates approaching 100%. Devastating ASF outbreaks and continuing epidemics starting in the Caucasus region and now in the Russian Federation, Europe, China, and other parts of Southeast Asia (2007 to date) highlight its significance. ASF strain Georgia-07 and its derivatives are now endemic in extensive regions of Europe and Asia and are "out of Africa" forever, a situation that poses a grave if not an existential threat to the swine industry worldwide. While our current concern is Georgia-07, other emerging ASFV strains will threaten for the indefinite future. Economic analysis indicates that an ASF outbreak in the U.S. would result in approximately $15 billion USD in losses, assuming the disease is rapidly controlled and the U.S. is able to reenter export markets within two years. ASF's potential to spread and become endemic in new regions, its rapid and efficient transmission among pigs, and the relative stability of the causative agent ASF virus (ASFV) in the environment all provide significant challenges for disease control. Effective and robust methods, including vaccines for ASF response and recovery, are needed immediately.

African Swine Fever Virus MGF-505-7R Negatively Regulates cGAS-STING-Mediated Signaling Pathway

African swine fever virus (ASFV) is a devastating infectious disease in pigs, severely threatening the global pig industry. To efficiently infect animals, ASFV must evade or inhibit fundamental elements of the innate immune system, namely the type I IFN response. In this study, we identified that ASFV MGF-505-7R protein exerts a negative regulatory effect on STING-dependent antiviral responses. MGF-505-7R interacted with STING and inhibited the cGAS-STING signaling pathway at STING level. MGF-505-7R overexpression either degraded STING or STING expression was reduced in ASFV-infected cells via autophagy, whereas STING expression was elevated in MGF-505-7R-deficient ASFV-infected cells. We further found that MGF-505-7R promoted the expression of the autophagy-related protein ULK1 to degrade STING, whereas ULK1 was elevated in MGF-505-7R-deficient ASFV-infected cells. Moreover, MGF-505-7R-deficient ASFV induced more IFN-β production than wild-type ASFV and was attenuated in replication compared with wild-type ASFV. The replicative ability of MGF-505-7R-deficient ASFV was also attenuated compared with wild-type. Importantly, MGF-505-7R-deficient ASFV was fully attenuated in pigs. Our results showed for the first time, to our knowledge, a relationship involving the cGAS-STING pathway and ASFV MGF-505-7R, contributing to uncover the molecular mechanisms of ASFV virulence and to the rational development of ASFV vaccines.

African Swine Fever Virus MGF-110-9L-deficient Mutant Has Attenuated Virulence in Pigs

African swine fever virus (ASFV) is the etiological agent of African swine fever (ASF), an often lethal disease in domestic and wild pigs. ASF represents a major threat to the swine industry worldwide. Currently, no commercial vaccine is available because of the complexity of ASFV or biosecurity concerns. Live attenuated viruses that are naturally isolated or genetically manipulated have demonstrated reliable protection against homologous ASFV strain challenge. In the present study, a mutant ASFV strain with the deletion of ASFV MGF-110-9L (ASFV-Δ9L) was generated from a highly virulent ASFV CN/GS/2018 parental strain, a genotype II ASFV. Relative to the parental ASFV isolate, deletion of the MGF-110-9L gene significantly decreased the ability of ASFV-Δ9L to replicate in vitro in primary swine macrophage cell cultures. The majority of animals inoculated intramuscularly with a low dose of ASFV-Δ9L (10 HAD₅₀) remained clinically normal during the 21-day observational period. Three of five ASFV-Δ9L-infected animals displayed low viremia titers and low virus shedding and developed a strong virus-specific antibody response, indicating partial attenuation of the ASFV-Δ9L strain in pigs. The findings imply the potential usefulness of the ASFV-Δ9L strain for further development of ASF control measures.

Emergence and prevalence of naturally occurring lower virulent African swine fever viruses in domestic pigs in China in 2020

African swine fever virus (ASFV) has been circulating in China for more than two years, and it is not clear whether the biological properties of the virus have changed. Here, we report on our surveillance of ASFVs in seven provinces of China, from June to December, 2020. A total of 22 viruses were isolated and characterized as genotype II ASFVs, with mutations, deletions, insertions, or short-fragment replacement occurring in all isolates compared with Pig/HLJ/2018 (HLJ/18), the earliest isolate in China. Eleven isolates had four different types of natural mutations or deletion in the EP402R gene and displayed a non-hemadsorbing (non-HAD) phenotype. Four isolates were tested for virulence in pigs; two were found to be as highly lethal as HLJ/18. However, two non-HAD isolates showed lower virulence but were highly transmissible; infection with 10⁶ TCID₅₀ dose was partially lethal and caused acute or sub-acute disease, whereas 10³ TCID₅₀ dose caused non-lethal, sub-acute or chronic disease, and persistent infection. The emergence of lower virulent natural mutants brings greater difficulty to the early diagnosis of ASF and creates new challenges for ASFV control.

Evaluation in Swine of a Recombinant Georgia 2010 African Swine Fever Virus Lacking the I8L Gene

African swine fever virus (ASFV) is the causative agent of African swine fever, a disease currently causing significant economic losses in Europe and Asia. Specifically, the highly virulent ASFV strain Georgia 2010 (ASFV-G) is producing disease outbreaks in this large geographical region. The ASFV genome encodes for over 150 genes, most of which are still not experimentally characterized. I8L is a highly conserved gene that has not been studied beyond its initial description as a virus ORF. Transcriptional analysis of swine macrophages infected with ASFV-G demonstrated that the I8L gene is transcribed early during the virus replication cycle. To assess the importance of I8L during ASFV-G replication in vitro and in vivo, as well as its role in virus virulence in domestic swine, we developed a recombinant virus lacking the I8L gene (ASFV-G-ΔI8L). Replication of ASFV-G-ΔI8L was similar to parental ASFV-G replication in primary swine macrophage cultures, suggesting that the I8L gene is not essential for ASFV-G replication in vitro. Similarly, replication of ASFV-G-ΔI8L in swine intramuscularly inoculated with 10² HAD₅₀ displayed replication kinetics similar to ASFV-G. In addition, animals inoculated with ASFV-G-ΔI8L presented with a clinical disease indistinguishable from that induced by the same dose of the virulent parental ASFV-G isolate. We conclude that deletion of the I8L gene from ASFV-G does not affect virus replication in vitro or in vivo, nor changes the disease outcome in swine.

Deletion of CD2-Like (CD2v) and C-Type Lectin-Like (EP153R) Genes from African Swine Fever Virus Georgia-∆9GL Abrogates Its Effectiveness as an Experimental Vaccine

African swine fever virus (ASFV) is currently the most dreaded infectious disease affecting the global swine production industry. There is no commercial vaccine available, making the culling of infected animals the current solution to control outbreaks. Effective experimental vaccines have been developed by deleting virus genes associated with virulence. Deletion of the ASFV 9GL gene (∆9GL) has resulted in the attenuation of different ASFV strains, although the degree of attenuation varies across isolates. Here, we investigated the possibility of the increased safety of the experimental vaccine strain ASFV-G-Δ9GL by deleting two additional virus genes involved in pathogenesis, CD2v, a CD2 like viral encoded gene from the EP402R open reading frame (ORF), and C-type lectin-like viral gene, encoded from the EP153R ORF. Two new recombinant viruses were developed, ASFV-G-Δ9GL/ΔCD2v and ASFV-G-Δ9GL/ΔCD2v/ΔEP153R, harboring two and three gene deletions, respectively. ASFV-G-Δ9GL/ΔCD2v/ΔEP153R, but not ASFV-G-Δ9GL/ΔCD2v, had a decreased ability to replicate in vitro in swine macrophage cultures when compared with parental ASFV-G-Δ9GL. Importantly, ASFV-G-Δ9GL/ΔCD2v and ASFV-G-Δ9GL/ΔCD2v/ΔEP153R induced almost undetectable viremia levels when inoculated into domestic pigs and failed to protect them against challenge with parental virulent ASFV-Georgia, while ASFV-G-Δ9GL offered robust protection during challenge. Therefore, the deletion of CD2-like and C-type lectin-like genes significantly decreased the protective potential of ASFV-G-Δ9GL as a vaccine candidate. This study constitutes an example of the unpredictability of genetic manipulation involving the simultaneous deletion of multiple genes from the ASFV genome.

Multifaceted Immune Responses to African Swine Fever Virus: Implications for Vaccine Development

African swine fever (ASF) is a highly contagious, often fatal viral disease caused by African swine fever virus (ASFV), leading to high fever, severe hemorrhages with high lethality in domestic pigs and wild boar. In 2007, ASF was reintroduced into Europe. Since then, ASF has spread to many European and Asian countries and now becomes a major concern to the swine industry worldwide. There have been various vaccine attempts, but no commercial ASF vaccines are available so far. A key hurdle in developing a safe and efficacious ASF vaccine is the limited understanding of innate and adaptive immune responses elicited by ASFV infection. Though several promising vaccine candidates have been described, more key scientific challenges remain unsolved. Here, we provide an overview of the current knowledge in innate and adaptive immune responses elicited by ASFV infection and different kinds of vaccine candidates. Additionally, the applications and prospects of vaccine candidates are discussed. Finally, we highlight the implications of these mechanisms for rational design of ASF vaccines.

X69R Is a Non-Essential Gene That, When Deleted from African Swine Fever, Does Not Affect Virulence in Swine

African swine fever virus (ASFV) is currently causing devastating outbreaks in Asia and Europe, and the ASFV strain Georgia (ASFV-G) is responsible for these outbreaks. ASFV-G is highly virulent and continues to be maintained in these outbreak areas, apparently without suffering significant genomic or phenotypic changes. When comparing the genome of ASFV-G to other isolates, a thus-far uncharacterized gene, X69R, is highly conserved and, interestingly, is similar to another ASFV uncharacterized gene, J64R. All sequenced ASFV isolates have one or both of these genes, X69R or J64R, suggesting that the presence of at least one of these genes may be necessary for ASFV replication and or virulence. The X69R gene is present in the ASFV-G genome while J64R is absent. To assess the importance of X69R in ASFV-G functionality, we developed a recombinant virus by deleting the X69R gene from the ASFV-G genome (ASFV-G-ΔX69R). ASFV-G-ΔX69R had the same replication kinetics in primary swine macrophage cultures as the parental ASFV-G, indicating that the X69R gene is not essential for ASFV-G viability or efficient replication in the main target cell during in vivo infection. In addition, swine intramuscularly inoculated with a low dose (10² HAD₅₀) of ASFV-G-ΔX69R developed a clinical disease indistinguishable from that induced by the same dose of the virulent parental ASFV-G isolate. Viremia values of ASFV-G-ΔX69R did not significantly differ from those detected in animals infected with parental virus. Therefore, deletion of the X69R gene from ASFV-G does not affect virus replication or virulence in swine.

P108 and T109 on E2 Glycoprotein Domain I Are Critical for the Adaptation of Classical Swine Fever Virus to Rabbits but Not for Virulence in Pigs

The classical swine fever virus (CSFV) live attenuated vaccine C-strain is adaptive to rabbits and attenuated in pigs, in contrast with the highly virulent CSFV Shimen strain. Previously, we demonstrated that P108 and T109 on the E2 glycoprotein (E2^P108-T109) in domain I (E2^DomainI) rather than R132, S133, and D191 in domain II (E2^DomainII) determine C-strain's adaptation to rabbits (ATR) (Y. Li, L. Xie, L. Zhang, X. Wang, C. Li, et al., Virology 519:197-206, 2018). However, it remains elusive whether these critical amino acids affect the ATR of the Shimen strain and virulence in pigs. In this study, three chimeric viruses harboring E2^P108-T109, E2^DomainI, or E2^DomainII of C-strain based on the non-rabbit-adaptive Shimen mutant vSM-HCLVE^rns carrying the E^rns glycoprotein of C-strain were generated and evaluated. We found that E2^P108-T109 or E2^DomainI but not E2^DomainII of C-strain renders vSM-HCLVE^rns adaptive to rabbits, suggesting that E2^P108-T109 in combination with the E^rns glycoprotein (E2^P108-T109-E^rns) confers ATR on the Shimen strain, creating new rabbit-adaptive CSFVs. Mechanistically, E2^P108-T109-E^rns of C-strain mediates viral entry during infection in rabbit spleen lymphocytes, which are target cells of C-strain. Notably, pig experiments showed that E2^P108-T109-E^rns of C-strain does not affect virulence compared with the Shimen strain. Conversely, the substitution of E2^DomainII and E^rns of C-strain attenuates the Shimen strain in pigs, indicating that the molecular basis of the CSFV ATR and that of virulence in pigs do not overlap. Our findings provide new insights into the mechanism of adaptation of CSFV to rabbits and the molecular basis of CSFV adaptation and attenuation.IMPORTANCE Historically, live attenuated vaccines produced by blind passage usually undergo adaptation in cell cultures or nonsusceptible hosts and attenuation in natural hosts, with a classical example being the classical swine fever virus (CSFV) lapinized vaccine C-strain, which was developed by hundreds of passages in rabbits. However, the mechanism of viral adaptation to nonsusceptible hosts and the molecular basis for viral adaptation and attenuation remain largely unknown. In this study, we demonstrated that P108 and T109 on the E2 glycoprotein together with the E^rns glycoprotein of the rabbit-adaptive C-strain confer adaptation to rabbits on the highly virulent CSFV Shimen strain by affecting viral entry during infection but do not attenuate the Shimen strain in pigs. Our results provide vital information on the different molecular bases of CSFV adaptation to rabbits and attenuation in pigs.

African swine fever vaccines: a promising work still in progress

Abstract

African swine fever (ASF), a disease of obligatory declaration to the World Organization for Animal Health (OIE), has contributed to poverty and underdevelopment of affected areas. The presence of ASF has been historically neglected in Africa, contributing to its uncontrolled expansion and favouring its spread to continental Europe on at least three occasions, the last one in 2007 through the Republic of Georgia. Since then, African swine fever virus (ASFV) has spread to neighbouring countries, reaching the European Union in 2014, China in the summer of 2018 and spreading through Southeast Asia becoming a global problem. Lack of available vaccines against ASF makes its control even more difficult, representing today the number one threat for the swine industry worldwide and negatively affecting the global commerce equilibrium.

Main body

In this review, we intend to put in perspective the reality of ASF vaccination today, taking into account that investment into ASF vaccine development has been traditionally unattractive, overall since ASF-free areas with large swine industries applied a non-vaccination policy for diseases listed by the OIE. The dramatic situation suffered in Asia and the increasing threat that ASF represents for wealthy countries with large swine industries, has dramatically changed the perspective that both private and public bodies have about ASF vaccinology, although this is controversial. The feasibility of modifying the ASFV genome has led to safe and efficacious experimental recombinant live attenuated viruses (LAVs). The main challenge today will be confirming the safety and efficacy of these technologies in the field, accelerating transfer to the industry for official registration and commercialization. The complexity of ASFV, together with the lack of knowledge about the mechanisms involved in protection and the specific antigens involved in it, requires further investment in research and development. Although far from the efficacy achieved by LAVs, subunit vaccines are the optimal choice for the future. If the world can wait for them or not is a contentious issue.

Conclusion

Despite their inherent disadvantages, LAVs will be the first technology to reach the market, while subunit vaccines will need much further research to become a successful commercial reality.

African swine fever vaccines: a promising work still in progress

ABSTRACT

African swine fever (ASF), a disease of obligatory declaration to the World Organization for Animal Health (OIE), has contributed to poverty and underdevelopment of affected areas. The presence of ASF has been historically neglected in Africa, contributing to its uncontrolled expansion and favouring its spread to continental Europe on at least three occasions, the last one in 2007 through the Republic of Georgia. Since then, African swine fever virus (ASFV) has spread to neighbouring countries, reaching the European Union in 2014, China in the summer of 2018 and spreading through Southeast Asia becoming a global problem. Lack of available vaccines against ASF makes its control even more difficult, representing today the number one threat for the swine industry worldwide and negatively affecting the global commerce equilibrium.

MAIN BODY

In this review, we intend to put in perspective the reality of ASF vaccination today, taking into account that investment into ASF vaccine development has been traditionally unattractive, overall since ASF-free areas with large swine industries applied a non-vaccination policy for diseases listed by the OIE. The dramatic situation suffered in Asia and the increasing threat that ASF represents for wealthy countries with large swine industries, has dramatically changed the perspective that both private and public bodies have about ASF vaccinology, although this is controversial. The feasibility of modifying the ASFV genome has led to safe and efficacious experimental recombinant live attenuated viruses (LAVs). The main challenge today will be confirming the safety and efficacy of these technologies in the field, accelerating transfer to the industry for official registration and commercialization. The complexity of ASFV, together with the lack of knowledge about the mechanisms involved in protection and the specific antigens involved in it, requires further investment in research and development. Although far from the efficacy achieved by LAVs, subunit vaccines are the optimal choice for the future. If the world can wait for them or not is a contentious issue.

CONCLUSION

Despite their inherent disadvantages, LAVs will be the first technology to reach the market, while subunit vaccines will need much further research to become a successful commercial reality.

Production of Recombinant African Swine Fever Viruses: Speeding Up the Process

African swine fever (ASF) is a devastating disease in pigs, with no vaccines for control. The genetic manipulation of African swine fever virus (ASFV) is often tedious and time consuming. Here, we describe a method to manipulate the virus genome to produce gene deletion viruses in a much-reduced time. This method combines the conventional homologous recombination with fluorescent-activated cells sorting (FACS), to isolate and purify viruses expressing fluorescent reporter genes. With three rounds of single cell isolation via FACS and two rounds of limiting dilution, we deleted two additional genes, EP153R and EP402R, from Benin 97/1 ASFV lacking the DP148R gene. By combining different fluorescent markers, this method has the potential to greatly facilitate studies on understanding ASFV gene functions and develop candidate live-attenuated vaccines.

A Single Amino Acid Substitution in the Matrix Protein (M51R) of Vesicular Stomatitis New Jersey Virus Impairs Replication in Cultured Porcine Macrophages and Results in Significant Attenuation in Pigs

In this study, we explore the virulence of vesicular stomatitis New Jersey virus (VSNJV) in pigs and its potential relationship with the virus’s ability to modulate innate responses. For this purpose, we developed a mutant of the highly virulent strain NJ0612NME6, containing a single amino acid substitution in the matrix protein (M51R). The M51R mutant of NJ0612NME6 was unable to suppress the transcription of genes associated with the innate immune response both in primary fetal porcine kidney cells and porcine primary macrophage cultures. Impaired viral growth was observed only in porcine macrophage cultures, indicating that the M51 residue is required for efficient replication of VSNJV in these cells. Furthermore, when inoculated in pigs by intradermal scarification of the snout, M51R infection was characterized by decreased clinical signs including reduced fever and development of less and smaller secondary vesicular lesions. Pigs infected with M51R had decreased levels of viral shedding and absence of RNAemia compared to the parental virus. The ability of the mutant virus to infect pigs by direct contact remained intact, indicating that the M51R mutation resulted in a partially attenuated phenotype capable of causing primary lesions and transmitting to sentinel pigs. Collectively, our results show a positive correlation between the ability of VSNJV to counteract the innate immune response in swine macrophage cultures and the level of virulence in pigs, a natural host of this virus. More studies are encouraged to evaluate the interaction of VSNJV with macrophages and other components of the immune response in pigs.

Development of a Highly Effective African Swine Fever Virus Vaccine by Deletion of the I177L Gene Results in Sterile Immunity against the Current Epidemic Eurasia Strain

African swine fever virus (ASFV) is the etiological agent of a contagious and often lethal disease of domestic pigs that has significant economic consequences for the swine industry. The disease is devastating the swine industry in Central Europe and East Asia, with current outbreaks caused by circulating strains of ASFV derived from the 2007 Georgia isolate (ASFV-G), a genotype II ASFV. In the absence of any available vaccines, African swine fever (ASF) outbreak containment relies on the control and culling of infected animals. Limited cross-protection studies suggest that in order to ensure a vaccine is effective, it must be derived from the current outbreak strain or at the very least from an isolate with the same genotype. Here, we report the discovery that the deletion of a previously uncharacterized gene, I177L, from the highly virulent ASFV-G produces complete virus attenuation in swine. Animals inoculated intramuscularly with the virus lacking the I177L gene, ASFV-G-ΔI177L, at a dose range of 10² to 10⁶ 50% hemadsorbing doses (HAD₅₀), remained clinically normal during the 28-day observational period. All ASFV-G-ΔI177L-infected animals had low viremia titers, showed no virus shedding, and developed a strong virus-specific antibody response; importantly, they were protected when challenged with the virulent parental strain ASFV-G. ASFV-G-ΔI177L is one of the few experimental vaccine candidate virus strains reported to be able to induce protection against the ASFV Georgia isolate, and it is the first vaccine capable of inducing sterile immunity against the current ASFV strain responsible for recent outbreaks.IMPORTANCE Currently, there is no commercially available vaccine against African swine fever. Outbreaks of this disease are devastating the swine industry from Central Europe to East Asia, and they are being caused by circulating strains of African swine fever virus derived from the Georgia 2007 isolate. Here, we report the discovery of a previously uncharacterized virus gene, which when deleted completely attenuates the Georgia isolate. Importantly, animals infected with this genetically modified virus were protected from developing ASF after challenge with the virulent parental virus. Interestingly, ASFV-G-ΔI177L confers protection even at low doses (10² HAD₅₀) and remains completely attenuated when inoculated at high doses (10⁶ HAD₅₀), demonstrating its potential as a safe vaccine candidate. At medium or higher doses (10⁴ HAD₅₀), sterile immunity is achieved. Therefore, ASFV-G-ΔI177L is a novel efficacious experimental ASF vaccine protecting pigs from the epidemiologically relevant ASFV Georgia isolate.

Deletion of CD2-like gene from the genome of African swine fever virus strain Georgia does not attenuate virulence in swine

The CD2-like African swine fever virus (ASFV) gene 8DR, (also known as EP402R) encodes for a structural transmembrane glycoprotein that has been shown to mediate hemadsorption and be involved in host immunomodulation as well as the induction of protective immune response. In addition, several natural ASFV isolates showing decreased virulence in swine has been shown to be non-hemadsorbing suggesting an association between altered or deleted forms of 8DR and virus attenuation. Here we demonstrate that deletion of 8DR gene from the genome of ASFV Georgia2010 isolate (ASFV-G-Δ8DR) does not significantly alter the virulence of the virus. ASFV-G-Δ8DR inoculated intramuscularly or intranasally (in a range of 10² to 10⁴ TCID₅₀) produced a clinical disease in domestic pigs indistinguishable from that induced by the same doses of the virulent parental ASFV Georgia2010 isolate. In addition, viremia values in ASFV-G-Δ8DR do not differ from those detected in animals infected with parental virus. Therefore, deletion of 8DR gene is not associated with a noticeable decrease in virulence of the ASFV Georgia isolate.

The MGF360-16R ORF of African Swine Fever Virus Strain Georgia Encodes for a Nonessential Gene That Interacts with Host Proteins SERTAD3 and SDCBP

African swine fever virus (ASFV) causes a contagious and frequently lethal disease of pigs with significant economic consequences to the swine industry. The ASFV genome encodes for more than 160 genes, but only a few of them have been studied in detail. Here we report the characterization of open reading frame (ORF) MGF360-16R. Kinetic studies of virus RNA transcription demonstrated that the MGF360-16R gene is transcribed as a late virus protein. Analysis of host-protein interactions for the MGF360-16R gene using a yeast two-hybrid screen identified SERTA domain containing 3 (SERTAD3) and syndecan-binding protein (SDCBP) as host protein binding partners. SERTAD3 and SDCBP are both involved in nuclear transcription and SDCBP has been shown to be involved in virus traffic inside the host cell. Interaction between MGF360-16R and SERTAD3 and SDCBP host proteins was confirmed in eukaryotic cells transfected with plasmids expressing MGF360-16R and SERTAD3 or SDCBP fused to fluorescent tags. A recombinant ASFV lacking the MGF360-16R gene (ASFV-G-ΔMGF360-16R) was developed from the highly virulent field isolate Georgia2007 (ASFV-G) and was used to show that MGF360-16R is a nonessential gene. ASFV-G-ΔMGF360-16R had a similar replication ability in primary swine macrophage cell cultures when compared to its parental virus ASFV-G. Experimental infection of domestic pigs showed that ASFV-G-ΔMGF360-16R is as virulent as the parental virus ASFV-G.

[African swine fever]

African swine fever (ASF) is a hemorrhagic infectious disease of Suids, which is endemic in sub-Saharan area of African continent. ASF is usually circulating sub-symptomatically among wild species of Suidae family, such as warthogs and bush pigs, by mediating Ornithodoros soft ticks. Domestic pigs (Sus scrofa) are, however, highly sensitive to the infection and show severe clinical signs with a high mortality rate, resulting a huge impact on pork production. Currently, there is no treatment or vaccine available. The etiological agent, ASFV, is highly resistant to environmental conditions, and resides in unheated pork meat or pork meat products for a long period, which may be a chance of its long-distance spread. Since August 2018, ASFV has been circulating in East and Southeast Asian countries and may possibly be introduced into Japan. Here, I describe the outline of the disease and the etiology of the pathogen in order to remind the importance of "awareness" and "preparedness" for the disease.

Rapid Sequence-Based Characterization of African Swine Fever Virus by Use of the Oxford Nanopore MinION Sequence Sensing Device and a Companion Analysis Software Tool

African swine fever virus (ASFV) is the causative agent of a severe and highly contagious viral disease of pigs that poses serious economic consequences to the swine industry due to the high mortality rate and impact on international trade. There is no effective vaccine to control African swine fever (ASF), and therefore, efficient disease control is dependent on early detection and diagnosis of ASFV. The large size of the ASFV genome (∼180 kb) has historically hindered efforts to rapidly obtain a full-genome sequence. Rapid acquisition of data is critical for characterization of the isolate and to support epidemiological efforts. Here, we investigated the capacity of the Oxford Nanopore MinION sequence sensing device to act as a rapid sequencing tool. When coupled with our novel companion software script, the African swine fever fast analysis sequencing tool (ASF-FAST), the analysis of output data was performed in real time. Complete ASFV genome sequences were generated from cell culture isolates and blood samples obtained from experimentally infected pigs. Removal of the host-methylated DNA from the extracted nucleic acid facilitated rapid ASFV sequence identification, with reads specific to ASFV detected within 6 min after the initiation of sequencing. Regardless of the starting material, sufficient sequence was available for complete genome resolution (up to 100%) within 10 min. Overall, this paper highlights the use of Nanopore sequencing technology in combination with the ASF-FAST software for the purpose of rapid and real-time resolution of the full ASFV genome from a diagnostic sample.

Mechanisms of African swine fever virus pathogenesis and immune evasion inferred from gene expression changes in infected swine macrophages

African swine fever (ASF) is a swine disease caused by a large, structurally complex, double-stranded DNA virus, African swine fever virus (ASFV). In domestic pigs, acute infection by highly virulent ASF viruses causes hemorrhagic fever and death. Previous work has suggested that ASFV pathogenesis is primarily mediated by host cytokines produced by infected monocytes and macrophages. To better understand molecular mechanisms mediating virus pathogenesis and immune evasion, we used transcriptome analysis to identify gene expression changes after ASFV infection in ex vivo swine macrophages. Our results suggest that the cytokines of TNF family including FASLG, LTA, LTB, TNF, TNFSF4, TNFSF10, TNFSF13B and TNFSF18 are the major causative cytokine factors in ASF pathogenesis via inducing apoptosis. Other up-regulated proinflammatory cytokines (IL17F and interferons) and down-regulated anti-inflammatory cytokine (IL10) may also significantly contribute to ASF pathogenesis and cause excessive tissue inflammatory responses. The differential expression of genes also indicates that ASFV could evade both the innate and adaptive immune responses by (i) inhibiting MHC Class II antigen processing and presentation, (ii) avoiding CD8+ T effector cells and neutrophil extracellular traps via decreasing expression of neutrophil/CD8+ T effector cell-recruiting chemokines, (iii) suppressing M1 activation of macrophages, (iv) inducing immune suppressive cytokines, and (v) inhibiting the processes of macrophage autophagy and apoptosis. These results provide novel information to further investigate and better understand the mechanism of pathogenesis and immune evasion of this devastating swine disease.

Current status and evolving approaches to African swine fever vaccine development

African swine fever (ASF) is a highly lethal haemorrhagic disease of swine caused by African swine fever virus (ASFV), a unique and genetically complex virus. The disease continues to be a huge burden to the pig industry in Africa, Europe and recently in Asia, especially China. The purpose of this review was to recapitulate the current scenarios and evolving trends in ASF vaccine development. The unavailability of an applicable ASF vaccine is partly due to the complex nature of the virus, which encodes various proteins associated with immune evasion. Moreover, the incomplete understanding of immune protection determinants of ASFV hampers the rational vaccine design. Developing an effective ASF vaccine continues to be a challenging task due to many undefined features of ASFV immunobiology. Recent attempts on DNA and live attenuated ASF vaccines have been reported with promising efficacy, and especially live attenuated vaccines have been proved to provide complete homologous protection. Single-cycle viral vaccines have been developed for various diseases such as Rift Valley fever and bluetongue, and the rational extension of these strategies could be helpful for developing single-cycle ASF vaccines. Therefore, live attenuated vaccines in short term and single-cycle vaccines in long term would be the next generation of ASF vaccines.