African swine fever virus replication and genomics

Thonningianin A disrupts pA104R-DNA binding and inhibits African swine fever virus replication

African swine fever is a highly lethal disease caused by the African swine fever virus (ASFV), posing a significant threat to the global pig industry, wherease no approved treatments are currently available. The ASFV DNA-binding protein, pA104R, plays a critical role in viral genome packaging and replication, making it a key target for drug discovery. Through structure-based virtual screening, we identified a polyphenolic compound, thonningianin A, which disrupts the pA104R-DNA binding and significantly inhibits ASFV replication. Mechanistic study revealed that thonningianin A binds to the DNA-binding region of pA104R, forming strong hydrogen bonds with H100 and occupying the vital DNA-binding residues K92, R94, and K97. In addition, we resolved the high-resolution (1.8 Å) structure of pA104R (PDB ID 9JS5), providing valuable insights for future drug screening. Together, these results demonstrate that thonningianin A holds great potential for the development of anti-ASFV drug, as a herb extract with favourable pharmacokinetic properties and safety.

Establishment of an immunoperoxidase monolayer assay for the detection of African swine fever virus antibodies

African swine fever (ASF) is a lethal infectious disease affecting domestic and wild pigs, caused by the African swine fever virus (ASFV), with a mortality rate of up to 100%. The evolving prevalence and variation of ASFV has led to the emergence of low-virulence strains, which induced chronic infections and posed challenges in nucleic acid-based diagnostics due to potential false negatives. This underscores the urgent need for reliable antibody monitoring to facilitate early diagnosis. In this study, based on a highly attenuated BK2258 cell-adapted strain HLJ18/BK33, we established an immunoperoxidase monolayer assay (IPMA) for ASFV antibodies detection. After optimization using a total of 608 pig sera, the performance of the assay was better than that of the commercial iELISA with higher sensitivity and specificity. The newly established IPMA method demonstrated high specificity with no cross-reactivity with positive sera for six other important porcine pathogens. The IPMA method developed in the present study could serve as the potential gold standard for serological diagnosis and evaluation of other detection methods for ASFV antibodies, owing to its high sensitivity and specificity. Furthermore, the IPMA method will provide a new and effective strategy for ASFV monitoring, prevention and control in China.

Establishment of a highly sensitive porcine alveolar macrophage cell line for African swine fever virus

African swine fever (ASF) caused by the African swine fever virus (ASFV) is a significant threat to domestic pig populations because of its highly contagious nature and associated morbidity and mortality. The lack of an appropriate cell line for ASFV propagation has significantly hindered the development of a safe and effective vaccine. In this study, we aimed to identify a cell line that is highly receptive to ASFV by evaluating various genes to determine their ability to support ASFV infection and replication. Our investigation revealed the efficient infection of a porcine alveolar macrophage cell line iPAM, upon stable overexpression of the transmembrane protein 107 (TMEM107). An isolated monoclonal cell line iPAMpCDH-TMEM107-B6 that was derived from the parental iPAM cell line exhibited increased susceptibility to ASFV infection. Notably, iPAMpCDH-TMEM107-B6 cells concurrently expressed ASFV B646L and ASFV p30 proteins after infection with ASFV. Biological characterization of iPAMpCDH-TMEM107-B6 revealed an enhanced proliferative capacity without compromised phagocytic function, indicating the retention of key cellular traits following genetic modification. The iPAMpCDH-TMEM107-B6 cell line has significant potential for ASFV research and will facilitate tasks such as isolation, replication, and genetic manipulation. The establishment of ASFV-sensitive cell lines provides an in vitro research platform for ASFV investigations, thereby advancing our understanding of the pathogenic mechanisms and aiding in vaccine development efforts.

Characterization of a swine-derived single-chain fragment variable targeting the conserved immunodominant epitope of African swine fever virus p30

Early detection and monitoring of antibodies against ASFV are essential for effectively reducing the risk of viral transmission. The ASFV structural protein p30 is an optimal target for diagnostic applications; however, its genetic variation across different genotypes presents a challenge. Therefore, identifying immunodominant epitopes on p30 that are consistently recognized by swine antibodies is critical for developing accurate and sensitive detection methods. In this study, we constructed a phage display library presenting scFv derived from an ASFV-infected pig and successfully isolated a swine-derived scFv against p30, termed scFvp30. The epitope recognized by scFvp30 was further identified as a highly conserved, immunodominant epitope located on the surface loop region of p30 (117SSFETLFEQ125), and the binding was mediated by hydrogen bonds and π-stacking interactions between p30 and scFvp30. Additionally, a bELISA based on scFvp30 was developed, which effectively detects ASFV-positive sera. The identification of the ASFV p30 immunodominant epitope and its specific swine-derived scFv lays the foundation for further investigations into the conserved structure and function of ASFV p30, as well as for elucidating ASFV pathogenesis. In addition, it also provides a target for the development of diagnostic methods to detect p30 protein antibodies across different genotypes ASFV.

African swine fever virus I177L induces host inflammatory responses by facilitating the TRAF6-TAK1 axis and NLRP3 inflammasome assembly

African swine fever virus (ASFV) is the pathogen of African swine fever (ASF), and its infection causes a lethal disease in pigs, with severe pathological lesions. These changes indicate excessive inflammatory responses in infected pigs, which is the main cause of death, but the ASFV proteins worked in this physiological process and the mechanisms underlying ASFV-induced inflammation remain unclear. Here, we identify that viral I177L works in these inflammatory responses. Mechanistically, I177L facilitates TRAF6 ubiquitination that enhances its binding to TAK1, which promotes TAK1 ubiquitination and phosphorylation. These processes depend on the E3 ubiquitin ligase activity of TRAF6. The upregulation of I177L to TRAF6-TAK1 interaction and TAK1 activation is responsible for I177L's activated effect on the NF-κB signaling pathway. Additionally, I177L promotes assembly of the NLRP3 inflammasome and ASC oligomerization, thus leading to the activation of the NLRP3 inflammasome and the production and secretion of mature IL-1β. TAK1 inhibition efficiently reverses ASFV-activated NF-κB signaling and inflammatory responses and suppresses ASFV replication. Furthermore, I177L-deficient ASFV induces milder inflammatory responses in pigs compared with parental ASFV, which still protects pigs against ASFV challenge. The finding confirms ASFV I177L as an important proinflammatory protein in vitro and in vivo and reveals a key mechanism underlying ASFV-mediated inflammatory responses for the first time, which enriches our knowledge of the complex ASFV, thus benefiting our understanding of the interplay between ASFV infection and the host's inflammatory responses.IMPORTANCEAfrican swine fever (ASF) is a devastating viral disease in pigs, and excessive inflammatory responses induced by ASFV mainly cause death. Thus, the study of the proinflammatory virulent proteins and the detailed mechanisms are important to ASF control. Here, I177L was demonstrated to be an essential protein in ASFV-mediated inflammation, which performs by simultaneously activating the NF-κB signaling and the NLRP3 inflammasome. The finding elucidates the molecular mechanism underlying ASFV-activated inflammatory responses for the first time. It provides a theoretical foundation for reducing the high mortality caused by excessive inflammation and opens new avenues for small-molecule drug development and vaccine design targeting ASFV.

DNAJA3 interacts with ASFV MGF360-14L protein and reduces MGF360-14L antagonistic role on Beta interferon production

African swine fever (ASF) is a devastating infectious disease caused by African swine fever virus (ASFV). Many multiple structural and non-structural proteins of ASFV have been confirmed to evade the host's immune response. In this study, the interaction of non-structural protein MGF360-14L with DnaJ heat shock protein family (Hsp40) member A3 (DNAJA3) were firstly detected by yeast two-hybrid screening, further confirmed by communoprecipitation and colocalization, meanwhile we also found that MGF360-14L was localized in the cytoplasm. The DNAJA3 (amino acids 296 to 453) and the MGF360-14L (amino acids 1 to 119) were shown to be critical for the interaction of DNAJA3 with MGF360-14L. Over-expression of DNAJA3 dramatically dampened MGF360-14L expression, and induced lysosomal degradation of MGF360-14L. Our study have previously demonstrated that the MGF360-14L induced ubiquitin degradation of IRF3 and thus inhibited the production of IFN-β. Further research showed that MGF360-14L can significantly enhance the ubiquitination-mediated degradation of IRF3 and strengthen the suppression of IFN-β in DNAJA3-knockout cells. These findings suggest that the DNAJA3 played a negative regulatory role for the inhibition of MGF360-14L on the IFN-β, further study indicated that DNAJA3 plays an important antiviral role against ASFV by both degrading MGF360-14L and restoring of IFN-β.

Lipid nanoparticle-encapsulated DNA vaccine encoding African swine fever virus p54 antigen elicits robust immune responses in pigs

African swine fever virus (ASFV) is one of the most significant viral pathogens affecting swine production worldwide. While several live attenuated ASF vaccines have been approved for clinical application in certain countries, there is a concern that the vaccine viruses might revert to virulence. Subunit vaccines containing one or a few viral immunogens provide a safer alternative. DNA plasmids are highly stable, easy to produce in large quantities at low cost, and safe for use in animals. However, unencapsulated DNA vaccines often exhibited low immunogenicity, largely due to the inefficient cellular entry of the plasmid DNA, leading to low protein expression. In this study, we used ASFV p54 as a model antigen to investigate the feasibility of using lipid nanoparticles (LNP) as nanocarriers to enhance the immunogenicity of DNA vaccines. Pigs immunized with the p54 LNP-DNA vaccine elicited high titers of p54-specific antibodies and T-cell responses after the second immunization. Using ELISAs based on an overlapping peptide library, we identified three antigenic areas within p54. Additionally, we noted that pigs vaccinated with the p54 LNP-DNA vaccine exhibited a similar antibody profile as those vaccinated with an experimental live attenuated vaccine or infected with a wild-type ASFV strain. The results highlight the promising potential of LNP-DNA as an effective platform for developing gene-based vaccines against ASFV.

Structure-function insights into the dual role of African swine fever virus pB318L: A typical geranylgeranyl-diphosphate synthase and a nuclear import protein

African swine fever virus (ASFV) pB318L is an important protein for viral replication that acts as a membrane-bound trans-geranylgeranyl-diphosphate synthase (GGPPS) catalyzing the condensation of isopentenyl diphosphate (IPP) with allylic diphosphates. Recently we solved the crystal structure pB318L lacking N-terminal transmembrane region and performed a preliminary structural analysis. In this study, structure-based mutagenesis study and geranylgeranyl pyrophosphate (GGPP) production assay further revealed the key residues for the GGPPS activity. Structural comparison showed pB318L displays a strong similarity to typical GGPPSs instead of protein prenyltransferases. The phylogenetic analysis indicated pB318L may share a common ancestor with the GGPPSs from Brassicaceae plants rather than from its natural host. The subcellular localization analysis showed pB318L is localized in both nucleus and cytoplasm (including the endoplasmic reticulum membrane and mitochondria outer membrane). A unique N-terminal nuclear localization signal (NLS) following the transmembrane region was discovered in pB318L and the NLS was confirmed to be required for the nuclear import. We further revealed the NLS plays an essential role in the interaction with nuclear transporter karyopherin subunit alpha 1 (KPNA1). Their interaction may suppress signal transducers and activators of transcription 1 (STAT1) translocation and subsequently competitively inhibit nuclear import of IFN-stimulated gene factor 3 (ISGF3) complex. Our biochemical, structural and cellular analyses provide novel insights to pB318L that acts as an essential GGPPS that promotes viral replication and as a nuclear import protein that may be involved in immune evasion of ASFV.

ASFV pS183L protein negatively regulates RLR-mediated antiviral signalling by blocking MDA5 oligomerisation

The retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are major sensors against viral infection, but their roles in DNA virus infection largely remain unknown. This study found that a previously uncharacterised protein, pS183L, negatively regulates RLR signalling by suppressing MDA5 oligomerisation. Specifically, we showed that the overexpression of pS183L suppresses MDA5 but not cGAS-STING or RIG-I-induced IFN-β activation. Consistently, pS183L inhibited high molecular weight poly (I:C) activated IFN-β production. Furthermore, we demonstrated that pS183L interacts with CARDs and the MDA5 Helicase domain, consequently blocking MDA5 oligomerisation and the MDA5-MAVS interaction. Taken together, we concluded that pS183L blocks MDA5 oligomerisation through protein-protein interaction and thus disrupts MDA5-mediated IFN-β signalling.

A synthetic genomics-based African swine fever virus engineering platform

African swine fever (ASF) is a deadly viral disease in domestic pigs that has a large global economic impact for the swine industry. It is present in Africa, Europe, Asia, and in the Caribbean island of Hispaniola. There are no effective treatments or broadly licensed vaccines to prevent disease. Efforts to counteract ASF have been hampered because of the lack of convenient tools to engineer its etiological agent, ASF virus (ASFV), largely due to its large noninfectious genome. Here, we report the use of synthetic genomics methodology to develop a reverse genetics system for ASFV using a CRISPR-Cas9-inhibited self-helper virus to reconstitute live recombinant ASFV from synthetic genomes to rapidly generate a variety of combinatorial mutants of ASFV. The method will substantially facilitate the development of therapeutics or subunit and live-attenuated vaccines for ASF. This synthetic genomics-based approach has wide-ranging impact because it can be applied to rapidly develop reverse genetics tools for emerging viruses with noninfectious genomes.

DNA-RNA hybrids in inflammation: sources, immune response, and therapeutic implications

Cytoplasmic DNA-RNA hybrids are emerging as important immunogenic nucleic acids, that were previously underappreciated. DNA-RNA hybrids, formed during cellular processes like transcription and replication, or by exogenous pathogens, are recognized by pattern recognition receptors (PRRs), including cGAS, DDX41, and TLR9, which trigger immune responses. Post-translational modifications (PTMs) including ubiquitination, phosphorylation, acetylation, and palmitoylation regulate the activity of PRRs and downstream signaling molecules, fine-tuning the immune response. Targeting enzymes involved in DNA-RNA hybrid metabolism and PTMs regulation offers therapeutic potential for inflammatory diseases. Herein, we discuss the sources, immune response, and therapeutic implications of DNA-RNA hybrids in inflammation, highlighting the significance of DNA-RNA hybrids as potential targets for the treatment of inflammation.

African swine fever virus vaccine strain Asfv-G-∆I177l reverts to virulence and negatively affects reproductive performance

ASFV-G-ΔI177L is a modified-live African swine fever virus (ASFV) strain that has been incorporated into a commercially available vaccine. Its safety in pregnant sows and genetic stability in an in vivo passaging experiment were investigated. Upon inoculation of two pregnant sows with ASFV-G-ΔI177L, one developed moderate ASF-related clinical signs. In terms of reproductive performance, 43% of the offspring was born dead and the live-born piglets developed ASF-specific clinical signs, became viremic, and only 17% survived until the end of study. During passaging in pigs, ASFV-G-ΔI177L reverted to virulence with severe ASF-specific clinical signs at passages 3 and 4, associated with increased viremia. Whole genome sequencing identified C257L mutations as a potential driver of increased replication fitness and virulence. The data show that ASFV-G-ΔI177L is not genetically stable and, therefore not safe for use in ASF vaccines and suggest that ASF vaccine candidates should be tested for safety in pregnant animals.

ASFV p30 interacts with CCAR2 and MATR3 to promote ASFV replication

African swine fever (ASF) is a highly contagious and lethal disease caused by the African swine fever virus (ASFV). Currently, effective vaccines are not available for the prevention and control of ASF. ASFV is susceptible to mutations as it has a large genome and encodes numerous proteins. In addition to evading the host immune response, ASFV utilizes host proteins to regulate its replication. The ASFV p30 protein is involved in virus internalization into the host cell and is expressed throughout the viral replication cycle, influencing viral replication. This study identified the host proteins that interact with p30 using mass spectrometry analysis. Immunoprecipitation analysis confirmed that the ASFV p30 protein interacted with the host proteins CCAR2 and MATR3, co-localizing with them in the cytoplasm. CCAR2 and MATR3 promoted ASFV replication. Conversely, ASFV infection upregulated the expression of CCAR2 and MATR3 in the host. Thus, the ASFV p30 protein regulates ASFV replication by interacting with CCAR2 and MATR3.

Biochemical characterization and inhibitor potential of African swine fever virus thymidine kinase

African Swine Fever (ASF) is a highly contagious disease affecting both domestic pigs and wild boars. In domestic pigs, ASF is a rapidly-progressing disease with a mortality rate reaching 100 %, causing tremendous economic loss in affected areas. ASFV is caused by African Swine Fever Virus (ASFV), which is a large, enveloped double-stranded DNA virus belonging to the Asfarviridae family. ASFV has a remarkably large genome size that encodes more than 150 open reading frames. Among the virally encoded enzymes, thymidine kinase (ASFV-TK) has been shown to be critical for the efficient replication and virulence of ASFV. Here, we report the bioinformatics analysis and biochemical characterization of ASFV-TK. Amino acid sequence analysis revealed that ASFV-TK can be classified as a type II thymidine kinase. Kinetics characterization revealed a maximum velocity (Vmax) and Michaelis constants (Km) that are within the same range as previously characterized type II enzymes. ASFV-TK is competitively inhibited by the feedback inhibitor thymidine 5'-triphosphate and can use 3'-azido-3'-deoxythymidine (AZT) as a substrate with kinetics parameters comparable to those obtained with natural substrates, suggesting that nucleosides and nucleotide analogs could be explored as anti-ASFV agents.

Transcriptome Profiling Reveals That the African Swine Fever Virus C315R Exploits the IL-6 STAT3 Signaling Axis to Facilitate Virus Replication

African swine fever (ASF) is an acute and highly contagious disease that has caused great losses in the past years. It is caused by African swine fever virus (ASFV), which is a large DNA virus encoding about 165 genes. It has been shown that the purified extracellular ASFV is internalized by both constitutive macropinocytosis and clathrin-mediated endocytosis, and the virus utilizes apoptotic bodies for infection and cell cell transmission. The ASFV-encoded RNA polymerase subunit C315R is thought to play an important role in ASFV replication and transcription. However, its involvement in ASFV infection, particularly in host response, remains only partially understood. In this study, the role of C315R in enhancing ASFV replication was investigated through RNA-Seq transcriptomic analysis, which was based on 3D4/21 cells transfected the plasmid expressing HA-tagged C315R or the empty vector. Our findings revealed that C315R significantly upregulates the expression of inflammatory mediators with a particular emphasis on IL-6. The most differentially expressed genes (DEGs) were predominantly associated with the TNF, IL-17, MAPK, and JAK STAT signaling pathways. RNA-seq results were validated through RT-PCR. Subsequently, we observed that ASFV infection increases IL-6 expression and STAT3 phosphorylation, which is regulated by the ASFV C315R protein. Notably, inhibiting STAT3 phosphorylation with specific inhibitors suppressed ASFV replication. In conclusion, our study demonstrates that the ASFV C315R protein actives STAT3 phosphorylation through promoting the transcription of IL-6 to facilitate virus replication. These findings highlight C315R as a positive regulator in the IL-6 STAT3 signaling axis during ASFV infection.

Development of Real-Time and Lateral Flow Dipstick Recombinase Polymerase Amplification Assays for the Rapid Field Diagnosis of MGF-505R Gene-Deleted Mutants of African Swine Fever Virus

Pigs are susceptible to the deadly infectious disease known as African swine fever (ASF), which is brought on by the African swine fever virus (ASFV). As such, prompt and precise disease detection is essential. Deletion of the virulence-related genes MGF-505/360 and EP402R generated from the virulent genotype II virus significantly reduces its virulence, and animal tests using one of the recombinant viruses show great lethality and transmissibility in pigs. The isothermal technique known as recombinase polymerase amplification (RPA) is perfect for rapid in-field detection. To accurately identify ASFV MGF-505R gene-deleted mutants and assess the complex infection situation of ASF, RPA assays in conjunction with real-time fluorescent detection (real-time RPA assay) and lateral flow dipstick (RPA-LFD assay) were created. These innovative methods allow for the direct detection of ASFV from pigs, offering in-field pathogen detection, timely disease management, and satisfying animal quarantine requirements. The specific primers and probes were designed against conserved regions of ASFV B646L and MGF-505R genes. Using recombinant plasmid DNA containing ASFV MGF-505R gene-deleted mutants as a template, the sensitivity of both ASF real-time RPA and ASF RPA-LFD assays were demonstrated to be 10 copies per reaction within 20 min at 37 °C. Neither assay had cross-reactions with CSFV, PRRSV, PPV, PRV, ot PCV2, common viruses seen in pigs, indicating that these methods were highly specific for ASFV. The evaluation of the performance of ASFV real-time RPA and ASFV RPA-LFD assays with clinical samples (n = 453) demonstrated their ability to specifically detect ASFV or MGF-505R gene-deleted mutants in samples of pig feces, ham, fresh pork, and blood. Both assays exhibited the same diagnostic rate as the WOAH-recommended real-time fluorescence PCR, highlighting their reliability and validity. These assays offer a simple, cost-effective, rapid, and sensitive method for on-site identification of ASFV MGF-505R gene-deleted mutants. As a promising alternative to real-time PCR, they have the potential to significantly enhance the prevention and control of ASF in field settings.

The Deletion of the MGF360-10L/505-7R Genes of African Swine Fever Virus Results in High Attenuation but No Protection Against Homologous Challenge in Pigs

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF), a severe hemorrhagic disease with a mortality rate reaching 100%. Despite extensive research on ASFV mechanisms, no safe and effective vaccines or antiviral treatments have been developed. Live attenuated vaccines generated via gene deletion are considered to be highly promising. We developed a novel recombinant ASFV strain by deleting MGF360-10L and MGF505-7R, significantly reducing virulence in pigs. In the inoculation experiment, pigs were infected with 104 50% hemadsorption doses (HAD50) of the mutant strain. All the animals survived the observation period without showing ASF-related clinical signs. Importantly, no significant viral infections were detected in the cohabitating pigs. In the virus challenge experiment, all pigs succumbed after being challenged with the parent strain. RNA-seq analysis showed that the recombinant virus induced slightly higher expression of natural immune factors than the parent ASFV; however, this level was insufficient to provide immune protection. In conclusion, our study demonstrates that deleting MGF360-10L and MGF505-7R from ASFV CN/GS/2018 significantly reduces virulence but fails to provide protection against the parent strain.

20 years of research on giant viruses

Some twenty years ago, the discovery of the first giant virus, Acanthamoeba polyphaga mimivirus (now mimivirus bradfordmassiliense species), paved the way for the discovery of more than 10 new families of protist-infecting DNA viruses with unexpected diversity in virion shape and size, gene content, genome topology and mode of replication. Following their brief description, we examine how the historical concepts of virology have held up in the light of this new knowledge. Although the initial emphasis was on the gigantism of the newly described viruses infecting amoebae, the subsequent discovery of viruses with intermediate virion and genome sizes gradually re-established a continuum between the smallest and largest viruses within the phylum Nucleocytoviricota.

African Swine Fever Vaccine Candidate ASFV-G-ΔI177L/ΔLVR Protects Against Homologous Virulent Challenge and Exhibits Long-Term Maintenance of Antibodies

African swine fever virus (ASFV) has substantially spread worldwide, resulting in significant economic losses in the swine industry. Despite extensive research, no ASF vaccine has surpassed the effectiveness of live attenuated vaccines. For instance, the live attenuated vaccine ASFV-G-ΔI177L/ΔLVR has demonstrated good efficacy and safety, along with prolonged persistence of ASF antibodies after vaccination. Therefore, we aimed to evaluate its potential for protection against highly virulent homologous ASF viruses based on changes in the farm environment. To this end, we challenged domestic pigs with a virulent field strain of ASFV following intramuscular immunization with ASFV-G-ΔI177L/ΔLVR. We further assessed its genomic stability and long-term antibody persistence in immunized domestic pigs. All vaccinated pigs exhibited high antibody positivity, with higher levels of antibodies observed at the time of challenge. These high ASF vaccine antibodies were maintained for approximately 2 months after vaccination. In addition, no organ or tissue damage was observed in the vaccinated animals. Our findings demonstrate the applicability of this vaccine candidate in the prevention of ASFV infection in the swine industry.

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.

Serologic differentiation between wild-type and cell-adapted African swine fever virus infections: A novel DIVA strategy using the MGF100-1L protein

African swine fever virus (ASFV) poses a significant threat to the global swine industry and requires improved control strategies. Here, we developed a Differentiating Infected from Vaccinated Animals (DIVA) assay based on the MGF100-1L protein, which is absent in a cell-adapted ASFV strain lacking several multigene family (MGF) genes. We analyzed seven deleted genes, including MGF genes, from the right variable region of the ASFV genome against sera from convalescent pigs. MGF100-1L showed significant reactivity and was produced as a recombinant protein for use in an enzyme-linked immunosorbent assay (ELISA). The assay, with a cut-off value of 0.284, successfully differentiated between naive and infected pigs with 100% accuracy. More importantly, pigs infected with the cell-adapted ASFV showed no significant change in ELISA readouts after 27 days post-infection. However, when these pigs were subsequently challenged with wild-type virus, MGF100-1L reactivity increased significantly by 21 days post-challenge. This study demonstrates the potential of MGF100-1L as a DIVA marker for ASFV, which offers a promising tool to distinguish between infections with wild-type ASFV and those with cell-adapted variants lacking specific MGF genes, thereby improving ASFV surveillance and control strategies.

The CP123L protein of African swine fever virus is a membrane-associated, palmitoylated protein required for viral replication

African swine fever (ASF) is a highly contagious and often lethal disease caused by African swine fever virus (ASFV) in pigs. Protein palmitoylation is a prevalent posttranslational lipid modification that can modulate viral replication. In this study, we investigated the palmitoylation of ASFV proteins. The results revealed that the CP123L protein (pCP123L) of ASFV was palmitoylated at the cysteine residue at position 18 (C18). To further elucidate the functional significance of this posttranslational modification, abolishing palmitoylation through a cysteine-to-serine mutation at C18 (C18S) of pCP123L (pCP123L/C18S) or treatment with 2-bromopalmitate (2-BP), a palmitoylation inhibitor, led to altered cytomembrane localization and migration rate of pCP123L. Furthermore, depalmitoylation achieved through 2-BP treatment significantly suppressed ASFV replication and exerted a profound impact on virus budding. Remarkably, blocking pCP123L palmitoylation via the C18S mutation resulted in decreased replication of ASFV. Our study represents the first evidence for the presence of palmitoylation in ASFV proteins and underscores its crucial role in viral replication.

IMPORTANCE

African swine fever (ASF) poses a significant threat to the global pig industry. The causative agent of ASF is African swine fever virus (ASFV), which encodes more than 165 proteins. Protein palmitoylation, a common posttranslational lipid modification, can modulate viral infection. To date, the ASFV proteins that undergo palmitoylation and their impacts on viral replication remain elusive. In this study, the CP123L protein (pCP123L) of ASFV was identified as a palmitoylated protein, and the cysteine residue at position 18 of pCP123L is responsible for its palmitoylation. Notably, our findings demonstrate that palmitoylation plays significant roles in ASFV protein functions and facilitates viral replication.

Monoclonal Antibodies Targeting Porcine Macrophages Are Able to Inhibit the Cell Entry of Macrophage-Tropic Viruses (PRRSV and ASFV)

Porcine reproductive and respiratory syndrome virus (PRRSV) and African swine fever virus (ASFV) cause serious economic losses to the swine industry worldwide. Both viruses show a tropism for macrophages, based on the use of specific entry mediators (e.g., Siglec-1 and CD163). Identifying additional mediators of viral entry is essential for advancing antiviral and vaccine development. In this context, monoclonal antibodies (mAbs) are valuable tools. This study employed a library of 166 mAbs targeting porcine alveolar macrophages (PAMs) to identify candidates capable of blocking early infection stages, including viral binding, internalization, and fusion. Immunofluorescence analysis revealed 74 mAbs with cytoplasmic staining and 70 mAbs with membrane staining. Fifteen reacted with blood monocytes as determined by flow cytometry. mAb blocking assays were performed at 4 °C and 37 °C to analyze the ability of mAbs to block PRRSV and/or ASFV infections in PAMs. The mAb 28C10 significantly blocked PRRSV (96% at 4 °C and 80% at 37 °C) and ASFV (64% at 4 °C and 81% at 37 °C) infections. The mAb 28G10B6 significantly blocked PRRSV (86% at 4 °C and 74% at 37 °C) and partially blocked ASFV (35% at 4 °C and 64% at 37 °C) infections. mAb 26B8F5-I only partially blocked PRRSV infection (65% at 4 °C and 46% at 37 °C). Western blotting and mass spectrometry identified the corresponding proteins as Siglec-1 (28C10; 250 kDa), MYH9 (28G10B6; 260 kDa), and ANXA1 (26B8F5-I; 37 kDa). Our findings are indicative that Siglec-1, MYH9, and ANXA1 play a role in PRRSV/ASFV entry into macrophages.

Progress in African Swine Fever Vector Vaccine Development

African swine fever (ASF) is a highly lethal, infectious, hemorrhagic fever disease, characterized by an acute mortality rate approaching 100%. It is highly contagious, and results in significant losses to the global hog industry as it spreads. Despite incremental progress in research on the African swine fever virus (ASFV), a safe and effective commercial vaccine has yet to be developed. Vector vaccines, a promising type of vaccine, offer unique advantages, and are a primary focus in ASFV vaccine research. This paper focuses on the characteristics of viral, bacterial, and yeast vector vaccines; elucidates the immunological mechanisms associated with antigens; lists the types of antigens that have significant potential; discusses the feasibility of using exogenously expressed cytokines to enhance the protective power of vector vaccines; and, finally, discusses the types of vectors that are commonly used and the latest advances in this field.

Genetic profile of the whole genome sequence of African swine fever virus from the first outbreak in Malaysian Borneo

African swine fever (ASF), a severe and highly contagious haemorrhagic viral disease of pigs, is becoming a major threat not only in Malaysia but around the world. The first confirmed case of ASF in Malaysia was reported in February 2021. Despite the emergence of ASF in Malaysia, genetic information on this causative pathogen for the local livestock is still limited. This study aimed to genetically characterize the African swine fever virus (ASFV) responsible for the 2021 outbreak in Malaysia. The genome of the ASFV isolated during the first outbreak in Malaysia was analysed as ASFV/Sabah/Malaysia/1160/21 which has 190,594 base pairs, with a nitrogenous bases (GC) content of 40.33% and 195 predicted Open Reading Frames (ORF). The complete genome sequence was compared with other annotated ASFV genomes retrieved from database of National Center for Biotechnology Information (NCBI) to obtain information based on target gene B646L, E183L, intergenic region (IGR) between I73R and I329L (IGRI73R-I329L), EP402R and B602L. The ASFV/Sabah/Malaysia/1160/21 genome had a high similarity percentage to the reference genome, Georgia 2007 and all Southeast Asian strains. Phylogenetic analysis revealed that the ASFV strain belonged to genotype II, serogroup 8, CVR1 and showed high characteristics of IGR variant II based on IGRI73R-I329L. This study expands our understanding of genetic diversity and provides significant insights into the genomic characteristics and variation of ASFV strains that are circulating in Malaysia.

Genome-Wide Approach Identifies Natural Large-Fragment Deletion in ASFV Strains Circulating in Italy During 2023

African swine fever (ASF), characterized by high mortality rates in infected animals, remains a significant global veterinary and economic concern, due to the widespread distribution of ASF virus (ASFV) genotype II across five continents. In this study, ASFV strains collected in Italy during 2022-2023 from two geographical clusters, North-West (Alessandria) and Calabria, were fully sequenced. In addition, an in vivo experiment in pigs was performed. Complete genomic sequencing of 30 strains revealed large-fragment deletions and translocations. In Alessandria, five samples showed two different deletions in the 5' genomic region: a ~4340 bp deletion (positions ~9020-13,356 in Georgia 2007/1) in four samples and a 2162 bp deletion (positions 17,837-19,998) in one sample. Another strain showed a truncation of 1950 bp at the 3' end. In Calabria, strains showed a 5137 bp deletion (positions 10,755-15,891) and ~2 kb truncations in the 3' region. Two strains showed a translocation from the region 1-2244 to positions 188,631-190,584. In vivo characterization of the deleted strain 22489.4_2312/RC/2023 revealed identical disease progression to the wild-type strain, with severe ASF symptoms in inoculated pigs. This study is the first to report natural deleted strains of ASFV in Italy, revealing unique genomic deletions distinct from those in previously known strains.

Structural basis of RNA polymerase complexes in African swine fever virus

African swine fever virus is highly contagious and causes a fatal infectious disease in pigs, resulting in a significant global impact on pork supply. The African swine fever virus RNA polymerase serves as a crucial multifunctional protein complex responsible for genome transcription and regulation. Therefore, it is essential to investigate its structural and functional characteristics for the prevention and control of African swine fever. Here, we determine the structures of endogenous African swine fever virus RNA polymerase in both nucleic acid-free and elongation states. The African swine fever virus RNA polymerase shares similarities with the core of typical RNA polymerases, but possesses a distinct subunit M1249L. Notably, the dynamic binding mode of M1249L with RNA polymerase, along with the C-terminal tail insertion of M1249L in the active center of DNA-RNA scaffold binding, suggests the potential of M1249L to regulate RNA polymerase activity within cells. These results are important for understanding the transcription cycle of the African swine fever virus and for developing antiviral strategies.

Resurgence of Type III IGR Between I73R and I329L in Wild Boars With African Swine Fever in South Korea in 2023

The African swine fever virus (ASFV) causes African swine fever (ASF), a highly lethal disease affecting domestic pigs and wild boars. Since its initial outbreak in October 2019 in Yeoncheon, Gyeonggi Province, ASF has continued to spread in South Korea. This study aimed to differentiate closely related ASFV strains through the analysis of the intergenic region (IGR) between I73R and I329L genes. In 2019, genetic analysis confirmed one IGR I type case and two IGR III type cases in Paju, followed by two more IGR III type cases in 2020. After a period of detecting only the IGR II type, IGR III type cases re-emerged in Pohang and Cheong-song in November and December 2023. Genetic analysis using B646L, CP204, B602L, and EP402R genes confirmed that the IGR III strains belong to genotype II and serogroup 8, similar to the Georgia/2007/1 strain but differing in IGR type. Since the first occurrence of ASF in wild boars in South Korea, we have continuously monitored the introduction and variation of ASFV. As a result, we reconfirmed the presence of the IGR III type ASFV in 2023, 3 years and 8 months later, in a different area from where it was last detected. This finding would not have been possible without the continuous monitoring of ASFV introduction and genetic variation. We emphasize the critical role of regular monitoring based on molecular markers and comprehensive genomic analysis in enhancing the effectiveness of ASFV control and prevention.

Pacmanvirus isolated from the Lost City hydrothermal field extends the concept of transpoviron beyond the family Mimiviridae

The microbial sampling of submarine hydrothermal vents remains challenging, with even fewer studies focused on viruses. Here we report what is to our knowledge the first isolation of a eukaryotic virus from the Lost City hydrothermal field, by co-culture with the laboratory host Acanthamoeba castellanii. This virus, named pacmanvirus lostcity, is closely related to previously isolated pacmanviruses (strains A23 and S19), clustering in a divergent clade within the long-established family Asfarviridae. The icosahedral particles of this virus are 200 nm in diameter, with an electron-dense core surrounded by an inner membrane. The viral genome of 395 708 bp (33% G + C) has been predicted to encode 473 proteins. However, besides these standard properties, pacmanvirus lostcity was found to be associated with a new type of selfish genetic element, 7 kb in length, whose architecture and gene content are reminiscent of those of transpovirons, hitherto specific to the family Mimiviridae. As in previously described transpovirons, this selfishg genetic element propagates as an episome within its host virus particles and exhibits partial recombination with its genome. In addition, an unrelated episome with a length of 2 kb was also found to be associated with pacmanvirus lostcity. Together, the transpoviron and the 2-kb episome might participate in exchanges between pacmanviruses and other DNA virus families. It remains to be elucidated if the presence of these mobile genetic elements is restricted to pacmanviruses or was simply overlooked in other members of the Asfarviridae.

Plasmacytoid dendritic cell sensing of African swine fever virus-infected macrophages results in STING-dependent robust interferon-α production

While several African swine fever virus (ASFV)-encoded proteins potently interfere with the cGAS-STING (cyclic GMP-AMP synthetase-stimulator of interferon genes) pathway at different levels to suppress interferon (IFN) type I production in infected macrophages, systemic IFN-α is induced during the early stages of AFSV infection in pigs. The present study elucidates a mechanism by which such responses can be triggered, at least in vitro. We demonstrate that infection of monocyte-derived macrophages (MDMs) by ASFV genotype 2 strains is highly efficient but immunologically silent with respect to IFN type I, IFN-stimulated gene induction, and tumor necrosis factor production. Additionally, ASFV does not directly activate plasmacytoid dendritic cells (pDCs). However, coculturing pDCs with ASFV-infected MDMs results in a strong pDC response characterized by high levels of IFN-α and tumor necrosis factor. IFN type I, in turn, promoted interleukin-1 receptor antagonist production by macrophages. Similar to the sensing of infected cells by other viruses, pDC activation required integrin-mediated cognate interactions with ASFV-infected MDMs to form an interferogenic synapse. Inhibitor studies indicated that the activation of pDCs requires the STING pathway and the formation of gap junctions. While IL-4-polarized macrophages showed increased susceptibility, IFN-γ-polarized ASFV-infected macrophages induced higher pDC activation. Pretreatment of pDCs with IFN-β and IFN-γ also enhanced IFN-α production in response to ASFV-infected macrophages, highlighting the influence of the immunological microenvironment. These findings suggest that the IFN-α detected during ASFV infection in pigs may be a result of pDC sensing ASFV-infected macrophages.

Construction of the First Russian Recombinant Live Attenuated Vaccine Strain and Evaluation of Its Protection Efficacy Against Two African Swine Fever Virus Heterologous Strains of Serotype 8

Background/Objectives: The spread of African swine fever virus (ASFV) has led to major economic losses to pork worldwide. In Russia, there are no developed or registered vaccines against ASFV genotype II, which is associated with numerous ASFV outbreaks in populations of domestic pigs and wild boars in the country. Methods: We introduced deletions of the six MGF360 and MGF505 genes of the ASFV virulent Stavropol_01/08 strain, isolated in Russia in 2008. Results: We show here that this deletion did lead to full attenuation of the ASFV virulent Stavropol_01/08 strain. Animals intramuscularly inoculated with 104 HAD50 of ΔMGF360/505_Stav developed a strong immune response and short period of viremia (at 3-7 days post-inoculation). Recombinant ΔMGF360/505_Stav strain provides complete protection of pigs against the ASFV parental Stavropol_01/08 strain (103 HAD50). Therefore, in our experiment, we did not detect the genome of both the virulent and the recombinant strains in the blood and organs post-challenge with the Stavropol_01/08. In contrast, we found only partial protection (40%) of the ΔMGF360/505_Stav-immunized pigs against challenge with the ASFV heterologous Rhodesia strain. Additionally, the surviving animals had a prolonged fever, and their condition was depressed for most of the experiment. Conclusions: Thus, the ASFV recombinant ΔMGF360/505_Stav strain is the first live attenuated vaccine (LAV) in Russia that induces complete protection in pigs challenged with the highly virulent, epidemiologically relevant strains genotype II and serotype 8. However, this ASF LAV is not able to provide a high level of protection against other variants of serotype 8.

African swine fever virus enhances viral replication by increasing intracellular reduced glutathione levels, which suppresses stress granule formation

African swine fever virus (ASFV) is a DNA virus that has significantly impacted the global swine industry. Currently, there are no effective therapies or vaccines against ASFV. Stress granules (SGs), known for their antiviral properties, are not induced during ASFV infection, even though reactive oxygen species (ROS) are generated. The mechanism by which ASFV regulates SGs formation remains unclear. This study demonstrates that ASFV antagonises SGs formation and increases intracellular levels of reduced glutathione (GSH) levels. The use of the GSH inhibitor BSO and the activator NAC confirmed that the ASFV-induced increase in GSH helps to suppress SGs formation and influences viral replication. Additionally, this study revealed that ASFV enhances GSH by upregulating the antioxidant transcription factor NRF2, as well as factors involved in GSH synthesis and regeneration, such as GCLC, and those related to the ferroptosis pathway, such as SLC7A11. Furthermore, the study uncovered that ASFV manipulates intracellular GSH levels by activating the mitochondrial protein AIFM1. This regulatory mechanism helps the virus inhibit the formation of intracellular SGs, thereby creating an optimal environment for viral replication. These findings provide new insights into the molecular strategies employed by ASFV.

Cell entry mechanisms of African swine fever virus

African swine fever virus (ASFV) is a highly complex virus that poses a significant threat to the global swine industry. However, little is known about the mechanisms of ASFV cell entry because ASFV has a multilayered structure and a genome encoding over 150 proteins. This review aims to elucidate the current knowledge on cell entry mechanisms of ASFV and the cellular and viral proteins involved. Experimental evidence suggests that ASFV utilises multiple pathways for entry, which may be cell or tissue type dependent, but the intricate nature of ASFV has hindered the identification of cellular and viral proteins involved in this process. Therefore, further research into the molecular virology of ASFV is essential to advance our understanding of the ASFV entry mechanisms, which will pave the way for innovative strategies to combat this formidable pathogen.

Development and characterization of high-efficiency cell-adapted live attenuated vaccine candidate against African swine fever

African swine fever (ASF), a contagious and lethal haemorrhagic disease of domestic pigs and wild boars, poses a significant threat to the global pig industry. Although experimental vaccine candidates derived from naturally attenuated, genetically engineered, or cell culture-adapted ASF virus have been tested, no commercial vaccine is accepted globally. We developed a safe and effective cell-adapted live attenuated vaccine candidate (ASFV-MEC-01) by serial passage of a field isolate in CA-CAS-01-A cells. ASFV-MEC-01, isolated via repeated plaque purification using next-generation sequencing analysis, was obtained at passage 18 and showed significant attenuation in 4- and 6-week-old pigs. ASFV-MEC-01 conferred 100% protection against challenge with lethal parental ASFV, which correlated with high ASFV-specific humoral and cellular immune responses. Additionally, ASFV-MEC-01 was not detected in blood until 28 days post-inoculation. Global transcriptome analysis showed that ASFV-MEC-01 lacking 12 genes triggered stronger innate antiviral responses than the parental virus, as exemplified by high levels of mRNA encoding interferon regulatory and inflammatory genes in PAM cells. Ectopic expression of most deleted genes increased replication of DNA viruses by suppressing production of interferons and pro-inflammatory cytokines. Among the genes deleted from ASFV-MEC-01, MGF100-1R interacted specifically with the scaffold dimerization domain of TBK1, thereby preventing TBK1 dimerization and impairing TBK1-mediated type I IFN and NF-κB signalling. These results suggest that attenuation of ASFV-MEC-01 may be mediated by induction of stronger type I IFN and NF-κB signalling within the host innate immune system. Thus, ASFV-MEC-01 could be a safe and effective live attenuated ASFV vaccine candidate with commercial potential.

Identification of cepharanthine as an effective inhibitor of African swine fever virus replication

African swine fever virus (ASFV) causes highly contagious swine disease, African swine fever (ASF), thereby posing a severe socioeconomic threat to the global pig industry and underscoring that effective antiviral therapies are urgently required. To identify safe and efficient anti-ASFV compounds, a natural compound library was screened by performing an established cell-based ELISA in an ASFV-infected porcine alveolar macrophage (PAM) model. In total, 6 effective anti-ASFV compounds with low cytotoxicity were identified. Cepharanthine (CEP), a bisbenzylisoquinoline alkaloid, was the most potent inhibitor effect with an IC50 of 0.3223 μM. To further investigate the mechanism through which CEP inhibits ASFV replication, transcriptome profiles were generated in PAMs treated with CEP and/or infected with ASFV. ASFV infection dramatically altered immune response-associated gene expression. CEP treatment upregulated the expression of cholesterol biosynthesis-related genes, regardless of infection status. According to time-of-addition experiments, CEP primarily exerts its antiviral effect during the early stages of ASFV infection, specifically by inhibiting viral entry. Transcriptomic analysis suggested that CEP blocks ASFV entry through the clathrin-mediated endocytosis pathway by increasing EHD2 gene expression in macrophages. Disrupting EHD2 with small interfering RNA promoted ASFV entry into clathrin-positive vesicles. Finally, the protective effect of CEP in vivo was evaluated using ASFV-infected pigs. CEP could provide partial protection against ASFV infection, as indicated by an increase in survival time from 9.67 days to 16.67 days. Our findings imply that CEP exhibits potential antiviral activity against ASFV infection in PAMs, positioning it as a promising therapeutic strategy for ASF.

The African swine fever virus MGF300-4L protein is associated with viral pathogenicity by promoting the autophagic degradation of IKKβ and increasing the stability of IκBα

African swine fever (ASF) is a highly contagious, often fatal viral disease caused by African swine fever virus (ASFV), which imposes a substantial economic burden on the global pig industry. When screening for the virus replication-regulating genes in the left variable region of the ASFV genome, we observed a notable reduction in ASFV replication following the deletion of the MGF300-4L gene. However, the role of MGF300-4L in ASFV infection remains unexplored. In this study, we found that MGF300-4L could effectively inhibit the production of proinflammatory cytokines IL-1β and TNF-α, which are regulated by the NF-κB signaling pathway. Mechanistically, we demonstrated that MGF300-4L interacts with IKKβ and promotes its lysosomal degradation via the chaperone-mediated autophagy. Meanwhile, the interaction between MGF300-4L and IκBα competitively inhibits the binding of the E3 ligase β-TrCP to IκBα, thereby inhibiting the ubiquitination-dependent degradation of IκBα. Remarkably, although ASFV encodes other inhibitors of NF-κB, the MGF300-4L gene-deleted ASFV (Del4L) showed reduced virulence in pigs, indicating that MGF300-4L plays a critical role in ASFV pathogenicity. Importantly, the attenuation of Del4L was associated with a significant increase in the production of IL-1β and TNF-α early in the infection of pigs. Our findings provide insights into the functions of MGF300-4L in ASFV pathogenicity, suggesting that MGF300-4L could be a promising target for developing novel strategies and live attenuated vaccines against ASF.

African swine fever virus modulates the endoplasmic reticulum stress-ATF6-calcium axis to facilitate viral replication

African swine fever (ASF), caused by African swine fever virus (ASFV), is a devastating infectious disease of domestic pigs and wild boar, which threatens the global pig industry. Endoplasmic reticulum (ER) is a multifunctional signaling organelle in eukaryotic cells that is involved in protein synthesis, processing, posttranslational modification and quality control. As intracellular parasitic organisms, viruses have evolved several strategies to modulate ER functions to favor their life cycles. We have previously demonstrated that the differentially expressed genes associated with unfolded protein response (UPR), which represents a response to ER stress, are significantly enriched upon ASFV infection. However, the correlation between the ER stress or UPR and ASFV replication has not been illuminated yet. Here, we demonstrated that ASFV infection induces ER stress both in target cells and in vivo, and subsequently activates the activating transcription factor 6 (ATF6) branch of the UPR to facilitate viral replication. Mechanistically, ASFV infection disrupts intracellular calcium (Ca2+) homeostasis, while the ATF6 pathway facilitates ASFV replication by increasing the cytoplasmic Ca2+ level. More specifically, we demonstrated that ASFV infection triggers ER-dependent Ca2+ release via the inositol triphosphate receptor (IP3R) channel. Notably, we showed that the ASFV B117L protein plays crucial roles in ER stress and the downstream activation of the ATF6 branch, as well as the disruption of Ca2+ homeostasis. Taken together, our findings reveal for the first time that ASFV modulates the ER stress-ATF6-Ca2+ axis to facilitate viral replication, which provides novel insights into the development of antiviral strategies for ASFV.

Pathogenicity and virulence of African swine fever virus

African swine fever (ASF) is a devastating disease with a high impact on the pork industry worldwide. ASF virus (ASFV) is a very complex pathogen, the sole member of the family Asfaviridae, which induces a state of immune suppression in the host through infection of myeloid cells and apoptosis of lymphocytes. Moreover, haemorrhages are the other main pathogenic effect of ASFV infection in pigs, related to the infection of endothelial cells, as well as the activation and structural changes of this cell population by proinflammatory cytokine upregulation within bystander monocytes and macrophages. There are still many gaps in the knowledge of the role of proteins produced by the ASFV, which is related to the difficulty in producing a safe and effective vaccine to combat the disease, although few candidates have been approved for use in Southeast Asia in the past couple of years.

EP152R-mediated endoplasmic reticulum stress contributes to African swine fever virus infection via the PERK-eIF2α pathway

African swine fever virus (ASFV) is a large, icosahedral, double-stranded DNA virus in the Asfarviridae family and the causative agent of African swine fever (ASF). ASFV causes a hemorrhagic fever with high mortality rates in domestic and wild pigs. ASFV contains an open reading frame named EP152R, previous research has shown that EP152R is an essential gene for virus rescue in swine macrophages. However, the detailed functions of ASFV EP152R remain elusive. Herein, we demonstrate that EP152R, a membrane protein located in the endoplasmic reticulum (ER), induces ER stress and swelling, triggering the PERK/eIF2α pathway, and broadly inhibiting host protein synthesis in vitro. Additionally, EP152R strongly promotes immune evasion, reduces cell proliferation, and alters cellular metabolism. These results suggest that ASFV EP152R plays a critical role in the intracellular environment, facilitating viral replication. Furthermore, virus-level experiments have shown that the knockdown of EP152R or PERK inhibitors efficiently affects viral replication by decreasing viral gene expression. In summary, these findings reveal a series of novel functions of ASFV EP152R and have important implications for understanding host-pathogen interactions.

Transcription regulation of African swine fever virus: dual role of M1249L

African swine fever virus (ASFV), which poses significant risks to the global economy, encodes a unique host-independent transcription system. This system comprises an eight-subunit RNA polymerase (vRNAP), temporally expressed transcription factors and transcript associated proteins, facilitating cross-species transmission via intermediate host. The protein composition of the virion and the presence of transcription factors in virus genome suggest existence of distinct transcription systems during viral infection. However, the precise mechanisms of transcription regulation remain elusive. Through analyses of dynamic transcriptome, vRNAP-associated components and cell-based assay, the critical role of M1249L in viral transcription regulation has been highlighted. Atomic-resolution structures of vRNAP-M1249L supercomplex, exhibiting a variety of conformations, have uncovered the dual functions of M1249L. During early transcription, M1249L could serve as multiple temporary transcription factors with C-terminal domain acting as a switcher for activation/inactivation, while during late transcription it aids in the packaging of the transcription machinery. The structural and functional characteristics of M1249L underscore its vital roles in ASFV transcription, packaging, and capsid assembly, presenting novel opportunities for therapeutic intervention.

Host Innate and Adaptive Immunity Against African Swine Fever Virus Infection

Africa swine fever virus (ASFV) is the causative agent of African swine fever (ASF), a highly contagious hemorrhagic disease that can result in up to 100% lethality in both wild and domestic swine, regardless of breed or age. The ongoing ASF pandemic poses significant threats to the pork industry and food security, with serious implications for the sanitary and socioeconomic system. Due to the limited understanding of ASFV pathogenesis and immune protection mechanisms, there are currently no safe and effective vaccines or specific treatments available, complicating efforts for prevention and control. This review summarizes the current understanding of the intricate interplay between ASFV and the host immune system, encompassing both innate and adaptive immune responses to ASFV infection, as well as insights into ASFV pathogenesis and immunosuppression. We aim to provide comprehensive information to support fundamental research on ASFV, highlighting existing gaps and suggesting future research directions. This work may serve as a theoretical foundation for the rational design of protective vaccines against this devastating viral disease.

Identification of linear B cell epitopes on the E146L protein of African swine fever virus with monoclonal antibodies

The outbreak and spread of African swine fever virus (ASFV) have caused considerable economic losses to the pig industry worldwide. Currently, to promote the development of effective ASF vaccines, especially subunit vaccines, more antigenic protein targets are urgently needed. In this work, six transmembrane proteins (I329L, E146L, C257L, EP153R, I177L, and F165R) were expressed in mammalian cell lines and screened with pig anti-ASFV serum. It was found that the E146L protein was an immunodominant protein antigen among the six selected proteins. Moreover, the E146L protein induced antibody responses in all immunized pigs. To gain insight into the antigenic characteristics of the E146L protein, three monoclonal antibodies (mAbs; 12H12, 15G1, and 15H10) were generated by immunizing BALB/c mice with the purified E146L protein. The epitopes of the mAbs were further finely mapped through a peptide fusion protein expression strategy. Finally, the epitopes of the mAbs were identified as 48PDESSIAYMRFRN61 of the mAb 12H12, 138TLTGLQRII146 of the mAb 15G1, and 30GWSPFKYSKGNT41 of the mAb 15H10. Furthermore, the epitope of mAb 15H10 was validated as the immunodominant epitope with ASFV-infected pig sera. The chemically synthesized mAb 15H10 epitope peptide (EP1) exhibited the most extensive immunoreactivity with artificially or naturally ASFV-infected pig sera. The epitope 15H10 is located on the surface of the E146L protein and is highly conserved. These findings provide insight into the structure and function of the E146L protein of ASFV.

TRIM28 regulates the coagulation cascade inhibited by p72 of African swine fever virus

In 2018, African swine fever virus (ASFV) emerged in China, causing extremely serious economic losses to the domestic pig industry. Infection with ASFV can cause disseminated coagulation, leading to the consumption of platelets and coagulation factors and severe bleeding. However, the mechanism of virus-induced coagulation has yet to be established. In our study, ASFV downregulated the coagulation process, as detected by D-dimer (D2D) and Factor X (F10) expression in pigs challenged with ASFV HLJ/18. In vitro, ASFV infection increased Factor IX (F9) and Factor XII (F12) expression while downregulating F10 expression in porcine alveolar macrophages (PAMs). African swine fever virus induced both intrinsic and extrinsic coagulation cascades. In addition, several encoded proteins affect the expression of the crucial coagulation protein F10, and among the encoded proteins, p72 inhibits the activity and expression of F10. Proteomic analysis also revealed that p72 is involved in the coagulation cascade. p72 can interact with F10, and its inhibitory functional domains include amino acids 423-432 and amino acids 443-452. Finally, we found that F10 and p72 interact with tripartite motif-containing protein 28 (TRIM28). TRIM28 knockdown resulted in a decrease in F10 expression. Importantly, TRIM28 contributes to the reduction in F10 protein expression regulated by p72. Our findings revealed an inhibitory effect of the viral protein p72 on the ASFV infection-induced coagulation cascade and revealed a role of TRIM28 in reducing F10 expression, revealing a molecular mechanism of ASFV-associated coagulation.

Antiviral Activity of Plant-Based Additives Against African Swine Fever Virus (ASFV) in Feed Ingredients

BACKGROUND

African swine fever (ASF) is one of the deadliest swine diseases with haemorrhagic symptoms and a high mortality rate. Plant-derived additives are potential antiviral agents against viruses due to their environmental and user-friendly properties.

OBJECTIVES

This study aims to evaluate the efficacy of plant-based additives (Phyto.A04 and Phyto.B) compared to an organic acid blend (OAB) in inactivating ASF virus (ASFV) in cell culture and feed.

METHODS

ASFV-spiked feed was treated with individual or combined additives such as OAB, Phyto.A04 and Phyto.B. The viability of ASFV after treatment of ASFV-spiked feed with additives was then confirmed by both methods, real-time PCR and cell culture.

RESULTS

The results of the in vitro test with cell cultures showed that all three additives (OAB, Phyto.A04 and Phyto.B) exerted a strong virucidal effect on ASFV in porcine alveolar macrophage cells. OAB at a concentration of 0.3% reduced the virus concentration from 4.48 log10 HAD50/mL after 1 day of treatment (day 1) to 3.29 log10 HAD50/mL after 3 days of treatment (day 3) and remained undetected after 7 days of treatment (day 7). In Phyto.A04 with 1%, the virus was only detectable on day 1 (3.53 log10 HAD50/mL). Phyto.B with 0.01% and 0.05% both showed good efficacy in completely inhibiting virus presence on days 3 and 7.

CONCLUSIONS

All additives, OAB, Phyto.A04 and Phyto.B, were able to inactivate ASFV in a dose-dependent manner, as confirmed by cell culture and PCR methods. The combination of additives at different concentrations consistently improved the virucidal results.

Enhanced Recovery and Detection of Highly Infectious Animal Disease Viruses by Virus Capture Using Nanotrap® Microbiome A Particles

This study reports the use of Nanotrap® Microbiome A Particles (NMAPs) to capture and concentrate viruses from diluted suspensions to improve their recovery and sensitivity to detection by real-time PCR/RT-PCR (qPCR/RT-qPCR). Five highly infectious animal disease viruses including goatpox virus (GTPV), sheeppox virus (SPPV), lumpy skin disease virus (LSDV), peste des petits ruminants virus (PPRV), and African swine fever virus (ASFV) were used in this study. After capture, the viruses remained viable and recoverable by virus isolation (VI) using susceptible cell lines. To assess efficacy of recovery, the viruses were serially diluted in phosphate-buffered saline (PBS) or Eagle's Minimum Essential Medium (EMEM) and then subjected to virus capture using NMAPs. The NMAPs and the captured viruses were clarified on a magnetic stand, reconstituted in PBS or EMEM, and analyzed separately by VI and virus-specific qPCR/RT-qPCR. The PCR results showed up to a 100-fold increase in the sensitivity of detection of the viruses following virus capture compared to the untreated viruses from the same dilutions. Experimental and clinical samples were subjected to virus capture using NMAPs and analyzed by PCR to determine diagnostic sensitivity (DSe) that was comparable (100%) to that determined using untreated (-NMAPs) samples. NMAPs were also used to capture spiked viruses from EDTA whole blood (EWB). Virus capture from EWB was partially blocked, most likely by hemoglobin (HMB), which also binds NMAPs and outcompetes the viruses. The effect of HMB could be removed by either dilution (in PBS) or using HemogloBind™ (Biotech Support Group; Monmouth Junction, NJ, USA), which specifically binds and precipitates HMB. Enhanced recovery and detection of viruses using NMAPs can be applicable to other highly pathogenic animal viruses of agricultural importance.

Modulation of Autophagy-Lysosome Axis by African Swine Fever Virus and Its Encoded Protein pEP153R

The autophagy-lysosome axis is an evolutionarily conserved intracellular degradation pathway which constitutes an important component of host innate immunity against microbial infections. Here, we show that African swine fever virus (ASFV), one of most devastating pathogens to the worldwide swine industry, can reshape the autophagy-lysosome axis by recruiting the critical lysosome membrane proteins (LAMP1 and LAMP2) to viral factories while inhibiting autophagic induction in macrophages. The screening of viral membrane proteins led to the identification of several ASFV membrane proteins, exemplified by viral protein pEP153R, that could significantly alter the subcellular localization of LAMP1/2 when expressed alone in transfected cells. Further analysis showed that pEP153R was also a component of viral factories and could induce endoplasmic reticulum (ER) retention of LAMP1/2, leading to the inhibition of the fusion of autophagosomes with lysosomes. Interestingly, the ASFV mutant lacking EP153R could still actively recruit LAMP into viral factories (VFs) and inhibit autophagic flux, indicating the existence of a functional redundancy of other viral proteins in the absence of pEP153R and highlighting the complexity of ASFV replication biology. Taken together, our results reveal novel information about the interplay of ASFV with the autophagy-lysosome axis and a previously unrecognized function of ASFV protein pEP153R in regulating the cellular autophagic process.

Development of quantum dot-based immunochromatographic strip for detection of antibodies against ASFV pp62

ASFV is the only known double-stranded insect-borne DNA virus, which can rapidly infect domestic pigs and wild boars with ticks as transmission medium. Since it was first discovered in 1921, it quickly spread to all parts of the world and brought huge economic losses to the pig industry all over the world. At present, there is still no safe and effective vaccine for ASFV. Here, we developed a quantum-dot labeled antibody test strip for the detection of antibodies against ASFV pp62. The pp62 protein was labeled with quantum dots, and the antibody test strip was developed uses it in a detection mode of labeled antigen-SPA interceptor-monoclonal antibody quality control. The test strip showed high sensitivity, the positive detection limit of the strip was 1: 106 by continuous multiple dilution using the positive standard serum of ASFV antibody as reference. The test strip showed good specificity, and there was no cross reaction with other swine diseases virus (PCV2, PRRSV, CSFV, PPV). Using the detection results of commercialized kit for African swine fever virus as reference, 80 ASFV antibody negative serum and 4 different ASFV antibody positive serum were detected using the ASFV pp62 quantum-dot labeled antibody test strip. The results were consistent with the commercial kit. This study provides a new detection method for the prevention and control of African swine fever.

African swine fever virus pCP312R interacts with host RPS27A to shut off host protein translation and promotes viral replication

African swine fever virus (ASFV) severely threatens the global economy and food security. ASFV encodes >150 genes, but the functions of most of them have yet to be characterized in detail. Here we explored the function of the ASFV CP312R gene and found that CP312R plays an essential role in ASFV replication. Knockout of the CP312R gene terminated viral replication and CP312R knockdown substantially suppressed ASFV infection in vitro. Furthermore, we resolved the crystal structure of pCP312R to 2.3 Å resolution and found that pCP312R has the potential to bind nucleic acids. LC-MS analysis and co-immunoprecipitation assay revealed that pCP312R interacts with RPS27A, a component of the 40S ribosomal subunit. Confocal microscopy showed the interaction between pCP312R and RPS27A leaded to a modification in the subcellular localization of this host protein, which suppresses host protein translation. Renilla-Glo luciferase assay and Ribopuromycylation analysis evidenced that knockout of RPS27A completely aborted the shutoff activity of pCP312R, and trans-complementation of RPS27A recovered pCP312R shutoff activity in RPS27A-knockout cells. Our findings shed light on the function of ASFV CP312R gene in virus infection, which triggers inhibition of host protein synthesis.

A novel live DNA tagging system for African swine fever virus shows that bisbenzimide Hoechst 33342 can effectively block its replication

African swine fever virus (ASFV) infection causes a frequently fatal disease in domestic swine that has affected more than 50 countries worldwide since 2021, with a major impact on animal welfare and economy. The development of effective vaccines or antivirals against this disease are urgently required for its effective control. Live detection of viral replication has been used as a tool for the screening and characterization of antiviral compounds in other dsDNA genome containing viruses. Here, we have adapted the ANCHOR fluorescent DNA labelling system to ASFV by constructing and characterizing a novel recombinant virus. We show that this virus is viable and effectively tags viral DNA replication sites, which can be detected and quantified in real time. Further, we have used high content cell microscopy to test the antiviral activity of bisbenzimide compounds and show that Hoechst 33342 has specific anti-ASFV activity. We expect this novel tool to be useful both in the further study of ASFV replication as in the screening of new specific antiviral compounds.