Architecture of African swine fever virus and implications for viral assembly

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.

Insights and progress on epidemic characteristics, pathogenesis, and preventive measures of African swine fever virus: A review

The African swine fever virus (ASFV) is the only giant double-stranded DNA virus known to be transmitted by insect vectors. It can infect pigs and cause clinical signs such as high fever, bleeding, and splenomegaly, which has been classified as a reportable disease by the WOAH. In 2018, African swine fever (ASF) was introduced into China and rapidly spread to several countries in the Asia-Pacific region, with morbidity and mortality rates reaching 100 percent, resulting in significant economic losses to the global pig industry. Because ASFV has large genomes and a complex escape host mechanism, there are currently no safe and effective drugs or vaccines against it. Therefore, it is necessary to optimize vaccination procedures and find effective treatments by studying the epidemiology of ASFV to reduce economic losses. This article reviews research progress on pathogenesis, genome, proteome and transcriptome, pathogenic mechanisms, and comprehensive control measures of ASFV infection.

An attenuated African swine fever virus expressing the E2 glycoprotein of classical swine fever virus protects pigs against challenge of both viruses

African swine fever (ASF) and classical swine fever (CSF) are highly contagious diseases with high morbidity and mortality rates resulting in an enormous impact on the global pig industry. A bivalent vaccine that simultaneously protects against both ASF and CSF is highly desirable. We previously developed a seven-gene-deleted African swine fever virus (ASFV) attenuated vaccine candidate (HLJ/18-7GD) that provides complete protection against homologous strains. Herein, we constructed a recombinant virus HLJ/18-7GD-E2 by inserting the classical swine fever virus (CSFV) E2 gene into the HLJ/18-7GD via homologous recombination. After continuous in vitro passaging, Western blotting analysis showed that the E2 gene was expressed and stably maintained within the ASFV genome. Next, the immunogenicity and protective efficacy of the recombinant HLJ/18-7GD-E2 virus was evaluated in pigs. The results revealed that a single dose of 106 TCID50 of HLJ/18-7GD-E2 induced an efficient immune response and provided complete protection against lethal challenges with ASFV or CSFV. These results demonstrate that recombinant ASFV expressing the CSFV E2 protein has potential as a bivalent live attenuated vaccine, providing solid protection against ASFV and CSFV infection in pigs.

Hijacking the autophagy-apoptosis crosstalk: African swine fever virus orchestrates immune evasion via host remodeling for viral pathogenesis

African swine fever (ASF) is an acute, highly fatal hemorrhagic disease of domestic and wild pigs caused by African swine fever virus (ASFV). ASFV, a large double-stranded DNA virus of the Asfarviridae family, is highly infectious and pathogenic. Through modulation of host apoptosis and autophagy pathways, the virus subverts innate immune surveillance to promote its replication and dissemination. Following ASFV infection, domestic pigs may exhibit 100 % morbidity and mortality rates with highly virulent strains, constituting a major threat to the global pork industry. Nowadays, ASF is listed as a notifiable terrestrial animal disease by the World Organisation for Animal Health (WOAH). Therefore, elucidating ASFV's pathogenic mechanisms, particularly its molecular regulation of apoptosis and autophagy, is crucial for developing effective ASF control and prevention strategies. This review comprehensively summarizes the pathogenic mechanisms of ASFV, with particular focus on the autophagy-apoptosis crosstalk and viral manipulation of these cellular processes. These insights not only improve our understanding of ASFV-mediated immune evasion mechanisms but also provide valuable references for developing ASF control strategies targeting apoptosis and autophagy pathways.

Selection, Design, and Immunogenicity Studies of ASFV Antigens for Subunit mRNA Cocktail Vaccines with Specific Immune Response Profiles

Development of safe and effective subunit vaccines for controlling African Swine Fever Virus (ASFV) infection has been hampered by a lack of protective viral antigens, complex virion structures, and multiple mechanisms of infection. Here, we selected ASFV antigens based on their localization on the virion, known functions, and homologies to the subunits of the protective vaccinia virus vaccine. We also engineered viral capsid proteins for inducing optimal antibody responses and designed T cell-directed antigens for inducing broad and robust cellular immunity. The selected antigens in lipid nanoparticle-mRNA formulations were evaluated for immunogenicity in both mice and pigs with concordant results. Different antigens induced divergent immune response profiles, including the levels of IgG and T cell responses and effector functions of antisera. We further developed a computational approach to combine antigens into cocktails for inducing specific immune response profiles and to validate candidate cocktail vaccines in mice. Our results provide a basis for further evaluation of candidate subunit mRNA vaccines in challenge studies.

LbCas12a-based DNA POCT facilitates fast genotyping on farm

Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 12a (CRISPR/Cas12a) detection system is now widely used for nucleic acid detection and disease diagnosis. However, there are still fewer detections for single nucleotide polymorphisms (SNPs) and limited diversified detection systems for pathogen and SNP sites detection, which greatly limits their applications. Obviously, the development of a more diversified and convenient suite of detection tools is essential to unlock the full potential of CRISPR/Cas12a technology and to expand its applications across a wider range of scenarios. We have successfully developed an integrated CRISPR/Cas12a assay system. This system introduces crRNA during protein expression, reducing the number of steps and reaction time by adding only a fluorescent reporter gene and target DNA during subsequent detection. It enables on-site visualization of the assay in combination with a Recombinase polymerase amplification (RPA) reaction. Combined with the RPA reaction, we are able to rapidly detect African swine fever virus (ASFV) pathogens with high specificity. The system also enables genotyping of the SNP site of the porcine prolificacy-associated estrogen receptor (ESR) gene and the sheep prolificacy-associated Fecundity booroola (FecB) gene. Visualization is possible up to a final concentration of 3 nM, and effective differentiation of low concentrations within the concentration range of the assay. The integrated CRISPR/Cas12a assay system we developed has a robust design that ensures high-fidelity genotyping and pathogen detection are no longer restricted to the lab, allowing for rapid field analysis, which is crucial for timely interventions in agricultural and clinical settings. In addition, it has the advantages of low cost, easy operation and visualization of results.

Advances in African swine fever virus molecular biology and host interactions contributing to new tools for control

African swine fever virus (ASFV) causes a frequently fatal hemorrhagic disease in domestic pigs and wild boar. The spread from Africa to Georgia in 2007 initiated a pandemic affecting many European and most Asian countries. This has had a very high socio-economic impact and threatens global food security. The virus is a large, complex, cytoplasmic DNA virus, the only member of the Asfarviridae family and codes for 170-190 proteins. Many of these have unknown functions and do not resemble other viruses or host proteins. This complexity has hindered the development of vaccines and other tools for control. The intensity of research has increased since the spread of ASFV in Europe and Asia, leading to rapid advances in knowledge. This review summarizes recent research, including the determination by cryogenic electron microscopy of the virus capsid structure and virion proteome. Novel information on the virus replication cycle, including mechanisms of virus entry into cells and the identification of host endosomal proteins important for entry, is summarized. Multiple, novel virus immune evasion proteins and their targets in the type I interferon response and inflammation pathways have been identified. The potential for the application of this knowledge to developing novel control tools, including modified live vaccines and other interventions targeting critical virus processes or host interactions, is discussed.

Virtual Screening and Molecular Dynamics Simulation Targeting the ATP Domain of African Swine Fever Virus Type II DNA Topoisomerase

African Swine Fever Virus (ASFV) Topo II ATPase domain, resistant to conventional inhibitors (e.g., ICRF-187) due to M18/W19 steric clashes, was targeted via hierarchical virtual screening (Schrödinger) of the Chembridge library combined with MM/GBSA calculations. Five ligands (10012949, 40242484, 46712145, 15880207, and 33688815) showed high affinity, with 46712145 adopting symmetrical π-π stacking, hydrogen bonds, and alkyl interactions to bypass steric hindrance. Molecular dynamics simulations (100 ns) revealed ligand-induced flexibility, evidenced by elevated RMSD/Rg values versus the free protein. DCCM analysis highlighted enhanced anti-correlated motions between GHKL motifs and sensor domains in chain B/C, suggesting stabilization of a non-catalytic conformation to inhibit ATP hydrolysis. Free energy landscape (FEL) analysis showed 46712145 occupying a broad, shallow energy basin, enabling conformational adaptability, contrasting the narrow deep well of the free protein. This study proposes a symmetric ligand design strategy and conformational capture mechanism to block ATPase activity. Compound 46712145 demonstrates stable binding and dynamic regulation, providing a novel lead scaffold for anti-ASFV drug development. These findings establish a structural framework for combating ASFV through targeted ATPase inhibition.

Epitope mapping targeting the K205R protein of African swine fever virus using nanobody as a novel tool

African swine fever (ASF) is a highly infectious and lethal swine disease, leading to enormous losses in the pig industry. K205R, a non-structural protein of ASF virus (ASFV), is abundantly expressed at the early stages of viral infection and induces a strong immune response. In our previous study, five strains of K205R-specific nanobodies (Nbs) were screened through phage display technology, among which Nb1, Nb14, Nb35, and Nb82 exhibited good affinity. In the present study, the above four Nbs were successfully expressed in HEK293T cells and exhibited strong reactivity. Four Nbs recognized linear B-cell epitopes of K205R in both prokaryotic and eukaryotic expression systems. Besides, four Nbs specifically reacted with the K205R protein of ASFV-infected cells. Two epitopes 1MVEPR5 and 188RTQF191 were further identified, with highly conserved in different ASFV strains, and could interact with inactivated ASFV-positive sera, indicating that the two epitopes were natural linear B-cell epitopes. Moreover, structural analysis indicated that both epitopes were exposed on the surface of the K205R molecule. Notably, the identified epitope 188RTQF191 was first reported. Overall, these findings provide valuable insights for K205R as an effective diagnostic tool and vaccine development.IMPORTANCEAfrican swine fever (ASF) is the number one killer affecting the pig industry, and there are no effective strategies for prevention. The ASFV K205R protein is prominently expressed in the early stages of viral infection, triggering a robust immune response. The full understanding of K205R protein epitopes provides a theoretical basis for the development of vaccine-candidate proteins. Nanobodies exhibit superior capability in detecting concealed epitopes of antigens compared with traditional antibodies. Here, we identify two epitopes 1MVEPR5 and 188RTQF191 based on nanobodies as a tool. Notably, the epitope188RTQF191 is being reported for the first time. These epitopes are highly conserved in different ASFV strains and represent natural linear B-cell epitopes. This study opens up nanobodies as a new tool for the identification of epitopes and also provides a direct material basis for the development of ASFV vaccines.

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 ASF monitoring, prevention and control in China.

Tracking the intracellular localization of African swine fever virus genomic DNA using DNAscope in situ hybridization

The genome of African swine fever virus (ASFV) is characterized by a single linear double-stranded DNA molecule. However, the question of whether ASFV genomic DNA enters the nucleus has been a subject of debate. In this study, DNAscope in situ hybridization (ISH) technology was employed to trace the genomic DNA of ASFV. Specific probes targeting the DNA of the ASFV HLJ/18 strain were designed and synthesized, facilitating the successful detection of ASFV DNA in ASFV-infected porcine alveolar macrophages (PAMs). Detection of ASFV genomic DNA commenced as early as 2 h post-infection (hpi), with a progressive increase in DNA signals over time. At 8 hpi, ASFV DNA predominantly localized around nuclear virus factories (VFs), and with continued viral replication increased, the fluorescence signals associated with the detected DNA intensified and extended to additional cytoplasmic areas beyond the VFs. After 20 hpi, the fluorescence signals reached a state of stabilization. Furthermore, confocal Z-stack imaging revealed that ASFV DNA signals were absent from the nucleus throughout all the stages of ASFV infection, providing substantial evidence regarding ASFV genome localization.

Development and efficacy of a novel mRNA cocktail for the delivery of African swine fever virus antigens and induction of immune responses

African swine fever (ASF) is a highly lethal infectious disease affecting pigs. Although several vaccine formulations have been developed to protect from ASF virus (ASFV) infection, none have yet provided complete protection without side effects or the risk of progressing to chronic infection. mRNA vaccines offer unparalleled advantages in terms of safety and ability to induce immune responses. In this study, we designed six mRNA vaccines encoding key antigenic proteins of ASFV- B602L, CD2V, EP153R, P30, P54, and P72 and combined them into an mRNA cocktail for vaccination in mice and pigs. Our findings suggest that the mRNA cocktail is capable of provoking robust multivalent humoral and cellular immune responses while maintaining a favorable safety profile. Thus, it may serve as a potential approach for controlling ASF transmission, contributing to the ongoing efforts to develop effective and safe ASFV vaccines. The administration of the mRNA cocktail induced both humoral and cellular immune responses in mice and pigs, suggesting a potential for future ASFV vaccine development.IMPORTANCEThis study explores an mRNA vaccine encoding six critical African swine fever virus (ASFV) antigens (B602L, CD2V, EP153R, P30, P54, P72), demonstrating its ability to induce robust humoral and cellular immune responses in both mice and pigs. This innovative approach serves as a significant advancement in ASFV vaccine development by addressing safety and efficacy concerns. The findings suggest that the mRNA cocktail developed in this study represents a step forward in ASFV vaccine research and development. This strategy holds promise for contributing to ASFV control by offering possibly safer and more effective alternatives to conventional vaccines. This could significantly impact ASF management and prevention strategies globally, ultimately benefiting animal health and reducing economic losses.

Generation of Chimeric African Swine Fever Viruses Through In Vitro and In Vivo Intergenotypic Gene Complementation

Background/Objectives: African swine fever (ASF), a fatal febrile hemorrhagic disease in domestic pigs and Eurasian wild boars, is caused by ASF virus (ASFV). ASF continues to spread across the globe, causing a significant impact on the world's pig industry. Recently, highly virulent chimeric ASFV (chASFV) strains with recombined genomes of the p72 genotype I and II viruses have been reported in China, Vietnam and Russia. Methods: In order to understand the propensity of ASFV genome for recombination, we attempted to experimentally generate chASFVs both in vitro and in vivo employing two distinct attenuated ASFV strains: OUR T88/3 (genotype I) and AQSΔB119L (genotype II). Results: When IPKM cells were co-infected with ASFV OUR T88/3 and AQSΔB119L strains, three genetically distinct chASFV emerged. When pigs were inoculated with the individual chASFV isolates, all pigs developed acute ASF. When four pigs were co-infected with ASFV OUR T88/3 and AQSΔB119L, all of them developed acute ASF and died or were euthanized. Three chASFV strains were successfully isolated from splenic homogenates from each pig. Conclusions: Our research indicates that genotype I and II chASFV with diverse genomes can be easily generated experimentally both in vitro and in vivo.

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.

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.

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.

The capsid protein p72 specific mAb and the corresponding novel epitope based ELISAs for detection of ASFV infection

The African Swine Fever Virus (ASFV) poses a significant threat to the global pig farming industry. The major icosahedral capsid protein p72 plays a key role in the assembly of virus particles and infection process. As one major antigen of ASFV, p72 has been widely utilized as a marker for diagnosing infection. In this study, five p72 specific monoclonal antibodies (mAbs) were generated through the immunization of mice followed by cell fusion. Among the five hybridomas, clones 1B7, 2F3, 5D3, and 5D4 recognized a novel epitope of 101FHDMVGHHILGACH114, while clone 1D7 recognized a new epitope, 239GPLLCNIHDL248. Both linear epitopes were found to be highly conserved across all genotypes I and II ASFV isolates, with the first located at the pseudo hexagonal base of the p72 trimer and the latter situated at the FGN insertion loop. The antigenic epitopes were capable of competing with the binding of corresponding mAbs in p72 protein based ELISA, and peptide based ELISA effectively detected ASFV antibodies in clinical samples. Furthermore, we developed and optimized a sandwich ELISA using the mAb 2F3 for efficient detection of ASFV p72 antigen. Our study not only provides valuable tools for assessing p72 function assay, but also lays the foundation for serological diagnosis, prevention and control of ASF.

Genomic and structural insights into Jyvaskylavirus, the first giant virus isolated from Finland

Giant viruses of protists are a diverse and likely ubiquitous group of organisms. Here, we describe Jyvaskylavirus, the first giant virus isolated from Finland. This clade B marseillevirus was found in Acanthamoeba castellanii from a composting soil sample in Jyväskylä, Central Finland. Its genome shares similarities with other marseilleviruses. Helium ion microscopy and electron microscopy of infected cells unraveled stages of the Jyvaskylavirus life cycle. We reconstructed the Jyvaskylavirus particle to 6.3 Å resolution using cryo-electron microscopy. The ~2500 Å diameter virion displays structural similarities to other Marseilleviridae giant viruses. The capsid comprises of 9240 copies of the major capsid protein, encoded by open reading frame (ORF) 184, which possesses a double jellyroll fold arranged in trimers forming pseudo-hexameric capsomers. Below the capsid shell, the internal membrane vesicle encloses the genome. Through cross-structural and -sequence comparisons with other Marseilleviridae using AI-based software in model building and prediction, we elucidated ORF142 as the penton protein, which plugs the 12 vertices of the capsid. Five additional ORFs were identified, with models predicted and fitted into densities that either cap the capsomers externally or stabilize them internally. The isolation of Jyvaskylavirus suggests that these viruses may be widespread in the boreal environment and provide structural insights extendable to other marseilleviruses.

Assembly of Matryoshka-Type Protein Nanocages for Compartmentalized Oxygen Sensing

Oxygen permeability is a critical property of protein nanocages (PNCs) that impacts or dictates the functions of PNCs. However, it remains challenging to determine it experimentally. Here, we report compartmentalized oxygen sensing inside PNCs by assembling matryoshka-type structures through interfacial engineering, namely, one PNC containing another smaller one functionalized with small-molecule oxygen probes. Oxygen in the lumen of the outer PNCs can be probed conveniently via phosphorescence spectrometry. This method enabled the analysis of two representative PNCs, MS2 virus-like particles and Thermotoga maritima encapsulin, revealing the former is oxygen permeable, while the latter is oxygen impermeable. This study establishes a general approach for measuring the oxygen permeability of PNC shells, which can provide an experimental basis for understanding the working mechanisms of PNCs and inspire applications like oxygen-sensitive or oxygen-responsive sensing, catalysis, and delivery. Also, the tunable nested PNCs may serve as platforms for designing hierarchical or compartmentalized devices or organelles.

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.

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.

Ca@Pb12: A Viable All-Main-Group-Metal Endofullerene That Imitates the Transition Metals

Herein, we report the Ca@Pb12 cluster, an all-main-group-metal endofullerene possessing the highly symmetric icosahedral (Ih) Platonic solid structure. The endohedral environment endows the Ca atom with concrete transition-metal-like bonding behaviors, as evidenced by the 18e (in 3d104s24p6 configuration) valence shell structure and five strong Pb12 → Ca(3d) dative bonds, whose orbital interaction energy (ΔEorb, -134.2 kcal/mol) represents 54.9% of total ΔEorb (-244.4 kcal/mol) between Ca2+ and [Pb12]2-. Moreover, since the favorable orbital interactions can compensate for the enlarged steric repulsion during encapsulation, Ca@Pb12 is thermodynamically stable, dynamically rigid (up to 1300 K), and electronically robust, as verified by large HOMO-LUMO gaps (4.58 eV), high vertical detachment energy (6.39 eV), and low electron affinity (-0.71 eV). It is the first viable all-main-group-metal endofullerene, where the guest alkaline earth metal exhibits the typical bonding behaviors of transition metals.

Dispersion Forces-Driven Hierarchical Assembly of Protein-Like Lanthanide Octamers and Emergent CPL

Hierarchical self-assembly driven by non-covalent interactions is a prevalent strategy employed by nature to construct sophisticated biomacromolecules, such as proteins. However, the construction of protein-like superstructures that rely on weaker dispersion forces-driven hierarchical assembly remains largely unexplored. Here, we report the first example of dispersion forces driving the high-order assembly of the lanthanide trinuclear circular helicate [HNEt₃]₃[Eu₃(LL)₆] (ΔΔΔ-1) into a protein-like lanthanide octamer ((ΔΔΔ-1)₈-2). Within the octamer, the forty-eight (48) menthol groups on the ligands and eighty-four (84) 1,4-dioxane solvent molecules contribute to enhanced dispersion forces through conformational adaptation and size-matching effects. These enhanced dispersion forces not only drive the formation of the hierarchical superstructure but also result in a four-level chirality transfer from the menthol to the octamer. Benefiting from the homochirality of Eu3+, the octamer is endowed the strong circularly polarized emission (|glum|=0.34, Φoverall=41 %). This understanding of how dispersion forces drive hierarchical self-assembly provides a foundation for the directed fabrication of more fascinating superstructures.

From structure prediction to function: defining the domain on the African swine fever virus CD2v protein required for binding to erythrocytes

African swine fever virus (ASFV) is a high-consequence pathogen posing a substantial threat to global food security. This large DNA virus encodes more than 150 open reading frames, many of which are uncharacterized. The EP402R gene encodes CD2v, a glycoprotein expressed on the surface of infected cells and the only viral protein known to be present in the virus external envelope. This protein mediates binding of erythrocytes to both cells and virions. This interaction is known to prolong virus persistence in blood thus facilitating viral transmission. The sequence of the extracellular domain of CD2v shows similarity with that of mammalian CD2 proteins and is therefore likely to feature two immunoglobulin (Ig)-like domains. A combination of protein structure modeling and extensive mutagenesis was used to identify residues mediating binding of transiently expressed CD2v to erythrocytes. The N-terminal Ig-like domain AGFCC'C″ β sheet was identified as the putative CD2v erythrocyte-binding area. This region differed from the putative CD58 ligand binding site of host CD2, suggesting that CD2v may bind to a ligand(s) other than CD58. An attenuated genotype I ASFV was constructed by replacing the wild-type EP402R gene for a mutant form expressing CD2v bearing a single amino acid substitution, which abrogated the binding to erythrocytes. Pigs immunized with the recombinant virus developed early antibody and cellular responses, low levels of viremia, mild clinical signs post-immunization, and high levels of protection against challenge. These findings improve our understanding of virus-host interactions and provide a promising approach to modified live vaccine development.

IMPORTANCE

A better understanding of the interactions between viruses and their hosts is a crucial step in the development of strategies for controlling viral diseases, such as vaccines and antivirals. African swine fever, a pig disease with fatality rates approaching 100%, causes very substantial economic losses in affected countries, and new control measures are clearly needed. In this study, we characterized the interaction between the ASFV CD2v protein and host erythrocytes. The interaction plays a key role in viral persistence in blood since it can allow the virus to "hide" from the host immune system. We identified the amino acids in the viral protein that mediate the interaction with erythrocytes and used this information to construct a mutant virus that is no longer able to bind these cells. This virus induces strong immune responses that provide high levels of protection against infection with the deadly parental virus.

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.

Luteolin inhibits BHV-1 replication and alleviates virus-induced inflammatory responses by regulating PI3K/AKT pathway

Bovine herpesvirus type 1 (BHV-1) seriously affects the production safety of the cattle industry and leads to epidemics worldwide. Luteolin (Lut), a flavonoid substance, can be found in vegetables, fruits, and herbs and possesses various biological properties. Here, we found that Lut can dose-dependently and significantly inhibit the cytopathic effects of BHV-1, decrease the viral titer, and suppress the BHV-1 gB gene and VP8 protein levels on bovine nasal turbinate osteoblasts (BT) and bovine kidney epithelial cells (MDBK). Mechanistic studies revealed that Lut can stably bind to the active sites of PI3K and AKT, and inhibit the PI3K/AKT pathway. Interestingly, 740Y-P (an agonist of the PI3K/AKT pathway) significantly attenuated the anti-BHV-1 effects of Lut. Further studies on the anti-inflammatory effects of Lut revealed that it attenuated BHV-1-induced activation of the NFκB pathway, which significantly suppressed the expression of TNF-α, IL-1β, IL-6, and IL-8 and increased the expression levels of IL-4 and IL-10. The PI3K/AKT pathway was also found to be involved in the anti-inflammatory effects of Lut. These results confirm the inhibitory effect of Lut on BHV-1 replication, which lays the foundation for further studies on the prevention and control of BHV-1.

Autophagy promotes p72 degradation and capsid disassembly during the early phase of African swine fever virus infection

During viral infections, autophagy functions as a cell-intrinsic defense mechanism by facilitating the delivery of virions or viral components to the endosomal/lysosomal pathway for degradation. In this study, we report that internalized African swine fever virus (ASFV) virions enter autolysosomes during the early phase of viral infection. Autophagy selectively targets the major capsid protein p72 within the ASFV virion. The ASFV p72 protein undergoes modification through ubiquitination at the C-terminus, a process mediated by the E3 ubiquitin ligase Stub1. Subsequently, ubiquitinated p72 is recognized by the autophagy receptor SQSTM1/p62 through its ubiquitin-binding domain. Stub1 facilitates the ubiquitination and degradation of p72 in an HSPA8-dependent manner via selective autophagy. Autophagy plays a critical role in disassembling ASFV virions and further promotes the release of ASFV genomic DNA. These findings support the notion that autophagy is involved in and contributes to the capsid disassembly of ASFV, providing valuable insights into this essential viral process.IMPORTANCEAfrican swine fever (ASF), a highly contagious disease caused by the ASF virus (ASFV), affects domestic pigs and wild boars, with a mortality rate of up to 100%. The ASF epidemic poses a persistent threat to the global pig industry. Currently, no effective vaccines or antiviral drugs are available for prevention and control. In this study, we discovered that autophagy promotes the degradation of p72 and the disassembly of the capsid during the early phase of ASFV infection. Mechanically, Stub1 facilitates the polyubiquitination of ASFV p72 through the chaperone HSPA8. The polyubiquitinated p72 then interacts with the autophagy receptor SQSTM1/p62, leading to its degradation via the selective autophagy pathway. These findings reveal the mechanism of p72 degradation through autophagy and provide new insights into the capsid disassembly process of ASFV.

A Mutant of Africa Swine Fever Virus Protein p72 Enhances Antibody Production and Regulates the Production of Cytokines

African swine fever virus (ASFV) is a severe threat to the global pig industry, and domestic pigs mostly develop severe clinical manifestations upon viral invasion. Currently, there is no available vaccine against ASFV. Its capsid structural protein p72 is one of the immuno-dominant proteins. In this study, we unexpectedly obtained a p72 mutant protein (p72∆377-428) which deleted the aa 377-428 within p72 and had stable and high expression in E. coli. Using SWISS-MODEL 1.0 software, the prediction showed that p72∆377-428 was quite distinct from the wild-type p72 protein in structure. p72∆377-428 induced stronger antibody production in mice on day 42 and 56 post immunization and could recognize ASFV-infected swine sera. p72∆377-428 reduced IFN-γ production in the splenocytes from p72∆377-428-immunized mice and p72∆377-428-treated swine macrophages compared to p72. p72∆377-428 also decreased the production of pro-inflammatory cytokine genes, including IL-1β, IL-6, and IL-12, compared to p72 in mice. Further, we found that p72∆377-428 reduced the induction of pro-inflammatory cytokine genes by inhibiting AKT phosphorylation and HIF1α expression. Taken together, these findings have implications for immunological function and the corresponding mechanism of ASFV p72, and our study indicates that p72∆377-428 could serve as a novel candidate for ASFV vaccines and diagnostic reagents.

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.

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.

Construction of a recombinant African swine fever virus with firefly luciferase and eGFP reporter genes and its application in high-throughput antiviral drug screening

African Swine Fever (ASF) is a highly lethal and contagious disease in pigs caused by African Swine Fever Virus (ASFV), which primarily infects domestic pigs and wild boars, with a mortality rate of up to 100%. Currently, there are no commercially available vaccines or drugs that are both safe and effective against ASFV. The ASFV 0428C strain was continuously passaged in Vero cells, and the adapted ASFV demonstrated efficient replication in Vero cells. The adapted ASFV was used as the parental virus, and an expression cassette encoding a dual reporter gene for firefly luciferase (Fluc) and enhanced green fluorescent protein (eGFP) was inserted into the ASFV genome using CRISPR/Cas9 gene editing technology to construct a recombinant ASFV variant (rASFV-FLuc-eGFP). rASFV-Fluc-eGFP was genetically stable, effectively infected porcine alveolar macrophages (PAM) and Vero cells, and expressed Fluc and eGFP concurrently. This study provides a tool for investigating the infection and pathogenic mechanisms of ASFV, as well as for screening essential host genes and antiviral drugs. Additionally, a high-throughput screening model of antiviral drugs was established based on rASFV-FLuc-eGFP in passaged cells, 218 compounds from the FDA-approved compound library were screened, and 5 candidate compounds with significant inhibitory effects in Vero cells were identified. The inhibitory effects on ASFV were further validated in both Vero and PAM cells, resulting in the identification of Salvianolic acid C (SAC), which demonstrated inhibitory effects and safety in both cell types. SAC is a candidate drug for the prevention and control of ASFV and shows promising application prospects.

Propidium Monoazide Integrated With qPCR Enables Rapid and Universal Detection of Infectious Porcine Reproductive and Respiratory Syndrome Viruses

Infectious porcine reproductive and respiratory syndrome virus (PRRSV) causes PRRS, but noninfectious PRRSV cannot. PCR and ELISA are commonly used for PRRSV detection but they cannot discriminate PRRSV infectivity. Virus isolation is a gold standard to determine virus infectivity. However, it is time-consuming. Therefore, we developed a propidium monoazide (PMA) qPCR assay for rapid and universal detection of infectious PRRSV in this study. After comparing the inactivation efficacies of distinct disinfectants, ultraviolet (UV) light, and heat, heat at 72°C for 15 min was determined as an effective strategy for PRRSV inactivation, which was confirmed by virus isolation and immunofluorescence assay (IFA) detection. In addition, PMA pretreatment parameters were optimized, including PMA concentration (5 μM), PMA binding time (25 min), PMA binding temperature (37°C), and photolysis time (25 min). The optimal concentration of primers and probes adapted from our previous study was redetermined. The optimized PMA-qPCR assay exhibited satisfied specificity, sensitivity, and reproducibility. Furthermore, the new PMA-qPCR was applied on the detection of 270 clinical samples (including 57 environmental feces, 177 lungs, 33 lymph nodes [LN], and 3 sera) and compared with previously developed qPCR. Eighty samples were qPCR positive, while only 63 samples were PMA-qPCR positive. No virus could be isolated in the 17 qPCR-positive but PMA-qPCR-negative clinical samples; meanwhile, PRRSV could be isolated in representative PMA-qPCR-positive samples, supporting that only live PRRSV isolates in distinct samples could be detected by this PMA-qPCR assay. In conclusion, this study provides the first PMA-qPCR assay for rapid and universal detection of infectious PRRSV, offering an alternative and effective method for PRRSV diagnosis, prevention, and control.

Identification of a New Conserved Antigenic Epitope by Specific Monoclonal Antibodies Targeting the African Swine Fever Virus Capsid Protein p17

African swine fever (ASF) has widely spread around the world in the last 100 years since its discovery. The African swine fever virus (ASFV) particles are made of more than 150 proteins, with the p17 protein encoded by the D117L gene serving as one of the major capsid proteins and playing a crucial role in the virus's morphogenesis and immune evasion. Thus, monoclonal antibody (mAb) targeting p17 is important for the research and detection of ASFV infection. Here, we produced two specific mAbs against p17, designated as 1G2 and 6G3, respectively, and both have been successfully used in enzyme-linked immunosorbent assay (ELISA), Western blotting, and immunofluorescence assay. Moreover, we found that both 1G2 and 6G3 mAbs recognize a novel epitope of 72-78 amino acids of p17 protein, highly conserved across all genotype I and II strains. Based on this epitope, an indirect ELISA has been established to effectively detect antibodies during ASFV infection, and it exhibits high consistency with commercial ASFV ELISA kits. In summary, the production of the specific p17 mAbs and the identification of the recognized epitope will significantly promote the serological diagnosis of ASFV.

Virus Infection Induces Immune Gene Activation with CTCF-anchored Enhancers and Chromatin Interactions in Pig Genome

Chromatin organization is important for gene transcription in pig genome. However, its three-dimensional (3D) structure and dynamics are much less investigated than those in human. Here, we applied the long-read chromatin interaction analysis by paired-end tag sequencing (ChIA-PET) method to map the whole-genome chromatin interactions mediated by CCCTC-binding factor (CTCF) and RNA polymerase II (RNAPII) in porcine macrophage cells before and after polyinosinic-polycytidylic acid [Poly(I:C)] induction. Our results reveal that Poly(I:C) induction impacts the 3D genome organization in the 3D4/21 cells at the fine-scale chromatin loop level rather than at the large-scale domain level. Furthermore, our findings underscore the pivotal role of CTCF-anchored chromatin interactions in reshaping chromatin architecture during immune responses. Knockout of the CTCF-binding locus further confirms that the CTCF-anchored enhancers are associated with the activation of immune genes via long-range interactions. Notably, the ChIA-PET data also support the spatial relationship between single nucleotide polymorphisms (SNPs) and related gene transcription in 3D genome aspect. Our findings in this study provide new clues and potential targets to explore key elements related to diseases in pigs and are also likely to shed light on elucidating chromatin organization and dynamics underlying the process of mammalian infectious diseases.

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.

A novel conserved B-cell epitope in pB602L of African swine fever virus

African swine fever virus (ASFV) is a complex DNA virus and the only member of the Asfarviridae family. It causes high mortality and severe economic losses in pigs. The ASFV pB602L protein plays a key role in virus assembly and functions as a molecular chaperone of the major capsid protein p72. In addition, pB602L is an important target for the development of diagnostic tools for African swine fever (ASF) because it is a highly immunogenic antigen against ASFV. In this study, we expressed and purified ASFV pB602L and validated its immunogenicity in serum from naturally infected pigs with ASFV. Furthermore, we successfully generated an IgG2a κ subclass monoclonal antibody (mAb 7E7) against pB602L using hybridoma technology. Using western blot and immunofluorescence assays, mAb 7E7 specifically recognized the ASFV Pig/HLJ/2018/strain and eukaryotic recombinant ASFV pB602L protein in vitro. The 474SKENLTPDE482 epitope in the ASFV pB602L C-terminus was identified as the minimal linear epitope for mAb 7E7 binding, with dozens of truncated pB602l fragments characterized by western blot assay. We also showed that this antigenic epitope sequence has a high conservation and antigenic index. Our study contributes to improved vaccine and antiviral development and provides new insights into the serologic diagnosis of ASF. KEY POINTS: • We developed a monoclonal antibody against ASFV pB602L, which can specifically recognize the ASFV Pig/HLJ/2018/ strain. • This study found one novel conserved B-cell epitope 474SKENLTPDE482. • In the 3D structure, 474SKENLTPDE482 is exposed on the surface of ASFV pB602L, forming a curved linear structure.

Structural insights into Semiliki forest virus receptor binding modes indicate novel mechanism of virus endocytosis

The Very Low-Density Lipoprotein Receptor (VLDLR) is an entry receptor for the prototypic alphavirus Semliki Forest Virus (SFV). However, the precise mechanisms underlying the entry of SFV into cells mediated by VLDLR remain unclear. In this study, we found that of the eight class A (LA) repeats of the VLDLR, only LA2, LA3, and LA5 specifically bind to the native SFV virion while synergistically promoting SFV cell attachment and entry. Furthermore, the multiple cryo-electron microscopy structures of VLDLR-SFV complexes and mutagenesis studies have demonstrated that under physiological conditions, VLDLR primarily binds to E1-DIII of site-1, site-2, and site-1' at the twofold symmetry axes of SFV virion through LA2, LA3, and LA5, respectively. These findings unveil a novel mechanism for viral entry mediated by receptors, suggesting that conformational transitions in VLDLR induced by multivalent binding of LAs facilitate cellular internalization of SFV, with significant implications for the design of antiviral therapeutics.

Screening and identification of linear B-cell epitopes on structural proteins of African Swine Fever Virus

This study aims to screen and identify linear B-cell epitopes on the structural proteins of African Swine Fever Virus (ASFV) to assist in the development of peptide-based vaccines. In experiments, 66 peptides of 12 structural proteins of ASFV were predicted as potential linear B-cell epitopes using bioinformatics tools and were designed; the potential epitope proteins carried the GST tag were expressed, purified, and subjected to antigenicity analysis with porcine antiserum against ASFV, and further identified based on their immunogenicity in mice. A total of 22 potential linear B-cell epitopes showed immunoreactivity and immunogenicity. Of these epitopes, 13 epitopes were firstly identified including 4 epitopes located in p72 (352-363, 416-434, 424-439, 496-530 aa), 3 epitopes located in pE248R (121-136, 138-169, 158-185 aa), and only one epitope of each protein of pH108R (33-46 aa), p17 (63-86 aa), pE120R (65-117 aa), pE199L (175-189 aa), p12 (36-56 aa) as well as pB438L (211-230 aa). Notably, the immunoreactivity of the epitopes from the 63-86 aa of p17 and the 65-117 aa of pE120R were the highest amongst identified epitopes, while the immunogenicity of epitopes from the 36-56 aa of p12, the 211-230 aa of pB438L, the 352-363 aa of p72 and the 63-86 aa of p17 were the best strong. The other 9 epitopes are partly overlapped with previous researches. These epitopes identified here will further enrich the database of ASFV epitope, as well as help to develop safe, effective epitope-based ASF vaccines and ASF diagnostic reagents.

A Nanobody-based TRIM-away targets the intracellular protein degradation of African swine fever virus

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF), a hemorrhagic illness with high fatality rates in domestic pigs that has resulted in a substantial socio-economic loss and threatens the global pork industry. Very few safe and efficient vaccines or compounds against ASF are commercially available, thus developing new antiviral strategies is urgently required. Targeted protein degradation (TPD) has emerged as one of the most innovative strategies for drug discovery. In this study, we generate Nanobody-based TRIM-aways specifically binding with and targeting ASFV-encoded structural proteins p30, p54, and p72 for degradation. Furthermore, nanobody-based trim-aways exhibit robust viral structural protein degradation capabilities in ASFV-infected iPAM and MA104 cells through both proteasomal and lysosomal pathways, concurrently demonstrating potent anti-ASFV activity with less viral production. Our study highlights the Nanobody-based TRIM-away targeting viral protein degradation as a potential candidate for the development of a novel antiviral strategy against ASF.

Rapid discrimination of African swine fever virus nucleic acid and virions using BenzoNuclease

African swine fever (ASF) is an acute and severe infectious disease caused by the African Swine Fever Virus (ASFV). ASFV exhibits significant resistance and stability in the environment, which, coupled with its double-stranded DNA and large genome, predisposes it to contaminate laboratory samples. This characteristic can lead to false-positive results in swine farm settings even days after disinfection, as detectable through polymerase chain reaction (PCR) or real-time fluorescent quantitative PCR (qPCR) assays. To meet the demand for efficient clinical methods capable of discriminating between ASFV nucleic acid and ASFV virions, this study aims to ascertain the efficacy of the nuclease "BenzoNuclease" in distinguishing ASFV nucleic acid (ASFV-DNA) from ASFV virions. BenzoNuclease is a versatile nucleic acid enzyme with the capacity to degrade nearly all forms of DNA and RNA. Initially, this research established a highly sensitive general PCR detection method for ASFV. Subsequently, a positive control was constructed using the M13 bacteriophage to substitute for active ASFV, facilitating the development of an improved qPCR method. It is important to note that common disinfectants have the potential to deactivate BenzoNuclease. However, in an environment simulating actual production applications, residual disinfectants do not interfere with the enzymatic efficacy of BenzoNuclease, thus not affecting the detection capabilities of this method. Positive clinical samples from pig farms, upon testing with the improved method, revealed that three samples were positive, indicating the presence of viral particles, while the remaining samples were negative, indicating the presence of nucleic acids. This provides an additional new option for sample testing in pig farms.

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.

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

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

How Does African Swine Fever Virus Evade the cGAS-STING Pathway?

African swine fever (ASF), a highly infectious and devastating disease affecting both domestic pigs and wild boars, is caused by the African swine fever virus (ASFV). ASF has resulted in rapid global spread of the disease, leading to significant economic losses within the swine industry. A significant obstacle to the creation of safe and effective ASF vaccines is the existing knowledge gap regarding the pathogenesis of ASFV and its mechanisms of immune evasion. The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway is a major pathway mediating type I interferon (IFN) antiviral immune response against infections by diverse classes of pathogens that contain DNA or generate DNA in their life cycles. To evade the host's innate immune response, ASFV encodes many proteins that inhibit the production of type I IFN by antagonizing the cGAS-STING signaling pathway. Multiple proteins of ASFV are involved in promoting viral replication by protein-protein interaction during ASFV infection. The protein QP383R could impair the function of cGAS. The proteins EP364R, C129R and B175L could disturb the function of cyclic guanosine monophosphate-adenosine monophosphate (cGAMP). The proteins E248R, L83L, MGF505-11L, MGF505-7R, H240R, CD2v, E184L, B175L and p17 could interfere with the function of STING. The proteins MGF360-11L, MGF505-7R, I215L, DP96R, A151R and S273R could affect the function of TANK Binding Kinase 1 (TBK1) and IκB kinase ε (IKKε). The proteins MGF360-14L, M1249L, E120R, S273R, D129L, E301R, DP96R, MGF505-7R and I226R could inhibit the function of Interferon Regulatory Factor 3 (IRF3). The proteins MGF360-12L, MGF505-7R/A528R, UBCv1 and A238L could inhibit the function of nuclear factor kappa B (NF-Κb).

Fluorescence immunochromatographic detection of antibodies to the p72 protein of African swine fever virus

The African swine fever virus (ASFV), a highly contagious pathogen responsible for African swine fever (ASF), causes significant economic losses in the global pork industry. Due to its large and complex structure, ASFV remains refractory to commercial vaccine development, necessitating the creation of rapid, sensitive, and specific diagnostic tools for disease control. In this study, quantum dots were conjugated to ASFV p72 protein to establish a fluorescent immunochromatographic assay for detecting ASFV-specific antibodies. The assay test strips contained four adjacent pads arranged sequentially: a sample-application pad, a pad containing mobile antigen-probe conjugate, a nitrocellulose readout pad featuring a test line containing immobilised staphylococcal protein A and a control line containing immobilised monoclonal antibodies against the ASFV p72 protein, and an absorbent pad driving the directional flow of liquid via capillary action. The resulting fluorescence immunochromatographic assay demonstrated highly sensitive and specific ASFV antibody detection in under 15 min. Specificity testing showed no cross-reactivity with serum antibodies against other viruses and sensitivity surpassing that of commercial ASFV antibody colloidal gold immunochromatographic test strips. This novel approach offers rapid detection, excellent specificity, and high sensitivity, and supports the future development of fluorescent immunochromatographic test strips for ASFV antibody detection.

Immune responses induced by a recombinant C-strain of classical swine fever virus expressing the F317L protein of African swine fever virus

African swine fever (ASF), a highly infectious and devastating disease affecting both domestic pigs and wild boars, owes its etiology to African swine fever virus (ASFV). ASFV encodes more than 165 proteins. However, novel immunogenic proteins remain unknown. This study aimed to determine the antigenicity of the F317L protein (pF317L) of ASFV. The results revealed that pF317L was able to react with convalescent pig sera, indicating that pF317L could be a candidate antigen. The antigenic potential of pF317L expressed by rHCLV-F317L, a recombinant virus in the backbone of C-strain (a lapinized live attenuated classical swine fever virus) was further investigated in rabbits and pigs. The results revealed that antibodies and cell-mediated immune responses against pF317L were induced in either rabbits or pigs inoculated with rHCLV-F317L. Importantly, anti-pF317L antibodies from rabbits or pigs immunized with rHCLV-F317L significantly inhibited ASFV replication in vitro. In conclusion, pF317L demonstrates favorable immunogenic properties, positioning it as a promising candidate for the development of protective antigens in the ongoing endeavor to formulate efficacious ASF vaccine strategies.