African Swine Fever Virus Biology and Vaccine Approaches

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.

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.

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.

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.

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.

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.

Navigating the threat of African swine fever: a comprehensive review

African swine fever (ASF) is caused by Asfivirus and has become one of the most important diseases of swine in recent years. ASF was an endemic disease of the sub-Saharan Africa but later spread to various parts of the world. The infection in ticks and wild swine, alongside global pork trade, drives its spread and persistence. Once introduced to an area, the disease is difficult to eliminate due to sylvatic, domestic, and tick-swine transmission cycles. Because of the existence of various modes of transmission of the ASF virus, biosecurity measures have not been very successful. The line of treatment is not of much use and the outcome of this disease is usually fatal. The prognosis or the recovery of the animal depends on the virulence of the strain involved. Development of vaccines has been attempted but to date has not been very successful. This review focuses on the basic context of ASF, the challenges associated with it, and the options that might be available to prevent its occurrence which includes the different vaccine development strategies tried and tested till now.

A rosin-functionalized plastic surface inactivates African swine fever virus

African swine fever virus (ASFV) causes a severe hemorrhagic disease in pigs, leading to up to 100% case fatality. The virus May persist on solid surfaces for long periods; thus, fomites, such as contaminated clothing, footwear, farming tools, equipment, and transport vehicles, May contribute to the indirect transmission of the virus. Here, a plastic surface functionalized with tall oil rosin was tested against ASFV. The rosin-functionalized plastic reduced ASFV infectious virus titers by 1.3 log10 after 60 min of contact time and killed all detectable viruses after 120 min, leading to a ~ 6 log10 reduction. In contrast, the infectious virus titer of ASFV in contact with low-density polyethylene (LDPE) plastic reduced <1 log10 after 120 min. Transmission electron microscopy (TEM) showed significant morphological changes in the virus after 2 h of contact with the rosin-functionalized plastic surface, but no changes were observed with the LDPE plastic. The use of antiviral plastic in the farming sector could reduce the spread of ASFV through fomites and could thus be part of an integrated program to control ASFV.

African swine fever virus MGF360-4L protein attenuates type I interferon response by suppressing the phosphorylation of IRF3

African swine fever (ASF) is a highly contagious and lethal disease of swine caused by African swine fever virus (ASFV), and the mortality rate caused by virulent stains can approach 100%. Many ASFV viral proteins suppress the interferon production to evade the host's innate immune responses. However, whether ASFV MGF360-4L could inhibit type I interferon (IFN-I) signaling pathway and the underlying molecular mechanisms remain unknown. Our study, indicated that ASFV MGF360-4L could negatively regulates the cGAS-STING mediated IFN-I signaling pathway. Overexpressing ASFV MGF360-4L could inhibit the cGAS/STING signaling pathway by inhibiting the interferon-β promoter activity, which was induced by cGAS/STING, TBK1, and IRF3-5D, and further reduced the transcriptional levels of ISG15, ISG54, ISG56, STAT1, STAT2, and TYK2. Confocal microscopy and immunoprecipitation revealed that MGF360-4L co-localized and interacted with IRF3, and WB revealed that ASFV MGF360-4L suppressed the phosphorylation of IRF3. 4L-F2 (75-162 aa) and 4L-F3 (146-387 aa) were the crucial immunosuppressive domains and sites. Altogether, our study reveals ASFV MGF360-4L inhibited cGAS-STING mediated IFN-I signaling pathways, which provides insights into an evasion strategy of ASFV involving in host's innate immune responses.

Assessment of reference genes for qRT-PCR normalization to elucidate host response to African swine fever infection

Viral infection disrupts the normal regulation of the host gene's expression. In order to normalise the expression of dysregulated host genes upon virus infection, analysis of stable reference housekeeping genes using quantitative real-time-PCR (qRT-PCR) is necessary. In the present study, healthy and African swine fever virus (ASFV) infected porcine tissues were assessed for the expression stability of five widely used housekeeping genes (HPRT1, B2M, 18 S rRNA, PGK1 and H3F3A) as reference genes using standard algorithm. Total RNA from each tissue sample (lymph node, spleen, kidney, heart and liver) from healthy and ASFV-infected pigs was extracted and subsequently cDNA was synthesized, and subjected to qRT-PCR. Stability analysis of reference genes expression was performed using the Comparative delta CT, geNorm, BestKeeper and NormFinder algorithm available at RefFinder for the different groups. Direct Cycle threshold (CT) values of samples were used as an input for the web-based tool RefFinder. HPRT1 in spleen, 18 S rRNA in liver and kidney and H3F3A in heart and lymph nodes were found to be stable in the individual healthy tissue group (group A). The majority of the ASFV-infected organs (liver, kidney, heart, lymph node) exhibited H3F3A as stable reference gene with the exception of the ASFV-infected spleen, where HPRT1 was found to be the stable gene (group B). HPRT1 was found to be stable in all combinations of all CT values of both healthy and ASFV-infected porcine tissues (group C). Of five different reference genes investigated for their stability in qPCR analysis, the present study revealed that the 18 S rRNA, H3F3A and HPRT1 genes were optimal reference genes in healthy and ASFV-infected different porcine tissue samples. The study revealed the stable reference genes found in healthy as well as ASF-infected pigs and these reference genes identified through this study will form the baseline data which will be very useful in future investigations on gene expression in ASFV-infected pigs.

In Vivo Study of Inoculation Approaches and Pathogenicity in African Swine Fever

African swine fever is an extremely infectious viral disease that can cause nearly 100% mortality in domestic pigs. In this study, we isolated an ASFV strain HB31A and characterized it using hemadsorption assay, immunofluorescence assay, and electron microscopy. We then performed animal experiments on 20-day-old pigs through intramuscular and oronasal inoculations with HB31A. Pigs in the intramuscular group exhibited more consistent clinical disease, with an incubation period of 4.33 ± 0.47 days and a 100% mortality rate within 6.67 (±0.47) days post-inoculation (dpi). In contrast, the oronasal group experienced a longer course of disease, with an incubation period of 6.00 ± 0.82 days. Two out of three pigs in the oronasal group died at 8 and 10 dpi, while the surviving pig exhibited chronic disease and persistent infection, intermittently excreting ASFV through the oral, nasal, and rectal pathways. Virus DNA was found in oral, nasal, and rectal swabs at 1-3 dpi in the intramuscular group and at 3-5 dpi in the oronasal group. In summary, HB31A is highly lethal to domestic pigs, and field-infected pigs have the potential to develop non-lethal, chronic disease and persistent infection, with intermittent viral shedding, even when infected with a highly virulent strain. These findings offer a valuable understanding of the viral dynamics and pathogenicity of ASFV and highlight the difficulties in diagnosing, preventing, and controlling African swine fever.

Development and evaluation of two rapid lateral flow assays for on-site detection of African swine fever virus

Introduction

African swine fever (ASF) is a lethal and highly contagious transboundary animal disease with the potential for rapid international spread. In the absence of a widely available and definitively proven vaccine, rapid and early detection is critical for ASF control. The quick and user-friendly lateral flow assay (LFA) can easily be performed by following simple instructions and is ideal for on-site use. This study describes the development and validation of two LFAs for the rapid detection of ASF virus (ASFV) in pig serum.

Methods

The highly immunogenic antigens (p30 and p72) of ASFV Georgia 2007/1 (genotype II) were expressed in plants (Nicotiana benthamiana) and were used to immunize BALB/c mice to generate specific monoclonal antibodies (mAbs) against the p30 and p72 proteins. mAbs with the strongest binding ability to each protein were used to develop p30_LFA and p72_LFA for detecting the respective ASFV antigens. The assays were first evaluated using a spike-in test by adding the purified p30 or p72 protein to a serum sample from a healthy donor pig. Further validation of the tests was carried out using serum samples derived from experimentally infected domestic pigs, field domestic pigs, and feral pigs, and the results were compared with those of ASFV real-time PCR.

Results

p30_LFA and p72_LFA showed no cross-reaction with common swine viruses and delivered visual results in 15 min. When testing with serially diluted proteins in swine serum samples, analytical sensitivity reached 10 ng/test for p30_LFA and 20 ng/test for p72_LFA. Using real-time PCR as a reference, both assays demonstrated high sensitivity (84.21% for p30_LFA and 100% for p72_LFA) with experimentally ASFV-infected pig sera. Specificity was 100% for both LFAs using a panel of PBS-inoculated domestic pig sera. Excellent specificity was also shown for field domestic pig sera (100% for p30_LFA and 93% for p72_LFA) and feral pig sera (100% for both LFAs).

Conclusion

The results obtained in this study suggest that p30_LFA and p72_LFA hold promise as rapid, sensitive, user-friendly, and field-deployable tools for ASF control, particularly in settings with limited laboratory resources.

Bis-benzylisoquinoline alkaloids inhibit African swine fever virus internalization and replication by impairing late endosomal/lysosomal function

African swine fever (ASF), caused by the African swine fever virus (ASFV), is a highly infectious disease afflicting domestic pigs and wild boars. It exhibits an alarming acute infection fatality rate of up to 100%. Regrettably, no commercial vaccines or specific drugs for combating this disease are currently available. This study evaluated the anti-ASFV activities in porcine alveolar macrophages, 3D4/21 cells, and PK-15 cells of four bis-benzylisoquinoline alkaloids (BBAs): cepharanthine (CEP), tetrandrine, fangchinoline, and iso-tetrandrine. Furthermore, we demonstrated that CEP, which exhibited the highest selectivity index (SI = 81.31), alkalized late endosomes/lysosomes, hindered ASFV endosomal transport, disrupted virus uncoating signals, and thereby inhibited ASFV internalization. Additionally, CEP disrupted ASFV DNA synthesis, leading to the inhibition of viral replication. Moreover, berbamine was labeled with NBD to synthesize a fluorescent probe to study the cellular location of these BBAs. By co-staining with Lyso-Tracker and lysosome-associated membrane protein 1, we demonstrated that BBAs target the endolysosomal compartments for the first time. Our data together indicated that BBAs are a class of natural products with significant inhibitory effects against ASFV infection. These findings suggest their potential efficacy as agents for the prevention and control of ASF, offering valuable references for the identification of potential drug targets.IMPORTANCEThe urgency and severity of African swine fever (ASF) underscore the critical need for effective interventions against this highly infectious disease, which poses a grave threat to domestic pigs and wild boars. Our study reveals the potent anti-African swine fever virus (ASFV) efficacy of bis-benzylisoquinoline alkaloids (BBAs), particularly evident in the absence of progeny virus production under a 5 µM concentration treatment. The structural similarity among cepharanthine, tetrandrine, fangchinoline, and iso-tetrandrine, coupled with their analogous inhibitory stages and comparable selectivity indexes, strongly suggests a shared antiviral mechanism within this drug category. Further investigation revealed that BBAs localize to lysosomes and inhibit the internalization and replication of ASFV by disrupting the endosomal/lysosomal function. These collective results have profound implications for ASF prevention and control, suggesting the potential of the investigated agents as prophylactic and therapeutic measures. Furthermore, our study offers crucial insights into identifying drug targets and laying the groundwork for innovative interventions.

The evolutionary and genetic patterns of African swine fever virus

African swine fever (ASF) is a serious animal disease, and has spread to Africa, Europe and Asia, causing massive economic losses. African swine fever virus (ASFV) is transmitted from a reservoir host (warthog) to domestic pigs via a sylvatic cycle (transmission between warthogs and soft ticks) and a domestic cycle (transmission between domestic pigs) and survives by expressing a variety of genes related to virus-host interactions. We evaluated differences in codon usage patterns among ASFV genotypes and clades and explored the common and specific evolutionary and genetic characteristics of ASFV sequences. We analysed the evolutionary relationships, nucleotide compositions, codon usage patterns, selection pressures (mutational pressure and natural selection) and viral adaptation to host codon usage based on the coding sequences (CDS) of key functional genes of ASFV. AT bias was detected in the six genes analysed, irrespective of clade. The AT bias of genes (A224L, A179L, EP153R) encoding proteins involved in interaction with host cells after infection was high; among them, the AT bias of EP153R was the greatest at 78.3%. A large number of overrepresented codons were identified in EP153R, whereas there were no overrepresented codons with a relative synonymous codon usage (RSCU) value of ≥3 in B646L. In most genes, the pattern of selection pressure was similar for each clade, but in EP153R, diverse patterns of selection pressure were captured within the same clade and genotype. As a result of evaluating host adaptation based on the codon adaptation index (CAI), for B646L, E183L, CP204L and A179L, the codon usage patterns in all sequences were more similar to tick than domestic pig or wild boar. However, EP153R showed the lowest average CAI value of 0.52 when selecting tick as a reference set. The genes analysed in this study showed different magnitudes of selection pressure at the clade and genotype levels, which is likely to be related to the function of the encoded proteins and may determine key evolutionary traits of viruses, such as the level of genetic variation and host range. The diversity of codon adaptations at the genetic level in ASFV may account for differences in translational selection in ASFV hosts and provides insight into viral host adaptation and co-evolution.

African Swine Fever Virus Immunosuppression and Virulence-Related Gene

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

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

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

Six underreported viral diseases of domesticated and wild swine in Africa: Implications and perspectives

Pig production is increasing annually in Africa as it is recognized as a significant source of income, livelihood and food security, particularly in rural communities. Understanding the circulating swine pathogens is crucial for the success of this emerging industry. Although there is extensive data available on the African swine fever virus due to its devastating impact on pig production, knowledge about the presence of other viral swine pathogens on the continent is still extremely limited. This review discusses what is currently known about six swine pathogens in Africa: classical swine fever virus, porcine reproductive and respiratory syndrome virus, porcine circovirus-2, porcine circovirus-3, porcine parvovirus-1, and pseudorabies virus. Gaps in our knowledge are identified and topics of future focus discussed.

Advances in research on the efficacy of traditional Chinese herbal medicine in combating African swine fever

African swine fever (ASF) is a highly contagious disease of domestic and wild pigs caused by African swine fever virus (ASFV). The mortality rate associated with ASF is remarkably high, almost approaching 100%. Since the introduction of ASF into China in 2018, its rapid spread has caused marked economic losses in the country’s swine industry. To date, there are no safe and effective commercial vaccines or antiviral drugs against ASF; thus, there is an urgent need to develop novel prevention and control strategies. Traditional Chinese medicine (TCM), which comprises various herbs that are abundant in various potential functional components, holds great promise for the prevention and control of ASF. Here, we provide a comprehensive review of the advancements in TCM and the effects of its compound formulas against ASF, including the antiviral abilities, immunoregulatory activities, and practical application of these formulas for the prevention and control of ASF. We specifically examined the potential and constraints of natural product-derived extracts and TCM formulas in combating ASFV. This review aims to offer insights and ideas for the holistic management and containment of ASF.

Nanobodies against African swine fever virus p72 and CD2v proteins as reagents for developing two cELISAs to detect viral antibodies

African swine fever virus (ASFV) poses a significant threat to the global swine industry. Currently, there are no effective vaccines or treatments available to combat ASFV infection in pigs. The primary means of controlling the spread of the disease is through rapid detection and subsequent elimination of infected pig. Recently, a lower virulent ASFV isolate with a deleted EP402R gene (CD2v-deleted) has been reported in China, which further complicates the control of ASFV infection in pig farms. Furthermore, an EP402R-deleted ASFV variant has been developed as a potential live attenuated vaccine candidate strain. Therefore, it is crucial to develop detection methods that can distinguish wild-type and EP402R-deleted ASFV infections. In this study, two recombinant ASFV-p72 and -CD2v proteins were expressed using a prokaryotic system and used to immunize Bactrian camels. Subsequently, eight nanobodies against ASFV-p72 and ten nanobodies against ASFV-CD2v were screened. Following the production of these nanobodies with horse radish peroxidase (HRP) fusion proteins, the ASFV-p72-Nb2-HRP and ASFV-CD2v-Nb22-HRP fusions were selected for the development of two competitive ELISAs (cELISAs) to detect anti-ASFV antibodies. The two cELISAs exhibited high sensitivity, good specificity, repeatability, and stability. The coincidence rate between the two cELISAs and commercial ELISA kits was 98.6% and 97.6%, respectively. Collectively, the two cELISA for detecting antibodies against ASFV demonstrated ease of operation, a low cost, and a simple production process. The two cELISAs could determine whether pigs were infected with wild-type or CD2v-deleted ASFV, and could play an important role in monitoring ASFV infections in pig farms.

Function investigation of p11.5 in ASFV infection

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

Identification of host proteins that interact with African swine fever virus pE301R

African swine fever virus (ASFV) infection poses enormous threats and challenges to the global pig industry; however, no effective vaccine is available against ASFV, attributing to the huge viral genome (approximately189 kb) and numerous encoding products (>150 genes) due to the limited understanding on the molecular mechanisms of viral pathogenesis. Elucidating the host-factor/viral-protein interaction network will reveal new targets for developing novel antiviral therapies. Using proteomic analysis, we identified 255 cellular proteins that interact with the ASFV-encoded pE301R protein when transiently expressed in HEK293T cells. Gene ontology (GO) annotation, Kyoto Encyclopedia of Genes and Genomes (KEGG) database enrichment, and protein-protein interaction (PPI) network analyses revealed that pE301R-interacting host proteins are potentially involved in various biological processes, including protein translation and folding, response to stimulation, and mitochondrial transmembrane transport. The interactions of two putative cellular proteins (apoptosis inducing factor mitochondria associated 1 (AIFM1) and vimentin (VIM)) with pE301R-apoptosis inducing factor have been verified by co-immunoprecipitation. Our study revealed the inhibitory role of pE301R in interferon (IFN) induction that involves VIM sequestration by pE301R, identified interactions between ASFV pE301R and cellular proteins, and predicted the potential function of pE301R and its associated biological processes, providing valuable information to enhance our understanding of viral protein function, pathogenesis, and potential candidates for the prevention and control of ASFV infection.

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

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

African swine fever virus MGF505-6R attenuates type I interferon production by targeting STING for degradation

African swine fever (ASF) is an acute hemorrhagic and devastating infectious disease affecting domestic pigs and wild boars. It is caused by the African swine fever virus (ASFV), which is characterized by genetic diversity and sophisticated immune evasion strategies. To facilitate infection, ASFV encodes multiple proteins to antagonize host innate immune responses, thereby contributing to viral virulence and pathogenicity. The molecular mechanisms employed by ASFV-encoded proteins to modulate host antiviral responses have not been comprehensively elucidated. In this study, it was observed that the ASFV MGF505-6R protein, a member of the multigene family 505 (MGF505), effectively suppressed the activation of the interferon-beta (IFN-β) promoter, leading to reduced mRNA levels of antiviral genes. Additional evidence has revealed that MGF505-6R antagonizes the cGAS-STING signaling pathway by interacting with the stimulator of interferon genes (STING) for degradation in the autophagy-lysosomal pathway. The domain mapping revealed that the N-terminal region (1-260aa) of MGF505-6R is the primary domain responsible for interacting with STING, while the CTT domain of STING is crucial for its interaction with MGF505-6R. Furthermore, MGF505-6R also inhibits the activation of STING by reducing the K63-linked polyubiquitination of STING, leading to the disruption of STING oligomerization and TANK binding kinase 1 (TBK1) recruitment, thereby impairing the phosphorylation and nuclear translocation of interferon regulatory factor 3 (IRF3). Collectively, our study elucidates a novel strategy developed by ASFV MGF505-6R to counteract host innate immune responses. This discovery may offer valuable insights for further exploration of ASFV immune evasion mechanisms and antiviral strategies.

A novel easy-to-desorb eluant contributes to address environmental contamination of African swine fever virus

African swine fever virus (ASFV) is a highly pathogenic and rapidly disseminated virus with strong viability in the environment, suggesting the importance of environmental detection for prevention and control in all the pig industry. However, the detection results of environmental swabs cannot always reflect the accurate status of viral pollution, leading to persistent ASFV environmental contamination. In this study, we developed an ASFV eluant with higher environmental ASFV detection efficiency relative to 0.85% saline solution, which obtains the patent certificate issued by the China Intellectual Property Office (patent number:202010976050.9). qPCR analysis showed that in the environmental swab samples, the number of viral copies was 100 times higher for the ASFV eluant treatment than the traditional eluant treatment (0.85% saline solution). And besides, the high sensitivity of the ASFV eluant had be verified in a slaughterhouse environmental sampling detection. In soil samples, the ASFV eluent showed the same extraction effect as the TIANamp Soil DNA Kit, in contrast to no extraction effect for 0.85% saline solution. Simultaneously, this eluent could protect ASFV from degradation and allow the transportation of samples at ambient temperature without refrigeration. In clinical practice, we monitored the environmental contamination condition of the ASFV in a large-scale pig farm. The results shown that the ASFV load decreased after every disinfection in environment. This study provides an effective solution for surveilling the potential threat of ASFV in environment.

African swine fever virus A137R protein inhibits NF-κB activation via suppression of MyD88 signaling in PK15 and 3D4/21 cells in vitro

African swine fever (ASF) is an infectious disease with high mortality caused by African swine fever virus (ASFV), which poses a great threat to the global swine industry. ASFV has evolved multiple strategies to evade host antiviral innate immunity by perturbing inflammatory responses and interferon production. However, the molecular mechanisms underlying manipulation of inflammatory responses by ASFV proteins are not fully understood. Here, we report that A137R protein of ASFV is a key suppressor of host inflammatory responses. Ectopic expression of ASFV A137R in HEK293T cells significantly inhibited the activation of IL-8 and NF-κB promoters triggered by Sendai virus (SeV), influenza A virus (IAV), or vesicular stomatitis virus (VSV). Accordingly, forced A137R expression caused a significant decrease in the production of several inflammatory cytokines such as IL-8, IL-6 and TNF-α in the cells infected with SeV or IAV. Similar results were obtained from experiments using A137R overexpressing PK15 and 3D4/21 cells infected with SeV or VSV. Furthermore, we observed that A137R impaired the activation of MAPK and NF-κB signaling pathways, as enhanced expression of A137R significantly decreased the phosphorylation of JNK, p38 and p65 respectively upon viral infection (SeV or IAV) and IL-1β treatment. Mechanistically, we found that A137R interacted with MyD88, and dampened MyD88-mediated activation of MAPK and NF-κB signaling. Together, these findings uncover a critical role of A137R in restraining host inflammatory responses, and improve our understanding of complicated mechanisms whereby ASFV evades innate immunity.

Saccharomyces cerevisiae oral immunization in mice using multi-antigen of the African swine fever virus elicits a robust immune response

African swine fever virus (ASFV) is one of the most complex viruses. ASFV is a serious threat to the global swine industry because no commercial vaccines against this virus are currently available except in Vietnam. Moreover, ASFV is highly stable in the environment and can survive in water, feed, and aerosols for a long time. ASFV is transmitted through the digestive and respiratory tract. Mucosal immunity is the first line of defense against ASFV. Saccharomyces cerevisiae (SC), which has been certified by the U.S. Food and Drug Administration and has a generally recognized as safe status in the food industry, was used for oral immunization in this study. ASFV antigens were effectively expressed in recombinant SC strains with high DNA copy numbers and stable growth though surface display technology and chromosome engineering (δ-integration). The recombinant SC strains containing eight ASFV antigens-KP177R, E183L, E199L, CP204L, E248R, EP402R, B602L, and B646L- induced strong humoral and mucosal immune responses in mice. There was no antigenic competition, and these antigens induced Th1 and Th2 cellular immune responses. Therefore, the oral immunization strategy using recombinant SC strains containing multiple ASFV antigens demonstrate potential for future testing in swine, including challenge studies to evaluate its efficacy as a vaccine against ASFV.

On-site detection and differentiation of African swine fever virus variants using an orthogonal CRISPR-Cas12b/Cas13a-based assay

The African swine fever virus (ASFV) and its variants have induced substantial economic losses in China, prompting a critical need for efficient detection methods. Several PCR-based methods have been developed to discriminate between wild-type ASFV and gene-deleted variants. However, the requirement for sophisticated equipment and skilled operators limits their use in field settings. Here, we developed a CRISPR-Cas12b/Cas13a-based detection assay that can identify ASFV variants with minimal equipment requirements and a short turnaround time. The assay utilizes the distinct DNA/RNA collateral cleavage preferences of Cas12b/Cas13a to detect two amplified targets from multiplex recombinase polymerase amplification (RPA) in a single tube, and the results can be visualized through fluorescent or lateral-flow readouts. When tested with clinical samples in field settings, our assay successfully detected all ASFV-positive samples in less than 60 min. This assay provides a rapid on-site surveillance tool for detecting ASFV and its emerging variants.

Enhanced detection of African swine fever virus in samples with low viral load using digital PCR technology

Detection of low viral load samples has long been a challenge for African swine fever (ASF) prevention and control. This study aimed to compare the detection efficacy of droplet digital PCR(ddPCR) and quantitative PCR(qPCR) for African swine fever virus (ASFV) at different viral loads, with a focus on assessing the accuracy of ddPCR in detecting low viral load samples. The results revealed that ddPCR had a detection limit of 1.97 (95% CI 1.48 - 4.12) copies/reaction and was 18.99 times more sensitive than qPCR (detection limit: 37.42, 95% CI 29.56 - 69.87 copies/reaction). In the quantification of high, medium, and low viral load samples, ddPCR showed superior stability with lower intra- (2.06% - 7.58%) and inter-assay (3.83% - 7.50%) coefficients of variation than those of qPCR (intra-assay: 8.08%-29.86%; inter-assay: 9.27%-34.58%). Bland-Altman analysis indicated acceptable consistency between ddPCR and qPCR for high and medium viral load samples; however, discrepancies were observed for low viral load samples, where two samples (2/24, 8.33%) exhibited deviations beyond the acceptable range (-46.18 copies/reaction). Moreover, ddPCR demonstrated better performance in detecting ASFV in clinical samples from asymptomatic pigs and environmental samples, with qPCR showing false negative rates of 7.69% (2/26) and 27.27% (12/44), respectively. McNemar analysis revealed significant differences between the two methods (P = 0.000) for samples with a viral load <100 copies/reaction. The results of this study demonstrate that ddPCR has better detection limits and adaptability than qPCR, allowing for a more accurate detection of ASFV in early-stage infections and low-concentration environmental samples. These findings highlight the potential of ddPCR in the prevention and control of ASF.

Recombinant Vaccine Strain ASFV-G-Δ9GL/ΔUK Produced in the IPKM Cell Line Is Genetically Stable and Efficacious in Inducing Protection in Pigs Challenged with the Virulent African Swine Fever Virus Field Isolate Georgia 2010

We have previously reported that the recombinant African Swine Fever (ASF) vaccine candidate ASFV-G-Δ9GL/ΔUK efficiently induces protection in domestic pigs challenged with the virulent strain Georgia 2010 (ASFV-G). As reported, ASFV-G-Δ9GL/ΔUK induces protection, while intramuscularly (IM), administered at doses of 104 HAD50 or higher, prevents ASF clinical disease in animals infected with the homologous ASFV g strain. Like other recombinant vaccine candidates obtained from ASFV field isolates, ASFV-G-Δ9GL/ΔUK stocks need to be produced in primary cultures of swine macrophages, which constitutes an important limitation in the production of large virus stocks at the industrial level. Here, we describe the development of ASFV-G-Δ9GL/ΔUK stocks using IPKM (Immortalized Porcine Kidney Macrophage) cells, which are derived from swine macrophages. We show that ten successive passages of ASFV-G-Δ9GL/ΔUK in IPKM cells induced small changes in the virus genome. The produced virus, ASFV-G-Δ9GL/ΔUKp10, presented a similar level of replication in swine macrophages cultures to that of the original ASFV-G-Δ9GL/ΔUK (ASFV-G-Δ9GL/ΔUKp0). The protective efficacy of ASFV-G-Δ9GL/ΔUKp10 was evaluated in pigs that were IM-inoculated with either 104 or 106 HAD50 of ASFV-G-Δ9GL/ΔUKp10. While animals inoculated with 104 HAD50 present a partial protection against the experimental infection with the virulent parental virus ASFV-G, those inoculated with 106 HAD50 were completely protected. Therefore, as was just recently reported for another ASF vaccine candidate, ASFV-G-ΔI177L, IPKM cells are an effective alternative to produce stocks for vaccine strains which only grow in swine macrophages.

KP177R-based visual assay integrating RPA and CRISPR/Cas12a for the detection of African swine fever virus

Introduction

Early detection of the virus in the environment or in infected pigs is a critical step to stop African swine fever virus (ASFV) transmission. The p22 protein encoded by ASFV KP177R gene has been shown to have no effect on viral replication and virulence and can serve as a molecular marker for distinguishing field virus strains from future candidate KP177R deletion vaccine strains.

Methods

This study established an ASFV detection assay specific for the highly conserved ASFV KP177R gene based on recombinase polymerase amplification (RPA) and the CRISPR/Cas12 reaction system. The KP177R gene served as the initial template for the RPA reaction to generate amplicons, which were recognized by guide RNA to activate the trans-cleavage activity of Cas12a protein, thereby leading to non-specific cleavage of single-stranded DNA as well as corresponding color reaction. The viral detection in this assay could be determined by visualizing the results of fluorescence or lateral flow dipstick (LFD) biotin blotting for color development, and was respectively referred to as fluorescein-labeled RPA-CRISPR/Cas12a and biotin-labeled LFD RPA-CRISPR/Cas12a. The clinical samples were simultaneously subjected to the aforementioned assay, while real-time quantitative PCR (RT-qPCR) was employed as a control for determining the diagnostic concordance rate between both assays.

Results

The results showed that fluorescein- and biotin-labeled LFD KP177R RPA-CRISPR/Cas12a assays specifically detected ASFV, did not cross-react with other swine pathogens including PCV2, PEDV, PDCoV, and PRV. The detection assay established in this study had a limit of detection (LOD) of 6.8 copies/μL, and both assays were completed in 30 min. The KP177R RPA-CRISPR/Cas12a assay demonstrated a diagnostic coincidence rate of 100% and a kappa value of 1.000 (p < 0.001), with six out of ten clinical samples testing positive for ASFV using both KP177R RPA-CRISPR/Cas12a and RT-qPCR, while four samples tested negative in both assays.

Discussion

The rapid, sensitive and visual detection assay for ASFV developed in this study is suitable for field application in swine farms, particularly for future differentiation of field virus strains from candidate KP177R gene-deleted ASFV vaccines, which may be a valuable screening tool for ASF eradication.

Characterization of the monoclonal antibody and the immunodominant B-cell epitope of African swine fever virus pA104R by using mouse model

The African swine fever virus (ASFV) structural protein pA104R is the only histone-like protein encoded by eukaryotic viruses. pA104R is an essential DNA-binding protein required for DNA replication and genome packaging of ASFV, which are vital for pathogen survival and proliferation. pA104R is an important target molecule for diagnosing, treating, and immune prevention of ASFV. This study characterized monoclonal antibodies (mAbs) against pA104R and found them to recognize natural pA104R in ASFV strains with different genotypes, showing high conservation. Confirmation analyses of pA104R epitopes using mAbs indicated the presence of immunodominant B-cell epitopes, and further characterization showed the high antigenic index and surface accessibility coefficients of the identified epitope. Furthermore, the pA104R protein functions through the polar interactions between the binding amino acid sites; however, these interactions may be blocked by the recognition of generated mAbs. Characterizing the immunodominant B-cell epitope of the ASFV critical proteins, such as pA104R, may contribute to developing sensitive diagnostic tools and vaccine candidate targets.IMPORTANCEAfrican swine fever (ASF) is a highly pathogenic, lethal, and contagious viral disease affecting domestic pigs and wild boars. As no effective vaccine or other treatments have been developed, the control of African swine fever virus (ASFV) relies heavily on virus detection and diagnosis. A potential serological target is the structural protein pA104R. However, the molecular basis of pA104R antigenicity remains unclear, and a specific monoclonal antibody (mAb) against this protein is still unavailable. In this study, mAbs against pA104R were characterized and found to recognize natural pA104R in ASFV strains with different genotypes. In addition, confirmation analyses of pA104R epitopes using mAbs indicated the presence of immunodominant B-cell epitopes, and further characterization showed the high antigenic index and surface accessibility coefficients of the identified epitope. Characteristics of the immunodominant B-cell epitope of ASFV proteins, such as pA104R, may contribute to developing sensitive diagnostic tools and identifying vaccine candidate targets.

ASFV epitope mapping by high density peptides microarrays

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

Establishment and characterization of a novel indirect ELISA method based on ASFV antigenic epitope-associated recombinant protein

African Swine Fever (ASF) is an acute and highly lethal disease in pigs caused by African Swine Fever Virus (ASFV). Viral proteins have been commonly used as antigenic targets for the development of ASF diagnostic methods. However, the prokaryotic expression of viral proteins has deficiencies such as instability, insolubility, and high cost in eukaryotic situations. This study screened and verified ASFV-encoded p72, p54, and p30 protein antigenic epitopes. Subsequently, a novel antigenic epitope-associated recombinant protein was designed based on an ideal structural protein and expressed in Escherichia coli (E. coli). Western blot analysis indicated that the recombinant protein could specifically react with the monoclonal antibody (mAb) of p72 and polyclonal antibodies of p54 and p30, respectively. Next, an ASF indirect ELISA (iELISA) method was established based on the recombinant protein, which has no specific reaction with sera of other important pig viral diseases. Meanwhile, it shows a sensitivity to detecting dilutions of ASF-positive reference serum up to 1:6400. The clinical sample detection results showed a high coincidence rate of 98 % with a commercial competition ELISA kit. In conclusion, we established a novel specific, and sensitive ASF serologic detection method that opens new avenues for ASF serodiagnostic method development.

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

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

African Swine Fever and Its Control Measures in Wild Boar: A "De Iure Condito" Analysis in the European Union

Recently, the African swine fever (ASF) epizootic has been reported in domestic pigs and wild boars in several European Union Member States (EU MS) and epidemiological evidence has accumulated which indicates that wild boar play a key role in maintaining and spreading the disease. Thanks to the experience gained when managing ASF outbreaks in Sardinia (Italy) and Eastern Europe, Directive 2002/60 CE was issued. This directive represented an important step forward in controlling the disease, particularly the risk of spreading the virus to wild animals. Since 2021, according to Regulation (EU) 2016/429, which is also called "Animal Health Law-AHL", when the MS competent authority suspects or confirms ASF (a cat. A listed disease) in wild animals, it is mandatory to conduct surveillance in the wild boar population and implement the necessary control measures. Within AHL, Regulations (EU) 2020/687 and 2023/594 established special ASF control measures in kept and wild porcine animals, and their products and by-products, focusing on and underlying old and new responsibilities that vets (both public and private ones) have to accomplish under the new regulations. The new change in the legal framework deals with specific measures to be applied in the wild and represents a great challenge for MS veterinary services. Some of these measures have been well established in the last two decades, particularly those related to application in the farming system, while other measures are still new to veterinary health management and require a holistic approach in terms of intensity, considering all geographical, ecological, productive, cultural and social features of the involved EU MS. In this contribution, the authors intend to focus on specific measures which have been issued in order to limit or stop the spread of ASF in a wild, "boundless" ecosystem. These measures expand the field of competence of the official veterinary service to wild areas in addition to farm activity.

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

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

Establishment of a p30-based lateral flow assay for African swine fever virus detection

African swine fever virus (ASFV) has continuously devastated the global pig industry. Viral persistence causes problems in large pig farms and kills small farms. Timely diagnostic tools play an important role in controlling outbreaks and minimizing losses. In this study, we developed a lateral flow assay to detect ASFV on-site. The VDRG® ASFV Ag Rapid Kit was established using two monoclonal antibodies (mAbs) against the p30 protein. The conjunction pad of the kit was coated with a mixture of the mAb and colloidal gold. This rapid kit was capable of detecting 11.5 ng of antigen and 0.16 HAD50 of virus from samples, in 20 min for the entire procedure. It passed cross-specific tests using common viruses that cause infectious diseases in pigs. ASFV was detected after 4 days in experimental infection in pigs by the kit. The specificity and sensitivity of the kit for clinical samples were 99.88% and 84.52% (93.8% for samples with a Ct value below 30), respectively. Finally, the kit can detect 100% positive herd outbreaks. The VDRG® ASFV Ag Rapid Kit presents a useful point-of-care tool for ASFV detection.

Identification of Linear Epitopes in the C-Terminal Region of ASFV p72 Protein

African swine fever, which is induced by the African swine fever virus (ASFV), poses a significant threat to the global pig industry due to its high lethality in domestic pigs and wild boars. Despite the severity of the disease, there is a lack of effective vaccines and drugs against the ASFV. The p72 protein, constituting 31 to 33% of the total virus particle mass, serves as the primary capsid protein of ASFV. It is a crucial antigen for the development of ASF subunit vaccines and serological diagnostic methods. In this investigation, 27 monoclonal antibodies (mAbs) were generated through mouse immunization with the truncated C-terminal p72 protein expressed by Escherichia coli. Among these, six mAbs exhibited binding to the p72 trimer, with their respective recognized epitopes identified as 542VTAHGINLIDKF553, 568GNAIKTP574, and 584FALKPREEY592. All three epitopes were situated within the interval sequences of functional units of the C-terminal jelly-roll barrel of p72. Notably, two epitopes, 568GNAIKTP574 and 584FALKPREEY592, were internal to the p72 trimer, while the epitope 542VTAHGINLIDKF553 was exposed on the surface of the trimer and consistently conserved across all ASFV genotypes. These findings enhance our comprehension of the antigenic function and structure of the p72 protein, facilitating the utilization of p72 in the development of diagnostic techniques for ASFV.

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

IMPORTANCE

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

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

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

African Swine Fever Virus pF778R Attenuates Type I Interferon Response by Impeding STAT1 Nuclear Translocation

African swine fever virus (ASFV) is an extensive and intricate double-stranded DNA virus with approximately 100% lethality in domestic swine. There is no effective vaccine to combat this virus, and this has led to substantial economic losses in the swine industry. ASFV encodes various proteins that impede interferon-based immune defenses in the host by employing diverse mechanisms. However, the roles of most of these proteins remain unknown. Therefore, understanding the immune evasion mechanisms employed by ASFV may facilitate the development of effective measures against the virus. In this study, we discovered a negative regulation of the type I interferon (IFN) response by the ASFV ribonuclease reductase large subunit pF778R. This novel type Ⅰ IFN response antagonist significantly inhibits IFN-α-induced interferon-stimulated response element promoter activation, precludes the upregulation of various interferon-stimulated genes, and prevents STAT1 nuclear translocation. Mechanistically, pF778R did not affect the protein levels of crucial molecules in the JAK/STAT signaling pathway or engage in direct interactions. However, pF778R expression impedes type I IFN responses mediated by the JAK/STAT signaling pathway. Further investigations revealed that pF778R did not interfere with STAT1 phosphorylation or dimerization, but it inhibited IFN signaling by weakening the nuclear accumulation of activated STAT1. The critical role of the ASFV protein pF778R in evading IFN-I-mediated innate immunity highlights a unique mode of ASFV evasion and provides insights into the pathogenic mechanism of the virus.

Innate immune escape and adaptive immune evasion of African swine fever virus: A review

African swine fever virus (ASFV) causes hemorrhagic fever in domestic and wild pigs. The continued spread of the virus in Africa, Europe and Asia threatens the global pig industry. The lack of an effective vaccine limits disease control. ASFV has evolved a variety of encoded immune escape proteins and can evade host adaptive immunity, inducing cellular inflammation, autophagy, or apoptosis in host cells. Frequent persistent infections hinder the development of a viral vaccine and impose technical barriers. Currently, knowledge of the virulence-related genes, main pathogenic genes and immunoregulatory mechanism of ASFV is not comprehensive. We explain that ASFV invades the host to regulate its inflammatory response, interferon production, antigen presentation and cellular immunity. Furthermore, we propose potential ideas for ASFV vaccine target design, such as knocking out high-virulence genes in ASFV and performing data mining to identify the main genes that induce antiviral responses. To support a rational strategy for vaccine development, a better understanding of how ASFV interacts with the host and regulates the host's response to infection is needed. We review the current knowledge about ASFV targeting of host innate and adaptive immunity and the mechanisms by which the affected immune pathways are suppressed.

Immunogenic response of recombinant pseudorabies virus carrying B646L and B602L genes of African swine fever virus in mice

African swine fever (ASF) is an acute infectious disease that poses a high lethality risk to domestic pigs and wild boars, causing substantial economic losses to the global pig industry. The prevention and control of ASF remain challenging, necessitating the urgent development of a safe and effective vaccine. This study focused on the essential structural protein p72 of ASFV (encoded by the B646L gene) and its chaperone protein pB602L (encoded by the B602L gene) as the target antigenic proteins. Based on CRISPR/Cas9 gene-editing technology, we constructed a live attenuated recombinant pseudorabies virus vector expressing the p72 and pB602L proteins (designated as rPRVXJ-EGFP/B602L/B646L), and assessed its immunization effect in mice. The recombinant virus rPRVXJ-EGFP/B602L/B646L successfully proliferated and demonstrated stable expression of the p72 and pB602L proteins in BHK-21 cells. Moreover, it exhibited excellent safety when used in mice and induced specific humoral and cellular immune responses targeting p72 and pB602L. In addition, it provided complete protection (100%) against the virulent PRV strain (PRV-XJ). These results indicate that the recombinant virus rPRVXJ-EGFP/B602L/B646L possesses robust immunogenicity and safety in mice. In conclusion, PRV represents a promising viral vector for expressing ASFV gene, and our study serves as an essential reference for the development of viral vector vaccines against ASFV.

Confirming the absence of parental African swine fever virus as a potential contaminant of recombinant live attenuated ASF vaccines

African swine fever (ASF) is a devastating disease that is currently producing a panzootic significantly impacting the swine industry worldwide. One of the major challenges for advancing the development of ASF vaccines has been the absence of international standards for ASF vaccine purity, potency, safety, and efficacy. To date, the most effective experimental vaccines have been live attenuated strains of viruses. Most of these promising vaccine candidates have been developed by deleting virus genes involved in the process of viral pathogenesis and disease production. This approach requires genomic modification of a parental virus field strain through a process of homologous recombination followed by purification of the recombinant attenuated virus. In this scenario, it is critical to confirm the absence of any parental virulent virus in the final virus stock used for vaccine production. We present here a protocol to establish the purity of virus stock using the live attenuated vaccine candidates ASFV-G-ΔMGF, ASFV-G-Δ9 GLΔUK and ASFV-G-ΔI177L. Procedures described here includes inoculation in susceptible pigs followed by the assessment of the obtained material by differential qPCRs that allows the identification of vaccine virus from ASFV field isolates. This protocol is proposed as a model to ensure that master seed virus stock used for vaccine production does not contain residual parental virulent virus. Procedures described here includes a passage in susceptible pigs followed by the assessment of the obtained material by differential qPCRs that allows the identification of vaccine virus from ASFV field isolates.

Dihydromyricetin inhibits African swine fever virus replication by downregulating toll-like receptor 4-dependent pyroptosis in vitro

African swine fever (ASF), caused by ASF virus (ASFV) infection, poses a huge threat to the pork industry owing to ineffective preventive and control measures. Hence, there is an urgent need to develop strategies, including antiviral drugs targeting ASFV, for preventing ASFV spread. This study aimed to identify novel compounds with anti-ASFV activity. To this end, we screened a small chemical library of 102 compounds, among which the natural flavonoid dihydromyricetin (DHM) exhibited the most potent anti-ASFV activity. DHM treatment inhibited ASFV replication in a dose- and time-dependent manner. Furthermore, it inhibited porcine reproductive and respiratory syndrome virus and swine influenza virus replication, which suggested that DHM exerts broad-spectrum antiviral effects. Mechanistically, DHM treatment inhibited ASFV replication in various ways in the time-to-addition assay, including pre-, co-, and post-treatment. Moreover, DHM treatment reduced the levels of ASFV-induced inflammatory mediators by regulating the TLR4/MyD88/MAPK/NF-κB signaling pathway. Meanwhile, DHM treatment reduced the ASFV-induced accumulation of reactive oxygen species, further minimizing pyroptosis by inhibiting the ASFV-induced NLRP3 inflammasome activation. Interestingly, the effects of DHM on ASFV were partly reversed by treatment with polyphyllin VI (a pyroptosis agonist) and RS 09 TFA (a TLR4 agonist), suggesting that DHM inhibits pyroptosis by regulating TLR4 signaling. Furthermore, targeting TLR4 with resatorvid (a specific inhibitor of TLR4) and small interfering RNA against TLR4 impaired ASFV replication. Taken together, these results reveal the anti-ASFV activity of DHM and the underlying mechanism of action, providing a potential compound for developing antiviral drugs targeting ASFV.

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

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

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

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