Structural Insight into African Swine Fever Virus A179L-Mediated Inhibition of Apoptosis

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.

Mastering Death: The Roles of Viral Bcl-2 in dsDNA Viruses

Proteins of the Bcl-2 family regulate cellular fate via multiple mechanisms including apoptosis, autophagy, senescence, metabolism, inflammation, redox homeostasis, and calcium flux. There are several regulated cell death (RCD) pathways, including apoptosis and autophagy, that use distinct molecular mechanisms to elicit the death response. However, the same proteins/genes may be deployed in multiple biochemical pathways. In apoptosis, Bcl-2 proteins control the integrity of the mitochondrial outer membrane (MOM) by regulating the formation of pores in the MOM and apoptotic cell death. A number of prosurvival genes populate the genomes of viruses including those of the pro-survival Bcl-2 family. Viral Bcl-2 proteins are sequence and structural homologs of their cellular counterparts and interact with cellular proteins in apoptotic and autophagic pathways, potentially allowing them to modulate these pathways and determine cellular fate.

African swine fever virus proteins against host antiviral innate immunity and their implications for vaccine development

African swine fever virus (ASFV) poses a significant threat to global swine populations, necessitating a profound understanding of viral strategies against host antiviral innate immunity. This review synthesizes current knowledge regarding ASFV proteins and their intricate interactions with host defenses. Noteworthy findings encompass the modulation of interferon signaling, manipulation of inflammatory pathways, and the impact on cellular apoptosis. The implications of these findings provide a foundation for advancing vaccine strategies against ASFV. In conclusion, this review consolidates current knowledge, emphasizing the adaptability of ASFV in subverting host immunity. Identified research gaps underscore the need for continued exploration, presenting opportunities for developing targeted vaccines. This synthesis provides a roadmap for future investigations, aiming to enhance our preparedness against the devastating impact of ASFV on global swine populations.

Deletion of the gene for the African swine fever virus BCL-2 family member A179L increases virus uptake and apoptosis but decreases virus spread in macrophages and reduces virulence in pigs

IMPORTANCE

African swine fever virus (ASFV) causes a lethal disease of pigs with high economic impact in affected countries in Africa, Europe, and Asia. The virus encodes proteins that inhibit host antiviral defenses, including the type I interferon response. Host cells also activate cell death through a process called apoptosis to limit virus replication. We showed that the ASFV A179L protein, a BCL-2 family apoptosis inhibitor, is important in reducing apoptosis in infected cells since deletion of this gene increased cell death and reduced virus replication in cells infected with the A179L gene-deleted virus. Pigs immunized with the BeninΔA179L virus showed no clinical signs and a weak immune response but were not protected from infection with the deadly parental virus. The results show an important role for the A179L protein in virus replication in macrophages and virulence in pigs and suggest manipulation of apoptosis as a possible route to control infection.

Functional Landscape of African Swine Fever Virus-Host and Virus-Virus Protein Interactions

Viral replication fully relies on the host cell machinery, and physical interactions between viral and host proteins mediate key steps of the viral life cycle. Therefore, identifying virus-host protein-protein interactions (PPIs) provides insights into the molecular mechanisms governing virus infection and is crucial for designing novel antiviral strategies. In the case of the African swine fever virus (ASFV), a large DNA virus that causes a deadly panzootic disease in pigs, the limited understanding of host and viral targets hinders the development of effective vaccines and treatments. This review summarizes the current knowledge of virus-host and virus-virus PPIs by collecting and analyzing studies of individual viral proteins. We have compiled a dataset of experimentally determined host and virus protein targets, the molecular mechanisms involved, and the biological functions of the identified virus-host and virus-virus protein interactions during infection. Ultimately, this work provides a comprehensive and systematic overview of ASFV interactome, identifies knowledge gaps, and proposes future research directions.

African Swine Fever Virus Interaction with Host Innate Immune Factors

African swine fever virus (ASFV) adversely affects pig farming owing to its 100% mortality rate. The condition is marked by elevated body temperature, bleeding, and ataxia in domestic pigs, whereas warthogs and ticks remain asymptomatic despite being natural reservoirs for the virus. Breeding ASFV-resistant pigs is a promising solution for eradicating this disease. ASFV employs several mechanisms to deplete the host antiviral response. This review explores the interaction of ASFV proteins with innate host immunity and the various types of machinery encompassed by viral proteins that inhibit and induce different signaling pathways, such as cGAS-STING, NF-κB, Tumor growth factor-beta (TGF-β), ubiquitination, viral inhibition of apoptosis, and resistance to ASFV infection. Prospects for developing a domestic pig that is resistant to ASFV are also discussed.

Structure and function of African swine fever virus proteins: Current understanding

African swine fever virus (ASFV) is a highly infectious and lethal double-stranded DNA virus that is responsible for African swine fever (ASF). ASFV was first reported in Kenya in 1921. Subsequently, ASFV has spread to countries in Western Europe, Latin America, and Eastern Europe, as well as to China in 2018. ASFV epidemics have caused serious pig industry losses around the world. Since the 1960s, much effort has been devoted to the development of an effective ASF vaccine, including the production of inactivated vaccines, attenuated live vaccines, and subunit vaccines. Progress has been made, but unfortunately, no ASF vaccine has prevented epidemic spread of the virus in pig farms. The complex ASFV structure, comprising a variety of structural and non-structural proteins, has made the development of ASF vaccines difficult. Therefore, it is necessary to fully explore the structure and function of ASFV proteins in order to develop an effective ASF vaccine. In this review, we summarize what is known about the structure and function of ASFV proteins, including the most recently published findings.

African swine fever virus ubiquitin-conjugating enzyme pI215L inhibits IFN-I signaling pathway through STAT2 degradation

African swine fever virus (ASFV) is the causative agent of one of the most lethal diseases affecting domestic pig and wild boar, which is endangering the swine industry due to its rapid expansion. ASFV has developed different mechanisms to evade the host immune response, including inhibition of type I IFN (IFN-I) production and signaling, since IFN-I is a key element in the cellular antiviral response. Here, we report a novel mechanism of evasion of the IFN-I signaling pathway carried out by the ASFV ubiquitin-conjugating enzyme pI215L. Our data showed that pI215L inhibited IFN-stimulated response element (ISRE) activity and the consecutive mRNA induction of the IFN-stimulated genes ISG15 and IFIT1 through the ubiquitination and proteasomal degradation of STAT2. Additionally, by immunofluorescence, co-immunoprecipitation and nucleus-cytoplasm fractionation approaches, we have confirmed the interaction and colocalization of STAT2 and pI215L, in ectopic experiments and during ASFV infection. Moreover, expression of the catalytic mutant (I215L-C85A) did not inhibit the induction of ISG15 and IFIT1, nor the activity of ISRE. Furthermore, we confirmed that STAT2 degradation by pI215L is dependent on its catalytic activity, since expression of the pI215L-C85A mutant did not affect STAT2 levels, compared to the wild-type protein. Yet, our data reveal that the interaction of pI215L with STAT2 does not require the integrity of its catalytic domain since the pI215L-C85A mutant co-immunoprecipitates with STAT2. All these findings reveal, for the first time, the involvement of E2-ubiquitin-conjugating enzyme activity of pI215L in the immune response modulation.

African Swine Fever Virus Host-Pathogen Interactions

African swine fever virus is a complex double-stranded DNA virus that exhibits tropism for cells of the mononuclear phagocytic system. Virus replication is a multi-step process that involves the nucleus of the host cell as well the formation of large perinuclear sites where progeny virions are assembled prior to transport to, and budding through, the plasma membrane. Like many viruses, African swine fever virus reorganises the cellular architecture to facilitate its replication and has evolved multiple mechanisms to avoid the potential deleterious effects of host cell stress response pathways. However, how viral proteins and virus-induced structures trigger cellular stress pathways and manipulate the subsequent responses is still relatively poorly understood. African swine fever virus alters nuclear substructures, modulates autophagy, apoptosis and the endoplasmic reticulum stress response pathways. The viral genome encodes for at least 150 genes, of which approximately 70 are incorporated into the virion. Many of the non-structural genes have not been fully characterised and likely play a role in host range and modifying immune responses. As the field moves towards approaches that take a broader view of the effect of expression of individual African swine fever genes, we summarise how the different steps in virus replication interact with the host cell and the current state of knowledge on how it modulates the resulting stress responses.

Infection Characteristics and Transcriptomics of African Swine Fever Virus in Bama Minipigs

Animal experiments on African swine fever virus (ASFV) are vital to the study of ASFV; however, ASFV can only infect pigs, and animal experiments need to be performed in animal biosafety level 3 (ABSL-3) laboratories, meaning that many small ABSL-3 laboratories are unable to carry out in vivo ASFV experiments. Therefore, miniaturized experimental animals for ASFV infection are urgently needed. Here, we successfully isolated genotype II of ASFV SY-1 from wild boars and evaluated ASFV-infected Bama minipigs in a negative-pressure isolator of a small ABSL-3 laboratory. The pathological changes of ASFV-infected Bama minipigs were consistent with characteristic lesions of ASFV-infected domestic pigs and wild boars. All pigs died 5 to 14 days postinfection (dpi) through intramuscular injection. Viral genomic DNA from nasal, oral, and rectal swab samples was first detectable at 2 to 4 dpi. The common differentially expressed genes were clustered in the immune-related, metabolic, and inflammatory response pathways from the spleen and inguinal lymph node samples comparing infected to mock. In summary, these results demonstrated that the Bama minipig was an appropriate model for ASFV infection in small ABSL-3 laboratories that can accelerate the research of vaccines and antiviral drugs and uncover pathogenic mechanisms of ASFV infection. IMPORTANCE African swine fever virus (ASFV) can only infect pigs rather than other animals. However, the domestic pigs cannot be kept in small ABSL-3 laboratories for a long time due to the characteristics of rapid growth and large size, which hinder ASFV research, including research of vaccines, antiviral drugs, and mechanisms. In contrast, Bama minipigs have unique advantages consisting of low growth and small size. In the research, Bama minipigs were used to evaluate the characteristics of ASFV infection in small ABSL-3 laboratories. The pathological changes, viral shedding, and gene regulation were consistent with those of domestic pigs infected with ASFV. Therefore, Bama minipigs can be a suitable model for ASFV infection in small ABSL-3 laboratories.

Complete Structural Predictions of the Proteome of African Swine Fever Virus Strain Georgia 2007

Here, we announce the predicted structures of the 193 proteins encoded by African swine fever virus (ASFV) strain Georgia 2007 (ASFV-G). Previously, only the structures of 16 ASFV proteins were elucidated.

Crystal Structures of Epstein-Barr Virus Bcl-2 Homolog BHRF1 Bound to Bid and Puma BH3 Motif Peptides

Apoptosis is a powerful defense mechanism used by multicellular organisms to counteract viral infection. In response to premature host cell suicide, viruses have evolved numerous countermeasures to ensure cell viability to optimize their replication by encoding proteins homologous in structure and function to cellular pro-survival Bcl-2 proteins. Epstein-Barr virus (EBV), a member of the Gammaherpesviridae, encodes the Bcl-2 homolog BHRF1, a potent inhibitor of Bcl-2-mediated apoptosis. BHRF1 acts by directly targeting Bid and Puma, two proapoptotic proteins of the Bcl-2 family. Here, we determined the crystal structures of BHRF1 bound to peptides spanning the Bcl-2 binding motifs (Bcl-2 homology 3 motif, BH3) of Bid and Puma. BHRF1 engages BH3 peptides using the canonical ligand-binding groove of its Bcl-2 fold and maintains a salt bridge between an Arg residue with a conserved Asp residue in the BH3 motif mimicking the canonical ionic interaction seen in host Bcl-2:BH3 motif complexes. Furthermore, both Bid and Puma utilize a fifth binding pocket in the canonical ligand binding groove of BHRF1 to provide an additional hydrophobic interaction distinct from the interactions previously seen with Bak and Bim. These findings provide a structural basis for EBV-mediated suppression of host cell apoptosis and reveal the flexibility of virus encoded Bcl-2 proteins in mimicking key interactions from the endogenous host signaling pathways.

Proteome Analysis of Swine Macrophages after Infection with Two Genotype II African Swine Fever Isolates of Different Pathogenicity

Since the introduction of a highly pathogenic genotype II isolate of the African swine fever virus (ASFV) into Georgia in 2007, African swine fever (ASF) has gone panzootic. Outbreaks have been reported in Europe, Asia and, more recently, Latin America. Thus, ASFV has become a major threat to the pig industry worldwide, as broadly applicable vaccines are not available. While the majority of ASFV strains show high virulence in domestic pigs and wild boar, variations within the ASFV genome have resulted in the emergence of attenuated strains with low or moderate virulence. However, the molecular basis of the differences in virulence has not yet been discovered. To reveal virulence-associated protein expression patterns, we analysed the proteomes of the natural target cells of ASFV, primary porcine macrophages, after infection with two genotype II ASFV strains displaying high (Armenia 2008) and moderate (Estonia 2014) virulence using quantitative mass spectrometry. Very similar expression patterns were observed for the viral genes, and any differences were limited to the deletions within the Estonia 2014 genome. In addition to the canonical ASFV proteins, twelve novel protein products from recently described transcripts were confirmed in both isolates. Pathway analyses showed that both isolates evoked a similar host proteome response, despite their difference in virulence. However, subtle differences in the manipulation of the proteins involved in the proinflammatory response mediated by the MAPK14/p38 signalling cascade were observed.

African Swine Fever Virus: A Review

African swine fever (ASF) is a viral disease with a high fatality rate in both domestic pigs and wild boars. ASF has greatly challenged pig-raising countries and also negatively impacted regional and national trade of pork products. To date, ASF has spread throughout Africa, Europe, and Asia. The development of safe and effective ASF vaccines is urgently required for the control of ASF outbreaks. The ASF virus (ASFV), the causative agent of ASF, has a large genome and a complex structure. The functions of nearly half of its viral genes still remain to be explored. Knowledge on the structure and function of ASFV proteins, the mechanism underlying ASFV infection and immunity, and the identification of major immunogenicity genes will contribute to the development of an ASF vaccine. In this context, this paper reviews the available knowledge on the structure, replication, protein function, virulence genes, immune evasion, inactivation, vaccines, control, and diagnosis of ASFV.

African Swine Fever Virus pI215L Inhibits Type I Interferon Signaling by Targeting Interferon Regulatory Factor 9 for Autophagic Degradation

African swine fever virus (ASFV) is the etiological agent of a highly lethal hemorrhagic disease in domestic pigs and wild boars that has significant economic consequences for the pig industry. The type I interferon (IFN) signaling pathway is a pivotal component of the innate antiviral response, and ASFV has evolved multiple mechanisms to antagonize this pathway and facilitate infection. Here, we reported a novel function of ASFV pI215L in inhibiting type I IFN signaling. Our results showed that ASFV pI215L inhibited IFN-stimulated response element (ISRE) promoter activity and subsequent transcription of IFN-stimulated genes (ISGs) by triggering interferon regulatory factor 9 (IRF9) degradation. Additionally, we found that catalytically inactive pI215L mutations retained the ability to block type I IFN signaling, indicating that this only known viral E2 ubiquitin-conjugating enzyme mediates IFR9 degradation in a ubiquitin-conjugating activity-independent manner. By coimmunoprecipitation, confocal immunofluorescence, and subcellular fractionation approaches, we demonstrated that pI215L interacted with IRF9 and impaired the formation and nuclear translocation of IFN-stimulated gene factor 3 (ISGF3). Moreover, further mechanism studies supported that pI215L induced IRF9 degradation through the autophagy-lysosome pathway in both pI215L-overexpressed and ASFV-infected cells. These findings reveal a new immune evasion strategy evolved by ASFV in which pI215L acts to degrade host IRF9 via the autophagic pathway, thus inhibiting the type I IFN signaling and counteracting the host innate immune response.

IMPORTANCE African swine fever virus (ASFV) causes a highly contagious and lethal disease in pigs and wild boars that is currently present in many countries, severely affecting the global pig industry. Despite extensive research, effective vaccines and antiviral strategies are still lacking, and many fundamental questions regarding the molecular mechanisms underlying host innate immunity escape remain unclear. In this study, we identified ASFV pI215L, the only known viral E2 ubiquitin-conjugating enzyme, which is involved in antagonizing the type I interferon signaling. Mechanistically, pI215L interacted with interferon regulatory factor 9 for autophagic degradation, and this degradation was independent of its ubiquitin-conjugating activity. These results increase the current knowledge regarding ASFV evasion of innate immunity, which may instruct future research on antiviral strategies and dissection of ASFV pathogenesis.

Structural Details of BH3 Motifs and BH3-Mediated Interactions: an Updated Perspective

Apoptosis is a mechanism of programmed cell death crucial in organism development, maintenance of tissue homeostasis, and several pathogenic processes. The B cell lymphoma 2 (BCL2) protein family lies at the core of the apoptotic process, and the delicate balance between its pro- and anti-apoptotic members ultimately decides the cell fate. BCL2 proteins can bind with each other and several other biological partners through the BCL2 homology domain 3 (BH3), which has been also classified as a possible Short Linear Motif and whose distinctive features remain elusive even after decades of studies. Here, we aim to provide an updated overview of the structural features characterizing BH3s and BH3-mediated interactions (with a focus on human proteins), elaborating on the plasticity of BCL2 proteins and the motif properties. We also discussed the implication of these findings for the discovery of interactors of the BH3-binding groove of BCL2 proteins and the design of mimetics for therapeutic purposes.

Structural Insight into KsBcl-2 Mediated Apoptosis Inhibition by Kaposi Sarcoma Associated Herpes Virus

Numerous large DNA viruses have evolved sophisticated countermeasures to hijack the premature programmed cell death of host cells post-infection, including the expression of proteins homologous in sequence, structure, or function to cellular Bcl-2 proteins. Kaposi sarcoma herpes virus (KSHV), a member of the gammaherpesvirinae, has been shown to encode for KsBcl-2, a potent inhibitor of Bcl-2 mediated apoptosis. KsBcl-2 acts by directly engaging host pro-apoptotic Bcl-2 proteins including Bak, Bax and Bok, the BH3-only proteins; Bim, Bid, Bik, Hrk, Noxa and Puma. Here we determined the crystal structures of KsBcl-2 bound to the BH3 motif of pro-apoptotic proteins Bid and Puma. The structures reveal that KsBcl-2 engages pro-apoptotic BH3 motif peptides using the canonical ligand binding groove. Thus, the presence of the readily identifiable conserved BH1 motif sequence “NWGR” of KsBcl-2, as well as highly conserved Arg residue (R86) forms an ionic interaction with the conserved Asp in the BH3 motif in a manner that mimics the canonical ionic interaction seen in host Bcl-2:BH3 motif complexes. These findings provide a structural basis for KSHV mediated inhibition of host cell apoptosis and reveal the flexibility of virus encoded Bcl-2 proteins to mimic key interactions from endogenous host signalling pathways.

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

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

Immune Escape Mechanism and Vaccine Research Progress of African Swine Fever Virus

African swine fever virus (ASFV) is the causative agent of the epidemic of African swine fever (ASF), with virulent strains having a mortality rate of up to 100% and presenting devastating impacts on animal farming. Since ASF was first reported in China in 2018, ASFV still exists and poses a potential threat to the current pig industry. Low-virulence and genotype I strains of ASFV have been reported in China, and the prevention and control of ASF is more complicated. Insufficient understanding of the interaction of ASFV with the host immune system hinders vaccine development. Physical barriers, nonspecific immune response and acquired immunity are the three barriers of the host against infection. To escape the innate immune response, ASFV invades monocytes/macrophages and dendritic cells, thereby inhibiting IFN expression, regulating cytokine expression and the body’s inflammatory response process. Meanwhile, in order to evade the adaptive immune response, ASFV inhibits antigen presentation, induces the production of non-neutralizing antibodies, and inhibits apoptosis. Recently, significant advances have been achieved in vaccine development around the world. Live attenuated vaccines (LAVs) based on artificially deleting specific virulence genes can achieve 100% homologous protection and partial heterologous protection. The key of subunit vaccines is identifying the combination of antigens that can effectively provide protection and selecting carriers that can effectively deliver the antigens. In this review, we introduce the epidemic trend of ASF and the impact on the pig industry, analyze the interaction mechanism between ASFV and the body’s immune system, and compare the current status of potential vaccines in order to provide a reference for the development of effective ASF vaccines.

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

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

African Swine Fever Virus pE199L Induces Mitochondrial-Dependent Apoptosis

African swine fever (ASF) is a severe hemorrhagic disease in swine characterized by massive lymphocyte depletion and cell death, with apoptosis and necrosis in infected lymphoid tissues. However, the molecular mechanism regarding ASFV-induced cell death remains largely unknown. In this study, 94 ASFV-encoded proteins were screened to determine the viral proteins involved in cell death in vitro, and pE199L showed the most significant effect. Ectopic expression of pE199L in porcine cells (CRL-2843) and human cells (HEK293T and HeLa cells) induced cell death remarkably, showing obvious shrinking, blistering, apoptotic bodies, and nuclear DNA fragments. Meanwhile, cell death was markedly alleviated when the expression of pE199L was knocked down during ASFV infection. Additionally, the expression of pE199L caused a loss of mitochondrial membrane potential, release of cytochrome C, and caspase-9 and -3/7 activation, indicating that the mitochondrial apoptotic pathway was involved in pE199L-induced apoptosis. Further investigations showed that pE199L interacted with several anti-apoptotic BCL-2 subfamily members (such as BCL-XL, MCL-1, BCL-W, and BCL-2A1) and competed with BAK for BCL-XL, which promoted BAK and BAX activation. Taken together, ASFV pE199L induces the mitochondrial-dependent apoptosis, which may provide clues for a comprehensive understanding of ASFV pathogenesis.

Structures and Functional Diversities of ASFV Proteins

African swine fever virus (ASFV), the causative pathogen of the recent ASF epidemic, is a highly contagious double-stranded DNA virus. Its genome is in the range of 170~193 kbp and encodes 68 structural proteins and over 100 non-structural proteins. Its high pathogenicity strains cause nearly 100% mortality in swine. Consisting of four layers of protein shells and an inner genome, its structure is obviously more complicated than many other viruses, and its multi-layered structures play different kinds of roles in ASFV replication and survival. Each layer possesses many proteins, but very few of the proteins have been investigated at a structural level. Here, we concluded all the ASFV proteins whose structures were unveiled, and explained their functions from the view of structures. Those structures include ASFV AP endonuclease, dUTPases (E165R), pS273R protease, core shell proteins p15 and p35, non-structural proteins pA151R, pNP868R (RNA guanylyltransferase), major capsid protein p72 (gene B646L), Bcl-2-like protein A179L, histone-like protein pA104R, sulfhydryl oxidase pB119L, polymerase X and ligase. These novel structural features, diverse functions, and complex molecular mechanisms promote ASFV to escape the host immune system easily and make this large virus difficult to control.

Designing BH3-Mimetic Peptide Inhibitors for the Viral Bcl-2 Homologues A179L and BHRF1: Importance of Long-Range Electrostatic Interactions

Viruses have evolved strategies to prevent apoptosis of infected cells at early stages of infection. The viral proteins (vBcl-2s) from specific viral genes adopt a helical fold that is structurally similar to that of mammalian antiapoptotic Bcl-2 proteins and exhibit little sequence similarity. Hence, vBcl-2 homologues are attractive targets to prevent viral infection. However, very few studies have focused on developing inhibitors for vBcl-2 homologues. In this study, we have considered two vBcl-2 homologues, A179L from African swine fever virus and BHRF1 from Epstein-Barr virus. We generated two sets of 8000 randomized BH3-like sequences from eight wild-type proapoptotic BH3 peptides. During this process, the four conserved hydrophobic residues and an Asp residue were retained at their respective positions, and all other positions were substituted randomly without any bias. We constructed 8000 structures each for A179L and BHRF1 in complex with BH3-like sequences. Histograms of interaction energies calculated between the peptide and the protein resulted in negatively skewed distributions. The BH3-like peptides with high helical propensities selected from the negative tail of the respective interaction energy distributions exhibited more favorable interactions with A179L and BHRF1, and they are rich in basic residues. Molecular dynamics studies and electrostatic potential maps further revealed that both acidic and basic residues favorably interact with A179L, while only basic residues have the most favorable interactions with BHRF1. As in mammalian homologues, the role of long-range interactions and nonhotspot residues has to be taken into account while designing specific BH3-mimetic inhibitors for vBcl-2 homologues.

Recent advances in cell homeostasis by African swine fever virus-host interactions

African swine fever (ASF) is an acute hemorrhagic disease caused by the infection of domestic swine and wild boar by the African swine fever virus (ASFV), with a mortality rate close to 90-100%. ASFV has been spreading in the world and poses a severe economic threat to the swine industry. There is no high effective vaccine commercially available or drug for this disease. However, attenuated ASFV isolates may infect pigs by chronic infection, and the infected pigs will not be lethal, which may indicate that pigs can produce protective immunity to resistant ASFV. Immunity acquisition and virus clearances are the central pillars to maintain the host normal cell activities and animal survival dependent on virus-host interactions, which has offered insights into the biology of ASFV. This review is organized around general themes including native immunity, endoplasmic reticulum stress, cell apoptosis, ubiquitination, autophagy regarding the intricate relationship between ASFV protein-host. Elucidating the multifunctional role of ASFV proteins in virus-host interactions can provide more new insights on the initial virus sensing, clearance, and cell homeostasis, and contribute to understanding viral pathogenesis and developing novel antiviral therapeutics.

Regulation and Evasion of Host Immune Response by African Swine Fever Virus

African swine fever (ASF) is an acute lethal hemorrhagic viral disease in domestic pigs and wild boars; is widely epidemic in Africa, Europe, Asia, and Latin America; and poses a huge threat to the pig industry worldwide. ASF is caused by the infection of the ASF virus (ASFV), a cytoplasmic double-stranded DNA virus belonging to the Asfarviridae family. Here, we review how the virus regulates the host immune response and its mechanisms at different levels, including interferon modulation, inflammation, apoptosis, antigen presentation, and cellular immunity.

Structure of African Swine Fever Virus and Associated Molecular Mechanisms Underlying Infection and Immunosuppression: A Review

African swine fever (ASF) is an acute, highly contagious, and deadly infectious disease. The mortality rate of the most acute and acute ASF infection is almost 100%. The World Organization for Animal Health [Office International des épizooties (OIE)] lists it as a legally reported animal disease and China lists it as class I animal epidemic. Since the first diagnosed ASF case in China on August 3, 2018, it has caused huge economic losses to animal husbandry. ASF is caused by the African swine fever virus (ASFV), which is the only member of Asfarviridae family. ASFV is and the only insect-borne DNA virus belonging to the Nucleocytoplasmic Large DNA Viruses (NCLDV) family with an icosahedral structure and an envelope. Till date, there are still no effective vaccines or antiviral drugs for the prevention or treatment of ASF. The complex viral genome and its sophisticated ability to regulate the host immune response may be the reason for the difficulty in developing an effective vaccine. This review summarizes the recent findings on ASFV structure, the molecular mechanism of ASFV infection and immunosuppression, and ASFV-encoded proteins to provide comprehensive proteomic information for basic research on ASFV. In addition, it also analyzes the results of previous studies and speculations on the molecular mechanism of ASFV infection, which aids the study of the mechanism of clinical pathological phenomena, and provides a possible direction for an intensive study of ASFV infection mechanism. By summarizing the findings on molecular mechanism of ASFV- regulated host cell immune response, this review provides orientations and ideas for fundamental research on ASFV and provides a theoretical basis for the development of protective vaccines against ASFV.

African Swine Fever Virus Induces STAT1 and STAT2 Degradation to Counteract IFN-I Signaling

African swine fever virus (ASFV) causes a serious disease in domestic pigs and wild boars and is currently expanding worldwide. No safe and efficacious vaccines against ASFV are available, which threats the swine industry worldwide. African swine fever virus (ASFV) is a complex dsDNA virus that displays multiple mechanisms to counteract the host innate immune response, whose efficacy might determine the different degrees of virulence displayed by attenuated and virulent ASFV strains. Here we report that infection with both virulent Arm/07/CBM/c2 and attenuated NH/P68 strains prevents interferon-stimulated gene (ISG) expression in interferon (IFN)-treated cells by counteracting the JAK/STAT pathway. This inhibition results in an impaired nuclear translocation of the interferon-stimulated gene factor 3 (ISGF3) complex, as well as in the proteasome-dependent STAT2 degradation and caspase 3-dependent STAT1 cleavage. The existence of two independent mechanisms of control of the JAK/STAT pathway, suggests the importance of preventing this pathway for successful viral replication. As ASFV virulence is likely associated with the efficacy of the IFN signaling inhibitory mechanisms, a better understanding of these IFN antagonistic properties may lead to new strategies to control this devastating pig disease.

Autophagy impairment by African swine fever virus

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

Structural Investigation of Orf Virus Bcl-2 Homolog ORFV125 Interactions with BH3-Motifs from BH3-Only Proteins Puma and Hrk

Numerous viruses have evolved sophisticated countermeasures to hijack the early programmed cell death of host cells in response to infection, including the use of proteins homologous in sequence or structure to Bcl-2. Orf virus, a member of the parapoxviridae, encodes for the Bcl-2 homolog ORFV125, a potent inhibitor of Bcl-2-mediated apoptosis in the host. ORFV125 acts by directly engaging host proapoptotic Bcl-2 proteins including Bak and Bax as well as the BH3-only proteins Hrk and Puma. Here, we determined the crystal structures of ORFV125 bound to the BH3 motif of proapoptotic proteins Puma and Hrk. The structures reveal that ORFV125 engages proapoptotic BH3 motif peptides using the canonical ligand binding groove. An Arg located in the structurally equivalent BH1 region of ORFV125 forms an ionic interaction with the conserved Asp in the BH3 motif in a manner that mimics the canonical ionic interaction seen in host Bcl-2:BH3 motif complexes. These findings provide a structural basis for Orf virus-mediated inhibition of host cell apoptosis and reveal the flexibility of virus encoded Bcl-2 proteins to mimic key interactions from endogenous host signalling pathways.

Regulation of innate immune responses by cell death-associated caspases during virus infection

Viruses are obligate intracellular pathogens that rely on cellular machinery for successful replication and dissemination. The host cells encode a number of different strategies to sense and restrict the invading viral pathogens. Caspase-mediated programmed cell death pathways that are triggered by virus infection, such as apoptosis and pyroptosis, provide a means for the infected cells to limit viral proliferation, leading to suicidal cell death (apoptosis) or lytic cell death and alerting uninfected cells to mount anti-viral responses (pyroptosis). However, some viruses can employ activated caspases to dampen the anti-viral responses and facilitate viral replication through cleavage of critical molecules of the innate immune pathways. The regulation of innate immune responses by caspase activation during virus infection has recently become an important topic. In this review, we briefly introduce the characteristics of different classes of caspases and the cell death pathways regulated by these caspases. We then describe how viruses trigger or dampen caspase activation during infection and how these activated caspases regulate three major innate immune response pathways of viral infections: the retinoic acid-inducible gene I-like receptor, toll-like receptor and cyclic GMP-AMP synthase-stimulator of interferon genes pathways.

Crystal structures of ORFV125 provide insight into orf virus-mediated inhibition of apoptosis

Premature apoptosis of cells is a strategy utilized by multicellular organisms to counter microbial threats. Orf virus (ORFV) is a large double-stranded DNA virus belonging to the poxviridae. ORFV encodes for an apoptosis inhibitory protein ORFV125 homologous to B-cell lymphoma 2 or Bcl-2 family proteins, which has been shown to inhibit host cell encoded pro-apoptotic Bcl-2 proteins. However, the structural basis of apoptosis inhibition by ORFV125 remains to be clarified. We show that ORFV125 is able to bind to a range of peptides spanning the BH3 motif of human pro-apoptotic Bcl-2 proteins including Bax, Bak, Puma and Hrk with modest to weak affinity. We then determined the crystal structures of ORFV125 alone as well as bound to the highest affinity ligand Bax BH3 motif. ORFV125 adopts a globular Bcl-2 fold comprising 7 α-helices, and utilizes the canonical Bcl-2 binding groove to engage pro-apoptotic host cell Bcl-2 proteins. In contrast with a previously predicted structure, ORFV125 adopts a domain-swapped dimeric topology, where the α1 helix from one protomer is swapped into a neighbouring unit. Furthermore, ORFV125 differs from the conserved architecture of the Bcl-2 binding groove and instead of α3 helix forming one of the binding groove walls, ORFV125 utilizes an extended α2 helix that comprises the equivalent region of helix α3. This results in a subtle variation of previously observed dimeric Bcl-2 architectures in other poxvirus and human encoded Bcl-2 proteins. Overall, our results provide a structural and mechanistic basis for orf virus-mediated inhibition of host cell apoptosis.

A novel function of African Swine Fever Virus pE66L in inhibition of host translation by the PKR/eIF2α pathway

African swine fever virus (ASFV) is one of the most contagious and lethal viruses infecting pigs. This virus is endemic in many countries and has very recently spread to China, but no licensed vaccines or treatments are currently available. Despite extensive research, the basic question of how ASFV-encoded proteins inhibit host translation remains. Here, we examined how ASFV interfered with host translation and optimized viral gene expression. We found that 14 ASFV proteins inhibited Renilla luciferase (Rluc) activity greater than 5-fold, and the protein with the strongest inhibitory effect was pE66L, which was not previously reported. Combined with bioinformatical analysis and biochemical experiment, we determined that the transmembrane (TM) domain (amino acids 13-34) of pE66L was required for the inhibition of host gene expression. Notably, we constructed a recombinant plasmid with the TM domain linked to enhanced green fluorescent protein (EGFP) and further demonstrated that this domain broadly inhibited protein synthesis. Confocal and biochemical analyses indicated that the TM domain might help proteins locate to the endoplasmic reticulum (ER) to suppress translation though the PKR/eIF2α pathway. Deletion of the E66L gene had little effect on virus replication in macrophages, but significantly recovered host gene expression. Taken together, our findings complement studies on the host translation of ASFV proteins and suggest that ASFV pE66L induces host translation shutoff, which is dependent on activation of the PKR/eIF2α pathway.Importance African swine fever virus (ASFV) is a member of the nucleocytoplasmic large DNA virus superfamily that predominantly replicates in the cytoplasm of infected cells. The ASFV double-stranded DNA genome varies in length from approximately 170 to 193 kbp depending on the isolate and contains between 150 and 167 open reading frames (ORFs), of which half the encoded proteins have not been explored. Our study showed that 14 proteins had an obvious inhibitory effect on Renilla luciferase (Rluc) gene synthesis, with pE66L showing the most significant effect. Furthermore, the transmembrane (TM) domain of pE66L broadly inhibited host protein synthesis in a PKR/eIF2a pathway-dependent manner. Loss of pE66L during ASFV infection had little effect on virus replication, but significantly recovered host protein synthetic. Based on the above results, our findings expand our view of ASFV in determining the fate of host-pathogen interactions.

The Main DNA Viruses Significantly Affecting Pig Livestock

Swine DNA viruses have developed unique mechanisms for evasion of the host immune system, infection and DNA replication, and finally, construction and release of new viral particles. This article reviews four classes of DNA viruses affecting swine: porcine circoviruses, African swine fever virus, porcine parvoviruses, and pseudorabies virus. Porcine circoviruses belonging to the Circoviridae family are small single-stranded DNA viruses causing different diseases in swine including poly-weaning multisystemic wasting syndrome, porcine dermatitis and nephropathy syndrome, and porcine respiratory disease complex. African swine fever virus, the only member of the Asfivirus genus in the Asfarviridae family, is a large double-stranded DNA virus and for its propensity to cause high mortality, it is currently considered the most dangerous virus in the pig industry. Porcine parvoviruses are small single-stranded DNA viruses belonging to the Parvoviridae family that cause reproductive failure in pregnant gilts. Pseudorabies virus, or suid herpesvirus 1, is a large double-stranded DNA virus belonging to the Herpesviridae family and Alphaherpesvirinae subfamily. Recent findings including general as well as genetic classification, virus structure, clinical syndromes and the host immune system responses and vaccine protection are described for all four swine DNA virus classes.

Poxviral Strategies to Overcome Host Cell Apoptosis

Apoptosis is a form of cellular suicide initiated either via extracellular (extrinsic apoptosis) or intracellular (intrinsic apoptosis) cues. This form of programmed cell death plays a crucial role in development and tissue homeostasis in multicellular organisms and its dysregulation is an underlying cause for many diseases. Intrinsic apoptosis is regulated by members of the evolutionarily conserved B-cell lymphoma-2 (Bcl-2) family, a family that consists of pro- and anti-apoptotic members. Bcl-2 genes have also been assimilated by numerous viruses including pox viruses, in particular the sub-family of chordopoxviridae, a group of viruses known to infect almost all vertebrates. The viral Bcl-2 proteins are virulence factors and aid the evasion of host immune defenses by mimicking the activity of their cellular counterparts. Viral Bcl-2 genes have proved essential for the survival of virus infected cells and structural studies have shown that though they often share very little sequence identity with their cellular counterparts, they have near-identical 3D structures. However, their mechanisms of action are varied. In this review, we examine the structural biology, molecular interactions, and detailed mechanism of action of poxvirus encoded apoptosis inhibitors and how they impact on host–virus interactions to ultimately enable successful infection and propagation of viral infections.

The structural basis of Bcl-2 mediated cell death regulation in hydra

Apoptosis is regulated by evolutionarily conserved signaling pathways to remove damaged, diseased or unwanted cells. Proteins homologous to the B-cell lymphoma 2 (Bcl-2) family of proteins, the primary arbiters of mitochondrially mediated apoptosis, are encoded by the cnidarian Hydra vulgaris. We mapped interactions between pro-survival and pro-apoptotic Bcl-2 proteins of H. vulgaris by affinity measurements between Hy-Bcl-2-4, the sole confirmed pro-survival Bcl-2 protein, with BH3 motif peptides of two Bcl-2 proteins from hydra that displayed pro-apoptotic activity, Hy-Bak1 and Hy-BH3-only-2, and the BH3 motif peptide of the predicted pro-apoptotic protein Hy-Bax. In addition to peptides from hydra encoded pro-apoptotic proteins, Hy-Bcl-2-4 also engaged BH3 motif peptides from multiple human pro-apoptotic Bcl-2 proteins. Reciprocally, human pro-survival Bcl-2 proteins Bcl-2, Bcl-xL, Bcl-w, Mcl-1 and A1/Bfl-1 bound to BH3 spanning peptides from hydra encoded pro-apoptotic Hy-Bak1, Hy-BH3-only and Hy-Bax. The molecular details of the interactions were determined from crystal structures of Hy-Bcl-2-4 complexes with BH3 motif peptides of Hy-Bak1 and Hy-Bax. Our findings suggest that the Bcl-2 family in hydra may function in a manner analogous to the Bcl-2 family in humans, and less like the worm Caenorhabditis elegans where evolutionary gene deletion has simplified the apoptotic program. Combined, our results demonstrate the powerful conservation of the interaction pattern between hydra and human Bcl-2 family members. Furthermore, our data reveal mechanistic differences in the mode of binding between hydra and sponges such as Geodia cydonium, with hydra encoded Bcl-2 resembling the more promiscuous pro-apoptotic Bcl-2 members found in mammals compared with its sponge counterpart.

African swine fever: Etiology, epidemiological status in Korea, and perspective on control

African swine fever (ASF), caused by the ASF virus, a member of the Asfarviridae family, is one of the most important diseases in the swine industry due to its clinical and economic impacts. Since the first report of ASF a century ago, ample information has become available, but prevention and treatment measures are still inadequate. Two waves of epizootic outbreaks have occurred worldwide. While the first wave of the epizootic outbreak was controlled in most of the infected areas, the second wave is currently active in the European and Asian continents, causing severe economic losses to the pig industry. There are different patterns of spreading in the outbreaks between those in European and Asian countries. Prevention and control of ASF are very difficult due to the lack of available vaccines and effective therapeutic measures. However, recent outbreaks in South Korea have been successfully controlled on swine farms, although feral pigs are periodically being found to be positive for the ASF virus. Therefore, we would like to share our story regarding the preparation and application of control measures. The success in controlling ASF on farms in South Korea is largely due to the awareness and education of swine farmers and practitioners, the early detection of infected animals, the implementation of strict control policies by the government, and widespread sharing of information among stakeholders. Based on the experience gained from the outbreaks in South Korea, this review describes the current understanding of the ASF virus and its pathogenic mechanisms, epidemiology, and control.

Current State of Global African Swine Fever Vaccine Development under the Prevalence and Transmission of ASF in China

African swine fever (ASF) is a highly lethal contagious disease of swine caused by African swine fever virus (ASFV). At present, it is listed as a notifiable disease reported to the World Organization for Animal Health (OIE) and a class one animal disease ruled by Chinese government. ASF has brought significant economic losses to the pig industry since its outbreak in China in August 2018. In this review, we recapitulated the epidemic situation of ASF in China as of July 2020 and analyzed the influencing factors during its transmission. Since the situation facing the prevention, control, and eradication of ASF in China is not optimistic, safe and effective vaccines are urgently needed. In light of the continuous development of ASF vaccines in the world, the current scenarios and evolving trends of ASF vaccines are emphatically analyzed in the latter part of the review. The latest research outcomes showed that attempts on ASF gene-deleted vaccines and virus-vectored vaccines have proven to provide complete homologous protection with promising efficacy. Moreover, gaps and future research directions of ASF vaccine are also discussed.

Structural insight into tanapoxvirus-mediated inhibition of apoptosis

Premature programmed cell death or apoptosis of cells is a strategy utilized by multicellular organisms to counter microbial threats. Tanapoxvirus (TANV) is a large double-stranded DNA virus belonging to the poxviridae that causes mild monkeypox-like infections in humans and primates. TANV encodes for a putative apoptosis inhibitory protein 16L. We show that TANV16L is able to bind to a range of peptides spanning the BH3 motif of human proapoptotic Bcl-2 proteins and is able to counter growth arrest of yeast induced by human Bak and Bax. We then determined the crystal structures of TANV16L bound to three identified interactors, Bax, Bim and Puma BH3. TANV16L adopts a globular Bcl-2 fold comprising 7 α-helices and utilizes the canonical Bcl-2 binding groove to engage proapoptotic host cell Bcl-2 proteins. Unexpectedly, TANV16L is able to adopt both a monomeric and a domain-swapped dimeric topology where the α1 helix from one protomer is swapped into a neighbouring unit. Despite adopting two different oligomeric forms, the canonical ligand binding groove in TANV16L remains unchanged from monomer to domain-swapped dimer. Our results provide a structural and mechanistic basis for tanapoxvirus-mediated inhibition of host cell apoptosis and reveal the capacity of Bcl-2 proteins to adopt differential oligomeric states whilst maintaining the canonical ligand binding groove in an unchanged state. DATABASE: Structural data are available in the Protein Data Bank (PDB) under the accession numbers 6TPQ, 6TQQ and 6TRR.

Crystal structures of the sheeppox virus encoded inhibitor of apoptosis SPPV14 bound to the proapoptotic BH3 peptides Hrk and Bax

Programmed death of infected cells is used by multicellular organisms to counter viral infections. Sheeppox virus encodes for SPPV14, a potent inhibitor of Bcl-2-mediated apoptosis. We reveal the structural basis of apoptosis inhibition by determining crystal structures of SPPV14 bound to BH3 motifs of proapoptotic Bax and Hrk. The structures show that SPPV14 engages BH3 peptides using the canonical ligand-binding groove. Unexpectedly, Arg84 from SPPV14 forms an ionic interaction with the conserved Asp in the BH3 motif in a manner that replaces the canonical ionic interaction seen in almost all host Bcl-2:BH3 motif complexes. These results reveal the flexibility of virus-encoded Bcl-2 proteins to mimic key interactions from endogenous host signalling pathways to retain BH3 binding and prosurvival functionality.

Classical swine fever virus Npro antagonises IRF3 to prevent IFN-independent TLR3 and RIG-I-mediated apoptosis

Classical swine fever virus (CSFV) is the causative agent of classical swine fever, a notifiable disease of economic importance that causes severe leukopenia, fever and haemorrhagic disease in domesticated pigs and wild boar across the globe. CSFV has been shown to antagonise the induction of type I IFN, partly through a function of its N-terminal protease (N^pro) which binds IRF3 and targets it for proteasomal degradation. Additionally, N^pro has been shown to antagonise apoptosis triggered by the dsRNA-homolog poly(I:C), however the exact mechanism by which this is achieved has not been fully elucidated. In this study we confirm the ability of N^pro to inhibit dsRNA-mediated apoptosis and show that N^pro is also able to antagonise Sendai virus-mediated apoptosis in PK-15 cells. Gene edited PK-15 cell lines were used to show the dsRNA-sensing pathogen recognition receptors (PRRs) TLR3 and RIG-I specifically respond to poly(I:C) and SeV respectively, subsequently triggering apoptosis through pathways that converge on IRF3 and culminate in the cleavage of caspase-3. Importantly, this IRF3-mediated apoptosis was found to be dependent on transcription-independent functions of IRF3 and also on Bax, a pro-apoptotic Bcl-2 family protein, through a direct interaction between the two proteins. Deletion of IRF3, stable expression of N^pro and infection with wild-type CSFV were found to antagonise the mitochondrial localisation of Bax, a key hallmark of the intrinsic, mitochondrial pathway of apoptosis. Together, these findings show that N^pro's putative interaction with IRF3 is involved not only in its antagonism of type I IFN, but also dsRNA-mediated mitochondrial apoptosis.Importance Responsible for severe haemorrhagic disease in domestic pigs and wild boar, classical swine fever is recognised by the World Organisation for Animal Health (OIE) and European Union as a notifiable disease of economic importance. Persistent infection, immunotolerance and early dissemination of the virus at local sites of infection have been linked to the antagonism of type I IFN induction by N^pro This protein may further contribute to these phenomena by antagonising the induction of dsRNA-mediated apoptosis. Ultimately, apoptosis is an important innate mechanism by which cells counter viruses at local sites of infection, thus preventing wider spread and dissemination within the host, potentially also contributing to the onset of persistence. Elucidation of the mechanism by which Npro antagonises the apoptotic response will help inform the development of rationally-designed live-attenuated vaccines and antivirals for control of outbreaks in typically CSFV-free countries.

A unique DNA-binding mode of African swine fever virus AP endonuclease

African swine fever virus (ASFV) is highly contagious and can cause lethal disease in pigs. ASFV is primarily replicated in the cytoplasm of pig macrophages, which is oxidative and caused constant damage to ASFV genome. ASFV AP endonuclease (AsfvAP) catalyzes DNA cleavage reaction at the abasic site and is a key enzyme of ASFV base excision repair (BER) system. Although it plays an essential role in ASFV survival in host cells, the basis underlying substrate binding and cleavage by AsfvAP remains unclear. Here, we reported the structural and functional studies of AsfvAP, showing that AsfvAP adopts a novel DNA-binding mode distinct from other APs. AsfvAP possesses many unique structural features, including one narrower nucleotide-binding pocket at the active site, the C16-C20 disulfide bond-containing region, and histidine-rich loop. As indicated by our mutagenesis, in vitro binding and cleavage assays, these features are important for AsfvAP to suit the acidic and oxidative environment. Owing to their functional importance, these unique features could serve as targets for designing small molecule inhibitors that could disrupt the repair process of ASFV genome and help fight against this deadly virus in the future.

The Bcl-2 Family: Ancient Origins, Conserved Structures, and Divergent Mechanisms

Intrinsic apoptosis, the response to intracellular cell death stimuli, is regulated by the interplay of the B-cell lymphoma 2 (Bcl-2) family and their membrane interactions. Bcl-2 proteins mediate a number of processes including development, homeostasis, autophagy, and innate and adaptive immune responses and their dysregulation underpins a host of diseases including cancer. The Bcl-2 family is characterized by the presence of conserved sequence motifs called Bcl-2 homology motifs, as well as a transmembrane region, which form the interaction sites and intracellular location mechanism, respectively. Bcl-2 proteins have been recognized in the earliest metazoans including Porifera (sponges), Placozoans, and Cnidarians (e.g., Hydra). A number of viruses have gained Bcl-2 homologs and subvert innate immunity and cellular apoptosis for their replication, but they frequently have very different sequences to their host Bcl-2 analogs. Though most mechanisms of apoptosis initiation converge on activation of caspases that destroy the cell from within, the numerous gene insertions, deletions, and duplications during evolution have led to a divergence in mechanisms of intrinsic apoptosis. Currently, the action of the Bcl-2 family is best understood in vertebrates and nematodes but new insights are emerging from evolutionarily earlier organisms. This review focuses on the mechanisms underpinning the activity of Bcl-2 proteins including their structures and interactions, and how they have changed over the course of evolution.

[African swine fever]

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

Current status and evolving approaches to African swine fever vaccine development

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

Crystal Structure of African Swine Fever Virus A179L with the Autophagy Regulator Beclin

Subversion of programmed cell death-based host defence systems is a prominent feature of infections by large DNA viruses. African swine fever virus (ASFV) is a large DNA virus and sole member of the Asfarviridae family that harbours the B-cell lymphoma 2 or Bcl-2 homolog A179L. A179L has been shown to bind to a range of cell death-inducing host proteins, including pro-apoptotic Bcl-2 proteins as well as the autophagy regulator Beclin. Here we report the crystal structure of A179L bound to the Beclin BH3 motif. A179L engages Beclin using the same canonical ligand-binding groove that is utilized to bind to pro-apoptotic Bcl-2 proteins. The mode of binding of Beclin to A179L mirrors that of Beclin binding to human Bcl-2 and Bcl-x_L as well as murine γ-herpesvirus 68. The introduction of bulky hydrophobic residues into the A179L ligand-binding groove via site-directed mutagenesis ablates binding of Beclin to A179L, leading to a loss of the ability of A179L to modulate autophagosome formation in Vero cells during starvation. Our findings provide a mechanistic understanding for the potent autophagy inhibitory activity of A179L and serve as a platform for more detailed investigations into the role of autophagy during ASFV infection.

African swine fever virus evasion of host defences

African swine fever virus causes a haemorrhagic fever in domestic pigs and wild boar. The continuing spread in Africa, Europe and Asia threatens the global pig industry. The lack of a vaccine limits disease control. To underpin rational strategies for vaccine development improved knowledge is needed of how the virus interacts with and modulates the host's responses to infection. The virus long double-stranded DNA genome codes for more than 160 proteins of which many are non-essential for replication in cells but can have important roles in evading the host's defences. Here we review knowledge of the pathways targeted by ASFV and the mechanisms by which these are inhibited. The impact of deleting single or multiple ASFV genes on virus replication in cells and infection in pigs is summarised providing information on strategies for rational development of modified live vaccines.

The Genetics of Life and Death: Virus-Host Interactions Underpinning Resistance to African Swine Fever, a Viral Hemorrhagic Disease

Pathogen transmission from wildlife hosts to genetically distinct species is a major driver of disease emergence. African swine fever virus (ASFV) persists in sub-Saharan Africa through a sylvatic cycle between warthogs and soft ticks that infest their burrows. The virus does not cause disease in these animals, however transmission of the virus to domestic pigs or wild boar causes a hemorrhagic fever that is invariably fatal. ASFV transmits readily between domestic pigs and causes economic hardship in areas where it is endemic. The virus is also a significant transboundary pathogen that has become established in Eastern Europe, and has recently appeared in China increasing the risk of an introduction of the disease to other pig producing centers. Although a DNA genome mitigates against rapid adaptation of the virus to new hosts, extended epidemics of African swine fever (ASF) can lead to the emergence of viruses with reduced virulence. Attenuation in the field leads to large deletions of genetic material encoding genes involved in modulating host immune responses. Therefore resistance to disease and tolerance of ASFV replication can be dependent on both virus and host factors. Here we describe the different virus-host interfaces and discuss progress toward understanding the genetic determinants of disease outcome after infection with ASFV.

Inhibition of cGAS-STING-TBK1 signaling pathway by DP96R of ASFV China 2018/1

African swine fever virus (ASFV) is a highly pathogenic large DNA virus that causes African swine fever (ASF) in domestic pigs and European wild boars with mortality rate up to 100%. The DP96R gene of ASFV encodes one of the viral virulence factors, yet its action mechanism remains unknown. In this study, we report that DP96R of ASFV China 2018/1 strain subverts type I IFN production in cGAS sensing pathway. DP96R inhibited the cGAS/STING, and TBK1 but not IRF3-5D mediated IFN-β and ISRE promoters activation. Furthermore, DP96R selectively blocked the activation of NF-κB promoter induced by cGAS/STING, TBK1, and IKKβ, but not by overexpression of p65. Moreover, DP96R inhibited phosphorylation of TBK1 stimulated by cGAS/STING activation, and TBK1-induced antiviral response. Finally, truncated mutation analysis demonstrated that the region spanning amino acids 30 to 96 of DP96R was responsible for the inhibitory activity. To our knowledge, this is for the first time that DP96R of ASFV China 2018/1 is reported to negatively regulate type I IFN expression and NF-κB signaling by inhibiting both TBK1 and IKKβ, which plays an important role in virus immune evasion.

A structural investigation of NRZ mediated apoptosis regulation in zebrafish

Bcl-2 family proteins play a crucial role in regulating apoptosis, a process critical for development, eliminating damaged or infected cells, host-pathogen interactions and in disease. Dysregulation of Bcl-2 proteins elicits an expansive cell survival mechanism promoting cell migration, invasion and metastasis. Through a network of intra-family protein-protein interactions Bcl-2 family members regulate the release of cell death factors from mitochondria. NRZ is a novel zebrafish pro-survival Bcl-2 orthologue resident on mitochondria and the endoplasmic reticulum (ER). However, the mechanism of NRZ apoptosis inhibition has not yet been clarified. Here we examined the interactions of NRZ with pro-apoptotic members of the Bcl-2 family using a combination of isothermal calorimetry and mutational analysis of NRZ. We show that NRZ binds almost all zebrafish pro-apoptotic proteins and displays a broad range of affinities. Furthermore, we define the structural basis for apoptosis inhibition of NRZ by solving the crystal structure of both apo-NRZ and a holo form bound to a peptide spanning the binding motif of the pro-apoptotic zBad, a BH3-only protein orthologous to mammalian Bad. The crystal structure of NRZ revealed that it adopts the conserved Bcl-2 like fold observed for other cellular pro-survival Bcl-2 proteins and employs the canonical ligand binding groove to bind Bad BH3 peptide. NRZ engagement of Bad BH3 involves the canonical ionic interaction between NRZ R86 and Bad D104 and an additional ionic interaction between NRZ D79 and Bad R100, and substitution of either NRZ R86 or D79 to Ala reduces the binding to Bad BH3 tenfold or more. Our findings provide a detailed mechanistic understanding for NRZ mediated anti-apoptotic activity in zebrafish by revealing binding to both Bad and Noxa, suggesting that NRZ is likely to occupy a unique mechanistic role in zebrafish apoptosis regulation by acting as a highly promiscuous pro-apoptotic Bcl-2 binder.