The laminin receptor is a cellular attachment receptor for classical Swine Fever virus

Porcine low-density lipoprotein receptor plays an important role in classical swine fever virus infection

Several cellular factors have been reported to be required for replication of classical swine fever virus (CSFV), a member of the genus Pestivirus within the family Flaviviridae. However, many steps of its replication cycle are still poorly understood. The low-density lipoprotein receptor (LDLR) is involved in cell entry and post-entry processes of different viruses including other members of the Flaviviridae. In this study, the relevance of LDLR in replication of CSFV and another porcine pestivirus, Bungowannah pestivirus (BuPV), was investigated by antibody-mediated blocking of LDLR and genetically engineered porcine cell lines providing altered LDLR expression levels. An LDLR-specific antibody largely blocked infection with CSFV, but had only a minor impact on BuPV. Infections of the genetically modified cells confirmed an LDLR-dependent replication of CSFV. Compared to wild type cells, lower and higher expression of LDLR resulted in a 3.5-fold decrease or increase in viral titers already 20 h post infection. Viral titers were 25-fold increased in LDLR-overexpressing cells compared to cells with reduced LDLR expression at 72 h post infection. The varying LDLR expression levels had no clear effect on permissivity to BuPV. A decoy receptor assay using recombinant soluble LDLR provided no evidence that LDLR may function as a receptor for CSFV or BuPV. Differences in their dependency on LDLR suggest that CSFV and BuPV likely use different mechanisms to interact with their host cells. Moreover, this study reveals similarities in the replication cycles of CSFV and other members of the family Flaviviridae that are dependent on LDLR.

Identification and functional characterization of laminin receptor in the mud crab, Scylla paramamosain, in response to MCDV-1 challenge

Laminin receptor (LR), which mediating cell adhesion to the extracellular matrix, plays a crucial role in cell signaling and regulatory functions. In the present study, a laminin receptor gene (SpLR) was cloned and characterized from the mud crab (Scylla paramamosain). The full length of SpLR contained an open reading frame (ORF) of 960 bp encoding 319 amino acids, a 5' untranslated region (UTR) of 66 bp and a 3' UTR of 49 bp. The predicted protein comprised two Ribosomal-S2 domains and a 40S-SA-C domain. The mRNA of SpLR was highly expressed in the gill, followed by the hepatopancreas. The expression of SpLR was up-regulated after mud crab dicistrovirus-1(MCDV-1) infection. Knocking down SpLR in vivo by RNA interference significantly down-regulated the expression of the immune genes SpJAK, SpSTAT, SpToll1, SpALF1 and SpALF5. This study shown that the expression level of SpToll1 and SpCAM in SpLR-interfered group significantly increased after MCDV-1 infection. Moreover, silencing of SpLR in vivo decreased the MCDV-1 replication and increased the survival rate of mud crabs after MCDV-1 infection. These findings collectively suggest a pivotal role for SpLR in the mud crab's response to MCDV-1 infection. By influencing the expression of critical innate immune factors and impacting viral replication dynamics, SpLR emerges as a key player in the intricate host-pathogen interaction, providing valuable insights into the molecular mechanisms underlying MCDV-1 pathogenesis in mud crabs.

The laminin receptor is a receptor for Micropterus salmoides rhabdovirus

Micropterus salmoides rhabdovirus (MSRV) is an important pathogen of largemouth bass. Despite extensive research, the functional receptors of MSRV remained unknown. This study identified the host protein, laminin receptor (LamR), as a cellular receptor facilitating MSRV entry into host cells. Our results demonstrated that LamR directly interacts with MSRV G protein, playing a pivotal role in the attachment and internalization processes of MSRV. Knockdown of LamR with siRNA, blocking cells with LamR antibody, or incubating MSRV virions with soluble LamR protein significantly reduced MSRV entry. Notably, we found that LamR mediated MSRV entry via clathrin-mediated endocytosis. Additionally, our findings revealed that MSRV G and LamR were internalized into cells and co-localized in the early and late endosomes. These findings highlight the significance of LamR as a cellular receptor facilitating MSRV binding and entry into target cells through interaction with the MSRV G protein.

IMPORTANCE

Despite the serious epidemic caused by Micropterus salmoides rhabdovirus (MSRV) in largemouth bass, the precise mechanism by which it invades host cells remains unclear. Here, we determined that laminin receptor (LamR) is a novel target of MSRV, that interacts with its G protein and is involved in viral attachment and internalization, transporting with MSRV together in early and late endosomes. This is the first report demonstrating that LamR is a cellular receptor in the MSRV life cycle, thus contributing new insights into host-pathogen interactions.

Host Subcellular Organelles: Targets of Viral Manipulation

Viruses have evolved sophisticated mechanisms to manipulate host cell processes and utilize intracellular organelles to facilitate their replication. These complex interactions between viruses and cellular organelles allow them to hijack the cellular machinery and impair homeostasis. Moreover, viral infection alters the cell membrane's structure and composition and induces vesicle formation to facilitate intracellular trafficking of viral components. However, the research focus has predominantly been on the immune response elicited by viruses, often overlooking the significant alterations that viruses induce in cellular organelles. Gaining a deeper understanding of these virus-induced cellular changes is crucial for elucidating the full life cycle of viruses and developing potent antiviral therapies. Exploring virus-induced cellular changes could substantially improve our understanding of viral infection mechanisms.

The Interaction between the DOCK7 Protein and the E2 Protein of Classical Swine Fever Virus Is Not Involved with Viral Replication or Pathogenicity

The classical swine fever virus (CSFV) particle consists of three glycoproteins, all of which have been shown to be important proteins involved in many virus functions, including interaction with several host proteins. One of these proteins, E2, has been shown to be directly involved with adsorption to the host cell and important for virus virulence. Using the yeast two-hybrid system, we have previously shown that CSFV E2 specifically interacts with the (DOCK7) dedicator of cytokinesis, a scaffolding protein. In this report, the interaction between E2 and DOCK7 was evaluated. To confirm the yeast two-hybrid results and to determine that DOCK7 interacts in swine cells with E2, we performed co-immunoprecipitation and proximity ligation assay (PLA). After demonstrating the protein interaction in swine cells, E2 amino acid residues Y65, V283, and T149 were determined to be critical for interaction with Dock7 by using a random mutated library of E2 and a reverse yeast two-hybrid approach. That disruption of these three residues with mutations Y65F, V283D, and T149A abrogated the Dock7-E2 protein interaction. These mutations were then introduced into a recombinant CSFV, E2DOCK7v, by a reverse genomics approach using the highly virulent CSFV Brescia isolate as a backbone. E2DOCKv was shown to have similar growth kinetics in swine primary macrophages and SK6 cell cultures to the parental Brescia strain. Similarly, E2DOCK7v demonstrated a similar level of virulence to the parental Brescia when inoculated in domestic pigs. Animals intranasally inoculated with 105 TCID50 developed a lethal form of clinical disease with virological and hematological kinetics changes indistinguishable from that produced by the parental strain. Therefore, interaction between CSFV E2 and host DOCK7 is not critically involved in the process of virus replication and disease production.

Host cell factors involved in classical swine fever virus entry

Classical swine fever virus (CSFV) is an ancient pathogen that continues to pose a threat to animal agriculture worldwide. The virus belongs to the genus Pestivirus and the family Flaviviridae. It causes a multisystemic disease that affects only pigs and is responsible for significant economic losses. CSFV infection is probably a multistep process that involves the proteins in the virus envelope and more than one receptor in the membrane of permissive cells. To date, the cellular receptors essential for CSFV entry and their detailed functions during this process remains unknown. All the viral envelope proteins Erns, E1 and E2 are involved in the entry process to some extent and the experimental approaches conducted until now have helped to unveil their contributions. This review aims to provide an overview of current knowledge on cellular molecules described to be involved in CSFV entry, including complement regulatory protein 46 (CD46), heparan sulphate (HS), Laminin receptor, Integrin ß3, Annexin II, MERKT and ADAM17. This knowledge would not only help to understand the molecular mechanisms involved in pestivirus infection, but also provide a rational basis for the development of nonvaccinal alternatives for CSFV control.

Channeling the Natural Properties of Sindbis Alphavirus for Targeted Tumor Therapy

Sindbis alphavirus vectors offer a promising platform for cancer therapy, serving as valuable models for alphavirus-based treatment. This review emphasizes key studies that support the targeted delivery of Sindbis vectors to tumor cells, highlighting their effectiveness in expressing tumor-associated antigens and immunomodulating proteins. Among the various alphavirus vectors developed for cancer therapy, Sindbis-vector-based imaging studies have been particularly extensive. Imaging modalities that enable the in vivo localization of Sindbis vectors within lymph nodes and tumors are discussed. The correlation between laminin receptor expression, tumorigenesis, and Sindbis virus infection is examined. Additionally, we present alternative entry receptors for Sindbis and related alphaviruses, such as Semliki Forest virus and Venezuelan equine encephalitis virus. The review also discusses cancer treatments that are based on the alphavirus vector expression of anti-tumor agents, including tumor-associated antigens, cytokines, checkpoint inhibitors, and costimulatory immune molecules.

Attachment, Entry, and Intracellular Trafficking of Classical Swine Fever Virus

Classical swine fever virus (CSFV), which is a positive-sense, single-stranded RNA virus with an envelope, is a member of the Pestivirus genus in the Flaviviridae family. CSFV causes a severe and highly contagious disease in pigs and is prevalent worldwide, threatening the pig farming industry. The detailed mechanisms of the CSFV life cycle have been reported, but are still limited. Some receptors and attachment factors of CSFV, including heparan sulfate (HS), laminin receptor (LamR), complement regulatory protein (CD46), MER tyrosine kinase (MERTK), disintegrin, and metalloproteinase domain-containing protein 17 (ADAM17), were identified. After attachment, CSFV internalizes via clathrin-mediated endocytosis (CME) and/or caveolae/raft-dependent endocytosis (CavME). After internalization, CSFV moves to early and late endosomes before uncoating. During this period, intracellular trafficking of CSFV relies on components of the endosomal sorting complex required for transport (ESCRT) and Rab proteins in the endosome dynamics, with a dependence on the cytoskeleton network. This review summarizes the data on the mechanisms of CSFV attachment, internalization pathways, and intracellular trafficking, and provides a general view of the early events in the CSFV life cycle.

Classical Swine Fever Virus Structural Glycoprotein E2 Interacts with Host Protein ACADM during the Virus Infectious Cycle

The E2 glycoprotein is one of the four structural proteins of the classical swine fever virus (CSFV) particle. E2 has been shown to be involved in many virus functions, including adsorption to host cells, virus virulence and interaction with several host proteins. Using a yeast two-hybrid screen, we have previously shown that the CSFV E2 specifically interacts with swine host protein medium-chain-specific acyl-Coenzyme A dehydrogenase (ACADM), an enzyme that catalyzes the initial step of the mitochondrial fatty acid beta-oxidation pathway. Here, we show that interaction between ACADM and E2 also happens in swine cells infected with CSFV using two different procedures: coimmunoprecipitation and a proximity ligation assay (PLA). In addition, the amino acid residues in E2 critically mediating the interaction with ACADM, M49 and P130 were identified via a reverse yeast two-hybrid screen using an expression library composed of randomly mutated versions of E2. A recombinant CSFV, E2ΔACADMv, harboring substitutions at residues M49I and P130Q in E2, was developed via reverse genomics from the highly virulent Brescia isolate. E2ΔACADMv was shown to have the same kinetics growth in swine primary macrophages and SK6 cell cultures as the parental Brescia strain. Similarly, E2ΔACADMv demonstrated a similar level of virulence when inoculated to domestic pigs as the parental Brescia. Animals intranasally inoculated with 10⁵ TCID₅₀ developed a lethal form of clinical disease with virological and hematological kinetics changes undistinguishable from those produced by the parental strain. Therefore, interaction between CSFV E2 and host ACADM is not critically involved in the processes of virus replication and disease production.

Different impact of bovine complement regulatory protein 46 (CD46bov) as a cellular receptor for members of the species Pestivirus H and Pestivirus G

The genus Pestivirus within the family Flaviviridae comprises highly relevant animal pathogens such as bovine viral diarrhoea virus 1 and 2 (BVDV-1 and -2) classified into the two species Pestivirus A and Pestivirus B, respectively. First described in 2004, HoBi-like pestiviruses (HoBiPeV) represent emerging bovine pathogens that belong to a separate species (Pestivirus H), but share many similarities with BVDV-1 and -2. Additionally, two giraffe pestivirus (GPeV) strains both originating from Kenya represent another distinct species (Pestivirus G), whose members replicate very efficiently in bovine cells. In this study, we investigated the role of bovine complement regulatory protein 46 (CD46bov), the receptor of BVDV-1 and -2, in the entry of HoBiPeV and GPeV. For this purpose, bovine CD46-knockout and CD46-rescue cell lines were generated by CRISPR/Cas9 technology and subsequent trans-complementation, respectively. Our results provide strong evidence that the impact of CD46bov differs between viruses belonging to Pestivirus H and viruses representing Pestivirus G: CD46bov revealed to be a major cellular entry factor for HoBiPeV strain HaVi-20. In contrast, GPeV strain PG-2 presented as largely independent of CD46bov, suggesting a different entry mechanism involving other molecular determinants which remain to be identified. In addition, we demonstrated that, similar to BVDV-1 and -2, virus isolates of both Pestivirus H and Pestivirus G are able to adapt to cell culture conditions by using heparan sulfate to enter the host cell. In conclusion, our findings show that different bovine pestiviruses use diverse mechanisms of host cell entry.

Generation of PCBP1-deficient pigs using CRISPR/Cas9-mediated gene editing

Classical swine fever virus (CSFV), a classic swine fever pathogen, causes severe economic losses worldwide. Poly (rC)-binding protein 1 (PCBP1), which interacts with N^pro of CSFV, plays a vital role in CSFV growth. We are the first to report the generation of PCBP1-deficient pigs via gene-editing technology. The PCBP1-deficient pigs exhibited normal birth weight and reproductive-performance traits and developed normally. Viral challenge experiments indicated that primary cells isolated from F₀- and F₁-generation pigs exhibited significantly reduced CSFV infection. Additional mechanistic exploration further confirmed that the PCBP1 deficiency-mediated antiviral effect is related to the activation of type I interferon (IFN). Besides showing that a gene-editing strategy could be used to generate PCBP1-deficient pigs, our study introduces a valuable animal model for further investigating the infection mechanisms of CSFV that will help to develop better antiviral solutions.

•

Reduced CSFV infection in PCBP1-deficient cells is related to activated ISGs expression

•

PCBP1-deficient pigs were successfully generated via gene-editing technology

•

Primary cells isolated from PCBP1-deficient pigs exhibited reduced CSFV infection

Interaction network of African swine fever virus structural protein p30 with host proteins

African swine fever virus (ASFV) is a complex nucleocytoplasmic large DNA virus (NCLDV) that causes a lethal hemorrhagic disease that is currently threatening the global pig industry. ASFV structural protein p30 is a membrane phosphoprotein that suggests it may play a regulatory role, possibly in signal transduction. Despite its significance in internalization into host cells, the interaction between p30 and host proteins is relatively unknown. In this study, we describe the application of a DUALmembrane yeast two-hybrid assay to screen a primary porcine alveolar macrophages cDNA library and analyze the interactome of p30 protein. Our data identify seven host cellular proteins (DAB2, RPSA, OAS1, PARP9, CAPG, ARPC5, and VBP1) that putatively interact with the p30. We further verified the interaction between p30 and host proteins by laser confocal microscopy, co-immunoprecipitation, and GST-pulldown assay. To further understand the relationship between host proteins and p30, we drew the interaction network diagram and analyzed the functional enrichment of each host protein. Enrichment analysis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes indicated that host proteins were mainly related to endocytosis, actin cytoskeleton regulation, and innate immunity. Collectively, we identified the interaction between p30 and host cell protein using a membrane protein yeast two-hybrid system, which increases our knowledge of the interaction between ASFV and the host and informs future research on antiviral strategies.

Genome-wide transcriptome profiling of CSF virus challenged monocyte-derived macrophages provides distinct insights into immune response of Landrace and indigenous Ghurrah pigs

The present study was undertaken to characterize the distinct immune response in indigenous Ghurrah and exotic Landrace pigs by challenging monocyte-derived macrophages (MDMs) with CSF virus under in-vitro conditions and assessing the variations in the transcriptome profile at 48 h post-infection (hpi). RNA-sequencing was carried out in infected and non-infected MDMs of Ghurrah (n = 3) and Landrace (n = 3) piglets prior- as well as post-stimulation. MDMs of Ghurrah showed greater immune regulation in response to CSF infection with 518 significantly differentially expressed genes (DEG) in infected versus non-infected MDMs, as compared to only 31 DEGs in Landrace MDMs. In Landrace, the principal regulators of inflammation (IL1α, IL1β and TNF) were upregulated in infected cells while in Ghurrah, these were downregulated. Overall, macrophages from indigenous Ghurrah showed more immunological dysregulation in response to virulent CSF virus infection as compared to the exotic Landrace pigs.

Current Status of Genetically Modified Pigs That Are Resistant to Virus Infection

Pigs play an important role in agriculture and biomedicine. The globally developing swine industry must address the challenges presented by swine-origin viruses, including ASFV (African swine fever virus), PRRSV (porcine reproductive and respiratory syndrome virus), PEDV (porcine epidemic diarrhea virus), PRV (pseudorabies virus), CSFV (classical swine fever virus), TGEV (transmissible gastroenteritis virus), et al. Despite sustained efforts by many government authorities, these viruses are still widespread. Currently, gene-editing technology has been successfully used to generate antiviral pigs, which offers the possibility for increasing animal disease tolerance and improving animal economic traits in the future. Here, we summarized the current advance in knowledge regarding the host factors in virus infection and the current status of genetically modified pigs that are resistant to virus infection in the world. There has not been any report on PEDV-resistant pigs, ASFV-resistant pigs, and PRV-resistant pigs owing to the poor understanding of the key host factors in virus infection. Furthermore, we summarized the remaining problems in producing virus-resistant pigs, and proposed several potential methods to solve them. Using genome-wide CRISPR/Cas9 library screening to explore the key host receptors in virus infection may be a feasible method. At the same time, exploring the key amino acids of host factors in virus infection with library screening based on ABEs and CBEs (Bes) may provide creative insight into producing antiviral pigs in the future.

ADAM17 Is an Essential Factor for the Infection of Bovine Cells with Pestiviruses

The entry of BVDV into bovine cells was studied using CRIB cells (cells resistant to infection with bovine viral diarrhea virus [BVDV]) that have evolved from MDBK cells by a spontaneous loss of susceptibility to BVDV. Recently, larger genetic deletions were reported but no correlation of the affected genes and the resistance to BVDV infection could be established. The metalloprotease ADAM17 was reported as an essential attachment factor for the related classical swine fever virus (CSFV). To assess whether ADAM17 might be involved in the resistance of CRIB-1 cells to pestiviruses, we analyzed its expression in CRIB-1 and MDBK cells. While ADAM17 protein was detectable in MBDK cells, it was absent from CRIB-1 cells. No functional full-length ADAM17 mRNA could be detected in CRIB cells and genetic analysis revealed the presence of two defective alleles. Transcomplementation of functional ADAM17 derived from MDBK cells in CRIB-1 cells resulted in a nearly complete reversion of their resistance to pestiviral infection. Our results demonstrate that ADAM17 is a key cellular factor for the pestivirus resistance of CRIB-1 cells and establishes its essential role for a broader range of pestiviruses.

Cellular ESCRT components are recruited to regulate the endocytic trafficking and RNA replication compartment assembly during classical swine fever virus infection

As the important molecular machinery for membrane protein sorting in eukaryotic cells, the endosomal sorting and transport complexes (ESCRT-0/I/II/III and VPS4) usually participate in various replication stages of enveloped viruses, such as endocytosis and budding. The main subunit of ESCRT-I, Tsg101, has been previously revealed to play a role in the entry and replication of classical swine fever virus (CSFV). However, the effect of the whole ESCRT machinery during CSFV infection has not yet been well defined. Here, we systematically determine the effects of subunits of ESCRT on entry, replication, and budding of CSFV by genetic analysis. We show that EAP20 (VPS25) (ESCRT-II), CHMP4B and CHMP7 (ESCRT-III) regulate CSFV entry and assist vesicles in transporting CSFV from Clathrin, early endosomes, late endosomes to lysosomes. Importantly, we first demonstrate that HRS (ESCRT-0), VPS28 (ESCRT-I), VPS25 (ESCRT-II) and adaptor protein ALIX play important roles in the formation of virus replication complexes (VRC) together with CHMP2B/4B/7 (ESCRT-III), and VPS4A. Further analyses reveal these subunits interact with CSFV nonstructural proteins (NS) and locate in the endoplasmic reticulum, but not Golgi, suggesting the role of ESCRT in regulating VRC assembly. In addition, we demonstrate that VPS4A is close to lipid droplets (LDs), indicating the importance of lipid metabolism in the formation of VRC and nucleic acid production. Altogether, we draw a new picture of cellular ESCRT machinery in CSFV entry and VRC formation, which could provide alternative strategies for preventing and controlling the diseases caused by CSFV or other Pestivirus.

ESCRT machinery can be responsible for virus budding and participate in regulating virus entry. However, little has been reported on its effects on VRC formation. Here, we uncover the novel roles of ESCRT-III and VPS4A in VRC assembly and update the additional subunits involved in the intracellular trafficking of CSFV. These data indicate that the ESCRT machinery promotes CSFV replication by forming VRC, which making it become nuclease-insensitive to avoid the recognition by the host antiviral surveillance system and the destruction of the viral RNA. Furthermore, we first demonstrate that the roles of ESCRT components in the formation of VRC in swine Pestivirus. Our findings highlight the growing evidence of diverse interactions between ESCRT subunits and viral factors of Flaviviridae family, and provide alternative strategies for preventing and controlling the diseases caused by CSFV or other Pestivirus.

Glycosaminoglycan binding by arboviruses: a cautionary tale

Arboviruses are medically important arthropod-borne viruses that cause a range of diseases in humans from febrile illness to arthritis, encephalitis and hemorrhagic fever. Given their transmission cycles, these viruses face the challenge of replicating in evolutionarily divergent organisms that can include ticks, flies, mosquitoes, birds, rodents, reptiles and primates. Furthermore, their cell attachment receptor utilization may be affected by the opposing needs for generating high and sustained serum viremia in vertebrates such that virus particles are efficiently collected during a hematophagous arthropod blood meal but they must also bind sufficiently to cellular structures on divergent organisms such that productive infection can be initiated and viremia generated. Sulfated polysaccharides of the glycosaminoglycan (GAG) groups, primarily heparan sulfate (HS), have been identified as cell attachment moieties for many arboviruses. Original identification of GAG binding as a phenotype of arboviruses appeared to involve this attribute arising solely as a consequence of adaptation of virus isolates to growth in cell culture. However, more recently, naturally circulating strains of at least one arbovirus, eastern equine encephalitis, have been shown to bind HS efficiently and the GAG binding phenotype continues to be associated with arbovirus infection in published studies. If GAGs are attachment receptors for many naturally circulating arboviruses, this could lead to development of broad-spectrum antiviral therapies through blocking of the virus-GAG interaction. This review summarizes the available data for GAG/HS binding as a phenotype of naturally circulating arbovirus strains emphasizing the importance of avoiding tissue culture amplification and artifactual phenotypes during their isolation.

Plasticity of the lettuce infectious yellows virus minor coat protein (CPm) in mediating the foregut retention and transmission of a chimeric CPm mutant by whitefly vectors

Transmission of the crinivirus, lettuce infectious yellows virus (LIYV), is determined by a minor coat protein (CPm)-mediated virion retention mechanism located in the foregut of its whitefly vector. To better understand the functions of LIYV CPm, chimeric CPm mutants engineered with different lengths of the LIYV CPm amino acid sequence and that of the crinivirus, lettuce chlorosis virus (LCV), were constructed based on bioinformatics and sequence alignment data. The 485 amino acid-long chimeric CPm of LIYV mutant, CPmP-1, contains 60 % (from position 3 to 294) of LCV CPm amino acids. The chimeric CPm of mutants CPmP-2, CPmP-3 and CPmP-4 contains 46 (position 3 to 208), 51 (position 3 to 238) and 41 % (position 261 to 442) of LCV CPm amino acids, respectively. All four mutants moved systemically, expressed the chimeric CPm and formed virus particles. However, following acquisition feeding of the virus preparations, only CPmP-1 was retained in the foreguts of a significant number of vectors and transmitted. In immuno-gold labelling transmission electron microscopy (IGL-TEM) analysis, CPmP-1 particles were distinctly labelled by antibodies directed against the LCV but not LIYV CPm. In contrast, CPmP-4 particles were not labelled by antibodies directed against the LCV or LIYV CPm, while CPmP-2 and -3 particles were weakly labelled by anti-LIYV CPm but not anti-LCV CPm antibodies. The unique antibody recognition and binding pattern of CPmP-1 was also displayed in the foreguts of whitefly vectors that fed on CPmP-1 virions. These results are consistent with the hypothesis that the chimeric CPm of CPmP-1 is incorporated into functional virions, with the LCV CPm region being potentially exposed on the surface and accessible to anti-LCV CPm antibodies.

Classical swine fever virus infection suppresses claudin-1 expression to facilitate its replication in PK-15 cells

Classical swine fever (CSF) is one of the most epidemic viral diseases in swine industry. The causative pathogen is CSF virus (CSFV), a small enveloped RNA virus of Flaviviridae family. Claudin-1 was reported to be involved in the infections of a number of viruses, including many from Flaviviridae family, but no studies have investigated the role of porcine claudin-1 during CSFV infection in PK-15 cells. In this study, on the one hand, we demonstrated that CSFV infection reduced the claudin-1 expression at both mRNA and protein levels; on the other hand, CSFV infection was enhanced after claudin-1 knockdown, but inhibited by claudin-1 overexpression in a dose-dependent manner. Furthermore, negative correlation was demonstrated between the claudin-1 expression and CSFV titer. In conclusion, claudin-1 might be a barrier for CSFV infection in PK-15 cells, while CSFV bypasses the barrier through lysosome mediated degradation of claudin-1, which could be repressed by bafilomycin A1. Although the elaborate mechanisms how claudin-1 plays its roles in CSFV infection require further investigations, this study may advance our understanding of the molecular host-pathogen interaction mechanisms underlying CSFV infection and suggests enhancement of porcine claudin-1 as a potential preventive or therapeutic strategy for CSF control.

Genome variability of classical swine fever virus during the 2018-2020 epidemic in Japan

Although RNA viruses exhibit extensive sequence diversity, the mutation rate must be limited to ensure protein functions that maintain the viral life cycle. Here, we compared the whole genome sequences of 150 isolates of classical swine fever virus (CSFV), obtained from a single epidemic that occurred in Japan during 2018-2020. After the detection of the first case, the disease spread among both farm pigs and wild boars and caused severe impact on the pig industry. To evaluate the diversification of the CSFV genome that eliminated mutations negatively affecting viral transmission, the substitution sets inherited by at least two isolates were separately evaluated as shared single nucleotide variants (SNVs) or shared single amino acid variants (SAVs). Comparisons of 12 protein-coding regions indicated that the percentages of SNVs and SAVs in the multifunctional nonstructural protein NS3 were the lowest, and shared SAVs were not detected in another nonstructural protein, NS4A. This demonstrated purifying negative selection suppressing changes in the protein sequences of NS3 and NS4A during virus transmission in the field. In contrast, a high possibility of nonsynonymous substitution among shared SNVs was detected only in genes encoding the secreted protein E^rns and the nonstructural protein NS2, suggesting positive selection during the epidemic. Mapping of shared SAVs to the three-dimensional structure of E^rns revealed that shared SAVs were not present in the substrate-binding sites but were instead localized to the peripheral region of the protein. These data will support efforts toward the development of diagnostic methods, recombinant vaccines, and antiviral agents targeting conserved and indispensable viral genes.

Structural Glycoprotein E2 of Classical Swine Fever Virus Critically Interacts with Host Protein Torsin-1A during the Virus Infectious Cycle

The classical swine fever virus (CSFV) glycoprotein E2 is the major structural component of the virus particle. E2 is involved in several functions, such as virus adsorption to the cell, the elicitation of protective immune responses, and virus virulence in swine. Using a yeast two-hybrid system, we previously identified the swine host protein Torsin-1A, an ATPase protein residing in the endoplasmic reticulum and inner nucleus membrane of the cell, as a specific binding partner for E2. The interaction between Torsin-1A and E2 proteins was confirmed to occur in CSFV-infected swine cells using three independent methods: coimmunoprecipitation, confocal microscopy, and proximity ligation assay (PLA). Furthermore, the E2 residue critical to mediate the protein-protein interaction with Torsin-1A was identified by a reverse yeast two-hybrid assay using a randomly mutated E2 library. A recombinant CSFV E2 mutant protein with a Q316L substitution failed to bind swine Torsin-1A in the yeast two-hybrid model. In addition, a CSFV infectious clone harboring the E2 Q316L substitution, although expressing substantial levels of E2 protein, repetitively failed to produce virus progeny when the corresponding RNA was transfected into susceptible SK6 cells. Importantly, PLA analysis of the transfected cells demonstrated an abolishment of the interaction between E2 Q316L and Torsin-1A, indicating a critical role for that interaction during CSFV replication.IMPORTANCE Structural glycoprotein E2 is an important structural component of the CSFV particle. E2 is involved in several virus functions, particularly virus-host interactions. Here, we characterized the interaction between CSFV E2 and swine protein Torsin-1A during virus infection. The critical amino acid residue in E2 mediating the interaction with Torsin-1A was identified and the effect of disrupting the E2-Torsin-1A protein-protein interaction was studied using reverse genetics. It is shown that the amino acid substitution abrogating E2-Torsin-1A interaction constitutes a lethal mutation, demonstrating that this virus-host protein-protein interaction is a critical factor during CSFV replication. This highlights the potential importance of the E2-Torsin-1A protein-protein interaction during CSFV replication and provides a potential pathway toward blocking virus replication, an important step toward the potential development of novel virus countermeasures.

Porcine Complement Regulatory Protein CD46 Is a Major Receptor for Atypical Porcine Pestivirus but Not for Classical Swine Fever Virus

Pestiviruses comprise animal pathogens such as classical swine fever virus (CSFV) and bovine viral diarrhea virus (BVDV) that cause notifiable diseases with great economic impact. Several additional pestivirus species affecting animal health were recently identified, including atypical porcine pestivirus (APPV).

Pestiviruses such as bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV) belong to the family Flaviviridae and represent pathogens of outstanding veterinary relevance. Pestiviruses enter cells via receptor-mediated endocytosis. For entry in bovine cells, complement regulatory protein CD46_bov serves as a cellular receptor for BVDV. In this study, the role of porcine CD46_pig in cellular entry was investigated for the recently discovered atypical porcine pestivirus (APPV), CSFV, and Bungowannah virus (BuPV) in order to elucidate the observed differences in host cell tropism. A cell culture-adapted APPV variant, which shows enhanced viral replication in vitro, was generated and demonstrated a strict tropism of APPV for porcine cells. One of the porcine cell lines displayed areas of CD46_pig-expressing cells and areas of nonexpressing cells, and one single cell line revealed not to express any CD46_pig. The CD46_pig-deficient porcine lymphoma cell line, known to facilitate CSFV replication, was the only porcine cell line nonpermissive to APPV, indicating a significant difference in the entry mechanism of APPV and CSFV. Infection experiments with a set of genetically engineered CD46_pig knockout cells confirmed that CD46_pig is a major receptor of APPV as CD46_bov is for BVDV. In contrast, it is apparently not an essential determinant in host cell entry of other porcine pestiviruses such as CSFV and BuPV. Existence of a CD46_pig-independent entry mechanism illustrates that the pestiviral entry process is more diverse than previously recognized.

IMPORTANCE Pestiviruses comprise animal pathogens such as classical swine fever virus (CSFV) and bovine viral diarrhea virus (BVDV) that cause notifiable diseases with great economic impact. Several additional pestivirus species affecting animal health were recently identified, including atypical porcine pestivirus (APPV). APPV is associated with health problems in piglets and is highly abundant in pig populations worldwide. Complement control protein CD46 serves as a receptor for diverse bacterial and viral pathogens, including particular adenoviruses, herpesviruses, measles virus (MeV), and BVDV. Porcine CD46 (CD46_pig) was suggested to be a major receptor for CSFV. Here, we identified remarkable differences in relevance of CD46_pig during entry of porcine pestiviruses. Resembling BVDV, efficient APPV infection in cell culture depends on CD46_pig, while other porcine pestiviruses can efficiently enter and infect cells in the absence of CD46_pig. Thus, the study provides insights into the entry process of these pathogens and may help to understand differences in their biology.

Anti-Classical Swine Fever Virus Strategies

Classical swine fever (CSF), caused by CSF virus (CSFV), is a highly contagious swine disease with high morbidity and mortality, which has caused significant economic losses to the pig industry worldwide. Biosecurity measures and vaccination are the main methods for prevention and control of CSF since no specific drug is available for the effective treatment of CSF. Although a series of biosecurity and vaccination strategies have been developed to curb the outbreak events, it is still difficult to eliminate CSF in CSF-endemic and re-emerging areas. Thus, in addition to implementing enhanced biosecurity measures and exploring more effective CSF vaccines, other strategies are also needed for effectively controlling CSF. Currently, more and more research about anti-CSFV strategies was carried out by scientists, because of the great prospects and value of anti-CSFV strategies in the prevention and control of CSF. Additionally, studies on anti-CSFV strategies could be used as a reference for other viruses in the Flaviviridae family, such as hepatitis C virus, dengue virus, and Zika virus. In this review, we aim to summarize the research on anti-CSFV strategies. In detail, host proteins affecting CSFV replication, drug candidates with anti-CSFV effects, and RNA interference (RNAi) targeting CSFV viral genes were mentioned and the possible mechanisms related to anti-CSFV effects were also summarized.

ADAM17 is an essential attachment factor for classical swine fever virus

Classical swine fever virus (CSFV) is an important pathogen in the swine industry. Virion attachment is mediated by envelope proteins Erns and E2, and E2 is indispensable. Using a pull-down assay with soluble E2 as the bait, we demonstrated that ADAM17, a disintegrin and metalloproteinase 17, is essential for CSFV entry. Loss of ADAM17 in a permissive cell line eliminated E2 binding and viral entry, but compensation with pig ADAM17 cDNA completely rescued these phenotypes. Similarly, ADAM17 silencing in primary porcine fibroblasts significantly impaired virus infection. In addition, human and mouse ADAM17, which is highly homologous to pig ADAM17, also mediated CSFV entry. The metalloproteinase domain of ADAM17 bound directly to E2 protein in a zinc-dependent manner. A surface exposed region within this domain was mapped and shown to be critical for CSFV entry. These findings clearly demonstrate that ADAM17 serves as an essential attachment factor for CSFV.

Classical swine fever virus: the past, present and future

Classical swine fever (CSF) is among the most relevant viral epizootic diseases of swine. Due to its severe economic impact, CSF is notifiable to the world organisation for animal health. Strict control policies, including systematic stamping out of infected herds with and without vaccination, have permitted regional virus eradication. Nevertheless, CSF virus (CSFV) persists in certain areas of the world and has re-emerged regularly. This review summarizes the basic established knowledge in the field and provides a comprehensive and updated overview of the recent advances in fundamental CSFV research, diagnostics and vaccine development. It covers the latest discoveries on the genetic diversity of pestiviruses, with implications for taxonomy, the progress in understanding disease pathogenesis, immunity against acute and persistent infections, and the recent findings in virus-host interactions and virulence determinants. We also review the progress and pitfalls in the improvement of diagnostic tools and the challenges in the development of modern and efficacious marker vaccines compatible with serological tests for disease surveillance. Finally, we highlight the gaps that require research efforts in the future.

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

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

Porcine RACK1 negatively regulates the infection of classical swine fever virus and the NF-κB activation in PK-15 cells

Classical swine fever (CSF) is one of the main viral diseases of swine worldwide. The causative pathogen is CSF virus (CSFV), a small enveloped RNA virus of the genus Pestivirus. Activation of NF-κB is a hallmark of most viral infections and the viral pathogens frequently kidnap NF-κB pathway for their own advantages, however, it is unclear or even controversial about whether CSFV infection can activate NF-κB signal pathway. RACK1 was shown as an interacting host protein with CSFV NS5A protein, but no studies so far have clearly defined the role of RACK1 during CSFV infection and NF-κB activation. In this study, to properly address these open questions, using RT-qPCR, western blot, indirect fluorescence staining, siRNA knockdown and protein overexpression techniques, we demonstrated that CSFV infection reduced the RACK1 expression at both mRNA and protein levels in PK-15 cells. Downregulation of cellular RACK1 enhanced CSFV infection and subsequent NF-κB activation, while RACK1 overexpression inhibited CSFV infection and the NF-κB activation. In conclusion, RACK1 is a negative cellular regulator for CSFV infection and NF-κB activation in PK-15 cells. Our work addressed a novel aspect concerning the regulation of innate antiviral immune response during CSFV infection. This study may provide some insights into the molecular mechanisms of CSFV infection in swine. However, the elaborate mechanism by which CSFV regulates NF-κB activation and how RACK1 plays its roles in CSFV infection and NF-κB induction require further in-depth studies.

SERTA Domain Containing Protein 1 (SERTAD1) Interacts with Classical Swine Fever Virus Structural Glycoprotein E2, Which Is Involved in Virus Virulence in Swine

E2 is the major structural glycoprotein of the classical swine fever virus (CSFV). E2 has been shown to be involved in important virus functions such as replication and virulence in swine. Using the yeast two-hybrid system, we previously identified several host proteins specifically interacting with CSFV E2. Here, we analyze the protein interaction of E2 with SERTA domain containing protein 1 (SERTAD1), a factor involved in the stimulation of the transcriptional activities of different host genes. We have confirmed that the interaction between these two proteins occurs in CSFV-infected swine cells by using a proximity ligation assay and confocal microscopy. Amino acid residues in the CSFV E2 protein that are responsible for mediating the interaction with SERTAD1 were mapped by a yeast two-hybrid approach using a randomly mutated E2 library. Using that information, a recombinant CSFV mutant (E2ΔSERTAD1v) that harbors substitutions in those residues mediating the protein-interaction with SERTAD1 was developed and used to study the role of the E2-SERTAD1 interaction in viral replication and virulence in swine. CSFV E2ΔSERTAD1v, when compared to the parental BICv, showed a clearly decreased ability to replicate in the SK6 swine cell line and a more severe replication defect in primary swine macrophage cultures. Importantly, 80% of animals infected with E2ΔSERTAD1v survived infection, remaining clinically normal during the 21-day observational period. This result would indicate that the ability of CSFV E2 to bind host SERTAD1 protein during infection plays a critical role in virus virulence.

Swine Host Protein Coiled-Coil Domain-Containing 115 (CCDC115) Interacts with Classical Swine Fever Virus Structural Glycoprotein E2 during Virus Replication

Interactions between the major structural glycoprotein E2 of classical swine fever virus (CSFV) with host proteins have been identified as important factors affecting virus replication and virulence. Previously, using the yeast two-hybrid system, we identified swine host proteins specifically interacting with CSFV E2. In this report, we use a proximity ligation assay to demonstrate that swine host protein CCDC115 interacts with E2 in CSFV-infected swine cells. Using a randomly mutated E2 library in the context of a yeast two-hybrid methodology, specific amino acid mutations in the CSFV E2 protein responsible for disrupting the interaction with CCDC115 were identified. A recombinant CSFV mutant (E2ΔCCDC115v) harboring amino acid changes disrupting the E2 protein interaction with CCDC115 was produced and used as a tool to assess the role of the E2–CCDC115 interaction in viral replication and virulence in swine. CSFV E2ΔCCDC115v showed a slightly decreased ability to replicate in the SK6 swine cell line and a greater replication defect in primary swine macrophage cultures. A decreased E2–CCDC115 interaction detected by PLA is observed in cells infected with E2ΔCCDC115v. Importantly, animals intranasally infected with 10⁵ TCID₅₀ of E2ΔCCDC115v experienced a significantly longer survival period when compared with those infected with the parental Brescia strain. This result would indicate that the ability of CSFV E2 to bind host CCDC115 protein during infection plays an important role in virus replication in swine macrophages and in virus virulence during the infection in domestic swine.

MERTK is a host factor that promotes classical swine fever virus entry and antagonizes innate immune response in PK-15 cells

Classical swine fever virus (CSFV) is a member of the genus Pestivirus in the Flaviviridae family. To date, the host factors required for CSFV entry remain poorly characterized. To identify the functional membrane protein(s) involved in CSFV infection, we analyzed the transcriptomic data from previous studies describing gene expression profiles for CSFV, and found twelve novel candidate proteins. One of these proteins, MERTK, significantly reduced CSFV protein expression by RNA interference screening using a recombinant CSFV that contains a luciferase reporter to measure CSFV protein expression. Furthermore, our results demonstrated that either anti-MERTK antibodies or soluble MERTK ectodomain could reduce CSFV infection in PK-15 cells in a dose-dependent manner. Mechanistically, MERTK interacted with the E2 protein of CSFV and facilitated virus entry. After virus entry, MERTK downregulates of mRNA expression of IFN-β and promotes CSFV infection. Interestingly, the soluble MERTK ectodomain could also reduce the infection of bovine viral diarrhea virus (BVDV), another pestivirus. Taken together, our results suggested that MERTK is a CSFV entry factor that synergistically dampens innate immune responses in PK-15 cells and is also involved in BVDV infection.

Foot-and-Mouth Disease Virus Capsid Protein VP1 Interacts with Host Ribosomal Protein SA To Maintain Activation of the MAPK Signal Pathway and Promote Virus Replication

Foot-and-mouth disease virus (FMDV) is the causative agent of foot-and-mouth disease, a highly contagious, economically important viral disease. The structural protein VP1 plays significant roles during FMDV infection. Here, we identified that VP1 interacted with host ribosomal protein SA (RPSA). RPSA is a viral receptor for dengue virus and classical swine fever virus infections. However, the incubation of susceptible cells using the anti-RPSA antibodies did not block the infection of FMDV. Overexpression of porcine RPSA in the insusceptible cells could not trigger FMDV infection, suggesting that RPSA was not responsible for FMDV entry and infection. On the contrary, we found that overexpression of RPSA suppressed FMDV replication, and knockdown of RPSA enhanced FMDV replication. We further determined that FMDV infection activated the mitogen-activated protein kinase (MAPK) pathway and demonstrated that MAPK pathway activation was critically important for FMDV replication. RPSA negatively regulated MAPK pathway activation during FMDV infection and displayed an antiviral function. FMDV VP1 interacted with RPSA to abrogate the RPSA-mediated suppressive role in MAPK pathway activation. Together, our study indicated that MAPK pathway activation was required for FMDV replication and that host RPSA played a negatively regulatory role on MAPK pathway activation to suppress FMDV replication. FMDV VP1 bound to RPSA to promote viral replication by repressing RPSA-mediated function and maintaining the activation of MAPK signal pathway.IMPORTANCE Identification of virus-cell interactions is essential for making strategies to limit virus replication and refine the models of virus replication. This study demonstrated that FMDV utilized the MAPK pathway for viral replication. The host RPSA protein inhibited FMDV replication by suppressing the activation of the MAPK pathway during FMDV infection. FMDV VP1 bound to RPSA to repress the RPSA-mediated regulatory effect on MAPK pathway activation. This study revealed an important implication of the MAPK pathway for FMDV infection and identified a novel mechanism by which FMDV VP1 has evolved to interact with RPSA and maintain the activation of the MAPK pathway, elucidating new information regarding the signal reprogramming of host cells by FMDV.

Signature of genome wide gene expression in classical swine fever virus infected macrophages and PBMCs of indigenous vis-a-vis crossbred pigs

The genetic basis of differential host immune response vis-à-vis transcriptome profile was explored in PBMCs of indigenous (Ghurrah) and crossbred pigs after classical swine fever vaccination and in monocyte derived macrophages (MDMs) challenged with virulent classical swine fever (CSF) virus. The humoral immune response (E2 antibody) was higher (74.87%) in crossbred than indigenous pigs (58.20%) at 21st days post vaccination (21dpv). The rate of reduction of ratio of CD4⁺/CD8⁺ was higher in crossbred pigs than indigenous pigs at 7th days post vaccination (7dpv). The immune genes IFIT1, IFIT5, RELA, NFKB2, TNF and LAT2 were up regulated at 7dpv in RNA seq data set and was in concordance during qRT-PCR validation. The Laminin Subunit Beta 1 (LAMB1) was significantly (p ≤ 0.05) down-regulated in MDMs of indigenous pigs and consequently a significantly (p ≤ 0.01) higher copy number of virulent CSF virus was evidenced in macrophages of crossbred pigs than indigenous pigs. Activation of LXR:RXR pathway at 60 h post infection (60hpi) in MDMs of indigenous versus crossbred pigs inhibited nuclear translocation of NF-κB, resulted into transrepression of proinflammatory genes. But it helped in maintenance of HDL level by lowering down cholesterol/LDL level in MDMs of indigenous pigs. The key immune genes (TLR2, TLR4, IL10, IL8, CD86, CD54, CASP1) of TREM1 signaling pathway were upregulated at 7dpv in PBMCs but those genes were downregulated at 60hpi in MDMs indigenous pigs. Using qRT-PCR, the validation of differentially expressed, immunologically important genes (LAMB1, OAS1, TLR 4, TLR8 and CD86) in MDMs revealed that expression of these genes were in concordance with RNA-seq data.

Identification of Chicken CD74 as a Novel Cellular Attachment Receptor for Infectious Bursal Disease Virus in Bursa B Lymphocytes

Infectious bursal disease virus (IBDV) is an important member of the Birnaviridae family, causing severe immunosuppressive disease in chickens. The major capsid protein VP2 is responsible for the binding of IBDV to the host cell and its cellular tropism. In order to find proteins that potentially interact with IBDV VP2, a liquid chromatography-mass spectrometry (LC-MS) assay was conducted, and the host chicken CD74 protein was identified. Here, we investigate the role of chicken CD74 in IBDV attachment. Coimmunoprecipitation assays indicated that the extracellular domain of CD74 interacted with the VP2 proteins of multiple IBDV strains. Knockdown and overexpression experiments showed that CD74 promotes viral infectivity. Confocal assays showed that CD74 overexpression allows the attachment of IBDV and subvirus-like particles (SVPs) to the cell surface of nonpermissive cells, and quantitative PCR (qPCR) analysis further confirmed the attachment function of CD74. Anti-CD74 antibody, soluble CD74, depletion of CD74 by small interfering RNA (siRNA), and CD74 knockdown in the IBDV-susceptible DT40 cell line significantly inhibited IBDV binding, suggesting a pivotal role of this protein in virus attachment. These findings demonstrate that CD74 is a novel important receptor for IBDV attachment to the chicken B lymphocyte cell line DT40.IMPORTANCE CD74 plays a pivotal role in the correct folding and functional stability of major histocompatibility complex class II (MHC-II) molecules and in the presentation of antigenic peptides, acting as a regulatory factor in the antigen presentation process. In our study, we demonstrate a novel role of CD74 during IBDV infection, showing that chicken CD74 plays a significant role in IBDV binding to target B cells by interacting with the viral VP2 protein. This is the first report demonstrating that CD74 is involved as a novel attachment receptor in the IBDV life cycle in target B cells, thus contributing new insight into host-pathogen interactions.

Structural Glycoprotein E2 of Classical Swine Fever Virus Interacts with Host Protein Dynactin Subunit 6 (DCTN6) during the Virus Infectious Cycle

The E2 protein in classical swine fever (CSF) virus (CSFV) is the major virus structural glycoprotein and is an essential component of the viral particle. E2 has been shown to be involved in several functions, including virus adsorption, induction of protective immunity, and virulence in swine. Using the yeast two-hybrid system, we previously identified a swine host protein, dynactin subunit 6 (DCTN6) (a component of the cell dynactin complex), as a specific binding partner for E2. We confirmed the interaction between DCTN6 and E2 proteins in CSFV-infected swine cells by using two additional independent methodologies, i.e., coimmunoprecipitation and proximity ligation assays. E2 residues critical for mediating the protein-protein interaction with DCTN6 were mapped by a reverse yeast two-hybrid approach using a randomly mutated E2 library. A recombinant CSFV mutant, E2ΔDCTN6v, harboring specific substitutions in those critical residues was developed to assess the importance of the E2-DCTN6 protein-protein interaction for virus replication and virulence in swine. CSFV E2ΔDCTN6v showed reduced replication, compared with the parental virus, in an established swine cell line (SK6) and in primary swine macrophage cultures. Remarkably, animals infected with CSFV E2ΔDCTN6v remained clinically normal during the 21-day observation period, which suggests that the ability of CSFV E2 to bind host DCTN6 protein efficiently during infection may play a role in viral virulence.IMPORTANCE Structural glycoprotein E2 is an important component of CSFV due to its involvement in many virus activities, particularly virus-host interactions. Here, we present the description and characterization of the protein-protein interaction between E2 and the swine host protein DCTN6 during virus infection. The E2 amino acid residues mediating the interaction with DCTN6 were also identified. A recombinant CSFV harboring mutations disrupting the E2-DCTN6 interaction was created. The effect of disrupting the E2-DCTN6 protein-protein interaction was studied using reverse genetics. It was shown that the same amino acid substitutions that abrogated the E2-DCTN6 interaction in vitro constituted a critical factor in viral virulence in the natural host, domestic swine. This highlights the potential importance of the E2-DCTN6 protein-protein interaction in CSFV virulence and provides possible mechanisms of virus attenuation for the development of improved CSF vaccines.

Recombinant pseudorabies virus expressing E2 of classical swine fever virus (CSFV) protects against both virulent pseudorabies virus and CSFV

Both classical swine fever (CSF) and pseudorabies are highly contagious, economically significant diseases of swine in China. Although vaccination with the C-strain against classical swine fever virus (CSFV) is widely carried out and severe outbreaks of CSF seldom occur in China, CSF is sporadic in many pig herds and novel sub-subgenotypes of CSFV endlessly emerge. Thus, new measures are needed to eradicate CSFV from Chinese farms. The emergence of a pseudorabies virus (PRV) variant also posed a new challenge for the control of swine pseudorabies. Here, the recombinant PRV strain JS-2012-ΔgE/gI-E2 expressing E2 protein of CSFV was developed by inserting the E2 expression cassette into the intergenic region between the gG and gD genes of the gE/gI-deletion PRV variant strain JS-2012-ΔgE/gI. The recombinant virus was stable when passaged in vitro. A single vaccination of JS-2012-ΔgE/gI-E2 via intramuscular injection fully protected against lethal challenges of PRV and CSFV. Vaccination of piglets with the recombinant JS-2012-ΔgE/gI-E2 in the presence of high levels of maternally derived antibodies (Abs) to PRV can provide partial protection against lethal challenge of CSFV. Vaccination of the recombinant PRV JS-2012-ΔgE/gI-E2 strain did not induce the production of Abs to the gE protein of PRV or to the CSFV proteins other than E2. Thus, JS-2012-ΔgE/gI-E2 appears to be a promising recombinant marker vaccine candidate against PRV and CSFV for the control and eradication of the PRV variant and CSFV.

The Lentiviral System Construction for Highly Expressed Porcine Stearoyl-CoA Desaturase-1 and Functional Characterization in Stably Transduced Porcine Swine Kidney Cells

The most highly regulated and abundant fatty acid in animal tissue is oleic acid (18:1n9). Oleic acid is synthesized by the Δ9 desaturase, stearoyl-CoA desaturase-1 (SCD1), which is responsible for the synthesis of the putative cytokine palmitoleic acid (16:1n7) and 18:2 cis-9, trans-11 conjugated linoleic acid. Owing to the importance of SCD1 in lipid metabolism, we generated porcine swine kidney (SK6) transgenic cell lines for sustained overexpression or knockdown of porcine stearoyl-CoA desaturase-1 (pSCD1) in an inducible manner by utilizing a lentiviral expression system. We successfully validated these cell culture models for expression and functionality of pSCD1 by documenting that the pSCD-transduced cells overexpressed pSCD1 protein and mRNA. Additionally, the pSCD1-transduced cells increased the conversion of palmitate (16:0) to palmitoleic acid nearly fourfold. The lentiviral vectors utilized in this study can be further used to generate transgenic animals to document the effects of the overexpression of SCD1 on obesity and steatosis.

Roles of secretory glycoproteins in particle formation of Flaviviridae viruses

The family Flaviviridae comprises four genera, namely, Flavivirus, Pestivirus, Pegivirus, and Hepacivirus. These viruses have similar genome structures, but the genomes of Pestivirus and Flavivirus encode the secretory glycoproteins E^rns and NS1, respectively. E^rns plays an important role in virus particle formation and cell entry, whereas NS1 participates in the formation of replication complexes and virus particles. Conversely, apolipoproteins are known to participate in the formation of infectious particles of hepatitis C virus (HCV) and various secretory glycoproteins play a similar role in HCV particles formation, suggesting that there is no strong specificity for the function of secretory glycoproteins in infectious-particle formation. In addition, recent studies have shown that host-derived apolipoproteins and virus-derived E^rns and NS1 play comparable roles in infectious-particle formation of both HCV and pestiviruses. In this review, we summarize the roles of secretory glycoproteins in the formation of Flaviviridae virus particles.

Interaction of Structural Glycoprotein E2 of Classical Swine Fever Virus with Protein Phosphatase 1 Catalytic Subunit Beta (PPP1CB)

Classical swine fever virus (CSFV) E2 protein, the major virus structural glycoprotein, is an essential component of the viral envelope. E2 is involved in virus absorption, induction of a protective immune response and is critical for virulence in swine. Using the yeast two-hybrid system, we identified protein phosphatase 1 catalytic subunit beta (PPP1CB), which is part of the Protein Phosphatase 1 (PP1) complex, as a specific binding host partner for E2. We further confirmed the occurrence of this interaction in CSFV-infected swine cells by using two independent methodologies: Co-immunoprecipitation and Proximity Ligation Assay. In addition, we demonstrated that pharmacological activation of the PP1 pathway has a negative effect on CSFV replication while inhibition of the PP1 pathway or knockdown of PPP1CB by siRNA had no observed effect. Overall, our data suggests that the CSFV E2 and PPP1CB protein interact in infected cells, and that activation of the PP1 pathway decreases virus replication.

Comparative proteomic analysis provides new insight into differential transmission of two begomoviruses by a whitefly

Background

Viruses in the genus Begomovirus (Family Geminiviridae) include many important economic plant viruses transmitted by whiteflies of the Bemisia tabaci species complex. In general, different begomoviruses may be acquired and transmitted by the same whitefly species with different efficiencies. For example, the species Mediterranean (MED) in this whitefly species complex transmits tomato yellow leaf curl virus (TYLCV) at a higher efficiency than papaya leaf curl China virus (PaLCuCNV). However, the proteomic responses of whitefly to the infection of different begomoviruses remain largely unknown.

Methods

We used iTRAQ-based proteomics coupled with RT-qPCR to investigate and compare responses of the MED whitefly to the infection of TYLCV and PaLCuCNV.

Results

Totally, 259, 395 and 74 differently expressed proteins (DEPs) were identified in the comparisons of TYLCV-infected vs. un-infected, PaLCuCNV-infected vs. un-infected, and TYLCV-infected vs. PaLCuCNV-infected whiteflies, respectively. These proteins appear associated with catabolic process, metabolic process, transport, defense response, cell cycle, and receptor. The comparisons of TYLCV-infected vs. un-infected and PaLCuCNV-infected vs. un-infected shared some similar DEPs, indicating possible involvement of laminin subunit alpha, dystroglycan, integrin alpha-PS2 and cuticle proteins in viral transport as well as the role of putative defense proteins 3 and PITH in anti-viral response. However, 20S proteasome subunits associated with regulation of virus degradation and accumulation were up-regulated in PaLCuCNV-infected but not in TYLCV-infected whiteflies, which may be related to the constraints of PaLCuCNV accumulation in MED.

Conclusions

These findings provide valuable clues for unravelling the roles of some whitefly proteins in begomovirus transmission.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1138-4) contains supplementary material, which is available to authorized users.

CRISPR/Cas9-Mediated Hitchhike Expression of Functional shRNAs at the Porcine miR-17-92 Cluster

Successful RNAi applications depend on strategies allowing stable and persistent expression of minimal gene silencing triggers without perturbing endogenous gene expression. In this study, we proposed an endogenous microRNA (miRNA) cluster as a novel integration site for small hairpin RNAs (shRNAs). We successfully integrated exogenous shRNAs at the porcine miRNA-17-92 (pmiR-17-92) cluster via a CRISPR/Cas9-mediated knock-in strategy. The anti-EGFP or anti-CSFV shRNAs could be stably and effectively expressed at the control of the endogenous promoter of the pmiR-17-92 cluster. Importantly, we confirmed that hitchhike expression of anti- classical swine fever (CSFV) shRNA had no effect on cell growth, blastocyst development and endogenous pmiR-17-92 expression in selected transgene (TG) porcine fetal fibroblasts (PFFs) clones. Moreover, these TG PFFs could inhibit the replication of CSFV by half and could be further used for generation of transgenic pigs. Taken together, these results show that our RNA interference (RNAi) expression strategy benefits numerous applications, from miRNA, genome and transgenic research, to gene therapy.

Antiviral Role of IFITM Proteins in Classical Swine Fever Virus Infection

The proteins IFITM1, IFITM2, and IFITM3 are host effectors against a broad range of RNA viruses whose roles in classical swine fever virus (CSFV) infection had not yet been reported. We investigated the effect of these proteins on CSFV replication in mammalian cells. The proteins were overexpressed and silenced using lentiviruses. Confocal microscopy was used to determine the distribution of these proteins in the cells, and immunofluorescence colocalization analysis was used to evaluate the relationship between IFITMs and the CSFV endosomal pathway, including early endosomes, late endosomes, and lysosomes. IFITM1, IFITM2, or IFITM3 overexpression significantly inhibited CSFV replication, whereas protein knockdown enhanced CSFV replication. In porcine alveolar macrophages (PAMs), IFITM1 was mainly located at the cell surface, whereas IFITM2 and IFITM3 were mainly located in the cytoplasm. Following CSFV infection, the distribution of IFITM1 changed. IFITM1, IFITM2, and IFITM3 colocalization with Lamp1, IFITM2 with Rab5 and Rab7, and IFITM3 with Rab7 were observed in CSFV-infected cells. Collectively, these results provide insights into the possible mechanisms associated with the anti-CSFV action of the IFITM family.

Genetically modified pigs are protected from classical swine fever virus

Classical swine fever (CSF) caused by classical swine fever virus (CSFV) is one of the most detrimental diseases, and leads to significant economic losses in the swine industry. Despite efforts by many government authorities to stamp out the disease from national pig populations, the disease remains widespread. Here, antiviral small hairpin RNAs (shRNAs) were selected and then inserted at the porcine Rosa26 (pRosa26) locus via a CRISPR/Cas9-mediated knock-in strategy. Finally, anti-CSFV transgenic (TG) pigs were produced by somatic nuclear transfer (SCNT). Notably, in vitro and in vivo viral challenge assays further demonstrated that these TG pigs could effectively limit the replication of CSFV and reduce CSFV-associated clinical signs and mortality, and disease resistance could be stably transmitted to the F1-generation. Altogether, our work demonstrated that RNA interference (RNAi) technology combining CRISPR/Cas9 technology offered the possibility to produce TG animal with improved resistance to viral infection. The use of these TG pigs can reduce CSF-related economic losses and this antiviral strategy may be useful for future antiviral research.

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), and is a highly contagious, often fatal porcine disease that causes significant economic losses. Due to the economic importance of this virus to the pig industry, the biology and pathogenesis of CSFV have been investigated extensively. Despite efforts by many government authorities to stamp out the disease from national pig populations, the disease remains widespread, and it is only a matter of time before the virus is reintroduced and the next round of disease outbreaks occurs. These findings highlight the necessity and urgency for developing effective approaches to eradicate the challenging CSFV. In this study, we successfully produced anti-CSFV TG pigs by combining RNAi technology and CRISPR/Cas9 technologies, and viral challenge results confirmed that these TG pigs could effectively limit the replication of CSFV in vivo and in vitro. Additionally, we confirmed that the disease resistance traits in the TG founders were stably transmitted to their F1-generation offspring. Altogether, our work reported the combinational application of CRISPR/Cas9 and RNA interference (RNAi) technology in the generation of anti-CSFV TG pigs, it provided an alternative strategy to change the virus. The results of this study suggested that these TG pigs offered potential benefits over commercial vaccination and reduced CSFV-related economic losses.

P27 (MBOV_RS03440) is a novel fibronectin binding adhesin of Mycoplasma bovis

Mycoplasma bovis, one of the major pathogens of bovine respiratory disease, binds to respiratory epithelial cells resulting in severe pneumonia and tissue damage. This study was designed to identify the adhesive function of a putative 27-kDa M. bovis lipoprotein, encoded by the gene MBOV_RS03440 and designated as P27. The gene was cloned and overexpressed to produce antibodies against the recombinant P27 (rP27). The western blot and flow cytometry assay confirmed P27 to be a surface-localized protein, while ELISA confirmed it to be an immunogenic protein. Confocal immunofluorescence microscopy demonstrated that rP27 bound to embryonic bovine lung (EBL) cell monolayers in a dose-dependent manner. Furthermore, anti-rP27 antiserum inhibited the attachment of M. bovis to EBL cells demonstrating the binding specificity of P27 to EBL cells. The attachment of rP27 to EBL cells was mediated by fibronectin (Fn), an extracellular matrix component. The interaction between rP27 and Fn was qualitatively and quantitatively monitored by ligand immunoblot assay, ELISA, and biolayer interferometry. Collectively, these results indicate that P27 is a novel Fn-binding, immunogenic adhesive protein of M. bovis, thereby contributing to the further understanding of the molecular pathogenesis of M. bovis.

The E2 glycoprotein is necessary but not sufficient for the adaptation of classical swine fever virus lapinized vaccine C-strain to the rabbit

Classical swine fever virus (CSFV) C-strain was developed through hundreds of passages of a highly virulent CSFV in rabbits. To investigate the molecular basis for the adaptation of C-strain to the rabbit (ACR), a panel of chimeric viruses with the exchange of glycoproteins E^rns, E1, and/or E2 between C-strain and the highly virulent Shimen strain and a number of mutant viruses with different amino acid substitutions in E2 protein were generated and evaluated in rabbits. Our results demonstrate that Shimen-based chimeras expressing E^rns-E1-E2, E^rns-E2 or E1-E2 but not E^rns-E1, E^rns, E1, or E2 of C-strain can replicate in rabbits, indicating that E2 in combination with either E^rns or E1 confers the ACR. Notably, E2 and the amino acids P108 and T109 in Domain I of E2 are critical in ACR. Collectively, our data indicate that E2 is crucial in mediating the ACR, which requires synergistic contribution of E^rns or E1.

A laminin-receptor-like protein regulates white spot syndrome virus infection by binding to the viral envelope protein VP28 in red claw crayfish Cherax quadricarinatus

White spot syndrome virus (WSSV) is a lethal pathogen of shrimp and many other crustaceans, which has been causing huge economic losses in global aquaculture. Laminin receptor (LR) is a cell surface receptor which participates in the interactions between cells as well as cells and extracellular matrix. Previously, we found that a CqLR-like gene was responsive to WSSV infection in the hematopoietic tissue (Hpt) cells from red claw crayfish Cherax quadricarinatus. To further reveal the role of CqLR-like gene involved in WSSV infection, the full-length cDNA of CqLR-like gene was cloned with 1000 bp, and the open reading frame encoded 308 amino acids with a conserved laminin-binding domain. Importantly, both the WSSV entry and viral replication were strongly reduced in Hpt cells after loss-of-function of CqLR-like gene by gene silencing. Protein interaction assay demonstrated that the recombinant CqLR-like protein could bind to WSSV virion in vitro by enzyme-linked immunosorbent assay and the binding affinity was in a dose-dependent manner. Furthermore, recombinant CqLR-like protein was found to bind to WSSV envelop protein VP28, but not other envelop proteins tested including VP19, VP24, and VP26, by pull down assay in HEK293T cells. In regarding to that LR is mainly localized on many types of cells' membrane, these data together suggested that CqLR-like protein was likely to function as a putative recognition molecule towards WSSV and act in the viral entry into a crustacean host cell, which may benefit the elucidation of the WSSV pathogenesis and further the pharmaceutical target for the possibly effective control of WSSV disease.

Complex Virus-Host Interactions Involved in the Regulation of Classical Swine Fever Virus Replication: A Minireview

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is one of the most devastating epizootic diseases of pigs in many countries. Viruses are small intracellular parasites and thus rely on the cellular factors for replication. Fundamental aspects of CSFV-host interactions have been well described, such as factors contributing to viral attachment, modulation of genomic replication and translation, antagonism of innate immunity, and inhibition of cell apoptosis. However, those host factors that participate in the viral entry, assembly, and release largely remain to be elucidated. In this review, we summarize recent progress in the virus-host interactions involved in the life cycle of CSFV and analyze the potential mechanisms of viral entry, assembly, and release. We conclude with future perspectives and highlight areas that require further understanding.

Host-derived apolipoproteins play comparable roles with viral secretory proteins Erns and NS1 in the infectious particle formation of Flaviviridae

Amphipathic α-helices of exchangeable apolipoproteins have shown to play crucial roles in the formation of infectious hepatitis C virus (HCV) particles through the interaction with viral particles. Among the Flaviviridae members, pestivirus and flavivirus possess a viral structural protein E^rns or a non-structural protein 1 (NS1) as secretory glycoproteins, respectively, while Hepacivirus including HCV has no secretory glycoprotein. In case of pestivirus replication, the C-terminal long amphipathic α-helices of E^rns are important for anchoring to viral membrane. Here we show that host-derived apolipoproteins play functional roles similar to those of virally encoded E^rns and NS1 in the formation of infectious particles. We examined whether E^rns and NS1 could compensate for the role of apolipoproteins in particle formation of HCV in apolipoprotein B (ApoB) and ApoE double-knockout Huh7 (BE-KO), and non-hepatic 293T cells. We found that exogenous expression of either E^rns or NS1 rescued infectious particle formation of HCV in the BE-KO and 293T cells. In addition, expression of apolipoproteins or NS1 partially rescued the production of infectious pestivirus particles in cells upon electroporation with an E^rns-deleted non-infectious RNA. As with exchangeable apolipoproteins, the C-terminal amphipathic α-helices of E^rns play the functional roles in the formation of infectious HCV or pestivirus particles. These results strongly suggest that the host- and virus-derived secretory glycoproteins have overlapping roles in the viral life cycle of Flaviviridae, especially in the maturation of infectious particles, while E^rns and NS1 also participate in replication complex formation and viral entry, respectively. Considering the abundant hepatic expression and liver-specific propagation of these apolipoproteins, HCV might have evolved to utilize them in the formation of infectious particles through deletion of a secretory viral glycoprotein gene.

The family Flaviviridae consists of 4 genera, namely Flavivirus, Pestivirus, Pegivirus, and Hepacivirus. Flaviviruses and pestiviruses can infect various species and tissues; however, infection of pegivirus and hepacivirus is observed in a strikingly restricted range of tissue and hosts. Although all the Flaviviridae viruses possess a similar genome structure, hepatitis C virus (HCV) from Hepacivirus encodes no secretory glycoprotein, such as E^rns of pestivirus and NS1 of flavivirus. The apolipoproteins, one of the host secretory glycoproteins, play important roles in the formation of infectious HCV particles through the interaction with viral particles. The data presented here show that the host-derived apolipoproteins and viral-derived E^rns and NS1 have overlapping roles in the maturation of infectious particles of Flaviviridae. Considering an abundant expression of apolipoproteins in the liver and their liver-specific propagation, HCV might have evolved to utilize the apolipoproteins in the formation of infectious particles through deletion of a gene encoding a secretory viral glycoprotein. The data of this manuscript also suggest that utilization of host factors in the viral life cycle is closely associated with the tissue- and species-specificities and evolution among Flaviviridae viruses.