Interaction of classical swine fever virus with membrane-associated heparan sulfate: role for virus replication in vivo and virulence

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.

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.

Host Cell Receptors Implicated in the Cellular Tropism of BVDV

Bovine viral diarrhea virus (BVDV) is one of the most hazardous viruses, which causes huge economic losses in the cattle industry around the world. In recent years, there has been a continuous increase in the diversity of pestivirus worldwide. As a member of the genus Pestivirus in the Flaviviridae family, BVDV has a wide range of host animals including cattle, goat, sheep, pig, camel and other cloven-hoofed animals, and it has multi-tissue tropism as well. The recognition of their permissive cells by viruses via interaction with the cellular receptors is a prerequisite for successful infection. So far, little is known about the cellular receptors essential for BVDV entry and their detailed functions during BVDV infection. Thus, discovery of the cellular receptors involved in the entry of BVDV and other pestiviruses is significant for development of the novel intervention. The viral envelope glycoprotein E^rns and E2 are crucial determinants of the cellular tropism of BVDV. The cellular proteins bound with E^rns and E2 potentially participate in BVDV entry, and their abundance might determine the cellular tropism of BVDV. Here, we summarize current knowledge regarding the cellular molecules have been described for BVDV entry, such as, complement regulatory protein 46 (CD46), heparan sulfate (HS), the low-density lipoprotein (LDL) receptor, and a disintegrin and metalloproteinase 17 (ADAM17). Furthermore, we focus on their implications of the recently identified cellular receptors for pestiviruses in BVDV life cycle. This knowledge provides a theoretical basis for BVDV prevention and treatment by targeting the cellular receptors essential for BVDV infection.

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.

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.

Fifty Shades of Erns: Innate Immune Evasion by the Viral Endonucleases of All Pestivirus Species

The genus Pestivirus, family Flaviviridae, includes four historically accepted species, i.e., bovine viral diarrhea virus (BVDV)-1 and -2, classical swine fever virus (CSFV), and border disease virus (BDV). A large number of new pestivirus species were identified in recent years. A common feature of most members is the presence of two unique proteins, N^pro and E^rns, that pestiviruses evolved to regulate the host’s innate immune response. In addition to its function as a structural envelope glycoprotein, E^rns is also released in the extracellular space, where it is endocytosed by neighboring cells. As an endoribonuclease, E^rns is able to cleave viral ss- and dsRNAs, thus preventing the stimulation of the host’s interferon (IFN) response. Here, we characterize the basic features of soluble E^rns of a large variety of classified and unassigned pestiviruses that have not yet been described. Its ability to form homodimers, its RNase activity, and the ability to inhibit dsRNA-induced IFN synthesis were investigated. Overall, we found large differences between the various E^rns proteins that cannot be predicted solely based on their primary amino acid sequences, and that might be the consequence of different virus-host co-evolution histories. This provides valuable information to delineate the structure-function relationship of pestiviral endoribonucleases.

Viral Traits and Cellular Knock-Out Genotype Affect Dependence of BVDV on Bovine CD46

The role of bovine CD46 in the host cell entry of BVDV has been established for more than a decade. By generating novel MDBK CD46 knock-out clones, we confirm previously reported data on the CD46 motives important for BVDV binding and the importance of the G479R exchange within BVDV E^rns to gain independence of bovine CD46 during entry. The comparison of different knock-out genotypes revealed a high variability of cellular susceptibility for a BVDV encoding the G479R exchange. These data highlight the effect of clonal selection of knock-outs on virus susceptibility, which should be considered when planning knock-out experiments.

A double deletion prevents replication of the pestivirus bovine viral diarrhea virus in the placenta of pregnant heifers

In contrast to wild type bovine viral diarhea virus (BVDV) specific double deletion mutants are not able to establish persistent infection upon infection of a pregnant heifer. Our data shows that this finding results from a defect in transfer of the virus from the mother animal to the fetus. Pregnant heifers were inoculated with such a double deletion mutant or the parental wild type virus and slaughtered pairwise on days 6, 9, 10 and 13 post infection. Viral RNA was detected via qRT-PCR and RNAscope analyses in maternal tissues for both viruses from day 6 p.i. on. However, the double deletion mutant was not detected in placenta and was only found in samples from animals infected with the wild type virus. Similarly, high levels of wild type viral RNA were present in fetal tissues whereas the genome of the double deletion mutant was not detected supporting the hypothesis of a specific inhibition of mutant virus replication in the placenta. We compared the induction of gene expression upon infection of placenta derived cell lines with wild type and mutant virus via gene array analysis. Genes important for the innate immune response were strongly upregulated by the mutant virus compared to the wild type in caruncle epithelial cells that establish the cell layer on the maternal side at the maternal–fetal interface in the placenta. Also, trophoblasts which can be found on the fetal side of the interface showed significant induction of gene expression upon infection with the mutant virus although with lower complexity. Growth curves recorded in both cell lines revealed a general reduction of virus replication in caruncular epithelial cells compared to the trophoblasts. Compared to the wild type virus this effect was dramtic for the mutant virus that reached only a TCID₅₀ of 1.0 at 72 hours post infection.

Here we report on animal studies elucidating mechanisms preventing the transfer of a double deletion mutant of a pestivirus to the fetus in pregnant heifers. This mutant lacks both known factors engaged in blocking the innate immune response to pestiviral infection. As shown also in earlier studies, this mutant was not detected in the fetuses at any of the tested time points in contrast to the wild-type (wt) virus. However, similar to the wt the mutant was detected in a large variety of different maternal tissues. The only exception was the placenta where only wt but not mutant virus was detected. Using gene array analyses we showed that infection of two cell lines derived either from the maternal or the fetal site of the maternal-fetal interface with the mutant virus induces a significant antiviral gene expression response. The reaction of cells from the maternal side was more complex and virus replication in these cells was reduced, almost completly blocking the mutant virus. These results support the hypothesis that replication of the mutant virus is blocked in the placenta due to a highly active innate immune response and the prevention of replication also blocks transfer of the virus to the fetus.

Positively Charged Amino Acids in the Pestiviral Erns Control Cell Entry, Endoribonuclease Activity and Innate Immune Evasion

The genus Pestivirus, family Flaviviridae, includes four economically important viruses of livestock, i.e., bovine viral diarrhea virus-1 (BVDV-1) and -2 (BVDV-2), border disease virus (BDV) and classical swine fever virus (CSFV). E^rns and N^pro, both expressed uniquely by pestiviruses, counteract the host's innate immune defense by interfering with the induction of interferon (IFN) synthesis. The structural envelope protein E^rns also exists in a soluble form and, by its endoribonuclease activity, degrades immunostimulatory RNA prior to their activation of pattern recognition receptors. Here, we show that at least three out of four positively-charged residues in the C-terminal glycosaminoglycan (GAG)-binding site of BVDV-E^rns are required for efficient cell entry, and that a positively charged region more upstream is not involved in cell entry but rather in RNA-binding. Moreover, the C-terminal domain on its own determines intracellular targeting, as GFP fused to the C-terminal amino acids of E^rns was found at the same compartments as wt E^rns. In summary, RNase activity and uptake into cells are both required for E^rns to act as an IFN antagonist, and the C-terminal amphipathic helix containing the GAG-binding site determines the efficiency of cell entry and its intracellular localization.

The Erns Carboxyterminus: Much More Than a Membrane Anchor

Pestiviruses express the unique essential envelope protein E^rns, which exhibits RNase activity, is attached to membranes by a long amphipathic helix, and is partially secreted from infected cells. The RNase activity of E^rns is directly connected with pestivirus virulence. Formation of homodimers and secretion of the protein are hypothesized to be important for its role as a virulence factor, which impairs the host's innate immune response to pestivirus infection. The unusual membrane anchor of E^rns raises questions with regard to proteolytic processing of the viral polyprotein at the E^rns carboxy-terminus. Moreover, the membrane anchor is crucial for establishing the critical equilibrium between retention and secretion and ensures intracellular accumulation of the protein at the site of virus budding so that it is available to serve both as structural component of the virion and factor controlling host immune reactions. In the present manuscript, we summarize published as well as new data on the molecular features of E^rns including aspects of its interplay with the other two envelope proteins with a special focus on the biochemistry of the E^rns membrane anchor.

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.

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.

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.

Foot-and-mouth disease virus VP1 target the MAVS to inhibit type-I interferon signaling and VP1 E83K mutation results in virus attenuation

VP1, a pivotal capsid protein encoded by the foot-and-mouth disease virus (FMDV), plays an important role in receptor-mediated attachment and humoral immune responses. Previous studies show that amino acid changes in the VP1 protein of cell culture-adapted strains of FMDV alter the properties of the virus. In addition, FMDV VP1 modulates host IFN signal transduction. Here, we examined the ability of cell culture-adapted FMDV VP1(83K) and wild-type FMDV VP1(83E) to evade host immunity by blocking mitochondrial antiviral signaling protein (MAVS)/TNF Receptor Associated Factor 3 (TRAF3) mediated cellular innate responses. Wild-type FMDV VP1(83E) interacted specifically with C-terminal TRAF3-binding site within MAVS and this interaction inhibited binding of TRAF3 to MAVS, thereby suppressing interferon-mediated responses. This was not observed for cell culture-adapted FMDV VP1(83K). Finally, chimeric FMDV harboring VP1(83K) showed very low pathogenicity in pigs. Collectively, these data highlight a critical role of VP1 with respect to suppression of type-I IFN pathway and attenuation of FMDV by the E83K mutation in VP1.

Foot-and-Mouth disease (FMD), a highly contagious viral disease of cloven-hoofed animals, causes huge economic losses. To generate a FMD vaccine, cell culture-adapted strains of FMDV that show improved growth properties and allow repeated passage are needed. Generally, adaptation of field-isolated FMDV is accompanied by changes in viral properties, including amino acid mutations. A VP1 E83K mutation in cell culture-adapted FMDV was identified previously; here, we examined the impact of VP1 E83K on virus pathogenicity and type-I IFN pathway. Cell culture-adapted FMDV O1 Manisa, and highly virulent strain of O/Andong/SKR/2010, acquired the E83K mutation in the VP1 protein, which attenuated the virus via disposing VP1 mediate negative regulation ability of host cellular IFN responses. The data suggest a rational approach to viral propagation in cell culture and virus attenuation, which could be utilized for future development of FMDV vaccines.

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.

A CRISPR/Cas9 Generated Bovine CD46-knockout Cell Line-A Tool to Elucidate the Adaptability of Bovine Viral Diarrhea Viruses (BVDV)

Bovine viral diarrhea virus (BVDV) entry into a host cell is mediated by the interaction of the viral glycoprotein E2 with the cellular transmembrane CD46 receptor. In this study, we generated a stable Madin-Darby Bovine Kidney (MDBK) CD46-knockout cell line to study the ability of different pestivirus A and B species (BVDV-1 and -2) to escape CD46-dependent cell entry. Four different BVDV-1/2 isolates showed a clearly reduced infection rate after inoculation of the knockout cells. However, after further passaging starting from the remaining virus foci on the knockout cell line, all tested virus isolates were able to escape CD46-dependency and grew despite the lack of the entry receptor. Whole-genome sequencing of the escape-isolates suggests that the genetic basis for the observed shift in infectivity is an amino acid substitution of an uncharged (glycine/asparagine) for a charged amino acid (arginine/lysine) at position 479 in the E^RNS in three of the four isolates tested. In the fourth isolate, the exchange of a cysteine at position 441 in the E^RNS resulted in a loss of E^RNS dimerization that is likely to influence viral cell-to-cell spread. In general, the CD46-knockout cell line is a useful tool to analyze the role of CD46 for pestivirus replication and the virus-receptor interaction.

Proteome Analysis Reveals Syndecan 1 Regulates Porcine Sapelovirus Replication

Porcine sapelovirus A (PSV) is a single stranded, positive-sense, non-enveloped RNA virus that causes enteritis, pneumonia, polioencephalomyelitis, and reproductive disorders in pigs. Research on PSV infection and interaction with host cells is unclear. In this study, we applied tandem mass tag proteomics analysis to investigate the differentially expressed proteins (DEPs) in PSV-infected pig kidney (PK)-15 cells and explored the interactions between PSV and host cells. Here we mapped 181 DEPs, including 59 up-regulated and 122 down-regulated DEPs. Among them, osteopontin (SPP1), induced protein with tetratricopeptide repeats 5 (IFIT5), ISG15 ubiquitin-like modifier (ISG15), vinculin (VCL), and syndecan-1 (SDC1) were verified significantly changed using RT-qPCR. Additionally, overexpression of SDC1 promoted PSV viral protein (VP)1 synthesis and virus titer, and silencing of SDC1 revealed the opposite results. Our findings show that SDC1 is a novel host protein and plays crucial roles in regulating PSV replication.

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.

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.

N-terminus of Classical swine fever virus strain TD96 glycoprotein Erns contains a potential heparin-binding domain

Classical swine fever virus (CSFV) envelope glycoprotein E^rns has been shown to bind to cell surface sulphated-heparin-like glycosaminoglycans (GAGs), which participate in cell attachment of the virus. In this study, the CSFV E^rns gene was codon optimized for expression in the yeast Pichia pastoris. A C-terminally truncated E^rns recombinant protein lacking the previously identified heparin-binding domain (HBD) bound to heparin column, suggesting the presence of another HBD in CSFV E^rns. Sequence analyses of the CSFV E^rns coding region revealed a common potential N-terminal HBD at residues 301－311. Site-directed mutagenesis of the basic amino acids at K³⁰³ and K³⁰⁶ significantly reduced the heparin-binding affinity of the protein. Further mutations of both T³¹⁰ and H³¹¹ had little effect. Thus, a novel potential heparin-binding site near the N-terminus of CSFV strain TD96 E^rns has been detected, and the two basic amino acids K³⁰³ and K³⁰⁶ are crucial for binding activity to heparin matrix and cell-surface GAGs.

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.

Detection of classical swine fever virus E2 gene in cattle serum samples from cattle herds of Meghalaya

The present study focused on the detection and genetic characterisation of 5' untranslated region (5'UTR) and E2 gene of classical swine fever virus (CSFV, family Flaviviridae, genus Pestivirus) from bovine population of the northeastern region of India. A total of 134 cattle serum samples were collected from organised cattle farms and were screened for CSFV antigen with a commercial antigen capture enzyme linked immunosorbent assay (Ag-ELISA) and reverse transcription-polymerase chain reaction (RT-PCR). A total of 10 samples were positive for CSFV antigen by ELISA, while all of them were positive in PCR for 5'UTR region. Full length E2 region of CSFV were successfully amplified from two positive samples and used for subsequent phylogenetic analysis and determination of protein 3D structure which showed similarity with reported CSFV isolate from Assam of sub-genogroup 2.1, with minor variations in protein structure.

Npro His49 and Erns Lys412 mutations in pig bovine viral diarrhea virus type 2 synergistically enhance the cellular antiviral response

Type I interferons are major components of the innate immune response of hosts, and accordingly, many viruses have evolved mechanisms to modulate the host response during infection. Bovine viral diarrhea virus (BVDV) nonstructural protein N^pro and structural protein E^rns play important roles in inhibiting type I interferon. The aim of this study was to explore the epistatic effects of amino acid mutations in N^pro and E^rns in porcine ST cells to characterize the immune response induced by BVDV-2. Plasmids with mutant amino acids His49 (H49), Glu22 (E22) in N^pro, and His300 (H300), Lys412 (K412) in E^rns which had been changed to Alanine (A) had similar effects on type I interferon production in MDBK and ST cells, but resulted in much greater ISG15, OAS, and Mx production in ST cells. The rescued vASH/N^proH49E^rnsK412 virus showed the best efficiency with respect to modulating antiviral cytokines, indicating that the amino acids N^pro H49 and E^rns K412 had highly synergistic effects in abolishing the ability to inhibit type I interferon. These findings have importance practical implications owing to the increasing prevalence of BVDV infections, including persistent infections, in domestic pigs.

Identification of the linear ligand epitope on classical swine fever virus that interacts with porcine kidney 15 cells

Binding of the viral ligand to a specific receptor is the first step of virus entry into target cells. The envelope proteins E^rns, E1, and E2 of classical swine fever virus (CSFV) are involved in the interaction with host cell receptors to mediate CSFV infection. The aim of this investigation was to identify epitopes that bind to porcine kidney (PK)-15 cells to prevent CSFV infection. Ten peptides representing E^rns, E1, and E2 were synthesized. Immunohistochemical study showed that the SE24 peptide, which is derived from the E2 amino acid sequence, could effectively bind to PK-15 cells. Similarly, a flow cytometry assay demonstrated that SE24 binding to PK-15 cells could be blocked by CSFV. The binding of SE24 with PK-15 cells leads to decreased CSFV infection of PK-15 cells in a dose-dependent manner. These results suggest a potential new strategy for the prevention and control of CSFV infection that requires further investigation.

Classical Swine Fever-An Updated Review

Classical swine fever (CSF) remains one of the most important transboundary viral diseases of swine worldwide. The causative agent is CSF virus, a small, enveloped RNA virus of the genus Pestivirus. Based on partial sequences, three genotypes can be distinguished that do not, however, directly correlate with virulence. Depending on both virus and host factors, a wide range of clinical syndromes can be observed and thus, laboratory confirmation is mandatory. To this means, both direct and indirect methods are utilized with an increasing degree of commercialization. Both infections in domestic pigs and wild boar are of great relevance; and wild boars are a reservoir host transmitting the virus sporadically also to pig farms. Control strategies for epidemic outbreaks in free countries are mainly based on classical intervention measures; i.e., quarantine and strict culling of affected herds. In these countries, vaccination is only an emergency option. However, live vaccines are used for controlling the disease in endemically infected regions in Asia, Eastern Europe, the Americas, and some African countries. Here, we will provide a concise, updated review on virus properties, clinical signs and pathology, epidemiology, pathogenesis and immune responses, diagnosis and vaccination possibilities.

Complete genome sequence of a novel sub-subgenotype 2.1g isolate of classical swine fever virus from China

Current subgenotype 2.1 isolates of classical swine fever virus (CSFV) play a dominant role in CSF outbreaks in China, and a novel sub-subgenotype 2.1g of CSFV was recently identified, but the complete genome sequence of this new sub-subgenotype has not been reported. In this study, complete genome of 2.1g isolate GD19/2011 collected from Guangdong province of China in 2011 was sequenced. It was found to be 12,298 nucleotides (nt) in length, including a 375-nt 5'UTR, a 11,697-nt opening reading frame (ORF), and a 227-nt 3'UTR. GD19/2011 shared 91.0-93.7 % and 95.6-97.5 % nt and amino acid sequence identity, respectively, with other subgenotype 2.1 isolates. The topology of a phylogenetic tree constructed based on complete genome sequences of GD19/2011 and other CSFV isolates was identical to that obtained with full-length E2 gene sequences, but it was significantly different from those obtained with the 5'UTR and core sequences. Serial passages of GD9/2011 in PK-15 cells generated a highly cell-adapted virus stock with an infectious titer of 10(7.8) TCID50/ml at the 12(th) passage in which two amino acid substitutions, S476R and N2494S, were observed in comparison with the complete polyprotein sequence of the original isolate from kidney tissue, GD19/2011. This is the first report of the complete genome sequence of a 2.1g isolate, and the GD19/2011 isolate will be useful for further analysis of the evolution and virulence of CSFV isolates.

In vitro adaptation and genome analysis of a sub-subgenotype 2.1c isolate of classical swine fever virus

Classical swine fever (CSF) still causes substantial economic losses in the pig industry in China. This study reports the isolation and characterization of a field CSF virus named GD53/2011 from pig kidney tissue collected during a CSF outbreak in Guangdong province, China. Phylogenetic analysis based on the full-length E2 gene sequence revealed that this isolate belongs to CSFV sub-subgenotype 2.1c. To further understand the replication characteristics, GD53/2011 was subsequently adapted in PK-15 cells, and its full-length genome was sequenced. After adaptation in PK-15 cells, the titer of GD53/2011 was significantly increased from 10(3.39) TCID50/ml at passage 6 (F6) to 10(8.50) TCID50/ml at passage 46 (F46) with the peak titer obtained at 48 h post-inoculation. Sequence comparison revealed that the E(rns) gene at passages 6, 15, and 25 of GD53/2011 was identical to that in the original tissue, but one amino acid substitution (S476R) was detected at passages 35 and 46. Furthermore, E2 gene sequences at passages 6, 15, 25, 35, and 46 was found identical to that in the original tissue, indicating that the E2 gene was stable during CSF virus adaptation in PK-15 cells. Full-length protein sequence comparison of GD53/2011 with other 2.1 sub-subgenotype isolates showed that Core and NS5A, rather than E2, are more genetically variable. Taken together, a field CSFV strain GD53/2011 was isolated, fully sequenced, and adapted to high growth titer in PK-15 cells, which might be suitable for future studies on CSFV infection, replication, and vaccine development.

Different evolutionary patterns of classical swine fever virus envelope proteins

Classical swine fever virus (CSFV) is the causative agent of classical swine fever, which is a highly contagious disease of the domestic pig as well as wild boar. The proteins E(rns), E1, and E2 are components of the viral envelope membrane. They are also implicated in virus attachment and entry, replication, and (or) anti-immune response. Here, we studied the genetic variations of these envelope proteins in the evolution of CSFV. The results reveal that the envelope proteins underwent different evolutionary fates. In E(rns) and E1, but not E2, a number of amino acid sites experienced functional divergence. Furthermore, the diversification in E(rns) and E1 was generally episodic because the divergence-related changes of E1 only occurred with the separation of 2 major groups of CSFV and that of E(rns) took place with the division of 1 major group. The major divergence-related sites of E(rns) are located on one of the substrate-binding regions of the RNase domain and C-terminal extension. These functional domains have been reported to block activation of the innate immune system and attachment and entry into host cells, respectively. Our results may shed some light on the divergent roles of the envelope proteins.

Structures and Functions of Pestivirus Glycoproteins: Not Simply Surface Matters

Pestiviruses, which include economically important animal pathogens such as bovine viral diarrhea virus and classical swine fever virus, possess three envelope glycoproteins, namely E^rns, E1, and E2. This article discusses the structures and functions of these glycoproteins and their effects on viral pathogenicity in cells in culture and in animal hosts. E2 is the most important structural protein as it interacts with cell surface receptors that determine cell tropism and induces neutralizing antibody and cytotoxic T-lymphocyte responses. All three glycoproteins are involved in virus attachment and entry into target cells. E1-E2 heterodimers are essential for viral entry and infectivity. E^rns is unique because it possesses intrinsic ribonuclease (RNase) activity that can inhibit the production of type I interferons and assist in the development of persistent infections. These glycoproteins are localized to the virion surface; however, variations in amino acids and antigenic structures, disulfide bond formation, glycosylation, and RNase activity can ultimately affect the virulence of pestiviruses in animals. Along with mutations that are driven by selection pressure, antigenic differences in glycoproteins influence the efficacy of vaccines and determine the appropriateness of the vaccines that are currently being used in the field.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

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

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), a highly contagious, economically important viral disease in many countries. The E(rns) and E2 envelope glycoproteins are responsible for the binding to and entry into the host cell by CSFV. To date, only one cellular receptor, heparan sulfate (HS), has been identified as being involved in CSFV attachment. HS is also present on the surface of various cells that are nonpermissive to CSFV. Hence, there must be another receptor(s) that has been unidentified to date. In this study, we used a set of small interfering RNAs (siRNAs) against a number of porcine cell membrane protein genes to screen cellular proteins involved in CSFV infection. This approach resulted in the identification of several proteins, and of these, the laminin receptor (LamR) has been demonstrated to be a cellular receptor for several viruses. Confocal analysis showed that LamR is colocalized with CSFV virions on the membrane, and a coimmunoprecipitation assay indicated that LamR interacts with the CSFV E(rns) protein. In inhibition assays, anti-LamR antibodies, soluble laminin, or LamR protein significantly inhibited CSFV infection in a dose-dependent manner. Transduction of PK-15 cells with a recombinant lentivirus expressing LamR yielded higher viral titers. Moreover, an attachment assay demonstrated that LamR functions during virus attachment. We also demonstrate that LamR acts as an alternative attachment receptor, especially in SK6 cells. These results indicate that LamR is a cellular attachment receptor for CSFV.

IMPORTANCE

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), an economically important viral disease affecting the pig industry in many countries. To date, only heparan sulfate (HS) has been identified to be an attachment receptor for CSFV. Here, using RNA interference screening with small interfering RNAs (siRNAs) against a number of porcine membrane protein genes, we identified the laminin receptor (LamR) to be another attachment receptor. We demonstrate the involvement of LamR together with HS in virus attachment, and we elucidate the relationship between LamR and HS. LamR also serves as an attachment receptor for many viral pathogens, including dengue virus, a fatal human flavivirus. The study will help to enhance our understanding of the life cycle of flaviviruses and the development of antiviral strategies for flaviviruses.

Porcine complement regulatory protein CD46 and heparan sulfates are the major factors for classical swine fever virus attachment in vitro

Classical swine fever virus (CSFV) is the causative agent of a severe multi-systemic disease of pigs. While several aspects of virus-host-interaction are known, the early steps of infection remain unclear. For the closely related bovine viral diarrhea virus (BVDV), a cellular receptor is known: bovine complement regulatory protein CD46. Given that these two pestiviruses are closely related, porcine CD46 is also a candidate receptor for CSFV. In addition to CD46, cell-culture-adapted CSFV strains have been shown to use heparan sulfates as an additional cellular factor. In the present study, the interaction of field-type and cell-culture-adapted CSFV with a permanent porcine cell line or primary macrophages was assessed using anti-porcine CD46 monoclonal antibodies and a heparan-sulfate-blocking compound, DSTP-27. The influence of receptor blocking was assessed using virus titration and quantitative PCR. Treatment of cells with monoclonal antibodies against porcine CD46 led to a reduction of viral growth in both cell types. The effect was most pronounced with field-type CSFV. The blocking could be enhanced by addition of DSTP-27, especially for cell-culture-adapted CSFV. The combined use of both blocking agents led to a significant reduction of viral growth but was also not able to abolish infection completely. The results obtained in this study showed that both porcine CD46 and heparan sulfates play a major role in the initial steps of CSFV infection. Additional receptors might also play a role for attachment and entry; however, their impact is obviously limited in vitro in comparison to CD46 and heparan sulfates.

The mechanism of HCV entry into host cells

Hepatitis C virus (HCV) is an enveloped, positive strand RNA virus classified within the Flaviviridae family and is a major cause of liver disease worldwide. HCV life cycle and propagation are tightly linked to several aspects of lipid metabolism. HCV propagation depends on and also shapes several aspects of lipid metabolism such as cholesterol uptake and efflux through different lipoprotein receptors during its entry into cells, lipid metabolism modulating HCV genome replication, lipid droplets acting as a platform for recruitment of viral components, and very low density lipoprotein assembly pathway resulting in incorporation of neutral lipids and apolipoproteins into viral particles. During the first steps of infection, HCV enters hepatocytes through a multistep and slow process. The initial capture of HCV particles by glycosaminoglycans and/or lipoprotein receptors is followed by coordinated interactions with the scavenger receptor class B type I, a major receptor of high-density lipoprotein, the CD81 tetraspanin, and the tight junction proteins Claudin-1 and Occludin. This tight concert of receptor interactions ultimately leads to uptake and cellular internalization of HCV through a process of clathrin-dependent endocytosis. Over the years, the identification of the HCV entry receptors and cofactors has led to a better understanding of HCV entry and of the narrow tropism of HCV for the liver. Yet, the role of the two HCV envelope glycoproteins, E1 and E2, remains ill-defined, particularly concerning their involvement in the membrane fusion process. Here, we review the current knowledge and advances addressing the mechanism of HCV cell entry within hepatocytes and we highlight the challenges that remain to be addressed.

Integrin β3 is required in infection and proliferation of classical swine fever virus

Classical Swine Fever (CSF) is a highly infectious fatal pig disease, resulting in huge economic loss to the swine industry. Integrins are membrane-bound signal mediators, expressed on a variety of cell surfaces and are known as receptors or co-receptors for many viruses. However, the role of integrin β3 in CSFV infection is unknown. Here, through quantitive PCR, immunofluorescence (IFC) and immunocytohistochemistry (ICC), we revealed that ST (swine testicles epithelial) cells have a prominent advantage in CSFV proliferation as compared to EC (swine umbilical vein endothelial cell), IEC (swine intestinal epithelial cell) and PK (porcine kidney epithelial) cells. Meanwhile, ST cells had remarkably more integrin β3 expression as compared to EC, IEC and PK cells, which was positively correlated with CSFV infection and proliferation. Integrin β3 was up-regulated post CSFV infection in all the four cell lines, while the CSFV proliferation rate was decreased in integrin β3 function-blocked cells. ShRNA1755 dramatically decreased integrin β3, with a deficiency of 96% at the mRNA level and 80% at the protein level. CSFV proliferation was dramatically reduced in integrin β3 constantly-defected cells (ICDC), with the deficiencies of 92.6%, 99% and 81.7% at 24 h, 48 h and 72 h post CSFV infection, respectively. These results demonstrate that integrin β3 is required in CSFV infection and proliferation, which provide a new insight into the mechanism of CSFV infection.

Interplay between basic residues of hepatitis C virus glycoprotein E2 with viral receptors, neutralizing antibodies and lipoproteins

Positively-charged amino acids are located at specific positions in the envelope glycoprotein E2 of the hepatitis C virus (HCV): two histidines (H) and four arginines (R) in two conserved WHY and one RGERCDLEDRDR motifs, respectively. Additionally, the E2 hypervariable region 1 (HVR1) is rich in basic amino acids. To investigate the role(s) of these residues in HCV entry, we subjected to comparative infection and sedimentation analysis cell culture-produced (HCVcc, genotype 2a) wild type virus, a panel of alanine single-site mutants and a HVR1-deletion variant. Initially, we analyzed the effects of these mutations on E2-heparan sulfate (HS) interactions. The positive milieu of the HVR1, formulated by its basic amino acids (key residues the conserved H³⁸⁶ and R⁴⁰⁸), and the two highly conserved basic residues H⁴⁸⁸ and R⁶⁴⁸ contributed to E2-HS interactions. Mutations in these residues did not alter the HCVcc-CD81 entry, but they modified the HCVcc-scavenger receptor class B type I (SR-BI) dependent entry and the neutralization by anti-E2 or patients IgG. Finally, separation by density gradients revealed that mutant viruses abolished partially or completely the infectivity of low density particles, which are believed to be associated with lipoproteins. This study shows that there exists a complex interplay between the basic amino acids located in HVR1 and other conserved E2 motifs with the HS, the SR-BI, and neutralizing antibodies and suggests that HCV-associated lipoproteins are implicated in these interactions.

Crystal structure of the pestivirus envelope glycoprotein E(rns) and mechanistic analysis of its ribonuclease activity

Pestiviruses, which belong to the Flaviviridae family of RNA viruses, are important agents of veterinary diseases causing substantial economical losses in animal farming worldwide. Pestivirus particles display three envelope glycoproteins at their surface: E(rns), E1, and E2. We report here the crystal structure of the catalytic domain of E(rns), the ribonucleolytic activity of which is believed to counteract the innate immunity of the host. The structure reveals a three-dimensional fold corresponding to T2 ribonucleases from plants and fungi. Cocrystallization experiments with mono- and oligonucleotides revealed the structural basis for substrate recognition at two binding sites previously identified for T2 RNases. A detailed analysis of poly-U cleavage products using (31)P-NMR and size exclusion chromatography, together with molecular docking studies, provides a comprehensive mechanistic picture of E(rns) activity on its substrates and reveals the presence of at least one additional nucleotide binding site.

Propagation of classical swine fever virus in vitro circumventing heparan sulfate-adaptation

Amplification of natural virus isolates in permanent cell lines can result in adaptation, in particular enhanced binding to heparan sulfate (HS)-containing glycosaminoglycans present on most vertebrate cells. This has been reported for several viruses, including the pestivirus classical swine fever virus (CSFV), the causative agent of a highly contagious hemorrhagic disease in pigs. Propagation of CSFV in cell culture is essential in virus diagnostics and research. Adaptation of CSFV to HS-binding has been related to amino acid changes in the viral E(rns) glycoprotein, resulting in viruses with altered replication characteristics in vitro and in vivo. Consequently, a compound blocking the HS-containing structures on cell surfaces was employed to monitor conversion from HS-independency to HS-dependency. It was shown that the porcine PEDSV.15 cell line permitted propagation of CSFV within a limited number of passages without adaptation to HS-binding. The selection of HS-dependent CSFV mutants was also prevented by propagation of the virus in the presence of DSTP 27. The importance of these findings can be seen from the altered ratio of cell-associated to secreted virus upon acquisition of enhanced HS-binding affinity, a phenotype proposed previously to be related to virulence in the natural host.

High-resolution epitope mapping for monoclonal antibodies to the structural protein Erns of classical swine fever virus using peptide array and random peptide phage display approaches

The structural glycoprotein E(rns) (an envelope protein with RNase activity) of classical swine fever virus (CSFV) is not well characterized with respect to its antigenic structure and organization. Here, we investigated the antigenic sites on E(rns) by raising mAbs against the Escherichia coli expressed E(rns) of CSFV strain Alfort/187 and defined the B-cell epitopes recognized by these antibodies. Eighteen mAbs to E(rns) were identified and they were classified as either immunoglobulin subclass G1 or G2b. Using an array of overlapping 12-mer peptides, spanning aa 27-227 of E(rns), the epitopes for 12 mAbs were mapped to a high resolution of six to eight residues, which cluster in five discrete locations, ¹³GIWPEKIC³⁸ (group I), ⁶⁵NYTCCKLQ⁷² (group II), ¹²⁷QARNRPTT¹³⁴ (group III), ¹⁴⁵SFAGTVIE¹⁵² (group IV) and ¹⁶¹VEDILY¹⁶⁶ (group V). Two mAbs recognize two or more antigenic determinants, including the group II epitope. The epitopes for four other mAbs could not be mapped using the overlapping 12-mer peptides. Random peptide phage display with one mAb from each of all the groups except group V further identified some conserved residues that may be critical for binding antibodies, i.e. Trp³³ in the epitope of group I, Leu⁷¹ in the epitope of group II, Gln¹²⁷ and Apn¹³⁰ in the epitope of group III, and Ser¹⁴⁵ and Gly¹⁴⁸ in the epitope of group IV. This study has provided new insights into the structure and organization of epitopes on the CSFV E(rns) and valuable epitope information for the rational design of vaccines, drugs and diagnostic immunoassays for CSFV.

A novel function of heparan sulfate in the regulation of cell-cell fusion

Despite the important contribution of cell-cell fusion in the development and physiology of eukaryotes, little is known about the mechanisms that regulate this process. Our study shows that glycosaminoglycans and more specifically heparan sulfate (HS) expressed on the cell surface and extracellular matrix may act as negative regulator of cell-cell fusion. Using herpes simplex virus type-1 as a tool to enhance cell-cell fusion, we demonstrate that the absence of HS expression on the cell surface results in a significant increase in cell-cell fusion. An identical phenomenon was observed when other viruses or polyethylene glycol was used as fusion enhancer. Cells deficient in HS biosynthesis showed increased activity of two Rho GTPases, RhoA and Cdc42, both of which showed a correlation between increased activity and increased cell-cell fusion. This could serve as a possible explanation as to why HS-deficient cells showed significantly enhanced cell-cell fusion and suggests that HS could regulate fusion via fine tuning of RhoA and Cdc42 activities.

Mutation of cysteine 171 of pestivirus E rns RNase prevents homodimer formation and leads to attenuation of classical swine fever virus

Pestiviruses represent important pathogens of farm animals that have evolved unique strategies and functions to stay within their host populations. E(rns), a structural glycoprotein of pestiviruses, exhibits RNase activity and represents a virulence factor of the viruses. E(rns) forms disulfide linked homodimers that are found in virions and virus-infected cells. Mutation or deletion of cysteine 171, the residue engaged in intermolecular disulfide bond formation, results in loss of dimerization as tested in coprecipitation and native protein gel electrophoresis analyses. Nevertheless, stable virus mutants with changes affecting cysteine codon 171 could be recovered in tissue culture. These mutants grew almost as well as the parental viruses and exhibited an RNase-positive phenotype. E(rns) dimerization-negative mutants of classical swine fever virus were found to be attenuated in pigs even though the virus clearly replicated and induced a significant neutralizing antibody response in the animals.

Formation of bovine viral diarrhea virus E1-E2 heterodimers is essential for virus entry and depends on charged residues in the transmembrane domains

The envelope of bovine viral diarrhea virus (BVDV) contains the glycoproteins Erns, E1 and E2. Complementation of a recombinant vesicular stomatitis virus (VSV) with BVDV glycoproteins resulted in infectious pseudotyped viruses. To elucidate the specific role of each of the single envelope glycoproteins during viral entry, pseudotypes were generated bearing the BVDV envelope proteins in different combinations. Pseudoviruses that contained E1 and E2 but not Erns were infectious, indicating that Erns is dispensable for virus entry. VSV/BVDV pseudotypes with chimeric proteins (the ectodomain of the BVDV glycoprotein and the transmembrane domain of the VSV-G protein) were not infectious. The fact that E1-E2 heterodimers were not detected if one of the proteins was chimeric indicated that the heterodimers are crucial for BVDV entry. It was shown by site-directed mutagenesis that the charged amino acids in the transmembrane domains of BVDV E1 (lysine and arginine) and the charged amino acid in the transmembrane domain of E2 (arginine) play a key role in heterodimer formation. Pseudoviruses bearing the mutation E2-R/A, where the charged amino acid was substituted by alanine, were not infectious, supporting the hypothesis that E1-E2 heterodimers are essential for BVDV entry.

Functionality of Chimeric E2 Glycoproteins of BVDV and CSFV in Virus Replication

An intriguing difference between the E2 glycoprotein of CSFV and the other groups of pestiviruses (nonCSFV) is a lack of two cysteine residues on positions cysteine 751 and 798. Other groups of pestivirus are not restricted to one species as swine, whereas CSFV is restricted to swine and wild boar. We constructed chimeric CSFV/BVDV E2 genes based on a 2D model of E2 proposed by van Rijn et al. (van Rijn et al. 1994, J Virol 68, 3934–42) and confirmed their expression by immunostaining of plasmid-transfected SK6 cells. No equivalents for the antigenic units B/C and A were found on E2 of BVDVII. This indicates major structural differences in E2. However, the immunodominant BVDVII domain A, containing epitopes with essential amino acids between position 760–764, showed to be dependent on the presence of the region defined by amino acids 684 to 796. As for the A domain of CSFV, the BVDVII A-like domain seemed to function as a separate unit. These combined domains in E2 proved to be the only combination which was functional in viral background of CSFV C-strain. The fitness of this virus (vflc36BVDVII 684–796) seemed to be reduced compared to vflc9 (with the complete antigenic region of BVDVII).

The pestivirus glycoprotein Erns is anchored in plane in the membrane via an amphipathic helix

E(rns) is a structural glycoprotein of pestiviruses found to be attached to the virion and to membranes within infected cells via its COOH terminus, although it lacks a hydrophobic anchor sequence. The COOH-terminal sequence was hypothesized to fold into an amphipathic alpha-helix. Alanine insertion scanning revealed that the ability of the E(rns) COOH terminus to bind membranes is considerably reduced by the insertion of a single amino acid at a wide variety of positions. Mutations decreasing the hydrophobicity of the apolar face of the putative helix led to reduction of membrane association. Proteinase K protection assays showed that E(rns) translated in vitro in the presence of microsomal membranes was protected, whereas a mutant with an artificial transmembrane region and a short cytosolic tag was shortened by the protease treatment. A tag fused to the COOH terminus of wild type E(rns) was not accessible for antibodies within digitonin-permeabilized cells, but the variant with the tag located downstream of the artificial transmembrane region was detected under the same conditions. These results are in accordance with the model that the COOH-terminal membrane anchor of E(rns) represents an amphipathic helix embedded in plane into the membrane. The integrity of the membrane anchor was found to be important for recovery of infectious virus.

The roles of CD81 and glycosaminoglycans in the adsorption and uptake of infectious HCV particles

Because appropriate cell-culture systems or small-animal models have been lacking, the early steps in the HCV life cycle have been difficult to study. A cell culture system was developed recently that allows production of infectious HCV. In this study, infectious HCV particles produced in cultured cells were used. To clarify the role of CD81 in HCV attachment and entry, the effect of anti-CD81 antibody was examined. The antibody blocked HCV virion entry but not particle attachment. Only the fraction bound to a heparin affinity column and eluted with 0.3 M NaCl productively infected Huh7 cells, indicating that infectious HCV particles bind to heparin. Both heparin treatment of the virus particles and heparinase treatment of the Huh7 cells reduced virus-cell binding without substantially inhibiting HCV infectivity. Finally, to confirm the role of both heparin sulfate proteoglycan (HSPG) and CD81 in HCV entry, the effects of heparinase I and anti-CD81 antibody were analyzed. No productive RNA replication was detected in the Huh7 cells in the presence of both heparinase I and anti-CD81 antibody. In conclusion, these data suggested that both HSPG and CD81 are important for HCV entry. HSPG may play a role in the initial cell surface binding of infectious HCV particles and CD81 is conceivably correlated with HCV entry after viral attachment.

Interaction between respiratory syncytial virus and glycosaminoglycans, including heparan sulfate

Glycosaminoglycans (GAGs), including heparan sulfate (HS), are expressed on the surface of nearly all cells, linked to transmembrane proteins. These GAGs are sulfated to varying extents, lending a negative charge, and are used by a large number of viruses to initiate infection of immortalized cell lines. Here we describe the rationale and methods for analyzing GAG usage by one such virus, respiratory syncytial virus (RSV). The protocols presented allow the determination of which GAG(s) is employed by the virus, which GAG modification(s) is important, and whether the important GAG is on the cell or on the virus. We also discuss the finding that many viruses are selected for GAG usage during passage in culture and present a method for rapidly determining whether GAG usage is characteristic of a wild virus or is limited to laboratory-adapted virus.

Role of the low-density lipoprotein receptor in entry of bovine viral diarrhea virus

Among several proposed cellular receptors for bovine viral diarrhea virus (BVDV), the low-density lipoprotein (LDL) receptor is of special interest because it is also considered a receptor for the related hepatitis C virus. It has been reported that an anti-LDL receptor monoclonal antibody blocked the infection of bovine cells by BVDV and that the resistance of bovine CRIB cells (cells resistant to infection with BVDV) (E. F. Flores and R. O. Donis, Virology 208:565-575, 1995) to BVDV infection was due to a lack of the LDL receptor (V. Agnello et al., Proc. Natl. Acad. Sci. USA 96:12766-12771, 1999). In connection with our studies on BVDV entry, we reevaluated the putative role of the LDL receptor as a cellular receptor for BVDV. It was first clearly demonstrated that neither of two monoclonal antibodies against the LDL receptor inhibited BVDV infection of two bovine cell lines. Furthermore, the LDL receptor was detected on the surface of CRIB cells. The functionality of the LDL receptor on CRIB cells was demonstrated by the internalization of fluorescently labeled LDL. In conclusion, at present no experimental evidence supports an involvement of the LDL receptor in BVDV invasion.

Identification of a novel virulence determinant within the E2 structural glycoprotein of classical swine fever virus

Classical swine fever virus (CSFV) E2 glycoprotein contains a discrete epitope (TAVSPTTLR, residues 829-837 of CSFV polyprotein) recognized by monoclonal antibody (mAb) WH303, used to differentiate CSFV from related ruminant pestiviruses, Bovine Viral Diarrhea Virus (BVDV) and Border Disease Virus (BDV), that infect swine without causing disease. Progressive mutations were introduced into mAb WH303 epitope in CSFV virulent strain Brescia (BICv) to obtain the homologous amino acid sequence of BVDV strain NADL E2 (TSFNMDTLA). In vitro growth of mutants T1v (TSFSPTTLR), T2v (TSFNPTTLR), T3v (TSFNMTTLR) was similar to parental BICv, while mutants T4v (TSFNMDTLR) and T5v (TSFNMDTLA) exhibited a 10-fold decrease in virus yield and reduced plaque size. In vivo, T1v, T2v or T3v induced lethal disease, T4v induced mild and transient disease and T5v induced mild clinical signs. Protection against BICv challenge was observed at 3 and 21 days post-T5v infection. These results indicate that E2 residues TAVSPTTLR play a significant role in CSFV virulence.

Characterization of herpes simplex virus type 1 recombinants that express and incorporate high levels of HCV E2-gC chimeric proteins

We report the construction of two HSV-1 recombinants encoding chimeric forms of the E2 glycoprotein of HCV-1a composed of the ectodomain of E2 (aa384-611 or 384-711) fused to different parts of the transmembrane and cytoplasmic domain of the HSV-1 gC glycoprotein (gC). The parental HSV-1, known as KgBpK(-)gC(-), is deleted for gC and the main heparan sulphate (HS) binding domain of gB, and it exhibits impaired binding (ca. 80%) to HS compared to the wild type virus KOS [Laquerre, S., Argnani, R., Anderson, D.B., Zucchini, S., Manservigi, R., Glorioso, J.C., 1998. Heparan sulphate proteoglycan binding by herpes simplex virus type 1 glycoproteins B and C, which differ in their contributions to virus attachment, penetration, and cell-to-cell spread. J. Virol. 72, 6119-6130]. We show that gC:E2 proteins are efficiently expressed and transported to the cell surface. We also demonstrate that HSV-1 can incorporate both gC:E2 chimeric proteins into particles and show that incorporation of both chimeric molecules in the viral envelope partially restored binding (ca. 20%) of the HSV-1 recombinants to heparan sulphate. Finally, we showed that the gC:E2ScaI chimeric glycoprotein was able to bind a recombinant form of hCD81 and virion-expressed gC:E2ScaI permitted the binding of the HSV-1 recombinant virus to the hCD81 molecule.

Viral and cellular determinants of the hepatitis C virus envelope-heparan sulfate interaction

Cellular binding and entry of hepatitis C virus (HCV) are the first steps of viral infection and represent a major target for antiviral antibodies and novel therapeutic strategies. We have recently demonstrated that heparan sulfate (HS) plays a key role in the binding of HCV envelope glycoprotein E2 to target cells (Barth et al., J. Biol. Chem. 278:41003-41012, 2003). In this study, we characterized the HCV-HS interaction and analyzed its inhibition by antiviral host immune responses. Using recombinant envelope glycoproteins, virus-like particles, and HCV pseudoparticles as model systems for the early steps of viral infection, we mapped viral and cellular determinants of HCV-HS interaction. HCV-HS binding required a specific HS structure that included N-sulfo groups and a minimum of 10 to 14 saccharide subunits. HCV envelope binding to HS was mediated by four viral epitopes overlapping the E2 hypervariable region 1 and E2-CD81 binding domains. In functional studies using HCV pseudoparticles, we demonstrate that HCV binding and entry are specifically inhibited by highly sulfated HS. Finally, HCV-HS binding was markedly inhibited by antiviral antibodies derived from HCV-infected individuals. In conclusion, our results demonstrate that binding of the viral envelope to a specific HS configuration represents an important step for the initiation of viral infection and is a target of antiviral host immune responses in vivo. Mapping of viral and cellular determinants of HCV-HS interaction sets the stage for the development of novel HS-based antiviral strategies targeting viral attachment and entry.