Cytopathogenic and noncytopathogenic RNA replicons of classical swine fever virus

New Generation Self-Replicating RNA Vaccines Derived from Pestivirus Genome

While mRNA vaccines have shown their worth, they have the same failing as inactivated vaccines, namely they have limited half-life, are non-replicating, and therefore limited to the size of the vaccine payload for the amount of material translated. New advances averting these problems are combining replicon RNA (RepRNA) technology with nanotechnology. RepRNA are large self-replicating RNA molecules (typically 12-15 kb) derived from viral genomes defective in at least one essential structural protein gene. They provide sustained antigen production, effectively increasing vaccine antigen payloads over time, without the risk of producing infectious progeny. The major limitations with RepRNA are RNase-sensitivity and inefficient uptake by dendritic cells (DCs), which need to be overcome for efficacious RNA-based vaccine design. We employed biodegradable delivery vehicles to protect the RepRNA and promote DC delivery. Condensing RepRNA with polyethylenimine (PEI) and encapsulating RepRNA into novel Coatsome-replicon vehicles are two approaches that have proven effective for delivery to DCs and induction of immune responses in vivo.

Generation and Efficacy of Two Chimeric Viruses Derived from GPE- Vaccine Strain as Classical Swine Fever Vaccine Candidates

A previous study proved that vGPE^- mainly maintains the properties of classical swine fever (CSF) virus, which is comparable to the GPE^- vaccine seed and is a potentially valuable backbone for developing a CSF marker vaccine. Chimeric viruses were constructed based on an infectious cDNA clone derived from the live attenuated GPE^- vaccine strain as novel CSF vaccine candidates that potentially meet the concept of differentiating infected from vaccinated animals (DIVA) by substituting the glycoprotein E^rns of the GPE^- vaccine strain with the corresponding region of non-CSF pestiviruses, either pronghorn antelope pestivirus (PAPeV) or Phocoena pestivirus (PhoPeV). High viral growth and genetic stability after serial passages of the chimeric viruses, namely vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns, were confirmed in vitro. In vivo investigation revealed that two chimeric viruses had comparable immunogenicity and safety profiles to the vGPE^- vaccine strain. Vaccination at a dose of 10^4.0 TCID₅₀ with either vGPE^-/PAPeV E^rns or vGPE^-/PhoPeV E^rns conferred complete protection for pigs against the CSF virus challenge in the early stage of immunization. In conclusion, the characteristics of vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns affirmed their properties, as the vGPE^- vaccine strain, positioning them as ideal candidates for future development of a CSF marker vaccine.

Different Types of Vaccines against Pestiviral Infections: "Barriers" for "Pestis"

The genus Pestivirus of the family Flaviviridae mainly comprises classical swine fever virus (CSFV), bovine viral diarrhea virus 1 (BVDV-1), BVDV-2, border disease virus (BDV), and multiple new pestivirus species such as atypical porcine pestivirus (APPV), giraffe pestivirus, and antelope pestivirus. Pestiviruses cause infectious diseases, resulting in tremendous economic losses to animal husbandry. Different types of pestivirus vaccines have been developed to control and prevent these important animal diseases. In recent years, pestiviruses have shown great potential as viral vectors for developing multivalent vaccines. This review analyzes the advantages and disadvantages of various pestivirus vaccines, including live attenuated pestivirus strains, genetically engineered marker pestiviruses, and pestivirus-based multivalent vaccines. This review provides new insights into the development of novel vaccines against emerging pestiviruses, such as APPV and ovine pestivirus.

Characteristics of Classical Swine Fever Virus Variants Derived from Live Attenuated GPE- Vaccine Seed

The GPE^- strain is a live attenuated vaccine for classical swine fever (CSF) developed in Japan. In the context of increasing attention for the differentiating infected from vaccinated animals (DIVA) concept, the achievement of CSF eradication with the GPE^- proposes it as a preferable backbone for a recombinant CSF marker vaccine. While its infectious cDNA clone, vGPE^-, is well characterized, 10 amino acid substitutions were recognized in the genome, compared to the original GPE^- vaccine seed. To clarify the GPE^- seed availability, this study aimed to generate and characterize a clone possessing the identical amino acid sequence to the GPE^- seed. The attempt resulted in the loss of the infectious GPE^- seed clone production due to the impaired replication by an amino acid substitution in the viral polymerase NS5B. Accordingly, replication-competent GPE^- seed variant clones were produced. Although they were mostly restricted to propagate in the tonsils of pigs, similarly to vGPE^-, their type I interferon-inducing capacity was significantly lower than that of vGPE^-. Taken together, vGPE^- mainly retains ideal properties for the CSF vaccine, compared with the seed variants, and is probably useful in the development of a CSF marker vaccine.

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.

Autonomously Replicating RNAs of Bungowannah Pestivirus: ERNS Is Not Essential for the Generation of Infectious Particles

Autonomously replicating subgenomic Bungowannah virus (BuPV) RNAs (BuPV replicons) with deletions of the genome regions encoding the structural proteins C, E^RNS, E1, and E2 were constructed on the basis of an infectious cDNA clone of BuPV. Nanoluciferase (Nluc) insertion was used to compare the replication efficiencies of all constructs after electroporation of in vitro-transcribed RNA from the different clones. Deletion of C, E1, E2, or the complete structural protein genome region (C-E^RNS-E1-E2) prevented the production of infectious progeny virus, whereas deletion of E^RNS still allowed the generation of infectious particles. However, those ΔE^RNS viral particles were defective in virus assembly and/or egress and could not be further propagated for more than three additional passages in porcine SK-6 cells. These "defective-in-third-cycle" BuPV ΔE^RNS mutants were subsequently used to express the classical swine fever virus envelope protein E2, the N-terminal domain of the Schmallenberg virus Gc protein, and the receptor binding domain of the Middle East respiratory syndrome coronavirus spike protein. The constructs could be efficiently complemented and further passaged in SK-6 cells constitutively expressing the BuPV E^RNS protein. Importantly, BuPVs are able to infect a wide variety of target cell lines, allowing expression in a very wide host spectrum. Therefore, we suggest that packaged BuPV ΔE^RNS replicon particles have potential as broad-spectrum viral vectors.IMPORTANCE The proteins N^PRO and E^RNS are unique for the genus Pestivirus, but only N^PRO has been demonstrated to be nonessential for in vitro growth. While this was also speculated for E^RNS, it has always been previously shown that pestivirus replicons with deletions of the structural proteins E^RNS, E1, or E2 did not produce any infectious progeny virus in susceptible host cells. Here, we demonstrated for the first time that BuPV E^RNS is dispensable for the generation of infectious virus particles but still important for efficient passaging. The E^RNS-defective BuPV particles showed clearly limited growth in cell culture but were capable of several rounds of infection, expression of foreign genes, and highly efficient trans-complementation to rescue virus replicon particles (VRPs). The noncytopathic characteristics and the absence of preexisting immunity to BuPV in human populations and livestock also provide a significant benefit for a possible use, e.g., as a vector vaccine platform.

A Polyuridine Insertion in the 3' Untranslated Region of Classical Swine Fever Virus Activates Immunity and Reduces Viral Virulence in Piglets

Classical swine fever (CSF), a highly contagious viral disease of pigs, is still endemic in some countries of Asia and Central and South America. Considering that the 3′ untranslated region (3′ UTR) plays an important role in flavivirus replication, the present study showed for the first time that a long polyuridine sequence acquired in the 3′ UTR by an endemic CSFV isolate can activate immunity, control viral replication, and modulate disease in piglets. Our findings provide new avenues for the development of novel vaccines against infections with CSF virus and other flaviviruses. Knowledge of molecular virulence determinants is also relevant for future development of rapid and efficient diagnostic tools for the prediction of the virulence of field isolates and for efficient CSF control.

Low-virulence classical swine fever virus (CSFV) strains make CSF eradication particularly difficult. Few data are available on the molecular determinants of CSFV virulence. The aim of the present study was to assess a possible role for CSFV virulence of a unique, uninterrupted 36-uridine (poly-U) sequence found in the 3′ untranslated region (3′ UTR) of the low-virulence CSFV isolate Pinar de Rio (PdR). To this end, a pair of cDNA-derived viruses based on the PdR backbone were generated, one carrying the long poly-U insertion in the 3′ UTR (vPdR-36U) and the other harboring the standard 5 uridines at this position (vPdR-5U). Two groups of 20 5-day-old piglets were infected with vPdR-36U and vPdR-5U. Ten contact piglets were added to each group. Disease progression, virus replication, and immune responses were monitored for 5 weeks. The vPdR-5U virus was significantly more virulent than the vPdR-36U virus, with more severe disease, higher mortality, and significantly higher viral loads in serum and body secretions, despite similar replication characteristics in cell culture. The two viruses were transmitted to all contact piglets. Ninety percent of the piglets infected with vPdR-36U seroconverted, while only one vPdR-5U-infected piglet developed antibodies. The vPdR-5U-infected piglets showed only transient alpha interferon (IFN-α) responses in serum after 1 week of infection, while the vPdR-36U-infected piglets showed sustained IFN-α levels during the first 2 weeks. Taken together, these data show that the 3′ UTR poly-U insertion acquired by the PdR isolate reduces viral virulence and activates the innate and humoral immune responses without affecting viral transmission.

IMPORTANCE Classical swine fever (CSF), a highly contagious viral disease of pigs, is still endemic in some countries of Asia and Central and South America. Considering that the 3′ untranslated region (3′ UTR) plays an important role in flavivirus replication, the present study showed for the first time that a long polyuridine sequence acquired in the 3′ UTR by an endemic CSFV isolate can activate immunity, control viral replication, and modulate disease in piglets. Our findings provide new avenues for the development of novel vaccines against infections with CSF virus and other flaviviruses. Knowledge of molecular virulence determinants is also relevant for future development of rapid and efficient diagnostic tools for the prediction of the virulence of field isolates and for efficient CSF control.

In Vivo Dynamics of Reporter Flaviviridae Viruses

Recombinant viruses possessing reporter proteins have been generated for virus research. In the case of the family Flaviviridae, we recently generated recombinant viruses, including the hepatitis C virus of the genus Hepacivirus, Japanese encephalitis virus (JEV) of the genus Flavivirus, and bovine viral diarrhea virus of the genus Pestivirus; all three viruses possess an 11-amino-acid subunit derived from NanoLuc luciferase (HiBiT). Here, we further developed the recombinant viruses and investigated their utility in vivo Recombinant viruses harboring HiBiT in the E, NS1, or NS3 protein constructed based on the predicted secondary structure, solvent-accessible surface area, and root mean square fluctuation of the proteins exhibited comparable replication to that of the wild-type virus in vitro The recombinant JEV carrying HiBiT in the NS1 protein exhibited propagation in mice comparable to that of the parental virus, and propagation of the recombinant was monitored by the luciferase activity. In addition, the recombinants of classical swine fever virus (CSFV) possessing HiBiT in the E^rns or E2 protein also showed propagation comparable to that of the wild-type virus. The recombinant CSFV carrying HiBiT in E^rns exhibited similar replication to the parental CSFV in pigs, and detection of viral propagation of this recombinant by luciferase activity was higher than that by quantitative PCR (qPCR). Taken together, these results demonstrated that the reporter Flaviviridae viruses generated herein are powerful tools for elucidating the viral life cycle and pathogeneses and provide a robust platform for the development of novel antivirals.IMPORTANCE In vivo applications of reporter viruses are necessary to understand viral pathogenesis and provide a robust platform for antiviral development. In developing such applications, determination of an ideal locus to accommodate foreign genes is important, because insertion of foreign genes into irrelevant loci can disrupt the protein functions required for viral replication. Here, we investigated the criteria to determine ideal insertion sites of foreign genes from the protein structure of viral proteins. The recombinant viruses generated by our criteria exhibited propagation comparable to that of parental viruses in vivo Our proteomic approach based on the flexibility profile of viral proteins may provide a useful tool for constructing reporter viruses, including Flaviviridae viruses.

Crystal Structure of Classical Swine Fever Virus NS5B Reveals a Novel N-Terminal Domain

Classical swine fever virus (CSFV) is the cause of classical swine fever (CSF). Nonstructural protein 5B (NS5B) is an RNA-dependent RNA polymerase (RdRp) that is a key enzyme initiating viral RNA replication by a de novo mechanism. It is also an attractive target for the development of anti-CSFV drugs. To gain a better understanding of the mechanism of CSFV RNA synthesis, here, we solved the first crystal structure of CSFV NS5B. Our studies show that the CSFV NS5B RdRp contains the characteristic finger, palm, and thumb domains, as well as a unique N-terminal domain (NTD) that has never been observed. Mutagenesis studies on NS5B validated the importance of the NTD in the catalytic activity of this novel RNA-dependent RNA polymerase. Moreover, our results shed light on CSFV infection.IMPORTANCE Pigs are important domesticated animals. However, a highly contagious viral disease named classical swine fever (CSF) causes devastating economic losses. Classical swine fever virus (CSFV), the primary cause of CSF, is a positive-sense single-stranded RNA virus belonging to the genus Pestivirus, family Flaviviridae Genome replication of CSFV depends on an RNA-dependent RNA polymerase (RdRp) known as NS5B. However, the structure of CSFV NS5B has never been reported, and the mechanism of CSFV replication is poorly understood. Here, we solve the first crystal structure of CSFV NS5B and analyze the functions of the characteristic finger, palm, and thumb domains. Additionally, our structure revealed the presence of a novel N-terminal domain (NTD). Biochemical studies demonstrated that the NTD of CSFV NS5B is very important for RdRp activity. Collectively, our studies provide a structural basis for future rational design of anti-CSFV drugs, which is critically important, as no effective anti-CSFV drugs have been developed.

Immunogenicity of a live attenuated classical swine fever vaccine in pigs under field conditions

A study was conducted for assessing the immunogenicity of live classical swine fever (CSF) vaccine in pigs under field conditions. Pre immunization blood samples and post immunization (PI) blood samples at 30, 60, 90, and 120 days post immunization (DPI) were collected. Th1 response was assessed by the expression of IFN-γ, IL- 2 and IL-12, and serum antibody titres and IL-10 expression were studied for Th2 response. The mean±SE of IFN- γ gene response as fold change (2ΔΔCt values) at 30, 60, 90, and 120 days post immunization (DPI) were 9.61±0.65, 3.49±0.44, 2.22±0.407, and 3.12±1.35 respectively. IFN-γ response showed significantly increased fold change during PI period. Significant peak increase in the expression level was observed at 30 DPI. IL-2 gene response showed significantly increased fold change during PI (Mean expression at 30, 60, 90, and 120 DPI was 4.41, 2.61, 3.291 and 2.09 respectively) and peak increase was observed at 30 DPI. IL-12 response also showed significant increase during PI with a peak expression at 30 DPI (Mean fold changes at 30, 60, 90, and 120 days post immunization were 4.92, 2.97, 2.72, and 3.61 respectively). Maximum IL-10 expression was observed at 60 DPI (Mean fold change at 30, 60, 90, and 120 days post immunization were 3.364±0.723, 11.56±1.64, 4.73±1.01, and 2.599±1.05 respectively). Pre- and post-immunization antibody responses were monitored by enzyme linked immunosorbent assay. Antibody titres increased consistently from 30 days after primary immunization until 120 days. Montanide (gel 01) coupled PK-15 adapted live, CSV vaccine induced a good Th1 response followed by Th2 response.

Uncoupling of Protease trans-Cleavage and Helicase Activities in Pestivirus NS3

The nonstructural protein NS3 from the Flaviviridae family is a multifunctional protein that contains an N-terminal protease and a C-terminal helicase, playing essential roles in viral polyprotein processing and genome replication. Here we report a full-length crystal structure of the classical swine fever virus (CSFV) NS3 in complex with its NS4A protease cofactor segment (PCS) at a 2.35-Å resolution. The structure reveals a previously unidentified ∼2,200-Ų intramolecular protease-helicase interface comprising three clusters of interactions, representing a "closed" global conformation related to the NS3-NS4A cis-cleavage event. Although this conformation is incompatible with protease trans-cleavage, it appears to be functionally important and beneficial to the helicase activity, as the mutations designed to perturb this conformation impaired both the helicase activities in vitro and virus production in vivo Our work reveals important features of protease-helicase coordination in pestivirus NS3 and provides a key basis for how different conformational states may explicitly contribute to certain functions of this natural protease-helicase fusion protein.IMPORTANCE Many RNA viruses encode helicases to aid their RNA genome replication and transcription by unwinding structured RNA. Being naturally fused to a protease participating in viral polyprotein processing, the NS3 helicases encoded by the Flaviviridae family viruses are unique. Therefore, how these two enzyme modules coordinate in a single polypeptide is of particular interest. Here we report a previously unidentified conformation of pestivirus NS3 in complex with its NS4A protease cofactor segment (PCS). This conformational state is related to the protease cis-cleavage event and is optimal for the function of helicase. This work provides an important basis to understand how different enzymatic activities of NS3 may be achieved by the coordination between the protease and helicase through different conformational states.

Classical swine fever virus nonstructural protein p7 modulates infectious virus production

The classical swine fever virus (CSFV) nonstructural protein p7 is crucial for virus production, yet precisely how the p7 modulates this process is unclear. In this study, we first identified the interactions of p7 with E2 and NS2. The key binding regions of both p7 and NS2 mapped to the first transmembrane (TM1) domain of two proteins. Three amino acid substitutions in the TM1 region of p7 (p7^{TDI18/19/20AAA}, p7^{EVV21/22/23AAA} and p7^{YFY25/26/30AAA}) impaired infectious virus production and reduced the interaction of p7 with the NS2 protein. The E2p7 processing and mature p7, but not the E2p7 precursor, are essential for infectious virus production. Bicistronic mutants (pSM/E2/IRES) with single substitutions at residues 1 to 9 of p7 exhibited a significantly increased infectious CSFV titer compared to their counterparts in the context of pSM. Viral genomic RNA copies of the mutants exhibited similar levels compared with the wt CSFV. Our results demonstrated that CSFV p7 and its precursor E2p7 modulate viral protein interactions and infectious virus production without influencing viral RNA replication.

Self-replicating RNA vaccine functionality modulated by fine-tuning of polyplex delivery vehicle structure

The major limitations with large and complex self-amplifying RNA vaccines (RepRNA) are RNase-sensitivity and inefficient translation in dendritic cells (DCs). Condensing RepRNA with polyethylenimine (PEI) gave positive in vitro readouts, but was largely inferior to virus-like replicon particles (VRP) or direct electroporation. In the present study, we improved such polyplex formulation and determined that fine-tuning of the polyplex structure is essential for ensuring efficacious translation. Thereby, three parameters dominate: (i) PEI molecular weight (MW); (ii) RepRNA:PEI (weight:weight) ratio; and (iii) inclusion of cell penetrating peptides (CPPs). Seven commercially available linear PEIs (MW 2,500-250,000) were classified as strong, intermediate or low for their aptitude at complexing and protecting RepRNA for delivery into porcine blood DCs. Inclusion of (Arg)₉ or TAT(57-57) CPPs further modified the translation readouts, but varied for different gene expressions. Dependent on the formulation, translation of the gene of interest (GOI) inserted into the RepRNA (luciferase, or influenza virus hemagglutinin or nucleoprotein) could decrease, while the RepRNA structural gene (E2) translation increased. This was noted in the porcine SK6 cell line, as well as both porcine and, for the first time, human DCs. Two formulations - [Rep/PEI-4,000 (1:3)] and [Rep/PEI-40,000 (1:2)/(Arg)₉] were efficacious in vivo in mice and pigs, where specific CD8⁺ T and CD4⁺ T-cell responses against the GOI-encoded antigen were observed for the first time. The results demonstrate that different polyplex formulations differ in their interaction with the RepRNA such that only certain genes can be translated. Thus, delivery of these large self-replicating RNA molecules require definition with respect to translation of different genes, rather than just the GOI as is the norm, for identifying optimal delivery for the desired immune activation in vivo.

Self-Replicating RNA Vaccine Delivery to Dendritic Cells

Most current vaccines are either inactivated pathogen-derived or protein/peptide-based, although attenuated and vector vaccines have also been developed. The former induce at best moderate protection, even as multimeric antigen, due to limitations in antigen loads and therefore capacity for inducing robust immune defense. While attenuated and vector vaccines offer advantages through their replicative nature, drawbacks and risks remain with potential reversion to virulence and interference from preexisting immunity. New advances averting these problems are combining self-amplifying replicon RNA (RepRNA) technology with nanotechnology. RepRNA are large self-replicating RNA molecules (12-15 kb) derived from viral genomes defective in at least one structural protein gene. They provide sustained antigen production, effectively increasing vaccine antigen payloads over time, without the risk of producing infectious progeny. The major limitation with RepRNA is RNase-sensitivity and inefficient uptake by dendritic cells (DCs)-absolute requirements for efficacious vaccine design. We employed biodegradable delivery vehicles to protect the RepRNA and promote DC delivery. Encapsulating RepRNA into chitosan nanoparticles, as well as condensing RepRNA with polyethylenimine (PEI), cationic lipids, or chitosans, has proven effective for delivery to DCs and induction of immune responses in vivo.

Generation and evaluation of a chimeric classical swine fever virus expressing a visible marker gene

Classical swine fever virus (CSFV) is a noncytopathogenic virus, and the incorporation of an enhanced green fluorescent protein (EGFP) tag into the viral genome provides a means of direct monitoring of viral infection without immunostaining. It is well established that the 3' untranslated region (3'-UTR) of the CSFV plays an important role in viral RNA replication. Although CSFV carrying a reporter gene and chimeric CSFV have been generated and evaluated, a chimeric CSFV with a visible marker has not yet been reported. Here, we generated and evaluated a chimeric virus containing the EGFP tag and the 3'-UTR from vaccine strain HCLV (C-strain) in the genetic background of the highly virulent CSFV Shimen strain. The chimeric marker CSFV was fluorescent and had an approximately 100-fold lower viral titer, lower replication level of viral genome, and weaker fluorescence intensity than the recombinant CSFV with only the EGFP tag or the parental virus. Furthermore, the marker chimera was avirulent and displayed no viremia in inoculated pigs, which were completely protected from lethal CSFV challenge as early as 15 days post-inoculation. The chimeric marker virus was visible in vitro and attenuated in vitro and in vivo, which suggests that CSFV can be engineered to produce attenuated variants with a visible marker to facilitate in vitro studies of CSFV infection and replication and to develop of novel vaccines against CSF.

The N-terminus of classical swine fever virus (CSFV) nonstructural protein 2 modulates viral genome RNA replication

Pestivirus nonstructural protein 2 (NS2) is a multifunctional, hydrophobic protein with an important but poorly understood role in viral RNA replication and infectious virus production. In the present study, based on sequence analysis, we mutated several representative conserved residues within the N-terminus of NS2 of classical swine fever virus (CSFV) and investigated how these mutations affected viral RNA replication and infectious virus production. Our results demonstrated that the mutation of two aspartic acids, NS2/D60A or NS2/D60K and NS2/D78K, in the N-terminus of NS2 abolished infectious virus production and that the substitution of arginine for alanine at position 100 (NS2/R100A) resulted in significantly decreased viral titer. The serial passage of cells containing viral genomic RNA molecules generated the revertants NS2/A60D, NS2/K60D and NS2/K78D, leading to the recovery of infectious virus. In the context of the NS2/R100A mutant, the NS2/I90L mutation compensated for infectious virus production. The regulatory roles of the indicated amino acid residues were identified to occur at the viral RNA replication level. These results revealed a novel function for the NS2 N-terminus of CSFV in modulating viral RNA replication.

Intracellular membrane association of the N-terminal domain of classical swine fever virus NS4B determines viral genome replication and virulence

Classical swine fever virus (CSFV) causes a highly contagious disease in pigs that can range from a severe haemorrhagic fever to a nearly unapparent disease, depending on the virulence of the virus strain. Little is known about the viral molecular determinants of CSFV virulence. The nonstructural protein NS4B is essential for viral replication. However, the roles of CSFV NS4B in viral genome replication and pathogenesis have not yet been elucidated. NS4B of the GPE-  vaccine strain and of the highly virulent Eystrup strain differ by a total of seven amino acid residues, two of which are located in the predicted trans-membrane domains of NS4B and were described previously to relate to virulence, and five residues clustering in the N-terminal part. In the present study, we examined the potential role of these five amino acids in modulating genome replication and determining pathogenicity in pigs. A chimeric low virulent GPE- -derived virus carrying the complete Eystrup NS4B showed enhanced pathogenicity in pigs. The in vitro replication efficiency of the NS4B chimeric GPE-  replicon was significantly higher than that of the replicon carrying only the two Eystrup-specific amino acids in NS4B. In silico and in vitro data suggest that the N-terminal part of NS4B forms an amphipathic α-helix structure. The N-terminal NS4B with these five amino acid residues is associated with the intracellular membranes. Taken together, this is the first gain-of-function study showing that the N-terminal domain of NS4B can determine CSFV genome replication in cell culture and viral pathogenicity in pigs.

The N-terminal domain of Npro of classical swine fever virus determines its stability and regulates type I IFN production

The viral protein Npro is unique to the genus Pestivirus within the family Flaviviridae. After autocatalytic cleavage from the nascent polyprotein, Npro suppresses type I IFN (IFN-α/β) induction by mediating proteasomal degradation of IFN regulatory factor 3 (IRF-3). Previous studies found that the Npro-mediated IRF-3 degradation was dependent of a TRASH domain in the C-terminal half of Npro coordinating zinc by means of the amino acid residues C112, C134, D136 and C138. Interestingly, four classical swine fever virus (CSFV) isolates obtained from diseased pigs in Thailand in 1993 and 1998 did not suppress IFN-α/β induction despite the presence of an intact TRASH domain. Through systematic analyses, it was found that an amino acid mutation at position 40 or mutations at positions 17 and 61 in the N-terminal half of Npro of these four isolates were related to the lack of IRF-3-degrading activity. Restoring a histidine at position 40 or both a proline at position 17 and a lysine at position 61 based on the sequence of a functional Npro contributed to higher stability of the reconstructed Npro compared with the Npro from the Thai isolate. This led to enhanced interaction of Npro with IRF-3 along with its degradation by the proteasome. The results of the present study revealed that amino acid residues in the N-terminal domain of Npro are involved in the stability of Npro, in interaction of Npro with IRF-3 and subsequent degradation of IRF-3, leading to downregulation of IFN-α/β production.

Visualization of the Npro protein in living cells using biarsenically labeling tetracysteine-tagged classical swine fever virus

Real-time fluorescence imaging of viral proteins in living cells is a valuable means to study virus-host interactions, and tetracysteine (TC)-biarsenical technology has been used in several viruses but not in classical swine fever virus (CSFV). Here, we generated CSFV mutants vSMTC385 or vSMTC412 bearing the small TC tag (CCPGCC) in the N-terminal region of the N(pro) protein. The mutants showed growth characteristics indistinguishable from that of the wild-type virus, and retained similar N(pro) subcellular localization to that of the parent virus. Furthermore, labeling with membrane-permeable biarsenical dye resulted in the fluorescent N(pro) protein in the context of virus infection. Finally, we showed that N(pro) was localized in the cytoplasm of CSFV-infected cells at 27 h post-infection (hpi) and present in the nucleus at 48 hpi, and the nuclear import and export was clearly observed from 36.5 to 37 hpi. Interestingly, our results demonstrated that N(pro) transported across the nuclear pores by passive diffusion, which might be prevented by exogenous interferon regulatory factor 3 interacting with N(pro). Taken together, biarsenical labeling allows real-time visualization of the nucleus import and export of the fluorescent N(pro) protein in CSFV-infected living cells.

Npro of classical swine fever virus contributes to pathogenicity in pigs by preventing type I interferon induction at local replication sites

Classical swine fever (CSF) caused by CSF virus (CSFV) is a highly contagious disease of pigs. The viral protein Npro of CSFV interferes with alpha- and beta-interferon (IFN-α/β) induction by promoting the degradation of interferon regulatory factor 3 (IRF3). During the establishment of the live attenuated CSF vaccine strain GPE-, Npro acquired a mutation that abolished its capacity to bind and degrade IRF3, rendering it unable to prevent IFN-α/β induction. In a previous study, we showed that the GPE- vaccine virus became pathogenic after forced serial passages in pigs, which was attributed to the amino acid substitutions T830A in the viral proteins E2 and V2475A and A2563V in NS4B. Interestingly, during the re-adaptation of the GPE- vaccine virus in pigs, the IRF3-degrading function of Npro was not recovered. Therefore, we examined whether restoring the ability of Npro to block IFN-α/β induction of both the avirulent and moderately virulent GPE--derived virus would enhance pathogenicity in pigs. Viruses carrying the N136D substitution in Npro regained the ability to degrade IRF3 and suppress IFN-α/β induction in vitro. In pigs, functional Npro significantly reduced the local IFN-α mRNA expression in lymphoid organs while it increased quantities of IFN-α/β in the circulation, and enhanced pathogenicity of the moderately virulent virus. In conclusion, the present study demonstrates that functional Npro influences the innate immune response at local sites of virus replication in pigs and contributes to pathogenicity of CSFV in synergy with viral replication.

Application of real-time quantitative polymerase chain reaction to monitoring infection of classic swine fever virus and determining optimal harvest time in large-scale production

Due to the non-cytopathogenic replication of classical swine fever virus (CSFV) in cell culture, large-scale production of CSFV using bioreactor system remains the problem of monitoring the time of maximum virus production for optimal harvest. In this study, we proposed the application of real-time quantitative PCR assay to monitoring the progress of CSFV infection and yield determination in large scale. The region of NS5B of CSFV responsible for CSFV genome replication was used for the designation of primers and probe. Viral titers determined by the real-time quantitative PCR assay were compared with the conventional cell-culture based method of immunofluorescent staining. Results from large scale production show that a similar profile of CSFV production was successfully outlined by real-time quantitative PCR and virus yields were comparable to the results from immunofluorescent staining assay. By using this method, an optimal harvesting time of the production could be rapidly and precisely determined leading to an improvement in virus harvest.

Efficient sensing of infected cells in absence of virus particles by plasmacytoid dendritic cells is blocked by the viral ribonuclease E(rns.)

Plasmacytoid dendritic cells (pDC) have been shown to efficiently sense HCV- or HIV-infected cells, using a virion-free pathway. Here, we demonstrate for classical swine fever virus, a member of the Flaviviridae, that this process is much more efficient in terms of interferon-alpha induction when compared to direct stimulation by virus particles. By employment of virus replicon particles or infectious RNA which can replicate but not form de novo virions, we exclude a transfer of virus from the donor cell to the pDC. pDC activation by infected cells was mediated by a contact-dependent RNA transfer to pDC, which was sensitive to a TLR7 inhibitor. This was inhibited by drugs affecting the cytoskeleton and membrane cholesterol. We further demonstrate that a unique viral protein with ribonuclease activity, the viral E^rns protein of pestiviruses, efficiently prevented this process. This required intact ribonuclease function in intracellular compartments. We propose that this pathway of activation could be of particular importance for viruses which tend to be mostly cell-associated, cause persistent infection, and are non-cytopathogenic.

Plasmacytoid dendritic cells (pDC) represent the most potent producers of interferon type I and are therefore of major importance in antiviral defences. A TLR7-dependent induction of interferon-α in pDC by infected cells in the absence of virions has been demonstrated for hepatitis C virus. Here, we show that this pathway is also very efficient for classical swine fever virus, a pestivirus that is also a member of the Flaviviridae. Our data indicate a transfer of RNA from the donor cell to pDC in a cell-contact-dependent manner requiring intact lipid rafts and cytoskeleton of the donor cell. Importantly, we demonstrate that the enigmatic viral E^rns protein unique to pestiviruses efficiently prevents this pathway of pDC activation. This novel function of E^rns is dependent on its RNase activity within intracellular compartments. The present study underlines the importance of pDC activation by infected cells and identifies a novel pathway of virus escaping the interferon system. Considering that E^rns is required for pestiviruses to establish persistent infection of foetuses after transplacental virus transmission resulting in the development of immunotolerant animals, this report also points on a possible role of pDC in preventing immunotolerance after viral infection of foetuses.

Analysis of classical swine fever virus RNA replication determinants using replicons

Self-replicating RNAs (replicons), with or without reporter gene sequences, derived from the genome of the Paderborn strain of classical swine fever virus (CSFV) have been produced. The full-length viral cDNA, propagated within a bacterial artificial chromosome, was modified by targeted recombination within Escherichia coli. RNA transcripts were produced in vitro and introduced into cells by electroporation. The translation and replication of the replicon RNAs could be followed by the accumulation of luciferase (from Renilla reniformis or Gaussia princeps) protein expression (where appropriate), as well as by detection of CSFV NS3 protein production within the cells. Inclusion of the viral E2 coding region within the replicon was advantageous for replication efficiency. Production of chimeric RNAs, substituting the NS2 and NS3 coding regions (as a unit) from the Paderborn strain with the equivalent sequences from the highly virulent Koslov strain or the vaccine strain Riems, blocked replication. However, replacing the Paderborn NS5B coding sequence with the RNA polymerase coding sequence from the Koslov strain greatly enhanced expression of the reporter protein from the replicon. In contrast, replacement with the Riems NS5B sequence significantly impaired replication efficiency. Thus, these replicons provide a system for determining specific regions of the CSFV genome required for genome replication without the constraints of maintaining infectivity.

Real-time SPR characterization of the interactions between multi-epitope proteins and antibodies against classical swine fever virus

The envelope glycoprotein E2 is the major immunodominant protein of the classical swine fever virus and can induce neutralizing antibodies and protective host-immune responses in infected swine. We designed, expressed, and purified multi-epitope protein (GST-BT22) that contains a tandem repeat of the E2 antigenic-determinant residues 693-704, 770-780, and 826-843, each of which is separated by a GGSSGG sequence. In the same manner, we also designed, expressed, and purified a second protein (GST-BT23) that contains a C-terminal sequence consisting of residues 1446-1460 from the classical swine fever virus nonstructural protein NS2-3 separated from the GST-BT22 sequence by a GGSSGG sequence. Western blotting of GST-BT22 and GST-BT23 with serum from a swine that had been experimentally infected with the virus showed that the proteins reacted with anti-serum, whereas GST did not. Surface plasmon resonance was used to quantify the affinities of GST-BT22 and GST-BT23 for serum antibodies (K(a) = 4.31 × 10(8) and 5.01 × 10(8), respectively). GST, used as a control, was reacted an order of magnitude less strongly than did GST-BT22 and GST-BT23. Surface plasmon resonance, therefore, appears to be a sensitive and precise method for epitope evaluation and can be used to characterize the immunogenicity of a recombinant protein.

Real-time analysis of the interaction of a multiple-epitope peptide with antibodies against classical swine fever virus using surface plasmon resonance

The E2 envelope glycoprotein is the major immunodominant protein of classical swine fever virus (CSFV), and can induce neutralizing antibodies and protective immune responses in infected swine. We developed a tandem-repeat multiple-epitope recombinant protein that contains two copies of each of the regions of E2 spanned by residues 693-704, 770-780, and 826-843, coupled by two copies of the region spanned by residues 1446-1460 of the CSFV nonstructural protein NS2-3. The chemically synthesized gene was expressed in Escherichia coli as a fusion with glutathione S-8 (GST), named GST-BT21. After it was purified with Glutathione Sepharose 4B, we used Western blotting to characterize the construct and surface plasmon resonance to analyze its affinity and specific interaction with CSFV-positive serum. Purified GST-BT21 protein displayed excellent immunoreactivity with antiserum against CSFV (Tian et al., 2012), and surface plasmon resonance confirmed the specific affinity between BT21, but not GST, and antibodies in serum from animals infected with CSFV. Surface plasmon resonance is a sensitive and precise method for epitope evaluation, and it can be used to characterize the immunogenicity and functions of recombinant proteins.

Interactive cellular proteins related to classical swine fever virus non-structure protein 2 by yeast two-hybrid analysis

Classical swine fever is caused by the classical swine fever virus (CSFV), which has a special affinity to endothelial cells. This fever is characterized by hemorrhage and necrosis of vascular injury. Very little information is available on the interaction between vascular endothelial cells and CSFV. In the current report, the cDNA library of swine umbilical vein endothelial cell (SUVEC) was constructed by the switching mechanism at 5' end of the RNA transcript approach. The yeast two-hybrid (Y2H) system was adopted to screen non-structure 2 protein (NS2) interactive proteins in the SUVEC line. Alignment with the NCBI database revealed 11 interactive proteins: GOPC, HNRNPH1, DNAJA1, ATP6, CSDE1, CNDP2, FANCL, TMED4, DNAJA4, MOAP1, and PNMA1. These proteins were mostly related to apoptosis, stress response and oxidation reduction, or metabolism. In the protein interaction network constructed based on proteins with NS2, the more important proteins were MOAP1, DNAJA1, GOPC, FANCL, TMED4, and CSDE1. The interactions detected by the Y2H should be regarded only as preliminary indications. However, the CSFV NS2 interactive proteins in the SUVEC cDNA library obtained in the current study provides valuable information for gaining a better understanding of the host protein-virus interactions of the CSFV NS2 protein.

Selection of classical swine fever virus with enhanced pathogenicity reveals synergistic virulence determinants in E2 and NS4B

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), a highly contagious disease of pigs. There are numerous CSFV strains that differ in virulence, resulting in clinical disease with different degrees of severity. Low-virulent and moderately virulent isolates cause a mild and often chronic disease, while highly virulent isolates cause an acute and mostly lethal hemorrhagic fever. The live attenuated vaccine strain GPE(-) was produced by multiple passages of the virulent ALD strain in cells of swine, bovine, and guinea pig origin. With the aim of identifying the determinants responsible for the attenuation, the GPE(-) vaccine virus was readapted to pigs by serial passages of infected tonsil homogenates until prolonged viremia and typical signs of CSF were observed. The GPE(-)/P-11 virus isolated from the tonsils after the 11th passage in vivo had acquired 3 amino acid substitutions in E2 (T830A) and NS4B (V2475A and A2563V) compared with the virus before passages. Experimental infection of pigs with the mutants reconstructed by reverse genetics confirmed that these amino acid substitutions were responsible for the acquisition of pathogenicity. Studies in vitro indicated that the substitution in E2 influenced virus spreading and that the changes in NS4B enhanced the viral RNA replication. In conclusion, the present study identified residues in E2 and NS4B of CSFV that can act synergistically to influence virus replication efficiency in vitro and pathogenicity in pigs.

[Pestivirus]

Members of the genus Pestivirus, are causative agents of economically important diseases for livestock and wild animals that occur worldwide, such as bovine viral diarrhea, classical swine fever, and border disease of sheep. Pestivirus have novel insertions of host genes in the viral genome and functions of unique viral proteins, N(pro) and E(rns), related to the pathogenicity although genomic structure is closely related to the other viruses of Flaviviridae family, especially hepatitis C virus. In this review, recent studies on the molecular basis of pathogenicity of pestivirus infections were summarized.

Experimental infection of Bama miniature pigs with a highly virulent classical swine fever virus

Background

Currently, larger domestic pigs are only animals widely used in vaccine evaluation and pathogenicity study of classical swine fever virus (CSFV). This study was aimed to create an alternative animal experimental infection model of CSFV.

Results

Twenty specific-pathogen-free Bama miniature pigs were randomly divided into two groups and rooms, infected and non-infected, and the pigs in the infected group were inoculated intramuscularly with 10⁴, 10⁵ or 10⁶ TCID₅₀ (median tissue culture infective dose) CSFV Shimen strain (n = 5 × 3) or left uninoculated to serve as in-contact pigs (n = 3). The uninfected control pigs (n = 2) were housed in a separate room. Clinical signs, body temperature, viraemia, tissue antigen distribution, pathological changes and seroconversion were monitored. Clinical signs were observed as early as 2 days post-inoculation (dpi) in all infected pigs (though mild in contact pigs), but not non-infected control pigs. All inoculated pigs showed viraemia by 6 dpi. The in-contact pigs showed lower levels of viraemia. At 10 dpi, seroconversion was noted in five of the 15 inoculated pigs. All inoculated or one in-contact pigs died by 15 dpi.

Conclusions

These results show that Bama miniature pigs support productive CSFV infection and display clinical signs and pathological changes consistent with CSFV infections observed in larger domestic pigs.

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.

Classical swine fever virus NS2 protein promotes interleukin-8 expression and inhibits MG132-induced apoptosis

Classical swine fever (CSF) caused by virulent strains of classical swine fever virus (CSFV) is a hemorrhagic disease of pigs and is characterized by disseminated intravascular coagulation, thrombocytopenia, and immunosuppression. Until now, the role of the NS2 protein produced by CSFV in the pathogenesis of CSF is not well understood. In this report, we investigated the function of CSFV NS2 by examining its effects on the pro-inflammatory CXC chemokine, interleukin-8 (IL-8) expression, and cell survival. Stable swine umbilical vein endothelial cell line (SUVEC) expressing CSFV NS2 were established and showed that CSFV NS2 expressing SUVEC cells express approximately 16-fold higher levels of IL-8 as compared to control vector GFP-expressing cells, GFP-E2 expressing cells, and untransfected cells. Further studies showed that CSFV NS2 induced endoplasmic reticulum stress and activated the nuclear transcription factor kappa B (NF-κB), which is responsible for the up-regulation of IL-8 and the anti-apoptotic protein, Bcl-2, expression. In addition, the GFPNS2-expressing SUVEC cells were resistant to MG132-induced apoptosis. This study suggested that CSFV NS2 plays an important role in the inflammatory response and in persistent CSFV infection. These findings provide novel information on the function of the poorly characterized CSFV NS2.

Biosynthesis of classical swine fever virus nonstructural proteins

Proteolytic processing of polyproteins is considered a crucial step in the life cycle of most positive-strand RNA viruses. An enhancement of NS2-3 processing has been described as a major difference between the noncytopathogenic (non-CP) and the cytopathogenic (CP) biotypes of pestiviruses. The effects of accelerated versus delayed NS2-3 processing on the maturation of the other nonstructural proteins (NSP) have never been compared. In this study, we analyzed the proteolytic processing of NSP in Classical swine fever virus (CSFV). Key to the investigation was a panel of newly developed monoclonal antibodies (MAbs) that facilitated monitoring of all nonstructural proteins involved in virus replication (NS2, NS3, NS4A, NS5A, and NS5B). Applying these MAbs in Western blotting and radioimmunoprecipitation allowed an unambiguous identification of the mature proteins and precursors in non-CP CSFV-infected cells. Furthermore, the kinetics of processing were determined by pulse-chase analyses for non-CP CSFV, CP CSFV, and a CP CSFV replicon. A slow but constant processing of NS4A/B-5A/B occurred in non-CP CSFV-infected cells, leading to balanced low-level concentrations of mature NSP. In contrast, the turnover of the polyprotein precursors was three times faster in CP CSFV-infected cells and in cells transfected with a CP CSFV replicon, causing a substantial increase of mature NSP concentrations. We conclude that a delayed processing not only of NS3 but further of all NSP represents a hallmark of regulation in non-CP pestiviruses.

Identification of an NTPase motif in classical swine fever virus NS4B protein

Classical swine fever (CSF) is a highly contagious and often fatal disease of swine caused by CSF virus (CSFV), a positive-sense single-stranded RNA virus within the Pestivirus genus of the Flaviviridae family. Here, we have identified conserved sequence elements observed in nucleotide-binding motifs (NBM) that hydrolyze NTPs within the CSFV non-structural (NS) protein NS4B. Expressed NS4B protein hydrolyzes both ATP and GTP. Substitutions of critical residues within the identified NS4B NBM Walker A and B motifs significantly impair the ATPase and GTPase activities of expressed proteins. Similar mutations introduced into the genetic backbone of a full-length cDNA copy of CSFV strain Brescia rendered no infectious viruses or viruses with impaired replication capabilities, suggesting that this NTPase activity is critical for the CSFV cycle. Recovered mutant viruses retained a virulent phenotype, as parental strain Brescia, in infected swine. These results have important implications for developing novel antiviral strategies against CSFV infection.

Immunogenic and replicative properties of classical swine fever virus replicon particles modified to induce IFN-α/β and carry foreign genes

Virus replicon particles (VRP) are genetically engineered infectious virions incapable of generating progeny virus due to partial or complete deletion of at least one structural gene. VRP fulfil the criteria of a safe vaccine and gene delivery system. With VRP derived from classical swine fever virus (CSF-VRP), a single intradermal vaccination protects from disease. Spreading of the challenge virus in the host is however not completely abolished. Parameters that are critical for immunogenicity of CSF-VRP are not well characterized. Considering the importance of type I interferon (IFN-α/β) to immune defence development, we generated IFN-α/β-inducing VRP to determine how this would influence vaccine efficacy. We also evaluated the effect of co-expressing granulocyte macrophage colony-stimulating factor (GM-CSF) in the vaccine context. The VRP were capable of long-term replication in cell culture despite the presence of IFN-α/β. In vivo, RNA replication was essential for the induction of an immune response. IFN-α/β-inducing and GM-CSF-expressing CSF-VRP were similar to unmodified VRP in terms of antibody and peripheral T-cell responses, and in reducing the blood levels of challenge virus RNA. Importantly, the IFN-α/β-inducing VRP did show increased efficacy over the unmodified VRP in terms of B-cell and T-cell responses, when tested with secondary immune responses by in vitro restimulation assay.

Enhanced immunity against classical swine fever in pigs induced by prime-boost immunization using an alphavirus replicon-vectored DNA vaccine and a recombinant adenovirus

Classical swine fever (CSF) - caused by the classical swine fever virus (CSFV) - is a fatal disease of pigs that is responsible for extensive losses to the swine industry worldwide. We had demonstrated previously that a prime-boost vaccination strategy using an alphavirus (Semliki Forest virus, SFV) replicon-vectored DNA vaccine (pSFV1CS-E2) and a recombinant adenovirus (rAdV-E2) expressing the E2 glycoprotein of CSFV induced enhanced immune responses in a mouse model. In this study, we evaluated further the efficacy of the heterologous prime-boost immunization approach in pigs, the natural host of CSFV. The results showed that the pigs (n=5) receiving pSFV1CS-E2/rAdV-E2 heterologous prime-boost immunization developed significantly higher titers of CSFV-specific neutralizing antibodies and comparable CD4(+) and CD8(+) T-cell proliferation, compared to the pigs receiving double immunizations with rAdV-E2 alone. When challenged with virulent CSFV Shimen strain, the pigs of the heterologous prime-boost group did not show clinical symptoms or viremia, which were observed in one of the 5 pigs immunized with rAdV-E2 alone and all the 5 control pigs immunized with an empty adenovirus. The results demonstrate that the heterologous DNA prime and recombinant adenovirus boost strategy can induce solid protective immunity.

Generation and efficacy evaluation of a recombinant adenovirus expressing the E2 protein of classical swine fever virus

Classical swine fever virus (CSFV) is the causative agent of classical swine fever (CSF), which causes significant economic losses to the pig industry worldwide. The E2 glycoprotein of CSFV is the main target for neutralizing antibodies. This study was aimed to develop a recombinant human adenovirus type 5 expressing the CSFV E2 gene (rAdV-E2) and evaluate its efficacy in rabbits and pigs. The results showed that the rabbits and the pigs immunized with the rAdV-E2 developed high-level CSFV-specific neutralizing antibodies. The rAdV-E2-immunized rabbits were protected from fever induced by infection with C-strain, which is pathogenic to the rabbit, and the rAdV-E2-immunized pigs were protected from lethal challenge with highly virulent Shimen strain. This indicates that the recombinant adenovirus can be an attractive candidate vaccine for preventing CSF.

Zinc binding in pestivirus N(pro) is required for interferon regulatory factor 3 interaction and degradation

Pestiviruses, such as bovine viral diarrhea virus and classical swine fever virus (CSFV), use the viral protein N(pro) to subvert host cell antiviral responses. N(pro) is the first protein encoded by the single large open reading frame of the pestivirus positive-sense RNA genome and has an autoproteolytic activity, cleaving itself off from the polyprotein. N(pro) also targets interferon regulatory factor 3 (IRF3), a transcription factor for alpha/beta interferon genes, and promotes its proteasomal degradation, a process that is independent of the proteolytic activity of N(pro). We determined that N(pro) contains a novel metal-binding TRASH motif consisting of Cys-X(21)-Cys-X(3)-Cys (where X is any amino acid) at its C-terminus. We also found that N(pro) coordinates a single zinc atom as determined by graphite furnace-atomic absorption spectrophotometry and inductively coupled plasma-mass spectrometry. Mutational and biochemical analyses show that the cysteine residues in the TRASH motif are required for zinc binding and protein stability. Individual substitutions of the cysteines in the TRASH motif of CSFV N(pro) abolished the interaction of N(pro) with IRF3 and resulted in the loss of virus-mediated IRF3 degradation in CSFV-infected cells. Thus, the zinc-binding ability of N(pro) in pestiviruses appears to be essential for the virus-mediated degradation of IRF3.

Sub-genomic replicon and virus-like particles of Omsk hemorrhagic fever virus

Omsk hemorrhagic fever virus (OHFV) is a member of the tick-borne encephalitis serocomplex of flaviviruses, and causes hemorrhagic disease in humans. To investigate the molecular mechanisms involved in OHFV pathogenesis, we constructed several subgenomic OHFV replicons containing large deletions in the structural region. Replicon RNA was introduced into BHK cells by transfection and the production of viral proteins was monitored by IFA. GFP and luciferase genes were inserted into the OHFV replicon, and these reporter genes were expressed in cells harboring replicating replicon RNA. OHFV replicons were packaged into single-round infectious virus-like particles (VLPs) by sequential transfection with replicon RNA and a plasmid expressing the viral structural proteins. Reporter genes were expressed in cells infected with VLPs, and the infection was inhibited by neutralizing antibodies. These replicon and VLP systems will be useful tools for investigating the molecular mechanism of OHFV pathogenicity.

Classical swine fever virus can remain virulent after specific elimination of the interferon regulatory factor 3-degrading function of Npro

Pestiviruses prevent alpha/beta interferon (IFN-alpha/beta) production by promoting proteasomal degradation of interferon regulatory factor 3 (IRF3) by means of the viral N(pro) nonstructural protein. N(pro) is also an autoprotease, and its amino-terminal coding sequence is involved in translation initiation. We previously showed with classical swine fever virus (CSFV) that deletion of the entire N(pro) gene resulted in attenuation in pigs. In order to elaborate on the role of the N(pro)-mediated IRF3 degradation in classical swine fever pathogenesis, we searched for minimal amino acid substitutions in N(pro) that would specifically abrogate this function. Our mutational analyses showed that degradation of IRF3 and autoprotease activity are two independent but structurally overlapping functions of N(pro). We describe two mutations in N(pro) that eliminate N(pro)-mediated IRF3 degradation without affecting the autoprotease activity. We also show that the conserved standard sequence at these particular positions is essential for N(pro) to interact with IRF3. Surprisingly, when these two mutations are introduced independently in the backbones of highly and moderately virulent CSFV, the resulting viruses are not attenuated, or are only partially attenuated, in 8- to 10-week-old pigs. This contrasts with the fact that these mutant viruses have lost the capacity to degrade IRF3 and to prevent IFN-alpha/beta induction in porcine cell lines and monocyte-derived dendritic cells. Taken together, these results demonstrate that contrary to previous assumptions and to the case for other viral systems, impairment of IRF3-dependent IFN-alpha/beta induction is not a prerequisite for CSFV virulence.