Bluetongue virus proteins and particles and their role in virus entry, assembly, and release

An insight into G-quadruplexes: Identification and potential therapeutic targets in livestock viruses

G-quadruplexes (G4s) are non-canonical nucleic acids secondary structures that involve in the regulation of some key biological processes, such as replication, transcription, and translation. G4s have been extensively described in the genomes of human and related diseases. In recent years, G4s were identified in several livestock viruses, including those of the emerging epidemics, like Nipah virus (NiV). Since their discovery, G4s have been developed as the potential antiviral targets, and the employment of G4 ligands or interacting proteins has helped to expound the viral infectivity and pathogenesis through G4-mediated mechanisms, and highlight the potential as therapeutic approaches. However, the comprehensively studies of G4s in livestock viruses have not been summarized. This review delves into the reported literatures of G4s in livestock viruses, particular focus on the presence, biophysical identification, and possible function of G4s in viral genome, summarizing the G4 ligands, interacted proteins and aptamers on antiviral applications. The strengths and the challenges of G4 targeting in this field are also discussed. Therefore, this review will shed new light on the future development of highly potent and targeting antiviral therapy.

Identification of three novel linear B-cell epitopes on VP7 of African horse sickness virus using monoclonal antibodies

African horse sickness (AHS) is an acute and subacute infectious disease of equine species caused by the African horse sickness virus (AHSV). The VP7 of AHSV is a group-specific protein conserved in all serotypes and is an excellent candidate for the serological diagnosis and an AHS vaccine component. However, to date, B-cell epitopes on the AHSV VP7 recognized by humoral immune responses remain unclear. This study expressed the recombinant AHSV VP7 soluble in Escherichia coli and purified it for mouse immunization. Four monoclonal antibodies (mAbs) were screened and identified by hybridoma cell fusion, clonal purification, and immunological assays. The B-cell epitopes, recognized by monoclonal antibodies 4B5, 3G10, 3D7, and 4D6, were identified by a series of truncated overlapping peptides expressed as glutathione S-transferase (GST)-fusion proteins. The results revealed that 4B5 recognized the 124VQTGRYAGA132 motif, 3G10 recognized the 140RYYVPQGRT148 motif, while 3D7 and 4D6 recognized the 292QPINPPIFP300 motif. Amino acid sequence alignment indicated that three novel B-cell epitopes were conserved among various AHSV serotypes but unconserved in other orbiviruses, such as the bluetongue and epidemic hemorrhagic disease viruses. This study informs on the antigenic epitopes of AHSV VP7, facilitating future investigations into the serological diagnosis method and epitope-based vaccines against AHSV.

Whole-transcriptome analyses of ovine lung microvascular endothelial cells infected with bluetongue virus

Bluetongue virus (BTV) infection induces profound and intricate changes in the transcriptional profile of the host to facilitate its survival and replication. However, there have been no whole-transcriptome studies on ovine lung microvascular endothelial cells (OLMECs) infected with BTV. In this study, we comprehensively analysed the whole-transcriptome sequences of BTV-1 serotype-infected and mock-infected OLMECs and subsequently performed bioinformatics differential analysis. Our analysis revealed 1215 differentially expressed mRNA transcripts, 82 differentially expressed long noncoding RNAs (lncRNAs) transcripts, 63 differentially expressed microRNAs (miRNAs) transcripts, and 42 differentially expressed circular RNAs (circRNAs) transcripts. Annotation from Gene Ontology, enrichment from the Kyoto Encyclopedia of Genes and Genomes, and construction of endogenous competing RNA network analysis revealed that the differentially expressed RNAs primarily participated in viral sensing and signal transduction pathways, antiviral and immune responses, inflammation, and extracellular matrix (ECM)-related pathways. Furthermore, protein‒protein interaction network analysis revealed that BTV may regulate the conformation of ECM receptor proteins and change their biological activity through a series of complex mechanisms. Finally, on the basis of real-time fluorescence quantitative polymerase chain reaction results, the expression trends of the differentially expressed RNA were consistent with the whole-transcriptome sequencing data, such as downregulation of the expression of COL4A1, ITGA8, ITGB5, and TNC and upregulation of the expression of CXCL10, RNASEL, IRF3, IRF7, and IFIHI. This study provides a novel perspective for further investigations of the mechanism of the ECM in the BTV-host interactome and the pathogenesis of lung microvascular endothelial cells.

Detection, Characterization and Sequencing of BTV Serotypes Circulating in Cuba in 2022

In Cuba, despite a high sero-prevalence of bluetongue virus (BTV), circulating serotypes remain unknown. The aim of this study was to identify circulating BTV serotypes in farms throughout the western region of Cuba. Blood samples were collected from 200 young cattle and sheep between May and July 2022 for virological analyses (PCR, viral isolation and virus neutralization) and genome sequencing. The results confirmed viral circulation, with viro-prevalence of 25% for BTV. The virus was isolated from 18 blood samples and twelve BTV serotypes were identified by sequencing RT-PCR products targeting the segment 2 of the BTV genome (BTV-1, 2, 3, 6, 10, 12, 13, 17, 18, 19, 22 and 24). Finally, the full genome sequences of 17 Cuban BTV isolates were recovered using a Sequence Independent Single Primer Amplification (SISPA) approach combined to MinION Oxford Nanopore sequencing technology. All together, these results highlight the co-circulation of a wide diversity of BTV serotypes in a quite restricted area and emphasize the need for entomological and livestock surveillance, particularly in light of recent changes in the global distribution and nature of BTV infections.

Recent advances in Reovirales viruses reverse genetics research

Reovirales are segmented double-strand RNA viruses with a broad host range that pose a serious threat to human and animal health. However, there are numerous viral species within the Reovirales, some of which have lagged behind other RNA viruses in the study of their biology due to the lack of an effective reverse genetics (RG) system. The RG systems are the most powerful tools for studying viral protein function, viral gene expression regulation, viral pathogenesis, and the generation of engineered vaccines. Recently, several entirely plasmid-based RG systems have been developed for several members of the Reovirales. This review outlines the development and future direction of the RG system for the best studied Reovirales viruses.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

A Brief Overview of Potential Treatments for Viral Diseases Using Natural Plant Compounds: The Case of SARS-Cov

The COVID-19 pandemic, as well as the more general global increase in viral diseases, has led researchers to look to the plant kingdom as a potential source for antiviral compounds. Since ancient times, herbal medicines have been extensively applied in the treatment and prevention of various infectious diseases in different traditional systems. The purpose of this review is to highlight the potential antiviral activity of plant compounds as effective and reliable agents against viral infections, especially by viruses from the coronavirus group. Various antiviral mechanisms shown by crude plant extracts and plant-derived bioactive compounds are discussed. The understanding of the action mechanisms of complex plant extract and isolated plant-derived compounds will help pave the way towards the combat of this life-threatening disease. Further, molecular docking studies, in silico analyses of extracted compounds, and future prospects are included. The in vitro production of antiviral chemical compounds from plants using molecular pharming is also considered. Notably, hairy root cultures represent a promising and sustainable way to obtain a range of biologically active compounds that may be applied in the development of novel antiviral agents.

Inhibition of the IFN Response by Bluetongue Virus: The Story So Far

Bluetongue virus (BTV) is the prototypical orbivirus that belongs to the Reoviridae family. BTV infection produces a disease in ruminants, particularly in sheep, that results in economic losses through reduced productivity. BTV is transmitted by the bite of Culicoides spp. midges and is nowadays distributed globally throughout subtropical and even temperate regions. As most viruses, BTV is susceptible to the IFN response, the first line of defense employed by the immune system to combat viral infections. In turn, BTV has evolved strategies to counter the IFN response and promote its replication. The present review we will revise the works describing how BTV interferes with the IFN response.

Vaccination With Recombinant Adenoviruses Expressing the Bluetongue Virus Subunits VP7 and VP2 Provides Protection Against Heterologous Virus Challenge

Bluetongue virus (BTV) is the causative agent of a disease that affects domestic and wild ruminants and leads to critical economic losses. BTV is an arbovirus from the Reoviridae family that is typically transmitted by the bite of infected Culicoides midges. BTV possesses multiple serotypes (up to 28 have been described), and immunity to one serotype offers little cross-protection to other serotypes. The design of vaccines that provide protection across multiple serotypes is therefore highly desirable to control this disease. We previously reported that a recombinant replication-defective human adenovirus serotype 5 (Ad5) that expresses the VP7 inner core protein of BTV serotype 8 (Ad5VP7-8) induced T-cell responses and provided protection. In the present work, we evaluated as BTV vaccine the combination of Ad5VP7-8 with another recombinant Ad5 that expresses the outer core protein VP2 from BTV-1 (Ad5VP2-1). The combination of Ad5VP2-1 and Ad5VP7-8 protected against homologous BTV challenge (BTV-1 and BTV-8) and partially against heterologous BTV-4 in a murine model. Cross-reactive anti-BTV immunoglobulin G (IgG) were detected in immunized animals, but no significant titers of neutralizing antibodies were elicited. The Ad5VP7-8 immunization induced T-cell responses that recognized all three serotypes tested in this study and primed cytotoxic T lymphocytes specific for VP7. This study further confirms that targeting antigenic determinant shared by several BTV serotypes using cellular immunity could help develop multiserotype BTV vaccines.

The VP3 Protein of Bluetongue Virus Associates with the MAVS Complex and Interferes with the RIG-I-Signaling Pathway

Bluetongue virus (BTV), an arbovirus transmitted by Culicoides biting midges, is a major concern of wild and domestic ruminants. While BTV induces type I interferon (alpha/beta interferon [IFN-α/β]) production in infected cells, several reports have described evasion strategies elaborated by this virus to dampen this intrinsic, innate response. In the present study, we suggest that BTV VP3 is a new viral antagonist of the IFN-β synthesis. Indeed, using split luciferase and coprecipitation assays, we report an interaction between VP3 and both the mitochondrial adapter protein MAVS and the IRF3-kinase IKKε. Overall, this study describes a putative role for the BTV structural protein VP3 in the control of the antiviral response.

Bluetongue virus non-structural protein 3 (NS3) and NS4 coordinatively antagonize type Ⅰ interferon signaling by targeting STAT1

Previous studies have pointed out that bluetongue virus (BTV) down-regulates the expression levels of type Ⅰ interferon (IFN-Ⅰ) and inhibits IFN-Ⅰ signaling by targeting on the Janus tyrosine kinase (JAK)-signal transducer and activator of transcription protein (STAT) pathway. However, individual viral protein could not effectively block IFN-Ⅰ signaling. There is a need to explore the underlying mechanisms by which viral proteins of BTV coordinate to antagonize the IFN-Ⅰ signaling. We investigated the coordinative role of BTV-1 nonstructural protein 3 (NS3) and NS4 in counteracting IFN-Ⅰ signaling in the JAK-STAT pathway by directly interacting with STAT1. The NS3 and NS4 targeted the SH2 domain of STAT1 to inhibit its phosphorylation, heterodimerization, nuclear translocation, as well as activation of downstream genes of the JAK-STAT pathway. NS3 and NS4 impaired STAT1 phosphorylation induced by IFN-Ⅰ in a dose dependent manner. Overall, this study confirmed that NS3 and NS4 of BTV participate in interfering with IFN-Ⅰ signaling process. Also, a new mechanism employed by BTV to evade host innate immune responses was revealed.

Roles of MOV10 in Animal RNA Virus Infection

Animal epidemic diseases caused by RNA viruses are the primary threat to the livestock industry, and understanding the mechanisms of RNA virus clearance from target cells is critical to establish an effective method to reduce economic losses. As an SF-1, ATP-dependent RNA helicase in the UPF1p family, MOV10 participates in the RNA degradation of multiple viruses mediated via miRNA pathways and therefore contributes to a decrease in the replication of RNA viruses. This review primarily focuses on the bioactivity of MOV10, the mechanism of RNA virus removal, and the potential roles of MOV10 in RNA virus clearance. In addition, clues are provided to reduce animal diseases caused by RNA viruses.

Virus-induced autophagic degradation of STAT2 as a mechanism for interferon signaling blockade

The mammalian interferon (IFN) signaling pathway is a primary component of the innate antiviral response, and viral pathogens have evolved multiple mechanisms to antagonize this pathway and to facilitate infection. Bluetongue virus (BTV), an orbivirus of the Reoviridae family, is transmitted by midges to ruminants and causes a disease that produces important economic losses and restriction to animal trade and is of compulsory notification to the World Organization for Animal Health (OIE). Here, we show that BTV interferes with IFN-I and IFN-II responses in two ways, by blocking STAT1 phosphorylation and by degrading STAT2. BTV-NS3 protein, which is involved in virion egress, interacts with STAT2, and induces its degradation by an autophagy-dependent mechanism. This STAT2 degradative process requires the recruitment of an E3-Ub-ligase to NS3 as well as NS3 K63 polyubiquitination. Taken together, our study identifies a new mechanism by which a virus degrades STAT2 for IFN signaling blockade, highlighting the diversity of mechanisms employed by viruses to subvert the IFN response.

Diagnosing bluetongue virus in domestic ruminants: current perspectives

This review provides an overview of current and potential new diagnostic techniques against bluetongue virus (BTV), an Orbivirus transmitted by arthropods that affects ruminants. Bluetongue is a disease currently notifiable to the World Organization for Animal Health (OIE), causing great economic losses due to decreased trade associated with bluetongue outbreaks and high mortality and morbidity. BTV cross-reacts with many antigenically related viruses including viruses that causes African Horse sickness and epizootic haemorrhagic disease of deer. Therefore, reliable diagnostic approaches to detect BTV among these other antigenically related viruses are used or being developed. The antigenic determinant for differentiation of virus species/serogroups among orbiviruses is the VP7 protein, meanwhile VP2 is serotype specific. Serologically, assays are established in many laboratories, based mainly on competitive ELISA or serum neutralization assay (virus neutralization assay [VNT]) although new techniques are being developed. Virus isolation from blood or semen is, additionally, another means of BTV diagnosis. Nevertheless, most of these techniques for viral isolation are time-consuming and expensive. Currently, reverse-transcription polymerase chain reaction (RT-PCR) panels or real-time RT-PCR are widely used methods although next-generation sequencing remains of interest for future virus diagnosis.

Recall T cell responses to bluetongue virus produce a narrowing of the T cell repertoire

In most viral infections, recall T cell responses are critical for protection. The magnitude of these secondary responses can also affect the CD8 and CD4 epitope repertoire diversity. Bluetongue virus (BTV) infection in sheep elicits a T cell response that contributes to viremia control and could be relevant for cross-protection between BTV serotypes. Here, we characterized CD4⁺ and CD8⁺ T cell responses during primary and recall responses. During primary immune responses, both CD4⁺ and CD8⁺ T cell populations expanded by 14 days post-infection (dpi). CD4⁺ T cell populations showed a lower peak of expansion and prolonged contraction phase compared to CD8⁺ T cell populations. Recall responses to BTV challenge led to BTV-specific expansion and activation of CD8⁺ but not of CD4⁺ T cells. The evolution of the BTV-specific TCR repertoire was also characterized in response to VP7 peptide stimulation. Striking differences in repertoire development were noted over the time-course of infection. During primary responses, a broader repertoire was induced for MHC-I and MHC-II epitopes. However, during memory responses, a narrowed repertoire was activated towards a dominant motif in VP7 comprising amino acids 139–291. Monocytes were also examined, and expanded during acute infection resolution. In addition, pro-inflammatory cytokine levels increased after BTV inoculation and persisted throughout the experiment, indicative of a prolonged inflammatory state during BTV infections. These findings could have implications for vaccine design as the narrowing memory T cell repertoire induced after BTV re-infection could lead to the development of protective immunodominant TCR repertoires that differs between individual sheep.

Novel serotype of bluetongue virus in South America and first report of epizootic haemorrhagic disease virus in Ecuador

Bluetongue virus (BTV) and Epizootic haemorrhagic disease virus (EHDV) are closely related Orbiviruses that affect domestic and wild ruminants. In Ecuador previous serological studies reported the presence of BTV; however, no data are available about the presence of EHDV. In this study, 295 cattle without symptoms of infection were sampled from two farms located in Andean and Amazonian regions and from a slaughterhouse in the coastal region. ELISA analyses showed high prevalence of BTV (98.9%) and EHDV (81.3%) antibodies, and RT-qPCRs revealed the presence of EHDV (24.1%) and BTV (10.2%) genomes in cattle blood samples. Viral isolation allowed to identify EHDV serotype 1 (EHDV1) and BTV serotypes 9 (BTV9), 13 and 18. These findings suggest that BTV and EHDV are enzootic diseases in Ecuador.

Structural Protein VP2 of African Horse Sickness Virus Is Not Essential for Virus Replication In Vitro

The Reoviridae family consists of nonenveloped multilayered viruses with a double-stranded RNA genome consisting of 9 to 12 genome segments. The Orbivirus genus of the Reoviridae family contains African horse sickness virus (AHSV), bluetongue virus, and epizootic hemorrhagic disease virus, which cause notifiable diseases and are spread by biting Culicoides species. Here, we used reverse genetics for AHSV to study the role of outer capsid protein VP2, encoded by genome segment 2 (Seg-2). Expansion of a previously found deletion in Seg-2 indicates that structural protein VP2 of AHSV is not essential for virus replication in vitro In addition, in-frame replacement of RNA sequences in Seg-2 by that of green fluorescence protein (GFP) resulted in AHSV expressing GFP, which further confirmed that VP2 is not essential for virus replication. In contrast to virus replication without VP2 expression in mammalian cells, virus replication in insect cells was strongly reduced, and virus release from insect cells was completely abolished. Further, the other outer capsid protein, VP5, was not copurified with virions for virus mutants without VP2 expression. AHSV without VP5 expression, however, could not be recovered, indicating that outer capsid protein VP5 is essential for virus replication in vitro Our results demonstrate for the first time that a structural viral protein is not essential for orbivirus replication in vitro, which opens new possibilities for research on other members of the Reoviridae family.

IMPORTANCE

Members of the Reoviridae family cause major health problems worldwide, ranging from lethal diarrhea caused by rotavirus in humans to economic losses in livestock production caused by different orbiviruses. The Orbivirus genus contains many virus species, of which bluetongue virus, epizootic hemorrhagic disease virus, and African horse sickness virus (AHSV) cause notifiable diseases according to the World Organization of Animal Health. Recently, it has been shown that nonstructural proteins NS3/NS3a and NS4 are not essential for virus replication in vitro, whereas it is generally assumed that structural proteins VP1 to -7 of these nonenveloped, architecturally complex virus particles are essential. Here we demonstrate for the first time that structural protein VP2 of AHSV is not essential for virus replication in vitro Our findings are very important for virologists working in the field of nonenveloped viruses, in particular reoviruses.

Impaired cellular energy metabolism contributes to bluetongue-virus-induced autophagy

Bluetongue virus (BTV) has been found to trigger autophagy to favor its replication, but the underlying mechanisms have not been clarified. Here, we show that cellular energy metabolism is involved in BTV-induced autophagy. Cellular ATP synthesis was impaired by BTV1 infection, causing metabolic stress, which was responsible for activation of autophagy, since the conversion of LC3 and aggregation of GFP-LC3 (autophagy markers) were suppressed when infection-caused energy depletion was reversed via MP (metabolic substrate) treatment. The reduced virus yields with MP further supported this view. Overall, our findings suggest that BTV1-induced disruption of cellular energy metabolism contributes to autophagy, and this provides new insights into BTV-host interactions.

Dissection and integration of the autophagy signaling network initiated by bluetongue virus infection: crucial candidates ERK1/2, Akt and AMPK

Bluetongue virus (BTV), a complex double-stranded segmented RNA virus, has been found to initiate cellular autophagy for its own benefit. Here, with a view to understanding the underlying mechanisms, we first systematically dissected the exact signaling network in BTV-induced autophagy. We found that the activity of mTOR, a crucial pivot, was inhibited by BTV1 infection, subsequently leading to downstream p70S6K suppression and autophagy initiation. We then explored the upstream regulators of mTOR and analyzed their activities via a series of assays. We found BTV1-induced autophagy to be independent of the ERK1/2 signaling pathway. However, the BTV1-induced inhibition of PI3K/Akt was found to be partially responsible for mTOR inactivation and subsequent autophagy initiation. Furthermore, we found unexpectedly that AMPK seemed to play a more important role in BTV1-induced autophagy. Elevated [Ca²⁺]_cyto-mediated activation of CaMKKβ exactly managed the activation of AMPK, which then positively regulated autophagy through suppressing mTOR. We must emphasize that TSC2 is a fatal mediator between upstream Akt or AMPK and downstream mTOR through its phosphorylation. Taken together, our data suggested that the BTV1-induced inhibition of the Akt-TSC2-mTOR pathway and the upregulation of the AMPK-TSC2-mTOR pathway both contributed to autophagy initiation and further favored virus replication.

Balance of RNA sequence requirement and NS3/NS3a expression of segment 10 of orbiviruses

Orbiviruses are insect-transmitted, non-enveloped viruses with a ten-segmented dsRNA genome of which the bluetongue virus (BTV) is the prototype. Viral non-structural protein NS3/NS3a is encoded by genome segment 10 (Seg-10), and is involved in different virus release mechanisms. This protein induces specific release via membrane disruptions and budding in both insect and mammalian cells, but also the cytopathogenic release that is only seen in mammalian cells. NS3/NS3a is not essential for virus replication in vitro with BTV Seg-10 containing RNA elements essential for virus replication, even if protein is not expressed. Recently, new BTV serotypes with distinct NS3/NS3a sequence and cell tropism have been identified. Multiple studies have hinted at the importance of Seg-10 in orbivirus replication, but the exact prerequisites are still unknown. Here, more insight is obtained with regard to the needs for orbivirus Seg-10 and the balance between protein expression and RNA elements. Multiple silent mutations in the BTV NS3a ORF destabilized Seg-10, resulting in deletions and sequences originating from other viral segments being inserted, indicating strong selection at the level of RNA during replication in mammalian cells in vitro. The NS3a ORFs of other orbiviruses were successfully exchanged in BTV1 Seg-10, resulting in viable chimeric viruses. NS3/NS3a proteins in these chimeric viruses were generally functional in mammalian cells, but not in insect cells. NS3/NS3a of the novel BTV serotypes 25 and 26 affected virus release from Culicoides cells, which might be one of the reasons for their distinct cell tropism.

Protective Efficacy in Sheep of Adenovirus-Vectored Vaccines against Bluetongue Virus Is Associated with Specific T Cell Responses

Bluetongue virus (BTV) is an economically important Orbivirus of the Reoviridae family that causes a hemorrhagic disease in ruminants. Its control has been achieved by inactivated-vaccines that have proven to protect against homologous BTV challenge although unable to induce long-term immunity. Therefore, a more efficient control strategy needs to be developed. Recombinant adenovirus vectors are lead vaccine candidates for protection of several diseases, mainly because of their potency to induce potent T cell immunity. Here we report the induction of humoral and T-cell mediated responses able to protect animals against BTV challenge by recombinant replication-defective human adenovirus serotype 5 (Ad5) expressing either VP7, VP2 or NS3 BTV proteins. First we used the IFNAR(-/-) mouse model system to establish a proof of principle, and afterwards we assayed the protective efficacy in sheep, the natural host of BTV. Mice were completely protected against BTV challenge, developing humoral and BTV-specific CD8+- and CD4+-T cell responses by vaccination with the different rAd5. Sheep vaccinated with Ad5-BTV-VP2 and Ad5-BTV-VP7 or only with Ad5-BTV-VP7 and challenged with BTV showed mild disease symptoms and reduced viremia. This partial protection was achieved in the absence of neutralizing antibodies but strong BTV-specific CD8+ T cell responses in those sheep vaccinated with Ad5-BTV-VP7. These data indicate that rAd5 is a suitable vaccine vector to induce T cell immunity during BTV vaccination and provide new data regarding the relevance of T cell responses in protection during BTV infection.

Non-structural protein NS3/NS3a is required for propagation of bluetongue virus in Culicoides sonorensis

BACKGROUND

Bluetongue virus (BTV) causes non-contagious haemorrhagic disease in ruminants and is transmitted by Culicoides spp. biting midges. BTV encodes four non-structural proteins of which NS3/NS3a is functional in virus release. NS3/NS3a is not essential for in vitro virus replication. However, deletion of NS3/NS3a leads to delayed virus release from mammalian cells and largely reduces virus release from insect cells. NS3/NS3a knockout BTV in sheep causes no viremia, but induces sterile immunity and is therefore proposed to be a Disabled Infectious Single Animal (DISA) vaccine candidate. In the absence of viremia, uptake of this vaccine strain by blood-feeding midges would be highly unlikely. Nevertheless, unintended replication of vaccine strains within vectors, and subsequent recombination or re-assortment resulting in virulent phenotypes and transmission is a safety concern of modified-live vaccines.

METHODS

The role of NS3/NS3a in replication and dissemination of BTV1, expressing VP2 of serotype 2 within colonized Culicoides sonorensis midges was investigated. Virus strains were generated using reverse genetics and their growth was examined in vitro. A laboratory colony of C. sonorensis, a known competent BTV vector, was fed or injected with BTV with or without expressing NS3/NS3a and replication in the midge was examined using RT PCR. Crossing of the midgut infection barrier was examined by separate testing of midge heads and bodies.

RESULTS

Although the parental NS3/NS3a expressing strain was not able to replicate and disseminate within C. sonorensis after oral feeding, this virus was able to replicate efficiently when the midgut infection barrier was bypassed by intrathoracic injection, whereas the NS3/NS3a knockout mutant was unable to replicate. This demonstrates that NS3/NS3a is required for viral replication within Culicoides.

CONCLUSION

The lack of viremia and the inability to replicate within the vector, clearly demonstrate the inability of NS3/NS3a knockout DISA vaccine strains to be transmitted by midges.

Autophagy Activated by Bluetongue Virus Infection Plays a Positive Role in Its Replication

Bluetongue virus (BTV) is an important pathogen of wild and domestic ruminants. Despite extensive study in recent decades, the interplay between BTV and host cells is not clearly understood. Autophagy as a cellular adaptive response plays a part in many viral infections. In our study, we found that BTV1 infection triggers the complete autophagic process in host cells, as demonstrated by the appearance of obvious double-membrane autophagosome-like vesicles, GFP-LC3 dots accumulation, the conversion of LC3-I to LC3-II and increased levels of autophagic flux in BSR cells (baby hamster kidney cell clones) and primary lamb lingual epithelial cells upon BTV1 infection. Moreover, the results of a UV-inactivated BTV1 infection assay suggested that the induction of autophagy was dependent on BTV1 replication. Therefore, we investigated the role of autophagy in BTV1 replication. The inhibition of autophagy by pharmacological inhibitors (3-MA, CQ) and RNA interference (siBeclin1) significantly decreased viral protein synthesis and virus yields. In contrast, treating BSR cells with rapamycin, an inducer of autophagy, promoted viral protein expression and the production of infectious BTV1. These findings lead us to conclude that autophagy is activated by BTV1 and contributes to its replication, and provide novel insights into BTV-host interactions.

Structure-based identification of functional residues in the nucleoside-2'-O-methylase domain of Bluetongue virus VP4 capping enzyme

Bluetongue virus (BTV) encodes a single capping protein, VP4, which catalyzes all reactions required to generate cap1 structures on nascent viral transcripts. Further, structural analysis by X-ray crystallography indicated each catalytic reaction is arranged as a discrete domain, including a nucleoside-2'-O-methyltransferase (2'-O MTase). In this study, we have exploited the structural information to identify the residues that are important for the catalytic activity of 2'-O MTase of VP4 and their influence on BTV replication. The effect of these mutations on GMP binding, guanylyltransferase (GTase) and methylase activities were analysed by a series of in vitro biochemical assays using recombinant mutant proteins; subsequently their effects on virus replication were assessed by introducing the same mutations in replicating viral genome using a reverse genetics system. Our data showed that single substitution mutations in the catalytic tetrad K-D-K-E were sufficient to abolish 2'-O MTase activity in vitro and to completely abrogate BTV replication in cells; although these mutants retained the upstream GMP binding, GTase and guanine-N7-methyltransferase activities. Mutations of the surrounding substrate-binding pocket (predicted to recruit cap0) had variable effects on in vitro VP4 capping activity. Only triple but not single substitution mutations of these residues in genome resulted in reduced virus replication kinetics. This is the first report investigating the importance of 2'-O MTase function for any member of the Reoviridae and highlights the significance of K-D-K-E tetrad and surrounding residues for the efficiency of 2'-O MTase activity and in turn, for virus fitness.

Application of bluetongue Disabled Infectious Single Animal (DISA) vaccine for different serotypes by VP2 exchange or incorporation of chimeric VP2

Bluetongue is a disease of ruminants caused by the bluetongue virus (BTV). Bluetongue outbreaks can be controlled by vaccination, however, currently available vaccines have several drawbacks. Further, there are at least 26 BTV serotypes, with low cross protection. A next-generation vaccine based on live-attenuated BTV without expression of non-structural proteins NS3/NS3a, named Disabled Infectious Single Animal (DISA) vaccine, was recently developed for serotype 8 by exchange of the serotype determining outer capsid protein VP2. DISA vaccines are replicating vaccines but do not cause detectable viremia, and induce serotype specific protection. Here, we exchanged VP2 of laboratory strain BTV1 for VP2 of European serotypes 2, 4, 8 and 9 using reverse genetics, without observing large effects on virus growth. Exchange of VP2 from serotype 16 and 25 was however not possible. Therefore, chimeric VP2 proteins of BTV1 containing possible immunogenic regions of these serotypes were studied. BTV1, expressing 1/16 chimeric VP2 proteins was functional in virus replication in vitro and contained neutralizing epitopes of both serotype 1 and 16. For serotype 25 this approach failed. We combined VP2 exchange with the NS3/NS3a negative phenotype in BTV1 as previously described for serotype 8 DISA vaccine. DISA vaccine with 1/16 chimeric VP2 containing amino acid region 249-398 of serotype 16 raised antibodies in sheep neutralizing both BTV1 and BTV16. This suggests that DISA vaccine could be protective for both parental serotypes present in chimeric VP2. We here demonstrate the application of the BT DISA vaccine platform for several serotypes and further extend the application for serotypes that are unsuccessful in single VP2 exchange.

Bluetongue virus without NS3/NS3a expression is not virulent and protects against virulent bluetongue virus challenge

Bluetongue is a disease in ruminants caused by the bluetongue virus (BTV), and is spread by Culicoides biting midges. Bluetongue outbreaks cause huge economic losses and death in sheep in several parts of the world. The most effective measure to control BTV is vaccination. However, both commercially available vaccines and recently developed vaccine candidates have several shortcomings. Therefore, we generated and tested next-generation vaccines for bluetongue based on the backbone of a laboratory-adapted strain of BTV-1, avirulent BTV-6 or virulent BTV-8. All vaccine candidates were serotyped with VP2 of BTV-8 and did not express NS3/NS3a non-structural proteins, due to induced deletions in the NS3/NS3a ORF. Sheep were vaccinated once with one of these vaccine candidates and were challenged with virulent BTV-8 3 weeks after vaccination. The NS3/NS3a knockout mutation caused complete avirulence for all three BTV backbones, including for virulent BTV-8, indicating that safety is associated with the NS3/NS3a knockout phenotype. Viraemia of vaccine virus was not detected using sensitive PCR diagnostics. Apparently, the vaccine viruses replicated only locally, which will minimize spread by the insect vector. In particular, the vaccine based on the BTV-6 backbone protected against disease and prevented viraemia of challenge virus, showing the efficacy of this vaccine candidate. The lack of NS3/NS3a expression potentially enables the differentiation of infected from vaccinated animals, which is important for monitoring virus spread in vaccinated livestock. The disabled infectious single-animal vaccine for bluetongue presented here is very promising and will be the subject of future studies.

VP2, VP7, and NS1 proteins of bluetongue virus targeted in avian reovirus muNS-Mi microspheres elicit a protective immune response in IFNAR(-/-) mice

Vaccination is critical for controlling the spread of bluetongue virus (BTV). The inactivated BTV vaccines that are now being used in Europe are effective in preventing outbreaks of BTV but secondary effects associated to repetitive inoculation of aluminum-containing adjuvants and the need to develop safer, cross-reactive, and more efficacious vaccines with differential diagnostic capability have re-stimulated the interest in developing improved vaccination strategies against BTV. We have engineered a subunit BTV vaccine candidate based on proteins VP2, VP7, and NS1 of BTV-4 incorporated into avian reovirus (ARV) muNS-Mi microspheres (MS-VP2/MS-VP7/MS-NS1). IFNAR(-/-) mice immunized with MS-VP2/MS-VP7/MS-NS1 without adjuvant generated significant levels of neutralizing antibodies specific to BTV-4. In addition, vaccination stimulated specific T cell responses, predominantly CD4+, against the virus. Immunized mice were fully protected against a homologous challenge with a lethal dose of BTV-4 and partially cross-protected against a heterologous challenge with a lethal dose of BTV-1. These results support MS-VP2/MS-VP7/MS-NS1 as a promising subunit vaccine candidate against multiple serotypes of BTV as well as the use of microspheres as an alternative delivery method with potent intrinsic adjuvant activity.

Dual modulation of type I interferon response by bluetongue virus

Bluetongue virus (BTV) is a double-stranded RNA (dsRNA) virus that causes an economically important disease in ruminants. BTV infection is a strong inducer of type I interferon (IFN-I) in multiple cell types. It has been shown recently that BTV and, more specifically, the nonstructural protein NS3 of BTV are able to modulate the IFN-I synthesis pathway. However, nothing is known about the ability of BTV to counteract IFN-I signaling. Here, we investigated the effect of BTV on the IFN-I response pathway and, more particularly, the Janus tyrosine kinase (JAK)/signal transducer and activator of transcription protein (STAT) signaling pathway. We found that BTV infection triggered the expression of IFN-stimulated genes (ISGs) in A549 cells. However, when BTV-infected cells were stimulated with external IFN-I, we showed that activation of the IFN-stimulated response element (ISRE) promoter and expression of ISGs were inhibited. We found that this inhibition involved two different mechanisms that were dependent on the time of infection. After overnight infection, BTV blocked specifically the phosphorylation and nuclear translocation of STAT1. This inhibition correlated with the redistribution of STAT1 in regions adjacent to the nucleus. At a later time point of infection, BTV was found to interfere with the activation of other key components of the JAK/STAT pathway and to induce the downregulation of JAK1 and TYK2 protein expression. Overall, our study indicates for the first time that BTV is able to interfere with the JAK/STAT pathway to modulate the IFN-I response.

IMPORTANCE

Bluetongue virus (BTV) causes a severe disease in ruminants and has an important impact on the livestock economy in areas of endemicity such as Africa. The emergence of strains, such as serotype 8 in Europe in 2006, can lead to important economic losses due to commercial restrictions and prophylactic measures. It has been known for many years that BTV is a strong inducer of type I interferon (IFN-I) in vitro and in vivo in multiple cell types. However, the ability of BTV to interact with the IFN-I system remains unclear. Here, we report that BTV is able to modulate the IFN-I response by interfering with the Janus tyrosine kinase (JAK)/signal transducer and activator of transcription protein (STAT) signaling pathway. These findings contribute to knowledge of how BTV infection interferes with the host's innate immune response and becomes pathogenic. This will also be important for the design of efficacious vaccine candidates.

RNA elements in open reading frames of the bluetongue virus genome are essential for virus replication

Members of the Reoviridae family are non-enveloped multi-layered viruses with a double stranded RNA genome consisting of 9 to 12 genome segments. Bluetongue virus is the prototype orbivirus (family Reoviridae, genus Orbivirus), causing disease in ruminants, and is spread by Culicoides biting midges. Obviously, several steps in the Reoviridae family replication cycle require virus specific as well as segment specific recognition by viral proteins, but detailed processes in these interactions are still barely understood. Recently, we have shown that expression of NS3 and NS3a proteins encoded by genome segment 10 of bluetongue virus is not essential for virus replication. This gave us the unique opportunity to investigate the role of RNA sequences in the segment 10 open reading frame in virus replication, independent of its protein products. Reverse genetics was used to generate virus mutants with deletions in the open reading frame of segment 10. Although virus with a deletion between both start codons was not viable, deletions throughout the rest of the open reading frame led to the rescue of replicating virus. However, all bluetongue virus deletion mutants without functional protein expression of segment 10 contained inserts of RNA sequences originating from several viral genome segments. Subsequent studies showed that these RNA inserts act as RNA elements, needed for rescue and replication of virus. Functionality of the inserts is orientation-dependent but is independent from the position in segment 10. This study clearly shows that RNA in the open reading frame of Reoviridae members does not only encode proteins, but is also essential for virus replication.

Vector independent transmission of the vector-borne bluetongue virus

Bluetongue is an economically important disease of ruminants. The causative agent, Bluetongue virus (BTV), is mainly transmitted by insect vectors. This review focuses on vector-free BTV transmission, and its epizootic and economic consequences. Vector-free transmission can either be vertical, from dam to fetus, or horizontal via direct contract. For several BTV-serotypes, vertical (transplacental) transmission has been described, resulting in severe congenital malformations. Transplacental transmission had been mainly associated with live vaccine strains. Yet, the European BTV-8 strain demonstrated a high incidence of transplacental transmission in natural circumstances. The relevance of transplacental transmission for the epizootiology is considered limited, especially in enzootic areas. However, transplacental transmission can have a substantial economic impact due to the loss of progeny. Inactivated vaccines have demonstrated to prevent transplacental transmission. Vector-free horizontal transmission has also been demonstrated. Since direct horizontal transmission requires close contact of animals, it is considered only relevant for within-farm spreading of BTV. The genetic determinants which enable vector-free transmission are present in virus strains circulating in the field. More research into the genetic changes which enable vector-free transmission is essential to better evaluate the risks associated with outbreaks of new BTV serotypes and to design more appropriate control measures.

Ovine and murine T cell epitopes from the non-structural protein 1 (NS1) of bluetongue virus serotype 8 (BTV-8) are shared among viral serotypes

Bluetongue virus (BTV) is a non-enveloped dsRNA virus that causes a haemorrhagic disease mainly in sheep. It is an economically important Orbivirus of the Reoviridae family. In order to estimate the importance of T cell responses during BTV infection, it is essential to identify the epitopes targeted by the immune system. In the present work, we selected potential T cell epitopes (3 MHC-class II-binding and 8 MHC-class I binding peptides) for the C57BL/6 mouse strain from the BTV-8 non-structural protein NS1, using H2^b-binding predictive algorithms. Peptide binding assays confirmed all MHC-class I predicted peptides bound MHC-class I molecules. The immunogenicity of these 11 predicted peptides was then determined using splenocytes from BTV-8-inoculated C57BL/6 mice. Four MHC-class I binding peptides elicited specific IFN-γ production and generated cytotoxic T lymphocytes (CTL) in BTV-8 infected mice. CTL specific for 2 of these peptides were also able to recognise target cells infected with different BTV serotypes. Similarly, using a combination of IFN-γ ELISPOT, intracellular cytokine staining and proliferation assays, two MHC-class II peptides were identified as CD4⁺ T cell epitopes in BTV-8 infected mice. Importantly, two peptides were also consistently immunogenic in sheep infected with BTV-8 using IFN-γ ELISPOT assays. Both of these peptides stimulated CD4⁺ T cells that cross-reacted with other BTV serotypes. The characterisation of these T cell epitopes can help develop vaccines protecting against a broad spectrum of BTV serotypes and differentiate infected from vaccinated animals.

Bluetongue virus nonstructural protein NS3/NS3a is not essential for virus replication

Orbiviruses form the largest genus of the family Reoviridae consisting of at least 23 different virus species. One of these is the bluetongue virus (BTV) and causes severe hemorrhagic disease in ruminants, and is transmitted by bites of Culicoides midges. BTV is a non-enveloped virus which is released from infected cells by cell lysis and/or a unique budding process induced by nonstructural protein NS3/NS3a encoded by genome segment 10 (Seg-10). Presence of both NS3 and NS3a is highly conserved in Culicoides borne orbiviruses which is suggesting an essential role in virus replication. We used reverse genetics to generate BTV mutants to study the function of NS3/NS3a in virus replication. Initially, BTV with small insertions in Seg-10 showed no CPE but after several passages these BTV mutants reverted to CPE phenotype comparable to wtBTV, and NS3/NS3a expression returned by repair of the ORF. These results show that there is a strong selection for functional NS3/NS3a. To abolish NS3 and/or NS3a expression, Seg-10 with one or two mutated start codons (mutAUG1, mutAUG2 and mutAUG1+2) were used to generate BTV mutants. Surprisingly, all three BTV mutants were generated and the respective AUG^Met→GCC^Ala mutations were maintained. The lack of expression of NS3, NS3a, or both proteins was confirmed by westernblot analysis and immunostaining of infected cells with NS3/NS3a Mabs. Growth of mutAUG1 and mutAUG1+2 virus in BSR cells was retarded in both insect and mammalian cells, and particularly virus release from insect cells was strongly reduced. Our findings now enable research on the role of RNA sequences of Seg-10 independent of known gene products, and on the function of NS3/NS3a proteins in both types of cells as well as in the host and insect vector.

Type I interferon limits the capacity of bluetongue virus to infect hematopoietic precursors and dendritic cells in vitro and in vivo

Hematopoietic stem cells (HSCs) give rise to progenitors with potential to produce multiple cell types, including dendritic cells (DCs). DCs are the principal antigen-presenting cells and represent the crucial link between innate and adaptive immune responses. Bluetongue virus (BTV), an economically important Orbivirus of the Reoviridae family, causes a hemorrhagic disease mainly in sheep and occasionally in other species of ruminants. BTV is transmitted between its mammalian hosts by certain species of biting midges (Culicoides spp.) and is a potent alpha interferon (IFN-α) inducer. In the present report, we show that BTV infects cells of hematopoietic origin but not HSCs in immunocompetent sheep. However, BTV infects HSCs in the absence of type I IFN (IFN-I) signaling in vitro and in vivo. Infection of HSCs in vitro results in cellular death by apoptosis. Furthermore, BTV infects bone marrow-derived DCs (BM-DCs), interfering with their development to mature DCs in the absence of type I IFN signaling. Costimulatory molecules CD80 and CD86 and costimulatory molecules CD40 and major histocompatibility complex class II (MHC-II) are affected by BTV infection, suggesting that BTV interferes with DC antigen-presenting capacity. In vivo, different DC populations are also affected during the course of infection, probably as a result of a direct effect of BTV replication in DCs and the production of infectious virus. These new findings suggest that BTV infection of HSCs and DCs can impair the immune response, leading to persistence or animal death, and that this relies on IFN-I.

NS3 of bluetongue virus interferes with the induction of type I interferon

Upon infection with Bluetongue virus (BTV), an arthropod-borne virus, type I interferon (IFN-I) is produced in vivo and in vitro. IFN-I is essential for the establishment of an antiviral cellular response, and most if not all viruses have elaborated strategies to counteract its action. In this study, we assessed the ability of BTV to interfere with IFN-I synthesis and identified the nonstructural viral protein NS3 as an antagonist of the IFN-I system.

Sensing and control of bluetongue virus infection in epithelial cells via RIG-I and MDA5 helicases

Bluetongue virus (BTV), an arthropod-borne member of the Reoviridae family, is a double-stranded RNA virus that causes an economically important livestock disease that has spread across Europe in recent decades. Production of type I interferon (alpha/beta interferon [IFN-α/β]) has been reported in vivo and in vitro upon BTV infection. However, the cellular sensors and signaling pathways involved in this process remain unknown. Here we studied the mechanisms responsible for the production of IFN-β in response to BTV serotype 8. Upon BTV infection of A549 cells, expression of IFN-β and other proinflammatory cytokines was strongly induced at both the protein and mRNA levels. This response appeared to be dependent on virus replication, since exposure to UV-inactivated virus failed to induce IFN-β. We also demonstrated that BTV infection activated the transcription factors IFN regulatory factor 3 and nuclear factor κB. We investigated the role of several pattern recognition receptors in this response and showed that expression of IFN-β was greatly reduced after small-interfering-RNA-mediated knockdown of the RNA helicase encoded by retinoic acid-inducible gene I (RIG-I) or melanoma differentiation-associated gene 5 (MDA5). In contrast, silencing of MyD88, Toll-like receptor 3, or the recently described DexD/H-box helicase DDX1 sensor had no or a weak effect on IFN-β induction, suggesting that the RIG-I-like receptor pathway is specifically engaged for BTV sensing. Moreover, we also showed that overexpression of either RIG-I or MDA5 impaired BTV expression in infected A549 cells. Overall, this indicates that RIG-I and MDA5 can both contribute to the recognition and control of BTV infection.

Bluetongue viruses based on modified-live vaccine serotype 6 with exchanged outer shell proteins confer full protection in sheep against virulent BTV8

Since 1998, Bluetongue virus (BTV)-serotypes 1, 2, 4, 9, and 16 have invaded European countries around the Mediterranean Basin. In 2006, a huge BT outbreak started after incursion of BTV serotype 8 (BTV8) in North-Western Europe. IN 2008, BTV6 and BTV11 were reported in the Netherlands and Germany, and in Belgium, respectively. In addition, Toggenburg orbivirus (TOV) was detected in 2008 in Swiss goats, which was recognized as a new serotype of BTV (BTV25). The (re-)emergency of BTV serotypes needs a rapid response to supply effective vaccines. Reverse genetics has been developed for BTV1 and more recently also for BTV6. This latter strain, BTV6/net08, is closely related to live-attenuated vaccine for serotype 6 as determined by full genome sequencing. Here, we used this strain as backbone and exchanged segment 2 and 6, respectively Seg-2 (VP2) and Seg-6 (VP5), for those of BTV serotype 1 and 8 using reverse genetics. These so-called 'serotyped' vaccine viruses, as mono-serotype and multi-serotype vaccine, were compared for their protective capacity in sheep. In general, all vaccinated animals developed a neutralizing antibody response against their respective serotype. After challenge at three weeks post vaccination with cell-passaged, virulent BTV8/net07 (BTV8/net07/e1/bhkp3) the vaccinated animals showed nearly no clinical reaction. Even more, challenge virus could not be detected, and seroconversion or boostering after challenge was negligible. These data demonstrate that all sheep were protected from a challenge with BTV8/net07, since sheep of the control group showed viremia, seroconversion and clinical signs that are specific for Bluetongue. The high level of cross-protection is discussed.

Novel virostatic agents against bluetongue virus

Bluetongue virus (BTV), a member in the family Reoviridae, is a re-emerging animal disease infecting cattle and sheep. With its recent outbreaks in Europe, there is a pressing need for efficacious antivirals. We presented here the identification and characterization of a novel virostatic molecule against BTV, an aminothiophenecarboxylic acid derivative named compound 003 (C003). The virostatic efficacy of C003 could be improved via chemical modification, leading to a de novo synthesized compound 052 (C052). The 50% effective concentrations (EC(50)) of C003 and C052 were determined at 1.76 ± 0.73 µM and 0.27 ± 0.12 µM, respectively. The 50% cytotoxicity concentration (CC(50)) of C003 was over 100 µM and the CC(50) of C052 was at 82.69 µM. Accordingly, the 50% selective index (SI(50)) of C003 and C052 against BTV was over 57 and 306, respectively. The inhibitory effect of C003/C052 on BTV-induced apoptosis was also confirmed via the inhibition of caspase-3/-7 activation post BTV infection. C003/C052 could inhibit BTV induced CPE even when added as late as 24 h.p.i., indicating that they might act at late stage of viral life-cycle. C003/C052 could reduce over two-logs of both the progeny virus production and the number of genomic viral RNA copies. Interestingly, both the activation of host autophagy and viral protein expression were inhibited post BTV infection when cells were treated with C003 and C052, suggesting that C003/C052 might act as virostatic agents via inhibiting host autophagy activation. Although further investigations might be needed to pin down the exact mechanism of C003/C052, our finding suggested that these compounds might be potent lead compounds with potential novel mechanism of action against BTV.

Rescue of recent virulent and avirulent field strains of bluetongue virus by reverse genetics

Since 1998, Bluetongue virus (BTV)-serotypes 1, 2, 4, 9, and 16 have invaded European countries around the Mediterranean Basin. In 2006, a huge BT-outbreak started after incursion of BTV-serotype 8 (BTV8) in North-Western Europe. More recently, BTV6 and BTV11 were reported in North-Western Europe in 2008. These latter strains are closely related to live-attenuated vaccine, whereas BTV8 is virulent and can induce severe disease in ruminants, including cattle. In addition, Toggenburg orbivirus (TOV) was detected in 2008 in Swiss goats, which was recognized as a new serotype of BTV (BTV25). The (re-)emergency of known and unknown BTV-serotypes needs a rapid response to supply effective vaccines, and research to study this phenomenon. Recently, orbivirus research achieved an important breakthrough by the establishment of reverse genetics for BTV1. Here, reverse genetics for two recent BTV strains representing virulent BTV8 and avirulent BTV6 was developed. For this purpose, extensive sequencing of full-genomes was performed, resulting in the consensus sequences of BTV8/net07 and BTV6/net08. The recovery of ‘synthetic BTV’, respectively rgBTV8 and rgBTV6, completely from T7-derived RNA transcripts was confirmed by silent mutations by which these ‘synthetic BTVs’ could be genetically distinguished from wild type BTV, respectively wtBTV6 and wtBTV8. The in vitro and in vivo properties of rgBTV6 or rgBTV8 were comparable to the properties of their parent strains. The asymptomatic or avirulent properties of rgBTV6 and the virulence of rgBTV8 were confirmed by experimental infection of sheep. Reverse genetics of the vaccine-related BTV6 provides a perfect start to develop new generations of BT-vaccines. Reverse genetics of the virulent BTV8 will accelerate research on the special features of BTV8, like transmission by species of Culicoides in a moderate climate, transplacental transmission, and pathogenesis in cattle.

Bluetongue virus serotype 6 in Europe in 2008-Emergence and disappearance of an unexpected non-virulent BTV

Bluetongue viruses (BTVs) could invade N-W Europe similar to BTV serotype 8 (BTV8/net06), since the source and route of introduction of this virus has not been solved. Therefore, the Dutch survey for Bluetongue by PCR testing was extended by further analysis of PCR positives to identify the involved BTV. In late August 2008, BTV was reported with 12 nucleotide differences in the S10 amplicon (S10 genotyping). This virus was identified as serotype 6, here named BTV6/net08. Promptly, serotype specific real-time PCR tests were developed for serotypes 1, 6, and 8 (S2 genotyping). Agreement was found between results by S10- and S2 genotyping. Further, BTV1 was identified by both S10- and S2 genotyping in one imported animal. After initial discovery of BTV6 in the Netherlands, animals from 18 holdings tested PCR positive for BTV6/net08 in 2008. Remarkably only one or two PCR positive animals per holding were found. Serum neutralization tests did not result in the discovery of more BTV6 infected animals. Retrospective studies indicated no evidence for infections by BTV6/net08 prior to the first discovery. Experimental infections with BTV6/net08 did not cause clinical disease in sheep, calves and cattle, except for a very short fever in some animals. This clearly showed that the vaccine-related BTV6/net08 is not virulent. BTV6/net08 was not found by passive and active surveys in the years after its discovery. Apparently, BTV6/net08 was not efficiently transmitted by endemic species of Culicoides in N-W Europe, and disappeared without the need of any control measure.

Co-circulation of bluetongue and epizootic haemorrhagic disease viruses in cattle in Reunion Island

Bluetongue virus (BTV) and epizootic haemorrhagic disease virus (EHDV) in deer have already been isolated in Reunion Island and have caused more or less severe clinical signs in cattle (EHDV) or in sheep (BTV), as observed in 2003. In January 2009, cattle in Reunion Island showed clinical signs suggesting infection by one or the other of these arboviral diseases. A study was set up to determine the etiology of the disease. Analysis by reverse transcriptase-polymerase chain reaction (RT-PCR) performed on blood samples from 116 cattle from different districts of the island detected the presence of the EHDV genome in 106 samples and, in 5 of them, the simultaneous occurrence of BTV and EHDV. One strain of EHDV (7 isolates) and one of BTV were isolated in embryonated eggs and a BHK-21 cell culture. Group and subgroup primer-pairs were designed on the segment 2 sequences available in GenBank to identify and type the EHDV strains. Phylogenetic analysis of the genomic segment 2 (encoding the VP2 serotype-specific protein) of the isolates confirmed the serotypes of these two orbiviruses as BTV-2 and EHDV-6 and allowed them to be compared with previously isolated strains.

Determinants of bluetongue virus virulence in murine models of disease

Bluetongue is a major infectious disease of ruminants that is caused by bluetongue virus (BTV). In this study, we analyzed virulence and genetic differences of (i) three BTV field strains from Italy maintained at either a low (L strains) or high (H strains) passage number in cell culture and (ii) three South African "reference" wild-type strains and their corresponding live attenuated vaccine strains. The Italian BTV L strains, in general, were lethal for both newborn NIH-Swiss mice inoculated intracerebrally and adult type I interferon receptor-deficient (IFNAR(-/-)) mice, while the virulence of the H strains was attenuated significantly in both experimental models. Similarly, the South African vaccine strains were not pathogenic for IFNAR(-/-) mice, while the corresponding wild-type strains were virulent. Thus, attenuation of the virulence of the BTV strains used in this study is not mediated by the presence of an intact interferon system. No clear distinction in virulence was observed for the South African BTV strains in newborn NIH-Swiss mice. Full genomic sequencing revealed relatively few amino acid substitutions, scattered in several different viral proteins, for the strains found to be attenuated in mice compared to the pathogenic related strains. However, only the genome segments encoding VP1, VP2, and NS2 consistently showed nonsynonymous changes between all virulent and attenuated strain pairs. This study established an experimental platform for investigating the determinants of BTV virulence. Future studies using reverse genetics will allow researchers to precisely map and "weight" the relative influences of the various genome segments and viral proteins on BTV virulence.

Enter the kill zone: initiation of death signaling during virus entry

Infection of host cells by a variety of viruses results in programmed cell death or apoptosis. In many cases, early events in virus replication that occur prior to synthesis of viral proteins and replication of viral genomes directly or indirectly activate signaling pathways that culminate in cell death. Using examples of viruses for which prodeath signaling is better defined, this review will describe how cell entry steps including virus attachment to receptors, virus uncoating in endosomes, and events that occur following membrane penetration lead to apoptosis. The relevance and physiologic consequences of early induction of prodeath signaling to viral pathogenesis also will be discussed.

Genetic modification of Bluetongue virus by uptake of "synthetic" genome segments

Since 1998, several serotypes of Bluetongue virus (BTV) have invaded several southern European countries. In 2006, the unknown BTV serotype 8 (BTV8/net06) unexpectedly invaded North-West Europe and has resulted in the largest BT-outbreak ever recorded. More recently, in 2008 BTV serotype 6 was reported in the Netherlands and Germany. This virus, BTV6/net08, is closely related to modified-live vaccine virus serotype 6, except for genome segment S10. This genome segment is closer related to that of vaccine virus serotype 2, and therefore BTV6/net08 is considered as a result of reassortment. Research on orbiviruses has been hampered by the lack of a genetic modification method. Recently, reverse genetics has been developed for BTV based on ten in vitro synthesized genomic RNAs. Here, we describe a targeted single-gene modification system for BTV based on the uptake of a single in vitro synthesized viral positive-stranded RNA. cDNAs corresponding to BTV8/net06 genome segments S7 and S10 were obtained by gene synthesis and cloned downstream of the T7 RNA-polymerase promoter and upstream of a unique site for a restriction enzyme at the 3'-terminus for run-off transcription. Monolayers of BSR cells were infected by BTV6/net08, and subsequently transfected with purified in vitro synthesized, capped positive-stranded S7 or S10 RNA from BTV8/net06 origin. "Synthetic" reassortants were rescued by endpoint dilutions, and identified by serotype-specific PCR-assays for segment 2, and serogroup-specific PCRs followed by restriction enzyme analysis or sequencing for S7 and S10 segments. The targeted single-gene modification system can also be used to study functions of viral proteins by uptake of mutated genome segments. This method is also useful to generate mutant orbiviruses for other serogroups of the genus Orbivirus for which reverse genetics has not been developed yet.

Emergence of bluetongue serotypes in Europe, part 1: description and validation of four real-time RT-PCR assays for the serotyping of bluetongue viruses BTV-1, BTV-6, BTV-8 and BTV-11

The control of bluetongue virus (BTV) in Central-Western Europe is greatly complicated by the coexistence of several BTV serotypes. Rapid, sensitive and specific assays are therefore needed to correctly identify the currently circulating BTV serotypes in field samples. In the present study, four serotype-specific real-time RT-PCR assays (RT-qPCR) are described for the detection of the BTV-1, BTV-6, BTV-8 and BTV-11 serotypes. The analytical sensitivity of the BTV-1/S2, BTV-6/S2, BTV-8/S2 and BTV-11/S2 serotype-specific RT-qPCR assays is comparable to the earlier described serogroup-specific pan-BTV/S5 RT-qPCR assay. In silico and in vitro analyses indicated that none of the assays cross-react with viruses which are symptomatically or genetically related to BTV and only detect the intended BTV serotypes. All assays exhibited a linear range of at least 0.05-3.80 log(10) TCID(50) ml(-1) and a PCR-efficiency approaching the ideal amplification factor of two per PCR cycle. Both intra- and inter-run variations were found to be low with a total coefficient of variation of 1-2% for clear positive samples and <10% for very weak positive samples. Finally, the performance of the described assays was compared with commercially available kits for the detection of BTV-1, BTV-6 and BTV-8. Three in-house assays gave exactly the same diagnostic result (positive/negative) as the commercial assays and can thus be used interchangeably. Together with the earlier described serogroup-specific pan-BTV/S5, the serotype-specific RT-qPCR assays form a flexible and properly validated set of tools to detect and differentiate the BTV serotypes currently circulating in Central-Western Europe.

Bluetongue virus: dissection of the polymerase complex

Bluetongue is a vector-borne viral disease of ruminants that is endemic in tropical and subtropical countries. Since 1998 the virus has also appeared in Europe. Partly due to the seriousness of the disease, bluetongue virus (BTV), a member of genus Orbivirus within the family Reoviridae, has been a subject of intense molecular study for the last three decades and is now one of the best understood viruses at the molecular and structural levels. BTV is a complex non-enveloped virus with seven structural proteins arranged in two capsids and a genome of ten double-stranded (ds) RNA segments. Shortly after cell entry, the outer capsid is lost to release an inner capsid (the core) which synthesizes capped mRNAs from each genomic segment, extruding them into the cytoplasm. This requires the efficient co-ordination of a number of enzymes, including helicase, polymerase and RNA capping activities. This review will focus on our current understanding of these catalytic proteins as derived from the use of recombinant proteins, combined with functional assays and the in vitro reconstitution of the transcription/replication complex. In some cases, 3D structures have complemented this analysis to reveal the fine structural detail of these proteins. The combined activities of the core enzymes produce infectious transcripts necessary and sufficient to initiate BTV infection. Such infectious transcripts can now be synthesized wholly in vitro and, when introduced into cells by transfection, lead to the recovery of infectious virus. Future studies thus hold the possibility of analysing the consequence of mutation in a replicating virus system.

The Aedes albopictus inhibitor of apoptosis 1 gene protects vertebrate cells from bluetongue virus-induced apoptosis

We sequenced and characterized the inhibitor of apoptosis (iap) 1 gene from Aedes albopictus, designated as Aaiap1. The Aaiap1 gene rescued Spodoptera frugiperda (Sf9) cells from apoptosis when cotransfected with the Drosophila pro-apoptotic hid gene. The antiapoptotic function of the Aaiap1 gene was evaluated in the bluetongue virus (BTV)-induced apoptosis system. BTV infection induced apoptosis in vertebrate cells via the intrinsic apoptotic pathway. This was shown by the translocation of cytochrome C and the second mitochondria-derived activator of caspase (Smac, also known as DIABLO) from the mitochondria and the subsequent activation of caspase-9 and -3. Stable expression of the Aaiap1 gene in derivative baby hamster kidney cells delayed BTV-induced apoptosis by 24 h and reduced the BTV progeny yield by 10-fold. This study provides the first evidence that the mosquito AaIAP1 protein possesses antiapoptotic activity.