Plasmid DNA-based reverse genetics as a platform for manufacturing of bluetongue vaccine

Control of bluetongue disease is predominantly through vaccination with licensed inactivated or live-attenuated vaccines (LAVs). Manufacturing of LAVs in endemic countries requires formulation with a high number of serotypes for effective protection. Herein, we evaluated a plasmid DNA-based reverse genetics platform for manufacturing a multivalent vaccine. The synthetic vaccine was characterized by a common BTV1 backbone, with exchange of outer capsid proteins. Recombinant South African vaccine serotypes 1, 5, and 14 were rescued by exchanging the VP2 protein on the backbone. BTV6 rescue was achieved following the exchange of VP2 and VP5 proteins. The particle sizes were comparable to commercial vaccines of respective serotypes. BTV1, BTV5, and BTV6 had distinct growth profiles compared to commercial vaccines, while BTV14 was indistinguishable. Stability and shelf-life determination under various storage conditions showed that commercial vaccines were more stable. Formulated antigens were evaluated for vaccine safety and immunogenicity in sheep. Serotyped BTV1 monovalent vaccine was safe, as no clinical signs were observed. Neutralizing antibodies (nAbs) were induced on day 14 and peaked at 32 on day 28. The multivalent synthetic vaccine containing four serotypes elicited BTV6 nAbs from day 21 with a titer of 52, which decreased to 33 by day 42. BTV1 elicited a weak immune response with a titer of 1 on day 42. No nAbs were detected against BTV5 and BTV14. This is a first report comparing reverse genetics-derived antigens with commercial vaccines. Data generated on production yields, stability, and immunogenicity demonstrated that some serotypes can be implemented as novel synthetic vaccines using this platform.IMPORTANCEVaccination is the most effective control strategy for viral diseases that affect livestock. To date, only live-attenuated and inactivated vaccines have been licensed for control of bluetongue (BT). This study demonstrated the use of reverse genetics as a possible platform for BTV vaccine production. Data generated in the study contribute toward the advancement of an alternative manufacturing platform for licensing of BT vaccines. Information on production yields and stability of synthetic vaccines in comparison to the conventional products demonstrated that optimization is required for some serotypes to fully translate the reverse genetics platform for manufacturing the BTV vaccine. The study highlighted the safety and immunogenicity of vaccines manufactured using the plasmid DNA-based reverse-genetics platform.

Vaccine candidates based on MVA viral vectors expressing VP2 or VP7 confer full protection against Epizootic hemorrhagic disease virus in IFNAR(-/-) mice

Epizootic hemorrhagic disease (EHD), caused by Epizootic hemorrhagic disease virus (EHDV), is an emerging and severe livestock disease. Recent incursion and distribution of EHDV in Europe have outlined the need for vaccine research against this viral disease. In this work, we report modified vaccinia virus Ankara (MVA)-vectored vaccines designed to express protein VP2 of EHDV-8 or protein VP7 of EHDV-2. Prime boost immunization of adult IFNAR(-/-) mice with the MVA-VP2 vaccine candidate induced high titers of EHDV-8-specific neutralizing antibodies (NAbs) and conferred full protection against homologous lethal challenge with EHDV-8. However, no heterologous protection was observed after lethal challenge with EHDV-6. In contrast, the MVA-VP7 vaccine candidate elicited strong cytotoxic CD8+ T-cell responses against VP7 and conferred complete protection against lethal challenge with either EHDV-8 or EHDV-6 in IFNAR(-/-) mice in the absence of NAbs, being the first multiserotype vaccine candidate against EHDV. Moreover, we expressed recombinant proteins VP2 and VP7 of EHDV in the baculovirus expression system, which were used to analyze the potential DIVA (differentiating infected from vaccinated animals) character of these vaccine candidates.IMPORTANCEEmergence and re-emergence of arthropod-borne viruses are major concerns for both human and animal health. The most recent example is the fast expansion of EHDV-8 through Europe. Besides, EHDV-8 relates with a high prevalence of pathologic cases in cattle populations. No vaccine is currently available in Europe, and vaccine research against this arboviral disease is negligible. In this work, we present novel DIVA vaccine candidates against EHDV, and most importantly, we identified the protein VP7 of EHDV as an antigen capable of inducing multiserotype protection, one of the major challenges in vaccine research against orbiviruses.

Vaccination as a Strategy to Prevent Bluetongue Virus Vertical Transmission

Bluetongue virus (BTV) produces an economically important disease in ruminants of compulsory notification to the OIE. BTV is typically transmitted by the bite of Culicoides spp., however, some BTV strains can be transmitted vertically, and this is associated with fetus malformations and abortions. The viral factors associated with the virus potency to cross the placental barrier are not well defined. The potency of vertical transmission is retained and sometimes even increased in live attenuated BTV vaccine strains. Because BTV possesses a segmented genome, the possibility of reassortment of vaccination strains with wild-type virus could even favor the transmission of this phenotype. In the present review, we will describe the non-vector-based BTV infection routes and discuss the experimental vaccination strategies that offer advantages over this drawback of some live attenuated BTV vaccines.

Pentavalent Disabled Infectious Single Animal (DISA)/DIVA Vaccine Provides Protection in Sheep and Cattle against Different Serotypes of Bluetongue Virus

Bluetongue (BT) is a midge-borne OIE-notifiable disease of ruminants caused by the bluetongue virus (BTV). There are at least 29 BTV serotypes as determined by serum neutralization tests and genetic analyses of genome segment 2 encoding serotype immunodominant VP2 protein. Large parts of the world are endemic for multiple serotypes. The most effective control measure of BT is vaccination. Conventionally live-attenuated and inactivated BT vaccines are available but have their specific pros and cons and are not DIVA compatible. The prototype Disabled Infectious Single Animal (DISA)/DIVA vaccine based on knockout of NS3/NS3a protein of live-attenuated BTV, shortly named DISA8, fulfills all criteria for modern veterinary vaccines of sheep. Recently, DISA8 with an internal in-frame deletion of 72 amino acid codons in NS3/NS3a showed a similar ideal vaccine profile in cattle. Here, the DISA/DIVA vaccine platform was applied for other serotypes, and pentavalent DISA/DIVA vaccine for “European” serotypes 1, 2, 3, 4, 8 was studied in sheep and cattle. Protection was demonstrated for two serotypes, and neutralization Ab titers indicate protection against other included serotypes. The DISA/DIVA vaccine platform is flexible in use and generates monovalent and multivalent DISA vaccines to combat specific field situations with respect to Bluetongue.

An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India

Bluetongue (BT) is an economically important, non-contagious viral disease of domestic and wild ruminants. BT is caused by BT virus (BTV) and it belongs to the genus Orbivirus and family Reoviridae. BTV is transmitted by Culicoides midges and causes clinical disease in sheep, white-tailed deer, pronghorn antelope, bighorn sheep, and subclinical manifestation in cattle, goats and camelids. BT is a World Organization for Animal Health (OIE) listed multispecies disease and causes great socio-economic losses. To date, 28 serotypes of BTV have been reported worldwide and 23 serotypes have been reported from India. Transplacental transmission (TPT) and fetal abnormalities in ruminants had been reported with cell culture adopted live-attenuated vaccine strains of BTV. However, emergence of BTV-8 in Europe during 2006, confirmed TPT of wild-type/field strains of BTV. Diagnosis of BT is more important for control of disease and to ensure BTV-free trade of animals and their products. Reverse transcription polymerase chain reaction, agar gel immunodiffusion assay and competitive enzyme-linked immunosorbent assay are found to be sensitive and OIE recommended tests for diagnosis of BTV for international trade. Control measures include mass vaccination (most effective method), serological and entomological surveillance, forming restriction zones and sentinel programs. Major hindrances with control of BT in India are the presence of multiple BTV serotypes, high density of ruminant and vector populations. A pentavalent inactivated, adjuvanted vaccine is administered currently in India to control BT. Recombinant vaccines with DIVA strategies are urgently needed to combat this disease. This review is the first to summarise the seroprevalence of BTV in India for 40 years, economic impact and pathobiology.

The Bluetongue Disabled Infectious Single Animal (DISA) Vaccine Platform Based on Deletion NS3/NS3a Protein Is Safe and Protective in Cattle and Enables DIVA

The bluetongue virus (BTV) is transmitted by Culicoides biting midges and causes bluetongue (BT), an OIE-notifiable disease of ruminants. At least 29 BTV serotypes are described as determined by the outer shell proteins VP2 and VP5. Vaccination is the most effective control measure. Inactivated and live-attenuated vaccines (LAVs) are currently available. These vaccines have their specific pros and cons, and both are not DIVA vaccines. The BT Disabled Infectious Single Animal (DISA) vaccine platform is based on LAV without nonessential NS3/NS3a expression and is applicable for many serotypes by the exchange of outer shell proteins. The DISA vaccine is effective and completely safe. Further, transmission of the DISA vaccine by midges is blocked (DISA principle). Finally, the DISA vaccine enables DIVA because of a lack of antibodies against the immunogenic NS3/NS3a protein (DIVA principle). The deletion of 72 amino acids (72aa) in NS3/NS3a is sufficient to block virus propagation in midges. Here, we show that a prototype DISA vaccine based on LAV with the 72aa deletion enables DIVA, is completely safe and induces a long-lasting serotype-specific protection in cattle. In conclusion, the in-frame deletion of 72-aa codons in the BT DISA/DIVA vaccine platform is sufficient to fulfil all the criteria for modern veterinary vaccines.

Reverse genetics approaches: a novel strategy for African horse sickness virus vaccine design

African horse sickness (AHS) is a devastating disease caused by African horse sickness virus (AHSV) and transmitted by arthropods between its equine hosts. AHSV is endemic in sub-Saharan Africa, where polyvalent live attenuated vaccine is in use even though it is associated with safety risks. This review article summarizes and compares new strategies to generate safe and effective AHSV vaccines based on protein, virus like particles, viral vectors and reverse genetics technology. Manipulating the AHSV genome to generate synthetic viruses by means of reverse genetic systems has led to the generation of potential safe vaccine candidates that are under investigation.

A protective bivalent vaccine against Rift Valley fever and bluetongue

Rift Valley fever (RVF) and bluetongue (BT) are two important ruminant diseases transmitted by arthropods. Both viruses have shown important geographic spread leading to endemicity of BT virus (BTV) in Africa and Europe. In this work, we report a dual vaccine that simultaneously induces protective immune responses against BTV and RVFV based on modified vaccinia Ankara virus (MVA) expressing BTV proteins VP2, NS1, or a truncated form of NS1 (NS1-Nt), and RVFV Gn and Gc glycoproteins. IFNAR^(-/-) mice immunized with two doses of MVA-GnGc-VP2 developed a significant neutralizing antibody response against BTV-4 and RVFV. Furthermore, the homologous prime-boost immunization with MVA-GnGc-NS1 or MVA-GnGc-NS1-Nt triggered neutralizing antibodies against RVFV and NS1-specific cytotoxic CD8+ T cells in mice. Moreover, all mice immunized with MVA-GnGc-NS1 or MVA-GnGc-NS1-Nt remained healthy after lethal challenge with RVFV or BTV-4. The homologous prime-boost vaccination with MVA-GnGc-NS1, which was the best immunization strategy observed in mice, was assayed in sheep. Clinical signs and viremia were absent or highly reduced in vaccinated sheep after challenge with BTV-4 or RVFV. These results indicate that MVA-GnGc-NS1 vaccination elicits immune protection against RVFV and BTV in sheep.

Highly efficient vaccines for Bluetongue virus and a related Orbivirus based on reverse genetics

Bluetongue virus (BTV) reverse genetics (RG), available since 2007, has allowed the dissection of the virus replication cycle, including discovery of a primary replication stage. This information has allowed the generation of Entry-Competent-Replication-Abortive (ECRA) vaccines, which enter cells and complete primary replication but fail to complete the later stage. A series of vaccine trials in sheep and cattle either with a single ECRA serotype or a cocktail of multiple ECRA serotypes have demonstrated that these vaccines provide complete protection against virulent virus challenge without cross-serotype interference. Similarly, an RG system developed for the related African Horse Sickness virus, which causes high mortality in equids has provided AHSV ECRA vaccines that are protective in horses. ECRA vaccines were incapable of productive replication in animals despite being competent for cell entry. This technology allows rapid generation of emerging Orbivirus vaccines and offers immunogenicity and safety levels that surpass attenuated or recombinant routes.

Prospects of Next-Generation Vaccines for Bluetongue

Bluetongue (BT) is a haemorrhagic disease of wild and domestic ruminants with a huge economic worldwide impact on livestock. The disease is caused by BT-virus transmitted by Culicoides biting midges and disease control without vaccination is hardly possible. Vaccination is the most feasible and cost-effective way to minimize economic losses. Marketed BT vaccines are successfully used in different parts of the world. Inactivated BT vaccines are efficacious and safe but relatively expensive, whereas live-attenuated vaccines are efficacious and cheap but are unsafe because of under-attenuation, onward spread, reversion to virulence, and reassortment events. Both manufactured BT vaccines do not enable differentiating infected from vaccinated animals (DIVA) and protection is limited to the respective serotype. The ideal BT vaccine is a licensed, affordable, completely safe DIVA vaccine, that induces quick, lifelong, broad protection in all susceptible ruminant species. Promising vaccine candidates show improvement for one or more of these main vaccine standards. BTV protein vaccines and viral vector vaccines have DIVA potential depending on the selected BTV antigens, but are less effective and likely more costly per protected animal than current vaccines. Several vaccine platforms based on replicating BTV are applied for many serotypes by exchange of serotype dominant outer shell proteins. These platforms based on one BTV backbone result in attenuation or abortive virus replication and prevent disease by and spread of vaccine virus as well as reversion to virulence. These replicating BT vaccines induce humoral and T-cell mediated immune responses to all viral proteins except to one, which could enable DIVA tests. Most of these replicating vaccines can be produced similarly as currently marketed BT vaccines. All replicating vaccine platforms developed by reverse genetics are classified as genetic modified organisms. This implies extensive and expensive safety trails in target ruminant species, and acceptance by the community could be hindered. Nonetheless, several experimental BT vaccines show very promising improvements and could compete with marketed vaccines regarding their vaccine profile, but none of these next generation BT vaccines have been licensed yet.

Vector competence is strongly affected by a small deletion or point mutations in bluetongue virus

BACKGROUND

Transmission of vector-borne virus by insects is a complex mechanism consisting of many different processes; viremia in the host, uptake, infection and dissemination in the vector, and delivery of virus during blood-feeding leading to infection of the susceptible host. Bluetongue virus (BTV) is the prototype vector-borne orbivirus (family Reoviridae). BTV serotypes 1-24 (typical BTVs) are transmitted by competent biting Culicoides midges and replicate in mammalian (BSR) and midge (KC) cells. Previously, we showed that genome segment 10 (S10) encoding NS3/NS3a protein is required for virus propagation in midges. BTV serotypes 25-27 (atypical BTVs) do not replicate in KC cells. Several distinct BTV26 genome segments cause this so-called 'differential virus replication' in vitro.

METHODS

Virus strains were generated using reverse genetics and their growth was examined in vitro. The midge feeding model has been developed to study infection, replication and disseminations of virus in vivo. A laboratory colony of C. sonorensis, a known competent BTV vector, was fed or injected with BTV variants and propagation in the midge was examined using PCR testing. Crossing of the midgut infection barrier was examined by separate testing of midge heads and bodies.

RESULTS

A 100 nl blood meal containing ±10^5.3 TCID₅₀/ml of BTV11 which corresponds to ±20 TCID₅₀ infected 50% of fully engorged midges, and is named one Midge Alimentary Infective Dose (MAID₅₀). BTV11 with a small in-frame deletion in S10 infected blood-fed midge midguts but virus release from the midgut into the haemolymph was blocked. BTV11 with S1[VP1] of BTV26 could be adapted to virus growth in KC cells, and contained mutations subdivided into 'corrections' of the chimeric genome constellation and mutations associated with adaptation to KC cells. In particular one amino acid mutation in outer shell protein VP2 overcomes differential virus replication in vitro and in vivo.

CONCLUSION

Small changes in NS3/NS3a or in the outer shell protein VP2 strongly affect virus propagation in midges and thus vector competence. Therefore, spread of disease by competent Culicoides midges can strongly differ for very closely related viruses.

Novel Function of Bluetongue Virus NS3 Protein in Regulation of the MAPK/ERK Signaling Pathway

Bluetongue virus (BTV) is an arbovirus transmitted by blood-feeding midges to a wide range of wild and domestic ruminants. In this report, we showed that BTV, through its nonstructural protein NS3 (BTV-NS3), is able to activate the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, as assessed by phosphorylation levels of ERK1/2 and the translation initiation factor eukaryotic translation initiation factor 4E (eIF4E). By combining immunoprecipitation of BTV-NS3 and mass spectrometry analysis from both BTV-infected and NS3-transfected cells, we identified the serine/threonine-protein kinase B-Raf (BRAF), a crucial player in the MAPK/ERK pathway, as a new cellular interactor of BTV-NS3. BRAF silencing led to a significant decrease in the MAPK/ERK activation by BTV, supporting a model wherein BTV-NS3 interacts with BRAF to activate this signaling cascade. This positive regulation acts independently of the role of BTV-NS3 in counteracting the induction of the alpha/beta interferon response. Furthermore, the intrinsic ability of BTV-NS3 to bind BRAF and activate the MAPK/ERK pathway is conserved throughout multiple serotypes/strains but appears to be specific to BTV compared to other members of Orbivirus genus. Inhibition of MAPK/ERK pathway with U0126 reduced viral titers, suggesting that BTV manipulates this pathway for its own replication. Altogether, our data provide molecular mechanisms that unravel a new essential function of NS3 during BTV infection.IMPORTANCE Bluetongue virus (BTV) is responsible of the arthropod-borne disease bluetongue (BT) transmitted to ruminants by blood-feeding midges. In this report, we found that BTV, through its nonstructural protein NS3 (BTV-NS3), interacts with BRAF, a key component of the MAPK/ERK pathway. In response to growth factors, this pathway promotes cell survival and increases protein translation. We showed that BTV-NS3 enhances the MAPK/ERK pathway, and this activation is BRAF dependent. Treatment of MAPK/ERK pathway with the pharmacologic inhibitor U0126 impairs viral replication, suggesting that BTV manipulates this pathway for its own benefit. Our results illustrate, at the molecular level, how a single virulence factor has evolved to target a cellular function to increase its viral replication.

Identification and genomic characterization of the first isolate of bluetongue virus serotype 5 detected in Australia

Bluetongue virus (BTV), transmitted by midges (Culicoides sp), is distributed worldwide and causes disease in ruminants. In particular, BT can be a debilitating disease in sheep causing serious trade and socio-economic consequences at both local and global levels. Across Australia, a sentinel cattle herd surveillance program monitors the BTV activity. Prior to 2014, BTV-1, -2, -3, -7, -9, -15, -16, -20, -21 and -23 had been isolated in Australia, but no bluetongue disease has occurred in a commercial Australian flock. We routinely use a combination of serology, virus isolation, RT-PCR and next generation and conventional nucleotide sequencing technologies to detect and phylogenetically characterize incursions of novel BTV strains into Australia. Screening of Northern Territory virus isolates in 2015 revealed BTV-5, a serotype new to Australia. We derived the complete genome of this isolate and determined its phylogenetic relationship with exotic BTV-5 isolates. Gene segments 2, 6, 7 and 10 exhibited a close relationship with the South African prototype isolate RSArrrr/5. This was the first Australian isolation of a Western topotype of segment 10. Serological surveillance data highlighted the antigenic cross-reactivity between BTV-5 and BTV-9. Phylogenetic investigation of segments 2 and 6 of these serotypes confirmed their unconventional relationships within the BTV serogroup. Our results further highlighted a need for a revision of the current serologically based system for BTV strain differentiation and importantly, implied a potential for genome segments of pathogenic Western BTV strains to rapidly enter Southeast Asia. This emphasized a need for continued high-level surveillance of vectors and viruses at strategic locations in the north of Australia The expansion of routine characterization and classification of BTV to a whole genome approach is recommended, to better monitor the presence and level of establishment of novel Western topotype segments within the Australian episystem.

Diagnostic DIVA tests accompanying the Disabled Infectious Single Animal (DISA) vaccine platform for African horse sickness

African Horse Sickness Virus (AHSV) (Orbivirus genus, Reoviridae family) causes high mortality in naïve domestic horses with enormous economic and socio-emotional impact. There are nine AHSV serotypes showing limited cross neutralization. AHSV is transmitted by competent species of Culicoides biting midges. AHS is a serious threat beyond the African continent as endemic Culicoides species in moderate climates transmit the closely related prototype bluetongue virus. There is a desperate need for safe and efficacious vaccines, while DIVA (Differentiating Infected from Vaccinated) vaccines would accelerate control of AHS. Previously, we have shown that highly virulent AHSV with an in-frame deletion of 77 amino acids (aa) in NS3/NS3a is completely safe, does not cause viremia and shows protective capacity. This deletion mutant is a promising DISA (Disabled Infectious Single Animal) vaccine platform, since exchange of serotype specific virus proteins has been shown for all nine serotypes. Here, we show that a prototype NS3 competitive ELISA is DIVA compliant to AHS DISA vaccine platforms. Epitope mapping of NS3/NS3a shows that more research is needed to evaluate this prototype serological DIVA assay regarding sensitivity and specificity, in particular for AHSVs expressing antigenically different NS3/NS3a proteins. Further, an experimental panAHSV PCR test targeting genome segment 10 is developed that detects reference AHSV strains, whereas AHS DISA vaccine platforms were not detected. This DIVA PCR test completely guarantees genetic DIVA based on in silico and in vitro validation, although test validation regarding diagnostic sensitivity and specificity has not been performed yet. In conclusion, the prototype NS3 cELISA and the PCR test described here enable serological and genetic DIVA accompanying AHS DISA vaccine platforms.

Bluetongue: control, surveillance and safe movement of animals

The performance of different bluetongue control measures related to both vaccination and protection from bluetongue virus (BTV) vectors was assessed. By means of a mathematical model, it was concluded that when vaccination is applied on 95% of animals even for 3 years, bluetongue cannot be eradicated and is able to re‐emerge. Only after 5 years of vaccination, the infection may be close to the eradication levels. In the absence of vaccination, the disease can persist for several years, reaching an endemic condition with low level of prevalence of infection. Among the mechanisms for bluetongue persistence, the persistence in the wildlife, the transplacental transmission in the host, the duration of viraemia and the possible vertical transmission in vectors were assessed. The criteria of the current surveillance scheme in place in the EU for demonstration of the virus absence need revision, because it was highlighted that under the current surveillance policy bluetongue circulation might occur undetected. For the safe movement of animals, newborn ruminants from vaccinated mothers with neutralising antibodies can be considered protected against infection, although a protective titre threshold cannot be identified. The presence of colostral antibodies interferes with the vaccine immunisation in the newborn for more than 3 months after birth, whereas the minimum time after vaccination of animal to be considered immune can be up to 48 days. The knowledge about vectors ecology, mechanisms of over‐wintering and criteria for the seasonally vector‐free period was updated. Some Culicoides species are active throughout the year and an absolute vector‐free period may not exist at least in some areas in Europe. To date, there is no evidence that the use of insecticides and repellents reduce the transmission of BTV in the field, although this may reduce host/vector contact. By only using pour‐on insecticides, protection of animals is lower than the one provided by vector‐proof establishments.

This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1182/full, http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1171/full

B-cell epitopes of African horse sickness virus serotype 4 recognised by immune horse sera

Identifying antigenic proteins and mapping their epitopes is important for the development of diagnostic reagents and recombinant vaccines. B-cell epitopes of African horse sickness virus (AHSV) have previously been mapped on VP2, VP5, VP7 and NS1, using mouse, rabbit and chicken monoclonal antibodies. A comprehensive study of the humoral immune response of five vaccinated horses to AHSV-4 antigenic peptides was undertaken. A fragmented-genome phage display library expressing a repertoire of AHSV-4 peptides spanning the entire genome was constructed. The library was affinity selected for binders on immobilised polyclonal immunoglobulin G (IgG) isolated from horse sera collected pre- and post-immunisation with an attenuated AHSV-4 monovalent vaccine. The DNA inserts of binding phages were sequenced with Illumina high-throughput sequencing. The data were normalised using pre-immune IgG-selected sequences. More sequences mapped to the genes coding for NS3, VP6 and VP5 than to the other genes. However, VP2 and VP5 each had more antigenic regions than each of the other proteins. This study identified a number of epitopes to which the horse’s humoral immune system responds during immunisation with AHSV-4.

Replication-Deficient Particles: New Insights into the Next Generation of Bluetongue Virus Vaccines

Bluetongue virus (BTV) is endemic in many parts of the world, often causing severe hemorrhagic disease in livestock. To date, at least 27 different serotypes have been recognized. Vaccination against all serotypes is necessary to protect susceptible animals and to prevent onward spread of the virus by insect vectors. In our previous studies, we generated replication-deficient (disabled infectious single-cycle [DISC]) virus strains for a number of serotypes and reported preliminary data on their protective efficacy in animals. In this report, to advance the DISC vaccines to the marketplace, we investigated different parameters of these DISC vaccines. First, we demonstrated the genetic stabilities of these vaccine strains and also the complementing cell line. Subsequently, the optimal storage conditions of vaccines, including additives, temperature, and desiccation, were determined and their protective efficacies in animals confirmed. Furthermore, to test if mixtures of different vaccine strains could be tolerated, we tested cocktails of DISC vaccines in combinations of three or six different serotypes in sheep and cattle, the two natural hosts of BTV. Groups of sheep vaccinated with a cocktail of six different vaccines were completely protected from challenge with individual virulent serotypes, both in early challenge and after 5 months of challenge without any clinical disease. There was no interference in protection between the different vaccines. Protection was also achieved in cattle with a mixture of three vaccine strains, albeit at a lesser level than in sheep. Our data support and validate the suitability of these virus strains as the next-generation vaccines for BTV.

IMPORTANCE Bluetongue (BT) is a debilitating and in many cases lethal disease that affects ruminants of economic importance. Classical vaccines that afford protection against bluetongue virus, the etiological agent, are not free from secondary and undesirable effects. A surge in new approaches to produce highly attenuated, safer vaccines was evident after the development of the BTV reverse-genetics system that allows the introduction of targeted mutations in the virus genome. We targeted an essential gene to develop disabled virus strains as vaccine candidates. The results presented in this report further substantiate our previous evidence and support the suitability of these virus strains as the next-generation BTV vaccines.

Bluetongue Disabled Infectious Single Animal (DISA) vaccine: Studies on the optimal route and dose in sheep

Bluetongue (BT) is a disease of ruminants caused by bluetongue virus (BTV) transmitted by biting midges of the Culicoides genus. Outbreaks have been controlled successfully by vaccination, however, currently available BT vaccines have several shortcomings. Recently, we have developed BT Disabled Infectious Single Animal (DISA) vaccines based on live-attenuated BTV without expression of dispensable non-structural NS3/NS3a protein. DISA vaccines are non-pathogenic replicating vaccines, do not cause viremia, enable DIVA and are highly protective. NS3/NS3a protein is involved in virus release, cytopathogenic effect and suppression of Interferon-I induction, suggesting that the vaccination route can be of importance. A standardized dose of DISA vaccine for serotype 8 has successfully been tested by subcutaneous vaccination. We show that 10 and 100times dilutions of this previously tested dose did not reduce the VP7 humoral response. Further, the vaccination route of DISA vaccine strongly determined the induction of VP7 directed antibodies (Abs). Intravenous vaccination induced high and prolonged humoral response but is not practical in field situations. VP7 seroconversion was stronger by intramuscular vaccination than by subcutaneous vaccination. For both vaccination routes and for two different DISA vaccine backbones, IgM Abs were rapidly induced but declined after 14days post vaccination (dpv), whereas the IgG response was slower. Interestingly, intramuscular vaccination resulted in an initial peak followed by a decline up to 21dpv and then increased again. This second increase is a steady and continuous increase of IgG Abs. These results indicate that intramuscular vaccination is the optimal route. The protective dose of DISA vaccine has not been determined yet, but it is expected to be significantly lower than of currently used BT vaccines. Therefore, in addition to the advantages of improved safety and DIVA compatibility, the novel DISA vaccines will be cost-competitive to commercially available live attenuated and inactivated vaccines for Bluetongue.

Current and next-generation bluetongue vaccines: Requirements, strategies, and prospects for different field situations

Bluetongue virus (BTV) causes the hemorrhagic disease bluetongue (BT) in ruminants. The best way to control outbreaks is vaccination. Currently, conventionally modified-live and inactivated vaccines are commercially available, which have been successfully used to control BT, but nonetheless have their specific shortcomings. Therefore, there is a need for improved BT vaccines. The ideal BT vaccine is efficacious, safe, affordable, protective against multiple serotypes and enables the differentiation of infected from vaccinated animals. Different field situations require specific vaccine profiles. Single serotype outbreaks in former BT-free areas need rapid onset of protection against viremia of the respective serotype. In contrary, endemic multiple serotype situations require long-lasting protection against all circulating serotypes. The ideal BT vaccine for all field situations does not exist and balancing between vaccine properties is needed. Many new vaccines candidates, ranging from non-replicating subunits to replicating next-generation reverse genetics based vaccines, have been developed. Some have been tested extensively in large numbers of ruminants, whereas others were developed recently and have only been tested in vitro and in mice models. Most vaccine candidates are promising, but have their specific shortcomings and advantages. In this review, current and next-generation BT vaccines are discussed in the light of prerequisites for different field situations.

Requirements and comparative analysis of reverse genetics for bluetongue virus (BTV) and African horse sickness virus (AHSV)

BACKGROUND

Bluetongue virus (BTV) and African horse sickness virus (AHSV) are distinct arthropod borne virus species in the genus Orbivirus (Reoviridae family), causing the notifiable diseases Bluetongue and African horse sickness of ruminants and equids, respectively. Reverse genetics systems for these orbiviruses with their ten-segmented genome of double stranded RNA have been developed. Initially, two subsequent transfections of in vitro synthesized capped run-off RNA transcripts resulted in the recovery of BTV. Reverse genetics has been improved by transfection of expression plasmids followed by transfection of ten RNA transcripts. Recovery of AHSV was further improved by use of expression plasmids containing optimized open reading frames.

RESULTS

Plasmids containing full length cDNA of the 10 genome segments for T7 promoter-driven production of full length run-off RNA transcripts and expression plasmids with optimized open reading frames (ORFs) were used. BTV and AHSV were rescued using reverse genetics. The requirement of each expression plasmid and capping of RNA transcripts for reverse genetics were studied and compared for BTV and AHSV. BTV was recovered by transfection of VP1 and NS2 expression plasmids followed by transfection of a set of ten capped RNAs. VP3 expression plasmid was also required if uncapped RNAs were transfected. Recovery of AHSV required transfection of VP1, VP3 and NS2 expression plasmids followed by transfection of capped RNA transcripts. Plasmid-driven expression of VP4, 6 and 7 was also needed when uncapped RNA transcripts were used. Irrespective of capping of RNA transcripts, NS1 expression plasmid was not needed for recovery, although NS1 protein is essential for virus propagation. Improvement of reverse genetics for AHSV was clearly demonstrated by rescue of several mutants and reassortants that were not rescued with previous methods.

CONCLUSIONS

A limited number of expression plasmids is required for rescue of BTV or AHSV using reverse genetics, making the system much more versatile and generally applicable. Optimization of reverse genetics enlarge the possibilities to rescue virus mutants and reassortants, and will greatly benefit the control of these important diseases of livestock and companion animals.

Defeating Bluetongue virus: new approaches in the development of multiserotype vaccines

Bluetongue virus (BTV) is a global threat to domestic and wild ruminants, causing massive economic losses throughout the world. New serotypes of the virus are rapidly emerging in different continents, unfortunately there is little cross-protection between BTV serotypes. The eradication of the virus from a region is particularly complicated in areas where multiple serotypes circulate for a long time. The present review summarizes the actual concerns about the spread of the virus and relevant approaches to develop efficient vaccines against BTV, in particular those focused on a multiserotype design.

Diagnostic Tools for Bluetongue and Epizootic Hemorrhagic Disease Viruses Applicable to North American Veterinary Diagnosticians

This review provides an overview of current and potential new diagnostic tests for bluetongue (BT) and epizootic hemorrhagic disease (EHD) viruses compiled from international participants of the Orbivirus Gap Analysis Workshop, Diagnostic Group. The emphasis of this review is on diagnostic tools available to North American veterinary diagnosticians. Standard diagnostic tests are readily available for BT/EHD viruses, and there are described tests that are published in the World Organization for Animal Health (OIE) Terrestrial Manual. There is however considerable variation in the diagnostic approach to these viruses. Serological assays are well established, and many laboratories are experienced in running these assays. Numerous nucleic acid amplification assays are also available for BT virus (BTV) and EHD virus (EHDV). Although there is considerable experience with BTV reverse-transcriptase PCR (RT-PCR), there are no standards or comparisons of the protocols used by various state and federal veterinary diagnostic laboratories. Methods for genotyping BTV and EHDV isolates are available and are valuable tools for monitoring and analyzing circulating viruses. These methods include RT-PCR panels or arrays, RT-PCR and sequencing of specific genome segments, or the use of next-generation sequencing. In addition to enabling virus characterization, use of advanced molecular detection methods, including DNA microarrays and next-generation sequencing, significantly enhance the ability to detect unique virus strains that may arise through genetic drift, recombination, or viral genome segment reassortment, as well as incursions of new virus strains from other geographical areas.

A Review of Knowledge Gaps and Tools for Orbivirus Research

Although recognized as causing emerging and re-emerging disease outbreaks worldwide since the late 1800 s, there has been growing interest in the United States and Europe in recent years in orbiviruses, their insect vectors, and the diseases they cause in domestic livestock and wildlife. This is due, in part, to the emergence of bluetongue (BT) in northern Europe in 2006-2007 resulting in a devastating outbreak, as well as severe BT outbreaks in sheep and epizootic hemorrhagic disease (EHD) outbreaks in deer and cattle in the United States. Of notable concern is the isolation of as many as 10 new BT virus (BTV) serotypes in the United States since 1999 and their associated unknowns, such as route of introduction, virulence to mammals, and indigenous competent vectors. This review, based on a gap analysis workshop composed of international experts on orbiviruses conducted in 2013, gives a global perspective of current basic virological understanding of orbiviruses, with particular attention to BTV and the closely related epizootic hemorrhagic disease virus (EHDV), and identifies a multitude of basic virology research gaps, critical for predicting and preventing outbreaks.

Inter-segment complementarity in orbiviruses: a driver for co-ordinated genome packaging in the Reoviridae?

The process by which eukaryotic viruses with segmented genomes select a complete set of genome segments for packaging into progeny virus particles is not understood. In this study a model based on the association of genome segments through specific RNA-RNA interactions driven by base pairing was formalized and tested in the Orbivirus genus of the Reoviridae family. A strategy combining screening of the genomic sequences for inter-segment complementarity with direct functional testing of inter-segment RNA-RNA interactions using reverse genetics is described in the type species of the Orbivirus genus, Bluetongue virus (BTV). Two examples, involving four of the ten BTV genomic segments, of specific inter-segment interaction motifs whose maintenance is essential for the generation of infectious virus, were identified. Equivalent inter-segment complementarities were found between the identified regions of the orthologous genome segments of all orbiviruses, including phylogenetically distant species. Specific interaction of the participating RNA segments was confirmed in vitro using electrophoretic mobility shift assays, with the interactions inhibited using oligonucleotides complementary to the interaction motif of one of the interacting partners, and also through mutagenesis of the motifs. In each example, the base pairing rather than the absolute sequence was critical to the formation of a functional inter-segment interaction, with mutations only being tolerated in rescued virus if compensating changes were made in the interacting partner to restore uninterrupted base pairing. The absolute sequence of the complementarity motifs varied between species, indicating that this newly identified phenomenon may contribute to the observed lack of reassortment between Orbivirus species.

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.

Development of a competitive ELISA for NS3 antibodies as DIVA test accompanying the novel Disabled Infectious Single Animal (DISA) vaccine for Bluetongue

Recently, we have developed a novel vaccine for Bluetongue named BT Disabled Infectious Single Animal (DISA) vaccine. Due to the lack of non-essential NS3/NS3a protein, BT DISA vaccine is a replicating vaccine, but without the inherent risks of live-attenuated vaccines, such as residual virulence or reversion to virulence by mutations, reassortment with field virus, horizontal spread by vectors and vertical transmission. The immune response induced by BT DISA vaccines is rapidly induced, highly protective and serotype specific which is dependent on the immunodominant and serotype determining VP2 protein. The BT DISA vaccine platform provides the replacement of exclusively VP2 from different serotypes in order to safely formulate multivalent cocktail vaccines. The lack of NS3/NS3a directed antibodies by BT DISA vaccination enables differentiation of infected from vaccinated animals (DIVA principle). A highly conserved immunogenic site corresponding to the late domain was mapped in the N-terminal region of NS3. We here established an NS3-specific competitive ELISA (NS3 cELISA) as serological DIVA test accompanying BT DISA vaccines. To this end, NS3 protein missing putative transmembrane regions was produced in large amounts in bacteria and used as antigen in the NS3 cELISA which was investigated with a variety of sera. The NS3 cELISA displayed a high sensitivity and specificity similar to the commercially available VP7-specific cELISA. Results of previously performed vaccination-challenge trials with BT DISA vaccines clearly demonstrate the DIVA system based on the NS3 cELISA and BT vaccine free of NS3 protein.

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.

Bluetongue in small ruminants: An opinionated review, with a brief appraisal of the 2014 outbreak of the disease in Greece and the south-east Europe

Bluetongue is an arthropod-borne viral disease of ruminants, especially of sheep, caused by Bluetongue virus, which belongs to the genus Orbivirus of the family Reoviridae and is classified into 26 antigenically distinct serotypes. Once thought to be restricted in Africa and parts of the Middle East, bluetongue has now become a concern in sheep-rearing countries around the world. In the past 10 years, severe outbreaks have occurred in Europe with important economic consequences; of these, the 2006-20008 outbreak in Europe was caused by a serotype 8 strain and the 2014 outbreak in Greece and the other countries of south-east Europe was caused by a serotype 4 strain, suggested to be a reassortant strain with genome segments from lineages of serotype 1, 2 and 4. Immunisation campaigns can be implemented for successful control and limiting of the disease. Nevertheless, in both of the above outbreaks, late application of vaccinations led to a wide spread of the disease, which subsequently resulted in significant losses in livestock in the affected regions. In view of that, standardisation of control measures in the future will be beneficial for efficiently limiting outbreaks of the disease.

Turnover Rate of NS3 Proteins Modulates Bluetongue Virus Replication Kinetics in a Host-Specific Manner

Bluetongue virus (BTV) is an arbovirus transmitted to livestock by midges of the Culicoides family and is the etiological agent of a hemorrhagic disease in sheep and other ruminants. In mammalian cells, BTV particles are released primarily by virus-induced cell lysis, while in insect cells they bud from the plasma membrane and establish a persistent infection. BTV possesses a ten-segmented double-stranded RNA genome, and NS3 proteins are encoded by segment 10 (Seg-10). The viral nonstructural protein 3 (NS3) plays a key role in mediating BTV egress as well as in impeding the in vitro synthesis of type I interferon in mammalian cells. In this study, we asked whether genetically distant NS3 proteins can alter BTV-host interactions. Using a reverse genetics approach, we showed that, depending on the NS3 considered, BTV replication kinetics varied in mammals but not in insects. In particular, one of the NS3 proteins analyzed harbored a proline at position 24 that leads to its rapid intracellular decay in ovine but not in Culicoides cells and to the attenuation of BTV virulence in a mouse model of disease. Overall, our data reveal that the genetic variability of Seg-10/NS3 differentially modulates BTV replication kinetics in a host-specific manner and highlight the role of the host-specific variation in NS3 protein turnover rate.

IMPORTANCE

BTV is the causative agent of a severe disease transmitted between ruminants by biting midges of Culicoides species. NS3, encoded by Seg-10 of the BTV genome, fulfills key roles in BTV infection. As Seg-10 sequences from various BTV strains display genetic variability, we assessed the impact of different Seg-10 and NS3 proteins on BTV infection and host interactions. In this study, we revealed that various Seg-10/NS3 proteins alter BTV replication kinetics in mammals but not in insects. Notably, we found that NS3 protein turnover may vary in ovine but not in Culicoides cells due to a single amino acid residue that, most likely, leads to rapid and host-dependent protein degradation. Overall, this study highlights that genetically distant BTV Seg-10/NS3 influence BTV biological properties in a host-specific manner and increases our understanding of how NS3 proteins contribute to the outcome of BTV infection.

Rapid mapping of functional cis-acting RNA elements by recovery of virus from a degenerate RNA population: application to genome segment 10 of bluetongue virus

The regulatory elements which control the processes of virus replication and gene expression in the Orbivirus genus are uncharacterized in terms of both their locations within genome segments and their specific functions. The reverse genetics system for the type species, Bluetongue virus, has been used in combination with RNA secondary structure prediction to identify and map the positions of cis-acting regions within genome segment 10. Through the simultaneous introduction of variability at multiple nucleotide positions in the rescue RNA population, the functional contribution of these positions was used to map regions containing cis-acting elements essential for virus viability. Nucleotides that were individually lethal when varied mapped within a region of predicted secondary structure involving base pairing between the 5' and 3' ends of the transcript. An extended region of predicted perfect base pairing located within the 3' untranslated region of the genome segment was also found to be required for virus viability. In contrast to the identification of individually lethal mutations, gross alteration of the composition of this predicted stem region was possible, providing the base-pairing potential between the two strands was maintained, identifying a structural feature predicted to be conserved throughout the Orbivirus genus. The approach of identifying cis-acting sequences through sequencing the recovered virus following the rescue of a degenerate RNA population is broadly applicable to viruses where reverse genetics is available.

The effect of glycosylation on cytotoxicity of Ibaraki virus nonstructural protein NS3

The cytotoxicity of Ibaraki virus nonstructural protein NS3 was confirmed, and the contribution of glycosylation to this activity was examined by using glycosylation mutants of NS3 generated by site-directed mutagenesis. The expression of NS3 resulted in leakage of lactate dehydrogenase to the culture supernatant, suggesting the cytotoxicity of this protein. The lack of glycosylation impaired the transport of NS3 to the plasma membrane and resulted in reduced cytotoxicity. Combined with the previous observation that NS3 glycosylation was specifically observed in mammalian cells (Urata et al., Virus Research 2014), it was suggested that the alteration of NS3 cytotoxicity through modulating glycosylation is one of the strategies to achieve host specific pathogenisity of Ibaraki virus between mammals and vector arthropods.

VP2 Exchange and NS3/NS3a Deletion in African Horse Sickness Virus (AHSV) in Development of Disabled Infectious Single Animal Vaccine Candidates for AHSV

African horse sickness virus (AHSV) is a virus species in the genus Orbivirus of the family Reoviridae. There are nine serotypes of AHSV showing different levels of cross neutralization. AHSV is transmitted by species of Culicoides biting midges and causes African horse sickness (AHS) in equids, with a mortality rate of up to 95% in naive horses. AHS has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climates appear to be competent vectors for the related bluetongue virus (BTV). To control AHS, live-attenuated vaccines (LAVs) are used in Africa. We used reverse genetics to generate “synthetic” reassortants of AHSV for all nine serotypes by exchange of genome segment 2 (Seg-2). This segment encodes VP2, which is the serotype-determining protein and the dominant target for neutralizing antibodies. Single Seg-2 AHSV reassortants showed similar cytopathogenic effects in mammalian cells but displayed different growth kinetics. Reverse genetics for AHSV was also used to study Seg-10 expressing NS3/NS3a proteins. We demonstrated that NS3/NS3a proteins are not essential for AHSV replication in vitro. NS3/NS3a of AHSV is, however, involved in the cytopathogenic effect in mammalian cells and is very important for virus release from cultured insect cells in particular. Similar to the concept of the bluetongue disabled infectious single animal (BT DISA) vaccine platform, an AHS DISA vaccine platform lacking NS3/NS3a expression was developed. Using exchange of genome segment 2 encoding VP2 protein (Seg-2[VP2]), we will be able to develop AHS DISA vaccine candidates for all current AHSV serotypes.

IMPORTANCE African horse sickness virus is transmitted by species of Culicoides biting midges and causes African horse sickness in equids, with a mortality rate of up to 95% in naive horses. African horse sickness has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climates are supposed to be competent vectors. By using reverse genetics, viruses of all nine serotypes were constructed by the exchange of Seg-2 expressing the serotype-determining VP2 protein. Furthermore, we demonstrated that the nonstructural protein NS3/NS3a is not essential for virus replication in vitro. However, the potential spread of the virus by biting midges is supposed to be blocked, since the in vitro release of the virus was strongly reduced due to this deletion. VP2 exchange and NS3/NS3a deletion in African horse sickness virus were combined in the concept of a disabled infectious single animal vaccine for all nine serotypes.

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.

Sequential packaging of RNA genomic segments during the assembly of Bluetongue virus

Bluetongue virus (BTV), a member of the Orbivirus genus within the Reoviridae family, has a genome of 10 double-stranded RNA segments, with three distinct size classes. Although the packaging of the viral genome is evidently highly specific such that every virus particle contains a set of 10 RNA segments, the order and mechanism of packaging are not understood. In this study we have combined the use of a cell-free in vitro assembly system with a novel RNA–RNA interaction assay to investigate the mechanism of single-stranded (ss) RNAs packaging during nascent capsid assembly. Exclusion of single or multiple ssRNA segments in the packaging reaction or their addition in different order significantly altered the outcome and suggested a particular role for the smallest segment, S10. Our data suggests that genome packaging probably initiates with the smallest segment which triggers RNA–RNA interaction with other smaller segments forming a complex network. Subsequently, the medium to larger size ssRNAs are recruited until the complete genome is packaging into the capsid. The untranslated regions of the smallest RNA segment, S10, is critical for the instigation of this process. We suggest that the selective packaging observed in BTV may also apply to other members of the Reoviridae family.

VP2-serotyped live-attenuated bluetongue virus without NS3/NS3a expression provides serotype-specific protection and enables DIVA

Bluetongue virus (BTV) causes Bluetongue in ruminants and is transmitted by Culicoides biting midges. Vaccination is the most effective measure to control vector borne diseases; however, there are 26 known BTV serotypes showing little cross protection. The BTV serotype is mainly determined by genome segment 2 encoding the VP2 protein. Currently, inactivated and live-attenuated Bluetongue vaccines are available for a limited number of serotypes, but each of these have their specific disadvantages, including the inability to differentiate infected from vaccinated animals (DIVA). BTV non-structural proteins NS3 and NS3a are not essential for virus replication in vitro, but are important for cytopathogenic effect in mammalian cells and for virus release from insect cells in vitro. Recently, we have shown that virulent BTV8 without NS3/NS3a is non-virulent and viremia in sheep is strongly reduced, whereas local in vivo replication leads to seroconversion. Live-attenuated BTV6 without NS3/NS3a expression protected sheep against BTV challenge. Altogether, NS3/NS3a knockout BTV6 is a promising vaccine candidate and has been named Disabled Infectious Single Animal (DISA) vaccine. Here, we show serotype-specific protection in sheep by DISA vaccine in which only genome segment 2 of serotype 8 was exchanged. Similarly, DISA vaccines against other serotypes could be developed, by exchange of only segment 2, and could therefore safely be combined in multi-serotype cocktail vaccines with respect to reassortment between vaccine viruses. Additionally, NS3 antibody responses are raised after natural BTV infection and NS3-based ELISAs are therefore appropriate tools for DIVA testing accompanying the DISA vaccine. To enable DIVA, we developed an experimental NS3 ELISA. Indeed, vaccinated sheep remained negative for NS3 antibodies, whereas seroconversion for NS3 antibodies was associated with viremia after heterologous BTV challenge.