Synthesis of bluetongue virus (BTV) corelike particles by a recombinant baculovirus expressing the two major structural core proteins of BTV

Development of a Plant-Expressed Subunit Vaccine against Brucellosis

Brucellosis is an important bacterial disease of livestock and the most common zoonotic disease. The current vaccines are effective but unsafe, as they result in animal abortions and are pathogenic to humans. Virus-like particles are being investigated as molecular scaffolds for foreign antigen presentation to the immune system. Here, we sought to develop a new-generation vaccine by presenting selected Brucella melitensis T cell epitopes on the surface of Orbivirus core-like particles (CLPs) and transiently expressing these chimeric particles in Nicotiana benthamiana plants. We successfully demonstrated the assembly of five chimeric CLPs in N. benthamiana plants, with each CLP presenting a different T cell epitope. The safety and protective efficacy of three of the highest-yielding CLPs was investigated in a mouse model of brucellosis. All three plant-expressed chimeric CLPs were safe when inoculated into BALB/c mice at specific antigen doses. However, only one chimeric CLP induced protection against the virulent Brucella strain challenge equivalent to the protection induced by the commercial Rev1 vaccine. Here, we have successfully shown the assembly, safety and protective efficacy of plant-expressed chimeric CLPs presenting B. melitensis T cell epitopes. This is the first step in the development of a safe and efficacious subunit vaccine against brucellosis.

Multi-Gene Recombinant Baculovirus Expression Systems: From Inception to Contemporary Applications

Many protein expression systems are primarily utilised to produce a single, specific recombinant protein. In contrast, most biological processes such as virus assembly rely upon a complex of several interacting proteins rather than the activity of a sole protein. The high complexity of the baculovirus genome, coupled with a multiphase replication cycle incorporating distinct transcriptional steps, made it the ideal system to manipulate for high-level expression of a single, or co-expression of multiple, foreign proteins within a single cell. We have developed and utilised a series of recombinant baculovirus systems to unravel the sequential assembly process of a complex non-enveloped model virus, bluetongue virus (BTV). The high protein yields expressed by the baculovirus system not only facilitated structure-function analysis of each viral protein but were also advantageous to crystallography studies and supported the first atomic-level resolution of a recombinant viral protein, the major BTV capsid protein. Further, the formation of recombinant double-shelled virus-like particles (VLPs) provided insights into the structure-function relationships among the four major structural proteins of the BTV whilst also representing a potential candidate for a viral vaccine. The baculovirus multi-gene expression system facilitated the study of structurally complex viruses (both non-enveloped and enveloped viruses) and heralded a new generation of viral vaccines.

A blocking ELISA based on virus-like nanoparticles chimerized with an antigenic epitope of ASFV P54 for detecting ASFV antibodies

African swine fever virus (ASFV) is a highly lethal pathogen of domestic and wild pigs. Due to no vaccines or drugs available, early accurate diagnosis and eradication of infected animals are the most important measures for ASFV prevention and control. Bluetongue virus (BTV) core-like particles (CLPs) are non-infectious hollow nanoparticles assembled from the BTV VP3 and VP7 proteins, which could be used as a platform for presenting foreign epitopes. In this study, the secondary structure of BTV VP7 protein was analyzed and predicted using the IEDB Analysis resource. Based on the prediction results of the VP7 protein, the chimeric CLPs with an ASFV P54 epitope were successfully prepared through the BAC-to-BAC baculovirus expression system and sucrose gradient centrifugation. Based on the chimeric CLPs and mAb 2E4 against AFSV P54 epitope, a blocking ELISA for detecting AFSV antibodies was established, and its reaction conditions were optimized. Through comprehensive evaluation of the method, the results showed the chimeric CLPs-based blocking ELISA displayed the best detection performance, with an AUC of 0.9961, a sensitivity of 97.65%, and a specificity of 95.24% in ROC analysis. Compared with western blot and a commercial c-ELISA for detecting anti-ASFV antibodies, this method had an excellent agreement of 96.35% (kappa value = 0.911) and 97.76% (kappa value = 0.946) with the other tests, respectively. This ELISA also had high repeatability, with CV < 10%, and no cross-reaction with the serum antibodies against other swine viruses or Orbivirus. In brief, this was the first report on developing a blocking ELISA based on virus-like nanoparticles chimerized with an antigenic epitope of ASFV P54 for serological diagnosis of ASFV.

A multidisciplinary approach to the identification of the protein-RNA connectome in double-stranded RNA virus capsids

How multi-segmented double-stranded RNA (dsRNA) viruses correctly incorporate their genomes into their capsids remains unclear for many viruses, including Bluetongue virus (BTV), a Reoviridae member, with a genome of 10 segments. To address this, we used an RNA-cross-linking and peptide-fingerprinting assay (RCAP) to identify RNA binding sites of the inner capsid protein VP3, the viral polymerase VP1 and the capping enzyme VP4. Using a combination of mutagenesis, reverse genetics, recombinant proteins and in vitro assembly, we validated the importance of these regions in virus infectivity. Further, to identify which RNA segments and sequences interact with these proteins, we used viral photo-activatable ribonucleoside crosslinking (vPAR-CL) which revealed that the larger RNA segments (S1-S4) and the smallest segment (S10) have more interactions with viral proteins than the other smaller segments. Additionally, using a sequence enrichment analysis we identified an RNA motif of nine bases that is shared by the larger segments. The importance of this motif for virus replication was confirmed by mutagenesis followed by virus recovery. We further demonstrated that these approaches could be applied to a related Reoviridae member, rotavirus (RV), which has human epidemic impact, offering the possibility of novel intervention strategies for a human pathogen.

Nanoparticle- and Microparticle-Based Vaccines against Orbiviruses of Veterinary Importance

Bluetongue virus (BTV) and African horse sickness virus (AHSV) are widespread arboviruses that cause important economic losses in the livestock and equine industries, respectively. In addition to these, another arthropod-transmitted orbivirus known as epizootic hemorrhagic disease virus (EHDV) entails a major threat as there is a conducive landscape that nurtures its emergence in non-endemic countries. To date, only vaccinations with live attenuated or inactivated vaccines permit the control of these three viral diseases, although important drawbacks, e.g., low safety profile and effectiveness, and lack of DIVA (differentiation of infected from vaccinated animals) properties, constrain their usage as prophylactic measures. Moreover, a substantial number of serotypes of BTV, AHSV and EHDV have been described, with poor induction of cross-protective immune responses among serotypes. In the context of next-generation vaccine development, antigen delivery systems based on nano- or microparticles have gathered significant attention during the last few decades. A diversity of technologies, such as virus-like particles or self-assembled protein complexes, have been implemented for vaccine design against these viruses. In this work, we offer a comprehensive review of the nano- and microparticulated vaccine candidates against these three relevant orbiviruses. Additionally, we also review an innovative technology for antigen delivery based on the avian reovirus nonstructural protein muNS and we explore the prospective functionality of the nonstructural protein NS1 nanotubules as a BTV-based delivery platform.

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.

Virus-like particles: preparation, immunogenicity and their roles as nanovaccines and drug nanocarriers

Virus-like particles (VLPs) are virus-derived structures made up of one or more different molecules with the ability to self-assemble, mimicking the form and size of a virus particle but lacking the genetic material so they are not capable of infecting the host cell. Expression and self-assembly of the viral structural proteins can take place in various living or cell-free expression systems after which the viral structures can be assembled and reconstructed. VLPs are gaining in popularity in the field of preventive medicine and to date, a wide range of VLP-based candidate vaccines have been developed for immunization against various infectious agents, the latest of which is the vaccine against SARS-CoV-2, the efficacy of which is being evaluated. VLPs are highly immunogenic and are able to elicit both the antibody- and cell-mediated immune responses by pathways different from those elicited by conventional inactivated viral vaccines. However, there are still many challenges to this surface display system that need to be addressed in the future. VLPs that are classified as subunit vaccines are subdivided into enveloped and non- enveloped subtypes both of which are discussed in this review article. VLPs have also recently received attention for their successful applications in targeted drug delivery and for use in gene therapy. The development of more effective and targeted forms of VLP by modification of the surface of the particles in such a way that they can be introduced into specific cells or tissues or increase their half-life in the host is likely to expand their use in the future. Recent advances in the production and fabrication of VLPs including the exploration of different types of expression systems for their development, as well as their applications as vaccines in the prevention of infectious diseases and cancers resulting from their interaction with, and mechanism of activation of, the humoral and cellular immune systems are discussed in this review.

Production and Easy One-Step Purification of Bluetongue Recombinant VP7 from Infected Sf9 Supernatant for an Immunoenzymatic Assay (ELISA)

Bluetongue (BT) is non-contagious, vector-borne viral disease of domestic and wild ruminants, transmitted by midges (Culicoides spp.) and is caused by Bluetongue virus (BTV). BTV is the type species of the Orbivirus genus within the Reoviridae family and possesses a genome consisting of 10 double-stranded RNA segments encoding 7 structural and 4 nonstructural proteins. Viral Protein 7 (VP7) is the major sera group-specific protein and is a good antigen candidate for immunoenzymatic assays for the BT diagnosis. In our work, BTV-2 recombinant VP7 (BTV-2 recVP7), expressed in Spodoptera frugiperda (Sf9) cells using a baculovirus system, was produced and purified by affinity chromatography from the supernatant of infected cell culture. The use of the supernatant allowed us to obtain a high quantity of recombinant protein with high purity level by an easy one-step procedure, rather than the multistep purification from the pellet. RecVP7-BTV2 was detected using a MAb anti-BTV in Western blot and it was used to develop an immunoenzymatic assay.

Bluetongue virus assembly and exit pathways

Bluetongue virus (BTV) is an insect-vectored emerging pathogen of wild ruminants and livestock in many parts of the world. The virion particle is a complex structure of consecutive layers of protein surrounding a genome of 10 double-stranded (ds) RNA segments. BTV has been studied extensively as a model system for large, nonenveloped dsRNA viruses. A combination of recombinant proteins and particles together with reverse genetics, high-resolution structural analysis by X-ray crystallography and cryo-electron microscopy techniques have been utilized to provide an order for the assembly of the capsid shell and the protein sequestration required for it. Further, a reconstituted in vitro assembly system and RNA-RNA interaction assay, have defined the individual steps required for the assembly and packaging of the 10-segmented RNA genome. In addition, various microscopic techniques have been utilized to illuminate the stages of virus maturation and its egress via multiple pathways. These findings have not only given an overall understanding of BTV assembly and morphogenesis but also indicated that similar assembly and egress pathways are likely to be used by related viruses and provided an informed starting point for intervention or prevention.

Solubilisation and purification of recombinant bluetongue virus VP7 expressed in a bacterial system

Bluetongue virus (BTV) is an Orbivirus that has a profound economic impact due to direct loss of livestock as well as movement bans in an attempt to prevent the spread of the disease to susceptible areas. BTV VP7, along with VP3, forms the inner capsid core of the virus where it acts as the barrier between the outer layer and the inner core housing the genetic material. Purification of BTV VP7 has proven to be problematic and expensive mainly due to its insolubility is several expression systems. To overcome this, in this paper we present a protocol for the solubilisation of BTV VP7 from inclusion bodies expressed in E.coli, and subsequent purification using nickel affinity chromatography. The purified protein was then characterised using native PAGE, far ultraviolet circular dichroism (far-UV CD) and intrinsic fluorescence and found to have both secondary and tertiary structure even in the presence of 5 M urea. Both tertiary and secondary structure was further shown to be to be maintained at least to 42 °C in 5 M urea.

Interaction between a Unique Minor Protein and a Major Capsid Protein of Bluetongue Virus Controls Virus Infectivity

Among the Reoviridae family of double-stranded RNA viruses, only members of the Orbivirus genus possess a unique structural protein, termed VP6, within their particles. Bluetongue virus (BTV), an important livestock pathogen, is the prototype Orbivirus. BTV VP6 is an ATP-dependent RNA helicase, and it is indispensable for virus replication. In the study described in this report, we investigated how VP6 might be recruited to the virus capsid and whether the BTV structural protein VP3, which forms the internal layer of the virus capsid core, is involved in VP6 recruitment. We first demonstrated that VP6 interacts with VP3 and colocalizes with VP3 during capsid assembly. A series of VP6 mutants was then generated, and in combination with immunoprecipitation and size exclusion chromatographic analyses, we demonstrated that VP6 directly interacts with VP3 via a specific region of the C-terminal portion of VP6. Finally, using our reverse genetics system, mutant VP6 proteins were introduced into the BTV genome and interactions between VP6 and VP3 were shown in a live cell system. We demonstrate that BTV strains possessing a mutant VP6 are replication deficient in wild-type BSR cells and fail to recruit the viral replicase complex into the virus particle core. Taken together, these data suggest that the interaction between VP3 and VP6 could be important in the packaging of the viral genome and early stages of particle formation.

IMPORTANCE The orbivirus bluetongue virus (BTV) is the causative agent of bluetongue disease of livestock, often causing significant economic and agricultural impacts in the livestock industry. In the study described in this report, we identified the essential region and residues of the unique orbivirus capsid protein VP6 which are responsible for its interaction with other BTV proteins and its subsequent recruitment into the virus particle. The nature and mechanism of these interactions suggest that VP6 has a key role in packaging of the BTV genome into the virus particle. As such, this is a highly significant finding, as this new understanding of BTV assembly could be exploited to design novel vaccines and antivirals against bluetongue disease.

Bluetongue virus structure and assembly

Bluetongue virus (BTV) is an insect-vectored emerging pathogen of wild ruminants and livestock in many parts of the world. The virion particle is a complex structure of consecutive layers of protein surrounding a genome of ten double-stranded (ds) RNA segments. BTV has been studied as a model system for large, non-enveloped dsRNA viruses. Several new techniques have been applied to define the virus-encoded enzymes required for RNA replication to provide an order for the assembly of the capsid shell and the protein sequestration required for it. Further, a reconstituted in vitro system has defined the individual steps of the assembly and packaging of the genomic RNA. These findings illuminate BTV assembly and indicate the pathways that related viruses might use to provide an informed starting point for intervention or prevention.

Engineering Recombinant Virus-like Nanoparticles from Plants for Cellular Delivery

Understanding capsid assembly following recombinant expression of viral structural proteins is critical to the design and modification of virus-like nanoparticles for biomedical and nanotechnology applications. Here, we use plant-based transient expression of the Bluetongue virus (BTV) structural proteins, VP3 and VP7, to obtain high yields of empty and green fluorescent protein (GFP)-encapsidating core-like particles (CLPs) from leaves. Single-particle cryo-electron microscopy of both types of particles revealed considerable differences in CLP structure compared to the crystal structure of infection-derived CLPs; in contrast, the two recombinant CLPs have an identical external structure. Using this insight, we exploited the unencumbered pore at the 5-fold axis of symmetry and the absence of encapsidated RNA to label the interior of empty CLPs with a fluorescent bioconjugate. CLPs containing 120 GFP molecules and those containing approximately 150 dye molecules were both shown to bind human integrin via a naturally occurring Arg-Gly-Asp motif found on an exposed loop of the VP7 trimeric spike. Furthermore, fluorescently labeled CLPs were shown to interact with a cell line overexpressing the surface receptor. Thus, BTV CLPs present themselves as a useful tool in targeted cargo delivery. These results highlight the importance of detailed structural analysis of VNPs in validating their molecular organization and the value of such analyses in aiding their design and further modification.

Entry of Bluetongue Virus Capsid Requires the Late Endosome-specific Lipid Lysobisphosphatidic Acid

The entry of viruses into host cells is one of the key processes of infection. The mechanisms of cellular entry for enveloped virus have been well studied. The fusion proteins as well as the facilitating cellular lipid factors involved in the viral fusion entry process have been well characterized. The process of non-enveloped virus cell entry, in comparison, remains poorly defined, particularly for large complex capsid viruses of the family Reoviridae, which comprises a range of mammalian pathogens. These viruses enter cells without the aid of a limiting membrane and thus cannot fuse with host cell membranes to enter cells. Instead, these viruses are believed to penetrate membranes of the host cell during endocytosis. However, the molecular mechanism of this process is largely undefined. Here we show, utilizing an in vitro liposome penetration assay and cell biology, that bluetongue virus (BTV), an archetypal member of the Reoviridae, utilizes the late endosome-specific lipid lysobisphosphatidic acid for productive membrane penetration and viral entry. Further, we provide preliminary evidence that lipid lysobisphosphatidic acid facilitates pore expansion during membrane penetration, suggesting a mechanism for lipid factor requirement of BTV. This finding indicates that despite the lack of a membrane envelope, the entry process of BTV is similar in specific lipid requirements to enveloped viruses that enter cells through the late endosome. These results are the first, to our knowledge, to demonstrate that a large non-enveloped virus of the Reoviridae has specific lipid requirements for membrane penetration and host cell entry.

Generation of virus-like particles for emerging epizootic haemorrhagic disease virus: Towards the development of safe vaccine candidates

Epizootic haemorrhagic disease virus (EHDV) is an insect-transmitted pathogen which causes high mortality in deer populations and may also cause high morbidity in cattle. EHDV belongs to the Orbivirus genus and is closely related to the prototype Bluetongue virus (BTV). To date seven distinct serotypes have been recognized. However, a live-attenuated vaccine is commercially available against only one serotype namely EHDV-2, which has been responsible for multiple outbreaks in North America, Canada, Asia and Australia. Here we expressed four major capsid proteins (VP2, VP3, VP5 and VP7) of EHDV-1 using baculovirus multiple gene expression systems and demonstrated that three-layered VLPs were assembled mimicking the authentic EHDV particles but lacking the viral genomic RNA segments and the transcriptase complex (TC). Antibodies generated with VLPs not only neutralized EHDV-1 infection in cell culture but also showed cross neutralizing reactivity against two other serotypes, EHDV-2 and EHDV-6. For proof of concept, we demonstrated that EHDV-2 VLPs could be generated rapidly by expressing the EHDV-2 variable outer capsid proteins (VP2, VP5) together with EHDV-1 VP3 and VP7, the two inner capsid proteins, which are highly conserved among the 7 serotypes. Data presented in this study validate the VLPs as a potential vaccine and demonstrate that a vaccine could be developed rapidly in the event of an outbreak of a new serotype.

Molecular pharming - VLPs made in plants

Plant-based expression offers a safe, inexpensive and potentially limitless way to produce therapeutics in a quick and flexible manner. Plants require only simple inorganic nutrients, water, carbon dioxide and sunlight for efficient growth. Virus-like particles (VLPs) are convincing look-alikes of viruses but without carrying infectious genomic material. However, they can still elicit a very potent immune response which makes them ideal vaccine candidates. In this review the different methods of plant expression are described together with the most recent developments in the field of transiently-expressed plant-made VLPs.

Transient Bluetongue virus serotype 8 capsid protein expression in Nicotiana benthamiana

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Expression of BTV-8 capsid genes results in CLPs and VLPs in Nicotiana benthamiana.

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Density of infiltrated Agrobacterium cells influences protein expression levels.

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CLPs/VLPs can be purified from leaf extracts using density gradient centrifugation.

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CLPs/VLPs are present in paracrystalline arrays within the plant cell cytoplasm.

Bluetongue virus (BTV) causes severe disease in domestic and wild ruminants, and has recently caused several outbreaks in Europe. Current vaccines include live-attenuated and inactivated viruses; while these are effective, there is risk of reversion to virulence by mutation or reassortment with wild type viruses. Subunit or virus-like particle (VLP) vaccines are safer options: VLP vaccines produced in insect cells by expression of the four BTV capsid proteins are protective against challenge; however, this is a costly production method. We investigated production of BTV VLPs in plants via Agrobacterium-mediated transient expression, an inexpensive production system very well suited to developing country use. Leaves infiltrated with recombinant pEAQ-HT vectors separately encoding the four BTV-8 capsid proteins produced more proteins than recombinant pTRA vectors. Plant expression using the pEAQ-HT vector resulted in both BTV-8 core-like particles (CLPs) and VLPs; differentially controlling the concentration of infiltrated bacteria significantly influenced yield of the VLPs. In situ localisation of assembled particles was investigated by using transmission electron microscopy (TEM) and it was shown that a mixed population of core-like particles (CLPs, consisting of VP3 and VP7) and VLPs were present as paracrystalline arrays in the cytoplasm of plant cells co-expressing all four capsid proteins.

Meeting report VLPNPV: Session 5: Plant based technology

The VLPNPV 2014 Conference that was convened at the Salk institute was the second conference of its kind to focus on advances in production, purification, and delivery of virus-like particles (VLPs) and nanoparticles. Many exciting developments were reported and discussed in this interdisciplinary arena, but here we report specifically on the contributions of plant-based platforms to VLP vaccine technology as reported in the section of the conference devoted to the topic as well in additional presentations throughout the meeting. The increasing popularity of plant production platforms is due to their lower cost, scalability, and lack of contaminating animal pathogens seen with other systems. Reports include production of complex VLPs consisting of 4 proteins expressed at finely-tuned expression levels, a prime-boost strategy for HIV vaccination using plant-made VLPs and a live viral vector, and the characterization and development of plant viral nanoparticles for use in cancer vaccines, drug delivery, and bioimaging.

Virus-Like Particles, a Versatile Subunit Vaccine Platform

Virus-like particles (VLPs) can be spontaneously formed after expression of self-polymerising viral capsid proteins. VLPs structurally resemble their native source virus, maintaining immunological relevance by retaining formation of immunogenic motifs with natural conformation. The absence of the virus genome renders VLPs safe for administration as a subunit vaccine. VLPs can target both arms of the immune response, with some VLPs initiating production of specific antibodies and others activating cytotoxic T cells. VLPs are also exceptionally versatile, conferring protection against the host virus or acting as a scaffold for antigenic molecules. In addition, VLP can support intraparticulate encapsulation for immunomodulation and gene delivery. VLP vaccines have been developed for prophylactic protection against infectious organisms, and therapeutic treatment of conditions such as Alzheimer’s disease, hypertension, and cancer. With an expanding list of vaccine candidates, VLP vaccines are a promising field with a wide range of applications.

Bluetongue virus capsid assembly and maturation

Maturation is an intrinsic phase of the viral life cycle and is often intertwined with egress. In this review we focus on orbivirus maturation by using Bluetongue virus (BTV) as a representative. BTV, a member of the genus Orbivirus within the family Reoviridae, has over the last three decades been subjected to intense molecular study and is thus one of the best understood viruses. BTV is a non-enveloped virus comprised of two concentric protein shells that encapsidate 10 double-stranded RNA genome segments. Upon cell entry, the outer capsid is shed, releasing the core which does not disassemble into the cytoplasm. The polymerase complex within the core then synthesizes transcripts from each genome segment and extrudes these into the cytoplasm where they act as templates for protein synthesis. Newly synthesized ssRNA then associates with the replicase complex prior to encapsidation by inner and outer protein layers of core within virus-triggered inclusion bodies. Maturation of core occurs outside these inclusion bodies (IBs) via the addition of the outer capsid proteins, which appears to be coupled to a non-lytic, exocytic pathway during early infection. Similar to the enveloped viruses, BTV hijacks the exocytosis and endosomal sorting complex required for trafficking (ESCRT) pathway via a non-structural glycoprotein. This exquisitely detailed understanding is assembled from a broad array of assays, spanning numerous and diverse in vitro and in vivo studies. Presented here are the detailed insights of BTV maturation and egress.

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.

The molecular biology of Bluetongue virus replication

The members of Orbivirus genus within the Reoviridae family are arthropod-borne viruses which are responsible for high morbidity and mortality in ruminants. Bluetongue virus (BTV) which causes disease in livestock (sheep, goat, cattle) has been in the forefront of molecular studies for the last three decades and now represents the best understood orbivirus at a molecular and structural level. The complex nature of the virion structure has been well characterised at high resolution along with the definition of the virus encoded enzymes required for RNA replication; the ordered assembly of the capsid shell as well as the protein and genome sequestration required for it; and the role of host proteins in virus entry and virus release. More recent developments of Reverse Genetics and Cell-Free Assembly systems have allowed integration of the accumulated structural and molecular knowledge to be tested at meticulous level, yielding higher insight into basic molecular virology, from which the rational design of safe efficacious vaccines has been possible. This article is centred on the molecular dissection of BTV with a view to understanding the role of each protein in the virus replication cycle. These areas are important in themselves for BTV replication but they also indicate the pathways that related viruses, which includes viruses that are pathogenic to man and animals, might also use providing an informed starting point for intervention or prevention.

Bioengineering virus-like particles as vaccines

Virus-like particle (VLP) technology seeks to harness the optimally tuned immunostimulatory properties of natural viruses while omitting the infectious trait. VLPs that assemble from a single protein have been shown to be safe and highly efficacious in humans, and highly profitable. VLPs emerging from basic research possess varying levels of complexity and comprise single or multiple proteins, with or without a lipid membrane. Complex VLP assembly is traditionally orchestrated within cells using black-box approaches, which are appropriate when knowledge and control over assembly are limited. Recovery challenges including those of adherent and intracellular contaminants must then be addressed. Recent commercial VLPs variously incorporate steps that include VLP in vitro assembly to address these problems robustly, but at the expense of process complexity. Increasing research activity and translation opportunity necessitate bioengineering advances and new bioprocessing modalities for efficient and cost-effective production of VLPs. Emerging approaches are necessarily multi-scale and multi-disciplinary, encompassing diverse fields from computational design of molecules to new macro-scale purification materials. In this review, we highlight historical and emerging VLP vaccine approaches. We overview approaches that seek to specifically engineer a desirable immune response through modular VLP design, and those that seek to improve bioprocess efficiency through inhibition of intracellular assembly to allow optimal use of existing purification technologies prior to cell-free VLP assembly. Greater understanding of VLP assembly and increased interdisciplinary activity will see enormous progress in VLP technology over the coming decade, driven by clear translational opportunity.

A method for rapid production of heteromultimeric protein complexes in plants: assembly of protective bluetongue virus-like particles

Plant expression systems based on nonreplicating virus-based vectors can be used for the simultaneous expression of multiple genes within the same cell. They therefore have great potential for the production of heteromultimeric protein complexes. This work describes the efficient plant-based production and assembly of Bluetongue virus-like particles (VLPs), requiring the simultaneous expression of four distinct proteins in varying amounts. Such particles have the potential to serve as a safe and effective vaccine against Bluetongue virus (BTV), which causes high mortality rates in ruminants and thus has a severe effect on the livestock trade. Here, VLPs produced and assembled in Nicotiana benthamiana using the cowpea mosaic virus-based HyperTrans (CPMV-HT) and associated pEAQ plant transient expression vector system were shown to elicit a strong antibody response in sheep. Furthermore, they provided protective immunity against a challenge with a South African BTV-8 field isolate. The results show that transient expression can be used to produce immunologically relevant complex heteromultimeric structures in plants in a matter of days. The results have implications beyond the realm of veterinary vaccines and could be applied to the production of VLPs for human use or the coexpression of multiple enzymes for the manipulation of metabolic pathways.

Use of baculovirus expression system for generation of virus-like particles: successes and challenges

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A brief overview of principles and applications of BES.

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Generation of VLPs using BES.

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Major properties of BES: promoting generation of VLPs.

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Bioprocess considerations for generation of VLPs.

The baculovirus expression system (BES) has been one of the versatile platforms for the production of recombinant proteins requiring multiple post-translational modifications, such as folding, oligomerization, phosphorylation, glycosylation, acylation, disulfide bond formation and proteolytic cleavage. Advances in recombinant DNA technology have facilitated application of the BES, and made it possible to express multiple proteins simultaneously in a single infection and to produce multimeric proteins sharing functional similarity with their natural analogs. Therefore, the BES has been used for the production of recombinant proteins and the construction of virus-like particles (VLPs), as well as for the development of subunit vaccines, including VLP-based vaccines. The VLP, which consists of one or more structural proteins but no viral genome, resembles the authentic virion but cannot replicate in cells. The high-quality recombinant protein expression and post-translational modifications obtained with the BES, along with its capacity to produce multiple proteins, imply that it is ideally suited to VLP production. In this article, we critically review the pros and cons of using the BES as a platform to produce both enveloped and non-enveloped VLPs.

Developments in virus-like particle-based vaccines for HIV

Virus-like particles (VLPs) hold great promise for the development of effective and affordable vaccines. VLPs, indeed, are suitable for presentation and efficient delivery to antigen-presenting cells of linear as well as conformational antigens. This will ultimately result in a crosspresentation with both MHC class I and II molecules to prime CD4(+) T-helper and CD8(+) cytotoxic T cells. This review describes an update on the development and use of VLPs as vaccine approaches for HIV.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

Minimum requirements for bluetongue virus primary replication in vivo

The replication mechanism of bluetongue virus (BTV) has been studied by an in vivo reverse genetics (RG) system identifying the importance of certain BTV proteins for primary replication of the virus. However, a unique in vitro cell-free virus assembly system was subsequently developed, showing that it did not require the same set of viral components, which is indicative of differences in these two systems. Here, we studied the in vivo primary replicase complex more in-depth to determine the minimum components of the complex. We showed that while NS2 is an essential component of the primary replication stage during BTV infection, NS1 is not an essential component but may play a role in enhancing BTV protein synthesis. Furthermore, we demonstrated that VP7, a major structural protein of the inner core, is not required for primary replication but appears to stabilize the replicase complex. In contrast, VP3, the other major structural core protein, is an essential component of the complex, together with the three minor enzymatic proteins (VP1, VP4, and VP6) of the core. In addition, our data have demonstrated that the smallest minor protein, VP6, which is known to possess an RNA-dependent helicase activity, may also act as an RNA translocator during assembly of the primary replicase complex.

Use of bacterial artificial chromosomes in baculovirus research and recombinant protein expression: current trends and future perspectives

The baculovirus expression system is one of the most successful and widely used eukaryotic protein expression methods. This short review will summarise the role of bacterial artificial chromosomes (BACS) as an enabling technology for the modification of the virus genome. For many years baculovirus genomes have been maintained in E. coli as bacterial artificial chromosomes, and foreign genes have been inserted using a transposition-based system. However, with recent advances in molecular biology techniques, particularly targeting reverse engineering of the baculovirus genome by recombineering, new frontiers in protein expression are being addressed. In particular, BACs have facilitated the propagation of disabled virus genomes that allow high throughput protein expression. Furthermore, improvement in the selection of recombinant viral genomes inserted into BACS has enabled the expression of multiprotein complexes by iterative recombineering of the baculovirus genome.

Developments in virus-like particle-based vaccines for infectious diseases and cancer

Virus-like particles hold great promise for the development of effective and affordable vaccines. Indeed, virus-like particles are suitable for presentation and efficient delivery of linear as well as conformational antigens to antigen-presenting cells. This will ultimately result in optimal B-cell activation and cross-presentation with both MHC class I and II molecules to prime CD4(+) T-helper as well as CD8(+) cytotoxic T cells. This article provides an update on the development and use of virus-like particles as vaccine approaches for infectious diseases and cancer.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

Protective efficacy of Bluetongue virus-like and subvirus-like particles in sheep: presence of the serotype-specific VP2, independent of its geographic lineage, is essential for protection

There have been multiple separate outbreaks of Bluetongue (BT) disease of ruminants in Europe since 1998, often entering via the Mediterranean countries of Italy, Spain and Greece. BT is caused by an orbivirus, Bluetongue virus (BTV), a member of the family Reoviridae. BTV is a non-enveloped double-capsid virus, which encodes 7 structural proteins (VP1-VP7) and several non-structural proteins (NS1, NS2, NS3/3a and NS4) from ten double-stranded RNA segments of the genome. In this report, we have prepared BTV virus-like particles (VLPs, composed of VP2, VP3, VP5 and VP7) and sub-viral, inner core-like particles (CLPs, VP3 and VP7) using a recombinant baculovirus expression system. We compared the protective efficacy of VLPs and CLPs in sheep and investigated the importance of geographical lineages of BTV in the development of vaccines. The Greek crossbred Karagouniko sheep, which display mild to sub-clinical BT, were vaccinated with VLPs or CLPs of BTV-1, derived from western lineage and were challenged with virulent BTV-1 from an eastern lineage. All VLP-vaccinated animals developed a neutralising antibody response to BTV-1 from both lineages prior to challenge. Moreover, post-challenged animals had no clinical manifestation or viraemia and the challenged virus replication was completely inhibited. In contrast, CLP-vaccinated animals did not induce any neutralising antibody response but developed the group specific VP7 antibodies. CLPs also failed to prevent the clinical manifestation and virus replication, but in comparison to controls, the severity of disease manifestation and viraemia was mitigated. The data demonstrated that the outer capsid was essential for complete protection, while the geographical origin of the BTV was not critical for development of a serotype specific vaccine.

Arbovirus vaccines; opportunities for the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is a well-established technology for the production of heterologous viral (glyco)proteins in cultured cells, applicable for basic scientific research as well as for the development and production of vaccines and diagnostics. Arboviruses form an emerging group of medically important viral pathogens that are transmitted to humans and animals via arthropod vectors, mostly mosquitoes, ticks or midges. Few arboviral vaccines are currently available, but there is a growing need for safe and effective vaccines against some highly pathogenic arboviruses such as Chikungunya, dengue, West Nile, Rift Valley fever and Bluetongue viruses. This comprehensive review discusses the biology and current state of the art in vaccine development for arboviruses belonging to the families Togaviridae, Flaviviridae, Bunyaviridae and Reoviridae and the potential of the baculovirus-insect cell expression system for vaccine antigen production The members of three of these four arbovirus families have enveloped virions and display immunodominant glycoproteins with a complex structure at their surface. Baculovirus expression of viral antigens often leads to correctly folded and processed (glyco)proteins able to induce protective immunity in animal models and humans. As arboviruses occupy a unique position in the virosphere in that they also actively replicate in arthropod cells, the baculovirus-insect cell expression system is well suited to produce arboviral proteins with correct folding and post-translational processing. The opportunities for recombinant baculoviruses to aid in the development of safe and effective subunit and virus-like particle vaccines against arboviral diseases are discussed.

In vitro reconstitution of Bluetongue virus infectious cores

Bluetongue virus (BTV) is a vector-borne, nonenveloped icosahedral particle that is organized in two capsids, an outer capsid of two proteins, VP2 and VP5, and an inner capsid (or core) composed of two major proteins, VP7 and VP3, in two layers. The VP3 layer (subcore) encloses viral transcription complex (VP1 polymerase, VP4 capping enzyme, VP6 helicase) and a 10-segmented double-stranded (dsRNA) genome. Although much is known about the BTV capsids, the order of the core assembly and the mechanism of genome packaging remain unclear. Here, we established a cell-free system to reconstitute subcore and core structures with the proteins and ssRNAs, demonstrating that reconstituted cores are infectious in insect cells. Furthermore, we showed that the BTV ssRNAs are essential to drive the assembly reaction and that there is a distinct order of internal protein recruitment during the assembly process. The in vitro engineering of infectious BTV cores is unique for any member of the Reoviridae and will facilitate future studies of RNA-protein interactions during BTV core assembly.

Oncolytic bluetongue viruses: promise, progress, and perspectives

Humans are sero-negative toward bluetongue viruses (BTVs) since BTVs do not infect normal human cells. Infection and selective degradation of several human cancer cell lines but not normal ones by five US BTV serotypes have been investigated. We determined the susceptibilities of many normal and human cancer cells to BTV infections and made comparative kinetic analyses of their cytopathic effects, survival rates, ultra-structural changes, cellular apoptosis and necrosis, cell cycle arrest, cytokine profiles, viral genome, mRNAs, and progeny titers. The wild-type US BTVs, without any genetic modifications, could preferentially infect and degrade several types of human cancer cells but not normal cells. Their selective and preferential BTV-degradation of human cancer cells is viral dose–dependent, leading to effective viral replication, and induced apoptosis. Xenograft tumors in mice were substantially reduced by a single intratumoral BTV injection in initial in vivo experiments. Thus, wild-type BTVs, without genetic modifications, have oncolytic potentials. They represent an attractive, next generation of oncolytic viral approach for potential human cancer therapy combined with current anti-cancer agents and irradiation.

Conformational HIV-1 envelope on particulate structures: a tool for chemokine coreceptor binding studies

The human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein gp120 presents conserved binding sites for binding to the primary virus receptor CD4 as well as the major HIV chemokine coreceptors, CCR5 and CXCR4.

Concerted efforts are underway to understand the specific interactions between gp120 and coreceptors as well as their contribution to the subsequent membrane fusion process.

The present review summarizes the current knowledge on this biological aspect, which represents one of the key and essential points of the HIV-host cell interplay and HIV life cycle. The relevance of conformational HIV-1 Envelope proteins presented on Virus-like Particles for appropriate assessment of this molecular interaction, is also discussed.

Virus-like particles as a vaccine delivery system: myths and facts

Vaccines against viral disease have traditionally relied on attenuated virus strains or inactivation of infectious virus. Subunit vaccines based on viral proteins expressed in heterologous systems have been effective for some pathogens, but have often suffered from poor immunogenicity due to incorrect protein folding or modification. In this chapter we focus on a specific class of viral subunit vaccine that mimics the overall structure of virus particles and thus preserves the native antigenic conformation of the immunogenic proteins. These virus-like particles (VLPs) have been produced for a wide range of taxonomically and structurally distinct viruses, and have unique advantages in terms of safety and immunogenicity over previous approaches. With new VLP vaccines for papillomavirus beginning to reach the market place we argue that this technology has now ‘come-of-age’ and must be considered a viable vaccine strategy.

Bluetongue virus infection alters the impedance of monolayers of bovine endothelial cells as a result of cell death

Bluetongue virus (BTV) is the cause of bluetongue, an emerging, arthropod-transmitted disease of ungulates. Bluetongue is characterized by vascular injury with hemorrhage, tissue infarction and widespread edema, lesions that are consistent with those of the so-called viral hemorrhagic fevers. To further investigate the pathogenesis of vascular injury in bluetongue, we utilized an electrical impedance assay and immunofluorescence staining to compare the effects of BTV infection on cultured bovine endothelial cells (bPAEC) with those of inducers of cell death (Triton X-100) and interendothelial gap formation (tissue necrosis factor [TNF]). The data confirm that the adherens junctions of BTV-infected bPAECs remained intact until 24h post-infection, and that loss of monolayer impedance precisely coincided with onset of virus-induced cell death. In contrast, recombinant bovine TNF-alpha caused rapid loss of bPAEC monolayer impedance that was associated with interendothelial gap formation and redistribution of VE-cadherin, but without early cell death. The data from these in vitro studies are consistent with a pathogenesis of bluetongue that involves virus-induced vascular injury leading to thrombosis, hemorrhage and tissue necrosis. However, the contribution of cytokine-induced interendothelial gap formation with subsequent edema and hypovolemic shock contributes to the pathogenesis of bluetongue remains to be fully characterized.

Virus-like particle vaccines and adjuvants: the HPV paradigm

Complex antigen structures currently represent the most-studied approach for prophylactic as well as therapeutic vaccines. Different types of complex vaccines, including virus-like particles and virosomes, have been developed depending on the nature of the viral pathogen they are trying to replicate (enveloped vs naked) or the modality to express antigenic epitopes (i.e., the binding of envelope protein on liposomic structures). The complex structure of these vaccines provides them with some adjuvanted properties, not uniformly present for all virus-like particle types. The further inclusion of specific adjuvants in vaccine preparations can modify the presentation modality of such particles to the immune system with a specific Th1 versus Th2 polarization efficacy. A paradigm of the relevance of these new adjuvants are the immunological results obtained with the inclusion of monophosphoryl lipid A adjuvant in the formulation of L1-based human papillomavirus-naked virus-like particles to reduce a Th1 cellular immunity impairment, peculiar for alum-derived adjuvants, along with the induction of highly enhanced humoral and memory B-cellular immunity.

Multigene expression of protein complexes by iterative modification of genomic Bacmid DNA

Background

Many cellular multi-protein complexes are naturally present in cells at low abundance. Baculovirus expression offers one approach to produce milligram quantities of correctly folded and processed eukaryotic protein complexes. However, current strategies suffer from the need to produce large transfer vectors, and the use of repeated promoter sequences in baculovirus, which itself produces proteins that promote homologous recombination. One possible solution to these problems is to construct baculovirus genomes that express each protein in a complex from a separate locus within the viral DNA. However current methods for selecting such recombinant genomes are too inefficient to routinely modify the virus in this way.

Results

This paper reports a method which combines the lambda red and bacteriophage P1 Cre-recombinase systems to efficiently generate baculoviruses in which protein complexes are expressed from multiple, single-locus insertions of foreign genes. This method is based on an 88 fold improvement in the selection of recombinant viruses generated by red recombination techniques through use of a bipartite selection cassette. Using this system, seven new genetic loci were identified in the AcMNPV genome suitable for the high level expression of recombinant proteins. These loci were used to allow the recovery two recombinant virus-like particles with potential biotechnological applications (influenza A virus HA/M1 particles and bluetongue virus VP2/VP3/VP5/VP7 particles) and the mammalian chaperone and cancer drug target CCT (16 subunits formed from 8 proteins).

Conclusion

1. Use of bipartite selections can significantly improve selection of modified bacterial artificial chromosomes carrying baculovirus DNA. Furthermore this approach is sufficiently robust to allow routine modification of the virus genome. 2. In addition to the commonly used p10 and polyhedrin loci, the ctx, egt, 39k, orf51, gp37, iap2 and odv-e56 loci in AcMNPV are all suitable for the high level expression of heterologous genes. 3. Two protein, four protein and eight protein complexes including virus-like particles and cellular chaperone complexes can be produced using the new approach.

Prospects for improved bluetongue vaccines

Bluetongue has been recognized as a viral disease of livestock for more than 100 years. Repeated incursions of Bluetongue into Europe since 1998 have been particularly devastating for highly sensitive European fine-wool sheep breeds, and have resulted in a resurgence of interest in vaccine manufacture. Fortunately, the virus and its serology are well understood and vaccination prevents the disease. However, current vaccines are not without their problems, and many new approaches are being tested to improve the safety and breadth of protection afforded. This review describes the leading technologies for improved bluetongue vaccines and looks ahead to how advances in other viral vaccines might be applied to this disease.

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.

Production, characterization and assay application of a purified, baculovirus-expressed, serogroup specific bluetongue virus antigen

The predominant serodiagnostic assay used in many countries to detect bluetongue virus (BTV) infections is a competitive enzyme-linked immunosorbent assay (c-ELISA) which employs two critical reagents: a cell culture-derived BTV antigen and group-specific monoclonal antibody (Mab). Ongoing difficulties have been reported by laboratories in the production and quality control of the native antigen reagent which relies on the presence of adequate molar quantities and appropriate presentation of the major BTV core protein VP7. To address this important issue, a recombinant baculovirus was constructed containing a cDNA copy of genome segment 7 of BTV serotype 11 and used to infect insect cells which, in turn, expressed high levels of theVP7 protein with an estimated molecular mass of 39 kDa. In its purified form, this recombinant protein could be detected by group-specific Mabs designated 3.17.A3 and 8A3B.6 produced against BTV serotypes 1 and 17, respectively, as well as by polyclonal bovine antibodies raised against North American and South African BTV serotypes. No reactivity was observed by Western blot analysis with these two Mabs suggesting that the common antigenic determinants, on the BTV VP7 protein, were mainly conformational. It was interesting to note that the purified recombinant VP7 protein demonstrated a greater degree of reactivity with Mab 8A3B.6 compared to that exhibited with Mab 3.17.A3 when evaluated in an ELISA. Due to its antigenic similarity to the native antigen, the recombinant protein was found to be a suitable replacement for use in a c-ELISA to detect BTV-specific antibodies with the added advantage that it could be consistently produced and was, therefore, amenable to quality control testing for purity, stability and other standards.

Functional mapping of bluetongue virus proteins and their interactions with host proteins during virus replication

Bluetongue virus (BTV) is a double-stranded RNA (dsRNA) virus which is transmitted by blood-feeding gnats to wild and domestic ruminants, causing high morbidity and often high mortality. Partly due to this BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels. BTV, like the other members of the Reoviridae family is a complex non-enveloped virus with seven structural proteins and a RNA genome consisting of 10 dsRNA segments of different sizes. In virus infected cells, three other virus encoded nonstructural proteins are synthesized. Significant recent advances have been made in understanding the structure-function relationships of BTV proteins and their interactions during virus assembly. By combining structural and molecular data it has been possible to make progress on the fundamental mechanisms used by the virus to invade, replicate in, and escape from, susceptible host cells. Data obtained from studies over a number of years have defined the key players in BTV entry, replication, assembly and egress. Specifically, it has been possible to determine the complex nature of the virion through three dimensional structure reconstructions; atomic structure of proteins and the internal capsid; the definition of the virus encoded enzymes required for RNA replication; the ordered assembly of the capsid shell and the protein sequestration required for it; and the role of three NS proteins in virus replication, assembly and release. Overall, this review demonstrates that the integration of structural, biochemical and molecular data is necessary to fully understand the assembly and replication of this complex RNA virus.