A comparison of six different bluetongue virus isolates by cross-hybridization of the dsRNA genome segments

Immunisation with bacterial expressed VP2 and VP5 of bluetongue virus (BTV) protect α/β interferon-receptor knock-out (IFNAR(-/-)) mice from homologous lethal challenge

BTV-4 structural proteins VP2 (as two domains: VP2D1 and VP2D2), VP5 (lacking the first 100 amino acids: VP5Δ1-100) and full-length VP7, expressed in bacteria as soluble glutathione S-transferase (GST) fusion-proteins, were used to immunise Balb/c and α/β interferon receptor knock-out (IFNAR(-/-)) mice. Neutralising antibody (NAbs) titres (expressed as log10 of the reciprocal of the last dilution of mouse serum which reduced plaque number by ≥50%) induced by the VP2 domains ranged from 1.806 to 2.408 in Balb/c and IFNAR(-/-) mice. The immunised IFNAR(-/-) mice challenged with a homologous live BTV-4 survived and failed to develop signs of infection (ocular discharge and apathy). Although subsequent attempts to isolate virus were unsuccessful (possibly reflecting presence of neutralising antibodies), a transient/low level viraemia was detected by real time RT-PCR. In contrast, mice immunised with the two VP2 domains with or without VP5Δ1-100 and VP7, then challenged with the heterologous serotype, BTV-8, all died by day 7 post-infection. We conclude that immunisation with bacterially-expressed VP2 domains can induce strong serotype-specific NAb responses. Bacterial expression could represent a cost effective and risk-free alternative to the use of live or inactivated vaccines, particularly if viruses prove to be difficult to propagate in cell culture (like BTV-25). A vaccine based on bacterially expressed VP2 and VP5 of BTV is also DIVA-compatible.

Full genome sequence of bluetongue virus serotype 4 from China

The complete genomic sequence of a bluetongue virus serotype 4 (BTV-4) strain (strain YTS-4), isolated from sentinel cattle in Yunnan Province, China, is reported here. This work is the first to document the complete genomic sequence of a BTV-4 strain from China. The sequence information will help determine the geographic origin of Chinese BTV-4 and provide data to facilitate future analyses of the genetic diversity and phylogenetic relationships of BTV strains.

Complete genome sequence of bluetongue virus serotype 16 of goat origin from India

In this article, we document the first complete genome sequence of an isolate of bluetongue virus serotype 16 (BTV16) from a goat in India. The virus was isolated from an in-contact goat from an animal farm in Chennai where clinical disease occurs in sheep. The total size of the genome is 19,185 bp. The information provided for full-length sequences of all 10 segments will help in understanding the geographical origin and transmission of the Indian isolate of BTV16 as well as its comparison with global isolates of BTV16 of sheep, cattle, and other host species origins.

Genetic and phylogenetic characterization of genome segments 2 and 6 of bluetongue virus isolates in Japan from 1985 to 2008

This study conducted genetic and phylogenetic analyses of genome segments 2 and 6 (Seg-2 and Seg-6), which encode serotype-specific structural proteins of the outer capsid, of bluetongue virus (BTV) isolated in Japan from 1985 to 2008. The Japanese strains of BTV were clearly sorted into six groups by several genetic characteristics of Seg-2, including segment length, ORF length and 5′- and 3′-terminal sequences, and were identified as serotypes 2, 3, 9, 12, 16 and 21 by phylogenetic comparisons with Seg-2 of reference and field strains of serotypes 1–24. In contrast, phylogenetic comparisons of Seg-6 also revealed some variations among the Japanese strains and partial correlations of the serotypes between the Japanese strains and the reference or field strains. Thus, the results revealed that at least six serotypes of BTV were isolated in Japan and that there were some variations in the genetic and phylogenetic characteristics of Seg-2 and Seg-6 among the Japanese strains, suggesting that BTV of several different origins has appeared sporadically in Japan. These data will be beneficial for understanding BTV epidemiology and taking better control measures against bluetongue in Japan and its neighbouring countries in the Asia-Pacific region.

Identification and differentiation of the twenty six bluetongue virus serotypes by RT-PCR amplification of the serotype-specific genome segment 2

Bluetongue (BT) is an arthropod-borne viral disease, which primarily affects ruminants in tropical and temperate regions of the world. Twenty six bluetongue virus (BTV) serotypes have been recognised worldwide, including nine from Europe and fifteen in the United States. Identification of BTV serotype is important for vaccination programmes and for BTV epidemiology studies. Traditional typing methods (virus isolation and serum or virus neutralisation tests (SNT or VNT)) are slow (taking weeks, depend on availability of reference virus-strains or antisera) and can be inconclusive. Nucleotide sequence analyses and phylogenetic comparisons of genome segment 2 (Seg-2) encoding BTV outer-capsid protein VP2 (the primary determinant of virus serotype) were completed for reference strains of BTV-1 to 26, as well as multiple additional isolates from different geographic and temporal origins. The resulting Seg-2 database has been used to develop rapid (within 24 h) and reliable RT-PCR-based typing assays for each BTV type. Multiple primer-pairs (at least three designed for each serotype) were widely tested, providing an initial identification of serotype by amplification of a cDNA product of the expected size. Serotype was confirmed by sequencing of the cDNA amplicons and phylogenetic comparisons to previously characterised reference strains. The results from RT-PCR and sequencing were in perfect agreement with VNT for reference strains of all 26 BTV serotypes, as well as the field isolates tested. The serotype-specific primers showed no cross-amplification with reference strains of the remaining 25 serotypes, or multiple other isolates of the more closely related heterologous BTV types. The primers and RT-PCR assays developed in this study provide a rapid, sensitive and reliable method for the identification and differentiation of the twenty-six BTV serotypes, and will be updated periodically to maintain their relevance to current BTV distribution and epidemiology (http://www.reoviridae.org/dsRNA_virus_proteins/ReoID/rt-pcr-primers.htm).

Complete genomic sequence of bluetongue virus serotype 16 from China

We report here the complete genomic sequence of the Chinese bluetongue virus serotype 16 (BTV16) strain BN96/16. This work is the first to document the complete genomic sequence (segments 1 to 10) of a BTV16 strain. The sequence information provided herein will help determine the geographic origin of BTV16 and define the phylogenetic relationship of BTV16 to other BTV strains.

Complete genomic sequence of bluetongue virus serotype 1 from China

We report here the complete genomic sequence of the Chinese bluetongue virus serotype 1 (BTV1) strain SZ97/1. This work is the first to document the complete genomic sequence of a BTV1 strain from China and represents the second complete sequence of BTV1 in the world. The sequence information provided here will help determine the geographic origin of Chinese BTV1 and provide data to facilitate future analyses of the genetic diversity and phylogenetic relationships of BTV strains.

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

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

Full genome characterisation of bluetongue virus serotype 6 from the Netherlands 2008 and comparison to other field and vaccine strains

In mid September 2008, clinical signs of bluetongue (particularly coronitis) were observed in cows on three different farms in eastern Netherlands (Luttenberg, Heeten, and Barchem), two of which had been vaccinated with an inactivated BTV-8 vaccine (during May-June 2008). Bluetongue virus (BTV) infection was also detected on a fourth farm (Oldenzaal) in the same area while testing for export. BTV RNA was subsequently identified by real time RT-PCR targeting genome-segment (Seg-) 10, in blood samples from each farm. The virus was isolated from the Heeten sample (IAH "dsRNA virus reference collection" [dsRNA-VRC] isolate number NET2008/05) and typed as BTV-6 by RT-PCR targeting Seg-2. Sequencing confirmed the virus type, showing an identical Seg-2 sequence to that of the South African BTV-6 live-vaccine-strain. Although most of the other genome segments also showed very high levels of identity to the BTV-6 vaccine (99.7 to 100%), Seg-10 showed greatest identity (98.4%) to the BTV-2 vaccine (RSAvvv2/02), indicating that NET2008/05 had acquired a different Seg-10 by reassortment. Although Seg-7 from NET2008/05 was also most closely related to the BTV-6 vaccine (99.7/100% nt/aa identity), the Seg-7 sequence derived from the blood sample of the same animal (NET2008/06) was identical to that of the Netherlands BTV-8 (NET2006/04 and NET2007/01). This indicates that the blood contained two different Seg-7 sequences, one of which (from the BTV-6 vaccine) was selected during virus isolation in cell-culture. The predominance of the BTV-8 Seg-7 in the blood sample suggests that the virus was in the process of reassorting with the northern field strain of BTV-8. Two genome segments of the virus showed significant differences from the BTV-6 vaccine, indicating that they had been acquired by reassortment event with BTV-8, and another unknown parental-strain. However, the route by which BTV-6 and BTV-8 entered northern Europe was not established.

A review of RT-PCR technologies used in veterinary virology and disease control: sensitive and specific diagnosis of five livestock diseases notifiable to the World Organisation for Animal Health

Real-time, reverse transcription polymerase chain reaction (rRT-PCR) has become one of the most widely used methods in the field of molecular diagnostics and research. The potential of this format to provide sensitive, specific and swift detection and quantification of viral RNAs has made it an indispensable tool for state-of-the-art diagnostics of important human and animal viral pathogens. Integration of these assays into automated liquid handling platforms for nucleic acid extraction increases the rate and standardisation of sample throughput and decreases the potential for cross-contamination. The reliability of these assays can be further enhanced by using internal controls to validate test results. Based on these advantageous characteristics, numerous robust rRT-PCRs systems have been developed and validated for important epizootic diseases of livestock. Here, we review the rRT-PCR assays that have been developed for the detection of five RNA viruses that cause diseases that are notifiable to the World Organisation for Animal Health (OIE), namely: foot-and-mouth disease, classical swine fever, bluetongue disease, avian influenza and Newcastle disease. The performance of these tests for viral diagnostics and disease control and prospects for improved strategies in the future are discussed.

Development and initial evaluation of a real-time RT-PCR assay to detect bluetongue virus genome segment 1

Since 1998, multiple strains of bluetongue virus (BTV), belonging to six different serotypes (types 1, 2, 4, 8, 9 and 16) have caused outbreaks of disease in Europe, causing one of the largest epizootics of bluetongue ever recorded, with the deaths of >1.8 million animals (mainly sheep). The persistence and continuing spread of BTV in Europe and elsewhere highlights the importance of sensitive and reliable diagnostic assay systems that can be used to rapidly identify infected animals, helping to combat spread of the virus and disease. BTV has a genome composed of 10 linear segments of dsRNA. We describe a real-time RT-PCR assay that targets the highly conserved genome segment 1 (encoding the viral polymerase--VP1) that can be used to detect all of the 24 serotypes, as well as geographic variants (different topotypes) within individual serotypes of BTV. After an initial evaluation using 132 BTV samples including representatives of all 24 BTV serotypes, this assay was used by the European Community Reference Laboratory (CRL) at IAH Pirbright to confirm the negative status of 2,255 animals imported to the UK from regions that were considered to be at risk during the 2006 outbreak of BTV-8 in Northern Europe. All of these animals were also negative by competition ELISA to detect BTV specific antibodies and none of them developed clinical signs of infection. These studies have demonstrated the value of the assay for the rapid screening of field samples.

Design of primers and use of RT-PCR assays for typing European bluetongue virus isolates: differentiation of field and vaccine strains

Bluetongue virus (BTV) is the causative agent of bluetongue, a disease of ruminant livestock that occurs almost worldwide between latitudes 3 degrees S and 5 degrees N. There are 24 serotypes of BTV (currently identified by serum neutralization assays). Since 1998, eight strains of six BTV serotypes (1, 2, 4, 8, 9 and 16) have invaded Europe. The most variable BTV protein is major outer-capsid component VP2, encoded by segment 2 (Seg-2) of the double-stranded RNA virus genome. VP2 represents the major target for neutralizing (and protective) antibodies that are generated in response to BTV infection, and is therefore the primary determinant of virus serotype. RT-PCR primers and assays targeting Seg-2 have been developed for rapid identification (within 24 h) of the six European BTV types. These assays are sensitive, specific and show perfect agreement with the results of conventional virus-neutralization methods. Previous studies have identified sequence variations in individual BTV genome segments that allow different isolates to be grouped on the basis of their geographical origins (topotypes). The assays described in this paper can detect any of the BTV isolates of the homologous serotype that were tested from different geographical origins (different Seg-2 topotypes). Primers were also identified that could be used to distinguish members of these different Seg-2 topotypes, as well as field and vaccine strains of most of the European BTV serotypes. The serotype-specific assays (and primers) showed no cross-amplification when they were evaluated with multiple isolates of the most closely related BTV types or with reference strains of the remaining 24 serotypes. Primers developed in this study will be updated periodically to maintain their relevance to current BTV distribution and epidemiology (http://www.iah.bbsrc.ac.uk/dsRNA_virus_proteins/ReoID/rt-pcr-primers.htm).

Analysis and phylogenetic comparisons of full-length VP2 genes of the 24 bluetongue virus serotypes

The outer capsid protein VP2 of Bluetongue virus (BTV) is a target for the protective immune response generated by the mammalian host. VP2 contains the majority of epitopes that are recognized by neutralizing antibodies and is therefore also the primary determinant of BTV serotype. Full-length cDNA copies of genome segment 2 (Seg-2, which encodes VP2) from the reference strains of each of the 24 BTV serotypes were synthesized, cloned and sequenced. This represents the first complete set of full-length BTV VP2 genes (from the 24 serotypes) that has been analysed. Each Seg-2 has a single open reading frame, with short inverted repeats adjacent to conserved terminal hexanucleotide sequences. These data demonstrated overall inter-serotype variations in Seg-2 of 29 % (BTV-8 and BTV-18) to 59 % (BTV-16 and BTV-22), while the deduced amino acid sequence of VP2 varied from 22.4 % (BTV-4 and BTV-20) to 73 % (BTV-6 and BTV-22). Ten distinct Seg-2 lineages (nucleotypes) were detected, with greatest sequence similarities between those serotypes that had previously been reported as serologically ‘related’. Fewer similarities were observed between different serotypes in regions of VP2 that have been reported as antigenically important, suggesting that they may play a role in the neutralizing antibody response. The data presented form an initial basis for BTV serotype identification by sequence analyses and comparison of Seg-2, and for development of molecular diagnostic assays for individual BTV serotypes (by RT-PCR).

The core of bluetongue virus binds double-stranded RNA

Double-stranded RNA (dsRNA) viruses conceal their genome from the host to avoid triggering unfavorable cellular responses. The crystal structure of the core of one such virus, bluetongue virus, reveals an outer surface festooned with dsRNA. This may represent a deliberate strategy to sequester dsRNA released from damaged particles to prevent host cell shutoff.

Taxonomy of African horse sickness viruses

Nine distinct genera are currently recognised within the virus family Reoviridae, which include a total of 63 virus groups or species (species = virus group = electropherotype or serogroup), comprising 214 virus serotypes or subtypes, as well as 20 provisional types or subtypes, most of which (149 + 9 tentative) are assigned to the genus Orbivirus [5, 9, 16]. The 19 species of orbiviruses (serogroups), were established principally on antigenic (serologic) grounds but many of these placements have been supported by molecular analyses. This introductory paper defines the taxonomy and classification of these viruses and establishes guidelines for use in other paper to be presented at this symposium and elsewhere.

The orbivirus genus. Diversity, structure, replication and phylogenetic relationships

The general properties of the orbiviruses have been examined at the physical, structural and molecular level. At the structural level, the orbiviruses (with the exception of the Kemerovo serogroup) appear similar. The replicative events are also similar, however differences in the ultrastructure of virus-specific structures and their association with components of the host cell have been observed. Further research in this area may be used to differentiate between the serogroups and even some serotypes, of orbiviruses. At the molecular level the properties of the genome can be used to determine relationships between members of the orbivirus genus. These relationships are revealed using a variety of techniques including serology and gene sequence analysis. Not only are the different serological responses to gene products present in the mature virus particle used for differential diagnosis, but the gene sequences themselves can also be utilized. Understanding of the relationships between these viruses is progressing to the point that insights into orbivirus molecular epidemiology is now possible.

Development of recombinant vaccines against bluetongue

Bluetongue virus is the aetiological agent of bluetongue, a disease of domestic and wild ruminants. Twenty-four serotypes are recognized. Novel subunit vaccines, that complement existing modified live polyvalent vaccines, are being developed. Serotype-specific viral neutralizing antibodies that are able to protect sheep against virulent homologous virus challenge can be induced by immunizing with the BTV outer capsid protein VP2 purified from virions or with VP2 expressed by baculovirus recombinants. Presentation of VP2 on virus-like particles, which assemble upon co-expression of the four major structural viral proteins (VP2, VP5, VP3 and VP7), improves the protective effect of VP2. Sheep immunized with core-like particles, comprised of VP3 and VP7, developed only limited clinical signs after virulent virus challenge, demonstrating that not only the outer capsid proteins, but also the core proteins are involved in protection against bluetongue.

Variation amongst the neutralizing epitopes of bluetongue viruses isolated in the United States in 1979-1981

Neutralizing epitopes present on field isolates of bluetongue virus (BTV) serotypes 10, 11, 13 and 17 were evaluated with a panel of polyclonal and neutralizing monoclonal antibodies (MAbs). A total of 91 field isolates were evaluated, including 15 isolates of BTV-10, 29 isolates of BTV-11, 26 isolates of BTV-13, and 21 isolates of BTV-17. The viruses were isolated from cattle, goats, sheep, elk and deer in Idaho, Louisiana, Nebraska and, predominantly, California, in the years 1979, 1980 and 1981. The isolates were analyzed and compared using a panel of neutralizing MAbs which included five MAbs raised against BTV-2, seven against BTV-10, five against BTV-13, and six against BTV-17. Neutralization patterns obtained with the MAb panel and individual field isolates were compared to those obtained with prototype viruses of each serotype. All field isolates were neutralized by at least some of the MAbs raised against the prototype virus of the same serotype. All field isolates of BTV-10 were neutralized by the seven MAbs raised to BTV-10, whereas the field isolates of BTV-11, BTV-13 and BTV-17 were not consistently neutralized by all of the MAbs raised against the prototype virus of the same serotype. Variation in neutralizing epitopes recognized by the MAb panel was most pronounced amongst the field isolates of BTV-17. A one-way cross neutralization was evident between BTV-10 and BTV-17 as all field isolates of BTV-17 were neutralized by four of the MAbs raised against BTV-10. In contrast, no BTV-10 isolates were neutralized by the MAbs raised against BTV-17. Differences in the MAb neutralization patterns of field isolates of BTV-11, BTV-13 and BTV-17 suggest that the immunogenic domain responsible for their neutralization is plastic, such that individual epitopes within the domain may vary in their significance to the neutralization of different viruses, even of the same serotype. The apparent conservation of neutralizing epitopes on field isolates of BTV-10 suggests that the field isolates may be derived from the modified-live vaccine strain of BTV-10.

Molecular comparison of VP3 from bluetongue and epizootic hemorrhagic disease viruses

The complete nucleic acid sequence of gene 3 from epizootic hemorrhagic disease of deer (EHD) virus serotype 1 was determined. The 2768 bp sequence encodes a single protein that contains 899 amino acids and has a molecular weight of 103 kDa. The predicted protein sequence has 94.7% identity with EHD virus serotype 2 and greater than 77% identity with the related bluetongue viruses serotypes 1, 10 and 17 VP3 proteins. The relevance of these data to studies of recombinant DNA diagnostics and genetic relatedness is discussed.

Genetic variation and evolutionary relationships amongst bluetongue viruses endemic in the United States

The genetic variation and evolutionary relationships amongst the five serotypes of bluetongue virus (BTV) endemic to the United States were investigated by oligonucleotide fingerprint analysis. The viruses analyzed include prototype viruses of the five U.S. serotypes, and 32 viruses isolated from domestic and wild ruminants from the U.S. in the years 1979-1981. With the exception of serotype 2, most genes encoding the viral core and non-structural proteins were demonstrated to be highly conserved both within and between serotypes and some also appear to have reassorted in nature. Gene segments 2 and 6, which encode the outer capsid proteins VP2 and VP5 respectively, were more variable and were not consistently linked as serotype determination was dependent solely on gene segment 2. Gene segment 2 was the most variable gene between serotypes, but it was highly conserved within serotypes and stable over time. This suggests that the emergence of new BTV serotypes, which would require the stable incorporation of numerous mutations, must be a very slow process. Fingerprint comparisons further suggested that BTV serotypes 10, 11, 13 and 17 have evolved together in the U.S. over a considerable period of time, whereas serotype 2, which is genetically distinct, has evolved elsewhere and is most likely a recent introduction to North America.

Phylogenetic analyses of the complete nucleotide sequence of the capsid protein (VP3) of Australian epizootic haemorrhagic disease of deer virus (serotype 2) and cognate genes from other orbiviruses

The complete nucleotide sequence of the minor capsid protein (VP3) of epizootic haemorrhagic disease of deer virus (EHDV; Australian serotype 2) was determined using a combination of cloning and sequencing methods. Gene segment 3 that coded for the EHDV VP3 capsid protein was 2768 nucleotides in length with a coding region of 2697 nucleotides flanked by 5' and 3' non-coding regions of 17 and 53 nucleotides, respectively. A protein of 899 amino acids (Mr 103,160) having no overall charge at neutral pH was deduced from the nucleotide sequence. Comparisons with equivalent regions from the other Australian EHDV serotypes showed the VP3 genes and the segments that coded for them were similar, varying by a maximum of 5%. Comparisons with known cognate genes from bluetongue viruses showed that their VP3 genes and the proteins translated from them were remarkably similar to those of EHDV, having approximately 70% to 80% homology at either level, respectively. In an attempt to delineate the evolution of orbiviruses, we have obtained sequence data from the VP3 genes from representative members of all Australian orbiviruses now known. Computer analyses of this data enabled a phylogenetic tree for the orbiviruses to be proposed that incorporated the concept of topotypes.

Detection and characterisation of bluetongue virus using the polymerase chain reaction

Pairs of oligodeoxynucleotide primers whose sequences were based on those of RNA segment 3 that encodes the bluetongue virus serogroup-reactive protein VP3, were synthesized for three BTVs from different geographic regions of the world and for seven Australian orbiviruses. Each pair of primers was then tested for the synthesis of cDNA and in subsequent polymerase chain reactions (PCR) with all ten virus groups. All primers were serogroup-specific at low or high stringency. One pair of primers was specifically designed for its ability to serogroup a BTV isolate irrespective of its geographic origin. At either high or low stringency, this primer-pair resulted in a common and specific PCR product for each of the BTVs tested but not for the other orbiviruses. Eight pairs of primers based on RNA2 sequences (the gene segment encoding the serotype-specific protein VP2) were also synthesized for the eight Australian serotypes of BTV. Each primer-pair was serotype-specific at low or high stringency except for the BTV16A pair, which cross-reacted with BTV3A and also gave a non-specific product that differed in Mr from the authentic PCR product. Using the PCR and BTV1A RNA3-based primers, BTV1A was detected in blood samples from two sheep at 9 days post inoculation. Virus was found in the platelet, buffy-coat and packed red blood cell fractions, but not in whole blood.

Bluetongue virus evolution: sequence analyses of the genomic S1 segments and major core protein VP7

The S1 segments, encoding the group-specific antigen, VP7, from the five United States prototype BTV serotypes were cloned as full-length entities. The nucleotide and deduced amino acid sequences of segment S1 of BTV-2 were determined and compared with BTV-10, -11, -13, and -17, completing the sequencing of this cognate gene segment from all five US BTV serotypes. Each segment is 1156 bp long and contains an open reading frame encoding the 349-amino acid VP7 protein. Most (greater than 94%) of the amino acids of VP7 among the serotypes are conserved, including the location (position 255) of a single lysine residue. Secondary structure analyses of VP7 predict a putative eight-stranded beta-barrel between amino acid positions 150 and 250, a structure similar to that observed in ssRNA viruses. The S1 genes are flanked by conserved 5' and 3' noncoding regions. Stem-loop structures are predicted at the 3' end of each gene (nucleotide positions 1058-1097). The S1 segments of BTV-2, -10, -11, and -17 have greater than 93% of the nucleotides conserved, while less than 80% of their bases are identical with BTV-13. Analyses of nucleotide mismatches in each codon position of the VP7 open reading frame, transition frequencies, and evolutionary distances show that of the five, BTV-13 is the most distantly related and that BTV-10 and -17 are the most closely related serotypes. Evolutionary distance calculations of segment L2 from BTV-10, -11, and -17 concur with these observations. Comparison of this relationship with hybridization data of segment M3, which codes for VP5, suggests that BTV-17 has evolved by a combination of genetic drift and genomic reassortment. The data also indicate that the five US BTV serotypes are derived from two distinct gene pools. Evolution distances were used to estimate an evolution rate of 2.2 x 10(-3) nucleotide substitution/site/year for BTV segment S1. This rate is similar to the genes of retroviruses and implies an absence of RNA polymerase proofreading activity for dsRNA viruses.

Development of the polymerase chain reaction for the detection of bluetongue virus in tissue samples

Total genomic dsRNA, extracted from purified core particles of bluetongue virus serotype 1 from South Africa (BTV1SA), was used as template to optimise a polymerase chain reaction (PCR) for the detection of bluetongue virus RNA. Pairs of oligonucleotides complementary to the 3' termini of eight of the ten genome segments were tested. Those representing the 5' termini of genome segment 7 gave the best amplification results producing a single DNA band with the same mobility during agarose gel electrophoresis as genome segment 7. It was confirmed by cloning and sequence analysis, that this PCR-amplified DNA contained both terminal regions of genome segment 7 and therefore represented full length cDNA. Using these segment 7 oligonucleotides it was not only possible to detect routinely as few as 6 molecules of segment 7 dsRNA per sample, but also to detect purified dsRNAs from isolates of other BTV serotypes (1 Australia (AUS), 2, 3, 4, 10, 16 and 20). However, with the exception of Tilligery virus, isolates from other Orbivirus serogroups tested all gave negative results (African horse sickness, epizootic haemorrhagic disease, Palyam, Warrego and Eubenangee). The PCR was also used to analyse red blood cells (RBC) and buffy coat samples from cattle infected with BTV4. Positive results were obtained from samples taken 7 days post-infection (p.i.) (containing 1.6 x 10(3) TCID50 of virus/ml of whole blood) and from the RBC sample only, taken 14 days p.i. (16 TCID50/ml). However, at 28 days p.i. (less than 1.6 TCID50/ml) BTV RNA was not detected using the PCR in either sample.

Relationships amongst bluetongue viruses revealed by comparisons of capsid and outer coat protein nucleotide sequences

Sequence data from the gene segments coding for the capsid protein. VP3, of all eight Australian bluetongue virus serotypes were compared. The high degree of nucleotide sequence homology for VP3 genes amongst BTV isolates from the same geographic region supported previous studies (Gould, 1987; 1988b, c; Gould et al., 1988b) and was proposed as a basis for "topotyping" a bluetongue virus isolate (Gould et al., 1989). The complete nucleotide sequences which coded for the VP2 outer coat proteins of South African BTV serotypes 1 and 3 (vaccine strains) were determined and compared to cognate gene sequences from North American and Australian BTVs. These VP2 comparisons demonstrated that BTVs of the same serotype, but from different geographical regions, were closely related at the nucleotide and amino acid levels. However, close inter-relationships were also demonstrated amongst other BTVs irrespective of serotype or geographic origin. These data enabled phylogenic relationships of the BTV serotypes to be analysed using VP2 nucleotide sequences as a determinant.

Identification of a bluetongue virus serotype 1-specific ovine helper T-cell determinant in outer capsid protein VP2

Ovine T-cell lines (including one clone [101A]), which are specific for Bluetongue virus serotype 1 (BTV1), have been established and characterized. Although these T-cell lines react with different isolates of BTV1 (including those from South Africa, Australia, Nigeria, and Cameroon), they do not react with heterologous BTV serotypes. Antigen specificity of these T-cells was studied using purified virus particles, infectious subviral particles (ISVP) and cores, or using individual BTV structural proteins that were either isolated by SDS-PAGE or expressed by recombinant strains of vaccinia virus. The results showed that each of the T-cell lines reacted with outer capsid protein VP2 (the BTV protein exhibiting most serotype-specific variation and the major neutralization antigen). However, all of the uncloned T-cell lines also reacted with either the core structural proteins or the outer capsid protein VP5. In contrast, the T-cell clone 101A only reacted with outer capsid protein VP2. Cell surface marker analysis showed that 101A has a helper T-cell phenotype (CD5+, CD4+, CD8-, T-19-). The T-cell lines and clone 101A all produced large amounts of interleukin 2 (IL-2) when stimulated with purified BTV1 virus particles, or with VP2 (up to 120 IU/ml from 2 x 10(5) T-cells). BTV serotype-specific antigenic sites, for B cells and at least one site for ovine helper T-cells, are therefore located within VP2.

Expression of the outer capsid protein, VP2, from a full length cDNA clone of genome segment 2 of bluetongue serotype 1 from South Africa, using both Sp6 and vaccinia expression systems and a comparison of the nucleic acid sequence of this segment with those of other serotypes

Genome segment 2 of bluetongue virus serotype 1 from South AfricA (BTV-1SA) was purified from a preparation of all ten dsRNA segments. This dsRNA was used as a template to make a full-length DNA copy of segment 2, which was then cloned into pUC19. The cDNA insert was transferred into a bacterial expression vector (pGEM; PROMEGA) and, by means of in vitro transcription and translation systems, used to synthesise a polypeptide of similar size to VP2 (as analyzed by PAGE). The cDNA insert was also transferred into a vaccinia virus vector using homologous recombination. The resulting recombinant virus when transfected into TK- cells produced a protein that co-migrated with VP2 of bluetongue virus. Immunoprecipitation of these polypeptides, synthesised by in vitro and in vivo techniques, using BTV-1SA antisera, confirmed that they were virus specific. Nucleotide sequence analysis of the cDNA demonstrated that genome segment 2 is 2940 base pairs in length. The positive sense (+ ve) RNA strand contains an open reading frame, coding for a polypeptide of 961 amino acids, which is flanked by 3' and 5' terminal non-coding regions of 37 and 17 nucleotides, respectively. Comparison with published data shows that genome segment 2 of BTV-1SA is identical in these characteristics to segment 2 of BTV-1 from Australia (BTV-1AUS) but differs from isolates of the five American serotypes of BTV (BTV-2, -10, -11, -13 and -17). However, there is a higher level of homology, in both the nucleotide and the amino acid sequence of genome segment 2 and protein VP2 respectively, between the two isolates of BTV-1 and the American isolate of BTV-2, than there is between BTV-2 and the other American serotypes. The significance of this similarity is discussed.

Analysis of the roles of bluetongue virus outer capsid proteins VP2 and VP5 in determination of virus serotype

Analyses of reassortant and parental strains of BTV serotypes 3 and 10, in serum neutralization tests, confirmed the major role of outer capsid protein VP2 in determination of virus serotype and its involvement in serum neutralization. However, a reassortant BTV strain (R70), containing protein VP5 derived from BTV 3 and VP2 derived from BTV 10, cross-neutralized with both parental virus strains (BTV 3 and BTV 10). It is concluded that VP5 also plays some part in serotype determination of these virus isolates, as analyzed by serum-neutralization, but its role may be less significant than that of VP2.

Comparison of virologic and serologic responses of lambs and calves infected with bluetongue virus serotype 10

Four lambs and 3 calves, seronegative to bluetongue virus (BTV), were inoculated intravenously with a highly plaque-purified strain of BTV Serotype 10. A single calf and lamb served as controls and were inoculated with uninfected cell culture lysate. All BTV-inoculated lambs exhibited mild clinical manifestations of bluetongue, whereas infected calves were asymptomatic. Viremia persisted in BTV-infected lambs for 35-42 days, and for 42-56 days in BTV-infected calves. Neutralizing antibodies were first detected in sera collected at Day 14 post-inoculation (PI) from 2 BTV-infected calves and all 4 infected lambs, and at Day 28 PI in the remaining calf. The appearance of neutralizing antibody in serum did not coincide with clearance of virus from blood; BTV and specific neutralizing antibody coexisted in peripheral blood of infected lambs and calves for as long as 28 days. The sequential development, specificity and intensity of virus protein-specific humoral immune responses of lambs and calves were evaluated by immunoprecipitation of [35S]-labelled proteins in BTV-infected cell lysates by sera collected from inoculated animals at bi-weekly intervals PI. Sera from infected lambs and calves reacted most consistently with BTV structural proteins VP2 and VP7, and nonstructural protein NS2, and less consistently with structural protein VP5, and nonstructural protein NS1. Lambs developed humoral immune responses to individual BTV proteins more rapidly than calves, and one calf had especially weak virus protein-specific humoral immune responses; viremia persisted longer in this calf than any other animal in the study. The clearance of virus from the peripheral blood of BTV-infected lambs and calves is not caused simply by the production of virus-specific neutralizing antibody, however the intensity of humoral immune responses to individual BTV proteins might influence the duration of viremia in different animals.

Sequence analysis and in vitro expression of a cDNA clone of genome segment 5 from bluetongue virus, serotype 1 from South Africa

A full length copy of genome segment 5 of bluetongue virus serotype 1 from South Africa (BTV-1SA) was assembled from two incomplete cDNA clones. The complete nucleotide sequence was determined (1635 nucleotides in length) and an open reading frame coding for 527 amino acids was found, which was flanked by a 5' non-coding region of 25 nucleotides and a 3' non-coding region of 29 nucleotides. The cDNA clone was transferred to an Sp6 expression vector from which an RNA transcript was obtained. This transcript, when translated in vitro in a reticulocyte lysate system, produced a protein that co-migrated during electrophoresis with both protein VP5 from disrupted virus particles and VP5 translated from denatured viral dsRNA. The protein synthesized from the cDNA clone was precipitable with antisera raised against BTV-1SA virus particles and with antisera raised against a synthetic peptide, the sequence of which was obtained from the predicted amino acid sequence of BTV-1SA protein VP5. These antisera also precipitated protein VP5 translated from denatured viral dsRNA. Collectively these data indicate that the cDNA clone encodes an authentic VP5 protein product. The amino acid sequence of BTV-1SA VP5, when compared to other published sequences for VP5, contained highly conserved regions interrupted by variable domains. If two isolates of the same serotype are compared, (BTV-1SA and BTV-1AUS) only two variable regions are apparent. However, if the amino acid sequences of VP5 from two different serotypes are compared, (BTV-1SA and BTV-10), eight variable regions are detectable (two of which are in the same position as the variable regions within a serotype). The implications of these variations in the outer coat protein, VP5, are discussed.

Nucleotide and deduced amino acid sequences of the non-structural protein, NS1, of Australian and South African bluetongue virus serotype 1

The sequence of the sense strand of RNA segment 5 of both Australian and South African bluetongue virus (BTV) serotype 1 has been determined and found to be 1771 and 1773 nucleotides in length, respectively. Both coding sequences of 1656 nucleotides were flanked by a 5' non-coding sequence of 34 nucleotides and 3' non-coding regions of 78 and 80 nucleotides, respectively. The methionine codons at residues 35-37 were assumed to initiate the synthesis of 64.6 or 64.415 kDa proteins which had calculated net charges of +5 or +4 at neutral pH, respectively. The encoded NS1 proteins had a very high molar ratio of cysteine residues. A variable region of approximately 45 nucleotides at the 3'-terminus of RNA segment 5 of South African and Australian BTV-1 and the RNA segment 6 of the North American BTV-10 was shown to be unusually rich in A + T residues (approximately 80-82%) compared with other BTV gene segments so far sequenced which have between 52 and 56% A + T. These regions were thought to be responsible for the variable migration of RNA 5 segments on electrophoresis in polyacrylamide gels in the presence of urea. This variability in the apparent molecular weight of RNA 5 segments was not restricted to BTV amongst Australian orbiviruses tested, nor was the apparent molecular weight for RNA 5 identical for different isolates of the same BTV serotype, indicating that this A + T rich region was highly variable. Comparison of the nucleotide and amino acid sequence divergence of the Australian and South African BTV RNA segments 5 to that for the North American BTV-10 RNA segment 6 (which codes for NS1) revealed the same relationships as those found for the core protein VP3 gene sequences, in that although all NS1 proteins were very similar in their amino acid sequences, their genes were more variable. The Australian NS1 sequence differed from both the South African and North American genes by 20% at the nucleotide level, whereas the North American and South African sequences diverged by only 11%. Hybridization analyses showed that RNA segment 5 DNA probes were capable of delineating the geographical origin of a BTV isolate, as had been observed for VP3 probes; however, other probes were also generated which were capable of unambiguously differentiating BTV isolates from other orbiviruses tested.

Analysis of genome segments from six different isolates of bluetongue virus using RNA-RNA hybridisation: a generalised coding assignment for bluetongue viruses

Nucleic acid probes prepared directly from bluetongue virus (BTV) genomic double-stranded RNA (dsRNA) have been used to identify the functionally equivalent genome segments from six distinct isolates of BTV after their separation in both agarose and polyacrylamide gel electrophoresis systems. Variations in the rate, and in one case the order, of migration of the equivalent genome segments from different viruses was detected in the polyacrylamide gel system. However, the genomic dsRNA profiles of eleven BTV isolates were found to be identical when analysed by agarose gel electrophoresis. Functionally equivalent genome segments from the six viruses that were analysed were found to migrate in identical relative positions in this gel system. From these data we propose a modified version of the protein coding assignments published for BTV 1 South Africa (Mertens et al., 1984) in which the identification of the genome segments would be based upon their order of migration in the agarose rather than the polyacrylamide gel system. The modified coding assignments, unlike the original assignments, would be applicable to all of those viruses analysed and appear likely to be valid for all normal BTV isolates.