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

Serological Cross-Reactions between Expressed VP2 Proteins from Different Bluetongue Virus Serotypes

Bluetongue (BT) is a severe and economically important disease of ruminants that is widely distributed around the world, caused by the bluetongue virus (BTV). More than 28 different BTV serotypes have been identified in serum neutralisation tests (SNT), which, along with geographic variants (topotypes) within each serotype, reflect differences in BTV outer-capsid protein VP2. VP2 is the primary target for neutralising antibodies, although the basis for cross-reactions and serological variations between and within BTV serotypes is poorly understood. Recombinant BTV VP2 proteins (rVP2) were expressed in Nicotiana benthamiana, based on sequence data for isolates of thirteen BTV serotypes (primarily from Europe), including three 'novel' serotypes (BTV-25, -26 and -27) and alternative topotypes of four serotypes. Cross-reactions within and between these viruses were explored using rabbit anti-rVP2 sera and post BTV-infection sheep reference-antisera, in I-ELISA (with rVP2 target antigens) and SNT (with reference strains of BTV-1 to -24, -26 and -27). Strong reactions were generally detected with homologous rVP2 proteins or virus strains/serotypes. The sheep antisera were largely serotype-specific in SNT, but more cross-reactive by ELISA. Rabbit antisera were more cross-reactive in SNT, and showed widespread, high titre cross-reactions against homologous and heterologous rVP2 proteins in ELISA. Results were analysed and visualised by antigenic cartography, showing closer relationships in some, but not all cases, between VP2 topotypes within the same serotype, and between serotypes belonging to the same 'VP2 nucleotype'.

Isolation and Characterization of Bluetongue Virus Recovered from Blood Samples by Immunoaffinity Purification

An immunoaffinity chromatography (IAC) method was optimized for the selective capture of bluetongue virus (BTV) from blood samples and isolation of the virus in cell culture. The antibody against BTV core particles (lacking the outer capsid proteins VP2 and VP5) was used for the optimization of IAC technique. The antibody against BTV core particle was conjugated with Protein A-virus complex and the complex was dissociated using elution buffer (4 M MgCl₂ with 75 mM HEPES, pH 6.5). The optimized IAC method specifically purified the BTV without capturing other commonly infecting small ruminant's viruses like gaotpox virus (GTPV), sheeppox virus (SPPV), Peste des petits ruminants virus (PPRV) and Foot and mouth disease virus (FMDV). The blood samples (n = 22), positive for BTV antigen in sandwich-ELISA were attempted for virus isolation in the BHK-21 cell using the optimized IAC method. A total of seven BTV were isolated by selective capturing of the virion particles. The isolated viruses were characterized by RNA-PAGE, sequence analysis and serum neutralization test (SNT). Electropherotypic analysis of viral dsRNA in the RNA-PAGE revealed the presence of ten dsRNA segments characteristic of BTV. Out of seven isolates, four isolates were identified as BTV-1 and three isolates were identified as BTV-16 based on nucleotide sequences of segment-2. Phylogenetic analysis of segment-2 nucleotide sequence segregated BTV-1 and BTV-16 isolates to monophyletic cluster at close proximity to other eastern topotype. In SNT, hyperimmune serum (HIS) against BTV-1 neutralized the four BTV-1 isolates up to a titer > 256 and HIS against BTV-16 neutralized the three BTV-16 isolates up to a titer > 128. The IAC technique will be useful for the selective capture of BTV from mixed infection (BTV with other small ruminant's viruses) and isolation from blood sample having low viral load by enrichment.

A review of potential bluetongue virus vaccine strategies

Bluetongue (BT) is an economically important, non-zoonotic arboviral disease of certain wild and domestic species of cloven-hooved ungulates. Bluetongue virus (BTV) is the causative agent and the occurrence of BTV infection is distinctly seasonal in temperate regions of the world, and dependent on the presence of vector biting midges (e.g. Culicoides sonorensis in much of North America). In recent years, severe outbreaks have occurred throughout Europe and BTV is endemic in most tropical and temperate regions of the world. Several vaccines have been licensed for commercial use, including modified live (live-attenuated) and inactivated products, and this review summarizes recent strategies developed for BTV vaccines with emphasis on technologies suitable for differentiating naturally infected from vaccinated animals. The goal of this review is to evaluate realistic vaccine strategies that might be utilized to control or prevent future outbreaks of BT.

A Review of Knowledge Gaps and Tools for Orbivirus Research

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

Vaccines for Prevention of Bluetongue and Epizootic Hemorrhagic Disease in Livestock: A North American Perspective

Bluetongue (BT) and epizootic hemorrhagic disease (EHD) are noncontagious, insect-transmitted diseases of domestic and wild ruminants caused by related but distinct viruses. There are significant gaps in our scientific knowledge and available countermeasures to control an outbreak of orbivirus-induced disease, whether BT or EHD. Both BT virus (BTV) and EHD virus (EHDV) cause hemorrhagic fevers in susceptible ruminants; however, BT is principally a disease of domestic livestock whereas EHD is principally a disease of certain species of wild, non-African ungulates, notably white-tailed deer. The live-attenuated (modified live virus [MLV]) vaccines available in the United States for use in small ruminant livestock do provide good protection against clinical disease following infection with the homologous virus serotype. Although there is increasing justification that the use of MLV vaccines should be avoided if possible, these are the only vaccines currently available in the United States. Specifically, MLVs are used in California to protect sheep against infection with BTV serotypes 10, 11, and 17, and a MLV to BTV serotype 10 is licensed for use in sheep throughout the United States. These MLV vaccines may need to continue to be used in the immediate future for protective immunization of sheep and goats against BT. There are currently no licensed vaccines available for EHD in the United States other than autogenous vaccines. If there is a need to rapidly develop a vaccine to meet an emerging crisis associated with either BTV or EHDV infections, development of an inactivated virus vaccine in a conventional adjuvanted formulation will likely be required. With two doses of vaccine (and in some instances just one dose), inactivated vaccines can provide substantial immunity to the epizootic serotype of either BTV or EHDV. This strategy is similar to that used in the 2006-2008 BTV serotype 8 outbreaks in northern Europe that provided vaccine to the field within 2 years of the initial incursion (by 2008). Further research and development are warranted to provide more efficacious and effective vaccines for control of BTV and EHDV infections.

VP2-segment sequence analysis of some isolates of bluetongue virus recovered in the Mediterranean basin during the 1998-2003 outbreak

The complete nucleotide sequences of the VP2 segments of bluetongue virus (BTV) isolates recovered from Italy, Greece and Israel, from 1998 to 2003, were determined. Phylogenetic analysis of these sequences, those from related viruses and the South African vaccine strains, were used to determine the probable geographic origin of BTV incursions into Italy. Results indicated that viruses from each of the four serotypes isolated in Italy (2, 4, 9 and 16) possibly had a different origin. Analysis of the bluetongue virus serotype 2 (BTV-2) isolates gave evidence that this serotype probably moved from Tunisia. BTV-4 results showed probable incursion from the southwest and not from Greece or Israel. BTV-9 isolates clearly have an eastern origin (most probably Greece), whereas BTV-16 isolates are indistinguishable from the BTV-16 live attenuated vaccine strain. The phylogenetic findings were supported by polyacrylamide gel electrophoresis (PAGE) analysis of the complete amplified genome of each isolate except for BTV-16 Italian field isolate, which showed a slightly different PAGE profile. A combination of the complete VP2 sequencing and PAGE analysis of complete genomes, allowed not only phylogenetic analysis, but also vaccine detection and assessment of reassortment events.

Changes in the outer capsid proteins of bluetongue virus serotype ten that abrogate neutralization by monoclonal antibodies

Six neutralizing monoclonal antibodies (Mabs) and nine neutralization resistant viral variants (escape-mutant viruses (EMVs)) were used to further characterize the neutralization determinants of bluetongue virus serotype 10 (BTV10). The EMVs were produced by sequential passage of a highly cell culture adapted United States prototype strain of BTV10 in the presence of individual neutralizing Mabs. Mabs were characterized by neutralization and immune precipitation assays, and phenotypic properties of EMVs were characterized by neutralization assay. Sequencing of the gene segments encoding outer capsid proteins VP2 and VP5 identified mutations responsible for the altered phenotypic properties exhibited by individual EMVs. Amino acid substitutions in VP2 were responsible for neutralization resistance in most EMVs, whereas an amino acid substitution in VP5, without any change in VP2, was responsible for the neutralization resistance of one EMV. The data confirm that VP2 contains the major neutralization determinants of BTV, and that VP5 also can influence neutralization of the virus. The considerable plasticity of the neutralization determinants of BTV has significant implications for future development of non-replicating vaccines.

Sequence comparison of the L2 and S10 genes of bluetongue viruses from the United States and the People's Republic of China

Bluetongue virus (BTV) infection of ruminants is endemic throughout much of the US and China. The S10 and a portion of the L2 gene segments of Chinese prototype strains of BTV serotypes 1, 2, 3, 4, 12, 15, and 16 were sequenced and compared to the same genes of prototype and field strains of BTV from the US. Phylogenetic analysis of the S10 gene segregated the Chinese viruses into a monophyletic group distinct from the US viruses, whereas similar analysis of the L2 gene segregated strains of BTV according to serotype, regardless of geographic origin.

Phylogenetic analysis of the S10 gene of field and laboratory strains of bluetongue virus from the United States

The sequence of the S10 gene segment of the United States prototype strains of BTV serotypes 10, 11, 13, and 17 obtained from the American Type Culture Collection (ATCC), the commercial modified live virus vaccine strains of BTV serotypes 10, 11, and 17, and 20 field isolates of BTV serotypes 10, 11, 13, and 17 was determined to better define the molecular epidemiology of BTV infection in the US. All S10 gene segments were 822 nucleotides in length with two in-frame initiation codons (nucleotides 20 to 22 and 59 to 61) and a single termination codon (nucleotides 707 to 709), thus all S10 genes were predicted to encode two proteins (NS3, NS3A). Nucleotide differences between the S10 genes from field isolates of BTV ranged from zero (100% identity) to 142 (81.8% identity). The sequences of the S10 gene segments from the US prototype ATCC strains of BTV 10 and 11 were very different from the previously published sequences of putative US prototype viruses of the same serotypes (Lee and Roy, 1986; Hwang et al., 1992). Comparison of the predicted NS3/NS3A proteins encoded by the S10 gene showed little variation between the various viruses (from 93 to 100% identity). This apparent conservation of NS3/NS3A amongst different strains and serotypes of BTV likely is a reflection of functional constraints on the protein that tolerate little variation. The various US isolates of BTV segregate into two distinct monophyletic groups based on their S10 gene sequences and clustering of viruses was independent of serotype, year of isolation, geographical origin, and of host species of isolation. The S10 sequence data also show that viruses that segregated within each of these two monophyletic groups co-circulated in the western US between 1953 and 1990, and that reassortment of the S10 gene segment likely occurs in nature. Comparison of dendograms derived from sequence analysis of the S3 (de Mattos et al., 1996)and the S10 gene segments from the same viruses also indicates that the S10 gene segment evolves and reassorts independently of the S3 gene segment.

Phylogenetic comparison of the S3 gene of United States prototype strains of bluetongue virus with that of field isolates from California

To better define the molecular epidemiology of bluetongue virus (BTV) infection, the genetic characteristics and phylogenetic relationships of the S3 genes of the five U.S. prototype strains of BTV, the commercially available serotype 10 modified live virus vaccine, and 18 field isolates of BTV serotypes 10, 11, 13, and 17 obtained in California during 1980, 1981, 1989, and 1990 were determined. With the exception of the S3 gene of the U.S. prototype strain of BTV serotype 2 (BTV 2), these viruses had an overall sequence homology of between 95 and 100%. Phylogenetic analyses segregated the prototype U.S. BTV 2 strain to a unique branch (100% bootstrap value), whereas the rest of the viruses clustered in two main monophyletic groups that were not correlated with their serotype, year of isolation, or geographical origin. The lack of consistent association between S3 gene sequence and virus serotype likely is a consequence of reassortment of BTV gene segments during natural mixed infections of vertebrate and invertebrate hosts. The prototype strain of BTV 13, which is considered an introduction to the U.S. like BTV 2, presents an S3 gene which is highly homologous to those of some isolates of BTV 10 and especially to that of the vaccine strain. This finding strongly suggests that the U.S. prototype strain of BTV 13 is a natural reassortant. The different topologies of the phylogenetic trees of the L2 and S3 genes of the various viruses indicate that these two genome segments evolve independently. We conclude that the S3 gene segment of populations of BTV in California is formed by different consensus sequences which cocirculate and which cannot be grouped by serotype.

Vaccines for bluetongue

Isolation of 8 serotypes of bluetongue virus (BTV) in Australia has led to widespread debate on how to prepare for an outbreak of bluetongue disease and the type of vaccine best suited to control bluetongue in Australia. This article describes the vaccine options under consideration by research workers and animal health administrators. The most widely discussed options are live attenuated virus, killed virus and virus-like particles (VLP) generated by recombinant baculoviruses. Attenuated virus vaccines are cheap and easy to produce and are administered in a single dose. They replicate in sheep without causing significant clinical effects and provide protection against challenge with virulent virus of the same serotype. The possibility that insects could acquire vaccine virus by feeding on vaccinated animals and transmit it to sheep or cattle cannot be eliminated. This poses a risk because attenuated viruses are teratogenic if ewes are infected in the first half of pregnancy. In addition, vaccine virus replication in insects and ruminants may lead to a reversion to virulence. Killed virus vaccines have been shown to be efficacious in small laboratory trials and cannot be transmitted to other animals in the field, but are significantly more expensive to produce than attenuated viruses and require at least 2 doses with adjuvant to elicit an immune response. More work is needed to properly assess their effectiveness and determine their cost of production. Recombinant VLP contain the 4 major structural proteins of BTV but no nucleic acid. VLP are relatively easy to isolate, but it is unlikely that the purification methods currently used in laboratories will be adapted for use commercially. Despite the enthusiasm of recent years, little commercial progress appears to have been made. Although scientific research in Australia and overseas has provided a number of options for development of bluetongue vaccines, the decisions on which to use in an outbreak are complex and will require, not only consideration of factors discussed here, but also agreement from industry and government.

The pathogenesis and immunology of bluetongue virus infection of ruminants

Bluetongue (BLU) virus is transmitted from infected to susceptible ruminants by hematophagous vector midges (Culicoides species). Cattle are important reservoir hosts of the virus because infection typically is asymptomatic and characterized by prolonged cell associated viremia, and because at least some species of insect vector preferentially feed on cattle. Interaction of BLU virus with the cell membrane of erythrocytes in infected cattle likely facilitates both prolonged viremia as well as infection of the insect vector. BLU disease is most common in sheep and some wildlife species. A variety of host, agent and environmental factors clearly can influence expression of disease in these species. The pathogenesis of BLU virus infection of cattle and sheep is remarkably similar, thus the basis for expression of disease in sheep but not cattle remains to be firmly established. Some difference in susceptibility of endothelial cells to infection in the two species is one potential explanation. Ruminants develop a variety of antiviral responses after BLU virus infection. Antibodies to outer capsid protein VP2 are responsible for virus neutralization, and confer resistance to reinfection with the homologous serotype of BLU virus. Antibodies to epitopes on proteins which are common to all viruses of the BLU serogroup form the basis of current diagnostic serologic tests. Cell mediated responses have been incompletely characterized, in part because BLU virus replicates within dividing lymphocytes and virus-mediated cytolysis inhibits in vitro blastogenesis. Immunological competence of ruminants to BLU virus arises prior to midgestation, and suggestions that persistent immune tolerant BLU virus infection occurs after in utero exposure of cattle have not been substantiated and are not consistent with recent findings.

Heterogeneity of the L2 gene of field isolates of bluetongue virus serotype 17 from the San Joaquin Valley of California

Genome segment 2 (L2) from six field isolates of bluetongue virus (BTV) serotype 17 was sequenced by cycling sequencing after the amplification of the viral cDNA by the polymerase chain reaction. The viruses were isolated from sheep, cattle and a goat in the San Joaquin Valley of California during the years 1981 and 1990. These viruses exhibit divergent patterns of neutralization with BTV 17-specific monoclonal antibodies. The six L2 genes of the BTV 17 field isolates all encode a protein of 955 amino acids. Similarity of the nucleotide sequences of the L2 genes with respect to the prototype strain ranges between 93.8% and 95.1%, whereas the similarity between the field isolates ranges from 96.8% to 99.1%. Although very closely related, the L2 gene of each virus is distinct. Furthermore, mutations in the L2 gene of field isolates of BTV do not consistently follow a linear pattern of accumulation over time. Some amino acid changes in the VP2 protein of field strains were conserved over time, whereas others were not correlated with the year of isolation and some substitutions were unique to individual viruses. The predicted VP2s constitute a group of non-identical, but closely related proteins. Phylogenetic analyses suggest that the viral variants which co-circulate in the San Joaquin Valley could evolve by different evolutionary pathways.