Morphogenesis of a bluetongue virus variant with an amino acid alteration at a neutralization site in the outer coat protein, VP2

An Early Block in the Replication of the Atypical Bluetongue Virus Serotype 26 in Culicoides Cells Is Determined by Its Capsid Proteins

Arboviruses such as bluetongue virus (BTV) replicate in arthropod vectors involved in their transmission between susceptible vertebrate-hosts. The "classical" BTV strains infect and replicate effectively in cells of their insect-vectors (Culicoides biting-midges), as well as in those of their mammalian-hosts (ruminants). However, in the last decade, some "atypical" BTV strains, belonging to additional serotypes (e.g., BTV-26), have been found to replicate efficiently only in mammalian cells, while their replication is severely restricted in Culicoides cells. Importantly, there is evidence that these atypical BTV are transmitted by direct-contact between their mammalian hosts. Here, the viral determinants and mechanisms restricting viral replication in Culicoides were investigated using a classical BTV-1, an "atypical" BTV-26 and a BTV-1/BTV-26 reassortant virus, derived by reverse genetics. Viruses containing the capsid of BTV-26 showed a reduced ability to attach to Culicoides cells, blocking early steps of the replication cycle, while attachment and replication in mammalian cells was not restricted. The replication of BTV-26 was also severely reduced in other arthropod cells, derived from mosquitoes or ticks. The data presented identifies mechanisms and potential barriers to infection and transmission by the newly emerged "atypical" BTV strains in Culicoides.

VP2 Gene-Based Molecular Evolutionary Patterns of Major Circulating Bluetongue Virus Serotypes Isolated during 2014-2018 from Telangana and Andhra Pradesh States of India

INTRODUCTION

Bluetongue disease is an economically important viral disease of livestock caused by bluetongue virus (BTV) having multiple serotypes. It belongs to the genus Orbivirus of family Reoviridae and subfamily Sedoreovirinae. The genome of BTV is 10 segmented dsRNA that codes for 7 structural and 4 nonstructural proteins, of which VP2 was reported to be serotype-specific and a major antigenic determinant.

OBJECTIVE

It is important to know the circulating serotypes in a particular geographical location for effective control of the disease. The present study unravels the molecular evolution of the circulating BTV serotypes during 2014-2018 in Telangana and Andhra Pradesh states of India.

METHODS

Multiple sequence alignment with available BTV serotypes in GenBank and phylogenetic analysis were performed for the partial VP2 sequences of major circulating BTV serotypes during the study period.

RESULTS

The multiple sequence alignment of circulating serotypes with respective reference isolates revealed variations in antigenic VP2. The phylogenetic analysis revealed that the major circulating serotypes were grouped into eastern topotypes (BTV-1, BTV-2, BTV-4, and BTV-16) and Western topotypes (BTV-5, BTV-12, and BTV-24).

CONCLUSION

Our study strengthens the need for development of an effective vaccine, which can induce the immune response for a range of serotypes within and in between topotypes.

Multiple Routes of Bluetongue Virus Egress

Bluetongue virus (BTV) is an arthropod-borne virus infecting livestock. Its frequent emergence in Europe and North America had caused significant agricultural and economic loss. BTV is also of scientific interest as a model to understand the mechanisms underlying non-enveloped virus release from mammalian and insect cells. The BTV particle, which is formed of a complex double-layered capsid, was first considered as a lytic virus that needs to lyse the infected cells for cell to cell transmission. In the last decade, however, a more in-depth focus on the role of the non-structural proteins has led to several examples where BTV particles are also released through different budding mechanisms at the plasma membrane. It is now clear that the non-structural protein NS3 is the main driver of BTV release, via different interactions with both viral and cellular proteins of the cell sorting and exocytosis pathway. In this review, we discuss the most recent advances in the molecular biology of BTV egress and compare the mechanisms that lead to lytic or non-lytic BTV release.

Multiple genome segments determine virulence of bluetongue virus serotype 8

Bluetongue virus (BTV) causes bluetongue, a major hemorrhagic disease of ruminants. In order to investigate the molecular determinants of BTV virulence, we used a BTV8 strain minimally passaged in tissue culture (termed BTV8L in this study) and a derivative strain passaged extensively in tissue culture (BTV8H) in in vitro and in vivo studies. BTV8L was pathogenic in both IFNAR(-/-) mice and in sheep, while BTV8H was attenuated in both species. To identify genetic changes which led to BTV8H attenuation, we generated 34 reassortants between BTV8L and BTV8H. We found that partial attenuation of BTV8L in IFNAR(-/-) mice was achieved by simply replacing genomic segment 2 (Seg2, encoding VP2) or Seg10 (encoding NS3) with the BTV8H homologous segments. Fully attenuated viruses required at least two genome segments from BTV8H, including Seg2 with either Seg1 (encoding VP1), Seg6 (encoding VP6 and NS4), or Seg10 (encoding NS3). Conversely, full reversion of virulence of BTV8H required at least five genomic segments of BTV8L. We also demonstrated that BTV8H acquired an increased affinity for glycosaminoglycan receptors during passaging in cell culture due to mutations in its VP2 protein. Replication of BTV8H was relatively poor in interferon (IFN)-competent primary ovine endothelial cells compared to replication of BTV8L, and this phenotype was determined by several viral genomic segments, including Seg4 and Seg9. This study demonstrated that multiple viral proteins contribute to BTV8 virulence. VP2 and NS3 are primary determinants of BTV pathogenesis, but VP1, VP5, VP4, VP6, and VP7 also contribute to virulence.

IMPORTANCE

Bluetongue is one of the major infectious diseases of ruminants, and it is listed as a notifiable disease by the World Organization for Animal Health (OIE). The clinical outcome of BTV infection varies considerably and depends on environmental and host- and virus-specific factors. Over the years, BTV serotypes/strains with various degrees of virulence (including nonpathogenic strains) have been described in different geographical locations. However, no data are available to correlate the BTV genotype to virulence. This study shows that BTV virulence is determined by different viral genomic segments. The data obtained will help to characterize thoroughly the pathogenesis of bluetongue. The possibility to determine the pathogenicity of virus isolates on the basis of their genome sequences will help in the design of control strategies that fit the risk posed by new emerging BTV strains.

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.

Definition of neutralizing sites on African horse sickness virus serotype 4 VP2 at the level of peptides

The antigenic structure of African horse sickness virus (AHSV) serotype 4 capsid protein VP2 has been determined at the peptide level by PEPSCAN analysis in combination with a large collection of polyclonal antisera and monoclonal antibodies. VP2, the determinant for the virus serotype and an important target in virus neutralization, was found to contain 15 antigenic sites. A major antigenic region containing 12 of the 15 sites was identified in the region between residues 223 and 400. A second domain between residues 568 and 681 contained the three remaining sites. These sites were used for the synthesis of peptides, which were later tested in rabbits. Of the 15 synthetic peptides, three were able to induce neutralizing antibodies for AHSV-4, defining two neutralizing epitopes, 'a' and 'b', between residues 321 and 339, and 377 and 400, respectively. A combination of peptides representing both sites induced a more effective neutralizing response. Still, the relatively low neutralization titres make the possibility of producing a synthetic vaccine for AHSV unlikely. The complex protein-protein interaction of the outer shell of the viral capsid would probably require the presence of either synthetic peptides in the correct conformation or peptide segments from the different proteins VP2, VP5 and VP7.

Co-replication of a reovirus and a polydnavirus in the ichneumonid parasitoid Hyposoter exiguae

A recently established colony of the ichneumonid parasitoid, Hyposoter exiguae, was found to carry both a reovirus (HeRV) and a polydnavirus (HePDV). Morphogenesis of these viruses was observed in all cells comprising the ovarian calyx epithelium, apparently without detrimental effect to the parasitoid. While polydnavirus replication in H. exiguae was restricted to the calyx region, HeRV was detected in ovarioles, oviducts, midguts, malpighian tubules, and accessory glands associated with the male reproductive system. In addition, HeRV was able to infect the fat body of parasitized host larvae and to establish a persistent infection in vitro. Electron microscopy revealed that both viruses were released into the calyx fluid compartment exclusively by budding, a phenomenon rarely observed among the Reoviridae; HeRV envelopes thus obtained, however, appeared to be subsequently shed within the oviducts. HeRV particles were concentrated to near homogeneity by differential centrifugation; mature virions consisted of seven to eight structural polypeptides and 10 dsRNA genome segments. Prominent spikes were observed at the vertices of icosahedral core particles. Most, but not all, individuals comprising the H. exiguae colony appeared to be infected with HeRV, suggesting a commensal relationship between wasp and virus; however, while this association is of obvious benefit to the virus, it seems unlikely that any advantage accrues to the parasitoid which carries it.

Cloning, sequencing and expression of the gene that encodes the major neutralisation-specific antigen of African horsesickness virus serotype 9

A marked improvement in the efficiency of cloning the large double stranded RNA (dsRNA) genome segments of African horsesickness virus (AHSV) was achieved when the dsRNA polyadenylation step was carried out with undenatured rather than strand-separated dsRNA. It is a prerequisite to use dsRNA of very high purity because in the presence of even trace amounts of single stranded RNA, the dsRNA appears to be poorly polyadenylated as judged by its effectiveness as a template for oligo-dT-primed cDNA synthesis. The full-length VP2 gene of AHSV-9, cloned by this approach, was sequenced and it was found to show the highest percentage identity (60%) to VP2 of AHSV-6, providing an explanation of why these two serotypes show some cross protection. The VP2 protein was also expressed in Spodoptera frugiperda (Sf9) cells by means of a baculovirus recombinant. The yield of the expressed VP2 was high, but the protein was found to be largely insoluble. Nine smaller, truncated VP2 peptides were subsequently expressed in insect cells, but no significant improvement in solubility of the peptides, as compared to that of the full-sized protein, was observed. A western immunoblot analysis of the overlapping peptides indicated the presence of a strong linear epitope located within a large hydrophilic domain between amino acids 369 and 403.

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.

Characterisation of Wongorr virus, an Australian orbivirus

Sequence analyses of VP3 gene segments of Wongorr virus isolates from the Northern Territory of Australia were compared with the cognate gene segments from Picola and Paroo River viruses. Previous serological investigations had demonstrated some relationships between these viruses, however VP3 gene sequence and phylogenetic analyses placed these viruses within the same serogroup which was distinct from other described orbivirus serogroups. A polymerase chain reaction (PCR) was developed for the detection of this serogroup and used to identify and determine partial sequence data for other isolates of the virus. Wongorr virus and the other tick and mosquito-borne orbiviruses (Kemerovo and Corriparta), were more closely related than the Culicoides transmitted orbiviruses, such as bluetongue (BTV) and African horse sickness virus (AHSV) which were shown to be on a separate branch of the orbivirus phylogenetic tree.

Phylogenetic relationships of the VP2 protein of a virulent isolate of bluetongue virus (BTV-23) compared to those of 6 other BTV serotypes

To determine the genetic relationship of the virulent Australian bluetongue virus serotype 23 with that of other serotypes and to identify the extent and nature of the antigenic variation among seven serotypes of bluetongue virus (BTV), the complete nucleotide sequence was determined for cDNA clones representing the L2 dsRNA of BTV-23, the gene that codes for the outer capsid neutralization antigen (VP2). The predicted amino acid sequence of the protein was compared with the VP2 sequences of the five USA serotypes (BTV-2, -10, -11, -13 and -17) as well as with an Australian isolate of BTV-1. The comparisons revealed that the VP2 of BTV-23 is most closely related to that of BTV-1, sharing 52% identical and 72% similar sequences, also that the VP2 of the two Australian serotypes are more closely related to that of BTV-2 than to the other four USA serotypes. Only 22% identical sequences are shared by all seven VP2 molecules; however, when homologous substitutions are considered the similarity index was as high as 48%. In addition, the conserved regions that have been identified previously for other VP2 molecules are also conserved in BTV-23.

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.

The use of immuno-gold silver staining in bluetongue virus adsorption and neutralisation studies

The immuno-gold-silver staining (IGSS) technique was used in scanning electron microscopy for the detection and semi-quantitation of low copy antigens on the surface of cells. The methodology was exploited in experiments designed to examine the interaction of small numbers of virus particles with the surface of susceptible host cells. Using bluetongue virus (BTV) as an example, IGSS procedures confirmed that maximum adsorption occurred within 60 min and that adsorbed virus particles were distributed randomly on the surface of the cell. Neutralising antibody did not prevent binding of BTV to the plasma membrane, but abrogated virus uptake. The use of IGSS in the study of virus-cell interactions was validated by transmission electron microscopy and classical biochemical experiments utilising radioactively-labelled virus.

Comparison of immunogold methodologies for the detection of low copy number viral antigens in bluetongue virus (BTV)-infected cells

Cells infected with bluetongue virus (BTV) were prepared for immunocytochemistry by freeze substitution, the progressive lowering of temperature technique and the Tokuyasu method. Sections containing virus-infected cells were incubated with specific monoclonal antibodies and colloidal gold probes to detect virus antigens of varying copy number; these BTV proteins were structural proteins VP2 and VP7 and the non-structural protein NS2. Protocols compared in this study represented those used in laboratories which handle infectious agents and as such, all samples were pre-fixed with minimum concentrations of glutaraldehyde to inactivate the virus. No statistical difference was found between the gold-labelling of sections prepared by the progressive lowering of temperature technique and freeze substitution. The results showed that cryo-sections yielded the best signal-to-noise ratio for all proteins examined in this study and were therefore the most sensitive system for the detection of low copy number proteins. The data and associated inferences relate to the system described in this paper and possibly other analogous systems.

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.

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.

The amino acid sequence of the outer coat protein VP2 of neutralizing monoclonal antibody-resistant, virulent and attenuated bluetongue viruses

Monoclonal antibodies which reacted with four different epitopes were used to select neutralization-resistant variants of Australian bluetongue virus serotype 1 (BTV1AUS; isolate CS156). Nucleotide sequencing of the VP2 outer coat protein gene of these variants showed that two of them contained alterations within the previously defined neutralization site at amino acids 328 to 335 (Gould et al., 1988). Comparison of VP2 sequences of several BTV serotypes, in addition to nucleotide sequence changes in a number of variants, suggested that this neutralization site was larger and contained 19 amino acids, the conformation of which could be affected by other regions of the VP2 protein. Nucleotide sequencing of neutralization-resistant variants revealed a total of four other regions of VP2 affecting the ability of monoclonal antibodies to neutralize the virus and these results support the notion that the neutralization site in VP2 was conformation dependent. The complete nucleotide sequence of the VP2 gene of virulent BTV1AUS (C5156) was determined directly from viral nucleic acid isolated from the blood of a sheep suffering clinical bluetongue disease. Comparison of the VP2 sequence of this virulent virus with that previously published for an avirulent, laboratory strain (Gould, 1988), indicated that the passage of virulent virus approximately 20 times in tissue culture over the last decade, not only led to attenuation but resulted in the appearance of ten nucleotide changes in the VP2 gene. Six of these nucleotide changes were silent, two resulted in conservative amino acid substitutions and two generated radical amino acid changes. However, in a separate experiment, a single passage of the virulent virus in tissue culture while leading to attenuation did not result in a nucleotide change in the VP2 outer coat protein gene.

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.

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.

Monoclonal antibodies to bluetongue virus define two neutralizing epitopes and a hemagglutinating epitope

Monoclonal antibodies were used to characterize neutralizing epitopes on VP2 of bluetongue virus serotype 10 (BTV-10). Six neutralizing monoclonal antibodies that immune precipitated VP2 demonstrated two distinct patterns of reactivity in the competitive enzyme-linked immune absorbent assay (ELISA). These results suggest that there are at least two distinct domains of neutralization on VP2 of BTV. Monoclonal antibodies defining the two domains were serotype-restricted in plaque neutralization, immune precipitation or ELISA. One of the two neutralizing domains also demonstrated significant hemagglutinating activity. Both cattle and sheep infected with BTV-10 produce antibodies to the two neutralizing epitopes.

Virological applications of the grid-cell-culture technique

Whole mounts of intact virus-infected cells have been used for several decades to examine virus-cell relationships and virus structure. The general concept of studying virus structure in association with the host cell has recently been expanded to reveal interactions between viruses and the cytoskeleton. The procedure permits utilization of immuno-gold protocols using both the transmission and scanning electron microscopes. The grid-cell-culture technique is reviewed to explain how it can be exploited to provide valuable information about virus structure and replication in both diagnostic and research laboratories. The use of the technique at the research level is discussed using bluetongue virus as a model. The procedure can provide basic structural information about intact virions and additional data on the intracellular location of viruses and virus-specific structures and about the mode of virus release from infected cells. Application of immunoelectron microscopy reveals information on the protein composition of not only released virus particles but also cell surface and cytoskeletal-associated viruses and virus-specific structures. Collectively, this simple and physically gentle technique has provided information which would otherwise be difficult to obtain.

The release of bluetongue virus from infected cells and their superinfection by progeny virus

Immunoelectron microscopy using an anti-VP2 monoclonal antibody complexed to colloidal gold has been used to study the mechanism of bluetongue virus (BTV) release from infected cells. Examination of the BTV-infected cell surface revealed that viruses are released both as enveloped particles, by budding through the plasma membrane, and as nonenveloped particles by "extrusion" through the membrane. Particles being released and those remaining on the cell surface retain an association with the cortical layer of the cytoskeleton. Analyses of virus particles released from infected cells and the intracellular viruses in the cytosol and attached to the cytoskeleton indicate that although the three populations have similar particle to infectivity ratios they differ in their ability to bind gold-labeled anti-VP2 antibody. The fact that released viruses bind less antibody than intracellular viruses suggests that virus release from infected cells may be associated with either a loss of VP2 or a rearrangement of the virus outer coat which obscures a proportion of the reactive epitopes on the virus surface. Electron microscopic observations also indicated that, in addition to virus release, events at the plasma membrane resulted in the uptake of progeny virus by endocytosis. Elevation of intraendosomal/lysosomal pH by lysomotropic bases and an acidic ionophore inhibited BTV replication when added to cells concurrently with the virus. Addition of such agents to infected cells at 4 hr p.i. decreased both the maximum titer of released virus and the rate at which virus antigen was synthesized in infected cells. Addition of anti-BTV antiserum 4 hr p.i. also resulted in a decreased rate of intracellular virus antigen accumulation. These results suggest that superinfection of BTV-infected cells by progeny virions effectively increases the multiplicity of infection and enhances the kinetics of BTV replication.