A competitive ELISA for the detection of group-specific antibody to equine encephalosis virus

A polyclonal antibody-based, group-specific, competitive ELISA (C-ELISA) for the detection of antibodies to equine encephalosis virus (EEV) was developed. The assay measures the competition between a specific guinea pig antiserum and a test serum, for a pre-titrated EEV antigen. The C-ELISA detected antibodies to the seven known EEV serotypes. Reference antisera raised against other arboviruses did not cross react with EEV antigen. Negative sera from horses in the United Kingdom were used to establish the baseline for a negative population. Negative and positive populations of South African horses, selected on the basis of virus neutralisation were assayed subsequently. Optimal test parameters, where sensitivity≅specificity≅100%, were calculated by two-graph receiver operator characteristic (TG-ROC) analysis to be at a cut-off value of 29.5% inhibition. Results show the EEV C-ELISA described to be sensitive, specific and reliable. Used in conjunction with ELISAs available for African horse sickness virus (AHSV), differential serological diagnosis between EEV and AHSV can be achieved.

A group-specific, indirect sandwich ELISA for the detection of equine encephalosis virus antigen

A polyclonal antibody-based, group-specific, indirect, sandwich ELISA (S-ELISA) for the detection of equine encephalosis virus (EEV) antigen was developed. Purified EEV particles were titrated in the S-ELISA and the limit of detection was determined to be approximately 9.0 ng of antigen/ml (0.45 ng/well). Positive S-ELISA reactions were recorded with seven serologically distinct EEV serotypes. No cross-reactions were recorded with other arboviruses including African horse sickness virus (AHSV) serotypes 1-9, bluetongue serotypes 1-24, epizootic haemorrhagic disease serotypes 1-8 and isolate 318, and selected isolates of Palyam, Eubenangee, Corriparta, Warrego, Akabane and bovine ephemeral fever viruses. The assay proved to be sensitive and specific for the rapid detection of EEV in cell cultures and in homogenated suckling mouse brain (MB). The data generated in this study suggest that the ELISA will be valuable for epidemiological studies of EE and will assist in making a reliable differential diagnosis between EEV and AHSV infections.

A possible overwintering mechanism for bluetongue virus in the absence of the insect vector

Bluetongue virus (BTV) and several other Orbivirus species are transmitted between mammalian hosts via bites from adults of certain species of Culicoides midges. However, BTV can survive for 9-12 months (typically during the winter), in the absence of adult vectors, with no detectable cases of viraemia, disease or seroconversion in the host. The survival of the virus from one 'vector season' to the next is called 'overwintering' but the mechanism involved is not fully understood. It is demonstrated that BTV can persistently infect ovine gammadelta T-cells in vitro, a process that may also occur during infection and viraemia in mammalian hosts, thus providing a mechanism for virus persistence. Interaction of persistently BTV-infected gammadelta T-cells with antibody to the gammadelta T-cell-specific surface molecule WC-1 resulted in conversion to a lytic infection and increased virus release. Skin fibroblasts induce a similar conversion, indicating that they express a counter ligand for WC-1. Feeding of Culicoides midges induces skin inflammation, which is accompanied by recruitment of large numbers of activated gammadelta T-cells. The interaction of persistently infected gammadelta T-cells with skin fibroblasts would result in increased virus production at 'biting sites', favouring transmission to the insect vector. This suggested mechanism might also involve up-regulation of the WC-1 ligand at inflamed sites. It has been shown previously that cleavage of virus surface proteins by protease enzymes (which may also be associated with inflammation) generates infectious subvirus particles that have enhanced infectivity (100 times) for the insect vector.

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.

The highly ordered double-stranded RNA genome of bluetongue virus revealed by crystallography

The concentration of double-stranded RNA within the bluetongue virus core renders the genome segments liquid crystalline. Powder diffraction rings confirm this local ordering with a 30 A separation between strands. Determination of the structure of the bluetongue virus core serotype 10 and comparison with that of serotype 1 reveals most of the genomic double-stranded RNA, packaged as well-ordered layers surrounding putative transcription complexes at the apices of the particle. The outer layer of RNA is sufficiently well ordered by interaction with the capsid that a model can be built and extended to the less-ordered inner layers, providing a structural framework for understanding the mechanism of this complex transcriptional machine. We show that the genome segments maintain local order during transcription.

Structural studies of orbivirus particles

We are using crystallographic methods to investigate the structure of AHSV and BTV. Our initial approach was to investigate the structure of the major protein component of the viral core, VP7(T13). This trimeric protein has been studied in several crystal forms from both orbiviruses and reveals a structure made up of conserved domains, capable of conformational changes and possessing a cleavage site. Further crystallographic analyses of native particles have provided a picture of the VP7(T13) and VP3(T2) layers of the BTV core. The VP7(T13) layer consists of 260 trimers arranged rather symmetrically and possessing very similar structures, thereby following the rules of quasi equivalence. The VP3(T2) layer is thin and contains 120 copies of 100 kDa protein arranged as 60 approximate dimers. This type of icosahedral construction has not been observed before and appears to contain a genome which is highly ordered. We anticipate that all of these features will be common to AHSV.

VP7 from African horse sickness virus serotype 9 protects mice against a lethal, heterologous serotype challenge

An established mouse model system was used to evaluate the effectiveness of the major outer core protein VP7 of African horse sickness virus (AHSV) serotype 9 as a subunit vaccine. Balb C mice were immunised with VP7 crystals purified from AHSV infected BHK cells. In groups of mice, each of which was immunised with > or = 1.5 micrograms of the protein in Freund's adjuvant, > or = 80% of mice survived challenge with a virulent strain of a heterologous AHSV serotype (AHSV 7), that killed > or = 80% of the mice in the uninoculated control groups. This level of protection was significantly greater than that observed in mice inoculated with equivalent amounts of either denatured VP7 (50% survival), or GST/VP7 fusion protein (50-70% survival), or which were vaccinated with AHSV 9 (40-50% survival). The VP7 protein folding, or its assembly into crystals, are thought to play some role in the effectiveness of the protective response observed. Titres of circulating antibodies against AHSV VP7 were determined by competitive ELISA but did not appear to correlate with the levels of protection observed. Passive transfer of these antibodies to syngeneic recipients also failed to protect Balb C mice from the AHSV 7 challenge. The observed protection is therefore unlikely to be due to an antibody mediated immune response.

Development of a mouse model system, coding assignments and identification of the genome segments controlling virulence of African horse sickness virus serotypes 3 and 8

Attenuated (att) and wild type (wt) strains of the nine AHSV serotypes were evaluated for virulence in adult Balb C mice. Although most were avirulent in this system, isolates of AHSV 1att, 3wt, 3att, 4wt, 5att, 7att and 8att caused some mortality when administered via an intranasal route. After plaque cloning, only the attenuated vaccine strain of AHSV 7att caused any mortality via an intravenous route. AHSV 3att and AHSV 8wt were virulent (V) and avirulent (AV) (respectively) in the mouse model and were selected as parental strains for production of genome segment reassortants. These progeny virus strains were plaque cloned, then characterised to identify the genome segments that influence virulence of AHSV in the mouse model. Three virulence phenotypes were observed: fully virulent (V); fully avirulent (A); and a novel intermediate virulence (N) not expressed by either parental strain. Genome segment 2 (encoding outer capsid protein VP2) from the avirulent parent appeared to have a controlling influence in production of the A phenotype. Reassortants with the V phenotype all contained segment 2 from the virulent parent, however in each case they also contained genome segments 5 and 10, also from AHSV 3 (V). Genome segments 5 and 10 encode the smaller outer capsid protein VP5 and the non structural proteins NS3/NS3a, respectively. A combination of genome segments 2, 5 and 6 from the avirulent parent and segment 10 from the virulent parent were found in each of the virus strains with the N phenotype. However, comparison of two reassortants (A79 and A790), which differ only in a single segment, showed that replacement of genome segment 10 from the avirulent parent with that from the virulent parent, conferred the N phenotype on A790.

The atomic structure of the bluetongue virus core

The structure of the core particle of bluetongue virus has been determined by X-ray crystallography at a resolution approaching 3.5 A. This transcriptionally active compartment, 700 A in diameter, represents the largest molecular structure determined in such detail. The atomic structure indicates how approximately 1,000 protein components self-assemble, using both the classical mechanism of quasi-equivalent contacts, which are achieved through triangulation, and a different method, which we term geometrical quasi-equivalence.

African horsesickness virus VP7 sub-unit vaccine protects mice against a lethal, heterologous serotype challenge

An established mouse model was used to evaluate the effectiveness of the major outer core protein of African horsesickness virus (AHSV), VP7, as a subunit vaccine. Adult female BALB/c mice were immunized with VP7 crystals purified from BHK cells infected with AHSV serotype 9 (AHSV-9), using three inoculations in Freund's adjuvant. Eighty to one hundred per cent of the immunized mice were protected against a heterologous challenge with a known lethal dose of AHSV-7. The protected immunized mice did not develop any clinical signs characteristic of virulent AHSV infection in this model during the study. In contrast, 80-100% mortality was observed in the non-immunized mice that received the same challenge virus. Subsequent studies indicated that a single inoculation of 1.5 micrograms purified AHSV VP7 in Freund's complete adjuvant was sufficient to protect at least 90% of mice from AHSV-7 challenge. If the antigen was presented in the absence of Freund's complete adjuvant, 70% of the mice were still protected by one inoculation of VP7 crystals. Titres of circulating antibody against AHSV VP7, determined by competitive ELISA, did not appear to correlate with protection and passive antibody transfer from immunized BALB/c mice failed to protect syngeneic recipients from AHSV-7 challenge. Therefore, the observed protection is unlikely to be due to an antibody-mediated immune response. The number of viraemic mice and the duration of viraemia post-challenge was significantly reduced in vaccinated mice compared to non-vaccinated controls. However, the levels of viraemia were similar.

Monoclonal antibody based competitive ELISA for the detection of antibodies against epizootic haemorrhagic disease of deer virus

A monoclonal antibody based competitive enzyme-linked immunosorbent assay (MC-ELISA) for the detection of antibodies against epizootic haemorrhagic disease of deer viruses (EHDV) is described. Test sera were competed with a monoclonal antibody against the VP7 protein of EHDV serotype 1. The assay was capable of detecting antibodies to all serotypes of EHDV but unlike the agar gel immunodiffusion (AGID) test, gave no cross-reactions with antibodies against bluetongue, Palyam or Tilligery viruses. The MC-ELISA was more sensitive than a polyclonal based ELISA reported previously (Thevasagayam et al., 1995b) and would be ideal for epidemiological surveys since it is suitable for the examination of antisera from all animal species without the need for individual anti-species enzyme conjugates.

Enhanced infectivity of modified bluetongue virus particles for two insect cell lines and for two Culicoides vector species

Previous studies (Mertens et al., Virology 157, 375-386, 1987) have shown that removal of the outer capsid layer from bluetongue virus (BTV) significantly reduces (approximately x 10(-4)) the infectivity of the resultant core particle for mammalian cells (BHK 21 cells). In contrast, the studies reported here, using a cell line (KC cells) derived from a species of Culicoides that can act as a vector for BTV (Culicoides variipennis), demonstrated a much higher infectivity of core particles than that in mammalian cells (approximately x 10(3)). This increase resulted in a specific infectivity for cores that was only 20-fold less than that of purified disaggregated virus particles (stored in the presence of 0.1% sodium-N-lauroylsarcosine (NLS)). Removal of this detergent caused intact virus particle aggregation and (as previously reported) resulted in an approximately 1 log10 drop in the specific infectivity of those virus particles which remained in suspension. In consequence the specific infectivity of core particles for the KC cells was directly comparable to that of the intact but aggregated virus. These data are compared with the results from oral infectivity studies using two vector species (C. variipennis and Culicoides nubeculosus), which showed similar infection rates at comparable concentrations of purified cores, or of the intact but aggregated virus particles (NLS was toxic to adult flies). The role of the outer core proteins (VP7) in cell attachment and penetration, as an alternative route of initiation of infection, is discussed. Previous studies (Mertens et al., Virology 157, 375-386, 1987) also showed that the outer capsid layer of BTV can be modified by proteases (including trypsin or chymotrypsin), thereby generating infectious subviral particles (ISVP). The specific infectivity of ISVP for mammalian cells (BHK21 cells) was shown to be similar to that of disaggregated virus particles. In contrast, we report a significantly higher specific infectivity of ISVP but not of the intact virus (approximately x 100) for two insect cell lines (KC cells and C6/36 mosquito cells (derived from Aedes albopictus)). In oral infection studies with adults of the two vector species, ISVP produced the same infection rate at approximately 100-fold lower concentrations than either core particles or the intact but aggregated virus particles. The importance of mammalian host serum proteases, or insect gut proteases, in modification of the intact virus particle to form ISVP and their role in initiation of infection and the vector status of the insect is discussed.

Detection and differentiation of epizootic haemorrhagic disease of deer and bluetongue viruses by serogroup-specific sandwich ELISA

A serogroup specific sandwich ELISA was developed for the detection of epizootic haemorrhagic disease of deer viruses (EHDV) in infected insects and tissue culture preparations. Polyclonal rabbit antiserum against purified EHDV core particles was used to capture viral antigen and specific binding detected using guinea pig antisera against EHDV core particles followed by anti-guinea pig immunoglobulin enzyme-labelled conjugate. The assay is EHDV specific and detects all 8 serotypes. No cross-reactions were found with related viruses such as bluetongue (BTV), Palyam, Tilligery or African horse sickness virus (AHSV). A similar serogroup specific sandwich ELISA was also developed for BTV. The assays showed a similar sensitivity in detecting the respective EHDV or BTV antigens in a pool of 500 midges where only 2 were infected. These assays allow a simple and rapid means of detecting and differentiating members of these closely related serogroups. The sensitivity of the tests will allow more extensive studies on vector competence and virus/vector distribution.

Competitive ELISA for the detection of antibodies against epizootic haemorrhagic disease of deer virus

A competitive enzyme-linked immunosorbent assay is described for the detection of antibodies against epizootic haemorrhagic disease of deer viruses (EHDV). Test antisera were tested against a guinea-pig antiserum raised against EHDV core particles. The assay detected antibodies to all serotypes of EHDV, but unlike the agar gel immunodiffusion (AGID) test, gave no cross-reactions with antibodies against bluetongue, Palyam and Tilligery viruses. The C-ELISA would be ideal for use in epidemiological surveys since it is suitable for the examination of antisera from all susceptible species without the need for individual species-specific enzyme conjugates.

Proteolytic cleavage of VP2, an outer capsid protein of African horse sickness virus, by species-specific serum proteases enhances infectivity in Culicoides

Purified African horse sickness virus (AHSV) was fed, as part of a blood meal, to adult females from a susceptible colony of Culicoides variipennis, established in the insectories at the Institute for Animal Health, Pirbright Laboratory, UK. The meal consisted of heparinized blood obtained from ovine, bovine, equine (horse and donkey) or canine sources spiked with AHSV serotype 9 (AHSV9). The infectivity levels observed for C. variipennis varied significantly, according to the source of the blood sample. Comparison of the protein profiles obtained from AHSV9 incubated with the individual serum of plasma samples indicated that some species-specific serum proteases were able to cleave the outer capsid protein, VP2. The blood samples containing serum proteases that were able to cleave VP2 also showed an increase in infectivity for the insect vector when spiked with purified AHSV.

Crystallization and preliminary X-ray analysis of the core particle of bluetongue virus

Core particles of bluetongue virus serotype 1 (South Africa) have been crystallized. The crystals, which grow up to 0.8 mm in diameter, belong to a primitive orthorhombic space group and have point group symmetry 222. The unit cell dimensions are 754 x 796 x 823 A3 and the crystallographic asymmetric unit contains one-half of a core particle. The best crystals diffract strongly to 4.8 A Bragg spacings, which is the maximum resolution to which we can measure data with the detectors available, suggesting that useful diffraction extends well beyond this. Core particles of serotype 10 have also been crystallized but the crystals have yet to be analyzed by X-ray diffraction.

Modeling toxicity and response in carboplatin-based combination chemotherapy

Data from women with advanced ovarian cancer (International Federation of Gynecology and Obstetrics stage III or IV) were analyzed to evaluate the pharmacokinetic/pharmacodynamic relationships of carboplatin-based combination chemotherapy. With the equation area under the plasma concentration versus time curve (AUC) = dose/(creatinine clearance + 25), carboplatin AUC was calculated in each of up to six treatment cycles in 224 women with advanced ovarian cancer who had been randomized to receive carboplatin 300 mg/m2 plus cyclophosphamide 600 mg/m2. In addition, for each patient, the predicted nadir count (obtained by rearranging the University of Maryland single-agent carboplatin dosing formula) was compared with the actual observed nadir count, received and relative received dose intensities were calculated, and carboplatin exposure intensity was defined. Relationships were sought between these treatment indices and the clinical outcomes of time to progression and survival. When combined with cyclophosphamide 600 mg/m2, any carboplatin AUC was found to be associated with greater myelotoxicity and a higher likelihood of both leukopenia and thrombocytopenia occurring than had been determined for single-agent carboplatin. Furthermore, the platelet nadir in 83% of patients was equal to or below that predicted to result from the same dose of single-agent carboplatin. There was a relatively narrow range of received dose intensities within this patient population, but carboplatin exposure intensity was calculated as being distributed over a two-fold range within the population. Therefore, received carboplatin dose intensity underestimates the range of plasma drug exposure associated with a fixed dosing regimen of carboplatin. However, there were no consistent relationships between received dose intensity, relative received dose intensity, or carboplatin exposure intensity and the clinical outcomes of time to progression or survival. The relationships between carboplatin exposure and the pharmacodynamic measures of toxicity and response are likely to require definition in each regimen that includes carboplatin and for each tumor type treated.

Purification and properties of virus particles, infectious subviral particles, cores and VP7 crystals of African horsesickness virus serotype 9

Methods were developed for the purification, at high yield, of four different particle types of African horsesickness virus serotype 9 (AHSV-9). These products included virus particles purified on CsCl gradients which contain proteins apparently directly comparable to those of bluetongue virus (VP1 to VP7); virus particles purified on sucrose gradients which also contain, as a variable component, protein NS2; infectious subviral particles (ISVPs), containing chymotrypsin cleavage products of VP2; and cores, obtained by treating purified ISVPs with 1 M-MgCl2 to remove the components of the outer capsid layer (VP5 and VP2 cleavage products). Additional protein bands migrating with apparent M(r)s lower than that of VP5 were detected during SDS-PAGE analysis of virus particles. These appear to be conformational variants of VP5 and are identified as VP5' and VP5". BHK-21 cells infected with this strain of AHSV-9 produce large quantities of flat, usually hexagonal crystals of VP7, a major group antigen and core protein; these were also purified. Either 20 mg of virus particles, 20 mg of ISVPs or 10 mg of cores as well as 20 mg of VP7 crystals could be purified from approximately 8 x 10(9) infected cells. None of the preparations of particles or crystals showed any detectable contamination with BHK-21 cell proteins or antigens, as determined by SDS-PAGE or indirect ELISA. Virus particle and ISVP preparations had similar specific infectivities for BHK-21 cells (approximately 1 x 10(9) TCID50/A260 unit) but the infectivity of cores was approximately 10(5)-fold lower.

Impact of cyclophosphamide on relationships between carboplatin exposure and response or toxicity when used in the treatment of advanced ovarian cancer

PURPOSE

To determine (1) the impact of cyclophosphamide 600 mg/m2 on previously defined relationships between carboplatin area under the plasma concentration versus time curve (AUC) and indices of toxicity and response in women with advanced ovarian cancer; and (2) the relationships between indices of cumulative drug exposure and clinical outcomes.

METHODS

Carboplatin AUC = dose/(creatinine clearance [CCr] + 25) and was calculated in 224 women who received carboplatin 300 mg/m2 and cyclophosphamide 600 mg/m2. The likelihood of grade 3 or greater myelotoxicity at any carboplatin AUC was compared with the likelihood of myelotoxicity at the same single-agent carboplatin AUC. The nadir count predicted using the University of Maryland single-agent carboplatin dosing formula was compared with the nadir count observed. Received and relative-received dose-intensity were calculated. Carboplatin exposure-intensity was defined by substituting cumulative carboplatin exposure for total dose. Relationships were sought between these indices and therapeutic outcomes.

RESULTS

The incidence of leukopenia and thrombocytopenia at any carboplatin AUC was greater for the two-drug combination than for single-agent carboplatin. The platelet nadir in 83% of patients was less than or equal to the nadir predicted for the same single-agent carboplatin AUC. Despite a narrow range of received dose-intensities, carboplatin exposure-intensity was distributed over a twofold range. There were no relationships between received and relative-received dose-intensity or carboplatin exposure-intensity and time to progression or survival.

CONCLUSION

Any carboplatin AUC when administered with cyclophosphamide 600 mg/m2 produces greater myelotoxicity than the same AUC of single-agent carboplatin. Received carboplatin dose-intensity underestimates the range of plasma drug exposure resulting from a fixed carboplatin dosing regimen. Whether higher carboplatin exposures can improve outcome requires prospective validation.

The expressed VP4 protein of bluetongue virus binds GTP and is the candidate guanylyl transferase of the virus

A minor core protein, VP4, of bluetongue virus serotype 10 (BTV-10) has been synthesized in insect cells infected with a genetically manipulated recombinant baculovirus. When insect cells were coinfected by this recombinant virus and a recombinant baculovirus expressing the two major core proteins (VP3 and VP7) of the virus, core-like particles (CLPs) consisting of all three proteins were formed. Purified CLPs reacted with [32P]GTP which was covalently bound to VP4 only. Similarly reconstituted CLPs with VP1 or VP6 did not form covalent complexes with [32P]GTP. The virion-derived VP4 was also shown to have GTP-binding activity. The covalent binding of GTP indicates that expressed VP4 not only is biologically active but also is the candidate guanylyl transferase of the virus. The optimum reaction conditions for GTP binding by VP4 have been investigated.

Relationships between carboplatin exposure and tumor response and toxicity in patients with ovarian cancer

PURPOSE

The study was undertaken to define the relationship between tumor response and carboplatin area under the curve (AUC) in patients with ovarian cancer; to study the relationship between carboplatin AUC and myelosuppression in the same population; to establish the true impact of carboplatin AUC, prior therapy, and pretreatment platelet and WBC counts on toxicity; and to define an optimal carboplatin exposure for treating patients with ovarian cancer.

METHODS

With the equation AUC = dose/(glomerular filtration rate [GFR]+25), carboplatin AUC (course 1) was calculated for 1,028 patients (450 previously untreated) who received single-agent carboplatin (40 to 1,000 mg/m2) for advanced ovarian cancer. GFR was measured (chromium-51-edathamil [51Cr-EDTA] or creatinine clearance) in all patients.

RESULTS

Regression analysis showed that carboplatin AUC, prior treatment, and Eastern Cooperative Oncology Group grade performance status (PS) are predictors of tumor response, thrombocytopenia, and leukopenia. Pretreatment platelet and WBC counts are additional predictors of thrombocytopenia and leukopenia, respectively. Although the likelihood of tumor response increased with increasing carboplatin AUC, this relationship was nonlinear. In all patient subsets, the likelihood of complete response (CR) or overall response did not increase significantly above a carboplatin AUC of 5 to 7 mg/mL x minutes. At any given carboplatin AUC, thrombocytopenia occurred more frequently than leukopenia, although both approached 100% as carboplatin AUC increased. Both thrombocytopenia and leukopenia were more frequent in pretreated than in untreated patients regardless of pretreatment count. At any carboplatin AUC, the influence of PS on likelihood of response and toxicity was profound.

CONCLUSION

Carboplatin dosing by AUC will lead to more predictable toxicity, and increasing carboplatin AUC above 5 to 7 mg/mL x minutes does not improve the likelihood of response but does increase myelotoxicity. Therefore, careful evaluation of high-dose carboplatin therapy in a prospective, randomized trial is needed before such treatment becomes accepted practice.

The detection of African horse sickness virus antigens and antibodies in young Equidae

Four ponies were each inoculated with a different serotype of African horse sickness virus (AHSV) which had been passaged through cell culture in order to achieve attenuation. Three of the ponies died suddenly after showing mild clinical signs, the fourth pony remained clinically normal and was killed at day 38. Infectious AHSV was isolated from blood samples collected at intervals from all four ponies. Positive antigen ELISA reactions were only observed with blood samples from two of the ponies on the two days preceding death. Specific AHSV antibodies were detected by ELISA in serum samples from the other two ponies although one eventually died. African horse sickness viral antigens were detected by ELISA in post-mortem tissue samples collected from all four ponies. No infectious virus could be detected in tissue samples taken post-mortem from the pony which survived African horse sickness (AHS) infection. In the event of a suspected outbreak of AHS it is recommended that sera and heparinized blood should be tested for specific antibodies and AHSV antigen respectively. When available, post-mortem tissues, including spleen, heart, lung and liver, should also be tested for AHSV antigen. Although the ELISA used for the detection of AHSV antigen is highly sensitive and specific, negative ELISA results should be confirmed by virus isolation attempts.

A serogroup specific enzyme-linked immunosorbent assay for the detection and identification of African horse sickness viruses

A serogroup specific, indirect, sandwich ELISA was developed for the rapid detection of African horse sickness virus and viral antigens in field samples or in infected tissue cultures. The assay was shown to be highly sensitive and capable of providing confirmation of clinical diagnosis within one day. The results demonstrated that this ELISA will be useful for epidemiological surveillance of insect and mammalian host populations.

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.

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.

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

The relationship between six different isolates of BTV was analyzed by cross-hybridization of genomic dsRNA using blotting and probe techniques (using an alkali fragmented probe made from BTV dsRNA). The viruses compared in this way included BTV serotype 1 from South Africa, serotypes 3 and 4 from Cyprus, serotype 10 from North America, and serotypes 1 and 20 from Australia. Under the hybridization and washing conditions used, which were calculated to allow stable duplex formation between RNA molecules containing greater than 90% sequence homology, two of the genome segments (segments 2 and either 5 or 6, which encode the two major outer capsid proteins VP2 and VP5) appeared to contain serotype-specific RNA sequences. Significant cross-hybridization between these segments from different serotypes was detected only with serotypes 4 and 20, which are known to have a particularly close antigenic relationship. The amounts of homologous sequence that were detected in segments other than 2 and 5 between different viruses indicated some correlation between their geographical origins and a degree of relatedness, which is independent of the virus serotype. High levels of sequence homology were detected between the isolates from Cyprus and Africa and to a slightly lesser extent from North America, suggesting a common ancestry. These results also indicated that within the limited number of viruses studied, the Australian isolates form a separate interrelated group of bluetongue viruses.

Purification and properties of virus particles, infectious subviral particles, and cores of bluetongue virus serotypes 1 and 4

Effective purification methods have been developed for virus particles, infectious subviral particles (ISVP), and virus cores of bluetongue virus (BTV) serotypes 1 and 4. The purified particles were analysed by indirect ELISA or PAGE using either silver staining, or fluorography of [35S]methionine-labelled preparations. No significant contamination with host cell proteins, or with the majority of BTV nonstructural proteins was detectable in any of the particle preparations. In addition to the two major outer capsid and five core proteins previously described, the purified virus particles of both serotypes were consistently found to contain small amounts of BTV protein NS2, previously regarded as exclusively nonstructural. This protein could be removed from the particle surface by treatment with a combination of chymotrypsin and sodium N-lauroyl sarcosinate, which also resulted in the cleavage of the larger of the two major outer capsid components (protein VP2). Two of the cleavage products of VP2 and the whole of the other major outer capsid component (protein VP5) formed a modified outer capsid layer in the resultant ISVP. These subviral particles were as or more infectious than the intact virus particles but had lost haemagglutinating activity. The core-associated RNA polymerase remained inactive in ISVP.

Two initiation sites for foot-and-mouth disease virus polyprotein in vivo

Typically, the translation of eukaryotic mRNAs into protein is initiated at a single site. However, we have recently shown that not one but two primary products, P20a and P16, are translated from the 5' end of the coding region of the genome of foot-and-mouth disease virus (FMDV). In this paper we show by partial protease digestion of these proteins that they differ only at their N termini, thus confirming the presence of two initiation sites for translation of FMDV RNA. Sequence analysis of two subtypes of the virus (A10 and A12) confirms the presence of two initiator AUG codons in the expected position on the genome. By correlation with protein synthesis data from these subtypes it appears that the relative use of each initiation site is dependent on its surrounding nucleotide sequence. In addition, the ratio of the two proteins when synthesized in vitro differs markedly from that when they are synthesized in vivo, suggesting the presence of a control mechanism for synthesis of P20a in vivo which may be absent in vitro. We also show that the cleavage site between these two proteins and the structural protein precursor, P88, is located closer to the N terminus of the polyprotein than has previously been reported.

Multiple proteases in foot-and-mouth disease virus replication

Translation of foot-and-mouth disease virus RNA in a rabbit reticulocyte lysate for short time intervals resulted in the production of the peptides P20a , P16, and P88 (Lab, Lb, and P1) (R. R. Rueckert , Recommendations of the 3rd European Study Group on Molecular Biology of Picornavirus, Urbino , Italy, 1983). If further translation was prevented, the structural protein precursor P88 was not cleaved, even after prolonged incubation. This result indicates that the mechanism of the cleavage between P20a -P16 and P88 and of that between P88 and P52 (P2) differs from the mechanism of the secondary cleavages which produce the structural proteins. Furthermore, treatment of foot-and-mouth disease virus-infected cells with the protease inhibitor D-valyl phenylalanyl lysyl chloromethyl ketone prevented the in vivo cleavage between P20a -P16 and P88 but had no effect on any of the other cleavage events. These results suggest that the cleavage of the foot-and-mouth disease virus polyprotein utilizes two different host proteases.

Caliciviridae

The caliciviruses, as a proposed family Caliciviridae, have a distinct virion morphology with cup-shaped depressions on a spherical capsid surface. The viruses have single-stranded RNA, which has a molecular weight about 2.6 x 10(6) and is infectious. The RNA is covalently linked to a small protein. A single major polypeptide is found in the capsid. A subgenomic RNA, molecular weight about 1 x 10(6), coding for the capsid polypeptide is found in infected cells. Caliciviruses infecting swine, pinnipeds and cats have been characterized. Viruses which are morphologically identical to the known caliciviruses have been identified in human feces; these viruses have been shown to be associated with gastroenteritis, but they have not yet been propagated in the laboratory.

Presence of a covalently linked protein on calicivirus RNA

The infective RNA of the calicivirus, vesicular exanthema virus, has been shown to contain a protein which is apparently linked to the RNA by a covalent bond. The protein remained bound to the RNA after boiling with SDS-mercaptoethanol-urea or treating with formamide-dimethylsulphoxide but was removed by incubating with proteinase K. The mol. wt. of the protein was estimated to be about 1o X 1O(3) by electrophoresis in highly cross-linked polyacrylamide gels. The infectivity of the RNA was destroyed by removal of the protein with proteinase K.

A model for vesicular exanthema virus, the prototype of the calicivirus group

The structure of vesicular exanthema virus, the prototype member of the calicivirus group, has been studied in more detail. The RNA comprises 18% of mol. wt. of about 2.8 x 10(6), based on polyacrylamide gel electrophoresis experiments in the presence of formaldehyde. The virus contains one major polypeptide, mol. wt. 70 x 10(3) as determined by polyacrylamide gel electrophoresis and by chromatography on Sepharose 6B in the presence of 6 M-guanidine. Further evidence for the presence of a single major polypeptide was obtained by tryptic peptide analysis of 35S-methionine labelled virus. The mol. wt. of a protein oligomer produced by adjusting the pH of virus suspensions to 3.5 was c. 200 x 10(3). On the basis of these data we propose a T = 3 model for the virus capsid incorporating 180 copies of the virus protein.