Expression in Escherichia coli and sequencing of the coding region for the capsid protein of Dutch maedi-visna virus strain ZZV 1050: application of recombinant protein in enzyme-linked immunosorbent assay for the detection of caprine and ovine lentiviruses

Argentinian small ruminant lentivirus (SRLV) p55gag antigen fused to maltose binding protein to use in SRLV serological confirmatory diagnosis

The complete gag gene from small ruminant lentiviruses (SRLV) encodes for a polyprotein of 55 kDa, known as p55gag. p55gag presents multiple antigenic epitopes, which can be recognized by antibodies, increasing the opportunity to detect SRLV-positive animals. Therefore, this polyprotein is considered an excellent candidate to use in diagnostic tests to detect antibodies against SRLV. Different studies have suggested that the selection of the recombinant antigen, which must be representative of the virus strains circulating in the test population, is crucial to avoid false negative results. Thus, the use of proteins from different viral strains isolated from goats or sheep of a given region or country may be a useful strategy to increase the ability to detect SRLV-infected animals. In the present study, the pMAL-p5X vector was used to express and purify p55gag (now called rp55gag for recombinant polyprotein 55 gag). The cloned gene was inserted downstream from the malE gene of Escherichia coli, which encodes a maltose-binding protein (MBP), resulting in the expression of an MBP fusion protein. The complete gag gene was amplified by RT-PCR. Finally, after digestion, the product was cloned into the pMAL-p5X vector and used to transform E. coli ER2325 cells. After the purification of MBP-rp55gag by affinity chromatography, the eluted fraction was observed by SDS-PAGE and Western Blot (WB). The WB was carried out with 85 serum samples from small ruminants previously analysed and compared by two commercial ELISAs. The results show that 76 of the serum samples were concordant with those by both ELISAs. Regarding the other nine serum samples, which showed discordant results between both ELISAs, were positive by WB. The results thus show that the rp55gag could be considered as an antigen in a confirmatory diagnostic assay to detect SRLV by WB. For this purpose, a future study with a high number of sera to determine the test specificity and sensitivity, using the p55gag of the circulating strain in Argentina will be necessary.

A polyclonal antibody against a recombinantly expressed Triticum aestivum RHT-D1A protein

Background

Reduced height-1 dwarfing alleles affect DELLA proteins belonging to a family of putative transcriptional regulators that modulate plant growth and development. The Arabidopsis thaliana genome encodes five DELLA proteins, whereas monocot plants, such as rice, barley, and wheat, each have a single DELLA protein. In wheat, wild-type Rht-B1a and Rht-D1a genes encode DELLA proteins and have many alleles that contain lesions. Among them, Rht-B1b and Rht-D1b are the most common mutant dwarfing alleles, which have played a key part in the creation of high-yielding wheat varieties. Despite their fundamental roles in plant biology, until now, DELLA proteins in wheat have been mainly researched regarding the phenotypic effect of defective Rht mutants on yield-related traits, without studies on the underlying mechanisms. The RHT-1 protein has yet to be detected in wheat tissues, owing to a lack of appropriate molecular tools for characterization of RHT function and protein interactions in signal transduction. This study is focused on the production of a polyclonal antibody to the wheat RHT-D1A protein.

Results

To generate the anti-RHT-D1A antibody, we expressed and purified soluble 6xHis-tagged RHT-D1A. The purified recombinant RHT-D1A was injected into New Zealand white rabbits to generate polyclonal antiserum. The polyclonal anti-RHT-D1A antibody was purified by ammonium sulfate precipitation, followed by affinity chromatography on protein A–agarose beads. The purified polyclonal antibody was demonstrated to be effective in immunoblotting, western blot hybridization, and immunoprecipitation. In wheat seedling extracts, the polyclonal antibody recognized a protein with a molecular mass close to the predicted molecular weight of the endogenous RHT-D1A protein. We also demonstrated that RHT-D1A disappears in response to exogenous and endogenous gibberellic acid.

Conclusion

The purified polyclonal antibody raised against the recombinant RHT-D1A protein is sufficiently specific and sensitive and could be a useful tool for future insights into upstream and downstream components of DELLA-regulatory mechanisms in wheat plants.

Development of TaqMan-based qPCR method for detection of caprine arthritis-encephalitis virus (CAEV) infection

A specific and sensitive two-step TaqMan real-time PCR has been developed for rapid diagnosis of caprine arthritis-encephalitis virus (CAEV) infection by using a set of specific primers and a TaqMan probe targeting a highly conserved region within the gene encoding the viral capsid protein (CA). The assay successfully detected CAEV proviral DNA in total DNA extracts originating from cell culture, whole blood samples and isolated PBMCs, with a lower detection limit of 10² copies and a linear dynamic range of 10⁵ to 10¹⁰ copies/ml. There was no cross-reaction with other animal viruses (e.g., goat pox virus, bovine leukemia virus, bovine mucosal disease virus, swine influenza virus and Nipah virus). When applied in parallel with serological AGID and conventional PCR for detection of CAEV in field samples, this assay exhibited a higher sensitivity than these traditional methods, and 7.8 % of the 308 specimens collected in the Shanxi and Tianjin regions of China from 1993 to 2011 were found to be positive. Thus, the TaqMan qPCR assay provides a fast, specific and sensitive means for detecting CAEV proviral DNA in goat specimens and should be useful for large-scale detection in eradication programs and epidemiological studies.

Small ruminant lentiviruses: genetic variability, tropism and diagnosis

Small ruminant lentiviruses (SRLV) cause a multisystemic chronic disease affecting animal production and welfare. SRLV infections are spread across the world with the exception of Iceland. Success in controlling SRLV spread depends largely on the use of appropriate diagnostic tools, but the existence of a high genetic/antigenic variability among these viruses, the fluctuant levels of antibody against them and the low viral loads found in infected individuals hamper the diagnostic efficacy. SRLV have a marked in vivo tropism towards the monocyte/macrophage lineage and attempts have been made to identify the genome regions involved in tropism, with two main candidates, the LTR and env gene, since LTR contains primer binding sites for viral replication and the env-encoded protein (SU ENV), which mediates the binding of the virus to the host's cell and has hypervariable regions to escape the humoral immune response. Once inside the host cell, innate immunity may interfere with SRLV replication, but the virus develops counteraction mechanisms to escape, multiply and survive, creating a quasi-species and undergoing compartmentalization events. So far, the mechanisms of organ tropism involved in the development of different disease forms (neurological, arthritic, pulmonary and mammary) are unknown, but different alternatives are proposed. This is an overview of the current state of knowledge on SRLV genetic variability and its implications in tropism as well as in the development of alternative diagnostic assays.

Genetic characterization of small ruminant lentiviruses circulating in naturally infected sheep and goats in Ontario, Canada

Maedi-visna virus (MVV) and caprine arthritis encephalitis virus (CAEV) are related members of a group of small ruminant lentiviruses (SRLVs) that infect sheep and goats. SRLVs are endemic in many countries, including Canada. However, very little is known about the genetic characteristics of Canadian SRLVs, particularly in the province of Ontario. Given the importance of surveillance and eradication programs for the control of SRLVs, it is imperative that the diagnostic tests used to identify infected animals are sensitive to local strains of SRLVs. The aim of this work was to characterize SRLV strains circulating in Ontario and to evaluate the variability of the immunodominant regions of the Gag protein. In this study, the nearly complete gag sequence of 164 SRLVs, from 130 naturally infected sheep and 32 naturally infected goats from Ontario, was sequenced. Animals belonged to distantly located single and mixed species (sheep and goats) farms. Ovine lentiviruses from the same farm tended to cluster more closely together than did caprine lentiviruses from the same farm. Sequence analysis revealed a higher degree of heterogeneity among the caprine lentivirus sequences with an average inter-farm pairwise DNA distance of 10% and only 5% in the ovine lentivirus group. Interestingly, amplification of SRLVs from ELISA positive sheep was successful in 81% of cases, whereas amplification of SRLV proviral DNA was only possible in 55% of the ELISA positive goat samples; suggesting that a significant portion of caprine lentiviruses circulating in Ontario possess heterogeneity at the primer binding sites used in this study. Sequences of sheep and goat SRLVs from Ontario were assembled into phylogenetic trees with other known SRLVs and were found to belong to sequence groups A2 and B1, respectively, as defined by Shah et al. (2004a). A novel caprine lentivirus with a pairwise genetic difference of 15.6-25.4% relative to other group B subtypes was identified. Thus we suggest the designation of a novel subtype, B4, within the caprine lentivirus-like cluster. Lastly, we demonstrate evidence of recombination between ovine lentiviruses. These results emphasize the broad genetic diversity of SRLV strains circulating in the province of Ontario and show that the gag region is suitable for phylogenetic studies and may be applied to monitor SRLV eradication programs.

Application of purified recombinant antigenic spike fragments to the diagnosis of avian infectious bronchitis virus infection

The spike (S) protein, containing two subunits, S1 and S2, is the major immunity-eliciting antigen of avian infectious bronchitis virus (IBV), a highly contagious disease of chickens. Several immunogenic regions, mainly located within the S1 subunit, have been identified. Nonetheless, these immune-dominant regions were defined using selected monoclonal antibodies or using a short peptide approach that involves only certain limited regions of the S protein. In addition, some immune-dominant regions are located in hypervariable regions (HVRs) which are not present in all serotypes. Hence, the aim of this study was to determine a broader range of antigenic regions that have strong antibody eliciting ability; these could then be applied for development of an IBV-diagnostic tool. Initially, the S1 and part of the S2 subunit protein (24-567 amino acids) were expressed as five fragments in prokaryotic system. The antigenicity was confirmed using IBV immunized sera. Performance of the S subfragments was evaluated by ELISA using a panel of field chicken sera with known IBV titres determined by a commercial kit. This indicated that, among the five antigenic recombinant proteins, the region S-E showed the highest specificity and sensitivity, namely 95.38 % and 96.29 %, respectively. The κ value for the in-house ELISA using the S-E fragment compared to a commercial kit was 0.9172, indicating a high agreement between these two methods. As region S-E harbors strong immunogenicity within the spike protein, it has the potential to be exploited as an antigen when developing a cost-effective ELISA-based diagnosis tool.

Recombinant protein-based viral disease diagnostics in veterinary medicine

Identification of pathogens or antibody response to pathogens in human and animals modulates the treatment strategies for naive population and subsequent infections. Diseases can be controlled and even eradicated based on the epidemiology and effective prophylaxis, which often depends on development of efficient diagnostics. In addition, combating newly emerging diseases in human as well as animal healthcare is challenging and is dependent on developing safe and efficient diagnostics. Detection of antibodies directed against specific antigens has been the method of choice for documenting prior infection. Other than zoonosis, development of inexpensive vaccines and diagnostics is a unique problem in animal healthcare. The advent of recombinant DNA technology and its application in the biotechnology industry has revolutionized animal healthcare. The use of recombinant DNA technology in animal disease diagnosis has improved the rapidity, specificity and sensitivity of various diagnostic assays. This is because of the absence of host cellular proteins in the recombinant derived antigen preparations that dramatically decrease the rate of false-positive reactions. Various recombinant products are used for disease diagnosis in veterinary medicine and this article discusses recombinant-based viral disease diagnostics currently used for detection of pathogens in livestock and poultry.

Comparison of immunoassay and real-time PCR methods for the detection of Jembrana disease virus infection in Bali cattle

A sensitive diagnostic assay for the detection of infections with the bovine lentivirus Jembrana disease virus (JDV) is required in Indonesia to control the spread of Jembrana disease. Immunoassays are used routinely but are compromised by cross-reactive epitopes in the capsid (CA) protein of JDV and the genetically related bovine immunodeficiency virus (BIV). JDV gag-specific primers were tested in a real-time PCR assay to detect proviral DNA in peripheral blood mononuclear cells from 165 cattle from the Tabanan district of Bali. JDV-specific amplicons were detected in 9% of the cattle and only 33% of the real-time PCR positive cattle were seropositive. The delayed seroconversion that occurs after infection with JDV could explain the low concordance between these assays but other factors may be responsible. BIV proviral DNA was not detected in any of the PBMC DNA samples. A high concordance value of 98.6% was found between the JDV plasma-derived antigen Western blot and the JDV p26-his recombinant protein ELISA. Only 21% of the seropositive cattle had detectable levels of proviral DNA suggesting that the proviral load in recovered cattle is low. A combination of real-time PCR and JDV p26-his ELISA is recommended for the detection of infection with JDV in Indonesia.

Serum containing ovine IgG2 antibody specific for maedi visna virus envelope glycoprotein mediates antibody dependent cellular cytotoxicity

Antibody-dependent cell-mediated cytotoxicity (ADCC) specific for maedi visna virus (MVV) has never been described. The IgG antibody response to MVV is restricted to an IgG1 response whilst MVV specific IgG2 is never seen in persistently infected sheep. To determine whether the isotypic restriction of the antibody response is responsible for the lack of ADCC, an ADCC assay was developed using polyclonal serum raised to recombinant MVV ENV protein. Sheep immunised with a recombinant GST:SUenv fusion protein in complete Freund's adjuvant produced an antibody response which contained IgG1 and IgG2 antibodies. The activity of this serum in an ADCC assay was compared to serum from persistently infected sheep. Serum from immunised sheep mediated ADCC reactions whilst no activity was ever seen in persistently infected sheep serum. IgG2 may therefore be the possible effector isotype for ADCC reactions against MVV. Failure of the IgG2 dependent ADCC system in vivo may contribute to the persistence of MVV-infected macrophages in vivo.

Development of a semi-nested PCR using degenerate primers for the generic detection of small ruminant lentivirus proviral DNA

A PCR assay was developed for the reliable detection of small ruminant lentivirus (SRLV) proviral DNA. The method involved the use of degenerate deoxyinosine-substituted primers and a second semi-nested PCR step that increased the polyvalency and sensitivity of the detection, respectively. Primers were designed from the pol gene conserved motifs of 85 SRLV isolates and were evaluated using different SRLV isolates together with Maedi-Visna virus (MVV) and caprine arthritis-encephalitis virus (CAEV) reference strains. The method successfully detected SRLV proviral DNA in total DNA extracts originating from whole blood samples, separated peripheral blood mononuclear cells (PBMCs) and tissue cultures. The semi-nested PCR was compared with the agar gel immunodiffusion test and proved to be highly sensitive, specific and capable of detecting many SRLV variants in infected or suspect animals. Therefore, it would be useful in the diagnosis of natural SRLV infections, in eradication programs and epidemiological studies. Whole blood samples can be used directly, thus alleviating the need for PBMC separation, and thereby enables a simple, fast and cost-effective analysis of a large number of samples.

The MHC-haplotype influences primary, but not memory, immune responses to an immunodominant peptide containing T- and B-cell epitopes of the caprine arthritis encephalitis virus Gag protein

In this report, we describe a short peptide, containing a T helper- and a B-cell epitope, located in the Gag protein of the caprine arthritis encephalitis virus (CAEV). This T-cell epitope is capable of inducing a robust T-cell proliferative response in vaccinated goats with different genetic backgrounds and to provide help for a strong antibody response to the B-cell epitope, indicating that it may function as a universal antigen-carrier for goat vaccines. The primary immune response of goats homozygous for MHC class I and II genes showed an MHC-dependent partitioning in rapid-high and slow-low responses, whereas the memory immune response was strong in both groups, demonstrating that a vaccine based on this immunodominant T helper epitope is capable to overcome genetic differences.

First partial characterisation of small ruminant lentiviruses from Greece

Small ruminant lentivirus (SRLV) infections are widespread in Greece, but SRLVs have never been isolated and characterized. In this study, we present the sequence of a 574-nucleotide (191-amino acid) region of the gag gene of SRLV strains from four sheep and one goat from a single geographic area of Greece. All five sequences appeared to be closely related at both nucleotide (2.1-14.2% variation) and deduced amino acid (1.6-4.2% variation) level. Greek SRLV strains were closer to ovine prototypic strains (average divergence 16.8%) than to the caprine strain CAEV-Co (21% divergence). By amino acid composition, the Greek SRLVs were on the average more than twice as distant from CAEV-Co as from other ovine strains. Phylogenetic analysis suggested that Greek strains segregate into a unique group, separate from, but related to, other ovine prototype sequences.

Genetic and antigenic characterization of the matrix protein of two genetically distinct ovine lentiviruses

Small Ruminant Lentiviruses (SRLV) are a group of non-oncogenic retroviruses including Maedi-Visna virus (MVV) and Caprine Arthritis-Encephalitis virus (CAEV), which cause a chronic, multisystemic disease in sheep and goats, respectively. Phylogenetic analyses of SRLV are based in most cases on partial pol sequences. Several reports indicate that the species specificity of these viruses is not as strict as previously thought; MVV-like viruses have been found in goat populations and vice versa. Recently, the sequencing of some Italian ovine isolates has shown the presence of a new cluster more similar to classical caprine isolates (CAEV-like). Few data are available on the variability of structural proteins involved in the antibody response of infected animals. In this study, the gag gene of two genetically distinct ovine isolates, namely the MVV-like It-561 and the CAEV-like It-Pi1, was sequenced and the epitopes of matrix protein (MA) were mapped. Recombinant MAs and their subunits from both ovine aforementioned strains were tested against a panel of sheep and goat sera. Reactive epitopes were found in all three subunits of MA, although the central subunit displayed a more consistent reactivity. Epitope mapping of this subunit demonstrated that the amino acid sequence of at least one immunodominant epitope was quite different in the two strains. This antigenic variability may affect the sensitivity of a single strain-based immunoassay and suggests that both SRLV genotypes should be used in the development of future diagnostic tests, to avoid viral strain selection during the eradication programmes.

Diagnostic tests for small ruminant lentiviruses

Maedi visna virus and caprine arthritis encephalitis virus are closely related retroviruses that cause chronic inflammatory disease in small ruminants. The infections are characterised by insidious onset and slow progression. Diagnosis of infection is usually by serological testing. A variety of assays are available for this purpose, though the relative sensitivity and specificity of these assays has not been compared systematically. Here we review recent developments in laboratory diagnostic methods and their use in field diagnosis. The results suggest that a combination of ELISA and PCR might afford optimal detection of SRLV infection.

Development of recombinant capsid antigen/transmembrane epitope fusion proteins for serological diagnosis of animal lentivirus infections

Among animal lentiviruses, Feline immunodeficiency virus (FIV), Equine infectious anaemia virus (EIAV) and Small ruminant lentiviruses (SRLV) are important pathogens associated with a variety of clinical pictures including immunodeficiency, anaemia, arthritis, pneumonia. The detection of viral antibody response represents a practical diagnostic approach in all lentivirus infections since they remain detectable long life. Capsid antigen (CA) is the major viral core protein and specific antibodies against this antigen are usually first recognised in infected sheep, goat and horse, remaining detectable for long period. Transmembrane (TM) domain of envelope glycoprotein contains a well conserved motif known to form an immunodominant epitope in several lentiviruses. In this study a simple strategy was developed to express the entire CA and the TM epitope in a single fusion protein from equine, feline and small ruminant lentiviruses in prokaryotic system and evaluated the diagnostic utility of a purified preparation in an indirect ELISA for each of the three infections. Results demonstrate that, for FIV and SRLV infections, the combination of CA and TM fractions increases the sensitivity of diagnostic tests based only on CA. The corresponding CA/TM antigen from EIAV showed excellent agreement with Coggins test.

Genetic heterogeneity of small ruminant lentiviruses involves immunodominant epitope of capsid antigen and affects sensitivity of single-strain-based immunoassay

The pol and gag gene fragments of small ruminant lentivirus field isolates collected in the last decade in Italy were amplified, sequenced, and analyzed. Phylogenetic analysis revealed that the majority of ovine isolates form a distinct cluster more similar to caprine lentivirus prototypes than to the visna virus prototype. These findings confirm and extend those reported by Leroux et al. (Arch. Virol., 142:1125-1137, 1997). Moreover, we observed that a variable region of Gag, included in the fragment analyzed, corresponded to one of the three major capsid antigen epitopes, which suggests that the antibody response to this epitope may be type specific. To test this hypothesis, two recombinant peptides, derived from the Icelandic prototype K1514 and this novel genotype, were expressed and used in an enzyme-linked immunosorbent assay to screen a panel of ovine and caprine sera collected from different geographical locations in Italy. Several sera reacted in a type-specific manner, indicating that in a diagnostic setting the combination of at least these two type-specific peptides is necessary to cover a wide range of infections. Additionally, these results support the hypothesis of cross-species transmission based on the phylogenetic analysis described above. This has implications for the control and eradication of small ruminant lentivirus infections.

Early detection of maedi-visna (ovine progressive pneumonia) virus seroconversion in field sheep samples

The aim of this work was to investigate whether an enzyme-linked immunosorbent assay (ELISA) was useful for early detection of maedi-visna virus (MVV) infection in sheep under field conditions. An ELISA based on p25 recombinant protein and a gp46 synthetic peptide was used. Sequentially obtained serum samples (n = 1,941) were studied for 4 years. ELISA results were compared with those of the agar gel immunodiffusion (AGID) test, and results of both tests were compared with a reference result established using consensus scores for at least 2 of 3 serologic techniques (AGID, ELISA, and western blotting, which was used to resolve result discrepancies between the other 2 techniques). A total of 247 discrepancies were observed between ELISA and AGID. Of these, 131 were due to an earlier detection of 120 sera by the ELISA and 11 sera by AGID. The remaining discrepancies (116) were due to the presence of false reactions in both tests. Fewer false-negative results were found by ELISA than with AGID (6 vs. 69 sera, respectively), whereas the number of false-positive results was virtually the same for ELISA and AGID (21 vs. 20, respectively). In relation to the reference result, ELISA sensitivity and specificity were 97.8% and 98.2%, respectively, whereas values for AGID were 76.3% and 98.3%, respectively. The agreement between ELISA and the reference result was higher than that between AGID and the reference result (K value: 0.96 and 0.77, respectively). A variation in the ELISA signal (based on optical density) was observed during the study period, suggesting different antibody levels throughout the animal's life. The ELISA was useful for detecting MVV-infected sheep in field conditions and has potential for use in control and eradication programs.

Detection of antibodies to ovine lentivirus using recombinant capsid and transmembrane proteins

The coding sequences of the capsid protein p25 and transmembrane protein of Maedi-Visna virus were amplified using polymerase chain reaction and cloned into the plasmid expression vector pRSET-B. Both DNA constructs expressed proteins tagged with polyhistidine. The recombinant proteins were purified using Ni-NTA agarose and used in immunoblot to detect antibodies against Maedi-Visna virus. A total of 260 ovine serum specimens was analysed. The total number of p25-positive sera was 111 (42.7%). Higher sensitivity was achieved with rTM antigen, which detected antibodies in 118 (45.4%) sera. The combination of both recombinant proteins as antigens resulted in higher sensitivity of serological detection compared to whole virus antigen.

Comparison of a maedi-visna virus CA-TM fusion protein ELISA with other assays for detecting sheep infected with North American ovine lentivirus strains

A maedi-visna virus CA-TM fusion protein ELISA (MVV ELISA) was evaluated for the detection of antibody in sheep infected with North American ovine lentivirus (OvLV). The results of the MVV ELISA were compared with other assays for OvLV antibody and with viral infection in an intensively studied group of 38 sheep with a high prevalence of OvLV infection and disease. The sensitivity, specificity, and concordance of assays for OvLV antibody (MVV ELISA, indirect ELISA, Western blot, and AGID), virus (virus isolation, PCR, antigen ELISA), and OvLV-induced disease in each animal were compared with OvLV infection status as defined by a positive result in two or more of the assays. Five sheep met the criteria for absence of OvLV infection. The sensitivity of the MVV ELISA in detecting OvLV infected sheep was 64%, whereas the sensitivity of the other three tests for antibody ranged from 85 to 94%. All the antibody assays were 100% specific in this group of animals. Of the assays for virus, the PCR test had the highest sensitivity and the best concordance with OvLV infection, but it also had the lowest specificity of any of the virus or antibody assays. Among the antibody tests, the concordance of the MVV ELISA compared most favorably with the AGID test for detecting OvLV-infected sheep. Analysis of serum samples from 28 lambs experimentally-infected with one of three North American strains of OvLV suggested that there were no significant strain differences detectable by antibody assay. Twenty virus-inoculated lambs were positive by both the MVV ELISA and the AGID test, five lambs were MVV ELISA negative and AGID test positive, and three lambs were MVV ELISA positive and AGID test negative. No pre-inoculation samples were positive by either assay. In a longitudinal study involving seven lambs, antibodies to OvLV were detected by AGID 3-5 weeks post-inoculation, but were not detected by MVV ELISA until 5-10 weeks post-inoculation. Among 128 naturally and experimentally-infected sheep that were seropositive in the AGID test, the overall sensitivity of the MVV ELISA was higher in the naturally infected sheep (84%) than in the experimentally infected sheep (69%). The data indicated that the MVV ELISA represents a less sensitive, but specific alternative for the detection of OvLV antibodies.

A new sensitive serological assay for detection of lentivirus infections in small ruminants

Lentivirus infections in small ruminants represent an economic problem affecting several European countries with important sheep-breeding industries. Programs for control and eradication of these infections are being initiated and require reliable screening assays. This communication describes the construction and evaluation of a new serological screening enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to maedi-visna virus (MVV) in sheep and to caprine arthritis encephalitis virus (CAEV) in goats. The solid phase is sensitized with a combination of the major core protein p25 of MVV produced in Escherichia coli and a peptide derived from the immunodominant region of the viral transmembrane protein gp46. The peptide carries an N-terminal biotin residue and is complexed with streptavidin prior to being coated. The new assay was evaluated with 2,336 sheep serum samples from different European countries with large differences in the levels of prevalence of MVV infections, and the results have been compared to those of the standard agar gel immunodiffusion test. Discrepant samples were analyzed by Western blotting with viral lysate, and most sera could be classified unambiguously. The estimated overall sensitivity of the new ELISA was 99.4% (95% confidence interval [CI], 98.4 to 99. 8%) and the specificity was 99.3% (95% CI, 98.7 to 99.6%). A limited set of goat sera (n = 212) was also analyzed, with similar results. These data indicate that the new assay is a reliable tool that can be used in control and eradication programs for small ruminant lentivirus infections.

Development of a monoclonal antibody blocking-ELISA for detection of antibodies against Maedi-Visna virus

A monoclonal antibody (MAb) blocking ELISA (Blck-ELISA) was developed to detect antibodies against Maedi-Visna virus (MVV) in sheep sera. The assay employs a MAb directed against the envelope protein p90 of the virus in a sandwich blocking procedure. To determine whether the MAb was a potential antibody for developing a Blck-ELISA, a collection of three hundred sera obtained from several sheep flocks known to be infected with MVV were used to examine the sensitivity of the Blck-ELISA. A total of 50 serum samples originating from a flock free of MVV were tested to assess the specificity of the assay. The results were compared with a commercial indirect ELISA (I-ELISA) and samples giving a conflicting or doubtful result were tested by immunoblot. The Blck-ELISA proved to be specific, sensitive and it showed high reproducibility and low variability.

Characterization of the immunodominant cross-reacting epitope of visna maedi virus and caprine arthritis-encephalitis virus capsid antigen

Maedi visna (MV) and caprine arthritis-encephalitis (CAE) are two retroviral infections distributed world wide. Antigenic cross reactions between the viruses have been demonstrated in gag and env encoded structural proteins. Antigens from ovine lentiviruses are easier to produce in cell culture systems and therefore have been used in the development of diagnostic tests for both infections. Antigenically relevant epitopes have been characterised in the transmembrane protein, but little information is available on the immunodominant and cross reacting epitopes in the major capsid antigen (p25). In this study four different recombinant subunits of ovine lentivirus p25 were tested against sera from infected goats and a detailed characterisation of the immunodominant subunit was carried out. Highest ELISA absorbances were obtained with a 29 amino acid subunit located in the N'-terminal half of p25. Through the analysis of overlapping peptides spanning this region we identified a 17 amino acid sequence that can be used in the development of a highly standardized synthetic peptide-based assay.

Maedi-visna virus and caprine arthritis-encephalitis virus: distinct species or quasispecies and its implications for laboratory diagnosis

The lentiviruses responsible for causing maedi-visna or ovine progressive pneumonia in sheep and caprine arthritis-encephalitis in goats have long been considered distinct, albeit related, viral species. Evidence, primarily in the form of nucleic acid sequence data, suggests this distinction may not be as absolute as once thought. These lentiviruses might better be viewed in the context of viral quasispecies whose individual members exhibit varying host range and pathogenic capabilities. Implications for diagnostic testing and control of these diseases are discussed.

Priming with tat-deleted caprine arthritis encephalitis virus (CAEV) proviral DNA or live virus protects goats from challenge with pathogenic CAEV

We previously reported that infection of goats with caprine arthritis encephalitis virus (CAEV) tat- proviral DNA or virus results in persistent infection, since the animals seroconverted and direct virus isolation from cultures of blood-derived macrophages was positive. In this study we wanted to determine whether goats injected with CAEV tat- proviral DNA or virus were protected against challenge with the pathogenic homologous virus and to investigate whether CAEV tat- was still pathogenic. All animals injected with CAEV tat- became infected as indicated by seroconversion and virus isolation. Challenge at 8 or 9 months postinfection demonstrated protection in four of four animals injected with CAEV tat- but did not in three of three mock-inoculated challenged goats. Challenge virus was undetectable in the blood macrophages of protected animals during a period of 6 or 10 months postchallenge. In two of four protected animals, however, we were able to detect the challenge wild-type virus by reverse transcriptase PCR on RNA directly extracted from synovial membrane cells surrounding the inoculation site. This result suggests that protection was achieved without complete sterilizing immunity. Animals injected with CAEV tat- and mock challenged developed inflammatory lesions in the joints, although these lesions were not as severe as those in CAEV wild-type-injected goats. These results confirm the dispensable role of Tat in CAEV replication in vivo for the establishment of infection and pathogenesis and demonstrate in another lentivirus infection model the efficacy of live attenuated viruses to induce resistance to superinfection.

Caracterização molecular parcial do gene gag de amostras do vírus da artrite-encefalite caprina (CAEV) isoladas de animais naturalmente infectados no Rio Grande do Sul, Brasil

Realizou-se a análise de parte do gene gag, que codifica para as proteínas do capsídeo viral, de 5 amostras de CAEV isolados de animais naturalmente infectados do Rio Grande do Sul, Brasil. As amostras foram analisadas por PCR e clivagem com enzimas de restrição (DdeI, HaeIII e NdeI). Fragmentos de aproximadamente 600 pb foram amplificados na PCR e submetidos à digestão enzimática. Os perfis obtidos foram comparados com as seqüências gag de 6 lentivírus de pequenos ruminantes.Os resultados obtidos permitiram separar as amostras em 3 grupos distintos. Os fragmentos observados foram diferentes dos descritos previamente.

PCR detection of lentiviral GAG segment DNA in the white blood cells of sheep and goats

A PCR assay for the detection of small ruminant lentiviral gag DNA (provirus) in the white blood cells of sheep and goats was developed and compared with a serological test (AGIDT). A sample of the DNA prepared from the white blood cells in 3 ml of blood from 208 sheep and goats from 18 different flocks was subjected to PCR assay. One of 85 animals from flocks accredited under the Dutch national MVV/CAEV control programme was positive by PCR while none was positive by AGIDT. In infected flocks, the AGIDT appeared slightly more sensitive, but preliminary results show that the sensitivity of the PCR assay may be further improved by increasing the number of monocytes tested. The PCR assay, however, was clearly more sensitive in detecting animals in the early stages of infection. With the use of a set of mixed primers and probes, the assay was able to detect the variety of CAEV and MVV strains occurring in the field.

Serologic diagnosis of ovine lentiviruses by whole virus ELISA and AGID test

Enzyme-linked immunosorbent assay (ELISA) and agar gel immunodiffusion (AGID) are the most widely used serological tests for Maedi-Visna diagnostics. The purpose of the present study was to develop an indirect whole virus ELISA and an immunodiffusion test and compare their sensitivity. A total of 747 ovine serum specimens were analysed for antibodies against this ovine lentivirus. The number of positive results in the ELISA was 430 (57.56%). In the AGID test, a positive result was found in 380 samples (50.87%). In the group of discordant results 78 (10.4%) samples tested positive by the ELISA and negative by the AGID test and 28 sera (3.7%) were found to be positive by the AGID test and negative by the ELISA. The data in this report show the ELISA to be more sensitive than the AGID test, but accurate serological diagnostics should be based on a combination of the two tests.

North American and French caprine arthritis-encephalitis viruses emerge from ovine maedi-visna viruses

The full extent of genetic diversity among small ruminant lentiviruses (SRLVs), i.e., caprine arthritis encephalitis viruses (CAEVs) and maedi-visna viruses (MVVs), remains unknown. This is due in part to the fact that few sequences of CAEV are available. To contribute to this knowledge, gag, pol, and env nucleotide sequences from an SRLV named CA680 originating from a goat from western France were determined. This analysis revealed that this virus is closely related to the Cork and 63 CAEV American isolates. Mismatched amino acids between the CA680 virus and prototype CAEVs ranged from 6.7, 0. 7, and 17.5% for gag, pol, and SU sequences, respectively. The differences between the CA680 virus and MVV prototypes ranged from 16.5, 12.5, and 32.3% for the protein sequences, respectively. A screening using a heteroduplex mobility assay (HMA) adapted to SRLVs revealed that 6 of 10 caprine virus field isolates were closely related to CA680, indicating that this latter isolate was a prototype of CAEVs common in the west of France. Phylogenetic trees drawn using CA, RT, or SU sequences of numerous SRLVs and rooted with EIAV sequences revealed that CA680 and CAEV prototypes, all infectious for goat, clustered in one group. From these HMA and phylogenetic analyses, it appears that U.S. and French caprine SRLVs form a clade that had emerged from a much more diverse group containing all SRLVs infectious for sheep. These ovine SRLVs form a more ancient group in which the EIAV is rooted.

Application of recombinant bovine viral diarrhea virus proteins in the diagnosis of bovine viral diarrhea infection in cattle

The National Animal Disease Laboratory (NADL) vaccine strain of bovine viral diarrhea virus (BVDV) genes for gp48 and p80 were expressed in Escherichia coli. The BVDV-NADL gene for gp62 was integrated into a baculovirus genome for expression in Spodoptera frugiperda (Sf-9) insect ovarian cells. The antigenicity of baculovirus expressed BVDV protein was detected by anti-BVDV specific antibodies in an enzyme-linked immunosorbent assay (ELISA), indirect immunofluorescent assay (IFA) and radio-immunoprecipitation (RIP). The recombinant proteins isolated from bacteria showed antigenic properties when analyzed by ELISA and immunoblotting using BVDV antibodies. The recombinant proteins were then used in ELISA or IFA to detect BVDV infection by testing 54 independent bovine serum samples. The baculovirus-expressed BVDV protein was used as an ELISA and IFA antigen, and the bacteria-expressed proteins were used as ELISA antigens. BVDV-NADL-infected Madin-Darby bovine kidney (MDBK) cell monolayers served as a control antigen. Statistical analysis showed a high degree of correlation between the reactivity of recombinants and natural antigens in ELISA using bovine sera. The results of ELISA or IFA proved there is a high degree of correlation with the virus neutralization. In the comparative ELISA assays, the insect-cell-mediated expression revealed greater specificity and sensitivity than the bacterial expression or the natural BVDV antigens produced by cell cultures.

Maedi-Visna and ovine progressive pneumonia

Maedi-Visna and ovine progressive pneumonia are disease of sheep that are caused by ovine lentivirus and characterized by chronic inflammation of the lungs, mammary glands, joints, and central nervous system. Although tremendous progress in research has led to a better understanding of the pathogenesis of these diseases, many questions still remain. Much of the mystery is the result of the complexity of the ovine lentivirus genome and the intricate interactions of the virus with the host during replication. Discoveries in molecular virology are shedding light on these interactions and novel approaches to prevent and control lentivirus infections are being explored. There is hope that some of these approaches will eventually be used to eradicate these diseases.

Detection of Maedi-Visna virus antibodies using a single fusion transmembrane-core p25 recombinant protein ELISA and a modified receiver-operating characteristic analysis to determine cut-off values

The core p25 and transmembrane (TM) genes of Maedi-Visna virus (MVV) were cloned individually into the pGEX-2T expression vector. Both proteins were expressed as a combined fusion protein in frame with glutathione S-transferase (GST). The purified recombinant antigens (GST-TM and GST-TM-p25) were used to develop a MVV ELISA. A preliminary assessment of the diagnostic potential of the recombinant antigens (GST-TM and GST-TM-p25) was made by testing the antigens against 46 seropositive and 46 seronegative sheep and comparing the results with a commercial p25 ELISA kit. A two-graph receiver operating characteristic (TG-ROC) analysis program was used to interpret the data. The GST-TM-p25 ELISA was more sensitive than the commercial assay which is based on the p25 antigen alone and more specific than the GST-TM ELISA.

Ovine lentivirus antibody detection in serum, colostrum and milk using a recombinant transmembrane protein ELISA

An enzyme-linked immunosorbent assay (ELISA) was used to detect antibodies against ovine lentivirus (OLV) in serum, colostrum, and milk from naturally infected sheep. The assay used OLV recombinant transmembrane envelope protein (rTM) as a test antigen. Matched serum/colostrum and serum/milk samples were collected at 24h, 4 weeks (mid-lactation), and 8 weeks (weaning) post-lambing. Among 129 paired samples collected at 24 h post-lambing, there was overall test agreement (concordance) of 82.9% and a kappa value of 0.658 between serum and colostrum rTM ELISA results. Among 130 mid-lactation samples, the milk ELISA had 100% specificity and 64.9% sensitivity relative to the serum ELISA, there was concordance of 79.2%, and a kappa value of 0.602. At mid-lactation, the serum agar gel immunodiffusion test had a sensitivity of 0.390 and 0.560 relative to the serum and milk rTM ELISAs, respectively. Matched serum and milk rTM ELISA results at weaning were very similar to those at mid-lactation. Finally, increased occurrence and severity of subclinical mastitis at weaning was found in ELISA-seropositive compared with ELISA-seronegative ewes. Both subclinical mastitis and ewe OLV infection had a negative impact on lamb growth and weaning weights. Compared with blood, colostrum and milk are easier and less expensive to sample and store. These results suggest that rTM ELISA testing of colostrum and milk could be used to supplement serologic testing in OLV screening or eradication programs.

Evaluation of two recombinant Maedi-visna virus proteins for use in an enzyme-linked immunosorbent assay for the detection of serum antibodies to ovine lentiviruses

Defined segments of the gag polyprotein and transmembrane envelope glycoprotein from Maedi-visna virus were expressed as glutathione S-transferase fusion proteins in Escherichia coli and evaluated singly and in combination for use in an enzyme-linked immunosorbent assay (ELISA). Two hundred sixty field serum specimens from 15 sheep flocks were tested in parallel with recombinant and whole-virus antigens, and the relative sensitivities and specificities of the recombinant antigens were calculated. When the recombinant gag and transmembrane proteins were used in combination, a sensitivity of 97.4% and a specificity of 99.4% relative to whole-virus antigen were observed, indicating the utility of these proteins in diagnostic testing.

Comparison of ovine lentivirus detection by conventional and recombinant serological methods

Recombinant (r) transmembrane protein (TM), major capsid protein P25, and matrix protein P16 of ovine lentivirus (OLV) were used as solid phase antigens in enzyme-linked immunosorbent assays (ELISAs) for the detection of specific antibodies against OLV in sheep sera. Sensitivity, specificity, and agreement of these three recombinant assays were compared with each other and with two currently available conventional OLV serological assays, the agar gel immunodiffusion (AGID) test and a whole-virus (WV) ELISA. Field sera from a total of 412 Midwestern United States sheep were tested and compared by the five OLV detection methods, including visibly healthy sheep selected for public sale (Group A, n = 171), samples from a breeding flock of Finnsheep and Finn-cross ewes (Group B, n = 184) and moribund sheep with clinical signs associated with OLV (Group C, n = 57). The rTM ELISA was the most sensitive OLV detection assay, both overall and within each group. Sera from 48.1% (198/412) of field samples were rTM ELISA positive. By contrast, positive rates for the rP25, rP16, and WV ELISAs and AGID test were 34.2%, 32.3%, 36.9%, and 26.9%, respectively. The rTM ELISA reactivity was 36.8% for Group A sera, 50.0% for Group B sera, and 75.4% for Group C sera. Among the 21 Group C sheep possessing OLV lung lesions at necropsy, 20 (95.2%) were rTM ELISA positive. The greatest test agreement occurred between the rP25 and the rP16 ELISAs. The data suggest that the recombinant TM immunoassay is the most accurate and sensitive of the five methods evaluated for the detection of serum anti-OLV antibodies in sheep, both at the subclinical infection and overt clinical disease stages.

Conventional immunoassays underestimate anti-GST antibody titre

This paper highlights problems associated with the quantitation of serum antibody levels to recombinant glutathione S-transferase (GST). Measurement of anti-GST antibodies in conventional immunoassays, where GST is bound directly to the ELISA plate, was found to substantially underestimate the amount of GST-specific antibody levels in test sera. This insensitivity in immunoassay of anti-GST antibodies can be overcome by using any one of several recombinant GST fusion proteins as the coating antigen in ELISA rather than simply GST. Comparison of anti-GST antibody titres assessed by the two procedures indicated that use of unfused GST underestimated the anti-GST antibody titre by more than ten-fold.

Use of rubella virus E1 fusion proteins for detection of rubella virus antibodies

Two glutathione S-transferase fusion proteins containing 44 (p1503) and 75 (p1509) amino acid residues of the rubella virus E1 glycoprotein were expressed in Escherichia coli with the aim of producing a recombinant rubella virus antigen for use in serological assays. p1503 contained three neutralizing and hemagglutinating epitopes (G. M. Terry, L. M. Ho-Terry, P. Londesborough, and K. R. Rees, Arch. Virol. 98:189-197, 1988); p1509 contained the putative neutralization domain described by Mitchell et al. (L. A. Mitchell, T. Zhang, M. Ho, D. Decarie, A. Tingle, M. Zrein, and M. Lacroix, J. Clin. Microbiol. 30:1841-1847, 1992) in addition to the three epitopes present in p1503. Both fusion proteins were soluble and affinity purified on glutathione-Sepharose 4B. In Western blots (immunoblots), p1503 and p1509 reacted with human sera containing rubella virus-specific immunoglobulin G. When used as antigens in indirect enzyme immunoassays to detect rubella virus-specific immunoglobulin G, p1503 correctly identified the rubella virus antibody status of 43 (76.8%) and p1509 correctly identified that of 48 (85.7%) of 56 serum samples received for routine rubella virus antibody screening. The results obtained with p1509 compare well with those obtained with a latex agglutination assay.

An ELISA based on whole virus for the detection of antibodies to small-ruminant lentiviruses

A new ELISA kit was developed, based on highly purified whole-virus antigen derived from the Swiss maedi-visna virus strain OLV. The sensitivity, specificity and accuracy of this assay were compared with that of an established ELISA based on recombinant GAG (group-specific antigens)-GST (glutathione S-transferase) fusion protein expressed in E. coli (GAG-GST ELISA). The whole-virus ELISA exhibits at least comparable specificity (99.3%) but higher sensitivity (98.6 versus 86.3%) and agreement with the 'true' status beyond chance in the detection of antiviral antibodies in serum from goats. Antibodies in milk samples are detected with higher specificity (98.9 versus 97.8%) but lower sensitivity (91.4 versus 98.2%) than in GAG-GST ELISA. The specificity of the new ELISA in the detection of antibodies in serum might be superior, since a set of 40 samples falsely rated positive in GAG-GST ELISA in routine diagnostic work was negative in the new ELISA. In both assays, milk samples can be tested instead of serum, although with slightly reduced sensitivity in the new ELISA. The major advantage of the new test kit is the low number of equivocal samples needing confirmation in a supplementary test. Results obtained with sheep sera indicate that the new ELISA kit is also suitable for the detection of antibodies to maedi-visna virus.

Antibody reactivity to the immunodominant epitopes of the caprine arthritis-encephalitis virus gp38 transmembrane protein associates with the development of arthritis

High titers of antibodies to caprine arthritis-encephalitis virus (CAEV) envelope (Env) glycoproteins are found in infected goats developing a progressive arthritis. In order to identify linear B epitopes of the CAEV Env, which may be involved in the immunopathology of arthritis, we constructed a lambda gt11 Env expression library. By combining library screening with sera from naturally infected Swiss goats with an enzyme immunoassay with overlapping peptides (pepscan), four group-specific epitopes could be precisely defined in the transmembrane envelope proteins: TM1 to TM4, including a conserved structure (TM3) that corresponds to the immunodominant epitope of human immunodeficiency virus type 1 and other lentiviruses. A panel of 190 CAEV naturally infected goat serum samples, obtained from animals with defined clinical status, was tested for reactivity to synthetic peptides corresponding to the TM epitopes in an enzyme-linked immunosorbent assay. Antibody reactivity to two epitopes was highly associated (TM3, P = 0.002, and TM4, P < 0.001) with the presence of clinically detectable arthritis. Such an association is absent for anti-Gag antibody. Antibodies to the immunodominant structures of the TM glycoprotein could thus have an important role in the immunopathogenic process leading to disease.

Detection of antibodies to caprine arthritis-encephalitis virus using recombinant gag proteins

The coding sequences of the core proteins p17 and p28 of caprine arthritis-encephalitis virus (CAEV) were amplified using the polymerase chain reaction and cloned into the plasmid expression vector p-GEX-2T. Both p17 and p28 were expressed as fusion proteins with glutathione S-transferase. The recombinant proteins were affinity purified from induced bacterial lysates using glutathione-agarose beads. The purified proteins were used in an enzyme-linked immunosorbent assay (ELISA) to detect antibodies against CAEV in goat sera and milk samples. Three different ELISA tests were developed based on p17, p28 or the combination of these two recombinant proteins (p17 + p28). A comparison was made to an ELISA based on purified whole virus particles and to agar immunodiffusion test (AGID). Sera with conflicting results in the different ELISA tests were examined by Western blotting. There was a high correlation between the ELISA tests based on p17 + p28 recombinant proteins and whole virus ELISA, with an estimated kappa value of 0.92. Only 72-75% of the sera that tested positive in these two ELISA tests were positive in AGID. Antibodies to CAEV were detected in significantly more animals when serum samples were tested compared to milk samples. Based on the time and materials required to prepare the reagents, the recombinant based ELISA test was less expensive than the whole virus ELISA.

Plasma amino acid dysregulation after lentiviral infection

The absence of AIDS-like symptoms in HIV-infected chimpanzees and SIV-infected African Green monkeys (AGMs) may provide important clues about the pathogenic mechanism of AIDS and about mechanisms of resistance. HIV-infected persons and SIV-infected rhesus macaques have, on the average, markedly decreased cysteine, cystine, and glutathione levels and elevated plasma glutamate concentrations. Glutamate inhibits the membrane transport of cystine and a combination of low plasma glutamate and high cystine levels was found to be correlated with high CD4+ T cell numbers even in HIV-negative healthy human individuals. We have now found that glutamate and cystine levels are also correlated with CD4+ T cell numbers in chimpanzees. But infection of chimpanzees, AGMs, and goats with HIV-1, SIV, and caprine arthritis encephalitis virus (CAEV), respectively, does not induce significant changes in plasma cystine or glutamate levels, although infected AGMs and goats have, on the average, significantly elevated plasma levels of the biochemically related amino acid proline.

Occurrence of caprine leucocyte class I and II antigens in Saanen goats affected by caprine arthritis (CAE)

The distribution of CLA class I and II antigens in CAE virus-infected, diseased and healthy Saanen goats has been investigated. Three class I antigens and two class II antigens showed a statistically different frequency in the two groups.

Application of the polymerase chain reaction (PCR) in veterinary diagnostic virology

The polymerase chain reaction has become an important diagnostic tool for the veterinary virologist. Conventional methods for detecting viral diseases can be laborious or ineffective. In many cases PCR can provide a rapid and accurate test. In this article we explain the basic principles of PCR and supply a reference list of its uses in diagnostic veterinary virology.