Serologic response of horses to the structural proteins of equine arteritis virus

Identification of Equine Arteritis Virus Immunodominant Epitopes Using a Peptide Microarray

Using the commercially available PEPperCHIP® microarray platform, a peptide microarray was developed to identify immunodominant epitopes for the detection of antibodies against Equine arteritis virus (EAV). For this purpose, the whole EAV Bucyrus sequence was used to design a total of 1250 peptides that were synthesized and spotted onto a microarray slide. A panel of 28 serum samples representing a selection of EAV strains was tested using the microarray. Of the 1250 peptides, 97 peptides (7.76%) showed reactivity with the EAV-positive samples. No single peptide was detected by all the positive serum samples. Seven peptides repeatedly showed reactivity above the cut-off and were considered to have diagnostic potential. Five of these peptides were within the immunodominant GP5 protein and two were within the replicase polyprotein regions NSP2 and NSP10, located in ORF1. The diagnostic sensitivity of the seven peptides selected was low, ranging from 5% to 55%; however, the combined diagnostic sensitivity and specificity of the seven peptides was 90% and 100%, respectively. This data demonstrate that multiple peptide sequences would be required to design a comprehensive serological test to cover the diversity of the EAV strains and the individual immune responses of horses.

Equine Arteritis Virus Elicits a Mucosal Antibody Response in the Reproductive Tract of Persistently Infected Stallions

Equine arteritis virus (EAV) has the ability to establish persistent infection in the reproductive tract of the stallion (carrier) and is continuously shed in its semen. We have recently demonstrated that EAV persists within stromal cells and a subset of lymphocytes in the stallion accessory sex glands in the presence of a significant local inflammatory response. In the present study, we demonstrated that EAV elicits a mucosal antibody response in the reproductive tract during persistent infection with homing of plasma cells into accessory sex glands. The EAV-specific immunoglobulin isotypes in seminal plasma included IgA, IgG1, IgG3/5, and IgG4/7. Interestingly, seminal plasma IgG1 and IgG4/7 possessed virus-neutralizing activity, while seminal plasma IgA and IgG3/5 did not. However, virus-neutralizing IgG1 and IgG4/7 in seminal plasma were not effective in preventing viral infectivity. In addition, the serological response was primarily mediated by virus-specific IgM and IgG1, while virus-specific serum IgA, IgG3/5, IgG4/7, and IgG6 isotype responses were not detected. This is the first report characterizing the immunoglobulin isotypes in equine serum and seminal plasma in response to EAV infection. The findings presented herein suggest that while a broader immunoglobulin isotype diversity is elicited in seminal plasma, EAV has the ability to persist in the reproductive tract, in spite of local mucosal antibody and inflammatory responses. This study provides further evidence that EAV employs complex immune evasion mechanisms during persistence in the reproductive tract that warrant further investigation.

Antiviral Innate Immune Response Interferes with the Formation of Replication-Associated Membrane Structures Induced by a Positive-Strand RNA Virus

Infection with nidoviruses like corona- and arteriviruses induces a reticulovesicular network of interconnected endoplasmic reticulum (ER)-derived double-membrane vesicles (DMVs) and other membrane structures. This network is thought to accommodate the viral replication machinery and protect it from innate immune detection. We hypothesized that the innate immune response has tools to counteract the formation of these virus-induced replication organelles in order to inhibit virus replication. Here we have investigated the effect of type I interferon (IFN) treatment on the formation of arterivirus-induced membrane structures. Our approach involved ectopic expression of arterivirus nonstructural proteins nsp2 and nsp3, which induce DMV formation in the absence of other viral triggers of the interferon response, such as replicating viral RNA. Thus, this setup can be used to identify immune effectors that specifically target the (formation of) virus-induced membrane structures. Using large-scale electron microscopy mosaic maps, we found that IFN-β treatment significantly reduced the formation of the membrane structures. Strikingly, we also observed abundant stretches of double-membrane sheets (a proposed intermediate of DMV formation) in IFN-β-treated samples, suggesting the disruption of DMV biogenesis. Three interferon-stimulated gene products, two of which have been reported to target the hepatitis C virus replication structures, were tested for their possible involvement, but none of them affected membrane structure formation. Our study reveals the existence of a previously unknown innate immune mechanism that antagonizes the viral hijacking of host membranes. It also provides a solid basis for further research into the poorly understood interactions between the innate immune system and virus-induced replication structures.

IMPORTANCE

Viruses with a positive-strand RNA genome establish a membrane-associated replication organelle by hijacking and remodeling intracellular host membranes, a process deemed essential for their efficient replication. It is unknown whether the cellular innate immune system can detect and/or inhibit the formation of these membrane structures, which could be an effective mechanism to delay viral RNA replication. In this study, using an expression system that closely mimics the formation of arterivirus replication structures, we show for the first time that IFN-β treatment clearly reduces the amount of induced membrane structures. Moreover, drastic morphological changes were observed among the remaining structures, suggesting that their biogenesis was impaired. Follow-up experiments suggested that host cells contain a hitherto unknown innate antiviral mechanism, which targets this common feature of positive-strand RNA virus replication. Our study provides a strong basis for further research into the interaction of the innate immune system with membranous viral replication organelles.

Conserved arginine residues in the carboxyl terminus of the equine arteritis virus E protein may play a role in heparin binding but may not affect viral infectivity in equine endothelial cells

Equine arteritis virus (EAV), the causative agent of equine viral arteritis, has relatively broad cell tropism in vitro. In horses, EAV primarily replicates in macrophages and endothelial cells of small blood vessels. Until now, neither the cellular receptor(s) nor the mechanism(s) of virus attachment and entry have been determined for this virus. In this study, we investigated the effect of heparin on EAV infection in equine endothelial cells (EECs). Heparin, but not other glycosaminoglycans, could reduce EAV infection up to 93 %. Sequence analysis of the EAV E minor envelope protein revealed a conserved amino acid sequence (52 RSLVARCSRGARYR 65) at the carboxy terminus of the E protein, which was predicted to be the heparin-binding domain. The basic arginine (R) amino acid residues were subsequently mutated to glycine by site-directed mutagenesis of ORF2a in an E protein expression vector and an infectious cDNA clone of EAV. Two single mutations in E (R52G and R57G) did not affect the heparin-binding capability, whereas the E double mutation (R52,60G) completely eliminated the interaction between the E protein and heparin. Although the mutant R52,60G EAV did not bind heparin, the mutations did not completely abolish infectivity, indicating that heparin is not the only critical factor for EAV infection. This also suggested that other viral envelope protein(s) might be involved in attachment through heparin or other cell-surface molecules, and this warrants further investigation.

OCORRÊNCIA DE ANTICORPOS CONTRA O HERPESVÍRUS EQUINO E VIRUS DA ARTERITE EQUINA EM REBANHOS EQUINOS DO ESTADO DO RIO DE JANEIRO

<title>Resumo</title><p>A infecção pelo herpesvírus equino (EHV) e o vírus da arterite equina (EAV) tem sido associada a perdas econômicas importantes para a equideocultura em nível mundial. Estudos sorológicos têm demonstrado a circulação destes agentes no Brasil. O presente trabalho teve como objetivo principal estudar a ocorrência de anticorpos específicos contra o EAV e EHV em cavalos oriundos de diferentes regiões do estado do Rio de Janeiro. Para este propósito, foram coletadas amostras de soro de 581 animais não vacinados e de 44 éguas provenientes de um plantel vacinado regularmente contra a rinopneunonite equina. Todas as amostras foram submetidas ao teste de soroneutralização para a pesquisa de anticorpos específicos contra cada um dos vírus. Os resultados demonstraram 29,6% (172/581) de animais soropositivos para o EHV (títulos entre 2 e ≥ 256) e 0,79% (05/630) para o EAV (títulos entre 2 e 4.096). Considerando a sorologia positiva em animais não imunizados, os achados demonstram a indução de anticorpos específicos após exposição natural aos respectivos vírus, indicando uma possível circulação desses agentes nos rebanhos analisados.</p>

Equine arteritis virus gP5 protein induces apoptosis in cultured insect cells

Equine Arteritis Virus (EAV) has been shown to induce apoptosis in vitro but the induction of this mechanism has not been previously associated with any viral gene product. In this work, we found a cytotoxicity effect of the EAV gP5 protein on baculovirus-insect cells and a low yield of protein recovery. Besides, different morphological features by electron transmission microscopy, DNA fragmentation in agarose gel, TUNEL analysis and caspase 3 activity were found. All these findings indicate that the EAV gP5 protein induces apoptosis in insect cells.

Equine arteritis virus

Equine arteritis virus (EAV) is the causative agent of equine viral arteritis (EVA), a respiratory and reproductive disease of equids. There has been significant recent progress in understanding the molecular biology of EAV and the pathogenesis of its infection in horses. In particular, the use of contemporary genomic techniques, along with the development and reverse genetic manipulation of infectious cDNA clones of several strains of EAV, has generated significant novel information regarding the basic molecular biology of the virus. Therefore, the objective of this review is to summarize current understanding of EAV virion architecture, replication, evolution, molecular epidemiology and genetic variation, pathogenesis including the influence of host genetics on disease susceptibility, host immune response, and potential vaccination and treatment strategies.

Arterivirus molecular biology and pathogenesis

Arteriviruses are positive-stranded RNA viruses that infect mammals. They can cause persistent or asymptomatic infections, but also acute disease associated with a respiratory syndrome, abortion or lethal haemorrhagic fever. During the past two decades, porcine reproductive and respiratory syndrome virus (PRRSV) and, to a lesser extent, equine arteritis virus (EAV) have attracted attention as veterinary pathogens with significant economic impact. Particularly noteworthy were the 'porcine high fever disease' outbreaks in South-East Asia and the emergence of new virulent PRRSV strains in the USA. Recently, the family was expanded with several previously unknown arteriviruses isolated from different African monkey species. At the molecular level, arteriviruses share an intriguing but distant evolutionary relationship with coronaviruses and other members of the order Nidovirales. Nevertheless, several of their characteristics are unique, including virion composition and structure, and the conservation of only a subset of the replicase domains encountered in nidoviruses with larger genomes. During the past 15 years, the advent of reverse genetics systems for EAV and PRRSV has changed and accelerated the structure-function analysis of arterivirus RNA and protein sequences. These systems now also facilitate studies into host immune responses and arterivirus immune evasion and pathogenesis. In this review, we have summarized recent advances in the areas of arterivirus genome expression, RNA and protein functions, virion architecture, virus-host interactions, immunity, and pathogenesis. We have also briefly reviewed the impact of these advances on disease management, the engineering of novel candidate live vaccines and the diagnosis of arterivirus infection.

Cyclophilin inhibitors block arterivirus replication by interfering with viral RNA synthesis

Virus replication strongly depends on cellular factors, in particular, on host proteins. Here we report that the replication of the arteriviruses equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) is strongly affected by low-micromolar concentrations of cyclosporine A (CsA), an inhibitor of members of the cyclophilin (Cyp) family. In infected cells, the expression of a green fluorescent protein (GFP) reporter gene inserted into the PRRSV genome was inhibited with a half-maximal inhibitory concentration (IC(50)) of 5.2 μM, whereas the GFP expression of an EAV-GFP reporter virus was inhibited with an IC(50) of 0.95 μM. Debio-064, a CsA analog that lacks its undesirable immunosuppressive properties, inhibited EAV replication with an IC(50) that was 3-fold lower than that of CsA, whereas PRRSV-GFP replication was inhibited with an IC(50) similar to that of CsA. The addition of 4 μM CsA after infection prevented viral RNA and protein synthesis in EAV-infected cells, and CsA treatment resulted in a 2.5- to 4-log-unit reduction of PRRSV or EAV infectious progeny. A complete block of EAV RNA synthesis was also observed in an in vitro assay using isolated viral replication structures. The small interfering RNA-mediated knockdown of Cyp family members revealed that EAV replication strongly depends on the expression of CypA but not CypB. Furthermore, upon fractionation of intracellular membranes in density gradients, CypA was found to cosediment with membranous EAV replication structures, which could be prevented by CsA treatment. This suggests that CypA is an essential component of the viral RNA-synthesizing machinery.

Development and characterization of an infectious cDNA clone of the modified live virus vaccine strain of equine arteritis virus

A stable full-length cDNA clone of the modified live virus (MLV) vaccine strain of equine arteritis virus (EAV) was developed. RNA transcripts generated from this plasmid (pEAVrMLV) were infectious upon transfection into mammalian cells, and the resultant recombinant virus (rMLV) had 100% nucleotide identity to the parental MLV vaccine strain of EAV. A single silent nucleotide substitution was introduced into the nucleocapsid gene (pEAVrMLVB), enabling the cloned vaccine virus (rMLVB) to be distinguished from parental MLV vaccine as well as other field and laboratory strains of EAV by using an allelic discrimination real-time reverse transcription (RT)-PCR assay. In vitro studies revealed that the cloned vaccine virus rMLVB and the parental MLV vaccine virus had identical growth kinetics and plaque morphologies in equine endothelial cells. In vivo studies confirmed that the cloned vaccine virus was very safe and induced high titers of neutralizing antibodies against EAV in experimentally immunized horses. When challenged with the heterologous EAV KY84 strain, the rMLVB vaccine virus protected immunized horses in regard to reducing the magnitude and duration of viremia and virus shedding but did not suppress the development of signs of EVA, although these were reduced in clinical severity. The vaccine clone pEAVrMLVB could be further manipulated to improve the vaccine efficacy as well as to develop a marker vaccine for serological differentiation of EAV naturally infected from vaccinated animals.

Chimeric viruses containing the N-terminal ectodomains of GP5 and M proteins of porcine reproductive and respiratory syndrome virus do not change the cellular tropism of equine arteritis virus

Equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) are members of family Arteriviridae; they are highly species specific and differ significantly in cellular tropism in cultured cells. In this study we examined the role of the two major envelope proteins (GP5 and M) of EAV and PRRSV in determining their cellular tropism. We generated three viable EAV/PRRSV chimeric viruses by swapping the N-terminal ectodomains of these two proteins from PRRSV IA1107 strain into an infectious cDNA clone of EAV (rMLVB4/5 GP5ecto, rMLVB4/5/6 Mecto and rMLVB4/5/6 GP5&Mecto). The three chimeric viruses could only infect EAV susceptible cell lines but not PRRSV susceptible cells in culture. Therefore, these data unequivocally demonstrate that the ectodomains of GP5 and M are not the major determinants of cellular tropism, further supporting the recent findings that the minor envelope proteins are the critical proteins in mediating cellular tropism (Tian et al., 2012).

Ultrastructural characterization of arterivirus replication structures: reshaping the endoplasmic reticulum to accommodate viral RNA synthesis

Virus-induced membrane structures support the assembly and function of positive-strand RNA virus replication complexes. The replicase proteins of arteriviruses are associated with double-membrane vesicles (DMVs), which were previously proposed to derive from the endoplasmic reticulum (ER). Using electron tomography, we performed an in-depth ultrastructural analysis of cells infected with the prototypic arterivirus equine arteritis virus (EAV). We established that the outer membranes of EAV-induced DMVs are interconnected with each other and with the ER, thus forming a reticulovesicular network (RVN) resembling that previously described for the distantly related severe acute respiratory syndrome (SARS) coronavirus. Despite significant morphological differences, a striking parallel between the two virus groups, and possibly all members of the order Nidovirales, is the accumulation in the DMV interior of double-stranded RNA, the presumed intermediate of viral RNA synthesis. In our electron tomograms, connections between the DMV interior and cytosol could not be unambiguously identified, suggesting that the double-stranded RNA is compartmentalized by the DMV membranes. As a novel approach to visualize and quantify the RNA content of viral replication structures, we explored electron spectroscopic imaging of DMVs, which revealed the presence of phosphorus in amounts equaling on average a few dozen copies of the EAV RNA genome. Finally, our electron tomograms revealed a network of nucleocapsid protein-containing protein tubules that appears to be intertwined with the RVN. This potential intermediate in nucleocapsid formation, which was not observed in coronavirus-infected cells, suggests that arterivirus RNA synthesis and assembly are coordinated in intracellular space.

Characterization of equine humoral antibody response to the nonstructural proteins of equine arteritis virus

Equine arteritis virus (EAV) replicase consists of two polyproteins (pp1a and pp1ab) that are encoded by open reading frames (ORFs) 1a and 1b of the viral genome. These two replicase polyproteins are posttranslationally processed by three ORF 1a-encoded proteinases to yield at least 13 nonstructural proteins (nsp1 to nsp12, including nsp7α and 7β). These nsps are expressed in EAV-infected cells, but the equine immune response they induce has not been studied. Therefore, the primary purpose of this study was to evaluate the humoral immune response of horses to each of the nsps following EAV infection. Individual nsp coding regions were cloned and expressed in both mammalian and bacterial expression systems. Each recombinant protein was used in an immunoprecipitation assay with equine serum samples from horses (n = 3) that were experimentally infected with three different EAV strains (VB, KY77, and KY84), from stallions (n = 4) that were persistently infected with EAV, and from horses (n = 4) that were vaccinated with the modified live-virus (MLV) vaccine strain. Subsequently, protein-antibody complexes were subjected to Western immunoblotting analysis with individual nsp-specific rabbit antisera, mouse anti-His antibody, or anti-FLAG tag antibody. Nsp2, nsp4, nsp5, and nsp12 were immunoprecipitated by most of the sera from experimentally or persistently infected horses, while sera from vaccinated horses did not react with nsp5 and reacted weakly with nsp4. However, serum samples from vaccinated horses were able to immunoprecipitate nsp2 and nsp12 proteins consistently. Information from this study will assist ongoing efforts to develop improved methods for the serologic diagnosis of EAV infection in horses.

Curing of HeLa cells persistently infected with equine arteritis virus by a peptide-conjugated morpholino oligomer

A significant consequence of equine arteritis virus (EAV) infection of horses is persistence of the virus in a variable percentage of infected stallions. We recently established an in vitro model of EAV persistence in cell culture for the purpose of furthering our understanding of EAV biology in general and viral persistence in the stallion in particular. In this study we investigated whether persistently infected HeLa cells could be cured of EAV infection by treatment with an antisense peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) designed to target the 5'-terminal region of the EAV genome. We found that persistently infected HeLa cells passaged three times in the presence of 5-10 microM EAV-specific PPMO produced no detectable virus. The PPMO-cured HeLa cells were free of infectious virus, viral antigen and EAV RNA as measured by plaque assay, indirect immunofluorescence assay and RT-PCR, respectively. Furthermore, when re-challenged with EAV at several passages after discontinuation of PPMO treatments, PPMO-cured HeLa cells were found to be refractory to re-infection and to the re-establishment of viral persistence. While these findings demonstrate that PPMO can be used to eliminate persistent EAV infection in cell culture, the efficacy of PPMO against EAV in vivo remains to be addressed.

Arterivirus Nsp1 modulates the accumulation of minus-strand templates to control the relative abundance of viral mRNAs

The gene expression of plus-strand RNA viruses with a polycistronic genome depends on translation and replication of the genomic mRNA, as well as synthesis of subgenomic (sg) mRNAs. Arteriviruses and coronaviruses, distantly related members of the nidovirus order, employ a unique mechanism of discontinuous minus-strand RNA synthesis to generate subgenome-length templates for the synthesis of a nested set of sg mRNAs. Non-structural protein 1 (nsp1) of the arterivirus equine arteritis virus (EAV), a multifunctional regulator of viral RNA synthesis and virion biogenesis, was previously implicated in controlling the balance between genome replication and sg mRNA synthesis. Here, we employed reverse and forward genetics to gain insight into the multiple regulatory roles of nsp1. Our analysis revealed that the relative abundance of viral mRNAs is tightly controlled by an intricate network of interactions involving all nsp1 subdomains. Distinct nsp1 mutations affected the quantitative balance among viral mRNA species, and our data implicate nsp1 in controlling the accumulation of full-length and subgenome-length minus-strand templates for viral mRNA synthesis. The moderate differential changes in viral mRNA abundance of nsp1 mutants resulted in similarly altered viral protein levels, but progeny virus yields were greatly reduced. Pseudorevertant analysis provided compelling genetic evidence that balanced EAV mRNA accumulation is critical for efficient virus production. This first report on protein-mediated, mRNA-specific control of nidovirus RNA synthesis reveals the existence of an integral control mechanism to fine-tune replication, sg mRNA synthesis, and virus production, and establishes a major role for nsp1 in coordinating the arterivirus replicative cycle.

Plus-strand RNA viruses, a major group of plant and animal pathogens, employ a variety of gene expression strategies. In some groups, the genome is translated into a single polyprotein precursor comprising all viral proteins, while the expression of genomes containing multiple open reading frames commonly depends on the production of additional, subgenomic mRNAs. These serve to translate the open reading frames that are inaccessible to host cell ribosomes engaged in genome translation. Arteriviruses and coronaviruses secure the expression of their structural protein genes by generating an extensive nested set of subgenomic mRNAs, which are copied from a set of complementary minus-strand templates. The production of these subgenome-length minus strands involves a unique mechanism of discontinuous RNA synthesis that essentially competes with the production of the full-length minus strand, the template for genome replication. We describe here that arterivirus non-structural protein 1 (nsp1) modulates the accumulation of minus-strand RNAs to control the relative abundance of both genome-length and subgenomic mRNAs, thereby ensuring efficient production of new virus particles. We found that specific nsp1 mutants with imbalanced mRNA levels and low virus production rapidly acquire additional nsp1 mutations that rescue these defects. Thus, a single arterivirus protein plays a decisive role in the integral control of replication, sg mRNA synthesis, and virus production.

Myristoylation of the arterivirus E protein: the fatty acid modification is not essential for membrane association but contributes significantly to virus infectivity

The envelope of equine arteritis virus (EAV) contains two glycoprotein complexes (GP2b/GP3/GP4 and GP5/M) and the small, non-glycosylated E protein. As E is essential for the production of infectious progeny but dispensable for assembly and release of virus-like particles, it probably mediates virus entry into cells, putatively in concert with the GP2b/GP3/GP4 complex. The E protein contains a central hydrophobic domain and a conserved potential site for N-terminal myristoylation, a hydrophobic modification usually pivotal for membrane targeting of the modified protein. Here, it was shown by radiolabelling that E is myristoylated at glycine-2, both in transfected cells as a fusion protein with yellow fluorescent protein (YFP) and in virus particles. Biochemical fractionation revealed that E–YFP with an inactivated acylation site was still completely membrane-bound, indicating that the putative transmembrane domain of E mediates membrane targeting. Confocal microscopy showed that both myristoylated and non-myristoylated E–YFP were localized to the endoplasmic reticulum and Golgi complex, the membranes from which EAV buds. The presence of a myristoylation inhibitor during replication of EAV, whilst completely blocking E acylation, reduced virus titres by 1.5 log10. Similarly, a mutant EAV with non-myristoylatable E grew to a titre five- to sevenfold lower than that of the wild-type virus and exhibited a reduced plaque size. Western blotting of cell-culture supernatants showed that N and M, the major structural proteins of EAV, are released in similar amounts by cells transfected with wild-type and mutant genomes. Thus, E myristoylation is not required for budding of particles and probably has a function during virus entry.

Identification of an additional neutralization determinant of equine arteritis virus

We recently established an in vitro model of equine arteritis virus (EAV) persistence in HeLa cells. The objective of this study was to determine whether viral variants with novel neutralization phenotypes emerged during persistent EAV infection of HeLa cells, as occurs during viral persistence in carrier stallions. Viruses recovered from persistently infected HeLa cells had different neutralization phenotypes than the virus in the original inoculum, as determined by neutralization assays using EAV-specific monoclonal antibodies and polyclonal equine antisera raised against different strains of EAV. Comparative sequence analyses of the entire structural protein genes (ORFs 2a, 2b, and 3-7) of these viruses, coupled with construction of chimeric viruses utilizing an infectious cDNA clone of EAV, confirmed that the alterations in neutralization phenotype were caused by amino acid changes in the GP5 protein encoded by ORF5. Site-directed mutagenesis studies unequivocally confirmed that amino acid 98 in the GP5 protein was responsible for the altered neutralization phenotype of these viruses. Amino acid 98 in the GP5 protein, which has not previously been identified as a neutralization determinant of EAV, should be included in an expanded neutralization site D (amino acids 98-106).

Persistent equine arteritis virus infection in HeLa cells

The horse-adapted virulent Bucyrus (VB) strain of equine arteritis virus (EAV) established persistent infection in high-passage-number human cervix cells (HeLa-H cells; passages 170 to 221) but not in low-passage-number human cervix cells (HeLa-L cells; passages 95 to 115) or in several other cell lines that were evaluated. However, virus recovered from the 80th passage of the persistently infected HeLa-H cells (HeLa-H-EAVP80) readily established persistent infection in HeLa-L cells. Comparative sequence analysis of the entire genomes of the VB and HeLa-H-EAVP80 viruses identified 16 amino acid substitutions, including 4 in the replicase (nsp1, nsp2, nsp7, and nsp9) and 12 in the structural proteins (E, GP2, GP3, GP4, and GP5). Reverse genetic studies clearly showed that substitutions in the structural proteins but not the replicase were responsible for the establishment of persistent infection in HeLa-L cells by the HeLa-H-EAVP80 virus. It was further demonstrated that recombinant viruses with substitutions in the minor structural proteins E and GP2 or GP3 and GP4 were unable to establish persistent infection in HeLa-L cells but that recombinant viruses with combined substitutions in the E (Ser53-->Cys and Val55-->Ala), GP2 (Leu15-->Ser, Trp31-->Arg, Val87-->Leu, and Ala112-->Thr), GP3 (Ser115-->Gly and Leu135-->Pro), and GP4 (Tyr4-->His and Ile109-->Phe) proteins or with a single point mutation in the GP5 protein (Pro98-->Leu) were able to establish persistent infection in HeLa-L cells. In summary, an in vitro model of EAV persistence in cell culture was established for the first time. This system can provide a valuable model for studying virus-host cell interactions, especially virus-receptor interactions.

Development of a fluorescent-microsphere immunoassay for detection of antibodies specific to equine arteritis virus and comparison with the virus neutralization test

The development and validation of a microsphere immunoassay (MIA) to detect equine antibodies to the major structural proteins of equine arteritis virus (EAV) are described. The assay development process was based on the cloning and expression of genes for full-length individual major structural proteins (GP5 amino acids 1 to 255 [GP5(1-255)], M(1-162), and N(1-110)), as well as partial sequences of these structural proteins (GP5(1-116), GP5(75-112), GP5(55-98), M(88-162), and N(1-69)) that constituted putative antigenic regions. Purified recombinant viral proteins expressed in Escherichia coli were covalently bound to fluorescent polystyrene microspheres and analyzed with the Luminex xMap 100 instrument. Of the eight recombinant proteins, the highest concordance with the virus neutralization test (VNT) results was obtained with the partial GP5(55-98) protein. The MIA was validated by testing a total of 2,500 equine serum samples previously characterized by the VNT. With the use of an optimal median fluorescence intensity cutoff value of 992, the sensitivity and specificity of the assay were 92.6% and 92.9%, respectively. The GP5(55-98) MIA and VNT outcomes correlated significantly (r = 0.84; P < 0.0001). Although the GP5(55-98) MIA is less sensitive than the standard VNT, it has the potential to provide a rapid, convenient, and more economical test for screening equine sera for the presence of antibodies to EAV, with the VNT then being used as a confirmatory assay.

Arterivirus subgenomic mRNA synthesis and virion biogenesis depend on the multifunctional nsp1 autoprotease

Many groups of plus-stranded RNA viruses produce additional, subgenomic mRNAs to regulate the expression of part of their genome. Arteriviruses and coronaviruses (order Nidovirales) are unique among plus-stranded RNA viruses for using a mechanism of discontinuous RNA synthesis to produce a nested set of 5'- and 3'-coterminal subgenomic mRNAs, which serve to express the viral structural protein genes. The discontinuous step presumably occurs during minus-strand synthesis and joins noncontiguous sequences copied from the 3'- and 5'-proximal domains of the genomic template. Nidovirus genome amplification ("replication") and subgenomic mRNA synthesis ("transcription") are driven by 13 to 16 nonstructural proteins (nsp's), generated by autocatalytic processing of two large "replicase" polyproteins. Previously, using a replicon system, the N-terminal nsp1 replicase subunit of the arterivirus equine arteritis virus (EAV) was found to be dispensable for replication but crucial for transcription. Using reverse genetics, we have now addressed the role of nsp1 against the background of the complete EAV life cycle. Mutagenesis revealed that nsp1 is in fact a multifunctional regulatory protein. Its papain-like autoprotease domain releases nsp1 from the replicase polyproteins, a cleavage essential for viral RNA synthesis. Several mutations in the putative N-terminal zinc finger domain of nsp1 selectively abolished transcription, while replication was either not affected or even increased. Other nsp1 mutations did not significantly affect either replication or transcription but still dramatically reduced the production of infectious progeny. Thus, nsp1 is involved in at least three consecutive key processes in the EAV life cycle: replicase polyprotein processing, transcription, and virion biogenesis.

Development and characterization of an infectious cDNA clone of the virulent Bucyrus strain of Equine arteritis virus

Strains of Equine arteritis virus (EAV) differ in the severity of the disease that they induce in horses. Infectious cDNA clones are potentially useful for identification of genetic determinants of EAV virulence; to date, two clones have been derived from a cell culture-adapted variant of the original (Bucyrus) isolate of EAV, and it has previously been shown that recombinant virus derived from one of these (rEAV030) is attenuated in horses. A complete cDNA copy of the genome of the virulent Bucyrus strain of EAV has now been assembled into a plasmid vector. In contrast to rEAV030, recombinant progeny virus derived from this clone caused severe disease in horses, characterized by pyrexia, oedema, leukopenia, high-titre viraemia and substantial nasal shedding of virus. The availability of infectious cDNA clones that produce recombinant viruses of different virulence to horses will facilitate characterization of the virulence determinants of EAV through reverse genetics.

A complex zinc finger controls the enzymatic activities of nidovirus helicases

Nidoviruses (Coronaviridae, Arteriviridae, and Roniviridae) encode a nonstructural protein, called nsp10 in arteriviruses and nsp13 in coronaviruses, that is comprised of a C-terminal superfamily 1 helicase domain and an N-terminal, putative zinc-binding domain (ZBD). Previously, mutations in the equine arteritis virus (EAV) nsp10 ZBD were shown to block arterivirus reproduction by disrupting RNA synthesis and possibly virion biogenesis. Here, we characterized the ATPase and helicase activities of bacterially expressed mutant forms of nsp10 and its human coronavirus 229E ortholog, nsp13, and correlated these in vitro activities with specific virus phenotypes. Replacement of conserved Cys or His residues with Ala proved to be more deleterious than Cys-for-His or His-for-Cys replacements. Furthermore, denaturation-renaturation experiments revealed that, during protein refolding, Zn2+ is essential for the rescue of the enzymatic activities of nidovirus helicases. Taken together, the data strongly support the zinc-binding function of the N-terminal domain of nidovirus helicases. nsp10 ATPase/helicase deficiency resulting from single-residue substitutions in the ZBD or deletion of the entire domain could not be complemented in trans by wild-type ZBD, suggesting a critical function of the ZBD in cis. Consistently, no viral RNA synthesis was detected after transfection of EAV full-length RNAs encoding ATPase/helicase-deficient nsp10 into susceptible cells. In contrast, diverse phenotypes were observed for mutants with enzymatically active nsp10, which in a number of cases correlated with the activities measured in vitro. Collectively, our data suggest that the ZBD is critically involved in nidovirus replication and transcription by modulating the enzymatic activities of the helicase domain and other, yet unknown, mechanisms.

The immune response to equine arteritis virus: potential lessons for other arteriviruses

The members of the family Arteriviridae, genus Arterivirus, include equine arteritis virus (EAV), porcine reproductive and respiratory syndrome virus (PRRSV), lactate dehydrogenase-elevating virus (LDV) of mice, and simian hemorrhagic fever virus (SHFV). PRRSV is the newest member of the family (first isolated in North America and Europe in the early 1990s), whereas the other three viruses were recognized earlier (EAV in 1953, LDV in 1960, and SHFV in 1964). Although arterivirus infections are strictly species-specific, the causative agents share many biological and molecular properties, including their virion morphology, replication strategy, unique properties of their structural proteins, and their ability to establish distinctive persistent infections in their natural hosts. The arteriviruses are each antigenically distinct and cause different disease syndromes in their natural hosts. Similarly, the mechanism(s) responsible for the prolonged and/or persistent infections that characterize infections with each arterivirus in their natural hosts are remarkably different. The objective of this review is to compare and contrast the immune response to EAV with that to the other three arteriviruses, and emphasize the potential relevance of apparent similarities and differences in the neutralization characteristics of each virus.

Characterization of lactate dehydrogenase-elevating virus ORF6 protein expressed by recombinant baculoviruses

Lactate dehydrogenase-elevating virus (LDV) has a strict species-specificity and can replicate only in a subset of mouse primary macrophages in vitro. Because it is difficult to grow and purify sufficient quantities of LDV virions from the primary macrophages, it has been difficult to further characterize LDV envelope proteins. A few expression systems have been reported for structural analysis of the nonglycosylated envelope protein M/VP-2, however, very few studies of the antigenicity of M/VP-2 have been reported. We cloned and expressed the ORF6 gene, which encodes the M/VP-2, as a fusion protein with a polyhistidine metal-binding tag (6 x His-tag) in Autographa californica nuclear polyhedrosis virus (baculovirus) under the control of the polyhedrin promoter. In Western blotting analysis, the expressed protein was similar in size to the native M/VP-2 plus 6 x His-tag. The usefulness of the baculovirus-expressed LDV ORF6 protein for analysis of the immunogenicity of LDV M/VP-2 was discussed.

Characterization of the neutralization determinants of equine arteritis virus using recombinant chimeric viruses and site-specific mutagenesis of an infectious cDNA clone

We have used an infectious cDNA clone of equine arteritis virus (EAV) and reverse genetics technology to further characterize the neutralization determinants in the GP5 envelope glycoprotein of the virus. We generated a panel of 20 recombinant viruses, including 10 chimeric viruses that each contained the ORF5 (which encodes GP5) of different laboratory, field, and vaccine strains of EAV, a chimeric virus containing the N-terminal ectodomain of GP5 of a European strain of porcine reproductive and respiratory syndrome virus, and 9 mutant viruses with site-specific substitutions in their GP5 proteins. The neutralization phenotype of each recombinant chimeric/mutant strain of EAV was determined with EAV-specific monoclonal antibodies and EAV strain-specific polyclonal equine antisera and compared to that of their parental viruses from which the substituted ORF5 was derived. The data unequivocally confirm that the GP5 ectodomain contains critical determinants of EAV neutralization. Furthermore, individual neutralization sites are conformationally interactive, and the interaction of GP5 with the unglycosylated membrane protein M is likely critical to expression of individual epitopes in neutralizing conformation. Substitution of individual amino acids within the GP5 ectodomain usually resulted in differences in neutralization phenotype of the recombinant viruses, analogous to differences in the neutralization phenotype of field strains of EAV and variants generated during persistent infection of EAV carrier stallions.

The serologic response of horses to equine arteritis virus as determined by competitive enzyme-linked immunosorbent assays (c-ELISAs) to structural and non-structural viral proteins

In an effort to further characterize the humoral immune response of horses to equine arteritis virus (EAV), direct and competitive enzyme-linked immunosorbent assays (c-ELISAs) were developed using monoclonal and polyclonal anti-sera to structural (G(L), N and M) and non-structural (nsp1) viral proteins. A nsp1-specific monoclonal antibody was produced to facilitate development of a c-ELISA to this protein. Data obtained using the various c-ELISAs confirm that the M protein is a major target of the antibody response of horses to EAV. However, none of the c-ELISAs that were developed were as sensitive in detecting EAV-specific antibodies in horse sera as the existing serum neutralization test.

Importance of M-protein C terminus as substrate antigen for serodetection of equine arteritis virus infection

Equine arteritis virus (EAV), an enveloped positive-stranded RNA virus, is the prototype of the arterivirus group. In a previous paper (A. Kheyar, S. Martin, G. St.-Laurent, P. J. Timoney, W. H. McCollum, and D. Archambault, Clin. Diagn. Lab. Immunol. 4:648-652, 1997), we have shown that the unglycosylated membrane (M) protein, which is composed of 162 amino acids (aa), is a major target of equine antibody to EAV. In order to determine the antigenic regions of the M protein, the cDNA encoding the M protein of EAV was inserted into the procaryotic expression vector pGEX-4T-1 to produce recombinant glutathione S-transferase-M fusion protein. Various deletion mutant clones, which covered the entire sequence of the M protein, were then generated by inverse PCR and expressed in Escherichia coli to examine, by a Western blot assay, the antigenic reactivity of the clone-derived truncated M proteins with sera from horses either experimentally or naturally infected with EAV. Deletion of the hydrophobic N-terminal 87 aa did not abolish immune reactivity of the protein with serum antibodies to EAV, thereby demonstrating the antigenicity of the C-terminal region (aa 88 to 162) of the M protein. Further truncations of the M-protein C-terminal domain defined particular linear epitope-containing amino acid sequence regions. However, only the M-protein C-terminal region was readily recognized by all EAV-specific horse antisera tested in this study. Based on these findings, only the M-protein C-terminal polypeptide composed of aa 88 to 162 is necessary to identify horse serum antibodies specific to the EAV M protein. Thus, this polypeptide might be useful for serodetection of EAV-infected animals.

Nuclear localization of non-structural protein 1 and nucleocapsid protein of equine arteritis virus

RNA synthesis (genome replication and subgenomic mRNA transcription) directed by equine arteritis virus (EAV; family Arteriviridae, order Nidovirales) occurs on modified cytoplasmic membranes to which most viral replicase subunits localize. Remarkably, a fraction of non-structural protein 1 (nsp1), a protein essential for transcription but dispensable for genome replication, is present in the host cell nucleus, in particular during the earlier stages of infection. Expression of GFP-tagged fusion proteins revealed that nsp1 is actively imported into the nucleus. Although the signals responsible for nsp1 transport could not be identified, our studies revealed that another EAV protein with a partially nuclear localization, the nucleocapsid (N) protein, utilizes the CRM1-mediated nuclear export pathway. Inactivation of this pathway with the drug leptomycin B resulted in the unexpected and immediate nuclear retention of all N protein molecules, thus revealing that the protein shuttles between cytoplasm and nucleus before playing its role in cytoplasmic virus assembly.

Evaluation of a prototype sub-unit vaccine against equine arteritis virus comprising the entire ectodomain of the virus large envelope glycoprotein (G(L)): induction of virus-neutralizing antibody and assessment of protection in ponies

An Escherichia coli-expressed recombinant protein (6hisG(L)ecto) comprising the entire ectodomain (aa 18-122) of equine arteritis virus (EAV) glycoprotein G(L), the immunodominant viral antigen, induced higher neutralizing antibody titres than other G(L)-derived polypeptides when compared in an immunization study in ponies. The potential of the recombinant G(L) ectodomain to act as a sub-unit vaccine against EAV was evaluated further in three groups of four ponies vaccinated with doses of 35, 70 or 140 microg of protein. All vaccinated animals developed a virus-neutralizing antibody (VNAb) response with peak titres 1-2 weeks after the administration of a booster on week 5 (VNAb titres of 1.8-3.1), 13 (VNAb titres of 1.4-2.9) or 53 (VNAb titres of 1.2-2.3). Vaccinated and unvaccinated control ponies were infected with EAV at different times post-vaccination to obtain information about the degree of protection relative to the levels of pre-challenge VNAb. Vaccination conferred varying levels of protection, as indicated by reduced or absent pyrexia, viraemia and virus excretion from the nasopharynx. The degree of protection correlated well with the levels of pre-challenge VNAb and, in particular, with levels of virus excretion. These results provide the first evidence that a sub-unit vaccine protects horses against EAV. The use of the sub-unit vaccine in combination with a differential diagnostic test based on other EAV antigens would enable serological discrimination between naturally infected and vaccinated equines.

Diseases of humans and their domestic mammals: pathogen characteristics, host range and the risk of emergence

Pathogens that can be transmitted between different host species are of fundamental interest and importance from public health, conservation and economic perspectives, yet systematic quantification of these pathogens is lacking. Here, pathogen characteristics, host range and risk factors determining disease emergence were analysed by constructing a database of disease-causing pathogens of humans and domestic mammals. The database consisted of 1415 pathogens causing disease in humans, 616 in livestock and 374 in domestic carnivores. Multihost pathogens were very prevalent among human pathogens (61.6%) and even more so among domestic mammal pathogens (livestock 77.3%, carnivores 90.0%). Pathogens able to infect human, domestic and wildlife hosts contained a similar proportion of disease-causing pathogens for all three host groups. One hundred and ninety-six pathogens were associated with emerging diseases, 175 in humans, 29 in livestock and 12 in domestic carnivores. Across all these groups, helminths and fungi were relatively unlikely to emerge whereas viruses, particularly RNA viruses, were highly likely to emerge. The ability of a pathogen to infect multiple hosts, particularly hosts in other taxonomic orders or wildlife, were also risk factors for emergence in human and livestock pathogens. There is clearly a need to understand the dynamics of infectious diseases in complex multihost communities in order to mitigate disease threats to public health, livestock economies and wildlife.

A zinc finger-containing papain-like protease couples subgenomic mRNA synthesis to genome translation in a positive-stranded RNA virus

The genome expression of positive-stranded RNA viruses starts with translation rather than transcription. For some viruses, the genome is the only viral mRNA and expression is regulated primarily at the translational level and by limited proteolysis of polyproteins. Other virus groups also generate subgenomic mRNAs later in the reproductive cycle. For nidoviruses, subgenomic mRNA synthesis (transcription) is discontinuous and yields a 5' and 3' coterminal nested set of mRNAs. Nidovirus transcription is not essential for genome replication, which relies on the autoprocessing products of two replicase polyproteins that are translated from the genome. We now show that the N-terminal replicase subunit, nonstructural protein 1 (nsp1), of the nidovirus equine arteritis virus is in fact dispensable for replication but crucial for transcription, thereby coupling replicase expression and subgenomic mRNA synthesis in an unprecedented manner. Nsp1 is composed of two papain-like protease domains and a predicted N-terminal zinc finger, which was implicated in transcription by site-directed mutagenesis. The structural integrity of nsp1 is essential, suggesting that the protease domains form a platform for the zinc finger to operate in transcription.

A zinc finger-containing papain-like protease couples subgenomic mRNA synthesis to genome translation in a positive-stranded RNA virus

The genome expression of positive-stranded RNA viruses starts with translation rather than transcription. For some viruses, the genome is the only viral mRNA and expression is regulated primarily at the translational level and by limited proteolysis of polyproteins. Other virus groups also generate subgenomic mRNAs later in the reproductive cycle. For nidoviruses, subgenomic mRNA synthesis (transcription) is discontinuous and yields a 5' and 3' coterminal nested set of mRNAs. Nidovirus transcription is not essential for genome replication, which relies on the autoprocessing products of two replicase polyproteins that are translated from the genome. We now show that the N-terminal replicase subunit, nonstructural protein 1 (nsp1), of the nidovirus equine arteritis virus is in fact dispensable for replication but crucial for transcription, thereby coupling replicase expression and subgenomic mRNA synthesis in an unprecedented manner. Nsp1 is composed of two papain-like protease domains and a predicted N-terminal zinc finger, which was implicated in transcription by site-directed mutagenesis. The structural integrity of nsp1 is essential, suggesting that the protease domains form a platform for the zinc finger to operate in transcription.

Genetic manipulation of arterivirus alternative mRNA leader-body junction sites reveals tight regulation of structural protein expression

To express its structural proteins, the arterivirus Equine arteritis virus (EAV) produces a nested set of six subgenomic (sg) RNA species. These RNA molecules are generated by a mechanism of discontinuous transcription, during which a common leader sequence, representing the 5' end of the genomic RNA, is attached to the bodies of the sg RNAs. The connection between the leader and body parts of an mRNA is formed by a short, conserved sequence element termed the transcription-regulating sequence (TRS), which is present at the 3' end of the leader as well as upstream of each of the structural protein genes. With the exception of RNA3, only one body TRS was previously assumed to be used to join the leader and body of each EAV sg RNA. Here we show that for the synthesis of two other sg RNAs, RNA4 and RNA5, alternative leader-body junction sites that differ substantially in transcriptional activity are used. By site-directed mutagenesis of an EAV infectious cDNA clone, the alternative TRSs used to generate RNA3, -4, and -5 were inactivated, which strongly influenced the corresponding RNA levels and the production of infectious progeny virus. The relative amounts of RNA produced from alternative TRSs differed significantly and corresponded to the relative infectivities of the virus mutants. This strongly suggested that the structural proteins that are expressed from these RNAs are limiting factors during the viral life cycle and that the discontinuous step in sg RNA synthesis is crucial for the regulation of their expression. On the basis of a theoretical analysis of the predicted RNA structure of the 3' end of the EAV genome, we propose that the local secondary RNA structure of the body TRS regions is an important factor in the regulation of the discontinuous step in EAV sg mRNA synthesis.

Expression of the two major envelope proteins of equine arteritis virus as a heterodimer is necessary for induction of neutralizing antibodies in mice immunized with recombinant Venezuelan equine encephalitis virus replicon particles

RNA replicon particles derived from a vaccine strain of Venezuelan equine encephalitis virus (VEE) were used as a vector for expression of the major envelope proteins (G(L) and M) of equine arteritis virus (EAV), both individually and in heterodimer form (G(L)/M). Open reading frame 5 (ORF5) encodes the G(L) protein, which expresses the known neutralizing determinants of EAV (U. B. R. Balasuriya, J. F. Patton, P. V. Rossitto, P. J. Timoney, W. H. McCollum, and N. J. MacLachlan, Virology 232:114-128, 1997). ORF5 and ORF6 (which encodes the M protein) of EAV were cloned into two different VEE replicon vectors that contained either one or two 26S subgenomic mRNA promoters. These replicon RNAs were packaged into VEE replicon particles by VEE capsid protein and glycoproteins supplied in trans in cells that were coelectroporated with replicon and helper RNAs. The immunogenicity of individual replicon particle preparations (pVR21-G(L), pVR21-M, and pVR100-G(L)/M) in BALB/c mice was determined. All mice developed antibodies against the recombinant proteins with which they were immunized, but only the mice inoculated with replicon particles expressing the G(L)/M heterodimer developed antibodies that neutralize EAV. The data further confirmed that authentic posttranslational modification and conformational maturation of the recombinant G(L) protein occur only in the presence of the M protein and that this interaction is necessary for induction of neutralizing antibodies.

The arterivirus replicase is the only viral protein required for genome replication and subgenomic mRNA transcription

Equine arteritis virus (EAV) (ARTERIVIRIDAE:) encodes several structural proteins. Whether any of these also function in viral RNA synthesis is unknown. For the related mouse hepatitis coronavirus (MHV), it has been suggested that the nucleocapsid protein (N) is involved in viral RNA synthesis. As described for MHV, we established that the EAV N protein colocalizes with the viral replication complex, suggesting a role in RNA synthesis. Using an infectious cDNA clone, point mutations and deletions were engineered in the EAV genome to disrupt the expression of each of the structural genes. All structural proteins, including N, were found to be dispensable for genome replication and subgenomic mRNA transcription. We also constructed a mutant in which translation of the intraleader ORF was disrupted. This mutant had a wild-type phenotype, indicating that, at least in cell culture, the product of this ORF does not play a role in the EAV replication cycle.

Monoclonal antibodies directed against conserved epitopes on the nucleocapsid protein and the major envelope glycoprotein of equine arteritis virus

We recently developed a highly effective immunization procedure for the generation of monoclonal antibodies (MAbs) directed against the porcine reproductive and respiratory syndrome virus (E. Weiland, M. Wieczorek-Krohmer, D. Kohl, K. K. Conzelmann, and F. Weiland, Vet. Microbiol. 66:171-186, 1999). The same method was used to produce a panel of 16 MAbs specific for the equine arteritis virus (EAV). Ten MAbs were directed against the EAV nucleocapsid (N) protein, and five MAbs recognized the major viral envelope glycoprotein (G(L)). Two of the EAV G(L)-specific MAbs and one antibody of unknown specificity neutralized virus infectivity. A comparison of the reactivities of the MAbs with 1 U.S. and 22 newly obtained European field isolates of EAV demonstrated that all N-specific MAbs, the three nonneutralizing anti-G(L) MAbs, and the weakest neutralizing MAb (MAb E7/d15-c9) recognized conserved epitopes. In contrast, the two MAbs with the highest neutralization titers bound to 17 of 23 (MAb E6/A3) and 10 of 23 (MAb E7/d15-c1) of the field isolates. Ten of the virus isolates reacted with only one of these two MAbs, indicating that they recognized different epitopes. The G(L)-specific MAbs and the strongly neutralizing MAb of unknown specificity (MAb E6/A3) were used for the selection of neutralization-resistant (NR) virus variants. The observation that the E6/A3-specific NR virus variants were neutralized by MAb E7/d15-c1 and that MAb E6/A3 blocked the infectivity of the E7/d15-c1-specific NR escape mutant confirmed that these antibodies reacted with distinct antigenic sites. Immunoelectron microscopy revealed for the first time that the antigenic determinants recognized by the anti-G(L) MAbs were localized on the virion surface. Surprisingly, although the immunofluorescence signal obtained with the neutralizing antibodies was relatively weak, they mediated binding of about three times as much gold granules to the viral envelope than the nonneutralizing anti-G(L) MAbs.

The predicted metal-binding region of the arterivirus helicase protein is involved in subgenomic mRNA synthesis, genome replication, and virion biogenesis

Equine arteritis virus (EAV), the prototype Arterivirus, is a positive-stranded RNA virus that expresses its replicase in the form of two large polyproteins of 1,727 and 3,175 amino acids. The functional replicase subunits (nonstructural proteins), which drive EAV genome replication and subgenomic mRNA transcription, are generated by extensive proteolytic processing. Subgenomic mRNA transcription involves an unusual discontinuous step and generates the mRNAs for structural protein expression. Previously, the phenotype of mutant EAV030F, which carries a single replicase point mutation (Ser-2429-->Pro), had implicated the nsp10 replicase subunit (51 kDa) in viral RNA synthesis, and in particular in subgenomic mRNA transcription. nsp10 contains an N-terminal (putative) metal-binding domain (MBD), located just upstream of the Ser-2429-->Pro mutation, and a helicase activity in its C-terminal part. We have now analyzed the N-terminal domain of nsp10 in considerable detail. A total of 38 mutants, most of them carrying specific single point mutations, were tested in the context of an EAV infectious cDNA clone. Variable effects on viral genome replication and subgenomic mRNA transcription were observed. In general, our results indicated that the MBD region, and in particular a set of 13 conserved Cys and His residues that are assumed to be involved in zinc binding, is essential for viral RNA synthesis. On the basis of these data and comparative sequence analyses, we postulate that the MBD may employ a rather unusual mode of zinc binding that could result in the association of up to four zinc cations with this domain. The region containing residue Ser-2429 may play the role of "hinge spacer," which connects the MBD to the rest of nsp10. Several mutations in this region specifically affected subgenomic mRNA synthesis. Furthermore, one of the MBD mutants was replication and transcription competent but did not produce infectious progeny virus. This suggests that nsp10 is involved in an as yet unidentified step of virion biogenesis.

Arterivirus discontinuous mRNA transcription is guided by base pairing between sense and antisense transcription-regulating sequences

To generate an extensive set of subgenomic (sg) mRNAs, nidoviruses (arteriviruses and coronaviruses) use a mechanism of discontinuous transcription. During this process, mRNAs are generated that represent the genomic 5' sequence, the so-called leader RNA, fused at specific positions to different 3' regions of the genome. The fusion of the leader to the mRNA bodies occurs at a short, conserved sequence element, the transcription-regulating sequence (TRS), which precedes every transcription unit in the genome and is also present at the 3' end of the leader sequence. Here, we have used site-directed mutagenesis of the infectious cDNA clone of the arterivirus equine arteritis virus to show that sg mRNA synthesis requires a base-pairing interaction between the leader TRS and the complement of a body TRS in the viral negative strand. Mutagenesis of the body TRS of equine arteritis virus RNA7 reduced sg RNA7 transcription severely or abolished it completely. Mutations in the leader TRS dramatically influenced the synthesis of all sg mRNAs. The construction of double mutants in which a mutant leader TRS was combined with the corresponding mutant RNA7 body TRS resulted in the specific restoration of mRNA7 synthesis. The analysis of the mRNA leader-body junctions of a number of mutants with partial transcriptional activity provided support for a mechanism of discontinuous minus-strand transcription that resembles similarity-assisted, copy-choice RNA recombination.

Detection of antibodies to equine arteritis virus by enzyme linked immunosorbant assays utilizing G(L), M and N proteins expressed from recombinant baculoviruses

Indirect enzyme linked immunosorbant assays (ELISAs) utilizing the three major structural proteins (M, N, and G(L)) of equine arteritis virus (EAV) expressed from recombinant baculoviruses were developed. A large panel of sera collected from uninfected horses, and from animals experimentally and naturally infected with EAV or vaccinated with the modified live virus vaccine against equine viral arteritis, were used to characterize the humoral immune response of horses to the three major EAV structural proteins. The data suggest that the M protein was the major target of the equine antibody response to EAV. The responses of individual animals varied and ELISAs that utilized individual EAV structural proteins were not reliable for detecting antibodies in all sera that contained neutralizing antibodies to EAV. An ELISA based on a cocktail of all three EAV structural proteins, however, was used successfully to detect antibodies in most equine sera that were positive in the standard serum neutralization assay following natural or experimental EAV infection (100% specificity, 92.3% sensitivity). In contrast, this ELISA did not reliably detect antibodies in the sera of vaccinated horses. EAV frequently causes a persistent infection in stallions and all sera from carrier stallions evaluated in this study had obvious reactivity with the N protein, whereas seropositive non-carrier stallions, mares and geldings did not respond consistently to the N protein.