Comparison of the structural protein coding sequences of the VR-2332 and Lelystad virus strains of the PRRS virus

Comparison of molecular and biological characteristics of a modified live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine (ingelvac PRRS MLV), the parent strain of the vaccine (ATCC VR2332), ATCC VR2385, and two recent field isolates of PRRSV

The objectives of this study were to compare the molecular and biological characteristics of recent porcine reproductive and respiratory syndrome virus (PRRSV) field isolates to those of a modified live virus (MLV) PRRS vaccine and its parent strain. One hundred seventeen, 4-week-old pigs were randomly assigned to six groups. Group 1 (n = 20) served as sham-inoculated negative controls, group 2 (n = 19) was inoculated with Ingelvac PRRS MLV vaccine, group 3 (n = 20) was inoculated with the parent strain of the vaccine (ATCC VR2332), group 4 (n = 19) was inoculated with vaccine-like PRRSV field isolate 98-38803, group 5 (n = 19) was inoculated with PRRSV field isolate 98-37120, and group 6 (n = 20) was inoculated with known high-virulence PRRSV isolate ATCC VR2385. The levels of severity of gross lung lesions (0 to 100%) among the groups were significantly different at both 10 (P < 0.0001) and 28 days postinoculation (p.i.) (P = 0.002). At 10 days p.i., VR2332 (26.5% +/- 4.64%) and VR2385 (36.4% +/- 6.51%) induced gross lesions of significantly greater severity than 98-38803 (0.0% +/- 0.0%), 98-37120 (0.8% +/- 0.42%), Ingelvac PRRS MLV (0.9% +/- 0.46%), and negative controls (2.3% +/- 1.26%). At 28 days p.i., 98-37120 (17.2% +/- 6.51%) induced gross lesions of significantly greater severity than any of the other viruses. Analyses of the microscopic-interstitial-pneumonia-lesion scores (0 to 6) revealed that VR2332 (2.9 +/- 0.23) and VR2385 (3.1 +/- 0.35) induced significantly more severe lesions at 10 days p.i. At 28 days p.i., VR2385 (2.5 +/- 0.27), VR2332 (2.3 +/- 0.21), 98-38803 (2.6 +/- 0.29), and 98-37120 (3.0 +/- 0.41) induced significantly more severe lesions than Ingelvac PRRS MLV (0.7 +/- 0.17) and controls (0.7 +/- 0.15). The molecular analyses and biological characterizations suggest that the vaccine-like isolate 98-38803 (99.5% amino acid homology based on the ORF5 gene) induces microscopic pneumonia lesions similar in type to, but different in severity and time of onset from, those observed with virulent strains VR2385 and the parent strain of the vaccine. Our data strongly suggest that isolate 98-38803 is a derivative of Ingelvac PRRS MLV and that the isolate is pneumovirulent.

Chimeric arteriviruses generated by swapping of the M protein ectodomain rule out a role of this domain in viral targeting

Arteriviruses are enveloped, positive-strand RNA viruses for which the two major envelope proteins GP(5) and M occur as disulfide-linked heterodimers. These were assumed to serve the viral targeting functions, but recent ectodomain swapping studies with equine arteritis virus (EAV) indicate that the GP(5) protein does not determine arteriviral tropism. Here, we focused on the short, 13- to 18-residue ectodomain of the M protein. Using an infectious cDNA clone of the Lelystad virus isolate of porcine reproductive and respiratory syndrome virus (PRRSV), we substituted the genomic sequence encoding the M ectodomain by that of murine lactate dehydrogenase-elevating virus, EAV, and the US PRRSV-isolate, VR2332. Viable viruses with a chimeric M protein were obtained in all three cases, but for the latter two only after removal of the genomic overlap between the M and GP(5) genes. Characterization of the chimeric viruses revealed that they could be distinguished immunologically from wild-type virus, that they were genetically stable in vitro, but that they were impaired in their growth, reaching lower titers than the parental virus. The latter appeared to be due to an increased particle-to-infectivity ratio of the chimeric virus particles. Interestingly, the chimeric viruses had retained their ability to infect porcine cells and had not acquired tropism for cells susceptible to the viruses from which the foreign ectodomains were derived. We conclude that the surface structures composed by the arterivirus M and GP(5) ectodomains do not determine viral tropism.

Construction and immunogenicity of recombinant Mycobacterium bovis BCG expressing GP5 and M protein of porcine reproductive respiratory syndrome virus

Mycobacterium bovis BCG was used to express a truncated form of GP5 (lacking the first 30 NH(2)-terminal residues) and M protein of porcine reproductive and respiratory syndrome virus (PRRSV). The PRRSV proteins were expressed in BCG under control of the mycobacterial hsp60 gene promoter either in the mycobacterial cytoplasm (BCGGP5cyt and BCGMcyt) or as MT19-fusion proteins on the mycobacterial surface (BCGGP5surf and BCGMsurf). Mice inoculated with BCGGP5surf and BCGMsurf developed antibodies against the viral proteins at 30 days post-inoculation (dpi) as detected by ELISA and Western blot. By 60 dpi, the animals developed titer of neutralizing antibodies of 8. A PRRSV-specific gamma interferon response was also detected in splenocytes of recombinant BCG-inoculated mice at 60 and 90 dpi. These results indicate that BCG was able to express antigens of PRRSV and elicit an immune response against the viral proteins in mice.

Mechanical transmission of porcine reproductive and respiratory syndrome virus throughout a coordinated sequence of events during cold weather

Using a field-based model, mechanical transmission of porcine reproductive and respiratory syndrome virus (PRRSV) wa assessed throughout a coordinated sequence of events that replicated common farm worker behavior during cold weather (< 0 degrees C). The model involved fomites (boots and containers), vehicle sanitation, transport, and the movement of personnel. A field strain of PRRSV was inoculated into carriers consisting of snow and water, and carriers were adhered to the undercarriage of a vehicle. The vehicle was driven approximately 50 km to a commercial truck washing facility where the driver's boots contacted the carriers during washing, introducing the virus to the vehicle interior. The vehicle was then driven 50 km to a simulated farm site, and the driver's boots mechanically spread virus into the farm anteroom. Types of containers frequently employed in swine farms (styrofoam semen cooler, metal toolbox, plastic lunch pail, and cardboard animal health product shipping parcel) contacted drippings from footwear on the anteroom floor. The truck wash floor, vehicle cab floor mats, boot soles, anteroom floor, and the ventral surface of containers were sampled to track the virus throughout the model. Ten replicates were conducted, along with sham-inoculated controls. At multiple sampling points PRRSV nucleic acid was detected in 8 of 10 replicates. In each of the 8 PCR-positive replicates, infectious PRRSV was detected on the surfaces of containers by virus isolation or swine bioassay. All sham-inoculated controls were negative. These results indicate that mechanical transmission of PRRSV can occur during coordinated sequence of events in cold weather.

Phosphorylation of the porcine reproductive and respiratory syndrome virus nucleocapsid protein

Porcine reproductive and respiratory syndrome virus (PRRSV) is a cytoplasmic RNA virus with the unique or unusual feature of having a nucleocapsid (N) protein that is specifically transported to the nucleolus of virus-infected cells. In this communication, we show that the N protein is a phosphoprotein. Phosphoamino acid analysis of authentic and recombinant N proteins demonstrated that serine residues were exclusively phosphorylated. The pattern of phosphorylated N protein cellular distribution in comparison with that of [(35)S]methionine-labeled N protein suggested that phosphorylation does not influence subcellular localization of the protein. Time course studies showed that phosphorylation occurred during, or shortly after, synthesis of the N protein and that the protein remained stably phosphorylated throughout the life cycle of the virus to the extent that phosphorylated N protein was found in the mature virion. Two-dimensional electrophoresis and acid-urea gel electrophoresis showed that one species of the N protein is predominant in virus-infected cells, suggesting that multiple phosphorylated isoforms of N do not exist.

Identification of radically different variants of porcine reproductive and respiratory syndrome virus in Eastern Europe: towards a common ancestor for European and American viruses

We determined 22 partial porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 sequences, representing pathogenic field strains mainly from Poland and Lithuania, and two currently available European-type live PRRSV vaccines. Also, the complete ORF7 of two Lithuanian and two Polish strains was sequenced. We found that Polish, and in particular Lithuanian, PRRSV sequences were exceptionally different from the European prototype, the Lelystad virus, and in addition showed a very high national diversity. The most diverse present-day European-type PRRSV sequences were from Poland (2000) and Lithuania (2000), and exhibited only 72.2% nucleotide identity in the investigated ORF5 sequence. While all sequences determined in the present study were clearly of European type, inclusion of the new Lithuanian sequences in the genealogy resulted in a common ancestor for the European type virus significantly closer to the American-type PRRSV than previously seen. In addition, the length of the ORF7 of the Lithuanian strains was 378 nucleotides, and thus intermediate between the sizes of the prototypical EU-type (387 nucleotides) and US-type (372 nucleotides) ORF7 lengths. These findings for the Lithuanian PRRSV sequences provide support for the hypothesis that the EU and US genotypes of PRRSV evolved from a common ancestor. Also, this is the first report of ORF7 protein size polymorphism in field isolates of EU-type PRRSV.

Functional properties of the predicted helicase of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a member of the positive-strand RNA virus family Arteriviridae. Although considerable research has focused on this important pathogen, little is known about the function of most PRRSV proteins. To examine characteristics of putative nonstructural proteins (nsp) encoded in ORF1b, which have been identified by nucleotide similarity to domains of equine arteritis virus, defined genomic regions were cloned and expressed in the pRSET expression system. One region, nsp10, encoded a protein with a putative helicase domain and was further examined for functional helicase-like activities. PRRSV nsp10 was found to possess a thermolabile and pH-sensitive NTPase activity that was modulated by polynucleotides and to unwind dsRNA in a 5' to 3' polarity. These results provide the first evidence of the functional properties of PRRSV helicase and further support the finding that nidovirus helicases possess properties that distinguish them from other viral helicases.

Mechanical transmission of porcine reproductive and respiratory syndrome virus by mosquitoes, Aedes vexans (Meigen)

The objective of this study was to determine whether porcine reproductive and respiratory syndrome virus (PRRSV) could be transmitted to naive pigs by mosquitoes following feeding on infected pigs. During each of 4 replicates, mosquito-to-pig contact took place on days 5, 6, and 7 after PRRSV infection of the donor pig. A total of 300 mosquitoes [Aedes vexans (Meigen)] were allowed to feed on each viremic donor pig, housed in an isolation room. After 30 to 60 s, feeding was interrupted, and the mosquitoes were manually transferred in small plastic vials and allowed to feed to repletion on a naïve recipient pig housed in another isolation room. Prior to contact with the recipient pig, the mosquitoes were transferred to clean vials. Swabs were collected from the exterior surface of all vials, pooled, and tested for PRRSV. Separate personnel handled the donor pig, the recipient pig, and the vial-transfer procedure. Transmission of PRRSV from the donor to the recipient pig occurred in 2 out of 4 replicates. The PRRSV isolated from the infected recipient pigs was nucleic-acid-sequenced and found to be 100% homologous with the virus used to infect the donor pigs. Homogenates of mosquito tissues collected in all replicates were positive by either polymerase chain reaction or swine bioassay. All control pigs remained PRRSV negative, and PRRSV was not detected on the surface of the vials. This study indicates that mosquitoes (A. vexans) can serve as mechanical vectors of PRRSV.

Assessing the duration of persistence and shedding of porcine reproductive and respiratory syndrome virus in a large population of breeding-age gilts

Porcine reproductive and respiratory syndrome virus (PRRSV) is an RNA virus in the order Nidovirales, family Arteriviridae, genus Arterivirus. The virus induces a prolonged viremia, replicates in macrophages, and produces persistent infection. The purpose of this study was to determine if PRRSV could persist for 90 d or more in a large population of breeding-age gilts housed under environmental conditions typical of commercial swine production and to determine if experimentally infected gilts could shed virus to naive sentinel gilts beyond 90 d postinfection. Using the intranasal route, we inoculated 120 PRRSV-naïve gilts, 4 mo of age, with 5 mL of cell culture fluid containing a total dose of 10(2.4) TCID50 of a field isolate (MN-30100) of PRRSV. The index gilts were organized into 3 groups (A, B, and C), 40 gilts per group. To assess the dynamics of the experimental infection, a monitor group of 30 index gilts was blood-tested on days 0, 3, 7, 14, 30, 60, 90, 120, 150, and 180 postinfection. PRRSV viremia was detected with the polymerase chain reaction (PCR) on days 3, 7, and 14 and by virus isolation (VI) on days 7 and 14. PRRSV antibodies were detected from day 14 by enzyme-linked immunosorbent assay (ELISA). To assess shedding, 30 PRRSV-naïve sentinel gilts were commingled with the index gilts on day 90 postinfection and tested by PCR, VI, and ELISA every 15 d until 180 d postinfection; all samples were negative. To assess persistence, 40 index and 10 sentinel gilts were slaughtered at 120 (group A), 150 (group B), or 180 (group C) d postinfection. Evidence of PRRSV was not detected by PCR or VI in any tissue samples from the 120 index gilts. These results indicate that persistence and shedding of PRRSV are of short duration in breeding-age gilts.

Evaluation of aerosol transmission of porcine reproductive and respiratory syndrome virus under controlled field conditions

The aim of this study was to determine whether porcine reproductive and respiratory syndrome virus (PRRSV) could be transmitted by aerosol under field conditions. A total of 210 five-month-old PRRSV-negative pigs were housed in a mechanically ventilated finishing facility containing 11 pens. Pen 1 contained 10 pigs (indirect contact controls) and pen 2 remained empty, providing a barrier of 2.5 m from the remaining pigs in pens 3 to 11. Fifteen or 16 of the pigs in each of pens 3 to 11 were infected experimentally with a field isolate of PRRSV and the other six or seven pigs served as direct contact controls. Five days after the pigs were infected, two trailers containing 10 five-week-old PRRSV-naive sentinel pigs were placed along each side of the building; one was placed 1 m from the exhaust fans on one side of the building, and the other was placed 30 m from the fans on the other side, and the sentinel pigs remained in the trailers for 72 hours. They were then moved to separate buildings on the same site, 30 and 80 m, respectively, from the infected barn, and their PRRSV status was monitored for 21 days. The direct and indirect contact control pigs became infected with PRRSV but the sentinel pigs did not.

Evolution of porcine reproductive and respiratory syndrome virus during sequential passages in pigs

Porcine reproductive and respiratory syndrome (PRRS) viruses are recognized as possessing a high degree of genetic and antigenic variability. Viral diversity has led to questions regarding the association of virus mutation and persistent infection in the host and has raised concerns vis-à-vis protective immunity, the ability of diagnostic assays to detect novel variants, and the possible emergence of virulent strains. The purpose of this study was to describe ongoing changes in PRRS virus during replication in pigs under experimental conditions. Animals were inoculated with a plaque-cloned virus derived from VR-2332, the North American PRRS virus prototype. Three independent lines of in vivo replication were maintained for 367 days by pig-to-pig passage of virus at 60-day intervals. A total of 315 plaque-cloned viruses were recovered from 21 pigs over the 367-day observation period and compared to the original plaque-cloned virus by virus neutralization assay, monoclonal antibody analysis, and sequencing of open reading frames (ORFs) 1b (replicase), 5 (major envelope protein), and 7 (nucleocapsid) of the genome. Variants were detected by day 7 postinoculation, and multiple variants were present concurrently in every pig sampled over the observation period. Sequence analysis showed ORFs 1b and 7 to be highly conserved. In contrast, sequencing of ORF 5 disclosed 48 nucleotide variants which corresponded to 22 amino acid variants. Although no epitopic changes were detected under the conditions of this experiment, PRRS virus was shown to evolve continuously in infected pigs, with different genes of the viral genome undergoing various degrees of change.

Characterization of regions in the GP5 protein of porcine reproductive and respiratory syndrome virus required to induce apoptotic cell death

Expression of the GP5 protein of porcine reproductive and respiratory syndrome virus in mammalian cells using a recombinant vaccinia virus has been shown to induce strong cytotoxicity due to apoptotic death. We have now developed a transient expression system that allows the observation and quantitation of the cell death due to GP5 synthesis, taking advantage of the reduction that this protein induces in the expression of two different co-transfected reporter genes. In this way, we are able to study the regions in GP5 implicated in apoptosis induction. The first 119 aminoacids constitute a region capable of fully inducing apoptosis, aminoacids 90-119 having a fundamental role. On the contrary, the C-terminal region is unable by itself of cell death induction and, moreover, is dispensable for this phenotype. We have also observed that induction of apoptosis is independent of cleavage of the N-terminal putative signal sequence in GP5 or N-glycosylation of this protein.

Genetic variation and phylogenetic analyses of the ORF5 gene of acute porcine reproductive and respiratory syndrome virus isolates

Swine herds in the US have experienced recent outbreaks of a severe form of porcine reproductive and respiratory syndrome (designated acute or atypical PRRS) characterized by abortion and high mortality in pregnant sows. Most of the affected herds had been vaccinated with modified live-vaccines (MLVs) against PRRS. To explore the possible mechanism of the emergence of acute PRRS, the open reading frame 5 (ORF5) gene encoding the major envelope protein (GP5) of acute PRRSV isolates was characterized. The complete ORF5 gene of eight acute PRRSV isolates from herds experiencing acute PRRS outbreaks in Iowa and North Carolina was amplified and sequenced. Sequence analyses revealed that these acute PRRSV isolates shared 88-95% nucleotide and 88-96% amino acid sequence identities to each other, 87-97% nucleotide and 84-96% amino acid sequence identities with other North American PRRSV isolates and the MLVs. Most of the amino acid substitutions locate in the putative signal sequence and two short hypervariable regions at the amino terminus. The ORF5 gene sequence of the acute PRRSV isolate 98-37120-2 from a non-vaccinated swine herd in Iowa is very closely related to that of the RespPRRS MLV, with 97% nucleotide and 96% amino acid sequence identities. Phylogenetic analysis revealed that all eight acute PRRSV isolates are clustered within the North American genotype. Several minor branches that are not associated with geographic origins were also identified within the North American genotype. One acute PRRSV isolate (98-37120-2) is clustered with the RespPRRS MLV and several Danish isolates that were confirmed to be derived from the RespPRRS MLV. The ORF5 gene sequences of other seven acute isolates are more related to those of several earlier PRRSV isolates and the PrimePac MLV than to that of the RespPRRS MLV. Our results showed that the acute PRRSV isolates analyzed in this study differed from each other in ORF5 genes, although they all clustered within the North American genotype. The data from this study do not fully support the hypothesis that the emergence of acute PRRS is due to reversion of MLVs to a pathogenic phenotype, as only one of the eight acute isolates was shown to be very closely related to the RespPRRS MLV.

Viable porcine arteriviruses with deletions proximal to the 3' end of the genome

In order to obtain attenuated live vaccine candidates of porcine reproductive and respiratory syndrome virus (PRRSV), a series of deletions was introduced at the 3' end of the viral genome using an infectious cDNA clone of the Lelystad virus isolate. RNA transcripts from the full-length cDNA clones were transfected into BHK-21 cells. The culture supernatant of these cells was subsequently used to infect porcine alveolar macrophages to detect the production of progeny virus. It is shown that C-terminal truncation of the nucleocapsid (N) protein, encoded by ORF7, was tolerated for up to six amino acids without blocking the production of infectious virus. Mutants containing larger deletions produced neither virus nor virus-like particles containing viral RNA. Deletion analysis of the 3' UTR immediately downstream of ORF7 showed that infectious virus was still produced after removal of seven nucleotides behind the stop codon of ORF7. Deletion of 32 nucleotides in this region abolished RNA replication and, consequently, no infectious virus was formed. Serial passage on porcine alveolar macrophages demonstrated that the viable deletion mutants were genetically stable at the site of mutation. In addition, the deletions did not affect the growth properties of the recombinant viruses in vitro, while their antigenic profiles were similar to that of wild-type virus. Immunoprecipitation experiments with the six-residue N protein-deletion mutant confirmed that the truncated protein was indeed smaller than the wild-type N protein. The deletion mutants produced in this study are interesting candidate vaccines to prevent PRRS disease in pigs.

A molecular clock dates the common ancestor of European-type porcine reproductive and respiratory syndrome virus at more than 10 years before the emergence of disease

The disease caused by porcine reproductive and respiratory syndrome virus (PRRSV) emerged independently and almost simultaneously in Europe (1990) and North America (1987). The original reservoir of the virus and the date it entered the pig populations is not known. In this study, we demonstrate an accurate molecular clock for the European PRRSV ORF 3 gene, place the root in the genealogy, estimate the rate of nucleotide substitution, and date the most recent common viral ancestor of the data set to 1979; more than 10 years before the onset of the European epidemic. Based on these findings, we conclude that PRRSV virus most likely entered the pig population some time before the epidemic emergence of the virus, and hence, that emergence of European-type PRRSV is not the result of a recent species transmission event. Together, our results show that ORF3 sequencing is a valuable epidemiologic tool for examining the emergence and spread of PRRSV in Europe. As such, the panel of well-characterized and highly divergent ORF3 sequences described in this study provides a reference point for future molecular epidemiologic studies.

Quantitative TaqMan RT-PCR for the detection and differentiation of European and North American strains of porcine reproductive and respiratory syndrome virus

Since two different types of porcine reproductive and respiratory syndrome virus (PRRSV), the European (EU) and the North American (US) strain, occur or coexist in European swine herds, their rapid and reliable detection and differentiation is essential for disease surveillance. A quantitative TaqMan reverse transcription-polymerase chain reaction (RT-PCR) is described for PRRSV detection and strain differentiation. Sensitivity and specificity were compared with a conventional PRRSV RT-PCR and to the detection of both PRRSV types in cell cultures and both were found to be equal or superior to the reference methods. Reproducibility was tested and proved that the assay was very reliable. Standard dilutions included in each test allowed absolute quantitation of the amount of viral RNA. The TaqMan assay described below is time-saving, easy to handle, exhibits a decreased risk of cross-contamination and is highly sensitive and specific. It is, therefore, considered to be a powerful tool for the rapid detection and differentiation of PRRSV.

Identification of genetically diverse sequences (ORF 5) of porcine reproductive and respiratory syndrome virus in a swine herd

The ability of genetically diverse strains of porcine reproductive and respiratory syndrome virus (PRRSV) to coexist in a 1750-sow farm was assessed through the case study describing a chronically infected farm, and also by an animal experiment involving the use of swine bioassay. The case study employed a program of monitoring sera from suckling, nursery, and finishing pigs for the presence of PRRSV by polymerase chain reaction (PCR) and virus isolation (VI). The swine bioassay tested homogenates, consisting of lymphoid and pulmonary tissues, collected from 60 breeding animals from the same farm. The open reading frame (ORF) 5 portion of selected positive PRRSV detected from sera or tissues were nucleic acid sequenced and their phylogenies compared. The results indicated the presence of 3 genetically diverse groups, designated PRRSV-A, -B, and -C. Sequence heterology ranged from 5.8 to 11% between groups. Sequence homology ranged from 98.7 to 99.8% within groups. Swine bioassay verified the presence of PRRSV-A in 1 of 60 animals, and no evidence of strains B or C were detected. This paper indicates that based on the evaluation of ORF 5, genetically diverse strains of PRRSV appear to coexist, although the frequency and significance of this observation is not understood.

Transmission of porcine reproductive and respiratory syndrome virus from persistently infected sows to contact controls

The objective of this study was to determine if porcine reproductive and respiratory syndrome virus (PRRSV) could persist in non-pregnant sows and if persistently infected sows could transmit virus to naive contact controls. Twelve PRRSV-naive, non-pregnant sows (index sows) were infected with a field isolate of PRRSV and housed in individual isolation rooms for 42 to 56 days postinfection. Following this period, 1 naive contact sow was placed in each room divided by a gate allowing nose-to-nose contact with a single index sow. Index sows were not viremic at the time of contact sow entry. Virus nucleic acid was detected by polymerase chain reaction, and infectious virus was detected by virus isolation in sera from 3 of the 12 contact sows at 49, 56, and 86 days postinfection. All 3 infected contacts developed PRRSV antibodies. Virus nucleic acid was detected in tissues of all of the 12 index sows at 72 or 86 days postinfection. Nucleic acid sequencing indicated that representative samples from index and infected contacts were homologous (> 99%) to the PRRSV used to infect index sows at the onset of the study. This study demonstrates that PRRSV can persist in sows and that persistently infected sows can transmit virus to naive contact animals.

Construction of chimeric arteriviruses reveals that the ectodomain of the major glycoprotein is not the main determinant of equine arteritis virus tropism in cell culture

The recent development of arterivirus full-length cDNA clones makes possible the construction of chimeric arteriviruses for fundamental and applied studies. Using an equine arteritis virus (EAV) infectious cDNA clone, we have engineered chimeras in which the ectodomains of the two major envelope proteins, the glycoprotein GP(5) and the membrane protein M, were replaced by sequences from envelope proteins of related and unrelated RNA viruses. Using immunofluorescence microscopy, we monitored the transport of the hybrid GP(5) and M proteins to the Golgi complex, which depends on their heterodimerization and is a prerequisite for virus assembly. The only viable chimeras were those containing the GP(5) ectodomain from the porcine (PRRSV) or mouse (LDV) arteriviruses, which are both considerably smaller than the corresponding sequence of EAV. Although the two viable GP(5) chimeras were attenuated, they were still able to infect baby hamster kidney (BHK-21) and rabbit kidney (RK-13) cells. These cells can be infected by EAV, but not by either PRRSV or LDV. This implies that the ectodomain of the major glycoprotein GP(5), which has been postulated to be involved in receptor recognition, is not the main determinant of EAV tropism in cell culture.

A 10-kDa structural protein of porcine reproductive and respiratory syndrome virus encoded by ORF2b

The major structural proteins of porcine reproductive and respiratory syndrome virus (PRRSV) are derived from ORFs 5, 6, and 7. Western blots of sucrose gradient-purified virions and PRRSV-infected MARC-145 cells, probed with immune pig serum, showed the presence of an additional 10-kDa protein. Nucleotide sequence analysis of North American PRRSV isolate SDSU-23983 revealed a small ORF within ORF2, named ORF2b, which, when translated, produced a 73-amino-acid nonglycosylated protein. Recombinant 2b protein expressed by a baculovirus clone, AcVR2, comigrated with the 10-kDa virus-associated protein. The loss of 10-kDa protein immunoreactivity after absorption of immune sera with lysates from AcVR2-infected insect cells demonstrated that the 2b and 10-kDa proteins are immunologically similar. Immunoblots were also used for the detection of anti-2b activity in serum samples from experimentally infected adult pigs. Antibodies against PRRSV were apparent by 14 days postinfection, followed by anti-2b activity and serum neutralizing activity. The putative ORF2b start codon is only 6 nucleotides downstream of the adenine of the ORF2a start codon. The expression of ORF2a and 2b as enhanced green fluorescent fusion proteins showed that both proteins were translated; however, the ORF2b was preferentially expressed. These results suggest that the 2b protein is virion associated and the principal product of ORF2.

Diagnostic investigation of chronic porcine reproductive and respiratory syndrome virus in a breeding herd of pigs

Forty-five sows and 15 boars were selected at random from a breeding herd known to be chronically infected with porcine reproductive and respiratory syndrome virus (PRRSV) and lymphoid, immune-privileged, and non-lymphoid/non-immune-privileged tissues were tested for the presence of the virus by PCR, virus isolation, and immunohistochemistry. The virus was isolated from the lateral retropharyngeal lymph node of one sow; the isolate was nucleic acid sequenced and determined to be of field origin, and it was inoculated into two PRRSV-naive pregnant sows (A and B) at 95 days of gestation. They were necropsied 14 days later and samples of maternal and fetal tissue and blood samples were collected. Sow A had 10 fresh, six partially autolysed, and two mummified fetuses, and sow B had six fresh and viable fetuses. Viral nucleic acid was detected by PCR in tissue pools from each sow and also from pooled fetal tissues, and the virus was isolated from fetal pools from sow A.

The role of pulmonary intravascular macrophages in porcine reproductive and respiratory syndrome virus infection

The objective of this article is to summarize the current state of knowledge of the complex interaction of porcine reproductive and respiratory syndrome virus (PRRSV) and porcine pulmonary intravascular macrophages (PIMs). PIMs play an important role in pulmonary surveillance, and in the past few years we have investigated their role in PRRSV infection. PRRSV antigens and nucleic acids have been demonstrated in PIMs both in vitro and in vivo. Examination of cultured PIMs infected with PRRSV revealed the accumulation of viral particles in the smooth-walled vesicles. PRRSV-infected PIMs in vitro yielded a virus titer similar to pulmonary alveolar macrophages. PRRSV infection induces either apoptosis or cell lysis of PIMs. The in vitro bactericidal activity of PRRSV-infected PIMs is significantly decreased. Phagocytic activity of PIMs, as measured by pulmonary copper clearance, is significantly decreased in PRRSV-infected pigs. This evidence supports the hypothesis that PRRSV-induced damage to PIMs results in increased susceptibility to bacteremic diseases. Recent studies with PRRSV and Streptococcus suis coinfection confirmed that PRRSV predisposes pigs to S. suis infection and bacteremia. These results could explain the increase in bacterial respiratory diseases and septicemias observed in PRRSV-infected pigs.

Porcine reproductive and respiratory syndrome virus: description of persistence in individual pigs upon experimental infection

We studied the persistence of porcine reproductive and respiratory syndrome virus (PRRSV) in individual experimentally infected pigs, during a period of up to 150 days postinfection (dpi). The results of this study suggest that the persistence of PRRSV involves continuous viral replication but that it is not a true steady-state persistent infection. The virus eventually clears the body and seems to do it in most of the animals by 150 dpi or shortly thereafter. High genetic stability was seen for several regions of the persistent PRRSV's genome, although some consistent mutations in the genes of envelope glycoproteins and M protein were also observed.

Variations in the major envelope glycoprotein GP5 of Czech strains of porcine reproductive and respiratory syndrome virus

The major envelope glycoprotein genes (ORF5) of seven Czech isolates of porcine reproductive and respiratory syndrome virus (PRRSV) were amplified and their nucleotide sequences were determined. ORF5 displayed nucleotide and amino acid identities of 87.5-100% and 87. 6-100%, respectively, among the isolates. In a phylogenetic tree, all European isolates were grouped in a genotype distinct from that of reference American strains (VR-2332, IAF-Klop). Among the European isolates, two different clades were identified. Two Czech isolates (V-501 and V-503) and Italian strain PRRSV 2156 fell into one clade. The remaining European strains comprised the second clade. Surprisingly, two separately clustered strains (V-501 and V-516) were isolated from the same herd. Additionally, the possible effect of in vitro cultivation on the nucleotide sequence was analysed. Nine point mutations in the ORF5 region resulted from 152 in vitro passages of the V-502 isolate in MARC-145 cells.

Heteroclite subgenomic RNAs are produced in porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome virus (PRRSV) was shown to produce atypical subgenomic RNAs that contain open reading frame la nucleotides and are present under a wide variety of culture conditions, including high and low multiplicities of infection, in simian and porcine host cells, and during infection with cell-adapted and wild-type PRIRSV strains. Sequence analysis demonstrated that they are heterogeneous in 5-3' junction sequence and size and may code for different predicted fusion proteins. This is the first report of these novel RNA5 in arteriviruses and we have termed them heteroclite (meaning 'deviating from common forms or rules") subgenomic RNAs. The unique properties of these subgenomic RNAs include (a) apparent association with normal virus infection and stability during serial passage, (b) packaging of heteroclite RNAs into virus-like particles, (c) short, heterogeneous sequences which may mediate the generation of these RNAs, (d) a primary structure which consists of the two genomic termini with one large internal deletion, and (eJ little apparent interference with parental virus replication. These subgenomic RNA5 may be critical to, or a necessary side product of, viral replication. The expression of these novel RNA species support the template-switching model of similarity-assisted RNA recombination. In summary, PRRSV readily undergoes nonhomologous RNA recombination to generate heteroclite sub-genomic RNA5.

Antigenic variation and genotype of isolates of porcine reproductive and respiratory syndrome virus in Korea

A panel of three anti-glycoprotein 5 (gp5) protein monoclonal antibodies (mAbs) (15, 28 and 246) and three anti-nucleocapsid (N) protein mAbs (SDOW17, VO17 and EP147) was used to investigate, by an indirect fluorescent antibody test, the antigenic variations of 50 Korean isolates of porcine reproductive and respiratory syndrome virus (PRRSV), and compare them with a us ATCC vR2332-derived attenuated vaccine strain and the reference European Lelystad strain of PRRSV. A multiplex PCR assay for the differentiation of European and North American genotypes of PRRSV was used to determine the genotype of the 50 Korean isolates. Forty-six (92 per cent) of the 50 Korean isolates shared the epitopes recognised by the anti-N protein mAb SDOW17. No reactivity to the anti-gp5 and anti-N protein mAbs was observed with the other four isolates. Six distinct patterns could be identified on the basis of their reactivities with the anti-PRRSV mAbs. All 50 isolates were identified as North American genotypes by the differential PCR.

Heterogeneity of porcine reproductive and respiratory syndrome virus: implications for current vaccine efficacy and future vaccine development

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a major problem to the pork industry worldwide. Increasing data indicate that PRRSV strains differ in virulence in infected pigs and are biologically, antigenically, and genetically heterogeneous. It is evident that the current vaccines, based on a single PRRSV strain, are not effective in protecting against infections with the genetically diverse field strains of PRRSV. The recent outbreaks of atypical or acute PRRS in vaccinated pigs have raised a serious concern about the efficacy of the current vaccines and provided the impetus for developing more effective vaccines. Special attention in this review is given to published work on antigenic, pathogenic and genetic variations of PRRSV and its potential implications for vaccine efficacy and development. Although there are ample data documenting the heterogeneous nature of PRRSV strains, information regarding how the heterogeneity is generated and what clinical impact it may have is very scarce. The observed heterogeneity will likely pose a major obstacle for effective prevention and control of PRRS. There remains an urgent need for fundamental research on this virus to understand the basic biology and the mechanism of heterogeneity and pathogenesis of PRRSV.

Neuropathogenicity and sensitivity to antibody neutralization of lactate dehydrogenase-elevating virus are determined by polylactosaminoglycan chains on the primary envelope glycoprotein

Common strains of lactate dehydrogenase-elevating virus (LDV, an arterivirus), such as LDV-P and LDV-vx, are highly resistant to antibody neutralization and invariably establish a viremic, persistent, yet asymptomatic, infection in mice. Other LDV strains, LDV-C and LDV-v, have been identified that, in contrast, are highly susceptible to antibody neutralization and are incapable of a high viremic persistent infection, but at the same time have gained the ability to cause paralytic disease in immunosuppressed C58 and AKR mice. Our present results further indicate that these phenotypic differences represent linked properties that correlate with the number of N-glycosylation sites associated with the single neutralization epitope on the short ectodomain of the primary envelope glycoprotein, VP-3P. The VP-3P ectodomains of LDV-P/vx possess three N-glycosylation sites, whereas those of LDV-C/v lack the two N-terminal sites. We have now isolated four independent neutralization escape variants of neuropathogenic LDV-C and LDV-v on the basis of their ability to establish a high viremic persistent infection in mice. The VP-3P ectodomains of all four variants had specifically regained two N-glycosylation sites concomitant with decreased immunogenicity of the neutralization eptitope and decreased sensitivity to antibody neutralization as well as loss of neuropathogenicity.

Genetic, geographical and temporal variation of porcine reproductive and respiratory syndrome virus in Illinois

Porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 gene sequences were generated by RT-PCR from 55 field isolates collected in Illinois and eastern Iowa. Spatial and temporal patterns of genetic variation in the virus were examined on a local geographical scale in order to test the hypothesis that the genetic similarity of PRRSV isolates (measured as their percentage pairwise ORF5 nucleotide similarity) was positively correlated with their geographical proximity. Levels of genetic variability in the Illinois/eastern Iowa PRRSV sample were similar to levels of variability seen across broader geographical regions within North America. The genetic similarity of isolates did not correlate with their geographical distance. These results imply that the movement of PRRSV onto farms does not generally occur via distance-limited processes such as wind or wildlife vectors, but more typically occurs via the long-distance transport of animals or semen. Genetic distances between PRRSV isolates collected from the same farms at different times increased as the time separating the collection events increased. This result implies rapid movement of new genetic types of PRRSV into and out of farms. PRRSV ORF5 displayed a pattern of third-codon-position diversity bias that was not evident in a geographically comparable sample of pseudorabies virus (a swine alphaherpesvirus) gC gene sequences. This result provides evidence that PRRSV ORF5 is experiencing stabilizing selection against structural novelty. Despite high genetic variability at all geographical levels, PRRSV ORF5 nevertheless contained potentially antigenic regions that were invariant at the amino acid level. These regions should make effective vaccine targets if they prove to be immunogenic.

The localization of porcine reproductive and respiratory syndrome virus nucleocapsid protein to the nucleolus of infected cells and identification of a potential nucleolar localization signal sequence

The nucleocapsid (N) protein of porcine reproductive and respiratory syndrome virus (PRRSV) possesses two regions in the N-terminal half of the protein that are enriched in basic amino acids. Presumably, these basic regions are important for packaging the RNA genome within the nucleocapsid of the virus. The PSORT computer program identified the same regions as nuclear localization signal (NLS) sequence motifs. N protein localization to the nucleus of infected MARC-145 and porcine pulmonary macrophages was observed following staining with SDOW-17 and SR-30 anti-N monoclonal antibodies. Furthermore, the co-localization of SR-30 antibody with human ANA-N autoimmune serum identified the nucleolus as the primary site for N protein localization within the nucleus. The localization of the N protein in the absence of infection was studied by following fluorescence in MARC-145 cells transfected with a plasmid, which expressed the nucleocapsid protein fused to an enhanced green fluorescent protein (N-EGFP). Similar to infected cells, N-EGFP localized to the cytoplasm and the nucleolus. Results following the transfection of cells with pEGFP fused to truncated portions of the N gene identified a region containing the second basic stretch of amino acids as the nucleolar localization signal (NoLS) sequence. Another outcome following transfection was the rapid disappearance of cells that expressed high levels of N-EGFP. However, cell death did not correlate with localization of N-EGFP to the nucleolus.

Characterization of antibody response to porcine reproductive and respiratory syndrome virus ORF5 product following infection and evaluation of its diagnostic use in pigs

The sensitivity and specificity of recombinant open reading frame 5 products used in the Western blotting assay for confirmation of porcine reproductive and respiratory syndrome virus (PRRSV) serologic status were evaluated. The recombinant antigen-based assays were specifically compared with a commercial enzyme-linked immunosorbent assay (ELISA) for PRRSV antibodies using 1) PRRSV antibody-negative reference sera (n = 30), 2) naturally infected pig sera (n = 40), 3) sequential sera obtained from 24 experimentally infected pigs, and 4) sera submitted to 3 state diagnostic laboratories (n = 200). The recombinant antigen assay yielded an average increased sensitivity of 10% over the commercial PRRSV ELISA. The negative controls (group 1 sera) showed no difference between the 2 assays. This comparison confirmed that the recombinant antigen-specific assay was more sensitive than the commercial ELISA and is well suited for routine confirmation of the presence of PRRSV antibodies.

Heterologous challenge with porcine reproductive and respiratory syndrome (PRRS) vaccine virus: no evidence of reactivation of previous European-type PRRS virus infection

In Denmark, a porcine reproductive and respiratory syndrome virus (PRRSV) control programme, comprising vaccination of seropositive herds with a live American type PRRSV vaccine, was started in 1996. In several of these herds, spread of vaccine virus from vaccinated 3-18 week old pigs to non-vaccinated sows was demonstrated by the isolation of vaccine virus from fetuses and stillborn piglets. Surprisingly, sows infected with the American type vaccine strain consistently exhibited significantly stronger serological responses towards European type PRRSV than American type PRRSV. In order to elucidate whether the unexpectedly strong serological reaction towards European-type PRRSV in American type PRRSV infected sows was due to a booster reaction, or reactivation of an unrecognized, latent infection in the sows with European type PRRSV, a challenge study with the vaccine was carried out. In this study, the stronger serological response towards European type PRRSV than towards American type PRRSV was reproduced, and reactivation of the previous natural infection with European PRRSV could neither be demonstrated by virus isolation nor by RT-PCR. So, the increase in antibody titers towards European PRRSV in previously European PRRSV infected pigs after challenge with the vaccine strain seems to be the result of a boosting effect on the immune system, induced by the heterologous vaccine PRRSV strain.

Recombination between North American strains of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV), a recently discovered arterivirus swine pathogen, was shown to undergo homologous recombination. Co-infection of MA-104 cells with two culture-adapted North American PRRSV strains resulted in recombinant viral particles containing chimeric ORF 3 and ORF 4 proteins. Nucleotide sequence analysis of cloned recombinant PCR products, encompassing 1182 bases of the 15.4 kb viral genome, revealed six independent recombination events. Recombinant products persisted in culture for at least three passages, indicating continuous formation of recombinant viruses, growth of recombinant viruses in competition with parental viruses, or both. The frequency of recombination was estimated from <2% up to 10% in the 1182 b fragment analyzed, which is similar to recombination frequencies observed in coronaviruses. An apparent example of natural ORF 5 recombination between naturally occurring wild type viruses was also found, indicating that recombination is likely an important genetic mechanism contributing to PRRSV evolution.

Monoclonal antibodies to the GP5 of porcine reproductive and respiratory syndrome virus are more effective in virus neutralization than monoclonal antibodies to the GP4

The arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) contains six structural proteins the roles of which are not completely understood. In a preceding study, immunization with the dutch isolate I10 of PRRSV had led to the development of MAbs against four structural proteins [Wieczorek-Krohmer, M., 1994. Herstellung und Charakterisierung von monoklonalen Antikörpern gegen das Virus des Porzinen Reproduktiven und Respiratorischen Syndroms (PRRSV). Inaugural-Dissertation, Ludwig-Maximilians-Universität, München] here finally identified by reaction with individual plasmid-expressed PRRSV proteins as products of ORFs 3 (GP3), 4 (GP4), 5 (GP5) and 7 (N). Surprisingly, the MAbs against GP5 revealed the presence of two antigenically distinct virus populations in the isolate I10, the population PRRSV-'PPV', isolated from plaques and the PRRSV-'EPV', gained by end point dilution. MAbs against GP3, GP4 and N reacted with both I10 populations as well as with natural PRRSV isolates. However, the anti-GP5 MAbs exclusively recognized PRRSV-'PPV'. In this study immunization of mice with both separated I10 populations confirmed that solely PRRSV-'PPV' possesses the property to induce an immune response ultimately leading to the establishment of MAbs against GP5. Whereas the 15 anti-GP5 MAbs (derived from four independent fusions) reacted exclusively with PRRSV-'PPV' of the isolate I10, anti-GP4 MAbs detected their target antigen on various isolates of European origin and were able to neutralize them. As indicated by competition assays and selection of neutralization-resistant virus mutants, all GP5 MAbs are directed against a single antigenic site on the ORF 5 protein. Both groups of neutralizing antibodies bound to the surface of purified virions demonstrating that the recognized epitopes represent surface structures of the virion envelope. However, anti-GP5 MAbs mediated the binding of more gold granules than anti-GP4 MAbs. Comparison of the neutralizing effect of anti-GP4 and anti-GP5 MAbs revealed the anti-GP5 MAbs as the more efficient antibodies. For the complete neutralization of about 100 ID50 of PRRSV-'PPV' anti-GP5 culture supernatant was effective up to a dilution of 1:1280 whereas the most effective anti-GP4 antibodies exhibited a comparable effect only up to 1:64. These results indicate that PRRSV GP5 in principle is a major target for neutralizing antibodies, as is found for other arteriviruses, but that in nature 'ORF 5 escape mutants' may develop as easily as in vitro.

Porcine reproductive and respiratory syndrome virus infection in neonatal pigs characterised by marked neurovirulence

Neonatal pigs from three herds of pigs were somnolent and inappetent and had microscopic lesions characterised by severe meningoencephalitis, necrotic interstitial pneumonia and gastric muscular inflammation. Porcine reproductive and respiratory syndrome virus (PRRSV) infection was diagnosed and confirmed by virus isolation, fluorescent antibody examination of frozen lung sections, serology, immunohistochemistry and in situ hybridisation. Each herd had a history of PRRSV infection and was using or had used a modified-live vaccine. The isolates from the affected pigs were genetically distinct from the modified-live vaccine strain of the virus when compared by restriction enzyme analysis and nucleotide sequencing of PRRSV open reading frames 5 and 6. The virus was identified in macrophages or microglia of brain lesions by immunohistochemical staining of brain sections with an anti-PRRSV monoclonal antibody and an anti-macrophage antibody. The replication of the virus in the brain was verified by in situ hybridisation. The meningoencephalitis induced by the virus in pigs from each of the herds was unusually severe and the brain lesions were atypical when compared with other descriptions of encephalitis induced by the virus, which should therefore be considered as a possible diagnosis for neonatal pigs with severe meningoencephalitis. In addition, field isolates of the virus which are capable of causing disease can emerge and coexist with modified-live vaccine virus in some pig herds.

Porcine reproductive and respiratory syndrome virus comparison: divergent evolution on two continents

Porcine reproductive and respiratory syndrome virus (PRRSV) is a recently described arterivirus responsible for disease in swine worldwide. Comparative sequence analysis of 3'-terminal structural genes of the single-stranded RNA viral genome revealed the presence of two genotypic classes of PRRSV, represented by the prototype North American and European strains, VR-2332 and Lelystad virus (LV), respectively. To better understand the evolution and pathogenicity of PRRSV, we obtained the 12,066-base 5'-terminal nucleotide sequence of VR-2332, encoding the viral replication activities, and compared it to those of LV and other arteriviruses. VR-2332 and LV differ markedly in the 5' leader and sections of the open reading frame (ORF) 1a region. The ORF 1b sequence was nearly colinear but varied in similarity of proteins encoded in identified regions. Furthermore, molecular and biochemical analysis of subgenomic mRNA (sgmRNA) processing revealed extensive variation in the number of sgmRNAs which may be generated during infection and in the lengths of noncoding sequence between leader-body junctions and the translation-initiating codon AUG. In addition, VR-2332 and LV select different leader-body junction sites from a pool of similar candidate sites to produce sgmRNA 7, encoding the viral nucleocapsid protein. The presence of substantial variations across the entire genome and in sgmRNA processing indicates that PRRSV has evolved independently on separate continents. The near-simultaneous global emergence of a new swine disease caused by divergently evolved viruses suggests that changes in swine husbandry and management may have contributed to the emergence of PRRS.

Seroneutralization of porcine reproductive and respiratory syndrome virus correlates with antibody response to the GP5 major envelope glycoprotein

To determine the structural protein of the porcine reproductive and respiratory syndrome virus (PRRSV) involved in the production of neutralizing antibodies following clinical infection, correlation was studied between virus neutralization capability of convalescent pig sera and antibody response to the open reading frames (ORFs) 3-, 4-, 5-, and 7-encoded proteins GP3, GP4, GP5, and N, respectively. Individual virus genes were cloned into the pGEX-4T-1 vector, and the recombinant viral proteins were expressed in Escherichia coli fused to the glutathione S-transferase (GST) protein. The resulting GST-ORF3, GST-ORF4, GST-ORF5, and GST-ORF7 recombinant fusion proteins were purified by electroelution and used as antigens for serologic testing by indirect enzyme-linked immunosorbent assay and western immunoblotting. The overall antibody (IgG and IgM) titers to PRRSV of pooled convalescent pig sera were first determined by indirect immunofluorescence, and then sera with specific IgG titers > 1:1,024 were tested for their specific virus neutralization activity and reactivity to individual recombinant fusion proteins. Except for the early immune response (as revealed by the presence of specific IgM), neutralizing titers were correlated with anti-GP5 titers but not with anti-GP3 and anti-GP4 titers. The correlation between virus neutralization and anti-GP5 titers was significant (r = 0.811, P < or = 0.001).

Detection of porcine reproductive and respiratory syndrome virus by reverse transcription-polymerase chain reaction using different regions of the viral genome

Serologic studies have revealed strain variability between American and European isolates and among American isolates of porcine reproductive and respiratory syndrome virus (PRRSV). The objective of this study was to develop an assay for the routine diagnosis of PRRSV in field specimens using reverse transcription-polymerase chain reaction (RT-PCR) amplification of conserved genomic regions. Twenty-four field isolates of PRRSV from different regions of the USA were analyzed in the study. Six primer pairs from open reading frames (ORFs) 4, 6, and 7 of the American strain (ATCC VR-2332) and from ORF 1b of the Lelystad strain were used for the amplification of the viral genome by PCR. Amplification products of the expected sizes were obtained from all isolates by PCR amplification of ORF 7, the gene encoding the nucleocapsid protein. Oligonucleotide primers designed to amplify ORFs 4 and 6 detected 92% and 96% of the isolates, respectively, whereas primers for the amplification of ORF 1b detected 88% of all isolates. The specificity of the amplified products of ORF 7 from 7 field isolates and 2 reference strains was confirmed by chemiluminescent hybridization using an internal digoxigenin-labeled DNA probe. Sequence analysis of this region indicated variation in the nucleotide sequence of 2 isolates that did not hybridize with the internal probe. These results indicate that ORF 7 may serve as a potential target for the detection of PRRSV strains by RT-PCR and that genomic variability should be considered when nucleic acid hybridization is used to confirm the specificity of PCR amplification for diagnostic purposes.

Localization and fine mapping of antigenic sites on the nucleocapsid protein N of porcine reproductive and respiratory syndrome virus with monoclonal antibodies

The purpose of this study was to analyze the antigenic structure of the nucleocapsid protein N of the Lelystad virus isolate of porcine reproductive and respiratory syndrome virus (PRRSV) and to identify antigenic differences between this prototype European isolate and other North American isolates. To do this, we generated a panel of monoclonal antibodies (mAbs) directed against the N protein of Lelystad virus and tested them in competition assays with other N-specific mAbs described previously (Drew et al., 1995; Nelson et al., 1993; van Nieuwstadt et al., 1996). Four different competition groups of mAbs were identified. Pepscan analysis with solid-phase dodecapeptides was used to identify specific antigenic regions in the N protein that were bound by the mAbs. In this pepscan analysis, we found that the mAb of the first competition group reacted with linear peptides whose core sequences consisted of amino acids 2-12 (site A), the mAbs of the second group reacted with peptides whose core sequences consisted of amino acids 25-30 (site B), and the mAb of the third group reacted with peptides whose core sequences consisted of amino acids 40-46 (site C). However, the fourth group of mAbs binding to an antigenic region, provisionally designated as domain D, reacted very weakly or did not react at all with solid-phase dodecapeptides. To further characterize the structure of the epitopes in domain D, we produced chimeric constructs composed of the N protein sequences of Lelystad virus and another arterivirus lactate dehydrogenase-elevating virus, which was used because its N protein has similarity in amino acid sequence and hydropathicity profile but does not react with our mAbs. When the mAbs specific to domain D were tested for binding to the chimeric N proteins expressed by Semliki Forest virus, we found that the regions between amino acids 51-67 and amino acids 80-90 are involved in the formation or are part of the epitopes in domain D. Therefore, we conclude that the N protein contains four distinct antigenic regions. The epitopes mapped to sites A-C are linear, whereas the epitopes mapped to domain D are more conformation dependent or discontinuous. Sites A and C contain epitopes that are conserved in European but not in North American isolates; site B contains epitopes that are conserved in European and North American isolates; and site D contains epitopes that are either conserved or not conserved in European and North American isolates. The antigenic regions identified here might be important for the development of diagnostic test for PRRSV in particular tests that discriminate between different antigenic types of PRRSV.

Sensitive detection and typing of porcine reproductive and respiratory syndrome virus by RT-PCR amplification of whole viral genes

Following the recent use of a live vaccine against porcine reproductive and respiratory syndrome virus (PRRSV) in Denmark, both American (vaccine) and European-type PRRSV now coexist in Danish herds. This situation highlighted a requirement for supplementary tests for precise virus-typing. As a result, we developed a RT-PCR assay able to detect as well as type PRRSV. To provide maximal sequence information, complete viral open reading frames (ORFs 5 and 7) were targeted for amplification. The RT-PCR test was able to amplify complete PRRSV ORFs from complex materials such as boar semen containing as little as 1 TCID50 ml(-1) of PRRSV. Typing of viruses was accomplished by any one of three strategies: (i) use of type-specific PCR primers, (ii) size determination of ORF 7 amplicons, (iii) DNA sequencing. All three typing strategies showed complete concordance with the currently used method of typing with monoclonal antibodies (MAbs) when used on a panel of PRRSV field isolates covering the period 1992-1997. The ORF 7-based test had particularly desirable characteristics, namely, highly sensitive detection of PRRSV without apparent type bias, typing of the detected virus, discrimination between pure and mixed virus populations, and semi-quantitative assessment of type ratios in mixed populations, all in a single PCR reaction. In addition, the obtained sequence data were used to predict two simple and rapid strategies (single-enzyme restriction length polymorphy analysis and oligonucleotide hybridization) for confirmation of the specificity of ORF 7 RT-PCR reactions. As such, the RT-PCR assay provides a new, powerful diagnostic tool to study the population dynamics between present and emerging PRRSV-types.

Identification of a common antigenic site in the nucleocapsid protein of European and North American isolates of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) nucleocapsid (N) protein has been identified as the most immunodominant viral protein. The N protein genes from two PRRSV isolates Olot/91 (European) and Quebec 807/94 (North American) were cloned and expressed in Escherichia coli using the pET3x system. The antigenic structure of the PRRSV N protein was dissected using seven monoclonal antibodies (MAbs) and overlapping fragments of the protein expressed in E.coli. Three antigenic sites were found. Four MAbs recognized two discontinuous epitopes that were present in the partially folded protein or at least a large fragment comprising the first 78 residues, respectively. The other three MAbs revealed the presence of a common antigenic site localized in the central region of the protein (amino acids 50 to 66). This hydrophillic region is well conserved among different isolates of European and North American origin. However, since this epitope is not recognized by many pig sera, it is not adequate for diagnostic purposes. Moreover, none of the N protein fragments were able to mimic the antigenicity of the entire N protein.

Genetic variation in the PRRS virus

Porcine reproductive and respiratory syndrome (PRRS) is characterized by late-term abortions and stillbirths in sows and respiratory difficulties in nursery pigs. The disease appeared in Europe and North America at approximately the same time between 1985 and 1990. The PRRS virus was isolated shortly thereafter and demonstrated unexpectedly profound differences between European (Lelystad) and North American (VR2332) isolates as measured by serological crossreactivity and nucleotide sequence similarity. In order to determine the amount of genetic variation in the PRRS virus and to understand the molecular mechanisms of viral evolution, nucleotide sequences of PRRS virus strains were determined. Comparisons among ten U.S. strains showed that variation in primary nucleotide sequence between isolates ranged from 2.5% to 7.9% for ORFs 2-7. In contrast, Lelystad virus was, on average, 35% different from US clones. These results provided direct molecular evidence that US and European PRRSV isolates represented genetically distinct groups of the same viral family. A further analysis of more than 150 isolates in the United States and Canada demonstrated that the PRRS virus in North America represents a single large and diverse genetic group that is distinct from European forms of the virus.

Sequence analysis of the nucleocapsid protein gene of the porcine reproductive and respiratory syndrome virus Taiwan MD-001 strain

The 3'-portion of the genome from a Taiwan isolate of porcine reproductive and respiratory syndrome (PRRS) virus, strain MD-001, was cloned and sequenced. The resultant 549 nucleotides contained an open reading frame with a coding capacity of 123 amino acids (predicted Mr 13,600). The predicted protein corresponds to the nucleocapsid protein, the gene product of ORF7. Comparative sequence analysis of several known PRRSV strains indicated that this protein showed the highest degree of amino acid similarity to the US VR2332 and the Canadian IAF-Exp91 strains (92.7%) and the least to the Dutch Lelystad strain (56.5%). The phylogenic trees constructed on the basis of the known PRRSV nucleotide sequences indicated that MD-001 strain belongs to the North American strain cluster and that it is distinct from the European virus.

Neuropathogenicity and susceptibility to immune response are interdependent properties of lactate dehydrogenase-elevating virus (LDV) and correlate with the number of N-linked polylactosaminoglycan chains on the ectodomain of the primary envelope glycoprotein

We have developed differential RT-PCR methods to distinguish different isolates of LDV and have purified several quasispecies by repeated end point dilution in mice. They fall into two groups, each possessing two or more members. Group A viruses are non-neuropathogenic, highly resistant to in vitro neutralization by antibodies and efficient in establishment of a life-long, persistently viremic infection in mice despite a detectable immune response. Group B viruses, on the other hand, are neuropathogenic, much more sensitive to antibody neutralization and have an impaired ability to establish a high viremia persistent infection in immune competent mice. These properties seem to be interdependent and correlate with the number of N-glycosylation sites on the short (about 30 amino acid long) ectodomain of the primary envelope glycoprotein, VP-3P, which probably is part of the attachment site for the LDV receptor on permissive cells and harbors an epitope(s) reacting with neutralizing antibodies. Group A viruses possess three closely spaced N-linked polylactosaminoglycan chains, whereas group B viruses lack the two N-terminal ones. We postulate that lack of these polylactosaminoglycan chains endows group B viruses with the ability to interact with a receptor on anterior horn neurons resulting in neuropathogenesis. At the same time, it increases an interaction with neutralizing antibodies thus impeding the infection of macrophages newly generated during the persistent phase of infection which is essential for the continued rounds of replication of the virus.

Subgenomic RNA7 is transcribed with different leader-body junction sites in PRRSV (strain VR2332) infection of CL2621 cells

Porcine reproductive and respiratory syndrome virus (PRRSV), like all members of the order Nidoviridae, is expressed in the infected cell as a nested set of subgenomic (sg) RNAs with a common 5'-leader sequence. We have determined that the 5'-leader sequence for the US prototype strain (VR2332, Collins, et al., 1992) is distinct from the European prototype strain [Lelystad (LV); Wensvoort, et al., 1991, Meulenberg et al., 1993a], yet these two strains use almost the same sequence for downstream sites of 5'-leader-body junction formation. Analysis of VR2332 genomic sequence identified several potential 5'-leader-body junction sequences upstream of open reading frame (ORF) 7, coding for the nucleocapsid protein, that could be used for generation of VR2332 sgRNA7 transcripts. Sequence determinations of RT-PCR-generated cDNA clones of sgRNA7 identified two species of RNA7 transcripts in infected cells, one utilizing a leader-body junction sequence (AUAACC) 123 nucleotides upstream of the AUG start site and one utilizing a sequence (UAAACC) 9 nucleotides upstream of the AUG start site for ORF7 translation.