Comparative sequence analysis of open reading frames 2 to 7 of the modified live vaccine virus and other North American isolates of the porcine reproductive and respiratory syndrome virus

Novel Nsp2 deletion based on molecular epidemiology and evolution of porcine reproductive and respiratory syndrome virus in Shandong Province from 2013 to 2014

Porcine reproductive and respiratory syndrome (PRRS) is an economically important swine disease affecting swine worldwide. In this study, a total of 385 samples were collected from Shandong pig farms during 2013 and 2014, when pigs were not inoculated with any vaccine. Results indicated that, out of 385 samples, 47 (12.21%) were PRRSV-RNA-positive. The gene sequence analysis of 12 ORF5, 12 ORF7, and 8 Nsp2 of these samples was used to determine the molecular epidemiology of PRRSV in different parts of China's Shandong Province. The phylogenetic tree based on these 3 genes indicated that the Chinese PRRSV strains could be divided into five subgroups and two large groups. The 8 study strains were clustered into subgroup IV, another 4 strains into subgroup I. The first 8 strains shared considerable homology with VR-2332 in ORF5 (96-97.5%), the other 4 strains shared considerable homology with JXA1 (94-98%). Phylogenetic tree of GP5 showed that the eight isolates formed a tightly novel clustered branch, subgroup V, which resembled but differed from isolate VR-2332. When examined using Nsp2 alone, the first 8 strains showed considerable homology with a U.S. vaccine strain, Ingelvac MLV (89.6-98.4%). One novel pattern of deletion was observed in Nsp2. The genetic diversity of genotype 2 PRRSV tended to vary in the field. The emergence of novel variants will probably be the next significant branch of PRRSV study.

Assessment of the safety and efficacy of an attenuated live vaccine based on highly pathogenic porcine reproductive and respiratory syndrome virus

The safety and efficacy of the JXA1-R vaccine, an attenuated strain of highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV), were examined using an intramuscular challenge model in piglets. The JXA1-R vaccine was obtained by passing HP-PRRSV JXA1 through Marc-145 cells (82nd passage). Genomic sequence comparisons showed that strain JXA1-R and its parental strain, JXA1, differ by 47 amino acids, and most of these differences are scattered throughout the PRRSV genome. Four-week-old PRRSV-free piglets were inoculated intramuscularly with JXA1-R vaccine (10(3.0), 10(4.0), 10(5.0), 10(6.0), and 10(7.0) 50% tissue culture infective doses [TCID50]/ml for groups 1 to 5, respectively) and then challenged intramuscularly with the 5th passage virus of JXA1 virus (JXA1-F5, 3 ml × 10(4.5) TCID50/ml) 28 days after inoculation. The humoral immune response, swine growth, clinical signs, and differential organ lesions were monitored. The results showed that all vaccinated piglets had a perceptible humoral immune response to vaccination after day 7, which then promptly increased, almost reaching the maximum sample/positive (S/P) ratio value at 28 days postimmunization. Viremia detection indicated that the viral replication levels of the challenge virus in the immunized groups (immunization doses ≥10(4.0)/ml) were significantly lower than that of the virus-challenged unvaccinated control group. Piglets in groups 2 to 5 were effectively protected against lethal HP-PRRSV infection and did not show any obvious changes in body temperature or clinical signs of disease at any point during the experiment. However, two of five piglets in group 1 showed mild pathological lesions and transitory high fever. These results suggest that JXA1-R (TCID50/ml ≥10(4.0)) is sufficiently attenuated and can provide effective protection against the lethal wild-type HP-PRRSV.

Nsp9 and Nsp10 contribute to the fatal virulence of highly pathogenic porcine reproductive and respiratory syndrome virus emerging in China

Atypical porcine reproductive and respiratory syndrome (PRRS), which is caused by the Chinese highly pathogenic PRRS virus (HP-PRRSV), has resulted in large economic loss to the swine industry since its outbreak in 2006. However, to date, the region(s) within the viral genome that are related to the fatal virulence of HP-PRRSV remain unknown. In the present study, we generated a series of full-length infectious cDNA clones with swapped coding regions between the highly pathogenic RvJXwn and low pathogenic RvHB-1/3.9. Next, the in vitro and in vivo replication and pathogenicity for piglets of the rescued chimeric viruses were systematically analyzed and compared with their backbone viruses. First, we swapped the regions including the 5′UTR+ORF1a, ORF1b, and structural proteins (SPs)-coding region between the two viruses and demonstrated that the nonstructural protein-coding region, ORF1b, is directly related to the fatal virulence and increased replication efficiency of HP-PRRSV both in vitro and in vivo. Furthermore, we substituted the nonstructural protein (Nsp) 9-, Nsp10-, Nsp11- and Nsp12-coding regions separately; or Nsp9- and Nsp10-coding regions together; or Nsp9-, Nsp10- and Nsp11-coding regions simultaneously between the two viruses. Our results indicated that the HP-PRRSV Nsp9- and Nsp10-coding regions together are closely related to the replication efficiency in vitro and in vivo and are related to the increased pathogenicity and fatal virulence for piglets. Our findings suggest that Nsp9 and Nsp10 together contribute to the fatal virulence of HP-PRRSV emerging in China, helping to elucidate the pathogenesis of this virus.

PRRS is a considerable threat to the pig industry worldwide. A large-scale atypical PRRS caused by highly pathogenic PRRSV (HP-PRRSV) that emerged in 2006 has resulted in considerable economic loss to Chinese pig production. The disease is characterized by a high body temperature (41°C–42°C), morbidity and by mortality of the affected pigs. Although the genomic marker, the 30-amino-acid deletion in its Nsp2-coding region has been previously verified to have no relation to its increased pathogenicity, the genomic region(s) associated with the fatal virulence of HP-PRRSV remain unclear. A series of chimeric viruses with swapped coding regions between HP- and LP-PRRSV were constructed, and their growth abilities and pathogenicities in piglets were analyzed. Our results demonstrated that Nsp9 and Nsp10 together contribute to the replication efficiency and the fatal virulence of HP-PRRSV for piglets. Our finding is not only the first unambiguous illumination concerning the key virulence determinant of Chinese HP-PRRSV but it also provides a novel insight for understanding the molecular pathogenesis of this virus and for designing new drugs and vaccines against PRRSV infection in the future.

A novel baculovirus vector shows efficient gene delivery of modified porcine reproductive and respiratory syndrome virus antigens and elicits specific immune response

Porcine reproductive and respiratory syndrome (PRRS) is an economically devastating epizootic of porcine species. Current vaccines are inadequate to control the disease burden and outbreaks in the field. We report a novel baculovirus vaccine vector with White spot syndrome virus immediate early 1 shuttle promoter, with strong activity in both insect cells and mammalian cells, for immunization against PRRSV. The insect cell cultured baculovirus vector produces PRRSV envelope glycoproteins ORF2a, ORF3, ORF4 and ORF5, which are similar to the antigens in the infectious PRRS virion, and these antigens are stably incorporated on the surface of the baculovirus. Further, the baculovirus vector efficiently transduces these antigens in cells of porcine origin, thereby simulating a live infection. The baculovirus vectored PRRSV antigens, upon inoculation in mice, elicits robust neutralizing antibodies against the infective PRRS virus. Further, the experiments indicate that hitherto under emphasized ORF2a and ORF4 are important target antigens for neutralizing PRRSV infectivity.

Genetic diversity and evolutionary characterization of Chinese porcine reproductive and respiratory syndrome viruses based on NSP2 and ORF5

To more fully understand the extent of genetic diversity of PRRSV in China, we analyzed the Nsp2 and ORF5 gene sequences of 35 representative PRRSV isolates from 2008 to 2012. Sequence analysis revealed that the Nsp2 and ORF5 genes have undergone genetic variation. Furthermore, the isolate FJLYDX04 contains five insertions at positions 599 to 603 and is the first isolate from China reported to have an insertion in Nsp2. Our results suggest that the highly pathogenic PRRSV has become the dominant strain in China and that Chinese PRRSV has undergone rapid evolution and can circumvent immune responses induced by currently used vaccines.

Genetic diversity of the ORF5 gene of porcine reproductive and respiratory syndrome virus isolates in southwest China from 2007 to 2009

To gain insight into the molecular epidemiology and possible mechanisms of genetic variation of porcine reproductive and respiratory syndrome (PRRS) in Yunnan Province of China, the ORF5 gene of 32 PRRSV isolates from clinical samples collected from 2007 to 2009 were sequenced and analyzed. Nucleotide and amino acid analyses were carried out on 32 isolates and representative strains of the North American genotype, European genotype and two representative Chinese isolates. Results revealed that these isolates share 86.9–99.0% nucleotide and 87.5–98.0% amino acid identity with VR-2332 the prototypical North American PRRSV, 61.7–62.9% and 54.3–57.8% with Lelystad virus (LV) the representative strain of European genotype, 91.2–95.4% and 90.0–94.5% with CH-1a that was isolated in mainland China in 1996, 88.1–99.3% and 85.5–99.0% with JX-A1 the representative strain of High pathogenic PRRSV in China, and 86.2–99.8% and 85.5–100.0% between isolated strains of different years, respectively. Phylogenetic analysis revealed that all 32 PRRSV isolates belonged to the North American genotype and were further divided into two different subgenotypes. Subgenotype 1 comprised twenty two Yunnan isolates which divided into two branches. Subgenotype 2 comprised ten isolates which closely related to the RespPRRS vaccine and its parent strain VR-2332. The functional domains of GP5 such as the signal peptide, ectodomain, transmembrane regions and endodomain were identified and some motifs in GP5 with known functions, such as primary neutralizing epitope (PNE) and decoy epitope were also further analyzed. Our study shown the great genetic diversity of PRRSV in southwest China, rendering the guide for control and prevention of this disease.

Expression of the nucleocapsid protein of porcine reproductive and respiratory syndrome virus in soybean seed yields an immunogenic antigenic protein

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is a serious disease of swine and contributes to severe worldwide economic losses in swine production. Current vaccines against PRRS rely on the use of an attenuated-live virus; however, these are unreliable. Thus, alternative effective vaccines against PRRS are needed. Plant-based subunit vaccines offer viable, safe, and environmentally friendly alternatives to conventional vaccines. In this study, efforts have been undertaken to develop a soybean-based vaccine against PRRSV. A construct carrying a synthesized PRRSV-ORF7 antigen, nucleocapsid N protein of PRRSV, has been introduced into soybean, Glycine max (L.) Merrill. cvs. Jack and Kunitz, using Agrobacterium-mediated transformation. Transgenic plants carrying the sORF7 transgene have been successfully generated. Molecular analyses of T(0) plants confirmed integration of the transgene and transcription of the PRRSV-ORF7. Presence of a 15-kDa protein in seeds of T(1) transgenic lines was confirmed by Western blot analysis using PRRSV-ORF7 antisera. The amount of the antigenic protein accumulating in seeds of these transgenic lines was up to 0.65% of the total soluble protein (TSP). A significant induction of a specific immune response, both humoral and mucosal, against PRRSV-ORF7 was observed following intragastric immunization of BALB/c female mice with transgenic soybean seeds. These findings provide a 'proof of concept', and serve as a critical step in the development of a subunit plant-based vaccine against PRRS.

Genetic and phenotypic characterization of a 2006 United States porcine reproductive and respiratory virus isolate associated with high morbidity and mortality in the field

The objective of this study was to characterize a porcine reproductive and respiratory syndrome virus (PRRSV) isolated from United States pigs experiencing high morbidity (50%) and mortality (20%). The PRRSV isolate, designated NC16845b, was characterized through phenotypic analysis and genomic sequencing and compared to Type 2 PRRSV isolates VR-2332, MN184 and VR-2385. NC16845b demonstrated slower replication in vitro compared to the three other isolates and grew to a peak titer of 5.4×10(5) plaque forming units (PFU) per ml at 60 h post inoculation, which was 4- to 13-fold less than the peak titer of the other three viruses. NC16845b plaques were intermediate size averaging 3.3mm in diameter that was larger than MN184 plaques and smaller than VR-2385 and VR-2332. Using Northern blot analysis, viral and subgenomic RNA were detected that demonstrated variable levels of hybridization in some open reading frames (ORF) compared to the other viruses. NC16845b is 15,389 nucleotides in length and ORF 5 restriction fragment length polymorphism (RFLP) analysis demonstrated a 1-18-2 pattern. Among all available Type 2 complete genome sequences, NC16845b showed the highest nucleotide homology (91.2%) to atypical PRRSV strain JA142. Compared to prototype VR-2332, NC16845b demonstrated marked nucleotide variability within non-structural protein (nsp) 1β and nsp2, and a nucleotide deletion of 24 bases in nsp2. Sequence homology with VR-2332 and MN184 was 88.4% and 82.9%, respectively; homology with the ORF2-7 of VR-2385 was 90.4%. Collectively, these data indicate that, compared to prototype Type 2 PRRSV isolates, NC16845b exhibited slower in vitro growth properties, had regions of heterogeneity within ORF1a that corresponded to at least two individual virus quasispecies, and also contained a continuous 8 amino acid deletion in the nsp2 protein.

Plasmid containing CpG motifs enhances the efficacy of porcine reproductive and respiratory syndrome live attenuated vaccine

Porcine reproductive and respiratory syndrome (PRRS) is now among the most important swine diseases that affect the Chinese swine industry. Both killed and live attenuated vaccines are currently used against the disease, but neither of them could provide full protection after vaccination. In the present study, the adjuvanticity of a plasmid containing CpG motifs (pUC18-CpG) was introduced to enhance the efficacy of a commercial PRRS live attenuated vaccine. After vaccination, PRRSV-specific antibodies, PRRSV-specific cytokines, and clinical parameters were studied and compared between different vaccinated groups. During a following challenge study, co-administration of pUC18-CpG with the vaccine could confer higher protection rate. Our results have shown that co-administration of pUC18-CpG with the vaccine could elicit more potent adaptive immune response and provide better protection.

The role of porcine reproductive and respiratory syndrome (PRRS) virus structural and non-structural proteins in virus pathogenesis

Porcine reproductive and respiratory syndrome (PRRS) is an economically devastating viral disease affecting the swine industry worldwide. The etiological agent, PRRS virus (PRRSV), possesses a RNA viral genome with nine open reading frames (ORFs). The ORF1a and ORF1b replicase-associated genes encode the polyproteins pp1a and pp1ab, respectively. The pp1a is processed in nine non-structural proteins (nsps): nsp1α, nsp1β, and nsp2 to nsp8. Proteolytic cleavage of pp1ab generates products nsp9 to nsp12. The proteolytic pp1a cleavage products process and cleave pp1a and pp1ab into nsp products. The nsp9 to nsp12 are involved in virus genome transcription and replication. The 3′ end of the viral genome encodes four minor and three major structural proteins. The GP2a, GP3and GP4(encoded by ORF2a, 3 and 4), are glycosylated membrane associated minor structural proteins. The fourth minor structural protein, the E protein (encoded by ORF2b), is an unglycosylated membrane associated protein. The viral envelope contains two major structural proteins: a glycosylated major envelope protein GP5(encoded by ORF5) and an unglycosylated membrane M protein (encoded by ORF6). The third major structural protein is the nucleocapsid N protein (encoded by ORF7). All PRRSV non-structural and structural proteins are essential for virus replication, and PRRSV infectivity is relatively intolerant to subtle changes within the structural proteins. PRRSV virulence is multigenic and resides in both the non-structural and structural viral proteins. This review discusses the molecular characteristics, biological and immunological functions of the PRRSV structural and nsps and their involvement in the virus pathogenesis.

Comparative analysis of complete nucleotide sequence of porcine reproductive and respiratory syndrome virus (PRRSV) isolates in Thailand (US and EU genotypes)

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) is a causative agent of Porcine Reproductive and Respiratory Syndrome (PRRS). In this study, the complete nucleotide sequences of the selected two Thai PRRSV isolates, EU (01CB1) and US (01NP1) genotypes were determined since both isolates are the Thai prototypes.

RESULTS

01CB1 and 01NP1 contain 14,943 and 15,412 nucleotides, respectively. The viruses compose 2 untranslated regions (5' UTR and 3' UTR) and 8 open reading frames (ORFs) designated as ORF1a, ORF1b and ORF2-7. Phylogenetic analysis of full length of the viruses also showed that the 01CB1 and 01NP1 were grouped into the EU and US genotype, respectively. In order to determine the genetic variation and genetic relatedness among PRRSV isolates, the complete nucleotide sequences of PRRSV isolated in Thailand, 01CB1 and 01NP1 were compared with those of 2 EU strains (Lelystad, and EuroPRRSV), 6 US strains (MLV, VR2332, PA8, 16244B, SP and HUN4). Our results showed that the 01CB1 genome shares approximately 99.2% (Lelystad) and 95.2% (EuroPRRSV) nucleotide identity with EU field strains. While, the 01NP1 genome has 99.9% nucleotide identity with a live vaccine strain (MLV) and 99.5% and 98.5% nucleotide identity with 2 other US isolates, VR2332 and 16244B, respectively. In addition, ORF5 nucleotide sequences of 9 PRRS viruses recovered in Thailand during 2002-2008 were also included in this study. Phylogenetic analysis of ORF5 showed high similarity among EU and US genotypes of the recent Thai PRRS viruses (2007-2008 viruses) with 01CB1 and 01NP1.

CONCLUSION

Overall, the results suggested that the Thai EU isolate (01CB1) may evolve from the EU prototype, Lelystad virus, whereas the Thai US isolate (01NP1) may originate and evolve from the vaccine virus or its derivatives. Interestingly, the US-MLV vaccine was not available in the Thai market in 2001. The Vaccine-like virus might have persisted in the imported pigs or semen and later spread in the Thai swine industry. This report is the first report of complete nucleotide sequences of the Thai PRRS viruses both EU and US genotypes.

Molecular mutations associated with the in vitro passage of virulent porcine reproductive and respiratory syndrome virus

Mutants of a highly pathogenic, porcine reproductive, and respiratory syndrome virus (PRRSV), JXA1 strain, were prepared by continuous in vitro passage. Genomic sequence comparisons were made between mutants obtained at different passages and the parental strain JXA1. The mutant strain obtained at passage 80 contained a 12 nucleotide insertion and 108 nucleotide mutations that resulted in 45 amino acid changes. Most of these changes (89%) occurred between passage 10 and 45 and were genetically stable for the next 35-70 passages. A comparison of the mutants, their parental strain, and several American PRRSV strains, identified 13 characteristic amino acid changes. These sites, as well as the distinct 12 nucleotide insertion, represent possible genetic markers for the evaluation of live vaccine applications, particularly for additional studies of the safety and potency of live PRRSV vaccines.

Identification of virulence determinants of porcine reproductive and respiratory syndrome virus through construction of chimeric clones

In order to determine virulence associated genes in porcine reproductive and respiratory syndrome virus (PRRSV), a series of chimeric viruses were generated where specific genomic regions of a highly virulent PRRSV infectious clone (FL12) were replaced with their counterparts of an attenuated vaccine strain (Prime Pac). Initial genome-wide scanning using a sow reproductive failure model indicated that non-structural (ORF 1a and 1b) and structural (ORF2-7) genomic regions appear to be sites where virulence determinants of PRRSV may reside. These results thus confirm the multigenic character of PRRSV virulence. Additional chimeras containing each individual structural ORFs (2 through 7) of Prime Pac and ORF5 of Neb-1 (parental strain of Prime Pac) within the FL12 backbone were generated and tested individually for further mapping of virulence determinants. Our results allow to conclude that NSP3-8 and ORF5 are the location of major virulence determinants, while other virulence determinants may also be contained in NSP1-3, NSP10-12 and ORF2.

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.

High-level expression of the ORF6 gene of porcine reproductive and respiratory syndrome virus (PRRSV) in Pichia pastoris

High-level expression of the ORF6 gene of porcine reproductive and respiratory syndrome virus (PRRSV) has been proved very difficult. In this work, we cloned and sequenced the ORF6 gene of PRRSV and found that it could not be expressed in Pichia pastoris strain GS115. Then, the ORF6 gene was modified and synthesized based on the codon bias, poly (A) signal of yeast expression system and secondary structure of 5′-end mRNA of foreign gene. The modified gene was inserted into the yeast expression vector pPICZαA, induced and expressed by the same methods. The recombinant protein with a molecular mass of approximately 23 kDa was screened by SDS-PAGE and identified by Western blot with convalescent sera of animals infected with CH-1a strain of PRRSV. The results indicated that it was similar to the native protein. The expression level of the recombinant protein could attain 2.0 g/L. In the meanwhile, the optimal conditions for expression were determined. It provides an additional means for studying the structural and functional characteristics of PRRSV ORF6 gene.

Molecular epidemiology of porcine reproductive and respiratory syndrome virus (PRRSV) in Québec

Porcine reproductive and respiratory syndrome virus (PRRSV) strains identified in samples from 226 field cases originating from Québec herds and submitted over a 4-year period (March 1998-July 2002) were studied. Sequencing of PRRSV strains was performed on the ORF5 gene amplified product and restriction fragment length polymorphism (RFLP) patterns for enzymes MluI, HincII and SacII were determined on these sequences. Twenty-four other PRRSV isolates including three vaccine strains were also included for comparison purposes and the total of 250 PRRSV strains were used in a phylogenetic analysis. Clinical and epidemiological data were collected through a questionnaire for each of the submitted field cases. About 75% of the cases were submitted during autumn and winter. Over 60% of the cases were submitted for reproductive problems, 33% for respiratory problems and 6% for increased PRRSV serological titers in the herd in absence of clinical signs. In 69% of the cases there was a PRRS vaccination program for the herd. However, only 26% of the animals from which samples were obtained had been vaccinated themselves. The genomic analysis of this large number of strains revealed a great variability of PRRSV ORF5 with 59% of amino acid positions being polymorphic. A total of 29 RFLP patterns were obtained. The main RFLP patterns obtained were 1-8-4 (28%), 1-4-4 (16%), 1-2-4 (9%) and 1-11-4 (9%). The global findings derived from the molecular analysis of 226 PRRSV strains suggest that PRRSV circulating in Québec represent a different sub-population of strains. Vaccine-like strains were identified in 10% of the cases. A phylogenetic tree enabled the identification of 44 groupings comprising two to 23 strains each. Of the 250 sequences analyzed, 183 (73%) could be included in one of these groupings. The data collected from the questionnaires were used to establish epidemiological links between strains within groupings. The main relationships between strains within a grouping were the introduction of infected animals (19%) and area spread (33%). In 40% of the cases from which an area spread was suspected, herds were located within 3 km from another. Aerosol transmission was suspected in several cases, more than half of which belonged to different owners. In 41 herds, more than one strain (2-8) were identified over a period varying from 3 months to 4 years. Data indicated that a PRRSV strain can persist in a herd up to 3.5 years displaying as little as 2% variation in ORF5 during this time. In 78% of the herds with multiple submissions genetically different strains were identified; often within 1 year of the original identification. These genetically distinct strains were often associated with a recrudescence of moderate to severe clinical signs. Coexistence on the same farm of different PRRSV strains was also observed.

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.

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.

Antibody and cellular immune responses of swine following immunisation with plasmid DNA encoding the PRRS virus ORF's 4, 5, 6 and 7

The objectives of this study was to investigate the role of DNA vaccines in the generation of an immune response and that elicited against individually encoded proteins of PRRSV. The genomic regions encoding ORF s 4, 5, 6 and 7 of the PRRS virus vaccine strain were cloned into the mammalian expression vector pc DNA 3.1 (+). Inoculations with the recombinant plasmids resulted in detection of PRRS virus-specific antibodies in 71 per cent of the immunized animals by ELISA, virus neutralization and/or Western blotting assays. In addition, cellular immune responses were detected in 86 per cent of the immunized pigs by interferon gamma assay and/or proliferation assay. Pigs in the control group had no detectable immune response to PRRS virus. The results obtained demonstrated that DNA immunization against PRRS virus results in the production of both humoral and cell mediated immune responses in pigs. The results also indicate that neutralization epitopes for PRRS virus are present on the viral envelope glycoproteins encoded by ORF 4 and ORF 5.

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.