GP5 ectodomain epitope of porcine reproductive and respiratory syndrome virus, strain Lelystad virus

Investigation of the distribution and origin of porcine reproductive and respiratory syndrome virus 1 in the swine production chain: A retrospective study of three farms in Thailand

Background and Aim

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRS virus (PRRSV), is a global issue that affects Thai swine as well. In Thailand, PRRSV-2 predominates over PRRSV-1. The origin of PRRSV-1 transmission remains undiscovered. This study traced the source of infected pigs responsible for disease transmission among three pig-fattening farms and analyzed the spread of PRRSV-1.

Materials and Methods

A total of 696 swine samples from breeding and pig-fattening farms in Thailand were screened for PRRSV using open reading frames (ORF7) reverse transcription polymerase chain reaction (RT-PCR). Positive samples were identified as PRRSV-1 using ORF5 RT-PCR. The analysis included the study of nucleotide homology, GP5 amino acid sequences, and N-linked glycosylation patterns to assess the spread of PRRSV-1 across these farms.

Results

Genetic examination identified 28 PRRSV-1-positive samples, of which 13 were chosen as representatives. These strains were categorized into three groups based on breeding farm pig houses and showed distinct distribution patterns across pig-fattening farms. Group 1 included piglets transferred from pig house A to Nakhon Pathom, Chonburi, and Sa Kaeo. Groups 2 and 3 showed transfers from pig houses F and H to Chonburi and Sa Kaeo farms. All 13 PRRSV-1 strains were categorized into PRRSV-1 subtype 1/clade H. N-linked glycosylation analysis revealed that nearly all PRRSV-1 strains exhibited a conserved glycosylation pattern at amino acid positions N37, N46, and N53. This pattern is consistent with the glycosylation profile of the previous Thai PRRSV-1 subtype 1/clade H.

Conclusion

The present study highlights the persistent presence of PRRSV-1 in Thai swine, which leads to sporadic outbreaks. The molecular genetic analysis identified three primary strain groups dispersed throughout the pig production system, emphasizing the importance of regular monitoring for new PRRSV strains in this herd. Understanding the PRRSV-1 distribution in swine farms is vital for veterinarians. This knowledge supports strategies for eradicating the virus and managing swine health effectively in Thailand.

Construction of an infectious cloning system of porcine reproductive and respiratory syndrome virus and identification of glycoprotein 5 as a potential determinant of virulence and pathogenicity

Porcine reproductive and respiratory syndrome virus (PRRSV) infection of pigs causes a variety of clinical manifestations, depending on the pathogenicity and virulence of the specific strain. Identification and characterization of potential determinant(s) for the pathogenicity and virulence of these strains would be an essential step to precisely design and develop effective anti-PRRSV intervention. In this study, we report the construction of an infectious clone system based on PRRSV vaccine strain SP by homologous recombination technique, and the rescue of a chimeric rSP-HUB2 strain by replacing the GP5 and M protein-coding region from SP strain with the corresponding region from a highly pathogenic strain PRRSV-HUB2. The two recombinant viruses were shown to be genetically stable and share similar growth kinetics, with rSP-HUB2 exhibiting apparent growth and fitness advantages. Compared to in cells infected with PRRSV-rSP, infection of cells with rSP-HUB2 showed significantly more inhibition of the induction of type I interferon (IFN-β) and interferon stimulator gene 56 (ISG56), and significantly more promotion of the induction of proinflammatory cytokines IL-6, IL-8, ISG15 and ISG20. Further overexpression, deletion and mutagenesis studies demonstrated that amino acid residue F16 in the N-terminal region of the GP5 protein from HUB2 was a determinant for the phenotypic difference between the two recombinant viruses. This study provides evidence that GP5 may function as a potential determinant for the pathogenicity and virulence of highly pathogenic PRRSV.

Molecular Characteristics and Pathogenicity of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) 1 in Taiwan during 2019-2020

Two variants of porcine reproductive and respiratory syndrome virus (PRRSV), PRRSV 1 and PRRSV 2, have caused abortion in pregnant sows and respiratory distress in nursery pigs worldwide. PRRSV 2 has been thoroughly researched in Taiwan since 1993; however, the first case of PRRSV 1 was not reported until late 2018. To decipher the genetic characteristics of PRRSV 1 in Taiwan, open reading frame 5 (ORF5) genes of PRRSV 1 strains collected from 11 individual pig farms in 2019-2020 were successfully sequenced. All Taiwanese ORF5 sequences were closely related to Spanish-like PRRSV strains, which are considered to share a common evolutionary origin with the strain used for the PRRSV 1 vaccine. Analyses of amino acid (aa) and non-synonymous substitutions showed that genetic variations resulted in numerously specific codon mutations scattered across the neutralizing epitopes within the ORF5 gene. The PRRSV 1 challenge experiment disclosed the pathogenetic capability of the NPUST2789 isolate in nursery pigs. These findings provide comprehensive knowledge of the molecular diversity of the PRRSV 1 variant in local Taiwanese fields and facilitate the development of suitable immunization programs against this disease.

Predicting Antigenic Distance from Genetic Data for PRRSV-Type 1: Applications of Machine Learning

The control of porcine reproductive and respiratory syndrome (PRRS) remains a significant challenge due to the genetic and antigenic variability of the causative virus (PRRSV). Predominantly, PRRSV management includes using vaccines and live virus inoculations to confer immunity against PRRSV on farms. While understanding cross-protection among strains is crucial for the continued success of these interventions, understanding how genetic diversity translates to antigenic diversity remains elusive. We developed machine learning algorithms to estimate antigenic distance in silico, based on genetic sequence data, and identify differences in specific amino acid sites associated with antigenic differences between viruses. First, we obtained antigenic distance estimates derived from serum neutralization assays cross-reacting PRRSV monospecific antisera with virus isolates from 27 PRRSV1 viruses circulating in Europe. Antigenic distances were weakly to moderately associated with ectodomain amino acid distance for open reading frames (ORFs) 2 to 4 (ρ < 0.2) and ORF5 (ρ = 0.3), respectively. Dividing the antigenic distance values at the median, we then categorized the sera-virus pairs into two levels: low and high antigenic distance (dissimilarity). In the machine learning models, we used amino acid distances in the ectodomains of ORFs 2 to 5 and site-wise amino acid differences between the viruses as potential predictors of antigenic dissimilarity. Using mixed-effect gradient boosting models, we estimated the antigenic distance (high versus low) between serum-virus pairs with an accuracy of 81% (95% confidence interval, 76 to 85%); sensitivity and specificity were 86% and 75%, respectively. We demonstrate that using sequence data we can estimate antigenic distance and potential cross-protection between PRRSV1 strains. IMPORTANCE Understanding cross-protection between cocirculating PRRSV1 strains is crucial to reducing losses associated with PRRS outbreaks on farms. While experimental studies to determine cross-protection are instrumental, these in vivo studies are not always practical or timely for the many cocirculating and emerging PRRSV strains. In this study, we demonstrate the ability to rapidly estimate potential immunologic cross-reaction between different PRRSV1 strains in silico using sequence data routinely collected by production systems. These models can provide fast turn-around information crucial for improving PRRS management decisions such as selecting vaccines/live virus inoculation to be used on farms and assessing the risk of outbreaks by emerging strains on farms previously exposed to certain PRRSV strains and vaccine development among others.

Infection dynamics, transmission, and evolution after an outbreak of porcine reproductive and respiratory syndrome virus

The present study was aimed at describing the infection dynamics, transmission, and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) after an outbreak in a 300-sow farrow-to-wean farm that was implementing a vaccination program. Three subsequent batches of piglets (9-11 litters/batch) were followed 1.5 (Batch 1), 8 (Batch 2), and 12 months after (Batch 3) from birth to 9 weeks of age. The RT-qPCR analysis showed that shortly after the outbreak (Batch 1), one third of sows were delivering infected piglets and the cumulative incidence reached 80% by 9 weeks of age. In contrast, in Batch 2, only 10% animals in total got infected in the same period. In Batch 3, 60% litters had born-infected animals and cumulative incidence rose to 78%. Higher viral genetic diversity was observed in Batch 1, with 4 viral clades circulating, of which 3 could be traced to vertical transmission events, suggesting the existence of founder viral variants. In Batch 3 though only one variant was found, distinguishable from those circulating previously, suggesting that a selection process had occurred. ELISA antibodies at 2 weeks of age were significantly higher in Batch 1 and 3 compared to Batch 2, while low levels of neutralizing antibodies were detected in either piglets or sows in all batches. In addition, some sows present in Batch 1 and 3 delivered infected piglets twice, and the offspring were devoid of neutralizing antibodies at 2 weeks of age. These results suggest that a high viral diversity was featured at the initial outbreak followed by a phase of limited circulation, but subsequently an escape variant emerged in the population causing a rebound of vertical transmission. The presence of unresponsive sows that had vertical transmission events could have contributed to the transmission. Moreover, the records of contacts between animals and the phylogenetic analyses allowed to trace back 87 and 47% of the transmission chains in Batch 1 and 3, respectively. Most animals transmitted the infection to 1-3 pen-mates, but super-spreaders were also identified. One animal that was born-viremic and persisted as viremic for the whole study period did not contribute to transmission.

Using Alphafold2 to Predict the Structure of the Gp5/M Dimer of Porcine Respiratory and Reproductive Syndrome Virus

Porcine reproductive and respiratory syndrome virus is a positive-stranded RNA virus of the family Arteriviridae. The Gp5/M dimer, the major component of the viral envelope, is required for virus budding and is an antibody target. We used alphafold2, an artificial-intelligence-based system, to predict a credible structure of Gp5/M. The short disulfide-linked ectodomains lie flat on the membrane, with the exception of the erected N-terminal helix of Gp5, which contains the antibody epitopes and a hypervariable region with a changing number of carbohydrates. The core of the dimer consists of six curved and tilted transmembrane helices, and three are from each protein. The third transmembrane regions extend into the cytoplasm as amphiphilic helices containing the acylation sites. The endodomains of Gp5 and M are composed of seven β-strands from each protein, which interact via β-strand seven. The area under the membrane forms an open cavity with a positive surface charge. The M and Orf3a proteins of coronaviruses have a similar structure, suggesting that all four proteins are derived from the same ancestral gene. Orf3a, like Gp5/M, is acylated at membrane-proximal cysteines. The role of Gp5/M during virus replication is discussed, in particular the mechanisms of virus budding and models of antibody-dependent virus neutralization.

Isolation and genomic characterization of a Chinese NADC34-like PRRSV isolated from Jiangsu province

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important causative agents to swine industry, which has been epidemic more than 30 years. The emergence and recombination of new virus strains bring great challenges to the prevention and control of PRRSV. In the present study, we reported and characterized a novel PRRSV strain, designated as JS2021NADC34, which was for the first time isolated from clinical samples in Jiangsu province, China. Phylogenetic analysis demonstrated that JS2021NADC34 belonging to sublineage 1.5 of PRRSV-2 and was highly related to NADC34-like strains. Genetically, JS2021NADC34 strain had a continuous 100 aa depletion in NSP2, as compared to VR-2332 strain, which was consistent with most reported NADC34-like strains. Moreover, there were several amino acid substitutions occurred in the antigenic regions of GP2-GP5. Similar to other reported NADC34-like PRRSV in China, JS2021NADC34 had no recombination with other domestic strains, which indicates this sublineage of PRRSV may be directly transported from the United States and have not undergone extensive mutation and recombination with local strains yet.

Porcine reproductive and respiratory syndrome virus: phylogenetic analysis of circulating strains in the Republic of Ireland from 2016 to 2017

In order to investigate the genetic diversity of porcine reproductive and respiratory syndrome virus (PRRSV) strains currently circulating in the Republic of Ireland (ROI), the ORF5 gene from 17 field strains originating from four vaccinating commercial herds was sequenced and phylogenetically analysed. High genetic variability was observed between farms at the nucleotide (86.3-95.2%) and amino acid (85.5-96%) levels. Phylogenetic analysis confirmed that all field strains belonged to the European species (type 1) and clustered into three separate groups within the subtype 1 subgroup. This variation may pose challenges for diagnosis and prophylactic control of PRRSV through vaccination in the ROI.

Characterization of porcine reproductive and respiratory syndrome virus (ORF5 RFLP 1-7-4 viruses) in northern China

The porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS). Disease outbreaks caused by NADC30-like PRRSV strains were a bit prevalent in China in recent years. In the present study, two newly emerged PRRSV strains, which were designated as PRRSV-ZDXYL-China-2018-1 and PRRSV-ZDXYL-China-2018-2 strains were found from piglets' lung tissues in Northern China. The virus belongs to lineage 1 of the PRRSV genotype 2 and is closely related to US strains that possess the open reading frame (ORF5) restriction fragment length polymorphism (RFLP) 1-7-4. The two strains were identified from infected weaning piglet herds in Zhaodong City, Heilongjiang province of China. The complete genome of the PRRSV-ZDXYL-China-2018-1 and PRRSV-ZDXYL-China-2018-2 strains were 15093 nt and 15110 nt, and shared 96.7%-97.0% and 97.1%-97.4% similarities with the US identified, ISU10 and NADC34 strains respectively. Then the PRRSV-ZDXYL-China-2018-1 strain was successfully isolated from the clinical sample. Our results demonstrate, that the emergence of ORF5 RFLP 1-7-4-like PRRSVs in China, could pose a significant challenge to PRRSV epidemic prevention.

High-frequency mutation and recombination are responsible for the emergence of novel porcine reproductive and respiratory syndrome virus in northwest China

Porcine reproductive and respiratory syndrome (PRRS) is one of the most highly infectious diseases in the pig industry, resulting in enormous economic losses worldwide. In this study, a PRRS virus (PRRSV) strain was isolated from primary porcine alveolar macrophage cells in Xinjiang in northwest China. This new strain was sequenced and designated as XJzx1-2015, and its sequence was then compared to those of other representative PRRSV strains from around the world. Complete genomic characterisation showed that the full-length nucleotide sequence of XJzx1-2015 exhibited low-level similarity to NB/04 (91.6%), JXA1 (90.5%), CH-1a (90.2%), VR-2332 (86.9%), QYYZ (85.7%), and JL580 (82.2%), with the highest similarity to HK13 (91.7%) sequence identity. Nonstructural protein 2 (NSP2) and glycosylated protein (GP) 2 of XJzx1-2015 had deletions of five and two amino acids, respectively, corresponding to strain VR-2332 positions 475-479 and 173-174. Phylogenetic analysis based on complete genome sequences showed that XJzx1-2015 and four other strains from China formed a new subgenotype closely related to other sublineage 8.7 (JXA1-like) strains belonging to the North American genotype. However, phylogenetic analysis based on NSP2 and GP5 showed that XJzx1-2015 clustered with sublineage 8.7 (JXA1-like, CH-1a-like) and lineage 3 (QYYZ-like) strains, respectively. Recombination analysis indicated that XJzx1-2015 is an intersubgenotype recombinant of CH-1a-like and QYYZ-like strains. Overall, our findings demonstrate that XJzx1-2015 is a novel PRRSV strain with a significantly high frequency of mutation and a recombinant between lineage 3 and sublineage 8.7 identified in northwest China. These results provide important insights into PRRSV evolution.

Evaluating an automated clustering approach in a perspective of ongoing surveillance of porcine reproductive and respiratory syndrome virus (PRRSV) field strains

Porcine reproductive and respiratory syndrome virus (PRRSV) has a major economic impact on the swine industry. The important genetic diversity needs to be considered for disease management. In this regard, information on the circulating endemic strains and their dispersal patterns through ongoing surveillance is beneficial. The objective of this project was to classify Quebec PRRSV ORF5 sequences in genetic clusters and evaluate stability of clustering results over a three-year period using an in-house automated clustering system. Phylogeny based on maximum likelihood (ML) was first inferred on 3661 sequences collected in 1998-2013 (Run 1). Then, sequences collected between January 2014 and September 2016 were sequentially added into 11 consecutive runs, each one covering a three-month period. For each run, detection of clusters, which were defined as groups of ≥15 sequences having a≥70% rapid bootstrap support (RBS) value, was automated in Python. Cluster stability was described for each cluster and run based on the number of sequences, RBS value, maximum pairwise distance and agreement in sequence assignment to a specific cluster. First and last run identified 29 and 33 clusters, respectively. In the last run, about 77% of the sequences were classified by the system. Most clusters were stable through time, with sequences attributed to one cluster in Run 1 staying in the same cluster for the 11 remaining runs. However, some initial groups were further subdivided into subgroups with time, which is important for monitoring since one specific wild-type cluster increased from 0% in 2007 to 45% of all sequences in 2016. This automated classification system will be integrated into ongoing surveillance activities, to facilitate communication and decision-making for stakeholders of the swine industry.

Genetic variation of highly pathogenic Indian porcine reproductive and respiratory syndrome viruses after introduction in 2013

To study its possible link to pathogenicity, the genomic variation in full ORF5 and ORF7 genes, and their encoded proteins in 26 field HP-PRRSV isolates from three major HP-PRRS outbreaks occurred in India, since 2013 was analysed. Sequence analysis and phylogenetic tree revealed involvement of genetically different strain in each outbreak of India rather persistence of a single strain. Analysis and comparison of N protein amino acid sequences of HP-PRRSV with VR2332 revealed consistent mutation at position 15D to N or K and 46 K to R in all the HP-PRRSV. GP5 protein showed consistent mutations at 29 positions from that of VR2332. The potential Nglycosylation sites in GP5 was found variable from 4–5 with one additional N-glycan moiety around the neutralizing epitope B. However, the ‘decoy’ epitope A was found highly conserved in all the HP-PRRSV.

Porcine reproductive and respiratory disease virus: Evolution and recombination yields distinct ORF5 RFLP 1-7-4 viruses with individual pathogenicity

Recent cases of porcine reproductive and respiratory syndrome virus (PRRSV) infection in United States swine-herds have been associated with high mortality in piglets and severe morbidity in sows. Analysis of the ORF5 gene from such clinical cases revealed a unique restriction fragment polymorphism (RFLP) of 1-7-4. The genome diversity of seventeen of these viruses (81.4% to 99.8% identical; collected 2013-2015) and the pathogenicity of 4 representative viruses were compared to that of SDSU73, a known moderately virulent strain. Recombination analyses revealed genomic breakpoints in structural and nonstructural regions of the genomes with evidence for recombination events between lineages. Pathogenicity varied between the isolates and the patterns were not consistent. IA/2014/NADC34, IA/2013/ISU-1 and IN/2014/ISU-5 caused more severe disease, and IA/2014/ISU-2 did not cause pyrexia and had little effect on pig growth. ORF5 RFLP genotyping was ineffectual in providing insight into isolate pathogenicity and that other parameters of virulence remain to be identified.

The complex co-translational processing of glycoprotein GP5 of type 1 porcine reproductive and respiratory syndrome virus

GP5 and M, the major membrane proteins of porcine reproductive and respiratory syndrome virus (PRRSV), are the driving force for virus budding and a target for antibodies. We studied co-translational processing of GP5 from an European PRRSV-1 strain. Using mass spectrometry, we show that in virus particles of a Lelystad variant, the signal peptide of GP5 was absent due to cleavage between glycine-34 and asparagine-35. This cleavage site removes an epitope for a neutralizing monoclonal antibody, but leaves intact another epitope recognized by neutralizing pig sera. Upon ectopic expression of this GP5 in cells, signal peptide cleavage was however inefficient. Complete cleavage occurred when cysteine-24 was changed to proline or an unused glycosylation site involving asparagine-35 was mutated. Insertion of proline at position 24 also caused carbohydrate attachment to asparagine-35. Glycosylation sites introduced downstream of residue 35 were used, but did not inhibit signal peptide processing. Co-expression of the M protein rescued this processing defect in GP5, suggesting a novel function of M towards GP5. We speculate that a complex interplay of the co-translational modifications of GP5 affect the N-terminal structure of the mature proteins and hence its antigenicity.

Heterologous expression of porcine reproductive and respiratory syndrome virus glycoprotein 5 in Bordetella bronchisepticaaroA mutant

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important disease around the globe. Protection against this virus remains problematic. Here, we evaluated antibody (IgG & IgA) inducibility of a heterologous PRRSV glycoprotein 5 (GP5) expressed in a live attenuated Bordetella bronchisepticaaroA mutant strain (BBS-GP5). Mice and pigs were primed with recombinant GP5 (rGP5) subcutaneously followed by boosting with live BBS-GP5. As a result, anti-GP5 IgG was induced in both mice (P<0.001) and pigs (P<0.1). Pigs were challenged with live PRRSV (VR2332). Viral RNA was found to be significantly (P<0.01) removed in the vaccinated pig group. Overall, BBS-GP5 is a good candidate as a live attenuated vaccine against PRRSV infection.

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Modulates Interferon-β Expression Mainly Through Attenuating Interferon-Regulatory Factor 3 Phosphorylation

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) that emerged from classic PRRSV causes more severe damage to the swine industry. The earlier reports indicating inhibition of interferon-β (IFN-β) expression by PRRSV through total blockage of IFN-regulatory factor 3 (IRF3) nuclear translocation made us investigate the mechanism of IFN-β expression in HP-PRRSV infection. For this purpose, the IRF3 nuclear translocation in the control group [Poly (I:C)] and test group [Poly (I:C)+HP-PRRSV] was detected by immunofluorescence, and the results showed that IRF3 nuclear translocation in cells with PRRSV was weaker than cells without PRRSV, which was different from the previous study. In addition, the IFN-β mRNA and protein expression was observed to be inhibited by HP-PRRSV along with decreased IRF3 mRNA and total protein, and IRF3 nuclear translocation of test group was suppressed in MARC-145 and porcine alveolar macrophage cells in comparison with the control group. The quantity of phosphorylated IRF3 protein was also reduced after HP-PRRSV infection. However, CREB-binding protein (CBP) expression did not change between the control and test group. These results indicate that the inhibition of IFN-β expression is mainly due to the quantitative change in the amount of phosphorylated IRF3 in the cytoplasm, but not dependent on the complete blockage of IRF3 nuclear translocation or the restraining of CBP expression in the nucleus by HP-PRRSV.

Evaluation of the use of non-pathogenic porcine circovirus type 1 as a vaccine delivery virus vector to express antigenic epitopes of porcine reproductive and respiratory syndrome virus

We previously demonstrated that the C-terminus of the capsid gene of porcine circovirus type 2 (PCV2) is an immune reactive epitope displayed on the surface of virions. Insertion of foreign epitope tags in the C-terminus produced infectious virions that elicited humoral immune responses against both PCV2 capsid and the inserted epitope tags, whereas mutation in the N terminus impaired viral replication. Since the non-pathogenic porcine circovirus type 1 (PCV1) shares similar genomic organization and significant sequence identity with pathogenic PCV2, in this study we evaluated whether PCV1 can serve as a vaccine delivery virus vector. Four different antigenic determinants of porcine reproductive and respiratory syndrome virus (PRRSV) were inserted in the C-terminus of the PCV1 capsid gene, the infectivity and immunogenicity of the resulting viruses are determined. We showed that an insertion of 12 (PRRSV-GP2 epitope II, PRRSV-GP3 epitope I, and PRRSV-GP5 epitope I), and 14 (PRRSV-GP5 epitope IV) amino acid residues did not affect PCV1 replication. We successfully rescued and characterized four chimeric PCV1 viruses expressing PRRSV linear antigenic determinants (GP2 epitope II: aa 40-51, ASPSHVGWWSFA; GP3 epitope I: aa 61-72, QAAAEAYEPGRS; GP5 epitope I: aa 35-46, SSSNLQLIYNLT; and GP5 epitope IV: aa 187-200, TPVTRVSAEQWGRP). We demonstrated that all chimeric viruses were stable and infectious in vitro and three chimeric viruses were infectious in vivo. An immunogenicity study in pigs revealed that PCV1-VR2385EPI chimeric viruses elicited neutralizing antibodies against PRRSV-VR2385. The results have important implications for further evaluating PCV1 as a potential vaccine delivery vector.

A porcine reproductive and respiratory syndrome virus vaccine candidate based on PRRSV glycoprotein 5 and the Toll-like receptor 5 agonist Salmonella typhimurium flagellin

Glycoprotein 5 (GP5) from porcine reproductive and respiratory syndrome virus (PRRSV) is a key inducer of neutralizing antibodies. A truncated GP5 gene lacking the signal peptide and transmembrane sequences was amplified via an overlap PCR method and inserted into prokaryotic expression vectors, pET32a or pGEX-6p-1, to add an His or GST tag, respectively. His-tagged GP5 was induced with IPTG, verified by SDS-PAGE and Western blotting, and purified to serve as an immunogen accompanied with the Salmonella typhimurium flagellin (FliC), a Toll-like receptor 5 (TLR5) agonist. Levels of TLR5 and cytokine mRNAs in spleens of mice following injection with FliC were detected by qRT-PCR to verify the activation of innate immunity. FliC was used as an adjuvant and administered with the GP5 to C57BL/6 mice via intraperitoneal injection. Coadministration of GP5 with FliC induced a significantly enhanced GP5-specific IgG and IFN-γ response compared with administration of GP5 alone, and the GP5-specific titer in the GP5 + FliC coadministration group was elevated almost twofold after the third immunization. These results indicate that FliC is an effective adjuvant, increasing the induction of antibodies against GP5 with the induction of both humoral and cellular immune responses.

Molecular epidemiology of porcine reproductive and respiratory syndrome viruses isolated from 1991 to 2013 in Taiwan

Porcine reproductive and respiratory syndrome virus (PRRSV) was first identified in Taiwan in 1991, but the genetic diversity and evolution of PRRSV has not been thoroughly investigated over the past 20 years. The aim of this study was to bridge the gap in understanding of its molecular epidemiology. A total of 31 PRRSV strains were collected and sequenced. The sequences were aligned using the MUSCLE program, and phylogenetic analysis were performed by the maximum-likelihood method and the neighbor-joining method using MEGA 5.2 software. In the early 1990s, two prototype strains, WSV and MD001 of the North American genotype, were first identified. Over the years, both viruses evolved separately. The population dynamics of PRRSV revealed that the strains of the MD001 group were predominant in Taiwan. Evolution was manifested in changes in the nsp2 and ORF5 genes. In addition, a suspected newly invading exotic strain was recovered in 2013, suggesting that international spread is still taking place and that it is affecting the population dynamics. Overall, the results provide an important basis for vaccine development for the control and prevention of PRRS.

Expression of antigenic epitopes of porcine reproductive and respiratory syndrome virus (PRRSV) in a modified live-attenuated porcine circovirus type 2 (PCV2) vaccine virus (PCV1-2a) as a potential bivalent vaccine against both PCV2 and PRRSV

Co-infection of pigs in the field with porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) is common and poses a major concern in effective control of PCV2 and PRRSV. We previously demonstrated that insertion of foreign epitope tags in the C-terminus of PCV2 ORF2 produced infectious virions that elicited humoral immune responses against both PCV2 capsid and inserted epitope tags. In this study, we aimed to determine whether the non-pathogenic chimeric virus PCV1-2a, which is the basis for the licensed PCV2 vaccine Fostera PCV, can express PRRSV antigenic epitopes, thus generating dual immunity as a potential bivalent vaccine against both PCV2 and PPRSV. Four different linear B-cell antigenic epitopes of PRRSV were inserted into the C-terminus of the capsid gene of the PCV1-2a vaccine virus. We showed that insertion of 12 (PRRSV-GP2 epitope II, PRRSV-GP3 epitope I, and PRRSV-GP5 epitope I), and 14 (PRRSV-GP5 epitope IV) amino acid residues did not impair the replication of the resulting PCV1-2a-PRRSVEPI chimeric viruses in vitro. The four chimeric PCV1-2a viruses expressing PRRSV B-cell linear epitopes were successfully rescued and characterized. An immunogenicity study in pigs revealed that two of the four chimeric viruses, PCV1-2a-PRRSVEPIGP3IG and PCV1-2a-PRRSVEPIEPIGP5IV, elicited neutralizing antibodies against PRRSV VR2385 as well as PCV2 (strains PCV2a, PCV2b, and mPCV2b). The results have important implications for exploring the potential use of PCV1-2a vaccine virus as a live virus vector to develop bivalent MLVs against both PCV2 and PRRSV.

Glycoprotein 5 of porcine reproductive and respiratory syndrome virus strain SD16 inhibits viral replication and causes G2/M cell cycle arrest, but does not induce cellular apoptosis in Marc-145 cells

Cell apoptosis is common after infection with porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV GP5 has been reported to induce cell apoptosis. To further understand the role of GP5 in PRRSV induced cell apoptosis, we established Marc-145 cell lines stably expressing full-length GP5, GP5(Δ84-96) (aa 84-96 deletion), and GP5(Δ97-119) (aa 97-119 deletion). Cell proliferation, cell cycle progression, cell apoptosis and virus replication in these cell lines were evaluated. Neither truncated nor full-length GP5 induced cell apoptosis in Marc-145 cells. However, GP5(Δ97-119), but not full-length or GP5(Δ84-96), induced a cell cycle arrest at the G2/M phase resulting in a reduction in the growth of Marc-145 cells. Additionally, GP5(Δ84-96) inhibited the replication of PRRSV in Marc-145 cells through induction of IFN-β. These findings suggest that PRRSV GP5 is not responsible for inducing cell apoptosis in Marc-145 cells under these experimental conditions; however it has other important roles in virus/host cell biology.

A porcine reproductive and respiratory syndrome virus (PRRSV) vaccine candidate based on the fusion protein of PRRSV glycoprotein 5 and the Toll-like Receptor-5 agonist Salmonella Typhimurium FljB

BACKGROUND

Porcine reproductive and respiratory syndrome (PRRS) is characterized by severe reproductive failure and severe pneumonia in neonatal pigs and is caused by PRRS virus (PRRSV). Glycoprotein 5 (GP5) from PRRSV is a key inducer of neutralizing antibodies. Flagellin, a toll-like receptor 5 (TLR-5) agonist, is an effective inducer of innate immune responses. This study presents a novel PRRSV vaccine candidate based on the adjuvant effect of Salmonella Typhimurium FljB fused with PRRSV GP5.

RESULTS

A truncated rGP5 gene lacking the signal peptide and transmembrane sequences was amplified and inserted into prokaryotic expression vectors, pColdI or pGEX-6p-1. Salmonella Typhimurium flagellin fljB was amplified and inserted into the plasmid pCold-rGP5, generating recombinant plasmid pCold-rGP5-fljB. Histidine (His)-tagged rGP5 and fusion protein rGP5-FljB were induced with isopropyl-β-d-thiogalactoside, verified by SDS-PAGE and western blotting, and purified via Ni-NTA affinity columns. The TLR-5-specific bioactivity of fusion protein rGP5-FljB was determined by detecting the expression levels of the cytokine IL-8 in HEK293-mTLR5 cells by sandwich ELISA. The purified endotoxin-free proteins were administered intraperitoneally in a C3H/HeJ mouse model. The results show that immunization with the fusion protein rGP5-FljB induced a significantly enhanced GP5-specific and PRRSV-specific IgG response that persisted for almost 5 weeks. Co-administration of the rGP5 with R848 or Alum also yielded a higher IgG response than administration of rGP5 alone. The IgG1/IgG2a ratio in the rGP5-FljB immunization group was significantly higher (9-fold) than that in the rGP5 alone group and was equivalent to the response in the rGP5 + Alum immunization group, suggesting a strong Th2 immune response was induced by the fusion protein.

CONCLUSIONS

Purified fusion protein rGP5-FljB is capable of activating the innate immune response, as demonstrated by the results of our TLR-5-specific bioactivity assay, and FljB has adjuvant activity, as shown by the results from our administration of rGP5-FljB in a mouse model. Our findings confirm that FljB could serve as an excellent adjuvant for the production of GP5-specific and PRRSV-specific IgG antibodies as part of an induction of a robust humoral immune response.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Observation of high recombination occurrence of Porcine Reproductive and Respiratory Syndrome Virus in field condition

Recombination in Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a well-documented phenomenon. A high recombination frequency has been reported in experimental conditions both in vitro and in vivo, and its role in driving viral evolution has been postulated by several authors. However field evidences are rare, mainly obtained from large-scale sampling and typically represented by single sequences rather than by groups of circulating "recombinant progenies". The present work was aimed to investigate the gray area between experimental studies and large-scale epidemiological investigations. The study was performed on ORF5, ORF7 and concatenated sequences obtained in our laboratory or available in GenBank collected between 2009 and 2012 in northern Italy. Six independent recombinant strains out of 66 concatenated sequences (∼9%) were found, demonstrating a high recombination frequency respect to previous field studies but comparable to in vitro experiments. In silico analysis let speculate that this new strain displayed physicochemical features diverse enough to potentially alter its immunological properties. Taken altogether, the results of our study support previous experimental evidences that depict PRRSV to be extremely prone to recombination. The limited temporal and geographical spread of recombinant strains however states in favor of a limited fitness of the recombinant progeny compared to parental strains and the marginal role of this phenomenon in PRRSV evolution.

Antigenic structures stably expressed by recombinant TGEV-derived vectors

Coronaviruses (CoVs) are positive-stranded RNA viruses with potential as immunization vectors, expressing high levels of heterologous genes and eliciting both secretory and systemic immune responses. Nevertheless, its high recombination rate may result in the loss of the full-length foreign gene, limiting their use as vectors. Transmissible gastroenteritis virus (TGEV) was engineered to express porcine reproductive and respiratory syndrome virus (PRRSV) small protein domains, as a strategy to improve heterologous gene stability. After serial passage in tissue cultures, stable expression of small PRRSV protein antigenic domains was achieved. Therefore, size reduction of the heterologous genes inserted in CoV-derived vectors led to the stable expression of antigenic domains. Immunization of piglets with these TGEV vectors led to partial protection against a challenge with a virulent PRRSV strain, as immunized animals showed reduced clinical signs and lung damage. Further improvement of TGEV-derived vectors will require the engineering of vectors with decreased recombination rate.

ORF5-based evolutionary and epidemiological dynamics of the type 1 porcine reproductive and respiratory syndrome virus circulating in Korea

This study applied a number of advanced genetic analysis tools to investigate the evolutionary trajectories and epidemiological dynamics of Korean type 1 PRRSV based on variations in the ORF5 gene over a long-term period from 2005 to 2013. Maximum likelihood phylogenetic analysis performed on large, worldwide ORF5 sequences (n=1127) strongly suggested no further introduction of genetically novel type 1 PRRSV into Korean pig farms, with the identification of only two clusters (I and II) in circulation to date. Using a codon-based extension of the Bayesian relaxed clock model, this study was able to distinguish between synonymous and non-synonymous substitutions and demonstrated that, while the absolute rates of synonymous substitution (E[S]) were similar between clusters I and II, the absolute rate of non-synonymous substitution (E[N]) was significantly different between the clusters. Cluster I was found to have an elevated E[N]/E[S] ratio relative to cluster II on the internal branches, compared to the external branches. Additionally, many fewer sites were predicted under diversifying selection in cluster II than in cluster I. Utilizing the Bayesian skyride method and the novel Bayesian birth-death skyline plot method, this study provided insights into the epidemiological dynamics of type 1 PRRSV in Korea by revealing that each cluster experienced a unique epidemic growth and by uncovering correlations between the effective population size and effective reproductive number.

Genetic and antigenic characterization of complete genomes of Type 1 Porcine Reproductive and Respiratory Syndrome viruses (PRRSV) isolated in Denmark over a period of 10 years

Porcine Reproductive and Respiratory Syndrome (PRRS) caused by the PRRS virus (PRRSV) is considered one of the most devastating swine diseases worldwide. PRRS viruses are divided into two major genotypes, Type 1 and Type 2, with pronounced diversity between and within the genotypes. In Denmark more than 50% of the herds are infected with Type 1 and/or Type 2 PRRSV. The main objective of this study was to examine the genetic diversity and drift of Type 1 viruses in a population with limited introduction of new animals and semen. A total of 43 ORF5 and 42 ORF7 nucleotide sequences were obtained from viruses collected from 2003 to February 2013. Phylogenetic analysis of ORF5 nucleotide sequences showed that the Danish isolates formed two major clusters within the subtype 1. The nucleotide identity to the subtype 1 protogenotype Lelystad virus (LV) spanned 84.9-98.8% for ORF5 and 90.7-100% for ORF7. Among the Danish viruses the pairwise nucleotide identities in ORF5 and ORF7 were 81.2-100% and 88.9-100%, respectively. Sequencing of the complete genomes, including the 5'- and 3'-end nucleotides, of 8 Danish PRRSV Type 1 showed that the genome lengths differed from 14,876 to 15,098 nucleotides and the pairwise nucleotide identity among the Danish viruses was 86.5-97.3% and the identity to LV was 88.7-97.9%. The study strongly indicated that there have been at least two independent introductions of Type 1 PRRSV in Denmark and analysis of the full genomes revealed a significant drift in several regions of the virus.

Arterivirus molecular biology and pathogenesis

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

Dynamics and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 following modified live PRRSV vaccination in a PRRSV-infected herd

The objective of this study was to investigate the dynamics and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 following the use of a modified live PRRSV (MLV) vaccine. A PRRSV-positive farm with coexistence of types 1 and 2 and no history of MLV vaccination was investigated. Vaccination with a type 2 MLV (Ingelvac PRRS MLV, Boehringer Ingelheim, USA) was implemented. All sows were vaccinated at monthly intervals for two consecutive months and then every third month. Piglets were vaccinated once at 7-10 days of age and weaned to nursery facilities at 21-23 days of age. Serum samples were collected monthly before and after vaccination from four population groups, including replacement gilts and suckling, nursery and finishing pigs, and assayed by PCR. After a year of blood collection, amplified products were sequenced, resulting in 277 complete ORF5 gene sequences from 145 type 1 and 132 type 2 isolates. Prior to and following vaccination, both type 1 and type 2 PRRSV were isolated and found to coexist in an individual pig. Each genotype evolved separately without influencing the strain development of the other. Although the substitution rates of both genotypes were relatively similar, MLV vaccination appears to increase the heterogenicity of type 2 PRRSV, resulting in the emergence of three novel type 2 PRRSV clusters in the herd, including an MLV-like cluster, which disappeared within the month following whole-herd vaccination. Two additional clusters included one related to the MLV vaccine and one related to the endemic cluster of the herd.

Oral immunogenicity of porcine reproductive and respiratory syndrome virus antigen expressed in transgenic banana

Porcine reproductive and respiratory syndrome virus (PRRSV) is a persistent threat of economically significant influence to the swine industry worldwide. Recombinant DNA technology coupled with tissue culture technology is a viable alternative for the inexpensive production of heterologous proteins in planta. Embryogenic cells of banana cv. 'Pei chiao' (AAA) have been transformed with the ORF5 gene of PRRSV envelope glycoprotein (GP5) using Agrobacterium-mediated transformation and have been confirmed. Recombinant GP5 protein levels in the transgenic banana leaves were detected and ranged from 0.021%-0.037% of total soluble protein. Pigs were immunized with recombinant GP5 protein by orally feeding transgenic banana leaves for three consecutive doses at a 2-week interval and challenged with PRRSV at 7 weeks postinitial immunization. A vaccination-dependent gradational increase in the elicitation of serum and saliva anti-PRRSV IgG and IgA was observed. Furthermore, significantly lower viraemia and tissue viral load were recorded when compared with the pigs fed with untransformed banana leaves. The results suggest that transgenic banana leaves expressing recombinant GP5 protein can be an effective strategy for oral delivery of recombinant subunit vaccines in pigs and can open new avenues for the production of vaccines against PRRSV.

Genetic diversity of the ORF5 gene of porcine reproductive and respiratory syndrome virus (PRRSV) genotypes I and II in Thailand

To investigate the genetic diversity of porcine reproductive and respiratory syndrome virus (PRRSV) in Thailand, 279 ORF5 sequences of PRRSV collected during 2010-2011 from 102 swine herds in five swine-producing areas were analyzed. The co-existence of European (EU) and North American (NA) genotypes was observed in 98 % of herds investigated and was evident at the pig level. Both genotypes have evolved separately with a temporal influence on strain development. Novel introductions influence the genetic diversity of the NA genotype. Although Thai EU and NA isolates develop their own clusters that are separate from those of other countries, there was no geographic influence on strain development within Thailand.

Immunisation of pigs with a major envelope protein sub-unit vaccine against porcine reproductive and respiratory syndrome virus (PRRSV) results in enhanced clinical disease following experimental challenge

Disease exacerbation was observed in pigs challenged with virulent porcine reproductive and respiratory syndrome virus (PRRSV) following immunisation with a recombinant GP5 sub-unit PRRSV vaccine (rGP5) produced in E. coli. Eighteen animals were divided into three experimental groups: group A were immunised twice IM with rGP5, 21 days apart; group B acted as positive controls (challenged but not immunised); and group C were negative controls. Pigs in groups A and B were challenged 21 days after the second immunisation of the group A animals. Following challenge, three pigs given rGP5 exhibited more severe clinical signs than the positive controls, including respiratory distress and progressive weight-loss. Although not statistically significant, the more severe disease exhibited by group A animals may suggest previous immunisation as a contributory factor. The mechanisms of these findings remain unclear and no association could be established between the severity of disease, non-neutralising antibody concentrations and tissue viral loads.

Porcine Reproductive and Respiratory Syndrome Virus isolates differ in their susceptibility to neutralization

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is highly heterogenic. This heterogeneity has an effect on antigenic composition of PRRSV and might create differences in sensitivity to neutralization between isolates. The sensitivity to neutralization could be an important feature of PRRSV isolates because it is likely that isolates resistant to neutralization pose a significant challenge for the development of vaccines that elicit broad protective immunity. Nonetheless, little information is available for understanding or categorizing the viral neutralization phenotype of PRRSV isolates. Consequently, the main purpose of this study was to determine whether PRRSV isolates differ in their susceptibility to neutralization and if they can be classified in different categories based on their neutralization phenotype. For this purpose, a panel of 39 PRRSV isolates and a set of 30 hyperimmune monospecific sera were used in cross-neutralization assays. The results of this study indicate that PRRSV isolates differ in their sensitivity to neutralization and k-means clustering system allowed classifying the isolates in four different categories according to their neutralization phenotype: highly sensitive, sensitive, moderately sensitive and resistant to neutralization. Further analyses using two additional clustering systems that considered individual data for the classification of the isolates confirmed that classification obtained by k-means is accurate in most cases and that only in a few instances classification is less stringent. Sequences of GP3, GP4 and GP5 were analyzed but no correlation could be found between the sequence of previously identified neutralizing epitopes or the number of N-linked glycosylation sites in different proteins and the neutralization phenotype of the isolates. These data provide the first systematic assessment of overall neutralization sensitivities of a panel of diverse PRRSV isolates. The classification of the isolates provides a useful tool to facilitate the systematic characterization of neutralizing antibody production elicited by new vaccine candidates.

Genetic and immunobiological diversities of porcine reproductive and respiratory syndrome genotype I strains

Genetic diversity of porcine reproductive and respiratory syndrome virus (PRRSV) has been based on ORF5/GP5 and ORF7/N protein variations. Complete viral genome studies are limited and focused on a single or a few set of strains. Moreover, there is a general tendency to extrapolate results obtained from a single isolate to the overall PRRSV population. In the present study, six genotype-I isolates of PRRSV were sequenced from ORF1a to ORF7. Phylogenetic comparisons and the variability degree of known linear B-epitopes were done considering other available full-length genotype-I sequences. Cytokine induction of all strains was also evaluated in different cellular systems. Non structural protein 2 (nsp2) was the most variable part of the virus with 2 out of 6 strains harboring a 74 aa deletion. Deletions were also found in ORF3 and ORF4. Phylogenetic analyses showed that isolates could be grouped differently depending on the ORF examined and the highest similarity with the full genome cluster was found for the nsp9. Interestingly, most of predicted linear B-epitopes in the literature, particularly in nsp2 and GP4 regions, were found deleted or varied in some of our isolates. Moreover, 4 strains, those with deletions in nsp2, induced TNF-α and 3 induced IL-10. These results underline the high genetic diversity of PRRSV mainly in nsp1, nsp2 and ORFs 3 and 4. This variability also affects most of the known linear B-epitopes of the virus. Accordingly, different PRRSV strains might have substantially different immunobiological properties. These data can contribute to the understanding of PRRSV complexity.

In vivo growth of porcine reproductive and respiratory syndrome virus engineered nsp2 deletion mutants

Prior studies on PRRSV strain VR-2332 non-structural protein 2 (nsp2) had shown that as much as 403 amino acids could be removed from the hypervariable region without losing virus viability in vitro. We utilized selected nsp2 deletion mutants to examine in vivo growth. Young swine (4 pigs/group; 5 control swine) were inoculated intramuscularly with one of 4 nsp2 deletion mutants (rΔ727–813, rΔ543–726, rΔ324–523, rΔ324–726) or full-length recombinant virus (rVR-2332). Serum samples were collected on various days post-inoculation and analyzed by HerdChek* ELISA, PRRSV real time RT-PCR, gamma interferon (IFN-γ) ELISA, and nucleotide sequence analysis of the entire nsp2 coding region. Tracheobronchial lymph node weight compared to body weight was recorded for each animal and used as a clinical measurement of viral pathogenesis. Results showed that all deletion mutants grew less robustly than full-length recombinant virus, yet all but the large deletion virus (rΔ324–726) recovered to parental viral RNA levels by study end. Swine receiving the rΔ727–813 mutants had a significant decrease in lymph node enlargement compared to rVR-2332. While swine infection with rVR-2332 caused a rapid rise in serum IFN-γ levels, the IFN-γ protein produced by infection with 3 of the 4 deletion mutant viruses was significantly reduced, perhaps due to differences in viral growth kinetics. The rΔ543–726 nsp2 mutant virus, although growth impaired, mimicked rVR-2332 in inducing a host serum IFN-γ response but exhibited a 2-week delay. Targeted sequencing showed that all deletions were stable in the region coding for nsp2 after one swine passage. The data suggested that the selected nsp2 deletion mutants were growth attenuated in swine, altered the induction of serum IFN-γ, an innate cytokine of unknown function in PRRSV clearance, and pointed to a domain that may influence tracheobronchial lymph node size.

Certainties, doubts and hypotheses in porcine reproductive and respiratory syndrome virus immunobiology

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most costly pathogens for the swine industry. Since its emergence some 20 years ago, much has been learned about the immunobiology of PRRSV. Although vaccines are available, they do not provide full and universal protection against PRRSV infection. In the present review, current knowledge on the virus's immunobiology will be discussed including: role of viral receptors, innate immune response to the virus, regulation of the immune response by PRRSV, and the characteristics and role of adaptive immunity. In addition, some hypotheses for future research in this area are presented.

Novel strategies and approaches to develop the next generation of vaccines against porcine reproductive and respiratory syndrome virus (PRRSV)

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important swine pathogen. Since its discovery in the early 1990s, tremendous progresses have been made in understanding the molecular biology and pathogenesis of PRRSV. Although modified live-attenuated vaccines (MLVs) and inactivated vaccines against PRRSV have been available for more than a decade, the disease remains difficult to control. The efficacies of these vaccines especially against heterologous strains remain questionable: the MLVs were generally effective against homologous strains but variable in success against heterologous strains, and the outcomes of inactivated vaccines in the field are not very promising. With the development of PRRSV reverse genetics systems and the acquisition of new understanding on anti-PRRSV immunity, rational design of the next generation of PRRSV vaccines can now be explored. In this review, we discussed the recent advances in anti-PRRSV immunity and vaccinology, the recent progresses in PRRSV vaccine development particularly the reverse genetics system-based vaccine development, and provided a perspective on potential novel strategies and approaches that may be applicable to the development of the next generation of PRRSV vaccines.

Immunogenicity of recombinant GP5 protein of porcine reproductive and respiratory syndrome virus expressed in tobacco plant

The aim of the study was to evaluate the immunogenicity of the ORF5-encoded major envelop glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) expressed in tobacco plant as a potential pig oral vaccine in protection against PRRSV infection. Six-week-old PRRSV-free pigs were fed four times orally with 50g of chopped fresh GP5 transgenic tobacco leaves (GP5-T) (GP5 reaching 0.011% of total soluble protein) or wild-type tobacco leaves (W-T) each on days 0, 14, 28, and 42. Samples of serum, saliva, and peripheral blood mononuclear cells (PBMCs) were collected on days -1, 6, 13, 20, 27, 34, 41, and 48 after the initial oral vaccination. A similar vaccination-dependent gradual increase in the responses of serum and saliva anti-PRRSV total IgG and IgA, respectively, and in the levels of PRRSV-specific blastogenic response of PBMCs was seen in GP5-T-treated pigs; all statistically significant elevations occurred after the 2nd vaccination and were revealed after 20 days post-initial oral vaccination (DPIOV). Pigs fed on GP5-T also developed serum neutralizing antibodies to PRRSV at a titer of 1:4-1:8 after the 4th vaccination by 48 DPIOV. No detectable anti-PRRSV antibody responses and PRRSV-specific blastogenic response were seen in W-T-treated pigs. The present study has demonstrated that pigs fed on GP5-T could develop specific mucosal as well as systemic humoral and cellular immune responses against PRRSV. The results also support that transgenic plant as GP5-T can be an effective system for oral delivery of recombinant subunit vaccines in pigs.

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.

Heterologous expression of fused genes encoding the glycoprotein 5 from PRRSV: a way for producing functional protein in prokaryotic microorganism

Based on the bioinformatics analysis of the gene encoding glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) isolate HH08, two gene fragments were amplified by polymerase chain reaction (PCR), deleting the signal peptide and transmembrane sequences in GP5 gene. Both gene fragments were designated GP5a and GP5b, respectively. They were ligated with a linker and cloned into prokaryotic expression vector, pET-30a. Expression of the protein of interest was induced by isopropyl beta-d-1-thiogalactopyranoside. The purified protein was used as an immunogen to elicit antibody in rabbit. The immunoreactivity of the protein was determined using ELISA and Western blot. Biologically active GP5 and anti-GP5 antibody inhibited cell infection by PRRSV. Moreover, the antibody produced in this study was capable of detecting the cell infection by PRRSV and distinguishing this virus from other viruses.

Challenges for porcine reproductive and respiratory syndrome virus (PRRSV) vaccinology

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a threat for the pig industry. Vaccines have been developed, but these failed to provide sustainable disease control, in particular against genetically unrelated strains. Here we give an overview of current knowledge and gaps in our knowledge that may be relevant for the development of a future generation of more effective vaccines. PRRSV replicates in cells of the monocyte/macrophage lineage, induces apoptosis and necrosis, interferes with the induction of a proinflammatory response, only slowly induces a specific antiviral response, and may cause persistent infections. The virus appears to use several evasion strategies to circumvent both innate and acquired immunity, including interference with antigen presentation, antibody-mediated enhancement, reduced cell surface expression of viral proteins, and shielding of neutralizing epitopes. In particular the downregulation of type I interferon-alpha production appears to interfere with the induction of acquired immunity. Current vaccines are ineffective because they suffer both from the immune evasion strategies of the virus and the antigenic heterogeneity of field strains. Future vaccines therefore must "uncouple" the immune evasion and apoptogenic/necrotic properties of the virus from its immunogenic properties, and they should induce a broad immune response covering the plasticity of its major antigenic sites. Alternatively, the composition of the vaccine should be changed regularly to reflect presently and locally circulating strains. Preferably new vaccines should also allow discriminating infected from vaccinated pigs to support a virus elimination strategy. Challenges in vaccine development are the incompletely known mechanisms of immune evasion and immunity, lack of knowledge of viral sequences that are responsible for the pathogenic and immunosuppressive properties of the virus, lack of knowledge of the forces that drive antigenic heterogeneity and its consequences for immunogenicity, and a viral genome that is relatively intolerant for subtle changes at functional sites.

Monoclonal antibodies and conserved antigenic epitopes in the C terminus of GP5 protein of the North American type porcine reproductive and respiratory syndrome virus

Glycoprotein 5 (GP5) is the major glycoprotein of porcine reproductive and respiratory syndrome virus (PRRSV). In this study, the gene encoding rtGP5, lacking signal peptide sequence, was expressed as GST-fusion protein in E. coli. Fifteen monoclonal antibodies (MAbs) against rtGP5 were developed and used to probe a series of GP5 peptides by ELISA, in which two MAbs specifically recognized the epitope GP5EP3 (146-156aa), four recognized GP5EP5 (164-180aa) and nine recognized GP5EP7 (192-200aa). After precise analysis by sequential deletion of the terminal amino acid residues, the three minimal epitopes (R(152)LYRWR(156), E(169)GHLIDLKRV(178) and Q(196)WGRL(200)) were determined, which were highly conserved among the North American type isolates, with the exception of one amino acid mutation (L(200) to P(200)). Mutational analysis showed that the mutant (Q(196)WGRP(200)) could be recognized by four of nine anti-GP5EP7 MAbs, indicating Q(196)WGRP(200) was also one minimal epitope. Western blot analysis showed that GP5EP5 and GP5EP7 (L(200) or P(200)) could be recognized by PRRSV-positive sera of CH-1a and/or BJ-4, suggesting GP5EP5 and GP5EP7 (L(200) or P(200)) were antigenic epitopes in the PRRSV-infected pigs. MAbs against GP5EP3, GP5EP5, and GP5EP7 could react with MARC-145 cells infected with the North American type isolates from China in IFA. However, very interestingly, when the highly pathogenic PRRSV, represented by HUN4, was passaged in MARC-145 cells, MAbs against GP5EP7 did not react with HUN4-F20-HUN4-F112 (20-112th passage virus), where Q(196)WGRL(200) had mutated to R(196)WGRL(200). Due to no mutations observed in GP5EP3 and GP5EP5, MAbs against GP5EP3 and GP5EP5 could recognize HUN4-F20-HUN4-F112. All the results herein might deepen the understanding of the antigen structure of in the C terminus of GP5 and facilitate the development of diagnostic antigens of the North American type PRRSV in China.

Influence of time on the genetic heterogeneity of Spanish porcine reproductive and respiratory syndrome virus isolates

The aim of the present study was to establish the degree of diversity of porcine reproductive and respiratory virus (PRRSV) isolates that circulate in the same geographical area in different years. Nucleotide sequences of open reading frame (ORF) 5 were determined for 28 Spanish field PRRSV isolates from different years and three European-type modified live virus vaccines. Sequences were aligned using Clustal W software and a phylogenetic tree constructed using the neighbour joining method. The results of pairwise homology comparisons of nucleotide and deduced amino acid sequences of these PRRSV isolates indicate a tendency for heterogeneity to increase with time. The study of the phylogenetic tree revealed that Spanish PRRSV isolates constitute two well-defined clades and a group of unrelated sequences. The observed heterogeneity does not appear to be due to temporal evolution exclusively. Early and recent isolates group themselves into different clusters independently of the time of isolation, indicating the co-circulation of different variants and the maintenance of variants of the original isolates in the field.

Genetic variation in open reading frame 5 gene of porcine reproductive and respiratory syndrome virus in Taiwan

In an effort to understand the genetic variation in porcine reproductive and respiratory syndrome virus (PRRSV) isolates in Taiwan, 40 isolates obtained between 2004 and 2006 were analyzed for their sequences of open reading frame 5. After reverse transcription polymerase chain reaction, the amplified open reading frame 5 fragments were analyzed by restriction fragment length polymorphism and sequence comparison. The results showed that all the Taiwanese isolates belonged to the North American genotype. Multiple patterns obtained from restriction fragment length polymorphism, 83-99% nucleotide similarity and 84-99% deduced amino acid similarity suggested a high level of genetic variation and PRRSV was not a single invasion to Taiwan. Moreover, vaccine-like isolates were isolated from the field, implying that some field isolates might originate from vaccine virus.

Antigen-specific B-cell responses to porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome virus (PRRSV) causes an acute, viremic infection of 4 to 6 weeks, followed by a persistent infection lasting for several months. We characterized antibody and B-cell responses to viral proteins in acute and persistent infection to better understand the immunological basis of the prolonged infection. The humoral immune response to PRRSV was robust overall and varied among individual viral proteins, with the important exception of a delayed and relatively weak response to envelope glycoprotein 5 (GP5). Memory B cells were in secondary lymphoid organs, not in bone marrow or Peyer's patches, in contrast to the case for many mammalian species. Potent anti-PRRSV memory responses were elicited to recall antigen in vitro, even though a second infection did not increase the B-cell response in vivo, suggesting that productive reinfection does not occur in vivo. Antibody titers to several viral proteins decline over time, even though abundant antigen is known to be present in lymphoid tissues, possibly indicating ineffective antigen presentation. The appearance of antibodies to GP5 is delayed relative to the resolution of viremia, suggesting that anti-GP5 antibodies are not crucial for resolving viremia. Lastly, viral infection had no immunosuppressive effect on the humoral response to a second, unrelated antigen. Taking these data together, the active effector and memory B-cell responses to PRRSV are robust, and over time the humoral immune response to PRRSV is effective. However, the delayed response against GP5 early in infection may contribute to the prolonged acute infection and the establishment of persistence.