Epitope mapping porcine reproductive and respiratory syndrome virus by phage display: the nsp2 fragment of the replicase polyprotein contains a cluster of B-cell epitopes

Research Progress of Porcine Reproductive and Respiratory Syndrome Virus NSP2 Protein

Porcine reproductive and respiratory syndrome virus (PRRSV) is globally prevalent and seriously harms the economic efficiency of pig farming. Because of its immunosuppression and high incidence of mutant recombination, PRRSV poses a great challenge for disease prevention and control. Nonstructural protein 2 (NSP2) is the most variable functional protein in the PRRSV genome and can generate NSP2N and NSP2TF variants due to programmed ribosomal frameshifts. These variants are broad and complex in function and play key roles in numerous aspects of viral protein maturation, viral particle assembly, regulation of immunity, autophagy, apoptosis, cell cycle and cell morphology. In this paper, we review the structural composition, programmed ribosomal frameshift and biological properties of NSP2 to facilitate basic research on PRRSV and to provide theoretical support for disease prevention and control and therapeutic drug development.

Pathogenicity characterization of PRRSV-1 181187-2 isolated in China

PRRSV-1 has caused more clinical infections in pigs in Chinese swine herds in recent years, however, the pathogenicity of PRRSV-1 in China is unclear. In order to study the pathogenicity of PRRSV-1, in this study, a PRRSV-1 strain, 181187-2, was isolated in primary alveolar macrophage (PAM) cells from a farm where abortions had been reported in China. The complete genome of 181187-2 was 14932 bp excluding Poly A, with 54-amino acid continuous deletion in the Nsp2 gene and 1 amino deletion in ORF3 gene compared with LV. Additionally, the piglets inoculated with strain 181187-2 by intranasal and intranasal plus intramuscular injection, animal experiments showed clinical symptoms including transient fever and depression, with no death. The obvious histopathological lesions including interstitial pneumonia and lymph node hemorrhage, and there were no significant differences in clinical symptoms and histopathological lesions with different challenge ways. Our results indicated that PRRSV -1 181187-2 was a moderately pathogenic strain in piglets.

Genomic Analysis of Porcine Reproductive and Respiratory Syndrome Virus 1 Revealed Extensive Recombination and Potential Introduction Events in China

Simple Summary

Porcine reproductive and respiratory syndrome, caused by the porcine reproductive and respiratory syndrome virus, is considered one of the most devastating swine diseases worldwide. Porcine reproductive and respiratory syndrome virus 1 was first isolated in China in 2006, and there have been few reports concerning its genetic characteristics in China. We hope to find out the regularity of genetic diversity, recombination, and evolution of the virus by analyzing all available genomic sequences during 1991–2018. We found that high-frequency recombination regions were concentrated in non-structural protein 2 and structural proteins 2 to 4 and extensive deletions in non-structural protein 2; phylogenetic analysis revealed four independent introductions in China. Our results suggest that attention should be paid to the prevention and control of porcine reproductive and respiratory syndrome virus 1 and the rational use of vaccine strains. These results will help us to understand the recombination of porcine reproductive and respiratory syndrome virus and strengthen viral inspection before mixing herds of swine to reduce the probability of novel recombinant variants. Moreover, our study might form the basis of monitoring and control measures to prevent the spread of this economically important virus.

Abstract

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is considered one of the most devastating swine diseases worldwide. PRRSV-1 was first isolated in China in 2006. However, there were few reports concerning the genetic characteristics of PRRSV-1 in China. In this study, three PRRSV-1 strains (HL85, HeB3, and HeB47) were detected by a general RT-qPCR method from clinical samples in 2018. HeB47 was identified as a recombinant between the BJEU06-1 and CReSA228-like strains. To further analyze the recombination and deletion features of PRRSV-1, all the available 88 complete genome sequences (isolated in 19 countries) from 1991 to 2018 in GenBank were analyzed. The high-frequency recombination regions were concentrated in NSP2 and GP2 to GP4. More importantly, phylogenetic analysis of PRRSV-1 revealed four independent introductions in China. Therefore, it is necessary to strengthen the important monitoring of breeding pigs and pork products and epidemiological surveys on pig farms to prevent the further spread of PRRSV-1.

Characterization of a new NSP2-deletion NADC34-Like Porcine Reproductive and Respiratory Syndrome Virus in China

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is the causative agent of Porcine Reproductive and Respiratory Syndrome (PRRS), which has caused huge economic losses to the pig industry worldwide. PRRSV NADC34-Like PRRSV 2020-Acheng-1 strain, which caused high morbidity and high mortality were isolated from dead piglets (high-throughput sequencing to show that only PRRSV and TGEV) on a farm in northeastern China. The full-length genome sequence of 2020-Acheng-1 shares 95.6% nucleotide homology with NADC34 PRRSV without any gene insertion, but has a unique 17 amino acid (469aa to 486aa) deletion in Nsp2 compared with all NADC34-Like strains in NCBI and there are unique 100 amino acid deletions. In addition, difference degree of changes in signal peptide, trans-membrane region (TM), main neutralizing epitope (PNE), non-neutralizing epitope and N-glycosylation site were observed in GP5 of 2020-Acheng-1 and other PRRSV-2 strains, we only found a change in the fifteenth amino acid of signal peptide of in GP5 of 2020-Acheng-1 with NADC34 strains. Recombination analysis showed that 2020-Acheng-1 strain did not have any recombination events with representative PRRSV-2 strains in China. This study provided valuable evidence for understanding the role of NADC34-Like strain that impact on pathogenicity.

Characterization of Two Immunodominant Antigenic Peptides in NSP2 of PRRSV-2 and Generation of a Marker PRRSV Strain Based on the Peptides

Porcine reproductive and respiratory syndrome (PRRS) is a widespread disease with great economic importance in the pig industry. Although vaccines against the PRRS virus (PRRSV) have been employed for more than 20 years, differentiating infected from vaccinated animals remains challenging. In this study, all 907 non-structural protein 2 (NSP2) full-length sequences of PRRSV-2 available from GenBank were aligned. Two peptides, at positions 562–627 (m1B) and 749–813 (m2B) of NSP2, were selected, and their potential for use in differential diagnosis was assessed. Both m1B and m2B were recognized by PRRSV-positive pig serum in peptide-coated enzyme-linked immunosorbent assays. Further epitope identification yielded five overlapping short peptides for the immunodominant regions of m1B and m2B. Using the infectious clone of PRRSV HuN4-F112 as a template, the deletion mutants, rHuN4-F112-m1B, rHuN4-F112-m2B, and rHuN4-F112-C5-m1B-m2B, were generated and successfully rescued in Marc-145 cells. Growth kinetics revealed that the deletion of m1B and m2B did not significantly affect virus replication. Hence, m1B and m2B show potential as molecular markers for developing a PRRSV vaccine.

Perspectives on passive antibody therapy and peptide-based vaccines against emerging pathogens like SARS-CoV-2

The current epidemic of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is raising awareness of the need to act faster when dealing with new pathogens. Exposure to an emerging pathogen generates an antibody response that can be used for preventing and treating the infection. These antibodies might have a high specificity to a target, few side effects, and are useful in the absence of an effective vaccine for treating immunocompromised individuals. The approved antibodies against the receptor-binding domain (RBD) of the viral spike protein of SARS-CoV-2 (e.g., regdanvimab, bamlanivimab, etesevimab, and casirivimab/imdevimab) have been selected from the antibody repertoire of B cells from convalescent patients using flow cytometry, next-generation sequencing, and phage display. This encourages use of these techniques especially phage display, because it does not require expensive types of equipment and can be performed on the lab bench, thereby making it suitable for labs with limited resources. Also, the antibodies in blood samples from convalescent patients can be used to screen pre-made peptide libraries to identify epitopes for vaccine development. Different types of vaccines against SARS-CoV-2 have been developed, including inactivated virus vaccines, mRNA-based vaccines, non-replicating vector vaccines, and protein subunits. mRNA vaccines have numerous advantages over existing vaccines, such as efficacy, ease of manufacture, safety, and cost-effectiveness. Additionally, epitope vaccination may constitute an attractive strategy to induce high levels of antibodies against a pathogen and phages might be used as immunogenic carriers of such peptides. This is a point worth considering further, as phage-based vaccines have been shown to be safe in clinical trials and phages are easy to produce and tolerate high temperatures. In conclusion, identification of the antibody repertoire of recovering patients, and the epitopes they recognize, should be an attractive alternative option for developing therapeutic and prophylactic antibodies and vaccines against emerging pathogens.

Hyper-phosphorylation of nsp2-related proteins of porcine reproductive and respiratory syndrome virus

Viruses exploit phosphorylation of both viral and host proteins to support viral replication. In this study, we demonstrate that porcine reproductive and respiratory syndrome virus replicase nsp2, and two nsp2-related −2/−1 frameshifting products, nsp2TF and nsp2N, are hyper-phosphorylated. By mapping phosphorylation sites, we subdivide an extended, previously uncharacterized region, located between the papain-like protease-2 (PLP2) domain and frameshifting site, into three distinct domains. These domains include two large hypervariable regions (HVR) with putative intrinsically disordered structures, separated by a conserved and partly structured interval domain that we defined as the inter-HVR conserved domain (IHCD). Abolishing phosphorylation of the inter-species conserved residue serine⁹¹⁸, which is located within the IHCD region, abrogates accumulation of viral genomic and subgenomic RNAs and recombinant virus production. Our study reveals the biological significance of phosphorylation events in nsp2-related proteins, emphasizes pleiotropic functions of nsp2-related proteins in the viral life cycle, and presents potential links to pathogenesis.

The nsp2 Hypervariable Region of Porcine Reproductive and Respiratory Syndrome Virus Strain JXwn06 Is Associated with Viral Cellular Tropism to Primary Porcine Alveolar Macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) poses a major threat to global pork production and has been notorious for its rapid genetic evolution in the field. The nonstructural protein 2 (nsp2) replicase protein represents the fastest evolving region of PRRSV, but the underlying biological significance has remained poorly understood. By deletion mutagenesis, we discovered that the nsp2 hypervariable region plays an important role in controlling the balance of genomic mRNA and a subset of subgenomic mRNAs. More significantly, we revealed an unexpected link of the nsp2 hypervariable region to viral tropism. Specifically, a mutant of the Chinese highly pathogenic PRRSV strain JXwn06 carrying a deletion spanning nsp2 amino acids 323 to 521 (nsp2Δ323-521) in its hypervariable region was found to lose infectivity in primary porcine alveolar macrophages (PAMs), although it could replicate relatively efficiently in the supporting cell line MARC-145. Consequently, this mutant failed to establish an infection in piglets. Further dissection of the viral life cycle revealed that the mutant had a defect (or defects) lying in the steps between virus penetration and negative-stranded RNA synthesis. Taken together, our results reveal novel functions of nsp2 in the PRRSV life cycle and provide important insights into the mechanisms of PRRSV RNA synthesis and cellular tropism.IMPORTANCE The PRRSV nsp2 replicase protein undergoes rapid and broad genetic variations in its middle region in the field, but the underlying significance has remained enigmatic. Here, we demonstrate that the nsp2 hypervariable region not only plays an important regulatory role in maintaining the balance of different viral mRNA species but also regulates PRRSV tropism to primary PAMs. Our results reveal novel functions for PRRSV nsp2 and have important implications for understanding the mechanisms of PRRSV RNA synthesis and cellular tropism.

Genetic diversity of porcine reproductive and respiratory syndrome virus 1 in the United States of America from 2010 to 2018

Porcine reproductive and respiratory syndrome virus 1 (PRRSV-1) was first detected in the United States of America (USA) in 1999, several strains were also recognized soon later, and these isolates are typically called North American (NA) PRRSV-1. However, few reports have characterized PRRSV-1 viruses in the USA. We explored the genetic characteristics and diversity of PRRSV-1 viruses circulating in the USA. PRRSV-1 PCR-positive samples collected from seven states in 2010-2018 (n = 27) were subjected to next-generation sequencing. The 27 PRRSV-1 viruses had 88.4-91.3% nucleotide identity to the PRRSV-1 Lelystad-virus strain (the type 1 prototype strain) and 87.4-89.8% to the previously reported NA PRRSV-1 viruses. Individual proteins had several unique genetic characteristics and only one of the 27 tested samples had the characteristic 17-amino acid (aa) deletion in Nsp2, a genetic marker of NA PRRSV-1 viruses described previously. Fourteen isolates displayed a 3-aa C-terminal truncation in the highly conserved Nsp12 gene; 16 samples had a 21- or 18-aa C-terminal truncation in GP3 gene; and one was observed with a 1-aa deletion at the overlapping region of GP3 and GP4. In addition, the GP5 protein in most isolates, excluding one exception, demonstrated similar genetic variation as other reported NA PRRSV-1 isolates. All tested isolates clustered within subtype 1 together with other available NA PRRSV-1 viruses. Collectively, our results provide up-to-date information on PRRSV-1 viruses circulating in the USA in the past 9 years although the number of PRRSV-1 isolates included in this study is limited. These PRRSV-1 viruses have undergone gradual genetic variation and exhibited some previously undescribed genetic characteristics and diversity, which complicates the diagnosis and control of NA PRRSV-1.

Genetic analysis of porcine productive and respiratory syndrome virus between 2013 and 2014 in Southern parts of China: identification of several novel strains with amino acid deletions or insertions in nsp2

Background

Porcine respiratory and reproductive syndrome virus (PRRSV) is one of the most economically significant pathogens in the Chinese swine industry. ORF5 and nsp2 are highly variable regions of the PRRSV genome. Therefore, nsp2 and GP5 are often selected for investigation of variations and phylogenetic analyses for their genetic diversities. Knowledge of the molecular evolution of PRRSV field strains may contribute to the control of PRRS in China.

Results

The results of multiple sequence alignments of GP5 showed that there is 84.5–100% aa identity among the 56 strains in this study. These strains shared 84.5–99.0% aa identity with the prototypical type 2 PRRSV VR-2332 and 56.6–59.2% with strain LV, prototypical type 1 PRRSV. Phylogenetic analysis showed there is considerable diversity among PRRSV ORF5 and the existence of two lineages (5 and 8). Most of the strains were classified into lineage 8 with multiple sub-lineages (3, 4 and 6). Moreover, PRRSV strains with 5 novel patterns of deletions or insertions in the nsp2 region were found.

Conclusions

Phylogenetic analysis based on ORF5 sequences indicated the diversity of PRRSV in southern parts of China and the strains with 30 aa deletion in nsp2 are dominant in the porcine population. Also, new PRRSV strains with different patterns of deletions or insertions in nsp2 are emerging. The data presented here constitute a useful basis for further epidemiological studies regarding the heterogeneity of PRRSV strains in China and provide a basis for the prevention of PRRS in southern parts of China.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1906-9) contains supplementary material, which is available to authorized users.

Evaluation of a Recombinant Mouse X Pig Chimeric Anti-Porcine DEC205 Antibody Fused with Structural and Nonstructural Peptides of PRRS Virus

Activation of the immune system using antigen targeting to the dendritic cell receptor DEC205 presents great potential in the field of vaccination. The objective of this work was to evaluate the immunogenicity and protectiveness of a recombinant mouse x pig chimeric antibody fused with peptides of structural and nonstructural proteins of porcine respiratory and reproductive syndrome virus (PRRSV) directed to DEC205⁺ cells. Priming and booster immunizations were performed three weeks apart and administered intradermally in the neck area. All pigs were challenged with PRRSV two weeks after the booster immunization. Immunogenicity was evaluated by assessing the presence of antibodies anti-PRRSV, the response of IFN-γ-producing CD4⁺ cells, and the proliferation of cells. Protection was determined by assessing the viral load in the blood, lungs, and tonsils using qRT-PCR. The results showed that the vaccine exhibited immunogenicity but conferred limited protection. The vaccine group had a lower viral load in the tonsils and a significantly higher production of antibodies anti-PRRSV than the control group (p < 0.05); the vaccine group also produced more CD4⁺IFN-γ⁺ cells in response to peptides from the M and Nsp2 proteins. In conclusion, this antigenized recombinant mouse x pig chimeric antibody had immunogenic properties that could be enhanced to improve the level of protection and vaccine efficiency.

Differential recognition of peptides within feline coronavirus polyprotein 1 ab by sera from healthy cats and cats with feline infectious peritonitis

The aim of the study was to identify peptides within the polyprotein (Pp) 1 ab that are differentially recognised by cats with either enteric or systemic disease following infection with feline coronavirus. Overlapping 12-mer peptides (n = 28,426) across the entire Pp1ab were arrayed on peptide chips and reacted with pooled sera from coronavirus seropositive cats and from one seronegative cat. Eleven peptides were further tested in ELISA with individual serum samples, and three were selected for further screening. Two peptides (16433 and 4934) in the nsp3 region encoding the papain 1 and 2 proteases were identified for final testing. Peptide 4934 reacted equally with positive sera from healthy cats and cats with feline infectious peritonitis (FIP), while peptide 16433 was recognized predominantly by FIP-affected cats. The value of antibody tests based on these peptides in differentiating between the enteric and FIP forms of feline coronavirus infection remains to be determined.

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Cats develop antibodies to polyprotein 1 ab (Pp1ab) of feline coronavirus.

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This is most evident for cats with feline infectious peritonitis (FIP).

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Differences exist in responses to selected peptides between FIP and non-FIP cats.

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Such differences may be utilised for development of a serological test for FIP.

Challenges for Porcine Reproductive and Respiratory Syndrome (PRRS) Vaccine Design: Reviewing Virus Glycoprotein Interactions with CD163 and Targets of Virus Neutralization

One of the main participants associated with the onset and maintenance of the porcine respiratory disease complex (PRDC) syndrome is porcine reproductive and respiratory syndrome virus (PRRSV), an RNA virus that has plagued the swine industry for 30 years. The development of effective PRRS vaccines, which deviate from live virus designs, would be an important step towards the control of PRRS. Potential vaccine antigens are found in the five surface proteins of the virus, which form covalent and multiple noncovalent interactions and possess hypervariable epitopes. Consequences of this complex surface structure include antigenic variability and escape from immunity, thus presenting challenges in the development of new vaccines capable of generating broadly sterilizing immunity. One potential vaccine target is the induction of antibody that disrupts the interaction between the macrophage CD163 receptor and the GP2, GP3, and GP4 heterotrimer that protrudes from the surface of the virion. Studies to understand this interaction by mapping mutations that appear following the escape of virus from neutralizing antibody identify the ectodomain regions of GP5 and M as important immune sites. As a target for antibody, GP5 possesses a conserved epitope flanked by N-glycosylation sites and hypervariable regions, a pattern of conserved epitopes shared by other viruses. Resolving this apparent conundrum is needed to advance PRRS vaccine development.

Differential evolution of antigenic regions of porcine reproductive and respiratory syndrome virus 1 before and after vaccine introduction

Porcine reproductive and respiratory syndrome virus (PRRSV) is a widespread viral pathogen that has caused tremendous economic losses throughout most pig-producing countries. Nowadays, both PRRSV-1 and PRRSV-2 co-circulate in Korean pig populations, and commercial modified live vaccine (MLV) is predominantly used to control PRRS. Specifically, control strategy using only PRRSV-2 MLV that was used since 1995 cannot prevent the spread of PRRSV-1 and damage from its infection, which led to the first introduction of two additional PRRSV-1 vaccines in 2014. Despite the wide implementation with PRRSV-1 vaccines, there is a lack of knowledge about the currently circulating Korean PRRSV-1 strains. Whole structural genes of PRRSV-1 before (11) and after (17) the introduction of vaccine were compared to determine the genetic evolutionary features of PRRSV. Genetic and phylogenetic analysis indicated that Korean PRRSV-1 shared 91.5 ± 1.7% nucleotide identity but formed a unique clade based on ORF2-7 phylogeny. Bioinformatics showed increased genetic heterogeneity, enhanced diversifying selection, and the emergence of novel glycosylation sites within neutralizing epitopes of minor structural proteins after vaccine introduction. Taken together, our data provide novel insight into the evolution of minor structural proteins of PRRSV-1 in the vaccination era.

Bottlenecks in the transmission of porcine reproductive and respiratory syndrome virus (PRRSV1) to naïve pigs and the quasi-species variation of the virus during infection in vaccinated pigs

This paper describes the results of two experiments regarding porcine reproductive and respiratory syndrome virus (PRRSV1): the first one studied the existence of bottlenecks in an experimental one-to-one model of transmission in pigs; while the second analysed the differences between viral quasi-species in vaccinated pigs that developed shorter or longer viraemias after natural challenge. Serum samples, as well as the initial inoculum, were deep-sequenced and a viral quasi-species was constructed per sample. For the first experiment, the results consistently reported a reduction in the quasi-species diversity after a transmission event, pointing to the existence of bottlenecks during PRRSV1 transmission. However, despite the identified preferred and un-preferred transmitted variants not being randomly distributed along the virus genome, it was not possible to identify any variant producing a structural change in any viral protein. In contrast, the mutations identified in GP2, nsp9 and M of the second experiment pointed to changes in the amino acid charges and the viral RNA-dependent RNA polymerase structure. The fact that the affected proteins are known targets of the immunity against PRRSV, plus the differential level of neutralizing antibodies present in pigs developing short or long viraemias, suggests that the immune response selected those changes.

Phages bearing specific peptides with affinity for porcine reproductive and respiratory syndrome virus GP4 protein prevent cell penetration of the virus

Porcine reproductive and respiratory syndrome (PRRS) has caused significant economic losses to the pig industry worldwide over the last 30 years. GP4 is a minor highly glycosylated structural protein composed of 187 and 183 amino acids in types I and II porcine reproductive and respiratory syndrome virus (PRRSV), respectively. The GP4 protein co-localizes with cluster of differentiation 163 (CD163), the major receptor on the target cell membrane, to mediate PRRSV internalization and disassembly. However, it remains to be established whether blocking interactions between GP4 and host cells can inhibit viral proliferation. In the present study, recombinant GP4 protein prepared and purified using the Escherichia coli system effectively recognized PRRSV-positive serum. Phage display biopanning on GP4 protein showed that the specific phages obtained could distinguish PRRSV from the other viruses. The exogenous peptide WHEYPLVWLSGY displayed on one of the candidate phages showed high affinity for GP4 protein and exerted a significant inhibitory effect on PRRSV penetration in vitro. Moreover, the N-terminus of GP4 was predicted as the critical receptor binding site and the beginning of the fifth scavenger receptor cysteine-rich domain of CD163 as the critical ligand recognition site based on sequence alignment and model prediction analyses. The current study expands our understanding of PRRSV GP4 and its receptor CD163 and provides a fresh perspective for the development of novel peptide-based viral inhibition reagents.

Genomic sequence and virulence of a novel NADC30-like porcine reproductive and respiratory syndrome virus isolate from the Hebei province of China

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), which results in immense economic losses in the swine industry. Outbreaks of disease caused by NADC30-like PRRSV are of great concern in China. Here, a novel variant, NADC30-like PRRSV strain HB17A, was analyzed and its pathogenicity in pigs was examined. The full-length genome sequence of HB17A shared 83.6-95.1% nucleotide similarity with NADC30-like and NADC30 PRRSV without any gene insertions, but with a unique 2-amino acid deletion in Nsp2. A phylogenetic analysis showed that HB17A clustered with NADC30 strains. Different degrees of variation in the signal peptide, transmembrane region (TM), primary neutralizing epitope (PNE), non-neutral epitopes, and N-glycosylation sites were observed in GP5. Challenge experiments showed that HB17A infection resulted in persistent fever, moderate respiratory clinical signs, low levels of viremia and viral loads in serum, and mild gross and microscopic lung lesions. Moreover, IFN-γ, IL-6, and IL-10 cytokine levels were significantly elevated in serum, but the levels of IFN-α and IL-2 were similar to those of the negative controls. HB17A was less pathogenic but was secreted longer in nasal discharge than HP-PRRSV FZ06A. Our findings indicate that HB17A is a novel NADC30-like strain with certain deletions and mutations but with no evidence of genomic recombination. This strain exhibits intermediate virulence in pigs. This research will be help define the evolutionary characteristics of Chinese NADC30-like PRRSV.

Glycoprotein 3 of Porcine Reproductive and Respiratory Syndrome Virus Exhibits an Unusual Hairpin-Like Membrane Topology

Glycoprotein 3 (GP3) of the arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) consists of a cleaved signal peptide, a highly glycosylated domain, a short hydrophobic region, and an unglycosylated C-terminal domain. GP3 is supposed to form a complex with GP2 and GP4 in virus particles, but secretion of GP3 from cells has also been reported. We analyzed the membrane topology of GP3 from various PRRSV strains. A fraction of the protein is secreted from transfected cells, GP3 from PRRSV-1 strains to a greater extent than GP3 from PRRSV-2 strains. This secretion behavior is reversed after exchange of the variable C-terminal domain. A fluorescence protease protection assay shows that the C terminus of GP3, fused to green fluorescent protein (GFP), is resistant to proteolytic digestion in permeabilized cells. Furthermore, glycosylation sites inserted into the C-terminal part of GP3 are used. Both experiments indicate that the C terminus of GP3 is translocated into the lumen of the endoplasmic reticulum. Deletion of the conserved hydrophobic region greatly enhances secretion of GP3, and fusion of this domain to GFP promotes membrane anchorage. Bioinformatics suggests that the hydrophobic region forms an amphipathic helix. Accordingly, exchanging only a few amino acids in its hydrophilic face prevents secretion of GP3 and in its hydrophobic face enhances it. Exchanging the latter amino acids in the context of the viral genome did not affect release of virions, but released particles were not infectious. In sum, GP3 exhibits an unusual hairpin-like membrane topology that might explain why a fraction of the protein is secreted.IMPORTANCE PRRSV is the most important pathogen in the pork industry. It causes persistent infections that lead to reduced weight gain of piglets; highly pathogenic strains even kill 90% of an infected pig population. PRRSV cannot be eliminated from pig farms by vaccination due to the large amino acid variability between the existing strains, especially in the glycoproteins. Here, we analyzed basic structural features of GP3 from various PRRSV strains. We show that the protein exhibits an unusual hairpin-like membrane topology; membrane anchoring might occur via an amphipathic helix. This rather weak membrane anchor explains why a fraction of the protein is secreted from cells. Interestingly, PRRSV-1 strains secrete more GP3 than PRRSV-2. We speculate that secreted GP3 plays a role during PRRSV infection of pigs: it might serve as a decoy to distract antibodies away from virus particles.

MicroRNA-like viral small RNA from porcine reproductive and respiratory syndrome virus negatively regulates viral replication by targeting the viral nonstructural protein 2

Many viruses encode microRNAs (miRNAs) that are small non-coding single-stranded RNAs which play critical roles in virus-host interactions. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically impactful viruses in the swine industry. The present study sought to determine whether PRRSV encodes miRNAs that could regulate PRRSV replication. Four viral small RNAs (vsRNAs) were mapped to the stem-loop structures in the ORF1a, ORF1b and GP2a regions of the PRRSV genome by bioinformatics prediction and experimental verification. Of these, the structures with the lowest minimum free energy (MFE) values predicted for PRRSV-vsRNA1 corresponded to typical stem-loop, hairpin structures. Inhibition of PRRSV-vsRNA1 function led to significant increases in viral replication. Transfection with PRRSV-vsRNA1 mimics significantly inhibited PRRSV replication in primary porcine alveolar macrophages (PAMs). The time-dependent increase in the abundance of PRRSV-vsRNA1 mirrored the gradual upregulation of PRRSV RNA expression. Knockdown of proteins associated with cellular miRNA biogenesis demonstrated that Drosha and Argonaute (Ago2) are involved in PRRSV-vsRNA1 biogenesis. Moreover, PRRSV-vsRNA1 bound specifically to the nonstructural protein 2 (NSP2)-coding sequence of PRRSV genome RNA. Collectively, the results reveal that PRRSV encodes a functional PRRSV-vsRNA1 which auto-regulates PRRSV replication by directly targeting and suppressing viral NSP2 gene expression. These findings not only provide new insights into the mechanism of the pathogenesis of PRRSV, but also explore a potential avenue for controlling PRRSV infection using viral small RNAs.

Effect of an 88-amino-acid deletion in nsp2 of porcine reproductive and respiratory syndrome virus on virus replication and cytokine responses in vitro

Previously, a spontaneous 88-amino-acid (aa) deletion in nsp2 was associated with cell-adaptation of porcine reproductive and respiratory syndrome virus (PRRSV) strain JXM100, which arose during passaging of the highly pathogenic PRRSV (HP-PRRSV) strain JX143 in MARC-145 cells. Here, to elucidate the biological role of this deletion, we specifically deleted the region of a cDNA clone of HP-PRRSV strain JX143 (pJX143) corresponding to these 88 amino acids. The effect of the deletion on virus replication in cultured cells and transcriptional activation of inflammatory cytokines and chemokines in pulmonary alveolar macrophages (PAMs) was examined. Mutant virus with the 88-aa deletion in nsp2 (rJX143-D88) had faster growth kinetics and produced larger plaques in MARC-145 cells than the parental virus (rJX143), suggesting that the deletion enhanced virus replication in MARC-145 cells. In contrast, the overall yield of rJX143 was almost 1 log higher than that of rJX143-D88, suggesting that the 88-aa deletion in nsp2 decreased the production of infectious viruses in PAMs. Infection with the mutant virus with the 88-aa deletion resulted in increased mRNA expression of type I interferon (IFN-α and IFN-β) and chemokines genes. In addition, the mRNA expression of antiviral genes (ISG15, ISG54 and PKR) regulated by the IFN response was upregulated in PAMs infected with the mutant virus rJX143-D88. Our results demonstrate that virus-specific host immunity can be enhanced by modifying certain nsp2 epitope regions. These findings provide important insights for understanding virus pathogenesis and development of future vaccines.

Serodiagnosis, targeting nonstructural protein 4, of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) causes great economic losses to the swine industry worldwide. Typically, an N protein-coated indirect enzyme-linked immunosorbent assay (N-coated iELISA) is used to detect PRRSV antibodies. Non-structural protein (NSP) 4 is essential to the PRRSV life cycle and contains B-cell epitopes. Yet, no specific antibody against NSP4 has been detected in clinical samples. In this study, we developed an NSP4-coated iELISA and compared its effectiveness with the N-coated iELISA. The NSP4-coated iELISA was developed with a cut-off value of 0.406 at an optical density of 450 nm by testing a panel of 70 PRRSV positive and 80 PRRSV negative pig serum samples, which generated a specificity and sensitivity of 100%. Agreement between the NSP4-coated and N-coated iELISAs was 92.2%. Interestingly, 50 serum samples, mostly from pigs vaccinated with the HP-PRRSV live strain, tested positive for PRRSV antibodies with the NSP4-coated iELISA, but were negative with the N-coated iELISA. These results were further confirmed by western blot analysis and another iELISA based on the N-terminus of NSP2 (NSP2-1-coated iELISA). The agreement between the results of western blot analysis with the NSP4-coated and NSP2-1-coated iELISA analyses were 92% and 96.1%, respectively, showing that the developed NSP4-coated iELISA is a useful tool to discriminate a false negative from a true negative response to the HP-PRRSV vaccine.

Whole genome characterization of a novel porcine reproductive and respiratory syndrome virus 1 isolate: Genetic evidence for recombination between Amervac vaccine and circulating strains in mainland China

Genotype 1 porcine reproductive and respiratory syndrome virus (PRRSV 1) have been continuously isolated in China in recent years. Complete genome sequences of these isolates are important to investigate the prevalence and evolution of Chinese PRRSV 1. Herein, we describe the isolation of a novel PRRSV 1 isolate, denominated HLJB1, in the Heilongjiang province of China. Complete genome sequencing of HLJB1 showed that it shares 90.66% and 58.21% nucleotide identities with PRRSV 1 and 2 prototypic strains Lelystad virus and ATCC VR-2332, respectively. HLJB1 has a unique 5-amino-acid insertion in nsp2, which has never been described in other PRRSV 1 isolates. Whole genome-based phylogenetic analysis revealed that all Chinese PRRSV 1 isolates are clustered in pan-European subtype 1 and can be divided into four subgroups. HLJB1 resides in the subgroup of BJEU06-1-like isolates but is also closely related to the Amervac-like isolates. Additionally, recombination analyses suggested that HLJB1 is a recombinant from the Amervac vaccine and the BJEU06-1 isolate. To our best knowledge, our results provide the first genetic evidence for recombination between Amervac vaccine and circulating strains. These findings are also beneficial for studying the origin and evolution of PRRSV 1 in China.

GP5 of porcine reproductive and respiratory syndrome virus (PRRSV) as a target for homologous and broadly neutralizing antibodies

Virus neutralization (VN) responses range from narrowly focused antibodies with only homologous neutralizing activity against the virus used for infection, to antibodies that can neutralize both Type 1 and Type 2 viruses, referred to as broadly neutralizing antibody (bnAb). Even though neutralizing epitopes are likely distributed among several structural glycoproteins, this paper focuses on the ectodomain region of GP5 as a model system for investigating the role for neutralizing and non-neutralizing antibodies in protection and disease. Epitope B within GP5 possesses several features common to broadly neutralizing epitopes. In the proposed model, accessibility of antibody to Epitope B is blocked by homologous neutralizing and non-neutralizing antibodies, which bind flanking hypervariable domains. Additional mechanisms for blocking the accessibility of bnAb include conformational alterations within the GP5-M heterodimer and glycan shielding. This model explains how the continuous escape from homologous neutralization provides a mechanism for persistence. The proposed mechanism for immune evasion is not unique to PRRSV, but can be found in other persistent viruses, such as hepatitis C virus (HCV).

Genomic sequence and virulence evaluation of MN184A-like porcine reproductive and respiratory syndrome virus in Japan

In this study, a porcine reproductive and respiratory syndrome virus (PRRSV) that was isolated from a 9-week-old diseased pig on a farm in Japan with a high mortality rate during 2007-2008 was characterized. This unique isolate, designated as Jpn5-37, did not have a high nucleotide identity in open reading frame 5 against any Japanese isolates. Among all available type 2 PRRSV complete genome sequences, Jpn5-37 shared the highest nucleotide identity (93.6%) with virulent strain MN184A. The genomic characteristics of Jpn5-37 were highly conserved with respect to the virulent MN184A, including a continuous eight amino acid deletion in the nonstructural protein 2 region. Moreover, virus distribution, viremia and the gross and microscopic characteristics of lesions were investigated in pigs 10 days post-inoculation to elucidate the pathogenicity of the isolate. Intranasal inoculation was found to rapidly result in viremia and dissemination of the Jpn5-37 isolate to several tissues in a similar manner to EDRD1; however, the amounts of Jpn5-37 RNA in serum were significantly greater. Similarly, the quantities of Jpn5-37 viral RNA in all organs tested tended to be higher than with EDRD1 infection. Mean rectal temperatures were significantly higher in the Jpn5-37-inoculated than in the control group at 4 and 6 days post infection (dpi) and in the EDRD1-inoculated group at 6 and 8 dpi. These results suggest that the Jpn5-37 strain replicates and is more efficiently distributed to the organs than is EDRD1 under the same conditions.

Emergence of a novel highly pathogenic porcine reproductive and respiratory syndrome virus in China

From 2014 to 2015, four novel highly pathogenic PRRS virus (HP-PRRSV) strains named 14LY01-FJ, 14LY02-FJ 15LY01-FJ, and 15LY02-FJ were isolated from high morbidity (100%) and mortality (40%-80%) in piglets and sows in Fujian Province. To further our knowledge about these novel virus strains, we characterized their complete genomes and determined their pathogenicity in piglets. Full-length genome sequencing analysis showed that these four isolates were closely related to type 2 (North American type, NA-type) isolates, with 88.1%-96.3% nucleotide similarity, but only 60.6%-60.8% homology to the Lelystad virus (LV) (European type, EU-type). The full length of the four isolates was determined to be 15017 or 15018 nucleotides (nt), excluding the poly(A) tail. Furthermore, the four isolates had three discontinuous deletions (aa 322-432, aa 483, and aa 504-522) within hypervariable region II (HV-II) of Nsp2, as compared to the reference strain VR-2332. This deletion pattern in the four isolates is consistent with strain MN184 and strain NADC30 isolated from America. Phylogenetic and molecular evolutionary analyses indicated that these virulent strains originated from a natural recombination event between the JXA1-like HP-PRRSV (JXA-1 is one of the earliest Chinese HP-PRRSV strains; sublineage 8.7) and the NADC30-like (lineage 1) PRRSV. Animal experiments demonstrated that these four strains caused significant weight loss and severe histopathological lung lesions as compared to the negative control group. High mortality rate (40% or 80%) was found in piglets infected with any one of the four strains, similar to that found with other Chinese HP-PRRSV strains. This study showed that the novel variant PRRSV was HP-PRRSV, and it is therefore critical to monitor PRRSV evolution in China and develop a method for controlling PRRS.

Complete genomic characterization of two European-genotype porcine reproductive and respiratory syndrome virus isolates in Fujian province of China

Porcine reproductive and respiratory syndrome (PRRS) is considered one of the most devastating swine diseases worldwide, resulting in immense economic losses. PRRS virus (PRRSV) is divided into two major genotypes, European (type 1) and the North American (type 2). Type 1 PRRSV have recently emerged in Fujian province (South China), and this might have a significant impact on the Chinese pig industry. From 2013 to 2014, two type 1 PRRSV strains, named FJEU13 and FJQEU14, were isolated from piglets and sows with respiratory problems and reproductive disorders in Fujian province. The full genome length of the two isolates was 14,869-15,062 nucleotides (nt), excluding the poly(A) tail. These isolates shared 86.0-89.9% sequence identity with the prototypic strains Lelystad virus (LV) and 82.8-92% with Chinese type 1 PRRSV strains, but only 59.9-60.1% with the North American reference strain VR-2332. However, they were 82.9% identical to each other. Nonstructural protein 2 (Nsp2) and ORF3-ORF5 were the most variable regions when compared to other type 1 PRRSV strains. Nsp2 and ORF3 contained multiple discontinuous deletions and a 204-bp deletion in NSP2 in isolate FJQEU14, which has never been described in other Chinese type 1 PRRSV strains. All of these results might be useful for understanding the epidemic status of type 1 PRRSV in China.

Arteriviruses, Pegiviruses, and Lentiviruses Are Common among Wild African Monkeys

Nonhuman primates (NHPs) are a historically important source of zoonotic viruses and are a gold-standard model for research on many human pathogens. However, with the exception of simian immunodeficiency virus (SIV) (family Retroviridae), the blood-borne viruses harbored by these animals in the wild remain incompletely characterized. Here, we report the discovery and characterization of two novel simian pegiviruses (family Flaviviridae) and two novel simian arteriviruses (family Arteriviridae) in wild African green monkeys from Zambia (malbroucks [Chlorocebus cynosuros]) and South Africa (vervet monkeys [Chlorocebus pygerythrus]). We examine several aspects of infection, including viral load, genetic diversity, evolution, and geographic distribution, as well as host factors such as age, sex, and plasma cytokines. In combination with previous efforts to characterize blood-borne RNA viruses in wild primates across sub-Saharan Africa, these discoveries demonstrate that in addition to SIV, simian pegiviruses and simian arteriviruses are widespread and prevalent among many African cercopithecoid (i.e., Old World) monkeys.

IMPORTANCE

Primates are an important source of viruses that infect humans and serve as an important laboratory model of human virus infection. Here, we discover two new viruses in African green monkeys from Zambia and South Africa. In combination with previous virus discovery efforts, this finding suggests that these virus types are widespread among African monkeys. Our analysis suggests that one of these virus types, the simian arteriviruses, may have the potential to jump between different primate species and cause disease. In contrast, the other virus type, the pegiviruses, are thought to reduce the disease caused by human immunodeficiency virus (HIV) in humans. However, we did not observe a similar protective effect in SIV-infected African monkeys coinfected with pegiviruses, possibly because SIV causes little to no disease in these hosts.

Modified-live PRRSV subtype 1 vaccine UNISTRAIN® PRRS provides a partial clinical and virological protection upon challenge with East European subtype 3 PRRSV strain Lena

Background

Western European porcine reproductive and respiratory syndrome virus (PRRSV) strains cause limited and mild clinical signs whereas more virulent strains are circulating in Eastern Europe. The emergence of such highly virulent strains in Western Europe might result in severe clinical problems and a financial disaster. In this context, the efficacy of the commercial modified-live PRRSV subtype 1 vaccine UNISTRAIN^® PRRS was tested upon challenge with the East European subtype 3 PRRSV strain Lena.

Results

The mean duration of fever was shortened and the number of fever days was significantly lower in vaccinated pigs than in control pigs. Moreover, a lower number of vaccinated animals showed fever, respiratory disorders and conjunctivitis. The mean virus titers in the nasal secretions post challenge (AUC) were significantly lower in the vaccinated group than in the control group. The duration of viremia was slightly shorter (not significantly different) in the vaccinated group as compared to the control group.

Conclusions

Vaccination of pigs with the modified-live vaccine UNISTRAIN^® PRRS provides a partial clinical and virological protection against the PRRSV subtype 3 strain Lena.

Advance in phage display technology for bioanalysis

Phage display technology has emerged as a powerful tool for target gene expression and target-specific ligand selection. It is widely used to screen peptides, proteins and antibodies with the advantages of simplicity, high efficiency and low cost. A variety of targets, including ions, small molecules, inorganic materials, natural and biological polymers, nanostructures, cells, bacteria, and even tissues, have been demonstrated to generate specific binding ligands by phage display. Phages and target-specific ligands screened by phage display have been widely used as affinity reagents in therapeutics, diagnostics and biosensors. In this review, comparisons of different types of phage display systems are first presented. Particularly, microfluidic-based phage display, which enables screening with high throughput, high efficiency and integration, is highlighted. More importantly, we emphasize the advances in biosensors based on phages or phage-derived probes, including nonlytic phages, lytic phages, peptides or proteins screened by phage display, phage assemblies and phage-nanomaterial complexes. However, more efficient and higher throughput phage display methods are still needed to meet an explosion in demand for bioanalysis. Furthermore, screening of cyclic peptides and functional peptides will be the hotspot in bioanalysis.

Influence of the amino acid residues at 70 in M protein of porcine reproductive and respiratory syndrome virus on viral neutralization susceptibility to the serum antibody

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is mainly responsible for the significant economic losses in pig industry in the world. The adaptive immune responses of the host act as an important source of selective pressure in the evolutionary process of the virus. In the previous study, we confirmed that the amino acid (aa) residues at 102 and 104 sites in GP5 played an important role in escaping from the neutralizing antibodies (NAbs) against highly pathogenic PRRSV (HP-PRRSV). In this study, we further analyzed the aa mutants affecting neutralization susceptibility of NAbs in other structure proteins in NAbs resistant variants.

Methods

Based on the different aa residues of the structural proteins between the resistant virus BB20s and the parent virus BB, 12 recombinant PRRSV strains containing these aa residue substitutions were constructed using reverse genetic techniques. The neutralizing antibody (NA) titers of the recombinant strains were tested on MARC-145 and porcine alveolar macrophages (PAMs). And the NAbs binding abilities of parent and rescued viruses were tested by using ELISA method.

Results

By using the neutralization assay, it was revealed that the NA titer of N4 serum with rBB/Ms was significantly lower than that with rBB. Meanwhile, NA titer of the serum with rBB20s/M was significantly higher than that with rBB20s. The ELISA binding results showed that rBB/Ms had higher binding inability to N4 than did rBB. And alignment of M protein revealed that the variant aa residue lysine (K) at 70 was also existed in field type 2 and vaccine PRRSV strains.

Conclusions

The aa residue at 70 in M protein of PRRSV played an important role in regulating neutralization susceptibility to the porcine serum NAbs. It may be helpful for monitoring the antigen variant strains in the field and developing new vaccine against PRRSV in the future.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0505-7) contains supplementary material, which is available to authorized users.

Efficacy of commercial genotype 1 porcine reproductive and respiratory syndrome virus (PRRSV) vaccine against field isolate of genotype 2 PRRSV

Although several recent studies have found that type 1 porcine reproductive and respiratory syndrome virus (PRRSV) modified live virus (MLV) vaccine showed appreciable levels of cross-protection against type 2 PRRSV infection, the possibility of cross-protection between two genotype of PRRSV is still controversial. To determine potential protective efficacy against hetero-genotype field strain of PRRSV and to improve understandings of the mechanisms underlying performance improvement after infection in vaccinated animals, piglets were vaccinated with type 1 PRRSV MLV vaccine and challenged with type 2 field strain of PRRSV. As a result, vaccinated animals gained on average 8.45 kg in comparison to 4.77 kg measured in non-vaccinated animals during a 3-week period after viral challenge, which shows using a certain PRRSV vaccine could be clinically effective against heterologous genotypic virus challenge. In vaccinated animals, viremia was reduced and cleared rapidly, whilst viral load was much higher and reduced more slowly, indicating rebound viremia in non-vaccinated animals. The titers of neutralizing antibody against the type 2 PRRSV did not exceed the protective level in any animal from both vaccinated and control groups. Instead, antibody avidity of vaccinated animals was much higher than in the control group clearly. Furthermore, a strong negative correlation between antibody avidity and viremia was noted in 80% of vaccinated animals. Through those results from tests evaluating degree of antibody maturation and its relevance with clearing viremia, it could be suggested that non-neutralizing antibodies induced by vaccination prior to challenge might play a key role in protection against PRRSV infection, especially in early time course.

Advancement and applications of peptide phage display technology in biomedical science

Combinatorial phage library is a powerful research tool for high-throughput screening of protein interactions. Of all available molecular display techniques, phage display has proven to be the most popular approach. Screening phage-displayed random peptide libraries is an effective means of identifying peptides that can bind target molecules and regulate their function. Phage-displayed peptide libraries can be used for (i) B-cell and T-cell epitope mapping, (ii) selection of bioactive peptides bound to receptors or proteins, disease-specific antigen mimics, peptides bound to non-protein targets, cell-specific peptides, or organ-specific peptides, and (iii) development of peptide-mediated drug delivery systems and other applications. Targeting peptides identified using phage display technology may be useful for basic research and translational medicine. In this review article, we summarize the latest technological advancements in the application of phage-displayed peptide libraries to applied biomedical sciences.

Characterization of two Austrian porcine reproductive and respiratory syndrome virus (PRRSV) field isolates reveals relationship to East Asian strains

Porcine reproductive and respiratory syndrome virus (PRRSV) causes major problems for the swine industry worldwide. Due to Austria's central location in Europe, a large number of animals are transported through the country. However, little is known about current PRRSV strains and epidemiology. We determined full-length genome sequences of two Austrian field isolates (AUT13-883 and AUT14-440) from recent PRRSV outbreaks and of a related German isolate (GER09-613). Phylogenetic analysis revealed that the strains belong to European genotype 1 subtype 1 and form a cluster together with a South Korean strain. Remarkably, AUT14-440 infected the simian cell line MARC-145 without prior adaptation. In addition, this isolate showed exceptional deletions in nonstructural protein 2, in the overlapping region of glycoprotein 3 and 4 and in the 3' untranslated region. Both Austrian isolates caused similar lung lesions but only pigs infected with AUT14-440 developed clear clinical signs of infection. Taken together, the genetic and biological characterization of two novel Austrian PRRSV field isolates revealed similarities to East Asian strains. This stresses the necessity for a more detailed analysis of current PRRSV strains in Europe beyond the determination of short ORF5 and ORF7 sequences.

Immunity raised by recent European subtype 1 PRRSV strains allows better replication of East European subtype 3 PRRSV strain Lena than that raised by an older strain

Stable spatial distribution of porcine reproductive and respiratory syndrome (PRRSV)-1 subtypes in Europe is accompanied by a strong population immunity induced by local PRRSV strains. In the present study, it was examined if the immunity induced by three West European subtype 1 PRRSV strains (2007 isolate 07V063 and 2013 isolates 13V091 and 13V117) offers protection against the highly virulent East European subtype 3 PRRSV strain Lena. The number of fever days was greater (p < 0.05) in the control group (7.6 ± 1.7 days) compared to the immune groups (07V063-immune: 4.0 ± 1.2 days, 13V091-immune: 4.6 ± 1.1 days, 13V117-immune: 4.0 ± 2.9 days). In all groups, protection was characterized by reduction (p < 0.05) of AUC values of nasal shedding (control: 14.6, 07V063-immune: 3.4, 13V091-immune: 8.9, 13V117-immune: 8.0) and viremia (control: 28.1, 07V063-immune: 5.4, 13V091-immune: 9.0, 13V117-immune: 8.3). Reduction of respiratory disease, nasal shedding (mean AUC and mean peak values) and viremia (mean AUC and mean peak values) was more pronounced in 07V063-immune (p < 0.05) than in 13V091-immune and 13V117-immune animals. Inoculation with subtype 1 PRRSV strains caused priming of the Lena-specific virus neutralization antibody response. Upon challenge with Lena, we observed a very strong serological booster effect for neutralizing antibodies against strains used for the first inoculation. Our results indicate that inoculation with subtype 1 PRRSV strains can partially protect against antigenically divergent subtype 3 strains. The lower protection level elicited by recently isolated subtype 1 PRRSV strains may impair the outcome of the spatial expansion of subtype 3 strains from East Europe to West Europe.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV): Pathogenesis and Interaction with the Immune System

This review addresses important issues of porcine reproductive and respiratory syndrome virus (PRRSV) infection, immunity, pathogenesis, and control. Worldwide, PRRS is the most economically important infectious disease of pigs. We highlight the latest information on viral genome structure, pathogenic mechanisms, and host immunity, with a special focus on immune factors that modulate PRRSV infections during the acute and chronic/persistent disease phases. We address genetic control of host resistance and probe effects of PRRSV infection on reproductive traits. A major goal is to identify cellular/viral targets and pathways for designing more effective vaccines and therapeutics. Based on progress in viral reverse genetics, host transcriptomics and genomics, and vaccinology and adjuvant technologies, we have identified new areas for PRRS control and prevention. Finally, we highlight the gaps in our knowledge base and the need for advanced molecular and immune tools to stimulate PRRS research and field applications.

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.

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.

Innate and adaptive immunity against Porcine Reproductive and Respiratory Syndrome Virus

Many highly effective vaccines have been produced against viruses whose virulent infection elicits strong and durable protective immunity. In these cases, characterization of immune effector mechanisms and identification of protective epitopes/immunogens has been informative for the development of successful vaccine programs. Diseases in which the immune system does not rapidly clear the acute infection and/or convalescent immunity does not provide highly effective protection against secondary challenge pose a major hurdle for clinicians and scientists. Porcine reproductive and respiratory syndrome virus (PRRSV) falls primarily into this category, though not entirely. PRRSV causes a prolonged infection, though the host eventually clears the virus. Neutralizing antibodies can provide passive protection when present prior to challenge, though infection can be controlled in the absence of detectable neutralizing antibodies. In addition, primed pigs (through natural exposure or vaccination with a modified-live vaccine) show some protection against secondary challenge. While peripheral PRRSV-specific T cell responses have been examined, their direct contribution to antibody-mediated immunity and viral clearance have not been fully elucidated. The innate immune response following PRRSV infection, particularly the antiviral type I interferon response, is meager, but when provided exogenously, IFN-α enhances PRRSV immunity and viral control. Overall, the quality of immunity induced by natural PRRSV infection is not ideal for informing vaccine development programs.

The epitopes necessary for protection may be identified through natural exposure or modified-live vaccines and subsequently applied to vaccine delivery platforms to accelerate induction of protective immunity following vaccination. Collectively, further work to identify protective B and T cell epitopes and mechanisms by which PRRSV eludes innate immunity will enhance our ability to develop more effective methods to control and eliminate PRRS disease.

Effect of amino acids residues 323-433 and 628-747 in Nsp2 of representative porcine reproductive and respiratory syndrome virus strains on inflammatory response in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that is responsible for large economic losses in the swine industry worldwide. In PRRSV strains, many genetic variations occur in the central hypervariable region (HV2) of the Nsp2 gene, which encodes non-structural protein 2. For example, PRRSV strains VR2332, Em2007, MN184C, and TJM-F92 contained variations in the Nsp2 sequences and exhibited differing levels of virulence in adult pigs. However, the role of HV2 with respect to PRRSV immunity is unclear. In this study, four recombinant PRRSV strains (rBB/+30aa, rBB/Δ68aa, rBB/Δ111aa, and rBB/Δ120aa) were rescued using a highly pathogenic type 2 PRRSV cDNA clone (pBB). All rescued strains displayed similar growth characteristics to the parental rBB virus in pulmonary alveolar macrophages (PAMs). Expression levels of inflammatory cytokines IL-β, IL-6, and TNF-α were significantly lower, at the mRNA and protein level, for groups infected with rBB/Δ111aa and rBB/Δ120aa than those in the rBB group. Levels of these inflammatory cytokines in the rBB/+30aa and rBB/Δ68aa groups were not significantly different with those in the rBB group. Phosphorylation levels of IκB were decreased to a greater extent in the rBB/Δ111aa and rBB/Δ120aa groups compared with those in the rBB/+30aa, rBB/Δ68aa, and rBB groups. Our results indicate that amino acids 323-433 and 628-747 of Nsp2 failed to exert significant effects on PRRSV replication in PAMs, but modulated the expression of inflammatory cytokines in vitro.

The amino acid residues at 102 and 104 in GP5 of porcine reproductive and respiratory syndrome virus regulate viral neutralization susceptibility to the porcine serum neutralizing antibody

Porcine reproductive and respiratory syndrome virus (PRRSV) is mainly responsible for the heavy economic losses in pig industry in the world. A number of neutralizing epitopes have been identified in the viral structural proteins GP3, GP4, GP5 and M. In this study, the important amino acid (aa) residues of HP-PRRSV strain BB affecting neutralization susceptibility of antibody were examined using resistant strains generated under neutralizing antibody (NAb) pressure in MARC-145 cells, reverse genetic technique and virus neutralization assay. HP-PRRSV strain BB was passaged under the pressure of porcine NAb serum in vitro. A resistant strain BB34s with 102 and 104 aa substitutions in GP5, which have been predicted to be the positive sites for pressure selection (Delisle et al., 2012), was cloned and identified. To determine the effect of the two aa residues on neutralization, eight recombinant PRRSV strains were generated, and neutralization assay results confirmed that the aa residues 102 and 104 in GP5 played an important role in NAbs against HP-PRRSV in MARC-145 cells and porcine alveolar macrophages. Alignment of GP5 sequences revealed that the variant aa residues at 102 and 104 were frequent among type 2 PRRSV strains. It may be helpful for understanding the mechanism regulating the neutralization susceptibility of PRRSV to the NAbs and monitoring the antigen variant strains in the field.

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.

Different clinical, virological, serological and tissue tropism outcomes of two new and one old Belgian type 1 subtype 1 porcine reproductive and respiratory virus (PRRSV) isolates

In this study, the pathogenic behavior of PRRSV 13V091 and 13V117, isolated in 2013 from two different Belgian farms with enzootic respiratory problems shortly after weaning in the nursery, were compared with the Belgian strain 07V063 isolated in 2007. Full-length genome sequencing was performed to identify their origin. Twelve weeks-old pigs were inoculated intranasally (IN) with 13V091, 13V117 or 07V063 (9 pigs/group). At 10 days post inoculation (dpi), 4 animals from each group were euthanized and tissues were collected for pathology, virological and serological analysis. 13V091 infection resulted in the highest respiratory disease scores and longest period of fever. Gross lung lesions were more pronounced for 13V091 (13%), than for 13V117 (7%) and 07V063 (11%). The nasal shedding and viremia was also most extensive with 13V091. The 13V091 group showed the highest virus replication in conchae, tonsils and retropharyngeal lymph nodes. 13V117 infection resulted in the lowest virus replication in lymphoid tissues. 13V091 showed higher numbers of sialoadhesin⁻ infected cells/mm² in conchae, tonsils and spleen than 13V117 and 07V063. Neutralizing antibody response with 07V063 was stronger than with 13V091 and 13V117. It can be concluded that (i) 13V091 is a highly pathogenic type 1 subtype 1 PRRSV strain that replicates better than 07V063 and 13V117 and has a strong tropism for sialoadhesin⁻ cells and (ii) despite the close genetic relationship between 13V117 and 07V063, 13V117 has an increased nasal replication and shedding, but a decreased replication in lymphoid tissues compared to 07V063.

Immunological features of the non-structural proteins of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is currently one of the most important viruses affecting the swine industry worldwide. Despite the large number of papers published each year, the participation of non-structural proteins (nsps) in the immune response is not completely clear. nsps have been involved in the host innate immune response, specifically, nsp1α/β, nsp2, nsp4 and nsp11 have been associated with the immunomodulation capability of the virus. To date, only participation by nsp1, nsp2, nsp4 and nsp7 in the humoral immune response has been reported, with the role of other nsps being overlooked. Furthermore, nsp1, nsp2, nsp5, nsp7 nsp9, nsp10, nsp11 have been implicated in the induction of IFN-γ and probably in the development of the cell-mediated immune response. This review discusses recent reports involving the participation of nsps in the modulation of the innate immune response and their role in the induction of both the humoral and cellular immune responses.

Engineering the PRRS virus genome: updates and perspectives

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We review PRRSV infectious clones and their applications.

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14 infectious clones are available so far for genotypes I and II.

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Genomic mutations, insertions, deletions, and replacements are successful.

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We discuss advances and utilization of PRRSV reverse genetics and future potential.

Porcine reproductive and respiratory syndrome virus (PRRSV) is endemic in most pig producing countries worldwide and causes enormous economic losses to the pork industry. Infectious clones for PRRSV have been constructed, and so far at least 14 different infectious clones are available representing both genotypes I and II. Two strategies have been taken for progeny reconstitution: RNA transfection and DNA transfection. Mutations, insertions, deletions, and replacements of the viral genome have been employed to study the structure function relationship, foreign gene expression, functional complementation, and virulence determinants. Essential regions and non-essential regions for viral replication have been identified in both the coding regions and non-encoding regions. Foreign sequences have successfully been inserted into the nsp2 and N regions and in the space between ORF1b and ORF2a. Chimeras between member viruses in the family Arteriviridae have also been constructed and utilized to study cell tropism and functional complementation. This review discusses the advances and utilization of PRRSV reverse genetics and its potential for future research.