Insertion and deletion in a non-essential region of the nonstructural protein 2 (nsp2) of porcine reproductive and respiratory syndrome (PRRS) virus: effects on virulence and immunogenicity

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.

Nonstructural Protein 2 Is Critical to Infection Efficiency of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus on PAMs and Influence Virulence In Vivo

Porcine reproductive and respiratory syndrome (PRRS) is an important viral disease, causing significant economic losses to the swine industry worldwide. Atypical cases caused by highly pathogenic PRRS virus (HP-PRRSV) emerged in 2006 in China. The vaccine strain HuN4-F112 has been developed from the wild-type HP-PRRSV HuN4 through repeated passages on MARC-145 cells. However, the mechanisms of attenuation have yet to be defined. Previous studies have shown that the vaccine strain HuN4-F112 could not effectively replicate in porcine alveolar macrophages (PAMs). In the present study, a series of chimeric and mutant PRRSVs were constructed to investigate regions associated with the virus attenuation. Firstly, the corresponding genome regions (ORF1a, ORF1b and ORFs 2-7) were exchanged between two infectious clones of HuN4 and HuN4-F112, and then the influence of small regions in ORF1a and ORF2-7 was evaluated, then influence of specific amino acids on NSP2 was tested. NSP2 was determined to be the key gene that regulated infection efficiency on PAMs, and amino acids at 893 and 979 of NSP2 were the key amino acids. The results of in vivo study indicated that NSP2 was not only important for infection efficiency in vitro, but also influenced the virulence, which was indicated by the results of survival rate, temperature, viremia, lung score and tissue score.

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 four types of MLV-derived porcine reproductive and respiratory syndrome viruses isolated in unvaccinated pigs from 2016 to 2020

Modified live vaccines (MLVs) have been utilized to combat porcine reproductive and respiratory syndrome (PRRS), which raises a serious concern about the MLV-derived PRRS virus (PRRSV) isolates. During the routine investigation of PRRSV in China, four lung samples collected from unvaccinated diseased pigs from 2016 to 2020 were detected as PRRSV positive. The PRRSVs shared high ORF5 identities to CH-1R, JXA1-R, TJM-F92 and RespPRRS MLV vaccines, respectively. The viruses were isolated in Marc-145 cells and denominated as SD1612-1, JS1703-21, JSTZ1907-714 and JSYC20-05-1. Genome comparison confirmed that these isolates share the highest genomic homologies to CH-1R (97.96%), JXA1-R (99.64%), TJM-F92 (99.00%) and RespPRRS MLV (99.57%) than any other known isolates. Genome-based phylogenetic analysis showed that SD1612-1 and CH-1R, JS1703-21 and JXA1-R, JSTZ1907-714 and TJM-F92, JSYC20-05-1 and RespPRRS MLV were grouped in the same branches. In addition, amino acids unique to corresponding vaccine attenuations were also identified in our isolates. Noticeably, amino-acids potentially associated with the virulence revision from MLV strains to parental virulent viruses were also identified in the MLV-derived isolates. Our results confirm that the four types of MLV-derived isolates are circulating and evolving in Chinese swine herds for years, which highlights the necessity for the fair use of PRRS MLVs.

Porcine Reproductive and Respiratory Syndrome Virus Reverse Genetics and the Major Applications

Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive sense, single-stranded RNA virus that is known to infect only pigs. The virus emerged in the late 1980s and became endemic in most swine producing countries, causing substantial economic losses to the swine industry. The first reverse genetics system for PRRSV was reported in 1998. Since then, several infectious cDNA clones for PRRSV have been constructed. The availability of these infectious cDNA clones has facilitated the genetic modifications of the viral genome at precise locations. Common approaches to manipulate the viral genome include site-directed mutagenesis, deletion of viral genes or gene fragments, insertion of foreign genes, and swapping genes between PRRSV strains or between PRRSV and other members of the Arteriviridae family. In this review, we describe the approaches to construct an infectious cDNA for PRRSV and the ten major applications of these infectious clones to study virus biology and virus–host interaction, and to design a new generation of vaccines with improved levels of safety and efficacy.

Generation of a Recombinant Porcine Reproductive and Respiratory Syndrome Virus Stably Expressing Two Marker Genes

Porcine reproductive and respiratory syndrome virus (PRRSV) has been used as a gene expression vector in the development of vaccines. Most of these recombinant PRRSV vectors express only a single foreign gene through either an internal insertion in the hypervariable region of nsp2 or expression cassette and some of these recombinant vectors are genetically unstable. Here, we combined internal insertion in nsp2 and expression cassette methods to generate a novel recombinant PRRSV stably expressing the red fluorescence protein (RFP) and the green fluorescence protein (GFP) genes. Biological characteristic analysis of the recombinant PRRSV carrying the two marker genes, rGX-RFP-GFP, showed that it displayed similar growth kinetics and yet it yielded less infectious viruses when compared to the parental virus rGXAM. Co-expression of both the RFP and GFP was observed using confocal fluorescence microscopy when the rGX-RFP-GFP viruses infected MARC-145 cells. Furthermore, the PRRSV-based two-marker gene expression vector is genetically stable during 20 serial passages in MARC-145 cells. These data demonstrate that it is possible to express two interested immunogens from a single PRRSV vector.

Whole-Genome Sequencing of Porcine Reproductive and Respiratory Syndrome Virus from Field Clinical Samples Improves the Genomic Surveillance of the Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major economic concern worldwide. There are currently large data sets available about the ORF5 gene of the virus, with thousands of sequences available, but little data are currently available on the full-length genome of PRRSV. We hypothesized that whole-genome sequencing (WGS) of the PRRSV genome would allow better epidemiological monitoring than ORF5 gene sequencing. PRRSV PCR-positive serum, oral fluid, and tissue clinical samples submitted to the diagnostic laboratory for routine surveillance or diagnosis of PRRSV infection in Québec, Canada, swine herds were used. The PRRSV reverse transcription-quantitative PCR Cq values of the processed samples varied between 11.5 and 34.34. PRRSV strain genomes were isolated using a poly (A)-tail method and were sequenced with a MiSeq Illumina sequencer. Ninety-two full-length PRRSV genomes were obtained from 88 clinical samples out of 132 tested samples, resulting in a PRRSV WGS success rate of 66.67%. Three important deletions in ORF1a were found in most wild-type (i.e., not vaccine-like) strains. The importance of these deletions remains undetermined. Two different full-length PRRSV genomes were found in four different samples (three serum samples and one pool of tissues), suggesting a 4.55% PRRSV strain coinfection prevalence in swine. Moreover, six PRRSV whole genomes (6.52% of PRRSV strains) were found to cluster differently than they did under the ORF5 classification method. Overall, WGS of PRRSV enables better strain classification and/or interpretation of results in 9.10% of clinical samples than ORF5 sequencing, as well as allowing interesting research avenues.

Generation of a porcine reproductive and respiratory syndrome virus expressing a marker gene inserted between ORF4 and ORF5a

In recent years, the availability of reverse genetics systems for porcine reproductive and respiratory syndrome virus (PRRSV) has created new perspectives for the use of recombinant viruses as expression vectors. Most of these recombinant PRRSV vectors express foreign genes through either an independent transcription unit inserted in ORF1b and ORF2, or in ORF7 and the 3' UTR. The aim of this study was to find an alternative site for foreign gene insertion into the PRRSV genome. Here, we constructed an infectious cDNA clone for a cell-adapted PRRSV strain, GXNN1396-P96. This cDNA-clone-derived recombinant virus (rGXAM) was comparable in its growth kinetics in MARC-145 cells to the parental virus, GX1396-P96. Using the infectious cDNA-clone, we inserted an independent transcription unit in ORF4 and ORF5a to generate a novel PRRSV-based recombinant virus expressing the green fluorescent protein (GFP) gene. Biological characterization of the recombinant virus, rGX45BSTRS-GFP, showed that it maintained similar growth characteristics but produced fewer infectious virions than the parental PRRSV. These data demonstrate that the ORF4 and ORF5a site is able to tolerate the insertion of foreign genes.

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.

Identification of Two Porcine Reproductive and Respiratory Syndrome Virus Variants Sharing High Genomic Homology but with Distinct Virulence

Porcine reproductive and respiratory syndrome virus (PRRSV) causes huge economic loss to the global swine industry. Even though several control strategies have been applied, PRRS is still not effectively controlled due to the continuous emergence of new variants and limited cross-protection by current vaccines. During the routine epidemiological investigation in 2017, two PRRSV variants were identified from a severe abortion farm and a clinically healthy farm, respectively. The viruses were isolated and denominated as XJ17-5 and JSTZ1712-12. Genomic sequencing indicated that their genomes are both 14,960 bp in length sharing 99.45% nucleotide identity. Sequence alignments identified a discontinuous 30-amino-acid deletion and a continuous 120-amino-acid deletion in nsp2 of both isolates. Genome-based phylogenetic analysis confirmed that XJ17-5 and JSTZ1712-12 belong to the HP-PRRSV subtype but form a new branch with other isolates containing the same 150-amino-acid deletion in nsp2. Pathogenic analysis showed that XJ17-5 is highly virulent causing 60% mortality, while JSTZ1712-12 is avirulent for piglets. Furthermore, fragment comparisons identified 34-amino-acid differences between XJ17-5 and JSTZ1712-12 that might be associated with the distinct virulence. The identification of highly homologous HP-PRRSV variants with new genetic feature and distinct virulence contributes to further analyze the pathogenesis and evolution of PRRSV in the field.

An Attenuated Highly Pathogenic Chinese PRRS Viral Vaccine Confers Cross Protection to Pigs against Challenge with the Emerging PRRSV NADC30-Like Strain

A novel PRRSV strain was isolated in China that was genetically similar to the NADC30 strain which is reported to have spread throughout China. The objective of the present study was to evaluate the cross-protective efficacy of the live vaccine TJM-F92 in young pigs against challenge with a NADC30-like strain, HN201605. Twenty-five PRRSV- and antibody-free pigs were randomly divided into the following five groups: Vac/ChA, Unvac/ChA, Vac/ChB, Unvac/ChB and the mock. The pigs in groups Vac/ChA and Vac/ChB were inoculated intramuscularly with 1 mL TJM-F92 (10^5.0 TCID₅₀/mL). At 28 days post vaccination (0 days post challenge), groups Vac/ChA and Unvac/ChA were inoculated intranasally with 10^4.5 TCID₅₀/mL PRRSV strain TJ F3 (2 mL/pig), while groups Vac/ChB and Unvac/ChB were inoculated, using the same route, with the same dose of the NADC30-like strain HN201605 F3. Protective effects of the PRRSV strain were observed in all pigs in the Vac/ChA and Vac/ChB groups. Neither high fever nor signs of clinical disease were observed through the experiment in these groups, whereas pigs in Unvac/ChA group exhibited serious clinical symptoms, pathological lesions, and weight loss. In Unvac/ChB group, pigs developed milder clinical symptoms, which demonstrated that the NADC30-like strain HN201605 had moderate pathogenicity. The results suggest that the MLV vaccine strain TJM-F92 is an effective and safe vaccine candidate for use in China.

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.

Development of monoclonal antibody for differentiating porcine reproductive and respiratory syndrome virus and identification of a novel non-structural protein 2 epitope peptide

Porcine reproductive and respiratory syndrome virus (PRRSV) nucleocapsid protein (NP) is the immunodominant region of PRRSV viral proteins. Non-structural protein 2 (Nsp2) and its hypervariable region play an essential role in the differential diagnosis of PRRSV. Western blot and immunofluorescence assay (IFA) analyses found that 2 out of 18 monoclonal antibodies (MAbs) recognized the NP and that 5 of 11 MAbs recognized Nsp2-120aa. IFA data demonstrated that 2 MAbs raised against the NP have a positive reaction to PRRSV; either HP-PRRSV, classic PRRSV or the vaccine strain at 1:100 dilution. Two MAbs raise against Nsp2-120aa also react positively with the classic PRRSV nor HP-PRRSV, but not with the PRRSV vaccine strain TJM-F92. Epitope mapping using truncated proteins identified a novel Nsp2-120aa epitope. In addition, we show that MAb BR/PNsp2-2A20 recognizes a 20 amino acid peptide (707) GRFEFLPKMILETPPPHPCG (727) of Nsp2. Based on our findings, we propose that MAb BR/PNsp2-2A20, raised against Nsp2-120aa of PRRSV, as a candidate specific diagnostic MAb for differentiation of the PRRSV virulent strains infected pig from vaccine strain TJM-F92 inoculated ones. The MAbs developed here have potential for use in diagnostic and research tools, including immunofluorescence assay, enzyme-linked immunosorbent assay and Western blotting.

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.

Induction of Apoptosis by the Nonstructural Protein 4 and 10 of Porcine Reproductive and Respiratory Syndrome Virus

Infection by most viruses triggers apoptosis in host cells, and viruses manipulate this cell response to promote viral replication, virus spread, and cell killing. Porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to induce apoptosis both in vitro and in vivo, while the regulatory roles of PRRSV-encoded products in apoptosis are not fully understood. In the present study, we first showed a biphasic apoptosis regulation by a highly pathogenic PRRSV strain JXwn06. It was indicated that PRRSV infection delays apoptosis at early infection but activates apoptosis at late infection in MARC-145 cells. In PRRSV-infected MARC-145 cells, procaspase-8, -9 and -12 were activated at late infection, demonstrating the involvements of death receptor pathway, mitochondrial pathway and endoplasmic reticulum (ER) stress pathway in inducing apoptosis. PRRSV was also shown to induce a similar apoptosis process in pulmonary alveolar macrophages (PAMs) with an early initiation. Next, the PRRSV-encoded apoptosis inducers were screened, indicating that the nonstructural protein (Nsp) 4 and Nsp10 of PRRSV are pro-apoptotic. In the presence of Nsp4, it was confirmed that procaspase-8, -9 and -12 were cleaved, and Nsp4 facilitates the cleavage of procaspase-9 by activating B-cell lymphoma 2 interacting mediator of cell death (Bim), a pro-apoptotic protein. In addition, Nsp4 was shown to induce the degradation of an anti-apoptotic protein, B-cell lymphoma-extra large (Bcl-xL). Nsp10 was shown to activate procaspase-8 and -9 but procaspase-12 and to upregulate the expression of BH3-only pro-apoptotic protein BH3 interacting-domain death agonist (Bid) and its active form, truncated Bid (tBid). Clearly, the participation of both activated caspase-8 and Bid is required for Nsp10-induced apoptosis, indicating a crosstalk between extrinsic- and mitochondria-dependent pathways. Together, our findings suggest that PRRSV infection regulates apoptosis in a two-phase manner and activates all three apoptotic pathways; the Nsp4 and Nsp10 of PRRSV function as apoptosis inducers with different molecular basis.

Live porcine reproductive and respiratory syndrome virus vaccines: Current status and future direction

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) was reported in the late 1980s. PRRS still is a huge economic concern to the global pig industry with a current annual loss estimated at one billion US dollars in North America alone. It has been 20 years since the first modified live-attenuated PRRSV vaccine (PRRSV-MLV) became commercially available. PRRSV-MLVs provide homologous protection and help in reducing shedding of heterologous viruses, but they do not completely protect pigs against heterologous field strains. There have been many advances in understanding the biology and ecology of PRRSV; however, the complexities of virus-host interaction and PRRSV vaccinology are not yet completely understood leaving a significant gap for improving breadth of immunity against diverse PRRS isolates. This review provides insights on immunization efforts using infectious PRRSV-based vaccines since the 1990s, beginning with live PRRSV immunization, development and commercialization of PRRSV-MLV, and strategies to overcome the deficiencies of PRRSV-MLV through use of replicating viral vectors expressing multiple PRRSV membrane proteins. Finally, powerful reverse genetics systems (infectious cDNA clones) generated from more than 20 PRRSV isolates of both genotypes 1 and 2 viruses have provided a great resource for exploring many innovative strategies to improve the safety and cross-protective efficacy of live PRRSV vaccines. Examples include vaccines with diminished ability to down-regulate the immune system, positive and negative marker vaccines, multivalent vaccines incorporating antigens from other porcine pathogens, vaccines that carry their own cytokine adjuvants, and chimeric vaccine viruses with the potential for broad cross-protection against heterologous strains. To combat this devastating pig disease in the future, evaluation and commercialization of such improved live PRRSV vaccines is a shared goal among PRRSV researchers, pork producers and biologics companies.

Antiviral regulation in porcine monocytic cells at different activation states

Monocytic cells, including macrophages and dendritic cells, exist in different activation states that are critical to the regulation of antimicrobial immunity. Many pandemic viruses are monocytotropic, including porcine reproductive and respiratory syndrome virus (PRRSV), which directly infects subsets of monocytic cells and interferes with antiviral responses. To study antiviral responses in PRRSV-infected monocytic cells, we characterized inflammatory cytokine responses and genome-wide profiled signature genes to investigate response pathways in uninfected and PRRSV-infected monocytic cells at different activation states. Our findings showed suppressed interferon (IFN) production in macrophages in non-antiviral states and an arrest of lipid metabolic pathways in macrophages at antiviral states. Importantly, porcine monocytic cells at different activation states were susceptible to PRRSV and responded differently to viral infection. Based on Gene Ontology (GO) analysis, two approaches were used to potentiate antiviral activity: (i) pharmaceutical modulation of cellular lipid metabolism and (ii) in situ PRRSV replication-competent expression of interferon alpha (IFN-α). Both approaches significantly suppressed exogenous viral infection in monocytic cells. In particular, the engineered IFN-expressing PRRSV strain eliminated exogenous virus infection and sustained cell viability at 4 days postinfection in macrophages. These findings suggest an intricate interaction of viral infection with the activation status of porcine monocytic cells. An understanding and integration of antiviral infection with activation status of monocytic cells may provide a means of potentiating antiviral immunity.

IMPORTANCE

Activation statuses of monocytic cells, including monocytes, macrophages (Mϕs), and dendritic cells (DCs), are critically important for antiviral immunity. Unfortunately, the activation status of porcine monocytic cells or how cell activation status functionally interacts with antiviral immunity remains largely unknown. This is a significant omission because many economically important porcine viruses are monocytotropic, including our focus, PRRSV, which alone causes nearly $800 million economic loss annually in the U.S. swine industries. PRRSV is ideal for deciphering how monocytic cell activation statuses interact with antiviral immunity, because it directly infects subsets of monocytic cells and subverts overall immune responses. In this study, we systematically investigate the activation status of porcine monocytic cells to determine the intricate interaction of viral infection with activation statuses and functionally regulate antiviral immunity within the framework of the activation paradigm. Our findings may provide a means of potentiating antiviral immunity and leading to novel vaccines for PRRS prevention.

A new porcine reproductive and respiratory syndrome virus strain with highly conserved molecular characteristics in its parental and attenuated strains

Porcine reproductive and respiratory syndrome virus (PRRSV) causes substantial economic losses to many swine-producing regions. In this study, PRRSV strain NT0801-F80 was derived from its parental isolate NT0801 by 80 passages in Marc-145 cells. Experimental infection of piglets clearly demonstrated that strain NT0801-F80 is less virulent than NT0801. However, whole genome sequencing showed that the genomes of the parental and attenuated strains are highly conserved compared with those of four other pairs of virulent parental/attenuated vaccine strains (VR2332 and RespPRRS MLV, JA142 and Ingelvac(®) ATP MLV, CH-1a and CH-1R, and JXA1 and JXAR). The attenuated strain NT0801-F80 has only 21 nucleotide changes, producing only 14 amino acid changes in NSP2, GP2, GP3, and GP5, compared with those aa sequences of the virulent parental strain. These mutated aa in the attenuated virus may be involved in virulence. These data provide valuable information on the attenuation mechanism of PRRSV that should be useful in future research.

Genomic analysis and pathogenic characteristics of Type 2 porcine reproductive and respiratory syndrome virus nsp2 deletion strains isolated in Korea

Porcine reproductive and respiratory syndrome virus (PRRSV) is a globally ubiquitous swine virus that exhibits genetic and pathogenic heterogeneity among isolates. The present study was conducted to determine the complete genome sequence and pathogenicity of two Korean type 2 PRRSV nonstructural protein 2 (nsp2) deletion mutants, CA-2 and KNU-12-KJ4. The full-length genomes of CA-2 and KNU-12-KJ4 were determined to be 15,018 and 15,019 nucleotides in length, excluding the poly(A) tail, respectively, which were 393- or 392-nucleotide shorter than that of the type 2 NA prototype strain VR-2332 due to the presence of notable large deletions within the nsp2 gene. The genomes of CA-2 and KNU-12-KJ4 consisted of a 189- or 190-nucleotide 5' untranslated region (UTR), a 14,677-nucleotide protein-coding region, and a 151-nucleotide 3' UTR. Whole genome evaluation revealed that the nucleotide sequences of CA-2 and KNU-12-KJ4 are most similar to each other (10.7% sequence divergence), and then to the Korean strain CA-1 (11.3% sequence divergence) and the US strain MN184C (13.1% sequence divergence), respectively. To evaluate the in vitro immunity of nsp2 deletion variants, we sought to explore alteration of inflammatory cytokine and chemokine expression in PAM-pCD163 cells infected with each virus strain using quantitative real-time RT-PCR. Cytokine genes including IL-8, IL-10, and TNF-α, and chemokines such as MCP-1 and RANTES were found to be significantly elevated in nsp2 deletion virus-infected PAM cells. In contrast, expression of interferons (IFN-β, γ, and λ) and antiviral genes including ISG-15, -54, and -56 were unchanged or down-regulated in PAM cells infected with the nsp2 deletion mutants. Animal studies to assess the pathogenicity of nsp2 deletion PRRSVs demonstrated that both CA-2 and KNU-12-KJ4 strains notably produce weight loss in infected pigs. Furthermore, the nsp2 deletion mutants replicated well in pigs with significantly increased and prolonged viremia kinetics. Taken together, our results indicate that, among the three isolates, the outcome of in vitro and in vivo infection by CA-2 and KNU-12-KJ4 is comparable, suggesting that the large nsp2 deletion may be one of the viral genetic determinants contributing to PRRSV pathogenicity.

Characterization of a serologic marker candidate for development of a live-attenuated DIVA vaccine against porcine reproductive and respiratory syndrome virus

DIVA (differentiating infected from vaccinated animals) vaccines have proven extremely useful for control and eradication of infectious diseases in livestock. We describe here the characterization of a serologic marker epitope, so-called epitope-M201, which can be a potential target for development of a live-attenuated DIVA vaccine against porcine reproductive and respiratory syndrome virus (PRRSV). Epitope-M201 is located at the carboxyl terminus (residues 161-174) of the viral M protein. The epitope is highly immunodominant and well-conserved among type-II PRRSV isolates. Rabbit polyclonal antibodies prepared against this epitope are non-neutralizing; thus, the epitope does not seem to contribute to the protective immunity against PRRSV infection. Importantly, the immunogenicity of epitope-M201 can be disrupted through the introduction of a single amino acid mutation which does not adversely affect the viral replication. All together, our results provide an important starting point for the development of a live-attenuated DIVA vaccine against type-II PRRSV.

Sequence analysis of NSP9 gene of 25 PRRSV strains from Guangdong province, subtropical southern China

In order to evaluate the trend in the prevalence of porcine reproductive and respiratory syndrome on farms, 25 porcine reproductive and respiratory syndrome virus (PRRSV) strains were collected from 2009 to 2012 from 11 districts in the Guangdong province. The complete gene sequences of NSP9 from the 25 PRRSV strains were amplified, sequenced, and then compared with the published sequences of other strains. The results showed that these sequences shared 97.6-99.4 % of the same nucleotides and 97.5-99.4 % of the same amino acid sequence identities with JXA1, 91.4-93.1 % of the same nucleotides and 95.4-92.6 % of the same amino acid sequence identities with VR2332, and 95.1-98.4 % of the same nucleotides and 95.3-97.0 % of the same amino acid sequences with the classical Chinese vaccine strain Ch-1a. Phylogenetic analyses showed that all of these 25 strains belonged to the North American genotype and were further divided into three sub-genotypes. This basic data allowed us to evaluate the prevalence of PRRS in the Guangdong province and the variation and evolution of the nsp9 gene in PRRSV.

Porcine reproductive and respiratory syndrome virus nonstructural protein 2 contributes to NF-κB activation

Background

Nuclear factor-kappaB (NF-κB) is an inducible transcription factor that plays a key role in inflammation and immune responses, as well as in the regulation of cell proliferation and survival. Previous studies by our group and others have demonstrated that porcine reproductive and respiratory syndrome virus (PRRSV) infection could activate NF-κB in MARC-145 cells and alveolar macrophages. The nucleocapsid (N) protein was identified as an NF-κB activator among the structural proteins encoded by PRRSV; however, it remains unclear whether the nonstructural proteins (Nsps) contribute to NF-κB activation. In this study, we identified which Nsps can activate NF-κB and investigated the potential mechanism(s) by which they act.

Results

By screening the individual Nsps of PRRSV strain WUH3, Nsp2 exhibited great potential to activate NF-κB in MARC-145 and HeLa cells. Overexpression of Nsp2 induced IκBα degradation and nuclear translocation of NF-κB. Furthermore, Nsp2 also induced NF-κB-dependent inflammatory factors, including interleukin (IL)-6, IL-8, COX-2, and RANTES. Compared with the Nsp2 of the classical PRRSV strain, the Nsp2 of highly pathogenic PRRSV (HP-PRRSV) strains that possess a 30 amino acid (aa) deletion in Nsp2 displayed greater NF-κB activation. However, the 30-aa deletion was demonstrated to not be associated with NF-κB activation. Further functional domain analyses revealed that the hypervariable region (HV) of Nsp2 was essential for NF-κB activation.

Conclusions

Taken together, these data indicate that PRRSV Nsp2 is a multifunctional protein participating in the modulation of host inflammatory response, which suggests an important role of Nsp2 in pathogenesis and disease outcomes.

Replication-competent recombinant porcine reproductive and respiratory syndrome (PRRS) viruses expressing indicator proteins and antiviral cytokines

Porcine reproductive and respiratory syndrome virus (PRRSV) can subvert early innate immunity, which leads to ineffective antimicrobial responses. Overcoming immune subversion is critical for developing vaccines and other measures to control this devastating swine virus. The overall goal of this work was to enhance innate and adaptive immunity following vaccination through the expression of interferon (IFN) genes by the PRRSV genome. We have constructed a series of recombinant PRRS viruses using an infectious PRRSV cDNA clone (pCMV-P129). Coding regions of exogenous genes, which included Renilla luciferase (Rluc), green and red fluorescent proteins (GFP and DsRed, respectively) and several interferons (IFNs), were constructed and expressed through a unique subgenomic mRNA placed between ORF1b and ORF2 of the PRRSV infectious clone. The constructs, which expressed Rluc, GFP, DsRed, efficiently produced progeny viruses and mimicked the parental virus in both MARC-145 cells and porcine macrophages. In contrast, replication of IFN-expressing viruses was attenuated, similar to the level of replication observed after the addition of exogenous IFN. Furthermore, the IFN expressing viruses inhibited the replication of a second PRRS virus co-transfected or co-infected. Inhibition by the different IFN subtypes corresponded to their anti-PRRSV activity, i.e., IFNω5 ° IFNα1 > IFN-β > IFNδ3. In summary, the indicator-expressing viruses provided an efficient means for real-time monitoring of viral replication thus allowing high‑throughput elucidation of the role of host factors in PRRSV infection. This was shown when they were used to clearly demonstrate the involvement of tumor susceptibility gene 101 (TSG101) in the early stage of PRRSV infection. In addition, replication‑competent IFN-expressing viruses may be good candidates for development of modified live virus (MLV) vaccines, which are capable of reversing subverted innate immune responses and may induce more effective adaptive immunity against PRRSV infection.

Mutations in the genome of the highly pathogenic porcine reproductive and respiratory syndrome virus potentially related to attenuation

A live-attenuated highly pathogenic porcine reproductive and respiratory syndrome (HP-PRRS) virus (HP-PRRSV) TJM vaccine strain was derived from the HP-PRRSV TJ strain by passage 92 times in the African green monkey kidney epithelial cell line Marc-145. We found that the virulence of the TJ strain to piglets was decreased greatly from passage 19. To identify mutations associated with attenuation of the TJM vaccine strain, we determined the nucleotide changes that arose during Marc-145 passage of the HP-PRRSV TJ virus. The TJM strain contains a 360 nucleotide (120 amino acids) deletion and a 118 nucleotide mutation that resulted in 48 amino acid changes. Analysis of the complete nucleotide sequences of intermediate passage-level viruses F19, F46 and F78 showed that 31 (64.6%) of the 48 amino acid mutations occurred in F19, 7 (14.6%) occurred in F46, 7 (14.6%) occurred in F78 and 3 (6.3%) occurred in F92. The 120 amino acid deletion occurred from F19 to TJM. Therefore, we hypothesized that the 31 amino acid mutations distributed in nsp1β, nsp2-nsp5, nsp7, nsp9, nsp10, GP4 and GP5 and the continuous 120 amino acid deletion in the nsp2 region from F19 provide a strong potential molecular basis for the observed attenuated phenotype.

Large scale parallel pyrosequencing technology: PRRSV strain VR-2332 nsp2 deletion mutant stability in swine

Fifteen porcine reproductive and respiratory syndrome virus (PRRSV) isolate genomes were derived simultaneously using 454 pyrosequencing technology. The viral isolates sequenced were from a recent swine study, in which engineered Type 2 prototype PRRSV strain VR-2332 mutants, with 87, 184, 200, and 403 amino acid deletions in the second hypervariable region of nsp2, were found to be stable in the nsp2 coding region after in vivo infection (Faaberg et al., 2010). Furthermore, 3 of 4 mutants achieved replication kinetics similar to wt virus by study end. We hypothesized that other mutations elsewhere in the virus may have contributed to their replication fitness in swine. To further assess the stability of the engineered viruses, all sequenced genomes were compared and contrasted. No specific mutations occurred in all nsp2 deletion mutant genomes that were not also seen in the parent genome of Type 2 PRRSV strain VR-2332. Second site (non-nsp2) deletions and/or insertions were not evident after replication in swine. The number of point mutations seen increased slightly with deletion size, but even the largest deletion (403 aa) had very few consensus mutations. Thus, our findings provide further substantiation that the nsp2 deletion mutant genomes were genetically stable after in vivo passage.

Establishment of a DNA-launched infectious clone for a highly pneumovirulent strain of type 2 porcine reproductive and respiratory syndrome virus: identification and in vitro and in vivo characterization of a large spontaneous deletion in the nsp2 region

A highly pneumovirulent strain of porcine reproductive and respiratory syndrome virus (PRRSV), ATCC VR2385, was isolated from a pig exhibiting typical PRRS in the early 90s. While passaging the virus in monkey kidney cells, we identified a large spontaneous deletion of a 435-bp in the nsp2 gene. To assess the biological significance of this spontaneous deletion, we first determined the full-length genomic sequence of this virus and established a DNA-launched infectious clone of the passage 14 virus containing the 435-bp nsp2 deletion (designated as pIR-VR2385-CA). The full-length viral genome engineered with two ribozyme elements at both ends was placed under the control of the eukaryotic CMV promoter. The infectious virus was successfully rescued from pIR-VR2385-CA DNA-transfected BHK-21 cells. To characterize the biological and pathological significance of this large nsp2 deletion, we subsequently constructed another DNA-launched infectious clone, pIR-VR2385-R, in which we restored the deleted 435-bp nsp2 sequence back to the pIR-VR2385-CA backbone. The growth characteristics of the two rescued viruses (VR2385-CA and VR2385-R) were compared, and the results showed that the VR2385-CA virus with the nsp2 deletion replicated more efficiently in vitro (1.0-1.5 log titer higher) than the VR2385-R virus with the restored nsp2 sequence but the VR2385-CA virus exhibited a significantly reduced serum viral RNA load in vivo. A comparative pathogenicity study in pigs (n=10) revealed that the nsp2 deletion had no effect on virus virulence, and the restored nsp2 sequence in the VR2385-R virus remains stable during virus replication in pigs. The results from this study indicates that the spontaneous nsp2 deletion plays a role for enhanced PRRSV replication in vitro but has no effect on the pathogenicity of the virus.

Cytokine profiles and phenotype regulation of antigen presenting cells by genotype-I porcine reproductive and respiratory syndrome virus isolates

The present study examined the immunological response of antigen presenting cells (APC) to genotype-I isolates of porcine reproductive and respiratory syndrome virus (PRRSV) infection by analysing the cytokine profile induced and evaluating the changes taking place upon infection on immunologically relevant cell markers (MHCI, MHCII, CD80/86, CD14, CD16, CD163, CD172a, SWC9). Several types of APC were infected with 39 PRRSV isolates. The results show that different isolates were able to induce different patterns of IL-10 and TNF-α. The four possible phenotypes based on the ability to induce IL-10 and/or TNF-α were observed, although different cell types seemed to have different capabilities. In addition, isolates inducing different cytokine-release profiles on APC could induce different expression of cell markers.

The PRRSV replicase: exploring the multifunctionality of an intriguing set of nonstructural proteins

Our knowledge about the structure and function of the nonstructural proteins (nsps) encoded by the arterivirus replicase gene has advanced in recent years. The continued characterization of the nsps of the arterivirus prototype equine arteritis virus has not only corroborated several important functional predictions, but also revealed various novel features of arteriviral replication. For porcine reproductive and respiratory syndrome virus (PRRSV), based on bioinformatics predictions and experimental studies, a processing map for the pp1a and pp1ab replicase polyproteins has been developed. Crystal structures have been resolved for two of the PRRSV nonstructural proteins that possess proteinase activity (nsp1α and nsp4). The functional characterization of the key enzymes for arterivirus RNA synthesis, the nsp9 RNA polymerase and nsp10 helicase, has been initiated. In addition, progress has been made on nsp functions relating to the regulation of subgenomic mRNAs synthesis (nsp1), the induction of replication-associated membrane rearrangements (nsp2 and nsp3), and an intriguing replicative endoribonuclease (nsp11) for which the natural substrate remains to be identified. The role of nsps in viral pathogenesis and host immunity is also being explored, and specific nsps (including nsp1α/β, nsp2, nsp4, nsp7, and nsp11) have been implicated in the modulation of host immune responses to PRRSV infection. The nsp3–8 region was identified as containing major virulence factors, although mechanistic information is scarce. The biological significance of PRRSV nsps in virus-host interactions and the technical advancements in engineering the PRRSV genome by reverse genetics are also reflected in recent developments in the area of vaccines and diagnostic assays.

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.

Vaccine efficacy of porcine reproductive and respiratory syndrome virus chimeras

The vaccine efficacy of six PRRSV Type 2 infectious clones, including five chimeras and a strain-specific deletion mutant, were examined using a respiratory challenge model in growing swine. The chimeras were constructed from different combinations of a licensed modified live vaccine (Ingelvac PRRS MLV) and a virulent field isolate (wt MN184) which differ by 14.3% on a nucleotide basis, while the deletion mutant tested had a broad deletion in the nsp2 region of strain MN184. The appearance of antibodies and virus characterization revealed regions of the genome that could influence PRRSV replication in vivo. Swine growth, clinical signs and lung lesions were also monitored. Average daily weight gain was negatively and directly impacted by some vaccines, and after challenge, vaccination with different constructs led to variable weight gain. We determined that 3 of the tested chimeras, including two previously published chimeras [1] and one in which strain MN184 ORF5-6 was placed on the background of Ingelvac PRRS MLV were able to prevent lung consolidation to a similar extent as traditionally prepared cell-passaged attenuated vaccines. The study suggested that only specific chimeras can attenuate clinical signs in swine and that attenuation cannot be directly linked to primary virus replication. Additionally, the strain MN184 deletion mutant was not found to have been sufficiently attenuated nor efficacious against heterologous challenge with strain JA-142.

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.