Identification of dispensable nucleotide sequence in 3' untranslated region of porcine reproductive and respiratory syndrome virus

Reverse genetics construction and pathogenicity of a novel recombinant NADC30-like PRRSV isolated in China

China has the largest pig herd in the world which accounts for more than 50% of the global pig population. Over the past three decades, the porcine reproductive and respiratory syndrome virus (PRRSV) has caused significant economic loss to the Chinese swine industry. Currently, the prevalent PRRSV strains in the field are extremely complicated, and the NADC30-like strains, NADC34-like strains, and novel recombinant viruses have become a great concern to PRRS control in China. In this study, a novel NADC30-like PRRSV, named GS2022, was isolated from the lung of a dead pig collected from a farm that experienced a PRRS outbreak. The complete genome of GS2022 shares the highest identity with the NADC30 strain and contains a discontinuous deletion of 131 aa in nsp2. Novel deletion and insertion have been identified in ORF7 and 3'UTR. Recombination analysis revealed that the GS2022 is a potential recombinant of NADC30-like and JXA1-like strains. Both inter-lineage and intra-lineage recombination events were predicted to be involved in the generation of the GS2022. An infectious cDNA clone of GS2022 was assembled to generate the isogenic GS2022 (rGS2022). The growth kinetics of rGS2022 were almost identical to those of GS2022. The pathogenicity of the GS2022 and rGS2022 was evaluated using a nursery piglet model. In the infection groups, the piglets exhibited mild clinical symptoms, including short periods of fever and respiratory diseases. Both gross lesions and histopathological lesions were observed in the lungs and lymph nodes of the infected piglets. Therefore, we reported a novel recombinant NADC30-like PRRSV strain with moderate pathogenicity in piglets. These results provide new information on the genomic characteristics and pathogenicity of the NADC30-like PRRSV in China.

ZNF283, a Krüppel-associated box zinc finger protein, inhibits RNA synthesis of porcine reproductive and respiratory syndrome virus by interacting with Nsp9 and Nsp10

Porcine reproductive and respiratory syndrome virus (PRRSV) is a viral pathogen with substantial economic implications for the global swine industry. The existing vaccination strategies and antiviral drugs offer limited protection. Replication of the viral RNA genome encompasses a complex series of steps, wherein a replication complex is assembled from various components derived from both viral and cellular sources, as well as from the viral genomic RNA template. In this study, we found that ZNF283, a Krüppel-associated box (KRAB) containing zinc finger protein, was upregulated in PRRSV-infected Marc-145 cells and porcine alveolar macrophages and that ZNF283 inhibited PRRSV replication and RNA synthesis. We also found that ZNF283 interacts with the viral proteins Nsp9, an RNA-dependent RNA polymerase, and Nsp10, a helicase. The main regions involved in the interaction between ZNF283 and Nsp9 were determined to be the KRAB domain of ZNF283 and amino acids 178-449 of Nsp9. The KRAB domain of ZNF283 plays a role in facilitating Nsp10 binding. In addition, ZNF283 may have an affinity for the 3' untranslated region of PRRSV. These findings suggest that ZNF283 is an antiviral factor that inhibits PRRSV infection and extend our understanding of the interactions between KRAB-containing zinc finger proteins and viruses.

Inducible miR-150 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication by Targeting Viral Genome and Suppressor of Cytokine Signaling 1

Hosts exploit various approaches to defend against porcine reproductive and respiratory syndrome virus (PRRSV) infection. microRNAs (miRNAs) have emerged as key negative post-transcriptional regulators of gene expression and have been reported to play important roles in regulating virus infection. Here, we identified that miR-150 was differentially expressed in virus permissive and non-permissive cells. Subsequently, we demonstrated that PRRSV induced the expression of miR-150 via activating the protein kinase C (PKC)/c-Jun amino-terminal kinases (JNK)/c-Jun pathway, and overexpression of miR-150 suppressed PRRSV replication. Further analysis revealed that miR-150 not only directly targeted the PRRSV genome, but also facilitated type I IFN signaling. RNA immunoprecipitation assay demonstrated that miR-150 targeted the suppressor of cytokine signaling 1 (SOCS1), which is a negative regulator of Janus activated kinase (JAK)/signal transducer and activator of the transcription (STAT) signaling pathway. The inverse correlation between miR-150 and SOCS1 expression implies that miR-150 plays a role in regulating ISG expression. In conclusion, miR-150 expression is upregulated upon PRRSV infection. miR-150 feedback positively targets the PRRSV genome and promotes type I IFN signaling, which can be seen as a host defensive strategy.

Research progress in the development of porcine reproductive and respiratory syndrome virus as a viral vector for foreign gene expression and delivery

INTRODUCTION

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease of swine characterized by respiratory disorders in growing and finishing pigs and reproductive failure in pregnant sows. PRRSV has been recognized as one of the most economically significant pathogens affecting the global pig industry.

AREAS COVERED

Currently, commercially available vaccines, including traditional killed virus (KV) vaccines and modified live virus (MLV) vaccines, are the cardinal approaches to prevent and control porcine reproductive and respiratory syndrome virus (PRRSV) infection. However, the protective efficacy of these vaccines is not satisfactory, resulting in the continuous evolution and recurrent appearance of the virus as well as the emergence of new variants. A safe and effective vaccine against PRRSV is in dire need. Here, we review the research progress in recent years in the development and use of PRRSV as a viral vector to express foreign genes, and their potential application in gene delivery and vaccine development.

EXPERT OPINION

The potential of using PRRSV-based vectors to express multiple antigens would be particularly instrumental for the development of a new generation of multivalent vaccines against PRRSV and other porcine viruses.

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.

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

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

Emergence of two novel recombinant porcine reproductive and respiratory syndrome viruses 2 (lineage 3) in Southwestern China

The lineage 3 of porcine reproductive and respiratory syndrome virus 2 (PRRSV-2) was first reported in mainland China in 2010 and it has spread rapidly in recent years. Here, two novel lineage 3 strains of PRRSV-2 were isolated from diseased pigs in Southwestern China during 2017-2018, and were designated as GZgy17 and SCya18. The complete genomes of the two isolates were then determined, and sequence alignment revealed that GZgy17 had the same discontinuous 30-amino acid (aa) deletion in NSP2 as JXA1, while SCya18 contained the discontinuous 131-aa deletion in NSP2 identical to that of NADC30, when compared to the strain VR-2332. Notably, GZgy17 contained an additional 19-aa deletion in NSP2, and SCya18 had a unique 3-nt deletion in its 3'UTR. Homology and phylogenetic analysis showed that GZgy17 and SCya18 shared low nucleotide homology (91.2-92.0%) with QYYZ and were classified into a new cluster of lineage 3 strains based on ORF5 genotyping. Recombination analyses revealed that GZgy17 and SCya18 both originated from a SH/CH/2016-like (lineage 3) strain and had recombined with a JXA1-like (lineage 8) and a NADC30-like (lineage 1) strain, respectively. Furthermore, we compared the virulence of the two strains in 4-week-old piglets. The results showed that GZgy17 caused mortality rates of 20% and exhibited higher pathogenicity in piglets compared to SCya18. Our findings suggest that recombination might be responsible for the variations in pathogenicity of lineage 3 strains of PRRSV-2 and highlight the importance of surveillance of this lineage in China.

Utilizing host endogenous microRNAs to negatively regulate the replication of porcine reproductive and respiratory syndrome virus in MARC-145 cells

MicroRNAs (miRNAs) contribute to gene regulation at the post-transcriptional level and are capable of mRNA silencing by binding to target sites exhibiting high degrees of complementarity. Therefore, cloning host miRNA-recognition sequences into the genome of RNA viruses represents a rational strategy for manipulating viral replication. Here, we performed deep sequencing to obtain small-RNA (sRNA)-expression profiles from in vitro-cultured MARC-145 cells post infection with porcine reproductive and respiratory syndrome virus (PRRSV) and chose six candidate miRNAs of different abundance (miR-21, miR-140-3p, miR-185, miR-26a, miR-505, and miR-199a) for further study. Based on the full-length cDNA clone p7USC, we constructed a number of PRRSV mutants that provided complementary base-pairing target sites for the miRNAs in 3′ untranslated regions. Our results showed that all low- and moderate- abundant miRNA-target mutants showed similar growth properties, whereas the highest-abundant miRNA-target mutant blocked both viral transcription and replication. Discontinuous mutations in high-abundant miRNA-target sites subsequently recovered viral viability and propagation. These results demonstrated the copy number of endogenous miRNAs and the extent of sRNA complementarity were key factors to silence potential mRNA expression/translation, thereby determining PRRSV viability. Interestingly, the mutant containing miR-140-target sites (v140-t) showed strong suppression of viral replication from P1 to P3 in vitro, as shown by virus titer, plaque morphology, and qRT-PCR assays. To assess genetic stability, sequencing of v140-t (P1, P3, P5 and P10) revealed spontaneous mutations preferentially located among several nucleotides near the 3′ end of the insertion region and corresponding to the “seed region” of miR-140-3p, explaining the induced viral repression and the direction of virus evolution. This approach provided a general silencing strategy for limiting PRRSV replication by endogenous miRNAs in MARC-145 cells.

Whole Genome Analysis of Two Novel Type 2 Porcine Reproductive and Respiratory Syndrome Viruses with Complex Genome Recombination between Lineage 8, 3, and 1 Strains Identified in Southwestern China

Recombination among porcine reproductive and respiratory syndrome viruses (PRRSVs) is thought to contribute to the emergence of new PRRSV variants. In this study, two newly emerged PRRSV strains, designated SCcd16 and SCya17, are isolated from lung tissues of piglets in Southwestern China. Genome comparative analysis reveals that SCcd16/SCya17 exhibit 93.1%/93.2%, 86.9%/87.0%, 85.3%/85.7%, and 83.6%/82.0% nucleotide similarity to PRRSVs JXA1, VR-2332, QYYZ and NADC30, respectively. They only exhibit 44.8%/45.1% sequence identity with LV (PRRSV-1), indicating that both emergent strains belong to the PRRSV-2 genotype. Genomic sequence alignment shows that SCcd16 and SCya17 have the same discontinuous 30-amino acid (aa) deletion in Nsp2 of the highly pathogenic Chinese PRRSV strain JXA1, when compared to strain VR-2332. Notably, SCya17 shows a unique 5-nt deletion in its 3’-UTR. Phylogenetic analysis shows that both of the isolates are classified in the QYYZ-like lineage based on ORF5 genotyping, whereas they appear to constitute an inter-lineage between JXA1-like and QYYZ-like lineages based on their genomic sequences. Furthermore, recombination analyses reveal that the two newly emerged PRRSV isolates share the same novel recombination pattern. They have both likely originated from multiple recombination events between lineage 8 (JXA1-like), lineage 1 (NADC30-like), and lineage 3 (QYYZ-like) strains that have circulated in China recently. The genomic data from SCcd16 and SCya17 indicate that there is on going evolution of PRRSV field strains through genetic recombination, leading to outbreaks in the pig populations in Southwestern China.

Identification of the RNA Pseudoknot within the 3' End of the Porcine Reproductive and Respiratory Syndrome Virus Genome as a Pathogen-Associated Molecular Pattern To Activate Antiviral Signaling via RIG-I and Toll-Like Receptor 3

Once infected by viruses, cells can detect pathogen-associated molecular patterns (PAMPs) on viral nucleic acid by host pattern recognition receptors (PRRs) to initiate the antiviral response. Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), characterized by reproductive failure in sows and respiratory diseases in pigs of different ages. To date, the sensing mechanism of PRRSV has not been elucidated. Here, we reported that the pseudoknot region residing in the 3' untranslated regions (UTR) of the PRRSV genome, which has been proposed to regulate RNA synthesis and virus replication, was sensed as nonself by retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 3 (TLR3) and strongly induced type I interferons (IFNs) and interferon-stimulated genes (ISGs) in porcine alveolar macrophages (PAMs). The interaction between the two stem-loops inside the pseudoknot structure was sufficient for IFN induction, since disruption of the pseudoknot interaction powerfully dampened the IFN induction. Furthermore, transfection of the 3' UTR pseudoknot transcripts in PAMs inhibited PRRSV replication in vitro Importantly, the predicted similar structures of other arterivirus members, including equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV), also displayed strong IFN induction activities. Together, in this work we identified an innate recognition mechanism by which the PRRSV 3' UTR pseudoknot region served as PAMPs of arteriviruses and activated innate immune signaling to produce IFNs that inhibit virus replication. All of these results provide novel insights into innate immune recognition during virus infection.IMPORTANCE PRRS is the most common viral disease in the pork industry. It is caused by PRRSV, a positive single-stranded RNA virus, whose infection often leads to persistent infection. To date, it is not yet clear how PRRSV is recognized by the host and what is the exact mechanism of IFN induction. Here, we investigated the nature of PAMPs on PRRSV and the associated PRRs. We found that the 3' UTR pseudoknot region of PRRSV, which has been proposed to regulate viral RNA synthesis, could act as PAMPs recognized by RIG-I and TLR3 to induce type I IFN production to suppress PRRSV infection. This report is the first detailed description of pattern recognition for PRRSV, which is important in understanding the antiviral response of arteriviruses, especially PRRSV, and extends our knowledge on virus recognition.

Porcine reproductive and respiratory syndrome virus expressing E2 of classical swine fever virus protects pigs from a lethal challenge of highly-pathogenic PRRSV and CSFV

Porcine reproductive and respiratory syndrome (PRRS) and classical swine fever (CSF) are economically significant diseases that affect the swine industry worldwide. However, the current vaccination strategy, which uses two single live attenuated vaccines, can result in interference for each other. In addition, the universally used CSFV vaccine C-strain does not allow for differentiation of infected and vaccinated animals. In this study, rPRRSV-E2, PRRS virus (PRRSV) expressing CSF virus (CSFV) E2, was constructed by reverse genetics. The E2 gene of CSFV was inserted between ORF1b and ORF2 in the genome of the PRRS vaccine virus, HuN4-F112. A copy of transcriptional regulatory sequence 6 was inserted at the 3' terminal of the exogenous gene to produce CSFV E2 as a unique subgenomic mRNA transcript. The rPRRSV-E2 was stable for at least 25 serial cell passages. Single-shot intramuscular immunization of rPRRSV-E2 into pigs induced PRRSV-specific and CSFV-specific antibodies and fully protected pigs from lethal challenge with highly-pathogenic PRRSV and CSFV. These results demonstrate that a novel strategy for recombinant PRRSV production is effective, and suggest that rPRRSV-E2 is a promising live, virus-vectored vaccine against PRRS and a marker vaccine against CSF.

Label-Free Quantitative Proteomic Analysis of Differentially Expressed Membrane Proteins of Pulmonary Alveolar Macrophages Infected with Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus and Its Attenuated Strain

Significant differences exist between the highly pathogenic (HP) porcine reproductive and respiratory syndrome virus (PRRSV) and its attenuated pathogenic (AP) strain in the ability to infect host cells. The mechanisms by which different virulent strains invade host cells remain relatively unknown. In this study, pulmonary alveolar macrophages (PAMs) are infected with HP‐PRRSV (HuN4) and AP‐PRRSV (HuN4‐F112) for 24 h, then harvested and subjected to label‐free quantitative MS. A total of 2849 proteins are identified, including 95 that are differentially expressed. Among them, 26 proteins are located on the membrane. The most differentially expressed proteins are involved in response to stimulus, metabolic process, and immune system process, which mainly have the function of binding and catalytic activity. Cluster of differentiation CD163, vimentin (VIM), and nmII as well as detected proteins are assessed together by string analysis, which elucidated a potentially different infection mechanism. According to the function annotations, PRRSV with different virulence may mainly differ in immunology, inflammation, immune evasion as well as cell apoptosis. This is the first attempt to explore the differential characteristics between HP‐PRRSV and its attenuated PRRSV infected PAMs focusing on membrane proteins which will be of great help to further understand the different infective mechanisms of HP‐PRRSV and AP‐PRRSV.

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

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

Novel porcine reproductive and respiratory syndrome virus strains in the United States with deletions in untranslated regions

Porcine reproductive and respiratory syndrome (PRRS) still causes major problems for the swine industry worldwide. Here, we report the detection and genomic characterization of two novel PRRS virus (PRRSV) strains from the United States with deletions in untranslated regions (UTRs). The OH155-2015 strain has two single-nucleotide deletions in the 5' UTR, whereas the OH28372-2013 strain has a 13-nt deletion in the 3' UTR. In addition, OH155-2015 and OH28372-2013 have a unique deletion and mutations in the NSP2 and N gene, respectively. Our study highlights the importance of continued monitoring of PRRSV using whole-genome sequencing.

Cysteine residues of the porcine reproductive and respiratory syndrome virus ORF5a protein are not essential for virus viability

ORF5a protein was recently identified as a novel structural protein in porcine reproductive and respiratory syndrome virus (PRRSV). The ORF5a protein possesses two cysteines at positions 29 and 30 that are highly conserved among type 2 PRRSV. In this study, the significance of the ORF5a protein cysteine residues on virus replication was determined based on a type 2 PRRSV cDNA clone (pAJXM). Each cysteine was substituted by serine or glycine and the mutations were introduced into pAJXM. We found that the replacement of cysteine to glycine at position 30 was lethal for virus viability, but all serine mutant clones produced infectious progeny viruses. This data indicated that cysteine residues in the ORF5a protein were not essential for replication of type 2 PRRSV. The bimolecular fluorescence complementation (BiFC) and Co-immunoprecipitation (Co-IP) assay were used to study ORF5a protein interacted with other enveloped proteins. These results showed that ORF5a protein interacted non-covalently with itself and interacted with GP4 and 2b protein. The replacement of cysteine to glycine at position 30 affected the ORF5a protein interacted non-covalently with itself, which may account for the lethal phenotype of mutants carrying substitution of cysteine to glycine at position 30.

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.

Conserved nucleotides in the terminus of the 3' UTR region are important for the replication and infectivity of porcine reproductive and respiratory syndrome virus

The 3' untranslated region (3' UTR), including the poly (A) tail, reportedly plays an important role in arterivirus replication, but the roles of the cis-acting elements present in the 3' UTR of porcine reproductive and respiratory syndrome virus (PRRSV) remain largely unknown. In the present study, PCR-based mutagenic analysis was conducted on the 3' UTR of PRRSV infectious full-length cDNA clone pAPRRS to investigate the structure and function of the conserved terminal nucleotides between the poly (A) tail and the 3' UTR region. Our findings indicated that the conservation of the primary sequence of the 3' terminal nucleotides, rather than the surrounding secondary structure, was vital for viral replication and infectivity. Four nucleotides (nt) (5'-(15517)AAUU(15520)-3') at the 3' proximal end of the 3' UTR and the dinucleotide 5'-AU-3' exerted an important regulatory effect on viral viability. Of the five 3'-terminal nucleotides of the 3' UTR (5'-(15503)AACCA(15507)-3'), at least three, including the last dinucleotide (5'-CA-3'), were essential for maintaining viral infectivity. Taken together, the 3'-terminal conserved sequence plays a critical role in PRRSV replication and may function as a contact site for specific assembly of the replication complex.

Replacement of the heterologous 5' untranslated region allows preservation of the fully functional activities of type 2 porcine reproductive and respiratory syndrome virus

The 5' untranslated region (UTR) is believed to be vital for the replication of porcine reproductive and respiratory syndrome virus (PRRSV), yet its functional mechanism remains largely unknown. In this study, to define the cis-acting elements for viral replication and infectivity, The 5' UTR swapping chimeric clones pTLV8 and pSHSP5 were constructed based on two different genotypes full-length infectious cDNA clone pAPRRS and pSHE backbones. Between them, vTLV8 could be rescued from pTLV8 and had similar virological properties to vAPRRS, including phenotypic characteristic and RNA synthesis level. However, pSHSP5 exhibited no evidence of infectivity. Taken together, the results presented here demonstrate that only the 5' UTR of type 1 PRRSV did not affect the infectivity and replication of type 2 PRRSV in vitro. The 5' UTR of type 2 PRRSV could be functionally replaced by its counterpart from type 1.

Genomic sequencing reveals mutations potentially related to the overattenuation of a highly pathogenic porcine reproductive and respiratory syndrome virus

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) continues to evolve when serially passaged in Marc-145 cells. In this study, we analyzed the genomic and antigenic variants of HP-PRRSV strain JXA1 during in vitro passage. Protective efficacies of JXA1 from passages 100, 110, 120, 140, and 170 against the high-virulence parental virus were evaluated by inoculating pigs with each of these viruses and then challenging with JXA1 from passage 5 at 28 days postimmunization. We found that the antigenicities of JXA1 from passages after 110 were significantly reduced. Inoculation with JXA1 from passages after 110 provided only insufficient protection against the parental strain challenge, indicating that the immunogenicity of JXA1 is significantly decreased when it is in vitro passaged for 110 times and more. To identify the genomic variants that emerged during the overattenuation, eight complete genomes of highly passaged JXA1 were sequenced. One guanine deletion in the 5' untranslated region (UTR), two nucleotide substitutions in the 3' UTR, and 65 amino acid mutations in nonstructural and structural proteins that accompanied with the attenuation and overattenuation were determined. Genomic sequencing of in vitro serially passaged HP-PRRSV first identified the mutations potentially correlated with the overattenuation of a HP-PRRSV strain. These results facilitate the research aimed at elucidating the mechanisms for PRRSV genomic and antigenic changes and may also contribute to developing a safe and effective PRRSV vaccine.

Complete genome sequence of an overattenuated highly pathogenic porcine reproductive and respiratory syndrome virus

JXA1-P170 is an overattenuated highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) that has been passaged in vitro 170 times. Vaccination with JXA1-P170 cannot protect pigs against JXA1 challenge. Compared with the parental virus JXA1, JXA1-P170 contains 1 nucleotide (nt) deletion and 113 nt mutations leading to 59 amino acid substitutions. Here we announce the first complete genome sequence of the overattenuated HP-PRRSV.

Mutational analysis of the SDD sequence motif of a PRRSV RNA-dependent RNA polymerase

The subgenomic mRNA transcription and genomic replication of the porcine reproductive and respiratory syndrome virus (PRRSV) are directed by the viral replicase. The replicase is expressed in the form of two polyproteins and is subsequently processed into smaller nonstructural proteins (nsps). nsp9, containing the viral replicase, has characteristic sequence motifs conserved among the RNA-dependent RNA polymerases (RdRp) of positive-strand (PS) RNA viruses. To test whether the conserved SDD motif can tolerate other conserved motifs of RNA viruses and the influence of every residue on RdRp catalytic activity, many amino acids substitutions were introduced into it. Only one nsp9 substitution, of serine by glycine (S3050G), could rescue mutant viruses. The rescued virus was genetically stable. Alteration of either aspartate residue was not tolerated, destroyed the polymerase activity, and abolished virus transcription, but did not eliminate virus replication. We also found that the SDD motif was essentially invariant for the signature sequence of PRRSV RdRp. It could not accommodate other conserved motifs found in other RNA viral polymerases, except the GDD motif, which is conserved in all the other PS RNA viruses. These findings indicated that nidoviruses are evolutionarily related to other PS RNA viruses. Our studies support the idea that the two aspartate residues of the SDD motif are critical and essential for PRRSV transcription and represent a sequence variant of the GDD motif in PS RNA viruses.

Cis-acting structural element in 5' UTR is essential for infectivity of porcine reproductive and respiratory syndrome virus

It is believed that the genomic 5' untranslated region (UTR) of Arterivirus plays crucial roles in viral genomic replication, subgenomic mRNA transcription and protein translation, yet the structure and function still remain largely unknown. In this study, we conducted serial nucleotide truncation, ranging from 1 to 190 nucleotides, to the 5' UTR of the porcine reproductive and respiratory syndrome virus (PRRSV) infectious full-length cDNA clone pAPRRS. In vitro synthetic RNAs were transfected into MARC-145 cells for further genetic and virologic analysis. Our results demonstrated that the first three nucleotides of PRRSV 5' UTR were dispensable for virus viability, which however was repaired with foreign sequences. In order to assess if the primary sequence or structural element play more important regulatory roles, the CMV promoter-driven 5' UTR truncation mutant cDNA clones were directly transfected into the BHK-21 cell lines. We found that PRRSV tolerated the first 16 nucleotides sequence alteration of 5' UTR without losing virus viability. However, these revertant viruses contained a range of non-templated with unknown origin exogenous nucleotides in the repaired 5' end. Further analyses revealed that the 5' proximal stem-loop 1 (SL1) in the highly structured 5' UTR was invariably required for virus infectivity. Taken together, we conclude that authentic 5'-proximal primary sequence is nonessential, but the resultant structural elements are probably indispensable for PRRSV infectivity.

A 5'-proximal stem-loop structure of 5' untranslated region of porcine reproductive and respiratory syndrome virus genome is key for virus replication

Background

It has been well documented that the 5' untranslated region (5' UTR) of many positive-stranded RNA viruses contain key cis-acting regulatory sequences, as well as high-order structural elements. Little is known for such regulatory elements controlling porcine arterivirus replication. We investigated the roles of a conserved stem-loop 2 (SL2) that resides in the 5'UTR of the genome of a type II porcine reproductive and respiratory syndrome virus (PRRSV).

Results

We provided genetic evidences demonstrating that 1) the SL2 in type II PRRSV 5' UTR, N-SL2, could be structurally and functionally substituted by its counterpart in type I PRRSV, E-SL2; 2) the functionality of N-SL2 was dependent upon the G-C rich stem structure, while the ternary-loop size was irrelevant to RNA synthesis; 3) serial deletions showed that the stem integrity of N-SL2 was crucial for subgenomic mRNA synthesis; and 4) when extensive base-pairs in the stem region was deleted, an alternative N-SL2-like structure with different sequence was utilized for virus replication.

Conclusion

Taken together, we concluded that the phylogenetically conserved SL2 in the 5' UTR was crucial for PRRSV virus replication, subgenomic mRNA synthesis in particular.