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

Engineering a live-attenuated porcine reproductive and respiratory syndrome virus vaccine to prevent RNA recombination by rewiring transcriptional regulatory sequences

Recombination is a significant factor driving the evolution of RNA viruses. The prevalence and variation of porcine reproductive and respiratory syndrome virus (PRRSV) in China have been increasing in complexity due to extensive interlineage recombination. When this recombination phenomenon occurs in live vaccine strains, it becomes increasingly difficult to prevent and control PRRSV. Reverse genetic manipulation to engineer a different transcriptional regulatory sequence (TRS) circuit introduces genetic traps into the viral genome that are lethal to recombinant RNA progeny viruses. In this study, major interlineage recombination patterns were identified between lineage 1 (L1) PRRSVs and lineage 8 (L8) PRRSVs in China, from 2019 to 2023. The recombinant mutant virus, vA-TRSall, was constructed and successfully rescued by rewiring the entire TRS circuit without changing the amino acid-coding sequence in the genome of the PRRSV live vaccine strain vHuN4-F112. The vA-TRSall, with a brand new TRS circuit, provided effective immune protection against the highly pathogenic L8 PRRSV (vHuN4) and epidemic NADC30-like L1 PRRSV (vZJqz21). Recombination analysis in vitro and in vivo showed that, compared with the vHuN4-F112 and vZJqz21 co-infection groups, the incidence rates of mutation breakpoints and template-switching recombination in the vA-TRSall and vZJqz21 co-infected groups were effectively reduced. The results have enriched our understanding of the critical role of TRS circuits in PRRSV recombination mechanisms and indicate a successful redesign that can endow PRRSV live vaccines with recombination-resistant capabilities.

IMPORTANCE

Porcine reproductive and respiratory syndrome viruses (PRRSVs) are genetically diverse, and this is due in part to their extensive recombination. Live vaccines are widely used to prevent and control PRRS in China. However, owing to the wide variety of live vaccines, non-standard use, and the wild viruses prevalent on pig farms, new strains, generated via RNA recombination, are continuously emerging. Vaccine strains are also involved in PRRSV recombination, which leads to the emergence of new variants and alterations in virulence and pathogenesis. A recombination-resistant genome was engineered by rewiring the entire transcriptional regulatory sequence (TRS) circuit of the live PRRSV vaccine strain. Theoretically, after clinical application, once the virus recombines with the genome of the epidemic strain, the base pairing between the two sets of TRS circuits should be disrupted, resulting in a fatal genetic trap for the generation of an RNA recombinant progeny virus. Therefore, the remodeled PRRSV TRS mutant generated in this study can serve as a recombination-resistant platform for the rational design of safe PRRS vaccines in the future.

Molecular mechanism of autophagy in porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome virus (PRRSV), a significant pathogen affecting the swine industry globally, has been shown to manipulate host cell processes, including autophagy, to facilitate its replication and survival within the host. Autophagy, an intracellular degradation process crucial for maintaining cellular homeostasis, can be hijacked by viruses for their own benefit. During PRRSV infection, autophagy plays a complex role, both as a defense mechanism of the host and as a tool exploited by the virus. This review explores the current understanding of the molecular mechanisms underlying autophagy induction under PRRSV infection, its impact on virus replication, and the potential implications for viral pathogenesis and antiviral strategies. By synthesizing the latest research findings, this article aims to enhance our understanding of the intricate relationship between autophagy and PRRSV, paving the way for novel therapeutic approaches against this swine pathogen.

Genetic and Pathogenic Characteristics of an Emerging Highly Virulent Recombinant Lineage Korean Clade C PRRSV Strain

A strain of porcine reproductive and respiratory syndrome virus (PRRSV) was isolated from lung tissue of a pig showing severe respiratory clinical signs from a farm in Gyeongsang province of South Korea. This PRRSV strain, designated as SNUVR220803, was classified within the lineage Korean clade C (LKC) based on a phylogenetic analysis of the ORF5 gene. A whole-genome analysis was conducted on the SNUVR220803 strain, which appears to be a recombinant between the PRRSV strains K07-2273 (part of LKC lineage) and Ingelvac MLV (part of Lineage 5). The Nsp2 amino acid sequence of this strain features a deletion of four additional amino acids, setting it apart from the typical Korean clades A, B, and C lineages. An animal inoculation experiment was conducted with 24 pigs divided into three groups: 12 pigs in the inoculated group, six in the sentinel group, and six in the negative control group. Inoculated pigs exhibited persisting hyperthermia (≥40.3°C) for 5 days, palpebral edema, and cyanosis. Subsequently, these pigs suffered from severe respiratory distress and cachexia, leading to a mortality rate of 58.3% (7 out of 12 pigs) at 14 days postinoculation (dpi). Body weight decreased post-SNUVR220803 strain infection in both the inoculated and sentinel groups. Gross pathology revealed noncollapsed lungs and serous effusion in the pericardial and peritoneal cavities. Microscopic analysis revealed severe interstitial pneumonia, while immunohistochemistry confirmed the presence of PRRSV antigen in the lungs, lymph nodes, thymus, kidneys, and the heart. Additionally, the levels of cytokines such as tumor necrosis factor-α (TNF-α), interferon-α (IFN-α), and IL-10 were significantly elevated in the plasma of infected pigs. These observations indicate that the LKC recombinant strain, combined with Lineage 5, possesses high virulence and infectivity as characterized by distinctive exudative lesions.

Identification of Pathogen-Specific Novel Sources of Genetic Resistance Against Ascochyta Blight and Identification of Their Underlying Genetic Control

Global chickpea production is restricted by Ascochyta blight caused by the necrotrophic fungi Ascochyta rabiei. Developing locally adapted disease-resistant cultivars is an economically and environmentally sustainable approach to combat this disease. However, the lack of genetic variability in cultivated chickpeas and breeder-friendly markers poses a significant challenge to Ascochyta blight-resistant breeding efforts in chickpeas. In this study, we screened the mini-core germplasm of Cicer reticulatum against a local pathotype of A. rabiei. A modified mini-dome screening approach resulted in the identification of five accessions showing a high level of resistance. The mean disease score of resistant accessions ranged between 1.75 ± 0.3 and 2.88 ± 0.4 compared to susceptible accessions, where the mean disease score ranged between 3.59 ± 0.62 and 8.86 ± 0.14. Genome-wide association study revealed a strong association on chromosome 5, explaining ∼58% of the phenotypic variance. The underlying region contained two candidate genes (Cr_14190.1_v2 and Cr_14189.1_v2), the characterization of which showed the presence of a DNA-binding domain (cl28899 and cd18793) in Cr_14190.1_v2 and its orthologs in C. arietinum, whereas Cr_14190.1_v2 carried an additional N-terminal domain (cl31759). qPCR expression analysis in resistant and susceptible accessions revealed ∼3- and ∼110-fold higher transcript abundance for Cr_14189.1 and Cr_14190.1, respectively.

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.

Genomic characteristics of a novel emerging PRRSV branch in sublineage 8.7 in China

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused serious economic losses to the pig industry worldwide. During the continuous monitoring of PRRSV, a new PRRSV strain type with novel characteristics was first identified in three different regions of Shandong Province. These strains presented a novel deletion pattern (1 + 8 + 1) in the NSP2 region and belonged to a new branch in sublineage 8.7 based on the ORF5 gene phylogenetic tree. To further study the genomic characteristics of the new-branch PRRSV, we selected a sample from each of the three farms for whole-genome sequencing and sequence analysis. Based on the phylogenetic analysis of the whole genome, these strains formed a new independent branch in sublineage 8.7, which showed a close relationship with HP-PRRSV and intermediate PRRSV according to nucleotide and amino acid homology but displayed a completely different deletion pattern in NSP2. Recombinant analysis showed that these strains presented similar recombination patterns, all of which involved recombination with QYYZ in the ORF3 region. Furthermore, we found that the new-branch PRRSV retained highly consistent nucleotides at positions 117-120 (AGTA) of a quite conserved motif in the 3'-UTR; showed similar deletion patterns in the 5'-UTR, 3'-UTR and NSP2; retained characteristics consistent with intermediate PRRSV and exhibited a gradual evolution trend. The above results showed that the new-branch PRRSV strains may have the same origin and be similar to HP-PPRSV also evolved from intermediate PRRSV, but are distinct strains that evolved simultaneously with HP-PRRSV. They persist in some parts of China through rapid evolution, recombine with other strains and have the potential to become epidemic strains. The monitoring and biological characteristics of these strains should be further studied.

A rescued NADC30-like virus by reverse genetic manipulation exhibits moderate virulence and a promising application perspective

NADC30-like porcine reproductive and respiratory syndrome virus (PRRSV), which is highly homologous to the NADC30 strain isolated in the United States. The NADC30-like PRRSV was first reported in 2014 in China, where it spread and gradually caused an epidemic. Currently, growing research has shown that NADC30-like strains have greater propensity to recombine with other PRRSV strains, particularly the PPRSV vaccine virus used clinically, making the prevention and control of PRRSV highly complex. To carry out an in-depth molecular biology and virulence analysis, a full-length infectious clone of the NADC30-like strain was successfully constructed and rescued by reverse genetic manipulation. The rescued virus, rZJqz, was indistinguishable from its parental virus, ZJqz21, based on virological characteristics. Further animal experiments demonstrated that rZJqz retained similar pathogenicity and induced the typical clinical symptoms and viral shedding observed in the ZJqz21 challenge model. Together, these results provide a useful tool for further study of the biological characteristics and pathogenicity of NADC30-like strains. Moreover, these findings also provide a solid foundation for studying the recombination of different PRRSVs and developing new and effective universal vaccines in the future.

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.

Immune duration of a recombinant PRRSV vaccine expressing E2 of CSFV

Classical swine fever virus (CSFV) and Porcine reproductive and respiratory syndrome virus (PRRSV) are both important pathogens which seriously harm the economic swine industry worldwide. We have previously demonstrated that rPRRSV-E2 is a promising live, virus-vectored vaccine that provides 100% protection against highly pathogenic PRRSV (HP-PRRSV) and CSFV. Here, we evaluated the duration of immunity (DOI) of the vaccine strain, rPRRSV-E2. Vaccine or cell culture medium was administered to piglets at 4 weeks of age. All immunized piglets developed high levels of antibodies, which could maintain for up to 23 weeks, against PRRSV and CSFV. All immunized pigs were well protected from the challenge of HP-PRRSV or CSFV at 20 weeks and 24 weeks post vaccination. The vaccine protection rate was still 100% at 24 weeks after immunization. The immune efficacy results showed that the immune duration of rPRRSV-E2 could be up to 5 months.

Evaluation of immune efficacy of recombinant PRRSV vectored vaccine rPRRSV-E2 in piglets with maternal derived antibodies

Currently live attenuated porcine reproductive and respiratory syndrome (PRRS) and classical swine fever (CSF) vaccines are widely used in Chinese swine herds. However, the mutual effects of vaccination procedures and severe stress caused by successive vaccinations harm piglets and make it difficult to stimulate robust and effective immune responses. In our previous study, a recombinant PRRS virus (PRRSV) vectored vaccine candidate rPRRSV-E2, which expresses CSF virus (CSFV) E2 protein, has been demonstrated being able to protect piglets against lethal challenge of highly-pathogenic (HP)-PRRSV and CSFV. In this study, we determine whether preexisting maternally derived antibodies (MDA) interfere with the immune efficacy of rPRRSV-E2. 8 experimental groups of piglets, with or without PRRSV MDAs or CSFV MDAs were immunized with a single dose of 10⁵ TCID₅₀ rPRRSV-E2 or DMEM and challenged with HP-PRRSV or CSFV. Clinical characteristics, PRRSV- or CSFV-specific antibodies, viremia and pathological changes were monitored, examined and analyzed. The results showed that rPRRSV-E2-vaccinated piglets, either with or without MDAs directed against PRRSV or CSFV were completely protected from the lethal challenge of HP-PRRSV or CSFV. These results demonstrate that the MDAs do not interfere with the immune efficacy of rPRRSV-E2, which indicates that rPRRSV-E2 could have great significance in the effective prevention and control of HP-PRRSV and CSFV.

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.

Recombinant porcine reproductive and respiratory syndrome virus expressing luciferase genes provide a new indication of viral propagation in both permissive and target cells

Porcine reproductive and respiratory syndrome virus (PRRSV) has a condensed single-stranded positive-sense RNA genome that contains several overlapping regions. The transcription regulatory sequence (TRS) is the important cis-acting element participating in PRRSV discontinuous transcription process. Based on reverse genetic system of type 2 highly pathogenic PRRSV cell-passage attenuated strain pHuN4-F112, firefly luciferase or Renilla luciferase genes were inserted between ORF1b and ORF2. An extra TRS6 was embedded behind the foreign luciferase genes. pA-Fluc and pA-Rluc were constructed and successfully rescued in MARC-145 cells. The phenotypical characteristics of the progeny virus were indistinguishable from those of vHuN4-F112 and were genetically stable for at least 25 cell passages. Mutant virus-infected cells were lysed at different time points to assess luciferase activities and measure foreign gene expression levels. The results showed identical variations in the luciferase activities of the recombinants in MARC-145 cells, indicating that they were suitable for monitoring viral propagation in PRRSV-permissive cell cultures. They were also used to infect pulmonary alveolar macrophages, which yielded similar variations in luciferase activities. Therefore, vA-Fluc and vA-Rluc present powerful new tools to monitor PRRSV propagation in both passaged and target cells.

Cellular miR-130b inhibits replication of porcine reproductive and respiratory syndrome virus in vitro and in vivo

MicroRNAs (miRNAs) can impact viral infections by binding to sequences with partial complementarity on viral RNA transcripts, usually resulting in the repression of virus replication. In the present study, we identified a potential binding site for miR-130 in the 5' untranslated region (bps 155-162) of the porcine reproductive and respiratory syndrome virus (PRRSV) genome. We found that the delivery of multiple miR-130 family mimics, especially miR-130b, resulted in inhibition of PRRSV replication in vitro. miR-130 was effective in inhibiting the replication of multiple type 2 PRRSV strains, but not against vSHE, a classical type 1 strain. miR-130 over-expression did not induce IFN-α or TNF-α expression in either uninfected or PRRSV-infected porcine alveolar macrophages. Results from luciferase reporter assays indicated that miR-130 directly targeted the PRRSV 5' UTR. Intranasal inoculation of piglets with miR-130b exhibited antiviral activity in vivo and partially protected piglets from an otherwise lethal challenge with HP-PRRSV strain vJX143. Overall, these results demonstrate the importance of the miR-130 family in modulating PRRSV replication and also provide a scientific basis for using cellular miRNAs in anti-PRRSV therapies.

Inhibition of porcine reproductive and respiratory syndrome virus replication with exosome-transferred artificial microRNA targeting the 3' untranslated region

Porcine reproductive and respiratory syndrome (PRRS) is an economically important swine disease. As part of the development of RNA interference (RNAi) strategy against the disease, in this study a recombinant adenovirus (rAd) expressing the artificial microRNA (amiRNA) targeting the 3' untranslated region (UTR) was used to investigate the exosome-mediated amiRNA transfer from different pig cell types to porcine alveolar macrophages (PAMs). Quantitative RT-PCR showed that the sequence-specific amiRNA was expressed in and secreted via exosomes from the rAd-transduced pig kidney cell line PK-15, PAM cell line 3D4/163, kidney fibroblast cells (PFCs) and endometrial endothelial cells (PEECs) with different secretion efficiencies. Fluorescent microscopy revealed that the dye-labeled amiRNA-containing exosomes of different cell origins were efficiently taken up by all of the five types of pig cells tested, including primary PAMs. Quantitative RT-PCR showed that the amiRNA-containing exosomes of different cell origins were taken up by primary PAMs in both time- and dose-dependent manners. Both quantitative RT-PCR and viral titration assays showed that the exosome-delivered amiRNA had potent anti-viral effects against three different PRRSV strains. These data suggest that the exosomes derived from pig cells could serve as an efficient miRNA transfer vehicle, and that the exosome-delivered amiRNA had potent anti-viral effects against different PRRSV strains.

PRRSV structure, replication and recombination: Origin of phenotype and genotype diversity

Porcine reproductive and respiratory disease virus (PRRSV) has the intrinsic ability to adapt and evolve. After 25 years of study, this persistent pathogen has continued to frustrate efforts to eliminate infection of herds through vaccination or other elimination strategies. The purpose of this review is to summarize the research on the virion structure, replication and recombination properties of PRRSV that have led to the extraordinary phenotype and genotype diversity that exists worldwide.

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Review of structure, replication and recombination of porcine reproductive and respiratory syndrome virus.

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Homologous recombination to produce conventional subgenomic messenger RNA as well as heteroclite RNA.

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Discussion of structure, replication and recombination mechanisms that have yielded genotypic and phenotypic diversity.

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.

Complete Genome Sequence of a Porcine Reproductive and Respiratory Syndrome Virus Variant with a New Deletion in the 5′ Untranslated Region

The GX1002 strain is a highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) characterized by a continuous 2-nucleotide deletion at positions 119 and 120 in the 5′ untranslated region. This differs from prevalent HP-PRRSVs in China, which have a deletion of only 1 nucleotide at position 119. Here we report the complete genome sequence of the GX1002 strain.

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.