Differentiation of U.S. and European isolates of porcine reproductive and respiratory syndrome virus by monoclonal antibodies

Recombination and pathogenicity analysis of NADC30-like and QYYZ-like PRRSV strains in South China

Since 2010, the Lineage 1 (NADC30-like) and Lineage 3 (QYYZ-like) strains of PRRSV-2 have become widespread in China, undergoing recombination and resulting in varying virulent and immune-evading mutants that have severely impacted the pig farming industry. In this study, we isolated one NADC30-like strain (GXHX20211106) and one QYYZ-like strain (GXGG202007) from pig serum samples collected in southern China. Comparative genomic analysis revealed that GXHX20211106 shares 90 % identity with NADC30, while GXGG202007 shows 91.4 % similarity to QYYZ. Both strains exhibit characteristic amino acids deletions or insertions in the Nsp2 region. Phylogenetic analyses further classified GXHX20211106 within Lineage 1.8 (NADC30-like) and GXGG202007 in Lineage 3 (QYYZ-like). Recombination analysis demonstrated that GXHX20211106 resulted from recombination between NADC30-like and HP-PRRSV-like strains, while GXGG202007 originated from recombination events involving QYYZ-like, VR-2332-like, and HP-PRRSV-like strains. Pathogenicity studies in piglets indicated that both isolates caused moderate clinical signs, with GXHX20211106 showing higher virulence compared to GXGG202007. No deaths were recorded in the infected piglets. These findings highlight the critical role of recombination in shaping PRRSV virulence and underscore the need for ongoing surveillance and control measures to mitigate the impact of recombinant PRRSV strains in southern China.

Identification of new antigenic epitopes of porcine reproductive and respiratory syndrome virus nsp12 protein using monoclonal antibodies

Porcine reproductive and respiratory syndrome virus (PRRSV), a member of the arteritis virus family, significantly impacts the swine industry due to its high infectiousness. nsp12, the nonstructural protein encoded by PRRSV, is a membrane-associated protein, with limited knowledge about its antigenic properties and functions. In this study, we used the expressed and purified nsp12 protein as antigen to immunize mice, and successfully screened five positive hybridoma cell lines that stably secrete anti-nsp12 monoclonal antibodies using immunological assays such as indirect ELISA and IFA. The antigenic epitopes recognized by the five monoclonal antibodies were identified using the fusion expression of peptides derived from the overlapping truncators of the nsp12 gene. The results showed that monoclonal antibodies 3G11 and 9C2 recognized the antigenic epitope 93TWGFESDTAY102, 2E3 recognized 115DYNDAFRARQ124, and 10G6 and 2A4 recognized 142PGPVIEPTL150. Furthermore, the three newly identified antigenic epitopes were all immunodominant and located on the surface of nsp12 protein. Notably, the antigenic epitopes 93-102 aa are all highly conserved across PRRSV-2 strains, making them suitable targets for PRRSV-2 detection. In conclusion, our findings advance the understanding of the antigenic properties of the PRRSV nsp12 protein and facilitate the development of assays for PRRSV detection.

Linear epitopes of PRRSV-1 envelope proteins ectodomains are not correlated with broad neutralization

BACKGROUND

Neutralizing antibodies against PRRSV are capable of conferring protection against viral reinfection, but they tend to be strain specific and usually have poor cross-reactivity. Nonetheless, it has been described that there are individuals capable of efficiently neutralizing viruses of different origin, so it is expected that there are conserved neutralizing epitopes relevant for broad neutralization. However, although immunodominant regions and neutralizing epitopes have been described in different envelope proteins, their role in broad neutralization is unknown. The main objective of this study was to determine whether the linear epitopes existing in the ectodomains of PRRSV envelope proteins play a role in cross-neutralization.

RESULTS

A pepscan analysis was carried out using synthetic peptides against the ectodomains of PRRSV envelope proteins and PRRSV-hyperimmune sera of different cross-reactivity. The results obtained confirm the existence of antigenic regions in the ectodomains of the GP2, GP3, GP4 and GP5 that tend to be relatively conserved among different PRRSV isolates. Nonetheless, these antigenic regions have poor immunogenicity since they are only recognized by a limited number of sera. Furthermore, no differences were found between the reactivity of sera with broad cross-neutralization capacity and sera with poor heterologous neutralization activity, which indicate that linear epitopes existing in the ectodomains of PRRSV envelope proteins are not relevant for the development of broadly reactive neutralizing antibodies. Subsequently, some selected peptides were used in competition assays with the virus for binding to the cell receptors and in seroneutralization inhibition assays by incubation with hyperimmune sera. Firstly, some peptides that interfere with virus infectivity were identified in competition assays, but only in the case of one viral isolate, which points to the possible existence of a strain-dependent inhibition. However, the results of the seroneutralization inhibition assay indicate that, under the conditions of our study, none of the peptides used was capable of inhibiting virus neutralization by the hyperimmune sera.

CONCLUSIONS

The results obtained indicate that the linear peptides analyzed in this study do not play a major role in the induction of broadly reactive neutralizing antibodies, which could probably depend on conformational neutralizing.

Determination of the frequency of individuals with broadly cross-reactive neutralizing antibodies against PRRSV in the sow population under field conditions

BACKGROUND

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a significant swine pathogen, yet the immune response components contributing to protection remain incompletely understood. Broadly reactive neutralizing antibodies (bNAs) may play a crucial role in preventing reinfections by heterologous viruses, although their occurrence is considered low under both field and experimental conditions. This study aimed to assess the frequency of sows exhibiting bNAs against PRRSV under field conditions and to analyze the epidemiological factors influencing the occurrence of these elite neutralizers. Blood samples were collected from breeding sows across eleven unrelated pig farms, with samples categorized by parity. Serum obtained was utilized in virus neutralization assays (VNs) against six PRRSV field isolates and two MLV strains.

RESULTS

Approximately 7% of the sows exhibited neutralization activity against all viruses in the panel, with a geometric mean of the titer (GMT) of NAs at or exceeding 4 log2. Exclusion of the PRRSV-2 isolate from the panel increased the proportion of elite neutralizers to around 15%. Farm-specific analysis revealed significant variations in both GMT of NAs and proportion of elite neutralizers. PRRSV unstable farms and those with a PRRS outbreak in the last 12 months displayed higher GMT of NAs compared to stable farms without recent outbreaks. The GMT of NAs showed a gradual, albeit moderate, increase with the parity of the sows. Parity's impact on bNA response was consistently observed in stable farms but not necessarily in unstable farms or those with recent outbreaks. Finally, the results indicated that vaccinated animals had higher NA titers against the vaccine virus used in the farm than against field viruses.

CONCLUSION

bNAs against heterologous isolates induced by PRRSV infection under field conditions are generally low, often falling below titers necessary for protection against reproductive failure. However, a subset of sows (approximately 15%) can be considered elite neutralizers, efficiently recognizing various PRRSV strains. Repeated exposures to PRRSV play a crucial role in eliciting these bNAs, with a higher frequency observed in unstable farms and those with recent outbreaks. In stable farms, parity only marginally influences bNA titers, highlighting its limited role compared to the impact of PRRSV exposure history.

Molecular epizootiology of porcine reproductive and respiratory syndrome virus in the Xinjiang Uygur Autonomous Region of China

Rapid evolution of porcine reproductive and respiratory syndrome virus (PRRSV) is the bottleneck for effective prevention and control of PRRS. Thus, understanding the prevalence and genetic background of PRRSV strains in swine-producing regions is important for disease prevention and control. However, there is only limited information about the epizootiological situation of PRRS in the Xinjiang Uygur Autonomous Region, China. In this study, blood or lung tissue samples were collected from 1,411 PRRS-suspected weaned pigs from 9 pig farms in Changji, Shihezi, and Wujiaqu cities between 2020 and 2022. The samples were first tested by RT-quantitative PCR, yielding a PRRSV-2 positive rate of 53.6%. Subsequently, 36 PRRSV strains were isolated through initial adaptation in bone marrow-derived macrophages followed by propagation in grivet monkey Marc-145 cells. Furthermore, 28 PRRSV-positive samples and 20 cell-adapted viruses were selected for high-throughput sequencing (HTS) to obtain the entire PRRSV genome sequences. Phylogenetic analysis based on the nucleotide sequences of the ORF5 gene of the PRRSV strains identified in this study grouped into sub-lineages 1.8 and 8.7 the former being the dominant strain currently circulating in Xinjiang. However, the NSP2 proteins of the Xinjiang PRRSV strains shared the same deletion patterns as sub-lineage 1.8 prototype strain NADC30 with the exception of 4 strains carrying 2-3 additional amino acid deletions. Further analysis confirmed that recombination events had occurred in 27 of 37 PRRSVs obtained here with the parental strains belonging to sub-lineages 1.8 and 8.7, lineages 3 and 5, with the recombination events having occurred most frequently in the 5' and 3' termini of ORF1a and 5' terminus of ORF1b.

Fidelity Characterization of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus and NADC30-like Strain

The porcine reproductive and respiratory syndrome virus (PRRSV) has significantly impacted the global pork industry for over three decades. Its high mutation rates and frequent recombination greatly intensifies its epidemic and threat. To explore the fidelity characterization of Chinese highly pathogenic PRRSV JXwn06 and the NADC30-like strain CHsx1401, self-recombination and mutation in PAMs, MARC-145 cells, and pigs were assessed. In vitro, CHsx1401 displayed a higher frequency of recombination junctions and a greater diversity of junction types than JXwn06. In vivo, CHsx1401 exhibited fewer junction types yet maintained a higher junction frequency. Notably, JXwn06 showed more accumulation of mutations. To pinpoint the genomic regions influencing their fidelity, chimeric viruses were constructed, with the exchanged nsp9-10 regions between JXwn06 and CHsx1401. The SJn9n10 strain, which incorporates JXwn06's nsp9-10 into the CHsx1401 genome, demonstrated reduced sensitivity to nucleotide analogs compared to CHsx1401. Conversely, compared with JXwn06, the JSn9n10 strain showed increased sensitivity to these inhibitors. The swapped nsp9-10 also influences the junction frequency and accumulated mutations as their donor strains. The results indicate a propensity for different types of genetic variations between these two strains and further highlight the nsp9-10 region as a critical determinant of their fidelity.

PRRSV inhibited the proliferation of CSFV by inducing IL-1β maturation via NLRP3 inflammasome activation

PRRSV and CSFV are both common infectious pathogens in porcine populations, posing significant threats to the healthy development of the porcine industry. Vaccine immunization is the main way to prevent and control these two diseases. Increasing studies have demonstrated that there is an interaction between PRRSV co-infection and CSFV vaccine immune failure. To investigate the effect of PRRSV infection on CSFV proliferation and its molecular mechanism, the proliferation dynamics of PRRSV/CSFV, the NLRP3 inflammasome components, and IL-1β expression levels were detected in PRRSV/CSFV alone- or co-infection. Subsequently, the relationship between inflammasome activation, IL-1β expression, and CSFV proliferation was analyzed through the construction of an inflammasome activation model, specific siRNA interference, and specific inhibitor treatment. The results showed that CSFV infection had a poor regulatory effect on NLRP3 inflammasome activation and IL-1β maturation, but PRRSV and CSFV co-infection could significantly up-regulate the expression of NLRP3 and ASC, induce Caspase-1 activation, and promote IL-1β maturation. It was further determined that NLRP3 inflammasome components played important roles in IL-1β maturation and inhibiting CSFV proliferation by PRRSV. Additional experiments indicated that PRRSV replication is essential for NLRP3 inflammasome activation, IL-1β maturation, and CSFV proliferation inhibition. More importantly, NLRP3 inflammasome activation is regulated by the TLR4-MyD88-NF-κB pathways. In conclusion, PRRSV infection induced IL-1β maturation by activating the NLRP3 inflammasome through the TLR4-MyD88-NF-κB pathways and then inhibited the proliferation of CSFV. These data further improved the theoretical basis for PRRSV inducing inflammatory factors and leading to the failure of CSFV immunization.

One-Step Assembly of a PRRSV Infectious cDNA Clone and a Convenient CRISPR/Cas9-Based Gene-Editing Technology for Manipulation of PRRSV Genome

Porcine reproductive and respiratory syndrome (PRRS) has been a persistent challenge for the swine industry for over three decades due to the lack of effective treatments and vaccines. Reverse genetics systems have been extensively employed to build rapid drug screening platforms and develop genetically engineered vaccines. Herein, we rescued recombinant PRRS virus (rPRRSV) WUH3 using an infectious cDNA clone of PRRSV WUH3 acquired through a BstXI-based one-step-assembly approach. The rPRRSV WUH3 and its parental PRRSV WUH3 share similar plaque sizes and multiple-step growth curves. Previously, gene-editing of viral genomes depends on appropriate restrictive endonucleases, which are arduous to select in some specific viral genes. Thus, we developed a restrictive endonucleases-free method based on CRISPR/Cas9 to edit the PRRSV genome. Using this method, we successfully inserted the exogenous gene (EGFP gene as an example) into the interval between ORF1b and ORF2a of the PRRSV genome to generate rPRRSV WUH3-EGFP, or precisely mutated the lysine (K) at position 150 of PRRSV nsp1α to glutamine (Q) to acquire rPRRSV WUH3 nsp1α-K150Q. Taken together, our study provides a rapid and convenient method for the development of genetically engineered vaccines against PRRSV and the study on the functions of PRRSV genes.

Progress in PRRSV Infection and Adaptive Immune Response Mechanisms

Since its discovery, Porcine reproductive and respiratory syndrome (PRRS) has had a huge impact on the farming industry. The virus that causes PRRS is Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), and because of its genetic diversity and the complexity of the immune response, the eradication of PRRS has been a challenge. To provide scientific references for PRRSV control and vaccine development, this study describes the processes of PRRSV-induced infection and escape, as well as the host adaptive immune response to PRRSV. It also discusses the relationship between PRRSV and the adaptive immune response.

Application of codon pair deoptimization for ORF7-induced attenuation of type I porcine reproductive and respiratory syndrome virus without reduced immune responses

Codon pair deoptimization (CPD) attenuated type I porcine reproductive and respiratory syndrome virus (PRRSV). Infectious clones covering the full genome of a Korean type I PRRSV (E38) were synthesized, and CPD induced nine synonymous mutants of NSP1 (n = 1) and ORF7 (n = 8). In a trial to rescue live viruses from infectious clones, only four clones with mutations at nt 177 downstream of ORF7 were rescued, which showed a substantial decrease in cellular replication ability. The rescue-failed clones had two common mutation sites with a high minimum free energy and significantly modified RNA secondary structure relative to the original virus. In infected pigs, CPD viruses demonstrated significantly lower replication ability and pathogenicity than the original virus. However, immune response level induced by the attenuated viruses and the original virus was similar. This is the first study to demonstrate that type I PRRSV virulence can be attenuated through CPD application to ORF7.

Molecular Characterization of the Nsp2 and ORF5s of PRRSV Strains in Sichuan China during 2012-2020

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that poses a serious threat to the global pig industry. Sichuan Province is one of the largest pig breeding provinces in China. There is a lack of reports on the continuous surveillance and systematic analysis of prevalent strains of PRRSV in Sichuan Province in recent years. To fill this gap, a total of 539 samples were collected from 13 breeding regions in Sichuan during 2012-2020. The detection result showed that the positive rate of PRRSV was 52.32% (282/539). The ORF5s and Nsp2 were obtained and further analyzed, with Chinese reference strains downloaded from the GenBank. Phylogenetic analysis showed that the PRRSV strains sequenced in this study belonged to PRRSV-1 and PRRSV-2 (lineage 1, 3, 5 and 8). In total, 168 PRRSV-2 strains were selected for ORF5 analyses, and these strains were classified into sub-lineage 8.7 (HP-PRRSV), sub-lineage 5.1 (classical PRRSV), sub-lineage 1.8 (NADC30-like), sub-lineage 1.5 (NADC34-like) and sub-lineage 3.5 (QYYZ-like), accounting for 60.71% (102/168), 11.31% (19/168), 18.45% (31/168), 2.97% (5/168) and 6.55% (11/168) of the total analyzed strains, respectively. The Nsp2 of identified PRRSV strains exhibited a nucleotide identity of 44.5-100%, and an amino acid identity of 46.82-100%. The ORF5 of the identified PRRSV strains exhibited a nucleotide identity of 81.3-100%, and an amino acid identity of 78.5-100%. A sequence analysis of ORF5 revealed that the mutation sites of GP5 were mainly concentrated in HVR1 and HVR2 and the virulence sites. In summary, the HP-PRRSV, NADC30-like PRRSV, Classic-PRRSV, QYYZ-like PRRSV, NADC34-like PRRSV and PRRSV-1 strains exist simultaneously in pigs in Sichuan. NADC30-like PRRSV was gradually becoming the most prevalent genotype currently in Sichuan province. This study suggested that PRRSV strains in Sichuan were undergoing genomic divergence.

Genetic characterization and pathogenicity of a novel recombinant PRRSV from lineage 1, 8 and 3 in China failed to infect MARC-145 cells

The diversity of porcine reproductive and respiratory syndrome virus (PRRSV) in China is increasing rapidly along with mutation and recombination. Recombination could occur between inter- and intra-lineage of PRRSV, which accelerated the complexity of pathogenicity and cell tropism of the recombinant strain. In the present study, a novel PRRSV strain named HN-YL1711 was isolated from a pig farm suffering from severe respiratory difficulty in Henan province, China. The whole genomic sequence analysis indicated that the genome of HN-YL1711 was 15018 nt. It shared 86%, 87.3%, 88.1%, 91.1%, 84.2%, and 84.1% nucleotide similarities with PRRSVs VR2332, CH1a, JXA1, NADC30, QYYZ, and GM2, respectively. Based on phylogenetic analysis of Nsp2, ORF5 and complete genomes, HN-YL1711 was classified into lineage 1 of PRRSV. However, seven genomic break points were detected in recombination analysis, which indicated that the HN-YL1711 originated from multiple recombination among NADC30-like (major parent, lineage 1), JXA1-like (minor parent, lineage 8), and QYYZ-like (minor parent, lineage 3) PRRSV. Porcine alveolar macrophages (PAMs), 3D4/21-CD163 and MARC-145 cells were used to explore the viral adaptation of HN-YL1711. The results indicated that it could infect the PAMs but failed to infect MARC-145 cells. Challenge experiments showed that HN-YL1711 exhibits intermediate virulence in pigs, compared with HP-PRRSV JXA1 and LP-PRRSV CH1a. Taken together, our findings suggest that recombination remains an important factor in PRRSV evolution and that recombination further complicates the cell tropism and pathogenicity of PRRSV.

A swine arterivirus deubiquitinase stabilizes two major envelope proteins and promotes production of viral progeny

Arteriviruses are enveloped positive-strand RNA viruses that assemble and egress using the host cell’s exocytic pathway. In previous studies, we demonstrated that most arteriviruses use a unique -2 ribosomal frameshifting mechanism to produce a C-terminally modified variant of their nonstructural protein 2 (nsp2). Like full-length nsp2, the N-terminal domain of this frameshift product, nsp2TF, contains a papain-like protease (PLP2) that has deubiquitinating (DUB) activity, in addition to its role in proteolytic processing of replicase polyproteins. In cells infected with porcine reproductive and respiratory syndrome virus (PRRSV), nsp2TF localizes to compartments of the exocytic pathway, specifically endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and Golgi complex. Here, we show that nsp2TF interacts with the two major viral envelope proteins, the GP5 glycoprotein and membrane (M) protein, which drive the key process of arterivirus assembly and budding. The PRRSV GP5 and M proteins were found to be poly-ubiquitinated, both in an expression system and in cells infected with an nsp2TF-deficient mutant virus. In contrast, ubiquitinated GP5 and M proteins did not accumulate in cells infected with the wild-type, nsp2TF-expressing virus. Further analysis implicated the DUB activity of the nsp2TF PLP2 domain in deconjugation of ubiquitin from GP5/M proteins, thus antagonizing proteasomal degradation of these key viral structural proteins. Our findings suggest that nsp2TF is targeted to the exocytic pathway to reduce proteasome-driven turnover of GP5/M proteins, thus promoting the formation of GP5-M dimers that are critical for arterivirus assembly.

Arteriviruses are a rapidly expanding family of positive-stranded RNA viruses, which includes economically important veterinary pathogens like equine arteritis virus (EAV) and two species of porcine reproductive and respiratory syndrome virus (PRRSV-1 and PRRSV-2). In our previous studies, we uncovered an unprecedented arterivirus gene expression mechanism: a highly efficient -2 programmed ribosomal frameshift (PRF) that is controlled by an interaction of viral protein nsp1ß with specific RNA sequences and host poly(C) binding proteins. It is used by PRRSVs, and most other arteriviruses, to efficiently produce a previously unknown nonstructural protein variant, nsp2TF. In this study, we demonstrate that PRRSV nsp2TF interacts with the two major arteriviral envelope proteins, GP5 and M, whose heterodimerization in the secretory pathway is a key step in envelope protein trafficking and virus assembly. Our findings suggest that nsp2TF promotes arterivirus assembly by antagonizing the ubiquitination-dependent proteasomal degradation of GP5 and M proteins. This mechanism is based on the DUB activity of the PLP2 protease domain located within the N-terminal region of nsp2TF. To our knowledge, this is the first study to demonstrate that viruses can express a DUB that functions specifically to counteract the ubiquitination and degradation of key viral structural proteins.

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.

The Performance of Seven Molecular Methods for the Detection of PRRSV

Porcine Reproductive and Respiratory Syndrome is a viral disease of swine characterized by reproductive failure of breeding animals and respiratory disorders in all categories. The first PRRS case in Serbia was recorded in 2001 after illegal import of boar semen. PRRS is economically the most important disease due to significant direct and indirect losses. Today, for routine diagnosis of PRRS in infected herds serological methods (ELISA) and molecular methods are used. Although modern diagnostic techniques are very robust, exceptional diversity of the viral strains is often the obstacle for an accurate diagnosis. To estimate the performance of seven different methods for PRRSV genome detection, twenty samples were used. However, none of the methods was able to detect all PRRSV strains. The best sensitivity was obtained by combining two methods. Until today, there is no absolutely accurate test which enables the detection of all circulating strains.

Cell-mediated immune response and protective efficacy of porcine reproductive and respiratory syndrome virus modified-live vaccines against co-challenge with PRRSV-1 and PRRSV-2

Cell-mediated immunity (CMI), IL-10, and the protective efficacy of modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccines (MLV) against co-challenge with PRRSV-1 and PRRSV-2 (HP-PRRSV) were investigated. Seventy, PRRSV-free, 3-week old, pigs were allocated into 7 groups. Six groups were intramuscularly vaccinated with MLV, including Porcilis (PRRSV-1 MLV, MSD Animal Health, The Netherlands), Amervac (PRRSV-1 MLV, Laboratorios Hipra, Spain), Fostera (PRRSV-2 MLV, Zoetis, USA), Ingelvac PRRS MLV and Ingelvac PRRS ATP (PRRSV-2, Boehringer Ingelheim, USA), and Prime Pac PRRS (PRRSV-2 MLV, MSD Animal Health, The Netherlands). Unvaccinated pigs were left as control. Lymphocyte proliferative response, IL-10 and IFN-γ production were determined. At 35 days post-vaccination (DPV), all pigs were inoculated intranasally with 2 ml of each PRRSV-1 (10^5.4 TCID₅₀/ml) and PRRSV-2 (10^5.2 TCID₅₀/ml, HP-PRRSV). Following challenge, sera were quantitatively assayed for PRRSV RNA. Pigs were necropsied at 7 days post-challenge. Viremia, macro- and microscopic lung lesion together with PRRSV antigen presence were evaluated in lung tissues. The results demonstrated that, regardless of vaccine genotype, CMI induced by all MLVs was relatively slow. Increased production of IL-10 in all vaccinated groups was observed at 7 and 14 DPV. Pigs in Amervac, Ingelvac MLV and Ingelvac ATP groups had significantly higher levels of IL-10 compared to Porcilis, Fostera and Prime Pac groups at 7 and 14 DPV. Following challenge, regardless to vaccine genotype, vaccinated pigs had significantly lower lung lesion scores and PRRSV antigens than those in the control group. Both PRRSV-1 and PRRSV-2 RNA were significantly reduced. Prime Pac pigs had lowest PRRSV-1 and PRRSV-2 RNA in serum, and micro- and macroscopic lung lesion scores (p < 0.05) compared to other vaccinated groups. In conclusion, PRRSV MLVs, regardless of vaccine genotype, can reduce viremia and lung lesions following co-challenge with PRRSV-1 and PRRSV-2 (HP-PRRSV). The main difference between PRRSV MLV is the production of IL-10 following vaccination.

Antiviral effect of 25-hydroxycholesterol against porcine reproductive and respiratory syndrome virus in vitro

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that causes economically huge losses to the pig industry worldwide. Current control of PRRSV infection remains inadequate although various means have been implemented. Thus, investigating novel antiviral therapeutics to combat PRRSV infection is essential. In the present study, the antiviral effect in vitro of 25-hydroxycholesterol (25HC) against PRRSV was investigated.

METHODS

Cell viability assay was performed to examine the impact of 25HC on the cell viability. Indirect immunofluorescence assay and virus titration were utilized to evaluate the levels of PRRSV growth. Viral attachment assay, penetration assay and release assay were conducted to investigate the antiviral mechanism of 25HC against PRRSV. Real-time RT-PCR assay was used to analyse the effect of 25HC on the genome synthesis of PRRSV.

RESULTS

We demonstrated that the growth of PRRSV was significantly inhibited in 25HC-pretreated cells and PRRSV-infected cells by 25HC. Moreover, 25HC could impair the attachment and entry of PRRSV in vitro, but not affect viral genome synthesis and virion release.

CONCLUSIONS

Our findings clearly indicate that 25HC can exert antiviral effect against PRRSV infection in vitro, suggesting that 25HC might be a novel potential agent to control PRRSV infection.

Determination of antibody induction by highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) vaccine: A comparison of two ELISA kits

Two commercial porcine reproductive and respiratory syndrome virus (PRRSV) antibody ELISA kits (IDEXX and LSI) are currently in extensive use. To determine which kit is more suitable for the evaluation of HP-PRRSV vaccine efficacy, the two kits were used to test 546 serum samples. The agreement between the results was unsatisfactory, with a kappa statistic of 0.681 and a linear correlation coefficient of 0.665. In tests of samples from experimentally vaccinated and PRRSV-negative herds, IDEXX-ELISA identified antibody-positive conversion earlier and showed a higher specificity compared to LSI-ELISA. The serological profile obtained by neutralization testing was closer to that obtained by IDEXX-ELISA than by LSI-ELISA in the late immunization period. The findings reveal that IDEXX-ELISA is the more suitable for the evaluation of antibody response to HP-PRRSV vaccine and for guiding vaccination strategies.

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.

Genetic Characterization and Pathogenicity of a Novel Recombined Porcine Reproductive and Respiratory Syndrome Virus 2 among Nadc30-Like, Jxa1-Like, and Mlv-Like Strains

Recombination among porcine reproductive and respiratory syndrome viruses (PRRSVs), coupled with point mutations, insertions, and deletions occurring in the genome, is considered to contribute to the emergence of new variants. Here, we report the complete genome sequences of a PRRSV field strain, designated SCN17, isolated from a RespPRRS MLV-vaccinated piglet in China in 2017. Sequence alignment revealed that SCN17 had discontinuous 131-amino acid (111 + 1 + 19-aa) deletion in the NSP2-coding region identical to that of NADC30 when compared to VR-2332. Notably, the strain, SCN17, contained an additional 1-aa deletion in NSP2, a 1-aa deletion in ORF5, and a unique 3-nt deletion in the 3′-UTR. Phylogenetic analysis showed that SCN17 clustered into NADC30-like lineage based on ORF5 genotyping, whereas it belonged to an inter-lineage between the NADC30-like and VR-2332-like lineages as established based on the full-length genome. Importantly, the SCN17 was identified as a novel virus recombined between a NADC30-like (moderately pathogenic), a JXA1-like (highly pathogenic), and an attenuated vaccine strain, RespPRRS MLV (parental strain VR-2332). Furthermore, we tested its pathogenicity in piglets. SCN17 infection caused a persistent fever, moderate interstitial pneumonia, and increased the viremia and antibody levels in the inoculated piglets. Of note, all SCN17-infected piglets survived throughout the study. The new virus was showed to be a moderately virulent isolate and have lower pathogenicity than HP-PRRSV strain, SCwhn09CD. Our results provide evidence for the continuing evolution of PRRSV field strain by genetic recombination and mutation leading to outbreaks in the vaccinated pig populations in China.

A new recombined porcine reproductive and respiratory syndrome virus virulent strain in China

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important swine diseases worldwide. In the present study, a new virulent strain of PRRS virus (PRRSV), GDsg, was isolated in Guangdong province, China, and caused high fever, high morbidity, and high mortality in sows and piglets. The genome of this new strain was 15,413 nucleotides (nt) long, and comparative analysis revealed that GDsg shared 82.4% to 94% identity with type 2 PRRSV strains, but only 61.5% identity with type 1 PRRSV Lelystad virus strain. Phylogenetic analysis indicated that type 2 PRRSV isolates include five subgenotypes (I, II, III, IV, and V), which are represented by NADC30, VR-2332, GM2, CH-1a, and HuN4, respectively. Moreover, GDsg belongs to a newly emerging type 2 PRRSV subgenotype III. More interestingly, the newly isolated GDsg strain has multiple discontinuous nt deletions, 131 (19 + 18 + 94) at position 1404–1540 and a 107 nt insertion in the NSP2 region. Most importantly, the GDsg strain was identified as a virus recombined between low pathogenic field strain QYYZ and vaccine strain JXA1-P80. In conclusion, a new independent subgenotype and recombinant PRRSV strain has emerged in China and could be a new threat to the swine industry of China.

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&rsquo;-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.

Transcriptome Analysis Reveals Dynamic Gene Expression Profiles in Porcine Alveolar Macrophages in Response to the Chinese Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important swine pathogens and causes reproductive failure in sows and respiratory disease in growing pigs. PRRSV mainly infects porcine alveolar macrophages (PAMs), leading to the subversion of innate and adaptive immunity of pigs. The transcriptome analysis of gene expression profiles in PRRSV-infected PAMs is essential for understanding the pathogenesis of PRRSV. Here we performed next-generation RNA sequencing and a comprehensive bioinformatics analysis to characterize the dynamic transcriptome landscapes in PAMs following PRRSV infection. Totally 38222 annotated mRNAs, 12987 annotated long noncoding RNAs (lncRNAs), and 17624 novel lncRNAs in PRRSV-infected PAMs were identified through a transcripts computational identification pipeline. The differentially expressed mRNAs and lncRNAs during PRRSV infection were characterized. Several differentially expressed transcripts were validated using qRT-PCR. Analyses on dynamic overrepresented GO terms and KEGG pathways in PRRSV-infected PAMs at different time points were performed. Meanwhile the genes involved in IFN-related signaling pathways, proinflammatory cytokines and chemokines, phagocytosis, and antigen presentation and processing were significantly downregulated, indicating the aberrant function of PAMs during PRRSV infection. Moreover, the differentially and highly expressed lncRNA XR_297549.1 was predicted to both cis-regulate and trans-regulate its neighboring gene, prostaglandin-endoperoxide synthase 2 (PTGS2), indicating its role in inflammatory response. Our findings reveal the transcriptome profiles and differentially expressed mRNAs and lncRNAs in PRRSV-infected PAMs in vitro, providing valuable information for further exploration of PRRSV pathogenesis.

Efficacy of live attenuated porcine reproductive and respiratory syndrome virus 2 strains to protect pigs from challenge with a heterologous Vietnamese PRRSV 2 field strain

Background

Effective vaccines against porcine reproductive and respiratory syndrome virus (PRRSV), especially against highly pathogenic (HP) PRRSV are still missing. The objective of this study was to evaluate the protective efficacy of an experimental live attenuated PRRSV 2 vaccine, composed of two strains, against heterologous challenge with a Vietnamese HP PRRSV 2 field strain. For this reason, 20 PRRSV negative piglets were divided into two groups. The pigs of group 1 were vaccinated with the experimental vaccine, group 2 remained unvaccinated. All study piglets received an intranasal challenge of the HP PRRSV 2 on day 0 of the study (42 days after vaccination). Blood samples were taken on days 7 and 21 after vaccination and on several days after challenge. On day 28 after challenge, all piglets were euthanized and pathologically examined.

Results

On days 7 and 21 after vaccination, a PRRSV 2 viraemia was seen in all piglets of group 1 which remained detectable in seven piglets up to 42 days after vaccination. On day 3 after challenge, all piglets from both groups were positive in PRRSV 2 RT-qPCR. From day 7 onwards, viral load and number of PRRSV 2 positive pigs were lower in group 1 than in group 2. All pigs of group 1 seroconverted after PRRSV 2 vaccination. PRRSV antibodies were detected in serum of all study pigs from both groups from day 14 after challenge onwards. In group 2, moderate respiratory symptoms with occasional coughing were seen following the challenge with HP PRRSV 2. Pigs of group 1 remained clinically unaffected. Interstitial pneumonia was found in four piglets of group 1 and in all ten piglets of group 2. Histopathological findings were more severe in group 2.

Conclusions

It was thus concluded that the used PRRSV 2 live experimental vaccine provided protection from clinical disease and marked reduction of histopathological findings and viral load in pigs challenged with a Vietnamese HP PRRSV 2 field strain.

Identification of a Novel Recombinant Type 2 Porcine Reproductive and Respiratory Syndrome Virus in China

Since the emergence of NADC30-like porcine reproductive and respiratory syndrome virus (PRRSV) in China in 2013, PRRSVs have undergone rapid evolution. In this study, a novel variant of PRRSV strain (designated SCcd17) was successfully isolated from piglets with clinical signs in Sichuan Province in China in 2017, and the complete genomic sequence was determined. The genome of this new isolate was 15,015 nucleotides (nt) long, and comparative analysis revealed that SCcd17 exhibited 90.2%, 85.2%, 84.9%, and 84.0% nucleotide similarity to PRRSVs NADC30, JXA1, CH-1a, and VR-2332, respectively. Phylogenetic analysis indicated that the SCcd17 strain was classified into the NADC30-like sub-genotype, in which all the strains contained the unique discontinuous 131-amino acid deletion in nonstructural protein 2 (nsp2) when compared to VR-2332-like viruses. Notably, extensive amino acid substitutions were observed in nsp2 and a unique single amino acid deletion at position 33 of the GP5 is being described for the first time. Strikingly, recombination analysis revealed that SCcd17 was the result of recombination between the NADC30-like, JXA1-like, and VR-2332-like strains at five recombination breakpoints: nsp1α (nt 641), nsp3 (nt 5141), nsp10 (nt 9521), open reading frame 3 (ORF3) (nt 12,581), and ORF4 (nt 13,021). The genomic data of SCcd17 will be helpful for understanding the role of genomic recombination in the evolution of PRRSV.

Characterization of the interactome of the porcine reproductive and respiratory syndrome virus glycoprotein-5

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in the swine industry, causing reproductive failure in sows and respiratory disorders in piglets. Glycosylated protein 5 (GP5) is a major envelope protein of the virus. It is essential for virus particle assembly and involved in viral pathogenesis. In the present study, we identified the host cellular proteins that interact with GP5 by performing immunoprecipitation in MARC-145 cells infected by a recombinant PRRSV containing a FLAG-tag insertion in GP5. In total, 122 cellular proteins were identified by LC-MS/MS. Gene Ontology and KEGG databases were used to map these proteins to different cellular processes, locations and functions. Interestingly, 10.24% of identified cellular proteins were involved in the process of translation. Follow up experiments demonstrated that expression of GP5 in transfected cells led to inhibition of translation of reporter genes. Interaction between GP5 and ATP synthase subunit alpha (ATP5A) was further confirmed by co-immunoprecipitation suggesting a possible role of GP5 in regulation of ATP production in cells. These data contribute to a better understanding of GP5's role in viral pathogenesis and virus-host interactions.

Inhibition of proanthocyanidin A2 on porcine reproductive and respiratory syndrome virus replication in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) is a widely prevalent and endemic swine pathogen that causes significant economic losses for the global pig industry annually. Currently, the most prevalent strategy for PRRSV control remains the prevention of virus transmission, with highly effective therapeutic agents and vaccines still lacking. Proanthocyanidin A2 (PA2) belongs to the family of tea polyphenols, which have been reported to exhibit a range of biological activities including anti-oxidative, cardio-protective, anti-tumoural, anti-bacterial, anti-viral, and anti-inflammatory effects in vitro as well as in vivo. Here, we demonstrate that PA2 exhibits potent anti-viral activity against PRRSV infection in Marc-145 cells. Similar inhibitory effects were also found in porcine alveolar macrophages, the primary target cell type of PRRSV infection in pigs in vivo. For traditional type II PRRSV CH-1a strain and high pathogenic GD-XH strain and GD-HD strain, PA2 exhibited broad-spectrum and comparable inhibitory activities in vitro with EC50 ranging from 2.2 to 3.2 μg/ml. Treatment of PRRSV-infected Marc-145 cells with PA2 significantly inhibited viral RNA synthesis, viral protein expression and progeny virus production in a dose-dependent manner. In addition, PA2 treatment reduced gene expressions of cytokines (TNF-α, IFN-α, IL-1β and IL-6) induced by PRRSV infection in PAMs. Mechanistically, PA2 inhibited PRRSV replication by targeting multiple pathways including blockade of viral entry and progeny virus release. Altogether, our findings suggest that PA2 has the potential to serve as a novel prophylactic and therapeutic strategies against PRRSV infection.

Porcine reproductive and respiratory syndrome virus neutralizing antibodies provide in vivo cross-protection to PRRSV1 and PRRSV2 viral challenge

Vaccine control and prevention of porcine reproductive and respiratory syndrome (PRRS), the most important disease of swine, is difficult to achieve. However, the discovery of broadly neutralizing antibody activity against porcine reproductive and respiratory syndrome virus (PRRSV) under typical field conditions opens the door to new immunologic approaches for robust protection. We show here that passive administration of purified immunoglobulins with neutralizing antibodies reduced PRRSV2 infection by up to 96%, and PRRSV1 infection by up to 87%, whereas immune immunoglobulins lacking neutralizing activity had no effect on viral infection. Hence, immune competence of passive immunoglobulin transfer was associated specifically with antibody neutralizing activity. Current models of PRRSV infection implicate a minor envelope glycoprotein (GP) complex including GP2, GP3, and GP4, as critical to permissive cell infection. However, conserved peptides comprising the putative cell attachment structure did not attenuate neutralization or viral infection. The results show that immunological approaches aimed at induction of broadly neutralizing antibodies may substantially enhance immune protection against PRRSV. The findings further show that naturally occurring viral isolates are able to induce protective humoral immunity against unrelated PRRSV challenge, thus removing a major conceptual barrier to vaccine development.

First-time detection of porcine reproductive and respiratory syndrome virus (PRRSV) infection in Uruguay

Within the last two decades, several high-impact viruses have emerged in the global swine population, including porcine reproductive and respiratory syndrome virus (PRRSV). In Uruguay, the more recent serological survey for PRRSV and other notifiable diseases such as Aujeszky's disease virus (ADV) and classical swine fever virus (CSFV) dated from year 2000. The main purpose of this study was to update our information on the infection status of PRRSV, ADV and CSFV in Uruguayan pig herds, in order to keep informed about the epidemiological situation of these notifiable infections in the country. For serological testing, a total of 524 swine serum samples collected during the period 2014-2016 were assayed by commercial ELISAs. Our results revealed the (unexpected) presence of PRRSV antibodies in Uruguayan domestic swine herds and confirmed the absence of ADV and CSFV antibodies in all of the assessed samples. Following such initial finding, PRRSV antibodies were further investigated in 23 retrospective samples collected during 2010-2014. Thirteen of these 23 samples resulted seropositive. Subsequently, a molecular detection approach in frozen serum samples was implemented to confirm PRRSV infection, and viral RNA was identified by reverse transcription-nested polymerase chain reaction (RT-nPCR). Fourteen of 86 evaluated 2014-2016 samples resulted positive for viral RNA, while molecular analysis of four retrospective samples also revealed the presence of PRRSV type 2. Viral isolation of selected samples was carried out in porcine alveolar macrophages (PAM) and MARC 145 simian kidney cells, and the virus identity was confirmed by cytopathic effect (CPE) and immunofluorescence assay (IFA) using specific monoclonal antibodies for PRRSV nucleocapsid. Data reported here evidence for the first time the circulation of PRRSV type 2 in Uruguay, and retrospective serology results suggest that the virus has been infecting pigs in this country at least since 2011.

Nonstructural proteins nsp2TF and nsp2N of porcine reproductive and respiratory syndrome virus (PRRSV) play important roles in suppressing host innate immune responses

Recently, we identified a unique -2/-1 ribosomal frameshift mechanism in PRRSV, which yields two truncated forms of nonstructural protein (nsp) 2 variants, nsp2TF and nsp2N. Here, in vitro expression of individual PRRSV nsp2TF and nsp2N demonstrated their ability to suppress cellular innate immune responses in transfected cells. Two recombinant viruses were further analyzed, in which either nsp2TF was C-terminally truncated (vKO1) or expression of both nsp2TF and nsp2N was knocked out (vKO2). Host cellular mRNA profiling showed that a panel of cellular immune genes, in particular those involved in innate immunity, was upregulated in cells infected with vKO1 and vKO2. Compared to the wild-type virus, vKO1 and vKO2 expedited the IFN-α response and increased NK cell cytotoxicity, and subsequently enhanced T cell immune responses in infected pigs. Our data strongly implicate nsp2TF/nsp2N in arteriviral immune evasion and demonstrate that nsp2TF/nsp2N-deficient PRRSV is less capable of counteracting host innate immune responses.

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In vitro expression of PRRSV nsp2TF/nsp2N affects cellular innate immune response.

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Nsp2TF/nsp2N suppressed immune genes expression in PRRSV-infected cells.

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Nsp2TF/nsp2N-deficient mutants showed attenuated growth in vivo.

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Nsp2TF/nsp2N-deficient mutants induced augmented immune responses in infected pigs.

Genomic analysis of a recombinant NADC30-like porcine reproductive and respiratory syndrome virus in china

Recently, NADC30-like porcine reproductive and respiratory syndrome viruses (PRRSVs), which are genetically similar to the NADC30 strain isolated in the United States of America in 2008, have become prevalent in China. Here, a novel variant PRRSV strain named HNhx was successfully isolated on porcine alveolar macrophages from Henan province and the full-length genome sequence was determined. Phylogenetic analysis indicated that HNhx strain was classified into the NADC30-like PRRSV subgroup, in which all the strains had the unique discontinuous 131-amino acid deletion relative to that of the nonstructural protein 2 (Nsp2) of the VR2332 strain. Genetically, HNhx shared 92.9% nucleotide similarity to NADC30. Furthermore, HNhx strain contained extensive amino acid mutations in GP5. In particular, the S32H, N33D, D34N, and S36G variations resulted in that HNhx lost all the putative N-linked glycosylation sites at amino acid positions 30, 32, 33, 34, and 35. Recombination analysis revealed that HNhx was the result of recombination between the NADC30 strain and the highly pathogenic PRRSV vaccine strain circulating in China in Nsp4 (nt 5261) to Nsp9 (nt 7911). The novel genome data of HNhx will be helpful for understanding the evolution and epidemiology of PRRSV in China.

Porcine reproductive and respiratory syndrome virus suppresses post-transcriptionally the protein expression of IFN-β by upregulating cellular microRNAs in porcine alveolar macrophages in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized to inhibit the response of type I interferon (IFN) both in vivo and in vitro. However, the post-transcriptional mechanism by which PRRSV suppresses type I IFN induction in virus-infected host cells remains unclear. The present study first demonstrated that PRRSV inhibited post-transcriptionally the protein induction of IFN-β in primary porcine alveolar macrophages (PAMs) during early infection, and the inhibition effect mediated by the Chinese highly pathogenic (HP)-PRRSV was stronger. Next, we analyzed the cellular microRNA (miRNA)-modulated protein expression of porcine IFN-β by dual firefly/Renilla luciferase reporter assay, transfection of miRNA mimics and inhibitor assay and polyinosinic-polycytidylic acid (poly I:C) treatment of PAMs, showing that porcine miRNAs including let-7b, miR-26a, miR-34a and miR-145 are able to inhibit IFN-β protein expression in primary PAMs by directly targeting sequences within the porcine IFN-β 3'UTR locating at 160-181, 9-31, 27-47 and 12-32 bp, respectively. Finally, we confirmed that let-7b, miR-26a, miR-34a and miR-145, were upregulated in PRRSV-infected PAMs early in vitro, and the expression level of these miRNAs in HP-PRRSV JXwn06-infected PAMs were higher than those in low pathogenic PRRSV HB-1/3.9-infected PAMs. The endogenous cellular miRNA-mediated inhibition of IFN-β induction in PRRSV-infected PAMs early could be relieved by miRNA antagonists. Taken together, our findings suggest for the first time that PRRSV can suppress post-transcriptionally protein expression of IFN-β by upregulating cellular miRNAs in PAMs in vitro, providing novel insight into mechanisms in relation to the PRRSV-mediated immunomodulation of porcine innate immunity.

Molecular cloning of porcine Siglec-3, Siglec-5 and Siglec-10, and identification of Siglec-10 as an alternative receptor for porcine reproductive and respiratory syndrome virus (PRRSV)

In recent years, several entry mediators have been characterized for porcine reproductive and respiratory syndrome virus (PRRSV). Porcine sialoadhesin [pSn, also known as sialic acid-binding immunoglobulin-type lectin (Siglec-1)] and porcine CD163 (pCD163) have been identified as the most important host entry mediators that can fully coordinate PRRSV infection into macrophages. However, recent isolates have not only shown a tropism for sialoadhesin-positive cells, but also for sialoadhesin-negative cells. This observation might be partly explained by the existence of additional receptors that can support PRRSV binding and entry. In the search for new receptors, recently identified porcine Siglecs (Siglec-3, Siglec-5 and Siglec-10), members of the same family as sialoadhesin, were cloned and characterized. Only Siglec-10 was able to significantly improve PRRSV infection and production in a CD163-transfected cell line. Compared with sialoadhesin, Siglec-10 performed equally effectively as a receptor for PRRSV type 2 strain MN-184, but it was less capable of supporting infection with PRRSV type 1 strain LV (Lelystad virus). Siglec-10 was demonstrated to be involved in the endocytosis of PRRSV, confirming the important role of Siglec-10 in the entry process of PRRSV. In conclusion, it can be stated that PRRSV may use several Siglecs to enter macrophages, which may explain the strain differences in the pathogenesis.

A recombinant type 2 porcine reproductive and respiratory syndrome virus between NADC30-like and a MLV-like: Genetic characterization and pathogenicity for piglets

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important pathogen for swine industry worldwide. The recombination occurring among PRRSV strains has been recognized as one of important molecular mechanisms for the evolution of PRRSV. Current prevalence of PRRSV NADC30-like causing clinical disease outbreaks is highly concerned in China. In the present study, the genetic characterization of a recombinant type 2 PRRSV (designated TJnh1501) was analyzed and its pathogenicity for piglets was examined. Our study showed that each region of TJnh1501 genome had 96.67-100% nucleotide and 96.5-100% amino acid identities with a Chinese highly pathogenic PRRSV-derived modified-live virus (MLV)-like except for its nonstructural protein 2 (nsp2)-coding region; while its nsp2-coding region shared higher nucleotide (84.44-85.85%) and amino acid (82.44-84.79%) identities with NADC30 and NADC30-like CHsx1401, and in particular, the highly variable region of nsp2 exhibited characteristic 131-aa deletion identical to NADC30 and NADC30-like CHsx1401. Meanwhile, we identified two recombination breakpoints located in the nt1737 and nt3506 of nsp2-coding region, which had higher nucleotide homology with NADC30 and NADC30-like CHsx1401. Moreover, TJnh1501 infection could cause persistent fever, moderate respiratory clinical signs, higher viremia, and obvious gross and microscopic lung lesions in piglets. The virus was shown to have lower pathogenicity than HP-PRRSV JXwn06, but higher than NADC30-like CHsx1401 for piglets. Our findings reveal that TJnh1501 is a recombinant type 2 PRRSV from the recombinant event between NADC30-like and MLV-like derived from the Chinese highly pathogenic PRRSV, and it exhibits intermediate virulence for pigs. This study adds valuable evidence for understanding the role of genomic recombination in the evolution of PRRSV.

Interleukin-2 enhancer binding factor 2 interacts with the nsp9 or nsp2 of porcine reproductive and respiratory syndrome virus and exerts negatively regulatory effect on the viral replication

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) causes reproductive failures in sows and respiratory diseases in growing pigs, resulting in huge economic loss for the pig production worldwide. The nonstructural protein 9 (nsp9) and nonstructural protein 2 (nsp2) of PRRSV are known to play important roles in viral replication. Cellular interleukin-2 enhancer binding factor 2 (ILF2) participates in many cellular pathways and involves in life cycle of some viruses. In the present study, we analyzed the interaction of cellular ILF2 with the nsp9 and nsp2 of PRRSV in vitro and explored the effect of ILF2 on viral replication.

Methods

The interaction of ILF2 with the nsp9 or nsp2 of PRRSV was analyzed in 293FT cells and MARC-145 cells by co-immunoprecipitation (Co-IP) and the co-localization of ILF2 with the nsp9 or nsp2 of PRRSV in MARC-145 cell and pulmonary alveolar macrophages (PAMs) was examined by confocal immunofluorescence assay. The effect of ILF2 knockdown and over-expression on PRRSV replication was explored in MARC-145 cells by small interfering RNA (siRNA) and lentivirus transduction, respectively.

Results

The interaction of ILF2 with nsp9 or nsp2 was first demonstrated in 293FT cells co-transfected with ILF2-expressing plasmid and nsp9-expressing plasmid or nsp2-expressing plasmid. The interaction of endogenous ILF2 with the nsp9 or nsp2 of PRRSV was further confirmed in MARC-145 cells transduced with GFP-nsp9-expressing lentiviruses or infected with PRRSV JXwn06. The RdRp domain of nsp9 was shown to be responsible for its interaction with ILF2, while three truncated nsp2 were shown to interact with ILF2. Moreover, we observed that ILF2 partly translocated from the nucleus to the cytoplasm and co-localized with nsp9 and nsp2 in PRRSV-infected MARC-145 cells and PAMs. Finally, our analysis indicated that knockdown of ILF2 favored the replication of PRRSV, while over-expression of ILF2 impaired the viral replication in MARC-145 cells.

Conclusion

Our findings are the first to confirm that the porcine ILF2 interacts with the nsp9 and nsp2 of PRRSV in vitro, and exerts negatively regulatory effect on the replication of PRRSV. Our present study provides more evidence for understanding the roles of the interactions between cellular proteins and viral proteins in the replication of PRRSV.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0794-5) contains supplementary material, which is available to authorized users.

Porcine reproductive and respiratory syndrome virus (PRRSV) in pig meat

Porcine reproductive and respiratory syndrome, caused by the porcine reproductive and respiratory syndrome virus (PRRSV), is an economically important disease in the swine industry. Previous studies demonstrated the presence of the virus in pig meat and its transmissibility by oral consumption. This study further analyzed the infectivity of PRRSV in commercial pig meat. Fresh bottom meat pieces (n = 1500) randomly selected over a period of 2 y from a pork ham boning plant located in Quebec, Canada, were tested by reverse transcriptase polymerase chain reaction (RT-PCR). Each trimmed meat was stored in the plant freezer, subsampled weekly for up to 15 wk, and tested with quantitative RT-PCR to determine the viral load. Meat infectivity was evaluated using specific pathogen-free piglets, each fed with approximately 500 g of meat at the end of the storage time. Genotype-specific RT-PCR confirmed the presence of PRRSV mainly during cold weather in 0.73% of the fresh meat pieces. Wild and vaccine strains of genotype 2 were detected. Porcine reproductive and respiratory syndrome virus nucleic acid was stable in meat stored at around -20°C during the 15 wk. Serological and molecular analysis showed the transmission of infection by a majority of PRRSV positive meat pieces (5/9) fed orally to naïve recipients. The results confirmed a low prevalence of PRRSV in market's pig meat, and virus transmissibility by oral consumption to naïve recipients even after several weeks of storage in a commercial freezer. It occurred mainly with meat harboring the highest PRRSV RNA copies, in the range of 109 copies per 500 g of meat, with both wild type and vaccine-related strains.

Emergence of mosaic recombinant strains potentially associated with vaccine JXA1-R and predominant circulating strains of porcine reproductive and respiratory syndrome virus in different provinces of China

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused several outbreaks in China since 2006. However, the genetic diversity of PRRSV in China has greatly increased by rapid evolution or recombination events. Modified live-attenuated vaccines are widely used to control this disease worldwide. Although the risk and inefficacy of the vaccine has been reported, the genetic diversity between epidemic field strains and vaccine strains in China has not been completely elucidated.

Methods

A total of 293 clinical samples were collected from 72 pig farms in 16 provinces of China in 2015 for PRRSV detection. A total of 28 infected samples collected from 24 pig farms in nine provinces were further selected for immunohistochemical analysis and whole genome sequencing of PRRSV. Phylogenetic analysis and recombination screening were performed with the full genome sequences of the 28 strains and other 623 reference sequences of PRRSV.

Results

Of 293 clinical samples, 117 (39.93%) were positive for PRRSV by RT-PCR. Phylogenetic results showed that the 28 strains were nested into sublineage 10.5 (classic highly pathogenic [HP]-PRRSV), sublineage 10.6 (HP-PRRSV-like strains and related recombinants), sublineage 10.7 (potential vaccine JXA1-R-like strains), and lineage 9 (NADC30-like strains and recombinants of NADC30-like strains), respectively, suggesting that multiple subgenotypes of PRRSV currently circulate in China. Recombination analyses showed that nine of 28 isolates and one isolate from other laboratory were potential complicated recombinants between the vaccine JXA1-R-like strains and predominant circulating strains.

Conclusions

These results indicated an increase in recombination rates of PRRSV under current vaccination pressure and a more pressing situation for PRRSV eradication and control in China.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0735-3) contains supplementary material, which is available to authorized users.

Construction and immunogenicity of a recombinant swinepox virus expressing a multi-epitope peptide for porcine reproductive and respiratory syndrome virus

To characterize neutralizing mimotopes, phages were selected from a 12-mer phage display library using three anti-porcine reproductive and respiratory syndrome virus (PRRSV) neutralizing monoclonal antibodies: (1) A1; (2) A2; and (3) A7. Of these, A2 and A7 recognize the mimotope, P2, which contains the SRHDHIH motif, which has conserved consensus sequences from amino acid positions 156 to 161 in the N-terminal ectodomain of GP3. The artificial multi-epitope gene, mp2, was designed by combining three repeats of the mimotope P2. The resulting sequence was inserted into the swinepox virus (SPV) genome to construct a recombinant swinepox virus (rSPV-mp2). The rSPV-mp2 was able to stably express the multi-epitope peptide, mP2, in vitro. The rSPV-mp2 immunized pigs exhibited a significantly shorter fever duration compared with the wtSPV treated group (P < 0.05). There was an enhanced humoral and cellular immune response, decreased number of PRRSV genomic copies, and a significant reduction in the gross lung pathology (P < 0.05) was observed following PRRSV infection in rSPV-mp2-immunized animals. The results suggest that the recombinant rSPV-mp2 provided pigs with significant protection against PRRSV infection.

Cross reactivity of immune responses to porcine reproductive and respiratory syndrome virus infection

Because porcine reproductive and respiratory syndrome virus (PRRSV) exhibits extensive genetic variation among field isolates, characterizing the extent of cross reactivity of immune responses, and most importantly cell-mediated immunity (CMI), could help in the development of broadly cross-protective vaccines. We infected 12 PRRSV-naïve pigs with PRRSV strain FL12 and determined the number of interferon (IFN)-γ secreting cells (SC) by ELISpot assay using ten type 2 and one type 1 PRRSV isolates as recall antigens. The number of IFN-γ SC was extremely variable among animals, and with exceptions, late to appear. Cross reactivity of IFN-γ SC among type 2 isolates was broad, and we found no evidence of an association between increased genetic distance among isolates and the intensity of the CMI response. Comparable to IFN-γ SC, total antibodies evaluated by indirect immunofluorescence assay (IFA) were cross reactive, however, neutralizing antibody titers could only be detected against the strain used for infection. Finally, we observed a moderate association between homologous IFN-γ SC and neutralizing antibodies.

Heterogeneous antigenic properties of the porcine reproductive and respiratory syndrome virus nucleocapsid

Porcine reproductive and respiratory syndrome virus (PRRSV) is an arterivirus responsible for a widespread contagious disease of domestic pigs with high economic impact. Switzerland is one of the rare PRRSV-free countries in Europe, although sporadic outbreaks have occurred in the past. The PRRSV isolate IVI-1173 from the short outbreak in Switzerland in 2012 was entirely sequenced, and a functional full-length cDNA clone was constructed. Genetic and antigenic characterization of IVI-1173 revealed the importance of amino acid 90 of the nucleocapsid protein N as part of a conformational epitope. IVI-1173 was not detected by SDOW17, a monoclonal antibody against N widely used to detect PRRSV-infected cells. Substitution of alanine at position 90 of N [N(A₉₀)] with a threonine [N(T₉₀)] restored reactivity of vIVI1173-N(T₉₀) to SDOW17 completely. The relevance of this amino acid for the conformational SDOW17 epitope of PRRSV N was further confirmed by the opposite substitution in a functional cDNA clone of the genotype 2 isolate RVB-581. Finally, N proteins from ten genotype 1 strains differing from threonine at position 90 were analysed for reactivity with SDOW17. N(A₉₀) totally disrupted or severely affected the epitope in 7 out of 8 strains tested. Based on these findings, 225 genotype 1 strains were screened for the prevalence of N(A₉₀). N(A₉₀) is rare in classical subtype 1 and in subtype 3 strains, but is frequent in Russian subtype 1 (70%) and in subtype 2 (45%) isolates. In conclusion, this study highlights the variable antigenic properties of N among genotype 1 PRRSV strains.

A Simple and Rapid Chromatographic Strip Test for Detection of Antibody to Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a major economic problem for swine industries worldwide despite several disease-reduction strategies such as age-segregated early weaning and all-in-all-out pig movement. Routine diagnosis of PRRSV is carried out by the combined use of an antibody-detecting enzyme-linked immunosorbent assay (ELISA), immunofluorescence, reverse transcription-polymerase chain reaction, and virus isolation. These assays require specialized laboratory equipment in addition to multistep sample handling and sample preparation. The objective of this study was to evaluate a simple pen-side assay (BioSign™ PRRSV) for rapid detection of PRRSV antibody based on a lateral flow chromatographic strip immunoassay system. This assay uses Escherichia coli–expressed viral nucleocapsid protein antigen for detecting antibodies against PRRSV in swine sera. In this report, the authors describe the evaluation of this assay using sera from both clinical samples and experimentally infected piglets. The results were compared with those of a standard, commercially available antibody ELISA (HerdChek®PRRS ELISA) and an indirect immunofluorescence assay using the same serum samples. The BioSign™ PRRSV assay was capable of detecting antibodies in sera known to contain antibodies to PRRSV, resulting in 93.2% sensitivity for samples from experimentally infected pigs and 98.7% sensitivity for clinical serum samples. For sera that did not contain antibodies to PRRSV, the specificity was found to be 98.5% and 99.2% for clinical and experimental serum samples, respectively.

Development of monoclonal antibodies and serological assays including indirect ELISA and fluorescent microsphere immunoassays for diagnosis of porcine deltacoronavirus

Background

A novel porcine deltacoronavirus (PDCoV), also known as porcine coronavirus HKU15, was reported in China in 2012 and identified in the U.S. in early 2014. Since then, PDCoV has been identified in a number of U.S. states and linked with clinical disease including acute diarrhea and vomiting in the absence of other identifiable pathogens. Since PDCoV was just recently linked with clinical disease, few specific antibody-based reagents were available to assist in diagnosis of PDCoV and limited serological capabilities were available to detect an antibody response to this virus. Therefore, the overall objective of this project was to develop and validate selected diagnostic reagents and assays for PDCoV antigen and antibody detection.

Results

The nucleoprotein of PDCoV was expressed as a recombinant protein and purified for use as an antigen to immunize mice for polyclonal, hyperimmune sera and monoclonal antibody (mAb) production. The resulting mAbs were evaluated for use in fluorescent antibody staining methods to detect PDCoV infected cells following virus isolation attempts and for immunohistochemistry staining of intestinal tissues of infected pigs. The same antigen was used to develop serological tests to detect the antibody response to PDCoV in pigs following infection. Serum samples from swine herds with recent documentation of PDCoV infection and samples from expected naïve herds were used for initial assay optimization. The tests were optimized in a checkerboard fashion to reduce signal to noise ratios using samples of known status. Statistical analysis was performed to establish assay cutoff values and assess diagnostic sensitivities and specificities. At least 629 known negative serum samples and 311 known positive samples were evaluated for each assay. The enzyme linked immunosorbent assay (ELISA) showed diagnostic sensitivity (DSe) of 96.1 % and diagnostic specificity (DSp) of 96.2 %. The fluorescent microsphere immunoassay (FMIA) showed a DSe of 95.8 % and DSp of 98.1 %. Both ELISA and FMIA detected seroconversion of challenged pigs between 8–14 days post-infection (DPI). An indirect fluorescent antibody (IFA) test was also developed using cell culture adapted PDCoV for comparative purposes.

Conclusion

These new, specific reagents and serological assays will allow for improved diagnosis of PDCoV. Since many aspects of PDCoV infection and transmission are still not fully understood, the reagents and assays developed in this project should provide valuable tools to help understand this disease and to aid in the control and surveillance of porcine deltacoronavirus outbreaks.

The Chinese highly pathogenic porcine reproductive and respiratory syndrome virus infection suppresses Th17 cells response in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to immunomodulate innate and adaptive immunity of pigs. The Chinese highly pathogenic PRRSV (HP-PRRSV) infection causes severe bacterial secondary infection in pigs. However, the mechanism in relation to the bacterial secondary infection induced by HP-PRRSV remains unknown. In the present study, Th17 cells response in peripheral blood, lungs, spleens and lymph nodes of piglets were analyzed, and bacterial loads in lungs of piglets were examined upon HP-PRRSV infection. Meanwhile the changes of CD4(+) and CD8(+) T cells in peripheral blood of the inoculated piglets were analyzed. The results showed that HP-PRRSV-inoculated piglets exhibited a suppressed Th17 cells response in peripheral blood and a reduced number of Th17 cells in lungs, and higher bacterial loads in lungs, compared with low pathogenic PRRSV. Moreover, HP-PRRSV obviously resulted in severe depletion of porcine T cells in peripheral blood at the early stage of infection. These findings indicate that HP-PRRSV infection suppresses the response of Th17 cells that play an important role in combating bacterial infections, suggesting a possible correlation between the suppression of Th17 cells response in vivo and bacterial secondary infection induced by HP-PRRSV. Our present study adds a novel insight into better understanding of the pathogenesis of the Chinese HP-PRRSV.

Porcine Reproductive and Respiratory Syndrome Virus Utilizes Nanotubes for Intercellular Spread

Intercellular nanotube connections have been identified as an alternative pathway for cellular spreading of certain viruses. In cells infected with porcine reproductive and respiratory syndrome virus (PRRSV), nanotubes were observed connecting two distant cells with contiguous membranes, with the core infectious viral machinery (viral RNA, certain replicases, and certain structural proteins) present in/on the intercellular nanotubes. Live-cell movies tracked the intercellular transport of a recombinant PRRSV that expressed green fluorescent protein (GFP)-tagged nsp2. In MARC-145 cells expressing PRRSV receptors, GFP-nsp2 moved from one cell to another through nanotubes in the presence of virus-neutralizing antibodies. Intercellular transport of viral proteins did not require the PRRSV receptor as it was observed in receptor-negative HEK-293T cells after transfection with an infectious clone of GFP-PRRSV. In addition, GFP-nsp2 was detected in HEK-293T cells cocultured with recombinant PRRSV-infected MARC-145 cells. The intercellular nanotubes contained filamentous actin (F-actin) with myosin-associated motor proteins. The F-actin and myosin IIA were identified as coprecipitates with PRRSV nsp1β, nsp2, nsp2TF, nsp4, nsp7-nsp8, GP5, and N proteins. Drugs inhibiting actin polymerization or myosin IIA activation prevented nanotube formation and viral clusters in virus-infected cells. These data lead us to propose that PRRSV utilizes the host cell cytoskeletal machinery inside nanotubes for efficient cell-to-cell spread. This form of virus transport represents an alternative pathway for virus spread, which is resistant to the host humoral immune response.

IMPORTANCE

Extracellular virus particles transmit infection between organisms, but within infected hosts intercellular infection can be spread by additional mechanisms. In this study, we describe an alternative pathway for intercellular transmission of PRRSV in which the virus uses nanotube connections to transport infectious viral RNA, certain replicases, and certain structural proteins to neighboring cells. This process involves interaction of viral proteins with cytoskeletal proteins that form the nanotube connections. Intercellular viral spread through nanotubes allows the virus to escape the neutralizing antibody response and may contribute to the pathogenesis of viral infections. The development of strategies that interfere with this process could be critical in preventing the spread of viral infection.

The Attenuation Phenotype of a Ribavirin-Resistant Porcine Reproductive and Respiratory Syndrome Virus Is Maintained during Sequential Passages in Pigs

In a previous study, ribavirin-resistant porcine reproductive and respiratory syndrome virus (PRRSV) mutants (RVRp13 and RVRp22) were selected, and their resistance against random mutation was shown in cultured cells. In the present study, these ribavirin-resistant mutants were evaluated in terms of their genetic and phenotypic stability during three pig-to-pig passages in comparison with modified live virus (MLV) (Ingelvac PRRS MLV). Pigs challenged with RVRp22 had significantly lower (P< 0.05) viral loads in sera and tissues than pigs challenged with MLV or RVRp13 at the first passage, and the attenuated replication of RVRp22 was maintained until the third passage. Viral loads in sera and tissues dramatically increased in pigs challenged with MLV or RVRp13 during the second passage. Consistently, all five sequences associated with the attenuation of virulent PRRSV in RVRp13 and MLV quickly reverted to wild-type sequences during the passages, but two attenuation sequences were maintained in RVRp22 even after the third passage. In addition, RVRp22 showed a significantly lower (P< 0.001) mutation frequency in nsp2, which is one of the most variable regions in the PRRSV genome, than MLV. Nine unique mutations were found in open reading frames (ORFs) 1a, 2, and 6 in the RVRp22 genome based on full-length sequence comparisons with RVRp13, VR2332 (the parental virus of RVRp13 and RVRp22), and MLV. Based on these results, it was concluded that RVRp22 showed attenuated replication in pigs; further, because of the high genetic stability of RVRp22, its attenuated phenotype was stable even after three sequential passages in pigs.

IMPORTANCE

PRRSV is a rapidly evolving RNA virus. MLV vaccines are widely used to control PRRS; however, there have been serious concerns regarding the use of MLV as a vaccine virus due to the rapid reversion to virulence during replication in pigs. As previously reported, ribavirin is an effective antiviral drug against many RNA viruses. Ribavirin-resistant mutants reemerged by escaping lethal mutagenesis when the treatment concentration was sublethal, and those mutants were genetically more stable than parental viruses. In a previous study, two ribavirin-resistant PRRSV mutants (RVRp13 and RVRp22) were selected, and their higher genetic stability was shown in vitro Consequently, in the present study, both of the ribavirin-resistant mutants were evaluated in terms of their genetic and phenotypic stability in vivo RVRp22 was found to exhibit higher genetic and phenotypic stability than MLV, and nine unique mutations were identified in the RVRp22 genome based on a full-length sequence comparison with the RVRp13, VR2332, and MLV genomes.

Mutations in a Highly Conserved Motif of nsp1β Protein Attenuate the Innate Immune Suppression Function of Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) nonstructural protein 1β (nsp1β) is a multifunctional viral protein, which is involved in suppressing the host innate immune response and activating a unique -2/-1 programmed ribosomal frameshifting (PRF) signal for the expression of frameshifting products. In this study, site-directed mutagenesis analysis showed that the R128A or R129A mutation introduced into a highly conserved motif ((123)GKYLQRRLQ(131)) reduced the ability of nsp1β to suppress interferon beta (IFN-β) activation and also impaired nsp1β's function as a PRF transactivator. Three recombinant viruses, vR128A, vR129A, and vRR129AA, carrying single or double mutations in the GKYLQRRLQ motif were characterized. In comparison to the wild-type (WT) virus, vR128A and vR129A showed slightly reduced growth abilities, while the vRR129AA mutant had a significantly reduced growth ability in infected cells. Consistent with the attenuated growth phenotype in vitro, pigs infected with nsp1β mutants had lower levels of viremia than did WT virus-infected pigs. Compared to the WT virus in infected cells, all three mutated viruses stimulated high levels of IFN-α expression and exhibited a reduced ability to suppress the mRNA expression of selected interferon-stimulated genes (ISGs). In pigs infected with nsp1β mutants, IFN-α production was increased in the lungs at early time points postinfection, which was correlated with increased innate NK cell function. Furthermore, the augmented innate response was consistent with the increased production of IFN-γ in pigs infected with mutated viruses. These data demonstrate that residues R128 and R129 are critical for nsp1β function and that modifying these key residues in the GKYLQRRLQ motif attenuates virus growth ability and improves the innate and adaptive immune responses in infected animals.

IMPORTANCE

PRRSV infection induces poor antiviral innate IFN and cytokine responses, which results in weak adaptive immunity. One of the strategies in next-generation vaccine construction is to manipulate viral proteins/genetic elements involved in antagonizing the host immune response. PRRSV nsp1β was identified to be a strong innate immune antagonist. In this study, two basic amino acids, R128 and R129, in a highly conserved GKYLQRRLQ motif were determined to be critical for nsp1β function. Mutations introduced into these two residues attenuated virus growth and improved the innate and adaptive immune responses of infected animals. Technologies developed in this study could be broadly applied to current commercial PRRSV modified live-virus (MLV) vaccines and other candidate vaccines.