A 10-kDa structural protein of porcine reproductive and respiratory syndrome virus encoded by ORF2b

An Expeditious Neutralization Assay for Porcine Reproductive and Respiratory Syndrome Virus Based on a Recombinant Virus Expressing Green Fluorescent Protein

Due to the extensive genetic and antigenic variation in Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), as well as its rapid mutability and evolution, PRRS prevention and control can be challenging. An expeditious and sensitive neutralization assay for PRRSV is presented to monitor neutralizing antibodies (NAbs) in serum during vaccine research. Here, a PRRSV expressing eGFP was successfully rescued with reverse genetics based on the infectious clone HuN4-F112-eGFP which we constructed. The fluorescent protein expressions of the reporter viruses remained stable for at least five passages. Based on this reporter virus, the neutralization assay can be easily used to evaluate the level of NAbs by counting cells with green fluorescence. Compared with the classical CPE assay, the newly developed assay increases sensitivity by one- to four-fold at the early antibody response stage, thus saving 2 days of assay waiting time. By using this assay to unveil the dynamics of neutralizing antibodies against PRRSV, priming immunity through either a single virulent challenge or only vaccination could produce limited NAbs, but re-infection with PRRSV would induce a faster and stronger NAb response. Overall, the novel HuN4-F112-eGFP-based neutralization assay holds the potential to provide a highly efficient platform for evaluating the next generation of PRRS vaccines.

Genetic background influences pig responses to porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly infectious and economically significant virus that causes respiratory and reproductive diseases in pigs. It results in reduced productivity and increased mortality in pigs, causing substantial economic losses in the industry. Understanding the factors affecting pig responses to PRRSV is crucial to develop effective control strategies. Genetic background has emerged as a significant determinant of susceptibility and resistance to PRRSV in pigs. This review provides an overview of the basic infection process of PRRSV in pigs, associated symptoms, underlying immune mechanisms, and roles of noncoding RNA and alternative splicing in PRRSV infection. Moreover, it emphasized breed-specific variations in these aspects that may have implications for individual treatment options.

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.

Research progress on the E protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is an economically important disease impacting the global pig industry, and it is characterized by reproductive disorder in sows and respiratory disorder in pigs of all ages. The PRRSV E protein is a nonglycosylated structural protein encoded by the ORF2b gene. The E protein is not necessary for the assembly of virus particles, but deletion of the E protein leads to transmissible virus particles not being produced. To better understand the structure and function of the E protein, we reviewed its genetic and evolutionary analysis, characteristics, subcellular localization and topology, ion channel activity, cellular immune response, additional biological functions, interactions with host proteins, interactions with PRRSV proteins, roles in infection, pathogenicity, and drugs. Therefore, this review can provide a theoretical basis for gaining an in-depth understanding of the E protein of PRRSV-2.

Simultaneous expression of three reporter proteins from a porcine reproductive and respiratory syndrome virus-based vector

The mechanism of discontinuous transcription for the synthesis of a series of sub-genomic mRNAs to express the structural proteins of porcine reproductive and respiratory syndrome virus (PRRSV) potentially allows for the simultaneous expression of multiple foreign genes. This can occur by insertion of multiple novel independent transcription units between the ORF sequences of the PRRSV genome. Here, an expression cassette consisting of a red fluorescent protein (RFP) gene flanked at its 3' end by transcription-regulating sequences (TRS) and an expression cassette consisting of an iLOV gene flanked at its 5' end by TRS, was constructed. The resulting expression cassette containing a RFP and an iLOV gene were introduced between ORF1b and 2 as well as ORF7 and 3'UTR, respectively, in an infectious PRRSV cDNA clone. Transfection of the resulting clone (pGX-12RFP-73iLOV) into cells resulted in the recovery of a recombinant virus (rGX-12RFP-73iLOV). Simultaneous expression of RFP and iLOV was observed in MARC-145 cells infected with rGX-RFP-iLOV. To test the ability of the PRRSV genome to express all three reporter genes simultaneously, an expression cassette containing the Gluc gene and one containing the iLOV gene were also inserted in between ORF1b and 2 as well as ORF7 and 3'UTR, respectively. This was performed in a recently obtained infectious PRRSV cDNA clone carrying a RFP gene in nsp2. Transfection of the construct (pGX-R-Gluc-iLOV) carrying the three reporter genes into cells allowed the rescue of the recombinant reporter virus (rGX-R-Gluc-iLOV) which showed similar growth characteristics to the parental virus but yielded 100-fold less infectious viruses. Fluorescence microscopy of cells infected with rGX-R-Gluc-iLOV demonstrated the presence of both RFP and iLOV genes. Gluc activities in supernatants harvested at different time points from cells infected with recombinant viruses carrying Gluc showed increased levels of Gluc activity as the infection progressed. This indicated that Gluc gene as well as its activity were acceptable parameters to monitor viral propagation. Our results indicate that it is possible to introduce at least three foreign proteins simultaneously in a PRRSV-based vector and such studies will prove invaluable in our future understanding of these viruses.

Genetic characterization of porcine reproductive and respiratory syndrome virus from Eastern China during 2017-2022

Porcine reproductive and respiratory syndrome (PRRS) is an immunosuppressive disease caused by PRSS virus (PRRSV). PRRSV mainly causes reproductive disorders in pregnant sows and respiratory diseases in piglets. Recently, it has emerged as one of the most important diseases of the pig industry across the globe. In this study, we have collected 231 samples from differently sized pig farms in Eastern China from 2017 to 2022 to investigate the epidemic characteristics of the disease. All samples were screened by RT-PCR and analyzed further using Nsp2 and ORF5 genes. The result showed that the positive rate of PRRSV was 24% (54/231). Phylogenetic analysis (13 positive samples) revealed that all isolates belonged to genotype 2, and they were mainly distributed in four lineages (i.e., lineage 1, 3, 5, and 8). Nsp2 is the most variable protein among all PRRSV NSPs, several isolates from this study had amino acid deletions within Nsp2 compared to that of strain VR-2332. The major structural protein glycoprotein (GP5) protein is encoded by ORF5. Epitope analysis of the 13 isolated strains and additional reference strains revealed that all 13 strains had some mutations on the decoy epitope, the primary neutralizing epitope, T cell epitopes, and B cell epitopes. This study showed that the prevalent PRRSV strain in Eastern China was still HP-PRRSV, while the proportion of NADC30-like and NADC34-like strains have increased. This study further enriches the epidemiological data of PRRS in Eastern China and provides a theoretical basis for vaccine development and prevention and control of the disease across the region.

Heterologous vaccine immunogenicity, efficacy, and immune correlates of protection of a modified-live virus porcine reproductive and respiratory syndrome virus vaccine

Although porcine reproductive and respiratory syndrome virus (PRRSV) vaccines have been available in North America for almost 30 years, many vaccines face a significant hurdle: they must provide cross-protection against the highly diverse PRRSV strains. This cross-protection, or heterologous vaccine efficacy, relies greatly on the vaccine’s ability to induce a strong immune response against various strains—heterologous immunogenicity. Thus, this study investigated vaccine efficacy and immunogenicity of a modified live virus (MLV) against four heterologous type 2 PRRSV (PRRSV-2) strains. In this study, 60 pigs were divided into 10 groups. Half were MOCK-vaccinated, and the other half vaccinated with the Prevacent® PRRS MLV vaccine. Four weeks after vaccination, groups were challenged with either MOCK, or four PRRSV-2 strains from three different lineages—NC174 or NADC30 (both lineage 1), VR2332 (lineage 5), or NADC20 (lineage 8). Pre-and post-challenge, lung pathology, viral loads in both nasal swabs and sera, anti-PRRSV IgA/G, neutralizing antibodies, and the PRRSV-2 strain-specific T-cell response were evaluated. At necropsy, the lung samples were collected to assess viral loads, macroscopical and histopathological findings, and IgA levels in bronchoalveolar lavage. Lung lesions were only induced by NC174, NADC20, and NADC30; within these, vaccination resulted in lower gross and microscopic lung lesion scores of the NADC20 and NADC30 strains. All pigs became viremic and vaccinated pigs had decreased viremia upon challenge with NADC20, NADC30, and VR2332. Regarding vaccine immunogenicity, vaccination induced a strong systemic IgG response and boosted the post-challenge serum IgG levels for all strains. Furthermore, vaccination increased the number of animals with neutralizing antibodies against three of the four challenge strains—NADC20, NADC30, and VR2332. The heterologous T-cell response was also improved by vaccination: Not only did vaccination increase the induction of heterologous effector/memory CD4 T cells, but it also improved the heterologous CD4 and CD8 proliferative and/or IFN-γ response against all strains. Importantly, correlation analyses revealed that the (non-PRRSV strain-specific) serum IgG levels and the PRRSV strain-specific CD4 T-cell response were the best immune correlates of protection. Overall, the Prevacent elicited various degrees of efficacy and immunogenicity against four heterologous and phylogenetically distant strains of PRRSV-2.

Genetic diversity of porcine reproductive and respiratory syndrome virus and evaluation of three one-step real-time RT-PCR assays in Korea

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses in the global swine industry. Frequent genetic variations in this virus cause difficulties in controlling and accurately diagnosing PRRSV.

Methods

In this study, we investigated the genetic characteristics of PRRSV-1 and PRRSV-2 circulating in Korea from January 2018 to September 2021 and evaluated three one-step real-time reverse transcription polymerase chain reaction (RT-PCR) assays.

Results

A total of 129 lung samples were collected, consisting of 47 samples for PRRSV-1, 62 samples for PRRSV-2, and 20 PRRSV-negative samples. Nucleotide sequence analysis of open reading frames (ORFs) 5, ORF6, and ORF7 genes from PRRSV samples showed that PRRSV-1 belonged to subgroup A (43/47, 91.49%) and subgroup C (4/47, 8.51%), whereas PRRSV-2 was classified as lineage 1 (25/62, 40.32%), Korean lineage (Kor) C (13/62, 20.97%), Kor B (10/62, 16.13%), lineage 5 (9/62, 14.52%), and Kor A (5/62, 8.06%). Amino acid sequence analysis showed that the neutralizing epitope and T cell epitope of PRRSV-1, and the decoy epitope region and hypervariable regions of PRRSV-2 had evolved under positive selection pressure. In particular, the key amino acid substitutions were found at positions 102 and 104 of glycoprotein 5 (GP5) in some PRRSV-2, and at positions 10 and 70 of membrane protein (M) in most PRRSV-2. In addition, one-step real-time RT-PCR assays, comprising two commercial tests and one test recommended by the World Organization for Animal Health (OIE), were evaluated.

Conclusion

The results revealed that two of the real-time RT-PCR assays had high sensitivities and specificities, whereas the real-time RT-PCR assay of the OIE had low sensitivity due to mismatches between nucleotides of Korean PRRSVs and forward primers. In this study, we genetically characterized recent PRRSV occurrences and evaluated three one-step real-time RT-PCR assays used in Korea.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03407-0.

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

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

Recombination between the Fostera MLV-like Strain and the Strain Belonging to Lineage 1 of Porcine Reproductive and Respiratory Syndrome Virus in Korea

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in the swine industry worldwide. In Korea, Fostera PRRS commercial modified live virus (MLV) vaccines have been used since 2014 to control the PRRSV infection. In this study, two PRRSV-2 strains (20D160-1 and 21R2-63-1) were successfully isolated, and their complete genomic sequences were determined. Genetic analysis showed that the two isolates have recombination events between the P129-like strain derived from the Fostera PRRS MLV vaccine and the strain of lineage 1. The 20D160-1 indicated that partial ORF2 to partial ORF4 of the minor parental KNU-1902-like strain, which belongs to Korean lineage C (Kor C) of lineage 1, was inserted into the major parental P129-like strain. The 21R2-63-1 revealed that partial ORF1b of the P129-like strain was inserted into the backbone of the NADC30-like strain. This study is the first to report natural recombinant strains between Fostera PRRS MLV-like strain and the field strain in Korea. These results may have significant implications for MLV evolution and the understanding of PRRSV genetic diversity, while highlighting the need for continuous surveillance of PRRSV.

A Novel Motif in the 3′-UTR of PRRSV-2 Is Critical for Viral Multiplication and Contributes to Enhanced Replication Ability of Highly Pathogenic or L1 PRRSV

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) with enhanced replication capability emerged in China and has become dominant epidemic strain since 2006. Up to now, the replication-regulated genes of PRRSV have not been fully clarified. Here, by swapping the genes or elements between HP-PRRSV and classical PRRSV based on infectious clones, NSP1, NSP2, NSP7, NSP9 and 3′-UTR are found to contribute to the high replication efficiency of HP-PRRSV. Further study revealed that mutations at positions 117th or 119th in the 3′-UTR are significantly related to replication efficiency, and the nucleotide at position 120th is critical for viral rescue. The motif composed by 117–120th nucleotides was quite conservative within each lineage of PRRSV; mutations in the motif of HP-PRRSV and currently epidemic lineage 1 (L1) PRRSV showed higher synthesis ability of viral negative genomic RNA, suggesting that those mutations were beneficial for viral replication. RNA structure analysis revealed that this motif maybe involved into a pseudoknot in the 3′-UTR. The results discovered a novel motif, 117–120th nucleotide in the 3′-UTR, that is critical for replication of PRRSV-2, and mutations in the motif contribute to the enhanced replicative ability of HP-PRRSV or L1 PRRSV. Our findings will help to understand the molecular basis of PRRSV replication and find the potential factors resulting in an epidemic strain of PRRSV.

The Local and Systemic Humoral Immune Response Against Homologous and Heterologous Strains of the Type 2 Porcine Reproductive and Respiratory Syndrome Virus

The humoral immune response plays a crucial role in the combat and protection against many pathogens including the economically most important, highly prevalent, and diverse pig pathogen PRRSV – the Porcine Reproductive and Respiratory Syndrome Virus. In addition to viremia and viral shedding analyses, this study followed the local and systemic humoral immune response of pigs for 63 days upon inoculation with one of three types of Type-2 PRRSV (PRRSV-2) strains – one modified live virus (MLV) vaccine strain, and two lineage 1 PRRSV-2 strains, NC134 and NC174. The local response was analyzed by quantifying immunoglobulin (Ig)A in nasal swabs. The systemic response was studied by the quantification of IgG with ELISA and homo- and heterologous neutralizing antibodies (NAs) utilizing a novel method of flow cytometry. In all PRRSV-2 inoculated groups, viral nasal shedding started at 3 dpi, peaked between 3 and 7 days post inoculation, and was cleared at 28–35 dpi with sporadic rebounds thereafter. The local IgA response started 4–7 days after viral shedding occurred and showed a bi-phasic course with peaks at 14 dpi and at 28–35 dpi. Of note, the NC134 and NC174 strains induced a much stronger local IgA response. As reported earlier, main viremia lasted from 7 dpi to 28 dpi (NC174), 42 dpi (NC134) or until the end of the study (MLV). Similar to the local IgA response, the systemic IgG response started 4–7 days after viremia; but in contrast to viremia, serum IgG levels stayed high for all PRRSV-2 inoculated groups until the end of the study. A significant finding was that while the serum NA response in the MLV group was delayed by 28 days, serum NAs in pigs infected with our two NC134 and NC174 strains could be detected as early as 7 dpi (NC134) and 14 dpi (NC174). Compared to homologous NA responses, the NA responses against heterologous strains was strong but slightly delayed between our lineage 1 one strains or non-existent between the MLV and lineage 1 strains. This study improves our understanding of the relationship between local and systemic infections and the humoral immune response induced by PRRSV-2 infection or MLV vaccination. Our data also provide novel insights into the timeline of the development of homologous and heterologous NA levels – by both MLV vaccination or infection with two strains from the currently prevalent PRRSV-2 lineage 1.

Newly-designed primer pairs for the detection of type 2 porcine reproductive and respiratory syndrome virus genes

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease, caused by PRRS virus (PRRSV), that critically affects the swine industry. While the detection of PRRSV genes plays a key role in PRRS control, the PRRSV genome is known to undergo frequent mutation. Nevertheless, primer pairs widely used for the detection of PRRSV genes were designed between 1995 and 2010. The reliability of these primer pairs for the detection of currently circulating PRRSVs is therefore questionable. Here, we investigated the sensitivity of the previously reported primer pairs to detect PRRSV genes that have been recently isolated or detected in Japan. In addition, based on nucleotide sequences from the recent Japanese PRRSVs, we designed four new primer pairs for the detection of PRRSV genes. The sensitivity and specificity of the new primer pairs were evaluated by quantitative reverse transcription PCR using RNA extracted from PRRSV isolates, swine serum, and oral fluid specimens collected from PRRS-affected pigs, and swine sera collected from a PRRSV-free pig farm in Japan. One of novel primer pairs used in our study exhibited greater sensitivity than the previously reported primer pairs, and is thus more reliable for the detection of PRRSV genes.

Maternal Autogenous Inactivated Virus Vaccination Boosts Immunity to PRRSV in Piglets

Maternal-derived immunity is a critical component for the survival and success of offspring in pigs to protect from circulating pathogens such as Type 2 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2). The purpose of this study is to investigate the transfer of anti-PRRSV immunity to piglets from gilts that received modified-live virus (MLV) alone (treatment (TRT) 0), or in combination with one of two autogenous inactivated vaccines (AIVs, TRT 1+2). Piglets from these gilts were challenged with the autogenous PRRSV-2 strain at two weeks of age and their adaptive immune response (IR) was evaluated until 4 weeks post inoculation (wpi). The systemic humoral and cellular IR was analyzed in the pre-farrow gilts, and in piglets, pre-inoculation, and at 2 and 4 wpi. Both AIVs partially protected the piglets with reduced lung pathology and increased weight gain; TRT 1 also lowered piglet viremia, best explained by the AIV-induced production of neutralizing antibodies in gilts and their transfer to the piglets. In piglets, pre-inoculation, the main systemic IFN-γ producers were CD21α⁺ B cells. From 0 to 4 wpi, the role of these B cells declined and CD4 T cells became the primary systemic IFN-γ producers. In the lungs, CD8 T cells were the primary and CD4 T cells were the secondary IFN-γ producers, including a novel subset of porcine CD8α⁻CCR7⁻ CD4 T cells, potentially terminally differentiated CD4 T_EMRA cells. In summary, this study demonstrates that maternal AIV vaccination can improve protection of pre-weaning piglets against PRRSV-2; it shows the importance of transferring neutralizing antibodies to piglets, and it introduces two novel immune cell subsets in pigs—IFN-γ producing CD21α⁺ B cells and CD8α⁻CCR7⁻ CD4 T cells.

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

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

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

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

Evaluation of Antibody Response Directed against Porcine Reproductive and Respiratory Syndrome Virus Structural Proteins

Luciferase-immunoprecipitation system (LIPS), a liquid phase immunoassay, was used to evaluate antibody responses directed against the structural proteins of PRRSV in pigs that were experimentally infected with virulent PRRSV strains. First, the viral N protein was used as a model antigen to validate the assay. The LIPS results were highly comparable to that of the commercial IDEXX PRRS X3 ELISA. Subsequently, the assay was applied to simultaneously measure antibody reactivity against all eight structural proteins of PRRSV. The highest immunoreactivities were detected against GP3, M, and N proteins while the lowest reactivity was detected against ORF5a protein. Comparative analysis of the kinetics of antibody appearance revealed that antibodies specific to N protein appeared earlier than antibodies against GP3. Finally, the assay was applied to measure immunoreactivities of clinical serum samples against N and GP3. The diagnostic sensitivity of the LIPS with N protein was superior to that of the LIPS with GP3. Collectively, the results provide additional information about the host antibody response to PRRSV infection.

Full Genomic Analysis of New Variants of Porcine Reproductive and Respiratory Syndrome Virus Revealed Multiple Recombination Events Between Different Lineages and Sublineages

Porcine reproductive and respiratory syndrome virus (PRRSV) has had a devastating impact on the pig industry in China, and monitoring its genetic diversity is important for epidemiological surveillance and understanding its evolution. Here, we determine the complete genome sequences of two PRRSV strains, GXYL1403 and GXNN1839. Comparative, phylogenetic, and recombination detection program analyses show that the two isolates are recombinant strains with large-fragment amino acid deletions in nsp2. GXYL1403 possesses a unique deletion region of 124 amino acids in nsp2, and GXNN1839 contains a deletion of 131 amino acids in nsp2 as compared with VR2332. Further analysis of the full-length sequence suggests that GXYL1403 is a natural recombinant between sublineages 8.1 (CH-1a like) and 8.3 (JXA1-like). The recombination site of GXYL1403 is located in nsp9–nsp12 (8961nt−11181nt). GXNN1839 is a natural recombinant between the lineage 5 (VR-2332-like) and lineage 1 (NADC30-like) strains. The recombination events occurred in nsp9 (7872nt-8162nt) and in ORF2 (12587nt−13282nt) in the genome of GXNN1839. These results provide new evidence that PRRSV strains circulating in the environment have undergone recombination among the different lineages or sublineages of field strains, and these add to our understanding of RNA combination events that occur in PRRSV.

Host Transcriptional Response to Persistent Infection with a Live-Attenuated Porcine Reproductive and Respiratory Syndrome Virus Strain

Both virulent and live-attenuated porcine reproductive and respiratory syndrome virus (PRRSV) strains can establish persistent infection in lymphoid tissues of pigs. To investigate the mechanisms of PRRSV persistence, we performed a transcriptional analysis of inguinal lymphoid tissue collected from pigs experimentally infected with an attenuated PRRSV strain at 46 days post infection. A total of 6404 differentially expressed genes (DEGs) were detected of which 3960 DEGs were upregulated and 2444 DEGs were downregulated. Specifically, genes involved in innate immune responses and chemokines and receptors associated with T-cell homing to lymphoid tissues were down regulated. As a result, homing of virus-specific T-cells to lymphoid tissues seems to be ineffective, evidenced by the lower frequencies of virus-specific T-cell in lymphoid tissue than in peripheral blood. Genes associated with T-cell exhaustion were upregulated. Likewise, genes involved in the anti-apoptotic pathway were upregulated. Collectively, the data suggested that the live-attenuated PRRSV strain establishes a pro-survival microenvironment in lymphoid tissue by suppressing innate immune responses, T-cell homing, and preventing cell apoptosis.

Hepatitis B core antigen-based vaccine demonstrates cross-neutralization against heterologous North American Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2) strains

The U.S. swine industry have been bearing the financial impact of Porcine Reproductive and Respiratory Syndrome (PRRS) for decades. Absent of a safe and efficacious vaccine to combat PRRS virus's genetic heterogeneity, it remains a costly disease on pig farms across the country. We have developed virus-like-particle (VLP) based vaccines that incorporate 4 PRRSV epitopes in the hepatitis B core antigen (HBcAg) backbone. Administration of the vaccines in female BALB/C mice resulted in extremely significant PRRSV epitope specific antibody response. One vaccine candidate GP3-4 was able to mount a significant viral neutralizing response against both parental PRRSV strain VR2385 and heterologous PRRSV strain NADC20, showing a promising potential for cross-protection against PRRSV.

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

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

Genetic signatures of the immune-escaping type 2 porcine reproductive and respiratory syndrome virus in farms with a robust vaccination program

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important porcine viruses worldwide. Recently, severe PRRS outbreaks had occurred in two farms located in eastern and southern Thailand where stringent vaccination had been routinely practiced. Genetic analysis of GP5 identified two highly virulent PRRSVs designated as NA/TH/S001/2015 and NA/TH/E001/2016 from the southern and eastern farms, respectively. Both incidences were the first outbreaks of severe PRRSV since the implementation of the modified live virus (MLV) vaccine, indicating the concurrent emergence of immune-escape viruses. The genetics of the two PRRSV variants, the previous studied sequences from Thailand, and the reference strains were characterized with a focus on the GP5 and NSP2 genes. The results indicated that NA/TH/S001/2015 and NA/TH/E001/2016 shared less than 87% nucleotide similarity to the MLV and PRRSV type 2, lineages 1 and 8.7 (NA), respectively. A comparative analysis of the retrospective GP5 sequences categorized the PRRSVs into five groups based on the clinical outcomes, and both of the novel PRRSV strains were in the same group. Epitope A, T cell epitope, and N-linked glycosylation patterns within GP5 of both PRRSV variants were highly variable and significantly differed from those of MLV. As observed in highly virulent type 2 strains, NA/TH/S001/2015 contained a single amino acid deletion at position 33 in the hypervariable region 1 (HV-1) of GP5. Amino acid analysis of the hypervariable region of NSP2 revealed that NA/TH/E001/2016 had a unique deletion pattern that included two discontinuous deletions: a 127-amino acid deletion from residues 301 to 427 and a single amino acid deletion at position 470. These results indicate the emergence of two novel PRRSV strains and highlight the common genetic characteristics of the immune-escaping PRRSV variants.

Genetic characterization of a novel recombinant PRRSV2 from lineage 8, 1 and 3 in China with significant variation in replication efficiency and cytopathic effects

There are four major porcine reproductive and respiratory syndrome virus 2 (PRRSV2) lineages circulating in China based on classification system, including lineages 1 (NADC30-like), 3 (QYYZ-like), 5.1 (VR2332-like) and 8 (JXA1-like/CH-1a-Like), which leads to the potential recombination. In the present study, a novel variant of PRRSV2 strain named JS18-3 was isolated from piglets suffering severe breathing difficulties in Jiangsu Province of China in 2018. Full-length genome analysis indicated that JS18-3 shared 86.5%, 87.9%, 84.2%, 82.2% and 86.4% nucleotide similarity with PRRSVs CH-1a, JXA1, VR2332, QYYZ and NADC30, respectively. 4871-6635 of JS18-3 shared the highest identity of 99.3% in nucleotide sequence with HP-PRRSV representative strain JXA1 indicating ongoing evolution to HP-PRRSV. JS18-3 was classified into classical lineage 8 of PRRSV2 based on phylogenetic analysis of complete genome and ORF5. Genomic break points in structural (ORF3) and non-structural (NSP2, NSP3) regions of genomes were detected in recombination analysis. JS18-3 is a recombinant isolate from lineages 8, 1 and 3. Replication enhancement and severe cytopathic effects caused by JS18-3 were observed in Marc-145 cells and porcine alveolar macrophages (PAMs) as compared to JX07, a typical strain of lineage 8. Pathogenicity results indicated that piglets inoculated with JS18-3 presented persistent fever, dyspnoea, serious microscopic lung lesions and lymph node congestion. The study suggests that lineage 8 of PRRSV2 is involved in continuing evolution by genetic recombination and mutation leading to outbreaks in vaccinated pigs in China.

Efficacy Evaluation of Two Commercial Vaccines Against a Recombinant PRRSV2 Strain ZJnb16-2 From Lineage 8 and 3 in China

From 2010, novel recombinant lineage 3 of porcine reproductive and respiratory syndrome virus 2 (PRRSV2) has continuously emerged China, which has brought about clinical outbreaks of the disease. Previously, a PRRSV2 strain named ZJnb16-2 was identified as a recombinant virus from lineage 8 and 3. In this study, two modified-live vaccines VR2332 MLV and HuN4-F112, which belong to lineage 5 and 8 respectively, were used for efficacy evaluation against the challenge of ZJnb16-2. Piglets vaccinated with HuN4-F112 exhibited temporary fever, higher average daily weight gain, and mild clinical signs as compared to VR2332 MLV vaccinated and unvaccinated piglets upon ZJnb16-2 challenge. Both vaccines could inhibit virus replication in piglets at 21days post challenge (DPC). Cross-reactivity of interferon (IFN)-γ secreting cells against ZJnb16-2 were detected in both vaccinated piglets. The number of IFN-γ secreting cells against ZJnb16-2 in the vaccination group exhibited sustaining elevation after challenge. Results demonstrated that both vaccines provided partial protection against ZJnb16-2 infection. A cross-neutralization antibody against ZJnb16-2 was not detected in any vaccinated piglet before challenge. A low neutralizing antibody titer against ZJnb16-2 was detected after challenge. Besides, all the vaccinated piglets suffered from different degrees of lung pathological lesions, indicating neither VR2332 MLV nor HuN4-F112 provided full protection against ZJnb16-2. This study provides valuable guidelines to control the recombinant virus from lineage 8 and 3 infection with MLV vaccines in the field.

A comparison of two commercially available porcine reproductive and respiratory syndrome virus (PRRSV) modified-live virus vaccines analyzing the growth performance in 1-day-old vaccinated swine located on endemic farms co-circulating PRRSV-1 and PRRSV-2

The objective of this study was to compare the efficacy of a porcine reproductive and respiratory syndrome virus (PRRSV)-1 and PRRSV-2 modified-live virus (MLV) vaccines when administered at 1 day of age under field conditions. The piglets elicited anti-PRRSV antibodies at 1 day of age even in the presence of maternally derived antibodies. The number of PRRSV-2 genomic copies in the sera of pigs from the PRRSV-2 MLV-vaccinated pigs was significantly (P<0.05) lower when compared to PRRSV-1 MLV-vaccinated pigs. The average daily gain in PRRSV-2 MLV-vaccinated pigs was significantly (P<0.05) higher when compared to both PRRSV-1 MLV-vaccinated and unvaccinated pigs. This study demonstrated that vaccination as early as 1 day of age was effective against PRRSV infection.

Efficacy of concurrent vaccination with modified-live PRRSV-1 and PRRSV-2 vaccines against heterologous dual PRRSV-1 and PRRSV-2 challenge in late term pregnancy gilts

The objective of this study was to evaluate the effect of concurrent vaccination with a porcine reproductive and respiratory syndrome virus (PRRSV)-1 modified-live virus (MLV) vaccine and a PRRSV-2 MLV vaccine against a dual heterologous PRRSV-1 and PRRSV-2 challenge in late term pregnancy gilts. Gilts were concurrently administered PRRSV-1 and PRRSV-2 MLV vaccines at 21 days prior to breeding at separate anatomical sites and were inoculated intranasally with both PRRSV types at 93 days of gestation. Vaccinated gilts had a higher number of live-born and weaned pigs, and a decrease in stillbirths compared to the unvaccinated control group following a dual challenge. Concurrent vaccination resulted also in the reduction of both PRRSV-1 and PRRSV-2 viremia which correlated with an increase in the number of PRRSV-1 and PRRSV-2 specific interferon-γ secreting cells (IFN-γ-SC). We believe the T cell responses contributed to the reduction of both PRRSV-1 and PRRSV-2 viremia. The results presented here demonstrate that concurrent vaccination with PRRSV-1 and PRRSV-2 MLV vaccines improves reproductive performance, reduces viremia of PRRSV-1 and PRRSV-2, and induces protective T cell reactions against dual PRRSV-1 and PRRSV-2 challenge in late term pregnancy gilts without local and systemic adverse reactions related to concurrent vaccination.

Nsp2 and GP5-M of Porcine Reproductive and Respiratory Syndrome Virus Contribute to Targets for Neutralizing Antibodies

Porcine reproductive and respiratory syndrome virus (PRRSV) is characterized by its genetic variation and limited cross protection among heterologous strains. Even though several viral structural proteins have been regarded as inducers of neutralizing antibodies (NAs) against PRRSV, the mechanism underlying limited cross-neutralization among heterologous strains is still controversial. In the present study, examinations of NA cross reaction between a highly pathogenic PRRSV (HP-PRRSV) strain, JXwn06, and a low pathogenic PRRSV (LP-PRRSV) strain, HB-1/3.9, were conducted with viral neutralization assays in MARC-145 cells. None of the JXwn06-hyperimmuned pigs' sera could neutralize HB-1/3.9 in vitro and vice versa. To address the genetic variation between these two viruses that are associated with limited cross-neutralization, chimeric viruses with coding regions swapped between these two strains were constructed. Viral neutralization assays indicated that variations in nonstructural protein 2 (nsp2) and structural proteins together contribute to weak cross-neutralization activity between JXwn06 and HB-1/3.9. Furthermore, we substituted the nsp2-, glycoprotein2 (GP2)-, GP3-, and GP4-coding regions together, or nsp2-, GP5-, and membrane (M) protein-coding regions simultaneously between these two viruses to construct chimeric viruses to test cross-neutralization reactivity with hyperimmunized sera induced by their parental viruses. The results indicated that the swapped nsp2 and GP5-M viruses increased the neutralization reactivity with the donor strain antisera in MARC-145 cells. Taken together, these results show that variations in nsp2 and GP5-M correlate with the limited neutralization reactivity between the heterologous strains HP-PRRSV JXwn06 and LP-PRRSV HB-1/3.9.

Molecular characterization of type 1 porcine reproductive and respiratory syndrome viruses (PRRSV) isolated in the Netherlands from 2014 to 2016

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of a devastating pig disease present all over the world. The remarkable genetic variation of PRRSV, makes epidemiological and molecular analysis of circulating viruses highly important to review current diagnostic tools and vaccine efficacy. Monitoring PRRS viruses supports modern herd management by explaining the source of found viruses, either internally or externally from the herd. No epidemiological or molecular study has been published on circulating PRRS-viruses in the Netherlands, since the early nineties. Therefore, the objective of this study is to investigate circulating PRRS-viruses in the Netherlands in 2014, 2015 and 2016 on a molecular level by sequencing ORF2, ORF3, ORF4, ORF5, ORF6 and ORF7. The results demonstrate that the 74 PRRSV strains belong to PRRSV-1, but the diversity among strains is high, based on nucleotide identity, individual ORF length and phylogenetic trees of individual ORFs. Furthermore, the data presented here show that the phylogenetic topology of some viruses is ORF dependent and suggests recombination. The identity of the strain of interest might be misinterpreted and wrong conclusions may be drawn in a diagnostic and epidemiological perspective, when only ORF5 is analyzed, as performed in many routine sequencing procedures.

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

Background

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

Results

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

Conclusions

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

Electronic supplementary material

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

Emergence of Two different recombinant PRRSV strains with low neutralizing antibody susceptibility in China

PRRSV causes major economic loss in global swine industry. 41 of 131 (31.29%) tissue samples collected from pig farms in central and east China from 2016 to 2017 were confirmed as PRRSV positive in RT-PCR. Base on phylogenetic analysis for ORF5 and ORF6, 3 isolates closely related to QYYZ strain form a new subgroup IV, while 3 other ones were clustered into subgroup III, represented by NADC30. Numerous amino acid substitutions involved in viral neutralization susceptibility were identified in GP5 among these isolates. Two emerging PRRSV strains (ZJnb16-2, SDbz16-2) were successfully isolated and sequenced. ZJnb16-2 was identified as a recombinant virus between strain QYYZ and JXA1 while SDbz16-2 was an inter-subgenotype recombinant virus of strains NADC30 and JXA1. As shown in the pathogenicity evaluation in piglets, ZJnb16-2 is highly pathogenic while SDbz16-2 is mild. Hyper-immune sera against major vaccine strains HUN4-F112 and JK-100 failed to neutralize either ZJnb16-2 or SDbz16-2. Only 0.8–2.0% of pig serum samples which were confirmed as PRRSV-positive with commercial ELISA kits presented neutralization reactivity against either ZJnb16-2 or SDbz16-2. The study confirmed that the viral genomic recombination contributes to the emergence of new pathogenic PRRSVs in China, which may escape from the protective immunity elicited by the conventional vaccines, highlighting the necessity in updates of vaccine strains and the need for a universal vaccine against PRRSV.

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

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

Porcine endogenous retrovirus envelope coated baculoviral DNA vaccine against porcine reproductive and respiratory syndrome virus

PERV is a major virus concerning xenotransplantation study. However, the interesting part is that PERV is present in all kinds of pigs without pathogenicity and immune response. Furthermore, since pig cells have receptors for PERV, the gene delivery system using PERV envelope is highly likely to develop into an excellent viral vector in pigs. We developed a recombinant baculovirus with a modified surface for expressing the porcine endogenous retrovirus (PERV) envelope. Porcine reproductive and respiratory syndrome virus (PRRSV) infection is a severe concern in the porcine industry due to reproduction failure and respiratory symptoms. GP5 and M proteins are major immunogenic proteins of PRRSV. Using PERV-modified baculovirus (Ac mPERV) as a delivery vector, we constructed a dual antigen (GP5 and M)-encoding DNA vaccine system, Ac mPERV-C5/C6. Intramuscular immunization in mice and pigs, Ac mPERV-C5/C6 induced comparative high humoral and cellular immune responses. Our results support further development of Ac mPERV-C5/C6 as a potential PRRSV vaccine in the porcine industry. In addition, the Ac mPERV system may be applied to the generation of other effective DNA vaccines against porcine viral diseases.

In vitro immune responses of porcine alveolar macrophages reflect host immune responses against porcine reproductive and respiratory syndrome viruses

BACKGROUND

Currently, an in vitro immunogenicity screening system for the immunological assessment of potential porcine reproductive and respiratory syndrome virus (PRRSV) vaccine candidates is highly desired. Thus, in the present study, two genetically divergent PRRSVs were characterized in vitro and in vivo to identify an in vitro system and immunological markers that predict the host immune response. Porcine alveolar macrophages (PAMs) and peripheral blood mononuclear cells (PBMCs) collected from PRRSV-negative pigs were used for in vitro immunological evaluation, and the response of these cells to VR2332c or JA142c were compared with those elicited in pigs challenged with the same viruses.

RESULTS

Compared with VR2332c or mock infection, JA142c induced increased levels of type I interferons and pro-inflammatory cytokines (TNF-α, IL-1α/β, IL-6, IL-8, and IL-12) in PAMs, and these elevated levels were comparable to the cytokine induction observed in PRRSV-challenged pigs. Furthermore, significantly greater numbers of activated CD4⁺ T cells, type I helper T cells, cytotoxic T cells and total IFN-γ⁺ cells were observed in JA142c-challenged pigs than in VR2332c- or mock-challenged pigs.

CONCLUSIONS

Based on these results, the innate immune response patterns (particularly IFN-α, TNF-α and IL-12) to specific PRRSV strains in PAMs might reflect those elicited by the same viruses in pigs.

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

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

Independent evolution of porcine reproductive and respiratory syndrome virus 2 with genetic heterogeneity in antigenic regions of structural proteins in Korea

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important pathogen that affects the global swine industry. The continuous evolution of this virus has made control and prevention difficult, which emphasizes the importance of monitoring currently circulating PRRSV strains. In this study, we investigated the genetic characteristics of whole structural genes of 35 PRRSV-2 isolates that circulated between 2012 and 2017 in Korea. Genetic and phylogenetic analysis demonstrated that a recently identified PRRSV-2 shared a relatively low level of nucleotide sequence identity that ranged from 86.2% to 92.8%; however, they were clustered into four distinct Korean field clades, except KU-N1702, in ORF2-7-based phylogeny. KU-N1702 was closely related to the NADC30-like strains that were identified in the USA and China. Amino acid sequence analysis showed that the GP5 neutralizing epitope was conserved among the KU viruses. In contrast, the viruses had genetic mutations in key residues for viral neutralization within GP5 and M. For minor structural proteins, neutralizing epitopes, aa 41-55 of GP2, 61-75 of GP3, and 51-65 of GP4, were variable among the KU viruses. Bioinformatics demonstrated diversifying evolution within the GP2 and GP4 neutralizing epitopes and the emergence of a novel glycosylation site within the GP3 and GP4 neutralizing epitopes. Taken together, these data provide evidence that Korean PRRSV-2 evolved independently in Korea, with genetic heterogeneity in antigenic regions of structural proteins.

Porcine reproductive and respiratory syndrome virus nsp1β and nsp11 antagonize the antiviral activity of cholesterol-25-hydroxylase via lysosomal degradation

Porcine reproductive and respiratory syndrome virus (PRRSV) is an immunosuppressive pathogen which has been recognized to modulate the host interferon (IFN) systems. Cholesterol-25-hydroxylase (CH25 H) is an important interferon-stimulated gene (ISG)-encoded polytopic membrane protein that significantly inhibits the replication of many viruses. In the current study, we showed that PRRSV infection induced the down-regulation of the endogenous CH25H in porcine alveolar macrophages (PAMs), and then discovered that the nonstructural protein (nsp) 1β and nsp11 of PRRSV could mediate the reduction of porcine CH25H d in HEK 293FT cells. Next, the amino acids including His-159 in nsp1β, and His-129, His-144 and Lys-173 in nsp11 were determined to play crucial roles in the reduction of CH25H. Furthermore, we confirmed that the nsp1β and nsp11 mediated the degradation of CH25H by lysosomal pathway in HEK 293FT cells. Finally, it was demonstrated that the anti-PRRSV activity of CH25H could be antagonized by nsp1β and nsp11 in MARC-145 cells. Our findings suggest a manner of antagonizing the antiviral activity of CH25H by PRRSV, and provide novel insight into the understanding of PRRSV's ability of escaping the innate immunity of host.

Cross-protection of a modified-live porcine reproductive and respiratory syndrome virus (PRRSV)-2 vaccine against a heterologous PRRSV-1 challenge in late-term pregnancy gilts

We have evaluated the cross-protection of a modified-live virus (MLV) vaccine based on porcine reproductive and respiratory syndrome virus (PRRSV)-2, against a heterologous PRRSV-1 challenge in late term pregnancy gilts. Gilts were vaccinated 42 days prior to breeding and then challenged intranasally with PRRSV-1 at 93 days of gestation. No local or systemic adverse effects related to vaccination were observed in the vaccinated gilts throughout the study. Vaccination resulted in a longer gestation period, a higher number of live-born and weaned piglets, and a significant decrease in the number of stillborn piglets compared to the unvaccinated group. The PRRSV-2 MLV vaccine was also able to significantly reduce PRRSV-1 viremia. At the time of PRRSV-1 challenge, vaccinated gilts had significantly higher PRRSV-1 specific interferon-γ secreting cells but low neutralizing antibody titers against PRRSV-1 compared to unvaccinated gilts. This correlated with a reduction of PRRSV-1 viremia, indicating that cell-mediated rather than humoral immunity played a role in PRRSV-1 clearance from the blood. Fetal thymic tissues from vaccinated pregnant gilts had fewer PRRSV-1 positive cells compared to unvaccinated gilts. Taken together these results indicate that vaccination of gilts with PRRSV-2 MLV vaccine can provide cross-protection against PRRSV-1 challenge and improve reproductive performance.

Clover-tagged porcine reproductive and respiratory syndrome virus infectious clones for rapid detection of virus neutralizing antibodies

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is a widespread disease that affects domestic pigs of all ages. Accurate and rapid detection of PRRSV specific neutralizing antibodies levels in a pig herd is beneficial for the evaluation of the herd's immunity to combat the specific viral infection. However, the current methods for viral detection, including fluorescent focus neutralization (FFN) and cytopathic effect (CPE) reduction neutralizing assays, are subjective and time-consuming. Therefore, a Clover-tagged PRRSV virus neutralization assay were developed that instrumentally measures the fluorescence signal of Clover stably expressing by a PRRSV infectious clone for at least 10 passages. Herein, the results showed that the proposed Clover-tagged PRRSV neutralization assay is reliable using instrumental measurements of the fluorescence signal of Clover and allows for rapid detection of neutralizing antibodies against PRRSV. The assay was evaluated by testing swine sera from experimental and field samples, and comparisons were made with the traditional FFN and CPE reduction assays. These results suggest that the Clover-tagged PRRSV infectious clone offers a fast and reliable testing method for neutralizing antibodies and could permit high-throughput screening of new antiviral agents.

Geographic distribution and molecular analysis of porcine reproductive and respiratory syndrome viruses circulating in swine farms in the Republic of Korea between 2013 and 2016

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) causes devastating disease characterized by reproductive failure and respiratory problems in the swine industry. To understand the recent prevalence and genetic diversity of field PRRSVs in the Republic of Korea, open reading frames (ORFs) 5 and 7 of PRRSV field isolates from 631 PRRS-affected swine farms nationwide in 2013-2016 were analyzed along with 200 Korean field viruses isolated in 2003-2010, and 113 foreign field and vaccine strains.

RESULTS

Korean swine farms were widely infected with PRRSVs of a single type (38.4 and 37.4% for Type 1 and Type 2 PRRSV, respectively) or both types (24.2%) with up to approximately 83% nucleotide sequence similarity to prototype PRRSVs (Lelystad or VR2332). Phylogenetic analysis based on the ORF5 nucleotide sequence revealed that Korean Type 1 field isolates were classified as subgroups A, B, and C under subtype 1, while Korean Type 2 field isolates were classified as lineages 1 and 5 as well as three Korean lineages (kor A, B, and C) with the highest infection prevalence in subgroup A (50.5%) and lineage 5 (15.3%) for Type 1 and Type 2 PRRSV, respectively, among ORF5-positive farms. In particular, the lineages kor B and C were identified as novel lineages in this study, and lineage kor B comprised only the field viruses isolated from Gyeongnam Province in 2014-2015, establishing regionally unique genetic characteristics. It has also recently been confirmed that commercialized vaccine-like viruses (subgroup C) of Type 1 PRRSV and NADC30-like viruses of Type 2 PRRSV (lineage 1) are spreading rapidly in Korean swine farms. The Korean field viruses were also expected to be antigenically variable as shown in the high diversity of neutralizing epitopes and N-glycosylation sites.

CONCLUSIONS

This up-to-date information regarding recent field PRRSVs should be taken into consideration when creating strategies for the application of PRRS control measures, including vaccination in the field.

A modified-live porcine reproductive and respiratory syndrome virus (PRRSV)-1 vaccine protects late-term pregnancy gilts against heterologous PRRSV-1 but not PRRSV-2 challenge

The objective of this study was to determine the efficacy of a commercially available porcine reproductive and respiratory syndrome virus (PRRSV)-1 modified-live virus (MLV) vaccine against PRRSV-1 and PRRSV-2 challenge in late-term pregnancy gilts. Gilts were vaccinated with the PRRSV-1 MLV vaccine at 4 weeks prior to breeding and then challenged intranasally with PRRSV-1 or PRRSV-2 at 93 days of gestation. After PRRSV-1 challenge, vaccinated pregnant gilts had a significantly longer gestation period, significantly higher numbers of live-born and weaned piglets and a significantly lower number of stillborn piglets at birth compared to unvaccinated pregnant gilts. No significant improvement in reproductive performance was observed between vaccinated and unvaccinated pregnant gilts following PRRSV-2 challenge. Vaccinated pregnant gilts also exhibited a significantly improved reproductive performance after challenge with PRRSV-1 compared to vaccinated pregnant gilts following PRRSV-2 challenge. The PRRSV-1 MLV vaccine was able to reduce PRRSV-1 but not PRRSV-2 viremia in pregnant gilts. Vaccinated gilts also showed a significantly higher number of PRRSV-1-specific IFN-γ-secreting cells (IFN-γ-SC) compared to PRRSV-2-specific IFN-γ-SC. The data presented here suggest that the vaccination of pregnant gilts with a PRRSV-1 MLV vaccine provides good protection against PRRSV-1 but only limited protection against PRRSV-2 challenge in late-term pregnancy gilts based on improvement of reproductive performance, reduction in viremia and induction of IFN-γ-SC.

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.

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

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

Nonstructural protein 9 residues 586 and 592 are critical sites in determining the replication efficiency and fatal virulence of the Chinese highly pathogenic porcine reproductive and respiratory syndrome virus

The highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has caused huge economic losses to the swine industry in China. Understanding the molecular basis in relation to the virulence of HP-PRRSV is essential for effectively controlling clinical infection and disease. In the current study, we constructed and rescued a serial of mutant viruses in nsp9 and nsp10 based on the differential amino acid sites between HP-PRRSV JXwn06 and LP-PRRSV HB-1/3.9. The replication efficiency in pulmonary alveolar macrophages (PAMs) and the pathogenicity of the mutant viruses for piglets were analyzed. Our results showed that the mutation of Thr to Ala in 586 and Ser to Thr in 592 of nsp9 decreased the replication efficiency of HP-PRRSV in PAMs, and could attenuate its virulence for piglets, suggesting that the residues 586 and 592 of nsp9 are critical sites natively in determining the fatal virulence of the Chinese HP-PRRSV for piglets.

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

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

Development of a broadly protective modified-live virus vaccine candidate against porcine reproductive and respiratory syndrome virus

Modified-live virus (MLV) vaccines are widely used to protect pigs against porcine reproductive and respiratory syndrome virus (PRRSV). However, current MLV vaccines do not confer adequate levels of heterologous protection, presumably due to the substantial genetic diversity of PRRSV isolates circulating in the field. To overcome this genetic variation challenge, we recently generated a synthetic PRRSV strain containing a consensus genomic sequence of PRRSV-2. We demonstrated that our synthetic PRRSV strain confers unprecedented levels of heterologous protection. However, the synthetic PRRSV strain at passage 1 (hereafter designated CON-P1) is highly virulent and therefore, is not suitable to be used as a vaccine in pigs. In the present study, we attenuated CON-P1 by continuously passaging the virus in MARC-145 cells, a non-natural host cell line. Using a young pig model, we demonstrated that the synthetic virus at passages 90 and 122 (designated as CON-P90 and CON-P122, respectively) were fully attenuated, as evidenced by the significantly reduced viral loads in serum and tissues and the absence of lung lesion in the infected pigs. Most importantly, CON-P90 confers similar levels of heterologous protection as its parental strain CON-P1. Taken together, the results indicate that CON-P90 is an excellent candidate for the formulation of next generation of PRRSV MLV vaccines with improved levels of heterologous protection.

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.

Cloning and molecular characterization of the ORF5 gene from a PRRSV-SN strain from Southwest China

To monitor the genetic variation of PRRSV, the ORF5 gene of the PRRSV-SN strain found in Suining City, Sichuan Province, was cloned and sequenced. The results showed that the PRRSV-SN strain was a highly pathogenic PRRSV (HP-PRRSV) variant strain with the North American (NA) genotype. Homology analysis showed that the ORF5 gene of the PRRSV-SN isolate shared 89.4% (86.5%) nucleotide (amino acid) sequence similarity with the North American strain VR-2332, 98.8% (96%) similarity with JXA1, and 63.8% (57.7%) similarity with the European type representative strain Lelystad virus. Phylogenetic analysis showed that PRRSV-SN belongs to the NA genotype and has the same subtype as other highly pathogenic PRRSV strains. Amino acid sequence analysis showed that compared with the VR2332 strain, PRRSV-SN has different degrees of variation in the signal peptide, transmembrane region (TM), primary neutralizing epitope (PNE), non-neutral epitopes and N-glycosylation sites. Antigenicity analysis showed that the PRRSV-SN ORF5 gene products and JXA1 have similar antigenic characteristics, and the antigenic epitopes are mainly located in aa30-39, aa50-60, aa128-141, aa146-155 and aa161-183 regions. In contrast, the antigenic characteristics of PRRSV-SN are quite different from those of the VR2332 strain. The main differences were that the PRRSV-SN strain was significantly narrower than the VR2332 strain in the aa30-39 and the aa50-60 regions but was significantly wider in the aa136-141 region. The results of this study showed that the epidemic strains that cause PRRSV outbreaks in the farm are still mainly JXA1 variants, but due to the more frequent use of live vaccine immunizations, the genes of the PRRSV epidemic strain still show constant variation. Vaccination with live PRRSV should be reduced, and surveillance of PRRSV strains should be enhanced.

Genetic and biological characterization of a Porcine Reproductive and Respiratory Syndrome Virus 2 (PRRSV-2) causing significant clinical disease in the field

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is the cause of severe reproductive and respiratory disease in swine worldwide. In Denmark, both PRRSV-1 and PRRSV-2 are circulating and approximately 35% of pig herds are seropositive for PRRSV. In November 2010, a pig herd in the Northern part of Denmark experienced an infection with PRRSV-2 with clinical signs that were much more severe than normally reported from current Danish PRRSV-2 affected herds. Due to the clinical observations of reproductive failure in sows and high mortality in piglets, it was speculated that a new, more pathogenic or vaccine evading PRRSV strain had emerged in Denmark. The overall aim of the present study was to perform a genetic and biological characterization of the virus isolated from the diseased herd. Complete genome sequencing of isolates from this herd revealed that although the case strain had some unique genetic features including a deduced 3 amino acid deletion, it was in overall very similar to the other PRRS-2 viruses circulating in Denmark. In an experimental trial in growing pigs, no overt clinical signs or pathology were observed following intranasal inoculation with the new virus isolate. Virus shedding, acute phase protein responses and serological responses were comparable to those seen after experimental challenge with a Danish PRRSV-2 reference strain isolated in 1997. Vaccination with a commercial modified live PRRSV-2 vaccine had a clear reducing effect on virus shedding, magnitude, and duration of viremia and viral load in the lungs. Overall, the results indicate that the severe disease observed in the field was contributed by additional factors in combination with the PRRS virus infection.