Assessment of the safety and efficacy of an attenuated live vaccine based on highly pathogenic porcine reproductive and respiratory syndrome virus

Protective evaluation of the commercialized porcine reproductive and respiratory syndrome virus vaccines in piglets challenged by NADC34-like strain

In China, the porcine reproductive and respiratory syndrome virus (PRRSV) has undergone several variations over the decades and contributed to the diversity of the clinical epidemic PRRSV strains. This has complicated the prevention and control of PRRS. In particular, the efficacy of the currently available commercial vaccines against the highly pathogenic NADC34-like strains is unclear. Therefore, the objective of this study was to evaluate the protection efficacy of three commercial PRRS modified-live virus (MLV) vaccines derived from classical PRRS VR2332 MLV and R98 MLV against challenge with a heterologous NADC34-like PRRSV strain, JS2021NADC34, which has high pathogenicity in pigs. PRRSV- and antibody-free piglets were immunized with the PRRS VR2332 MLV vaccine or either of two R98 MLV vaccines (from different manufacturers) and were challenged with the JS2021NADC34 strain 28 days after immunization. Rectal temperature, clinical symptoms, viremia and viral shedding from the nose, gross lesions in the thymus and lungs, microscopic lesions and viral distribution in the lungs, as well as the humoral immune response and mortality rates were recorded over a 14-day post-challenge period. The results showed that PRRS VR2332 MLV had better efficacy against the JS2021NADC34 challenge than PRRS R98 MLV, with vaccinated piglets in the former group showing transient and mild symptoms, mild pathological lesions in the lungs, mild thymic atrophy, and low viral levels in sera and nasal swabs, as well as better growth performance and a 100% survival rate. In contrast, two PRRS R98 MLVs exhibited limited efficacy against the JS2021NADC34 challenge, with the piglets in two R98 groups showing obvious clinical symptoms and pathological changes in the lungs and thymus; moreover, there were two deaths caused by PRRS in two R98 groups, respectively. Despite this, the mortality rate was lower than that of the unvaccinated piglets that were challenged with JS2021NADC34. The cumulative results demonstrate that PRRS VR2332 MLV was partly effective against the highly pathogenic PRRSV NADC34-like strain based on the observations over the 14-day post-challenge period. Thus, it might be a viable option among the commercially available vaccines for control of NADC34-like virus infections in swine herds.

Recombination of Porcine Reproductive and Respiratory Syndrome Virus: Features, Possible Mechanisms, and Future Directions

Recombination is a pervasive phenomenon in RNA viruses and an important strategy for accelerating the evolution of RNA virus populations. Recombination in the porcine reproductive and respiratory syndrome virus (PRRSV) was first reported in 1999, and many case reports have been published in recent years. In this review, all the existing reports on PRRSV recombination events were collected, and the genotypes, parental strains, and locations of the recombination breakpoints have been summarized and analyzed. The results showed that the recombination pattern constantly changes; whether inter- or intra-lineage recombination, the recombination hotspots vary in different recombination patterns. The virulence of recombinant PRRSVs was higher than that of the parental strains, and the emergence of virulence reversion was caused by recombination after using MLV vaccines. This could be attributed to the enhanced adaptability of recombinant PRRSV for entry and replication, facilitating their rapid propagation. The aim of this paper was to identify common features of recombinant PRRSV strains, reduce the recombination risk, and provide a foundation for future research into the mechanism of PRRSV recombination.

Recombination and Mutation in a New HP-PRRSV Strain (SD2020) from China

A new HP-PRRSV strain (SD2020) was isolated from pigs with suspected highly pathogenic porcine reproductive and respiratory syndrome disease in a pig farm in Shandong Province, China, and its genome was sequenced. This pig farm has been using the VR-2332 vaccine strain to immunize pigs for a long time. The phylogenic and single nucleotide polymorphisms (SNPs) analysis of the viruses isolated from dead pigs showed that SD2020 was a natural recombinant virus of the VR-2332 vaccine strain and the JXA1 similar strain, and that two splicing fragments highly homologous to JXA1 in the virus genome were probably derived from the JXA1 wild strain and JXA1-R vaccine strain, respectively. Therefore, the possible recombination events of SD2020 and its mutation site might be related to high pathogenicity.

Porcine reproductive and respiratory syndrome virus reinfection causes the distribution of porcine interleukin-4 in close proximity to B lymphocytes within lymphoid follicles and a reduction in B and T lymphocytes

Interleukin 4 (IL-4) plays a major role in T-lymphocyte development and is thought to be a central regulator as a cofactor in resting B-lymphocyte proliferation. Primary infection with porcine reproductive and respiratory syndrome virus (PRRSV) induces minimal IL-4 production, whereas an IL-4 response occurs in the peripheral blood of piglets reinfected by PRRSV. The locations and interaction partners for the massive volume of IL-4 triggered by PRRSV reinfection remain unclear. This study aimed to investigate the characteristics of IL-4 secretion and location changes in peripheral immune organs induced by PRRSV infection and reinfection. Our results show that PRRSV reinfection induced higher levels of IL-4 mRNA and protein expression in the peripheral immune organs (e.g., lymph node and spleen) and peripheral blood compared with PRRSV primary infection. Importantly, we found that, following PRRSV reinfection, an obvious large-scale migration of IL-4 occurred in the lymph nodes. During PRRSV primary infection, IL-4 was mainly concentrated around the lymphoid follicles and paracortical regions of the lymph node and also located in the marginal area and periarterial lymphatic sheath region of the spleen. During PRRSV reinfection, the now abundant IL-4 gathered into the lymphoid follicles of the lymph node and spleen. Notably, IL-4 changed its location state from scattered and sparse during primary infection to clinging to B lymphocytes in the lymphoid follicles during reinfection. During reinfection, IL-4 was often co-localized with T and B lymphocytes; furthermore, the percentages of several T lymphocyte subsets, N protein-specific antibody levels, and viral load in the peripheral blood or lymph tissues underwent remarkable variation. Another important finding of this study was that the numbers of B lymphocytes and T lymphocytes in the lymphoid nodes were significantly reduced after PRRSV infection or reinfection, presumably due to PRRSV-induced acute bone marrow failure and autophagy in thymic epithelial cells. This study revealed the characteristics of IL-4 migration and distribution in the peripheral lymph organs induced by PRRSV reinfection and provides valuable clues for further exploration of the interactions between IL-4, B lymphocytes, and T lymphocytes during PRRSV infection and reinfection.

Evolution Characterization and Pathogenicity of a Porcine Reproductive and Respiratory Syndrome Virus Isolate from a Pig Farm in Shandong Province, China

In recent years, porcine reproductive and respiratory syndrome virus (PRRSV) strains have been experiencing extensive recombination in Chinese swine farms. This recombination usually happens in NADC30/34 strains and highly pathogenic (HP) PRRSV strains. This study identified a new PRRSV isolate that shared 99% and 99.1% nucleotide identity with CH-1a and CH-1R at the genomic level, respectively. After purification by viral plaque assay, this isolate was named PRRSV CSR1801. The isolate did not experience any recombination with other PRRSV strains common in swine herd epidemics in China, which means it still maintains the stable features of the classical PRRSV strain and did not easily recombine with other PRRSV strains. Further analysis of the pathogenicity of the PRRSV isolate CSR1801 was performed in piglets. The results indicated that none of the inoculated piglets showed the typical clinical manifestations of PRRS, which presented with runny noses, rough back hair, rectal temperatures always below 40.5 °C, and no deaths. Additionally, no obvious histopathological lesions such as severe interstitial pneumonia could be observed in the lungs of the piglets. Hence, the PRRSV isolate CSR1801 should be classified as a classical-like PRRSV strain. This classical PRRSV strain showed genetic stability and maintained low pathogenicity. This study may provide new clues for further understanding the genetic evolution and pathogenicity of PRRSV and may also be an important reference for the prevention and control of PRRS in swine farms.

Lineage 1 Porcine Reproductive and Respiratory Syndrome Virus Attenuated Live Vaccine Provides Broad Cross-Protection against Homologous and Heterologous NADC30-Like Virus Challenge in Piglets

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that endangers the swine industry worldwide. Recently, lineage 1 PRRSVs, especially NADC30-like PRRSVs, have become the major endemic strains in many pig-breeding countries. Since 2016, NADC30-like PRRSV has become the predominant strain in China. Unfortunately, current commercial vaccines cannot provide sufficient protection against this strain. Here, an attenuated lineage 1 PRRSV strain, named SD-R, was obtained by passaging an NADC30-like PRRSV strain SD in Marc-145 cells for 125 passages. Four-week-old PRRSV-free piglets were vaccinated intramuscularly with 105.0TCID50 SD-R and then challenged intramuscularly (2 mL) and intranasally (2 mL) with homologous NADC30-like PRRSV SD (1 × 105.0TCID50/mL) and heterologous NADC30-like PRRSV HLJWK108-1711 (1 × 105.0TCID50/mL). The results showed that antibodies against specific PRRSVs in 5 of 5 immunized piglets were positive after a 14-day post-vaccination and did not develop fever or clinical diseases after NADC30-like PRRSV challenges. Additionally, compared with challenge control piglets, vaccinated piglets gained significantly more weight and showed much milder pathological lesions. Furthermore, the viral replication levels of the immunized group were significantly lower than those of the challenge control group. These results demonstrate that lineage 1 PRRSV SD-R is a good candidate for an efficacious vaccine, providing complete clinical protection for piglets against NADC30-like PRRSVs.

Recent Advances in Porcine Reproductive and Respiratory Syndrome Virus NADC30-Like Research in China: Molecular Characterization, Pathogenicity, and Control

The name porcine reproductive and respiratory syndrome virus (PRRSV) NADC30-like was first coined in 2015. It originated from the NADC30 strain that was introduced into China by importing breeding pigs and has since undergone mutations or recombination, resulting in variant viruses. Following widespread outbreaks in China in recent years, these NADC30-like strains have presented major health challenges in swine production systems. Outcomes induced by PRRSV NADC30-like infection are highly variable, ranging from inapparent to severe, depending on the recombination between NADC30 and field PRRSV strains prevalent in swine farms. Vaccines and strict biosecurity measures have been explored to fight this disease; however, current PRRSV commercially modified-live virus vaccines (MLVs) have the potential to revert to virulence and only provide limited or no cross-protection efficacy against NADC30-like strains. PRRSVs will remain an ongoing challenge to the swine industry until safe and effective vaccines or antiviral reagents are developed.

The Novel PRRSV Strain HBap4-2018 with a Unique Recombinant Pattern Is Highly Pathogenic to Piglets

Currently, various porcine reproductive and respiratory syndrome virus (PRRSV) variants emerged worldwide with different genetic characteristics and pathogenicity, increasing the difficulty of PRRS control. In this study, a PRRSV strain named HBap4-2018 was isolated from swine herds suffering severe respiratory disease with high morbidity in Hebei Province of China in 2018. The genome of HBap4-2018 is 15,003 nucleotides in length, and compared with NADC30-like PRRSV, nsp2 of HBap4-2018 has an additional continuous deletion of five amino acids. Phylogenetic analysis based on complete genome and ORF5 showed that HBap4-2018 belonged to lineage 8 of PRRSV-2, which was characterized by highly variable genome. However, HBap4-2018 was classified into lineage 1 based on phylogenetic analysis of nsp2, sharing higher amino acid homology (85.3%-85.5%) with NADC30-like PRRSV. Further analysis suggested that HBap4-2018 was a novel natural recombinant PRRSV with three recombinant fragments in the genome, of which highly pathogenic PRRSV (HP-PRRSV) served as the major parental strains, while NADC30-like PRRSV served as the minor parental strains. Five recombination break points were identified in nsp2, nsp3, nsp5, nsp9 and ORF6, respectively, presenting a novel recombinant pattern in the genome. Piglets inoculated with HBap4-2018 presented typical clinical signs with a mortality rate of 60%. High levels of viremia and obvious macroscopic and histopathological lesions in the lungs were observed, revealing the high pathogenicity of HBap4-2018 in piglets.

Evaluation of Four Commercial Vaccines for the Protection of Piglets against the Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus (hp-PRRSV) QH-08 Strain

Vaccination is the best way to prevent economic losses from highly pathogenic porcine reproductive and respiratory syndrome virus (hp-PRRSV) disease. However, the commercially available vaccines need to periodically evaluate their efficacy against infections caused by new hp-PRRSV variants. Therefore, the objective of this study was to evaluate the efficacy of four (two modified live vaccines (MLV) and two inactivated) PRRSV commercial vaccines in piglets challenged with QH-08 and to estimate the genetic distance of the vaccine strains from recently isolated (QH-08) filed strain. Randomly, piglets (n = 5) allocated in groups 1–4 were immunized with Ingelvac PRRS MLV, CH-1a, JXA1, and JXA1-RMLV vaccines, whereas the infected and non-infected control piglets in groups 5 and 6 (n = 3), respectively, were subjected to PBS. Results indicated that JXA1 and JXA1-R MLV vaccines showed complete protection, but Ingelvac PRRS MLV and CH-1α vaccines revealed partial protection against the QH-08 PRRSV challenge. Similarly, vaccinated and challenged pigs showed lower macroscopic and microscopic lesions than the pigs in group 5. Our findings demonstrated a new insight that the variation in ORF1a and 1b coding sequence could significantly affect PRRSV vaccines efficacy. In conclusion, QH-08 is a good candidate for the design and development of an innovative PRRSV vaccine that ultimately helps in the control and prevention strategies.

Identification of potential SLA-I-restricted CTL epitopes within the M protein of porcine reproductive and respiratory syndrome virus

CD8+ cytotoxic T lymphocytes (CTLs), are essential for clearance of porcine reproductive and respiratory syndrome virus (PRRSV) infection and regulation of host immune responses. Identification of SLA I-restricted CD8+ CTL epitopes would facilitate PRRSV vaccine development. Here, cells isolated from peripheral blood mononuclear cells (PBMCs) of PRRSV-immunized Large White pigs (JXA1-R strain) were screened for immunodominant PRRSV-2 M protein T cell epitopes via ELISPOT assay. Of nine immunodominant epitopes detected, eight elicited significant IFN-γ secretion responses that varied among individual pigs and according to epitope. To predict which epitopes harbored potential CTL epitopes, swine leukocyte antigen (SLA) class I genes of Large White pigs were cloned and sequenced, yielding fourteen distinct SLA class I gene sequences. Based on ELISPOT and SLA genotyping results, SLA-restricted binding of the fourteen predicted class I proteins to peptides derived from the eight immunodominant epitopes were predicted in-silico. After evaluation of 42 pET-peptide-SLA-I-β2m complexes containing predicted restricted peptides, extracellular SLA class I domains and β2m, ELISA testing of 33 peptide-SLA-I-β2m complexes detected four complexed peptides. These four peptides were evaluated using in vitro complex refolding assays that confirmed that M_2-5 and M_6-1 peptides each formed complexes with SLA-2*0502 and sβ2m, while M_9-1 formed a complex with SLA-2*1201 and sβ2m. ELISPOT results confirmed these three 9-mer potential CTL epitopes efficiently stimulated IFN-γ secretion when presented by SLA class I molecules specified here. This study describes effective CTL epitope identification methods for use in future investigations of swine cellular immunity toward T cell-based vaccine development.

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A "Leaky" Vaccine with Debatable Efficacy and Safety

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.

A broadly neutralizing monoclonal antibody induces broad protection against heterogeneous PRRSV strains in piglets

Neutralizing antibodies (NAbs) have attracted attention as tools for achieving PRRSV control and prevention, but viral antigenic variation undermines the abilities of NAbs elicited by attenuated PRRSV vaccines to confer full protection against heterogeneous PRRSV field isolates. As demonstrated in this study, the monoclonal antibody (mAb) mAb-PN9cx3 exhibited broad-spectrum recognition and neutralizing activities against PRRSV-1 and PRRSV-2 strains in vitro. Furthermore, in vivo experiments revealed that the administration of two 10-mg doses of mAb-PN9cx3 before and after the inoculation of piglets with heterologous PRRSV isolates (HP-PRRSV-JXA1 or PRRSV NADC30-like strain HNhx) resulted in significant reduction of the PRRSV-induced pulmonary pathological changes and virus loads in porcine alveolar macrophages (PAMs) compared with the results obtained with mAb-treated isotype controls. Moreover, minimal hilar lymph node PRRSV antigen levels were observed in mAb-PN9cx3-treated piglets. A transcriptome profile analysis of PAMs extracted from lung tissues of piglets belonging to different groups (except for antibody-isotype controls) indicated that mAb-PN9cx3 treatment reversed the PRRSV infection-induced alterations in expression profiles. A gene ontology (GO) enrichment analysis of these genes traced their functions to pathways that included the immune response, inflammatory response, and response to steroid hormone, and their functions in oogenesis and positive regulation of angiogenesis have been implicated in PRRSV pathogenesis. Overall, NADC30-like HNhx infection affected more gene pathways than HP-PRRSV infection. In conclusion, our research describes a novel immunologic approach involving the use of mAbs that confer cross-protection against serious illness resulting from infection with heterogeneous PRRSV-2 isolates, which is a feat that has not yet been achieved through vaccination. Ultimately, mAb-PN9cx3 will be a powerful addition to our current arsenal for achieving PRRSV prevention and eradication.

The infectious cDNA clone of commercial HP-PRRS JXA1-R-attenuated vaccine can be a potential effective live vaccine vector

Multiple commercial porcine reproductive and respiratory syndrome (PRRS) modified live vaccines are currently utilized in Chinese swine herds due to the limited cross-protection of vaccines and coexistence of different PRRS viruses. In this study, an infectious cDNA clone of the highly pathogenic PRRS (HP-PRRS) vaccine JXA1-R strain was generated. We successfully rescued the virus from direct in vitro DNA transfection of rJXA1-R clone, which has similar growth kinetics to the parental JXA1-R virus in Marc-145 cells. To further evaluate the potential use of the cloned rJXA1-R virus as a live vector for foreign gene expression, the enhanced green fluorescent protein (EGFP) was inserted between non-structural and structural genes. Our results showed that the dynamic expression of EGFP can be visualized by live cell imaging system during the infection in Marc-145 cells. The availability of our cloned JXA1-R viruses provides a crucial platform to study the fundamental biology of HP-PRRS virus vaccine and also serves as a potential effective vector for developing live vector vaccines against swine pathogens.

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

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

Quantitative Proteomic Analysis of MARC-145 Cells Infected with a Mexican Porcine Reproductive and Respiratory Syndrome Virus Strain Using a Label-Free Based DIA approach

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease characterized by severe reproductive failure in sows, acute respiratory disorders in growing pigs, and high mortality in piglets. The causative agent of this syndrome is the PRRS virus (PRRSV), an RNA virus belonging to the Arteriviridae family. To date, several quantitative approaches of proteomics have been applied to analyze the gene expression profiles during PRRSV infection in PAMs and MARC-145 cells, and few proteins have been consistent among independent studies, probably due to the differences in the levels of virulence of different PRRSV strains used and/or due to analytical conditions. In this study, total proteins isolated from noninfected and infected MARC-145 cells with a Mexican PRRSV strain were relatively quantified using label-free based DIA approach in combination with ion-mobility separation. As a result, 1456 quantified proteins were found to be shared between the control and infected samples. Afterward, these proteins were filtered, and 699 of them were considered without change. Also, 17 proteins were up-regulated and 19 proteins were down-regulated during the PRSSV infection. Bioinformatic analysis revealed that many of the differentially expressed proteins are involved in processes like antigen processing, presentation of antigens, response to viruses, response to IFNs, and innate immune response, among others. The present work is the first one which provides a detailed proteomic analysis through label-free based DIA approach in MARC-145 cells during the infection with a Mexican PRRSV strain.

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.

Novel nucleotide variants in SLA-DOB and CD4 are associated with immune traits in pregnant sows

Reinforcing the immunity of pregnant sows can not only improve their own health condition but also increase the survival rate and healthy status of their piglets. This study aims to find single-nucleotide polymorphism (SNP) and molecular markers that are associated with the immune traits of pregnant sows. SLA-DOB and CD4 were selected as candidate genes, and blood samples were randomly collected from pregnant Landrace sows and used to detect T-lymphocyte subsets, interferon alpha, interleukin 6, Toll-like receptor 3, serum antibody immunoglobulin G, and porcine reproductive and respiratory syndrome virus-specific antibody. Then, association analyses were conducted for the polymorphic sites of candidate genes with immune traits. We found 12 mutations in the two genes and conducted an association study with eight of them. Our results indicated that among the eight mutations, SNP1, SNP2, and SNP3 of the SLA-DOB gene and Ins9, SNP10, and SNP11 in the CD4 gene are newly discovered mutations. Except for SNP1, SNP3, and SNP11, the other five SNPs are associated with at least one immune trait tested. Especially, SNP2 and Ins9 are significantly associated with at least one of the T-lymphocyte subgroups and at least one antibody. These novel mutations have potential important effects on the polymorphic loci of the above immune traits in pregnant sows. The results suggest that the SLA-DOB and CD4 genes and their genetic mutations can be considered as important candidate genes and mutations for the immunity of pregnant sows.

High reversion potential of a cell-adapted vaccine candidate against highly pathogenic porcine reproductive and respiratory syndrome

Modified live vaccine (MLV) based on highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) is prone to quick reversion of virulence upon circulating in host animals. The objective of this study was to evaluate the virulence reversion potential of HP-PRRSV MLV and to identify elements within the HP-PRRSV genome contributing to this phenomenon. A blind passage, cell-adaptation strategy was attempted to attenuate a HP-PRRSV strain JX143, which was isolated during the atypical PRRS outbreak in 2006. Two attenuated candidates passage 87 (JXM87) and passage 105 (JXM105) used as MLVs showed the best balance of safety and efficacy in 4 week-old piglets (unpublished data). Two studies were performed during which the candidates were assessed for reversion to virulence through five back passages in susceptible piglets (21 ± 3 days of age). Both study results showed increase in clinical signs, pyrexia and lung lesions as well as decreased average daily weight gain as of passage 3 in susceptible pigs clearly, and it indicated that both candidates regained virulence, irrespective of the passage level. Increase in respective parameters was accompanied by increase in viremia in piglets: JXM87 virus titer increased from Passage 1 (P1) 4.40 Lg TCID₅₀/mL to P4 5.75 Lg TCID₅₀/mL, and JXM105 virus titer increased from P1 3.78 Lg TCID₅₀/mL to P4 6.42 Lg TCID₅₀/mL. Next generation sequencing (NGS) was performed on clinical samples (serum, lung tissue) from P4 animals. Sequence analysis comparing P4 materials with their parental strains revealed 10 amino acid mutations in 4 proteins for JXM87 and 14 amino acid mutations in 9 proteins for JXM105, respectively. Interestingly, five amino acid mutations were identical for the two candidates, which were located in nsp1β, GP5a and nsp10 coding regions, suggesting nsp1β, GP5a and nsp10 could contribute to virulence in HP-PRRSV.

Evolutionary analysis of six isolates of porcine reproductive and respiratory syndrome virus from a single pig farm: MLV-evolved and recombinant viruses

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens with economic significance for swine industry globally. The virus is characterized by its genetically extraordinary variability and diversified strains due to high mutation frequency and recombination rates. In the current study, we analyzed the genomes of six PRRSV strains isolated in different years from a single pig farm practicing HP-PRRSV-derived modified-live virus (MLV) vaccination in China, and in detail dissected their evolutionary relationship to MLV vaccines. The results show that all the six isolates belong to type 2 PRRSV, and are clustered into two lineages (lineage 1 and 8) based on their ORF5 genes. Further comparative analyses of genomes and the characteristic amino acid sites show that both HeN1201 and HeN1502 are MLV-like strains evolved from the MLV HuN4-F112. Recombination analyses reveal that HeN1301 is a recombinant virus emerged from the recombination event between two MLV-like strains evolved from the MLV TJM-F92 and HuN4-F112, and HeN1501 is a recombinant virus between two MLV-like strains evolved from HuN4-F112, while HeN1401 and HeN1601 are recombinant viruses derived from the recombination between NADC30-like and a MLV-like strain evolved from TJM-F92. Our findings provide more evidence for the fact that MLV-derived from the Chinese HP-PRRSV can be evolved and reversed in the field and enrich the data about the recombination between the viruses evolved from MLV and newly infected strain of PRRSV. This study stresses the necessity for pig producers to restrain the use of HP-PRRSV MLV vaccines in the field.

Efficacy of a type 2 PRRSV modified live vaccine (PrimePac™ PRRS) against a Thai HP-PRRSV challenge

The Chinese highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has caused a severe threat to the pig population in Southeast Asian countries. The purpose of this study was to investigate the efficacy of a type 2 PRRSV modified live vaccine (PrimePac™ PRRS, lineage 7) against a Thai HP-PRRSV (10PL01, lineage 8). Three-week-old PRRSV-free pigs were randomly assigned into three groups. Vaccinated challenged group (group 1, n = 16) was immunized with PrimePac™ PRRS vaccine at 3 weeks old. The unvaccinated challenged group (group 2, n = 16) was injected with PBS at 3 weeks old, and unvaccinated unchallenged group (group 3, n = 10) was served as a negative control. At 9 weeks old, all groups, except the negative control group, were challenged with the Thai HP-PRRSV. All pigs were monitored daily during 10 days post-infection (dpi) and were necropsied at 10 and 17 dpi. The results revealed that vaccinated challenged pigs showed significantly lower (p < 0.05) mean rectal temperatures, clinical respiratory scores, lung lesion scores, and levels of virus load in serum and lung tissue compared with the unvaccinated challenged pigs. Moreover, vaccinated challenged pigs exhibited PRRSV-specific serum neutralizing antibodies at the end of the experiment. Our findings indicated that the studied type 2 PRRSV vaccine provided partial protection against the Thai HP-PRRSV infection based on the body temperature, levels of viremia, and the severity of lung lesions. These results demonstrated that partial protection of PrimePac™ PRRS vaccine might be useful for controlling HP-PRRSV infection in the endemic area.

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.

CD163 knockout pigs are fully resistant to highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe economic losses to current swine production worldwide. Highly pathogenic PRRSV (HP-PRRSV), originated from a genotype 2 PRRSV, is more virulent than classical PRRSV and further exacerbates the economic impact. HP-PRRSV has become the predominant circulating field strain in China since 2006. CD163 is a cellular receptor for PRRSV. The depletion of CD163 whole protein or SRCR5 region (interaction site for the virus) confers resistance to infection of several PRRSV isolates in pigs or cultured host cells. In this study, we described the generation of a CD163 knockout (KO) pig in which the CD163 protein was ablated by using CRISPR/Cas9 gene targeting and somatic cell nuclear transfer (SCNT) technologies. Challenge with HP-PRRSV TP strain showed that CD163 KO pigs are completely resistant to viral infection manifested by the absence of viremia, antibody response, high fever or any other PRRS-associated clinical signs. By comparison, wild-type (WT) controls displayed typical signs of PRRSV infection and died within 2 weeks after infection. Deletion of CD163 showed no adverse effects to the macrophages on immunophenotyping and biological activity as hemoglobin-haptoglobin scavenger. The results demonstrated that CD163 knockout confers full resistance to HP-PRRSV infection to pigs without impairing the biological function associated with the gene.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): porcine reproductive and respiratory syndrome (PRRS)

Porcine reproductive and respiratory syndrome (PRRS) has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of PRRS to be listed, Article 9 for the categorisation of PRRS according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to PRRS. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, PRRS can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL. The disease would comply with the criteria as in Sections 4 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (d) and (e) of Article 9(1). The animal species to be listed for PRRS according to Article 8(3) criteria are domestic pigs and wild boar.

Pathogenesis and control of the Chinese highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has remained a major threat to the worldwide swine industry ever since its first discovery in the early 1990s. Under the selective pressures in the field, this positive-stranded RNA virus undergoes rapid genetic evolution that eventually leads to emergence in 2006 of the devastating Chinese highly pathogenic PRRSV (HP-PRRSV). The atypical nature of HP-PRRSV has caused colossal economic losses to the swine producers in China and the surrounding countries. In this review, we summarize the recent advances in our understanding of the pathogenesis, evolution and ongoing field practices on the control of this troubling virus in China.

Positive immunomodulatory effects of heterologous DNA vaccine- modified live vaccine, prime-boost immunization, against the highly-pathogenic PRRSV infection

Porcine reproductive and respiratory syndrome virus (PRRSV) infection is one of the most important swine pathogens, and causes a major economic impact worldwide. Recently, a new variant type 2 PRRSV, highly pathogenic PRRSV (HP-PRRSV) has emerged and continued to circulate in Southeast Asia region. Currently, commercially available PRRSV vaccines, modified live PRRS vaccines (MLV) are not able to provide complete protection against HP-PRRSV and been reported to induce negative immunomodulatory effects. Interestingly, a novel DNA vaccine was developed and successfully used to improve PRRSV-specific immune responses following MLV vaccination. To investigate the efficacy of a heterologous DNA-MLV prime-boost immunization against the HP-PRRSV infection, an experimental vaccinated-challenged study was conducted. Two-week-old, PRRSV-seronegative, crossbred pigs (5-8 pigs/group) were allocated into 5 groups. At day -14 (D-14), the treatment group (DNA-MLV) was immunized with a DNA vaccine encoding PRRSV-truncated nucleocapsid protein (pORF7t), followed by a commercial modified live type 2 PRRS vaccine (MLV) at D0. The other groups included the group that received PBS at D-14 followed by MLV at D0 (MLV), pORF7t at D-14 (DNA), PBS at D0 (PBS) and the negative control group. At D42, all groups, except the negative control group, were challenged with HP-PRRSV (strain 10PL1). The results demonstrated that pigs that received MLV, regardless of the DNA priming, exhibited less clinical signs and faster viral clearance. Following HP-PRRSV challenge, the DNA-MLV group exhibited improved PRRSV-specific immunity, as observed by increased neutralizing antibody titers and PRRSV-specific IFN-γ production, and reduced IL-10 and PRRSV-specific Treg productions. However, neither the prime-boost immunization nor the MLV was able to induce complete clinical protection against HP-PRRSV infection. In conclusion, improved immunological responses, but not clinical protection, were achieved by DNA-MLV prime-boost immunization. This study highlights the potential use of heterologous prime-boost vaccination regimen, where DNA can be incorporated with other vaccine candidates, for improving anti-PRRSV immunity that may eventually lead induction of complete PRRSV protection.

Immune responses to modified live virus vaccines developed from classical or highly pathogenic PRRSV following challenge with a highly pathogenic PRRSV strain

Modified live virus vaccines (MLVs) are used on swine farms to control porcine reproductive and respiratory syndrome virus (PRRSV). MLVs from classical PRRSV (C-PRRSV) provide some protection against emergent highly pathogenic PRRSV (HP-PRRSV). This study characterized the protective efficacy and immune response to MLVs from C-PRRSV (CH-1R) or HP-PRRSV (HuN4-F112) in a challenge using HP-PRRSV (HuN4). The outcomes were clinical signs of disease, pathological changes in the thymus and lungs, viremia, and humoral and cellular immune responses. CH-1R provided some protection against challenge with HuN4, while HuN4-F112 was protective in the HuN4 challenge. Compared to unvaccinated piglets, the vaccinated piglets had milder symptoms and fewer pathological changes in the lung and thymus. Piglets vaccinated with HuN4-F112 had higher antibody titers and lower viral loads than piglets vaccinated with CH-1R post challenge. The differences in outcome between the MLVs suggested that underlying differences in the immune responses might warrant further study.

Pigs immunized with Chinese highly pathogenic PRRS virus modified live vaccine are protected from challenge with North American PRRSV strain NADC-20

Modified live virus (MLV) vaccines developed to protect against PRRSV circulating in North America (NA) offer limited protection to highly pathogenic (HP) PRRSV strains that are emerging in Asia. MLV vaccines specific to HP-PRRSV strains commercially available in China provide protection to HP-PRRSV; however, the efficacy of these HP-PRRSV vaccines to current circulating NA PRRS viruses has not been reported. The aim of this study is to investigate whether pigs vaccinated with attenuated Chinese HP-PRRSV vaccine (JXA1-R) are protected from infection by NA PRRSV strain NADC-20. We found that pigs vaccinated with JXA1-R were protected from challenges with HV-PRRSV or NADC-20 as shown by fewer days of clinical fever, reduced lung pathology scores, and lower PRRS virus load in the blood. PRRSV-specific antibodies, as measured by IDEXX ELISA, appeared one week after vaccination and virus neutralizing antibodies were detected four weeks post vaccination. Pigs vaccinated with JXA1-R developed broadly neutralizing antibodies with high titers to NADC-20, JXA1-R, and HV-PRRSV. In addition, we also found that IFN-α and IFN-β occurred at higher levels in the lungs of pigs vaccinated with JXA1-R. Taken together, our studies provide the first evidence that JXA1-R can confer protection in pigs against the heterologous NA PRRSV strain NADC-20.