Chinese and Vietnamese strains of HP-PRRSV cause different pathogenic outcomes in United States high health swine

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

Porcine reproductive and respiratory syndrome virus (PRRSV) is an RNA virus that causes reproductive failure in sows and respiratory problems in piglets. PRRSV infection leads to substantial pig mortality and causing huge economic losses so that disease outbreaks caused by the new PRRSV strain from other regions have caused great concern in China. In this study, we analysed the pathogenicity of the novel ORF5 RFLP 1-7-4-like PRRSV strain, named PRRSV-ZDXYL-China-2018-1 in pigs. The viral challenge test showed that PRRSV-ZDXYL-China-2018-1 infection can cause persistent fever, moderate dyspnoea, serum viraemia and interstitial pneumonia in piglets. The levels of viral loads in serum and PRRSV-specific antigen were also detected in lung tissues were used one-step Taq-Man RT-qPCR and Immunohistochemistry, respectively. At 28dpi, the level of specific antibodies was increased among infected piglets. Importantly, the new virus appeared be a moderately virulent isolate with pathogenicity compared to HP-PRRSV strain LQ (JXA1-like strain). Histological examination revealed severe monocyte haemorrhage and interstitial pneumonia associated with monocyte infiltration in the lung tissue of pigs infected with PRRSV-ZDXYL-China-2018-1 and LQ-JXA1 strains. Immunohistochemistry (IHC) results showed positive brown-red epithelial cells and macrophages in pig lungs. Therefore, it is critical to establish an effective strategy to control the spread of PRRSV in China.

A novel protein chip for simultaneous detection of antibodies against four epidemic swine viruses in China

BACKGROUND

At present, pig industry in China is faced with the complex situation of mixed infection caused by multiple pathogens. It is urgent to develop some new high-throughput molecular diagnosis assays to simultaneously detect pathogens or antibodies. Biochip array technology has made it possible to screen thousands of samples simultaneously; it has been twice named as one of the top 10 scientific and technological breakthroughs. Studies have reported encouraging results using protein biochips for detecting antibodies against avian infectious bronchitis virus and ruminant bluetongue virus, but the research of this technology for the diagnosis of swine diseases is still sparse.

RESULTS

In this study, a novel protein chip was developed that can simultaneously detect the antibodies of four important swine viruses as follow, classical swine fever virus (CSFV), porcine parvovirus (PPV), Japanese encephalitis virus (JEV), and porcine reproductive and respiratory syndrome virus (PRRSV). Four prokaryotic expression plasmids pET-32a-E2 of CSFV, -VP2 of PPV, -EDIII of JEV, and -N of PRRSV were induced by IPTG (Isopropyl β-D-1-Thiogalactopyranoside) and overexpressed in E.coli, respectively. The purified proteins were identified by Western blotting and then printed on epoxy-coated glass slides. The optimized parameters of this diagnostic chip showed that the spotting concentrations of E2、VP2、EDIII、N proteins were 0.2, 0.4, 0.4, and 0.4 mg/mL. The optimal primary and secondary antibody dilutions were 1:50 and 1: 600. Compared with the commercial ELISA (Enzyme-linked immunosorbent assay) kits, the positive and negative coincidence rates of this chip were 95.8% ~ 100 and 86.2% ~ 100%, as well as, no cross-reaction.

CONCLUSION

This protein chip provided a fast, specific, and sensitive method for simultaneous detection of antibodies in clinical serum samples. Compared with traditional methods, this protein chip can monitor very small amount of serum.

Insights into the Porcine Reproductive and Respiratory Syndrome Virus Viral Ovarian Tumor Domain Protease Specificity for Ubiquitin and Interferon Stimulated Gene Product 15

Porcine reproductive and respiratory syndrome (PRRS) is a widespread economically devastating disease caused by PRRS virus (PRRSV). First recognized in the late 1980s, PRRSV is known to undergo somatic mutations and high frequency viral recombination, which leads to many diverse viral strains. This includes differences within viral virulence factors, such as the viral ovarian tumor domain (vOTU) protease, also referred to as the papain-like protease 2. These proteases down-regulate innate immunity by deubiquitinating proteins targeted by the cell for further processing and potentially also acting against interferon-stimulated genes (ISGs). Recently, vOTUs from vaccine derivative Ingelvac PRRS modified live virus (MLV) and the highly pathogenic PRRSV strain JXwn06 were biochemically characterized, revealing a marked difference in activity toward K63 linked polyubiquitin chains and a limited preference for interferon-stimulated gene product 15 (ISG15) substrates. To extend our research, the vOTUs from NADC31 (low virulence) and SDSU73 (moderately virulent) were biochemically characterized using a myriad of ubiquitin and ISG15 related assays. The K63 polyubiquitin cleavage activity profiles of these vOTUs were found to track with the established pathogenesis of MLV, NADC31, SDSU73, and JXwn06 strains. Fascinatingly, NADC31 demonstrated significantly enhanced activity toward ISG15 substrates compared to its counterparts. Utilizing this information and strain-strain differences within the vOTU encoding region, sites were identified that can modulate K63 polyubiquitin and ISG15 cleavage activities. This information represents the basis for new tools to probe the role of vOTUs in the context of PRRSV pathogenesis.

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

Background

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

Results

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

Conclusions

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

Progress toward an enhanced vaccine: Eight marked attenuated viruses to porcine reproductive and respiratory disease virus

Recombinant viruses of strain Ingelvac® PRRS porcine reproductive and respiratory syndrome virus (PRRSV) modified live virus vaccine were produced with two individual small in-frame deletions in nonstructural protein 2 (nsp2; Δ23 and Δ87) and also the same deletions supplanted with foreign tags (Δ23-V5, Δ23-FLAG, Δ23-S, Δ87-V5, Δ87-FLAG, Δ87-S). The viruses, but one (Δ87-FLAG), were stable for 10 passages and showed minimal effects on in vitro growth. Northern hybridization showed that the Δ23-tagged probe detected intracellular viral genome RNA as well as shorter RNAs that may represent heteroclite species, while the Δ87-tagged probe detected predominantly only genome length RNAs. When the tagged viruses were used to probe nsp2 protein in infected cells, perinuclear localization similar to native nsp2 was seen. Dual infection of Δ23-S and Δ87-S viruses allowed some discrimination of individual tagged nsp2 protein, facilitating future research. The mutants could potentially also be used to differentiate infected from vaccinated animals.

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

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

Comparison of Asian porcine high fever disease isolates of porcine reproductive and respiratory syndrome virus to United States isolates for their ability to cause disease and secondary bacterial infection in swine

Epidemiologic data from Asian outbreaks of highly-pathogenic (HP) porcine reproductive and respiratory syndrome virus (PRRSV) suggest that disease severity was associated with both the virulence of the PRRSV isolates and secondary bacterial infections. Previous reports have indicated that U.S. isolates of PRRSV predispose to secondary bacterial infections as well, but the severity of disease that occurred in Asia in pigs infected with these HP-PRRSV strains has not been reported in the U.S. The objectives of this research were to compare the pathogenesis of Asian and U.S. PRRSV isolates with regard to their ability to cause disease and predispose to secondary bacterial infections in swine. To address these objectives groups of pigs were infected with 1 of 2 Asian HP-PRRSV strains (rJXwn06 or rSRV07) or 1 of 2 U.S. PRRSV strains (SDSU73 or VR-2332) alone or in combination with Streptococcus suis, Haemophilus parasuis, and Actinobacillus suis. Pigs infected with rJXwn06 exhibited the most severe clinical disease while the pigs infected with rSRV07 and SDSU73 exhibited moderate clinical disease, and pigs infected with VR-2332 exhibited minimal clinical signs. The frequency of secondary bacterial pneumonia was associated with the clinical severity induced by the PRRSV strains evaluated. The levels of proinflammatory cytokines in the serum were often lower for pigs coinfected with virus and bacteria compared to pigs infected with PRRSV alone indicating an alteration in the immune response in coinfected pigs. Combined our results demonstrate that severity of disease appears to be dependent on virulence of the PRRSV strain, and development of secondary bacterial infection.

Interleukin-1 receptor antagonist: an early immunomodulatory cytokine induced by porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) infection poorly induces pro-inflammatory cytokines (IL-1, IL-6 and TNF-α) and type I IFN production during the early phase of infection. Our microarray analysis indicated strong upregulation of the IL1RA gene in type 2 PRRSV -infected monocyte-derived dendritic cells. Interleukin-1 receptor antagonist (IL-1Ra) is an early inhibitory cytokine that suppresses pro-inflammatory cytokines and T-lymphocyte responses. To investigate the induction of IL-1Ra by PRRSV, monocyte-derived dendritic cells were cultured with type 2 PRRSV or other swine viruses. PRRSV increased both IL1RA gene expression and IL-1Ra protein production in the culture. The enhanced production of IL-1Ra was further confirmed in PRRSV-cultured PBMC and PRRSV-exposed pigs by flow cytometry. Myeloid cell population appeared to be the major IL-1Ra producer both in vitro and in vivo. In contrast to the type 2 PRRSV, the highly pathogenic (HP)- PRRSV did not upregulate IL1RA gene expression in vitro. To determine the kinetics of PRRSV-induced IL1RA gene expression in relation to other pro-inflammatory cytokine genes, PRRSV-negative pigs were vaccinated with a commercially available type 2 modified-live PRRS vaccine or intranasally inoculated with HP-PRRSV. In modified-live PRRS vaccine pigs, upregulation of IL1RA, but not IL1B and IFNA, gene expression was observed from 2 days post- vaccination. Consistent with the in vitro findings, upregulation of IL1RA gene expression was not observed in the HP-PRRSV-infected pigs throughout the experiment. This study identified IL-1Ra as an early immunomodulatory mediator that could be involved in the immunopathogenesis of PRRSV infections.

Enhancement of innate immunity with granulocyte colony-stimulating factor did not mitigate disease in pigs infected with a highly pathogenic Chinese PRRSV strain

Porcine reproductive and respiratory syndrome virus (PRRSV) is responsible for one of the most economically important diseases in swine worldwide. It causes reproductive failure in sows and pneumonia in pigs that predisposes them to secondary bacterial infections. Methods to control PRRSV and/or limit secondary bacterial infections are desired to reduce the impact of this virus on animal health. Neutrophils play a major role in combatting infection; they can act as phagocytes as well as produce and release lytic enzymes that have potent antimicrobial effects leading to the destruction and clearance of bacterial pathogens. Granulocyte-colony stimulating factor (G-CSF) is a cytokine that controls the production, differentiation and function of granulocytes (including neutrophils) from the bone marrow. Recent work from our laboratory has shown that encoding porcine G-CSF in a replication-defective adenovirus (Ad5-G-CSF) and delivering a single dose to pigs induced a neutrophilia lasting more than two weeks. As secondary bacterial infection is a common occurrence following PRRSV infection, particularly following challenge with highly pathogenic (HP)-PRRSV, the aim of the current study was to evaluate the effectiveness of a single prophylactic dose of adenovirus-encoded G-CSF to mitigate secondary bacterial disease associated with HP-PRRSV infection. Administration of Ad5-G-CSF induced a significant neutrophilia as expected. However, between 1 and 2days following HP-PRRSV challenge the number of circulating neutrophils decreased dramatically in the HP-PRRSV infected group, but not the non-infected Ad5-G-CSF group. Ad5-G-CSF administration induced monocytosis as well, which was also reduced by HP-PRRSV challenge. There was no difference in the progression of disease between the Ad5-G-CSF and Ad5-empty groups following HP-PRRSV challenge, with pneumonia and systemic bacterial infection occurring in both treatment groups. Given the impact of HP-PRRSV infection on the neutrophilia induced by the Ad5-G-CSF administration, additional studies are warranted to evaluate the timing of Ad5-G-CSF induced neutrophilia and multiple G-CSF inoculations on protection against secondary bacterial infection following PRRSV infection. Nevertheless, this study may provide insight into the pathogenesis of HP-PRRSV.

Pathogenesis of highly pathogenic porcine reproductive and respiratory syndrome virus in Chinese Tibetan swine

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) was first characterized in 2006 in China, and it causes great economic losses to the Chinese swine production industry. A China Landrace pig, the Tibetan pig, which has striking phenotypic and physiological differences from lowland pigs, is mainly distributed in the Tibetan highlands of China. The susceptibility of the Tibetan pig to HP-PRRSV has not been reported. In this study, 15 4-week-old Tibetan piglets were divided into three groups, and their susceptibility to HP-PRRSV was examined in the highland region. Five pigs in group 1 were inoculated intranasally with HP-PRRSV strain BB0907. At 2days post-inoculation, five other pigs were introduced into this group and then removed to a separated room to serve as contact group 2. Meanwhile, five pigs in group 3 were mock infected and used as controls. The results showed that the pigs in the inoculated and contact groups showed high fevers and clear clinical signs, including depression, anorexia, lethargy, sticky eye secretions, and hind limb paralysis, with high mortality. The main symptom was interstitial pneumonia. Viremia appeared on days 4 to 14 post-infection. HP-PRRSV infection resulted in inflammatory responses within the first week of infection, as evidenced by the expression of tumor necrosis factor alpha, interleukin (IL)-1β, IL-6, and IL-10. All the data indicate that the Tibetan pig is susceptible to HP-PRRSV infection. Thus, it is necessary to investigate and prevent PRRSV infections in the highland region in China.

Pathogenicity comparison between highly pathogenic and NADC30-like porcine reproductive and respiratory syndrome virus

The pathogenicity of HNjz15, an NADC30-like strain of porcine reproductive and respiratory syndrome virus (PRRSV), was investigated and compared to that of a highly pathogenic PRRSV JAX1 strain. Six-week-old pigs infected with each virus showed typical clinical symptoms, including high fever and respiratory disorders. Pigs infected with JXA1 had more-severe clinical manifestations than pigs infected with HNjz15. HNjz15 replicated in vivo with kinetics similar to those of JXA1 but induced a lower level of PRRSV-specific antibody at the beginning of virus infection. Histopathologically, JXA1 infection led to more-severe lung lesions and broader organ tropism than HNjz15 did. Different from what was observed with the previously reported NADC30-like PRRSV JL580 strain, all HNjz15-infected pigs survived until the end of the study. All of these results indicated that NADC30-like PRRSV HNjz15 is virulent to pigs but is less pathogenic than the JXA1 and JL580 PRRSV strains.

Live porcine reproductive and respiratory syndrome virus vaccines: Current status and future direction

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) was reported in the late 1980s. PRRS still is a huge economic concern to the global pig industry with a current annual loss estimated at one billion US dollars in North America alone. It has been 20 years since the first modified live-attenuated PRRSV vaccine (PRRSV-MLV) became commercially available. PRRSV-MLVs provide homologous protection and help in reducing shedding of heterologous viruses, but they do not completely protect pigs against heterologous field strains. There have been many advances in understanding the biology and ecology of PRRSV; however, the complexities of virus-host interaction and PRRSV vaccinology are not yet completely understood leaving a significant gap for improving breadth of immunity against diverse PRRS isolates. This review provides insights on immunization efforts using infectious PRRSV-based vaccines since the 1990s, beginning with live PRRSV immunization, development and commercialization of PRRSV-MLV, and strategies to overcome the deficiencies of PRRSV-MLV through use of replicating viral vectors expressing multiple PRRSV membrane proteins. Finally, powerful reverse genetics systems (infectious cDNA clones) generated from more than 20 PRRSV isolates of both genotypes 1 and 2 viruses have provided a great resource for exploring many innovative strategies to improve the safety and cross-protective efficacy of live PRRSV vaccines. Examples include vaccines with diminished ability to down-regulate the immune system, positive and negative marker vaccines, multivalent vaccines incorporating antigens from other porcine pathogens, vaccines that carry their own cytokine adjuvants, and chimeric vaccine viruses with the potential for broad cross-protection against heterologous strains. To combat this devastating pig disease in the future, evaluation and commercialization of such improved live PRRSV vaccines is a shared goal among PRRSV researchers, pork producers and biologics companies.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Development of a genome copy specific RT-qPCR assay for divergent strains of type 2 porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) became a significant pathogen of swine upon its emergence in the late 1980s and since then has exemplified a rapidly evolving, constantly re-emerging pathogen. In addition to the challenges faced in development of vaccines and diagnostics, research on the basic molecular pathogenesis of PRRSV is also restrained by the ability to accurately and comparatively quantitate levels of replication in different tissues and between strains. This is further complicated by the presence of non-genomic RNA within infected tissues which are generally detected with equivalent efficiency by RT-qPCR based techniques, thereby introducing inherent error in these measurements that may differ significantly by tissue and strain. To address this, an RT-qPCR based technique was developed which targets the viral RNA-dependent RNA polymerase gene (nsp9) which is unique to genomic RNA, being absent from all subgenomic and heteroclite RNAs. This assay targets a region of considerable sequence conservation, and based on sequence only, should be quantitative for approximately 40% of all Type 2 PRRSV strains in GenBank for which nsp9 sequence is available. The assay was demonstrated to be linear over nine orders of magnitude (10(10)-10(2) copies) and can be readily adapted for multiplex detection of additional divergent PRRSV strains. This assay will add significantly to the ability to assess and compare PRRSV replication in a variety of tissues and between divergent strains, including highly pathogenic strains of considerable concern to the global pork industry.

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

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

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

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

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

A comparative study of the local cytokine response in the lungs of pigs experimentally infected with different PRRSV-1 strains: upregulation of IL-1α in highly pathogenic strain induced lesions

Porcine reproductive and respiratory syndrome viruses (PRRSV) show high genetic differences both among and within genotypes. Recently, several highly pathogenic PRRSV (HP-PRRSV) strains have been described. This study compares and characterizes the production of cytokines by pulmonary macrophages in pigs experimentally infected with four different PRRSV-1 strains: two low-virulent strains, Lelystad (LV) and a British field strain (215-06); a HP strain (SU1-bel) from Belarus and the attenuated vaccine strain DV (Porcilis(®) PRRS). Animals were clinically monitored and post-mortem examinations were performed at 3, 7 and 35 days post-infection (dpi). Lung samples were processed for histopathological and immunohistochemical studies by using specific antibodies against PRRSV, IL1-α, IL-6, TNF-α, IL-10 and IFN-γ. SU1-bel infected animals presented the highest mean scores for clinical observations, gross and microscopic lesions as well as for PRRSV expression compared with the other infected groups (p≤0.027). These animals displayed the highest expression of IL1-α at 7dpi, together with the highest score for lung pathology, whereas LV, 215-06 and DV inoculated animals only showed a transient enhancement in some of these cytokines. SU1-bel-infected pigs showed a positive correlation between the amount of PRRSV antigen and IL-1α expression (r=0.645, p<0.001). The highest expression of IL-10 was detected in 215-06-infected animals (p≤0.004), with a positive correlation with the numbers of virus-infected cells (r=0.375, p≤0.013). In conclusion, the HP-PRRSV SU1-bel strain replicated more efficiently in the lung of infected animals and induced a higher expression of IL-1α than the other PRRSV-1-infected groups, which may have played a key role in the onset of the clinical signs and interstitial pneumonia.

Comparison of experimental infection with northern and southern Vietnamese strains of highly pathogenic porcine reproductive and respiratory syndrome virus

The aim of this study was to compare the virulence of northern and southern Vietnamese strains of highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) as assessed by the level of viral replication, gross and microscopical lung lesions and virus distribution in experimentally infected pigs. The northern and southern Vietnamese HP-PRRSV strains share 96.7% (non-structural protein 2) and 99.3% (open reading frame 5) nucleotide identity. On experimental challenge, approximately 50% of pigs infected with northern Vietnamese HP-PRRSV died, while death was not observed in any pigs infected with southern Vietnamese HP-PRRSV. Mean viral titres (expressed as log(10)TCID(50)/ml) were significantly (P <0.05) higher in sera and lungs from pigs infected with the northern Vietnamese HP-PRRSV than from those infected with the southern Vietnamese strain at multiple time points. Lung lesion scores and PRRSV antigen within pulmonary and lymphoid lesions were significantly (P <0.05) higher in pigs infected with northern Vietnamese HP-PRRSV than in those receiving southern Vietnamese HP-PRRSV at multiple time points. PRRSV antigens were observed in cardiac myocytes, gastric and renal tubular epithelial cells and astrocytes and microglia of white matter in the brain from pigs infected with the northern Vietnamese HP-PRRSV strain only. Thus, genetic similarity did not predict the degree of virulence of these strains. Northern Vietnamese HP-PRRSV was more virulent and had extended tissue tropism when compared with southern Vietnamese HP-PRRSV.

Efficacy of Type 2 PRRSV vaccine against Chinese and Vietnamese HP-PRRSV challenge in pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant reproductive losses in the sow herd and respiratory disease in growing pigs. The virus belongs to the family Arteriviridae and there are two major genotypes. Type 1 is represented by Lelystad virus, the European prototype virus, and Type 2 is represented by the North American prototype virus, VR-2332. Depending on husbandry, immune status of the herd, and virulence of the isolate, the severity of disease and magnitude of economic loss can be variable. Vaccine use is not always successful indicating a lack of cross-protection between vaccine strains and circulating wild-type viruses. To date, there is no clear method to demonstrate if a vaccine confers protection against a specific isolate except for empirical animal studies. In 2006, a new lineage of Type 2 PRRSV emerged in Chinese swine herds that were suffering dramatic losses resulting in those viruses being described as "Highly Pathogenic PRRSV" (HP-PRRSV). Experimental reproduction of severe disease with HP-PRRSV isolates and virus derived from HP-PRRSV clones demonstrated the causal role of this virus. Recently, partial heterologous protection has been reported for Type 1 and Type 2 attenuated PRRSV vaccines against challenge by different Chinese HP-PRRSV isolates providing some hope for reducing economic loss. This paper reports the efficacy of a commercially available Type 2 attenuated vaccine in young pigs against heterologous challenge with a Chinese and Vietnamese HP-PRRSV isolate. When compared to unvaccinated pigs, vaccination decreased the length of viremia and viral titer, diminished the time of high fever and reduced macroscopic lung scores following homologous and heterologous PRRSV challenge. These results demonstrate the potential use of vaccine as an aid in the control of HP-PRRSV outbreaks.

The vOTU domain of highly-pathogenic porcine reproductive and respiratory syndrome virus displays a differential substrate preference

Arterivirus genus member Porcine reproductive and respiratory syndrome virus (PRRSV) causes an economically devastating disease, recently exacerbated by the emergence of highly pathogenic strains (HP-PRRSV). Within the nonstructural protein 2 of PRRSV is a deubiquitinating enzyme domain belonging to the viral ovarian tumor (vOTU) protease superfamily. vOTUs, which can greatly vary in their preference for their host ubiquitin (Ub) and Ub-like substrates such as interferon stimulated gene 15 (ISG15), have been implicated as a potential virulence factor. Since various strains of PRRSV have large variations in virulence, the specificity of vOTUs from two PRRSV strains of varying virulence were determined. While both vOTUs showed de-ubiquitinating activity and markedly low deISGylating activity, HP-PRRSV demonstrated a strong preference for lysine 63-linked poly-Ubiquitin, tied to innate immune response regulation. This represents the first report of biochemical activity unique to HP-PRRSV that has implications for a potential increase in immunosuppression and virulence.

Highly efficient expression of interleukin-2 under the control of rabbit β-globin intron II gene enhances protective immune responses of porcine reproductive and respiratory syndrome (PRRS) DNA vaccine in pigs

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) had caused catastrophic losses in swine industry in China. The current inactivated vaccine provided only limited protection, and the attenuated live vaccine could protect piglets against the HP-PRRSV but there was a possibility that the attenuated virus returned to high virulence. In this study, the eukaryotic expression vector pVAX1^© was modified under the control of rabbit β-globin intron II gene and the modified vector pMVAX1^© was constructed. Porcine interleukin-2 (IL-2) and GP3-GP5 fusion protein of HP-PRRSV strain SD-JN were highly expressed by pMVAX1^©. Mice inoculated with pMVAX1^©-GP35 developed significantly higher PRRSV-specific antibody responses and T cell proliferation than those vaccinated with pVAX1^©-GP35. pMVAX1^©-GP35 was selected as PRRS DNA vaccine candidate and co-administrated with pVAX1^©-IL-2 or pMVAX1^©-IL-2 in pigs. pMVAX1^©-IL-2+pMVAX1^©-GP35 could provide enhanced PRRSV-specific antibody responses, T cell proliferation, Th1-type and Th2-type cytokine responses and CTL responses than pMVAX1^©-GP35 and pVAX1^©-IL-2+pMVAX1^©-GP35. Following homologous challenge with HP-PRRSV strain SD-JN, similar with attenuated PRRS vaccine group, pigs inoculated with pMVAX1^©-IL-2+pMVAX1^©-GP35 showed no clinical signs, almost no lung lesions and no viremia, as compared to those in pMVAX1^©-GP35 and pVAX1^©-IL-2+pMVAX1^©-GP35 groups. It indicated that pMVAX1^©-IL-2 effectively increases humoral and cell mediated immune responses of pMVAX1^©-GP35. Co-administration of pMVAX1^©-IL-2 and pMVAX1^©-GP35 might be attractive candidate vaccines for preventing HP-PRRSV infections.

Highly divergent strains of porcine reproductive and respiratory syndrome virus incorporate multiple isoforms of nonstructural protein 2 into virions

Viral structural proteins form the critical intermediary between viral infection cycles within and between hosts, function to initiate entry, participate in immediate early viral replication steps, and are major targets for the host adaptive immune response. We report the identification of nonstructural protein 2 (nsp2) as a novel structural component of the porcine reproductive and respiratory syndrome virus (PRRSV) particle. A set of custom α-nsp2 antibodies targeting conserved epitopes within four distinct regions of nsp2 (the PLP2 protease domain [OTU], the hypervariable domain [HV], the putative transmembrane domain [TM], and the C-terminal region [C]) were obtained commercially and validated in PRRSV-infected cells. Highly purified cell-free virions of several PRRSV strains were isolated through multiple rounds of differential density gradient centrifugation and analyzed by immunoelectron microscopy (IEM) and Western blot assays using the α-nsp2 antibodies. Purified viral preparations were found to contain pleomorphic, predominantly spherical virions of uniform size (57.9 nm ± 8.1 nm diameter; n = 50), consistent with the expected size of PRRSV particles. Analysis by IEM indicated the presence of nsp2 associated with the viral particle of diverse strains of PRRSV. Western blot analysis confirmed the presence of nsp2 in purified viral samples and revealed that multiple nsp2 isoforms were associated with the virion. Finally, a recombinant PRRSV genome containing a myc-tagged nsp2 was used to generate purified virus, and these particles were also shown to harbor myc-tagged nsp2 isoforms. Together, these data identify nsp2 as a virion-associated structural PRRSV protein and reveal that nsp2 exists in or on viral particles as multiple isoforms.