Pathogenesis of porcine reproductive and respiratory syndrome virus infection in gnotobiotic pigs

Antiviral activity and underlying mechanisms of baicalin against porcine reproductive and respiratory syndrome virus in vitro

Porcine reproductive and respiratory syndrome (PRRS) is a major challenge for the global swine industry, causing huge economic losses worldwide. To date, there are no effective measures to prevent and control the spread of PRRS virus (PRRSV). Baicalin (BA) is a natural flavonoid with various pharmacological effects, including antiviral, anti-inflammatory, antioxidant and immunomodulatory. Here, we demonstrate that BA exhibits potent anti-PRRSV activity in vitro, BA concentrations in the range of 5-20 μg/mL significantly inhibited PRRSV infection in a dose-dependent manner and were independent of PRRSV strain. Mechanistically, BA inhibited PRRSV replication by directly interacting with virions, thereby affecting multiple stages of the virus life cycle. Meanwhile, the preventive effect of BA on PRRSV could be realized by inhibiting CD151 and CD163 expression. Furthermore, BA reduced the PRRSV-induced expression of PAMs cytokines (IFN-α, IL-6, IL-8, and TNF-α), suggesting that BA-induced antiviral cytokines may help BA inhibit PRRSV infection. Taken together, BA can be used as an inhibitor of PRRSV infection in vitro, which provides a theoretical basis for the clinical application of BA and the prevention and control of PRRSV infection, which is worthy of further in vivo studies in swine.

PRRSV alters m6A methylation and alternative splicing to regulate immune, extracellular matrix-associated function

The alternative splicing and N6-methyladenosine (m6A) modifications occurring during porcine reproductive and respiratory syndrome virus (PRRSV) infections remain poorly understood. Transcriptome and MeRIP-seq analyses were performed to identify the gene expression changes, splicing and m6A modifications in the lungs of PRRSV-infected pigs. In total, 1624 differentially expressed genes (DEGs) were observed between PRRSV-infected and uninfected pigs. We observed significant alterations in alternative splicing (54,367 events) and m6A modifications (2265 DASEs) in numerous genes, including LMO7, SLC25A27, ZNF185, and ECM1, during PRRSV infection. LMO7 and ZNF185 exhibited alternative splicing variants and reduced mRNA expression levels following PRRSV infection. Notably, LMO7 inhibited c-JUN, SMAD3, and FAK expression, whereas ZNF185 affected the expression of FAK, CDH1, and GSK3β downstream. Additionally, ECM1 influenced FAK expression by targeting ITGB3 and AKT2, suggesting its involvement in extracellular matrix accumulation through the ITGB3-AKT2/FAK pathway. These changes may facilitate viral invasion and replication by modulating the expression of genes and proteins participating in crucial cellular processes associated with immunity and the extracellular matrix. We highlight the importance of these genes and their associated pathways in PRRSV infections and suggest that targeting these may be a promising therapeutic approach for treating viral infections.

Pathogenicity characterization of PRRSV-1 181187-2 isolated in China

PRRSV-1 has caused more clinical infections in pigs in Chinese swine herds in recent years, however, the pathogenicity of PRRSV-1 in China is unclear. In order to study the pathogenicity of PRRSV-1, in this study, a PRRSV-1 strain, 181187-2, was isolated in primary alveolar macrophage (PAM) cells from a farm where abortions had been reported in China. The complete genome of 181187-2 was 14932 bp excluding Poly A, with 54-amino acid continuous deletion in the Nsp2 gene and 1 amino deletion in ORF3 gene compared with LV. Additionally, the piglets inoculated with strain 181187-2 by intranasal and intranasal plus intramuscular injection, animal experiments showed clinical symptoms including transient fever and depression, with no death. The obvious histopathological lesions including interstitial pneumonia and lymph node hemorrhage, and there were no significant differences in clinical symptoms and histopathological lesions with different challenge ways. Our results indicated that PRRSV -1 181187-2 was a moderately pathogenic strain in piglets.

Cell-penetrating porcine single-chain antibodies (transbodies) against nonstructural protein 1β (NSP1β) of porcine reproductive and respiratory syndrome virus inhibit virus replication

Porcine reproductive and respiratory syndrome virus (PRRSV) causes porcine reproductive and respiratory syndrome (PRRS) worldwide, especially in domestic pigs, with an enormous economic impact, estimated at $664 million in losses every year to the pig industry. Current vaccines confer limited protection, and no direct-acting anti-PRRS treatment is available. Non-structural protein (NSP) 1β, a cysteine-like protease (CLPro) of PRRSV plays an essential role in viral polyprotein processing, subgenomic RNA synthesis, and evasion of host innate immunity. Therefore, agents that interfere with the bioactivity of NSP1β would be expected to inhibit virus replication. In this study, a porcine single-chain antibody (scFv)-phage display library was constructed and used as a tool for production of NSP1β-specific porcine scFvs (pscFvs). The pscFvs to NSP1β were linked to a cell-penetrating peptide to form cell-penetrating pscFvs (transbodies), which could be internalized and inhibit PRRSV replication in infected cells. A computer simulation indicated that the effective pscFvs used several residues in multiple complementarity determining regions (CDRs) to interact with multiple residues in the CLPro and C-terminal motifs, which might explain the mechanism of pscFv-mediated inhibition of virus replication. Although experiments are needed to determine the antiviral mechanism of the transbodies, the current data indicate that transbodies can potentially be applied for treatment and prevention of PRRSV infection.

TAT Nanobody Exerts Antiviral Effect against PRRSV In Vitro by Targeting Viral Nucleocapsid Protein

Porcine reproductive and respiratory syndrome (PRRS) is caused by the PRRS virus (PRRSV), which has brought huge economic losses to the pork industry worldwide since its first discovery in the late 1980s in North America. To date, there are no effective commercial vaccines or therapeutic drugs available for controlling the spread of PRRSV. Due to their unique advantages of high affinity and high specificity, nanobodies (Nbs) have received increasing attention in the process of disease diagnosis and treatment. Trans-activator transcription (TAT) can serve as a vector to carry specific proteins into cells by passing through cell membranes. In our previous study, a specific Nb against the PRRSV nucleocapsid (N) protein was screened using phage display technology. For this study, we developed a novel recombinant protein constituting a TAT-conjugated Nb, which we call TAT-Nb1. The target cell entry efficiency of TAT-Nb1 and its effect on PRRSV infection and replication were then investigated. Our results indicate that TAT delivered Nb1 into Marc-145 cells and porcine alveolar macrophages (PAMs) in a dose- and time-dependent manner. Furthermore, TAT-Nb1 dose-dependently suppressed PRRSV infection and replication, where this antiviral effect was independent of PRRSV strain. Co-immunoprecipitation results revealed that Nb1 efficiently interacted with the N protein of PRRSV. Taken together, the presented results suggest that TAT-Nb1 can effectively suppress PRRSV replication, and it may be considered as a new anti-PRRSV candidate drug.

Streptococcus suis contributes to inguinal lymph node lesions in piglets after highly pathogenic porcine reproductive and respiratory syndrome virus infection

The swine pathogens porcine reproductive and respiratory syndrome virus (PRRSV) and Streptococcus suis have both been reported to cause damage to the immune organs. Inguinal lymph node (ILN) injury has been reported in PRRSV-infected pigs with secondary S. suis infection, but not much is known about the mechanism. In this study, secondary S. suis infection after highly pathogenic (HP)-PRRSV infection caused more severe clinical symptoms, mortality, and ILN lesions. Histopathological lesions were seen in ILNs with a marked decrease in lymphocyte numbers. Terminal deoxynucleotidyl transferase (TdT)-mediated de-oxyuridine triphosphate (dUTP)-biotin nick end-labeling (TUNEL) assays revealed that HP-PRRSV strain HuN4 alone induced ILN apoptosis, but dual-infection with S. suis strain BM0806 induced greater levels of apoptosis. Besides, we found that some HP-PRRSV-infected cells underwent apoptosis. Furthermore, anti-caspase-3 antibody staining confirmed that ILN apoptosis was mainly induced by a caspase-dependent pathway. Pyroptosis was also observed in HP-PRRSV-infected cells, and there was more pyroptosis in piglets infected with HP-PRRSV alone compared with those with secondary S. suis infection, and HP-PRRSV-infected cells underwent pyroptosis. Altogether, this is the first report to identify pyroptosis in ILNs and which signaling pathway is related to ILN apoptosis in single or dual-infected piglets. These results contribute to a better understanding of the pathogenic mechanisms during secondary S. suis infection.

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.

Porcine reproductive and respiratory syndrome virus RNA detection in tongue tips from dead animals

The control of porcine reproductive and respiratory syndrome virus (PRRSV) hinges on monitoring and surveillance. The objective of this study was to assess PRRSV RNA detection by RT-PCR in tongue tips from dead suckling piglets compared to serum samples, processing fluids, and family oral fluids. Tongue tips and serum samples were collected from three PRRSV-positive breeding herd farms (farms A, B, and C) of three different age groups: newborns (<24 h), processing (2 to 7 days of age), and weaning (18 to 22 days of age). Additionally, processing fluids and family oral fluids were collected from 2–7 days of age and weaning age, respectively. In farms A and B, PRRSV RNA was detected in tongue tips from all age groups (100 and 95%, respectively). In addition, PRRSV RNA was detected in pooled serum samples (42 and 27%), processing fluids (100 and 50%), and family oral fluids (11 and 22%). Interestingly, the average Ct value from tongue tips was numerically lower than the average Ct value from serum samples in the newborn age. In farm C, PRRSV RNA was only detected in serum samples (60%) and family oral fluids (43%), both from the weaning age. Further, no PRRSV RNA was detected in tongue tips when pooled serum samples from the same age group tested PRRSV RNA-negative. Taken together, these results demonstrate the potential value of tongue tips for PRRSV monitoring and surveillance.

Frequency of porcine circovirus 3 detection and histologic lesions in clinical samples from swine in the United States

Porcine circovirus 3 (PCV3) is widespread in pigs worldwide. Diverse clinical signs and lesions have been associated with PCV3, but the role of PCV3 as a cause of disease in swine remains unclear. We investigated the association of PCV3 with clinical signs and histologic lesions in 730 diagnostic swine cases between February 2016 and January 2018. The cases contained 2,177 samples submitted from 474 sites located in 21 states in the United States. PCR assay results were positive for PCV3 for 577 of 2,177 (27%) samples, 255 of 730 (35%) cases, 181 of 474 (38%) sites, and 17 of 21 (81%) states. We detected PCV3 in 19 of 28 specimen types and in pigs of all ages and clinical presentations, including healthy pigs, with the highest detection rate in adult pigs. PCV3 detection was not associated with respiratory, gastrointestinal, or CNS signs, weight loss, or sudden death. Of 58 types of histologic lesions evaluated, PCV3 detection was associated with myocarditis, cardiac vasculitis, and interstitial pneumonia in growing pigs. A high PCV3 detection rate was observed in aborted fetuses.

Virological and Histopathological Findings in Boars Naturally Infected With Porcine Reproductive and Respiratory Syndrome Virus Type 1

Major geographical transmission of porcine reproductive and respiratory syndrome virus (PRRSV) occurs via semen when a boar stud is infected. This happened in Denmark in 2019, providing an opportunity to compare previous experimental PRRSV boar studies with natural PRRSV-1 infection in boars. The aim of this study was to investigate the association between the presence of PRRSV RNA in serum, semen, testicles, and epididymis of boars naturally infected with PRRSV and to describe the histological lesions in the testes and epididymis combined with direct visualisation of PRRSV-infected cells by immunohistochemical staining (IHC). The exact timing of infection of each boar was not determined, but based on serology the boars were divided into two groups: acute and late infections. All boars included were sampled the same day. In this study, 35 boars and 10 healthy boars from another PRRSV-negative boar stud were included as histological controls. PRRSV RNA was found most often in serum (51%) and least frequently in semen (22%) and was more often detected in the reproductive tract in the acute phase of infection (p < 0.0001; RR: 2.58). Mononuclear cells and multinuclear giant cells were present in the adluminal compartment of the testis and epididymis in PRRSV-infected boars, but not in control boars (p < 0.05), which supports the hypothesis that macrophages are involved in the venereal spread of the virus.

PRRSV Induces HMGB1 Phosphorylation at Threonine-51 Residue to Enhance Its Secretion

Porcine reproductive and respiratory syndrome virus (PRRSV) induces secretion of high mobility group box 1 (HMGB1) to mediate inflammatory response that is involved in the pulmonary injury of infected pigs. Our previous study indicates that protein kinase C-delta (PKC-delta) is essential for HMGB1 secretion in PRRSV-infected cells. However, the underlying mechanism in HMGB1 secretion induced by PRRSV infection is still unclear. Here, we discovered that the phosphorylation level of HMGB1 in threonine residues increased in PRRSV-infected cells. A site-directed mutagenesis study showed that HMGB1 phosphorylation at threonine-51 was associated with HMGB1 secretion induced by PRRSV infection. Co-immunoprecipitation (co-IP) of HMGB1 failed to precipitate PKC-delta, but interestingly, mass spectrometry analysis of the HMGB1 co-IP product showed that PRRSV infection enhanced HMGB1 binding to ribosomal protein S3 (RPS3), which has various extra-ribosomal functions. The silencing of RPS3 by siRNA blocked HMGB1 secretion induced by PRRSV infection. Moreover, the phosphorylation of HMGB1 at threonine-51 was correlated with the interaction between HMGB1 and RPS3. In vivo, PRRSV infection also increased RPS3 levels and nuclear accumulation in pulmonary alveolar macrophages. These results demonstrate that PRRSV may induce HMGB1 phosphorylation at threonine-51 and increase its interaction with RPS3 to enhance HMGB1 secretion. This finding provides insights into the pathogenesis of PRRSV infection.

Safety of PRRSV-2 MLV vaccines administrated via the intramuscular or intradermal route and evaluation of PRRSV transmission upon needle-free and needle delivery

Two distinct experiments (Exp) were conducted to evaluate the shedding and efficacy of 2 modified live porcine reproductive and respiratory syndrome virus (PRRSV) type 2 vaccines (MLV) when administered intramuscularly (IM) or intradermally (ID) (Exp A), and the potential of PRRSV transmission using a needle-free device (Exp B). One-hundred fifty-four, 3-week-old castrated-male, pigs were procured from a PRRSV-free herd. In Exp A, 112 pigs were randomly allocated into 4 groups of 21 pigs including IM/Ingelvac MLV (G1), IM/Prime Pac (G2), ID/Prime Pac (G3), and non-vaccination (G4). Twenty-eight remaining pigs were served as non-vaccination, age-matched sentinel pigs. G1 was IM vaccinated once with Ingelvac PRRS MLV (Ing) (Boehringer Ingelheim, Germany). G2 and G3 were IM and ID vaccinated once with a different MLV, Prime Pac PRRS (PP) (MSD Animal Health, The Netherlands), respectively. Following vaccination, an antibody response, IFN-γ-SC, and IL-10 secretion in supernatants of stimulated PBMC were monitored. Sera, tonsils, nasal swabs, bronchoalveolar lavage, urines, and feces were collected from 3 vaccinated pigs each week to 42 days post-vaccination (DPV) and assayed for the presence of PRRSV using virus isolation and qPCR. Age-matched sentinel pigs were used to evaluate the transmission of vaccine viruses and were introduced into vaccinated groups from 0 to 42 DPV. Seroconversion was monitored. In Exp B, 42 pigs were randomly allocated into 5 groups of 3 pigs each including IM/High (T1), ID/High (T2), IM/Low (T3), ID/Low (T4), and NoChal. Twenty-seven remaining pigs were left as non-challenge, age-matched sentinel pigs. The T1 and T2, and T3 and T4 groups were intranasally challenged at approximately 26 days of age with HP-PRRSV-2 at high (10⁶) and low (10³ TCID₅₀/ml) doses, respectively. At 7 days post-challenge, at the time of the highest viremia levels of HP-PRRSV-2, T1 and T2, and T3 and T4 groups were IM and ID injected with Diluvac Forte using needles and a need-less device (IDAL 3G, MSD Animal Health, The Netherlands), respectively. Same needles or needle-less devices were used to inject the same volume of Diluvac Forte into sentinel pigs. Seroconversion of sentinels was evaluated. The results demonstrated that PP vaccinated groups (G2 and G3), regardless of the route of vaccination, had ELISA response significantly lower than G1 at 7 and 14 DPV. PP-vaccinated groups (G2 and G3) had significantly higher IFN-γ-SC and lower IL-10 secretion compared to the Ing-vaccinated group (G1). The two different MLV when administered intramuscularly demonstrated the difference in virus distribution and shedding patterns. PP-vaccinated pigs had significantly shortened viremia than the Ing-vaccinated pigs. However, ID-vaccinated pigs had lower virus distribution in organs and body fluids without virus shedding to sentinel pigs. In Exp B, regardless of the challenge dose, sentinel pigs intradermally injected with the same needle-less device used to inject challenged pigs displayed no seroconversion. In contrast, sentinel pigs intramuscularly injected with the same needle used to inject challenged pigs displayed seroconversion. The results demonstrated the transmission of PRRSV by using a needle, but not by using a needle-less device. In conclusion, our results demonstrated that ID vaccination might represent an alternative to improve vaccine efficacy and safety, and may be able to reduce the shedding of vaccine viruses and reduce the iatrogenic transfer of pathogens between animals with shared needles.

Interferon-Induced Transmembrane Protein 3 Is a Virus-Associated Protein Which Suppresses Porcine Reproductive and Respiratory Syndrome Virus Replication by Blocking Viral Membrane Fusion

Porcine reproductive and respiratory syndrome (PRRS), which is caused by PRRS virus (PRRSV), is of great economic significance to the swine industry. Due to the complicated immune escape mechanisms of PRRSV, there are no effective vaccines or therapeutic drugs currently available against PRRS. Identification of cellular factors and underlying mechanisms that establish an effective antiviral state against PRRSV can provide unique strategies for developing antiviral vaccines or drugs. As an interferon (IFN)-stimulated gene, the role of IFN-induced transmembrane 3 (IFITM3) in PRRSV infection has not been reported as of yet. In the present study, it was shown that IFITM3 can exert a potent anti-PRRSV effect, and PRRS virions are trafficked to IFITM3-containing cell vesicles, where viral membrane fusion is impaired by cholesterol accumulation that is induced by IFITM3. Additionally, both endogenous and exogenous IFITM3 are incorporated into newly assembled progeny virions, and this decreased their intrinsic infectivity.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection eliminates production of type I interferons (IFNs) in host cells, which triggers an antiviral immune response through the induction of downstream IFN-stimulated genes (ISGs), thus escaping the fate of host-mediated clearance. The IFN-induced transmembrane 3 (IFITM3) has recently been identified as an ISG and plays a pivotal role against enveloped RNA viruses by restricting cell entry. However, the role of IFITM3 in PRRSV replication is unknown. The present study demonstrated that overexpression of IFITM3 suppresses PRRSV replication, while silencing of endogenous IFITM3 prominently promoted PRRSV replication. Additionally, it was shown that IFITM3 undergoes S-palmitoylation and ubiquitination modification, and both posttranslational modifications contribute to the anti-PRRSV activity of IFITM3. Further study showed that PRRSV particles are transported into endosomes and then into lysosomes during the early stages of infection, and confocal microscopy results revealed that PRRSV particles are transported to IFITM3-positive cellular vesicles. By using a single virus particle fluorescent labeling technique, we confirmed that IFITM3 can restrict PRRSV membrane fusion by inducing accumulation of cholesterol in cellular vesicles. Additionally, we found that both endogenous and exogenous IFITM3 are incorporated into newly producing PRRS virions and diminish viral intrinsic infectivity. By using cell coculture systems, we found that IFITM3 effectively restricted PRRSV intercellular transmission, which may have been caused by disrupted membrane fusion and reduced viral infectivity. In conclusion, our results demonstrate, for the first time, that swine IFITM3 interferes with the life cycle of PRRSV, and possibly other enveloped arteritis viruses, at multiple steps.

IMPORTANCE Porcine reproductive and respiratory syndrome (PRRS), which is caused by PRRS virus (PRRSV), is of great economic significance to the swine industry. Due to the complicated immune escape mechanisms of PRRSV, there are no effective vaccines or therapeutic drugs currently available against PRRS. Identification of cellular factors and underlying mechanisms that establish an effective antiviral state against PRRSV can provide unique strategies for developing antiviral vaccines or drugs. As an interferon (IFN)-stimulated gene, the role of IFN-induced transmembrane 3 (IFITM3) in PRRSV infection has not been reported as of yet. In the present study, it was shown that IFITM3 can exert a potent anti-PRRSV effect, and PRRS virions are trafficked to IFITM3-containing cell vesicles, where viral membrane fusion is impaired by cholesterol accumulation that is induced by IFITM3. Additionally, both endogenous and exogenous IFITM3 are incorporated into newly assembled progeny virions, and this decreased their intrinsic infectivity.

Upregulation of HMGB1 secretion in lungs of pigs infected by highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) remains a major driver for substantial economic losses to the swine industry across the world. Pulmonary inflammatory injury is a common manifestation in infected pigs. Previous studies reported that PRRS virus (PRRSV) induces secretion of high mobility group box 1 (HMGB1), a proinflammatory factor, in cultured cells. The objective of this study was to evaluate whether HMGB1 secretion is associated with PRRSV-induced pulmonary inflammatory responses in the early stage of infection in vivo. Three-week-old piglets were inoculated with either HuN4, a highly pathogenic PRRSV (HP-PRRSV) strain, or CH1R, an avirulent PRRSV vaccine strain. Necropsy was performed at 7 days post-infection. The results showed that HuN4 significantly induced the secretion of HMGB1 and inflammatory cytokines (IL-1β, IL-6) into the bronchoalveolar lavage fluid (BALF). HuN4 infection induced severe interstitial pneumonia in the pigs. In contrast, pigs infected by CH1R had mild lung inflammation with minimal HMGB1 secretion. In addition, high viral load of HuN4 was detected in both pulmonary alveolar macrophages (PAMs) and lung tissue, whereas viral RNA of CH1R was confined to PAMs. In consistent with the pneumonia development, HuN4 induced inflammatory cytokines in both PAMs and lung tissue, while their expression in CH1R-infected pigs confined only to PAMs. These results indicate that the HuN4-induced HMGB1 secretion into BALF may enhance the pulmonary inflammatory response and exacerbate the lung injury. This finding provides insights to the inflammatory response and pathogenesis of the HP-PRRSV infection.

Histopathological Lesions Accompanied with First-Time Isolation of a PRRSV-2 Strain in Greece

Genotype 2 strains of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2) have been reported sporadically in Europe. Even if, PRRSV-2 reported to be genetically homogenous in Europe due to the introduction of an MLV vaccine strain, independent introductions of PRRSV-2 field strains have been reported. The aim of the present study was to report the complete genome sequence and evaluate the histopathological lesions of a PRRSV-2 strain, isolated for the first time in Greece. During a routine blood sampling in a commercial pig farm, the results revealed positive samples in weaners of 40-60 days for the PRRSV-2, using real-time polymerase chain reaction. The clinical picture was characterized from respiratory symptoms in weaners, as well as coughing and poor performance at finishing stage and less than 3% mortality rate from weaning stage to finishing stage. The use of ORF5 for PRRSV phylogenetic analysis of the isolated PRRSV strain, named "x1544-1 strain", was successfully determined, belonging to the genotype PRRSV-2. Comparison of the obtained sequence revealed nucleotide sequence identity >98% with PRRSV-2 strain VR2332 and other related strains from Denmark and China. The histopathological evaluation revealed diffuse interstitial pneumonia, multifocal interstitial nephritis, while in the lymphoid organs, follicular and paracortical hyperplasia, coexisting with necrosis and depletion of germ cells were detected. The results of current study undersign the importance for veterinary practitioners to have up-to-date access to phylogenetic data linked to phenotypic information to follow-up the control and prevention strategies against PRRSV.

Identification of enterobacteria in viscera of pigs afflicted with porcine reproductive and respiratory syndrome and other viral co-infections

In order to investigate enterobacteria presence involved in the secondary infections in Porcine Reproductive and Respiratory Syndrome (PRRS) pigs with different viral co-infections, we identified enterobacteria for guiding clinical treatment. Twenty-one diseased pigs were diagnosed with the PRRS virus (PRRSV) and other 7 virus primers by PCR/RT-PCR in the lung and spleen samples. Enterobacteria were isolated using MacConkey agar from 5 visceral samples of PRRS pigs, and identified by 16S rDNA sequencing. PRRSV was positive in 100% of the lung samples and 81.0% of the spleen samples. Seven diseased pigs were diagnosed with only PRRSV infection (33.3%), 7 pigs with PRRSV and 1 or 2 other viruses (33.3%) and 7 pigs with PRRSV and more than 2 types of other viruses (33.3%). PRRSV was more inclined to co-infect pigs with porcine group A rotavirus (PARV) with the co-infection rate of 52.4% (11/21). Approximately 13 types of bacteria were successfully isolated from lung, spleen, liver, kidney and lymph node samples of different PRRS pigs. Enterobacteria were isolated in 100% of lung, liver and lymph samples from pigs infected with PRRSV alone. However, the isolation rates were significantly decreased in the more than 3 viruses co-infection group. Escherichia coli was the most prevalent bacterium, followed by Morganella, Proteus, Shigella, Salmonella, Klebsiella and Aeromonas. Most of the isolated enterobacteria were opportunistic pathogens. Therefore, timely combination with antimicrobial agents is necessary for effective treatment of PRRS-infected pigs.

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PRRSV was more inclined to co-infect with PARV.

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Pigs co-infected with more virus, less enterobacteria were isolated from their viscera.

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Enterobacteria isolated from viscera of PRRS pigs were opportunistic pathogens.

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coli, Morganella and Proteus were the most prevalent enterobacteria.

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Secondary infection of enterobacteria should be considered in PRRS treatment.

Full-genome analysis and pathogenicity of a genetically distinct Russian PRRSV-1 Tyu16 strain

Porcine reproductive and respiratory syndrome virus-1 (PRRSV-1) strains from Eastern Europe have a high diversity. All three known subtypes (1, 2, 3) of PRRSV-1 have been detected in Russia. There are two different groups of viruses belonging to the subtype 1: pan-European subtype 1 strains, and insufficiently studied Russian strains. The main objective of this study was to characterize the full genomic structure of the atypical Tyu16 strain of the Russian group subtype 1 PRRSV-1 and to assess its pathogenicity. Complete sequencing of the Tyu16 strain revealed that it did not belong to any existing subtype. Comparison of the whole genome sequence of the Tyu16 strain with that of PRRSV-1 prototype strains revealed 78.1 % (subtype 1 Lelystad), 78.1 % (subtype 2 WestSib13) and 77.7 % (subtype 3 Lena) nucleotide identity level, respectively. The coding sequence of different parts of the Tyu16 strain genome demonstrated a varying percentage identity to the different reference PRRSV-1 strains, which may indicate recombination events in its evolutionary history. We assume that among PRRSV-1 isolates, the Tyu16 is the closest relative to the common ancestor of PRRSV-1 and PRRSV-2. Low pathogenicity of the Tyu16 was demonstrated by experimental infection of 70-day-old piglets. Infected animals showed fever not exceeding 7 days, dyspnea in two out of five pigs and reduced weight gain. The virus shedding was undetectable and viremia was at low level.

Evaluation of local and systemic immune responses in pigs experimentally challenged with porcine reproductive and respiratory syndrome virus

The host-associated defence system responsible for the clearance of porcine reproductive and respiratory syndrome virus (PRRSV) from infected pigs is currently poorly understood. To better understand the dynamics of host–pathogen interactions, seventy-five of 100 pigs infected with PRRSV-JA142 and 25 control pigs were euthanized at 3, 10, 21, 28 and 35 days post-challenge (dpc). Blood, lung, bronchoalveolar lavage (BAL) and bronchial lymph node (BLN) samples were collected to evaluate the cellular immune responses. The humoral responses were evaluated by measuring the levels of anti-PRRSV IgG and serum virus-neutralizing (SVN) antibodies. Consequently, the highest viral loads in the sera and lungs of the infected pigs were detected between 3 and 10 dpc, and these resulted in moderate to mild interstitial pneumonia, which resolved accompanied by the clearance of most of the virus by 28 dpc. At peak viremia, the frequencies of alveolar macrophages in infected pigs were significantly decreased, whereas the monocyte-derived DC/macrophage and conventional DC frequencies were increased, and these effects coincided with the early induction of local T-cell responses and the presence of proinflammatory cytokines/chemokines in the lungs, BAL, and BLN as early as 10 dpc. Conversely, the systemic T-cell responses measured in the peripheral blood mononuclear cells were delayed and significantly induced only after the peak viremic stage between 3 and 10 dpc. Taken together, our results suggest that activation of immune responses in the lung could be the key elements for restraining PRRSV through the early induction of T-cell responses at the sites of virus replication.

Nonstructural Protein 11 of Porcine Reproductive and Respiratory Syndrome Virus Induces STAT2 Degradation To Inhibit Interferon Signaling

Interferons (IFNs) play a crucial role in host antiviral response by activating the JAK/STAT (Janus kinase/signal transducer and activator of transcription) signaling pathway to induce the expression of myriad genes. STAT2 is a key player in the IFN-activated JAK/STAT signaling. Porcine reproductive and respiratory syndrome virus (PRRSV) is an important viral pathogen, causing huge losses to the swine industry. PRRSV infection elicits a meager protective immune response in pigs. The objective of this study was to investigate the effect of PRRSV on STAT2 signaling. Here, we demonstrated that PRRSV downregulated STAT2 to inhibit IFN-activated signaling. PRRSV strains of both PRRSV-1 and PRRSV-2 species reduced the STAT2 protein level, whereas the STAT2 transcript level had minimal change. PRRSV reduced the STAT2 level in a dose-dependent manner and shortened STAT2 half-life significantly from approximately 30 to 5 h. PRRSV-induced STAT2 degradation could be restored by treatment with the proteasome inhibitor MG132 and lactacystin. In addition, PRRSV nonstructural protein 11 (nsp11) was identified to interact with and reduce STAT2. The N-terminal domain (NTD) of nsp11 was responsible for STAT2 degradation and interacted with STAT2 NTD and the coiled-coil domain. Mutagenesis analysis showed that the amino acid residue K59 of nsp11 was indispensable for inducing STAT2 reduction. Mutant PRRSV with the K59A mutation generated by reverse genetics almost lost the ability to reduce STAT2. Together, these results demonstrate that PRRSV nsp11 antagonizes IFN signaling via mediating STAT2 degradation and provide further insights into the PRRSV interference of the innate immunity.IMPORTANCE PRRSV infection elicits a meager protective immune response in pigs. One of the possible reasons is that PRRSV antagonizes interferon induction and its downstream signaling. Interferons are key components in the innate immunity and play crucial roles against viral infection and in the activation of adaptive immune response via JAK/STAT signaling. STAT2 is indispensable in the JAK/STAT signaling since it is also involved in activation of antiviral activity in the absence of STAT1. Here, we discovered that PRRSV nsp11 downregulates STAT2. Interestingly, the N-terminal domain of nsp11 is responsible for inducing STAT2 degradation and directly interacts with STAT2 N-terminal domain. We also identified a crucial amino acid residue K59 in nsp11 since a mutation of it led to loss of the ability to downregulate STAT2. A mutant PRRSV with mutation of K59 had minimal effect on STAT2 reduction. Our data provide further insights into PRRSV interference with interferon signaling.

Direct Interaction Between CD163 N-Terminal Domain and MYH9 C-Terminal Domain Contributes to Porcine Reproductive and Respiratory Syndrome Virus Internalization by Permissive Cells

Porcine reproductive and respiratory syndrome virus (PRRSV) has a highly restricted tropism for cells of the monocyte-macrophage lineage, including porcine alveolar macrophages (PAMs). PRRSV entry into permissive cells involves several mediators in addition to two required host cell receptors, CD163 and MYH9. It is unknown whether CD163 directly interacts and/or cooperates with MYH9 to facilitate PRRSV infection. In this study, CD163 and MYH9 were co-immunoprecipitated from PAMs regardless of PRRSV infection status. Further truncation analysis indicated that the CD163 N-terminal region, containing scavenger receptor cysteine-rich domains 1 to 4 (SRCR1-4), directly interacts with the MYH9 C-terminal domain region without involvement of other adaptor proteins. Meanwhile, non-permissive HEK293T cells that stably expressed truncated swine CD163 SRCR1-4 domain did not support virus attachment. However, virus attachment to cells stably expressing SRCR5-CT domain was demonstrated to occur without appreciable virus internalization. The involvement of the SRCR1-4 domain in virus internalization was further demonstrated by the fact that incubation of recombinant SRCR1-4 protein with PAMs abolished subsequent virus internalization by permissive cells. These results demonstrated that CD163 SRCR1-4 interacts with the MYH9 C–terminal domain to facilitate PRRSV virion internalization in permissive cells, thus expanding our understanding of PRRSV cell-invasion mechanisms.

Characterization of age-related susceptibility of macrophages to porcine reproductive and respiratory syndrome virus

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is the most economically important disease affecting swine production worldwide. The severity and susceptibility of PRRSV infection varies with age. Nursery pigs have been shown to be more susceptible to PRRSV infection and a more severe and prolonged infection is observed as compared to growing or adult pigs. However, antibody responses to PRRSV are observed independent of age. Swine are the only known hosts of PRRSV, infection is restricted to cells of monocytic lineage, and fully differentiated porcine alveolar macrophages are the primary target of natural infection. Pulmonary intravascular macrophages from young pigs have been shown to be more susceptible to infection than those from adult pigs. A better understanding of why young pigs and macrophages from young pigs are more susceptible to PRRSV infection is critical to identify mechanisms of infection that can be explored for enhanced treatment or prevention of disease. This study examined PRRSV susceptibility of porcine alveolar macrophages isolated from the lungs of pigs of different age groups, and the presence of cell surface receptors to determine if differences correlated with infection level. The younger the pigs were, the more susceptible the macrophage were to PRRSV infection, but no differences in cellular receptor expression were observed between pigs of different ages. Resistance to infection is likely related to intracellular innate immune mechanisms rather than receptor-mediated entry.

Tandem 3' UTR Patterns and Gene Expression Profiles of Marc-145 Cells During PRRSV Infection

Porcine reproductive and respiratory syndrome virus (PRRSV) causes substantial economic losses to the global pig industry. Alternative polyadenylation (APA) is a mechanism that diversifies gene expression, which is important for tumorigenesis, development, and cell differentiation. However, it is unclear whether APA plays a role in the course of PRRSV infection. To address this issue, in this study we carried out a whole-genome transcriptome analysis of PRRSV-infected Marc-145 African green monkey kidney cells and identified 185 APA switching genes and 393 differentially expressed genes (DEGs). Most of these genes were involved in cellular process, metabolism, and biological regulation, and there was some overlap between the two gene sets. DEGs were found to be more directly involved in the antiviral response than APA genes. These findings provide insight into the dynamics of host gene regulation during PRRSV infection and a basis for elucidating the pathogenesis of PRRSV.

Investigating porcine parvoviruses genogroup 2 infection using in situ polymerase chain reaction

Background

Porcine parvovirus 2 (PPV2) was detected in swine serum without showing any relationship with disease. The emergence of the virus seemed to be a unique event until other genetically highly similar parvoviruses were identified in China and, later in 2012, the presence of the virus was also described in Europe. PPV2 is widely distributed in pig populations where it is suspected to be involved in respiratory conditions, based on its frequent detection in lung samples. In order to investigate the potential pathogenic involvement of PPV2, 60 dead pigs were examined from two farms. They were necropsied and tested for PPV2 and PCV2 (Porcine circovirus type 2) by PCR; by Brown and Brenn (B&B) staining for bacteria; by immunohistochemistry (IHC) to detect CD3, Swine leukocyte antigen class II DQ (SLAIIDQ), lysozyme, porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza (SIV), Mycoplasma hyopneumoniae (Mhyo); and by in situ hybridization (ISH) to detect ssDNA and dsDNA of PCV2. PPV2 positive samples were subjected to in situ polymerase chain reaction (IS-PCR) including double staining method to detect PPV2 and host cell markers. To calculate statistical difference we used GENMOD or LOGISTIC procedures in Statistical Analysis System (SAS®).

Results

We found that the PPV2 was localized mostly in lymphocytes in lungs, lymph nodes and liver. Neither CD3 antigen nor lysozyme was expressed by these infected cells. In contrast, low levels of SLAIIDQ were expressed by infected cells, suggesting that PPV2 may have a specific tropism for immature B lymphocytes and/or NK lymphocytes though possibly not T lymphocytes.

Conclusion

The overall conclusion of this study indicates that PPV2 may contribute to the pathogenesis of pneumonia.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1487-z) contains supplementary material, which is available to authorized users.

Long duration of immunity against a type 1 heterologous PRRS virus challenge in pigs immunised with a novel PRRS MLV vaccine: a randomised controlled study

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is widespread in commercial pig farms worldwide, and has a significant cost to the swine industry. Herd owners need a vaccine that will confer long-lasting immunity to prevent PRRSV infection and transmission. The studies described here evaluated duration of immunity conferred by a European-derived PRRS (isolate 94,881) modified live virus (MLV) vaccine, Ingelvac PRRSFLEX® EU, at 20, 24, and 26 weeks post-vaccination. Primary endpoints were the assessment of gross and histological lung lesions and viral RNA load in lung tissue 10 days following heterologous PRRSV challenge. Secondary endpoints included clinical observations, average daily weight gain (ADWG) and viral RNA load in serum 10 days post-challenge. Three blinded, vaccination-challenge efficacy studies were performed using separate cohorts of pigs (n = 56 per study). Pigs received either Ingelvac PRRSFLEX® EU (Group 1) or placebo (Groups 2 and 3). Groups 1 and 2 were subsequently challenged with heterologous European PRRSV isolate 205,817 at 20, 24 or 26 weeks post-vaccination.

Results

Mean gross lung lesion scores were significantly lower in Group 1 than in Group 2 at 24 and 26 weeks (p <  0.0001), but not at 20 weeks (p = 0.299). Significantly lower mean histological lung lesion scores were observed in Group 1 versus Group 2 at 20 (p = 0.0065), 24 (p <  0.0001) and 26 weeks (p <  0.0001). Mean viral RNA load in lung tissue was significantly lower in Group 1 than in Group 2 (p <  0.0001) at 20 (p <  0.0001), 24 (p <  0.0001) and 26 weeks (p <  0.0001). Cumulative viral RNA loads in serum during days 1–10 post-challenge were significantly lower in Group 1 than in Group 2 (p <  0.0001) in all studies. A significant increase in ADWG was observed in Group 1 compared with Group 2 at 20 weeks (p = 0.0027) and 24 weeks (p = 0.0004), but not at 26 weeks (p = 0.1041). There were no significant differences in clinical signs post-challenge in any study.

Conclusion

These results suggest that Ingelvac PRRSFLEX® EU confers long-term immunity to European heterologous PRRSV, which is maintained up to 26 weeks after vaccination, corresponding to the expected lifespan of commercial pigs.

Molecular Cloning and Identification of the 2'-5' Oligoadenylate Synthetase 2 Gene in Chinese Domestic Pigs Through Bioinformatics Analysis, and Determination of Its Antiviral Activity Against Porcine Reproductive and Respiratory Syndrome Virus Infection

An interferon-mediated antiviral protein, 2'-5' oligoadenylate synthetase 2, plays an important role in the antiviral response of interferons. In this study, 2'-5' oligoadenylate synthetase 2 genes were cloned from Chinese domestic pigs. Bioinformatics analysis revealed that the 2024-bp long open reading fame encodes 707 amino acids. There are two conserved regions in this protein: the nucleotidyltransferase domain, and the 2'-5' oligoadenylate synthetase domain (OAS). Genetic evolution analysis showed that the 2'-5' oligoadenylate synthetase 2 gene in domestic pigs is closely related to that of cattle. There are multiple antigenic sites, no signal peptide, and no transmembrane region in the gene, which is predicted to be a hydrophilic protein. Secondary structures were found to be mainly alpha helix-based; its tertiary structure is close to that of humans and cattle, but not that of mice. Tissue distribution results indicated that this protein is distributed in multiple organs, with high distribution in the liver; it is mainly localized in the cytoplasm. PRRSV infection, interferon-beta, and Poly(I: C) treatment all promoted 2'-5' oligoadenylate synthetase 2 gene expression. Overexpression and RNA silencing of porcine OAS2 inhibited and promoted PRRSV replication in cells, respectively. The inhibitory effect of porcine OAS2 was mainly dependent on RNase L, similar to what was predicted. This study has laid the foundation for future antiviral studies in pig, and provided a new way of preventing and treating PRRSV in the future.

Porcine reproductive and respiratory syndrome virus induces HMGB1 secretion via activating PKC-delta to trigger inflammatory response

Porcine reproductive and respiratory syndrome virus (PRRSV) causes inflammatory injuries in infected pigs. PRRSV induces secretion of high mobility group box 1 (HMGB1) that enhances inflammatory response. However, the mechanism of PRRSV-induced HMGB1 secretion is unknown. Here, we discovered PRRSV induced HMGB1 secretion via activating protein kinase C-delta (PKCδ). HMGB1 secretion was positively correlated with PKCδ activation in PRRSV-infected cells in a dose and time-dependent manner. Suppression of PKCδ with inhibitor and siRNA significantly blocked PRRSV-induced HMGB1 translocation and secretion, which indicates PKCδ activation is essential for the PRRSV-mediated HMGB1 secretion. In addition, PKCδ knockdown in PRRSV-infected cells led to downregulation of inflammatory cytokines, including IL-1beta and IL-6. Moreover, PRRSV E and pORF5a proteins were found to activate PKCδ and consequent HMGB1 secretion. These results demonstrate PRRSV activates PKCδ to induce HMGB1 secretion via E and pORF5a. This finding provides insights on the inflammatory response and pathogenesis of PRRSV infection.

Genotype 2 Strains of Porcine Reproductive and Respiratory Syndrome Virus Dysregulate Alveolar Macrophage Cytokine Production via the Unfolded Protein Response

Porcine reproductive and respiratory syndrome virus (PRRSV) infects alveolar macrophages (AMϕ), causing dysregulated alpha interferon (IFN-α) and tumor necrosis factor alpha (TNF-α) production through a mechanism(s) yet to be resolved. Here, we show that AMϕ infected with PRRSV secreted a reduced quantity of IFN-α following exposure of the cell to synthetic double-stranded RNA (dsRNA). This reduction did not correlate with reduced IFNA1 gene transcription. Rather, it coincided with two events that occurred late during infection and that were indicative of translational attenuation, specifically, the activation of eukaryotic translation initiation factor 2α (eIF2α) and the appearance of stress granules. Notably, the typical rapid production of TNF-α by AMϕ exposed to lipopolysaccharide (LPS) was suppressed or enhanced by PRRSV, depending on when the LPS exposure occurred after virus infection. If exposure was delayed until 6 h postinfection (hpi) so that the development of the cytokine response coincided with the time in which phosphorylation of eIF2α by the stress sensor PERK (protein kinase RNA [PKR]-like ER kinase) occurred, inhibition of TNF-α production was observed. However, if LPS exposure occurred at 2 hpi, prior to a detectable onset of eIF2α phosphorylation, a synergistic response was observed due to the earlier NF-κB activation via the stress sensor IRE1α (inositol-requiring kinase 1α). These results suggest that the asynchronous actions of two branches of the unfolded protein response (UPR), namely, IRE1α, and PERK, activated by ER stress resulting from the virus infection, are associated with enhancement or suppression of TNF-α production, respectively.IMPORTANCE The activation of AMϕ is controlled by the microenvironment to deter excessive proinflammatory cytokine responses to microbes that could impair lung function. However, viral pneumonias frequently become complicated by secondary bacterial infections, triggering severe inflammation, lung dysfunction, and death. Although dysregulated cytokine production is considered an integral component of the exacerbated inflammatory response in viral-bacterial coinfections, the mechanism responsible for this event is unknown. Here, we show that PRRSV replication in porcine AMϕ triggers activation of the IRE1α branch of the UPR, which causes a synergistic TNF-α response to LPS exposure. Thus, the severe pneumonias typically observed in pigs afflicted with PRRSV-bacterial coinfections could result from dysregulated, overly robust TNF-α production in response to opportunistic pathogens that is not commensurate with the typical restrained reaction by uninfected AMϕ. This notion could help in the design of therapies to mitigate the severity of viral and bacterial coinfections.

Generation of murine macrophage-derived cell lines expressing porcine CD163 that support porcine reproductive and respiratory syndrome virus infection

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) exhibits a highly restricted tropism for cells of the monocyte-macrophage lineage, utilizing porcine CD163 (pCD163) as an indispensable cellular receptor for infection. Transfection the gene of pCD163 into several non-permissive cell lines followed by protein expression confers susceptibility to PRRSV. A lack of specialized porcine antibody tools for use with existing porcine-derived primary cells and cell lines has hampered studies of both PRRSV pathogenesis and virus triggering of immune response cascades. Therefore, we constructed PRRSV-susceptible murine alveolar macrophage-derived MH-S and peritoneal macrophage-like RAW264.7 cell lines by achieving pCD163 cell surface expression in these cells. We then evaluated PRRSV susceptibility and cytokine expression patterns induced upon PRRSV infection of these pCD163-expressing cell lines.

Results

Growth of MH-S^CD163 and RAW264.7^CD163 cells was indistinguishable from growth of un-transfected parental cell lines. Meanwhile, various stages of the PRRSV replication cycle, including viral particle attachment, internalization, disassembly and infection were confirmed in both pCD163-transfected cell lines. Analysis of PRRSV replication using immunofluorescence staining of virus and viral titration of cell lysates demonstrated that both MH-S^CD163 and RAW264.7^CD163 cells supported replication of various genotype 2 PRRSV isolates. Moreover, PRRSV replication in MH-S^CD163 cells was similar to that observed in porcine alveolar macrophages (PAMs) and was more efficient than in RAW264.7^CD163 cells. However, peak virus titers in MH-S^CD163 cells were attained at 60 h post-infection (pi) versus 48 hpi in PAMs. Analysis of cytokine expression showed that post-PRRSV infection, mRNA expression patterns of anti-inflammatory cytokines (IL-4 and IL-10) and pro-inflammatory cytokines (TNF-α and IFN-γ) in MH-S^CD163 cells were more similar to those observed in PAMs versus levels in RAW264.7^CD163 cells.

Conclusions

MH-S and RAW264.7 cells were not susceptible to PRRSV infection until transfection and subsequent expression of pCD163 were achieved in these cell lines. The PRRSV-susceptible MH-S^CD163 cell line efficiently supported viral replication of various genotype 2 PRRSV isolates and exhibited similar cytokine expression patterns as observed in PAMs. In conclusion, this work describes the development of new tools to further understand PRRSV pathogenesis and immune response mechanisms to PRRSV infection.

Electronic supplementary material

The online version of this article (10.1186/s12896-017-0399-5) contains supplementary material, which is available to authorized users.

Antagonizing cytokine-mediated JAK-STAT signaling by porcine reproductive and respiratory syndrome virus

Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway is activated by myriad cytokines, which are involved in regulation of cell growth, proliferation, differentiation, apoptosis, angiogenesis, immunity and inflammatory response. Because of its significance in immune response, JAK-STAT pathway is often targeted by pathogens, including porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV causes reproductive failure in sows and respiratory disease in pigs of all ages. A typical feature of the immune response to PRRSV infection in pigs is delayed production and low titer of virus neutralizing antibodies, and weak cell-mediated immune response. One of the possible reasons for the weak protective immune response is that PRRSV interferes with cytokine-mediated JAK-STAT signaling. PRRSV inhibits interferon-activated JAK-STAT signaling by blocking nuclear translocation of STAT1 and STAT2. The mechanism is that PRRSV non-structural protein 1β (nsp1β) induces degradation of karyopherin α1 (KPNA1), a critical adaptor in nucleo-cytoplasmic transport. PRRSV also antagonizes IL6-activated JAK-STAT3 signaling via inducing degradation of STAT3. In this review, we briefly introduce JAK-STAT signaling, summarize the PRRSV interference with it, and provide perspective on the perturbation in the context of PRRSV-elicited immune response.

Porcine 2', 5'-oligoadenylate synthetase 2 inhibits porcine reproductive and respiratory syndrome virus replication in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) is acknowledged a fulminating infectious pathogen affecting the pig farming industry, and current vaccines and drugs could hardly inhibit this virus. The 2', 5'-oligoadenylate synthetase (OASs) have antiviral activities, but the role(s) played by porcine OAS2 in protection against PRRSV infection are unknown. Here we found that endogenous expression of the porcine OAS2 gene could be promoted by interferon (IFN)-beta or PRRSV infection in porcine alveolar macrophages. Knockdown of porcine OAS2 led to increases in PRRSV replication, and OAS2 expression suppressed replication of PRRSV in a retinoic acid inducible gene I (RIG-I)-dependent manner, anti-PRRSV activity of porcine OAS2 would be lost if RNase L and OAS2 were both silenced. This discovery illustrates a pathway that porcine OAS2 responses to host anti-PRRSV function.

The middle half genome of interferon-inducing porcine reproductive and respiratory syndrome virus strain A2MC2 is essential for interferon induction

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to antagonize the innate immune response. An atypical PRRSV strain A2MC2 is capable of inducing synthesis of type I interferons (IFNs) in cultured cells. Here, we show that the middle half of the A2MC2 genome is needed for triggering the IFN synthesis. First, a cDNA infectious clone of this atypical strain was constructed as a DNA-launched version. Virus recovery was achieved from the infectious clone and the recovered virus, rA2MC2, was characterized. The rA2MC2 retained the feature of IFN induction in cultured cells. Infection of pigs with the rA2MC2 virus caused viremia similar to that of the wild-type virus. Chimeric infectious clones were constructed by swapping genomic fragments with a cDNA clone of a moderately virulent strain VR-2385 that antagonizes IFN induction. Analysis of the rescued chimeric viruses demonstrated that the middle two fragments, ranging from nt4545 to nt12709 of the A2MC2 genome, were needed for the IFN induction, whereas the chimeric viruses containing any one of the two A2MC2 fragments failed to do so. The results and the cDNA infectious clone of the IFN-inducing A2MC2 will facilitate further study of its biology, ultimately leading towards the development of an improved vaccine against PRRS.

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.

Detection of a novel sapelovirus in central nervous tissue of pigs with polioencephalomyelitis in the USA

An approximately 3,000 finishing swine operation in the United States experienced an outbreak of an atypical neurologic disease in 11-weeks-old pigs with an overall morbidity of 20% and case fatality rate of 30%. The clinical onset and progression of signs in affected pigs varied but included inappetence, compromised ambulation, ataxia, incoordination, mental dullness, paresis, paralysis and decreased response to environmental stimuli. Tissues from affected pigs were submitted for diagnostic investigation. Histopathologic examination of the cerebrum, cerebellum and spinal cord revealed severe lymphoplasmacytic and necrotizing polioencephalomyelitis with multifocal areas of gliosis and neuron satellitosis, suggestive of a neurotropic viral infection. Bacterial pathogens were not isolated by culture of neurologic tissue from affected pigs. Samples tested by polymerase chain reaction (PCR) were negative for pseudorabies virus and atypical porcine pestivirus. Immunohistochemistry for porcine reproductive and respiratory syndrome virus, porcine circovirus and Listeria was negative. Porcine sapelovirus (PSV) was identified in spinal cord by a nested PCR used to detect porcine enterovirus, porcine teschovirus and PSV. Next-generation sequencing of brainstem and spinal cord samples identified PSV and the absence of other or novel pathogens. In addition, Sapelovirus A mRNA was detected in neurons and nerve roots of the spinal cord by in situ hybridization. The PSV is genetically novel with an overall 94% amino acid identity and 86% nucleotide identity to a recently reported sapelovirus from Korea. This is the first case report in the United States associating sapelovirus with severe polioencephalomyelitis in pigs.

Porcine Reproductive and Respiratory Syndrome Virus Antagonizes JAK/STAT3 Signaling via nsp5, Which Induces STAT3 Degradation

Signal transducer and activator of transcription 3 (STAT3) is a pleiotropic signaling mediator of many cytokines, including interleukin-6 (IL-6) and IL-10. STAT3 is known to play critical roles in cell growth, proliferation, differentiation, immunity and inflammatory responses. The objective of this study was to determine the effect of porcine reproductive and respiratory syndrome virus (PRRSV) infection on the STAT3 signaling since PRRSV induces a weak protective immune response in host animals. We report here that PRRSV infection of MARC-145 cells and primary porcine pulmonary alveolar macrophages led to significant reduction of STAT3 protein level. Several strains of both PRRSV type 1 and type 2 led to a similar reduction of STAT3 protein level but had a minimal effect on its transcripts. The PRRSV-mediated STAT3 reduction was in a dose-dependent manner as the STAT3 level decreased, along with incremental amounts of PRRSV inocula. Further study showed that nonstructural protein 5 (nsp5) of PRRSV induced the STAT3 degradation by increasing its polyubiquitination level and shortening its half-life from 24 h to ∼3.5 h. The C-terminal domain of nsp5 was shown to be required for the STAT3 degradation. Moreover, the STAT3 signaling in the cells transfected with nsp5 plasmid was significantly inhibited. These results indicate that PRRSV antagonizes the STAT3 signaling by accelerating STAT3 degradation via the ubiquitin-proteasomal pathway. This study provides insight into the PRRSV interference with the JAK/STAT3 signaling, leading to perturbation of the host innate and adaptive immune responses.

IMPORTANCE

The typical features of immune responses in PRRSV-infected pigs are delayed onset and low levels of virus neutralizing antibodies, as well as weak cell-mediated immunity. Lymphocyte development and differentiation rely on cytokines, many of which signal through the JAK/STAT signaling pathway to exert their biological effects. Here, we discovered that PRRSV antagonizes the JAK/STAT3 signaling by inducing degradation of STAT3, a master transcription activator involved in multiple cellular processes and the host immune responses. The nsp5 protein of PRRSV is responsible for the accelerated STAT3 degradation. The PRRSV-mediated antagonizing STAT3 could lead to suppression of a broad spectrum of cytokines and growth factors to allow virus replication and spread in host animals. This may be one of the reasons for the PRRSV interference with the innate immunity and its poor elicitation of protective immunity. This finding provides insight into PRRSV pathogenesis and its interference with the host immune responses.

Estimation of the transmission parameters for swine influenza and porcine reproductive and respiratory syndrome viruses in pigs from weaning to slaughter under natural conditions

In the present study, the transmission parameters of swine influenza virus (SIV) and porcine reproductive and respiratory virus (PRRSV) have been calculated using the basic reproductive rate (R) parameter in two commercial pig farms (F1 and F2). In order to do this, a serological (PRRSV genotype 1 and SIV) and virological (SIV) follow-up of a batch of animals was carried out weekly from 3 weeks of age until the age of slaughter on each farm. Results of the analysis for SIV and PRRSV showed different transmission profiles depending on the farm, the pathogen, and time of transmission. In F1, transmission of both viruses was detected throughout the sampling. The R_t (R for a given period of time) value for SIV ranged from 1.5 [0.9-2.3] to 3.6 [2.3-4.9] from farrowing to the beginning of the fattening period, and the R_t value for PRRSV was 3.3 [2.9-4.3] to 3.5 [2.8-4.1] from farrowing until the slaughter age. These results indicated that both viruses were transmitted enzootically in that farm for these periods of time. A different transmission pattern with a higher incidence was also observed during the fattening period in F1 (after 15 weeks of age) for SIV, coinciding with the entrance of a new subtype. In this case, R value for SIV reached 3.3 [1.65-4.9]. On the other hand, in F2, SIV and PRRSV seemed to be restricted to the fattening period. R reached a value of 6.4 [4.1-8.8] for SIV and 7.1 [3.5-10.6] for PRRSV. These findings suggest a different origin of the virus, as well as a more epidemic circulation, especially for SIV, where most of the new cases were observed in a one week period. In conclusion, the present study offers a reliable estimation of the range of R_t values for SIV and genotype 1 PRRSV transmission under field conditions, suggesting that enzootic circulations of both viruses are similar in terms of transmission, probably higher for PRRSV, but also that transmission of SIV is more efficient (or epidemic) than transmission of a genotype 1 PRRSV isolate in naïve animals given the new cases observed in only in F2.

Sustaining Interferon Induction by a High-Passage Atypical Porcine Reproductive and Respiratory Syndrome Virus Strain

Porcine reproductive and respiratory syndrome virus (PRRSV) strain A2MC2 induces type I interferons in cultured cells. The objective of this study was to attenuate this strain by serial passaging in MARC-145 cells and assess its virulence and immunogenicity in pigs. The A2MC2 serially passaged 90 times (A2MC2-P90) retains the feature of interferon induction. The A2MC2-P90 replicates faster with a higher virus yield than wild type A2MC2 virus. Infection of primary pulmonary alveolar macrophages (PAMs) also induces interferons. Sequence analysis showed that the A2MC2-P90 has genomic nucleic acid identity of 99.8% to the wild type but has a deletion of 543 nucleotides in nsp2. The deletion occurred in passage 60. The A2MC2-P90 genome has a total of 35 nucleotide variations from the wild type, leading to 26 amino acid differences. Inoculation of three-week-old piglets showed that A2MC2-P90 is avirulent and elicits immune response. Compared with Ingelvac PRRS^® MLV strain, A2MC2-P90 elicits higher virus neutralizing antibodies. The attenuated IFN-inducing A2MC2-P90 should be useful for development of an improved PRRSV vaccine.

Review on the transmission porcine reproductive and respiratory syndrome virus between pigs and farms and impact on vaccination

Porcine reproductive and respiratory syndrome (PRRS) is considered to be one of the most costly diseases affecting intensive pig production worldwide. Control of PRRS is a complex issue and involves a combination of measures including monitoring, diagnosis, biosecurity, herd management, and immunization. In spite of the numerous studies dealing with PRRS virus epidemiology, transmission of the infection is still not fully understood. The present article reviews the current knowledge on PRRSV transmission between and within farm, and the impact of vaccination on virus transmission.

Vascular Lesions in Nine Göttingen Minipigs with Thrombocytopenic Purpura Syndrome

Tissues from 9 Göttingen minipigs, aged 7 weeks to 1 year, with clinically diagnosed thrombocytopenic purpura syndrome were examined microscopically. All pigs had a history of spontaneous cutaneous purpura that was generally accompanied by disseminated visceral hemorrhages. Hematologic abnormalities included anemia (8 out of 9 pigs) and thrombocytopenia (7 out of 9 pigs), with platelet counts consistently below 20,000/μ. Microscopically, degenerative vascular lesions with morphologic features of arteriosclerosis were present in all 9 pigs. Vascular lesions affected small- to medium-sized muscular arteries and arterioles in various organs and extraparenchymal tissues; vessels of the renal pelvis and coronary arteries were consistently involved. Microscopic lesions in small- to medium-sized muscular arteries consisted of neointimal proliferation, medial thickening, luminal stenosis, thrombosis, disruption and fragmentation of the internal elastic lamina, necrosis of the tunica media, and medial deposits of myxoid matrix material. Microscopic lesions in arterioles included concentric laminar thickening of vessel walls (onion-skin pattern), endothelial cell hypertrophy, smooth muscle cell vacuolation, necrosis of the tunica media, thrombosis, and partial to complete luminal stenosis. Arteritis and/or periarteritis were also noted in 4 out of 9 pigs. Additional microscopic lesions included membranoproliferative glomerulonephritis (3 out of 9), myocardial microinfarcts (4 out of 7), renal interstitial fibrosis (2 out of 9), extramedullary hematopoiesis (6 out of 9), and intracapillary hyaline thrombi (2 out of 9). Degenerative vascular lesions have not been previously described in Göttingen minipigs with thrombocytopenic purpura syndrome. The etiopathogenesis of both the vascular lesions and thrombocytopenic purpura syndrome is currently unknown.

Synthetic Toll-like receptor 7 ligand inhibits porcine reproductive and respiratory syndrome virus infection in primary porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV), a common viral pathogen, causes huge annual economic losses to the swine industry worldwide. After triggering by specific ligands, the Toll-like receptor 7 (TLR7), a type of pattern-recognition receptor (PRR), induces antiviral cytokines production. Previously, we synthesized an adenine analog, designated SZU101, a TLR7-specific ligand. In this study, we assessed the inhibitory effect of SZU101 on PRRSV infection in vitro. SZU101 significantly suppressed PRRSV infection in primary porcine alveolar macrophages (PAMs) in a dose-dependent manner. Moreover, SZU101-induced inhibition involved NF-κB pathway activation in PAMs to initiate expression of TLR7-mediated cytokines and induce expression of downstream signaling IFN-stimulated genes (ISGs). Chloroquine, a TLR7 inhibitor, and BAY 11-7082, an NF-κB inhibitor, reversed both the SZU101-induced antiviral effect and induction of cytokine genes and ISGs expression. Therefore, SZU101 antiviral effects depend at least in part on TLR7-NF-κB signaling pathway. Additionally, administration of SZU101 enhanced the humoral and cell-mediated immune responses against PRRSV antigens in mice. Given these results, SZU101 holds promise as an antiviral agent and a vaccine adjuvant to prevent PRRSV infection in pigs.

Molecular identification and genetic diversity of open reading frame 7 field isolated porcine reproductive and respiratory syndrome in North Sumatera, Indonesia, in the period of 2008-2014

AIM

Molecular identification and genetic diversity of open reading frame 7 (ORF7) of field isolated porcine reproductive and respiratory syndrome virus (PRRSV) in North Sumatera, Indonesia, in the period of 2008-2014.

MATERIALS AND METHODS

A total of 47 PRRSV samples were collected from the death case of pigs. The samples were collected from different districts in the period of 2008-2014 from North Sumatera province. Two pairs of primer were designed to amplify ORF7 of Type 1 and 2 PRRSV based on the sequence of reference viruses VR2332 and Lelystad. Viral RNAs were extracted from samples using PureLink™ micro-to-Midi total RNA purification system (Invitrogen). To amplify the ORF7 of PRRSV, the synthesis cDNA and DNA amplification were performed by reverse transcription polymerase chain reaction (RT-PCR) and nested PCR method. Then the DNA sequencing of PCR products and phylogenetic analysis were accomplished by molecular evolutionary genetics analysis version 6.0 software program.

RESULTS

RT-: PCR and nested PCR used in this study had successfully detected of 18 samples positive PRRS virus with the amplification products at 703bp and 508bp, respectively. Sequencing of the ORF7 shows that 18 PRRS viruses isolated from North Sumatera belonged to North American (NA). JXA1 Like and classic NA type viruses. Several mutations were detected, particularly in the area of nuclear localization signal (NLS1) and in NLS2. In the local viruses, which were related closed to JXA1 virus; there are two differences in amino acids in position 12 and 43 of ORF7. Our tested viruses showed that the amino acid positions 12 and 43 are Asparagine and Arginine, while the reference virus (VR2332, Lelystad, and JXA1) occupied both by Lysine. Based on differences in two amino acids at position 12 and 43 showed that viruses from North Sumatera has its own uniqueness and related closed to highly pathogenic PRRS (HP-PRRS) virus (JXA1).

CONCLUSION

The results demonstrated that North Sumatera type PRRS virus has caused PRRS outbreaks in pig in North Sumatera between 2008 and 2014. The JAX1 like viruses had unique amino acid residue in position 12 and 43 of asparagine and lysine, and these were genetic determinants of North Sumatera viruses compared to other PRRS viruses.

Downregulation of protein kinase PKR activation by porcine reproductive and respiratory syndrome virus at its early stage infection

The interferon-induced double-strand RNA activated protein kinase (PKR) plays an important role in antiviral response. The objective of this study was to assess the effect of porcine reproductive and respiratory syndrome virus (PRRSV) on PKR activation. Here we report that PRRSV inhibited PKR activation during its early stage infection of primary pulmonary alveolar macrophages (PAMs). PRRSV infection led to lower level of phosphorylated PKR in comparison with mock-infected cells. The PKR inhibition was sustained until 10h post infection in the presence of polyI:C, a synthetic analog of double-stranded RNA activating PKR. PKR-mediated phosphorylation of the eukaryotic translation initiation factor eIF2α was also lower in the PRRSV-infected PAMs during the early stage infection. Interestingly, inactivated PRRSV was capable to inhibit the PKR activation until 6h post infection. This suggests that structural components of PRRSV virions were responsible for the inhibition, although PRRSV replication was needed for longer inhibition. These results indicate that the downregulation of PKR activation during early infection stage should be essential for PRRSV to avoid the antiviral response to initiate replication. This finding contributes to our understanding on PRRSV interaction with host innate immune response and reveal a target for control of PRRSV infection.

The Interferon-Induced Mx2 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication

Porcine reproductive and respiratory syndrome virus (PRRSV) causes one of the most economically important diseases of swine in the world. Current vaccination strategies provide only limited protection against PRRSV infection. Recently, myxovirus resistance 2 (Mx2) has been identified as a novel interferon (IFN)-induced, innate immunity restriction factor that inhibits some viral infections. However, the role of Mx2 in PRRSV infection is not well understood. In this study, we cloned the full-length monkey Mx2 (mMx2) complementary DNA (cDNA) from IFN-β-treated African green monkey Marc-145 cells, and found that overexpression of mMx2 inhibited PRRSV replication in Marc-145 cells. IFN-β induced expression of mMx2 in Marc-145 cells and suppressed PRRSV replication in a dose-dependent manner. Knockdown of mMx2 impaired the antiviral activity mediated by IFN-β. Confocal imaging and immunoprecipitation assays indicated that mMx2 interacted with PRRSV N protein in virus-infected cells. Furthermore, we showed that GTPase activity of mMx2 is necessary, but that the first N-terminal 51 amino acids are dispensable for antiviral activity. Finally, porcine Mx2 was also found to have the antiviral activity against PRRSV in Marc-145 cells. We conclude that mMx2 protein inhibits PRRSV replication by interaction with the viral N protein.

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.

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

The safety and efficacy of the JXA1-R vaccine, an attenuated strain of highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV), were examined using an intramuscular challenge model in piglets. The JXA1-R vaccine was obtained by passing HP-PRRSV JXA1 through Marc-145 cells (82nd passage). Genomic sequence comparisons showed that strain JXA1-R and its parental strain, JXA1, differ by 47 amino acids, and most of these differences are scattered throughout the PRRSV genome. Four-week-old PRRSV-free piglets were inoculated intramuscularly with JXA1-R vaccine (10(3.0), 10(4.0), 10(5.0), 10(6.0), and 10(7.0) 50% tissue culture infective doses [TCID50]/ml for groups 1 to 5, respectively) and then challenged intramuscularly with the 5th passage virus of JXA1 virus (JXA1-F5, 3 ml × 10(4.5) TCID50/ml) 28 days after inoculation. The humoral immune response, swine growth, clinical signs, and differential organ lesions were monitored. The results showed that all vaccinated piglets had a perceptible humoral immune response to vaccination after day 7, which then promptly increased, almost reaching the maximum sample/positive (S/P) ratio value at 28 days postimmunization. Viremia detection indicated that the viral replication levels of the challenge virus in the immunized groups (immunization doses ≥10(4.0)/ml) were significantly lower than that of the virus-challenged unvaccinated control group. Piglets in groups 2 to 5 were effectively protected against lethal HP-PRRSV infection and did not show any obvious changes in body temperature or clinical signs of disease at any point during the experiment. However, two of five piglets in group 1 showed mild pathological lesions and transitory high fever. These results suggest that JXA1-R (TCID50/ml ≥10(4.0)) is sufficiently attenuated and can provide effective protection against the lethal wild-type HP-PRRSV.

Changes in leukocyte subsets of pregnant gilts experimentally infected with porcine reproductive and respiratory syndrome virus and relationships with viral load and fetal outcome

In spite of more than two decades of extensive research, the understanding of porcine reproductive and respiratory syndrome virus (PRRSv) immunity is still incomplete. A PRRSv infection of the late term pregnant female can result in abortions, early farrowings, fetal death, and the birth of weak, congenitally infected piglets. The objectives of the present study were to investigate changes in peripheral blood mononuclear cell populations in third trimester pregnant females infected with type 2 PRRSv (NVSL 97–7895) and to analyze potential relationships with viral load and fetal mortality rate. PRRSv infection caused a massive, acute drop in total leukocyte counts affecting all PBMC populations by two days post infection. Except for B cells, cell counts started to rebound by day six post infection. Our data also show a greater decrease of naïve B cells, T-helper cells and cytolytic T cells than their respective effector or memory counterparts. Absolute numbers of T cells and γδ T cells were negatively associated with PRRSv RNA concentration in gilt serum over time. Additionally, absolute numbers of T helper cells may be predictive of fetal mortality rate. The preceding three leukocyte populations may therefore be predictive of PRRSv-related pathological outcomes in pregnant gilts. Although many questions regarding the immune responses remain unanswered, these findings provide insight and clues that may help reduce the impact of PRRSv in pregnant gilts.

Molecular characterization of the porcine S100A6 gene and analysis of its expression in pigs infected with highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV)

Our previous microarray study revealed that S100A6 was significantly upregulated in porcine alveolar macrophages (PAMs) infected with highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV). In the present study, we cloned both cDNA and genomic DNA sequences of the gene. Transient transfection indicated that the porcine S100A6 protein was located in the nucleus and cytoplasm. Reverse transcription polymerase chain reaction (RT-PCR) revealed that the porcine S100A6 gene was highly expressed in the kidney and subcutaneous fat. Polyinosinic-polycytidylic acid [poly (I:C)] induced porcine S100A6 gene expression in PK-15 cells. Quantitative real-time PCR (Q-PCR) analysis further showed that the porcine S100A6 gene was upregulated in different cells and tissues of Tongcheng pigs infected with HP-PRRSV. Chromosome walking obtained the porcine S100A6 promoter region and then luciferase reporter assays confirmed its regulatory activities. We observed a putative NF-κB binding site in the core promoter region, which may explain the upregulation of porcine S100A6 in response to PRRSV. Transfection of NF-κB (p65 subunit) intensely induced the promoter activity of the porcine S100A6 gene, while an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC), inhibited this activity. Furthermore, compared to its wild type, the promoter activity was significantly reduced when it contained a mutant NF-κB binding site. All these results provide a solid foundation to further investigate how S100A6 is involved in PRRSV infection.

Analysis of mRNA expression for genes associated with regulatory T lymphocytes (CD25, FoxP3, CTLA4, and IDO) after experimental infection with bovine viral diarrhea virus of low or high virulence in beef calves

Immunosuppression caused by bovine viral diarrhea virus (BVDV) has been associated with lymphocyte depletion, leukopenia and impairment of leukocyte function; however, no work has been done on the relationship between BVDV and regulatory T lymphocytes (Tregs). The objective of this study was to compare the mRNA expression of genes associated with Tregs (CD25, FoxP3, CTLA4, and IDO), after experimental infection of beef calves with low (LV) or high (HV) virulence BVDV. Thirty BVDV-naïve calves were randomly assigned to three groups. Calves were intra-nasally inoculated with LV (n=10, strain SD-1) or HV (n=10, strain 1373) BVDV or BVDV-free cell culture medium (control, n=10). Quantitative RT-PCR was used to determine the expression of target genes in tracheo-bronchial lymph nodes and spleen on day 5 post-infection. The mRNA expression of CD25 was up-regulated in tracheo-bronchial lymph nodes of LV (P<0.05), but not in HV compared to the control group. The expression of FoxP3 and CTLA4 was not increased in tracheo-bronchial lymph nodes of either of the BVDV-inoculated groups. A dramatic up-regulation of IDO mRNA was observed in tracheo-bronchial lymph nodes of LV (P<0.05), but not HV compared to the control calves. In conclusion, experimental infection with BVDV did not provide evidence of Treg activation based on expression of FoxP3 and CTL4. Differential expression of CD25 and IDO mRNA on day 5 post-infection with HV or LV BVDV might reflect temporal differences in transcription occurring during the immune response elicited by these viral strains, or differences in viral infectivity of the host cells.