Infection of porcine reproductive and respiratory syndrome virus suppresses the antibody response to classical swine fever virus vaccination

Genetic characterization and pathogenicity of two recombinant PRRSV-2 strains from lineages 1, 3, 5, and 8 emerged in China

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major economic threat to the global swine industry. Currently, NADC30-like PRRSV has undergone complex recombination with local Chinese strains, which has exacerbated the evolution of PRRSV. Recently, new recombinant PRRSV-2 strains from four lineages (lineages 1, 3, 5, and 8) have emerged in China. However, information on the pathogenicity of the novel isolate in China remains limited. To further our knowledge about the isolate, FJLIUY2017 and PRRSV2/CN/G8/2018 were selected to analyze their pathogenicity for piglets.

METHODS

The PRRSV FJLIUY2017 and PRRSV2/CN/G8/2018 strains were isolated by porcine alveolar macrophages (PAMs) and MARC-145CD163. Complete genomic sequence analyses were conducted using the DNASTAR 7.0 software and the phylogenetic tree was constructed with MEGA 7.0. Recombination events were detected using RDP V4.10 and SIMPLOT software 3.5.1. Five PRRSV-free per group were inoculated with 2 mL (2 × 105 TCID50) of the FJLIUY-2017 and PRRSV2/CN/G8/2018. Clinical signs of disease were recorded daily after challenge. Blood samples were collected from all piglets on days 0, 4, 7, 11, and 14 dpi for analysis of viral load by IFA and PRRSV-specific antibody levels by ELISA kit. Lung gross and microscopic lesions of the inoculated piglets were examined by scoring system for lung lesion.

RESULTS

Full-length genome analysis revealed that FJLIUY2017 and PRRSV2/CN/G8/2018 share 89.2% identity with each other, and in particular, they had a low degree of homology (< 92%) with PRRSV sequences available in GenBank. Phylogenetic and recombination analyses revealed that the two strains were recombinant viruses from lineages 1, 3, 5.1, and 8.7 strains. Animal studies indicated that FJLIUY-2017 resulted in the typical clinical signs of PRRSV, including persistent fever, higher viremia, severe lung lesions, and 20% mortality, whereas PRRSV2/CN/G8/2018 caused moderate clinical symptoms and no mortality during the challenge period. Hyper-immune sera against the major vaccine strains JXA1-R (lineage 8) and Ingelvac PRRS MLV (Lineage 5) failed to neutralize two strains.

CONCLUSIONS

FJLIUY-2017 caused persistent fever, higher viremia, 20% mortality and exhibited higher pathogenicity in piglets compared to PRRSV2/CN/G8/2018. Our results suggest that recombination between different PRRSV-2 lineages can result in the development of PRRSV variants with increased pathogenicity.

Evaluation of Immune Status of Pigs against Classical Swine Fever for Three Years after the Initiation of Vaccination in Gifu Prefecture, Japan

In 2018, classical swine fever (CSF) reemerged in Gifu Prefecture, Japan, after 26 years of absence, and vaccination of domestic pigs using a live attenuated vaccine was initiated in 2019. Because the vaccine efficacy in piglets is influenced by the maternal antibody levels, vaccination should be administered at the optimal age by assuming the antibody level in sows. In this study, the shift in the antibody titer distribution in sows due to the initiation of vaccination to naïve herds and its influence on the vaccine-induced immunity rate in fattening pigs were investigated for 3 years. The results indicated that higher antibody titers were induced in first-generation sows after vaccine initiation because they were immunologically naïve, but the distribution of antibody titers shifted to lower levels along with their replacement with second-generation sows. The average vaccination age of fattening pigs became earlier year by year, and the vaccine-induced antibody rate was almost ≥80%. Based on the estimation of the optimal age for vaccination, it was found that vaccination at a younger age may reduce the risk of CSF infection. Taken together, the risk of CSF outbreaks can be reduced by administering vaccines at the optimal age based on the sequential monitoring of the sow's immune status.

Exploring Appropriate Strategies for Vaccination against Classical Swine Fever under a Dynamic Change in Antibody Titer in Sows after Starting Vaccination in a Japanese Farm Setting

After 26 years of absence in Japan, a classical swine fever (CSF) outbreak occurred at a domestic pig farm in 2018. Vaccination against the CSF virus with a live attenuated vaccine at pig farms was restarted in October 2019, which was 13 years after the 2006 ban on vaccination. An individual-based simulation model for CSF antibody dynamics was developed to determine an effective CSF vaccination strategy for pig populations. In creating a simulated pig herd, the optimal vaccination age of piglets and the effect of vaccinating piglets twice were evaluated. Additionally, the herd immunity was monitored every 6 months for 4 years after the start of vaccination, and the effects of intensive sow replacement policies were assessed. The simulation results indicated that the vaccination age should be delayed relative to the age used before the 2006 ban on vaccination and shifted earlier, from 8 weeks to 6 weeks, as time elapses. The simulations indicated a tradeoff in protection between the weaning period (i.e., maternally derived antibodies) and the fattening period (i.e., by vaccine-induced antibodies). Mixing sows with high and low antibody titers, particularly sows that received the first vaccination and those born after the start of vaccination, resulted in a high variation in antibody titer among pigs on the farm. This study also clarified the positive effect of intensive sow replacement strategies on shortening the period in which sows show diverse titers. Differences in sow replacement rates among farms and/or the time lag in starting vaccination in different prefectures result in heterogeneity in herd immunity in Japan; thus, herd immunity status should be examined at every farm using this simulation model.

A New Porcine Reproductive and Respiratory Syndrome Virus with N-Linked Glycosylation Site Deletion in GP5 44th Amino Acid from JXA1, NADC30-Like, and JM Triparental Recombination

Porcine reproductive and respiratory syndrome virus (PRRSV) is a significant pathogen causing substantial financial losses in the global swine industry. The prevention of PRRSV is hampered due to frequent gene recombination among different strains of PRRSV. In this study, a new PRRSV strain, PRRSV-HQ-2020, was identified from nursery piglets in Yunnan Province, China, in 2020. The complete genome analysis revealed that PRRSV-HQ-2020 is highly similar to JXA1-like (lineage 8.7 PRRSV, isolated from China in 2008) in the 5'UTR, nsp1-9, and nsp11 coding regions. Additionally, it has a resemblance to JM (lineage 3 PRRSV, isolated from Taiwan, China, in 2010) in the nsp12-M coding region and NADC30 (lineage 1.8 PRRSV, isolated from North American in 2008) in the nsp10, N, and 3'UTR, suggesting a natural recombination event. Furthermore, recombination analyses showed three interlineage recombination events among lineages 8.7, 1.8, and 3. Notably, the GP5 protein of PRRSV-HQ-2020 exhibited a crucial mutation at position 44, leading to the deletion of a key glycosylation site. These findings provide direct evidence for the natural occurrence of recombination events among three lineages of PRRSV-2 in Chinese swine herds, leading to the emergence of unique genetic properties of PRRSV variants, and providing a theoretical basis for developing better PRRSV prevention strategies.

Comparison of the cross-protection of PPRSV sublineage 8.7 MLV vaccines against the recombinant NADC30-like strain

The emergence of recombinant porcine reproductive and respiratory syndrome virus (PRRSV) has caused a substantial threat to the swine industry in recent years. However, the protective efficacy of different sublineage 8.7 PRRSV modified-live virus (MLV) vaccines against emerging strains were still obscure. In this study, a broad epidemiological investigation of PRRSV showed the prevalence of NADC30-like strain increased in Shandong Province, China from 2018 to 2020. Through piglet trial for vaccination and challenge with recombinant NADC30-like SDlz1601 strain, CH-1R MLV vaccine showed better protective effect than JXA1-R and TJM-F92 MLV vaccines in terms of clinical score and pathological observation. Moreover, all three MLV vaccines could reduce virus loads in the serum of piglets. This study provides valuable insights into the prevalence of the NADC30-like strain and the protective effect of PRRS MLV vaccines against recombinant NADC30-like strains, which could help to improve the prevention and control of PRRSV infections.

Vaccination Failures in Pigs-The Impact of Chosen Factors on the Immunisation Efficacy

Infectious diseases that often lead to economic losses still pose a severe problem in the pig production sector. Because of increasing restrictions on antibiotic usage, vaccines may become one of the major approaches to controlling infectious diseases; much research has proved that they could be very efficient. Nevertheless, during their life, pigs are exposed to various factors that can interfere with vaccination efficacy. Therefore, in the present paper, we reviewed the influence of chosen factors on the pig immunisation process, such as stress, faecal microbiota, host genetics, the presence of MDAs, infections with immunosuppressive pathogens, and treatment with antibiotics and mycotoxins. Many of them turned out to have an adverse impact on vaccine efficacy.

Comparative efficacy evaluation of different CSF vaccines in pigs with CSF maternally derived antibodies

Classical swine fever (CSF) is a highly contagious and important swine disease in China. Sporadic outbreaks with mild clinical signs are still being reported despite massive vaccination with the CSF C-strain vaccine. One possible reason for vaccine failure could be interference from maternally derived antibodies (MDAs) during vaccination in the field. The aim of this study was to evaluate the efficacy of different CSF vaccines in the presence of MDAs and to assess the different vaccination schemes in the field. The results demonstrated that vaccination with a single dose of C-strain-PK vaccine protected pigs against severe clinical signs and significantly reduced viremia. The impact of MDAs was negligible. The interference was also mild during a prime and boost vaccination scheme using the C-strain-ST vaccine. In contrast, a significant influence of MDAs on the efficacy of the subunit E2 vaccine in a one-dose vaccination scheme was observed, with pigs showing severe clinical signs, CSF-associated death, typical pathological lesions and a high level of viremia after challenge, despite robust E2 antibody induction. A field vaccination and challenge study further confirmed the superior effectiveness of a single dose of C-strain-PK vaccine in the presence of MDAs in comparison to a routine prime and boost vaccination scheme applied in the field, with pigs having fever, chronic signs, significant viremia and shedding after challenge. Delaying the vaccination time from the age of 28 days to 45 days, when MDA was low, was beneficial for improving the clinical protection and immunity induced by vaccines. Altogether, the results presented here emphasize that a high-quality vaccine and a scientific design of the vaccination scheme based on serological surveillance are essential pillars to control and eliminate CSF in China.

Attenuation of porcine deltacoronavirus disease severity by porcine reproductive and respiratory syndrome virus coinfection in a weaning pig model

Porcine deltacoronavirus (PDCoV) is a potentially emerging zoonotic pathogen that causes severe diarrhea in young pigs, with a risk of fatal dehydration. Its pathogenicity on neonatal piglet has been previously reported, however, it is less known if the coinfection with immunosuppressive pathogens can influence PDCoV disease manifestation. Here, a coinfection model of PDCoV and porcine reproductive and respiratory syndrome virus (PRRSV), a global-spread immunosuppressive virus, was set to study their interaction. Weaning pigs in the coinfection group were intranasally inoculated with PRRSV NADC30-like virus and latterly orally inoculated with PDCoV at three day-post-inoculation (DPI). Unexpectedly, compared with pigs in the PDCoV single-infected group, the coinfected pigs did not show any obvious diarrhea, as PDCoV fecal shedding, average daily weight gain (ADWG), gross and microscopic lesions and PDCoV IHC scores consistently indicated that PRRSV coinfection lessened PDCoV caused diarrhea. Additionally, three proinflammatory cytokines TNF-α, IL-1 and IL-6, which can be secreted by PRRSV infected macrophages, were detected to be highly expressed at the intestine from both PRRSV infected groups. By adding to PDCoV-infected cells, these three cytokines were further confirmed to be able to inhibit the PDCoV replication post its cellular entry. Meanwhile, the inhibition effect of the supernatant from PRRSV-infected PAMs could be obviously blocked by the antagonist of these three cytokines. In conclusion, PRRSV coinfection increased TNF-α, IL-1, and IL-6 in the microenvironment of intestines, which inhibits the PDCoV proliferation, leading to lessened severity of diarrhea. The findings provide some new insight into the pathogenesis and replication regulation of PDCoV.

Different clinical presentations of subgenotype 2.1 strain of classical swine fever infection in weaned piglets and adults, and long-term cross-protection conferred by a C-strain vaccine

Classical swine fever is an important swine disease in China, and sporadic outbreaks with mild clinical signs despite compulsory vaccination have raised questions about the virulence and pathogenicity of prevalent subgenotype 2.1 strains, and the ability of C-strain vaccines to cross-protect against them. To investigate this, three field isolates were evaluated in experimentally infected piglets and compared with the highly virulent reference Shimen strain. Clinical signs for the field strains ranged from mild to severe, and mortality ranged from 0 to 80 %. These data show differences in virulence among the subgenotype 2.1 field isolates and support the use of field strain GD191 as a genotype 2 challenge virus to assess efficacy of C-strain vaccines. In contrast to the historical genotype 1 strain, which caused acute infection with significant virus shedding in non-vaccinated animals, the subgenotype 2.1 GD191 strain produced different clinical manifestations in weaned piglets and adults. Adult pigs showed subclinical infection with viral shedding, whereas weaned piglets showed overt signs of infection. Efficacy of, and duration of immunity conferred by a C-strain vaccine were assessed using the reference Shimen strain and field isolate GD191 at 12 and 15 months after vaccination. A robust antibody response and sterilising protection were seen in all vaccinated animals and lasted up to 15 months post-vaccination. This study confirms that C-strain vaccines confer both clinical and virological protection against the historical genotype 1 Shimen strain and cross-protection against the prevalent genotype 2 field strain.

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

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

Serodynamic Analysis of the Piglets Born from Sows Vaccinated with Modified Live Vaccine or E2 Subunit Vaccine for Classical Swine Fever

Classical swine fever (CSF) caused by the CSF virus (CSFV) is one of the most important swine diseases, resulting in huge economic losses to the pig industry worldwide. Systematic vaccination is one of the most effective strategies for the prevention and control of this disease. Two main CSFV vaccines, the modified live vaccine (MLV) and the subunit E2 vaccine, are recommended. In Taiwan, CSF cases have not been reported since 2006, although systemic vaccination has been practiced for 70 years. Here, we examined the sero-dynamics of the piglets born from sows that received either the CSFV MLV or the E2 vaccine and investigated in the field the correlation between the porcine reproductive and respiratory syndrome virus (PRRSV) loads and levels of CSFV antibody. A total of 1398 serum samples from 42 PRRSV-positive farms were evaluated to determine the PRRSV loads by real-time PCR and to detect CSFV antibody levels by commercial ELISA. Upon comparing the two sow vaccination protocols (CSFV MLV vaccination at 4 weeks post-farrowing versus E2 vaccination at 4-5 weeks pre-farrowing), the lowest levels of CSFV antibody were found in piglets at 5-8 and 9-12 weeks of age for the MLV and E2 groups, respectively. Meanwhile, the appropriate time window for CSFV vaccination of offspring was at 5-8 and 9-12 weeks of age in the MLV and E2 groups, respectively. There was a very highly significant negative correlation between the PRRSV load and the level of CSFV antibody in the CSFV MLV vaccination group (P < 0.0001). The PRRSV detection rate in the pigs from the MLV group (27.78%) was significantly higher than that in pigs from the E2 group (21.32%) (P = 0.011). In addition, there was a significant difference (P = 0.019) in the PRRSV detection rate at 5-8 weeks of age between the MLV (42.15%) and E2 groups (29.79%). Our findings indicate that the vaccination of CSFV MLV in piglets during the PRRSV susceptibility period at 5-8 weeks of age may be overloading the piglet's immune system and should be a critical concern for industrial pork production in the field.

Identification of Two Porcine Reproductive and Respiratory Syndrome Virus Variants Sharing High Genomic Homology but with Distinct Virulence

Porcine reproductive and respiratory syndrome virus (PRRSV) causes huge economic loss to the global swine industry. Even though several control strategies have been applied, PRRS is still not effectively controlled due to the continuous emergence of new variants and limited cross-protection by current vaccines. During the routine epidemiological investigation in 2017, two PRRSV variants were identified from a severe abortion farm and a clinically healthy farm, respectively. The viruses were isolated and denominated as XJ17-5 and JSTZ1712-12. Genomic sequencing indicated that their genomes are both 14,960 bp in length sharing 99.45% nucleotide identity. Sequence alignments identified a discontinuous 30-amino-acid deletion and a continuous 120-amino-acid deletion in nsp2 of both isolates. Genome-based phylogenetic analysis confirmed that XJ17-5 and JSTZ1712-12 belong to the HP-PRRSV subtype but form a new branch with other isolates containing the same 150-amino-acid deletion in nsp2. Pathogenic analysis showed that XJ17-5 is highly virulent causing 60% mortality, while JSTZ1712-12 is avirulent for piglets. Furthermore, fragment comparisons identified 34-amino-acid differences between XJ17-5 and JSTZ1712-12 that might be associated with the distinct virulence. The identification of highly homologous HP-PRRSV variants with new genetic feature and distinct virulence contributes to further analyze the pathogenesis and evolution of PRRSV in the field.

The impact of porcine circovirus associated diseases on live attenuated classical swine fever vaccine in field farm applications

Porcine circovirus associated diseases (PCVADs) are among the most important diseases affecting the worldwide swine industry. Vaccination against porcine circovirus type 2 (PCV2) infection has been utilized for disease control and effectively reduces clinical signs of PCVADs. To evaluate the efficacy of the PCV2 vaccine in field farms, we conducted a trial using conventional pigs immunized with the subunit PCV2 vaccine followed by PCV2 challenge. Immunized pigs demonstrated lower serum viral loads, less viral antigen staining in lymph nodes, and higher average daily weight gain, confirming the protective efficacy of the vaccine. However, low levels of PCV2 infection were still detected in vaccinated pigs after challenge, suggesting that the PCV2 vaccine was unable to eradicate the virus, which could lead to asymptomatic PCV2 subclinical infection (PCV2-SI) in pig farms. Additionally, PCV2 infection is a risk factor for impaired pig immune response development during the weaning to growth stages, which is a crucial period to receive vaccines against classical swine fever (CSF). Therefore, the impact of PCV2-SI or PCV2-systemic disease (PCV2-SD) on live attenuated CSF vaccine was investigated. After PCV2 challenge, there was no difference in levels of classical swine fever virus (CSFV) neutralizing antibodies (NA) between pigs with PCV2-SD and PCV2-SI, suggesting that the efficacy of CSF vaccine was compromised. Moreover, results of long-term monitoring of CSFV NA titers in PCV2-SI pigs with minimized interference by maternally-derived antibodies suggested that serum PCV2 viral loads greater than 10² copies/mL may compromise the efficacy of CSF vaccine. Overall, a conventional pig model was established to demonstrate the impaired efficacy of the subunit PCV2 vaccine and its impact on the CSF vaccine in vaccination-challenge trials. Additionally, the impaired efficacy of the PCV2 vaccine resulted in increased PCV2-SI, eventually leading to compromised the live attenuated CSF vaccine induced NA response in field farm applications.

TNF-α induced by porcine reproductive and respiratory syndrome virus inhibits the replication of classical swine fever virus C-strain

Porcine productive and respiratory syndrome virus (PRRSV) and classical swine fever virus (CSFV) both are major pathogens of swine that pose a great threat to the Chinese pig industry. It has been found that PRRSV infection can lead to vaccination failure of CSFV C strain-derived modified live vaccine (CSFV-C) by interfering with the immune responses to the latter. To investigate whether PRRSV can suppress CSFV-C replication, we created a 3D4/21-based cell line PAM39 that is susceptible to both viruses by expressing PRRSV receptors CD163 and CD169, and then investigated their interplay under the condition of either sequential or simultaneous co-infection. The most significant suppressive effect came from the sequential infection when the cells were first infected by PRRSV and then followed by CSFV-C at an interval of 6 h. In addition, this effect was independent of PRRSV strains. Mechanistically, PRRSV induced an elevated level of a subset of pro-inflammatory cytokines, especially tumor necrosis factor (TNF-α), through the nuclear factor κB (NF-κB) signaling pathway to inhibit the replication of CSFV-C in vitro. Thus, our studies provide an alternative explanation on PRRSV-induced CSFV vaccination failure, and this has an important implication in CSF vaccination and control.

Negative Immunomodulatory Effects of Type 2 Porcine Reproductive and Respiratory Syndrome Virus-Induced Interleukin-1 Receptor Antagonist on Porcine Innate and Adaptive Immune Functions

Impaired innate and adaptive immune responses are evidenced throughout the course of PRRSV infection. We previously reported that interleukin-1 receptor antagonist (IL-1Ra) was involved in PRRSV-induced immunosuppression during an early phase of infection. However, the exact mechanism associated with PRRSV-induced IL-1Ra immunomodulation remains unknown. To explore the immunomodulatory properties of PRRSV-induced IL-1Ra on porcine immune functions, monocyte-derived dendritic cells (MoDC) and leukocytes were cultured with type 2 PRRSV, and the immunological role of IL-1Ra was assessed by addition of anti-porcine IL-1Ra Ab. The results demonstrated that PRRSV-induced IL-1Ra reduced phagocytosis, surface expression of MHC II (SLA-DR) and CD86, as well as downregulation of IFNA and IL1 gene expression in the MoDC culture system. Interestingly, IL-1Ra secreted by the PRRSV-infected MoDC also inhibited T lymphocyte differentiation and proliferation, but not IFN-γ production. Although PRRSV-induced IL-1Ra was not directly linked to IL-10 production, it contributed to the differentiation of regulatory T lymphocytes (Treg) within the culture system. Taken together, our results demonstrated that PRRSV-induced IL-1Ra downregulates innate immune functions, T lymphocyte differentiation and proliferation, and influences collectively with IL-10 in the Treg induction. The immunomodulatory roles of IL-1Ra elucidated in this study increase our understanding of the immunobiology of PRRSV.

Emergence of a novel highly pathogenic recombinant virus from three lineages of porcine reproductive and respiratory syndrome virus 2 in China 2017

A novel porcine reproductive and respiratory syndrome virus 2 (PRRSV2) was isolated from diseased piglets in Shandong, China in 2017 and denominated as SD17-38. ORF5 sequencing showed that SD17-38 contains a unique serine/asparagine deletion at position 33 and an asparagine insertion at position 60 of GP5, which has never been described. The SD17-38 complete genome was then determined, and genome-based phylogenetic analysis showed that SD17-38 is clustered with NADC30-like isolates. Sequence alignment and recombination analyses by RDP4 and SimPlot all indicated that SD17-38 is a recombinant virus from NADC30 (lineage 1), BJ-4 (lineage 5) and TJ (lineage 8) isolates. Animal challenge study in 4-week piglets showed that SD17-38 causes high fever (≥41°C), 100% morbidity and 40% mortality. In addition, significantly lower weight gain and severe histopathological lung lesions could be observed in SD17-38-infected pigs. In particular, the unique deletion and insertion in GP5 were stable during the challenge study. This study provides direct evidence for the natural occurrence of recombination events among three lineages of PRRSV2 in Chinese swine herds, resulting in the emergence of novel PRRSV variant with unique genetic property and high pathogenicity.

NADC30-Like Porcine Reproductive and Respiratory Syndrome in China

NADC30-like porcine reproductive and respiratory syndrome virus (PRRSV) has widely spread in China and become locally dominant virus strain in some provinces. Although they are not pathogenic as highly pathogenic PRRSV (HP-RRRSV) that outbreaks since 2006, NADC30-like PRRSVs distinguished themselves by high incidence of recombination with other virus strains which lead to change of virulence. The outbreaks of NADC30-like PRRSV in the vaccinated pig herds suggested that current commercial PRRSV vaccines cannot provide complete protection to the infection. In this review, we have described in detail the current situation of NADC30 PRRSV including epidemiology, genomic characterization, pathogenicity, and efficacy of current commercial vaccines in China.

UK Pigs at the Time of Slaughter: Investigation into the Correlation of Infection with PRRSV and HEV

Hepatitis E virus (HEV) and porcine reproductive and respiratory syndrome virus (PRRSV) and are both globally prevalent in the pig population. While HEV does not cause clinical disease in pigs, its zoonotic potential has raised concerns in the food safety sector. PRRS has become endemic in the United Kingdom (UK) since its introduction in 1991, and continues to cause considerable economic losses to the swine industry. A better understanding of the current prevalence and diversity of PRRSV and HEV in the UK, and their potential association, is needed to assess risks and target control measures appropriately. This study used plasma, tonsil, and cecal content samples previously collected from pigs in 14 abattoirs in England and Northern Ireland to study the prevalence of several pathogens including PRRSV and HEV. The diversity of PRRSV strains detected in these samples was analyzed by sequencing open reading frame 5 (ORF5), revealing no substantial difference in PRRSV strains from these clinically unaffected pigs relative to those from clinical cases of disease in the UK. Despite the potential immuno-modulatory effect of PRRSV infection, previously demonstrated to affect Salmonella and HEV shedding profiles, no significant association was found between positive PRRSV status and positive HEV status.

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.

Up-regulation of IL-10 upon PRRSV vaccination impacts on the immune response against CSFV

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection negatively impacts the efficacy of classical swine fever virus (CSFV) vaccine. This paper investigates whether the PRRSV vaccine also impacts the CSFV vaccine and if the impact is time-related. Forty-eight piglets born from four sows were divided into five groups (G1-G5). The piglets in G1 to G4 were given PRRSV vaccine at 14, 21, 28 and 35days of age. The G5 group was not vaccinated with the PRRSV. All pigs were given the CSFV vaccine at 35days of age. Immune responses to the CSFV vaccine were evaluated by testing CSFV-specific sera antibodies, lymphocyte proliferation and cytokine secretion. The results demonstrate that the PRRSV vaccine significantly reduces the immune responses of the CSFV vaccination when immunised both vaccines at the same time or with only a one week interval. The PRRSV vaccination induced higher levels of IL-10 expression in the first week and this may be why the CSFV vaccination is immunosuppressed. The findings indicate that a time interval of more than one week is necessary for vaccinated CSF after the PRRSV immunisation.

Impact of porcine reproductive and respiratory syndrome virus and porcine circovirus-2 infection on the potency of the classical swine fever vaccine (LOM strain)

The classical swine fever (CSF) vaccine, which is derived from the LOM strain of the CSF virus (CSFV), induces protective immunity against CSFV infection. However, several factors influence vaccine efficacy. Evidence suggests that infection by porcine reproductive and respiratory syndrome virus (PRRSV) and/or porcine circovirus 2 (PCV2) reduces the efficacy of several vaccines. Here, we examined the effect of PRRSV or PCV2 alone or co-infection by PRRSV/PCV2 on the potency of the LOM vaccine in pigs. Neither CSFV antibody levels nor the period during which CSFV antigens were detectable in LOM-vaccinated pigs were negatively affected by infection by PRRSV or PCV2. However, co-infection with PRRSV/PCV2 may affect the replication or activity of the CSF vaccine virus in pigs vaccinated with the LOM strain, although CSFV antibody levels were not negatively affected. Nevertheless, the LOM vaccine afforded complete protection against a virulent strain of CSFV.

The 15N and 46R Residues of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nucleocapsid Protein Enhance Regulatory T Lymphocytes Proliferation

Porcine reproductive and respiratory syndrome virus (PRRSV) negatively modulates host immune responses, resulting in persistent infection and immunosuppression. PRRSV infection increases the number of PRRSV-specific regulatory T lymphocytes (Tregs) in infected pigs. However, the target antigens for Tregs proliferation in PRRSV infection have not been fully understood. In this study, we demonstrated that the highly pathogenic PRRSV (HP-PRRSV) induced more CD4⁺CD25⁺Foxp3⁺ Tregs than classical PRRSV (C-PRRSV) strain. Of the recombinant GP5, M and N proteins of HP-PRRSV expressed in baculovirus expression systems, only N protein induced Tregs proliferation. The Tregs assays showed that three amino-acid regions, 15–21, 42–48 and 88–94, in N protein played an important role in induction of Tregs proliferation with synthetic peptides covering the whole length of N protein. By using reverse genetic methods, it was firstly found that the 15N and 46R residues in PRRSV N protein were critical for induction of Tregs proliferation. The phenotype of induced Tregs closely resembled that of transforming-growth-factor-β-secreting T helper 3 Tregs in swine. These data should be useful for understanding the mechanism of immunity to PRRSV and development of infection control strategies in the future.

Increased pathogenicity of European porcine reproductive and respiratory syndrome virus is associated with enhanced adaptive responses and viral clearance

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important diseases of swine worldwide. Since its first emergence in 1987 the PRRS virus (PRRSV) has become particularly divergent with highly pathogenic strains appearing in both Europe and Asia. However, the underlying mechanisms of PRRSV pathogenesis are still unclear. This study sets out to determine the differences in pathogenesis between subtype 1 and 3 strains of European PRRSV (PRRSV-I), and compare the immune responses mounted against these strains. Piglets were infected with 3 strains of PRRSV-I: Lelystad virus, 215-06 a British field strain and SU1-bel from Belarus. Post-mortem examinations were performed at 3 and 7 days post-infection (dpi), and half of the remaining animals in each group were inoculated with an Aujeszky's disease (ADV) vaccine to investigate possible immune suppression resulting from PRRSV infection. The subtype 3 SU1-bel strain displayed greater clinical signs and lung gross pathology scores compared with the subtype 1 strains. This difference did not appear to be caused by higher virus replication, as viraemia and viral load in broncho-alveolar lavage fluid (BALF) were lower in the SU1-bel group. Infection with SU1-bel induced an enhanced adaptive immune response with greater interferon (IFN)-γ responses and an earlier PRRSV-specific antibody response. Infection with PRRSV did not affect the response to vaccination against ADV. Our results indicate that the increased clinical and pathological effect of the SU1-bel strain is more likely to be caused by an enhanced inflammatory immune response rather than higher levels of virus replication.

The epidemic status and genetic diversity of 14 highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) isolates from China in 2009

A high-mortality swine disease, the highly pathogenic porcine reproductive and respiratory syndrome (HP-PRRS), reappeared in some regions of China in 2009. To explore the possible mechanisms underlying the emergence of HP-PRRSV and more fully understand the extent of the genetic diversity of this virus in China, the complete genome of 14 isolates from 10 provinces in China from 2009 were analyzed. Full-length genome sequencing analysis showed that the 14 isolates were closely related to HP-PRRSV, with 98.0-98.9% nucleotide similarity, although 2 of the 14 strains exhibited a new, discontinuous 29-amino acid deletion in the Nsp2 gene. Furthermore, amino acid analysis of the GP5 protein indicated that the 14 isolates had a concurrent mutation in a decoy epitope and different mutations in glycosylation sites. Additionally, the antigenic drift in GP3 and a 1-nucleotide deletion in both the 5'-UTR and 3'-UTR, which are found in almost all highly pathogenic Chinese PRRSV isolates, were examined in all 14 isolates. The phylogenetic analysis showed that the 14 strains belonged to the North American genotype and were clustered in a subgroup with other HP-PRRSV isolates that have been found in China since 2006. However, compared with other Chinese HP-PRRSV isolates collected in 2006-2008, the phylogenetic tree showed that the 14 isolates had a closer relationship with each other. These results indicated that HP-PRRSV remained an extensive pandemic, affecting swine farms in China in 2009 and revealed new genetic diversity.

Taming PRRSV: revisiting the control strategies and vaccine design

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a threat, causing economically significant impacts on the swine industry worldwide. In this article, we share the information related to the Thai PRRSV and review the available options for PRRS control strategies. Unfortunately, the traditional control strategies and conventional vaccines fail to provide sustainable disease control, as they suffer from both antigenic heterogeneity and various immune evasion strategies of PRRSV. Induction of interleukin (IL)-10 following PRRSV infection is believed to be a focal mechanism leading to the unique immunological outcomes and interference of PRRS vaccine efficacy. It is likely that the nucleocapsid protein plays an important role in induction of IL-10 following PRRSV infection. We propose that removal or reduction of the PRRSV-induced, negative immunomodulatory effects especially during the first 2 weeks following infection is essential to establish proper anti-PRRSV immunity. In other word, incorporation of the "taming strategy" will be needed to reduce PRRSV-induced immunomodulatory effects, and to ensure maximal vaccine-induced immunity in the face of viral exposure. Any PRRSV vaccine that can induce cross-protective immunity and simultaneously eliminate the immunoinhibitory effects of PRRSV would be ideal. In addition, tracking of the inhibitory parameters, following the PRRSV challenge should be included in the vaccine testing protocol. Therefore, the future of PRRSV vaccine development relies tremendously on the basic knowledge of host-virus interactions and the communication between the basic and clinical PRRSV research fields.

Induction of inducible CD4+CD25+Foxp3+ regulatory T lymphocytes by porcine reproductive and respiratory syndrome virus (PRRSV)

Increases in numbers or activities of regulatory T lymphocytes (Tregs) have been linked to the establishments of several persistent infections. It has been previously shown that porcine reproductive and respiratory syndrome virus (PRRSV) can negatively modulate the host immune responses, resulting in persistent infection and secondary immunodeficiency. Recently, the existence of porcine CD4(+)CD25(+) Tregs has been demonstrated. We investigated the effect of PRRSV on the CD4(+)CD25(+) Tregs. The CD4(+)CD25(+)Foxp3(+) T lymphocytes in the peripheral blood mononuclear cells (PBMCs) were identified, using the anti-human anti-Foxp3 monoclonal antibody. In vitro culture of porcine PBMC in the presence of PRRSV, but not classical swine fever virus, significantly increased the numbers of Foxp3(+) lymphocytes, particularly in the CD4(+)CD25(high) subpopulation. The time-course study revealed that PRRSV significantly increased the numbers of viral-specific CD4(+)CD25(high)Foxp3(+) subpopulation in the culture starting from 12h through the end of the observation period. Consistent to the results obtained by flow cytometry, enhanced Foxp3 gene expression was observed in the PBMC cultured with PRRSV in a time-course manner. The presence of monocyte-derived DC in the co-culture significantly enhanced the induction of CD4(+)CD25(+) Foxp3(+) T lymphocytes. The PRRSV-induced CD4(+)CD25(high) T lymphocytes exhibited suppressive activity when co-cultured with PHA-activated, autologous peripheral blood leukocytes, indicating the suppressive activity of the PRRSV-specific Tregs. In addition, PRRSV exposure significantly increased the numbers of PRRSV-specific CD4(+)CD25(+)Foxp3(+) subpopulation in the PBMC of infected pigs at 10 days post-infection. In summary, the results indicated that PRRSV could increase the numbers of viral-specific, inducible regulatory T lymphocytes in the porcine PBMC, both in vitro and in vivo. The findings suggested the novel immunomodulatory mechanism induced by PRRSV.

Simultaneous detection of porcine circovirus type 2, classical swine fever virus, porcine parvovirus and porcine reproductive and respiratory syndrome virus in pigs by multiplex polymerase chain reaction

A multiplex polymerase chain reaction (PCR) was designed for the simultaneous detection of four viruses involved in reproductive and respiratory failure in pigs: porcine circovirus type 2 (PCV-2), porcine parvovirus (PPV), classical swine fever virus (CSFV) and porcine reproductive and respiratory syndrome virus (PRRSV). Each of the four pairs of oligonucleotide primers exclusively amplified the targeted fragment of the specific viruses. The sensitivity of the multiplex PCR using purified plasmid constructs containing the specific viral target fragments was 2.58x10(7), 2.64x10(5), 2.66x10(7) and 2.73x10(5) copies for PRRSV, PCV-2, CSFV and PPV, respectively. Using the multiplex PCR, co-infections with these four viruses were identified in 26/76 (34.2%) piglets born from sows with reproductive failure in China. This method is a rapid, sensitive and cost-effective diagnostic tool for the routine surveillance of viral infections in pigs.

Simultaneous detection of Classical swine fever virus and North American genotype Porcine reproductive and respiratory syndrome virus using a duplex real-time RT-PCR

Classical swine fever and porcine reproductive and respiratory syndrome are both notifiable diseases of the World Organization for Animal Health (OIE). The two diseases exhibit indistinguishable clinical symptoms and sometimes co-exist in swine herds. In this study, a duplex real-time RT-PCR for simultaneous detection of Classical swine fever virus (CSFV) and North American (NA) genotype Porcine reproductive and respiratory syndrome virus (PRRSV) based on two differently labeled TaqMan probes was developed and evaluated. The detection limit of the assay was 3.2 TCID(50) or 13 RNA copies for CSFV and 1.8 TCID(50) or 10 RNA copies for PRRSV, about 50 times more sensitive than conventional RT-PCRs. The duplex real-time RT-PCR was capable of specifically detecting different subgroups of wild-type CSFV and different strains of NA-genotype PRRSV, whereas a number of non-CSFV/PRRSV porcine viruses and bovine pestivirus were tested negative. Out of 155 field samples, 16 were tested positive for CSFV, 73 were positive for PRRSV, and 13 were co-infected with the two viruses. These results were 99.4% in agreement with those using conventional RT-PCRs. Therefore, the assay provides sensitive and simultaneous detection and differentiation of CSFV and PRRSV.

Emergence of fatal PRRSV variants: unparalleled outbreaks of atypical PRRS in China and molecular dissection of the unique hallmark

Porcine reproductive and respiratory syndrome (PRRS) is a severe viral disease in pigs, causing great economic losses worldwide each year. The causative agent of the disease, PRRS virus (PRRSV), is a member of the family Arteriviridae. Here we report our investigation of the unparalleled large-scale outbreaks of an originally unknown, but so-called "high fever" disease in China in 2006 with the essence of PRRS, which spread to more than 10 provinces (autonomous cities or regions) and affected over 2,000,000 pigs with about 400,000 fatal cases. Different from the typical PRRS, numerous adult sows were also infected by the "high fever" disease. This atypical PRRS pandemic was initially identified as a hog cholera-like disease manifesting neurological symptoms (e.g., shivering), high fever (40-42 degrees C), erythematous blanching rash, etc. Autopsies combined with immunological analyses clearly showed that multiple organs were infected by highly pathogenic PRRSVs with severe pathological changes observed. Whole-genome analysis of the isolated viruses revealed that these PRRSV isolates are grouped into Type II and are highly homologous to HB-1, a Chinese strain of PRRSV (96.5% nucleotide identity). More importantly, we observed a unique molecular hallmark in these viral isolates, namely a discontinuous deletion of 30 amino acids in nonstructural protein 2 (NSP2). Taken together, this is the first comprehensive report documenting the 2006 epidemic of atypical PRRS outbreak in China and identifying the 30 amino-acid deletion in NSP2, a novel determining factor for virulence which may be implicated in the high pathogenicity of PRRSV, and will stimulate further study by using the infectious cDNA clone technique.

Factors critical for successful vaccination against classical swine fever in endemic areas

Classical swine fever (CSF) or hog cholera, caused by the classical swine fever virus (CSFV), is one of the most important viral diseases that cause serious economic loss to the swine industry worldwide. During the past 5 years, several techniques for measuring porcine cell-mediated immunity (CMI) were applied, in conjunction with other conventional techniques, to study factors that influence the induction of CSFV-specific immunity. Information, obtained from a series of experiments, demonstrated cell-mediated immune responses in providing protective immunity against CSF infection. Although it has been confirmed that commercially available modified live CSF vaccines are able to induce complete protection in vaccinated pigs, several factors including maternal immunity, the age of primary vaccination, vaccination protocol and complications caused by other pathogens, can greatly affect the effectiveness of CSF vaccines in the field.

Porcine circovirus type 2 infection decreases the efficacy of a modified live porcine reproductive and respiratory syndrome virus vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV)-induced pneumonia is a major problem, and vaccination is used to reduce losses associated with PRRSV. Porcine circovirus type 2 (PCV2) causes lymphoid depletion, and there is concern that this adversely affects the immune response. The objective of this study was to investigate the effect of PCV2 infection on the efficacy of modified live virus (MLV) PRRSV vaccine. Sixty-nine 2-week-old pigs were randomly assigned to one of seven groups of 9 to 10 pigs each. At 6 weeks of age, pigs in groups 4, 5, and 6 were inoculated intranasally with PCV2 ISU-40895. At 8 weeks of age, groups 3, 4, 6, and 7 were vaccinated with a PRRSV MLV vaccine. At 12 weeks of age, groups 2, 3, and 4 were challenged with PRRSV SDSU73. All pigs were necropsied 14 days after PRRSV challenge. PCV2-infected, PRRSV-vaccinated, and PRRSV-challenged pigs had significantly (P < 0.05) more-severe macroscopic lung lesions than did the PRRSV-vaccinated and PRRSV-challenged pigs that were not exposed to PCV2 prior to PRRSV vaccination. Nonvaccinated PRRSV-infected pigs had a significantly (P < 0.001) higher incidence of PRRSV antigen in lungs than did all other groups except the group infected with PCV2 prior to PRRSV vaccination and challenge. The nonvaccinated PRRSV-challenged group and the group challenged with PCV2 prior to PRRSV vaccination and challenge had significantly (P < 0.001) lower average daily weight gain than did the control and the vaccinated groups. This work suggests that PCV2 infection has an adverse effect on the development of protective immunity induced by PRRSV vaccine.

First Results of Detection of PRRSV and CSFV RNA by SYBR Green I-based Quantitative PCR

Porcine reproductive and respiratory syndrome (PRRS) and classical swine fever (CSF) cause significant economic losses to the swine industry worldwide. As both diseases cause similar symptoms, rapid and reliable detection of these diseases is essential for disease surveillance. A quantitative SYBR Green I-based reverse transcription-polymerase chain reaction (RT-PCR) is described for simultaneous and differential diagnosis. The established RT-PCR for the quantitation of PRRSV and CSFV cDNA was found to provide a broad dynamic range, detecting from 10(3) to 10(11) and 10(2) to 10(11) copies of cDNA per reaction, respectively. Sensitivity and specificity of this method were compared with those of conventional RT-PCR and both were equal or superior to the reference method. Reproducibility was tested and the assay was proved very reliable. The assay is timesaving, easy to handle, and highly sensitive and specific. Therefore, it is a powerful tool for detecting PRRSV and CSFV simultaneously for routine outbreak investigation.