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

In vitro and in vivo antiviral effects of CLEVir-X against porcine reproductive and respiratory syndrome virus

The aim of this study was to investigate the in vitro and in vivo antiviral effects of CLEVir-X, against porcine reproductive and respiratory syndrome virus (PRRSV). CLEVir-X is a nucleoside analogue and a dialdehyde form of xanthosine. CLEVir-X demonstrated antiviral action during the in vitro portion of this experiment with its inosine monophosphate dehydrogenase (IMPDH) inhibition against PRRSV. The anti-PRRSV effect of CLEVir-X was recovered through supplementation with guanosine. This suggests that PRRSV replication may be regulated through IMPDH and its guanosine biosynthetic pathway. CLEVir-X treatment in cultures resulted in mutation frequency increase of up to 7.8-fold within the viral genomes (e.g. ORF6) compared to their parallel, untreated cultures. The incorporation of CLEVir-X into the viral genome causes lethal mutagenesis and subsequent decrease in specific infectivity. During the in vivo antiviral experiment, 21-day-old pigs began oral administration of 5 mL of phosphate buffered saline containing CLEVir-X (with purity of 68 % and dosage of 40 mg/kg body weight). This treatment was provided twice daily at 9:00AM and 5:00PM for 14 days. Pigs were simultaneously intranasally inoculated with PRRSV at the beginning of CLEVir-X treatment (21 days of age). Several beneficial effects from the oral administration of CLEVir-X were observed including reduction of body temperature, alleviation of respiratory clinical signs, decreased PRRSV load in both blood and lung tissues, and mitigation of lung interstitial pneumonia lesions. The results of the present study demonstrated that CLEVir-X has mutagenic and nonmutagenic modes of antiviral action against PRRSV based on both in vitro and in vivo antiviral experiments.

Boosting PRRSV-Specific Cellular Immunity: The Immunological Profiling of an Fc-Fused Multi-CTL Epitope Vaccine in Mice

The continuously evolving PRRSV has been plaguing pig farms worldwide for over 30 years, with conventional vaccines suffering from insufficient protection and biosecurity risks. To address these challenges, we identified 10 PRRSV-specific CTL epitopes through enzyme-linked immunospot assay (ELISPOT) and constructed a multi-epitope peptide (PTE) by linking them in tandem. This PTE was then fused with a modified porcine Fc molecule to create the recombinant protein pFc-PTE. Our findings indicate that pFc-PTE effectively stimulates PRRSV-infected specific splenic lymphocytes to secrete high levels of interferon-gamma (IFN-γ) and is predicted to be non-toxic and non-allergenic. Compared to PTE alone, pFc-PTE not only induced a comparable cellular immune response in mice but also extended the duration of the immune response to at least 10 weeks post-immunization. Additionally, pFc-PTE predominantly induced a Th1 immune response, suggesting its potential advantage in enhancing cellular immunity. Consequently, pFc-PTE holds promise as a novel, safe, and potent candidate vaccine for PRRSV and may also provide new perspectives for vaccine design against other viral diseases.

Synthetic Peptides Elicit Humoral Response against Porcine Reproductive and Respiratory Syndrome Virus in Swine

The aim of this study was to analyze the immunogenic response elicited in swine by two synthetic peptides derived from GP5 to understand the role of lineal B epitopes in the humoral and B-cell-mediated response against the porcine reproductive and respiratory syndrome virus (PRRSV). For inoculation, twenty-one-day-old pigs were allocated into six groups: control, vehicle, vaccinated (Ingelvac-PRRSV, MLV®), non-vaccinated and naturally infected, GP5-B and GP5-B3. At 2 days post-immunization (dpi), the GP5-B3 peptide increased the serum concentrations of cytokines associated with activate adaptive cellular immunity, IL-1β (1.15 ± 1.15 to 10.17 ± 0.94 pg/mL) and IL-12 (323.8 ± 23.3 to 778.5 ± 58.11 pg/mL), compared to the control group. The concentration of IgGs anti-GP5-B increased in both cases at 21 and 42 dpi compared to that at 0 days (128.3 ± 8.34 ng/mL to 231.9 ± 17.82 and 331 ± 14.86 ng/mL), while IgGs anti-GP5-B3 increased at 21 dpi (105.1 ± 19.06 to 178 ± 15.09 ng/mL) and remained at the same level until 42 dpi. Also, antibody-forming/Plasma B cells (CD2+/CD21-) increased in both cases (9.85 ± 0.7% to 13.67 ± 0.44 for GP5-B and 15.72 ± 1.27% for GP5-B3). Furthermore, primed B cells (CD2-/CD21+) from immunized pigs showed an increase in both cases (9.62 ± 1.5% to 24.51 ± 1.3 for GP5-B and 34 ± 2.39% for GP5-B3) at 42 dpi. Conversely the naïve B cells from immunized pigs decreased compared with the control group (8.84 ± 0.63% to 6.25 ± 0.66 for GP5-B and 5.78 ± 0.48% for GP5-B3). Importantly, both GP5-B and GP5-B3 peptides exhibited immunoreactivity against serum antibodies from the vaccinated group, as well as the non-vaccinated and naturally infected group. In conclusion, GP5-B and GP5-B3 peptides elicited immunogenicity mediated by antigen-specific IgGs and B cell activation.

Current Status of Vaccines for Porcine Reproductive and Respiratory Syndrome: Interferon Response, Immunological Overview, and Future Prospects

Porcine reproductive and respiratory syndrome (PRRS) remains a formidable challenge for the global pig industry. Caused by PRRS virus (PRRSV), this disease primarily affects porcine reproductive and respiratory systems, undermining effective host interferon and other immune responses, resulting in vaccine ineffectiveness. In the absence of specific antiviral treatments for PRRSV, vaccines play a crucial role in managing the disease. The current market features a range of vaccine technologies, including live, inactivated, subunit, DNA, and vector vaccines, but only modified live virus (MLV) and killed virus (KV) vaccines are commercially available for PRRS control. Live vaccines are promoted for their enhanced protective effectiveness, although their ability to provide cross-protection is modest. On the other hand, inactivated vaccines are emphasized for their safety profile but are limited in their protective efficacy. This review updates the current knowledge on PRRS vaccines' interactions with the host interferon system, and other immunological aspects, to assess their current status and evaluate advents in PRRSV vaccine development. It presents the strengths and weaknesses of both live attenuated and inactivated vaccines in the prevention and management of PRRS, aiming to inspire the development of innovative strategies and technologies for the next generation of PRRS vaccines.

A Comprehensive Review of Our Understanding and Challenges of Viral Vaccines against Swine Pathogens

Pig farming has become a strategically significant and economically important industry across the globe. It is also a potentially vulnerable sector due to challenges posed by transboundary diseases in which viral infections are at the forefront. Among the porcine viral diseases, African swine fever, classical swine fever, foot and mouth disease, porcine reproductive and respiratory syndrome, pseudorabies, swine influenza, and transmissible gastroenteritis are some of the diseases that cause substantial economic losses in the pig industry. It is a well-established fact that vaccination is undoubtedly the most effective strategy to control viral infections in animals. From the period of Jenner and Pasteur to the recent new-generation technology era, the development of vaccines has contributed significantly to reducing the burden of viral infections on animals and humans. Inactivated and modified live viral vaccines provide partial protection against key pathogens. However, there is a need to improve these vaccines to address emerging infections more comprehensively and ensure their safety. The recent reports on new-generation vaccines against swine viruses like DNA, viral-vector-based replicon, chimeric, peptide, plant-made, virus-like particle, and nanoparticle-based vaccines are very encouraging. The current review gathers comprehensive information on the available vaccines and the future perspectives on porcine viral vaccines.

Isolation and pathogenicity comparison of two novel natural recombinant porcine reproductive and respiratory syndrome viruses with different recombination patterns in Southwest China

Porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses in the swine industry. Frequent mutations and recombinations account for PRRSV immune evasion and the emergence of novel strains. In this study, we isolated and characterized two novel PRRSV-2 strains from Southwest China exhibiting distinct recombination patterns. They were designated SCABTC-202305 and SCABTC-202309. Phylogenetic results indicated that SCABTC-202305 was classified as lineage 8, and SCABTC-202309 was classified as lineage 1.8. Amino acid mutation analysis identified unique amino acid substitutions and deletions in ORF5 and Nsp2 genes. The results of the recombination analysis revealed that SCABTC-202305 is a recombinant with JXA1 as the major parental strain and NADC30 as the minor parental strain. At the same time, SCABTC-202309 is identified as a recombinant with NADC30 as the major parental strain and JXA1 as the minor parental strain. In this study, we infected piglets with SCABTC-202305, SCABTC-202309, or mock inoculum (control) to study the pathogenicity of these isolates. Although both isolated strains were pathogenic, SCABTC-202305-infected piglets exhibited more severe clinical signs and higher mortality, viral load, and antibody response than SCABTC-202309-infected piglets. SCABTC-202305 also caused more extensive lung lesions based on histopathology. Our findings suggest that the divergent pathogenicity observed between the two novel PRRSV isolates may be attributed to variations in the genetic information encoded by specific genomic regions. Elucidating the genetic determinants governing PRRSV virulence and transmissibility will inform efforts to control this devastating swine pathogen.IMPORTANCEPorcine reproductive and respiratory syndrome virus (PRRSV) is one of the most critical pathogens impacting the global swine industry. Frequent mutations and recombinations have made the control of PRRSV increasingly difficult. Following the NADC30-like PRRSV pandemic, recombination events involving PRRSV strains have further increased. We isolated two novel field PRRSV recombinant strains, SCABTC-202305 and SCABTC-202309, exhibiting different recombination patterns and compared their pathogenicity in animal experiments. The isolates caused higher viral loads, persistent fever, marked weight loss, moderate respiratory clinical signs, and severe histopathologic lung lesions in piglets. Elucidating correlations between recombinant regions and pathogenicity in these isolates can inform epidemiologic tracking of emerging strains and investigations into viral adaptive mechanisms underlying PRRSV immunity evasion. Our findings underscore the importance of continued genomic surveillance to curb this economically damaging pathogen.

Diltiazem HCl suppresses porcine reproductive and respiratory syndrome virus infection in susceptible cells and in swine

Porcine reproductive and respiratory syndrome virus (PRRSV) is a pathogen for swine, resulting in substantial economic losses to the swine industry. However, there has been little success in developing effective vaccines or drugs for PRRSV control. In the present study, we discovered that Diltiazem HCl, an inhibitor of L-type Ca2+ channel, effectively suppresses PRRSV replication in MARC-145, PK-15CD163 and PAM cells in dose-dependent manner. Furthermore, it demonstrates a broad-spectrum activity against both PRRSV-1 and PRRSV-2 strains. Additionally, we explored the underlying mechanisms and found that Diltiazem HCl -induced inhibition of PRRSV associated with regulation of calcium ion homeostasis in susceptible cells. Moreover, we evaluated the antiviral effects of Diltiazem HCl in PRRSV-challenged piglets, assessing rectal temperature, viremia, and gross and microscopic lung lesions. Our results indicate that Diltiazem HCl treatment alleviates PRRSV-induced rectal temperature spikes, pulmonary pathological changes, and serum viral load. In conclusion, our data suggest that Diltiazem HCl could serve as a novel therapeutic drug against PRRSV infection.

The mRNA Vaccine Expressing Single and Fused Structural Proteins of Porcine Reproductive and Respiratory Syndrome Induces Strong Cellular and Humoral Immune Responses in BalB/C Mice

PRRS is a viral disease that profoundly impacts the global swine industry, causing significant economic losses. The development of a novel and effective vaccine is crucial to halt the rapid transmission of this virus. There have been several vaccination attempts against PRRSV using both traditional and alternative vaccine design development approaches. Unfortunately, there is no currently available vaccine that can completely control this disease. Thus, our study aimed to develop an mRNA vaccine using the antigens expressed by single or fused PRRSV structural proteins. In this study, the nucleotide sequence of the immunogenic mRNA was determined by considering the antigenicity of structural proteins and the stability of spatial structure. Purified GP5 protein served as the detection antigen in the immunological evaluation. Furthermore, cellular mRNA expression was detected by immunofluorescence and western blotting. In a mice experiment, the Ab titer in serum and the activation of spleen lymphocytes triggered by the antigen were detected by ELISA and ICS, respectively. Our findings demonstrated that both mRNA vaccines can significantly stimulate cellular and humoral immune responses. More specifically, the GP5-mRNA exhibited an immunological response that was similar to that of the commercially available vaccine when administered in high doses. To conclude, our vaccine may show promising results against the wild-type virus in a natural host.

Prevalence and genetic evolution of porcine reproductive and respiratory syndrome virus in commercial fattening pig farms in China

BACKGROUND

To investigate the prevalence and evolution of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) at commercial fattening pig farms, a total of 1397 clinical samples were collected from a single fattening cycle at seven pig farms in five provinces of China from 2020 to 2021.

RESULTS

The RT‒PCR results revealed that PRRSV was present on all seven farms, and the percentage of PRRSV-positive individuals was 17.54-53.33%. A total of 344 partial NSP2 gene sequences and 334 complete ORF5 gene sequences were obtained from the positive samples. The statistical results showed that PRRSV-2 was present on all seven commercial fattening farms, and PRRSV-1 was present on only one commercial fattening farm. A total of six PRRSV-2 subtypes were detected, and five of the seven farms had two or more PRRSV-2 subtypes. L1.8 (L1C) PRRSV was the dominant epidemic strain on five of the seven pig farms. Sequence analysis of L1.8 (L1C) PRRSV from different commercial fattening pig farms revealed that its consistency across farms varied substantially. The amino acid alignment results demonstrated that there were 131 aa discontinuous deletions in NSP2 between different L1.8 (L1C) PRRSV strains and that the GP5 mutation in L1.8 (L1C) PRRSV was mainly concentrated in the peptide signal region and T-cell epitopes. Selection pressure analysis of GP5 revealed that the use of the PRRSV MLV vaccine had no significant episodic diversifying effect on L1.8 (L1C) PRRSV.

CONCLUSION

PRRSV infection is common at commercial fattening pig farms in China, and the percentage of positive individuals is high. There are multiple PRRSV subtypes of infection at commercial fattening pig farms in China. L1.8 (L1C) is the main circulating PRRSV strain on commercial fattening pig farms. L1.8 (L1C) PRRSV detected at different commercial fattening pig farms exhibited substantial differences in consistency but similar molecular characteristics. The pressure on the GP5 of L1.8 (L1C) PRRSV may not be directly related to the use of the vaccines.

Protective efficacy of a candidate live attenuated vaccine derived from the SD-R strain of lineage 1 porcine reproductive and respiratory syndrome virus against a lethal challenge with HP-PRRSV HuN4 in piglets

IMPORTANCE

Both highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) and NADC30-like PRRSV have caused tremendous economic losses to the Chinese pig industry. In this study, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative HP-PRRSV strain (HuN4). The control piglets in the challenge experiment displayed obvious clinical symptoms of PRRSV infection, with a mortality rate up to 40%. In contrast, all the piglets in the vaccinated challenged group survived, and only some pigs had transient fever. The daily gain of SD-R immunized group piglets was significantly increased, and the pathological changes were significantly reduced. In addition, the viral replication levels in the serum of the immunized group were significantly lower than those of the challenged control group. The live attenuated vaccine SD-R strain can provide protection against HP-PRRSV challenge, indicating that the SD-R strain is a promising vaccine candidate for use in the swine industry.

Development, Evaluation, and Clinical Application of PRRSV-2 Vaccine-like Real-Time RT-PCR Assays

In this study, we developed and validated (1) singleplex real-time RT-PCR assays for specific detection of five PRRSV-2 MLV vaccine viruses (Ingelvac MLV, Ingelvac ATP, Fostera, Prime Pac, and Prevacent) and (2) a four-plex real-time RT-PCR assay (IngelvacMLV/Fostera/Prevacent/XIPC) including the internal positive control XIPC for detecting and distinguishing the three most commonly used vaccines in the USA (Prevacent, Ingelvac MLV, and Fostera). The singleplex and 4-plex vaccine-like PCRs and the reference PCR (VetMAXTM PRRSV NA&EU, Thermo Fisher Scientific, Waltham, MA, USA) did not cross-react with non-PRRSV swine viral and bacterial pathogens. The limits of detection of vaccine-like PCRs ranged from 25 to 50 genomic copies/reactions. The vaccine-like PCRs all had excellent intra-assay and inter-assay repeatability. Based on the testing of 531 clinical samples and in comparison to the reference PCR, the diagnostic sensitivity, specificity, and agreement were in the respective range of 94.67-100%, 100%, and 97.78-100% for singleplex PCRs and 94.94-100%, 100%, and 97.78-100% for the 4-plex PCR, with a CT cutoff of 37. In addition, 45 PRRSV-2 isolates representing different genetic lineages/sublineages were tested with the vaccine-like PCRs and the results were verified with sequencing. In summary, the vaccine-like PCRs specifically detect the respective vaccine-like viruses with comparable performances to the reference PCR, and the 4-plex PCR allows to simultaneously detect and differentiate the three most commonly used vaccine viruses in the same sample. PRRSV-2 vaccine-like PCRs provide an additional tool for detecting and characterizing PRRSV-2.

Development of a one-step reverse transcription-quantitative polymerase chain reaction assay for the detection of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) is an important disease that severely affects the swine industry and, therefore, warrants rapid and accurate diagnosis for its control. Despite the progress in developing diagnostic tools, including polymerase chain reaction (PCR)-based methods such as reverse transcription quantitative PCR (RT-qPCR) to diagnose PRRSV infection, its diagnosis at the genetic level is challenging because of its high genetic variability. Nevertheless, RT-qPCR is the easiest and fastest method for diagnosing PRRSV. Therefore, this study aimed to develop an RT-qPCR assay for rapid and accurate diagnosis of PRRSV by encompassing all publicly available PRRSV sequences. The developed assay using highly specific primers and probes could detect up to 10 copies of PRRSV-1 and -2 subtypes. Furthermore, a comparison of the performance of the developed assay with those of two commercial kits widely used in South Korea demonstrated the higher efficiency of the developed assay in detecting PRRSV infections in field samples. For PRRSV-1 detection, the developed assay showed a diagnostic agreement of 97.7% with the results of ORF5 sequencing, while for commercial kits, it showed 95.3% and 72.1% agreement. For PRRSV-2, the developed assay showed a diagnostic agreement of 97.7%, whereas the commercial kits showed 93% and 90.7% agreement. In conclusion, we developed an assay with higher accuracy than those of the tested commercial kits, which will contribute markedly to global PRRSV control.

Evaluation of dynamics of immune responses and protective efficacy in piglets immunized with an inactivated porcine reproductive and respiratory syndrome vaccine candidate

Porcine Reproductive and Respiratory Syndrome (PRRS) is an important viral disease of swine that causes significant mortality in piglets and production losses in adult pigs. In this study, we investigated the protective efficacy of an inactivated PRRS virus vaccine candidate and evaluated the differences in PRRSV specific anamnestic response in piglets when challenged with live PRRSV at two different intervals post-immunization. Six-week-old piglets were immunized intramuscularly with an inactivated, Montanide ISA-206 adjuvanted Indian PRRSV isolate, followed by a booster dose at 21 days post-immunization. Homologous live PRRS virus challenge was done on 60 and 180 days post-booster (dpb). We assessed humoral and cell-mediated immune responses at various intervals post-immunization and after challenge. Viraemia, virus shedding in nasal secretions and lung lesion scores were studied to assess the efficacy of the vaccine candidate. All the immunized pigs developed PRRSV-specific antibodies upon booster dose administration. Neutralizing antibody (NA) titres before challenge, in most animals, ranged between 0 and 4. Potentially protective NA titre of 8 was observed in serum of seven out of the 12 immunized piglets after challenge, across the immunized groups. A significant increase in the mean T-helper, T-cytotoxic, memory or activated T-helper and NK cell populations was observed in immunized piglets challenged at 180 dpb, from 4 to 11 dpc, 5 to 11 dpc, 5 to 7 dpc and 6 to 11 dpc, respectively as compared to the challenge controls. Protective efficacy of the inactivated PRRSV antigen against the homologous virus challenge was evidenced by earlier onset of PRRSV specific virus neutralizing antibodies and cell mediated immune responses, reduced viremia, nasal virus shedding and severity of lung lesions in immunized piglets as compared to unimmunized controls post-challenge. Our results indicated that the inactivated PRRSV antigen elicited better virus specific anamnestic immune responses in piglets when challenged at six months after the single booster dose, due to age related increase in antigen-specific memory T helper cell responses, as compared to those challenged at 2 months post booster.

Therapeutic efficacy of tylvalosin combined with Poria cocos polysaccharides against porcine reproductive and respiratory syndrome

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important infectious diseases of pigs worldwide. Vaccination and various management measures have been implemented to control PRRS. However, due to high genetic diversity and insufficient understanding of the pathogenesis and immunological mechanisms, PRRS is still a challenge to the pig industry. Therefore, it is important to develop novel strategies to combat PRRS virus (PRRSV) infection. In this study, our data show that tylvalosin, a third-generation animal-specific macrolide, could inhibit PRRSV replication in MARC-145 cells, and suppress the PRRSV-induced NF-κB activation and cytokines expression. The pig infection experiment further demonstrated that tylvalosin could significantly reduce the virus loads in serum and tissues, and alleviate lung lesions of pigs infected with highly pathogenic PRRSV strains. The fever and loss of daily gain (LoDG) of the pigs were decreased as well. Considering the feature of immune suppression of PRRSV, a combination of tylvalosin with the immunopotentiator Poria cocos polysaccharides (PCP) was developed. Pig experiment showed this combination had a better therapeutic efficacy against PRRSV infection than tylvalosin and PCP alone in attenuating lung lesions, alleviating fever, and suppressing cytokines production. This study suggests that tylvalosin has significant antiviral and anti-inflammatory effects against PRRSV infection, and the combination of tylvalosin and PCP provides a promising strategy for PRRS treatment.

Research Progress on the Development of Porcine Reproductive and Respiratory Syndrome Vaccines

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease in the pig industry, but its pathogenesis is not yet fully understood. The disease is caused by the PRRS virus (PRRSV), which primarily infects porcine alveolar macrophages and disrupts the immune system. Unfortunately, there is no specific drug to cure PRRS, so vaccination is crucial for controlling the disease. There are various types of single and combined vaccines available, including live, inactivated, subunit, DNA, and vector vaccines. Among them, live vaccines provide better protection, but cross-protection is weak. Inactivated vaccines are safe but have poor immune efficacy. Subunit vaccines can be used in the third trimester of pregnancy, and DNA vaccines can enhance the protective effect of live vaccines. However, vector vaccines only confer partial protection and have not been widely used in practice. A PRRS vaccine that meets new-generation international standards is still needed. This manuscript provides a comprehensive review of the advantages, disadvantages, and applicability of live-attenuated, inactivated, subunit, live vector, DNA, gene-deletion, synthetic peptide, virus-like particle, and other types of vaccines for the prevention and control of PRRS. The aim is to provide a theoretical basis for vaccine research and development.

Infection dynamics and incidence of wild-type porcine reproductive and respiratory syndrome virus in growing pig herds in the U.S. Midwest

Porcine reproductive and respiratory syndrome virus (PRRSV) infections greatly impact the health and productivity of growing pigs. The introduction and persistence of wild-type PRRSV (WT-PRRSV) strains in growing pig populations is poorly understood. In an observational prospective cohort study, we monitored and surveyed 63 wean-to-finish (WTF) herds across 10 companies located in medium to high pig dense areas in the U.S. Midwest. All herds received weaned pigs from PRRSV-negative or positive-stable breeding herds. Herds were monitored monthly using oral fluids collected following a fixed spatial sampling regime and samples were tested by PRRSV ELISA, RT-PCR and ORF5 sequencing. In most (90%) of the herds, pigs were vaccinated with PRRSV modified-live vaccines either at processing, weaning or shortly after weaning. Wild type PRRSV (WT-PRRSV) infections were defined by the criterion of having more than 2% nucleotide differences in the ORF-5 region compared with reference vaccine strain sequences. Wild type PRRSV was detected in 42% of the herds with infections being more prevalent in the mid to late growing period, with a mean of 20 weeks post placement. Nineteen distinct WT-PRRSV were identified in seven out of 10 production companies with an average of 3 distinct WT-PRRSV strains per company. Vaccinated WTF herds with and without WT-PRRSV detection were compared to each other showing different PCR and ELISA infection patterns. Close-out mortality in vaccinated herds with WT-PRRSV was numerically higher (6.5%) than mortality in those sites where WT-PRRSV was not detected (5.0%) (p = 0.07). Mortality was also higher (10.5%) when WT-PRRSV was detected earlier at eight weeks post-placement compared to late finishing at 20 and 25 weeks post-placement, 2.9% and 4.5% respectively (p = 0.017). Overall, this study sheds light on WT-PRRSV infection dynamics in vaccinated populations of growing pigs, reinforces the importance of biosecurity practices in this phase of production and calls for better understanding of risk factors associated with PRRSV introductions in growing pig sites.

Recent Progress in Studies of Porcine Reproductive and Respiratory Syndrome Virus 1 in China

Due to the high incidence of PRRSV mutation and recombination, PRRSV infection is difficult to prevent and control in China and worldwide. Two species of PRRSV, Betaarterivirus suid 1 (PRRSV-1) and Betaarterivirus suid 2 (PRRSV-2), exist in China, and PRRSV-1 has always received less attention in China. However, the number of PRRSV-1 strains detected in China has increased recently. To date, PRRSV-1 has spread to more than 23 regions in China. Based on the phylogenetic analysis of ORF5 and the whole genome of PRRSV-1, Chinese PRRSV-1 can be divided into at least seven independent subgroups. Among them, BJEU06-1-like has become the mainstream subgroup in some regions of China. This subgroup of strains has a 5-aa (4 + 1) characteristic discontinuous deletion pattern at aa 357~aa 360 and aa 411 in Nsp2. Previous studies have indicated that the pathogenicity of PRRSV-1 in China is mild, but recent studies found that the pathogenicity of PRRSV-1 was enhanced in China. Therefore, the emergence of PRRSV-1 deserves attention, and the prevention and control of PRRSV-1 infection in China should be strengthened. PRRSV infection is usually prevented and controlled by a combination of virus monitoring, biosafety restrictions, herd management measures and vaccination. However, the use of PRRSV-1 vaccines is currently banned in China. Thus, we should strengthen the monitoring of PRRSV-1 and the biosafety management of pig herds in China. In this review, we summarize the prevalence of PRRSV-1 in China and clarify the genomic characteristics, pathogenicity, vaccine status, and prevention and control management system of PRRSV-1 in China. Consequently, the purpose of this review is to provide a basis for further development of prevention and control measures for PRRSV-1.

Evaluating anti-viral effect of Tylvalosin tartrate on porcine reproductive and respiratory syndrome virus and analyzing the related gene regulation by transcriptomics

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important pathogen, characterized by its genetic and antigenic variation. The PRRSV vaccine is widely used, however, the unsatisfied heterologic protection and the risk of reverse virulence raise the requirement to find some new anti-PRRSV strategies for disease control. Tylvalosin tartrate is used to inhibit PRRSV in the field non-specifically, however, the mechanism is still less known.

METHODS

The antiviral effects of Tylvalosin tartrates from three producers were evaluated in a cell inoculation model. Their safety and efficacy concentrations, and effecting stage during PRRSV infection were analyzed. And, the Tylvalosin tartrates regulated genes and pathways which are potentially related to the anti-viral effect were further explored by using transcriptomics analysis. Last, the transcription level of six anti-virus-related DEGs was selected to confirm by qPCR, and the expression level of HMOX1, a reported anti-PRRSV gene, was proved by western blot.

RESULTS

The safety concentrations of Tylvalosin tartrates from three different producers were 40 µg/mL (Tyl A, Tyl B, and Tyl C) in MARC-145 cells and 20 µg/mL (Tyl A) or 40 µg/mL (Tyl B and Tyl C) in primary pulmonary alveolar macrophages (PAMs) respectively. Tylvalosin tartrate can inhibit PRRSV proliferation in a dose-dependent manner, causing more than 90% proliferation reduction at 40 µg/mL. But it shows no virucidal effect, and only achieves the antiviral effect via long-term action on the cells during the PRRSV proliferation. Furthermore, GO terms and KEGG pathway analysis was carried out based on the RNA sequencing and transcriptomic data. It was found that the Tylvalosin tartrates can regulate the signal transduction, proteolysis, and oxidation-reduction process, as well as some pathways such as protein digestion and absorption, PI3K-Akt signaling, FoxO signaling, and Ferroptosis pathways, which might relate to PRRSV proliferation or host innate immune response, but further studies still need to confirm it. Among them, six antivirus-related genes HMOX1, ATF3, FTH1, FTL, NR4A1, and CDKN1A were identified to be regulated by Tylvalosin tartrate, and the increased expression level of HMOX1 was further confirmed by western blot.

CONCLUSIONS

Tylvalosin tartrate can inhibit PRRSV proliferation in vitro in a dose-dependent manner. The identified DEGs and pathways in transcriptomic data will provide valuable clues for further exploring the host cell restriction factors or anti-PRRSV target.

Codon Pair Deoptimization (CPD)-Attenuated PRRSV-1 Vaccination Exhibit Immunity to Virulent PRRSV Challenge in Pigs

Commercially used porcine respiratory and reproductive syndrome (PRRS) modified live virus (MLV) vaccines provide limited protection with heterologous viruses, can revert back to a virulent form and they tend to recombine with circulating wild-type strains. Codon pair deoptimization (CPD) is an advanced method to attenuate a virus that overcomes the disadvantages of MLV vaccines and is effective in various virus vaccine models. The CPD vaccine against PRRSV-2 was successfully tested in our previous study. The co-existence of PRRSV-1 and -2 in the same herd demands protective immunity against both viruses. In this study, live attenuated PRRSV-1 was constructed by recoding 22 base pairs in the ORF7 gene of the E38 strain. The efficacy and safety of the CPD live attenuated vaccine E38-ORF7 CPD to protect against virulent PRRSV-1 were evaluated. Viral load, and respiratory and lung lesion scores were significantly reduced in animals vaccinated with E38-ORF7 CPD. Vaccinated animals were seropositive by 14 days post-vaccination with an increased level of interferon-γ secreting cells. In conclusion, the codon-pair-deoptimized vaccine was easily attenuated and displayed protective immunity against virulent heterologous PRRSV-1.

Toll-like Receptor-Mediated Immunomodulation of Th1-Type Response Stimulated by Recombinant Antigen of Type 2 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2)

PRRSV infects CD163-positive macrophages and skews their polarization toward an M2 phenotype, followed by T-cell inactivation. In our previous study, we found that recombinant protein A1 antigen derived from PRRSV-2 was a potential vaccine or adjuvant for immunization against PRRSV-2 infection due to its ability to repolarize macrophages into M1 subtype, thereby reducing CD163 expression for viral entry and promoting immunomodulation for Th1-type responses, except for stimulating Toll-like receptor (TLR) activation. The aim of our current study was to evaluate the effects of another two recombinant antigens, A3 (ORF6L5) and A4 (NLNsp10L11), for their ability to trigger innate immune responses including TLR activation. We isolated pulmonary alveolar macrophages (PAMs) from 8- to 12-week-old specific pathogen free (SPF) piglets and stimulated them with PRRSV (0.01 MOI and 0.05 MOI) or antigens. We also investigated the T-cell differentiation by immunological synapse activation of PAMs and CD4+ T-cells in the cocultured system. To confirm the infection of PRRSV in PAMs, we checked the expression of TLR3, 7, 8, and 9. Our results showed that the expression of TLR3, 7, and 9 were significantly upregulated in PAMs by A3 antigen induction, similar to the extent of PRRSV infection. Gene profile results showed that A3 repolarizes macrophages into the M1 subtype potently, in parallel with A1, as indicated by significant upregulation of proinflammatory genes (TNF-α, IL-6, IL-1β and IL-12). Upon immunological synapse activation, A3 potentially differentiated CD4 T cells into Th1 cells, determined by the expression of IL-12 and IFN-γ secretion. On the contrary, antigen A4 promoted regulatory T cell (T-reg) differentiation by significant upregulation of IL-10 expression. Finally, we concluded that the PRRSV-2 recombinant protein A3 provided better protection against PRRSV infection, suggested by its capability to reeducate immunosuppressive M2 macrophages into proinflammatory M1 cells. As M1 macrophages are prone to be functional antigen-presenting cells (APCs), they can call for TLR activation and Th1-type immune response within the immunological synapse.

Testable Candidate Immune Correlates of Protection for Porcine Reproductive and Respiratory Syndrome Virus Vaccination

Porcine reproductive and respiratory syndrome virus (PRRSV) is an on-going problem for the worldwide pig industry. Commercial and experimental vaccinations often demonstrate reduced pathology and improved growth performance; however, specific immune correlates of protection (CoP) for PRRSV vaccination have not been quantified or even definitively postulated: proposing CoP for evaluation during vaccination and challenge studies will benefit our collective efforts towards achieving protective immunity. Applying the breadth of work on human diseases and CoP to PRRSV research, we advocate four hypotheses for peer review and evaluation as appropriate testable CoP: (i) effective class-switching to systemic IgG and mucosal IgA neutralizing antibodies is required for protective immunity; (ii) vaccination should induce virus-specific peripheral blood CD4+ T-cell proliferation and IFN-γ production with central memory and effector memory phenotypes; cytotoxic T-lymphocytes (CTL) proliferation and IFN-γ production with a CCR7- phenotype that should migrate to the lung; (iii) nursery, finishing, and adult pigs will have different CoP; (iv) neutralizing antibodies provide protection and are rather strain specific; T cells confer disease prevention/reduction and possess greater heterologous recognition. We believe proposing these four CoP for PRRSV can direct future vaccine design and improve vaccine candidate evaluation.

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

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

Evaluation of Intradermal PRRSV MLV Vaccination of Suckling Piglets on Health and Performance Parameters under Field Conditions

Porcine reproductive and respiratory syndrome virus (PRRSV) causes respiratory disease in weaning and growing pigs. A vaccination against PRRSV is one of the most important control measures. This trial aimed to evaluate the effect of the intradermal (ID) administration of a PRRSV-1 modified live virus (MLV) vaccine in comparison to the intramuscular (IM) administration on the piglets’ health and performance. A total of 187 suckling piglets of a PRRSV-positive commercial farrow-to-finish farm were assigned to four groups: group A—PRRSV ID, group B—PRRSV IM, group C—control ID, and group D—control IM. At 2 weeks of age, all the study piglets were either vaccinated with a PRRSV-1 MLV vaccine or injected with the vaccine adjuvant (controls). The collected blood serum samples were tested by ELISA and qRT-PCR. The side effects, body weight (BW), average daily gain (ADG), mortality rate, and lung and pleurisy lesions scores (LLS, PLS) were also recorded. The ELISA results indicated that the vaccination induced an important seroconversion at 4 and 7 weeks. Significant differences in the qRT-PCR results were noticed only at 10 weeks in group A vs. group C (p < 0.01) and group B vs. group C (p < 0.05). High viral loads, as evidenced by the qRT-PCR Ct values, were noticed in animals of both non-vaccinated groups at 7, 10, and 13 weeks. An ID vaccination has a positive impact on the BW at the piglets’ slaughter, while both an ID and IM vaccination had a positive impact on the ADG. The mortality rate was lower in vaccinated groups at the finishing stage. The LLS and PLS were significantly lower in the vaccinated groups. In conclusion, our study demonstrated that the ID vaccination of suckling piglets with a PRRSV-1 MLV vaccine has a positive effect on the piglets’ health and performance, including an improved BW and a lower LLS and PLS index at their slaughter, as well as a decreased mortality rate at the growing/finishing stage.

Porcine Reproductive and Respiratory Syndrome Virus nsp1β Stabilizes HIF-1α to Enhance Viral Replication

Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has been devastating the swine industry worldwide since the late 1980s. Severe interstitial pneumonia is the typical pathological characteristic of PRRSV-infected swine. Accumulating evidence has suggested that hypoxia-inducible factor 1α (HIF-1α) plays vital roles in the development of inflammation and the viral life cycle. However, the role and the underlying mechanism of HIF-1α in PRRSV infection remain elusive. Here, we found that PRRSV infection elevated HIF-1α expression. Furthermore, overexpression of HIF-1α increased PRRSV replication, whereas knockdown of HIF-1α inhibited PRRSV infection. Our further mechanistic analysis revealed that PRRSV-encoded nonstructural protein 1β (nsp1β) promoted HIF-1α transcription via its N-terminal nuclease activity and degraded the polyubiquitin chain of HIF-1α via its C-terminal deubiquitylation (DUB) enzyme activity, collectively stabilizing HIF-1α. Meanwhile, nsp1β interacted with both HIF-1α and von Hippel-Lindau tumor suppressor (pVHL) to form a ternary complex, which may have hindered pVHL-mediated ubiquitination degradation of HIF-1α by impairing the interaction between HIF-1α and pVHL. Interestingly, pVHL also stabilized nsp1β via K63-linked ubiquitination, forming a positive feedback loop to stabilize HIF-1α. Taken together, these results indicate that PRRSV infection stabilizes HIF-1α to facilitate viral proliferation and that viral nsp1β plays a vital role in enhancing the expression and stabilization of HIF-1α. The regulation of HIF-1α may have great therapeutic potential for the development of novel drugs against PRRSV. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) has devastated the swine industry worldwide for over 30 years and shows no signs of slowing down. In this study, we found that PRRSV infection elevated hypoxia-inducible factor 1α (HIF-1α) expression. In addition, overexpressed HIF-1α contributed to PRRSV replication, whereas knockdown of HIF-1α reduced PRRSV growth. The PRRSV-encoded nonstructural protein 1β (nsp1β) exerted a stabilizing effect on HIF-1α through its nuclease protease and papain-like cysteine protease enzymatic domains. PRRSV nsp1β also interacted with von Hippel-Lindau tumor suppressor (pVHL) and HIF-1α, whereby nsp1β impaired the interaction between HIF-1α and pVHL. This work deepens our understanding of the molecular mechanisms involved in PRRSV infection and provides new insights for the development of HIF-1α-based anti-PRRSV therapies.

Attenuated Porcine Reproductive and Respiratory Syndrome Virus Regains Its Fatal Virulence by Serial Passaging in Pigs or Porcine Alveolar Macrophages To Increase Its Adaptation to Target Cells

Porcine reproductive and respiratory syndrome (PRRS) is a globally important disease threatening the pork industry, and modified live-virus (MLV) vaccines are widely used for its prevention. However, PRRS MLV shows high potential for reversion to virulence, leading to a major concern about its safety. Yet the revertant mechanism is still poorly understood. Here, attenuated virus JXwn06-P80, derived from the highly pathogenic PRRS virus (PRRSV) strain JXwn06 by serial passaging in MARC-145 cells, was reversely passaged in pigs through intranasal inoculation to mimic natural infection for 13 rounds, and the pathogenicity of viruses at the 3rd, 5th, 9th, 10th, and 11th passages was evaluated in pigs. From the 9th passage, the viruses caused mortality, which was related to their increased adaptability and replication efficiency (100 times higher than those of JXwn06-P80) in porcine alveolar macrophage (PAM) target cells. Similarly, JXwn06-P80 could also regain fatal virulence through reverse passage in PAMs for 25 or more passages, indicating that the increased adaptability in PAMs directly contributes to its regained fatal virulence. Next, the full-genome sequences were analyzed to explore the genetic evolutionary processes during adaptation both in vivo and in vitro. Finally, by a reverse genetic operation, four reverse mutation sites, NSP12-W121R, ORF2b (open reading frame 2b)-H9D, ORF5-H15L, and ORF5-V189L, were finally identified to partially contribute to the ability of the virus to adapt to PAMs, which may be related to virulence reversion during reverse passage. These findings provided direct scientific evidence for the virulence reversion of PRRS MLV and provided valuable clues for exploring its molecular mechanism. IMPORTANCE Reversion to virulence of a live attenuated vaccine is a public concern; however, direct scientific evidence is limited, and the mechanism is still poorly understood. Here, we present direct evidence for the reversion to virulence of PRRS MLV after serial passaging in pigs or target cells and found a correlation between virulence reversion and increased replication fitness in primary PAMs. The genetic evolutionary process during adaptation will provide valuable clues for exploring the molecular mechanism of PRRS MLV virulence reversion and offer important implications for understanding the reversion mechanisms of other vaccines.

Itaconate derivative 4-OI inhibits PRRSV proliferation and associated inflammatory response

Itaconate, a metabolite of the tricarboxylic acid (TCA) cycle produced by immunoresponsive gene 1 (IRG1) via catalyzation of cis-aconitate, plays important roles in metabolism and immunity. Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has devastated the swine industry worldwide for over 30 years. Here, we found that 4-octyl itaconate (4-OI), a cell-permeable itaconate derivative, dose-dependently inhibited PRRSV proliferation by interfering with viral attachment, replication, and release. Furthermore, 4-OI suppressed the PRRSV-induced inflammatory response by enhancing nuclear factor erythroid 2-related factor 2 (Nrf2) signaling. Interestingly, PRRSV infection caused a reduction in itaconate abundance and simultaneously led to an accumulation of cis-aconitate, the upstream metabolite of itaconate, and both of these effects were accomplished by downregulating IRG1 expression. Taken together, these results demonstrate that 4-OI not only inhibits PRRSV replication but also suppresses PRRSV-induced inflammatory responses, indicating that 4-OI is a promising drug candidate for combating PRRSV.

Analysis of Recombinant Characteristics Based on 949 PRRSV-2 Genomic Sequences Obtained from 1991 to 2021 Shows That Viral Multiplication Ability Contributes to Dominant Recombination

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important diseases affecting the pig-raising industry. The PRRS virus (PRRSV) has high genetic diversity, partly owing to viral recombination. Some individual recombinant type 2 PRRSV (PRRSV-2) strains have been detected; however, the sequence composition characteristics of recombination hot spots and potential driving forces for recombinant PRRSV-2 are still unreported. Therefore, all available genomic sequences of PRRSV-2 (n = 949, including 29 genomes sequenced in this study) from 11 countries from 1991 to 2021 were collected and analyzed. The results revealed that the dominant major recombinant parent has been converted from lineage 3 (L3) to L1 since 2012. The recombination hot spots were located at nucleotides (nt) 7900 to 8200 (in NSP9, encoding viral RNA-dependent RNA polymerase) and nt 12500 to nt 13300 (in ORF2-ORF4, mean ORF2 to ORF4); no AU-rich characteristics were found in the recombination hot spots. Based on infectious clones of L1 and L8 PRRSV-2, recombinant PRRSVs were generated by switching complete or partial NSP9 (harboring the recombination hot spot). The results showed that recombinant PRRSVs based on the L1 backbone, but not the L8 backbone, acquired a higher replication capacity in pig primary alveolar macrophages. These findings will help to understand the reason behind the dominance of L1-based recombination in PRRSV-2 strains and provide new clues for an in-depth study of the recombination mechanism of PRRSV-2. IMPORTANCE Recombination is an important driver of the genetic shifts that are tightly linked to the evolution of RNA viruses. Viral recombination contributes substantially to the emergence of new variants, alterations in virulence, and pathogenesis. PRRSV is genetically diverse, partly because of extensive recombination. In this study, we analyzed interlineage recombination based on available genomic sequences of PRRSV-2 from 1991 to 2021. The study revealed the temporal and geographical distribution of recombinant PRRSVs and the recombination hot spot's location and showed that artificially constructed recombinant PRRSVs (harboring a high-frequency region) had more viral genomic copies than their parental virus, indicating that dominant recombination was shaped by a tendency to benefit viral replication. This finding will enrich our understanding of PRRSV recombination and provide new clues for an in-depth study of the recombination mechanism.

Integrated time-series transcriptomic and metabolomic analyses reveal different inflammatory and adaptive immune responses contributing to host resistance to PRRSV

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly contagious disease that affects the global pig industry. To understand mechanisms of susceptibility/resistance to PRRSV, this study profiled the time-serial white blood cells transcriptomic and serum metabolomic responses to PRRSV in piglets from a crossbred population of PRRSV-resistant Tongcheng pigs and PRRSV-susceptible Large White pigs. Gene set enrichment analysis (GSEA) illustrated that PRRSV infection up-regulated the expression levels of marker genes of dendritic cells, monocytes and neutrophils and inflammatory response, but down-regulated T cells, B cells and NK cells markers. CIBERSORT analysis confirmed the higher T cells proportion in resistant pigs during PRRSV infection. Resistant pigs showed a significantly higher level of T cell activation and lower expression levels of monocyte surface signatures post infection than susceptible pigs, corresponding to more severe suppression of T cell immunity and inflammatory response in susceptible pigs. Differentially expressed genes between resistant/susceptible pigs during the course of infection were significantly enriched in oxidative stress, innate immunity and humoral immunity, cell cycle, biotic stimulated cellular response, wounding response and behavior related pathways. Fourteen of these genes were distributed in 5 different QTL regions associated with PRRSV-related traits. Chemokine CXCL10 levels post PRRSV infection were differentially expressed between resistant pigs and susceptible pigs and can be a promising marker for susceptibility/resistance to PRRSV. Furthermore, the metabolomics dataset indicated differences in amino acid pathways and lipid metabolism between pre-infection/post-infection and resistant/susceptible pigs. The majority of metabolites levels were also down-regulated after PRRSV infection and were significantly positively correlated to the expression levels of marker genes in adaptive immune response. The integration of transcriptome and metabolome revealed concerted molecular events triggered by the infection, notably involving inflammatory response, adaptive immunity and G protein-coupled receptor downstream signaling. This study has increased our knowledge of the immune response differences induced by PRRSV infection and susceptibility differences at the transcriptomic and metabolomic levels, providing the basis for the PRRSV resistance mechanism and effective PRRS control.

Using a concurrent challenge with porcine circovirus 2 and porcine reproductive and respiratory syndrome virus to compare swine vaccination programs

The objectives of the present study were to evaluate the immune response of six commercial vaccines against PRRSV-2 and PCV2, administered as monovalent or combined products via intramuscular (IM) or intradermal (ID) routes. Seventy-two, 3-week-old pigs were randomly allocated into 8 treatments with 9 pigs each: IMPP0/PCVMH7, IDPP0/PCVMH7, IMING0/PCVMH7, IMPP0/PCVMH0, IDPP0/PCVMH0, IMTRF0, NV/CH, and NV/NC. IMPP0/PCVMH0 and IMPP0/PCVMH7 groups were IM vaccinated once with Prime Pac PRRS (MSD Animal Health, The Netherlands) at 0 days post-vaccination (DPV), followed by single IM vaccination with Porcilis PCV M Hyo (MSD Animal Health, The Netherlands) either at 0 or 7 DPV, respectively. IDPP0/PCVMH0 and IDPP0/PCVMH7 groups were ID vaccinated once with Prime Pac PRRS (MSD Animal Health, The Netherlands) at 0 DPV, followed by a single concurrent ID injection of Porcilis PCV ID (MSD Animal Health, The Netherlands) and Porcilis M Hyo ID ONCE (MSD Animal Health, The Netherlands) either at 0 or 7 DPV, respectively. The IMING0/PCVMH7 group was IM vaccinated once with Ingelvac PRRS MLV (Boehringer Ingelheim, Germany) at 0 DPV, and subsequently IM vaccinated with Ingelvac CircoFLEX (Boehringer Ingelheim, Germany) and Ingelvac MycoFLEX (Boehringer Ingelheim, Germany) at 7 DPV. The IMTRF0 group was IM vaccinated once with combined products of Ingelvac PRRS MLV (Boehringer Ingelheim, Germany), Ingelvac CircoFLEX (Boehringer Ingelheim, Germany), and Ingelvac MycoFLEX (Boehringer Ingelheim, Germany) at 0 DPV. The NV/CH and NV/NC groups were left unvaccinated. At 28 DPV (0 days post-challenge, DPC), pigs were intranasally inoculated with a 4 ml of mixed cell culture inoculum containing HP-PRRSV-2 (10^5.6 TCID₅₀/ml) and PCV2d (10^5.0 TCID₅₀/ml). Antibody response, IFN-γ-secreting cells (SC), and IL-10 secretion in supernatants of stimulated PBMC were monitored. Sera were collected and quantified for the PRRSV RNA and PCV2 DNA using qPCR. Three pigs from each group were necropsied at 7 DPC, lung lesions were evaluated. Tissues were collected and performed immunohistochemistry (IHC). Our study demonstrated that concurrent vaccination via the ID or the IM route did not introduce additional reactogenicity. We found no interference with the induction of immune response between vaccination timing. In terms of an immune response, ID vaccination resulted in significantly lower IL-10 levels and higher IFN-γ-SC values compared to the IM-vaccinated groups. In terms of clinical outcomes, only one IM-vaccinated group showed significantly better efficacy when antigens were injected separately compared with concurrently. While the vaccines were ID delivered, these effects disappeared. Our findings confirm that concurrent vaccination of PRRSV-2 MLV and PCV2 via either the IM or the ID routes could be a viable immunization strategy to assist with the control of PRDC. In situations where maximal efficacy is required, over all other factors, concurrent vaccination is possible with the ID route but might not be an ideal strategy if using the IM route.

Phylodynamic and phylogeographic reconstruction of porcine reproductive and respiratory syndrome virus (PRRSV) in Europe: Patterns and determinants

Porcine reproductive and respiratory syndrome (PRRS) is among the most devastating diseases affecting the pig industry. Despite vaccines having been available for decades, the remarkable genetic variability of this virus, leading to poor cross-protection, has limited their efficacy, and other measures must be adopted to effectively control the viral circulation. Some recent studies have investigated the factors involved in viral spreading and persistence, at least at the local level. However, despite the topic's relevance, no statistically grounded evidence is currently available evaluating the variables more involved in porcine reproductive and respiratory syndrome virus (PRRSV) epidemiological success at a broader scale, such as the European scale. In the present study, an extensive phylodynamic and phylogeographic analysis was performed on more than 1000 ORF5 sequences to investigate the history, dynamics and spreading patterns of PRRSV within European borders. Moreover, several potential predictors, representative of swine population features and trade, human population, economy and geographic characteristics, were evaluated through a specifically designed generalized linear model (GLM) to assess their weight on viral migration rate between countries over time. Although pig stock density, mean PRRSV strain genetic diversity, investments in agriculture (including a likely role of vaccination) and farmer education were involved to a certain extent, the major determinant was proven to be by far the live pig trade. Providing a robust depiction of PRRSV European molecular epidemiology patterns and determinants, the present study could contribute to a more rational allocation of limited resources based on an effective prioritization of control measures.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSv): A Cross-Sectional Study on ELISA Seronegative, Multivaccinated Sows

Vaccination against Porcine Reproductive and Respiratory Syndrome virus (PRRSv) is widely used to control clinical disease, but the effectiveness appears in some cases to be suboptimal. Field reports have stated the presence of routinely PRRSv-vaccinated but ELISA seronegative sows: the ELISA non-responders. The real extent of this phenomenon (prevalence–origin–consequences) was not yet investigated. In this study, the prevalence of ELISA non-responders was assessed by measuring PRRSv-specific antibodies in 1400 sows, originating from 70 PRRSv-vaccinating sow herds, using IDEXX ELISA (ELISA 1) and CIVTEST E/S ELISA (ELISA 2). Neutralizing antibodies (NAbs) were quantified in a virus neutralization assay. Univariable logistic regression was used to identify herd risk factors for the presence of ELISA non-responders. The global prevalence of non-responders varied from 3.5% (ELISA 1) to 4.1% (ELISA 2), the herd-level prevalence was 40% and the within-herd prevalence ranged from 5% to 20% (ELISA 1) and from 5% to 30% (ELISA 2). The ELISA non-responders had significantly lower NAbs than the ELISA responders. Herds using the combination of one modified live vaccine and one killed vaccine had a significantly reduced risk of having ELISA non-responders. A first assessment of the prevalence and possible consequences of ELISA non-responders has been provided by this study. The clinical importance, origin and underlying immunological mechanisms warrant further research.

Role of transcription factors in porcine reproductive and respiratory syndrome virus infection: A review

Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease caused by the PRRS virus that leads to reproductive disorders and severe dyspnoea in pigs, which has serious economic impacts. One of the reasons PRRSV cannot be effectively controlled is that it has developed countermeasures against the host immune response, allowing it to survive and replicate for long periods. Transcription Factors acts as a bridge in the interactions between the host and PRRSV. PRRSV can create an environment conducive to PRRSV replication through transcription factors acting on miRNAs, inflammatory factors, and immune cells. Conversely, some transcription factors also inhibit PRRSV proliferation in the host. In this review, we systematically described how PRRSV uses host transcription factors such as SP1, CEBPB, STATs, and AP-1 to escape the host immune system. Determining the role of transcription factors in immune evasion and understanding the pathogenesis of PRRSV will help to develop new treatments for PRRSV.

Characterization of PRRSV in clinical samples and the corresponding cell culture isolates

Isolation of porcine reproductive and respiratory syndrome virus (PRRSV) in cell culture is a primary means of obtaining virus isolates for autogenous vaccine production and other applications. However, it has not been well characterized whether cell culture isolate and the virus in clinical sample are equivalent. This study compared PRRSV ORF5 sequences from 1,024 clinical samples (995 PRRSV-2, 26 PRRSV-1, and 3 PRRSV-1 and PRRSV-2 PCR-positive) and their isolates in MARC-145 and/or ZMAC cells. For 3 PRRSV-1 and PRRSV-2 PCR-positive clinical samples, both PRRSV-1 and PRRSV-2 were isolated in ZMAC cells whereas either PRRSV-1 or PRRSV-2, but not both, was isolated in MARC-145 cells, with isolate sequences matching the respective viruses in clinical samples. Twenty-six PRRSV-1 and most of 995 PRRSV-2 PCR-positive clinical samples had matching viral ORF5 sequences with their cell culture isolates. However, 14 out of 995 PRRSV-2 cases (1.4%) had non-matching viral sequences between clinical samples and MARC-145 isolates although viral sequences from clinical samples and ZMAC isolates matched. This is concerning because, if the MARC-145 isolate is directly used for autogenous vaccine production without sequencing confirmation against the virus in the clinical sample, it is possible that the produced autogenous vaccine does not include the desired wild-type virus strain found on the farm and instead contains vaccine-like virus. Vaccine-specific PCR and next-generation sequencing performed on six selected cases indicated presence of ≥2 PRRSV-2 strains (mixed infection) in such clinical samples. In summary, PRRSV ORF5 sequences from clinical samples and cell culture isolates matched each other for majority of the cases. However, PRRSV sequences between clinical sample and MARC-145 cell culture isolate could occasionally be different when the clinical sample contains ≥2 PRRSV-2 strains. Characterizing PRRSV sequences from clinical samples and cell culture isolates should be conducted before using isolates for producing autogenous vaccines or other applications. This article is protected by copyright. All rights reserved.

PRRSV Infection Induces Gasdermin D-Driven Pyroptosis of Porcine Alveolar Macrophages through NLRP3 Inflammasome Activation

For more than 3 decades, mounting evidence has associated porcine reproductive and respiratory syndrome virus (PRRSV) infection with late-term abortions and stillbirths in sows and respiratory disease in piglets, causing enormous economic losses to the global swine industry. However, to date, the underlying mechanisms of PRRSV-triggered cell death have not been well clarified, especially in the pulmonary inflammatory injury characterized by the massive release of pro-inflammatory factors. Here, we demonstrated that PRRSV infection triggered gasdermin D-mediated host pyroptosis in vitro and in vivo. Mechanistically, PRRSV infection triggered disassembly of the trans-Golgi network (TGN); the dispersed TGN then acted as a scaffold for NLRP3 activation through phosphatidylinositol-4-phosphate. In addition, PRRSV replication-transcription complex (RTC) formation stimulated TGN dispersion and pyroptotic cell death. Furthermore, our results indicated that TMEM41B, an endoplasmic reticulum (ER)-resident host protein, functioned as a crucial host factor in the formation of PRRSV RTC, which is surrounded by the intermediate filament network. Collectively, these findings uncover new insights into clinical features as previously unrecognized mechanisms for PRRSV-induced pathological effects, which may be conducive to providing treatment options for PRRSV-associated diseases and may be conserved during infection by other highly pathogenic viruses. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the pathogens responsible for major economic losses in the global swine industry. Characterizing the detailed process by which PRRSV induces cell death pathways will help us better understand viral pathogenesis and provide implications for therapeutic intervention against PRRSV. Here, we showed that PRRSV infection induces GSDMD-driven host pyroptosis and IL-1β secretion through NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome activation in vitro and in vivo. Furthermore, the molecular mechanisms of PRRSV-induced NLRP3 inflammasome activation and pyroptosis are elucidated here. The dispersed trans-Golgi network (TGN) induced by PRRSV serves as a scaffold for NLRP3 aggregation into multiple puncta via phosphatidylinositol 4-phosphate (PtdIns4P). Moreover, the formation of PRRSV replication-transcription complex is essential for TGN dispersion and host pyroptosis. This research advances our understanding of the PRRSV-mediated inflammatory response and cell death pathways, paving the way for the development of effective treatments for PRRSV diseases.

Effect of Killed PRRSV Vaccine on Gut Microbiota Diversity in Pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important pathogens affecting the global swine industry. Vaccination is still a main strategy for PRRSV control; however, host factors associated with vaccine efficacy remain poorly understood. Growing evidence suggests that mucosa-associated microbiomes may play a role in the responses to vaccination. In this study, we investigated the effects of a killed virus vaccine on the gut microbiome diversity in pigs. Fecal microbial communities were longitudinally assessed in three groups of pigs (vaccinated/challenged with PRRSV, unvaccinated/challenged with PRRSV, and unvaccinated/unchallenged) before and after vaccination and after viral challenge. We observed significant interaction effects between viral challenge and vaccination on both taxonomic richness and community diversity of the gut microbiota. While some specific taxonomic alterations appear to be enhanced in vaccinated/challenged pigs, others appeared to be more consistent with the levels in control animals (unvaccinated/unchallenged), indicating that vaccination incompletely protects against viral impacts on the microbiome. The abundances of several microbial taxa were further determined to be correlated with the level of viral load and the amount of PRRSV reactive CD4+ and CD8+ T-cells. This study highlights the potential roles of gut microbiota in the response of pigs to vaccination, which may pave the road for the development of novel strategies to enhance vaccine efficacy.

Molecular characterization of porcine reproductive and respiratory syndrome virus (PRRSv) identified from slaughtered pigs in northern Uganda

Background

A cross sectional study was conducted to detect and characterize species of porcine reproductive and respiratory syndrome virus (PRRSv) identified from slaughtered pigs in Lira district, northern Uganda. The study was conducted from March to September 2019 in three selected slaughter slabs. Pigs brought for slaughter were randomly sampled. At necropsy, lungs were extracted from the thoracic cavity and examined for pneumonic lesions. Seventy-three (73) pigs with gross lung lesions were sampled, from which one hundred and one (101) tissue samples were taken. A real-time reverse transcriptase PCR (RT-qPCR) was used to characterize PRRSv species.

Results

A total of 20 samples tested positive for PRRSv. The respective prevalence of PRRSv type 1 and type 2 were 24.65% (n = 18) and 2.73% (n = 2) respectively. Of the pigs sampled (n = 73), only two pigs, 2.73% (n = 2) tested positive to both species. The likelihood of PRRSv detection decreased with pig age, but increased with gross pneumonic pathology.

Conclusions

This study demonstrated dual circulation of both species in northern Uganda. The association between PRRSv and lung pathology suggests that it may be an important cause of lung disease in pigs in Uganda and hence loss of production. This calls for further investigations on potential economic impacts of PRRSv on pig productivity. These findings contribute to discussions about the need of surveillance and possible vaccination strategies against PRRSv in Uganda.

A novel amino acid site of N protein could affect the PRRSV-2 replication by regulating the viral RNA transcription

Background

Finding the key amino acid sites that could affect viral biological properties or protein functions has always been a topic of substantial interest in virology. The nucleocapsid (N) protein is one of the principal proteins of the porcine reproductive and respiratory syndrome virus (PRRSV) and plays a vital role in the virus life cycle. The N protein has only 123 or 128 amino acids, some of key amino acid sites which could affect the protein functions or impair the viral biological characteristics have been identified. In this research, our objective was to find out whether there are other novel amino acid sites of the N protein can affect N protein functions or PRRSV-2 replication.

Results

In this study, we found mutated the serine⁷⁸ and serine ⁹⁹of the nucleocapsid (N) protein can reduce the N-induced expression of IL-10 mRNA; Then, by using reverse genetics system, we constructed and rescued the mutant viruses, namely, A78 and A99.The IFA result proved that the mutations did not affect the rescue of the PRRSV-2. However, the results of the multistep growth kinetics and qPCR assays indicated that, compared with the viral replication ability, the titres and gRNA levels of A78 were significantly decreased compared with the wild-type. Further study showed that a single amino acid change from serine to alanine at position 78 of the N protein could abrogates the level of viral genomic and subgenomic RNAs. It means the mutation could significant decrease the viral replication efficiency in vitro.

Conclusions

Our results suggest that the serine⁷⁸ of N protein is a key site which could affect the N protein function and PRRSV replication ability.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03274-9.

A rescued NADC30-like virus by reverse genetic manipulation exhibits moderate virulence and a promising application perspective

NADC30-like porcine reproductive and respiratory syndrome virus (PRRSV), which is highly homologous to the NADC30 strain isolated in the United States. The NADC30-like PRRSV was first reported in 2014 in China, where it spread and gradually caused an epidemic. Currently, growing research has shown that NADC30-like strains have greater propensity to recombine with other PRRSV strains, particularly the PPRSV vaccine virus used clinically, making the prevention and control of PRRSV highly complex. To carry out an in-depth molecular biology and virulence analysis, a full-length infectious clone of the NADC30-like strain was successfully constructed and rescued by reverse genetic manipulation. The rescued virus, rZJqz, was indistinguishable from its parental virus, ZJqz21, based on virological characteristics. Further animal experiments demonstrated that rZJqz retained similar pathogenicity and induced the typical clinical symptoms and viral shedding observed in the ZJqz21 challenge model. Together, these results provide a useful tool for further study of the biological characteristics and pathogenicity of NADC30-like strains. Moreover, these findings also provide a solid foundation for studying the recombination of different PRRSVs and developing new and effective universal vaccines in the future.

Porcine Reproductive and Respiratory Syndrome Virus Adapts Antiviral Innate Immunity via Manipulating MALT1

To fulfill virus replication and persistent infection in hosts, viruses have to find ways to compromise innate immunity, including timely impedance on antiviral RNases and inflammatory responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is a major swine pathogen causing immune suppression. MALT1 is a central immune regulator in both innate and adaptive immunity. In this study, MALT1 was confirmed to be induced rapidly upon PRRSV infection and mediate the degradation of two anti-PRRSV RNases, MCPIP1 and N4BP1, relying on its proteolytic activity, consequently facilitating PRRSV replication. Multiple PRRSV nsps, including nsp11, nsp7β, and nsp4, contributed to MALT1 elicitation. Interestingly, the elevated expression of MALT1 began to decrease once intracellular viral expression reached a high enough level. Higher infection dose brought earlier MALT1 inflection. Further, PRRSV nsp6 mediated significant MALT1 degradation via ubiquitination-proteasome pathway. Downregulation of MALT1 suppressed NF-κB signals, leading to the decrease in proinflammatory cytokine expression. In conclusion, MALT1 expression was manipulated by PRRSV in an elaborate manner to antagonize precisely the antiviral effects of host RNases without excessive and continuous activation of inflammatory responses. These findings throw light on the machinery of PRRSV to build homeostasis in infected immune system for viral settlement.

Pattern of Antibiotic Consumption in Two Italian Production Chains Differing by the Endemic Status for Porcine Reproductive and Respiratory Syndrome

The aim of this case study was to quantify antibiotic (AB) use in Italian weaning (W) and fattening (F) units differentiated for porcine reproductive and respiratory syndrome (PRRS) occurrence. Farms were classified as either PRRS negative (–) or PRRS positive (+) based on the circulation of the virus among the animals. In all the farms, the modified live PRRS virus (PRRSV) vaccine was provided to all the animals. In the PRRS– farms, the level of circulating antibodies was low, and the disease, in its clinical form, did not occur. In the PRRS+ farms, the level of circulating antibodies against the virus was high, and the disease was recurrent. Data regarding AB consumption were collected from 2017 to 2020, and the active compounds (ACs) were expressed as milligrams of AC/total kilogram of body weight (BW) produced. Each AC was classified into one of four categories according to the European Medicines Agency classification of ABs for prudent and responsible use in animals: Avoid, Restrict, Caution, and Prudence. Data regarding the ACs in each category were analyzed using a linear model that included production phase, PRRS status, and their interaction as factors. Performance parameters, average age of the pigs at the end of each phase, daily live weight gain, feed-to-gain ratio, total losses, cost index, and medication costs were significantly influenced by the PRRS chain. The use of class B ABs was not affected by production phase or PRRS status. Conversely, for class C ABs, interaction between the two factors (p = 0.02) was observed; W/PRRS+ and F/PRRS+ showed the greatest AB use for this class (p = 0.003). For class D ABs, the interaction was significant (p = 0.01); class C and D ABs were used more in the weaning (p = 0.07) than in the fattening phase (p = 0.003). For the weaning phase, the use of class C and D ABs was greater in the PRRS+ than in the PRRS– chain (p < 0.01). In conclusion, PRRS status affected the growth of pigs and economic performance. Moreover, PRRS status significantly influenced the use of ABs during all the growing periods with the greatest impact being on the weaning phase.

Epidemiological and Genetic Characteristics of Porcine Reproductive and Respiratory Syndrome Virus in South China Between 2017 and 2021

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a major threat to the swine industry in China and has caused enormous losses every year. To monitor the epidemiological and genetic characteristics of PRRSV in South China, 6,795 clinical samples from diseased pigs were collected between 2017 and 2021, and 1,279 (18.82%) of them were positive for PRRSV by RT-PCR detecting the ORF5 gene. Phylogenetic analysis based on 479 ORF5 sequences revealed that a large proportion of them were highly-pathogenic PRRSVs (409, 85.39%) and PRRSV NADC30-like strains (66, 13.78%). Furthermore, 93.15% of these highly-pathogenic strains were found to be MLV-derived. We next recovered 11 PRRSV isolates from the positive samples and generated the whole genome sequences of them. Bioinformatic analysis showed that seven isolates were MLV-derived. Besides, six isolates were found to be recombinant strains. These eleven isolates contained different types of amino acid mutations in their GP5 and Nsp2 proteins compared to those of the PRRSVs with genome sequences publicly available in GenBank. Taken together, our findings contribute to understanding the prevalent status of PRRSV in South China and provide useful information for PRRS control especially the use of PRRSV MLV vaccines.

Development of a Chimeric Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)-2 Vaccine Candidate Expressing Hypo-Glycosylated Glycoprotein-5 Ectodomain of Korean Lineage-1 Strain

Vaccination is a practical method to provide protection against porcine reproductive and respiratory syndrome virus (PRRSV), but current PRRSV vaccines show limited efficacy against divergent field strains. Lineage 1 PRRSV includes virulent strains such as NADC30 and MN184 and now has become one of the most prevalent viruses in Korea. Accordingly, there is an urgent need to develop a new vaccine for Korean lineage-1 strains. In this study, a vaccine candidate against Korean lineage-1 PRRSV, vCSL1-GP5-N33D, was developed by reverse genetics technology. vCSL1-GP5-N33D was designed as a hypo-glycosylated chimeric virus containing the glycoprotein 5 ectodomain region of the Korean lineage-1 wild-type strain. An inactivated vaccine of vCSL1-GP5-N33D was applied to a PRRS-endemic farm and elicited high serum virus neutralization (SVN) antibody titers. The vaccinated group induced SVN antibody titers of 4.40 (log₂) ± 2.46, which were approximately 2-fold higher than those of the negative control at 8-weeks post-vaccination. Moreover, 60% of pigs in the vaccinated group displayed SVN antibody titers of ≥5 (log₂), while none of the pigs in the negative control exhibited SVN antibody titers of ≥5 (log₂). The overall results of the animal experiment suggest that the vCSL1-GP5-N33D inactivated vaccine is a promising vaccine candidate.

Mapping the Key Residues within the Porcine Reproductive and Respiratory Syndrome Virus nsp1α Replicase Protein Required for Degradation of Swine Leukocyte Antigen Class I Molecules

The nonstructural protein 1α (nsp1α) of the porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to target swine leukocyte antigen class I (SLA-I) for degradation, but the molecular details remain unclear. In this report, we further mapped the critical residues within nsp1α by site-directed mutagenesis. We identified a cluster of residues (i.e., Phe17, Ile81, Phe82, Arg86, Thr88, Gly90, Asn91, Phe94, Arg97, Thr160, and Asn161) necessary for this function. Interestingly, they are all located in a structurally relatively concentrated region. Further analysis by reverse genetics led to the generation of two viable viral mutants, namely, nsp1α-G90A and nsp1α-T160A. Compared to WT, nsp1α-G90A failed to co-localize with either chain of SLA-I within infected cells, whereas nsp1α-T160A exhibited a partial co-localization relationship. Consequently, the mutant nsp1α-G90A exhibited an impaired ability to downregulate SLA-I in infected macrophages as demonstrated by Western blot, indirect immunofluorescence, and flow cytometry analysis. Consistently, the ubiquitination level of SLA-I was significantly reduced in the conditions of both infection and transfection. Together, our results provide further insights into the mechanism underlying PRRSV subversion of host immunity and have important implications in vaccine development.

An Assessment of Diagnostic Assays and Sample Types in the Detection of an Attenuated Genotype 5 African Swine Fever Virus in European Pigs over a 3-Month Period

African swine fever virus causes hemorrhagic disease in swine. Attenuated strains are reported in Africa, Europe, and Asia. Few studies on the diagnostic detection of attenuated ASF viruses are available. Two groups of pigs were inoculated with an attenuated ASFV. Group 2 was also vaccinated with an attenuated porcine reproductive and respiratory syndrome virus vaccine. Commercially available ELISA, as well as extraction and qPCR assays, were used to detect antibodies in serum and oral fluids (OF) and nucleic acid in buccal swabs, tonsillar scrapings, OF, and blood samples collected over 93 days, respectively. After 12 dpi, serum (88.9% to 90.9%) in Group 1 was significantly better for antibody detection than OF (0.7% to 68.4%). Group 1′s overall qPCR detection was highest in blood (48.7%) and OF (44.2%), with the highest detection in blood (85.2%) from 8 to 21 days post inoculation (dpi) and in OF (83.3%) from 1 to 7 dpi. Group 2′s results were not significantly different from Group 1, but detection rates were lower overall. Early detection of attenuated ASFV variants requires active surveillance in apparently healthy animals and is only reliable at the herd level. Likewise, antibody testing will be needed to prove freedom from disease.

The mycotoxin deoxynivalenol (DON) can deteriorate vaccination efficacy against porcine reproductive and respiratory syndrome virus (PRRSV) at subtoxic levels

Background

Feedgrain contamination with mycotoxins, including deoxynivalenol (DON, “vomitoxin”) is relatively frequently encountered. Pigs are particularly sensitive to the toxicity of DON. To assess the interplay between DON and porcine reproductive and respiratory syndrome virus (PRRSV), we performed an experimental DON exposure–PRRSV vaccination–challenge infection trial. Three-week-old piglets were divided into four groups. Groups I, II and III (10 animals/group) were vaccinated with a PRRSV modified live vaccine and 2 weeks later challenged with a heterologous field strain. While group I was not supplemented with DON, animals in groups II and III received DON for 4 weeks prior to challenge infection at levels that can be encountered in pig feed, employing a low-dose or high-dose regime (group II: 40 µg DON/kg body weight per day; group III: 80 µg DON/kg body weight per day, corresponding to approx. 1 or 2 mg DON/kg feed, respectively). Eight animals (group IV; unvaccinated, not DON exposed) served as control animals for the challenge infection.

Results

We assessed clinical signs, virus load in serum and various organs as well as antibody titres in the animals. All vaccinated animals mounted an efficient PRRSV-specific antibody response within 2 weeks, except for 20% of the animals receiving the higher DON dose. Upon virus challenge, the vaccinated animals in group I were protected from clinical signs. Vaccinated DON-exposed animals in group II and III were protected from clinical signs to a lesser extent. Clinical signs in group III receiving the higher dose of DON were as severe as in the (unvaccinated, not DON exposed) control group IV. The animals of group III also displayed lower antibody titres compared with the animals in group I and II.

Conclusions

The experimental vaccination/challenge study therefore revealed that exposure of pigs to DON for a period of 4 weeks deteriorates the efficacy of vaccination against clinical signs of PRRS.

Efficacy of the Synergy Between Live-Attenuated and Inactivated PRRSV Vaccines Against a NADC30-Like Strain of Porcine Reproductive and Respiratory Syndrome Virus in 4-Week Piglets

The NADC30-like strain of porcine reproductive and respiratory syndrome virus (PRRSV) is a novel strain responsible for substantial economic losses to swine production in China. This study evaluated the cross-protective efficacy of the synergy between live-attenuated and inactivated PRRSV vaccines compared with a single vaccination with PRRS modified-live virus (MLV) vaccine against challenge with NADC30-like strain, v2016/ZJ/09-03. A total of 45 PRRSV free pigs were randomly divided into five groups: (1) strict control (SC); (2) positive control (PC); (3) single MLV dose (M1); (4) primed intramuscularly with MLV and boosted with killed vaccine 3 weeks later (MK1); and (5) intramuscular prime MLV boosted subcutaneously with killed vaccine B 3 weeks later (MK2). Serological tests in MK groups revealed no differences in both anti-N and anti-GP protein antibodies compared with M1 group, and failed to provide further protection against clinical signs, virus shedding, and gross lesions. However, the viremic titer, gross lung lesions, and average daily weight gain were significantly improved in the MLV vaccinated groups, suggesting that MLV provides substantial cross-protection against the NADC30-like virus. Thus, as a booster, the killed vaccine confers minimal additional protection in NADC30-like infected piglets.

TRIM26-mediated degradation of nucleocapsid protein limits porcine reproductive and respiratory syndrome virus-2 infection

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRSV, has ranked among the most economically important veterinary infectious diseases globally. Recently, tripartite motif (TRIMs) family members have arisen as novel restriction factors in antiviral immunity. Noteworthy, TRIM26 was reported as a binding partner of IRF3, TBK1, TAB1, and NEMO, yet its role in virus infection remains controversial. Herein, we showed that TRIM26 bound N protein by the C-terminal PRY/SPRY domain. Moreover, ectopic expression of TRIM26 impaired PRRSV replication and induced degradation of N protein. The anti-PRRSV activity was independent of the nuclear localization signal (NLS). Instead, deletion of the RING domain, or the PRY/SPRY portion, abrogated the antiviral function. Finally, siRNA depletion of TRIM26 resulted in enhanced production of viral RNA and virus yield in porcine alveolar macrophages (PAMs) after PRRSV infection. Overexpression of an RNAi-resistant TRIM26 rescue-plasmid led to the acquisition of PRRSV restriction in TRIM26-knockdown cells. Together, these data add TRIM26 as a potential target for drug design against PRRSV.

WGS- versus ORF5-Based Typing of PRRSV: A Belgian Case Study

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of one of the most widespread and economically devastating diseases in the swine industry. Typing circulating PRRSV strains by means of sequencing is crucial for developing adequate control strategies. Most genetic studies only target the highly variable open reading frame (ORF) 5, for which an extensive database is available. In this study, we performed whole-genome sequencing (WGS) on a collection of 124 PRRSV-1 positive serum samples that were collected over a 5-year period (2015–2019) in Belgium. Our results show that (nearly) complete PRRSV genomes can be obtained directly from serum samples with a high success rate. Analysis of the coding regions confirmed the exceptionally high genetic diversity, even among Belgian PRRSV-1 strains. To gain more insight into the added value of WGS, we performed phylogenetic cluster analyses on separate ORF datasets as well as on a single, concatenated dataset (CDS) containing all ORFs. A comparison between the CDS and ORF clustering schemes revealed numerous discrepancies. To explain these differences, we performed a large-scale recombination analysis, which allowed us to identify a large number of potential recombination events that were scattered across the genome. As PRRSV does not contain typical recombination hot-spots, typing PRRSV strains based on a single ORF is not recommended. Although the typing accuracy can be improved by including multiple regions, our results show that the full genetic diversity among PRRSV strains can only be captured by analysing (nearly) complete genomes. Finally, we also identified several vaccine-derived recombinant strains, which once more raises the question of the safety of these vaccines.

Porcine Reproductive and Respiratory Syndrome Virus nsp11 Antagonizes Broad Antiviral Effects of MCPIP1 by Inducing Interleukin-17 Expression

Monocyte chemotactic protein-induced protein 1 (MCPIP1) is an inflammatory regulator in immune response and has broad antiviral effects by targeting viral RNA. Porcine reproductive and respiratory syndrome virus (PRRSV), a major viral pathogen in pigs, causes immune suppression leading to coinfection of swine pathogens, but the mechanisms are not fully clarified. In this study, MCPIP1 expression was found to be significantly upregulated in lungs of PRRSV-infected piglets, as well as in Marc-145 and porcine pulmonary alveolar macrophage (PAM) cells upon PRRSV stimulation. MCPIP1 overexpression significantly inhibited PRRSV replication, while MCPIP1 knockdown increased the virus titer. Various mutations in RNase functional domains of MCPIP1 impaired the inhibitory activity against PRRSV, while those in deubiquitinase domains failed to do so. MCPIP1 expression started to decrease from 60 h after PRRSV infection in PAMs. Meanwhile, infection with higher dose of PRRSV further downregulated MCPIP1, indicating the antagonizing effects from PRRSV against MCPIP1. Moreover, it was confirmed that MCPIP1 expression was downregulated in 3D4 cells with either interleukin-17 (IL-17) or nsp11 overexpression, while IL-17 inhibitor abolished the decrease of MCPIP1 caused by nsp11, indicating nsp11 employs IL-17 induction to inhibit MCPIP1. Furthermore, the PRRSV nsp11 mutant with a deficiency in IL-17 induction showed the recovered expression of MCPIP1 in infected cells, inspiring a strategy for virus attenuation. This is the first report about the role of MCPIP1 against PRRSV and the function of PRRSV nsp11 against innate immunity to facilitate virus replication via IL-17. The study not only illuminates PRRSV infection machinery but also enlightens alternative antiviral strategies, such as vaccine candidates. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) suppresses the innate immunity and leads to coinfection of swine pathogens. Monocyte chemotactic protein-induced protein 1 (MCPIP1) is a broad-spectrum host antiviral protein. Therefore, to further clarify the mechanism of PRRSV against innate immunity, we explored the relationship between MCPIP1 and PRRSV infection. The results showed that MCPIP1 inhibited PRRSV infection in the early stage of virus infection. Importantly, PRRSV nsp11 subsequently employed IL-17 induction to suppress MCPIP1 expression and antagonized anti-PRRSV effects. Furthermore, PRRSV with mutation of nsp11 S74A failed to induce MCPIP1 reduction. These findings confirmed the function of MCPIP1 against PRRSV and revealed that PRRSV nsp11 plays an important role in virus against innate immunity. This study enlightens a new strategy to develop safer attenuated vaccines against PRRSV by nsp11 mutation.