Key Gaps in the Knowledge of the Porcine Respiratory Reproductive Syndrome Virus (PRRSV)

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.

Caffeic acid phenethyl ester: an effective antiviral agent against porcine reproductive and Respiratory Syndrome Virus

Porcine Reproductive and Respiratory Syndrome (PRRS) presents a formidable viral challenge in swine husbandry. Confronting the constraints of existing veterinary pharmaceuticals and vaccines, this investigation centers on Caffeic Acid Phenethyl Ester (CAPE) as a prospective clinical suppressant for the Porcine Reproductive and Respiratory Syndrome Virus (PRRSV). The study adopts an integrated methodology to evaluate CAPE's antiviral attributes. This encompasses a dual-phase analysis of CAPE's interaction with PRRSV, both in vitro and in vivo, and an examination of its influence on viral replication. Varied dosages of CAPE were subjected to empirical testing in animal models to quantify its efficacy in combating PRRSV infections. The findings reveal a pronounced antiviral potency, notably in prophylactic scenarios. As a predominant component of propolis, CAPE stands out as a promising candidate for clinical suppression, showing exceptional effectiveness in pre-exposure prophylaxis regimes. This highlights the potential of CAPE in spearheading cutting-edge strategies for the management of future PRRSV outbreaks.

Research progress on the N protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease caused by the porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV exhibits genetic diversity and complexity in terms of immune responses, posing challenges for eradication. The nucleocapsid (N) protein of PRRSV, an alkaline phosphoprotein, is important for various biological functions. This review summarizes the structural characteristics, genetic evolution, impact on PRRSV replication and virulence, interactions between viral and host proteins, modulation of host immunity, detection techniques targeting the N protein, and progress in vaccine development. The discussion provides a theoretical foundation for understanding the pathogenic mechanisms underlying PRRSV virulence, developing diagnostic techniques, and designing effective vaccines.

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.

PRRSV degrades MDA5 via dual autophagy receptors P62 and CCT2 to evade antiviral innate immunity

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major economically devastating pathogen that has evolved various strategies to evade innate immunity. Downregulation of antiviral interferon largely promotes PRRSV immunoevasion by utilizing cytoplasmic melanoma differentiation-associated gene 5 (MDA5), a receptor that senses viral RNA. In this study, the downregulated transcription and expression levels of porcine MDA5 in PRRSV infection were observed, and the detailed mechanisms were explored. We found that the interaction between P62 and MDA5 is enhanced due to two factors: the phosphorylation modification of the autophagic receptor P62 by the upregulated kinase CK2α and the K63 ubiquitination of porcine MDA5 catalyzed by the E3 ubiquitinase TRIM21 in PRRSV-infected cells. As a result of these modifications, the classic P62-mediated autophagy is triggered. Additionally, porcine MDA5 interacts with the chaperonin containing TCP1 subunit 2 (CCT2), which is enhanced by PRRSV nsp3. This interaction promotes the aggregate formation and autophagic clearance of MDA5-CCT2-nsp3 independently of ubiquitination. In summary, enhanced MDA5 degradation occurs in PRRSV infection via two autophagic pathways: the binding of MDA5 with the autophagy receptor P62 and the aggrephagy receptor CCT2, leading to intense innate immune suppression. The research reveals a novel mechanism of immune evasion in PRRSV infection and provides fundamental insights for the development of new vaccines or therapeutic strategies.

Emerging role of extracellular vesicles in veterinary practice: novel opportunities and potential challenges

Extracellular vesicles are nanoscale vesicles that transport signals between cells, mediating both physiological and pathological processes. EVs facilitate conserved intercellular communication. By transferring bioactive molecules between cells, EVs coordinate systemic responses, regulating homeostasis, immunity, and disease progression. Given their biological importance and involvement in pathogenesis, EVs show promise as biomarkers for veterinary diagnosis, and candidates for vaccine production, and treatment agents. Additionally, different treatment or engineering methods could be used to boost the capability of extracellular vesicles. Despite the emerging veterinary interest, EV research has been predominantly human-based. Critical knowledge gaps remain regarding isolation protocols, cargo loading mechanisms, in vivo biodistribution, and species-specific functions. Standardized methods for veterinary EV characterization and validation are lacking. Regulatory uncertainties impede veterinary clinical translation. Advances in fundamental EV biology and technology are needed to propel the veterinary field forward. This review introduces EVs from a veterinary perspective by introducing the latest studies, highlighting their potential while analyzing challenges to motivate expanded veterinary investigation and translation.

Effects of increasing omega-3 fatty acids on growth performance, immune response, and mortality in nursery pigs

Three experiments evaluated omega-3 fatty acids, provided by O3 trial feed, on nursery pig growth performance, mortality, and response to an LPS immune challenge or natural Porcine reproductive and respiratory virus (PRRSV) outbreak. In experiment 1, 350 pigs (241 × 600, DNA; initially 5.8 kg) were used. Pens of pigs were randomly assigned to one of the five dietary treatments containing increasing omega-3 fatty acids (0%, 1%, 2%, 3%, and 4% O3 trial feed) with 14 replications per treatment. On day 25, two pigs per pen were injected intramuscularly with 20 μg Escherichia coli LPS per kg BW and one pig per pen was injected with saline as a control. Body temperature was taken from all three pigs prior to and 2, 4, 6, and 12 h post-LPS challenge. Serum IL-1β and TNF-α concentrations were determined in LPS-challenged pigs 24 h prior and 4 h post-LPS challenge. There was no interaction between treatment and time for change in body temperature (P > 0.10). Overall, increasing the O3 trial feed did not affect (P > 0.10) ADG, ADFI, G:F, IL-1β, or TNF-α. In experiment 2, 1,056 pigs (PIC TR4 × [Fast LW × PIC L02] initially 7.3 kg) were used. Pens of pigs were randomly assigned to one of the four dietary treatments containing increasing omega-3 fatty acids (0%, 0.75%, 1.5%, and 3% O3 trial feed) with 12 replications per treatment. Oral fluids tested negative on days 7 and 14, but then positive for North American PRRSV virus via PCR on days 21, 28, 35, and 42. Overall, increasing O3 trial feed increased (linear, P < 0.001) ADG, ADFI, and G:F and decreased (linear, P = 0.027) total removals and mortality. In experiment 3, 91,140 pigs (DNA 600 × PIC 1050; initially 5.1 kg), originating from PRRSV-positive sow farms, were used across eight nursery sites. Each site contained five barns with two rooms per barn and ~1,100 pigs per room. Rooms of pigs were blocked by nursery site and allocated within sow source to one of the two dietary treatments (control or 3% O3 trial feed) with 40 replications per treatment. Oral fluids from 61 of the 80 rooms tested positive for North American PRRSV virus 1 wk postweaning and 78 of the 80 rooms tested positive 3 wk after weaning. Overall, O3 trial feed did not affect ADG, ADFI, or G:F but increased (P < 0.001) total removals and mortalities. In summary, increasing omega-3 fatty acids, sourced by O3 trial feed, did not improve growth performance or immune response in healthy pigs given an LPS challenge. However, it appears that if omega-3 fatty acids are fed prior to a natural PRRSV break (as in experiment 2), growth performance may be improved, and mortality reduced.

Using a novel structure/function approach to select diverse swine major histocompatibility complex 1 alleles to predict epitopes for vaccine development

MOTIVATION

Swine leukocyte antigens (SLAs) (i.e. swine major histocompatibility complex proteins) conduct a fundamental role in swine immunity. To generate a protective vaccine across an outbred species, such as pigs, it is critical that epitopes that bind to diverse SLA alleles are used in the vaccine development process. We introduced a new strategy for epitope prediction.

RESULTS

We employed molecular dynamics simulation to identify key amino acids for interactions with epitopes. We developed an algorithm wherein each SLA-1 is compared to a crystalized reference allele with unique weighting for non-conserved amino acids based on R group and position. We then performed homology modeling and electrostatic contact mapping to visualize how relatively small changes in sequences impacted the charge distribution in the binding site. We selected eight diverse SLA-1 alleles and performed homology modeling followed, by protein-peptide docking and binding affinity analyses, to identify porcine reproductive and respiratory syndrome virus matrix protein epitopes that bind with high affinity to these alleles. We also performed docking analysis on the epitopes identified as strong binders using NetMHCpan 4.1. Epitopes predicted to bind to our eight SLA-1 alleles had equivalent or higher energetic interactions than those predicted to bind to the NetMHCpan 4.1 allele repertoire. This approach of selecting diverse SLA-1 alleles, followed by homology modeling, and docking simulations, can be used as a novel strategy for epitope prediction that complements other available tools and is especially useful when available tools do not offer a prediction for SLAs/major histocompatibility complex.

AVAILABILITY AND IMPLEMENTATION

The data underlying this article are available in the online Supplementary Material.

miR-204 suppresses porcine reproductive and respiratory syndrome virus (PRRSV) replication via inhibiting LC3B-mediated autophagy

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) has been regarded as a persistent challenge for the swine farms worldwide. microRNAs (miRNAs) play key roles in regulating almost every important biological process, including virus-host interaction. In this study, we found that miR-204 was highly expressed in cells that were not permissive to PRRSV infection compared with cells susceptible to PRRSV infection. Subsequently, we demonstrated that overexpression of miR-204 significantly inhibited PRRSV replication in porcine alveolar macrophages (PAMs). Through bioinformatic analysis, we found that there existed a potential binding site of miR-204 on the 3'UTR of microtubule associated protein 1 light chain 3B (MAP1LC3B, LC3B), a hallmark of autophagy. Applying experiments including luciferase reporter assay and UV cross-linking and immunoprecipitation (CLIP) assay, we demonstrated that miR-204 directly targeted LC3B, thereby downregulating autophagy. Meanwhile, we investigated the interplay between autophagy and PRRSV replication in PAMs, confirming that PRRSV infection induces autophagy, which in turn facilitates viral replication. Overall, we verify that miR-204 suppresses PRRSV replication via inhibiting LC3B-mediated autophagy in PAMs. These findings will provide a novel potential approach for us to develop antiviral therapeutic agents and controlling measures for future PRRSV outbreaks.

Complex interplay between PRRSV-1 genetic diversity, coinfections and antimicrobial use influences performance parameters in post-weaning pigs

Porcine reproductive and respiratory syndrome (PRRS) is one of the main diseases of pigs, leading to large economic losses in swine production worldwide. PRRSV high mutation rate and low cross-protection between strains make PRRS control challenging. Through a semi-longitudinal approach, we analysed the relationships among performance parameters, PRRSV-1 genetic diversity, coinfections and antimicrobial use (AMU) in pig nurseries. We collected data over the course of five years in five PRRS-positive nurseries belonging to an Italian multisite operation, for a total of 86 batches and over 200,000 weaners involved. The farm experienced a severe PRRS outbreak in the farrowing unit at the onset of the study, but despite adopting vaccination of all sows, batch-level losses in nurseries in the following years remained constantly high (mean±SE: 11.3 ± 0.5 %). Consistently with previous studies, our phylogenetic analysis of ORF 7 sequences highlighted the peculiarity of strains circulating in Italy. Greater genetic distances between the strain circulating in a weaners' batch and strains from the farrowing unit and the previous batch were associated with increased mortality (p < 0.0001). All the respiratory and enteric coinfections contributed to an increase in losses (all p < 0.026), with secondary infections by Streptococcus suis and enteric bacteria also inducing an increase in AMU (both p < 0.041). Our findings highlight that relying solely on sows' vaccination is insufficient to contain PRRS losses, and the implementation of rigorous biosecurity measures is pivotal to limit PRRSV circulation among pig flows and consequently minimise the risk of exposure to genetically diverse strains that would increase production costs.

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.

A novel chimeric vaccine candidate for porcine reproductive and respiratory syndrome virus (PRRSV) I and II elicits neutralizing antibodies against both types

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important virus within the swine industry. The virus causes respiratory disease and reproductive failure. Two species of PRRSV-I and II are co-dominant, yet no effective vaccination strategy has been developed to protect against these two types. With an aim to develop a chimeric vaccine strain to protect against both types, in this study, a chimeric porcine reproductive and respiratory syndrome virus (PRRSV) type I and II was rescued using reverse genetics for the first time. Four chimeric infectious clones were designed based on the genomic arrangement of the structural proteins. However, only the clone carrying the transcriptional regulatory sequence (TRS) and ORF6 of a PRRSV-I and ORF6 of a PRRSV-II generated a viable recombinant virus, suggesting that concurrent expression of ORF6 from both parental viruses is essential for the recovery of type I and II chimeric PRRSV. The chimeric virus showed significantly lower replication ability than its parental strains in vitro, which was improved by serial passaging. In vivo, groups of pigs were inoculated with either the chimeric virus, one of the parental strains, or PBS. The chimeric virus replicated in pig tissue and was detected in serum 7 days post-inoculation. Serum neutralization tests indicated that pigs inoculated with the chimeric virus elicited neutralizing antibodies that inhibited infection with strains of both species and with greater coverage than the parental viruses. In conclusion, the application of this technique to construct a chimeric PRRSV holds promise for the development of a highly effective modified live vaccine candidate. This is particularly significant since there are currently no approved commercial divalent vaccines available to combat PRRSV-I and II co-infections.

Research Progress on NSP11 of Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome (PRRS) is a virulent infectious disease caused by the PRRS virus (PRRSV). The non-structural protein 11 (NSP11) of PRRSV is a nidovirus-specific endonuclease (NendoU), which displays uridine specificity and catalytic functions conserved throughout the entire NendoU family and exerts a wide range of biological effects. This review discusses the genetic evolution of NSP11, its effects on PRRSV replication and virulence, its interaction with other PRRSV and host proteins, its regulation of host immunity, the conserved characteristics of its enzyme activity (NendoU), and its diagnosis, providing an essential theoretical basis for in-depth studies of PRRSV pathogenesis and vaccine design.

Progress in PRRSV Infection and Adaptive Immune Response Mechanisms

Since its discovery, Porcine reproductive and respiratory syndrome (PRRS) has had a huge impact on the farming industry. The virus that causes PRRS is Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), and because of its genetic diversity and the complexity of the immune response, the eradication of PRRS has been a challenge. To provide scientific references for PRRSV control and vaccine development, this study describes the processes of PRRSV-induced infection and escape, as well as the host adaptive immune response to PRRSV. It also discusses the relationship between PRRSV and the adaptive immune response.

Bergamottin Inhibits PRRSV Replication by Blocking Viral Non-Structural Proteins Expression and Viral RNA Synthesis

The porcine reproductive and respiratory syndrome virus (PRRSV) causes economic losses in the swine industry worldwide. However, current vaccines cannot provide effective protection against PRRSV, and PRRSV-specific treatments for infected herds are still unavailable. In this study, we found that bergamottin showed strong inhibitory effects against PRRSV replication. Bergamottin inhibited PRRSV at the stage of the replication cycle. Mechanically, bergamottin promoted the activation of IRF3 and NF-κB signaling, leading to the increased expression of proinflammatory cytokines and interferon, which inhibited viral replication to some extent. In addition, bergamottion could reduce the expression of the non-structural proteins (Nsps), leading to the interruption of replication and transcription complex (RTC) formation and viral dsRNA synthesis, ultimately restraining PRRSV replication. Our study identified that bergamottin possesses potential value as an antiviral agent against PRRSV in vitro.

Simultaneous co-infection with swine influenza A and porcine reproductive and respiratory syndrome viruses potentiates adaptive immune responses

Porcine respiratory disease is multifactorial and most commonly involves pathogen co-infections. Major contributors include swine influenza A (swIAV) and porcine reproductive and respiratory syndrome (PRRSV) viruses. Experimental co-infection studies with these two viruses have shown that clinical outcomes can be exacerbated, but how innate and adaptive immune responses contribute to pathogenesis and pathogen control has not been thoroughly evaluated. We investigated immune responses following experimental simultaneous co-infection of pigs with swIAV H3N2 and PRRSV-2. Our results indicated that clinical disease was not significantly exacerbated, and swIAV H3N2 viral load was reduced in the lung of the co-infected animals. PRRSV-2/swIAV H3N2 co-infection did not impair the development of virus-specific adaptive immune responses. swIAV H3N2-specific IgG serum titers and PRRSV-2-specific CD8β⁺ T-cell responses in blood were enhanced. Higher proportions of polyfunctional CD8β⁺ T-cell subset in both blood and lung washes were found in PRRSV-2/swIAV H3N2 co-infected animals compared to the single-infected groups. Our findings provide evidence that systemic and local host immune responses are not negatively affected by simultaneous swIAV H3N2/PRRSV-2 co-infection, raising questions as to the mechanisms involved in disease modulation.

Sanguinarine Exhibits Antiviral Activity against Porcine Reproductive and Respiratory Syndrome Virus via Multisite Inhibition Mechanisms

Porcine reproductive and respiratory syndrome virus (PRRSV), the etiological agent of PRRS, is prevalent worldwide, causing substantial and immense economic losses to the global swine industry. While current commercial vaccines fail to efficiently control PRRS, the development of safe and effective antiviral drugs against PRRSV is urgently required. Alkaloids are natural products with wide pharmacological and biological activities. Herein, sanguinarine, a benzophenanthridine alkaloid that occurs in many plants such as Macleaya cordata, was demonstrated as a potent antagonist of PRRSV. Sanguinarine attenuated PRRSV proliferation by targeting the internalization, replication, and release stages of the viral life cycle. Furthermore, ALB, AR, MAPK8, MAPK14, IGF1, GSK3B, PTGS2, and NOS2 were found as potential key targets related to the anti-PRRSV effect of sanguinarine as revealed by network pharmacology and molecular docking. Significantly, we demonstrated that the combination of sanguinarine with chelerythrine, another key bioactive alkaloid derived from Macleaya cordata, improved the antiviral activity. In summary, our findings reveal the promising potential of sanguinarine as a novel candidate for the development of anti-PRRSV agents.

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.

Development of a Multiplex Crystal Digital RT-PCR for Differential Detection of Classical, Highly Pathogenic, and NADC30-like Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) type 1 (European genotype) and PRRSV type 2 (North American genotype) are prevalent all over the world. Nowadays, the North American genotype PRRSV (NA-PRRSV) has been widely circulating in China and has caused huge economic losses to the pig industry. In recent years, classical PRRSV (C-PRRSV), highly pathogenic PRRSV (HP-PRRSV), and NADC30-like PRRSV (NL-PRRSV) have been the most common circulating strains in China. In order to accurately differentiate the circulating strains of NA-PRRSV, three pairs of specific primers and corresponding probes were designed for the Nsp2 region of C-PRRSV, HP-PRRSV, and NL-PRRSV. After optimizing the annealing temperature, primer concentration, and probe concentration, a multiplex real-time quantitative RT-PCR (qRT-PCR) and a multiplex Crystal digital RT-PCR (cdRT-PCR) for the differential detection of C-PRRSV, HP-PRRSV, and NL-PRRSV were developed. The results showed that the two assays illustrated high sensitivity, with a limit of detection (LOD) of 3.20 × 10⁰ copies/μL for the multiplex qRT-PCR and 3.20 × 10^-1 copies/μL for the multiplex cdRT-PCR. Both assays specifically detected the targeted viruses, without cross-reaction with other swine viruses, and indicated excellent repeatability, with coefficients of variation (CVs) of less than 1.26% for the multiplex qRT-PCR and 2.68% for the multiplex cdRT-PCR. Then, a total of 320 clinical samples were used to evaluate the application of these assays, and the positive rates of C-PRRSV, HP-PRRSV, and NL-PRRSV by the multiplex qRT-PCR were 1.88%, 21.56%, and 9.69%, respectively, while the positive rates by the multiplex cdRT-PCR were 2.19%, 25.31%, and 11.56%, respectively. The high sensitivity, strong specificity, excellent repeatability, and reliability of these assays indicate that they could provide useful tools for the simultaneous and differential detection of the circulating strains of C-PRRSV, HP-PRRSV, and NL-PRRSV in the field.

LGP2 Promotes Type I Interferon Production To Inhibit PRRSV Infection via Enhancing MDA5-Mediated Signaling

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in the global pig industry, which modulates the host's innate antiviral immunity to achieve immune evasion. RIG-I-like receptors (RLRs) sense viral RNA and activate the interferon signaling pathway. LGP2, a member of the RLR family, plays an important role in regulating innate immunity. However, the role of LGP2 in virus infection is controversial. Whether LGP2 has a role during infection with PRRSV remains unclear. Here, we found that LGP2 overexpression restrained the replication of PRRSV, while LGP2 silencing facilitated PRRSV replication. LGP2 was prone to interact with MDA5 and enhanced viral RNA enrichment and recognition by MDA5, thus promoting the activation of RIG-I/IRF3 and NF-κB signaling pathways and reinforcing the expression of proinflammatory cytokines and type I interferon during PRRSV infection. Meanwhile, there was a decreased protein expression of LGP2 upon PRRSV infection in vitro. PRRSV Nsp1 and Nsp2 interacted with LGP2 and promoted K63-linked ubiquitination of LGP2, ultimately leading to the degradation of LGP2. These novel findings indicate that LGP2 plays a role in regulating PRRSV replication through synergistic interaction with MDA5. Moreover, targeting LGP2 is responsible for PRRSV immune evasion. Our work describes a novel mechanism of virus-host interaction and provides the basis for preventing and controlling PRRSV. IMPORTANCE LGP2, a member of retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), shows higher-affinity binding to RNA and work synergism with RIG-I or MDA5. However, LGP2 has divergent responses to different viruses, which remains controversial in antiviral immune responses. Here, we present the detailed process of LGP2 in positively regulating the anti-PRRSV response. Upon PRRSV infection, LGP2 was prone to bind to MDA5 and enhanced MDA5 signaling, manifesting the enrichment of viral RNA on MDA5 and the activation of downstream IRF3 and NF-κB, which results in increased proinflammatory cytokines and type I interferon expression, ultimately inhibiting PRRSV at the early stage of infection. Moreover, PRRSV Nsp1 and Nsp2 interacted with LGP2 via ubiquitin-proteasome pathways, thus blocking LGP2-mediated immune response. This research helps us understand the host recognition and innate antiviral response to PRRSV infection by neglected pattern recognition receptors, which sheds light on the detailed mechanism of virus-host interaction.

Identificationof a novel linear epitope on the porcine reproductive and respiratory syndrome virus nucleocapsid protein, as recognized by a specific monoclonal antibody

Introduction

Porcine reproductive and respiratory syndrome virus (PRRSV) remains one of the most threatening pathogens of swine. The nucleocapsid (N) protein is the major structural protein of the virus and has been used as a PRRSV diagnostic antigen due to its high level of inherent immunogenicity.

Methods

The recombinant PRRSV N protein was generated by the prokaryotic expressing system and used to immunized mice. Monoclonal antibodies against PRRSV were produced and validated by western blot analysis and indirect immunofluorescence analysis. In this study, the linear epitope of a specific monoclonal antibody mAb (N06) was subsequently identified by enzyme-linked immunosorbent assays (ELISA) using the synthesized overlapping peptides as antigens.

Results

According to the results of western blot analysis and indirect immunofluorescence analysis, mAb (N06) was capable of recognizing the native form as well as the denatured form of PRRSV N protein. The results of ELISA showed that mAb N06 recognized the epitope NRKKNPEKPHFPLATE, which was consistent with BCPREDS predictions of antigenicity.

Conclusion

All the data suggested that the mAb (N06) can be used as diagnostic reagents for PRRSV detection, while the recognized linear epitope can be useful in epitope-based vaccines development, which is helpful for the control of local PRRSV infections in swine.

Proteomic Comparison of Three Wild-Type Pseudorabies Virus Strains and the Attenuated Bartha Strain Reveals Reduced Incorporation of Several Tegument Proteins in Bartha Virions

Pseudorabies virus (PRV) is a member of the alphaherpesvirus subfamily and the causative agent of Aujeszky's disease in pigs. Driven by the large economic losses associated with PRV infection, several vaccines and vaccine programs have been developed. To this day, the attenuated Bartha strain, generated by serial passaging, represents the golden standard for PRV vaccination. However, a proteomic comparison of the Bartha virion to wild-type (WT) PRV virions is lacking. Here, we present a comprehensive mass spectrometry-based proteome comparison of the attenuated Bartha strain and three commonly used WT PRV strains: Becker, Kaplan, and NIA3. We report the detection of 40 structural and 14 presumed nonstructural proteins through a combination of data-dependent and data-independent acquisition. Interstrain comparisons revealed that packaging of the capsid and most envelope proteins is largely comparable in-between all four strains, except for the envelope protein pUL56, which is less abundant in Bartha virions. However, distinct differences were noted for several tegument proteins. Most strikingly, we noted a severely reduced incorporation of the tegument proteins IE180, VP11/12, pUS3, VP22, pUL41, pUS1, and pUL40 in Bartha virions. Moreover, and likely as a consequence, we also observed that Bartha virions are on average smaller and more icosahedral compared to WT virions. Finally, we detected at least 28 host proteins that were previously described in PRV virions and noticed considerable strain-specific differences with regard to host proteins, arguing that the potential role of packaged host proteins in PRV replication and spread should be further explored. IMPORTANCE The pseudorabies virus (PRV) vaccine strain Bartha-an attenuated strain created by serial passaging-represents an exceptional success story in alphaherpesvirus vaccination. Here, we used mass spectrometry to analyze the Bartha virion composition in comparison to three established WT PRV strains. Many viral tegument proteins that are considered nonessential for viral morphogenesis were drastically less abundant in Bartha virions compared to WT virions. Interestingly, many of the proteins that are less incorporated in Bartha participate in immune evasion strategies of alphaherpesviruses. In addition, we observed a reduced size and more icosahedral morphology of the Bartha virions compared to WT PRV. Given that the Bartha vaccine strain elicits potent immune responses, our findings here suggest that differences in protein packaging may contribute to its immunogenicity. Further exploration of these observations could aid the development of efficacious vaccines against other alphaherpesvirus vaccines such as HSV-1/2 or EHV-1.

Potential Novel N-Glycosylation Patterns Associated with the Emergence of New Genetic Variants of PRRSV-2 in the U.S

Glycosylation of proteins is a post-translational process where oligosaccharides are attached to proteins, potentially altering their folding, epitope availability, and immune recognition. In Porcine reproductive and respiratory syndrome virus-type 2 (PRRSV-2), positive selection pressure acts on amino acid sites potentially associated with immune escape through glycan shielding. Here, we describe the patterns of potential N-glycosylation sites over time and across different phylogenetic lineages of PRRSV-2 to better understand how these may contribute to patterns of coexistence and emergence of different lineages. We screened 19,179 PRRSV GP5 sequences (2004−2021) in silico for potential N-glycosylated sites. The emergence of novel combinations of N-glycosylated sites coincided with past PRRSV epidemics in the U.S. For lineage L1A, glycosylation at residues 32, 33, 44, 51, and 57 first appeared in 2012, but represented >62% of all L1A sequences by 2015, coinciding with the emergence of the L1A 1-7-4 strain that increased in prevalence from 8 to 86% of all L1A sequences from 2012 to 2015. The L1C 1-4-4 strain that emerged in 2020 also had a distinct N-glycosylation pattern (residues 32, 33, 44, and 51). From 2020 to 2021, this pattern was responsible for 44−47% of the L1C sequences, contrasting to <5% in years prior. Our findings support the hypothesis that antigenic evolution contributes to the sequential dominance of different PRRSV strains and that N-glycosylation patterns may partially account for antigenic differences amongst strains. Further studies on glycosylation and its effect on PRRSV GP5 folding are needed to further understand how glycosylation patterns shape PRRSV occurrence.

Porcine respiratory disease complex: Dynamics of polymicrobial infections and management strategies after the introduction of the African swine fever

A few decades ago, porcine respiratory disease complex (PRDC) exerted a major economic impact on the global swine industry, particularly due to the adoption of intensive farming by the latter during the 1980's. Since then, the emerging of porcine reproductive and respiratory syndrome virus (PRRSV) and of porcine circovirus type 2 (PCV2) as major immunosuppressive viruses led to an interaction with other endemic pathogens (e.g., Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Streptococcus suis, etc.) in swine farms, thereby exacerbating the endemic clinical diseases. We herein, review and discuss various dynamic polymicrobial infections among selected swine pathogens. Traditional biosecurity management strategies through multisite production, parity segregation, batch production, the adoption of all-in all-out production systems, specific vaccination and medication protocols for the prevention and control (or even eradication) of swine diseases are also recommended. After the introduction of the African swine fever (ASF), particularly in Asian countries, new normal management strategies minimizing pig contact by employing automatic feeding systems, artificial intelligence, and robotic farming and reducing the numbers of vaccines are suggested. Re-emergence of existing swine pathogens such as PRRSV or PCV2, or elimination of some pathogens may occur after the ASF-induced depopulation. ASF-associated repopulating strategies are, therefore, essential for the establishment of food security. The "repopulate swine farm" policy and the strict biosecurity management (without the use of ASF vaccines) are, herein, discussed for the sustainable management of small-to-medium pig farms, as these happen to be the most potential sources of an ASF re-occurrence. Finally, the ASF disruption has caused the swine industry to rapidly transform itself. Artificial intelligence and smart farming have gained tremendous attention as promising tools capable of resolving challenges in intensive swine farming and enhancing the farms' productivity and efficiency without compromising the strict biosecurity required during the ongoing ASF era.

Influence of PRRSV-1 vaccination and infection on mononuclear immune cells at the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most devastating viruses for the global swine industry. Infection during late gestation causes reproductive failure but the local immune response in utero remains poorly understood. In this study, an experimental PRRSV-infection model with two different PRRSV-1 field isolates was used to investigate the immune cell phenotypes at the maternal-fetal interface during late gestation. In addition, phenotypic changes induced by a modified live virus (MLV, ReproCyc® PRRS EU) vaccine were studied. Vaccinated (n = 12) and non-vaccinated pregnant gilts (n = 12) were challenged with either one of the PRRSV-1 field isolates (low vs. high virulent, LV or HV) or sham-inoculated at day 84 of gestation. Twenty-one days post infection all gilts were euthanized and the fetal preservation status for all fetuses per litter was assessed. Leukocytes from the maternal-fetal interface were isolated and PRRSV-induced changes were investigated using ex vivo phenotyping by flow cytometry. PRRSV load in tissue from the maternal endometrium (ME) and fetal placenta (FP) was determined by RT-qPCR. In the ME, a vast increase in CD8β T cells with CD8αposCD27dim early effector phenotype was found for fetuses from the non-vaccinated LV and HV-challenged gilts, compared to non-treated and vaccinated-only controls. HV-challenged fetuses also showed significant increases of lymphocytes with effector phenotypes in the FP, including NKp46pos NK cells, CD8αhigh γδ T cells, as well as CD8αposCD27pos/dim CD4 and CD8 T cells. In vaccinated animals, this common activation of effector phenotypes was more confined and the fetal preservation status significantly improved. Furthermore, a negative correlation between the viral load and CD163highCD169pos mononuclear phagocytic cells was observed in the FP of HV-infected animals. These results suggest that the strong expansion of effector lymphocytes in gilts that were only infected causes immune-pathogenesis rather than protection. In contrast, the attenuated MLV seems to dampen this effect, yet presumably induces memory cells that limit reproductive failure. This work provides valuable insights into changes of local immune cell phenotypes following PRRSV vaccination and infection.

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.

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.

Metagenomic Analysis of RNA Fraction Reveals the Diversity of Swine Oral Virome on South African Backyard Swine Farms in the uMgungundlovu District of KwaZulu-Natal Province

Numerous RNA viruses have been reported in backyard swine populations in various countries. In the absence of active disease surveillance, a persistent knowledge gap exists on the diversity of RNA viruses in South African backyard swine populations. This is the first study investigating the diversity of oral RNA virome of the backyard swine in South Africa. We used three samples of backyard swine oral secretion (saliva) collected from three distantly located backyard swine farms (BSFs) in the uMgungundlovu District, KwaZulu-Natal, South Africa. Total viral RNA was extracted and used for the library preparation for deep sequencing using the Illumina HiSeq X instrument. The FASTQ files containing paired-end reads were analyzed using Genome Detective v 1.135. The assembled nucleotide sequences were analyzed using the PhyML phylogenetic tree. The genome sequence analysis identified a high diversity of swine enteric viruses in the saliva samples obtained from BSF2 and BSF3, while only a few viruses were identified in the saliva obtained from BSF1. The swine enteric viruses belonged to various animal virus families; however, two fungal viruses, four plant viruses, and five unclassified RNA viruses were also identified. Specifically, viruses of the family Astroviridae, according to the number of reads, were the most prevalent. Of note, the genome sequences of Rotavirus A (RVA) and Rotavirus C (RVC) at BSF2 and RVC and Hepatitis E virus (HEV) at BSF3 were also obtained. The occurrence of various swine enteric viruses in swine saliva suggests a high risk of diarrhoeic diseases in the backyard swine. Of note, zoonotic viruses in swine saliva, such as RVA, RVC, and HEV, indicate a risk of zoonotic spillover to the exposed human populations. We recommend the implementation of biosecurity to ensure sustainable backyard swine farming while safeguarding public health.

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.

Isolation and immortalization of macrophages derived from fetal porcine small intestine and their susceptibility to porcine viral pathogen infections

Macrophages are a heterogeneous population of cells that are present in all vertebrate tissues. They play a key role in the innate immune system, and thus, in vitro cultures of macrophages provide a valuable model for exploring their tissue-specific functions and interactions with pathogens. Porcine macrophage cultures are often used for the identification and characterization of porcine viral pathogens. Recently, we have developed a simple and efficient method for isolating primary macrophages from the kidneys and livers of swine. Here, we applied this protocol to fetal porcine intestinal tissues and demonstrated that porcine intestinal macrophages (PIM) can be isolated from mixed primary cultures of porcine small intestine-derived cells. Since the proliferative capacity of primary PIM is limited, we attempted to immortalize them by transferring the SV40 large T antigen and porcine telomerase reverse transcriptase genes using lentiviral vectors. Consequently, immortalized PIM (IPIM) were successfully generated and confirmed to retain various features of primary PIM. We further revealed that IPIM are susceptible to infection by the African swine fever virus and the porcine reproductive and respiratory syndrome virus and support their replication. These findings suggest that the IPIM cell line is a useful tool for developing in vitro models that mimic the intestinal mucosal microenvironments of swine, and for studying the interactions between porcine pathogens and host immune cells.

A Multiplex Crystal Digital PCR for Detection of African Swine Fever Virus, Classical Swine Fever Virus, and Porcine Reproductive and Respiratory Syndrome Virus

African swine fever (ASF), classical swine fever (CSF), and porcine reproductive and respiratory syndrome (PRRS) are highly infectious diseases of domestic pigs and wild boars. The co-infections of ASF virus (ASFV), CSF virus (CSFV), and PRRS virus (PRRSV) have been reported in different pig farms. Early differential detection and diagnosis of ASFV, CSFV, and PRRSV in the clinical samples is very important for the effective prevention and control of these diseases. A multiplex crystal digital PCR (dPCR) was developed for differential detection of ASFV, CSFV, and PRRSV in this study, targeting p72, 5' untranslated region (UTR), and ORF7 genes, respectively. The different reaction conditions were optimized, and the specificity, sensitivity, and repeatability of the assay were evaluated. The results showed that the multiplex crystal dPCR was able to accurately and differentially detect ASFV, CSFV, and PRRSV with a limit of detection of 4.69 × 10⁻¹ copies/μl, respectively, and could not detect other porcine viruses, i.e., foot-and-mouth disease virus (FMDV), Senecavirus A (SVA), atypical porcine pestivirus (APPV), pseudorabies virus (PRV), porcine circovirus type 2 (PCV2), and porcine parvovirus (PPV). The assay showed excellent repeatability and reproducibility, with coefficients of variation (CV) of the intra- and inter-assay from 0.09 to 1.40%, and from 0.64 to 2.26%, respectively. The 289 clinical samples from different pig herds in Guangxi province, China, were tested by the multiplex crystal dPCR and a reference multiplex real-time quantitative RT-PCR (qRT-PCR) established previously in our laboratory. The positive rates of ASFV, CSFV, and PRRSV were 30.10, 13.49, and 22.49% by the multiplex crystal dPCR, and 24.57, 8.65, and 18.34% by the multiplex qRT-PCR, with coincidence rates of 94.66, 95.16, and 95.84%, respectively. The results indicated that the established multiplex crystal dPCR was a specific, sensitive, and accurate method for the detection and quantification of ASFV, CSFV, and PRRSV. This is the first report on the multiplex dPCR for detecting ASFV, CSFV, and PRRSV.

Inducible miR-150 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication by Targeting Viral Genome and Suppressor of Cytokine Signaling 1

Hosts exploit various approaches to defend against porcine reproductive and respiratory syndrome virus (PRRSV) infection. microRNAs (miRNAs) have emerged as key negative post-transcriptional regulators of gene expression and have been reported to play important roles in regulating virus infection. Here, we identified that miR-150 was differentially expressed in virus permissive and non-permissive cells. Subsequently, we demonstrated that PRRSV induced the expression of miR-150 via activating the protein kinase C (PKC)/c-Jun amino-terminal kinases (JNK)/c-Jun pathway, and overexpression of miR-150 suppressed PRRSV replication. Further analysis revealed that miR-150 not only directly targeted the PRRSV genome, but also facilitated type I IFN signaling. RNA immunoprecipitation assay demonstrated that miR-150 targeted the suppressor of cytokine signaling 1 (SOCS1), which is a negative regulator of Janus activated kinase (JAK)/signal transducer and activator of the transcription (STAT) signaling pathway. The inverse correlation between miR-150 and SOCS1 expression implies that miR-150 plays a role in regulating ISG expression. In conclusion, miR-150 expression is upregulated upon PRRSV infection. miR-150 feedback positively targets the PRRSV genome and promotes type I IFN signaling, which can be seen as a host defensive strategy.

Molecular Evolution of Porcine Reproductive and Respiratory Syndrome Virus Field Strains from Two Swine Production Systems in the Midwestern United States from 2001 to 2020

Porcine reproductive and respiratory syndrome virus (PRRSV) poses an extensive economic threat to the United States swine industry. The high degree of PRRSV genetic and antigenic variability challenges existing vaccination programs. We evaluated the ORF5 sequence of 1,931 PRRSV-2 strains detected from >300 farms managed by two pork production systems in the midwestern United States from 2001 to 2020 to assess the genetic diversity and molecular characteristics of heterologous PRRSV-2 strains. Phylogenetic analysis was performed on ORF5 sequences and classified using the global PRRSV classification system. N-glycosylation and the global and local selection pressure in the putative GP5 encoded by ORF5 were estimated. The PRRSV-2 sequences were classified into lineage 5 (L5; n = 438[22.7%]) or lineage 1 (L1; n = 1,493[77.3%]). The L1 strains belonged to one of three subclades: L1A (n = 1,225[63.4%]), L1B (n = 69[3.6%]), and L1C/D (n = 199[10.3%]). 10 N-glycosylation sites were predicted, and positions N44 and N51 were detected in most GP5 sequences (n = 1,801[93.3%]). Clade-specific N-glycosylation sites were observed: 57th in L1A, 33rd in L1B, 30th and 34th in L1C/D, and 30th and 33rd in L5. We identified nine and 19 sites in GP5 under significant positive selection in L5 and L1, respectively. The 13th, 151st, and 200th positive selection sites were exclusive to L5. Heterogeneity of N-glycosylation and positive selection sites may contribute to varying the evolutionary processes of PRRSV-2 strains circulating in these swine production systems. L1A and L5 strains denoted excellence in adaptation to the current swine population by their extensive positive selection sites with higher site-specific selection pressure. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is known for its high genetic and antigenic variability. In this study, we evaluated the ORF5 sequences of PRRSV-2 strains circulating in two swine production systems in the midwestern United States from 2001 to 2020. All the field strains were classified into four major groups based on genetic relatedness, where one group is closely related to the Ingelvac PRRS MLV strain. Here, we systematically compared differences in the ORF5 polymorphisms, N-glycosylation sites, and local and global evolutionary dynamics between different groups. Sites 44 and 51 were common for N-glycosylation in most amino acid sequences (n = 1,801, 93.3%). We identified that the L5 sequences had more positive selection pressure compared to the L1 strains. Our findings will provide valuable insights into the evolutionary mechanisms of PRRSV-2 and these molecular changes may lead to suboptimal effectiveness of Ingelvac PRRS MLV vaccine.

Investigating Multi-Target Antiviral Compounds by Screening of Phytochemicals From Neem (Azadirachta indica) Against PRRSV: A Vetinformatics Approach

Porcine reproductive and respiratory syndrome virus (PRRSV) is a global health problem for pigs. PRRSV is highly destructive and responsible for significant losses to the swine industry. Vaccines are available but incapable of providing adequate and long-term protection. As a result, effective and safe strategies are urgently needed to combat the virus. The scavenger receptor cysteine-rich domain 5 (SRCR5) in porcine CD163, non-structural protein 4 (Nsp4), and Nsp10 are known to play significant roles in PRRSV infection and disease development. Therefore, we targeted these proteins to identify multi-target antiviral compounds. To identify potent inhibitors, molecular docking of neem phytochemicals was conducted; three compounds [7-deacetyl-7-oxogedunin (CID:1886), Kulactone (CID:15560423), and Nimocin (CASID:104522-76-1)] were selected based on the lowest binding energy and multi-target inhibitory nature. The efficacy and safety of the selected compounds were revealed through the pharmacokinetics analysis and toxicity assessment. Moreover, 100 ns molecular dynamics (MD) simulation was performed to evaluate the stability and dynamic behavior of target proteins and their docked complexes with selected compounds. Besides, molecular mechanics Poisson–Boltzmann surface area method was used to estimate the binding free energy of each protein-ligand complex obtained from the MD simulations and validate the affinities of selected compounds to target proteins. Based on our analysis, we concluded that the identified multi-target compounds can be utilized as lead compounds for the development of natural drugs against PRRSV. If further validated in clinical studies, these compounds can be used individually or in combination against the virus.

Paraoxonase-1 Facilitates PRRSV Replication by Interacting with Viral Nonstructural Protein-9 and Inhibiting Type I Interferon Pathway

Paraoxonase-1 (PON1), an esterase with specifically paraoxonase activity, has been proven to be involved in inflammation and infection. Porcine reproductive and respiratory syndrome virus (PRRSV) is still a major concern in pigs and causes severe economic losses to the swine industry worldwide. In this study, the role of PON1 was investigated in porcine alveolar macrophages (PAMs) during PRRSV infection. The results showed that PRRSV replication downregulated PON1, and the knockdown of PON1 significantly decreased PRRSV replication. Similarly, PON1 overexpression could enhance PRRSV replication. Interestingly, we observed that PON1 interacted with PRRSV nonstructural protein 9 (Nsp9), the RNA-dependent RNA polymerase, and the knockdown of PON1 lowered the RNA binding ability of Nsp9, suggesting that PON1 can facilitate Nsp9 function in viral replication. In addition, the knockdown of PON1 expression led to the amplification of type I interferon (IFN) genes and vice versa. In summary, our data demonstrate that PON1 facilitates PRRSV replication by interacting with Nsp9 and inhibiting the type I IFN signaling pathway. Hence, PON1 may be an additional component of the anti-PRRSV defenses.

Combined Subcutaneous-Intranasal Immunization With Epitope-Based Antigens Elicits Binding and Neutralizing Antibody Responses in Serum and Mucosae Against PRRSV-2 and SARS-CoV-2

New vaccine design approaches, platforms, and immunization strategies might foster antiviral mucosal effector and memory responses to reduce asymptomatic infection and transmission in vaccinated individuals. Here, we investigated a combined parenteral and mucosal immunization scheme to induce local and serum antibody responses, employing the epitope-based antigens 3BT and NG19m. These antigens target the important emerging and re-emerging viruses PRRSV-2 and SARS-CoV-2, respectively. We assessed two versions of the 3BT protein, which contains conserved epitopes from the GP5 envelope protein of PRRSV-2: soluble and expressed by the recombinant baculovirus BacDual-3BT. On the other hand, NG19m, comprising the receptor-binding motif of the S protein of SARS-CoV-2, was evaluated as a soluble recombinant protein only. Vietnamese mini-pigs were immunized employing different inoculation routes: subcutaneous, intranasal, or a combination of both (s.c.-i.n.). Animals produced antigen-binding and neut1ralizing antibodies in serum and mucosal fluids, with varying patterns of concentration and activity, depending on the antigen and the immunization schedule. Soluble 3BT was a potent immunogen to elicit binding and neutralizing antibodies in serum, nasal mucus, and vaginal swabs. The vectored immunogen BacDual-3BT induced binding antibodies in serum and mucosae, but PRRSV-2 neutralizing activity was found in nasal mucus exclusively when administered intranasally. NG19m promoted serum and mucosal binding antibodies, which showed differing neutralizing activity. Only serum samples from subcutaneously immunized animals inhibited RBD-ACE2 interaction, while mini-pigs inoculated intranasally or via the combined s.c.-i.n. scheme produced subtle neutralizing humoral responses in the upper and lower respiratory mucosae. Our results show that intranasal immunization, alone or combined with subcutaneous delivery of epitope-based antigens, generates local and systemic binding and neutralizing antibodies. Further investigation is needed to evaluate the capability of the induced responses to prevent infection and reduce transmission.

PRRSV Non-Structural Proteins Orchestrate Porcine E3 Ubiquitin Ligase RNF122 to Promote PRRSV Proliferation

Ubiquitination plays a major role in immune regulation after viral infection. An alternatively spliced porcine E3 ubiquitin ligase RNF122 promoted PRRSV infection and upregulated in PRRSV-infected PAM cells was identified. We characterized the core promoter of RNF122, located between −550 to −470 bp upstream of the transcription start site (TSS), which displayed significant differential transcriptional activities in regulating the transcription and expression of RNF122. The transcription factor HLTF was inhibited by nsp1α and nsp7 of PRRSV, and the transcription factor E2F complex regulated by nsp9. Together, they modulated the transcription and expression of RNF122. RNF122 could mediate K63-linked ubiquitination to raise stability of PRRSV nsp4 protein and thus promote virus replication. Moreover, RNF122 also performed K27-linked and K48-linked ubiquitination of MDA5 to degrade MDA5 and inhibit IFN production, ultimately promoted virus proliferation. In this study, we illustrate a new immune escape mechanism of PRRSV that enhances self-stability and function of viral nsp4, thus, regulating RNF122 expression to antagonize IFNα/β production. The present study broadens our knowledge of PRRSV-coding protein modulating transcription, expression and modification of host protein to counteract innate immune signaling, and may provide novel insights for the development of antiviral drugs.

mRNA Vaccine Development for Emerging Animal and Zoonotic Diseases

In the prevention and treatment of infectious diseases, mRNA vaccines hold great promise because of their low risk of insertional mutagenesis, high potency, accelerated development cycles, and potential for low-cost manufacture. In past years, several mRNA vaccines have entered clinical trials and have shown promise for offering solutions to combat emerging and re-emerging infectious diseases such as rabies, Zika, and influenza. Recently, the successful application of mRNA vaccines against COVID-19 has further validated the platform and opened the floodgates to mRNA vaccine's potential in infectious disease prevention, especially in the veterinary field. In this review, we describe our current understanding of the mRNA vaccines and the technologies used for mRNA vaccine development. We also provide an overview of mRNA vaccines developed for animal infectious diseases and discuss directions and challenges for the future applications of this promising vaccine platform in the veterinary field.

Two immune-based methods using immortalized porcine kidney macrophages for quantifying neutralizing activity against porcine reproductive and respiratory syndrome virus-2

Porcine reproductive and respiratory syndrome virus (PRRSV) causes a serious infectious disease in pigs in farms worldwide. Neutralizing antibody titer is an effective index for evaluating immunity to PRRSV; however, PRRSV has different neutralizing cross-reactivity between strains. Therefore, quantitative measurement of neutralizing antibody titers against field PRRSV strains would be required to evaluate whether neutralizing antibodies in pigs could possess neutralizing activity against individual or multiple strains. Immune-based methods, such as image cytometry (ICM) and cell-based enzyme-linked immune sorbent assay (ELISA), are quantitative and can be used to evaluate many samples. Using immortalized porcine kidney macrophages (IPKMs), which are highly susceptible to infection from field PRRSV-2 strains compared with other cell lines, immune-based methods could enable the evaluation of the neutralizing activity of porcine serum against field strains of PRRSV-2 that are difficult to isolate in conventional cells. In summary, we adapted two methods, namely ICM and cell-based ELISA, to IPKMs for quantitative neutralizing antibody titer measurements. Two immune-based methods using IPKMs are adequate for quantifying neutralizing activity of porcine serum against PRRSV-2, including field strains.

Heterologous Challenge with PRRSV-1 MLV in Pregnant Vaccinated Gilts: Potential Risk on Health and Immunity of Piglets

Simple Summary

Modified live virus (MLV) vaccines are considered as the key component to control the Porcine Reproductive and Respiratory Syndrome Virus (PRRSV). The majority of pig farms apply the ‘mass’ vaccination strategy in breeding female animals. However, this PRRS MLV vaccination protocol involves the risk of inoculation of sows in the last stage of gestation, resulting in possible infection of the fetus as the virus can efficiently cross the placenta during the last period of pregnancy. Thus, we evaluated the ability of the vaccine virus to act as a pathogenic strain, to be transmitted to fetuses and to affect the health status of neonatal piglets. The results indicated that the study gilts transmitted the vaccine virus to their offspring, as well as that the PRRSV-infected piglets showed a poor clinical performance. Consequently, the pig farms that apply PRRS MLV vaccination in a routine blanket vaccination strategy must avoid inoculating pregnant gilts the last week before their parturition.

Abstract

The objective of the present study was to evaluate the potential risks of the four commercial PRRS-1 MLV vaccines in pregnant vaccinated gilts at the last stage of gestation under field conditions. The study was conducted at four pig farms, including 25 gilts from each farm (25 × 4 = 100 gilts), which were equally allocated to five different study groups. A PRRS-1 MLV vaccination was applied on the 100th day of their pregnancy with the different commercial vaccines that are available in the Greek market. The results indicated virus congenital infection and viremia in piglets (20/200 = 10% PRRSV infected piglets), and detection of PRRSV-specific antibodies (181/200 = 90.5% piglets found with PRRSV antibodies). The subsequent phylogenetic analyses revealed high percentages of similarity between the PRRSV-1 strain detected in infected litters and the PRRSV-1 vaccine strain to which the study gilts had been previously exposed to. Health status analyses of trial piglets resulted in differences between litters from vaccinated sows and litters from non-vaccinated sows at 110th day of gestation as regards the number of weak-born piglets, mummies, and piglets with splay-leg and/or respiratory symptoms. The current study’s results indicate several potential dangers of the PRRS MLV vaccination in late gestation.

Tumor Susceptibility Gene 101 (TSG101) Contributes to Virion Formation of Porcine Reproductive and Respiratory Syndrome Virus via Interaction with the Nucleocapsid (N) Protein along with the Early Secretory Pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses to global swine industry. As an intracellular obligate pathogen, PRRSV exploits host cellular machinery to establish infection. The endocytic sorting complex required for transport (ESCRT) system has been shown to participate in different life cycle stages of multiple viruses. In the current study, a systematic small interference RNA (siRNA) screening assay identified that certain ESCRT components contributed to PRRSV infection. Among them, tumor susceptibility gene 101 (TSG101) was demonstrated to be important for PRRSV infection by knockdown and overexpression assays. TSG101 was further revealed to be involved in virion formation rather than viral attachment, internalization, RNA replication and nucleocapsid (N) protein translation within the first round of PRRSV life cycle. In detail, TSG101 was determined to specially interact with PRRSV N protein and take effect on its subcellular localization along with the early secretory pathway. Taken together, these results provide evidence that TSG101 is a pro-viral cellular factor for PRRSV assembly, which will be a promising target to interfere with the viral infection. IMPORTANCE PRRSV infection results in a serious swine disease affecting pig farming in the world. However, efficient prevention and control of PRRSV is hindered by its complicated infection process. Up to now, our understanding of PRRSV assembly during infection is especially limited. Here, we identified that TSG101, an ESCRT-I subunit, facilitated virion formation of PRRSV via interaction with the viral N protein along with the early secretory pathway. Our work actually expands the knowledge of PRRSV infection and provides a novel therapeutic target for prevention and control of the virus.

Immunogenic and antigenic analysis of recombinant NSP1 and NSP11 of PRRS virus

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is an enveloped RNA virus in the order Nidovirales, family Arteriviridae, genus Betaarterivirus. Antibodies against nonstructural proteins (NSPs) from this virus can be found in pigs starting 4 days postinfection and they remain detectable for several months.

Objective

The goal of this study was to evaluate the immunogenicity and antigenic properties of recombinant proteins NSP1 and NSP11 expressed in Escherichia coli cells, as well as to assess the neutralization activity that they elicit.

Methods

We obtained the complete ORF‐1 genes coding for NSP1 and NSP11 from PRRSV using the VR‐2332 strain. Cloning was performed with the pET23a(+) vector with a histidine tag (His6), linearized by restriction enzyme digestion; the expression of the NSP1 and NSP11 clones was induced in OverExpress C41(DE3) chemically competent cells. Recombinant proteins were used to generate hyperimmune sera and we perform serological assays to confirm neutralizing antibodies.

Results

The expressed recombinant NSP1 and NSP11 were found to be immunogenic when injected in pigs, as well as demonstrated higher specificity in recognition of antigen in field sera from pigs positive infected with PRRSV. Furthermore, both NSP1 and NSP11 recombinant proteins elicited PRRSV neutralizing antibodies.

Conclusions

In this study, we demonstrated the immune humoral response to NSP 1 and NSP11, and neutralizing‐antibody response to PRRSV VR2332 strain in sera from hyperimmunized pigs.

miR-541-3p Promoted Porcine Reproductive and Respiratory Syndrome Virus 2 (PRRSV-2) Replication by Targeting Interferon Regulatory Factor 7

Porcine reproductive and respiratory syndrome (PRRS) is a disease caused by PRRS virus (PRRSV), which seriously harms the pig industry. Revealing the mechanism by which PRRSV inhibits immune response will help prevent and control PRRS. Here, we found that PRRSV-2 may hijack host miR-541-3p to inhibit host innate immune response. Firstly, this work showed that miR-541-3p mimics could facilitate the replication of PRRSV-2 and the results of the quantitative real time polymerase chain reaction (qRT-PCR) showed that PRRSV-2 could up-regulate the expression of miR-541-3p in MARC-145 cells. Since previous studies have shown that type I interferon could effectively inhibit the replication of PRRSV-2, the present work explored whether miR-541-3p regulated the expression of type I interferon and found that miR-541-3p could negatively regulate the transcription of type I interferon by targeting interferon regulatory factor 7 (IRF7). More importantly, PRRSV-2 infection could down-regulate the expression of IRF7 and over-expression of IRF7 could down-regulate the replication of PRRSV-2 in MARC-145 cells. In conclusion, PRRSV-2 infection up-regulated the expression of miR-541-3p to promote its replication in MARC-145 cells, since miR-541-3p can negatively regulate the transcription of type I interferon by targeting IRF7.

Generation and Evaluation of Recombinant Baculovirus Coexpressing GP5 and M Proteins of Porcine Reproductive and Respiratory Syndrome Virus Type 1

Porcine reproductive and respiratory syndrome virus (PRRSV) is the pathogen of the porcine reproductive and respiratory syndrome, which is one of the most economically devastating diseases of the swine industry. However, whether the inactivated vaccine and modified live attenuated vaccines are effective in disease control is still controversial. Although several groups developed PRRSV virus-like particles (VLPs) as a vaccine against PRRSV, all these VLP-based vaccines targeted PRRSV-2, but not PRRSV-1 or both. Therefore, it is urgent to produce VLPs against PRRSV-1. In this study, we rescued recombinant baculovirus expressing GP5 and M proteins of PRRSV-1 through the Bac-to-Bac^® baculovirus expression system. Thereafter, PRRSV VLP was obtained efficiently in the recombinant baculovirus-infected High Five insect cells. Moreover, the PRRSV VLP and PRRSV VLP+A5 could efficiently trigger specific humoral immune responses and B cellular immune responses through intranasal immunization. The combination of PRRSV VLP and A5 adjuvant could improve the level of the immune response. The PRRSV-1 VLPs generated in this study have greater potential for vaccine development to control PRRSV-1 infection.

Further host-genomic characterization of total antibody response to PRRSV vaccination and its relationship with reproductive performance in commercial sows: genome-wide haplotype and zygosity analyses

Background

The possibility of using antibody response (S/P ratio) to PRRSV vaccination measured in crossbred commercial gilts as a genetic indicator for reproductive performance in vaccinated crossbred sows has motivated further studies of the genomic basis of this trait. In this study, we investigated the association of haplotypes and runs of homozygosity (ROH) and heterozygosity (ROHet) with S/P ratio and their impact on reproductive performance.

Results

There was no association (P-value ≥ 0.18) of S/P ratio with the percentage of ROH or ROHet, or with the percentage of heterozygosity across the whole genome or in the major histocompatibility complex (MHC) region. However, specific ROH and ROHet regions were significantly associated (P-value ≤ 0.01) with S/P ratio on chromosomes 1, 4, 5, 7, 10, 11, 13, and 17 but not (P-value ≥ 0.10) with reproductive performance. With the haplotype-based genome-wide association study (GWAS), additional genomic regions associated with S/P ratio were identified on chromosomes 4, 7, and 9. These regions harbor immune-related genes, such as SLA-DOB, TAP2, TAPBP, TMIGD3, and ADORA. Four haplotypes at the identified region on chromosome 7 were also associated with multiple reproductive traits. A haplotype significantly associated with S/P ratio that is located in the MHC region may be in stronger linkage disequilibrium (LD) with the quantitative trait loci (QTL) than the previously identified single nucleotide polymorphism (SNP) (H3GA0020505) given the larger estimate of genetic variance explained by the haplotype than by the SNP.

Conclusions

Specific ROH and ROHet regions were significantly associated with S/P ratio. The haplotype-based GWAS identified novel QTL for S/P ratio on chromosomes 4, 7, and 9 and confirmed the presence of at least one QTL in the MHC region. The chromosome 7 region was also associated with reproductive performance. These results narrow the search for causal genes in this region and suggest SLA-DOB and TAP2 as potential candidate genes associated with S/P ratio on chromosome 7. These results provide additional opportunities for marker-assisted selection and genomic selection for S/P ratio as genetic indicator for litter size in commercial pig populations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12711-021-00676-5.

Simultaneous Infection With Porcine Reproductive and Respiratory Syndrome and Influenza Viruses Abrogates Clinical Protection Induced by Live Attenuated Porcine Reproductive and Respiratory Syndrome Vaccination

The porcine respiratory disease complex (PRDC) is responsible for significant economic losses in the pig industry worldwide. Porcine reproductive and respiratory syndrome virus (PRRSV) and swine influenza virus are major viral contributors to PRDC. Vaccines are cost-effective measures for controlling PRRS, however, their efficacy in the context of co-infections has been poorly investigated. In this study, we aimed to determine the effect of PRRSV-2 and swine influenza H3N2 virus co-infection on the efficacy of PRRSV modified live virus (MLV) vaccination, which is widely used in the field. Following simultaneous challenge with contemporary PRRSV-2 and H3N2 field isolates, we found that the protective effect of PRRS MLV vaccination on clinical disease and pathology was abrogated, although viral load was unaffected and antibody responses were enhanced. In contrast, co-infection in non-immunized animals reduced PRRSV-2 viremia and H3N2 virus load in the upper respiratory tract and potentiated T cell responses against both PRRSV-2 and H3N2 in the lung. Further analysis suggested that an upregulation of inhibitory cytokines gene expression in the lungs of vaccinated pigs may have influenced responses to H3N2 and PRRSV-2. These findings provide important insights into the effect of viral co-infections on PRRS vaccine efficacy that may help identify more effective vaccination strategies against PRDC in the field.

Development of a multiplex qRT-PCR assay for detection of African swine fever virus, classical swine fever virus and porcine reproductive and respiratory syndrome virus

BACKGROUND

African swine fever virus (ASFV), classical swine fever virus (CSFV), and porcine reproductive and respiratory syndrome virus (PRRSV) are still prevalent in many regions of China. Co-infections make it difficult to distinguish their clinical symptoms and pathological changes. Therefore, a rapid and specific method is needed for the differential detection of these pathogens.

OBJECTIVES

The aim of this study was to develop a multiplex real-time quantitative reverse transcription polymerase chain reaction (multiplex qRT-PCR) for the simultaneous differential detection of ASFV, CSFV, and PRRSV.

METHODS

Three pairs of primers and TaqMan probes targeting the ASFV p72 gene, CSFV 5' untranslated region, and PRRSV ORF7 gene were designed. After optimizing the reaction conditions, including the annealing temperature, primer concentration, and probe concentration, multiplex qRT-PCR for simultaneous and differential detection of ASFV, CSFV, and PRRSV was developed. Subsequently, 1,143 clinical samples were detected to verify the practicality of the assay.

RESULTS

The multiplex qRT-PCR assay could specifically and simultaneously detect the ASFV, CSFV, and PRRSV with a detection limit of 1.78 × 10⁰ copies for the ASFV, CSFV, and PRRSV, but could not amplify the other major porcine viruses, such as pseudorabies virus, porcine circovirus type 1 (PCV1), PCV2, PCV3, foot-and-mouth disease virus, porcine parvovirus, atypical porcine pestivirus, and Senecavirus A. The assay had good repeatability with coefficients of variation of intra- and inter-assay of less than 1.2%. Finally, the assay was used to detect 1,143 clinical samples to evaluate its practicality in the field. The positive rates of ASFV, CSFV, and PRRSV were 25.63%, 9.36%, and 17.50%, respectively. The co-infection rates of ASFV+CSFV, ASFV+PRRSV, CSFV+PRRSV, and ASFV+CSFV+PRRSV were 2.45%, 2.36%, 1.57%, and 0.17%, respectively.

CONCLUSIONS

The multiplex qRT-PCR developed in this study could provide a rapid, sensitive, specific diagnostic tool for the simultaneous and differential detection of ASFV, CSFV, and PRRSV.