High reversion potential of a cell-adapted vaccine candidate against highly pathogenic porcine reproductive and respiratory syndrome

A chimeric strain of porcine reproductive and respiratory syndrome virus 2 derived from HP-PRRSV and NADC30-like PRRSV confers cross-protection against both strains

Porcine reproductive and respiratory syndrome (PRRS) is one of the most significant swine viral infectious diseases worldwide. Vaccination is a key strategy for the control and prevention of PRRS. At present, the NADC30-like PRRSV strain has become the predominant epidemic strain in China, superseding the HP-PRRSV strain. The existing commercial vaccines offer substantial protection against HP-PRRSV, but their efficacy against NADC30-like PRRSV is limited. The development of a novel vaccine that can provide valuable cross-protection against both NADC30-like PRRSV and HP-PRRSV is highly important. In this study, an infectious clone of a commercial MLV vaccine strain, GD (HP-PRRSV), was first generated (named rGD). A recombinant chimeric PRRSV strain, rGD-SX-5U2, was subsequently constructed by using rGD as a backbone and embedding several dominant immune genes, including the NSP2, ORF5, ORF6, and ORF7 genes, from an NADC30-like PRRSV isolate. In vitro experiments demonstrated that chimeric PRRSV rGD-SX-5U2 exhibited high tropism for MARC-145 cells, which is of paramount importance in the production of PRRSV vaccines. Moreover, subsequent in vivo inoculation and challenge experiments demonstrated that rGD-SX-5U2 confers cross-protection against both HP-PRRSV and NADC30-like PRRSV, including an improvement in ADG levels and a reduction in viremia and lung tissue lesions. In conclusion, our research demonstrated that the chimeric PRRSV strain rGD-SX-5U2 is a novel approach that can provide broad-spectrum protection against both HP-PRRSV and NADC30-like PRRSV. This may be a significant improvement over previous MLV vaccinations.

Virucidal effects of eucalyptus essential oil on porcine reproductive and respiratory syndrome virus

Currently, the efficacy of vaccination for preventing and controlling PRRSV is insufficient. Therefore, there is an urgent need for novel effective preventive strategies. This study aimed to investigate the antiviral effect of Eucalyptus essential oil (EEO) against PRRSV in vitro. Marc-145 cells were infected with PRRSV (rJXA1-R), and the toxicity of EEO in the cells was measured using the Cell Counting Kit-8 method. Additionally, the antiviral effect of EEO on PRRSV-infected cells was assessed using three treatment methods: drug administration post-PRRSV inoculation (post-treatment), drug administration before PRRSV inoculation (pre-treatment), and simultaneous drug administration and PRRSV inoculation (co-treatment). The EEO could not inhibit virus adsorption and/or replication since post-treatment and pre-treatment did not prevent viral infectivity. However, EEO exerted a significant virucidal effect on PRRSV. When PRRSV-infected cells were treated with 0.0156, 0.0312, and 0.0625% EEO, the cell survival rates were 55.37, 118.96, and 121.67%, respectively, and the titer of progeny virions decreased from 5.77 Log10TCID50 to 5.21 Log10TCID50, 0.55 Log10TCID50, and less than 0.167 Log10TCID50, respectively (where TCID50 is the 50% tissue culture infected dose). The fluorescence intensity of the PRRSV N protein significantly decreased in the indirect immunofluorescence assay. When cells were co-treated with EEO (0.0625%) and PRRSV (1000 TCID50) for 15 min, the viral particles were inactivated, and PRRSV (1000 TCID50) particles loss infectivity when the co-treatment time reached 60 min. In a word, EEO has no obvious therapeutic effect on PRRSV infection, but it can effectively inactivate virus particles and make them lose the ability to infect cells. These findings provide insights for the development and use of EEO to treat PRRS.

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.

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.

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.

A Synthetic Biology Approach for Vaccine Candidate Design against Delta Strain of SARS-CoV-2 Revealed Disruption of Favored Codon Pair as a Better Strategy over Using Rare Codons

The SARS-CoV-2 delta variant (B.1.617.2) appeared for the first time in December 2020 and later spread worldwide. Currently available vaccines are not so efficacious in curbing the viral pathogenesis of the delta strain of COVID; therefore, the development of a safe and effective vaccine is required. In the present study, we envisaged molecular patterns in the structural genes' spike, nucleoprotein, membrane, and envelope of the SARS-CoV-2 delta variant. The study was based on determining compositional features, dinucleotide odds ratio, synonymous codon usage, positive and negative codon contexts, rare codons, and insight into relatedness between the human host isoacceptor tRNA and preferred codons from the structural genes. We found specific patterns, including a significant abundance of T nucleotide over all other three nucleotides. The underrepresentation of GpA, GpG, CpC, and CpG dinucleotides and the overrepresentation of TpT, ApA, CpT, and TpG were observed. A preference towards ACT- (Thr), AAT- (Asn), TTT- (Phe), and TTG- (Leu) initiated codons and aversion towards CGG (Arg), CCG (Pro), and CAC (His) was present in the structural genes of the delta strain. The interaction between the host tRNA pool and preferred codons of the envisaged structural genes revealed that the virus preferred the codons for those suboptimal numbers of isoacceptor tRNA were present. We see this as a strategy adapted by the virus to keep the translation rate low to facilitate the correct folding of viral proteins. The information generated in the study helps design the attenuated vaccine candidate against the SARS-CoV-2 delta variant using a synthetic biology approach. Three strategies were tested: changing TpT to TpA, introducing rare codons, and disrupting favored codons. It found that disrupting favored codons is a better approach to reducing virus fitness and attenuating SARS-CoV-2 delta strain using structural genes.

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.

Protective efficacy of intranasal inactivated pseudorabies vaccine is improved by combination adjuvant in mice

Pseudorabies virus (PRV) not only causes great economic loss to the pig industry but also seriously threatens the biosafety of other mammals, including humans. Since 2011, PRV mutant strains have emerged widely in China, and the classical Bartha-K61 vaccine cannot confer complete protection for pigs. PRV mainly infects pigs via the respiratory tract. Intranasal immunization with PRV has received more attention because intranasal vaccination elicits systemic and mucosal immune responses. To induce systemic and mucosal immune responses against PRV, we developed a combination adjuvant as a delivery system for intranasal vaccine, which was formulated with MONTANIDE™ Gel 01 and CVCVA5. In comparison to naked antigen of inactivated PRV, single Gel 01 adjuvanted inactivated antigen and single CVCVA5 adjuvanted inactivated antigen, intranasal inactivated PRV vaccine formulated with the combination adjuvant induced greater mucosal IgA immunity and serum antibody responses (IgG, IgG1, and IgG2a). Furthermore, the production of the Th1-type cytokine IFN-γ and the Th2-type cytokine IL-4 indicated that the cellular and humoral responses to the intranasal vaccine were improved by the combination adjuvant. In addition, the intranasal vaccine formulated with the combination adjuvant induced long-term T lymphocyte memory with increased central (CD62L⁺CD44⁺) and effector (CD62L^–CD44⁺) memory subsets of both CD4 and CD8 T cells in nasal-associated lymphoid tissue. Intranasal challenge with virulent PRV in mice showed that the protective efficacy of the intranasal PRV vaccine was improved by the combination adjuvant compared with the other single-adjuvanted vaccines. In summary, these data demonstrated that Gel 01 combined with the CVCVA5 adjuvant induced a synergistic effect to improve mucosal immunity and protective efficacy of the intranasally inactivated PRV vaccine in mice. It represents a promising vaccination approach against PRV infection.

Epidemiological Survey of Four Reproductive Disorder Associated Viruses of Sows in Hunan Province during 2019–2021

Porcine reproductive disorders have been considered as the major factors that threaten pig industries worldwide. In this study, 407 aborted-fetus samples were obtained from 89 pig farms in Hunan province, to investigate the prevalence of four viruses associated with porcine reproductive disease, including porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2), pseudorabies virus (PRV), and classical swine fever virus (CSFV). Meanwhile, the target gene sequences of representative PRRSV (ORF5), PCV2 (ORF2), CSFV (E2), and PRV (gE) strains were amplified, sequenced, and analyzed. The results showed that the positive rates of PRRSV, PCV2, PRV, and CSFV among the collected samples were 26.29% (107/407), 52.83% (215/407), 6.39% (26/407), and 12.29% (50/407), respectively. Moreover, co-infection with two and three pathogens were frequently identified, with PCV2/PRRSV, PRRSV/CSFV, PRRSV/PRV, PCV2/CSFV, PCV2/PRV, and PRRSV/PCV2/CSFV mix infection rates of 9.09%, 3.19%, 2.95%, 3.69%, 2.21%, and 0.49%, respectively. Moreover, ORF5-based phylogenetic analysis showed that 9, 4, and 24 of 37 PRRSV strains belonged to the PRRSV2 lineages 1, 5, and 8, respectively. ORF2-based phylogenetic analysis revealed that PCV2d and PCV2b were prevalent in Hunan province, with the proportions of 87.5% (21/24) and 12.5% (3/24), respectively. An E2-based phylogenetic tree showed that all 13 CSFV strains were clustered with 2.1 subgenotypes, these isolates were composed of 2.1b (10/13) and 2.1c (3/13) sub-subgenotypes. A gE-based phylogenetic tree showed that all six PRV strains belonged to the genotype II, which were genetically closer to variant PRV strains. Collectively, the present study provides the latest information on the epidemiology and genotype diversity of four viruses in sows with reproductive diseases in Hunan province, China, which would contribute to developing effective strategies for disease control.

Rapid metagenomic identification of two major swine pathogens with real-time nanopore sequencing

Metagenomic next-generation sequencing (mNGS) is a rapid deep-sequencing diagnostic tool for the unbiased identification of pathogens. In this study, we established a nanopore-sequencing-based mNGS protocol to detect two major viral pathogens of swine, Porcine reproductive and respiratory syndrome virus (PRRSV) and Porcine epidemic diarrhea virus (PEDV). Samples were spiked with the serially diluted viruses as standard references to define the specific protocols. The utility of the method was evaluated with key parameters. The limits of detection for PRRSV and PEDV were 2.3 × 10² and 9.0 × 10⁴ copies per reaction, respectively, and good correlations between PCR quantification cycle value and the mapped read count (log value) were observed. Only the nanopore reads could be assembled de novo into nearly full-length of the PRRSV genome, with 99.9% pairwise identity, and 90.0% genome coverage for PEDV. The established protocol was validated in PRRSV-positive clinical samples. The results for PRRSV-positive tissue and serum samples tested with mNGS protocol were 100% concordant with quantitative PCR results. The protocol also recognized infections of single or multiple viruses in a single sample. In conclusion, we have established a nanopore-sequencing-based mNGS protocol that efficiently identifies and characterizes viral pathogen(s) in a variety of clinical sample types.

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.

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.

Interpretable machine learning applied to on-farm biosecurity and porcine reproductive and respiratory syndrome virus

Effective biosecurity practices in swine production are key in preventing the introduction and dissemination of infectious pathogens. Ideally, on-farm biosecurity practices should be chosen by their impact on bio-containment and bio-exclusion; however, quantitative supporting evidence is often unavailable. Therefore, the development of methodologies capable of quantifying and ranking biosecurity practices according to their efficacy in reducing disease risk has the potential to facilitate better-informed choices of biosecurity practices. Using survey data on biosecurity practices, farm demographics, and previous outbreaks from 139 herds, a set of machine learning algorithms were trained to classify farms by porcine reproductive and respiratory syndrome virus status, depending on their biosecurity practices and farm demographics, to produce a predicted outbreak risk. A novel interpretable machine learning toolkit, MrIML-biosecurity, was developed to benchmark farms and production systems by predicted risk and quantify the impact of biosecurity practices on disease risk at individual farms. By quantifying the variable impact on predicted risk, 50% of 42 variables were associated with fomite spread while 31% were associated with local transmission. Results from machine learning interpretations identified similar results, finding substantial contribution to predicted outbreak risk from biosecurity practices relating to the turnover and number of employees, the surrounding density of swine premises and pigs, the sharing of haul trailers, distance from the public road and farm production type. In addition, the development of individualized biosecurity assessments provides the opportunity to better guide biosecurity implementation on a case-by-case basis. Finally, the flexibility of the MrIML-biosecurity toolkit gives it the potential to be applied to wider areas of biosecurity benchmarking, to address biosecurity weaknesses in other livestock systems and industry-relevant diseases.

Evolution of pathogen tolerance and emerging infections: A missing experimental paradigm

Researchers worldwide are repeatedly warning us against future zoonotic diseases resulting from humankind's insurgence into natural ecosystems. The same zoonotic pathogens that cause severe infections in a human host frequently fail to produce any disease outcome in their natural hosts. What precise features of the immune system enable natural reservoirs to carry these pathogens so efficiently? To understand these effects, we highlight the importance of tracing the evolutionary basis of pathogen tolerance in reservoir hosts, while drawing implications from their diverse physiological and life-history traits, and ecological contexts of host-pathogen interactions. Long-term co-evolution might allow reservoir hosts to modulate immunity and evolve tolerance to zoonotic pathogens, increasing their circulation and infectious period. Such processes can also create a genetically diverse pathogen pool by allowing more mutations and genetic exchanges between circulating strains, thereby harboring rare alive-on-arrival variants with extended infectivity to new hosts (i.e., spillover). Finally, we end by underscoring the indispensability of a large multidisciplinary empirical framework to explore the proposed link between evolved tolerance, pathogen prevalence, and spillover in the wild.

Abundance of Lactobacillus in porcine gut microbiota is closely related to immune response following PRRSV immunization

Increasing evidence shows that gut microbiota plays a critical role in host immune system development and immune regulation, thus the composition of gut microbiota may affect how individuals respond to immunizations. Currently, little evidence is available on the correlation between porcine gut microbiota and vaccine immune response. Here, we investigated the influence of gut microbiota on immune response in pigs to porcine reproductive and respiratory syndrome virus (PRRSV) vaccine. Based on the antibody levels for PRRSV, the immunized pigs were divided into three groups (high, low, and others), and followed by virulent PRRSV challenge. The comprehensive analysis of microbial composition revealed that gut microbiota was similar in the richness and diversity among different groups before immunization. After immunization, the richness and diversity of gut microbial community in the high group were still similar to the low group, although there was a decrease in community diversity overtime. Interestingly, the antibody titer was positively correlated with the abundance of Lactobacillus in gut microbiota in immunized pigs. Further analysis indicated that gut microbial composition might be correlated to the clinical parameters such as body weight and rectal temperature after virus challenge. Taken together, our findings suggest that certain specific members of gut microbiota, such as Lactobacillus may serve as a mechanism for regulating the immune response following immunization in pigs.

Effects of glycyrrhizin on the growth cycle and ATPase activity of PRRSV-2-infected MARC-145 cells

Porcine reproductive and respiratory syndrome (PRRS) is a viral infectious disease caused by the porcine reproductive and respiratory syndrome virus (PRRSV) and is devastating the swine industry. MARC-145 cells, an African green monkey kidney cell line, are sensitive to PRRSV-2, and are often used for in vitro studies on PRRSV-2. Preliminary research has shown that glycyrrhizin, an important active component extracted from traditional Chinese medicinal licorice, significantly inhibits the proliferation of PRRSV-2 in MARC-145 cells; however, the in-depth molecular mechanism remains unclear. By determining the cell growth cycle, this study found that PRRSV-2 infection first increased the content of G1-phase MARC-145 cells and then decreased the content of G1-phase cells. Moreover, glycyrrhizin affected the role of PRRSV-2 in regulating the cell cycle. Furthermore, PRRSV-2 had the highest proliferation titer in G0/G1-phase MARC-145 cells, and glycyrrhizin reduced the content of PRRSV-2 in synchronized MARC-145 cells. According to the results of ATPase detection, PRRSV-2 infection weakened the Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase activities in MARC-145 cells, while glycyrrhizin significantly enhanced their activities in PRRSV-2-infected MARC-145 cells. The above results provide theoretical support toward clarifying the mechanism by which glycyrrhizin inhibits the proliferation of PRRSV-2 in MARC-145 cells. Moreover, these results offer references for the development and use of glycyrrhizin and the clinical treatment of PRRSV-2 infection.

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A "Leaky" Vaccine with Debatable Efficacy and Safety

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.

Characterization of four types of MLV-derived porcine reproductive and respiratory syndrome viruses isolated in unvaccinated pigs from 2016 to 2020

Modified live vaccines (MLVs) have been utilized to combat porcine reproductive and respiratory syndrome (PRRS), which raises a serious concern about the MLV-derived PRRS virus (PRRSV) isolates. During the routine investigation of PRRSV in China, four lung samples collected from unvaccinated diseased pigs from 2016 to 2020 were detected as PRRSV positive. The PRRSVs shared high ORF5 identities to CH-1R, JXA1-R, TJM-F92 and RespPRRS MLV vaccines, respectively. The viruses were isolated in Marc-145 cells and denominated as SD1612-1, JS1703-21, JSTZ1907-714 and JSYC20-05-1. Genome comparison confirmed that these isolates share the highest genomic homologies to CH-1R (97.96%), JXA1-R (99.64%), TJM-F92 (99.00%) and RespPRRS MLV (99.57%) than any other known isolates. Genome-based phylogenetic analysis showed that SD1612-1 and CH-1R, JS1703-21 and JXA1-R, JSTZ1907-714 and TJM-F92, JSYC20-05-1 and RespPRRS MLV were grouped in the same branches. In addition, amino acids unique to corresponding vaccine attenuations were also identified in our isolates. Noticeably, amino-acids potentially associated with the virulence revision from MLV strains to parental virulent viruses were also identified in the MLV-derived isolates. Our results confirm that the four types of MLV-derived isolates are circulating and evolving in Chinese swine herds for years, which highlights the necessity for the fair use of PRRS MLVs.

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

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

Efficacy of two porcine reproductive and respiratory syndrome (PRRS) modified-live virus (MLV) vaccines against heterologous NADC30-like PRRS virus challenge

The emergence of novel and variant porcine reproductive and respiratory syndrome virus (PRRSV) strains has made controlling this disease a challenge in China. Several NADC30-like PRRSV outbreaks have occurred in mainland China since 2013. The objective of the present study was to evaluate the cross-protection efficacy of two commercial PRRS modified-live virus (MLV) vaccines, derived from classical PRRSV (VR2332) and highly pathogenic (HP) PRRSV (TJM-F92), against an increasingly circulating NADC30-like lineage in pigs. Thirty-five PRRSV- and antibody-free pigs were randomly divided into the following four groups: strict control (SC), negative control (NC), Boehringer control (BC), and Zoetis control (ZC) groups. The NADC30-like PRRSV used in this study caused fever, clinical respiratory signs, and gross and microscopic lung lesions in inoculated pigs in the NC group. Vaccination with the VR2332 vaccine significantly reduced the percentage of viremic pigs as well as gross lung lesions and improved average daily weight gain compared to the ZC and NC groups, suggesting that this MLV vaccine provides cross-protection against the NADC30-like virus. There were no significant differences in the efficacy of the two MLV vaccines based on clinical scores, immunological responses, or pathological outcomes. This study demonstrated that VR2332 MLV was effective against circulating NADC30-like PRRSV and could be used to control NADC30-like virus infections in the field.

Intranasal inoculations of naked or PLGA-PEI nanovectored DNA vaccine induce systemic and mucosal antibodies in pigs: A feasibility study

Mucosa are the routes of entry of most pathogens into animals' organisms. Reducing the important global burden of mucosal infectious diseases in livestock animals is required in the field of veterinary public health. For veterinary respiratory pathogens, one possible strategy is the development of intranasal (IN) DNA vaccination. The aim of this study was to assess the feasibility of IN DNA vaccination in pigs, an important species in livestock production industry, and a source of zoonotic diseases. To achieve this goal, we used a DNA vaccine against pseudorabies virus (PrV) encoding the immunogenic glycoprotein B (pcDNA3-gB plasmid). When pigs were inoculated with the naked DNA vaccine through the IN route, PrV-specific IgG and IgA type antibodies were detected in porcine sera. Interestingly, mucosal salivary IgA antibodies against PrV were also detected, at similar levels to those measured following intramuscular injection (positive controls). Furthermore, the IN delivery of pcDNA3-gB combined with PLGA-PEI nanoparticles resulted in similar levels of antibodies but was associated with an increase in the duration of detection of mucosal IgA for 2 out of 3 pigs. Our results suggest that there is room to improve the efficacy of IN DNA vaccination in pigs through optimization of IN inoculations, for example by using nanoparticles such as PLGA-PEI. Further studies will be dedicated to optimizing and testing the protective potential of IN DNA vaccination procedures against PrV.

Administration with Vaccinia Virus Encoding Canine Parvovirus 2 vp2 Elicits Systemic Immune Responses in Mice and Dogs

Canine parvovirus type 2 (CPV2) is a highly contagious cause of serious and often fatal disease in young dogs. Despite the widespread availability of attenuated vaccines, safer, more stable, and more effective CPV2 vaccine candidates are still under exploration. Vaccinia virus (VV) has already been proved to be a safe, stable, and effective vaccine vector. In this study, we generated a VV-based CPV2 vaccine candidate (VV_-CPV-VP2) and then evaluated its immunogenicity in mice and dogs. The exogenous vp2 gene of CPV2, which replaced the major virulence gene hemagglutinin (ha) of VV, expressed efficiently and stably in vitro. Subsequently, intramuscular immunization of mice induced robust and lasting systemic immune responses, including neutralizing antibody against both CPV2a and CPV2b, and CPV2-VP2-specific interferon gamma (IFN-γ) secreting T cell. In addition, administration with a high-dose of VV_-CPV-VP2 did not cause significant side effects for mice, thus indicating marked safety of this vaccine candidate. Most importantly, a single-dose vaccination of VV_-CPV2-VP2 elicited substantial antibody responses and provided comparable protection for dogs with attenuated CPV2 vaccine. Collectively, this study demonstrated that VV_-CPV2-VP2 could be used as a promising vaccine candidate preventing CPV2 from infection for dogs.

High genetic diversity of Chinese porcine reproductive and respiratory syndrome viruses from 2016 to 2019

High genetic diversity and limited cross-protection are two major reasons for ineffective control of porcine reproductive and respiratory syndrome virus (PRRSV) infection. Therefore, it's important to dynamically monitor the prevalence of PRRSV for adopting appropriate control strategy. In this study, we analyzed PRRSV infection by detecting 712 clinical samples collected from 2016 to 2019 in China. Totally 100 samples were detected as PRRSV positive, including 2 and 98 samples were infected with PRRSV1 and PRRSV2, respectively. In addition, two out of the 98 PRRSV2 positive samples were co-infected with two distinct viruses. ORF5-based phylogenetic analysis showed that JXA1-like HP-PRRSV2 (lineage 8) and NADC30-like PRRSV2 (lineage 1) isolates are currently predominant, but QYYZ-like PRRSV2, CH-1a-like PRRSV2 and PRRSV1 isolates also co-exist in Chinese swine herds. In addition, two commercial MLV-derived viruses (TJM-F92-like and JXA1-R-like) were frequently detected. GP5 alignment also detected insertion and deletion in the extravirion domain. Our study presents the up-to-date PRRSV infection status and highlights the high genetic diversity of PRRSV currently circulating in China.

Construction and immunological evaluation of recombinant Newcastle disease virus vaccines expressing highly pathogenic porcine reproductive and respiratory syndrome virus GP3/GP5 proteins in pigs

Highly pathogenic porcine reproductive and respiratory syndrome (HP-PRRS) poses a significant threat to the pig industry, for which vaccination is considered to be an effective means of prevention and control. Here, we developed two recombinant Newcastle disease virus (NDV) LaSota-vectored PRRS candidate vaccines, rLaSota-GP5 and rLaSota-GP3-GP5, using reverse genetic techniques. The two recombinant viruses exhibited a high degree of genetic stability after 10 successive generations in chicken embryos. There was no significant difference in pathogenicity compared with the rLaSota parent strain in poultry, mice and pigs. The recombinant viruses could not be detected in the feeding environment of immunized pigs, but could be detected in the organs and tissues of pigs for no more than 10 days after immunization. Importantly, in contrast to rLaSota-GP5, rLaSota-GP3-GP5 elicited both significant humoral and cellular immune responses in pigs. In particular, the neutralizing antibody titer in the rLaSota-GP3-GP5 group was 1.51 times significantly higher than that of the commercial vaccine group at 42 days post-immunization. At the same time, there was significant difference in the level of IFN-γ between the rLaSota-GP3-GP5 group and the commercial vaccine group. Furthermore, the viral load in the organs and tissues of rLaSota-GP3-GP5-immunized pigs was substantially lower than that of unimmunized pigs after being challenged with HP-PRRS virus GD strain. These results suggest that rLaSota-GP3-GP5 is a safe and promising candidate vaccine, and there is potential for further development of a recombinant virus vaccine for PRRS using NDV.