Immunogenicity of the highly pathogenic porcine reproductive and respiratory syndrome virus GP5 protein encoded by a synthetic ORF5 gene

Development of a Ferritin Protein Nanoparticle Vaccine with PRRSV GP5 Protein

Porcine reproductive and respiratory syndrome virus (PRRSV) presents a significant threat to the global swine industry. The development of highly effective subunit nanovaccines is a promising strategy for preventing PRRSV variant infections. In this study, two different types of ferritin (Ft) nanovaccines targeting the major glycoprotein GP5, named GP5m-Ft and (Bp-IVp)3-Ft, were constructed and evaluated as vaccine candidates for PRRSV. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) demonstrated that both purified GP5m-Ft and (Bp-IVp)3-Ft proteins could self-assemble into nanospheres. A comparison of the immunogenicity of GP5m-Ft and (Bp-IVp)3-Ft with an inactivated PRRSV vaccine in BALB/c mice revealed that mice immunized with GP5m-Ft exhibited the highest ELISA antibody levels, neutralizing antibody titers, the lymphocyte proliferation index, and IFN-γ levels. Furthermore, vaccination with the GP5m-Ft nanoparticle effectively protected piglets against a highly pathogenic PRRSV challenge. These findings suggest that GP5m-Ft is a promising vaccine candidate for controlling PRRS.

Genetic Characterization and Pathogenicity of a Recombinant Porcine Reproductive and Respiratory Syndrome Virus Strain in China

Since it was first reported in 2013, the NADC30-like PRRSV has been epidemic in China. Hubei Province is known as China's key hog-exporting region. To understand the prevalence and genetic variation of PRRSV, herein, we detected and analyzed 317 lung tissue samples from pigs with respiratory disease in Hubei Province, and demonstrated that the NADC30-like strain was the second-most predominant strain during 2017-2018, following the highly pathogenic PRRSV (HP-PRRSV). Additionally, we isolated a new NADC30-like PRRSV strain, named CHN-HB-2018, which could be stably passaged in Marc-145 cells. Genetic characterization analysis showed that compared with the NADC30 strain, the CHN-HB-2018 strain had several amino acid variations in glycoprotein (GP) 3, GP5, and nonstructural protein 2 (NSP2). Moreover, the CHN-HB-2018 strain showed a unique 5-amino acid (aa) deletion in NSP2, which has not previously been reported. Gene recombination analysis identified the CHN-HB-2018 strain as a potentially recombinant PRRSV of the NADC30-like strain and HP-PRRSV. Animal experiments indicated that the CHN-HB-2018 strain has a mild pathogenicity, with no mortality and only mild fever observed in piglets. This study contributes to defining the evolutionary characteristics of PRRSV and its molecular epidemiology in Hubei Province, and provides a potential candidate strain for PRRSV vaccine development.

Pathological observation and transcriptomic analysis of thymus injury in PRRSV-infected piglets

The thymus, the central immune organ in mammals, plays an important role in immune defense. Porcine reproductive and respiratory syndrome virus (PRRSV) infection in piglets can cause thymus injury and immunosuppression. However, the mechanisms of thymus injury remain unknown. This study was aimed at investigating the specific manifestations of thymus injury through the construction of a PRRSV-infected piglet model and histopathological observation. In this study, fourteen 40-day-old PRRSV-free piglets were randomly divided into two groups, eleven of which were intramuscularly injected with 3 mL of PRRSV WUH3 virus suspension (106 PFU /mL) in the infection group, and three of which were sham-inoculated with 3 mL of RPMI-1640 medium in the control group. Clinical necropsy and samples collection were performed on day 8 after artificial infection. With the Illumina platform, the transcriptomes of piglet thymus tissues from infected and control piglets were sequenced to explore the relationships of differentially expressed genes (DEGs) and signaling pathways with thymus injury. The immune organs of PRRSV-infected piglets were severely damaged. The histopathological findings in the thymus indicated that PRRSV infection was associated with a large decrease in lymphocytes, cell necrosis and cell apoptosis; an increase in blood vessels and macrophages; thymic corpuscle hyperplasia; and interstitial widening of the thymic lobules. The transcriptomic analysis results revealed that the Gene Ontology functions of DEGs were enriched primarily in biological processes such as angiogenesis, regulation of angiogenesis and positive regulation of cell migration. Moreover, greater numbers of blood vessels and macrophages were observed in the thymus in PRRSV-infected than control piglets. KEGG pathway enrichment analysis revealed that the DEGs were significantly enriched in the Toll-like receptor signaling pathway, chemokine signaling pathway, IL-17 signaling pathway and TNF signaling pathway. The expression of TLR8, IRF5, the chemokines CCL2, CCL3L1 and CCL5; and their receptors CCR1, CCR2 and CCR5 was significantly up-regulated in PRRSV infection, thus suggesting that these cytokines were associated with the pathological processes of thymus injury.

Dispersion of synonymous codon usage patterns in hepatitis E virus genomes derived from various hosts

Hepatitis E virus (HEV) is an important zoonotic pathogen infecting a wide range of host species. It has a positive-sense, single-stranded RNA genome encoding three open reading frames (ORFs). Synonymous codon usages of viruses essentially determine their survival and adaptation to susceptible hosts. To better understand the interplay between the ever-expanding host range and synonymous codon usages of HEV, we quantified the dispersion of synonymous codon usages of HEV genomes isolated from different hosts via V_s calculation and information entropy. HEV ORFs show species-specific synonymous codon usage patterns. Ruminant-derived HEV ORFs own the most synonymous codons with stable usage patterns (V_s value <0.1) which leads to the stable overall codon usage patterns (R value being close to zero). Swine-derived HEV ORFs own more concentrated synonymous codons than those from wild boar. Compared with HEV strains isolated from other hosts, the human-derived HEV exhibits a distinct pattern at the overall codon usage (R < 0). Generally, ORF1 contains more synonymous codons with stable usage patterns (V_s  < 0.1) than those of ORFs 2 and 3. Moreover, ORF3 contains more synonymous codons with varied patterns (V_s  > 1.0) than ORFs 1 and 2. The host factor serving as one of the evolutionary dynamics probably influences synonymous codon usage patterns of the HEV genome. Taken together, synonymous codons with stable usage patterns in ORF1 might help to sustain the infection, while that with varied usage patterns in ORF3 may facilitate cross-species infection and expand the host range.

MiR-142-5p/FAM134B Axis Manipulates ER-Phagy to Control PRRSV Replication

Porcine reproductive and respiratory syndrome virus (PRRSV) can replicate its RNA genome in endoplasmic reticulum (ER) and utilize ER to facilitate its assembly and maturation. To maintain ER homeostasis, host cells initiate reticulophagy (known as ER-phagy) to effectively digest the stressed ER. In this study, we found that PRRSV infection subverted ER-phagy by downregulating ER-phagy receptor FAM134B. PRRSV-induced miR-142-5p directly targeted FAM134B and significantly promoted PRRSV replication. Meanwhile, siRNA-mediated depletion of FAM134B protein and overexpression of FAM134B mutant protein significantly disrupted ER-phagy and facilitated PRRSV replication. Furthermore, our results showed that FAM134B-mediated ER-phagy activated type I interferon signaling to inhibit PRRSV replication. Overall, this study reveals the important role of ER-phagy in PRRSV replication in a FAM134B-dependent manner. Our findings provide an insight into the pathogenesis of PRRSV and offer a theoretical basis for further development of antiviral therapeutic targets.

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.

Molecular detection and characterization of highly pathogenic porcine reproductive and respiratory syndrome virus from a natural outbreak in wild pigs, Mizoram, India

This study reports for the first time a natural outbreak of highly pathogenic porcine reproductive and respiratory syndrome (HP-PRRS) caused by HP-PRRS virus (HP-PRRSV) in wild pigs characterized by sudden onset of depression, anorexia, respiratory distress, and high fever. The disease has caused severe haemorrhagic pneumonia, haemorrhagic lymphadenitis, enlarged spleen with areas of infarction, and petechial haemorrhages on the myocardium and on the surface of kidneys. HP-PRRSV was detected in representative tissue samples by reverse transcription-PCR, and the field strain was isolated in the MA104 cell line. The phylogenetic analyses based on the whole genome sequences and nucleotide sequences of open reading frame 5 (ORF5) gene showed close grouping with the subtype IV of lineage 8/8.7 of PRRSV II, which represents the HP-PRRSV strains that predominate in the pig population of China since 2010. The amino acid sequence analysis of the ORF5 gene revealed the replacement of leucine (L) at position 39 to isoleucine (I) in the primary neutralizing epitope. Among the four potential N glycosylation sites, the N34 was mutated and found to be restricted to only three N glycosylation sites. The present findings have indicated that HP-PRRSV can cause fatal outbreaks and may emerge as a major threat to the wild pig population.

Structural Characterization of the Helicase nsp10 Encoded by Porcine Reproductive and Respiratory Syndrome Virus

Currently, an effective therapeutic treatment for porcine reproductive and respiratory syndrome virus (PRRSV) remains elusive. PRRSV helicase nsp10 is an important component of the replication transcription complex that plays a crucial role in viral replication, making nsp10 an important target for drug development. Here, we report the first crystal structure of full-length nsp10 from the arterivirus PRRSV, which has multiple domains: an N-terminal zinc-binding domain (ZBD), a 1B domain, and helicase core domains 1A and 2A. Importantly, our structural analyses indicate that the conformation of the 1B domain from arterivirus nsp10 undergoes a dynamic transition. The polynucleotide substrate channel formed by domains 1A and 1B adopts an open state, which may create enough space to accommodate and bind double-stranded RNA (dsRNA) during unwinding. Moreover, we report a unique C-terminal domain structure that participates in stabilizing the overall helicase structure. Our biochemical experiments also showed that deletion of the 1B domain and C-terminal domain significantly reduced the helicase activity of nsp10, indicating that the four domains must cooperate to contribute to helicase function. In addition, our results indicate that nidoviruses contain a conserved helicase core domain and key amino acid sites affecting helicase function, which share a common mechanism of helicase translocation and unwinding activity. These findings will help to further our understanding of the mechanism of helicase function and provide new targets for the development of antiviral drugs.IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is a major respiratory disease agent in pigs that causes enormous economic losses to the global swine industry. PRRSV helicase nsp10 is a multifunctional protein with translocation and unwinding activities and plays a vital role in viral RNA synthesis. Here, we report the first structure of full-length nsp10 from the arterivirus PRRSV at 3.0-Å resolution. Our results show that the 1B domain of PRRSV nsp10 adopts a novel open state and has a unique C-terminal domain structure, which plays a crucial role in nsp10 helicase activity. Furthermore, mutagenesis and structural analysis revealed conservation of the helicase catalytic domain across the order Nidovirales (families Arteriviridae and Coronaviridae). Importantly, our results will provide a structural basis for further understanding the function of helicases in the order Nidovirales.

Genome-wide analysis of long noncoding RNA and mRNA profiles in PRRSV-infected porcine alveolar macrophages

Porcine reproductive and respiratory syndrome (PRRS), which is caused by PRRS virus (PRRSV), is one of the most globally devastating swine diseases. It is essential to develop new strategy to control PRRS via an understanding of mechanisms that PRRSV utilizes to interfere with the host's innate immunity. In this study, we deeply sequenced and analyzed long noncoding RNA (lncRNA) and mRNA expression profiles of the porcine alveolar macrophages (PAMs) after PRRSV infection. 126 lncRNAs and 753 mRNAs were differentially expressed between PRRSV-infected and control PAMs. The co-expressed genes of down-regulated lncRNAs were significantly enriched within NF-kappa B and toll-like receptor signaling pathways. Co-expression network analysis indicated that part of the dysregulated lncRNAs associated with the interferon-induced genes. These dysregulated lncRNAs may play an important role in the host's innate immune responses to PRRSV infection. However, further research is required to characterize the function of these lncRNAs.

MicroRNA transcriptome analysis of poly I:C-stimulated and PRRSV-infected porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe reproductive failure in sows, respiratory diseases, and high mortality in piglets, which results in serious economic losses to the swine industry worldwide. Previous studies have described that PRRSV could suppress the host immune system and had antiapoptotic activity in its initial phase of infection. Polyinosinic-polycytidylic acid (poly I:C), a synthesized analogue of viral double-strand RNA, activates innate immunity responses and induces apoptosis in cells. Therefore, we performed miRNA transcriptome analysis of poly I:C-stimulated and PRRSV-infected porcine alveolar macrophages (PAMs) using deep sequencing technology, to compare the different miRNA profiles between the statuses of innate immune activation and inactivation. After sequencing, 267 known mature miRNAs and 64 novel miRNAs were observed in PAMs, and a total of 197 miRNAs were significantly differently expressed in poly I:C-stimulated PAMs, compared with mock control cells. Thirty-three of them were also significantly alerted in PRRSV-infected PAMs. This indicated that PRRSV only slightly alerted the miRNA expression profile of host cells compared with poly I:C-stimulated PAMs, which confirmed that PRRSV could suppress host innate immune responses during the early stages of infection. Among the differentially expressed miRNAs, we found that ssc-miR-27b-3p could significantly inhibit PRRSV RNA and protein replication in MARC-145 cells and PAMs. Its antiviral mechanism needs further research in the future.

Genomic sequence and virulence of a novel NADC30-like porcine reproductive and respiratory syndrome virus isolate from the Hebei province of China

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), which results in immense economic losses in the swine industry. Outbreaks of disease caused by NADC30-like PRRSV are of great concern in China. Here, a novel variant, NADC30-like PRRSV strain HB17A, was analyzed and its pathogenicity in pigs was examined. The full-length genome sequence of HB17A shared 83.6-95.1% nucleotide similarity with NADC30-like and NADC30 PRRSV without any gene insertions, but with a unique 2-amino acid deletion in Nsp2. A phylogenetic analysis showed that HB17A clustered with NADC30 strains. Different degrees of variation in the signal peptide, transmembrane region (TM), primary neutralizing epitope (PNE), non-neutral epitopes, and N-glycosylation sites were observed in GP5. Challenge experiments showed that HB17A infection resulted in persistent fever, moderate respiratory clinical signs, low levels of viremia and viral loads in serum, and mild gross and microscopic lung lesions. Moreover, IFN-γ, IL-6, and IL-10 cytokine levels were significantly elevated in serum, but the levels of IFN-α and IL-2 were similar to those of the negative controls. HB17A was less pathogenic but was secreted longer in nasal discharge than HP-PRRSV FZ06A. Our findings indicate that HB17A is a novel NADC30-like strain with certain deletions and mutations but with no evidence of genomic recombination. This strain exhibits intermediate virulence in pigs. This research will be help define the evolutionary characteristics of Chinese NADC30-like PRRSV.

Genomic characterization and pathogenic study of two porcine reproductive and respiratory syndrome viruses with different virulence in Fujian, China

Two strains of porcine reproductive and respiratory syndrome virus (PRRSV) were isolated in 2006 and 2016 and designated as FZ06A and FZ16A, respectively. Inoculation experiments showed that FZ06A caused 100% morbidity and 60% mortality, while FZ16A caused 100% morbidity without death. By using genomic sequence and phylogenetic analyses, close relationships between a Chinese highly pathogenic PRRSV strain and the FZ06A and FZ16A strains were observed. Based on the achieved results, multiple genomic variations in Nsp2, a unique N-glycosylation site (N³³→K³³), and a K151 amino acid (AA) substitution for virulence in the GP5 of FZ16A were detected; except the 30 AA deletion in the Nsp2-coding region. Inoculation experiments were conducted and weaker virulence of FZ16A than FZ06A was observed. Based on our results, a 30 AA deletion in the Nsp2-coding region is an unreliable genomic indicator of a high virulence PRRSV strain. The Nsp2 and GP5 differences, in addition to the virulence difference between these two highly pathogenic PRRSV strains, have the potential to be used to establish a basis for further study of PRRSV virulence determinants and to provide data useful in the development of vaccines against this economically devastating disease.

Recent advances in veterinary applications of structural vaccinology

The deployment of effective veterinary vaccines has had a major impact on improving food security and consequently human health. Effective vaccines were essential for the global eradication of Rinderpest and the control and eradication of foot-and-mouth disease in some regions of the world. Effective vaccines also underpin the development of modern intensive food production systems such as poultry and aquaculture. However, for some high consequence diseases there are still significant challenges to develop effective vaccines. There is a strong track record in veterinary medicine of early adoption of new technologies to produce vaccines. Here we provide examples of new technologies to interrogate B cell responses and using structural biology to improve antigens.

Targeted Delivery of GP5 Antigen of PRRSV to M Cells Enhances the Antigen-Specific Systemic and Mucosal Immune Responses

Efficient delivery of antigens through oral immunization is a first and critical step for successful induction of mucosal immunity, which can provide protection against pathogens invading the mucosa. Membranous/microfold cells (M cells) within the mucosa can transcytose internalized antigen without degradation and thus play an important role in initiating antigen-specific mucosal immune responses through inducing secretory IgA production. In this research, we modified poly (D, L-lactide-co-glycolide) (PLGA) nanoparticles (NPs) with Ulex europaeus agglutinin 1 (UEA-1) and successfully prepared an oral vaccine delivery system, UEA-1/PLGA NPs. PLGA NPs were prepared using a standard double emulsion solvent evaporation technique, which can protect the entrapped PRRSV DNA vaccine [pcDNA3.1-SynORF5 (synthetic ORF5)] or subunit vaccine ORF5-encoded glycoprotein (GP5) from exposure to the gastrointestinal (GI) tract and release the plasmids in a controlled manner. With UEA-1 modification, the UEA-1/PLGA NPs can be effectively transported by M-cells. We investigated immune response induced by UEA-1/PLGA-SynORF5 or UEA-1/PLGA-GP5 following inoculation in mice and piglets. Compared with PLGA-SynORF5 or PLGA-GP5 NPs, UEA-1/PLGA-SynORF5, or UEA-1/PLGA-GP5 NPs stimulated significantly increased serum IgG levels and augmented intestinal IgA levels in mice and piglets (P < 0.05). Our findings indicate UEA-1/PLGA NPs can be applied as a promising and universally robust oral vaccine delivery system.

Identification of two antiviral inhibitors targeting 3C-like serine/3C-like protease of porcine reproductive and respiratory syndrome virus and porcine epidemic diarrhea virus

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Identification of antiviral inhibitors that target PRRSV nsp4 (3CLSP).

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Suppression of PRRSV replication by compounds 2, 3 and 5.

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Compounds 2 and 3 exhibit broad-spectrum antiviral activity against PEDV.

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Compounds 2 and 3 may block the combination of proteases and substrates.

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine epidemic diarrhea virus (PEDV) are highly virulent and contagious porcine pathogens that cause tremendous economic losses to the swine industry worldwide. Currently, there is no effective treatment for PRRSV and PEDV, and commercial vaccines do not induce sterilizing immunity. In this study, we screened a library of 1000 compounds and identified two specific inhibitors, designated compounds 2 and 3, which target the PRRSV 3C-like serine protease (3CLSP). First, we evaluated the inhibitory effects of compounds 2 and 3 on PRRSV 3CLSP activity. Next, we determined the anti-PRRSV capacity of compounds 2 and 3 in MARC-145 cells and obtained EC₅₀ and CC₅₀ values of 57 μM (CC₅₀ = 479.9 μM) and 56.8 μM (CC₅₀ = 482.8 μM), respectively. Importantly, compounds 2 and 3 also targeted the PEDV 3C-like protease (3CL protease) and inhibited PEDV replication, showing EC₅₀ and CC₅₀ values of 100 μM (CC₅₀ = 533.8 μM) and 57.9 μM (CC₅₀ = 522.3 μM), respectively. Finally, our results indicated that the active sites (His39 in 3CLSP and His41 in 3CL protease) were conservative, and contacted compounds 2 and 3 via hydrogen bonds and hydrophobic forces in the putative substrate-binding models. In summary, compounds 2 and 3 exhibit broad-spectrum antiviral activity and may facilitate the development of antiviral drugs against PRRSV and PEDV.

Cloning and molecular characterization of the ORF5 gene from a PRRSV-SN strain from Southwest China

To monitor the genetic variation of PRRSV, the ORF5 gene of the PRRSV-SN strain found in Suining City, Sichuan Province, was cloned and sequenced. The results showed that the PRRSV-SN strain was a highly pathogenic PRRSV (HP-PRRSV) variant strain with the North American (NA) genotype. Homology analysis showed that the ORF5 gene of the PRRSV-SN isolate shared 89.4% (86.5%) nucleotide (amino acid) sequence similarity with the North American strain VR-2332, 98.8% (96%) similarity with JXA1, and 63.8% (57.7%) similarity with the European type representative strain Lelystad virus. Phylogenetic analysis showed that PRRSV-SN belongs to the NA genotype and has the same subtype as other highly pathogenic PRRSV strains. Amino acid sequence analysis showed that compared with the VR2332 strain, PRRSV-SN has different degrees of variation in the signal peptide, transmembrane region (TM), primary neutralizing epitope (PNE), non-neutral epitopes and N-glycosylation sites. Antigenicity analysis showed that the PRRSV-SN ORF5 gene products and JXA1 have similar antigenic characteristics, and the antigenic epitopes are mainly located in aa30-39, aa50-60, aa128-141, aa146-155 and aa161-183 regions. In contrast, the antigenic characteristics of PRRSV-SN are quite different from those of the VR2332 strain. The main differences were that the PRRSV-SN strain was significantly narrower than the VR2332 strain in the aa30-39 and the aa50-60 regions but was significantly wider in the aa136-141 region. The results of this study showed that the epidemic strains that cause PRRSV outbreaks in the farm are still mainly JXA1 variants, but due to the more frequent use of live vaccine immunizations, the genes of the PRRSV epidemic strain still show constant variation. Vaccination with live PRRSV should be reduced, and surveillance of PRRSV strains should be enhanced.

Porcine Interferon Stimulated Gene 12a Restricts Porcine Reproductive and Respiratory Syndrome Virus Replication in MARC-145 Cells

Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe losses in the global pig industry. In the present study, we investigated the molecular characterization of porcine interferon stimulated gene 12a (ISG12A) and confirmed its anti-PRRSV ability for the first time. We found that porcine ISG12A was localized in mitochondria and significantly decreased the number of cells in G2/S phase. Porcine ISG12A mRNA was up-regulated in cells/tissues of Tongcheng (TC) pigs and Large White (LW) pigs after PRRSV challenge. More importantly, the ectopic overexpression of ISG12A could significantly suppress PRRSV replication at 24, 36 and 48 h post challenge (hpc), which was confirmed by detecting PRRSV ORF7 mRNA with quantitative reverse transcription polymerase chain reaction (qRT-PCR) and PRRSV N protein with indirect immunofluorescence assay (IFA) in MARC-145 cells. Meanwhile, knockdown of endogenic ISG12A could obviously facilitate PRRSV replication in MARC-145 cells at 36 hpc. The results will lead to a better understanding of the interaction between host immune system and PRRSV, which may help us develop novel therapeutic tools to control PRRSV.

Genetic characterization of 11 porcine reproductive and respiratory syndrome virus isolates in South China from 2014 to 2015

Background

Porcine reproductive and respiratory syndrome (PRRS) has leaded to an enormous loss per year to the swine industry, its etiology porcine reproductive and respiratory syndrome virus (PRRSV) is a highly mutated virus in pigs. To fully understand the genetic characteristics of PRRSV genome in South China, this study collected the lung samples infected with PRRSV in Guangdong and Hainan province from 2014 to 2015 and tried to isolate the PRRSV. Finally, the complete genomes of isolated strains were sequenced and analyzed.

Methods

Virus isolation was performed in MARC-145 cells. The 13 fragments of PRRSV genome were amplified by RT-PCR and the complete PRRSV genome sequence was obtained by SeqMan program of DNASTAR7.0 software. Nucleotide and deduced amino acid (AA) sequences of NSP2 and ORF5 were aligned using the MegAlign program of DNASTAR7.0 software to determine sequence homology. A phylogenetic tree was constructed using MEGA5.2 software with the neighbor-joining method to analyze the evolutionary relationship.

Results

11 PRRSV strains were isolated in South China from 2014 to 2015. All the isolated strains clustered into subgenotype V along with the HP-PRRSV representative strains JXA1, HuN4 and JXwn06. The subgenotype V was furtherly divided into two groups. AA sequence alignment analysis indicated that all the isolated strains had 1 AA deletion and 29 AA continuous deletion at position 481 and 533-561. Notably, GDHY strain had another 120 AA continuous deletion at position 629-748. All the isolated strains had an A137S mutation in the residue A137 of GP5 which was considered to differentiate vaccine strains. All the isolated strains had a L39I mutation in the primary neutralizing epitope (PNE) of GP5. Except GDHZ had a N34T mutation, all the other isolated strains had conserved N30, N44 and N51 glycosylation sites in the four potential N-glycosylation sites (N30, N34, N44 and N51) of GP5.

Conclusions

Our study showed that the prevalent strains in this region were highly pathogenic PRRS virus-like. Moreover, one new strain having another 120 amino acids continuous deletion except the discontinuous 30 (29+1) amino acids deletion in NSP2 region had emerged. Besides, the isolated strains had extensive amino acids substitutions in the putative signal, extravirion and intravirion regions of GP5. These results showed that PRRSV has undergone extensive variation in South China, providing some theoretical basis for researching effective vaccince to better controling the PRRSV in this area.

Assessment of the efficacy of two novel DNA vaccine formulations against highly pathogenic Porcine Reproductive and Respiratory Syndrome Virus

Since May 2006, a highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has emerged and prevailed in mainland China, affecting over 2 million pigs. Commercial PRRSV killed and modified live vaccines cannot provide complete protection against HP-PRRSV due to genetic variation. Development of more effective vaccines against the emerging HP-PRRSV is urgently required. In our previous studies, two formulations of DNA vaccines (pcDNA3.1-PoIFN-λ1-SynORF5 and BPEI/PLGA-SynORF5) based on the HP-PRRSV were constructed and shown to induce enhanced humoral and cellular immune responses in mice. The objective of this study was to evaluate the immune response induced by these novel formulations in piglets. PcDNA3.1-PoIFN-λ1-SynORF5 and BPEI/PLGA-SynORF5 vaccines induced significantly enhanced GP5-specific antibody and PRRSV-specific neutralizing antibody in pigs compared with the pcDNA3.1-SynORF5 parental construct. Though IFN-γ levels and lymphocyte proliferation responses induced by the two DNA vaccine formulations were comparable to that induced by the pcDNA3.1-SynORF5 construct, each of the novel formulations provided efficient protection against challenge with HP-PRRSV. Non-severe clinical signs and rectal temperatures were observed in pigs immunized with BPEI/PLGA-SynORF5 compared with other groups. Thus, these novel DNA constructs may represent promising candidate vaccines against emerging HP-PRRSV.

Porcine GPX1 enhances GP5-based DNA vaccination against porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has been causing huge economic losses to the swine industry worldwide. Commercial PRRSV vaccines including killed and modified live ones are available. However the protective efficacy of these vaccines is incomplete. Thus, it is in urgent need to develop safer and more effective PRRSV vaccines. In this study, we constructed a recombinant plasmid co-expressing porcine glutathione peroxidase-1 (GPX1) and the envelope glycoprotein (GP5) encoding-gene of PRRSV (pcDNA3.1-GPX1-LSynORF5), and investigated the immune response induced following inoculation of mice and piglets. Significantly enhanced GP5-specific antibody, PRRSV-specific neutralizing antibody, IFN-γ level, as well as lymphocyte proliferation response, were induced in mice and pigs immunized with the DNA construct encoding GPX1 and GP5 compared with those inoculated with a construct encoding PRRSV GP5 only (pcDNA3.1-SynORF5). The enhanced cellular immune response in pigs induced by pcDNA3.1-GPX1-LSynORF5 was comparable to that induced by the attenuated virus vaccine JXA1-R, although the humoral immune response induced by the plasmid was much lower than the response induced by JXA1-R. Following the challenge with highly pathogenic PRRSV, less-severe clinical signs and rectal temperatures were observed in pigs immunized with the GPX1-GP5 construct compared with the control group. However, the viraemia of groups immunized with plasmid was more severe than that inoculated with JXA1-R, and it is likely that this could be attributed to the poor humoral response induced by the GPX1-GP5 construct. These results demonstrated that inclusion of GPX1 in a PRRSV DNA vaccine leads to an adjuvant effect, enhancing the humoral and cellular responses following vaccination.

A Dimerization-Dependent Mechanism Drives the Endoribonuclease Function of Porcine Reproductive and Respiratory Syndrome Virus nsp11

Porcine reproductive and respiratory syndrome virus (PRRSV) RNA endoribonuclease nsp11 belongs to the XendoU superfamily and plays a crucial role in arterivirus replication. Here, we report the first crystal structure of the arterivirus nsp11 protein from PRRSV, which exhibits a unique structure and assembles into an asymmetric dimer whose structure is completely different from the hexameric structure of coronavirus nsp15. However, the structures of the PRRSV nsp11 and coronavirus nsp15 catalytic domains were perfectly superimposed, especially in the “active site loop” (His129 to His144) and “supporting loop” (Val162 to Thr179) regions. Importantly, our biochemical data demonstrated that PRRSV nsp11 exists mainly as a dimer in solution. Mutations of the major dimerization site determinants (Ser74 and Phe76) in the dimerization interface destabilized the dimer in solution and severely diminished endoribonuclease activity, indicating that the dimer is the biologically functional unit. In the dimeric structure, the active site loop and supporting loop are packed against one another and stabilized by monomer-monomer interactions. These findings may help elucidate the mechanism underlying arterivirus replication and may represent great potential for the development of antiviral drugs.

IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is a member of the family Arteriviridae, order Nidovirales. PRRSV is a major agent of respiratory diseases in pigs, causing tremendous economic losses to the swine industry worldwide. The PRRSV nsp11 endoribonuclease plays a vital role in arterivirus replication, but its precise roles and mechanisms of action are poorly understood. Here, we report the first dimeric structure of the arterivirus nsp11 from PRRSV at 2.75-Å resolution. Structural and biochemical experiments demonstrated that nsp11 exists mainly as a dimer in solution and that nsp11 may be fully active as a dimer. Mutagenesis and structural analysis revealed NendoU active site residues, which are conserved throughout the order Nidovirales (families Arteriviridae and Coronaviridae) and the major determinants of dimerization (Ser74 and Phe76) in Arteriviridae. Importantly, these findings may provide a new structural basis for antiviral drug development.

Differences of immune responses between Tongcheng (Chinese local breed) and Large White pigs after artificial infection with highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is one of the severest infectious diseases of pigs throughout the world. Pigs of different breeds infected with PRRS virus (PRRSV) have been reported to vary in their immune responses. Here, the differences of immune responses to highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) were investigated by artificially infecting Tongcheng (TC) pigs (a Chinese indigenous breed) and Large White (LW) pigs with PRRSV WUH3. Compared to LW pigs, TC pigs showed less severe symptoms and lower level of viral load. The routine blood test results indicated that TC pigs were relatively steady in terms of erythrocyte, leukocyte and platelet. Additionally, PRRSV infection induced higher IFN-γ activity in TC pigs, but stimulated an excessive level of IL-10 and IL-12p40 in LW pigs. Our study provides direct evidence that TC pigs have stronger resistance to early PRRSV infection than LW pigs, suggesting that the resistance of pigs to PRRSV is likely associated with breed differences.

Challenges in Veterinary Vaccine Development and Immunization

In approaching the development of a veterinary vaccine, researchers must choose from a bewildering array of options that can be combined to enhance benefit. The choice and combination of options is not just driven by efficacy, but also consideration of the cost, practicality, and challenges faced in licensing the product. In this review we set out the different choices faced by veterinary vaccine developers, highlight some issues, and propose some pressing needs to be addressed.

The nonstructural protein 11 of porcine reproductive and respiratory syndrome virus inhibits NF-κB signaling by means of its deubiquitinating activity

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PRRSV Nsp11 possesses DUB activity.

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Nsp11 specifically cleave K48-linked, but not K63-linked polyubiquitin chains.

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DUB activity is responsible for the ability of Nsp11 to inhibit NF-κB activation.

Since its emergence in the late 1980s, porcine reproductive and respiratory syndrome (PRRS) has been devastating the swine industry worldwide. The causative agent is an Arterivirus, referred to as PRRS virus (PRRSV). The pathogenic mechanisms of PRRS are poorly understood, but are believed to correlate with the ability of PRRSV to inhibit immune responses of the host. However, precisely how the virus is capable of doing so remains obscure. In this study, we showed that PRRSV infection led to reduced ubiquitination of cellular proteins. Screening all of the 12 nonstructural proteins (Nsps) encoded by PRRSV revealed that, apart from the Nsp2 which contains the deubiqintinating (DUB) ovarian tumor (OTU) domain, Nsp11, which encodes a unique and conserved endoribonuclease (NendoU) throughout the Nidovirus order, also possesses DUB activity. In vivo assay demonstrated that Nsp11 specifically removed lysine 48 (K48)-linked polyubiquitin chains and the conserved sites C112, H144, D173, K180, and Y219 were critical for its DUB activity. Remarkably, DUB activity was responsible for the capacity of Nsp11 to inhibit nuclear factor κB (NF-κB) activation. Mutations abrogating the DUB activity of Nsp11 toward K48-linked polyubiquitin chains of IκBα nullified the suppressive effect on NF-κB. Our data add Nsp11 to the list of DUBs encoded by PRRSV and uncover a novel mechanism by which PRRSV cripples host innate immune responses.

Complete Genomic Characterization of Porcine Reproductive and Respiratory Syndrome Virus Strain HB-XL

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causal agent of a serious disease of swine. Here, we report the genome sequence of PRRSV strain HB-XL isolated from a pig farm with a clinical outbreak of porcine reproductive and respiratory syndrome. The genome is 15,323 bp long and has nine open reading frames (GenBank: KP162169). Comparative and phylogenetic analysis showed that HB-XL belongs to the highly pathogenic PRRSV (HP-PRRSV) subfamily in the family PRRSV. The viral nonstructural protein 2 (Nsp2) of the HB-XL strain contained 30 discontinuous amino acid (AA) deletions relative to that of the Nsp2 of the VR2332 strain. The AA substitutions R13 and R151 suggested high virulence of the HB-XL strain. The unique mutations in glycoprotein 5 (GP5) and Nsp2 revealed that HB-XL might be a novel variant PRRSV strain recombined with vaccine strains. However, the low morbidity and mortality in the pig herd from which HB-XL was isolated indicate that the virulence of the virus was weak, so it has potential as a future vaccine strain.

Inactivated and subunit vaccines against porcine reproductive and respiratory syndrome: Current status and future direction

Within a few years of its emergence in the late 1980s, the PRRS virus had spread globally to become the foremost infectious disease concern for the pork industry. Since 1994, modified live-attenuated vaccines against porcine reproductive and respiratory syndrome virus (PRRSV-MLV) have been widely used, but have failed to provide complete protection against emerging and heterologous field strains of the virus. Moreover, like many other MLVs, PRRSV-MLVs have safety concerns including vertical and horizontal transmission of the vaccine virus and several documented incidences of reversion to virulence. Thus, the development of efficacious inactivated vaccines is warranted for the control and eradication of PRRS. Since the early 1990s, researchers have been attempting to develop inactivated PRRSV vaccines, but most of the candidates have failed to elicit protective immunity even against homologous virus challenge. Recent research findings relating to both inactivated and subunit candidate PRRSV vaccines have shown promise, but they need to be pursued further to improve their heterologous efficacy and cost-effectiveness before considering commercialization. In this comprehensive review, we provide information on attempts to develop PRRSV inactivated and subunit vaccines. These includes various virus inactivation strategies, adjuvants, nanoparticle-based vaccine delivery systems, DNA vaccines, and recombinant subunit vaccines produced using baculovirus, plant, and replication-deficient viruses as vector vaccines. Finally, future directions for the development of innovative non-infectious PRRSV vaccines are suggested. Undoubtedly there remains a need for novel PRRSV vaccine strategies targeted to deliver cross-protective, non-infectious vaccines for the control and eradication of PRRS.

Production and evaluation of virus-like particles displaying immunogenic epitopes of porcine reproductive and respiratory syndrome virus (PRRSV)

Porcine reproductive and respiratory syndrome (PRRS) is the most significant infectious disease currently affecting the swine industry worldwide. Several inactivated and modified live vaccines (MLV) have been developed to curb PRRSV infections. However, the efficacy and safety of these vaccines are unsatisfactory, and hence, there is a strong demand for the development of new PRRS universal vaccines. Virus-like particle (VLP)-based vaccines are gaining increasing acceptance compared to subunit vaccines, as they present the antigens in a more veritable conformation and are readily recognized by the immune system. Hepatitis B virus core antigen (HBcAg) has been successfully used as a carrier for more than 100 viral sequences. In this study, hybrid HBcAg VLPs were generated by fusion of the conserved protective epitopes of PRRSV and expressed in E. coli. An optimized purification protocol was developed to obtain hybrid HBcAg VLP protein from the inclusion bodies. This hybrid HBcAg VLP protein self-assembled to 23-nm VLPs that were shown to block virus infection of susceptible cells when tested on MARC 145 cells. Together with the safety of non-infectious and non-replicable VLPs and the low cost of production through E. coli fermentation, this hybrid VLP could be a promising vaccine candidate for PRRS.

MiR-506 inhibits PRRSV replication in MARC-145 cells via CD151

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important diseases of swine, which is caused by PRRS virus (PRRSV). CD151, one of PRRSV entry mediators, determines the cell susceptibility for PRRSV. Emerging evidence indicates that the host microRNAs (miRNAs) play key roles in modulating virus infection and viral pathogenesis. In the present study, targeting porcine CD151 miRNAs were identified, and their function during PRRSV infection in MARC-145 cells was further verified. We found that miR-506 could directly target porcine CD151 3'-UTR mRNA by luciferase reporter assay. Overexpression of miR-506 significantly decreased CD151 expression at both mRNA and protein levels. Furthermore, overexpression of miR-506 reduced cellular PRRSV replication and virus release in MARC-145 cells. Our results suggested that miR-506 could inhibit PRRSV replication by directly targeting PRRSV receptor of CD151 in MARC-145 cells. However, the molecular mechanisms of miR-506 and its function in vivo need further investigation.

Protective humoral immune response induced by an inactivated porcine reproductive and respiratory syndrome virus expressing the hypo-glycosylated glycoprotein 5

Porcine reproductive and respiratory syndrome (PRRS) causes significant economic losses to the swine industry worldwide. Although inactivated and live vaccines are commercially available for the control of PRRS, both types of vaccine have not always proven successful in terms of generating a protective immune response, particularly in the case of inactivated vaccines. In this study, we tested whether an inactivated vaccine could induce a humoral immune response to PRRS during a homologous challenge. Amino acid substitutions were introduced into glycoprotein (GP) 5 of the FL12 strain of the PRRS virus (PRRSV) using site-directed mutagenesis with a pFL12 infectious clone. The substitutions led to double deglycosylation in the putative glycosylation moieties on GP5. The mutant virus was subsequently inactivated with binary ethylenimine. The efficacy of the inactivated mutant virus was compared with that of the inactivated wild-type PRRSV. Only the inactivated mutant PRRSV induced serum neutralizing antibodies at six weeks post-vaccination. The group that was administered the inactivated mutant virus twice exhibited a significantly increased neutralizing antibody titer after a challenge with the virulent homologous strain and exhibited more rapid clearing of viremia compared to other groups, including the groups that were administered either the inactivated mutant or wild-type virus only once and the group that was administered the inactivated wild-type virus twice. Histopathological examination of lung tissue sections revealed that the group that was administered the inactivated mutant virus twice exhibited significantly thinner alveolar septa, whereas the thickness of the alveolar septa of the other groups were markedly increased due to lymphocyte infiltration. These results indicated that the deglycosylation of GP5 enhanced the immunogenicity of the inactivated mutant PRRSV and that twice administrations of the inactivated mutant virus conferred better protection against the homologous challenge. These findings suggest that the inactivated PRRSV that expresses a hypo-glycosylated GP5 is a potential inactivated vaccine candidate and a valuable tool for controlling PRRS for the swine industry.

Recombinant Kluyveromyces lactis expressing highly pathogenic porcine reproductive and respiratory syndrome virus GP5 elicits mucosal and cell-mediated immune responses in mice

Currently, killed-virus and modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccines are used to control porcine reproductive and respiratory syndrome. However, both types of vaccines have inherent drawbacks; accordingly, the development of novel PRRSV vaccines is urgently needed. Previous studies have suggested that yeast possesses adjuvant activities, and it has been used as an expression vehicle to elicit immune responses to foreign antigens. In this report, recombinant Kluyveromyces lactis expressing GP5 of HP-PRRSV (Yeast-GP5) was generated and immune responses to this construct were analyzed in mice. Intestinal mucosal PRRSV-specific sIgA antibody and higher levels of IFN-γ in spleen CD4⁺ and CD8⁺ T cells were induced by oral administration of Yeast-GP5. Additionally, Yeast-GP5 administered subcutaneously evoked vigorous cell-mediated immunity, and PRRSV-specific lymphocyte proliferation and IFN-γ secretion were detected in the splenocytes of mice. These results suggest that Yeast-GP5 has the potential for use as a vaccine for PRRSV in the future.

Construction and immunogenicity of DNA vaccines encoding fusion protein of porcine IFN- λ 1 and GP5 gene of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has been mainly responsible for the catastrophic economic losses in pig industry worldwide. The commercial vaccines only provide a limited protection against PRRSV infection. Thus, the focus and direction is to develop safer and more effective vaccines in the research field of PRRS. The immune modulators are being considered to enhance the effectiveness of PRRSV vaccines. IFN-λ1 belongs to type III interferon, a new interferon family. IFN-λ1 is an important cytokine with multiple functions in innate and acquired immunity. In this study, porcine IFN-λ1 (PoIFN-λ1) was evaluated for its adjuvant effects on the immunity of a DNA vaccine carrying the GP5 gene of PRRSV. Groups of mice were immunized twice at 2-week interval with 100 μg of the plasmid DNA vaccine pcDNA3.1-SynORF5, pcDNA3.1-PoIFN-λ1-SynORF5, and the blank vector pcDNA3.1, respectively. The results showed that pcDNA3.1-PoIFN-λ1-SynORF5 can significantly enhance GP5-specific ELISA antibody, PRRSV-specific neutralizing antibody, IFN-γ level, and lymphocyte proliferation rather than the responses induced by pcDNA3.1-SynORF5. Therefore, type III interferon PoIFN-λ1 could enhance the immune responses of DNA vaccine of PRRSV, highlighting the potential value of PoIFN-λ1 as a molecular adjuvant in the prevention of PRRSV infection.

Molecular evolution of porcine reproductive and respiratory syndrome virus isolates from central China

To investigate the genetic diversity of prevailing porcine reproductive and respiratory syndrome virus (PRRSV) in Henan Province of China, 61 ORF5 gene sequences, originating from Henan Province during 2003-2010, were subjected to amino acid variation and phylogenetic analysis. The analyzed PRRSV ORF5 sequences carried evidence of one unique recombination event. Phylogenetic analysis revealed that all Henan isolates belonged to type 2 genotype and were divided into two subgroups. The dominant isolates had shifted from subgroup 1 to subgroup 2 during 2003-2010. Amino acid variation analysis of the glycoprotein 5 revealed that Henan PRRSV strains tended to accumulate more substitutions within the N-terminus and hypervariable region. Selective pressure analysis revealed evidence that some ORF5 sites have likely evolved in response to immune pressure.

Immunogenic and protective properties of GP5 and M structural proteins of porcine reproductive and respiratory syndrome virus expressed from replicating but nondisseminating adenovectors

Porcine reproductive and respiratory syndrome virus (PRRSV) is responsible for significant economic losses in the porcine industry. Currently available commercial vaccines do not allow optimal and safe protection. In this study, replicating but nondisseminating adenovectors (rAdV) were used for the first time in pigs for vaccinal purposes. They were expressing the PRRSV matrix M protein in fusion with either the envelope GP5 wild-type protein (M-GP5) which carries the major neutralizing antibody (NAb)-inducing epitope or a mutant form of GP5 (M-GP5m) developed to theoretically increase the NAb immune response. Three groups of fourteen piglets were immunized both intramuscularly and intranasally at 3-week intervals with rAdV expressing the green fluorescent protein (GFP, used as a negative control), M-GP5 or M-GP5m. Two additional groups of pigs were primed with M-GP5m-expressing rAdV followed by a boost with bacterially-expressed recombinant wild-type GP5 or were immunized twice with a PRRSV inactivated commercial vaccine. The results show that the rAdV expressing the fusion proteins of interest induced systemic and mucosal PRRSV GP5-specific antibody response as determined in an ELISA. Moreover the prime with M-GP5m-expressing rAdV and boost with recombinant GP5 showed the highest antibody response against GP5. Following PRRSV experimental challenge, pigs immunized twice with rAdV expressing either M-GP5 or M-GP5m developed partial protection as shown by a decrease in viremia overtime. The lowest viremia levels and/or percentages of macroscopic lung lesions were obtained in pigs immunized twice with either the rAdV expressing M-GP5m or the PRRSV inactivated commercial vaccine.

MiR-125b reduces porcine reproductive and respiratory syndrome virus replication by negatively regulating the NF-κB pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has been devastating the swine industry worldwide since the late 1980s. To investigate the impact of cellular microRNAs (miRNAs) on the replication of PRRSV, we screened 10 highly conserved miRNAs implicated in innate immunity or antiviral function and identified miR-125b as an inhibitor of PRRSV replication. Virus titer and western blot assays demonstrated that miR-125b reduced PRRSV replication and viral gene expression in a dose-dependent manner in both MARC-145 cell line and primary porcine alveolar macrophages. Mechanistically, miR-125b did not target the PRRSV genome. Rather, it inhibited activation of NF-κB, which we found to be required for PRRSV replication. PRRSV, in turn, down-regulated miR-125b expression post-infection to promote viral replication. Collectively, miR-125b is an antiviral host factor against PRRSV, but it is subject to manipulation by PRRSV. Our study reveals an example of manipulation of a cellular miRNA by an arterivirus to re-orchestrate host gene expression for viral propagation and sheds new light on targeting host factors to develop effective control measures for PRRS.

Porcine reproductive and respiratory syndrome virus vaccines: current status and strategies to a universal vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of PRRS, the most significant infectious disease currently affecting swine industry worldwide. In the United States alone, the economic losses caused by PRRS amount to more than 560 million US dollars every year. Due to immune evasion strategies and the antigenic heterogeneity of the virus, current commercial PRRSV vaccines (killed-virus and modified-live vaccines) are of unsatisfactory efficacy, especially against heterologous infection. Continuous efforts have been devoted to develop better PRRSV vaccines. Experimental PRRSV vaccines, including live attenuated vaccines, recombinant vectors expressing PRRSV viral proteins, DNA vaccines and plant-made subunit vaccines, have been developed. However, the genetic and antigenic heterogeneity of the virus limits the value of almost all of the PRRSV vaccines tested. Developing a universal vaccine that can provide broad protection against circulating PRRSV strains has become a major challenge for current vaccine development. This paper reviews current status of PRRSV vaccine development and discusses strategies to develop a universal PRRSV vaccine.

Molecular characterization of porcine SARM1 and its role in regulating TLRs signaling during highly pathogenic porcine reproductive and respiratory syndrome virus infection in vivo

Toll-like receptors (TLRs) are important pattern-recognition receptors (PRRs) that trigger innate immune response and mediate acquired immunity. Evidence has shown that SARM1 (sterile-α and TIR motif containing protein 1) is one of five TIR domain-containing adaptor proteins involved in TLRs signaling transduction. In the present study, a full-length cDNA sequence was cloned for the porcine SARM1 gene, which contains nine exons. Using the radiation hybrid mapping approach, we assigned the porcine gene to SSC12 q13. Under the normal condition, porcine SARM1 was highly expressed in brain and spleen. Polyinosinic-polycytidylic acid (poly (I:C)) weakly induced the porcine SARM1 expression in the early stimulation. We found that porcine SARM1 protein is localized in mitochondria and attenuates NF-κB activation induced by stimulation and infection. The quantitative real-time PCR (Q-PCR) analysis showed that the expression of porcine SARM1 significantly decreased in several tissues of Tongcheng pigs infected with highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV). Gene-interaction network analysis for porcine SARM1 in porcine alveolar macrophages (PAMs) showed that down-regulation of SARM1 gene in infected Tongcheng pig may modulate TRIF-depend TLRs signaling and regulate the expression of disease-resistant genes and inflammatory genes. Our findings provide evidence that porcine SARM1 may play an important role in immune regulation with PRRSV infection.

Enhanced immunogenicity induced by an alphavirus replicon-based pseudotyped baculovirus vaccine against porcine reproductive and respiratory syndrome virus

Pseudotyped baculovirus has emerged as a promising vector for vaccine development and gene therapy. Alphaviruses, such as Semliki Forest virus (SFV), have also received considerable attention for use as expression vectors because of their self-replicating properties. In this study, pseudotyped baculovirus containing the hybrid cytomegalovirus (CMV) promoter/SFV replicon was used as a vector to co-express the GP5 and M proteins of porcine reproductive and respiratory syndrome virus (PRRSV). The immunogenicity of the resulting recombinant baculovirus (BV-SFV-5m6) was compared with the pseudotyped baculovirus vaccine (BV-CMV-5m6), in which the expression of GP5 and M were driven by the CMV promoter only. In vitro, BV-SFV-5m6 exhibited enhanced expression of foreign proteins and also caused apoptosis in transduced cells. After immunization in BALB/c mice, BV-SFV-5m6 induced strong GP5-specific ELISA antibodies and neutralizing antibodies against homologous and heterologous viruses, along with dose sparing. Further analysis of the cell-mediated immune response showed that BV-SFV-5m6 elicited a Th1-dominant immune response that was greater than that elicited by BV-CMV-5m6. Taken together, the results of this study indicate that a baculovirus containing the hybrid CMV promoter/alphavirus replicon can be utilized as an alternative strategy to develop an efficacious vaccine against PRRSV infection.

Porcine reproductive and respiratory syndrome virus nonstructural protein 2 contributes to NF-κB activation

Background

Nuclear factor-kappaB (NF-κB) is an inducible transcription factor that plays a key role in inflammation and immune responses, as well as in the regulation of cell proliferation and survival. Previous studies by our group and others have demonstrated that porcine reproductive and respiratory syndrome virus (PRRSV) infection could activate NF-κB in MARC-145 cells and alveolar macrophages. The nucleocapsid (N) protein was identified as an NF-κB activator among the structural proteins encoded by PRRSV; however, it remains unclear whether the nonstructural proteins (Nsps) contribute to NF-κB activation. In this study, we identified which Nsps can activate NF-κB and investigated the potential mechanism(s) by which they act.

Results

By screening the individual Nsps of PRRSV strain WUH3, Nsp2 exhibited great potential to activate NF-κB in MARC-145 and HeLa cells. Overexpression of Nsp2 induced IκBα degradation and nuclear translocation of NF-κB. Furthermore, Nsp2 also induced NF-κB-dependent inflammatory factors, including interleukin (IL)-6, IL-8, COX-2, and RANTES. Compared with the Nsp2 of the classical PRRSV strain, the Nsp2 of highly pathogenic PRRSV (HP-PRRSV) strains that possess a 30 amino acid (aa) deletion in Nsp2 displayed greater NF-κB activation. However, the 30-aa deletion was demonstrated to not be associated with NF-κB activation. Further functional domain analyses revealed that the hypervariable region (HV) of Nsp2 was essential for NF-κB activation.

Conclusions

Taken together, these data indicate that PRRSV Nsp2 is a multifunctional protein participating in the modulation of host inflammatory response, which suggests an important role of Nsp2 in pathogenesis and disease outcomes.

Cellular membrane cholesterol is required for porcine reproductive and respiratory syndrome virus entry and release in MARC-145 cells

Cholesterol represents one of the key constituents of small, dynamic, sterol- and sphingolipid-enriched domains on the plasma membrane. It has been reported that many viruses depend on plasma membrane cholesterol for efficient infection. In this study, the role of the plasma membrane cholesterol in porcine reproductive and respiratory syndrome virus (PRRSV) infection of MARC-145 cells was investigated. Pretreatment of MARC-145 cells with methyl-β-cyclodextrin (MβCD), a drug used to deplete cholesterol from cellular membrane, significantly reduced PRRSV infection in a dose-dependent manner. This inhibition was partially reversed by supplementing exogenous cholesterol following MβCD treatment, suggesting that the inhibition of PRRSV infection was specifically mediated by removal of cellular cholesterol. Further detailed studies showed that depletion of cellular membrane cholesterol significantly inhibited virus entry, especially virus attachment and release. These results indicate that the presence of cholesterol in the cellular membrane is a key component of PRRSV infection.

Induction of autophagy enhances porcine reproductive and respiratory syndrome virus replication

Autophagy is an evolutionarily conserved lysosome-dependent degradation pathway that acts in the maintenance of cellular homeostasis and plays important functions in viral replication and pathogenesis. In this study, we investigated the role of autophagy in the replication of porcine reproductive and respiratory syndrome virus (PRRSV), an agent that has caused devastating losses in the international swine industry since the late 1980s. Using protein quantification and microscopy, we observed that PRRSV infection results in LC3-I/II conversion, an increased accumulation of punctate GFP-LC3-expressing cells, and a higher number of autophagosome-like double-membrane vesicles in the cytoplasm of host cells. Inhibition of autophagy using 3-methyladenine (3-MA) or small interfering RNAs targeting ATG7 and Beclin-1 led to a significant reduction in PRRSV titers and protein expression. Conversely, induction of autophagy by rapamycin resulted in increased viral replication. These results demonstrate that PRRSV infection induces autophagy which, in turn, enhances viral replication efficiency.

Molecular characterization of transcriptome-wide interactions between highly pathogenic porcine reproductive and respiratory syndrome virus and porcine alveolar macrophages in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) infects mainly the porcine alveolar macrophages (PAMs) and causes porcine reproductive and respiratory syndrome (PRRS). Previous studies have analyzed the global gene expression profiles of lung tissue in vivo and PAMs in vitro following infection with PRRSV, however, transcriptome-wide understanding of the interaction between highly pathogenic PRRSV (HP-PRRSV) and PAMs in vivo has not yet been established. In this study, we employed Affymetrix microarrays to investigate the gene expression patterns of PAMs isolated from Tongcheng piglets (a Chinese indigenous breed) after infection with HP-PRRSV. During the infection, Tongcheng piglets exhibited typical clinical signs, e.g. fever, asthma, coughing, anorexia, lethargy and convulsion, but displayed mild regional lung damage at 5 and 7 dpi. Microarray analysis revealed that HP-PRRSV infection has affected PAMs in expression of the important genes involved in cytoskeleton and exocytosis organization, protein degradation and folding, intracellular calcium and zinc homeostasis. Several potential antiviral strategies might be employed in PAMs, including upregulating IFN-induced genes and increasing intracellular zinc ion concentration. And inhibition of the complement system likely attenuated the lung damage during HP-PRRSV infection. Transcriptomic analysis of PAMs in vivo could lead to a better understanding of the HP-PRRSV-host interaction, and to the identification of novel antiviral therapies and genetic components of swine tolerance/susceptibility to HP-PRRS.