Recombinant swine beta interferon protects swine alveolar macrophages and MARC-145 cells from infection with Porcine reproductive and respiratory syndrome virus

Antiviral activity and underlying mechanisms of baicalin against porcine reproductive and respiratory syndrome virus in vitro

Porcine reproductive and respiratory syndrome (PRRS) is a major challenge for the global swine industry, causing huge economic losses worldwide. To date, there are no effective measures to prevent and control the spread of PRRS virus (PRRSV). Baicalin (BA) is a natural flavonoid with various pharmacological effects, including antiviral, anti-inflammatory, antioxidant and immunomodulatory. Here, we demonstrate that BA exhibits potent anti-PRRSV activity in vitro, BA concentrations in the range of 5-20 μg/mL significantly inhibited PRRSV infection in a dose-dependent manner and were independent of PRRSV strain. Mechanistically, BA inhibited PRRSV replication by directly interacting with virions, thereby affecting multiple stages of the virus life cycle. Meanwhile, the preventive effect of BA on PRRSV could be realized by inhibiting CD151 and CD163 expression. Furthermore, BA reduced the PRRSV-induced expression of PAMs cytokines (IFN-α, IL-6, IL-8, and TNF-α), suggesting that BA-induced antiviral cytokines may help BA inhibit PRRSV infection. Taken together, BA can be used as an inhibitor of PRRSV infection in vitro, which provides a theoretical basis for the clinical application of BA and the prevention and control of PRRSV infection, which is worthy of further in vivo studies in swine.

Defining correlates of protection for mammalian livestock vaccines against high-priority viral diseases

Enhancing livestock biosecurity is critical to safeguard the livelihoods of farmers, global and local economies, and food security. Vaccination is fundamental to the control and prevention of exotic and endemic high-priority infectious livestock diseases. Successful implementation of vaccination in a biosecurity plan is underpinned by a strong understanding of correlates of protection-those elements of the immune response that can reliably predict the level of protection from viral challenge. While correlates of protection have been successfully characterized for many human viral vaccines, for many high-priority livestock viral diseases, including African swine fever and foot and mouth disease, they remain largely uncharacterized. Current literature provides insights into potential correlates of protection that should be assessed during vaccine development for these high-priority mammalian livestock viral diseases. Establishment of correlates of protection for biosecurity purposes enables immune surveillance, rationale for vaccine development, and successful implementation of livestock vaccines as part of a biosecurity strategy.

FADD promotes type I interferon production to suppress porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome (PRRS) is an epidemic animal infectious disease worldwide, causing huge economic losses to the global swine industry. Fas-associated death domain (FADD) was previously reported to be an adaptor protein that functions in transferring the apoptotic signals regulated by the death receptors. In the current study, we unravel its unidentified role in promoting type I interferon (IFN) production during PRRS virus (PRRSV) infection. We identified that FADD inhibited PRRSV infection via promotion of type I IFN transcription. Overexpression of FADD suppressed the replication of PRRSV, while knockout of FADD increased viral titer and nucleocapsid protein expression. Mechanistically, FADD promoted mitochondrial antiviral signaling protein (MAVS)-mediated production of IFN-β and some IFN-stimulated genes (ISGs). Furthermore, FADD exerted anti-PRRSV effects in a MAVS-dependent manner and increased the type I IFN signaling during PRRSV infection. This study highlights the importance of FADD in PRRSV replication, which may have implications for the future control of PRRS.

Recombinant Lactococcus lactis Expressing Human LL-37 Prevents Deaths from Viral Infections in Piglets and Chicken

Novel antibiotic substitutes are increasingly in demand in the animal husbandry industry. An oral recombinant Lactococcus lactis (L. lactis) expressing human LL-37 (oral LL-37) was developed and its safety and antiviral effectiveness in vivo was tested. In addition to impairing liposome integrity, LL-37 polypeptide from recombinant L. lactis could prevent the host cell infection by a variety of viruses, including recombinant SARS, SARS-CoV-2, Ebola virus, and vesicular stomatitis virus G. Subchronic toxicity studies performed on Sprague-Dawley rats showed that no cumulative toxicity was found during short-term intervention. Oral LL-37 treatment after the onset of fever could reduce mortality in piglets infected with porcine reproductive and respiratory syndrome virus. Moreover, body weight gain of piglets receiving treatment was progressively restored, and nucleic acid positive rebound was not undetected after discontinuation. Oral LL-37 consistently increased the lifespan of chickens infected with Newcastle viruses. These findings suggested a potential use of recombinantly modified microorganisms in veterinary medicine.

Porcine reproductive and respiratory syndrome virus degrades DDX10 via SQSTM1/p62-dependent selective autophagy to antagonize its antiviral activity

Porcine reproductive and respiratory syndrome virus (PRRSV) is a typical immunosuppressive virus devastating the global swine industry. DEAD-box helicases (DDXs) are a family of ATP-dependent RNA helicases that are predominantly implicated in modulating cellular RNA metabolism. Meanwhile, a growing number of studies have suggested that some DDXs are associated with innate immunity and virus infection, so they are considered potential antiviral targets. Herein, we screened 40 DDXs and found that ectopic expression of DDX10 exhibited a significant anti-PRRSV effect, while DDX10 knockdown promoted PRRSV proliferation. Further analysis revealed that DDX10 positively regulates type I interferon production, which may contribute to its anti-PRRSV effect. Interestingly, PRRSV infection promoted DDX10 translocation from the nucleus to the cytoplasm for macroautophagic/autophagic degradation to block the antiviral effect of DDX10. By screening PRRSV-encoded proteins, we found that the viral envelope (E) protein interacted with DDX10. In line with the autophagic degradation of DDX10 during PRRSV infection, E protein could induce autophagy and reduce DDX10 expression in wild-type cells, but not in ATG5 or ATG7 knockout (KO) cells. When further screening the cargo receptors for autophagic degradation, we found that SQSTM1/p62 (sequestosome 1) interacted with both DDX10 and E protein, and E protein-mediated DDX10 degradation was almost entirely blocked in SQSTM1 KO cells, demonstrating that E protein degrades DDX10 by promoting SQSTM1-mediated selective autophagy. Our study reveals a novel mechanism by which PRRSV escapes host antiviral innate immunity through selective autophagy, providing a new target for developing anti-PRRSV drugs.Abbreviations: ACTB: actin beta; ATG: autophagy related; co-IP: co-immunoprecipitation; CQ: chloroquine; DDX10: DEAD-box helicase 10; E: envelope; EGFP: enhanced green fluorescent protein; hpi: hours post infection; hpt: hours post transfection; IFA: indirect immunofluorescence assay; IFN-I: type I IFN; IFNB/IFN-β: interferon beta; IRF3: interferon regulatory factor 3; ISGs: interferon-stimulated genes; KO: knockout; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; mAb: monoclonal antibody; MOI: multiplicity of infection; NBR1: NBR1 autophagy cargo receptor; NFKB/NF-κB: nuclear factor kappa B; OPTN: optineurin; ORF: open reading frame; PRRSV: porcine reproductive and respiratory syndrome virus; SeV: sendai virus; siRNA: small interfering RNA; SQSTM1/p62: sequestosome 1; TCID50: 50% tissue culture infective dose; WT: wild type.

A New Long Noncoding RNA, MAHAT, Inhibits Replication of Porcine Reproductive and Respiratory Syndrome Virus by Recruiting DDX6 To Bind to ZNF34 and Promote an Innate Immune Response

Long noncoding RNAs (lncRNAs) have increasingly been recognized as being integral to cellular processes, including the antiviral immune response. Porcine reproductive and respiratory syndrome virus (PRRSV) is costly to the global swine industry. To identify PRRSV-related lncRNAs, we performed RNA deep sequencing and compared the profiles of lncRNAs in PRRSV-infected and uninfected Marc-145 cells. We identified a novel lncRNA called MAHAT (maintaining cell morphology-associated and highly conserved antiviral transcript; LTCON_00080558) that inhibits PRRSV replication. MAHAT binds and negatively regulates ZNF34 expression by recruiting and binding DDX6, an RNA helicase forming a complex with ZNF34. Inhibition of ZNF34 expression results in increased type I interferon expression and decreased PRRSV replication. This finding reveals a novel mechanism by which PRRSV evades the host antiviral innate immune response by downregulating the MAHAT-DDX6-ZNF34 pathway. MAHAT could be a host factor target for antiviral therapies against PRRSV infection. IMPORTANCE Long noncoding RNAs (lncRNAs) play important roles in viral infection by regulating the transcription and expression of host genes, and interferon signaling pathways. Porcine reproductive and respiratory syndrome virus (PRRSV) causes huge economic losses in the swine industry worldwide, but the mechanisms of its pathogenesis and immunology are not fully understood. Here, a new lncRNA, designated MAHAT, was identified as a regulator of host innate immune responses. MAHAT negatively regulates the expression of its target gene, ZNF34, by recruiting and binding DDX6, an RNA helicase, forming a complex with ZNF34. Inhibition of ZNF34 expression increases type I interferon expression and decreases PRRSV replication. This finding suggests that MAHAT has potential as a new target for developing antiviral drugs against PRRSV infection.

Modulation of Alveolar Macrophages by Postimmunobiotics: Impact on TLR3-Mediated Antiviral Respiratory Immunity

Beneficial microbes with immunomodulatory capacities (immunobiotics) and their non-viable forms (postimmunobiotics) could be effectively utilized in formulations towards the prevention of respiratory viral infections. In this study, novel immunobiotic strains with the ability to increase antiviral immunity in porcine alveolar macrophages were selected from a library of Lactobacillus gasseri. Postimmunobiotics derived from the most remarkable strains were also evaluated in their capacity to modulate the immune response triggered by Toll-like receptor 3 (TLR3) in alveolar macrophages and to differentially regulate TLR3-mediated antiviral respiratory immunity in infant mice. We provide evidence that porcine alveolar macrophages (3D4/31 cells) are a useful in vitro tool for the screening of new antiviral immunobiotics and postimmunobiotics by assessing their ability to modulate the expression IFN-β, IFN-λ1, RNAseL, Mx2, and IL-6, which can be used as prospective biomarkers. We also demonstrate that the postimmunobiotics derived from the Lactobacillus gasseri TMT36, TMT39 and TMT40 (HK36, HK39 or HK40) strains modulate the innate antiviral immune response of alveolar macrophages and reduce lung inflammatory damage triggered by TLR3 activation in vivo. Although our findings should be deepened and expanded, the results of the present work provide a scientific rationale for the use of nasally administered HK36, HK39 or HK40 to beneficially modulate TLR3-triggerd respiratory innate immune response.

DEAD-Box RNA Helicase 21 (DDX21) Positively Regulates the Replication of Porcine Reproductive and Respiratory Syndrome Virus via Multiple Mechanisms

The porcine reproductive and respiratory syndrome virus (PRRSV) remains a persistent hazard in the global pig industry. DEAD (Glu-Asp-Ala-Glu) box helicase 21 (DDX21) is a member of the DDX family. In addition to its function of regulating cellular RNA metabolism, DDX21 also regulates innate immunity and is involved in the replication cycle of some viruses. However, the relationship between DDX21 and PRRSV has not yet been explored. Here, we found that a DDX21 overexpression promoted PRRSV replication, whereas knockdown of DDX21 reduced PRRSV proliferation. Mechanistically, DDX21 promoted PRRSV replication independently of its ATPase, RNA helicase, and foldase activities. Furthermore, overexpression of DDX21 stabilized the expressions of PRRSV nsp1α, nsp1β, and nucleocapsid proteins, three known antagonists of interferon β (IFN-β). Knockdown of DDX21 activated the IFN-β signaling pathway in PRRSV-infected cells, suggesting that the effect of DDX21 on PRRSV-encoded IFN-β antagonists may be a driving factor for its contribution to viral proliferation. We also found that PRRSV infection enhanced DDX21 expression and promoted its nucleus-to-cytoplasm translocation. Screening PRRSV-encoded proteins showed that nsp1β interacted with the C-terminus of DDX21 and enhanced the expression of DDX21. Taken together, these findings reveal that DDX21 plays an important role in regulating PRRSV proliferation through multiple mechanisms.

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

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

Screening of Porcine Innate Immune Adaptor Signaling Revealed Several Anti-PRRSV Signaling Pathways

Porcine reproductive and respiratory syndrome virus (PRRSV) causes PRRS and is known to effectively suppress host innate immunity. The current strategies for controlling PRRSV are limited and complete understanding of anti-PRRSV innate immunity is needed. Here, we utilized nine porcine innate immune signaling adaptors which represent all currently known innate immune receptor signaling pathways for screening of anti-PRRSV activity. The analysis of PRRSV N gene transcription and protein expression both suggested that the multiple ectopic adaptors exhibited varying degrees of anti-PRRSV activities, with TRIF and MAVS most effective. To better quantify the PRRSV replication, the GFP signal of PRRSV from reverse genetics were measured by flow cytometry and similarly varying anti-PRRSV activities by different signaling adaptors were observed. Based on the screening data, and considering the importance of viral nucleic acid in innate immune response, endogenous TRIF, MAVS and STING were selected for further examination of anti-PRRSV activity. Agonist stimulation assay showed that MAVS and STING signaling possessed significant anti-PRRSV activities, whereas siRNA knockdown assay showed that TRIF, MAVS and STING are all involved in anti-PRRSV activity, with TLR3-TRIF displaying discrepancy in anti-PRRSV infection. Nevertheless, our work suggests that multiple pattern recognition receptor (PRR) signaling pathways are involved in anti-PRRSV innate immunity, which may have implications for the development of future antiviral strategies.

Transcriptome sequencing analysis of porcine alveolar macrophages infected with PRRSV strains to elucidate virus pathogenicity and immune evasion strategies

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) causes a serious disease to the swine industry worldwide. To understand the mechanisms of HP-PRRSV infection, RNA-seq-based transcriptome analyses were performed on porcine alveolar macrophages (PAMs) infected with a HP-PRRSV strain (TJ), a less virulent strain of a classical lineage (CH-1a), and a vaccine strain TJM-F92. Gene ontology, Kyoto Encyclopedia of Genes and Genomes analyses indicate that TJM-F92 led to significant up-regulation of gene expression for proteins associated with membrane-bound organelles. The differentially expressed genes of HP-PRRSV TJ-infected PAM cells were up-regulated in the special G-protein coupled receptor. The six cytokines were tested by real time Reverse Transcription-Polymerase Chain Reaction (RT-PCR). The relative expression levels showed the same trend of expression difference. Significant up-regulation of TMEM173 plays an important role in the cytosolic DNA-sensing pathway and the RIG-I-like receptor signaling pathway in TJM-F92 infected PAM cells. These data provide new insight into PRRSV pathogenicity and immune evasion strategies.

The Innate Immune DNA Sensing cGAS-STING Signaling Pathway Mediates Anti-PRRSV Function

Porcine reproductive and respiratory syndrome virus (PRRSV) modulates host innate immunity which plays a key role against PRRSV infection. As a RNA virus, PRRSV is mainly sensed by innate immune RNA receptors, whereas the role of innate immune DNA sensors in the PRRSV infection has not been elucidated. Here, we investigated the roles of DNA sensing cGAS-STING pathway in both PRRSV infected Marc-145 cells and porcine macrophages. The results show that in Marc-145 cells, the stable expression of STING with or without stimulations exhibited anti-PRRSV activity, and STING knockout heightened PRRSV infection. In CD163-3D4/21 porcine macrophages, either expression of STING or stimulation of cGAS-STING signaling obviously suppressed PRRSV infection, whereas in STING knockdown macrophages, the PRRSV infection was upregulated. Our results clearly demonstrate that the host cGAS-STING signal exerts an important antiviral role in PRRSV infection.

Cryptotanshinone protects porcine alveolar macrophages from infection with porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRS virus (PRRSV) infection, imposes enormous economic impact to the world pork industry. Currently there is no effective treatment to prevent PRRSV infection in swine. We report that the natural compound cryptotanshinone (Cpt) effectively inhibits the infection of various strains of PRRSV to porcine alveolar macrophages (PAMs), the primary cell target of PRRSV in vivo. Mechanistically, Cpt inhibits the activation of signal transducer and activator of transcription 3 (STAT3), and blocks the interleukin 10 (IL-10) stimulated as well as the basal level CD163 expression in PAMs. Cpt-treatment of PAMs is effective when applied either before or after PRRSV infection, with the combined pre- and post-PRRSV infection treatment resulting in the most significant, dose-dependent inhibition of PRRSV infection. Cpt inhibited both type I/II PRRSV infection in PAMs. Our study identified a new approach to prevent/treat PRRSV infection of pigs with natural compounds.

Two coupled mutations abolished the binding of CEBPB to the promoter of CXCL14 that displayed an antiviral effect on PRRSV by activating IFN signaling

Porcine reproductive and respiratory syndrome (PRRS) is the most economically important infectious disease of pigs worldwide. Our previous study revealed that Tongcheng (TC) pigs display higher resistance to PRRS than Largewhite (LW) pigs, but the genetic mechanism remains unknown. Here, we first confirmed that CXCL14 was downregulated in lungs and porcine alveolar macrophages (PAMs) responding to PRRS virus (PRRSV) infection, but the decline in LW pigs was more obvious than that in TC pigs. Then, we found that the overexpression of CXCL14 activated type-I interferon (IFN-I) signaling by upregulating interferon beta (IFNB), which plays a major role in the antiviral effect. To further decipher the mechanism underlying its differential expression, we characterized the core promoter of CXCL14 as being located from -145 to 276 bp of the transcription start site (TSS) and identified two main haplotypes that displayed significant differential transcriptional activities. We further identified two coupled point mutations that altered the binding status of CEBPB and were responsible for the differential expression in TC and LW pigs. The regulatory effect of CEBPB on CXCL14 was further confirmed by RNA interference (RNAi) and chromatin immunoprecipitation (ChIP), providing crucial clues for deciphering the mechanism of CXCL14 downregulation in unusual conditions. The present study revealed the potential antiviral effect of CXCL14, occurring via activation of interferon signaling, and suggested that CXCL14 contributes to the PRRS resistance of TC pigs.

Molecular and Cellular Mechanisms for PRRSV Pathogenesis and Host Response to Infection

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PRRSV has evolved to arm with various strategies to modify host antiviral response.

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Viral modulation of homeostatic cellular processes provides favorable conditions for PRRSV survival during infection.

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PRRSV modulation of cellular processes includes pathways for interferons, apoptosis, microRNAs, cytokines, autophagy, and viral genome recombination.

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous amounts of economic losses to the swine industry for more than three decades, but its control is still unsatisfactory. A significant amount of information is available for host cell-virus interactions during infection, and it is evident that PRRSV has evolved to equip various strategies to disrupt the host antiviral system and provide favorable conditions for survival. The current study reviews viral strategies for modulations of cellular processes including innate immunity, apoptosis, microRNAs, inflammatory cytokines, and other cellular pathways.

miR-382-5p promotes porcine reproductive and respiratory syndrome virus (PRRSV) replication by negatively regulating the induction of type I interferon

Previous studies have indicated that inhibition of type I interferon production may be an important reason for porcine reproductive and respiratory syndrome virus (PRRSV) to achieve immune escape, revealing the mechanism of inhibiting the production of type I interferon will help design novel strategies for controlling PRRS. Here, we found that PRRSV infection upregulated the expression of miR-382-5p, which in turn inhibited polyI:C-induced the production of type I interferon by targeting heat shock protein 60 (HSP60), thus facilitating PRRSV replication in MARC-145 cells. Furthermore, we found that HSP60 could interact with mitochondrial antiviral signaling protein (MAVS), an important signal transduction protein for inducing production of type I interferon, and promote polyI:C-mediated the production of type I interferon in a MAVS-dependent manner. Finally, we also found that HSP60 could inhibit PRRSV replication in a MAVS-dependent manner, which indicated that HSP60 was a novel antiviral protein against PRRSV replication. In conclusion, the study demonstrated that miR-382-5p was upregulated during PRRSV infection and may promote PRRSV replication by negatively regulating the production of type I interferon, which also indicated that miR-382-5p and HSP60 might be the potential therapeutic targets for anti-PRRSV.

IFI16 Inhibits Porcine Reproductive and Respiratory Syndrome Virus 2 Replication in a MAVS-Dependent Manner in MARC-145 Cells

Porcine reproductive and respiratory syndrome virus (PRRSV) is a single-stranded positive-sense RNA virus, and the current strategies for controlling PRRSV are limited. Interferon gamma-inducible protein 16 (IFI16) has been reported to have a broader role in the regulation of the type I interferons (IFNs) response to RNA and DNA viruses. However, the function of IFI16 in PRRSV infection is unclear. Here, we revealed that IFI16 acts as a novel antiviral protein against PRRSV-2. IFI16 could be induced by interferon-beta (IFN-β). Overexpression of IFI16 could significantly suppress PRRSV-2 replication, and silencing the expression of endogenous IFI16 by small interfering RNAs led to the promotion of PRRSV-2 replication in MARC-145 cells. Additionally, IFI16 could promote mitochondrial antiviral signaling protein (MAVS)-mediated production of type I interferon and interact with MAVS. More importantly, IFI16 exerted anti-PRRSV effects in a MAVS-dependent manner. In conclusion, our data demonstrated that IFI16 has an inhibitory effect on PRRSV-2, and these findings contribute to understanding the role of cellular proteins in regulating PRRSV replication and may have implications for the future antiviral strategies.

Fetal cytokine response to porcine reproductive and respiratory syndrome virus-2 infection

To understand the fetal immune response to porcine reproductive and respiratory virus-2 (PRRSV) and to evaluate the association with fetal viability, pregnant gilts were challenged on gestation day 85 and euthanized 21 days post infection. Based on preservation status and viral load in serum and thymus, fetuses were classified as either uninfected-viable (UNIF), high viral load viable (HV-VIA), or high viral load meconium stained (HV-MEC) and were compared with age matched control (CON) fetuses derived from mock infected gilts. Gene expression of IFNB, IFNG, CCL2, CCL5, CXCL10 and IL10, were all found to be significantly upregulated in the thymus and spleen of both high viral load groups. UNIF fetuses remained largely unaffected, with only small upregulations in IFNA and IL10 in the thymus, and IFNA, CCL5 and CXCL10 in the spleen. Regarding fetal viability, expression of CCL5 was significantly elevated in the thymus and spleen of HV-MEC compared to HV-VIA fetuses. The concentrations of IFNα, IFNγ, TNFα and CCL2 were elevated in the sera of all infected fetuses, whereas IFNβ was below the detection limit in all fetal sera. Additional gene expression analysis in the thymus showed significant downregulation of CDK1, CDK2 and CDK4, and upregulation of the inhibitor CDKN1A, suggesting altered regulation of cell cycle progression. Collectively, these results show near complete compartmentalization of the fetal immune response to infected fetuses and suggest this immune response is not a major contributor to fetal death.

Porcine reproductive and respiratory syndrome virus counteracts type I interferon-induced early antiviral state by interfering IRF7 activity

Porcine reproductive and respiratory syndrome (PRRS) is an economically important disease with a significant impact on the pig industry. It is caused by PRRS virus (PRRSV), which predominantly infects and replicates in porcine pulmonary alveolar macrophages (PAMs). We pretreated PAMs with porcine interferon (IFN)-α to induce an antiviral state within the cells and subsequently infected them with highly pathogenic PRRSV. Changes in global gene expression in IFN-α-pretreated PAMs in response to PRRSV infection were determined by RNA-sequence analysis and confirmed by real-time PCR. We found that IRF7 and other antiviral interferon stimulating genes (ISG)s were suppressed by PRRSV infection. Further studies demonstrated that PRRSV could down-regulate the expression of IRF7 by the non-structure protein 7 (nsp7). In conclusion, PRRSV infection had a strong immunosuppressive effect of IFN. PRRSV nsp7 inhibits the expression of IRF7, thereby down-regulating the expression of IFN and downstream ISGs and facilitated the virus to replicate.

Identification of the RNA Pseudoknot within the 3' End of the Porcine Reproductive and Respiratory Syndrome Virus Genome as a Pathogen-Associated Molecular Pattern To Activate Antiviral Signaling via RIG-I and Toll-Like Receptor 3

Once infected by viruses, cells can detect pathogen-associated molecular patterns (PAMPs) on viral nucleic acid by host pattern recognition receptors (PRRs) to initiate the antiviral response. Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), characterized by reproductive failure in sows and respiratory diseases in pigs of different ages. To date, the sensing mechanism of PRRSV has not been elucidated. Here, we reported that the pseudoknot region residing in the 3' untranslated regions (UTR) of the PRRSV genome, which has been proposed to regulate RNA synthesis and virus replication, was sensed as nonself by retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 3 (TLR3) and strongly induced type I interferons (IFNs) and interferon-stimulated genes (ISGs) in porcine alveolar macrophages (PAMs). The interaction between the two stem-loops inside the pseudoknot structure was sufficient for IFN induction, since disruption of the pseudoknot interaction powerfully dampened the IFN induction. Furthermore, transfection of the 3' UTR pseudoknot transcripts in PAMs inhibited PRRSV replication in vitro Importantly, the predicted similar structures of other arterivirus members, including equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV), also displayed strong IFN induction activities. Together, in this work we identified an innate recognition mechanism by which the PRRSV 3' UTR pseudoknot region served as PAMPs of arteriviruses and activated innate immune signaling to produce IFNs that inhibit virus replication. All of these results provide novel insights into innate immune recognition during virus infection.IMPORTANCE PRRS is the most common viral disease in the pork industry. It is caused by PRRSV, a positive single-stranded RNA virus, whose infection often leads to persistent infection. To date, it is not yet clear how PRRSV is recognized by the host and what is the exact mechanism of IFN induction. Here, we investigated the nature of PAMPs on PRRSV and the associated PRRs. We found that the 3' UTR pseudoknot region of PRRSV, which has been proposed to regulate viral RNA synthesis, could act as PAMPs recognized by RIG-I and TLR3 to induce type I IFN production to suppress PRRSV infection. This report is the first detailed description of pattern recognition for PRRSV, which is important in understanding the antiviral response of arteriviruses, especially PRRSV, and extends our knowledge on virus recognition.

Porcine reproductive and respiratory syndrome virus suppresses post-transcriptionally the protein expression of IFN-β by upregulating cellular microRNAs in porcine alveolar macrophages in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized to inhibit the response of type I interferon (IFN) both in vivo and in vitro. However, the post-transcriptional mechanism by which PRRSV suppresses type I IFN induction in virus-infected host cells remains unclear. The present study first demonstrated that PRRSV inhibited post-transcriptionally the protein induction of IFN-β in primary porcine alveolar macrophages (PAMs) during early infection, and the inhibition effect mediated by the Chinese highly pathogenic (HP)-PRRSV was stronger. Next, we analyzed the cellular microRNA (miRNA)-modulated protein expression of porcine IFN-β by dual firefly/Renilla luciferase reporter assay, transfection of miRNA mimics and inhibitor assay and polyinosinic-polycytidylic acid (poly I:C) treatment of PAMs, showing that porcine miRNAs including let-7b, miR-26a, miR-34a and miR-145 are able to inhibit IFN-β protein expression in primary PAMs by directly targeting sequences within the porcine IFN-β 3'UTR locating at 160-181, 9-31, 27-47 and 12-32 bp, respectively. Finally, we confirmed that let-7b, miR-26a, miR-34a and miR-145, were upregulated in PRRSV-infected PAMs early in vitro, and the expression level of these miRNAs in HP-PRRSV JXwn06-infected PAMs were higher than those in low pathogenic PRRSV HB-1/3.9-infected PAMs. The endogenous cellular miRNA-mediated inhibition of IFN-β induction in PRRSV-infected PAMs early could be relieved by miRNA antagonists. Taken together, our findings suggest for the first time that PRRSV can suppress post-transcriptionally protein expression of IFN-β by upregulating cellular miRNAs in PAMs in vitro, providing novel insight into mechanisms in relation to the PRRSV-mediated immunomodulation of porcine innate immunity.

Inhibition of NF-κB activity by the porcine epidemic diarrhea virus nonstructural protein 1 for innate immune evasion

Porcine epidemic diarrhea virus emerged in the US is known to suppress the type I interferons response during infection. In the present study using porcine epithelial cells, we showed that PEDV inhibited both NF-κB and proinflammatory cytokines. PEDV blocked the p65 activation in infected cells and suppressed the PRD II-mediated NF-κB activity. Of the total of 22 viral proteins, nine proteins were identified as NF-κB antagonists, and nsp1 was the most potent suppressor of proinflammatory cytokines. Nsp1 interfered the phosphorylation and degradation of IκBα, and thus blocked the p65 activation. Mutational studies demonstrated the essential requirements of the conserved residues of nsp1 for NF-κB suppression. Our study showed that PEDV inhibited NF-κB activity and nsp1 was a potent NF-κB antagonist for suppression of both IFN and early production of pro-inflammatory cytokines.

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PEDV inhibits type I IFNs and NF-κB-mediated pro-inflammatory cytokines.

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PEDV blocks p65 nuclear translocation in virus-infected cells.

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Among 22 viral proteins, nsp1, nsp3, nsp5, nsp7, nsp14, nsp15, nsp16, ORF3, and E are NF-κB antagonists.

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Nsp1 suppresses pro-inflammatory cytokines and p65 activation by blocking IκBα phosphorylation.

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The conserved residues of nsp1 are crucial for NF-κB suppression.

The integrity of PRRSV nucleocapsid protein is necessary for up-regulation of optimal interleukin-10 through NF-κB and p38 MAPK pathways in porcine alveolar macrophages

Porcine reproductive and respiratory syndrome (PRRS), a highly contagious disease, has been constantly causing huge economic losses all over the world. PRRS virus (PRRSV) infection results in immunosuppression and IL-10 up-regulation. The relationship between them is still in dispute. Previous studies demonstrated the protein of PRRSV nucleocapsid (N) protein is able to up-regulate IL-10, yet the underlying molecular mechanisms remain unknown. In this study, the expression kinetics of IL-10 up-regulation induced by PRRSV N protein were analyzed in immortalized porcine alveolar macrophages (PAMs). N protein induced IL-10 expression in a time- and dose-dependent manner. Inhibition experiments of signaling pathways suggested NF-κB and p38 MAPK pathways are both involved in N protein-induced IL-10 up-regulation. Besides, the integrity of N protein is essential for significant IL-10 up-regulation. This research is beneficial for further understanding of the interplay between PRRSV and host immune system.

MicroRNA 373 Facilitates the Replication of Porcine Reproductive and Respiratory Syndrome Virus by Its Negative Regulation of Type I Interferon Induction

MicroRNAs (miRNAs) play an important role in the regulation of immune responses. Previous studies have indicated that dysregulating the miRNAs leads to the immunosuppression of porcine reproductive and respiratory syndrome virus (PRRSV). However, it is not clear how PRRSV regulates the expression of host miRNA, which may lead to immune escape or promote the replication of the virus. The present work suggests that PRRSV upregulated the expression of miR-373 through elevating the expression of specificity protein 1 (Sp1) in MARC-145 cells. Furthermore, this work demonstrated that miR-373 promoted the replication of PRRSV, since miR-373 was a novel negative miRNA for the production of beta interferon (IFN-β) by targeting nuclear factor IA (NFIA), NFIB, interleukin-1 receptor-associated kinase 1 (IRAK1), IRAK4, and interferon regulatory factor 1 (IRF1). We also found that both NFIA and NFIB were novel proteins for inducing the production of IFN-β, and both of them could inhibit the replication of PRRSV. In conclusion, PRRSV upregulated the expression of miR-373 by elevating the expression of Sp1 and hijacked the host miR-373 to promote the replication of PRRSV by negatively regulating the production of IFN-β.

IMPORTANCE

PRRSV causes one of the most economically devastating diseases of swine, and there is no effective method for controlling PRRSV. It is not clear how PRRSV inhibits the host's immune response and induces persistent infection. Previous studies have shown that PRRSV inhibited the production of type I IFN, and the treatment of type I IFN could efficiently inhibit the replication of PRRSV, so it will be helpful to design new methods of controlling PRRSV by understanding the molecular mechanism by which PRRSV modulated the production of IFN. The current work shows that miR-373, upregulated by PRRSV, promotes PRRSV replication, since miR-373 impaired the production of IFN-β by targeting NFIA, NFIB, IRAK1, IRAK4, and IRF1, and both NFIA and NFIB were antiviral proteins to PRRSV. In conclusion, this paper revealed a novel mechanism of PRRSV that impaired the production of type I IFN by upregulating miR-373 expression in MARC-145 cells.

The activation of the IFNβ induction/signaling pathway in porcine alveolar macrophages by porcine reproductive and respiratory syndrome virus is variable

BACKGROUND

It has been recognized that the expression of type I interferon (IFNα/β) may be suppressed during infection with porcine reproductive, respiratory syndrome virus (PRRSV). This causes profound negative effects on both the innate and adaptive immunity of the host resulting in persistence of infection.

OBJECTIVE

Test the effects of PRRSV infection of porcine alveolar macrophages (PAMs), the main target cell, on the expression of interferon beta (IFNβ) and downstream signaling events.

METHODS

In order to examine those effects, PAMs harvested from lungs of healthy PRRSV-free animals were infected with virulent, attenuated, infectious clone-derived chimeric viruses, or field PRRS virus strains. Culture supernatants from the infected PAMs were tested for IFNβ protein expression by means of indirect ELISA and for bioactivity by a vesicular stomatitis virus plaque reduction assay. The expression of the Mx protein was assayed to ascertain signaling events.

RESULTS

These experiments demonstrated that PRRSV does induce variably, the expression of bioactive IFNβ protein in the natural host cell. To further elucidate the effects of PRRSV infection on IFNβ signaling, Mx-1 an interferon stimulated gene (ISG), was also tested for expression. Interestingly, Mx-1 expression by infected PAMs generally correlated with IFNβ production.

CONCLUSION

The results of this study demonstrate that the induction of IFNβ and signaling in PAMs after PRRSV infection is variable.

Systematic evaluation of sericin protein as a substitute for fetal bovine serum in cell culture

Fetal bovine serum (FBS) shows obvious deficiencies in cell culture, such as low batch to batch consistency, adventitious biological contaminant risk, and high cost, which severely limit the development of the cell culture industry. Sericin protein derived from the silkworm cocoon has become increasingly popular due to its diverse and beneficial cell culture characteristics. However, systematic evaluation of sericin as a substitute for FBS in cell culture medium remains limited. In this study, we conducted cellular morphological, physiological, and transcriptomic evaluation on three widely used mammalian cells. Compared with cells cultured in the control, those cultured in sericin-substitute medium showed similar cellular morphology, similar or higher cellular overall survival, lower population doubling time (PDT), and a higher percentage of S-phase with similar G2/G1 ratio, indicating comparable or better cell growth and proliferation. At the transcriptomic level, differentially expressed genes between cells in the two media were mainly enriched in function and biological processes related to cell growth and proliferation, reflecting that genes were activated to facilitate cell growth and proliferation. The results of this study suggest that cells cultured in sericin-substituted medium perform as well as, or even better than, those cultured in FBS-containing medium.

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Modulates Interferon-β Expression Mainly Through Attenuating Interferon-Regulatory Factor 3 Phosphorylation

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) that emerged from classic PRRSV causes more severe damage to the swine industry. The earlier reports indicating inhibition of interferon-β (IFN-β) expression by PRRSV through total blockage of IFN-regulatory factor 3 (IRF3) nuclear translocation made us investigate the mechanism of IFN-β expression in HP-PRRSV infection. For this purpose, the IRF3 nuclear translocation in the control group [Poly (I:C)] and test group [Poly (I:C)+HP-PRRSV] was detected by immunofluorescence, and the results showed that IRF3 nuclear translocation in cells with PRRSV was weaker than cells without PRRSV, which was different from the previous study. In addition, the IFN-β mRNA and protein expression was observed to be inhibited by HP-PRRSV along with decreased IRF3 mRNA and total protein, and IRF3 nuclear translocation of test group was suppressed in MARC-145 and porcine alveolar macrophage cells in comparison with the control group. The quantity of phosphorylated IRF3 protein was also reduced after HP-PRRSV infection. However, CREB-binding protein (CBP) expression did not change between the control and test group. These results indicate that the inhibition of IFN-β expression is mainly due to the quantitative change in the amount of phosphorylated IRF3 in the cytoplasm, but not dependent on the complete blockage of IRF3 nuclear translocation or the restraining of CBP expression in the nucleus by HP-PRRSV.

Extracellular expression and antiviral activity of a bovine interferon-alpha through codon optimization in Pichia pastoris

Interferons (IFNs) are the primary line of defense against infectious agents. In particular, IFN-α is an important antiviral cytokine and has a wide range of immune-modulating functions. Porcine and human IFN-α have been successfully prepared and play important roles in the prevention and therapy of viral diseases. To date, there has been limited applied research on bovine IFN-α. To achieve high-level expression of recombinant bovine IFN-α (bIFN-α) in Pichia pastoris for large-scale application, the bIFN-α gene was optimized and synthesized on the basis of codon bias of P. pastoris. Optimized bIFN-α (opti-bIFN-α) was successfully expressed in P. pastoris and directly secreted into the culture supernatant. The amount of extracellular soluble opti-bIFN-α was observed to be 200μg/mL in a shake flask. Expression efficiency of opti-bIFN-α was found to be about three times that of wild-type bIFN-α when the expression yield was compared at the same copies of the targeted gene. In addition, both the original cultural supernatant and purified opti-bIFN-α showed strong antiviral activity in MDBK cells (2×10(6)AU/mL and 1×10(7)AU/mg, respectively) and IBRS-2 cells (3×10(5)AU/mL and 1.5×10(6)AU/mg, respectively) against a recombinant vesicular stomatitis virus expressing the green fluorescence protein. In this study, we demonstrated high-level extracellular expression of opti-bIFN-α by P. pastoris. To the best of our knowledge, the opti-bIFN-α yield observed in this study is the highest to be reported to date. Our results demonstrated that the extracellular opti-bIFN-α with strong antiviral activity could be easily prepared and purified at a low cost and that it may be a potential biological therapeutic drug against bovine viral infections.

Suppression of type I interferon production by porcine epidemic diarrhea virus and degradation of CREB-binding protein by nsp1

Type I interferons (IFN-α/β) are the major components of the innate immune response of hosts, and in turn many viruses have evolved to modulate the host response during infection. We found that the IFN-β production was significantly suppressed during PEDV infection in cells. To identify viral IFN antagonists and to study their suppressive function, viral coding sequences for the entire structural and nonstructural proteins were cloned and expressed. Of 16 PEDV nonstructural proteins (nsps), nsp1, nsp3, nsp7, nsp14, nsp15 and nsp16 were found to inhibit the IFN-β and IRF3 promoter activities. The sole accessory protein ORF3, structure protein envelope (E), membrane (M), and nucleocapsid (N) protein were also shown to inhibit such activities. PEDV nsp1 did not interfere the IRF3 phosphorylation and nuclear translocation but interrupted the enhanceosome assembly of IRF3 and CREB-binding protein (CBP) by degrading CBP. A further study showed that the CBP degradation by nsp1 was proteasome-dependent. Our data demonstrate that PEDV modulates the host innate immune responses by degrading CBP and suppressing ISGs expression.

Glycoprotein 5 of porcine reproductive and respiratory syndrome virus strain SD16 inhibits viral replication and causes G2/M cell cycle arrest, but does not induce cellular apoptosis in Marc-145 cells

Cell apoptosis is common after infection with porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV GP5 has been reported to induce cell apoptosis. To further understand the role of GP5 in PRRSV induced cell apoptosis, we established Marc-145 cell lines stably expressing full-length GP5, GP5(Δ84-96) (aa 84-96 deletion), and GP5(Δ97-119) (aa 97-119 deletion). Cell proliferation, cell cycle progression, cell apoptosis and virus replication in these cell lines were evaluated. Neither truncated nor full-length GP5 induced cell apoptosis in Marc-145 cells. However, GP5(Δ97-119), but not full-length or GP5(Δ84-96), induced a cell cycle arrest at the G2/M phase resulting in a reduction in the growth of Marc-145 cells. Additionally, GP5(Δ84-96) inhibited the replication of PRRSV in Marc-145 cells through induction of IFN-β. These findings suggest that PRRSV GP5 is not responsible for inducing cell apoptosis in Marc-145 cells under these experimental conditions; however it has other important roles in virus/host cell biology.

Cellular microRNA miR-26a suppresses replication of porcine reproductive and respiratory syndrome virus by activating innate antiviral immunity

Porcine reproductive and respiratory syndrome (PRRS) has caused large economic losses in the swine industry in recent years. Current PRRS vaccines fail to effectively prevent and control this disease. Consequently, there is a need to develop new antiviral strategies. MicroRNAs play critical roles in intricate host-pathogen interaction networks, but the involvement of miRNAs during PRRS virus (PRRSV) infection is not well understood. In this study, pretreatment with miR-26a induced a significant inhibition of PRRSV replication and remission of the cytopathic effect in MARC-145 cells, and this antiviral effect was sustained for at least 120 h. Luciferase reporter analysis showed that the PRRSV genome was not the target of miRNA-26a. Instead, RNA-seq analysis demonstrated that miR-26a significantly up-regulated innate anti-viral responses, including activating the type I interferon (IFN) signaling pathway and promoting the production of IFN-stimulated genes. These findings suggest that delivery of miR-26a may provide a potential strategy for anti-PRRSV therapies.

Comparative analysis of cytokine transcript profiles within mediastinal lymph node compartments of pigs after infection with porcine reproductive and respiratory syndrome genotype 1 strains differing in pathogenicity

Porcine reproductive and respiratory syndrome virus (PRRSV) induces a weak immune response enabling it to persist in different organs of infected pigs. This has been attributed to the ability of PRRSV to influence the induction of cytokine responses. In this study, we investigated the cytokine transcriptional profiles in different compartments of the mediastinal lymph node of pigs infected with three genotype 1 PRRSV strains of differing pathogenicity: the low virulence prototype Lelystad virus (LV), and UK field strain 215–06 and the highly virulent subtype 3 SU1-Bel isolate from Belarus. We have used a combination of laser capture micro-dissection (LCM) followed by real time quantitative PCR (RT-qPCR) and immunohistochemical (IHC) detection of immune cell markers (CD3, CD79a and MAC387) and RT-qPCR quantification of PRRSV and cytokine transcripts. Compared to mock infected pigs, we found a significant downregulation of TNF-α and IFN-α in follicular and interfollicular areas of the mediastinal lymph node from 3 days post-infection (dpi) in animals infected with all three strains. This was accompanied by a transient B cell depletion and T cell and macrophage infiltration in the follicles together with T cell depletion in the interfollicular areas. A delayed upregulation of IFN-γ and IL-23p19 was observed mainly in the follicles. The PRRSV load was higher in all areas and time-points studied in the animals infected with the SU1-Bel strain. This paper describes the first application of LCM to study the cytokine transcript profiles and virus distribution in different compartments of the lymph node of pigs.

Host miR-26a suppresses replication of porcine reproductive and respiratory syndrome virus by upregulating type I interferons

MicroRNAs (miRNAs) play important roles in viral infections, especially by modulating the expression of cellular factors essential to viral replication or the host innate immune response to infection. To identify host miRNAs important to controlling porcine reproductive and respiratory syndrome virus (PRRSV) infection, we screened 15 miRNAs that were previously implicated in innate immunity or antiviral functions. Over-expression of the miR-26 family strongly inhibited PRRSV replication in vitro, as shown by virus titer assays, Western blotting, and qRT-PCR assays. MiR-26a inhibited the replication of both type 1 and type 2 PRRSV strains. Mutating the seed region of miR-26 restored viral titers. Luciferase reporters showed that miR-26a does not target the PRRSV genome directly but instead affects the expression of type I interferon and the IFN-stimulated genes MX1 and ISG15 during PRRSV infection. These results demonstrate the important role of miR-26a in modulating PRRSV infection and also support the possibility of using host miR-26a to achieve RNAi-mediated antiviral therapeutic strategies.

Regulation and evasion of antiviral immune responses by porcine reproductive and respiratory syndrome virus

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Five PRRSV viral proteins are shown to inhibit type I IFN induction and signaling by targeting different intracellular signaling intermediates.

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PRRSV regulates the expression of IL-10 and TNFα.

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PRRSV modulates apoptosis during infection.

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MicroRNAs might play significant roles in subverting immunity for PRRSV.

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PRRSV escapes from adaptive immunity by impairing antigen presentation, activating Tregs, and ADE.

Virus infection of mammalian cells triggers host innate immune responses to restrict viral replication and induces adaptive immunity for viral elimination. In order to survive and propagate, viruses have evolved sophisticated mechanisms to subvert host defense system by encoding proteins that target key components of the immune signaling pathways. Porcine reproductive and respiratory syndrome virus (PRRSV), a RNA virus, impairs several processes of host immune responses including interfering with interferon production and signaling, modulating cytokine expression, manipulating apoptotic responses and regulating adaptive immunity. In this review, we highlight the molecular mechanisms of how PRRSV interferes with the different steps of initial antiviral host responses to establish persistent infection in pigs. Dissection of the PRRSV–host interaction is the key in understanding PRRSV pathogenesis and will provide a basis for the rational design of vaccines.

Porcine reproductive and respiratory syndrome virus nonstructural protein 4 antagonizes beta interferon expression by targeting the NF-κB essential modulator

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly infectious pathogen that causes severe diseases in pigs and great economic losses to the swine industry worldwide. Type I interferons (IFNs) play a crucial role in antiviral immunity. In the present study, we demonstrated that infection with the highly pathogenic PRRSV strain JXwn06 antagonized type I IFN expression induced by poly(I·C) in both porcine alveolar macrophages (PAMs) and blood monocyte-derived macrophages (BMo). Subsequently, we showed that the inhibition of poly(I·C)-induced IFN-β production by PRRSV was dependent on the blocking of NF-κB signaling pathways. By screening PRRSV nonstructural and structural proteins, we demonstrated that nonstructural protein 4 (nsp4), a viral 3C-like serine protease, significantly suppressed IFN-β expression. Moreover, we verified that nsp4 inhibited NF-κB activation induced by signaling molecules, including RIG-I, VISA, TRIF, and IKKβ. nsp4 was shown to target the NF-κB essential modulator (NEMO) at the E349-S350 site to mediate its cleavage. Importantly, nsp4 mutants with defective protease activity abolished its ability to cleave NEMO and inhibit IFN-β production. These findings might have implications for our understanding of PRRSV pathogenesis and its mechanisms for evading the host immune response.

IMPORTANCE

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major agent of respiratory diseases in pigs. Like many other viruses, PRRSV has evolved a variety of strategies to evade host antiviral innate immunity for survival and propagation. In this study, we show that PRRSV nsp4 is a novel antagonist of the NF-κB signaling pathway, which is responsible for regulating the expression of type I interferons and other crucial cytokines. We then investigated the underlying mechanism used by nsp4 to suppress NF-κB-mediated IFN-β production. We found that nsp4 interfered with the NF-κB signaling pathway through the cleavage of NEMO (a key regulator of NF-κB signaling) at the E349-S350 site, leading to the downregulation of IFN-β production induced by poly(I·C). The data presented here may help us to better understand PRRSV pathogenesis.

Antagonizing interferon-mediated immune response by porcine reproductive and respiratory syndrome virus

Interferons (IFNs) are important components in innate immunity involved in the first line of defense to protect host against viral infection. Porcine reproductive and respiratory syndrome virus (PRRSV) leads to severe economic losses for swine industry since being first identified in early 1990s. PRRSV interplays with host IFN production and IFN-activated signaling, which may contribute to the delayed onset and low level of neutralizing antibodies, as well as weak cell-mediated immune response in infected pigs. PRRSV encodes several proteins that act as antagonists for the IFN signaling. In this review, we summarized the various strategies used by PRRSV to antagonize IFN production and thwart IFN-activated antiviral signaling, as well as the variable interference with IFN-mediated immune response by different PRRSV strains. Thorough understanding of the interaction between PRRSV and host innate immune response will facilitate elucidation of PRRSV pathogenesis and development of a better strategy to control PRRS.

Equine arteritis virus does not induce interferon production in equine endothelial cells: identification of nonstructural protein 1 as a main interferon antagonist

The objective of this study was to investigate the effect of equine arteritis virus (EAV) on type I interferon (IFN) production. Equine endothelial cells (EECs) were infected with the virulent Bucyrus strain (VBS) of EAV and expression of IFN-β was measured at mRNA and protein levels by quantitative real-time RT-PCR and IFN bioassay using vesicular stomatitis virus expressing the green fluorescence protein (VSV-GFP), respectively. Quantitative RT-PCR results showed that IFN-β mRNA levels in EECs infected with EAV VBS were not increased compared to those in mock-infected cells. Consistent with quantitative RT-PCR, Sendai virus- (SeV-) induced type I IFN production was inhibited by EAV infection. Using an IFN-β promoter-luciferase reporter assay, we subsequently demonstrated that EAV nsps 1, 2, and 11 had the capability to inhibit type I IFN activation. Of these three nsps, nsp1 exhibited the strongest inhibitory effect. Taken together, these data demonstrate that EAV has the ability to suppress the type I IFN production in EECs and nsp1 may play a critical role to subvert the equine innate immune response.

Analysis of the binding sites of porcine sialoadhesin receptor with PRRSV

Porcine reproductive and respiratory syndrome virus (PRRSV) can infect pigs and cause enormous economic losses to the pig industry worldwide. Porcine sialoadhesin (pSN) and CD163 have been identified as key viral receptors on porcine alveolar macrophages (PAM), a main target cell infected by PRRSV. In this study, the protein structures of amino acids 1-119 from the pSN and cSN (cattle sialoadhesin) N-termini (excluding the 19-amino acid signal peptide) were modeled via homology modeling based on mSN (mouse sialoadhesin) template structures using bioinformatics tools. Subsequently, pSN and cSN homology structures were superposed onto the mSN protein structure to predict the binding sites of pSN. As a validation experiment, the SN N-terminus (including the wild-type and site-directed-mutant-types of pSN and cSN) was cloned and expressed as a SN-GFP chimera protein. The binding activity between SN and PRRSV was confirmed by WB (Western blotting), FAR-WB (far Western blotting), ELISA (enzyme-linked immunosorbent assay) and immunofluorescence assay. We found that the S107 amino acid residue in the pSN N-terminal played a crucial role in forming a special cavity, as well as a hydrogen bond for enhancing PRRSV binding during PRRSV infection. S107 may be glycosylated during PRRSV infection and may also be involved in forming the cavity for binding PRRSV along with other sites, including W2, Y44, S45, R97, R105, W106 and V109. Additionally, S107 might also be important for pSN binding with PRRSV. However, the function of these binding sites must be confirmed by further studies.

Porcine reproductive and respiratory syndrome virus inhibition of interferon-β transcription by IRF3-independent mechanisms in MARC-145 cells in early infection

Interferon β is an important antiviral molecule whose expression is triggered through recognition of viral components by pattern recognition receptors via a cascade of signaling molecules, while viruses could target these molecules to evade from innate immunity. IFN regulatory factor 3 (IRF3) plays a crucial role in innate immune responses. Here, we demonstrate that PRRSV infection did not induce IFN-β gene transcription in MARC-145 cells, but inhibited poly (I:C) stimulated IFN-β gene transcription instead. Such inhibition is time-dependent with the progression of PRRSV infection. We also show that the inhibition of IFN-β transcription in the early stage of infection could not be due to inhibition of phosphorylation and nuclear translocation of IRF3, though significant decrease of p-IRF3 and its nuclear translocation in PRRSV-infected and poly (I:C) cells was observed later at 48 h post-infection. The different patterns of inhibition for IFN-β transcription and IRF3 phosphorylation have important implications as to the mechanism(s) by which PRRSV suppresses the type I IFN signaling at early stage of infection. There could be mechanism(s) other than effecting on IRF3 or molecules upstream that require further investigation.

swIFN-β promotes genetic mutation of porcine reproductive and respiratory syndrome virus in Marc-145

This result, for the first time, demonstrates that under swine interferon-β (swIFN-β) immune pressure molecular variation of porcine reproductive and respiratory syndrome virus (PRRSV) accelerates. PRRSV-A1 strain was continuously propagated in Marc-145 cells primed with swIFN-β for 20 passages to develop RRRSV-A1βf20 strains while PRRSV-A1f20 was control without swIFN-β respectively. NSP2, ORF3, ORF5 and ORF6 genes of these strains were amplified, cloned and sequenced. Sequencing analysis indicated that PRRSV-A1βf20 had 43 nucleotide mutations while PRRSV-A1f20 had only 14 in NSP2, ORF3, ORF5 and ORF6. The ratio of mutative nucleotides between them was 307%. Amino acid variations were 24 in RRRSV-A1βf20 and 5 in PRRSV-A1, so that mutation in RRRSV-A1βf20 appeared to be 4.8 times faster than that in PRRSV-A1. The ratio of non-synonymous (NS) mutations to synonymous (S) mutations, named NS/S, was 3 in RRRSV-A1βf20 and 0.625 in PRRSV-A1f20. The nucleotide mutation rate of NSP2 ORF3 ORF5 and ORF6 of PRRSV-A1βf20 were 3.8%, 2.3%, 4.5%, 0.57% and their NS/S were 5, 2, 6 and 1 respectively, which is much higher than those in PRRSV-A1f20. Thus the mutative frequency of PRRSV passaged under swIFN-β immune pressure was significantly faster than that without swIFN-β. The immuno-pressure of swIFN-β accelerated genetic variation on PRRSV ORF5, NSP2 and ORF3, but not on the ORF6 gene. PRRSV NSP2 and ORF5 genes may play a key role in escaping from inhibition of swIFN-β.

Reactomes of porcine alveolar macrophages infected with porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) has devastated pig industries worldwide for many years. It is caused by a small RNA virus (PRRSV), which targets almost exclusively pig monocytes or macrophages. In the present study, five SAGE (serial analysis of gene expression) libraries derived from 0 hour mock-infected and 6, 12, 16 and 24 hours PRRSV-infected porcine alveolar macrophages (PAMs) produced a total 643,255 sequenced tags with 91,807 unique tags. Differentially expressed (DE) tags were then detected using the Bayesian framework followed by gene/mRNA assignment, arbitrary selection and manual annotation, which determined 699 DE genes for reactome analysis. The DAVID, KEGG and REACTOME databases assigned 573 of the DE genes into six biological systems, 60 functional categories and 504 pathways. The six systems are: cellular processes, genetic information processing, environmental information processing, metabolism, organismal systems and human diseases as defined by KEGG with modification. Self-organizing map (SOM) analysis further grouped these 699 DE genes into ten clusters, reflecting their expression trends along these five time points. Based on the number one functional category in each system, cell growth and death, transcription processes, signal transductions, energy metabolism, immune system and infectious diseases formed the major reactomes of PAMs responding to PRRSV infection. Our investigation also focused on dominant pathways that had at least 20 DE genes identified, multi-pathway genes that were involved in 10 or more pathways and exclusively-expressed genes that were included in one system. Overall, our present study reported a large set of DE genes, compiled a comprehensive coverage of pathways, and revealed system-based reactomes of PAMs infected with PRRSV. We believe that our reactome data provides new insight into molecular mechanisms involved in host genetic complexity of antiviral activities against PRRSV and lays a strong foundation for vaccine development to control PRRS incidence in pigs.

The zinc-finger domain was essential for porcine reproductive and respiratory syndrome virus nonstructural protein-1α to inhibit the production of interferon-β

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused one of the most economically devastating and pandemic diseases of swine. Previous studies have documented that PRRSV nonstructural protein-1α (nsp1α) was an interferon antagonist, but the mechanism by which nsp1α inhibited the interferon (IFN)-β production was unclear. Here, by site-directed mutagenesis of the predicted zinc-coordinating residues of the zinc-finger (ZF) domain of nsp1α or by deletion of the ZF domain of nsp1α, we explored whether the ZF domain was required for nsp1α to disrupt the IFN-β production. The results showed that both mutagenesis of the predicted zinc-coordinating residues of the ZF domain and deletion of the ZF domain made nsp1α lose its interferon antagonism activity. In conclusion, our present work indicated that the ZF domain of nsp1α was necessary for nsp1α to inhibit the IFN-β induction.

Pathogenic characteristics of three genotype II porcine reproductive and respiratory syndrome viruses isolated from China

Background

We examined differences in pathogenicity in pigs from China that had been experimentally infected with porcine reproductive and respiratory syndrome virus (PRRSV).

Methods

We compared pathogenic characteristics of a field isolate (GX-1/2008F), two PRRSV isolates (HN-1/2008, YN-1/2008) propagated in cells, and GX-1/2008F that had been propagated in cells (GX-1/2008). The clinical courses, along with humoral and cell-mediated responses, were monitored for 21 days post-infection (DPI). Animals were sacrificed and tissue samples used for gross pathological, histopathological and ultrastructure examination.

Results

At 2–3 DPI, animals infected with cell-propagated viruses exhibited signs of coughing, anorexia and fever. However their rectal temperature did not exceed 40.5°C. Viremia was detectable as early as 3 DPI in animals infected with HN-1/2008 and YN-1/2008. Animals inoculated with GX-1/2008F displayed clinical signs at 6 DPI; the rectal temperature of two animals in this group exceeded 41.0°C, with viremia first detected at 7 DPI. Seroconversion for all challenged pigs, except those infected with GX-1/2008, was seen as early as 7 DPI. All of these pigs had fully seroconverted by 11 DPI. All animals challenged with GX-1/2008 remained seronegative until the end of the experiment. Innate immunity was inhibited, with levels of IFN-α and IL-1 not significantly different between control and infected animals. The cytokines IFN-γ and IL-6 transiently increased during acute infection. All virus strains caused gross lesions including multifocal interstitial pneumonia and hyperplasia of lymph nodes. Inflammation of the stomach and small intestine was also observed. Lesions in the group infected with GX-1/2008F were more serious than in other groups. Transmission electron microscopy revealed that alveolar macrophages, plasmacytes and lymphocytes had fractured cytomembranes, and hepatocytes had disrupted organelles and swollen mitochondria.

Conclusions

The pathogenicity of the PRRSV field isolate became attenuated when propagated in MARC-145 cells. Tissue tropism of highly pathogenic strains prevailing in China was altered compared with classical PRRSV strains. The observed damage to immune cells and modulation of cytokine production could be mechanisms that PRRSV employs to evade host immune responses.

Recent progress in studies of arterivirus- and coronavirus-host interactions

Animal coronaviruses, such as infectious bronchitis virus (IBV), and arteriviruses, such as porcine reproductive and respiratory syndrome virus (PRRSV), are able to manifest highly contagious infections in their specific native hosts, thereby arising in critical economic damage to animal industries. This review discusses recent progress in studies of virus-host interactions during animal and human coronavirus and arterivirus infections, with emphasis on IBV-host cell interactions. These interactions may be directly involved in viral replication or lead to the alteration of certain signaling pathways, such as cell stress response and innate immunity, to facilitate viral replication and pathogenesis.

Replication-competent recombinant porcine reproductive and respiratory syndrome (PRRS) viruses expressing indicator proteins and antiviral cytokines

Porcine reproductive and respiratory syndrome virus (PRRSV) can subvert early innate immunity, which leads to ineffective antimicrobial responses. Overcoming immune subversion is critical for developing vaccines and other measures to control this devastating swine virus. The overall goal of this work was to enhance innate and adaptive immunity following vaccination through the expression of interferon (IFN) genes by the PRRSV genome. We have constructed a series of recombinant PRRS viruses using an infectious PRRSV cDNA clone (pCMV-P129). Coding regions of exogenous genes, which included Renilla luciferase (Rluc), green and red fluorescent proteins (GFP and DsRed, respectively) and several interferons (IFNs), were constructed and expressed through a unique subgenomic mRNA placed between ORF1b and ORF2 of the PRRSV infectious clone. The constructs, which expressed Rluc, GFP, DsRed, efficiently produced progeny viruses and mimicked the parental virus in both MARC-145 cells and porcine macrophages. In contrast, replication of IFN-expressing viruses was attenuated, similar to the level of replication observed after the addition of exogenous IFN. Furthermore, the IFN expressing viruses inhibited the replication of a second PRRS virus co-transfected or co-infected. Inhibition by the different IFN subtypes corresponded to their anti-PRRSV activity, i.e., IFNω5 ° IFNα1 > IFN-β > IFNδ3. In summary, the indicator-expressing viruses provided an efficient means for real-time monitoring of viral replication thus allowing high‑throughput elucidation of the role of host factors in PRRSV infection. This was shown when they were used to clearly demonstrate the involvement of tumor susceptibility gene 101 (TSG101) in the early stage of PRRSV infection. In addition, replication‑competent IFN-expressing viruses may be good candidates for development of modified live virus (MLV) vaccines, which are capable of reversing subverted innate immune responses and may induce more effective adaptive immunity against PRRSV infection.

Inhibitory effects of recombinant porcine interferon-α on high- and low-virulence porcine reproductive and respiratory syndrome viruses

The inhibitory effects of recombinant porcine interferon alpha (rPoIFN-α) on the propagation of low-virulence PRRSV (lvPRRSV) in MARC-145 cells, and on the progress and severity of high virulence PRRSV (hvPRRSV)-induced infections in pigs, were determined. Pre-treatment of MARC-145 cells with increasing concentrations of rPoIFN-α prior to infection with lvPRRSV decreased the observed cytopathic effects (CPEs) in a concentration-dependent manner. Viral propagation and antibody response were temporarily delayed in swine treated with rPoIFN-α either at the same time as the hvPRRSV challenge was administered or post-challenge. Exposure of challenged animals to rPoIFN-α after the onset of disease symptoms alleviated associated hyperthermia. Variations in lymphocyte subsets indicated that rPoIFN-α treatment might alleviate damage to the immune system or enhance propagation of host cytotoxic T-lymphocytes when the treatment was applied simultaneously with the virus or 1dpc, respectively.