Identification of amino acid residues important for anti-IFN activity of porcine reproductive and respiratory syndrome virus non-structural protein 1

Infection dynamics, transmission, and evolution after an outbreak of porcine reproductive and respiratory syndrome virus

The present study was aimed at describing the infection dynamics, transmission, and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) after an outbreak in a 300-sow farrow-to-wean farm that was implementing a vaccination program. Three subsequent batches of piglets (9-11 litters/batch) were followed 1.5 (Batch 1), 8 (Batch 2), and 12 months after (Batch 3) from birth to 9 weeks of age. The RT-qPCR analysis showed that shortly after the outbreak (Batch 1), one third of sows were delivering infected piglets and the cumulative incidence reached 80% by 9 weeks of age. In contrast, in Batch 2, only 10% animals in total got infected in the same period. In Batch 3, 60% litters had born-infected animals and cumulative incidence rose to 78%. Higher viral genetic diversity was observed in Batch 1, with 4 viral clades circulating, of which 3 could be traced to vertical transmission events, suggesting the existence of founder viral variants. In Batch 3 though only one variant was found, distinguishable from those circulating previously, suggesting that a selection process had occurred. ELISA antibodies at 2 weeks of age were significantly higher in Batch 1 and 3 compared to Batch 2, while low levels of neutralizing antibodies were detected in either piglets or sows in all batches. In addition, some sows present in Batch 1 and 3 delivered infected piglets twice, and the offspring were devoid of neutralizing antibodies at 2 weeks of age. These results suggest that a high viral diversity was featured at the initial outbreak followed by a phase of limited circulation, but subsequently an escape variant emerged in the population causing a rebound of vertical transmission. The presence of unresponsive sows that had vertical transmission events could have contributed to the transmission. Moreover, the records of contacts between animals and the phylogenetic analyses allowed to trace back 87 and 47% of the transmission chains in Batch 1 and 3, respectively. Most animals transmitted the infection to 1-3 pen-mates, but super-spreaders were also identified. One animal that was born-viremic and persisted as viremic for the whole study period did not contribute to transmission.

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

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

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

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

Porcine Reproductive and Respiratory Syndrome Virus: Immune Escape and Application of Reverse Genetics in Attenuated Live Vaccine Development

Porcine reproductive and respiratory syndrome virus (PRRSV), an RNA virus widely prevalent in pigs, results in significant economic losses worldwide. PRRSV can escape from the host immune response in several processes. Vaccines, including modified live vaccines and inactivated vaccines, are the best available countermeasures against PRRSV infection. However, challenges still exist as the vaccines are not able to induce broad protection. The reason lies in several facts, mainly the variability of PRRSV and the complexity of the interaction between PRRSV and host immune responses, and overcoming these obstacles will require more exploration. Many novel strategies have been proposed to construct more effective vaccines against this evolving and smart virus. In this review, we will describe the mechanisms of how PRRSV induces weak and delayed immune responses, the current vaccines of PRRSV, and the strategies to develop modified live vaccines using reverse genetics systems.

Porcine Reproductive and Respiratory Syndrome Virus Reverse Genetics and the Major Applications

Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive sense, single-stranded RNA virus that is known to infect only pigs. The virus emerged in the late 1980s and became endemic in most swine producing countries, causing substantial economic losses to the swine industry. The first reverse genetics system for PRRSV was reported in 1998. Since then, several infectious cDNA clones for PRRSV have been constructed. The availability of these infectious cDNA clones has facilitated the genetic modifications of the viral genome at precise locations. Common approaches to manipulate the viral genome include site-directed mutagenesis, deletion of viral genes or gene fragments, insertion of foreign genes, and swapping genes between PRRSV strains or between PRRSV and other members of the Arteriviridae family. In this review, we describe the approaches to construct an infectious cDNA for PRRSV and the ten major applications of these infectious clones to study virus biology and virus–host interaction, and to design a new generation of vaccines with improved levels of safety and efficacy.

Nsp1α of Porcine Reproductive and Respiratory Syndrome Virus Strain BB0907 Impairs the Function of Monocyte-Derived Dendritic Cells via the Release of Soluble CD83

Porcine reproductive and respiratory syndrome virus (PRRSV), a virulent pathogen of swine, suppresses the innate immune response and induces persistent infection. One mechanism used by viruses to evade the immune system is to cripple the antigen-processing machinery in monocyte-derived dendritic cells (MoDCs). In this study, we show that MoDCs infected by PRRSV express lower levels of the major histocompatibility complex (MHC)-peptide complex proteins TAP1 and ERp57 and are impaired in their ability to stimulate T cell proliferation and increase their production of CD83. Neutralization of sCD83 removes the inhibitory effects of PRRSV on MoDCs. When MoDCs are incubated with exogenously added sCD83 protein, TAP1 and ERp57 expression decreases and T lymphocyte activation is impaired. PRRSV nonstructural protein 1α (Nsp1α) enhances CD83 promoter activity. Mutations in the ZF domain of Nsp1α abolish its ability to activate the CD83 promoter. We generated recombinant PRRSVs with mutations in Nsp1α and the corresponding repaired PRRSVs. Viruses with Nsp1α mutations did not decrease levels of TAP1 and ERp57, impair the ability of MoDCs to stimulate T cell proliferation, or increase levels of sCD83. We show that the ZF domain of Nsp1α stimulates the secretion of CD83, which in turn inhibits MoDC function. Our study provides new insights into the mechanisms of immune suppression by PRRSV.IMPORTANCE PRRSV has a severe impact on the swine industry throughout the world. Understanding the mechanisms by which PRRSV infection suppresses the immune system is essential for a robust and sustainable swine industry. Here, we demonstrated that PRRSV infection manipulates MoDCs by interfering with their ability to produce proteins in the MHC-peptide complex. The virus also impairs the ability of MoDCs to stimulate cell proliferation, due in large part to the enhanced release of soluble CD83 from PRRSV-infected MoDCs. The viral nonstructural protein 1 (Nsp1) is responsible for upregulating CD83 promoter activity. Amino acids in the ZF domain of Nsp1α (L5-2A, rG45A, G48A, and L61-6A) are essential for CD83 promoter activation. Viruses with mutations at these sites no longer inhibit MoDC-mediated T cell proliferation. These findings provide novel insights into the mechanism by which the adaptive immune response is suppressed during PRRSV infection.

Transcriptional profiles of PBMCs from pigs infected with three genetically diverse porcine reproductive and respiratory syndrome virus strains

Porcine reproductive and respiratory syndrome virus is the cause of reproductive failure in sows and respiratory disease in young pigs, which has been considered as one of the most costly diseases to the worldwide pig industry for almost 30 years. This study used microarray-based transcriptomic analysis of PBMCs from experimentally infected pigs to explore the patterns of immune dysregulation after infection with two East European PRRSV strains from subtype 2 (BOR and ILI) in comparison to a Danish subtype 1 strain (DAN). Transcriptional profiles were determined at day 7 post infection in three tested groups of pigs and analysed in comparison with the expression profile of control group. Microarray analysis revealed differential regulation (> 1.5-fold change) of 4253 and 7335 genes in groups infected with BOR and ILI strains, respectively, and of 12518 genes in pigs infected with Danish strain. Subtype 2 PRRSV strains showed greater induction of many genes, especially those involved in innate immunity, such as interferon stimulated antiviral genes and inflammatory markers. Functional analysis of the microarray data revealed a significant up-regulation of genes involved in processes such as acute phase response, granulocyte and agranulocyte adhesion and diapedesis, as well as down-regulation of genes enrolled in pathways engaged in protein synthesis, cell division, as well as B and T cell signaling. This study provided an insight into the host response to three different PRRSV strains at a molecular level and demonstrated variability between strains of different pathogenicity level.

Interplay between Janus Kinase/Signal Transducer and Activator of Transcription Signaling Activated by Type I Interferons and Viral Antagonism

Interferons (IFNs), which were discovered a half century ago, are a group of secreted proteins that play key roles in innate immunity against viral infection. The major signaling pathway activated by IFNs is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, which leads to the expression of IFN-stimulated genes (ISGs), including many antiviral effectors. Viruses have evolved various strategies with which to antagonize the JAK/STAT pathway to influence viral virulence and pathogenesis. In recent years, notable progress has been made to better understand the JAK/STAT pathway activated by IFNs and antagonized by viruses. In this review, recent progress in research of the JAK/STAT pathway activated by type I IFNs, non-canonical STAT activation, viral antagonism of the JAK/STAT pathway, removing of the JAK/STAT antagonist from viral genome for attenuation, and the potential pathogenesis roles of tyrosine phosphorylation-independent non-canonical STATs activation during virus infection are discussed in detail. We expect that this review will provide new insight into the understanding the complexity of the interplay between JAK/STAT signaling and viral antagonism.

Identification of viral genes associated with the interferon-inducing phenotype of a synthetic porcine reproductive and respiratory syndrome virus strain

We recently generated a fully synthetic porcine reproductive and respiratory syndrome virus strain (designated as PRRSV-CON), which confers unprecedented levels of heterologous protection. We report herein that the synthetic PRRSV-CON possesses a unique phenotype in that it induces type-I interferons (IFNs) instead of suppressing these cytokines as most of the naturally occurring PRRSV isolates do. Through gain- and loss- of-function studies, the IFN-inducing phenotype of PRRSV-CON was mapped to the 3.3kb genomic fragment encoding three viral nonstructural proteins: nsp1α, nsp1β and the N-terminal part of nsp2. Further studies indicated that a cooperation among these 3 proteins was required for effective induction of IFNs. Collectively, this study constitutes the first step toward understanding the mechanisms by which the synthetic PRRSV-CON confers heterologous protection.

Porcine reproductive and respiratory syndrome virus vaccine does not fit in classical vaccinology

All vaccines are developed to elicit an effective immune response in vaccinated animals such as innate, humoral and cell mediated response to protect animal health. Quality and intensity of the immune responses are differing by characteristics of the vaccine formulation and nature of the infectious agent. Modified live virus vaccines showed advantages over killed vaccines in terms of rapid immune response, duration of the immunity and better cell mediated protection mechanism. The porcine reproductive and respiratory syndrome virus (PRRSV) is relatively newly emerging (1986 in United States, 1990 in Europe) viral pathogen in pigs and tremendous effort has been made to protect pigs from this economically devastating disease such as developing killed, modified live, recombinant protein based and DNA vaccines. However, only cell culture attenuated virus vaccine is practiced with arguably limited efficacy. The PRRSV vaccine did not clear virus from infected pigs nor prevent re-infection of the virus. The vaccine showed very limited innate immune response, low anamnestic immune response and negligible cell mediated immune response. Despite of the current developed scientific technology, there still remain many questions to solve a most important pig disease worldwide.

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

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) was reported in the late 1980s. PRRS still is a huge economic concern to the global pig industry with a current annual loss estimated at one billion US dollars in North America alone. It has been 20 years since the first modified live-attenuated PRRSV vaccine (PRRSV-MLV) became commercially available. PRRSV-MLVs provide homologous protection and help in reducing shedding of heterologous viruses, but they do not completely protect pigs against heterologous field strains. There have been many advances in understanding the biology and ecology of PRRSV; however, the complexities of virus-host interaction and PRRSV vaccinology are not yet completely understood leaving a significant gap for improving breadth of immunity against diverse PRRS isolates. This review provides insights on immunization efforts using infectious PRRSV-based vaccines since the 1990s, beginning with live PRRSV immunization, development and commercialization of PRRSV-MLV, and strategies to overcome the deficiencies of PRRSV-MLV through use of replicating viral vectors expressing multiple PRRSV membrane proteins. Finally, powerful reverse genetics systems (infectious cDNA clones) generated from more than 20 PRRSV isolates of both genotypes 1 and 2 viruses have provided a great resource for exploring many innovative strategies to improve the safety and cross-protective efficacy of live PRRSV vaccines. Examples include vaccines with diminished ability to down-regulate the immune system, positive and negative marker vaccines, multivalent vaccines incorporating antigens from other porcine pathogens, vaccines that carry their own cytokine adjuvants, and chimeric vaccine viruses with the potential for broad cross-protection against heterologous strains. To combat this devastating pig disease in the future, evaluation and commercialization of such improved live PRRSV vaccines is a shared goal among PRRSV researchers, pork producers and biologics companies.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Simian hemorrhagic fever virus: Recent advances

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SHFV induces hemorrhagic fever in macaques but not in African nonhuman primates.

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SHFV infection of macaque but not baboon cells induces proinflammatory cytokines.

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Unique N- and C-terminal genes encoded by SHFV were functionally analyzed.

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PLP1γ can cleave at upstream sites as well as at the expected downstream site.

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Eight minor structural proteins are required for infectious virus production.

The simian hemorrhagic fever virus (SHFV) genome differs from those of other members of the family Arteriviridae in encoding three papain-like one proteases (PLP1α, PLP1β and PLP1γ) at the 5′ end and two adjacent sets of four minor structural proteins at the 3′ end. The catalytic Cys and His residues and cleavage sites for each of the SHFV PLP1s were predicted and their functionality was tested in in vitro transcription/translation reactions done with wildtype or mutant polyprotein constructs. Mass spectrometry analyses of selected autoproteolytic products confirmed cleavage site locations. The catalytic Cys of PLP1α is unusual in being adjacent to an Ala instead of a Typ. PLP1γ cleaves at both downstream and upstream sites. Intermediate precursor and alternative cleavage products were detected in the in vitro transcription/translation reactions but only the three mature nsp1 proteins were detected in SHFV-infected MA104 cell lysates with SHFV nsp1 protein-specific antibodies. The duplicated sets of SHFV minor structural proteins were predicted to be functionally redundant. A stable, full-length, infectious SHFV-LVR cDNA clone was constructed and a set of mutant infectious clones was generated each with the start codon of one of the minor structural proteins mutated. All eight of the minor structural proteins were found to be required for production of infectious extracellular virus. SHFV causes a fatal hemorrhagic fever in macaques but asymptomatic, persistent infections in natural hosts such as baboons. SHFV infections were compared in macrophages and myeloid dendritic cells from baboons and macaques. Virus yields were higher from macaque cells than from baboon cells. Macrophage cultures from the two types of animals differed dramatically in the percentage of cells infected. In contrast, similar percentages of myeloid dendritic cells were infected but virus replication was efficient in the macaque cells but inefficient in the baboon cells. SHFV infection induced the production of pro-inflammatory cytokines, including IL-1β, IL-6, IL-12/23(p40), TNF-α and MIP-1α, in macaque cells but not baboon cells.

Full-length genome sequence analysis of a Hungarian porcine reproductive and respiratory syndrome virus isolated from a pig with severe respiratory disease

Here, we report the isolation of a type 1 porcine reproductive and respiratory syndrome virus (PRRSV) strain from a clinical outbreak of severe respiratory problems and high fever. Next-generation sequencing was used to determine the complete genome sequence of the isolate (9625/2012). The virus belongs to a new branch within subtype 1, clade D, and shows the highest similarity to PRRSV Olot/1991 and to the Amervac vaccine strain. Mutation analysis of 9625/2012 revealed no evidence of recombination but did show a high proportion of amino acid substitutions in the putative neutralizing epitopes, suggesting an important role of selective immune pressure in the evolution of PRRSV 9625/2012.

Engineering the PRRS virus genome: updates and perspectives

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We review PRRSV infectious clones and their applications.

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14 infectious clones are available so far for genotypes I and II.

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Genomic mutations, insertions, deletions, and replacements are successful.

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We discuss advances and utilization of PRRSV reverse genetics and future potential.

Porcine reproductive and respiratory syndrome virus (PRRSV) is endemic in most pig producing countries worldwide and causes enormous economic losses to the pork industry. Infectious clones for PRRSV have been constructed, and so far at least 14 different infectious clones are available representing both genotypes I and II. Two strategies have been taken for progeny reconstitution: RNA transfection and DNA transfection. Mutations, insertions, deletions, and replacements of the viral genome have been employed to study the structure function relationship, foreign gene expression, functional complementation, and virulence determinants. Essential regions and non-essential regions for viral replication have been identified in both the coding regions and non-encoding regions. Foreign sequences have successfully been inserted into the nsp2 and N regions and in the space between ORF1b and ORF2a. Chimeras between member viruses in the family Arteriviridae have also been constructed and utilized to study cell tropism and functional complementation. This review discusses the advances and utilization of PRRSV reverse genetics and its potential for future research.

Modulation of innate immune signaling by nonstructural protein 1 (nsp1) in the family Arteriviridae

Arteriviruses infect immune cells and may cause persistence in infected hosts. Inefficient induction of pro-inflammatory cytokines and type I IFNs are observed during infection of this group of viruses, suggesting that they may have evolved to escape the host immune surveillance for efficient survival. Recent studies have identified viral proteins regulating the innate immune signaling, and among these, nsp1 (nonstructural protein 1) is the most potent IFN antagonist. For porcine reproductive and respiratory syndrome virus (PRRSV), individual subunits (nsp1α and nsp1β) of nsp1 suppress type I IFN production. In particular, PRRSV-nsp1α degrades CREB (cyclic AMP responsive element binding)-binding protein (CBP), a key component of the IFN enhanceosome, whereas PRRSV-nsp1β degrades karyopherin-α1 which is known to mediate the nuclear import of ISGF3 (interferon-stimulated gene factor 3). All individual subunits of nsp1 of PRRSV, equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV) appear to contain IFN suppressive activities. As with PRRSV-nsp1α, CBP degradation is evident by LDV-nsp1α and partly by SHFV-nsp1γ. This review summarizes the biogenesis and the role of individual subunits of nsp1 of arteriviruses for innate immune modulation.

Functional analyses of the three simian hemorrhagic fever virus nonstructural protein 1 papain-like proteases

The N-terminal region of simian hemorrhagic fever virus (SHFV) nonstructural polyprotein 1a is predicted to encode three papain-like proteases (PLP1α, PLP1β, and PLP1γ). Catalytic residues and cleavage sites for each of the SHFV PLP1s were predicted by alignment of the SHFV PLP1 region sequences with each other as well as with those of other arteriviruses, and the predicted catalytic residues were shown to be proximal by homology modeling of the SHFV nsp1s on porcine respiratory and reproductive syndrome virus (PRRSV) nsp1 crystal structures. The functionality of the predicted catalytic Cys residues and cleavage sites was tested by analysis of the autoproteolytic products generated in in vitro transcription/translation reactions done with wild-type or mutant SHFV nsp1 constructs. Cleavage sites were also analyzed by mass spectroscopy analysis of selected immunoprecipitated cleavage products. The data showed that each of the three SHFV PLP1s is an active protease. Cys63 was identified as the catalytic Cys of SHFV PLP1α and is adjacent to an Ala instead of the canonical Tyr observed in other arterivirus PLP1s. SHFV PLP1γ is able to cleave at both downstream and upstream nsp1 junction sites. Although intermediate precursor polyproteins as well as alternative products generated by each of the SHFV PLP1s cleaving at sites within the N-terminal region of nsp1β were produced in the in vitro reactions, Western blotting of SHFV-infected, MA104 cell lysates with SHFV nsp1 protein-specific antibodies detected only the three mature nsp1 proteins.

IMPORTANCE

SHFV is unique among arteriviruses in having three N-terminal papain-like protease 1 (PLP1) domains. Other arteriviruses encode one or two active PLP1s. This is the first functional study of the SHFV PLP1s. Analysis of the products of in vitro autoprocessing of an N-terminal SHFV nonstructural 1a polypeptide fragment showed that each of the three SHFV PLP1s is active, and the predicted catalytic Cys residues and cleavage sites for each PLP1 were confirmed by testing mutant constructs. Several unique features of the SHFV PLP1s were discovered. The SHFV PLP1α catalytic Cys63 is unique among arterivirus PLP1s in being adjacent to an Ala instead of a Trp. Other arterivirus PLP1s cleave only in cis at a single downstream site, but SHFV PLP1γ can cleave at both the downstream nsp1γ-nsp2 and upstream nsp1β-nsp1γ junctions. The three mature nsp1 proteins were produced both in the in vitro reactions and in infected cells.

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.

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.

Biogenesis of non-structural protein 1 (nsp1) and nsp1-mediated type I interferon modulation in arteriviruses

Type I interferons (IFNs-α/β) play a key role for the antiviral state of host, and the porcine arterivirus; porcine reproductive and respiratory syndrome virus (PRRSV), has been shown to down-regulate the production of IFNs during infection. Non-structural protein (nsp) 1 of PRRSV has been identified as a viral IFN antagonist, and the nsp1α subunit of nsp1 has been shown to degrade the CREB-binding protein (CBP) and to inhibit the formation of enhanceosome thus resulting in the suppression of IFN production. The study was expanded to other member viruses in the family Arteriviridae: equine arteritis virus (EAV), murine lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV). While PRRSV-nsp1 and LDV-nsp1 were auto-cleaved to produce the nsp1α and nsp1β subunits, EAV-nsp1 remained uncleaved. SHFV-nsp1 was initially predicted to be cleaved to generate three subunits (nsp1α, nsp1β, and nsp1γ), but only two subunits were generated as SHFV-nsp1αβ and SHFV-nsp1γ. The papain-like cysteine protease (PLP) 1α motif in nsp1α remained inactive for SHFV, and only the PLP1β motif of nsp1β was functional to generate SHFV-nsp1γ subunit. All subunits of arterivirus nsp1 were localized in the both nucleus and cytoplasm, but PRRSV-nsp1β, LDV-nsp1β, EAV-nsp1, and SHFV-nsp1γ were predominantly found in the nucleus. All subunits of arterivirus nsp1 contained the IFN suppressive activity and inhibited both interferon regulatory factor 3 (IRF3) and NF-κB mediated IFN promoter activities. Similar to PRRSV-nsp1α, CBP degradation was evident in cells expressing LDV-nsp1α and SHFV-nsp1γ, but no such degradation was observed for EAV-nsp1. Regardless of CBP degradation, all subunits of arterivirus nsp1 suppressed the IFN-sensitive response element (ISRE)-promoter activities. Our data show that the nsp1-mediated IFN modulation is a common strategy for all arteriviruses but their mechanism of action may differ from each other.

Equine arteritis virus

Equine arteritis virus (EAV) is the causative agent of equine viral arteritis (EVA), a respiratory and reproductive disease of equids. There has been significant recent progress in understanding the molecular biology of EAV and the pathogenesis of its infection in horses. In particular, the use of contemporary genomic techniques, along with the development and reverse genetic manipulation of infectious cDNA clones of several strains of EAV, has generated significant novel information regarding the basic molecular biology of the virus. Therefore, the objective of this review is to summarize current understanding of EAV virion architecture, replication, evolution, molecular epidemiology and genetic variation, pathogenesis including the influence of host genetics on disease susceptibility, host immune response, and potential vaccination and treatment strategies.

Arterivirus molecular biology and pathogenesis

Arteriviruses are positive-stranded RNA viruses that infect mammals. They can cause persistent or asymptomatic infections, but also acute disease associated with a respiratory syndrome, abortion or lethal haemorrhagic fever. During the past two decades, porcine reproductive and respiratory syndrome virus (PRRSV) and, to a lesser extent, equine arteritis virus (EAV) have attracted attention as veterinary pathogens with significant economic impact. Particularly noteworthy were the 'porcine high fever disease' outbreaks in South-East Asia and the emergence of new virulent PRRSV strains in the USA. Recently, the family was expanded with several previously unknown arteriviruses isolated from different African monkey species. At the molecular level, arteriviruses share an intriguing but distant evolutionary relationship with coronaviruses and other members of the order Nidovirales. Nevertheless, several of their characteristics are unique, including virion composition and structure, and the conservation of only a subset of the replicase domains encountered in nidoviruses with larger genomes. During the past 15 years, the advent of reverse genetics systems for EAV and PRRSV has changed and accelerated the structure-function analysis of arterivirus RNA and protein sequences. These systems now also facilitate studies into host immune responses and arterivirus immune evasion and pathogenesis. In this review, we have summarized recent advances in the areas of arterivirus genome expression, RNA and protein functions, virion architecture, virus-host interactions, immunity, and pathogenesis. We have also briefly reviewed the impact of these advances on disease management, the engineering of novel candidate live vaccines and the diagnosis of arterivirus infection.

Characterization of a serologic marker candidate for development of a live-attenuated DIVA vaccine against porcine reproductive and respiratory syndrome virus

DIVA (differentiating infected from vaccinated animals) vaccines have proven extremely useful for control and eradication of infectious diseases in livestock. We describe here the characterization of a serologic marker epitope, so-called epitope-M201, which can be a potential target for development of a live-attenuated DIVA vaccine against porcine reproductive and respiratory syndrome virus (PRRSV). Epitope-M201 is located at the carboxyl terminus (residues 161-174) of the viral M protein. The epitope is highly immunodominant and well-conserved among type-II PRRSV isolates. Rabbit polyclonal antibodies prepared against this epitope are non-neutralizing; thus, the epitope does not seem to contribute to the protective immunity against PRRSV infection. Importantly, the immunogenicity of epitope-M201 can be disrupted through the introduction of a single amino acid mutation which does not adversely affect the viral replication. All together, our results provide an important starting point for the development of a live-attenuated DIVA vaccine against type-II PRRSV.

Porcine reproductive and respiratory syndrome virus Nsp1β inhibits interferon-activated JAK/STAT signal transduction by inducing karyopherin-α1 degradation

Porcine reproductive and respiratory syndrome virus (PRRSV) inhibits the interferon-mediated antiviral response. Type I interferons (IFNs) induce the expression of IFN-stimulated genes by activating phosphorylation of both signal transducer and activator of transcription 1 (STAT1) and STAT2, which form heterotrimers (interferon-stimulated gene factor 3 [ISGF3]) with interferon regulatory factor 9 (IRF9) and translocate to the nucleus. PRRSV Nsp1β blocks the nuclear translocation of the ISGF3 complex by an unknown mechanism. In this study, we discovered that Nsp1β induced the degradation of karyopherin-α1 (KPNA1, also called importin-α5), which is known to mediate the nuclear import of ISGF3. Overexpression of Nsp1β resulted in a reduction of KPNA1 levels in a dose-dependent manner, and treatment of the cells with the proteasome inhibitor MG132 restored KPNA1 levels. Furthermore, the presence of Nsp1β induced an elevation of KPNA1 ubiquitination and a shortening of its half-life. Our analysis of Nsp1β deletion constructs showed that the N-terminal domain of Nsp1β was involved in the ubiquitin-proteasomal degradation of KPNA1. A nucleotide substitution resulting in an amino acid change from valine to isoleucine at residue 19 of Nsp1β diminished its ability to induce KPNA1 degradation and to inhibit IFN-mediated signaling. Interestingly, infection of MARC-145 cells by PRRSV strains VR-2332 and VR-2385 also resulted in KPNA1 reduction, whereas infection by an avirulent strain, Ingelvac PRRS modified live virus (MLV), did not. MLV Nsp1β had no effect on KPNA1; however, a mutant with an amino acid change at residue 19 from isoleucine to valine induced KPNA1 degradation. These results indicate that Nsp1β blocks ISGF3 nuclear translocation by inducing KPNA1 degradation and that valine-19 in Nsp1β correlates with the inhibition.