Modulation of type I interferon induction by porcine reproductive and respiratory syndrome virus and degradation of CREB-binding protein by non-structural protein 1 in MARC-145 and HeLa cells

Knock down of transforming growth factor beta improves expressions of co-stimulatory molecules, type I interferon-regulated genes, and pro-inflammatory cytokine in PRRSV-inoculated monocyte-derived macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) induces a poor innate immune response following infection. This study evaluates the effects of transforming growth factor beta 1 (TGFβ1) up-regulated by PRRSV on gene expressions of co-stimulatory molecules, type I interferon (IFN), type I IFN-regulated genes (IRGs), pattern recognition receptors, and pro-inflammatory cytokines in PRRSV-inoculated monocyte-derived macrophages (MDMs). Phosphorothioate-modified antisense oligodeoxynucleotides (AS ODNs) specific to various regions of porcine TGFβ1 mRNA were synthesized, and those specific to the AUG region efficiently knockdown TGFβ1 mRNA expression and protein translation. Transfection of TGFβAS ODNs in MDMs inoculated with either classical PRRSV-2 (cPRRSV-2) or highly pathogenic PRRSV-2 (HP-PRRSV-2) significantly reduced TGFβ1 mRNA expression and significantly increased mRNA expressions of CD80, CD86, IFNβ, IRGs (i.e. IFN regulatory factor 3 (IRF3), IRF7, myxovirus resistance 1, osteopontin, and stimulator of IFN genes), Toll-like receptor 3, and tumor necrosis factor-alpha. Transfection of TGFβAS ODNs in MDMs inoculated with HP-PRRSV-2 also significantly increased mRNA expressions of IFNα, IFNγ, and 2'-5'-oligoadenylate synthetase 1. The quantity of PRRSV-2 RNA copy numbers was significantly reduced in MDMs transfected with TGFβAS ODNs as compared to untransfected MDMs. Recombinant porcine TGFβ1 (rTGFβ1) and recombinant porcine IFNα (rIFNα) sustained and reduced the yields of PRRSV-2 RNA copy numbers in PRRSV-2 inoculated MDMs, respectively. These findings demonstrate a strategy of PRRSV for innate immune suppression via an induction of TGFβ expression. These findings also suggest TGFβ as a potential parameter that future PRRSV vaccine and vaccine adjuvant candidates should take into consideration.

Innate Immune Evasion of PRRSV nsp11 through Degradation of the HDAC2 by Its Endoribonuclease Activity

Porcine reproductive and respiratory syndrome virus (PRRSV), a member of the Arteriviridae family, represents a persistent menace to the global pig industry, causing reproductive failure and respiratory disease in pigs. In this study, we delved into the role of histone deacetylases (HDAC2) during PRRSV infection. Our findings revealed that HDAC2 expression is downregulated upon PRRSV infection. Notably, suppressing HDAC2 activity through specific small interfering RNA led to an increase in virus production, whereas overexpressing HDAC2 effectively inhibited PRRSV replication by boosting the expression of IFN-regulated antiviral molecules. Furthermore, we identified the virus's nonstructural protein 11 (nsp11) as a key player in reducing HDAC2 levels. Mutagenic analyses of PRRSV nsp11 revealed that its antagonistic effect on the antiviral activity of HDAC2 is dependent on its endonuclease activity. In summary, our research uncovered a novel immune evasion mechanism employed by PRRSV, providing crucial insights into the pathogenesis of this virus and guiding the development of innovative prevention strategies against PRRSV infection.

Type I and II interferons, transcription factors and major histocompatibility complexes were enhanced by knocking down the PRRSV-induced transforming growth factor beta in monocytes co-cultured with peripheral blood lymphocytes

The innate and adaptive immune responses elicited by porcine reproductive and respiratory syndrome virus (PRRSV) infection are known to be poor. This study investigates the impact of PRRSV-induced transforming growth factor beta 1 (TGFβ1) on the expressions of type I and II interferons (IFNs), transcription factors, major histocompatibility complexes (MHC), anti-inflammatory and pro-inflammatory cytokines in PRRSV-infected co-cultures of monocytes and peripheral blood lymphocytes (PBL). Phosphorothioate-modified antisense oligodeoxynucleotide (AS ODN) specific to the AUG region of porcine TGFβ1 mRNA was synthesized and successfully knocked down TGFβ1 mRNA expression and protein translation. Monocytes transfected with TGFβAS1 ODN, then simultaneously co-cultured with PBL and inoculated with either classical PRRSV-2 (cPRRSV-2) or highly pathogenic PRRSV-2 (HP-PRRSV-2) showed a significant reduction in TGFβ1 mRNA expression and a significant increase in the mRNA expressions of IFNα, IFNγ, MHC-I, MHC-II, signal transducer and activator of transcription 1 (STAT1), and STAT2. Additionally, transfection of TGFβAS1 ODN in the monocyte and PBL co-culture inoculated with cPRRSV-2 significantly increased the mRNA expression of interleukin-12p40 (IL-12p40). PRRSV-2 RNA copy numbers were significantly reduced in monocytes and PBL co-culture transfected with TGFβAS1 ODN compared to the untransfected control. The yields of PRRSV-2 RNA copy numbers in PRRSV-2-inoculated monocytes and PBL co-culture were sustained and reduced by porcine TGFβ1 (rTGFβ1) and recombinant porcine IFNα (rIFNα), respectively. These findings highlight the strategy employed by PRRSV to suppress the innate immune response through the induction of TGFβ expression. The inclusion of TGFβ as a parameter for future PRRSV vaccine and vaccine adjuvant candidates is recommended.

Porcine interferon-α linked to the porcine IgG-Fc induces prolonged and broad-spectrum antiviral effects against foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is an economically important disease, and the FMD virus (FMDV) can spread rapidly in susceptible animals. FMD is usually controlled through vaccination. However, commercial FMD vaccines are only effective 4-7 days after vaccination. Furthermore, FMDV comprises seven serotypes and various topotypes, and these aspects should be considered when selecting a vaccine. Antiviral agents could provide rapid and broad protection against FMDV. Therefore, this study aimed to develop a fusion protein of consensus porcine interferon-α and Fc portion of porcine antibody IgG (poIFN-α-Fc) using a baculovirus expression system to develop a novel antiviral agent against FMDV. We measured the antiviral effects of the poIFN-α-Fc protein against FMDV and the enhanced duration in vitro and in vivo. The broad-spectrum antiviral effects were tested against seven FMDV serotypes, porcine reproductive and respiratory syndrome virus (PRRSV), and bovine enterovirus (BEV). Furthermore, the early protective effects and neutralizing antibody levels were tested by co-injecting poIFN-α-Fc and an FMD-inactivated vaccine into mice or pigs. Sustained antiviral effects in pig sera and mice were observed, and pigs injected with a combination of the poIFN-α-Fc and an inactivated FMD vaccine were protected against FMDV in a dose-dependent manner at 2- and 4-days post-vaccination. In addition, combined with the inactivated FMD vaccine, poIFN-α-Fc increased the neutralizing antibody levels in mice. Therefore, poIFN-α-Fc is a potential broad-spectrum antiviral and adjuvant candidate that can be used with inactivated FMD vaccines to protect pigs against FMDV.

A clinically attenuated double-mutant of porcine reproductive and respiratory syndrome virus-2 that does not prompt overexpression of proinflammatory cytokines during co-infection with a secondary pathogen

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to suppress the type I interferon (IFNs-α/β) response during infection. PRRSV also activates the NF-κB signaling pathway, leading to the production of proinflammatory cytokines during infection. In swine farms, co-infections of PRRSV and other secondary bacterial pathogens are common and exacerbate the production of proinflammatory cytokines, contributing to the porcine respiratory disease complex (PRDC) which is clinically a severe disease. Previous studies identified the non-structural protein 1β (nsp1β) of PRRSV-2 as an IFN antagonist and the nucleocapsid (N) protein as the NF-κB activator. Further studies showed the leucine at position 126 (L126) of nsp1β as the essential residue for IFN suppression and the region spanning the nuclear localization signal (NLS) of N as the NF-κB activation domain. In the present study, we generated a double-mutant PRRSV-2 that contained the L126A mutation in the nsp1β gene and the NLS mutation (ΔNLS) in the N gene using reverse genetics. The immunological phenotype of this mutant PRRSV-2 was examined in porcine alveolar macrophages (PAMs) in vitro and in young pigs in vivo. In PAMs, the double-mutant virus did not suppress IFN-β expression but decreased the NF-κB-dependent inflammatory cytokine productions compared to those for wild-type PRRSV-2. Co-infection of PAMs with the mutant PRRSV-2 and Streptococcus suis (S. suis) also reduced the production of NF-κB-directed inflammatory cytokines. To further examine the cytokine profiles and the disease severity by the mutant virus in natural host animals, 6 groups of pigs, 7 animals per group, were used for co-infection with the mutant PRRSV-2 and S. suis. The double-mutant PRRSV-2 was clinically attenuated, and the expressions of proinflammatory cytokines and chemokines were significantly reduced in pigs after bacterial co-infection. Compared to the wild-type PRRSV-2 and S. suis co-infection control, pigs coinfected with the double-mutant PRRSV-2 exhibited milder clinical signs, lower titers and shorter duration of viremia, and lower expression of proinflammatory cytokines. In conclusion, our study demonstrates that genetic modification of the type I IFN suppression and NF-κB activation functions of PRRSV-2 may allow us to design a novel vaccine candidate to alleviate the clinical severity of PRRS-2 and PRDC during bacterial co-infection.

Suppression of TRIM19 by arterivirus nonstructural protein 1 promotes viral replication

Tripartite motif (TRIM)-containing proteins are a family of regulatory proteins that can participate in the induction of antiviral cytokines and antagonize viral replication. Promyelocytic leukemia (PML) protein is known as TRIM19 and is a major scaffold protein organizing the PML nuclear bodies (NBs). PML NBs are membrane-less organelles in the nucleus and play a diverse role in maintaining cellular homeostasis including antiviral response. Porcine reproductive and respiratory syndrome virus (PRRSV), a member virus of the family Arteriviridae, inhibits type I interferon (IFN) response during infection, and nonstructural protein 1 (nsp1) of the virus has been identified as a potent IFN antagonist. We report that the numbers of PML NBs per nucleus were significantly downregulated during infection of PRRSV. The overexpression of all six isoforms of PML suppressed the PRRSV replication, and conversely, the silencing of PML gene expression enhanced the PRRSV replication. The suppression of PML NBs by the nsp1 protein was common in other member viruses of the family, represented by equine arteritis virus, lactate dehydrogenase elevating virus of mice, and simian hemorrhagic fever virus. Our study unveils a conserved viral strategy in arteriviruses for innate immune evasion.

The Identification of Host Proteins That Interact with Non-Structural Proteins-1α and -1β of Porcine Reproductive and Respiratory Syndrome Virus-1

Porcine reproductive and respiratory syndrome viruses (PRRSV-1 and -2) are the causative agents of one of the most important infectious diseases affecting the global pig industry. Previous studies, largely focused on PRRSV-2, have shown that non-structural protein-1α (NSP1α) and NSP1β modulate host cell responses; however, the underlying molecular mechanisms remain to be fully elucidated. Therefore, we aimed to identify novel PRRSV-1 NSP1-host protein interactions to improve our knowledge of NSP1-mediated immunomodulation. NSP1α and NSP1β from a representative western European PRRSV-1 subtype 1 field strain (215-06) were used to screen a cDNA library generated from porcine alveolar macrophages (PAMs), the primary target cell of PRRSV, using the yeast-2-hybrid system. This identified 60 putative binding partners for NSP1α and 115 putative binding partners for NSP1β. Of those taken forward for further investigation, 3 interactions with NSP1α and 27 with NSP1β were confirmed. These proteins are involved in the immune response, ubiquitination, nuclear transport, or protein expression. Increasing the stringency of the system revealed NSP1α interacts more strongly with PIAS1 than PIAS2, whereas NSP1β interacts more weakly with TAB3 and CPSF4. Our study has increased our knowledge of the PRRSV-1 NSP1α and NSP1β interactomes, further investigation of which could provide detailed insight into PRRSV immunomodulation and aid vaccine development.

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.

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

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

Reprogramming viral immune evasion for a rational design of next-generation vaccines for RNA viruses

Type I interferons (IFNs-α/β) are antiviral cytokines that constitute the innate immunity of hosts to fight against viral infections. Recent studies, however, have revealed the pleiotropic functions of IFNs, in addition to their antiviral activities, for the priming of activation and maturation of adaptive immunity. In turn, many viruses have developed various strategies to counteract the IFN response and to evade the host immune system for their benefits. The inefficient innate immunity and delayed adaptive response fail to clear of invading viruses and negatively affect the efficacy of vaccines. A better understanding of evasion strategies will provide opportunities to revert the viral IFN antagonism. Furthermore, IFN antagonism-deficient viruses can be generated by reverse genetics technology. Such viruses can potentially serve as next-generation vaccines that can induce effective and broad-spectrum responses for both innate and adaptive immunities for various pathogens. This review describes the recent advances in developing IFN antagonism-deficient viruses, their immune evasion and attenuated phenotypes in natural host animal species, and future potential as veterinary vaccines.

Evasion of interferon-mediated immune response by arteriviruses

IFN is the most potent antiviral cytokine required for the innate and adaptive immune responses, and its expression can help the host defend against viral infection. Arteriviruses have evolved strategies to antagonize the host cell’s innate immune responses, interfering with IFN expression by interfering with RIG, blocking PRR, obstructing IRF-3/7, NF-κB, and degrading STAT1 signaling pathways, thereby assisting viral immune evasion. Arteriviruses infect immune cells and may result in persistence in infected hosts. In this article, we reviewed the strategies used by Arteriviruses to antagonize IFN production and thwart IFN-activated antiviral signaling, mainly including structural and nonstructural proteins of Arteriviruses encoding IFN antagonists directly or indirectly to disrupt innate immunity. This review will certainly provide a better insight into the pathogenesis of the arthritis virus and provide a theoretical basis for developing more efficient vaccines.

Oral Supplementation of Houttuynia cordata Extract Reduces Viremia in PRRSV-1 Modified-Live Virus-Vaccinated Pigs in Response to the HP-PRRSV-2 Challenge

This study evaluated the in vitro antiviral activities and the ex vivo immunomodulatory effects of Houttuynia cordata Thunb. (HC) ethanolic extracts in response to porcine reproductive and respiratory syndrome virus (PRRSV). In addition, this study evaluated the in vivo effects of oral supplementation of HC extract on immune responses to and cross-protective efficacy of PRRSV-1 modified-live virus (MLV) vaccine against the highly pathogenic (HP)-PRRSV-2 challenge. In vitro experiments demonstrated that HC extracted in either 50%, 70%, or 95% ethanol (referred to as HC50, HC70, and HC95, respectively) significantly interfered with PRRSV replication in MARC-145 cells. Ex vivo experiments revealed that all HC extracts significantly enhanced mRNA expressions of type I interferon-regulated genes, type I and II interferon (IFN), and pro- and anti-inflammatory cytokines in HP-PRRSV-2-inoculated monocyte-derived macrophages. An in vivo experiment included four groups of six pigs (4 weeks old; n = 24). Group 1 and group 2 were vaccinated with the PRRSV-1 MLV vaccine at 0 dpv (day post vaccination). Group 2 also received oral administration of HC50 extract at 0–49 dpv. Group 3 received the PRRSV-1 MLV vaccine solvent at 0 dpv, while group 4 served as strict control. Groups 1–3 were challenged intranasally with HP-PRRSV-2 at 28 dpv and immune-related and clinical parameters were monitored weekly until 49 dpv. Compared to group 1, group 2 demonstrated significantly increased IFN regulatory factor 3 mRNA expression of PRRSV-recalled peripheral blood mononuclear cells, and significantly reduced HP-PRRSV-2 viremia. No difference in PRRSV-specific antibody responses, rectal temperature, clinical scores, and average daily weight gain was detected. Our study reports the immunomodulatory and anti-PRRSV potentials of HC extract in PRRSV-1 MLV-vaccinated/HP-PRRSV-2 challenged pigs.

Porcine Reproductive and Respiratory Syndrome Virus Adapts Antiviral Innate Immunity via Manipulating MALT1

To fulfill virus replication and persistent infection in hosts, viruses have to find ways to compromise innate immunity, including timely impedance on antiviral RNases and inflammatory responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is a major swine pathogen causing immune suppression. MALT1 is a central immune regulator in both innate and adaptive immunity. In this study, MALT1 was confirmed to be induced rapidly upon PRRSV infection and mediate the degradation of two anti-PRRSV RNases, MCPIP1 and N4BP1, relying on its proteolytic activity, consequently facilitating PRRSV replication. Multiple PRRSV nsps, including nsp11, nsp7β, and nsp4, contributed to MALT1 elicitation. Interestingly, the elevated expression of MALT1 began to decrease once intracellular viral expression reached a high enough level. Higher infection dose brought earlier MALT1 inflection. Further, PRRSV nsp6 mediated significant MALT1 degradation via ubiquitination-proteasome pathway. Downregulation of MALT1 suppressed NF-κB signals, leading to the decrease in proinflammatory cytokine expression. In conclusion, MALT1 expression was manipulated by PRRSV in an elaborate manner to antagonize precisely the antiviral effects of host RNases without excessive and continuous activation of inflammatory responses. These findings throw light on the machinery of PRRSV to build homeostasis in infected immune system for viral settlement.

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.

The long non-coding RNA LNC_000397 negatively regulates PRRSV replication through induction of interferon-stimulated genes

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most significant threats to the global swine industry. It is of great importance to understand viral-host interactions to develop novel antiviral strategies. Long non-coding RNAs (lncRNAs) have emerged as critical factors regulating host antiviral immune responses. However, lncRNAs participating in virus-host interactions during PRRSV infection remain largely unexplored.

Method

RNA transcripts of porcine alveolar macrophages (PAMs) infected with two different PRRSV strains, GSWW/2015 and VR2332, at 24 h post-infection were sequenced by high-throughput sequencing. Four programs namely, CNCI, CPC, PFAM, and phyloCSF, were utilized to predict the coding potential of transcripts. mRNAs co-localized or co-expressed with differentially expressed lncRNAs were considered as their targets. Fuction of lncRNAs was predicted by GO and KEGG analysis of their target mRNAs. The effect of LNC_000397 on PRRSV replication was validated by knockdown its expression using siRNA. Target genes of LNC_000397 were identified by RNA-Sequencing and validated by RT-qPCR.

Result

In this study, we analyzed lncRNA and mRNA expression profiles of PRRSV GSWW/2015 and VR2332 infected porcine alveolar macrophages. A total of 1,147 novel lncRNAs were characterized, and 293 lncRNAs were differentially expressed. mRNAs co-localized and co-expressed with lncRNAs were enriched in pathogen-infection-related biological processes such as Influenza A and Herpes simplex infection. Functional analysis revealed the lncRNA, LNC_000397, which was up-regulated by PRRSV infection, negatively regulated PRRSV replication. Knockdown of LNC_000397 significantly impaired expression of antiviral ISGs such as MX dynamin-like GTPase 1 (MX1), ISG15 Ubiquitin-like modifier (ISG15), and radical S-adenosyl methionine domain containing 2 (RSAD2).

Conclusions

LNC_000397 negatively regulated PRRSV replication by inducing interferon-stimulated genes (ISGs) expression. Our study is the first report unveiling the role of host lncRNA in regulating PRRSV replication, which might be beneficial for the development of novel antiviral therapeutics.

Ribosome profiling of porcine reproductive and respiratory syndrome virus reveals novel features of viral gene expression

The arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses to the swine industry worldwide. Here we apply ribosome profiling (RiboSeq) and parallel RNA sequencing (RNASeq) to characterise the transcriptome and translatome of both species of PRRSV and to analyse the host response to infection. We calculated programmed ribosomal frameshift (PRF) efficiency at both sites on the viral genome. This revealed the nsp2 PRF site as the second known example where temporally regulated frameshifting occurs, with increasing -2 PRF efficiency likely facilitated by accumulation of the PRF-stimulatory viral protein, nsp1β. Surprisingly, we find that PRF efficiency at the canonical ORF1ab frameshift site also increases over time, in contradiction of the common assumption that RNA structure-directed frameshift sites operate at a fixed efficiency. This has potential implications for the numerous other viruses with canonical PRF sites. Furthermore, we discovered several highly translated additional viral ORFs, the translation of which may be facilitated by multiple novel viral transcripts. For example, we found a highly expressed 125-codon ORF overlapping nsp12, which is likely translated from novel subgenomic RNA transcripts that overlap the 3' end of ORF1b. Similar transcripts were discovered for both PRRSV-1 and PRRSV-2, suggesting a potential conserved mechanism for temporally regulating expression of the 3'-proximal region of ORF1b. We also identified a highly translated, short upstream ORF in the 5' UTR, the presence of which is highly conserved amongst PRRSV-2 isolates. These findings reveal hidden complexity in the gene expression programmes of these important nidoviruses.

Porcine Reproductive and Respiratory Syndrome Virus Evades Antiviral Innate Immunity via MicroRNAs Regulation

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important diseases in pigs, leading to significant economic losses in the swine industry worldwide. MicroRNAs (miRNAs) are small single-stranded non-coding RNAs involved in regulating gene expressions at the post-transcriptional levels. A variety of host miRNAs are dysregulated and exploited by PRRSV to escape host antiviral surveillance and help virus infection. In addition, PRRSV might encode miRNAs. In this review, we will summarize current progress on how PRRSV utilizes miRNAs for immune evasions. Increasing knowledge of the role of miRNAs in immune evasion will improve our understanding of PRRSV pathogenesis and help us develop new treatments for PRRSV-associated diseases.

Porcine Reproductive and Respiratory Syndrome Virus nsp11 Antagonizes Broad Antiviral Effects of MCPIP1 by Inducing Interleukin-17 Expression

Monocyte chemotactic protein-induced protein 1 (MCPIP1) is an inflammatory regulator in immune response and has broad antiviral effects by targeting viral RNA. Porcine reproductive and respiratory syndrome virus (PRRSV), a major viral pathogen in pigs, causes immune suppression leading to coinfection of swine pathogens, but the mechanisms are not fully clarified. In this study, MCPIP1 expression was found to be significantly upregulated in lungs of PRRSV-infected piglets, as well as in Marc-145 and porcine pulmonary alveolar macrophage (PAM) cells upon PRRSV stimulation. MCPIP1 overexpression significantly inhibited PRRSV replication, while MCPIP1 knockdown increased the virus titer. Various mutations in RNase functional domains of MCPIP1 impaired the inhibitory activity against PRRSV, while those in deubiquitinase domains failed to do so. MCPIP1 expression started to decrease from 60 h after PRRSV infection in PAMs. Meanwhile, infection with higher dose of PRRSV further downregulated MCPIP1, indicating the antagonizing effects from PRRSV against MCPIP1. Moreover, it was confirmed that MCPIP1 expression was downregulated in 3D4 cells with either interleukin-17 (IL-17) or nsp11 overexpression, while IL-17 inhibitor abolished the decrease of MCPIP1 caused by nsp11, indicating nsp11 employs IL-17 induction to inhibit MCPIP1. Furthermore, the PRRSV nsp11 mutant with a deficiency in IL-17 induction showed the recovered expression of MCPIP1 in infected cells, inspiring a strategy for virus attenuation. This is the first report about the role of MCPIP1 against PRRSV and the function of PRRSV nsp11 against innate immunity to facilitate virus replication via IL-17. The study not only illuminates PRRSV infection machinery but also enlightens alternative antiviral strategies, such as vaccine candidates. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) suppresses the innate immunity and leads to coinfection of swine pathogens. Monocyte chemotactic protein-induced protein 1 (MCPIP1) is a broad-spectrum host antiviral protein. Therefore, to further clarify the mechanism of PRRSV against innate immunity, we explored the relationship between MCPIP1 and PRRSV infection. The results showed that MCPIP1 inhibited PRRSV infection in the early stage of virus infection. Importantly, PRRSV nsp11 subsequently employed IL-17 induction to suppress MCPIP1 expression and antagonized anti-PRRSV effects. Furthermore, PRRSV with mutation of nsp11 S74A failed to induce MCPIP1 reduction. These findings confirmed the function of MCPIP1 against PRRSV and revealed that PRRSV nsp11 plays an important role in virus against innate immunity. This study enlightens a new strategy to develop safer attenuated vaccines against PRRSV by nsp11 mutation.

Evasion of Antiviral Innate Immunity by Porcine Reproductive and Respiratory Syndrome Virus

Innate immunity is the front line for antiviral immune responses and bridges adaptive immunity against viral infections. However, various viruses have evolved many strategies to evade host innate immunity. A typical virus is the porcine reproductive and respiratory syndrome virus (PRRSV), one of the most globally devastating viruses threatening the swine industry worldwide. PRRSV engages several strategies to evade the porcine innate immune responses. This review focus on the underlying mechanisms employed by PRRSV to evade pattern recognition receptors signaling pathways, type I interferon (IFN-α/β) receptor (IFNAR)-JAK-STAT signaling pathway, and interferon-stimulated genes. Deciphering the antiviral immune evasion mechanisms by PRRSV will enhance our understanding of PRRSV’s pathogenesis and help us to develop more effective methods to control and eliminate PRRSV.

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.

The Function of the PRRSV-Host Interactions and Their Effects on Viral Replication and Propagation in Antiviral Strategies

Porcine reproductive and respiratory syndrome virus (PRRSV) affects the global swine industry and causes disastrous economic losses each year. The genome of PRRSV is an enveloped single-stranded positive-sense RNA of approximately 15 kb. The PRRSV replicates primarily in alveolar macrophages of pig lungs and lymphatic organs and causes reproductive problems in sows and respiratory symptoms in piglets. To date, studies on how PRRSV survives in the host, the host immune response against viral infections, and pathogenesis, have been reported. PRRSV vaccines have been developed, including inactive virus, modified live virus, attenuated live vaccine, DNA vaccine, and immune adjuvant vaccines. However, there are certain problems with the durability and effectiveness of the licensed vaccines. Moreover, the high variability and fast-evolving populations of this RNA virus challenge the design of PRRSV vaccines, and thus effective vaccines against PRRSV have not been developed successfully. As is well known, viruses interact with the host to escape the host’s immune response and then replicate and propagate in the host, which is the key to virus survival. Here, we review the complex network and the mechanism of PRRSV–host interactions in the processes of virus infection. It is critical to develop novel antiviral strategies against PRRSV by studying these host–virus interactions and structures to better understand the molecular mechanisms of PRRSV immune escape.

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

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.

Oral supplementation of quercetin in PRRSV-1 modified-live virus vaccinated pigs in response to HP-PRRSV-2 challenge

This study evaluated the immunomodulatory effect of quercetin on improving cross protection of porcine reproductive and respiratory syndrome virus-1 (PRRSV-1) modified-live virus (MLV) vaccine against highly pathogenic (HP)-PRRSV-2 challenge. Ex vivo experiments demonstrated that quercetin significantly enhanced type I interferon-regulated genes (IRGs) and type I and II interferon (IFN), and significantly decreased pro- and anti-inflammatory cytokine expressions in HP-PRRSV-inoculated monocyte-derived macrophages. In vivo experiments divided pigs (4-week-old; n = 24) into four groups of six pigs. Group 1 and group 2 were immunized with PRRSV-1 MLV vaccine at 0 dpv (day post vaccination). Group 2 also received oral administration of quercetin at 0-49 dpv. Group 3 was injected with PRRSV-1 MLV vaccine solvent at 0 dpv. Group 4 served as strict control. Group 1-3 were challenged intranasally with HP-PRRSV at 28 dpv and immune and clinical parameters were monitored weekly until 49 dpv. Group 1 demonstrated significantly reduced HP-PRRSV viremia, rectal temperature and clinical scores, and significantly improved average daily weight gain (ADWG), compared to group 3. Group 2 demonstrated significantly increased IFN regulatory factor 3, stimulator of IFN genes, IFNα, and significantly decreased transforming growth factor beta (TGFβ) mRNA expressions, compared to group 1. The animals demonstrated significantly reduced HP-PRRSV viremia, but did not demonstrate any further improved PRRSV-specific antibody responses, rectal temperature, clinical scores, and ADWG as compared to group 1. Our findings suggest that quercetin up-regulates IRGs, IFNα, and down-regulates TGFβ mRNA expressions which may contribute to further reducing number of viremic pigs and HP-PRRSV viremia which were conferred by PRRSV-1 MLV vaccine. Our findings also suggest that quercetin may serve as an effective oral immunomodulator for improving cell-mediated immune defense to HP-PRRSV.

RNA sequencing analyses of gene expressions in a canine macrophages cell line DH82 infected with canine distemper virus

Virulent morbillivirus infections, including Meals Virus (MeV) and Canine Distemper Virus (CDV), caused severe immune suppression and leukopenia, while attenuated vaccine strains developed protective host immune responses. However, the detailed molecular foundations of host antiviral responses were poorly characterized. In order to better understand the interactions between attenuated vaccine and host antiviral responses, the global gene expression changes in CDV-11-infected DH82 cells, a macrophage-derived cell line from canine, were investigated by transcriptomic analysis, and portions of results were confirmed with quantitative RT-PCR. The results exhibited that 372 genes significantly up-regulated (p < .01) and 119 genes were significantly down-regulated (p < .01) in CDV-infected macrophages DH82 at 48 h p.i.. The enriched functions of the significantly up-regulated (p < .01) genes were closely associated with interferon stimulated genes (ISGs), chemokine genes and pro-inflammatory factor genes. Gene ontology and pathway analysis of differentially expressed genes (DEGs) revealed that the most significantly involved pathways in CDV-infected DH82 cells were NF-κB and TNF signaling pathway, cytokine-cytokine receptor interaction, and pathogen associated molecular patterns (PAMPs), such as Toll-like, RIG-I-like and NOD-like receptor signalings. Thus, the findings indicated that pattern recognition receptors (PRRs) possibly mediated host innate and protective antiviral immune responses in CDV-11 infected DH82 cells.

S100A9 regulates porcine reproductive and respiratory syndrome virus replication by interacting with the viral nucleocapsid protein

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses to the pig industry worldwide over the last 30 years, yet the associated viral-host interactions remain poorly understood. S100A9 is a damage-associated molecular pattern of the S100 protein family. Here, we found that PRRSV infection stimulated S100A9 expression in porcine alveolar macrophages (PAMs) and Marc-145 cells. S100A9 inhibited PRRSV replication via cellular Ca²⁺ dependent manner. The viral nucleocapsid (N) protein co-localized with S100A9 in the cytoplasm, and directly interacted at amino acid 78 of S100A9 and amino acids 36-37 of N protein. Moreover, we also found that the mutant S100A9 (E78Q) protein exhibited decreased antiviral activity against PRRSV compared with the parent S100A9. Recombinant PRRSV rBB (36/37) with two mutations in amino acid 36-37 in the N protein exhibited greater replication than the parent PRRSV BB0907 in S100A9-overexpressed PAM and Marc-145 cells. Thus, S100A9 may restrict PRRSV proliferation by interacting with the viral N protein.

Nonstructural Protein 11 of Porcine Reproductive and Respiratory Syndrome Virus Induces STAT2 Degradation To Inhibit Interferon Signaling

Interferons (IFNs) play a crucial role in host antiviral response by activating the JAK/STAT (Janus kinase/signal transducer and activator of transcription) signaling pathway to induce the expression of myriad genes. STAT2 is a key player in the IFN-activated JAK/STAT signaling. Porcine reproductive and respiratory syndrome virus (PRRSV) is an important viral pathogen, causing huge losses to the swine industry. PRRSV infection elicits a meager protective immune response in pigs. The objective of this study was to investigate the effect of PRRSV on STAT2 signaling. Here, we demonstrated that PRRSV downregulated STAT2 to inhibit IFN-activated signaling. PRRSV strains of both PRRSV-1 and PRRSV-2 species reduced the STAT2 protein level, whereas the STAT2 transcript level had minimal change. PRRSV reduced the STAT2 level in a dose-dependent manner and shortened STAT2 half-life significantly from approximately 30 to 5 h. PRRSV-induced STAT2 degradation could be restored by treatment with the proteasome inhibitor MG132 and lactacystin. In addition, PRRSV nonstructural protein 11 (nsp11) was identified to interact with and reduce STAT2. The N-terminal domain (NTD) of nsp11 was responsible for STAT2 degradation and interacted with STAT2 NTD and the coiled-coil domain. Mutagenesis analysis showed that the amino acid residue K59 of nsp11 was indispensable for inducing STAT2 reduction. Mutant PRRSV with the K59A mutation generated by reverse genetics almost lost the ability to reduce STAT2. Together, these results demonstrate that PRRSV nsp11 antagonizes IFN signaling via mediating STAT2 degradation and provide further insights into the PRRSV interference of the innate immunity.IMPORTANCE PRRSV infection elicits a meager protective immune response in pigs. One of the possible reasons is that PRRSV antagonizes interferon induction and its downstream signaling. Interferons are key components in the innate immunity and play crucial roles against viral infection and in the activation of adaptive immune response via JAK/STAT signaling. STAT2 is indispensable in the JAK/STAT signaling since it is also involved in activation of antiviral activity in the absence of STAT1. Here, we discovered that PRRSV nsp11 downregulates STAT2. Interestingly, the N-terminal domain of nsp11 is responsible for inducing STAT2 degradation and directly interacts with STAT2 N-terminal domain. We also identified a crucial amino acid residue K59 in nsp11 since a mutation of it led to loss of the ability to downregulate STAT2. A mutant PRRSV with mutation of K59 had minimal effect on STAT2 reduction. Our data provide further insights into PRRSV interference with interferon signaling.

PP2A Facilitates Porcine Reproductive and Respiratory Syndrome Virus Replication by Deactivating irf3 and Limiting Type I Interferon Production

Protein phosphatase 2A (PP2A), a major serine/threonine phosphatase in mammalian cells, is known to regulate the kinase-driven intracellular signaling pathways. Emerging evidences have shown that the PP2A phosphatase functions as a bona-fide therapeutic target for anticancer therapy, but it is unclear whether PP2A affects a porcine reproductive and respiratory syndrome virus infection. In the present study, we demonstrated for the first time that inhibition of PP2A activity by either inhibitor or small interfering RNA duplexes in target cells significantly reduced their susceptibility to porcine reproductive and respiratory syndrome virus (PRRSV) infection. Further analysis revealed that inhibition of PP2A function resulted in augmented production of type I interferon (IFN). The mechanism is that inhibition of PP2A activity enhances the levels of phosphorylated interferon regulatory factor 3, which activates the transcription of IFN-stimulated genes. Moreover, inhibition of PP2A activity mainly blocked PRRSV replication in the early stage of viral life cycle, after virus entry but before virus release. Using type I IFN receptor 2 specific siRNA in combination with PP2A inhibitor, we confirmed that the effect of PP2A on viral replication within target cells was an interferon-dependent manner. Taken together, these findings demonstrate that PP2A serves as a negative regulator of host cells antiviral responses and provides a novel therapeutic target for virus infection.

Porcine Reproductive and Respiratory Syndrome Virus Nonstructural Protein 1 Beta Interacts with Nucleoporin 62 To Promote Viral Replication and Immune Evasion

Porcine reproductive and respiratory syndrome virus (PRRSV) blocks host mRNA nuclear export to the cytoplasm, and nonstructural protein 1 beta (nsp1β) of PRRSV has been identified as the protein that disintegrates the nuclear pore complex. In the present study, the molecular basis for the inhibition of host mRNA nuclear export was investigated. Nucleoporin 62 (Nup62) was found to bind to nsp1β, and the region representing the C-terminal residues 328 to 522 of Nup62 was determined to be the binding domain for nsp1β. The nsp1β L126A mutant in the SAP domain did not bind to Nup62, and in L126A-expressing cells, host mRNA nuclear export occurred normally. The vL126A mutant PRRSV generated by reverse genetics replicated at a lower rate, and the titer was lower than for wild-type virus. In nsp1β-overexpressing cells or small interfering RNA (siRNA)-mediated Nup62 knockdown cells, viral protein synthesis increased. Notably, the production of type I interferons (IFN-α/β), IFN-stimulated genes (PKR, OAS, Mx1, and ISG15 genes), IFN-induced proteins with tetratricopeptide repeats (IFITs) 1 and 2, and IFN regulatory factor 3 decreased in these cells. As a consequence, the growth of vL126A mutant PRRSV was rescued to the level of wild-type PRRSV. These findings are attributed to nuclear pore complex (NPC) disintegration by nsp1β, resulting in increased viral protein production and decreased host protein production, including antiviral proteins in the cytoplasm. Our study reveals a new strategy of PRRSV for immune evasion and enhanced replication during infection.IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) causes PRRS and is known to effectively suppress host innate immunity. The PRRSV nsp1β protein blocks host mRNA nuclear export, which has been shown to be one of the viral mechanisms for inhibition of antiviral protein production. nsp1β binds to the cellular protein nucleoporin 62 (Nup62), and as a consequence, the nuclear pore complex (NPC) is disintegrated and the nucleocytoplasmic trafficking of host mRNAs and host proteins is blocked. We show the dual benefits of Nup62 and nsp1β binding for PRRSV replication: the inhibition of host antiviral protein expression and the exclusive use of host translation machinery by the virus. Our study unveils a novel strategy of PRRSV for immune evasion and enhanced replication during infection.

Porcine reproductive and respiratory syndrome virus nsp1β and nsp11 antagonize the antiviral activity of cholesterol-25-hydroxylase via lysosomal degradation

Porcine reproductive and respiratory syndrome virus (PRRSV) is an immunosuppressive pathogen which has been recognized to modulate the host interferon (IFN) systems. Cholesterol-25-hydroxylase (CH25 H) is an important interferon-stimulated gene (ISG)-encoded polytopic membrane protein that significantly inhibits the replication of many viruses. In the current study, we showed that PRRSV infection induced the down-regulation of the endogenous CH25H in porcine alveolar macrophages (PAMs), and then discovered that the nonstructural protein (nsp) 1β and nsp11 of PRRSV could mediate the reduction of porcine CH25H d in HEK 293FT cells. Next, the amino acids including His-159 in nsp1β, and His-129, His-144 and Lys-173 in nsp11 were determined to play crucial roles in the reduction of CH25H. Furthermore, we confirmed that the nsp1β and nsp11 mediated the degradation of CH25H by lysosomal pathway in HEK 293FT cells. Finally, it was demonstrated that the anti-PRRSV activity of CH25H could be antagonized by nsp1β and nsp11 in MARC-145 cells. Our findings suggest a manner of antagonizing the antiviral activity of CH25H by PRRSV, and provide novel insight into the understanding of PRRSV's ability of escaping the innate immunity of host.

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.

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.

Transcriptome Analysis Reveals Dynamic Gene Expression Profiles in Porcine Alveolar Macrophages in Response to the Chinese Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically important swine pathogens and causes reproductive failure in sows and respiratory disease in growing pigs. PRRSV mainly infects porcine alveolar macrophages (PAMs), leading to the subversion of innate and adaptive immunity of pigs. The transcriptome analysis of gene expression profiles in PRRSV-infected PAMs is essential for understanding the pathogenesis of PRRSV. Here we performed next-generation RNA sequencing and a comprehensive bioinformatics analysis to characterize the dynamic transcriptome landscapes in PAMs following PRRSV infection. Totally 38222 annotated mRNAs, 12987 annotated long noncoding RNAs (lncRNAs), and 17624 novel lncRNAs in PRRSV-infected PAMs were identified through a transcripts computational identification pipeline. The differentially expressed mRNAs and lncRNAs during PRRSV infection were characterized. Several differentially expressed transcripts were validated using qRT-PCR. Analyses on dynamic overrepresented GO terms and KEGG pathways in PRRSV-infected PAMs at different time points were performed. Meanwhile the genes involved in IFN-related signaling pathways, proinflammatory cytokines and chemokines, phagocytosis, and antigen presentation and processing were significantly downregulated, indicating the aberrant function of PAMs during PRRSV infection. Moreover, the differentially and highly expressed lncRNA XR_297549.1 was predicted to both cis-regulate and trans-regulate its neighboring gene, prostaglandin-endoperoxide synthase 2 (PTGS2), indicating its role in inflammatory response. Our findings reveal the transcriptome profiles and differentially expressed mRNAs and lncRNAs in PRRSV-infected PAMs in vitro, providing valuable information for further exploration of PRRSV pathogenesis.

Type I interferon suppression-negative and host mRNA nuclear retention-negative mutation in nsp1β confers attenuation of porcine reproductive and respiratory syndrome virus in pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) has the ability to suppress the type I interferons (IFNs-α/β) induction to facilitate its survival during infection, and the nsp1 protein of PRRSV has been identified as the potent IFN antagonist. The nsp1β subunit of nsp1 has also been shown to block the host mRNA nuclear export as one of the mechanisms to suppress host antiviral protein production. The SAP motif in nsp1β is the functional motif for both IFN suppression and host mRNA nuclear retention, and using infectious clones, two mutant viruses vL126A and vL135A have been generated. These mutants retain the infectivity, but the phenotype is negative for both IFN suppression and host mRNA nuclear retention due to the loss of the SAP motif. To examine the pathogenic role of IFN suppression in pigs, 40 piglets were allotted to four groups and each group was intramuscularly infected with vL126A, vL135A, wild-type (WT) PRRSV, and placebo. Pigs infected with vL126A or vL135A exhibited mild clinical signs with low viral titers and short duration of viremia. The levels of PRRSV-specific antibody remained comparable in all infected groups but the neutralizing antibody titers were high in vL126A-infected or vL135A-infected pigs. The IFN-α concentration was also high in pigs infected with the SAP mutants. Reversion to WT sequence was observed in the SAP motif in some animals, and the revertants regained the function to suppress IFN production and host mRNA nuclear export, indicating strong selection pressure in the SAP motif of nsp1β. Together, our data demonstrate that the IFN antagonism and host mRNA nuclear retention mediated by nsp1β contributes to viral virulence, and loss of these functions confers PRRSV attenuation.

Proteome analysis of differential protein expression in porcine alveolar macrophages regulated by porcine reproductive and respiratory syndrome virus nsp1β protein

Porcine reproductive and respiratory syndrome virus (PRRSV), an acute infectious disease agent in swine, causes enormous economic losses to the global swine industry. PRRSV nonstructural protein 1β (nsp1β) plays a critical role in viral subgenomic mRNA synthesis and host immune regulation. However, the global changes of cellular gene expression in natural target cells regulated by the nsp1β have not yet been identified. Here, isobaric tags for relative and absolute quantification (iTRAQ) labeling coupled with liquid chromatography-tandem mass spectrometry were used to quantitatively identify cellular proteins in porcine alveolar macrophage (PAM) 3D4/21 cells transduced with recombinant lentivirus expressing PRRSV nsp1β that are differentially expressed compared with PAM 3D4/21 cells transduced with recombinant lentivirus expressing GFP. Of the 425 cellular proteins detected as differentially expressed, 186 were upregulated and 239 were downregulated. Based on the identities of the differentially expressed cellular proteins and the essential role of nsp1β in interferon (IFN) activation and inflammatory factor antagonism during PRRSV infection, we propose a potential mechanism in which nsp1β inhibits IFN induction and nuclear factor κB (NF-κB) signaling pathways. Our results suggest that mitochondrial antiviral signaling (MAVS) protein and translocases of outer membrane complex 70 (TOM70), involved in type I IFN induction, were downregulated, while protein phosphatase 1A (PPM1A), related to the inhibition of NF-κB pathway activation, was upregulated in nsp1β-overexpressed PAM 3D4/21 cells. These data provide valuable information for better understanding the potential biological function of nsp1β during PRRSV infection and the mechanism of virus escape from host immune surveillance of viral replication.

Saponin Quil A up-regulates type I interferon-regulated gene and type I and II interferon expressions which are suppressed by porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) suppresses innate immune response following infection of myeloid antigen-presenting cells. Poor innate immune response results in weak and delayed PRRSV-specific adaptive immunity, and facilitates PRRSV replication, pathogenesis, and persistent infection. Numerous efforts have been made to enhance the effective innate and adaptive immune defenses to PRRSV, however, only a few attempts have so far elicited satisfactory results. The present study aims to evaluate in vitro the potential of saponin quil A to enhance the expression of type I interferon (IFN)-regulated gene, type I and II IFNs, and pro-inflammatory cytokines in PRRSV-inoculated peripheral blood mononuclear cells (PBMC). Naïve PBMC from four PRRSV-seronegative pigs were inoculated with PRRSV and subsequently stimulated with quil A in the absence or presence of either polyinosinic:polycytidylic acid (poly IC) or lipopolysaccharide (LPS). The mRNA expression levels of myxovirus resistance 1 (Mx1), interferon regulatory factor 3 (IRF3), IRF7, 2'-5'-oligoadenylatesynthetase 1 (OAS1), stimulator of interferon genes (STING), osteopontin (OPN), IFNα, IFNβ, IFNγ, interleukin-2 (IL-2), IL-10, IL-13, tumor necrosis factor alpha (TNFα), and transforming growth factor beta (TGFβ) were evaluated by real-time PCR. Compared with uninoculated PBMC, PRRSV significantly suppressed expression of all immune parameters except IL-2, IL-10, IL-13, and TGFβ. When compared with PRRSV-inoculated PBMC, stimulation with quil A significantly enhanced Mx1, IRF3, IRF7, OAS1, STING, IFNβ, and IFNγ mRNA expressions, and significantly reduced TGFβ mRNA expression. Our findings thus suggest that quil A has a potential to up-regulate the expression of type I IFN-regulated gene and type I and II IFNs which are suppressed by PRRSV. Therefore, it may serve as an effective immunostimulator for potentiating the innate immune defense to PRRSV.

Swine IRF3/IRF7 attenuates inflammatory responses through TLR4 signaling pathway

To explore the role of IRF3/IRF7 during inflammatory responses, we investigated the effects of swine IRF3/IRF7 on TLR4 signaling pathway and inflammatory factors expression in porcine kidney epithelial PK15 cell lines. We successfully constructed eukaryotic vectors PB-IRF3 and PB-IRF7, transfected these vectors into PK15 cells and observed GFP under a fluorescence microscope. In addition, RT-PCR was also used to detect transfection efficiency. We found that IRF3/IRF7 was efficiently overexpressed in PK15 cells. Moreover, we evaluated the effects of IRF3/IRF7 on the TLR4 signaling pathway and inflammatory factors by RT-PCR. Transfected cells were treated with lipopolysaccharide (LPS) alone, or in combination with a TBK1 inhibitor (LiCl). We revealed that IRF3/IRF7 enhanced IFNα production, and decreased IL-6 mRNA expression. Blocking the TBK1 pathway, inhibited the changes in IFNα, but not IL-6 mRNA. This illustrated that IRF3/IRF7 enhanced IFNα production through TLR4/TBK1 signaling pathway and played an anti-inflammatory role, while IRF3/IRF7 decreased IL-6 expression independent of the TBK1 pathway. Trends in MyD88, TRAF6, TBK1 and NFκB mRNA variation were similar in all treatments. LPS increased MyD88, TRAF6, TBK1 and NFκB mRNA abundance in PBR3/PBR7 and PBv cells, while LiCl blocked the LPS-mediated effects. The levels of these four factors in PBR3/PBR7 cells were higher than those in PBv. These results demonstrated that IRF3/IRF7 regulated the inflammatory response through the TLR4 signaling pathway. Overexpression of swine IRF3/IRF7 in PK15 cells induced type I interferons production, and attenuated inflammatory responses through TLR4 signaling pathway.

The viral innate immune antagonism and an alternative vaccine design for PRRS virus

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PRRS virus has evolved to suppress the antiviral innate immunity during infection.

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Type I interferons are potent antiviral cytokines and function to stimulate the adaptive immune responses.

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Six viral proteins have been identified as interferon antagonists and characterized for their molecular actions.

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Interferon antagonism-negative viruses are attenuated and have been proven induce protective immunity.

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Interferon suppression-negative PRRS virus may serve as an alternative vaccine for PRRS.

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically significant diseases in the swine industry worldwide. The current vaccines are less satisfactory to confer protections from heterologous infections and long-term persistence, and the need for better vaccines are urgent. The immunological hallmarks in PRRSV-infected pigs include the unusually poor production of type I interferons (IFNs-α/β) and the aberrant and delayed adaptive immune responses, indicating that PRRSV has the ability to suppress both innate and adaptive immune responses in the host. Type I IFNs are the potent antiviral cytokines and recent studies reveal their pleiotropic functions in the priming of expansion and maturation of adaptive immunity. Thus, IFN antagonism-negative PRRSV is hypothesized to be attenuated and to build effective and broad- spectrum innate and adaptive immune responses in pigs. Such vaccines are promising alternatives to traditional vaccines for PRRSV.

Nuclear imprisonment of host cellular mRNA by nsp1β protein of porcine reproductive and respiratory syndrome virus

Positive-strand RNA genomes function as mRNA for viral protein synthesis which is fully reliant on host cell translation machinery. Competing with cellular protein translation apparatus needs to ensure the production of viral proteins, but this also stifles host innate defense. In the present study, we showed that porcine reproductive and respiratory syndrome virus (PRRSV), whose replication takes place in the cytoplasm, imprisoned host cell mRNA in the nucleus, which suggests a novel mechanism to enhance translation of PRRSV genome. PRRSV nonstructural protein (nsp) 1β was identified as the nuclear protein playing the role for host mRNA nuclear retention and subversion of host protein synthesis. A SAP (SAF-A/B, Acinus, and PIAS) motif was identified in nsp1β with the consensus sequence of ₁₂₆-LQxxLxxxGL-₁₃₅. In situ hybridization unveiled that SAP mutants were unable to cause nuclear retention of host cell mRNAs and did not suppress host protein synthesis. In addition, these SAP mutants reverted PRRSV-nsp1β-mediated suppression of interferon (IFN) production, IFN signaling, and TNF-α production pathway. Using reverse genetics, a series of SAP mutant PRRS viruses, vK124A, vL126A, vG134A, and vL135A were generated. No mRNA nuclear retention was observed during vL126A and vL135A infections. Importantly, vL126A and vL135A did not suppress IFN production. For other arteriviruses, mRNA nuclear accumulation was also observed for LDV-nsp1β and SHFV-nsp1β. EAV-nsp1 was exceptional and did not block the host mRNA nuclear export.

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PRRS virus blocks host mRNA nuclear export to the cytoplasm.

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PRRSV nsp1β is the viral protein responsible for host mRNA nuclear retention.

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SAP domain in nsp1β is essential for host mRNA nuclear retention and type I interferon suppression.

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Mutation in the SAP domain of nsp1β causes the loss of function.

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Host mRNA nuclear retention by nsp1β is common in the family Arteriviridae, except equine arteritis virus.

Porcine Reproductive and Respiratory Syndrome Virus nsp1α Inhibits NF-κB Activation by Targeting the Linear Ubiquitin Chain Assembly Complex

Linear ubiquitination, a newly discovered posttranslational modification, is catalyzed by the linear ubiquitin chain assembly complex (LUBAC), which is composed of three subunits: one catalytic subunit HOIP and two accessory molecules, HOIL-1L and SHARPIN. Accumulating evidence suggests that linear ubiquitination plays a crucial role in innate immune signaling and especially in the activation of the NF-κB pathway by conjugating linear polyubiquitin chains to NF-κB essential modulator (NEMO, also called IKKγ), the regulatory subunit of the IKK complex. Porcine reproductive and respiratory syndrome virus (PRRSV), an Arterivirus that has devastated the swine industry worldwide, is an ideal model to study the host's disordered inflammatory responses after viral infection. Here, we found that LUBAC-induced NF-κB and proinflammatory cytokine expression can be inhibited in the early phase of PRRSV infection. Screening the PRRSV-encoded proteins showed that nonstructural protein 1α (nsp1α) suppresses LUBAC-mediated NF-κB activation and its CTE domain is required for the inhibition. Mechanistically, nsp1α binds to HOIP/HOIL-1L and impairs the interaction between HOIP and SHARPIN, thus reducing the LUBAC-dependent linear ubiquitination of NEMO. Moreover, PRRSV infection also blocks LUBAC complex formation and NEMO linear-ubiquitination, the important step for transducing NF-κB signaling. This unexpected finding demonstrates a previously unrecognized role of PRRSV nsp1α in modulating LUBAC signaling and explains an additional mechanism of immune modulation by PRRSV.

IMPORTANCE

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important veterinary infectious diseases in countries with intensive swine industries. PRRS virus (PRRSV) infection usually suppresses proinflammatory cytokine expression in the early stage of infection, whereas it induces an inflammatory storm in the late stage. However, precisely how the virus is capable of doing so remains obscure. In this study, we found that by blocking the interaction of its catalytic subunit HOIP and accessory molecule SHARPIN, PRRSV can suppress NF-κB signal transduction in the early stage of infection. Our findings not only reveal a novel mechanism evolved by PRRSV to regulate inflammatory responses but also highlight the important role of linear ubiquitination modification during virus infection.

Nonstructural Protein 11 of Porcine Reproductive and Respiratory Syndrome Virus Suppresses Both MAVS and RIG-I Expression as One of the Mechanisms to Antagonize Type I Interferon Production

Type I interferons (IFN-α/β) play a key role in antiviral defense, and porcine reproductive and respiratory syndrome virus (PRRSV) is known to down-regulate the IFN response in virus-infected cells and pigs. In this study, we showed that the overexpression of nsp11 of PRRSV induced a strong suppression of IFN production. Nsp11 suppressed both IRF3 and NF-κB activities when stimulated with a dsRNA analogue and TNF-α, respectively. This suppression was RLR dependent, since the transcripts and proteins of MAVS and RIG-I, two critical factors in RLR-mediated pathway, were both found to be reduced in the presence of overexpressed nsp11. Since nsp11 is an endoribonuclease (EndoU), the structure function relationship was examined using a series of nsp11 EndoU mutant plasmids. The mutants that impaired the EndoU activity failed to suppress IFN and led to the normal expression of MAVS. Seven single amino acid substitutions (4 in subdomain A and 3 in subdomain B) plus one insertion (frame-shift in nsp11) were then introduced into PRRSV infectious cDNA clones to generate nsp11 mutant viruses. Unfortunately, all EndoU knock-out nsp11 mutant viruses appeared replication-defective and no progenies were produced. Three mutations in EndoU subdomain A expressed the N and nsp2/3 proteins but their infectivity diminished after 2 passages. Taken together, our data show that PRRSV nsp11 endoribonuclease activity is critical for both viral replication and IFN antagonism. More importantly, the endoribonuclease activity of nsp11 demonstrates the substrate specificity towards MAVS and RIG-I (transcripts and proteins) over p65 and IRF3 in the context of gene transfection and overexpression. This is likely a mechanism of nsp11 suppression of type I IFN production.

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.

Viral infection causes a shift in the self peptide repertoire presented by human MHC class I molecules

PURPOSE

MHC class I presentation of peptides allows T cells to survey the cytoplasmic protein milieu of host cells. During infection, presentation of self peptides is, in part, replaced by presentation of microbial peptides. However, little is known about the self peptides presented during infection, despite the fact that microbial infections alter host cell gene expression patterns and protein metabolism.

EXPERIMENTAL DESIGN

The self peptide repertoire presented by HLA-A*01;01, HLA-A*02;01, HLA-B*07;02, HLA-B*35;01, and HLA-B*45;01 (where HLA is human leukocyte antigen) was determined by tandem MS before and after vaccinia virus infection.

RESULTS

We observed a profound alteration in the self peptide repertoire with hundreds of self peptides uniquely presented after infection for which we have coined the term "self peptidome shift." The fraction of novel self peptides presented following infection varied for different HLA class I molecules. A large part (approximately 40%) of the self peptidome shift arose from peptides derived from type I interferon-inducible genes, consistent with cellular responses to viral infection. Interestingly, approximately 12% of self peptides presented after infection showed allelic variation when searched against approximately 300 human genomes.

CONCLUSION AND CLINICAL RELEVANCE

Self peptidome shift in a clinical transplant setting could result in alloreactivity by presenting new self peptides in the context of infection-induced inflammation.

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.

Interferon regulatory factor 3 is a key regulation factor for inducing the expression of SAMHD1 in antiviral innate immunity

SAMHD1 is a type I interferon (IFN) inducible host innate immunity restriction factor that inhibits an early step of the viral life cycle. The underlying mechanisms of SAMHD1 transcriptional regulation remains elusive. Here, we report that inducing SAMHD1 upregulation is part of an early intrinsic immune response via TLR3 and RIG-I/MDA5 agonists that ultimately induce the nuclear translocation of the interferon regulation factor 3 (IRF3) protein. Further studies show that IRF3 plays a major role in upregulating endogenous SAMHD1 expression in a mechanism that is independent of the classical IFN-induced JAK-STAT pathway. Both overexpression and activation of IRF3 enhanced the SAMHD1 promoter luciferase activity, and activated IRF3 was necessary for upregulating SAMHD1 expression in a type I IFN cascade. We also show that the SAMHD1 promoter is a direct target of IRF3 and an IRF3 binding site is sufficient to render this promoter responsive to stimulation. Collectively, these findings indicate that upregulation of endogenous SAMHD1 expression is attributed to the phosphorylation and nuclear translocation of IRF3 and we suggest that type I IFN induction and induced SAMHD1 expression are coordinated.

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nsp4 Cleaves VISA to Impair Antiviral Responses Mediated by RIG-I-like Receptors

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most significant etiological agents in the swine industry worldwide. It has been reported that PRRSV infection can modulate host immune responses, and innate immune evasion is thought to play a vital role in PRRSV pathogenesis. In this study, we demonstrated that highly pathogenic PRRSV (HP-PRRSV) infection specifically down-regulated virus-induced signaling adaptor (VISA), a unique adaptor molecule that is essential for retinoic acid induced gene-I (RIG-I) and melanoma differentiation associated gene 5 (MDA5) signal transduction. Moreover, we verified that nsp4 inhibited IRF3 activation induced by signaling molecules, including RIG-I, MDA5, VISA, and TBK1, but not IRF3. Subsequently, we demonstrated that HP-PRRSV nsp4 down-regulated VISA and suppressed type I IFN induction. Importantly, VISA was cleaved by nsp4 and released from mitochondrial membrane, which interrupted the downstream signaling of VISA. However, catalytically inactive mutant of nsp4 abolished its ability to cleave VISA. Interestingly, nsp4 of typical PRRSV strain CH-1a had no effect on VISA. Taken together, these findings reveal a strategy evolved by HP-PRRSV to counteract anti-viral innate immune signaling, which complements the known PRRSV-mediated immune-evasion mechanisms.

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.