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

Intervention strategies targeting virus and host factors against porcine reproductive and respiratory syndrome virus: A systematic review

Porcine reproductive and respiratory syndrome (PRRS) caused by porcine reproductive and respiratory syndrome virus (PRRSV) causes considerable economic losses to the global swine industry every year and seriously hinders the healthy development of this industry. Although tremendous efforts have been made over the past 30 years toward the development of prevention and control strategies against PRRSV infection, to date, treatments with proven efficacy have yet to be available due to our incomplete understanding of the molecular basis and complexity of the infection machinery. This review systematically discusses recent advances in the research and development of anti-PRRSV therapies targeting different stages of the viral life cycle. Furthermore, this review puts forward novel intervention targets and research approaches based on our in-depth exploration of virus-host interactions and the latest biological technologies, which have the potential to complement or transform current anti-PRRSV strategies and become breakthrough points for the control of PRRS in the future.

Research progress on the pattern recognition receptors involved in porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically devastating infectious diseases of pigs globally. The pathogen, porcine reproductive and respiratory syndrome virus (PRRSV), is an enveloped positive-stranded RNA virus, which is considered to be the key triggers for the activation of effective innate immunity through pattern recognition receptor (PRR)-dependent signaling pathways. Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs) and Cytoplasmic DNA receptors (CDRs) are used as PRRs to identify distinct but overlapping microbial components. The innate immune system has evolved to recognize RNA or DNA molecules from microbes through pattern recognition receptors (PRRs) and to induce defense response against infections, including the production of type I interferon (IFN-I) and inflammatory cytokines. However, PRRSV is capable of continuous evolution through gene mutation and recombination to evade host immune defenses and exploit host cell mechanisms to synthesize and transport its components, thereby facilitating successful infection and replication. This review presents the research progress made in recent years in the study of these PRRs and their associated adapters during PRRSV infection.

Pseudorabies virus usurps non-muscle myosin heavy chain IIA to dampen viral DNA recognition by cGAS for antagonism of host antiviral innate immunity

Alphaherpesvirus pseudorabies virus (PRV) causes severe economic losses to the global pig industry and has garnered increasing attention due to its broad host range including humans. PRV has developed a variety of strategies to antagonize host antiviral innate immunity. However, the underlying mechanisms have not been fully elucidated. In our previous work, we demonstrated that non-muscle myosin heavy chain IIA (NMHC-IIA), a multifunctional cytoskeleton protein, attenuates innate immune responses triggered by RNA viruses. In the current study, we reported a previously unrecognized role of NMHC-IIA in counteracting PRV-induced cyclic GMP-AMP synthase (cGAS)-dependent type I interferon (IFN-I) production. Mechanistically, PRV infection led to an elevation of NMHC-IIA, strengthening the interaction between poly (ADP-ribose) polymerase 1 (PARP1) and cGAS. This interaction impeded cGAS recognition of PRV DNA and hindered downstream signaling activation. Conversely, inhibition of NMHC-IIA by Blebbistatin triggered innate immune responses and enhanced resistance to PRV proliferation both in vitro and in vivo. Taken together, our findings unveil that PRV utilizes NMHC-IIA to antagonize host antiviral immune responses via impairing DNA sensing by cGAS. This in-depth understanding of PRV immunosuppression not only provides insights for potential PRV treatment strategies but also highlights NMHC-IIA as a versatile immunosuppressive regulator usurped by both DNA and RNA viruses. Consequently, NMHC-IIA holds promise as a target for the development of broad-spectrum antiviral drugs.IMPORTANCECyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) axis plays a vital role in counteracting alphaherpesvirus infections. Alphaherpesviruses exploit various strategies for antagonizing cGAS-STING-mediated antiviral immune responses. However, limited examples of pseudorabies virus (PRV)-caused immunosuppression have been documented. Our findings reveal a novel role of non-muscle myosin heavy chain IIA (NMHC-IIA) in suppressing PRV-triggered innate immune responses to facilitate viral propagation both in vitro and in vivo. In detail, NMHC-IIA recruits poly (ADP-ribose) polymerase 1 (PARP1) to augment its interaction with cGAS, which impairs cGAS recognition of PRV DNA. Building on our previous demonstration of NMHC-IIA's immunosuppressive role during RNA virus infections, these findings indicate that NMHC-IIA acts as a broad-spectrum suppressor of host antiviral innate immunity in response to both DNA and RNA viruses. Therefore, NMHC-IIA will be a promising target for the development of comprehensive antiviral strategies.

NS7a of SADS-CoV promotes viral infection via inducing apoptosis to suppress type III interferon production

Swine acute diarrhea syndrome coronavirus (SADS-CoV) is a newly discovered swine coronavirus with potential cross-species transmission risk. Although SADS-CoV-induced host cell apoptosis and innate immunity antagonization has been revealed, underlying signaling pathways remain obscure. Here, we demonstrated that infection of SADS-CoV induced apoptosis in vivo and in vitro, and that viral protein NS7a is mainly responsible for SADS-CoV-induced apoptosis in host cells. Furthermore, we found that NS7a interacted with apoptosis-inducing factor mitochondria associated 1 (AIFM1) to activate caspase-3 via caspase-6 in SADS-CoV-infected cells, and enhanced SADS-CoV replication. Importantly, NS7a suppressed poly(I:C)-induced expression of type III interferon (IFN-λ) via activating caspase-3 to cleave interferon regulatory factor 3 (IRF3), and caspase-3 inhibitor protects piglets against SADS-CoV infection in vivo. These findings reveal how SADS-CoV induced apoptosis to inhibit innate immunity and provide a valuable clue to the development of effective drugs for the clinical control of SADS-CoV infection.IMPORTANCEOver the last 20 years, multiple animal-originated coronaviruses, including severe acute respiratory syndrome coronavirus (SARS-CoV), middle east respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2, have caused millions of deaths, seriously jeopardized human health, and hindered social development, indicating that the study of animal-originated coronaviruses with potential for cross-species transmission is particularly important. Bat-originated swine acute diarrhea syndrome coronavirus (SADS-CoV), discovered in 2017, can not only cause fatal diarrhea in piglets, but also infect multiple human cells, with a potential risk of cross-species transmission, but its pathogenesis is unclear. In this study, we demonstrated that NS7a of SADS-CoV suppresses IFN-λ production via apoptosis-inducing factor mitochondria associated 1 (AIFM1)-caspase-6-caspase-3-interferon regulatory factor 3 (IRF3) pathway, and caspase-3 inhibitor (Z-DEVD-FMK) can effectively inhibit SADS-CoV replication and protect infected piglets. Our findings in this study contribute to a better understanding of SADS-CoV-host interactions as a part of the coronaviruses pathogenesis and using apoptosis-inhibitor as a drug as potential therapeutic approaches for prevention and control of SADS-CoV infection.

IFI16 Positively Regulates RIG-I-Mediated Type I Interferon Production in a STING-Independent Manner

Previous studies have shown that interferon gene-stimulating protein (STING) is essential for IFN-γ-inducible protein 16 (IFI16) as the DNA sensor and RNA sensor to induce transcription of type I interferon (IFN-I) and is essential for IFI16 to synergize with DNA sensor GMP-AMP (cGAMP) synthase (cGAS) in induction of IFN-I transcription. While other and our previous studies have shown that IFI16 enhanced retinoic acid-inducible gene I (RIG-I)-, which was an RNA sensor, and mitochondrial antiviral signaling (MAVS)-, which was the adaptor protein of RIG-I, induced production of IFN-I, so we wonder whether IFI16 regulates the signal pathway of RNA-RIG-I-MAVS-IFN-I in a STING-dependent manner. We used HEK 293T cells, which did not express endogenous STING and were unable to mount an innate immune response upon DNA transfection and found that IFI16 could enhance RIG-I- and MAVS-mediated induction of IFN-I in a STING-independent way. Furthermore, we found that upregulation of the expression of NF-kappa-B essential modulator (NEMO) by IFI16 was not the mechanism that IFI16 regulated the induction of IFN-I. In conclusion, we found that IFI16 regulated the signal pathway of RNA-RIG-I-MAVS-IFN-I in a STING-independent manner.

Evasion strategies of porcine reproductive and respiratory syndrome virus

During the co-evolution of viruses and their hosts, viruses have developed various strategies for overcoming host immunological defenses so that they can proliferate efficiently. Porcine reproductive and respiratory syndrome virus (PRRSV), a significant virus to the swine industry across the world, typically establishes prolonged infection via diverse and complicated mechanisms, which is one of the biggest obstacles for controlling the associated disease, porcine reproductive and respiratory syndrome (PRRS). In this review, we summarize the latest research on how PRRSV circumvents host antiviral responses from both the innate and adaptive immune systems and how this virus utilizes other evasion mechanisms, such as the manipulation of host apoptosis and microRNA. A thorough understanding of the exact mechanisms of PRRSV immune evasion will help with the development of novel antiviral strategies against PRRSV.

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.

Inducible miR-150 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication by Targeting Viral Genome and Suppressor of Cytokine Signaling 1

Hosts exploit various approaches to defend against porcine reproductive and respiratory syndrome virus (PRRSV) infection. microRNAs (miRNAs) have emerged as key negative post-transcriptional regulators of gene expression and have been reported to play important roles in regulating virus infection. Here, we identified that miR-150 was differentially expressed in virus permissive and non-permissive cells. Subsequently, we demonstrated that PRRSV induced the expression of miR-150 via activating the protein kinase C (PKC)/c-Jun amino-terminal kinases (JNK)/c-Jun pathway, and overexpression of miR-150 suppressed PRRSV replication. Further analysis revealed that miR-150 not only directly targeted the PRRSV genome, but also facilitated type I IFN signaling. RNA immunoprecipitation assay demonstrated that miR-150 targeted the suppressor of cytokine signaling 1 (SOCS1), which is a negative regulator of Janus activated kinase (JAK)/signal transducer and activator of the transcription (STAT) signaling pathway. The inverse correlation between miR-150 and SOCS1 expression implies that miR-150 plays a role in regulating ISG expression. In conclusion, miR-150 expression is upregulated upon PRRSV infection. miR-150 feedback positively targets the PRRSV genome and promotes type I IFN signaling, which can be seen as a host defensive strategy.

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

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

Interaction of HnRNP F with the guanine-rich segments in viral antigenomic RNA enhances porcine reproductive and respiratory syndrome virus-2 replication

Background

Heterogeneous nuclear ribonucleoprotein (HnRNP) F is a member of HnRNP family proteins that participate in splicing of cellular newly synthesized mRNAs by specifically recognizing tandem guanine-tracts (G-tracts) RNA sequences. Whether HnRNP F could recognize viral-derived tandem G-tracts and affect virus replication remain poorly defined.

Methods

The effect of HnRNP F on porcine reproductive and respiratory syndrome virus (PRRSV) propagation was evaluated by real-time PCR, western blotting, and plaque-forming unit assay. The association between HnRNP F and PRRSV guanine-rich segments (GRS) were analyzed by RNA pulldown and RNA immunoprecipitation. The expression pattern of HnRNP F was investigated by western blotting and nuclear and cytoplasmic fractionation.

Results

Knockdown of endogenous HnRNP F effectively blocks the synthesis of viral RNA and nucleocapsid (N) protein. Conversely, overexpression of porcine HnRNP F has the opposite effect. Moreover, RNA pulldown and RNA immunoprecipitation assays reveal that the qRMM1 and qRRM2 domains of HnRNP F recognize the GRS in PRRSV antigenomic RNA. Finally, HnRNP F is redistributed into the cytoplasm and forms a complex with guanine-quadruplex (G4) helicase DHX36 during PRRSV infection.

Conclusions

These findings elucidate the potential functions of HnRNP F in regulating the proliferation of PRRSV and contribute to a better molecular understanding of host-PRRSV interactions.

A Novel Motif in the 3′-UTR of PRRSV-2 Is Critical for Viral Multiplication and Contributes to Enhanced Replication Ability of Highly Pathogenic or L1 PRRSV

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) with enhanced replication capability emerged in China and has become dominant epidemic strain since 2006. Up to now, the replication-regulated genes of PRRSV have not been fully clarified. Here, by swapping the genes or elements between HP-PRRSV and classical PRRSV based on infectious clones, NSP1, NSP2, NSP7, NSP9 and 3′-UTR are found to contribute to the high replication efficiency of HP-PRRSV. Further study revealed that mutations at positions 117th or 119th in the 3′-UTR are significantly related to replication efficiency, and the nucleotide at position 120th is critical for viral rescue. The motif composed by 117–120th nucleotides was quite conservative within each lineage of PRRSV; mutations in the motif of HP-PRRSV and currently epidemic lineage 1 (L1) PRRSV showed higher synthesis ability of viral negative genomic RNA, suggesting that those mutations were beneficial for viral replication. RNA structure analysis revealed that this motif maybe involved into a pseudoknot in the 3′-UTR. The results discovered a novel motif, 117–120th nucleotide in the 3′-UTR, that is critical for replication of PRRSV-2, and mutations in the motif contribute to the enhanced replicative ability of HP-PRRSV or L1 PRRSV. Our findings will help to understand the molecular basis of PRRSV replication and find the potential factors resulting in an epidemic strain of PRRSV.

Antiviral activity of porcine interferon delta 8 against pesudorabies virus in vitro

Recently, pseudorabies virus (PRV) was isolated from human cases, and infected patients presented with respiratory dysfunction and acute neurological symptoms. However, there was no available effective drug to prevent the progression of PRV infection. In the present study, we screened a stably Drosophila S2 cell line which can secretory express a novel type I IFNs-interferon delta 8 (IFN-δ8) and the yield was about 10 mg/L. After purification, recombinant IFN-δ8 was demonstrated to be acid-stable, heat-stable, and nontoxic to PK-15 and 3D4/21 cells. Antiviral effects of IFN-δ8 against PRV were tested in vitro. Our results showed both pre- and post-treatment, recombinant PoIFN-δ8 exerted a significant protective effect against PRV infection in PK-15 and 3D4/21 cells. In addition, PoIFN-δ8 remarkably increased the expression of eight IFN-stimulated genes (ISGs), including ISG15, OAS1, PKR, MX1, CH25H, IFITM1, IFITM2 and IFITM3, to resist virus infection. These findings highlight the significance of IFN-δ8 that might serve as an antiviral agent for the prevention of PRV infection, and maybe expand the potential function of IFN antiviral drugs in the future.

Drug repurposing approach to fight COVID-19

Currently, there are no treatment options available for the deadly contagious disease, coronavirus disease 2019 (COVID-19). Drug repurposing is a process of identifying new uses for approved or investigational drugs and it is considered as a very effective strategy for drug discovery as it involves less time and cost to find a therapeutic agent in comparison to the de novo drug discovery process. The present review will focus on the repurposing efficacy of the currently used drugs against COVID-19 and their mechanisms of action, pharmacokinetics, dosing, safety, and their future perspective. Relevant articles with experimental studies conducted in-silico, in-vitro, in-vivo, clinical trials in humans, case reports, and news archives were selected for the review. Number of drugs such as remdesivir, favipiravir, ribavirin, lopinavir, ritonavir, darunavir, arbidol, chloroquine, hydroxychloroquine, tocilizumab and interferons have shown inhibitory effects against the SARS-CoV2 in-vitro as well as in clinical conditions. These drugs either act through virus-related targets such as RNA genome, polypeptide packing and uptake pathways or target host-related pathways involving angiotensin-converting enzyme-2 (ACE2) receptors and inflammatory pathways. Using the basic knowledge of viral pathogenesis and pharmacodynamics of drugs as well as using computational tools, many drugs are currently in pipeline to be repurposed. In the current scenario, repositioning of the drugs could be considered the new avenue for the treatment of COVID-19.

The Roles of Apoptosis in Swine Response to Viral Infection and Pathogenesis of Swine Enteropathogenic Coronaviruses

Apoptosis is a tightly regulated mechanism of cell death that plays important roles in various biological processes including biological evolution, multiple system development, anticancer, and viral infections. Swine enteropathogenic coronaviruses invade and damage villous epithelial cells of the small intestine causing severe diarrhea with high mortality rate in suckling piglets. Transmissible gastroenteritis virus (TGEV), Porcine epidemic diarrhea virus (PEDV), Porcine deltacoronavirus (PDCoV), and Swine acute diarrhea syndrome coronavirus (SADS-CoV) are on the top list of commonly-seen swine coronaviruses with a feature of diarrhea, resulting in significant economic losses to the swine industry worldwide. Apoptosis has been shown to be involved in the pathogenesis process of animal virus infectious diseases. Understanding the roles of apoptosis in host responses against swine enteropathogenic coronaviruses infection contribute to disease prevention and control. Here we summarize the recent findings that focus on the apoptosis during swine coronaviruses infection, in particular, TGEV, PEDV, PDCoV, and SADS-CoV.

The Impact of BPI Expression on Escherichia coli F18 Infection in Porcine Kidney Cells

Simple Summary

Escherichia coli frequently causes bacterial diarrhea in piglets. Vaccine development and improved feeding and animal management strategies have reduced the incidence of bacterial diarrhea in piglets to some extent. However, current breeding strategies also have the potential to improve piglet resistance to diarrhea at a genetic level. This study sought to advance the current understanding of the functional and regulatory mechanisms whereby the candidate gene bactericidal/permeability-increasing protein (BPI) regulates piglet diarrhea at the cellular level.

Abstract

The efficacy and regulatory activity of bactericidal/permeability-increasing protein (BPI) as a mediator of Escherichia coli (E. coli) F18 resistance remains to be defined. In the present study, we evaluated lipopolysaccharide (LPS)-induced changes in BPI gene expression in porcine kidney (PK15) cells in response to E. coli F18 exposure. We additionally generated PK15 cells that overexpressed BPI to assess the impact of this gene on Toll-like receptor 4 (TLR4) signaling and glycosphingolipid biosynthesis-related genes. Through these analyses, we found that BPI expression rose significantly following LPS exposure in response to E. coli F18ac stimulation (p < 0.01). Colony count assays and qPCR analyses revealed that E. coli F18 adherence to PK15 cells was markedly suppressed following BPI overexpression (p < 0.01). BPI overexpression had no significant effect on the mRNA-level expression of genes associated with glycosphingolipid biosynthesis or TLR4 signaling. BPI overexpression suppressed the LPS-induced TLR4 signaling pathway-related expression of proinflammatory cytokines (IFN-α, IFN-β, MIP-1α, MIP-1β and IL-6). Overall, our study serves as an overview of the association between BPI and resistance to E. coli F18 at the cellular level, offering a framework for future investigations of the mechanisms whereby piglets are able to resist E. coli F18 infection.

Porcine enteric alphacoronavirus Inhibits IFN-α, IFN-β, OAS, Mx1, and PKR mRNA Expression in Infected Peyer's Patches in vivo

Porcine enteric alphacoronavirus (PEAV) is a newly identified swine enteropathogenic coronavirus that causes watery diarrhea in neonatal piglets. The pathogenesis and host immune responses of PEAV infection are not fully characterized. The reason lies in the stomach environment, which would degrade cell-cultured live viruses via oral infection, making it difficult to establish an effective infection model to study the pathogenesis and host immune responses in pigs with a mature immune system. To solve this problem, in this study, coated PEAV-loaded microspheres were developed by centrifugal granulation-fluidized bed coating and demonstrated as an effective oral delivery system/animal infection model to protect PEAV virion against the complex gastrointestinal environment in vitro and to cause infection in weaned piglets in vivo. Weaned piglets orally inoculated with coated PEAV-loaded microspheres developed diarrhea and virus RNA was detected in rectal swabs from one to seven days post inoculation. In addition, microscopic lesions in the small intestine were observed, and viral antigens were also detected in the small intestines with PEAV immunohistochemical staining. Importantly, PEAV significantly inhibited mRNA expression of IFN-α, IFN-β, OAS, Mx1, and PKR, the genes involved in modulation of the host immune responses, in infected Peyer's patches, indicating that PEAV can overcome the antiviral response to cause damage when infection occurs. Collectively, our research successfully established a PEAV animal infection model in weaned piglets and suggested that the observed gene expression profile might help explain immunological changes associated with PEAV infection.

Avian Influenza A Virus Infects Swine Airway Epithelial Cells without Prior Adaptation

Pigs play an important role in the interspecies transmission of influenza A viruses (IAV). The porcine airway epithelium contains binding sites for both swine/human IAV (α2,6-linked sialic acids) and avian IAV (α2,3-linked sialic acids) and therefore is suited for adaptation of viruses from other species as suggested by the “mixing vessel theory”. Here, we applied well-differentiated swine airway epithelial cells to find out whether efficient infection by avian IAV requires prior adaption. Furthermore, we analyzed the influence of the sialic acid-binding activity and the virus-induced detrimental effects. Surprisingly, an avian IAV H1N1 strain circulating in European poultry and waterfowl shows increased and prolonged viral replication without inducing a strong innate immune response. This virus could infect the lower respiratory tract in our precision cut-lung slice model. Pretreating the cells with poly (I:C) and/or JAK/STAT pathway inhibitors revealed that the interferon-stimulated innate immune response influences the replication of avian IAV in swine airway epitheliums but not that of swine IAV. Further studies indicated that in the infection by IAVs, the binding affinity of sialic acid is not the sole factor affecting the virus infectivity for swine or human airway epithelial cells, whereas it may be crucial in well-differentiated ferret tracheal epithelial cells. Taken together, our results suggest that the role of pigs being the vessel of interspecies transmission should be reconsidered, and the potential of avian H1N1 viruses to infect mammals needs to be characterized in more detail.

Porcine reproductive and respiratory syndrome virus up-regulates sialoadhesin via IFN-STAT signaling to facilitate its infection

Porcine reproductive and respiratory syndrome (PRRS) has caused huge economic losses to global swine industry. Porcine sialoadhesin (poSn) was previously reported to be a putative receptor for the causative agent, PRRS virus (PRRSV). In the current study, we first observed that PRRSV infection up-regulated expression of poSn in a dose- and time-dependent manner. Subsequently, we found that PRRSV-triggered transcription of type I interferons (IFNs) was involved in poSn up-regulation through the IFN-signal transducer and activator of transcription (STAT) signaling cascade. Interestingly, poSn up-regulation was shown to promote PRRSV infection during post-entry process. Taken together, this work deepens our understanding of PRRSV pathogenesis and provides a novel idea on its establishment of persistent infection, which will be interesting to unravel the detailed mechanisms in the future.

Porcine sialoadhesin suppresses type I interferon production to support porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome virus (PRRSV) is a significant threat to the global swine industry. Porcine sialoadhesin (poSn) has been previously shown to mediate PRRSV attachment and internalization. In the current study, we report its unidentified role in antagonism of type I interferon (IFN) production during PRRSV infection. We determined that poSn facilitated PRRSV infection via inhibition of type I IFN transcription. Mechanistically, poSn interacted with a 12 kDa DNAX-activation protein (DAP12), which was dependent on residues 51–57 within DAP12 transmembrane domain (TMD). PRRSV exploited the poSn-DAP12 pathway to attenuate activation of nuclear factor-kappa B (NF-κB). More importantly, the poSn-DAP12 pathway was involved in inhibiting poly (I:C)-triggered IFN production. All these results reveal a novel role of poSn in suppressing host antiviral responses, which deepens our understanding of PRRSV pathogenesis.

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

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

Porcine Reproductive and Respiratory Syndrome Virus Enhances Self-Replication via AP-1-Dependent Induction of SOCS1

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous economic losses in the swine industry since its emergence in the late 1980s. PRRSV exploits various strategies to evade immune responses and establish chronic persistent infections. Suppressor of cytokine signaling (SOCS) 1, a member of the SOCS family, is a crucial intracellular negative regulator of innate immunity. In this study, it was shown that SOCS1 can be co-opted by PRRSV to evade host immune responses, facilitating viral replication. It was observed that PRRSV induced SOCS1 production in porcine alveolar macrophages, monkey-derived Marc-145 cells, and porcine-derived CRL2843-CD163 cells. SOCS1 inhibited the expression of IFN-β and IFN-stimulated genes, thereby markedly enhancing PRRSV replication. It was observed that the PRRSV N protein has the ability to upregulate SOCS1 production and that nuclear localization signal-2 (NLS-2) is essential for SOCS1 induction. Moreover, SOCS1 upregulation was dependent on p38/AP-1 and JNK/AP-1 signaling pathways rather than classical type I IFN signaling pathways. In summary, to our knowledge, the findings of this study uncovered the molecular mechanism that underlay SOCS1 induction during PRRSV infection, providing new insights into viral immune evasion and persistent infection.

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 deltacoronavirus induces TLR3, IL-12, IFN-α, IFN-β and PKR mRNA expression in infected Peyer's patches in vivo

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PDCoV infection caused severe diarrhea, virus shedding and intestinal lesion in weaned piglets.

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PDCoV could induce TLR3 mRNA expression in infected Peyer's patches from weaned piglets.

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PDCoV obviously induced IL-12, IFN-α, IFN-β, and PKR mRNA expression in infected Peyer's patches from weaned piglets.

Porcine deltacoronavirus (PDCoV) is a newly identified swine enteropathogenic coronavirus that causes watery diarrhea in piglets and results in significant economic losses to the pig industry. Currently there are no effective treatments or vaccines for PDCoV. In particular, the pathogenesis of PDCoV infection is still largely unknown. In this study, we reported that inoculating conventional weaned piglets with 1 × 10⁹ TCID₅₀ of the PDCoV CHN-GD-2016 strain by oral feeding could cause severe diarrhea. Virus RNA was detected in rectal swabs from 1 to 7 days post inoculation. In addition, microscopic lesions in small intestine were observed, and viral antigen also detected in the small intestines with PDCoV immunohistochemical staining. Importantly, PDCoV significantly induced mRNA expression of TLR3, IL-12, IFN-α, IFN-β, and PKR, the genes involved in modulation of the host immune responses, in infected Peyer's patches at 3 d.p.i., indicating that Peyer's patches play an important role in PDCoV immune responses in vivo. Collectively, our findings suggest that the observed gene expression profile might help explain immunological and pathological changes associated with PDCoV infection.