Kobuvirus VP3 protein restricts the IFN-β-triggered signaling pathway by inhibiting STAT2-IRF9 and STAT2-STAT2 complex formation

Impaired interferon response in senecavirus A infection and identification of 3Cpro as an antagonist

Senecavirus A (SVA), a picornavirus, causes vesicular diseases and epidemic transient neonatal losses in swine, resulting in a multifaceted economic impact on the swine industry. SVA counteracts host antiviral response through multiple strategies facilitatng viral infection and transmission. However, the mechanism of how SVA modulates interferon (IFN) response remains elusive. Here, we demonstrate that SVA 3C protease (3Cpro) blocks the transduction of Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway to antagonize type I IFN response. Mechanistically, 3Cpro selectively cleaves and degrades STAT1 and STAT2 while does not target JAK1, JAK2, and IRF9, through its protease activity. Notably, SVA 3Cpro cleaves human and porcine STAT1 on a Leucine (L)-Aspartic acid (D) motif, specifically L693/D694. In the case of STAT2, two cleavage sites were identified: glutamine (Q) 707 was identified in both human and porcine, while the second cleavage pattern differed, with residues 754-757 (Valine-Leucine-Glutamine-Serine motifs) in human STAT2 and Q758 in porcine STAT2. These cleavage patterns by SVA 3Cpro partially differ from previously reported classical motifs recognized by other picornaviral 3Cpro, highlighting the distinct characteristics of SVA 3Cpro. Together, these results reveal a mechanism by which SVA 3Cpro antagonizes IFN-induced antiviral response but also expands our knowledge about the substrate recognition patterns for picornaviral 3Cpro.IMPORTANCESenecavirus A (SVA), the only member in the Senecavirus genus within the Picornaviridae family, causes vesicular diseases in pigs that are clinically indistinguishable from foot-and-mouth disease (FMD), a highly contagious viral disease listed by the World Organization for Animal Health (WOAH). Interferon (IFN)-mediated antiviral response plays a pivotal role in restricting and controlling viral infection. Picornaviruses evolved numerous strategies to antagonize host antiviral response. However, how SVA modulates the JAK-STAT signaling pathway, influencing the type I IFN response, remains elusive. Here, we identify that 3Cpro, a protease of SVA, functions as an antagonist for the IFN response. 3Cpro utilizes its protease activity to cleave STAT1 and STAT2, thereby diminishing the host IFN response to promote SVA infection. Our findings underscore the significance of 3Cpro as a key virulence factor in the antagonism of the type I signaling pathway during SVA infection.

Isolation and Characteristics of a Novel Aichivirus D from Yak

Aichivirus D (AiV-D) is a newly emerging Kobuvirus detected in bovine and sheep, and information is limited regarding its biological significance and prevalence. This study aimed to explore both the prevalence and characteristics of AiV-D in yaks. From May to August 2021, 117 fecal samples were collected from yaks with diarrhea in three provinces of China's Qinghai-Tibet Plateau, 15 of which were selected and pooled for metagenomic analysis. A high abundance of AiV-D sequences was obtained. Of the 117 diarrhea samples, 29 (24.8%) tested AiV-D-positive, including 33.3% (14/42) from Sichuan, 21.1% (8/38) from Qinghai, and 18.9% (7/37) from Tibet, respectively, suggesting a wide geographical distribution of the AiV-D in yaks in the Qinghai-Tibet Plateau. Furthermore, three AiV-D strains were successfully isolated using Vero cells. Significantly, the AiV-D strain could cause diarrhea, intestinal bleeding, and inflammation in yak calves via oral inoculation. The virus was distributed in the ileum, jejunum, duodenum, colon, cecum, and rectum. Based on phylogenetic analysis of the genome and capsid protein P1 (VP0, VP3, and VP1 genes), the yak AiV-D strains likely represent a novel genotype of AiV-D. On the whole, this study identified a novel genotype of AiV-D from yaks, which was successfully isolated, and confirmed that this virus is a diarrhea pathogen in yaks and has a wide geographical distribution in the Qinghai-Tibet Plateau. Our results expand the host range of AiV-D and the pathogen spectrum of yaks and have significant implications for diagnosing and controlling diarrhea in yaks. IMPORTANCE In this study, we identified and successfully isolated a novel genotype of AiV-D from yaks. Animal infection confirmed that this virus can cause diarrhea, intestinal bleeding, and inflammation in yak calves via oral inoculation. The virus was distributed in the ileum, jejunum, cecum, duodenum, colon, and rectum. All of these results have significant implications for diagnosing and controlling diarrhea in yaks. These novel AiV-D strains have a wide geographical distribution in yaks from the Qinghai-Tibet Plateau in China. In addition to expanding the host range of AiV-D and the pathogen spectrum of yaks, these findings can increase knowledge of the prevalence and diversity of AiV-D.

A Systematic Review: Is Porcine Kobuvirus Causing Gastrointestinal Disease in Young Pigs?

Since porcine kobuvirus (PKV) was first described in 2008, researchers have speculated whether the virus is of clinical importance. This systematic literature review answers the question: Is porcine kobuvirus a cause of gastrointestinal disease in young pigs? A case-control study showed that PKV was not associated with neonatal diarrhea. A cohort study suffered from a very small sample size (n = 5), and in an experimental trial, the effect of PKV inoculation could not be separated from the effect of being inoculated with porcine epidemic diarrhea virus. In 13 poorly defined observational studies, more than 4000 young pigs had been assigned a diarrhea status and their feces analyzed for PKV. Unfortunately, the studies lacked well-characterized unbiased samples, and thus the strongest possible inference from these studies was that a very strong association between PKV and diarrhea is unlikely. PKV was commonly detected in non-diarrheic pigs, and this could indicate that PKV is not a sufficient cause in itself or that reinfection of individuals with some immunological protection due to previous infections is common. Conclusively, there is a lack of good evidence of PKV being a cause of gastrointestinal disease, but the sparse available evidence suggests that PKV is of limited clinical importance.

STAT and Janus kinase targeting by human herpesvirus 8 interferon regulatory factor in the suppression of type-I interferon signaling

Human herpesvirus 8 (HHV-8), also known as Kaposi’s sarcoma (KS)-associated herpesvirus, is involved etiologically in AIDS-associated KS, primary effusion lymphoma (PEL), and multicentric Castleman’s disease, in which both viral latent and lytic functions are important. HHV-8 encodes four viral interferon regulatory factors (vIRFs) that are believed to contribute to viral latency (in PEL cells, at least) and/or to productive replication via suppression of cellular antiviral and stress signaling. Here, we identify vIRF-1 interactions with signal transducer and activator of transcription (STAT) factors 1 and 2, interferon (IFN)-stimulated gene factor 3 (ISGF3) cofactor IRF9, and associated signal transducing Janus kinases JAK1 and TYK2. In naturally infected PEL cells and in iSLK epithelial cells infected experimentally with genetically engineered HHV-8, vIRF-1 depletion or ablation, respectively, led to increased levels of active (phosphorylated) STAT1 and STAT2 in IFNβ-treated, and untreated, cells during lytic replication and to associated cellular-gene induction. In transfected 293T cells, used for mechanistic studies, suppression by vIRF-1 of IFNβ-induced phospho-STAT1 (pSTAT1) was found to be highly dependent on STAT2, indicating vIRF-1-mediated inhibition and/or dissociation of ISGF3-complexing, resulting in susceptibility of pSTAT1 to inactivating dephosphorylation. Indeed, coprecipitation experiments involving targeted precipitation of ISGF3 components identified suppression of mutual interactions by vIRF-1. In contrast, suppression of IFNβ-induced pSTAT2 was effected by regulation of STAT2 activation, likely via detected inhibition of TYK2 and its interactions with STAT2 and IFN type-I receptor (IFNAR). Our identified vIRF-1 interactions with IFN-signaling mediators STATs 1 and 2, co-interacting ISGF3 component IRF9, and STAT-activating TYK2 and the suppression of IFN signaling via ISGF3, TYK2-STAT2 and TYK2-IFNAR disruption and TYK2 inhibition represent novel mechanisms of vIRF function and HHV-8 evasion from host-cell defenses.

Viral interferon regulatory factors (vIRFs) encoded by Kaposi’s sarcoma- and lymphoma-associated human herpesvirus 8 (HHV-8) are mediators of protection from cellular antiviral responses and therefore are considered to be pivotal for successful de novo infection, latency establishment and maintenance, and productive (lytic) replication. Identification and characterization of their interactions with cellular proteins, the functional consequences of these interactions, and the operation of these mechanisms in the context of infection has the potential to enable the development of novel antiviral strategies targeted to these interactions and mechanisms. In this report we identify vIRF-1 interactions with transcription factors STAT1 and STAT2, the co-interacting component, IRF9, of the antiviral interferon (IFN)-induced transcription complex ISGF3, and the ability of vIRF-1 to inhibit activation and functional associations of IFN-I receptor- and STAT1/2-kinase TYK2, suppress STAT1/2 activation, and dissociate STAT1 from IFN-induced ISGF3 to blunt IFN signaling and promote STAT1 inactivation. These interactions and activities, which mediate suppression of innate cellular defenses against virus replication, represent novel properties among vIRFs and could potentially be exploited for antiviral and therapeutic purposes.

First detection and molecular characteristics of caprine kobuvirus in goats in China

Caprine kobuvirus (CKoV), a member of the genus Kobuvirus, has only been identified in South Korea and Italy until now. In this study, 24 goat diarrheic fecal samples were collected from 3 farms in Sichuan province, China, and 87.5% (21/24) samples were detected as CKoV positive by RT-PCR. Meanwhile, full-length VP0, VP3, and VP1 genes were simultaneously cloned from 17 clinical samples. Phylogenetic analysis showed that all CKoV strains were most closely related to porcine kobuvirus based on amino acid (aa) sequences of VP0 and VP3 proteins, but CKoV strains were closely related to with Aichivirus B strains (ferret, bovine, and sheep kobuvirus) based on aa sequences of the VP1 protein. Interestingly, compared with known CKoV strains in the GenBank database, Chinese CKoV strains have unique amino acid changes in VP0 and VP1 proteins. Moreover, the first Chinese CKoV nearly complete genome was successfully obtained from a diarrheic fecal sample, named SWUN/F11/2019. Compared with the two known CKoV strains, five aa mutations (S60A, L252I, V267T, I, V 306 L, V331I) were found in the VP0 gene and 7 aa mutations (S57N, G, T243A, V244I, T, A248V, L, S251A, R252H, and M255L) were found in VP1 in the SWUN/F11/2019 genome. This was the first report of the detection and molecular characteristics of CKoV from goats in China, which could be helpful for improving the understanding of the prevalence and genetic evolution of CKoV.

Innate immune evasion by picornaviruses

The family Picornaviridae comprises a large number of viruses that cause disease in broad spectrum of hosts, which have posed serious public health concerns worldwide and led to significant economic burden. A comprehensive understanding of the virus-host interactions during picornavirus infections will help to prevent and cure these diseases. Upon picornavirus infection, host pathogen recognition receptors (PRRs) sense viral RNA to activate host innate immune responses. The activated PRRs initiate signal transduction through a series of adaptor proteins, which leads to activation of several kinases and transcription factors, and contributes to the consequent expression of interferons (IFNs), IFN-inducible antiviral genes, as well as various inflammatory cytokines and chemokines. In contrast, to maintain viral replication and spread, picornaviruses have evolved several elegant strategies to block innate immune signaling and hinder host antiviral response. In this review, we will summarize the recent progress of how the members of family Picornaviridae counteract host immune response through evasion of PRRs detection, blocking activation of adaptor molecules and kinases, disrupting transcription factors, as well as counteraction of antiviral restriction factors. Such knowledge of immune evasion will help us better understand the pathogenesis of picornaviruses, and provide insights into developing antiviral strategies and improvement of vaccines.

The Vif protein of caprine arthritis encephalitis virus inhibits interferon production

Caprine arthritis-encephalitis (CAE) is a chronic progressive infectious disease caused by caprine arthritis-encephalitis virus (CAEV) that seriously threatens the goat industry. Chronic infection and life-long multi-tissue inflammation are the typical features of the disease. Innate antiviral immunity is essential for the host defense system that rapidly recognizes and eliminates invading viruses. Interferon β (IFN-β) is important for innate immunity and regulates immunity against a broad spectrum of viruses. To investigate the details of the IFN-β response to CAEV infection, the effects of six viral proteins and the molecular mechanisms by which they affect IFN-β production were analyzed. Overexpression of DU and Vif promote virus proliferation and inhibit the production of IFN-β. qRT-PCR and luciferase reporter assays showed that overexpression of Vif inhibits the expression of luciferase under the control of the ISRE, NF-κB or IFN-β promoter but does not affect the expression of IFN-β activated by IRF3, indicating that Vif negatively regulates IFN-β production by affecting upstream signal transduction of IRF3. Amino acids 149-164 of Vif were found to be necessary for the inhibitory effect of IFN-β production. Our results indicate that CAEV evades surveillance and clearance by intracellular innate immunity by downregulating IFN-β production.

Molecular detection and genomic characteristics of bovine kobuvirus from dairy calves in China

In this study, 96 diarrheic and 77 non-diarrheic fecal samples from dairy calves were collected from 14 dairy farms in 4 provinces to investigate the molecular prevalence and genomic characteristics of Bovine Kobuvirus (BKoV) in China. The results showed that the BKoV positive rate for the diarrheic feces (35.42%) was significantly higher than that for the non-diarrheic feces (11.69%, p < 0.001). Interestingly, three potential novel VP1 lineages were identified from 15 complete VP1 sequences, and a unique triple nucleotide insertion which can result in an aa insertion, was first observed in the 11/12 VP0 fragments with 660 bp long in this study, compared with known BKoV VP0 sequences. Moreover, the first Chinese BKoV genome was successfully obtained from a diarrheic fecal sample, named CHZ/CHINA. The open reading frame (ORF) of the genome from strain CHZ/China shares 87.4%–88.3% nucleotide (nt) and 93.7%–96.4% amino acid (aa) identity, compared with the three known genomes of BKoV. Interestingly, phylogenetic tree based on aa sequences of these genomes showed that CHZ/CHINA was clustered into an independent branch, suggesting the strain may represent a novel BKoV strain. The findings contribute to better understanding the molecular characteristics and evolution of BKoV.

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Three potential novel VP1 lineages in BKoV.

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A unique VP0 sequence type in BKoV.

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The first BKoV genome from China which may represent a novel BKoV strain.

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Contributing to better understanding the molecular characteristics of BKoV.

The Strategy of Picornavirus Evading Host Antiviral Responses: Non-structural Proteins Suppress the Production of IFNs

Viral infections trigger the innate immune system to produce interferons (IFNs), which play important role in host antiviral responses. Co-evolution of viruses with their hosts has favored development of various strategies to evade the effects of IFNs, enabling viruses to survive inside host cells. One such strategy involves inhibition of IFN signaling pathways by non-structural proteins. In this review, we provide a brief overview of host signaling pathways inducing IFN production and their suppression by picornavirus non-structural proteins. Using this strategy, picornaviruses can evade the host immune response and replicate inside host cells.