Differential Antagonism of Human Innate Immune Responses by Tick-Borne Phlebovirus Nonstructural Proteins

Pathogenesis and virulence of Heartland virus

Heartland virus (HRTV), an emerging tick-borne pathogenic bunyavirus, has been a concern since 2012, with an increasing incidence, expanding geographical distribution, and high pathogenicity in the United States. Infection from HRTV results in fever, thrombocytopenia, and leucopenia in humans, and in some cases, symptoms can progress to severe outcomes, including haemorrhagic disease, multi-organ failure, and even death. Currently, no vaccines or antiviral drugs are available for treatment of the HRTV disease. Moreover, little is known about HRTV-host interactions, viral replication mechanisms, pathogenesis and virulence, further hampering the development of vaccines and antiviral interventions. Here, we aimed to provide a brief review of HRTV epidemiology, molecular biology, pathogenesis and virulence on the basis of published article data to better understand this virus and provide clues for further study.

Current insights into human pathogenic phenuiviruses and the host immune system

Phenuiviruses are a class of segmented negative-sense single-stranded RNA viruses, typically consisting of three RNA segments that encode four distinct proteins. The emergence of pathogenic phenuivirus strains, such as Rift Valley fever phlebovirus (RVFV) in sub-Saharan Africa, Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) in East and Southeast Asia, and Heartland Virus (HRTV) in the United States has presented considerable challenges to global public health in recent years. The innate immune system plays a crucial role as the initial defense mechanism of the host against invading pathogens. In addition to continued research aimed at elucidating the epidemiological characteristics of phenuivirus, significant advancements have been made in investigating its viral virulence factors (glycoprotein, non-structural protein, and nucleoprotein) and potential host-pathogen interactions. Specifically, efforts have focused on understanding mechanisms of viral immune evasion, viral assembly and egress, and host immune networks involving immune cells, programmed cell death, inflammation, nucleic acid receptors, etc. Furthermore, a plethora of technological advancements, including metagenomics, metabolomics, single-cell transcriptomics, proteomics, gene editing, monoclonal antibodies, and vaccines, have been utilized to further our understanding of phenuivirus pathogenesis and host immune responses. Hence, this review aims to provide a comprehensive overview of the current understanding of the mechanisms of host recognition, viral immune evasion, and potential therapeutic approaches during human pathogenic phenuivirus infections focusing particularly on RVFV and SFTSV.

SFTSV nucleoprotein mediates DNA sensor cGAS degradation to suppress cGAS-dependent antiviral responses

Cyclic GMP-AMP synthase (cGAS) is an important DNA pattern recognition receptor that senses double-stranded DNA derived from invading pathogens or self DNA in cytoplasm, leading to an antiviral interferon response. A tick-borne Bunyavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), is an RNA virus that causes a severe emerging viral hemorrhagic fever in Asia with a high case fatality rate of up to 30%. However, it is unclear whether cGAS interacts with SFTSV infection. In this study, we found that SFTSV infection upregulated cGAS RNA transcription and protein expression, indicating that cGAS is an important innate immune response against SFTSV infection. The mechanism of cGAS recognizing SFTSV is by cGAS interacting with misplaced mitochondrial DNA in the cytoplasm. Depletion of mitochondrial DNA significantly inhibited cGAS activation under SFTSV infection. Strikingly, we found that SFTSV nucleoprotein (N) induced cGAS degradation in a dose-dependent manner. Mechanically, N interacted with the 161-382 domain of cGAS and linked the cGAS to LC3. The cGAS-N-LC3 trimer was targeted to N-induced autophagy, and the cGAS was degraded in autolysosome. Taken together, our study discovered a novel antagonistic mechanism of RNA viruses, SFTSV is able to suppress the cGAS-dependent antiviral innate immune responses through N-hijacking cGAS into N-induced autophagy. Our results indicated that SFTSV N is an important virulence factor of SFTSV in mediating host antiviral immune responses.

IMPORTANCE

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne RNA virus that is widespread in East and Southeast Asian countries with a high fatality rate of up to 30%. Up to now, many cytoplasmic pattern recognition receptors, such as RIG-I, MDA5, and SAFA, have been reported to recognize SFTSV genomic RNA and trigger interferon-dependent antiviral responses. However, current knowledge is not clear whether SFTSV can be recognized by DNA sensor cyclic GMP-AMP synthase (cGAS). Our study demonstrated that cGAS could recognize SFTSV infection via ectopic mitochondrial DNA, and the activated cGAS-stimulator of interferon genes signaling pathway could significantly inhibit SFTSV replication. Importantly, we further uncovered a novel mechanism of SFTSV to inhibit innate immune responses by the degradation of cGAS. cGAS was degraded in N-induced autophagy. Collectively, this study illustrated a novel virulence factor of SFTSV to suppress innate immune responses through autophagy-dependent cGAS degradation.

Physiological functions of RIG-I-like receptors

RIG-I-like receptors (RLRs) are crucial for pathogen detection and triggering immune responses and have immense physiological importance. In this review, we first summarize the interferon system and innate immunity, which constitute primary and secondary responses. Next, the molecular structure of RLRs and the mechanism of sensing non-self RNA are described. Usually, self RNA is refractory to the RLR; however, there are underlying host mechanisms that prevent immune reactions. Studies have revealed that the regulatory mechanisms of RLRs involve covalent molecular modifications, association with regulatory factors, and subcellular localization. Viruses have evolved to acquire antagonistic RLR functions to escape the host immune reactions. Finally, the pathologies caused by the malfunction of RLR signaling are described.

Pathological and virological findings of type I interferon receptor knockout mice upon experimental infection with Heartland virus

Heartland virus (HRTV) causes generalized symptoms, severe shock, and multiple organ failure. We previously reported that interferon-α/β receptor knockout (IFNAR-/-) mice infected intraperitoneally with 1 × 107 tissue culture-infective dose (TCID50) of HRTV died, while those subcutaneously infected with the same dose of HRTV did not. The pathophysiology of IFNAR-/- mice infected with HRTV and the mechanism underlying the difference in disease severity, which depends on HRTV infection route, were analyzed in this study. The liver, spleen, mesenteric and axillary lymph nodes, and gastrointestinal tract of intraperitoneally (I.P.) infected mice had pathological changes; however, subcutaneously (S.C.) infected mice only had pathological changes in the axillary lymph node and gastrointestinal tract. HRTV RNA levels in the mesenteric lymph node, lung, liver, spleen, kidney, stomach, intestine, and blood were significantly higher in I.P. infected mice than those in S.C. infected mice. Chemokine ligand-1 (CXCL-1), tumor necrosis factor (TNF)-α, interleukin (IL)-12, interferon (IFN)-γ, and IL-10 levels in plasma of I.P. infected mice were higher than those of S.C. infected mice. These results indicated that high levels of viral RNA and the induction of inflammatory responses in HRTV-infected IFNAR-/- mice may be associated with disease severity.

Non-Structural Protein-W61 as a Novel Target in Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV): An In-Vitro and In-Silico Study on Protein-Protein Interactions with Nucleoprotein and Viral Replication

The non-structural protein (NSs) and nucleoprotein (NP) of the severe fever with thrombocytopenia syndrome virus (SFTSV) encoded by the S segment are crucial for viral pathogenesis. They reside in viroplasm-like structures (VLS), but their interaction and their significance in viral propagation remain unclear. Here, we investigated the significance of the association between NSs and NP during viral infection through in-silico and in-vitro analyses. Through in-silico analysis, three possible binding sites were predicted, at positions C6S (Cystein at 6th position to Serine), W61Y (Tryptophan 61st to Tyrosine), and S207T (Serine 207th to Threonine), three mutants of NSs were developed by site-directed mutagenesis and tested for NP interaction by co-immunoprecipitation. NSsW61Y failed to interact with the nucleoprotein, which was substantiated by the conformational changes observed in the structural analyses. Additionally, molecular docking analysis corroborated that the NSW61Y mutant protein does not interact well compared to wild-type NSs. Over-expression of wild-type NSs in HeLa cells increased the SFTSV replication by five folds, but NSsW61Y exhibited 1.9-folds less viral replication than wild-type. We demonstrated that the W61Y alteration was implicated in the reduction of NSs-NP interaction and viral replication. Thus, the present study identified a critical NSs site, which could be targeted for development of therapeutic regimens against SFTSV.

Emerging Tick-Borne Dabie bandavirus: Virology, Epidemiology, and Prevention

Severe Fever with Thrombocytopenia Syndrome (SFTS), caused by Dabie bandavirus (SFTSV), is an emerging infectious disease first identified in China. Since its discovery, infections have spread throughout East Asian countries primarily through tick bites but also via transmission between animals and humans. The expanding range of ticks, the primary vectors for SFTSV, combined with migration patterns of tick-carrying birds, sets the stage for the global spread of this virus. SFTSV rapidly evolves due to continuous mutation and reassortment; currently, no approved vaccines or antiviral drugs are available. Thus, the threat this virus poses to global health is unmistakable. This review consolidates the most recent research on SFTSV, including its molecular characteristics, transmission pathways through ticks and other animals, as well as the progress in antiviral drug and vaccine development, encompassing animal models and clinical trials.

Genome-informed investigation of the molecular evolution and genetic reassortment of severe fever with thrombocytopenia syndrome virus

BACKGROUND

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a viral pathogen causing significant clinical signs from mild fever with thrombocytopenia to severe hemorrhages. World Health Organization has paid special attention to the dramatic increase in human SFTS cases in China, Japan, and South Korea since the 2010s. The present study investigated the molecular evolution and genetic reassortment of SFTSVs using complete genomic sequences.

METHODS/PRINCIPAL FINDING

We collected the complete genome sequences of SFTSVs globally isolated until 2019 (L segment, n = 307; M segment, n = 326; and S segment, n = 564) and evaluated the evolutionary profiles of SFTSVs based on phylogenetic and molecular selection pressure analyses. By employing a time-scaled Bayesian inference method, we found the geographical heterogeneity of dominant SFTSV genotypes in China, Japan, and South Korea around several centuries before and locally spread by tick-born spillover with infrequent long-distance transmission. Purifying selection predominated the molecular evolution of SFTSVs with limited gene reassortment and fixed substitution, but almost all three gene segments appeared to harbor at least one amino acid residue under positive selection. Specifically, the nonstructural protein and glycoprotein (Gn/Gc) genes were preferential selective targets, and the Gn region retained the highest number of positively selected residues.

CONCLUSION/SIGNIFICANCE

Here, the large-scale genomic analyses of SFTSVs improved prior knowledge of how this virus emerged and evolved in China, Japan, and South Korea. Our results highlight the importance of SFTSV surveillance in both human and non-human reservoirs at the molecular level to fight against fatal human infection with the virus.

The NSs protein of severe fever with thrombocytopenia syndrome virus differentially inhibits the type 1 interferon response among animal species

Severe fever with thrombocytopenia syndrome virus (SFTSV), which has been reported in China, Korea, Japan, Vietnam, and Taiwan, is a causative agent of severe fever thrombocytopenia syndrome (SFTS). This virus has a high mortality and induces thrombocytopenia and leukocytopenia in humans, cats, and aged ferrets, whereas immunocompetent adult mice infected with SFTSV never show symptoms. Anti-SFTSV antibodies have been detected in several animals- including goats, sheep, cattle, and pigs. However, there are no reports of SFTS in these animals. Previous studies have reported that the nonstructural protein NSs of SFTSV inhibits the type I interferon (IFN-I) response through the sequestration of human signal transducer and activator of transcription (STAT) proteins. In this study, comparative analysis of the function of NSs as IFN antagonists in human, cat, dog, ferret, mouse, and pig cells revealed a correlation between pathogenicity of SFTSV and the function of NSs in each animal. Furthermore, we found that the inhibition of IFN-I signaling and phosphorylation of STAT1 and STAT2 by NSs depended on the binding ability of NSs to STAT1 and STAT2. Our results imply that the function of NSs in antagonizing STAT2 determines the species-specific pathogenicity of SFTSV.

Virus-host interactions during tick-borne bunyavirus infection

The Bunyavirales order is the largest grouping of RNA viruses, comprising emerging and re-emerging human, plant and animal pathogens. Bunyaviruses have a global distribution and many members of the order are transmitted by arthropods. They have evolved a plethora of mechanisms to manipulate the regulatory processes of the infected cell to facilitate their own replicative cycle, in hosts of disparate phylogenies. Interest in virus-vector interactions is growing rapidly. However, current understanding of tick-borne bunyavirus cellular interaction is heavily biased to studies conducted in mammalian systems. In this short review, we summarise current understandings of how tick-borne bunyaviruses utilise major cellular pathways (innate immunity, apoptosis and RNAi responses) in mammalian or tick cells to facilitate virus replication.

Reverse Genetics System for Heartland Bandavirus: NSs Protein Contributes to Heartland Bandavirus Virulence

Heartland bandavirus (HRTV), which is an emerging tick-borne virus first identified in Missouri in 2009, causes fever, fatigue, decreased appetite, headache, nausea, diarrhea, and muscle or joint pain in humans. HRTV is genetically close to Dabie bandavirus, which is the causative agent of severe fever with thrombocytopenia syndrome (SFTS) in humans and is known as SFTS virus (SFTSV). The generation of infectious HRTV entirely from cloned cDNAs has not yet been reported. The absence of a reverse genetics system for HRTV has delayed efforts to understand its pathogenesis and to generate vaccines and antiviral drugs. Here, we developed a reverse genetics system for HRTV, which employs an RNA polymerase I-mediated expression system. A recombinant nonstructural protein (NSs)-knockout HRTV (rHRTV-NSsKO) was generated. We found that NSs interrupted signaling associated with innate immunity in HRTV-infected cells. The rHRTV-NSsKO was highly attenuated, indicated by the apparent absence of symptoms in a mouse model of HRTV infection. Moreover, mice immunized with rHRTV-NSsKO survived a lethal dose of HRTV. These findings suggest that NSs is a virulence factor of HRTV and that rHRTV-NSsKO could be a vaccine candidate for HRTV. IMPORTANCE Heartland bandavirus (HRTV) is a tick-borne virus identified in the United States in 2009. HRTV causes fever, fatigue, decreased appetite, headache, nausea, diarrhea, and muscle or joint pain in humans. FDA-approved vaccines and antiviral drugs are unavailable. The lack of a reverse genetics system hampers efforts to develop such antiviral therapeutics. Here, we developed a reverse genetics system for HRTV that led to the generation of a recombinant nonstructural protein (NSs)-knockout HRTV (rHRTV-NSsKO). We found that NSs interrupted signaling associated with innate immunity in HRTV-infected cells. Furthermore, rHRTV-NSsKO was highly attenuated and immunogenic in a mouse model. These findings suggest that NSs is a virulence factor of HRTV and that rHRTV-NSsKO could be a vaccine candidate for HRTV.

NSs of the mildly virulent sandfly fever Sicilian virus is unable to inhibit interferon signaling and upregulation of interferon-stimulated genes

Phleboviruses (order Bunyavirales, family Phenuiviridae) are globally emerging arboviruses with a wide spectrum of virulence. Sandfly fever Sicilian virus (SFSV) is one of the most ubiquitous members of the genus Phlebovirus and associated with a self-limited, incapacitating febrile disease in travellers and military troops. The phleboviral NSs protein is an established virulence factor, acting as antagonist of the antiviral interferon (IFN) system. Consistently, we previously reported that SFSV NSs targets the induction of IFN mRNA synthesis by specifically binding to the DNA-binding domain of the IFN transcription factor IRF3. Here, we further characterized the effect of SFSV and its NSs towards IFN induction, and evaluated its potential to affect the downstream IFN-stimulated signalling and the subsequent transactivation of antiviral interferon-stimulated genes (ISGs). We found that SFSV dampened, but did not entirely abolish type I and type III IFN induction. Furthermore, SFSV NSs did not affect IFN signalling, resulting in substantial ISG expression in infected cells. Hence, although SFSV targets IRF3 to reduce IFN induction, it is not capable of entirely disarming the IFN system in the presence of high basal IRF3 and/or IRF7 levels, and we speculate that this significantly contributes to its low level of virulence.

Regulation of antiviral innate immune signaling and viral evasion following viral genome sensing

A harmonized balance between positive and negative regulation of pattern recognition receptor (PRR)-initiated immune responses is required to achieve the most favorable outcome for the host. This balance is crucial because it must not only ensure activation of the first line of defense against viral infection but also prevent inappropriate immune activation, which results in autoimmune diseases. Recent studies have shown how signal transduction pathways initiated by PRRs are positively and negatively regulated by diverse modulators to maintain host immune homeostasis. However, viruses have developed strategies to subvert the host antiviral response and establish infection. Viruses have evolved numerous genes encoding immunomodulatory proteins that antagonize the host immune system. This review focuses on the current state of knowledge regarding key host factors that regulate innate immune signaling molecules upon viral infection and discusses evidence showing how specific viral proteins counteract antiviral responses via immunomodulatory strategies.

Nonstructural Protein NSs Hampers Cellular Antiviral Response through LSm14A during Severe Fever with Thrombocytopenia Syndrome Virus Infection

The nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) plays multiple functions in the virus life cycle. Proteomic screening for host proteins interacting with NSs identified the cellular protein LSm14A. LSm14A, a member of the LSm family involved in RNA processing in the processing bodies, binds to viral RNA or synthetic homolog and mediates IFN regulatory factor 3 activation and IFN-β induction. NSs interacted with and colocalized with LSm14A, and this interaction effectively inhibited downstream phosphorylation and dimerization of IFN regulatory factor 3, resulting in the suppression of antiviral signaling and IFN induction in several cell types of human origin. Knockdown of NSs resulted in the suppression of SFTSV replication in host cells. Viral RNA bound to LSm14A-NSs protein complex during the interaction. A newly discovered LRRD motif of NSs functioned to interact with LSm14A. Altogether, our data demonstrated a mechanism used by SFTSV to inhibit host innate immune response.

Unraveling the Underlying Interaction Mechanism Between Dabie bandavirus and Innate Immune Response

The genus Bandavirus consists of seven tick-borne bunyaviruses, among which four are known to infect humans. Dabie bandavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), poses serious threats to public health worldwide. SFTSV is a tick-borne virus mainly reported in China, South Korea, and Japan with a mortality rate of up to 30%. To date, most immunology-related studies focused on the antagonistic role of SFTSV non-structural protein (NSs) in sequestering RIG-I-like-receptors (RLRs)-mediated type I interferon (IFN) induction and type I IFN mediated signaling pathway. It is still elusive whether the interaction of SFTSV and other conserved innate immune responses exists. As of now, no specific vaccines or therapeutics are approved for SFTSV prevention or treatments respectively, in part due to a lack of comprehensive understanding of the molecular interactions occurring between SFTSV and hosts. Hence, it is necessary to fully understand the host-virus interactions including antiviral responses and viral evasion mechanisms. In this review, we highlight the recent progress in understanding the pathogenesis of SFTS and speculate underlying novel mechanisms in response to SFTSV infection.

The Role of Non-Structural Protein NSs in the Pathogenesis of Severe Fever with Thrombocytopenia Syndrome

Viral non-structural proteins, such as NSs of the newly emerging severe fever with thrombocytopenia syndrome virus, are well established virulence factors, mediating viral pathogenesis and disease progression through various mechanisms. NSs has been described as a potent interferon antagonist and NF-κB agonist, two divergent signaling pathways in many immune responses upon a viral encounter. In this review, we highlight the many mechanisms used by NSs on the host that promote viral replication and hyper-inflammation. Understanding these host-pathogen interactions is crucial for antiviral therapy development.

A Look into Bunyavirales Genomes: Functions of Non-Structural (NS) Proteins

In 2016, the Bunyavirales order was established by the International Committee on Taxonomy of Viruses (ICTV) to incorporate the increasing number of related viruses across 13 viral families. While diverse, four of the families (Peribunyaviridae, Nairoviridae, Hantaviridae, and Phenuiviridae) contain known human pathogens and share a similar tri-segmented, negative-sense RNA genomic organization. In addition to the nucleoprotein and envelope glycoproteins encoded by the small and medium segments, respectively, many of the viruses in these families also encode for non-structural (NS) NSs and NSm proteins. The NSs of Phenuiviridae is the most extensively studied as a host interferon antagonist, functioning through a variety of mechanisms seen throughout the other three families. In addition, functions impacting cellular apoptosis, chromatin organization, and transcriptional activities, to name a few, are possessed by NSs across the families. Peribunyaviridae, Nairoviridae, and Phenuiviridae also encode an NSm, although less extensively studied than NSs, that has roles in antagonizing immune responses, promoting viral assembly and infectivity, and even maintenance of infection in host mosquito vectors. Overall, the similar and divergent roles of NS proteins of these human pathogenic Bunyavirales are of particular interest in understanding disease progression, viral pathogenesis, and developing strategies for interventions and treatments.

Zika Virus Infection Leads to Demyelination and Axonal Injury in Mature CNS Cultures

Understanding how Zika virus (Flaviviridae; ZIKV) affects neural cells is paramount in comprehending pathologies associated with infection. Whilst the effects of ZIKV in neural development are well documented, impact on the adult nervous system remains obscure. Here, we investigated the effects of ZIKV infection in established mature myelinated central nervous system (CNS) cultures. Infection incurred damage to myelinated fibers, with ZIKV-positive cells appearing when myelin damage was first detected as well as axonal pathology, suggesting the latter was a consequence of oligodendroglia infection. Transcriptome analysis revealed host factors that were upregulated during ZIKV infection. One such factor, CCL5, was validated in vitro as inhibiting myelination. Transferred UV-inactivated media from infected cultures did not damage myelin and axons, suggesting that viral replication is necessary to induce the observed effects. These data show that ZIKV infection affects CNS cells even after myelination-which is critical for saltatory conduction and neuronal function-has taken place. Understanding the targets of this virus across developmental stages including the mature CNS, and the subsequent effects of infection of cell types, is necessary to understand effective time frames for therapeutic intervention.

Immunocompetent mouse model for Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever (CCHF) is a severe tick-borne febrile illness with wide geographic distribution. CCHF is caused by infection with the Crimean-Congo hemorrhagic fever virus (CCHFV) and case fatality rates can be as high as 30%. Despite causing severe disease in humans, our understanding of the host and viral determinants of CCHFV pathogenesis are limited. A major limitation in the investigation of CCHF has been the lack of suitable small animal models. Wild-type mice are resistant to clinical isolates of CCHFV and consequently, mice must be deficient in type I interferon responses to study the more severe aspects of CCHFV. We report here a mouse-adapted variant of CCHFV that recapitulates in adult, immunocompetent mice the severe CCHF observed in humans. This mouse-adapted variant of CCHFV significantly improves our ability to study host and viral determinants of CCHFV-induced disease in a highly tractable mouse model.

Host restriction of emerging high-pathogenic bunyaviruses via MOV10 by targeting viral nucleoprotein and blocking ribonucleoprotein assembly

Bunyavirus ribonucleoprotein (RNP) that is assembled by polymerized nucleoproteins (N) coating a viral RNA and associating with a viral polymerase can be both the RNA synthesis machinery and the structural core of virions. Bunyaviral N and RNP thus could be assailable targets for host antiviral defense; however, it remains unclear which and how host factors target N/RNP to restrict bunyaviral infection. By mass spectrometry and protein-interaction analyses, we here show that host protein MOV10 targets the N proteins encoded by a group of emerging high-pathogenic representatives of bunyaviruses including severe fever with thrombocytopenia syndrome virus (SFTSV), one of the most dangerous pathogens listed by World Health Organization, in RNA-independent manner. MOV10 that was further shown to be induced specifically by SFTSV and related bunyaviruses in turn inhibits the bunyaviral replication in infected cells in series of loss/gain-of-function assays. Moreover, animal infection experiments with MOV10 knockdown corroborated the role of MOV10 in restricting SFTSV infection and pathogenicity in vivo. Minigenome assays and additional functional and mechanistic investigations demonstrate that the anti-bunyavirus activity of MOV10 is likely achieved by direct impact on viral RNP machinery but independent of its helicase activity and the cellular interferon pathway. Indeed, by its N-terminus, MOV10 binds to a protruding N-arm domain of N consisting of only 34 amino acids but proving important for N function and blocks N polymerization, N-RNA binding, and N-polymerase interaction, disabling RNP assembly. This study not only advances the understanding of bunyaviral replication and host restriction mechanisms but also presents novel paradigms for both direct antiviral action of MOV10 and host targeting of viral RNP machinery.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging high-pathogenic bunyavirus listed by the World Health Organization as a top priority pathogen for research and development. Although SFTSV and related bunyaviruses emerging globally have raised serious public health concerns, specific antivirals or vaccines are currently unavailable and little is known on the virus-host interactions and viral replication mechanism. The nucleoprotein (N) is essential for bunyavirus replication by driving assembly of ribonucleoprotein (RNP), the RNA synthesis machinery and structural core of virions. Here we show that N proteins of SFTSV and related bunyaviruses can be targeted by host factor MOV10 in RNA-independent manner. Further, MOV10 can be induced specifically by the bunyaviruses and in turn restrict the viral replication and pathogenicity in vitro and in vivo. The anti-bunyavirus activity of MOV10 is independent of its helicase region and cellular interferon pathway. Instead, by its N-terminus, MOV10 binds to a protruding N-arm domain of N and blocks N polymerization, N-RNA binding, and N-polymerase interaction, disabling RNP assembly. This study provides a delicate model for host targeting of viral RNP machinery and sheds light on bunyaviral replication and host restriction mechanisms, which may promote specific antiviral therapy development.

eIF2B as a Target for Viral Evasion of PKR-Mediated Translation Inhibition

RNA-activated protein kinase (PKR) is one of the most powerful antiviral defense factors of the mammalian host. PKR acts by phosphorylating mRNA translation initiation factor eIF2α, thereby converting it from a cofactor to an inhibitor of mRNA translation that strongly binds to initiation factor eIF2B. To sustain synthesis of their proteins, viruses are known to counteract this on the level of PKR or eIF2α or by circumventing initiation factor-dependent translation altogether. Here, we report a different PKR escape strategy executed by sandfly fever Sicilian virus (SFSV), a member of the increasingly important group of phleboviruses. We found that the nonstructural protein NSs of SFSV binds to eIF2B and protects it from inactivation by PKR-generated phospho-eIF2α. Protein synthesis is hence maintained and the virus can replicate despite ongoing full-fledged PKR signaling in the infected cells. Thus, SFSV has evolved a unique strategy to escape the powerful antiviral PKR.

RNA-activated protein kinase (PKR) is a major innate immune factor that senses viral double-stranded RNA (dsRNA) and phosphorylates eukaryotic initiation factor (eIF) 2α. Phosphorylation of the α subunit converts the eIF2αβγ complex into a stoichiometric inhibitor of eukaryotic initiation factor eIF2B, thus halting mRNA translation. To escape this protein synthesis shutoff, viruses have evolved countermechanisms such as dsRNA sequestration, eIF-independent translation by an internal ribosome binding site, degradation of PKR, or dephosphorylation of PKR or of phospho-eIF2α. Here, we report that sandfly fever Sicilian phlebovirus (SFSV) confers such a resistance without interfering with PKR activation or eIF2α phosphorylation. Rather, SFSV expresses a nonstructural protein termed NSs that strongly binds to eIF2B. Although NSs still allows phospho-eIF2α binding to eIF2B, protein synthesis and virus replication are unhindered. Hence, SFSV encodes a unique PKR antagonist that acts by rendering eIF2B resistant to the inhibitory action of bound phospho-eIF2α.

The Andes Orthohantavirus NSs Protein Antagonizes the Type I Interferon Response by Inhibiting MAVS Signaling

The small messenger RNA (SmRNA) of the Andes orthohantavirus (ANDV), a rodent-borne member of the Hantaviridae family of viruses of the Bunyavirales order, encodes a multifunctional nucleocapsid (N) protein and for a nonstructural (NSs) protein of unknown function. We have previously shown the expression of the ANDV-NSs, but only in infected cell cultures. In this study, we extend our early findings by confirming the expression of the ANDV-NSs protein in the lungs of experimentally infected golden Syrian hamsters. Next, we show, using a virus-free system, that the ANDV-NSs protein antagonizes the type I interferon (IFN) induction pathway by suppressing signals downstream of the melanoma differentiation-associated protein 5 (MDA5) and the retinoic acid-inducible gene 1 (RIG-I) and upstream of TBK1. Consistent with this observation, the ANDV-NSs protein antagonized mitochondrial antiviral-signaling protein (MAVS)-induced IFN-β, NF-κB, IFN-regulatory factor 3 (IRF3), and IFN-sensitive response element (ISRE) promoter activity. Results demonstrate that ANDV-NSs binds to MAVS in cells without disrupting the MAVS-TBK-1 interaction. However, in the presence of the ANDV-NSs ubiquitination of MAVS is reduced. In summary, this study provides evidence showing that the ANDV-NSs protein acts as an antagonist of the cellular innate immune system by suppressing MAVS downstream signaling by a yet not fully understand mechanism. Our findings reveal new insights into the molecular regulation of the hosts' innate immune response by the Andes orthohantavirus.IMPORTANCE Andes orthohantavirus (ANDV) is endemic in Argentina and Chile and is the primary etiological agent of hantavirus cardiopulmonary syndrome (HCPS) in South America. ANDV is distinguished from other hantaviruses by its unique ability to spread from person to person. In a previous report, we identified a novel ANDV protein, ANDV-NSs. Until now, ANDV-NSs had no known function. In this new study, we established that ANDV-NSs acts as an antagonist of cellular innate immunity, the first line of defense against invading pathogens, hindering the cellular antiviral response during infection. This study provides novel insights into the mechanisms used by ANDV to establish its infection.

The Severe Fever with Thrombocytopenia Syndrome Virus NSs Protein Interacts with CDK1 To Induce G2 Cell Cycle Arrest and Positively Regulate Viral Replication

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly identified phlebovirus associated with severe hemorrhagic fever in humans. While many viruses subvert the host cell cycle to promote viral growth, it is unknown whether this is a strategy employed by SFTSV. In this study, we investigated how SFTSV manipulates the cell cycle and the effect of the host cell cycle on SFTSV replication. Our results suggest that cells arrest at the G₂/M transition following infection with SFTSV. The accumulation of cells at the G₂/M transition did not affect virus adsorption and entry but did facilitate viral replication. In addition, we found that SFTSV NSs, a nonstructural protein that forms viroplasm-like structures in the cytoplasm of infected cells and promotes virulence by modulating the interferon response, induces a large number of cells to arrest at the G₂/M transition by interacting with CDK1. The interaction between NSs and CDK1, which is inclusion body dependent, inhibits formation and nuclear import of the cyclin B1-CDK1 complex, thereby leading to cell cycle arrest. Expression of a CDK1 loss-of-function mutant reversed the inhibitive effect of NSs on the cell cycle, suggesting that this protein is a potential antiviral target. Our study provides new insight into the role of a specific viral protein in SFTSV replication, indicating that NSs induces G₂/M arrest of SFTSV-infected cells, which promotes viral replication.IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne pathogen that causes severe hemorrhagic fever. Although SFTSV poses a serious threat to public health and was recently isolated, its pathogenesis remains unclear. In particular, the relationship between SFTSV infection and the host cell cycle has not been described. Here, we show for the first time that both asynchronized and synchronized SFTSV-susceptible cells arrest at the G₂/M checkpoint following SFTSV infection and that the accumulation of cells at this checkpoint facilitates viral replication. We also identify a key mechanism underlying SFTSV-induced G₂/M arrest, in which SFTSV NSs interacts with CDK1 to inhibit formation and nuclear import of the cyclin B1-CDK1 complex, thus preventing it from regulating cell cycle progression. Our study highlights the key role that NSs plays in SFTSV-induced G₂/M arrest.

Revisiting new tick-associated viruses: what comes next?

Tick-borne viral infections continue to cause diseases with considerable impact on humans, livestock, companion animals and wildlife. Many lack specific therapeutics and vaccines are available for only a few. Tick-borne viruses will continue to emerge, facilitated by anthroponotic factors related to the modern lifestyle. We persistently identify and are obliged to cope with new examples of emerging tick-borne viral diseases and novel viruses today. Many new strains have been detected in vertebrates and arthropods, some causing severe diseases likely to challenge public and veterinary health. This manuscript aims to provide a narrative overview of recently-described tick-associated viruses, with perspectives on changing paradigms in identification, screening and control.

Cross-genotype protection of live-attenuated vaccine candidate for severe fever with thrombocytopenia syndrome virus in a ferret model

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging viral pathogen discovered in 2009. The virus is present in countries of East Asia and is transmitted through the bite of an infected Haemaphysalis longicornis tick. SFTSV disease is associated with high morbidity and is often fatal. Despite the incidence of disease, no antiviral therapy or vaccine has been approved for use. Here, we report and assess 2 live attenuated viruses as vaccine candidates in our recently described ferret model of infection. We show that the viruses caused no clinical disease or mortality in healthy animals. Immunized animals mounted a robust humoral immune response to a single dose of virus, and this response protected the animals from a lethal challenge.

Severe fever with thrombocytopenia syndrome (SFTS) virus (SFTSV) is an emerging tick-borne virus classified within the Banyangvirus genus. SFTS disease has been reported throughout East Asia since 2009 and is characterized by high fever, thrombocytopenia, and leukopenia and has a 12 to 30% case fatality rate. Due to the recent emergence of SFTSV, there has been little time to conduct research into preventative measures aimed at combatting the virus. SFTSV is listed as one of the World Health Organization’s Prioritized Pathogens for research into antiviral therapeutics and vaccine development. Here, we report 2 attenuated recombinant SFTS viruses that induce a humoral immune response in immunized ferrets and confer complete cross-genotype protection to lethal challenge. Animals infected with rHB29NSsP₁₀₂A or rHB2912aaNSs (both genotype D) had a reduced viral load in both serum and tissues and presented without high fever, thrombocytopenia, or mortality associated with infection. rHB29NSsP₁₀₂A- or rHB2912aaNSs-immunized animals developed a robust anti-SFTSV immune response against cross-genotype isolates of SFTSV. This immune response was capable of neutralizing live virus in a focus-reduction neutralization test (FRNT) and was 100% protective against a cross-genotype lethal challenge with the CB1/2014 strain of SFTSV (genotype B). Thus, using our midsized, aged ferret infection model, we demonstrate 2 live attenuated vaccine candidates against the emerging pathogen SFTSV.

Immune Modulation and Immune-Mediated Pathogenesis of Emerging Tickborne Banyangviruses

In the last decade, the emergence of several, novel tickborne viruses have caused significant disease in humans. Of interest are the tickborne banyangviruses: Severe fever with thrombocytopenia syndrome virus (SFTSV), Heartland virus (HRTV), and Guertu virus (GTV). SFTSV and HRTV infection in humans cause viral hemorrhagic fever-like disease leading to mortality rates ranging from 6–30% of the cases. The systemic inflammatory response syndrome (SIRS) associated with SFTSV infection is hypothesized to contribute significantly to pathology seen in patients. Despite the severe disease caused by HRTV and SFTSV, there are no approved therapeutics or vaccines. Investigation of the immune response during and following infection is critical to the generation of fully protective vaccines and/or supportive treatments, and overall understanding of viral immune evasion mechanisms may aid in the development of a new class of therapeutics.

Species-Specific Pathogenicity of Severe Fever with Thrombocytopenia Syndrome Virus Is Determined by Anti-STAT2 Activity of NSs

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel emerging virus that has been identified in China, South Korea, and Japan, and it induces thrombocytopenia and leukocytopenia in humans with a high case fatality rate. SFTSV is pathogenic to humans, while immunocompetent adult mice and golden Syrian hamsters infected with SFTSV never show apparent symptoms. However, mice deficient for the gene encoding the α chain of the alpha- and beta-interferon receptor (Ifnar1^-/- mice) and golden Syrian hamsters deficient for the gene encoding signal transducer and activator of transcription 2 (Stat2^-/- hamsters) are highly susceptible to SFTSV infection, with infection resulting in death. The nonstructural protein (NSs) of SFTSV has been reported to inhibit the type I IFN response through sequestration of human STAT proteins. Here, we demonstrated that SFTSV induces lethal acute disease in STAT2-deficient mice but not in STAT1-deficient mice. Furthermore, we discovered that NSs cannot inhibit type I IFN signaling in murine cells due to an inability to bind to murine STAT2. Taken together, our results imply that the dysfunction of NSs in antagonizing murine STAT2 can lead to inefficient replication and the loss of pathogenesis of SFTSV in mice.IMPORTANCE Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by SFTSV, which has been reported in China, South Korea, and Japan. Here, we revealed that mice lacking STAT2, which is an important factor for antiviral innate immunity, are highly susceptible to SFTSV infection. We also show that SFTSV NSs cannot exert its anti-innate immunity activity in mice due to the inability of the protein to bind to murine STAT2. Our findings suggest that the dysfunction of SFTSV NSs as an IFN antagonist in murine cells confers a loss of pathogenicity of SFTSV in mice.

Heartland virus antagonizes type I and III interferon antiviral signaling by inhibiting phosphorylation and nuclear translocation of STAT2 and STAT1

Heartland virus (HRTV) is a pathogenic phlebovirus recently identified in the United States and related to severe fever with thrombocytopenia syndrome virus (SFTSV) emerging in Asia. We previously reported that SFTSV disrupts host antiviral responses directed by interferons (IFNs) and their downstream regulators, signal transducer and activator of transcription (STAT) proteins. However, whether HRTV infection antagonizes the IFN-STAT signaling axis remains unclear. Here, we show that, similar to SFTSV, HRTV also inhibits IFN-α- and IFN-λ-mediated antiviral responses. As expected, the nonstructural protein (NSs) of HRTV (HNSs) robustly antagonized both type I and III IFN signaling. Protein interaction analyses revealed that a common component downstream of type I and III IFN signaling, STAT2, is the target of HNSs. Of note, the DNA-binding and linker domains of STAT2 were required for an efficient HNSs-STAT2 interaction. Unlike the NSs of SFTSV (SNSs), which blocks both STAT2 and STAT1 nuclear accumulation, HNSs specifically blocked IFN-triggered nuclear translocation only of STAT2. However, upon HRTV infection, IFN-induced nuclear translocation of both STAT2 and STAT1 was suppressed, suggesting that STAT1 is an additional HRTV target for IFN antagonism. Consistently, despite HNSs inhibiting phosphorylation only of STAT2 and not STAT1, HRTV infection diminished both STAT2 and STAT1 phosphorylation. These results suggest that HRTV antagonizes IFN antiviral signaling by dampening both STAT2 and STAT1 activities. We propose that HNSs-specific targeting of STAT2 likely plays an important role but is not all of the "tactics" of HRTV in its immune evasion.

NSs Protein of Sandfly Fever Sicilian Phlebovirus Counteracts Interferon (IFN) Induction by Masking the DNA-Binding Domain of IFN Regulatory Factor 3

Sandfly fever Sicilian virus (SFSV) is one of the most widespread and frequently identified members of the genus Phlebovirus (order Bunyavirales, family Phenuiviridae) infecting humans. Being transmitted by Phlebotomus sandflies, SFSV causes a self-limiting, acute, often incapacitating febrile disease ("sandfly fever," "Pappataci fever," or "dog disease") that has been known since at least the beginning of the 20th century. We show that, similarly to other pathogenic phleboviruses, SFSV suppresses the induction of the antiviral type I interferon (IFN) system in an NSs-dependent manner. SFSV NSs interfered with the TBK1-interferon regulatory factor 3 (IRF3) branch of the RIG-I signaling pathway but not with NF-κB activation. Consistently, we identified IRF3 as a host interactor of SFSV NSs. In contrast to IRF3, neither the IFN master regulator IRF7 nor any of the related transcription factors IRF2, IRF5, and IRF9 were bound by SFSV NSs. In spite of this specificity for IRF3, NSs did not inhibit its phosphorylation, dimerization, or nuclear accumulation, and the interaction was independent of the IRF3 activation or multimerization state. In further studies, we identified the DNA-binding domain of IRF3 (amino acids 1 to 113) as sufficient for NSs binding and found that SFSV NSs prevented the association of activated IRF3 with the IFN-β promoter. Thus, unlike highly virulent phleboviruses, which either destroy antiviral host factors or sequester whole signaling chains into inactive aggregates, SFSV modulates type I IFN induction by directly masking the DNA-binding domain of IRF3.IMPORTANCE Phleboviruses are receiving increased attention due to the constant discovery of new species and the ongoing spread of long-known members of the genus. Outbreaks of sandfly fever were reported in the 19th century, during World War I, and during World War II. Currently, SFSV is recognized as one of the most widespread phleboviruses, exhibiting high seroprevalence rates in humans and domestic animals and causing a self-limiting but incapacitating disease predominantly in immunologically naive troops and travelers. We show how the nonstructural NSs protein of SFSV counteracts the upregulation of the antiviral interferon (IFN) system. SFSV NSs specifically inhibits promoter binding by IFN transcription factor 3 (IRF3), a molecular strategy which is unique among phleboviruses and, to our knowledge, among human pathogenic RNA viruses in general. This IRF3-specific and stoichiometric mechanism, greatly distinct from the ones exhibited by the highly virulent phleboviruses, correlates with the intermediate level of pathogenicity of SFSV.

Two Conserved Amino Acids within the NSs of Severe Fever with Thrombocytopenia Syndrome Phlebovirus Are Essential for Anti-interferon Activity

The nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome phlebovirus (SFTSV) sequesters TANK-binding kinase 1 (TBK1) into NSs-induced cytoplasmic structures to inhibit the phosphorylation and nuclear translocation of interferon (IFN) regulatory factor 3 (IRF3) and subsequent interferon beta (IFN-β) production. Although the C-terminal region of SFTSV NSs (NSs_66-249) has been linked to the formation of NSs-induced cytoplasmic structures and inhibition of host IFN-β responses, the role of the N-terminal region in antagonizing host antiviral responses remains to be defined. Here, we demonstrate that two conserved amino acids at positions 21 and 23 in the SFTSV and heartland virus (HRTV) NSs are essential for suppression of IRF3 phosphorylation and IFN-β mRNA expression following infection with SFTSV or recombinant influenza virus lacking the NS1 gene. Surprisingly, formation of SFTSV/HRTV NSs-induced cytoplasmic structures is not essential for inhibition of host antiviral responses. Rather, an association between SFTSV/HRTV NSs and TBK1 is required for suppression of mitochondrial antiviral signaling protein (MAVS)-mediated activation of IFN-β promoter activity. Although SFTSV NSs did not prevent the ubiquitination of TBK1, it associates with TBK1 through its N-terminal kinase domain (residues 1 to 307) to block the autophosphorylation of TBK1. Furthermore, we found that both wild-type NSs and the 21/23A mutant (NSs in which residues at positions 21 and 23 were replaced with alanine) of SFTSV suppressed NLRP3 inflammasome-dependent interleukin-1β (IL-1β) secretion, suggesting that the importance of these residues is restricted to TBK1-dependent IFN signaling. Together, our findings strongly implicate the two conserved amino acids at positions 21 and 23 of SFTSV/HRTV NSs in the inhibition of host interferon responses.IMPORTANCE Recognition of viruses by host innate immune systems plays a critical role not only in providing resistance to viral infection but also in the initiation of antigen-specific adaptive immune responses against viruses. Severe fever with thrombocytopenia syndrome (SFTS) is a newly emerging infectious disease caused by the SFTS phlebovirus (SFTSV), a highly pathogenic tick-borne phlebovirus. The 294-amino-acid nonstructural protein (NSs) of SFTSV associates with TANK-binding kinase 1 (TBK1), a key regulator of host innate antiviral immunity, to inhibit interferon beta (IFN-β) production and enhance viral replication. Here, we demonstrate that two conserved amino acids at positions 21 and 23 in the NSs of SFTSV and heartland virus, another tick-borne phlebovirus, are essential for association with TBK1 and suppression of IFN-β production. Our results provide important insight into the molecular mechanisms by which SFTSV NSs helps to counteract host antiviral strategies.

Nonstructural protein of severe fever with thrombocytopenia syndrome phlebovirus targets STAT2 and not STAT1 to inhibit type I interferon-stimulated JAK-STAT signaling

The nonstructural protein NSs of severe fever with thrombocytopenia syndrome phlebovirus blocks type I interferon (IFN)-stimulated JAK-STAT signaling. However, there is continuing controversy as to whether NSs targets STAT1 or STAT2 or both for this blockade. The present study was designed to gain a further understanding of the blockade mechanism. Immunoprecipitation experiments revealed a stronger interaction of NSs with STAT2 than with any other component constituting the JAK-STAT pathway. Expression of NSs resulted in the formation of cytoplasmic inclusion bodies (IBs), and affected cytoplasmic distribution of STAT2. STAT2 was relocated to NSs-induced IBs. Consequently, NSs inhibited IFN-α-stimulated tyrosine phosphorylation and nuclear translocation of STAT2. These inhibitory effects as well as the signaling blockade activity were not observed in NSs mutant proteins lacking the STAT2-binding ability. In contrast, NSs affected neither subcellular distribution nor phosphorylation of STAT1 in response to IFN-α and IFN-γ, demonstrating that NSs has little physical and functional interactions with STAT1. Taken together, these results suggest that NSs sequesters STAT2 into NSs-induced IBs, thereby blocking type I IFN JAK-STAT signaling.

A novel tick-borne phlebovirus, closely related to severe fever with thrombocytopenia syndrome virus and Heartland virus, is a potential pathogen

Tick-borne viral diseases have attracted much attention in recent years because of their increasing incidence and threat to human health. Severe fever with thrombocytopenia syndrome phlebovirus (SFTSV) and Heartland virus (HRTV) were recently identified as tick-borne phleboviruses (TBPVs) in Asia and the United States, respectively, and are associated with severe human diseases with similar clinical manifestations. In this study, we report the first identification and isolation of a novel TBPV named Guertu virus (GTV) from Dermacentor nuttalli ticks in Xinjiang Province, China, where TBPVs had not been previously discovered. Genome sequence and phylogenetic analyses showed that GTV is closely related to SFTSV and HRTV and was classified as a member of the genus Phlebovirus, family Phenuiviridae, order Bunyavirales. In vitro and in vivo investigations of the properties of GTV demonstrated that it was able to infect animal and human cell lines and can suppress type I interferon signaling, similar to SFTSV, that GTV nucleoprotein (NP) can rescue SFTSV replication by replacing SFTSV NP, and that GTV infection can cause pathological lesions in mice. Moreover, a serological survey identified antibodies against GTV from serum samples of individuals living in Guertu County, three of which contained neutralizing antibodies, suggesting that GTV can infect humans. Our findings suggested that this virus is a potential pathogen that poses a threat to animals and humans. Further studies and surveillance of GTV are recommended to be carried out in Xinjiang Province as well as in other locations.

Control of Lyme borreliosis and other Ixodes ricinus-borne diseases

Lyme borreliosis (LB) and other Ixodes ricinus-borne diseases (TBDs) are diseases that emerge from interactions of humans and domestic animals with infected ticks in nature. Nature, environmental and health policies at (inter)national and local levels affect the risk, disease burden and costs of TBDs. Knowledge on ticks, their pathogens and the diseases they cause have been increasing, and resulted in the discovery of a diversity of control options, which often are not highly effective on their own. Control strategies involving concerted actions from human and animal health sectors as well as from nature managers have not been formulated, let alone implemented. Control of TBDs asks for a “health in all policies” approach, both at the (inter)national level, but also at local levels. For example, wildlife protection and creating urban green spaces are important for animal and human well-being, but may increase the risk of TBDs. In contrast, culling or fencing out deer decreases the risk for TBDs under specific conditions, but may have adverse effects on biodiversity or may be societally unacceptable. Therefore, in the end, nature and health workers together must carry out tailor-made control options for the control of TBDs for humans and animals, with minimal effects on the environment. In that regard, multidisciplinary approaches in environmental, but also medical settings are needed. To facilitate this, communication and collaboration between experts from different fields, which may include patient representatives, should be promoted.