Roles of viroplasm-like structures formed by nonstructural protein NSs in infection with severe fever with thrombocytopenia syndrome virus

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.

Severe fever with thrombocytopenia syndrome virus induces lactylation of m6A reader protein YTHDF1 to facilitate viral replication

Severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging infectious pathogen with a high fatality rate, is an enveloped tripartite segmented single-stranded negative-sense RNA virus. SFTSV infection is characterized by suppressed host innate immunity, proinflammatory cytokine storm, failure of B-cell immunity, and robust viral replication. m6A modification has been shown to play a role in viral infections. However, interactions between m6A modification and SFTSV infection remain poorly understood. Through MeRIP-seq, we identify m6A modifications on SFTSV RNA. We show that YTHDF1 can bind to m6A modification sites on SFTSV, decreasing the stability of SFTSV RNA and reducing the translation efficiency of SFTSV proteins. The SFTSV virulence factor NSs increases lactylation of YTHDF1 and YTHDF1 degradation, thus facilitating SFTSV replication. Our findings indicate that the SFTSV protein NSs induce lactylation to inhibit YTHDF1 as a countermeasure to host's YTHDF1-mediated degradation of m6A-marked viral mRNAs.

Downregulation of transcription 1 hinders the replication of Dabie bandavirus by promoting the expression of TLR7, TLR8, and TLR9 signaling pathway

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a bunyavirus that causes SFTS, with a case fatality rate of up to 30 %. The innate immune system plays a crucial role in the defense against SFTSV; however, the impact of viral propagation of STFSV on the innate immune system remains unclear. Although proteomics analysis revealed that the expression of the downregulator of transcription 1 (DR1) increased after SFTSV infection, the specific change trend and the functional role of DR1 during viral infection remain unelucidated. In this study, we demonstrate that DR1 was highly expressed in response to SFTSV infection in HEK 293T cells using qRT-PCR and Western blot analysis. Furthermore, viral replication significantly increased the expression of various TLRs, especially TLR9. Our data indicated that DR1 positively regulated the expression of TLRs in HEK 293T cells, DR1 overexpression highly increased the expression of numerous TLRs, whereas RNAi-mediated DR1 silencing decreased TLR expression. Additionally, the myeloid differentiation primary response gene 88 (MyD88)-dependent or TIR-domain-containing adaptor inducing interferon-β (TRIF)-dependent signaling pathways were highly up- and downregulated by the overexpression and silencing of DR1, respectively. Finally, we report that DR1 stimulates the expression of TLR7, TLR8, and TLR9, thereby upregulating the TRIF-dependent and MyD88-dependent signaling pathways during the SFTSV infection, attenuating viral replication, and enhancing the production of type I interferon and various inflammatory factors, including IL-1β, IL-6, and IL-8. These results imply that DR1 defends against SFTSV replication by inducing the expression of TLR7, TLR8, and TLR9. Collectively, our findings revealed a novel role and mechanism of DR1 in mediating antiviral responses and innate immunity.

Hsp90 β is critical for the infection of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS) caused by the SFTS virus (SFTSV) is an emerging disease in East Asia with a fatality rate of up to 30%. However, the viral-host interaction of SFTSV remains largely unknown. The heat-shock protein 90 (Hsp90) family consists of highly conserved chaperones that fold and remodel proteins and has a broad impact on the infection of many viruses. Here, we showed that Hsp90 is an important host factor involved in SFTSV infection. Hsp90 inhibitors significantly reduced SFTSV replication, viral protein expression, and the formation of inclusion bodies consisting of nonstructural proteins (NSs). Among viral proteins, NSs appeared to be the most reduced when Hsp90 inhibitors were used, and further analysis showed that their translation was affected. Co-immunoprecipitation of NSs with four isomers of Hsp90 showed that Hsp90 β specifically interacted with them. Knockdown of Hsp90 β expression also inhibited replication of SFTSV. These results suggest that Hsp90 β plays a critical role during SFTSV infection and could be a potential target for the development of drugs against SFTS.

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.

Developing and characterizing monoclonal antibodies of Guertu bandavirus nucleoprotein for developing methods of Guertu bandavirus and severe fever with thrombocytopenia syndrome virus detection

Belonging to genus Bandavirus in Phenuiviridae family, Guertu bandavirus (GTV) is a potential pathogen closely related to severe fever with thrombocytopenia syndrome virus (SFTSV) and heartland virus (HRTV) associated with human diseases. Although the medical significance of GTV is not clear, there was serological evidence suggesting past infection with this virus has occurred, indicating its potential threat to human health. So, it is important to prepare for detection of GTV infection so as to control virus transmission and promote disease diagnosis and treatment. This study is aimed at obtaining monoclonal antibodies (mAbs) against GTV nucleoprotein (NP) and evaluating their activities in recognizing viral antigens from genetic-related bandaviruses, SFTSV and HRTV. Eight mAbs were obtained and four of them (22G1, 25C2, 25E2, and 26F8) recognize linear epitopes of GTV NP. The four mAbs showed cross-reactivity to SFTSV but were unable to react with HRTV. Two fine epitopes were identified by the four mAbs, ENP1 (194YNSFRDPLHAAV205) and ENP2 (226GPDGLP231), which are highly conserved in the NPs of GTV and SFTSV but are distinct in HRTV NP. The features of epitopes, including their hydrophilicity, antibody accessibility, flexibility, antigenicity, and spatial locations, were predicted and analyzed, and their potential functional impacts on virus infection and replication and their use for virus detection were discussed. Our results promote the understanding of the molecular basis of GTV and SFTSV NP in inducing antibody responses. The NP-specific mAbs generated in this study are promising fundamental materials for developing viral antigen detection methods for GTV and SFTSV.

The interplay between lipid droplets and virus infection

As an intracellular parasite, the virus usurps cellular machinery and modulates cellular metabolism pathways to replicate itself in cells. Lipid droplets (LDs) are universally conserved energy storage organelles that not only play vital roles in maintaining lipid homeostasis but are also involved in viral replication. Increasing evidence has demonstrated that viruses take advantage of cellular lipid metabolism by targeting the biogenesis, hydrolysis, and lipophagy of LD during viral infection. In this review, we summarize the current knowledge about the modulation of cellular LD by different viruses, with a special emphasis on the Hepatitis C virus, Dengue virus, and SARS-CoV-2.

The SFTSV Nonstructural Proteins Induce Autophagy to Promote Viral Replication via Interaction with Vimentin

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly identified phlebovirus associated with severe hemorrhagic fever in humans. Studies have shown that SFTSV nucleoprotein (N) induces BECN1-dependent autophagy to promote viral assembly and release. However, the function of other SFTSV proteins in regulating autophagy has not been reported. In this study, we identify SFTSV NSs, a nonstructural protein that forms viroplasm-like structures in the cytoplasm of infected cells as the virus component mediating SFTSV-induced autophagy. We found that SFTSV NSs-induced autophagy was inclusion body independent, and most phenuivirus NSs had autophagy-inducing effects. Unlike N protein-induced autophagy, SFTSV NSs was key in regulating autophagy by interacting with the host's vimentin in an inclusion body-independent manner. NSs interacted with vimentin and induced vimentin degradation through the K48-linked ubiquitin-proteasome pathway. This negatively regulating Beclin1-vimentin complex formed and promoted autophagy. Furthermore, we identified the NSs-binding domain of vimentin and found that overexpression of wild-type vimentin antagonized the induced effect of NSs on autophagy and inhibited viral replication, suggesting that vimentin is a potential antiviral target. The present study shows a novel mechanism through which SFTSV nonstructural protein activates autophagy, which provides new insights into the role of NSs in SFTSV infection and pathogenesis. IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly emerging tick-borne pathogen that causes multifunctional organ failure and even death in humans. As a housekeeping mechanism for cells to maintain steady state, autophagy plays a dual role in viral infection and the host's immune response. However, the relationship between SFTSV infection and autophagy has not been described in detail yet. Here, we demonstrated that SFTSV infection induced complete autophagic flux and facilitated viral proliferation. We also identified a key mechanism underlying NSs-induced autophagy, in which NSs interacted with vimentin to inhibit the formation of the Beclin1-vimentin complex and induced vimentin degradation through K48-linked ubiquitination modification. These findings may help us understand the new functions and mechanisms of NSs and may aid in the identification of new antiviral targets.

Liquid Phase Partitioning in Virus Replication: Observations and Opportunities

Viruses frequently carry out replication in specialized compartments within cells. The effect of these structures on virus replication is poorly understood. Recent research supports phase separation as a foundational principle for organization of cellular components with the potential to influence viral replication. In this review, phase separation is described in the context of formation of viral replication centers, with an emphasis on the nonsegmented negative-strand RNA viruses. Consideration is given to the interplay between phase separation and the critical processes of viral transcription and genome replication, and the role of these regions in pathogen-host interactions is discussed. Finally, critical questions that must be addressed to fully understand how phase separation influences viral replication and the viral life cycle are presented, along with information about new approaches that could be used to make important breakthroughs in this emerging field.

Immune escape mechanisms of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS), which is caused by SFTS virus (SFTSV), poses a serious threat to global public health, with high fatalities and an increasing prevalence. As effective therapies and prevention strategies are limited, there is an urgent need to elucidate the pathogenesis of SFTS. SFTSV has evolved several mechanisms to escape from host immunity. In this review, we summarize the mechanisms through which SFTSV escapes host immune responses, including the inhibition of innate immunity and evasion of adaptive immunity. Understanding the pathogenesis of SFTS will aid in the development of new strategies for the treatment of this disease.

Nonstructural Protein NSs Activates Inflammasome and Pyroptosis through Interaction with NLRP3 in Human Microglial Cells Infected with Severe Fever with Thrombocytopenia Syndrome Bandavirus

Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne febrile disease caused by SFTS virus (SFTSV), or Dabie bandavirus, in the Phenuiviridae family. Clinically neurological disorders in SFTS have been commonly reported, but their neuropathogenesis has rarely been studied. Microglia are a type of neuroglia accounting for 10 to 12% of all cells in the brain. As resident immune cells, microglial cells are the first line of immune defense present in the central nervous system (CNS). Here, we report that SFTSV was able to infect microglial cells and stimulate interleukin 1β (IL-1β) secretion in the brains of infected neonatal BALB/c mice. We characterized the cell death induced in infected human microglial HMC3 cells, also susceptible to SFTSV, and found that the NOD-like receptor protein 3 (NLRP3) inflammasome was activated, leading to secretion of IL-1β and pyroptosis. Knockdown of NLRP3 or inhibition of the NLRP3 inflammasome activation suppressed the viral replication, suggesting that the activation of the NLRP3 inflammasome may support SFTSV replication in microglial cells. Viral nonstructural protein NSs, a known modulator of immune responses, interacted and colocalized with NLRP3 for the inflammasome activation. It appeared that the N-terminal fragment, amino acids 1 to 66, of NSs was critical to promote the assembly of the inflammasome complex by interacting with NLRP3 for its activation in microglial cells. Our findings provide evidence that SFTSV may cause neurological disorders through infecting microglia and activating the inflammasome through its nonstructural protein NSs for neural cell death and inflammation. This study may have revealed a novel mechanism of SFTSV NSs in dysregulating host response. IMPORTANCE Encephalitis or encephalopathy during severe fever with thrombocytopenia syndrome (SFTS) is considered a critical risk factor leading to high mortality, but there have been no studies to date on the pathogenesis of encephalitis or encephalopathy caused by SFTS virus. Here, we report that SFTSV infection can active the NLRP3 inflammasome and induce IL-1β secretion in the brains of infected newborn mice. In infected human HMC3 microglia, SFTSV activated the NLRP3 inflammasome via the viral nonstructural protein NSs through interaction with its N-terminal fragment. Notably, our findings suggest that the activation of the NLRP3 inflammasome may promote SFTSV replication in infected microglial cells. This study may reveal a novel mechanism by SFTSV to dysregulate host responses through its nonstructural protein, which could help us understand viral neuropathogenesis in SFTS patients.

Activation of the NLRP3 inflammasome and elevation of interleukin-1β secretion in infection by sever fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging phlebovirus that causes a hemorrhagic fever known as the severe fever with thrombocytopenia syndrome (SFTS). Inflammasomes are a molecular platform that are assembled to process pro-caspase 1 and subsequently promote secretion of interleukin (IL)-1β/IL-18 for proinflammatory responses induced upon infection. We hypothesize that inflammasome activation and pyroptosis induced in SFTS results in elevated levels of IL-1β/IL-18 responsible for high fever and hemorrhage in the host, characteristic of SFTS. Here we report that IL-1β secretion was elevated in SFTS patients and infected mice and IL-1β levels appeared to be reversibly associated to disease severity and viral load in patients’ blood. Increased caspase-1 activation, IL-1β/IL-18 secretion, cell death, and processing of gasdermin D were detected, indicating that pyroptosis was induced in SFTSV-infected human peripheral blood monocytes (PBMCs). To characterize the mechanism of pyroptosis induction, we knocked down several NOD-like receptors (NLRs) with respective shRNAs in PBMCs and showed that the NLR family pyrin domain containing 3 (NLRP3) inflammasome was critical for processing pro-caspase-1 and pro-IL-1β. Our data with specific inhibitors for NLRP3 and caspase-1 further showed that activation of the NLRP3 inflammasome was key to caspase-1 activation and IL-1β secretion which may be inhibitory to viral replication in PBMCs infected with SFTSV. The findings in this study suggest that the activation of the NLPR3 inflammasome and pyroptosis, leading to IL-1β/IL-18 secretion during the SFTSV infection, could play important roles in viral pathogenesis and host protection. Pyroptosis as part of innate immunity might be essential in proinflammatory responses and pathogenicty in humans infected with this novel phlebovirus.

Structure and function of negative-strand RNA virus polymerase complexes

Viruses with negative-strand RNA genomes (NSVs) include many highly pathogenic and economically devastating disease-causing agents of humans, livestock, and plants-highlighted by recent Ebola and measles virus epidemics, and continuously circulating influenza virus. Because of their protein-coding orientation, NSVs face unique challenges for efficient gene expression and genome replication. To overcome these barriers, NSVs deliver a large and multifunctional RNA-dependent RNA polymerase into infected host cells. NSV-encoded polymerases contain all the enzymatic activities required for transcription and replication of their genome-including RNA synthesis and mRNA capping. Here, we review the structures and functions of NSV polymerases with a focus on key domains responsible for viral replication and gene expression. We highlight shared and unique features among polymerases of NSVs from the Mononegavirales, Bunyavirales, and Articulavirales orders.

Deconstructing virus condensation

Viruses have evolved precise mechanisms for using the cellular physiological pathways for their perpetuation. These virus-driven biochemical events must be separated in space and time from those of the host cell. In recent years, granular structures, known for over a century for rabies virus, were shown to host viral gene function and were named using terms such as viroplasms, replication sites, inclusion bodies, or viral factories (VFs). More recently, these VFs were shown to be liquid-like, sharing properties with membrane-less organelles driven by liquid–liquid phase separation (LLPS) in a process widely referred to as biomolecular condensation. Some of the best described examples of these structures come from negative stranded RNA viruses, where micrometer size VFs are formed toward the end of the infectious cycle. We here discuss some basic principles of LLPS in connection with several examples of VFs and propose a view, which integrates viral replication mechanisms with the biochemistry underlying liquid-like organelles. In this view, viral protein and RNA components gradually accumulate up to a critical point during infection where phase separation is triggered. This yields an increase in transcription that leads in turn to increased translation and a consequent growth of initially formed condensates. According to chemical principles behind phase separation, an increase in the concentration of components increases the size of the condensate. A positive feedback cycle would thus generate in which crucial components, in particular nucleoproteins and viral polymerases, reach their highest levels required for genome replication. Progress in understanding viral biomolecular condensation leads to exploration of novel therapeutics. Furthermore, it provides insights into the fundamentals of phase separation in the regulation of cellular gene function given that virus replication and transcription, in particular those requiring host polymerases, are governed by the same biochemical principles.

Non-structural Proteins of Severe Fever With Thrombocytopenia Syndrome Virus Suppress RNA Synthesis in a Transcriptionally Active cDNA-Derived Viral RNA Synthesis System

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by the tick-borne SFTS bunyavirus (SFTSV) resulting in a high fatality rate up to 30%. SFTSV is a negative-strand RNA virus containing three single-stranded RNA genome segments designated as L, M, and S, which respectively, encode the RNA-dependent RNA polymerase (RdRp), glycoproteins Gn and Gc, and nucleoprotein (N) and non-structural proteins (NSs). NSs can form inclusion bodies (IBs) in infected and transfected cells. A previous study has provided a clue that SFTSV NSs may be involved in virus-like or viral RNA synthesis; however, the details remain unclear. Our work described here reveals that SFTSV NSs can downregulate virus-like RNA synthesis in a dose-dependent manner within a cDNA-derived viral RNA synthesis system, i.e., minigenome (−) and minigenome (+) systems based on transfection, superinfection, and luciferase reporter activity determination; meanwhile, NSs also show a weak inhibitory effect on virus replication. By using co-immunoprecipitation (Co-IP) and RT-PCR combined with site-directed mutagenesis, we found that NSs suppress virus-like RNA or virus replication through interacting with N but not with RdRp, and the negative regulatory effect correlates closely with the IB structure it formed but is not associated with its role of antagonizing host innate immune responses. When the cytoplasmic structure of IB formed by SFTSV NSs was deprived, the inhibitory effect of NSs on virus-like RNA synthesis would weaken and even disappear. Similarly, we also evaluated other bandavirus NSs that cannot form IB in neither infected nor transfected cells, and the results showed that the NSs of Heartland bandavirus (HRTV) did not show a significant inhibitory effect on virus-like RNA synthesis within a minigenome system. Our findings provide experimental evidence that SFTSV NSs participate in regulating virus-like or viral RNA synthesis and the negative effect may be due to the NSs–N interaction.

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.

shRNA targeting nonstructural protein inhibits the replication of severe fever with thrombocytopenia syndrome virus

Aim: To explore whether shRNA targeting nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) could inhibit SFTSV replication in Vero cells. Materials & methods: SFTSV used in this experiment was propagated in Vero cells and stored at -20°C. shRNA plasmid against NSs of SFTSV was transfected to Vero cells and infected with SFTSV, after which western blotting and tissue culture infective dose (TCID50) were used to measure the virus titers. Results: shRNA against NSs protein decreased the expression of NSs and inhibited the replication of SFTSV. Conclusion: The constructed SFTSV NSs-shRNA plasmid could inhibit the replication of SFTSV. It was concluded that SFTSV NSs-shRNA could inhibit virus replication for at least 72 h. shRNA-mediated antiviral effects were dose-dependent.

New Perspectives on the Biogenesis of Viral Inclusion Bodies in Negative-Sense RNA Virus Infections

Infections by negative strand RNA viruses (NSVs) induce the formation of viral inclusion bodies (IBs) in the host cell that segregate viral as well as cellular proteins to enable efficient viral replication. The induction of those membrane-less viral compartments leads inevitably to structural remodeling of the cellular architecture. Recent studies suggested that viral IBs have properties of biomolecular condensates (or liquid organelles), as have previously been shown for other membrane-less cellular compartments like stress granules or P-bodies. Biomolecular condensates are highly dynamic structures formed by liquid-liquid phase separation (LLPS). Key drivers for LLPS in cells are multivalent protein:protein and protein:RNA interactions leading to specialized areas in the cell that recruit molecules with similar properties, while other non-similar molecules are excluded. These typical features of cellular biomolecular condensates are also a common characteristic in the biogenesis of viral inclusion bodies. Viral IBs are predominantly induced by the expression of the viral nucleoprotein (N, NP) and phosphoprotein (P); both are characterized by a special protein architecture containing multiple disordered regions and RNA-binding domains that contribute to different protein functions. P keeps N soluble after expression to allow a concerted binding of N to the viral RNA. This results in the encapsidation of the viral genome by N, while P acts additionally as a cofactor for the viral polymerase, enabling viral transcription and replication. Here, we will review the formation and function of those viral inclusion bodies upon infection with NSVs with respect to their nature as biomolecular condensates.

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.

Preparation of a polyclonal antibody against the non-structural protein, NSs of SFTSV

Severe fever with thrombocytopenia syndrome virus (SFTSV) is newly discovered virus which is the member of the order Bunyavirales, family phenuiviridae, phlebovirus genus. Its genome is composed of 3 segments of negative-sense RNA L, M and S. NSs is a non structure protein encoded by S segment which is important for viral replication and virulence. NSs protein of SFTSV is only involved in the regulation of host innate immune responses and suppression of IFN-promoter activities. So, the exact functions of this protein need to be studied deeply. To understand the exact role of NSs from SFTSV in viral replication and host immune response, a qualified antibody against this protein is required. In this study, NSs gene of SFTSV, was cloned into a bacterial expression vector (pGEX-6P-1) and the recombinant plasmid was transformed into Escherichia coli BL21 (DE3) cells. The SFTSV NSs fusion protein was purified using Glutathione Sepharose 4B and utilized as an antigen to immunize rabbits and obtain an anti-SFTSV NSs polyclonal antibody. Proper expression of the fusion protein and polyclonal antibody specificity were confirmed by western blotting and immunofluorescence analyses. The polyclonal antibody recognized NSs from SFTSV specifically. This is the first report that NSs can form viroplasm-like structures not only in infected cells but also in transfected cells with NSs plasmids. This polyclonal antibody will be useful for future studies of NSs functions.

SFTSV Infection Induced Interleukin-1β Secretion Through NLRP3 Inflammasome Activation

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne virus that causes hemorrhagic fever. Previous studies showed that SFTSV-infected patients exhibited elevated levels of pro-inflammatory cytokines like interleukin-1β (IL-1β), indicating that SFTSV infection may activate inflammasomes. However, the detailed mechanism remains poorly understood. Herein, we found that SFTSV could stimulate the IL-1β secretion in the infected human peripheral blood mononuclear cells (PBMCs), human macrophages, and C57/BL6 mice. We demonstrate that the maturation and secretion of IL-1β during SFTSV infection is mediated by the nucleotide and oligomerization domain, leucine-rich repeat-containing protein family, pyrin-containing domain 3 (NLRP3) inflammasome. This process is dependent on protease caspase-1, a component of the NLRP3 inflammasome complex. For the first time, our study discovered the role of NLRP3 in response to SFTSV infection. This finding may lead to the development of novel drugs to impede the pathogenesis of SFTSV infection.

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.

Interactions of Viral Proteins from Pathogenic and Low or Non-Pathogenic Orthohantaviruses with Human Type I Interferon Signaling

Rodent-borne orthohantaviruses are asymptomatic in their natural reservoir, but they can cause severe diseases in humans. Although an exacerbated immune response relates to hantaviral pathologies, orthohantaviruses have to antagonize the antiviral interferon (IFN) response to successfully propagate in infected cells. We studied interactions of structural and nonstructural (NSs) proteins of pathogenic Puumala (PUUV), low-pathogenic Tula (TULV), and non-pathogenic Prospect Hill (PHV) viruses, with human type I and III IFN (IFN-I and IFN-III) pathways. The NSs proteins of all three viruses inhibited the RIG-I-activated IFNβ promoter, while only the glycoprotein precursor (GPC) of PUUV, or its cleavage product Gn/Gc, and the nucleocapsid (N) of TULV inhibited it. Moreover, the GPC of both PUUV and TULV antagonized the promoter of IFN-stimulated responsive elements (ISRE). Different viral proteins could thus contribute to inhibition of IFNβ response in a viral context. While PUUV and TULV strains replicated similarly, whether expressing entire or truncated NSs proteins, only PUUV encoding a wild type NSs protein led to late IFN expression and activation of IFN-stimulated genes (ISG). This, together with the identification of particular domains of NSs proteins and different biological processes that are associated with cellular proteins in complex with NSs proteins, suggested that the activation of IFN-I is probably not the only antiviral pathway to be counteracted by orthohantaviruses and that NSs proteins could have multiple inhibitory functions.

Severe Fever with Thrombocytopenia Syndrome Virus NSs Interacts with TRIM21 To Activate the p62-Keap1-Nrf2 Pathway

Nuclear factor erythroid 2-related factor 2 (Nrf2) dissociates from its inhibitor, Keap1, upon stress signals and subsequently induces an antioxidant response that critically controls the viral life cycle and pathogenesis. Besides intracellular Fc receptor function, tripartite motif 21 (TRIM21) E3 ligase plays an essential role in the p62-Keap1-Nrf2 axis pathway for redox homeostasis. Specifically, TRIM21-mediated p62 ubiquitination abrogates p62 oligomerization and sequestration activity and negatively regulates the Keap1-Nrf2-mediated antioxidant response. A number of viruses target the Nrf2-mediated antioxidant response to generate an optimal environment for their life cycle. Here we report that a nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) interacts with and inhibits TRIM21 to activate the Nrf2 antioxidant signal pathway. Mass spectrometry identified TRIM21 to be a binding protein for NSs. NSs bound to the carboxyl-terminal SPRY subdomain of TRIM21, enhancing p62 stability and oligomerization. This facilitated p62-mediated Keap1 sequestration and ultimately increased Nrf2-mediated transcriptional activation of antioxidant genes, including those for heme oxygenase 1, NAD(P)H quinone oxidoreductase 1, and CD36. Mutational analysis found that the NSs-A46 mutant, which no longer interacted with TRIM21, was unable to increase Nrf2-mediated transcriptional activation. Functionally, the NS wild type (WT), but not the NSs-A46 mutant, increased the surface expression of the CD36 scavenger receptor, resulting in an increase in phagocytosis and lipid uptake. A combination of reverse genetics and assays with Ifnar ^-/- mouse models revealed that while the SFTSV-A46 mutant replicated similarly to wild-type SFTSV (SFTSV-WT), it showed weaker pathogenic activity than SFTSV-WT. These data suggest that the activation of the p62-Keap1-Nrf2 antioxidant response induced by the NSs-TRIM21 interaction contributes to the development of an optimal environment for the SFTSV life cycle and efficient pathogenesis.IMPORTANCE Tick-borne diseases have become a growing threat to public health. SFTSV, listed by the World Health Organization as a prioritized pathogen, is an emerging phlebovirus, and fatality rates among those infected with this virus are high. Infected Haemaphysalis longicornis ticks are the major source of human SFTSV infection. In particular, the recent spread of this tick to over 12 states in the United States has increased the potential for outbreaks of this disease beyond Far East Asia. Due to the lack of therapies and vaccines against SFTSV infection, there is a pressing need to understand SFTSV pathogenesis. As the Nrf2-mediated antioxidant response affects viral life cycles, a number of viruses deregulate Nrf2 pathways. Here we demonstrate that the SFTSV NSs inhibits the TRIM21 function to upregulate the p62-Keap1-Nrf2 antioxidant pathway for efficient viral pathogenesis. This study not only demonstrates the critical role of SFTSV NSs in viral pathogenesis but also suggests potential future therapeutic approaches to treat SFTSV-infected patients.

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.

Interferon-γ-Directed Inhibition of a Novel High-Pathogenic Phlebovirus and Viral Antagonism of the Antiviral Signaling by Targeting STAT1

Severe fever with thrombocytopenia syndrome (SFTS) is a life-threatening infectious disease caused by a novel phlebovirus, SFTS virus (SFTSV). Currently, there is no vaccine or antiviral available and the viral pathogenesis remains largely unknown. In this study, we demonstrated that SFTSV infection results in substantial production of serum interferon-γ (IFN-γ) in patients and then that IFN-γ in turn exhibits a robust anti-SFTSV activity in cultured cells, indicating the potential role of IFN-γ in anti-SFTSV immune responses. However, the IFN-γ anti-SFTSV efficacy was compromised once viral infection had been established. Consistently, we found that viral nonstructural protein (NSs) expression counteracts IFN-γ signaling. By protein interaction analyses combined with mass spectrometry, we identified the transcription factor of IFN-γ signaling pathway, STAT1, as the cellular target of SFTSV for IFN-γ antagonism. Mechanistically, SFTSV blocks IFN-γ-triggered STAT1 action through (1) NSs-STAT1 interaction-mediated sequestration of STAT1 into viral inclusion bodies and (2) viral infection-induced downregulation of STAT1 protein level. Finally, the efficacy of IFN-γ as an anti-SFTSV drug in vivo was evaluated in a mouse infection model: IFN-γ pretreatment but not posttreatment conferred significant protection to mice against lethal SFTSV infection, confirming IFN-γ's anti-SFTSV effect and viral antagonism against IFN-γ after the infection establishment. These findings present a picture of virus-host arm race and may promote not only the understanding of virus-host interactions and viral pathogenesis but also the development of antiviral therapeutics.

Alterations of Gut Microbiome in the Patients With Severe Fever With Thrombocytopenia Syndrome

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne infectious disease caused by SFTS virus, and the number of SFTS cases increased year by year in China. Previous studies had indicated that gut microbiome closely associated with human health and diseases, including infection diseases, liver diseases, gastrointestinal diseases and metabolic diseases. The aim of this study is to investigate the alterations and involvements of gut microbial in SFTS patients. We compared the gut microbiome of 26 SFTS patients between 20 health controls using the Illumina MiSeq sequencing platform. Reduced gut microbiota diversity and dramatic shifts of fecal microbial composition in SFTS patients were observed compared with health controls. In the intestinal microbial of SFTS patients, the Lachnospiraceae and Ruminococcaceae which could produce short-chain fatty acids were clearly dropped compared with health people, meanwhile, Sutterella which have anti-inflammation properties were reduced too. On the contrary, some common opportunistic pathogens like Enterococcus and Streptococcus and endotoxin-producing bacteria Escherichia which could rise the risk of infections were increased in SFTS patients than healthy people, in addition lactate-producing bacteria Lactobacillaceae also significantly increased in SFTS patients. In addition, research findings on the correlation between gut microbiota and biochemical data found that the changes of gut microbiota of SFTS patients were closely associated with clinical symptoms, key serum enzymes, infection and mortality. These alterations of gut microbiome in SFTS patients suggest the potential contributions of gut microbial to the pathogenesis of SFTS.

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.

Targeting of severe fever with thrombocytopenia syndrome virus structural proteins to the ERGIC (endoplasmic reticulum Golgi intermediate compartment) and Golgi complex

Severe fever with thrombocytopenia syndrome phlebovirus (SFTSV) is a newly emerged phlebovirus identified in China, Japan, and South Korea. Phlebovirus glycoproteins (GP) play a key role in targeting viral structural components to the budding compartments in the ER-Golgi intermediate compartment (ERGIC) and Golgi complex. However, the role of SFTSV GP in targeting structural proteins to the ERGIC and Golgi complex remains unresolved. In this study, we show that SFTSV GP plays a significant role in targeting RNA-dependent RNA polymerase (L) and nucleocapsid protein (NP) to the budding sites. Confocal microscopy was used to investigate the subcellular localization of SFTSV structural proteins. In SFTSV-infected cells, GP and L localized to the ER, ERGIC and Golgi complex, whereas NP localized to the ERGIC and Golgi complex. In addition, GP colocalized with L and NP in infected cells. In cells singly transfected with GP, L or NP, GP localized to the same subcellular compartments as in infected cells. However, L or NP alone did not localize to the ER, ERGIC, or Golgi complex. Cotransfection experiments showed that GP altered the localization of L to the ERGIC and Golgi complex but not that of NP. Interestingly, plasmid-expressed NP fused with a hemagglutinin tag localized to the ERGIC and Golgi complex when expressed in SFTSV-infected cells and colocalised with GP, suggesting that GP plays a role in the subcellular localization of L and NP in infected cells. Thus, the SFTSV structural components start to assemble at the ERGIC to Golgi complex. GP is required for transporting L and NP to the ERGIC and Golgi complex. In addition, targeting of NP requires interaction with other factors besides GP.

The cholesterol, fatty acid and triglyceride synthesis pathways regulated by site 1 protease (S1P) are required for efficient replication of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by the SFTS virus (SFTSV), which has a high mortality rate. Currently, no licensed vaccines or therapeutic agents have been approved for use against SFTSV infection. Here, we report that the cholesterol, fatty acid, and triglyceride synthesis pathways regulated by S1P is involved in SFTSV replication, using CHO-K1 cell line (SRD-12B) that is deficient in site 1 protease (S1P) enzymatic activity, PF-429242, a small compound targeting S1P enzymatic activity, and Fenofibrate and Lovastatin, which inhibit triglyceride and cholesterol synthesis, respectively. These results enhance our understanding of the SFTSV replication mechanism and may contribute to the development of novel therapies for SFTSV infection.

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.

SFTS phlebovirus promotes LC3-II accumulation and nonstructural protein of SFTS phlebovirus co-localizes with autophagy proteins

Autophagy is essential for eukaryotic cell homeostasis and can perform both anti-viral and pro-viral roles depending on the kinds of viruses, cell types and cell environment. Severe fever with thrombocytopenia syndrome phlebovirus (SFTSV) is a newly discovered tick-borne virus in the Phenuiviridae family that causes a severe hemorrhagic fever disease in East Asia. In this study we determined interactions between SFTSV and autophagy. Our results showed that LC3-II (microtubule associated protein 1 light chain 3-II) protein accumulated from 4 h to 24 h after SFTSV infection compared to mock-infected Vero cells, and the use of E64d and pepstatin A did not affect the expression of LC3-II protein, which indicated that the increased LC3-II may be the result of inhibition of autophagic degradation caused by SFTSV infection. However, knockdown of LC3B promotes SFTSV replication, which indicated a negative role of LC3B protein in SFTSV replication. We also detected co-localization of SFTSV non-structure (NSs) protein with LC3B, p62 and Lamp2b respectively in SFTSV infected Vero cells, which indicated the possibility of selective autophagy or chaperone-mediated autophagy involving in SFTSV infection. Our results indicated that SFTSV infection promotes LC3 accumulation and several proteins of the autophagy pathway co-localize with NSs protein during SFTSV infection.

Heartland virus NSs protein disrupts host defenses by blocking the TBK1 kinase-IRF3 transcription factor interaction and signaling required for interferon induction

Heartland virus (HRTV) is a pathogenic phlebovirus related to the severe fever with thrombocytopenia syndrome virus (SFTSV), another phlebovirus causing life-threatening disease in humans. Previous findings have suggested that SFTSV can antagonize the host interferon (IFN) system via viral nonstructural protein (NSs)-mediated sequestration of antiviral signaling proteins into NSs-induced inclusion bodies. However, whether and how HRTV counteracts the host innate immunity is unknown. Here, we report that HRTV NSs (HNSs) also antagonizes IFN and cytokine induction and bolsters viral replication, although no noticeable inclusion body formation was observed in HNSs-expressing cells. Furthermore, HNSs inhibited the virus-triggered activation of IFN-β promoter by specifically targeting the IFN-stimulated response element but not the NF-κB response element. Consistently, HNSs blocked the phosphorylation and nuclear translocation of IFN regulatory factor 3 (IRF3, an IFN-stimulated response element-activating transcription factor). Reporter gene assays next showed that HNSs blockades the antiviral signaling mediated by RIG-I-like receptors likely at the level of TANK-binding kinase 1 (TBK1). Indeed, HNSs strongly interacts with TBK1 as indicated by confocal microscopy and pulldown analyses, and we also noted that the scaffold dimerization domain of TBK1 is required for the TBK1-HNSs interaction. Finally, pulldown assays demonstrated that HNSs expression dose-dependently diminishes a TBK1-IRF3 interaction, further explaining the mechanism for HNSs function. Collectively, these data suggest that HNSs, an antagonist of host innate immunity, interacts with TBK1 and thereby hinders the association of TBK1 with its substrate IRF3, thus blocking IRF3 activation and transcriptional induction of the cellular antiviral responses.

The Emergence of Severe Fever with Thrombocytopenia Syndrome Virus

Severe fever with thrombocytopenia syndrome (SFTS) is a newly recognized hemorrhagic fever disease found throughout Asia with a case fatality rate between 12% and 30%. Since 2009, SFTS has been reported in China throughout 14 Chinese Provinces. In addition, SFTS has been recognized in South Korea and Japan with the first confirmed cases reported in 2012. A similar disease, caused by the closely related Heartland virus, was also reported in the United States in 2009. SFTS is caused by SFTS virus, a novel tick-borne virus in the family Bunyaviridae, genus Phlebovirus. Unlike other mosquito- and sandfly-borne bunyaviruses, SFTS virus has not been extensively studied due to its recent emergence and many unknowns regarding its pathogenesis, life cycle, transmission, and options for therapeutics remains. In this review, we report the most current findings in SFTS virus research.

Mapping of Transcription Termination within the S Segment of SFTS Phlebovirus Facilitated Generation of NSs Deletant Viruses

SFTS phlebovirus (SFTSV) is an emerging tick-borne bunyavirus that was first reported in China in 2009. Here we report the generation of a recombinant SFTSV (rHB29NSsKO) that cannot express the viral nonstructural protein (NSs) upon infection of cells in culture. We show that rHB29NSsKO replication kinetics are greater in interferon (IFN)-incompetent cells and that the virus is unable to suppress IFN induced in response to viral replication. The data confirm for the first time in the context of virus infection that NSs acts as a virally encoded IFN antagonist and that NSs is dispensable for virus replication. Using 3' rapid amplification of cDNA ends (RACE), we mapped the 3' end of the N and NSs mRNAs, showing that the mRNAs terminate within the coding region of the opposite open reading frame. We show that the 3' end of the N mRNA terminates upstream of a 5'-GCCAGCC-3' motif present in the viral genomic RNA. With this knowledge, and using virus-like particles, we could demonstrate that the last 36 nucleotides of the NSs open reading frame (ORF) were needed to ensure the efficient termination of the N mRNA and were required for recombinant virus rescue. We demonstrate that it is possible to recover viruses lacking NSs (expressing just a 12-amino-acid NSs peptide or encoding enhanced green fluorescent protein [eGFP]) or an NSs-eGFP fusion protein in the NSs locus. This opens the possibility for further studies of NSs and potentially the design of attenuated viruses for vaccination studies.IMPORTANCE SFTS phlebovirus (SFTSV) and related tick-borne viruses have emerged globally since 2009. SFTSV has been shown to cause severe disease in humans. For bunyaviruses, it has been well documented that the nonstructural protein (NSs) enables the virus to counteract the human innate antiviral defenses and that NSs is one of the major determinants of virulence in infection. Therefore, the use of reverse genetics systems to engineer viruses lacking NSs is an attractive strategy to rationally attenuate bunyaviruses. Here we report the generation of several recombinant SFTS viruses that cannot express the NSs protein or have the NSs open reading frame replaced with a reporter gene. These viruses cannot antagonize the mammalian interferon (IFN) response mounted to virus infection. The generation of NSs-lacking viruses was achieved by mapping the transcriptional termination of two S-segment-derived subgenomic mRNAs, which revealed that transcription termination occurs upstream of a 5'-GCCAGCC-3' motif present in the virus genomic S RNA.

Downregulation of Interferon-β and Inhibition of TLR3 Expression are associated with Fatal Outcome of Severe Fever with Thrombocytopenia Syndrome

Severe Fever with Thrombocytopenia Syndrome (SFTS) is an emerging infectious disease with high mortality and increasing prevalence in the East Asia. Though the etiological agent has been identified as a novel Bunyavirus, cellular mechanisms of viral pathogenesis and host immune response to SFTS virus infection remain unknown. A comprehensive study was conducted on a cohort of 70 patients on clinical manifestations, viral loads, modulation of cytokines, serum interferon level, immune related gene expression in peripheral blood cells, and dynamic changes of circulating dendritic cells during the acute phase of SFTSV infection. We found that high level viremia, reduced platelets, coagulation dysfunction, multi-organ injuries, elevated IL-6 and TNF-α were closely associated with the aggravation of SFTS. In addition, we demonstrated strong correlations between disease severity and the decline of serum IFN-β and IL-1β level, reduction of myeloid dendritic cells (mDCs) and suppressed Toll like receptor 3 expression in monocytes and mDCs. In general, dysfunction of innate immune response and cytokine storm are both involved in the pathogenesis of SFTS. Reduction of myeloid DCs contributes to the fatal outcome of SFTS virus infection, and the regulation of TLR3 could probably be the mechanism.

Lipid droplet functions beyond energy storage

Lipid droplets are cytoplasmic organelles that store neutral lipids and are critically important for energy metabolism. Their function in energy storage is firmly established and increasingly well characterized. However, emerging evidence indicates that lipid droplets also play important and diverse roles in the cellular handling of lipids and proteins that may not be directly related to energy homeostasis. Lipid handling roles of droplets include the storage of hydrophobic vitamin and signaling precursors, and the management of endoplasmic reticulum and oxidative stress. Roles of lipid droplets in protein handling encompass functions in the maturation, storage, and turnover of cellular and viral polypeptides. Other potential roles of lipid droplets may be connected with their intracellular motility and, in some cases, their nuclear localization. This diversity highlights that lipid droplets are very adaptable organelles, performing different functions in different biological contexts. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

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

In recent years, several newly discovered tick-borne viruses causing a wide spectrum of diseases in humans have been ascribed to the Phlebovirus genus of the Bunyaviridae family. The nonstructural protein (NSs) of bunyaviruses is the main virulence factor and interferon (IFN) antagonist. We studied the molecular mechanisms of IFN antagonism employed by the NSs proteins of human apathogenic Uukuniemi virus (UUKV) and those of Heartland virus (HRTV) and severe fever with thrombocytopenia syndrome virus (SFTSV), both of which cause severe disease. Using reporter assays, we found that UUKV NSs weakly inhibited the activation of the beta interferon (IFN-β) promoter and response elements. UUKV NSs weakly antagonized human IFN-β promoter activation through a novel interaction with mitochondrial antiviral-signaling protein (MAVS), confirmed by coimmunoprecipitation and confocal microscopy studies. HRTV NSs efficiently antagonized both IFN-β promoter activation and type I IFN signaling pathways through interactions with TBK1, preventing its phosphorylation. HRTV NSs exhibited diffused cytoplasmic localization. This is in comparison to the inclusion bodies formed by SFTSV NSs. HRTV NSs also efficiently interacted with STAT2 and impaired IFN-β-induced phosphorylation but did not affect STAT1 or its translocation to the nucleus. Our results suggest that a weak interaction between STAT1 and HRTV or SFTSV NSs may explain their inability to block type II IFN signaling efficiently, thus enabling the activation of proinflammatory responses that lead to severe disease. Our findings offer insights into how pathogenicity may be linked to the capacity of NSs proteins to block the innate immune system and illustrate the plethora of viral immune evasion strategies utilized by emerging phleboviruses. IMPORTANCE Since 2011, there has been a large expansion in the number of emerging tick-borne viruses that have been assigned to the Phlebovirus genus. Heartland virus (HRTV) and SFTS virus (SFTSV) were found to cause severe disease in humans, unlike other documented tick-borne phleboviruses such as Uukuniemi virus (UUKV). Phleboviruses encode nonstructural proteins (NSs) that enable them to counteract the human innate antiviral defenses. We assessed how these proteins interacted with the innate immune system. We found that UUKV NSs engaged with innate immune factors only weakly, at one early step. However, the viruses that cause more severe disease efficiently disabled the antiviral response by targeting multiple components at several stages across the innate immune induction and signaling pathways. Our results suggest a correlation between the efficiency of the virus protein/host interaction and severity of disease.

Severe fever with thrombocytopenia syndrome virus inhibits exogenous Type I IFN signaling pathway through its NSs invitro

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by a novel bunyavirus (SFTS virus, SFTSV). At present there is still no specific antiviral treatment for SFTSV; To understand which cells support SFTSV life cycle and whether SFTSV infection activates host innate immunity, four different cell lines (Vero, Hela, Huh7.5.1, and Huh7.0) were infected with SFTSV. Intracellular/extracellular viral RNA and expression of IFNα, and IFNß were detected by real-time RT- PCR following infection. To confirm the role of non-structural protein (NSs) of SFTSV in exogenous IFNα-induced Jak/STAT signaling, p-STAT1 (Western Blot), ISRE activity (Luciferase assay) and ISG expression (real-time PCR) were examined following IFNα stimulation in the presence or absence of over-expression of NSs in Hela cells. Our study showed that all the four cell lines supported SFTSV life cycle and SFTSV activated host innate immunity to produce type I IFNs in Hela cells but not in Huh7.0, Huh7.5.1 or Vero cells. NSs inhibited exogenous IFNα-induced Jak/STAT signaling as shown by decreased p-STAT1 level, suppressed ISRE activity and down-regulated ISG expression. Suppression of the exogenous Type I IFN-induced Jak/STAT signaling by NSs might be one of the mechanisms of SFTSV to evade host immune surveillance.

Current status of severe fever with thrombocytopenia syndrome in China

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by SFTS virus (SFTSV). SFTSV is associated with a high mortality rate and has been reported in China, South Korea and Japan. SFTSV undergoes rapid changes owing to evolution, gene mutations, and reassortment between different strains of SFTSV. In this review, we summarize the recent cases and general properties of SFTS, focusing on the epidemiology, genetic diversity, clinical features, and diagnostics of SFTSV in China. From 2010 to October 2016, SFTS cases were reported in 23 provinces of China, with increased numbers yearly. Infection and death cases are mainly found in central China, where the Haemaphysalis longicornis ticks are spread. The national average mortality rate of SFTS infection was 5.3%, with higher risk to elder people. The main epidemic period was from May to July, with a peak in May. Thus, SFTS reminds a significant public health problem, and development of prophylactic vaccines and effective antiviral drugs will be highly needed.

Rift Valley fever virus NSs protein functions and the similarity to other bunyavirus NSs proteins

Rift Valley fever is a mosquito-borne zoonotic disease that affects both ruminants and humans. The nonstructural (NS) protein, which is a major virulence factor for Rift Valley fever virus (RVFV), is encoded on the S-segment. Through the cullin 1-Skp1-Fbox E3 ligase complex, the NSs protein promotes the degradation of at least two host proteins, the TFIIH p62 and the PKR proteins. NSs protein bridges the Fbox protein with subsequent substrates, and facilitates the transfer of ubiquitin. The SAP30-YY1 complex also bridges the NSs protein with chromatin DNA, affecting cohesion and segregation of chromatin DNA as well as the activation of interferon-β promoter. The presence of NSs filaments in the nucleus induces DNA damage responses and causes cell-cycle arrest, p53 activation, and apoptosis. Despite the fact that NSs proteins have poor amino acid similarity among bunyaviruses, the strategy utilized to hijack host cells are similar. This review will provide and summarize an update of recent findings pertaining to the biological functions of the NSs protein of RVFV as well as the differences from those of other bunyaviruses.

Phleboviruses and the Type I Interferon Response

The genus Phlebovirus of the family Bunyaviridae contains a number of emerging virus species which pose a threat to both human and animal health. Most prominent members include Rift Valley fever virus (RVFV), sandfly fever Naples virus (SFNV), sandfly fever Sicilian virus (SFSV), Toscana virus (TOSV), Punta Toro virus (PTV), and the two new members severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV). The nonstructural protein NSs is well established as the main phleboviral virulence factor in the mammalian host. NSs acts as antagonist of the antiviral type I interferon (IFN) system. Recent progress in the elucidation of the molecular functions of a growing list of NSs proteins highlights the astonishing variety of strategies employed by phleboviruses to evade the IFN system.

Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNA-expressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection.

A novel viral RNA helicase with an independent translation enhancement activity

RNA helicases have not been identified among negative sense RNA viruses. In this study, it is shown that Nonstructural protein (NSs) of Groundnut bud necrosis virus (GBNV) acts as a Mg(2+) - and ATP-dependent bipolar RNA helicase. Biophysical and biochemical analysis of the deletion mutants (NΔ124 NSs, CΔ80 NSs) revealed that both the N- and C-terminal residues are required for substrate binding, oligomerization and helicase activity, but are dispensable for ATPase activity. Interestingly, NSs could enhance the translation of RNA (~ 10-fold) independent of its helicase activity. This is the first report of a RNA helicase from negative strand RNA viruses.

Rotavirus replication and the role of cellular lipid droplets: New therapeutic targets?

Rotaviruses (RVs) are a major cause of acute gastroenteritis in infants and young children worldwide. These viruses infect the villous epithelium of the small intestine. Part of their replication occurs in cytoplasmic inclusion bodies termed viroplasms. Viroplasms and the lipid droplets (LDs) of cellular organelles are known to interact both physically and functionally. Compounds interfering with the homoeostasis of LDs significantly decrease the production of infectious RV progeny. There is considerable scope for more detailed exploration of such compounds as potential antiviral agents for a disease for which at present no specific therapy exists.

Extracellular Vesicles Mediate Receptor-Independent Transmission of Novel Tick-Borne Bunyavirus

Severe fever with thrombocytopenia syndrome (SFTS) virus is a newly recognized member of the genus Phlebovirus in the family Bunyaviridae. The virus was isolated from patients presenting with hemorrhagic manifestations and an initial case fatality rate of 12 to 30% was reported. Due to the recent emergence of this pathogen, there is limited knowledge on the molecular virology of SFTS virus. Recently, we reported that the SFTS virus NSs protein inhibited the activation of the beta interferon (IFN-β) promoter. Furthermore, we also found that SFTS virus NSs relocalizes key components of the IFN response into NSs-induced cytoplasmic structures. Due to the important role these structures play during SFTS virus replication, we conducted live cell imaging studies to gain further insight into the role and trafficking of these cytoplasmic structures during virus infection. We found that some of the SFTS virus NSs-positive cytoplasmic structures were secreted to the extracellular space and endocytosed by neighboring cells. We also found that these secreted structures isolated from NSs-expressing cells and SFTS virus-infected cells were positive for the viral protein NSs and the host protein CD63, a protein associated with extracellular vesicles. Electron microscopy studies also revealed that the isolated CD63-immunoprecipitated extracellular vesicles produced during SFTS virus infection contained virions. The virions harbored within these structures were efficiently delivered to uninfected cells and were able to sustain SFTS virus replication. Altogether, these results suggest that SFTS virus exploits extracellular vesicles to mediate virus receptor-independent transmission to host cells and open the avenue for novel therapeutic strategies against SFTS virus and related pathogens.

IMPORTANCE

SFTS virus is novel bunyavirus associated with hemorrhagic fever illness. Currently, limited information is available about SFTS virus. In the present study, we demonstrated that extracellular vesicles produced by SFTS virus-infected cells harbor infectious virions. We sought to determine whether these "infectious" extracellular vesicles can mediate transmission of the virus and confirmed that the SFTS virions were efficiently transported by these secreted structures into uninfected cells and were able to sustain efficient replication of SFTS virus. These results have significant impact on our understanding of how the novel tick-borne phleboviruses hijack cellular machineries to establish infection and point toward a novel mechanism for virus replication among arthropod-borne viruses.

Host Responses and Regulation by NFκB Signaling in the Liver and Liver Epithelial Cells Infected with A Novel Tick-borne Bunyavirus

Infection in humans by severe fever with thrombocytopenia syndrome virus (SFTSV), a novel bunyavirus transmitted by ticks, is often associated with pronounced liver damage, especially in fatal cases. Little has been known, however, about how liver cells respond to SFTSV and how the response is regulated. In this study we report that proinflammatory cytokines were induced in liver tissues of C57/BL6 mice infected with SFTSV, which may cause tissue necrosis in mice. Human liver epithelial cells were susceptible to SFTSV and antiviral interferon (IFN) and IFN-inducible proteins were induced upon infection. We observed that infection of liver epithelial cells led to significant increases in proinflammatory cytokines and chemokines, including IL-6, RANTES, IP-10, and MIP-3a, which were regulated by NFκB signaling, and the activation of NFκB signaling during infection promoted viral replication in liver epithelial cells. Viral nonstructural protein NSs was inhibitory to the induction of IFN-β, but interestingly, NFκB activation was enhanced in the presence of NSs. Therefore, NSs plays dual roles in the suppression of antiviral IFN-β induction as well as the promotion of proinflammatory responses. Our findings provide the first evidence for elucidating host responses and regulation in liver epithelial cells infected by an emerging bunyavirus.