Identification of severe fever with thrombocytopenia syndrome virus isolates in the northwest of Hubei Province, China.

Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne viral disease that is increasingly affecting human being worldwide. The clinical manifestations and mortality rates of SFTS can vary depending on the geographic region and the specific genotype of the SFTS virus (SFTSV). From July 2022 to August 2023, we collected serum samples from 83 patients with suspected SFTSV infection in the northwest of Hubei Province, China. From which, 13 patients tested positive for SFTSV. Phylogenetic analysis of the SFTSV L, M, and S gene segments was performed using the maximum likelihood method to determine the genetic diversity of the isolates. At least 2 SFTSV genotypes (A and F) were identified in the northwest of Hubei Province. The clinical manifestations and laboratory findings on the first day of admission were investigated. Results showed that bleeding and disturbance of consciousness, and significant elevated AST and APTT, are valuable for assessing the prognosis for SFTS patients. This study disclosed the genomic sequences and genotypes of SFTSV spreading in the northwest of Hubei Province for the first time, providing information of genetically etiology for SFTS in the local district. Furthermore, certain symptoms and/or laboratory findings may indicate adverse clinical outcomes, highlighting the importance of identifying the symptoms and monitoring specific laboratory markers. Future research is needed to investigate the threshold values of these markers and to closely observe the indicative symptoms in order to early identify and timely management of critically ill patients within clinical settings.

Clinical and etiological characteristics of severe hemorrhagic fever caused by coinfection of hantaan orthohantavirus and severe fever with thrombocytopenia syndrome virus.

Severe fever with thrombocytopenia syndrome (SFTS) and hemorrhagic fever with renal syndrome (HFRS) usually have different infection routes, and coinfection is relatively rare. This study examines the clinical and etiological characteristics of coinfection by these two pathogens to provide important references for clinical diagnosis and treatment. Blood samples from 22 clinically diagnosed patients with HFRS were collected for molecular detection of HFRS and common tick and mouse borne diseases. Inoculate the blood of six severe and critically patients into cells to isolate and proliferate potential viruses, and retest the cell culture to determine the pathogen. In addition, complete data were collected from these 22 HFRS and concurrent SFTS patients, and white blood cells (WBCs), platelet (PLT), blood urea nitrogen (BUN), creatinine (Cr) and other data were compared and analyzed. A total of 31 febrile patients, including 22 HFRS patients and 9 SFTS patients, were collected from September 2021 to October 2022. Among these HFRS patients, 11 were severe or critical. Severe and critical HFRS patients were characterized by rodent exposure history, pharyngeal and conjunctival hyperemia, abnormal WBC and PLT counts, and elevated BUN and Cr values. Virus isolation and molecular detection on blood samples from 6 patients showed that three of the six severe patients were positive for hantaan virus (HTNV), and two of the three HTNV positives were also positive for SFTS bunyavirus (SFTSV). The two coinfected patients exhibited different clinical and laboratory characteristics compared to those infected by either virus alone. Coinfection of HTNV and SFTSV leads to severe and complex hemorrhagic fever. Laboratory characteristics, such as the indicators of WBC, PLT, BUN, and Cr, may differ between HFRS and SFTS. These findings have implications and provide references for the diagnosis and treatment of coinfected cases.

Bunyavirus SFTSV nucleoprotein exploits TUFM-mediated mitophagy to impair antiviral innate immunity.

Severe fever with thrombocytopenia syndrome is an emerging viral hemorrhagic fever caused by a tick-borne bunyavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), with a high case fatality. We previously found that SFTSV nucleoprotein (NP) induces macroautophagy/autophagy to facilitate virus replication. However, the role of NP in antagonizing host innate immunity remains unclear. Mitophagy, a selected form of autophagy, eliminates damaged mitochondria to maintain mitochondrial homeostasis. Here, we demonstrate that SFTSV NP triggers mitophagy to degrade MAVS (mitochondrial antiviral signaling protein), thereby blocking MAVS-mediated antiviral signaling to escape the host immune response. Mechanistically, SFTSV NP translocates to mitochondria by interacting with TUFM (Tu translation elongation factor, mitochondrial), and mediates mitochondrial sequestration into phagophores through interacting with LC3, thus inducing mitophagy. Notably, the N-terminal LC3-interacting region (LIR) motif of NP is essential for mitophagy induction. Collectively, our results demonstrated that SFTSV NP serves as a novel virulence factor, inducing TUFM-mediated mitophagy to degrade MAVS and evade the host immune response.Abbreviation: 3-MA: 3-methyladenine; ACTB: actin beta; co-IP: co-immunoprecipitation; CQ: chloroquine; DAPI: 4',6-diamidino-2-phenylindole, dihydrochloride; DMSO: dimethyl sulfoxide; FCCP: carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone; GFP: green fluorescent protein; HTNV: Hantan virus; IAV: influenza A virus; IFN: interferon; LAMP1: lysosomal associated membraneprotein 1; LIR: LC3-interacting region; MAP1LC3B/LC3B: microtubule associatedprotein 1 light chain 3 beta; MAVS: mitochondrial antiviral signaling protein; Mdivi-1: mitochondrial division inhibitor 1; MOI: multiplicity of infection; MT-CO2/COXII: mitochondrially encoded cytochrome C oxidase II; NP: nucleoprotein; NSs: nonstructural proteins; poly(I:C): polyinosinic:polycytidylic acid; RIGI: RNA sensor RIG-I; RLR: RIGI-like receptor; SFTSV: severe fever withthrombocytopenia syndrome virus; TCID50: 50% tissue culture infectiousdose; TIMM23: translocase of inner mitochondrial membrane 23; TOMM20:translocase of outer mitochondrial membrane 20; TUFM: Tu translation elongationfactor, mitochondrial.

Antiviral activity of vitamin D derivatives against severe fever with thrombocytopenia syndrome virus in vitro and in vivo.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne virus that causes the severe fever thrombocytopenia syndrome, which manifests as fever and haemorrhage, accompanied by severe neurological complications. To date, no specific antiviral drugs have been approved for this indication. Herein, we investigated whether vitamin D derivatives inhibit SFTSV both in vitro and in vivo. An in vitro study demonstrated that vitamin D derivatives significantly suppressed viral RNA replication, plaque formation, and protein expression in a dose-dependent manner. Subsequently, in vivo studies revealed that doxercalciferol and alfacalcidol were associated with increased survival and reduced viral RNA load in the blood. Time-of-addition assay suggested that vitamin D derivatives primarily acted during the post-entry phase of SFTSV infection. However, cytopathic effect protective activity was not observed in RIG-I immunodeficient cell line Huh7.5, and the administration of vitamin D derivatives did not improve the survival rates or reduce the blood viral loads in adult A129 mice. Further transcriptome exploration into the antiviral mechanism revealed that alfacalcidol stimulates host innate immunity to exert antiviral effects. To expand the application of vitamin D derivatives, in vitro and in vivo drug combination assays were performed, which highlighted the synergistic effects of vitamin D derivatives and T-705 on SFTSV. The combination of alfacalcidol and T-705 significantly enhanced the therapeutic effects in mice. This study highlights the potential of vitamin D derivatives against SFTSV and suggests that they may have synergistic effects with other compounds used in the treatment of SFTSV infection.

The hnRNP A2B1 is important for the replication of SFTSV and other RNA viruses.

The heterogeneous nuclear ribonucleoprotein (hnRNP A2B1) is a key component of the hnRNP complex involving RNA modulation in eukaryotic cells and it has also been reported to be involved in the replication of the hepatitis E virus, influenza A virus, and hepatitis B virus. However, it is not clear whether the role of the hnRNP A2B1 in viral replication is conserved among RNA viruses and what is the mechanism of hnRNP A2B1 in RNA virus replication. In this study, we first used severe fever with thrombocytopenia syndrome virus (SFTSV), a tick-borne RNA virus that causes a severe viral hemorrhagic fever as well as other RNA viruses including VSV-GFP, SeV, EV71, and ZIKV to demonstrate that knockout hnRNPA2B1 gene inhibited viral RNA replication and overexpression of hnRNP A2B1 could restore the RNA levels of all tested RNA viruses. These results suggest that hnRNPA2B1 upregulation of viral replication is conserved among RNA viruses. Next, we demonstrated that hnRNP A2B1 was translocated from the nucleus to the cytoplasm under RNA virus infection including SFTSV, VSV-GFP, SeV, EV71, and ZIKV, suggesting translocation of hnRNP A2B1 from the nucleus to the cytoplasm is crucial for RNA virus replication. We then used SFTSV as a model to demonstrate the mechanism of hnRNP A2B1 in the promotion of RNA virus replication. We found that overexpression of SFTSV nucleoprotein can also cause hnRNP A2B1 translocation from the nucleus to the cytoplasm and that the SFTSV NP interacted with the RNA recognition motif 1 domain of hnRNP A2B1. We further demonstrated that the hnRNP A2B1 interacted with the 5' UTR of SFTSV RNA. In conclusion, we revealed that the hnRNP A2B1 upregulation of viral RNA replication is conserved among RNA viruses; the mechanism of hnRNP A2B1 in promotion of SFTSV viral RNA replication is that SFTSV NP interacted with the hnRNPA2B1 to retain it in the cytoplasm where the hnRNP A2B1 interacted with the 5' UTR of SFTSV RNA to promote the viral RNA replication.IMPORTANCESevere fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne RNA virus with a high mortality rate of up to 30%. In this study, we first used SFTSV as a model to demonstrate that the role of hnRNPA2B1 in viral replication is conserved in SFTSV. Then we used other RNA viruses, including VSV-GFP, SeV, EV71, and ZIKV, to repeat the experiment and demonstrated the same results as SFTSV in all tested RNA viruses. By knocking out the hnRNPA2B1 gene, SFTSV RNA replication was inhibited, and overexpression of hnRNPA2B1 restored RNA levels of SFTSV and other tested RNA viruses. We revealed a novel mechanism where the SFTSV nucleoprotein interacts with hnRNPA2B1, retaining it in the cytoplasm. This interaction promotes viral RNA replication by binding to the 5' UTR of SFTSV RNA. The findings suggest that targeting hnRNPA2B1 could be a potential strategy for developing broad-spectrum antiviral therapies, given its conserved role across different RNA viruses. This research provides significant insights into the replication mechanisms of RNA viruses and highlights potential targets for antiviral interventions.

Characterization of high-affinity antibodies against the surface Gc protein of Dabie bandavirus / severe fever with thrombocytopenia syndrome virus.

Severe fever with thrombocytopenia syndrome virus (SFTSV) or Dabie bandavirus is an emerging pathogen responsible for SFTS. It is considered a novel threat to human health, given the high associated fatality. SFTSV is a segmented negative-strand RNA virus containing three single-stranded RNAs, with the M segment encoding the glycoproteins Gn and Gc. Gc is vital for viral entry into the host cell surface, along with the Gn protein. As the Gc is the surface-exposable antigen from virions, it is a critical diagnostic marker of infection. Although various SFTSV Gn or N protein-based sero-diagnostic methods have been developed, there are no commercially available sero-diagnostic kits. Therefore, we generated monoclonal antibodies (mAbs) against SFTSV Gc and explored their application in serum diagnostic tests to develop sensitive serodiagnostic tools covering broad-range genotypes (A to F). First, 10 SFTSV Gc antibody-binding fragments (Fabs) were isolated using a phage display system and converted into human IgGs. Enzyme-linked immunosorbent assays (ELISA) of the SFTSV and Rift Valley fever virus (RVFV: same genus as SFTSV) Gc antigens showed that all antibodies attached to the SFTSV Gc protein had high affinity. An immunofluorescence assay (IFA), to verify the cross-reactivity of seven antibodies with high affinities for various SFTSV genotypes (A, B2, B3, D, and F) and detect mAb binding with intact Gc proteins, revealed that five IgG type mAbs were bound to intact Gc proteins of various genotypes. Six high-affinity antibodies were selected using ELISA and IFA. The binding capacity of the six antibodies against the SFTSV Gc antigen was measured using surface plasmon resonance. All antibodies had high binding capacity. Consequently, these antibodies serve as valuable markers in the serological diagnosis of SFTSV.

Prevalence, Isolation, and Molecular Characterizationof Severe Fever with Thrombocytopenia Syndrome Virus in Cattle from the Republic of Korea.

Background: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease caused by Bandavirus dabieense. Initially identified in China, this disease has spread throughout Asian countries via tick bites and animal-to-human transmission. However, reports of the prevalence of SFTS virus (SFTSV) in cattle in Korea are lacking. This study aimed to investigate SFTSV infections in grazing cattle in the Republic of Korea (ROK). Materials and Methods: In total, 845 grazing cattle serum samples were collected over 2 years (2019 and 2020) in the ROK, and viral RNA was extracted using a kit. One-step RT-nested PCR was performed to amplify the S-segment of SFTSV. Positive serum samples were used to isolate SFTSV in Vero E6 cells, and the full sequences were analyzed. A phylogenetic tree was constructed using the maximum-likelihood method with MEGA X. In addition, immunoglobulin G antibodies against SFTSV were investigated using an enzyme-linked immunosorbent assay. Results: Here, 4.0% of serum samples (34/845) were positive for SFTSV S-segments, and one virus isolate was cultured in Vero E6 cells. Phylogenetic analysis based on the partial S-segment classified 4 SFTSV isolates as the B-2 genotype, 9 as the B-3 genotype, 18 as an unclassified B genotype, and 3 as the D genotype. One cultured virus was classified as the B-2 genotype based on SFTSV L-, M-, and S-segments. Antibody detection results showed that 21.1% of serum samples (161/763) were positive for SFTSV. Conclusion: To the best of our knowledge, this is the first study performed to identify the prevalence of SFTSV in grazing cattle in the ROK. Our findings indicate the necessity for more intensive and continuous SFTSV monitoring, not only in cattle but also in other animals, to comprehend the genetic diversity of the virus and its potential eco-epidemiological impact on human health.

Thrombocytopenia in SFTS due to platelets with altered function undergoing cell death pathways.

BACKGROUND: Thrombocytopenia is the major clinical feature associated with the severity of SFTS, but the mechanism by which it occurs remains unclear. METHODS: RNA transcriptome analyses were performed on platelets purified from SFTS patients and SFTSV-infected mice. The functions of differentially expressed genes (DEGs) in the platelets were characterized. ELISA, flow cytometry, and qRT-PCR were used to measure the levels of platelet activation, SFTSV infection in platelets, formation of neutrophil extracellular traps (NETs), transcription of DEGs and percent of platelets undergoing cell death. RESULTS: Enhanced neutrophil activation and interferon (IFN) signaling involved in the viral life cycle were common platelet responses in SFTS, which may consume increasing numbers of platelets. Other functional changes may be associated with different outcomes of SFTS. SFTSV infection led to platelet destruction by pyroptosis, apoptosis, necroptosis, and autophagy. In contrast to SFTS patients, platelets in SFTSV-infected mice mainly play a role in adaptive immunity, and platelet death was not as severe as in humans. CONCLUSIONS: The altered functions of platelets, such as mediating leukocyte activation and undergoing cell death, contribute to thrombocytopenia in SFTS patients. The different mechanisms of thrombocytopenia in mice, suggest that platelet functions should be considered in experimental animal models.

Differential short-term and long-term metabolic and cytokine responses to infection of severe fever with thrombocytopenia syndrome virus.

INTRODUCTION: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by the SFTS virus (SFTSV), which has a wide geographic distribution. The primary clinical manifestations of SFTS are fever and thrombocytopenia, with multiorgan failure being the leading cause of death. While most patients recover with treatment, little is known about the potential long-term metabolic effects of SFTSV infection. OBJECTIVES: This study aimed to shed light on dysregulated metabolic pathways and cytokine responses following SFTSV infection, which pose significant risks to the short-term and long-term health of affected individuals. METHODS: Fourteen laboratory-confirmed clinical SFTS cases and thirty-eight healthy controls including 18 SFTSV IgG-positive and 20 IgG-negative individuals were recruited from Taizhou city of Zhejiang province, Eastern China. Inclusion criteria of healthy controls included residing in the study area for at least one year, absence of fever or other symptoms in the past two weeks, and no history of SFTS diagnosis. Ultrahigh-performance liquid chromatography-mass spectrometry (UHPLC-MS) was used to obtain the relative abundance of plasma metabolites. Short-term metabolites refer to transient alterations present only during SFTSV infection, while long-term metabolites persistently deviate from normal levels even after recovery from SFTSV infection. Additionally, the concentrations of 12 cytokines were quantified through fluorescence intensity measurements. Differential metabolites were screened using orthogonal projections to latent structures discriminant analysis (OPLS-DA) and the Wilcoxon rank test. Metabolic pathway analysis was performed using MetaboAnalyst. Between-group differences of metabolites and cytokines were examined using the Wilcoxon rank test. Correlation matrices between identified metabolites and cytokines were analyzed using Spearman's method. RESULTS AND CONCLUSIONS: We screened 122 long-term metabolites and 108 short-term metabolites by analytical comparisons and analyzed their correlations with 12 cytokines. Glycerophospholipid metabolism (GPL) was identified as a significant short-term metabolic pathway suggesting that the activation of GPL might be linked to the self-replication of SFTSV, whereas pentose phosphate pathway and alanine, aspartate, and glutamate metabolism were indicated as significant long-term metabolic pathways playing a role in combating long-standing oxidative stress in the patients. Furthermore, our study suggests a new perspective that α-ketoglutarate could serve as a dietary supplement to protect recovering SFTS patients.

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.

Seroprevalence study in humans and molecular detection in Rhipicephalus sanguineus ticks of severe fever with thrombocytopenia syndrome virus in Thailand.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne virus with a mortality rate of up to 30%. First identified in China in 2009, it was later reported in other Asian countries, including Thailand in 2020. SFTSV has been detected in several tick species, including Rhipicephalus sanguineus, known for infesting dogs. We conducted a seroprevalence study of SFTSV in Bangkok and Nong Khai, Thailand, by analyzing 1162 human samples collected between 2019 and 2023. The testing method relied on IgG detection using ELISA and confirmed though a virus seroneutralization test. The results indicated that out of the participants, 12 (1.1%) tested positive for anti-SFTSV IgG antibodies; however, none exhibited positive results in the seroneutralization assay. Additionally, molecular detection of SFTSV, Crimean-Congo hemorrhagic fever (CCHF), Coxiella spp., Bartonella spp., and Rickettsia spp. was performed on 433 Rh. sanguineus ticks collected from 49 dogs in 2023 in Chachoengsao Province, Thailand. No evidence of these pathogens was found in ticks. These findings highlight the importance of exploring viral cross-reactivity. Furthermore, it is important to conduct additional studies to isolate SFTSV from animals and ticks in order to identify the potential transmission routes contributing to human and animal infections in Thailand.

Bispecific antibodies targeting two glycoproteins on SFTSV exhibit synergistic neutralization and protection in a mouse model.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with a high fatality rate of up to 30% caused by SFTS virus (SFTSV). However, no specific vaccine or antiviral therapy has been approved for clinical use. To develop an effective treatment, we isolated a panel of human monoclonal antibodies (mAbs). SF5 and SF83 are two neutralizing mAbs that recognize two viral glycoproteins (Gn and Gc), respectively. We found that their epitopes are closely located, and we then engineered them as several bispecific antibodies (bsAbs). Neutralization and animal experiments indicated that bsAbs display more potent protective effects than the parental mAbs, and the cryoelectron microscopy structure of a bsAb3 Fab-Gn-Gc complex elucidated the mechanism of protection. In vivo virus passage in the presence of antibodies indicated that two bsAbs resulted in less selective pressure and could efficiently bind to all single parental mAb-escape mutants. Furthermore, epitope analysis of the protective mAbs against SFTSV and RVFV indicated that they are all located on the Gn subdomain I, where may be the hot spots in the phleboviruses. Collectively, these data provide potential therapeutic agents and molecular basis for the rational design of vaccines against SFTSV infection.

The Effect of Tryptophan-to-Tyrosine Mutation at Position 61 of the Nonstructural Protein of Severe Fever with Thrombocytopenia Syndrome Virus on Viral Replication through Autophagosome Modulation.

In our prior investigations, we elucidated the role of the tryptophan-to-tyrosine substitution at the 61st position in the nonstructural protein NSsW61Y in diminishing the interaction between nonstructural proteins (NSs) and nucleoprotein (NP), impeding viral replication. In this study, we focused on the involvement of NSs in replication via the modulation of autophagosomes. Initially, we examined the impact of NP expression levels, a marker for replication, upon the infection of HeLa cells with severe fever thrombocytopenia syndrome virus (SFTSV), with or without the inhibition of NP binding. Western blot analysis revealed a reduction in NP levels in NSsW61Y-expressing conditions. Furthermore, the expression levels of the canonical autophagosome markers p62 and LC3 decreased in HeLa cells expressing NSsW61Y, revealing the involvement of individual viral proteins on autophagy. Subsequent experiments confirmed that NSsW61Y perturbs autophagy flux, as evidenced by reduced levels of LC3B and p62 upon treatment with chloroquine, an inhibitor of autophagosome-lysosome fusion. LysoTracker staining demonstrated a decrease in lysosomes in cells expressing the NS mutant compared to those expressing wild-type NS. We further explored the mTOR-associated regulatory pathway, a key regulator affected by NS mutant expression. The observed inhibition of replication could be linked to conformational changes in the NSs, impairing their binding to NP and altering mTOR regulation, a crucial upstream signaling component in autophagy. These findings illuminate the intricate interplay between NSsW61Y and the suppression of host autophagy machinery, which is crucial for the generation of autophagosomes to facilitate viral replication.

Serologic evidence of human exposure to the severe fever with thrombocytopenia syndrome virus and associated viruses in Kenya.

BACKGROUND: Although the diverse communities of tick-borne viruses (TBVs) have recently been proposed, the threat of infection and exposure to TBVs among humans across Kenya has been poorly understood. OBJECTIVE: Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne viral agent associated with the epidemic of severe fever with thrombocytopenia syndrome (SFTS) disease in East Asian countries. This study investigated the seroprevalence of SFTSV among humans in Kenya. METHODS: Serum samples were collected from 459 healthy people in Kenya and tested for anti-SFTSV antibodies, which were further confirmed by immunofluorescence assays. Micro neutralization assays were performed to identify neutralising antibodies against SFTSV and SFTSV-related viruses. RESULTS: A high seroprevalence (162/459, 35.3%) of SFTSV was found in the samples from nine of the ten surveyed counties in Kenya, with higher rates in the eastern plateau forelands, semiarid and arid areas, and coastal areas than in the area aside Rift valley. The seropositive rate was slightly higher in women than in men and was significantly higher in the 55-64 age group. Neutralising activity against SFTSV was detected in four samples, resulting in a rate of 0.9%. No cross-neutralising activity against the SFTSV-related Guertu virus and Heartland virus was detected in the anti-SFTSV positive serum samples. CONCLUSION: The results provide serologic evidence of human exposure to SFTSV in Kenya and extend our understanding of SFTSV prevalence from Asia to Africa. The findings suggest an increasing threat of exposure to emerging TBVs and the need to investigate tick viromes in Kenya.

Bunyavirus SFTSV NSs utilizes autophagy to escape the antiviral innate immune response.

Severe fever with thrombocytopenia syndrome virus (SFTSV) nonstructural protein (NSs) is an important viral virulence factor that sequesters multiple antiviral proteins into inclusion bodies to escape the antiviral innate immune response. However, the mechanism of the NSs restricting host innate immunity remains largely elusive. Here, we found that the NSs induced complete macroautophagy/autophagy by interacting with the CCD domain of BECN1, thereby promoting the formation of a BECN1-dependent autophagy initiation complex. Importantly, our data showed that the NSs sequestered antiviral proteins such as TBK1 into autophagic vesicles, and therefore promoted the degradation of TBK1 and other antiviral proteins. In addition, the 8A mutant of NSs reduced the induction of BECN1-dependent autophagy flux and degradation of antiviral immune proteins. In conclusion, our results indicated that SFTSV NSs sequesters antiviral proteins into autophagic vesicles for degradation and to escape antiviral immune responses.

Outbreak of Natural Severe Fever with Thrombocytopenia Syndrome Virus Infection in Farmed Minks, China.

We isolated severe fever with thrombocytopenia syndrome virus (SFTSV) from farmed minks in China, providing evidence of natural SFTSV infection in farmed minks. Our findings support the potential role of farmed minks in maintaining SFTSV and are helpful for the development of public health interventions to reduce human infection.

The Tick Saliva Peptide HIDfsin2 TLR4-Dependently Inhibits the Tick-Borne Severe Fever with Thrombocytopenia Syndrome Virus in Mouse Macrophages.

Ticks transmit a variety of pathogens to their hosts by feeding on blood. The interactions and struggle between tick pathogens and hosts have evolved bilaterally. The components of tick saliva can directly or indirectly trigger host biological responses in a manner that promotes pathogen transmission; however, host cells continuously develop strategies to combat pathogen infection and transmission. Moreover, it is still unknown how host cells develop their defense strategies against tick-borne viruses during tick sucking. Here, we found that the tick saliva peptide HIDfsin2 enhanced the antiviral innate immunity of mouse macrophages by activating the Toll-like receptor 4 (TLR4) signaling pathway, thereby restricting tick-borne severe fever with thrombocytopenia syndrome virus (SFTSV) replication. HIDfsin2 was identified to interact with lipopolysaccharide (LPS), a ligand of TLR4, and then depolymerize LPS micelles into smaller particles, effectively enhancing the activation of the nuclear factor kappa-B (NF-κB) and type I interferon (IFN-I) signaling pathways, which are downstream of TLR4. Expectedly, TLR4 knockout completely eliminated the promotion effect of HIDfsin2 on NF-κB and type I interferon activation. Moreover, HIDfsin2 enhanced SFTSV replication in TLR4-knockout mouse macrophages, which is consistent with our recent report that HIDfsin2 hijacked p38 mitogen-activated protein kinase (MAPK) to promote the replication of tick-borne SFTSV in A549 and Huh7 cells (human cell lines) with low expression of TLR4. Together, these results provide new insights into the innate immune mechanism of host cells following tick bites. Our study also shows a rare molecular event relating to the mutual antagonism between tick-borne SFTSV and host cells mediated by the tick saliva peptide HIDfsin2 at the tick-host-virus interface.

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.

Direct transmission of severe fever with thrombocytopenia syndrome virus from farm-raised fur animals to workers in Weihai, China.

BACKGROUND: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease. SFTS virus (SFTSV) is transmitted by tick bites and contact with the blood or body fluids of SFTS patients. Animal-to-human transmission of SFTS has been reported in Japan, but not in China. In this study, the possible transmission route of two patients who fed and cared for farm-raised fur animals in a mink farm was explored. METHOD: An epidemiological investigation and a genetic analysis of patients, animals and working environment were carried out. RESULTS: It was found that two patients had not been bitten by ticks and had no contact with patients infected with SFTS virus, but both of them had skinned the dying animals. 54.55% (12/22) of the farm workers were positive for SFTS virus antibody. By analyzing the large, medium and small segments sequences, the viral sequences from the two patients, animals and environments showed 99.9% homology. CONCLUSION: It is suspected that the two patients may be directly infected by farm-raised animals, and that the virus may have been transmitted by aerosols when skinning dying animals. Transmission by direct blood contacts or animal bites cannot be ignored.

Vaccine Development for Severe Fever with Thrombocytopenia Syndrome Virus in Dogs.

Severe fever with thrombocytopenia syndrome (SFTS) is a life-threatening viral zoonosis. The causative agent of this disease is the Dabie bandavirus, which is usually known as the SFTS virus (SFTSV). Although the role of vertebrates in SFTSV transmission to humans remains uncertain, some reports have suggested that dogs could potentially transmit SFTSV to humans. Consequently, preventive measures against SFTSV in dogs are urgently needed. In the present study, dogs were immunized three times at two-week intervals with formaldehyde-inactivated SFTSV with two types of adjuvants. SFTSV (KCD46) was injected into all dogs two weeks after the final immunization. Control dogs showed viremia from 2 to 4 days post infection (dpi), and displayed white pulp atrophy in the spleen, along with a high level of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling assay (TUNEL) positive area. However, the inactivated SFTSV vaccine groups exhibited rare pathological changes and significantly reduced TUNEL positive areas in the spleen. Furthermore, SFTSV viral loads were not detected at any of the tested dpi. Our results indicate that both adjuvants can be safely used in combination with an inactivated SFTSV formulation to induce strong neutralizing antibodies. Inactivated SFTSV vaccines effectively prevent pathogenicity and viremia in dogs infected with SFTSV. In conclusion, our study highlighted the potential of inactivated SFTSV vaccination for SFTSV control in dogs.