CCR2 is a host entry receptor for severe fever with thrombocytopenia syndrome virus

A viral necrosome mediates direct RIPK3 activation to promote inflammatory necroptosis

Necroptosis is an inflammatory programmed cell death pathway triggered by RIPK3 activation through one of the upstream RHIM-domain-containing proteins including RIPK1, TRIF, and ZBP1. Whether necroptosis can be activated independent of the upstream signaling pathways leading to inflammatory pathogenesis remains ambiguous. Here, we revealed a mechanism in which a viral protein mediates direct RIPK3 activation resulting in severe inflammatory pathogenesis in patients. The nonstructural protein NSs of a pathogenic hemorrhagic virus, SFTSV, interacts with the RIPK3 kinase domain and forms biocondensate to promote RIPK3 autophosphorylation and necroptosis activation in an RHIM-independent manner. In parallel, sequestration of RIPK3 within the NSs-RIPK3 condensate inhibited RIPK3-mediated apoptosis and promoted viral replication. Infection with an SFTSV NSs mutant virus not forming NSs condensate triggered pronounced apoptosis resulting in reduced viral replication and decreased fatality in vivo. Blocking SFTSV-triggered necroptosis through depletion of MLKL or treatment with a RIPK3-kinase inhibitor reduced viral inflammatory pathogenesis and fatality in vivo. In contrast, blocking SFTSV-triggered apoptosis through depletion of RIPK3 resulted in enhanced viral replication and increased fatality in vivo. The virus-triggered necroptosis correlated with severe inflammatory pathogenesis and lethality in virus-infected patients. The NSs-RIPK3 condensate may represent a necroptosis activation mechanism that promotes viral pathogenesis.

Metformin as antiviral therapy protects hyperglycemic and diabetic patients

Viral infections disrupt glucose metabolism; however, their impact on disease prognosis in highly pathogenic viruses remains largely unknown. There is an additional need to investigate the antiviral mechanisms of glucose-lowering therapeutics. Here, our multicenter clinical study shows that hyperglycemia and pre-existing diabetes are independent risk factors for mortality in patients infected with severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging and highly pathogenic bunyavirus. SFTSV infection triggers gluconeogenesis, which, in turn, inhibits AMPK activity and subsequent interferon I (IFN-I) responses, thereby facilitating viral replication. In vitro and animal studies further reveal that metformin inhibits SFTSV replication by suppressing autophagy through the AMPK-mTOR pathway, contributing to protection against lethal SFTSV infection in mice. Importantly, our large cohort study demonstrates that metformin reduces viremia and SFTSV-related mortality in patients with hyperglycemia or pre-existing diabetes, contrasting with the disadvantageous effect of insulin. These findings highlight the promising therapeutic potential of metformin in treating viral infections, particularly among individuals with hyperglycemia or diabetes.

IMPORTANCE

Severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging tick-borne bunyavirus, causes severe hemorrhagic fever with a high mortality rate. Previous studies have shown metabolic disturbances, particularly hyperglycemia, in SFTSV-infected individuals. However, the mechanism underlying this metabolic derangement remains unclear, and further investigation is needed to determine whether glucose-lowering therapeutics could be beneficial for SFTSV-infected patients. In this study, our multicenter clinical data show that hyperglycemia and pre-existing diabetes are independent risk factors for mortality in patients with SFTSV infection. Furthermore, we observed that SFTSV infection triggers gluconeogenesis, which promotes viral replication through the regulation of the AMPK-IFN-I signaling pathway. Notably, metformin significantly reduces viremia and SFTSV-related mortality in patients with hyperglycemia or pre-existing diabetes, attributed to its inhibitory effect on autophagy through the AMPK-mTOR pathway. Therefore, our study uncovers the interaction between SFTSV infection and glucose metabolic disorder and highlights the promising therapeutic potential of metformin for treating SFTSV infection.

Genome-wide CRISPR screening identifies LRP1 as an entry factor for SFTSV

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease caused by the SFTS virus (SFTSV), which has high mortality rates and poses a significant threat to public health. To identify potential therapeutic targets against SFTSV, we conduct genome-wide knockout screening, which identifies the previously known host factor CCR2, and reveals prolow-density lipoprotein receptor-related protein 1 (LRP1) as an entry factor for SFTSV. Knockdown or knockout of LRP1 significantly attenuate SFTSV infection in mouse embryonic fibroblasts (MEFs). Additionally, inhibition of LRP1 suppresses SFTSV pseudovirus infection in MEFs, suggesting its role in viral entry. The interaction between the SFTSV glycoprotein Gn and LRP1 via the CLI and CLII domains is revealed by co-IP and surface plasmon resonance (SPR). Moreover, LRP1 antagonists and neutralizing antibodies effectively attenuate SFTSV infection in MEFs. Administration of an LRP1-neutralizing antibody in a lethal male mouse model reduces the viral load, mitigates tissue damage, and improves survival. This study identifies LRP1 as a host entry receptor for SFTSV, providing a target for therapeutic strategy development.

SFTSV NSs interacts with AGO2 to regulate the RNAi pathway for viral replication

RNA interference (RNAi) is a posttranscriptional gene silencing mechanism acting as an antiviral defense in eukaryotes. During viral replication, intermediate double-stranded RNAs are processed into virus-derived small interfering RNAs (vsiRNAs) by the host enzyme, DICER. These vsiRNAs are incorporated into the RNA-induced silencing complex (RISC), where AGO2 cleaves viral genomic RNAs. However, viruses have evolved mechanisms to suppress this pathway. Here, we report that the nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) interacts with RISC to suppress the RNAi pathway. NSs forms a ternary complex by interacting with both DICER and AGO2 of the RNAi pathway. The interaction between NSs and DICER, facilitated by the two RNase III domains of DICER, is disrupted in the absence of AGO2 or the DICER-interacting domain (PIWI) of AGO2, indicating a direct interaction between NSs and AGO2. Functional assays using shRNA- and siRNA-mediated silencing of GFP signal, along with co-localization studies, demonstrated that NSs competes with siRNA to interact with AGO2, thereby abolishing RNAi activity. Mutational analysis identified an NSs-A26 mutant that no longer interacts with AGO2 and is unable to suppress RNAi activity, suggesting that NSs acts as a viral suppressor of RNAi (VSR) for SFTSV. Viral infection led to the generation of vsiRNA and showed higher replication in AGO2-/- cells compared to wild-type (WT) cells, confirming the antiviral role of the RNAi pathway against SFTSV infection. These data suggest that the NSs-AGO2 interaction suppresses RNAi, counteracting the antiviral RNAi pathway, thereby facilitating SFTSV infection and pathogenesis.IMPORTANCERNA interference (RNAi) is the main antiviral defense pathway in plants and insects but is not predominant in mammals. While RNAi's role in countering severe fever with thrombocytopenia syndrome virus (SFTSV) infection has been studied in ticks, its role in humans is unknown. The SFTSV nonstructural protein (NSs) forms inclusion bodies (IBs) in the host, sequestering multiple antiviral proteins and facilitating pathogenesis, contributing to SFTSV's high mortality rate. Our study found that SFTSV NSs directly interacts with AGO2, a key RNAi protein, hindering its function. A novel NSs mutant failed to interact with AGO2 and lost its RNAi suppression ability, highlighting NSs as a viral suppressor of RNAi (VSR). Infection studies confirmed the RNAi pathway's critical role in combating SFTSV infection. This study demonstrates NSs's role in viral infection and suggests potential therapeutic approaches.

Verteporfin Inhibits Severe Fever with Thrombocytopenia Syndrome Virus Infection via Inducing the Degradation of the Viral Gn Protein

Background: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel tick-borne bunyavirus, causing the hemorrhagic infectious disease of SFTS, with a case fatality rate up to 30% due to the absence of effective therapeutic interventions. Therefore, it is urgent to develop safe and effective therapeutic drugs to control this viral hemorrhagic fever. Methods: The activity of verteporfin (VP), screened from an FDA-approved drugs library, against SFTSV, was systematically evaluated in Huh7 cells in a wide range of concentrations. We performed time-of-addition experiments with VP, along with binding, endocytosis, and membrane fusion assays, to determine which part of the SFTSV life cycle VP has its effect on. The potential targets of VP were detected by a drug affinity responsive target stability (DARTS) assay. Results: VP exhibited a potent anti-SFTSV activity by blocking the initial viral binding to the target cells during viral entry via significantly inducing the degradation of the viral Gn protein. Conclusions: The VP-induced inhibition of SFTSV binding, the first step of viral invasion, suggested that VP might be an ideal and potent anti-SFTSV agent due to its prophylaxis and therapeutic effects on viral infection.

Urine of Cats with Severe Fever with Thrombocytopenia Syndrome: A Potential Source of Infection Transmission

Severe fever with thrombocytopenia syndrome (SFTS), caused by infection with the SFTS virus, is an emerging fatal tick-borne zoonosis endemic to East Asia. Although SFTS is a tick-borne disease, the virus can be transmitted from animals with SFTS without a tick bite. Direct transmission of the SFTS virus from animals to humans has been reported; however, the transmission route is unclear in some cases. Therefore, this study focused on the possibility of SFTS virus transmission through urine and attempted to isolate the infectious virus from the urine of animals with SFTS. Since more efficient cell isolation is needed to determine whether the SFTS virus is present, we first expressed dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN), the major receptor for the virus, in Vero cells (Vero-DC-SIGN cells) using a retroviral vector. When inoculated with equal amounts of the SFTS virus strain and SFTS-virus-infected animal serum, Vero-DC-SIGN cells had 42-136% and 20-85% more foci, respectively, than their parent Vero cells. After confirming that Vero-DC-SIGN cells were more suitable for the isolation of the SFTS virus, we investigated whether it could be isolated from the urine of eight cats and two dogs with SFTS. The virus was isolated from 25 μL of urine from two cats with SFTS. Considering that cats excrete 50-100 mL of urine per day, the transmission of the SFTS virus via the urine of cats with SFTS cannot be ruled out. Individuals examining or caring for cats suspected of having SFTS should be aware of the possibility of viral transmission via urine.

The Role of Orthobunyavirus Glycoprotein Gc in the Viral Life Cycle: From Viral Entry to Egress

Orthobunyavirus refers to the virus members within the Genus Orthobunyavirus, which is the largest virus genus in the Family Peribunyaviridae and even Class Bunyaviricetes. To date, over 130 species of Orthobunyaviruses have been identified worldwide. Orthobunyaviruses mainly infect arthropods, while some species are capable of being transmitted to mammals, including humans, via intermediate vectors. As emerging and re-emerging pathogens, orthobunyavirus poses a significant threat to both human and veterinary public health worldwide. Currently, there are no commercial vaccines against orthobunyavirus. The structure of orthobunyavirus is relatively simple, consisting of a typical tri-segmented negative-sense RNA genome that encodes four structural proteins (L, Gn, Gc, and N) and two non-structural proteins (NSm and NSs). The highly glycosylated Gc protein, which has a complex conformation and forms polymers embedded in the viral envelope, plays a critical role in inducing neutralizing antibodies throughout the orthobunyavirus infection cycle from entry to egress. This review provides a comprehensive summary of the virus-encoded Gc protein and its role in the virus life cycle from viral entry to egress, offering researchers with valuable integrated information for further investigations.

Safety, Immunogenicity, and Efficacy of a Recombinant Vesicular Stomatitis Virus Vectored Vaccine Against Severe Fever with Thrombocytopenia Syndrome Virus and Heartland Bandavirus

BACKGROUND

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a recently emerged tickborne virus in east Asia with over 18,000 confirmed cases. With a high case fatality ratio, SFTSV has been designated a high priority pathogen by the WHO and the NIAID. Despite this, there are currently no approved therapies or vaccines to treat or prevent SFTS. Vesicular stomatitis virus (VSV) represents an FDA-approved vaccine platform that has been considered for numerous viruses due to its low sero-prevalence in humans, ease in genetic manipulation, and promiscuity in incorporating foreign glycoproteins into its virions.

METHODS

In this study, we developed a recombinant VSV (rVSV) expressing the SFTSV glycoproteins Gn/Gc (rVSV-SFTSV) and assessed its safety, immunogenicity, and efficacy in C57BL/6, Ifnar-/-, and AG129 mice.

RESULTS

We demonstrate that rVSV-SFTSV is safe when given to immunocompromised animals and is not neuropathogenic when injected intracranially into young immunocompetent mice. Immunization of wild type (C57BL/6) and Ifnar-/- mice with rVSV-SFTSV resulted in high levels of neutralizing antibodies and protection in a lethal SFTSV challenge model. Additionally, passive transfer of sera from immunized Ifnar-/- mice into naïve animals was protective when given pre- or post-exposure. Finally, we demonstrate that immunization with rVSV-SFTSV cross protects AG129 mice against challenge with the closely related Heartland bandavirus despite negligible neutralizing titers to the virus.

CONCLUSIONS

Taken together, these data suggest that rVSV-SFTSV is a promising vaccine candidate for SFTSV and Heartland bandavirus with a favorable safety profile.

A global lipid map of severe fever with thrombocytopenia syndrome virus infection reveals glycerophospholipids as novel prognosis biomarkers

Severe fever with thrombocytopenia syndrome (SFTS) is a rapidly progressing infectious disease caused by a novel bunyavirus characterized by high fever, thrombocytopenia, and multiple organ damage. While lipids play an important role in viral infections, the specific alterations in lipid metabolism during SFTSV infection remain unclear. This study aimed to elucidate the global lipid metabolic profiles of SFTS patients with mild, severe, and fatal outcomes. A total of 60 SFTS patients, consisting of 30 mild, 15 severe and 15 fatal patients, and 30 healthy controls, were enrolled for the investigation of global lipidomics in serum using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Our findings revealed global alterations in the lipid signature induced by SFTSV infection and further confirmed that the glycerophospholipid metabolism pathway was profoundly affected during the progression of mild, severe, and fatal outcomes in SFTS patients. Importantly, LysoPC (20:0) and LysoPC (P-16:0) are strongly correlated with the clinical parameters of SFTSV infection. Furthermore, we demonstrated the substantial prognostic value of LysoPC (20:0) and LysoPC (P-16:0) by receiver operating characteristic (ROC) curve analysis, providing evidence for their remarkable value as prognostic biomarkers for predicting SFTS clinical outcomes. In particular, LysoPC (20:0) and LysoPC (P-16:0), along with APTT, yielded superior prognostic performance for fatal SFTS [area under the curve (AUC) = 98.4%], outperforming routine clinical parameters. Collectively, our findings revealed the lipidomic landscape after SFTSV infection, which offers new insights into the mechanisms of SFTS disease progression and suggests that targeting lipid metabolism may serve as a potential therapeutic strategy.

IMPORTANCE

This study systematically investigated the lipid landscape profile of SFTS-infected patients with different clinical outcomes. Our results revealed a global alteration in the lipid signature, particularly the glycerophospholipid metabolic pathway, induced by SFTSV infection. Notably, LysoPC (20:0) and LysoPC (P-16:0) presented remarkable prognostic value as novel biomarkers for SFTSV infection and may contribute to the prognosis of SFTS progression and appropriate interventions.

Pathogenesis and virulence of Heartland virus

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

Current insights into human pathogenic phenuiviruses and the host immune system

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

A review of virus host factor discovery using CRISPR screening

The emergence of genome-scale forward genetic screening techniques, such as Haploid Genetic screen and clustered regularly interspaced short palindromic repeats (CRISPR) knockout screen has opened new horizons in our understanding of virus infection biology. CRISPR screening has become a popular tool for the discovery of novel host factors for several viruses due to its specificity and efficiency in genome editing. Here, we review how CRISPR screening has revolutionized our understanding of virus-host interactions from scientific and technological viewpoints. A summary of the published screens conducted thus far to uncover virus host factors is presented, highlighting their experimental design and significant findings. We will outline relevant methods for customizing the CRISPR screening process to answer more specific hypotheses and compile a glossary of conducted CRISPR screens to show their design aspects. Furthermore, using flaviviruses and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as examples, we hope to offer a broad-based perspective on the capabilities of CRISPR screening to serve as a reference point to guide future unbiased discovery of virus host factors.

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.

Taurolithocholic acid protects against viral haemorrhagic fever via inhibition of ferroptosis

Bile acids are microbial metabolites that can impact infection of enteric and hepatitis viruses, but their functions during systemic viral infection remain unclear. Here we show that elevated levels of the secondary bile acid taurolithocholic acid (TLCA) are associated with reduced fatality rates and suppressed viraemia in patients infected with severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging tick-borne haemorrhagic fever virus. TLCA inhibits viral replication and mitigates host inflammation during SFTSV infection in vitro, and indirectly suppresses SFTSV-mediated induction of ferroptosis by upregulating fatty acid desaturase 2 via the TGR5-PI3K/AKT-SREBP2 axis. High iron and ferritin serum levels during early infection were correlated with decreased TLCA levels and fatal outcomes in SFTSV-infected patients, indicating potential biomarkers. Furthermore, treatment with either ferroptosis inhibitors or TLCA protected mice from lethal SFTSV infection. Our findings highlight the therapeutic potential of bile acids to treat haemorrhagic fever viral infection.

An atlas of genetic effects on cellular composition of the tumor microenvironment

Deciphering the composition of the tumor microenvironment (TME) is critical for understanding tumorigenesis and to design immunotherapies. In the present study, we mapped genetic effects on cell-type proportions using single-cell and bulk RNA sequencing data, identifying 3,494 immunity quantitative trait loci (immunQTLs) across 23 cancer types from The Cancer Genome Atlas. Functional annotation revealed regulatory potential and we further assigned 1,668 genes that regulate TME composition. We constructed a combined immunQTL map by integrating data from European and Chinese colorectal cancer (CRC) samples. A polygenic risk score that incorporates these immunQTLs and hits on a genome-wide association study outperformed in CRC risk stratification within 447,495 multiethnic individuals. Using large-scale population cohorts, we identified that the immunQTL rs1360948 is associated with CRC risk and prognosis. Mechanistically, the rs1360948-G-allele increases CCL2 expression, recruiting regulatory T cells that can exert immunosuppressive effects on CRC progression. Blocking the CCL2-CCR2 axis enhanced anti-programmed cell death protein 1 ligand therapy. Finally, we have established a database (CancerlmmunityQTL2) to serve the research community and advance our understanding of immunogenomic interactions in cancer pathogenesis.

A broadly protective antibody targeting glycoprotein Gn inhibits severe fever with thrombocytopenia syndrome virus infection

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging bunyavirus that causes severe viral hemorrhagic fever and thrombocytopenia syndrome with a fatality rate of up to 30%. No licensed vaccines or therapeutics are currently available for humans. Here, we develop seven monoclonal antibodies (mAbs) against SFTSV surface glycoprotein Gn. Mechanistic studies show that three neutralizing mAbs (S2A5, S1G3, and S1H7) block multiple steps during SFTSV infection, including viral attachment and membrane fusion, whereas another neutralizing mAb (B1G11) primarily inhibits the viral attachment step. Epitope binning and X-ray crystallographic analyses reveal four distinct antigenic sites on Gn, three of which have not previously been reported, corresponding to domain I, domain II, and spanning domain I and domain II. One of the most potent neutralizing mAbs, S2A5, binds to a conserved epitope on Gn domain I and broadly neutralizes infection of six SFTSV strains corresponding to genotypes A to F. A single dose treatment of S2A5 affords both pre- and post-exposure protection of mice against lethal SFTSV challenge without apparent weight loss. Our results support the importance of glycoprotein Gn for eliciting a robust humoral response and pave a path for developing prophylactic and therapeutic antibodies against SFTSV infection.

RVFV virulence factor NSs triggers the mitochondrial MCL-1-BAK axis to activate pathogenic NLRP3 pyroptosis

Infection of Rift Valley fever virus (RVFV), a highly pathogenic mosquito-borne zoonotic virus, triggers severe inflammatory pathogenesis but the underlying mechanism of inflammation activation is currently unclear. Here, we report that the non-structural protein NSs of RVFV triggers mitochondrial damage to activate the NLRP3 inflammasome leading to viral pathogenesis in vivo. It is found that the host transcription inhibition effect of NSs causes rapid down-regulation of myeloid cell leukemia-1(MCL-1), a pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) protein family. MCL-1 down-regulation led to BAK activation in the mitochondria, which triggered mtROS production and release of oxidized mitochondrial DNA (ox-mtDNA) into the cytosol. Cytosolic ox-mtDNA binds and activates the NLRP3 inflammasome triggering NLRP3-GSDMD pyroptosis in RVFV infected cells. A NSs mutant virus (RVFV-NSsRM) that is compromised in inducing transcription inhibition did not trigger MCL-1 down-regulation nor NLRP3-GSDMD pyroptosis. RVFV infection of the Nlrp3-/- mouse model demonstrated that the RVFV-triggered NLRP3 pyroptosis contributed to RVFV inflammatory pathogenesis and fatal infection in vivo. Infection with the RVFV-NSsRM mutant virus similarly showed alleviated inflammatory pathogenesis and reduced fatality rate. Taken together, these results revealed a mechanism by which a virulence factor activates the mitochondrial MCL-1-BAK axis through inducing host transcription inhibition to trigger NLRP3-dependent inflammatory pathogenesis.

A full-length glycoprotein mRNA vaccine confers complete protection against severe fever with thrombocytopenia syndrome virus, with broad-spectrum protective effects against bandaviruses

Highly pathogenic viruses from family Phenuiviridae, which are mainly transmitted by arthropods, have intermittently sparked epidemics worldwide. In particular, tick-borne bandaviruses, such as severe fever with thrombocytopenia syndrome virus (SFTSV), continue to spread in mountainous areas, resulting in an average mortality rate as high as 10.5%, highlighting the urgency and importance of vaccine development. Here, an mRNA vaccine developed based on the full-length SFTSV glycoprotein, containing both the receptor-binding domain and the fusion domain, was shown to confer complete protection against SFTSV at a very low dose by triggering a type 1 helper T cell-biased cellular immune response in rodents. Moreover, the vaccine candidate elicited long-term immunity and protection against SFTSV for at least 5 months. Notably, it provided complete cross-protection against other bandaviruses, such as the Heartland virus and Guertu virus, in lethal challenge models. Further research revealed that the conserved epitopes among bandaviruses within the full-length SFTSV glycoprotein may facilitate broad-spectrum protection mediated by the cellular immune response. Collectively, these findings demonstrate that the full-length SFTSV glycoprotein mRNA vaccine is a promising vaccine candidate for SFTSV and other bandaviruses, and provide guidance for the development of broad-spectrum vaccines from conserved antigens and epitopes.

IMPORTANCE

Tick-borne bandaviruses, such as SFTSV and Heartland virus, sporadically trigger outbreaks in addition to influenza viruses and coronaviruses, yet there are no specific vaccines or therapeutics against them. mRNA vaccine technology has advantages in terms of enabling in situ expression and triggering cellular immunity, thus offering new solutions for vaccine development against intractable viruses, such as bandaviruses. In this study, we developed a novel vaccine candidate for SFTSV by employing mRNA vaccination technology and using a full-length glycoprotein as an antigen target. This candidate vaccine confers complete and durable protection against SFTSV at a notably low dose while also providing cross-protection against Heartland virus and Guertu virus. This study highlights the prospective value of full-length SFTSV-glycoprotein-based mRNA vaccines and suggests a potential strategy for broad-spectrum bandavirus vaccines.

N-glycosylation of viral glycoprotein is a novel determinant for the tropism and virulence of highly pathogenic tick-borne bunyaviruses

Severe fever with thrombocytopenia syndrome (SFTS) virus, a tick-borne bunyavirus, causes a severe/fatal disease termed SFTS; however, the viral virulence is not fully understood. The viral non-structural protein, NSs, is the sole known virulence factor. NSs disturbs host innate immune responses and an NSs-mutant SFTS virus causes no disease in an SFTS animal model. The present study reports a novel determinant of viral tropism as well as virulence in animal models, within the glycoprotein (GP) of SFTS virus and an SFTS-related tick-borne bunyavirus. Infection with mutant SFTS viruses lacking the N-linked glycosylation of GP resulted in negligible usage of calcium-dependent lectins in cells, less efficient infection, high susceptibility to a neutralizing antibody, low cytokine production in macrophage-like cells, and reduced virulence in Ifnar-/- mice, when compared with wildtype virus. Three SFTS virus-related bunyaviruses had N-glycosylation motifs at similar positions within their GP and a glycan-deficient mutant of Heartland virus showed in vitro and in vivo phenotypes like those of the SFTS virus. Thus, N-linked glycosylation of viral GP is a novel determinant for the tropism and virulence of SFTS virus and of a related virus. These findings will help us understand the process of severe/fatal diseases caused by tick-borne bunyaviruses.

MCP-3 as a prognostic biomarker for severe fever with thrombocytopenia syndrome: a longitudinal cytokine profile study

Introduction

Severe fever with thrombocytopenia syndrome (SFTS) is characterized by a high mortality rate and is associated with immune dysregulation. Cytokine storms may play an important role in adverse disease regression, this study aimed to assess the validity of MCP-3 in predicting adverse outcomes in SFTS patients and to investigate the longitudinal cytokine profile in SFTS patients.

Methods

The prospective study was conducted at Yantai Qishan Hospital from May to November 2022. We collected clinical data and serial blood samples during hospitalization, patients with SFTS were divided into survival and non-survival groups based on the clinical prognosis.

Results

The levels of serum 48 cytokines were measured using Luminex assays. Compared to healthy controls, SFTS patients exhibited higher levels of most cytokines. The non-survival group had significantly higher levels of 32 cytokines compared to the survival group. Among these cytokines, MCP-3 was ranked as the most significant variable by the random forest (RF) model in predicting the poor prognosis of SFTS patients. Additionally, we validated the predictive effects of MCP-3 through receiver operating characteristic (ROC) curve analysis with an AUC of 0.882 (95% CI, 0.787-0.978, P <0.001), and the clinical applicability of MCP-3 was assessed favorably based on decision curve analysis (DCA). The Spearman correlation analysis indicated that the level of MCP-3 was positively correlated with ALT, AST, LDH, α-HBDH, APTT, D-dimer, and viral load (P<0.01).

Discussion

For the first time, our study identified and validated that MCP-3 could serve as a meaningful biomarker for predicting the fatal outcome of SFTS patients. The longitudinal cytokine profile analyzed that abnormally increased cytokines were associated with the poor prognosis of SFTS patients. Our study provides new insights into exploring the pathogenesis of cytokines with organ damage and leading to adverse effects.

Isolation and Identification of Severe Fever with Thrombocytopenia Syndrome Virus from Farmed Mink in Shandong, China

Severe fever with thrombocytopenia syndrome (SFTS) virus, recently named Bandavirus dabieense, belongs to the genus Bandavirus of family Phenuiviridae, and it causes SFTS in humans with clinical symptoms including fever, thrombocytopenia, gastrointestinal symptoms, and leukocytopenia. However, there are few reports on the pathogenesis of SFTSV in animals. This study first isolated the SFTSV strain SD22-2 from sick-farmed mink. Viral metagenomics was used to detect SFTSV nucleotide in the clinical specimens obtained from symptomatic minks. Then, we isolated the virus using Vero and DH82 cells, and Real-Time Quantitative PCR (RT-qPCR), indirect immunofluorescence assay, transmission electron microscopy, and Western blotting identified it. Meanwhile, phylogenetic analysis based on partial L, M, and S segment sequences indicated that the mink-origin SFTSV strain SD22-2 belonged to genotype D and was genetically close to the HB2016-003 strain isolated from humans. Taken together, we isolated and identified an SFTSV from farmed mink that may be the reservoir hosts of SFTSV. We should pay more attention to farmed minks and biosecurity practices, and active surveillance at fur farms must be reviewed and enhanced.

Antiviral immunity of severe fever with thrombocytopenia syndrome: current understanding and implications for clinical treatment

Dabie Banda virus (DBV), a tick-borne pathogen, was first identified in China in 2009 and causes profound symptoms including fever, leukopenia, thrombocytopenia and multi-organ dysfunction, which is known as severe fever with thrombocytopenia syndrome (SFTS). In the last decade, global incidence and mortality of SFTS increased significantly, especially in East Asia. Though previous studies provide understandings of clinical and immunological characteristics of SFTS development, comprehensive insight of antiviral immunity response is still lacking. Here, we intensively discuss the antiviral immune response after DBV infection by integrating previous ex- and in-vivo studies, including innate and adaptive immune responses, anti-viral immune responses and long-term immune characters. A comprehensive overview of potential immune targets for clinical trials is provided as well. However, development of novel strategies for improving the prognosis of the disease remains on challenge. The current review may shed light on the establishment of immunological interventions for the critical disease SFTS.

Current Progress of Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) Vaccine Development

SFTSV is an emerging tick-borne virus causing hemorrhagic fever with a case fatality rate (CFR) that can reach up to 27%. With endemic infection in East Asia and the recent spread of the vector tick to more than 20 states in the United States, the SFTSV outbreak is a globally growing public health concern. However, there is currently no targeted antiviral therapy or licensed vaccine against SFTSV. Considering the age-dependent SFTS pathogenesis and disease outcome, a sophisticated vaccine development approach is required to safeguard the elderly population from lethal SFTSV infection. Given the recent emergence of SFTSV, the establishment of animal models to study immunogenicity and protection from SFTS symptoms has only occurred recently. The latest research efforts have applied diverse vaccine development approaches-including live-attenuated vaccine, DNA vaccine, whole inactivated virus vaccine, viral vector vaccine, protein subunit vaccine, and mRNA vaccine-in the quest to develop a safe and effective vaccine against SFTSV. This review aims to outline the current progress in SFTSV vaccine development and suggest future directions to enhance the safety and efficacy of these vaccines, ensuring their suitability for clinical application.

SFTSV Gn-Head mRNA vaccine confers efficient protection against lethal viral challenge

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne virus, causing thrombocytopenia and hemorrhagic fever, with a fatality rate ranging from 12% to 30%. SFTSV possesses Gn and Gc glycoproteins, which are responsible for host cell receptor attachment and membrane fusion, respectively, to infect host cells. We have previously reported a protein subunit vaccine candidate (sGn-H-FT) of the SFTSV soluble Gn head region (sGn-H) fused with self-assembling ferritin (FT) nanoparticles, displaying strong protective immunogenicity. In this study, we present messenger RNA (mRNA) vaccine candidates encoding sGn-H or sGn-H-FT, both of which exhibit potent in vivo immunogenicity and protection capacity. Mice immunized with either sGn-H or sGn-H-FT mRNA lipid nanoparticle (LNP) vaccine produced strong total antibodies and neutralizing antibodies (NAbs) against sGn-H. Importantly, NAb titers remained high for an extended period. Finally, mice immunized with sGn-H or sGn-H-FT mRNA LNP vaccine were fully protected from a lethal dose of SFTSV challenge, showing no fatality. These findings underscore the promise of sGn-H and sGn-H-FT as vaccine antigen candidates capable of providing protective immunity against SFTSV infection.