Bunyavirus SFTSV exploits autophagic flux for viral assembly and egress

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.

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.

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.

Autophagy and Apoptosis in Rabies Virus Replication

Rabies virus (RABV) is a single-stranded negative-sense RNA virus belonging to the Rhabdoviridae family and Lyssavirus genus, which is highly neurotropic and can infect almost all warm-blooded animals, including humans. Autophagy and apoptosis are two evolutionarily conserved and genetically regulated processes that maintain cellular and organismal homeostasis, respectively. Autophagy recycles unnecessary or dysfunctional intracellular organelles and molecules in a cell, whereas apoptosis eliminates damaged or unwanted cells in an organism. Studies have shown that RABV can induce both autophagy and apoptosis in target cells. To advance our understanding of pathogenesis of rabies, this paper reviews the molecular mechanisms of autophagy and apoptosis induced by RABV and the effects of the two cellular events on RABV replication.

Rift Valley Fever Virus-Infection, Pathogenesis and Host Immune Responses

Rift Valley Fever Virus is a mosquito-borne phlebovirus causing febrile or haemorrhagic illness in ruminants and humans. The virus can prevent the induction of the antiviral interferon response through its NSs proteins. Mutations in the NSs gene may allow the induction of innate proinflammatory immune responses and lead to attenuation of the virus. Upon infection, virus-specific antibodies and T cells are induced that may afford protection against subsequent infections. Thus, all arms of the adaptive immune system contribute to prevention of disease progression. These findings will aid the design of vaccines using the currently available platforms. Vaccine candidates have shown promise in safety and efficacy trials in susceptible animal species and these may contribute to the control of RVFV infections and prevention of disease progression in humans and ruminants.

Rift Valley Fever Virus Nucleoprotein Triggers Autophagy to Dampen Antiviral Innate Immune Responses

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus that causes severe and potentially fatal hemorrhagic fever in humans. Autophagy is a self-degradative process that can restrict viral replication at multiple infection steps. In this study, we evaluated the effects of RVFV-triggered autophagy on viral replication and immune responses. Our results showed that RVFV infection triggered autophagosome formation and induced complete autophagy. Impairing autophagy flux by depleting autophagy-related gene 5 (ATG5), ATG7, or sequestosome 1 (SQSTM1) or treatment with autophagy inhibitors markedly reduced viral RNA synthesis and progeny virus production. Mechanistically, our findings demonstrated that the RVFV nucleoprotein (NP) C-terminal domain interacts with the autophagy receptor SQSTM1 and promotes the SQSTM1-microtubule-associated protein 1 light chain 3 B (LC3B) interaction and autophagy. Deletion of the NP C-terminal domain impaired the interaction between NP and SQSTM1 and its ability to trigger autophagy. Notably, RVFV-triggered autophagy promoted viral infection in macrophages but not in other tested cell types, including Huh7 hepatocytes and human umbilical vein endothelial cells, suggesting cell type specificity of this mechanism. It was further revealed that RVFV NP-triggered autophagy dampens antiviral innate immune responses in infected macrophages to promote viral replication. These results provide novel insights into the mechanisms of RVFV-triggered autophagy and indicate the potential of targeting the autophagy pathway to develop antivirals against RVFV. IMPORTANCE We showed that RVFV infection induced the complete autophagy process. Depletion of the core autophagy genes ATG5, ATG7, or SQSTM1 or pharmacologic inhibition of autophagy in macrophages strongly suppressed RVFV replication. We further revealed that the RVFV NP C-terminal domain interacted with SQSTM1 and enhanced the SQSTM1/LC3B interaction to promote autophagy. RVFV NP-triggered autophagy strongly inhibited virus-induced expression of interferon-stimulated genes in infected macrophages but not in other tested cell types. Our study provides novel insights into the mechanisms of RVFV-triggered autophagy and highlights the potential of targeting autophagy flux to develop antivirals against this virus.

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.

Recent Advances in the Study of the Immune Escape Mechanism of SFTSV and Its Therapeutic Agents

Sever fever with thrombocytopenia syndrome (SFTS) is a new infectious disease that has emerged in recent years and is widely distributed, highly contagious, and lethal, with a mortality rate of up to 30%, especially in people with immune system deficiencies and elderly patients. SFTS is an insidious, negative-stranded RNA virus that has a major public health impact worldwide. The development of a vaccine and the hunt for potent therapeutic drugs are crucial to the prevention and treatment of Bunyavirus infection because there is no particular treatment for SFTS. In this respect, investigating the mechanics of SFTS-host cell interactions is crucial for creating antiviral medications. In the present paper, we summarized the mechanism of interaction between SFTS and pattern recognition receptors, endogenous antiviral factors, inflammatory factors, and immune cells. Furthermore, we summarized the current therapeutic drugs used for SFTS treatment, aiming to provide a theoretical basis for the development of targets and drugs against SFTS.

Severe fever with thrombocytopenia syndrome virus replicates in platelets and enhances platelet activation

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) virus (SFTSV) infection causes an emerging hemorrhagic fever in East Asia with a high mortality rate. Thrombocytopenia is a consistent feature of SFTS illness, but the mechanism remains elusive.

OBJECTIVES

We aimed to better understand the role of platelets in the pathophysiology of SFTSV infection, including the development of thrombocytopenia.

METHODS

Using platelets from healthy volunteers and patients with SFTS, we evaluated the functional changes in platelets against SFTSV infection. We investigated the direct effect of glycoprotein VI on platelet-SFTSV interaction by quantitative real-time PCR, molecular docking, surface plasmon resonance spectrometry, flow cytometry, western blot, and platelet functional studies in vitro. Interactions of SFTSV and platelet-SFTSV complexes with macrophages were also determined by scanning electron microscope, quantitative real-time PCR, and flow cytometry.

RESULTS

This study is the first to demonstrate that platelets are capable of harboring and producing SFTSV particles. Structural and functional studies found that SFTSVs bind platelet glycoprotein VI to potentiate platelet activation, including platelet aggregation, adenosine triphosphate release, spreading, clot retraction, coagulation, phosphatidylserine exposure, thrombus formation, and adherence. In vitro mechanistic studies highlighted that the interaction of platelets with human THP-1 cells promoted SFTSV clearance and suppressed cytokine production in macrophages. However, unwanted SFTSV replication in macrophages reciprocally aggravated SFTSV persistence in the circulation, which may contribute to thrombocytopenia and other complications during SFTSV infection.

CONCLUSION

These findings together highlighted the pathophysiological role of platelets in initial intrinsic defense against SFTSV infections, as well as intertwined processes with host immunity, which can also lead to thrombocytopenia and poor prognosis.

The role of autophagy in viral infections

Autophagy is an evolutionarily conserved catabolic cellular process that exerts antiviral functions during a viral invasion. However, co-evolution and co-adaptation between viruses and autophagy have armed viruses with multiple strategies to subvert the autophagic machinery and counteract cellular antiviral responses. Specifically, the host cell quickly initiates the autophagy to degrade virus particles or virus components upon a viral infection, while cooperating with anti-viral interferon response to inhibit the virus replication. Degraded virus-derived antigens can be presented to T lymphocytes to orchestrate the adaptive immune response. Nevertheless, some viruses have evolved the ability to inhibit autophagy in order to evade degradation and immune responses. Others induce autophagy, but then hijack autophagosomes as a replication site, or hijack the secretion autophagy pathway to promote maturation and egress of virus particles, thereby increasing replication and transmission efficiency. Interestingly, different viruses have unique strategies to counteract different types of selective autophagy, such as exploiting autophagy to regulate organelle degradation, metabolic processes, and immune responses. In short, this review focuses on the interaction between autophagy and viruses, explaining how autophagy serves multiple roles in viral infection, with either proviral or antiviral functions.

SFTS bunyavirus NSs protein sequestrates mTOR into inclusion bodies and deregulates mTOR-ULK1 signaling, provoking pro-viral autophagy

Autophagy is emerging as a critical player in host defense against diverse infections, in addition to its conserved function to maintain cellular homeostasis. Strikingly, some pathogens have evolved strategies to evade, subvert or exploit different steps of the autophagy pathway for their lifecycles. Here, we present a new viral mechanism of manipulating autophagy for its own benefit with severe fever with thrombocytopenia syndrome bunyavirus (SFTSV, an emerging high-pathogenic virus) as a model. SFTSV infection triggers autophagy, leading to complete autophagic flux. Mechanistically, we show that the nonstructural protein of SFTSV (NSs) interacts with mTOR, the pivotal regulator of autophagy, by targeting its kinase domain and captures mTOR into viral inclusion bodies (IBs) induced by NSs itself. Furthermore, NSsimpairs mTOR-mediated phosphorylation of unc-51-like kinase 1 (ULK1) at Ser757, disrupting the inhibitory effect of mTOR on ULK1 activity and thus contributing to autophagy induction. Pharmacologic treatment and Beclin-1 knockout experimental results establish that, in turn, autophagy enhances SFTSV infection and propagation. Moreover, the minigenome reporter system reveals that SFTSV ribonucleoprotein (the transcription and replication machinery) activity can be bolstered by autophagy. Additionally, we found that the NSs proteins of SFTSV-related bunyaviruses have a conserved function of targeting mTOR. Taken together, we unravel a viral strategy of inducing pro-viral autophagy by interacting with mTOR, sequestering mTOR into IBs and hence provoking the downstream ULK1 pathway, which presents a new paradigm for viral manipulation of autophagy and may help inform future development of specific antiviral therapies against SFTSV and related pathogens.

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.

The Endless Wars: Severe Fever With Thrombocytopenia Syndrome Virus, Host Immune and Genetic Factors

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging arboviral infectious disease with a high rate of lethality in susceptible humans and caused by severe fever with thrombocytopenia syndrome bunyavirus (SFTSV). Currently, neither vaccine nor specific antiviral drugs are available. In recent years, given the fact that both the number of SFTS cases and epidemic regions are increasing year by year, SFTS has become a public health problem. SFTSV can be internalized into host cells through the interaction between SFTSV glycoproteins and cell receptors and can activate the host immune system to trigger antiviral immune response. However, SFTSV has evolved multiple strategies to manipulate host factors to create an optimal environment for itself. Not to be discounted, host genetic factors may be operative also in the never-ending winning or losing wars. Therefore, the identifications of SFTSV, host immune and genetic factors, and their interactions are critical for understanding the pathogenic mechanisms of SFTSV infection. This review summarizes the updated pathogenesis of SFTS with regard to virus, host immune response, and host genetic factors to provide some novel perspectives of the prevention, treatment, as well as drug and vaccine developments.