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

Abstract

Severe fever with thrombocytopenia syndrome (SFTS) virus is a newly recognized member of the genus Phlebovirus in the family Bunyaviridae. The virus was isolated from patients presenting with hemorrhagic manifestations and an initial case fatality rate of 12 to 30% was reported. Due to the recent emergence of this pathogen, there is limited knowledge on the molecular virology of SFTS virus. Recently, we reported that the SFTS virus NSs protein inhibited the activation of the beta interferon (IFN-␤) promoter. Furthermore, we also found that SFTS virus NSs relocalizes key components of the IFN response into NSs-induced cytoplasmic structures. Due to the important role these structures play during SFTS virus replication, we conducted live cell imaging studies to gain further insight into the role and trafficking of these cytoplasmic structures during virus infection. We found that some of the SFTS virus NSs-positive cytoplasmic structures were secreted to the extracellular space and endocytosed by neighboring cells. We also found that these secreted structures isolated from NSs-expressing cells and SFTS virus-infected cells were positive for the viral protein NSs and the host protein CD63, a protein associated with extracellular vesicles. Electron microscopy studies also revealed that the isolated CD63-immunoprecipitated extracellular vesicles produced during SFTS virus infection contained virions. The virions harbored within these structures were efficiently delivered to uninfected cells and were able to sustain SFTS virus replication. Altogether, these results suggest that SFTS virus exploits extracellular vesicles to mediate virus receptor-independent transmission to host cells and open the avenue for novel therapeutic strategies against SFTS virus and related pathogens. IMPORTANCESFTS virus is novel bunyavirus associated with hemorrhagic fever illness. Currently, limited information is available about SFTS virus. In the present study, we demonstrated that extracellular vesicles produced by SFTS virus-infected cells harbor infectious virions. We sought to determine whether these "infectious" extracellular vesicles can mediate transmission of the virus and confirmed that the SFTS virions were efficiently transported by these secreted structures into uninfected cells and were able to sustain efficient replication of SFTS virus. These results have significant impact on our understanding of how the novel tick-borne phleboviruses hijack cellular machineries to establish infection and point toward a novel mechanism for virus replication among arthropod-borne viruses. The Bunyaviridae family comprises five genera, including Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. The majority of viruses within this family (with the exception of Hantaviruses) are considered arthropod-borne viruses and are important causes of morbidity and mortality around the world. These viruses are associated with a range of clinical symptoms characterized by febrile illness and in the most severe cases fatal hepa...

Structure of a phleboviral envelope glycoprotein reveals a consolidated model of membrane fusion

“…Following receptor recognition, the virion is endocytosed into the host cell (13)(14)(15), and the metastable Gc orchestrates fusion of endosomal and viral membranes, facilitating release of viral RNA into the cytosol. Interestingly, SFTSV is also capable of cell entry via extracellular vesicles, which likely allows evasion of the host immune system (16). Structural studies of the cognate Gc from RVFV (17) in the prefusion conformation revealed that the phleboviral Gc forms a class II architecture, which has been also observed for envelope glycoproteins from positive-sense RNA viruses from the Togaviridae and Flaviridae families (18,19).…”

Extracellular vesicles are the Trojan horses of viral infection

“…Moreover the Ago2-mi122 complex is found to traffic along with the HCV genome, which potentially facilitates replication relative to infection by free HCV particles [26•]. SFTS virus infected cells release entire virions in exosomes and these exosomes are not only infectious but the infection is through a yet to be determined receptor-independent pathway such as direct fusion of exosomes with host cells [28]. Other viruses like RVFV, HIV, HTLV-1 appear to also use exosomes to indirectly favor their transmission to other cells by packaging segments of viral nucleic acids and proteins within exosomes that modulate the survival of neighboring uninfected cells [29•,30,31].…”

Analogies Between Cancer-Derived Extracellular Vesicles and Enveloped Viruses with an Emphasis on Human Breast Cancer

“…One such example involves the HIV-1 Gag protein, which was localized to endosome-like regions and released into exosomes from infected Jurkat and K562 chronic myelogenous leukemia cells [95]. Viruses can take advantage of these EVs and use them as a means to promote infectivity, as shown by viral transfer of sever fever with thrombocytopenia syndrome virus through EVs to infect host HeLa cells [96]. In addition to proteins, viral nucleic acids can be transferred to host cells via EVs, which was demonstrated through the transfer of functional EBV miRNA to host monocyte-derived cells and resulting downregulation of miRNA targets [97].…”