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

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

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