Insect-borne non-enveloped bluetongue virus utilizes discrete small vesicles for non-lytic release and cell-to-cell transmission

Bluetongue virus (BTV) is one of the most economically relevant orbiviruses and is the only example of a large complex, but non-enveloped arbovirus. In addition to cell lysis, BTV is known to employ a 'budding' process analogous to that used by enveloped viruses for cell exit, in which the viral glycosylated NS3 protein plays a key role. Recent reports have demonstrated that BTV can also induce non-lytic release via extracellular vesicles (EVs), however, details of the type and origin of the EV used and the role of NS3 in the process remain incompletely understood. In this study we undertook biochemical studies on the non-lytic release of BTV particles in different forms of EVs from several types of host cells and complemented this by comprehensive microscopic analyses using fluorescence microscopy, transmission electron microscopy and electron cryo-tomography. We discovered that BTV particles use both large EVs (LEVs) and smaller size EVs (SEVs) for non-lytic release and that, in each cell type studied, SEV fractions were particularly enriched with NS3. Non-enveloped BTV particles initially released in SEVs were highly infectious and promote efficient cell-to-cell transmission. This discovery highlights the complex mechanisms utilized by a non-enveloped arbovirus for egress and the significance of different EV types in this process.