Multiple Routes of Bluetongue Virus Egress

Abstract

Bluetongue virus (BTV) is an arthropod-borne virus infecting livestock. Its frequent emergence in Europe and North America had caused significant agricultural and economic loss. BTV is also of scientific interest as a model to understand the mechanisms underlying non-enveloped virus release from mammalian and insect cells. The BTV particle, which is formed of a complex double-layered capsid, was first considered as a lytic virus that needs to lyse the infected cells for cell to cell transmission. In the last decade, however, a more in-depth focus on the role of the non-structural proteins has led to several examples where BTV particles are also released through different budding mechanisms at the plasma membrane. It is now clear that the non-structural protein NS3 is the main driver of BTV release, via different interactions with both viral and cellular proteins of the cell sorting and exocytosis pathway. In this review, we discuss the most recent advances in the molecular biology of BTV egress and compare the mechanisms that lead to lytic or non-lytic BTV release.

Keywords: arthropod-borne virus; bluetongue virus; extracellular vesicles; non-structural protein; virus egress.

Figure 1

BTV particles purified from the supernatant of infected ovine PT cells (derived from domestic sheep) present different phenotypes. (A–C) Transmission electron microscopy of BTV virus particles that are (A) naked, (B) transiently associated with lipid membranes after budding at the plasma membrane, (C) or cloaked in EVs. Scale bars represent 200 nm, white head arrows indicate the lipid membranes, and black arrows indicate BTV particles.

Figure 2

Current model of BTV egress mechanisms from in vitro infected mammalian cells. After the assembly of core particles in VIBs, NS3 (in blue) is responsible for trafficking the BTV particles toward MVBs. Virions are then transferred in secretory lysosomes where the pH is increased from acidic to neutral range, thus preventing virion degradation. Lysosomes are then secreted and BTV is transmitted to another cell as a pool of virions cloaked in EVs. Progeny BTV virions are also exported to the plasma membrane, mediated by the interaction between NS3 and outer capsid proteins of BTV, and between NS3 and the cellular ESCRT machinery. BTV virions are then released by budding at the plasma membrane, afterwards only a fraction of virions keeps a transient envelope. In addition, apoptosis triggered by the infection leads to cell death and the release of free virus particles between 12–24h post-infection. ER: Endoplasmic reticulum, Golgi: Golgi apparatus, VIBs: Viral inclusion bodies, and NS3 is represented in blue.