It is a long-standing paradigm in the field of virology that naked viruses cause lysis of infected cells to release progeny virus. However, recent data indicate that naked virus types of the Picornaviridae and Hepeviridae families can also leave cells via an alternative route involving enclosure in fully host-derived lipid bilayers. The resulting particles resemble extracellular vesicles (EV), which are 50 nm-1 μm vesicles released by all cells. These EV contain lipids, proteins, and RNA, and generally serve as vehicles for intercellular communication in various (patho)physiological processes. EV can act as carriers of naked viruses and as invisibility cloaks to evade immune attacks. However, the exact combination of virions and host-derived molecules determines how these virus-containing EV affect spread of infection and/or triggering of antiviral immune responses. An underexposed aspect in this research area is that infected cells likely release multiple types of virus-induced and constitutively released EV with unique molecular composition and function. Utrecht University researchers identify virus-, cell-, and environment-specific factors that shape the EV population released by naked virus-infected cells. In addition, current findings on the formation and molecular composition of EV induced by different virus types will be compared and placed in the context of the widely proven heterogeneity of EV populations and biases caused by different EV isolation methodologies. Close interactions between the fields of EV biology and virology will help to further delineate the intricate relationship between EV and naked viruses and its relevance for viral life cycles and outcomes of viral infections.
Multiple factors can influence the composition of EV-virus isolates
The figure presents a schematic overview of factors identified in the EV- and EV-virus-fields that affect the molecular composition of EV isolates. First, EV(-virus) production itself can vary based on factors relating to the producing cell, including the nature of the cell (intrinsic factors) and its environmentally determined condition (extrinsic factors). Upon infection, these factors coalesce with the properties of the virus in a time-sensitive manner to govern the production and release of virions, EV-virus, and other EV by the infected cell. Secondly, the heterogeneous population of released EV can undergo ‘EV-editing’ by engaging with factors encountered in the extracellular environment. These factors can either bind to or disrupt EV membranes to modify the existing particles. Additional variation in the composition of EV isolates is introduced during isolation and purification steps. Depending on the centrifugal force applied in pre-clearing steps to remove cell debris, subsets of larger EV may either be depleted in this step or may be co-isolated in subsequent steps. The different techniques applied to isolate EV-virus are based on different principles related to physical, affinity or biochemical characteristics of EV. These EV isolation techniques therefore differ in the efficiency with which EV can be separated from contaminating naked virions and protein aggregates, or may specifically enrich for certain EV subtypes