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. 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. 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. In this review, we identify virus-, cell-, and environment-specific factors that shape the EV population released by naked virus-infected cells.
Nonlytic spread of naked viruses
Covering up a naked virus
Some naked viruses apparently are more modest than we believed. Examples of viruses that are enveloped include dengue virus, influenza virus , and measles virus. Recently it was discovered that hepatitis A virus HAV particles are released from cells in membrane vesicles containing virus particles. However virus in the feces, which is transmitted to other hosts, is not enveloped. Viral envelopes typically contain viral glycoproteins, such as the HA protein of influenza viruses, which serve important functions during replication, such as attachment to cell receptors.
When Viruses get Naked
How do viruses spread from cell to cell? Enveloped viruses acquire their surrounding membranes by budding: either through the plasma membrane or an internal membrane of infected cells. Thus, a newly budded enveloped virus finds itself either in the extracellular milieu or in a lumenal compartment from which it can exit the cell by conventional secretion. On the other hand, naked viruses such as poliovirus, nodavirus, adenovirus, and SV40 lack an external membrane.
Some viruses have a lipid envelope surrounding their protein capsid, which they get from the host cell membrane during the budding process. The envelope acts as an anchor for viral glycoproteins, which facilitate entry of the newly budded virus into a new cell by recognizing and binding host cell receptors. Until recently, individual members of a viral species were thought to be either enveloped or naked, but not both. However, accumulating evidence suggests that some viruses that were thought to be exclusively naked can exit the cell by hijacking the cellular vesicular transport system. Hepatitis A virus HAV spreads by both the fecal-oral route by ingestion of food contaminated with fecal matter and the blood route such as by use of contaminated needles.