Extracellular Virions: The Advance Guard of Poxvirus Infections

Most poxviruses produce multiple types of virions: intracellular mature virions (MVs), wrapped virions (WVs), and extracellular virions (EVs). The EVs represent the culmination of a morphogenetic pathway including the development of MVs, which are then transformed into WVs, and finally transformed into EVs. Each EV consists of one form of MV wrapped within a second lipid membrane containing at least four, and usually six, EV-specific viral proteins [reviewed in 1,2]. Although EVs are the end product of the viral morphogenetic pathway they often comprise<1% of the total progeny virions [3]. The conservation of the ability to produce EVs amongst most poxviruses indicates that EVs are generally advantageous for poxvirus replication in vivo. While EVs are not essential for virus infection either in vitro or in vivo, viruses lacking the capacity to efficiently produce EVs are usually highly attenuated in vivo [reviewed in 4]. These attributes suggest that EVs have specialized roles distinct from those of infectious MVs in viral replication.