Replication and morphogenesis of the turbot aquareovirus (TRV) in cell culture

The ultrastructural aspects of the replication cycle of the turbot aquareovirus (TRV) were studied using CHSE-214 cells. Although this virus in general followed the typical pattern of reovirus replication and morphogenesis, it presented some important differences. Internalisation of virions into the host cell occurred by direct penetration through the plasma membrane (even at temperatures below 4°C), and this step was independent of trypsin treatment. Two size classes of viral particles were detected in the cytoplasm of infected cells: (i) particles of 30 nm diameter, probably corresponding to cores, located inside cytoplasmic viroplasms, and (ii) 45 nm diameter particles, located in vesicles of the endoplasmic reticulum, probably corresponding to single-shelled virions. Double-shelled complete viral particles (75–80 nm diameter) were only detected at the exterior of the cell. Cores formed inside the viroplasms were transformed into 45 nm immature virions by budding from viroplasms to cytoplasmic vesicles. The vesicles carried those subviral particles towards the periphery of the cell. Final maturation occurred by budding of the immature virions through the plasma membrane. During this process, incomplete virions acquired an external protein layer, to form the 75 nm double-shelled complete virions released to the exterior. Thus, release of TRV viral progeny was not necessarily associated with cell lysis. Replication of TRV was cytoplasmic, and some of the ultrastructural changes caused by the viral replication included development of many dense fibrillar bodies, and cell cytoskeleton alterations.