Adenoviruses: Classification, structure, resistance and replication
The History of Adenovirus Discovery
These viruses were discovered by W. Rowe and coworkers in 1953. Primary investigation of adenoviruses revealed their ability to persist and develop the cytopathic effect in adenoids and tonsils, so they were named adenoviruses.
Classification of Adenoviruses
Adenoviruses pertain to the family Adenoviridae that encompasses 5 distinct genera. Currently known 7 human pathogenic species (human adenviruses A-G) are included into genus Mastadenovirus, which contains adenoviruses, affecting mammals.
From 100 adenoviral human serotypes known, above 50 serotypes are pathogenic for humans. They are clustered into several groups.
Structure of Adenoviruses
Adenoviruses are DNA-containing naked viruses. Viral DNA is the linear double-stranded molecule, which is attached to the protein at the end of genome. DNA is packed into the core of the virion.
Virus size varies in the range of 70-100 nm. The virions are of icosahedral symmetry.
Adenoviral capsid is composed of 252 capsomeres, where 12 capsomers are pentons, (polyhedrons, based on pentagon structure) and other 240 are hexons (units, based on hexagons). Spike-like capsid structures, known as fibers, are connected with pentons.
Fibers develop hemagglutinating activity, and pentons display cytopathic effect in the cell cultures.
Pentons, hexons and fibers are the major antigens of adenoviruses, containing group- and type-specific epitopes on their surface. Fibers carry type-specific epitopes that are used for virus serotyping.
Virions of adenoviruses are highly stable in the environment. Protein capsid protects them from unfavorable influences. They stay viable in water for weeks, on dry inanimate surface – up to 3 months
Adenoviruses easily resist freezing. Heating at a temperature of 56oC inactivates them in 30 minutes; 60oC – in less than 10 minutes. They are insusceptible to ether and detergents, but destroyed by chlorine-containing disinfectants and formaldehyde.
Viral Replication Cycle
Adenoviruses propagate within epithelial cells. Viral attachment is promoted by fibers. They bind to the specific cell receptors. Viral penetration is facilitated by penton interaction with cellular integrins. Adenoviruses are transported rapidly from the endosomes towards the cell nucleus, where the uncoating is finished.
About 20 early nonstructural proteins are synthesized, providing DNA replication. After DNA replication, the late proteins are translated.
Capsomeres are self-assembled in the nucleus. The duration of adenovirus replication cycle is about 24 hours.