adenovirus

adenovirus
Name	Adenovirus
Caption	Schematic of an adenovirus virion
Taxon	Adenoviridae
Subdivision ranks	Genera
Subdivision	Mastadenovirus, Aviadenovirus, Atadenovirus, Siadenovirus, Ichtadenovirus

adenovirus. Adenoviruses are a family of non-enveloped, double-stranded DNA viruses first isolated from human adenoid tissue in the1953 by Maurice Hilleman and Wallace Rowe. They are classified within the family Adenoviridae and are known for their ability to cause a wide range of illnesses, primarily respiratory, ocular, and gastrointestinal infections in humans and animals. Their stable icosahedral capsid and efficient gene delivery mechanisms have also made them invaluable tools in molecular biology, vaccine development, and gene therapy.

Structure and classification

The virion possesses a non-enveloped, icosahedral capsid approximately 90-100 nm in diameter, composed of 252 capsomeres. Major structural proteins include the hexon, which forms the facets, the penton base at the vertices, and a fiber protein protruding from each penton. The linear, double-stranded DNA genome is associated with core proteins. Classification within the International Committee on Taxonomy of Viruses system divides the family into five genera: Mastadenovirus (infecting mammals), Aviadenovirus (birds), Atadenovirus, Siadenovirus, and Ichtadenovirus. Human adenoviruses, belonging to Mastadenovirus, are further categorized into over 100 types (e.g., Human adenovirus C, Human adenovirus B) based on serology, genomic sequencing, and hemagglutination properties.

Replication cycle

The replication cycle begins with attachment via the fiber knob to specific host cell receptors, such as the Coxsackievirus and adenovirus receptor or CD46. This is followed by internalization via clathrin-mediated endocytosis and endosomal escape. The virion traffics to the nuclear pore complex, where the viral genome is imported into the cell nucleus. Transcription occurs in a regulated cascade using the host RNA polymerase II, producing early, intermediate, and late gene products. Early proteins, like E1A, modulate the host cell cycle and promote viral replication. Genome replication proceeds via a protein-primed mechanism facilitated by the viral DNA polymerase. Late phase involves structural protein synthesis, assembly of progeny virions in the nucleus, and cell lysis mediated by the adenovirus death protein to release new infectious particles.

Pathogenesis and disease

Adenoviruses are opportunistic pathogens causing self-limiting but often acute illnesses. Clinical manifestations depend on the viral type and host immune status. Common syndromes include acute febrile respiratory illness (pharyngitis, pneumonia), keratoconjunctivitis (notably epidemic keratoconjunctivitis), acute gastroenteritis, and cystitis. In immunocompromised individuals, such as recipients of hematopoietic stem cell transplantation or those with AIDS, infections can be severe, leading to disseminated disease, hepatitis, or encephalitis. Certain types, like Human adenovirus 36, have been studied for a potential association with obesity in some models. The virus can establish persistent, asymptomatic infections in lymphoid tissues like the tonsils and adenoids.

Epidemiology and transmission

Adenoviruses are endemic worldwide, with infections occurring year-round and often in closed settings like daycare centers, military barracks, and hospitals. Transmission primarily occurs via direct contact, the fecal-oral route, respiratory droplets, and occasionally through contaminated fomites or water sources, such as in swimming pools causing pharyngoconjunctival fever. Outbreaks of respiratory disease have been documented by the Centers for Disease Control and Prevention among military recruits. The World Health Organization notes the global distribution of various serotypes, with some geographical variations in predominant strains causing disease.

Prevention and treatment

No specific antiviral therapy is universally approved; management is typically supportive. The nucleoside analog cidofovir is used off-label for severe infections in immunocompromised patients under monitoring by institutions like the National Institutes of Health. Prevention in community settings relies on good hygiene, including handwashing and disinfection of surfaces. Live, oral vaccines against types Human adenovirus 4 and Human adenovirus 7 have been used exclusively by the United States Armed Forces to prevent outbreaks of acute respiratory disease among recruits. These vaccines are not available for the general civilian population.

Applications in research and medicine

Due to their efficient transduction of dividing and non-dividing cells, adenoviruses are widely employed as vectors. They are a cornerstone in gene therapy trials for conditions like severe combined immunodeficiency and cancers, pioneered by researchers such as James M. Wilson. The ChAdOx1 nCoV-19 vaccine, developed by the University of Oxford and AstraZeneca against COVID-19, utilizes a chimpanzee adenovirus vector. In basic research, they have been instrumental in discovering RNA splicing by Phillip Sharp and Richard Roberts and in studying cell cycle regulation and apoptosis. Their use is also explored in oncolytic virotherapy platforms.

Category:Viruses Category:Infectious diseases