What Are Adenoviruses?

Adenoviruses are a family of non-enveloped double-stranded DNA viruses that infect vertebrate hosts and are characterized by an icosahedral protein capsid approximately 90 nanometers in diameter with distinctive fiber projections at each vertex. First isolated from human adenoid tissue in 1953, they cause a range of respiratory, ocular, and gastrointestinal infections in humans, with more than 100 serotypes identified across mammalian and avian species. In biomedical engineering and clinical medicine, adenoviruses are studied primarily for their utility as vectors in gene therapy and vaccine development, a role enabled by their ability to infect a wide variety of dividing and non-dividing cells with high efficiency and to deliver DNA payloads to the cell nucleus without integrating into the host genome.

The non-integrating behavior of adenoviral vectors is both an advantage and a constraint: it minimizes the risk of insertional mutagenesis (a concern with integrating vectors), but it also means that therapeutic transgene expression is transient in proliferating tissues, which limits applications requiring lifelong correction.

Viral Structure and Biology

The adenovirus capsid is assembled from three major proteins: hexon, which forms the facets of the icosahedral shell; penton base, which anchors the fiber proteins at each vertex; and the fiber itself, whose knob domain binds to host cell surface receptors to initiate infection. Upon receptor binding, the virus is endocytosed and escapes from the endosome through a pH-dependent membrane disruption mechanism, delivering its genome to the nucleus through the nuclear pore complex. The genome is approximately 36 kilobases and is organized into early and late transcription units, with the early genes (E1, E2, E3, E4) controlling host cell reprogramming and the late genes encoding structural proteins. In replication-defective vectors, the E1 region is deleted and replaced with the therapeutic transgene, eliminating the virus's ability to replicate in normal cells. The molecular biology of adenovirus replication is reviewed in PMC's archive of adenovirus vector research, which covers vector design, safety, and clinical translation.

Adenoviruses as Gene Therapy Vectors

Recombinant adenovirus vectors have been used in more than 600 clinical trials, more than any other viral vector class, for applications including cancer gene therapy, genetic disease correction, and vaccination. Their high transduction efficiency and ability to accept inserts up to approximately 8 kilobases make them practical for a wide range of therapeutic transgenes. Gendicine, an adenovirus vector carrying the p53 tumor suppressor gene, was approved in China in 2003 for treatment of head and neck cancer, becoming the first approved gene therapy product. A limitation of first-generation adenovirus vectors is immunogenicity: the capsid proteins trigger potent innate and adaptive immune responses that can reduce the duration of transgene expression and complicate re-dosing. High-capacity or "gutless" adenovirus vectors address this by removing all viral coding sequences, reducing immunogenicity and increasing insert capacity to approximately 36 kilobases, as described in a Lancet eBioMedicine review of viral and non-viral gene therapy vectors.

Vaccine Development

Adenovirus vectors have become important platforms for prophylactic vaccines because they generate strong cellular and humoral immune responses to the transgene antigen without requiring adjuvants. The COVID-19 vaccines developed by Oxford-AstraZeneca (ChAdOx1) and Johnson & Johnson (Ad26.COV2.S) used chimpanzee and human adenovirus serotypes, respectively, to deliver the SARS-CoV-2 spike protein gene, demonstrating large-scale manufacturability and global deployment feasibility. Adenovirus-based vaccines are also in development for HIV, tuberculosis, malaria, and Ebola. The clinical safety profile of adenovirus vaccines and gene therapy vectors across more than 30 years of trials is summarized in the PMC review of adenoviral clinical safety data, which covers immunogenicity management and the rare thrombotic complications associated with some adenovirus COVID-19 vaccines.

Applications

Adenoviruses have applications in a range of fields, including:

• Oncology, where replication-selective oncolytic adenoviruses are engineered to destroy tumor cells
• Rare genetic disease research, where transient adenoviral gene delivery is used to study gene function in animal models
• Vaccine development, where adenovirus platforms offer rapid antigen-switching for emerging pathogens
• Basic virology and cell biology, where adenoviruses serve as model systems for studying viral entry, nuclear import, and gene expression
• Biomanufacturing process development, where adenovirus production at scale informs broader viral vector manufacturing practices