Gene therapy uses modified viruses to deliver therapeutic genetic material into human cells. Adeno-Associated Virus (AAV) and Adenovirus are two commonly studied viral vectors. Although they share a similar name, these non-enveloped DNA viruses are fundamentally distinct. Their differences in structure, life cycle, and interaction with the immune system dictate their unique applications in the clinic.
Comparing Structure and Basic Biology
Adenovirus and AAV differ significantly in structure and replication strategies. Adenovirus is medium-sized, measuring 90 to 100 nanometers in diameter, and houses a linear, double-stranded DNA (dsDNA) genome. As an independent virus, Adenovirus can replicate on its own within a host cell, and its larger size allows it to carry a substantial genetic payload.
AAV is much smaller, measuring 20 to 25 nanometers in diameter. Its genome is a linear, single-stranded DNA (ssDNA) molecule. AAV is classified as a “Dependoparvovirus” because it cannot replicate productively without a “helper virus,” such as Adenovirus or Herpesvirus, which provides necessary replication factors. Both viruses possess an icosahedral protein shell (capsid), but the Adenovirus capsid is more complex than the AAV structure.
Cellular Delivery and Gene Expression
The life cycle of the therapeutic genetic material differs markedly for the two vectors. Adenovirus delivers its dsDNA cargo into the nucleus, where it remains separate from the host chromosomes in a transient, extrachromosomal state (an episome). Since this episomal DNA is not replicated with the host cell’s own DNA, gene expression is short-lived, especially in frequently dividing cells. Consequently, the therapeutic effect is diluted over time as cells turn over.
AAV delivers its ssDNA genome to the nucleus, where it must first be converted into a stable double-stranded form. This new dsDNA molecule then circularizes or forms stable, concatenated structures that persist as episomes. This stable episomal form allows AAV to provide long-lasting, sustained gene expression, particularly in non-dividing cells (e.g., muscle, brain, or eye tissue). Its ability to maintain expression for years is a safety advantage, though the gene is eventually lost in dividing cells.
Host Immunity and Safety Profile
The host’s immune response is a primary factor influencing the safety and clinical utility of these vectors. Adenovirus triggers a robust and rapid innate immune response, often manifesting as flu-like symptoms, fever, and inflammation. This strong reaction limits the therapy’s effectiveness by clearing the vector and can lead to significant systemic toxicity, especially at high doses. The body also develops strong antibodies against the Adenovirus capsid, preventing re-administration of the same vector type.
AAV generally has a milder safety profile and is considered non-pathogenic. However, the body still mounts an immune response, primarily by producing neutralizing antibodies against the AAV capsid. Since many people have been naturally exposed to wild-type AAVs, pre-existing immunity can render the therapy ineffective. Like Adenovirus, new antibodies formed after treatment prevent subsequent dosing with the same AAV serotype, constraining long-term management of chronic diseases.
Fatalities have occurred in clinical trials for both vectors, highlighting the need for careful dose management and immunosuppression. High doses of AAV have been linked to severe adverse events, including liver toxicity. Researchers are actively working to engineer both AAV and Adenovirus capsids to make them less visible to the immune system and overcome safety and re-dosing challenges.
Current Uses and Practical Constraints
The distinct biological properties of AAV and Adenovirus have steered their use toward different clinical applications. Adenovirus can carry a large genetic payload (up to 36 kilobases), making it suitable for transferring large genes that exceed AAV’s limited capacity. The strong immune stimulation caused by Adenovirus makes it a preferred platform for developing vaccines (e.g., against COVID-19) and for oncolytic therapy, where the virus destroys cancer cells.
AAV’s ability to achieve long-term, stable gene expression in non-dividing cells, combined with its low toxicity, has made it the leading vector for in vivo gene therapy targeting specific organs. AAV is favored for treating inherited diseases of the eye, central nervous system, and liver, and several AAV-based therapies have received regulatory approval. The primary constraint for AAV remains its small packaging capacity, limited to approximately 4.7 kilobases, severely restricting the size of the therapeutic gene that can be delivered.