What is Ad5? A Common Virus and Scientific Tool

Adenovirus type 5, or Ad5, is a member of a family of viruses that commonly infect humans and other vertebrates. For most people, an encounter with this virus results in a mild, self-limiting illness, such as the common cold or pink eye. Researchers have learned to harness the virus’s natural ability to enter human cells, turning it into a vehicle for delivering genetic material. This has led to applications ranging from vaccine development to gene therapy.

The Biology of Adenovirus Type 5

Adenoviruses are non-enveloped viruses, meaning they lack a fatty outer layer and are instead protected by a protein shell called a capsid. This capsid has a 20-sided icosahedral shape and is constructed from several proteins, including the hexon and penton base proteins. Protruding from each of the 12 vertices are long fiber proteins that attach to host cells. Inside this protein coat lies the virus’s genetic blueprint: a linear, double-stranded DNA genome.

The lifecycle of Ad5 begins when its fiber proteins bind to receptors on a human cell’s surface. The virus is then taken into the cell through receptor-mediated endocytosis. Once inside, it travels to the cell’s nucleus and injects its DNA. The host cell’s machinery is co-opted to read the viral DNA and produce new viral components, which self-assemble into thousands of new virus particles that are released to infect more cells. In humans, this process typically manifests as respiratory infections, conjunctivitis, or gastroenteritis.

Ad5 as a Tool in Gene Therapy and Vaccines

Ad5’s efficiency at entering cells and depositing its DNA makes it an attractive candidate for use as a viral vector, a delivery vehicle engineered to carry genetic information into cells. To create an Ad5 vector, scientists remove viral genes required for replication, such as the E1 gene region. This renders the virus incapable of causing disease or multiplying on its own, creating a safe delivery system. The space created by removing these genes can then be filled with a genetic payload.

This technology has been explored in gene therapy, where Ad5 vectors can deliver correct copies of faulty genes to combat genetic disorders. They can also introduce genes into cancer cells to make them more susceptible to treatment. Some Ad5 vectors are designed as oncolytic viruses, which selectively infect and destroy tumor cells while sparing healthy tissue.

The platform has also been used in vaccine development. For vaccines, the vector is engineered to carry a gene that codes for an antigen—a specific protein from a pathogen, such as Ebola or SARS-CoV-2. When the Ad5 vector introduces this gene into human cells, the cells begin to produce the pathogen’s protein. This protein is then recognized by the immune system, which builds a protective response, including antibodies and T-cells, without ever being exposed to the actual dangerous pathogen.

The Impact of Pre-existing Ad5 Immunity

A hurdle for the use of Ad5 vectors is pre-existing immunity. Because Ad5 is a common virus, many adults have been exposed to it, often during childhood. This prior exposure leads to the development of immune memory, which includes neutralizing antibodies (NAbs) and virus-specific T-cells. These immune components are primed to recognize and attack Ad5 upon subsequent encounters.

When an Ad5-based vector is administered to someone with pre-existing immunity, their immune system identifies it. Neutralizing antibodies, which primarily target the hexon and fiber proteins of the capsid, bind to the vector and prevent it from entering target cells. This neutralization process can reduce the efficacy of the therapy or vaccine by clearing the payload before it can work.

This immune response not only blunts the vector’s effectiveness but can also trigger an inflammatory reaction and transient side effects. The high prevalence of Ad5 immunity, with rates exceeding 80% in some regions, poses a challenge for researchers. It limits the patient population that can benefit from unmodified Ad5-based treatments.

Advancements in Ad5 Vector Design

To navigate pre-existing immunity, scientists have developed strategies to modify Ad5 vectors. One approach is altering the viral capsid, the primary target of neutralizing antibodies. This can involve “cloaking” the vector with polymers or modifying capsid proteins like the hexon and fiber. By swapping parts of Ad5’s proteins with segments from rarer adenoviruses, the resulting chimeric vector can evade pre-existing antibodies.

Another strategy is serotype switching. Instead of using Ad5, researchers build vectors based on less common human adenovirus serotypes, like Ad26 or Ad35, or even adenoviruses from other species, like chimpanzees (ChAd). Because most people have not been exposed to these alternatives, they lack the immunity that would neutralize the vector. This approach was used in the development of several COVID-19 vaccines.

A third approach is the creation of “gutless” or high-capacity adenoviral vectors. Here, all viral coding genes are removed, leaving only the packaging signals. This maximizes space for therapeutic genes and reduces the vector’s ability to trigger an immune response, as it no longer produces viral proteins after entering the cell.