Use of Viral Vectors
Viruses are inherently capable of binding to mammalian cells and transferring genetic information into those cells. Each virus has evolved to utilize the host cell's machinery in order to replicate itself. The amount of genetic material that can be packaged in each virus is determined, in part, by the structure and volume of its capsid (shell).Using viruses for vectors takes advantage of this cell targeting and gene expression system. In designing gene vectors, the virus is generally engineered so it cannot reproduce (replication-deficient). This is accomplished by removing a gene from the virus genome that is critical for replication. Removing this gene also creates space to allow the insertion of the gene desired for expression("gene of interest"). This vector can now be reproduced by incubating it with cells that can compensate for the gene that was deleted, allowing the virus to replicate within the cell (packaging cell line). In some cases, another virus can supply the missing replication machinery (helper virus). The goal is to end up with a large number of viral particles with the "gene of interest", but to not allow the virus to exert any pathogenic properties associated with the whole or "wild-type" virus.
There are two basic biosafety concerns regarding research using viral vectors. First, it is impossible to completely control cellular processes to ensure that a replication-deficient virus will not naturally gain back genes that it requires for replication (become replication-competent). If a virus becomes replication-competent, it may re-acquire any pathogenic characteristics associated with the "wild-type" virus and could cause illness. In addition, the "gene of interest" may be acquired by viruses or cells not expected to be associated with that gene. Because of these concerns, biosafety containment recommendations are made according to the properties of the "wild type" virus, and must also take into consideration the nature and necessary containment of the "gene of interest".
Adenoviral Vectors
Wild-type adenoviruses most commonly cause respiratory illness, but may also cause other illnesses such as gastroenteritis and conjunctivitis. Symptoms of respiratory illness caused by adenovirus infection range from the common cold to pneumonia, croup, and bronchitis. Patients with compromised immune systems are especially susceptible to severe complications of adenovirus infection. Adenoviruses are highly stable in even adverse conditions, allowing prolonged survival outside the body. Adenoviruses are most commonly transmitted by direct contact, such as a handshake. Persons who are infected may shed the virus for months or years, without experiencing any symptoms.
Adenoviral vectors used in research are rendered replication-deficient by removal of the E1a gene, which is essential for the virus to replicate. If the virus cannot reproduce, it cannot sustain the illnesses listed above. However, E1a-deleted virus may still be capable of low-level replication. Other genes may also be removed to minimize the likelihood of recombination to a replication-competent virus. E1a deletion mutants can be grown in the packaging cell line, HEK-293, that stably expresses the E1a region, allowing replication of the viral vector plus "gene of interest".
Biological Safety Issues for Research with Adenoviral Vectors
Replication-deficient adenovirus may theoretically recombine with the wild-type virus to result in a replication-competent virus. Because the wild-type virus is very common, this possibility must be considered when assigning biosafety levels for work with adenoviral vectors. The National Institutes of Health has assigned any adenovirus to Risk Group 2 (agents that are associated with human disease which is rarely serious and for which preventive or therapeutic interventions are often available). As such, Biosafety Level 2 containment is generally required for work with this vector. Please be aware, however, that this containment level could be increased depending on the nature of the "gene of interest".
Courtesy of Vanderbilt Environmental Health and Safety
For additional information on specific viral vectors, please click on the Stanford links in the menu to the left.