Choosing the Right Viral Vector
Adenovirus vs. AAV
Adenoviruses and Adeno-Associated Viruses (AAVs) are two types of viral vectors used for gene delivery. Both of these recombinant viral systems have the ability to infect a broad range of hosts, including dividing and non-dividing cells, without integrating with the host genome. However, there are several key distinctions between them, including: packaging capacity, level, onset and duration of gene expression, and immune response.
Adenoviruses have a capacity of ~8.5 kilobases, high levels of protein expression, and transient gene expression. The onset of expression can occur as early as 16-24 hours after infection. The high immune response from the target cells are the main limitation of adenoviral systems. Despite this, they are still widely used in research, due to their highly efficient transduction of most tissue.
AAVs have a packaging capacity of ~4.5 kilobases, relatively low levels of protein expression, and the potential for long lasting gene expression. The tropism of AAV can also be increased via different serotypes. The primary disadvantage of AAV is its smaller packaging size for gene of interest, as well as a much later onset of expression (2-7 days for in vitro and 3-21 days for in vivo). However, this delivery system triggers very low levels of immune response.
| Adenovirus | AAV | |
|---|---|---|
| Tropism | Dividing and Non-Dividing Cells | Dividing and Non-Dividing Cells |
| Genome Integration | No | No |
| Packaging Capacity | ~8.5 kb | ~4.5 kb |
| Protein Expression | High | Low |
| Gene Expression | Transient | Potentially Long Lasting |
| Target Cell’s Immune Response | High | Very Low |
| Onset of Expression | 16-24 Hours | 2-7 Days (in vitro) 3-21 Days (in vivo) |
| Biosafety Level | BSL-2 | BSL-1 |
Ad5 vs. Ad(RGD)
Most cells can be infected using the standard Ad5, but there are some exceptions. Adenovirus infection is initiated by recognition of the native Ad5 receptor, coxsackievirus-adenovirus receptor (CAR), on target cells by the carboxy-terminal portion (i.e. knob) of the fiber protein. Therefore, the infection efficiency of adenoviruses is very low for cells with few or no CAR receptors.
The Arg-Gly-Asp (RGD) motif was introduced in the surface-exposed loops of adenovirus fiber knobs. This enables the virus to bypass CAR and mediate cell entry via RGD binding integrins. RGD adenoviruses can be used for “retargeting” cells in which regular adenoviruses don’t work well. The infection effeciency of RGD modified adenoviruses can be tens or hundreds of times higher than standard Ad5 in some cells, including human or mouse macrophages, T cells, synoviocytes, islet grafts, adipocytes, MEF, etc.
AAV Serotypes
| Tissue Target | Recommended Serotypes |
|---|---|
| Central Nervous System | AAV1, AAV2, AAV5, AAV8, AAV9, AAV-DJ, AAV-DJ9 |
| Cardiac | AAV1, AAV8, AAV9, AAV-DJ9 |
| Liver | AAV8, AAV-DJ |
| Lung | AAV6, AAV9 |
| Kidney | AAV2 |
| Pancreas | AAV8 |
| Photoreceptor | AAV2, AAV5, AAV8 |
| RPE | AAV1, AAV5, AAV8 |
| Skeletal Muscle | AAV1, AAV6, AAV8, AAV9 |
There are several factors that affect tropism, such as type of study, tissue target, delivery method, species, etc. Here, you’ll find our recommendations for what serotypes work best with specific tissues for in vivo studies. Keep in mind, however, that no single serotype is 100% optimal for every tissue target. For a more comprehensive look at serotypes, please see our AAV Serotype Selection Guide.
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