AAV2 VS AAV8

Because of its efficient gene delivery capability to target cells, non-pathogenic adeno-associated virus (AAV) is frequently used as a viral vector in clinical trials for gene therapy. AAV2 and AAV8 stand out among the various AAV serotypes because of their distinctive features which prompt extensive analysis to assess their benefits and drawbacks for different gene therapy approaches.

Structure and Development

AAV2

• Structure

The AAV2 serotype stands out as the most extensively examined member of the AAV family. AAV2 naturally targets skeletal muscle cells as well as neurons vascular smooth muscle cells and liver cells. The AAV2 capsid structure consists of three viral proteins VP1, VP2 and VP3 which organize into an eight-stranded β-barrel formation with extended loops present between each β-strand. The surface of AAV2 capsid contains specific amino acid sequences which bind to cellular receptors including heparan sulfate proteoglycan (HSPG) to start viral entry into host cells.

• Development

Scientists identified AAV2 in 1965 when it appeared as a contaminant in adenovirus preparations from rhesus monkeys and later demonstrated that it posed no pathogenic threat to humans. AAV2 became the first AAV serotype to be mass-produced because of its exceptional promise in gene therapy applications and it has found wide use in both research and clinical settings.

AAV8

• Structure

Researchers successfully extracted AAV8 as a new serotype from rhesus monkey tissue samples. AAV8 capsid structure shares features with AAV2 and AAV4 through its eight-stranded β-barrel formation yet displays unique amino acid sequences on the surface of the capsid. AAV8 experiences reduced basic charge distribution in AAV2's HSPG-binding region which prevents it from binding to heparan sulfate.

• Development

AAV8 was first isolated in 2002, later studies demonstrated its superior transduction efficiency in liver tissues compared to other AAV serotypes. The identification of AAV8's properties positioned it as a top viral vector candidate for liver-directed gene therapies to treat conditions like hemophilia and familial hypercholesterolemia.

Figure 1. Structure and conservation of adeno-associated viruses. (Source: Rayaprolu V, et al. 2013)

Comparison Between AAV2 and AAV8

Tissue Tropism

• AAV2 exhibits its main cell targeting activities within the liver and retina and extends to the central nervous system. While AAV2 remains a standard vector in liver gene therapy its transduction efficiency remains inferior to that of AAV8. During ophthalmic gene therapy procedures researchers found that AAV2 targets retinal cells strongly yet AAV8 proves superior in transducing photoreceptor cells.
• AAV8 demonstrates superior hepatocyte transduction efficiency across mouse, canine and non-human primate models compared to other AAV serotypes. AAV8 displays efficient transduction capabilities in several tissues including skeletal muscle, cardiac muscle along with kidney and pancreas.

Receptor Binding and Cellular Entry Mechanisms

• AAV2: AAV2 achieves viral entry through initial HSPG cell surface binding and requires further attachment to αVβ5 integrin and the fibroblast growth factor receptor 1 (FGFR1) co-receptors.
• AAV8: The receptor-binding mechanism of AAV8 is different from AAV2 because it does not attach to HSPG and instead uses liver-specific receptors (LR) to enter cells.

Figure 2. Comparison of serum human α-1-antitrypsin (hAAT) levels among the different rAAVs and delivery routes. (Source: Wang L, et al. 2009)

Transduction Efficiency

• The transduction efficiency of AAV8 in liver cells reaches levels 10 to 100 times greater than those achieved by AAV2.
• AAV2 results in lower photoreceptor transduction efficiency compared to AAV8 when injected subretinally.

Immunogenicity

• AAV2: AAV2 demonstrates a relatively mild immune response in human hosts. As the first AAV serotype to receive extensive research attention and clinical use AAV2 now encounters widespread pre-existing neutralizing antibodies within the human population. Pre-existing immunity to AAV2 restricts its repeated application in gene therapy treatments.
• AAV8: AAV8 demonstrates reduced immunogenicity compared to AAV2 along with a lower presence of pre-existing anti-AAV8 antibodies among humans. AAV8 achieves superior immune evasion capabilities which enhances its suitability for in vivo gene therapy applications.

Vector Design and Modifications

Comparative studies of AAV2 and AAV8 capsid proteins reveal structural differences that enable researchers to investigate their functional properties and create more efficient chimeric viral vectors. Scientists created AAV2/8 chimeric vectors that improve liver transduction efficiency by adding AAV8's liver-targeting functional region to the AAV2 capsid.

Packaging Capacity

• AAV2: AAV2 can package approximately 4.7 kb of genetic material which restricts the size of genes for delivery.
• AAV8: AAV8 matches AAV2 in terms of packaging capacity but achieves greater transduction efficiency which offsets this restriction.

Clinical Applications

• AAV2: Clinical trials using AAV2 study potential treatments for both cystic fibrosis as well as hemophilia and muscular dystrophy. Research and clinical applications of ophthalmic gene therapy extensively utilize AAV2 in trials that focus on treating Leber congenital amaurosis and choroidal disorders. The restricted gene delivery capability of AAV2 continues to constrain its wider application.
• AAV8: Liver-directed gene therapy shows significant advantages with the AAV8 serotype which also yields promising clinical trial results for both hemophilia and familial hypercholesterolemia. Its high efficiency for liver transduction together with low immunogenicity makes it the best viral vector choice for disease treatment.

Figure 3. Characterization of mouse liver transgene expression mediated by AAV8, AAV2 and a chimeric vector containing variable regions VII and IX from AAV8 in an otherwise fully AAV2 backbone. (Source: Tenney RM, et al. 2014)

Overall, AAV2 and AAV8 serve as key AAV serotypes that find widespread use in gene therapy applications. AAV2 naturally targets multiple tissues yet shows reduced transduction efficiency when targeting the liver. AAV8 achieves superior liver transduction results and lower immunogenicity making it the preferred option for liver-targeted therapies. The selection of an appropriate AAV vector demands a thorough evaluation of the strengths and weaknesses of each serotype to achieve optimal therapeutic outcomes for specific gene therapy applications.

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