AAV9 capsids (ELISA Control)

AAV is an important vector for gene therapy because of its low toxicity, low immunogenicity, and continuous expression in vivo. AAV was first discovered from adenoviral preparations and is a nonenveloped linear single-stranded DNA virus with an icosahedral protein capsid. The coat consists of three structural proteins, VP1, VP2, and VP3, in a ratio of 1:1:10. The genome is flanked at both ends by inverted terminal repeat sequences (ITRs) and flanked by two open reading frames (ORFs), rep and cap. Recombinant AAV is generated by deleting the rep and cap genes between ITRs and inserting the target transgene at the deletion site and is commonly used in research and clinical settings.

Figure 1. Genome structure of (a) wild-type AAV and (b) recombinant AAV
(Source: Liu D, et al. 2021)

The discovery that AAV has different serotypes, which have different tissue propensities, has also facilitated the research and development of AAV-targeted gene delivery methods. Genomic differences between the various AAV serotypes are mainly found in variable regions of the viral capsid sequence, especially VP3, which plays a crucial role in determining tropism. AAV9 is tissue-tropically expressed in the heart and central nervous system (CNS) in humans and has been widely used in basic and clinical studies of gene delivery to the CNS. AAV9 primarily conducts neurons when injected directly into the brain and can propagate retrogradely or retrogradely along axons within cells, whereas it naturally crosses the blood-brain barrier when injected intravascularly, bringing hope for non-invasive treatment of CNS diseases.

Clinical applications of AAV urgently require production and purification systems capable of producing large quantities of high-quality recombinant AAV (rAAV) to meet safety, efficacy, stability, and cost requirements. The inability of rAAV to self-amplify by reinfection requires the expression of both the helper virus genes and the rAAV-specific rep and cap genes for replication and packaging of the ITR-flanked vector genome at the time of production. Currently developed rAAV production strategies are categorized into transient transfection and viral infection. Among transfection-based rAAV production, plasmid cotransfection of HEK293 cells is the most widely used production system. Production of rAAV by viral infection requires the use of three different viruses, including Ad and HSV, the natural coviruses of AAV, and baculoviruses. The generation of rAAV also involves mammalian cells or insect cells. Each approach has its own strengths in terms of flexibility, quality and scalability.

Adeno-associated virus 9 capsids
Adeno-associated virus type 9 capsids