Understanding AAV Packaging

In the field of gene therapy, adeno-associated virus (AAV) has emerged as a highly promising vector. But why does AAV require packaging? What are the underlying concepts, and why is packaging so crucial?

1. Wild-type vs. Recombinant AAV

Wild-type AAV

Wild-type AAV represents a non-enveloped single-stranded DNA virus with a diameter range from 18 to 25 nanometers. The virus is classified under the Dependoparvovirus genus of the Parvoviridae family. Adeno-associated virus requires a helper virus like adenovirus or herpesvirus to replicate which its name specifies.

The natural AAV genome includes two open reading frames that produce three Cap proteins which form the structure and four Rep proteins which regulate activity. The AAV genome features inverted terminal repeats positioned at its terminal regions. Essential cis-acting regulatory elements for viral life cycle functions such as genome replication initiation sites, packaging signals, and integration signals are contained in these sequences.

Figure 1. Genome organization of wild-type AAV (Source: Merten O-W, 2024)

Recombinant AAV Vector

Recombinant AAV (rAAV) vectors originate from non-pathogenic wild-type AAV and serve as gene delivery systems. Scientists have identified more than 12 distinct AAV serotypes along with over 100 different variants. The strong safety profile together with reduced immune response and extended gene expression duration makes rAAV vectors optimal for in vivo gene function studies.

3.  Principles of AAV Packaging

The essential viral proteins Rep and Cap support AAV packaging through their roles in genome replication and capsid assembly. Initiating genome replication and packaging functions are essential roles of the ITR sequences.

Role of Rep Proteins

• Viral genome replication: Rep proteins recognize and bind to the ITR sequences of the AAV genome, initiating the viral DNA replication process. During packaging, Rep proteins ensure that sufficient copies of the target gene are replicated for subsequent virus particle assembly.
• Regulation of gene expression: Rep proteins also regulate the transcription of AAV genes by interacting with viral promoters and enhancers. This controls the synthesis of viral proteins and the assembly of virus particles.

Role of Cap Proteins

• Capsid formation: The AAV capsid is predominantly made of Cap proteins. The capsid is made up of three structural proteins, VP1, VP2, and VP3, which are organized into an icosahedral structure in a precise ratio (usually 1:1:10). Cap proteins wrap the replicated AAV genome, resulting in a fully produced virus particle during packaging.
• Determining serotype and tissue tropism: Different AAV serotypes possess unique Cap protein sequences and structures which determine their tissue and cell type infection capabilities. The Cap protein serves as the determinant for both serotype and tissue tropism of AAV which facilitates gene delivery to precise target cells.

5. Evolution of AAV Packaging Technologies

Early Transfection-Based Methods

Initially, AAV was produced using transfection-based methods. These methods were simple and flexible, allowing easy switching between different transgenes and serotypes. However, they also had limitations, including:

• Large amounts of plasmid DNA required for production,
• Variable transfection efficiency,
• Challenges in clinical-scale manufacturing.

Infection-Based Systems

To overcome these limitations, infection-based systems were developed:

• Baculovirus System:

The first baculovirus-based AAV production system used three different recombinant baculoviruses to infect SF9 insect cells. Later, the system was improved by reducing the number of baculoviruses needed and optimizing genetic constructs.

• Herpesvirus System:

A system based on two replication-deficient herpesvirus vectors was later developed. This system provided the Rep-Cap genes and AAV vector genome, integrated into the herpesvirus genome.

• Adenovirus System:

In early AAV production, adenovirus was widely used as a helper virus. This method involved transfecting HEK293 cells with two plasmids followed by adenovirus infection. However, the triple-transfection system later replaced it, eliminating adenoviral contamination in AAV products.

Stable Packaging and Producer Cell Lines

• HeLa and A549-based Cell Lines:

In the late 1990s and early 2000s, the first stable packaging and producer cell lines were developed. These were mainly based on HeLa or A549 cells, engineered to stably express Rep/Cap proteins.

• HEK293-based Cell Lines:

Since HEK293 cells express the E1a gene, they became the preferred platform for AAV production. The first HEK293-based AAV producer cell line was developed, followed by inducible producer cell lines like the TESSA system and CEVEC's Alpha cell line.

Insect Cell-Based Packaging Systems

The SF9/Baculovirus system was further optimized for large-scale AAV production. A two-component system using recombinant SF9 cells and recombinant baculoviruses was developed. Later, the OneBac system was introduced, capable of producing AAV serotypes 1–12.

In summary, the development of AAV packaging technologies has significantly advanced the field of gene therapy. From early transfection-based methods to modern stable packaging and producer cell lines, each innovation has brought us closer to efficient, cost-effective AAV production.

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