Adeno-associated viruses (AAVs) are small, naturally occurring, non-pathogenic vehicles for gene delivery. They are a preferred choice for gene therapy due to their safety, low immunogenicity, and ability to provide long-term gene expression in non-dividing cells. Different “serotypes” of AAVs exist, significantly influencing their application.
Understanding AAV Serotypes
AAV serotypes are naturally occurring variants of the Adeno-associated virus, distinguished by their unique capsid structure, the outer protein shell. Thirteen human and primate serotypes are currently identified. All AAV capsids share a similar basic icosahedral structure, containing both constant and variable regions.
Constant regions of AAV capsid proteins are highly similar across serotypes, playing a role in capsid assembly, maintenance, and genome packaging. Variable regions, in contrast, show significant sequence variations. These differences in variable regions create diversity in receptor binding domains, antigenic sites, and surface loops, dictating each serotype’s unique biological properties. The capsid, formed by 60 copies of three viral proteins (VP1, VP2, and VP3), determines how the virus interacts with different cells and the host’s immune system.
How Serotypes Guide Gene Delivery
AAV serotypes differ significantly in their functional properties, particularly concerning tissue tropism and immunogenicity. Tissue tropism refers to the natural preference of each serotype for infecting specific cell types or tissues. This specificity arises because the unique capsid proteins bind to different receptors on the surface of target cells, allowing selective entry.
For example, AAV2 binds to heparan sulfate proteoglycan (HSPG) as a primary receptor, along with integrins and fibroblast growth factor receptor 1 (FGFR-1). In contrast, AAV9 capsids interact with galactose, while AAV1, AAV4, AAV5, and AAV6 capsids interact with sialic acid proteoglycans. This differential preference for cellular receptors dictates the tropism of a given AAV serotype. For instance, some serotypes may preferentially target liver cells, others muscle, and some may even cross the blood-brain barrier to reach brain tissue.
The body’s immune system recognizes and responds differently to various AAV capsids, a phenomenon known as immunogenicity. Prior exposure to a specific AAV serotype, or a closely related one, can lead to the development of pre-existing antibodies that can neutralize the therapeutic virus. These neutralizing antibodies can prevent the virus from effectively delivering its therapeutic gene to target cells. Studies indicate that a significant portion of the human population, ranging from 40% to 80%, has pre-existing antibodies against known wild-type AAV serotypes such as AAV1 and AAV2, with immunity often developing in childhood. Even low levels of these antibodies can hinder successful gene transfer.
Choosing the Right AAV for Therapy
Selecting the appropriate AAV serotype for a gene therapy application involves careful consideration of several practical factors. The target organ or tissue is a primary determinant, as the desired delivery location dictates the choice of serotype based on its tropism. For instance, AAV2 is frequently used for ocular and central nervous system (CNS) disorders, while AAV8 is often employed for blood disorders and liver-targeted gene therapy. AAV9 is a common choice for neurological conditions due to its ability to cross the blood-brain barrier.
The immune status of the patient is another significant factor in serotype selection. Given the prevalence of pre-existing antibodies against certain AAV serotypes, patients are often screened for these antibodies before treatment. If a patient has high levels of neutralizing antibodies against a particular serotype, an alternative serotype with different capsid properties and less pre-existing immunity may be chosen. This “serotype-switching” strategy relies on finding an alternative AAV serotype that still possesses the desired tissue tropism.
The method of administration also influences serotype selection. For local injections into immune-privileged organs like the eye or central nervous system, the impact of pre-existing antibodies is less severe, offering a possibility for repeat dosing. However, systemic delivery of AAV vectors is more challenging when pre-existing anti-capsid antibodies are present. While natural serotypes are powerful tools, ongoing research focuses on engineering new AAV variants with enhanced properties, such as improved tissue specificity or reduced immunogenicity.