Gene therapy offers the potential to treat genetic diseases by delivering therapeutic genetic material to target cells. However, effective gene delivery must overcome several biological barriers, including immune recognition, cellular uptake, intracellular trafficking, and efficient expression of the therapeutic payload. Viral and non-viral vectors have therefore been developed to address these challenges.
Among viral vectors, adeno-associated virus (AAV) has become an important platform for in vivo gene delivery. AAV vectors can transduce both dividing and non-dividing cells and support sustained transgene expression. Different AAV capsids also exhibit distinct tissue tropism, providing opportunities to develop vectors for specific organs and cell types.
AAV-based approaches nevertheless have important limitations. These include the relatively small packaging capacity of AAV, pre-existing immunity to some capsids, immune responses following administration, and challenges associated with achieving efficient and sufficiently specific delivery to certain tissues. These limitations have driven continued efforts to improve AAV vector design and performance.
Why Are NHPs Important in AAV Research?
Nonhuman primates (NHPs) can provide valuable translational information during the development of AAV-based therapies because of their similarities to humans in anatomy, physiology, genetics, and immune biology. NHP studies can be used to evaluate vector biodistribution, tissue transduction, transgene expression, pharmacological activity, and immune responses in vivo.
Importantly, AAV capsid performance can vary substantially between species. A vector that shows strong tropism in rodents may not exhibit the same distribution or transduction profile in primates. NHP studies can therefore provide additional information when assessing whether an AAV vector has characteristics suitable for further development.
Developing Novel AAV Capsids
One major area of AAV research is the development of capsids with improved tissue tropism and transduction efficiency. Researchers have used approaches such as directed evolution, rational design, and peptide display to generate AAV variants with altered biological properties.
These approaches aim to improve delivery to specific tissues, including the central nervous system, liver, heart, and retina, while potentially reducing unwanted tissue distribution or immune recognition. NHP studies can help determine whether the properties observed during earlier-stage screening translate to a larger animal model with greater physiological relevance to humans.
Tissue-Specific and Regulated Gene Expression
Vector performance depends not only on capsid selection but also on how the therapeutic gene is expressed after delivery. Promoter and regulatory elements can be engineered to influence the level and cellular specificity of transgene expression.
Tissue- or cell-specific promoters can help restrict expression to the intended target cells, while inducible regulatory systems can provide additional control over the timing or level of expression. These strategies may improve the therapeutic window by limiting expression in non-target tissues.
NHP studies can be useful for evaluating the distribution and specificity of transgene expression in vivo, particularly when promoter activity and tissue biology differ between species.
AAV and CRISPR-Cas9 Genome Editing
AAV vectors can also be used to deliver components of genome-editing systems such as CRISPR-Cas9. Rather than simply providing a functional gene, AAV-based genome editing aims to modify a specific genomic sequence within target cells.
Preclinical studies have investigated AAV-delivered genome-editing approaches for diseases including Duchenne muscular dystrophy, hemophilia, cystic fibrosis, and inherited retinal disorders. NHP studies can provide important information on delivery efficiency, editing activity, tissue distribution, and immune responses before clinical translation.
However, AAV-based CRISPR approaches face additional challenges, including the packaging limitations of AAV vectors, potential off-target editing, immune responses against vector components or editing proteins, and the need to achieve sufficient editing efficiency in the relevant target cells.
The Role of NHP Research in AAV Development
AAV vector development increasingly requires an integrated assessment of capsid biology, tissue distribution, transgene expression, pharmacological activity, and immunogenicity. NHP models can complement in vitro and rodent studies by providing translational information that may be difficult to obtain from other models.
The appropriate NHP species, administration route, dose, sampling strategy, and endpoints should be determined according to the vector, target tissue, therapeutic mechanism, and development stage. NHP research is therefore most informative when incorporated into a broader preclinical strategy designed to address specific development questions.
As AAV technologies continue to evolve, advances in capsid engineering, regulatory elements, and genome-editing systems may expand the potential applications of gene therapy. Well-designed NHP studies can help characterize these technologies and provide critical translational data to support their further development.
