Executive Summary

Pre-existing and induced immunity against adeno-associated virus (AAV) vectors represents a significant obstacle to achieving durable transgene expression and predictable clinical outcomes. The primary drivers of this response are the vector capsid and, to a lesser extent, the vector genome itself. Proactive, data-driven vector design, focusing on both capsid modification and genome optimization, provides the most effective path to mitigating immunogenicity and improving the safety and efficacy profile of a therapeutic candidate.

Frequently Asked Questions

Designing for Immune Evasion

The interaction between an AAV vector and the host immune system is a complex, multi-faceted process that can determine the success or failure of a gene therapy program. A vector that is rapidly neutralized or that triggers a potent T-cell response will fail to deliver a therapeutic benefit. Addressing immunogenicity must begin at the earliest stages of vector design.

Our approach is built on a deep understanding of AAV biology and immunology, leveraging decades of experience within our >100,000 sq ft facility. This foundation provides the strategic design guidance needed to accelerate development lifecycles and guide programs toward a typical 18-24 month IND timeline.

Capsid Engineering: The First Line of Defense

The AAV capsid is the interface with the host environment. Pre-existing NAbs, present in a significant portion of the human population due to natural AAV infection, can prevent the vector from reaching its target tissue. Capsid-derived peptides presented on the surface of transduced cells can also be targeted by CTLs.

Key strategies for capsid modification include:

• Rational Design: Site-directed mutagenesis to remove known T-cell epitopes from the capsid surface.

• Directed Evolution: Creating large libraries of capsid variants and selecting for those that evade neutralization by human serum.

• Serotype Switching: Utilizing novel or less common AAV serotypes that have a lower prevalence of pre-existing NAbs in the general population.

As research into T-cell responses to AAV has shown, understanding the mechanisms that lead to both deleterious effects and potential tolerance is fundamental to developing improved gene transfer protocols (PMID: 28323492).

Learn More: Watch the full webinar, Vector Ready: Where AAV projects begin and how they succeed, to explore capsid engineering in greater detail.

Genome Optimization: Reducing Innate Immune Triggers

While the capsid is the primary target of adaptive immunity, the vector genome can initiate the process by activating innate immune sensors. The TLR9 pathway is a well-characterized mechanism by which the host recognizes foreign DNA, such as that from a viral vector.

By engineering the transgene cassette, we can significantly reduce these innate immune triggers:

• Codon Optimization: In addition to improving translation efficiency, codon optimization can be used to eliminate CpG motifs without altering the protein sequence.

• CpG Motif Reduction: Systematically removing CpG dinucleotides from the promoter, transgene, and polyA signal regions of the vector genome dampens TLR9 activation.

This level of precise vector design allows for fine control over the resulting immune response. For instance, in a vaccine context, a well-designed AAV vector can be engineered to produce a potent and sustained immunogenic response, demonstrating the platform’s flexibility when immunogenicity is the desired outcome (PMID: 33442684). This principle of control is directly applicable when the goal is immune evasion for a gene replacement therapy.

Our scientific continuity and deep institutional knowledge are core to our process. As one partner noted, “The key people from UPenn Vector Core joined Franklin Biolabs and our partnership transitioned without interruption… Franklin Biolabs is a key collaborator in our AAV-vector based gene therapy candidate development and we hope to continue the partnership for years to come.”