Vector Affinity and Receptor Distribution Define Tissue-Specific Targeting in an Engineered AAV Capsid.

Unbiased in vivo selections of diverse capsid libraries can yield engineered capsids that overcome gene therapy delivery challenges like traversing the blood-brain barrier (BBB), but little is known about the parameters of capsid-receptor interactions that govern their improved activity. This hampers broader efforts in precision capsid engineering and is a practical impediment to ensuring the translatability of capsid properties between preclinical animal models and human clinical trials. In this work, we utilize the adeno-associated virus (AAV)-PHP.B-Ly6a model system to better understand the targeted delivery and BBB penetration properties of AAV vectors. This model offers a defined capsid-receptor pair that can be used to systematically define relationships between target receptor affinity and in vivo activity of engineered AAV vectors. Here, we report a high-throughput method for quantifying capsid-receptor affinity and demonstrate that direct binding assays can be used to organize a vector library into families with varied affinity for their target receptor. Our data indicate that efficient central nervous system transduction requires high levels of target receptor expression at the BBB, but it is not a requirement for receptor expression to be limited to the target tissue. We observed that enhanced receptor affinity leads to reduced transduction of off-target tissues but can negatively impact on-target cellular transduction and penetration of endothelial barriers. Together, this work provides a set of tools for defining vector-receptor affinities and demonstrates how receptor expression and affinity interact to impact the performance of engineered AAV vectors in targeting the central nervous system. IMPORTANCE Novel methods for measuring adeno-associated virus (AAV)-receptor affinities, especially in relation to vector performance in vivo, would be useful to capsid engineers as they develop AAV vectors for gene therapy applications and characterize their interactions with native or engineered receptors. Here, we use the AAV-PHP.B-Ly6a model system to assess the impact of receptor affinity on the systemic delivery and endothelial penetration properties of AAV-PHP.B vectors. We discuss how receptor affinity analysis can be used to isolate vectors with optimized properties, improve the interpretation of library selections, and ultimately translate vector activities between preclinical animal models and humans.

Context-Specific Function of the Engineered Peptide Domain of PHP.B.

One approach to improve the utility of adeno-associated virus (AAV)-based gene therapy is to engineer the AAV capsid to (i) overcome poor transport through tissue barriers and (ii) redirect the broadly tropic AAV to disease-relevant cell types. Peptide- or protein-domain insertions into AAV surface loops can achieve both engineering goals by introducing a new interaction surface on the AAV capsid. However, we understand little about the impact of insertions on capsid structure and the extent to which engineered inserts depend on a specific capsid context to function. Here, we examine insert-capsid interactions for the engineered variant AAV9-PHP.B. The 7-amino-acid peptide insert in AAV9-PHP.B facilitates transport across the murine blood-brain barrier via binding to the receptor Ly6a. When transferred to AAV1, the engineered peptide does not bind Ly6a. Comparative structural analysis of AAV1-PHP.B and AAV9-PHP.B revealed that the inserted 7-amino-acid loop is highly flexible and has remarkably little impact on the surrounding capsid conformation. Our work demonstrates that Ly6a binding requires interactions with both the PHP.B peptide and specific residues from the AAV9 HVR VIII region. An AAV1-based vector that incorporates a larger region of AAV9-PHP.B-including the 7-amino-acid loop and adjacent HVR VIII amino acids-can bind to Ly6a and localize to brain tissue. However, unlike AAV9-PHP.B, this AAV1-based vector does not penetrate the blood-brain barrier. Here we discuss the implications for AAV capsid engineering and the transfer of engineered activities between serotypes. IMPORTANCE Targeting AAV vectors to specific cellular receptors is a promising strategy for enhancing expression in target cells or tissues while reducing off-target transgene expression. The AAV9-PHP.B/Ly6a interaction provides a model system with a robust biological readout that can be interrogated to better understand the biology of AAV vectors' interactions with target receptors. In this work, we analyzed the sequence and structural features required to successfully transfer the Ly6a receptor-binding epitope from AAV9-PHP.B to another capsid of clinical interest, AAV1. We found that AAV1- and AAV9-based vectors targeted to the same receptor exhibited different brain-transduction profiles. Our work suggests that, in addition to attachment-receptor binding, the capsid context in which this binding occurs is important for a vector's performance.

Intranasal gene therapy to prevent infection by SARS-CoV-2 variants.

SARS-CoV-2 variants have emerged with enhanced pathogenicity and transmissibility, and escape from pre-existing immunity, suggesting first-generation vaccines and monoclonal antibodies may now be less effective. Here we present an approach for preventing clinical sequelae and the spread of SARS-CoV-2 variants. First, we affinity matured an angiotensin-converting enzyme 2 (ACE2) decoy protein, achieving 1000-fold binding improvements that extend across a wide range of SARS-CoV-2 variants and distantly related, ACE2-dependent coronaviruses. Next, we demonstrated the expression of this decoy in proximal airway when delivered via intranasal administration of an AAV vector. This intervention significantly diminished clinical and pathologic consequences of SARS-CoV-2 challenge in a mouse model and achieved therapeutic levels of decoy expression at the surface of proximal airways when delivered intranasally to nonhuman primates. Importantly, this long-lasting, passive protection approach is applicable in vulnerable populations such as the elderly and immune-compromised that do not respond well to traditional vaccination. This approach could be useful in combating COVID-19 surges caused by SARS-CoV-2 variants and should be considered as a countermeasure to future pandemics caused by one of the many pre-emergent, ACE2-dependent CoVs that are poised for zoonosis.

The GPI-Linked Protein LY6A Drives AAV-PHP.B Transport across the Blood-Brain Barrier.

Efficient delivery of gene therapy vectors across the blood-brain barrier (BBB) is the holy grail of neurological disease therapies. A variant of the neurotropic vector adeno-associated virus (AAV) serotype 9, called AAV-PHP.B, was shown to very efficiently deliver transgenes across the BBB in C57BL/6J mice. Based on our recent observation that this phenotype is mouse strain dependent, we used whole-exome sequencing-based genetics to map this phenotype to a specific haplotype of lymphocyte antigen 6 complex, locus A (Ly6a) (stem cell antigen-1 [Sca-1]), which encodes a glycosylphosphatidylinositol (GPI)-anchored protein whose function had been thought to be limited to the biology of hematopoiesis. Additional biochemical and genetic studies definitively linked high BBB transport to the binding of AAV-PHP.B with LY6A (SCA-1). These studies identify, for the first time, a ligand for this GPI-anchored protein and suggest a role for it in BBB transport that could be hijacked by viruses in natural infections or by gene therapy vectors to treat neurological diseases.