Adeno-associated Virus (AAV) Vector – Efficacy and Immunity
Adeno-associated virus (AAV) is a parvovirus with a single-stranded DNA genome of approximately 4.7 kb. It is a dependovirus that is unable to replicate in the absence of a helper virus such as adenovirus; thus, although it is a common natural infection, AAV is not associated with any known pathogenic infections in humans. Recombinant AAVs are modified by the removal of any DNA encoding for viral protein. Only the inverted terminal repeats (ITRs) required for packaging are retained from the viral genome, giving rAAV vectors a packaging capacity of about 5 kb for the promoter and gene of interest. Several factors make AAV an attractive vector for in vivo gene therapy, including its ability to infect nondividing cells, the maintenance of vector genomes as episomal concatemers (minimizing the risk of insertional mutagenesis), its relatively low immunogenicity and the wide variety of capsid serotypes that allow gene delivery to numerous target tissues and cell types.
Immune responses to AAV vectors have been observed in the clinic, posing a hurdle for gene therapy. The Herzog Lab continues to define the mechanisms of innate immunity to AAV vectors. For example, innate immunity in the liver was found to depend on sensing of the viral genome by the endosomal DNA receptor TLR9. Furthermore, TLR9-MyD88 pattern-recognition receptor pathway is uniquely capable of initiating CD8+ T cell responses against the viral capsid (while, for instance, TLR2, STING, or TLR4 have no effect). Importantly, both conventional (cDCs) and plasmacytoid dendritic cells (pDCs) are required for the cross-priming of capsid-specific CD8+ T cells, while other antigen presenting cells are not involved. TLR9 signaling is specifically required in pDCs but not in cDCs, indicating that sensing of the viral genome by pDCs activates cDCs in trans to cross-present capsid antigen during CD8+ T cell activation.
Cross-presentation and cross-priming also require IFN I signaling and can be inhibited by administering specific molecules to prevent induction of capsid-specific CD8+ T cells. The effects of innate immune pathways immune responses to vector and transgene product in different target organs are under active investigation.