Human Immune Responses to Adeno-Associated Virus (AAV) Vectors

Abstract

Recombinant adeno-associated virus (rAAV) vectors are one of the most promising in vivo gene delivery tools. Several features make rAAV vectors an ideal platform for gene transfer. However, the high homology with the parental wild-type virus, which often infects humans, poses limitations in terms of immune responses associated with this vector platform. Both humoral and cell-mediated immunity to wild-type AAV have been documented in healthy donors, and, at least in the case of anti-AAV antibodies, have been shown to have a potentially high impact on the outcome of gene transfer. While several factors can contribute to the overall immunogenicity of rAAV vectors, vector design and the total vector dose appear to be responsible of immune-mediated toxicities. While preclinical models have been less than ideal in predicting the outcome of gene transfer in humans, the current preclinical body of evidence clearly demonstrates that rAAV vectors can trigger both innate and adaptive immune responses. Data gathered from clinical trials offers key learnings on the immunogenicity of AAV vectors, highlighting challenges as well as the potential strategies that could help unlock the full therapeutic potential of in vivo gene transfer.

Keywords: AAV vectors; B cells; T cells; clinical trials; gene therapy.

FIGURE 1

Immunological barriers to gene transfer. (1) Pre-existing neutralizing antibodies to AAV vectors reduce gene transfer efficacy. (2) Capsids can be recognized by TLR2 at the surface of the cells thus triggering innate immune responses. (3) After endocytosis, the viral genome can stimulate TLR9-mediated innate immunity. (4) Transgene expression may be associated to the development of an immune response that impacts the outcome of the gene therapy. (5) Capsid proteins can be degraded by proteasome and presented on MHC class I. (6) Capsid proteins can be presented on MHC class II. (7) After presentation on MHC class I, capsid-specific cytotoxic CD8⁺ T cells can clear transduced cells. (8) After presentation on MHC class II, anti-capsid humoral response prevents further vector re-administration.

FIGURE 2

Factors influencing AAV capsid immunogenicity. The proteins of the capsid, the genome and the transgene product are the main potential immunogenic components of AAV vectors. Production of dsRNA driven by the promoter activity of ITRs can also trigger innate immunity. Additional host-dependent and vector-dependent factors can modulate the overall vector immunogenicity. These factors are mostly poorly understood, although innate immunity activators like CpG and vector dose appear to be important determinants of AAV vector immunogenicity.

FIGURE 3

Immunomonitoring in gene transfer. A broad range of assays can be implemented for immunomonitoring in gene transfer trials. Serum samples or other relevant samples like cerebrospinal fluid (CSF) can be used to monitor markers of innate immunity as well as to determine antibody titers before and after vector administration. The cell fraction of peripheral blood is frequently used for both B and T cell assays by ELISPOT. More complex technologies can also be useful for example to track T cell clones via TCR sequencing, or to define transcriptome changes at the single cell level. Additionally, high content flow-based assay can be applied for the simultaneous characterization of a large number of surface and intracellular markers. While a lot of information can be gathered by studying immune response to AAV in peripheral blood, access to tissue samples could potentially help better define the nature of the local immune response in a transduced tissue as well as its impact on vector genome persistence. As many questions remain on AAV immunogenicity, the field of AAV gene therapy research needs further efforts to resolve the complexity of capsid-related immune responses. The harmonization of patient immunomonitoring using standard guidelines, and quality controls to check immune assay performance over time and across clinical trials, would greatly facilitate the comparison of data, and subsequently the understanding of the complexity of anti-AAV immune responses.