Unraveling the Complex Story of Immune Responses to AAV Vectors Trial After Trial

Abstract

Over the past decade, vectors derived from adeno-associated virus (AAV) have established themselves as a powerful tool for in vivo gene transfer, allowing long-lasting and safe transgene expression in a variety of human tissues. Nevertheless, clinical trials demonstrated how B and T cell immune responses directed against the AAV capsid, likely arising after natural infection with wild-type AAV, might potentially impact gene transfer safety and efficacy in patients. Seroprevalence studies have evidenced that most individuals carry anti-AAV neutralizing antibodies that can inhibit recombinant AAV transduction of target cells following in vivo administration of vector particles. Likewise, liver- and muscle-directed clinical trials have shown that capsid-reactive memory CD8⁺ T cells could be reactivated and expanded upon presentation of capsid-derived antigens on transduced cells, potentially leading to loss of transgene expression and immune-mediated toxicities. In celebration of the 25th anniversary of the European Society of Gene and Cell Therapy, this review article summarizes progress made during the past decade in understanding and modulating AAV vector immunogenicity. While the knowledge generated has contributed to yield impressive clinical results, several important questions remain unanswered, making the study of immune responses to AAV a priority for the field of in vivo transfer.

Keywords: AAV vectors; T cells; antibody responses; gene therapy; immune responses.

Figure 1.

Initiation and reactivation of adaptive immune responses to adeno-associated virus (AAV). During natural infection with wild-type (WT) AAV, capsid-specific adaptive immune responses can be triggered, with the development of anti-AAV antibodies and the establishment of a pool of long-lasting capsid-reactive memory B and T lymphocytes. Upon in vivo administration of recombinant AAV (rAAV) vectors, pre-existing anti-AAV antibodies can neutralize vector particles, while memory lymphocytes can be reactivated and expanded, leading to the de novo production of anti-AAV antibodies or, potentially, to the destruction of transduced cells presenting capsid-derived antigens.

Figure 2.

Working model of capsid processing in hepatocyte and presentation to AAV-specific memory CD8⁺ T cells. (a) After administration, rAAV vectors enter hepatocytes via receptor-mediated endocytosis. (b) Following escape from the endosome and uncoating, vector DNA traffics to the nucleus where it drives the expression of the transgene. (c) Capsids are cleaved by the proteasome (or immune-proteasome) into short peptides. (d) Capsid-derived peptides are transported to the endoplasmic reticulum and loaded onto MHC class I molecules. (e) AAV-peptide/MHC complexes are transported to the plasmalemma, where they flag transduced hepatocytes as targets for AAV capsid-specific memory CD8⁺ T cells. (f) AAV-derived epitopes are presented to AAV capsid-specific memory CD8⁺ T cells through interaction between the TCR and the AAV-peptide/MHC complex. (g) Upon antigen recognition, AAV capsid-specific memory CD8⁺ T cells undergo expansion and differentiation into cytotoxic effector cells which can clear transduced hepatocytes through secretion of cytolytic factors or expression of death-inducing ligands.