AAV Vector Immunogenicity in Humans: A Long Journey to Successful Gene Transfer

Abstract

Gene therapy with adeno-associated virus (AAV) vectors has demonstrated safety and long-term efficacy in a number of trials across target organs, including eye, liver, skeletal muscle, and the central nervous system. Since the initial evidence that AAV vectors can elicit capsid T cell responses in humans, which can affect the duration of transgene expression, much progress has been made in understanding and modulating AAV vector immunogenicity. It is now well established that exposure to wild-type AAV results in priming of the immune system against the virus, with development of both humoral and T cell immunity. Aside from the neutralizing effect of antibodies, the impact of pre-existing immunity to AAV on gene transfer is still poorly understood. Herein, we review data emerging from clinical trials across a broad range of gene therapy applications. Common features of immune responses to AAV can be found, suggesting, for example, that vector immunogenicity is dose-dependent, and that innate immunity plays an important role in the outcome of gene transfer. A range of host-specific factors are also likely to be important, and a comprehensive understanding of the mechanisms driving AAV vector immunogenicity in humans will be key to unlocking the full potential of in vivo gene therapy.

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

Graphical abstract

Figure 1

Host Immune Responses against AAV Vectors Prior to vector administration, humans are exposed to wild-type AAV and therefore can develop both humoral and T cell-mediated immunity to the vector. Exposure to wild-type AAV can occur years prior to gene transfer, and together with host-specific factors can determine the overall immunological context of AAV vector delivery. Immediately after vector delivery, the vector in its components can trigger innate immune recognition. While no evidence of severe systemic inflammation has been observed in AAV trials immediately after vector delivery, some episodes of pyrexia have been documented, as well as toxicities potentially associated with complement activation. Later after vector administration, anti-capsid antibodies are produced and persist for several years after gene transfer. Capsid T cell activation has also been documented in several trials, in some cases correlating directly with loss of transgene expression. Transgene immune responses are also a potential immune-related risk in gene therapy, although to date they have been documented only in isolated trials.

Figure 2

Factors that Influence AAV Vector Immunogenicity The capsid, its 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 act as a trigger for innate immunity. Additional host-dependent and vector-dependent factors can modulate the overall vector immunogenicity. These factors are mostly poorly understood, although the presence of innate immunity activators such as CpG and vector dose seem to correlate with vector-related immunotoxicities in some trials.

Figure 3

Liver Gene Transfer Can Drive Transgene Immune Tolerance Tolerance to a variety of transgenes expressed in the liver is mediated by a variety of mechanisms. Tregs are a common denominator of liver tolerance, as they mediate the suppression of both humoral and T cell-mediated transgene immune responses. Additional mechanisms include anergy, exhaustion, and deletion of reactive T cells. Key to tolerance induction appears to be a robust transgene expression in hepatocytes. Adapted from Sherman et al.

Figure 4

Humoral Immune Responses to AAV Pre-existing immunity to AAV can block target tissue transduction when the vector is administered systemically directly into the bloodstream. While eradication of humoral immunity with immunosuppressive drugs can be challenging, as pharmacological targeting of B cells has inherent risks, pre-clinical evaluation of physical removal of antibodies with plasmapheresis has shown promising results. Isolation of target organs at the time of vector administration has also been explored. Recent data linked the acute toxicities observed following systemic administration of AAV vectors with complement activations. These toxicities may be mediated by anti-AAV antibodies and can be modulated by drugs targeting the complement activation pathways. Finally, after gene transfer, antibodies to AAV are induced and persist for the long term. Several potential approaches to vector readministration have been explored preclinically, with variable degrees of success.

Figure 5

AAV Vector Administration to the Eye Two main routes of vector administration to the eye have been explored. Subretinal administration has been tested in several trials and has a demonstrated safety and efficacy profile in humans. While the approach appears to be feasible, safe, and associated with a low vector immunogenicity profile, it required a surgical procedure that is relatively invasive. Conversely, intravitreal vector administration requires a simple procedure for vector delivery. This route of administration seems to result in higher vector immunogenicity, resulting in inflammation after vector delivery. The safety and efficacy profile of intravitreal delivery of AAV vectors is being evaluated in several trials.