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. Pre-existing Immunity to WT AAV in Humans Pre-existing Humoral ImmunitySeveral studies have investigated the seroprevalence of neutralizing antibodies directed against WT AAV in humans. [20][21][22][23][24][25] Seroprevalence varies geographically, with anti-AAV2 neutralizing antibodies displaying the highest prevalence, ranging from 30% to 60% of the population. Due to the broad cross-reactivity between AAV serotypes, 26 neutralizing antibodies recognizing virtually all serotypes can be found in almost all subjects. 27 This cross-reactivity reflects the amino acid sequence and structural homology across capsids of different AAV serotypes. 28 The prevalence of total anti-AAV antibodies is close to 70% of the population for AAV1 and AAV2, 45% for AAV6 and AAV9, and 38% for AAV8. 20 Importantly, titers of anti-AAV immunoglobulin G (IgG) antibodies correlate significantly with titers of

Engineering and In Vitro Selection of a Novel AAV3B Variant with High Hepatocyte Tropism and Reduced Seroreactivity

“… 25 , 48 , 49 , 50 , 51 Elimination of phosphorylation sites therefore enhances transfer to the nucleus and reduces presentation of peptides derived from proteasomal degradation, therefore reducing the likelihood of being targeted by capsid-specific CD8 + T cells. 9 , 50 , 52 , 53 When attempting to optimize the AAV3B capsid, elimination of a serine and a threonine site (S663V+T492V mutations) was found to be more efficacious 33 , 43 , 44 for transduction of human and non-human primate hepatocytes. In this study, we diversified the T 492 position to include either of the following positions: T/I/A/V, while leaving S 663 as in the WT AAV3B.…”

“…The first major limitation to systemic or intramuscular administration of AAV is the presence of pre-existing neutralizing antibodies (NAbs) against the vector capsid that can block cellular entry. 9 Most of the human population is seropositive for AAV, mostly due to previous subclinical exposure to the WT virus. 10 This is a major exclusion criterion for prospective patients, as even very low titers of NAbs in circulation can prevent vector entry and significantly limit effective gene transfer to the target organ.…”

Gene therapy in Duchenne muscular dystrophy: Identifying and preparing for the challenges ahead

“…This 'one-off' nature of gene therapy has ethical issues for the running of trials where a dose-escalation arm is included and a patient may receive a lower than efficacious dose and subsequently be unable to receive a repeat, higher dose that proved to be effective. Possibilities of re-dosing would need to be explored 6 .…”

Translating CRISPR-Cas Therapeutics: Approaches and Challenges

“…Liver gene therapy clinical trials have been tested for a small number of diseases, with hemophilia B studies taking center stage. 14 Ultimately, hemophilia patients need to maintain at least 1% plasma circulation for effective hemostasis in their lifetime. AAV-mediated transfer of factor IX (FIX) or factor VIII (FIIIV) in liver, especially in children, will result in dilution of the transgene as the hepatocytes divide.…”

“…selection of appropriate serotypes. 10 Efforts have been made to enhance the safety profile of AAVs by understanding how the viruses interact with and evade host immunity [11][12][13][14] and engineering new capsids to mitigate the immune response. 15 Although AAV capacity limits the size of packaged constructs to about 5 kb, recent studies have uncovered Cas proteins that are considerably smaller than Streptococcus pyogenes Cas9, some of which show functional promise.…”