Prevalence study of cellular capsid-specific immune responses to AAV2, 4, 5, 8, 9 and rh10 in healthy donors

Recombinant adeno-associated virus (rAAV) vectors appear, more than ever, to be efficient viral vectors for in vivo gene transfer as illustrated by the approvals of 7 drugs across Europe and the USA. Nevertheless, pre-existing immunity to AAV capsid in humans remains one of the major limits for a successful clinical translation. Whereas pre-existing humoral response to AAV capsid is well documented, the prevalence of pre-existing capsid-specific T cell responses still needs to be studied and characterized. Here, we investigated the prevalence of AAV-specific circulating T cells towards AAV2, 4, 5, 8, 9 and rh10 in a large cohort of healthy donors using the standard IFNγ ELISpot assay. We observed the highest prevalence of pre-existing cellular immunity to AAV9 serotype followed by AAV8, AAV4, AAV2, AAVrh10 and AAV5 independently of the donors' serological status. An in-depth analysis of T cell responses towards the 2 most prevalent serotypes 8 and 9 shows that IFNγ secretion is mainly mediated by CD8 T cells for both serotypes. A polyfunctional analysis reveals different cytokine profiles between AAV8 and AAV9. Surprisingly, no IL-2 secretion was mediated by anti-AAV9 immune cells suggesting that these cells may rather be exhausted or terminally differentiated than cytotoxic T cells. Altogether, these results suggest that pre-existing immunity to AAV may vary depending on the serotype and support the necessity of using multiparametric monitoring methods to better characterize anti-capsid cellular immunity and foresee its impact in rAAV-mediated clinical trials.