Adeno-associated virus (AAV)-mediated gene therapy may provide durable protection from bleeding events and reduce treatment burden for people with hemophilia A (HA). However, pre-existing immunity against AAV may limit transduction efficiency and hence treatment success. Global data on the prevalence of AAV serotypes are limited. In this global, prospective, noninterventional study, we determined the prevalence of pre-existing immunity against AAV2, AAV5, AAV6, AAV8, and AAVrh10 among people ≥12 years of age with HA and residual FVIII levels ≤2 IU/dL. Antibodies against each serotype were detected using validated, electrochemiluminescent-based enzyme-linked immunosorbent assays. To evaluate changes in antibody titers over time, 20% of participants were retested at 3 and 6 months. In total, 546 participants with HA were enrolled at 19 sites in 9 countries. Mean (standard deviation) age at enrollment was 36.0 (14.87) years, including 12.5% younger than 18 years, and 20.0% 50 years of age and older. On day 1, global seroprevalence was 58.5% for AAV2, 34.8% for AAV5, 48.7% for AAV6, 45.6% for AAV8, and 46.0% for AAVrh10. Considerable geographic variability was observed in the prevalence of pre-existing antibodies against each serotype, but AAV5 consistently had the lowest seroprevalence across the countries studied. AAV5 seropositivity rates were 51.8% in South Africa ( n = 56), 46.2% in Russia ( n = 91), 40% in Italy ( n = 20), 37.2% in France ( n = 86), 26.8% in the United States ( n = 71), 26.9% in Brazil ( n = 26), 28.1% in Germany ( n = 89), 29.8% in Japan ( n = 84), and 5.9% in the United Kingdom ( n = 17). For all serotypes, seropositivity tended to increase with age. Serostatus and antibody titer were generally stable over the 6-month sampling period. As clinical trials of AAV-mediated gene therapies progress, data on the natural prevalence of antibodies against various AAV serotypes may become increasingly important.

Summary.Prior treatment (priming) with a weakly immunogenic dose of TVpe 111 pneumococcal polysaccharide (SIII) results in the development of an antigen-specific state of unresponsiveness termed low-dose paralysis which is believed to be mediated by suppressor T cells. The present findings show that the passive administration of functionally distinct non-complement-fixing (NCF) IgM anti-SIII antibodies either in monoclonal form or from protein-A absorbed immune serum could significantly suppress the direct plaque-forming cell (PFC) response to an immunogenic dose of SIII administered concurrently. The degree of suppression was comparable with that induced by low-dose paralysis. Low-dose paralysis was consistently induced in athymic (nude) mice 4 days, but not 3 days, after priming with a low dose of SIII, and was associated with the delayed appearance of NCF-IgM anti-SIII in the serum of athymic mice. In contrast, low-dose paralysis was readily induced in normal BALB/c mice 3 days after priming when NCF-lgM anti-SlU antibodies were present. Comparable inhibiton of the direct anti-SIH PFC response was observed when Concanavalin A (Con A) or NCF-lgM anti-Sill serum was administered with Sill antigen. That Con A and NCF-lgM anti-SIII together did not produce additive suppression was attributed to the adsorption of NCF-lgM anti-SIII antibodies to Con A. Complement-dependent single radial haemolysis mediated by CF hybrid igM/A or CF-IgM anti-SIII serum was blocked by monoclonal NCF-lgM or IgA anti-SIII antibodies and indicated that each of the antibodies was specific for the same Slll-determinant. Evidence is presented to show that low-dose paralysis in the CF-IgM response to Sill is not mediated by suppressor T cells but can be attributed to highly avid NCF-lgM anti-SIII antibodies, formed preferentially to low doses of SIII, being able to reduce the immunogenicity of SIII administered subsequently.We propose that low-dose paralysis to Sill is the result of an immunobiological function of highly avid NCF-lgM anti-SIII antibodies which not only confer resistance against capsulated pneumococci but preferentially bind soluble Slll-antigen to reduce its immunogenicity and thereby protect specific CF-lgM positive B cells from being rendered tolerant by direct contact with higher doses of Sill antigen.