Pre-existing Anti-AAV9 antibodies in the Chinese healthy and rare disease populations: Implications for gene therapy

• The seroprevalence of anti-AAV9 NAbs in the Chinese population was 58.7 % (0 y-90 y). • The seroprevalence in adults was higher than in children (75.0 % vs 34.3 %). • The antibody titers (NAbs or TAbs) showed no strong correlation with age. • Both the positivity rate and titers showed a strong correlation between TAb and Nab. • Age between 6 m and 3 y is potentially the optimal age range for gene therapy. The adeno-associated virus 9 (AAV9) vector was particularly notable for its broad tissue tropism, making it a preferred vector for gene therapy. Goals: The study aimed to investigate the patterns of pre-existing immunity against AAV9 in the Chinese population. In this study, we conducted a serological research from November 2022 to June 2024. The study included 341 participants in total with age ranged from 0 to 90 years old: 270 healthy individuals, 30 pediatric patients and 41 adults with rare diseases. Total AAV9-binding antibodies (TAbs) and neutralizing antibodies (NAbs) were measured. The seroprevalence of anti-AAV9 NAbs showed no significant differences between healthy individuals and rare disease patients across both pediatric and adult groups. Newborns exhibited a high NAb-positive rate (64.3 %), while children aged 6 months to 3 years had the lowest prevalence (7.7 %). This rate progressively increased through childhood and adolescence. Overall, 58.7 % of the Chinese population aged 0–90 years tested positive for anti-AAV9 NAbs, with adults showing a significantly higher prevalence than children (75.0 % vs. 34.3 %). Additionally, 58.1 % of the population exhibited low levels of anti-AAV9 NAb titers (IC 50 ≤ 100). No significant sex-specific differences were observed, and antibody titers (NAbs or TAbs) showed no strong correlation with age. A strong correlation was identified between TAb and NAb positivity rates and titers. The optimal AAV9-based GT period was between 6 months and 3 years in that patients possessed lowest pre-existing immunity. Since TAbs had a strong association with NAbs, TAbs was considered as an alternative indicator to screen rare diseases.