Pre-Existing Anti-Adeno-Associated Virus Immunity in Gene Therapy: Mechanisms, Challenges, and Potential Solutions

Adeno-associated virus (AAV) vectors have emerged as versatile and promising tools in gene therapy due to their favorable safety profile, broad tissue tropism, and long-term gene expression. However, pre-existing immunity, especially in the form of neutralizing antibodies (NAbs) remains a significant barrier, reducing vector efficacy and restricting patient eligibility. This review provides a comprehensive overview of the immunological landscape affecting AAV gene therapy, including global seroprevalence, environmental influences, and antibody cross-reactivity stemming from natural parvovirus exposure or vaccination of animal research models.We detail the mechanisms underlying immune detection and vector clearance, covering innate pattern recognition receptors, complement activation, and adaptive immune effector functions such as antibody-dependent complement deposition, cytotoxicity, and phagocytosis.We further analyze how species, age, serotype, administration route, and target tissue contribute to immune susceptibility and variable transduction outcomes. To overcome these challenges, we propose a three-pronged classification of mitigation strategies: (1) immune-focused strategies, such as plasmapheresis, immunoadsorption, enzymatic antibody cleavage, corticosteroids, and B cell depletion; (2) delivery-focused strategies, which include targeting immune-privileged sites, localized or intrathecal delivery, and timing of vector administration; and (3) capsid-focused strategies, comprising rational capsid engineering and the use of decoy particles or empty capsids.We also discuss promising advances such as AAV-specific regulatory T cells and re-dosable AAV platforms. This strategic framework offers a roadmap for tailoring gene therapy approaches to individual immune profiles and improving the safety, efficacy, and accessibility of AAVbased therapeutics.