Pre-existing humoral immunity
Many individuals carry antibodies against AAV from prior natural exposure to wild-type virus. Even low levels of these antibodies can substantially reduce vector transduction or modify vector tropism. This is the single largest barrier to broad patient eligibility in clinical trials. Patients are routinely screened for anti-AAV antibodies before enrollment, and seropositivity to the therapeutic serotype typically results in exclusion.
Innate immune responses
AAV vectors carry pathogen-associated molecular patterns (PAMPs) that are recognized by innate immune receptors. The viral capsid can activate Toll-like receptor 2 (TLR2) on the cell surface, while unmethylated CpG motifs in the vector DNA genome are detected by TLR9 in endosomes. Cytosolic DNA sensor cGAS, through its adaptor STING, can detect AAV vector genomes and trigger type I interferon production. Additionally, MDA5 and RIG-I may detect RNA intermediates arising from aberrant transcription. Activation of these pathways triggers the production of pro-inflammatory cytokines and can promote downstream adaptive immune responses. Strategies to reduce innate activation include depleting CpG motifs from the vector genome and incorporating immunosuppressive sequences.
Complement activation
The complement system, a key component of innate immunity, has emerged as a particularly important concern in AAV gene therapy. In some patients receiving high vector doses, complement activation has led to thrombotic microangiopathy (TMA), characterized by hemolytic anemia, low platelet counts, and kidney damage. The leading hypothesis is that AAV particles adhering to cell surfaces activate the classical complement pathway through binding to C1q, ultimately leading to the formation of the membrane attack complex (MAC) that damages endothelial cells. This has been observed in treatments for SMA and Duchenne muscular dystrophy, and in rare cases has resulted in fatalities. Complement inhibitors such as eculizumab (targeting C5) and APL-9 (targeting C3) are being explored as prophylactic strategies. Monitoring and mitigating complement responses are active areas of research and are critical for improving the safety profile of high-dose systemic AAV therapies.
Adaptive T and B cell responses
After AAV vector administration, the adaptive immune system mounts responses against both the capsid and, in some cases, the transgene product. CD8+ T cells can recognize capsid-derived peptides presented on the surface of transduced cells via MHC class I molecules, leading to destruction of those cells and loss of transgene expression. Timely administration of corticosteroids has been effective in controlling these T cell responses in many patients. B cells produce anti-capsid antibodies that prevent successful re-administration of the vector, essentially making AAV gene therapy a one-shot treatment with current technology. Various immunosuppressive strategies, including rapamycin and rituximab, are being investigated to manage adaptive immune responses and potentially enable redosing.
Assessing AAV immunogenicity
Given the central role of immune responses in determining the safety and efficacy of AAV gene therapy, comprehensive immunogenicity testing is an essential part of both preclinical development and clinical monitoring. This includes measuring pre-existing NAb titers, tracking innate immune markers such as complement activation products, and monitoring adaptive responses, including anti-capsid T cells and anti-transgene antibodies. As the field moves toward higher doses and broader disease targets, robust immunogenicity assessment tools become increasingly important for patient stratification and safety management.
How Svar can help
We provide specialized solutions for gene therapy development, including custom potency assays, AAV safety testing, and mechanism-of-action (MoA)-reflective bioassays. Our expertise helps you meet regulatory requirements and accelerate innovation.
From anti-AAV NAb and TAb assays to functional complement activation assessments, biomarker profiling (TCC, C4d, C3a, C5a, and Properdin), and custom cell-based potency assays and services, we provide tailored analytical solutions supported by GMP-compliant CRO services.
