Figure 1. AAV vectors-triggered immunological liver injury and transgene loss. Hepatocyte injury mediated by CD8+ cytotoxic T lymphocytes (CTLs) constitutes the central mechanism underlying hepatotoxicity and the loss of coagulation factor expression. Activated AAV capsid-specific CTLs migrate to the liver, where they recognize and attack transduced hepatocytes presenting the corresponding peptide–MHC-I complexes. The attack mechanisms include the release of perforin and granzymes to directly induce hepatocyte apoptosis, as well as Fas/FasL pathway-mediated cell death. This specific killing leads to the elimination of hepatocytes containing the transgene genome, thereby resulting in elevated liver transaminases and decreased coagulation factor expression levels. Furthermore, the innate immune response is the earliest defensive reaction triggered upon exposure to AAV vectors. When AAV vectors are administered intravenously, pathogen-associated molecular patterns (PAMPs) such as viral capsid proteins and unmethylated CpG oligodeoxynucleotides within the vector genome are recognized by Toll-like receptors on the surface of hepatocytes, liver sinusoidal endothelial cells, and resident immune cells in the liver, such as Kupffer cells. This recognition activates downstream signaling pathways, leading to the activation of nuclear factor kappa B (NF-κB) and interferon regulatory factors (IRFs), which in turn promotes the release of type I interferons (IFN-α/β) and various inflammatory cytokines, such as IL-6 and TNF-α. Additionally, AAV capsid proteins, vector dosage and genome design, and pre-existing immunity are all important triggers of immune responses.