The likely trigger was immune-mediated liver toxicity—the AAV vector provokes inflammation in the liver. Immunosuppressive treatment did not prevent adverse outcomes. AAV-based gene therapies frequently lead to hepatotoxicity, ranging from mild transaminase increases to rare, fatal liver failure. AAV vectors delivered systemically naturally end up in the liver (hepatocytes are readily transduced). That makes the liver a major site of vector uptake and therefore of potential toxicity and possibly the production of too much product but generally it is regarded as immune reaction to the capsid (innate/complement or T-cell mediated) causing liver injury.