Chronic antigenic stimulation induced by an inactivated vaccine drives hepatic AA amyloidosis and liver rupture syndrome in laying hens

Recent epidemiological observations suggest a potential link between layer hemorrhagic liver rupture syndrome (LHLRS) and intensive vaccination protocols. However, the causal relationships and underlying mechanisms remain poorly understood. In this study, we developed an accelerated experimental model using an inactivated infectious coryza (IC) vaccine to evaluate the transition from acute hepatic inflammation to terminal AA amyloidosis (AA, amyloid A protein). Our results demonstrate that a single immunization is sufficient to trigger a rapid proinflammatory shift characterized by a burst of cytokines ( IL-6 and TNFα ) and robust serum amyloid A ( SAA ) upregulation. In contrast, repeated immunization (5 doses) drives a cumulative pathological state by preventing inflammation resolution. This sustained proinflammatory microenvironment and SAA overload led to a "tipping point" after the 3rd dose, where AA amyloid fibrils were deposited within hepatic sinusoids, culminating in severe hepatomegaly and fatal subcapsular hemorrhage. Mechanistically, IC vaccine immunization drives SAA overexpression through the TLR4 / NF-κB signaling pathway. Immunofluorescence colocalization confirmed that macrophages actively internalize SAA , a process that leads to fibril assembly under chronic stimulation. These findings suggest that LHLRS is a consequence of persistent inflammatory stress that overwhelms hepatic proteostatic capacity, providing a mechanistic framework for understanding vaccine-associated metabolic disorders in poultry.