Neutrophils induce astrocytic AQP4 expression via IL-1α and TNF, contributing to cerebral oedema in ischaemic stroke rats

During the acute phase (1-3 days) of cerebral infarction, neutrophils and macrophages accumulate at the infarction site, inducing inflammation and cerebral oedema. However, the role of neutrophils in oedema formation after ischaemic stroke remains unclear. This study examined neutrophil involvement in cerebral oedema using a transient middle cerebral artery occlusion (tMCAO) rat model, primary cultured neutrophils, and astrocytes. Brain specimens were stained with myeloperoxidase (MPO) and lymphocyte antigen 6 complexes, locus G (Ly6G), and the number of MPO⁺/Ly6G⁺ cells was counted. Neutrophil infiltration began in the leptomeninges at 3 h, reaching the ischaemic cortex by 6 h and the striatum by 24 h, peaking at 24-48 h before declining. Neutrophils attached to endothelial walls and infiltrated the brain parenchyma, correlating with oedema severity. Infiltrating neutrophils strongly expressed IL-1α and TNF in the ischaemic brain. Co-culturing LPS-activated neutrophils with astrocytes increased Aqp4 mRNA and protein expression, which was inhibited by IL-1RI and TNF antagonists. These findings suggest that activated neutrophils exacerbate cerebral oedema by inducing astrocytic AQP4 expression via IL-1α and TNF in peri-infarct and ischaemic core tissues.