Subarachnoid Hemorrhage Induces Gliosis and Increased Expression of the Pro-inflammatory Cytokine High Mobility Group Box 1 Protein

Subarachnoid hemorrhage (SAH) following cerebral aneurysm rupture is associated with high rates of morbidity and mortality. Surviving SAH patients often suffer from neurological impairment, yet little is currently known regarding the influence of subarachnoid blood on brain parenchyma. The objective of the present study was to examine the impact of subarachnoid blood on glial cells using a rabbit SAH model. The astrocyte-specific proteins, glial fibrillary acidic protein (GFAP) and S100B, were upregulated in brainstem from SAH model rabbits, consistent with the development of reactive astrogliosis. In addition to reactive astrogliosis, cytosolic expression of the proinflammatory cytokine, high-mobility group box 1 protein (HMGB1) was increased in brain from SAH animals. We found that greater than 90% of cells expressing cytosolic HMGB1 immunostained positively for Iba1, a specific marker for microglia and macrophages. Further, the number of Iba1-positive cells was similar in brain from control and SAH animals, suggesting the majority of these cells were likely resident microglial cells rather than infiltrating macrophages. These observations demonstrate SAH impacts brain parenchyma by activating astrocytes and microglia, triggering up-regulation of the pro-inflammatory cytokine HMGB1.