Pre-existing Gut Microbiome Dysbiosis Exacerbates Neuroinflammation and Vasospasm After Subarachnoid Hemorrhage in Mice

BACKGROUND AND OBJECTIVES: Delayed cerebral ischemia remains a major determinant of poor outcomes after aneurysmal subarachnoid hemorrhage (SAH), yet effective preventive strategies are limited. The gut-brain axis has emerged as an important modulator of post-SAH neuroinflammation and vascular dysfunction. We hypothesized that pre-existing gut microbiome dysbiosis (GMD) exacerbates neuroinflammation and vasospasm after SAH. METHODS: Male C57BL/6J mice underwent broad-spectrum antibiotic-induced gut microbiome depletion or control treatment, followed by endovascular perforation SAH or control surgery. Neurological function, body weight, and mortality were assessed longitudinally. Cerebral vasospasm was quantified by anterior cerebral artery morphometry. Endothelial activation and neuroinflammation were evaluated using intercellular adhesion molecule 1 and ionized calcium-binding adapter molecule 1 immunofluorescence. Immune cell populations in the brain and spleen were analyzed by flow cytometry, and serum cytokines were measured by multiplex assays. Gut microbiome composition was assessed using 16S rRNA sequencing in microbiota-intact mice. RESULTS: SAH alone caused minimal early changes in gut microbial diversity or composition, indicating that early post-SAH outcomes were not driven by SAH-induced dysbiosis. By contrast, pre-existing GMD did not affect initial SAH severity but significantly worsened post-SAH outcomes, including weight loss, neurological deficits, and cerebral vasospasm. Vasospasm severity correlated robustly with endothelial intercellular adhesion molecule 1 expression and cortical ionized calcium-binding adapter molecule 1-positive microglia/macrophages. GMD amplified central and peripheral inflammatory responses, characterized by increased CD86-positive macrophages and neutrophils in the brain and splenic macrophage expansion. Systemically, GMD altered cytokine profiles, with elevated CCL5 and reduced granulocyte colony-stimulating factor, and CCL5 levels correlated with both neuroinflammation and vasospasm severity. CONCLUSION: Pre-existing GMD exacerbates neurovascular inflammation, vasospasm, and neurological impairment after SAH through dysregulated central and systemic immune responses. These findings identify the gut-brain axis as a critical modulator of delayed cerebral ischemia-like pathology and suggest microbiome-targeted strategies as potential therapeutic approaches for SAH.