Neutrophils: Novel Contributors to Estrogen‐Dependent Intracranial Aneurysm Rupture Via Neutrophil Extracellular Traps

Background Intracranial aneurysms (IAs) are more prevalent in women than men, and aneurysmal subarachnoid hemorrhage disproportionately affects postmenopausal women. These sex differences suggest estrogen protects against IA progression that can lead to rupture, but the underlying mechanisms are not fully understood. Although studies have demonstrated estrogen regulates inflammatory processes that contribute to IA pathogenesis, the role of neutrophils remains to be characterized. Using a murine model, we tested our hypothesis that neutrophils contribute to IA pathophysiology in an estrogen‐dependent manner. Methods and Results We compared neutrophil infiltration in C57BL/6 female mice that develop IAs to those with a normal circle of Willis. Next, we investigated the estrogen‐dependent role of neutrophils in IA formation, rupture, and symptom‐free survival using a neutrophil depletion antibody. Finally, we studied the role of neutrophil extracellular trap formation (NETosis) as an underlying mechanism of aneurysm progression. Mice that developed aneurysms had increased neutrophil infiltration compared with those with a normal circle of Willis. In estrogen‐deficient female mice, both neutrophil depletion and NETosis inhibition decreased aneurysm rupture. In estrogen‐deficient female mice treated with estrogen rescue and estrogen‐intact female mice, neither neutrophil depletion nor NETosis inhibition affected IA formation, rupture, or symptom‐free survival. Conclusions Neutrophils contribute to aneurysm rupture in an estrogen‐dependent manner. NETosis appears to be an underlying mechanism for neutrophil‐mediated IA rupture in estrogen deficiency. Targeting NETosis may lead to the development of novel therapeutics to protect against IA rupture in the setting of estrogen deficiency.