Viral mimetic triggers haemorrhagic transformation in a childhood stroke model via neutrophil elastase

Childhood arterial ischemic stroke is distinct from adult and neonatal stroke in terms of both incidence and pathophysiology, with its underlying mechanisms yet to be comprehensively understood. Common infections can trigger arterial ischemic stroke in children. Viral infections induce inflammatory effects on arteries, contributing to cerebral arteriopathies. They drive stroke and strongly predict stroke recurrence and poorer outcomes, implicating sensitization of the neurovasculature by viral infection as a predisposing factor in childhood arterial ischemic stroke. Unfortunately, there are currently no available murine models for investigating this phenomenon. To examine the consequences of viral priming for childhood arterial ischemic stroke, we first developed a novel model that simulates virus-induced childhood cerebral arteriopathy by administering viral mimetic/TLR3-agonist polyinosinic:polycytidylic acid (Poly-IC) in juvenile mice and subsequently combined Poly-IC priming with transient middle cerebral artery occlusion three days later. Poly-IC administration rapidly and transiently altered gene expression in brain microvascular cells, priming endothelial and inflammatory responses in juvenile mice before stroke. This priming led to increased neutrophil accumulation in the brain, neutrophil elastase activation, and NETosis in injured regions after stroke, resulting in marked hemorrhagic transformation. Poly-IC priming altered the neutrophil phenotypes and caused long-term changes of vascular networks after stroke in juvenile mice. Inhibiting NE at the time of middle cerebral artery occlusion significantly reduced hemorrhagic transformation and overall injury by attenuating the initiation of the peripheral inflammatory response and limiting neutrophil elastase-mediated NETosis formation in the injured brain. Neutrophil elastase inhibition also altered neutrophil-monocyte interactions in injured juvenile brain. Together, these findings identify neutrophil elastase/NETosis inhibition as a potential strategy to minimize injury induced by TLR3-mediated viral priming in childhood arterial ischemic stroke.