Neutrophil-mediated dynamic capillary stalls in ischemic penumbra: persistent traffic jams after reperfusion contribute to injury

Ever since the introduction of thrombolysis and the subsequent expansion of endovascular treatments for acute ischemic stroke, it remains to be identified why the actual outcomes are less favorable despite recanalization. Here, by high spatio-temporal resolution imaging of capillary circulation in mice, we introduce the pathological phenomenon of dynamic flow stalls in cerebral capillaries, occurring persistently in the salvageable penumbra after recanalization. These stalls, which are distinct from permanent cellular plugs that can lead to no-flow, were temporarily and repetitively occurring in the capillary network, impairing the overall circulation like small focal traffic jams. In vivo microscopy in the ischemic penumbra revealed leukocytes traveling through capillary lumen or getting stuck, while red blood cell flow was being disturbed in the neighboring segments, within 3 hours after stroke onset. Stall dynamics could be modulated, by injection of an anti-Ly6G antibody specifically targeting neutrophils. By decreasing the number and duration of stalls, we were able to improve the blood flow in the penumbra within 2-24 hours after reperfusion, increase capillary oxygenation, decrease cellular damage and improve functional outcome. Thereby the dynamic microcirculatory stall phenomenon contributes to the ongoing penumbral injury and is a potential hyperacute stage mechanism adding on previous observations of detrimental effects of activated neutrophils in ischemic stroke. Significance This work provides in vivo evidence that, even in perfused capillaries, abnormal capillary flow patterns in the form of dynamic stalls can contribute to ongoing tissue injury in the salvageable penumbra in very early hours of cerebral ischemia. These events resembling micro traffic jams in a complex road network, are mediated by passage of neutrophils through the microcirculation and persist despite recanalization of the occluded artery.