Dynamic capillary stalls in reperfused ischemic penumbra contribute to injury: A hyperacute role for neutrophils in persistent traffic jams

Abstract

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 salvageable penumbra after reperfusion. These stalls, which are different from permanent cellular plugs of no-reflow, 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 slowly through capillary lumen or getting stuck, while red blood cell flow was being disturbed in the neighboring segments under reperfused conditions. Stall dynamics could be modulated, by injection of an anti-Ly6G antibody specifically targeting neutrophils. Decreased number and duration of stalls were associated with improvement in penumbral blood flow within 2–24 h after reperfusion along with increased capillary oxygenation, decreased cellular damage and improved functional outcome. Thereby, dynamic microcirculatory stall phenomenon can be a contributing factor to ongoing penumbral injury and is a potential hyperacute mechanism adding on previous observations of detrimental effects of activated neutrophils in ischemic stroke.

Origins of 1/f-like tissue oxygenation fluctuations in the murine cortex

“…The inter-RBC interval correlations show that RBCs tend to cluster together in time(Figure 7b). The rare occurrence of "stall" events in our data is consistent with previous work[92,94,128].…”

“…In addition to stalls in flow caused by vessel occlusion by leukocytes [87,88,92,94,128,129], the very nature of the RBC-plasma suspension will drive low-frequency fluctuations in flow and RBC heterogeneity [133,141]. RBCs have different rheological properties than the plasma [142,143], making the Newtonian descriptions of fluid flow that works for larger vessels inapplicable to the flow through capillaries.…”

“…Stalls, brief stoppages in blood flow through capillaries happen sporadically and continuously in the cortex due to transient blockage of blood flow by leukocytes [87][88][89][90][91][92], which are known to greatly increase vascular resistance [93]. These blockages likely drive changes in tissue oxygenation [94] and increased frequency of these stalls has been linked to neurodegenerative disorders [87,88,94].…”

“…Collectively, our results suggest that processes affect the location and frequency of stalling events and RBC heterogeneity will greatly impact the levels of oxygen in the tissues. Besides the aforementioned leukocytes caused "stall" events[87,88,92,94,128,129], the temporal dynamics of RBCs in the capillary may also reflect general features of the capillary network supply the same brain region[133,141]. It is also possible that the arterial blood pressure fluctuations[147], heart beat dynamics[148], upstream vessel diameter oscillations[44], rheological properties of flowing blood[142,143] and anatomical structures of capillary[149], and the dilation of capillaries and activity of pericytes[144,146,150,151] contribute to the stochastic RBC dynamics.…”

Dynamic light scattering imaging

“…Before the DLSI analysis, the ischemic core was identified on the basis of the OCTA and OCT signal attenuation images [see Fig. 3)] as the area with almost no capillary flow and strong signal attenuation (52,53).…”