The Role of Neutrophils in Ischemia/Reperfusion Injury following Acute Stroke

Inflammation is the body’s response to tissue damage, including brain tissue after stroke. Stroke is one of the leading causes of death and disability, affecting more that 795,000 Americans per year. A majority of strokes are ischemic strokes, where blood flow to the brain is obstructed. Most therapeutic interventions restore blood flow, but these therapies have a limited time frame in which they are effective. Moreover, some patients do not improve even with blood flow restoration. One likely explanation for the limited therapeutic benefit of blood flow restoration after stroke is the secondary damage caused by the acute inflammatory response. This is the so-called, “ischemia/reperfusion (I/R) injury.” Therefore, further understanding and characterization of the inflammatory response to stroke is critical to the development of new therapeutic interventions. Following an ischemic stroke, brain blood vessels respond to inflammatory signals and recruit leukocytes to the area of damage. Neutrophils (PMN) are the earliest responders to tissue damage in the central nervous system (CNS). Like other leukocytes, PMN interact with adhesion molecules on the endothelial cell surface and undergo transendothelial migration (TEM), squeezing between endothelial cells and migrating into the tissue bed. TEM is important because it is essentially irreversible, committing the cell to extravasation. Our research shows there is a large degree of spatiotemporal heterogeneity in PMN infiltration and TEM across the ischemic core and penumbra of a stroke. Therefore, PMN may have different roles in ischemia/ reperfusion injury, depending on the time after reperfusion and their location. We seek to understand the mechanisms controlling the PMN extravasation following ischemia and reperfusion (I/R) and the effect of interference with PMN extravasation on stroke outcomes. To understand the mechanisms controlling the PMN extravasation following I/R, our first aim will identify how inhibition of PMN extravasation during I/R injury in acute stroke affects the distribution of leukocytes. Our studies will not only identify potential differences between endothelial cells across ischemic brain regions and time, but also differences in leukocytes. Our second aim will determine the therapeutic effect of selectively blocking PMN extravasation following I/R. Our studies will be conducted at different time points after reperfusion, identifying the effect of inhibition on brain pathology and mouse behavior. PMN extravasation will be inhibited through two methods: use of anti-PECAM and the selective depletion of PMN. Completion of these studies will provide insight into the mechanisms regulating PMN response to I/R injury and potentially identify a therapeutic intervention that can be used at the relevant time frame.