MMP-9–Positive Neutrophil Infiltration Is Associated to Blood–Brain Barrier Breakdown and Basal Lamina Type IV Collagen Degradation During Hemorrhagic Transformation After Human Ischemic Stroke

Abstract

An abnormal expression of some matrix metalloproteinases (MMPs) is related with hemorrhagic transformation events after stroke. Our aim was to investigate MMP-2 and MMP-9 in the ischemic brain and its relation with blood-brain barrier breakdown after hemorrhagic transformation in human stroke. We assessed 5 cases of fatal ischemic strokes with hemorrhagic complications; brain samples were obtained from infarct, hemorrhagic, and contralateral tissue. MMP-9 and MMP-2 content was analyzed by zymography and immunohistochemistry was performed to localize MMP-9 and to assess collagen IV integrity in the basal lamina. Laser capture microdissection was performed to isolate blood-brain barrier vessels to study these MMPs. Overall, MMP-9 levels were higher both in hemorrhagic and nonhemorrhagic infarcted tissue compared to contralateral areas (P<0.0001 and P<0.05). Moreover, levels of the cleaved MMP-9 85kDa-form were significantly elevated in the hemorrhagic compared to nonhemorrhagic and contralateral areas (P=0.033 and P<0.0001). No changes were found for MMP-2 content. Immunostaining revealed a strong MMP-9-positive neutrophil infiltration surrounding brain microvessels associated with severe basal lamina type IV collagen degradation and blood extravasation. Microdissection confirmed that content of MMP-9 was similarly high in microvessel endothelium from hemorrhagic and infarcted areas compared to contralateral hemisphere vessels (P<0.05), pointing to neutrophils surrounding dissected microvessels as the main source of MMP-9 in hemorrhagic areas. Our results show a strong neutrophil infiltration in the infarcted and hemorrhagic areas with local high MMP-9 content closely related to basal lamina collagen IV degradation and blood-brain barrier breakdown. Microvessel and inflammatory MMP-9 response are associated with hemorrhagic complications after stroke.