Toll-Like Receptor 4 Mediates Hemorrhagic Transformation After Delayed Tissue Plasminogen Activator Administration in In Situ Thromboembolic Stroke

Abstract

Background and Purpose— Hemorrhagic transformation is the main complication of revascularization therapies after stroke. Toll-like receptor 4 (TLR4) is implicated in cerebral damage and inflammation in stroke. This study was designed to determine the role of TLR4 in hemorrhagic transformation development after tissue plasminogen activator (tPA) administration. Methods— Mice expressing (TLR4 +/+ ) or lacking functional TLR4 (TLR4 − /− ) were subjected to middle cerebral artery occlusion using an in situ thromboembolic model by thrombin injection into the middle cerebral artery, and tPA (10 mg/kg) was administered 20 minutes or 3 hours after ischemia. Infarct size, hemorrhages, IgG extravasation, matrix metalloproteinase 9 expression, and neutrophil infiltration were assessed 24 hours after ischemia. Results— In TLR4 +/+ , early reperfusion (tPA at 20 minutes) resulted infarct volume, whereas late recanalization (tPA at 3 hours) did not modify lesion size and increased the rate of the most severe hemorrhages. In TLR4 − /− mice, both early and late reperfusion did not modify lesion size. Importantly, late tPA administration did not result in worse hemorrhages and in an increased bleeding area as occurred in TLR4 +/+ group. In TLR4 − /− animals, late reperfusion produced a lesser increase in matrix metalloproteinase 9 expression when compared with TLR4 +/+ animals. Conclusions— Our results demonstrate TLR4 involvement in hemorrhagic transformation induced by delayed tPA administration, very likely by increasing matrix metalloproteinase 9 expression.