Background and Purpose-To test the role of fibrinolysis in stroke, we used a mouse model in which preformed 2.5-to 3-m-diameter fibrin microemboli are injected into the cerebral circulation. The microemboli lodge in the downstream precapillary vasculature and are susceptible to fibrinolysis. Methods-We injected various doses of microemboli into the internal carotid artery in mice and characterized their distribution, effects on cerebral blood flow, neurological deficit, infarct area, and spontaneous dissolution. By comparing wild-type and tissue plasminogen activator (tPA) knockout (tPA Ϫ/Ϫ ) mice, we analyzed the role of endogenous tPA in acute thrombotic stroke. Results-Microemboli cause dose-dependent brain injury. Although moderate doses of microemboli are followed by spontaneous reperfusion, they result in reproducible injury. Gene knockout of tPA markedly delays dissolution of cerebral emboli and restoration of blood flow and aggravates ischemic thrombotic infarction in the brain. Conclusions-We describe a microembolic model of stroke, in which degree of injury can be controlled by the dose of microemboli injected. Unlike vessel occlusion models, this model can be modulated to allow spontaneous fibrinolysis. Application to tPA Ϫ/Ϫ mice supports a key role of endogenous tPA in restoring cerebral blood flow and limiting infarct size after thrombosis.

Plasminogen activators (PAs; e.g., tissue-type, tPA) coupled to red blood cells (RBCs) display in vivo features useful for thromboprophylaxis: prolonged circulation, minimal extravasation, and preferential lysis of nascent versus preexisting clots. Yet, factors controlling the activity of RBC-bound PAs in vivo are not defined and may not mirror the profile of soluble PAs. We tested the role of RBC/PA binding to fibrin in fibrinolysis. RBC/ tPA and RBC/tPA variant with low fibrin affinity (rPA) bound to and lysed plasminogen-containing fibrin clots in vitro comparably. In contrast, when coinjected in mice with fibrin emboli lodging in pulmonary vasculature, only RBC/tPA accumulated in lungs, which resulted in a more extensive fibrinolysis versus RBC/rPA (p Ͻ 0.01). Reconciling this apparent divergence between in vitro and in vivo behaviors, RBC/tPA, but not RBC/rPA perfused over fibrin in vitro at physiological shear stress bound to fibrin clots and caused greater fibrinolysis versus RBC/rPA (p Ͻ 0.001). These results indicate that because of high fibrin affinity, RBC/tPA binding to clots endures hemodynamic stress, which enhances fibrinolysis. Behavior of RBC/PAs under hemodynamic pressure is an important predictor of their performance in vivo.