Leung PY, Hurst S, Berny-Lang MA, Verbout NG, Gailani D, Tucker EI, Wang RK, McCarty OJT, Gruber A. Translational Stroke Research 2012. Epub 2012 May 3. Blood coagulation factor XI (FXI) is an established risk factor for acute ischemic stroke (AIS) and thrombosis, but is also needed for normal hemostasis. Contact factor XII (FXII), an upstream activator of FXI, also contributes to experimental stroke, but is not required for hemostasis. We investigated whether selectively inhibiting FXII-mediated FXI activation, while leaving other FXI and FXII functions intact, could improve the outcome of experimental AIS in mice. Twenty-four hours before induction of AIS by placement of a filament into the internal carotid artery for 60 min, mice were anticoagulated with an antibody directed against the apple 2 domain of FXI. This antibody selectively reduces the prothrombotic activation of FXI by FXIIa but does not affect activated FXI or hemostatic activation of FXI by thrombin, thus leaving hemostasis intact in mice and primates. In this model of AIS/reperfusion injury, mice that received the antibody before AIS displayed less ischemic damage, manifested as reduced cerebral infarction and fibrin deposition (thrombosis), increased cortical reperfusion, and improved neurological behavior. Further, the antibody-anticoagulated mice had no detectable hemostasis impairment. Consistent with the neuroprotective phenotype of FXII-deficient mice, our data suggest that a single molecular event, FXII-mediated FXI activation, contributes to the development of experimental AIS.