Mice lacking factor XII (fXII) or factor XI (fXI) are resistant to experimentallyinduced thrombosis, suggesting fXIIa activation of fXI contributes to thrombus formation in vivo. It is not clear whether this reaction has relevance for thrombosis in primates. In 2 carotid artery injury models (FeCl 3 and Rose Bengal/laser), fXII-deficient mice are more resistant to thrombosis than fXI-or factor IX (fIX)-deficient mice, raising the possibility that fXII and fXI function in distinct pathways. Antibody 14E11 binds fXI from a variety of mammals and interferes with fXI activation by fXIIa in vitro. In mice, 14E11 prevented arterial occlusion induced by FeCl 3 to a similar degree to total fXI deficiency. 14E11 also had a modest beneficial effect in a tissue factor-induced pulmonary embolism model, indicating fXI and fXII contribute to thrombus formation even when factor VIIa/tissue factor initiates thrombosis. In baboons, 14E11 reduced plateletrich thrombus growth in collagen-coated grafts inserted into an arteriovenous shunt. These data support the hypothesis that fXIIa-mediated fXI activation contributes to thrombus formation in rodents and primates. Since fXII deficiency does not impair hemostasis, targeted inhibition of fXI activation by fXIIa may be a useful antithrombotic strategy associated with a low risk of bleeding complications. (Blood. 2010;116(19):3981-3989) IntroductionInitiation of fibrin formation by contact activation requires proteolytic conversion of plasma factor XII (fXII) to the protease factor XIIa (fXIIa) on a surface. 1-3 FXIIa activates the next zymogen in the coagulation cascade, factor XI (fXI), to factor XIa (fXIa), which in turn converts factor IX (fIX) to factor IXa␤ (fIXa␤). This series of reactions, referred to as the intrinsic pathway of coagulation, drives thrombin generation and fibrin formation in the activated partial thromboplastin time (aPTT) assay used by clinical laboratories. A role for fIX in hemostasis is not in question, as its deficiency causes the severe bleeding disorder hemophilia B. However, the importance of the intrinsic pathway, as a whole, to clot formation and stability at a site of injury is probably limited, as fXII deficiency is not associated with abnormal bleeding, 1,2 and fXI-deficient patients have a variable hemorrhagic disorder with milder symptoms than hemophiliacs. 2,4 Current models of thrombin generation address these phenotypic differences by incorporating additional mechanisms for protease activation. Thus, fIX is activated by the factor VIIa/tissue factor complex in addition to fXIa, 3,5 while fXI can be activated by thrombin. 3,6 Mice lacking fXII, like their human counterparts, do not have a demonstrable bleeding abnormality, 7 supporting the premise that fXIIa activation of fXI is not required for hemostasis. 8 Given this, it was surprising to observe that mice lacking fXII 9 or fXI 10 were resistant to arterial thrombotic occlusion. While this suggested contact activation might play an important role in pathologic coagulation, if not hemostasis...

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Summary Background Laminin is the most abundant non-collagenous protein in the basement membrane. Recent studies have shown that laminin supports platelet adhesion, activation and aggregation under flow conditions, highlighting a possible role for laminin in hemostasis. Objective To investigate the ability of laminin to promote coagulation and support thrombus formation under shear. Results and methods Soluble laminin accelerated factor (F) XII activation in a purified system, and shortened the clotting time of recalcified plasma in a FXI- and FXII-dependent manner. Laminin promoted phosphatidylserine exposure on platelets and supported platelet adhesion and fibrin formation in recalcified blood under shear flow conditions. Fibrin formation in laminin-coated capillaries was abrogated by an antibody that interferes with FXI activation by activated FXII, or an antibody that blocks activated FXI activation of FIX. Conclusion This study identifies a role for laminin in the initiation of coagulation and the formation of platelet-rich thrombi under shear conditions in a FXII-dependent manner.

Objective-Factor XI (FXI) promotes hemostasis and thrombosis through enhancement of thrombin generation and has been shown to play a critical role in the formation of occlusive thrombi in arterial injury models. The aim of this study was to investigate the mechanisms governing interactions between FXI and platelets. 2 Inherited FXI deficiency causes a mild bleeding diathesis and is protective against ischemic stroke, 3,4 whereas an elevated FXI plasma level is an independent risk factor for thrombotic diseases such as deep vein thrombosis. 5 Consistent with these observations, FXI plays a critical role in experimental thrombus growth in rabbits, mice, and primates. 6 -10 See accompanying article on page 1409 FXI circulates as a disulfide-linked homodimer in complex with plasma high molecular weight kininogen (HK). FXI shares high sequence homology (58% amino acid identity) with the functionally-distinct plasma protein prekallikrein, which also circulates in complex with HK. 11,12 Although the serine protease domain of each FXI subunit is similar to catalytic domains for other coagulation proteases, the noncatalytic portion contains 4 apple domains (A1 to A4), a feature shared only with prekallikrein. 12,13 The FXI A3 domain has been shown to contain binding sites for FIX and for the platelet receptor glycoprotein Ib-IX-V (GPIb). 14,15 FXI binding has been localized to the leucine-rich repeat (LRR) sequences on the NH 2 -terminal globular domain of GPIb␣, at a site distinct from the anionic thrombin-binding domain of GPIb␣. 16,17 It is unknown whether FXI-platelet binding is solely mediated by GPIb␣ or whether other platelet receptor(s) exist that can support interactions with FXI.GPIb␣ has been shown to form a complex on the platelet surface with apolipoprotein E receptor 2 (ApoER2, LRP8), 18 -20 a member of the low-density lipoprotein (LDL) family of receptors. ApoER2 initiates intracellular signaling through the adaptor protein disabled-1 (Dab-1) in platelets. 21 The extracellular domain of ApoER2 consists of 3 regions: (1) the type A-binding repeats of LDL-binding domains displaying a negatively-charged surface, which are responsible for receptor-ligand interactions; (2) type B repeats, which are homologous to regions in the epidermal growth factor precursor; and (3) the protein stack of O-linked sugar domains that separate the LDL-binding domains from the cellular surface. We have recently shown that platelet and leukocyte ApoER2 mediate interactions with the anticoagulant serine protease, activated protein C. 22,23 Here, we present the first evidence that identifies FXI as a ligand for ApoER2.