Histidine-rich glycoprotein (HRG) circulates in plasma at a concentration of 2M and binds plasminogen, fibrinogen, and thrombospondin. Despite these interactions, the physiologic role of HRG is unknown. Previous studies have shown that mice and humans deficient in HRG have shortened plasma clotting times. To better understand this phenomenon, we examined the effect of HRG on clotting tests. HRG prolongs the activated partial thromboplastin time in a concentrationdependent fashion but has no effect on tissue factor-induced clotting, localizing its effect to the contact pathway. Plasma immunodepleted of HRG exhibits a shortened activated partial thromboplastin time that is restored to baseline with HRG replenishment. To explore how HRG affects the contact pathway, we examined its binding to factors XII, XIIa, XI, and XIa. HRG binds factor XIIa with high affinity, an interaction that is enhanced in the presence of Zn 2؉ , but does not bind factors XII, XI, or XIa. In addition, HRG inhibits autoactivation of factor XII and factor XIIa-mediated activation of factor XI. These results suggest that, by binding to factor XIIa, HRG modulates the intrinsic pathway of coagulation, particularly in the vicinity of a thrombus where platelet release of HRG and Zn 2؉ will promote this interaction.(Blood. 2011;117(15): 4134-4141) IntroductionDespite the capacity of the intrinsic pathway to enhance thrombin generation, patients deficient in factor XII (FXII) do not bleed. 1 Even patients with FXI deficiency rarely have hemorrhagic complications, except with surgery or major trauma. 2 Because of these observations, it is well accepted that the contact system plays little part in hemostasis; and, by extension, it was also thought to be unimportant for thrombosis. However, a number of recent studies challenge this thinking. First, mice with deficiencies of highmolecular-weight kininogen (HK), bradykinin B2 receptor, FXII, or FXI are protected against injury-induced thrombosis, and an antibody against FXI inhibits thrombus formation in a baboon arteriovenous shunt model. [3][4][5][6][7] These observations raise the possibility that the contact pathway contributes to thrombogenesis. 8 Second, potential physiologic activators of FXII have been identified, which could initiate the contact system at sites of vascular injury. In addition to glycosaminoglycans and collagen, novel activators include polyphosphates and RNA. 9,10 Polyphosphates, which are released from the dense granules of platelets activated at sites of injury, trigger coagulation in a FXII-dependent fashion. Likewise, RNA released from the damaged vessel wall also can activate FXII, and RNAase administration to animals attenuates thrombosis at sites of injury, observations that have sparked a renewed interest in the contact pathway. 10 Consequently, it is important to better understand this pathway and how it is regulated.Histidine-rich glycoprotein (HRG) is an abundant plasma protein whose role is largely unknown. 11 HRG circulates at a concentration of approximately 2M. I...