title: Activated human platelets induce factor XIIa-mediated contact activation.
abstract: Earlier studies have shown that isolated platelets in buffer systems can promote activation of FXII or amplify contact activation, in the presence of a negatively charge substance or material. Still proof is lacking that FXII is activated by platelets in a more physiological environment. In this study we investigate if activated platelets can induce FXII-mediated contact activation and whether this activation affects clot formation in human blood. Human platelets were activated with a thrombin receptor-activating peptide, SFLLRN-amide, in platelet-rich plasma or in whole blood. FXIIa and FXIa in complex with preferentially antithrombin (AT) and to some extent C1-inhibitor (C1INH) were generated in response to TRAP stimulation. This contact activation was independent of surface-mediated contact activation, tissue factor pathway or thrombin. In clotting whole blood FXIIa-AT and FXIa-AT complexes were specifically formed, demonstrating that AT is a potent inhibitor of FXIIa and FXIa generated by platelet activation. Contact activation proteins were analyzed by flow cytometry and FXII, FXI, high-molecular weight kininogen, and prekallikrein were detected on activated platelets. Using chromogenic assays, enzymatic activity of platelet-associated FXIIa, FXIa, and kallikrein were demonstrated. Inhibition of FXIIa in non-anticoagulated blood also prolonged the clotting time. We conclude that platelet activation triggers FXII-mediated contact activation on the surface and in the vicinity of activated platelets. This leads specifically to generation of FXIIa-AT and FXIa-AT complexes, and contributes to clot formation. Activated platelets may thereby constitute an intravascular locus for contact activation, which may explain the recently reported importance of FXII in thrombus formation.
authors.affiliation: Departments of Oncology, Radiology, and Clinical Immunology, Division of Clinical Immunology, Rudbeck Laboratory C5, Uppsala University, SE-751 85 Uppsala, Sweden.
authors.forename: Jennie
authors.initials: J
authors.lastname: Bäck
authors.forename: Javier
authors.initials: J
authors.lastname: Sanchez
authors.forename: Graciela
authors.initials: G
authors.lastname: Elgue
authors.forename: Kristina Nilsson
authors.initials: KN
authors.lastname: Ekdahl
authors.forename: Bo
authors.initials: B
authors.lastname: Nilsson
mesh_terms: D000991:Antithrombins / Q000378:metabolism; D001777:Blood Coagulation; D001792:Blood Platelets / Q000201:enzymology / Q000502:physiology*; D002450:Cell Communication; D015956:Factor XIIa / Q000378:metabolism*; D015951:Factor Xa / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D007610:Kallikreins / Q000378:metabolism; D015539:Platelet Activation; D053657:Platelet-Rich Plasma / Q000201:enzymology / Q000502:physiology*; D015843:Serpins / Q000378:metabolism*
chemical_list: D000991:Antithrombins; D015843:Serpins; C088216:factor rXai; D007610:Kallikreins; D015956:Factor XIIa; D015951:Factor Xa