title: Extracellular histones promote thrombin generation through platelet-dependent mechanisms: involvement of platelet TLR2 and TLR4.
abstract: The release of histones from dying cells is associated with microvascular thrombosis and, because histones activate platelets, this could represent a possible pathogenic mechanism. In the present study, we assessed the influence of histones on the procoagulant potential of human platelets in platelet-rich plasma (PRP) and in purified systems. Histones dose-dependently enhanced thrombin generation in PRP in the absence of any trigger, as evaluated by calibrated automated thrombinography regardless of whether the contact phase was inhibited. Activation of coagulation required the presence of fully activatable platelets and was not ascribable to platelet tissue factor, whereas targeting polyphosphate with phosphatase reduced thrombin generation even when factor XII (FXII) was blocked or absent. In the presence of histones, purified polyphosphate was able to induce thrombin generation in plasma independently of FXII. In purified systems, histones induced platelet aggregation; P-selectin, phosphatidylserine, and FV/Va expression; and prothrombinase activity. Blocking platelet TLR2 and TLR4 with mAbs reduced the percentage of activated platelets and lowered the amount of thrombin generated in PRP. These data show that histone-activated platelets possess a procoagulant phenotype that drives plasma thrombin generation and suggest that TLR2 and TLR4 mediate the activation process.
authors.affiliation: Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, Oklahoma City, OK 73104, USA.
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Semeraro
authors.forename: Concetta T
authors.initials: CT
authors.lastname: Ammollo
authors.forename: James H
authors.initials: JH
authors.lastname: Morrissey
authors.forename: George L
authors.initials: GL
authors.lastname: Dale
authors.forename: Paul
authors.initials: P
authors.lastname: Friese
authors.forename: Naomi L
authors.initials: NL
authors.lastname: Esmon
authors.forename: Charles T
authors.initials: CT
authors.lastname: Esmon
mesh_terms: D000925:Anticoagulants / Q000494:pharmacology; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism*; D004247:DNA / Q000378:metabolism; D006493:Heparin / Q000494:pharmacology; D006657:Histones / Q000378:metabolism*; D006801:Humans; D015539:Platelet Activation / Q000187:drug effects; D053657:Platelet-Rich Plasma / Q000378:metabolism; D013917:Thrombin / Q000378:metabolism*; D051195:Toll-Like Receptor 2 / Q000378:metabolism*; D051197:Toll-Like Receptor 4 / Q000378:metabolism*
chemical_list: D000925:Anticoagulants; D006657:Histones; C495342:TLR2 protein, human; C495345:TLR4 protein, human; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D006493:Heparin; D004247:DNA; D013917:Thrombin