Actions of thrombin in the interstitium

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.

Activation of neutrophils by microbial or inflammatory stimuli results in the release of neutrophil extracellular traps (NETs) that are composed of DNA, histones, and antimicrobial proteins. In purified systems, cell-free DNA (CFDNA) activates the intrinsic pathway of coagulation, whereas histones promote thrombin generation through platelet-dependent mechanisms. However, the overall procoagulant effects of CFDNA/histone complexes as part of intact NETs are unknown. In this study, we examined the procoagulant potential of intact NETs released from activated neutrophils. We also determined the relative contribution of CFDNA and histones to thrombin generation in plasmas from patients with sepsis.

Animal and human studies support the importance of the coagulation cascade in pulmonary fibrosis. In a cohort of subjects with progressive idiopathic pulmonary fibrosis (IPF), we tested the hypothesis that treatment with warfarin at recognized therapeutic doses would reduce rates of mortality, hospitalization, and declines in FVC. This was a double-blind, randomized, placebo-controlled trial of warfarin targeting an international normalized ratio of 2.0 to 3.0 in patients with IPF. Subjects were randomized in a 1:1 ratio to warfarin or matching placebo for a planned treatment period of 48 weeks. International normalized ratios were monitored using encrypted home point-of-care devices that allowed blinding of study therapy. The primary outcome measure was the composite outcome of time to death, hospitalization (nonbleeding, nonelective), or a 10% or greater absolute decline in FVC. Due to a low probability of benefit and an increase in mortality observed in the subjects randomized to warfarin (14 warfarin versus 3 placebo deaths; P = 0.005) an independent Data and Safety Monitoring Board recommended stopping the study after 145 of the planned 256 subjects were enrolled (72 warfarin, 73 placebo). The mean follow-up was 28 weeks. This study did not show a benefit for warfarin in the treatment of patients with progressive IPF. Treatment with warfarin was associated with an increased risk of mortality in an IPF population who lacked other indications for anticoagulation.