Histones substitute FVa in an alternative prothrombinase that generates thrombin without phospholipids to propagate DIC.
Histones significantly reduce FXa requirement for initiation of thrombin generation to enable coagulation in hemophilia plasma.
Thrombin generation is pivotal to both physiological blood clot formation and pathological development of disseminated intravascular coagulation (DIC). In critical illness, extensive cell damage can release histones into the circulation, which can increase thrombin generation and cause DIC, but the molecular mechanism is not clear. Typically, thrombin is generated by the prothrombinase complex, comprising activated factor X (FXa), activated co-factor V (FVa) and phospholipids to cleave prothrombin in the presence of calcium. In this study, we found that in the presence of extracellular histones, an alternative prothrombinase could form without FVa and phospholipids. Histones directly bind to prothrombin fragments F1 and F2 specifically, to facilitate FXa cleavage of prothrombin to release active thrombin, unlike FVa which requires phospholipid surfaces to anchor the classical prothrombinase complex. In vivo, histone infusion into mice induced DIC, which was significantly abrogated when prothrombin fragments F1+F2 were infused prior to histones, to act as decoy. In a cohort of intensive care unit (ICU) patients with sepsis (n=144), circulating histone levels were significantly elevated in patients with DIC. These data suggest that histone-induced alternative prothrombinase without phospholipid anchorage may disseminate intravascular coagulation, and reveal a new molecular mechanism of thrombin generation and DIC development. In addition, histones significantly reduced the requirement for FXa in the coagulation cascade to enable clot formation in Factor VIII and IX-deficient plasma, as well as in Factor VIII-deficient mice. In conclusion, this study highlights a novel mechanism in coagulation with therapeutic potential in both targeting systemic coagulation activation as well as in correcting coagulation factor deficiency.