A New Procoagulant Mechanism Mediated by Fucoidans

Abstract 4420

Fucoidans are heterogeneous, polyanionic molecules with procoagulant activities in a wide concentration range. They have been described as non-anticoagulant sulfated polysaccharides (NASP) and shown to improve clotting in FVIII- and FIX-deficient plasma. In vitro characterization has suggested that fucoidans exert their procoagulant activity by inhibiting tissue factor pathway inhibitor (Liu et al. Thromb Haemost 2006; 95:68) and by accelerating thrombin-dependent FVa formation (Mutch et al. J Thromb Haemost 2007; 5 Suppl2). In our study we describe a new, previously unrecognized mechanism by which fucoidans act as procoagulant agents. The procoagulant activity of several fucoidans was characterized by calibrated automated thrombography in tissue factor (TF)-dependent experiments and by using coagulation factor-deficient plasmas. Spiking experiments with purified coagulation factors or inhibitory antibodies verified the mechanism.

Stimulation of thrombin generation (TG) by fucoidans requires anionic lipid surfaces like synthetic phospholipid vesicles which contain phosphatidylserine and is TF-dependent (0-20pM). However, stimulatory activity was most pronounced in the absence of TF. Control experiments with corn trypsin inhibitor or FXII-deficient plasma excluded any involvement of the contact system. Plasmas from patients with congenital coagulation factor deficiencies were screened for TG to identify the target coagulation factor by which fucoidans exert their procoagulant activities. In the absence of TF, plasmas deficient in coagulation factors from the common pathway do not support fucoidan-mediated thrombin generation, whereas FVII-deficient plasma does. FXI was identified as the most upstream factor of the intrinsic pathway which is required for fucoidan-stimulated thrombin generation, suggesting it to be the target for the procoagulant activities of fucoidan. Spiking 30nM FXI to FXI-deficient plasma restored fucoidan-mediated TG and addition of polyclonal FXI inhibitory antibodies to normal plasma abrogated TG. Fucoidan-dependent TG did not improve when FXIa (60pM) was added to FXI-deficient plasma, suggesting activation of FXI by fucoidan. The relevance of this mechanism in hemophilia A plasma was studied by addition of low levels of FVIII (0.2-10%) resulting in a FVIII concentration-dependent increase in fucoidan-mediated TG. These results highlight the requirement of a functional intrinsic pathway for this new mechanism of fucoidans. Our findings present FXI activation at low TF concentrations as a possible mechanism for fucoidan.