Inhibitors of coagulation factor XIa (FXIa) are currently being investigated as potential anticoagulant therapies. We hypothesize that circulating FXIa could be a potential target for these therapies. Using previous analyses of FXIa impurities in immune globulin products involved in thrombotic adverse events, we estimated that picomolar levels of FXIa can be thrombogenic. In an in vitro clot-growth assay, 0.1-3 pM of FXIa did not, by itself, activate clotting but increased the size of growing clots. Spatio-temporal reconstruction of thrombin activity inside the clot revealed that FXIa's effect was limited to the clot-plasma interface, in which FXIa produced a taller than standard wave of thrombin. Factor-depleted plasma and a panel of selective anti-FXIa antibodies showed that exogenous FXIa effects are (1) blocked by anti-FXIa antibodies, (2) independent of FXI activation inside the clot, and (3) larger than the contribution of in situ FXIa. In a thrombin generation (TG) assay, picomolar FXIa did not initiate TG but rather promoted TG triggered by tissue factor or thrombin, suggesting that the effect of FXIa on the thrombin wave is mediated by the elevation of thrombin-triggered TG. In circulating bovine blood, low doses of human FXIa did not initiate clotting but increased the size of stenosis-triggered thrombi. FXIa injection in mice enhanced TG in plasma for at least 6 hours ex vivo, confirming the persistence of circulating FXIa. Our findings suggest that picomolar levels of circulating FXIa may not be able to initiate thrombosis but can facilitate thrombus growth through the facilitation of TG inside the clot.
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