Saliva from persons with hemophilia A triggers coagulation via extrinsic tenase complexes

Human saliva contains extracellular vesicles (EVs). These EVs expose extrinsic tenase complexes of tissue factor (TF) and activated factor VII (FVIIa), and trigger blood coagulation. Here, we show that EVs exposing extrinsic tenase complexes are also present in saliva of persons with severe hemophilia A, i.e. persons with FVIII deficiency. Addition of these salivary EVs to autologous FVIII-deficient blood results in FXa generation, thereby compensating for the lack of FXa generation via intrinsic tenase (FVIIIa/FIXa) complexes. Consistently, in our retrospective analysis of persons with severe hemophilia A who do not perform prophylactic FVIII substitution, oropharyngeal mucosal bleedings are infrequent and self-limited. Conversely, in saliva from persons with severe FVII deficiency, in whom oropharyngeal bleedings are prevalent, functional extrinsic tenase complexes are absent, because EVs lack FVII. Saliva from persons with severe FVII deficiency is unable to restore blood coagulation, which is due to the absence of FVII in both their saliva and blood. Picomolar levels of recombinant FVIIa can restore the coagulant potential of saliva from FVII deficient persons. Taken together, our findings may explain the paucity of oropharyngeal bleedings in persons with hemophilia A as well as the occurrence of such bleedings in persons with severe FVII deficiency.