The influence of genetic variations on activation of blood coagulation: a possible explanations for the increased thrombotic risk in bullous pemphigoid

Background: Bullous pemphigoid (BP) is an inflammatory blistering skin disease mainly affecting elderly subjects and associated with an elevated thrombotic risk. It is caused by autoantibodies to hemidesmosomal proteins, with eosinophils participating in blister formation. Aims: Since eosinophils are a source of tissue factor (TF), the main initiator of blood coagulation, we evaluated the local and systemic activation of coagulation in BP. Methods: We studied 60 patients with active BP (sixteen re-evaluated during remission) and 60 sex- and age-matched healthy controls. The coagulation markers prothrombin fragment F1+2 and D-dimer were measured in plasma of all subjects and in both plasma and blister fluid of patients with BP. TF was evaluated immunohistochemically in skin specimens from 40 patients and in 20 normal samples. In lesional skin specimens from 5 BP patients, we evaluated TF mRNA expression by in situ hybridization as well as the colocalization of TF and eosinophil cationic protein (a classic eosinophil marker) by confocal microscopy using immunofluorescence techniques. Results: F1+2 and D-dimer levels were higher in plasma of patients with BP than in plasma of controls (p=0.0001 for both), and were very high in blister fluid (p=0.0001 vs plasma). Plasma and blister fluid F1+2 and D-dimer levels paralleled blood and tissue eosinophilia and disease severity. In the sixteen patients reevaluated during remission, there was a marked reduction in F1+2 (from 600±377 to 193±151 pmol/L; p=0.0001) and D-dimer (from 3225±3010 to 561±476 ng/ ml; p=0.0001). Immunohistochemistry revealed strong TF reactivity in BP skin (p=0.0001) confirmed by in situ hybridization. Colocalization studies by confocal microscopy identified eosinophils as a source of TF. Conclusions: The coagulation cascade is activated in BP most likely via TF expressed by eosinophils. The hypercoagulability correlates with the severity of the disease and possibly contributes to inflammation, tissue damage, blister formation and thrombotic risk in BP.