Activation of the contact system at the surface of Fusobacterium necrophorum: a possible virulence mechanism in Lemierre's syndrome.

Fusobacterium necrophorum causes Lemièrre's syndrome, a serious disease with septic thrombophlebitis of the internal jugular vein, pulmonary involvement and systemic inflammation. The contact system is a link between inflammation and coagulation and contact activation by the bacteria could therefore contribute to the abnormal coagulation and inflammation seen in patients with Lemièrre's syndrome. In this study, F. necrophorum was found to bind radiolabeled high molecular weight kininogen (HK), a central component of the contact system. Binding was inhibited by the addition of unlabeled HK and domain D5 of HK, but not by other components of the contact system, indicating a specific interaction mediated through the D5 region. Binding of HK was significantly reduced after pre-treatment of the bacteria with trypsin, suggesting that surface proteins are involved in HK binding. Incubation of the bacteria with human plasma resulted in a breakdown pattern of HK suggestive of bradykinin release, and bradykinin was also detected in the supernatant. In addition, we show that factor XI (FXI), another component of the contact system, binds to F. necrophorum, and that the bound FXI reconstitutes the activated partial thromboplastin time (aPTT) of FXI-deficient plasma. Thrombin activity was detected at the surface of the bacteria following incubation with plasma, indicating that the intrinsic pathway of coagulation is activated at the surface. This activity was completely blocked by inhibitors of the contact system. The combined results show that the contact system is activated at the surface of F. necrophorum, suggesting a pathogenic role for this system in Lemièrre's syndrome