Fibrinolysis and proteolysis in bacterial spread and tissue destruction.

In this PhD project, we have studied the role of the host hemostatic system and its manipulation by S. aureus in the outcome of S. aureus infections.

It has long been known that most S. aureus strains produce staphylokinase. We have shown that staphylokinase-mediated fibrinolytic and proteolytic activity permit the local invasion of S. aureus in skin and soft tissue through degradation of fibrin and extracellular matrix. The fibrin-specificity of staphylokinase limits its action to the vicinity of the fibrin-rich S. aureus abscess community, as both fibrin and bacterial surfaces protect staphylokinase from inhibition by alpha-2-antiplasmin. Indeed, the host protease inhibitor alpha-2-antiplasmin limited bacterial dissemination in later stages of skin infection. (Chapter 1, Research Question 1)

Fibrin is a central component of S. aureus biofilms in foreign body infections, such as catheter infections. We have shown that staphylokinase production is downregulated inside biofilms, but quorum sensing dependent upregulation of staphylokinase allows S. aureus to escape from its fibrin-rich biofilm. In catheter infections in vivo, we could confirm that staphylokinase action reduces biofilm on foreign bodies. As such, while staphylocoagulase activity and fibrin are crucial for the primary adhesion and the establishment of an S. aureus biofilm, staphylokinase activity takes its role in the later stages of biofilm disassembly. (Chapter 2, Research Question 2)

The manipulation of the fibrinolytic system by bacteria is a widespread phenomenon, which we have reviewed in Chapter 3.  Plasminogen activation offers pathogens the opportunity to degrade fibrin, invade host tissues and evade immune attacks.  To date, few therapeutic strategies have been derived from these observations. A first hypothetical option could be to use anti-proteases to limit unregulated proteolytic activity such as occurs in necrotizing fasciitis. We have studied a second option: using fibrinolytic coatings to prevent S. aureus device infections.

Apart from its manipulation of the fibrinolytic system, S. aureus is notable for its interaction with coagulation factors. The contribution of S. aureus coagulase activity to outcome in different infection models, and the promising effects of coagulase inhibition are reviewed in Chapter 4.  This provides a consistent preclinical evidence base to foster translational research of coagulase inhibition in a clinical trial.

We have completed the first trial of coagulase inhibition in S. aureus bacteremia, in 94 patients collected over a 3-year period.  Our results show that the feasibility of coagulase inhibition with available direct thrombin inhibitors is limited by their anticoagulant action.  However, the observation of faster resolution of coagulation disturbances associated with S. aureus infection in patients treated with direct thrombin inhibitors, stimulates further research on specific inhibitors of bacterial coagulase activity that do not interfere with host thrombin. (Chapter 5, Research Question 3)

In a final series of experiments, we have shown that the ratio between ADAMTS-13 and von Willebrand factor (VWF) modulates the course of S. aureus sepsis. Both high VWF levels and (relative) deficiency of ADAMTS-13 promote end organ microvascular thrombosis and mortality in S. aureus bloodstream infections.  It remains to be shown if proteolysis of the ultra-long VWF that renders the inflamed vessel wall ‘ultra-adhesive’ for platelets and neutrophils, could improve outcome in S. aureus bacteremia. (Chapter 6, Research Question 4)

In conclusion, we have gained further insight into the role of the hemostatic system in S. aureus infections. Our data could suggest future therapeutic endeavors to improve the management of these frequent and often difficult-to-treat infections. Some observations might be relevant and deserve exploration beyond S. aureus disease. For example, evaluation of the prevention of fibrin deposition on intravascular devices might also decrease other biofilm infections on these foreign bodies. Also, coagulation deregulation, and more specifically von Willebrand factor and the inflamed vessel wall, warrant further study as possible targets to mitigate the outcome of sepsis.