TO THE HEART OF STAPHYLOCOCCAL ENDOCARDITIS, NEW INSIGHTS IN THE PATHOGENESIS OF HEART VALVE INFECTIONS

Infection of the heart valves or infective endocarditis yearly affects between 2 and 12 per 100,000 individuals, killing close to 50.000 people each year. Remarkably, despite recent progress in medical technology, the mortality of this devastating disease has remained unchanged over the last decades. Depending on the causative pathogen, between 15 and 45% of endocarditis patients do not survive endocarditis. Moreover, not only has there been little progress in disease outcome, also efforts to prevent endocarditis, such as antibiotic prophylaxis, vaccination, anticoagulation therapy have mostly failed, leaving an unmet need for new therapeutic and preventive strategies. This thesis focuses on two important causes of endocarditis; Staphylococcus aureus (S. aureus) and Staphylococcus lugdunensis (S lugdunensis). S. aureus is currently the most frequent cause of infective endocarditis and causes an aggressive and rapidly fatal form of the disease. Coagulase negative staphylococci on the other hand are less virulent and seldom cause native valve endocarditis. Staphylococcus lugdunensis (S. lugdunensis) however is a notable exception to this rule. This coagulase negative staphylococcus has only been recently discovered but has proved to be an emerging pathogen that causes aggressive valve infections, similar to those caused by S. aureus. New preventive or therapeutic strategies for endocarditis are lacking mainly because the pathogenesis of this complex disease and especially that of its initial phase remains insufficiently understood. After penetrating into the bloodstream, bacteria are pumped throughout the circulation and leave the left ventricle at an astounding speed of 55 km/h. This high speed creates tremendous shear stress, a frictional force caused by the displacement of blood relative to the vessel wall that impedes the bacteria of adhering. Yet, somehow bacteria are able to slow down and latch on to the heart valve and must therefore have developed mechanisms to overcome shear stress. In this thesis we explore the mechanisms by which endocarditis pathogens overcome shear stress. We show that similarly to platelets, both S. aureus and S. lugdunensis bind von Willebrand Factor (VWF). VWF is a large multimeric protein that is secreted from and temporarily retained on damaged or inflamed vascular endothelium. In addition, circulating VWF multimers can accumulate in the subendothelial matrix if the vascular integrity is breached. When bound VWF is exposed to the blood flow, the resulting shear forces expose previously concealed binding domains, to which platelets can bind. Both S. aureus and S. lugdunensis were able to highjack this powerful mechanism to overcome shear stress, allowing their adhesion to the vessel wall and cardiac valves. This mechanism may explain why both S. aureus and S. lugdunensis are so proficient in causing endocarditis, compared to other staphylococci. In the second part, we dive deeper into the mechanisms behind early bacterial adhesion in S. aureus endocarditis. As normally the endothelium is fairly resistant to infection, S. aureus seldom infects healthy heart valves. Clinical observations however suggest that S. aureus can infect either damaged or inflamed heart valves. The underlying mechanisms nevertheless remained elusive, as until now there were no good animal models to study the early phase of endocarditis. Using 3D confocal microscopy, we therefore developed a new mouse model that allowed us to measure the early adhesion of fluorescent S. aureus to either damaged or inflamed aortic valves. With this model we confirmed that both cardiac valve damage and cardiac valve inflammation predisposed to bacterial adhesion, but by very distinct mechanisms. In case of cardiac valve damage, the endothelium was breached, leading to VWF and fibrin deposition unto the subendothelial matrix. S. aureus adhered directly to VWF and fibrin using known adhesins such as the VWF-binding protein and Clumping factor A, respectively. Platelets did not contribute significantly to this process. In contrast, in case of cardiac valve inflammation, widespread endothelial cell activation led to the release of endothelial cell-bound VWF. This VWF recruited large amounts of platelets, capturing S. aureus to the valve surface. Neither fibrinogen, nor the usual adhesins were essential in inflammation induced bacterial adhesion. To conclude, this thesis provides a unique glimpse into the early phase of staphylococcal endocarditis, something that is impossible to study in clinical or epidemiological trails. These findings can provide inspiration for the development of much-needed new preventive and therapeutic strategies for this devastating disease.

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