Abstract Number: OC 72.2
Meeting: ISTH 2022 Congress
Background: Infective endocarditis (IE) is characterized by an infected thrombus. How bacteria bypass the immune system and cause these complex thrombi remains unclear. Neutrophils, via release of neutrophil extracellular traps (NETs), lie at this interface between host defense and thrombosis.
Aims: We aimed to determine the role of neutrophils/NETs in IE.
Methods: We injected mice with Staphylococcus aureus i.v. and stimulated the aortic endothelium locally with histamine, resulting in IE on inflamed valves. We identified neutrophils and NETs in IE by immunostaining, performed antibody-mediated neutrophil depletion in WT animals, and determined the role of NETs by using neutrophil-specific PAD4-knockout mice. In addition, we used S. aureus deficient in nuclease (Δnuc) or both coagulase and von Willebrand factor-binding protein (Δcoa/Δvwb) to induce IE.
Results: Neutrophils and neutrophils releasing NETs were present in thrombi and within large cellular infiltrates in the surrounding aortic wall in mice with IE. Neutrophil depletion significantly increased endocarditis incidence and led to persistent bacteremia. However, incidence of IE and bacteremia was similar between NETosis-impaired mice and wild-type controls. As S. aureus nucleases degrade NETs, we tested Δnuc S. aureus in our model. This mutant did not affect endocarditis incidence, bacteremia and infiltration size. To investigate if bacteria shielded off neutrophils via a coagulase-induced fibrin layer, mice were infected with Δcoa/Δvwb S. aureus. These mice had improved survival, decreased bacteremia, smaller infiltrates, and decreased tissue destruction. Significantly more NETs were present inside infected thrombi induced by the mutant, which correlated to decreased bacteria and cell death in the surrounding vessel wall.
Conclusion(s): Neutrophils are protective against IE independent of NET release. S. aureus-induced fibrin likely shields neutrophils from entering the thrombus. In absence of this protective fibrin layer, NETs may be able to constrain the infection and hamper tissue damage, a key hallmark of valve destruction in IE.
To cite this abstract in AMA style:Meyers S, Lox M, Kraisin S, Martens C, Liesenborghs L, Frederix L, Missiakas D, Baatsen P, Vanassche T, Verhamme P, Martinod K. Neutrophils protect against Staphylococcus aureus endocarditis but the impact of NET release is negated by coagulases [abstract]. https://abstracts.isth.org/abstract/neutrophils-protect-against-staphylococcus-aureus-endocarditis-but-the-impact-of-net-release-is-negated-by-coagulases/. Accessed March 4, 2024.
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