Endocarditis Pathogen Promotes Vegetation Formation by Inducing Intravascular Neutrophil Extracellular Traps Through Activated Platelets

Background— Endocarditis-inducing streptococci form multilayered biofilms in complex with aggregated platelets on injured heart valves, but the host factors that interconnect and entrap these bacteria-platelet aggregates to promote vegetation formation were unclear. Methods and Results— In a Streptococcus mutans endocarditis rat model, we identified layers of neutrophil extracellular traps interconnecting and entrapping bacteria-platelet aggregates inside vegetation that could be reduced significantly in size along with diminished colonizing bacteria by prophylaxis with intravascular DNase I alone. The combination of activated platelets and specific immunoglobulin G–adsorbed bacteria are required to induce the formation of neutrophil extracellular traps through multiple activation pathways. Bacteria play key roles in coordinating the signaling through spleen tyrosine kinase, Src family kinases, phosphatidylinositol-3-kinase, and p38 mitogen-activated protein kinase pathways to upregulate the expression of P-selectin in platelets, while inducing reactive oxygen species–dependent citrullination in the arm of neutrophils. Neutrophil extracellular traps in turn serve as the scaffold to further enhance and entrap bacteria-platelet aggregate formation and expansion. Conclusions— Neutrophil extracellular traps promote and expand vegetation formation through enhancing and entrapping bacteria-platelet aggregates on the injured heart valves. (Circulation. 2015;131:571–581. DOI: 10.1161/CIRCULATIONAHA.114.011432.)