Katarina Kulhankova, Kyle J. Kinney, Jessica M. Stach, Françoise A. Gourronc, Isabella M. Grumbach, Aloysius J. Klingelhutz, Wilmara Salgado-Pabón

Staphylococcus aureus infective endocarditis (IE) is a fast-progressing and tissue destructive infection of the cardiac endothelium. The superantigens (SAgs) toxic shock syndrome toxin-1 (TSST-1), staphylococcal enterotoxin C (SEC), and the enterotoxin gene cluster (egc) play a novel and essential role in the etiology of S. aureus IE. Recent studies indicate that SAgs act at the infection site to cause tissue pathology and promote vegetation growth. The underlying mechanism of SAg involvement has not been clearly defined. In SAg-mediated responses, immune cell priming is considered a primary triggering event leading to endothelial cell activation and altered function. Utilizing immortalized human aortic endothelial cells (iHAECs), we demonstrate that TSST-1 directly activates iHAECs, as documented by upregulation of vascular and intercellular adhesion molecules (VCAM-1 and ICAM-1). TSST-1-mediated activation results in increased monolayer permeability and defects in vascular re-endothelialization. Yet, stimulation of iHAECs with TSST-1 fails to induce IL-8 and IL-6 production. Furthermore, simultaneous stimulation of iHAECs with TSST-1 and LPS inhibits LPS-mediated IL-8 and IL-6 secretion, even after pre-treatment with the pro-inflammatory cytokines TNFα or IL-1β. IL-8 suppression is not mediated by TSST-1 binding to its canonical receptor MHC-II, supporting current evidence for a non-hematopoietic interacting site on SAgs. Altogether the data suggest that TSST-1 differentially regulates cell-bound and secreted markers of endothelial cell activation that may result in dysregulated innate immune responses during S. aureus IE. Endothelial changes resulting from the action of SAgs can therefore directly contribute to the aggressive nature of S. aureus IE and development of life-threatening complications.