Interaction between extracellular adherence protein (Eap) from Staphylococcus aureus and the human host

Staphylococcus aureus is one of the most common agents causing bacterial infections in humans and animals. The emergence of extended antibiotic resistance among S. aureus strains as a worldwide epidemic has driven the development of alternative strategies to combat this microorganism. This thesis is based on the interaction between Extracellular Adherence Protein (Eap) from S. aureus and the human host. Eap is an extracellular protein capable of binding to several plasma proteins including fibronectin, fibrinogen and prothrombin. Adherence and internalization into host cells are key steps during the infectious process; adherence for successful colonization and internalization to shield the bacteria from host defense and antibiotic treatment. Using a mutant strain, Newman AH12 lacking the eap gene we were able to demonstrate a clear role for Eap in adherence and internalization. Strain Newman (wild type strain) could adhere to and become internalized better by eukaryotic cells than the isogenic mutant and externally added Eap enhanced adherence and internalization of strain Newman, Newman AH12, and clinical strains. Antibodies against Eap were able to block the adherence and internalization process in strain Newman. In the second part of this thesis we confirm the role of Eap as an immunomodulating protein. We could demonstrate that Eap inhibits the binding of neutrophils to the endothelium under static and dynamic flow conditions and also inhibits transendothelial migration of neutrophils in an in vitro model. We could also show that Eap blocked to the same extent as human ICAM-1 antibodies. This data together with previous reports suggest that Eap also in this setting elicits its effect mainly by binding to ICAM-1. On the other hand, we could also demonstrate a direct effect of Eap on peripheral blood mononuclear cells (PBMCs), which was concentration dependent. At low concentrations, Eap elicited a stimulatory effect on PBMC and at high concentrations it had an inhibitory effect through induction of apoptosis in T and B cells. Protective cellular immunity is the main mechanisms of defense used by the host to eliminate S. aureus infection. The presence of Eap during a S. aureus infection will have a major impact on this system by virtue of (i) Eap mediated adherence and internalization into host cells, (ii) inhibition of neutrophils migration to the site of infection (iii) inhibition of T and B cell proliferation. Hence, these studies imply that intervention directed against Eap could be a novel approach to prevent or improve therapy of Staphylococcal infections