Biological effects of extracellular fibrinogen binding protein (Efb) in staphylococcus aureus infection

Abstract

Staphylococcus aureus is a leading cause of human and animal infection. The increasing incidence of antibiotic resistance among strains has complicated treatment of these infections. In order to develop new treatment strategies, it is important to identify and characterise bacterial factors that contribute to infection. S. aureus produces a diverse array of virulence factors, among these Extracellular fibrinogen binding protein, Efb. Efb is one of several fibrinogen-binding proteins produced by S. aureus. Efb is produced in vivo during infection and contributes to the pathogenesis of severe S. aureus wound infection in an animal model. This thesis has been focussed on characterising the biological function of Efb in S. aureus infection.

We have shown that Efb interacts with the Aalpha chain of fibrinogen, at a site of functional importance for the interaction between fibrinogen and platelets. We have studied the effect of Efb on platelet function in vitro and in vivo. Efb bound specifically to a receptor on activated platelets, however fibrinogen was not involved in mediating this binding. Efb stimulated a novel type of fibrinogen binding to the platelet, which did not involve the normal platelet fibrinogen receptor, GPIIb/IIIa. We propose that fibrinogen can also bind to the platelet via Efb and this generates the novel fibrinogen binding mediated by Efb.

In the presence of Efb, platelet activation was diminished and Efb also inhibited platelet aggregation in response to various platelet agonists. This antiplatelet effect of Efb was confirmed in vivo. Intravenous Efb significantly prolonged bleeding time but had no effect on the coagulation system. This confirms that Efb specifically inhibits platelet function. Moreover, in a mouse model, intravenous Efb rescued 100% of animals from death due to acute thrombosis. This reflects the inability of the platelets to aggregate in the presence of Efb, an effect so powerful that Efb can counteract the massive thrombosis generated in this model.

We have also studied the immune response against Efb and characterised a protective antibody response. Hyperimmune IgG against Efb neutralised Efb and blocked the various biological effects of Efb in vitro. IgG against Efb blocked Efb binding to fibrinogen, neutralised Efb and returned platelet aggregation in the presence of Efb to normal. Furthermore, immunization with Efb protected against the development of severe infection in an animal model of S. aureus infection. The animals in the vaccinated group developed high titre, specific antibodies against Efb and had significantly less severe infection than those in the unvaccinated group. This establishes Efb as a worthy vaccine candidate for S. aureus infection.

Collectively, in this thesis we have elucidated the biological function of Efb and confirmed its importance in S. aureus wound infection. Efb is a powerful antiplatelet agent, which impairs haemostasis and wound healing. Efb also represents a worthwhile vaccine candidate for certain S. aureus infections.