| Summary: | Staphylococcus aureus, a Gram-positive bacterium colonizing nares, skin, and gastrointestinal tract, frequently invades the skin, soft tissues or bloodstream of humans. Even with surgical and antibiotic therapy, bloodstream infections are associated with significant mortality. The secretion of coagulases, proteins that associate with and activate the host hemostatic factor prothrombin, and bacterial surface display of agglutinins, proteins that bind polymerized fibrin, are key virulence strategies for the pathogenesis of S. aureus bloodstream infections, which culminate in the establishment of abscess lesions. Research on staphylococcal agglutination factors is a frontier for the characterization of protective vaccine antigens and the development of immune therapeutics aiming to prevent disease or improve outcome.
Using affinity chromatography with non-catalytic coagulase (Coa) and von Willebrand factor binding protein (vWbp) we identified novel ligands for these virulence factors in human plasma. vWbp bound to prothrombin, fibrinogen, fibronectin, and factor XIII (FXIII), whereas Coa co-purified with prothrombin, fibrinogen, and fibronectin. vWbp association with fibrinogen and factor XIII, but not fibronectin, required prothrombin and triggered the non-proteolytic activation of FXIII in vitro. S. aureu coagulation of human plasma was associated with the recruitment of prothrombin, FXIII, and fibronectin as well as the formation of cross-linked fibrin. FXIII activity in staphylococcal clots could be attributed to thrombin-dependent proteolytic activation as well as vWbp-mediated non-proteolytic activation of FXIII zymogen.
Pathogenic staphylococci express several different serine-aspartate (SD) repeat proteins that are modified by two glycosyltransferases, SdgA and SdgB. We characterized three genes of S. aureu, aggA, aggB (sdgA) and aggC (sdgB), and showed that aggA and aggC contribute to staphylococcal agglutination with fibrin fibrils in human plasma. We demonstrated that aggB (sdgA) and aggC (sdgB) are involved in GlcNAc modification of the ClfA SD repeats. However, only sdgB is essential for GlcNAc modification and an sdgB mutant is defective in the pathogenesis of sepsis in mice. Thus, GlcNAc modification of proteins promotes S. aureu replication in the bloodstream of mammalian hosts.
Lastly, we showed that the conserved repeat (R) domain of Coa enables bloodstream infections by directing fibrinogen to the staphylococcal surface, generating a protective fibrin shield that inhibits phagocytosis. The fibrin shield can be marked with R-specific antibodies, which trigger phagocytic killing of staphylococci and protect mice against lethal bloodstream infections caused by a broad spectrum of MRSA isolates. These findings challenge long-held paradigms on protective immunity and potentially explain the failure of vaccines targeting molecules on the surface of S. aureu.
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