title: Staphylococcus aureus induces platelet aggregation via a fibrinogen-dependent mechanism which is independent of principal platelet glycoprotein IIb/IIIa fibrinogen-binding domains.
abstract: Platelet aggregation by bacteria is felt to play an important role in the pathogenesis of infective endocarditis. However, the mechanisms involved in bacterium-induced platelet aggregation are not well-defined. In the present study, we examined the mechanisms by which Staphylococcus aureus causes rabbit platelet aggregation in vitro. In normal plasma, the kinetics of S. aureus-induced platelet aggregation were rapid and biphasic. The onset and magnitude of aggregation phase 1 varied with the bacterium-platelet ratio, with maximal aggregation observed at a ratio of 5:1. The onset of aggregation phase 2 was delayed in the presence of apyrase (an ADP hydrolase), suggesting that this later aggregation phase may be triggered by secreted ADP. The onset of aggregation phase 2 was delayed in the presence of prostaglandin I2-treated platelets, and this phase was absent when paraformaldehyde-fixed platelets were used, implicating platelet activation in this process. Platelet aggregation phase 2 was dependent on S. aureus viability and an intact bacterial cell wall, and it was mitigated by antibody directed against staphylococcal clumping factor (a fibrinogen-binding protein) and by the cyclooxygenase inhibitor indomethacin. Similarly, aggregation phase 2 was either delayed or absent in three distinct transposon-induced S. aureus mutants with reduced capacities to bind fibrinogen in vitro. In addition, a synthetic pentadecapeptide, corresponding to the staphylococcal binding domain in the C terminus of the fibrinogen delta-chain, blocked aggregation phase 2. However, phase 2 of aggregation was not inhibited by two synthetic peptides (alone or in combination) analogous to the two principal fibrinogen-binding domains on the platelet glycoprotein (GP) IIb/IIIa integrin receptor: (i) a recognition site on the IIIa molecule for the Arg-Gly-Asp (RGD) sequence of the fibrinogen alpha-chain and (ii) a recognition site on the IIb molecule for a dodecapeptide sequence of the fibrinogen delta-chain. This differs from ADP-induced platelet aggregation, which relies on an intact platelet GP IIb/IIIa receptor with an accessible RGD sequence and dodecapeptide recognition site for fibrinogen. Furthermore, a monoclonal antibody directed against the RGD recognition site on rabbit platelet GP IIb/IIIa receptors failed to inhibit rabbit platelet aggregation by S. aureus. Collectively, these data suggest that S. aureus-induced platelet aggregation requires bacterial binding to fibrinogen but is not principally dependent upon the two major fibrinogen-binding domains on the platelet GP IIb/IIIa integrin receptor, the RGD and dodecapeptide recognition sites.
authors.affiliation: Division of Adult Infectious Diseases, Harbor-UCLA Medical Center, Torrance 90509, USA.
authors.forename: A S
authors.initials: AS
authors.lastname: Bayer
authors.forename: P M
authors.initials: PM
authors.lastname: Sullam
authors.forename: M
authors.initials: M
authors.lastname: Ramos
authors.forename: C
authors.initials: C
authors.lastname: Li
authors.forename: A L
authors.initials: AL
authors.lastname: Cheung
authors.forename: M R
authors.initials: MR
authors.lastname: Yeaman
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001422:Bacterial Adhesion; D001665:Binding Sites; D001792:Blood Platelets / Q000382:microbiology; D005340:Fibrinogen / Q000502:physiology*; D007213:Indomethacin / Q000494:pharmacology; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000494:pharmacology; D015539:Platelet Activation; D010974:Platelet Aggregation; D010980:Platelet Membrane Glycoproteins / Q000502:physiology*; D011817:Rabbits; D013211:Staphylococcus aureus / Q000502:physiology*
chemical_list: D009842:Oligopeptides; D010980:Platelet Membrane Glycoproteins; D000255:Adenosine Triphosphate; D005340:Fibrinogen; C046617:arginyl-glycyl-aspartyl-serine; D007213:Indomethacin