STAPHYLOCOCCUS-AUREUS INDUCES PLATELET-AGGREGATION VIA A FIBRINOGEN-DEPENDENT MECHANISM WHICH IS INDEPENDENT OF PRINCIPAL PLATELET GLYCOPROTEIN IIB IIIA FIBRINOGEN-BINDING DOMAINS/

Platelet aggregation by bacteria is felt to play an important role in the pathogenesis of infective endocarditis, However, the mechanisms in volved in bacterium-induced platelet aggregation are not well-defined, In the present study, we examined the mechanisms by which Staphylococ cus aureus causes rabbit platelet aggregation in vitro. In normal plas ma, the kinetics of S. aureus-induced platelet aggregation were rapid and biphasic. The onset and magnitude of aggregation phase 1 varied wi th the bacterium-platelet ratio, with maximal aggregation observed at a ratio of 5:1. The onset of aggregation phase 2 was delayed in the pr esence of apyrase (an ADP hydrolase), suggesting that this later aggre gation phase may be triggered by secreted ADP, The onset of aggregatio n phase 2 was delayed in the presence of prostaglandin I-2-treated pla telets, and this phase was absent when paraformaldehyde-fixed platelet s were used, implicating platelet activation in this process, Platelet aggregation phase 2 was dependent on S. aureus viability and an intac t bacterial cell wall, and it was mitigated by antibody directed again st staphylococcal clumping factor (a fibrinogen-binding protein) and b y the cyclooxygenase inhibitor indomethacin, Similarly, aggregation ph ase 2 was either delayed or absent in three distinct transposon-induce d S. aureus mutants with reduced capacities to bind fibrinogen in vitr o, In addition, a synthetic pentadecapeptide, corresponding to the sta phylococcal binding domain in the C terminus of the fibrinogen delta-c hain, blocked aggregation phase 2, However, phase 2 of aggregation was not inhibited by two synthetic peptides (alone or in combination) ana logous 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 fibrino gen alpha-chain and (ii) a recognition site on the IIb molecule for a dodecapeptide sequence of the fibrinogen delta-chain. This differs fro m ADP-induced platelet aggregation, which relies on an intact platelet GP IIb/IIIa receptor with an accessible RGD sequence and dodecapeptid e recognition site for fibrinogen, Furthermore, a monoclonal antibody directed against the RGD recognition site on rabbit platelet GP IIb/II Ia receptors failed to inhibit rabbit platelet aggregation by S. aureu s. Collectively, these data suggest that S. aureus-induced platelet ag gregation requires bacterial binding to fibrinogen but is not principa lly dependent upon the two major fibrinogen-binding domains on the pla telet GP IIb/lIIa integrin receptor, the RGD and dodecapeptide recogni tion sites.