Evidence for the involvement of complement proteins in platelet aggregation by Streptococcus sanguis NCTC 7863

Abstract

We investigated the mechanisms of platelet aggregation by the type strain of Streptococcus sanguis (NCTC 7863). This species is one of the major aetiological agents of infective endocarditis. S. sanguis NCTC 7863 caused aggregation of normal human platelets in vitro following a lag period that varied between donors (7-19 min). Platelet aggregation was dependent on one or more plasma constituents and all the necessary factors gradually became bound to the bacterial surface during the lag period. The length of the lag period was determined by the plasma of the donor and not by a feature of their platelets. Platelet aggregation by S. sanguis NCTC 7863 could be inhibited by heating plasma at 56 degrees C, by treating plasma with cobra venom factor, or by incubating with soluble Complement Receptor 1, all of which inhibit or deplete complement. Complement activation required Mg2+, but not Ca2+ ions and the the cleavage fragment, Ba, of factor B was produced, indicating that the alternative pathway was operative. Zymosan- and S. sanguis-induced aggregation showed similarities, including the same variability in lag times among donors, and absorption of plasma with zymosan prevented the plasma from supporting platelet aggregation by S. sanguis, C3, C9 and vitronectin were found to bind to S. sanguis NCTC 7863, but the latter two were present at very low levels on a non-aggregating strain of S. sanguis, SK96. The rate of assembly of the C5b-9 complex on the NCTC 7863 bacterial surface correlated with the lag time. These data suggest a role for the complement pathway in platelet aggregation by the type strain of S. sanguis.

Platelet-Bacterial Interactions as Therapeutic Targets in Infective Endocarditis

“…Ford et al demonstrated that S. sanguinis strain NCTC 7863 induced aggregation of normal platelets suspended in plasma however removal of plasma proteins abolished platelet aggregation (Ford et al, 1996). The long lag time (12-15mins) of S. sanguinis strain 7863 was progressively shortened by incubating the bacteria in plasma for increasing lengths of time prior to addition to platelets.…”

Platelet Interactions with Bacteria

“…The long lag time is indicative of the time taken for complement assembly [38]. Inactivation of complement by cobra venom or heat treatment abolished aggregation [38]. It is not known precisely how complement activation triggers platelet activation but it is possible that there is a threshold of bacterial-platelet interactions (capable of inducing strong or weak signals) which must be surpassed before triggering platelet aggregation however this remains to be investigated.…”

“…The long lag time is indicative of the time taken for complement assembly [38]. Inactivation of complement by cobra venom or heat treatment abolished aggregation [38].…”

Platelet-Bacterial Interactions in the Pathogenesis of Infective Endocarditis — Part I: The Streptococcus

“…The lag time to aggregation in platelet rich plasma (PRP) in response to S. sanguinis 7863 is 7-19 minutes [111] and this variation was correlated to the rate of assembly of the C5b-9 complex on the surface of the bacteria as detected by flow cytometry [110]. Accordingly, the lag time to aggregation using bacterial cells preincubated with plasma before addition to PRP could be progressively shortened with extension of the incubation time [110]. Complement activation can be triggered by antigen-antibody complexes (classical pathway) or by binding of specific complement proteins (alternative pathway) or mannose binding protein (lectin pathway) to the microbial surface.…”

“…Roles for complement in bacterial-platelet interactions have been demonstrated in Staphylococcal aureus and S. sanguinis [102,107,110]. The lag time to aggregation in platelet rich plasma (PRP) in response to S. sanguinis 7863 is 7-19 minutes [111] and this variation was correlated to the rate of assembly of the C5b-9 complex on the surface of the bacteria as detected by flow cytometry [110]. Accordingly, the lag time to aggregation using bacterial cells preincubated with plasma before addition to PRP could be progressively shortened with extension of the incubation time [110].…”