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Evidence for the involvement of complement proteins in platelet aggregation by Streptococcus sanguis NCTC 7863
1Department of Haematology, Royal Hallamshire Hospital, Sheffield.
Insights
Streptococcus sanguis NCTC 7863 triggers human platelet aggregation via complement activation. This process, crucial in infective endocarditis, involves bacterial binding of complement factors, with lag times varying by donor plasma.
Area of Science:
- Microbiology
- Immunology
- Hematology
Background:
- Streptococcus sanguis is a key cause of infective endocarditis.
- Bacterial interactions with platelets are critical in disease pathogenesis.
- Mechanisms of Streptococcus sanguis-induced platelet aggregation remain incompletely understood.
Purpose of the Study:
- To elucidate the mechanisms by which Streptococcus sanguis NCTC 7863 induces human platelet aggregation.
- To identify the host factors involved in this aggregation process.
- To investigate the role of the complement system in Streptococcus sanguis-mediated platelet activation.
Main Methods:
- In vitro platelet aggregation assays using normal human platelets and plasma.
- Complement inhibition studies using heat, cobra venom factor, and soluble Complement Receptor 1.
- Detection of complement components (C3, C9) and vitronectin on bacterial surfaces via binding assays.
- Analysis of complement activation pathway markers (Factor B cleavage).
Main Results:
- Streptococcus sanguis NCTC 7863 induced platelet aggregation after a donor-variable lag period (7-19 min).
- Aggregation depended on plasma factors and complement activation via the alternative pathway (Mg2+ dependent, Factor B cleavage observed).
- Complement components C3 and C9, along with vitronectin, bound to the bacterial surface, with C5b-9 complex assembly correlating with lag time.
Conclusions:
- The complement system, particularly the alternative pathway, plays a significant role in Streptococcus sanguis-induced platelet aggregation.
- Bacterial binding of complement factors is essential for initiating platelet aggregation.
- These findings provide insights into the pathogenesis of infective endocarditis involving Streptococcus sanguis.
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.