Complement Component C3 Binds to Activated Normal Platelets without Preceding Proteolytic Activation and Promotes Binding to Complement Receptor 1

Abstract

It has been reported that complement is activated on the surface of activated platelets, despite the presence of multiple regulators of complement activation. To reinvestigate the mechanisms by which activated platelets bind to complement components, the presence of complement proteins on the surfaces of nonactivated and thrombin receptor-activating peptide-activated platelets was analyzed by flow cytometry and Western blot analyses. C1q, C4, C3, and C9 were found to bind to thrombin receptor-activating peptide-activated platelets in lepirudin-anticoagulated platelet-rich plasma (PRP) and whole blood. However, inhibiting complement activation at the C1q or C3 level did not block the binding of C3 to activated platelets. Diluting PRP and chelating divalent cations also had no effect, further indicating that the deposition of complement components was independent of complement activation. Furthermore, washed, activated platelets bound added C1q and C3 to the same extent as platelets in PRP. The use of mAbs against different forms of C3 demonstrated that the bound C3 consisted of C3(H2O). Furthermore, exogenously added soluble complement receptor 1 was shown to bind to this form of platelet-bound C3. These observations indicate that there is no complement activation on the surface of platelets under physiological conditions. This situation is in direct contrast to a number of pathological conditions in which regulators of complement activation are lacking and thrombocytopenia and thrombotic disease are the ultimate result. However, the generation of C3(H2O) represents nonproteolytic activation of C3 and after factor I cleavage may act as a ligand for receptor binding.

Complement inhibition decreases the procoagulant response and confers organ protection in a baboon model of Escherichia coli sepsis

“…We have detected a decreased C5b9 staining of platelets in compstatintreated groups compared with nontreated controls supporting the involvement of complement in platelet deposition in tissues and subsequent thrombocytopenia. It is known that C3a induces platelet activation and aggregation 27 and that C3 could bind to the platelet surface without 36 or with C5b9 formation. 26 Inhibition of complement activation by a potent inhibitor of C3 activation, a key event common to all 3 complement activation pathways, had organ-protective effects in both early and late The tissues were collected after euthanasia at T ϩ 24 hours.…”

Are there sufficient standards for the in vitro hemocompatibility testing of biomaterials?

“…Most commonly used anticoagulants are heparin for whole blood studies and sodium citrate for studies that focus on platelet-biomaterial interactions in vitro [52,55]. More recently, hirudin appears in an increasing number of hemocompatibility studies and clinically applied routine test systems [56,57]. The decision for a specific anticoagulant should be driven by the clinical background, the choice of tested blood (e.g.…”

“…These assays focus on the platelet-biomaterial interactions and are well suited to assess the thrombogenic potential of a material, even though, sodium citrate is not applied as a systemic anticoagulant. For in vitro studies focusing on complement activation, anticoagulation with hirudin (commercially available as lepirudin or bivalirudine [68]) appears to mimic physiological conditions more appropriately than other anticoagulants [56,57,69].…”

Up-regulated Complement 3 Production by Toll-like receptor 9/ Transforming Growth Factor-Beta 1/Complement 3 Pathway in Whole Blood Cells of Lupus Thrombocytopenia

“…4,27 Furthermore, complements are implicated in participation in antibody mediated destruction of thrombocytes in periphery of patients with active lupus. [28][29][30][31] Based on these studies, we hypothesize that whole blood cells' synthesis of C3 mediate local injury on platelets in SLE patients with TCP. However, we just observed the correlation but have not explored the causal relationship between the pathway and lupus TCP in current study.…”