Complement component C3 binds to activated normal platelets without preceding proteolytic activation and promotes binding to complement receptor 1.
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(H(2)O). 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(H(2)O) represents nonproteolytic activation of C3 and after factor I cleavage may act as a ligand for receptor binding.
Division of Clinical Immunology, Rudbeck Laboratory C5, Uppsala University, Uppsala, Sweden.
Osama A
OA
Hamad
Per H
PH
Nilsson
Diana
D
Wouters
John D
JD
Lambris
Kristina N
KN
Ekdahl
Bo
B
Nilsson
D000595:Amino Acid Sequence; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D015153:Blotting, Western; D002809:Chondroitin Sulfates / Q000378:metabolism; D003167:Complement Activation; D015922:Complement C1q / Q000378:metabolism; D003176:Complement C3 / Q000378:metabolism*; D003181:Complement C4 / Q000378:metabolism; D003186:Complement C9 / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D005343:Fibrinolytic Agents / Q000494:pharmacology; D005434:Flow Cytometry; D006629:Hirudins / Q000494:pharmacology; D006801:Humans; D009842:Oligopeptides / Q000494:pharmacology; D015539:Platelet Activation; D053657:Platelet-Rich Plasma / Q000187:drug effects / Q000378:metabolism; D011485:Protein Binding; D017463:Receptors, Complement 3b / Q000378:metabolism*; D011994:Recombinant Proteins / Q000494:pharmacology
C509991:CR1 protein, human; D003176:Complement C3; D003181:Complement C4; D003186:Complement C9; D005343:Fibrinolytic Agents; D006629:Hirudins; D009842:Oligopeptides; D017463:Receptors, Complement 3b; D011994:Recombinant Proteins; C096900:thrombin receptor-activating peptide SFLLRNPNDKY; D015922:Complement C1q; D002809:Chondroitin Sulfates; C083544:lepirudin