Contribution of chondroitin sulfate A to the binding of complement proteins to activated platelets.
BACKGROUND
Exposure of chondroitin sulfate A (CS-A) on the surface of activated platelets is well established. The aim of the present study was to investigate to what extent CS-A contributes to the binding of the complement recognition molecule C1q and the complement regulators C1 inhibitor (C1INH), C4b-binding protein (C4BP), and factor H to platelets.


RESULTS
Human blood serum was passed over Sepharose conjugated with CS-A, and CS-A-specific binding proteins were identified by Western blotting and mass spectrometric analysis. C1q was shown to be the main protein that specifically bound to CS-A, but C4BP and factor H were also shown to interact. Binding of C1INH was dependent of the presence of C1q and then not bound to CS-A from C1q-depleted serum. The specific interactions observed of these proteins with CS-A were subsequently confirmed by surface plasmon resonance analysis using purified proteins. Importantly, C1q, C4BP, and factor H were also shown to bind to activated platelets and this interaction was inhibited by a CS-A-specific monoclonal antibody, thereby linking the binding of C1q, C4BP, and factor H to exposure of CS-A on activated platelets. CS-A-bound C1q was also shown to amplify the binding of model immune complexes to both microtiter plate-bound CS-A and to activated platelets.


CONCLUSIONS
This study supports the concept that CS-A contributes to the binding of C1q, C4BP, and factor H to platelets, thereby adding CS-A to the previously reported binding sites for these proteins on the platelet surface. CS-A-bound C1q also seems to amplify the binding of immune complexes to activated platelets, suggesting a role for this molecule in immune complex diseases.
Hamad
Osama A
OA
Division of Clinical Immunology, Rudbeck Laboratory C5, Uppsala University, Uppsala, Sweden.
Nilsson
Per H
PH
Lasaosa
Maria
M
Ricklin
Daniel
D
Lambris
John D
JD
Nilsson
Bo
B
Ekdahl
Kristina Nilsson
KN
D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002809:Chondroitin Sulfates / Q000494:pharmacology*; D003174:Complement C1 Inactivator Proteins / Q000378:metabolism; D050718:Complement C1 Inhibitor Protein; D015922:Complement C1q / Q000378:metabolism; D050716:Complement C4b-Binding Protein / Q000378:metabolism; D017242:Complement Factor H / Q000378:metabolism; D003165:Complement System Proteins / Q000378:metabolism*; D006801:Humans; D015539:Platelet Activation / Q000187:drug effects*; D011485:Protein Binding / Q000187:drug effects
D003174:Complement C1 Inactivator Proteins; D050718:Complement C1 Inhibitor Protein; D050716:Complement C4b-Binding Protein; C469952:SERPING1 protein, human; D015922:Complement C1q; D017242:Complement Factor H; D002809:Chondroitin Sulfates; D003165:Complement System Proteins