The Journal of Immunology, Vol 138, Issue 5 1537-1541, Copyright © 1987 by American Association of Immunologists

ARTICLES

Human blood platelets possess specific binding sites for C1q

EI Peerschke and B Ghebrehiwet

Although platelet interactions with C1q are implied by the inhibitory effect of C1q on collagen-induced platelet aggregation, specific receptors have not as yet been identified. To address the question of platelet receptors for free C1q, direct radioligand binding studies were performed by using human blood platelets and purified, 125I- labeled C1q, and a monoclonal antibody (II1/D1) (IgM, lambda) directed against C1q receptors on peripheral blood leukocytes. Washed platelets bound both purified 125I-labeled C1q and II1/D1 in a specific and saturable manner under physiologic ionic strength conditions. At equilibrium, approximately 4000 molecules of C1q bound per platelet with an apparent dissociation constant of 3.5 X 10(-7) M. Maximum C1q binding was achieved in 5 min and correlated well with inhibition of collagen-induced platelet aggregation. Equilibrium binding of 125I- labeled II1/D1 to washed platelets required an incubation period of 15 to 30 min and II1/D1 concentrations approaching 50 micrograms/ml. Approximately 2000 molecules of II1/D1 bound per platelet, with an apparent dissociation constant of 2.8 X 10(-8) M. II1/D1 binding could be inhibited by the collagenous tail of C1q (c-C1q), suggesting that platelet receptors for these ligands are either the same or in close proximity. The data demonstrate that human blood platelets possess specific and saturable binding sites for free C1q that may function as collagen receptors, and may antigenically resemble C1q receptors on peripheral blood leukocytes.

This article has been cited by other articles:

				M. Sawada, H. Yanamoto, I. Nagata, N. Hashimoto, I. Nakahara, Y. Akiyama, H. Kikuchi, and R. L. Macdonald Prevention of Neointimal Formation by a Serine Protease Inhibitor, FUT-175, After Carotid Balloon Injury in Rats • Editorial Comment Stroke, March 1, 1999; 30(3): 644 - 650. [Abstract] [Full Text] [PDF]

				R. H. van den Berg, M. C. Faber-Krol, R. B. Sim, and M. R. Daha The First Subcomponent of Complement, C1q, Triggers the Production of IL-8, IL-6, and Monocyte Chemoattractant Peptide-1 by Human Umbilical Vein Endothelial Cells J. Immunol., December 15, 1998; 161(12): 6924 - 6930. [Abstract] [Full Text] [PDF]

				R. H. van den Berg, M. C. Faber-Krol, S. van Wetering, P. S. Hiemstra, and M. R. Daha Inhibition of Activation of the Classical Pathway of Complement by Human Neutrophil Defensins Blood, November 15, 1998; 92(10): 3898 - 3903. [Abstract] [Full Text] [PDF]

				R. R. Nepomuceno and A. J. Tenner C1qRP, the C1q Receptor That Enhances Phagocytosis, Is Detected Specifically in Human Cells of Myeloid Lineage, Endothelial Cells, and Platelets J. Immunol., February 15, 1998; 160(4): 1929 - 1935. [Abstract] [Full Text] [PDF]