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C1q (C1) Receptor on Human Platelets: Inhibition of Collagen-Induced Platelet Aggregation by C1q (C1) Molecules¹

From the Naval Medical Research Institute, National Naval Medical Center, and the National Institute of Arthritis, Metabolism and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20014

Abstract

Hemolytically active human C1q incubated with EA before the addition of complement inhibited the immune hemolysis. On the contrary, heat-inactivated preparation (30 min 56°C) was ineffective. Preincubation of EA with bovine collagen also resulted in a decreased hemolysis. When aggregation was measured by a turbidimetric method in citrated human platelet-rich plasma, it was found that hemolytically active human C1q (C1) alone does not induce platelet aggregation. However, in its presence the platelets failed to aggregate or exhibited a significantly reduced aggregation response to bovine collagen. The inhibition by C1q depended on the preincubation time with platelets. Heat treatment (30 min 56°C) destroyed the inhibitory action of C1q (C1). The effect of C1q proved to be highly specific because different C1q preparations at their inhibitory doses in collagen-induced platelet aggregation did not influence the response to other aggregating agents (bovine thrombin, ADP, horse anti-human thymocyte globulin, goat anti-baboon platelet antiserum). The results prove that collagen and C1q are capable of binding to the same site(s); namely, to those of EA and human platelets; furthermore, they suggest the presence of a receptor for C1q (C1) on human platelets.

Footnotes

¹ This work was supported by the Naval Medical Research and Development Command, Work Unit No. MR041.02.01.0029. The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the Navy Department or the naval service at large.

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