Secretion of the terminal complement proteins, C5-C9, by human platelets.
The terminal complement components, C8 and C9, and to a lesser extent C5, C6, and C7, but minimal amounts of C3, were shown to be associated with washed human platelets. In unactivated platelets, the complement components were detected in the platelet pellet by hemolytic assays after centrifugation and disruption of the platelets by freeze-thawing. However, after platelets had been activated by collagen, thrombin, or aggregated IgG to induce aggregation, the complement components were released into the supernatant. The rank order of hemolytic activity of C9, C8, C7, C6, and C5 detected in the supernatants of activated platelets was quite different from that found in serum from the same donors, in the same assays. In particular, the serum C7 hemolytic titer was more than twice the serum C9 hemolytic titer, whereas the activity of C9 detected from platelets was more than twice that of C7. This argues against a purely nonspecific uptake of these proteins by platelets from plasma. The functional role of terminal complement components released from platelets during activation is unknown, but it is tempting to speculate that these proteins may have a role in platelet-dependent immunological tissue injury. Because the C5b-9 membrane attack complex activates platelets, it is possible that release of terminal complement proteins serves to amplify platelet activation and may also play a role in diseases in which complement membrane attack complexes have been implicated.
Houle
J J
JJ
University of Rochester Medical Center, Department of Medicine, New York 14642.
Leddy
J P
JP
Rosenfeld
S I
SI
D001792:Blood Platelets / Q000502:physiology*; D003182:Complement C5 / Q000096:biosynthesis / Q000502:physiology; D003183:Complement C6 / Q000096:biosynthesis / Q000502:physiology; D003184:Complement C7 / Q000096:biosynthesis / Q000502:physiology; D003185:Complement C8 / Q000096:biosynthesis / Q000502:physiology; D003186:Complement C9 / Q000096:biosynthesis / Q000502:physiology; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000096:biosynthesis*; D006461:Hemolysis; D006801:Humans; D010974:Platelet Aggregation
D003182:Complement C5; D003183:Complement C6; D003184:Complement C7; D003185:Complement C8; D003186:Complement C9; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins