Similarities and dissimilarities between the binding ability of C1q and collagen

Based on chemical-structural similarities between C1q and collagen, we studied their activities in reactions which are typically induced in collagen or mediated by C1q. Human C1q suppressed the collagen-induced platelet aggregation in human platelet-rich plasmas. Both human C1q and a suspension in insoluble bovine collagen inhibited in time-dependent fashion the lysis of sensitized sheep erythrocytes (EA) by guinea-pig complement. They both agglutinated sheep erythrocytes (EA and EAC4) sensitized with rabbit haemolysin, mainly in the IgM type, polystyrene latex particles complexed with heat-denatured human IgG, a combination of horse, goat and sheep globulins, or deoxyribonucleoprotein. Heating of C1q and collagen (56 degrees C, 30 min), which disrupts the collagen fold into a random coil structure, almost completely abrogated all activities of C1q and considerably reduced those of collagen, suggesting that an intact triple helix is essential for their activities. In spite of their far-ranging similarities, C1q was more potent by weight in most reactions, showed evidence of a faster rate of binding to EA, and was more sensitive to heat treatment at 56 degrees C than was collagen. on the basis of the binding activities of collagen, a model is proposed according to which platelets, sensitized erythrocytes, aggregated gammaglobulins, immune complexes and deoxyribonucleoprotein might accumulate at the site of endothelial damage where blood and its components are exposed to collagenous substances. C1q is able to inhibit all these reactions, indicating that not only is C1q collagen-like in its behaviour, but that collagen also had C1q-like properties.