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Electronic free platelet counting was more sensitive than turbidimetry to detect collagen-induced platelet activation in human platelet-rich plasma. Purified human C1q exhibited a greater inhibitory effect on collagen-induced platelet aggregation in turbidimetry than free platelet counting. Because the change from small to large platelet aggregates is responsible for the continuing increase in light transmission, C1q was likely more capable of blocking the formation of large platelet aggregates than the formation of small aggregates from single platelets. The rate of change by collagen in light transmission and free platelet count was reduced in the presence of C1q but the timing of the peak response remained the same. Electronic platelet sizing revealed that the volume of single platelets transiently increased during the turbidimetric "lag phase". The mean, mode and median volume of the remaining free platelets then decreased, suggesting a selective loss of large, functionally more active platelets and/or platelet degranulation. C1q had no effect on the volume increment during the "lag phase", but reduced the subsequent fall in the volume of free platelets.

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.

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 degrees 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 degrees 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.