Modulation of platelet responses to collagen by C1q receptors

We recently described specific binding sites for C1q on human blood platelets that cross-react with antibodies against C1q receptors (C1qR) on lymphoblastoid cells. Inasmuch as C1q inhibits collagen-induced platelet aggregation, we compared the effects of C1qR occupancy by purified C1q, monoclonal (II1/B5) and polyclonal anti-C1qR antibodies on collagen-induced platelet adhesion, release, and aggregation. Washed platelets in buffered Tyrode's solution containing 2 mM magnesium were premcubated (30 min, 22°C) with antibodies, C1q, or appropriate control buffers and antiserum. Platelet aggregation and release measurements were made using ¹⁴C-serotonin-labeled platelets stimulated with type I collagen. Adhesion assays were performed in the presence of magnesium under static conditions at 22°C with ⁵¹Cr-labeled platelets and collagen-coated microtiter wells. Concentrations of II1/B5, polyclonal anti-C1q R antiserum, or C1q causing 82 to 92% (n = 7) inhibition of collagen-induced release and 67 to 98% inhibition of aggregation, failed to inhibit magnesium-dependent platelet adhesion to collagen. Inasmuch as divalent cation-independent platelet-collagen interactions have also been described, further studies were performed to compare the divalent cation requirement of C1qR occupancy by C1q and inhibition of platelet-collagen interactions. Whereas, C1q binding to platelets was divalent cation independent, neither C1q nor anti-C1qR antibodies prevented platelet adhesion to collagen in the presence of EDTA. These data suggest that under defined in vitro conditions, C1qR modulate collagen-induced platelet aggregation and secretion, but not platelet adhesion.