Key result
C1qR occupancy by C1q or anti-C1qR antibodies inhibited collagen-induced platelet aggregation and release, but did not inhibit platelet adhesion to collagen.
Under defined in vitro conditions, C1q receptors modulate collagen-induced platelet aggregation and secretion, but not platelet adhesion.
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ClqR effects on platelets remain in vitro only; leaves open any in vivo role in thrombosis or hemostasis.
We recently described specific binding sites for Clq on human blood platelets that cross-react with antibodies against Clq receptors (ClqR) on lymphoblastoid cells. Inasmuch as Clq inhibits collagen-induced platelet aggregation, we compared the effects of ClqR occupancy by purified Clq, monoclonal (IIl/B5) and polyclonal anti-ClqR antibodies on collagen-induced platelet adhesion, release, and aggregation. Washed platelets in buffered Tyrode's solution containing 2 mM magnesium were preincubated (30 min, 22 degrees C) with antibodies, Clq, 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 degrees C with 51Cr-labeled platelets and collagen-coated microtiter wells. Concentrations of IIl/B5, polyclonal anti-Clq R antiserum, or Clq 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 ClqR occupancy by Clq and inhibition of platelet-collagen interactions. Whereas, Clq binding to platelets was divalent cation independent, neither Clq nor anti-ClqR antibodies prevented platelet adhesion to collagen in the presence of EDTA. These data suggest that under defined in vitro conditions, ClqR modulate collagen-induced platelet aggregation and secretion, but not platelet adhesion.
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Peerschke et al. (1990) studied this question. C1qR occupancy by purified C1q and anti-C1qR antibodies vs. control buffers and antiserum was evaluated on collagen-induced platelet adhesion, release, and aggregation. C1qR occupancy by C1q or anti-C1qR antibodies inhibited collagen-induced platelet aggregation and release, but did not inhibit platelet adhesion to collagen.
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