Selective inhibition of platelet activation by the amyloid P-component of serum

One component of amyloid, protein AP, has a characteristic pentameric structure and is identical with a 9.5s serum alpha 1-globulin designated serum amyloid P-component or SAP. Another pentameric molecule, the acute-phase reactant C-reactive protein (CRP), shares major amino acid sequence homology with SAP although, in man, SAP is not an acute-phase reactant. Recently, we demonstrated that heat-aggregated CRP (H-CRP), like heat-aggregated IgG, activates platelets to reactions of aggregation, secretion, and generation of thromboxane A2. We report here that physiologic concentrations of SAP inhibit platelet aggregation stimulated by H-CRP. SAP must be present before platelet challenge with H-CRP to be effective. Native (unaggregated) CRP does not inhibit platelet activation induced by H-CRP, and the platelet inhibitory effect of SAP is restricted because platelet responses to each heat-aggregated IgG, acid-soluble collagen, DNA, ADP, and thrombin remain unaltered in the presence of SAP. Thus, human SAP seems to selectively modulate platelet reactivity to modified CRP, and as such to down-regulate at least one aspect of the biologic capacity of its acute-phase homologue.