Platelet agonist synergism by the acute phase reactant C-reactive protein

C-reactive protein is the prototypic acute phase reactant. A self-complexed form (H-CRP) can induce isolated platelets to undergo aggregation, secretion of dense and alpha-granule constituents, and generation of thromboxane A2, but fails to function in platelet-rich plasma (PRP) as a direct agonist. In contrast, when PRP was activated with an amount of adenosine diphosphate (ADP) that produced only reversible platelet aggregation, the presence of H-CRP resulted in irreversible aggregation and the secretion of adenosine triphosphate (ATP). Following a maximum stimulus with ADP alone, where platelet secretion occurred late during the aggregation response, the presence of H-CRP shifted and increased the secretory burst to a time simultaneous with the onset of aggregation. This hypersecretion required H-CRP to be present prior to platelet stimulation or to be added within 15 to 30 seconds following the addition of ADP. H-CRP also potentiated platelet activation stimulated with epinephrine, thrombin, and collagen. When the synergism generated in PRP by H-CRP in the presence of ADP or epinephrine was compared to the synergism similarly produced by aggregated human IgG, collagen, or thrombin, it more closely resembled that of collagen, as reflected by the kinetics and characteristics of synergism and sensitivity to creatine phosphate/creatine phosphokinase or 5,8,11,14-eicosatetraynoic acid. These data provide a philosophically ideal niche for the acute phase (and C-reactive protein) in that a platelet-directed activity associated with this acute phase reactant is not utilized unless platelets are otherwise challenged.