Specific binding of thrombin by human peripheral blood monocytes: possible role in the clearance of activated clotting factors from the circulation.

Abstract

The possibility that human PBM might specifically bind several proteins of the coagulation mechanism was studied in vitro. Purified preparations of human thrombin, AT III, TH-AT III complex, and prothrombin were radiolabeled with Na[125I] and incubated with human monocyte monolayers that had been isolated from whole blood by Ficoll-Paque sedimentation gradient and adherence to plastic tissue culture wells. Specificity of binding was determined by its suppression in the presence of excess concentrations of the appropriate nonradiolabeled protein. PBM were found to bind thrombin specifically but did not specifically bind AT III, TH-AT III complex, or prothrombin. Further characterization of thrombin binding by PBM showed that uptake was time- and temperature-dependent, saturable, and reversible, suggesting an active, receptor-mediated process. Scatchard analysis of [125I]thrombin binding in the presence of increasing concentrations of nonradiolabeled thrombin showed two populations of receptors: a high-affinity receptor with Kd = 3.4 x 10(-9) M and a low-affinity receptor with Kd = 1.3 x 10(-7) M. These studies indicate that PBM may play a role in the clearance of activated clotting factors from the circulation and serve as an experimental model to study the role of the RES as a defense against thrombosis.