Degree of polymer organization decreases the binding of a monoclonal antibody raised against the beta-chain amino terminus of fibrin.

UNLABELLED: Accurate non-invasive diagnosis of deep venous thrombosis and pulmonary embolism remains an elusive goal. Radiolabeled antibodies specific for the epitope exposed on the beta-chain of fibrin after fibrino-peptide B release (anti-beta) enabled in situ imaging of thrombi in experimental subjects with nuclear medicine techniques. When used in patients anticoagulated for thrombo-embolic disease, however, the antibody was unable to reliably image the thrombi. We postulated that the neoepitope on the beta-chain of fibrin is covered up as fibrin organizes into a polymer network and is therefore exposed to the antibody only during active incorporation of fibrin subunits. We determined the equilibrium binding kinetics of an anti-beta monoclonal antibody to fibrin in various stages of organization. The concentration of exposed epitopes on immobilized fibrin monomers was equal to the molar concentration of fibrin beta-chains. The percentage of beta-chains exposed to the antibodies markedly decreased as the fibrin network was allowed to organize, a process catalyzed by calcium.
CONCLUSIONS: The beta-chain amino terminus of fibrin is exposed transiently as subunits are added to the enlarging fibrin network. Anti-beta antibodies bind preferentially to actively enlarging fibrin polymers.