A Monoclonal Antibody That Recognizes a Neo-Antigen Exposed in the E Domain of Fibrin Monomer Complexed With Fibrinogen or Its Derivatives: Its Application to the Measurement of Soluble Fibrin in Plasma

Using urea-solubilized human fibrin monomer as an immunogan, we raised in mice a battery of monoclonal antibodies that reacted with the immunogen but not with urea-treated or native fibrinogen. Although they all failed to react with acid-solubilized fibrin monomer (acid-FM) alone, an antibody designated as IF-43 was found to recognize acid-FM, which was bound with fibrinogen or its derivatives to form a 1:2 complex of soluble fibrin. The epitope for this antibody, thus, appears to be exposed most probably by conformation changes induced in the acid-FM molecule upon formation of the complex. Because IF-43 was able to recognize fibrin-derived plasmic fragment E treated with urea but not the thrombin- and urea-treated amino-terminal disulfide knot of fibrinogen, the presence of the Aα (52-78) residue segment seems to be prerequisite for the epitope expression. The antibody was found to react with soluble fibrin monomer spiked to normal plasma dose-dependently up to 200 fig/ μL. By an aggregation assay using latex beads coated with IF-43, we found that concentrations of soluble fibrin monomer in plasma derived from patients with thrombotic diseases were mostly elevated, but not necessarily correlated with those of the D-dimer, reflecting another aspects of the disease. Furthermore, the soluble fibrin monomer in plasma derived from patients with thrombotic diseases was found to be depleted solely of the A peptides, but not the B peptides, based on its subunit polypeptide compositions lacking the β-chain on immunoblotting.