A monoclonal antibody recognizing an icosapeptide sequence in the heavy chain of human factor XII inhibits surface-catalyzed activation.

A monoclonal antibody (mAb B7C9) to human factor XI1 was raised in murine somatic cell using purified factor XI1 antigen.The purified antibody was subtyped IgGIK and had a K D of 9.8 n M for antigen factor XII.Functional studies indicated that mAb B7C9 blocks surface-mediated coagulant activity of factor XI1 but not the amidolytic activity of factor XIIa against the small substrate H-D-Pro-Phe-Arg-p-nitroanilide (S-2302), suggesting that the mAb B7C9 epitope is located at or near the surface binding domain of the heavy chain region of factor XII.Western blot analysis indicated that the antibody reacts with factor XI1 and the heavy chain of factor XIIa. Affinity isolation of factor XI1 peptides, produced after cleavage by kallikrein, resulted in three factor XI1 heavy chain domain segments that were identified in the known factor XI1 sequence by limited N-terminal analysis.The epitope was located to a 20-amino acid sequence of 2.5 kDa in the heavy chain of factor XI1 which is the putative surface binding region of factor XII.The 2.5-kDa peptide was synthesized and demonstrated to react with mAb B7C9.mAb B7C9 was immobilized on an affinity resin and was successfully utilized to purify functionally active factor XII from plasma.Human factor XII' is a single chain 80-kDa plasma glycoprotein.After activation, factor XI1 is responsible for initiating the intrinsic pathway of blood coagulation, participates in the release of kinins, is implicated in the activation of the fibrinolytic and complement systems, and stimulates neutrophils (1, 2).In uitro, activation of factor XI1 by kallikrein initially results in the cleavage of a single peptide bond.The product, with an identical molecular weight, factor XIIa, is composed of a light chain, 28 kDa, containing the catalytic site of the enzyme linked by a disulfide bond to a heavy chain, 50 kDa, containing the surface binding region of the molecule (3).Further proteolysis of factor XIIa is known to transform it t o a smaller enzyme, factor XIIf, of 32 kDa, which lacks most of the heavy chain of factor XIIa (3).The complete amino acid sequence of human factor XI1 has been determined * Preliminary reports of this study were presented at the annual national meeting of the American Federation for Clinical Research, Washington, D. C., May 3-6, 1985.The costs of publication of this article were defrayed in part by the payment of page charges.This ance with 18 U.S.C.Section 1734