Monoclonal antibodies against antithrombin III. Identification of their epitopes and effects on antithrombin III activities

Conformational change in antithrombin induced by heparin probed with a monoclonal antibody against the 1C/4B region

A murine monoclonal antibody (MAb) raised against a covalent antithrombin-heparin complex was used to probe the conformational change resulting when the serpin antithrombin binds to heparin. This MAb completely inhibited the progressive activity of antithrombin against thrombin. However, although the MAb remained bound to antithrombin in the presence of heparin, it did not significantly inhibit heparin cofactor activity against thrombin, and increasing concentrations of the antithrombin-binding pentasaccharide progressively unblocked the inhibitory action of the MAb. The MAb bound to antithrombin without affecting either heparin-binding affinity or heparin-induced fluorescence enhancement, and it did not convert antithrombin from inhibitor to substrate. The MAb failed to interact with reduced and S-carboxymethylated antithrombin, indicating the conformational nature of its epitope. Antithrombin variants with N-terminal substitutions (Arg47-->Cys or His, Leu99-->Phe, Arg129-->Gln) modifying heparin binding, and C-terminal substitutions affecting the reactive site (Arg393-->Cys) or resulting in substrate-variant antithrombin (Ala384-->Pro), were all recognized normally, as were normal reactive site cleaved antithrombin and the thrombin-antithrombin complex. However, interaction of the MAb with antithrombin was reduced by several substitution mutations (Phe402-->Cys, Phe402-->Ser, Phe402-->Leu, Ala404-->Thr, Pro407-->Thr) in the 402-407 sequence which codes for amino acid residues of strand 1C and the polypeptide leading to strand 4B. Pro429-->Leu also blocks recognition [Olds et al. (1992) Blood 79, 1206-1212], and this residue is believed to be spatially approximated to strand 1C.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies against human antithrombin III

Three monoclonal antibodies identified as D8, B11 and C5 of different specificities have been produced against human antithrombin III (AT). The apparent dissociation constants (Kd app) of the AT-antibody interaction were determined by ELISA method: Kd app (D8) = 2.4 nmole, Kd app (B11) = 13 nmole, Kd app (C5) = 24 nmole. All three antibodies reacted with isolated AT on immunoblots obtained with "native" PAGE. The D8 antibody also reacted with plasma and serum AT while B11 antibody reacted with serum thrombin-antithrombin (TAT) complexes as well.

Release of a small two-chain form of antithrombin III from a conformationally changed antithrombin III-thrombin complex

Reaction of antithrombin III (AT) with thrombin results in the formation of stable antithrombin III-thrombin (AT-T) complex with a Mr of 92.5-kDa, accompanied by the appearance of a proteolytically modified form of the inhibitor (ATM). Under these conditions AT-T is also transformed to a smaller complex (AT-TS). This smaller complex (81-kDa), a product of a conformational change at the AT moiety of the AT-T complex, is further transformed to a very small complex (AT-TVS) with a Mr of 71-kDa. Along with this process, AT-TS slowly dissociates to a free enzyme and a small, presumably two-chain product of AT (ATMS) with a Mr of 49-kDa. The newly described component, ATMS, naturally occurs in plasma and serum and accumulates significantly in plasma of patients suffering from cardiovascular disease.

Characterization of the cellular binding domain and the effects of monoclonal antibodies and thrombin inhibitors on the binding and internalization of the antithrombin-III--thrombin complex by cultured cells

Antithrombin III (AT) binds to cultured cells mainly as a complex with thrombin or other serine proteases rather than in its free form. This implies that, upon complex formation, a new determinant appears on the AT molecule which is recognized by the cells. Fragmentation of AT by cyanogen bromide exposes this determinant and an 8-kDa fragment is recognized by cultured cells. The binding of this fragment to cultured cells is inhibited by antithrombin-III-thrombin (AT-T) complex, but not by free AT. The putative cellular binding domain of AT-T is located over amino acid residues 253-314 of AT, in the large loop close to the carboxy-terminus of the molecule. The cell-associated AT-T is internalized and degraded, forming the thrombin-modified AT (ATM). This process is inhibited by lysosomal degradation inhibitors, such as chloroquine or benzamidine. Thus, the appearance of ATM in cells is not a result of its binding to the cell surface, but probably a result of lysosomal degradation of cell-associated AT-T. Another degradation product, with a molecular mass of 43 kDa, appears in cells along with the appearance of ATM. Hirudin, a specific inhibitor of thrombin, inhibits the cellular internalization of AT-T complexes, indicating a possible role for thrombin activity in the internalization process of complexes. Monoclonal antibodies (mAb) against AT, whose epitopes on the AT molecule have been located, affect the binding of AT-T to cultured cells. Two mAb, A10 whose epitope is found outside of the cellular binding site, and B108 whose epitope may overlap this sequence, totally inhibit the binding of the complex to cells. Inhibition of binding results either from steric hindrance or induced changes in the binding site.