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Title
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Epitope location on tissue factor determines the anticoagulant potency of monoclonal anti-tissue factor antibodies.
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Authors
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D.Kirchhofer,
P.Moran,
N.Chiang,
J.Kim,
M.A.Riederer,
C.Eigenbrot,
R.F.Kelley.
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Ref.
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Thromb Haemost, 2000,
84,
1072-1081.
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PubMed id
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Abstract
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Tissue factor (TF), the cellular cofactor for the serine protease factor VIIa
(F.VIIa), triggers blood coagulation and is involved in the pathogenesis of
various thrombosis-related disorders. Therefore, agents which specifically
target tissue factor, such as monoclonal antibodies, may provide promising new
antithrombotic therapy. We mapped the epitopes of several anti-TF antibodies
using a panel of soluble TF mutants. They bound to three distinct TF regions.
The epitope of the 7G11 antibody included Phe50 and overlapped with a TF-F.VIIa
light chain contact area. The common epitope of the antibodies 6B4 and HTF1
included residues Tyr94 and Phe76 both of which make critical contacts to the
catalytic domain of F.VIIa. The antibodies D3 and 5G6 had a common epitope
outside the TF-F.VIIa contact region. It included residues Lys 165, Lys 166,
Asn199, Arg200 and Lys201 and thus overlapped with the substrate interaction
region of tissue factor. The antibodies 5G6 and D3 were potent anticoagulants
when infused to flowing human blood in an ex-vivo thrombosis model. Plasma
fibrinopeptide A levels and fibrin deposition were completely inhibited. In
contrast, 6B4 was a weak inhibitor in this ex-vivo thrombosis model, and HTF1
displayed no inhibition at all. These disparate activities were also reflected
in TF-dependent F.X activation assays performed with human plasma. The potency
differences could neither be explained by the determined binding affinities nor
by the on-rates of antibodies. Therefore, the results suggest that antibody
binding epitope and hence the particular mechanism of inhibition, is the main
determinative factor of anticoagulant potency of anti-TF antibodies.
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