D. Kirchhofer et al., Epitope location on tissue factor determines the anticoagulant potency of monoclonal anti-tissue factor antibodies, THROMB HAEM, 84(6), 2000, pp. 1072-1081
Tissue factor (TF), the cellular cofactor for the serine protease factor VI
Ia (F.VIIa), triggers blood coagulation and is involved in the pathogenesis
of various thrombosis-related disorders. Therefore, agents which specifica
lly target tissue factor, such as monoclonal antibodies, may provide promis
ing new antithrombotic therapy. We mapped the epitopes of several anti-TF a
ntibodies 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 antib
odies 6B4 and HTF1 included residues Tyr94 and Phe76 both of which make cri
tical 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 res
idues Lys165, Lys166, Asn199, Arg200 and Lys201 and thus overlapped with th
e substrate interaction region of tissue factor. The anti bodies 5G6 and D3
were potent anticoagulants when infused to flowing human blood in an ex-vi
vo thrombosis model. Plasma fibrinopeptide A levels and fibrin deposition w
ere completely inhibited. In contrast, 6B4 was a weak inhibitor in this ex-
vivo thrombosis model, and HTF1 displayed no inhibition at all. These dispa
rate activities were also reflected in TF-dependent F.X activation assays p
erformed with human plasma. The potency differences could neither be explai
ned 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 an
ticoagulant potency of anti-TF antibodies.