Monoclonal antibody specific for tissue factor pathway inhibitor-factor Xa complex: Its characterization and application to plasmas from patients with disseminated intravascular coagulation and pre-disseminated intravascular coagulation

Tissue factor pathway inhibitor (TFPI), a Kunitz-type protease inhibitor with three tandem inhibitory domains (K1, K2 and K3), inhibits the initial reactions of the extrinsic blood coagulation pathway through its K1 and K2 domains. We prepared and characterized a monoclonal antibody (Mab8-1) against TFPI-factor Xa (TFPI-Xa) complex. The reactivities of Mab8-1 toward TFPI-Xa complex, TFPI without C-terminal (TFPI-C)-Xa complex, K1K2-Xa complex and K2K3-Xa complex were examined using a surface plasmon resonance analysis (Biacore®). The Biacore system allowed a quantitative analysis of antibody- antigen interaction, in real time, from which the association and dissociation rate constants could readily be obtained. The bindings of Mab8-1 to TFPI-Xa complex, TFPI-C-Xa complex and K2K3-Xa complex were each concentration-dependent. However, no binding of Mab8-1 to the K1K2-Xa complex was observed. The binding of Mab8-1 to TFPI or Xa was also not observed. These results suggested that the epitope for Mab8-1 was exposed in the K3 domain of TFPI, which was generated by the conformational change after the formation of TFPI-Xa complex. We then developed an enzyme-linked immunosorbent assay method specific for TFPI-Xa complex using Mab8-1, and we used this assay to measure plasma levels of TFPI-Xa. The normal range assessed from analyses of plasma from 30 normal healthy volunteers was 17.7- 66.7 with a mean of 35.5 ± 11.7 pmol/l. In order to asses the clinical implication of TFPI-Xa complex in the plasma of patients with thrombotic disorders, plasma concentrations were measured in 37 patients with disseminated intravascular coagulation (DIC) caused by a variety of underlying diseases. The TFPI-Xa antigen levels were significantly higher in the patients with DIC (51.9 ± 21.6 pmol/l) and the 36 patients with pre-DIC (55.1 ± 20.2 pmol/l) than in the 137 non-DIC patients (37.9 ± 13.1 pmol/l). In the patients with DIC or pre-DIC, there was no significant correlation between TFPI-Xa complex and the elevated levels of thrombin-antithrombin complex, plasmin-α2 plasmin inhibitor complex, D-dimer, soluble fibrin monomer, soluble thrombomodulin or tissue factor. These data indicate that the plasma level of TFPI-Xa seems to be a novel independent molecular marker of DIC and pre-DIC.