PB0187 - FIX binding to collagen IV generates thrombin via the intrinsic tenase

Background: Haemophilia B is a blood clotting disorder caused by deficiency in coagulation Factor IX (FIX). Patients are treated by infusion of FIX concentrates based on their plasma concentration. FIX binds reversibly to collagen IV in the basal lamina of blood vessels and may be the site of an extravascular store.

Aims: To determine if the FIX-collagen IV complex participates in coagulation.

Methods: Flat-bottomed plates were coated with collagen IV prior to application of recombinant FIX. Wells were blocked and washed to prevent unspecific binding and remove unbound protein. Thrombin generation (TG) was performed in FIX deficient plasma (FIX-DP) or pooled normal plasma (PNP) and measured using the Calibrated Automated Thrombogram. In some cases, 2 x 108 plt/mL isolated from a healthy donor, 1 pM tissue factor (TF) or 600 nM corn trypsin inhibitor (CTI) were incorporated. FIX activity was measured using a commercial chromogenic assay.

Results: No thrombin was generated by FIX-DP ± platelets on uncoated or collagen coated surfaces. However, on surfaces with the FIX-collagen complex present, FIX-DP + platelets generated a peak thrombin concentration comparable to PNP; 139 vs. 162 nM, respectively. Addition of 1 pM TF to FIX-DP + platelets shortened the lag time by 24-fold and increased the peak. Inhibition of the contact pathway by an excess of CTI confirmed that thrombin generation was not taking place via contact activation. FIX activity while in complex with collagen was confirmed by chromogenic assay to be approximately 33 µM.

Conclusion(s): Our data suggest that collagen may induce activation of FIX and raises the possibility that the intrinsic tenase may assemble on the collagen surface. Collagen IV binding of FIX may provide a long-lasting extravascular reservoir of FIX at a haemostatic and functional location, such as the joints.