September/October 1998
Volume 4 Issue 5

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EDITORIAL BOARD
A.Warner, Ph.D.
P. Biddinger, M.D.
C. Macpherson, M.D.
G. Retzinger, M.D., Ph.D.

Tissue Factor and Tissue Factor Inhibitor Pathway

Introduction
The tissue factor (TF) pathway of blood clotting is generally held to be the major physiological mechanism for triggering clotting in normal hemostasis (1). TF is a transmembrane glycoprotein which consists of three domains: a short cytoplasmic domain of 19 residues, a single transmembrane domain of 23 residues and a large extracellular domain of 219 residues. The extracellular domain is responsible for binding factor VII (F VII) which has affinity that promotes conversion of F VII to the enzymatically active form FVIIa. The resulting TF-FVIIa complex activates two serine protease zymogens, factors IX and X to IXa and Xa respectively. Factor Xa activates prothrombin to thrombin. This leads to fibrinogen cleavage and the deposition of polymerized fibrin.

Tissue Factor Pathway Inhibitor
Tissue factor pathway inhibitor (TFPI) plays a major role in regulating TF-induced coagulation. TFPI is primarily synthesized by the endothelium under normal physiologic conditions. There are at least three pools of TFPI in vivo:

• 80-85% remains associated with endothelial surface presumably bound to cell-surface glycosaminoglycans
• 10% is associated with lipoproteins in plasma
• 3% is present in platelets.

1. The middle domain of TFPI binds to factor Xa.
2. The first domain binds to factor VIIa in the TF-VIIa complex.

Tissue Factor and Tissue Factor Inhibitor Assays
Circulating blood normally is not exposed to cells expressing surface membrane TF activity, which is why activation of blood coagulation in healthy individuals is negligible. Once monocytes are exposed to a perturbing stimuli, they start to express TF activity.

Clotting assays, chromogenic methods, and flow cytometry have been used to measure monocyte TF activity. The results are difficult to interpret since purification of monocyte populations may artifactually activate the cells. There is evidence that monocyte TF expression is elevated in a variety of clinical conditions (Table 1). Despite its potential usefulness as a diagnostic tool, problems of methodology seem to preclude the use of monocyte TF assessment as a clinical laboratory test.

Plasma TFPI levels can be measured using a variety of assays. Chromogenic substrate assays have been described in which diluted test plasma is incubated with tissue factor, factor VIIa, and factor X. The amount of factor Xa generated is inversely proportional to the amount of TFPI present in the sample.. This functional assay has a sensitivity of 125 - 2,000 ng/ml. Other functional assays are based on a modified prothrombin time assay in which the clotting time is measured in the presence and absence of anti-TFPI antibody. A sandwich-type ELISA assay has been developed for the measurement of plasma TFPI antigenic levels with a sensitivity of 0-400 ng/ml. The immunologic assay offers the advantage of high sensitivity as well as specificity for TFPI, however, this assay does not determine if the TFPI being measured has anticoagulant activity (3).

Table 1. Conditions Associated with Elevation of Monocyte
Tissue Factor Expression.

Tissue Factor in Health and Disease
TF is constitutively expressed in a variety of normal human tissues, being particularly abundant in brain, lung and placenta. TF appears to form a protective lining around tissues and blood vessels ready to activate blood coagulation after vascular injury. No natural TF-deficient state has yet been described, leading to the postulate that its absence may not be compatible with survival.

TF has been implicated in the pathophysiology of DIC, atherosclerosis and malignancy (4). The condition in which the essential role of TF has been best documented, mainly on the basis of experimental studies, is endotoxin-induced DIC. The TF activity of circulating monocytes has been shown to be elevated in DIC.

In malignant disease, there is a high incidence of coagulation and fibrin deposition at the sites of tumor growth. Although the mechanisms underlying these phenomena involve multiple factors, TF is generally considered to be prominent and is thought to arise from the tumor cells.

TF antigen and activity are found in abundance in human atherosclerotic plaques, particularly in the lipid rich core. The macrophage is likely to be the major source of TF within the plaque. In acute arterial injury, smooth muscle cells appear to be the chief source of TF. The presence of TF within the plaque or at the site of vessel injury leads to rapid fibrin deposition.

Conclusion
TF is recognized as the most potent activator of blood coagulation. TF activity is increasingly being associated with the pathogenesis of various disease states. Several methodologies are available for clinical measurement of TF and TFPI activity, however they all await FDA approval.

References

1. Morrisey, J.H. Tissue factor interactions with factor VII: measurement and clinical significance of factor VIIa in plasma. Blood Coagulation and Fibrinolysis 6, Suppl. 1995;1:S14-9.
2. Bajaj, M.S. and Bajaj, P.S. Tissue factor pathway inhibitor: Potential therapeutic applications. Thrombosis and Haemostasis 1997;78: 471-7.
3. Bognacki, J. and Hammelburger, J. Functional and immunologic methods for the measurement of human tissue factor pathway inhibitor. Blood Coagulation and Fibrinolysis 6, Suppl. 1995;1:S65-2.
4. Semerano, N. and Colucci, M. Tissue factor in health and Disease. Thrombosis and Haemostasis 1997;78:759-4.