Truncated and Microparticle Free Soluble Tissue Factor Bound to Peripheral Monocytes Preferentially Activate Factor VII.

Soluble plasma tissue factor (TF) circulates in picomolar concentrations in healthy individuals and increases in a wide spectrum of diseases. This study tests the hypothesis that both truncated or long length TF antigens in low concentrations combine with monocytes or platelets to convert factor VII (fVII) to fVIIa. Both recombinant truncated TF (rsTF, kDa 33) and long length (47 kDa) plasma TF (pTF), obtained from pericardial wounds of patients having cardiac surgery using cardiopulmonary bypass, were studied in association with blood cells and TF bearing microparticles. Tissue factor was measured by ELISA. RsTF binds to erythrocytes, platelets, mononuclear cells and neutrophils. The rate of fVII conversion with rsTF (1.0 – 10³nmoles/L) is fastest with mononuclear cells, less with platelets, minimal with neutrophils and undetectable with erythrocytes. Both unstimulated and stimulated mononuclear cells or platelets in the presence of 3.5 pmoles/L rsTF or pTF convert fVII (10 nmoles/L) to fVIIa, but the amounts of fVIIa produced is greater in mononuclear cells. When leukocytes or platelets are absent, microparticles associated with 3.5 pmoles/L TF antigen, derived from pericardial wound plasma, do not activate fVII. Stimulated mononuclear cells convert nearly all available fVII (10 nmoles/L) to fVIIa with 3.5 pmoles/L pTF. Unstimulated mononuclear cells convert small amounts of fVII with as little as 1 pmole/L rsTF but no VIIa is produced by platelets, neutrophils or erythrocytes under these conditions. At all concentrations (0.001 to 1m moles/L) mononuclear cells more efficiently convert fVII to fVIIa than platelets. This study shows that stimulated mononuclear cells provide the most efficient phospholipid platform for activation of truncated or long length molecules of soluble TF at low concentrations of TF antigen. The results suggest a new mechanism by which soluble TF, which is increased in clinical inflammatory diseases, may lead to a prothrombotic state.