Thrombosis in primary antiphospholipid syndrome: a pivotal role for monocyte tissue factor expression

The antiphospholipid syndrome (APS) is a disorder of recurrent thrombosis, pregnancy loss, and thrombocytopenia associated with the production of anticardiolipin antibodies (aCL) and lupus anticoagulant (LAC). The mechanisms of thrombus formation remain unknown. Tissue factor (TF), an inducible cell glycoprotein, is a major initiator of coagulation in vivo. The present study was therefore undertaken to investigate TF expression and procoagulant activity on monocytes from patients with primary APS and its correlation with thrombotic events.

Three groups of patients were studied: group 1 comprised 23 primary APS patients with thrombosis, group 2 consisted of 10 primary APS patients without thrombosis, and group 3 contained 20 patients with thrombosis but without antiphospholipid antibodies. Twenty age- and sex-matched healthy blood donors were used as controls (group 4). Anticardiolipin antibodies were measured by enzyme-linked immunosorbent assay (ELISA) and LAC by standard methodology. Cell surface expression of TF on monocytes was assessed by flow cytometry. The amount of TF in cell lysates (TF(Ag)) and soluble TF(Ag) plasma levels were analyzed by ELISA, and the TF-related procoagulant activity (PCA-TF) on intact cells and cell lysates by a chromogenic assay. Levels of the cytokines that influence TF production, i.e., tumor necrosis factor alpha (TNF alpha) and interleukin-1beta (IL-1beta), were determined by ELISA.

Cell surface expression of TF was increased in group 1 (mean +/- SEM 50.2 +/- 4% positive cells) compared with group 2 (14.6 +/- 1.6%), group 3 (16.8 +/- 3.7%), and group 4 (14.1 +/- 1.6%). TF(Ag) levels were also elevated in group 1 (215.8 +/- 11.2 pg/10(6)) compared with group 2 (150.8 +/- 15.2), group 3 (101.4 +/- 14.8), and group 4 (80.32 +/- 5.5). PCA-TF on intact cells and cell lysates was significantly increased in group 1 (148.8 +/- 16.3 units/10(5) lysate cells, compared with 54.5 +/- 11.5, 38.6 +/- 9.7, and 22.5 +/- 3.1 in groups 2, 3, and 4, respectively). Among group 1 patients, there was a significant increase in the degree of TF expression in those positive for IgG aCL, but not in those positive for IgM aCL or LAC. TNF alpha and IL-1beta plasma levels did not differ significantly between any of the groups.

These results suggest that monocyte TF expression is directly involved in the pathogenesis of thrombotic complications in patients with the primary APS.