Induction of tissue factor-like activity in monocytes by anti-cardiolipin antibodies.

Anti-cardiolipin Abs (ACLA) are present in the sera of patients with antiphospholipid syndrome (APLS) and are associated with high incidence of thromboembolic phenomena, fetal loss, thrombocytopenia, and prolongation of the phospholipid-dependent coagulation assays (lupus anticoagulant). Recently, it has been shown that APLS can be induced experimentally by using ACLA. However, the pathophysiology of thrombus formation in this syndrome is unknown. Monocytes generate a potent procoagulant activity (PCA) after stimulation with various substances. Increased PCA has been found in monocytes from patients with diseases that are associated with high incidence of thromboembolic phenomena. In the present study, we report that the monoclonal ACLA that were shown previously by us to induce APLS stimulate mononuclear cells to generate a potent PCA. The PCA resembled tissue factor (TF) in that it accelerated clotting through the extrinsic coagulation pathway, was abolished by phospholipase C, and was inhibited by anti-TF mAbs. The induction of TF-like activity by ACLA in monocytes was dose- and time-dependent. It was induced in monocytes and monocytic cell lines, but not in lymphoid or myeloid cells, and did not require T lymphocytes for expression. The generation of PCA was dependent on protein synthesis inasmuch as it was prevented by adding puromycin to the system and was not affected by cytarabine. The TF-like activity that is induced by ACLA in monocytes may activate coagulation and thereby play a major role in the pathogenesis of thrombus formation in APLS.