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SummaryThe effect of sera and IgG from 12 patients with systemic lupus erythematosus (SLE) on the endothelial cell (EC) procoagulant activity (PCA) was investigated in an in vitro thrombosis model. Six of the 12 SLE sera contained antiphospholipid antibodies (aPL).EC were stimulated for 8 h at 37° C with or without 50 pM tumor necrosis factor (TNF) in culture medium containing 20% patient or control serum. Then the endothelial cell matrix (ECM) was isolated and subsequently exposed in a perfusion chamber to circulating normal whole blood, anticoagulated with low molecular weight heparin (LMWH). The PCA of the ECM was determined as the amount of generated fibrinopeptide A (FPA) in samples taken before and after perfusion. Furthermore, cross sections were made of the perfused matrix and analyzed for platelet adhesion and aggregate formation.All six aPL containing sera induced a small, but significant increase of ECM procoagulant activity. When added in combination with a low dose of TNF (50 pM), a synergistic enhancement of ECM procoagulant activity was found. The FPA generation was increased to 150–614% from the values obtained after stimulation with TNF and control serum. Also a shift towards the formation of larger platelet thrombi was observed. After stimulation with TNF and patient serum the surface of ECM covered with large aggregates (> 5 µm) was increased by 124–329% compared to the results obtained after stimulation with control serum and TNF. When patient sera were depleted from IgG the effects were strongly decreased.These data show that the potentiation of TNF-induced PCA formation by aPL containing sera from patients with SLE leads to enhanced thrombus formation in an in vitro thrombosis model. This may help explain the increased thrombotic tendency in these patients.

SummaryThe interrelationship of antibodies directed against cardiolipin (CL), double stranded DNA (dsDNA), endothelial cells (EC) and blood platelets was investigated. IgG fractions, reactive with these antigens, were isolated from the plasmas from 8 patients with systemic lupus erythematosus and tested for crossreactivity with anionic phospholipids (CL, phosphatidylserine, phos-phatidylinositol), zwitterionic phospholipids (phosphatidyl-ethanolamine, phosphatidylcholine), dsDNA, EC and platelets by enzyme-linked immunosorbent assays and for lupus anticoagulant (LAC) activity with a coagulation assay.Our results demonstrate the frequent occurrence of crossreactivity between antibodies to anionic phospholipids, EC and platelets. Crossreactivities between these antibodies and antibodies to dsDNA or zwitterionic phospholipids are exceptional. LAC activity was found in the anti-CL, anti-EC and anti-platelet fractions of only one patient. These findings support the hypothesis that subpopulations of antibodies directed against negatively charged phospholipids can bind to EC and blood platelets, which may have implications for the pathogenetic potential of antiphospholipid antibodies.

SummaryThe effect of 15 antiphospholipid antibody (APA) positive SLE sera, 13 APA negative SLE sera, 10 APA negative sera from patients with other connective tissue diseases (CTD) and 15 control sera on the expression of endothelial procoagulant activity (PCA) was studied. Endothelial cells (EC) were stimulated with tumor necrosis factor (TNF) and 20% serum for 4 hr and the surface PC A expression was measured. Without TNF, none of the sera stimulated PC A expression. With subop timal TNF stimulation, 14/15 APA positive SLE sera, 7/13 APA negative SLE sera, 2/10 CTD sera and 2/15 control sera enhanced PC A expression. This stimulating effect resided in the IgG fraction and was associated with the presence of APA, but not with a history of thrombosis. Purified APA had no PC A stimulating activity. PC A expression was inhibited by cycloheximide and heat treatment (30 min, 56° C) of serum.In conclusion, 21/28 (75%) SLE sera increase the TNF-induced endothelial PC A expression. Although this effect predominantly occurs with APA positive serum, a causative role of APA was not demonstrated.

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