AUTOANTIBODIES DIRECTED AGAINST THE EPIDERMAL GROWTH FACTOR-LIKE DOMAINS OF THROMBOMODULIN INHIBIT PROTEIN-C ACTIVATION IN-VITRO

No consensus has been obtained about the question whether autoantibodi es, in particular antiphospholipid antibodies (aPL), may cause thrombo sis by inhibiting thrombomodulin (TM) mediated protein C activation. I n order to clarify the mechanism by which autoantibodies inhibit TM-me diated protein C activation, we have screened 12 patients with autoimm une diseases for the presence of circulating autoantibodies inhibiting TM function. In a cross-sectional study we found that IgG fractions f rom two patients (who were aPL negative) inhibited TM mediated protein C activation in an assay system using purified components. A longitud inal study of six patients with a history of thrombosis of which two w ere aPL positive showed that all had at some time circulating antibodi es inhibiting TM function, suggesting that the presence of these antib odies is transient. Three different TMs were used to identify the epit ope of the antithrombomodulin antibodies (aTM): rabbit TM, which conta ins the entire TM molecule; Solulin, which contains the extracellular part of TM, and rEGF-TM, which contains the six epidermal growth facto r (EGF) domains of TM. We showed that the aTM inhibited protein C acti vation mediated by all three TMs, indicating that the aTM are directed against the region containing the EGF domains. When TM was incorporat ed in phospholipid vesicles, no inhibition by these aTM could be demon strated. In addition, protein C activation mediated by cultured endoth elial cells (EC) could not be inhibited by aTM. The lack of inhibition of TM in phospholipid vesicles and EC-TM by aTM suggests that aTM onl y inhibit soluble TM. In conclusion, we demonstrated the transient pre sence of circulating autoantibodies directed against the region of TM containing the EGF domains in SLE patients with a history of thromboti c complications. We postulate that the presence of antibodies to solub le TM may be, in addition to aPL, a risk factor for the occurrence of thrombosis in patients with autoimmune diseases.