Jd. Oosting et al., AUTOANTIBODIES DIRECTED AGAINST THE EPIDERMAL GROWTH FACTOR-LIKE DOMAINS OF THROMBOMODULIN INHIBIT PROTEIN-C ACTIVATION IN-VITRO, British Journal of Haematology, 85(4), 1993, pp. 761-768
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.