Anti-thrombomodulin antibodies and venous thrombosis

Thrombomodulin (TM) is a cell surface endothelial glycoprotein having anticoagulant properties. It inhibits thrombin, and activates protein C, leading to the inhibition of activated factors V and VIII. TM autoantibodies could theoretically predispose to thrombosis. We have tested 83 unselected patients with deep venous thrombosis, 36 males and 47 females aged from 1 to 70 years [mean +/- standard deviation (SD), 34.2 +/- 14.5 years] for the presence of IgG anti-TM in their plasmas. Tests were performed by enzyme-linked immunosorbent assay (ELISA) using recombinant human TM kindly provided by PAION GmbH (Aachen, Germany). Results are expressed as the optical density (OD) differences between coated and un-coated wells. Plasmas from 83 normal volunteer donors were used to define the cut-off value as the mean of absorbance of the control group + 3 SDs. The median OD of normal controls group was 0.024 (mean, 0.034; SD, 0.066; range, -0.048 to 0.309). The median OD obtained with plasmas from patients was 0.048 (mean, 0.114; SD, 0.215; range, -0.039 to 1.312) and was significantly higher than that of the control group (P < 0.0001). Choosing a cut-off value of 0.232 (mean OD of the control group + 3 SDs), 11 patients are considered as positive for IgG autoantibodies to TM and three normal controls are weakly positive. Selected plasma with IgG anti-TM and purified IgG were further tested by dot blot using recombinant purified TM and were found positive. Purified IgG of positive plasmas inhibits protein C activation by TM and thrombin, suggesting that anti-TM antibodies have a procoagulant effect. Interestingly, in our study, anti-TM antibodies are found in three of six patients with Budd-Chiari syndrome and four of eight patients with cerebral venous thrombosis. In conclusion, thrombomodulin autoantibodies could be a new interesting marker of thrombophilia easily detected by ELISA.