Reduced ADAMTS13 levels due to congenital deficiency or development of ADAMTS13 autoantibodies has been associated with the severe disorder, thrombotic thrombocytopenic purpura (TTP). ADAMTS13 autoantibodies, reduced activity and elevated level of ADAMTS13/FXI complex have also been reported in TTP and venous thromboembolism (VTE) (1,2). Recently decreased ADAMTS13 activity or high titers of ADAMTS13 autoantibodies were reported in antiphospholipid syndrome (APS) (3), an autoimmune disease associated with VTE, arterial thrombosis and recurrent pregnancy loss. The diagnosis of APS relies upon determination of Lupus anticoagulants (LA) and various antiphospholipid autoantibodies (e.g. anti-CL and anti-beta2GP1). In this study we measured ADAMTS13 autoantibody, antigen, activity and ADAMTS13/fXI complex in plasmas of patients previously evaluated for LA. All of the LA plasmas were STACLOT LA positive and DRVVT negative at the time of diagnosis. In this study we also tested each plasma for dilute Prothrombin time, dPT (ACTICLOT® dPT™ with confirm procedure (4)) and measured anti-CL (IgG and IgM) and anti-beta2GPI (IgG and IgM) antiphospholipid antibodies. ADAMTS13 activity was measured using a new FRET assay, ACTIFLUOR™ ADAMTS13 (1). ADAMTS13 autoantibodies (IgG and IgM) were measured using IMUBIND® ADAMTS13 Autoantibody ELISA and ADAMTS13/fXI complexes using IMUBIND ADAMTS13/fXI ELISA. D-dimer levels were also measured (IMUCLONE® FDP ELISA). In this cohort of LA plasmas, 48/100 tested positive in the dPT screen and 23/48 in dPT confirm. Medium titer of aCL IgM (24/100, 24%) and anti-beta2GPI IgM (11/100, 11%) were predominantly found. ADAMTS13 activity and antigen levels generally fell within the range from normal donors. Anti-ADAMTS13 IgG titer (>14 au/mL) was detected in 11/91 (12%) and anti-ADAMTS13 IgM (>85 au/mL) in 22/96 (23%) plasmas. A subset of thirty- four plasmas was tested for ADAMTS13/fXI complex, 16/34 (47%) had elevated level of ADAMTS13/fXI complex. In the 16 plasmas with elevated ADAMTS13/fXI complex, 13/16 (81%) had elevated aCL IgM and 1/16, elevated beta2GPI IgM. The D-dimer level, a key marker of VTE, was high in 11/16 (68%) of these ADAMTS13/fXI complex -containing plasmas. In a subset of thirty-one plasmas with positive dPT screen and assayed for D-dimer, elevated D-dimer was found in 71% (22/31), of which 50% (11/22) was dPT confirmed.
These findings support the hypothesis that ADAMTS13 autoantibodies and ADAMTS13/fXI represent novel biomarkers in thrombotic disorders including APS.
Disclosure:Employment: EG, NG, HJLF, CS and RSG are employed by American Diagnostica Inc, EE is ADI Summer research intern. JD and MB are employed by Quest Diagnostics.