Importance of Measuring Circulating ADAMTS13 Immune Complexes in Addition to Free Antibodies in Patients with Acquired TTP

Abstract 102

A severe functional deficiency of the von Willebrand factor (VWF) cleaving protease ADAMTS13 results in accumulation of uncleaved, highly adhesive VWF multimers in plasma. This VWF multimer accumulation leads to increased platelet aggregation and thrombus formation in the microcirculation. Anti-ADAMTS13 autoantibodies that either neutralize ADAMT13's activity or enhance protein clearance are the main cause of severe ADAMTS13 deficiency in acquired thrombotic thrombocytopenic purpura (TTP). Circulating anti-ADAMTS13 immune complexes (CIC) that may be a factor in disease pathogenesis have been described in plasma of TTP patients. We have developed an ELISA assay to specifically determine the levels of CIC in which the antigen portion is immobilized by a polyclonal anti-ADAMTS13 IgG and the immunoglobulin component detected by a class-specific antibody. Samples (individual or longitudinal) from a total of 100 patients with acute idiopathic TTP were used to compare IC levels with those of the free anti-ADAMTS13 antibodies, ADAMTS13 antigen (ADAMTS13:Ag), ADAMTS13 activity (ADAMTS13:Ac) and inhibitor. This analysis was to provide for the first time a complete picture of the autoantibody response in this population.

IgG-containing CIC were present in most of the patient samples, with IgG4-IC being the predominant type of CIC. A correlation between the subclasses of the free and complexed IgG anti-ADAMTS13 antibodies was observed, but the levels of CIC appeared to follow inverse kinetics to the free antibody and the inhibitory titer. We also found samples devoid of free ADAMTS13:Ag, but containing CIC, indicating that ADAMTS13, either newly synthesized or exogenously administered by plasma infusion during plasma exchange, had been quantitatively sequestered through formation of IC. ADAMTS13:Ag was also undetectable in some samples with high levels of free anti-ADAMTS13 antibodies but lacking CIC, suggesting that IC that had formed were already cleared at these time points.

Our study suggests that in addition to characterization of autoantibodies against ADAMTS13, ADAMTS13-specific CIC also need to be determined at time of diagnosis and during treatment to obtain a full picture of disease status. The formation of ICs capturing all freely available ADAMTS13 may have a high impact on treatment response, especially in patients who are refractory to treatment. The gathered data may thus help to timely implement alternative treatment strategies in patients unresponsive to standard treatments, thereby reducing unexpected disease relapses with fatal outcome.