Alterations of ADAMTS-13 Activity as a Common Indicator of the Endothelial Dysfunction Developing in Different Thrombotic Microangiopathies.

Injury to the endothelial cell is a key factor for the initiation and development of thrombotic microangiopathies (TMA). Endothelial dysfunction implies alterations of the von Willebrand factor (vWF) pathway. Severe deficiency of the vWF-cleaving protease activity has been traditionally associated with thrombotic thrombocytopenic purpura (TTP). However, there is ongoing evidence indicating that ADAMTS-13 may be a causal factor in the pathogenesis of other TMA.

We evaluated whether ADAMTS-13 deficiency and autoantibodies inactivating the protease were prevalent in TMA with different physiopathological origin. Patients included in the study presented TMA associated with: TTP (n=13), hemolytic uremic syndrome (HUS) (n=4), gastrointestinal angiodysplasia (GA) (n=11), or stem cell transplantation (SCT) (n=3). We measured ADAMTS-13 activity and antigen (ADAMTS-13:Ag), the presence and inhibitory activity of autoantibodies to the protease, vWF:Ag, and vWF:multimeric pattern. ADAMTS-13 activity and inhibitory activity of autoantibodies were analyzed using a modification of the immunoblotting assay described by Furlan et al (1996). ADAMTS-13:Ag and the presence of IgG were measured by ELISA kits.

The vWF:multimeric pattern was resolved by electrophoresis in agarose gels.

In patients with TTP, undetectable ADAMTS-13 activity was found in 10 out of 13 patients, 9 of which had IgG against the metalloprotease and reduced ADAMTS-13:Ag levels. Plasma exchange normalized the ADAMTS-13:Ag levels and decreased IgG title. Interestingly, one case of TTP with autoantibodies was associated with clopidogrel treatment. Levels of vWF:Ag in these patients varied from 50 to 180%.

As for patients with HUS, in 2 out of 4 ADAMTS-13 activity was absent and ADAMTS-13:Ag was reduced, but no IgG to the metalloprotease was detected. It should be pointed out that one of the cases of HUS was subsequent to cocaine abuse. In TMA associated with SCT and GA, ADAMTS-13 activity was around 20% in all the patients studied, although levels of the metalloprotease were within the control range (700–1400ng/ml). vWF:Ag levels were found to be significantly increased (to around 200%) in the former group of patients.

In summary, presence of autoantibodies to ADAMTS-13 concurred with decreased antigen levels, reinforcing the hypothesis of an accelerated clearance of the metalloprotease in vivo. Among the patients studied, we found a clopidogrel associated TTP and a cocaine caused HUS. Presence of antibodies to ADAMTS-13 associated with clopidogrel treatment would suggest an immunological mechanism, which cannot be ruled out in the case of cocaine associated HUS. Overall, our results indicate that the absence of ADAMTS-13 activity is not specific for TTP. A reduced activity of the metalloprotease may be an indicator of endothelial dysfuntion as a common feature in different TMA.