The active conformation of von Willebrand factor in patients with thrombotic thrombocytopenic purpura in remission.
BACKGROUND
Functional deficiency of ADAMTS13 in thrombotic thrombocytopenic purpura (TTP) patients is associated with circulating ultralarge von Willebrand factor (VWF) molecules that display spontaneous platelet-binding capacities. Upon remission, however, ADAMTS13 activity does not always return to baseline.


OBJECTIVE
To study ADAMTS13 and VWF-related features in TTP patients in remission.


METHODS
ADAMTS13 activity, anti-ADAMTS13 antibodies, VWF antigen, ultralarge VWF and levels of VWF that circulate in a glycoprotein Ibalpha-binding conformation were determined in plasma samples of 22 acquired TTP patients in remission between 1 month and 6 years after achieving remission. The composition of active multimers was investigated with a novel immunoprecipitation assay based on monoclonal antibody AU/VWF-a12, which specifically recognizes the active conformation of VWF.


RESULTS
ADAMTS13 activity was undetectable in 23% of the patients, even years after they had achieved remission, and lack of ADAMTS13 activity was associated with increased active VWF levels and the presence of ultralarge VWF multimers. Active VWF levels and ultralarge VWF were also associated with blood groups. Results from immunoprecipitation experiments revealed the full range of multimers to be present.


CONCLUSIONS
ADAMTS13 deficiency and the concurrent presence of ultralarge VWF and increased active VWF levels can be detected in TTP patients for years after they have achieved remission. Immunoprecipitation results suggest that the active conformation of VWF may be present in the lower molecular weight multimers, but future studies are necessary to confirm our findings.
Groot
E
E
Department of Clinical Chemistry and Haematology, University Medical Centre Utrecht, Utrecht, the Netherlands.
Fijnheer
R
R
Sebastian
S A E
SA
de Groot
Ph G
PG
Lenting
P J
PJ
D051722:ADAM Proteins / Q000378:metabolism; D000071120:ADAMTS13 Protein; D002478:Cells, Cultured; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D047468:Immunoprecipitation; D011487:Protein Conformation; D011697:Purpura, Thrombotic Thrombocytopenic / Q000097:blood*; D014841:von Willebrand Factor / Q000737:chemistry* / Q000378:metabolism
D014841:von Willebrand Factor; D051722:ADAM Proteins; D000071120:ADAMTS13 Protein; C099604:ADAMTS13 protein, human