Summary
Thrombotic thrombocytopenic purpura (TTP) is a microangiopathy often associated with
a severely decreased activity of ADAMTS13. In plasma of the majority of patients withTTP,
antibodies are present that inhibit the vonWillebrand factor (VWF) processing activity
of ADAMTS13.We describe a sensitive assay that monitors binding of recombinant ADAMTS13
to immobilized IgG derived from patient plasma. Analysis of fifteen patients with
TTP and severely reduced ADAMTS13 activity revealed that in all patients antibodies
directed toADAMTS13 were present. Levels of anti-ADAMTS13 antibodies varied considerably
among patients, specific antibody levels in plasma range from less than 100 ng/ml
to over 1 μg/ml. Longitudinal analysis in three patients revealed that anti-ADAMTS13
antibody levels declined with different kinetics. For further characterization of
anti- ADAMTS13 antibodies, we prepared a series of recombinan fragments corresponding
to the various ADAMTS13 domains. All seven TTP plasma samples tested, showed reactivity
of antibodies towards a fragment consisting of the disintegrin/ TSR1/cysteine-rich/spacer
domains. In one patient, we also observed reactivity towards the TSR2–8 repeats. No
binding of antibodies to propeptide, metalloprotease and CUB domains was detected.
To further delineate the binding site in the disintegrin/ TSR1/cysteine-rich/spacer
region, we prepared additional ADAMTS13 fragments. Antibodies directed towards the
cysteine- rich/spacer fragment were found in all plasma samples analyzed. No antibodies
reacting with the disintegrin/TSR1 domains were detected. A recombinant fragment comprising
the spacer domain was recognized by all patients samples analyzed, suggesting that
the 130-amino-acid spacer domain harbors a major binding site for anti-ADAMTS-13 antibodies.
Keywords
Von Willebrand factor - immunity (auto-) - acquired coagulation disorders