Acquired thrombotic thrombocytopenic purpura (TTP) is characterized by the presence of circulating antibodies directed towards ADAMTS13. This results in the formation of large platelet aggregates that occlude the microvasculature giving rise to thrombocytopenia and micro-angiopathic hemolytic anemia in patients with TTP who lack functional ADAMTS13. Antibodies directed towards the spacer domain of ADAMTS13 are present in the majority of patients with acquired TTP. We have previously isolated a panel of human monoclonal antibodies directed towards ADAMTS13 from the immunoglobulin repertoire of one patient with acquired TTP employing phage display. To further extend these observations we obtained a panel of human anti-ADAMTS13 antibodies from a second patient with acquired TTP. Sequence analysis revealed that the variable heavy chain domains of the majority of anti-ADAMTS13 antibodies were encoded by germline gene segment VH1–69. The VH1–69 encoded antibody II-1 and the previously described antibody I-9, which was also encoded by VH1–69 were selected for further study. Epitope-mapping studies employing a hybrid ADAMTS13 variant containing the spacer domain of ADAMTS1 showed that both antibodies were directed towards the spacer domain. Both antibodies inhibited ADAMTS13 activity as measured by cleavage of FRETS-VWF73 substrate and also inhibited cleavage of VWF multimers under static conditions as well as under flow on the surface of endothelial cells. We subsequently developed an assay to monitor the presence of VH1-69 encoded antibodies in plasma of patients with acquired TTP using the anti-idiotypic monoclonal antibody G8 which specifically reacts with human antibodies that use the VH1-69 gene segment. Analysis of a large cohort of patients with acquired TTP revealed that antibodies reactive with the spacer domain of ADAMTS13 express the VH1-69 derived idiotype that is recognized by monoclonal antibody G8. These results support the notion that VH1-69 derived human antibodies directed towards the spacer domain of ADAMTS13 develop in the majority of patients with acquired TTP.

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