A human monoclonal antibody against the distal carboxyl terminus of ADAMTS13 modulates its susceptibility to an inhibitor in thrombotic thrombocytopenic purpura.

BACKGROUND Immune thrombotic thrombocytopenic purpura (iTTP) is a potentially fatal thrombotic microangiopathy, resulting from a severe deficiency of plasma ADAMTS13 activity. Immunoglobulin (Ig) G-type autoantibodies are primarily responsible for the inhibition of plasma ADAMTS13 activity. However, the mechanism underlying autoantibody-mediated inhibition is not fully understood. OBJECTIVE The purpose of the present study is to determine the role of IgG autoantibodies against various carboxyl-terminal domains of ADAMTS13 in regulating ADAMTS13 activity and its inhibition. METHOD Various human monoclonal antibodies isolated by phage display, recombinant protein expression and purification, and biochemical analyses were employed for the study. Results Our results demonstrate for the first time that a human monoclonal antibody fragment, the single chain fragment of the variable region (scFv) isolated from a patient with acute iTTP that binds the distal carboxyl-terminus of ADAMTS13, is able to activate ADAMTS13 and increase the proteolytic cleavage of a FRETS-VWF73 substrate; moreover, binding of such a human monoclonal antibody against the carboxyl-terminus of ADAMTS13 to plasma ADAMTS13 appears to modulate inhibition by another human monoclonal antibody (i.e., scFv4-20), also isolated from an iTTP patient, that targets the spacer domain of ADAMTS13. These results provide new insights into our understanding of the pathogenesis of iTTP.