Submitted July 19, 2002
Accepted April 12, 2003
Drug induced thrombocytopenia: localization of the binding site of GPIX specific quinine-dependent antibodies
Parisa Asvadi, Zohra Ahmadi, and Beng H Chong*
Center for Thrombosis and Vascular Research and Department of Medicine, St. George Clinical School, The University of New South Wales, Sydney, NSW, Australia
Center for Thrombosis and Vascular Research and Department of Medicine, St. George Clinical School, The University of New South Wales, Sydney, NSW, Australia; National Heart Center, Singapore, Singapore
* Corresponding author; email: beng.chong{at}unsw.edu.au.
Immune thrombocytopenia is a common complication of therapy with a large number of drugs. The most widely studied drug-induced immune thrombocytopenia (DIT) is caused by quinine. In the majority of cases of DIT, antibodies bind to the platelet membrane glycoprotein (GP) Ib-IX complex in a drug-dependent fashion and bring about increased platelet clearance by the reticuloendothelial system resulting in thrombocytopenia. Here, we report the characterization of the quinine dependent antibody activity of sera from 13 quinine-induced thrombocytopenia patients. In our series of patients, GPIX was the most prevalent target of quinine-dependent antibodies. To identify the structural determinants of GPIX recognized by quinine dependent antibodies, 4 chimeric mouse/human GPIX constructs and stable CHO cell lines that expressed the chimeras in association with GPIba and b were produced. The analysis of 6 patient sera with the chimeric cell lines provided evidence for localization of the anti-GPIX quinine dependent antibody binding site to the C-ext region (aa 64-135) of human GPIX. Further characterization of the C-ext region of the GPIX indicated that replacement of the R110 and Q115 of the human GPIX with the corresponding residues from mouse (Q and E respectively) resulted in a significant reduction in the binding of GPIX antibodies in our series of patients, with R110
Q giving a more pronounced effect than Q115E. Hence these two residues (particularly R110), play an important role in the structure of the antigenic site on GPIX recognized by anti_GPIX antibodies.
