Submitted September 18, 2007
Accepted October 18, 2007
Fine specificity of drug-dependent antibodies reactive with a restricted domain of platelet GPIIIA
Julie A Peterson*, Tamara N Nelson, Adam J Kanack, and Richard H Aster
Blood Research Institute, BloodCenter of Wisconsin, Milwaukee, WI, United States
Departments of Medicine and Pathology, Medical College of Wisconsin, MIlwaukee, WI, United States
* Corresponding author; email: julie.peterson{at}bcw.edu.
Drug-induced immune thrombocytopenia is caused by drug-dependent antibodies (DDAbs) that bind tightly to platelet glycoproteins only when drug is present. How drugs mediate this interaction is not yet resolved. Several studies indicate that sites recognized by DDAbs tend to cluster in specific structural domains, suggesting they may recognize a limited number of distinct epitopes. To address this issue, we characterized the binding sites for 16 quinine-dependent antibodies thought on the basis of preliminary studies to be possibly specific for a single epitope on GPIIIa. Fourteen of the antibodies reacted with a 29kD GPIIIa fragment comprising only the GPIIIa hybrid and PSI homology domains. However, studies with mutant GPIIIa and the blocking monoclonal antibody AP3 showed that the 14 DDAbs recognize at least 6 and possibly more distinct, but overlapping structures involving GPIIIa residues 50-66. The findings suggest that even antibodies specific for restricted domains on a target glycoprotein may each have a slightly different fine specificity, i.e., "unique" epitopes recognized by DDAbs may be rare or non-existent. The observations are consistent with a recently proposed model in which drug reacts non-covalently with both target protein and antibody to promote binding of an otherwise non-reactive immunoglobulin.
