Glycoprotein Ib/IX complex is the target in rifampicin-induced immune thrombocytopenia

Thrombocytopenia is a major adverse effect of several drug treatments. Rifa mpicin has been recognized as a cause of immune thrombocytopenia during int ermittent high-dose therapy. We characterized the antibody of a patient who presented with purpura and thrombocytopenia during treatment of tuberculos is with rifampicin. Drug-dependent binding of the antibody to platelets was demonstrated by flow cytometry. In a glycoprotein-specific immunoassay, th e binding epitope of the IgG antibody was found in the glycoprotein Ib/IX c omplex, using four different monoclonal antibodies (mAbs) against various e pitopes on the GPIb/IX complex, as well as mAbs against GPIIb/IIIa, GPIa/II a and GPIV. By immunoprecipitation of biotin-labelled platelets, reactivity of the antibody with GPIb/IX was found only in the presence of the drug. T hese findings clearly demonstrate that rifampicin induces the formation of drug-dependent antibodies capable of causing thrombocytopenia. The binding site of the rifampicin-dependent antibody, located in the GPIb/IX complex, seems to be a favoured target for antibodies induced by different drugs.