Quinine-induced thrombocytopenia: drug-dependent GPIb/IX antibodies inhibit megakaryocyte and proplatelet production in vitro

The development of immune cytopenias is a well-recognized side effect of many drugs. Quinine- and quinidine-dependent antibodies are classic examples of drug-induced effects that cause severe, life-threatening thrombocytopenia. Whereas the effects of drug-dependent antibodies on platelets have been well documented, their effects on megakaryocyte (Mk) biology are still unclear. We analyzed sera from several quinine-induced thrombocytopenia (QITP) patients on highly pure Mks (98% glycoprotein IIb-positive [GPIIb+]; 92% GPIX+) derived from human CD34+ cells cultured with human thrombopoietin. We demonstrate by flow cytometry and confocal microscopy that QITP IgGs bind Mks efficiently in the presence of quinine. Incubation of day-4 Mks with QITP sera or purified IgG resulted in induction of apoptosis, a significant decrease in cell viability, and an increase in cell death. Furthermore, QITP sera preferentially reduced the number of late GPIX+/GPIbα+ Mks and the number of receptors per cell in the surviving population. Ploidy distribution, lobularity, and average cell size of Mks remained unchanged after treatment. In addition, treated Mks showed a marked decrease in their proplatelet production capacity, suggesting that drug-dependent antibodies hinder platelet production. Therefore, QITP antibodies considerably reduce the proplatelet production capabilities of Mks despite undetectable effects on DNA content, morphology, and cell size.