Monensin induces secretory granule-mediated cell death in eosinophils

Eosinophils contribute to the pathology of several types of disorders, in particular of allergic nature, and strategies to limit their actions are therefore warranted.We sought to evaluate the possibility of targeting the acidic, lysosome-like eosinophil granules as a potential means of inducing eosinophil cell death.To this end, we used monensin, an ionophoric drug that has previously been shown to permeabilize the secretory granules of mast cells, thereby inducing cell death.Our findings reveal that monensin induces cell death in human eosinophils, whereas neutrophils were less affected. Blockade of granule acidification reduced the effect of monensin on the eosinophils, demonstrating that granule acidity is an important factor in the mechanism of cell death. Furthermore, monensin caused an elevation of the granule pH, which was accompanied by a decrease of the cytosolic pH, hence indicating that monensin caused leakage of acidic contents from the granules into the cytosol. In agreement with a granule-targeting mechanism, transmission electron microscopy analysis revealed that monensin caused extensive morphological alterations of the eosinophil granules, as manifested by a marked loss of electron density. Eosinophil cell death in response to monensin was caspase-independent, but dependent on granzyme B, a pro-apoptotic serine protease known to be expressed by eosinophils.We conclude that monensin causes cell death of human eosinophils through a granule-mediated mechanism dependent on granzyme B.