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Mastocytosis is a KIT‐related myeloproliferative disease characterised by abnormal expansion of neoplastic mast cells (MC) in the skin or virtually any other organ system. The cutaneous form of adult‐onset mastocytosis is almost invariably combined with indolent systemic involvement for which curative therapy is yet not available. Here we evaluated a concept of depleting cutaneous MCs in mastocytosis lesions ex vivo by targeting their secretory granules. Skin biopsies from mastocytosis patients were incubated with or without mefloquine, an antimalarial drug known to penetrate into acidic organelles such as MC secretory granules. Mefloquine reduced the number of dermal MCs without affecting keratinocyte proliferation or epidermal gross morphology at drug concentrations up to 40 μM. Flow cytometric analysis of purified dermal MCs showed that mefloquine‐induced cell death was mainly due to apoptosis and accompanied by caspase‐3 activation. However, caspase inhibition provided only partial protection against mefloquine‐induced cell death, indicating predominantly caspase‐independent apoptosis. Further assessments revealed that mefloquine caused an elevation of granule pH and a corresponding decrease in cytosolic pH, suggesting drug‐induced granule permeabilisation. Extensive damage to the MC secretory granules was confirmed by transmission electron microscopy analysis. Further, blockade of granule acidification or serine protease activity prior to mefloquine treatment protected MCs from apoptosis, indicating that granule acidity and granule‐localised serine proteases play major roles in the execution of mefloquine‐induced cell death. Altogether, these findings reveal that mefloquine induces selective apoptosis of MCs by targeting their secretory granules and suggest that the drug may potentially extend its range of medical applications. [ABSTRACT FROM AUTHOR]

Mast cells (MCs) are known to have an aggravating impact on a range of debilitating human diseases, including allergic asthma,1,2 and strategies to dampen their harmful activities are therefore highly demanded.3,4 Conceptually, selective depletion of MCs would constitute an efficient regimen to accomplish this. Monensin induces selective mast cell apoptosis through a secretory granule-mediated pathway The panels show quantification of viable (Annexin V-/ Via-Probe-) cells within the lymphocyte (B), macrophage (C), and MC (D) populations (n = 3). [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

Mast cells are known to have a detrimental impact on numerous types of inflammatory skin diseases such as contact dermatitis, atopic eczema and cutaneous mastocytosis. Regimens that dampen skin mast cell-mediated activities can thus offer an attractive therapeutic option under such circumstances. As mast cells are known to secrete a large array of potentially pathogenic compounds, both from preformed stores in secretory lysosomes (granules) and after de novo synthesis, mere inhibition of degranulation or interference with individual mast cell mediators may not be sufficient to provide an effective blockade of harmful mast cell activities. An alternative strategy may therefore be to locally reduce skin mast cell numbers. Here, we explored the possibility of using lysosomotropic agents for this purpose, appreciating the fact that mast cell granules contain bioactive compounds prone to trigger apoptosis if released into the cytosolic compartment. Based on this principle, we show that incubation of human skin punch biopsies with the lysosomotropic agents siramesine or Leu-Leu methyl ester preferably ablated the mast cell population, without causing any gross adverse effects on the skin morphology. Subsequent analysis revealed that mast cells treated with lysosomotropic agents predominantly underwent apoptotic rather than necrotic cell death. In summary, this study raises the possibility of using lysosomotropic agents as a novel approach to targeting deleterious mast cell populations in cutaneous mastocytosis and other skin disorders negatively influenced by mast cells. [ABSTRACT FROM AUTHOR]