Induction of Mast Cell Apoptosis via Granule Permeabilization : A Novel Approach to Target Mast Cells

Abstract: Mast cells are densely granulated tissue-resident immune cells that play an important role in orchestrating inflammatory responses. Dysregulated increases in the numbers and activation status of mast cells can have deleterious consequences for the body in various inflammatory diseases. Mast cells are best-known for their detrimental roles in allergic diseases, e.g., asthma. Thus, strategies that target mast cells and their harmful activities in such pathological conditions are potentially attractive therapeutic options. An efficient strategy to accomplish a full blockade of the harmful events mediated by various mast cell mediators is to locally eliminate mast cell populations altogether by inducing their apoptosis.Using in vitro-cultured mast cells, we identified that mefloquine, an antimalarial drug with lysosomotropic activity, causes permeabilization of secretory granules, increased production of reactive oxygen species (ROS), release of granule-localized proteases into the cytosol and apoptotic cell death (Paper I). Moreover, intraperitoneal injections of mefloquine in mice resulted in a reduced peritoneal mast cell population in vivo.To evaluate the possibility of using lysosomotropic agents for selectively depleting human lung mast cells by induction of apoptosis, human lung specimens were used. Exposure of either intact human lung tissue, purified lung mast cells or mixed populations of lung cells to mefloquine revealed that human lung mast cells are highly susceptible to ROS-induced apoptosis in this setting. In contrast, other cell populations of the lung were largely refractory (Paper II).Lastly, in an attempt to gain a deeper insight into the mechanism underlying ROS production and the downstream events in response to lysosomotropic challenge, we identified that the mast cell secretory granules comprise major subcellular compartments for ROS production in response to mefloquine (Paper III). Lysosomal iron, granzyme B and the ERK1/2 MAP kinase signaling pathway were found to contribute to production of ROS in response to mefloquine. Furthermore, granule acidification was shown to be essential for mefloquine-mediated effects in mast cells, i.e., granule permeabilization, ROS production and cell death. Collectively, the present thesis introduces the possibility of inducing selective mast cell apoptosis via granule permeabilization as a novel strategy to target mast cells. Thus, this strategy has a potential to be used therapeutically to ameliorate mast cell-mediated detrimental effects in inflammatory diseases, such as asthma.