Novel Strategies to Target Mast Cells in Disease

Mast cells (MCs) are versatile effector cells of the immune system, characterized by a large content of secretory granules containing a variety of inflammatory mediators. They are implicated in the host protection toward various external insults, but are mostly well known for their detrimental impact on a variety of pathological conditions, including allergic disorders such as asthma and a range of additional disease settings. Based on this, there is currently a large demand for therapeutic regimens that can dampen the detrimental impact of MCs in these respective pathological conditions. This can be accomplished by several strategies, including targeting of individual mediators released by MCs, blockade of receptors for MC-released compounds, inhibition of MC activation, limiting mast cell growth or by inducing mast cell apoptosis. Here, we review the currently available and emerging regimens to interfere with harmful mast cell activities in asthma and other pathological settings and discuss the advantages and limitations of such strategies

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

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

DNA demethylation regulates gene expression in IgE-activated mouse mast cells

Letter to the Editor

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

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

Quantitative In-Depth Analysis of the Mouse Mast Cell Transcriptome Reveals Organ-Specific Mast Cell Heterogeneity

Mast cells (MCs) are primarily resident hematopoietic tissue cells that are localized at external and internal surfaces of the body where they act in the first line of defense. MCs are found in all studied vertebrates and have also been identified in tunicates, an early chordate. To obtain a detailed insight into the biology of MCs, here we analyzed the transcriptome of MCs from different mouse organs by RNA-seq and PCR-based transcriptomics. We show that MCs at different tissue locations differ substantially in their levels of transcripts coding for the most abundant MC granule proteins, even within the connective tissue type, or mucosal MC niches. We also demonstrate that transcript levels for the major granule proteins, including the various MC-restricted proteases and the heparin core protein, can be several orders of magnitude higher than those coding for various surface receptors and enzymes involved in protease activation, as well as enzymes involved in the synthesis of heparin, histamine, leukotrienes, and prostaglandins. Interestingly, our analyses revealed an almost complete absence in MCs of transcripts coding for cytokines at baseline conditions, indicating that cytokines are primarily produced by activated MCs. Bone marrow-derived MCs (BMMCs) are often used as equivalents of tissue MCs. Here, we show that these cells differ substantially from tissue MCs with regard to their transcriptome. Notably, they showed a transcriptome indicative of relatively immature cells, both with respect to the expression of granule proteases and of various enzymes involved in the processing/synthesis of granule compounds, indicating that care should be taken when extrapolating findings from BMMCs to the in vivo function of tissue-resident MCs. Furthermore, the latter finding indicates that the development of fully mature tissue-resident MCs requires a cytokine milieu beyond what is needed for in vitro differentiation of BMMCs. Altogether, this study provides a comprehensive quantitative view of the transcriptome profile of MCs resident at different tissue locations that builds nicely on previous studies of both the mouse and human transcriptome, and form a solid base for future evolutionary studies of the role of MCs in vertebrate immunity

Quantitative In-Depth Analysis of the Mouse Mast Cell Transcriptome Reveals Organ-Specific Mast Cell Heterogeneity

Mast cells (MCs) are primarily resident hematopoietic tissue cells that are localized at external and internal surfaces of the body where they act in the first line of defense. MCs are found in all studied vertebrates and have also been identified in tunicates, an early chordate. To obtain a detailed insight into the biology of MCs, here we analyzed the transcriptome of MCs from different mouse organs by RNA-seq and PCR-based transcriptomics. We show that MCs at different tissue locations differ substantially in their levels of transcripts coding for the most abundant MC granule proteins, even within the connective tissue type, or mucosal MC niches. We also demonstrate that transcript levels for the major granule proteins, including the various MC-restricted proteases and the heparin core protein, can be several orders of magnitude higher than those coding for various surface receptors and enzymes involved in protease activation, as well as enzymes involved in the synthesis of heparin, histamine, leukotrienes, and prostaglandins. Interestingly, our analyses revealed an almost complete absence in MCs of transcripts coding for cytokines at baseline conditions, indicating that cytokines are primarily produced by activated MCs. Bone marrow-derived MCs (BMMCs) are often used as equivalents of tissue MCs. Here, we show that these cells differ substantially from tissue MCs with regard to their transcriptome. Notably, they showed a transcriptome indicative of relatively immature cells, both with respect to the expression of granule proteases and of various enzymes involved in the processing/synthesis of granule compounds, indicating that care should be taken when extrapolating findings from BMMCs to the in vivo function of tissue-resident MCs. Furthermore, the latter finding indicates that the development of fully mature tissue-resident MCs requires a cytokine milieu beyond what is needed for in vitro differentiation of BMMCs. Altogether, this study provides a comprehensive quantitative view of the transcriptome profile of MCs resident at different tissue locations that builds nicely on previous studies of both the mouse and human transcriptome, and form a solid base for future evolutionary studies of the role of MCs in vertebrate immunity

Mast cell chymase regulates extracellular matrix remodeling-related events in primary human small airway epithelial cells

Background: Mast cells are implicated in the pathogenesis of asthma, but the underlying mechanisms are not fully elucidated. Under asthmatic conditions, mast cells can relocalize to the epithelial layer and may thereby affect the functional properties of the airway epithelial cells. Objectives: Activated mast cells release large quantities of proteases from their secretory granules, including chymase and tryptase. Here we investigated whether these proteases may affect airway epithelial cells. Methods: Primary small airway epithelial cells were treated with tryptase or chymase, and the effects on epithelial cell viability, proliferation, migration, cytokine output, and transcriptome were evaluated. Results: Airway epithelial cells were relatively refractory to tryptase. In contrast, chymase had extensive effects on multiple features of the epithelial cells, with a particular emphasis on processes related to extracellular matrix (ECM) remodeling. These included suppressed expression of ECM-related genes such as matrix metalloproteinases, which was confirmed at the protein level. Further, chymase suppressed the expression of the fibronectin gene and also caused degradation of fibronectin released by the epithelial cells. Chymase was also shown to suppress the migratory capacity of the airway epithelial cells and to degrade the cell-cell contact protein E-cadherin on the epithelial cell surface. Conclusion: Our findings suggest that chymase may affect the regulation of ECM remodeling events mediated by airway epithelial cells, with implications for the impact of mast cells in inflammatory lung diseases such as asthma

Quantitative In-Depth Analysis of the Mouse Mast Cell Transcriptome Reveals Organ-Specific Mast Cell Heterogeneity

Mast cells (MCs) are primarily resident hematopoietic tissue cells that are localized at external and internal surfaces of the body where they act in the first line of defense. MCs are found in all studied vertebrates and have also been identified in tunicates, an early chordate. To obtain a detailed insight into the biology of MCs, here we analyzed the transcriptome of MCs from different mouse organs by RNA-seq and PCR-based transcriptomics. We show that MCs at different tissue locations differ substantially in their levels of transcripts coding for the most abundant MC granule proteins, even within the connective tissue type, or mucosal MC niches. We also demonstrate that transcript levels for the major granule proteins, including the various MC-restricted proteases and the heparin core protein, can be several orders of magnitude higher than those coding for various surface receptors and enzymes involved in protease activation, as well as enzymes involved in the synthesis of heparin, histamine, leukotrienes, and prostaglandins. Interestingly, our analyses revealed an almost complete absence in MCs of transcripts coding for cytokines at baseline conditions, indicating that cytokines are primarily produced by activated MCs. Bone marrow-derived MCs (BMMCs) are often used as equivalents of tissue MCs. Here, we show that these cells differ substantially from tissue MCs with regard to their transcriptome. Notably, they showed a transcriptome indicative of relatively immature cells, both with respect to the expression of granule proteases and of various enzymes involved in the processing/synthesis of granule compounds, indicating that care should be taken when extrapolating findings from BMMCs to the in vivo function of tissue-resident MCs. Furthermore, the latter finding indicates that the development of fully mature tissue-resident MCs requires a cytokine milieu beyond what is needed for in vitro differentiation of BMMCs. Altogether, this study provides a comprehensive quantitative view of the transcriptome profile of MCs resident at different tissue locations that builds nicely on previous studies of both the mouse and human transcriptome, and form a solid base for future evolutionary studies of the role of MCs in vertebrate immunity