Approaches for Analyzing the Roles of Mast Cells and Their Proteases In Vivo

Stephen J. Galli*,┼,1; Mindy Tsai*; Thomas Marichal*,╬; Elena Tchougounova§; Laurent L. Reber*; Gunnar Pejler¶,#    * Department of Pathology, Stanford University School of Medicine, Stanford, California, USA
┼ Microbiology & Immunology, Stanford University School of Medicine, Stanford, California, USA
╬ GIGA-Research and Faculty of Veterinary Medicine, University of Liege, Liege, Belgium
§ Department of Immunology, Genetics, and Pathology, Rudbeck Laboratory, Uppsala University, Uppsala, Sweden
¶ Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, Sweden
# Department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden
1 Corresponding author: email address: sgalli@stanford.edu

Abstract

The roles of mast cells in health and disease remain incompletely understood. While the evidence that mast cells are critical effector cells in IgE-dependent anaphylaxis and other acute IgE-mediated allergic reactions seems unassailable, studies employing various mice deficient in mast cells or mast cell-associated proteases have yielded divergent conclusions about the roles of mast cells or their proteases in certain other immunological responses. Such "controversial" results call into question the relative utility of various older versus newer approaches to ascertain the roles of mast cells and mast cell proteases in vivo. This review discusses how both older and more recent mouse models have been used to investigate the functions of mast cells and their proteases in health and disease. We particularly focus on settings in which divergent conclusions about the importance of mast cells and their proteases have been supported by studies that employed different models of mast cell or mast cell protease deficiency. We think that two major conclusions can be drawn from such findings: (1) no matter which models of mast cell or mast cell protease deficiency one employs, the conclusions drawn from the experiments always should take into account the potential limitations of the models (particularly abnormalities affecting cell types other than mast cells) and (2) even when analyzing a biological response using a single model of mast cell or mast cell protease deficiency, details of experimental design are critical in efforts to define those conditions under which important contributions of mast cells or their proteases can be identified.

Keywords

Basophils

c-kit

Cre recombinase

Mouse model

Stem cell factor

1 Mast Cell Biology

1.1 Origin and tissue distribution of mast cells

Mast cells (MCs) are long-lived granulated cells derived from hematopoietic precursors; such MC progenitors ordinarily are found only in small numbers in the blood and complete their differentiation and maturation in the microenvironments of almost all vascularized tissues (Douaiher et al., 2014; Galli, Grimbaldeston, & Tsai, 2008; Gurish & Austen, 2012; Moon et al., 2010). Like cells in the monocyte lineage, mature MCs located in the tissues can proliferate after appropriate stimulation (Galli, Borregaard, & Wynn, 2011). In addition, increased recruitment, survival, and maturation of MC progenitors may also contribute to the local expansion of MC populations (Galli et al., 2008; Gurish & Austen, 2012). Stem cell factor (SCF), the ligand for Kit, is produced by structural cells in the tissues (and also by MCs) and plays a crucial role in MC development, survival, migration, and function (Douaiher et al., 2014; Galli, Zsebo, & Geissler, 1994; Gurish & Austen, 2012; Moon et al., 2010). Other growth factors (Galli et al., 2008; Gurish & Austen, 2012) that have been shown to influence MC growth and survival include interleukin (IL)-3, IL-4, IL-9, IL-10, IL-33, and TGF-ß. MCs are distributed throughout nearly all tissues, and often in close proximity to potential targets of their mediators such as epithelia and glands, smooth muscle and cardiac muscle cells, fibroblasts, blood and lymphatic vessels, and nerves. Mature MCs are particularly abundant in tissues and organs exposed to the external environment, such as the skin, the lung, and the gut (Galli et al., 2008).

1.2 The spectrum of mast cell-derived mediators

MCs can store and release upon degranulation and/or secrete de novo a wide spectrum of biologically active mediators, many of which also can be produced by other cell types. During IgE-associated biologic responses, the antigen-dependent cross-linking of antigen-specific IgE bound to Fc?RI on the plasma membrane of MCs induces the aggregation of Fc?RI, thereby activating downstream signaling events that lead to the secretion of biologically active products implicated in allergic reactions (Blank & Rivera, 2004; Boyce, 2007; Galli & Tsai, 2012; Metcalfe, Peavy, & Gilfillan, 2009; Rivera, Fierro, Olivera, & Suzuki, 2008). Following antigen binding, MCs very rapidly release into the extracellular space mediators pre-stored in their cytoplasmic granules, for example, vasoactive amines (histamine and serotonin), neutral proteases (tryptases, chymases, and carboxypeptidase A3 [CPA3]), proteoglycans (e.g., heparin), and some cytokines and growth factors by a process called degranulation. A second class of secreted products is generated by de novo synthesis of proinflammatory lipid mediators, such as prostaglandins and leukotrienes. Finally, MCs are also able to synthesize and secrete a large number of growth factors, cytokines, and chemokines, e.g., IL-1, IL-6, IL-10, and TNF-a, VEGF, angiopoietin-1, TGF-ß, and many others, with the types and amounts of such products that are released being influenced by factors such as the type and species of origin of the MCs, the nature of the stimulus inducing MC activation (Galli, Kalesnikoff, et al., 2005; Galli, Nakae and Tsai, 2005; Moon et al., 2010), and, in the case of IgE-dependent MC activation, whether the activation is by low- or high-affinity stimuli (Suzuki et al., 2014).

Notably, MCs can be activated to secrete biologically active products not only by IgE and specific antigen, but by a long list of other stimuli including physical agents, products of diverse pathogens (Abraham & St John, 2010), many innate danger signals (Supajatura et al., 2002), certain endogenous peptides and structurally similar peptides found in invertebrate and vertebrate venoms (Akahoshi et al., 2011; Metz et al., 2006; Schneider, Schlenner, Feyerabend, Wunderlin, & Rodewald, 2007), and products of innate and adaptive immune responses including products of complement activation (Schäfer et al., 2012), certain chemokines and cytokines (including IL-33; Enoksson et al., 2011; Lunderius-Andersson, Enoksson, & Nilsson, 2012), and immune complexes of IgG. The ability of MCs to secrete biologically active mediators can be modulated by many factors, including interactions with other granulocytes (Fantozzi et al., 1985), regulatory T cells (Gri et al., 2008), or lymphocytes (Gaudenzio et al., 2009), and certain cytokines, including the main MC development and survival growth factor, the Kit ligand, SCF (Galli, Kalesnikoff, et al., 2005; Galli, Nakae, et al., 2005; Galli, Zsebo, et al., 1994; Hill et al., 1996; Ito et al., 2012), as well as IL-33 (Komai-Koma et al., 2012) and interferon-? (Okayama, Kirshenbaum, & Metcalfe, 2000). Many mediators which can be produced by MCs have been shown to have various positive or negative effects on the function of diverse immune or structural cells, findings which indicate that MCs at least have the potential to influence inflammation, hemostasis, tissue remodeling, cancer, metabolism, reproduction, behavior, sleep, homeostasis, and many other biological responses (Galli et al., 2008; Gilfillan & Beaven, 2011; Kennelly, Conneely, Bouchier-Hayes, & Winter, 2011; Ribatti & Crivellato, 2011).

1.3 Phenotypic heterogeneity and functional plasticity

Many phenotypic and functional characteristics of MCs, such as proliferation, survival, and ability to store and/or secrete various products, as well as the magnitude and nature of their secretory responses to particular activation signals, can be modulated or "tuned" by many environmental and genetic factors (Galli, Kalesnikoff, et al., 2005; Galli, Nakae, et al., 2005). The properties of individual MCs thus may differ depending on the genetic background of the host and/or the local or systemic levels of factors that affect various aspects of MC biology. This "plasticity" of multiple aspects of MC phenotype can result in the development of phenotypically distinct populations of MCs in various anatomic sites and in different animal species. Such altered expression of...