Development of Mast Cells and Importance of Their Tryptase and Chymase Serine Proteases in Inflammation and Wound Healing

Abstract

Mast cells (MCs) are active participants in blood coagulation and innate and acquired immunity. This review focuses on the development of mouse and human MCs, as well as the involvement of their granule serine proteases in inflammation and the connective tissue remodeling that occurs during the different phases of the healing process of wounded skin and other organs. The accumulated data suggest that MCs, their tryptases, and their chymases play important roles in tissue repair. While MCs initially promote healing, they can be detrimental if they are chronically stimulated or if too many MCs become activated at the same time. The possibility that MCs and their granule serine proteases contribute to the formation of keloid and hypertrophic scars makes them potential targets for therapeutic intervention in the repair of damaged skin.

Introduction

Although mast cells (MCs) were discovered more than a century ago by Nobel Laureate Paul Ehrlich (Ehrlich, 1878), the importance of these immune cells in homeostasis and pathogen defense was appreciated only recently. MCs are not abundant in any tissue (Metcalfe, Baram, & Mekori, 1997), and they complete their development only after their poorly granulated progenitors home to tissues (Fig. 6.1). Thus, the inability to obtain sufficient numbers of in vivo-differentiated mature MCs for study greatly hindered our understanding of the importance of these cells and why they had been conserved for more than 500 million years of evolution. A contributing factor that prevented the identification of MC-restricted genes and transcripts was the observation that mature, in vivo-differentiated MCs contained very little mRNA relative to the contaminating cells in varied tissue preparations.

The discovery in the 1980s that the T-cell-derived factor interleukin (IL)-3 selectively promoted the viability, proliferation, and differentiation of a pluripotent population of mouse MCs from their hematopoietic progenitors that were free of contaminating cells finally facilitated the generation of large numbers of MCs for study (Razin et al., 1981, Razin, Ihle, et al., 1984, Schrader et al., 1981). These in vitro-generated mouse bone marrow-derived MCs (mBMMCs) were less mature than those in the jejunum, skin, and other connective tissues. Nevertheless, the ability to generate nontransformed MCs in vitro from wild-type (WT) and transgenic mice on different genetic backgrounds allowed detailed studies on the developmental control and functions of these cells at the molecular level. The resulting data led to a better understanding of the importance of mouse MCs and their human equivalents in acquired and innate immunity, inflammation, and blood coagulation. The observation that IL-3-developed mBMMCs contained more mRNA on a per cell basis than mature in vivo-differentiated MCs enabled the identification and cloning of many of the MC's mediators (e.g., mouse MC protease (mMCP)-5 (McNeil, Austen, Somerville, Gurish, & Stevens, 1991)), receptors (e.g., gp49B1/Lilrb4; Katz et al., 1996), and intracellular signaling proteins (e.g., RasGRP4; Yang et al., 2002). IL-3-developed mBMMCs have been used in nearly 1000 peer-reviewed publications. Since it was subsequently found that similar populations of MCs could be generated from fetal liver (Razin, Stevens, et al., 1984) and even embryonic stem cells (Tsai, Tam, Wedemeyer, & Galli, 2002), IL-3-dependent mouse MCs were particularly valuable for evaluating at the molecular level the functions of ubiquitously expressed proteins like Rac2 (Gu et al., 2002) and other intracellular signaling proteins that are critical for embryonic development. Thus, the in vitro method developed by Razin, Ihle, et al. (1984) for generating IL-3-dependent mouse MCs was a major technological advance.

The identification of “reaginic” immunoglobulin by the Ishizakas in the 1960s (Ishizaka, Ishizaka, & Hornbrook, 1966) led to the discovery that the IgE-dependent activation of MCs can result in life-threatening systemic anaphylaxis. The generation of mBMMCs and numerous variants (McGivney, Crews, Hirata, Axelrod, & Siraganian, 1981) of the transformed RBL-1 rat MC line (Eccleston, Leonard, Lowe, & Welford, 1973) allowed investigators to deduce the mechanisms at the molecular level by which these cells participate in IgE-dependent reactions. More recent studies revealed that MCs are involved in many non-IgE-dependent processes. In that regard, some populations of mouse and human MCs can be induced to degranulate by thrombin via protease-activated receptor-1 (Par-1) (Razin and Marx, 1984, Vliagoftis, 2002), by IgG complexes via FcγRIIa or FcγRIIIa (Malbec & Daeron, 2007), by ATP via P2X, P2Y, and adenosine receptors (Forsythe and Ennis, 1999, Kurashima et al., 2012, Sudo et al., 1996), and by complement-derived anaphylatoxins via the C3a and C5a receptors (el Lati et al., 1994, Erdei and Pecht, 1996) (Fig. 6.2). MCs express numerous Toll-like receptors (TLRs). While some populations of mouse and human MCs that have been examined so far do not degranulate when exposed to the TLR ligand lipopolysaccharide (LPS), the treated cells release numerous proinflammatory cytokines and chemokines (Matsushima et al., 2004, McCurdy et al., 2003). Whether or not MCs are active participants in the inflammation, proliferation, and/or remodeling stages of wound healing remains an area of investigation. In this review, we present recent literature that details the diverse functions of MCs and their protease mediators that help orchestrate this complex process.