Mast Cell Overview

Mast cells are predominantly tissue-resident granulocytes that, along with basophils, play a key role in allergy and anaphylaxis, as they are a predominant cellular source of histamine.  Originally believed to be tissue-resident basophils, they are now understood to be a separate lineage derived from a distinct progenitor cell.  Although most often associated with allergic responses, mast cells also play an important role in immune tolerance, as well as host defense against toxins and parasitic infections.

The development of mast cells remain poorly understood, as they are unique among hematopoietic cells in that they mature in peripheral tissues, rather than in lymphoid organs or in circulation.  Originally believed to be derived from basophils that had migrated to tissues from circulation, they have since been identified as a distinct lineage.  Immature mast cell progenitors are derived from the granulocyte/monocyte progenitor population and released from the bone marrow and complete their development as tissue-resident cells.  They express CD117 or c-Kit, a stem cell factor (SCF) receptor, throughout development, and the presence of this marker, along with the high-affinity IgE receptor (FcεR1), is typically used to identify mast cells from a heterogenous cell population. 

The balance between transcription factors C/EBPα and MITF is thought to be important for mast cell development, with upregulation of the former favoring basophil differentiation, and the latter driving mast cell differentiation from common progenitor cells. Cytokines important for the differentiation of mast cells include SCF and IL-3.  While SCF has been shown to be dispensable for their development and survival, IL-3 and its signaling mediator, STAT5, are considered essential.

Unlike basophils, phenotypic and functional subtypes of mast cells have been identified, with different categories for human and murine mast cells which are broadly defined based on location, protease secretion, and reactivity.  In mice, two main subsets have been described: mucosal-type mast cells (MMC) and connective tissue-type mast cells (CTMC).  Aside from location, differences in proteoglycan and protease expression are observed between these subtypes, which are relatively well-correlated with tissue distribution.

Human mast cells are broadly grouped into three categories based on the types of proteases released upon degranulation: tryptase-positive mast cells (MCT) which secrete tryptases, chymase-positive mast cells (MCC) which secrete chymases, and tryptase-chymase-positive mast cells (MCTC) which secrete both.  While human MCT share some characteristics with murine MMC and MCTM with CTMC, the correlation with tissue distribution is not observed in humans and most human tissues contain a mixed population of all three subtypes. 

Plasticity of these mast cell subtypes in response to different environmental cues has been observed, and different responses to stimuli within cultures of the same subtype are also documented.  These findings, along with the longevity of this lineage, suggest additional mast cell heterogeneity beyond the currently described subtypes that is yet to be discovered.

Mast cells are most often associated with allergic and anaphylactic responses but have also been demonstrated to play a protective role, particularly in immunity against parasites and environmental toxins such as venoms.  Like basophils, mast cells express FcεRI, which binds the Fc region of the IgE immunoglobulin secreted in response to parasitic infections and allergens.  Cross-linking of IgE with FcεRI results in degranulation and release of inflammatory mediators such as histamine, proteases, cytokines, and leukotrienes.  Mast cells share a variety of overlapping functions with basophils, and a diagram of these can be found in Figure 2.

Both basophils and mast cells release proteases upon degranulation, and while some of them, such as Granzyme B, are secreted by both cell types, many are unique to mast cells.  Chymases, tryptases, cathepsin G, and matrix metalloproteases (MMPs) are among the proteases specific to mast cells but not basophils.  The production of prostaglandins is also unique to mast cells, although other lipid mediators, including leukotrienes, are shared with basophils.  These molecules contribute to bronchoconstriction and vasodilation in allergic responses.  The two cell types also display highly divergent cytokine expression profiles; mast cells in particular produce and secrete a broad variety of pro-inflammatory cytokines in response to activation (Figure 2).

Mast cells are key mediators of cutaneous and mucosal allergies, which are triggered via IgE-FcεR1 signaling.  Binding of an allergen to IgE triggers mast cell degranulation, with symptoms ranging from mild to severe systemic anaphylaxis.  Mast cells are also implicated in the pathogenesis of some autoimmune disorders, such as rheumatoid arthritis. Their residence in the tissues most commonly affected by allergic responses has led to acceptance of the hypothesis that mast cells play a more central role in this pathogenesis than circulatory basophils.  However, evidence for this theory is largely based on mouse models, and the heterogeneity between cell populations and disease pathogenesis between species suggests that the specific roles for these cell types have yet to be clearly defined.  Basophils have a greater ability than mast cells to drive Th2-mediated inflammation due to their larger secretion of IL-4, so it is theorized that mast cells may contribute to the acute phase, while basophils play a greater role in promoting a late phase response.

Due to the overlapping functions of mast cells and basophils, therapeutic interventions for the treatment of allergic responses typically target the responses and mediators produced by both cell types.  Anti-IgE therapies have been shown to be effective against a variety of allergic disorders and prevent activation of both mast cells and basophils.  Therapies targeting the inflammatory mediators, such as antihistamines, are also broadly used to ameliorate the symptoms of allergic responses.

Isolating mast cells from tissues or the peritoneal cavity are labor-intensive and often suffer from poor yields.  The development of transformed mast cell lines has been used as models, but these cells are not perfect surrogates for endogenous phenotypes.  Protocols for the differentiation of mouse mast cells from bone marrow or human mast cells from bone marrow, cord blood, or fetal liver have also been described and typically involve culture with IL-3, SCF, or both, for an extended amount of time.  However, these cultured cells are also not ideal surrogates for tissue-resident mast cells, as the precise microenvironments in which these cells typically differentiate is difficult to recapitulate in vitro. The low frequencies of mast cells and their residencies in tissues have historically made their study difficult. It is further complicated by the observed heterogeneity and interspecies differences, which prevent extrapolation of mouse models and results directly to human biology.  The difficulty of mast cell investigation has resulted in the widespread use of the more easily accessible basophils as indicators of allergic sensitization. This test analyzes basophil degranulation in response to stimulation by flow cytometry.  A list of cellular markers for phenotyping mast cells can be found in Table 1 below.