A Guide to Mast Cell Markers

Mast cells are granulated, multifunctional leukocytes that reside primarily in tissues. In harmful situations and as a reaction to environmental changes, they function as first responders, communicating with other cells as part of the immunological response. They are also implicated in the pathogenesis of several disease states, which include chronic allergies and inflammation, autoimmune disorders, and cancer. As a guide for investigating mast cells, this article will focus on the characteristic protein markers commonly associated with this unique cell type.

Mast cell precursors existed within invertebrate chordates, occurring two hundred million years ago prior to the emergence of immunoglobulin (Ig) genes. These primitive mast cells contained granules and resembled today’s connective tissue mast cells with their ability to release histamines and prostaglandins upon activation. In mammals, mast cells are leukocytes derived from bone marrow and released into circulation as immature cells where they subsequently migrate to peripheral tissues to complete their differentiation. Found primarily in tissues that interact with the external environment, they exist in skin, lungs, and the gastrointestinal tract. The proteomes of mast cells are unrelated to those of eosinophils, neutrophils, or basophils, but are important effector cells with the ability to detect pathogens via cell surface and intracellular pattern recognition receptors and respond with degranulation and pro-inflammatory cytokine production to fight infection. Dependent upon tissue location, protease patterns, and response to stimuli, mast cells can be classified as connective tissue or mucosal type.

Figure: A brief overview of mast cell functions and characteristic proteins and molecules.

Markers for mast cell maturation

Mast cells originate from hematopoietic stem cells in the bone marrow before entering the blood as mast cell precursors (MCp). MCp will then mature within peripheral tissues in a highly regulated process. Mast cells recognize immunological, inflammatory, and environmental cues from receptors such as CD34, CD13, c-Kit, and the IgE receptor FcɛRI. When stimulated, they release a variety of preformed cytoplasmic granules containing substances like histamines or proteoglycans. Mast cells also produce prostaglandins (PGE₂ and PGD₂), leukotrienes (LTB4 and LTC4), and cytokines (IL-1β, IL-4, IL-6, IL-9, IL-10, and IL-33) causing local inflammation, enhanced vessel permeability, and stimulation of peripheral nerves resulting in symptoms of itching, sneezing, and coughing. 

Receptors such as c-Kit are responsible for mast cell survival, development, and maturation. In addition, it seems that IL-3 and IL-4 work together to promote mast cell growth and survival. In the mouse lung, the expression of integrin β7 has been reported as another marker of mast cell progenitor populations.

Markers for mast cell activation

Mast cell activation is induced by IgE-FcɛRI receptor cross-linking, which releases mediators such as histamines, heparin, PGD₂, LTC4, chymase, tryptase, and cytokines. This leads to a range of responses including smooth muscle contraction, increased vascular permeability, mucus production, and immune cell recruitment. Although mast cells are usually involved in host defense against pathogens and the promotion of wound healing, they have also been associated with severe allergic reactions, anaphylaxis, and several autoimmune diseases.

Similar to the maturation phase, common markers expressed upon activation include c-Kit, IL-3Rα, and FcɛRI. In mice, mast cells also express markers such as ENPP-3 and CD63. Receptors for various ligands including cytokines, chemokines (CCL2 and CCL5), and SCF are also involved in mast cell activation. Upon stimulation by pathogenic receptors, mast cells will express TLRs 2-5, 7, and 9. TLR stimulation further promotes the expression of MyD88, an adaptor protein. 

Markers for mast cell migration

Mature mast cells derive from mast cell progenitors, which serve as the pathway toward commitment. Migration of MCps to tissues is a regulated process stimulated by inflammation and leading to an increase in tissue mast cells. Markers found at this stage result from granulocyte release and include Lin^-, c-Kit⁺, CD34⁺, and FcγRII/III (CD32/CD16). Since these markers are shared with basophils, this suggests there is a close development relationship between the two types of cells.

Mast cells in tissues are very long-lived, surviving up to 12 weeks, for example, in the skin. They are capable of degranulation and re-granulation and display organ-specific increases in number during human diseases. 

Table of mast cell markers

The table below lists human and mouse proteins used for phenotyping different populations of mast cells as recently mentioned in the literature. Accompanying each listed marker are links to relevant antibodies and ELISA kits, as these immunodetection tools are routinely used in cell characterization studies via flow cytometry and immunostaining. The associated products are offered by a variety of manufacturers and can serve as a useful reference for mast cell immunophenotyping.

Note: *Some proteins are protein isoforms or multi-subunit protein complexes composed of several distinct genes. Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org (for human genes only). 

Other mast cell markers

Certain markers will appear depending upon organ or tissue location. When blood MCps migrate to intestinal tissue, markers include Lin^-, CD34⁺, β7 integrin, and FcɛRI. Assisting in homing to the intestinal region are markers such as VCAM-1 and CXCR2. Those two markers as well as CCL2 are expressed in lung. For cells mobilizing to the skin, markers include LTB4 and CCL2.

Several markers play a role in regulating one another. For example, when endothelial cells upregulate VCAM-1, that upregulation is mediated by TNFα. Also, the cytokine IL-9 can activate and support the growth of mast cells.

Allergens and respiratory viruses induce alarmin release such as IL-33 and TSLP. Il-33, a member of the IL-1 family accelerates maturation of CD34⁺ cells and induces secretion of Th2 cytokines and chemokines. IL-33 also promotes production of IL-13 and produces IL-2, which in turn expands the presence of CD4⁺, CD25⁺, and Foxp3⁺ and inhibits inflammation. The presence of these particular mast cell markers confirms the pathway associated with allergic asthma.

Developing a deeper understanding of the structure of mast cells while also determining their activation triggers could aid in the discovery of targeted therapies not only for asthma, but also for cancer and gastrointestinal diseases. 

References

1. Dahlin JS, Hallgren J. Mast cell progenitors: origin, development and migration to tissues. Mol Immunol. 2015;63(1):9-17. 

2. Plum T, Wang X, Rettel M, Krijgsveld J, Feyerabend TB, Rodewald HR. Human Mast Cell Proteome Reveals Unique Lineage, Putative Functions, and Structural Basis for Cell Ablation. Immunity. 2020;52(2):404-416.e5. 

3. da Silva EZ, Jamur MC, Oliver C. Mast cell function: a new vision of an old cell. J Histochem Cytochem. 2014;62(10):698-738. 

4. Kanagaratham C, El Ansari YS, Lewis OL, Oettgen HC. IgE and IgG Antibodies as Regulators of Mast Cell and Basophil Functions in Food Allergy. Front Immunol. 2020;11:603050. Published 2020 Dec 11. 

5. Méndez-Enríquez E, Hallgren J. Mast Cells and Their Progenitors in Allergic Asthma. Front Immunol. 2019;10:821. Published 2019 May 29. 

6. Ikuno T, Ito S, Inoue T. Human induced pluripotent stem cell-derived mast cells useful for in vitro mast cell activation assay exhibiting phenotypes and morphological characteristics of human mast cells. J Toxicol Sci. 2019;44(11):789-797. 

7. Oldford SA, Marshall JS. Mast cells as targets for immunotherapy of solid tumors. Mol Immunol. 2015;63(1):113-124. 

8. Yanase Y, Takahagi S, Ozawa K, Hide M. The Role of Coagulation and Complement Factors for Mast Cell Activation in the Pathogenesis of Chronic Spontaneous Urticaria. Cells. 2021;10(7):1759. Published 2021 Jul 12.