1.3.1 Mast cells and basophils

Mast cells and basophils play a central role in inflammatory and immediate allergic reactions. They are able to release potent inflammatory mediators, such as histamine, proteases, chemotactic factors, cytokines and metabolites of arachidonic acid that act on the vasculature, smooth muscle, connective tissue, mucous glands and inflammatory cells.

Mast cells settle in connective tissues and usually do not circulate in the blood stream.

Basophils are the smallest circulating granulocytes with relatively the least known function. They arise in the bone marrow, and following maturation and differentiation, are released into the blood circulation. If they are adequately stimulated they may settle in the tissues.

Both mast cells and basophils contain special cytoplasmic granules which store mediators of inflammation. The extracellular release of the mediators is known as degranulation and may be induced by:

(a)

physical destruction, such as high temperature, mechanical trauma, ionising irradiation, etc.;

(b)

chemical substances, such as toxins, venoms, proteases;

(c)

endogenous mediators, including tissue proteases, cationic proteins derived from eosinophils and neutrophils;

(d)

immune mechanisms which may be IgE-dependent or IgE-independent. The former is elicited by aggregation of IgE bound to high-afinity receptors (FcRI) on the surface of these cells. Specific antigen (allergen) is responsible for the IgE aggregation. In the IgE-independent way, the anafylatoxins C5a, C3a and C4a are formed during activation of complement. Then, the degranulation is triggered through C5a-receptors on the surface of mast cells and basophils.

There are two categories of inflammatory (anaphylactic) mediators in mast cells and basophils. Preformed mediators, stored in secretory granules and secreted upon cell activation, include a biogenic amine, typically histamine, proteoglycans, either heparin, over-sulphate chondroitin sulphates or both, and a spectrum of neutral proteases. Released histamine acts at H1, H2 and H3 receptors on cells and tissues, and is rapidly metabolized extracellularly. The proteoglycan, which imparts the metachromatic staining characteristic of mast cells when exposed to certain basic dyes such as toluidine blue, has two functions: it may package histamine and basic proteins into secretory granules, and in human mast cells it appears to regulate the stability of the protease called tryptase. Neutral proteases, which account for the vast majority of the granule protein, serve as markers of mast cells and of different types of mast cells.

Newly generated mediators, often absent in the resting mast cells, are typically produced during IgE-mediated activation, and consist of arachidonic acid metabolites, principally leukotriene C (LTC) and prostaglandin D (PGD) and cytokines. Of particular interest in humans is the production of tumour necrosis factor (TNF-), IL-4, IL-5 and IL-6. In the cytoplasma of both mastocytes and macrophages are special organelles -- lipid bodies -- where metabolism of arachidonic acid occur and where their products, including leukotrienes, may be stored.

Mast cells are heterogeneous -- two types of them, mucosal and connective tissue, were reported in rodent tissue back in the 1960's on the basis of histochemical and fixation characteristics that reflect, in part, whether heparin proteoglycan was present in secretory granules. Neutral proteases better reflect the heterogeneity or plasticity of mast cells in vivo and in vitro, particularly in humans where histochemical heterogeneity is less apparent (Table 1.2).

 
Table 1.2:  Predominant granule mediators of mast cells

In murine mast cells, five chymases ( mouse mast cell protease -- MMCP-1,-2,-3,-4 and-5), one mast cell carboxypeptidase and two tryptases (MMCP-6 and -7) have been reported.

In human mast cells, genes encoding two chymotryptic enzymes (chymase and cathepsin G-like protease) and one mast cell carboxypeptidase enzyme, and at least two genes encoding tryptase peptides have been detected. The gene encoding chymase resides on chromosome 14, closely linked to the gene encoding cathepsin G, an enzyme apparently expressed in mast cells and various myelomonocytic cells, and to the genes encoding granzymes, which are expressed in cytotoxic T lymphocytes and natural killer cells. Two types of mast cells have been found by immunohistochemical analyses. The MCtype contains tryptase, chymase, cathepsin G like protease and mast-cell carboxypeptidase, and predominates in normal skin and intestinal submucosa, whereas the MCtype contain only tryptase, and predominates in normal intestinal mucosa and lung alveolar wall. Nearly equivalent concentrations of each type are found in nasal mucosa. In MC cells, tryptase, chymase and mast-cell carboxypeptidase reside in macromolecular complexes with proteoglycan, but interestingly, tryptase reside in a separate complex from that in which chymase and mast-cell carboxypeptidase are found.

The biological function of mast cell neutral proteases, like mast cells themselves, remain to be fully clarified. In serum, elevated levels of tryptase are detected in systemic mast-cell disorders, such as anaphylaxis and mastocytosis. Ongoing mast-cell activation in asthma appear to be a charakteristic of this chronic inflammatory disease. It is detected by elevated levels of tryptase and PGD in bronchoalveolar lavage fluid, higher spontaneous release of histamine by mast cell obtained from the bronchoalveolar lavage fluid of asthmatics than non asthmatics, and ultrastructural analysis of mast cell in pulmonary tissue.

The number of basophils and mast cells increase at sites of inflammation. To reach these areas, basophils must migrate from the blood into tissue sites. A crucial step in this process is the adherence of cells to the endothelium. Cell adherence is mediated by several families of adhesion molecules and adhesion receptors in the surface of basophils and mast cells that can mediate binding to other cell and to the extracellular matrix (ECM) glycoproteins. Upon stimulation, basophils and mast cells release cytokines, including TNF- and IL-4, that can modulate adhesion molecules on endothelial cells. Activated endothelial cells express the intercellular adhesion molecule (ICAM-1), endothelial-leukocyte adhesion molecule (ELAM-1) and vascular cell adhesion molecule (VCAM-1) on their cell surface. Human basophils express integrins as receptors for these molecules.

Until recently, the effects of adherence on cell function were believed to result only from changes in cell shape and cytoskeletal organization. However, in addition to cell spreading, aggregated adhesion receptors transduce a variety of intracellular signals that regulate cell function. These signals include protein tyrosine phosphorylation, phosphoinositide hydrolysis, changes in intracellular pH or calcium concentration and the expression of several genes. The adhesion properties of basophils and mast cells regulate their migration, localization, proliferation and phenotype.

Different mechanisms could contribute to the increase in the number of mast cells at sites of tissue injury: mast cells or their progenitors could migrate to these sites; or resident mast-cell precursors could proliferate. Adhesion receptors and their ligands also play a role in the localization and migration of mast cells in normal tissues. ECM proteins that are the ligands for adhesion receptors are chemotactics for mast cells. Adherence of mast cells to fibroblasts, other cells or to ECM proteins can transduce signals that affect cell growth and differentiation.

The increase in the number of mast cells and basophils, and the enhanced secretion at sites of inflammation, can accelerate the elimination of the cause of tissue injury or, paradoxically, may lead to a chronic inflammatory response. Thus, manipulating mast-cell and basophil adhesion may be an important strategy for controlling the outcome of allergic and inflammatory responses.