IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 23. Hypersensitivity - Type I

MAST CELLS

It has long been known that there are interspecies variations in mast Cell Morphology, including staining characteristics, surface architecture, and The Mechanism of degranulation. The latter can be clearly demonstrated; in humans, granule membranes fuse with one another prior to exocytosis (compound exocytosis), whereas in rats, granules are released individually (Fig. 23.10).

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Fig. 23.10. Scanning and transmission Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF rat mast Cells. 1. Intact rat peritoneal mast cell with cell membrane-covered granules. Scanning Electron Microscopy, x1500. 2. Degranulation of a rat peritoneal mast cell following a 30-second incubation with anti-IgE Antibodies. Scanning electron microscopy, x1500. (Photographs kindly provided by Dr. T. Orr.) 3. Rat peritoneal mast cell containing electron-dense granules. 4. Following incubation with anti-IgE antibodies, cellular vacuolation occurs, accompanied by the exocytosis of granule contents. Transmission electron microscopy, x2700. (Photographs kindly provided by Dr. D. Lawson.)

Based on tissue localization, staining properties, and protease profiles, mast cells are divided into two main types: Connective Tissue mast cells (CTMC) and mucosal mast cells (MMC) (Figs. 23.11 and 23.12).

Functional differences also exist between these two mast cell populations, as they respond differently to secretagogues that stimulate degranulation (such as Ca²⁺ ionophores and compound 48/80) or inhibitors of histamine release (such as sodium cromoglycate).

Fig. 23.11. There are at least two distinct mast cell subpopulations: mucosal mast cells (MMC) and connective tissue mast cells (CTMC). Differences in their morphology and pharmacological responses point to distinct in vivo functional roles. MMCs participate in responses to parasitic infections and, likely, in allergic reactions. They are smaller and have a shorter lifespan than CTMCs; their function is T-cell dependent, they express higher levels of Fcε receptors on their surface, and their Cytoplasm contains IgE. Granules in both cell types contain histamine and serotonin, though the higher granule density in CTMCs may account for their greater histamine content. Both mast cell types produce major arachidonic acid (AA) metabolites—Prostaglandins and Leukotrienes—albeit in different proportions. For instance, MMCs produce leukotriene LTC₄ and prostaglandin PGD₂ at a ratio of 25:1, whereas CTMCs produce them at a ratio of 1:40. Various agents affect the degranulation of these two cell types differently. Sodium cromoglycate (SCG) and theophylline inhibit histamine release from CTMCs but not from MMCs (which may have important therapeutic implications for asthma Treatment). It should be noted that many of these findings are derived from rodent studies and may not be directly applicable to humans.

Fig. 23.12. Tryptase is a tetramer with a Molecular Weight of 134 kDa, accounting for up to 25% of the total mast cell protein. Chymase is a monomer with a molecular weight of 30 kDa. Based on The ratio of these proteases, mast cells are subdivided into TC_T and TC_CH phenotypes, which are distributed differently across human Tissues. Basophils contain extremely low levels of both proteases. (The subscripts "T" and "CH" denote the presence of tryptase and chymase, respectively.)

Classification of mast cells by tissue Location and morphology

In most tissues, CTMCs are located perivascularly. Although CTMCs from different sites share similar properties, cells from the peritoneal cavity and Skin, for example, can vary significantly in granule number and size, staining density, and pharmacological responsiveness (see Figs. 23.11 and 23.12). MMCs exhibit a different distribution; in humans, their highest concentration is found in the mucosa of the mid-gut and the Lungs.

During parasitic infections, such as Nippostrongylus brasiliensis in rats, the number of MMCs in the intestinal mucosa increases dramatically. Elevated numbers are also observed in Crohn's disease and Ulcerative Colitis, although the precise role of mast cells in these pathologies remains unclear.

It is hypothesized that mucosal MMC precursors originate in regional Lymph Nodes AND migrate to the gut via the Thoracic duct. MMC proliferation during parasite infection is clearly dependent on T-cell-derived cytokines, including IL-3 and IL-4. In contrast, CTMC clones can be generated in vitro from fibroblasts independently of T cells or T-cell factors.

Recent evidence indicates that MMCs and CTMCs develop from common precursor cells, with their ultimate phenotype being determined by microenvironmental factors.

CTMCs and MMCs contain characteristic proteases in their granules. Recent studies have cloned and sequenced the genes for several mast cell granule proteases. Based on the differential expression of two of these Enzymes—tryptase and chymase—CTMCs and MMCs can be further divided into subpopulations (see Fig. 23.12).

These proteases are of significant medical interest because tryptase can induce bronchial hyperreactivity, while chymase stimulates bronchial mucus secretion; both are hallmark features of asthma (see Fig. 23.20). Both proteases are also capable of cleaving vasoactive intestinal peptide (VIP), a mediator of bronchodilation. Furthermore, tryptase is a potent fibroblast growth factor and may serve as a molecular link connecting mast cell activation to tissue fibrosis.

Clinical studies of mast cells in asthma and hay fever

Recent clinical studies have demonstrated that during the pollen season (but not before it), individuals suffering from hay fever exhibit an infiltration of nasal mucosal MMCs into the epithelium. Similarly, increased numbers of mast cells (not yet fully characterized) are found in bronchoalveolar lavage fluid from asthma patients.

Because the bronchial mucosal surface is the primary site of contact with inhaled allergens, mast cell interaction with these allergens triggers mediator release and increases mucosal permeability. This leads to the secondary release of mediators from submucosal mast cells, thereby exacerbating clinical symptoms. The extent of degranulation can be assessed by measuring serum tryptase levels. Due to its molecular stability, tryptase serves as a more reliable biomarker of mast cell degranulation than histamine.

Pharmacological agents acting on mast cells can produce clinically significant effects. Elucidating The Nature of mast cells on the bronchoalveolar surface and their response to anti-allergic drugs could greatly aid in The Development of novel therapies. For example, in rats infected with the parasite Nippostrongylus brasiliensis, corticosteroids rapidly and markedly reduce the accumulation of MMCs in the intestine.

Interestingly, topical corticosteroid administration also blocks the seasonal increase in nasal mast cell numbers seen in hay fever sufferers during the pollen season. The mechanism behind this suppression remains unclear, but it is known that corticosteroids inhibit The production of cytokines—including IL-3 and IL-4, which act as mast cell growth factors—by T_H cells.

The effects of various drugs on mast cell degranulation can have profound functional and clinical implications. In rats, sodium cromoglycate and theophylline inhibit histamine release from CTMCs but not from MMCs. Due to mast cell heterogeneity and interspecies differences, these findings cannot be directly extrapolated to humans with certainty. The generation of pure human mast cell lines could greatly facilitate the development of effective treatments for allergic diseases, which have become a true scourge of modern humanity.

Other cells of The Immune System also bind IgE

Normal eosinophils and platelets sensitized with IgE acquire enhanced cytotoxicity against certain parasites, including schistosomes. In addition, in allergic individuals, these cells can be sensitized by circulating immune complexes containing IgE. Both eosinophils and platelets likely participate in allergic reactions because they contain various mediators and inflammatory Proteins capable of amplifying these responses. It has recently been established that eosinophils, macrophages, platelets, and Langerhans cells express both high- and low-affinity IgE receptors (Fig. 23.13).

Fig. 23.13. Compared with FcεRI of mast cells and basophils, the receptors of other cells (FcεRII) have a much lower affinity for IgE. The FcεRIIa receptor is constitutively expressed by normal B cells, whereas the expression of FcεRIIb on various cells is induced by IL-4. The FcεRIIb receptor is expressed on T cells, B cells, monocytes, and macrophages, as well as on cutaneous Langerhans cells. Langerhans cells, macrophages, and eosinophils also express high-affinity FcεRI.

Notably, bound IgE is present On the surface of cutaneous Langerhans cells in patients with atopic eczema, which may play an important role in antigen/allergen presentation to skin-infiltrating T cells and thereby trigger inflammatory skin reactions. Such IgE-bearing Langerhans cells are absent in the skin of healthy individuals or atopic patients without eczema.

Mast cell degranulation can be induced by various pathways

Following the binding of IgE to FcεRI on The surface of mast cells and basophils, cross-linking of this immunoglobulin can trigger cell degranulation. Such cross-linking is carried out by an allergen or other molecules and leads to the aggregation of Fcε receptors, which causes an influx of calcium into The Cell and, consequently, its degranulation.

Degranulation is also caused by direct cross-linking of receptors (Fig. 23.14). For example, Lectins such as PHA or ConA can cross-link IgE by interacting with carbohydrate residues within the Fc fragment. This may explain the cause of urticaria in some individuals with an allergy to strawberries—berries containing a high amount of lectin.

Fig. 23.14. Mast cells can be activated by immunological stimuli leading to the cross-linking of Fcε receptors, as well as by other agents such as anaphylatoxins and secretagogues (e.g., compound 48/80, melittin, and the calcium ionophore A23187). It has been established that several other substances, including codeine, morphine, and synthetic ACTH, act directly on mast cells. All these agents exert the same effect—they induce an influx of Ca²⁺ ions into the cell, which is a mandatory requirement for its degranulation. Microtubule formation and the movement of granules toward The cell membrane lead to their fusion with the membrane and the release of preformed mediators residing within the granules. Changes in The Plasma Membrane accompanied by the activation of phospholipase A2 result in the release of arachidonic acid, which is then acted upon by one of the enzymes (depending on the mast cell type)—lipoxygenase or cyclooxygenase. The resulting lipid metabolites of the cyclooxygenase pathway include prostaglandins (PGD₂) and thromboxanes, while those of the lipoxygenase pathway include leukotrienes (LTC₄, LTD₄, and the chemotactic LTB₄). Both preformed and newly formed granule lipid mediators exert Three types of effects.

Chemotactic agents attract many other cells to the site of mast cell activation, particularly eosinophils, neutrophils, and mononuclear cells, including lymphocytes. In addition, it has recently been established that certain preformed cytokines released by mast cells upon degranulation also serve as chemotactic signals for inflammatory cells.

Inflammatory activators can cause vasodilation, edema, and (with the participation of platelet-activating factor, PAF) The formation of microthrombi with local tissue damage. Tryptase, the major neutral protease of mast cells present in the human lung, can directly activate C3; this effect is blocked by heparin. Kininogenases released during inflammation convert kininogens into kinins, dilating small Blood Vessels and in turn promoting inflammation.

Spasmogens act directly on bronchial smooth Muscle, but can also enhance mucus secretion, leading to bronchial obstruction.

Certain substances are extremely potent in inducing mast cell degranulation. Probably the most important among these in vivo are the Complement component degradation products C3a and C5a. These anaphylatoxins also affect many other cells, including neutrophils, platelets, and macrophages. Substances capable of directly activating mast cells also include calcium ionophores, melittin, compound 48/80, and drugs such as synthetic ACTH, codeine, and morphine. All of them activate mast cells by triggering an influx of Calcium Ions. The anaphylactic response induced by these agents is identical to the IgE-mediated response, although these substances naturally act via IgE-independent mechanisms.

Degranulation leads to the release of preformed mediators and induces the synthesis of others from arachidonic acid. Antigen-induced calcium influx into mast cells has two main consequences. First, granule exocytosis occurs with the release of preformed mediators (mainly histamine in humans). Second, the formation of new mediators from arachidonic acid—prostaglandins and leukotrienes—is induced, which exert direct local effects on the tissue. In the lungs, they immediately cause bronchoconstriction, mucosal edema, and hypersecretion of mucus, leading to asthma (see Figs. 23.14 and 23.20).

It turns out that different mast cell populations produce different newly formed mediators. For example, antihistamines are clinically effective in rhinitis and urticaria, but not in asthma, in the development of which leukotrienes play a more important role.

Certain compounds block the release of mediators by increasing intracellular cAMP levels (for example, this effect is produced by the β-adrenoceptor agonist isoprenaline) or by preventing its breakdown by phosphodiesterase (such is the action of theophylline). The MECHANISM OF ACTION of sodium cromoglycate, which blocks histamine release from mast cells, remains unclear; it may consist in inhibiting the allergen-induced influx of calcium into cells, and this substance may also affect mediator release from other cells.