Mediators from mast cells and basophils

The secretory granules of these cells contain preformed mediators that are released when the cells are stimulated. The biogenic amines, such as histamine and serotonin, are major components of the secretory granules. Human mast cells contain hista mine but not serotonin; however, serotonin is also present in the mast cells of other species, e.g. rodents. Histamine is produced from the amino acid l-histi-dine by the cytoplasmic enzyme histidine decarboxylase and then stored in granules. Human mast cells contain 2-3 pg of histamine per cell. After histamine is released from basophils or mast cells it binds and activates specific receptors on other cells, such as bronchial and gastrointestinal smooth muscle. This results in contraction of these smooth muscles and in enhanced vascular permeability. Serotonin, or 5-hydroxytryptamine, is formed by decarboxylation of the hydroxylated tryptophan molecule; in rodents it causes edema and plays an important role in ana phylactic reactions. Neutral proteases are also a major component of the secretory granules of mast cells. Several different proteases have been found in mast cells, including tryptase, chymase and carboxypeptidase. Tryptase is in mast cells but not basophils. The characteristic metachromatic staining of mast cells is due to the presence of proteoglycans in their granules, i.e. heparin or chondroitin sulfate-type that differ in their carbohydrate linkage. Human mast cells contain heparin and chondroitin sulfate E, whereas basophils have chondroitin sulfate A. Proteoglycans are involved in packaging of preformed mediators in granules. Heparin is an anticoagulant and has several other biological effects. A number of other enzymes including /3-hexosaminidase, (3-glucuronidase, /3-d-galactosidase, arylsulfatase, superoxide dismutase and peroxidase are present and released from the secretory granules. Chemotactic factors for eosinophils, neutrophils and monocytes are also released from mast cells or basophils. These include histamine, which is chemotactic for eosinophils, and small and large peptides.

A number of lipid mediators are generated after these cells are stimulated. During secretion arachidonic acid is released from phospholipids as a result of the activation of phospholipase enzymes. The ara chidonic acid is then metabolized, either along the cycloxygenase pathway with the formation of the prostaglandins (predominantly PGD2 but also thromboxanes), or by the lipoxygenase pathway with the formation of the leukotrienes (predominantly LTC4). Platelet-activating factor is a low molecular weight phospholipid generated from alkyl phospholipids in the cell. These lipid mediators have a number of potent biological activities. Mast cell lines and basophils synthesize and release cytokines when stimulated, including IL-3, IL-4, IL-5, IL-6, IL-8, tumor necrosis factor a (TNFa), interferon y (IFNy), several chemokines and granulocyte-macrophage colony-stimulating factor (GM-CSF).

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