Introduction
Since Ehrlich’s first description of the characteristic staining properties of the eosinophil in 1879 many different functions have been attributed to this readily recognisable cell (Spry 1988). Accummulation of eosinophils in the tissues has been associated with parasitic and allergic diseases since the turn of the century. However their role in these disorders has only become more clear with advances in the understanding of the immune response at a cellular and biochemical level. The cytoplasm of the eosinophil contains granules which are responsible for its particular staining properties (the granules stain a red colour with eosin). The nucleus is lobulated in a similar fashion to that of the neutrophil. These cells are however derived from very different precursor cells within the bone marrow and have very different functions. The eosinophil granules contain toxic proteins which are responsible for many of the local effects that can result from eosinophil degranulation.
Eosinophil granules.
There are three types of granules present in the cytoplasm of eosinophils (Gleich and Loegering 1984):
1. Primary granules.
These are found in immature eosinophils and are thought to develop into secondary granules.
2. Secondary granules.
These are the predominant type in mature cells and consist of a matrix with a crystalline core.
3. Small granules.
These are less dense and contain arylsulphatase and acid phosphatase.
It is only relatively recently that the main component proteins of the secondary granules have been characterised.
Major basic protein (MBP).
The crystalline core of the secondary granule is a crystal of major basic protein (Gleich and Loegering 1984). This protein has a molecular weight of 14,(XX) Daltons. MBP is also present in the granules of basophils, which might be expected since they both appear to arise from a common precursor cell in the bone marrow. Since eosinophils are found in the tissues in parasitic infestations it is of interest that they have been found to have the ability to damage parasites in vitro. Major basic protein has been shown to be directly toxic to schistosomules (an intermediate stage in the life cycle of the Schistosoma) in vitro. It has also been shown to have the potential to damage mammalian cells in experiments on murine ascites tumour cells in vitro. Thus it may have a role in tissue damage associated with hypersensitivity reactions. As well as its direct toxic effects it is also able to stimulate mast cells to degranulate.
Eosinophilic cationic protein (ECP).
This protein was first isolated from the matrix of the granule by Olsson (Olsson and Venge 1974). Its molecular weight is 18-21,000 Daltons. ECP is cytotoxic to both mammalian and nonmammalian cells such as parasites. It can also induce
the Gordon phenomenon. (In 1933 M.H.Gordon injected extracts of eosinophil- rich lymph nodes from patients with Hodgkin’s disease into the brains of rabbits and produced paralysis. The active ingredient was shown to be produced by the eosinophils and was subsequently called eosinophil-derived neurotoxin (Gordon
1933, Fredens 1982).). Two antigenically distinct forms exist depending on whether the ECP is found intracellularly or extracellulary (Tai et al. 1984a). Like MBP it has the ability to stimulate histamine release from mast cells but it can also inhibit T lymphocyte responses.
Eosinophil-derived neurotoxin (EDN).
This 18,000 Dalton protein, like ECP, is found in the granule matrix. It is neurotoxic as shown in the Gordon phenomenon (Durack et al. 1981).
Eosinophil peroxidase (EPO).
EPO is formed of two chains: a heavy chain of 52,000 Daltons and a light one of 15,000 Daltons. It is found in the granule matrix. In conjunction with hydrogen peroxide and halide it can kill bacteria, parasites and mammalian cells. It can also initiate mast cell secretion.
Charcot Leyden Crystals (CLC)
These crystals can be extracted from eosinophils and may be shed in the sputum of asthmatics. They are pure crystals of a lysophospholipase which is located in the plasma membrane of the eosinophil. Lysophospholipase may have a role in removing potentially cytotoxic lysophospholipids formed by lysophospholipase A2.
Other granule-associated enzymes include the following (Kay 1985):
Arylsulphatase Ribonuclease
Phospholipase Cathepsin
Histaminase Acid phosphatase Beta glucuronidase
Eosinophils can also release lipid mediators which include platelet activating factor (PAF) and leukotriene C4. The latter is a potent vasoactive mediator which causes smooth muscle spasm and mucus secretion (Weller et al. 1983). In addition to generating inflammatory mediators themselves, eosinophils can induce mediator release from mast cells and basophils thus amplifying the inflammatory response.
Cell surface receptors and proteins
Interactions between different inflammatory cells and between inflammatory cells and locally released mediators are determined by receptor expression. There has been much study of such receptors in relation to the eosinophil (Kay 1985). Eosinophils express receptors for IgG, IgE and IgA on their cell membranes. There are also receptors for complement components C lq, C3b/C4b (CRl), iC3b (CR3) and C5a (Weller 1991). There are receptors for the cytokines interleukin 3 (lL-3), interleukin 5 (lL-5) and granulocyte monocyte colony stimulating factor (GM-CSF).
Recent interest in leucocyte adhesion molecules (which are involved in regulating the sites of leucocyte tissue migration) has been extended to the eosinophil. Although these glycoproteins appear to be expressed on the cell membrane of the eosinophil, the cell still has the ability to migrate into the tissues in an
independent fashion (Weller 1991).
It has also been shown that eosinophils express CD4 which acts as a signal transducer which on binding promotes migration rather than degranulation. The eosinophil can also synthesise HLA-DR and express it on its cell surface allowing it to present antigen to accessory inflammatory cells.
Eosinophil production and activation occurs principally under the influence of three cytokines for which it has receptors:- GM-CSF, lL-3 and lL-5. GM-CSF and lL-3 can stimulate other cells such as neutrophils, but lL-5 appears to have an effect that is relatively specific for eosinophils.
Functions o f the eosinophil
Originally the eosinophil was thought to have a role that was primarily one of modulating or dampening down inflammation occurring subsequent to the release of mast cell and other cell products in inflammation. The presence of enzymes that could deactivate mast cell products lent support to this idea (e.g. histaminase, arylsulphatase) (Goetzl et al. 1979). However the increased understanding of the structure and properties of the granule proteins, and the finding of granule protein products in the tissues and secretions of patients with eosinophil-associated
diseases, has resulted in the view that these cells are highly cytotoxic and may be producing much of the tissue damage in their own right (Weller 1991, Kay 1985). It seems that the primary role of eosinophils is host defence. Although the eosinophil is capable of phagocytosis it is much less efficient at this than neutrophils. Its toxic granule proteins and other mediators are capable of extracellular cytotoxicity and it seems that the eosinophil is best suited to this role. Thus they form an important defence mechanism against parasites which are too large to be phagocytosed.
Eosinophil-associated diseases
Apart from those diseases caused by parasitic infestation, eosinophils have also been shown to accumulate in the tissues in a wide variety of diseases (for review see Spry 1988) A significant role for eosinophils has been demonstrated in the pathogenesis of disorders as diverse as asthma, allergic rhinitis, endomyocardial fibrosis, allergic skin disorders and the idiopathic hypereosinophilic syndrome. Since mast cells are often intimately involved in reactions involving eosinophils their role in the inflammatory response will be discussed next.