Eosinophil | |
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Details | |
Pronunciation | /ˌiːoʊˈsɪnəfɪl/ ) [1] |
System | Immune system |
Identifiers | |
MeSH | D004804 |
TH | H2.00.04.1.02017 |
FMA | 62861 |
Anatomical terms of microanatomy |
Eosinophils, sometimes called eosinophiles or, less commonly, acidophils, are a variety of white blood cells and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. [2] Along with mast cells and basophils, they also control mechanisms associated with allergy and asthma. They are granulocytes that develop during hematopoiesis in the bone marrow before migrating into blood, after which they are terminally differentiated and do not multiply. [3]
These cells are eosinophilic or "acid-loving" due to their large acidophilic cytoplasmic granules, which show their affinity for acids by their affinity to coal tar dyes: Normally transparent, it is this affinity that causes them to appear brick-red after staining with eosin, a red dye, using the Romanowsky method. [4] The staining is concentrated in small granules within the cellular cytoplasm, which contain many chemical mediators, such as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases (DNase), lipase, plasminogen, and major basic protein. These mediators are released by a process called degranulation following activation of the eosinophil, and are toxic to both parasite and host tissues.
In normal individuals, eosinophils make up about 1–3% of white blood cells, and are about 12–17 micrometres in size with bilobed nuclei. [3] [5] While eosinophils are released into the bloodstream, they reside in tissue. [4] They are found in the medulla and the junction between the cortex and medulla of the thymus, and, in the lower gastrointestinal tract, ovaries, uterus, spleen, prostate, and lymph nodes, but not in the lungs, skin, esophagus, or some other internal organs[ vague ] under normal conditions. The presence of eosinophils in these latter organs is associated with disease. For instance, patients with eosinophilic asthma have high levels of eosinophils that lead to inflammation and tissue damage, making it more difficult for patients to breathe. [6] [7] Eosinophils persist in the circulation for 8–12 hours, and can survive in tissue for an additional 8–12 days in the absence of stimulation. [8] Pioneering work in the 1980s elucidated that eosinophils were unique granulocytes, having the capacity to survive for extended periods of time after their maturation as demonstrated by ex-vivo culture experiments. [9]
TH2 and ILC2 cells both express the transcription factor GATA-3, which promotes the production of TH2 cytokines, including the interleukins (ILs). [6] IL-5 controls the development of eosinophils in the bone marrow, as they differentiate from myeloid precursor cells. [6] [10] [11] [12] Their lineage fate is determined by transcription factors, including GATA and C/EBP. [3] Eosinophils produce and store many secondary granule proteins prior to their exit from the bone marrow. After maturation, eosinophils circulate in blood and migrate to inflammatory sites in tissues, or to sites of helminth infection in response to chemokines like CCL11 (eotaxin-1), CCL24 (eotaxin-2), CCL5 (RANTES), 5-hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid, and certain leukotrienes like leukotriene B4 (LTB4) and MCP1/4. Interleukin-13, another TH2 cytokine, primes eosinophilic exit from the bone marrow by lining vessel walls with adhesion molecules such as VCAM-1 and ICAM-1. [6] When eosinophils are activated, they undergo cytolysis, where the breaking of the cell releases eosinophilic granules found in extracellular DNA traps. [6] High concentrations of these DNA traps are known to cause cellular damage, as the granules they contain are responsible for the ligand-induced secretion of eosinophilic toxins which cause structural damage. [6] There is evidence to suggest that eosinophil granule protein expression is regulated by the non-coding RNA EGOT. [13]
Following activation, eosinophils effector functions include production of the following:
There are also eosinophils that play a role in fighting viral infections, which is evident from the abundance of RNases they contain within their granules, and in fibrin removal during inflammation. Eosinophils, along with basophils and mast cells, are important mediators of allergic responses and asthma pathogenesis and are associated with disease severity. They also fight helminth (worm) colonization and may be slightly elevated in the presence of certain parasites. Eosinophils are also involved in many other biological processes, including postpubertal mammary gland development, oestrus cycling, allograft rejection and neoplasia. [21] They have also been implicated in antigen presentation to T cells. [22]
Eosinophils are responsible for tissue damage and inflammation in many diseases, including asthma. [6] [7] High levels of interleukin-5 has been observed to up regulate the expression of adhesion molecules, which then facilitate the adhesion of eosinophils to endothelial cells, thereby causing inflammation and tissue damage. [7]
An accumulation of eosinophils in the nasal mucosa is considered a major diagnostic criterion for allergic rhinitis (nasal allergies).
Following activation by an immune stimulus, eosinophils degranulate to release an array of cytotoxic granule cationic proteins that are capable of inducing tissue damage and dysfunction. [23] These include:
Major basic protein, eosinophil peroxidase, and eosinophil cationic protein are toxic to many tissues. [21] Eosinophil cationic protein and eosinophil-derived neurotoxin are ribonucleases with antiviral activity. [24] Major basic protein induces mast cell and basophil degranulation, and is implicated in peripheral nerve remodelling. [25] [26] Eosinophil cationic protein creates toxic pores in the membranes of target cells, allowing potential entry of other cytotoxic molecules to the cell, [27] can inhibit proliferation of T cells, suppress antibody production by B cells, induce degranulation by mast cells, and stimulate fibroblast cells to secrete mucus and glycosaminoglycans. [28] Eosinophil peroxidase forms reactive oxygen species and reactive nitrogen intermediates that promote oxidative stress in the target, causing cell death by apoptosis and necrosis. [21]
Strong evidence indicates that blood eosinophil counts can predict the effectiveness of specific anti-inflammatory drugs. Despite their increasing use in clinical practice, data on "normal" blood eosinophil counts remain insufficient. Due to the right-skewed distribution of these counts, median values are more informative than mean values for determining normal levels. Few large-scale studies have reported median blood eosinophil counts, with the median for healthy individuals being 100 cells/μL and the 95th percentile at 420 cells/μL. Thus, it is now evident that the normal median blood eosinophil count in healthy adults is around 100 cells/μL, with counts above 400 cells/μL considered outside the normal range. Current cutoffs such as 150 or 300 cells/μL used in asthma or COPD management fall within the normal range. [29]
An increase in eosinophils, i.e., the presence of more than 500 eosinophils/microlitre of blood is called an eosinophilia, and is typically seen in people with a parasitic infestation of the intestines; autoimmune and collagen vascular disease (such as rheumatoid arthritis) and Systemic lupus erythematosus; malignant diseases such as eosinophilic leukemia, clonal hypereosinophilia, and Hodgkin lymphoma; lymphocyte-variant hypereosinophilia; extensive skin diseases (such as exfoliative dermatitis); Addison's disease and other causes of low corticosteroid production (corticosteroids suppress blood eosinophil levels); reflux esophagitis (in which eosinophils will be found in the squamous epithelium of the esophagus) and eosinophilic esophagitis; and with the use of certain drugs such as penicillin. But, perhaps the most common cause for eosinophilia is an allergic condition such as asthma. In 1989, contaminated L-tryptophan supplements caused a deadly form of eosinophilia known as eosinophilia-myalgia syndrome, which was reminiscent of the toxic oil syndrome in Spain in 1981.
Eosinophils play an important role in asthma as the number of accumulated eosinophils corresponds to the severity of asthmatic reaction. [7] Eosinophilia in mice models are shown to be associated with high interleukin-5 levels. [7] Furthermore, mucosal bronchial biopsies conducted on patients with diseases such as asthma have been found to have higher levels of interleukin-5 leading to higher levels of eosinophils. [7] The infiltration of eosinophils at these high concentrations causes an inflammatory reaction. [7] This ultimately leads to airway remodelling and difficulty of breathing. [7]
Eosinophils can also cause tissue damage in the lungs of asthmatic patients. [7] High concentrations of eosinophil major basic protein and eosinophil-derived neurotoxin that approach cytotoxic levels are observed at degranulation sites in the lungs as well as in the asthmatic sputum. [7]
Treatments used to combat autoimmune diseases and conditions caused by eosinophils include:
Monoclonal antibodies such as dupilumab and lebrikizumab target IL-13 and its receptor, which reduces eosinophilic inflammation in patients with asthma due to lowering the number of adhesion molecules present for eosinophils to bind to, thereby decreasing inflammation. [30] [31] Mepolizumab and benralizumab are other treatment options that target the alpha subunit of the IL-5 receptor, thereby inhibiting its function and reducing the number of developing eosinophils as well as the number of eosinophils leading to inflammation through antibody-dependent cell-mediated cytotoxicity and eosinophilic apoptosis. [32] [33] Lysosomotropic agents are an efficient means to target the lysosome-like eosinophil granules inducing eosinophil apoptosis. [34]
Within the fat (adipose) tissue of CCR2 deficient mice, there is an increased number of eosinophils, greater alternative macrophage activation, and a propensity towards type 2 cytokine expression. Furthermore, this effect was exaggerated when the mice became obese from a high fat diet. [35] Mouse models of eosinophilia from mice infected with T. canis showed an increase in IL-5 mRNA in mice spleen. [7] Mouse models of asthma from OVA show a higher TH2 response. [6] When mice are administered IL-12 to induce the TH1 response, the TH2 response becomes suppressed, showing that mice without TH2 cytokines are significantly less likely to express asthma symptoms. [6]
Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 5×108/L (500/μL). Hypereosinophilia is an elevation in an individual's circulating blood eosinophil count above 1.5 × 109/L (i.e. 1,500/μL). The hypereosinophilic syndrome is a sustained elevation in this count above 1.5 × 109/L (i.e. 1,500/μL) that is also associated with evidence of eosinophil-based tissue injury.
A mast cell is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a part of the immune and neuroimmune systems. Mast cells were discovered by Paul Ehrlich in 1877. Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing, angiogenesis, immune tolerance, defense against pathogens, and vascular permeability in brain tumors.
Basophils are a type of white blood cell. Basophils are the least common type of granulocyte, representing about 0.5% to 1% of circulating white blood cells. They are the largest type of granulocyte. They are responsible for inflammatory reactions during immune response, as well as in the formation of acute and chronic allergic diseases, including anaphylaxis, asthma, atopic dermatitis and hay fever. They also produce compounds that coordinate immune responses, including histamine and serotonin that induce inflammation, and heparin that prevents blood clotting, although there are less than that found in mast cell granules. Mast cells were once thought to be basophils that migrated from the blood into their resident tissues, but they are now known to be different types of cells.
Granulocytes are cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear, that is, they have varying shapes (morphology) of the nucleus ; and are referred to as polymorphonuclear leukocytes. In common terms, polymorphonuclear granulocyte refers specifically to "neutrophil granulocytes", the most abundant of the granulocytes; the other types have varying morphology. Granulocytes are produced via granulopoiesis in the bone marrow.
Hypereosinophilic syndrome is a disease characterized by a persistently elevated eosinophil count in the blood for at least six months without any recognizable cause, with involvement of either the heart, nervous system, or bone marrow.
Eosinophilic granulomatosis with polyangiitis (EGPA), formerly known as allergic granulomatosis, is an extremely rare autoimmune condition that causes inflammation of small and medium-sized blood vessels (vasculitis) in persons with a history of airway allergic hypersensitivity (atopy).
The interleukin 4 is a cytokine that induces differentiation of naive helper T cells (Th0 cells) to Th2 cells. Upon activation by IL-4, Th2 cells subsequently produce additional IL-4 in a positive feedback loop. IL-4 is produced primarily by mast cells, Th2 cells, eosinophils and basophils. It is closely related and has functions similar to IL-13.
Interleukin 5 (IL-5) is an interleukin produced by type-2 T helper cells and mast cells.
Interleukin 13 (IL-13) is a protein that in humans is encoded by the IL13 gene. IL-13 was first cloned in 1993 and is located on chromosome 5q31.1 with a length of 1.4kb. It has a mass of 13 kDa and folds into 4 alpha helical bundles. The secondary structural features of IL-13 are similar to that of Interleukin 4 (IL-4); however it only has 25% sequence identity to IL-4 and is capable of IL-4 independent signaling. IL-13 is a cytokine secreted by T helper type 2 (Th2) cells, CD4 cells, natural killer T cell, mast cells, basophils, eosinophils and nuocytes. Interleukin-13 is a central regulator in IgE synthesis, goblet cell hyperplasia, mucus hypersecretion, airway hyperresponsiveness, fibrosis and chitinase up-regulation. It is a mediator of allergic inflammation and different diseases including asthma., and atopic dermatitis.
Loeffler endocarditis is a form of heart disease characterized by a stiffened, poorly-functioning heart caused by infiltration of the heart by white blood cells known as eosinophils. Restrictive cardiomyopathy is a disease of the heart muscle which results in impaired diastolic filling of the heart ventricles, i.e. the large heart chambers which pump blood into the pulmonary or systemic circulation. Diastole is the part of the cardiac contraction-relaxation cycle in which the heart fills with venous blood after the emptying done during its previous systole.
Allergic inflammation is an important pathophysiological feature of several disabilities or medical conditions including allergic asthma, atopic dermatitis, allergic rhinitis and several ocular allergic diseases. Allergic reactions may generally be divided into two components; the early phase reaction, and the late phase reaction. While the contribution to the development of symptoms from each of the phases varies greatly between diseases, both are usually present and provide us a framework for understanding allergic disease.
Interleukin-25 (IL-25) – also known as interleukin-17E (IL-17E) – is a protein that in humans is encoded by the IL25 gene on chromosome 14. IL-25 was discovered in 2001 and is made up of 177 amino acids.
Degranulation is a cellular process that releases antimicrobial, cytotoxic, or other molecules from secretory vesicles called granules found inside some cells. It is used by several different cells involved in the immune system, including granulocytes. It is also used by certain lymphocytes such as natural killer (NK) cells and cytotoxic T cells, whose main purpose is to destroy invading microorganisms.
Eosinophilic gastroenteritis, also known as eosinophilic enteritis, is a rare and heterogeneous condition characterized by patchy or diffuse eosinophilic infiltration of gastrointestinal (GI) tissue, first described by Kaijser in 1937. Presentation may vary depending on location as well as depth and extent of bowel wall involvement and usually runs a chronic relapsing course. It can be classified into mucosal, muscular and serosal types based on the depth of involvement. Any part of the GI tract can be affected, and isolated biliary tract involvement has also been reported. The stomach is the organ most commonly affected, followed by the small intestine and the colon.
Eosinophil cationic protein (ECP) also known as ribonuclease 3 is a basic protein located in the eosinophil primary matrix. In humans, the eosinophil cationic protein is encoded by the RNASE3 gene.
Eosinophilic myocarditis is inflammation in the heart muscle that is caused by the infiltration and destructive activity of a type of white blood cell, the eosinophil. Typically, the disorder is associated with hypereosinophilia, i.e. an eosinophil blood cell count greater than 1,500 per microliter. It is distinguished from non-eosinophilic myocarditis, which is heart inflammation caused by other types of white blood cells, i.e. lymphocytes and monocytes, as well as the respective descendants of these cells, NK cells and macrophages. This distinction is important because the eosinophil-based disorder is due to a particular set of underlying diseases and its preferred treatments differ from those for non-eosinophilic myocarditis.
Clonal hypereosinophilia, also termed primary hypereosinophilia or clonal eosinophilia, is a grouping of hematological disorders all of which are characterized by the development and growth of a pre-malignant or malignant population of eosinophils, a type of white blood cell that occupies the bone marrow, blood, and other tissues. This population consists of a clone of eosinophils, i.e. a group of genetically identical eosinophils derived from a sufficiently mutated ancestor cell.
Lymphocyte-variant hypereosinophilia is a rare disorder in which eosinophilia or hypereosinophilia is caused by an aberrant population of lymphocytes. These aberrant lymphocytes function abnormally by stimulating the proliferation and maturation of bone marrow eosinophil-precursor cells termed colony forming unit-eosinophils or CFU-Eos.
Familial eosinophilia is a rare congenital disorder characterized by the presence of sustained elevations in blood eosinophil levels that reach ranges diagnostic of eosinophilia or, far more commonly, hypereosinophilia. Although high eosinophil levels are associated with certain diseases and thought to contribute to the tissue destruction found in many other eosinophilia-related diseases, clinical manifestations and tissue destruction related to the eosinophilia in familial eosinophilia is uncommon: this genetic disease typically has a benign phenotype and course compared to other congenital and acquired eosinophilic diseases.
Type 2 inflammation is a pattern of immune response. Its physiological function is to defend the body against helminths, but a dysregulation of the type 2 inflammatory response has been implicated in the pathophysiology of several diseases.