EDN promotes the activation of dendritic cells , which in turn trigger the proliferation of T cells both helper and cytotoxic and B cells via antigen presentation. Eosinophils themselves can also present antigens to T cells , The cytokines released from eosinophil-specific granules have various immunomodulatory effects.
For example, IL-4 and IL simulate the proliferation of B cells and IgE production , , and IL-6 enhances survival of plasma cells , Eosinophil-specific granules are also capable of releasing several chemokines. In addition, eosinophils express indoleamine 2,3-dioxygenase IDO , an enzyme that catalyzes the production of kynurenine, suppressing T H 1 activity and promoting a type-2 inflammatory milieu , Figure 3.
The immunomodulatory role of eosinophils in asthma. Eosinophils may influence other leukocytes both directly e. Eosinophils have traditionally been regarded as end-stage effector cells, responding to infections directly, i. However, research performed over the last 30 years has revealed additional roles fulfilled by the eosinophil, involving links with both the innate and adaptive immune systems.
These roles include antigen presentation and interaction with other parts of the immune system, such as the complement pathway Pattern recognition receptors on the cell membranes of eosinophils allow them to detect the presence of PAMPs such as lipopolysaccharide LPS and beta-glucans, cell wall components of bacteria and fungi, respectively The cysteine and serine proteases produced by mites and fungi activate eosinophils via protease-activated receptors 5.
The contents of eosinophil-specific granules are directly cytotoxic to pathogens. MBP causes disruption of cell membranes due to its highly basic nature ECP has antiviral activity and can also agglutinate Gram-negative bacteria by binding to LPS and peptidoglycans However, EDN significantly reduces the infectivity of respiratory syncytial virus group B, indicating a role in the immune response to viruses EPO facilitates the generation of toxic reactive oxygen species In addition to releasing cytotoxic proteins, eosinophils have been shown to phagocytose bacteria albeit less efficiently than neutrophils Eosinophils express receptors for various complement proteins, including C3a and C5a, which are known to promote eosinophil recruitment, extravasation, and activation , Complement is thought to facilitate eosinophil adherence to, and damage of, nematode larvae , although the development of secondary immunity is unaffected in complement-deficient mice As fungi are known to trigger the production of T H 2-associated cytokines i.
Such diseases include severe eosinophilic asthma, as well as related conditions e. The increased susceptibility to respiratory viral infections observed in patients with asthma has been linked to reduced production of type I and type III interferons , The lung is known to harbor communities of bacteria, known as the lung microbiome, during health, which are deranged in disease states including asthma , Data have recently been published suggesting a possible link between the level of eosinophilia and microbiome community structure in asthma Further dedicated studies, examining subject groups matched for baseline characteristics, are required.
Although eosinophils have been associated with asthma since their initial discovery, our understanding of their roles in health and disease has evolved significantly over time. However, its ability to modulate the innate and adaptive immune systems may be just as important.
An appreciation of the numerous receptors expressed by eosinophils offers some insight into the many different interactions this versatile cell is capable of. Not only is the eosinophil recruited to the lungs in the context of pro-inflammatory type-2 cytokines but it is also a promoter of the type-2 inflammatory milieu, taking on roles such as antigen presentation and cytokine-mediated modulation of local lymphocytes.
There is strong evidence that eosinophils contribute to airway remodeling in asthma. Mechanisms also exist by which eosinophils could promote AHR and mucus hypersecretion.
The development of new anti-eosinophilic drugs, capable of selective depletion of eosinophils, offers great potential to explore further questions relating to the role of eosinophils in asthma and the consequences of their eradication.
Research into variation in eosinophil-related gene expression between individuals may provide further insights regarding the relative contributions of eosinophils in different asthma phenotypes and the potential application of personalized medicine to this field.
CM performed the initial literature review and drafted the article. AM-G provided critical review and additional content. CM has attended an international conference with Boehringer Ingelheim. He has attended international conferences with Napp and Astra Zeneca and has consultancy agreements with Astra Zeneca and Vectura. Lambrecht BN, Hammad H. The immunology of asthma. Nat Immunol 16 1 — Eosinophils: changing perspectives in health and disease. Nat Rev Immunol 13 1 :9— Dahl R. Systemic side effects of inhaled corticosteroids in patients with asthma.
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Traffic 6 11 — A doctor can test a tissue sample taken from a person to find out if eosinophil levels are too high. They can also test mucus from the nose. Because an underlying condition causes eosinophilia, having high levels of eosinophils can cause varying symptoms in a person. A doctor will consider the underlying condition when treating eosinophilia.
Too many eosinophils cause a number of medical conditions. These conditions range from the occasionally annoying to the deadly. The following are examples of some but not all conditions that are caused by eosinophilia. Eosinophilic pneumonia occurs when a person has too many eosinophils in their lung tissue. The effects are:. A person can have acute eosinophilic pneumonia, which causes a sudden, quick progression of pneumonia.
It is unknown what causes this condition. Another form, known as chronic eosinophilic pneumonia, results in a more lingering illness. Causes include:. Eosinophilic esophagitis EoE is an allergic reaction in the food pipe, the thin tube that carries food from the mouth to the stomach.
An allergen triggers an immune system response that brings too many eosinophils to the food pipe. This causes symptoms like:. Eosinophilic granulomatosis with polyangiitis EGPA is a form of vasculitis. This causes inflammation in the blood vessels. Figure 2. Strategies for inhibition of eosinophili inflammation. Bitesize category Cells: Bite-sized Immunology. Related Articles Basophils. B Cells. Dendritic Cells.
Dendritic Cells Tissue-specific. Your body may produce more of them in response to Allergic disorders Skin conditions Parasitic and fungal infections Autoimmune diseases Some cancers Bone marrow disorders In some conditions, the eosinophils can move outside the bloodstream and build up in organs and tissues. Start Here. Also in Spanish. Diagnosis and Tests. Living With. Clinical Trials.
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