The three main processes responsible for the clinical features of asthma are well recognized: bronchoconstriction, mucus hypersecretion, and airway inflammation. However, the underlying pathophysiology responsible for these processes is complex and nuanced, involving multiple cell types and cytokines (1). Furthermore, the activity and clinical impact of each cellular and subcellular component varies considerably between individuals and can change over time, as well as in response to drug therapy and environmental/lifestyle influences.

Among these myriad cellular interactions and this extremely heterogeneous patient group, it is possible to identify certain key cells that are commonly involved—of which, arguably, the eosinophil is the most important.

Eosinophil precursors originate in the bone marrow and following differentiation traffic to the lungs (among other sites) via the bloodstream (2). While high concentrations of circulating eosinophils are often measured in asthmatic patients, of more clinical relevance is the lung tissue eosinophilia that is also frequently present.

The phenotype of “severe eosinophilic asthma” refers to a subgroup of asthmatic patients with evidence of eosinophilia that often require high maintenance doses of oral corticosteroids to maintain reasonable disease control. The notoriously non-specific mechanisms of action of corticosteroid therapy give rise to numerous well-documented adverse effects (3), which have driven decades of research focused on the development of targeted anti-eosinophil drug therapies. In order to understand how to better assist this group of patients, who currently have an unmet clinical need, it is helpful to understand the eosinophil itself, and the role that it plays in asthma. Targeted anti-eosinophil therapies will be touched upon but will be covered in greater detail by other reviews in this Research Topic.

Eosinophils are granulocytes, typically measuring 10–16 µm in diameter. They possess segmented (usually bi-lobed) nuclei and their nucleus: cytoplasm ratio is approximately 30%. Eosinophils stain with acidophilic dyes—a feature noted in 1879 by Paul Ehrlich, who first described eosinophils and appreciated their increased presence in patients with asthma and helminth infections, among other conditions (4). See Figure 1 for an overview of the eosinophil ultrastructure.

The presence of large specific granules, also known as secondary granules, is a characteristic feature that distinguishes eosinophils from other granulocytes (neutrophils and basophils). Specific granules consist of a dense crystalline core and a matrix, surrounded by a membrane (5). They contain a large number of mediators capable of inducing inflammation and/or tissue damage, including basic proteins, cytokines, chemokines, growth factors, and enzymes. The predominant substances are the proteins: major basic protein (MBP) is located in the core, while the matrix contains eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and eosinophil-derived neurotoxin (EDN) (6).

Primary granules tend to be smaller than specific granules. They are the principal location of Charcot–Leyden crystal protein (galactin-10): hexagonal bipyramidal crystals, which exhibit lysophospholipase activity and have been identified in tissues subject to eosinophilic inflammation (7).

Lipid bodies are particularly important when considering the role of eosinophils in asthma, due to their involvement in the production of eicosanoids, including cysteinal leukotrienes, prostaglandins, and thromboxane (2). Lipid bodies are a key site of arachidonic acid esterification and eicosanoid production due to their high concentrations of relevant enzymes such as cyclooxygenases, 5-lipoxygenase, and leukotriene C4-synthase (5).

Golgi bodies, endoplasmic reticulum, and mitochondria are also present and fulfill the fundamental duties of protein and adenosine triphosphate production within the eosinophil.

The histological appearance of eosinophils varies depending on the level of activation. For example, higher numbers of vesicles such as eosinophil sombrero vesicles may be seen when the cell is undergoing piecemeal degranulation (PMD), a process described in detail further on.

The varied roles of the eosinophil are reflected in its wide repertoire of surface molecules and receptors, which integrate eosinophils with both the innate and adaptive immune systems.

Eosinophils develop from pluripotent CD34+ granulocyte progenitor cells.

Differentiation and maturation occurs as follows: Myeloblast→Promyelocyte→Eosinophil myelocyte→Eosinophil metamyelocyte→Eosinophil.

Allergen challenge of mild asthmatics results in increased expression of IL-5Rα on CD34+ cells in the bone marrow, associated with blood and sputum eosinophilia (23). Eosinophil differentiation usually occurs in the bone marrow. However, eosinophil precursors have been isolated from the peripheral blood of atopic subjects at significantly higher concentrations compared to non-atopic controls (24). Increased numbers of CD34+/IL-5Rα+ eosinophil precursors have also been identified in bronchial biopsies of atopic asthmatics, compared to non-asthmatic control subjects (both atopic and non-atopic) (25). Eosinophil-lineage committed cells have also been identified in lung tissue in a mouse model of allergic airway inflammation (26). More recently, eosinophil progenitors isolated from the blood of patients with severe eosinophilic asthma have been shown to have an exaggerated clonogenic response to IL-5 in vitro, compared to eosinophil precursors from mild asthmatics, suggesting that in situ eosinophilopoiesis may have a clinically relevant role in severe eosinophilic asthma (27).

The differentiation of eosinophils is regulated by the transcription factors GATA-binding protein 1 (GATA-1), PU.1, and the CCAAT-enhancing binding protein (c/EBP) family. GATA-1 and PU.1 synergistically promote transcription of MBP (28). GATA-1 is thought to have the most important role, as disruption of the GATA-1 gene in mice results in a strain completely devoid of eosinophils (29).

The cytokines IL-3, IL-5, and GM-CSF also synergistically contribute to the development of mature eosinophils (30). IL-5 is the most eosinophil-specific and also promotes the release of eosinophils from the bone marrow to the bloodstream, acting synergistically with eotaxin (31, 32). Eosinophils are present in relatively low numbers in peripheral blood, usually accounting for no more than 5% of the total white blood cell count (33). They have a relatively short blood half-life of approximately 18 h (34). Migration to specific body sites, including the lungs and intestines, is mediated by eosinophil chemoattractants such as eotaxins. Eotaxins are a variety of CC-chemokines. There are three known subtypes: eotaxin-1 (CCL-11), eotaxin-2 (CCL-24), and eotaxin-3 (CCL-26). These bind to CCR3 receptors on the cell membranes of eosinophils and induce chemotaxis. 5-oxo 6, 8, 11, 14-eicosatetraenoic acid (5-oxo-ETE) is another eosinophil chemoattractant.

In vitro, the presence of prostaglandin-D2 (PGD2) has been shown to significantly enhance the chemoattractant effects of eotaxin-1 and 5-oxo-ETE on eosinophils and—unlike eotaxin-1 or 5-oxo-ETE—PGD2 retains its chemoattractant effect in the presence of blood or plasma (35). It is therefore proposed that PGD2 acts as the initial chemoattractant, triggering the migration of circulating eosinophils to specific tissues, where eotaxins and 5-oxo-ETE then predominate. PGD2 is released from activated mast cells (36) and acts via CRTh2 (chemoattractant receptor-homologous molecule expressed on T_H2 cells).

In allergic inflammation and asthma, circulating eosinophils adhere to the vascular endothelium and roll along it, before extravasating to the lung tissue. Initial tethering to the endothelium occurs as a result of the eosinophil cell membrane adhesion receptor P-selectin binding to P-selectin glycoprotein ligand-1 on the endothelium (37). Binding of the integrin VLA-4 to vascular cell adhesion molecule-1 promotes eosinophil activation and extravasation (37). IL-13 results in increased eosinophil expression of P-selectin and increased P-selectin mediated adhesion to endothelial cells (38).

The eosinophil’s ability to store several preformed cytotoxic mediators ready for rapid release upon appropriate stimulation facilitates a much quicker reaction to pro-inflammatory stimuli, compared to other cells, whose responses depend on upregulating the transcription of genes coding for such proteins.

The bronchial epithelium produces the cytokines IL-25, IL-33, and thymic stromal lymphopoietin, collectively known as the alarmins, in response to irritants such as allergens, pollutants, and pathogens. These cytokines trigger an inflammatory cascade involving, among others, T helper-2 (T_H2) cells and type-2 innate lymphoid cells (ILC2s), resulting in increased production of numerous cytokines including IL-4, IL-5, and IL-13, therefore prompting eosinophil activation (1, 39).

High mobility group box 1 protein, acting via receptors TLR-2, TLR-4, and RAGE, also promotes eosinophilia, although less is known regarding its mechanism of action (2).

Specific granule contents may be released via three different degranulation processes. Conventional exocytosis entails the specific granules fusing with the eosinophil cell membrane, resulting in the release of the entire contents of the specific granule. Alternatively, the eosinophil may be lysed (cytolysis), releasing all the cell contents, including the intact specific granules. These extracellular granules can be found in tissues affected by eosinophil-mediated disease and may subsequently release their contents in response to pro-inflammatory stimuli (40). However, the most common mechanism of eosinophil granulation is termed piecemeal degranulation (PMD). In this process, vesicles (both round and tubular) are released from specific granules and travel to the cell membrane to discharge their contents to the extracellular domain (41). The tubular vesicles tend to curl into a hoop-like morphology, giving rise to the term “eosinophil sombrero vesicles” (42). Vesicles with particular contents may be selectively released in response to particular cytokines, allowing eosinophils to supply a specific combination of cytotoxic mediators on demand (42, 43).

The activation of TLRs on eosinophils has been shown to promote adhesion and the release of certain cytokines and superoxides (44). Activation of TLR-2 and TLR-9 triggers eosinophil degranulation (44, 45). In vitro, eosinophils from atopic subjects have been shown to produce more IL-8 and EDN in response to stimulation of TLR-7 and TLR-9, compared to healthy controls (45).

Eosinophil survival is promoted by IL-3, IL-5, GM-CSF, and eotaxin (37). Activation of TLR-7 (the most abundant TLR subtype expressed by eosinophils) also enhances eosinophil survival (45).

Among the type-2 cytokines, IL-3, IL-5, and GM-CSF are particularly important for the initiation and perpetuation of eosinophilic airway inflammation. These three cytokines are closely linked, in that the genes for all three are all located on chromosome 5, and their receptors also share a common β-subunit (βc).

Monoclonal antibodies against IL-5 have been developed in order to treat eosinophil-mediated diseases such as eosinophilic asthma. Although inhibition of IL-5 activity in this manner (using mepolizumab) results in significant depletion of circulating eosinophils, the effect on bronchial tissue eosinophilia is less marked, with a median reduction of 55% (46). The residual tissue eosinophilia may reflect ongoing effects mediated by IL-3 and GM-CSF.

In a mouse model of allergic airways inflammation, allergen-induced lung tissue eosinophilia was abolished in mice bred to lack the common β-subunit, therefore incapable of responding to IL-3, IL-5, and GM-CSF (47). In the same study, lung tissue from βc-deficient mice was found to contain fewer myeloid dendritic cells, and the local T_H2 cells showed a reduced ability to proliferate and produce type-2 cytokines (47). These findings suggest a multifactorial role for the common β-subunit in the regulation of allergic airway inflammation.

In comparison to the roles that eosinophils play in diseases and infections, relatively little is known about their purpose in health. However, an increasing number of homeostatic mechanisms have been attributed to—or at least associated with—eosinophils in recent years. This has prompted a call for a fundamental change of the perception of eosinophils purely as cytotoxic effector cells (48, 49).

In health, eosinophils are found in the thymus, spleen, lymph nodes, and gastrointestinal (GI) tract (50). The number of eosinophils in the thymus declines with age (51). Eosinophils may have a role in T cell selection. In a mouse model of MHC I-restricted acute negative selection, eosinophil recruitment to the corticomedullary region of the thymus and association with apoptotic bodies has been demonstrated (52). Eosinophils also enhance the ability of macrophages to phagocytose apoptotic thymic cells (53).

Eosinophils migrate to the GI tract during embryonic development, i.e., prior to the development of any viable gut flora (54). In health, they are present throughout the GI tract—with the notable exception of the esophagus. Eosinophils contribute to the immune defense against gut microorganisms, due to multiple antimicrobial properties. (The antimicrobial properties of eosinophils are discussed in detail further on, with specific relation to respiratory pathogens.) Other potential homeostatic roles for eosinophils within the gut are not currently well defined but may relate to their ability to interact with the enteric neuronal system and increase smooth muscle reactivity (via release of MBP) (55).

In murine white adipose tissue, a positive correlation was identified between eosinophil counts and the numbers of arginase-1-expressing macrophages (56). Macrophages expressing arginase-1 are thought to contribute to glucose homeostasis, although macrophage classification is contentious (57). In a more recent study involving more than 9,000 human participants, the peripheral blood eosinophil percentage was found to be inversely associated with the risk of type-2 diabetes mellitus and insulin resistance (58).

Eosinophils have also been implicated in the regeneration of liver tissue (59) and skeletal muscle (60). The increased presence of eosinophils in preovulatory ovarian follicles (61) and in endometrium (62) has prompted speculation that they may have a role in tissue remodeling related to ovulation and menstruation.

Eosinophils also perform several important immunomodulatory functions, discussed in the following section.

Asthma pathophysiology is complex, and the relative contributions of the various cytokine networks involved vary between patients. Core features include airway hyperresponsiveness (AHR), mucus hypersecretion, tissue damage, and airway remodeling. See Figure 2 for an overview of the eosinophil’s role in asthma pathophysiology.

It has long been observed that eosinophil counts in peripheral blood and bronchoalveolar lavage (BAL) fluid are higher in asthmatics compared to healthy controls (63). Analysis of BAL fluid obtained from patients with atopic asthma reveals increased expression of T_H2 cytokines (64), including IL-5, which are strongly associated with eosinophilic inflammation (65). In general, the degree of eosinophilia correlates with disease severity and exacerbation frequency (63, 66). However, non-eosinophilic asthma phenotypes are also recognized (67). Peripheral blood eosinophilia may also occur in numerous other conditions (see Box 1).

Eosinophils have traditionally been regarded as end-stage effector cells, responding to infections directly, i.e., by releasing substances that are toxic to pathogens (in particular, helminths) and resulting in the unwanted secondary effect of human tissue damage. 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 (144).

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 (144). The cysteine and serine proteases produced by mites and fungi activate eosinophils via protease-activated receptors (5). TLR-7, the most common TLR expressed by eosinophils, is activated by viral single-stranded RNA (2).

The contents of eosinophil-specific granules are directly cytotoxic to pathogens. MBP causes disruption of cell membranes due to its highly basic nature (145). ECP has antiviral activity (146) and can also agglutinate Gram-negative bacteria by binding to LPS and peptidoglycans (147). EDN is only mildly toxic to helminths, compared to MBP and ECP (148). However, EDN significantly reduces the infectivity of respiratory syncytial virus group B, indicating a role in the immune response to viruses (149). EPO facilitates the generation of toxic reactive oxygen species (150).

In addition to releasing cytotoxic proteins, eosinophils have been shown to phagocytose bacteria (albeit less efficiently than neutrophils) (151). More recently, the “catapult-like” extrusion of “traps” consisting of mitochondrial DNA and eosinophil granule contents, in response to Gram-negative bacteria, has been observed (152).

Eosinophils express receptors for various complement proteins, including C3a and C5a, which are known to promote eosinophil recruitment, extravasation, and activation (153, 154). Complement is thought to facilitate eosinophil adherence to, and damage of, nematode larvae (155), although the development of secondary immunity is unaffected in complement-deficient mice (156).

As fungi are known to trigger the production of T_H2-associated cytokines (i.e., type-2 cytokines) and eosinophilia, it has been hypothesized that subclinical fungal infection/colonization of the airways may play a role in the genesis of diseases characterized by eosinophilia. Such diseases include severe eosinophilic asthma, as well as related conditions (e.g., chronic rhinosinusitis). One study of patients undergoing sinus surgery found that 74% of those with T_H2-associated conditions had evidence of airway surface mycosis, compared to just 16% of controls (157). However, potential confounding factors such as inhaled and/or systemic corticosteroid usage must be considered.

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 (158, 159). Eosinophils may contribute to this impairment by producing TGF-β, which has been shown to diminish the ability of bronchial epithelial cells to produce interferons in response to human rhinovirus in vitro (160).

The lung is known to harbor communities of bacteria, known as the lung microbiome, during health, which are deranged in disease states including asthma (161, 162). Data have recently been published suggesting a possible link between the level of eosinophilia and microbiome community structure in asthma (163). 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. The eosinophil’s status as a cytotoxic effector cell appears to be justified, due its capacity to release potent destructive basic proteins, capable of antimicrobial effects as well as host tissue damage. 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. AMG has attended advisory boards for Glaxo SmithKline, Novartis, Astra Zeneca, Boehringer Ingelheim and Teva. He has received speaker fees from Novartis, Astra Zeneca, Vectura, Boehringer Ingelheim and Teva. He has participated in research for which his institution has been renumerated with Hoffman La Roche, Glaxo SmithKline and Boehringer Ingelheim. He has attended international conferences with Napp and Astra Zeneca and has consultancy agreements with Astra Zeneca and Vectura.