The early history of the eosinophil

Interleukin-5 Antagonists Usher in a New Generation of Asthma Therapy

Asthma is the most common chronic respiratory disease in the USA. A subset of patients with asthma have refractory symptoms, persistent eosinophilic inflammation, and recurrent exacerbations despite maximal medical therapy. The monoclonal antibodies targeting the IL-5 pathway are a new class of medications designed to target severe eosinophilic asthma. There are two medications clinically available: mepolizumab and reslizumab, both of which target IL-5. A third medication, benralizumab, is currently under development and targets the IL-5 receptor. Clinical data suggest these medications can reduce asthma exacerbations and improve lung function in patients with peripheral eosinophilia and poorly controlled asthma despite maximal medical therapy. The anti-IL-5 medications are among the first targeted molecular therapies for asthma and will usher in an exciting new era in the treatment of severe asthma.

Paul Ehrlich and the Early History of Granulocytes

Paul Ehrlich's techniques, published between 1879 and 1880, for staining blood films using coal tar dyes, and his method of differential blood cell counting, ended years of speculation regarding the classification of white cells. Acidic and basic dyes had allowed him to recognize eosinophil and basophil granules, respectively, work that was a direct continuation of his discovery of the tissue mast cell described in his doctoral thesis. Ehrlich went on to develop neutral dyes that identified epsilon granules in neutrophils ("cells with polymorphous nuclei"). He also speculated, for the most part correctly, on the formation, function, and fate of blood neutrophils and eosinophils. Before Ehrlich, a number of important observations had been made on white cells and their role in health and disease. Among the most notable were William Hewson's studies of blood and lymph; the early descriptions of leukemia by Alfred Donné, John Hughes Bennett, Rudolf Virchow, and others; as well as seminal observations on inflammation by William Addison, Friedrich von Recklinghausen, and Julius Cohnheim. Eosinophils were almost certainly recognized previously by others. In 1846, Thomas Wharton Jones (1808-1891) described "granule blood-cells" in several species including humans. The term "granule cell" had also been used by Julius Vogel (1814-1880), who had previously observed similar cells in inflammatory exudates. Vogel, in turn, was aware of the work of Gottlieb Gluge (1812-1898), who had observed "compound inflammatory globules" in pus and serum that resembled eosinophils. Almost 20 years before Ehrlich developed his staining methods, Max Johann Schultze (1825-1874) performed functional experiments on fine and coarse granular cells using a warm stage microscopic technique and showed they had amoeboid movement and phagocytic abilities. Despite these earlier observations, it was Ehrlich's use of stains that heralded the modern era of studies of leukocyte biology and pathology.

Eosinophils in Autoimmune Diseases

Eosinophils are multifunctional granulocytes that contribute to initiation and modulation of inflammation. Their role in asthma and parasitic infections has long been recognized. Growing evidence now reveals a role for eosinophils in autoimmune diseases. In this review, we summarize the function of eosinophils in inflammatory bowel diseases, neuromyelitis optica, bullous pemphigoid, autoimmune myocarditis, primary biliary cirrhosis, eosinophilic granulomatosis with polyangiitis, and other autoimmune diseases. Clinical studies, eosinophil-targeted therapies, and experimental models have contributed to our understanding of the regulation and function of eosinophils in these diseases. By examining the role of eosinophils in autoimmune diseases of different organs, we can identify common pathogenic mechanisms. These include degranulation of cytotoxic granule proteins, induction of antibody-dependent cell-mediated cytotoxicity, release of proteases degrading extracellular matrix, immune modulation through cytokines, antigen presentation, and prothrombotic functions. The association of eosinophilic diseases with autoimmune diseases is also examined, showing a possible increase in autoimmune diseases in patients with eosinophilic esophagitis, hypereosinophilic syndrome, and non-allergic asthma. Finally, we summarize key future research needs.

Reslizumab and Eosinophilic Asthma: One Step Closer to Precision Medicine?

Human eosinophils represent approximately 1% of peripheral blood leukocytes. However, these cells have the propensity to leave the blood stream and migrate into inflamed tissues. Eosinophilic inflammation is present in a significant proportion of patients with severe asthma. Asthma is a chronic inflammatory disorder that affects more than 315 million people worldwide, with 10% having severe uncontrolled disease. Although the majority of patients can be efficiently treated, severe asthmatics continue to be uncontrolled and are at risk of exacerbations and even death. Interleukin-5 (IL-5) plays a fundamental role in eosinophil differentiation, maturation, activation and inhibition of apoptosis. Therefore, targeting IL-5 is an appealing approach to the treatment of patients with severe eosinophilic asthma. Reslizumab, a humanized anti-IL-5 monoclonal antibody, binds with high affinity to amino acids 89–92 of IL-5 that are critical for binding to IL-5 receptor α. Two phase III studies have demonstrated that reslizumab administration in adult patients with severe asthma and eosinophilia (≥400 cells/μL) improved lung function, asthma control, and symptoms. Thus, the use of blood eosinophils as a baseline biomarker could help to select patients with severe uncontrolled asthma who are likely to achieve benefits in asthma control with reslizumab. In conclusion, targeted therapy with reslizumab represents one step closer to precision medicine in patients with severe eosinophilic asthma.

Eosinophils, probiotics, and the microbiome

There is currently substantial interest in the therapeutic properties of probiotic microorganisms as recent research suggests that oral administration of specific bacterial strains may reduce inflammation and alter the nature of endogenous microflora in the gastrointestinal tract. Eosinophils are multifunctional tissue leukocytes, prominent among the resident cells of the gastrointestinal mucosa that promote local immunity. Recent studies with genetically altered mice indicate that eosinophils not only participate in maintaining gut homeostasis, but that the absence of eosinophils may have significant impact on the nature of the endogenous gut microflora and responses to gut pathogens, notably Clostridium difficile Furthermore, in human subjects, there is an intriguing relationship between eosinophils, allergic inflammation, and the nature of the lung microflora, notably a distinct association between eosinophil infiltration and detection of bacteria of the phylum Actinobacteria. Among topics for future research, it will be important to determine whether homeostatic mechanisms involve direct interactions between eosinophils and bacteria or whether they involve primarily eosinophil-mediated responses to cytokine signaling in the local microenvironment. Likewise, although is it clear that eosinophils can and do interact with bacteria in vivo, their ability to discern between pathogenic and probiotic species in various settings remains to be explored.

Interleukin-5 pathway inhibition in the treatment of eosinophilic respiratory disorders: evidence and unmet needs

PURPOSE OF REVIEW

Human eosinophils were first identified and named by Paul Ehrlich in 1879 on the basis of the cell's granular uptake of eosin. Although eosinophils represent approximately 1% of peripheral blood leukocytes, they have the propensity to leave the blood stream and migrate into inflamed tissues. Eosinophils and their mediators are critical effectors to asthma and eosinophilic granulomatosis with polyangiitis (EGPA). Eosinophils are equipped with a large number of cell-surface receptors and produce specific cytokines and chemokines.

RECENT FINDINGS

Eosinophils are the major source of interleukin-5 and highly express the interleukin-5Rα on their surface. Clinical trials evaluating monoclonal antibodies to interleukin-5 (mepolizumab and reslizumab) and its receptor interleukin-5Rα (benralizumab) have been or are underway in patients with eosinophilic asthma, EGPA and chronic obstructive pulmonary disease (COPD). Overall, targeting interleukin-5/interleukin-5Rα is associated with a marked decrease in blood and sputum eosinophilia, the number of exacerbations and improvement of some clinical parameters in adult patients with severe eosinophilic asthma. Pilot studies suggest that mepolizumab might be a glucocorticoid-sparing treatment in patients with EGPA. A preliminary study found that benralizumab did not reduce the exacerbations and did modify lung function in patients with eosinophilic COPD.

SUMMARY

The review examines recent advances in the biology of eosinophils and how targeting the interleukin-5 pathway might offer benefit to some patients with severe asthma, EGPA, and COPD. Interleukin-5/interleukin-5Rα-targeted treatments offer promises to patients with eosinophilic respiratory disorders.

Eosinophilic airway inflammation: role in asthma and chronic obstructive pulmonary disease

The chronic lung diseases, asthma and chronic obstructive pulmonary disease (COPD), are common affecting over 500 million people worldwide and causing substantial morbidity and mortality. Asthma is typically associated with Th2-mediated eosinophilic airway inflammation, in contrast to neutrophilic inflammation observed commonly in COPD. However, there is increasing evidence that the eosinophil might play an important role in 10-40% of patients with COPD. Consistently in both asthma and COPD a sputum eosinophilia is associated with a good response to corticosteroid therapy and tailored strategies aimed to normalize sputum eosinophils reduce exacerbation frequency and severity. Advances in our understanding of the multistep paradigm of eosinophil recruitment to the airway, and the consequence of eosinophilic inflammation, has led to the development of new therapies to target these molecular pathways. In this article we discuss the mechanisms of eosinophilic trafficking, the tools to assess eosinophilic airway inflammation in asthma and COPD during stable disease and exacerbations and review current and novel anti-eosinophilic treatments.

Eosinophils

Eosinophils have been traditionally understood as end-stage, primarily cytotoxic effector cells. Recent studies have had profound impact on this limited view and have led to new research on the functions and capabilities of this unique leukocyte lineage. Novel insights into eosinophil development, localization, modes of degranulation, and the nature of their granule contents have provided a better understanding of these cells as immunomodulatory mediators in health and disease.