Abstract
Eosinophils are typically a minority population of circulating granulocytes being released from the bone-marrow as terminally differentiated cells. Besides their function in the defense against parasites and in promoting allergic airway inflammation, regulatory functions have now been attributed to eosinophils in various organs. Although eosinophils are involved in the inflammatory response to allergens, it remains unclear whether they are drivers of the asthma pathology or merely recruited effector cells. Recent findings highlight the homeostatic and pro-resolving capacity of eosinophils and raise the question at what point in time their function is regulated. Similarly, eosinophils from different physical locations display phenotypic and functional diversity. However, it remains unclear whether eosinophil plasticity remains as they develop and travel from the bone marrow to the tissue, in homeostasis or during inflammation. In the tissue, eosinophils of different ages and origin along the inflammatory trajectory may exhibit functional diversity as circumstances change. Herein, we outline the inflammatory time line of allergic airway inflammation from acute, late, adaptive to chronic processes. We summarize the function of the eosinophils in regards to their resident localization and time of recruitment to the lung, in all stages of the inflammatory response. In all, we argue that immunological differences in eosinophils are a function of time and space as the allergic inflammatory response is initiated and resolved.
Highlights
Eosinophils represent a minority population of peripheral leukocytes of the innate immune system
At birth very few eosinophils are present in the lungs of mice, they are recruited by IL-5 from type-2 innate lymphoid cells (ILC2) under the influence of epithelium-derived IL-33 coinciding with the alveolarization phase at post-natal day (PND) 3
Our perspective on the lung has changed dramatically over the last decades, culminating in the view of the lung as a place where epithelial cells, stromal cells, and immune cells support a multifaceted frontline defense system focused on inducing tolerance, supporting highly-efficient injury-repair responses, as well as inflammatory responses, like asthma
Summary
Eosinophils represent a minority population of peripheral leukocytes of the innate immune system. Eosinophils are, under homeostatic conditions, distributed in many organs like the lung, spleen, and gastrointestinal tract, as well as in the blood, lamina propria and adipose tissue [5] As such, these cells are proposed to have a physiological function in each of these different organs, which is strengthened by evidence on the existence of multiple tissue specific subtypes of eosinophils based on distinct surface marker expression and functional characteristics [6, 7]. We emphasize the need to define eosinophils during acute, late, and chronic inflammatory responses, as well as resolution in lung inflammation in regards to both time and space
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