Phenotypic plasticity and targeting of Siglec‐F^highCD11c^low eosinophils to the airway in a murine model of asthma

Abstract

Eosinophil recruitment in asthma is a multistep process, involving both trans-endothelial migration to the lung interstitium and trans-epithelial migration into the airways. While the trans-endothelial step is well studied, trans-epithelial recruitment is less understood. To contrast eosinophil recruitment between these two compartments, we employed a murine kinetics model of asthma. Eosinophils were phenotyped by multicolor flow cytometry in digested lung tissue and bronchoalveolar lavage (BAL) simultaneously, 6 h after each ovalbumin (OVA) challenge. There was an early expansion of tissue eosinophils after OVA challenge followed by eosinophil buildup in both compartments and a shift in phenotype over the course of the asthma model. Gradual transition from a Siglec-F(med) CD11c(-) to a Siglec-F(high) CD11c(low) phenotype in lung tissue was associated with eosinophil recruitment to the airways, as all BAL eosinophils were of the latter phenotype. Secondary microarray analysis of tissue-activated eosinophils demonstrated upregulation of specific integrin and chemokine receptor signature suggesting interaction with the mucosa. Using adhesion assays, we demonstrated that integrin CD11c mediated adhesion of eosinophils to fibrinogen, a significant component of epithelial barrier repair and remodeling. To the best of our knowledge, this is the only report to date dissecting compartmentalization of eosinophil recruitment as it unfolds during allergic inflammation. By capturing the kinetics of eosinophil phenotypic change in both tissue and BAL using flow cytometry and sorting, we were able to demonstrate a previously undocumented association between phenotypic shift of tissue-recruited eosinophils and their trans-epithelial movement, which implicates the existence of a specific mechanism targeting these cells to mucosal airways.

Shaping eosinophil identity in the tissue contexts of development, homeostasis, and disease

“…In most homeostatic adult tissues, the numbers of resident eosinophils range from very limited to non‐existent. For instance, resident eosinophils in the healthy adult mouse lung average 1.5% of the total CD45 + hematopoietic cell population, although the turnover rate and functional significance of such resident populations is not well understood. Eosinophil numbers increase significantly in the following types of tissue microenvironments: (1) at homeostatic sites where epithelial cell turnover and stem cell activity are high (small intestine, endometrial lining of the uterus, bone marrow, thymus); (2) during developmental/morphogenetic events (ductal differentiation of mammary glands, development of Peyer's patches, postnatal lung development, beige fat biogenesis); (3) during processes of normal injury repair; (4) in diseases involving extensive tissue remodeling (helminthic parasite infections, acute lung injury (oxygen stress), fibrosis, cancers, allergic diseases of the gut, skin, and airway, and non‐atopic diseases with a significant remodeling component (eosinophilic esophagitis, chronic rhinosinusitis, AERD)); and (5) endocrinopathies.…”

“…Eosinophils are found in very low numbers at steady state but those numbers significantly increase during development and disease. Moreover, developmental and disease eosinophils are phenotypically distinct from true resident cells in homeostasis . Analogous to this, the homeostatic intestine undergoes active epithelial turnover, which also happens at a much slower rate in the steady state lung.…”

Eosinophil Activation Status in Separate Compartments and Association with Asthma

“…Some recent very interesting articles studied mouse lung tissue eosinophils and partly also human lung tissue eosinophils (87, 88). Abdala Valencia and colleagues reported that, after antigen challenge, mouse lung tissue eosinophils shifted from a surface phenotype with intermediate expression of Siglec-F and no or very low α X integrin (CD11c) to a Siglec-F-high/CD11c-low phenotype, and that BAL eosinophils were of the latter phenotype (87).…”

“…Abdala Valencia and colleagues reported that, after antigen challenge, mouse lung tissue eosinophils shifted from a surface phenotype with intermediate expression of Siglec-F and no or very low α X integrin (CD11c) to a Siglec-F-high/CD11c-low phenotype, and that BAL eosinophils were of the latter phenotype (87). Mesnil and others found that mouse steady-state pulmonary resident eosinophils were IL-5-independent and expressed an intermediate level of Siglec-F (in consistency with the first tissue phenotype in the Abdala Valencia publication), high L-selectin (CD62L), and low CD101 (immunoglobulin superfamily member 2) (88).…”

Homeostatic Eosinophils: Characteristics and Functions

“…Lung hEos represent an exception and rather resemble resting blood eosinophils. Indeed, both blood and lung eosinophils have a ring-shaped nucleus (as is the case for mammary hEos as well), express CD62L, display only intermediate levels of Siglec-F, and are negative for CD11c (8, 10, 17, 36, 37) (Figure 1). In mouse eosinophils, such characteristics, especially the presence of a ring-shaped nucleus, are considered a sign of cell immaturity (38, 39), suggesting that pulmonary hEos retain an immature phenotype when spreading into the lungs.…”