Circulating Human Eosinophils Share a Similar Transcriptional Profile in Asthma and Other Hypereosinophilic Disorders

Advancing toward a unified eosinophil signature from transcriptional profiling

Eosinophils are granulocytes that can accumulate in increased numbers in tissues and/or peripheral blood in disease. Phenotyping of eosinophils in health and disease has the potential to improve the precision of diagnosis and choice of therapies for eosinophilic-associated diseases. Transcriptional profiling of eosinophils has been plagued by cell fragility and difficulty isolating high-quality RNA. With several technological advances, single-cell RNA sequencing has become possible with eosinophils, at least from mice, while bulk RNA sequencing and microarrays have been performed in both murine and human samples. Anticipating more eosinophil transcriptional profiles in the coming years, we provide a summary of prior studies conducted on mouse and human eosinophils in blood and tissue, with a discussion of the advantages and potential pitfalls of various approaches. Common technical standards in studying eosinophil biology would help advance the field and make cross-study comparisons possible. Knowledge gaps and opportunities include identifying a minimal set of genes that define the eosinophil lineage, comparative studies between active disease and remission vs. homeostasis or development, especially in humans, and a comprehensive comparison between murine and human eosinophils at the transcriptional level. Characterizing such transcriptional patterns will be important to understanding the complex and diverse roles of eosinophils in both health and disease.

Eosinophil specialization is regulated by exposure to the esophageal epithelial microenvironment

Distinct subsets of eosinophils are reported in inflammatory and healthy tissues, yet the functions of uniquely specialized eosinophils and the signals that elicit them, particularly in eosinophilic esophagitis, are not well understood. Herein, we report an ex vivo system wherein freshly isolated human eosinophils were cocultured with esophageal epithelial cells and disease-relevant proinflammatory (IL-13) or profibrotic (TGF-β) cytokines. Compared with untreated cocultures, IL-13 increased expression of CD69 on eosinophils, whereas TGF-β increased expression of CD81, CD62L, and CD25. Eosinophils from IL-13-treated cocultures demonstrated increased secretion of GRO-α, IL-8, and macrophage colony-stimulating factor and also generated increased extracellular peroxidase activity following activation. Eosinophils from TGF-β-treated cocultures secreted increased IL-6 and exhibited increased chemotactic response to CCL11 compared with eosinophils from untreated or IL-13-treated coculture conditions. When eosinophils from TGF-β-treated cocultures were cultured with fibroblasts, they upregulated SERPINE1 expression and fibronectin secretion by fibroblasts compared with eosinophils that were cultured with granulocyte macrophage colony-stimulating factor alone. Translational studies revealed that CD62L was heterogeneously expressed by eosinophils in patient biopsy specimens. Our results demonstrate that disease-relevant proinflammatory and profibrotic signals present in the esophagus of patients with eosinophilic esophagitis cause distinct profiles of eosinophil activation and gene expression.

Transcriptional profiles of peripheral eosinophils in chronic obstructive pulmonary disease and asthma-An exploratory study

The role of eosinophilic inflammation in the pathogenesis of chronic obstructive pulmonary disease (COPD) remains ambiguous and likely differs from its role in asthma. The molecular processes underlying the differences between eosinophils from asthma and COPD have not been sufficiently studied. The objective of this study was to compare the transcriptomic profiles of blood eosinophils in COPD and asthma. Eosinophils were isolated from peripheral blood drawn from stable mild-to-moderate COPD and asthma patients. RNA was isolated from eosinophils and sequenced using an NGSelect RNA. The prepared libraries were sequenced on an Illumina platform. The study group included five patients with asthma and four patients with COPD. The RNA-Seq data analysis identified 26 differentially expressed genes between COPD and asthma (according to adjusted p-value). In total, 6 genes were upregulated (e.g. CCL3L1, CCL4L2, GPR82) and 20 were downregulated (e.g. JUN, IFITM3, DUSP1, GNG7) in peripheral eosinophils of COPD patients compared to asthma. The genes associated with signalling of IL-4 and IL-13 pathways were downregulated in COPD eosinophils compared to asthma. In conclusion, blood eosinophils from COPD and asthma patients present different transcriptomic profiles suggesting their different function in pathobiology of both obstructive airway diseases. These differences might indicate the direction of the search of targeted therapy in COPD.

Blood and Sputum Eosinophils of COPD Patients Are Differently Polarized than in Asthma

Different eosinophil subpopulations have been identified in asthma and other eosinophilic disorders. However, there is a paucity of data on eosinophil subpopulations in patients with chronic obstructive pulmonary disease (COPD). The aim of this study was to compare eosinophil phenotypes in blood and induced sputum in patients with COPD, asthma and controls. Stable patients with mild-to-moderate COPD (n = 15) and asthma (n = 14) with documented blood eosinophilia ≥100 cells/µL in the year prior to the study and the control group (n = 11) were included to the study. The blood and sputum eosinophil phenotypes were analyzed by flow cytometry. IL-5, IL-13, CCL5 and eotaxin-3 levels were measured in the induced sputum. The marker expression on blood eosinophils was similar among control, asthma and COPD groups. The expressions of CD125, CD193, CD14 and CD62L were higher on blood than on sputum eosinophils in all three groups. We found increased levels of CD193+ and CD66b+ sputum eosinophils from COPD patients, and an elevated level of CD11b+ sputum eosinophils in asthma compared to COPD patients. The results of our study suggest that the profile of marker expression on COPD sputum eosinophils differed from other groups, suggesting a distinct phenotype of eosinophils of COPD patients than in asthma or healthy subjects.

Blood and adipose-resident eosinophils are defined by distinct transcriptional profiles

Eosinophils are granular leukocytes of the innate immune system that play important functions in host defense. Inappropriate activation of eosinophils can occur in pathologies such as asthma and esophagitis. However, eosinophils also reside within adipose tissue, where they play homeostatic roles and are important in the activation of thermogenic beige fat. Here we performed bulk RNA sequencing in mouse adipose tissue-resident eosinophils isolated from both subcutaneous and gonadal depots, for the first time, and compared gene expression to blood eosinophils. We found a predominantly conserved transcriptional landscape in eosinophils between adipose depots that is distinct from blood eosinophils in circulation. Through exploration of differentially expressed transcription factors and transcription factors with binding sites enriched in adipose-resident eosinophil genes, we identified KLF, CEBP, and Fos/Jun family members that may drive functional specialization of eosinophils in adipose tissue. These findings increase our understanding of tissue-specific eosinophil heterogeneity, with implications for targeting eosinophil function to treat metabolic disorders such as obesity.

Genetic Determinants in Airways Obstructive Diseases: The Case of Asthma Chronic Obstructive Pulmonary Disease Overlap

Genome-wide association studies (GWAS) of asthma and chronic obstructive pulmonary disease (COPD) with ever-increasing sample sizes have found multiple genetic loci associated with either disease. However, there are few intersecting loci between asthma and COPD. GWAS specifically focused on asthma-COPD overlap (ACO) have been limited by smaller sample sizes and the lack of a consistent definition of ACO that has also hampered clinical and epidemiologic studies. Other genomic techniques, such as gene expression profiling, are feasible with smaller sample sizes. Genetic analyses of objective measures of airway reactivity and allergy/T2 inflammation biomarkers in COPD studies may be another strategy to overcome limitations in ACO definitions.

Eosinophil-mediated lung inflammation associated with elevated natural killer T cell response in COVID-19 patients

BACKGROUND/AIMS

Coronavirus disease 2019 (COVID-19) is associated with acute respiratory syndrome. The mechanisms underlying the different degrees of pneumonia severity in patients with COVID-19 remain elusive. This study provides evidence that COVID-19 is associated with eosinophil-mediated inflammation.

METHODS

We performed a retrospective case series of three patients with laboratory and radiologically confirmed COVID-19 pneumonia admitted to Chosun University Hospital. Demographic and clinical data on inflammatory cell lung infiltration and cytokine levels in patients with COVID-19 were collected.

RESULTS

Cytological analysis of sputum, tracheal aspirates, and bronchoalveolar lavage fluid (BALF) samples from all three patients revealed massive infiltration of polymorphonuclear cells (PMNs), such as eosinophils and neutrophils. All sputum and BALF specimens contained high levels of eosinophil cationic proteins. The infiltration of PMNs into the lungs, together with elevated levels of natural killer T (NKT) cells in BALF and peripheral blood samples from patients with severe pneumonia in the acute phase was confirmed by flow cytometry.

CONCLUSION

These results suggest that the lungs of COVID-19 patients can exhibit eosinophil-mediated inflammation, together with an elevated NKT cell response, which is associated with COVID-19 pneumonia.

Inflammatory signature in acute-on-chronic liver failure includes increased expression of granulocyte genes ELANE, MPO and CD177

Acute-on-Chronic Liver Failure (ACLF) is associated with innate immune dysfunction and high short-term mortality. Neutrophils have been identified to influence prognosis in ACLF. Neutrophil biology is under-evaluated in ACLF. Therefore, we investigated neutrophil-specific genes and their association with ACLF outcomes. This is an observational study. Enriched granulocytes, containing neutrophils, isolated from study participants in three groups- ACLF(n = 10), chronic liver disease (CLD, n = 4) and healthy controls (HC, n = 4), were analysed by microarray. Differentially expressed genes were identified and validated by qRT-PCR in an independent cohort of ACLF, CLD and HC (n = 30, 15 and 15 respectively). The association of confirmed overexpressed genes with ACLF 28-day non-survivors was investigated. The protein expression of selected neutrophil genes was confirmed using flow cytometry and IHC. Differential gene expression analysis showed 1140 downregulated and 928 upregulated genes for ACLF versus CLD and 2086 downregulated and 1091 upregulated genes for ACLF versus HC. Significant upregulation of neutrophilic inflammatory signatures were found in ACLF compared to CLD and HC. Neutrophil enriched genes ELANE, MPO and CD177 were highly upregulated in ACLF and their expression was higher in ACLF 28-day non-survivors. Elevated expression of CD177 protein on neutrophil surface in ACLF was confirmed by flow cytometry. IHC analysis in archival post mortem liver biopsies showed the presence of CD177+ neutrophils in the liver tissue of ACLF patients. Granulocyte genes ELANE, MPO and CD177 are highly overexpressed in ACLF neutrophils as compared to CLD or HC. Further, this three-gene signature is highly overexpressed in ACLF 28-day non-survivors.

Eosinophils as Drivers of Severe Eosinophilic Asthma: Endotypes or Plasticity?

Asthma is now recognized as a heterogeneous disease, encompassing different phenotypes driven by distinct pathophysiological mechanisms called endotypes. Common phenotypes of asthma, referred to as eosinophilic asthma, are characterized by the presence of eosinophilia. Eosinophils are usually considered invariant, terminally differentiated effector cells and have become a primary therapeutic target in severe eosinophilic asthma (SEA) and other eosinophil-associated diseases (EADs). Biological treatments that target eosinophils reveal an unexpectedly complex role of eosinophils in asthma, including in SEA, suggesting that "not all eosinophils are equal". In this review, we address our current understanding of the role of eosinophils in asthma with regard to asthma phenotypes and endotypes. We further address the possibility that different SEA phenotypes may involve differences in eosinophil biology. We discuss how these differences could arise through eosinophil "endotyping", viz. adaptations of eosinophil function imprinted during their development, or through tissue-induced plasticity, viz. local adaptations of eosinophil function through interaction with their lung tissue niches. In doing so, we also discuss opportunities, technical challenges, and open questions that, if addressed, might provide considerable benefits in guiding the choice of the most efficient precision therapies of SEA and, by extension, other EADs.

Bidirectional crosstalk between eosinophils and esophageal epithelial cells regulates inflammatory and remodeling processes

Eosinophils accumulate adjacent to epithelial cells in the mucosa of patients with eosinophilic esophagitis (EoE), yet the bidirectional communication between these cells is not well understood. Herein, we investigated the crosstalk between human eosinophils and esophageal epithelial cells. We report that blood-derived eosinophils have prolonged survival when cocultured with epithelial cells; 96 ± 1% and 30 ± 6% viability was observed after 7 and 14 days of coculture, respectively, compared with 1 ± 0% and 0 ± 0% of monoculture. In the presence of IL-13 and epithelial cells, eosinophils had greater survival (68 ± 1%) at 14 days compared with cocultures lacking IL-13. Prolonged eosinophil viability did not require cellular contact and was observed when eosinophils were cultured in conditioned media from esophageal epithelial cells; neutralizing GM-CSF attenuated eosinophil survival. The majority of eosinophil transcripts (58%) were dysregulated in cocultured eosinophils compared with freshly isolated cells. Analysis of epithelial cell transcripts indicated that exposure to eosinophils induced differential expression of a subset of genes that were part of the EoE esophageal transcriptome. Collectively, these results uncover a network of crosstalk between eosinophils and esophageal epithelial cells involving epithelial mediated eosinophil survival and reciprocal changes in cellular transcripts, events likely to occur in EoE.

The Release Kinetics of Eosinophil Peroxidase and Mitochondrial DNA Is Different in Association with Eosinophil Extracellular Trap Formation

Eosinophils are a subset of granulocytes characterized by a high abundance of specific granules in their cytoplasm. To act as effector cells, eosinophils degranulate and form eosinophil extracellular traps (EETs), which contain double-stranded DNA (dsDNA) co-localized with granule proteins. The exact molecular mechanism of EET formation remains unknown. Although the term “EET release” has been used in scientific reports, it is unclear whether EETs are pre-formed in eosinophils and subsequently released. Moreover, although eosinophil degranulation has been extensively studied, a precise time-course of granule protein release has not been reported until now. In this study, we investigated the time-dependent release of eosinophil peroxidase (EPX) and mitochondrial DNA (mtDNA) following activation of both human and mouse eosinophils. Unexpectedly, maximal degranulation was already observed within 1 min with no further change upon complement factor 5 (C5a) stimulation of interleukin-5 (IL-5) or granulocyte/macrophage colony-stimulating factor (GM-CSF)-primed eosinophils. In contrast, bulk mtDNA release in the same eosinophil populations occurred much slower and reached maximal levels between 30 and 60 min. Although no single-cell analyses have been performed, these data suggest that the molecular pathways leading to degranulation and mtDNA release are at least partially different. Moreover, based on these data, it is likely that the association between the mtDNA scaffold and granule proteins in the process of EET formation occurs in the extracellular space.

Learning gene networks underlying clinical phenotypes using SNP perturbation

Availability of genome sequence, molecular, and clinical phenotype data for large patient cohorts generated by recent technological advances provides an opportunity to dissect the genetic architecture of complex diseases at system level. However, previous analyses of such data have largely focused on the co-localization of SNPs associated with clinical and expression traits, each identified from genome-wide association studies and expression quantitative trait locus mapping. Thus, their description of the molecular mechanisms behind the SNPs influencing clinical phenotypes was limited to the single gene linked to the co-localized SNP. Here we introduce PerturbNet, a statistical framework for learning gene networks that modulate the influence of genetic variants on phenotypes, using genetic variants as naturally occurring perturbation of a biological system. PerturbNet uses a probabilistic graphical model to directly model the cascade of perturbation from genetic variants to the gene network to the phenotype network along with the networks at each layer of the biological system. PerturbNet learns the entire model by solving a single optimization problem with an efficient algorithm that can analyze human genome-wide data within a few hours. PerturbNet inference procedures extract a detailed description of how the gene network modulates the genetic effects on phenotypes. Using simulated and asthma data, we demonstrate that PerturbNet improves statistical power for detecting disease-linked SNPs and identifies gene networks and network modules mediating the SNP effects on traits, providing deeper insights into the underlying molecular mechanisms.

Eosinophil and mast cell Siglecs: From biology to drug target

Mast cells and eosinophils are innate immune cells involved in both acute and chronic inflammatory responses. Siglecs are a family of cell surface receptors that share sialic acid binding activity. Over the past 20 years, our knowledge of the expression and function of Siglecs on cells of the immune system and others has greatly expanded, as has our understanding of their signaling, ligands, and possible roles in disease pathophysiology. Because of this, Siglecs have garnered interest as potential drug targets using strategies ranging from biologics to ligand-directed nanoparticles. This mini-review will highlight the state of our knowledge regarding human eosinophil and mast cell Siglecs, their biology, what they recognize, tools developed for in vitro and preclinical experimentation, and the status of ongoing efforts to develop drugs that engage eosinophil and mast cell Siglecs for potential therapeutic benefit.

Influenza A virus directly modulates mouse eosinophil responses

Allergic asthma and influenza are common respiratory diseases with a high probability of co-occurrence. During the 2009 influenza pandemic, hospitalized patients with influenza experienced lower morbidity if asthma was an underlying condition. We have previously demonstrated that acute allergic asthma protects mice from severe influenza and have implicated eosinophils in the airways of mice with allergic asthma as participants in the antiviral response. However, very little is known about how eosinophils respond to direct exposure to influenza A virus (IAV) or the microenvironment in which the viral burden is high. We hypothesized that eosinophils would dynamically respond to the presence of IAV through phenotypic, transcriptomic, and physiologic changes. Using our mouse model of acute fungal asthma and influenza, we showed that eosinophils in lymphoid tissues were responsive to IAV infection in the lungs and altered surface expression of various markers necessary for cell activation in a niche-specific manner. Siglec-F expression was altered in a subset of eosinophils after virus exposure, and those expressing high Siglec-F were more active (IL-5Rα^hi CD62L^lo ). While eosinophils exposed to IAV decreased their overall transcriptional activity and mitochondrial oxygen consumption, transcription of genes encoding viral recognition proteins, Ddx58 (RIG-I), Tlr3, and Ifih1 (MDA5), were up-regulated. CD8⁺ T cells from IAV-infected mice expanded in response to IAV PB1 peptide-pulsed eosinophils, and CpG methylation in the Tbx21 promoter was reduced in these T cells. These data offer insight into how eosinophils respond to IAV and help elucidate alternative mechanisms by which they regulate antiviral immune responses during IAV infection.

Eosinophils and Respiratory Viruses

Eosinophils have been mainly associated with parasitic infection and pathologies such as asthma. Some patients with asthma present a high number of eosinophils in their airways. Since respiratory viruses are associated with asthma exacerbations, several studies have evaluated the role of eosinophils against respiratory viruses. Eosinophils contain and produce molecules with antiviral activity, including RNases and reactive nitrogen species. They can also participate in adaptive immunity, serving as antigen-presenting cells. Eosinophil antiviral response has been demonstrated against some respiratory viruses in vitro and in vivo, including respiratory syncytial virus and influenza. Given the implication of respiratory viruses in asthma, the eosinophil antiviral role might be an important factor to consider in this pathology.

Altered fecal bacterial composition correlates with disease activity in inflammatory bowel disease and the extent of IL8 induction

PURPOSE OF THE STUDY

In this study we investigated the presence and relative abundance of important genera of the gut microbiota in IBD patients and their role in induction of IL8 in a cell culture model.

PATIENTS AND METHODS

Stool samples of IBD patients and healthy controls were collected and relative diversity of thirteen bacterial families was measured using quantitative real-time PCR assay. Moreover, filtrate of the stool samples was used for treatment of HT-29 cell line to analyze involvement of diversity of the fecal bacterial communities in the extent of IL8 induction.

RESULTS

Bacteroides, Faecalibacterium prausnitzii, Prevotella spp., and Methanobrevibacterium were significantly less abundant in IBD patients (UC, N = 22; CD, N = 7) compared with control group (N = 29). Increase in relative amounts of Haemophilus, Streptococcus spp., and H. pylori were detected in IBD patients, which was not statistically significant. Relative decrease in amount of Bacteroides spp., Faecalibacterium prausnitzii, and Prevotella spp. were found in UC patients with disease activity score greater than 4; however, higher levels of Streptococcus and Haemophilus were detected in the patients who were at flares. A relationship between the reduction of Haemophilus spp. and higher BMI was shown in IBD patients. Expression of IL8 was significantly higher in the treated cells by the fecal inoculates of IBD patients. Increase in relative amounts of Enterobacteriacea showed a correlation with the higher level of IL8 induction in both groups.

CONCLUSIONS

These results showed that changes in the fecal microbiota composition could affect disease activity, BMI, and IL8 induction.

FACS - based isolation of human eosinophils allows purification of high quality RNA

Here we extensively describe a FACS-based protocol for isolating intact non-stained human eosinophils from peripheral blood; a stop forward from our recently published initial study. This method of purification could be accomplished in <3 h with only small volumes of whole blood necessary, even in healthy subjects generally exhibiting low levels of circulating eosinophils. Eosinophil activation during the isolation steps appeared to be minimal and this purification procedure yielded high quality RNA. Moreover, these FACS-isolated eosinophils had prolonged viability in culture and were suitable for further activation assays.

Metabolic Profiling of Human Eosinophils

Immune cells face constant changes in their microenvironment, which requires rapid metabolic adaptation. In contrast to neutrophils, which are known to rely near exclusively on glycolysis, the metabolic profile of human eosinophils has not been characterized. Here, we assess the key metabolic parameters of peripheral blood-derived human eosinophils using real-time extracellular flux analysis to measure extracellular acidification rate and oxygen consumption rate, and compare these parameters to human neutrophils. Using this methodology, we demonstrate that eosinophils and neutrophils have a similar glycolytic capacity, albeit with a minimal glycolytic reserve. However, compared to neutrophils, eosinophils exhibit significantly greater basal mitochondrial respiration, ATP-linked respiration, maximum respiratory capacity, and spare respiratory capacity. Of note, the glucose oxidation pathway is also utilized by eosinophils, something not evident in neutrophils. Furthermore, using a colorimetric enzymatic assay, we show that eosinophils have much reduced glycogen stores compared to neutrophils. We also show that physiologically relevant levels of hypoxia (PO₂ 3 kPa), by suppressing oxygen consumption rates, have a profound effect on basal and phorbol-myristate-acetate-stimulated eosinophil and neutrophil metabolism. Finally, we compared the metabolic profile of eosinophils purified from atopic and non-atopic subjects and show that, despite a difference in the activation status of eosinophils derived from atopic subjects, these cells exhibit comparable oxygen consumption rates upon priming with IL-5 and stimulation with fMLP. In summary, our findings show that eosinophils display far greater metabolic flexibility compared to neutrophils, with the potential to use glycolysis, glucose oxidation, and oxidative phosphorylation. This flexibility may allow eosinophils to adapt better to diverse roles in host defense, homeostasis, and immunomodulation.

Functions of tissue-resident eosinophils

Eosinophils are a prominent cell type in particular host responses such as the response to helminth infection and allergic disease. Their effector functions have been attributed to their capacity to release cationic proteins stored in cytoplasmic granules by degranulation. However, eosinophils are now being recognized for more varied functions in previously underappreciated diverse tissue sites, based on the ability of eosinophils to release cytokines (often preformed) that mediate a broad range of activities into the local environment. In this Review, we consider evolving insights into the tissue distribution of eosinophils and their functional immunobiology, which enable eosinophils to secrete in a selective manner cytokines and other mediators that have diverse, 'non-effector' functions in health and disease.

Eosinophils and eosinophil-associated diseases: An update

The goal of this series is to offer a survey of the latest literature for clinicians and scientists alike, providing a list of important recent advances relevant to the broad field of allergy and immunology. This particular assignment was to cover the topic of eosinophils. In an attempt to highlight major ideas, themes, trends, and advances relevant to basic and clinical aspects of eosinophil biology, a search of articles published since 2015 in the Journal of Allergy and Clinical Immunology and other high-impact journals was performed. Articles were then reviewed and organized, and then key findings were summarized. Given space limitations, many outstanding articles could not be included, but the hope is that what follows provides a succinct overview of recently published work that has significantly added to our knowledge of eosinophils and eosinophil-associated diseases.

Recent advances in understanding eosinophil biology

With the advent of novel therapies targeting eosinophils, there has been renewed interest in understanding the basic biology of this unique cell. In this context, murine models and human studies have continued to highlight the role of the eosinophil in homeostatic functions and immunoregulation. This review will focus on recent advances in our understanding of eosinophil biology that are likely to have important consequences on the development and consequences of eosinophil-targeted therapies. Given the breadth of the topic, the discussion will be limited to three areas of interest: the eosinophil life cycle, eosinophil heterogeneity, and mechanisms of cell-cell communication.

The epithelial barrier and immunoglobulin A system in allergy

Airway and intestinal epithelial layers represent first-line physical barriers, playing a key role in mucosal immunity. Barrier dysfunction, characterized by alterations such as disruption of cell-cell apical junctions and aberrant epithelial responses, probably constitutes early and key events for chronic immune responses to environmental antigens in the skin and in the gut. For instance, barrier dysfunction drives Th2 responses in atopic disorders or eosinophilic esophagitis. Such epithelial impairment is also a salient feature of allergic asthma and growing evidence indicates that barrier alterations probably play a driving role in this disease. IgA has been identified as the most abundant immunoglobulin in mucosa, where it acts as an active barrier through immune exclusion of inhaled or ingested antigens or pathogens. Historically, it has been thought to represent the serum factor underlying reaginic activity before IgE was discovered. Despite several studies about regulation and major functions of IgA at mucosal surfaces, its role in allergy remains largely unclear. This review aims at summarizing findings about epithelial functions and IgA biology that are relevant to allergy, and to integrate the emerging concepts and the recent developments in mucosal immunology, and how these could translate to clinical observations in allergy.

Lung-resident eosinophils represent a distinct regulatory eosinophil subset

Increases in eosinophil numbers are associated with infection and allergic diseases, including asthma, but there is also evidence that eosinophils contribute to homeostatic immune processes. In mice, the normal lung contains resident eosinophils (rEos), but their function has not been characterized. Here, we have reported that steady-state pulmonary rEos are IL-5-independent parenchymal Siglec-FintCD62L+CD101lo cells with a ring-shaped nucleus. During house dust mite-induced airway allergy, rEos features remained unchanged, and rEos were accompanied by recruited inflammatory eosinophils (iEos), which were defined as IL-5-dependent peribronchial Siglec-FhiCD62L-CD101hi cells with a segmented nucleus. Gene expression analyses revealed a more regulatory profile for rEos than for iEos, and correspondingly, mice lacking lung rEos showed an increase in Th2 cell responses to inhaled allergens. Such elevation of Th2 responses was linked to the ability of rEos, but not iEos, to inhibit the maturation, and therefore the pro-Th2 function, of allergen-loaded DCs. Finally, we determined that the parenchymal rEos found in nonasthmatic human lungs (Siglec-8+CD62L+IL-3Rlo cells) were phenotypically distinct from the iEos isolated from the sputa of eosinophilic asthmatic patients (Siglec-8+CD62LloIL-3Rhi cells), suggesting that our findings in mice are relevant to humans. In conclusion, our data define lung rEos as a distinct eosinophil subset with key homeostatic functions.