Bochner, Bruce S.

Abstract Eosinophilia and eosinophil activation are recurrent features in various reactive states and certain hematologic malignancies. In patients with hypereosinophilia (HE), HE‐induced organ damage is often encountered and may lead to the diagnosis of a hypereosinophilic syndrome (HES). A number of known mechanisms and etiologies contribute to the development of HE and HES. Based on these etiologies and the origin of eosinophils, HE and HES are divided into primary forms where eosinophils are clonal cells, reactive forms where an underlying reactive or neoplastic condition is detected and eosinophils are considered to be “non‐clonal” cells, and idiopathic HE and HES in which neither a clonal nor a reactive underlying pathology is detected. Since 2012, this classification and the related criteria have been widely accepted and regarded as standard. However, during the past few years, new developments in the field and an increasing number of markers and targets have created a need to update these criteria and the classification of HE and HES. To address this challenge, a Working Conference on eosinophil disorders was organized in 2021. In this conference, a panel of experts representing the relevant fields, including allergy, dermatology, hematology, immunology, laboratory medicine, and pathology, met and discussed new markers and concepts as well as refinements in definitions, criteria and classifications of HE and HES. The outcomes of this conference are presented in this article and should assist in the diagnosis and management of patients with HE and HES in daily practice and in the preparation and conduct of clinical trials.

AbstractBackgroundBiomarkers of eosinophilic disease activity, especially in the context of novel therapies that reduce blood eosinophil counts, are an unmet need. Absolute eosinophil count (AEC) does not accurately reflect tissue eosinophilia or eosinophil activation. Therefore, the aims of this study were to compare the reliability of plasma and urine eosinophil major basic protein 1, eosinophil cationic protein, eosinophil‐derived neurotoxin (EDN), and eosinophil peroxidase measurement and to evaluate the usefulness of eosinophil granule protein (EGP) measurement for the assessment of disease activity in patients with eosinophil‐associated diseases treated with mepolizumab, benralizumab, or dexpramipexole.MethodsEosinophil granule protein concentrations were measured in serum, plasma, and urine from healthy volunteers and patients with hypereosinophilic syndrome (HES), eosinophilic granulomatosis with polyangiitis (EGPA), and eosinophilic asthma using a multiplex assay.ResultsUrine EGP concentrations remained stable, whereas serum and plasma EGP concentrations increased significantly with delayed processing. Plasma (p) EDN, but not urine (u) EDN, concentration correlated with AEC and negatively correlated with prednisone dose. Both pEDN and uEDN decreased significantly following treatment of HES patients with benralizumab and EGPA patients with mepolizumab. uEDN appeared to increase with clinical relapse in both patient groups.ConclusionsMeasurement of EGP in urine is noninvasive and unaffected by cellular lysis. Although plasma and urine EDN concentrations showed a similar pattern following benralizumab and mepolizumab treatment, the lack of correlation between AEC or prednisone dose and uEDN concentrations suggests that measurement of uEDN may provide a potential biomarker of disease activity in patients with HES and EGPA.

AbstractEosinophil-associated diseases (EADs) are rare, heterogeneous disorders characterized by the presence of eosinophils in tissues and/or peripheral blood resulting in immunopathology. The heterogeneity of tissue involvement, lack of sufficient animal models, technical challenges in working with eosinophils, and lack of standardized histopathologic approaches have hampered progress in basic research. Additionally, clinical trials and drug development for rare EADs are limited by the lack of primary and surrogate endpoints, biomarkers, and validated patient-reported outcomes. Researchers with expertise in eosinophil biology and eosinophil-related diseases reviewed the state of current eosinophil research, resources, progress, and unmet needs in the field since the 2012 meeting of the NIH Taskforce on the Research of Eosinophil-Associated Diseases (TREAD). RE-TREAD focused on gaps in basic science, translational, and clinical research on eosinophils and eosinophil-related pathogenesis. Improved recapitulation of human eosinophil biology and pathogenesis in murine models was felt to be of importance. Characterization of eosinophil phenotypes, the role of eosinophil subsets in tissues, identification of biomarkers of eosinophil activation and tissue load, and a better understanding of the role of eosinophils in human disease were prioritized. Finally, an unmet need for tools for use in clinical trials was emphasized. Histopathologic scoring, patient- and clinician-reported outcomes, and appropriate coding were deemed of paramount importance for research collaborations, drug development, and approval by regulatory agencies. Further exploration of the eosinophil genome, epigenome, and proteome was also encouraged. Although progress has been made since 2012, unmet needs in eosinophil research remain a priority.Review on eosinophil biology and eosinophil-related disorders (2012–2017) with a focus on continued unmet needs in eosinophil-associated diseases.

Abstract Eosinophils play homeostatic roles in different tissues and are found in several organs at a homeostatic baseline, though their tissue numbers increase significantly in development and disease. The morphological, phenotypical, and functional plasticity of recruited eosinophils are influenced by the dynamic tissue microenvironment changes between homeostatic, morphogenetic, and disease states. Activity of the epithelial-mesenchymal interface, extracellular matrix, hormonal inputs, metabolic state of the environment, as well as epithelial and mesenchymal-derived innate cytokines and growth factors all have the potential to regulate the attraction, retention, in situ hematopoiesis, phenotype, and function of eosinophils. This review examines the reciprocal relationship between eosinophils and such tissue factors, specifically addressing: (1) tissue microenvironments associated with the presence and activity of eosinophils; (2) non-immune tissue ligands regulatory for eosinophil accumulation, hematopoiesis, phenotype, and function (with an emphasis on the extracellular matrix and epithelial–mesenchymal interface); (3) the contribution of eosinophils to regulating tissue biology; (4) eosinophil phenotypic heterogeneity in different tissue microenvironments, classifying eosinophils as progenitors, steady state eosinophils, and Type 1 and 2 activated phenotypes. An appreciation of eosinophil regulation by non-immune tissue factors is necessary for completing the picture of eosinophil immune activation and understanding the functional contribution of these cells to development, homeostasis, and disease. Review on the two-way communication between tissue and eosinophils, discussing eosinophil heterogeneity in the framework of biological processes occurring in different tissue states.

Abstract Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is a human cell surface protein expressed exclusively on eosinophils, mast cells, and basophils that, when engaged, induces eosinophil apoptosis and inhibits mast cell mediator release. This makes Siglec-8 a promising therapeutic target to treat diseases involving these cell types. However, preclinical studies of Siglec-8 targeting in vivo are lacking because this protein is only found in humans, apes, and some monkeys. Therefore, we have developed a mouse strain in which SIGLEC8 transcription is activated by Cre recombinase and have crossed this mouse with the eoCre mouse to achieve eosinophil-specific expression. We confirmed that Siglec-8 is expressed exclusively on the surface of mature eosinophils in multiple tissues at levels comparable to those on human blood eosinophils. Following ovalbumin sensitization and airway challenge, Siglec-8 knock-in mice generated a pattern of allergic lung inflammation indistinguishable from that of littermate controls, suggesting that Siglec-8 expression within the eosinophil compartment does not alter allergic eosinophilic inflammation. Using bone marrow from these mice, we demonstrated that, during maturation, Siglec-8 expression occurs well before the late eosinophil developmental marker C-C motif chemokine receptor 3, consistent with eoCre expression. Antibody ligation of the receptor induces Siglec-8 endocytosis and alters the phosphotyrosine profile of these cells, indicative of productive signaling. Finally, we demonstrated that mouse eosinophils expressing Siglec-8 undergo cell death when the receptor is engaged, further evidence that Siglec-8 is functional on these cells. These mice should prove useful to investigate Siglec-8 biology and targeting in vivo in a variety of eosinophilic disease models. We have developed a novel eosinophil-specific Siglec-8 knock-in mouse strain that facilitates the in vivo study of Siglec-8 targeting and biology.

Abstract Tissue structural cells are known in some situations to play a role in the presentation of antigen and in immunoregulation. We assessed the expression of B7 homologs, known to be involved in antigen presentation and lymphocyte costimulation, in human airway epithelial cells. Flow cytometry performed on the airway epithelial cell line BEAS-2B, as well as primary bronchial epithelial cells (PBEC), showed that B7-H2 was constitutively expressed on both BEAS-2B and PBEC, whereas B7-1 and B7-2 were undetectable on either epithelial cell type. B7-H2 expression was confirmed by Western blot using a specific antibody. Stimulation with various cytokines, including tumor necrosis factor-α, interferon-γ, and interleukin-4, slightly downregulated B7-H2 expression detected by flow cytometry, but did not significantly alter the apparent level of protein as assessed by Western blotting. Northern blotting detected mRNA for B7-H2 and B7-1, but not B7-2. B7-H2 was cloned from BEAS-2B cells and the sequence verified. Expression of B7-H2 mRNA was detected by real-time reverse transcriptase–polymerase chain reaction in PBEC from three independent donors. Immunohistochemical analysis of airway derived from autopsies revealed expression of B7-H2 in human airway epithelial cells. These results demonstrate that airway epithelial cells express the costimulatory molecule B7-H2, and suggest the possibility that B7-H2 may participate in antigen presentation by epithelial cells.