Decoding eosinophils: biological insights hidden in plain sight?

This is the authors' abstract. We don't add key points for this paper.

Noninvasive biomarkers are useful in multiple aspects of disease management, including risk assessment, diagnosis, choice of therapy, and assessing treatment responsiveness. Easy accessibility makes cellular and soluble constituents of peripheral blood an ideal biomarker source. In this issue of the Journal of Leukocyte Biology, Dezoteux et al. explores the utility of phenotypic signatures of circulating eosinophils to discriminate adult patients with moderate to severe atopic dermatitis (AD) from healthy individuals.1 Their findings contribute new insights that speak to the potential clinical value of evaluating multiparameter cellular attributes beyond absolute peripheral eosinophil counts in allergic disease. Eosinophils are granulocytes characterized by a dense cytoplasmic concentration of granules that contain a preformed myriad of cationic cytotoxic proteins (eg eosinophil peroxidase), cytokines, and chemokines. Typically present in low numbers in circulating blood of healthy individuals (<500 per microliter; < 5% of WBCs), eosinophils emerge from the bone marrow as mature cells and transit through the blood en route to specific tissue niches including the gastrointestinal tract, lung, adipose tissue, and thymus. Within the context of parasitic infections and type 2 inflammation, IL-5 is the major driver of extra physiologic levels of eosinophil expansion. Moderate or severe elevations in absolute counts of blood eosinophils are associated with several medical conditions including hypereosinophilic syndrome (HES), eosinophilic granulomatosis with polyangiitis, respiratory viral infections, eosinophilic asthma, and chronic obstructive pulmonary disease (COPD), and in some instances may be utilized for therapeutic decision making.2–4 For example, in asthma and COPD, contemporaneous peripheral eosinophilia serves as a surrogate marker of bronchial and/or lung eosinophilic inflammation, thereby identifying those patients who may benefit from type 2 and/or eosinophil-targeting therapies.