Eosinophils in Autoimmune Diseases

Eosinophils have been considered end-stage cells involved in host protection against parasites. However, numerous lines of evidence have now changed this perspective by showing that eosinophils are pleiotropic multifunctional leukocytes involved in initiation and propagation of diverse inflammatory responses, as well as modulators of innate and adaptive immunity. In this review, we summarize the biology of eosinophils, focusing on the growing properties of eosinophil-derived products, including the constituents of their granules as well as the mechanisms by which they release their pleiotropic mediators. We examine new views on the role of eosinophils in homeostatic function, including developmental biology and innate and adaptive immunity (as well as interaction with mast cells and T cells). The molecular steps involved in eosinophil development and trafficking are described, with special attention to the important role of the transcription factor GATA-1, the eosinophil-selective cytokine IL-5, and the eotaxin subfamily of chemokines. We also review the role of eosinophils in disease processes, including infections, asthma, and gastrointestinal disorders, and new data concerning genetically engineered eosinophil-deficient mice. Finally, strategies for targeted therapeutic intervention in eosinophil-mediated mucosal diseases are conceptualized.

Key Points Eosinophils have been traditionally perceived as terminally differentiated cytotoxic effector cells. Recent studies have provided a more sophisticated understanding of eosinophil effector functions and a more nuanced view of their contributions to the pathogenesis of various diseases, including asthma and respiratory allergies, eosinophilic gastrointestinal diseases, hypereosinophilic syndromes and parasitic infection. Eosinophils are granulocytes that develop in the bone marrow from pluripotent progenitors in response to cytokines, such as interleukin-5 (IL-5), IL-3 and granulocyte–macrophage colony-stimulating factor (GM-CSF). Mature eosinophils are released into the peripheral blood and enter tissues in response to cooperative signalling between IL-5 and eotaxin family chemokines. Eosinophils in peripheral blood and tissues are uniquely identified by their bilobed nuclei, their large specific granules that store cytokines, cationic proteins and enzymes, and their expression of the IL-5 receptor and CC-chemokine receptor 3 (CCR3). In addition, the receptors sialic acid-binding immunoglobulin-like lectin 8 (SIGLEC-8) and SIGLEC-F are expressed by human and mouse eosinophils, respectively. IL-5 has a central and profound role in all aspects of eosinophil development, activation and survival. IL-5 is produced by T helper 2 (TH2) cells, and more recently the contributions of the epithelium-derived innate cytokines thymic stromal lymphopoietin (TSLP), IL-25 and IL-33 in promoting eosinophilia via the induction of IL-5 have also been recognized. Although eosinophil responses are influenced by cytokines produced by T cells, eosinophils in turn modulate the functions of B and T cells. Eosinophils also communicate with a range of innate immune cells (such as mast cells, dendritic cells, macrophages and neutrophils). Eosinophils serve to bridge innate and adaptive immunity by regulating the production of chemoattractants and cytokines (including CC-chemokine ligand 17 (CCL17), CCL22, a proliferation-inducing ligand (APRIL) and IL-6) and via antigen presentation. Both successful and unsuccessful attempts to target eosinophils have yielded remarkable insights into their contribution to disease pathogenesis. Many eosinophil-associated inflammatory conditions have been shown to be heterogeneous in nature. As such, successful therapeutic strategies will depend on the correlation of disease activity with dysregulated eosinophil function as well as the identification of the crucial molecules that regulate eosinophil accumulation in the affected tissues. Supplementary information The online version of this article (doi:10.1038/nri3341) contains supplementary material, which is available to authorized users.