Eosinophils in Autoimmune Diseases

Abstract

Eosinophils are multifunctional granulocytes that contribute to initiation and modulation of inflammation. Their role in asthma and parasitic infections has long been recognized. Growing evidence now reveals a role for eosinophils in autoimmune diseases. In this review, we summarize the function of eosinophils in inflammatory bowel diseases, neuromyelitis optica, bullous pemphigoid, autoimmune myocarditis, primary biliary cirrhosis, eosinophilic granulomatosis with polyangiitis, and other autoimmune diseases. Clinical studies, eosinophil-targeted therapies, and experimental models have contributed to our understanding of the regulation and function of eosinophils in these diseases. By examining the role of eosinophils in autoimmune diseases of different organs, we can identify common pathogenic mechanisms. These include degranulation of cytotoxic granule proteins, induction of antibody-dependent cell-mediated cytotoxicity, release of proteases degrading extracellular matrix, immune modulation through cytokines, antigen presentation, and prothrombotic functions. The association of eosinophilic diseases with autoimmune diseases is also examined, showing a possible increase in autoimmune diseases in patients with eosinophilic esophagitis, hypereosinophilic syndrome, and non-allergic asthma. Finally, we summarize key future research needs.

Keywords: autoimmune diseases; bullous pemphigoid; eosinophilia; eosinophilic granulomatosis with polyangiitis; inflammatory bowel disease; innate immune system; myocarditis; neuromyelitis optica.

Figure 1

Cellular structure, receptors, and mediators of eosinophils. The pseudocolored composite electron micrograph of an eosinophil highlights cellular structures. Characteristic features of eosinophils include the multilobed nucleus, specific eosinophil granules, lipid bodies, and sombrero vesicles. Eosinophil granules contain cationic proteins, cytokines, growth factors, chemokines, and enzymes. The granule contents can be released upon stimulation. Lipid bodies are the place of synthesis for numerous lipid mediators. Granule contents can be released through sombrero vesicles. Eosinophils carry numerous cell surface receptors including chemokine receptors, Fc receptors, pattern recognition receptors, receptors for lipid mediators, cytokine receptors, complement receptors, and adhesion receptors. Abbreviations: 15-HETE, 15-hydroxyeicosatetraenoic acid; APRIL, a proliferation-inducing ligand; CCL, CC-chemokine ligand; CCR, CC-chemokine receptor; CXCL, CXC-chemokine ligand; CXCR, CXC-chemokine receptor; ECP, eosinophil cationic protein; EDN, eosinophil-derived neurotoxin; EGF, epidermal growth factor; EPX, eosinophil peroxidase; GMCSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; MBP, major basic protein; NGF, nerve growth factor; PDGF, platelet-derived growth factor; PAF, platelet-activating factor; SCF, stem cell factor; TGF, transforming growth factor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor. The electron micrograph was generously provided by Dr. Isabelle Coppens, Johns Hopkins University, Baltimore, MD, USA.

Figure 2

Possible mechanisms of eosinophil-mediated damage in different autoimmune diseases. In neuromyelitis optica, eosinophils damage astrocytes through antibody-dependent and complement-dependent cell-mediated cytotoxicity. Eosinophil degranulation in damaged bile ducts was shown for primary biliary cirrhosis. In bullous pemphigoid, eosinophils release proteases that degrade the dermal–epidermal anchoring complex. Eosinophil infiltration in the heart results in damage to the endocardium and myocardium either directly or indirectly through mast cells. Eosinophil-derived IL-4 can drive progression from autoimmune myocarditis to DCM. In inflammatory bowel diseases, eosinophils can damage the mucosa through multiple mechanisms. Eosinophil granule proteins damage epithelial cells and increase epithelial barrier permeability. Eosinophil-derived IL-22BP blocks the protective effects of IL-22 on epithelial cells. GM-CSF may prolong survival and activation of eosinophils in the intestine. In eosinophilic granulomatosis with polyangiitis, eosinophils damage nerves and blood vessels. Abbreviations: CNS, central nervous system; AQP4, aquaporin 4; FcR, Fc receptor; BP180, bullous pemphigoid 180; RBC, red blood cell; DCM, dilated cardiomyopathy; EPX, eosinophil peroxidase; CRH, corticotropin-releasing hormone; MBP, major basic protein; IL-22BP, IL-22-binding protein; GM-CSF, granulocyte-macrophage colony-stimulating factor; Th₁₇, T-helper 17 cell; Th₂, T-helper 2 cell.