The Emerging Role of Eosinophils as Multifunctional Leukocytes in Health and Disease

Eosinophils are terminally differentiated cytotoxic effector cells that have a role in parasitic infections and allergy by releasing their granule-derived cytotoxic proteins. However, an increasing number of recent observations indicate that eosinophils are not only associated with the pathogenesis of a wide range of diseases, but also contribute to the maintenance of homeostatic responses in previously underappreciated diverse tissues, such as the gastrointestinal (GI) tract and adipose tissue. In this review, we describe biological characteristics of eosinophils, as their developmental properties, permissive proliferation and survival, degranulation activity, and migration properties enable them to distribute to both homeostatic and inflamed tissues. We describe pathologic aspects of eosinophils with a role in asthma and in various GI diseases, including eosinophilic GI disorders, inflammatory bowel disease, and radiation-induced enteropathy. Finally, we discuss the beneficial role of eosinophils, which contribute to the resolution of pathogenic conditions and to the modulation of homeostatic biologic responses.

Optimization of Cytokine Milieu to Reproduce Atopic Dermatitis-related Gene Expression in HaCaT Keratinocyte Cell Line

Although atopic dermatitis (AD) is characterized by cytokine production predominantly mediated by T helper (Th) 2 cells, AD pathogenesis also involves innate immune and Th1 cells. To optimize the cytokine milieu required for accurate reproduction of AD-related gene expression profile in vitro, we evaluated the expression pattern of CCL22, CCL17, IL5, IL13, IL33, IL25, TSLP, FLG, and LOR in human lesional AD skin and cytokine-stimulated HaCaT cells. An increase in Th2 mediators (IL5, IL13, CCL22, CCL17, IL25, IL33, and TSLP) and a decrease in genes related to cornified cell envelope (filaggrin and loricrin) were observed in human AD lesions. Innate (tumor necrosis factor-α) and/or Th1/Th2 adaptive cytokines (interferon-γ/IL-4) were required for inducing these inflammatory changes in HaCaT cells, implying that a complex network of innate, Th1, and Th2 cytokines drives AD-like changes. Therefore, stimulation with various combinations of cytokines, beyond Th2 polarization, is necessary when HaCaT cell line is used to study genetic changes implicated in AD pathogenesis.

Terminally Differentiating Eosinophils Express Neutrophil Primary Granule Proteins as well as Eosinophil-specific Granule Proteins in a Temporal Manner

Neutrophils and eosinophils, 2 prominent granulocytes, are commonly derived from myelocytic progenitors through successive stages in the bone marrow. Our previous genome-wide transcriptomic data unexpectedly showed that genes encoding a multitude of neutrophil primary granule proteins (NPGPs) were markedly downregulated during the end period of eosinophilic terminal differentiation when cord blood (CB) cluster of differentiation (CD) 34+ cells were induced to differentiate toward the eosinophil lineage during a 24-day culture period. Accordingly, this study aimed to examine whether NPGP genes were expressed on the way to eosinophil terminal differentiation stage and to compare their expression kinetics with that of genes encoding eosinophil-specific granule proteins (ESGPs). Transcripts of all NPGP genes examined, including proteinase 3, myeloperoxidase, cathepsin G (CTSG), and neutrophil elastase, reached a peak at day 12 and sharply declined thereafter, while transcript of ESGP genes including major basic protein 1 (MBP1) attained maximum expression at days 18 or 24. Growth factor independent 1 (GFI1) and CCAAT/enhancer-binding protein α (C/EBPA), transactivators for the NPGP genes, were expressed immediately before the NPGP genes, whereas expression of C/EBPA, GATA1, and GATA2 kinetically paralleled that of eosinophil granule protein genes. The expression kinetics of NPGPs and ESGPs were duplicated upon differentiation of the eosinophilic leukemia cell line (EoL-1) immature eosinophilic cells. Importantly, confocal image analysis showed that CTSG was strongly coexpressed with MBP1 in differentiating CB eosinophils at days 12 and 18 and became barely detectable at day 24 and beyond. Our results suggest for the first time the presence of an immature stage where eosinophils coexpress NPGPs and ESGPs before final maturation.

Eosinophils Regulate Type 2 Immune Responses Following Infection with the Nematode Trichinella spiralis

Eosinophils are multifunctional leukocytes implicated in protection against helminth infections. Although eosinophils comprise between 1~5% of peripheral blood leukocytes, they primarily reside in the gastrointestinal tract under homeostatic conditions, and rapidly proliferate upon parasitic infection. Intestinal infection with Trichinella spiralis (T. spiralis) induces eosinophilia when the parasite enters the larval stages and larvae finally migrate to the skeletal muscle. Eosinophils are known to mediate parasite death through antibody-dependent cellular cytotoxicity. In this study, we aimed to address the functional significance of eosinophils in the intestinal phase of T. spiralis infection by analysis of immune responses in the Peyer's patch (PP) of infected BALB/c and eosinophil-ablated ΔdblGATA mice. Trafficking of eosinophils to the PP was significantly increased, with upregulation of interleukin-5 at 14 days post infection. Eosinophil deficiency led to a significant augmentation of serum immunoglobulin (Ig) M and IgG1 antibody levels. In accordance with this, IgG1+ B cells in the PP were substantially increased in ΔdblGATA mice compared to that in BALB/c mice. Transforming growth factor-β expression in the PP of infected ΔdblGATA mice was significantly decreased compared to that in BALB/c mice, whereas the number of T. spiralis larvae in the diaphragm was increased. Taken together, these findings indicate that eosinophils contribute to the regulation of Th2 immune responses, and protect the host from T. spiralis attempting to establish larvae in the skeletal muscle.

Eosinophils and Type 2 Cytokine Signaling in Macrophages Support the Biogenesis of Cold-induced Beige Fat

Brown adipose generates heat via oxidation of fatty acids by a mitochondrial uncoupling protein 1 (UCP1)-dependent process. In addition, a subpopulation of cells within subcutaneous white adipose tissue, known as beige adipocytes, also plays a role in thermogenesis. The biogenesis of beige adipocytes is induced by thermogenic signals, such as chronic cold exposure. Recently, it has been reported that eosinophils, type 2 cytokines of IL-4/13, and alternatively activated macrophages control the thermogenic cycle of beige adipocytes. Alternatively, activated macrophages induce UCP1+ beige adipocytes through secretion of catecholamines. These results define the role of type 2 immune responses in the regulation of energy homeostasis.

Eosinophils are Required for Immune Responses Induced by Oral Immunization

Eosinophils are multifunctional leukocytes that reside in several tissues, most abundantly in the small intestinal lamina propria under the steady state. To date, the phenotypic and functional characteristics of small intestinal eosinophils have remained poorly understood. In this study, we found that proliferation of ovalbumin (OVA)-specific CD4+ T cells isolated from the mesenteric lymph nodes of eosinophil-deficient DeltadblGATA mice were decreased relative to wild-type mice after oral immunization with OVA and cholera toxin (CT), the typical mucosal adjuvant that induces CD4+ T cell-dependent responses. DeltadblGATA mice showed reduced mucosal secretion of OVA-specific IgA and IgG1 while maintaining a systemic level of anti-OVA IgG1 upon oral immunization with OVA and CT. These findings suggest that eosinophils might have a role in the modulation of T cell-mediated immune responses including mucosal antibody responses in the gastrointestinal tract following oral immunization.

Comparative Analysis of Dibutyric cAMP and Butyric Acid on the Differentiation of Human Eosinophilic Leukemia EoL-1 Cells

Purification of enough numbers of circulating eosinophils is difficult because eosinophils account for less than 5% peripheral blood leukocytes. Human eosinophilic leukemia EoL-1 cells have been considered an in vitro source of eosinophils as they can differentiate into mature eosinophil-like cells when incubated with dibutyryl cAMP (dbcAMP) or butyric acid. In this study, the viability and phenotypic maturation of EoL-1 cells stimulated by either dbcAMP or butyric acid were comparatively analyzed. After treatment with 100 microM dbcAMP or 0.5 microM butyric acid, EoL-1 cells showed morphological signs of differentiation, although the number of nonviable EoL-1 cells was significantly increased following butyric acid treatment. Stimulation of EoL-1 cells with 0.5 microM butyric acid more effectively induced the expression of mature eosinophil markers than stimulation with dbcAMP. These results suggest that treatment of EoL-1 cells with 0.5 microM butyric acid for limited duration could be an effective strategy for inducing their differentiation. Considering that expression of CCR3 was not sufficient in EoL-1 cells stimulated with 0.5 microM butyric acid, treatment of the chemically stimulated EoL-1 cells with cytokines, which primarily support eosinophil maturation, would help to obtain differentiated EoL-1 cells with greater functional maturity.