Diversity of Adipose Tissue Immune Cells: Are All Eosinophils Created Equal?

In this issue of BioEssays, the article by Knights et al. provides a well-written dissection of the current knowledge gaps in the Immunometabolism field concerning the role of eosinophils in adipose tissue (AT) homeostasis and whole-body metabolism. Though it does not exhaustively describe all published studies on eosinophils in AT, it highlights several key studies to illustrate the field’s progression. The authors acknowledge that AT eosinophil function, not just number, needs to be more thoroughly investigated. With obesity and its comorbidities affecting a third of the US population and quickly rising world-wide, the study of AT has become more pertinent than ever. AT is now recognized as an important endocrine organ that stores and supplies the body with adequate energy, secretes hormones to influence vital processes such as hunger and energy expenditure, and stabilizes body temperature through insulation and thermogenesis. Obesity impairs AT function, due in part to a massive inflammatory overhaul. While the most prominent immunemodulation in AT during obesity is macrophage number and inflammatory state, many other immune cells are known to be altered, such as eosinophils. The study of AT eosinophils is less than a decade old with the first publication noting the presence of eosinophils in AT as recent as 2011. Since then a series of publications have shown positive effects of eosinophils on AT function, including weight loss, reduced AT inflammation, improved AT insulin sensitivity, and increased whole body glucose tolerance. However, not all publications have shown a positive role for eosinophils in AT health, thoroughly discussed in. For example, in the piece that this brief commentary accompanies, Knights et al. contemplate the implications of a recent paper from our lab in which elevating AT eosinophils did not rescue metabolic impairments during obesity. Knights and colleagues discuss the important new concept that eosinophil expression profiles should be examined since increasing eosinophils alone does not always improve AT health. In line with this thinking, our lab has been curious to determine if AT eosinophils exist on an E1/E2 spectrum, akin to M1/M2macrophage polarization and Th1 and Th2 T cells. Some evidence supports our idea of an E1/E2 spectrum: while eosinophils classically polarize M2-like