Can eosinophils in adipose tissue add fuel to the fire?

Over the last four decades the prevalence of obesity has risen dramatically worldwide. Obesity is a multifactorial disease often associated with development of insulin resistance, type 2 diabetes and cardiovascular disease, all maladies negatively affecting quality of life and expectancy.¹ Despite significant improvement in the understanding of the disease, we still lack successful treatment strategies. In a recent Nature Communications article, Knights et al.² explore the role of the transcription factor Krüppel-like factor 3 (KLF3) in thermogenesis and its effect on limiting weight gain.

Within the last two decades, adipose tissue (AT) has been identified as not only a storage for excess of nutrients (e.g. lipids), but also a true endocrine organ with a wide array of secreted bioactive molecules called adipokines with many functions including appetite regulation (e.g. leptin) and whole-body homeostasis (e.g. adiponectin).³ As such, AT is a key player in the pathogenesis of obesity, insulin resistance and its complications, and viewed as a potential target for future drug therapy development. Today, adipocytes are classified as white (WAT), brown and beige. While WAT stocks triglycerides as a future source of metabolic energy,³ brown AT has a high number of iron-rich mitochondria able to generate heat from fatty acid oxidation via activation of uncoupling protein 1 (UCP1),⁴ in a process called nonshivering thermogenesis. Brown AT is anatomically dispersed over several areas in rodents⁵ and humans.⁶ Recently, gene profiling of subcutaneous WAT depot in rodents identified a subset of cells similar to brown adipocytes, termed beige fat cells, with a very low basal level of UCP1.⁷ In beige adipocytes, a number of secreted factors⁸ and several AT-resident immune cell populations⁹ can activate UCP1 (referred as beiging), leading to heat generation via nonshivering thermogenesis. The discovery of nonshivering thermogenesis as a metabolic process to disperse energy, thereby limiting adiposity, has fueled many investigations in both animal models and humans. The role of immune cell populations in the regulation of WAT nonshivering thermogenesis is quickly evolving, creating opportunity for additional advancement of the knowledge in the immunometabolism field.

Recently, a novel function for eosinophils (EOS) in AT metabolism¹⁰ was identified, beyond the traditional knowledge of this white blood cell involved in allergy response and/or parasite infestations.¹¹ These studies suggest that EOS preserve insulin sensitivity in an animal model of diet-induced obesity (DIO). When compared with standard diet, the number of AT-resident EOS was significantly decreased in DIO animals, and inversely related to the animal’s body weight. Following these initial observations, subsequent studies suggested that AT-resident EOS could be important in the modulation of WAT thermogenic activity.¹² The authors proposed that EOS release interleukins 4 and 13, which in turn activate AT-resident M2 macrophages to secrete catecholamines. Catecholamines are a known stimulus to initiate nonshivering thermogenesis by activation of UCP1¹³ signaling in both brown AT and beige adipocytes. Since the publication of this study, the concept of macrophages secreting catecholamine has been widely debated.¹⁴ In addition to EOS and macrophages, group 2 innate lymphoid cells appear to increase the number and activity of beige adipocytes in subcutaneous fat via an interleukin-33 pathway and similar to AT-resident EOS, their number is decreased in DIO animal models.¹⁵

Knights et al.² explore the role of transcription KLF3 in thermogenesis and its effect on limiting weight gain in a DIO animal model. KLF3 is a member of a group of 15 transcription factors known to regulate development, activation, and/or repression of both stromal and hematopoietic cells.¹⁶ Previously, the same group reported for KLF3 knockout mice (KLF3^−/−) to be smaller in size and have a decreased AT content than their wild-type (WT) littermates.¹⁷ In the present study, the authors focused on changes in subcutaneous WAT related to an increase in content and function of beige adipocytes. They hypothesized KLF3 to play a contributory role in WAT beiging process mediated by the hematopoietic system. In support of their hypothesis, they showed that on standard diet total body fat content of KLF3^−/− mice is lower than that of WT animals and morphologic analysis of the subcutaneous AT depot in KLF3^−/− mice presented histological and molecular signatures of beige adipocytes. In addition, exposure to cold (4°C) temperatures resulted in an increased UCP1 and several mitochondrial protein subunits, in addition to increased mRNA expression of UCP1 and Elovl3 in the knockout mice. The authors concluded that KLF3 is a critical regulator of beige adipocyte activation.

As KLF3 affects many cells, including adipocytes¹⁸ themselves, the authors sought to define the isolated contribution of hematopoietic cells, specifically, to induce beige adipocytes and limit adiposity. To this end, they generated chimeras with bone marrow derived from either KLF3^−/− or WT. Both groups of animals were placed on high-fat diet for 11 weeks, resulting in a statistically significant decreased weight gain in the WT^KLF3−/− chimeras compared with the WT^WT chimera group. Under high-fat diet, significant changes in the mRNA level of UCP1 in subcutaneous fat were shown. Flow cytometry analysis of inguinal (i.e. subcutaneous) fat depot of knockout versus WT animals depicted an increased AT-resident EOS number, and a reduction in ST2⁺ group 2 innate lymphoid cells, whereas macrophages and other leukocytes remained unchanged. While a beiging role to AT-EOS had been previously described to be mediated via activation of the M₂ macrophage population, these data provided evidence for a novel and direct role of EOS in the beiging process in WAT. This was elegantly demonstrated, where in vitro exposure of mature adipocytes to KLF3^−/− EOS demonstrated a significant increase in messenger RNA levels of thermogenic genes, whereas exposure to WT EOS or media alone did not alter gene expression. This work represents clearly for the first time how EOS and EOS-derived factors (termed “eosinokines”), under KLF3 directly, and independent of macrophages, regulate adipocyte beiging, which in turn, limited weight gain in DIO animal models.

While the Knights et al.² animal model proves that KLF3 plays a role in hematopoietic cells, it does not rule out the role of KLF3 in other stromal and immune cells. It is possible to speculate that in future studies, specific deletion of KLF3 in cells other than the immune population will provide greater knowledge on their individual contribution to the beiging of subcutaneous AT. For example, the EOS expression of interleukin-33 and the reduction of ST2⁺ group 2 innate lymphoid cells were not anticipated based on the literature,^{14, 19} thus cell-specific deletion of KLF3 in these cells and adipocytes may further clarify the interaction between immune cells and adipocytes in the beiging processes. In addition, the authors showed that under high-fat diet, in the WT^KLF3−/− chimeras, a decrease in adiposity is associated with an increase in beige gene profiles (Ucp1, Cpt1b, Cd137) when compared with the WT^WT chimeras. This hints at a role for nonshivering thermogenesis in potential treatment for obesity and its complications, such as insulin resistance. Future studies assessing functional measures of AT metabolism, adipokines secretion or whole-body glucose homeostasis under high-fat condition could provide more insights into additional benefits from beige adipocytes thermogenesis on metabolism beyond modulation of body weight.

In summary, the observations made by Knights et al.² open the door to explore the potential for interactions between EOS and AT, independent or in parallel to the role assigned to EOS in activating alternatively AT macrophages in health and disease states (Figure 1). The novel concept of eosinokines is quite fascinating and deserves further consideration.

As the immunometabolism field gains more insight into the connection between the immune system and adipocyte biology, it is essential to consider the importance of translating these findings to human patients with obesity. Therapeutic strategies tempering AT inflammation have produced scant results.²⁰ It is time to explore whether the relationship between EOS and adipocytes exists in human health and disease. Such a connection would offer potential novel targets for drug therapy development. Currently, the role of human AT-resident EOS is largely unexplored; one study has described a rather increased number of EOS in human AT in patients with obesity,²¹ whereas another reported a decrease in AT-resident EOS in aging studies of human participants.²² Both reports have clear limitations in the methodology implemented to assess AT EOS number. However, a fluorescence-activated cell sorting-based approach²³ could potentially help to explore the role of AT-resident EOS in tissue metabolism and/or beiging processes in humans. Overall, the novel study by Knight et al. suggests how EOS, under the critical regulation of KLF3, through secreted factors (eosinokines) regulate adipocyte beiging and limited weight gain in DIO animal models, providing a potential future candidate to explore for drug therapy development targeting obesity and its complications.

Conflict of Interest

The authors declare no conflicts of interest.

Author Contribution

Elizabeth A Jacobsen: Conceptualization; Writing-original draft; Writing-review & editing. Eleanna De Filippis: Conceptualization; Writing-original draft; Writing-review & editing.