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Keywords

• Obesity

• Sex-differences

• RELMα

• Macrophages

• Eosinophils

Main Findings

The authors demonstrated that RELMα protects female mice from developing obesity from high-fat diet (HFD) feeding through driving an eosinophil-macrophage axis in adipose tissue. The data shown demonstrates that RELMα deficiency promotes weight gain in HFD-fed female mice, whereas WT female mice show no weight gain after 6 weeks of HFD-feeding. RELMα-knockout showed increased accumulation of pro-inflammatory CD11cHimacrophages and loss of eosinophils in visceral adipose depots. RELMα treatment or adoptive transfer of eosinophils restored protection from weight gain in HFD-fed female mice and decreased pro-inflammatory M1 macrophage accumulation. Single-cell RNA sequencing performed on adipose stromal cell fractions uncovered that macrophages from WT female mice were enriched for eosinophil chemotactic factors, including CCL24. Trajectory analysis revealed that macrophage differentiation was found to be dysregulated in RELMα KO females, promoting differentiation of pro-inflammatory macrophage populations. Collectively, these results highlight a sex-specific difference in RELMα function and demonstrates that a RELMα-eosinophils-macrophage axis in adipose tissue provides protection from weight gain from HFD-feeding in female mice.

Limitations

● This study utilizes the Diet-induced obesity (DIO) model where mice are fed a diet high in saturated fats to induce obesity. A caveat of this model is that it induces metabolic syndrome in male mice, including hyperglycaemia and insulin resistance, but not in female mice (PMID: 23049932). While this paper focuses on the role of RELMα in protecting female mice from weight gain due to HFD-feeding, no experiments are done to examine whether loss of RELMα promotes metabolic syndrome in female mice. The addition of glucose and insulin tolerance tests would benefit the results of this paper.

● Various studies have shown that 6 weeks of HFD-feeding is sufficient to induce metabolic syndrome in male mice, the timepoint utilized in this study for the single cell sequencing of SVF cell (PMID: 35747264s). Metabolic syndrome has been shown to have profound effects on immune cell activation/function, and thus may be responsible for some of the observed phenotypes. Earlier time points of HFD-feeding coupled with metabolic assessment assays could uncover a timepoint where female mice do not gain weight and also where male mice do not yet acquire metabolic syndrome.

● The authors analysed the SVF for expression levels of various cytokines, including GM-CSF and IL-5. Further analysis of classical Th2-skewing cytokines such as IL-4 and IL-13 would strengthen the overall conclusions of this paper, that a Th2 environment is protecting female mice from DIO. Furthermore, the role of ILC2s in the sex-dependant effects observed should be further interrogated as ILCs are a potent source of Th2-skewing cytokines and are regulated by lipid metabolism and sex-hormones (PMID: 28484078 and PMID: 32268121).

● The authors found that single cell sequencing was unable to identify eosinophil populations in the SVF. Since protection from DIO was restored through adoptively transferring sorted eosinophils into RELMα KO mice, further analysis of the effects of RELMα loss on eosinophils would be a plus. Eosinophils could be sorted from the SVF and sequenced alone or used for functional assays.

● Similarly, Eosinophil-depletion would provide further evidence on the role of eosinophils in protection from DIO in female mice.

● The authors report frequencies for flow cytometric analysis of various immune cell populations in the SVF, but no absolute numbers are reported.

● The authors analysed RELMα expression in SVF cells through flow cytometry but the signal appears very minimal, unlike what is observed in the IHC staining later on in their results. The methods section is missing the details on how the intracellular flow cytometry was performed. Incubation of the cells with golgi plug would likely increase the signal but also an FMO stain should be added and compared to stains in RELMa-KO cells.

Significance/Novelty

This article adds to the growing body of evidence on the role of Th2 immune responses in protecting female mice from diet-induced obesity. The proposed role of RELMα on mediating the Th2 immune response in adipose tissue, through promoting eosinophil infiltration and downregulating pro-inflammatory macrophage accumulation, provides new insights on RELMa function and also further reinforces the importance of eosinophils in protection from obesity.

Credit

Reviewed by Brendan Cordeiro (University of Toronto) as part of the cross-institutional journal club of the Immunology Institute of the Icahn School of Medicine, Mount Sinai and the Kennedy Institute of Rheumatology, University of Oxford.