Role of invariant natural killer T cell subsets in adipose tissue homeostasis

Abstract

Obesity and its related metabolic disorders have the potential to reduce the human life span by 5-20 years, being key drivers of deadly maladies such as cancer, diabetes, and cardiovascular disease. At the heart of these disorders is the metabolic dysfunction of adipose tissue, driven by overexpansion and inflammation.

Invariant natural killer T (iNKT) cells are innate-like T cells that recognize glycolipid antigens in the context of the major histocompatibility complex 1-like molecule CD1d. While in most cases iNKT cells direct proinflammatory responses against pathogens, iNKT cells in adipose tissue are highly anti-inflammatory and produce several cytokines required to expand homeostatic immune cells such as alternatively-activated macrophages and regulatory T cells. Chief among these cytokines is IL-10, driven by the transcription factor E4BP4 which is not expressed in iNKT cells outside of the adipose tissue. However, how adipose iNKT cells become regulatory and whether there is functional heterogeneity within the adipose iNKT compartment remain open questions.

We performed single-cell RNA sequencing on adipose iNKT cells from lean mice, and surprisingly found significant transcriptional and functional heterogeneity within this compartment. When we segregated these cells into two equal populations based on expression of the natural killer cell receptor NK1.1 we found that IL-10, as well as several other immunoregulatory cytokines were produced mainly by NK1.1NEG iNKT cells, while the NK1.1POS population dominantly produced IFNγ. Mechanistically, we found that exposure to adipose free fatty acids drove E4BP4 expression and IL-10 production downstream of the inositol-requiring enzyme 1α-X box binding protein 1 (IRE1α-XBP1) arm of the unfolded protein response, and that this pathway was chiefly active in the NK1.1NEG population. Correspondingly, only adoptive transfer of NK1.1NEG iNKT cells restored metabolic function and reduced inflammation in obese mice. Interestingly, we also identified a critical role for NK1.1POS iNKT cells and IFNγ in promoting metabolic homeostasis in lean adipose tissue, where they direct natural killer cell cytotoxicity against macrophages. In IFNγ- or iNKT- deficient mice, macrophages accumulate in lean fat and drive inflammation and metabolic dysfunction. Thus, two iNKT subsets utilize non-redundant pathways to preserve metabolic integrity in lean adipose tissue.