Title: Untangling iNKT Cell Function in Adipose Tissue Homeostasis
Abstract: iNKT cells are important regulatory cells in metabolic health and disease. In this issue of Cell Metabolism, LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar clarify the origin and heterogeneity of these unconventional T cells and identify the specific signals in adipose tissues that direct their function. iNKT cells are important regulatory cells in metabolic health and disease. In this issue of Cell Metabolism, LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar clarify the origin and heterogeneity of these unconventional T cells and identify the specific signals in adipose tissues that direct their function. The immune system plays a critical role in the maintenance of metabolic homeostasis. This role is increasingly well documented in adipose tissues, where diverse populations of immune cells including macrophages, type 2 innate lymphoid cells (ILC2), eosinophils, T regulatory (Treg) cells, γδ T cells, and invariant natural killer T (iNKT) cells have been shown to control local inflammation and metabolic function (Crosby and Kronenberg, 2018Crosby C.M. Kronenberg M. Tissue-specific functions of invariant natural killer T cells.Nat. Rev. Immunol. 2018; 18: 559-574Crossref PubMed Scopus (92) Google Scholar; LaMarche et al., 2018LaMarche N.M. Kohlgruber A.C. Brenner M.B. Innate T cells govern adipose tissue biology.J. Immunol. 2018; 201: 1827-1834Crossref PubMed Scopus (11) Google Scholar). During obesity-associated inflammation, both the number and function of these cell types skew toward a more inflammatory state, further promoting metabolic disease (Gregor and Hotamisligil, 2011Gregor M.F. Hotamisligil G.S. Inflammatory mechanisms in obesity.Annu. Rev. Immunol. 2011; 29: 415-445Crossref PubMed Scopus (2186) Google Scholar). Conversely, the fasted state is associated with anti-inflammatory functions of immune cells. This paradigm is well accepted when it comes to distinct phenotypes of macrophages. There is less consensus concerning the role of iNKT cells in adipose tissue biology. While iNKT cells are found throughout mouse and human tissues, the subset residing in adipose is unique in that it produces IL-10 (Lynch et al., 2015Lynch L. Michelet X. Zhang S. Brennan P.J. Moseman A. Lester C. Besra G. Vomhof-Dekrey E.E. Tighe M. Koay H.F. et al.Regulatory iNKT cells lack expression of the transcription factor PLZF and control the homeostasis of T(reg) cells and macrophages in adipose tissue.Nat. Immunol. 2015; 16: 85-95Crossref PubMed Scopus (220) Google Scholar; Sag et al., 2014Sag D. Krause P. Hedrick C.C. Kronenberg M. Wingender G. IL-10-producing NKT10 cells are a distinct regulatory invariant NKT cell subset.J. Clin. Invest. 2014; 124: 3725-3740Crossref PubMed Scopus (149) Google Scholar). Previous work has shown that the transcription factor E4BP4 drives this production of IL-10; however, the mechanism is unknown (Lynch et al., 2015Lynch L. Michelet X. Zhang S. Brennan P.J. Moseman A. Lester C. Besra G. Vomhof-Dekrey E.E. Tighe M. Koay H.F. et al.Regulatory iNKT cells lack expression of the transcription factor PLZF and control the homeostasis of T(reg) cells and macrophages in adipose tissue.Nat. Immunol. 2015; 16: 85-95Crossref PubMed Scopus (220) Google Scholar). Furthermore, the function of these cells both during homeostasis and metabolic disease is debated, and conflicting reports describe these cells as both anti- and pro-inflammatory. Several groups have observed that iNKT cells produce regulatory cytokines, supporting adipose Treg and macrophage populations and protecting against metabolic disease (Ji et al., 2012Ji Y. Sun S. Xu A. Bhargava P. Yang L. Lam K.S.L. Gao B. Lee C.H. Kersten S. Qi L. Activation of natural killer T cells promotes M2 macrophage polarization in adipose tissue and improves systemic glucose tolerance via interleukin-4 (IL-4)/STAT6 protein signaling axis in obesity.J. Biol. Chem. 2012; 287: 13561-13571Crossref PubMed Scopus (147) Google Scholar; Lynch et al., 2015Lynch L. Michelet X. Zhang S. Brennan P.J. Moseman A. Lester C. Besra G. Vomhof-Dekrey E.E. Tighe M. Koay H.F. et al.Regulatory iNKT cells lack expression of the transcription factor PLZF and control the homeostasis of T(reg) cells and macrophages in adipose tissue.Nat. Immunol. 2015; 16: 85-95Crossref PubMed Scopus (220) Google Scholar; Schipper et al., 2012Schipper H.S. Rakhshandehroo M. van de Graaf S.F.J. Venken K. Koppen A. Stienstra R. Prop S. Meerding J. Hamers N. Besra G. et al.Natural killer T cells in adipose tissue prevent insulin resistance.J. Clin. Invest. 2012; 122: 3343-3354Crossref PubMed Scopus (148) Google Scholar). However, other reports show that during obesity, iNKT cells promote proinflammatory and pathogenic immune responses (Satoh et al., 2016Satoh M. Hoshino M. Fujita K. Iizuka M. Fujii S. Clingan C.S. Van Kaer L. Iwabuchi K. Adipocyte-specific CD1d-deficiency mitigates diet-induced obesity and insulin resistance in mice.Sci. Rep. 2016; 6: 28473Crossref PubMed Scopus (29) Google Scholar; Wu et al., 2012Wu L. Parekh V.V. Gabriel C.L. Bracy D.P. Marks-Shulman P.A. Tamboli R.A. Kim S. Mendez-Fernandez Y.V. Besra G.S. Lomenick J.P. et al.Activation of invariant natural killer T cells by lipid excess promotes tissue inflammation, insulin resistance, and hepatic steatosis in obese mice.Proc. Natl. Acad. Sci. USA. 2012; 109: E1143-E1152Crossref PubMed Scopus (123) Google Scholar). Importantly, the study by LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar presents an explanation for these seemingly opposing functions of iNKT cells and proposes a mechanism of iNKT cell education within adipose tissues. LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar identify transcriptionally distinct populations of adipose tissue iNKT cells, the two largest clusters distinguished by NK1.1 expression. These populations, NK1.1NEG and NK1.1POS, appear to be functionally distinct; NK1.1POS iNKT cells produce IFNγ and TNF, and NK1.1NEG iNKT cells produce IL-10, IL-4, IL-13, IL-2, and IL-17. While these two populations have been described previously, their distinct functions have yet to be clarified. Additionally, the signals that enable these functions are not known. In addressing these unknown mechanisms, LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar find that the adipose environment drives PLZF downregulation, E4BP4 expression, and IL-10 production, independent of TCR stimulation via CD1d in NK1.1NEG cells. Furthermore, they identify free fatty acids (FFAs) as the signal that drives E4BP4 expression. They determine that intracellular lipids accumulate in NK1.1NEG cells, resulting in endoplasmic reticulum (ER) stress. Specifically, they show that lipid accumulation results in activation of the IRE1α:XBP1s axis of the unfolded protein response, one of three pathways engaged by cells undergoing ER stress. In NK1.1NEG iNKT cells, the authors find E4BP4 to be regulated by IRE1α:XBP1s, thus linking FFAs, ER stress, E4BP4, and IL-10 production (Figure 1). Next, this study shows the physiologic relevance of NK1.1NEG cells in obesity. The transfer of NK1.1NEG cells to mice on a high-fat diet results in the restoration of glycemic control, while transfer of NK1.1POS cells has no effect. The distinct functions of these two populations may explain past conflicting results finding both pro- and anti-inflammatory potential of iNKT cells. In addition, LaMarche et al., 2020LaMarche N.M. Kane H. Kohlgruber A.C. Dong H. Lynch L. Brenner M.B. Distinct iNKT cell populations use IFNγ or ER stress-induced IL-10 to control adipose tissue homeostasis.Cell Metab. 2020; 32 (this issue): 243-258Google Scholar demonstrate that NK1.1POS cells play a key role in promoting metabolic health in the non-obese setting. They show that iNKT cells are a significant source of IFNγ, and in the absence of IFNγ signaling, macrophage numbers in adipose of lean mice are increased and mice display systemic metabolic dysfunction. While IFNγ tends to be pro-inflammatory and can exacerbate insulin resistance and metabolic disease during obesity, in lean mice, this cytokine is protective. Lastly, they make the provocative discovery that this NK1.1POS population directs NK cell-mediated killing of adipose tissue macrophages in an IFNγ-dependent manner (Figure 1). This control of macrophage numbers may help to promote metabolic health in the non-obese state. One of the most intriguing findings from this study is that iNKT cells join several other immune cell populations, including T cells and macrophages, as those whose functions and phenotypes are largely influenced by their local environment. In the case of iNKT cells specifically, it is known that depending on the tissue, these cells respond differentially to activating signals by producing distinct cytokines. Surprisingly, NK1.1NEG and NK1.1POS cells have non-redundant functions despite residing in the same environment. The authors propose there are two factors involved (higher levels of ER stress inducers [intracellular lipids] and increased sensitivity to these stressors) that together result in heightened engagement of IRE1α:XBP1s signaling. The authors suggest that this is due to a stronger influence of developmental origin in the thymus on NK1.1POS cells and a larger impact of tissue environment on NK1.1NEG cells. However, it is also possible that NK1.1POS cells respond to a different environmental cue (rather than FFAs) and that this further drives the opposing pro-inflammatory function of these cells (Figure 1). Adipose tissue is unique in that it undergoes continual changes in response to the fed and fasted state. A possible interpretation of the data presented in this manuscript is that the two phenotypically distinct populations of iNKT cells may be relevant in these two opposing physiologic states of adipose tissue. Since these changes occur continuously and periodically, it makes sense that homeostatic functions would be controlled by a tissue-resident immune cell population such as iNKT cells. It would also be consistent with the role of E4BP4 described in this study, since E4BP4 plays a role in circadian control and may adjust cytokine production according to fasting/feeding phase. Indeed, the authors show an increase in the NK1.1NEG population during fasting, suggesting that the functional dichotomy observed in adipose tissue could be due to the changing metabolic needs of this particular tissue. It will be fascinating to see the full spectrum of functions imparted on tissue-resident immune cells by their microenvironment. It now seems the rule, rather than the exception. As additional environmental cues are identified, novel therapeutic strategies to adjust a given environment and thus activate or suppress particular immune cell subsets can be developed for the treatment of a wide array of inflammatory conditions, including metabolic disease. Distinct iNKT Cell Populations Use IFNγ or ER Stress-Induced IL-10 to Control Adipose Tissue HomeostasisLaMarche et al.Cell MetabolismJune 8, 2020In BriefLaMarche et al. reveal two pathways by which iNKT cell subsets control adipose tissue inflammation. NK1.1NEG iNKT cells dominantly produce anti-inflammatory IL-10, driven by intracellular lipid accumulation and IRE1α-XBP1s signaling. In contrast, NK1.1POS cells produce IFNγ, which, in lean adipose tissue, drives NK cell-mediated macrophage killing to limit pathogenic macrophage expansion. Full-Text PDF Open Archive