A Novel Mouse Model of iNKT Cell-deficiency Generated by CRISPR/Cas9 Reveals a Pathogenic Role of iNKT Cells in Metabolic Disease

Contribution of NKT cells and CD1d-expressing cells in obesity-associated adipose tissue inflammation

Natural killer T (NKT) cell are members of the innate-like T lymphocytes and recognizes lipid antigens presented by CD1d-expressing cells. Obesity-associated inflammation in adipose tissue (AT) leads to metabolic dysfunction, including insulin resistance. When cellular communication is properly regulated among AT-residing immune cells and adipocytes during inflammation, a favorable balance of Th1 and Th2 immune responses is achieved. NKT cells play crucial roles in AT inflammation, influencing the development of diet-induced obesity and insulin resistance. NKT cells interact with CD1d-expressing cells in AT, such as adipocytes, macrophages, and dendritic cells, shaping pro-inflammatory or anti-inflammatory microenvironments with distinct characteristics depending on the antigen-presenting cells. Additionally, CD1d may be involved in the inflammatory process independently of NKT cells. In this mini-review, we provide a brief overview of the current understanding of the interaction between immune cells, focusing on NKT cells and CD1d signaling, which control AT inflammation both in the presence and absence of NKT cells. We aim to enhance our understanding of the mechanisms of obesity-associated diseases.

Early Activation of iNKT Cells Increased Survival Time of BALB/c Mice in a Murine Model of Melioidosis

Melioidosis is an infectious disease caused by Burkholderia pseudomallei. High interferon gamma (IFN-γ) levels in naive mice were reported to mediate protection against B. pseudomallei infection. Invariant natural killer T (iNKT) cells can produce and secrete several cytokines, including IFN-γ. When iNKT cell-knockout (KO) BALB/c mice were infected with B. pseudomallei, their survival time was significantly shorter than wild-type mice. Naive BALB/c mice pretreated intraperitoneally with α-galactosylceramide (α-GalCer), an iNKT cell activator, 24 h before infection demonstrated 62.5% survival at the early stage, with prolonged survival time compared to nonpretreated infected control mice (14 ± 1 days versus 6 ± 1 days, respectively). At 4 h after injection with α-GalCer, treated mice showed significantly higher levels of serum IFN-γ, interleukin-4 (IL-4), IL-10, and IL-12 than control mice. Interestingly, the IFN-γ levels in the α-GalCer-pretreated group were decreased at 4, 24, and 48 h after infection, while they were highly increased in the control group. At 24 h postinfection in the α-GalCer group, bacterial loads were significantly lower in blood (no growth and 1,780.00 ± 51.21, P < 0.0001), spleens (no growth and 34,300 ± 1,106.04, P < 0.0001), and livers (1,550 ± 68.72 and 13,400 ± 1,066.67, P < 0.0001) than in the control group, but not in the lungs (15,300 ± 761.10 and 1,320 ± 41.63, P < 0.0001), and almost all were negative at 48 h postinfection. This study for the first time shows that early activation of iNKT cells by α-GalCer helps clearance of B. pseudomallei and prolongs mouse survival.

Adipose invariant NKT cells interact with CD1d-expressing macrophages to regulate obesity-related inflammation

Obesity is accompanied by and accelerated with chronic inflammation in adipose tissue, especially visceral adipose tissue (VAT). This low-level inflammation predisposes the host to the development of metabolic disease, most notably type 2 diabetes. We have focused on the capacity of glycolipid-reactive, CD1d-restricted natural killer T (NKT) cells to modulate obesity and its associated metabolic sequelae. We previously reported that CD1d knockout (KO) mice are partially protected against the development of obesity-associated insulin-resistance, and these findings were recapitulated in mice with an adipocyte-specific CD1d deficiency, suggesting that NKT cell-adipocyte interactions play a critical role in exacerbating disease. However, many other CD1d-expressing cells contribute to the in vivo responses of NKT cells to lipid antigens. In the present study, we examined the role of CD1d expression by macrophages (Mϕ) to the development of obesity-associated metabolic inflammation using LysMcre-cd1d1^f/f mice where the CD1d1 gene is disrupted in a Mϕ-specific manner. Unexpectedly, these animals contained a higher frequency of T-bet⁺ CD4⁺ T cells in VAT with increased production of Th1-cytokines that aggravated VAT inflammation. Mϕ from mutant mice displayed increased production of IL-12p40, suggesting M1 polarization. These findings indicate that interactions of CD1d on Mϕ with NKT cells play a beneficial role in obesity-associated VAT inflammation and insulin resistance with a sharp contrast to an aggravating role of CD1d on another type of antigen presenting cell, dendritic cells.

Complementary approaches to study NKT cells in cancer

NKT cells are a small but influential member of the T cell family, recognizing lipids presented by the non-classical MHC-like molecule CD1d rather than peptides presented by classical MHC molecules. They bridge between the innate and adaptive immune systems, serving as rapid responders but also allowing the T cell immune system to recognize lipid antigens, for example derived from tumors or bacteria. They also serve as potent regulatory cells, controlling other immune responses. Type I NKT cells use a semi-invariant T cell receptor (TCR) whereas type II use diverse TCRs. Most often, type I NKT cells promote tumor immunity whereas type II tend to suppress it, and the two subtypes crossregulate each other, forming an immunoregulatory axis. Lack of tools to study these important cells has limited the understanding of these, but newer tools have allowed great advances, especially in mouse models. These range from transgenic and knock-out mice to CD1d tetramers carrying ligands for type I or II NKT cells, to antibodies and NKT cell hybridomas. Here we describe these complementary tools and approaches and their use to study NKT cells and their role in the immunology and immunotherapy of cancer.

Adipose Tissue-Resident Immune Cells in Obesity and Type 2 Diabetes

Inflammation is an important contributor to the pathogenesis of obesity-related type 2 diabetes (T2D). Adipose tissue-resident immune cells have been observed, and the potential contribution of these cells to metabolic dysfunction has been appreciated in recent years. This review focused on adipose tissue-resident immune cells that are dysregulated in the context of obesity and T2D. We comprehensively overviewed emerging knowledge regarding the phenotypic and functional properties of these cells and local factors that control their development. We discussed their function in controlling the immune response cascade and disease progression. We also characterized the metabolic profiles of these cells to explain the functional consequences in obese adipose tissues. Finally, we discussed the potential therapeutic targeting of adipose tissue-resident immune cells with the aim of addressing novel therapeutic approaches for the treatment of this disease.

T Cell Receptor Expression Timing and Signal Strength in the Functional Differentiation of Invariant Natural Killer T Cells

The CD1d-restricted Vα14 invariant NKT (iNKT) cell lineage in mice (Vα24 in humans) represents an evolutionary conserved innate-like immune cell type that recognizes glycolipid antigens. Because of their unique ability to promptly secrete copious amounts of both pro-inflammatory and anti-inflammatory cytokines, typically produced by different T helper cell types, iNKT cells are implicated in the regulation of various pathologic conditions such as infection, allergy, autoimmune disease, maintenance of transplantation tolerance, and cancer. This striking multifaceted role in immune regulation is correlated with the presence of multiple functionally distinct iNKT cell subsets that can be distinguished based on the expression of characteristic surface markers and transcription factors. However, to date it, remains largely unresolved how this puzzling diversity of iNKT cell functional subsets emerges and what factors dictate the type of effector cell differentiation during the thymic differentiation considering the mono-specific nature of their T cell receptor (TCR) and their selecting molecule CD1d. Here, we summarize recent findings focusing on the role of TCR-mediated signaling and discuss possible mechanisms that may influence the sub-lineage choice of iNKT cells.

iNKT Cells Suppress Pathogenic NK1.1+CD8+ T Cells in DSS-Induced Colitis

T cells producing IFNγ play a pathogenic role in the development of inflammatory bowel disease (IBD). To investigate the functions of CD1d-dependent invariant natural killer T (iNKT) cells in experimental colitis induced in Yeti mice with dysregulated expression of IFNγ, we generated iNKT cell-deficient Yeti/CD1d KO mice and compared colitis among WT, CD1d KO, Yeti, and Yeti/CD1d KO mice following DSS treatment. We found that deficiency of iNKT cells exacerbated colitis and disease pathogenesis was mainly mediated by NK1.1⁺CD8⁺ T cells. Furthermore, the protective effects of iNKT cells correlated with up-regulation of regulatory T cells. Taken together, our results have demonstrated that CD1d-dependent iNKT cells and CD1d-independent NK1.1⁺CD8⁺ T cells reciprocally regulate the development of intestinal inflammatory responses mediated by IFNγ-dysregulation. These findings also identify NK1.1⁺CD8⁺ T cells as novel target cells for the development of therapeutics for human IBD.

Insights Into Mucosal-Associated Invariant T Cell Biology From Studies of Invariant Natural Killer T Cells

Mucosal-associated invariant T (MAIT) cells and invariant natural killer T (iNKT) cells are innate-like T cells that function at the interface between innate and adaptive immunity. They express semi-invariant T cell receptors (TCRs) and recognize unconventional non-peptide ligands bound to the MHC Class I-like molecules MR1 and CD1d, respectively. MAIT cells and iNKT cells exhibit an effector-memory phenotype and are enriched within the liver and at mucosal sites. In humans, MAIT cell frequencies dwarf those of iNKT cells, while in laboratory mouse strains the opposite is true. Upon activation via TCR- or cytokine-dependent pathways, MAIT cells and iNKT cells rapidly produce cytokines and show direct cytotoxic activity. Consequently, they are essential for effective immunity, and alterations in their frequency and function are associated with numerous infectious, inflammatory, and malignant diseases. Due to their abundance in mice and the earlier development of reagents, iNKT cells have been more extensively studied than MAIT cells. This has led to the routine use of iNKT cells as a reference population for the study of MAIT cells, and such an approach has proven very fruitful. However, MAIT cells and iNKT cells show important phenotypic, functional, and developmental differences that are often overlooked. With the recent availability of new tools, most importantly MR1 tetramers, it is now possible to directly study MAIT cells to understand their biology. Therefore, it is timely to compare the phenotype, development, and function of MAIT cells and iNKT cells. In this review, we highlight key areas where MAIT cells show similarity or difference to iNKT cells. In addition, we discuss important avenues for future research within the MAIT cell field, especially where comparison to iNKT cells has proven less informative.

New Genetically Manipulated Mice Provide Insights Into the Development and Physiological Functions of Invariant Natural Killer T Cells

Invariant natural killer T (iNKT) cells are a unique T cell subset that exhibits characteristics of both innate immune cells and T cells. They express Vα14-Jα18 (Trav11-Traj18) as an invariant chain of the T cell receptor (TCR) and are restricted to the MHC class I-like monomorphic antigen presenting molecule CD1d. iNKT cells are known as immune regulators that bridge the innate and acquired immune systems by rapid and massive production of a wide range of cytokines, which could enable them to participate in immune responses during various disease states. Thus, Traj18-deficient mice, Cd1d-deficient mice, or iNKT cell-overexpressing mice such as iNKT TCRα transgenic mice and iNKT cell cloned mice which contain a Vα14-Jα18 rearrangement in the TCRα locus are useful experimental models for the analysis of iNKT cells in vivo and in vitro. In this review, we describe the pros and cons of the various available genetically manipulated mice and summarize the insights gained from their study, including the possible roles of iNKT cells in obesity and diabetes.

Role of Natural Killer T Cells in the Development of Obesity and Insulin Resistance: Insights From Recent Progress

Natural killer T (NKT) cells play important roles in adipose tissue inflammation, and thus influence the development of diet-induced obesity and insulin resistance. The interactions between cluster of differentiation (CD)1d and NKT T cell receptor are thought to be critical in this process, as demonstrated in two NKT cell-deficient mouse models-systemic CD1d gene knockout (KO) and prototypic Jα18 KO mice. The latter lacks some repertoires besides invariant (i)NKT cells due to manipulation of the Jα18 gene segment; therefore, the role of iNKT vs. variant NKT cells must be reinterpreted considering the availability of new Jα18 KO mice. NKT cells have varied roles in the development of obesity; indeed, studies have reported contradictory results depending on the mouse model, diet, and rearing conditions, all of which could affect the microbiome. In this mini-review, we discuss these points considering recent findings from our laboratory and others as well as the role of NKT cells in the development of obesity and insulin resistance based on data obtained from studies on conditional CD1d1 KO and new Jα18 KO mice generated through gene editing.