Prevention of type 1 diabetes by invariant NKT cells is independent of peripheral CD1d expression

Cluster of differentiation molecules in the metabolic syndrome

Metabolic syndrome (MetS) represents a significant public health concern due to its association with an increased risk of cardiovascular disease, type 2 diabetes, and other serious health conditions. Despite extensive research, the underlying molecular mechanisms contributing to MetS pathogenesis remain elusive. This review aims to provide a comprehensive overview of the molecular mechanisms linking MetS and cluster of differentiation (CD) markers, which play critical roles in immune regulation and cellular signaling. Through an extensive literature review with a systematic approach, we examine the involvement of various CD markers in MetS development and progression, including their roles in adipose tissue inflammation, insulin resistance, dyslipidemia, and hypertension. Additionally, we discuss potential therapeutic strategies targeting CD markers for the management of MetS. By synthesizing current evidence, this review contributes to a deeper understanding of the complex interplay between immune dysregulation and metabolic dysfunction in MetS, paving the way for the development of novel therapeutic interventions.

Transient Depletion of Foxp3+ Regulatory T Cells Selectively Promotes Aggressive β Cell Autoimmunity in Genetically Susceptible DEREG Mice

Type 1 diabetes (T1D) represents a hallmark of the fatal multiorgan autoimmune syndrome affecting humans with abrogated Foxp3⁺ regulatory T (Treg) cell function due to Foxp3 gene mutations, but whether the loss of Foxp3⁺ Treg cell activity is indeed sufficient to promote β cell autoimmunity requires further scrutiny. As opposed to human Treg cell deficiency, β cell autoimmunity has not been observed in non-autoimmune-prone mice with constitutive Foxp3 deficiency or after diphtheria toxin receptor (DTR)-mediated ablation of Foxp3⁺ Treg cells. In the spontaneous nonobese diabetic (NOD) mouse model of T1D, constitutive Foxp3 deficiency did not result in invasive insulitis and hyperglycemia, and previous studies on Foxp3⁺ Treg cell ablation focused on Foxp3^DTR NOD mice, in which expression of a transgenic BDC2.5 T cell receptor (TCR) restricted the CD4⁺ TCR repertoire to a single diabetogenic specificity. Here we revisited the effect of acute Foxp3⁺ Treg cell ablation on β cell autoimmunity in NOD mice in the context of a polyclonal TCR repertoire. For this, we took advantage of the well-established DTR/GFP transgene of DEREG mice, which allows for specific ablation of Foxp3⁺ Treg cells without promoting catastrophic autoimmune diseases. We show that the transient loss of Foxp3⁺ Treg cells in prediabetic NOD.DEREG mice is sufficient to precipitate severe insulitis and persistent hyperglycemia within 5 days after DT administration. Importantly, DT-treated NOD.DEREG mice preserved many clinical features of spontaneous diabetes progression in the NOD model, including a prominent role of diabetogenic CD8⁺ T cells in terminal β cell destruction. Despite the severity of destructive β cell autoimmunity, anti-CD3 mAb therapy of DT-treated mice interfered with the progression to overt diabetes, indicating that the novel NOD.DEREG model can be exploited for preclinical studies on T1D under experimental conditions of synchronized, advanced β cell autoimmunity. Overall, our studies highlight the continuous requirement of Foxp3⁺ Treg cell activity for the control of genetically pre-installed autoimmune diabetes.

Helminth protection against type-1 diabetes: an insight into immunomodulatory effect of helminth-induced infection

Helminths are the old dirty friends of humans from decades and may live undetected by the immune system for years in the tissues. They have evolved as good experts at subverting the immune system. Despite of their pathogenicity, they provide protection to their host against certain inflammatory diseases such as diabetes by modulating the immune mechanisms. These parasites are extra-cellular and induce Th2 response which triggers the adaptive immune cells as well as innate immune cells to work synergistically allowing Tregs to work in a toll-like receptor-dependent manure. T-helper cells type-2 also secrete certain anti-inflammatory cytokines including IL-4, IL-10, IL-13 and TGF-β which also provide protection against type-1 diabetes. Several helminths such as T. crassiceps, S. venezuelensis, filarial worms, Schistosoma spp. and T. spiralis have been reported to prevent diabetes in mouse models as well as in some clinical trials. Immunomodulatory talent of helminths is receiving greater attention to prevent diabetes. Herein, an attempt has been made to review and highlight the possible immuno-modulatory mechanisms by which helminths provide protection against diabetes. Moreover, this review also emphasizes on the use of helminth-derived molecules or synthetic derivatives of helminth-antigens in clinical trials to overcome rapidly growing autoimmune disorders including diabetes.

Role of regulatory T cells in Schistosoma-mediated protection against type 1 diabetes

The prevalence of T1D in developed societies is partly based on the hygiene hypothesis, that is, the loss of exposure to infectious agents accompanies the loss of immune stimuli shaping the immune system during development. Indeed, the components of parasites, such as Schistosoma, have been reported to ameliorate or prevent the development of T1D, which might be associated with immune cell activity especially that of regulatory T cells (Tregs). Schistosoma infection can lead to the expansion of Treg. Herein, we provide a comprehensive overview of the involvement of Tregs in the response against Schistosoma infection and the mechanism of Schistosoma-associated host protection against T1D.

Immunomodulation by helminths: Similar impact on type 1 and type 2 diabetes?

The incidence of both type 1 (T1D) and type 2 diabetes (T2D) is drastically increasing, and it is predicted that the global prevalence of diabetes will reach almost 600 million cases by 2035. Even though the pathogenesis of both types of diabetes is distinct, the immune system is actively involved in both forms of the disease. Genetic and environmental factors determine the risk to develop T1D. On the other hand, sedentary life style, surplus of food intake and other lifestyle changes contribute to the increase of T2D incidence. Improved sanitation with high-quality medical treatment is such an environmental factor that has led to a continuous reduction of infectious diseases including helminth infections over the past decades. Recently, a growing body of evidence has implicated a negative association between helminth infections and diabetes in humans as well as animal models. In this review, we discuss studies that have provided evidence for the beneficial impact of helminth infections on T1D and T2D. Possible mechanisms are presented by which helminths prevent T1D onset by mitigating pancreatic inflammation and confer protection against T2D by improving insulin sensitivity, alleviating inflammation, augmenting browning of adipose tissue and improving lipid metabolism and insulin signalling.

NKT cell subsets as key participants in liver physiology and pathology

Natural killer T (NKT) cells are innate-like lymphocytes that generally recognize lipid antigens and are enriched in microvascular compartments of the liver. NKT cells can be activated by self- or microbial-lipid antigens and by signaling through toll-like receptors. Following activation, NKT cells rapidly secrete pro-inflammatory or anti-inflammatory cytokines and chemokines, and thereby determine the milieu for subsequent immunity or tolerance. It is becoming clear that two different subsets of NKT cells-type I and type II-have different modes of antigen recognition and have opposing roles in inflammatory liver diseases. Here we focus mainly on the roles of both NKT cell subsets in the maintenance of immune tolerance and inflammatory diseases in liver. Furthermore, how the differential activation of type I and type II NKT cells influences other innate cells and adaptive immune cells to result in important consequences for tissue integrity is discussed. It is crucial that better reagents, including CD1d tetramers, be used in clinical studies to define the roles of NKT cells in liver diseases in patients.

Cellular mechanisms of CCL22-mediated attenuation of autoimmune diabetes

Autoimmune destruction of insulin-producing β cells in type 1 diabetes and islet transplantation involves a variety of immune pathways but is primarily mediated by self-reactive T cells. Chemokines can modulate local immune responses in inflammation and tumors by recruiting immune cells. We have reported that expression of the chemokine CCL22 in pancreatic β cells in the NOD mouse prevents autoimmune attack by recruiting T regulatory cells (Tregs), protecting mice from diabetes. In this study we show that invariant NKT cells are also recruited to CCL22-expressing islet transplants and are required for CCL22-mediated protection from autoimmunity. Moreover, CCL22 induces an influx of plasmacytoid dendritic cells, which correlates with higher levels of IDO in CCL22-expressing islet grafts. In addition to its chemotactic properties, we found that CCL22 activates Tregs and promotes their ability to induce expression of IDO by dendritic cells. Islet CCL22 expression thus produces a tolerogenic milieu through the interplay of Tregs, invariant NKT cells, and plasmacytoid dendritic cells, which results in suppression of effector T cell responses and protection of β cells. The immunomodulatory properties of CCL22 could be harnessed for prevention of graft rejection and type 1 diabetes as well as other autoimmune disorders.

Different subsets of natural killer T cells may vary in their roles in health and disease

Natural killer T cells (NKT) can regulate innate and adaptive immune responses. Type I and type II NKT cell subsets recognize different lipid antigens presented by CD1d, an MHC class-I-like molecule. Most type I NKT cells express a semi-invariant T-cell receptor (TCR), but a major subset of type II NKT cells reactive to a self antigen sulphatide use an oligoclonal TCR. Whereas TCR-α dominates CD1d-lipid recognition by type I NKT cells, TCR-α and TCR-β contribute equally to CD1d-lipid recognition by type II NKT cells. These variable modes of NKT cell recognition of lipid-CD1d complexes activate a host of cytokine-dependent responses that can either exacerbate or protect from disease. Recent studies of chronic inflammatory and autoimmune diseases have led to a hypothesis that: (i) although type I NKT cells can promote pathogenic and regulatory responses, they are more frequently pathogenic, and (ii) type II NKT cells are predominantly inhibitory and protective from such responses and diseases. This review focuses on a further test of this hypothesis by the use of recently developed techniques, intravital imaging and mass cytometry, to analyse the molecular and cellular dynamics of type I and type II NKT cell antigen-presenting cell motility, interaction, activation and immunoregulation that promote immune responses leading to health versus disease outcomes.

Natural killer T cells: drivers or passengers in preventing human disease?

Natural killer T (NKT) cells are credited with regulatory roles in immunity against cancers, autoimmune diseases, allergies, and bacterial and viral infections. Studies in mice and observational research in patient groups have suggested that NKT cell-based therapies could be used to prevent or treat these diseases, yet the translation into clinical settings has been disappointing. We support the view that NKT cells have regulatory characteristics that could be exploited in clinical settings, but there are doubts about the natural roles of NKT cells in vivo and whether NKT cell defects are fundamental drivers of disease in humans. In this Opinion article, we discuss the uncertainties and opportunities regarding NKT cells in humans, and the potential for NKT cells to be manipulated to prevent or treat disease.

Plasmacytoid dendritic cells license regulatory T cells, upon iNKT-cell stimulation, to prevent autoimmune diabetes

Invariant NKT (iNKT)-cell stimulation with exogenous specific ligands prevents the development of type 1 diabetes (T1D) in NOD mice. Studies based on anti-islet T-cell transfer showed that iNKT cells prevent the differentiation of these T cells into effector T cells in the pancreatic lymph nodes (PLNs). We hypothesize that this defective priming could be explained by the ability of iNKT cells to induce tolerogenic dendritic cells (DCs) in the PLNs. We evaluated the effect of iNKT-cell stimulation on T1D development by transferring naïve diabetogenic BDC2.5 T cells into proinsulin 2(-/-) NOD mice treated with a long-lasting α-galactosylceramide regimen. In this context, iNKT cells induce the conversion of BDC2.5 T cells into Foxp3(+) Treg cells in the PLNs accumulating in the pancreatic islets. Furthermore, tolerogenic plasmacytoid DCs (pDCs) characterized by low MHC class II molecule expression and TGF-β production are critical in the PLNs for the recruitment of Treg cells into the pancreatic islets by inducing CXCR3 expression. Accordingly, pDC depletion in α-galactosylceramide-treated proinsulin 2(-/-) NOD mice abrogates the protection against T1D. These findings reveal that upon repetitive iNKT-cell stimulation, pDCs are critical for the recruitment of Treg cells in the pancreatic islets and the prevention of T1D development.

[Regulatory role of NKT cells in the prevention of type 1 diabetes]

Type 1 diabetes is an autoimmune disease resulting from the destruction of pancreatic β cells by the immune system. NKT cells are innate-like T cells that can exert potent immuno-regulatory functions. The regulatory role of NKT cells was initially proposed after the observed decreased frequency of this subset in mouse models of type 1 diabetes, as well as in patients developing various autoimmune pathologies. Increasing NKT cell frequency and function prevent the development of type 1 diabetes in mouse models. Several mechanisms including IL-4 and IL-10 production by NKT cells and the accumulation of tolerogenic dendritic cells are critical for the dampening of pathogenic anti-islet T cell responses by NKT cells. Importantly, these cells can at the same time prevent diabetes and promote efficient immune responses against infectious agents. These results strengthen the potential role of NKT cells as a key target for the development of therapeutic strategies against type 1 diabetes.

Functional education of invariant NKT cells by dendritic cell tuning of SHP-1

Invariant NKT (iNKT) cells play key roles in host defense by recognizing lipid Ags presented by CD1d. iNKT cells are activated by bacterial-derived lipids and are also strongly autoreactive toward self-lipids. iNKT cell responsiveness must be regulated to maintain effective host defense while preventing uncontrolled stimulation and potential autoimmunity. CD1d-expressing thymocytes support iNKT cell development, but thymocyte-restricted expression of CD1d gives rise to Ag hyperresponsive iNKT cells. We hypothesized that iNKT cells require functional education by CD1d(+) cells other than thymocytes to set their correct responsiveness. In mice that expressed CD1d only on thymocytes, hyperresponsive iNKT cells in the periphery expressed significantly reduced levels of tyrosine phosphatase SHP-1, a negative regulator of TCR signaling. Accordingly, heterozygous SHP-1 mutant mice displaying reduced SHP-1 expression developed a comparable population of Ag hyperresponsive iNKT cells. Restoring nonthymocyte CD1d expression in transgenic mice normalized SHP-1 expression and iNKT cell reactivity. Radiation chimeras revealed that CD1d(+) dendritic cells supported iNKT cell upregulation of SHP-1 and decreased responsiveness after thymic emigration. Hence, dendritic cells functionally educate iNKT cells by tuning SHP-1 expression to limit reactivity.

Natural Killer T cell obsession with self-antigens

Natural Killer T (NKT) cells are distinct lymphocyte lineages that recognize lipid antigens presented by the non-classical Major Histocompatibility Complex molecule CD1d. Two categories of NKT cells, type I and type II, have been described based on T-cell receptor expression and antigenic specificity. In both cases, increasing evidence suggest that recognition of self-antigens by these cells plays an important role not only in their development but also in their regulation of a broad range of immune responses. Here we review recent advances in our understanding of how and when NKT cell autoreactivity manifests itself, how the NKT T cell receptor engages self-antigens and the nature of these self-antigens.

NOD mice contain an elevated frequency of iNKT17 cells that exacerbate diabetes

Invariant natural killer T (iNKT) cells are a distinct lineage of innate-like T lymphocytes and converging studies in mouse models have demonstrated the protective role of iNKT cells in the development of type 1 diabetes. Recently, a new subset of iNKT cells, producing high levels of the pro-inflammatory cytokine IL-17, has been identified (iNKT17 cells). Since this cytokine has been implicated in several autoimmune diseases, we have analyzed iNKT17 cell frequency, absolute number and phenotypes in the pancreas and lymphoid organs in non-obese diabetic (NOD) mice. The role of iNKT17 cells in the development of diabetes was investigated using transfer experiments. NOD mice exhibit a higher frequency and absolute number of iNKT17 cells in the lymphoid organs as compared with C57BL/6 mice. iNKT17 cells infiltrate the pancreas of NOD mice where they express IL-17 mRNA. Contrary to the protective role of CD4(+) iNKT cells, the CD4(-) iNKT cell population, which contains iNKT17 cells, enhances the incidence of diabetes. Treatment with a blocking anti-IL-17 antibody prevents the exacerbation of the disease. This study reveals that different iNKT cell subsets play distinct roles in the regulation of type 1 diabetes and iNKT17 cells, which are abundant in NOD mice, exacerbate diabetes development.

The split personality of NKT cells in malignancy, autoimmune and allergic disorders

NKT cells are a heterogeneous subset of specialized, self-reactive T cells, with innate and adaptive immune properties, which allow them to bridge innate and adaptive immunity and profoundly influence autoimmune and malignant disease outcomes. NKT cells mediate these activities through their ability to rapidly express pro- and anti-inflammatory cytokines that influence the type and magnitude of the immune response. Not only do NKT cells regulate the functions of other cell types, but experimental evidence has found NKT cell subsets can modulate the functions of other NKT subsets. Depending on underlying mechanisms, NKT cells can inhibit or exacerbate autoimmunity and malignancy, making them potential targets for disease intervention. NKT cells can respond to foreign and endogenous antigenic glycolipid signals that are expressed during pathogenic invasion or ongoing inflammation, respectively, allowing them to rapidly react to and influence a broad array of diseases. In this article we review the unique development and activation pathways of NKT cells and focus on how these attributes augment or exacerbate autoimmune disorders and malignancy. We also examine the growing evidence that NKT cells are involved in liver inflammatory conditions that can contribute to the development of malignancy.

NKT cells are required for complete Freund's adjuvant-mediated protection from autoimmune diabetes

Autoimmune diabetes in NOD mice can be prevented by application of Ags derived from Mycobacterium tuberculosis in the form of bacillus Calmette-Guérin or CFA. Disease protection by CFA is associated with a reduction in the numbers of pathogenic β-cell specific, self-reactive CTLs, a phenomenon dependent on the presence and function of NK cells. However, the mechanisms by which NK cells are activated and recruited by heat-killed M. tuberculosis within CFA are unclear. In this study, we report that CFA-mediated NK cell activation and mobilization is dependent on CD1d expression. The administration of M. tuberculosis from CFA results in rapid NKT cell activation and IFN-γ secretion both in vitro and in vivo. CFA-induced NKT cell activation is intact in MyD88(-/-) mice suggesting that the mechanism is independent of TLR signaling. Furthermore, CD1d expression was found to be essential for both M. tuberculosis-triggered NKT cell activation and CFA-mediated protection of NOD mice from diabetes. Collectively, these findings reveal hitherto previously unidentified roles for NKT cells in the adjuvant-promoting effects of CFA on innate and adaptive immunity.

Viral infection prevents diabetes by inducing regulatory T cells through NKT cell-plasmacytoid dendritic cell interplay

Type 1 diabetes (T1D) is an autoimmune disease resulting from T cell-mediated destruction of insulin-producing β cells, and viral infections can prevent the onset of disease. Invariant natural killer T cells (iNKT cells) exert a regulatory role in T1D by inhibiting autoimmune T cell responses. As iNKT cell-plasmacytoid dendritic cell (pDC) cooperation controls viral replication in the pancreatic islets, we investigated whether this cellular cross talk could interfere with T1D development during viral infection. Using both virus-induced and spontaneous mouse models of T1D, we show that upon viral infection, iNKT cells induce TGF-β-producing pDCs in the pancreatic lymph nodes (LNs). These tolerogenic pDCs convert naive anti-islet T cells into Foxp3(+) CD4(+) regulatory T cells (T reg cells) in pancreatic LNs. T reg cells are then recruited into the pancreatic islets where they produce TGF-β, which dampens the activity of viral- and islet-specific CD8(+) T cells, thereby preventing T1D development in both T1D models. These findings reveal a crucial cooperation between iNKT cells, pDCs, and T reg cells for prevention of T1D by viral infection.

NKT and tolerance

NKT cells are innate-like αβ T cells that are conserved between humans and mice. They are distinct from conventional T cells as they recognize lipid antigens presented by the CD1d molecule. Most NKT cells expressed a highly restricted TCR repertoire and can be activated by α-galactosylceramide (α-GalCer) and detected by α-GalCer-loaded-CD1d tetramers. Upon activation, NKT cells respond in few hours by producing cytokines and stimulating many other cells of the innate and adaptive immune system. Over the last decade, many studies have analyzed the regulatory role of NKT cells that can either suppress or exacerbate immune functions. This chapter describes the tools and techniques required to study in vivo and in vitro the regulatory role of NKT cells in mouse as well as from human blood.

Mechanism of regulation of autoimmunity by iNKT cells

iNKT cells, CD1d dependent natural killer T cells are a unique population of T cells. The capacity of iNKT cells to produce regulatory cytokines first provided an indication of their regulatory potential. Later on, in experimental models as well as in patients afflicted with an auto-immune disease, such as Type 1 diabetes mellitus, multiple sclerosis, and systemic lupus erythematosus along with others, a deficit in iNKT cell number was observed, suggesting the role these cells may possibly have in the prevention of auto-immune diseases. More importantly, experimental strategies which focused on increasing the volume or stimulation of iNKT cells in laboratory animals, demonstrated an improved level of protection against the development of auto-immune diseases. This article reviews the mechanism of protection against autoimmunity by iNKT cells, discusses the obstacles against and indications for the potential use of iNKT cell manipulation in the treatment of human auto-immune diseases.

The dangerous liaison between iNKT cells and dendritic cells: does it prevent or promote autoimmune diseases?

Invariant natural killer T (iNKT) cells represent an important regulatory T-cell subset that perceives signals of danger and/or cellular distress and modulate the adaptive immune response accordingly. In the presence of pathogens, iNKT cells acquire an adjuvant function that is fundamental to boost anti-microbial and anti-tumor immunity. At the same time, iNKT cells can play a negative regulatory function to maintain peripheral T-cell tolerance toward self-antigens and to prevent autoimmune disease. Both these effects of iNKT cells involve the modulation of the activity of dendritic cells (DCs) through cell-cell interaction. Indeed, iNKT cells can either boost Th1 immunity by enhancing maturation of pro-inflammatory DCs or promote immune tolerance through the maturation of tolerogenic DCs. This dual action of iNKT cells opens questions on the modalities by which a single-cell subset can exert opposite effects on DCs and may even put in question the overall immunosuppressive properties of iNKT cells. This review presents the large body of evidence that shows the ability of iNKT cells to negatively regulate autoimmunity and to prevent autoimmune diseases including multiple sclerosis, type 1 diabetes, rheumatoid arthritis, and systemic lupus erythematosus. In addition, an update is provided on the mechanisms of iNKT-DCs interactions and how this can result in inflammatory or tolerogenic responses.

Regulation of type 1 diabetes, tuberculosis, and asthma by parasites

Helminth infection is a worldwide health problem. In addition to directly causing disease, helminthic infection also affects the incidence and progression of other diseases by exerting immune modulatory effects. In animal models, infection with helminthic parasites can prevent autoimmune diseases and allergic inflammatory diseases, but worsens protective immunity to certain infectious pathogens. In this review, we summarize current findings regarding the effects of helminth infection on type 1 diabetes, tuberculosis, and asthma and discuss possible mechanisms through which helminthic parasites modulate host immunity. Investigating these mechanisms could lead to treatment strategies that specifically modulate the immune response as well as address fundamental questions in immunobiology.

Clinical application of NKT cell biology in type I (autoimmune) diabetes mellitus

Type 1 natural killer T (NKT) cells are a population of CD1d-restricted, regulatory T cells that exhibit various NK cell characteristics and rapidly produce cytokines on stimulation with glycolipid antigen. In type I diabetes (TID), NKT cells are thought to have a tolerogenic function, evidenced by NKT cell numerical and functional deficiencies in the nonobese diabetic (NOD) mouse, which when corrected, can ameliorate disease. The mechanisms by which NKT cells can mediate their immunosuppressive effects in NOD mice are still poorly understood, which makes successful clinical translation of NKT- cell-based therapies challenging. However, new insights into the genetic control of NKT cell deficiencies have provided some understanding of the genes that may control NKT cell number and function, potentially offering a new avenue for assessing TID risk in humans. Here, we review the mechanisms by which NKT cells are thought to prevent TID, discuss the evidence for involvement of NKT cells in the regulation of human TID and examine the genetic control of NKT cell number and function. A greater understanding of these areas will increase the chances of successful clinical manipulation of NKT cells to prevent or treat TID.

NKT cell-plasmacytoid dendritic cell cooperation via OX40 controls viral infection in a tissue-specific manner

Invariant natural killer T (iNKT) cells promote immune responses to various pathogens, but exactly how iNKT cells control antiviral responses is unclear. Here, we showed that iNKT cells induced tissue-specific antiviral effects in mice infected by lymphocytic choriomeningitis virus (LCMV). Indeed, iNKT cells inhibited viral replication in the pancreas and liver but not in the spleen. In the pancreas, iNKT cells expressed the OX40 molecule and promoted type I interferon (IFN) production by plasmacytoid dendritic cells (pDCs) through OX40-OX40 ligand interaction. Subsequently, this iNKT cell-pDC cooperation attenuated the antiviral adaptive immune response in the pancreas but not in the spleen. The dampening of pancreatic anti-LCMV CD8(+) T cell response prevented tissue damage in transgenic mice expressing LCMV protein in islet beta cells. Thus, this study identifies pDCs as an essential partner of iNKT cells for mounting an efficient, nondeleterious antiviral response in peripheral tissue.

Invariant NKT cells regulate experimental autoimmune encephalomyelitis and infiltrate the central nervous system in a CD1d-independent manner

Invariant NKT cells are CD1d-restricted T cells specific for glycolipid Ags. Their activation or transgenic enrichment abrogates the development of experimental autoimmune encephalomyelitis (EAE). Herein, we demonstrate that in NKT-enriched mice the protection from EAE is associated with the infiltration of NKT cells in the CNS and the local expression of CD1d. This indicates that the CNS acquires the potential for local glycolipid presentation when exposed to inflammatory stress, permitting the triggering of NKT cells. To address the importance of CD1d-mediated Ag presentation, we used transgenic mice that express CD1d solely in the thymus. Interestingly, enrichment of NKT cells in these mice also conferred resistance to EAE, with an efficacy indistinguishable from that of NKT-enriched CD1d-sufficient mice. This protection was due to an abrogation of the encephalitogenic Th1 and Th17 response in the spleen, revealing that endogenous glycolipid presentation is dispensable for the regulatory function of NKT cells in EAE. Moreover, abrogating extrathymic CD1d expression failed to affect both the recruitment of NKT cells and their effector phenotype. CNS-infiltrating NKT cells were characterized by a cytotoxic IFN-gamma(high)IL-4(low)IL-10(low)granzyme B(high) profile, irrespective of the local expression of CD1d. Glycolipid Ag presentation is therefore dispensable for the control of autoimmune demyelination by NKT cells, underlining the importance of alternative cognate and/or soluble factors in the control of NKT cell function.

Understanding the behavior of invariant NKT cells in autoimmune diseases

Invariant NKT (iNKT) cells are a unique subset of lymphocytes that recognize glycolipid antigens presented by a monomorphic glycoprotein CD1d. Numerous works have shown that iNKT cells may serve as regulatory cells in autoimmune diseases including multiple sclerosis (MS). However, recent studies have revealed that the presence of iNKT cells accelerates some inflammatory conditions, implying that their protective role against autoimmunity is not predetermined. Here we review recent information concerning the mechanism of how iNKT cells intervene or promote autoimmune inflammation. Although iNKT cells are thought to be specific for a limited set of glycolipids, they may cross-react to self and non-self ligands. Regarding the response to non-self, it is now known that iNKT cells produce enormous amounts of proinflammatory cytokines during the course of infectious diseases, which is triggered by TCR ligation by microbial lipids, cytokines produced from APCs or both. Whereas the strongly activated iNKT cells play a beneficial role in combating environmental pathogens, they could play a deleterious role in autoimmunity by producing disease-promoting cytokines. However, iNKT cells in the steady state would retain an ability to produce anti-inflammatory cytokines, which is needed for terminating the ongoing inflammation. Though an initial trigger for their regulatory responses remains elusive, our recent work indicates that iNKT cells may start regulating inflammation after sensing the presence of IL-2 in addition to recognizing a ubiquitous endogenous ligand. Understanding of how iNKT cells regulate autoimmunity should lead to a more sophisticated strategy for controlling autoimmune diseases.

IL-4 inducibility in NKT cells, naïve CD4+ T cells and TCR-gamma delta T cells

NKT cells, naïve CD4(+) T cells, and TCR-gammadelta T cells belong to distinct T cell lineages but all express T cell receptors generated through random combinatorial joining of V-(D)-J genes. These distinct lineage T cells also possess the property of promptly activating the IL-4 gene upon T cell receptor stimulation. A comparative accounting of features as they pertain to IL-4 inducibility in these three distinct lineage T cells is provided here.