Antigen recognition by CD1d-restricted NKT T cell receptors

State of play in the molecular presentation and recognition of anti-tumor lipid-based analogues

The Natural Killer T cells (NKT) are a unique subset of T lymphocytes that recognize lipid-based antigens that are presented by the monomorphic MHC-I-like molecule, CD1d. Over 30 years ago, the discovery of the glycolipid α-Galactosylceramide (α-GalCer) from the marine sponge Agelas mauritianus, as a potent activator of the invariant Natural Killer T (iNKT) cells, has attracted great attention for its use in cancer immunotherapy. However, α-GalCer can initiate both pro-inflammatory T helper cell 1 (Th1) and anti-inflammatory Th2 type immune responses that can result in either enhanced or suppressed immunity in a somewhat unpredictable manner. Th1 polarized immune response is often correlated with an optimal anti-tumor immunity, and therefore α-GalCer did not fully offer the desired potential as an anti-tumor therapeutic. Over the past decades, considerable efforts have then been invested into the design and development of novel synthetic α-GalCer analogues that will direct a more efficient immune response towards the production of Th1 biased cytokines. In this minireview, we will discuss how subtle modifications in the chemical nature of a number of α-GalCer derivatives varied immune responses. Whilst some of these analogues showed potential in enhancing stability within CD1d and directing favourable immune responses for tumor immunotherapy, their responses in mice also highlighted the need for further research in humanized models to overcome translational challenges and optimize therapeutic efficacy.

Tumor antigen presentation and the associated signal transduction during carcinogenesis

Numerous developments have been achieved in the study and treatment of cancer throughout the decades that it has been common. After decades of research, about 100 different kinds of cancer have been found, each with unique subgroups within certain organs. This has significantly expanded our understanding of the illness. A mix of genetic, environmental, and behavioral variables contribute to the complicated and diverse process of cancer formation. Mutations, or changes in the DNA sequence, are crucial to the development of cancer. These mutations have the ability to downregulate the expression and function of Major Histocompatibility Complex class I (MHC I) and MHCII receptors, as well as activate oncogenes and inactivate tumor suppressor genes. Cancer cells use this tactic to avoid being recognized by cytotoxic CD8+T lymphocytes, which causes issues with antigen presentation and processing. This review goes into great length into the PI3K pathway, changes to MHC I, and positive impacts of tsMHC-II on disease-free survival and overall survival and the involvement of dendritic cells (DCs) in different tumor microenvironments. The vital functions that the PI3K pathway and its link to the mTOR pathway are highlighted and difficulties in developing effective cancer targeted therapies and feedback systems has also been mentioned, where resistance mechanisms include RAS-mediated oncogenic changes and active PI3K signalling.

Glycolipid antigen recognition by invariant natural killer T cells and its role in homeostasis and antimicrobial responses

Due to the COVID-19 pandemic, the importance of developing effective vaccines has received more attention than ever before. To maximize the effects of vaccines, it is important to select adjuvants that induce strong and rapid innate and acquired immune responses. Invariant natural killer T (iNKT) cells, which constitute a small population among lymphocytes, bypass the innate and acquired immune systems through the rapid production of cytokines after glycolipid recognition; hence, their activation could be used as a vaccine strategy against emerging infectious diseases. Additionally, the diverse functions of iNKT cells, including enhancing antibody production, are becoming more understood in recent years. In this review, we briefly describe the functional subset of iNKT cells and introduce the glycolipid antigens recognized by them. Furthermore, we also introduce novel vaccine development taking advantages of iNKT cell activation against infectious diseases.

Same yet different - how lymph node heterogeneity affects immune responses

Lymph nodes are secondary lymphoid organs in which immune responses of the adaptive immune system are initiated and regulated. Distributed throughout the body and embedded in the lymphatic system, local lymph nodes are continuously informed about the state of the organs owing to a constant drainage of lymph. The tissue-derived lymph carries products of cell metabolism, proteins, carbohydrates, lipids, pathogens and circulating immune cells. Notably, there is a growing body of evidence that individual lymph nodes differ from each other in their capacity to generate immune responses. Here, we review the structure and function of the lymphatic system and then focus on the factors that lead to functional heterogeneity among different lymph nodes. We will discuss how lymph node heterogeneity impacts on cellular and humoral immune responses and the implications for vaccination, tumour development and tumour control by immunotherapy.

Anti-CD1d treatment suppresses immunogenic maturation of lung dendritic cells dependent on lung invariant natural killer T cells in asthmatic mice

Our previous findings show that invariant natural killer T (iNKT)cells can promote immunogenic maturation of lung dendritic cells (LDCs) to enhance Th2 cell responses in asthma. It has been accepted that recognition of glycolipid antigens presented by CD1d molecules by the T cell receptors of iNKT cells leads to iNKT cell activation. Therefore, we examine the immunoregulatory influences of anti-CD1d treatment on Th2 cell response and immunogenic maturation of LDCs and subsequently explored whether these influences were dependent on lung iNKT cells in asthmatic mice. We discoveredthat in wild-type mice sensitized and challenged with house dust mite or ovalbumin (OVA), anti-CD1d treatment inhibited Th2 cell response and immunogenic maturation of LDCs. LDCs from asthmatic mice with anti-CD1d treatment had a markedly decreased influence on Th2 cell responses in vivo and in vitro. Furthermore, anti-CD1d treatment reduced the abundance and activation of lung iNKT cells in asthmatic mice. Moreover, in asthmatic iNKT cell-deficient Jα18-/- mice, anti-CD1d treatment did not influence Th2 cell responses and immunogenic maturation of LDCs. Meanwhile, the quantity of CD40L+ iNKT cells in asthmatic mice was significant decreased by anti-CD1d treatment. Finally, the inhibition of anti-CD1d treatment on LDC immunogenic maturation and Th2 cell responses in asthmatic mice was reversed by anti-CD40 treatment. Our data suggest that anti-CD1d treatment can suppress Th2 cell responses through inhibiting immunogenic maturation of LDCs dependent on lung iNKT cells, which couldbe partially related to the downregulation of CD40L expression on lung iNKT cells in asthmatic mice.

Nutraceuticals as Potential Therapeutic Modulators in Immunometabolism

Nutraceuticals act as cellular and functional modulators, contributing to the homeostasis of physiological processes. In an inflammatory microenvironment, these functional foods can interact with the immune system by modulating or balancing the exacerbated proinflammatory response. In this process, immune cells, such as antigen-presenting cells (APCs), identify danger signals and, after interacting with T lymphocytes, induce a specific effector response. Moreover, this conditions their change of state with phenotypical and functional modifications from the resting state to the activated and effector state, supposing an increase in their energy requirements that affect their intracellular metabolism, with each immune cell showing a unique metabolic signature. Thus, nutraceuticals, such as polyphenols, vitamins, fatty acids, and sulforaphane, represent an active option to use therapeutically for health or the prevention of different pathologies, including obesity, metabolic syndrome, and diabetes. To regulate the inflammation associated with these pathologies, intervention in metabolic pathways through the modulation of metabolic energy with nutraceuticals is an attractive strategy that allows inducing important changes in cellular properties. Thus, we provide an overview of the link between metabolism, immune function, and nutraceuticals in chronic inflammatory processes associated with obesity and diabetes, paying particular attention to nutritional effects on APC and T cell immunometabolism, as well as the mechanisms required in the change in energetic pathways involved after their activation.

Augmenting Granzyme B-Expressing NK Cells by Invariant NKT Ligand-Loaded APCs in Patients with Postoperative Early Stage Non-Small Cell Lung Cancer: Results of a Randomized Phase II Study

NK cells are major effector cells involved in the elimination of early tumors and prevent metastasis. They often have an impaired function in patients with cancer. Preclinical studies have demonstrated NK cell activation as the adjunctive effect of invariant NKT (iNKT) cells. Activation of iNKT cells after administration of the glycolipid ligand α-galactosylceramide, loaded with CD1d-expressing human PBMC-derived APCs (APC/Gal), is an attractive cancer therapy to optimize the use of NK cells. However, the subsets of NK cells that are activated following iNKT cell activation as well as the period of NK cell activation remain unclear. In this study, we report that the granzyme B-expressing NK cell response in postoperative lung cancer patients was enhanced 49 d after administration of APC/Gal in a phase II study. We found maximum IFN-γ production on day 49 in 13 out of 27 APC/Gal-treated patients. On day 49, 14 out of 27 patients (51.9%) had higher IFN-γ production by iNKT cells (>6-fold higher than the baseline level). This increment significantly correlated with granzyme B-expressing NK cells. Although IFN-γ production was lower in patients in the nontreated group, we detected maximum IFN-γ production 12 mo after the resection of lung cancer (9 out of 29 patients [31%]). These findings suggest that elimination of cancer cells leads to increased NK cell function, which can be further enhanced by APC/Gal therapy.

Role of Sphingolipids in Multiple Myeloma Progression, Drug Resistance, and Their Potential as Therapeutic Targets

Multiple myeloma (MM) is an incapacitating hematological malignancy characterized by accumulation of cancerous plasma cells in the bone marrow (BM) and production of an abnormal monoclonal protein (M-protein). The BM microenvironment has a key role in myeloma development by facilitating the growth of the aberrant plasma cells, which eventually interfere with the homeostasis of the bone cells, exacerbating osteolysis and inhibiting osteoblast differentiation. Recent recognition that metabolic reprograming has a major role in tumor growth and adaptation to specific changes in the microenvironmental niche have led to consideration of the role of sphingolipids and the enzymes that control their biosynthesis and degradation as critical mediators of cancer since these bioactive lipids have been directly linked to the control of cell growth, proliferation, and apoptosis, among other cellular functions. In this review, we present the recent progress of the research investigating the biological implications of sphingolipid metabolism alterations in the regulation of myeloma development and its progression from the pre-malignant stage and discuss the roles of sphingolipids in in MM migration and adhesion, survival and proliferation, as well as angiogenesis and invasion. We introduce the current knowledge regarding the role of sphingolipids as mediators of the immune response and drug-resistance in MM and tackle the new developments suggesting the manipulation of the sphingolipid network as a novel therapeutic direction for MM.

Adipocyte CD1d Gene Transfer Induces T Cell Expansion and Adipocyte Inflammation in CD1d Knockout Mice

CD1d, a lipid Ag-presenting molecule for invariant NKT (iNKT) cells, is abundantly expressed on adipocytes and regulates adipose homeostasis through iNKT cells. CD1d gene expression was restored in visceral adipose tissue adipocytes of CD1d knockout (KO) mice to investigate the interactions between adipocytes and immune cells within adipose tissue. We developed an adipocyte-specific targeting recombinant adeno-associated viral vector, with minimal off-target transgene expression in the liver, to rescue CD1d gene expression in visceral adipose tissue adipocytes of CD1d KO mice, followed by assessment of immune cell alternations in adipose tissue and elucidation of the underlying mechanisms of alteration. We report that adeno-associated virus-mediated gene transfer of CD1d to adipocytes in CD1d KO mice fails to rescue iNKT cells but leads to massive and selective expansion of T cells within adipose tissue, particularly CD8+ T effector cells, that is associated with adipocyte NLRP3 inflammasome activation, dysregulation of adipocyte functional genes, and upregulation of apoptotic pathway proteins. An NLRP3 inhibitor has no effect on T cell phenotypes whereas depletion of CD8+ T cells significantly attenuates inflammasome activation and abolishes the dysregulation of adipocyte functional genes induced by adipocyte CD1d. In contrast, adipocyte overexpression of CD1d fails to induce T cell activation in wild-type mice or in invariant TCR α-chain Jα18 KO mice that have a normal lymphocyte repertoire except for iNKT cells. Our studies uncover an adipocyte CD1d → CD8+ T cell → adipocyte inflammasome cascade, in which CD8+ T cells function as a key mediator of adipocyte inflammation likely induced by an allogeneic response against the CD1d molecule.

Structure-guided stabilization of pathogen-derived peptide-HLA-E complexes using non-natural amino acids conserves native TCR recognition

The nonpolymorphic class Ib molecule, HLA-E, primarily presents peptides from HLA class Ia leader peptides, providing an inhibitory signal to NK cells via CD94/NKG2 interactions. Although peptides of pathogenic origin can also be presented by HLA-E to T cells, the molecular basis underpinning their role in antigen surveillance is largely unknown. Here, we solved a co-complex crystal structure of a TCR with an HLA-E presented peptide (pHLA-E) from bacterial (Mycobacterium tuberculosis) origin, and the first TCR-pHLA-E complex with a noncanonically presented peptide from viral (HIV) origin. The structures provided a molecular foundation to develop a novel method to introduce cysteine traps using non-natural amino acid chemistry that stabilized pHLA-E complexes while maintaining native interface contacts between the TCRs and different pHLA-E complexes. These pHLA-E monomers could be used to isolate pHLA-E-specific T cells, with obvious utility for studying pHLA-E restricted T cells, and for the identification of putative therapeutic TCRs.

Insights Into the Emergence of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal Nocturnal Hemoglobinuria (PNH) is a disease as simple as it is complex. PNH patients develop somatic loss-of-function mutations in phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene (PIGA), required for the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. Ubiquitous in eukaryotes, GPI anchors are a group of conserved glycolipid molecules responsible for attaching nearly 150 distinct proteins to the surface of cell membranes. The loss of two GPI-anchored surface proteins, CD55 and CD59, from red blood cells causes unregulated complement activation and hemolysis in classical PNH disease. In PNH patients, PIGA-mutant, GPI (-) hematopoietic cells clonally expand to make up a large portion of patients’ blood production, yet mechanisms leading to clonal expansion of GPI (-) cells remain enigmatic. Historical models of PNH in mice and the more recent PNH model in rhesus macaques showed that GPI (-) cells reconstitute near-normal hematopoiesis but have no intrinsic growth advantage and do not clonally expand over time. Landmark studies identified several potential mechanisms which can promote PNH clonal expansion. However, to what extent these contribute to PNH cell selection in patients continues to be a matter of active debate. Recent advancements in disease models and immunologic technologies, together with the growing understanding of autoimmune marrow failure, offer new opportunities to evaluate the mechanisms of clonal expansion in PNH. Here, we critically review published data on PNH cell biology and clonal expansion and highlight limitations and opportunities to further our understanding of the emergence of PNH clones.

Distinct CD1d docking strategies exhibited by diverse Type II NKT cell receptors

Type I and type II natural killer T (NKT) cells are restricted to the lipid antigen-presenting molecule CD1d. While we have an understanding of the antigen reactivity and function of type I NKT cells, our knowledge of type II NKT cells in health and disease remains unclear. Here we describe a population of type II NKT cells that recognise and respond to the microbial antigen, α-glucuronosyl-diacylglycerol (α-GlcADAG) presented by CD1d, but not the prototypical type I NKT cell agonist, α-galactosylceramide. Surprisingly, the crystal structure of a type II NKT TCR-CD1d-α-GlcADAG complex reveals a CD1d F’-pocket-docking mode that contrasts sharply with the previously determined A’-roof positioning of a sulfatide-reactive type II NKT TCR. Our data also suggest that diverse type II NKT TCRs directed against distinct microbial or mammalian lipid antigens adopt multiple recognition strategies on CD1d, thereby maximising the potential for type II NKT cells to detect different lipid antigens.

Natural killer T (NKT) cells include type I that express semi-invariant T cell receptor (TCR), and type II that cover a broader repertoire. Here the authors describe the crystal structure of a type II NKT TCR complexed with CD1d/antigen to propose that type II NKT TCRs may adapt multiple CD1d docking modes to maximise antigen recognition efficacy.

Tissue-Specific Phenotype and Activation of iNKT Cells in Morbidly Obese Subjects: Interaction with Adipocytes and Effect of Bariatric Surgery

BACKGROUND

The immune response of visceral adipose tissue (VAT) in obesity, in particular the role of invariant natural killer T (iNKT) cells, has not yet been fully elucidated.

OBJECTIVE

To characterize iNKT cells and its activation status in VAT and peripheral blood mononuclear cells (PBMC) in morbidly obese subjects (MO), and to analyze their association with metabolic parameters.

SUBJECTS AND METHODS

Twenty non-obese and 20 MO subjects underwent Roux-en-Y gastric bypass (RYGB) and were studied before and 6 months after RYGB. VAT and PBMC were obtained.

RESULTS

A decrease in VAT iNKT cells from MO was found, however, not in PBMC. Visceral adipocytes from MO presented increased CD1d expression (p = 0.032). MO presented an increase in early activated CD69+ iNKT cells in PBMC before RYGB (p < 0.001), but not after RYGB nor in VAT, and an increase in later activated CD25+ iNKT in VAT (p = 0.046), without differences in PBMC. The co-expression of early and later markers (CD69+CD25+) in iNKT cells was increased in MO in VAT (p = 0.050) and PBMC (p = 0.006), decreasing after RYGB (p = 0.050). CD69+ iNKT and CD69+CD25+ iNKT cells in PBMC after RYGB correlated negatively with glucose, insulin, and insulin resistance levels.

CONCLUSIONS

There is a tissue-specific phenotype and activation of iNKT cells in VAT in morbid obesity, which could be involved in VAT immunometabolism dysregulation. Also, the increase in CD1d expression could be to offset the lack of VAT iNKT cells.

Structure-Function Implications of the Ability of Monoclonal Antibodies Against α-Galactosylceramide-CD1d Complex to Recognize β-Mannosylceramide Presentation by CD1d

iNKT cells are CD1d-restricted T cells recognizing lipid antigens. The prototypic iNKT cell-agonist α-galactosylceramide (α-GalCer) alongside compounds with similar structures induces robust proliferation and cytokine production of iNKT cells and protects against cancer in vivo. Monoclonal antibodies (mAbs) that detect CD1d-α-GalCer complexes have provided critical information for understanding of antigen presentation of iNKT cell agonists. Although most iNKT cell agonists with antitumor properties are α-linked glycosphingolipids that can be detected by anti-CD1d-α-GalCer mAbs, β-ManCer, a glycolipid with a β-linkage, induces strong antitumor immunity via mechanisms distinct from those of α-GalCer. In this study, we unexpectedly discovered that anti-CD1d-α-GalCer mAbs directly recognized β-ManCer-CD1d complexes and could inhibit β-ManCer stimulation of iNKT cells. The binding of anti-CD1d-α-GalCer mAb with β-ManCer-CD1d complexes was also confirmed by plasmon resonance and could not be explained by α-anomer contamination. The binding of anti-CD1d-α-GalCer mAb was also observed with CD1d loaded with another β-linked glycosylceramide, β-GalCer (C26:0). Detection with anti-CD1d-α-GalCer mAbs indicates that the interface of the β-ManCer-CD1d complex exposed to the iNKT cell TCR can assume a structure like that of CD1d-α-GalCer, despite its disparate carbohydrate structure. These results suggest that certain β-linked monoglycosylceramides can assume a structural display similar to that of CD1d-α-GalCer and that the data based on anti-CD1d-α-GalCer binding should be interpreted with caution.

Bimodal CD40/Fas-Dependent Crosstalk between iNKT Cells and Tumor-Associated Macrophages Impairs Prostate Cancer Progression

Heterotypic cellular and molecular interactions in the tumor microenvironment (TME) control cancer progression. Here, we show that CD1d-restricted invariant natural killer (iNKT) cells control prostate cancer (PCa) progression by sculpting the TME. In a mouse PCa model, iNKT cells restrained the pro-angiogenic and immunosuppressive capabilities of tumor-infiltrating immune cells by reducing pro-angiogenic TIE2⁺, M2-like macrophages (TEMs), and sustaining pro-inflammatory M1-like macrophages. iNKT cells directly contacted macrophages in the PCa stroma, and iNKT cell transfer into tumor-bearing mice abated TEMs, delaying tumor progression. iNKT cells modulated macrophages through the cooperative engagement of CD1d, Fas, and CD40, which promoted selective killing of M2-like and survival of M1-like macrophages. Human PCa aggressiveness associate with reduced intra-tumoral iNKT cells, increased TEMs, and expression of pro-angiogenic genes, underscoring the clinical significance of this crosstalk. Therefore, iNKT cells may control PCa through mechanisms involving differential macrophage modulation, which may be harnessed for therapeutically reprogramming the TME.

Inflammatory Phenotype of Intrahepatic Sulfatide-Reactive Type II NKT Cells in Humans With Autoimmune Hepatitis

Background: Natural Killer T (NKT) cells are CD1d-restricted innate-like T cells that can rapidly release stored cytokines upon recognition of lipid antigens. In mice, type I NKT cells seem to promote liver inflammation, whereas type II NKT cells seem to restrict hepatitis. Here, we aimed at characterizing the role of human type I and type II NKT in patients with autoimmune hepatitis (AIH).

Methods: NKT cells were analyzed by flow cytometry in peripheral blood and liver of AIH patients and control groups. α-galactosylceramide-loaded or sulfatide-loaded tetramers were used to detect type I or II NKT cells, respectively. Hepatic CD1d was stained by in situ-hybridization of liver biopsies.

Results and Conclusions: Type II NKT cells were more prevalent in human peripheral blood and liver than type I NKT cells. In AIH patients, the frequency of sulfatide-reactive type II NKT cells was significantly increased in peripheral blood (0.11% of peripheral blood leukocytes) and liver (3.78% of intrahepatic leukocytes) compared to healthy individuals (0.05% and 1.82%) and patients with drug-induced liver injury (0.06% and 2.03%; p < 0.05). Intrahepatic type II NKT cells of AIH patients had a different cytokine profile than healthy subjects with an increased frequency of TNFα (77.8% vs. 59.1%, p < 0.05), decreased IFNγ (32.7% vs. 63.0%, p < 0.05) and a complete lack of IL-4 expressing cells (0% vs. 2.1%, p < 0.05). T cells in portal tracts expressed significantly more CD1d-RNA in AIH livers compared to controls. This study supports that in contrast to their assumed protective role in mice, human intrahepatic, sulfatide-reactive type II NKT cells displayed a proinflammatory cytokine profile in patients with AIH. Infiltrating T cells in portal areas of AIH patients overexpressed CD1d and could thereby activate type II NKT cells.

Adipocytes harbor a glucosylceramide biosynthesis pathway involved in iNKT cell activation

BACKGROUND

Natural killer T (NKT) cells in adipose tissue (AT) contribute to whole body energy homeostasis.

RESULTS

Inhibition of the glucosylceramide synthesis in adipocytes impairs iNKT cell activity.

CONCLUSION

Glucosylceramide biosynthesis pathway is important for endogenous lipid antigen activation of iNKT cells in adipocytes.

SIGNIFICANCE

Unraveling adipocyte-iNKT cell communication may help to fight obesity-induced AT dysfunction. Overproduction and/or accumulation of ceramide and ceramide metabolites, including glucosylceramides, can lead to insulin resistance. However, glucosylceramides also fulfill important physiological functions. They are presented by antigen presenting cells (APC) as endogenous lipid antigens via CD1d to activate a unique lymphocyte subspecies, the CD1d-restricted invariant (i) natural killer T (NKT) cells. Recently, adipocytes have emerged as lipid APC that can activate adipose tissue-resident iNKT cells and thereby contribute to whole body energy homeostasis. Here we investigate the role of the glucosylceramide biosynthesis pathway in the activation of iNKT cells by adipocytes. UDP-glucose ceramide glucosyltransferase (Ugcg), the first rate limiting step in the glucosylceramide biosynthesis pathway, was inhibited via chemical compounds and shRNA knockdown in vivo and in vitro. β-1,4-Galactosyltransferase (B4Galt) 5 and 6, enzymes that convert glucosylceramides into potentially inactive lactosylceramides, were subjected to shRNA knock down. Subsequently, (pre)adipocyte cell lines were tested in co-culture experiments with iNKT cells (IFNγ and IL4 secretion). Inhibition of Ugcg activity shows that it regulates presentation of a considerable fraction of lipid self-antigens in adipocytes. Furthermore, reduced expression levels of either B4Galt5 or -6, indicate that B4Galt5 is dominant in the production of cellular lactosylceramides, but that inhibition of either enzyme results in increased iNKT cell activation. Additionally, in vivo inhibition of Ugcg by the aminosugar AMP-DNM results in decreased iNKT cell effector function in adipose tissue. Inhibition of endogenous glucosylceramide production results in decreased iNKT cells activity and cytokine production, underscoring the role of this biosynthetic pathway in lipid self-antigen presentation by adipocytes.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

MHC Molecules, T cell Receptors, Natural Killer Cell Receptors, and Viral Immunoevasins-Key Elements of Adaptive and Innate Immunity

Molecules encoded by the Major Histocompatibility Complex (MHC) bind self or foreign peptides and display these at the cell surface for recognition by receptors on T lymphocytes (designated T cell receptors-TCR) or on natural killer (NK) cells. These ligand/receptor interactions govern T cell and NK cell development as well as activation of T memory and effector cells. Such cells participate in immunological processes that regulate immunity to various pathogens, resistance and susceptibility to cancer, and autoimmunity. The past few decades have witnessed the accumulation of a huge knowledge base of the molecular structures of MHC molecules bound to numerous peptides, of TCRs with specificity for many different peptide/MHC (pMHC) complexes, of NK cell receptors (NKR), of MHC-like viral immunoevasins, and of pMHC/TCR and pMHC/NKR complexes. This chapter reviews the structural principles that govern peptide/MHC (pMHC), pMHC/TCR, and pMHC/NKR interactions, for both MHC class I (MHC-I) and MHC class II (MHC-II) molecules. In addition, we discuss the structures of several representative MHC-like molecules. These include host molecules that have distinct biological functions, as well as virus-encoded molecules that contribute to the evasion of the immune response.

IL13Rα2 expression identifies tissue-resident IL-22-producing PLZF+ innate T cells in the human liver

Innate lymphocytes are selectively enriched in the liver where they have important roles in liver immunology. Murine studies have shown that type I NKT cells can promote liver inflammation, whereas type II NKT cells have an anti-inflammatory role. In humans, type II NKT cells were found to accumulate in the gut during inflammation and IL13Rα2 was proposed as a marker for these cells. In the human liver, less is known about type I and II NKT cells. Here, we studied the phenotype and function of human liver T cells expressing IL13Rα2. We found that IL13Rα2 was expressed by around 1% of liver-resident memory T cells but not on circulating T cells. In support of their innate-like T-cell character, the IL13Rα2+ T cells had higher expression of promyelocytic leukaemia zinc finger (PLZF) compared to IL13Rα2- T cells and possessed the capacity to produce IL-22. However, only a minority of human liver sulfatide-reactive type II NKT cells expressed IL13Rα2. Collectively, these findings suggest that IL13Rα2 identifies tissue-resident intrahepatic T cells with innate characteristics and the capacity to produce IL-22.

Functions of CD1d-Restricted Invariant Natural Killer T Cells in Antimicrobial Immunity and Potential Applications for Infection Control

CD1d-restricted invariant natural killer T (iNKT) cells are innate-type lymphocytes that express a T-cell receptor (TCR) containing an invariant α chain encoded by the Vα14 gene in mice and Vα24 gene in humans. These iNKT cells recognize endogenous, microbial, and synthetic glycolipid antigens presented by the major histocompatibility complex (MHC) class I-like molecule CD1d. Upon TCR stimulation by glycolipid antigens, iNKT cells rapidly produce large amounts of cytokines, including interferon-γ (IFNγ) and interleukin-4 (IL-4). Activated iNKT cells contribute to host protection against a broad spectrum of microbial pathogens, and glycolipid-mediated stimulation of iNKT cells ameliorates many microbial infections by augmenting innate and acquired immunity. In some cases, however, antigen-activated iNKT cells exacerbate microbial infections by promoting pathogenic inflammation. Therefore, it is important to identify appropriate microbial targets for the application of iNKT cell activation as a treatment or vaccine adjuvant. Many studies have found that iNKT cell activation induces potent adjuvant activities promoting protective vaccine effects. In this review, we summarize the functions of CD1d-restricted iNKT cells in immune responses against microbial pathogens and describe the potential applications of glycolipid-mediated iNKT cell activation for preventing and controlling microbial infections.

Phospholipid signaling in innate immune cells

Phospholipids comprise a large body of lipids that define cells and organelles by forming membrane structures. Importantly, their complex metabolism represents a highly controlled cellular signaling network that is essential for mounting an effective innate immune response. Phospholipids in innate cells are subject to dynamic regulation by enzymes, whose activities are highly responsive to activation status. Along with their metabolic products, they regulate multiple aspects of innate immune cell biology, including shape change, aggregation, blood clotting, and degranulation. Phospholipid hydrolysis provides substrates for cell-cell communication, enables regulation of hemostasis, immunity, thrombosis, and vascular inflammation, and is centrally important in cardiovascular disease and associated comorbidities. Phospholipids themselves are also recognized by innate-like T cells, which are considered essential for recognition of infection or cancer, as well as self-antigens. This Review describes the major phospholipid metabolic pathways present in innate immune cells and summarizes the formation and metabolism of phospholipids as well as their emerging roles in cell biology and disease.

Endogenous lipid antigens for invariant natural killer T cells hold the reins in adipose tissue homeostasis

The global obesity epidemic and its associated co-morbidities, including type 2 diabetes, cardiovascular disease and certain types of cancers, have drawn attention to the pivotal role of adipocytes in health and disease. Besides their 'classical' function in energy storage and release, adipocytes interact with adipose-tissue-resident immune cells, among which are lipid-responsive invariant natural killer T (iNKT) cells. The iNKT cells are activated by lipid antigens presented by antigen-presenting cells as CD1d/lipid complexes. Upon activation, iNKT cells can rapidly secrete soluble mediators that either promote or oppose inflammation. In lean adipose tissue, iNKT cells elicit a predominantly anti-inflammatory immune response, whereas obesity is associated with declining iNKT cell numbers. Recent work showed that adipocytes act as non-professional antigen-presenting cells for lipid antigens. Here, we discuss endogenous lipid antigen processing and presentation by adipocytes, and speculate on how these lipid antigens, together with 'environmental factors' such as tissue/organ environment and co-stimulatory signals, are able to influence the fate of adipose-tissue-resident iNKT cells, and thereby the role of these cells in obesity and its associated pathologies.

Natural Killer T Cells in Cancer Immunotherapy

Natural killer T (NKT) cells are specialized CD1d-restricted T cells that recognize lipid antigens. Following stimulation, NKT cells lead to downstream activation of both innate and adaptive immune cells in the tumor microenvironment. This has impelled the development of NKT cell-targeted immunotherapies for treating cancer. In this review, we provide a brief overview of the stimulatory and regulatory functions of NKT cells in tumor immunity as well as highlight preclinical and clinical studies based on NKT cells. Finally, we discuss future perspectives to better harness the potential of NKT cells for cancer therapy.

Bacteroides are associated with GALT iNKT cell function and reduction of microbial translocation in HIV-1 infection

Invariant natural killer T (iNKT) cells are innate-like T cells that respond to lipid antigens presented by CD1d. These immunoregulatory cells have the capacity for rapid cytokine release after antigen recognition and are essential for the activation of multiple arms of the immune response. HIV-1 infection is associated with iNKT cell depletion in the peripheral blood; however, their role in the gastrointestinal-associated lymphoid tissue (GALT) is less well studied. Our results show that iNKT cells are found at a higher frequency in GALT compared with blood, particularly in HIV-1 elite controllers. The capacity of iNKT cells to produce interleukin-4 (IL-4) and IL-10 in the GALT was associated with less immune activation and lower markers of microbial translocation, whereas regulatory T cell frequency showed positive associations with immune activation. We hypothesized that the composition of the microbiota would influence iNKT cell frequency and function. We found positive associations between the abundance of several Bacteroides species and iNKT cell frequency and their capacity to produce IL-4 in the GALT but not in the blood. Overall, our results are consistent with the hypothesis that GALT iNKT cells, influenced by certain bacterial species, may have a key role in regulating immune activation in HIV-1 infection.

Glycolipid activators of invariant NKT cells as vaccine adjuvants

Natural Killer T cells (NKT cells) are a subpopulation of T lymphocytes with unique phenotypic properties and a remarkably broad range of immune effector and regulatory functions. One subset of these cells, known as invariant NKT cells (iNKT cells), has become a significant focus in the search for new and better ways to enhance immunotherapies and vaccination. These unconventional T cells are characterized by their ability to be specifically activated by a range of foreign and self-derived glycolipid antigens presented by CD1d, an MHC class I-related antigen presenting molecule that has evolved to bind and present lipid antigens. The development of synthetic α-galactosylceramides as a family of powerful glycolipid agonists for iNKT cells has led to approaches for augmenting a wide variety of immune responses, including those involved in vaccination against infections and cancers. Here we review the basic background biology of iNKT cells that is relevant to their potential for improving immune responses, and summarize recent work supporting the further development of glycolipid activators of iNKT cells as a new class of vaccine adjuvants.

Does an NKT-cell-based immunotherapeutic approach have a future in multiple myeloma?

Natural killer T (NKT) cells constitute a unique subset of innate-like T lymphocytes which differ from conventional T cells by recognizing lipid antigens presented by the non-polymorphic major histocompatibility complex (MHC) I-like molecule CD1d. Despite being a relatively infrequent population of lymphocytes, NKT cells can respond rapidly upon activation with glycosphingolipids by production of cytokines which aim to polarize different axes of the immune system. Due to their dual effector capacities, NKT cells can play a vital role in cancer immunity, infection, inflammation and autoimmune diseases. It is believed that modulation of their activity towards immune activation can be a useful tool in anti-tumor immunotherapeutic strategies. Here we summarize the characteristics of NKT cells and discuss their involvement in immunosurveillance. Furthermore, an update is given about their role and the progress that has been made in the field of multiple myeloma (MM). Finally, some challenges are discussed that are currently hampering further progress.

The Extended Family of CD1d-Restricted NKT Cells: Sifting through a Mixed Bag of TCRs, Antigens, and Functions

Natural killer T (NKT) cells comprise a family of specialized T cells that recognize lipid antigens presented by CD1d. Based on their T cell receptor (TCR) usage and antigen specificities, CD1d-restricted NKT cells have been divided into two main subsets: type I NKT cells that use a canonical invariant TCR α-chain and recognize α-galactosylceramide (α-GalCer), and type II NKT cells that use a more diverse αβ TCR repertoire and do not recognize α-GalCer. In addition, α-GalCer-reactive NKT cells that use non-canonical αβ TCRs and CD1d-restricted T cells that use γδ or δ/αβ TCRs have recently been identified, revealing further diversity among CD1d-restricted T cells. Importantly, in addition to their distinct antigen specificities, functional differences are beginning to emerge between the different members of the CD1d-restricted T cell family. In this review, while using type I NKT cells as comparison, we will focus on type II NKT cells and the other non-invariant CD1d-restricted T cell subsets, and discuss our current understanding of the antigens they recognize, the formation of stimulatory CD1d/antigen complexes, the modes of TCR-mediated antigen recognition, and the mechanisms and consequences of their activation that underlie their function in antimicrobial responses, anti-tumor immunity, and autoimmunity.

The glycobiology of the CD system: a dictionary for translating marker designations into glycan/lectin structure and function

The profile of cell surface molecules, the biochemical platform for cellular communication, can be likened to a molecular fingerprint. Historically, raising monoclonal antibodies by immunization with cells has been instrumental in obtaining tools suited for phenotyping and functional analysis. Initially for leukocyte antigens, the resulting cluster of differentiation (CD) nomenclature has become a popular system for classification. Glycans presented on proteins or lipids and receptors for carbohydrate structures (lectins) are part of the CD list. Our review presents biochemical and biomedical highlights of the respective CD entries.

Thymic low affinity/avidity interaction selects natural Th1 cells

Identification of intrathymic eomesodermin(+) (Eomes(+)) CD4 T cells creates a novel idea that there is more than one way for the generation of innate CD4 T cells. Promyelocytic leukemia zinc finger protein(+) T cells and natural Th17 cells are known to be generated by sensing a high and persistent TCR strength, whereas this is not the case for Eomes(+) CD4 T cells. These cells go through low-level signal during the entire maturation pathway, which subsequently leads to induction of high susceptibility to cytokine IL-4. This event seems to be a major determinant for the generation of this type of cell. These T cells are functionally equivalent to Th1 cells that are present in the periphery, and this event takes place both in transgenic and in wild-type mice. There is additional evidence that this type of Eomes(+) innate CD4 T cell is also present in human cord blood.

Evasion of early antiviral responses by herpes simplex viruses

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Type II NKT-TFH cells against Gaucher lipids regulate B-cell immunity and inflammation

Chronic inflammation including B-cell activation is commonly observed in both inherited (Gaucher disease [GD]) and acquired disorders of lipid metabolism. However, the cellular mechanisms underlying B-cell activation in these settings remain to be elucidated. Here, we report that β-glucosylceramide 22:0 (βGL1-22) and glucosylsphingosine (LGL1), 2 major sphingolipids accumulated in GD, can be recognized by a distinct subset of CD1d-restricted human and murine type II natural killer T (NKT) cells. Human βGL1-22- and LGL1-reactive CD1d tetramer-positive T cells have a distinct T-cell receptor usage and genomic and cytokine profiles compared with the classical type I NKT cells. In contrast to type I NKT cells, βGL1-22- and LGL1-specific NKT cells constitutively express T-follicular helper (TFH) phenotype. Injection of these lipids leads to an increase in respective lipid-specific type II NKT cells in vivo and downstream induction of germinal center B cells, hypergammaglobulinemia, and production of antilipid antibodies. Human βGL1-22- and LGL1-specific NKT cells can provide efficient cognate help to B cells in vitro. Frequency of LGL1-specific T cells in GD mouse models and patients correlates with disease activity and therapeutic response. Our studies identify a novel type II NKT-mediated pathway for glucosphingolipid-mediated dysregulation of humoral immunity and increased risk of B-cell malignancy observed in metabolic lipid disorders.

Metabolic activation of intrahepatic CD8+ T cells and NKT cells causes nonalcoholic steatohepatitis and liver cancer via cross-talk with hepatocytes

Hepatocellular carcinoma (HCC), the fastest rising cancer in the United States and increasing in Europe, often occurs with nonalcoholic steatohepatitis (NASH). Mechanisms underlying NASH and NASH-induced HCC are largely unknown. We developed a mouse model recapitulating key features of human metabolic syndrome, NASH, and HCC by long-term feeding of a choline-deficient high-fat diet. This induced activated intrahepatic CD8(+) T cells, NKT cells, and inflammatory cytokines, similar to NASH patients. CD8(+) T cells and NKT cells but not myeloid cells promote NASH and HCC through interactions with hepatocytes. NKT cells primarily cause steatosis via secreted LIGHT, while CD8(+) and NKT cells cooperatively induce liver damage. Hepatocellular LTβR and canonical NF-κB signaling facilitate NASH-to-HCC transition, demonstrating that distinct molecular mechanisms determine NASH and HCC development.

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.

Synthesis of a 2"-deoxy-β-GalCer

Structural studies of ternary complexes of CD1d/glycosyl ceramides/iNKT cells and CD1d/sulfatide/sulfatide reactive Type II NKT cells have shown how the polar moieties on the glycolipids interact with both the antigen presenting protein (CD1d) and the T cell receptors. However, these structures alone do not reveal the relative importance of these interactions. This study focuses on the synthesis of the previously unknown 2"-deoxy-β-galactosyl ceramide 2. This glycolipid is also evaluated for its ability to stimulate iNKT cells and sulfatide-reactive Type II NKT cells.

SAP-Dependent and -Independent Regulation of Innate T Cell Development Involving SLAMF Receptors

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) plays an essential role in the immune system mediating the function of several members of the SLAM family (SLAMF) of receptors, whose expression is essential for T, NK, and B-cell responses. Additionally, the expression of SAP in double-positive thymocytes is mandatory for natural killer T (NKT) cells and, in mouse, for innate CD8⁺ T cell development. To date, only two members of the SLAMF of receptors, Slamf1 and Slamf6, have been shown to positively cooperate during NKT cell differentiation in mouse. However, it is less clear whether other members of this family may also participate in the development of these innate T cells. Here, we show that Slamf[1 + 6]^−/− and Slamf[1 + 5 + 6]^−/−B6 mice have ~70% reduction of NKT cells compared to wild-type B6 mice. Unexpectedly, the proportion of innate CD8⁺ T cells slightly increased in the Slamf[1 + 5 + 6]^−/−, but not in the Slamf[1 + 6]^−/− strain, suggesting that Slamf5 may function as a negative regulator of innate CD8⁺ T cell development. Accordingly, Slamf5^−/− B6 mice showed an exclusive expansion of innate CD8⁺ T cells, but not NKT cells. Interestingly, the SAP-independent Slamf7^−/− strain showed an expansion of both splenic innate CD8⁺ T cells and thymic NKT cells. On the other hand, and similar to what was recently shown in Slamf3^−/− BALB/c mice, the proportions of thymic promyelocytic leukemia zinc finger (PLZF^hi) NKT cells and innate CD8⁺ T cells significantly increased in the SAP-independent Slamf8^−/− BALB/c strain. In summary, these results show that NKT and innate CD8⁺ T cell development can be regulated in a SAP-dependent and -independent fashion by SLAMF receptors, in which Slamf1, Slamf6, and Slamf8 affect development of NKT cells, and that Slamf5, Slamf7, and Slamf8 affect the development of innate CD8⁺ T cells.

Synthesis and evaluation of immunostimulant plasmalogen lysophosphatidylethanolamine and analogues for natural killer T cells

Plasmalogen lysophosphatidylethanolamine (pLPE) had been identified as a self antigen for natural killer T cells (NKT cells). It is very important in the development, maturation and activation of NKT cells in thymus. Besides, pLPE is a novel type of antigen for NKT cells. To evaluate the structure-activity relationship (SAR) of this new antigen, pLPE and its analogues referred to different aliphatic chains and linkages at the sn-1 position of the glycerol backbone were synthesized, and the biological activities of these analogues was characterized. It is discovered that the linkages between phosphate and lipid moiety are not important for the antigens' activities. The pLPE analogues 1, 3, 4, 7 and 9, which have additional double bonds on lipid parts, were identified as new NKT agonists. Moreover, the analogues 4, 7 and 9 were discovered as potent Th2 activators for NKT cells.

Recognition of vitamin B metabolites by mucosal-associated invariant T cells

The mucosal-associated invariant T-cell antigen receptor (MAIT TCR) recognizes MR1 presenting vitamin B metabolites. Here we describe the structures of a human MAIT TCR in complex with human MR1 presenting a non-stimulatory ligand derived from folic acid and an agonist ligand derived from a riboflavin metabolite. For both vitamin B antigens, the MAIT TCR docks in a conserved manner above MR1, thus acting as an innate-like pattern recognition receptor. The invariant MAIT TCR α-chain usage is attributable to MR1-mediated interactions that prise open the MR1 cleft to allow contact with the vitamin B metabolite. Although the non-stimulatory antigen does not contact the MAIT TCR, the stimulatory antigen does. This results in a higher affinity of the MAIT TCR for a stimulatory antigen in comparison with a non-stimulatory antigen. We formally demonstrate a structural basis for MAIT TCR recognition of vitamin B metabolites, while illuminating how TCRs recognize microbial metabolic signatures.

The versatility of the αβ T-cell antigen receptor

The T-cell antigen receptor is a heterodimeric αβ protein (TCR) expressed on the surface of T-lymphocytes, with each chain of the TCR comprising three complementarity-determining regions (CDRs) that collectively form the antigen-binding site. Unlike antibodies, which are closely related proteins that recognize intact protein antigens, TCRs classically bind, via their CDR loops, to peptides (p) that are presented by molecules of the major histocompatibility complex (MHC). This TCR-pMHC interaction is crucially important in cell-mediated immunity, with the specificity in the cellular immune response being attributable to MHC polymorphism, an extensive TCR repertoire and a variable peptide cargo. The ensuing structural and biophysical studies within the TCR-pMHC axis have been highly informative in understanding the fundamental events that underpin protective immunity and dysfunctional T-cell responses that occur during autoimmunity. In addition, TCRs can recognize the CD1 family, a family of MHC-related molecules that instead of presenting peptides are ideally suited to bind lipid-based antigens. Structural studies within the CD1-lipid antigen system are beginning to inform us how lipid antigens are specifically presented by CD1, and how such CD1-lipid antigen complexes are recognized by the TCR. Moreover, it has recently been shown that certain TCRs can bind to vitamin B based metabolites that are bound to an MHC-like molecule termed MR1. Thus, TCRs can recognize peptides, lipids, and small molecule metabolites, and here we review the basic principles underpinning this versatile and fascinating receptor recognition system that is vital to a host's survival.

A divergent approach to the synthesis of iGb3 sugar and lipid analogues via a lactosyl 2-azido-sphingosine intermediate

Isoglobotrihexosylceramide (iGb3, 1) is an immunomodulatory glycolipid that binds to CD1d and is presented to the T-cell receptor (TCR) of invariant natural killer T (iNKT) cells.

A bidirectional crosstalk between iNKT cells and adipocytes mediated by leptin modulates susceptibility for T cell mediated hepatitis

BACKGROUND & AIMS

Immunometabolism is an emerging field of clinical investigation due to the obesity epidemic worldwide. A reciprocal involvement of immune mediators in the body energy metabolism has been recognized for years, but is only partially understood. We hypothesized that the adipokine leptin could provide an important modulator of iNKT cells.

METHODS

The expression of leptin receptor (LR) on resting and activated iNKT cells was measured by flow cytometry. FACS-sorted hepatic iNKT cells were stimulated with anti-CD3/CD28Ab coated beads in the absence or presence of a neutralizing anti-leptin Ab. Furthermore, we evaluated the outcome of LR blocking nanobody treatment in ConA induced hepatitis and towards metabolic parameters in WT and iNKT cell deficient mice.

RESULTS

The LR is expressed on iNKT cells and leptin suppresses iNKT cell proliferation and cytokine production in vitro. LR deficient iNKT cells are hyper-responsive further enforcing the role of leptin as an important inhibitor of iNKT cell function. Consistently, in vivo blockade of LR signaling exacerbated ConA hepatitis in wild-type but not in iNKT cell deficient mice, through both Janus kinase (JAK)2 and mitogen-activated protein kinase (MAPK) dependent mechanisms. Moreover, LR inhibition altered fat pad features and was accompanied by insulin resistance, only in wild-type mice. Curiously, this interaction was strictly dependent on MAPK mediated LR signaling in iNKT cells and uncoupled from the more central effects of leptin.

CONCLUSIONS

Our data support a new concept of immune regulation by which leptin protects towards T cell mediated hepatitis via modulation of iNKT cells.

Stimulation of natural killer T cells by glycolipids

Natural killer T (NKT) cells are a subset of T cells that recognize glycolipid antigens presented by the CD1d protein. The initial discovery of immunostimulatory glycolipids from a marine sponge and the T cells that respond to the compounds has led to extensive research by chemists and immunologists to understand how glycolipids are recognized, possible responses by NKT cells, and the structural features of glycolipids necessary for stimulatory activity. The presence of this cell type in humans and most mammals suggests that it plays critical roles in antigen recognition and the interface between innate and adaptive immunity. Both endogenous and exogenous natural antigens for NKT cells have been identified, and it is likely that glycolipid antigens remain to be discovered. Multiple series of structurally varied glycolipids have been synthesized and tested for stimulatory activity. The structural features of glycolipids necessary for NKT cell stimulation are moderately well understood, and designed compounds have proven to be much more potent antigens than their natural counterparts. Nevertheless, control over NKT cell responses by designed glycolipids has not been optimized, and further research will be required to fully reveal the therapeutic potential of this cell type.

Invariant natural killer T cells in adipose tissue: novel regulators of immune-mediated metabolic disease

Adipose tissue (AT) represents a microenvironment where intersection takes place between immune processes and metabolic pathways. A variety of immune cells have been characterized in AT over the past decades, with the most recent addition of invariant natural killer T (iNKT) cells. As members of the T cell family, iNKT cells represent a subset that exhibits both innate and adaptive characteristics and directs ensuing immune responses. In disease conditions, iNKT cells have established roles that include disorders in the autoimmune spectrum in malignancies and infectious diseases. Recent work supports a role for iNKT cells in the maintenance of AT homeostasis through both immune and metabolic pathways. The deficiency of iNKT cells can result in AT metabolic disruptions and insulin resistance. In this review, we summarize recent work on iNKT cells in immune regulation, with an emphasis on AT-resident iNKT cells, and identify the potential mechanisms by which adipocytes can mediate iNKT cell activity.

Invariant natural killer T cells recognize a fungal glycosphingolipid that can induce airway hyperreactivity

Aspergillus fumigatusis a saprophytic fungus that is ubiquitous in the environment and commonly associated with allergic sensitization and severe asthma in humans. Although A. fumigatus is recognized by multiple microbial pattern recognition receptors, we identified and synthesized an A. fumigatus glycosphingolipid, asperamide B, that directly activated invariant natural killer T (iNKT) cells in vitro in a CD1d-restricted, MyD88- and dectin-1-independent fashion. Moreover, asperamide B, when loaded into CD1d, directly stained, and was sufficient to activate, iNKT cells. In vivo, asperamide B rapidly induced airway hyperreactivity, a cardinal feature of asthma, by activating pulmonary iNKT cells in an IL-33-ST2-dependent fashion. Asperamide B is thus the first fungal glycolipid found to directly activate iNKT cells. These results extend the range of microorganisms that can be directly detected by iNKT cells to the Kingdom of Fungi, and may explain the effectiveness of A. fumigatus in causing severe chronic respiratory diseases in humans.

Predisposed αβ T cell antigen receptor recognition of MHC and MHC-I like molecules?

The diverse αβ T cell receptor (TCR) repertoire exhibits versatility in its ability to generate antigen (Ag) receptors capable of interacting with polymorphic Major Histocompatibility Complex (MHC) molecules and monomorphic MHC-I like molecules, including the CD1 and MR1 families. Collectively, these evolutionarily related Ag-presenting molecules present peptides, lipids and vitamin B metabolites for T cell surveillance. Interestingly, whilst common TCR gene usage can underpin recognition of these distinct classes of Ags, it is unclear whether the 'rules' that govern αβTCR-Ag MHC interactions are shared. We highlight recent observations in the context of TCR biases towards MHC and MHC-I like molecules.

Immune system targeting by biodegradable nanoparticles for cancer vaccines

The concept of therapeutic cancer vaccines is based on the activation of the immune system against tumor cells after the presentation of tumor antigens. Nanoparticles (NPs) have shown great potential as delivery systems for cancer vaccines as they potentiate the co-delivery of tumor-associated antigens and adjuvants to dendritic cells (DCs), insuring effective activation of the immune system against tumor cells. In this review, the immunological mechanisms behind cancer vaccines, including the role of DCs in the stimulation of T lymphocytes and the use of Toll-like receptor (TLR) ligands as adjuvants will be discussed. An overview of each of the three essential components of a therapeutic cancer vaccine - antigen, adjuvant and delivery system - will be provided with special emphasis on the potential of particulate delivery systems for cancer vaccines, in particular those made of biodegradable aliphatic polyesters, such as poly(lactic-co-glycolic acid) (PLGA) and poly-ε-caprolactone (PCL). Some of the factors that can influence NP uptake by DCs, including size, surface charge, surface functionalization and route of administration, will also be considered.

The adaptable major histocompatibility complex (MHC) fold: structure and function of nonclassical and MHC class I-like molecules

The MHC fold is found in proteins that have a range of functions in the maintenance of an organism's health, from immune regulation to fat metabolism. Well adapted for antigen presentation, as seen for peptides in the classical MHC molecules and for lipids in CD1 molecules, the MHC fold has also been modified to perform Fc-receptor activity (e.g., FcRn) and for roles in host homeostasis (e.g., with HFE and ZAG). The more divergent MHC-like molecules, such as some of those that interact with the NKG2D receptor, represent the minimal MHC fold, doing away with the α3 domain and β2m while maintaining the α1/α2 platform domain for receptor engagement. Viruses have also co-opted the MHC fold for immune-evasive functions. The variations on the theme of a β-sheet topped by two semiparallel α-helices are discussed in this review, highlighting the fantastic adaptability of this fold for good and for bad.