Activation of iNKT cells by a distinct constituent of the endogenous glucosylceramide fraction

Lipidomics Study of Type 1 Diabetic Rats Using Online Phase Transition Trapping-Supercritical Fluid Extraction-Chromatography Coupled with Quadrupole Time-of-Flight Tandem Mass Spectrometry

Chromatography-mass spectrometry-based lipidomics represents an essential tool for elucidating lipid dysfunction mechanisms and is extensively employed in investigating disease mechanisms and identifying biomarkers. However, the detection of low-abundance lipids in biological matrices, along with cumbersome operational procedures, complicates comprehensive lipidomic analyses, necessitating the development of highly sensitive, environmentally friendly, and automated methods. In this study, an online phase transition trapping-supercritical fluid extraction-chromatography-mass spectrometry (PTT-SFEC-MS/MS) method was developed and successfully applied to plasma lipidomics analysis in Type 1 diabetes (T1D) rats. The PTT strategy captured entire extracts at the column head by converting CO2 from a supercritical state to a gaseous state, thereby preventing peak spreading, enhancing peak shape for precise quantification, and boosting sensitivity without any sample loss. This method utilized only 5 μL of plasma and accomplished sample extraction, separation, and detection within 27 min. Ultimately, 77 differential lipids were identified, including glycerophospholipids, sphingolipids, and glycerolipids, in T1D rat plasma. The results indicated that the progression of the disease might be linked to alterations in glycerophospholipid and sphingolipid metabolism. Our findings demonstrated a green, highly efficient, and automated method for the lipidomics analysis of biological samples, providing a scientific foundation for understanding the pathogenesis and diagnosis of T1D.

Die Kämpfe únd schláchten-the struggles and battles of innate-like effector T lymphocytes with microbes

The large majority of lymphocytes belong to the adaptive immune system, which are made up of B2 B cells and the αβ T cells; these are the effectors in an adaptive immune response. A multitudinous group of lymphoid lineage cells does not fit the conventional lymphocyte paradigm; it is the unconventional lymphocytes. Unconventional lymphocytes-here called innate/innate-like lymphocytes, include those that express rearranged antigen receptor genes and those that do not. Even though the innate/innate-like lymphocytes express rearranged, adaptive antigen-specific receptors, they behave like innate immune cells, which allows them to integrate sensory signals from the innate immune system and relay that umwelt to downstream innate and adaptive effector responses. Here, we review natural killer T cells and mucosal-associated invariant T cells-two prototypic innate-like T lymphocytes, which sense their local environment and relay that umwelt to downstream innate and adaptive effector cells to actuate an appropriate host response that confers immunity to infectious agents.

Roles and therapeutic potential of CD1d-Restricted NKT cells in inflammatory skin diseases

Natural killer T (NKT) cells are innate-like T lymphocytes that recognize glycolipid antigens rather than peptides. Due to their immunoregulatory properties, extensive work has been done to elucidate the immune functions of NKT cells in various immune contexts such as autoimmunity for more than two decades. In addition, as research on barrier immunity such as the mucosa-associated lymphoid tissue has flourished in recent years, the role of NKT cells to immunity in the skin has attracted substantial attention. Here, we review the contributions of NKT cells to regulating skin inflammation and discuss the factors that can modulate the functions of NKT cells in inflammatory skin diseases such as atopic dermatitis. This mini-review article will mainly focus on CD1d-dependent NKT cells and their therapeutic potential in skin-related immune diseases.

Innate and Innate-like Effector Lymphocytes in Health and Disease

Lymphocytes can be functionally partitioned into subsets belonging to the innate or adaptive arms of the immune system. Subsets of innate and innate-like lymphocytes may or may not express Ag-specific receptors of the adaptive immune system, yet they are poised to respond with innate-like speed to pathogenic insults but lack the capacity to develop classical immunological memory. These lymphocyte subsets display a number of common properties that permit them to integrate danger and stress signals dispatched by innate sensor cells to facilitate the generation of specialized effector immune responses tailored toward specific pathogens or other insults. In this review, we discuss the functions of distinct subsets of innate and innate-like lymphocytes. A better understanding of the mechanisms by which these cells are activated in different contexts, their interactions with other immune cells, and their role in health and disease may inform the development of new or improved immunotherapies.

Deconstructing iNKT cell development at single-cell resolution

Invariant natural killer T (iNKT) cells are increasingly regarded as disease biomarkers and immunotherapeutic targets. However, a greater understanding of their biology is necessary to effectively target these cells in the clinic. The discovery of iNKT1/2/17 cell effector subsets was a milestone in our understanding of iNKT cell development and function. Recent transcriptomic studies have uncovered an even greater heterogeneity and challenge our understanding of iNKT cell ontogeny and effector differentiation. We propose a refined model whereby iNKT cells differentiate through a dynamic and continuous instructive process that requires the accumulation and integration of various signals within the thymus or peripheral tissues. Within this framework, we question the existence of true iNKT2 cells and discuss the parallels between mouse and human iNKT cells.

Identification of T cell antigens in the 21st century, as difficult as ever

Identifying antigens recognized by T cells is still challenging, particularly for innate like T cells that do not recognize peptides but small metabolites or lipids in the context of MHC-like molecules or see non-MHC restricted antigens. The fundamental reason for this situation is the low affinity of T cell receptors for their ligands coupled with a level of degeneracy that makes them bind to similar surfaces on antigen presenting cells. Herein we will describe non-exhaustively some of the methods that were used to identify peptide antigens and briefly mention the high throughput methods more recently proposed for that purpose. We will then present how the molecules recognized by innate like T cells (NKT, MAIT and γδ T cells) were discovered. We will show that serendipity was instrumental in many cases.

NKT Cells Contribute to the Control of Microbial Infections

Innate (-like) T lymphocytes such as natural killer T (NKT) cells play a pivotal role in the recognition of microbial infections and their subsequent elimination. They frequently localize to potential sites of pathogen entry at which they survey extracellular and intracellular tissue spaces for microbial antigens. Engagement of their T cell receptors (TCRs) induces an explosive release of different cytokines and chemokines, which often pre-exist as constitutively expressed gene transcripts in NKT cells and underlie their poised effector state. Thus, NKT cells regulate immune cell migration and activation and subsequently, bridge innate and adaptive immune responses. In contrast to conventional T cells, which react to peptide antigens, NKT cells recognize lipids presented by the MHC class I like CD1d molecule on antigen presenting cells (APCs). Furthermore, each NKT cell TCR can recognize various antigen specificities, whereas a conventional T lymphocyte TCR reacts mostly only to one single antigen. These lipid antigens are either intermediates of the intracellular APC`s-own metabolism or originate from the cell wall of different bacteria, fungi or protozoan parasites. The best-characterized subset, the type 1 NKT cell subset expresses a semi-invariant TCR. In contrast, the TCR repertoire of type 2 NKT cells is diverse. Furthermore, NKT cells express a panoply of inhibitory and activating NK cell receptors (NKRs) that contribute to their primarily TCR-mediated rapid, innate like immune activation and even allow an adaption of their immune response in an adoptive like manner. Dueto their primary localization at host-environment interfaces, NKT cells are one of the first immune cells that interact with signals from different microbial pathogens. Vice versa, the mutual exchange with local commensal microbiota shapes also the biology of NKT cells, predominantly in the gastrointestinal tract. Following infection, two main signals drive the activation of NKT cells: first, cognate activation upon TCR ligation by microbial or endogenous lipid antigens; and second, bystander activation due to cytokines. Here we will discuss the role of NKT cells in the control of different microbial infections comparing pathogens expressing lipid ligands in their cell walls to infectious agents inducing endogenous lipid antigen presentation by APCs.

New Paradigm in NKT Cell Antigens: MCS-0208 (2-(Hydroxymethyl)phenylthio-phytoceramide) - an Aryl-Phytoceramide Compound with a Single Hydroxyl Group Stimulates NKT Cells

Natural Killer T (NKT) cells play an important role in the immune response and can be activated by glycolipids presented by CD1d protein. We present MCS‐0208, an unprecedented arylthioether‐phytoceramide able to induce potent invariant NKT (iNKT) cell activation, notably when tested in human iNKT cells. This arylsphingolipid analog has a simple phenyl group containing a single hydroxyl substituent as a surrogate of the sugar ring. The phenylthioether structure contrasts with α‐galactosylceramide (1), the prototypical glycolipid used to induce iNKT cell stimulation, where the galactose 2’‐OH and 3’‐OH substituents are accepted as the minimal footprint and considered critical for NKT T cell receptor (TCR) recognition. A computational study supports the recognition of aromatic compound by the CD1d and TCR proteins as radically new structures for NKT cell stimulation.

Ceramide glycosylation and related enzymes in cancer signaling and therapy

Ceramides, the core of the sphingolipid metabolism, draw wide attention as tumor suppressor, and act directly on mitochondria to trigger apoptotic cell death. Ceramide-based therapies are being developed by using promote ceramide generating agents. The ceramide metabolism balance is regulated by multifaceted factors in cancer development. Ceramide metabolic enzymes can increase the elimination of ceramide and counteract the anti-tumor effects of ceramide. However, recent research showed that these metabolic enzymes were highly expressed in several cancers. Especially ceramide glycosyltransferases, they catalyze ceramide glycosylation and synthesis the skeleton of glycosphingolipids (GSLs), play an important role in regulating tumor progression and have a significant correlation with the poor prognosis of cancer patients. To further understand the biological characteristics of ceramide metabolism in tumor, this review focuses on the role of ceramide glycosylation and related enzymes in cancer signaling and therapy. Besides, the research on multidrug resistance and potential inhibitors of ceramide glycosyltransferases are also discussed. Advance study on the structure of ceramide glycosyltransferases and ceramide glycosylation signaling pathway will open the path to new therapies and treatments.

Antigen-Presenting Cells in Food Tolerance and Allergy

Food allergy now affects 6%-8% of children in the Western world; despite this, we understand little about why certain people become sensitized to food allergens. The dominant form of food allergy is mediated by food-specific immunoglobulin E (IgE) antibodies, which can cause a variety of symptoms, including life-threatening anaphylaxis. A central step in this immune response to food antigens that differentiates tolerance from allergy is the initial priming of T cells by antigen-presenting cells (APCs), primarily different types of dendritic cells (DCs). DCs, along with monocyte and macrophage populations, dictate oral tolerance versus allergy by shaping the T cell and subsequent B cell antibody response. A growing body of literature has shed light on the conditions under which antigen presentation occurs and how different types of T cell responses are induced by different APCs. We will review APC subsets in the gut and discuss mechanisms of APC-induced oral tolerance versus allergy to food identified using mouse models and patient samples.

Natural Killer T Lymphocytes Integrate Innate Sensory Information and Relay Context to Effector Immune Responses

It is now appreciated that a group of lymphoid lineage cells, collectively called innate-like effector lymphocytes, have evolved to integrate information relayed by the innate sensory immune system about the state of the local tissue environment and to pass on this context to downstream effector innate and adaptive immune responses. Thereby, innate functions engrained into such innate-like lymphoid lineage cells during development can control the quality and magnitude of an immune response to a tissue-altering pathogen and facilitate the formation of memory engrams within the immune system. These goals are accomplished by the innate lymphoid cells that lack antigen-specific receptors, γδ T cell receptor (TCR)-expressing T cells, and several αβ TCR-expressing T cell subsets-such as natural killer T cells, mucosal-associated invariant T cells, et cetera. Whilst we briefly consider the commonalities in the origins and functions of these diverse lymphoid subsets to provide context, the primary topic of this review is to discuss how the semi-invariant natural killer T cells got this way in evolution through lineage commitment and onward ontogeny. What emerges from this discourse is the question: Has a "limbic immune system" emerged (screaming quietly in plain sight!) out of what has been dubbed "in-betweeners"?

Off-the-Shelf Allogeneic T Cell Therapies for Cancer: Opportunities and Challenges Using Naturally Occurring "Universal" Donor T Cells

Chimeric antigen receptor (CAR) engineered T cell therapies individually prepared for each patient with autologous T cells have recently changed clinical practice in the management of B cell malignancies. Even though CARs used to redirect polyclonal T cells to the tumor are not HLA restricted, CAR T cells are also characterized by their endogenous T cell receptor (TCR) repertoire. Tumor-antigen targeted TCR-based T cell therapies in clinical trials are thus far using “conventional” αβ-TCRs that recognize antigens presented as peptides in the context of the major histocompatibility complex. Thus, both CAR- and TCR-based adoptive T cell therapies (ACTs) are dictated by compatibility of the highly polymorphic HLA molecules between donors and recipients in order to avoid graft-versus-host disease and rejection. The development of third-party healthy donor derived well-characterized off-the-shelf cell therapy products that are readily available and broadly applicable is an intensive area of research. While genome engineering provides the tools to generate “universal” donor cells that can be redirected to cancers, we will focus our attention on third-party off-the-shelf strategies with T cells that are characterized by unique natural features and do not require genome editing for safe administration. Specifically, we will discuss the use of virus-specific T cells, lipid-restricted (CD1) T cells, MR1-restricted T cells, and γδ-TCR T cells. CD1- and MR1-restricted T cells are not HLA-restricted and have the potential to serve as a unique source of universal TCR sequences to be broadly applicable in TCR-based ACT as their targets are presented by the monomorphic CD1 or MR1 molecules on a wide variety of tumor types. For each cell type, we will summarize the stage of preclinical and clinical development and discuss opportunities and challenges to deliver off-the-shelf targeted cellular therapies against cancer.

Hypersensitivities following allergen antigen recognition by unconventional T cells

Conventional T cells recognise protein-derived antigens in the context of major histocompatibility complex (MHC) class Ia and class II molecules and provide anti-microbial and anti-tumour immunity. Conventional T cells have also been implicated in type IV (also termed delayed-type or T cell-mediated) hypersensitivity reactions in response to protein-derived allergen antigens. In addition to conventional T cells, subsets of unconventional T cells exist, which recognise non-protein antigens in the context of monomorphic MHC class I-like molecules. These include T cells that are restricted to the cluster of differentiation 1 (CD1) family members, known as CD1-restricted T cells, and mucosal-associated invariant T cells (MAIT cells) that are restricted to the MHC-related protein 1 (MR1). Compared with conventional T cells, much less is known about the immune functions of unconventional T cells and their role in hypersensitivities. Here, we review allergen antigen presentation by MHC-I-like molecules, their recognition by unconventional T cells, and the potential role of unconventional T cells in hypersensitivities. We also speculate on possible scenarios of allergen antigen presentation by MHC-I-like molecules to unconventional T cells, the hallmarks of such responses, and the expected frequencies of hypersensitivities within the human population.

Metabolic alterations in immune cells associate with progression to type 1 diabetes

AIMS/HYPOTHESIS

Previous metabolomics studies suggest that type 1 diabetes is preceded by specific metabolic disturbances. The aim of this study was to investigate whether distinct metabolic patterns occur in peripheral blood mononuclear cells (PBMCs) of children who later develop pancreatic beta cell autoimmunity or overt type 1 diabetes.

METHODS

In a longitudinal cohort setting, PBMC metabolomic analysis was applied in children who (1) progressed to type 1 diabetes (PT1D, n = 34), (2) seroconverted to ≥1 islet autoantibody without progressing to type 1 diabetes (P1Ab, n = 27) or (3) remained autoantibody negative during follow-up (CTRL, n = 10).

RESULTS

During the first year of life, levels of most lipids and polar metabolites were lower in the PT1D and P1Ab groups compared with the CTRL group. Pathway over-representation analysis suggested alanine, aspartate, glutamate, glycerophospholipid and sphingolipid metabolism were over-represented in PT1D. Genome-scale metabolic models of PBMCs during type 1 diabetes progression were developed by using publicly available transcriptomics data and constrained with metabolomics data from our study. Metabolic modelling confirmed altered ceramide pathways, known to play an important role in immune regulation, as specifically associated with type 1 diabetes progression.

CONCLUSIONS/INTERPRETATION

Our data suggest that systemic dysregulation of lipid metabolism, as observed in plasma, may impact the metabolism and function of immune cells during progression to overt type 1 diabetes.

DATA AVAILABILITY

The GEMs for PBMCs have been submitted to BioModels (www.ebi.ac.uk/biomodels/), under accession number MODEL1905270001. The metabolomics datasets and the clinical metadata generated in this study were submitted to MetaboLights (https://www.ebi.ac.uk/metabolights/), under accession number MTBLS1015.

Invariant NKT cells facilitate cytotoxic T-cell activation via direct recognition of CD1d on T cells

Invariant natural killer T (iNKT) cells are a major subset of NKT cells that recognize foreign and endogenous lipid antigens presented by CD1d. Although iNKT cells are characteristically autoreactive to self-antigens, the role of iNKT cells in the regulation of cytotoxic T lymphocytes (CTL) has been elucidated using α-galactosylceramide (α-GalCer), a strong synthetic glycolipid that is presented by professional antigen presenting cells (APCs), such as dendritic cells. Despite the well-known effects of α-GalCer and dendritic cells on lipid antigen presentation, the physiological role of endogenous antigens presented by CTLs during crosstalk with iNKT cells has not yet been addressed. In this study, we found that antigen-primed CTLs with transient CD1d upregulation could present lipid self-antigens to activate the iNKT cell production of IFN-γ. CTL-mediated iNKT cell activation in turn enhanced IFN-γ production and the proliferation and cytotoxicity of CTLs. We also found that the direct interaction of iNKT cells and CTLs enhanced the antitumor immune responses of CTLs. This partially explains the functional role of iNKT cells in CTL-mediated antitumor immunity. Our findings suggest that in the absence of exogenous iNKT cell ligands, iNKT cells enhanced the CTL production of IFN-γ and CTL proliferation and cytotoxicity via direct interaction with CD1d expressed on T cells without interacting with APCs.

Cancer-killing T cells engage in a form of molecular crosstalk with other specialized immune cells to enhance anti-tumor immune responses in mice. Se-Ho Park and colleagues from Korea University in Seoul, South Korea, studied a specialized population of immune cells known as invariant natural killer T (iNKT) cells, which serve as important mediators of tumor surveillance. They showed that iNKT cells directly interact with cancer-killing cytotoxic T cells through surface molecules and not through other immune cells. The resulting activation of iNKT cells leads to the production of a pro-inflammatory signaling molecule, which in turn enhances the proliferation and killing potential of the cytotoxic T cells, ultimately producing more potent tumor control in a mouse model of lymphoma. The findings could aid in the development of iNKT-based cancer immunotherapies.

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.

Bacterial immunogenic α-galactosylceramide identified in the murine large intestine: dependency on diet and inflammation

The glycosphingolipid, α-galactosylceramide (αGalCer), when presented by CD1d on antigen-presenting cells, efficiently activates invariant natural killer T (iNKT) cells. Thereby, it modulates immune responses against tumors, microbial and viral infections, and autoimmune diseases. Recently, the production of αGalCer by Bacteroidetes from the human gut microbiome was elucidated. Using hydrophilic interaction chromatography coupled to MS², we screened murine intestinal tracts to identify and quantify αGalCers, and we investigated the αGalCer response to different dietary and physiologic conditions. In both the cecum and the colon of mice, we found 1-15 pmol of αGalCer per milligram of protein; in contrast, mice lacking microbiota (germ-free mice) and fed identical diet did not harbor αGalCer. The identified αGalCer contained a β(R)-hydroxylated hexadecanoyl chain N-linked to C18-sphinganine, which differed from what has been reported with Bacteroides fragilis Unlike β-anomeric structures, but similar to αGalCers from B. fragilis, the synthetic form of the murine αGalCer induced iNKT cell activation in vitro. Last, we observed a decrease in αGalCer production in mice exposed to conditions that alter the composition of the gut microbiota, including Western type diet, colitis, and influenza A virus infection. Collectively, this study suggests that αGalCer is produced by commensals in the mouse intestine and reveals that stressful conditions causing dysbiosis alter its synthesis. The consequences of this altered production on iNKT cell-mediated local and systemic immune responses are worthy of future studies.

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.

Enhancing T cell responses and tumour immunity by vaccination with peptides conjugated to a weak NKT cell agonist

Activated NKT cells can stimulate antigen-presenting cells leading to enhanced peptide antigen-specific immunity. However, administration of potent NKT cell agonists like α-galactosylceramide (α-GalCer) can be associated with release of high levels of cytokines, and in some situations, hepatotoxicity. Here we show that it is possible to provoke sufficient NKT cell activity to stimulate strong antigen-specific T cell responses without these unwanted effects. This was achieved by chemically conjugating antigenic peptides to α-galactosylphytosphingosine (α-GalPhs), an NKT cell agonist with very weak activity based on structural characterisation and biological assays. Conjugation improved delivery to antigen-presenting cells in vivo, while use of a cathepsin-sensitive linker to release the α-GalPhs and peptide within the same cell promoted strong T cell activation and therapeutic anti-tumour responses in mice. The conjugates activated human NKT cells and enhanced human T cell responses to a viral peptide in vitro. Accordingly, we have demonstrated a means to safely exploit the immunostimulatory properties of NKT cells to enhance T cell activation for virus- and tumour-specific immunity.

TLR9-mediated dendritic cell activation uncovers mammalian ganglioside species with specific ceramide backbones that activate invariant natural killer T cells

CD1d-restricted invariant natural killer T (iNKT) cells represent a heterogeneous population of lipid-reactive T cells that are involved in many immune responses, mediated through T-cell receptor (TCR)–dependent and/or independent activation. Although numerous microbial lipid antigens (Ags) have been identified, several lines of evidence have suggested the existence of relevant Ags of endogenous origin. However, the identification of their precise nature as well as the molecular mechanisms involved in their generation are still highly controversial and ill defined. Here, we identified two mammalian gangliosides—namely monosialoganglioside GM3 and disialoganglioside GD3—as endogenous activators for mouse iNKT cells. These glycosphingolipids are found in Toll-like receptor-stimulated dendritic cells (DC) as several species varying in their N-acyl fatty chain composition. Interestingly, their ability to activate iNKT cells is highly dependent on the ceramide backbone structure. Thus, both synthetic GM3 and GD3 comprising a d18:1-C24:1 ceramide backbone were able to activate iNKT cells in a CD1d-dependent manner. GM3 and GD3 are not directly recognized by the iNKT TCR and required the Ag presenting cell intracellular machinery to reveal their antigenicity. We propose a new concept in which iNKT cells can rapidly respond to pre-existing self-molecules after stress-induced structural changes in CD1d-expressing cells. Moreover, these gangliosides conferred partial protection in the context of bacterial infection. Thus, this report identified new biologically relevant lipid self-Ags for iNKT cells.

Although the existence of self-antigens for invariant Natural Killer T (iNKT) cells is widely accepted, their precise nature remains a matter of debate. This study shows that two mammalian ganglioside species activate iNKT cells in a CD1d-dependent manner.

Invariant natural killer T (iNKT) cells are a population of unconventional T lymphocytes that activate rapidly during inflammation due to their innate-like features. They are unconventional since they do not react to peptidic antigens (Ags) presented by classical major histocompatibility complex (MHC) molecules; instead, they recognize lipid-based Ags in the context of the MHC class I-like molecule CD1d. While numerous Ags of microbial origins have been described, their endogenous Ags are far less understood and remain a matter of strong debate. Here, we report that engagement of an innate receptor on the Ag-presenting cells leads to modulation of their lipid metabolism. This results in an enrichment of particular glycosphingolipid species that differ in both the nonpolar tail and polar head structures. Among those, two species have the potential to activate iNKT cells in a CD1d-dependent manner after further intracellular modifications. Based on these data, we propose a concept that iNKT cells can rapidly respond to pre-existing self-molecules after stress-induced changes in CD1d-expressing cells. Given the presence of closely related molecules in some pathological conditions such as cancer, it will be interesting to evaluate the biological relevance of these Ags in disease states.

Innate T cells in the intensive care unit

Rapid onset of acute inflammation is a hallmark of critical illnesses that bring patients to the intensive care unit (ICU). In critical illness, innate T cells rapidly reach full activation and drive a robust acute inflammatory response. As "cellular adjuvants," innate T cells worsen inflammation and mortality in several common critical illnesses including sepsis, ischemia-reperfusion injury, stroke, and exacerbations of respiratory disease. Interestingly, innate T cell subsets can also promote a protective and anti-inflammatory response in sepsis, ischemia-reperfusion injury, and asthma. Therapies that target innate T cells have been validated in several models of critical illness. Here, we review the role of natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells and γδ T cells in clinical and experimental critical illness.

Self-glycerophospholipids activate murine phospholipid-reactive T cells and inhibit iNKT cell activation by competing with ligands for CD1d loading

Glycosphingolipids and glycerophospholipids bind CD1d. Glycosphingolipid-reactive invariant NKT-cells (iNKT) exhibit myriad immune effects, however, little is known about the functions of phospholipid-reactive T cells (PLT). We report that the normal mouse immune repertoire contains αβ T cells, which recognize self-glycerophospholipids such as phosphatidic acid (PA) in a CD1d-restricted manner and don't cross-react with iNKT-cell ligands. PA bound to CD1d in the absence of lipid transfer proteins. Upon in vivo priming, PA induced an expansion and activation of T cells in Ag-specific manner. Crystal structure of the CD1d:PA complex revealed that the ligand is centrally located in the CD1d-binding groove opening for TCR recognition. Moreover, the increased flexibility of the two acyl chains in diacylglycerol ligands and a less stringent-binding orientation for glycerophospholipids as compared with the bindings of glycosphingolipids may allow glycerophospholipids to readily occupy CD1d. Indeed, PA competed with α-galactosylceramide to load onto CD1d, leading to reduced expression of CD1d:α-galactosylceramide complexes on the surface of dendritic cells. Consistently, glycerophospholipids reduced iNKT-cell proliferation, expansion, and cytokine production in vitro and in vivo. Such superior ability of self-glycerophospholipids to compete with iNKT-cell ligands to occupy CD1d may help maintain homeostasis between the diverse subsets of lipid-reactive T cells, with important pathogenetic and therapeutic implications.

Unconventional T Cell Targets for Cancer Immunotherapy

Most studies on the immunotherapeutic potential of T cells have focused on CD8 and CD4 T cells that recognize peptide antigens (Ag) presented by polymorphic major histocompatibility complex (MHC) class I and MHC class II molecules, respectively. However, unconventional T cells, which interact with MHC class Ib and MHC-I like molecules, are also implicated in tumor immunity, although their role therein is unclear. These include unconventional T cells targeting MHC class Ib molecules such as HLA-E and its murine ortholog Qa-1b, natural killer T (NKT) cells, mucosal associated invariant T (MAIT) cells, and γδ T cells. Here, we review the current understanding of the roles of these unconventional T cells in tumor immunity and discuss why further studies into the immunotherapeutic potential of these cells is warranted.

Lipid-Reactive T Cells in Immunological Disorders of the Lung

Regulation of T cell-mediated immunity in the lungs is critical for prevention of immune-related lung disorders and for host protection from pathogens. While the prevalent view of pulmonary T cell responses is based on peptide recognition by antigen receptors, called T cell receptors (TCR), on the T cell surface in the context of classical major histocompatibility complex (MHC) molecules, novel pathways involving the presentation of lipid antigens by cluster of differentiation 1 (CD1) molecules to lipid-reactive T cells are emerging as key players in pulmonary immune system. Whereas, genetic conservation of group II CD1 (CD1d) in mouse and human genomes facilitated numerous in vivo studies of CD1d-restricted invariant natural killer T (iNKT) cells in lung diseases, the recent development of human CD1-transgenic mice has made it possible to examine the physiological roles of group I CD1 (CD1a-c) molecules in lung immunity. Here, we discuss current understanding of the biology of CD1-reactive T cells with a specific focus on their roles in several pulmonary disorders.

Innate T Cells Govern Adipose Tissue Biology

During the past 25 y, the immune system has appeared as a key regulator of adipose tissue biology and metabolic homeostasis. In lean animals, adipose-resident leukocytes maintain an anti-inflammatory microenvironment that preserves the proper functioning of the tissue. In this review, we describe two populations of innate T cells enriched in adipose tissue, invariant NKT and γδ T cells, and how they serve overlapping and nonredundant roles in controlling adipose tissue functions. These cells interact with and expand anti-inflammatory regulatory T cells and M2 macrophages, thereby driving a metabolically beneficial tissue milieu. Surprisingly, we have found that adipose invariant NKT and γδ T cells also promote weight loss and heat production in a process called "nonshivering thermogenesis." The data surrounding these two cell types highlight their powerful ability to regulate not only other leukocytes, but also tissue-wide processes that affect an entire organism.

Induction of antiinflammatory purinergic signaling in activated human iNKT cells

Invariant natural killer T (iNKT) cells are activated at sites of local tissue injury, or globally during vaso-occlusive episodes of sickle cell disease (SCD). Tissue damage stimulates production of CD1d-restricted lipid antigens that activate iNKT cells to produce Th1- and Th2-type cytokines. Here, we show that circulating iNKT cells in SCD patients express elevated levels of the ectonucleoside triphosphate diphosphosphohydrolase, CD39, as well the adenosine A2A receptor (A2AR). We also investigated the effects of stimulating cultured human iNKT cells on the expression of genes involved in the regulation of purinergic signaling. iNKT cell stimulation caused induction of ADORA2A, P2RX7, CD38, CD39, ENPP1, CD73, PANX1, and ENT1. Transcription of ADA, which degrades adenosine, was reduced. Induction of CD39 mRNA was associated with increased ecto-ATPase activity on iNKT cells that was blocked by POM1. Exposure of iNKT cells to A2AR agonists during stimulation reduced production of IFN-γ and enhanced production of IL-13 and CD39. Based on these findings, we define "purinergic Th2-type cytokine bias" as an antiinflammatory purinergic response to iNKT cell stimulation resulting from changes in the transcription of several genes involved in purine release, extracellular metabolism, and signaling.

Gut Sphingolipid Composition as a Prelude to Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) remains a major challenge in neonatology. Little is known about NEC pathophysiology apart from the presence of pre-event gut dysbiosis. Here, we applied broad range metabolomics to stools obtained 1-5 days before NEC developed from 9 cases (9 samples) and 19 (32 samples) controls matched for gestational age at birth and birth weight. The 764 identified metabolites identified six pathways that differ between cases and controls. We pursued sphingolipid metabolism because cases had decreased ceramides and increased sphingomyelins compared to controls, and because of the relevance of sphingolipids to human inflammatory disorders. Targeted analysis of samples from 23 cases and 46 controls confirmed the initial broad range observations. While metabolites provided only 73% accuracy of classification by machine learning, hierarchical clustering defined a sphingolipid associated grouping that contained 60% of the cases but only 13% of the controls, possibly identifying a pathophysiologically distinct subset of NEC. The clustering did not associate with any of the analyzed clinical and sample variables. We conclude that there are significant changes in sphingolipid metabolism components in pre-NEC stools compared to controls, but our data urge circumspection before using sphingolipids as broadly applicable predictive biomarkers.

The interaction between invariant Natural Killer T cells and the mucosal microbiota

The surface of mammalian bodies is colonized by a multitude of microbial organisms, which under normal conditions support the host and are considered beneficial commensals. This requires, however, that the composition of the commensal microbiota is tightly controlled and regulated. The host immune system plays an important role in the maintenance of this microbiota composition. Here we focus on the contribution of one particular immune cell type, invariant Natural Killer T (iNKT) cells, in this process. The iNKT cells are a unique subset of T cells characterized by two main features. First, they express an invariant T-cell receptor that recognizes glycolipid antigens presented by CD1d, a non-polymorphic major histocompatibility complex class I-like molecule. Second, iNKT cells develop as effector/memory cells and swiftly exert effector functions, like cytokine production and cytotoxicity, after activation. We outline the influence that the mucosal microbiota can have on iNKT cells, and how iNKT cells contribute to the maintenance of the microbiota composition.

New developments in patients with eosinophilic gastrointestinal diseases presented at the CEGIR/TIGERS Symposium at the 2018 American Academy of Allergy, Asthma & Immunology Meeting

The Consortium of Eosinophilic Gastrointestinal Diseases and the International Gastrointestinal Eosinophil Researchers organized a day-long symposium at the recent 2018 Annual Meeting of the American Academy of Allergy, Asthma & Immunology, which was coupled for the first time with the World Allergy Organization meeting to create an international platform. The symposium featured experts in many facets of eosinophilic gastrointestinal diseases, including allergy, immunology, gastroenterology, pathology, and nutrition, and was a well-attended event. The basic science, genetics, cellular immunology, and clinical features of the diseases, with a focus on epithelial, eosinophil, and mast cell responses, as well as current and emerging treatment options, were reviewed. Here we briefly review some of the highlights of the material presented at the meeting.

How Lipid-Specific T Cells Become Effectors: The Differentiation of iNKT Subsets

In contrast to peptide-recognizing T cells, invariant natural killer T (iNKT) cells express a semi-invariant T cell receptor that specifically recognizes self- or foreign-lipids presented by CD1d molecules. There are three major functionally distinct effector states for iNKT cells. Owning to these innate-like effector states, iNKT cells have been implicated in early protective immunity against pathogens. Yet, growing evidence suggests that iNKT cells play a role in tissue homeostasis as well. In this review, we discuss current knowledge about the underlying mechanisms that regulate the effector states of iNKT subsets, with a highlight on the roles of a variety of transcription factors and describe how each subset influences different facets of thymus homeostasis.

Immunometabolic Activation of Invariant Natural Killer T Cells

Invariant natural killer T (iNKT) cells are lipid-reactive T cells with profound immunomodulatory potential. They are unique in their restriction to lipid antigens presented in CD1d molecules, which underlies their role in lipid-driven disorders such as obesity and atherosclerosis. In this review, we discuss the contribution of iNKT cell activation to immunometabolic disease, metabolic programming of lipid antigen presentation, and immunometabolic activation of iNKT cells. First, we outline the role of iNKT cells in immunometabolic disease. Second, we discuss the effects of cellular metabolism on lipid antigen processing and presentation to iNKT cells. The synthesis and processing of glycolipids and other potential endogenous lipid antigens depends on metabolic demand and may steer iNKT cells toward adopting a Th1 or Th2 signature. Third, external signals such as toll-like receptor ligands, adipokines, and cytokines modulate antigen presentation and subsequent iNKT cell responses. Finally, we will discuss the relevance of metabolic programming of iNKT cells in human disease, focusing on their role in disorders such as obesity and atherosclerosis. The critical response to metabolic changes places iNKT cells at the helm of immunometabolic disease.

Deficiency of Invariant Natural Killer T Cells Does Not Protect Against Obesity but Exacerbates Atherosclerosis in Ldlr-/- Mice

Obesity is a chronic inflammatory state characterized by altered levels of adipose tissue immune cell populations. Natural killer T (NKT) cells are CD1d restricted lymphocyte subsets that recognize lipid antigens whose level decreases in obese adipose tissue. However, studies in mice with deficiency or increased levels of NKT cells have yielded contradictory results, so the exact role of these cells in obesity and adipose tissue inflammation is not yet established. We previously showed that Ldlr^−/− mice with excess invariant NKT (iNKT) cells demonstrate significant weight gain, adiposity, metabolic abnormalities, and atherosclerosis. The current study evaluates the effects of NKT cell deficiency on obesity, associated metabolic changes, and atherosclerosis in Jα18^−/−Ldlr^−/− (lacking iNKT cells) and Cd1d^−/−Ldlr^−/− (lacking invariant and type II NKT cells) mice, and control mice were fed an obesogenic diet (high fat, sucrose, cholesterol) for 16 weeks. Contrary to expectations, Ja18^−/−Ldlr^−/− mice gained significantly more weight than Ldlr^−/− or Cd1d^−/−Ldlr^−/− mice, developed hypertriglyceridemia, and had worsened adipose tissue inflammation. All the mice developed insulin resistance and hepatic triglyceride accumulation. Ja18^−/−Ldlr^−/− mice also had increased atherosclerotic lesion area. Our findings suggest that iNKT cells exacerbates the metabolic, inflammatory, and atherosclerotic features of diet-induced obesity. Further work is required to unravel the paradox of an apparently similar effect of iNKT cell surplus and depletion on obesity.

Critical Role for Very-Long Chain Sphingolipids in Invariant Natural Killer T Cell Development and Homeostasis

The role of sphingolipids (SLs) in the immune system has come under increasing scrutiny recently due to the emerging contributions that these important membrane components play in regulating a variety of immunological processes. The acyl chain length of SLs appears particularly critical in determining SL function. Here, we show a role for very-long acyl chain SLs (VLC-SLs) in invariant natural killer T (iNKT) cell maturation in the thymus and homeostasis in the liver. Ceramide synthase 2-null mice, which lack VLC-SLs, were susceptible to a hepatotropic strain of lymphocytic choriomeningitis virus, which is due to a reduction in the number of iNKT cells. Bone marrow chimera experiments indicated that hematopoietic-derived VLC-SLs are essential for maturation of iNKT cells in the thymus, whereas parenchymal-derived VLC-SLs are crucial for iNKT cell survival and maintenance in the liver. Our findings suggest a critical role for VLC-SL in iNKT cell physiology.

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.

Activation of invariant natural killer T cells stimulated with microbial α-mannosyl glycolipids

Some synthetic and bacterial glycolipids presented by CD1d specifically activate invariant NKT (iNKT) cells bearing an invariant Vα14-Jα18 (mouse) or Vα24-Jα18 (human) TCR. The antigenic glycolipids identified to date consist of two hydrophobic chains and an α-glycoside in which the 2′-OH group is in the cis orientation toward the anomeric group, namely, either an α-galactoside or an α-glucoside. Several microbial α-mannosyl glycolipids, in which the 2′-OH group is in the trans orientation, were herein examined to establish whether they have potential to activate iNKT cells. We found that α-mannnosyl1-3 (6′-O-acyl α-mannosyl)-1-1 monoacylglycerol and cholesteryl 6′-O-acyl α-mannoside, found in Saccharopolyspora and Candida albicans, respectively, induced the activation of iNKT cells, dependent on CD1d. In contrast, α-mannosyldiacylglycerol found in Streptococcus suis or α-mannosylceramide demonstrated markedly less antigenicity for iNKT cells. The potentially antigenic α-mannosyl glycolipids contributed to the protection of mice against infection with S. pneumoniae in which iNKT cells have previously been found to participate. Furthermore, these glycolipids induced the production of proinflammatory cytokines by macrophages, thereby suggesting their recognition by specific pattern recognition receptors (PRRs). Collectively, these results suggest that these microbial α-mannosyl glycolipids are capable of being recognized by both the invariant TCR and PRRs and inducing immune responses.

Glucosylceramide Synthase Is Involved in Development of Invariant Natural Killer T Cells

Invariant natural killer T (iNKT) cells represent a unique population of CD1d-restricted T lymphocytes expressing an invariant T cell receptor encoded by Vα14-Jα18 and Vα24-Jα18 gene segments in mice and humans, respectively. Recognition of CD1d-loaded endogenous lipid antigen(s) on CD4/CD8-double positive (DP) thymocytes is essential for the development of iNKT cells. The lipid repertoire of DP thymocytes and the identity of the decisive endogenous lipid ligands have not yet been fully elucidated. Glycosphingolipids (GSL) were implicated to serve as endogenous ligands. However, further in vivo investigations were hampered by early embryonal lethality of mice deficient for the key GSL-synthesizing enzyme glucosylceramide (GlcCer) synthase [GlcCer synthase (GCS), EC 2.4.1.80]. We have now analyzed the GSL composition of DP thymocytes and shown that GlcCer represented the sole neutral GSL and the acidic fraction was composed of gangliosides. Furthermore, we report on a mouse model that by combination of Vav-promoter-driven iCre and floxed GCS alleles (Vav^CreGCS^f/f) enabled an efficient depletion of GCS-derived GSL very early in the T cell development, reaching a reduction by 99.6% in DP thymocytes. Although the general T cell population remained unaffected by this depletion, iNKT cells were reduced by approximately 50% in thymus, spleen, and liver and showed a reduced proliferation and an increased apoptosis rate. The Vβ-chains repertoire and development of iNKT cells remained unaltered. The GSL-depletion neither interfered with expression of CD1d, SLAM, and Ly108 molecules nor impeded the antigen presentation on DP thymocytes. These results indicate that GlcCer-derived GSL, in particular GlcCer, contribute to the homeostatic development of iNKT cells.

The processing and presentation of lipids and glycolipids to the immune system

The recognition of CD1-lipid complexes by T cells was discovered 20 years ago and has since been an emerging and expanding field of investigation. Unlike protein antigens, which are presented on MHC class I and II molecules, lipids can only be presented by CD1 molecules, a unique family of MHC-like proteins whose singularity is a hydrophobic antigen-binding groove. The processing and loading of lipid antigens inside this groove of CD1 molecules require localization to endosomal and lysosomal subcellular compartments and their acidic pHs. This particular environment provides the necessary glycolytic enzymes and lipases that process lipid and glycolipid antigens, as well as a set of lipid transfer proteins that load the final version of the antigen inside the groove of CD1. The overall sequence of events needed for efficient presentation of lipid antigens is now understood and presented in this review. However, a large number of important details have been elusive. This elusiveness is linked to the inherent technical difficulties of studying lipids and the lipid-protein interface in vitro and in vivo. Here, we will expose some of those limitations and describe new approaches to address them during the characterization of lipids and glycolipids antigen presentation.

Structural determination of lipid antigens captured at the CD1d-T-cell receptor interface

Glycolipid antigens recognized by αβ T-cell receptors (TCRs) drive the activation of invariant natural killer T (iNKT) cells, a specialized subset of innate T lymphocytes. Glycolipids with α-linked anomeric carbohydrates have been identified as potent microbial lipid antigens for iNKT cells, and their unusual α-anomeric linkage has been thought to define a "foreign" lipid antigen motif. However, mammals use endogenous lipids to select iNKT cells, and there is compelling evidence for iNKT cell responses in various types of sterile inflammation. The nature of endogenous or environmental lipid antigens encountered by iNKT cells is not well defined. Here, we sought to identify lipid antigens in cow's milk, a prominent part of the human diet. We developed a method to directly capture lipid antigens within CD1d-lipid-TCR complexes, while excluding CD1d bound to nonantigenic lipids, followed by direct biochemical analysis of the lipid antigens trapped at the TCR-CD1d interface. The specific antigens captured by this "TCR trap" method were identified as α-linked monohexosylceramides by mass spectrometry fragmentation patterns that distinguished α- from β-anomeric monohexosylceramides. These data provide direct biochemical evidence for α-linked lipid antigens from a common dietary source.

Autoreactivity to Sulfatide by Human Invariant NKT Cells

Invariant NKT (iNKT) cells are innate-like lymphocytes that recognize lipid Ags presented by CD1d. The prototypical Ag, α-galactosylceramide, strongly activates human and mouse iNKT cells, leading to the assumption that iNKT cell physiology in human and mouse is similar. In this article, we report the surprising finding that human, but not mouse, iNKT cells directly recognize myelin-derived sulfatide presented by CD1d. We propose that sulfatide is recognized only by human iNKT cells because of the unique positioning of the 3-O-sulfated β-galactose headgroup. Surface plasmon resonance shows that the affinity of human CD1d-sulfatide for the iNKT cell receptor is relatively low compared with CD1d-α-galactosylceramide (K_D of 19-26 μM versus 1 μM). Apolipoprotein E isolated from human cerebrospinal fluid carries sulfatide that can be captured by APCs and presented by CD1d to iNKT cells. APCs from patients with metachromatic leukodystrophy, who accumulate sulfatides due to a deficiency in arylsulfatase-A, directly activate iNKT cells. Thus, we have identified sulfatide as a self-lipid recognized by human iNKT cells and propose that sulfatide recognition by innate T cells may be an important pathologic feature of neuroinflammatory disease and that sulfatide in APCs may contribute to the endogenous pathway of iNKT cell activation.

Intracellular metabolite β-glucosylceramide is an endogenous Mincle ligand possessing immunostimulatory activity

Sensing and reacting to tissue damage is a fundamental function of immune systems. Macrophage inducible C-type lectin (Mincle) is an activating C-type lectin receptor that senses damaged cells. Notably, Mincle also recognizes glycolipid ligands on pathogens. To elucidate endogenous glycolipids ligands derived from damaged cells, we fractionated supernatants from damaged cells and identified a lipophilic component that activates reporter cells expressing Mincle. Mass spectrometry and NMR spectroscopy identified the component structure as β-glucosylceramide (GlcCer), which is a ubiquitous intracellular metabolite. Synthetic β-GlcCer activated myeloid cells and induced production of inflammatory cytokines; this production was abrogated in Mincle-deficient cells. Sterile inflammation induced by excessive cell death in the thymus was exacerbated by hematopoietic-specific deletion of degrading enzyme of β-GlcCer (β-glucosylceramidase, GBA1). However, this enhanced inflammation was ameliorated in a Mincle-deficient background. GBA1-deficient dendritic cells (DCs) in which β-GlcCer accumulates triggered antigen-specific T-cell responses more efficiently than WT DCs, whereas these responses were compromised in DCs from GBA1 × Mincle double-deficient mice. These results suggest that β-GlcCer is an endogenous ligand for Mincle and possesses immunostimulatory activity.

Key Residues at Third CDR3β Position Impact Structure and Antigen Recognition of Human Invariant NK TCRs

The human invariant NK (iNK) TCR is largely composed of the invariant TCR Vα24-Jα18 chain and semivariant TCR Vβ11 chains with variable CDR3β sequences. The direct role of CDR3β in Ag recognition has been studied extensively. Although it was noted that CDR3β can interact with CDR3α, how this interaction might indirectly influence Ag recognition is not fully elucidated. We observed that the third position of Vβ11 CDR3 can encode an Arg or Ser residue as a result of somatic rearrangement. Clonotypic analysis of the two iNK TCR types with a single amino acid substitution revealed that the staining intensity by anti-Vα24 Abs depends on whether Ser or Arg is encoded. When stained with an anti-Vα24-Jα18 Ab, human primary invariant NKT cells could be divided into Vα24 low- and high-intensity subsets, and Arg-encoding TCR Vβ11 chains were more frequently isolated from the Vα24 low-intensity subpopulation compared with the Vα24 high-intensity subpopulation. The Arg/Ser substitution also influenced Ag recognition as determined by CD1d multimer staining and CD1d-restricted functional responses. Importantly, in silico modeling validated that this Ser-to-Arg mutation could alter the structure of the CDR3β loop, as well as the CDR3α loop. Collectively, these results indicate that the Arg/Ser encoded at the third CDR3β residue can effectively modulate the overall structure of, and Ag recognition by, human iNK TCRs.

Discrete TCR Binding Kinetics Control Invariant NKT Cell Selection and Central Priming

Invariant NKT (iNKT) cells develop and differentiate in the thymus, segregating into iNKT1/2/17 subsets akin to Th1/2/17 classical CD4⁺ T cells; however, iNKT TCRs recognize Ags in a fundamentally different way. How the biophysical parameters of iNKT TCRs influence signal strength in vivo and how such signals affect the development and differentiation of these cells are unknown. In this study, we manipulated TCRs in vivo to generate clonotypic iNKT cells using TCR retrogenic chimeras. We report that the biophysical properties of CD1d-lipid-TCR interactions differentially impacted the development and effector differentiation of iNKT cells. Whereas selection efficiency strongly correlated with TCR avidity, TCR signaling, cell-cell conjugate formation, and iNKT effector differentiation correlated with the half-life of CD1d-lipid-TCR interactions. TCR binding properties, however, did not modulate Ag-induced iNKT cytokine production. Our work establishes that discrete TCR interaction kinetics influence iNKT cell development and central priming.

Mechanisms and Consequences of Antigen Presentation by CD1

The CD1 proteins are a family of non-polymorphic and MHC class I-related molecules that present lipid antigens to subsets of T lymphocytes with innate- or adaptive-like immune functions. Recent studies have provided new insight into the identity of immunogenic CD1 antigens and the mechanisms that control the generation and loading of these antigens onto CD1 molecules. Furthermore, substantial progress has been made in identifying CD1-restricted T cells and decoding the diverse immunological functions of distinct CD1-restricted T cell subsets. These findings shed new light on the contributions of the CD1 antigen-presentation pathway to normal health and to a diverse array of pathologies, and provide a new impetus for exploiting this fascinating recognition system for the development of vaccines and immunotherapies.

Sphingolipids as Mediators in the Crosstalk between Microbiota and Intestinal Cells: Implications for Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) describes different illnesses characterized by chronic inflammation of the gastrointestinal tract. Although the pathogenic mechanisms leading to IBD are poorly understood, immune system disturbances likely underlie its development. Sphingolipids (SLs) have been identified as important players and promising therapeutic targets to control inflammation in IBD. Interestingly, it seems that microorganisms of the normal gut microbiota and probiotics are involved in sphingolipid function. However, there is a great need to investigate the role of SLs as intermediates in the crosstalk between intestinal immunity and microorganisms. This review focuses on recent investigations that describe some mechanisms involved in the regulation of cytokine profiles by SLs. We also describe the importance of gut microbiota in providing signaling molecules that favor the communication between resident bacteria and intestinal cells. This, in turn, modulates the immune response in the bowel and likely in other peripheral organs. The potential of SLs and gut microbiota as targets or therapeutic agents for IBD is also discussed.

Hallmarks of Tissue-Resident Lymphocytes

Although they are classically viewed as continuously recirculating through the lymphoid organs and blood, lymphocytes also establish residency in non-lymphoid tissues, most prominently at barrier sites, including the mucosal surfaces and skin. These specialized tissue-resident lymphocyte subsets span the innate-adaptive continuum and include innate lymphoid cells (ILCs), unconventional T cells (e.g., NKT, MAIT, γδ T cells, and CD8αα(+) IELs), and tissue-resident memory T (T(RM)) cells. Although these diverse cell types differ in the particulars of their biology, they nonetheless exhibit important shared features, including a role in the preservation of tissue integrity and function during homeostasis, infection, and non-infectious perturbations. In this Review, we discuss the hallmarks of tissue-resident innate, innate-like, and adaptive lymphocytes, as well as their potential functions in non-lymphoid organs.

Activation strategies for invariant natural killer T cells

Invariant natural killer T (iNKT) cells are a specialized T cell subset that plays an important role in host defense, orchestrating both innate and adaptive immune effector responses against a variety of microbes. Specific microbial lipids and mammalian self lipids displayed by the antigen-presenting molecule CD1d can activate iNKT cells through their semi-invariant αβ T cell receptors (TCRs). iNKT cells also constitutively express receptors for inflammatory cytokines typically secreted by antigen-presenting cells (APCs) after recognition of pathogen-associated molecular patterns (PAMPs), and they can be activated through these cytokine receptors either in combination with TCR signals, or in some cases even in the absence of TCR signaling. During infection, experimental evidence suggests that both TCR-driven and cytokine-driven mechanisms contribute to iNKT cell activation. While the relative contributions of these two signaling mechanisms can vary widely depending on the infectious context, both lipid antigens and PAMPs mediate reciprocal activation of iNKT cells and APCs, leading to downstream activation of multiple other immune cell types to promote pathogen clearance. In this review, we discuss the mechanisms involved in iNKT cell activation during infection, focusing on the central contributions of both lipid antigens and PAMP-induced inflammatory cytokines, and highlight in vivo examples of activation during bacterial, viral, and fungal infections.

Innate Invariant NKT Cell Recognition of HIV-1-Infected Dendritic Cells Is an Early Detection Mechanism Targeted by Viral Immune Evasion

Invariant NKT (iNKT) cells are innate-like T cells that respond rapidly with a broad range of effector functions upon recognition of glycolipid Ags presented by CD1d. HIV-1 carries Nef- and Vpu-dependent mechanisms to interfere with CD1d surface expression, indirectly suggesting a role for iNKT cells in control of HIV-1 infection. In this study, we investigated whether iNKT cells can participate in the innate cell-mediated immune response to HIV-1. Infection of dendritic cells (DCs) with Nef- and Vpu-deficient HIV-1 induced upregulation of CD1d in a TLR7-dependent manner. Infection of DCs caused modulation of enzymes in the sphingolipid pathway and enhanced expression of the endogenous glucosylceramide Ag. Importantly, iNKT cells responded specifically to rare DCs productively infected with Nef- and Vpu-defective HIV-1. Transmitted founder viral isolates differed in their CD1d downregulation capacity, suggesting that diverse strains may be differentially successful in inhibiting this pathway. Furthermore, both iNKT cells and DCs expressing CD1d and HIV receptors resided in the female genital mucosa, a site where HIV-1 transmission occurs. Taken together, these findings suggest that innate iNKT cell sensing of HIV-1 infection in DCs is an early immune detection mechanism, which is independent of priming and adaptive recognition of viral Ag, and is actively targeted by Nef- and Vpu-dependent viral immune evasion mechanisms.

CD1b-autoreactive T cells recognize phospholipid antigens and contribute to antitumor immunity against a CD1b+ T cell lymphoma

Adoptive immunotherapy for cancer treatment is an emerging field of study. Till now, several tumor-derived, peptide-specific T cell responses have been harnessed for treating cancers. However, the contribution of lipid-specific T cells in tumor immunity has been understudied. CD1 molecules, which present self- and foreign lipid antigens to T cells, are divided into group 1 (CD1a, CD1b, and CD1c) and group 2 (CD1d). Although the role of CD1d-restricted natural killer T cells (NKT) in several tumor models has been well established, the contribution of group 1 CD1-restricted T cells in tumor immunity remains obscure due to the lack of group 1 CD1 expression in mice. In this study, we used a double transgenic mouse model expressing human group 1 CD1 molecules (hCD1Tg) and a CD1b-restricted, self-lipid reactive T cell receptor (HJ1Tg) to study the potential role of group 1 CD1-restricted autoreactive T cells in antitumor response. We found that HJ1 T cells recognized phospholipids and responded more potently to lipid extracted from tumor cells than the equivalent amount of lipids extracted from normal cells. Additionally, the autoreactivity of HJ1 T cells was enhanced upon treatment with various intracellular toll-like receptor (TLR) agonists, including CpG oligodeoxynucleotides (ODN), R848, and poly (I:C). Interestingly, the adoptive transfer of HJ1 T cells conferred protection against the CD1b-transfected murine T cell lymphoma (RMA-S/CD1b) and CpG ODN enhanced the antitumor effect. Thus, this study, for the first time, demonstrates the antitumor potential of CD1b-autoreactive T cells and their potential use in adoptive immunotherapy.