Germline-encoded recognition of diverse glycolipids by natural killer T cells

Invariant natural killer T cells: an innate activation scheme linked to diverse effector functions

Invariant natural killer T (iNKT) cells exist in a 'poised effector' state, which enables them to rapidly produce cytokines following activation. Using a nearly monospecific T cell receptor, they recognize self and foreign lipid antigens presented by CD1d in a conserved manner, but their activation can catalyse a spectrum of polarized immune responses. In this Review, we discuss recent advances in our understanding of the innate-like mechanisms underlying iNKT cell activation and describe how lipid antigens, the inflammatory milieu and interactions with other immune cell subsets regulate the functions of iNKT cells in health and disease.

Lack of iGb3 and Isoglobo-Series Glycosphingolipids in Pig Organs Used for Xenotransplantation: Implications for Natural Killer T-Cell Biology

α-1,3-Terminated galactose residues on glycoproteins and glycosphingolipids are recognized by natural anti-α-1,3-galactose antibodies in human serum and cause hyperacute rejection in pig-to-human xenotransplantation. Genetic depletion of α-1,3-galactosyltransferase-1 in pigs abolishes the hyperacute rejection reaction. However, the isoglobotriosylceramide (iGb3) synthase in pigs may produce additional α-1,3-terminated galactose residues on glycosphingolipids. In both α-1,3-galactosyltranserase-1 knockout mice and pigs, cytotoxic anti-α-1,3-galactose antibodies could be induced; thus, a paradox exists that anti-α-1,3-galactose antibodies are present in animals with functional iGb3 synthases. Furthermore, iGb3 has been found to be an endogenous antigen for natural killer T (NKT) cells, an innate type of lymphocyte that may initiate the adaptive immune responses. It has been reasoned that iGb3 may trigger the activation of NKT cells and cause the rejection of α-1,3-galactosyltransferase-1-deficient organs through the potent stimulatory effects of NKT cells on adaptive immune cells (see ref.([20])). In this study, we examined the expression of iGb3 and the isoglobo-series glycosphingolipids in pig organs, including the heart, liver, pancreas, and kidney, by ion-trap mass spectrometry, which has a sensitivity of measuring 1% iGb3 among Gb3 isomers, when 5 μg/mL of the total iGb3/Gb3 mixture is present (see ref.([35])). We did not detect iGb3 or other isoglobo-series glycosphingolipids in any of these organs, although they were readily detected in mouse and human thymus and dendritic cells. The lack of iGb3 and isoglobo-series glycosphingolipids in pig organs indicates that iGb3 is unlikely to be a relevant immune epitope in xenotransplantation.

Implications of Lymphocyte Anergy to Glycolipids in Multiple Sclerosis (MS): iNKT Cells May Mediate the MS Infectious Trigger

Immunogenic lipids may play key roles in host defenses against infection and in generating autoimmune inflammation and organ-specific damage. In multiple sclerosis (MS) there are unequivocal autoimmune features and vulnerability to aggravation or induction by microbial or viral infection. We have found glycolipid-driven anergy of circulating lymphocytes in MS indicating that this immune response is affected in MS and the robust effects of iNKT activation with potent cellular and cytokine activities emphasizes its potential importance. Diverse glycolipids including the endogenous myelin acetylated-galactosylceramides (AcGalCer) can drive activation that could be critical to the inflammatory demyelination in the central nervous system and clinical consequences. The iNKT cells and their invariant or iTCR (Vα24Jα18Vβ11) receptor an innate defense–a discrete immune arm that is separate from peptide-driven acquired immune responses. This offers new possibilities for insight including a likelihood that the pattern recognition of exogenous microbial and myelin immunogens can overlap and cross-react especially in an inflammatory milieu.

Natural killer T cell deficiency in active adult-onset Still's Disease: correlation of deficiency of natural killer T cells with dysfunction of natural killer cells

OBJECTIVE

To examine the levels and functions of natural killer (NK) and natural killer T (NKT) cells, investigate relationships between NK and NKT cells, and determine the clinical relevance of NKT cell levels in patients with adult-onset Still's disease (AOSD).

METHODS

Patients with active untreated AOSD (n = 20) and age- and sex-matched healthy controls (n = 20) were studied. NK and NKT cell levels were measured by flow cytometry. Peripheral blood mononuclear cells were cultured in vitro with α-galactosylceramide (αGalCer). NK cytotoxicity against K562 cells and proliferation indices of NKT cells were estimated by flow cytometry.

RESULTS

Percentages and absolute numbers of NKT cells were significantly lower in the peripheral blood of AOSD patients than in that of healthy controls. Proliferative responses of NKT cells to αGalCer were also lower in patients, and this was found to be due to proinflammatory cytokines and NKT cell apoptosis. In addition, NK cytotoxicity was found to be significantly lower in patients than in healthy controls, but NK cell levels were comparable in the 2 groups. Notably, this NKT cell deficiency was found to be correlated with NK cell dysfunction and to reflect active disease status. Furthermore, αGalCer-mediated NK cytotoxicity, showing the interaction between NK and NKT cells, was significantly lower in AOSD patients than in healthy controls.

CONCLUSION

These findings demonstrate that NK and NKT cell functions are defective in AOSD patients and suggest that these abnormalities contribute to innate immune dysfunction in AOSD.

Recognition of CD1d-restricted antigens by natural killer T cells

Natural killer T (NKT) cells are innate-like T cells that rapidly produce a variety of cytokines following T cell receptor (TCR) activation and can shape the immune response in many different settings. There are two main NKT cell subsets: type I NKT cells are typically characterized by the expression of a semi-invariant TCR, whereas the TCRs expressed by type II NKT cells are more diverse. This Review focuses on the defining features and emerging generalities regarding how NKT cells specifically recognize self, microbial and synthetic lipid-based antigens that are presented by CD1d. Such information is vitally important to better understand, and fully harness, the therapeutic potential of NKT cells.

Molecular basis of lipid antigen presentation by CD1d and recognition by natural killer T cells

Together with peptides, T lymphocytes respond to hydrophobic molecules, mostly lipids, presented by the non-classical CD1 family (CD1a-e). These molecules have evolved complex and diverse binding grooves in order to survey different cellular compartments for self and exogenous antigens, which are then presented for recognition to T-cell receptors (TCRs) on the surface of T cells. In particular, most CD1d-presented antigens are recognized by a population of lymphocytes denominated natural killer T (NKT) cells, characterized by a strong immunomodulatory potential. Among NKT cells, two major subsets (type I and type II NKT cells) have been described, based on their TCR repertoire and antigen specificity. Here we review recent structural and biochemical studies that have shed light on the molecular details of CD1d-mediated antigen recognition by type I and II NKT cells, which are in many aspects distinct from what has been observed for peptide major histocompatibility complex-reactive TCRs.

Turned on by danger: activation of CD1d-restricted invariant natural killer T cells

CD1d-restricted invariant natural killer T (iNKT) cells bear characteristics of innate and adaptive lymphocytes, which allow them to bridge the two halves of the immune response and play roles in many disease settings. Recent work has characterized precisely how their activation is initiated and regulated. Novel antigens from important pathogens have been identified, as has an abundant self-antigen, β-glucopyranosylcaramide, capable of mediating an iNKT-cell response. Studies of the iNKT T-cell receptor (TCR)-antigen-CD1d complex show how docking between CD1d-antigen and iNKT TCR is highly conserved, and how small sequence differences in the TCR establish intrinsic variation in iNKT TCR affinity. The sequence of the TCR CDR3β loop determines iNKT TCR affinity for ligand-CD1d, independent of ligand identity. CD1d ligands can promote T helper type 1 (Th1) or Th2 biased cytokine responses, depending on the composition of their lipid tails. Ligands loaded into CD1d on the cell surface promote Th2 responses, whereas ligands with long hydrophobic tails are loaded endosomally and promote Th1 responses. This information is informing the design of synthetic iNKT-cell antigens. The iNKT cells may be activated by exogenous antigen, or by a combination of dendritic cell-derived interleukin-12 and iNKT TCR-self-antigen-CD1d engagement. The iNKT-cell activation is further modulated by recent foreign or self-antigen encounter. Activation of dendritic cells through pattern recognition receptors alters their antigen presentation and cytokine production, strongly influencing iNKT-cell activation. In a range of bacterial infections, dendritic cell-dependent innate activation of iNKT cells through interleukin-12 is the dominant influence on their activity.

T-cell receptor (TCR) interaction with peptides that mimic nickel offers insight into nickel contact allergy

T cell-mediated allergy to Ni(++) is one of the most common forms of allergic contact dermatitis, but how the T-cell receptor (TCR) recognizes Ni(++) is unknown. We studied a TCR from an allergic patient that recognizes Ni(++) bound to the MHCII molecule DR52c containing an unknown self-peptide. We identified mimotope peptides that can replace both the self-peptide and Ni(++) in this ligand. They share a p7 lysine whose εNH(2) group is surface-exposed when bound to DR52c. Whereas the TCR uses germ-line complementary-determining region (CDR)1/2 amino acids to dock in the conventional diagonal mode on the mimotope-DR52c complex, the interface is dominated by the TCR Vβ CDR3 interaction with the p7 lysine. Mutations in the TCR CDR loops have similar effects on the T-cell response to either the mimotope or Ni(++) ligand. We suggest that the mimotope p7 lysine mimics Ni(++) in the natural TCR ligand and that MHCII β-chain flexibility in the area around the peptide p7 position forms a common site for cation binding in metal allergies.

Human and mouse type I natural killer T cell antigen receptors exhibit different fine specificities for CD1d-antigen complex

Human and mouse type I natural killer T (NKT) cells respond to a variety of CD1d-restricted glycolipid antigens (Ags), with their NKT cell antigen receptors (NKT TCRs) exhibiting reciprocal cross-species reactivity that is underpinned by a conserved NKT TCR-CD1d-Ag docking mode. Within this common docking footprint, the NKT TCR recognizes, to varying degrees of affinity, a range of Ags. Presently, it is unclear whether the human NKT TCRs will mirror the generalities underpinning the fine specificity of the mouse NKT TCR-CD1d-Ag interaction. Here, we assessed human NKT TCR recognition against altered glycolipid ligands of α-galactosylceramide (α-GalCer) and have determined the structures of a human NKT TCR in complex with CD1d-4',4″-deoxy-α-GalCer and CD1d-α-GalCer with a shorter, di-unsaturated acyl chain (C20:2). Altered glycolipid ligands with acyl chain modifications did not affect the affinity of the human NKT TCR-CD1d-Ag interaction. Surprisingly, human NKT TCR recognition is more tolerant to modifications at the 4'-OH position in comparison with the 3'-OH position of α-GalCer, which contrasts the fine specificity of the mouse NKT TCR-CD1d-Ag recognition (4'-OH > 3'-OH). The fine specificity differences between human and mouse NKT TCRs was attributable to differing interactions between the respective complementarity-determining region 1α loops and the Ag. Accordingly, germline encoded fine-specificity differences underpin human and mouse type I NKT TCR interactions, which is an important consideration for therapeutic development and NKT cell physiology.

Identification of novel glycolipid ligands activating a sulfatide-reactive, CD1d-restricted, type II natural killer T lymphocyte

Sulfatide-reactive CD1d-restricted natural killer T (NKT) lymphocytes belong to the type II NKT cell subset with diverse TCRs, and have been found to regulate experimental auto-immune encephalomyelitis, tumor immunity, and experimental hepatitis in murine models. NKT cells can be activated by self-lipids presented by CD1d, manifested as autoreactivity. The identity of most of these self-lipids remains unknown. By isolating lipids from a CD1d-expressing, highly stimulatory antigen presenting cell, we identified isoforms of β-glucosylceramide (GlcCer), with sphingosine and fatty acid chain lengths of C24:0 and C16:0, that activated a sulfatide-reactive type II NKT cell hybridoma. A screen of structurally related glycosphingolipids demonstrated β-galactosylceramide (GalCer) as another ligand, and further, that the lysoforms were the most potent isoform of the glycosphingo-lipid ligands, followed by isoforms with a long fatty acid chain of C24. Thus, the same type II NKT cell was activated by several ligands, namely sulfatide, GlcCer, and GalCer. However, CD1d-dependent reactivity to antigen presenting cells lacking all GlcCer-based glycosphingolipids, or all glycosphingolipids, was maintained. This suggests that other endogenous, nonglycosphingolipid, lipid ligands contribute to steady-state autoreactivity by type II NKT cells.

Strategy of lipid recognition by invariant natural killer T cells: 'one for all and all for one'

Invariant natural killer T (iNKT) cells are evolutionarily conserved lipid-reactive T cells that bridge innate and adaptive immune responses. Despite a relatively restricted T-cell receptor (TCR) diversity, these cells respond to a variety of structurally distinct foreign (i.e. microbial or synthetic) as well as host-derived (self-) lipid antigens presented by the CD1d molecule. These multi-tasking lymphocytes are among the first responders in immunity, and produce an impressive array of cytokines and chemokines that can tailor the ensuing immune response. Accordingly, iNKT cells play important functions in autoimmune diseases, cancer, infection and inflammation. These properties make iNKT cells appealing targets in immune-based therapies. Yet, much has to be learned on the mechanisms that allow iNKT cells to produce polarized responses. Responses of iNKT cells are influenced by the direct signals perceived by the cells through their TCRs, as well as by indirect co-stimulatory (and potentially co-inhibitory) cues that they receive from antigen-presenting cells or the local milieu. A decade ago, biochemists and immunologists have started to describe synthetic lipid agonists with cytokine skewing potential, paving a new research avenue in the iNKT cell field. Yet how iNKT cells translate various antigenic signals into distinct functional responses has remained obscure. Recent findings have revealed a unique and innate mode of lipid recognition by iNKT cells, and suggest that both the lipid antigen presented and the diversity of the TCR modulate the strength of CD1d-iNKT TCR interactions. In this review, we focus on novel discoveries on lipid recognition by iNKT cells, and how these findings may help us to design effective strategies to steer iNKT cell responses for immune intervention.

NKT and MAIT invariant TCRα sequences can be produced efficiently by VJ gene recombination

Semi-invariant T cell receptors (TCRs) found on natural killer T (NKT) and mucosal-associated invariant T (MAIT) cells are characterized by the use of invariant variable (V) and joining (J) gene combinations in the TCR α-chain, as well as ubiquitous canonical TCRα amino acid sequences that are dominant in many individuals and similar across species. That they are so prevalent indicates that they occupy an important niche within the immune system. However, these TCRs are produced by a largely random gene recombination process, which seems a risky approach for the immune system to acquire these innate-like cells. We surveyed studies reporting NKT and MAIT TCRα sequences for six and four different species, respectively. Although the germline nature of the canonical human and mouse NKT and mouse MAIT TCRα sequences and an overlap of nucleotides between the mouse MAIT-related Vα and Jα genes have been noted in previous studies, in this study we demonstrate that, for all reported species, the canonical TCRα amino acid sequences can be encoded by at least one germline-derived nucleotide sequence. Moreover, these nucleotide sequences can utilize an overlap between the Vα and Jα genes in their production, which enables them to be produced by a large variety of recombination mechanisms. We investigated the role of these TCRα features in the production of the canonical NKT and MAIT TCRα sequences. In computer simulations of a random recombination process involving the invariant NKT and MAIT TCRα gene combinations for each species, the canonical NKT and MAIT TCRα sequences were the first or second most generated of all sequences with the CDR3α length restrictions associated with NKT and MAIT cells. These results suggest that the immune machinery enables the canonical NKT and MAIT TCRα sequences to be produced with great efficiency through the process of convergent recombination, ensuring their prevalence across individuals and species.

Interplay between carbohydrate and lipid in recognition of glycolipid antigens by natural killer T cells

Natural killer T (NKT) cells are a T cell subpopulation that were named originally based on coexpression of receptors found on natural killer (NK) cells, cells of the innate immune system, and by T lymphocytes. The maturation and activation of NKT cells requires presentation of glycolipid antigens by CD1d, a cell surface protein distantly related to the major histocompatibility complex (MHC)-encoded antigen presenting molecules. This specificity distinguishes NKT cells from most CD4(+) and CD8(+) T cells that recognize peptides presented by MHC class I and class II molecules. The rapid secretion of a large amount of both Th1 and Th2 cytokines by activated NKT cells endows them with the ability to play a vital role in the host immune defense against various microbial infections. In this review, we summarize progress on identifying the sources of microbe-derived glycolipid antigens recognized by NKT cells and the biochemical basis for their recognition.

Structural insight into MR1-mediated recognition of the mucosal associated invariant T cell receptor

Crystal structure and mutagenesis analyses suggest a MAIT TCR–MR1 docking mode distinct from the NKT TCR-CD1d docking mode.

Mucosal-associated invariant T (MAIT) cells express a semiinvariant αβ T cell receptor (TCR) that binds MHC class I–like molecule (MR1). However, the molecular basis for MAIT TCR recognition by MR1 is unknown. In this study, we present the crystal structure of a human Vα7.2Jα33-Vβ2 MAIT TCR. Mutagenesis revealed highly conserved requirements for the MAIT TCR–MR1 interaction across different human MAIT TCRs stimulated by distinct microbial sources. Individual residues within the MAIT TCR β chain were dispensable for the interaction with MR1, whereas the invariant MAIT TCR α chain controlled specificity through a small number of residues, which are conserved across species and located within the Vα-Jα regions. Mutagenesis of MR1 showed that only two residues, which were centrally positioned and on opposing sides of the antigen-binding cleft of MR1, were essential for MAIT cell activation. The mutagenesis data are consistent with a centrally located MAIT TCR–MR1 docking that was dominated by the α chain of the MAIT TCR. This candidate docking mode contrasts with that of the NKT TCR–CD1d-antigen interaction, in which both the α and β chain of the NKT TCR is required for ligation above the F′-pocket of CD1d.

Lysophospholipid presentation by CD1d and recognition by a human Natural Killer T-cell receptor

Invariant Natural Killer T (iNKT) cells use highly restricted αβ T cell receptors (TCRs) to probe the repertoire of lipids presented by CD1d molecules. Here, we describe our studies of lysophosphatidylcholine (LPC) presentation by human CD1d and its recognition by a native, LPC-specific iNKT TCR. Human CD1d presenting LPC adopts an altered conformation from that of CD1d presenting glycolipid antigens, with a shifted α1 helix resulting in an open A' pocket. Binding of the iNKT TCR requires a 7-Å displacement of the LPC headgroup but stabilizes the CD1d-LPC complex in a closed conformation. The iNKT TCR CDR loop footprint on CD1d-LPC is anchored by the conserved positioning of the CDR3α loop, whereas the remaining CDR loops are shifted, due in part to amino-acid differences in the CDR3β and Jβ segment used by this iNKT TCR. These findings provide insight into how lysophospholipids are presented by human CD1d molecules and how this complex is recognized by some, but not all, human iNKT cells.

Natural variations at position 93 of the invariant Vα24-Jα18 α chain of human iNKT-cell TCRs strongly impact on CD1d binding

Human invariant natural killer T (NKT) cell TCRs bind to CD1d via an "invariant" Vα24-Jα18 chain (iNKTα) paired to semi-invariant Vβ11 chains (iNKTβ). Single-amino acid variations at position 93 (p93) of iNKTα, immediately upstream of the "invariant" CDR3α region, have been reported in a substantial proportion of human iNKT-cell clones (4-30%). Although p93, a serine in most human iNKT-cell TCRs, makes no contact with CD1d, it could affect CD1d binding by altering the conformation of the crucial CDR3α loop. By generating recombinant refolded iNKT-cell TCRs, we show that natural single-nucleotide variations in iNKTα, translating to serine, threonine, asparagine or isoleucine at p93, exert a powerful effect on CD1d binding, with up to 28-fold differences in affinity between these variants. This effect was observed with CD1d loaded with either the artificial α-galactosylceramide antigens KRN7000 or OCH, or the endogenous glycolipid β-galactosylceramide, and its importance for autoreactive recognition of endogenous lipids was demonstrated by the binding of variant iNKT-cell TCR tetramers to cell surface expressed CD1d. The serine-containing variant showed the strongest CD1d binding, offering an explanation for its predominance in vivo. Complementary molecular dynamics modeling studies were consistent with an impact of p93 on the conformation of the CDR3α loop.

Structural basis for the recognition of C20:2-αGalCer by the invariant natural killer T cell receptor-like antibody L363

Natural killer T (NKT) cells express a semi-invariant Vα14 T cell receptor (TCR) and recognize structurally diverse antigens presented by the antigen-presenting molecule CD1d that range from phosphoglycerolipids to α- and β-anomeric glycosphingolipids, as well as microbial α-glycosyl diacylglycerolipids. Recently developed antibodies that are specific for the complex of the prototypical invariant NKT (iNKT) cell antigen αGalCer (KRN7000) bound to mouse CD1d have become valuable tools in elucidating the mechanism of antigen loading and presentation. Here, we report the 3.1 Å resolution crystal structure of the Fab of one of these antibodies, L363, bound to mCD1d complexed with the αGalCer analog C20:2, revealing that L363 is an iNKT TCR-like antibody that binds CD1d-presented αGalCer in a manner similar to the TCR. The structure reveals that L363 depends on both the L and H chains for binding to the glycolipid-mCD1d complex, although only the L chain is involved in contacts with the glycolipid antigen. The H chain of L363 features residue Trp-104, which mimics the TCR CDR3α residue Leu-99, which is crucial for CD1d binding. We characterized the antigen-specificity of L363 toward several different glycolipids, demonstrating that whereas the TCR can induce structural changes in both antigen and CD1d to recognize disparate lipid antigens, the antibody L363 can only induce the F' roof formation in CD1d but fails to reorient the glycolipid headgroup necessary for binding. In summary, L363 is a powerful tool to study mechanism of iNKT cell activation for structural analogs of KRN7000, and our study can aid in the design of antibodies with altered antigen specificity.

Vβ2 natural killer T cell antigen receptor-mediated recognition of CD1d-glycolipid antigen

Natural killer T cell antigen receptors (NKT TCRs) recognize lipid-based antigens (Ags) presented by CD1d. Although the TCR α-chain is invariant, NKT TCR Vβ exhibits greater diversity, with one (Vβ11) and three (Vβ8, Vβ7, and Vβ2) Vβ chains in humans and mice, respectively. With the exception of the Vβ2 NKT TCR, NKT TCRs possess canonical tyrosine residues within complementarity determining region (CDR) 2β that are critical for CD1d binding. Thus, how Vβ2 NKT TCR docks with CD1d-Ag was unclear. Despite the absence of the CDR2β-encoded tyrosine residues, we show that the Vβ2 NKT TCR engaged CD1d-Ag in a similar manner and with a comparable affinity and energetic footprint to the manner observed for the Vβ8.2 and Vβ7 NKT TCRs. Accordingly, the germline-encoded regions of the TCR β-chain do not exclusively dictate the innate NKT TCR-CD1d-Ag docking mode. Nevertheless, clear fine specificity differences for the CD1d-Ag existed between the Vβ2 NKT TCR and the Vβ8.2 and Vβ7 NKT TCRs, with the Vβ2 NKT TCR exhibiting greater sensitivity to modifications to the glycolipid Ag. Furthermore, within the Vβ2 NKT TCR-CD1d-αGalCer complex, the CDR2β loop mediated fewer contacts with CD1d, whereas the CDR1β and CDR3β loops contacted CD1d to a much greater extent compared with most Vβ11, Vβ8.2, and Vβ7 NKT TCRs. Accordingly, there is a greater interplay between the germline- and nongermline-encoded loops within the TCR β-chain of the Vβ2 NKT TCR that enables CD1d-Ag ligation.

Unique interplay between sugar and lipid in determining the antigenic potency of bacterial antigens for NKT cells

Structural and biophysical studies reveal the induced-fit mechanism underlying the stringent specificity of invariant natural killer T cells for unique glycolipid antigens from the pathogen Streptococcus pneumoniae.

Invariant natural killer T (iNKT) cells are an evolutionary conserved T cell population characterized by features of both the innate and adaptive immune response. Studies have shown that iNKT cells are required for protective responses to Gram-positive pathogens such as Streptococcus pneumoniae, and that these cells recognize bacterial diacylglycerol antigens presented by CD1d, a non-classical antigen-presenting molecule. The combination of a lipid backbone containing an unusual fatty acid, vaccenic acid, as well as a glucose sugar that is weaker or not stimulatory when linked to other lipids, is required for iNKT cell stimulation by these antigens. Here we have carried out structural and biophysical studies that illuminate the reasons for the stringent requirement for this unique combination. The data indicate that vaccenic acid bound to the CD1d groove orients the protruding glucose sugar for TCR recognition, and it allows for an additional hydrogen bond of the glucose with CD1d when in complex with the TCR. Furthermore, TCR binding causes an induced fit in both the sugar and CD1d, and we have identified the CD1d amino acids important for iNKT TCR recognition and the stability of the ternary complex. The studies show also how hydrogen bonds formed by the glucose sugar can account for the distinct binding kinetics of the TCR for this CD1d-glycolipid complex. Therefore, our studies illuminate the mechanism of glycolipid recognition for antigens from important pathogens.

Invariant natural killer T (iNKT) cells are an evolutionarily conserved population of immune cells that recognize lipid antigens. A protein called a T cell receptor for antigen (TCR) on the surface of these iNKT cells recognizes lipids bound to a protein called CD1d on the surface of antigen-presenting cells. Here we describe the three-dimensional structure of the complex that forms between CD1d and the iNKT TCR together with a glycolipid antigen from the infectious bacterium Streptococcus pneumoniae, which is a common cause of bacterial meningitis in adults and is responsible for many other pneumococcal infections. We determined the three-dimensional structure of the complex by X-ray crystallography. The data obtained allow us to understand the structural requirements that make this glycolipid a potent antigen for iNKT cells, and why the TCR of these cells recognizes a particular combination of hexose sugar and diacylglycerol lipid. Moreover, by mutating CD1d and using biophysical methods to study the mutant protein complexes, we analyzed the role of the protein–protein interface between CD1d and the TCR and found that it plays an important role in the stability, but not the formation, of the trimolecular complex containing glycolipid antigen.

NKT cell ligand recognition logic: molecular basis for a synaptic duet and transmission of inflammatory effectors

NKT cells that express the semi-invariant TCR are innate-like lymphocytes whose functions are regulated by self and foreign glycolipid ligands presented by the Ag-presenting, MHC class I-like molecule CD1d. Activation of NKT cells in vivo results in rapid release of copious amounts of effector cytokines and chemokines with which they regulate innate and adaptive immune responses to pathogens, certain types of cancers, and self-antigens. The nature of CD1d-restricted ligands, the manner in which they are recognized, and the unique effector functions of NKT cells suggest an immunoregulatory role for this T cell subset. Their ability to respond fast and our ability to steer NKT cell cytokine response to altered lipid ligands make them an important target for vaccine design and immunotherapies against autoimmune diseases. This review summarizes our current understanding of CD1d-restricted ligand recognition by NKT cells and how these innate-like lymphocytes regulate inflammation.

NKT TCR recognition of CD1d-α-C-galactosylceramide

NKT cells respond to a variety of CD1d-restricted glycolipid Ags that are structurally related to the prototypic Ag α-galactosylceramide (α-GalCer). A modified analog of α-GalCer with a carbon-based glycosidic linkage (α-C-GalCer) has generated great interest because of its apparent ability to promote prolonged, Th1-biased immune responses. In this study, we report the activation of spleen NKT cells to α-C-GalCer, and related C-glycoside ligands, is weaker than that of α-GalCer. Furthermore, the Vβ8.2 and Vβ7 NKT TCR affinity for CD1d-α-C-GalCer, and some related analogs, is ∼10-fold lower than that for the NKT TCR-CD1d-α-GalCer interaction. Nevertheless, the crystal structure of the Vβ8.2 NKT TCR-CD1d-α-C-GalCer complex is similar to that of the corresponding NKT TCR-CD1d-α-GalCer complex, although subtle differences at the interface provide a basis for understanding the lower affinity of the NKT TCR-CD1d-α-C-GalCer interaction. Our findings support the concept that for CD1d-restricted NKT cells, altered glycolipid ligands can promote markedly different responses while adopting similar TCR-docking topologies.

Evolutionarily conserved features contribute to αβ T cell receptor specificity

αβ T cell receptors (TCRs) bind specifically to foreign antigens presented by major histocompatibility complex proteins (MHC) or MHC-like molecules. Accumulating evidence indicates that the germline-encoded TCR segments have features that promote binding to MHC and MHC-like molecules, suggesting coevolution between TCR and MHC molecules. Here, we assess directly the evolutionary conservation of αβ TCR specificity for MHC. Sequence comparisons showed that some Vβs from distantly related jawed vertebrates share amino acids in their complementarity determining region 2 (CDR2). Chimeric TCRs containing amphibian, bony fish, or cartilaginous fish Vβs can recognize antigens presented by mouse MHC class II and CD1d (an MHC-like protein), and this recognition is dependent upon the shared CDR2 amino acids. These results indicate that features of the TCR that control specificity for MHC and MHC-like molecules were selected early in evolution and maintained between species that last shared a common ancestor more than 400 million years ago.

Invariant natural killer T cells recognize glycolipids from pathogenic Gram-positive bacteria

Natural killer T cells (NKT cells) recognize glycolipid antigens presented by CD1d. These cells express an evolutionarily conserved, invariant T cell antigen receptor (TCR), but the forces that drive TCR conservation have remained uncertain. Here we show that NKT cells recognized diacylglycerol-containing glycolipids from Streptococcus pneumoniae, the leading cause of community-acquired pneumonia, and group B Streptococcus, which causes neonatal sepsis and meningitis. Furthermore, CD1d-dependent responses by NKT cells were required for activation and host protection. The glycolipid response was dependent on vaccenic acid, which is present in low concentrations in mammalian cells. Our results show how microbial lipids position the sugar for recognition by the invariant TCR and, most notably, extend the range of microbes recognized by this conserved TCR to several clinically important bacteria.

Mutagenesis of beryllium-specific TCRs suggests an unusual binding topology for antigen recognition

Unconventional Ags, such as metals, stimulate T cells in a very specific manner. To delineate the binding landscape for metal-specific T cell recognition, alanine screens were performed on a set of Be-specific TCRs derived from the lung of a chronic beryllium disease patient. These TCRs are HLA-DP2-restricted and express nearly identical TCR Vβ5.1 chains coupled with different TCR α-chains. Site-specific mutagenesis of all amino acids comprising the CDRs of the TCRA and TCRB genes showed a dominant role for Vβ5.1 residues in Be recognition, with little contribution from the TCR α-chain. Solvent-exposed residues along the α-helices of the HLA-DP2 α- and β-chains were also mutated to alanine. Two β-chain residues, located near the proposed Be binding site of HLA-DP2, played a dominant role in T cell recognition with no contribution from the HLA-DP2 α-chain. These findings suggest that Be-specific T cells recognize Ag using an unconventional binding topology, with the majority of interactions contributed by TCR Vβ5.1 residues and the HLA-DP2 β1-chain. Thus, unusual docking topologies are not exclusively used by autoreactive T cells, but also for the recognition of unconventional metal Ags, such as Be.

New ways to turn on NKT cells

Godfrey and Rossjohn discuss the varied ways to turn on NKT cells in the context of recent findings.

Natural killer T (NKT) cells are CD1d-restricted, lipid antigen–reactive T cells with powerful immunoregulatory potential. The prototypic antigen for NKT cells is a marine sponge–derived glycolipid, α-galactosylceramide (α-GalCer), but this is not normally encountered in the mammalian environment. Thus, there is great interest in the identification of more physiological stimuli for NKT cells, and numerous studies have shown that NKT cells are capable of responding to a range of microbial lipid-based antigens. Two new studies expand our understanding of environmental NKT cell stimuli, with one showing that CD1d-restricted NKT cell antigens are present within common house dust extract (HDE), whereas the other shows that NKT cells can respond to innate stimuli irrespective of the presence of foreign microbial antigens. Collectively, these two investigations indicate that NKT cells are far more likely to encounter foreign antigens, or innate activating signals, than previously recognized, suggesting a more central role for these cells in the immune system.

Cutting edge: structural basis for the recognition of β-linked glycolipid antigens by invariant NKT cells

Invariant NKT (iNKT) cells expressing a semi-invariant Vα14 TCR recognize self and foreign lipid Ags when presented by the nonclassical MHCI homolog CD1d. Whereas the majority of known iNKT cell Ags are characterized by the presence of a single α-linked sugar, mammalian self Ags are β-linked glycosphingolipids, posing the interesting question of how the semi-invariant TCR can bind to such structurally distinct ligands. In this study, we show that the mouse iNKT TCR recognizes the complex β-linked Ag isoglobotrihexosylceramide (iGb3; Galα1-3-Galβ1-4-Glcβ1-1Cer) by forcing the proximal β-linked sugar of the trisaccharide head group to adopt the typical binding orientation of α-linked glycolipids. The squashed iGb3 orientation is stabilized by several interactions between the trisaccharide and CD1d residues. Finally, the formation of novel contacts between the proximal and second sugar of iGb3 and CDR2α residues of the TCR suggests an expanded recognition logic that can possibly distinguish foreign Ags from self Ags.

Recognition of β-linked self glycolipids mediated by natural killer T cell antigen receptors

The most potent foreign antigens for natural killer T cells (NKT cells) are α-linked glycolipids, whereas NKT cell self-reactivity involves weaker recognition of structurally distinct β-linked glycolipid antigens. Here we provide the mechanism for the autoreactivity of T cell antigen receptors (TCRs) on NKT cells to the mono- and tri-glycosylated β-linked agonists β-galactosylceramide (β-GalCer) and isoglobotrihexosylceramide (iGb3), respectively. In binding these disparate antigens, the NKT cell TCRs docked onto CD1d similarly, achieving this by flattening the conformation of the β-linked ligands regardless of the size of the glycosyl head group. Unexpectedly, the antigenicity of iGb3 was attributable to its terminal sugar group making compensatory interactions with CD1d. Thus, the NKT cell TCR molds the β-linked self ligands to resemble the conformation of foreign α-linked ligands, which shows that induced-fit molecular mimicry can underpin the self-reactivity of NKT cell TCRs to β-linked antigens.

A single T cell receptor bound to major histocompatibility complex class I and class II glycoproteins reveals switchable TCR conformers

Major histocompatibility complex class I (MHCI) and MHCII proteins differ in structure and sequence. To understand how T cell receptors (TCRs) can use the same set of variable regions to bind both proteins, we have presented a comparison of a single TCR bound to both MHCI and MHCII ligands. The TCR adopts similar orientations on both ligands with TCR amino acids thought to be evolutionarily conserved for MHC interaction occupying similar positions on the MHCI and MHCII helices. However, the TCR antigen-binding loops use different conformations when interacting with each ligand. Most importantly, we observed alternate TCR core conformations. When bound to MHCI, but not MHCII, Vα disengages from the Jα β strand, switching Vα's position relative to Vβ. In several other structures, either Vα or Vβ undergoes this same modification. Thus, both TCR V-domains can switch among alternate conformations, perhaps extending their ability to react with different MHC-peptide ligands.

A semi-invariant Vα10+ T cell antigen receptor defines a population of natural killer T cells with distinct glycolipid antigen-recognition properties

Type I natural killer T cells (NKT cells) are characterized by an invariant variable region 14-joining region 18 (V(α)14-J(α)18) T cell antigen receptor (TCR) α-chain and recognition of the glycolipid α-galactosylceramide (α-GalCer) restricted to the antigen-presenting molecule CD1d. Here we describe a population of α-GalCer-reactive NKT cells that expressed a canonical V(α)10-J(α)50 TCR α-chain, which showed a preference for α-glucosylceramide (α-GlcCer) and bacterial α-glucuronic acid-containing glycolipid antigens. Structurally, despite very limited TCRα sequence identity, the V(α)10 TCR-CD1d-α-GlcCer complex had a docking mode similar to that of type I TCR-CD1d-α-GalCer complexes, although differences at the antigen-binding interface accounted for the altered antigen specificity. Our findings provide new insight into the structural basis and evolution of glycolipid antigen recognition and have notable implications for the scope and immunological role of glycolipid-specific T cell responses.

Invariant NKT cells are required for airway inflammation induced by environmental antigens

House dust contains antigens capable of activating mouse and human iNKT cells, contributing to allergen-induced airway inflammation.

Invariant NKT cells (iNKT cells) are a unique subset of T lymphocytes that rapidly carry out effector functions. In this study, we report that a majority of sterile house dust extracts (HDEs) tested contained antigens capable of activating mouse and human iNKT cells. HDEs had adjuvant-like properties in an ovalbumin (OVA)-induced asthma model, which were dependent on Vα14i NKT cells, as vaccinated animals deficient for iNKT cells displayed significantly attenuated immune responses and airway inflammation. Furthermore, the administration of HDEs together with OVA mutually augmented the synthesis of cytokines by Vα14i NKT cells and by conventional CD4⁺ T cells in the lung, demonstrating a profound immune response synergy for both Th2 cytokines and IL-17A. These data demonstrate that iNKT cell antigens are far more widely dispersed in the environment than previously anticipated. Furthermore, as the antigenic activity in different houses varied greatly, they further suggest that iNKT cell responses to ambient antigens, particular to certain environments, might promote sensitization to conventional respiratory allergens.

A molecular basis for NKT cell recognition of CD1d-self-antigen

The antigen receptor for natural killer T cells (NKT TCR) binds CD1d-restricted microbial and self-lipid antigens, although the molecular basis of self-CD1d recognition is unclear. Here, we have characterized NKT TCR recognition of CD1d molecules loaded with natural self-antigens (Ags) and report the 2.3 Å resolution structure of an autoreactive NKT TCR-phosphatidylinositol-CD1d complex. NKT TCR recognition of self- and foreign antigens was underpinned by a similar mode of germline-encoded recognition of CD1d. However, NKT TCR autoreactivity is mediated by unique sequences within the non-germline-encoded CDR3β loop encoding for a hydrophobic motif that promotes self-association with CD1d. Accordingly, NKT cell autoreactivity may arise from the inherent affinity of the interaction between CD1d and the NKT TCR, resulting in the recognition of a broad range of CD1d-restricted self-antigens. This demonstrates that multiple self-antigens can be recognized in a similar manner by autoreactive NKT TCRs.

A molecular basis for the exquisite CD1d-restricted antigen specificity and functional responses of natural killer T cells

Natural killer T (NKT) cells respond to a variety of CD1d-restricted antigens (Ags), although the basis for Ag discrimination by the NKT cell receptor (TCR) is unclear. Here we have described NKT TCR fine specificity against several closely related Ags, termed altered glycolipid ligands (AGLs), which differentially stimulate NKT cells. The structures of five ternary complexes all revealed similar docking. Acyl chain modifications did not affect the interaction, but reduced NKT cell proliferation, indicating an affect on Ag processing or presentation. Conversely, truncation of the phytosphingosine chain caused an induced fit mode of TCR binding that affected TCR affinity. Modifications in the glycosyl head group had a direct impact on the TCR interaction and associated cellular response, with ligand potency reflecting the t(1/2) life of the interaction. Accordingly, we have provided a molecular basis for understanding how modifications in AGLs can result in striking alterations in the cellular response of NKT cells.

Natural killer T cells in health and disease

Natural killer T (NKT) cells are a subset of T lymphocytes that share surface markers and functional characteristics with both conventional T lymphocytes and natural killer cells. Most NKT cells express a semi-invariant T cell receptor that reacts with glycolipid antigens presented by the major histocompatibility complex class I-related protein CD1d on the surface of antigen-presenting cells. NKT cells become activated during a variety of infections and inflammatory conditions, rapidly producing large amounts of immunomodulatory cytokines. NKT cells can influence the activation state and functional properties of multiple other cell types in the immune system and, thus, modulate immune responses against infectious agents, autoantigens, tumors, tissue grafts and allergens. One attractive aspect of NKT cells is that their immunomodulatory activities can be readily harnessed with cognate glycolipid antigens, such as the marine sponge-derived glycosphingolipid alpha-galactosylceramide. These properties of NKT cells are being exploited for therapeutic intervention to prevent or treat cancer, infections, and autoimmune and inflammatory diseases.

Genetic and structural basis for selection of a ubiquitous T cell receptor deployed in Epstein-Barr virus infection

Despite the ∼10(18) αβ T cell receptor (TCR) structures that can be randomly manufactured by the human thymus, some surface more frequently than others. The pinnacles of this distortion are public TCRs, which exhibit amino acid-identical structures across different individuals. Public TCRs are thought to result from both recombinatorial bias and antigen-driven selection, but the mechanisms that underlie inter-individual TCR sharing are still largely theoretical. To examine this phenomenon at the atomic level, we solved the co-complex structure of one of the most widespread and numerically frequent public TCRs in the human population. The archetypal AS01 public TCR recognizes an immunodominant BMLF1 peptide, derived from the ubiquitous Epstein-Barr virus, bound to HLA-A*0201. The AS01 TCR was observed to dock in a diagonal fashion, grasping the solvent exposed peptide crest with two sets of complementarity-determining region (CDR) loops, and was fastened to the peptide and HLA-A*0201 platform with residue sets found only within TCR genes biased in the public response. Computer simulations of a random V(D)J recombination process demonstrated that both TCRα and TCRβ amino acid sequences could be manufactured easily, thereby explaining the prevalence of this receptor across different individuals. Interestingly, the AS01 TCR was encoded largely by germline DNA, indicating that the TCR loci already comprise gene segments that specifically recognize this ancient pathogen. Such pattern recognition receptor-like traits within the αβ TCR system further blur the boundaries between the adaptive and innate immune systems.

The Vα14 invariant natural killer T cell TCR forces microbial glycolipids and CD1d into a conserved binding mode

Invariant natural killer T cells (iNKT cells) rapidly produce effector cytokines. In this study, we report the first crystal structures of the iNKT cell T cell receptor (TCR) bound to two natural, microbial glycolipids presented by CD1d. Binding of the TCR induced CDR3-α-dependent structural changes in the F' roof of CD1d; these changes resemble those occurring in the absence of TCR engagement when the highly potent synthetic antigen α-galactosylceramide (α-GalCer) binds CD1d. Furthermore, in the Borrelia burgdorferi α-galactosyl diacylglycerol-CD1d complex, TCR binding caused a marked repositioning of the galactose sugar into an orientation that closely resembles α-GalCer. The TCR-dependent reorientation of the sugar, together with the induced CD1d fit, may explain the weaker potency of the microbial antigens compared with α-GalCer. We propose that the TCR of iNKT cells binds with a conserved footprint onto CD1d, regardless of the bound glycolipid antigen, and that for microbial antigens this unique binding mode requires TCR-initiated conformational changes.

Enzymatic synthesis of Gb3 and iGb3 ceramides

Gb3 and iGb3 are physiologically important trihexosylceramides with a terminal alpha-d-Galp-(1-->4)-beta-d-Galp- and alpha-d-Galp-(1-->3)-beta-d-Galp sequence, respectively. In particular iGb3 is attracting considerable attention as it is believed to serve as a ligand for natural killer T cells. Whether or not iGb3 is present in humans and which enzyme might be responsible for its synthesis is at present a matter of lively debate. In the current investigation we evaluated human blood group B galactosyltransferase (GTB) for its ability to catalyze the formation of iGb3 from lactosylceramide and UDP-Galp. GTB is a retaining glycosyltransferase that in vivo catalyzes the transfer of galactose from UDP-Galp donors to OH-3 of Galp on the H-antigen (alpha-l-Fucp-(1-->2)-beta-d-Galp) acceptor forming the blood group B antigen. GTB tolerates modifications in donor and acceptor substrates and its ability to accept lactosides as acceptors makes it a possible candidate for iGb3 production in humans. For comparison iGb3 and Gb3 were also synthesized from the same acceptor using an alpha-(1-->3)- and alpha-(1-->4)-specific galactosyltransferase, respectively. All the enzymes tested catalyzed the desired reactions. Product characterization by NMR analysis clearly differentiated between the alpha-Galp-(1-->3)-Galp and alpha-Galp-(1-->4)-Galp product, with the GTB product being identical to that of the alpha-(1-->3)-GalT-catalyzed reaction. The rate of transfer by GTB however was very low, only 0.001% of the rate obtained with a good substrate, H antigen disaccharide (octyl alpha-l-Fucp-(1-->2)-beta-d-Galp). This is too low to account for the possible formation of the iGb3 structure in humans in vivo.

Human mucosal associated invariant T cells detect bacterially infected cells

A first indication of the biological role of mucosal associated invariant T (MAIT) cells reveals that this discrete T cell subset is broadly reactive to bacterial infection. In particular MAIT cells recognize Mycobacterium tuberculosis-infected lung airway epithelial cells via the most evolutionarily conserved major histocompatibility molecule.

Control of infection with Mycobacterium tuberculosis (Mtb) requires Th1-type immunity, of which CD8⁺ T cells play a unique role. High frequency Mtb-reactive CD8⁺ T cells are present in both Mtb-infected and uninfected humans. We show by limiting dilution analysis that nonclassically restricted CD8⁺ T cells are universally present, but predominate in Mtb-uninfected individuals. Interestingly, these Mtb-reactive cells expressed the Vα7.2 T-cell receptor (TCR), were restricted by the nonclassical MHC (HLA-Ib) molecule MR1, and were activated in a transporter associated with antigen processing and presentation (TAP) independent manner. These properties are all characteristics of mucosal associated invariant T cells (MAIT), an “innate” T-cell population of previously unknown function. These MAIT cells also detect cells infected with other bacteria. Direct ex vivo analysis demonstrates that Mtb-reactive MAIT cells are decreased in peripheral blood mononuclear cells (PBMCs) from individuals with active tuberculosis, are enriched in human lung, and respond to Mtb-infected MR1-expressing lung epithelial cells. Overall, these findings suggest a generalized role for MAIT cells in the detection of bacterially infected cells, and potentially in the control of bacterial infection.

About one-third of the world's population is infected with Mycobacterium tuberculosis (Mtb), yet thanks to a robust immune response most infected people remain healthy. CD8 T cells are unique in detecting intracellular infections. Surprisingly, Mtb-reactive CD8 T cells are found in humans with no prior exposure to Mtb. We show that mucosal associated invariant T (MAIT) cells, which have no previously known in vivo function, make up a proportion of these Mtb-reactive CD8 T cells and detect Mtb-infected cells via a specific major histocompatibility molecule called MHC-related molecule 1, which is evolutionarily conserved among mammals. Mtb-reactive MAIT cells are enriched in lung and detect primary Mtb-infected lung epithelial cells from the airway where initial exposure to Mtb occurs. We go on to show that MAIT cells are not specific for Mtb since they can detect cells infected with a variety of other bacteria. Curiously, Mtb-reactive MAIT cells are absent in the blood of individuals with active tuberculosis. We postulate that MAIT cells are innate detectors of bacterial infection poised to play a role in control of intracellular infection.

Innate-like control of human iNKT cell autoreactivity via the hypervariable CDR3beta loop

Invariant Natural Killer T cells (iNKT) are a versatile lymphocyte subset with important roles in both host defense and immunological tolerance. They express a highly conserved TCR which mediates recognition of the non-polymorphic, lipid-binding molecule CD1d. The structure of human iNKT TCRs is unique in that only one of the six complementarity determining region (CDR) loops, CDR3beta, is hypervariable. The role of this loop for iNKT biology has been controversial, and it is unresolved whether it contributes to iNKT TCR:CD1d binding or antigen selectivity. On the one hand, the CDR3beta loop is dispensable for iNKT TCR binding to CD1d molecules presenting the xenobiotic alpha-galactosylceramide ligand KRN7000, which elicits a strong functional response from mouse and human iNKT cells. However, a role for CDR3beta in the recognition of CD1d molecules presenting less potent ligands, such as self-lipids, is suggested by the clonal distribution of iNKT autoreactivity. We demonstrate that the human iNKT repertoire comprises subsets of greatly differing TCR affinity to CD1d, and that these differences relate to their autoreactive functions. These functionally different iNKT subsets segregate in their ability to bind CD1d-tetramers loaded with the partial agonist alpha-linked glycolipid antigen OCH and structurally different endogenous beta-glycosylceramides. Using surface plasmon resonance with recombinant iNKT TCRs and different ligand-CD1d complexes, we demonstrate that the CDR3beta sequence strongly impacts on the iNKT TCR affinity to CD1d, independent of the loaded CD1d ligand. Collectively our data reveal a crucial role for CDR3beta for the function of human iNKT cells by tuning the overall affinity of the iNKT TCR to CD1d. This mechanism is relatively independent of the bound CD1d ligand and thus forms the basis of an inherent, CDR3beta dependent functional hierarchy of human iNKT cells.

Vgamma2Vdelta2 T Cell Receptor recognition of prenyl pyrophosphates is dependent on all CDRs

gammadelta T cells differ from alphabeta T cells in the Ags they recognize and their functions in immunity. Although most alphabeta TCRs recognize peptides presented by MHC class I or II, human gammadelta T cells expressing Vgamma2Vdelta2 TCRs recognize nonpeptide prenyl pyrophosphates. To define the molecular basis for this recognition, the effect of mutations in the TCR CDR was assessed. Mutations in all CDR loops altered recognition and cover a large footprint. Unlike murine gammadelta TCR recognition of the MHC class Ib T22 protein, there was no CDR3delta motif required for recognition because only one residue is required. Instead, the length and sequence of CDR3gamma was key. Although a prenyl pyrophosphate-binding site was defined by Lys109 in Jgamma1.2 and Arg51 in CDR2delta, the area outlined by critical mutations is much larger. These results show that prenyl pyrophosphate recognition is primarily by germline-encoded regions of the gammadelta TCR, allowing a high proportion of Vgamma2Vdelta2 TCRs to respond. This underscores its parallels to innate immune receptors. Our results also provide strong evidence for the existence of an Ag-presenting molecule for prenyl pyrophosphates.

Synthetic iNKT cell-agonists as vaccine adjuvants--finding the balance

The unique position of invariant natural killer T (iNKT) cells at the interface of the innate and adaptive arms of the immune response, combined with their ability to modulate the activity of antigen-presenting cells, has led to their intensive investigation as a means of augmenting the immune response both in vaccination strategies for microbial infections and in tumor immunotherapy. Several synthetic iNKT-cell agonists that have potential as vaccine adjuvants have been identified, but these are not without their limitations-strong agonists can lead to the undesirable effects associated with overstimulation of the immune system, whereas too weak agonists may provide insufficient iNKT cell help to stimulate maturation of dendritic cells and differentiation of B cells. In this article we explore strategies being investigated as means of increasing the specificity of and controlling the magnitude of the immune response generated by activation of iNKT cells with synthetic agonists.

Oligoclonality and innate-like features in the TCR repertoire of type II NKT cells reactive to a beta-linked self-glycolipid

TCR-mediated recognition of beta-linked self-glycolipids bound to CD1d is poorly understood. Here, we have characterized the TCR repertoire of a CD1d-restricted type II NKT cell subset reactive to sulfatide involved in the regulation of autoimmunity and antitumor immunity. The sulfatide/CD1d-tetramer(+) cells isolated from naïve mice show an oligoclonal TCR repertoire with predominant usage of the Valpha3/Valpha1-Jalpha7/Jalpha9 and Vbeta8.1/Vbeta3.1-Jbeta2.7 gene segments. The CDR3 regions of both the alpha- and beta-chains are encoded by either germline or nongermline gene segments of limited lengths containing several conserved residues. Presence of dominant clonotypes with limited TCR gene usage for both TCR alpha- and beta-chains in type II NKT cells reflects specific antigen recognition not found in the type I NKT cells but similar to the MHC-restricted T cells. Although potential CD1d-binding tyrosine residues in the CDR2beta region are conserved between most type I and type II NKT TCRs, CDR 1alpha and 3alpha regions differ significantly between the two subsets. Collectively, the TCR repertoire of sulfatide-reactive type II NKT cells exhibits features of both antigen-specific conventional T cells and innate-like cells, and these findings provide important clues to the recognition of beta-linked glycolipids by CD1d-restricted T cells in general.

Autoreactive natural killer T cells: promoting immune protection and immune tolerance through varied interactions with myeloid antigen-presenting cells

Natural killer T (NKT) cells are innate T lymphocytes that are restricted by CD1d antigen-presenting molecules and recognize lipids and glycolipids as antigens. NKT cells have attracted attention for their potent immunoregulatory effects. Like other types of regulatory lymphocytes, a high proportion of NKT cells appear to be autoreactive to self antigens. Thus, as myeloid antigen-presenting cells (APCs) such as monocytes, dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) constitutively express CD1d, NKT cells are able to interact with these APCs not only during times of immune activation but also in immunologically quiescent periods. The interactions of NKT cells with myeloid APCs can have either pro-inflammatory or tolerizing outcomes, and a central question is how the ensuing response is determined. Here we bring together published results from a variety of model systems to highlight three critical factors that influence the outcome of the NKT-APC interaction: (i) the strength of the antigenic signal delivered to the NKT cell, as determined by antigen abundance and/or T-cell receptor (TCR) affinity; (ii) the presence or absence of cytokines that costimulate NKT cells [e.g. interleukin (IL)-12, IL-18 and interferon (IFN)-alpha]; (iii) APC intrinsic factors such as differentiation state (e.g. monocyte versus DC) and Toll-like receptor (TLR) stimulation. Together with recent findings that demonstrate new links between NKT cell activation and endogenous lipid metabolism, these results outline a picture in which the functions of NKT cells are closely attuned to the existing biological context. Thus, NKT cells may actively promote tolerance until a critical level of danger signals arises, at which point they switch to activating pro-inflammatory immune responses.

iNKT cell autoreactivity: what is 'self' and how is it recognized?

Following stimulation through their T cell receptor, invariant natural killer T (iNKT) cells function as innate effector cells by rapidly releasing large amounts of effector cytokines and chemokines and therefore have an important role in modulating the ensuing immune response. iNKT cells recognize, and are activated by, diverse glycolipid antigens, many of which are found in microorganisms. However, iNKT cells also show some reactivity to 'self'. Here, I outline our current understanding of iNKT cell autoreactivity and propose that several self lipids are probably involved in the positive selection and autoreactivity of iNKT cells.

Alternative cross-priming through CCL17-CCR4-mediated attraction of CTLs toward NKT cell-licensed DCs

Cross-priming allows dendritic cells (DCs) to induce cytotoxic T cell (CTL) responses to extracellular antigens. DCs require cognate 'licensing' for cross-priming, classically by helper T cells. Here we demonstrate an alternative mechanism for cognate licensing by natural killer T (NKT) cells recognizing microbial or synthetic glycolipid antigens. Such licensing caused cross-priming CD8alpha(+) DCs to produce the chemokine CCL17, which attracted naive CTLs expressing the chemokine receptor CCR4. In contrast, DCs licensed by helper T cells recruited CTLs using CCR5 ligands. Thus, depending on the type of antigen they encounter, DCs can be licensed for cross-priming by NKT cells or helper T cells and use at least two independent chemokine pathways to attract naive CTLs. Because these chemokines acted synergistically, this can potentially be exploited to improve vaccinations.

Recent advances in processing and presentation of CD1 bound lipid antigens

It is well established that different populations of alphabeta T lymphocytes can recognize not only peptides in the context of MHC class I and class II molecules, but also foreign and self-lipids in association with CD1 proteins, which share structural similarities with MHC class I molecules. CD1 molecules are comprised of five isoforms, known as group 1 (CD1a, b, c, e) and group 2 (CD1d) CD1, presenting lipid antigens to conventional T lymphocytes or innate-like T cells bearing an invariant T cell receptor (TCR) and known as invariant NKT (iNKT) cells. During the last couple of years, several papers have been published describing important aspects of the mechanisms controlling the processing and presentation of endogenous and exogenous CD1 lipid antigens, which will be the main focus of this review.

Natural killer T cell recognition of lipid antigens

Natural killer T cells recognize lipid antigens in the context of CD1 molecules. Recent publications show that this mode of recognition differs substantially from that of classic T-cell receptor peptide-major histocompatibility complex interaction.

How invariant natural killer T cells respond to infection by recognizing microbial or endogenous lipid antigens

Invariant natural killer T (iNKT) cells have evolved to recognize CD1d-presented lipid antigens and are known to play important roles during infection with bacterial, viral, protozoan, and fungal pathogens. The limited antigen specificity and reactivity to self- and foreign antigens distinguish iNKT cells from MHC-restricted T cells and bear similarity to innate-like lymphocytes, such as NK cells, gammadelta T cells, MZB and B1-B cells. This review summarizes how direct recognition of microbial lipids or synergistic stimulation by self-lipids and pro-inflammatory cytokines results in activation of these innate-like iNKT cell during infection. iNKT cell activation in the absence of foreign antigen recognition is unique for cells bearing TCRs and underscores that not only the function but also the activation mechanism of iNKT cells is innate-like, and distinct from adaptive T cells. The different pathways of activation endow iNKT cells with the ability to respond rapidly to a wide variety of infectious agents and to contribute effectively to the early immune response during infection.