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

The molecular basis for recognition of CD1d/α-galactosylceramide by a human non-Vα24 T cell receptor

Human Vα24− CD1d-restricted T cells use variation in their CDR1α loop to respond to lipid antigens presented by CD1d, altering their specificities from that of invariant natural killer T cells.

CD1d-mediated presentation of glycolipid antigens to T cells is capable of initiating powerful immune responses that can have a beneficial impact on many diseases. Molecular analyses have recently detailed the lipid antigen recognition strategies utilized by the invariant Vα24-Jα18 TCR rearrangements of iNKT cells, which comprise a subset of the human CD1d-restricted T cell population. In contrast, little is known about how lipid antigens are recognized by functionally distinct CD1d-restricted T cells bearing different TCRα chain rearrangements. Here we present crystallographic and biophysical analyses of α-galactosylceramide (α-GalCer) recognition by a human CD1d-restricted TCR that utilizes a Vα3.1-Jα18 rearrangement and displays a more restricted specificity for α-linked glycolipids than that of iNKT TCRs. Despite having sequence divergence in the CDR1α and CDR2α loops, this TCR employs a convergent recognition strategy to engage CD1d/αGalCer, with a binding affinity (∼2 µM) almost identical to that of an iNKT TCR used in this study. The CDR3α loop, similar in sequence to iNKT-TCRs, engages CD1d/αGalCer in a similar position as that seen with iNKT-TCRs, however fewer actual contacts are made. Instead, the CDR1α loop contributes important contacts to CD1d/αGalCer, with an emphasis on the 4′OH of the galactose headgroup. This is consistent with the inability of Vα24− T cells to respond to α-glucosylceramide, which differs from αGalCer in the position of the 4′OH. These data illustrate how fine specificity for a lipid containing α-linked galactose is achieved by a TCR structurally distinct from that of iNKT cells.

Certain lineages of T cells can recognize lipids as stimulatory antigens when presented in the context of CD1 molecules. We know how most Natural Killer T (NKT) cells react with this unusual ligand because they use a single invariant T cell receptor (TCR) alpha chain to do the job. NKT cells place particular emphasis on their CDR3α and CDR2β loops in recognition of antigen—these complementarity determining regions (CDRs) are the hypervariable parts of the TCR that “complement” an antigen's shape. How do these other T cells recognize closely related yet distinct lipid antigens? Here we show that human CD1d-restricted T cells, typically called Vα24− T cells due to their use of diverse Vα domains in their TCRs, use similar molecular strategies to respond to lipid antigens presented by CD1d. To this end we present a 2.5 Å complex structure of a Vα24− TCR complexed with CD1d presenting the protypical lipid, α-galactosylceramide (αGalCer). The TCR examined in this study notably shifts its binding slightly, placing more emphasis on the interaction with the CDR1α loop as revealed through alanine scanning mutagenesis. This shift explains the inability of these T cells to respond to lipids that vary at this site of contact (the 4'OH), like the related α-linked glucosylceramide. These results provide a molecular basis for the fine-specificity of different CD1d-restricted T cell lineages.

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.

Type II natural killer T cells use features of both innate-like and conventional T cells to recognize sulfatide self antigens

Glycolipids presented by the major histocompatibility complex (MHC) class I homolog CD1d are recognized by natural killer T cells (NKT cells) characterized by either a semi-invariant T cell antigen receptor (TCR) repertoire (type I NKT cells or iNKT cells) or a relatively variable TCR repertoire (type II NKT cells). Here we describe the structure of a type II NKT cell TCR in complex with CD1d-lysosulfatide. Both TCR α-chains and TCR β-chains made contact with the CD1d molecule with a diagonal footprint, typical of MHC-TCR interactions, whereas the antigen was recognized exclusively with a single TCR chain, similar to the iNKT cell TCR. Type II NKT cell TCRs, therefore, recognize CD1d-sulfatide complexes by a distinct recognition mechanism characterized by the TCR-binding features of both iNKT cells and conventional peptide-reactive T cells.

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.

Structure-activity relationship studies of novel glycosphingolipids that stimulate natural killer T-cells

KRN7000, an anticancer drug candidate developed by Kirin Brewery Co. in 1995, is an α-galactosyl ceramide. It is a ligand making a complex with CD1d protein, and it stimulates invariant natural killer T (NKT) cells, which are one of the lineages of immunocytes. NKT cells activated by recognition of the CD1d/KRN7000 complex with its invariant T-cell receptor (TCR) can induce both protective and regulatory immune responses. To determine the recognition and activation mechanisms of NKT cells and to develop drug candidates more effective than KRN7000, a large number of analogs of KRN7000 have been synthesized. Some of them show potent bioactivities and have the potential of being utilized as therapeutic agents. In this review, structure-activity relationship studies of novel glycolipids which stimulate NKT cells efficiently are summarized.

Species-specific activity of glycolipid ligands for invariant NKT cells

The immunomodulatory glycolipid α-galactosylceramide (α-GalCer) binds to CD1d and exhibits potent activity as a ligand for invariant CD1d-restricted natural killer-like T cells (iNKT cells). Structural analogues of α-GalCer have been synthesised to determine which components are required for CD1d presentation and iNKT cell activation, however, to date the importance of the phytosphingosine 4-hydroxyl for iNKT cell activation has been disputed. To clarify this, we synthesised two 4-deoxy α-GalCer analogues (sphinganine and sphingosine) and investigated their ability to activate murine and human iNKT cells. Analysis revealed that the analogues possessed comparable activity to α-GalCer in stimulating murine iNKT cells, but were severely compromised in their ability to stimulate human iNKT cells. Here we determined that species-specific glycolipid activity was due to a lack of recognition of the analogues by the T-cell receptors on human iNKT cells rather than insufficient presentation of the analogues on human CD1d molecules. From these results we suggest that glycolipids developed for potent iNKT cell activity in humans should contain a phytosphingosine base.

Glycolipids that elicit IFN-γ-biased responses from natural killer T cells

Natural killer T (NKT) cells recognize glycolipids presented by CD1d. The first antigen described, α-galactosyl ceramide (αGalCer), is a potential anticancer agent whose activity depends upon IFN-γ secretion. We report two analogs of αGalCer based on a naturally occurring glycosphingolipid, plakoside A. These compounds induce enhanced IFN-γ that correlates with detergent-resistant binding to CD1d and an increased stability of the lipid-CD1d complexes on antigen-presenting cells. Structural analysis on one of the analogs indicates that it is more deeply bound inside the CD1d groove, suggesting tighter lipid-CD1d interactions. To our knowledge, this is the first example in which structural information provides an explanation for the increased lipid-CD1d stability, likely responsible for the Th1 bias. We provide insights into the mechanism of IFN-γ-inducing compounds, and because our compounds activate human NKT cells, they could have therapeutic utility.

Boosting the Immune Response: The Use of iNKT cell ligands as vaccine adjuvants

Natural killer T (NKT) cells comprise a small, but important T cell subset and are thought to bridge the innate and adaptive immune responses. The discovery of NKT cells and extensive research on their activating ligands have paved the way for modulation of these potent immunoregulatory cells in order to improve the outcome of various clinical conditions. Efforts to modulate NKT cell effector functions have ranged from therapy for influenza to anti-tumor immunotherapy. These approaches have also led to the use of NKT cell agonists such as α-Galactosylceramide (α-GalCer) and its analogs as vaccine adjuvants, an approach that is aimed at boosting specific B and T cell responses to a vaccine candidate by concomitant activation of NKT cells. In this review we will provide a comprehensive overview of the efforts made in using α-GalCer and its analogs as vaccine adjuvants. The diverse array of vaccination strategies used, as well as the role of NKT cell activating adjuvants will be discussed, with focus on vaccines against malaria, HIV, influenza and tumor vaccines. Collectively, these studies demonstrate the efficacy of NKT cell-specific agonists as adjuvants and further suggest that these compounds warrant serious consideration during the development of vaccination strategies.

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.

Structural and functional characterization of a novel nonglycosidic type I NKT agonist with immunomodulatory properties

Activation of type I NKT (iNKT) cells by CD1d-presented agonists is a potent immunotherapeutic tool. α-Galactosylceramide (α-GalCer) is the prototypic agonist, but its excessive potency with simultaneous production of both pro- and anti-inflammatory cytokines hampers its potential therapeutic use. In search for novel agonists, we have analyzed the structure and function of HS44, a synthetic aminocyclitolic ceramide analog designed to avoid unrestrained iNKT cell activation. HS44 is a weaker agonist compared with α-GalCer in vitro, although in vivo it induces robust IFN-γ production, and highly reduced but still functional Th2 response. The characteristic cytokine storm produced upon α-GalCer activation was not induced. Consequently, HS44 induced a very efficient iNKT cell-dependent antitumoral response in B16 animal model. In addition, intranasal administration showed the capacity to induce lung inflammation and airway hyperreactivity, a cardinal asthma feature. Thus, HS44 is able to elicit functional Th1 or Th2 responses. Structural studies show that HS44 binds to CD1d with the same conformation as α-GalCer. The TCR binds to HS44 similarly as α-GalCer, but forms less contacts, thus explaining its weaker TCR affinity and, consequently, its weaker recognition by iNKT cells. The ability of this compound to activate an efficient, but not massive, tailored functional immune response makes it an attractive reagent for immune manipulation.

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.

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.

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 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.

Galactose-modified iNKT cell agonists stabilized by an induced fit of CD1d prevent tumour metastasis

Invariant natural killer T (iNKT) cells are known to have marked immunomodulatory capacity due to their ability to produce copious amounts of effector cytokines. Here, we report the structure and function of a novel class of aromatic α-galactosylceramide structurally related glycolipids with marked Th1 bias in both mice and men, leading to superior tumour protection in vivo. The strength of the Th1 response correlates well with enhanced lipid binding to CD1d as a result of an induced fit mechanism that binds the aromatic substitution as a third anchor, in addition to the two lipid chains. This induced fit is in contrast to another Th1-biasing glycolipid, α-C-GalCer, whose CD1d binding follows a conventional key-lock principle. These findings highlight the previously unexploited flexibility of CD1d in accommodating galactose-modified glycolipids and broaden the range of glycolipids that can stimulate iNKT cells. We speculate that glycolipids can be designed that induce a similar fit, thereby leading to superior and more sustained iNKT cell responses in vivo.

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.

Introduction of aromatic group on 4'-OH of α-GalCer manipulated NKT cell cytokine production

The glycosphingolipid α-GalCer has been found to influence mammalian immune system significantly through the natural killer T cells. Unfortunately, the pre-clinical and clinical studies revealed several critical disadvantages that prevented the therapeutic application of α-GalCer in treating cancer and other diseases. Recently, the detailed illustration of the CD1d/α-GalCer/NKT TCR complex crystal structural, together with other latest structural and biological understanding on glycolipid ligands and NKT cells, provided a new platform for developing novel glycolipid ligands with optimized therapeutic effects. Here, we designed a series of novel aromatic group substituted α-GalCer analogues. The biological activity of these analogues was characterized and the results showed the unique substitution group manipulated the immune responses of NKT cells. Computer modeling and simulation study indicated the analogues had unique binding mode when forming CD1d/glycolipid/NKT TCR complex, comparing to original α-GalCer.