The length of lipids bound to human CD1d molecules modulates the affinity of NKT cell TCR and the threshold of NKT cell activation

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.

Structure-guided design of an invariant natural killer T cell agonist for optimum protection from type 1 diabetes in non-obese diabetic mice

Because invariant natural killer T (iNK T) cells link innate and adaptive immunity, the structure-dependent design of iNK T cell agonists may have therapeutic value as vaccines for many indications, including autoimmune disease. Previously, we showed that treatment of non-obese diabetic (NOD) mice with the iNK T cell activating prototypic glycolipid α-galactosylceramide (α-GalCer) protects them from type 1 diabetes (T1D). However, α-GalCer is a strong agonist that can hyperactivate iNK T cells, elicit several side effects and has shown only limited success in clinical trials. Here, we used a structure-guided design approach to identify an iNK T cell agonist that optimally protects from T1D with minimal side effects. Analyses of the kinetics and function of a panel of synthetic α-GalCer fatty acyl chain derivatives (C8:0-C16:0) were performed in NOD mice. C16:0 elicited the highest protection from insulitis and T1D, which was associated with a higher frequency and survival of iNK T cells and enhanced activity of tolerogenic dendritic cells (DCs) in draining pancreatic lymph nodes (PLN), inability to transactivate NK cells and a more rapid kinetics of induction and recovery of iNK T cells from anergy. We conclude that the length and structure of the acyl chain of α-GalCer regulates the level of protection against T1D in mice, and propose that the extent of this protection depends on the relative capacity of the acyl chain to accommodate an endogenous spacer lipid of appropriate length and structure. Thus, our findings with the α-GalCer C16:0 derivative suggest strongly that it be considered as a lead glycolipid candidate in clinical trials of T1D.

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.

Structural reorganization of the antigen-binding groove of human CD1b for presentation of mycobacterial sulfoglycolipids

The mechanisms permitting nonpolymorphic CD1 molecules to present lipid antigens that differ considerably in polar head and aliphatic tails remain elusive. It is also unclear why hydrophobic motifs in the aliphatic tails of some antigens, which presumably embed inside CD1 pockets, contribute to determinants for T-cell recognition. The 1.9-Å crystal structure of an active complex of CD1b and a mycobacterial diacylsulfoglycolipid presented here provides some clues. Upon antigen binding, endogenous spacers of CD1b, which consist of a mixture of diradylglycerols, moved considerably within the lipid-binding groove. Spacer displacement was accompanied by F' pocket closure and an extensive rearrangement of residues exposed to T-cell receptors. Such structural reorganization resulted in reduction of the A' pocket capacity and led to incomplete embedding of the methyl-ramified portion of the phthioceranoyl chain of the antigen, explaining why such hydrophobic motifs are critical for T-cell receptor recognition. Mutagenesis experiments supported the functional importance of the observed structural alterations for T-cell stimulation. Overall, our data delineate a complex molecular mechanism combining spacer repositioning and ligand-induced conformational changes that, together with pocket intricacy, endows CD1b with the required molecular plasticity to present a broad range of structurally diverse antigens.

Knock-down of HEXA and HEXB genes correlate with the absence of the immunostimulatory function of HSC-derived dendritic cells

In an attempt to investigate whether the genetic defect in the HEXA and HEXB genes (which causes the absence of the lysosomal β-N-acetyl-hexosaminidase), are related to the wide inflammation in GM2 gangliosidoses (Tay-Sachs and Sandhoff disease), we have chosen the dendritic cells (DCs) as a study model. Using the RNA interference approach, we generated an in vitro model of HEXs knock-down immunogenic DCs (i-DCs) from CD34(+)-haemopoietic stem cells (CD34(+)-HSCs), thus mimicking the Tay-Sachs (HEXA-/-) and Sandhoff (HEXB-/-) cells. We showed that the absence of β-N-acetyl-hexosaminidase activity does not alter the differentiation of i-DCs from HSCs, but it is critical for the activation of CD4(+)T cells because knock-down of HEXA or HEXB gene causes a loss of function of i-DCs. Notably, the silencing of the HEXA gene had a stronger immune inhibitory effect, thereby indicating a major involvement of β-N-acetyl-hexosaminidase A isoenzyme within this mechanism.

Avidity of CD1d-ligand-receptor ternary complex contributes to T-helper 1 (Th1) polarization and anticancer efficacy

Invariant natural killer T cell (NKT) cells (iNKT cells) produce both T-helper 1 (Th1) and T-helper 2 cytokines in response to α-Galactosylceramide (α-GalCer) stimulation and are thought to be the important effectors in the regulation of both innate and adaptive immunity involved in autoimmune disorders, microbial infections, and cancers. However, the anticancer effects of α-GalCer were limited in early clinical trial. In this study, several analogs of α-GalCer, containing phenyl groups in the lipid tails were found to stimulate murine and human iNKT cells to secrete Th1-skewed cytokines and exhibit greater anticancer efficacy in mice than α-GalCer. We explored the possibility of different Vβ usages of murine Vα14 iNKT or human Vα24 iNKT cells, accounting for differential cytokine responses. However, T-cell receptor Vβ analysis revealed no significant differences in Vβ usages by α-GalCer and these phenyl glycolipid analogs. On the other hand, these phenyl glycolipids showed greater binding avidity and stability for iNKT T-cell receptor when complexed with CD1d. These findings suggest that CD1d-phenyl glycolipid complexes may interact with the same population of iNKT cells but with higher avidity and stability to drive Th1 polarization. Thus, this study provides a key to the rational design of Th1 biased CD1d reactive glycolipids in the future.

New insights into the T cell synapse from single molecule techniques

T cell activation depends on extracellular ligation of the T cell receptor (TCR) by peptide-MHC complexes in a synapse between the T cell and an antigen-presenting cell. The process then requires the assembly of signalling complexes between the TCR and the adaptor protein linker for activation of T cells (LAT), and subsequent filamentous actin (F-actin)-dependent TCR cluster formation. Recent progress in each of these areas, made possible by the emergence of new techniques, has forced us to rethink our assumptions and consider some radical new models. These describe the receptor interaction parameters that control T cell responses and the mechanism by which LAT is recruited to the TCR signalling machinery. This is an exciting time in T cell biology, and further innovation in imaging and genomics is likely to lead to a greater understanding of how T cells are activated.

Identification of self-lipids presented by CD1c and CD1d proteins

The CD1 family consists of five proteins that are related to the peptide-presenting MHC class I family. T cells can recognize the presentation of both foreign and self-derived lipids on four CD1 family members. The identities of the self-lipids capable of stimulating autoreactive T cell responses remain elusive or controversial. Here, we employed mass spectrometry to analyze the lipid content of highly purified CD1c and CD1d protein samples. We report the identification of 11 novel self-lipids presented by CD1c and nine by CD1d. Rigorous controls provide strong evidence that the identified lipids were specifically loaded into the lipid-binding site of the CD1 molecules. The diverse but distinct population of lipids identified from each CD1 family member implies each present a different subset of self-lipids, and the enrichment of particular motifs indicates that the lipids that are presented by CD1 family members could be predicted. Finally, our results imply the CD1 system surveys the endoplasmic reticulum, Golgi apparatus, and/or secretory compartments, in addition to its well characterized surveillance of the endocytic and lysosomal compartments.

CD1b tetramers bind αβ T cell receptors to identify a mycobacterial glycolipid-reactive T cell repertoire in humans

Glucose monomycolate–loaded CD1b tetramers identify a subset of CD4⁺ T cells in patients with Mycobacterium tuberculosis infection.

Microbial lipids activate T cells by binding directly to CD1 and T cell receptors (TCRs) or by indirect effects on antigen-presenting cells involving induction of lipid autoantigens, CD1 transcription, or cytokine release. To distinguish among direct and indirect mechanisms, we developed fluorescent human CD1b tetramers and measured T cell staining. CD1b tetramer staining of T cells requires glucose monomycolate (GMM) antigens, is specific for TCR structure, and is blocked by a recombinant clonotypic TCR comprised of TRAV17 and TRBV4-1, proving that CD1b–glycolipid complexes bind the TCR. GMM-loaded tetramers brightly stain a small subpopulation of blood-derived cells from humans infected with Mycobacterium tuberculosis, providing direct detection of a CD1b-reactive T cell repertoire. Polyclonal T cells from patients sorted with tetramers are activated by GMM antigens presented by CD1b. Whereas prior studies emphasized CD8⁺ and CD4⁻CD8⁻ CD1b-restricted clones, CD1b tetramer-based studies show that nearly all cells express the CD4 co-receptor. These findings prove a cognate mechanism whereby CD1b–glycolipid complexes bind to TCRs. CD1b tetramers detect a natural CD1b-restricted T cell repertoire ex vivo with unexpected features, opening a new investigative path to study the human CD1 system.

Invariant natural killer T cells from children with versus without food allergy exhibit differential responsiveness to milk-derived sphingomyelin

BACKGROUND

A key immunologic feature of food allergy (FA) is the presence of a T(h)2-type cytokine bias. Ligation of the invariant natural killer T cell (iNKT) T-cell receptor (TCR) by sphingolipids presented via the CD1d molecule leads to copious secretion of T(h)2-type cytokines. Major food allergens (eg, milk, egg) are the richest dietary source of sphingolipids (food-derived sphingolipids [food-SLs]). Nonetheless, the role of iNKTs in FA is unknown.

OBJECTIVE

To investigate the role of iNKTs in FA and to assess whether food-SL-CD1d complexes can engage the iNKT-TCR and induce iNKT functions.

METHODS

PBMCs from 15 children with cow's milk allergy (MA), 12 children tolerant to cow's milk but with allergy to egg, and 13 healthy controls were incubated with α-galactosylceramide (αGal), cow's milk-sphingomyelin, or hen's egg-ceramide. iNKTs were quantified, and their cytokine production and proliferation were assessed. Human CD1d tetramers loaded with milk-sphingomyelin or egg-ceramide were used to determine food-SL binding to the iNKT-TCR.

RESULTS

Milk-sphingomyelin, but not egg-ceramide, can engage the iNKT-TCR and induce iNKT proliferation and T(h)2-type cytokine secretion. Children with FA, especially those with MA, had significantly fewer peripheral blood iNKTs and their iNKTs exhibited a greater T(h)2 response to αGal and milk-sphingomyelin than iNKTs of healthy controls.

CONCLUSION

iNKTs from children with FA, especially those with MA, are reduced in number and exhibit a T(h)2 bias in response to αGal and milk-sphingomyelin. These data suggest a potential role for iNKTs in FA.

Centriole polarisation to the immunological synapse directs secretion from cytolytic cells of both the innate and adaptive immune systems

BACKGROUND

Cytolytic cells of the immune system destroy pathogen-infected cells by polarised exocytosis of secretory lysosomes containing the pore-forming protein perforin. Precise delivery of this lethal hit is essential to ensuring that only the target cell is destroyed. In cytotoxic T lymphocytes (CTLs), this is accomplished by an unusual movement of the centrosome to contact the plasma membrane at the centre of the immunological synapse formed between killer and target cells. Secretory lysosomes are directed towards the centrosome along microtubules and delivered precisely to the point of target cell recognition within the immunological synapse, identified by the centrosome. We asked whether this mechanism of directing secretory lysosome release is unique to CTL or whether natural killer (NK) and invariant NKT (iNKT) cytolytic cells of the innate immune system use a similar mechanism to focus perforin-bearing lysosome release.

RESULTS

NK cells were conjugated with B-cell targets lacking major histocompatibility complex class I 721.221 cells, and iNKT cells were conjugated with glycolipid-pulsed CD1-bearing targets, then prepared for thin-section electron microscopy. High-resolution electron micrographs of the immunological synapse formed between NK and iNKT cytolytic cells with their targets revealed that in both NK and iNKT cells, the centrioles could be found associated (or 'docked') with the plasma membrane within the immunological synapse. Secretory clefts were visible within the synapses formed by both NK and iNKT cells, and secretory lysosomes were polarised along microtubules leading towards the docked centrosome. The Golgi apparatus and recycling endosomes were also polarised towards the centrosome at the plasma membrane within the synapse.

CONCLUSIONS

These results reveal that, like CTLs of the adaptive immune system, the centrosomes of NK and iNKT cells (cytolytic cells of the innate immune system) direct secretory lysosomes to the immunological synapse. Morphologically, the overall structure of the immunological synapses formed by NK and iNKT cells are very similar to those formed by CTLs, with both exocytic and endocytic organelles polarised towards the centrosome at the plasma membrane, which forms a focal point for exocytosis and endocytosis within the immunological synapse. We conclude that centrosomal polarisation provides a rapid, responsive and precise mechanism for secretory lysosome delivery to the immunological synapse in CTLs, NK cells and iNKT cells.

The immutable recognition of CD1d

CD1d presents lipid antigens to natural killer T cells. In this issue of Immunity, Wun et al. (2011) and Mallevaey et al. (2011) explore the molecular details of nonself lipid discrimination and self-recognition.

Binding strength and dynamics of invariant natural killer cell T cell receptor/CD1d-glycosphingolipid interaction on living cells by single molecule force spectroscopy

Invariant natural killer T (iNKT) cells are a population of T lymphocytes that play an important role in regulating immunity to infection and tumors by recognizing endogenous and exogenous CD1d-bound lipid molecules. Using soluble iNKT T cell receptor (TCR) molecules, we applied single molecule force spectroscopy for the investigation of the iNKT TCR affinity for human CD1d molecules loaded with glycolipids differing in the length of the phytosphingosine chain using either recombinant CD1d molecules or lipid-pulsed THP1 cells. In both settings, the dissociation of the iNKT TCR from human CD1d molecules loaded with the lipid containing the longer phytosphingosine chain required higher unbinding forces compared with the shorter phytosphingosine lipid. Our findings are discussed in the context of previous results obtained by surface plasmon resonance measurements. We present new insights into the energy landscape and the kinetic rate constants of the iNKT TCR/human CD1d-glycosphingolipid interaction and emphasize the unique potential of single molecule force spectroscopy on living cells.

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.

Stimulation of a shorter duration in the state of anergy by an invariant natural killer T cell agonist enhances its efficiency of protection from type 1 diabetes

We have reported previously that treatment of non-obese diabetic (NOD) mice with the invariant natural killer T (iNK T) cell agonist α-galactosylceramide C26:0 (α-GalCer) or its T helper type 2 (Th2)-biasing derivative α-GalCer C20:2 (C20:2) protects against type 1 diabetes (T1D), with C20:2 yielding greater protection. After an initial response to α-GalCer, iNK T cells become anergic upon restimulation. While such anergic iNK T cells can induce tolerogenic dendritic cells (DCs) that mediate protection from T1D, chronic administration of α-GalCer also results in long-lasting anergy accompanied by significantly reduced iNK T cell frequencies, which raises concerns about its long-term therapeutic use. In this study, our objective was to understand more clearly the roles of anergy and induction of tolerogenic DCs in iNK T cell-mediated protection from T1D and to circumvent potential complications associated with α-GalCer. We demonstrate that NOD iNK T cells activated during multi-dose (MD) treatment in vivo with C20:2 enter into and exit from anergy more rapidly than after activation by α-GalCer. Importantly, this shorter duration of iNK T cells in the anergic state promotes the more rapid induction of tolerogenic DCs and reduced iNK T cell death, and enables C20:2 stimulated iNK T cells to elicit enhanced protection from T1D. Our findings further that suggest C20:2 is a more effective therapeutic drug than α-GalCer for protection from T1D. Moreover, the characteristics of C20:2 provide a basis of selection of next-generation iNK T cell agonists for the prevention of T1D.

Mouse and human iNKT cell agonist β-mannosylceramide reveals a distinct mechanism of tumor immunity

Type 1 or invariant NKT (iNKT) cell agonists, epitomized by α-galactosylceramide, protect against cancer largely by IFN-γ-dependent mechanisms. Here we describe what we believe to be a novel IFN-γ-independent mechanism induced by β-mannosylceramide, which also defines a potentially new class of iNKT cell agonist, with an unusual β-linked sugar. Like α-galactosylceramide, β-mannosylceramide directly activates iNKT cells from both mice and humans. In contrast to α-galactosylceramide, protection by β-mannosylceramide was completely dependent on NOS and TNF-α, neither of which was required to achieve protection with α-galactosylceramide. Moreover, at doses too low for either alone to protect, β-mannosylceramide synergized with α-galactosylceramide to protect mice against tumors. These results suggest that treatment with β-mannosylceramide provides a distinct mechanism of tumor protection that may allow efficacy where other agonists have failed. Furthermore, the ability of β-mannosylceramide to synergize with α-galactosylceramide suggests treatment with this class of iNKT agonist may provide protection against tumors in humans.

Synthesis of truncated analogues of the iNKT cell agonist, α-galactosyl ceramide (KRN7000), and their biological evaluation

Stimulation of iNKT cells by α-galactosyl ceramide (α-GalCer), also known as KRN7000, and its truncated analogue OCH induces both Th1- and Th2-cytokines, with OCH inducing a Th2-cytokine bias. Skewing of the iNKT cells' response towards either a Th1- or Th2-cytokine profile offers potential therapeutic benefits. The length of both the acyl and the sphingosine chains in α-galactosyl ceramides is known to influence the cytokine release profile. We have synthesized analogues of α-GalCer with truncated sphingosine chains for biological evaluation, with particular emphasis on the Th1/Th2 distribution. Starting from a common precursor, d-lyxose, the sphingosine derivatives were synthesised via a straightforward Wittig condensation.

Α-galactosylceramide analogs with weak agonist activity for human iNKT cells define new candidate anti-inflammatory agents

CD1d-restricted natural killer T cells with invariant T cell receptor α chains (iNKT cells) are a unique lymphocyte subset that responds to recognition of specific lipid and glycolipid antigens. They are conserved between mice and humans and exert various immunoregulatory functions through their rapid secretion of a variety of cytokines and secondary activation of dendritic cells, B cells and NK cells. In the current study, we analyzed the range of functional activation states of human iNKT cells using a library of novel analogs of α-galactosylceramide (αGalCer), the prototypical iNKT cell antigen. Measurement of cytokines secreted by human iNKT cell clones over a wide range of glycolipid concentrations revealed that iNKT cell ligands could be classified into functional groups, correlating with weak versus strong agonistic activity. The findings established a hierarchy for induction of different cytokines, with thresholds for secretion being consistently lowest for IL-13, higher for interferon-γ (IFNγ), and even higher for IL-4. These findings suggested that human iNKT cells can be intrinsically polarized to selective production of IL-13 by maintaining a low level of activation using weak agonists, whereas selective polarization to IL-4 production cannot be achieved through modulating the strength of the activating ligand. In addition, using a newly designed in vitro system to assess the ability of human iNKT cells to transactivate NK cells, we found that robust secondary induction of interferon-γ secretion by NK cells was associated with strong but not weak agonist ligands of iNKT cells. These results indicate that polarization of human iNKT cell responses to Th2-like or anti-inflammatory effects may best be achieved through selective induction of IL-13 and suggest potential discrepancies with findings from mouse models that may be important in designing iNKT cell-based therapies in humans.

NKT cell costimulation: experimental progress and therapeutic promise

Invariant natural killer T (iNKT) cells are innate lymphocytes with unique specificity for glycolipid antigens and remarkable immunomodulatory properties. The role of costimulatory interactions in iNKT cell responses has recently come under scrutiny. Although iNKT cells and their prototype glycolipid agonist α-galactosylceramide (α-GalCer) have shown promise in several clinical trials conducted in patients with cancer or viral diseases, current iNKT cell-based therapies are far from effective. The concomitant targeting of T cell receptors (TCRs) and costimulatory molecules on iNKT cells represents an exciting new opportunity to optimize such therapeutic approaches. Here, we review recent advances in our understanding of iNKT cell costimulation and discuss potential treatment modalities based on the responsiveness of iNKT cells to disease-tailored glycolipids and select costimulatory ligands.

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.

Primary deficiency of microsomal triglyceride transfer protein in human abetalipoproteinemia is associated with loss of CD1 function

Abetalipoproteinemia (ABL) is a rare Mendelian disorder of lipid metabolism due to genetic deficiency in microsomal triglyceride transfer protein (MTP). It is associated with defects in MTP-mediated lipid transfer onto apolipoprotein B (APOB) and impaired secretion of APOB-containing lipoproteins. Recently, MTP was shown to regulate the CD1 family of lipid antigen-presenting molecules, but little is known about immune function in ABL patients. Here, we have shown that ABL is characterized by immune defects affecting presentation of self and microbial lipid antigens by group 1 (CD1a, CD1b, CD1c) and group 2 (CD1d) CD1 molecules. In dendritic cells isolated from ABL patients, MTP deficiency was associated with increased proteasomal degradation of group 1 CD1 molecules. Although CD1d escaped degradation, it was unable to load antigens and exhibited functional defects similar to those affecting the group 1 CD1 molecules. The reduction in CD1 function resulted in impaired activation of CD1-restricted T and invariant natural killer T (iNKT) cells and reduced numbers and phenotypic alterations of iNKT cells consistent with central and peripheral CD1 defects in vivo. These data highlight MTP as a unique regulator of human metabolic and immune pathways and reveal that ABL is not only a disorder of lipid metabolism but also an immune disease involving CD1.

Design of a potent CD1d-binding NKT cell ligand as a vaccine adjuvant

The glycolipid alpha-galactosylceramide (alpha-GalCer) has been shown to bind CD1d molecules to activate invariant natural killer T (iNKT) cells, and subsequently induce activation of various immune-competent cells, including dendritic cells, thereby providing a significant adjuvant effect for various vaccines. However, in phase I clinical trials, alpha-GalCer was shown to display only marginal biological activity. In our search for a glycolipid that can exert more potent stimulatory activity against iNKT cells and dendritic cells and produce an adjuvant effect superior to alpha-GalCer, we performed step-wise screening assays on a focused library of 25 alpha-GalCer analogues. Assays included quantification of the magnitude of stimulatory activity against human iNKT cells in vitro, binding affinity to human and murine CD1d molecules, and binding affinity to the invariant t cell receptor of human iNKT cells. Through this rigorous and iterative screening process, we have identified a lead candidate glycolipid, 7DW8-5, that exhibits a superior adjuvant effect than alpha-GalCer on HIV and malaria vaccines in mice.

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.

Therapy for minimal residual tumor disease: beta-galactosylceramide inhibits the growth of recurrent HPV16-associated neoplasms after surgery and chemotherapy

Natural killer T (NKT) cells are potent modulators of antitumor immunity. Their protective effects can be achieved upon their activation by glycolipid ligands presented in the context of the CD1d molecule. These CD1d-binding glycolipid antigens have been described as potent therapeutic agents against tumors, infections, as well as autoimmune diseases. Immunoregulatory and therapeutic effects of glycolipid ligands depend on their structure and modes of administration. Therefore, more studies are needed for optimization of the particular therapeutic settings. This study was focused on the tumor-inhibitory effects of 12 carbon acyl chain beta-galactosyl ceramide (C12 beta-D-Galactosyl Ceramide; beta-GalCer(C12)) on the growth of human papillomavirus type 16 (HPV16)-associated neoplasms transplanted in syngeneic mice. Treatment of tumor-bearing mice with beta-GalCer(C12) 3-14 days after tumor cell transplantation significantly inhibited the growth of the major histocompatibility complex (MHC) Class I-positive (TC-1), as well as MHC Class I-deficient (TC-1/A9) HPV16-associated tumors. Moreover, administration of beta-GalCer(C12) after surgical removal of TC-1 tumors inhibited the growth of tumor recurrences. Similar results were obtained in the treatment of tumors after chemotherapy. beta-GalCer(C12) treatment turned out to be also synergistic with immunotherapy based on administration of IL-12-producing cellular vaccines. These results suggest that beta-GalCer(C12), whose antitumor effects have so far not been studied in detail, can be effective for the treatment of minimal residual tumor disease as well as an adjuvant for cancer immunotherapy.

Improved localization of cellular membrane receptors using combined fluorescence microscopy and simultaneous topography and recognition imaging

The combination of fluorescence microscopy and atomic force microscopy has a great potential in single-molecule-detection applications, overcoming many of the limitations coming from each individual technique. Here we present a new platform of combined fluorescence and simultaneous topography and recognition imaging (TREC) for improved localization of cellular receptors. Green fluorescent protein (GFP) labeled human sodium-glucose cotransporter (hSGLT1) expressed Chinese Hamster Ovary (CHO) cells and endothelial cells (MyEnd) from mouse myocardium stained with phalloidin-rhodamine were used as cell systems to study AFM topography and fluorescence microscopy on the same surface area. Topographical AFM images revealed membrane features such as lamellipodia, cytoskeleton fibers, F-actin filaments and small globular structures with heights ranging from 20 to 30 nm. Combined fluorescence and TREC imaging was applied to detect density, distribution and localization of YFP-labeled CD1d molecules on alpha-galactosylceramide (alphaGalCer)-loaded THP1 cells. While the expression level, distribution and localization of CD1d molecules on THP1 cells were detected with fluorescence microscopy, the nanoscale distribution of binding sites was investigated with molecular recognition imaging by using a chemically modified AFM tip. Using TREC on the inverted light microscope, the recognition sites of cell receptors were detected in recognition images with domain sizes ranging from approximately 25 to approximately 160 nm, with the smaller domains corresponding to a single CD1d molecule.

Induction of Th1-biased cytokine production by alpha-carba-GalCer, a neoglycolipid ligand for NKT cells

NKT cells are characterized by their production of both T(h)1 and T(h)2 cytokines immediately after stimulation with alpha-galactosylceramide (alpha-GalCer), which is composed of alpha-galactopyranose linked to ceramide (itself composed of sphingosine and fatty-acyl chains); the chain length of the ceramide varies and this affects the ability of alpha-GalCer to stimulate cytokine production. However, the contribution of its galactopyranose sugar moiety remains unclear. We synthesized alpha-carba-GalCer, which has an alpha-linked carba-galactosyl moiety; here, the 5a'-oxygen atom of the D-galactopyranose ring of alpha-GalCer is replaced by a methylene group. The alpha-carba-GalCer was more stable and showed higher affinity to the NKT receptor. It thus enhanced and prolonged production of IL-12 and IFN-gamma compared with alpha-GalCer, resulting in augmented NKT cell-mediated adjuvant effects in vivo. The alpha-carba-GalCer, which has an ether linkage, was more resistant to degradation by liver microsomes than was alpha-GalCer, which has an acetal bond. Modulation of the sugar moiety in glycolipids might therefore provide optimal therapeutic reagents for protective immune responses against tumor or pathogens.

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.

Commensal microbiota and CD8+ T cells shape the formation of invariant NKT cells

Commensal bacteria play an important role in formation of the immune system, but the mechanisms involved are incompletely understood. In this study, we analyze CD1d-restricted invariant NKT (iNKT) cells in germfree mice and in two colonies of C57BL/6 mice termed conventional flora and restricted flora (RF), stably bearing commensal microbial communities of diverse but distinct composition. In germfree mice, iNKT cells were moderately reduced, suggesting that commensal microbiota were partially required for the antigenic drive in maintaining systemic iNKT cells. Surprisingly, even greater depletion of iNKT cell population occurred in RF mice. This was in part attributable to reduced RF levels of intestinal microbial taxa (Sphingomonas spp.) known to express antigenic glycosphingolipid products. However, memory and activated CD8(+) T cells were also expanded in RF mice, prompting us to test whether CD8(+) T cell activity might be further depleting iNKT cells. Indeed, iNKT cell numbers were restored in RF mice bearing the CD8alpha(-/-) genotype or in adult wild-type RF mice acutely depleted with anti-CD8 Ab. Moreover, iNKT cells were restored in RF mice bearing the Prf1(-/-) phenotype, a key component of cytolytic function. These findings indicate that commensal microbiota, through positive (antigenic drive) and negative (cytolytic depletion by CD8(+) T cells) mechanisms, profoundly shape the iNKT cell compartment. Because individuals greatly vary in the composition of their microbial communities, enteric microbiota may play an important epigenetic role in the striking differences in iNKT cell abundance in humans and therefore in their potential contribution to host immune status.

The immunological synapse, TCR microclusters, and T cell activation

T cell activation begins with the interaction between an antigen-specific T cell and an antigen-presenting cell (APC). This interaction results in the formation of the immunological synapse, which had been considered to be responsible for antigen recognition and T cell activation. Recent advances in imaging analysis have provided new insights into T cell activation. The T cell receptor (TCR) microclusters, TCRs, kinases, and adaptors are generated upon antigen recognition at the interfaces between the T cells and the APCs and serve as a fundamental signaling unit for T cell activation. CD28-mediated costimulation is also found to be regulated by the formation of microclusters. Therefore, the dynamic regulations of TCR and CD28 microcluster formation, migration, and interaction are the key events for the initiation of T cell-mediated immune responses. Comprehensive analyses of the composition and characteristics of the TCR microcluster have identified its dynamic features. This review will outline new discoveries of the microclusters and its related concept in T cell activation.

An alpha-galactosylceramide C20:2 N-acyl variant enhances anti-inflammatory and regulatory T cell-independent responses that prevent type 1 diabetes

Protection from type 1 diabetes (T1D), a T helper type 1 (Th1)-mediated disease, is achievable in non-obese diabetic (NOD) mice by treatment with alpha-galactosylceramide (alpha-GalCer) glycolipids that stimulate CD1d-restricted invariant natural killer T (iNK T) cells. While we have reported previously that the C20:2 N-acyl variant of alpha-GalCer elicits a Th2-biased cytokine response and protects NOD mice from T1D more effectively than a form of alpha-GalCer that induces mixed Th1 and Th2 responses, it remained to determine whether this protection is accompanied by heightened anti-inflammatory responses. We show that treatment of NOD mice with C20:2 diminished the activation of 'inflammatory' interleukin (IL)-12 producing CD11c(high)CD8+ myeloid dendritic cells (mDCs) and augmented the function of 'tolerogenic' DCs more effectively than treatment with the prototypical iNKT cell activator KRN7000 (alpha-GalCer C26:0) that induces Th1- and Th2-type responses. These findings correlate with a reduced capacity of C20:2 to sustain the early transactivation of T, B and NK cells. They may also explain our observation that C20:2 activated iNK T cells depend less than KRN7000 activated iNK T cells upon regulation by regulatory T cells for cytokine secretion and protection from T1D. The enhanced anti-inflammatory properties of C20:2 relative to KRN7000 suggest that C20:2 should be evaluated further as a drug to induce iNK T cell-mediated protection from T1D in humans.

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.

A double-edged sword: the role of NKT cells in malaria and HIV infection and immunity

NKT cells are known to play a role against certain microbial infections, including malaria and HIV, two major global infectious diseases. NKT cells exhibit either protective or pathogenic role against malaria. They are depleted by HIV infection and have a direct pathogenic role against many opportunistic infections common in end-stage AIDS. This review discusses the various features of the interaction between NKT cells and malaria parasites and HIV, and the potential to harness this interaction for therapeutic and vaccine strategies.

Mechanisms for glycolipid antigen-driven cytokine polarization by Valpha14i NKT cells

Certain glycolipid Ags for Valpha14i NKT cells can direct the overall cytokine balance of the immune response. Th2-biasing OCH has a lower TCR avidity than the most potent agonist known, alpha-galactosylceramide. Although the CD1d-exposed portions of OCH and alpha-galactosylceramide are identical, structural analysis indicates that there are subtle CD1d conformational differences due to differences in the buried lipid portion of these two Ags, likely accounting for the difference in antigenic potency. Th1-biasing C-glycoside/CD1d has even weaker TCR interactions than OCH/CD1d. Despite this, C-glycoside caused a greater downstream activation of NK cells to produce IFN-gamma, accounting for its promotion of Th1 responses. We found that this difference correlated with the finding that C-glycoside/CD1d complexes survive much longer in vivo. Therefore, we suggest that the pharmacokinetic properties of glycolipids are a major determinant of cytokine skewing, suggesting a pathway for designing therapeutic glycolipids for modulating invariant NKT cell responses.

Lipid and glycolipid antigens of CD1d-restricted natural killer T cells

In spite of their relatively limited antigen receptor repertoire, CD1d-restricted NKT cells recognize a surprisingly diverse range of lipid and glycolipid antigens. Recent studies of natural and synthetic CD1d-presented antigens provide an increasingly detailed picture of how the specific structural features of these lipids and glycolipids influence their ability to be presented to NKT cells and stimulate their diverse immunologic functions. Particularly for synthetic analogues of alpha-galactosylceramides which have been the focus of intense recent investigation, it is becoming clear that the design of glycolipid antigens with the ability to precisely control the specific immunologic activities of NKT cells is likely to be feasible. The emerging details of the mechanisms underlying the structure-activity relationship of NKT cell antigens will assist greatly in the design and production of immunomodulatory agents for the precise manipulation of NKT cells and the many other components of the immune system that they influence.

Molecular and immunogenic features of myelin lipids: incitants or modulators of multiple sclerosis?

Myelin lipids have long been thought to play intriguing roles in the pathogenesis of multiple sclerosis (MS). This review summarizes current understanding of the molecular basis of MS with emphasis on the: (i.) physico-chemical properties, organization and accessibility of the lipids and their distribution within the myelin multilayer; (ii.) characterization of myelin lipid structures, and structure-function relationships relevant to MS mechanisms, and; (iii.) immunogenic and other features of lipids in MS including molecular mimicry, lipid enzyme genetic knockouts, glycolipid-reactive NKT cells, and monoclonal antibody-induced remyelination. New findings associate anti-lipid antibodies with pathophysiological biomarkers and suggest clinical utility. The structure of CD1d-lipid complexed with the lipophilic invariant T cell receptor (iTCR) may be crucial to understanding MS pathogenesis, and design of lipid antigen-specific therapeutics. Novel immuno-modulatory tools for treatment of autoimmune diseases including MS in which there is both constraint of inflammation and stimulation of remyelination are now emerging.

Recognition of lyso-phospholipids by human natural killer T lymphocytes

Natural killer T (NKT) cells are a subset of T lymphocytes with potent immunoregulatory properties. Recognition of self-antigens presented by CD1d molecules is an important route of NKT cell activation; however, the molecular identity of specific autoantigens that stimulate human NKT cells remains unclear. Here, we have analyzed human NKT cell recognition of CD1d cellular ligands. The most clearly antigenic species was lyso-phosphatidylcholine (LPC). Diacylated phosphatidylcholine and lyso-phosphoglycerols differing in the chemistry of the head group stimulated only weak responses from human NKT cells. However, lyso-sphingomyelin, which shares the phosphocholine head group of LPC, also activated NKT cells. Antigen-presenting cells pulsed with LPC were capable of stimulating increased cytokine responses by NKT cell clones and by freshly isolated peripheral blood lymphocytes. These results demonstrate that human NKT cells recognize cholinated lyso-phospholipids as antigens presented by CD1d. Since these lyso-phospholipids serve as lipid messengers in normal physiological processes and are present at elevated levels during inflammatory responses, these findings point to a novel link between NKT cells and cellular signaling pathways that are associated with human disease pathophysiology.

Linking inflammation to natural killer T cell activation

Immune activation is often associated with inflammation, but inflammation's role in the expansion of antigen-specific immune responses remains unclear. This primer focuses on recent findings that show how specific natural killer T cells are activated by inflammatory messengers, thus illuminating the cellular and molecular links between immunity and inflammation.

Multiple tissue-specific isoforms of sulfatide activate CD1d-restricted type II NKT cells

The glycosphingolipid sulfatide (SO(3)-3Galbeta1Cer) is a demonstrated ligand for a subset of CD1d-restricted NKT cells, which could regulate experimental autoimmune encephalomyelitis, a murine model for multiple sclerosis, as well as tumor immunity and experimental hepatitis. Native sulfatide is a mixture of sulfatide isoforms, i.e. sulfatide molecules with different long-chain bases and fatty acid chain lengths and saturation. Here, we demonstrate that sulfatide-specific CD1d-restricted murine NKT hybridomas recognized several different sulfatide isoforms. These included the physiologically relevant isoforms C24:1 and C24:0, major constituents of the myelin sheet of the nervous system, and C16:0, prominent in the pancreatic islet beta-cells. The most potent sulfatide isoform was lysosulfatide (lacking a fatty acid). Shortened fatty acid chain length (C24:1 versus C18:1), or saturation of the long fatty acid (C24:0), resulted in reduced stimulatory capacity, and fatty acid hydroxylation abolished the response. Moreover, sulfatide was not responsible for the natural autoreactivity toward splenocytes by XV19 T hybridoma cells. Our results reveal a promiscuity in the recognition of sulfatide isoforms by a CD1d-restricted NKT-cell clone, and suggest that sulfatide, a major component of the myelin sheet and pancreatic beta-cells, is one of several natural ligands for type II CD1d-restricted NKT cells.

Structural evaluation of potent NKT cell agonists: implications for design of novel stimulatory ligands

Natural killer T (NKT) cells are a subset of T cells that are activated by CD1d-glycolipid complexes through a semi-invariant alphabeta T cell receptor (NKT TCR). Upon activation, NKT cells secrete regulatory cytokines that are implicated in T helper cell responses. alpha-Galactosylceramide (alpha-GalCer) is a potent NKT cell agonist when presented by CD1d. Phenyl ring substitutions of the alpha-GalCer fatty acid moiety were recently found to be superior in eliciting regulatory cytokines. Crystal structures of four new mouse CD1d-lipid complexes (five structures), a new PBS-25 complex, and CD1d with an endogenous ligand, at 1.6-1.9 A resolution, reveal that the alpha-GalCer phenyl analogues impart minor structural differences to the A'-pocket, while the sphingosine and galactose moieties, important for NKT TCR recognition, remain virtually unchanged. The observed differences in cytokine-release profiles appear to be associated with increased stability of the CD1d-glycolipid complexes rather than increased affinity for the NKT TCR. Furthermore, comparison of mouse CD1d-glycolipid complexes in different crystallographic space groups reveals considerable conformational variation, particularly above the F'-pocket, the primary site of interaction with the NKT TCR. We propose that modifications of the sphingosine moiety or other substitutions that decrease alpha-GalCer flexibility would stabilize the F'-pocket. Such compounds might then increase CD1d affinity for the NKT TCR and further enhance the stimulatory and regulatory properties of alpha-GalCer derivatives.