Crystal structure of mouse CD1d bound to the self ligand phosphatidylcholine: a molecular basis for NKT cell activation

Optimizing iNKT-driven immune responses against cancer by modulating CD1d in tumor and antigen presenting cells

Two major antigen processing pathways represent protein Ags through major histocompatibility complexes (MHC class I and II) or lipid Ags through CD1 molecules influence the tumor immune response. Invariant Natural Killer T cells (iNKT) manage a significant role in cancer immunotherapy. CD1d, found on antigen-presenting cells (APCs), presents lipid Ags to iNKT cells. In many cancers, the number and function of iNKT cell are compromised, leading to immune evasion. Additionally impaired motility of iNKT cells may contribute to poor tumor prognosis. Emerging evidences suggest that CD1d, itself also influences cancer progression. Patient databases further highlight the importance of CD1d expression in different cancers and its correlation with patient survival outcomes. The ability of iNKT cells to activate and enhance the immune response renders them an attractive target for cancer immunotherapy. This review discusses all the possible ways of cancer immune evasion and restoration of immune responses mediated by CD1d-iNKT interactions.

Atypical sideways recognition of CD1a by autoreactive γδ T cell receptors

CD1a is a monomorphic antigen-presenting molecule on dendritic cells that presents lipids to αβ T cells. Whether CD1a represents a ligand for other immune receptors remains unknown. Here we use CD1a tetramers to show that CD1a is a ligand for Vδ1+ γδ T cells. Functional studies suggest that two γδ T cell receptors (TCRs) bound CD1a in a lipid-independent manner. The crystal structures of three Vγ4Vδ1 TCR-CD1a-lipid complexes reveal that the γδ TCR binds at the extreme far side and parallel to the long axis of the β-sheet floor of CD1a's antigen-binding cleft. Here, the γδ TCR co-recognises the CD1a heavy chain and β2 microglobulin in a manner that is distinct from all other previously observed γδ TCR docking modalities. The 'sideways' and lipid antigen independent mode of autoreactive CD1a recognition induces TCR clustering on the cell surface and proximal T cell signalling as measured by CD3ζ phosphorylation. In contrast with the 'end to end' binding of αβ TCRs that typically contact carried antigens, autoreactive γδ TCRs support geometrically diverse approaches to CD1a, as well as antigen independent recognition.

Phosphatidylcholine-Derived Lipid Mediators: The Crosstalk Between Cancer Cells and Immune Cells

To become resistant, cancer cells need to activate and maintain molecular defense mechanisms that depend on an energy trade-off between resistance and essential functions. Metabolic reprogramming has been shown to fuel cell growth and contribute to cancer drug resistance. Recently, changes in lipid metabolism have emerged as an important driver of resistance to anticancer agents. In this review, we highlight the role of choline metabolism with a focus on the phosphatidylcholine cycle in the regulation of resistance to therapy. We analyze the contribution of phosphatidylcholine and its metabolites to intracellular processes of cancer cells, both as the major cell membrane constituents and source of energy. We further extended our discussion about the role of phosphatidylcholine-derived lipid mediators in cellular communication between cancer and immune cells within the tumor microenvironment, as well as their pivotal role in the immune regulation of therapeutic failure. Changes in phosphatidylcholine metabolism are part of an adaptive program activated in response to stress conditions that contribute to cancer therapy resistance and open therapeutic opportunities for treating drug-resistant cancers.

Choline chloride attenuates the allergic airway disease by inhibiting the lysophosphatidylcholine induced response in mouse model

Aims

Allergic airway disease manifestation is induced by lysophosphatidylcholine (LPC) through CD1d-restricted Natural killer T (NKT) cells. Choline chloride (ChCl) and LPC both have the “choline” moiety in their structure and this may interplay the effect in allergic airway disease pathway.

Main methods

To test the hypothesis, mice were sensitized with cockroach extract (CE); challenged with CE or exposed to LPC and were given ChCl 1hr later.

Key findings

A significant increase in Airway hyperresponsiveness (AHR), total and differential cell count, Th2 cytokines, 8-isoprostanes level in bronchoalveolar lavage fluid (BALF) and inflammation score based on lung histology were observed on challenge with CE or exposure to LPC (p ​< ​0.05) indicating LPC induced airway disease manifestation in mice. These parameters were reduced significantly after administering mice with ChCl (p ​< ​0.05). The inflammatory parameters were significantly increased in LPC exposed mice, not sensitized with CE, which were significantly decreased when mice were administered with ChCl demonstrating its role in the inhibition of LPC induced allergic airway disease manifestation. Docking of CD1d with LPC and ChCl indicated the competitive inhibition of LPC induced effect by ChCl. This was validated in vivo in the form of decreased CD1d-restricted NKT cells in BALF and lung of the immunized mice on ChCl administration. There was no effect of ChCl administration on CD1d expression in BALF and lung cells.

Significance

This study shows that ChCl attenuates the allergic response by inhibiting the LPC induced- NKT cell mediated AHR, inflammation and oxidative stress by competitive inhibition to LPC in binding to CD1d.

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ChCl down regulates LPC (critical for allergic manifestation) induced response.

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Results were validated in cockroach extract immunized mice model.

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In silico studies indicate competitive inhibition to LPC by ChCl in binding to CD1d.

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In silico results were also validated in vivo in terms of CD1d-restricted NKT cells.

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Study explains the mechanism of ChCl action against allergic disease.

What one lipid giveth, another taketh away

Acid sphingomyelinase deficiency, which prevents degradation of sphingomyelin (SM), causes lysosomal SM overload both in mice and in patients with Niemann–Pick disease A or B. Altered cellular SM homeostasis disrupts the development and function of natural killer T cells by obstructing the presentation of lipid agonists by CD1d molecules.

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

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

Molecular recognition of microbial lipid-based antigens by T cells

The immune system has evolved to protect hosts from pathogens. T cells represent a critical component of the immune system by their engagement in host defence mechanisms against microbial infections. Our knowledge of the molecular recognition by T cells of pathogen-derived peptidic antigens that are presented by the major histocompatibility complex glycoproteins is now well established. However, lipids represent an additional, distinct chemical class of molecules that when presented by the family of CD1 antigen-presenting molecules can serve as antigens, and be recognized by specialized subsets of T cells leading to antigen-specific activation. Over the past decades, numerous CD1-presented self- and bacterial lipid-based antigens have been isolated and characterized. However, our understanding at the molecular level of T cell immunity to CD1 molecules presenting microbial lipid-based antigens is still largely unexplored. Here, we review the insights and the molecular basis underpinning the recognition of microbial lipid-based antigens by T cells.

Differing roles of CD1d2 and CD1d1 proteins in type I natural killer T cell development and function

MHC class I-like CD1 molecules have evolved to present lipid-based antigens to T cells. Differences in the antigen-binding clefts of the CD1 family members determine the conformation and size of the lipids that are presented, although the factors that shape CD1 diversity remain unclear. In mice, two homologous genes, CD1D1 and CD1D2, encode the CD1d protein, which is essential to the development and function of natural killer T (NKT) cells. However, it remains unclear whether both CD1d isoforms are equivalent in their antigen presentation capacity and functions. Here, we report that CD1d2 molecules are expressed in the thymus of some mouse strains, where they select functional type I NKT cells. Intriguingly, the T cell antigen receptor repertoire and phenotype of CD1d2-selected type I NKT cells in CD1D1^-/- mice differed from CD1d1-selected type I NKT cells. The structures of CD1d2 in complex with endogenous lipids and a truncated acyl-chain analog of α-galactosylceramide revealed that its A'-pocket was restricted in size compared with CD1d1. Accordingly, CD1d2 molecules could not present glycolipid antigens with long acyl chains efficiently, favoring the presentation of short acyl chain antigens. These results indicate that the two CD1d molecules present different sets of self-antigen(s) in the mouse thymus, thereby impacting the development of invariant NKT cells.

Tailored design of NKT-stimulatory glycolipids for polarization of immune responses

Natural killer T (NKT) cell is a distinct population of T lymphocytes that can rapidly release massive amount of Th1 and Th2 cytokines upon the engagement of their T cell receptor with glycolipids presented by CD1d. The secreted cytokines can promote cell-mediated immunity to kill tumor cells and intracellular pathogens, or suppress autoreactive immune cells in autoimmune diseases. Thus, NKT cell is an attractive target for developing new therapeutics to manipulate immune system. The best-known glycolipid to activate NKT cells is α-galactosylceramide (α-GalCer), which has been used as a prototype for designing new NKT stimulatory glycolipids. Many analogues have been generated by modification of the galactosyl moiety, the acyl chain or the phytosphingosine chain of α-GalCer. Some of the analogues showed greater abilities than α-GalCer in polarizing immune responses toward Th1 or Th2 dominance. Among them, several analogues containing phenyl groups in the lipid tails were more potent in inducing Th1-skewed cytokines and exhibited greater anticancer efficacy than α-GalCer. Analyses of the correlation between structure and activity of various α-GalCer analogues on the activation of iNKT cell revealed that CD1d–glycolipid complexes interacted with the same population of iNKT cell expressing similar T-cell receptor Vβ as α-GalCer. On the other hand, those phenyl glycolipids with propensity for Th1 dominant responses showed greater binding avidity and stability than α-GalCer for iNKT T-cell receptor when complexed with CD1d. Thus, it is the avidity and stability of the ternary complexes of CD1d-glycolipid-iNKT TCR that dictate the polarity and potency of immune responses. These findings provide a key to the rationale design of immune modulating glycolipids with desirable Th1/Th2 polarity for clinical application. In addition, elucidation of α-GalCer-induced anergy, liver damage and accumulation of myeloid derived suppressor cells has offered explanation for its lacklustre anti-cancer activities in clinical trials. On other hand, the lack of such drawbacks in glycolipid analogues containing phenyl groups in the lipid tails of α-GalCer coupled with the greater binding avidity and stability of CD1d-glycolipid complex for iNKT T-cell receptor, account for their superior anti-cancer efficacy in tumor bearing mice. Further clinical development of these phenyl glycolipids is warranted.

Novel Adjuvant Based on the Pore-Forming Protein Sticholysin II Encapsulated into Liposomes Effectively Enhances the Antigen-Specific CTL-Mediated Immune Response

Vaccine strategies to enhance CD8⁺ CTL responses remain a current challenge because they should overcome the plasmatic and endosomal membranes for favoring exogenous Ag access to the cytosol of APCs. As a way to avoid this hurdle, sticholysin (St) II, a pore-forming protein from the Caribbean Sea anemone Stichodactyla helianthus, was encapsulated with OVA into liposomes (Lp/OVA/StII) to assess their efficacy to induce a CTL response. OVA-specific CD8⁺ T cells transferred to mice immunized with Lp/OVA/StII experienced a greater expansion than when the recipients were injected with the vesicles without St, mostly exhibiting a memory phenotype. Consequently, Lp/OVA/StII induced a more potent effector function, as shown by CTLs, in vivo assays. Furthermore, treatment of E.G7-OVA tumor-bearing mice with Lp/OVA/StII significantly reduced tumor growth being more noticeable in the preventive assay. The contribution of CD4⁺ and CD8⁺ T cells to CTL and antitumor activity, respectively, was elucidated. Interestingly, the irreversibly inactive variant of the StI mutant StI W111C, encapsulated with OVA into Lp, elicited a similar OVA-specific CTL response to that observed with Lp/OVA/StII or vesicles encapsulating recombinant StI or the reversibly inactive StI W111C dimer. These findings suggest the relative independence between StII pore-forming activity and its immunomodulatory properties. In addition, StII-induced in vitro maturation of dendritic cells might be supporting these properties. These results are the first evidence, to our knowledge, that StII, a pore-forming protein from a marine eukaryotic organism, encapsulated into Lp functions as an adjuvant to induce a robust specific CTL response.

The CD1 family: serving lipid antigens to T cells since the Mesozoic era

Class I-like CD1 molecules are in a family of antigen-presenting molecules that bind lipids and lipopeptides, rather than peptides for immune surveillance by T cells. Since CD1 lacks the high degree of polymorphism found in their major histocompatibility complex (MHC) class I molecules, different species express different numbers of CD1 isotypes, likely to be able to present structurally diverse classes of lipid antigens. In this review, we will present a historical overview of the structures of the different human CD1 isotypes and also discuss species-specific adaptations of the lipid-binding groove. We will discuss how single amino acid changes alter the shape and volume of the CD1 binding groove, how these minor changes can give rise to different numbers of binding pockets, and how these pockets affect the lipid repertoire that can be presented by any given CD1 protein. We will compare the structures of various lipid antigens and finally, we will discuss recognition of CD1-presented lipid antigens by antigen receptors on T cells (TCRs).

Group 1 CD1-restricted T cells and the pathophysiological implications of self-lipid antigen recognition

T cell responses are generally regarded as specific for protein-derived peptide antigens. This is based on the molecular paradigm dictated by the T cell receptor (TCR) recognition of peptide-major histocompatibility complexs, which provides the molecular bases of the specificity and restriction of the T cell responses. An increasing number of findings in the last 20 years have challenged this paradigm, by showing the existence of T cells specific for lipid antigens presented by CD1 molecules. CD1-restricted T cells have been proven to be frequent components of the immune system and to recognize exogenous lipids, derived from pathogenic bacteria, as well as cell-endogenous self-lipids. This represents a young and exciting area of research in immunology with intriguing biological bases and a potential direct impact on human health.

Recognition of Microbial Glycolipids by Natural Killer T Cells

T cells can recognize microbial antigens when presented by dedicated antigen-presenting molecules. While peptides are presented by classical members of the major histocompatibility complex (MHC) family (MHC I and II), lipids, glycolipids, and lipopeptides can be presented by the non-classical MHC member, CD1. The best studied subset of lipid-reactive T cells are type I natural killer T (iNKT) cells that recognize a variety of different antigens when presented by the non-classical MHCI homolog CD1d. iNKT cells have been shown to be important for the protection against various microbial pathogens, including B. burgdorferi, the causative agents of Lyme disease, and S. pneumoniae, which causes pneumococcal meningitis and community-acquired pneumonia. Both pathogens carry microbial glycolipids that can trigger the T cell antigen receptor (TCR), leading to iNKT cell activation. iNKT cells have an evolutionary conserved TCR alpha chain, yet retain the ability to recognize structurally diverse glycolipids. They do so using a conserved recognition mode, in which the TCR enforces a conserved binding orientation on CD1d. TCR binding is accompanied by structural changes within the TCR binding site of CD1d, as well as the glycolipid antigen itself. In addition to direct recognition of microbial antigens, iNKT cells can also be activated by a combination of cytokines (IL-12/IL-18) and TCR stimulation. Many microbes carry TLR antigens, and microbial infections can lead to TLR activation. The subsequent cytokine response in turn lower the threshold of TCR-mediated iNKT cell activation, especially when weak microbial or even self-antigens are presented during the cause of the infection. In summary, iNKT cells can be directly activated through TCR triggering of strong antigens, while cytokines produced by the innate immune response may be necessary for TCR triggering and iNKT cell activation in the presence of weak antigens. Here, we will review the molecular basis of iNKT cell recognition of glycolipids, with an emphasis on microbial glycolipids.

Role of Group 1 CD1-Restricted T Cells in Infectious Disease

The evolutionarily conserved CD1 family of antigen-presenting molecules presents lipid antigens rather than peptide antigens to T cells. CD1 molecules, unlike classical MHC molecules, display limited polymorphism, making CD1-restricted lipid antigens attractive vaccine targets that could be recognized in a genetically diverse human population. Group 1 CD1 (CD1a, CD1b, and CD1c)-restricted T cells have been implicated to play critical roles in a variety of autoimmune and infectious diseases. In this review, we summarize current knowledge and recent discoveries on the development of group 1 CD1-restricted T cells and their function in different infection models. In particular, we focus on (1) newly identified microbial and self-lipid antigens, (2) kinetics, phenotype, and unique properties of group 1 CD1-restricted T cells during infection, and (3) the similarities of group 1 CD1-restricted T cells to the closely related group 2 CD1-restricted T cells.

Role of Innate T Cells in Anti-Bacterial Immunity

Innate T cells are a heterogeneous group of αβ and γδ T cells that respond rapidly (<2 h) upon activation. These innate T cells also share a non MHC class I or II restriction requirement for antigen recognition. Three major populations within the innate T cell group are recognized, namely, invariant NKT cells, mucosal associated invariant T cells, and gamma delta T cells. These cells recognize foreign/self-lipid presented by non-classical MHC molecules, such as CD1d, MR1, and CD1a. They are activated during the early stages of bacterial infection and act as a bridge between the innate and adaptive immune systems. In this review, we focus on the functional properties of these three innate T cell populations and how they are purposed for antimicrobial defense. Furthermore, we address the mechanisms through which their effector functions are targeted for bacterial control and compare this in human and murine systems. Lastly, we speculate on future roles of these cell types in therapeutic settings such as vaccination.

Synthesis of a 2"-deoxy-β-GalCer

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

Design and Evaluation of ω-Hydroxy Fatty Acids Containing α-GalCer Analogues for CD1d-Mediated NKT Cell Activation

CD1d molecules recognize glycolipid antigens with straight chain fatty acid moieties. Although most of the residues in the CD1d binding groove are hydrophobic, some of the amino acids can form hydrogen bonds. Consequently, we have designed ω-hydroxy fatty acid-containing glycolipid derivatives of the prototypical CD1d ligand α-GalCer. The potency of the ω-hydroxy analogues of the proper length is comparable to that of α-GalCer. We propose, based on the biological results and molecular modeling studies, that a hydrogen bonding interaction is involved between the ω-hydroxy group and a polar amino acid residue in the hydrophobic binding groove.

Liposomes of phosphatidylcholine and cholesterol induce an M2-like macrophage phenotype reprogrammable to M1 pattern with the involvement of B-1 cells

Macrophages respond to endogenous and non-self stimuli acquiring the M1 or M2 phenotypes, corresponding to classical or alternative activation, respectively. The role of B-1 cells in the regulation of macrophage polarization through the secretion of interleukin (IL)-10 has been demonstrated. However, the influence of B-1 cells on macrophage phenotype induction by an immunogen that suppress their ability to secrete IL-10 has not been explored. Here, we studied the peritoneal macrophage pattern induced by liposomes comprised of dipalmitoylphosphatidylcholine (DPPC) and cholesterol (Chol) carrying ovalbumin (OVA) (Lp DPPC/OVA), and the involvement of B-1 cells in macrophage polarization. Peritoneal cells from BALB/c, B-1 cells-deficient BALB/xid and C57BL/6 mice immunized with Lp DPPC/OVA and OVA in soluble form (PBS/OVA) were analyzed and stimulated or not in vitro with lipopolysaccharide (LPS). Peritoneal macrophages from BALB/c and C57BL/6 mice immunized with Lp DPPC/OVA showed an M2-like phenotype as evidenced by their high arginase activity without LPS stimulation. Upon stimulation, these macrophages were reprogrammable toward the M1 phenotype with the upregulation of nitric oxide (NO) and a decrease in IL-10 secretion. In addition, high IFN-γ levels were detected in the culture supernatant of peritoneal cells from BALB/c and C57BL/6 mice immunized with Lp DPPC/OVA. Nevertheless, still high levels of arginase activity and undetectable levels of IL-12 were found, indicating that the switch to a classical activation state was not complete. In the peritoneal cells from liposomes-immunized BALB/xid mice, levels of arginase activity, NO, and IL-6 were below those from wild type animals, but the last two products were restored upon adoptive transfer of B-1 cells, together with an increase in IFN-γ secretion. Summarizing, we have demonstrated that Lp DPPC/OVA induce an M2-like pattern in peritoneal macrophages reprogrammable to M1 phenotype after LPS stimulation, with the involvement of B-1 cells.

Biology of CD1- and MR1-restricted T cells

Over the past 15 years, investigators have shown that 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 the nonclassical MHC class I-like molecules, CD1 proteins. In this review, we describe the most recent events in the field, with particular emphasis on (a) structural and functional aspects of lipid presentation by CD1 molecules, (b) the development of CD1d-restricted invariant natural killer T (iNKT) cells and transcription factors required for their differentiation, (c) the ability of iNKT cells to modulate innate and adaptive immune responses through their cross talk with lymphoid and myeloid cells, and (d) MR1-restricted and group I (CD1a, CD1b, and CD1c)-restricted T cells.

The versatility of the αβ T-cell antigen receptor

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

Therapeutic manipulation of natural killer (NK) T cells in autoimmunity: are we close to reality?

T cells reactive to lipids and restricted by major histocompatibility complex (MHC) class I-like molecules represent more than 15% of all lymphocytes in human blood. This heterogeneous population of innate cells includes the invariant natural killer T cells (iNK T), type II NK T cells, CD1a,b,c-restricted T cells and mucosal-associated invariant T (MAIT) cells. These populations are implicated in cancer, infection and autoimmunity. In this review, we focus on the role of these cells in autoimmunity. We summarize data obtained in humans and preclinical models of autoimmune diseases such as primary biliary cirrhosis, type 1 diabetes, multiple sclerosis, systemic lupus erythematosus, rheumatoid arthritis, psoriasis and atherosclerosis. We also discuss the promise of NK T cell manipulations: restoration of function, specific activation, depletion and the relevance of these treatments to human autoimmune diseases.

Phospholipid--driven gene regulation

Phospholipids (PLs), well known for their fundamental role in cellular structure, play critical signaling roles via their derivatives and cleavage products acting as second messengers in signaling cascades. Recent work has shown that intact PLs act as signaling molecules in their own right by modulating the activity of nuclear hormone transcription factors responsible for tuning genes involved in metabolism, lipid flux, steroid synthesis and inflammation. As such, PLs have been classified as novel hormones. This review highlights recent work in PL-driven gene regulation with a focus on the unique structural features of phospholipid-sensing transcription factors and what sets them apart from well known soluble phospholipid transporters.

Raising the roof: the preferential pharmacological stimulation of Th1 and th2 responses mediated by NKT cells

Natural killer T (NKT) cells serve as a bridge between the innate and adaptive immune systems, and manipulating their effector functions can have therapeutic significances in the treatment of autoimmunity, transplant biology, infectious disease, and cancer. NKT cells are a subset of T cells that express cell-surface markers characteristic of both natural killer cells and T cells. These unique immunologic cells have been demonstrated to serve as a link between the innate and adaptive immune systems through their potent cytokine production following the recognition of a range of lipid antigens, mediated through presentation of the major histocompatibility complex (MHC) class I like CD1d molecule, in addition to the NKT cell's cytotoxic capabilities upon activation. Although a number of glycolipid antigens have been shown to complex with CD1d molecules, most notably the marine sponge derived glycolipid alpha-galactosylceramide (α-GalCer), there has been debate as to the identity of the endogenous activating lipid presented to the T-cell receptor (TCR) via the CD1d molecule on antigen-presenting cells (APCs). This review aims to survey the use of pharmacological agents and subsequent structure-activity relationships (SAR) that have given insight into the binding interaction of glycolipids with both the CD1d molecules as well as the TCR and the subsequent immunologic response of NKT cells. These studies not only elucidate basic binding interactions but also pave the way for future pharmacological modulation of NKT cell responses.

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.

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.

Novel insights into lipid antigen presentation

T cells recognizing lipid antigens are present in large numbers in circulating blood. They exert multiple functions including immunoregulation, tumour surveillance and protection during infection. Here, we review the latest information on the mechanisms of lipid antigen presentation by CD1 molecules. Recent studies have provided insight into CD1 trafficking within the cell, lipid distribution and handling, CD1 maturation, lipid antigen processing and loading. The structural resolution of all human CD1 molecules has revealed unique features that correlate with function. Molecular mechanisms regulating CD1 expression and multiple evasion mechanisms evolved by viral and bacterial pathogens have been disclosed. With rapid progression, these studies have decoded lipid-specific immunity and have revealed the important immunological role of this type of antigen recognition.

Heteroaromatic Moieties in the Sphingosine Backbone of α-Galactosylceramides for Noncovalent Interactions with CD1d

A series of α-GalCer analogues containing heterocyclic and aromatic moieties in the sphingosine backbone were synthesized to improve the selectivity in the Th1/Th2 cytokine profile via noncovalent interaction with three aromatic residues at the binding pocket of CD1d. In vitro and in vivo biological evaluations revealed the treatment of α-GalCer analogue (6) induced the selective stimulation of natural killer T cells to facilitate the secretion of Th2 cytokines.

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.

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.

Crystal structure of human CD1e reveals a groove suited for lipid-exchange processes

CD1e is the only human CD1 protein existing in soluble form in the late endosomes of dendritic cells, where it facilitates the processing of glycolipid antigens that are ultimately recognized by CD1b-restricted T cells. The precise function of CD1e remains undefined, thus impeding efforts to predict the participation of this protein in the presentation of other antigens. To gain insight into its function, we determined the crystal structure of recombinant CD1e expressed in human cells at 2.90-Å resolution. The structure revealed a groove less intricate than in other CD1 proteins, with a significantly wider portal characterized by a 2 Å-larger spacing between the α1 and α2 helices. No electron density corresponding to endogenous ligands was detected within the groove, despite the presence of ligands unequivocally established by native mass spectrometry in recombinant CD1e. Our structural data indicate that the water-exposed CD1e groove could ensure the establishment of loose contacts with lipids. In agreement with this possibility, lipid association and dissociation processes were found to be considerably faster with CD1e than with CD1b. Moreover, CD1e was found to mediate in vitro the transfer of lipids to CD1b and the displacement of lipids from stable CD1b-antigen complexes. Altogether, these data support that CD1e could have evolved to mediate lipid-exchange/editing processes with CD1b and point to a pathway whereby the repertoire of lipid antigens presented by human dendritic cells might be expanded.

The structural and functional role of myelin fast-migrating cerebrosides: pathological importance in multiple sclerosis

A family of neutral glycosphingolipids containing a 3-O-acetyl-sphingosine galactosylceramide (3-SAG) has been characterized. Seven new derivatives of galactosylceramide (GalCer), designated as fast-migrating cerebrosides (FMCs) by TLC retention factor, have been identified. The simplest compounds - FMC-1 and FMC-2 - of this series have been characterized as the 3-SAG containing nonhydroxy and hydroxy fatty acyl, respectively. The next two - FMC-3 and FMC-4 - add 6-O-acetyl-galactose and the most complex glycosphingolipids, FMC-5, -6 and -7, are 2,3,4,6-tetra-O-acetyl-3-SAG. These hydrophobic myelin lipid biomarkers coappear with GalCer during myelinogenesis and disappear along with GalCer in de- or dys-myelinating disorders. Myelin lipid antigens, including FMCs, are keys to myelin biology, opening the possibility of new and novel immune modulatory tools for treatment of autoimmune diseases including multiple sclerosis.

Cardiolipin binds to CD1d and stimulates CD1d-restricted γδ T cells in the normal murine repertoire

Cardiolipin (CL), a major phospholipid in bacterial cell walls, is sequestered from the immune system in mammalian mitochondria and is, therefore, a potential danger signal. Based on growing evidence that phospholipids constitute natural ligands for CD1 and that CD1d-restricted T cells recognize phospholipids, we hypothesized that CD1d binds and presents CL and that T cells in the normal immune repertoire respond to CL in a CD1d-restricted manner. We determined the murine CD1d-CL crystal structure at 2.3 Å resolution and established through additional lipid loading experiments that CL, a tetra-acylated phospholipid, binds to murine CD1d with two alkyl chains buried inside the CD1d binding groove and the remaining two exposed into the solvent. We furthermore demonstrate the functional stimulatory activity of CL, showing that splenic and hepatic γδ T cells from healthy mice proliferate in vitro in response to mammalian or bacterial CL in a dose-dependent and CD1d-restricted manner, rapidly secreting the cytokines IFN-γ and RANTES. Finally, we show that hepatic γδ T cells are activated in vivo by CD1d-bearing dendritic cells that have been pulsed with CL, but not phosphatidylcholine. Together, these findings demonstrate that CD1d is able to bind and present CL to a subset of CL-responsive γδ T cells that exist in the spleen and liver of healthy mice and suggest that these cells could play a role in host responses to bacterial lipids and, potentially, self-CL. We propose that CL-responsive γδ T cells play a role in immune surveillance during infection and tissue injury.

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.

On/off TLR signaling decides proinflammatory or tolerogenic dendritic cell maturation upon CD1d-mediated interaction with invariant NKT cells

Invariant NKT (iNKT) cells play an effector/adjuvant function during antimicrobial and antitumoral immunity and a regulatory role to induce immune tolerance and prevent autoimmunity. iNKT cells that differentially modulate adaptive immunity do not bear a unique phenotype and/or specific cytokine secretion profile, thus opening questions on how a single T cell subset can exert opposite immunological tasks. In this study, we show that iNKT cells perform their dual roles through a single mechanism of action relying on the cognate interaction with myeloid dendritic cells (DCs) and leading to opposite effects depending on the presence of other maturation stimuli simultaneously acting on DCs. The contact of murine purified iNKT cells with immature autologous DCs directly triggers the tolerogenic maturation of DCs, rendering them able to induce regulatory T cell differentiation and prevent autoimmune diabetes in vivo. Conversely, the interaction of the same purified iNKT cells with DCs, in the presence of simultaneous TLR4 stimulation, significantly enhances proinflammatory DC maturation and IL-12 secretion. The different iNKT cell effects are mediated through distinct mechanisms and activation of different molecular pathways within the DC: CD1d signaling and activation of the ERK1/2 pathway for the tolerogenic action, and CD40-CD40L interaction and NF-κB activation for the adjuvant effect. Our data suggest that the DC decision to undergo proinflammatory or tolerogenic maturation results from the integration of different signals received at the time of iNKT cell contact and could have important therapeutic implications for exploiting iNKT cell adjuvant/regulatory properties in autoimmune diseases, infections, and cancer.

Crystal structure of bovine CD1b3 with endogenously bound ligands

The CD1 family of Ag-presenting molecules is able to display lipids to T cells by binding them within a hydrophobic groove connected to the protein surface. In particular, the CD1b isotype is capable of binding ligands with greatly varying alkyl chain lengths through a complex network of interconnected hydrophobic pockets. Interestingly, mycobacterial lipids such as glucose monomycolate exclusively bind to CD1b. We determined the crystal structure of one of the three expressed bovine CD1b proteins, CD1b3, in complex with endogenous ligands, identified by mass spectrometry as a mixture of phosphatidylcholine and phosphatidylethanolamine, and analyzed the ability of the protein to bind glycolipids in vitro. The structure reveals a complex binding groove architecture, similar to the human ortholog but with consequential differences. Intriguingly, in bovine CD1b3 only the A', C' and F' pockets are present, whereas the T' pocket previously described in human CD1b is closed. This different pocket conformation could affect the ability of boCD1b3 to recognize lipids with long acyl chains such as glucose monomycolate. However, even in the absence of a T' tunnel, bovine CD1b3 is able to bind mycolates from Rhodococcus ruber in vitro.

Invariant NKT cells limit activation of autoreactive CD1d-positive B cells

Faulty activation of autoreactive B cells is a hallmark of autoimmune diseases like systemic lupus erythematosus (SLE). An important feature restricting activation of autoreactive B cells is efficient removal of apoptotic material. Mounting evidence also connects a primary defect in invariant natural killer T (iNKT) cells to autoimmune disease development. However, exactly how this unconventional T cell subset is involved remains to be defined. Here, we identify a suppressive role for iNKT cells in a model where autoantibody production is triggered by an increased load of circulating apoptotic cells, resembling the situation in SLE patients. Absence or reduction of iNKT cells as well as absence of CD1d-expression on B cells, needed for direct iNKT-B cell interaction, leads to increased autoreactive B cell activation and symptoms of disease. The suppression mediated by the iNKT cells is observed before B cell entry into germinal centers and can be rescued by transferring iNKT cells to deficient mice. This links iNKT cells to handling of dying cells and identifies a novel peripheral tolerance checkpoint relevant for autoimmune disease. Thus, these observations connect two clinical observations in SLE patients previously considered to be unrelated and define a new target for immunotherapy.

Lipid binding orientation within CD1d affects recognition of Borrelia burgorferi antigens by NKT cells

Invariant natural killer T cells (iNKT cells) respond to CD1d-presented glycolipids from Borrelia burgdorferi, the causative agent of Lyme disease. Although mouse and human iNKT cells respond to different antigens based on subtle differences in their fatty acids, the mechanism by which fatty acid structure determines antigenic potency is not well understood. Here we show that the mouse and human CD1d present glycolipids having different fatty acids, based in part upon a difference at a single amino acid position that is involved in positioning the sugar epitope. CD1d also can bind nonantigenic lipids, however, but unexpectedly, mouse CD1d orients the two aliphatic chains of a nonantigenic lipid rotated 180 degrees, causing a dramatic repositioning of the exposed sugar. Therefore, our data reveal the biochemical basis for the high degree of antigenic specificity of iNKT cells for certain fatty acids, and they suggest how microbes could alter fatty acid biosynthesis as an immune evasion mechanism.

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

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

Antigen recognition by CD1d-restricted NKT T cell receptors

alphabeta T cell receptors (TCRs) have traditionally been viewed as receptors for peptide antigens presented by either Major Histocompatibility Complex (MHC) class I (for CD8 T cells) or MHC class II (for CD4 T cells) antigen-presenting molecules. However, it is now clear that some T cell lineages express TCRs that are specialized for recognition of lipid-based antigens presented by the MHC class I-like CD1 family. Recently, the molecular basis for the TCR recognition of glycolipid antigens presented by CD1d has revealed an evolutionarily conserved-docking mode that is distinct from that of peptide-based recognition. T cells carrying these receptors follow a unique developmental pathway that results not only in unconventional antigen specificity, but also seemingly exaggerated functional capabilities, which makes these cells and their antigens highly attractive targets for immunotherapeutic manipulation.

The cellular and biochemical rules of lipid antigen presentation

The recognition of both protein and lipid antigens follows similar strategies that rely on different molecular mechanisms. APC present lipid antigens exploiting the same mechanisms implicated in lipid translocation, lipoprotein assembly and lipid degradation. An important issue is how the lipid structure contributes to antigenicity. Lipid hydrophobicity influences the modes of internalization by APC, the trafficking through different membrane compartments, the binding to CD1 molecules and the stability of antigenic complexes. Some glycolipids with large hydrophilic parts require processing of the sugar moieties exerted by lysosomal hydrolases. Finally, extraction of lipids from membranes, their solubilization and loading on CD1 molecules are facilitated by the same lysosomal lipid-binding proteins that are also instrumental in lipid catabolism. More recent investigations reveal how lipid-specific immunity is regulated during infections. In this review we describe the main cellular and biochemical rules of lipid antigen presentation and discuss their implications in anti-microbial and autoimmune responses.

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.

Adaptability of the semi-invariant natural killer T-cell receptor towards structurally diverse CD1d-restricted ligands

The semi-invariant natural killer (NK) T-cell receptor (NKTcr) recognises structurally diverse glycolipid antigens presented by the monomorphic CD1d molecule. While the alpha-chain of the NKTcr is invariant, the beta-chain is more diverse, but how this diversity enables the NKTcr to recognise diverse antigens, such as an alpha-linked monosaccharide (alpha-galactosylceramide and alpha-galactosyldiacylglycerol) and the beta-linked trisaccharide (isoglobotriaosylceramide), is unclear. We demonstrate here that NKTcrs, which varied in their beta-chain usage, recognised diverse glycolipid antigens with a similar binding mode on CD1d. Nevertheless, the NKTcrs recognised distinct epitopic sites within these antigens, including alpha-galactosylceramide, the structurally similar alpha-galactosyldiacylglycerol and the very distinct isoglobotriaosylceramide. We also show that the relative roles of the CDR loops within the NKTcr beta-chain varied as a function of the antigen. Thus, while NKTcrs characteristically use a conserved docking mode, the NKTcr beta-chain allows these cells to recognise unique aspects of structurally diverse CD1d-restricted ligands.

Alpha versus beta: are we on the way to resolve the mystery as to which is the endogenous ligand for natural killer T cells?

Natural killer T (NKT) lymphocytes are a unique subset of cells that play a role in regulating the immune system. For the past decade, studies have focused upon attempts to define these cells and to determine the ligand(s) that are required for their development and peripheral activation. Many research groups have focused upon determining the mechanisms for activating or inhibiting NKT cells in an attempt to control immune-mediated disorders as well as infectious and malignant conditions by using different ligand structures. Alpha-anomeric glycolipids and phospholipids derived from mammalian, bacterial, protozoan and plant species have been suggested as potential ligands for these lymphocytes. Some of these ligands were structured in forms that can bind to CD1d molecules. The lack of alpha-anomeric glycosphingolipids in mammals and the modest effect of these ligands in human studies, along with recent data from animal models and humans on the NKT-dependent immunomodulatory effect of beta-glycosphingolipids, suggest that the beta-anomeric ligands have the potential to be the endogenous NKT ligand.

Natural killer T cells and the regulation of asthma

A crucial role has been suggested for invariant natural killer T cells (iNKT) in regulating the development of asthma, a complex and heterogeneous disease characterized by airway inflammation and airway hyperreactivity (AHR). iNKT cells constitute a unique subset of T cells responding to endogenous and exogenous lipid antigens, rapidly secreting a large amount of cytokines, which amplify both innate and adaptive immunity. Herein, we review recent studies showing a requirement for iNKT cells in various models of asthma in mice and monkeys as well as studies in human patients. Surprisingly, in several different murine models of asthma, distinct subsets of iNKT cells were required, suggesting that iNKT cells serve as a common critical pathogenic element for many different forms of asthma. The importance of iNKT cells in both allergic and non-allergic forms of asthma, which are independent of adaptive immunity and associated with airway neutrophils, may explain situations previously found to be incompatible with the Th2 paradigm of asthma.