Crystal structure of mouse CD1: An MHC-like fold with a large hydrophobic binding groove

New Directions for Natural Killer T Cells in the Immunotherapy of Cancer

Natural killer T (NKT) cells have been placed at the interface between innate and adaptive immunity by a long series of experiments that convincingly showed that beyond cytokine secretion and NK cell recruitment, NKT cells were coordinating dendritic cell and B cell maturation through direct membrane contacts and initiate productive responses. As such, NKT cells are the cellular adjuvant of many immune reactions and have functions that go much beyond what their name encapsulates. In addition, the initial discovery of the ligands of NKT cells is deeply linked to cancer biology and therapy. However, for a host of reasons, animal models in which agonists of NKT cells were used did not translate well to human cancers. A systematic reassessment of NKT cells role in tumorigenesis, especially spontaneous one, is now accessible using single cell analysis technologies both in mouse and man, and should be taken advantage of. Similarly, the migration, localization, phenotype of NKT cells following induced expansion after injection of an agonist can be examined at the single cell level. This technological revolution will help evaluate where and how NKT cells can be used in cancer.

CD1: From Molecules to Diseases

The human cluster of differentiation (CD)1 system for antigen display is comprised of four types of antigen-presenting molecules, each with a distinct functional niche: CD1a, CD1b, CD1c, and CD1d. Whereas CD1 proteins were thought solely to influence T-cell responses through display of amphipathic lipids, recent studies emphasize the role of direct contacts between the T-cell receptor and CD1 itself. Moving from molecules to diseases, new research approaches emphasize human CD1-transgenic mouse models and the study of human polyclonal T cells in vivo or ex vivo in disease states. Whereas the high genetic diversity of major histocompatibility complex (MHC)-encoded antigen-presenting molecules provides a major hurdle for designing antigens that activate T cells in all humans, the simple population genetics of the CD1 system offers the prospect of discovering or designing broadly acting immunomodulatory agents.

Four pathways of CD1 antigen presentation to T cells

CD1a, CD1b, CD1c and CD1d proteins migrate through distinct subcellular compartments of antigen presenting cells and so can be considered to take four separate pathways leading to display of lipid antigens to T cell receptors. This review discusses the intersection of CD1 trafficking and lipid antigen loading mechanisms in cells, highlighting key controversies relating to CD1 gene expression, size mismatches between antigens and CD1 binding clefts and unexpected mechanisms of T cell receptor-based recognition.

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

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

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.

Targeting Innate-Like T Cells in Tuberculosis

Peptide-specific conventional T cells have been major targets for designing most antimycobacterial vaccines. Immune responses mediated by conventional T cells exhibit a delayed onset upon primary infection and are highly variable in different human populations. In contrast, innate-like T cells quickly respond to pathogens and display effector functions without undergoing extensive clonal expansion. Specifically, the activation of innate-like T cells depends on the promiscuous interaction of highly conserved antigen-presenting molecules, non-peptidic antigens, and likely semi-invariant T cell receptors. In antimicrobial immune responses, mucosal-associated invariant T cells are activated by riboflavin precursor metabolites presented by major histocompatibility complex-related protein I, while lipid-specific T cells including natural killer T cells are activated by lipid metabolites presented by CD1 proteins. Multiple innate-like T cell subsets have been shown to be protective or responsive in mycobacterial infections. Through rapid cytokine secretion, innate-like T cells function in early defense and memory response, offering novel advantages over conventional T cells in the design of anti-tuberculosis strategies.

Type II Natural Killer T Cells that Recognize Sterol Carrier Protein 2 Are Implicated in Vascular Inflammation in the Rat Model of Systemic Connective Tissue Diseases

We previously generated a rat model that developed systemic connective tissue diseases, including synovitis, myositis, and small-vessel vasculitis (SVV), and established a vascular endothelial cell-reactive T-cell clone, VASC-1, from the model. VASC-1 was determined to be a type II natural killer T-cell clone. In this study, we attempted to identify the antigen recognized by VASC-1. The monkey-derived cell line COS-7 was used because VASC-1 does not bind naturally to COS-7, although the amino acid sequences are well conserved between monkey CD1d and rat CD1d. We generated 98 COS-7 clones transfected with miscellaneous rat cDNA and screened them for VASC-1 binding. Consequently, we found one clone, 4D2, which could bind to VASC-1. Sequencing identified the rat cDNA introduced into 4D2 as sterol carrier protein 2 (SCP2). When VASC-1 was co-cultured with SCP2 knockdown rat vascular endothelial cells, VASC-1 binding was reduced significantly. Moreover, we designed a series of rat SCP2 peptides and introduced them into COS-7 cells. On the basis of VASC-1 binding and proliferation, we revealed that the peptide rSCP2_518-532 included the epitope recognized by VASC-1. Furthermore, immunization with rSCP2_518-532 accelerated the development of SVV in the rat model. The collective findings suggest that type II natural killer T cells reactive with autologous SCP2 are implicated in vascular inflammation in the rat model.

Blocking of CD1d Decreases Trypanosoma cruzi-Induced Activation of CD4-CD8- T Cells and Modulates the Inflammatory Response in Patients With Chagas Heart Disease

The control of inflammatory responses to prevent the deadly cardiac pathology in human Chagas disease is a desirable and currently unattained goal. Double-negative (DN) T cells are important sources of inflammatory and antiinflammatory cytokines in patients with Chagas heart disease and those with the indeterminate clinical form of Chagas disease, respectively. Given the importance of DN T cells in immunoregulatory processes and their potential as targets for controlling inflammation-induced pathology, we studied the involvement of CD1 molecules in the activation and functional profile of Trypanosoma cruzi-specific DN T cells. We observed that parasite stimulation significantly increased the expression of CD1a, CD1b, CD1c, and CD1d by CD14(+) cells from patients with Chagas disease. Importantly, among the analyzed molecules, only CD1d expression showed an association with the activation of DN T cells, as well as with worse ventricular function in patients with Chagas disease. Blocking of CD1d-mediated antigen presentation led to a clear reduction of DN T-cell activation and a decrease in the expression of interferon γ (IFN-γ) by DN T cells. Thus, our results showed that antigen presentation via CD1d is associated with activation of DN T cells in Chagas disease and that CD1d blocking leads to downregulation of IFN-γ by DN T cells from patients with Chagas heart disease, which may be a potential target for preventing progression of inflammation-mediated dilated cardiomyopathy.

Gene conversion of the major histocompatibility complex class I Caja-G in common marmosets (Callithrix jacchus)

Currently, the amount of sequenced and classified MHC class I genes of the common marmoset is limited, in spite of the wide use of this species as an animal model for biomedical research. In this study, 480 clones of MHC class I G locus (Caja‐G) cDNA sequences were obtained from 21 common marmosets. Up to 10 different alleles were detected in each common marmoset, leading to the assumption that the Caja‐G loci duplicated in the marmoset genome. In the investigated population, four alleles occurred more often, giving evidence for higher immunological advantage of these alleles. In contrast to the human non‐classical MHC class I genes, Caja‐G shows high rates of polymorphism at the relevant peptide‐binding sites, despite its phylogenetic relationship to the non‐classical HLA‐G. Our results provide information for better understanding of the immunological properties of the common marmoset and confirm the theory of a gene conversion of the Caja‐G due to its detected plasticity and the absence of any known HLA‐A equivalent.

Donor-unrestricted T cells in the human CD1 system

The CD1 and MHC systems are specialized for lipid and peptide display, respectively. Here, we review evidence showing how cellular CD1a, CD1b, CD1c, and CD1d proteins capture and display many cellular lipids to T cell receptors (TCRs). Increasing evidence shows that CD1-reactive T cells operate outside two classical immunogenetic concepts derived from the MHC paradigm. First, because CD1 proteins are non-polymorphic in human populations, T cell responses are not restricted to the donor's genetic background. Second, the simplified population genetics of CD1 antigen-presenting molecules can lead to simplified patterns of TCR usage. As contrasted with donor-restricted patterns of MHC-TCR interaction, the donor-unrestricted nature of CD1-TCR interactions raises the prospect that lipid agonists and antagonists of T cells could be developed.

MR1 discovery

The moment of MR1 discovery is described. The MR1 gene is the first and the last reported human MHC-related gene intentionally isolated from the human genome composed of three billion base pairs. Evolutionary considerations formed the basis of its isolation. Some details surrounding the moment and some retrospective descriptions with various kinds of encounters are also included.

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).

CD1A, D and E gene polymorphisms in a North African population from Morocco

CD1 molecules are specialized in capturing and presenting lipids and glycolipids to distinct subsets of T and NKT cells. Glycolipid presentation could play a significant role in the immune response against microbial infections. There are five closely linked CD1 genes in humans, named CD1A, B, C, D, and E, which all show a limited polymorphism. In this study, exon 2 polymorphisms of CD1A, CD1D and CD1E were investigated and allele, genotype and haplotype frequencies of these loci were reported in a Moroccan population. A comparison with allele, genotype and haplotype frequencies observed in other geographic areas was also performed. Results confirmed the presence of ethnic differences in CD1 polymorphism, mainly in CD1D (in this population two additional CD1D variant alleles, CD1D(∗)03 and CD1D(∗)04, were described) and E genes. These data could be useful to evaluate a possible pathogenetic role of CD1 in diseases. Increasing the knowledge in this field may offer possibilities for the development of new immunotherapeutic approaches.

The Implication and Significance of Beta 2 Microglobulin: A Conservative Multifunctional Regulator

OBJECTIVE

This review focuses on the current knowledge on the implication and significance of beta 2 microglobulin (β2M), a conservative immune molecule in vertebrate.

DATA SOURCES

The data used in this review were obtained from PubMed up to October 2015. Terms of β2M, immune response, and infection were used in the search.

STUDY SELECTIONS

Articles related to β2M were retrieved and reviewed. Articles focusing on the characteristic and function of β2M were selected. The exclusion criteria of articles were that the studies on β2M-related molecules.

RESULTS

β2M is critical for the immune surveillance and modulation in vertebrate animals. The dysregulation of β2M is associated with multiple diseases, including endogenous and infectious diseases. β2M could directly participate in the development of cancer cells, and the level of β2M is deemed as a prognostic marker for several malignancies. It also involves in forming major histocompatibility complex (MHC class I or MHC I) or like heterodimers, covering from antigen presentation to immune homeostasis.

CONCLUSIONS

Based on the characteristic of β2M, it or its signaling pathway has been targeted as biomedical or therapeutic tools. Moreover, β2M is highly conserved among different species, and overall structures are virtually identical, implying the versatility of β2M on applications.

Species Specific Differences of CD1d Oligomer Loading In Vitro

CD1d molecules are MHC class I-like molecules that present glycolipids to iNKT cells. The highly conserved interaction between CD1d:α-Galactosylceramide (αGC) complexes and the iNKT TCR not only defines this population of αβ T cells but can also be used for its direct identification. Therefore, CD1d oligomers are a widely used tool for iNKT cell related investigations. To this end, the lipid chains of the antigen have to be inserted into the hydrophobic pockets of the CD1d binding cleft, often with help of surfactants. In this study, we investigated the influence of different surfactants (Triton X-100, Tween 20, Tyloxapol) on in vitro loading of CD1d molecules derived from four different species (human, mouse, rat and cotton rat) with αGC and derivatives carrying modifications of the acyl-chain (DB01-1, PBS44) and a 6-acetamido-6-deoxy-addition at the galactosyl head group (PBS57). We also compared rat CD1d dimers with tetramers and staining of an iNKT TCR transductant was used as readout for loading efficacy. The results underlined the importance of CD1d loading efficacy for proper analysis of iNKT TCR binding and demonstrated the necessity to adjust loading conditions for each oligomer/glycolipid combination. The efficient usage of surfactants as a tool for CD1d loading was revealed to be species-specific and depending on the origin of the CD1d producing cells. Additional variation of surfactant-dependent loading efficacy between tested glycolipids was influenced by the acyl-chain length and the modification of the galactosyl head group with PBS57 showing the least dependence on surfactants and the lowest degree of species-dependent differences.

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.

Lipid and small-molecule display by CD1 and MR1

The antigen-presenting molecules CD1 and MHC class I-related protein (MR1) display lipids and small molecules to T cells. The antigen display platforms in the four CD1 proteins are laterally asymmetrical, so that the T cell receptor (TCR)-binding surfaces are comprised of roofs and portals, rather than the long grooves seen in the MHC antigen-presenting molecules. TCRs can bind CD1 proteins with left-sided or right-sided footprints, creating unexpected modes of antigen recognition. The use of tetramers of human CD1a, CD1b, CD1c or MR1 proteins now allows detailed analysis of the human T cell repertoire, which has revealed new invariant TCRs that bind CD1b molecules and are different from those that define natural killer T cells and mucosal-associated invariant T cells.

Activation of human T cells by CD1 and self-lipids

Over two decades ago, it was discovered that the human T-cell repertoire contains T cells that do not recognize peptide antigens in the context of MHC molecules but instead respond to lipid antigens presented by CD1 antigen-presenting molecules. The ability of T cells to 'see' lipid antigens bound to CD1 enables these lymphocytes to sense changes in the lipid composition of cells and tissues as a result of infections, inflammation, or malignancies. Although foreign lipid antigens have been shown to function as antigens for CD1-restricted T cells, many CD1-restricted T cells do not require foreign antigens for activation but instead can be activated by self-lipids presented by CD1. This review highlights recent developments in the field, including the identification of common mammalian lipids that function as autoantigens for αβ and γδ T cells, a novel mode of T-cell activation whereby CD1a itself rather than lipids serves as the autoantigen, and various mechanisms by which the activation of CD1-autoreactive T cells is regulated. As CD1 can induce T-cell effector functions in the absence of foreign antigens, multiple mechanisms are in place to regulate this self-reactivity, and stimulatory CD1-lipid complexes appear to be tightly controlled in space and time.

Coevolution of T-cell receptors with MHC and non-MHC ligands

The structure and amino acid diversity of the T-cell receptor (TCR), similar in nature to that of Fab portions of antibodies, would suggest that these proteins have a nearly infinite capacity to recognize antigen. Yet all currently defined native T cells expressing an α and β chain in their TCR can only sense antigen when presented in the context of a major histocompatibility complex (MHC) molecule. This MHC molecule can be one of many that exist in vertebrates, presenting small peptide fragments, lipid molecules, or small molecule metabolites. Here we review the pattern of TCR recognition of MHC molecules throughout a broad sampling of species and T-cell lineages and also touch upon T cells that do not appear to require MHC presentation for their surveillance function. We review the diversity of MHC molecules and information on the corresponding T-cell lineages identified in divergent species. We also discuss TCRs with structural domains unlike that of conventional TCRs of mouse and human. By presenting this broad view of TCR sequence, structure, domain organization, and function, we seek to explore how this receptor has evolved across time and been selected for alternative antigen-recognition capabilities in divergent lineages.

A Subset of CD8αβ+ Invariant NKT Cells in a Humanized Mouse Model

Invariant NKT (iNKT) cells are unconventional innate-like T cells demonstrating potent antitumor function in conventional mouse models. However, the iNKT cell ligands have had limited efficacy in human antitumor clinical trials, mostly due to the profound differences in the properties and compositions of iNKT cells between the two species, including the presence of a CD8(+) subset of iNKT cells only in humans. To build reliable in vivo models for studying human iNKT cells, we recently developed the first humanized mouse model (hCD1d-KI) with human CD1d knocked in. To further humanize the mouse model, we now introduced the human invariant NKT TCRα-chain (Vα24Jα18) into the hCD1d-knockin mice. Similar to humans, this humanized mouse model developed a subset of CD8αβ(+) iNKT cells among other human-like iNKT subsets. The presence of the CD8αβ(+) iNKT cells in the thymus suggests that these cells developed in the thymus. In the periphery, these NKT cells showed a strong Th1-biased cytokine response and potent cytotoxicity for syngeneic tumor cells upon activation, as do human CD8αβ(+) iNKT cells. The low binding avidity of iNKT TCRs to the human CD1d/lipid complex and high prevalence of Vβ7 TCRβ among the CD8(+) iNKT cells strongly point to a low avidity-based developmental program for these iNKT cells, which included the suppression of Th-POK and upregulation of eomesodermin transcriptional factors. Our establishment of this extensively humanized mouse model phenotypically and functionally reflecting the human CD1d/iNKT TCR system will greatly facilitate the future design and optimization of iNKT cell-based immunotherapies.

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.

The Role of Invariant Natural Killer T Cells in Dendritic Cell Licensing, Cross-Priming, and Memory CD8(+) T Cell Generation

New vaccination strategies focus on achieving CD8⁺ T cell (CTL) immunity rather than on induction of protective antibody responses. While the requirement of CD4⁺ T (Th) cell help in dendritic cell (DC) activation and licensing, and in CTL memory induction has been described in several disease models, CTL responses may occur in a Th cell help-independent manner. Invariant natural killer T cells (iNKT cells) can substitute for Th cell help and license DC as well. iNKT cells produce a broad spectrum of Th1 and Th2 cytokines, thereby inducing a similar set of costimulatory molecules and cytokines in DC. This form of licensing differs from Th cell help by inducing other chemokines, while Th cell-licensed DCs produce CCR5 ligands, iNKT cell-licensed DCs produce CCL17, which attracts CCR4⁺ CD8⁺ T cells for subsequent activation. It has recently been shown that iNKT cells do not only enhance immune responses against bacterial pathogens or parasites but also play a role in viral infections. The inclusion of iNKT cell ligands in influenza virus vaccines enhanced memory CTL generation and protective immunity in a mouse model. This review will focus on the role of iNKT cells in the cross-talk with cross-priming DC and memory CD8⁺ T cell formation.

Synthesis of C-5″ and C-6″-modified α-GalCer analogues as iNKT-cell agonists

Alpha-galactosyl ceramide (α-GalCer) is a prototypical synthetic ligand of invariant natural killer T (iNKT) cells. Upon presentation by the MHC class I-like molecule CD1d, this glycolipid stimulates iNKT cells to secrete a vast amount of both pro-inflammatory Th1 and anti-inflammatory Th2 cytokines. Recently, we discovered that selected 6″-modified α-GalCer analogues may produce markedly Th1-biased responses due to the formation of either an additional anchor with CD1d or by establishing extra interactions with the T-cell receptor of iNKT cells. Here, we report a practical synthesis towards 6″-O-carbamate and galacturonamide analogues of α-GalCer and their evaluation as iNKT cell agonists in mice.

Increased CD1D polymorphism: identification of two novel alleles, CD1D*03 and *04, in individuals from Morocco

Two novel CD1D alleles were identified in unrelated individuals from Morocco. They differ each from the common CD1D*01 allele by one nucleotide substitution in exon 2 resulting in one amino acid change in the G-ALPHA1-LIKE domain. According to the IMGT unique numbering for G domain, CD1D*03 has one nucleotide transition c136 > t in codon 46, with an arginine-to-cysteine amino acid change (R46 > C) in the D-STRAND, whereas CD1D*04 has one transition c98 > t in codon 33, with a threonine-to-methionine amino acid change (T33 > M) in the C-STRAND. This suggests that CD1D is more polymorphic than previously assumed.

MHC structure and function - antigen presentation. Part 2

The second part of this review deals with the molecules and processes involved in the processing and presentation of the antigenic fragments to the T-cell receptor. Though the nature of the antigens presented varies, the most significant class of antigens is proteins, processed within the cell to be then recognized in the form of peptides, a mechanism that confers an extraordinary degree of precision to this mode of immune response. The efficiency and accuracy of this system is also the result of the myriad of mechanisms involved in the processing of proteins and production of peptides, in addition to the capture and recycling of alternative sources aiming to generate further diversity in the presentation to T-cells.

CD1 and mycobacterial lipids activate human T cells

For decades, proteins were thought to be the sole or at least the dominant source of antigens for T cells. Studies in the 1990s demonstrated that CD1 proteins and mycobacterial lipids form specific targets of human αβ T cells. The molecular basis by which T-cell receptors (TCRs) recognize CD1-lipid complexes is now well understood. Many types of mycobacterial lipids function as antigens in the CD1 system, and new studies done with CD1 tetramers identify T-cell populations in the blood of tuberculosis patients. In human populations, a fundamental difference between the CD1 and major histocompatibility complex systems is that all humans express nearly identical CD1 proteins. Correspondingly, human CD1 responsive T cells show evidence of conserved TCRs. In addition to natural killer T cells and mucosal-associated invariant T (MAIT cells), conserved TCRs define other subsets of human T cells, including germline-encoded mycolyl-reactive (GEM) T cells. The simple immunogenetics of the CD1 system and new investigative tools to measure T-cell responses in humans now creates a situation in which known lipid antigens can be developed as immunodiagnostic and immunotherapeutic reagents for tuberculosis disease.

Bee venom processes human skin lipids for presentation by CD1a

Venoms frequently co-opt host immune responses, so study of their mode of action can provide insight into novel inflammatory pathways. Using bee and wasp venom responses as a model system, we investigated whether venoms contain CD1-presented antigens. Here, we show that venoms activate human T cells via CD1a proteins. Whereas CD1 proteins typically present lipids, chromatographic separation of venoms unexpectedly showed that stimulatory factors partition into protein-containing fractions. This finding was explained by demonstrating that bee venom-derived phospholipase A2 (PLA2) activates T cells through generation of small neoantigens, such as free fatty acids and lysophospholipids, from common phosphodiacylglycerides. Patient studies showed that injected PLA2 generates lysophospholipids within human skin in vivo, and polyclonal T cell responses are dependent on CD1a protein and PLA2. These findings support a previously unknown skin immune response based on T cell recognition of CD1a proteins and lipid neoantigen generated in vivo by phospholipases. The findings have implications for skin barrier sensing by T cells and mechanisms underlying phospholipase-dependent inflammatory skin disease.

CD1d serves as a surface receptor for oxidized cholesterol induction of peroxisome proliferator-activated receptor-γ

OBJECTIVE

The cluster of differentiation-1d (CD1d) recognizes and presents the lipid antigens to NK-T lymphocytes. Atherosclerotic lesions contain atherogenic lipids, mainly cholesterol and its oxides. Peroxisome proliferator-activated receptor-γ (PPARγ) is also known to exist in atherosclerotic lesions, participating in regulation of lipid metabolism. The current study tested whether CD1d acts as a surface receptor that mediates induction and activation of PPARγ by oxysterols commonly found in atherosclerotic lesions.

METHODS AND RESULTS

CD1d overexpression in HEK 293 cells transfected with CD1d cDNA was confirmed by fluorescence, flow cytometry, Western blotting and mRNA expression. Tritiated ((3)H) 7-ketocholesterol (7K) was used for lipid binding assays. Radioactive assessment demonstrated an increased 7K-binding activity HEK 293 cells with CD1d overexpression. The 7K binding could be blocked by another oxysterol, 25-hydroxycholesterol, but not by native free cholesterol. Addition of CD1d:IgG dimer protein or an anti-CD1d antibody, but not control IgG, significantly diminished 7K binding to CD1d-expressing HEK 293 cells. CD1d deficiency markedly diminished the 7K-binding in macrophages and smooth muscle cells. Western blot and gel shift assays demonstrated that CD1d-mediated 7K binding induced expression and activation of PPARγ. The PPARγ agonist PGJ2 enhances the 7K stimulatory effect on PPARγ expression and activity but the antagonist GW9662 inhibits the 7K effect on the CD1d-expressing cells.

CONCLUSIONS

CD1d acts as a cell surface receptor that recognizes and binds oxysterols and initializes a pathway connecting oxysterol binding to PPARγ activation.

Discovery of NKT cells and development of NKT cell-targeted anti-tumor immunotherapy

Natural Killer T (NKT) cells are unique lymphocytes characterized by their expression of a single invariant antigen receptor encoded by Vα14Jα18 in mice and Vα24Jα18 in humans, which recognizes glycolipid antigens in association with the monomorphic CD1d molecule. NKT cells mediate adjuvant activity to activate both CD8T cells to kill MHC-positive tumor cells and NK cells to eliminate MHC-negative tumor at the same time in patients, resulting in the complete eradication of tumors without relapse. Therefore, the NKT cell-targeted therapy can be applied to any type of tumor and also to anyone individual, regardless of HLA type.Phase IIa clinical trials on advanced lung cancers and head and neck tumors have been completed and showed significantly prolonged median survival times with only the primary treatment. Another potential treatment option for the future is to use induced pluripotent stem cell (iPS)-derived NKT cells, which induced adjuvant effects on anti-tumor responses, inhibiting in vivo tumor growth in a mouse model.

γδ T cell surveillance via CD1 molecules

γδ T cells are a prominent epithelial-resident lymphocyte population, possessing multi-functional capacities in the repair of host tissue, pathogen clearance, and tumor surveillance. Although three decades have now passed since their discovery, the nature of γδ T cell receptor (TCR)-mediated ligand recognition remains poorly defined. Recent studies have provided structural insight into this recognition, demonstrating that γδ T cells survey both CD1 and the presented lipid, and in some cases are exquisitely lipid specific. We review these findings here, examining the molecular basis for and the functional relevance of this interaction. We discuss potential implications on the notion that non-classical major histocompatibility complex (MHC) molecules may function as important restricting elements of γδ TCR specificity, and on our understanding of γδ T cell activation and function.

γδ T Lymphocytes as a First Line of Immune Defense: Old and New Ways of Antigen Recognition and Implications for Cancer Immunotherapy

Among γδT cells, the Vδ1 subset, resident in epithelial tissues, is implied in the defense against viruses, fungi, and certain hematological malignancies, while the circulating Vδ2 subpopulation mainly respond to mycobacteria and solid tumors. Both subsets can be activated by stress-induced molecules (MIC-A, MIC-B, ULBPs) to produce pro-inflammatory cytokines and lytic enzymes and destroy bacteria or damaged cells. γδT lymphocytes can also recognize lipids, as those associated to M. tuberculosis, presented by the CD1 molecule, or phosphoantigens (P-Ag), either autologous, which accumulates in virus-infected cells, or microbial produced by prokaryotes and parasites. In cancer cells, P-Ag accumulate due to alterations in the mevalonate pathway; recently, butyrophilin 3A1 has been shown to be the presenting molecule for P-Ag. Of interest, aminobisphosphonates indirectly activate Vδ2 T cells inducing the accumulation of P-Ag. Based on these data, γδT lymphocytes are attractive effectors for cancer immunotherapy. However, the results obtained in clinical trials so far have been disappointing: this review will focus on the possible reasons of this failure as well as on suggestions for implementation of the therapeutic strategies.

The CD1 size problem: lipid antigens, ligands, and scaffolds

Whereas research on CD1d has emphasized a few glycosyl ceramides, the broader family of four human CD1 antigen-presenting molecules binds hundreds of distinct self-lipids. Individual lipid types bind within CD1 grooves in different ways, such that they partially fill the groove, match the groove volume, or protrude substantially from the groove. These differing modes of binding can now be connected to differing immunological functions, as individual lipids can act as stimulatory antigens, inhibitory ligands, or space-filling scaffolds. Because each type of CD1 protein folds to produce antigen-binding grooves with differing sizes and shapes, CD1a, CD1b, CD1c, CD1d, and CD1e have distinct mechanisms of capturing self-lipids and exchanging them for foreign lipids. The size discrepancy between endogeneous lipids and groove volume is most pronounced for CD1b. Recent studies show that the large CD1b cavity can simultaneously bind two self-lipids, the antigen, and its scaffold lipid, which can be exchanged for one large bacterial lipid. In this review, we will highlight recent studies showing how cells regulate lipid antigen loading and the roles CD1 groove structures have in control of the presentation of chemically diverse lipids to T cells.

Bcl-xL regulates CD1d-mediated antigen presentation to NKT cells by altering CD1d trafficking through the endocytic pathway

NKT cells are a unique subset of T cells that recognize glycolipid Ags presented in the context of CD1d molecules. NKT cells mount strong antitumor responses and are a major focus in developing effective cancer immunotherapy. It is known that CD1d molecules are constantly internalized from the cell surface, recycled through the endocytic compartments, and re-expressed on the cell surface. However, little is known about the regulation of CD1d-mediated Ag processing and presentation in B cell lymphoma. Prosurvival factors of the Bcl-2 family, such as Bcl-xL, are often upregulated in B cell lymphomas and are intimately linked to sphingolipid metabolism, as well as the endocytic compartments. We hypothesized that Bcl-xL can regulate CD1d-mediated Ag presentation to NKT cells. We found that overexpression or induction of Bcl-xL led to increased Ag presentation to NKT cells. Conversely, the inhibition or knockdown of Bcl-xL led to decreased NKT cell activation. Furthermore, knockdown of Bcl-xL resulted in the loss of CD1d trafficking to lysosome-associated membrane protein 1(+) compartments. Rab7, a late endosomal protein, was upregulated and CD1d molecules accumulated in the Rab7(+) late endosomal compartment. These results demonstrate that Bcl-xL regulates CD1d-mediated Ag processing and presentation to NKT cells by altering the late endosomal compartment and changing the intracellular localization of CD1d.

The effect of intracellular trafficking of CD1d on the formation of TCR repertoire of NKT cells

CD1 molecules belong to non-polymorphic MHC class I-like proteins and present lipid antigens to T cells. Five different CD1 genes (CD1a-e) have been identified and classified into two groups. Group 1 include CD1a-c and present pathogenic lipid antigens to αβ T cells reminiscence of peptide antigen presentation by MHC-I molecules. CD1d is the only member of Group 2 and presents foreign and self lipid antigens to a specialized subset of αβ T cells, NKT cells. NKT cells are involved in diverse immune responses through prompt and massive production of cytokines. CD1d-dependent NKT cells are categorized upon the usage of their T cell receptors. A major subtype of NKT cells (type I) is invariant NKT cells which utilize invariant Vα14-Jα18 TCR alpha chain in mouse. The remaining NKT cells (type II) utilize diverse TCR alpha chains. Engineered CD1d molecules with modified intracellular trafficking produce either type I or type II NKT cell-defects suggesting the lipid antigens for each subtypes of NKT cells are processed/generated in different intracellular compartments. Since the usage of TCR by a T cell is the result of antigen-driven selection, the intracellular metabolic pathways of lipid antigen are a key in forming the functional NKT cell repertoire.

Evolution of nonclassical MHC-dependent invariant T cells

TCR-mediated specific recognition of antigenic peptides in the context of classical MHC molecules is a cornerstone of adaptive immunity of jawed vertebrate. Ancillary to these interactions, the T cell repertoire also includes unconventional T cells that recognize endogenous and/or exogenous antigens in a classical MHC-unrestricted manner. Among these, the mammalian nonclassical MHC class I-restricted invariant T cell (iT) subsets, such as iNKT and MAIT cells, are now believed to be integral to immune response initiation as well as in orchestrating subsequent adaptive immunity. Until recently the evolutionary origins of these cells were unknown. Here we review our current understanding of a nonclassical MHC class I-restricted iT cell population in the amphibian Xenopus laevis. Parallels with the mammalian iNKT and MAIT cells underline the crucial biological roles of these evolutionarily ancient immune subsets.

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.

A divergent approach to the synthesis of iGb3 sugar and lipid analogues via a lactosyl 2-azido-sphingosine intermediate

Isoglobotrihexosylceramide (iGb3, 1) is an immunomodulatory glycolipid that binds to CD1d and is presented to the T-cell receptor (TCR) of invariant natural killer T (iNKT) cells.

NKT cells as an ideal anti-tumor immunotherapeutic

Human natural killer T (NKT) cells are characterized by their expression of an invariant T cell antigen receptor α chain variable region encoded by a Vα24Jα18 rearrangement. These NKT cells recognize α-galactosylceramide (α-GalCer) in conjunction with the MHC class I-like CD1d molecule and bridge the innate and acquired immune systems to mediate efficient and augmented immune responses. A prime example of one such function is adjuvant activity: NKT cells augment anti-tumor responses because they can rapidly produce large amounts of IFN-γ, which acts on NK cells to eliminate MHC negative tumors and also on CD8 cytotoxic T cells to kill MHC positive tumors. Thus, upon administration of α-GalCer-pulsed DCs, both MHC negative and positive tumor cells can be effectively eliminated, resulting in complete tumor eradication without tumor recurrence. Clinical trials have been completed in a cohort of 17 patients with advanced non-small cell lung cancers and 10 cases of head and neck tumors. Sixty percent of advanced lung cancer patients with high IFN-γ production had significantly prolonged median survival times of 29.3 months with only the primary treatment. In the case of head and neck tumors, 10 patients who completed the trial all had stable disease or partial responses 5 weeks after the combination therapy of α-GalCer-DCs and activated NKT cells. We now focus on two potential powerful treatment options for the future. One is to establish artificial adjuvant vector cells containing tumor mRNA and α-GalCer/CD1d. This stimulates host NKT cells followed by DC maturation and NK cell activation but also induces tumor-specific long-term memory CD8 killer T cell responses, suppressing tumor metastasis even 1 year after the initial single injection. The other approach is to establish induced pluripotent stem (iPS) cells that can generate unlimited numbers of NKT cells with adjuvant activity. Such iPS-derived NKT cells produce IFN-γ in vitro and in vivo upon stimulation with α-GalCer/DCs, and mediated adjuvant effects, suppressing tumor growth in vivo.

Forming a complex with MHC class I molecules interferes with mouse CD1d functional expression

CD1d molecules are structurally similar to MHC class I, but present lipid antigens as opposed to peptides. Here, we show that MHC class I molecules physically associate with (and regulate the functional expression of) mouse CD1d on the surface of cells. Low pH (3.0) acid stripping of MHC class I molecules resulted in increased surface expression of murine CD1d on antigen presenting cells as well as augmented CD1d-mediated antigen presentation to NKT cells. Consistent with the above results, TAP1-/- mice were found to have a higher percentage of type I NKT cells as compared to wild type mice. Moreover, bone marrow-derived dendritic cells from TAP1-/- mice showed increased antigen presentation by CD1d compared to wild type mice. Together, these results suggest that MHC class I molecules can regulate NKT cell function, in part, by masking CD1d.

Dynamics of the antigen-binding grooves in CD1 proteins: reversible hydrophobic collapse in the lipid-free state

CD1 proteins mediate the presentation of endogenous and foreign lipids on the cell surface for recognition by T cell receptors. To sample a diverse antigen pool, CD1 proteins are repeatedly internalized and recycled, assisted, in some cases, by lipid transfer proteins such as saposins. The specificity of each CD1 isoform is, therefore, conferred in part by its intracellular pathway but also by distinct structural features of the antigen-binding domain. Crystal structures of CD1-lipid complexes reveal hydrophobic grooves and pockets within these binding domains that appear to be specialized for different lipids. However, the mechanism of lipid loading and release remains to be characterized. Here we gain insights into this mechanism through a meta-analysis of the five human CD1 isoforms, in the lipid-bound and lipid-free states, using all-atom molecular dynamics simulations. Strikingly, for isoforms CD1b through CD1e, our simulations show the near-complete collapse of the hydrophobic cavities in the absence of the antigen. This event results from the spontaneous closure of the binding domain entrance, flanked by two α-helices. Accordingly, we show that the anatomy of the binding cavities is restored if these α-helices are repositioned extrinsically, suggesting that helper proteins encountered during recycling facilitate lipid exchange allosterically. By contrast, we show that the binding cavity of CD1a is largely preserved in the unliganded state because of persistent electrostatic interactions that keep the portal α-helices at a constant separation. The robustness of this binding groove is consistent with the observation that lipid exchange in CD1a is not dependent on cellular internalization.

Highly stereocontrolled total synthesis of β-D-mannosyl phosphomycoketide: a natural product from Mycobacterium tuberculosis

β-D-mannosyl phosphomycoketide (C32-MPM), a naturally occurring glycolipid found in the cell walls of Mycobacterium tuberculosis, acts as a potent antigen to activate T-cells upon presentation by CD1c protein. The lipid portion of C32-MPM contains a C32-mycoketide, consisting of a saturated oligoisoprenoid chain with five chiral methyl branches. Here we develop several stereocontrolled approaches to assemble the oligoisoprenoid chain with high stereopurity (>96%) using Julia-Kocienski olefinations followed by diimide reduction. By careful choice of olefination sites, we could derive all chirality from a single commercial compound, methyl (2S)-3-hydroxy-2-methylpropionate (>99% ee). Our approach is the first highly stereocontrolled method to prepare C32-MPM molecule with >96% stereopurity from a single >99% ee starting material. We anticipate that our methods will facilitate the highly stereocontrolled synthesis of a variety of other natural products containing chiral oligoisoprenoid-like chains, including vitamins, phytol, insect pheromones, and archaeal lipids.

Synthesis and biological activity of hydroxylated analogues of KRN7000 (α-galactosylceramide)

KRN7000 is one of the α-galactosylceramides, which has a 2-hexacosanoylamino-3,4-dihydroxyoctadecyl group. This compound, known as a ligand for the activation of CD1d mediated invariant natural killer T cells (iNKT cells) which release both T helper 1 (Th1) cytokines such as IFNγ and Th2 cytokines such as IL-4, has been anticipated as an antitumor drug, because of its strong secretion of IFNγ. This time, we focused on the hydroxylated analogues of KRN7000 which could be thought of as increasing hydrophilicity and showing bias to Th2 cytokine (IL-4) secretion. Therefore, they may become the drugs for autoimmune diseases for the following reasons: (i) compound OCH, one of the α-galactosylceramide analogues with a shorter sphingosine chain than KRN7000, increases hydrophilicity relative to KRN7000; and (ii) OCH is known to induce much more Th2 cytokines (IL-4) than Th1 cytokines from iNKT cells compared to KRN7000. Naturally, OCH has become one of the candidate drugs for autoimmune diseases. The more hydroxylated derivatives of KRN7000 are anticipated to induce Th2 bias. Therefore, eight analogues with 1-4 excess hydroxyl groups on the lipid chain of KRN7000 were synthesized to examine if they behave in the same way as OCH. As a result, three out of eight compounds biased largely to IL-4 secretion, and their effectiveness for experimental autoimmune encephalomyelitis (EAE) was examined. It was recognized that two compounds (†)RCAI-147/-160 showed good suppression of EAE symptoms.

The adaptable major histocompatibility complex (MHC) fold: structure and function of nonclassical and MHC class I-like molecules

The MHC fold is found in proteins that have a range of functions in the maintenance of an organism's health, from immune regulation to fat metabolism. Well adapted for antigen presentation, as seen for peptides in the classical MHC molecules and for lipids in CD1 molecules, the MHC fold has also been modified to perform Fc-receptor activity (e.g., FcRn) and for roles in host homeostasis (e.g., with HFE and ZAG). The more divergent MHC-like molecules, such as some of those that interact with the NKG2D receptor, represent the minimal MHC fold, doing away with the α3 domain and β2m while maintaining the α1/α2 platform domain for receptor engagement. Viruses have also co-opted the MHC fold for immune-evasive functions. The variations on the theme of a β-sheet topped by two semiparallel α-helices are discussed in this review, highlighting the fantastic adaptability of this fold for good and for bad.

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.