Highly conserved antigen-presenting function of CD1d molecules

Advanced omics techniques shed light on CD1d-mediated lipid antigen presentation to iNKT cells

Invariant natural killer T cells (iNKT cells) can be activated through binding antigenic lipid/CD1d complexes to their TCR. Antigenic lipids are processed, loaded, and displayed in complex with CD1d by lipid antigen presenting cells (LAPCs). The mechanism of lipid antigen presentation via CD1d is highly conserved with recent work showing adipocytes are LAPCs that, besides having a role in lipid storage, can activate iNKT cells and play an important role in systemic metabolic disease. Recent studies shed light on parameters potentially dictating cytokine output and how obesity-associated metabolic disease may affect such parameters. By following a lipid antigen's journey, we identify five key areas which may dictate cytokine skew: co-stimulation, structural properties of the lipid antigen, stability of lipid antigen/CD1d complexes, intracellular and extracellular pH, and intracellular and extracellular lipid environment. Recent publications indicate that the combination of advanced omics-type approaches and machine learning may be a fruitful way to interconnect these 5 areas, with the ultimate goal to provide new insights for therapeutic exploration.

Augmenting Granzyme B-Expressing NK Cells by Invariant NKT Ligand-Loaded APCs in Patients with Postoperative Early Stage Non-Small Cell Lung Cancer: Results of a Randomized Phase II Study

NK cells are major effector cells involved in the elimination of early tumors and prevent metastasis. They often have an impaired function in patients with cancer. Preclinical studies have demonstrated NK cell activation as the adjunctive effect of invariant NKT (iNKT) cells. Activation of iNKT cells after administration of the glycolipid ligand α-galactosylceramide, loaded with CD1d-expressing human PBMC-derived APCs (APC/Gal), is an attractive cancer therapy to optimize the use of NK cells. However, the subsets of NK cells that are activated following iNKT cell activation as well as the period of NK cell activation remain unclear. In this study, we report that the granzyme B-expressing NK cell response in postoperative lung cancer patients was enhanced 49 d after administration of APC/Gal in a phase II study. We found maximum IFN-γ production on day 49 in 13 out of 27 APC/Gal-treated patients. On day 49, 14 out of 27 patients (51.9%) had higher IFN-γ production by iNKT cells (>6-fold higher than the baseline level). This increment significantly correlated with granzyme B-expressing NK cells. Although IFN-γ production was lower in patients in the nontreated group, we detected maximum IFN-γ production 12 mo after the resection of lung cancer (9 out of 29 patients [31%]). These findings suggest that elimination of cancer cells leads to increased NK cell function, which can be further enhanced by APC/Gal therapy.

NKT Cells Contribute to the Control of Microbial Infections

Innate (-like) T lymphocytes such as natural killer T (NKT) cells play a pivotal role in the recognition of microbial infections and their subsequent elimination. They frequently localize to potential sites of pathogen entry at which they survey extracellular and intracellular tissue spaces for microbial antigens. Engagement of their T cell receptors (TCRs) induces an explosive release of different cytokines and chemokines, which often pre-exist as constitutively expressed gene transcripts in NKT cells and underlie their poised effector state. Thus, NKT cells regulate immune cell migration and activation and subsequently, bridge innate and adaptive immune responses. In contrast to conventional T cells, which react to peptide antigens, NKT cells recognize lipids presented by the MHC class I like CD1d molecule on antigen presenting cells (APCs). Furthermore, each NKT cell TCR can recognize various antigen specificities, whereas a conventional T lymphocyte TCR reacts mostly only to one single antigen. These lipid antigens are either intermediates of the intracellular APC`s-own metabolism or originate from the cell wall of different bacteria, fungi or protozoan parasites. The best-characterized subset, the type 1 NKT cell subset expresses a semi-invariant TCR. In contrast, the TCR repertoire of type 2 NKT cells is diverse. Furthermore, NKT cells express a panoply of inhibitory and activating NK cell receptors (NKRs) that contribute to their primarily TCR-mediated rapid, innate like immune activation and even allow an adaption of their immune response in an adoptive like manner. Dueto their primary localization at host-environment interfaces, NKT cells are one of the first immune cells that interact with signals from different microbial pathogens. Vice versa, the mutual exchange with local commensal microbiota shapes also the biology of NKT cells, predominantly in the gastrointestinal tract. Following infection, two main signals drive the activation of NKT cells: first, cognate activation upon TCR ligation by microbial or endogenous lipid antigens; and second, bystander activation due to cytokines. Here we will discuss the role of NKT cells in the control of different microbial infections comparing pathogens expressing lipid ligands in their cell walls to infectious agents inducing endogenous lipid antigen presentation by APCs.

Human invariant natural killer T cells promote tolerance by preferential apoptosis induction of conventional dendritic cells

Graft-versus-host disease (GvHD) is a major cause of morbidity and mortality after allogeneic hematopoietic cell transplantation. We recently showed in murine studies and in vitro human models that adoptively transferred invariant natural killer T (iNKT) cells protect from GvHD and promote graft-versus-leukemia effects. The cellular mechanisms underlying GvHD prevention by iNKT cells in humans, however, remain unknown. In order to study relevant cellular interactions, dendritic cells (DC) were either generated from monocytes or isolated directly from blood of healthy donors or GvHD patients and co-cultured in a mixed lymphocyte reaction (MLR) with T cells obtained from healthy donors or transplantation bags. Addition of culture-expanded iNKT cells to the MLR-induced DC apoptosis in a cell contact-dependent manner, thereby preventing T-cell activation and proliferation. Annexin V/propidium iodide staining and image stream assays showed that CD4⁺CD8^–, CD4^–CD8⁺ and double negative iNKT cells are similarly able to induce DC apoptosis. Further MLR assays revealed that conventional DC (cDC) but not plasmacytoid DC (pDC) could induce alloreactive T-cell activation and proliferation. Interestingly, cDC were also more susceptible to apoptosis induced by iNKT cells, which correlates with their higher CD1d expression, leading to a bias in favor of pDC. Remarkably, these results could also be observed in GvHD patients. We propose a new mechanism how ex vivo expanded human iNKT cells prevent alloreactivity of T cells. iNKT cells modulate T-cell responses by selective apoptosis of DC subsets, resulting in suppression of T-cell activation and proliferation while enabling beneficial immune responses through pDC.

Human CD4- invariant NKT lymphocytes regulate graft versus host disease

Despite increasing evidence for a protective role of invariant (i) NKT cells in the control of graft-versus-host disease (GVHD), the mechanisms underpinning regulation of the allogeneic immune response in humans are not known. In this study, we evaluated the distinct effects of human in vitro expanded and flow-sorted human CD4⁺ and CD4^- iNKT subsets on human T cell activation in a pre-clinical humanized NSG mouse model of xenogeneic GVHD. We demonstrate that human CD4^- but not CD4⁺ iNKT cells could control xenogeneic GVHD, allowing significantly prolonged overall survival and reduced pathological GVHD scores without impairing human T cell engraftment. Human CD4^- iNKT cells reduced the activation of human T cells and their Th1 and Th17 differentiation in vivo. CD4^- and CD4⁺ iNKT cells had distinct effects upon DC maturation and survival. Compared to their CD4⁺ counterparts, in co-culture experiments in vitro, human CD4^- iNKT cells had a higher ability to make contacts and degranulate in the presence of mouse bone marrow-derived DCs, inducing their apoptosis. In vivo we observed that infusion of PBMC and CD4^- iNKT cells was associated with decreased numbers of splenic mouse CD11c⁺ DCs. Similar differential effects of the iNKT cell subsets were observed on the maturation and in the induction of apoptosis of human monocyte-derived dendritic cells in vitro. These results highlight the increased immunosuppressive functions of CD4^- versus CD4⁺ human iNKT cells in the context of alloreactivity, and provide a rationale for CD4^- iNKT selective expansion or transfer to prevent GVHD in clinical trials.

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.

Histone deacetylase inhibitors enhance CD1d-dependent NKT cell responses to lymphoma

Histone deacetylases (HDACs) are a family of enzymes that influence expression of genes implicated in tumor initiation, progression, and anti-tumor responses. In addition to their canonical role in deacetylation of histones, HDACs regulate many non-canonical targets, such as Signal Transducer and Activator of Transcription 3 (STAT3). We hypothesize that tumors use epigenetic mechanisms to dysregulate CD1d-mediated antigen presentation, thereby impairing the ability of natural killer T (NKT) cells to recognize and destroy malignant cells. In this study, we pre-treated CD1d-expressing tumor cells with HDAC inhibitors (HDACi) and assessed CD1d-dependent NKT cell responses to mantle cell lymphoma (MCL). Pre-treatment with Trichostatin-A, a pan-HDACi, rapidly enhanced both CD1d- and MHC class II-mediated antigen presentation. Similarly, treatment of MCL cells with other HDACi resulted in enhanced CD1d-dependent NKT cell responses. The observed changes are due, at least in part, to an increase in both CD1D mRNA and CD1d cell surface expression. Mechanistically, we found that HDAC2 binds to the CD1D promoter. Knockdown of HDAC2 in tumor cells resulted in a significant increase in CD1d-mediated antigen presentation. In addition, treatment with HDACi inhibited STAT3 and STAT3-regulated inflammatory cytokine secretion by MCL cells. We demonstrated that MCL-secreted IL-10 inhibits CD1d-mediated antigen presentation and pre-treatment with TSA abrogates secretion of IL-10 by MCL. Taken together, our studies demonstrate the efficacy of HDACi in restoring anti-tumor responses to MCL through both cell-intrinsic and cell-extrinsic mechanisms and strongly implicate a role for HDACi in enhancing immune responses to cancer.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

From the Deep Sea to Everywhere: Environmental Antigens for iNKT Cells

Invariant natural killer T (iNKT) cells are a unique subset of innate T cells that share features with innate NK cells and adaptive memory T cells. The first iNKT cell antigen described was found 1993 in a marine sponge and it took over 10 years for other, bacterial antigens to be described. Given the paucity of known bacterial iNKT cell antigens, it appeared as if iNKT cells play a very specialist role in the protection against few, rare and unusual pathogenic bacteria. However, in the last few years several publications painted a very different picture, suggesting that antigens for iNKT cells are found almost ubiquitous in the environment. These environmental iNKT cell antigens can shape the distribution, phenotype and function of iNKT cells. Here, these recent findings will be reviewed and their implications for the field will be outlined.

Prevention of NKT cell activation accelerates cutaneous wound closure and alters local inflammatory signals

We previously reported that in the absence of NKT cells, wound closure was accelerated in a murine excisional punch wound model. Here, we explored whether purposefully inhibiting NKT cell activation had similar effects on wound closure and the dermal inflammatory response to injury. We found that prevention of NKT cell activation accelerated wound closure in a dose-responsive manner. If anti-CD1d was administered before wounding, NKT cell infiltration into cutaneous wounds was diminished without quantitative changes in cellular infiltrates. Furthermore, prevention of NKT cell activation transiently enhanced the local production of a subset of chemokines, including MIP-2, MCP-1, MIP-1α, and MIP-1β, and altered the relative expression of CD69 and CXCR2 on the surface of both circulating and wound NKT cells. Taken together, these findings suggest that wounding activates NKT cells via CD1d presentation of glycolipid antigen and help further define a role for NKT cells in the regulation of wound inflammation and closure. Many soluble factors have been targeted as potential wound healing therapies, but their clinical success has been limited. Given our findings, the NKT cell may be an attractive target for wound healing therapies.

CD1a and CD1d genes polymorphisms in breast, colorectal and lung cancers

CD1 molecules might contribute to anti-tumor immune response by presentation of tumor-derived lipid and glycolipid antigens to T cells and NKT cells. Polymorphisms in CD1 genes have been suggested to modify ligand binding of CD1 molecules and thereby change the antigen presenting ability of these molecules. The aim of this study was to investigate the exon 2 polymorphisms of CD1a and CD1d in several high incident cancers in Iran. For this purpose, 201 female breast cancer patients and 207 healthy women, 64 lung cancer patients and 95 healthy individuals and 109 patients with colorectal cancer and 109 healthy controls were recruited to this study. Using PCR-SSP method, no significant correlation was found in genotype and allele frequencies of CD1a between all three studied groups and their control counterparts. Moreover, a dominant frequency of CD1d 01 (A) allele was observed in the majority of studied individuals. No significant association between the CD1 polymorphisms and prognostic factors in breast, lung and colorectal cancers was detected. Our results highlight the conserved nature of CD1 genes and may point to the immuoregulatory functions of CD1 molecules in cancer that can be exerted through fine tuning of NK, T and NKT cells.

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

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

Human leukemic cells loaded with alpha-galactosylceramide (alpha-GalCer) activate murine NKT cells in situ

Invariant NKT cells (NKT) cells become activated after stimulation with antigen-presenting cells (APCs) loaded with the NKT cell ligand, alpha-galactosylceramide (alpha-GalCer). In this study, we investigated whether human APCs loaded with alpha-GalCer have the ability to activate NKT cells in mice. We found that human dendritic cells (DCs) loaded with alpha-GalCer (hDC/Gal) and injected into C57BL/6 mice stimulated the secretion of IFN-gamma by activated murine NKT cells. Furthermore, the number of transferred hDC/Gal correlated with the number of recovered IFN-gamma-producing spleen cells, indicating that the capacity of APCs to load alpha-GalCer can be measured by IFN-gamma release in an ELISPOT assay. Finally, alpha-GalCer-loaded human leukemic cell lines and primary leukemic cells injected into C57BL/6 mice also had the capacity to stimulate murine NKT cells in vivo. These results indicate that in vivo murine NKT cell responses can be used to quantitate the alpha-GalCer-loading capacity of human APCs. This method could be utilized to develop future immunotherapies in which NKT cells are targeted for activation.

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.

The crystal structure of avian CD1 reveals a smaller, more primordial antigen-binding pocket compared to mammalian CD1

The molecular details of glycolipid presentation by CD1 antigen-presenting molecules are well studied in mammalian systems. However, little is known about how these non-classical MHC class I (MHCI) molecules diverged from the MHC locus to create a more complex, hydrophobic binding groove that binds lipids rather than peptides. To address this fundamental question, we have determined the crystal structure of an avian CD1 (chCD1-2) that shares common ancestry with mammalian CD1 from approximately 310 million years ago. The chCD1-2 antigen-binding site consists of a compact, narrow, central hydrophobic groove or pore rather than the more open, 2-pocket architecture observed in mammalian CD1s. Potential antigens then would be restricted in size to single-chain lipids or glycolipids. An endogenous ligand, possibly palmitic acid, serves to illuminate the mode and mechanism of ligand interaction with chCD1-2. The palmitate alkyl chain is inserted into the relatively shallow hydrophobic pore; its carboxyl group emerges at the receptor surface and is stabilized by electrostatic and hydrogen bond interactions with an arginine residue that is conserved in all known CD1 proteins. In addition, other novel features, such as an A' loop that interrupts and segments the normally long, continuous alpha1 helix, are unique to chCD1-2 and contribute to the unusually narrow binding groove, thereby limiting its size. Because birds and mammals share a common ancestor, but the rate of evolution is slower in birds than in mammals, the chCD1-2-binding groove probably represents a more primordial CD1-binding groove.

Influence of a non-NK complex region of chromosome 6 on CD4+ invariant NK T cell homeostasis

The number and function of immunoregulatory invariant NKT (iNKT) cells are genetically controlled. A defect of iNKT cell ontogeny and function has been implicated as one causal factor of NOD mouse susceptibility to type 1 diabetes. Other factors of diabetes susceptibility, such as a decrease of regulatory T cell function or an increase in TLR1 expression, are corrected in diabetes-resistant Idd6 NOD.C3H 6.VIII congenic mice. Thus, we surmised that the iNKT cell defects found in NOD mice may also be rescued in congenic mice. Unexpectedly, we found, in both the thymus and the periphery, a 50% reduction in iNKT cell number in NOD.C3H 6.VIII mice as compared with NOD mice. This reduction only affected CD4(+) iNKT cells, and left the double negative iNKT cells unchanged. In parallel, the production of IL-4 and IFN-gamma following alpha-GalCer stimulation was proportionally reduced. Using three subcongenic strains, we have narrowed down the region controlling iNKT development within Idd6 (5.8 Mb) to Idd6.2 region (2.5 Mb). Idd6 region had no effect on NK cell number and in vivo cytotoxic activity. These results indicate that the role of iNKT cells in diabetes development is equivocal and more complex than initially considered. In addition, they bring strong evidence that the regulation of CD4(+) iNKT cell production is independent from that of DN iNKT cells, and involves genes of the Idd6 locus.

CD1a and CD1e allele frequencies in an Italian population from the Abruzzo region

CD1 is a small family comprising 5 MHC-like genes located on chromosome 1 and encoding glycoproteins termed CD1a, CD1b, CD1c, CD1d and CD1e. They are expressed mainly on the surface of dendritic cells, monocytes and some thymocytes and are specialized in presenting lipid antigens to T lymphocytes. The structure is similar to that of MHC class I molecules with 3 globular domains and the Beta2-microglobulin. It has been shown that all five human CD1 genes exhibit a limited number of polymorphisms in the alpha1 domain whose effects are still unknown. CD1e results to be the most polymorphic isoform with six CD1e alleles (01, 02 in exon 2 and 03, 04, 05, 06 in ex3) described to date. At this moment, few investigations on the allele frequencies of the CD1 genes have been reported and all additional information improves our knowledge on this new class of antigen-presenting molecules. In order to study possible allelic variations of exon 2 of human CD1a and CD1e genes, we analyzed, by a sensitive technique, the sequence-based typing (SBT), 114 DNA samples from unrelated healthy Italian individuals from the Abruzzo region. Our experimental findings indicate that the allele frequency distribution of both CD1a and CD1e genes is in accordance with that observed in other geographic areas and did not identify any new allele, thus confirming a very low polymorphism.

TCR-mediated recognition of glycolipid CD1 complexes

Populations of unconventional T lymphocytes that express alpha beta T cell antigen receptors (TCRs) have been characterized, including T cells reactive to glycolipids presented by CD1 molecules. The CD1 molecules have a structure broadly similar to major histocompatibility complex (MHC) class I and class II proteins, but because the antigens CD 1 presents are so different from peptides, it is possible that glycolipid reactive TCRs have properties that distinguish them from TCRs expressed by conventional T cells. Consistent with this possibility, CD1-reactive T cells have an unrestrained pattern of co-receptor expression, as they include CD4+, CD8+, and double-negative cells. Furthermore, unlike peptide-reactive T cells, there are populations of glycolipid-reactive T cells with invariant alpha chain TCRs that are conserved across species. There are also glycolipid reactive populations with more variable TCRs, however, suggesting that it may be difficult to make categorical generalizations about glycolipid reactive TCRs. Among the glycolipid reactive TCRs, the invariant TCR expressed by CD1d reactive NKT cells has been by far the most thoroughly studied, and in this article we emphasize the unique features of this antigen recognition system, including repertoire formation, fine specificity, TCR affinity, and TCR structure.

Tumor Cells Loaded with α-Galactosylceramide Induce Innate NKT and NK Cell-Dependent Resistance to Tumor Implantation in Mice

Dendritic cells (DCs) loaded with alpha-galactosylceramide (alpha-GalCer) are known to be active APCs for the stimulation of innate NKT and NK cell responses in vivo. In this study, we evaluated the capacity of non-DCs to present alpha-GalCer in vitro and in vivo, particularly tumor cells loaded with alpha-GalCer (tumor/Gal). Even though the tumor cells lacked expression of CD40, CD80, and CD86 costimulatory molecules, the i.v. injection of tumor/Gal resulted in IFN-gamma secretion by NKT and NK cells. These innate responses to tumor/Gal, including the induction of IL-12p70, were comparable to or better than alpha-GalCer-loaded DCs. B16 melanoma cells that were stably transduced to express higher levels of CD1d showed an increased capacity relative to wild-type B16 cells to present alpha-GalCer in vivo. Three different tumor cell lines, when loaded with alpha-GalCer, failed to establish tumors upon i.v. injection, and the mice survived for at least 6 mo. The resistance against tumor cells was independent of CD4 and CD8 T cells but dependent upon NKT and NK cells. Mice were protected from the development of metastases if the administration of live B16 tumor cells was followed 3 h or 3 days later by the injection of CD1d(high)-alpha-GalCer-loaded B16 tumor cells with or without irradiation. Taken together, these results indicate that tumor/Gal are effective APCs for innate NKT and NK cell responses, and that these innate immune responses are able to resist the establishment of metastases in vivo.

MTP regulated by an alternate promoter is essential for NKT cell development

Microsomal triglyceride transfer protein (MTP), an endoplasmic reticulum lipid transfer protein critical for apolipoprotein B (apoB) secretion, regulates CD1d antigen presentation. We identified MTP variant 1 (MTPv1), a novel splice variant of mouse MTP, by polymerase chain reaction and Northern analysis in non-apoB-secreting tissues, including thymocytes and antigen-presenting cells (APCs). Edman degradation of MTPv1 isolated from transfected cells revealed three unique residues; however, recombinant MTP and MTPv1 had an equivalent protein disulfide isomerase association, subcellular localization, triglyceride transfer, phospholipid transfer, response to inhibitors, and ability to support apoB secretion. MTP and MTPv1 efficiently transferred phosphatidylethanolamine to CD1d in vitro. NKT cells fail to develop in fetal thymic organ culture (FTOC) treated with MTP antagonists. MTP-inhibited FTOCs produced negligible numbers of CD1d tetramer-positive cells and exhibited marked defects in IL-4 production upon stimulation with anti-CD3 or alpha-galactosylceramide-pulsed APCs. CD1d expression on CD4(+)CD8(+) FTOC cells was unaffected by MTP inhibition. Thus, our results demonstrate that MTPv1 in thymocytes is critical to NKT cell development. We hypothesize that, when MTP is inactive, CD1d traffics to the cell surface and presents no lipid or a lipid that is incapable of mediating NKT cell selection and/or is refractory to lysosomal editing.

Improved Outcomes in NOD Mice Treated with a Novel Th2 Cytokine-Biasing NKT Cell Activator

Activation of CD1d-restricted invariant NKT (iNKT) cells by alpha-galactosylceramide (alphaGalCer) significantly suppresses development of diabetes in NOD mice. The mechanisms of this protective effect are complex, involving both Th1 and Th2 cytokines and a network of regulatory cells including tolerogenic dendritic cells. In the current study, we evaluated a newly described synthetic alphaGalCer analog (C20:2) that elicits a Th2-biased cytokine response for its impact on disease progression and immunopathology in NOD mice. Treatment of NOD mice with alphaGalCer C20:2 significantly delayed and reduced the incidence of diabetes. This was associated with significant suppression of the late progression of insulitis, reduced infiltration of islets by autoreactive CD8(+) T cells, and prevention of progressive disease-related changes in relative proportions of different subsets of dendritic cells in the draining pancreatic lymph nodes. Multiple favorable effects observed with alphaGalCer C20:2 were significantly more pronounced than those seen in direct comparisons with a closely related analog of alphaGalCer that stimulated a more mixed pattern of Th1 and Th2 cytokine secretion. Unlike a previously reported Th2-skewing murine iNKT cell agonist, the alphaGalCer C20:2 analog was strongly stimulatory for human iNKT cells and thus warrants further examination as a potential immunomodulatory agent for human disease.

Invariant natural killer T cells in the response to bacteria: the advent of specific antigens

Invariant natural killer T (iNKT) cells are a unique subset of T lymphocytes that have been implicated in diverse immune reactions, ranging from self-tolerance and development of autoimmunity to responses to pathogens and tumors. Although some degree of autoreactivity of iNKT cells has been shown, it remained controversial whether the T-cell antigen receptor expressed by these cells could recognize microbial antigens, hampering the investigation of their physiological role during tolerance and immunity. Several recent publications have now defined natural antigens for the majority of iNKT cells in some Proteobacteria and in Borrelia burgdorferi, demonstrating specificity of these cells for microbes in addition to self-reactivity. The characterization of natural antigens from bacteria, and the iNKT cell response to bacteria containing them, are decisive steps toward the clarification of the natural role of iNKT cells in host defense against pathogens, and will likely spur numerous findings in the near future.

CD1d and CD1d-restricted iNKT-cells play a pivotal role in contact hypersensitivity

CD1d-restricted T-cells are activated by glycolipids presented by the major histocompatibility complex class-Ib molecule CD1d, found on the surface of antigen-presenting cells (APC). This interaction between APC, most notably dendritic cells (DC), and CD1d-restricted T-cells is an important regulatory step in the initiation of adaptive immune responses. It is well known that DC play a crucial role in the induction of contact hypersensitivity (CHS), a frequently studied form of in vivo T-cell-mediated immunity. In this study, we show that CD1d-restricted T-cells are also necessary for CHS, because both wild-type mice treated systemically or topically with CD1d glycolipid antagonists and CD1d-restricted T-cell-null mice have markedly diminished CHS responses. Thus, pharmacologic antagonists of CD1d can be used as effective inhibitors of CHS, a prototype for a variety of delayed-type tissue hypersensitivity responses.

Modulation of CD1d-restricted NKT cell responses by using N-acyl variants of alpha-galactosylceramides

A form of alpha-galactosylceramide, KRN7000, activates CD1d-restricted Valpha14-invariant (Valpha14i) natural killer (NK) T cells and initiates multiple downstream immune reactions. We report that substituting the C26:0 N-acyl chain of KRN7000 with shorter, unsaturated fatty acids modifies the outcome of Valpha14i NKT cell activation. One analogue containing a diunsaturated C20 fatty acid (C20:2) potently induced a T helper type 2-biased cytokine response, with diminished IFN-gamma production and reduced Valpha14i NKT cell expansion. C20:2 also exhibited less stringent requirements for loading onto CD1d than KRN7000, suggesting a mechanism for the immunomodulatory properties of this lipid. The differential cellular response elicited by this class of Valpha14i NKT cell agonists may prove to be useful in immunotherapeutic applications.

Human CD1D gene has TATA boxless dual promoters: an SP1-binding element determines the function of the proximal promoter

CD1d presents lipid Ags to a specific population of NK T cells, which are involved in the host immune defense, suppression of autoimmunity, and the rejection of tumor cells. The transcriptional control mechanism that determines the regulation and the tissue distribution of CD1d remains largely unknown. After investigating 3.7 kb 5' upstream of the coding region, we found that human gene encoding CD1d molecule (CD1D) has TATA boxless dual promoters with multiple transcription initiation sites. The proximal promoter is located within the region of -106 to +24, and the distal promoter is located within the region of -665 to -202 with the A of the translational start codon defined as +1. The longest 5'-untranslated region derived from 5'-RACE and apparently generated by the distal promoter has 272 bp in length covering the genomic sequence of the proximal promoter. The region covering the proximal promoter gave a much higher luciferase activity in Jurkat cells than in K562 cells, whereas it was in reverse for the region covering the distal promoter, indicating a cell type sp. act. of the two promoters. Transcription factor SP1 plays a crucial role in the function of the proximal promoter. The analysis of the CD1D promoter region indicates that IFN-gamma, NF-IL-6, and T cell factor 1/lymphoid enhancer-binding factor 1 are most likely involved in the regulation of CD1d expression. The illustration of the dual CD1D gene promoters will help to reveal the regulatory factors that control CD1d expression and its tissue distribution for a better understanding of the cross-regulation between CD1d and NK T cells.

CD1d1 Displayed on Cell Size Beads Identifies and Enriches an NK Cell Population Negatively Regulated by CD1d1

NK cells destroy microbe-infected cells while sparing healthy cells, and are controlled, in part, by inhibitory receptors specific for class I Ag-presenting molecules. CD1d1, a beta(2)-microglobulin-associated class I-like molecule, binds glycolipids and stimulates NKT cells. We previously demonstrated that target cell lysis by IL-2-activated mouse NK cells is inhibited by target cell expression of CD1d1, suggesting that IL-2-activated NK cells may express a CD1d1-specific inhibitory receptor. We now report that a significant subset of mouse IL-2-activated NK cells specifically binds cell size beads displaying either naturally expressed or recombinant CD1d1. In contrast, although tetramers of soluble recombinant CD1d1 loaded with alpha-galactosylceramide identify NKT cells, binding of this reagent to resting or IL-2-activated NK cells was undetectable, even with activated NK cells sorted with CD1d1 beads. Cytotoxicity by the CD1d1 bead-separated NK subset was strongly inhibited by CD1d1, compared with the NK cell subset not bound to CD1d1 beads. An Ab that blocks NKT cell recognition of CD1d1 also reverses CD1d1 inhibition of NK lysis, suggesting that TCRs of NKT cells and NK inhibitory receptor(s) may interact with a similar site on CD1d1. These results provide direct evidence for a physical interaction of NK cells with CD1d1, mediated by a functional, CD1d1-specific low-affinity inhibitory NK receptor. Display of ligands on cell size beads to maximize multivalent interaction may offer an alternative approach to examine NK cell receptor-ligand interactions, particularly those of lower expression and/or lower affinity/avidity that may go undetected using tetrameric reagents.

Janus-like role of regulatory iNKT cells in autoimmune disease and tumour immunity

Invariant CD1D-restricted natural killer T (iNKT) cells function during innate and adaptive immunity and regulate numerous immune responses, such as autoimmune disease, tumour surveillance, infectious disease and abortions. However, the molecular basis of their functions and the nature of disease-associated defects of iNKT cells are unclear and have been the subject of recent controversy. Here, we review recent findings that underscore the potential importance of interactions between iNKT cells and dendritic cells (DCs) that indicate that iNKT cells regulate DC activity to shape both pro-inflammatory and tolerogenic immune responses. The ability to modulate iNKT-cell activity in vivo using the ligand alpha-galactosylceramide and to treat patients with autoimmune disease or cancer is evaluated also.

The CD1d natural killer T-cell antigen presentation pathway is highly conserved between humans and rhesus macaques

Natural killer T (NKT) cells play an important role in controlling cancers, infectious diseases and autoimmune diseases. Although the rhesus macaque is a useful primate model for many human diseases such as infectious and autoimmune diseases, little is known about their NKT cells. We analyzed V alpha 24TCR+ T cells from rhesus macaque peripheral blood mononuclear cells stimulated with alpha-galactosylceramide (alpha-GalCer) and interleukin-2. We found that rhesus macaques possess V alpha 24TCR+ T cells, suggesting that recognition of alpha-GalCer is highly conserved between rhesus macaques and humans. The amino acid sequences of the V-J junction for the V alpha 24TCR of rhesus macaque and human NKT cells are highly conserved (93% similarity), and the CD1d alpha1-alpha2 domains of both species are highly homologous (95.6%). These findings indicate that the rhesus macaque is a useful primate model for understanding the contribution of NKT cells to the control of human diseases.

Optimal T cell responses to Cryptococcus neoformans mannoprotein are dependent on recognition of conjugated carbohydrates by mannose receptors

Cryptococcosis is a leading cause of death among individuals with compromised T cell function. Soluble Cryptococcus neoformans mannoproteins (MP) have emerged as promising vaccine candidates due to their capacity to elicit delayed-type hypersensitivity and Th type 1-like cytokines, both critical to the clearance of this pathogenic yeast. In this study, the mechanisms responsible for the potent immunostimulatory properties of MP were explored. Using Chinese hamster ovary cells expressing human macrophage mannose receptor (MMR), we determined that MP is a MMR ligand. Functionally, competitive blockade of multilectin mannose receptors (MR) on APCs diminished MP-dependent stimulation of primary T cells from immunized mice and the MP-reactive CD4(+) T cell hybridoma, P1D6, by 72 and 99%, respectively. Removal of O-linked saccharides from MP by beta-elimination inhibited MP-dependent stimulation of P1D6 and primary T cells by 89 and 90%, respectively. In addition, MP-dependent stimulation of P1D6 was abrogated after digestion with proteinase K, suggesting the protein core of MP contributed the antigenic moiety presented by APC. Stimulation of P1D6 by MP also was abolished using APC obtained from invariant chain-deficient mice, demonstrating Ag presentation was MHC class II restricted. Our data suggest that MP is a ligand for the MMR and that T cell stimulation is functionally inhibited either by competitive blockade of MR or by removal of carbohydrate residues critical for recognition. The demonstration that efficient T cell responses to MP require recognition of terminal mannose groups by MMR provides both a molecular basis for the immunogenicity of cryptococcal MP and support for vaccination strategies that target MR.

CD1--the pathology perspective

CD1 molecules are a family of cell surface-associated glycoproteins now recognized as having a role in antigen presentation. These glycoproteins are distinct from yet have some similarities to classical major histocompatibility complex class I and class II molecules. The role of these molecules has been studied in detail over recent years, with an explosion of interest following the demonstration that they can present nonprotein antigens to certain subpopulations of T cells. The purpose of this paper is to provide an overview of current knowledge of the function of the CD1 family with specific emphasis on the potential role in the pathogenesis of certain diseases. Although much of the current research in this field has inevitably concentrated on mice and humans, this work also has potential significance for veterinary species.

Gene expression in NKT cells: defining a functionally distinct CD1d-restricted T cell subset

Since their discovery as cells bearing both TCRs and NK cell receptors, NKT cells have been intensively studied as a possible bridge between innate and adaptive immunity. Although their involvement in a wide variety of immune responses and in disease states have been well documented, molecular details of this functionality have been lacking. Recently, transcriptional profiling using microarrays has been applied to these cells, pinpointing gene-expression differences between this regulatory T cell subset and conventional T cells, and providing a framework for subset-specific therapeutic intervention in clinical settings.

Elevated proportion of natural killer T cells in periodontitis lesions: a common feature of chronic inflammatory diseases

Although periodontitis is a chronic inflammatory disease caused by a group of so-called periodontopathic bacteria, autoimmune mechanisms have also been implicated in the disease process. Recently, a unique subset of lymphocytes designated natural killer (NK) T cells expressing the Valpha24JalphaQ invariant T cell receptor (TCR) has been reported to have a regulatory role in certain autoimmune diseases. Therefore, we investigated the proportion of the invariant Valpha24JalphaQ TCR within the Valpha24 T cell population in periodontitis lesions and gingivitis lesions using single-strand conformation polymorphism methodology. NK T cells were identified with a specific JalphaQ probe whereas the total Valpha24 TCR was identified using an internal Calpha probe. NK T cells were a significant proportion of the total Valpha24 population both in periodontitis lesions and to a lesser extent in gingivitis lesions but not in the peripheral blood of either periodontitis patients or nondiseased controls. Using immunohistochemistry, some of Valpha24(+) cells in the periodontitis lesions seemed to associate with CD1d(+) cells, which are specific antigen-presenting cells for NK T cells. Although the mechanism underlying the elevation of NK T cells in periodontitis and in gingivitis lesions remains unclear, it can be postulated that NK T cells are recruited to a play regulatory role in the immune response to bacterial infection.

Presentation of self and microbial lipids by CD1 molecules

CD1 molecules present both self lipids and microbial lipids. Recent studies have elucidated novel antigenic structures that can be presented by CD1 for T cell stimulation, as well as new pathways for lipid-antigen presentation. Additionally, the development of lipid-CD1 tetramers now permits the tracking of CD1-reactive T cells during immune responses. Despite this, the roles of CD1-reactive T cells in both host defense and immune regulation remain to be unequivocally defined.

The molecular biology of CD1

CD1 were the first human differentiation antigens to be identified by monoclonal antibodies. In this review, we summarize some key results from the molecular study of CD1, with particular reference to their relationship to MHC antigens, and to the existence of two distinct groups of CD1 molecules.

Differential expression of ovine CD1

was examined using a combination of reverse transcription-polymerase chain reaction (RT-PCR) with sequence-specific primers and Northern and in situ hybridization techniques. The aim of the study was to establish the patterns of CD1 expression at the molecular level and address questions posed by previous studies in other species regarding expression patterns of CD1. A 'pan-CD1' probe based on the exon 4 (alpha 3) region was used in addition to isotype-specific probes for SCD1B (the exon 3 region of clone SCD1B42) and SCD1D (the exon 3 region of clone SCD1D). Widespread expression of CD1 (including thymus, peripheral blood lymphocytes, lung and intestine) was identified using both the exon 4 and SCD1D probe. SCD1B expression was more restricted, being identified in equivalent levels only in the thymus and in scattered populations of dendritic cells. These results highlight the difference in expression patterns between group 1 and group 2 CD1 family members and establish SCD1D as the CD1 family member with the widest pattern of expression, consistent with a differential role for the different CD1 family members.