DprE2 is a molecular target of the anti-tubercular nitroimidazole compounds pretomanid and delamanid

Mycobacterium tuberculosis is one of the global leading causes of death due to a single infectious agent. Pretomanid and delamanid are new antitubercular agents that have progressed through the drug discovery pipeline. These compounds are bicyclic nitroimidazoles that act as pro-drugs, requiring activation by a mycobacterial enzyme; however, the precise mechanisms of action of the active metabolite(s) are unclear. Here, we identify a molecular target of activated pretomanid and delamanid: the DprE2 subunit of decaprenylphosphoribose-2'-epimerase, an enzyme required for the synthesis of cell wall arabinogalactan. We also provide evidence for an NAD-adduct as the active metabolite of pretomanid. Our results highlight DprE2 as a potential antimycobacterial target and provide a foundation for future exploration into the active metabolites and clinical development of pretomanid and delamanid.

Structure of the priming arabinosyltransferase AftA required for AG biosynthesis of Mycobacterium tuberculosis

Arabinogalactan (AG) is an essential cell wall component in mycobacterial species, including the deadly human pathogen Mycobacterium tuberculosis. It plays a pivotal role in forming the rigid mycolyl-AG-peptidoglycan core for in vitro growth. AftA is a membrane-bound arabinosyltransferase and a key enzyme involved in AG biosynthesis which bridges the assembly of the arabinan chain to the galactan chain. It is known that AftA catalyzes the transfer of the first arabinofuranosyl residue from the donor decaprenyl-monophosphoryl-arabinose to the mature galactan chain (i.e., priming); however, the priming mechanism remains elusive. Herein, we report the cryo-EM structure of Mtb AftA. The detergent-embedded AftA assembles as a dimer with an interface maintained by both the transmembrane domain (TMD) and the soluble C-terminal domain (CTD) in the periplasm. The structure shows a conserved glycosyltransferase-C fold and two cavities converging at the active site. A metal ion participates in the interaction of TMD and CTD of each AftA molecule. Structural analyses combined with functional mutagenesis suggests a priming mechanism catalyzed by AftA in Mtb AG biosynthesis. Our data further provide a unique perspective into anti-TB drug discovery.

The proliferation of human mucosal-associated invariant T cells requires a MYC-SLC7A5-glycolysis metabolic axis

Mucosal-associated invariant T (MAIT) cells are an abundant population of innate T cells that recognize bacterial ligands and play a key role in host protection against bacterial and viral pathogens. Upon activation, MAIT cells undergo proliferative expansion and increase their production of effector molecules such as cytokines. In this study, we found that both mRNA and protein abundance of the key metabolism regulator and transcription factor MYC was increased in stimulated MAIT cells. Using quantitative mass spectrometry, we identified the activation of two MYC-controlled metabolic pathways, amino acid transport and glycolysis, both of which were necessary for MAIT cell proliferation. Last, we showed that MAIT cells isolated from people with obesity showed decreased MYC mRNA abundance upon activation, which was associated with defective MAIT cell proliferation and functional responses. Collectively, our data uncover the importance of MYC-regulated metabolism for MAIT cell proliferation and provide additional insight into the molecular basis for the functional defects of MAIT cells in obesity.

Assay development and inhibition of the Mt-DprE2 essential reductase from Mycobacterium tuberculosis

DprE2 is an essential enzyme in the synthesis of decaprenylphosphoryl-β-d-arabinofuranose (DPA) and subsequently arabinogalactan, and is a significant new drug target for M. tuberculosis. Two compounds from the GSK-177 box set, GSK301A and GSK032A, were identified through Mt-DprE2-target overexpression studies. The Mt-DprE1-DprE2 complex was co-purified and a new in vitro DprE2 assay developed, based on the oxidation of the reduced nicotinamide adenine dinucleotide cofactor of DprE2 (NADH/NADPH). The Mt-DprE1-DprE2 complex showed interesting kinetics in both the DprE1 resazurin-based assay, where Mt-DprE2 was found to enhance Mt-DprE1 activity and reduce substrate inhibition; and also in the DprE2 assay, which similarly exhibited substrate inhibition and a difference in kinetics of the two potential cofactors, NADH and NADPH. Although, no inhibition was observed in the DprE2 assay by the two GSK set compounds, spontaneous mutant generation indicated a possible explanation in the form of a pro-drug activation pathway, involving fgd1 and fbiC.

A humanized mouse model for in vivo evaluation of invariant Natural Killer T cell responses

Invariant natural killer T (iNKT) cells mediate immune responses when stimulated by glycolipid agonists presented by CD1d. In extensive studies of synthetic analogues of α-galactosyl ceramides, we identified numerous examples of significant differences in the recognition of specific glycolipids in wild type mice versus human iNKT cell clones or PBMC samples. To predict human iNKT cell responses more accurately in a mouse model, we derived a mouse line in which compound genetic modifications were used to express a human-like iNKT cell TCR along with human CD1d in place of the endogenous mouse proteins. Detailed transcriptional and phenotypic profiling demonstrated that these partially humanized mice developed an expanded population of T cells recognizing CD1d-presented glycolipid antigens, among which a subset characterized by expression of chemokine receptor CXCR6 had features characteristic of authentic iNKT cells. Responses to iNKT cell activating glycolipids in these mice generated cytokine production in vitro and in vivo that showed a pattern of fine specificity that closely resembled that of cultured human iNKT cell clones. Anti-tumor responses to variants of α-galactosyl ceramide in VαKI mice also correlated with their potency for stimulating human iNKT cells. This genetically modified mouse line provides a practical model for human presentation and recognition of iNKT cell activators in the context of a normally functioning immune system, and may furnish valuable opportunities for preclinical evaluation of iNKT cell-based therapies.

The P5-type ATPase ATP13A1 modulates major histocompatibility complex I-related protein 1 (MR1)-mediated antigen presentation

The monomorphic antigen-presenting molecule major histocompatibility complex-I-related protein 1 (MR1) presents small-molecule metabolites to mucosal-associated invariant T (MAIT) cells. The MR1-MAIT cell axis has been implicated in a variety of infectious and noncommunicable diseases, and recent studies have begun to develop an understanding of the molecular mechanisms underlying this specialized antigen presentation pathway. However, proteins regulating MR1 folding, loading, stability, and surface expression remain to be identified. Here, we performed a gene trap screen to discover novel modulators of MR1 surface expression through insertional mutagenesis of an MR1-overexpressing clone derived from the near-haploid human cell line HAP1 (HAP1.MR1). The most significant positive regulators identified included β2-microglobulin, a known regulator of MR1 surface expression, and ATP13A1, a P5-type ATPase in the endoplasmic reticulum (ER) not previously known to be associated with MR1-mediated antigen presentation. CRISPR/Cas9-mediated knockout of ATP13A1 in both HAP1.MR1 and THP-1 cell lines revealed a profound reduction in MR1 protein levels and a concomitant functional defect specific to MR1-mediated antigen presentation. Collectively, these data are consistent with the ER-resident ATP13A1 being a key posttranscriptional determinant of MR1 surface expression.

CD1a selectively captures endogenous cellular lipids that broadly block T cell response

We optimized lipidomics methods to broadly detect endogenous lipids bound to cellular CD1a proteins. Whereas membrane phospholipids dominate in cells, CD1a preferentially captured sphingolipids, especially a C42, doubly unsaturated sphingomyelin (42:2 SM). The natural 42:2 SM but not the more common 34:1 SM blocked CD1a tetramer binding to T cells in all human subjects tested. Thus, cellular CD1a selectively captures a particular endogenous lipid that broadly blocks its binding to TCRs. Crystal structures show that the short cellular SMs stabilized a triad of surface residues to remain flush with CD1a, but the longer lipids forced the phosphocholine group to ride above the display platform to hinder TCR approach. Whereas nearly all models emphasize antigen-mediated T cell activation, we propose that the CD1a system has intrinsic autoreactivity and is negatively regulated by natural endogenous inhibitors selectively bound in its cleft. Further, the detailed chemical structures of natural blockers could guide future design of therapeutic blockers of CD1a response.

Serial Stimulation of Invariant Natural Killer T Cells with Covalently Stabilized Bispecific T-cell Engagers Generates Antitumor Immunity While Avoiding Anergy

CD1d-restricted invariant natural killer T cells (iNKT cells) mediate strong antitumor immunity when stimulated by glycolipid agonists. However, attempts to develop effective iNKT cell agonists for clinical applications have been thwarted by potential problems with dose-limiting toxicity and by activation-induced iNKT cell anergy, which limits the efficacy of repeated administration. To overcome these issues, we developed a unique bispecific T-cell engager (BiTE) based on covalent conjugates of soluble CD1d with photoreactive analogues of the glycolipid α-galactosylceramide. Here we characterize the in vivo activities of iNKT cell-specific BiTEs and assess their efficacy for cancer immunotherapy in mouse models using transplantable colorectal cancer or melanoma tumor lines engineered to express human Her2 as a tumor-associated antigen. Systemic administration of conjugated BiTEs stimulated multiple iNKT cell effector functions including cytokine release, secondary activation of NK cells, and induction of dendritic cell maturation and also initiated epitope spreading for tumor-specific CD8⁺ cytolytic T-cell responses. The antitumor effects of iNKT-cell activation with conjugated BiTEs were further enhanced by simultaneous checkpoint blockade with antibodies to CTLA-4, providing a potential approach for combination immunotherapy. Multiple injections of covalently stabilized iNKT cell-specific BiTEs activated iNKT cells without causing iNKT cell anergy or exhaustion, thus enabling repeated administration for effective and nontoxic cancer immunotherapy regimens. SIGNIFICANCE: Covalently stabilized conjugates that engage the antigen receptors of iNKT cells and target a tumor antigen activate potent antitumor immunity without induction of anergy or depletion of the responding iNKT cells.

Chemical insights into the search for MAIT cells activators

Mucosal-associated invariant T cells (MAIT cells) represent a potential therapeutic target as they can tune or enhance immune responses. They recognise and become activated by antigens, presented by the monomorphic MHC-I related molecule, MR1. To assess the significance of MAIT cells in human diseases, a better understanding of the MAIT cell-MR1-antigen interaction is imperative. Easy access to MR1 ligands and MAIT cells activators can help achieve this. In this review, we summarise current literature that has identified the natural ligands and drug-like molecules that activate MAIT cells and provide insight into their key molecular interactions with MR1 and MAIT T cell receptors (TCRs). We focus on the progress made in synthesizing and isolating 5-amino-6-d-ribitylaminouracil (5-A-RU), a key precursor in the synthesis of the known natural ligands, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil(5-OP-RU) and 5-(2-oxoethylideneamino)-6-d-ribitylaminouracil (5-OE-RU), and also on the stabilisation and optimisation of the latter compounds.

Ligand-dependent downregulation of MR1 cell surface expression

The antigen-presenting molecule MR1 presents riboflavin-based metabolites to Mucosal-Associated Invariant T (MAIT) cells. While MR1 egress to the cell surface is ligand-dependent, the ability of small-molecule ligands to impact on MR1 cellular trafficking remains unknown. Arising from an in silico screen of the MR1 ligand-binding pocket, we identify one ligand, 3-([2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl]formamido)propanoic acid, DB28, as well as an analog, methyl 3-([2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl]formamido)propanoate, NV18.1, that down-regulate MR1 from the cell surface and retain MR1 molecules in the endoplasmic reticulum (ER) in an immature form. DB28 and NV18.1 compete with the known MR1 ligands, 5-OP-RU and acetyl-6-FP, for MR1 binding and inhibit MR1-dependent MAIT cell activation. Crystal structures of the MAIT T cell receptor (TCR) complexed with MR1-DB28 and MR1-NV18.1, show that these two ligands reside within the A'-pocket of MR1. Neither ligand forms a Schiff base with MR1 molecules; both are nevertheless sequestered by a network of hydrophobic and polar contacts. Accordingly, we define a class of compounds that inhibits MR1 cellular trafficking.

Structures of cell wall arabinosyltransferases with the anti-tuberculosis drug ethambutol

The arabinosyltransferases EmbA, EmbB, and EmbC are involved in Mycobacterium tuberculosis cell wall synthesis and are recognized as targets for the anti-tuberculosis drug ethambutol. In this study, we determined cryo-electron microscopy and x-ray crystal structures of mycobacterial EmbA-EmbB and EmbC-EmbC complexes in the presence of their glycosyl donor and acceptor substrates and with ethambutol. These structures show how the donor and acceptor substrates bind in the active site and how ethambutol inhibits arabinosyltransferases by binding to the same site as both substrates in EmbB and EmbC. Most drug-resistant mutations are located near the ethambutol binding site. Collectively, our work provides a structural basis for understanding the biochemical function and inhibition of arabinosyltransferases and the development of new anti-tuberculosis agents.

Tissue-specific shaping of the TCR repertoire and antigen specificity of iNKT cells

Tissue homeostasis is critically dependent on the function of tissue-resident lymphocytes, including lipid-reactive invariant natural killer T (iNKT) cells. Yet, if and how the tissue environment shapes the antigen specificity of iNKT cells remains unknown. By analysing iNKT cells from lymphoid tissues of mice and humans we demonstrate that their T cell receptor (TCR) repertoire is highly diverse and is distinct for cells from various tissues resulting in differential lipid-antigen recognition. Within peripheral tissues iNKT cell recent thymic emigrants exhibit a different TCR repertoire than mature cells, suggesting that the iNKT population is shaped after arrival to the periphery. Consistent with this, iNKT cells from different organs show distinct basal activation, proliferation and clonal expansion. Moreover, the iNKT cell TCR repertoire changes following immunisation and is shaped by age and environmental changes. Thus, post-thymic modification of the TCR-repertoire underpins the distinct antigen specificity for iNKT cells in peripheral tissues

Sterile activation of invariant natural killer T cells by ER-stressed antigen-presenting cells

Invariant NKT (iNKT) cells have the unique ability to shape immunity during antitumor immune responses and other forms of sterile and nonsterile inflammation. Recent studies have highlighted a variety of classes of endogenous and pathogen-derived lipid antigens that can trigger iNKT cell activation under sterile and nonsterile conditions. However, the context and mechanisms that drive the presentation of self-lipid antigens in sterile inflammation remain unclear. Here we report that endoplasmic reticulum (ER)-stressed myeloid cells, via signaling events modulated by the protein kinase RNA-like ER kinase (PERK) pathway, increase CD1d-mediated presentation of immunogenic endogenous lipid species, which results in enhanced iNKT cell activation both in vitro and in vivo. In addition, we demonstrate that actin cytoskeletal reorganization during ER stress results in an altered distribution of CD1d on the cell surface, which contributes to enhanced iNKT cell activation. These results define a previously unidentified mechanism that controls iNKT cell activation during sterile inflammation.

Mucosal-Associated Invariant T Cells Display Diminished Effector Capacity in Oesophageal Adenocarcinoma

Oesophageal adenocarcinoma (OAC) is an aggressive malignancy with poor prognosis, and incidence is increasing rapidly in the Western world. Mucosal-associated invariant T (MAIT) cells recognize bacterial metabolites and kill infected cells, yet their role in OAC is unknown. We aimed to elucidate the role of MAIT cells during cancer development by characterizing the frequency, phenotype, and function of MAIT cells in human blood and tissues, from OAC and its pre-malignant inflammatory condition Barrett's oesophagus (BO). Blood and tissues were phenotyped by flow cytometry and conditioned media from explanted tissue was used to model the effects of the tumor microenvironment on MAIT cell function. Associations were assessed between MAIT cell frequency, circulating inflammatory markers, and clinical parameters to elucidate the role of MAIT cells in inflammation driven cancer. MAIT cells were decreased in BO and OAC blood compared to healthy controls, but were increased in oesophageal tissues, compared to BO-adjacent tissue, and remained detectable after neo-adjuvant treatment. MAIT cells in tumors expressed CD8, PD-1, and NKG2A but lower NKG2D than BO cohorts. MAIT cells produced less IFN-γ and TNF-α in the presence of tumor-conditioned media. OAC cell line viability was reduced upon exposure to expanded MAIT cells. Serum levels of chemokine IP-10 were inversely correlated with MAIT cell frequency in both tumors and blood. MAIT cells were higher in the tumors of node-negative patients, but were not significantly associated with other clinical parameters. This study demonstrates that OAC tumors are infiltrated by MAIT cells, a type of CD8 T cell featuring immune checkpoint expression and cytotoxic potential. These findings may have implications for immunotherapy and immune scoring approaches.

Obesity Reduces mTORC1 Activity in Mucosal-Associated Invariant T Cells, Driving Defective Metabolic and Functional Responses

Obesity underpins the development of numerous chronic diseases, such as type II diabetes mellitus. It is well established that obesity negatively alters immune cell frequencies and functions. Mucosal-associated invariant T (MAIT) cells are a population of innate T cells, which we have previously reported are dysregulated in obesity, with altered circulating and adipose tissue frequencies and a reduction in their IFN-γ production, which is a critical effector function of MAIT cells in host defense. Hence, there is increased urgency to characterize the key molecular mechanisms that drive MAIT cell effector functions and to identify those which are impaired in the obesity setting. In this study, we found that MAIT cells significantly upregulate their rates of glycolysis upon activation in an mTORC1-dependent manner, and this is essential for MAIT cell IFN-γ production. Furthermore, we show that mTORC1 activation is dependent on amino acid transport via SLC7A5. In obese patients, using RNA sequencing, Seahorse analysis, and a series of in vitro experiments, we demonstrate that MAIT cells isolated from obese adults display defective glycolytic metabolism, mTORC1 signaling, and SLC7A5 aa transport. Collectively, our data detail the intrinsic metabolic pathways controlling MAIT cell cytokine production and highlight mTORC1 as an important metabolic regulator that is impaired in obesity, leading to altered MAIT cell responses.

Photoactivable Glycolipid Antigens Generate Stable Conjugates with CD1d for Invariant Natural Killer T Cell Activation

Activation of invariant natural killer T lymphocytes (iNKT cells) by α-galactosylceramide (α-GC) elicits a range of pro-inflammatory or anti-inflammatory immune responses. We report the synthesis and characterization of a series of α-GC analogues with acyl chains of varying length and a terminal benzophenone. These bound efficiently to the glycolipid antigen presenting protein CD1d, and upon photoactivation formed stable CD1d-glycolipid covalent conjugates. Conjugates of benzophenone α-GCs with soluble or cell-bound CD1d proteins retained potent iNKT cell activating properties, with biologic effects that were modulated by acyl chain length and the resulting affinities of conjugates for iNKT cell antigen receptors. Analysis by mass spectrometry identified a unique covalent attachment site for the glycolipid ligands in the hydrophobic ligand binding pocket of CD1d. The creation of covalent conjugates of CD1d with α-GC provides a new tool for probing the biology of glycolipid antigen presentation, as well as opportunities for developing effective immunotherapeutics.

MAIT cell clonal expansion and TCR repertoire shaping in human volunteers challenged with Salmonella Paratyphi A

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that can detect bacteria-derived metabolites presented on MR1. Here we show, using a controlled infection of humans with live Salmonella enterica serovar Paratyphi A, that MAIT cells are activated during infection, an effect maintained even after antibiotic treatment. At the peak of infection MAIT cell T-cell receptor (TCR)β clonotypes that are over-represented prior to infection transiently contract. Select MAIT cell TCRβ clonotypes that expand after infection have stronger TCR-dependent activation than do contracted clonotypes. Our results demonstrate that host exposure to antigen may drive clonal expansion of MAIT cells with increased functional avidity, suggesting a role for specific vaccination strategies to increase the frequency and potency of MAIT cells to optimize effector function.

Activation of Human Mucosal-Associated Invariant T Cells Induces CD40L-Dependent Maturation of Monocyte-Derived and Primary Dendritic Cells

Mucosal-associated invariant T (MAIT) cells are innate T cells that recognize intermediates of the vitamin B2 biosynthetic pathway presented by the monomorphic MR1 molecule. It remains unclear whether, in addition to their cytolytic activity that is important in antimicrobial defense, MAIT cells have immune-modulatory functions that could enhance dendritic cell (DC) maturation. In this study, we investigated the molecular mechanisms dictating the interactions between human MAIT cells and DCs and demonstrate that human MAIT cells mature monocyte-derived and primary DCs in an MR1- and CD40L-dependent manner. Furthermore, we show that MAIT cell–derived signals synergize with microbial stimuli to induce secretion of bioactive IL-12 by DCs. Activation of human MAIT cells in whole blood leads to MR1- and cytokine-dependent NK cell transactivation. Our results underscore an important property of MAIT cells, which can be of translational relevance to rapidly orchestrate adaptive immunity through DC maturation.

Disruption of Mycobacterial AftB Results in Complete Loss of Terminal β(1 → 2) Arabinofuranose Residues of Lipoarabinomannan

Lipoarabinomannan (LAM) and arabinogalactan (AG) are the two major mycobacterial cell wall (lipo)polysaccharides, which contain a structurally similar arabinan domain that is highly branched and assembled in a stepwise fashion by variety of arabinofuranosyltransferases (ArafT). In addition to playing an essential role in mycobacterial physiology, LAM and its biochemical precursor lipomannan possess potent immunomodulatory activities that affect the host immune response. In the search of additional mycobacterial ArafTs that participate in the synthesis of the arabinan segment of LAM, we disrupted aftB (MSMEG_6400) in Mycobacterium smegmatis. The deletion of chromosomal aftB locus could only be achieved in the presence of a rescue plasmid carrying a functional copy of aftB, strongly suggesting that it is essential for the viability of M. smegmatis. Isolation and detailed structural characterization of a LAM molecule derived from the conditional mutant deficient in AftB revealed the absence of terminal β(1 → 2)-linked arabinofuranosyl residues. Furthermore, we demonstrated that truncated LAM displays proinflammatory activity, which is due to its ability to activate Toll-like receptor 2. All together, our results indicate that AftB is an essential mycobacterial ArafT that plays a role in the synthesis of the arabinan domain of LAM.

MKAN27435 is required for the biosynthesis of higher subclasses of lipooligosaccharides in Mycobacterium kansasii

Lipooligosaccharides are glycolipids found in the cell wall of many mycobacterial species including the opportunistic pathogen Mycobacterium kansasii. The genome of M. kansasii ATCC12478 contains a cluster with genes orthologous to Mycobacterium marinum LOS biosynthesis genes. To initiate a genetic dissection of this cluster and demonstrate its role in LOS biosynthesis in M. kansasii, we chose MKAN27435, a gene encoding a putative glycosyltransferase. Using Specialized Transduction, a phage-based gene knockout tool previously used to generate null mutants in other mycobacteria, we generated a MKAN27435 null mutant. The mutant strain was found to be defective in the biosynthesis of higher LOS subspecies, viz LOS-IV, LOS-V, LOS-VI and LOS-VII. Additionally, a range of low abundance species were detected in the mutant strain and mass spectroscopic analysis indicated that these were shunt products generated from LOS-III by the addition of up to six molecules of a pentose.

Defining aspects of noncognate splenic iNKT cell help which contribute to humoral memory (INC6P.403)

Successful induction of pathogen-specific humoral B cell memory responses depends upon help from CD4+ T cells. We find that invariant natural killer T (iNKT) cells, glycolipid-specific, CD1d-restricted innate lymphocytes, can provide both cognate (direct) and noncognate (or indirect) helper signals to enhance B cell humoral responses. In fact, a subset of iNKT cells, termed iNKT follicular helper cells, are the cells which specifically act to promote germinal centers, facilitate optimal B cell memory, and plasma cell differentiation. While both cognate and noncognate iNKT cell help induce class-switched, antigen-specific humoral immune responses, only noncognate iNKT cell help drives formation of humoral memory and plasma cell development. We find that differences in the strength of the TcR signal received by iNKT cells, very early effector cytokine production by iNKT cells, T follicular helper cells, and T follicular regulatory cells all contribute to the observed differences in the ultimate humoral outcome. These findings help to identify the optimal requirements for iNKT induction of humoral memory, which has important implications for the application of glycolipid molecules as humoral adjuvants.

A single subset of dendritic cells controls the cytokine bias of natural killer T cell responses to diverse glycolipid antigens

Many hematopoietic cell types express CD1d and are capable of presenting glycolipid antigens to invariant natural killer T cells (iNKT cells). However, the question of which cells are the principal presenters of glycolipid antigens in vivo remains controversial, and it has been suggested that this might vary depending on the structure of a particular glycolipid antigen. Here we have shown that a single type of cell, the CD8α⁺ DEC-205⁺ dendritic cell, was mainly responsible for capturing and presenting a variety of different glycolipid antigens, including multiple forms of α-galactosylceramide that stimulate widely divergent cytokine responses. After glycolipid presentation, these dendritic cells rapidly altered their expression of various costimulatory and coinhibitory molecules in a manner that was dependent on the structure of the antigen. These findings show flexibility in the outcome of two-way communication between CD8α⁺ dendritic cells and iNKT cells, providing a mechanism for biasing toward either proinflammatory or anti-inflammatory responses.

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Complexes of antigenic glycolipids bound to CD1d have been visualized in situ

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A single DC subset predominates in presentation of a variety of glycolipids

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Antigen presentation to iNKT cells rapidly alters accessory molecules on APCs

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Reciprocal induction of CD70 and PD-L2 controls cytokine bias of iNKT cell responses

The mechanism of splenic invariant NKT cell activation dictates localization in vivo

Invariant NKT (iNKT) cells are glycolipid-specific innate lymphocytes emerging as critical players in the immune response to diverse infections and disease. iNKT cells are activated through cognate interactions with lipid-loaded APCs, by Ag-independent cytokine-mediated signaling pathways, or a combination of both. Although each of these modes of iNKT cell activation plays an important role in directing the humoral and cell-mediated immune response, the spatio-temporal nature of these interactions and the cellular requirements for activation are largely undefined. Combining novel in situ confocal imaging of αGalactosylceramide-loaded CD1d tetramer labeling to localize the endogenous iNKT cell population with cytokine reporter mice, we reveal the choreography of early murine splenic iNKT cell activation across diverse settings of glycolipid immunization and systemic infection with Streptococcus pneumoniae. We find that iNKT cells consolidate in the marginal zone and require dendritic cells lining the splenic marginal zone for activation following administration of cognate glycolipids and during systemic infection but not following exogenous cytokine administration. Although further establishing the importance of cognate iNKT cell interactions with APCs, we also show that noncognate iNKT-dependent mechanisms are sufficient to mediate effector outcomes, such as STAT signaling and dendritic cell licensing throughout the splenic parenchyma. Collectively, these data provide new insight into how iNKT cells may serve as a natural adjuvant in facilitating adaptive immune responses, irrespective of their tissue localization.

Expression of CD1c enhances human invariant NKT cell activation by α-GalCer

Invariant natural killer T (iNKT) cells are innate T lymphocytes that specifically recognize α-linked glycosphingolipids (α-GSLs) as antigens presented by CD1d molecules. Activating iNKT cells by administering α-GSLs improves disease outcomes in murine cancer models and, thus, there is great interest in the clinical potential of these lipids for treating human cancers. However, humans possess several other CD1 isoforms that are not present in mice and it is not clear whether these CD1 molecules, which also bind lipids, affect human iNKT cell responses. We demonstrate here that CD1c, which is co-expressed with CD1d on blood dendritic cells and on a fraction of B cells, is able to present α-galactosylceramide (α-GalCer) as a weak agonist to human iNKT cells, and that the presence of CD1c synergistically enhances α-GalCerdependent activation of iNKT cells by CD1d. Primary human B cells expressing CD1c induced stronger iNKT cell responses to α-GalCer than the CD1c- subset, and an antibody against CD1c inhibited iNKT cell cytokine secretion. These results suggest that therapeutic activation of human iNKT cells by α-GSLs will be driven preferentially by CD1c+ cell types. Thus, B cell neoplasias that co-express CD1c and CD1d may be particularly susceptible to α-GSL therapy, and cancer vaccines using α-GSLs as adjuvants may be most effective when presented by CD1c+ antigen-presenting cells.

α-Galactosylceramide analogues in cancer immunotherapeutics (P2102)

Natural Killer T cells (NKT) initiate potent anti-tumor responses upon activation by glycolipid antigens presented by CD1d, a non-classical MHC class I-like antigen presenting molecule. Anti-tumor responses are induced following NKT cell activation through their direct production of cytokines such as IFN-γ, and through a variety of indirect effects including dendritic cell maturation, the activation of NK cells and the cross-priming of tumor-specific CD8+ T cells. The glycolipid α-galactosylceramide (α-GC) is a CD1d ligand and potent NKT cell activator that enhances anti-tumor immune responses and decreases metastases in vivo. However, the response is short-lived, and causes liver toxicity and long term anergy of NKT cells. Here we report a novel class of chemically modified α-GCs, designated GCBs, that demonstrate unique binding properties to both mouse and human CD1d and enhanced NKT cell stimulation in vitro, as compared to previously described NKT cell activating ligands. A range of analyses have been carried out to characterize the structure and stability of GCB:CD1d complexes, revealing unique features that distinguish them from previously analyzed α-GC:CD1d complexes. Soluble GCB:CD1d complexes are currently being tested in vivo in mouse models for their ability to drive the sustained activation of NKT cells, which may provide a superior approach for development of improved cancer immunotherapies.

Endoplasmic reticulum glycoprotein quality control regulates CD1d assembly and CD1d-mediated antigen presentation

The non-classical major histocompatibility complex (MHC) homologue CD1d presents lipid antigens to innate-like lymphocytes called natural-killer T (NKT) cells. These cells, by virtue of their broad cytokine repertoire, shape innate and adaptive immune responses. Here, we have assessed the role of endoplasmic reticulum glycoprotein quality control in CD1d assembly and function, specifically the role of a key component of the quality control machinery, the enzyme UDP glucose glycoprotein glucosyltransferase (UGT1). We observe that in UGT1-deficient cells, CD1d associates prematurely with β2-microglobulin (β2m) and is able to rapidly exit the endoplasmic reticulum. At least some of these CD1d-β2m heterodimers are shorter-lived and can be rescued by provision of a defined exogenous antigen, α-galactosylceramide. Importantly, we show that in UGT1-deficient cells the CD1d-β2m heterodimers have altered antigenicity despite the fact that their cell surface levels are unchanged. We propose that UGT1 serves as a quality control checkpoint during CD1d assembly and further suggest that UGT1-mediated quality control can shape the lipid repertoire of newly synthesized CD1d. The quality control process may play a role in ensuring stability of exported CD1d-β2m complexes, in facilitating presentation of low abundance high affinity antigens, or in preventing deleterious responses to self lipids.

Invariant natural killer T cells are not affected by lysosomal storage in patients with Niemann-Pick disease type C

Invariant natural killer T (iNKT) cells are a specialised subset of T cells that are restricted to the MHC class I like molecule, CD1d. The ligands for iNKT cells are lipids, with the canonical superagonist being α-galactosylceramide, a non-mammalian glycosphingolipid. Trafficking of CD1d through the lysosome is required for the development of murine iNKT cells. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by dysfunction in either of two lysosomal proteins, NPC1 or NPC2, resulting in the storage of multiple lipids, including glycosphingolipids. In the NPC1 mouse model, iNKT cells are virtually undetectable, which is likely due to the inability of CD1d to be loaded with the selecting ligand due to defective lysosomal function and/or CD1d trafficking. However, in this study we have found that in NPC1 patients iNKT cells are present at normal frequencies, with no phenotypic or functional differences. In addi-tion, antigen-presenting cells derived from NPC1 patients are functionally competent to present several different CD1d/iNKT-cell ligands. This further supports the hypothesis that there are different trafficking requirements for the development of murine and human iNKT cells, and a functional lysosomal/late-endosomal compartment is not required for human iNKT-cell development.

Visualization of the splenic iNKT cell effector program reveals cross-talk with dendritic cells at the marginal zone (45.13)

Invariant Natural Killer T (iNKT) cells are an innate, CD1d-restricted T cell lineage that rapidly secretes cytokines such as interleukin (IL)-4 and interferon gamma upon activation. iNKT cells potentiate adaptive T cell immunity, in part, by activating or “licensing” dendritic cells (DCs). However, the choreography of DC licensing within secondary lymphoid organs remains unclear. Using a combination of gene reporter mice and mCD1d-specific tetramer staining in situ, we localize the endogenous splenic iNKT cell population and characterize their migration pattern and site of canonical cytokine production in response to cognate lipid antigen encounter. We show that iNKT cells are distributed throughout the spleen of naive mice, but upon activation with αGalCer or pathogen-derived lipids rapidly accumulate in the bridging channels and marginal zone where they secrete IL-4. Early iNKT cell cytokine production and migration depends on the presence of marginal zone DCs. iNKT activation acts reciprocally to upregulate CCR7 and relocalize marginal zone DCs to the splenic T cell zone, another means by which iNKTs may “license” DC-mediated adaptive immunity. Studies of iNKT migration and DC licensing during live bacterial infection are currently underway to further understand how iNKT cells exert their anti-microbial effects in vivo.

Invariant natural killer T cells direct B cell responses to cognate lipid antigen in an IL-21-dependent manner

Mouse invariant natural killer T cells (iNKT cells) provide cognate and noncognate help for lipid and protein-specific B cells, respectively. However, the long-term outcome for B cells after cognate help is provided by iNKT cells is unknown at present. Here we found that cognate iNKT cell help resulted in a B cell differentiation program characterized by extrafollicular plasmablasts, germinal-center formation, affinity maturation and a robust primary immunoglobulin G (IgG) antibody response that was uniquely dependent on iNKT cell-derived interleukin 21 (IL-21). However, cognate help from iNKT cells did not generate an enhanced humoral memory response. Thus, cognate iNKT cell help for lipid-specific B cells induces a unique signature that is a hybrid of classic T cell-dependent and T cell-independent type 2 B cell responses.

Cannabinoid receptor 2 positions and retains marginal zone B cells within the splenic marginal zone

In addition to other receptors, including sphingosine-1-phosphate receptor 1, cannabinoid receptor 2 positions mouse marginal zone B cells within the marginal zone and also prevents their loss to the blood.

Specialized B cells residing in the splenic marginal zone (MZ) continuously survey the blood for antigens and are important for immunity to systemic infections. However, the cues that uniquely attract cells to the MZ have not been defined. Previous work demonstrated that mice deficient in cannabinoid receptor 2 (CB2) have decreased numbers of MZ B cells but it has been unclear whether CB2 regulates MZ B cell development or positioning. We show that MZ B cells are highly responsive to the CB2 ligand 2-arachidonylglycerol (2-AG) and that CB2 antagonism rapidly displaces small numbers of MZ B cells to the blood. Antagonism for longer durations depletes MZ B cells from the spleen. In mice deficient in sphingosine-1-phosphate receptor function, CB2 antagonism causes MZ B cell displacement into follicles. Moreover, CB2 overexpression is sufficient to position B cells to the splenic MZ. These findings establish a role for CB2 in guiding B cells to the MZ and in preventing their loss to the blood. As a consequence of their MZ B cell deficiency, CB2-deficient mice have reduced numbers of CD1d-high B cells. We show that CB2 deficiency results in diminished humoral responses to a CD1d-restricted systemic antigen.

Lysosomal trafficking, antigen presentation, and microbial killing are controlled by the Arf-like GTPase Arl8b

Antigen presentation and microbial killing are critical arms of host defense that depend upon cargo trafficking into lysosomes. Yet, the molecular regulators of traffic into lysosomes are only partly understood. Here, using a lysosome-dependent immunological screen of a trafficking shRNA library, we identified the Arf-like GTPase Arl8b as a critical regulator of cargo delivery to lysosomes. Homotypic fusion and vacuole protein sorting (HOPS) complex members were identified as effectors of Arl8b and were dependent on Arl8b for recruitment to lysosomes, suggesting that Arl8b-HOPS plays a general role in directing traffic to lysosomes. Moreover, the formation of CD1 antigen-presenting complexes in lysosomes, their delivery to the plasma membrane, and phagosome-lysosome fusion were all markedly impaired in Arl8b silenced cells resulting in corresponding defects in T cell activation and microbial killing. Together, these results define Arl8b as a key regulator of lysosomal cellular and immunological functions.

A rapid fluorescence-based assay for classification of iNKT cell activating glycolipids

Structural variants of α-galactosylceramide (αGC) that activate invariant natural killer T cells (iNKT cells) are being developed as potential immunomodulatory agents for a variety of applications. Identification of specific forms of these glycolipids that bias responses to favor production of proinflammatory vs anti-inflammatory cytokines is central to current efforts, but this goal has been hampered by the lack of in vitro screening assays that reliably predict the in vivo biological activity of these compounds. Here we describe a fluorescence-based assay to identify functionally distinct αGC analogues. Our assay is based on recent findings showing that presentation of glycolipid antigens by CD1d molecules localized to plasma membrane detergent-resistant microdomains (lipid rafts) is correlated with induction of interferon-γ secretion and Th1-biased cytokine responses. Using an assay that measures lipid raft residency of CD1d molecules loaded with αGC, we screened a library of ∼200 synthetic αGC analogues and identified 19 agonists with potential Th1-biasing activity. Analysis of a subset of these novel candidate Th1 type agonists in vivo in mice confirmed their ability to induce systemic cytokine responses consistent with a Th1 type bias. These results demonstrate the predictive value of this novel in vitro assay for assessing the in vivo functionality of glycolipid agonists and provide the basis for a relatively simple high-throughput assay for identification and functional classification of iNKT cell activating glycolipids.

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

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

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

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

Calreticulin controls the rate of assembly of CD1d molecules in the endoplasmic reticulum

CD1d is an MHC class I-like molecule comprised of a transmembrane glycoprotein (heavy chain) associated with β₂-microglobulin (β₂m) that presents lipid antigens to NKT cells. Initial folding of the heavy chain involves its glycan-dependent association with calreticulin (CRT), calnexin (CNX), and the thiol oxidoreductase ERp57, and is followed by assembly with β₂m to form the heterodimer. Here we show that in CRT-deficient cells CD1d heavy chains convert to β₂m-associated dimers at an accelerated rate, indicating faster folding of the heavy chain, while the rate of intracellular transport after assembly is unaffected. Unlike the situation with MHC class I molecules, antigen presentation by CD1d is not impaired in the absence of CRT. Instead, there are elevated levels of stable and functional CD1d on the surface of CRT-deficient cells. Association of the heavy chains with the ER chaperones Grp94 and Bip is observed in the absence of CRT, and these may replace CRT in mediating CD1d folding and assembly. ER retention of free CD1d heavy chains is impaired in CRT-deficient cells, allowing their escape and subsequent expression on the plasma membrane. However, these free heavy chains are rapidly internalized and degraded in lysosomes, indicating that β₂m association is required for the exceptional resistance of CD1d to lysosomal degradation that is normally observed.

Acceptor substrate discrimination in phosphatidyl-myo-inositol mannoside synthesis: structural and mutational analysis of mannosyltransferase Corynebacterium glutamicum PimB'

Long term survival of the pathogen Mycobacterium tuberculosis in humans is linked to the immunomodulatory potential of its complex cell wall glycolipids, which include the phosphatidylinositol mannoside (PIM) series as well as the related lipomannan and lipoarabinomannan glycoconjugates. PIM biosynthesis is initiated by a set of cytosolic α-mannosyltransferases, catalyzing glycosyl transfer from the activated saccharide donor GDP-α-D-mannopyranose to the acceptor phosphatidyl-myo-inositol (PI) in an ordered and regio-specific fashion. Herein, we report the crystal structure of mannosyltransferase Corynebacterium glutamicum PimB' in complex with nucleotide to a resolution of 2.0 Å. PimB' attaches mannosyl selectively to the 6-OH of the inositol moiety of PI. Two crystal forms and GDP- versus GDP-α-d-mannopyranose-bound complexes reveal flexibility of the nucleotide conformation as well as of the structural framework of the active site. Structural comparison, docking of the saccharide acceptor, and site-directed mutagenesis pin regio-selectivity to a conserved Asp residue in the N-terminal domain that forces presentation of the correct inositol hydroxyl to the saccharide donor.

Synthesis and biological activity of alpha-glucosyl C24:0 and C20:2 ceramides

Alpha-glucosyl ceramides 4 and 5 have been synthesised and evaluated for their ability to stimulate the activation and expansion of human iNKT cells. The key challenge in the synthesis of both target molecules was the stereoselective synthesis of the alpha-glycosidic linkage. Of the methods examined, glycosylation using per-TMS-protected glucosyl iodide 16 was completely alpha-selective and provided gram quantities of amine 11, from which alpha-glucosyl ceramides 4 and 5 were obtained by N-acylation. alpha-GlcCer 4, containing a C24 saturated acyl chain, stimulated a marked proliferation and expansion of human circulating iNKT cells in short-term cultures. alpha-GlcCer 5, which contains a C20 11,14-cis-diene acyl chain (C20:2), induced extremely similar levels of iNKT cell activation and expansion.

Synthesis of a novel alpha-galactosyl ceramide haptenated-lipid antigen, a useful tool in demonstrating the involvement of iNKT cells in the production of antilipid antibodies

A new haptenated derivative of alpha-galactosyl ceramide (alpha-GalCer) has been synthesized to assist in the study of the mechanism of T cell help for the production of B cell antibodies. Our synthetic route provides access to an amine intermediate which can easily be extended to generate an array of compounds, useful in various ongoing studies. Herein, we also describe the biological evaluation of the nitrophenyl (NP) haptenated alpha-GalCer and demonstrate its importance in such mechanistic studies. For instance, in vitro studies showed that NP-alpha-GalCer stimulates both T and B cell proliferation while in vivo studies in immunized mice showed the production of IgG anti-NP antibodies after exposure to NP-alpha-GalCer. The interpretation of these results helps toward a better understanding of T cell help for the production of antibodies.

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

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

Synthesis and biological activity of alpha-L-fucosyl ceramides, analogues of the potent agonist, alpha-D-galactosyl ceramide KRN7000

Several L-fucoglycolipids are associated with diseases such as cancer, cystic fibrosis and rheumatoid arthritis. Activation of iNKT cells is known to lead to the production of cytokines that can help alleviate or exacerbate these conditions. alpha-Galactosyl ceramide (alpha-GalCer) is a known agonist of iNKT cells and it is believed that its fucosyl counterpart might have similar immunogenic properties. We herein report the synthesis of alpha-L-fucosyl ceramide derivatives and describe their biological evaluation. The key challenge in the synthesis of the target molecules involved the stereoselective synthesis of the alpha-glycosidic linkage. Of the methods examined, the per-TMS-protected glycosyl iodide donor was completely alpha-selective, and could be scaled up to provide gram quantities of the azide precursor 11, from which a range of N-acylated alpha-L-fucosyl ceramides were readily obtained and evaluated for ex vivo expansion of human iNKT cells.

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

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

Recognition of lyso-phospholipids by human natural killer T lymphocytes

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

Synthesis and biological activity of alpha-galactosyl ceramide KRN7000 and galactosyl (alpha1-->2) galactosyl ceramide

We herein report a faster and less cumbersome synthesis of the biologically attractive, alpha-galactosyl ceramide (alpha-GalCer), known as KRN7000, and its analogues. More importantly, the use of a silicon tethered intramolecular glycosylation reaction gave easy access to the diglycosyl ceramide Gal(alpha1-->2)GalCer, which has been shown to require uptake and processing to the biologically active alpha-GalCer derivative.

Distinct requirements for CD1d intracellular transport for development of V(alpha)14 iNKT cells

The positive selection of V(alpha)14 invariant (i)NKT cells in mice requires CD1d-mediated Ag presentation by CD4(+)CD8(+) thymocytes. Maturation of newly selected iNKT cells continues in the periphery and also involves CD1d expression. CD1d molecules acquire Ags for presentation in endosomal compartments, to which CD1d molecules have access through an intrinsic CD1d-encoded tyrosine motif and by association with the class II MHC chaperone, invariant chain. In this study, we report the generation of mice in which all CD1d is replaced by CD1d-enhanced yellow fluorescent fusion protein (EYFP). CD1d-EYFP molecules are stable, present lipid Ags, and have near normal subcellular distribution. CD1d-EYFP molecules mediated positive selection of V(alpha)14 iNKT cell precursors at decreased efficiency, caused a delay in their terminal maturation, and did not invoke V(alpha)14iNKT cell effector function as wild-type CD1d could. Using these mice, we show that the intrinsic CD1d-encoded sorting motif mediates thymic selection and activation of V(alpha)14 iNKT cells by professional APCs, while for peripheral terminal differentiation the intrinsic CD1d sorting motif is dispensable.

Invariant NKT cells reduce the immunosuppressive activity of influenza A virus-induced myeloid-derived suppressor cells in mice and humans

Infection with influenza A virus (IAV) presents a substantial threat to public health worldwide, with young, elderly, and immunodeficient individuals being particularly susceptible. Inflammatory responses play an important role in the fatal outcome of IAV infection, but the mechanism remains unclear. We demonstrate here that the absence of invariant NKT (iNKT) cells in mice during IAV infection resulted in the expansion of myeloid-derived suppressor cells (MDSCs), which suppressed IAV-specific immune responses through the expression of both arginase and NOS, resulting in high IAV titer and increased mortality. Adoptive transfer of iNKT cells abolished the suppressive activity of MDSCs, restored IAV-specific immune responses, reduced IAV titer, and increased survival rate. The crosstalk between iNKT and MDSCs was CD1d- and CD40-dependent. Furthermore, IAV infection and exposure to TLR agonists relieved the suppressive activity of MDSCs. Finally, we extended these results to humans by demonstrating the presence of myeloid cells with suppressive activity in the PBLs of individuals infected with IAV and showed that their suppressive activity is substantially reduced by iNKT cell activation. These findings identify what we believe to be a novel immunomodulatory role of iNKT cells, which we suggest could be harnessed to abolish the immunosuppressive activity of MDSCs during IAV infection.

Natural killer T-cell autoreactivity leads to a specialized activation state

Natural killer T (NKT) cells are innate-like T cells that recognize specific microbial antigens and also display autoreactivity to self-antigens. The nature of NKT-cell autoreactive activation remains poorly understood. We show here that the mitogen-activated protein kinase (MAPK) pathway is operative during human NKT-cell autoreactive activation, but calcium signaling is severely impaired. This results in a response that is biased toward granulocyte macrophage colony-stimulating factor (GM-CSF) secretion because this cytokine requires extracellular signal-regulated kinase (ERK) signaling but is not highly calcium dependent, whereas interferon-gamma (IFN-gamma), interleukin (IL)-4, and IL-2 production are minimal. Autoreactive activation was associated with reduced migration velocity but did not induce arrest; thus, NKT cells retained the ability to survey antigen presenting cells (APCs). IL-12 and IL-18 stimulated autoreactively activated NKT cells to secrete IFN-gamma, and this was mediated by Janus kinase-signal transducers and activators of transcription (JAK-STAT)-dependent signaling without induction of calcium flux. This pathway did not require concurrent contact with CD1d(+) APCs but was strictly dependent on preceding autoreactive stimulation that induced ERK activation. In contrast, NKT-cell responses to the glycolipid antigen alpha-galactosyl ceramide (alpha-GalCer) were dampened by prior autoreactive activation. These results show that NKT-cell autoreactivity induces restricted cytokine secretion and leads to altered basal activation that potentiates innate responsiveness to costimulatory cytokines while modulating sensitivity to foreign antigens.

Invariant NKT cells reduce the immunosuppressive activity of influenza A virus-induced myeloid-derived suppressor cells in mice and humans

Infection with influenza A virus (IAV) presents a substantial threat to public health worldwide, with young, elderly, and immunodeficient individuals being particularly susceptible. Inflammatory responses play an important role in the fatal outcome of IAV infection, but the mechanism remains unclear. We demonstrate here that the absence of invariant NKT (iNKT) cells in mice during IAV infection resulted in the expansion of myeloid-derived suppressor cells (MDSCs), which suppressed IAV-specific immune responses through the expression of both arginase and NOS, resulting in high IAV titer and increased mortality. Adoptive transfer of iNKT cells abolished the suppressive activity of MDSCs, restored IAV-specific immune responses, reduced IAV titer, and increased survival rate. The crosstalk between iNKT and MDSCs was CD1d- and CD40-dependent. Furthermore, IAV infection and exposure to TLR agonists relieved the suppressive activity of MDSCs. Finally, we extended these results to humans by demonstrating the presence of myeloid cells with suppressive activity in the PBLs of individuals infected with IAV and showed that their suppressive activity is substantially reduced by iNKT cell activation. These findings identify what we believe to be a novel immunomodulatory role of iNKT cells, which we suggest could be harnessed to abolish the immunosuppressive activity of MDSCs during IAV infection.

A Mild Pummerer-Like Reaction of Carbohydrate-Based Selenoethers and Thioethers Involving Linear Ozonide Acetates as Putative Intermediates

Pummerer-like rearrangements of carbohydrate-based heterocycles containing selenium and sulfur were investigated. To the best of our knowledge, this is the first report on the Pummerer rearrangement in selenoheterocycles. Ozonization of 1,4-anhydro-D-galactitol or 1,5-anhydroxylitol derivatives containing sulfur or selenium as the ring heteroatom gave unstable intermediates that were attributed to ozonides. These intermediates decomposed upon warming to give selenoxides or sulfoxides. Significantly, addition of acetic anhydride at low temperature to the ozonization reaction mixtures gave Pummerer-rearrangement products after warming to ambient temperature. However, when the isolated selenoxides or sulfoxides were treated with acetic anhydride, Pummerer rearrangement occurred but the sulfoxides required much higher reaction temperatures. The latter results are at variance with the former and are interpreted in terms of the rearrangement of the ozonide acetate intermediates in the former cases. To probe whether the rearrangement proceeded heterolytically via extrusion of singlet oxygen or homolytically via the generation of radical species, trapping experiments with rubrene and electron paramagnetic resonance (EPR) studies with the radical trap DMPO were performed. The results of these experiments are consistent with the intermediacy of radical species and suggest a new and milder synthetic method to generate Pummerer-type products.