New CD1d agonists: synthesis and biological activity of 6″-triazole-substituted α-galactosyl ceramides

1,5-disubstituted 1,2,3-triazolylated carbohydrates and nucleosides

In general, 1,5-disubstituted 1,2,3-triazolyl moiety is much less common in the synthesis and applications in comparison to its regioisomeric counterpart. Moreover, the synthesis of 1,5-disubstituted 1,2,3-triazoles are not so straightforward as is the case for copper catalyzed strategy of 1,4-disubstituted 1,2,3-triazoles. The preparation of 1,5-triazolylated carbohydrates and nucleosides are even more complex because of the difficulties in accessing the appropriate starting materials as well as the compatibility of reaction conditions with the various protecting groups. 1,5-Disubstitution regioisomeric triazoles of carbohydrates and nucleosides were traditionally obtained as minor products through straightforward heating of the mixture of azides and terminal alkynes. However, the separation of isomers was tedious or in some cases futile. On the other hand, regioselective synthesis using ruthenium catalysis triggered serious concern of residual metal content in therapeutically important ingredients. Therefore, serious efforts are being made by several groups to develop non-toxic metal based or completely metal-free synthesis of 1,5-disubstituted 1,2,3-triazoles. This article strives to summarize the pre-Click era as well as the post-2001 reports on the synthesis and potential applications of 1,5-disubstituted 1,2,3-triazoles in biological systems.

A structural perspective of how T cell receptors recognize the CD1 family of lipid antigen-presenting molecules

The CD1 family of antigen-presenting molecules adopt a major histocompatibility complex class I (MHC-I) fold. Whereas MHC molecules present peptides, the CD1 family has evolved to bind self- and foreign-lipids. The CD1 family of antigen-presenting molecules comprises four members-CD1a, CD1b, CD1c, and CD1d-that differ in their architecture around the lipid-binding cleft, thereby enabling diverse lipids to be accommodated. These CD1-lipid complexes are recognized by T cell receptors (TCRs) expressed on T cells, either through dual recognition of CD1 and lipid or in a new model whereby the TCR directly contacts CD1, thereby triggering an immune response. Chemical syntheses of lipid antigens, and analogs thereof, have been crucial in understanding the underlying specificity of T cell-mediated lipid immunity. This review will focus on our current understanding of how TCRs interact with CD1-lipid complexes, highlighting how it can be fundamentally different from TCR-MHC-peptide corecognition.

A Glycolipid-Peptide-Hapten Tricomponent Conjugate Vaccine Generates Durable Antihapten Antibody Responses in Mice

Eliciting an antihapten antibody response to vaccination typically requires the use of constructs where multiple copies of the hapten are covalently attached to a larger carrier molecule. The carrier is required to elicit T cell help via presentation of peptide epitopes on major histocompatibility complex (MHC) class II molecules; as such, attachment to full-sized proteins, alone or in a complex, is generally used to account for the significant MHC diversity in humans. While such carrier-based vaccines have proven extremely successful, particularly in protecting against bacterial diseases, they can be challenging to manufacture, and repeated use can be compromised by pre-existing immunity against the carrier. One approach to reducing these complications is to recruit help from type I natural killer T (NKT) cells, which exhibit limited diversity in their antigen receptors and respond to glycolipid antigens presented by the highly conserved presenting molecule CD1d. Synthetic vaccines for universal use can, therefore, be prepared by conjugating haptens to an NKT cell agonist such as α-galactosylceramide (αGalCer, KRN7000). An additional advantage is that the quality of NKT cell help is sufficient to overcome the need for an extra immune adjuvant. However, while initial studies with αGalCer-hapten conjugate vaccines report strong and rapid antihapten antibody responses, they can fail to generate lasting memory. Here, we show that antibody responses to the hapten 4-hydoxy-3-nitrophenyl acetyl (NP) can be improved through additional attachment of a fusion peptide containing a promiscuous helper T cell epitope (Pan DR epitope, PADRE) that binds diverse MHC class II molecules. Such αGalCer-hapten-peptide tricomponent vaccines generate strong and sustained anti-NP antibody titers with increased hapten affinity compared to vaccines without the helper epitope. The tricomponent vaccine platform is therefore suitable for further exploration in the pursuit of efficacious antihapten immunotherapies.

Recent Progress on Synthesis of Functionalized 1,5-Disubstituted Triazoles

Immediately after the invention of 'Click Chemistry' in 2002, the regioselective 1,2,3- triazole scaffolds resulted from respective organic azides and terminal alkynes under Cu(I) catalysis have been well recognized as the functional heterocyclic core at the centre of modern organic chemistry, medicinal chemistry, and material sciences. This CuAAC reaction has several notable features including excellent regioselectivity, high-to-excellent yields, easy to execute, short reaction time, modular in nature, mild condition, readily available starting materials, etc. Moreover, the resulting regioselective triazoles can serve as amide bond isosteres, a privileged functional group in drug discovery and development. More than hundreds of reviews had been devoted to the 'Click Chemistry' in special reference to 1,4-disubstituted triazoles, while only little efforts were made for an opposite regioisomer i.e., 1,5-disubstituted triazole. Herein, we have presented various classical approaches for an expeditious synthesis of a wide range of biologically relevant 1,5- disubstituted 1,2,3-triazole analogues. The syntheses of such a class of diversly functionalized triazoles have emerged as a crucial investigation in the domain of chemistry and biology. This tutorial review covers the literature assessment on the development of various synthetic protocols for the functionalized 1,5-disubstituted triazoles reported during the last 12 years.

An immunostimulatory glycolipid that blocks SARS-CoV-2, RSV, and influenza infections in vivo

Prophylactic vaccines for SARS-CoV-2 have lowered the incidence of severe COVID-19, but emergence of viral variants that are antigenically distinct from the vaccine strains are of concern and additional, broadly acting preventive approaches are desirable. Here, we report on a glycolipid termed 7DW8-5 that exploits the host innate immune system to enable rapid control of viral infections in vivo. This glycolipid binds to CD1d on antigen-presenting cells and thereby stimulates NKT cells to release a cascade of cytokines and chemokines. The intranasal administration of 7DW8-5 prior to virus exposure significantly blocked infection by three different authentic variants of SARS-CoV-2, as well as by respiratory syncytial virus and influenza virus, in mice or hamsters. We also found that this protective antiviral effect is both host-directed and mechanism-specific, requiring both the CD1d molecule and interferon-[Formula: see text]. A chemical compound like 7DW8-5 that is easy to administer and cheap to manufacture may be useful not only in slowing the spread of COVID-19 but also in responding to future pandemics long before vaccines or drugs are developed.

Chemical Synthesis and Molecular Interaction Analysis of α-Galactosyl Ceramide Derivatives as CD1d Ligands

CD1d is a non-classical major histocompatibility complex (MHC) protein, responsible for lipid antigen presentation, which presents lipids to natural killer T (NKT) cells. Various CD1d lipid ligands have been reported, including microbial and endogenous glycolipids/phospholipids. Among them, an α-galactosylceramide (α-GalCer), a representative CD1d ligand, is one of the most potent ligands and its derivatives have been developed. In this chapter, the chemistry of α-GalCer and its derivatives are described with an emphasis on their chemical syntheses and molecular interaction analysis with CD1d are described.

6″-Modifed α-GalCer-peptide conjugate vaccine candidates protect against liver-stage malaria

Self-adjuvanting vaccines consisting of peptide epitopes conjugated to immune adjuvants are a powerful way of generating antigen-specific immune responses. We previously showed that a Plasmodium-derived peptide conjugated to a rearranged form of α-galactosylceramide (α-GalCer) could stimulate liver-resident memory T (T_RM) cells that were effective killers of liver-stage Plasmodium berghei ANKA (Pba)-infected cells. To investigate if similar or even superior T_RM responses can be induced by modifying the α-GalCer adjuvant, we created new conjugate vaccine cadidates by attaching an immunogenic Plasmodium-derived peptide antigen to 6″-substituted α-GalCer analogues. Vaccine synthesis involved developing an efficient route to α-galactosylphytosphingosine (α-GalPhs), from which the prototypical iNKT cell agonist, α-GalCer, and its 6″-deoxy-6″-thio and -amino analogues were derived. Attaching a cathepsin B-cleavable linker to the 6″-modified α-GalCer created pro-adjuvants bearing a pendant ketone group available for peptide conjugation. Optimized reaction conditions were developed that allow for the efficient conjugation of peptide antigens to the pro-adjuvants via oxime ligation to create new glycolipid-peptide (GLP) conjugate vaccines. A single dose of the vaccine candidates induced acute NKT and Plasmodium-specific CD8⁺ T cell responses that generated potent hepatic T_RM responses in mice. Our findings demonstrate that attaching antigenic peptides to 6″-modifed α-GalCer generates powerful self-adjuvanting conjugate vaccine candidates that could potentially control hepatotropic infections such as liver-stage malaria.

Candidate vaccines comprised of peptide antigen conjugated to 6″-modified α-GalCer analogues generate potent hepatic T_RM responses in mice with a single dose inducing protective immunity against malaria in a Plasmodium sporozoite challenge model.

Natural and synthetic carbohydrate-based vaccine adjuvants and their mechanisms of action

Modern subunit vaccines based on homogeneous antigens offer more precise targeting and improved safety compared with traditional whole-pathogen vaccines. However, they are also less immunogenic and require an adjuvant to increase the immunogenicity of the antigen and potentiate the immune response. Unfortunately, few adjuvants have sufficient potency and low enough toxicity for clinical use, highlighting the urgent need for new, potent and safe adjuvants. Notably, a number of natural and synthetic carbohydrate structures have been used as adjuvants in clinical trials, and two have recently been approved in human vaccines. However, naturally derived carbohydrate adjuvants are heterogeneous, difficult to obtain and, in some cases, unstable. In addition, their molecular mechanisms of action are generally not fully understood, partly owing to the lack of tools to elucidate their immune-potentiating effects, thus hampering the rational development of optimized adjuvants. To address these challenges, modification of the natural product structure using synthetic chemistry emerges as an attractive approach to develop well-defined, improved carbohydrate-containing adjuvants and chemical probes for mechanistic investigation. This Review describes selected examples of natural and synthetic carbohydrate-based adjuvants and their application in synthetic self-adjuvanting vaccines, while also discussing current understanding of their molecular mechanisms of action.

Amide-Linked C4″-Saccharide Modification of KRN7000 Provides Potent Stimulation of Human Invariant NKT Cells and Anti-Tumor Immunity in a Humanized Mouse Model

Activation of invariant natural killer T (iNKT) cells by α-galactosylceramides (α-GalCers) stimulates strong immune responses and potent anti-tumor immunity. Numerous modifications of the glycolipid structure have been assessed to derive activating ligands for these T cells with altered and potentially advantageous properties in the induction of immune responses. Here, we synthesized variants of the prototypical α-GalCer, KRN7000, with amide-linked phenyl alkane substitutions on the C4″-position of the galactose ring. We show that these variants have weak iNKT cell stimulating activity in mouse models but substantially greater activity for human iNKT cells. The most active of the C4″-amides in our study showed strong anti-tumor effects in a partially humanized mouse model for iNKT cell responses. In silico analysis suggested that the tether length and degree of flexibility of the amide substituent affected the recognition by iNKT cell antigen receptors of the C4″-amide substituted glycolipids in complex with their antigen presenting molecule CD1d. Our findings establish the use of stable C4″-amide linked additions to the sugar moiety for further exploration of the immunological effects of structural modifications of iNKT cell activating glycolipids and highlight the critical need for more accurate animal models to assess these compounds for immunotherapeutic potential in humans.

Synthesis and biological activities of amino acids functionalized α-GalCer analogues

Invariant natural killer T-cells (iNKT-cells) are promising targets for manipulating the immune system, which can rapidly release a large amount of Th1 and Th2 cytokines upon the engagement of their T cell receptor with glycolipid antigens presented by CD1d. In this paper, we wish to report a novel series of α-GalCer analogues which were synthesized by incorporation of l-amino acid methyl esters in the C-6' position of glycolipid. The evaluation of these synthetic analogues for their capacities to stimulate iNKT-cells into producing Th1 and Th2 cytokines both in vitro and in vivo indicated that they were potent CD1d ligands and could stimulate murine spleen cells into a higher release of the Th1 cytokine IFN-γ in vitro. In vivo, Gly-α-GalCer (1) and Lys-α-GalCer (3) showed more Th1-biased responses than α-GalCer, especially analogue 3 showed the highest selectivity for IFN-γ production (IFN-γ/IL-4 = 5.32) compared with α-GalCer (IFN-γ/IL-4 = 2.5) in vivo. These novel α-GalCer analogues might be used as efficient X-ray crystallographic probes to reveal the relationship between glycolipids and CD1d proteins in α-GalCer/CD1d complexes and pave the way for developing new potent immunostimulating agents.

3,4-Dideoxy-3,3,4,4-tetrafluoro- and 4-OH epimeric 3-deoxy-3,3-difluoro-α-GalCer analogues: Synthesis and biological evaluation on human iNKT cells stimulation

iNKT cells recognize CD1d/α-galactosylceramide (α-GalCer) complexes via their invariant TCR receptor and stimulate the immune response. Many α-GalCer analogues have been investigated to interrogate this interaction. Following our previous work related to the modification of the hydrogen bond network between α-GalCer and CD1d, we have now focused our attention on the synthesis of 3-deoxy-3,3-difluoro- and 3,4-dideoxy-3,3,4,4-tetrafluoro-α-GalCer analogues, and studied their ability to stimulate human iNKT cells. In each case, deoxygenation at the indicated positions was accompanied by difluoro introduction in order to evaluate the resulting electronic effect on the stability of the ternary CD1d/Galcer/TCR complex which has been rationalized by modeling study. With deoxy-difluorination at the 3-position, the two epimeric 4-OH analogues were investigated to establish their capacity to compensate for the lack of the hydrogen bond donating group at the 3-position. The 3,4-dideoxytetrafluoro analogue was of interest to highlight the amide NH-bond hydrogen bond properties.

4"-O-Alkylated α-Galactosylceramide Analogues as iNKT-Cell Antigens: Synthetic, Biological, and Structural Studies

Invariant natural killer T-cells (iNKT) are a glycolipid-responsive subset of T-lymphocytes that fulfill a pivotal role in the immune system. The archetypical synthetic glycolipid, α-galactosylceramide (α-GalCer), whose molecular framework is inspired by a group of amphiphilic natural products, remains the most studied antigen for iNKT-cells. Nonetheless, the potential of α-GalCer as an immunostimulating agent is compromised by the fact that this glycolipid elicits simultaneous secretion of Th1- and Th2-cytokines. This has incited medicinal chemistry efforts to identify analogues that are able to perturb the Th1/Th2 balance. In this work, we present the synthesis of an extensive set of 4"-O-alkylated α-GalCer analogues, which were evaluated in vivo for their cytokine induction. We have found that conversion of the 4"-OH group to ether moieties decreases the immunogenic potential in mice relative to α-GalCer. Yet, the benzyl-modified glycolipids are able to produce a distinct pro-inflammatory immune response. The crystal structures suggest an extra hydrophobic interaction between the benzyl moiety and the α2-helix of CD1d.

Efficient Divergent Synthesis of New Immunostimulant 4″-Modified α-Galactosylceramide Analogues

A synthesis strategy for the swift generation of 4″-modified α-galactosylceramide (α-GalCer) analogues is described, establishing a chemical platform to comprehensively investigate the structure–activity relationships (SAR) of this understudied glycolipid part. The strategy relies on a late-stage reductive ring-opening of a p-methoxybenzylidene (PMP) acetal to regioselectively liberate the 4″-OH position. The expediency of this methodology is demonstrated by the synthesis of a small yet diverse set of analogues, which were tested for their ability to stimulate invariant natural killer T-cells (iNKT) in vitro and in vivo. The introduction of a p-chlorobenzyl ether yielded an analogue with promising immunostimulating properties, paving the way for further SAR studies.

An in silico approach for modelling T-helper polarizing iNKT cell agonists

Many analogues of the glycolipid alpha-galactosylceramide (α-GalCer) are known to activate iNKT cells through their interaction with CD1d-expressing antigen-presenting cells, inducing the release of Th1 and Th2 cytokines. Because of iNKT cell involvement and associated Th1/Th2 cytokine changes in a broad spectrum of human diseases, the design of iNKT cell ligands with selective Th1 and Th2 properties has been the subject of extensive research. This search for novel iNKT cell ligands requires refined structural insights. Here we will visualize the chemical space of 333 currently known iNKT cell activators, including several newly tested analogues, by more than 3000 chemical descriptors which were calculated for each individual analogue. To evaluate the immunological responses we analyzed five different cytokines in five different test-systems. We linked the chemical space to the immunological space using a system biology computational approach resulting in highly sensitive and specific predictive models. Moreover, these models correspond with the current insights of iNKT cell activation by α-GalCer analogues, explaining the Th1 and Th2 biased responses, downstream of iNKT cell activation. We anticipate that such models will be of great value for the future design of iNKT cell agonists.

Design and synthesis of galactose-6-OH-modified α-galactosyl ceramide analogues with Th2-biased immune responses

In this study, a simple type of O-6 analogue of KRN7000 was synthesized starting from galactosyl iodide and d-lyxose.

Stimulation of natural killer T cells by glycolipids

Natural killer T (NKT) cells are a subset of T cells that recognize glycolipid antigens presented by the CD1d protein. The initial discovery of immunostimulatory glycolipids from a marine sponge and the T cells that respond to the compounds has led to extensive research by chemists and immunologists to understand how glycolipids are recognized, possible responses by NKT cells, and the structural features of glycolipids necessary for stimulatory activity. The presence of this cell type in humans and most mammals suggests that it plays critical roles in antigen recognition and the interface between innate and adaptive immunity. Both endogenous and exogenous natural antigens for NKT cells have been identified, and it is likely that glycolipid antigens remain to be discovered. Multiple series of structurally varied glycolipids have been synthesized and tested for stimulatory activity. The structural features of glycolipids necessary for NKT cell stimulation are moderately well understood, and designed compounds have proven to be much more potent antigens than their natural counterparts. Nevertheless, control over NKT cell responses by designed glycolipids has not been optimized, and further research will be required to fully reveal the therapeutic potential of this cell type.