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

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.

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.

CD1d-dependent natural killer T-cells inactivation aggravates sepsis-induced myocardial injury via T lymphocytes infiltration and IL-6 production in mice

Myocardial edema mediated by endothelial dysfunction plays an important role in sepsis-induced cardiomyopathy (SIC); however, its mechanism is unclear. The current study aimed to provide evidence on the cardioprotection of CD1d-dependent natural killer T (NKT) cells and clarify the possible mechanism in a mouse model of sepsis. Wild-type (WT) and CD1d-dependent NKT-cells inactivation (CD1dko) mice were subjected to sepsis induced by intraperitoneal injection of lipopolysaccharide (LPS). The NKT-cells number and CD1d expression were both increased in the hearts and blood of WT mice after LPS treatment. Compared with WT mice, CD1dko mice exhibited remarkably accelerated LPS-induced mortality, cardiac dysfunction, myocardial injury, endothelial apoptosis, microvascular damage, microvascular permeability and cardiac edema. Mechanistically, CD1d deficiency further increased LPS-induced accumulation of T lymphocytes in the myocardium and upregulation of IL-6 protein levels. Administration of an IL-6 neutralizing antibody to CD1dko mice improved cardiac dysfunction, myocardial injury and edema induced by LPS. Our study identified that CD1d-dependent NKT-cells inactivation exacerbated SIC via T lymphocytes infiltration and IL-6 production. Hence, activation of CD1d-dependent NKT cells may be a potential candidate strategy for SIC treatment.

Harnessing NKT cells for vaccination

Natural killer T (NKT) cells are innate-like T cells capable of enhancing both innate and adaptive immune responses. When NKT cells are stimulated in close temporal association with co-administered antigens, strong antigen-specific immune responses can be induced, prompting the study of NKT cell agonists as novel immune adjuvants. This activity has been attributed to the capacity of activated NKT cells to act as universal helper cells, with the ability to provide molecular signals to dendritic cells and B cells that facilitate T cell and antibody responses, respectively. These signals can override the requirement for conventional CD4⁺ T cell help, so that vaccines can be designed without need to consider CD4⁺ T cell repertoire and major histocompatibility complex Class II diversity. Animal studies have highlighted some drawbacks of the approach, namely, concerns around induction of NKT cell hyporesponsiveness, which may limit vaccine boosting, and potential for toxicity. Here we highlight studies that suggest these obstacles can be overcome by targeted delivery in vivo. We also feature new studies that suggest activating NKT cells can help encourage differentiation of T cells into tissue-resident memory cells that play an important role in prophylaxis against infection, and may be required in cancer therapy.

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.

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.

Redirecting iNKT Cell Antitumor Immunity with α-GalCer/CD1d-scFv Fusion Proteins

Invariant natural killer T (iNKT) cells display important properties that could bridge the innate and adaptive immunity, and they have been shown to play key roles in cancer immunotherapy. However, administration of iNKT cell agonist αGalCer fails to induce sustained antitumor immunity due to the rapid anergy induction after an initial strong activation. To this end, we have designed a recombinant CD1d protein that is fused to an anti-TAA scFv, which is able to recruit iNKT cells to the tumor site and induce tumor regression. Importantly, recombinant CD1d fusion proteins loaded with α-GalCer demonstrated sustained activation of iNKT cells upon repeated injections and superior tumor control, as compared to α-GalCer treatment.

Using agonists for iNKT cells in cancer therapy

The capacity of α-galactosylceramide (α-GalCer) to act as an anti-cancer agent in mice through the specific stimulation of type I NKT (iNKT) cells has prompted extensive investigation to translate this finding to the clinic. However, low frequencies of iNKT cells in cancer patients and their hypo-responsiveness to repeated stimulation have been seen as barriers to its efficacy. Currently the most promising clinical application of α-GalCer, or its derivatives, is as stimuli for ex vivo expansion of iNKT cells for adoptive therapy, although some encouraging clinical results have recently been reported using α-GalCer pulsed onto large numbers of antigen presenting cells (APCs). In on-going preclinical studies, attempts to improve efficacy of injected iNKT cell agonists have focussed on optimising presentation in vivo, through encapsulation in particulate vectors, making structural changes that help binding to the presenting molecule CD1d, or injecting agonists covalently attached to recombinant CD1d. Variations on these same approaches are being used to enhance the APC-licencing function of iNKT cells in vivo to induce adaptive immune responses to associated tumour antigens. Looking ahead, a unique capacity of in vivo-activated iNKT cells to facilitate formation of resident memory CD8⁺ T cells is a new observation that could find a role in cancer therapy.

Design and Discovery of Covalent α-GalCer Derivatives as Potent CD1d Ligands

CD1d is a nonpolymorphic antigen-presenting protein responsible for the regulation of natural killer T (NKT) cell activation. α-Galactosyl ceramide (α-GalCer, KRN7000) is the representative CD1d ligand that can bind to the CD1d protein. The resulting complex is recognized by the T cell receptors of the NKT cell, inducing various immune responses. Previous structure-activity relationship studies of α-GalCer have revealed that the ability of NKT cells to induce cytokines depends on the ligand structure, and in particular, ligands that form more stable complexes with CD1d display potent activity. We focused on the Cys residue of the large hydrophobic pockets of CD1d (A' pocket) and developed α-GalCer derivatives containing groups that can form covalent bonds. The assay results revealed that these ligands displayed higher levels of cytokine production and Th2 cell-type cytokine polarization response. Furthermore, the LC-MS/MS analysis indicated that the chloroacetylamide-containing ligand was covalently bound to Cys12 of CD1d, which suggests that the enhanced activities result from the formation of a stable CD1d-ligand complex. To our knowledge, this is the first ligand that allows covalent bond formation to CD1d under physiological conditions.

Photocrosslinking probes for capture of carbohydrate interactions

Glycan-mediated interactions are essential in many biological processes and regulate a wide variety of cellular functions. However, characterizing these interactions is difficult because glycan biosynthesis is not template driven and because carbohydrate recognition events are usually of low affinity and transient. Photocrosslinking carbohydrate probes can form a covalent bond with molecules in close proximity on UV irradiation and are capable of capturing interactions between glycans and glycan-binding proteins in situ. Because of these advantages, multiple photocrosslinking carbohydrate probes have been designed and applied to study the biological functions of glycans. This review will discuss recent advances in the development of novel photocrosslinking functional groups and the design of photocrosslinking probes to detect interactions mediated by glycolipids, peptidoglycan, and multivalent carbohydrate ligands. These probes have demonstrated the potential to address some of the major challenges in the study of glycan-mediated interactions in both model systems and in more complex biological settings.

Bacterial immunogenic α-galactosylceramide identified in the murine large intestine: dependency on diet and inflammation

The glycosphingolipid, α-galactosylceramide (αGalCer), when presented by CD1d on antigen-presenting cells, efficiently activates invariant natural killer T (iNKT) cells. Thereby, it modulates immune responses against tumors, microbial and viral infections, and autoimmune diseases. Recently, the production of αGalCer by Bacteroidetes from the human gut microbiome was elucidated. Using hydrophilic interaction chromatography coupled to MS², we screened murine intestinal tracts to identify and quantify αGalCers, and we investigated the αGalCer response to different dietary and physiologic conditions. In both the cecum and the colon of mice, we found 1-15 pmol of αGalCer per milligram of protein; in contrast, mice lacking microbiota (germ-free mice) and fed identical diet did not harbor αGalCer. The identified αGalCer contained a β(R)-hydroxylated hexadecanoyl chain N-linked to C18-sphinganine, which differed from what has been reported with Bacteroides fragilis Unlike β-anomeric structures, but similar to αGalCers from B. fragilis, the synthetic form of the murine αGalCer induced iNKT cell activation in vitro. Last, we observed a decrease in αGalCer production in mice exposed to conditions that alter the composition of the gut microbiota, including Western type diet, colitis, and influenza A virus infection. Collectively, this study suggests that αGalCer is produced by commensals in the mouse intestine and reveals that stressful conditions causing dysbiosis alter its synthesis. The consequences of this altered production on iNKT cell-mediated local and systemic immune responses are worthy of future studies.

Development of α-GalCer Analogues with an α-Fluorocarbonyl Moiety as Th2-Selective Ligands of CD1d

A series of α-GalCer analogues containing an α-fluorocarbonyl moiety at the terminal position of the acyl chain were designed for targeting polar residues in the hydrophobic cavity of CD1d using a structure-based approach. The acyl chain length was efficiently adjusted by an asymmetric alkyne-alkyne cross coupling strategy, and the newly synthesized α-GalCer analogues showed the high Th2-selective activity of iNKT cells. The biased activity of ligands could be caused by the hydrogen-bonding interaction between ligands and CD1d according to the Th2-selective cytokine secretion and molecular docking studies.