Antigen specificity of invariant natural killer T-cells

Quantifying acyl-chain diversity in isobaric compound lipids containing monomethyl branched-chain fatty acids

Compound lipids comprise a diverse group of metabolites present in living systems, and metabolic- and environmentally-driven structural distinctions across this family are increasingly linked to biological function. However, methods for deconvoluting these often isobaric lipid species are lacking or require specialized instrumentation. Notably, acyl-chain diversity within cells may be influenced by nutritional states, metabolic dysregulation, or genetic alterations. Therefore, a reliable, validated method of quantifying structurally similar even-, odd-, and branched-chain acyl groups within intact compound lipids will be invaluable for gaining molecular insights into their biological functions. Here we demonstrate the chromatographic resolution of isobaric lipids containing distinct combinations of straight-chain and branched-chain acyl groups via ultra-high-pressure liquid chromatography (UHPLC)-mass spectrometry (MS) using a C30 liquid chromatography column. Using metabolically engineered adipocytes lacking branched-keto acid dehydrogenase A (Bckdha), we validate this approach through a combination of fatty acid supplementation and metabolic tracing using monomethyl branched-chain fatty acids and valine. We observe the resolution of numerous isobaric triacylglycerols and other compound lipids, demonstrating the resolving utility of this method. This approach adds to the toolbox for laboratories to quantify and characterize acyl chain diversity across the lipidome.

Quantifying acyl-chain diversity in isobaric compound lipids containing monomethyl branched-chain fatty acids

Compound lipids comprise a diverse group of metabolites present in living systems, and metabolic- and environmentally-driven structural distinctions across this family is increasingly linked to biological function. However, methods for deconvoluting these often isobaric lipid species are lacking or require specialized instrumentation. Notably, acyl-chain diversity within cells may be influenced by nutritional states, metabolic dysregulation, or genetic alterations. Therefore, a reliable, validated method of quantifying structurally similar even-, odd-, and branched-chain acyl groups within intact compound lipids will be invaluable for gaining molecular insights into their biological functions. Here we demonstrate the chromatographic resolution of isobaric lipids containing distinct combinations of straight-chain and branched-chain acyl groups via ultra-high-pressure liquid chromatography (UHPLC)-mass spectrometry (MS) using a C30 liquid chromatography column. Using metabolically-engineered adipocytes lacking branched-keto acid dehydrogenase A (Bckdha), we validate this approach through a combination of fatty acid supplementation and metabolic tracing using monomethyl branched-chain fatty acids and valine. We observe resolution of numerous isobaric triacylglycerols and other compound lipids, demonstrating the resolving utility of this method. This approach strengthens our ability to quantify and characterize the inherent diversity of acyl chains across the lipidome.

A Humanized Mouse Model Coupled with Computational Analysis Identifies Potent Glycolipid Agonist of Invariant NKT Cells

Invariant natural killer T (iNKT) cells play an important role in many innate and adaptive immune responses, with potential applications in cancer immunotherapy. The glycolipid KRN7000, an α-galactosylceramide, potently activates iNKT cells but has shown limited anticancer effects in human clinical trials conducted so far. In spite of almost three decades of structure-activity relationship studies, no alternative glycolipid has yet emerged as a superior clinical candidate. One reason for the slow progress in this area is that standard mouse models do not accurately reflect the specific ligand recognition by human iNKT cells and their requirements for activation. Here we evaluated a series of KRN7000 analogues using a recently developed humanized mouse model that expresses a human αTCR chain sequence and human CD1d. In this process, a more stimulatory, previously reported but largely overlooked glycolipid was identified, and its activity was probed and rationalized via molecular simulations.

Functional prominence of natural killer cells and natural killer T cells in pregnancy and infertility: A comprehensive review and update

During pregnancy, complicated connections are formed between a mother and a fetus. In a successful pregnancy, the maternal-fetal interface is affected by dynamic changes, and the fetus is protected against the mother's immune system. Natural killer (NK) cells are one of the immune system cells in the female reproductive system that play an essential role in the physiology of pregnancy. NK cells not only exist in peripheral blood (PB) but also can exist in the decidua. Studies have suggested multiple roles for these cells, including decidualization, control of trophoblast growth and invasion, embryo acceptance and maintenance by the mother, and facilitation of placental development during pregnancy. Natural killer T (NKT) cells are another group of NK cells that play a crucial role in the maintenance of pregnancy and regulation of the immune system during pregnancy. Studies show that NK and NKT cells are not only effective in maintaining pregnancy but also can be involved in infertility-related diseases. This review focuses on NK and NKT cells biology and provides a detailed description of the functions of these cells in implantation, placentation, and immune tolerance during pregnancy and their role in pregnancy complications.

Comprehensive Analysis of the ILCs and Unconventional T Cells in Virus Infection: Profiling and Dynamics Associated with COVID-19 Disease for a Future Monitoring System and Therapeutic Opportunities

This review is a comprehensive analysis of the effects of SARS-CoV-2 infection on Unconventional T cells and innate lymphoid cells (ILCs). COVID-19 affected patients show dysregulation of their adaptive immune systems, but many questions remain unsolved on the behavior of Unconventional cells and ILCs during infection, considering their role in maintaining homeostasis in tissue. Therefore, we highlight the differences that exist among the studies in cohorts of patients who in general were categorized considering symptoms and hospitalization. Moreover, we make a critical analysis of the presence of particular clusters of cells that express activation and exhausted markers for each group in order to bring out potential diagnostic factors unconsidered before now. We also focus our attention on studies that take into consideration recovered patients. Indeed, it could be useful to determine Unconventional T cells’ and ILCs’ frequencies and functions in longitudinal studies because it could represent a way to monitor the immune status of SARS-CoV-2-infected subjects. Possible changes in cell frequencies or activation profiles could be potentially useful as prognostic biomarkers and for future therapy. Currently, there are no efficacious therapies for SARS-CoV-2 infection, but deep studies on involvement of Unconventional T cells and ILCs in the pathogenesis of COVID-19 could be promising for targeted therapies.

NKT Cells Contribute to the Control of Microbial Infections

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

Mucosal Microbiota and Metabolome in the Ileum of Hu Sheep Offered a Low-Grain, Pelleted or Non-pelleted High-Grain Diet

Alterations in mucosal microbiota and metabolites are critical to intestinal homeostasis and host health. This study used a combination of 16S rRNA gene sequencing and liquid chromatography-mass spectrometry (LC/MS) to investigate mucosal microbiota and their metabolic profiles in the ileum of Hu sheep fed different diets. Here, we randomly allocated 15 Hu sheep to three diets, a non-pelleted low-grain diet (control diet; CON), a non-pelleted high-grain diet (HG), and a pelleted high-grain diet (HP). After 60 days of treatment, ileal mucosal samples were collected for microbiome and metabolome analysis. The results of principal coordinate analysis and permutation multivariate analysis showed that there was a tendency for microbial differentiation between the CON and HG groups (P < 0.1), although no significant difference between the HG and HP groups was observed (P > 0.05). Compared with the CON diet, the HG diet decreased (P < 0.05) the abundance of some probiotic species (e.g., Sphingomonas and Candidatus Arthromitus) and increased (P < 0.05) the abundance of acid-producing microbiota (e.g., Succiniclasticum, Nesterenkonia, and Alloprevotella) in the ileal mucosa. Compared with the HG diet, the HP diet decreased (P < 0.05) the abundance of Alloprevotella and increased (P < 0.05) the abundance of Mycoplasma in the ileal mucosa. Furthermore, partial least squares discriminant analysis and orthogonal partial least-squared discriminant analysis indicated that different dietary treatments resulted in different metabolic patterns in the ileal mucosa of the CON, HG, and HP groups. The HG diet altered (VIP > 1 and P < 0.05) the metabolic patterns of amino acids, fatty acids, and nucleotides/nucleosides (such as increased amounts of ornithine, tyrosine, cis-9-palmitoleic acid, and adenosine) compared with the CON diet. However, 10 differential metabolites (VIP > 1 and P < 0.05; including tyrosine, ornithine, and cis-9-palmitoleic acid) identified in the HG group exhibited a diametrically opposite trend in the HP group, suggesting that the HP diet could partially eliminate the changes brought upon by the HG diet. Collectively, our findings demonstrate that different diets altered the ileal mucosal microbiota and metabolites and provide new insight into the effects of high-grain diets on the intestinal health of ruminant animals.

Regulation and Functions of Protumoral Unconventional T Cells in Solid Tumors

The vast majority of studies on T cell biology in tumor immunity have focused on peptide-reactive conventional T cells that are restricted to polymorphic major histocompatibility complex molecules. However, emerging evidence indicated that unconventional T cells, including γδ T cells, natural killer T (NKT) cells and mucosal-associated invariant T (MAIT) cells are also involved in tumor immunity. Unconventional T cells span the innate-adaptive continuum and possess the unique ability to rapidly react to nonpeptide antigens via their conserved T cell receptors (TCRs) and/or to activating cytokines to orchestrate many aspects of the immune response. Since unconventional T cell lineages comprise discrete functional subsets, they can mediate both anti- and protumoral activities. Here, we review the current understanding of the functions and regulatory mechanisms of protumoral unconventional T cell subsets in the tumor environment. We also discuss the therapeutic potential of these deleterious subsets in solid cancers and why further feasibility studies are warranted.

Structure, metabolism and biological functions of steryl glycosides in mammals

Steryl glycosides (SGs) are sterols glycosylated at their 3β-hydroxy group. They are widely distributed in plants, algae, and fungi, but are relatively rare in bacteria and animals. Glycosylation of sterols, resulting in important components of the cell membrane SGs, alters their biophysical properties and confers resistance against stress by freezing or heat shock to cells. Besides, many biological functions in animals have been suggested from the observations of SG administration. Recently, cholesteryl glucosides synthesized via the transglycosidation by glucocerebrosidases (GBAs) were found in the central nervous system of animals. Identification of patients with congenital mutations in GBA genes or availability of respective animal models will enable investigation of the function of such endogenously synthesized cholesteryl glycosides by genetic approaches. In addition, mechanisms of the host immune responses against pathogenic bacterial SGs have partially been resolved. This review is focused on the biological functions of SGs in mammals taking into consideration their therapeutic applications in the future.

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.

Activation of invariant natural killer T cells stimulated with microbial α-mannosyl glycolipids

Some synthetic and bacterial glycolipids presented by CD1d specifically activate invariant NKT (iNKT) cells bearing an invariant Vα14-Jα18 (mouse) or Vα24-Jα18 (human) TCR. The antigenic glycolipids identified to date consist of two hydrophobic chains and an α-glycoside in which the 2′-OH group is in the cis orientation toward the anomeric group, namely, either an α-galactoside or an α-glucoside. Several microbial α-mannosyl glycolipids, in which the 2′-OH group is in the trans orientation, were herein examined to establish whether they have potential to activate iNKT cells. We found that α-mannnosyl1-3 (6′-O-acyl α-mannosyl)-1-1 monoacylglycerol and cholesteryl 6′-O-acyl α-mannoside, found in Saccharopolyspora and Candida albicans, respectively, induced the activation of iNKT cells, dependent on CD1d. In contrast, α-mannosyldiacylglycerol found in Streptococcus suis or α-mannosylceramide demonstrated markedly less antigenicity for iNKT cells. The potentially antigenic α-mannosyl glycolipids contributed to the protection of mice against infection with S. pneumoniae in which iNKT cells have previously been found to participate. Furthermore, these glycolipids induced the production of proinflammatory cytokines by macrophages, thereby suggesting their recognition by specific pattern recognition receptors (PRRs). Collectively, these results suggest that these microbial α-mannosyl glycolipids are capable of being recognized by both the invariant TCR and PRRs and inducing immune responses.

The Role of Lipids in Development of Allergic Responses

Most allergic diseases are caused by activation of Th2 type immune responses resulting in the production of specific IgE against proteins found in normally harmless substances such as pollen, mites, epithelia or food. Allergenic substances are composed, in addition to proteins, of other compounds such as carbohydrates and lipids. Those lipids are able to promote the development of Th2-type responses associated with allergy. There are lipids found in pollen, milk or insect venom that are specifically recognized by CD1 restricted unconventional T lymphocytes, which can promote allergic reactions. Furthermore, a large number of allergens are proteins containing hydrophobic parts that specifically bind lipids that are capable to favor allergenic immune responses. Also, lipids associated to substances like pollen, dander, epithelia or the bacteria can act on cells of the innate system, including dendritic cells, which in turn lead to the differentiation of Th2-type clones. Finally, lipids may also influence the ability of allergens to be exposed to the immune system within the oral, respiratory or intestinal mucosa where allergic response occurs with great frequency.

Infiltration of invariant natural killer T cells occur and accelerate brain infarction in permanent ischemic stroke in mice

Invariant natural killer T (iNKT) cells are a unique subset of T cells that have been implicated in inflammation, atopy, autoimmunity, infections, and cancer. Although iNKT cells have been extensively studied over the past decade, its role in the pathogenesis of ischemic brain injury is still largely unknown. In our study, we determined whether iNKT cells infiltration occur in a mouse model of permanent cerebral ischemia. C57BL6/J male mice were treated with either alpha-galactosylceramide (α-GalCer) or vehicle control before undergoing permanent middle cerebral artery occlusion (pMCAO). α-GalCer, a glycolipid antigen, specifically activates iNKT cells by a CD1d-restricted mechanism. Using flow cytometry, 10,000 leukocytes (CD45 high cells) from the ischemic hemisphere and peripheral blood respectively were analyzed to determine the number of NK1.1⁺CD3⁺ cells at 3, 12, 24 and 48h post-pMCAO. Cerebral infarct size, brain edema and morphological characteristics were measured at the stipulated time points by 2,3,5-triphenyltetrazolium chloride (TTC) staining, weighing, and H&E staining. The levels of IFN-γ and TNF-α in brain tissue and serum were assessed by immunohistochemistry and ELISA respectively. We found that the number of iNKT cells started increasing from 12h (PB sample) and 24h (ischemic hemisphere sample) respectively in the vehicle treated group. iNKT cells infiltration occurred at an earlier time-point compared in the α-GalCer treated group (T=3H vs T=12H in PB sample; T=12H vs T=24H in ischemic hemisphere sample). Brain water content at 12h and 24h was significantly higher in pMCAO+α-GalCer mice compared to pMCAO+vehicle mice which was in turn higher than mice that underwent sham surgery. Aggravated morphological abnormalities in HE-stained neurons and significantly increased neurons with pyknotic nuclei and cavitation in the ischemic region were observed at 24h in the pMCAO+α-GalCer and pMCAO+vehicle groups. Cerebral infarct volume, neurological deficit Scores and brain edema were significantly increased at 24h in the pMCAO+α-GalCer group compared to pMCAO+vehicle group. In the pMCAO+vehicle group, the serum concentrations of TNF-α and IFN-γ were increased at 12h and 24h post-pMCAO, and remained elevated up to 48h. In mice treated with pMCAO+α-GalCer, TNF-α and IFN-γ were both increased at 12h post-pMCAO, and remained elevated up to 48h. Immunohistochemistry showed that protein expression of TNF-α and IFN-γ in brain tissues was higher in α-GalCer-treated mice. Our results demonstrate that within 48h of focal permanent cerebral ischemia, iNKT cells infiltrate into the brain and contribute to brain infarction.

The two faces of invariant natural killer T cells

In this issue of the Biomedical Journal, we take a look at some of the immune system's most peculiar cells, invariant natural killer T cells, which have features of both innate and adaptive cells. We also highlight a clinical study revealing that high serum phosphate levels could show that it's time to start dialysis in patients with chronic kidney diseases. Finally, this issue also includes some case reports, including an unusual case of aspergillosis related to long-term inhaler use.