CD1b-mediated T cell recognition of a glycolipid antigen generated from mycobacterial lipid and host carbohydrate during infection

Effect of Mycolic Acids on Host Immunity and Lipid Metabolism

Mycolic acids constitute pivotal constituents within the cell wall structure of Mycobacterium tuberculosis. Due to their structural diversity, the composition of mycolic acids exhibits substantial variations among different strains, endowing them with the distinctive label of being the 'signature' feature of mycobacterial species. Within Mycobacterium tuberculosis, the primary classes of mycolic acids include α-, keto-, and methoxy-mycolic acids. While these mycolic acids are predominantly esterified to the cell wall components (such as arabinogalactan, alginate, or glucose) of Mycobacterium tuberculosis, a fraction of free mycolic acids are secreted during in vitro growth of the bacterium. Remarkably, different types of mycolic acids possess varying capabilities to induce foamy macro-phages and trigger immune responses. Additionally, mycolic acids play a regulatory role in the lipid metabolism of host cells, thereby exerting influence over the progression of tuberculosis. Consequently, the multifaceted properties of mycolic acids shape the immune evasion strategy employed by Mycobacterium tuberculosis. A comprehensive understanding of mycolic acids is of paramount significance in the pursuit of developing tuberculosis therapeutics and unraveling the intricacies of its pathogenic mechanisms.

CD1-mediated immune responses in mucosal tissues: molecular mechanisms underlying lipid antigen presentation system

The cluster of differentiation 1 (CD1) molecule differs from major histocompatibility complex class I and II because it presents glycolipid/lipid antigens. Moreover, the CD1-restricted T cells that recognize these self and foreign antigens participate in both innate and adaptive immune responses. CD1s are constitutively expressed by professional and nonprofessional antigen-presenting cells in mucosal tissues, namely, the skin, lung, and intestine. This suggests that CD1-reactive T cells are involved in the immune responses of these tissues. Indeed, evidence suggests that these cells play important roles in diverse diseases, such as inflammation, autoimmune disease, and infection. Recent studies elucidating the molecular mechanisms by which CD1 presents lipid antigens suggest that defects in these mechanisms could contribute to the activities of CD1-reactive T cells. Thus, improving our understanding of these mechanisms could lead to new and effective therapeutic approaches to CD1-associated diseases. In this review, we discuss the CD1-mediated antigen presentation system and its roles in mucosal tissue immunity.

Polysaccharides and glycolipids of Mycobacterium tuberculosis and their induced immune responses

Tuberculosis (TB) is a chronic infectious disease mainly caused by Mycobacterium tuberculosis (M. tuberculosis). The structures of polysaccharides and glycolipids at M. tuberculosis cell wall vary among different strains, which affect the physiology and pathogenesis of mycobacteria by activating or inhibiting innate and acquired immunity. Among them, some components such as lipomannan (LM) and lipoarabinomannan (LAM) activate innate immunity by recognizing some kinds of pattern recognition receptors (PRRs) like Toll-like receptors, while other components like mannose-capped lipoarabinomannan (ManLAM) could prevent innate immune responses by inhibiting the secretion of pro-inflammatory cytokines and maturation of phagosomes. In addition, many glycolipids can activate natural killer T (NKT) cells and CD1-restricted T cells to produce interferon-γ (IFN-γ). Furthermore, humoral immunity against cell wall components, such as antibodies against LAM, plays a role in immunity against M. tuberculosis infection. Cell wall polysaccharides and glycolipids of M. tuberculosis have potential applications as antigens and adjuvants for novel TB subunit vaccines.

αβ T-cell receptor recognition of self-phosphatidylinositol presented by CD1b

CD1 glycoproteins present lipid-based antigens to T-cell receptors (TCRs). A role for CD1b in T-cell-mediated autoreactivity was proposed when it was established that CD1b can present self-phospholipids with short alkyl chains (∼C34) to T cells; however, the structural characteristics of this presentation and recognition are unclear. Here, we report the 1.9 Å resolution binary crystal structure of CD1b presenting a self-phosphatidylinositol-C34:1 and an endogenous scaffold lipid. Moreover, we also determined the 2.4 Å structure of CD1b-phosphatidylinositol complexed to an autoreactive αβ TCR, BC8B. We show that the TCR docks above CD1b and directly contacts the presented antigen, selecting for both the phosphoinositol headgroup and glycerol neck region via antigen remodeling within CD1b and allowing lateral escape of the inositol moiety through a channel formed by the TCR α-chain. Furthermore, through alanine scanning mutagenesis and surface plasmon resonance, we identified key CD1b residues mediating this interaction, with Glu-80 abolishing TCR binding. We in addition define a role for both CD1b α1 and CD1b α2 molecular domains in modulating this interaction. These findings suggest that the BC8B TCR contacts both the presented phospholipid and the endogenous scaffold lipid via a dual mechanism of corecognition. Taken together, these data expand our understanding into the molecular mechanisms of CD1b-mediated T-cell autoreactivity.

Mycobacterium tuberculosis H37Rv Strain Increases the Frequency of CD3+TCR+ Macrophages and Affects Their Phenotype, but Not Their Migration Ability

In mycobacterial infections, the number of cells from two newly discovered subpopulations of CD3⁺ myeloid cells are increased at the infection site; one type expresses the T cell receptor (CD3⁺TCRαβ⁺) and the other does not (CD3⁺TCRαβ⁻). The role of Mycobacterium tuberculosis (Mtb) virulence in generating these subpopulations and the ability of these cells to migrate remains unclear. In this study, monocyte-derived macrophages (MDMs) infected in vitro with either a virulent (H37Rv) or an avirulent (H37Ra) Mtb strain were phenotypically characterized based on three MDM phenotypes (CD3⁻, CD3⁺TCRαβ⁺, and CD3⁺TCRαβ⁻); then, their migration ability upon Mtb infection was evaluated. We found no differences in the frequency of CD3⁺ MDMs at 24 h of infection with either Mtb strain. However, H37Rv infection increased the frequency of CD3⁺TCRαβ⁺ MDMs at a multiplicity of infection of 1 and altered the expression of CD1b, CD1c, and TNF on the surface of cells from both the CD3⁺ MDM subpopulations; it also modified the expression of CCR2, CXCR1, and CCR7, thus affecting CCL2 and IL-8 levels. Moreover, H37Rv infection decreased the migration ability of the CD3⁻ MDMs, but not CD3⁺ MDMs. These results confirm that the CD3⁺ macrophage subpopulations express chemokine receptors that respond to chemoattractants, facilitating cell migration. Together, these data suggest that CD3⁺ MDMs are a functional subpopulation involved in the immune response against Mtb.

A simple assay to quantify mycobacterial lipid antigen-specific T cell receptors in human tissues and blood

T cell receptors (TCRs) encode the history of antigenic challenge within an individual and have the potential to serve as molecular markers of infection. In addition to peptide antigens bound to highly polymorphic MHC molecules, T cells have also evolved to recognize bacterial lipids when bound to non-polymorphic CD1 molecules. One such subset, germline-encoded, mycolyl lipid-reactive (GEM) T cells, recognizes mycobacterial cell wall lipids and expresses a conserved TCR-ɑ chain that is shared among genetically unrelated individuals. We developed a quantitative PCR assay to determine expression of the GEM TCR-ɑ nucleotide sequence in human tissues and blood. This assay was validated on plasmids and T cell lines. We tested blood samples from South African subjects with or without tuberculin reactivity or with active tuberculosis disease. We were able to detect GEM TCR-ɑ above the limit of detection in 92% of donors but found no difference in GEM TCR-ɑ expression among the three groups after normalizing for total TCR-ɑ expression. In a cohort of leprosy patients from Nepal, we successfully detected GEM TCR-ɑ in 100% of skin biopsies with histologically confirmed tuberculoid and lepromatous leprosy. Thus, GEM T cells constitute part of the T cell repertoire in the skin. However, GEM TCR-ɑ expression was not different between leprosy patients and control subjects after normalization. Further, these results reveal the feasibility of developing a simple, field deployable molecular diagnostic based on mycobacterial lipid antigen-specific TCR sequences that are readily detectable in human tissues and blood independent of genetic background.

Tanshinones from Salvia miltiorrhiza inhibit Mycobacterium tuberculosis via disruption of the cell envelope surface and oxidative stress

The unique structure of Mycobacterium tuberculosis cell envelope provides impermeable barrier against environmental stimuli. In the situation that this barrier is disturbed Mycobacteria react at the transcriptional and translational level to redirect metabolic processes and to maintain integrity of the cell. In this work we aimed to explore the early metabolic response of M. tuberculosis to tanshinones, which are active antimycobacterial compounds of Salvia miltiorrhiza Bunge root. The investigation of the expression of sigma factors revealed the significant shifts in the general bacterial regulatory network, whereas LC-MS metabolomics evidenced the changes in the composition of bacterial cell envelope and indicated altered metabolic pathways. Tanshinones acted via the disruption of the cell envelope surface and generation of reactive oxygen species. Bacteria responded with overproduction of inner region of outer membrane, fluctuations in the production of glycerophosphoinositolglycans, as well as changes in the levels of mycobactins, accompanied by enrichment of metabolic pathways related to redox balance and repair of damages caused by tanshinones.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Multimodally profiling memory T cells from a tuberculosis cohort identifies cell state associations with demographics, environment and disease

Multimodal T cell profiling can enable more precise characterization of elusive cell states underlying disease. Here, we integrated single-cell RNA and surface protein data from 500,089 memory T cells to define 31 cell states from 259 individuals in a Peruvian tuberculosis (TB) progression cohort. At immune steady state >4 years after infection and disease resolution, we found that, after accounting for significant effects of age, sex, season and genetic ancestry on T cell composition, a polyfunctional type 17 helper T (TH17) cell-like effector state was reduced in abundance and function in individuals who previously progressed from Mycobacterium tuberculosis (M.tb) infection to active TB disease. These cells are capable of responding to M.tb peptides. Deconvoluting this state-uniquely identifiable with multimodal analysis-from public data demonstrated that its depletion may precede and persist beyond active disease. Our study demonstrates the power of integrative multimodal single-cell profiling to define cell states relevant to disease and other traits.

T Cells Specific for a Mycobacterial Glycolipid Expand after Intravenous Bacillus Calmette-Guérin Vaccination

Intradermal vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) protects infants from disseminated tuberculosis, and i.v. BCG protects nonhuman primates (NHP) against pulmonary and extrapulmonary tuberculosis. In humans and NHP, protection is thought to be mediated by T cells, which typically recognize bacterial peptide Ags bound to MHC proteins. However, during vertebrate evolution, T cells acquired the capacity to recognize lipid Ags bound to CD1a, CD1b, and CD1c proteins expressed on APCs. It is unknown whether BCG induces T cell immunity to mycobacterial lipids and whether CD1-restricted T cells are resident in the lung. In this study, we developed and validated Macaca mulatta (Mamu) CD1b and CD1c tetramers to probe ex vivo phenotypes and functions of T cells specific for glucose monomycolate (GMM), an immunodominant mycobacterial lipid Ag. We discovered that CD1b and CD1c present GMM to T cells in both humans and NHP. We show that GMM-specific T cells are expanded in rhesus macaque blood 4 wk after i.v. BCG, which has been shown to protect NHP with near-sterilizing efficacy upon M. tuberculosis challenge. After vaccination, these T cells are detected at high frequency within bronchoalveolar fluid and express CD69 and CD103, markers associated with resident memory T cells. Thus, our data expand the repertoire of T cells known to be induced by whole cell mycobacterial vaccines, such as BCG, and show that lipid Ag-specific T cells are resident in the lungs, where they may contribute to protective immunity.

The role of donor-unrestricted T-cells, innate lymphoid cells, and NK cells in anti-mycobacterial immunity

Vaccination strategies against mycobacteria, focusing mostly on classical T‐ and B‐cells, have shown limited success, encouraging the addition of alternative targets. Classically restricted T‐cells recognize antigens presented via highly polymorphic HLA class Ia and class II molecules, while donor‐unrestricted T‐cells (DURTs), with few exceptions, recognize ligands via genetically conserved antigen presentation molecules. Consequently, DURTs can respond to the same ligands across diverse human populations. DURTs can be activated either through cognate TCR ligation or via bystander cytokine signaling. TCR‐driven antigen‐specific activation of DURTs occurs upon antigen presentation via non‐polymorphic molecules such as HLA‐E, CD1, MR1, and butyrophilin, leading to the activation of HLA‐E–restricted T‐cells, CD1‐restricted T‐cells, mucosal‐associated invariant T‐cells (MAITs), and TCRγδ T‐cells, respectively. NK cells and innate lymphoid cells (ILCs), which lack rearranged TCRs, are activated through other receptor‐triggering pathways, or can be engaged through bystander cytokines, produced, for example, by activated antigen‐specific T‐cells or phagocytes. NK cells can also develop trained immune memory and thus could represent cells of interest to mobilize by novel vaccines. In this review, we summarize the latest findings regarding the contributions of DURTs, NK cells, and ILCs in anti–M tuberculosis, M leprae, and non‐tuberculous mycobacterial immunity and explore possible ways in which they could be harnessed through vaccines and immunotherapies to improve protection against Mtb.

Animal models for human group 1 CD1 protein function

The CD1 antigen presenting system is evolutionary conserved and found in mammals, birds and reptiles. Humans express five isoforms, of which CD1a, CD1b and CD1c represent the group 1 CD1-molecules. They are recognized by T cells that express diverse αβ-T cell receptors. Investigation of the role of group 1 CD1 function has been hampered by the fact that CD1a, CD1b and CD1c are not expressed by mice. However, other animals, such as guinea pigs or cattle, serve as alternative models and have established basic aspects of CD1-dependent, antimicrobial immune functions. Group 1 CD1 transgenic mouse models became available about ten years ago. In a series of seminal studies these mouse models coined the mechanistical understanding of the role of the corresponding CD1 restricted T cell responses. This review gives a short overview of available animal studies and the lessons that have been and still can be learned.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Cholesteryl 6-O-acyl-α-glucosides from diverse Helicobacter spp. signal through the C-type lectin receptor Mincle

Helicobacter spp. are Gram-negative bacteria that cause a spectrum of disease in the gut, biliary tree and liver. Many Helicobacter spp. produce a range of cholesteryl α-glucosides that have the potential to act as pathogen associated molecular patterns. We report a highly stereoselective α-glucosylation of cholesterol using 3,4,6-tri-O-acetyl-2-O-benzyl-d-glucopyranosyl N-phenyl-2,2,2-trifluoroacetimidate, which allowed the synthesis of cholesteryl α-glucoside (αCG) and representative Helicobacter spp. cholesteryl 6-O-acyl-α-glucosides (αCAGs; acyl = C12:0, 14:0, C16:0, C18:0, C18:1). All αCAGs, irrespective of the nature of their acyl chain composition, strongly agonised signalling through the C-type lectin receptor Mincle from human and mouse to similar degrees. By contrast, αCG only weakly signalled through human Mincle, and did not signal through mouse Mincle. These results provide a molecular basis for understanding of the immunobiology of non-pylori Helicobacter infections in humans and other animals.

Total Synthesis of a Mycolic Acid from Mycobacterium tuberculosis

In Mycobacterium tuberculosis, mycolic acids and their glycerol, glucose, and trehalose esters ("cord factor") form the main part of the mycomembrane. Despite their first isolation almost a century ago, full stereochemical evaluation is lacking, as is a scalable synthesis required for accurate immunological, including vaccination, studies. Herein, we report an efficient, convergent, gram-scale synthesis of four stereo-isomers of a mycolic acid and its glucose ester. Binding to the antigen presenting protein CD1b and T cell activation studies are used to confirm the antigenicity of the synthetic material. The absolute stereochemistry of the syn-methoxy methyl moiety in natural material is evaluated by comparing its optical rotation with that of synthetic material.

The mycobacterial cell envelope - a moving target

Mycobacterium tuberculosis, the leading cause of death due to infection, has a dynamic and immunomodulatory cell envelope. The cell envelope structurally and functionally varies across the length of the cell and during the infection process. This variability allows the bacterium to manipulate the human immune system, tolerate antibiotic treatment and adapt to the variable host environment. Much of what we know about the mycobacterial cell envelope has been gleaned from model actinobacterial species, or model conditions such as growth in vitro, in macrophages and in the mouse. In this Review, we combine data from different experimental systems to build a model of the dynamics of the mycobacterial cell envelope across space and time. We describe the regulatory pathways that control metabolism of the cell wall and surface lipids in M. tuberculosis during growth and stasis, and speculate about how this regulation might affect antibiotic susceptibility and interactions with the immune system.

CD3+ Macrophages Deliver Proinflammatory Cytokines by a CD3- and Transmembrane TNF-Dependent Pathway and Are Increased at the BCG-Infection Site

Macrophages are essential cells of the innate immune response against microbial infections, and they have the ability to adapt under both pro- and anti-inflammatory conditions and develop different functions. A growing body of evidence regarding a novel macrophage subpopulation that expresses CD3 has recently emerged. Here, we explain that human circulating monocytes can be differentiated into CD3⁺TCRαβ⁺ and CD3⁺TCRαβ⁻ macrophages. Both cell subpopulations express on their cell surface HLA family molecules, but only the CD3⁺TCRαβ⁺ macrophage subpopulation co-express CD1 family molecules and transmembrane TNF (tmTNF). CD3⁺TCRαβ⁺ macrophages secrete IL-1β, IL-6 IP-10, and MCP-1 by both tmTNF- and CD3-dependent pathways, while CD3⁺TCRαβ⁻ macrophages specifically produce IFN-γ, TNF, and MIP-1β by a CD3-dependent pathway. In this study, we also used a mouse model of BCG-induced pleurisy and demonstrated that CD3⁺ myeloid cells (TCRαβ⁺ and TCRαβ⁻ cells) are increased at the infection sites during the acute phase (2 weeks post-infection). Interestingly, cell increment was mediated by tmTNF, and the soluble form of TNF was dispensable. BCG-infection also induced the expression of TNF receptor 2 on CD3⁺ myeloid cells, which increased after BCG-infection, suggesting that the tmTNF/TNFRs axis plays an important role in the presence or function of these cells in tuberculosis.

A TCR β-Chain Motif Biases toward Recognition of Human CD1 Proteins

High-throughput TCR sequencing allows interrogation of the human TCR repertoire, potentially connecting TCR sequences to antigenic targets. Unlike the highly polymorphic MHC proteins, monomorphic Ag-presenting molecules such as MR1, CD1d, and CD1b present Ags to T cells with species-wide TCR motifs. CD1b tetramer studies and a survey of the 27 published CD1b-restricted TCRs demonstrated a TCR motif in humans defined by the TCR β-chain variable gene 4-1 (TRBV4-1) region. Unexpectedly, TRBV4-1 was involved in recognition of CD1b regardless of the chemical class of the carried lipid. Crystal structures of two CD1b-specific TRBV4-1⁺ TCRs show that germline-encoded residues in CDR1 and CDR3 regions of TRBV4-1-encoded sequences interact with each other and consolidate the surface of the TCR. Mutational studies identified a key positively charged residue in TRBV4-1 and a key negatively charged residue in CD1b that is shared with CD1c, which is also recognized by TRBV4-1 TCRs. These data show that one TCR V region can mediate a mechanism of recognition of two related monomorphic Ag-presenting molecules that does not rely on a defined lipid Ag.

The role of low molecular weight thiols in Mycobacterium tuberculosis

Low molecular weight (LMW) thiols are molecules with a functional sulfhydryl group that enable them to detoxify reactive oxygen species, reactive nitrogen species and other free radicals. Their roles range from their ability to modulate the immune system to their ability to prevent damage of biological molecules such as DNA and proteins by protecting against oxidative, nitrosative and acidic stress. LMW thiols are synthesized and found in both eukaryotes and prokaryotes. Due to their beneficial role to both eukaryotes and prokaryotes, their specific functions need to be elucidated, most especially in pathogenic prokaryotes such as Mycobacterium tuberculosis (M.tb), in order to provide a rationale for targeting their biosynthesis for drug development. Ergothioneine (ERG), mycothiol (MSH) and gamma-glutamylcysteine (GGC) are LMW thiols that have been shown to interplay to protect M.tb against cellular stress. Though ERG, MSH and GGC seem to have overlapping functions, studies are gradually revealing their unique physiological roles. Understanding their unique physiological role during the course of tuberculosis (TB) infection, would pave the way for the development of drugs that target their biosynthetic pathway. This review identifies the knowledge gap in the unique physiological roles of LMW thiols and proposes their mechanistic roles based on previous studies. In addition, it gives an update on identified inhibitors of their biosynthetic enzymes.

Validation of a CD1b tetramer assay for studies of human mycobacterial infection or vaccination

CD1 tetramers loaded with lipid antigens facilitate the identification of rare lipid-antigen specific T cells present in human blood and tissue. Because CD1 proteins are structurally non-polymorphic, these tetramers can be applied to genetically diverse human populations, unlike MHC-I and MHC-II tetramers. However, there are no standardized assays to quantify and characterize lipid antigen-specific T cells present within clinical samples. We incorporated CD1b tetramers loaded with the mycobacterial lipid glucose monomycolate (GMM) into a multi-parameter flow cytometry assay. Using a GMM-specific T-cell line, we demonstrate that the assay is linear, reproducible, repeatable, precise, accurate, and has a limit of detection of approximately 0.007%. Having formally validated this assay, we performed a cross-sectional study of healthy U.S. controls and South African adolescents with and without latent tuberculosis infection (LTBI). We show that GMM-specific T cells are specifically detected in South African subjects with LTBI and not in U.S. healthy controls. This assay can be expanded to include additional tetramers or phenotypic markers to characterize GMM-specific T cells in studies of mycobacterial infection, disease, or vaccination.

Cell envelope lipids in the pathophysiology of Mycobacterium tuberculosis

Mycobacterium tuberculosis is an intracellular bacterium that persists and replicates inside macrophages. The bacterium possesses an unusual lipid-rich cell envelope that provides a hydrophobic impermeable barrier against many environmental stressors and allows it to survive extremely hostile intracellular surroundings. Since the lipid-rich envelope is crucial for M. tuberculosis virulence, the components of the cell wall lipid biogenesis pathways constitute an attractive target for the development of vaccines and antimycobacterial chemotherapeutics. In this review, we provide a detailed description of the mycobacterial cell envelope lipid components and their contributions to the physiology and pathogenicity of mycobacteria. We also discussed the current status of the antimycobacterial drugs that target biosynthesis, export and regulation of cell envelope lipids.

Lipid structure influences the ability of glucose monocorynomycolate to signal through Mincle

Mincle (macrophage-inducible C-type lectin) is a C-type lectin receptor that provides the capacity for immune sensing of a range of pathogen- and commensal-derived glycolipids. Mincle can recognize mycolic and/or corynomycolic acid esters of trehalose, glycerol and glucose from mycobacteria and corynebacteria. While simple straight-chain long fatty acids (e.g. behenic acid) can substitute for mycolic acid on trehalose and glycerol and maintain robust signalling through Mincle, glucose monobehenate has been reported to be much less active than glucose monocorynomycolate (GMCM). We report the preparation of a range of analogues of GMCM to explore structural requirements in the lipid chain for signalling through Mincle. GMCM analogues bearing simple straight chain or branched fatty acid esters provided only weak signalling through human and mouse Mincle. A GMCM variant with a truncated (pentyl) α-chain provided attenuated signalling, whereas an analogue with an extended (tricosyl; C23) α-chain signalled as potently as GMCM. This work suggests that Mincle has the ability to survey mycolate-derived glycolipids from actinomycetes, distinguishing non-pathogenic (e.g. Rhodococcus spp.) and pathogenic (e.g. Mycobacterium tuberculosis) species on the basis of α-chain length. Finally, an α-phenyldodecyl analogue of GMCM possessed similar potency to GMCM and was only slightly less potent than trehalose dimycolate (cord factor), showing that large functional groups may be tolerated in the α-chain.

The Janus Face of NKT Cell Function in Autoimmunity and Infectious Diseases

Natural killer T cells (NKT) are a subset of T lymphocytes bridging innate and adaptive immunity. These cells recognize self and microbial glycolipids bound to non-polymorphic and highly conserved CD1d molecules. Three NKT cell subsets, type I, II, and NKT-like expressing different antigen receptors (TCR) were described and TCR activation promotes intracellular events leading to specific functional activities. NKT can exhibit different functions depending on the secretion of soluble molecules and the interaction with other cell types. NKT cells act as regulatory cells in the defense against infections but, on the other hand, their effector functions can be involved in the pathogenesis of several inflammatory disorders due to their exposure to different microbial or self-antigens, respectively. A deep understanding of the biology and functions of type I, II, and NKT-like cells as well as their interplay with cell types acting in innate (neuthrophils, innate lymphoid cells, machrophages, and dendritic cells) and adaptive immunity (CD4⁺,CD8⁺, and double negative T cells) should be important to design potential immunotherapies for infectious and autoimmune diseases.

CD1b-restricted GEM T cell responses are modulated by Mycobacterium tuberculosis mycolic acid meromycolate chains

Tuberculosis (TB), caused by Mycobacterium tuberculosis, remains a major human pandemic. Germline-encoded mycolyl lipid-reactive (GEM) T cells are donor-unrestricted and recognize CD1b-presented mycobacterial mycolates. However, the molecular requirements governing mycolate antigenicity for the GEM T cell receptor (TCR) remain poorly understood. Here, we demonstrate CD1b expression in TB granulomas and reveal a central role for meromycolate chains in influencing GEM-TCR activity. Meromycolate fine structure influences T cell responses in TB-exposed individuals, and meromycolate alterations modulate functional responses by GEM-TCRs. Computational simulations suggest that meromycolate chain dynamics regulate mycolate head group movement, thereby modulating GEM-TCR activity. Our findings have significant implications for the design of future vaccines that target GEM T cells.

CD1b-mycolic acid tetramers demonstrate T-cell fine specificity for mycobacterial lipid tails

Mycobacterium tuberculosis synthesizes a thick cell wall comprised of mycolic acids (MA), which are foreign antigens for human T cells. T-cell clones from multiple donors were used to determine the fine specificity of MA recognition by human αβ T cells. Most CD1-presented lipid antigens contain large hydrophilic head groups comprised of carbohydrates or peptides that dominate patterns of T-cell specificity. MA diverges from the consensus antigen motif in that it lacks a head group. Using multiple forms of natural and synthetic MA and MA-specific T-cells with different T-cell receptors, we found that, unlike antigens with larger head groups, lipid length strongly controlled T-cell responses to MA. In addition, the three forms of MA that naturally occur in M. tuberculosis that differ in modifications on the lipid tail, differ in their potency for activating MA-specific T-cell clones. Thus, naturally occurring MA forms should be considered as separate, partly cross-reactive antigens. Two of the three forms of MA could be loaded onto human CD1b proteins, creating working CD1b-MA tetramers. The creation of CD1b-MA tetramers represents a new tool for future studies that track the effector functions and kinetics of MA-specific T-cells ex vivo.

T cell receptor recognition of CD1b presenting a mycobacterial glycolipid

CD1 proteins present microbial lipids to T cells. Germline-encoded mycolyl lipid-reactive (GEM) T cells with conserved αβ T cell receptors (TCRs) recognize CD1b presenting mycobacterial mycolates. As the molecular basis underpinning TCR recognition of CD1b remains unknown, here we determine the structure of a GEM TCR bound to CD1b presenting glucose-6-O-monomycolate (GMM). The GEM TCR docks centrally above CD1b, whereby the conserved TCR α-chain extensively contacts CD1b and GMM. Through mutagenesis and study of T cells from tuberculosis patients, we identify a consensus CD1b footprint of TCRs present among GEM T cells. Using both the TCR α- and β-chains as tweezers to surround and grip the glucose moiety of GMM, GEM TCRs create a highly specific mechanism for recognizing this mycobacterial glycolipid.

Germline-encoded mycolyl lipid-reactive (GEM) T cells recognize CD1b proteins presenting mycobacterial mycolates via their T-cell receptors (TCRs). Here, the authors present the structure of this interaction and provide a molecular basis for the co-recognition of CD1b and a mycobacterial glycolipid.

Mycolic acid-specific T cells protect against Mycobacterium tuberculosis infection in a humanized transgenic mouse model

Group 1 CD1 molecules, CD1a, CD1b and CD1c, present lipid antigens from Mycobacterium tuberculosis (Mtb) to T cells. Mtb lipid-specific group 1 CD1-restricted T cells have been detected in Mtb-infected individuals. However, their role in protective immunity against Mtb remains unclear due to the absence of group 1 CD1 expression in mice. To overcome the challenge, we generated mice that expressed human group 1 CD1 molecules (hCD1Tg) and a CD1b-restricted, mycolic-acid specific TCR (DN1Tg). Using DN1Tg/hCD1Tg mice, we found that activation of DN1 T cells was initiated in the mediastinal lymph nodes and showed faster kinetics compared to Mtb Ag85B-specific CD4⁺ T cells after aerosol infection with Mtb. Additionally, activated DN1 T cells exhibited polyfunctional characteristics, accumulated in lung granulomas, and protected against Mtb infection. Therefore, our findings highlight the vaccination potential of targeting group 1 CD1-restricted lipid-specific T cells against Mtb infection.

DOI:http://dx.doi.org/10.7554/eLife.08525.001

Most cases of tuberculosis are caused by a bacterium called Mycobacterium tuberculosis, which is believed to have infected one third of the world’s population. Most of these infections are dormant and don’t cause any symptoms. However, active infections can be deadly if left untreated and often require six months of treatment with multiple antibiotics. One reason why these infections are so difficult to treat is because the M. tuberculosis cell walls contain fatty molecules known as mycolic acids, which make the bacteria less susceptible to antibiotics. These molecules also help the bacteria to subvert and then hide from the immune system.

The prevalence of the disease and the increasing problem of antibiotic resistance have spurred the search for an effective vaccine against tuberculosis. While most efforts have focused on using protein fragments in tuberculosis vaccines, some evidence suggests that human immune cells can recognize fatty molecules such as mycolic acids and that these cells could help manage and control M. tuberculosis infections. However, it has been difficult to determine whether these immune cells genuinely play a protective role against the disease because most vaccine research uses mouse models and mice do not have an equivalent of these immune cells.

Now, Zhao et al. have engineered a “humanized” mouse model that produces the fatty molecule-specific immune cells, and show that these mice do respond to the presence of mycolic acids. Infecting the genetically engineered mice with M. tuberculosis revealed that the fatty molecule-specific immune cells were quickly activated within lymph nodes at the center of the chest. These cells later accumulated at sites in the lung where the bacteria reside, and ultimately protected against M. tuberculosis infection. The results show that these specific immune cells can counteract M. tuberculosis, and highlight the potential of using mycolic acids to generate an effective vaccine that provides protection against tuberculosis.

DOI:http://dx.doi.org/10.7554/eLife.08525.002

Expression of CD1a and Type-1 Polarization Are Dissociated in Human Monocyte-Derived Dendritic Cells

Ex vivo generated monocyte-derived dendritic cell (moDC)-vaccines have long been touted as promising immunotherapeutic agents for cancer treatment, although the response rate generally remains low. The reasons for this are still unclear and confounded by the diversity in manufacturing protocols that may affect moDC function. Preclinical studies have shown that the stimulatory function of dendritic cells can be improved by engaging invariant NKT cells in vivo through the presentation of the glycolipid alpha-galactosylceramide via CD1d. However, expression of CD1d on moDC has been shown to be negatively correlated with expression of CD1a, which in turn has been suggested to be a surrogate marker for IL-12 secreting type-1 polarized moDC, the preferred functional characteristics for cancer vaccines. Here we challenge this notion by showing that plasma-derived lipids drive functional levels of CD1d expression, while CD1a expression can vary considerably in these cells without being correlated with a loss of polarization or immunogenicity.

Recognition of Microbial Glycolipids by Natural Killer T Cells

T cells can recognize microbial antigens when presented by dedicated antigen-presenting molecules. While peptides are presented by classical members of the major histocompatibility complex (MHC) family (MHC I and II), lipids, glycolipids, and lipopeptides can be presented by the non-classical MHC member, CD1. The best studied subset of lipid-reactive T cells are type I natural killer T (iNKT) cells that recognize a variety of different antigens when presented by the non-classical MHCI homolog CD1d. iNKT cells have been shown to be important for the protection against various microbial pathogens, including B. burgdorferi, the causative agents of Lyme disease, and S. pneumoniae, which causes pneumococcal meningitis and community-acquired pneumonia. Both pathogens carry microbial glycolipids that can trigger the T cell antigen receptor (TCR), leading to iNKT cell activation. iNKT cells have an evolutionary conserved TCR alpha chain, yet retain the ability to recognize structurally diverse glycolipids. They do so using a conserved recognition mode, in which the TCR enforces a conserved binding orientation on CD1d. TCR binding is accompanied by structural changes within the TCR binding site of CD1d, as well as the glycolipid antigen itself. In addition to direct recognition of microbial antigens, iNKT cells can also be activated by a combination of cytokines (IL-12/IL-18) and TCR stimulation. Many microbes carry TLR antigens, and microbial infections can lead to TLR activation. The subsequent cytokine response in turn lower the threshold of TCR-mediated iNKT cell activation, especially when weak microbial or even self-antigens are presented during the cause of the infection. In summary, iNKT cells can be directly activated through TCR triggering of strong antigens, while cytokines produced by the innate immune response may be necessary for TCR triggering and iNKT cell activation in the presence of weak antigens. Here, we will review the molecular basis of iNKT cell recognition of glycolipids, with an emphasis on microbial glycolipids.

Role of Group 1 CD1-Restricted T Cells in Infectious Disease

The evolutionarily conserved CD1 family of antigen-presenting molecules presents lipid antigens rather than peptide antigens to T cells. CD1 molecules, unlike classical MHC molecules, display limited polymorphism, making CD1-restricted lipid antigens attractive vaccine targets that could be recognized in a genetically diverse human population. Group 1 CD1 (CD1a, CD1b, and CD1c)-restricted T cells have been implicated to play critical roles in a variety of autoimmune and infectious diseases. In this review, we summarize current knowledge and recent discoveries on the development of group 1 CD1-restricted T cells and their function in different infection models. In particular, we focus on (1) newly identified microbial and self-lipid antigens, (2) kinetics, phenotype, and unique properties of group 1 CD1-restricted T cells during infection, and (3) the similarities of group 1 CD1-restricted T cells to the closely related group 2 CD1-restricted T cells.

Trehalolipid biosurfactants from nonpathogenic Rhodococcus actinobacteria with diverse immunomodulatory activities

Actinobacteria of the genus Rhodococcus produce trehalolipid biosurfactants with versatile biochemical properties and low toxicity. In recent years, these biosurfactants are increasingly studied as possible biomedical agents with expressed immunological activities. Applications of trehalolipids from Rhodococcus, predominantly cell-bound, in biomedicine are also attractive because their cost drawback could be less significant for high-value products. The review summarizes recent findings in immunomodulatory activities of trehalolipid biosurfactants from nonpathogenic Rhodococcus and related actinobacteria and compares their biomedical potential with well-known immunomodifying properties of trehalose dimycolates from Mycobacterium tuberculosis. Molecular mechanisms of trehalolipid interactions with immunocompetent cells are also discussed.

Corynomycolic acid-containing glycolipids signal through the pattern recognition receptor Mincle

Glucose monocorynomycolate is revealed to signal through both mouse and human Mincle. Glycerol monocorynomycolate is shown to selectively signal through human Mincle, with the activity residing predominantly in the 2′S-isomer.

Role of host- and pathogen-associated lipids in directing the immune response in mycobacterial infections, with emphasis on Mycobacterium avium subsp. paratuberculosis

Mycobacteria have a complex cell wall with a high lipid content that confers unique advantages for bacterial survival in the hostile host environment, leading to long-term infection. There is a wealth of evidence suggesting the role cell wall-associated lipid antigens play at the host-pathogen interface by contributing to bacterial virulence. One pathway that pathogenic mycobacteria use to subvert host immune pathways to their advantage is host cholesterol/lipid homeostasis. This review focuses on the possible role of pathogen- and host-associated lipids in the survival and persistence of pathogenic mycobacteria with emphasis on Mycobacterium avium subsp. paratuberculosis. We draw upon literature in diverse areas of infectious and metabolic diseases and explain a mechanism by which mycobacterial-induced changes in the host cellular energy state could account for phenomena that are a hallmark of chronic mycobacterial diseases.

Nonclassical T cells and their antigens in tuberculosis

T cells that recognize nonpeptidic antigens, and thereby are identified as nonclassical, represent important yet poorly characterized effectors of the immune response. They are present in large numbers in circulating blood and tissues and are as abundant as T cells recognizing peptide antigens. Nonclassical T cells exert multiple functions including immunoregulation, tumor control, and protection against infections. They recognize complexes of nonpeptidic antigens such as lipid and glycolipid molecules, vitamin B2 precursors, and phosphorylated metabolites of the mevalonate pathway. Each of these antigens is presented by antigen-presenting molecules other than major histocompatibility complex (MHC), including CD1, MHC class I-related molecule 1 (MR1), and butyrophilin 3A1 (BTN3A1) molecules. Here, we discuss how nonclassical T cells participate in the recognition of mycobacterial antigens and in the mycobacterial-specific immune response.

Targeted delivery of mycobacterial antigens to human dendritic cells via Siglec-7 induces robust T cell activation

Lipids from mycobacteria can be presented to human T cells by group 1 CD1 Ag-presenting molecules (CD1a, CD1b, and CD1c). Group 1 CD1-restricted T cells are activated by lipid Ags presented by myeloid dendritic cells (DCs), after which they generate antibacterial effector functions, including IFN-γ secretion and cytolysis. Thus, mycobacterial lipids are being investigated as components of novel vaccines for mycobacterial infections. In this study we show that the mycobacterial lipid Ag C80 glucose-6-monomycolate can be delivered to human CD1b(+) DCs via targeted liposomal nanoparticles, leading to robust group 1 CD1-restricted activation of T cells. Targeting was achieved by decorating the liposomes with a high-affinity glycan ligand of sialic acid-binding Ig-like lectin (Siglec)-7, a siglec receptor expressed on DCs that mediates rapid endocytosis and transport of its cargo to lysosomes. An Ab to Siglec-7 completely blocked the binding of targeted liposomes to human monocyte-derived DCs (Mo-DCs), demonstrating their targeting specificity. Mo-DCs pulsed with targeted liposomes containing C80 glucose-6-monomycolate more potently activated a CD1b-restricted T cell line relative to Mo-DCs pulsed with free lipid Ag or antigenic liposomes without Siglec-7 ligand. These data suggest that the endocytic function of Siglec-7 can be exploited to deliver glycolipid Ags to their target cell and increase the efficiency of display to T cells.

The T-Cell Response to Lipid Antigens of Mycobacterium tuberculosis

T-cells recognize lipid antigens presented by dedicated antigen-presenting molecules that belong to the CD1 family. This review discusses the structural properties of CD1 molecules, the nature of mycobacterial lipid antigens, and the phenotypic and functional properties of T-cells recognizing mycobacterial lipids. In humans, the five CD1 genes encode structurally similar glycoproteins that recycle in and thus survey different cellular endosomal compartments. The structure of the CD1-lipid-binding pockets, their mode of intracellular recycling and the type of CD1-expressing antigen-presenting cells all contribute to diversify lipid immunogenicity and presentation to T-cells. Mycobacteria produce a large variety of lipids, which form stable complexes with CD1 molecules and stimulate specific T-cells. The structures of antigenic lipids may be greatly different from each other and each lipid may induce unique T-cells capable of discriminating small lipid structural changes. The important functions of some lipid antigens within mycobacterial cells prevent the generation of negative mutants capable of escaping this type of immune response. T-cells specific for lipid antigens are stimulated in tuberculosis and exert protective functions. The mechanisms of antigen recognition, the type of effector functions and the mode of lipid-specific T-cell priming are discussed, emphasizing recent evidence of the roles of lipid-specific T-cells in tuberculosis.

Mycobacterial Lipidomics

Lipidomics is a distinct subspecialty of metabolomics concerned with hydrophobic molecules that organize into membranes. Most of the lipid classes present in Mycobacterium tuberculosis are found only in Actinobacteria and show extreme structural diversity. This article highlights the conceptual basis and the practical challenges associated with the mass spectrometry–based lipidomic study of M. tuberculosis to solve basic questions about the virulence of this lipid-laden organism.

Host-pathogen interactions during Mycobacterium tuberculosis infections

The intimate and persistent connection between Mycobacterium tuberculosis and its human host suggests that the pathogen has evolved extensive mechanisms to evade eradication by the immune system. In particular, the organism has adapted to replicate within phagocytic cells, especially macrophages, which are specialized to kill microbes. Over the past decade of M. tuberculosis research, the means to manipulate both the organism and the host has ushered in an exciting time that has uncovered some of the mechanisms of the innate macrophage-pathogen interactions that lie at the heart of M. tuberculosis pathogenesis, though many interactions likely still await discovery. In this chapter, we will delve into some of these advances, with an emphasis on the interactions that occur on the cellular level when M. tuberculosis cells encounter macrophages. In particular, we focus on two major aspects of M. tuberculosis biology regarding the proximal physical interface between the bacterium and host, namely the interactions with the phagosomal membrane as well as the distinctive mycobacterial cell wall. Importantly, some of the emerging paradigms in M. tuberculosis pathogenesis and host response represent common themes in bacterial pathogenesis, such as the role of host cell membrane perforation in intracellular survival and host response. However, the array of unique bacterial lipid mediators and their interaction with host cells highlights the unique biology of this persistent pathogen.

T-cell activation by transitory neo-antigens derived from distinct microbial pathways

T cells discriminate between foreign and host molecules by recognizing distinct microbial molecules, predominantly peptides and lipids. Riboflavin precursors found in many bacteria and yeast also selectively activate mucosal-associated invariant T (MAIT) cells, an abundant population of innate-like T cells in humans. However, the genesis of these small organic molecules and their mode of presentation to MAIT cells by the major histocompatibility complex (MHC)-related protein MR1 (ref. 8) are not well understood. Here we show that MAIT-cell activation requires key genes encoding enzymes that form 5-amino-6-d-ribitylaminouracil (5-A-RU), an early intermediate in bacterial riboflavin synthesis. Although 5-A-RU does not bind MR1 or activate MAIT cells directly, it does form potent MAIT-activating antigens via non-enzymatic reactions with small molecules, such as glyoxal and methylglyoxal, which are derived from other metabolic pathways. The MAIT antigens formed by the reactions between 5-A-RU and glyoxal/methylglyoxal were simple adducts, 5-(2-oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU) and 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU), respectively, which bound to MR1 as shown by crystal structures of MAIT TCR ternary complexes. Although 5-OP-RU and 5-OE-RU are unstable intermediates, they became trapped by MR1 as reversible covalent Schiff base complexes. Mass spectra supported the capture by MR1 of 5-OP-RU and 5-OE-RU from bacterial cultures that activate MAIT cells, but not from non-activating bacteria, indicating that these MAIT antigens are present in a range of microbes. Thus, MR1 is able to capture, stabilize and present chemically unstable pyrimidine intermediates, which otherwise convert to lumazines, as potent antigens to MAIT cells. These pyrimidine adducts are microbial signatures for MAIT-cell immunosurveillance.

TCR bias and affinity define two compartments of the CD1b-glycolipid-specific T Cell repertoire

Current views emphasize TCR diversity as a key feature that differentiates the group 1 (CD1a, CD1b, CD1c) and group 2 (CD1d) CD1 systems. Whereas TCR sequence motifs define CD1d-reactive NKT cells, the available data do not allow a TCR-based organization of the group 1 CD1 repertoire. The observed TCR diversity might result from donor-to-donor differences in TCR repertoire, as seen for MHC-restricted T cells. Alternatively, diversity might result from differing CD1 isoforms, Ags, and methods used to identify TCRs. Using CD1b tetramers to isolate clones recognizing the same glycolipid, we identified a previously unknown pattern of V gene usage (TRAV17, TRBV4-1) among unrelated human subjects. These TCRs are distinct from those present on NKT cells and germline-encoded mycolyl lipid-reactive T cells. Instead, they resemble the TCR of LDN5, one of the first known CD1b-reactive clones that was previously thought to illustrate the diversity of the TCR repertoire. Interdonor TCR conservation was observed in vitro and ex vivo, identifying LDN5-like T cells as a distinct T cell type. These data support TCR-based organization of the CD1b repertoire, which consists of at least two compartments that differ in TCR sequence motifs, affinity, and coreceptor expression.

Characterization of the receptors for mycobacterial cord factor in Guinea pig

Guinea pig is a widely used animal for research and development of tuberculosis vaccines, since its pathological disease process is similar to that present in humans. We have previously reported that two C-type lectin receptors, Mincle (macrophage inducible C-type lectin, also called Clec4e) and MCL (macrophage C-type lectin, also called Clec4d), recognize the mycobacterial cord factor, trehalose-6,6′-dimycolate (TDM). Here, we characterized the function of the guinea pig homologue of Mincle (gpMincle) and MCL (gpMCL). gpMincle directly bound to TDM and transduced an activating signal through ITAM-bearing adaptor molecule, FcRγ. Whereas, gpMCL lacked C-terminus and failed to bind to TDM. mRNA expression of gpMincle was detected in the spleen, lymph nodes and peritoneal macrophages and it was strongly up-regulated upon stimulation of zymosan and TDM. The surface expression of gpMincle was detected on activated macrophages by a newly established monoclonal antibody that also possesses a blocking activity. This antibody potently suppressed TNF production in BCG-infected macrophages. Collectively, gpMincle is the TDM receptor in the guinea pig and TDM-Mincle axis is involved in host immune responses against mycobacteria.

Mycolic acids: structures, biosynthesis, and beyond

Mycolic acids are major and specific lipid components of the mycobacterial cell envelope and are essential for the survival of members of the genus Mycobacterium that contains the causative agents of both tuberculosis and leprosy. In the alarming context of the emergence of multidrug-resistant, extremely drug-resistant, and totally drug-resistant tuberculosis, understanding the biosynthesis of these critical determinants of the mycobacterial physiology is an important goal to achieve, because it may open an avenue for the development of novel antimycobacterial agents. This review focuses on the chemistry, structures, and known inhibitors of mycolic acids and describes progress in deciphering the mycolic acid biosynthetic pathway. The functional and key biological roles of these molecules are also discussed, providing a historical perspective in this dynamic area.

T cell recognition of non-peptidic antigens in infectious diseases

The immune system has evolved to recognize a wide range of antigenic molecules of self and non-self origin. The stimulatory antigens form complexes with antigen-presenting molecules and directly interact with the T cell receptor (TCR). Peptidic antigens associate with major histocompatibility complex (MHC) molecules and therefore, are indicated as MHC-restricted. Non-peptidic antigens do not bind to MHC molecules and are presented by other classes of antigen-presenting molecules. These non-MHC restricted antigens include glycolipid molecules, phosphorylated metabolites of the mevalonate pathway and vitamin B2 precursors. T cells specific for non-peptidic antigens have important roles in host defense against infections, autoimmunity, allergies and tumour immunosurveillance. Hence, understanding the molecular interactions between the antigen presenting cell (APC) and the T cells with non-peptidic specificity is of great relevance. Here, we review current knowledge of this type of T cells, their TCR repertoire, the structural aspects of recognized antigens, the mode of antigen recognition, and their function with special emphasis on their role in infectious diseases.

A conserved human T cell population targets mycobacterial antigens presented by CD1b

T cell receptors (TCRs) pair in millions of combinations to create complex and personally unique T cell repertoires. Using tetramers to analyze CD1b-reactive TCRs, we detected T cells with highly stereotyped TCR α chains present among genetically unrelated tuberculosis patients. These germline-encoded mycolyl-reactive (GEM) T cells were defined by CD4 expression and rearrangement of TRAV1-2 to TRAJ9 with few N-region additions. TCR analysis by high throughput sequencing, binding and crystallography showed linkage of TCR α sequence motifs to high affinity antigen recognition. Thus, the CD1-reactive TCR repertoire is composed of at least two compartments, high affinity GEM TCRs and more diverse TCRs with low affinity for CD1b-lipid complexes. These data demonstrate high inter-donor conservation of TCRs, which likely results from selection by a non-polymorphic antigen presenting molecule and an immunodominant antigen.

Development of vizantin, a safe immunostimulant, based on the structure-activity relationship of trehalose-6,6'-dicorynomycolate

Vizantin, 6,6'-bis-O-(3-nonyldodecanoyl)-α,α'-trehalose, was developed as a safe immunostimulator on the basis of a structure-activity relationship (SAR) study with trehalose 6,6'-dicorynomycolate (TDCM). It was possible to synthesize vizantin on a large scale more easily than in the case of TDCM, and the compound exhibited more potent prophylactic effect on experimental lung metastasis of B16-F0 melanoma cells. Because vizantin stimulated human macrophages, it is a promising candidate for clinical application.

Towards understanding the functional diversity of cell wall mycolic acids of Mycobacterium tuberculosis

Mycolic acids constitute the waxy layer of the outer cell wall of Mycobacterium spp. and a few other genera. They are diverse in structure, providing a unique chromatographic foot-print for almost each of the more than 70 Mycobacterium species. Although mainly esterified to cell wall arabinogalactan, trehalose or glucose, some free mycolic acid is secreted during in vitro growth of Mycobacterium tuberculosis. In M. tuberculosis, α-, keto- and methoxy-mycolic acids are the main classes, each differing in their ability to attract neutrophils, induce foamy macrophages or adopt an antigenic structure for antibody recognition. Of interest is their particular relationship to cholesterol, discovered by their ability to attract cholesterol, to bind Amphotericin B or to be recognised by monoclonal antibodies that cross-react with cholesterol. The structural elements that determine this diverse functionality include the carboxylic acid in the mycolic motif, as well as the nature and stereochemistry of the two functional groups in the merochain. The functional diversity of mycolic acid classes implies that much information may be contained in the selective expression and secretion of mycolic acids to establish tuberculosis after infection of the host. Their cholesteroid nature may relate to how they utilize host cholesterol for their persistent survival.