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.

Identification of a Potent Microbial Lipid Antigen for Diverse NKT Cells

Semi-invariant/type I NKT cells are a well-characterized CD1d-restricted T cell subset. The availability of potent Ags and tetramers for semi-invariant/type I NKT cells allowed this population to be extensively studied and revealed their central roles in infection, autoimmunity, and tumor immunity. In contrast, diverse/type II NKT (dNKT) cells are poorly understood because the lipid Ags that they recognize are largely unknown. We sought to identify dNKT cell lipid Ag(s) by interrogating a panel of dNKT mouse cell hybridomas with lipid extracts from the pathogen Listeria monocytogenes. We identified Listeria phosphatidylglycerol as a microbial Ag that was significantly more potent than a previously characterized dNKT cell Ag, mammalian phosphatidylglycerol. Further, although mammalian phosphatidylglycerol-loaded CD1d tetramers did not stain dNKT cells, the Listeria-derived phosphatidylglycerol-loaded tetramers did. The structure of Listeria phosphatidylglycerol was distinct from mammalian phosphatidylglycerol because it contained shorter, fully-saturated anteiso fatty acid lipid tails. CD1d-binding lipid-displacement studies revealed that the microbial phosphatidylglycerol Ag binds significantly better to CD1d than do counterparts with the same headgroup. These data reveal a highly potent microbial lipid Ag for a subset of dNKT cells and provide an explanation for its increased Ag potency compared with the mammalian counterpart.

α-Galactosylceramide analogues in cancer immunotherapeutics (P2102)

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

Innate and cytokine-driven signals, rather than microbial antigens, dominate in natural killer T cell activation during microbial infection

Invariant natural killer T cells (iNKT cells) are critical for host defense against a variety of microbial pathogens. However, the central question of how iNKT cells are activated by microbes has not been fully explained. The example of adaptive MHC-restricted T cells, studies using synthetic pharmacological α-galactosylceramides, and the recent discovery of microbial iNKT cell ligands have all suggested that recognition of foreign lipid antigens is the main driver for iNKT cell activation during infection. However, when we compared the role of microbial antigens versus innate cytokine-driven mechanisms, we found that iNKT cell interferon-γ production after in vitro stimulation or infection with diverse bacteria overwhelmingly depended on toll-like receptor-driven IL-12. Importantly, activation of iNKT cells in vivo during infection with Sphingomonas yanoikuyae or Streptococcus pneumoniae, pathogens which are known to express iNKT cell antigens and which require iNKT cells for effective protection, also predominantly depended on IL-12. Constitutive expression of high levels of IL-12 receptor by iNKT cells enabled instant IL-12-induced STAT4 activation, demonstrating that among T cells, iNKT cells are uniquely equipped for immediate, cytokine-driven activation. These findings reveal that innate and cytokine-driven signals, rather than cognate microbial antigen, dominate in iNKT cell activation during microbial infections.

Crystal structure of bovine CD1b3 with endogenously bound ligands

The CD1 family of Ag-presenting molecules is able to display lipids to T cells by binding them within a hydrophobic groove connected to the protein surface. In particular, the CD1b isotype is capable of binding ligands with greatly varying alkyl chain lengths through a complex network of interconnected hydrophobic pockets. Interestingly, mycobacterial lipids such as glucose monomycolate exclusively bind to CD1b. We determined the crystal structure of one of the three expressed bovine CD1b proteins, CD1b3, in complex with endogenous ligands, identified by mass spectrometry as a mixture of phosphatidylcholine and phosphatidylethanolamine, and analyzed the ability of the protein to bind glycolipids in vitro. The structure reveals a complex binding groove architecture, similar to the human ortholog but with consequential differences. Intriguingly, in bovine CD1b3 only the A', C' and F' pockets are present, whereas the T' pocket previously described in human CD1b is closed. This different pocket conformation could affect the ability of boCD1b3 to recognize lipids with long acyl chains such as glucose monomycolate. However, even in the absence of a T' tunnel, bovine CD1b3 is able to bind mycolates from Rhodococcus ruber in vitro.

Identification of arylamine N-acetyltransferase inhibitors as an approach towards novel anti-tuberculars

New anti-tubercular drugs and drug targets are urgently needed to reduce the time for treatment and also to identify agents that will be effective against Mycobacterium tuberculosis persisting intracellularly. Mycobacteria have a unique cell wall. Deletion of the gene for arylamine N-acetyltransferase (NAT) decreases mycobacterial cell wall lipids, particularly the distinctive mycolates, and also increases antibiotic susceptibility and killing within macrophage of Mycobacterium bovis BCG. The nat gene and its associated gene cluster are almost identical in sequence in M. bovis BCG and M. tuberculosis. The gene cluster is essential for intracellular survival of mycobacteria. We have therefore used pure NAT protein for high-throughput screening to identify several classes of small molecules that inhibit NAT activity. Here, we characterize one class of such molecules-triazoles-in relation to its effects on the target enzyme and on both M. bovis BCG and M. tuberculosis. The most potent triazole mimics the effects of deletion of the nat gene on growth, lipid disruption and intracellular survival. We also present the structure-activity relationship between NAT inhibition and effects on mycobacterial growth, and use ligand-protein analysis to give further insight into the structure-activity relationships. We conclude that screening a chemical library with NAT protein yields compounds that have high potential as anti-tubercular agents and that the inhibitors will allow further exploration of the biochemical pathway in which NAT is involved.

Synthesis and biological evaluation of NAS-21 and NAS-91 analogues as potential inhibitors of the mycobacterial FAS-II dehydratase enzyme Rv0636

The identification of potential new anti-tubercular chemotherapeutics is paramount due to the recent emergence of extensively drug-resistant strains of Mycobacterium tuberculosis (XDR-TB). Libraries of NAS-21 and NAS-91 analogues were synthesized and evaluated for their whole-cell activity against Mycobacterium bovis BCG. NAS-21 analogues 1 and 2 demonstrated enhanced whole-cell activity in comparison to the parental compound, and an M. bovis BCG strain overexpressing the dehydratase enzyme Rv0636 was resistant to these analogues. NAS-91 analogues with ortho-modifications gave enhanced whole-cell activity. However, extension with biphenyl modifications compromised the whole-cell activities of both NAS-21 and NAS-91 analogues. Interestingly, both libraries demonstrated in vitro activity against fatty acid synthase II (FAS-II) but not FAS-I in cell-free extracts. In in vitro assays of FAS-II inhibition, NAS-21 analogues 4 and 5 had IC₅₀ values of 28 and 19 μg ml⁻¹, respectively, for the control M. bovis strain, and the M. bovis BCG strain overexpressing Rv0636 showed a marked increase in resistance. In contrast, NAS-91 analogues demonstrated moderate in vitro activity, although increased resistance was again observed in FAS-II activity assays with the Rv0636-overexpressing strain. Fatty acid methyl ester (FAME) and mycolic acid methyl ester (MAME) analysis of M. bovis BCG and the Rv0636-overexpressing strain revealed that the effect of the drug was relieved in the overexpressing strain, further implicating and potentially identifying Rv0636 as the target for these known FabZ dehydratase inhibitors. This study has identified candidates for further development as drug therapeutics against the mycobacterial FAS-II dehydratase enzyme.

Flavonoid inhibitors as novel antimycobacterial agents targeting Rv0636, a putative dehydratase enzyme involved in Mycobacterium tuberculosis fatty acid synthase II

Flavonoids comprise a large group of bioactive polyphenolic plant secondary metabolites. Several of these possess potent in vivo activity against Escherichia coli and Plasmodium falciparum, targeting enzymes involved in fatty acid biosynthesis, such as enoyl-ACP-reductase, beta-ketoacyl-ACP reductase and beta-hydroxyacyl-ACP dehydratase. Herein, we report that butein, isoliquirtigenin, 2,2',4'-trihydroxychalcone and fisetin inhibit the growth of Mycobacterium bovis BCG. Furthermore, in vitro inhibition of the mycolic-acid-producing fatty acid synthase II (FAS-II) of Mycobacterium smegmatis suggests a mode of action related to those observed in E. coli and P. falciparum. Through a bioinformatic approach, we have established the product of Rv0636 as a candidate for the unknown mycobacterial dehydratase, and its overexpression in M. bovis BCG conferred resistance to growth inhibition by butein and isoliquirtigenin, and relieved inhibition of fatty acid and mycolic acid biosynthesis in vivo. Furthermore, after overexpression of Rv0636 in M. smegmatis, FAS-II was less sensitive to these inhibitors in vitro. Overall, the data suggest that these flavonoids are inhibitors of mycobacterial FAS-II and in particular Rv0636, which represents a strong candidate for the beta-hydroxyacyl-ACP dehydratase enzyme of M. tuberculosis FAS-II.

Synthesis and biological evaluation of a C5-biphenyl thiolactomycin library

Fifteen novel C5 analogues of thiolactomycin (13 biphenyl analogues and two biphenyl mimics) have been synthesised and assessed for their in vitro mtFabH and whole cell Mycobacterium bovis BCG activity, respectively. Analysis of the 15 compounds revealed that six possessed enhanced in vitro activity in a direct mtFabH assay. Encouragingly analogues 11, 12 and 13 gave a significant enhancement in in vitro activity against mtFabH. Analogue 13 (5-(4-methoxycarbonyl-biphenyl-4-ylmethyl)-4-hydroxy-3,5-dimethyl-5H-thiophen-2-one) gave an IC(50) value of 3 microM compared to the parent drug thiolactomycin (75 microM) against mtFabH. The biological analysis of this library reaffirms the requirement for a linear pi-rich system containing hydrogen bond accepting substituents attached to the para-position of the C5 biphenyl analogue to generate compounds with enhanced activity.

EthA, a common activator of thiocarbamide-containing drugs acting on different mycobacterial targets

Many of the current antimycobacterial agents require some form of cellular activation unmasking reactive groups, which in turn will bind to their specific targets. Therefore, understanding the mechanisms of activation of current antimycobacterials not only helps to decipher mechanisms of drug resistance but may also facilitate the development of alternative activation strategies or of analogues that do not require such processes. Herein, through the use of genetically defined strains of Mycobacterium bovis BCG we provide evidence that EthA, previously shown to activate ethionamide, also converts isoxyl (ISO) and thiacetazone (TAC) into reactive species. These results were further supported by the development of an in vitro assay using purified recombinant EthA, which allowed direct assessment of the metabolism of ISO. Interestingly, biochemical analysis of [(14)C]acetate-labeled cultures suggested that all of these EthA-activated drugs inhibit mycolic acid biosynthesis via different mechanisms through binding to specific targets. This report is also the first description of the molecular mechanism of action of TAC, a thiosemicarbazone antimicrobial agent that is still used in the treatment of tuberculosis as a second-line drug in many developing countries. Altogether, the results suggest that EthA is a common activator of thiocarbamide-containing drugs. The broad specificity of EthA can now be used to improve the activation process of these drugs, which may help overcome the toxicity problems associated with clinical thiocarbamide use.

Symmetrical and unsymmetrical analogues of isoxyl; active agents against Mycobacterium tuberculosis

Symmetrical and unsymmetrical analogues of the antimycobacterial agent isoxyl have been synthesized and tested against Mycobacterium tuberculosis H37Rv and Mycobacterium bovis BCG, some showing an increased bactericidal effect. In particular, compounds 1-(p-n-butylphenyl)-3-(4-propoxy-phenyl) thiourea (10) and 1-(p-n-butylphenyl)-3-(4-n-butoxy-phenyl) thiourea (11) showed an approximate 10-fold increase in in vitro potency compared to isoxyl, paralleled by increased inhibition of mycolic acid biosynthesis in M. bovis BCG. Interestingly, these isoxyl analogues showed relatively poor inhibition of oleate production, suggesting that the modifications have changed the spectrum of biological activity.