Deciphering the molecular bases of Mycobacterium tuberculosis binding to the lectin DC-SIGN reveals an underestimated complexity

Immune Response to the Recombinant Apa Protein from Mycobacterium tuberculosis Expressed in Streptomyces lividans After Intranasal Administration in Mice. Induction of Protective Response to Tubercle Bacillus Aerosols Exposure

Identifying and evaluating potential vaccine candidates has become one of the main objectives to combat tuberculosis. Among them, mannosylated Apa antigen from Mycobacterium tuberculosis and the non-mannosylated protein expressed in Escherichia coli, have been studied. Although both proteins can induce a protective response in mice, it has been considered that native protein can be dispensed. In this work, we study the protective response induced by Apa expressed in E. coli and in Streptomyces lividans. The latter, like native is secreted as a double band of 45/47 kDa, however, only its 47 kDa band is mannosylated. Both antigens and BCG were intranasal administrated in mice, and animals were then challenged by aerosol with M. tuberculosis H37Rv. The results showed that both, Apa from S. lividans and E. coli conferred statistically significantly protection to animals compared to controls. The cytokine immune response was studied by an immunoassay after animals' immunization, revealing that Apa from S. lividans induced a statistically significant proliferation of T cell, as well as the expression of IFN-γ, IL-1β, IL-17 and IL-10. In contrast, non-proliferation was obtained with non-mannosylated protein, but induction of IL-12 and IL-17 was observed. Together, these results demonstrate that both proteins were able to modulate a specific immune response against M. tuberculosis, that could be driven by different mechanisms possibly associated with the presence or not of mannosylation. Furthermore, stimulation of cells from BCG-vaccinated animals with the proteins could be an important tool, to help define the use of a given subunit-vaccine after BCG vaccination.

Impact of Methylthioxylose Substituents on the Biological Activities of Lipomannan and Lipoarabinomannan in Mycobacterium tuberculosis

Two lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM), play various, albeit incompletely defined, roles in the interactions of mycobacteria with the host. Growing evidence points to the modification of LM and LAM with discrete covalent substituents as a strategy used by these bacteria to modulate their biological activities. One such substituent, originally identified in Mycobacterium tuberculosis (Mtb), is a 5-methylthio-d-xylose (MTX) sugar, which accounts for the antioxidative properties of LAM. The widespread distribution of this motif across Mtb isolates from several epidemiologically important lineages have stimulated interest in MTX-modified LAM as a biomarker of tuberculosis infection. Yet, several lines of evidence indicate that MTX may not be restricted to Mtb and that this motif may substitute more acceptors than originally thought. Using a highly specific monoclonal antibody to the MTX capping motif of Mtb LAM, we here show that MTX motifs not only substitute the mannoside caps of LAM but also the mannan core of LM in Mtb. MTX substituents were also found on the LM and LAM of pathogenic, slow-growing nontuberculous mycobacteria. The presence of MTX substituents on the LM and LAM from Mtb enhances the pro-apoptotic properties of both lipoglycans on LPS-stimulated THP-1 macrophages. A comparison of the cytokines and chemokines produced by resting and LPS-activated THP-1 cells upon exposure to MTX-proficient versus MTX-deficient LM further indicates that MTX substituents confer anti-inflammatory properties upon LM. These findings add to our understanding of the glycan-based strategies employed by slow-growing pathogenic mycobacteria to alter the host immune response to infection.

Development of a novel target-based cell assay, reporter of the activity of Mycobacterium tuberculosis protein-O-mannosyltransferase

The Protein-O-mannosyltransferase is crucial for the virulence of Mycobacterium tuberculosis, the etiological agent of tuberculosis. This enzyme, called MtPMT (Rv1002c), is responsible for the post-translational O-mannosylation of mycobacterial proteins. It catalyzes the transfer of a single mannose residue from a polyprenol phospho-mannosyl lipidic donor to the hydroxyl groups of selected Ser/Thr residues in acceptor proteins during their translocation across the membrane. Previously, we provided evidence that the loss of MtPMT activity causes the absence of mannoproteins in Mycobacterium tuberculosis, severely impacting its intracellular growth, as well as a strong attenuation of its pathogenicity in immunocompromised mice. Therefore, it is of interest to develop specific inhibitors of this enzyme to better understand mycobacterial infectious diseases. Here we report the development of a "target-based" phenotypic assay for this enzyme, assessing its O-mannosyltransferase activity in bacteria, in the non-pathogenic Mycobacterium smegmatis strain. Robustness of the quantitative contribution of this assay was evaluated by intact protein mass spectrometry, using a panel of control strains, overexpressing the MtPMT gene, carrying different key point-mutations. Then, screening of a limited library of 30 compounds rationally chosen allowed us to identify 2 compounds containing pyrrole analogous rings, as significant inhibitors of MtPMT activity, affecting neither the growth of the mycobacterium nor its secretion of mannoproteins. These molecular cores could therefore serve as scaffold for the design of new pharmaceutical agents that could improve treatment of mycobacterial diseases. We report here the implementation of a miniaturized phenotypic activity assay for a glycosyltransferase of the C superfamily.

Nanoscale clustering of mycobacterial ligands and DC-SIGN host receptors are key determinants for pathogen recognition

The bacterial pathogen Mycobacterium tuberculosis binds to the C-type lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin) on dendritic cells to evade the immune system. While DC-SIGN glycoconjugate ligands are ubiquitous among mycobacterial species, the receptor selectively binds pathogenic species from the M. tuberculosis complex (MTBC). Here, we unravel the molecular mechanism behind this intriguing selective recognition by means of a multidisciplinary approach combining single-molecule atomic force microscopy with Förster resonance energy transfer and bioassays. Molecular recognition imaging of mycobacteria demonstrates that the distribution of DC-SIGN ligands markedly differs between Mycobacterium bovis Bacille Calmette-Guérin (BCG) (model MTBC species) and Mycobacterium smegmatis (non-MTBC species), the ligands being concentrated into dense nanodomains on M. bovis BCG. Upon bacteria-host cell adhesion, ligand nanodomains induce the recruitment and clustering of DC-SIGN. Our study highlights the key role of clustering of both ligands on MTBC species and DC-SIGN host receptors in pathogen recognition, a mechanism that might be widespread in host-pathogen interactions.

Immunological hyporesponsiveness in tuberculosis: The role of mycobacterial glycolipids

Glycolipids constitute a major part of the cell envelope of Mycobacterium tuberculosis (Mtb). They are potent immunomodulatory molecules recognized by several immune receptors like pattern recognition receptors such as TLR2, DC-SIGN and Dectin-2 on antigen-presenting cells and by T cell receptors on T lymphocytes. The Mtb glycolipids lipoarabinomannan (LAM) and its biosynthetic relatives, phosphatidylinositol mannosides (PIMs) and lipomannan (LM), as well as other Mtb glycolipids, such as phenolic glycolipids and sulfoglycolipids have the ability to modulate the immune response, stimulating or inhibiting a pro-inflammatory response. We explore here the downmodulating effect of Mtb glycolipids. A great proportion of the studies used in vitro approaches although in vivo infection with Mtb might also lead to a dampening of myeloid cell and T cell responses to Mtb glycolipids. This dampened response has been explored ex vivo with immune cells from peripheral blood from Mtb-infected individuals and in mouse models of infection. In addition to the dampening of the immune response caused by Mtb glycolipids, we discuss the hyporesponse to Mtb glycolipids caused by prolonged Mtb infection and/or exposure to Mtb antigens. Hyporesponse to LAM has been observed in myeloid cells from individuals with active and latent tuberculosis (TB). For some myeloid subsets, this effect is stronger in latent versus active TB. Since the immune response in individuals with latent TB represents a more protective profile compared to the one in patients with active TB, this suggests that downmodulation of myeloid cell functions by Mtb glycolipids may be beneficial for the host and protect against active TB disease. The mechanisms of this downmodulation, including tolerance through epigenetic modifications, are only partly explored.

Mycobacterial Adhesion: From Hydrophobic to Receptor-Ligand Interactions

Adhesion is crucial for the infective lifestyles of bacterial pathogens. Adhesion to non-living surfaces, other microbial cells, and components of the biofilm extracellular matrix are crucial for biofilm formation and integrity, plus adherence to host factors constitutes a first step leading to an infection. Adhesion is, therefore, at the core of pathogens’ ability to contaminate, transmit, establish residency within a host, and cause an infection. Several mycobacterial species cause diseases in humans and animals with diverse clinical manifestations. Mycobacterium tuberculosis, which enters through the respiratory tract, first adheres to alveolar macrophages and epithelial cells leading up to transmigration across the alveolar epithelium and containment within granulomas. Later, when dissemination occurs, the bacilli need to adhere to extracellular matrix components to infect extrapulmonary sites. Mycobacteria causing zoonotic infections and emerging nontuberculous mycobacterial pathogens follow divergent routes of infection that probably require adapted adhesion mechanisms. New evidence also points to the occurrence of mycobacterial biofilms during infection, emphasizing a need to better understand the adhesive factors required for their formation. Herein, we review the literature on tuberculous and nontuberculous mycobacterial adhesion to living and non-living surfaces, to themselves, to host cells, and to components of the extracellular matrix.

Effect of Protein O-Mannosyltransferase (MSMEG_5447) on M. smegmatis and Its Survival in Macrophages

Protein O-mannosyltransferase (PMT) catalyzes an initial step of protein O-mannosylation of Mycobacterium tuberculosis (Mtb) and plays a crucial role for Mtb survival in the host. To better understand the role of PMT in the host innate immune response during mycobacterial infection, in this study, we utilized Mycobacterium smegmatis pmt (MSMEG_5447) gene knockout strain, ΔM5447, to infect THP-1 cells. Our results revealed that the lack of MSMEG_5447 not only impaired the growth of M. smegmatis in 7H9 medium but also reduced the resistance of M. smegmatis against lysozyme and acidic stress in vitro. Macrophage infection assay showed that ΔM5447 displayed attenuated growth in macrophages at 24 h post-infection. The production of TNF-α and IL-6 and the activation of transcription factor NF-κB were decreased in ΔM5447-infected macrophages, which were further confirmed by transcriptomic analysis. Moreover, ΔM5447 failed to inhibit phagosome–lysosome fusion in macrophages. These findings revealed that PMT played a role in modulating the innate immune responses of the host, which broaden our understanding for functions of protein O-mannosylation in mycobacterium–host interaction.

Generation of Liposomes to Study the Effect of Mycobacterium Tuberculosis Lipids on HIV-1 cis- and trans-Infections

Tuberculosis (TB) is the leading cause of death among HIV-1-infected individuals and Mycobacterium tuberculosis (Mtb) co-infection is an early precipitate to AIDS. We aimed to determine whether Mtb strains differentially modulate cellular susceptibility to HIV-1 infection (cis- and trans-infection), via surface receptor interaction by their cell envelope lipids. Total lipids from pathogenic (lineage 4 Mtb H37Rv, CDC1551 and lineage 2 Mtb HN878, EU127) and non-pathogenic (Mycobacterium bovis BCG and Mycobacterium smegmatis) Mycobacterium strains were integrated into liposomes mimicking the lipid distribution and antigen accessibility of the mycobacterial cell wall. The resulting liposomes were tested for modulating in vitro HIV-1 cis- and trans-infection of TZM-bl cells using single-cycle infectious virus particles. Mtb glycolipids did not affect HIV-1 direct infection however, trans-infection of both R5 and X4 tropic HIV-1 strains were impaired in the presence of glycolipids from M. bovis, Mtb H37Rv and Mtb EU127 strains when using Raji-DC-SIGN cells or immature and mature dendritic cells (DCs) to capture virus. SL1, PDIM and TDM lipids were identified to be involved in DC-SIGN recognition and impairment of HIV-1 trans-infection. These findings indicate that variant strains of Mtb have differential effect on HIV-1 trans-infection with the potential to influence HIV-1 disease course in co-infected individuals.

Evolution of Drug-Resistant Mycobacterium tuberculosis Strains and Their Adaptation to the Human Lung Environment

In the last two decades, multi (MDR), extensively (XDR), extremely (XXDR) and total (TDR) drug-resistant Mycobacterium tuberculosis (M.tb) strains have emerged as a threat to public health worldwide, stressing the need to develop new tuberculosis (TB) prevention and treatment strategies. It is estimated that in the next 35 years, drug-resistant TB will kill around 75 million people and cost the global economy $16.7 trillion. Indeed, the COVID-19 pandemic alone may contribute with the development of 6.3 million new TB cases due to lack of resources and enforced confinement in TB endemic areas. Evolution of drug-resistant M.tb depends on numerous factors, such as bacterial fitness, strain's genetic background and its capacity to adapt to the surrounding environment, as well as host-specific and environmental factors. Whole-genome transcriptomics and genome-wide association studies in recent years have shed some insights into the complexity of M.tb drug resistance and have provided a better understanding of its underlying molecular mechanisms. In this review, we will discuss M.tb phenotypic and genotypic changes driving resistance, including changes in cell envelope components, as well as recently described intrinsic and extrinsic factors promoting resistance emergence and transmission. We will further explore how drug-resistant M.tb adapts differently than drug-susceptible strains to the lung environment at the cellular level, modulating M.tb-host interactions and disease outcome, and novel next generation sequencing (NGS) strategies to study drug-resistant TB.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross‐talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll‐like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C‐type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

Potential Plasticity of the Mannoprotein Repertoire Associated to Mycobacterium tuberculosis Virulence Unveiled by Mass Spectrometry-Based Glycoproteomics

To date, Mycobacterium tuberculosis (Mtb) remains the world’s greatest infectious killer. The rise of multidrug-resistant strains stresses the need to identify new therapeutic targets to fight the epidemic. We previously demonstrated that bacterial protein-O-mannosylation is crucial for Mtb infectiousness, renewing the interest of the bacterial-secreted mannoproteins as potential drug-targetable virulence factors. The difficulty of inventorying the mannoprotein repertoire expressed by Mtb led us to design a stringent multi-step workflow for the reliable identification of glycosylated peptides by large-scale mass spectrometry-based proteomics. Applied to the differential analyses of glycoproteins secreted by the wild-type Mtb strain—and by its derived mutant invalidated for the protein-O-mannosylating enzyme PMTub—this approach led to the identification of not only most already known mannoproteins, but also of yet-unknown mannosylated proteins. In addition, analysis of the glycoproteome expressed by the isogenic recombinant Mtb strain overexpressing the PMTub gene revealed an unexpected mannosylation of proteins, with predicted or demonstrated functions in Mtb growth and interaction with the host cell. Since in parallel, a transient increased expression of the PMTub gene has been observed in the wild-type bacilli when infecting macrophages, our results strongly suggest that the Mtb mannoproteome may undergo adaptive regulation during infection of the host cells. Overall, our results provide deeper insights into the complexity of the repertoire of mannosylated proteins expressed by Mtb, and open the way to novel opportunities to search for still-unexploited potential therapeutic targets.

Identification of Secreted O-Mannosylated Proteins From BCG and Characterization of Immunodominant Antigens BCG_0470 and BCG_0980

Bacterial glycoproteins have been investigated as vaccine candidates as well as diagnostic biomarkers. However, they are poorly understood in Mycobacterium bovis strain bacille Calmette-Guérin (BCG), a non-pathogenic model of Mycobacterium tuberculosis. To understand the roles of secreted O-mannosylated glycoproteins in BCG, we conducted a ConA lectin-affinity chromatography and mass spectra analysis to identify O-mannosylated proteins in BCG culture filtrate. Subsequent screening of antigens was performed using polyclonal antibodies obtained from a BCG-immunized mouse, with 15 endogenous O-mannosylated proteins eventually identified. Of these, BCG_0470 and BCG_0980 (PstS3) were revealed as the immunodominant antigens. To examine the protective effects of the antigens, recombinant antigens proteins were first expressed in Mycobacterium smegmatis and Escherichia coli, with the purified proteins then used to boost BCG primed-mice. Overall, the treated mice showed a greater delayed-type hypersensitivity response in vivo, as well as stronger Th1 responses, including higher level of IFN-γ, TNF-α, and specific-IgG. Therefore, mannosylated proteins BCG_0470 and BCG_0980 effectively amplified the immune responses induced by BCG in mice. Together, our results suggest that the oligosaccharide chains containing mannose are the antigenic determinants of glycoproteins, providing key insight for future vaccine optimization and design.

C-type Lectins in Immunity to Lung Pathogens

The respiratory tract is tasked with responding to a constant and vast influx of foreign agents. It acts as an important first line of defense in the innate immune system and as such plays a crucial role in preventing the entry of invading pathogens. While physical barriers like the mucociliary escalator exert their effects through the clearance of these pathogens, diverse and dynamic cellular mechanisms exist for the activation of the innate immune response through the recognition of pathogen-associated molecular patterns (PAMPs). These PAMPs are recognized by pattern recognition receptors (PRRs) that are expressed on a number of myeloid cells such as dendritic cells, macrophages, and neutrophils found in the respiratory tract. C-type lectin receptors (CLRs) are PRRs that play a pivotal role in the innate immune response and its regulation to a variety of respiratory pathogens such as viruses, bacteria, and fungi. This chapter will describe the function of both activating and inhibiting myeloid CLRs in the recognition of a number of important respiratory pathogens as well as the signaling events initiated by these receptors.

The cell surface adhesins of Mycobacterium tuberculosis

Bacterial cell surface adhesins play a major role in facilitating host colonization and subsequent establishment of infection. The surface of Mycobacterium tuberculosis, owing to the complex architecture of its cell envelope, expresses numerous adhesins with varied chemical nature, including proteins, lipids, lipoproteins, glycoproteins and glycopolymers. Studies on mycobacterial adhesins show that they bind with multifarious host receptors and extracellular matrix (ECM) components. In this review we have highlighted the adhesins that are abundantly present on the mycobacterial surface and their interactions with host receptors. M. tuberculosis interacts with various host cell surface receptors such as toll like receptors, C-type lectin receptors, scavenger receptors, and Fc and complement receptors. Apart from these, ECM components like fibronectin, collagen, elastin, laminin, fibrillin and vitronectin also provide binding sites for surface adhesins of the tubercle bacilli. M. tuberculosis adhesins include proteins with and without signal peptide sequence and transmembrane proteins. Other surface adhesin macromolecules of M. tuberculosis comprises of lipids, glycolipids and glycopolymers. The interaction between the mycobacterial adhesins and their host receptors result in adhesion of the microbe to the host cells, induction of immune response and aid in the pathogenesis of the disease. A thorough understanding of the different M. tuberculosis surface adhesins and host receptors will provide a better picture of interaction between them at molecular level. The information gained on adhesins and host receptors will prove beneficial in developing novel therapeutic strategies such as the use of anti-adhesin molecules to hinder the adhesion of bacteria to the host cells, thereby preventing establishment of infection. The surface molecules discussed in this review will also benefit in identification of new drug targets, diagnostic markers or vaccine candidates against the deadly pathogen.

Molecular Insights into DC-SIGN Binding to Self-Antigens: The Interaction with the Blood Group A/B Antigens

The dendritic cell-specific intracellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) is an important receptor of the immune system. Besides its role as pathogen recognition receptor (PRR), it also interacts with endogenous glycoproteins through the specific recognition of self-glycan epitopes, like Le^X. However, this lectin represents a paradigmatic case of glycan binding promiscuity, and it also has been shown to recognize antigens with α1−α2 linked fucose, such as the histo blood group antigens, with similar affinities to Le^X. Herein, we have studied the interaction in solution between DC-SIGN and the blood group A and B antigens, to get insights into the atomic details of such interaction. With a combination of different NMR experiments, we demonstrate that the Fuc coordinates the primary Ca²⁺ ion with a single binding mode through 3-OH and 4-OH. The terminal αGal/αGalNAc affords marginal direct polar contacts with the protein, but provides a hydrophobic hook in which V351 of the lectin perfectly fits. Moreover, we have found that αGal, but not αGalNAc, is a weak binder itself for DC-SIGN, which could endow an additional binding mode for the blood group B antigen, but not for blood group A.

Immunoscreening of the M. tuberculosis F15/LAM4/KZN secretome library against TB patients' sera identifies unique active- and latent-TB specific biomarkers

Tuberculosis (TB) protein biomarkers are urgently needed for the development of point-of-care diagnostics, new drugs and vaccines. Mycobacterium tuberculosis extracellular and secreted proteins play an important role in host-pathogen interactions. Antibodies produced against M. tuberculosis proteins before the onset of clinical symptoms can be used in proteomic studies to identify their target proteins. In this study, M. tuberculosis F15/LAM4/KZN strain phage secretome library was screened against immobilized polyclonal sera from active TB patients (n = 20), TST positive individuals (n = 15) and M. tuberculosis uninfected individuals (n = 20) to select and identify proteins recognized by patients' antibodies. DNA sequence analysis from randomly selected latent TB and active TB specific phage clones revealed 118 and 96 ORFs, respectively. Proteins essential for growth, virulence and metabolic pathways were identified using different TB databases. The identified active TB specific biomarkers included five proteins, namely, TrpG, Alr, TreY, BfrA and EspR, with no human homologs, whilst latent TB specific biomarkers included NarG, PonA1, PonA2 and HspR. Future studies will assess potential applications of identified protein biomarkers as TB drug or vaccine candidates/targets and diagnostic markers with the ability to discriminate LTBI from active TB.

Mycobacteria and their sweet proteins: An overview of protein glycosylation and lipoglycosylation in M. tuberculosis

Post-translational modifications represent a key aspect of enzyme and protein regulation and function. Post-translational modifications are involved in signaling and response to stress, adaptation to changing environments, regulation of toxic and damaged proteins, proteins localization and host-pathogen interactions. Glycosylation in Mycobacterium tuberculosis (Mtb), is a post-translational modification often found in conjunction with acylation in mycobacterial proteins. Since the discovery of glycosylated proteins in the early 1980's, important advances in our understanding of the mechanisms of protein glycosylation have been made. The number of known glycosylated substrates in Mtb has grown through the years, yet many questions remain. This review will explore the current knowledge on protein glycosylation in Mtb, causative agent of Tuberculosis and number one infectious killer in the world. The mechanism and significance of this post-translational modification, as well as maturation, export and acylation of glycosylated proteins will be reviewed. We expect to provide the reader with an overall view of protein glycosylation in Mtb, as well as the significance of this post-translational modification to the physiology and host-pathogen interactions of this important pathogen. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD011081 and 10.6019/PXD011081.

Deciphering the molecular basis of mycobacteria and lipoglycan recognition by the C-type lectin Dectin-2

Dectin-2 is a C-type lectin involved in the recognition of several pathogens such as Aspergillus fumigatus, Candida albicans, Schistosoma mansonii, and Mycobacterium tuberculosis that triggers Th17 immune responses. Identifying pathogen ligands and understanding the molecular basis of their recognition is one of the current challenges. Purified M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) was shown to induce signaling via Dectin-2, an activity that requires the (α1 → 2)-linked mannosides forming the caps. Here, using isogenic M. tuberculosis mutant strains, we demonstrate that ManLAM is a bona fide and actually the sole ligand mediating bacilli recognition by Dectin-2, although M. tuberculosis produces a variety of cell envelope mannoconjugates, such as phosphatidyl-myo-inositol hexamannosides, lipomannan or manno(lipo)proteins, that bear (α1 → 2)-linked mannosides. In addition, we found that Dectin-2 can recognize lipoglycans from other bacterial species, such as Saccharotrix aerocolonigenes or the human opportunistic pathogen Tsukamurella paurometabola, suggesting that lipoglycans are prototypical Dectin-2 ligands. Finally, from a structure/function relationship perspective, we show, using lipoglycan variants and synthetic mannodendrimers, that dimannoside caps and multivalent interaction are required for ligand binding to and signaling via Dectin-2. Better understanding of the molecular basis of ligand recognition by Dectin-2 will pave the way for the rational design of potent adjuvants targeting this receptor.

Tuberculosis: Smart manipulation of a lethal host

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a global threat to human health. Development of drug resistance and co-infection with HIV has increased the morbidity and mortality caused by TB. Macrophages serve as primary defense against microbial infections, including TB. Upon recognition and uptake of mycobacteria, macrophages initiate a series of events designed to lead to generation of effective immune responses and clearance of infection. However, pathogenic mycobacteria utilize multiple mechanisms for manipulating macrophage responses to protect itself from being killed and to survive within these cells that are designed to kill them. The outcomes of mycobacterial infection are determined by several host- and pathogen-related factors. Significant advancements in understanding mycobacterial pathogenesis have been made in recent years. In this review, some of the important factors/mechanisms regulating mycobacterial survival inside macrophages are discussed.

Insights into Interactions of Mycobacteria with the Host Innate Immune System from a Novel Array of Synthetic Mycobacterial Glycans

An array of homogeneous glycans representing all the major carbohydrate structures present in the cell wall of the human pathogen Mycobacterium tuberculosis and other mycobacteria has been probed with a panel of glycan-binding receptors expressed on cells of the mammalian innate immune system. The results provide an overview of interactions between mycobacterial glycans and receptors that mediate uptake and survival in macrophages, dendritic cells, and sinusoidal endothelial cells. A subset of the wide variety of glycan structures present on mycobacterial surfaces interact with cells of the innate immune system through the receptors tested. Endocytic receptors, including the mannose receptor, DC-SIGN, langerin, and DC-SIGNR (L-SIGN), interact predominantly with mannose-containing caps found on the mycobacterial polysaccharide lipoarabinomannan. Some of these receptors also interact with phosphatidyl-myo-inositol mannosides and mannose-containing phenolic glycolipids. Many glycans are ligands for overlapping sets of receptors, suggesting multiple, redundant routes by which mycobacteria can enter cells. Receptors with signaling capability interact with two distinct sets of mycobacterial glycans: targets for dectin-2 overlap with ligands for the mannose-binding endocytic receptors, while mincle binds exclusively to trehalose-containing structures such as trehalose dimycolate. None of the receptors surveyed bind furanose residues, which often form part of the epitopes recognized by antibodies to mycobacteria. Thus, the innate and adaptive immune systems can target different sets of mycobacterial glycans. This array, the first of its kind, represents an important new tool for probing, at a molecular level, biological roles of a broad range of mycobacterial glycans, a task that has not previously been possible.

Protein O-mannosylation deficiency increases LprG-associated lipoarabinomannan release by Mycobacterium tuberculosis and enhances the TLR2-associated inflammatory response

Protein O-mannosylation is crucial for the biology of Mycobacterium tuberculosis but the key mannosylated protein(s) involved and its(their) underlying function(s) remain unknown. Here, we demonstrated that the M. tuberculosis mutant (Δpmt) deficient for protein O-mannosylation exhibits enhanced release of lipoarabinomannan (LAM) in a complex with LprG, a lipoprotein required for LAM translocation to the cell surface. We determined that LprG is O-mannosylated at a unique threonine position by mass spectrometry analyses of the purified protein. However, although replacement of this amino acid by an alanine residue completely abolished LprG O-mannosylation, the increased release of the LAM/LprG complex was preserved. We found that the increased secretion of this complex is due to enhanced LAM production in the Δpmt M. tuberculosis and M. smegmatis mutants relative to their wild-type counterparts. This abnormal release of LAM/LprG has functional consequences on the induction of inflammatory responses and provides a possible explanation for the reduced virulence of the M. tuberculosis Δpmt mutant.

Scrutiny of Mycobacterium tuberculosis 19 kDa antigen proteoforms provides new insights in the lipoglycoprotein biogenesis paradigm

Post-translational modifications (PTMs) are essential processes conditioning the biophysical properties and biological activities of the vast majority of mature proteins. However, occurrence of several distinct PTMs on a same protein dramatically increases its molecular diversity. The comprehensive understanding of the functionalities resulting from any particular PTM association requires a highly challenging full structural description of the PTM combinations. Here, we report the in-depth exploration of the natural structural diversity of the M. tuberculosis (Mtb) virulence associated 19 kDa lipoglycoprotein antigen (LpqH) using intact protein high-resolution mass spectrometry (HR-MS) coupled to liquid chromatography. Combined top-down and bottom-up HR-MS analyses of the purified Mtb LpqH protein allow, for the first time, to uncover a complex repertoire of about 130 molecular species resulting from the intrinsically heterogeneous combination of lipidation and glycosylation together with some truncations. Direct view on the co-occurring PTMs stoichiometry reveals the presence of functionally distinct LpqH lipidation states and indicates that glycosylation is independent from lipidation. This work allowed the identification of a novel unsuspected phosphorylated form of the unprocessed preprolipoglycoprotein totally absent from the current lipoglycoprotein biogenesis pathway and providing new insights into the biogenesis and functional determinants of the mycobacterial lipoglycoprotein interacting with the host immune PRRs.

Rhodococcus erythropolis as a host for expression, secretion and glycosylation of Mycobacterium tuberculosis proteins

Background

Glycosylation is one of the most abundant posttranslational polypeptide chain modification in nature. Although carbohydrate modification of protein antigens from many microbial pathogens constitutes important components of B cell epitopes, the role in T cell immunity is not completely understood. There is growing evidence about the importance of these modifications in host bacteria interactions in tuberculosis. It is known, that the sugars present in some Mycobacterium tuberculosis glycoproteins play an important role in both humoral and cellular immune response against the pathogen. Since this modification is lost in the recombinant proteins expressed in Escherichia coli, it is fundamental to search for host bacteria with the capacity to modify the foreign proteins. Amongst the bacteria that are likely to have this possibility are some members of Rhodococcus genus which are Gram-positive bacteria, with high GC-content and genetically very close related to M. tuberculosis.

Results

In this work, apa, pstS1 and lprG genes that coding for M. tuberculosis glycoproteins were cloned and expressed in Rhodococcus erythropolis. All recombinant proteins were mannosylated as demonstrated by their interaction with mannose binding lectin Concanavalin A. In addition, as native proteins recombinants Apa and PstS1 were secreted to the culture medium in contrast with LprG that was retained in the cell wall.

Conclusions

Together these results, point out R. erythropolis, as a new host for expression of M. tuberculosis glycoproteins.

Mycobacterial cell wall biosynthesis: a multifaceted antibiotic target

Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB), is recognized as a global health emergency as promoted by the World Health Organization. Over 1 million deaths per year, along with the emergence of multi- and extensively-drug resistant strains of Mtb, have triggered intensive research into the pathogenicity and biochemistry of this microorganism, guiding the development of anti-TB chemotherapeutic agents. The essential mycobacterial cell wall, sharing some common features with all bacteria, represents an apparent ‘Achilles heel’ that has been targeted by TB chemotherapy since the advent of TB treatment. This complex structure composed of three distinct layers, peptidoglycan, arabinogalactan and mycolic acids, is vital in supporting cell growth, virulence and providing a barrier to antibiotics. The fundamental nature of cell wall synthesis and assembly has rendered the mycobacterial cell wall as the most widely exploited target of anti-TB drugs. This review provides an overview of the biosynthesis of the prominent cell wall components, highlighting the inhibitory mechanisms of existing clinical drugs and illustrating the potential of other unexploited enzymes as future drug targets.

Secretome profiling of highly virulent Mycobacterium bovis 04-303 strain reveals higher abundance of virulence-associated proteins

Mycobacterium bovis is the causative agent of tuberculosis in farms, wildlife and causes sporadic disease in humans. Despite the high similitude in genome sequence between M. bovis strains, some strains like the wild boar 04-303 isolate show a highly virulent phenotype in animal models. Comparative studies will contribute to link protein expression with the virulence phenotype. In vitro, the 04-303 strain was more phagocytized by J774A.1 macrophages in comparison with 444 strain (a cow isolate with the same genotype) and BCG. The secretome of these strains showed a significant proportion of shared proteins (368 spots). Among the proteins only visualized in the secretome of the 04-303 strain, we identify the nine most abundant proteins by LC-MS/MS. The most relevant were EsxA and EsxB proteins, which are encoded in the RD1 region, deleted in BCG strains. These proteins are the major virulence factor of M. tuberculosis. The other proteins identified belong to functional categories of virulence, detoxification, and adaptation; lipid metabolism; and cell wall and cell processes. The relatively high proportion of proteins involved in the cell wall and cell process is consistent with the previously described variation among M. bovis genomes.

A glycomic approach reveals a new mycobacterial polysaccharide

Mycobacterium tuberculosis lipoarabinomannan (LAM) and biosynthetically related lipoglycans and glycans play an important role in host-pathogen interactions. Therefore, the elucidation of the complete biosynthetic pathways of these important molecules is expected to afford novel therapeutic targets. The characterization of biosynthetic enzymes and transporters involved in the formation and localization of these complex macromolecules in the bacterial cell envelope largely relies on genetic manipulation of mycobacteria and subsequent analyses of lipoglycan structural alterations. However, lipoglycans are present in relatively low amounts. Their purification to homogeneity remains tedious and time-consuming. To overcome these issues and to reduce the biomass and time required for lipoglycan purification, we report here the development of a methodology to efficiently purify lipoglycans by sodium deoxycholate-polyacrylamide gel electrophoresis. This faster purification method can be applied on a small amount of mycobacterial cells biomass (10-50 mg), resulting in tens of micrograms of purified lipoglycans. This amount of purified products was found to be sufficient to undertake structural analyses of lipoglycans and glycans carbohydrate domains by a combination of highly sensitive analytical procedures, involving cryoprobe NMR analysis of intact macromolecules and chemical degradations monitored by gas chromatography and capillary electrophoresis. This glycomic approach was successfully applied to the purification and structural characterization of a newly identified polysaccharide, structurally related to LAM, in the model fast-growing species Mycobacterium smegmatis.

The presence of a galactosamine substituent on the arabinogalactan of Mycobacterium tuberculosis abrogates full maturation of human peripheral blood monocyte-derived dendritic cells and increases secretion of IL-10

Slow-growing and pathogenic Mycobacterium spp. are characterized by the presence of galactosamine (GalN) that modifies the interior branched arabinosyl residues of the arabinogalactan (AG) that is a major heteropolysaccharide cell wall component. The availability of null mutants of the polyprenyl-phospho-N-acetylgalactosaminyl synthase (Rv3631, PpgS) and the (N-acetyl-) galactosaminyl transferase (Rv3779) of Mycobacterium tuberculosis (Mtb) has provided a means to elucidate the role of the GalN substituent of AG in terms of host-pathogen interactions. Comparisons of treating human peripheral blood monocyte-derived dendritic cells (hPMC-DCs) with wild-type, Rv3631 and Rv3779 mutant strains of Mtb revealed increased expression of DC maturation markers, decreased affinity for a soluble DC-SIGN probe, reduced IL-10 secretion and increased TLR-2-mediated NF-κB activation among GalN-deficient Mtb strains compared to GalN-producing strains. Analysis of surface expression of a panel of defined or putative DC-SIGN ligands on both WT strains or either Rv3631 or Rv3779 mutant did not show significant differences suggesting that the role of the GalN substituent of AG may be to modulate access of the bacilli to immunologically-relevant receptor domains on DCs or contribute to higher ordered pathogen associated molecular pattern (PAMP)/pattern recognition receptor (PRR) interactions rather than the GalN-AG components having a direct immunological effect per se.

Lipid metabolism and Type VII secretion systems dominate the genome scale virulence profile of Mycobacterium tuberculosis in human dendritic cells

Background

Mycobacterium tuberculosis continues to kill more people than any other bacterium. Although its archetypal host cell is the macrophage, it also enters, and survives within, dendritic cells (DCs). By modulating the behaviour of the DC, M. tuberculosis is able to manipulate the host’s immune response and establish an infection. To identify the M. tuberculosis genes required for survival within DCs we infected primary human DCs with an M. tuberculosis transposon library and identified mutations with a reduced ability to survive.

Results

Parallel sequencing of the transposon inserts of the surviving mutants identified a large number of genes as being required for optimal intracellular fitness in DCs. Loci whose mutation attenuated intracellular survival included those involved in synthesising cell wall lipids, not only the well-established virulence factors, pDIM and cord factor, but also sulfolipids and PGL, which have not previously been identified as having a direct virulence role in cells. Other attenuated loci included the secretion systems ESX-1, ESX-2 and ESX-4, alongside many PPE genes, implicating a role for ESX-5. In contrast the canonical ESAT-6 family of ESX substrates did not have intra-DC fitness costs suggesting an alternative ESX-1 associated virulence mechanism. With the aid of a gene-nutrient interaction model, metabolic processes such as cholesterol side chain catabolism, nitrate reductase and cysteine-methionine metabolism were also identified as important for survival in DCs.

Conclusion

We conclude that many of the virulence factors required for survival in DC are shared with macrophages, but that survival in DCs also requires several additional functions, such as cysteine-methionine metabolism, PGLs, sulfolipids, ESX systems and PPE genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1569-2) contains supplementary material, which is available to authorized users.

A mycobacterial phosphoribosyltransferase promotes bacillary survival by inhibiting oxidative stress and autophagy pathways in macrophages and zebrafish

Mycobacterium tuberculosis employs various strategies to modulate host immune responses to facilitate its persistence in macrophages. The M. tuberculosis cell wall contains numerous glycoproteins with unknown roles in pathogenesis. Here, by using Concanavalin A and LC-MS analysis, we identified a novel mannosylated glycoprotein phosphoribosyltransferase, encoded by Rv3242c from M. tuberculosis cell walls. Homology modeling, bioinformatic analyses, and an assay of phosphoribosyltransferase activity in Mycobacterium smegmatis expressing recombinant Rv3242c (MsmRv3242c) confirmed the mass spectrometry data. Using Mycobacterium marinum-zebrafish and the surrogate MsmRv3242c infection models, we proved that phosphoribosyltransferase is involved in mycobacterial virulence. Histological and infection assays showed that the M. marinum mimG mutant, an Rv3242c orthologue in a pathogenic M. marinum strain, was strongly attenuated in adult zebrafish and also survived less in macrophages. In contrast, infection with wild type and the complemented ΔmimG:Rv3242c M. marinum strains showed prominent pathological features, such as severe emaciation, skin lesions, hemorrhaging, and more zebrafish death. Similarly, recombinant MsmRv3242c bacteria showed increased invasion in non-phagocytic epithelial cells and longer intracellular survival in macrophages as compared with wild type and vector control M. smegmatis strains. Further mechanistic studies revealed that the Rv3242c- and mimG-mediated enhancement of intramacrophagic survival was due to inhibition of autophagy, reactive oxygen species, and reduced activities of superoxide dismutase and catalase enzymes. Infection with MsmRv3242c also activated the MAPK pathway, NF-κB, and inflammatory cytokines. In summary, we show that a novel mycobacterial mannosylated phosphoribosyltransferase acts as a virulence and immunomodulatory factor, suggesting that it may constitute a novel target for antimycobacterial drugs.

Manipulation of the endocytic pathway and phagocyte functions by Mycobacterium tuberculosis lipoarabinomannan

Lipoarabinomannan is a major immunomodulatory lipoglycan found in the cell envelope of Mycobacterium tuberculosis and related human pathogens. It reproduces several salient properties of M. tuberculosis in phagocytic cells, including inhibition of pro-inflammatory cytokine production, inhibition of phagolysosome biogenesis, and inhibition of apoptosis as well as autophagy. In this review, we present our current knowledge on lipoarabinomannan structure and ability to manipulate the endocytic pathway as well as phagocyte functions. A special focus is put on the molecular mechanisms employed and the signaling pathways hijacked. Available information is discussed in the context of M. tuberculosis pathogenesis.

Mycobacteriophage cell binding proteins for the capture of mycobacteria

Slow growing Mycobacteriumavium subsp. paratuberculosis (MAP) causes a deadly condition in cattle known as Johne's disease where asymptomatic carriers are the major source of disease transmission. MAP was also shown to be associated with chronic Crohn's disease in humans. Mycobacterium smegmatis is a model mycobacterium that can cause opportunistic infections in a number of human tissues and, rarely, a respiratory disease. Currently, there are no rapid, culture-independent, reliable and inexpensive tests for the diagnostics of MAP or M. smegmatis infections. Bacteriophages are viruses producing a number of proteins that effectively and specifically recognize the cell envelopes of their bacterial hosts. We demonstrate that the mycobacterial phage L5 minor tail protein Gp6 and lysin Gp10 are useful tools for the rapid capture of mycobacteria. Immobilized Gp10 was able to bind both MAP and M. smegmatis cells whereas Gp6 was M. smegmatis specific. Neither of the 2 proteins was able to capture E. coli, salmonella, campylobacter or Mycobacterium marinum cells. Gp6 was detected previously as a component of the phage particle and shows no homology to proteins with known function. Therefore, electrospray ionization mass spectrometry was used to determine whether recombinant Gp6 could bind to a number of chemically synthesized fragments of mycobacterial surface glycans. These findings demonstrate that mycobacteriophage proteins could be used as a pathogen capturing platform that can potentially improve the effectiveness of existing diagnostic methods.

Carbohydrate-dependent binding of langerin to SodC, a cell wall glycoprotein of Mycobacterium leprae

Langerhans cells participate in the immune response in leprosy by their ability to activate T cells that recognize the pathogen, Mycobacterium leprae, in a langerin-dependent manner. We hypothesized that langerin, the distinguishing C-type lectin of Langerhans cells, would recognize the highly mannosylated structures in pathogenic Mycobacterium spp. The coding region for the extracellular and neck domain of human langerin was cloned and expressed to produce a recombinant active trimeric form of human langerin (r-langerin). Binding assays performed in microtiter plates, by two-dimensional (2D) Western blotting, and by surface plasmon resonance demonstrated that r-langerin possessed carbohydrate-dependent affinity to glycoproteins in the cell wall of M. leprae. This lectin, however, yielded less binding to mannose-capped lipoarabinomannan (ManLAM) and even lower levels of binding to phosphatidylinositol mannosides. However, the superoxide dismutase C (SodC) protein of the M. leprae cell wall was identified as a langerin-reactive ligand. Tandem mass spectrometry verified the glycosylation of a recombinant form of M. leprae SodC (rSodC) produced in Mycobacterium smegmatis. Analysis of r-langerin affinity by surface plasmon resonance revealed a carbohydrate-dependent affinity of rSodC (equilibrium dissociation constant [KD] = 0.862 μM) that was 20-fold greater than for M. leprae ManLAM (KD = 18.69 μM). These data strongly suggest that a subset of the presumptively mannosylated M. leprae glycoproteins act as ligands for langerin and may facilitate the interaction of M. leprae with Langerhans cells.

The cell envelope glycoconjugates of Mycobacterium tuberculosis

Tuberculosis (TB) remains the second most common cause of death due to a single infectious agent. The cell envelope of Mycobacterium tuberculosis (Mtb), the causative agent of the disease in humans, is a source of unique glycoconjugates and the most distinctive feature of the biology of this organism. It is the basis of much of Mtb pathogenesis and one of the major causes of its intrinsic resistance to chemotherapeutic agents. At the same time, the unique structures of Mtb cell envelope glycoconjugates, their antigenicity and essentiality for mycobacterial growth provide opportunities for drug, vaccine, diagnostic and biomarker development, as clearly illustrated by recent advances in all of these translational aspects. This review focuses on our current understanding of the structure and biogenesis of Mtb glycoconjugates with particular emphasis on one of the most intriguing and least understood aspect of the physiology of mycobacteria: the translocation of these complex macromolecules across the different layers of the cell envelope. It further reviews the rather impressive progress made in the last 10 years in the discovery and development of novel inhibitors targeting their biogenesis.

Dectin-2 is a direct receptor for mannose-capped lipoarabinomannan of mycobacteria

Mycobacteria possess various immunomodulatory molecules on the cell wall. Mannose-capped lipoarabinomannan (Man-LAM), a major lipoglycan of Mycobacterium tuberculosis, has long been known to have both inhibitory and stimulatory effects on host immunity. However, the direct Man-LAM receptor that explains its pleiotropic activities has not been clearly identified. Here, we report that a C-type lectin receptor Dectin-2 (gene symbol Clec4n) is a direct receptor for Man-LAM. Man-LAM activated bone-marrow-derived dendritic cells (BMDCs) to produce pro- and anti-inflammatory cytokines, whereas it was completely abrogated in Clec4n(-/-) BMDCs. Man-LAM promoted antigen-specific T cell responses through Dectin-2 on DCs. Furthermore, Man-LAM induced experimental autoimmune encephalitis (EAE) as an adjuvant in mice, whereas Clec4n(-/-) mice were resistant. Upon mycobacterial infection, Clec4n(-/-) mice showed augmented lung pathology. These results demonstrate that Dectin-2 contributes to host immunity against mycobacterial infection through the recognition of Man-LAM.

Toward Understanding the Essence of Post-Translational Modifications for the Mycobacterium tuberculosis Immunoproteome

CD4⁺ T cells are prominent effector cells in controlling Mycobacterium tuberculosis (Mtb) infection but may also contribute to immunopathology. Studies probing the CD4⁺ T cell response from individuals latently infected with Mtb or patients with active tuberculosis using either small or proteome-wide antigen screens so far revealed a multi-antigenic, yet mostly invariable repertoire of immunogenic Mtb proteins. Recent developments in mass spectrometry-based proteomics have highlighted the occurrence of numerous types of post-translational modifications (PTMs) in proteomes of prokaryotes, including Mtb. The well-known PTMs in Mtb are glycosylation, lipidation, or phosphorylation, known regulators of protein function or compartmentalization. Other PTMs include methylation, acetylation, and pupylation, involved in protein stability. While all PTMs add variability to the Mtb proteome, relatively little is understood about their role in the anti-Mtb immune responses. Here, we review Mtb protein PTMs and methods to assess their role in protective immunity against Mtb.

The mycobacterial cell envelope-lipids

Mycobacterium tuberculosis (Mtb) lipids are indelibly imprinted in just about every key aspect of tuberculosis (TB) basic and translational research. Although the interest in these compounds originally stemmed from their abundance, structural diversity, and antigenicity, continued research in this field has been driven by their important contribution to TB pathogenesis and their interest from the perspective of drug, vaccine, diagnostic, and biomarker development. This article summarizes what is known of the roles of lipids in the physiology and pathogenicity of Mtb and the exciting developments that have occurred in recent years in identifying new lead compounds targeting their biogenesis.

The glycosylated Rv1860 protein of Mycobacterium tuberculosis inhibits dendritic cell mediated TH1 and TH17 polarization of T cells and abrogates protective immunity conferred by BCG

We previously reported interferon gamma secretion by human CD4⁺ and CD8⁺ T cells in response to recombinant E. coli-expressed Rv1860 protein of Mycobacterium tuberculosis (MTB) as well as protection of guinea pigs against a challenge with virulent MTB following prime-boost immunization with DNA vaccine and poxvirus expressing Rv1860. In contrast, a Statens Serum Institute Mycobacterium bovis BCG (BCG-SSI) recombinant expressing MTB Rv1860 (BCG-TB1860) showed loss of protective ability compared to the parent BCG strain expressing the control GFP protein (BCG-GFP). Since Rv1860 is a secreted mannosylated protein of MTB and BCG, we investigated the effect of BCG-TB1860 on innate immunity. Relative to BCG-GFP, BCG-TB1860 effected a significant near total reduction both in secretion of cytokines IL-2, IL-12p40, IL-12p70, TNF-α, IL-6 and IL-10, and up regulation of co-stimulatory molecules MHC-II, CD40, CD54, CD80 and CD86 by infected bone marrow derived dendritic cells (BMDC), while leaving secreted levels of TGF-β unchanged. These effects were mimicked by BCG-TB1860His which carried a 6-Histidine tag at the C-terminus of Rv1860, killed sonicated preparations of BCG-TB1860 and purified H37Rv-derived Rv1860 glycoprotein added to BCG-GFP, but not by E. coli-expressed recombinant Rv1860. Most importantly, BMDC exposed to BCG-TB1860 failed to polarize allogeneic as well as syngeneic T cells to secrete IFN-γ and IL-17 relative to BCG-GFP. Splenocytes from mice infected with BCG-SSI showed significantly less proliferation and secretion of IL-2, IFN-γ and IL-17, but secreted higher levels of IL-10 in response to in vitro restimulation with BCG-TB1860 compared to BCG-GFP. Spleens from mice infected with BCG-TB1860 also harboured significantly fewer DC expressing MHC-II, IL-12, IL-2 and TNF-α compared to mice infected with BCG-GFP. Glycoproteins of MTB, through their deleterious effects on DC may thus contribute to suppress the generation of a TH1- and TH17-dominated adaptive immune response that is vital for protection against tuberculosis.

Tuberculosis (TB), although recognized as an infectious disease for centuries, is still the leading cause of human deaths, claiming a million lives annually. Successful control of TB, either through drugs or effective preventive vaccines has not been achieved despite decades of research. We have studied the role for mannosylated protein Rv1860 of MTB in interfering with the early response of dendritic cells, which belong to the host's innate immune arsenal, to this mycobacterium. We were able to show that incorporating the gene coding for Rv1860 of MTB into the safe vaccine strain BCG resulted in loss of BCG's protective ability in the guinea pig animal model. Using primary mouse bone marrow derived dendritic cells in vitro as well as spleen dendritic cells from infected mice, we show in this study that exposure to mannosylated Rv1860 leads to loss of dendritic cell functions such as cytokine secretion and T cell activation. This leads to defective downstream T cell responses to the mycobacteria. We suggest that altering or extinguishing the expression of such glycoproteins by mycobacteria may be a strategy for developing better vaccines against TB.

Mannan core branching of lipo(arabino)mannan is required for mycobacterial virulence in the context of innate immunity

The causative agent of tuberculosis (TB), Mycobacterium tuberculosis, remains an important worldwide health threat. Although TB is one of the oldest infectious diseases of man, a detailed understanding of the mycobacterial mechanisms underlying pathogenesis remains elusive. Here, we studied the role of the α(1→2) mannosyltransferase MptC in mycobacterial virulence, using the Mycobacterium marinum zebrafish infection model. Like its M. tuberculosis orthologue, disruption of M. marinum mptC (mmar_3225) results in defective elongation of mannose caps of lipoarabinomannan (LAM) and absence of α(1→2)mannose branches on the lipomannan (LM) and LAM mannan core, as determined by biochemical analysis (NMR and GC-MS) and immunoblotting. We found that the M. marinum mptC mutant is strongly attenuated in embryonic zebrafish, which rely solely on innate immunity, whereas minor virulence defects were observed in adult zebrafish. Strikingly, complementation with the Mycobacterium smegmatis mptC orthologue, which restored mannan core branching but not cap elongation, was sufficient to fully complement the virulence defect of the mptC mutant in embryos. Altogether our data demonstrate that not LAM capping, but mannan core branching of LM/LAM plays an important role in mycobacterial pathogenesis in the context of innate immunity.

Physicochemical and biological characterization of synthetic phosphatidylinositol dimannosides and analogues

Native phosphatidylinositol mannosides (PIMs), isolated from the cell wall of Mycobacterium bovis, and synthetic PIM analogues have been reported to offer a variety of immunomodulating properties, including both suppressive and stimulatory activity. While numerous studies have examined the biological activity of these molecules, the aim of this research was to assess the physicochemical properties at a molecular level and correlate these characteristics with biological activity in a mouse model of airway eosinophilia. To accomplish this, we varied the flexibility and lipophilicity of synthetic PIMs by changing the polar headgroup (inositol- vs glycerol-based core) and the length of the acyl chains of the fatty acid residues (C0, C10, C16, and C18). A series of six phosphatidylinositol dimannosides (PIM2s) and phosphatidylglycerol dimannosides (PGM2s) were synthesized and characterized in this study. Langmuir monolayer studies showed that surface pressure-area (π-A) isotherms were greatly influenced by the length of the lipid acyl chains as well as the steric hindrance and volume of the headgroups. In aqueous solution, lipidated PIM2 and PGM2 compounds were observed to self-assemble into circular aggregates, as confirmed by dynamic light scattering and transmission electron microscopic investigations. Removal of the inositol ring but retention of the three-carbon glycerol unit maintained biological activity. We found that the deacylated PGM2, which did not show self-organization, had no effect on the eosinophil numbers but did have an impact on the expansion of OVA-specific CD4(+) Vα2Vβ5 T cells.

Bacterial protein-O-mannosylating enzyme is crucial for virulence of Mycobacterium tuberculosis

A posttranslational protein O-mannosylation process resembling that found in fungi and animals has been reported in the major human pathogen Mycobacterium tuberculosis (Mtb) and related actinobacteria. However, the role and incidence of this process, which is essential in eukaryotes, have never been explored in Mtb. We thus analyzed the impact of interrupting O-mannosylation in the nonpathogenic saprophyte Mycobacterium smegmatis and in the human pathogen Mtb by inactivating the respective putative protein mannosyl transferase genes Msmeg_5447 and Rv1002c. Loss of protein O-mannosylation in both mutant strains was unambiguously demonstrated by efficient mass spectrometry-based glycoproteomics analysis. Unexpectedly, although the M. smegmatis phenotype was unaffected by the lack of manno-proteins, the Mtb mutant had severely impacted growth in vitro and in cellulo associated with a strong attenuation of its pathogenicity in immunocompromised mice. These data are unique in providing evidence of the biological significance of protein O-mannosylation in mycobacteria and demonstrate the crucial contribution of this protein posttranslational modification to Mtb virulence in the host.

Synthesis and biological investigation of PIM mimics carrying biotin or a fluorescent label for cellular imaging

Phosphatidyl inositol mannosides (PIMs) are constituents of the mycobacterial cell wall; these glycolipids are known to exhibit potent inhibitory activity toward the LPS-induced production of cytokines by macrophages, and therefore have potential as anti-inflammatory agents. Recently, heterocyclic analogues of PIMs in which the inositol is replaced by a piperidine (aza-PIM mimics) or a tetrahydropyran moiety (oxa-PIM mimics) have been prepared by short synthetic sequences and shown to retain the biological activity of the parent PIM structures. In this investigation, the aza-PIM analogue was used as a convenient scaffold to link biotin or a fluorescent label (tetramethyl-rhodamine) by way of an aminocaproyl spacer, with the goal of using these conjugates for intracellular localization and for the study of the mechanism of their antiinflammatory action. The synthesis of these compounds is reported, as well as the evaluation of their activities as inhibitors of LPS-induced cytokine production by macrophages (TNFα, IL12p40); preliminary investigations by FACS and confocal microscopy indicated that PIM-biotin conjugate binds to macrophage membranes with rapid kinetics.

Lipoarabinomannan mannose caps do not affect mycobacterial virulence or the induction of protective immunity in experimental animal models of infection and have minimal impact on in vitro inflammatory responses

Mannose-capped lipoarabinomannan (ManLAM) is considered an important virulence factor of Mycobacterium tuberculosis. However, while mannose caps have been reported to be responsible for various immunosuppressive activities of ManLAM observed in vitro, there is conflicting evidence about their contribution to mycobacterial virulence in vivo. Therefore, we used Mycobacterium bovis BCG and M. tuberculosis mutants that lack the mannose cap of LAM to assess the role of ManLAM in the interaction of mycobacteria with the host cells, to evaluate vaccine-induced protection and to determine its importance in M. tuberculosis virulence. Deletion of the mannose cap did not affect BCG survival and replication in macrophages, although the capless mutant induced a somewhat higher production of TNF. In dendritic cells, the capless mutant was able to induce the upregulation of co-stimulatory molecules and the only difference we detected was the secretion of slightly higher amounts of IL-10 as compared to the wild type strain. In mice, capless BCG survived equally well and induced an immune response similar to the parental strain. Furthermore, the efficacy of vaccination against a M. tuberculosis challenge in low-dose aerosol infection models in mice and guinea pigs was not affected by the absence of the mannose caps in the BCG. Finally, the lack of the mannose cap in M. tuberculosis did not affect its virulence in mice nor its interaction with macrophages in vitro. Thus, these results do not support a major role for the mannose caps of LAM in determining mycobacterial virulence and immunogenicity in vivo in experimental animal models of infection, possibly because of redundancy of function.

Cyanovirin-N inhibits mannose-dependent Mycobacterium-C-type lectin interactions but does not protect against murine tuberculosis

Cyanovirin-N (CV-N) is a mannose-binding lectin that inhibits HIV-1 infection by blocking mannose-dependent target cell entry via C-type lectins. Like HIV-1, Mycobacterium tuberculosis expresses mannosylated surface structures and exploits C-type lectins to gain cell access. In this study, we investigated whether CV-N, like HIV-1, can inhibit M. tuberculosis infection. We found that CV-N specifically interacted with mycobacteria by binding to the mannose-capped lipoglycan lipoarabinomannan. Furthermore, CV-N competed with the C-type lectins DC-SIGN and mannose receptor for ligand binding and inhibited the binding of M. tuberculosis to dendritic cells but, unexpectedly, not to macrophages. Subsequent in vivo infection experiments in a mouse model demonstrated that, despite its activity, CV-N did not inhibit or delay M. tuberculosis infection. This outcome argues against a critical role for mannose-dependent C-type lectin interactions during the initial stages of murine M. tuberculosis infection and suggests that, depending on the circumstances, M. tuberculosis can productively infect cells using different modes of entry.

Identification of the Mycobacterium marinum Apa antigen O-mannosylation sites reveals important glycosylation variability with the M. tuberculosis Apa homologue

The 45/47 kDa Apa, an immuno-dominant antigen secreted by Mycobacterium tuberculosis is O-mannosylated at multiple sites. Glycosylation of Apa plays a key role in colonization and invasion of the host cells by M. tuberculosis through interactions of Apa with the host immune system C-type lectins. Mycobacterium marinum (M.ma) a fish pathogen, phylogenetically close to M. tuberculosis, induces a granulomatous response with features similar to those described for M. tuberculosis in human. Although M.ma possesses an Apa homologue, its glycosylation status is unknown, and whether this represents a crucial element in the pathophysiology induced by M.ma remains to be addressed. To this aim, we have identified two concanavalin A-reactive 45/47 kDa proteins from M.ma, which have been further purified by a two-step anion exchange chromatography process. Advanced liquid chromatography-nanoESI mass spectrometry-based proteomic analyses of peptides, derived from either tryptic digestion alone or in combination with the Asp-N endoproteinase, established that M.ma Apa possesses up to seven distinct O-mannosylated sites with mainly single mannose substitutions, which can be further extended at the Ser/Thr/Pro rich region near the N-terminus. This opens the way to further studies focussing on the involvement and biological functions of Apa O-mannosylation using the M.ma/zebrafish model.

Comparison of antibodies and carbohydrates to target vaccines to human dendritic cells via DC-SIGN

Vaccine efficacy is improved upon specific delivery to professional antigen (Ag) presenting cells, such as dendritic cells (DCs). Antigenicity and adjuvanticity of vaccine components can be enhanced by encapsulation within nanoparticle (NP) vaccine carriers that are targeted to the human DC-specific C-type lectin receptor DC-SIGN. Here we used two strategies to target vaccines components to DC-SIGN: 1) carbohydrates as natural receptor ligands and 2) receptor-specific antibodies (Abs). To determine the optimal targeting strategy, we coated NP vaccines harboring MHC class I or II-restricted Ags and the TLR ligands (TLRLs) poly I:C and resiquimod with either the DC-SIGN ligands Lewis-X (Le(x)), mannosylated lipoarabinomannan (ManLAM), glycosylated HIV protein gp120, or three distinct DC-SIGN Abs. Although, because of their lower MW, surface coating of NP vaccines with carbohydrates resulted in a higher number of surface molecules per NP than coating with Abs, NP vaccines carrying Abs were more effectively bound and internalized by human DCs than carriers harboring Le(x), ManLAM or gp120. Furthermore, NP vaccines harboring TLRLs triggered significant induction of DC maturation markers when compared to those without TLRLs, irrespective of the targeting moiety. Ab- and gp120-mediated targeting induced equally high levels of proinflammatory cytokines and increased presentation of the MHC class I-restricted epitope. By contrast, presentation of the MHC class II-restricted epitope was more efficient upon Ab-mediated targeting than when using gp120, Le(x) or ManLAM. From these findings we conclude that receptor-specific Abs are more effective than carbohydrates for DC-targeted vaccination strategies.