Inositol phosphate capping of the nonreducing termini of lipoarabinomannan from rapidly growing strains of Mycobacterium

Phylactic role of anti-lipoarabinomannan IgM directed against mannan core during mycobacterial infection in macrophages

Mycobacteria enter host phagocytes, such as macrophages by binding to several receptors on phagocytes. Several mycobacterial species, including Mycobacterium tuberculosis have evolved systems to evade host bactericidal pathways. Lipoarabinomannan (LAM) is an essential mycobacterial molecule for both binding to phagocytes and escaping from bactericidal pathways. Integrin CD11b plays critical roles as a phagocytic receptor and contributes to host defense by mediating both nonopsonic and opsonic phagocytosis. However, the mechanisms by which CD11b-mediated phagocytosis associates with LAM and drives the phagocytic process of mycobacteria remain to be fully elucidated. We recently identified TMDU3 as anti-LAM IgM antibody against the mannan core of LAM. The present study investigated the roles of CD11b and TMDU3 in macrophage phagocytosis of mycobacteria and subsequent bactericidal lysosomal fusion to phagosomes. CD11b knockout cells generated by a CRISPR/Cas9 system showed significant attenuation of the ability to phagocytose non-opsonized mycobacteria and LAM-conjugated beads. Moreover, recombinant human CD11b protein was found to bind to LAM. TMDU3 markedly inhibited macrophage phagocytosis of non-opsonized mycobacteria. This antibody slightly increased the phagocytosis of mycobacteria under opsonized conditions, whereas it significantly enhanced CD11b-mediated bactericidal functions. Taken together, these results show a novel phylactic role of anti-LAM IgM during mycobacterial infection in macrophages.

Collected Thoughts on Mycobacterial Lipoarabinomannan, a Cell Envelope Lipoglycan

The presence of lipoarabinomannan (LAM) in the Mycobacterium tuberculosis (Mtb) cell envelope was first reported close to 100 years ago. Since then, numerous studies have been dedicated to the isolation, purification, structural definition, and elucidation of the biological properties of Mtb LAM. In this review, we present a brief historical perspective on the discovery of Mtb LAM and the herculean efforts devoted to structurally characterizing the molecule because of its unique structural and biological features. The significance of LAM remains high to this date, mainly due to its distinct immunological properties in conjunction with its role as a biomarker for diagnostic tests due to its identification in urine, and thus can serve as a point-of-care diagnostic test for tuberculosis (TB). In recent decades, LAM has been thoroughly studied and massive amounts of information on this intriguing molecule are now available. In this review, we give the readers a historical perspective and an update on the current knowledge of LAM with information on the inherent carbohydrate composition, which is unique due to the often puzzling sugar residues that are specifically found on LAM. We then guide the readers through the complex and myriad immunological outcomes, which are strictly dependent on LAM's chemical structure. Furthermore, we present issues that remain unresolved and represent the immediate future of LAM research. Addressing the chemistry, functions, and roles of LAM will lead to innovative ways to manipulate the processes that involve this controversial and fascinating biomolecule.

Monoclonal antibodies to lipoarabinomannan/arabinomannan - characteristics and implications for tuberculosis research and diagnostics

Antibodies to the mycobacterial surface lipoglycan lipoarabinomannan (LAM) and its related capsular polysaccharide arabinomannan (AM) are increasingly important for investigations focused on both understanding mechanisms of protection against Mycobacterium tuberculosis (Mtb) and developing next-generation point-of-care tuberculosis (TB) diagnostics. We provide here an overview of the growing pipeline of monoclonal antibodies (mAbs) to LAM/AM. Old and new methodologies for their generation are reviewed and we outline and discuss their glycan epitope specificity and other features with implications for the TB field.

Performance of novel antibodies for lipoarabinomannan to develop diagnostic tests for Mycobacterium tuberculosis

Lipoarabinomannan (LAM), a component of the Mycobacterium tuberculosis (MTB) cell wall, is detectable in the urine of MTB infected patients with active tuberculosis (TB). LAM-specific antibodies (Igs) have been developed by a variety of traditional and recombinant methods for potential use in a rapid diagnostic test (RDT). We evaluated the analytical performance of the TB LAM Igs to identify pairs that offer superior performance over existing urine LAM tests. We assessed 25 new and 4 existing Igs in a matrixed format using a multiplex electrochemiluminescence-based liquid immunoassay. A total of 841 paired Ig combinations were challenged with in vitro cultured LAM (cLAM) derived from MTB strains representing diverse phylogenetic lineages, alongside urinary LAM (uLAM) from the urine of adults with active pulmonary TB. Analytical sensitivity of down-selected Ig pairs was determined using MTB Aoyama-B cLAM, while diagnostic accuracy was determined using clinical samples. When testing cLAM, the reactivity of Ig pairs was similar across MTB lineages 1-4 but lineage 5:6 had significantly more reactivity among Ig pairs. Overall, 41 Ig pairs had a strong binding affinity to cLAM, as compared to the reference pair of S4-20/A194-01, and 28 Ig pairs therein exhibited a strong affinity for both cLAM and uLAM. Retrospective testing on clinical urine specimens demonstrated varying sensitivities (12-80%) and specificities (14-100%). The five top pairs had a similar analytical limit of detection to the reference pair but in four instances, the sensitivity and specificity with clinical uLAM samples was poor. Overall, epitopes presented by uLAM are different from cLAM, which may affect antibody performance when testing uLAM in patient samples. Several new Ig pairs had similar ranges of high sensitivity to cLAM but overall, there were no new candidate Ig pairs identified in this round of screening with increased performance with uLAM as compared to an existing optimal pair.

Identification of anti-lipoarabinomannan antibodies against mannan core and their effects on phagocytosis of mycobacteria by human neutrophils

Mycobacterium tuberculosis (MTB) and M. avium-intracellulare complex (MAC) enter host phagocytes, such as neutrophils through lipoarabinomannan (LAM) binding to pattern-recognition receptors, inducing innate immune responses including phagocytosis. Phagocytosis of mycobacteria by human neutrophils depends on the binding of α(1 → 2)-monomannose branching α(1 → 6)-mannan core of LAM/lipomannan (LM), a common component among mycobacterial species, to lactosylceramide (LacCer)-enriched lipid microdomains. We investigated the binding specificities of several anti-LAM antibodies (Abs) to LAMs/LM and found anti-LAM monoclonal IgMs TMDU3 and LA066 were directed against mannan core. Each IgM showed different binding specificity to mannan core. Confocal and stimulated emission depletion microscopy revealed TMDU3 and LA066 strongly bind to MTB and MAC, respectively. Flow cytometric analysis revealed human neutrophils do not express Dectin-2, DC-SIGN or mannose receptor. Furthermore, neutrophil phagocytosis of mycobacteria was markedly inhibited by TMDU3 and LA066, respectively. Similarly, treatment of each mAb with neutrophils reduced the numbers of intracellular MAC. Together, our results suggest that the interaction of LacCer-enriched lipid microdomains with mannan core and its blocking are therapeutic or diagnostic targets for both TB and non-tuberculous mycobacteria infection.

Functional analysis and enzyme characterization of Mannose-1-phosphate guanylyl transferase (ManB) from Mycobacterium tuberculosis

Mycobacterium tuberculosis cell wall consist variety of mannose containing glycoconjugates including lipomannan (LM) and lipoarabinomannan (LAM). These lipoglycans are involved in cell wall integrity and play role in virulence of M. tuberculosis by modulating host immune response. GDP-mannose, required for the synthesis of lipoglycans, is catalyzed by enzyme Mannose-1-phosphate guanylyl transferase (ManB). The enzyme with similar function has been studied in variety of species of prokaryotes and eukaryotes. However, biological role of ManB and its enzymatic activity remains uncharacterized in M. tuberculosis. In present study, we elucidated the role of enzyme by constructing manB knockdown strain of M. tuberculosis H37Ra. The manB knockdown decreased the cell growth and also effected the morphology of M. tuberculosis by altering the permeability of cell membrane. These findings provide the understanding on ManB function and suggesting that ManB could be the potential target for novel anti-tuberculosis drug. Furthermore, we also characterized ManB enzyme by establishing 96 well plate colorimetric assay and determined the kinetic properties including initial velocity, optimum temperature, optimum pH and other kinetic parameters. Our established assay will be helpful for further high throughput screening of potential inhibitors against ManB.

Structural implications of lipoarabinomannan glycans from global clinical isolates in diagnosis of Mycobacterium tuberculosis infection

In Mycobacterium tuberculosis (Mtb), surface-exposed Lipoarabinomannan (LAM) is a key determinant of immunogenicity, yet its intrinsic heterogeneity confounds typical structure–function analysis. Recently, LAM gained a strong foothold as a validated marker for active tuberculosis (TB) infection and has shown great potential in new diagnostic efforts. However, no efforts have yet been made to model or evaluate the impact of mixed polyclonal Mtb infections (infection with multiple strains) on TB diagnostic procedures other than antibiotic susceptibility testing. Here, we selected three TB clinical isolates (HN878, EAI, and IO) and purified LAM from these strains to present an integrated analytical approach of one-dimensional and two-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy, as well as enzymatic digestion and site-specific mass spectrometry (MS) to probe LAM structure and behavior at multiple levels. Overall, we found that the glycan was similar in all LAM preparations, albeit with subtle variations. Succinates, lactates, hydroxybutyrate, acetate, and the hallmark of Mtb LAM-methylthioxylose (MTX), adorned the nonreducing terminal arabinan of these LAM species. Newly identified acetoxy/hydroxybutyrate was present only in LAM from EAI and IO Mtb strains. Notably, detailed LC/MS-MS unambiguously showed that all acyl modifications and the lactyl ether in LAM are at the 3-OH position of the 2-linked arabinofuranose adjacent to the terminal β-arabinofuranose. Finally, after sequential enzymatic deglycosylation of LAM, the residual glycan that has ∼50% of α−arabinofuranose -(1→5) linked did not bind to monoclonal antibody CS35. These data clearly indicate the importance of the arabinan termini arrangements for the antigenicity of LAM.

Lipoarabinomannan as a Point-of-Care Assay for Diagnosis of Tuberculosis: How Far Are We to Use It?

Tuberculosis (TB) is still a severe public health problem; the current diagnostic tests have limitations that delay treatment onset. Lipoarabinomannan (LAM) is a glycolipid that is a component of the cell wall of the bacillus Mycobacterium tuberculosis, the etiologic agent of TB. This glycolipid is excreted as a soluble form in urine. The World Health Organization has established that the design of new TB diagnostic methods is one of the priorities within the EndTB Strategy. LAM has been suggested as a biomarker to develop diagnostic tests based on its identification in urine, and it is one of the most prominent candidates to develop point-of-care diagnostic test because urine samples can be easily collected. Moreover, LAM can regulate the immune response in the host and can be found in the serum of TB patients, where it probably affects a wide variety of host cell populations, consequently influencing the quality of both innate and adaptive immune responses during TB infection. Here, we revised the evidence that supports that LAM could be used as a tool for the development of new point-of-care tests for TB diagnosis, and we discussed the mechanisms that could contribute to the low sensitivity of diagnostic testing.

A Siloxane-Bridged Glycosyl Donor Enables Highly Stereoselective β-Xylulofuranosylation

We report a siloxane-protected donor (7) for the highly stereoselective formation of β-(2,3-cis)-xylulofuranosyl bonds. Glycosylation reactions with 7 gave >80% yields, and only β-xylulofuranosides were isolated in all cases. The utility of 7 for the synthesis of complex glycans was shown by its successful application to the preparation of the repeating unit from the lipopolysaccharide O-antigen of Yersinia enterocolitica serovars O:5/O:5,27. This structure is a pentasaccharide with two β-xylulofuranose residues; using 7, both were introduced simultaneously with excellent stereocontrol.

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.

Antibiotics and resistance: the two-sided coin of the mycobacterial cell wall

Mycobacterium tuberculosis, the bacterium responsible for tuberculosis, is the global leading cause of mortality from an infectious agent. Part of this success relies on the unique cell wall, which consists of a thick waxy coat with tightly packed layers of complexed sugars, lipids and peptides. This coat provides a protective hydrophobic barrier to antibiotics and the host's defences, while enabling the bacterium to spread efficiently through sputum to infect and survive within the macrophages of new hosts. However, part of this success comes at a cost, with many of the current first- and second-line drugs targeting the enzymes involved in cell wall biosynthesis. The flip side of this coin is that resistance to these drugs develops either in the target enzymes or the activation pathways of the drugs, paving the way for new resistant clinical strains. This review provides a synopsis of the structure and synthesis of the cell wall and the major current drugs and targets, along with any mechanisms of resistance.

Lipoarabinomannan in sputum to detect bacterial load and treatment response in patients with pulmonary tuberculosis: Analytic validation and evaluation in two cohorts

BACKGROUND

Lipoarabinomannan (LAM) is a major antigen of Mycobacterium tuberculosis (MTB). In this report, we evaluated the ability of a novel immunoassay to measure concentrations of LAM in sputum as a biomarker of bacterial load prior to and during treatment in pulmonary tuberculosis (TB) patients.

METHODS AND FINDINGS

Phage display technology was used to isolate monoclonal antibodies binding to epitopes unique in LAM from MTB and slow-growing nontuberculous mycobacteria (NTM). Using these antibodies, a sandwich enzyme-linked immunosorbent assay (LAM-ELISA) was developed to quantitate LAM concentration. The LAM-ELISA had a lower limit of quantification of 15 pg/mL LAM, corresponding to 121 colony-forming units (CFUs)/mL of MTB strain H37Rv. It detected slow-growing NTMs but without cross-reacting to common oral bacteria. Two clinical studies were performed between the years 2013 and 2016 in Manila, Philippines, in patients without known human immunodeficiency virus (HIV) coinfection. In a case-control cohort diagnostic study, sputum specimens were collected from 308 patients (aged 17-69 years; 62% male) diagnosed as having pulmonary TB diseases or non-TB diseases, but who could expectorate sputum, and were then evaluated by smear microscopy, BACTEC MGIT 960 Mycobacterial Detection System (MGIT) and Lowenstein-Jensen (LJ) culture, and LAM-ELISA. Some sputum specimens were also examined by Xpert MTB/RIF. The LAM-ELISA detected all smear- and MTB-culture-positive samples (n = 70) and 50% (n = 29) of smear-negative but culture-positive samples (n = 58) (versus 79.3%; 46 positive cases by the Xpert MTB/RIF), but none from non-TB patients (n = 56). Among both LAM and MGIT MTB-culture-positive samples, log10-transformed LAM concentration and MGIT time to detection (TTD) showed a good inverse relationship (r = -0.803, p < 0.0001). In a prospective longitudinal cohort study, 40 drug-susceptible pulmonary TB patients (aged 18-69 years; 60% male) were enrolled during the first 56 days of the standard 4-drug therapy. Declines in sputum LAM concentrations correlated with increases of MGIT TTD in individual patients. There was a 1.29 log10 decrease of sputum LAM concentration, corresponding to an increase of 221 hours for MGIT TTD during the first 14 days of treatment, a treatment duration often used in early bactericidal activity (EBA) trials. Major limitations of this study include a relatively small number of patients, treatment duration up to only 56 days, lack of quantitative sputum culture CFU count data, and no examination of the correlation of sputum LAM to clinical cure.

CONCLUSIONS

These results indicate that the LAM-ELISA can determine LAM concentration in sputum, and sputum LAM measured by the assay may be used as a biomarker of bacterial load prior to and during TB treatment. Additional studies are needed to examine the predictive value of this novel biomarker on treatment outcomes.

A paucity of knowledge regarding nontuberculous mycobacterial lipids compared to the tubercle bacillus

All mycobacteria, including nontuberculous mycobacteria (NTM), synthesize an array of lipids including phosphatidylinositol mannosides (PIM), lipomannan (LM), and lipoarabinomannan (LAM). While absent from Mycobacterium tuberculosis (M. tb), glycopeptidolipids (GPL) are critical to the biology of NTM. M. tb and some NTM also synthesize trehalose-containing glycolipids and phenolic glycolipids (PGL), key membrane constituents with essential roles in metabolism. While lipids facilitate immune evasion, they also induce host immunity against tuberculosis. However, much less is known about the significance of NTM-derived PIM, LM, LAM, GPL, trehalose-containing glycolipids, and PGL as virulence factors, warranting further investigation. While culling the scientific literature on NTM lipids, it's evident that such studies were relatively few in number with the overwhelming majority of prior work dedicated to understanding lipids from the saprophyte Mycobacterium smegmatis. The identification and functional analysis of immune reactive NTM-derived lipids remain challenging, but such work is likely to yield a greater understanding of the pathogenesis of NTM lung disease. In this review, we juxtapose the vast literature of what is currently known regarding M. tb lipids to the lesser number of studies for comparable NTM lipids. But because GPL is the most widely recognized NTM lipid, we highlight its role in disease pathogenesis.

Lectins of Mycobacterium tuberculosis - rarely studied proteins

The importance of bacterial lectins for adhesion, pathogenicity, and biofilm formation is well established for many Gram-positive and Gram-negative bacteria. However, there is very little information available about lectins of the tuberculosis-causing bacterium, Mycobacterium tuberculosis (Mtb). In this paper we review previous studies on the carbohydrate-binding characteristics of mycobacteria and related Mtb proteins, discussing their potential relevance to Mtb infection and pathogenesis.

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.

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.

Characterization of the Antigenic Heterogeneity of Lipoarabinomannan, the Major Surface Glycolipid of Mycobacterium tuberculosis, and Complexity of Antibody Specificities toward This Antigen

Lipoarabinomannan (LAM), the major antigenic glycolipid of Mycobacterium tuberculosis, is an important immunodiagnostic target for detecting tuberculosis (TB) infection in HIV-1–coinfected patients, and is believed to mediate a number of functions that promote infection and disease development. To probe the human humoral response against LAM during TB infection, several novel LAM-specific human mAbs were molecularly cloned from memory B cells isolated from infected patients and grown in vitro. The fine epitope specificities of these Abs, along with those of a panel of previously described murine and phage-derived LAM-specific mAbs, were mapped using binding assays against LAM Ags from several mycobacterial species and a panel of synthetic glycans and glycoconjugates that represented diverse carbohydrate structures present in LAM. Multiple reactivity patterns were seen that differed in their specificity for LAM from different species, as well as in their dependence on arabinofuranoside branching and nature of capping at the nonreducing termini. Competition studies with mAbs and soluble glycans further defined these epitope specificities and guided the design of highly sensitive immunodetection assays capable of detecting LAM in urine of TB patients, even in the absence of HIV-1 coinfection. These results highlighted the complexity of the antigenic structure of LAM and the diversity of the natural Ab response against this target. The information and novel reagents described in this study will allow further optimization of diagnostic assays for LAM and may facilitate the development of potential immunotherapeutic approaches to inhibit the functional activities of specific structural motifs in LAM.

Disruption of Mycobacterial AftB Results in Complete Loss of Terminal β(1 → 2) Arabinofuranose Residues of Lipoarabinomannan

Lipoarabinomannan (LAM) and arabinogalactan (AG) are the two major mycobacterial cell wall (lipo)polysaccharides, which contain a structurally similar arabinan domain that is highly branched and assembled in a stepwise fashion by variety of arabinofuranosyltransferases (ArafT). In addition to playing an essential role in mycobacterial physiology, LAM and its biochemical precursor lipomannan possess potent immunomodulatory activities that affect the host immune response. In the search of additional mycobacterial ArafTs that participate in the synthesis of the arabinan segment of LAM, we disrupted aftB (MSMEG_6400) in Mycobacterium smegmatis. The deletion of chromosomal aftB locus could only be achieved in the presence of a rescue plasmid carrying a functional copy of aftB, strongly suggesting that it is essential for the viability of M. smegmatis. Isolation and detailed structural characterization of a LAM molecule derived from the conditional mutant deficient in AftB revealed the absence of terminal β(1 → 2)-linked arabinofuranosyl residues. Furthermore, we demonstrated that truncated LAM displays proinflammatory activity, which is due to its ability to activate Toll-like receptor 2. All together, our results indicate that AftB is an essential mycobacterial ArafT that plays a role in the synthesis of the arabinan domain of LAM.

Lipoarabinomannan binding to lactosylceramide in lipid rafts is essential for the phagocytosis of mycobacteria by human neutrophils

Pathogenic mycobacteria use virulence factors, including mannose-capped lipoarabinomannan (ManLAM), to survive in host phagocytic cells, such as neutrophils. We assessed the roles of lactosylceramide (LacCer, CDw17)-enriched lipid rafts in the phagocytosis of mycobacteria by human neutrophils and in the intracellular fate of phagocytosed mycobacteria. We showed that the association of the Src family kinase (SFK) Lyn with C24 fatty acid chain-containing LacCer was essential for the phagocytosis of mycobacteria by neutrophils. Assays with LacCer-containing liposomes, LacCer-coated plastic plates, and LAM-coated beads demonstrated that the phagocytosis of mycobacteria was mediated through the binding of LacCer to LAM. Both ManLAM from pathogenic species and phosphoinositol-capped LAM (PILAM) from nonpathogenic Mycobacterium smegmatis bound equivalently to LacCer to stimulate phagocytosis. However, PILAM from an M. smegmatis α1,2-mannosyltransferase deletion mutant (ΔMSMEG_4247), lacking the α1,2-monomannose side branches of the LAM mannan core, did not bind to LacCer or induce phagocytosis. An anti-LacCer antibody immunoprecipitated the SFK Hck from the phagosomes of neutrophils that internalized nonpathogenic mycobacteria but not from those that internalized pathogenic mycobacteria. Furthermore, knockdown of Hck by short inhibitory RNA abolished the fusion of lysosomes with phagosomes containing nonpathogenic mycobacteria. Further analysis showed that ManLAM, but not PILAM, inhibited the association of Hck with LacCer-enriched lipid rafts in phagosomal membranes, effectively blocking phagolysosome formation. Together, these findings suggest that pathogenic mycobacteria use ManLAM not only for binding to LacCer-enriched lipid rafts and entering neutrophils but also for disrupting signaling through Hck-coupled, LacCer-enriched lipid rafts and preventing phagolysosome formation.

Lipoarabinomannan, and its related glycolipids, induce divergent and opposing immune responses to Mycobacterium tuberculosis depending on structural diversity and experimental variations

Exposure to Mycobacterium tuberculosis (Mtb) may lead to active or latent tuberculosis, or clearance of Mtb, depending essentially on the quality of the host's immune response. This response is initiated through the interaction of Mtb cell wall surface components, mostly glycolipids, with cells of the innate immune system, particularly macrophages (Mφs) and dendritic cells (DCs). The way Mφs and DC alter their cytokine secretome, activate or inhibit different microbicidal mechanisms and present antigens and consequently trigger the T cell-mediated immune response impacts the host immune response against Mtb. Lipoarabinomannan (LAM) is one of the major cell wall components of Mtb. Mannosyl-capped LAM (ManLAM), and its related cell wall-associated types of glycolipids/lipoglycans, namely phosphatidylinositol mannosides (PIMs) and lipomannan (LM), exhibit important and distinct immunomodulatory properties. The structure, internal heterogeneity and abundance of these molecules vary between Mtb strains exhibiting distinct degrees of virulence. Thus ManLAM, LM and PIMs may be considered crucial Mtb-associated virulence factors in the pathogenesis of tuberculosis. Of particular relevance for this review, there is controversy about the specific immunomodulatory properties of these distinct glycolipids, particularly when tested as purified molecules in vitro. In addition to the variability in the glycolipid composition conflicting reports may also result from differences in the protocols used for glycolipid isolation and for in vitro experiments including immune cell types and procedures to generate them. Understanding the immunomodulatory properties of these cell wall glycolipids, how they differ between distinct Mtb strains, and how they influence the degree of Mtb virulence, is of utmost relevance to understand how the host mounts a protective or otherwise pathologic immune response. This is essential for the design of preventive strategies against tuberculosis. Thus, since clarifying the controversy on this matter is crucial we here review, summarize and discuss reported data from in vitro stimulation with the three major Mtb complex cell wall glycolipids (ManLAM, PIMs and LM) in an attempt to conciliate the conflicting findings.

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.

Mannosylated lipoarabinomannans from Mycobacterium avium subsp. paratuberculosis alters the inflammatory response by bovine macrophages and suppresses killing of Mycobacterium avium subsp. avium organisms

Analysis of the mechanisms through which pathogenic mycobacteria interfere with macrophage activation and phagosome maturation have shown that engagement of specific membrane receptors with bacterial ligands is the initiating event. Mannosylated lipoarabinomannan (Man-LAM) has been identified as one of the ligands that modulates macrophage function. We evaluated the effects of Man-LAM derived from Mycobacterium avium subsp. paratuberculosis (MAP) on bovine macrophages. Man-LAM induced a rapid and prolonged expression of IL-10 message as well as transient expression of TNF-α. Preincubation with Man-LAM for up to 16 h did not suppress expression of IL-12 in response to interferon-γ. Evaluation of the effect of Man-LAM on phagosome acidification, phagosome maturation, and killing of Mycobacterium avium subsp. avium (MAA) showed that preincubation of macrophages with Man-LAM before addition of MAA inhibited phagosome acidification, phagolysosome fusion, and reduced killing. Analysis of signaling pathways provided indirect evidence that inhibition of killing was associated with activation of the MAPK-p38 signaling pathway but not the pathway involved in regulation of expression of IL-10. These results support the hypothesis that MAP Man-LAM is one of the virulence factors facilitating survival of MAP in macrophages.

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.

Macrophages in tuberculosis: friend or foe

Tuberculosis (TB) remains one of the greatest threats to human health. The causative bacterium, Mycobacterium tuberculosis (Mtb), is acquired by the respiratory route. It is exquisitely human adapted and a prototypic intracellular pathogen of macrophages, with alveolar macrophages (AMs) being the primary conduit of infection and disease. The outcome of primary infection is most often a latently infected healthy human host, in whom the bacteria are held in check by the host immune response. Such individuals can develop active TB later in life with impairment in the immune system. In contrast, in a minority of infected individuals, the host immune response fails to control the growth of bacilli, and progressive granulomatous disease develops, facilitating spread of the bacilli via infectious aerosols coughed out into the environment and inhaled by new hosts. The molecular details of the Mtb-macrophage interaction continue to be elucidated. However, it is clear that a number of complex processes are involved at the different stages of infection that may benefit either the bacterium or the host. Macrophages demonstrate tremendous phenotypic heterogeneity and functional plasticity which, depending on the site and stage of infection, facilitate the diverse outcomes. Moreover, host responses vary depending on the specific characteristics of the infecting Mtb strain. In this chapter, we describe a contemporary view of the behavior of AMs and their interaction with various Mtb strains in generating unique immunologic lung-specific responses.

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.

Divergent effects of mycobacterial cell wall glycolipids on maturation and function of human monocyte-derived dendritic cells

BACKGROUND

Mycobacterium tuberculosis (Mtb) is able to evade the immune defenses and may persist for years, decades and even lifelong in the infected host. Mtb cell wall components may contribute to such persistence by modulating several pivotal types of immune cells. Dendritic cells (DCs) are the most potent antigen-presenting cells and hence play a crucial role in the initial immune response to infections by connecting the innate with the adaptive immune system.

PRINCIPAL FINDINGS

We investigated the effects of two of the major mycobacterial cell wall-associated types of glycolipids, mannose-capped lipoarabinomannan (ManLAM) and phosphatidylinositol mannosides (PIMs) purified from the Mtb strains H37Rv and Mycobacterium bovis, on the maturation and cytokine profiles of immature human monocyte-derived DCs. ManLAM from Mtb H37Rv stimulated the release of pro-inflammatory cytokines TNF, IL-12, and IL-6 and expression of co-stimulatory (CD80, CD86) and antigen-presenting molecules (MHC class II). ManLAM from M. bovis also induced TNF, IL-12 and IL-6 but at significantly lower levels. Importantly, while ManLAM was found to augment LPS-induced DC maturation and pro-inflammatory cytokine production, addition of PIMs from both Mtb H37Rv and M. bovis strongly reduced this stimulatory effect.

CONCLUSIONS

These results indicate that the mycobacterial cell wall contains macromolecules of glycolipid nature which are able to induce strong and divergent effects on human DCs; i.e while ManLAM is immune-stimulatory, PIMs act as powerful inhibitors of DC cytokine responses. Thus PIMs may be important Mtb-associated virulence factors contributing to the pathogenesis of tuberculosis disease. These findings may also aid in the understanding of some earlier conflicting reports on the immunomodulatory effects exerted by different ManLAM preparations.

Arabinogalactan and lipoarabinomannan biosynthesis: structure, biogenesis and their potential as drug targets

Mycobacterium tuberculosis, the etiological agent of TB, remains the leading cause of mortality from a single infectious organism. The persistence of this human pathogen is associated with its distinctive lipid-rich cell wall structure that is highly impermeable to hydrophilic chemical drugs. This highly complex and unique structure is crucial for the growth, viability and virulence of M. tuberculosis, thus representing an attractive target for vaccine and drug development. It contains a large macromolecular structure known as the mycolyl-arabinogalactan-peptidoglycan complex, as well as phosphatidyl-myo-inositol derived glycolipids with potent immunomodulatory activity, notably lipomannan and lipoarabinomannan. These cell wall components are often the targets of effective chemotherapeutic agents against TB, such as ethambutol. This review focuses on the structural details and biosynthetic pathways of both arabinogalactan and lipoarabinomannan, as well as the effects of potent drugs on these important (lipo)polysaccharides.

Comparative evaluation of profiles of antibodies to mycobacterial capsular polysaccharides in tuberculosis patients and controls stratified by HIV status

Despite the complexity of tuberculosis (TB) serology, antibodies (Abs) remain attractive biomarkers for TB. Recent evidence of a mycobacterial capsule that consists mainly of the polysaccharides arabinomannan (AM) and glucan provides new options for serologic targets. For this study, Ab responses to AM and glucan for 47 U.S. TB patients (33 HIV negative [HIV(-)], 14 HIV positive [HIV(+)]), 42 healthy controls, and 38 asymptomatic HIV(+) controls were evaluated by enzyme-linked immunosorbent assays (ELISAs). The results were compared with Ab responses to the mycobacterial glycolipid cell wall antigen lipoarabinomannan (LAM) and to the proteins malate synthase (MS) and MPT51. We found that the main immunoglobulin (Ig) isotype response to polysaccharides was IgG, predominantly of subclass IgG2. IgG responses to AM were significantly higher for HIV(-) and HIV(+) TB cases than for controls (P, <0.0001 and <0.01, respectively); significantly higher for HIV(-) than for HIV(+) TB cases (P, <0.01); and significantly higher in sputum smear-positive than smear-negative patients in both HIV(-) and HIV(+) cases (P, 0.01 and 0.02, respectively). In both TB groups, titers of Ab to glucan were significantly lower than titers of Ab to AM (P, <0.0001). IgG responses to AM and MS or to AM and MPT51 did not correlate with each other in HIV(-) TB patients, while they correlated significantly in HIV(+) TB patients (P, 0.01 and 0.05, respectively). We conclude that Ab responses to AM could contribute to the serodiagnosis of TB, especially for HIV(-) TB patients. This study also provides new and important insights into the differences in the profiles of Abs to mycobacterial antigens between HIV(-) and HIV(+) TB patients.

Lipoarabinomannan localization and abundance during growth of Mycobacterium smegmatis

Lipoarabinomannan (LAM) is a structurally heterogeneous amphipathic lipoglycan present in Mycobacterium spp. and other actinomycetes, which constitutes a major component of the cell wall and exhibits a wide spectrum of immunomodulatory effects. Analysis of Mycobacterium smegmatis subcellular fractions and spheroplasts showed that LAM and lipomannan (LM) were primarily found in a cell wall-enriched subcellular fraction and correlated with the presence (or absence) of the mycolic acids in spheroplast preparations, suggesting that LAM and LM are primarily associated with the putative outer membrane of mycobacteria. During the course of these studies significant changes in the LAM/LM content of the cell wall were noted relative to the age of the culture. The LAM content of the M. smegmatis cell wall was dramatically reduced as the bacilli approached stationary phase, whereas LM, mycolic acid, and arabinogalactan content appeared to be unchanged. In addition, cell morphology and acid-fast staining characteristics showed variations with growth phase of the bacteria. In the logarithmic phase, the bacteria were found to be classic rod-shaped acid-fast bacilli, while in the stationary phase M. smegmatis lost the characteristic rod shape and developed a punctate acid-fast staining pattern with carbolfuchsin. The number of viable bacteria was independent of LAM content and phenotype. Taken together, the results presented here suggest that LAM is primarily localized with the mycolic acids in the cell wall and that the cellular concentration of LAM in M. smegmatis is selectively modulated with the growth phase.

Lipoarabinomannan and related glycoconjugates: structure, biogenesis and role in Mycobacterium tuberculosis physiology and host-pathogen interaction

Approximately one third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. This bacterium has an unusual lipid-rich cell wall containing a vast repertoire of antigens, providing a hydrophobic impermeable barrier against chemical drugs, thus representing an attractive target for vaccine and drug development. Apart from the mycolyl–arabinogalactan–peptidoglycan complex, mycobacteria possess several immunomodulatory constituents, notably lipomannan and lipoarabinomannan. The availability of whole-genome sequences of M. tuberculosis and related bacilli over the past decade has led to the identification and functional characterization of various enzymes and the potential drug targets involved in the biosynthesis of these glycoconjugates. Both lipomannan and lipoarabinomannan possess highly variable chemical structures, which interact with different receptors of the immune system during host–pathogen interactions, such as Toll-like receptors-2 and C-type lectins. Recently, the availability of mutants defective in the synthesis of these glycoconjugates in mycobacteria and the closely related bacterium, Corynebacterium glutamicum, has paved the way for host–pathogen interaction studies, as well as, providing attenuated strains of mycobacteria for the development of new vaccine candidates. This review provides a comprehensive account of the structure, biosynthesis and immunomodulatory properties of these important glycoconjugates.

Lipoarabinomannan biosynthesis in Corynebacterineae: the interplay of two α(1→2)-mannopyranosyltransferases MptC and MptD in mannan branching

Lipomannan (LM) and lipoarabinomannan (LAM) are key Corynebacterineae glycoconjugates that are integral components of the mycobacterial cell wall, and are potent immunomodulators during infection. LAM is a complex heteropolysaccharide synthesized by an array of essential glycosyltransferase family C (GT-C) members, which represent potential drug targets. Herein, we have identified and characterized two open reading frames from Corynebacterium glutamicum that encode for putative GT-Cs. Deletion of NCgl2100 and NCgl2097 in C. glutamicum demonstrated their role in the biosynthesis of the branching α(1→2)-Manp residues found in LM and LAM. In addition, utilizing a chemically defined nonasaccharide acceptor, azidoethyl 6-O-benzyl-α-D-mannopyranosyl-(1→6)-[α-D-mannopyranosyl-(1→6)]₇-D-mannopyranoside, and the glycosyl donor C₅₀-polyprenol-phosphate-[¹⁴C]-mannose with membranes prepared from different C. glutamicum mutant strains, we have shown that both NCgl2100 and NCgl2097 encode for novel α(1→2)-mannopyranosyltransferases, which we have termed MptC and MptD respectively. Complementation studies and in vitro assays also identified Rv2181 as a homologue of Cg-MptC in Mycobacterium tuberculosis. Finally, we investigated the ability of LM and LAM from C. glutamicum, and C. glutamicumΔmptC and C. glutamicumΔmptD mutants, to activate Toll-like receptor 2. Overall, our study enhances our understanding of complex lipoglycan biosynthesis in Corynebacterineae and sheds further light on the structural and functional relationship of these classes of polysaccharides.

Organelle membrane proteomics reveals differential influence of mycobacterial lipoglycans on macrophage phagosome maturation and autophagosome accumulation

The mycobacterial cell wall component lipoarabinomannan (LAM) has been described as one of the key virulence factors of Mycobacterium tuberculosis. Modification of the terminal arabinan residues of this lipoglycan with mannose caps in M. tuberculosis or with phosphoinositol caps in Mycobacterium smegmatis results in distinct host immune responses. Given that M. tuberculosis typically persists in the phagosomal vacuole after being phagocytosed by macrophages, we performed a proteomic analysis of that organelle after treatment of macrophages with LAMs purified from the two mycobacterial species. The quantitative changes in phagosomal proteins suggested a distinct role for mannose-capped LAM in modulating protein trafficking pathways that contribute to the arrest of phagosome maturation. Enlightened by our proteomic data, we performed further experiments to show that only the LAM from M. tuberculosis inhibits accumulation of autophagic vacuoles in the macrophage, suggesting a new function for this virulence-associated lipid.

LAM has been described as one of the key virulence factors of Mycobacterium tuberculosis. Modification of the terminal arabinan residues of this lipoglycan with mannose caps in M. tuberculosis or with phosphoinositol caps in Mycobacterium smegmatis results in distinct host immune responses. Given that M. tuberculosis typically persists in the phagosomal vacuole after being phagocytosed by macrophages, we performed a proteomic analysis of that organelle after treatment of macrophages with LAMs purified from the two mycobacterial species.

The non-pathogenic mycobacteria M. smegmatis and M. fortuitum induce rapid host cell apoptosis via a caspase-3 and TNF dependent pathway

Background

The HIV pandemic raised the potential for facultative-pathogenic mycobacterial species like, Mycobacterium kansasii, to cause disseminating disease in humans with immune deficiencies. In contrast, non-pathogenic mycobacterial species, like M. smegmatis, are not known to cause disseminating disease even in immunocompromised individuals. We hypothesized that this difference in phenotype could be explained by the strong induction of an innate immune response by the non-pathogenic mycobacterial species.

Results

A comparison of two rapid-growing, non-pathogenic species (M. smegmatis and M. fortuitum) with two facultative-pathogenic species (M. kansasii and M. bovis BCG) demonstrated that only the non-pathogenic bacteria induced strong apoptosis in human THP-1 cells and murine bone marrow-derived macrophages (BMDM) and dendritic cells (BMDD). The phospho-myo-inositol modification of lipoarabinomannan (PI-LAM) isolated from non-pathogenic species may be one of the cell wall components responsible for the pro-inflammatory activity of the whole bacteria. Indeed, PI-LAM induces high levels of apoptosis and IL-12 expression compared to the mannosyl modification of LAM isolated from facultative-pathogenic mycobacteria. The apoptosis induced by non-pathogenic M. smegmatis was dependent upon caspase-3 activation and TNF secretion. Consistently, BALB/c BMDM responded by secreting large amounts of TNF upon infection with non-pathogenic but not facultative-pathogenic mycobacteria. Interestingly, C57Bl/6 BMDM do not undergo apoptosis upon infection with non-pathogenic mycobacteria despite the fact that they still induce an increase in TNF secretion. This suggests that the host cell signaling pathways are different between these two mouse genotypes and that TNF is necessary but not sufficient to induce host cell apoptosis.

Conclusion

These results demonstrate a much stronger induction of the innate immune response by non-pathogenic versus facultative-pathogenic mycobacteria as measured by host cell apoptosis, IL-12 and TNF cytokine induction. These observations lend support to the hypothesis that the strong induction of the innate immune response is a major reason for the lack of pathogenicity in fast-growing mycobacteria.

Controlled expression of branch-forming mannosyltransferase is critical for mycobacterial lipoarabinomannan biosynthesis

Lipomannan (LM) and lipoarabinomannan (LAM) are phosphatidylinositol-anchored glycans present in the mycobacterial cell wall. In Mycobacterium smegmatis, the mannan core of LM/LAM constitutes a linear chain of 20-25 alpha1,6-mannoses elaborated by 8-9 alpha1,2-monomannose side branches. At least two alpha1,6-mannosyltransferases mediate the linear mannose chain elongation, and one branching alpha1,2-mannosyltransferase (encoded by MSMEG_4247) transfers monomannose branches. An MSMEG_4247 deletion mutant accumulates branchless LAM and interestingly fails to accumulate LM, suggesting an unexpected role of mannose branching for LM synthesis or maintenance. To understand the roles of MSMEG_4247-mediated branching more clearly, we analyzed the MSMEG_4247 deletion mutant in detail. Our study showed that the deletion mutant restored the synthesis of wild-type LM and LAM upon the expression of MSMEG_4247 at wild-type levels. In striking contrast, overexpression of MSMEG_4247 resulted in the accumulation of dwarfed LM/LAM, although monomannose branching was restored. The dwarfed LAM carried a mannan chain less than half the length of wild-type LAM and was elaborated by an arabinan that was about 4 times smaller. Induced overexpression of an elongating alpha1,6-mannosyltransferase competed with the overexpressed branching enzyme, alleviating the dwarfing effect of the branching enzyme. In wild-type cells, LM and LAM decreased in quantity in the stationary phase, and the expression levels of branching and elongating mannosyltransferases were reduced in concert, presumably to avoid producing abnormal LM/LAM. These data suggest that the coordinated expressions of branching and elongating mannosyltransferases are critical for mannan backbone elongation.

Reduced transcript stabilization restricts TNF-alpha expression in RAW264.7 macrophages infected with pathogenic mycobacteria: evidence for an involvement of lipomannan

Despite the critical role that TNF-alpha plays in the containment of mycobacterial infection, the mechanisms involved in regulation of its expression by mycobacteria are poorly defined. We addressed this question by studying MAP, which causes a chronic enteritis in ruminants and is linked to human Crohn's disease. We found that in MAP infected macrophages, TNF-alpha gene expression was substantially lower than in macrophages infected with nonpathogenic MS or stimulated with LPS. TNF-alpha transcriptional one could not fully explain the differential TNF-alpha mRNA expression, suggesting that there must be a substantial contribution by post-transcriptional mechanisms.Accordingly, we found reduced TNF-alpha mRNA stability in MAP-infected macrophages. Further comparison of MAP- and MS-infected macrophages revealed that lower TNF-alpha mRNA stability combined with lower mRNA and protein expression in MAP-infected macrophages correlated with lower p38 MAPK phosphorylation. These findings were independent of viability of MAP and MS. We demonstrate that the major mycobacterial cell-wall lipoglycan LM of MAP and MS induced TNF-alpha mRNA transcription,but only the MS-LM induced p38 MAPK-dependent transcript stabilization. Overall, our data suggest that pathogenic mycobacteria cause weak p38 and TNF-alpha mRNA stabilization as a result of their structural cell-wall components such as LM and thereby, restrict TNF-alpha expression in macrophages.

A truncated lipoglycan from mycobacteria with altered immunological properties

Maintenance of cell-wall integrity in Mycobacterium tuberculosis is essential and is the target of several antitubercular drugs. For example, ethambutol targets arabinogalactan and lipoarabinomannan (LAM) biosynthesis through the inhibition of several arabinofuranosyltransferases. Apart from their role in cell-wall integrity, mycobacterial LAMs also exhibit important immunomodulatory activities. Here we report the isolation and detailed structural characterization of a unique LAM molecule derived from Mycobacterium smegmatis deficient in the arabinofuranosyltransferase AftC (AftC-LAM). This mutant LAM expresses a severely truncated arabinan domain completely devoid of 3,5-Araf-branching residues, revealing an intrinsic involvement of AftC in the biosynthesis of LAM. Furthermore, we found that ethambutol efficiently inhibits biosynthesis of the AftC-LAM arabinan core, unambiguously demonstrating the involvement of the arabinofuranosyltransferase EmbC in early stages of LAM-arabinan biosynthesis. Finally, we demonstrate that AftC-LAM exhibits an enhanced proinflammatory activity, which is due to its ability to activate Toll-like receptor 2 (TLR2). Overall, our efforts further describe the mechanism of action of an important antitubercular drug, ethambutol, and demonstrate a role for specific arabinofuranosyltransferases in LAM biosynthesis. In addition, the availability of sufficient amounts of chemically defined wild-type and isogenic truncated LAMs paves the way for further investigations of the structure-function relationship of TLR2 activation by mycobacterial lipoglycans.

Critical role of amino acid position 343 of surfactant protein-D in the selective binding of glycolipids from Mycobacterium tuberculosis

Surfactant protein D (SP-D), a lectin that recognizes carbohydrates via its C-type carbohydrate recognition domains (CRDs), regulates Mycobacterium tuberculosis (M.tb)-macrophage interactions via recognition of M.tb mannosylated cell wall components. SP-D binds to, agglutinates, and reduces phagocytosis and intracellular growth of M.tb. Species-specific variations in the CRD amino acid sequence contribute to carbohydrate recognition preferences and have been exploited to enhance the antimicrobial properties of SP-D in vitro. Here, we characterized the binding interaction between several wild-type and mutant SP-D neck + CRD trimeric subunits (NCRDs) and pathogenic and nonpathogenic mycobacterial species. Specific amino acid substitutions (i.e., the 343-amino-acid position) that flank the carbohydrate binding groove led to significant increases in binding of only virulent and attenuated M.tb strains and to a lesser extent M. marinum, whereas there was negligible binding to M. avium complex and M. smegmatis. Moreover, a nonconserved mutation at the critical 321-amino-acid position (involved in Ca(2+) coordination) abrogated binding to M.tb and M. marinum. We further characterized the binding of NCRDs to the predominant surface-exposed mannosylated lipoglycans of the M.tb cell envelope. Results showed a binding pattern that is dependent on the nature of the side chain of the 343-amino-acid position flanking the SP-D CRD binding groove and the nature of the terminal mannosyl sugar linkages of the mycobacterial lipoglycans. We conclude that the 343 position is critical in defining the binding pattern of SP-D proteins to M.tb and its mannosylated cell envelope components.

Lipoglycans of Mycobacterium tuberculosis: isolation, purification, and characterization

In this chapter, we describe in detail the steps involved in isolation and characterization of lipoglycans from Mycobacterium tuberculosis and Mycobacterium smegmatis. In addition, procedures involved in structural analysis such as immunoblotting with mAb CS-35 or CS-40, gas chromatography, gas chromatography/mass spectrometry, nuclear magnetic resonance spectroscopy, and endoarabinanase digestion followed by high-pH anion exchange chromatography and two-dimensional gel electrophoresis are presented.

Lipoarabinomannan of Mycobacterium: mannose capping by a multifunctional terminal mannosyltransferase

Biosynthesis of phosphatidylinositol (PI)-containing lipoarabinomannan (LAM) and lipomannan (LM) of Mycobacterium spp. follows a conserved pathway involving multiple membrane-associated, substrate-specific mannosyltransferases (ManTs) responsible for the sequential addition of alpha-mannopyranosyl (Manp) units donated by decaprenyl-P-Manp on the periplasmic side of the plasma membrane. Because of their receptor-binding and immunomodulatory properties, the alpha(1-->2)-linked di- and tri-Manp motifs that functionalize the nonreducing arabinan termini of LAM (ManLAM) in Mycobacterium tuberculosis are of crucial importance. We now show that the M. tuberculosis ManT, Rv2181, is required for the addition of these alpha(1-->2)-linked Manp residues but also at other locations of the LAM molecule. Structural analyses of the LM and LAM variants produced by a M. tuberculosis Rv2181 knockout mutant revealed the presence of but a single Manp residue on the nonreducing arabinan termini of LAM and also a complete absence of alpha(1-->2)-linked Man branching on the mannan backbones of LM and LAM. A recombinant strain was constructed in ManLAM-deficient Mycobacterium smegmatis that coexpressed Rv2181 and Rv1635c-the ManT responsible for the addition of the first Manp capping residue of ManLAM. Analysis revealed LAM termini fully capped with di- and tri-Manp motifs in addition to alpha(1-->2)Man branching on the mannan backbones of LM and LAM, confirming the involvement of the alpha(1-->2)ManT Rv2181 in the dual role of Man capping and mannan-core branching, and in the process generated a rapidly growing, ManLAM-containing strain, a tool for the study of the role of ManLAM in the pathogenesis of tuberculosis.