Isolation of a distinct Mycobacterium tuberculosis mannose-capped lipoarabinomannan isoform responsible for recognition by CD1b-restricted T cells

Mannose-capped lipoarabinomannan (ManLAM) is a complex lipoglycan abundantly present in the Mycobacterium tuberculosis cell envelope. Many biological properties have been ascribed to ManLAM, from directly interacting with the host and participating in the intracellular survival of M. tuberculosis, to triggering innate and adaptive immune responses, including the activation of CD1b-restricted T cells. Due to its structural complexity, ManLAM is considered a heterogeneous population of molecules which may explain its different biological properties. The presence of various modifications such as fatty acids, succinates, lactates, phosphoinositides and methylthioxylose in ManLAM have proven to correlate directly with its biological activity and may potentially be involved in the interactions between CD1b and the T cell population. To further delineate the specific ManLAM epitopes involved in CD1b-restricted T cell recognition, and their potential roles in mediating immune responses in M. tuberculosis infection, we established a method to resolve ManLAM into eight different isoforms based on their different isoelectric values. Our results show that a ManLAM isoform with an isoelectric value of 5.8 was the most potent in stimulating the production of interferon-γ in different CD1b-restricted T-cell lines. Compositional analyses of these isoforms of ManLAM revealed a direct relationship between the overall charge of the ManLAM molecule and its capacity to be presented to T cells via the CD1 compartment.

Mechanistic insights into the retaining glucosyl-3-phosphoglycerate synthase from mycobacteria

Considerable progress has been made in recent years in our understanding of the structural basis of glycosyl transfer. Yet the nature and relevance of the conformational changes associated with substrate recognition and catalysis remain poorly understood. We have focused on the glucosyl-3-phosphoglycerate synthase (GpgS), a "retaining" enzyme, that initiates the biosynthetic pathway of methylglucose lipopolysaccharides in mycobacteria. Evidence is provided that GpgS displays an unusually broad metal ion specificity for a GT-A enzyme, with Mg(2+), Mn(2+), Ca(2+), Co(2+), and Fe(2+) assisting catalysis. In the crystal structure of the apo-form of GpgS, we have observed that a flexible loop adopts a double conformation L(A) and L(I) in the active site of both monomers of the protein dimer. Notably, the L(A) loop geometry corresponds to an active conformation and is conserved in two other relevant states of the enzyme, namely the GpgS·metal·nucleotide sugar donor and the GpgS·metal·nucleotide·acceptor-bound complexes, indicating that GpgS is intrinsically in a catalytically active conformation. The crystal structure of GpgS in the presence of Mn(2+)·UDP·phosphoglyceric acid revealed an alternate conformation for the nucleotide sugar β-phosphate, which likely occurs upon sugar transfer. Structural, biochemical, and biophysical data point to a crucial role of the β-phosphate in donor and acceptor substrate binding and catalysis. Altogether, our experimental data suggest a model wherein the catalytic site is essentially preformed, with a few conformational changes of lateral chain residues as the protein proceeds along the catalytic cycle. This model of action may be applicable to a broad range of GT-A glycosyltransferases.

Pathogen-specific epitopes as epidemiological tools for defining the magnitude of Mycobacterium leprae transmission in areas endemic for leprosy

During recent years, comparative genomic analysis has allowed the identification of Mycobacterium leprae-specific genes with potential application for the diagnosis of leprosy. In a previous study, 58 synthetic peptides derived from these sequences were tested for their ability to induce production of IFN-γ in PBMC from endemic controls (EC) with unknown exposure to M. leprae, household contacts of leprosy patients and patients, indicating the potential of these synthetic peptides for the diagnosis of sub- or preclinical forms of leprosy. In the present study, the patterns of IFN-γ release of the individuals exposed or non-exposed to M. leprae were compared using an Artificial Neural Network algorithm, and the most promising M. leprae peptides for the identification of exposed people were selected. This subset of M. leprae-specific peptides allowed the differentiation of groups of individuals from sites hyperendemic for leprosy versus those from areas with lower level detection rates. A progressive reduction in the IFN-γ levels in response to the peptides was seen when contacts of multibacillary (MB) patients were compared to other less exposed groups, suggesting a down modulation of IFN-γ production with an increase in bacillary load or exposure to M. leprae. The data generated indicate that an IFN-γ assay based on these peptides applied individually or as a pool can be used as a new tool for predicting the magnitude of M. leprae transmission in a given population.

Real-time PCR and high-resolution melt analysis for rapid detection of Mycobacterium leprae drug resistance mutations and strain types

Drug resistance surveillance and strain typing of Mycobacterium leprae are necessary to investigate ongoing transmission of leprosy in regions of endemicity. To enable wider implementation of these molecular analyses, novel real-time PCR-high-resolution melt (RT-PCR-HRM) assays without allele-specific primers or probes and post-PCR sample handling were developed. For the detection of mutations within drug resistance-determining regions (DRDRs) of folP1, rpoB, and gyrA, targets for dapsone, rifampin, and fluoroquinolones, real-time PCR-HRM assays were developed. Wild-type and drug-resistant mouse footpad-derived strains that included three folP1, two rpoB, and one gyrA mutation types in a reference panel were tested. RT-PCR-HRM correctly distinguished the wild type from the mutant strains. In addition, RT-PCR-HRM analyses aided in recognizing samples with mixed or minor alleles and also a mislabeled sample. When tested in 121 sequence-characterized clinical strains, HRM identified all the folP1 mutants representing two mutation types, including one not within the reference panel. The false positives (<5%) could be attributed to low DNA concentration or PCR inhibition. A second set of RT-PCR-HRM assays for identification of three previously reported single nucleotide polymorphisms (SNPs) that have been used for strain typing were developed and validated in 22 reference and 25 clinical strains. Real-time PCR-HRM is a sensitive, simple, rapid, and high-throughput tool for routine screening known DRDR mutants in new and relapsed cases, SNP typing, and detection of minor mutant alleles in the wild-type background at lower costs than current methods and with the potential for quality control in leprosy investigations.

Stable extracellular RNA fragments of Mycobacterium tuberculosis induce early apoptosis in human monocytes via a caspase-8 dependent mechanism

The molecular basis of pathogen-induced host cell apoptosis is well characterized for a number of microorganisms. Mycobacterium tuberculosis is known to induce apoptosis and it was shown that live but not heat killed M. tuberculosis stimulates this biological pathway in monocytes. The dependence of this activity on live bacilli led us to hypothesize that products released or secreted by M. tuberculosis are the primary apoptotic factors for human monocytes. Thus, the culture filtrate of in vitro grown M. tuberculosis strain H37Rv was fractioned by conventional chromatography and the apoptosis-inducing activity of individual fractions was measured on human monocytes. The tests employed included measurement of cell membrane damage, caspase activation, and cytokine release. Small molecular weight RNAs of M. tuberculosis were recognized as the predominant apoptosis inducing factors. The RNA was comprised primarily of tRNA and rRNA fragments that stably accumulate in the culture filtrate during early log-phase growth. The RNA fragments signaled through a caspase-8 dependent, caspase-1 and TNF-α independent pathway that ultimately compromised the human monocytes' ability to control M. tuberculosis infection. These studies provide the first report of bacterial RNA inducing apoptosis. They also provide a foundation to pursue pathways for secretion or release of nucleic acids from M. tuberculosis and the impact of secreted RNA fragments on pathogenesis.

SNP genotypes of Mycobacterium leprae isolates in Thailand and their combination with rpoT and TTC genotyping for analysis of leprosy distribution and transmission

Based on the discovery of three single nucleotide polymorphisms (SNPs) in Mycobacterium leprae, it has been previously reported that there are four major SNP types associated with different geographic regions around the world. Another typing system for global differentiation of M. leprae is the analysis of the variable number of short tandem repeats within the rpoT gene. To expand the analysis of geographic distribution of M. leprae, classified by SNP and rpoT gene polymorphisms, we studied 85 clinical isolates from Thai patients and compared the findings with those reported from Asian isolates. SNP genotyping by PCR amplification and sequencing revealed that all strains like those in Myanmar were SNP type 1 and 3, with the former being predominant, while in Japan, Korea, and Indonesia, the SNP type 3 was found to be more frequent. The pattern of M. leprae distribution in Thailand and Myanmar is quite similar, except that SNP type 2 was not found in Thailand. In addition, the 3-copy hexamer genotype in the rpoT gene is shared among the isolates from these two neighboring countries. On the basis of these two markers, we postulate that M. leprae in leprosy patients from Myanmar and Thailand has a common historical origin. Further differentiation among Thai isolates was possible by assessing copy numbers of the TTC sequence, a more polymorphic microsatellite locus.

Comparison of three immunological tests for leprosy diagnosis and detection of subclinical infection

OBJECTIVE

Our aim was to compare the performance of three serological assays in leprosy patients and their household contacts utilising two quantitative ELISA tests using native PGL-I (PGL-1 ELISA), synthetic ND-O-HSA (ND-O-HSA ELISA), and the semi-quantitative lateral flow test (ML Flow).

METHODS

Comparisons among three immunological assays, PGL-I ELISA, ND-O-HSA ELISA, and ML Flow were performed in 154 leprosy patients, 191 household contacts and 52 health subjects.

RESULTS

The sensitivity results of the PGL-1, ND-O-HSA, and ML Flow were 68.83%, 63.84%, and 60.65%, respectively, with specificity of 98% for both ELISA assays. The native and synthetic PGL-I ELISA assays detected antibodies in 22.73% and 31.82% of the paucibacillary (PB) patients, respectively and the ML Flow test did not detect antibodies in this group. The ML Flow test was able to discriminate patients into PB or multibacillary (MB) forms, while the native PGL-I and ND-O-HSA was correlated with the bacillary load and the Ridley-Jopling clinical forms. In household contacts, the native PGL-I, ND-O-HSA, and ML Flow assays detected seropositivity of 25%, 17%, and 10%, respectively.

CONCLUSIONS

The use of ELISA and ML Flow tests are thus recommended as additional tools in the diagnosis and classification of the clinical forms, aiding in prescribing the correct treatment regimen to prevent subsequent nerve damage and disability, and besides, the PGL-I ELISA may be used to detect subclinical infection in leprosy.

The role of Mycobacterium leprae phenolic glycolipid I (PGL-I) in serodiagnosis and in the pathogenesis of leprosy

PGL-I (phenolic glycolipid I) emerged in the early 1980s on the one hand as part of intensive efforts to define the typing antigens of a host of Mycobacterium spp. and also from characterisation of the lipids of skin biopsies from highly bacillary positive lepromatous leprosy patients. PGL-I, despite its extreme lipophilicity due to its inherent phthiocerol dimycocerosyl component, is highly antigenic evoking high titre IgM antibodies in lepromatous leprosy patients, attributable largely to the unique 3,6-di-O-methyl-beta-D-glucosyl entity at the non-reducing terminus of its trisaccharide. PGL-I itself or in the form of semisynthetic neoglycoproteins containing the synthetic terminal disaccharide or the whole trisaccharide chemically conjugated to such as bovine or human serum albumin, has found its greatest utility in the serological diagnosis, confirmation and management of lepromatous leprosy. PGL-I has also been implicated in the tropism of M. leprae for Schwann cells, through specific binding to laminin, and to play an important role in downregulation of the inflammatory immune response and inhibition of dendritic cell maturation and activation, thereby facilitating the persistence of M. leprae/leprosy.

Investigation of ABC transporter from mycobacterial arabinogalactan biosynthetic cluster

Two genes from the "mycobacterial arabinogalactan biosynthetic cluster" spanning the region from Rv3779 to Rv3809c in the genome of Mycobacterium tuberculosis H37Rv were annotated as possible components of the ATP-binding cassette transporter. Rv3781 encodes a nucleotide-binding domain and Rv3783 determines production of a membrane-spanning domain. We have examined possible roles of these genes in mycobacterial cell wall biosynthesis through inactivation of their respective orthologs in Mycobacterium smegmatis mc(2)155, phenotypic characterization of the mutant strains via metabolic labeling with [U-(14)C]-glucose, cell-free reactions with UDP-[U-(14)C]-galactose monitoring galactan build-up and transcriptional analysis. Several lines of evidence suggest that this ABC transporter is involved in biosynthesis of arabinogalactan, although more investigation is needed to establish its precise role or the transported substrate.

Transmission of dapsone-resistant leprosy detected by molecular epidemiological approaches

Drug resistance surveillance identified six untreated leprosy patients in the Philippines with Mycobacterium leprae folP1 mutations which confer dapsone resistance. Five patients share a village of residence; four who carried the mutation, Thr53Val, were also linked by M. leprae variable-number tandem repeat (VNTR) strain types. In India, folP1 mutations were detected in two relapse patients with a history of dapsone treatment. Mutations were not found in the rifampin target gene rpoB. These findings indicate that dapsone resistance is being transmitted.

Invariant natural killer T cells recognize lipid self antigen induced by microbial danger signals

Invariant natural killer T cells (iNKT cells) have a prominent role during infection and other inflammatory processes, and these cells can be activated through their T cell antigen receptors by microbial lipid antigens. However, increasing evidence shows that they are also activated in situations in which foreign lipid antigens would not be present, which suggests a role for lipid self antigen. We found that an abundant endogenous lipid, β-D-glucopyranosylceramide (β-GlcCer), was a potent iNKT cell self antigen in mouse and human and that its activity depended on the composition of the N-acyl chain. Furthermore, β-GlcCer accumulated during infection and in response to Toll-like receptor agonists, contributing to iNKT cell activation. Thus, we propose that recognition of β-GlcCer by the invariant T cell antigen receptor translates innate danger signals into iNKT cell activation.

Serum metabolomics reveals higher levels of polyunsaturated fatty acids in lepromatous leprosy: potential markers for susceptibility and pathogenesis

BACKGROUND

Leprosy is a disease of the skin and peripheral nervous system caused by the obligate intracellular bacterium Mycobacterium leprae. The clinical presentations of leprosy are spectral, with the severity of disease determined by the balance between the cellular and humoral immune response of the host. The exact mechanisms that facilitate disease susceptibility, onset and progression to certain clinical phenotypes are presently unclear. Various studies have examined lipid metabolism in leprosy, but there has been limited work using whole metabolite profiles to distinguish the clinical forms of leprosy.

METHODOLOGY AND PRINCIPAL FINDINGS

In this study we adopted a metabolomics approach using high mass accuracy ultrahigh pressure liquid chromatography mass spectrometry (UPLC-MS) to investigate the circulatory biomarkers in newly diagnosed untreated leprosy patients. Sera from patients having bacterial indices (BI) below 1 or above 4 were selected, subjected to UPLC-MS, and then analyzed for biomarkers which distinguish the polar presentations of leprosy. We found significant increases in the abundance of certain polyunsaturated fatty acids (PUFAs) and phospholipids in the high-BI patients, when contrasted with the levels in the low-BI patients. In particular, the median values of arachidonic acid (2-fold increase), eicosapentaenoic acid (2.6-fold increase) and docosahexaenoic acid (1.6-fold increase) were found to be greater in the high-BI patients.

SIGNIFICANCE

Eicosapentaenoic acid and docosahexaenoic acid are known to exert anti-inflammatory properties, while arachidonic acid has been reported to have both pro- and anti-inflammatory activities. The observed increase in the levels of several lipids in high-BI patients may provide novel clues regarding the biological pathways involved in the immunomodulation of leprosy. Furthermore, these results may lead to the discovery of biomarkers that can be used to investigate susceptibility to infection, facilitate early diagnosis and monitor the progression of disease.

Detailed structural and quantitative analysis reveals the spatial organization of the cell walls of in vivo grown Mycobacterium leprae and in vitro grown Mycobacterium tuberculosis

The cell wall of mycobacteria consists of an outer membrane, analogous to that of gram-negative bacteria, attached to the peptidoglycan (PG) via a connecting polysaccharide arabinogalactan (AG). Although the primary structure of these components is fairly well deciphered, issues such as the coverage of the PG layer by covalently attached mycolates in the outer membrane and the spatial details of the mycolic acid attachment to the arabinan have remained unknown. It is also not understood how these components work together to lead to the classical acid-fast staining of mycobacteria. Because the majority of Mycobacterium tuberculosis bacteria in established experimental animal infections are acid-fast negative, clearly cell wall changes are occurring. To address both the spatial properties of mycobacterial cell walls and to begin to study the differences between bacteria grown in animals and cultures, the cell walls of Mycobacterium leprae grown in armadillos was characterized and compared with that of M. tuberculosis grown in culture. Most fundamentally, it was determined that the cell wall of M. leprae contained significantly more mycolic acids attached to PG than that of in vitro grown M. tuberculosis (mycolate:PG ratios of 21:10 versus 16:10, respectively). In keeping with this difference, more arabinogalactan (AG) molecules, linking the mycolic acids to PG, were found. Differences in the structures of the AG were also found; the AG of M. leprae is smaller than that of M. tuberculosis, although the same basic structural motifs are retained.

Synthetic UDP-furanoses as potent inhibitors of mycobacterial galactan biogenesis

UDP-galactofuranose (UDP-Galf) is a substrate for two types of enzymes, UDP-galactopyranose mutase and galactofuranosyltransferases, which are present in many pathogenic organisms but absent from mammals. In particular, these enzymes are involved in the biosynthesis of cell wall galactan, a polymer essential for the survival of the causative agent of tuberculosis, Mycobacterium tuberculosis. We describe here the synthesis of derivatives of UDP-Galf modified at C-5 and C-6 using a chemoenzymatic route. In cell-free assays, these compounds prevented the formation of mycobacterial galactan, via the production of short "dead-end" intermediates resulting from their incorporation into the growing oligosaccharide chain. Modified UDP-furanoses thus constitute novel probes for the study of the two classes of enzymes involved in mycobacterial galactan assembly, and studies with these compounds may ultimately facilitate the future development of new therapeutic agents against tuberculosis.

Analysis of antibody responses to Mycobacterium leprae phenolic glycolipid I, lipoarabinomannan, and recombinant proteins to define disease subtype-specific antigenic profiles in leprosy

A simple serodiagnostic test based on the Mycobacterium leprae-specific phenolic glycolipid I(PGL-I), for individuals with leprosy is nearly universally positive in leprosy patients with high bacillary loads but cannot be used as a stand-alone diagnostic test for the entire spectrum of the disease process. For patients with early infection with no detectable acid-fast bacilli in lesions or with low or no antibody titer to PGL-I, as in those at the tuberculoid end of the disease spectrum, this diagnostic approach has limited usefulness. To identify additional M. leprae antigens that might enhance the serological detection of these individuals, we have examined the reactivity patterns of patient sera to PGL-I, lipoarabinomannan (LAM), and six recombinant M. leprae proteins (ML1877, ML0841, ML2028, ML2038, ML0380, and ML0050) by Western blot analysis and enzyme-linked immunosorbent assay (ELISA). Overall, the responses to ML2028 (Ag85B) and ML2038 (bacterioferritin) were consistently high in both multibacillary and paucibacillary groups and weak or absent in endemic controls, while responses to other antigens showed considerable variability, from strongly positive to completely negative. This analysis has given a clearer understanding of some of the differences in the antibody responses between individuals at opposite ends of the disease spectrum, as well as illustrating the heterogeneity of antibody responses toward protein, carbohydrate, and glycolipid antigens within a clinical group. Correlating these response patterns with a particular disease state could allow for a more critical assessment of the form of disease within the leprosy spectrum and could lead to better patient management.

Molecular basis of phosphatidyl-myo-inositol mannoside biosynthesis and regulation in mycobacteria

Phosphatidyl-myo-inositol mannosides (PIMs) are unique glycolipids found in abundant quantities in the inner and outer membranes of the cell envelope of all Mycobacterium species. They are based on a phosphatidyl-myo-inositol lipid anchor carrying one to six mannose residues and up to four acyl chains. PIMs are considered not only essential structural components of the cell envelope but also the structural basis of the lipoglycans (lipomannan and lipoarabinomannan), all important molecules implicated in host-pathogen interactions in the course of tuberculosis and leprosy. Although the chemical structure of PIMs is now well established, knowledge of the enzymes and sequential events leading to their biosynthesis and regulation is still incomplete. Recent advances in the identification of key proteins involved in PIM biogenesis and the determination of the three-dimensional structures of the essential phosphatidyl-myo-inositol mannosyltransferase PimA and the lipoprotein LpqW have led to important insights into the molecular basis of this pathway.

Biosynthetic origin of the galactosamine substituent of Arabinogalactan in Mycobacterium tuberculosis

The arabinogalactan (AG) of slow growing pathogenic Mycobacterium spp. is characterized by the presence of galactosamine (GalN) modifying some of the interior branched arabinosyl residues. The biosynthetic origin of this substituent and its role(s) in the physiology and/or pathogenicity of mycobacteria are not known. We report on the discovery of a polyprenyl-phospho-N-acetylgalactosaminyl synthase (PpgS) and the glycosyltransferase Rv3779 from Mycobacterium tuberculosis required, respectively, for providing and transferring the GalN substrate for the modification of AG. Disruption of either ppgS (Rv3631) or Rv3779 totally abolished the synthesis of the GalN substituent of AG in M. tuberculosis H37Rv. Conversely, expression of ppgS in Mycobacterium smegmatis conferred upon this species otherwise devoid of ppgS ortholog and any detectable polyprenyl-phospho-N-acetylgalactosaminyl synthase activity the ability to synthesize polyprenyl-phospho-N-acetylgalactosamine (polyprenyl-P-GalNAc) from polyprenyl-P and UDP-GalNAc. Interestingly, this catalytic activity was increased 40-50-fold by co-expressing Rv3632, the encoding gene of a small membrane protein apparently co-transcribed with ppgS in M. tuberculosis H37Rv. The discovery of this novel lipid-linked sugar donor and the involvement of a the glycosyltransferase C-type glycosyltransferase in its transfer onto its final acceptor suggest that pathogenic mycobacteria modify AG on the periplasmic side of the plasma membrane. The availability of a ppgS knock-out mutant of M. tuberculosis provides unique opportunities to investigate the physiological function of the GalN substituent and the potential impact it may have on host-pathogen interactions.