Mycobacterial lipoarabinomannans modulate cytokine production in human T helper cells by interfering with raft/microdomain signalling

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.

Chemical Tools for Illumination of Tuberculosis Biology, Virulence Mechanisms, and Diagnosis

Tuberculosis (TB) remains one of the deadliest infectious diseases and begs the scientific community to up the ante for research and exploration of completely novel therapeutic avenues. Chemical biology-inspired design of tunable chemical tools has aided in clinical diagnosis, facilitated discovery of therapeutics, and begun to enable investigation of virulence mechanisms at the host-pathogen interface of Mycobacterium tuberculosis. This Perspective highlights chemical tools specific to mycobacterial proteins and the cell lipid envelope that have furnished rapid and selective diagnostic strategies and provided unprecedented insights into the function of the mycobacterial proteome and lipidome. We discuss chemical tools that have enabled elucidating otherwise intractable biological processes by leveraging the unique lipid and metabolite repertoire of mycobacterial species. Some of these probes represent exciting starting points with the potential to illuminate poorly understood aspects of mycobacterial pathogenesis, particularly the host membrane-pathogen interactions.

Silver Nanoparticles for the Therapy of Tuberculosis

Rapid emergence of aggressive, multidrug-resistant Mycobacteria strain represents the main cause of the current antimycobacterial-drug crisis and status of tuberculosis (TB) as a major global health problem. The relatively low-output of newly approved antibiotics contributes to the current orientation of research towards alternative antibacterial molecules such as advanced materials. Nanotechnology and nanoparticle research offers several exciting new-concepts and strategies which may prove to be valuable tools in improving the TB therapy. A new paradigm in antituberculous therapy using silver nanoparticles has the potential to overcome the medical limitations imposed in TB treatment by the drug resistance which is commonly reported for most of the current organic antibiotics. There is no doubt that AgNPs are promising future therapeutics for the medication of mycobacterial-induced diseases but the viability of this complementary strategy depends on overcoming several critical therapeutic issues as, poor delivery, variable intramacrophagic antimycobacterial efficiency, and residual toxicity. In this paper, we provide an overview of the pathology of mycobacterial-induced diseases, andhighlight the advantages and limitations of silver nanoparticles (AgNPs) in TB treatment.

A Virulence-Associated Glycolipid with Distinct Conformational Attributes: Impact on Lateral Organization of Host Plasma Membrane, Autophagy, and Signaling

Mycobacterium tuberculosis (Mtb) serves as the epitome of how lipids-next to proteins-are utilized as central effectors in pathogenesis. It synthesizes an arsenal of structurally atypical lipids (C60-C90) to impact various membrane-dependent steps involved in host interactions. There is a growing precedent to support insertion of these exposed lipids into the host membrane as part of their mode of action. However, the vital role of specific virulence-associated lipids in modulating cellular functions by altering the host membrane organization and associated signaling pathways remain unanswered questions. Here, we combined chemical synthesis, biophysics, cell biology, and molecular dynamics simulations to elucidate host membrane structure modifications and modulation of membrane-associated signaling using synthetic Mycobacterium tuberculosis sulfoglycolipids (Mtb SL). We reveal that Mtb SL reorganizes the host cell plasma membrane domains while showing higher preference for fluid membrane regions. This rearrangement is governed by the distinct conformational states sampled by SL acyl chains. Physicochemical assays with SL analogues reveal insights into their structure-function relationships, highlighting specific roles of lipid acyl chains and headgroup, along with effects on autophagy and cytokine profiles. Our findings uncover a mechanism whereby Mtb uses specific chemical moieties on its lipids to fine-tune host lipid interactions and confer control of the downstream functions by modifying the cell membrane structure and function. These findings will inspire development of chemotherapeutics against Mtb by counteracting their effects on the host-cell membrane.

Mycobacterial mannose-capped lipoarabinomannan: a modulator bridging innate and adaptive immunity

Mannose-capped lipoarabinomannan (ManLAM) is a high molecular mass amphipathic lipoglycan identified in pathogenic Mycobacterium tuberculosis (M. tb) and M. bovis Bacillus Calmette-Guérin (BCG). ManLAM, serves as both an immunogen and a modulator of the host immune system, and its critical role in mycobacterial survival during infection has been well-characterized. ManLAM can be recognized by various types of receptors on both innate and adaptive immune cells, including macrophages, dendritic cells (DCs), neutrophils, natural killer T (NKT) cells, T cells and B cells. MamLAM has been shown to affect phagocytosis, cytokine production, antigen presentation, T cell activation and polarization, as well as antibody production. Exploring the mechanisms underlying the roles of ManLAM during mycobacterial infection will aid in improving tuberculosis (TB) prevention, diagnosis and treatment interventions. In this review, we highlight the interaction between ManLAM and receptors, intracellular signalling pathways triggered by ManLAM and its roles in both innate and adaptive immune responses.

Dynamic Remodeling of the Host Cell Membrane by Virulent Mycobacterial Sulfoglycolipid-1

Lipids dictate membrane properties to modulate lateral membrane organization, lipid/protein diffusion and lipid-protein interactions, thereby underpinning proper functioning of cells. Mycobacterium tuberculosis harnesses the power of its atypical cell wall lipids to impact immune surveillance machinery centered at the host cell membrane. However, the role of specific virulent lipids in altering host cellular functions by modulating membrane organization and the associated signaling response are still pertinent unresolved questions. Here, combining membrane biophysics and cell biology, we elucidate how virulent Mtb sulfoglycolipids hijack the host cell membrane, affecting its order, fluidity, and stiffness along with manipulating the linked cytoskeleton. The functional outcome of this perturbation was assayed by monitoring membrane-associated autophagy signaling. These actions form a part of the overall response to commandeer host membrane-associated immune processes during infection. The findings on the mechanism of action of Mtb lipids on host cell membrane structure and downstream signaling will deepen the collective understanding of their functional aspects in membrane-dictated bacterial survival, pathogenesis and drug resistance and reveal suitable membrane driven-therapeutic intervention points and diagnostic tools.

Lipoarabinomannan Decreases Galectin-9 Expression and Tumor Necrosis Factor Pathway in Macrophages Favoring Mycobacterium tuberculosis Intracellular Growth

Lipoarabinomannan (LAM) is a lipid virulent factor secreted by Mycobacterium tuberculosis (Mtb). LAM can be found in the sputum and urine of patients with active tuberculosis. When human monocytes are differentiated into macrophages [monocyte-derived macrophages (MDM)] in the presence of LAM, MDM are poorly functional which may limit the immune response to Mtb infection. Our previous studies have shown that TIM3 and galectin (GAL)9 interaction induces anti-mycobacterial activity, and the expression levels of TIM3 and GAL9 are downregulated during Mtb infection. We postulated that LAM affects GAL9/TIM3 pathway, and, in consequence, the ability of the macrophage to control bacterial growth could be affected. In this work, we have generated MDM in the presence of LAM and observed that the expression of TIM3 was not affected; in contrast, GAL9 expression was downregulated at the transcriptional and protein levels. We observed that the cell surface and the soluble form of tumor necrosis factor (TNF) receptor 2 were decreased. We also found that when LAM-exposed MDM were activated with LPS, they produced less TNF, and the transcription factor proteinase-activated receptor-2 (PAR2), which is involved in host immune responses to infection, was not induced. Our data show that LAM-exposed MDM were deficient in the control of intracellular growth of Mtb. In conclusion, LAM-exposed MDM leads to MDM with impaired intracellular signal activation affecting GAL9, TNF, and PAR2 pathways, which are important to restrict Mtb growth.

Mycobacterium tuberculosis Membrane Vesicles Inhibit T Cell Activation

Mycobacterium tuberculosis utilizes multiple mechanisms to evade host immune responses, and inhibition of effector CD4⁺ T cell responses by M. tuberculosis may contribute to immune evasion. TCR signaling is inhibited by M. tuberculosis cell envelope lipoglycans, such as lipoarabinomannan and lipomannan, but a mechanism for lipoglycans to traffic from M. tuberculosis within infected macrophages to reach T cells is unknown. In these studies, we found that membrane vesicles produced by M. tuberculosis and released from infected macrophages inhibited the activation of CD4⁺ T cells, as indicated by reduced production of IL-2 and reduced T cell proliferation. Flow cytometry and Western blot demonstrated that lipoglycans from M. tuberculosis-derived bacterial vesicles (BVs) are transferred to T cells, where they inhibit T cell responses. Stimulation of CD4⁺ T cells in the presence of BVs induced expression of GRAIL, a marker of T cell anergy; upon restimulation, these T cells showed reduced ability to proliferate, confirming a state of T cell anergy. Furthermore, lipoarabinomannan was associated with T cells after their incubation with infected macrophages in vitro and when T cells were isolated from lungs of M. tuberculosis-infected mice, confirming the occurrence of lipoarabinomannan trafficking to T cells in vivo. These studies demonstrate a novel mechanism for the direct regulation of CD4⁺ T cells by M. tuberculosis lipoglycans conveyed by BVs that are produced by M. tuberculosis and released from infected macrophages. These lipoglycans are transferred to T cells to inhibit T cell responses, providing a mechanism that may promote immune evasion.

Detection of lipoarabinomannan (LAM) in urine is an independent predictor of mortality risk in patients receiving treatment for HIV-associated tuberculosis in sub-Saharan Africa: a systematic review and meta-analysis

BACKGROUND

Simple immune capture assays that detect mycobacterial lipoarabinomannan (LAM) antigen in urine are promising new tools for the diagnosis of HIV-associated tuberculosis (HIV-TB). In addition, however, recent prospective cohort studies of patients with HIV-TB have demonstrated associations between LAM in the urine and increased mortality risk during TB treatment, indicating an additional utility of urinary LAM as a prognostic marker. We conducted a systematic review and meta-analysis to summarise the evidence concerning the strength of this relationship in adults with HIV-TB in sub-Saharan Africa, thereby quantifying the assay's prognostic value.

METHODS

We searched MEDLINE and Embase databases using comprehensive search terms for 'HIV', 'TB', 'LAM' and 'sub-Saharan Africa'. Identified studies were reviewed and selected according to predefined criteria.

RESULTS

We identified 10 studies eligible for inclusion in this systematic review, reporting on a total of 1172 HIV-TB cases. Of these, 512 patients (44 %) tested positive for urinary LAM. After a variable duration of follow-up of between 2 and 6 months, overall case fatality rates among HIV-TB cases varied between 7 % and 53 %. Pooled summary estimates generated by random-effects meta-analysis showed a two-fold increased risk of mortality for urinary LAM-positive HIV-TB cases compared to urinary LAM-negative HIV-TB cases (relative risk 2.3, 95 % confidence interval 1.6-3.1). Some heterogeneity was explained by study setting and patient population in sub-group analyses. Five studies also reported multivariable analyses of risk factors for mortality, and pooled summary estimates demonstrated over two-fold increased mortality risk (odds ratio 2.5, 95 % confidence interval 1.4-4.5) among urinary LAM-positive HIV-TB cases, even after adjustment for other risk factors for mortality, including CD4 cell count.

CONCLUSIONS

We have demonstrated that detectable LAM in urine is associated with increased risk of mortality during TB treatment, and that this relationship remains after adjusting for other risk factors for mortality. This may simply be due to a positive test for urinary LAM serving as a marker of higher mycobacterial load and greater disease dissemination and severity. Alternatively, LAM antigen may directly compromise host immune responses through its known immunomodulatory effects. Detectable LAM in the urine is an independent risk factor for mortality among patients receiving treatment for HIV-TB. Further research is warranted to elucidate the underlying mechanisms and to determine whether this vulnerable patient population may benefit from adjunctive interventions.

Mannose-Capped Lipoarabinomannan from Mycobacterium tuberculosis Induces CD4+ T Cell Anergy via GRAIL

Mycobacterium tuberculosis cell wall glycolipid, lipoarabinomannan, can inhibit CD4(+) T cell activation by downregulating the phosphorylation of key proximal TCR signaling molecules: Lck, CD3ζ, ZAP70, and LAT. Inhibition of proximal TCR signaling can result in T cell anergy, in which T cells are inactivated following an Ag encounter, yet remain viable and hyporesponsive. We tested whether mannose-capped lipoarabinomannan (LAM)-induced inhibition of CD4(+) T cell activation resulted in CD4(+) T cell anergy. The presence of LAM during primary stimulation of P25 TCR-transgenic murine CD4(+) T cells with M. tuberculosis Ag85B peptide resulted in decreased proliferation and IL-2 production. P25 TCR-transgenic CD4(+) T cells primed in the presence of LAM also exhibited decreased response upon restimulation with Ag85B. The T cell anergic state persisted after the removal of LAM. Hyporesponsiveness to restimulation was not due to apoptosis, generation of Foxp3-positive regulatory T cells, or inhibitory cytokines. Acquisition of the anergic phenotype correlated with upregulation of gene related to anergy in lymphocytes (GRAIL) protein in CD4(+) T cells. Inhibition of human CD4(+) T cell activation by LAM also was associated with increased GRAIL expression. Small interfering RNA-mediated knockdown of GRAIL before LAM treatment abrogated LAM-induced hyporesponsiveness. In addition, exogenous IL-2 reversed defective proliferation by downregulating GRAIL expression. These results demonstrate that LAM upregulates GRAIL to induce anergy in Ag-reactive CD4(+) T cells. Induction of CD4(+) T cell anergy by LAM may represent one mechanism by which M. tuberculosis evades T cell recognition.

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.

Mannose-capped lipoarabinomannan from Mycobacterium tuberculosis preferentially inhibits sphingosine-1-phosphate-induced migration of Th1 cells

Chemokine receptor cross-desensitization provides an important mechanism to regulate immune cell recruitment at sites of inflammation. We previously reported that the mycobacterial cell wall glycophospholipid mannose-capped lipoarabinomannan (ManLAM) could induce human peripheral blood T cell chemotaxis. Therefore, we examined the ability of ManLAM to desensitize T cells to other chemoattractants as a potential mechanism for impaired T cell homing and delayed lung recruitment during mycobacterial infection. We found that ManLAM pretreatment inhibited in vitro migration of naive human or mouse T cells to the lymph node egress signal sphingosine-1-phosphate (S1P). Intratracheal administration of ManLAM in mice resulted in significant increases in T cells, primarily CCR5(+) (Th1) cells, in lung-draining lymph nodes. To investigate the selective CCR5 effect, mouse T cells were differentiated into Th1 or Th2 populations in vitro, and their ability to migrate to S1P with or without ManLAM pretreatment was analyzed. ManLAM pretreatment of Th1 populations inhibited S1P-induced migration but had no effect on Th2 cell S1P-directed migration, suggesting a differential effect by S1P on the two subsets. The PI3K/AKT inhibitor Ly294002 inhibited S1P-directed migration by Th1 cells, whereas the ERK inhibitor U0126 inhibited Th2 cell S1P-directed migration. These observations demonstrate that S1P-induced migratory responses in Th1 and Th2 lymphocytes occurs via different signaling pathways and suggests further that the production of ManLAM during Mycobacterium tuberculosis infection may function to sequester Th1 cells in lung-draining lymph nodes, thereby delaying their recruitment to the lung.

Mannose-capped Lipoarabinomannan from Mycobacterium tuberculosis induces soluble tumor necrosis factor receptor production through tumor necrosis factor alpha-converting enzyme activation

Primary Mycobacterium tuberculosis infection results in granuloma formation in lung tissue. A granuloma encapsulates mycobacterium-containing cells, thereby preventing dissemination and further infection. Tumor necrosis factor alpha (TNF-α) is a host-protective cytokine during M. tuberculosis infection due to its role in promoting and sustaining granuloma formation. TNF activity is regulated through the production of soluble TNF receptors (sTNFRI and sTNFRII). Therefore, we examined the potential production of endogenous sTNFRs during M. tuberculosis infection. Using the murine model of aerosol M. tuberculosis infection, we determined that levels of sTNFR production were elevated in bronchoalveolar lavage fluid 1 month following infection. An investigation of M. tuberculosis cell wall components identified that the known virulence factor mannose-capped lipoarabinomannan (ManLAM) was sufficient to induce sTNFR production, with sTNFRII being produced preferentially compared with sTNFRI. ManLAM stimulated the release of sTNFRs without TNF production, which corresponded to an increase in TNF-α-converting enzyme (TACE) activity. To determine the relevance of these findings, serum samples from M. tuberculosis-infected patients were tested and found to have an increase in the sTNFRII/sTNFRI ratio. These data identify a mechanism by which M. tuberculosis infection can promote the neutralization of TNF and furthermore suggest the potential use of the sTNFRII/sTNFRI ratio as an indicator of tuberculosis disease.

Comparison of interferon-γ-, interleukin (IL)-17- and IL-22-expressing CD4 T cells, IL-22-expressing granulocytes and proinflammatory cytokines during latent and active tuberculosis infection

In this study, we investigated the role and expression of T helper type 17 (Th17) cells and Th17 cytokines in human tuberculosis. We show that the basal proportion of interferon (IFN)-γ-, interleukin (IL)-17- and IL-22-expressing CD4(+) T cells and IL-22-expressing granulocytes in peripheral blood were significantly lower in latently infected healthy individuals and active tuberculosis patients compared to healthy controls. In contrast, CD4(+) T cells expressing IL-17, IL-22 and IFN-γ were increased significantly following mycobacterial antigens stimulation in both latent and actively infected patients. Interestingly, proinflammatory IFN-γ and tumour necrosis factor (TNF)-α were increased following antigen stimulation in latent infection. Similarly, IL-1β, IL-4, IL-8, IL-22 and TNF-α were increased in the serum of latently infected individuals, whereas IL-6 and TNF-α were increased significantly in actively infected patients. Overall, we observed differential induction of IL-17-, IL-22- and IFN-γ-expressing CD4(+) T cells, IL-22-expressing granulocytes and proinflammatory cytokines in circulation and following antigenic stimulation in latent and active tuberculosis.

Mycobacterium tuberculosis ManLAM inhibits T-cell-receptor signaling by interference with ZAP-70, Lck and LAT phosphorylation

Immune evasion is required for Mycobacterium tuberculosis to survive in the face of robust CD4(+) T cell responses. We have shown previously that M. tuberculosis cell wall glycolipids, including mannose capped lipoarabinomannan (ManLAM), directly inhibit polyclonal murine CD4(+) T cell activation by blocking ZAP-70 phosphorylation. We extended these studies to antigen-specific murine CD4(+) T cells and primary human T cells and found that ManLAM inhibited them as well. Lck and LAT phosphorylation also were inhibited by ManLAM without affecting their localization to lipid rafts. Inhibition of proximal TCR signaling was temperature sensitive, suggesting that ManLAM insertion into T cell membranes was required. Thus, M. tuberculosis ManLAM inhibits antigen-specific CD4(+) T cell activation by interfering with very early events in TCR signaling through ManLAM's insertion in T cell membranes.

Interaction between mannosylated lipoarabinomannan and dendritic cell-specific intercellular adhesion molecule-3 grabbing nonintegrin influences dendritic cells maturation and T cell immunity

The aim of the study was to investigate the interaction between manLAM and DC-SIGN influencing DCs maturation and downstream immune response using small interfering RNA-expressing lentiviral vectors to specifically knockdown DC-SIGN. Our data indicated that DC-SIGN knockdown alone in DCs did not affect the maturation or the immunological function of lipopolysacharide (LPS)-activated DCs. Surface molecules were dramatically down-regulated in DCs primed with manLAM but not in mock control DCs (P<0.05). Meanwhile, manLAM enhanced the production of the immunosuppressive cytokine IL-10 in DCs (P<0.05). The level of IFN-γ was significantly down-regulated in the supernatants of naive T cells after co-cultured with DCs primed with manLAM (P<0.05). We demonstrated that DCs primed with manLAM may partially impair maturation phenotypes and immune response in LPS-activated DCs. However, the alterations of DCs function and downstream immune response caused by manLAM were reversed by the knockdown of DC-SIGN.

Mycobacterial PIMs inhibit host inflammatory responses through CD14-dependent and CD14-independent mechanisms

Mycobacteria develop strategies to evade the host immune system. Among them, mycobacterial LAM or PIMs inhibit the expression of pro-inflammatory cytokines by activated macrophages. Here, using synthetic PIM analogues, we analyzed the mode of action of PIM anti-inflammatory effects. Synthetic PIM(1) isomer and PIM(2) mimetic potently inhibit TNF and IL-12 p40 expression induced by TLR2 or TLR4 pathways, but not by TLR9, in murine macrophages. We show inhibition of LPS binding to TLR4/MD2/CD14 expressing HEK cells by PIM(1) and PIM(2) analogues. More specifically, the binding of LPS to CD14 was inhibited by PIM(1) and PIM(2) analogues. CD14 was dispensable for PIM(1) and PIM(2) analogues functional inhibition of TLR2 agonists induced TNF, as shown in CD14-deficient macrophages. The use of rough-LPS, that stimulates TLR4 pathway independently of CD14, allowed to discriminate between CD14-dependent and CD14-independent anti-inflammatory effects of PIMs on LPS-induced macrophage responses. PIM(1) and PIM(2) analogues inhibited LPS-induced TNF release by a CD14-dependent pathway, while IL-12 p40 inhibition was CD14-independent, suggesting that PIMs have multifold inhibitory effects on the TLR4 signalling pathway.

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.

Mycobacterium tuberculosis lipoproteins directly regulate human memory CD4(+) T cell activation via Toll-like receptors 1 and 2

The success of Mycobacterium tuberculosis as a pathogen relies on its ability to regulate the host immune response. M. tuberculosis can manipulate adaptive T cell responses indirectly by modulating antigen-presenting cell (APC) function or by directly interacting with T cells. Little is known about the role of M. tuberculosis molecules in direct regulation of T cell function. Using a biochemical approach, we identified lipoproteins LprG and LpqH as major molecules in M. tuberculosis lysate responsible for costimulation of primary human CD4(+) T cells. In the absence of APCs, activation of memory CD4(+) T cells with LprG or LpqH in combination with anti-CD3 antibody induces Th1 cytokine secretion and cellular proliferation. Lipoprotein-induced T cell costimulation was inhibited by blocking antibodies to Toll-like receptor 2 (TLR2) and TLR1, indicating that human CD4(+) T cells can use TLR2/TLR1 heterodimers to directly respond to M. tuberculosis products. M. tuberculosis lipoproteins induced NF-κB activation in CD4(+) T cells in the absence of TCR co-engagement. Thus, TLR2/TLR1 engagement alone by M. tuberculosis lipoprotein triggered intracellular signaling, but upregulation of cytokine production and proliferation required co-engagement of the TCR. In conclusion, our results demonstrate that M. tuberculosis lipoproteins LprG and LpqH participate in the regulation of adaptive immunity not only by inducing cytokine secretion and costimulatory molecules in innate immune cells but also through directly regulating the activation of memory T lymphocytes.

Primary transcriptomes of Mycobacterium avium subsp. paratuberculosis reveal proprietary pathways in tissue and macrophages

Background

Mycobacterium avium subsp. paratuberculosis (MAP) persistently infects intestines and mesenteric lymph nodes leading to a prolonged subclinical disease. The MAP genome sequence was published in 2005, yet its transcriptional organization in natural infection is unknown. While prior research analyzed regulated gene sets utilizing defined, in vitro stress related or advanced surgical methods with various animal species, we investigated the intracellular lifestyle of MAP in the intestines and lymph nodes to understand the MAP pathways that function to govern this persistence.

Results

Our transcriptional analysis shows that 21%, 8% and 3% of the entire MAP genome was represented either inside tissues, macrophages or both, respectively. Transcripts belonging to latency and cell envelope biogenesis were upregulated in the intestinal tissues whereas those belonging to intracellular trafficking and secretion were upregulated inside the macrophages. Transcriptomes of natural infection and in vitro macrophage infection shared genes involved in transcription and inorganic ion transport and metabolism. MAP specific genes within large sequence polymorphisms of ancestral M. avium complex were downregulated exclusively in natural infection.

Conclusions

We have unveiled common and unique MAP pathways associated with persistence, cell wall biogenesis and virulence in naturally infected cow intestines, lymph nodes and in vitro infected macrophages. This dichotomy also suggests that in vitro macrophage models may be insufficient in providing accurate information on the events that transpire during natural infection. This is the first report to examine the primary transcriptome of MAP at the local infection site (i.e. intestinal tissue). Regulatory pathways that govern the lifecycle of MAP appear to be specified by tissue and cell type. While tissues show a "shut-down" of major MAP metabolic genes, infected macrophages upregulate several MAP specific genes along with a putative pathogenicity island responsible for iron acquisition. Many of these regulatory pathways rely on the advanced interplay of host and pathogen and in order to decipher their message, an interactome must be established using a systems biology approach. Identified MAP pathways place current research into direct alignment in meeting the future challenge of creating a MAP-host interactome.

Global network analysis of lipid-raft-related proteins reveals their centrality in the network and their roles in multiple biological processes

Lipid rafts are specialized cholesterol-enriched microdomains in the cell membrane. They have been known as a platform for protein-protein interactions and to take part in multiple biological processes. Nevertheless, how lipid rafts influence protein properties at the proteomic level is still an open question for researchers using traditional biochemical approaches. Here, by annotating the lipid raft localization of proteins in human protein-protein interaction networks, we performed a systematic analysis of the function of proteins related to lipid rafts. Our results demonstrated that lipid raft proteins and their interactions were critical for the structure and stability of the whole network, and that the interactions between them were significantly enriched. Furthermore, for each protein in the network, we calculated its "lipid raft dependency (LRD)," which indicates how close it is topologically associated with lipid rafts, and we then uncovered the connection between LRD and protein functions. Proteins with high LRD tended to be essential for mammalian development, and malfunction of these proteins was inclined to cause human diseases. Coordinated with their neighbors, high-LRD proteins participated in multiple biological processes and targeted many pathways in diseases pathogenesis. High-LRD proteins were also found to have tissue specificity of expression. In summary, our network-based analysis denotes that lipid raft proteins have higher centrality in the network, and that lipid-raft-related proteins have multiple functions and are probably concerned with many biological processes in disease development.

Diversity in Mycobacterium tuberculosis mannosylated cell wall determinants impacts adaptation to the host

Mycobacterium tuberculosis (the causal agent of TB) has co-evolved with humans for centuries. It infects via the airborne route and is a prototypic highly adapted intracellular pathogen of macrophages. Extensive sequencing of the M. tuberculosis genome along with recent molecular phylogenetic studies is enabling us to gain insight into the biologic diversity that exists among bacterial strains that impact the pathogenesis of latent infection and disease. The majority of the M. tuberculosis cell envelope is comprised of carbohydrates and lipids, and there is increasing evidence that these microbial determinants that are readily exposed to the host immune system play critical roles in disease pathogenesis. Studies from our laboratory and others have raised the possibility that M. tuberculosis is adapting to the human host by cloaking its cell envelope molecules with terminal mannosylated (i.e. Man-alpha-(1-->2)-Man) oligosaccharides that resemble the glycoforms of mammalian mannoproteins. These mannosylated biomolecules engage the mannose receptor (MR) on macrophages during phagocytosis and dictate the intracellular fate of M. tuberculosis by regulating formation of the unique vesicular compartment in which the bacterium survives. The MR is highly expressed on alveolar macrophages (predominant C-type lectin on human cells) and functions as a scavenger receptor to maintain the healthiness of the lung by clearing foreign particles and at the same time regulating dangerous inflammatory responses. Thus M. tuberculosis exploits MR functions to gain entry into the macrophage and survive. Key biochemical pathways and mycobacterial determinants involved in the development and maintenance of the M. tuberculosis phagosome are being identified. The phylogenetic diversity observed in M. tuberculosis strains that impact its cell wall structure together with the genetic diversity observed in human populations, including those elements that affect macrophage function, may help to explain the extraordinary evolutionary adaptation of this pathogen to the human host. Major developments in these areas are the focus of this review.

Bacterial manipulation of innate immunity to promote infection

The mammalian innate immune response provides a barrier against invading pathogens. Innate immune mechanisms are used by the host to respond to a range of bacterial pathogens in an acute and conserved fashion. Host cells express pattern recognition receptors that sense pathogen-associated molecular patterns. After detection, an arsenal of antimicrobial mechanisms is deployed to kill bacteria in infected cells. Innate immunity also stimulates antigen-specific responses mediated by the adaptive immune system. In response, pathogens manipulate host defence mechanisms to survive and eventually replicate. This Review focuses on the control of host innate immune responses by pathogenic intracellular bacteria.

Mycobacterium tuberculosis entry into mast cells through cholesterol-rich membrane microdomains

Cholesterol-enriched membrane microdomains (lipid rafts) play a role in the uptake of many pathogens. Mycobacteria are one of the intracellular pathogens that utilize lipid rafts in order to invade both phagocytic and non-phagocytic cells. However, the mechanism of Mycobacterium tuberculosis uptake by mast cell is not known. To address this issue, we investigated the interaction of M. tuberculosis (H37Rv strain) with mast cells. Confocal microscopy showed that interaction of mycobacterium with mast cell resulted in changes in the mast cell surface, with formation of pseudopod-like structure and activation with visibly extruded granules. Moreover, infection of mast cells with Mycobacteria induced cholesterol accumulation at the site of bacterial entry and around intracellular mycobacteria. Disruption of mast cells lipid rafts by cholesterol depletion markedly inhibited the mycobacterium entry. Intracellular multiplication of M. tuberculosis within mast cells was also observed. Overall, our results indicate that M. tuberculosis employs a cholesterol-dependent pathway to infect mast cells, which leads to degranulation and mast cell morphological changes. These results suggest that although mast cells are capable to respond to M. tuberculosis infection, entry of mycobacterium through lipid rafts may allow replication within mast cells.

Mycobacterium tuberculosis cell wall glycolipids directly inhibit CD4+ T-cell activation by interfering with proximal T-cell-receptor signaling

Immune evasion is required for Mycobacterium tuberculosis to survive in the face of robust adaptive CD4(+) T-cell responses. We have previously shown that M. tuberculosis can indirectly inhibit CD4(+) T cells by suppressing the major histocompatibility complex class II antigen-presenting cell function of macrophages. This study was undertaken to determine if M. tuberculosis could directly inhibit CD4(+) T-cell activation. Murine CD4(+) T cells were purified from spleens by negative immunoaffinity selection followed by flow sorting. Purified CD4(+) T cells were activated for 16 to 48 h with CD3 and CD28 monoclonal antibodies in the presence or absence of M. tuberculosis and its subcellular fractions. CD4(+) T-cell activation was measured by interleukin 2 production, proliferation, and expression of activation markers, all of which were decreased in the presence of M. tuberculosis. Fractionation identified that M. tuberculosis cell wall glycolipids, specifically, phosphatidylinositol mannoside and mannose-capped lipoarabinomannan, were potent inhibitors. Glycolipid-mediated inhibition was not dependent on Toll-like receptor signaling and could be bypassed through stimulation with phorbol 12-myristate 13-acetate and ionomycin. ZAP-70 phosphorylation was decreased in the presence of M. tuberculosis glycolipids, indicating that M. tuberculosis glycolipids directly inhibited CD4(+) T-cell activation by interfering with proximal T-cell-receptor signaling.

Activation requirements and responses to TLR ligands in human CD4+ T cells: comparison of two T cell isolation techniques

Direct regulation of T cell function by microbial ligands through Toll-like receptors (TLR) is an emerging area of T cell biology. Currently either immunomagnetic cell sorting (IMACS) or fluorescence-activated cell sorting (FACS), are utilized to isolate T-cell subsets for such studies. However, it is unknown to what extent differences in T cell purity between these isolation techniques influence T cell functional assays. We compared the purity, response to mitogen, activation requirements, and response to TLR ligands between human CD4(+) T cells isolated either by IMACS (IMACS-CD4(+)) or by IMACS followed by FACS (IMACS/FACS-CD4(+)). As expected, IMACS-CD4(+) were less pure than IMACS/FACS-CD4(+) (92.5%+/-1.4% versus 99.7%+/-0.2%, respectively). Consequently, IMACS-CD4(+) proliferated and produced cytokines in response to mitogen alone and had lower activation requirements compared to IMACS/FACS-CD4(+). In addition IMACS-CD4(+) but not IMACS/FACS-CD4(+) responses were upregulated by the TLR-4 ligand lipopolysaccharide (LPS). On the other hand, TLR-2 and TLR-5 engagement induced costimulation in both IMACS-CD4(+) and highly purified IMACS-/FACS-CD4(+). Altogether these results indicate that small differences in cell purity can significantly alter T cell responses to TLR ligands. This study stresses the importance of a stringent purification method when investigating the role of microbial ligands in T cell function.

How Mycobacterium tuberculosis subverts host immune responses

Mycobacterium tuberculosis is the causative agent of pulmonary tuberculosis which has infected one third of the mankind and causes 2-3 million deaths worldwide each year. The persistence of the infection ensues from the ability of M. tuberculosis to subvert host immune responses in favor of survival and growth of mycobacteria in macrophages. The mechanisms by which M. tuberculosis manipulates the host immune system have only recently come to light. These activities are attributed to lipoarabinomannans (LAM) and their precursors lipomannans (LM), two predominant glycolipids of M. tuberculosis cell wall. LM are able to skew anti-mycobacterial immune responses into un-protective ones, while LAM evoke immunosupression upon binding to macrophage and dendritic cell receptors specialized in binding to "self" host components. A newly emerging idea implicates plasma membrane rafts in LM and LAM signaling. Depending on acylation patterns, the glycolipids may either directly incorporate into the raft membrane via mannosylphosphatidylinositol anchors or interact with raft-associated proteins to affect the assembly of receptor signaling complexes.

Mycobacterial manipulation of vacuolar sorting

Approximately one-third of the world's population is infected with Mycobacterium tuberculosis, and the World Health Organization estimates 1.6 million deaths were caused by M. tuberculosis in 2005. The enormous worldwide burden of disease underscores the proficiency by which M. tuberculosis is able to evade eradication by the host, subverting innate and adaptive defences. At the cellular level, mycobacteria are able to modulate macrophage defences by altering phagosome maturation. This review focuses on the bacterial proteins and lipids that are important in establishing the mycobacterial replicative niche. While there is a detailed molecular description of the vacuole and an increasing number of bacterial effectors have been implicated in creating this compartment, exactly how they intersect host cell processes remains ill-defined. However, the emerging picture is that an array of lipid and protein effectors collaborate to create and maintain the mycobacterial phagosome.

Incorporation of Mycobacterium tuberculosis lipoarabinomannan into macrophage membrane rafts is a prerequisite for the phagosomal maturation block

Lipoarabinomannan (LAM) is one of the key virulence factors for Mycobacterium tuberculosis, the etiological agent of tuberculosis. During uptake of mycobacteria, LAM interacts with the cell membrane of the host macrophage and can be detected throughout the cell upon infection. LAM can inhibit phagosomal maturation as well as induce a proinflammatory response in bystander cells. The aim of this study was to investigate how LAM exerts its action on human macrophages. We show that LAM is incorporated into membrane rafts of the macrophage cell membrane via its glycosylphosphatidylinositol anchor and that incorporation of mannose-capped LAM from M. tuberculosis results in reduced phagosomal maturation. This is dependent on successful insertion of the glycosylphosphatidylinositol anchor. LAM does not, however, induce the phagosomal maturation block through activation of p38 mitogen-activated protein kinase, contradicting some previous suggestions.

A Mycobacterium tuberculosis-derived lipid inhibits membrane fusion by modulating lipid membrane domains

Tuberculosis is an infectious and potentially fatal disease caused by the acid-fast bacillus Mycobacterium tuberculosis (MTB). One hallmark of a tuberculosis infection is the ability of the bacterium to subvert the normal macrophage defense mechanism of the host immune response. Lipoarabinomannan (LAM), an integral component of the MTB cell wall, is released when MTBs are taken into phagosomes and has been reported to be involved in the inhibition of phago-lysosomal (P-L) fusion. However, the physical chemistry of the effects of LAM on lipid membrane structure relative to P-L fusion has not been studied. We produced membranes in vitro composed of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol to simulate phagosomal lipid membranes and quantified the effects of the addition of LAM to these membranes, using fluorescence resonance energy transfer assays and atomic force microscopy. We found that LAM inhibits vesicle fusion and markedly alters lipid membrane domain morphology and sphingomyelin-chollesterol/dioleoylphosphatidylcholine ratios. These data demonstrate that LAM induces a dramatic reorganization of lipid membranes in vitro and clarifies the role of LAM in the inhibition of P-L fusion and the survival of the MTB within the macrophage.

Phosphatidylinositol mannoside from Mycobacterium tuberculosis binds alpha5beta1 integrin (VLA-5) on CD4+ T cells and induces adhesion to fibronectin

The pathological hallmark of the host response to Mycobacterium tuberculosis is the granuloma where T cells and macrophages interact with the extracellular matrix (ECM) to control the infection. Recruitment and retention of T cells within inflamed tissues depend on adhesion to the ECM. T cells use integrins to adhere to the ECM, and fibronectin (FN) is one of its major components. We have found that the major M. tuberculosis cell wall glycolipid, phosphatidylinositol mannoside (PIM), induces homotypic adhesion of human CD4+ T cells and T cell adhesion to immobilized FN. Treatment with EDTA and cytochalasin D prevented PIM-induced T cell adhesion. PIM-induced T cell adhesion to FN was blocked with mAbs against alpha5 integrin chain and with RGD-containing peptides. Alpha5beta1 (VLA-5) is one of two major FN receptors on T cells. PIM was found to bind directly to purified human VLA-5. Thus, PIM interacts directly with VLA-5 on CD4+ T lymphocytes, inducing activation of the integrin, and promoting adhesion to the ECM glycoprotein, FN. This is the first report of direct binding of a M. tuberculosis molecule to a receptor on human T cells resulting in a change in CD4+ T cell function.

Mycobacterium tuberculosis inhibition of phagolysosome biogenesis and autophagy as a host defence mechanism

A marquee feature of the powerful human pathogen Mycobacterium tuberculosis is its macrophage parasitism. The intracellular survival of this microorganism rests upon its ability to arrest phagolysosome biogenesis, avoid direct cidal mechanisms in macrophages, and block efficient antigen processing and presentation. Mycobacteria prevent Rab conversion on their phagosomes and elaborate glycolipid and protein trafficking toxins that interfere with Rab effectors and regulation of specific organellar biogenesis in mammalian cells. One of the major Rab effectors affected in this process is the type III phosphatidylinositol 3-kinase hVPS34 and its enzymatic product phosphatidylinositol 3-phosphate (PI3P), a regulatory lipid earmarking organellar membranes for specific trafficking events. PI3P is also critical for the process of autophagy, recently recognized as an effector of innate and adaptive immunity. Induction of autophagy by physiological, pharmacological or immunological signals, including the major antituberculosis Th1 cytokine IFN-gamma and its downstream effector p47 GTPase LRG-47, can overcome mycobacterial phagosome maturation block and inhibit intracellular M. tuberculosis survival. This review summarizes the findings centred around the PI3P-nexus where the mycobacterial phagosome maturation block and execution stages of autophagy intersect.

DNA vaccines for therapy of tuberculosis: Where are we now?

DNA vaccines that were being investigated in mice for prophylactic use against tuberculosis were soon found also to be surprisingly effective as treatment against established infection. The immune system was stimulated to kill the bacteria, even including the persistent latent bacteria that are otherwise refractory to the immune system and antibacterial chemotherapeutic drugs alike. Subsequent results from a range of laboratories using diverse DNA vaccines in diverse murine models of infection have been very varied, ranging from enhanced pathology, through negligible effects, to major additive benefit from combined vaccine and chemotherapy. This review summarises the data and assesses future prospects.