Trehalose 6,6'-dimycolate and lipid in the pathogenesis of caseating granulomas of tuberculosis in mice

A Photoactivatable Free Mycolic Acid Probe to Investigate Mycobacteria-Host Interactions

Mycolic acids are long-chain, α-branched, β-hydroxylated fatty acid lipids that populate the outer mycomembrane of mycobacteria, including the pathogen Mycobacterium tuberculosis. Mycolic acids predominantly occur in the form of glycolipids, but nonglycosylated free mycolic acids (fMA), which are generated during mycomembrane remodeling, are major constituents of the M. tuberculosis biofilm extracellular matrix and promote host immune evasion during M. tuberculosis infection. However, our understanding of these processes is nascent, and there is limited information about the fMA-protein interactions involved. To facilitate such studies, we synthesized a fMA analogue probe (x-Alk-MA) containing a photo-cross-linking diazirine and a clickable alkyne to enable live-cell capture and analysis of protein interactors. The synthetic strategy featured asymmetric hydrogenation to establish the β-hydroxy group, diastereoselective alkylation to establish the α-branch, and late-stage modification to install the functional tags. In macrophages, x-Alk-MA recapitulated the cytokine response of native MA and selectively photolabeled TREM2, a host cell receptor for fMAs that suppresses macrophage activation and has been implicated in M. tuberculosis immune evasion. The synthetic strategy, chemical probes, and photolabeling methods disclosed herein should facilitate future studies aimed at understanding the roles of fMA in mycobacterial physiology and pathogenesis.

Isoniazid and nicotinic hydrazide hybrids mitigate trehalose-6,6'-dimycolate-induced inflammatory responses and pulmonary granulomas via Syk/PI3K pathways: A promising host-directed therapy for tuberculosis

Granulomas, dense clusters of immune cells and bacteria, are critical barriers in tuberculosis (TB) treatment. Recent advancements in TB management have highlighted granuloma control as a potential host-directed therapy (HDT) strategy. Although isoniazid (INH) is the first-line drug for TB therapy, its efficacy is limited to non-replicating Mycobacterium tuberculosis (Mtb) under granulomatous conditions, necessitating the development of more effective derivatives. In this study, hybrid compounds of isoniazid, designated as INH-D1 and INH-D2, were synthesized and evaluated for their effects on controlling inflammatory responses and pulmonary granuloma lesions induced by trehalose-6,6'-dimycolate (TDM), a glycolipid of Mtb. Both INH-D1 and INH-D2 demonstrated stronger inhibitory effects on inflammatory mediators (TNF-α, interleukin-6, co-stimulatory molecules, and MHC class I) in TDM-stimulated macrophages compared to original INH. These anti-inflammatory effects were mediated by the inhibition of Syk, p38, PI3K, and NF-κB transcription. INH-D1 and INH-D2 exhibited stronger binding energies to Syk and PI3Kα/β than INH, which are known as proximal kinases and key mediator in TDM-mediated inflammatory responses. Oral administration of INH-D2 successfully relieved TDM-induced pulmonary granuloma pathology by reducing innate immune cell infiltration, hypoxic conditions in the lungs, and systemic inflammation by decreasing serum cytokines and chemokines. In contrast, original INH and INH-D1 did not effectively alleviate pulmonary granuloma pathology. These findings demonstrate that the novel molecule INH-D2 is effective in treating pulmonary granulomas owing to its strong anti-inflammatory effects, highlighting it as a promising HDT candidate for the management of pulmonary tuberculosis, thereby providing a strategic alternative to standard anti-TB antibiotics.

Understanding the development of tuberculous granulomas: insights into host protection and pathogenesis, a review in humans and animals

Granulomas, organized aggregates of immune cells which form in response to Mycobacterium tuberculosis (Mtb), are characteristic but not exclusive of tuberculosis (TB). Despite existing investigations on TB granulomas, the determinants that differentiate host-protective granulomas from granulomas that contribute to TB pathogenesis are often disputed. Thus, the goal of this narrative review is to help clarify the existing literature on such determinants. We adopt the a priori view that TB granulomas are host-protective organelles and discuss the molecular and cellular determinants that induce protective granulomas and those that promote their failure. While reports about protective TB granulomas and their failure may initially seem contradictory, it is increasingly recognized that either deficiencies or excesses of the molecular and cellular components in TB granuloma formation may be detrimental to the host. More specifically, insufficient or excessive expression/representation of the following components have been reported to skew granulomas toward the less protective phenotype: (i) epithelioid macrophages; (ii) type 1 adaptive immune response; (iii) type 2 adaptive immune response; (iv) tumor necrosis factor; (v) interleukin-12; (vi) interleukin-17; (vii) matrix metalloproteinases; (viii) hypoxia in the TB granulomas; (ix) hypoxia inducible factor-1 alpha; (x) aerobic glycolysis; (xi) indoleamine 2,3-dioxygenase activity; (xii) heme oxygenase-1 activity; (xiii) immune checkpoint; (xiv) leukotriene A4 hydrolase activity; (xv) nuclear-factor-kappa B; and (xvi) transforming growth factor-beta. Rather, more precise and timely coordinated immune responses appear essential for eradication or containment of Mtb infection. Since there are several animal models of infection with Mtb, other species within the Mtb complex, and the surrogate Mycobacterium marinum - whether natural (cattle, elephants) or experimental (zebrafish, mouse, guinea pig, rabbit, mini pig, goat, non-human primate) infections - we also compared the TB granulomatous response and other pathologic lung lesions in various animals infected with one of these mycobacteria with that of human pulmonary TB. Identifying components that dictate the formation of host-protective granulomas and the circumstances that result in their failure can enhance our understanding of the macrocosm of human TB and facilitate the development of novel remedies - whether they be direct therapeutics or indirect interventions - to efficiently eliminate Mtb infection and prevent its pathologic sequelae.

The Impact of Trehalose Dimycolate on the Clinical Course of Mycobacterium avium Complex Pulmonary Disease

Rationale: The clinical implications of trehalose 6,6'-dimycolate (TDM) in nontuberculous mycobacterial pulmonary disease have not been studied. Objectives: To examine the presence of TDM in clinical isolates obtained from patients with Mycobacterium avium complex (MAC) pulmonary disease (PD) and its impact on disease severity and treatment outcomes. Methods: We analyzed clinical isolates from patients with diagnoses of MAC PD at Seoul National University Hospital between January 1, 2019, and December 31, 2021. The lipids were extracted from clinical isolates obtained at the time of diagnosis using mass spectrometry. Mass peaks between 300 and 3,500 m/z were obtained, and the peak patterns of the total lipids were analyzed. Results: TDM was identified in clinical isolates from 176 of 343 patients. Cavities were more prevalent in patients with TDM-negative isolates (19.8%) than in those with TDM-positive isolates (10.2%) (P = 0.015). The time to antibiotic treatment was shorter in patients with TDM-negative isolates (4 mo [interquartile range, 2-10 mo]) than in those with TDM-positive isolates (7 mo [interquartile range, 3-16 mo]) (P = 0.032). Patients with TDM-negative isolates had a significantly lower proportion of culture conversions (P = 0.012). TDM was associated with higher likelihood of culture conversion (adjusted hazard ratio, 2.29; P = 0.035). Conclusions: TDM-negative isolates were linked to a higher occurrence of cavities, earlier initiation of treatment, and worse treatment outcome in patients with MAC PD.

A novel humanized mouse model for HIV and tuberculosis co-infection studies

Background

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), continues to be a major public health problem worldwide. The human immunodeficiency virus (HIV) is another equally important life-threatening pathogen. HIV infection decreases CD4+ T cell levels markedly increasing Mtb co-infections. An appropriate animal model for HIV/Mtb co-infection that can recapitulate the diversity of the immune response in humans during co-infection would facilitate basic and translational research in HIV/Mtb infections. Herein, we describe a novel humanized mouse model.

Methods

The irradiated NSG-SGM3 mice were transplanted with human CD34+ hematopoietic stem cells, and the humanization was monitored by staining various immune cell markers for flow cytometry. They were challenged with HIV and/or Mtb, and the CD4+ T cell depletion and HIV viral load were monitored over time. Before necropsy, the live mice were subjected to pulmonary function test and CT scan, and after sacrifice, the lung and spleen homogenates were used to determine Mtb load (CFU) and cytokine/chemokine levels by multiplex assay, and lung sections were analyzed for histopathology. The mouse sera were subjected to metabolomics analysis.

Results

Our humanized NSG-SGM3 mice were able to engraft human CD34+ stem cells, which then differentiated into a full-lineage of human immune cell subsets. After co-infection with HIV and Mtb, these mice showed decrease in CD4+ T cell counts overtime and elevated HIV load in the sera, similar to the infection pattern of humans. Additionally, Mtb caused infections in both lungs and spleen, and induced granulomatous lesions in the lungs. Distinct metabolomic profiles were also observed in the tissues from different mouse groups after co-infections.

Conclusion

The humanized NSG-SGM3 mice are able to recapitulate the pathogenic effects of HIV and Mtb infections and co-infection at the pathological, immunological and metabolism levels and are therefore a reproducible small animal model for studying HIV/Mtb co-infection.

A Novel Humanized Mouse Model for HIV and Tuberculosis Co-infection Studies

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), continues to be a major public health problem worldwide. The human immunodeficiency virus (HIV) is another equally important life-threatening pathogen. Further, co-infections with HIV and Mtb have severe effects in the host, with people infected with HIV being fifteen to twenty-one times more likely to develop active TB. The use of an appropriate animal model for HIV/Mtb co-infection that can recapitulate the diversity of the immune response in humans would be a useful tool for conducting basic and translational research in HIV/Mtb infections. The present study was focused on developing a humanized mouse model for investigations on HIV-Mtb co-infection. Using NSG-SGM3 mice that can engraft human stem cells, our studies showed that they were able to engraft human CD34+ stem cells which then differentiate into a full-lineage of human immune cell subsets. After co-infection with HIV and Mtb, these mice showed decrease in CD4+ T cell counts overtime and elevated HIV load in the sera, similar to the infection pattern of humans. Additionally, Mtb caused infections in both lungs and spleen, and induced the development of granulomatous lesions in the lungs, detected by CT scan and histopathology. Distinct metabolomic profiles were also observed in the tissues from different mouse groups after co-infections. Our results suggest that the humanized NSG-SGM3 mice are able to recapitulate the effects of HIV and Mtb infections and co-infection in the human host at pathological, immunological and metabolism levels, providing a dependable small animal model for studying HIV/Mtb co-infection.

Temporal and cellular analysis of granuloma development in mycobacterial infected adult zebrafish

Because granulomas are a hallmark of tuberculosis pathogenesis, the study of the dynamic changes in their cellular composition and morphological character can facilitate our understanding of tuberculosis pathogenicity. Adult zebrafish infected with Mycobacterium marinum form granulomas that are similar to the granulomas in human patients with tuberculosis and therefore have been used to study host-mycobacterium interactions. Most studies of zebrafish granulomas, however, have focused on necrotic granulomas, while a systematic description of the different stages of granuloma formation in the zebrafish model is lacking. Here, we characterized the stages of granulomas in M. marinum-infected zebrafish, including early immune cell infiltration, nonnecrotizing granulomas, and necrotizing granulomas, using corresponding samples from patients with pulmonary tuberculosis as references. We combined hematoxylin and eosin staining and in situ hybridization to identify the different immune cell types and follow their spatial distribution in the different stages of granuloma development. The macrophages in zebrafish granulomas were shown to belong to distinct subtypes: epithelioid macrophages, foamy macrophages, and multinucleated giant cells. By defining the developmental stages of zebrafish granulomas and the spatial distribution of the different immune cells they contain, this work provides a reference for future studies of mycobacterial granulomas and their immune microenvironments.

Proteomics coupled transcriptomics reveals lipopolysaccharide inhibiting peroxisome proliferator-activated receptors signalling pathway to reduce lipid droplets accumulation in mouse liver

Lipid droplets (LDs) are multifunctional organelles consisting of a central compartment of non-polar lipids shielded from the cytoplasm by a phospholipid monolayer. The excessive accumulation of LDs in cells is closely related to the development and progression of many diseases in humans and animals, such as liver-related and cardiovascular diseases. Thus, regulating the LDs size and abundance is necessary to maintain metabolic homeostasis. This study found that lipopolysaccharide (LPS) stimulation reduced the LDs content in the mouse liver. We tried to explain the possible molecular mechanisms at the broad protein and mRNA levels, finding that inhibition of the peroxisome proliferator-activated receptors (PPAR) signalling pathway by LPS may be a critical factor in reducing LDs content.

New therapeutic strategies for Mycobacterium abscessus pulmonary diseases - untapping the mycolic acid pathway

INTRODUCTION

Treatment options against Mycobacterium abscessus infections are very limited. New compounds are needed to cure M. abscessus pulmonary diseases. While the mycolic acid biosynthetic pathway has been largely exploited for the treatment of tuberculosis, this metabolic process has been overlooked in M. abscessus, although it offers many potential drug targets for the treatment of this opportunistic pathogen.

AREAS COVERED

Herein, the authors review the role of the MmpL3 membrane protein and the enoyl-ACP reductase InhA involved in the transport and synthesis of mycolic acids, respectively. They discuss their importance as two major vulnerable drug targets in M. abscessus and report the activity of MmpL3 and InhA inhibitors. In particular, they focus on NITD-916, a direct InhA inhibitor against M. abscessus, particularly warranted in the context of multidrug resistance.

EXPERT OPINION

There is an increasing body of evidence validating the mycolic acid pathway as an attractive drug target to be further exploited for M. abscessus lung disease treatments. The NITD-916 studies provide a proof-of-concept that direct inhibitors of InhA are efficient in vitro, in macrophages and in zebrafish. Future work is now required to improve the activity and pharmacological properties of these inhibitors and their evaluation in pre-clinical models.

Prospective Subunit Nanovaccine against Mycobacterium tuberculosis Infection─Cubosome Lipid Nanocarriers of Cord Factor, Trehalose 6,6' Dimycolate

An improved vaccine is urgently needed to replace the now more than 100-year-old Bacillus Calmette-Guérin (BCG) vaccine against tuberculosis (TB) disease, which represents a significant burden on global public health. Mycolic acid, or cord factor trehalose 6,6' dimycolate (TDM), a lipid component abundant in the cell wall of the pathogen Mycobacterium tuberculosis (MTB), has been shown to have strong immunostimulatory activity but remains underexplored due to its high toxicity and poor solubility. Herein, we employed a novel strategy to encapsulate TDM within a cubosome lipid nanocarrier as a potential subunit nanovaccine candidate against TB. This strategy not only increased the solubility and reduced the toxicity of TDM but also elicited a protective immune response to control MTB growth in macrophages. Both pre-treatment and concurrent treatment of the TDM encapsulated in lipid monoolein (MO) cubosomes (MO-TDM) (1 mol %) induced a strong proinflammatory cytokine response in MTB-infected macrophages, due to epigenetic changes at the promoters of tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) in comparison to the untreated control. Furthermore, treatment with MO-TDM (1 mol %) cubosomes significantly improved antigen processing and presentation capabilities of MTB-infected macrophages to CD4 T cells. The ability of MO-TDM (1 mol %) cubosomes to induce a robust innate and adaptive response in vitro was further supported by a mathematical modeling study predicting the vaccine efficacy in vivo. Overall, these results indicate a strong immunostimulatory effect of TDM when delivered through the lipid nanocarrier, suggesting its potential as a novel TB vaccine.

The cell envelope of Mycobacterium abscessus and its role in pathogenesis

Mycobacterium abscessus is a nontuberculosis mycobacterium (NTM) that has shown an exponential rise in its ability to cause disease. Due to its ubiquitous presence in the environment, M. abscessus is widely implicated in secondary exacerbations of many nosocomial infections and genetic respiratory disorders, such as cystic fibrosis (CF). Contrary to other rapidly growing NTMs, the cell envelope of M. abscessus harbors several prominent features and undergoes modifications that are responsible for its pathogenesis. Compositional changes of the mycobacterial outer membrane (MOM) significantly decrease the presence of glycopeptidolipids (GPLs) and enable the transition from a colonizing, smooth morphotype into a virulent, rough morphotype. The GPLs are transported to the MOM by the Mycobacterial membrane proteins Large (MmpL), which further act as drug efflux pumps and confer antibiotic resistance. Lastly, M. abscessus possesses 2 type VII secretion systems (T7SS): ESX-3 and ESX-4, both of which have recently been implicated in host-pathogen interactions and virulence. This review summarizes the current knowledge of M. abscessus pathogenesis and highlights the clinically relevant association between the structure and functions of its cell envelope.

In the Thick of It: Formation of the Tuberculous Granuloma and Its Effects on Host and Therapeutic Responses

The defining pathology of tuberculosis is the granuloma, an organized structure derived from host immune cells that surrounds infecting Mycobacterium tuberculosis. As the location of much of the bacteria in the infected host, the granuloma is a central point of interaction between the host and the infecting bacterium. This review describes the signals and cellular reprogramming that drive granuloma formation. Further, as a central point of host-bacterial interactions, the granuloma shapes disease outcome by altering host immune responses and bacterial susceptibility to antibiotic treatment, as discussed herein. This new understanding of granuloma biology and the signaling behind it highlights the potential for host-directed therapies targeting the granuloma to enhance antibiotic access and tuberculosis-specific immune responses.

Concise Modular Synthesis and NMR Structural Determination of Gallium Mycobactin T

A modular synthesis of mycobactin T and its N-acetyl analogue is reported in a route that facilitates permutation of the lipid tails. A key feature is the generation of N(α)-Cbz-N(ε)-benzyloxy-N(ε)-Boc-lysine (A4) with methyl(trifluoromethyl)dioxirane in 59% yield. Selective hydroxamate N-acylation was achieved with acyl fluorides, enabling installation of lipids tails in the final step. O-Benzyl-dehydrocobactin T (B4) was prepared by modifying a known five-step sequence with an overall yield of 49%. 2-Hydroxyphenyl-4-carboxyloxazoline (C3) was prepared from 2-hydroxybenzoic acid and l-serine methyl ester in three steps with an overall yield of 55%. Ester coupling of A4 and B4 with EDCI afforded MbI-1 in 73% yield. Catalytic hydrogenation with Pd/BaSO₄ and 50 psi of H₂ simultaneously effected alkene reduction and debenzylation to afford MbI-2 in 96% yield. Fragment C3 was converted into acyl fluoride C4, which coupled with MbI-2 to afford MbI-3 in 51% yield. Finally, Boc-removal with HCl/EtOAc and treatment of the resultant hydroxylamine with stearyl fluoride furnished mycobactin T in 65% yield. Overall, the yield is 4% over 14 steps. The gallium mycobactin T-N-acetyl derivative (GaMbT-NAc) structure was determined by ¹H NMR. The structure shows an octahedral Ga and two internal hydrogen bonds between peptidic N-Hs and two of the oxygen atoms coordinating Ga.

Aggregation state of Mycobacterium tuberculosis impacts host immunity and augments pulmonary disease pathology

In vitro phagocytosis of Mycobacterium tuberculosis (Mtb) aggregates (Mtb-AG), rather than similar numbers of single bacilli (Mtb-SC), induces host macrophage death and favors bacterial growth. Here, we examined whether aggregation contributes to enhanced Mtb pathogenicity in vivo in rabbit lungs. Rabbits were exposed to infectious aerosols containing mainly Mtb-AG or Mtb-SC. The lung bacterial load, systemic immune response, histology, and immune cell composition were investigated over time. Genome-wide transcriptome analysis, cellular and tissue-level assays, and immunofluorescent imaging were performed on lung tissue to define and compare immune activation and pathogenesis between Mtb-AG and Mtb-SC infection. Lung bacillary loads, disease scores, lesion size, and structure were significantly higher in Mtb-AG than Mtb-SC infected animals. Differences in immune cell distribution and activation were noted in the lungs of the two groups of infected animals. Consistently larger lung granulomas with large aggregates of Mtb, extensive necrotic foci, and elevated matrix metalloproteases expression were observed in Mtb-AG infected rabbits. Our findings suggest that bacillary aggregation increases Mtb fitness for improved growth and accelerates lung inflammation and infected host cell death, thereby exacerbating disease pathology in the lungs.

A Mycobacterium species for Crohn's disease?

In ruminants Mycobacterium avium subspecies paratuberculosis (MAP) is the causative organism of a chronic granulomatous inflammatory bowel disease called Johne's disease (JD). Some researchers have hypothesised that MAP is also associated with Crohn's disease (CD), an inflammatory bowel disease in humans that shares some histological features of JD. Despite numerous attempts to demonstrate causality by researchers, direct microbiological evidence of MAP involvement in CD remains elusive. Importantly, it has not been possible to reliably and reproducibly demonstrate mycobacteria in the tissue of CD patients. Past attempts to visualise mycobacteria in tissue may have been hampered by the use of stains optimised for Mycobacterium tuberculosis complex (MTB) and the lack of reliable bacteriological culture media for both non-tuberculous mycobacteria (NTM) and cell-wall-deficient mycobacteria (CWDM). Here we describe a Ziehl-Neelsen (ZN) staining method for the demonstration of CWDM in resected tissue from patients with Crohn's disease, revealing the association of CWDM in situ with host tissue reactions, and posit this as a cause of the tissue inflammation. Using the ZN stain described we demonstrated the presence of CWDM in 18 out of 18 excised tissue samples from patients diagnosed as having Crohn's disease, and in zero samples out of 15 non-inflammatory bowel disease controls.

Roles for phthiocerol dimycocerosate lipids in Mycobacterium tuberculosis pathogenesis

The success of Mycobacterium tuberculosis as a pathogen is well established: tuberculosis is the leading cause of death by a single infectious agent worldwide. The threat of multi- and extensively drug-resistant bacteria has renewed global concerns about this pathogen and understanding its virulence strategies will be essential in the fight against tuberculosis. The current review will focus on phthiocerol dimycocerosates (PDIMs), a long-known and well-studied group of complex lipids found in the M. tuberculosis cell envelope. Numerous studies show a role for PDIMs in several key steps of M. tuberculosis pathogenesis, with recent studies highlighting its involvement in bacterial virulence, in association with the ESX-1 secretion system. Yet, the mechanisms by which PDIMs help M. tuberculosis to control macrophage phagocytosis, inhibit phagosome acidification and modulate host innate immunity, remain to be fully elucidated.

Mycobacterium tuberculosis Small RNA MTS1338 Confers Pathogenic Properties to Non-Pathogenic Mycobacterium smegmatis

Small non-coding RNAs play a key role in bacterial adaptation to various stresses. Mycobacterium tuberculosis small RNA MTS1338 is upregulated during mycobacteria infection of macrophages, suggesting its involvement in the interaction of the pathogen with the host. In this study, we explored the functional effects of MTS1338 by expressing it in non-pathogenic Mycobacterium smegmatis that lacks the MTS1338 gene. The results indicated that MTS1338 slowed the growth of the recombinant mycobacteria in culture and increased their survival in RAW 264.7 macrophages, where the MTS1338-expressing strain significantly (p < 0.05) reduced the number of mature phagolysosomes and changed the production of cytokines IL-1β, IL-6, IL-10, IL-12, TGF-β, and TNF-α compared to those of the control strain. Proteomic and secretomic profiling of recombinant and control strains revealed differential expression of proteins involved in the synthesis of main cell wall components and in the regulation of iron metabolism (ESX-3 secretion system) and response to hypoxia (furA, whiB4, phoP). These effects of MTS1338 expression are characteristic for M. tuberculosis during infection, suggesting that in pathogenic mycobacteria MTS1338 plays the role of a virulence factor supporting the residence of M. tuberculosis in the host.

Is It Time To Kill the Survival Curve? A Case for Disease Progression Factors in Microbial Pathogenesis and Host Defense Research

The molecular mechanisms of microbial virulence and host defense are most often studied using animal models and Koch's molecular postulates. A common rationale for these types of experiments is to identify therapeutic targets based on the assumption that microbial or host factors that confer extreme animal model survival phenotypes represent critical virulence and host defense factors. Yet null mutant strains of microbial (or host) factors often yield extreme survival curve phenotypes because they fail to establish an infection. The lack of infection and disease establishment prevents true assessment of the given factor's role(s) in disease progression. Here, we posit that the emphasis on extreme survival curve phenotypes in fungal infectious disease models is leading to missed opportunities to identify new fungal and host factors critical for disease progression. We simply do not yet have a sufficient understanding of fungal virulence and host defense mechanisms throughout the temporal course of an infection. We propose that there is a need to develop new approaches and to revisit tried and true methods to define infection site biology beyond the analysis of survival curve phenotypes. To stimulate these new approaches, we propose the (new) terms "disease initiation factor" and "disease progression factor" to distinguish functional roles at distinct temporal stages of an infection and give us targets to foster new discoveries.

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

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

Therapeutic Vaccination with Cationic Liposomes Formulated with Dioctadecyldimethylammonium and Trehalose Dibehenate (CAF01) and Peptide P10 Is Protective in Mice Infected with Paracoccidioides brasiliensis

The peptide P10 is a vaccine candidate for Paracoccidioidomycosis, a systemic mycosis caused by fungal species of the genus Paracoccidioides spp. We have previously shown that peptide P10 vaccination, in the presence of several different adjuvants, induced a protective cellular immune response mediated by CD4⁺ Th₁ lymphocytes that was associated with the increased production of IFN-γ in mice challenged with a virulent isolate of Paracoccidoides brasiliensis. Cationic liposomes formulated with dioctadecyldimethylammonium and trehalose dibehenate (DDA/TDB, termed also CAF01–cationic adjuvant formulation) have been developed for safe administration in humans and CAF01 liposomes are utilized as an adjuvant for modulating a robust Th₁/Th₁₇ cellular response. We evaluated the efficacy of the adsorption of peptide P10 to CAF01 cationic liposomes and used the generated liposomes to vaccinate C57Bl/6 mice infected with P. brasiliensis. Our results showed that P10 was efficiently adsorbed onto CAF01 liposomes. The vaccination of infected mice with cationic liposomes formulated with DDA/TDB 250/50 µg/mL and 20 µg of P10 induced an effective cellular immune response with increased levels of Th₁₇ cytokines, which correlated with significant decreases in the fungal burdens in lungs and protective granulomatous tissue responses. Hence, cationic liposomes of DDA/TDB 250/50 µg/mL with 20 µg of P10 are a promising therapeutic for safely and effectively improving the treatment of paracoccidioidomycosis.

The Pathogenesis of Tuberculosis-The Koch Phenomenon Reinstated

Research on the pathogenesis of tuberculosis (TB) has been hamstrung for half a century by the paradigm that granulomas are the hallmark of active disease. Human TB, in fact, produces two types of granulomas, neither of which is involved in the development of adult type or post-primary TB. This disease begins as the early lesion; a prolonged subclinical stockpiling of secreted mycobacterial antigens in foamy alveolar macrophages and nearby highly sensitized T cells in preparation for a massive necrotizing hypersensitivity reaction, the Koch Phenomenon, that produces caseous pneumonia that is either coughed out to form cavities or retained to become the focus of post-primary granulomas and fibrocaseous disease. Post-primary TB progresses if the antigens are continuously released and regresses when they are depleted. This revised paradigm is supported by nearly 200 years of research and suggests new approaches and animal models to investigate long standing mysteries of human TB and vaccines that inhibit the early lesion to finally end its transmission.

Trehalose and bacterial virulence

Trehalose is a disaccharide of two D-glucose molecules linked by a glycosidic linkage, which plays both structural and functional roles in bacteria. Trehalose can be synthesized and degraded by several pathways, and induction of trehalose biosynthesis is typically associated with exposure to abiotic stress. The ability of trehalose to protect against abiotic stress has been exploited to stabilize a range of bacterial vaccines. More recently, there has been interest in the role of this molecule in microbial virulence. There is now evidence that trehalose or trehalose derivatives play important roles in virulence of a diverse range of Gram-positive and Gram-negative pathogens of animals or plants. Trehalose and/or trehalose derivatives can play important roles in host colonization and growth in the host, and can modulate the interactions with host defense mechanisms. However, the roles are typically pathogen-specific. These findings suggest that trehalose metabolism may be a target for novel pathogen-specific rather than broad spectrum interventions.

Underestimated Manipulative Roles of Mycobacterium tuberculosis Cell Envelope Glycolipids During Infection

The Mycobacterium tuberculosis cell envelope has been evolving over time to make the bacterium transmissible and adaptable to the human host. In this context, the M. tuberculosis cell envelope contains a peripheral barrier full of lipids, some of them unique, which confer M. tuberculosis with a unique shield against the different host environments that the bacterium will encounter at the different stages of infection. This lipid barrier is mainly composed of glycolipids that can be characterized by three different subsets: trehalose-containing, mannose-containing, and 6-deoxy-pyranose-containing glycolipids. In this review, we explore the roles of these cell envelope glycolipids in M. tuberculosis virulence and pathogenesis, drug resistance, and further, how these glycolipids may dictate the M. tuberculosis cell envelope evolution from ancient to modern strains. Finally, we address how these M. tuberculosis cell envelope glycolipids are impacted by the host lung alveolar environment, their role in vaccination and masking host immunity, and subsequently the impact of these glycolipids in shaping how M. tuberculosis interacts with host cells, manipulating their immune response to favor the establishment of an infection.

[Immunological aspects of tuberculosis pathogenesis]

The morphological aspects of TB pathogenesis are well described in the publications. Much is also known about the main stages of development and formation of specific adaptive immunity. However, from our point of view, not enough attention is being paid to the involvement of the immune system in the pathogenesis of clinically relevant TB abnormalities, as well as various forms of the disease. Nevertheless, there is no doubt that the variety of clinical manifestations of any disease associated with the penetration of a foreign agent into the body, and Mycobacterium tuberculosis (MTB) in particular, is due to the collective interaction of the infectious agent and the individual response of the macroorganism to this infectious agent. The mosaic of such interactions usually imposes its own adjustments on the development of different forms of the process, its speed and direction, as well as the outcomes. Certainly, the response of a macroorganism to MTB is an integral part of pathogenesis and consists of many general components including the responses associated with the mechanisms of natural and acquired immunity. Intensity of these reactions depends on the characteristics of an agent (MTB) and a macroorganism. For the development of TB disease, massiveness of TB infection, dose and duration of MTB exposure to the human body, as well as virulence of MTB and the level of body's protection during the exposure play a very important role. TB pathogenesis is somewhat different in primary MTB infection and re - infection. With primary infection, 88-90% of individuals do not have clinical manifestations, and only the tuberculin skin test conversion signals the onset of infection. In some cases, without any use of anti-TB drugs limited abnormalities may result in spontaneous cure with the minimal residual changes in the lungs, intrathoracic lymph nodes and tissues of other organs, often in the form of calcifications and limited areas of fibrosis in more advanced cases. Only 10-12% of newly infected individuals develop TB with severe clinical manifestations requiring TB therapy. The absence of clinical manifestations of primary TB infection can be explained by a high level of natural resistance of the human body to tuberculosis, and sometimes can be an effect of acquired protection due to BCG vaccination. This review attempts to discuss the role of immune mechanisms in the pathogenesis both at the beginning of disease development, and in the process of its various manifestations. Issues of genetically determined resistance or susceptibility to TB are not being covered in detail in this manuscript.

Vaccination of European sea bass Dicentrarchus labrax with avirulent Mycobacterium marinum (iipA::kan mutant)

Mycobacteriosis is a chronic progressive disease affecting teleost fishes all over the world. No vaccine is commercially available against its main etiological agent, Mycobacterium marinum. The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to modulate M. marinum pathology. The innate and adaptive immune responses in sea bass (Dicentrarchus labrax) vaccinated with M. marinum iipA::kan mutant with (and without) the use of adjuvant, with (and without) a booster vaccination were monitored. The adjuvanted vaccine induced enhanced immune responses. TNF-α transcription levels were extremely high in spleen of the fish vaccinated with the addition of adjuvant in both fish vaccinated once and twice, followed by an IgM response highly specific for M. marinum. Also, histologically, granulomas started appearing in spleen and head-kidney tissues (but with no visible bacteria) within a month after vaccination, mainly with the adjuvanted vaccine. This was followed by reduction in pathology, as demonstrated by the lower number of granulomas (with visible bacteria), indicating that even heat-killed bacteria were able to elicit granulomatous formations. Adhesion of the internal organs and moderate pigmentation were observed in the perivisceral adipose tissue of nearly all vaccinated fish. Although the adjuvanted heat-killed avirulent iipA::kan mutant clearly induced a strong humoral and adaptive immune response, the booster treatment did not seem to have produced a significantly higher degree of protection from the disease compared to fish that received a single vaccination.

Kinase Targets for Mycolic Acid Biosynthesis in Mycobacterium tuberculosis

BACKGROUND

Mycolic acids (MAs) are the characteristic, integral building blocks for the mycomembrane belonging to the insidious bacterial pathogen Mycobacterium tuberculosis (M.tb). These C60-C90 long α-alkyl-β-hydroxylated fatty acids provide protection to the tubercle bacilli against the outside threats, thus allowing its survival, virulence and resistance to the current antibacterial agents. In the post-genomic era, progress has been made towards understanding the crucial enzymatic machineries involved in the biosynthesis of MAs in M.tb. However, gaps still remain in the exact role of the phosphorylation and dephosphorylation of regulatory mechanisms within these systems. To date, a total of 11 serine-threonine protein kinases (STPKs) are found in M.tb. Most enzymes implicated in the MAs synthesis were found to be phosphorylated in vitro and/or in vivo. For instance, phosphorylation of KasA, KasB, mtFabH, InhA, MabA, and FadD32 downregulated their enzymatic activity, while phosphorylation of VirS increased its enzymatic activity. These observations suggest that the kinases and phosphatases system could play a role in M.tb adaptive responses and survival mechanisms in the human host. As the mycobacterial STPKs do not share a high sequence homology to the human's, there have been some early drug discovery efforts towards developing potent and selective inhibitors.

OBJECTIVE

Recent updates to the kinases and phosphatases involved in the regulation of MAs biosynthesis will be presented in this mini-review, including their known small molecule inhibitors.

CONCLUSION

Mycobacterial kinases and phosphatases involved in the MAs regulation may serve as a useful avenue for antitubercular therapy.

Mycobacterial Evolution Intersects With Host Tolerance

Over the past 200 years, tuberculosis (TB) has caused more deaths than any other infectious disease, likely infecting more people than it has at any other time in human history. Mycobacterium tuberculosis (Mtb), the etiologic agent of TB, is an obligate human pathogen that has evolved through the millennia to become an archetypal human-adapted pathogen. This review focuses on the evolutionary framework by which Mtb emerged as a specialized human pathogen and applies this perspective to the emergence of specific lineages that drive global TB burden. We consider how evolutionary pressures, including transmission dynamics, host tolerance, and human population patterns, may have shaped the evolution of diverse mycobacterial genomes.

MmpL Proteins in Physiology and Pathogenesis of M. tuberculosis

Mycobacterium tuberculosis (Mtb) remains an important human pathogen. The Mtb cell envelope is a critical bacterial structure that contributes to virulence and pathogenicity. Mycobacterial membrane protein large (MmpL) proteins export bulky, hydrophobic substrates that are essential for the unique structure of the cell envelope and directly support the ability of Mtb to infect and persist in the host. This review summarizes recent investigations that have enabled insight into the molecular mechanisms underlying MmpL substrate export and the role that these substrates play during Mtb infection.

A FRET-Based Fluorogenic Trehalose Dimycolate Analogue for Probing Mycomembrane-Remodeling Enzymes of Mycobacteria

The mycobacterial outer membrane, or mycomembrane, is essential for the viability and virulence of Mycobacterium tuberculosis and related pathogens. The mycomembrane is a dynamic structure, whose chemical composition and biophysical properties can change during stress to give an advantage to the bacterium. However, the mechanisms that govern mycomembrane remodeling and their significance to mycobacterial pathogenesis are still not well characterized. Recent studies have shown that trehalose dimycolate (TDM), a major glycolipid of the mycomembrane, is broken down by the mycobacteria-specific enzyme TDM hydrolase (Tdmh) in response to nutrient deprivation, a process which appears to modulate the mycomembrane to increase nutrient acquisition, but at the expense of stress tolerance. Tdmh activity thus balances the growth of M. tuberculosis during infection in a manner that is contingent upon host immunity. Current methods to probe Tdmh activity are limited, impeding the development of inhibitors and the investigation of the role of Tdmh in bacterial growth and persistence. Here, we describe the synthesis and evaluation of FRET-TDM, which is a fluorescence-quenched analogue of TDM that is designed to fluoresce upon hydrolysis by Tdmh and potentially other trehalose ester-degrading hydrolases involved in mycomembrane remodeling. We found that FRET-TDM was efficiently activated in vitro by recombinant Tdmh, generating a 100-fold increase in fluorescence. FRET-TDM was also efficiently activated in the presence of whole cells of Mycobacterium smegmatis and M. tuberculosis, but the observed signal was predominantly Tdmh-independent, suggesting that physiological levels of Tdmh are low and that other mycobacterial enzymes also hydrolyze the probe. The latter notion was confirmed by employing a native protein gel-based fluorescence assay to profile FRET-TDM-activating enzymes from M. smegmatis lysates. On the other hand, FRET-TDM was capable of detecting the activity of Tdmh in cells when it was overexpressed. Together, our data demonstrate that FRET-TDM is a convenient and sensitive in vitro probe of Tdmh activity, which will be beneficial for Tdmh enzymatic characterization and inhibitor screening. In more complex samples, for example, live cells or cell lysates, FRET-TDM can serve as a tool to probe Tdmh activity at elevated enzyme levels, and it may facilitate the identification and characterization of related hydrolases that are involved in mycomembrane remodeling. Our study also provides insights as to how the structure of FRET-TDM or related fluorogenic probes can be optimized to achieve improved specificity and sensitivity for detecting mycobacteria.

The Importance of the Autopsy in Medicine: Perspectives of Pathology Colleagues

This article presents a perspective on the importance of the autopsy in medical practice and science based on experiences of the authors as physician-scientists involved in autopsy practice. Our perspectives are presented on the seminal contributions of the autopsy in the areas of cardiovascular disease, including congenital heart disease, atherosclerosis, coronary artery disease, and myocardial infarction, and infectious disease, including tuberculosis and viral infections. On the positive side of the future of the autopsy, we discuss the tremendous opportunities for important research to be done by application of advanced molecular biological techniques to formalin-fixed, paraffin-embedded tissue blocks obtained at autopsy. We also note with concern the countervailing forces impacting the influence of pathology in education and clinical practice at our academic medical centers, which also present impediments to increasing autopsy rates. Our challenge as academic pathologists, whose careers have been molded by involvement in the autopsy, is to counter these trends. The challenges are great but the benefits for medicine and society are enormous.

Rapid synthesis and immunogenicity of mycobacterial (1→5)-α-d-arabinofuranan

A rapid synthesis of the α(1→5) arabinofuranan polysaccharides, found on the outer surface of Mycobacterium tuberculosis (Mtb), is achieved by a regio- and stereocontrolled ring opening polymerization of β-d-arabinofuranose-1,2,5-orthobenzoate. The robust polymerization reaction allows the incorporation of an amine linker, which was used to conjugate with protein tetanus toxoid (TT) to further investigate its adjuvant activities. The synthetic arabinan, which is the glycan on the non-reducing end of Mtb lipoarabinomannan (LAM), was evaluated for its immunological properties in vitro and in vivo. Systemic inflammation and the promotion of innate immune response were observed in macrophages treated with the synthetic arabinan as an adjuvant through an increase in the production of TNF-α and IL-12. In vivo evaluation of IFN-γ, IL-2, and TNF-α productions in mice pre-immunized with the synthetic arabinan conjugated TT indicated great enhancements of the immunological responses when compared to that of TT alone.

Cyclopropane Modification of Trehalose Dimycolate Drives Granuloma Angiogenesis and Mycobacterial Growth through Vegf Signaling

Mycobacterial infection leads to the formation of characteristic immune aggregates called granulomas, a process accompanied by dramatic remodeling of the host vasculature. As granuloma angiogenesis favors the infecting mycobacteria, it may be actively promoted by bacterial determinants during infection. Using Mycobacterium marinum-infected zebrafish as a model, we identify the enzyme proximal cyclopropane synthase of alpha-mycolates (PcaA) as an important bacterial determinant of granuloma-associated angiogenesis. cis-Cyclopropanation of mycobacterial mycolic acids by pcaA drives the activation of host Vegf signaling within granuloma macrophages. Cyclopropanation of the mycobacterial cell wall glycolipid trehalose dimycolate is both required and sufficient to induce robust host angiogenesis. Inducible genetic inhibition of angiogenesis and Vegf signaling during granuloma formation results in bacterial growth deficits. Together, these data reveal a mechanism by which PcaA-mediated cis-cyclopropanation of mycolic acids promotes bacterial growth and dissemination in vivo by eliciting granuloma vascularization and suggest potential approaches for host-directed therapies.

The Pathogenesis of Tuberculosis: The Early Infiltrate of Post-primary (Adult Pulmonary) Tuberculosis: A Distinct Disease Entity

It has long been recognized that tuberculosis (TB) induces both protective and tissue damaging immune responses. This paper reviews nearly two centuries of evidence that protection and tissue damage are mediated by separate disease entities in humans. Primary TB mediates protective immunity to disseminated infection while post-primary TB causes tissue damage that results in formation of cavities. Both are necessary for continued survival of Mycobacterium tuberculosis (MTB). Primary TB has been extensively studied in humans and animals. Post-primary TB, in contrast, is seldom recognized or studied. It begins as an asymptomatic early infiltrate that may resolve or progress by bronchogenic spread to caseous pneumonia that either fragments to produce cavities or is retained to produce post-primary granulomas and fibrocaseous disease. Primary and post-primary TB differ in typical age of onset, histopathology, organ distribution, x-ray appearance, genetic predisposition, immune status of the host, clinical course and susceptibility to protection by BCG. MTB is a highly successful human parasite because it produces both primary and post-primary TB as distinct disease entities in humans. No animal reproduces this sequence of lesions. Recognition of these facts immediately suggests plausible solutions, animal models and testable hypotheses to otherwise inaccessible questions of the immunity and pathogenesis of TB.

Enhanced Permeability and Retention-like Extravasation of Nanoparticles from the Vasculature into Tuberculosis Granulomas in Zebrafish and Mouse Models

The enhanced permeability and retention (EPR) effect is the only described mechanism enabling nanoparticles (NPs) flowing in blood to reach tumors by a passive targeting mechanism. Here, using the transparent zebrafish model infected with Mycobacterium marinum we show that an EPR-like process also occurs allowing different types of NPs to extravasate from the vasculature to reach granulomas that assemble during tuberculosis (TB) infection. PEGylated liposomes and other NP types cross endothelial barriers near infection sites within minutes after injection and accumulate close to granulomas. Although ∼100 and 190 nm NPs concentrated most in granulomas, even ∼700 nm liposomes reached these infection sites in significant numbers. We show by confocal microscopy that NPs can concentrate in small aggregates in foci on the luminal side of the endothelium adjacent to the granulomas. These spots are connected to larger foci of NPs on the ablumenal side of these blood vessels. EM analysis suggests that NPs cross the endothelium via the paracellular route. PEGylated NPs also accumulated efficiently in granulomas in a mouse model of TB infection with Mycobacterium tuberculosis, arguing that the zebrafish embryo model can be used to predict NP behavior in mammalian hosts. In earlier studies we and others showed that uptake of NPs by macrophages that are attracted to infection foci is one pathway for NPs to reach TB granulomas. This study reveals that when NPs are designed to avoid macrophage uptake, they can also efficiently target granulomas via an alternative mechanism that resembles EPR.

Adjuvant effect in aquaculture fish of cell-wall glycolipids isolated from acid-fast bacteria

Mycobacteriosis and nocardiosis in cultured fish caused by infections with acid-fast bacteria, are responsible for large economic losses globally. In this study, we suggest a novel adjuvant using glycolipids that activates host immune systems. The immune response to glycolipids stimulation was investigated using ginbuna crucian carp. Ginbuna vaccinated with FKC (formalin-killed cells) + glycolipids isolated from Mycobacterium sp., upregulated inflammatory- and Th1-related cytokines, and a DTH (delayed-type hypersensitivity) response was confirmed only in ginbuna vaccinated with FKC + glycolipids. These observations suggest that glycolipids activated host innate and cell-mediated immunity. Subsequently, we evaluated the adjuvant effect of glycolipids against amberjack nocardiosis. In a challenge test, a higher survival rate was observed in amberjack vaccinated with FKC + glycolipids emulsified with conventional oil adjuvant than in fish vaccinated with FKC + oil adjuvant without glycolipids. Therefore, glycolipids potentially could be used as a practical, economical and safe adjuvant for aquaculture fish.

Pathogenesis and Animal Models of Post-Primary (Bronchogenic) Tuberculosis, A Review

Primary and post-primary tuberculosis (TB) are different diseases caused by the same organism. Primary TB produces systemic immunity. Post-primary TB produces cavities to support massive proliferation of organisms for transmission of infection to new hosts from a person with sufficient immunity to prevent systemic infection. Post-primary, also known as bronchogenic, TB begins in humans as asymptomatic bronchial spread of obstructive lobular pneumonia, not as expanding granulomas. Most lesions regress spontaneously. However, some undergo caseation necrosis that is coughed out through the necrotic bronchi to form cavities. Caseous pneumonia that is not expelled through the bronchi is retained to become the focus of fibrocaseous disease. No animal reproduces this entire process. However, it appears that many mammals utilize similar mechanisms, but fail to coordinate them as do humans. Understanding this makes it possible to use human tuberculous lung sections to guide manipulation of animals to produce models of particular human lesions. For example, slowly progressive and reactivation TB in mice resemble developing human bronchogenic TB. Similarly, bronchogenic TB and cavities resembling those in humans can be induced by bronchial infection of sensitized rabbits. Granulomas in guinea pigs have characteristics of both primary and post primary TB. Mice can be induced to produce a spectrum of human like caseating granulomas. There is evidence that primates can develop bronchogenic TB. We are optimistic that such models developed by coordinated study of human and animal tissues can be used with modern technologies to finally address long-standing questions about host/parasite relationships in TB, and support development of targeted therapeutics and vaccines.

Natural history of Mycobacterium fortuitum pulmonary infection presenting with migratory infiltrates: a case report with microbiological analysis

Background

Presence of Mycobacterium fortuitum in respiratory tracts usually indicates mere colonization or transient infection, whereas true pulmonary infection occurs in patients with gastroesophageal disease. However, little is known about the diagnostic indications for true M. fortuitum pulmonary infection and the natural history of the disease.

Case presentation

A 59-year-old man was referred to our hospital for treatment against M. fortuitum pulmonary infection. Fifteen years before the referral, he underwent total gastrectomy, after which he experienced esophageal reflux symptoms. After the referral, the patient was closely monitored without antimicrobial therapy because of mild symptoms and no pathological evidence of M. fortuitum pulmonary infection. During the observation, chest imaging showed migratory infiltrates. Two years after the referral, his lung biopsy specimen revealed foamy macrophages and multinucleated giant cells, indicating lipoid pneumonia. However, he was continually monitored without any treatment because there was no evidence of nontuberculous mycobacterial infection. Four years after the referral, he developed refractory pneumonia despite receiving adequate antibiotic therapy. After confirmation of granulomatous lesions, multiple antimicrobial therapy for M. fortuitum resulted in a remarkable improvement with no exacerbation for over 5 years. Random amplified polymorphic DNA polymerase chain reaction analysis revealed identical M. fortuitum strains in seven isolates from six sputum and one intestinal fluid specimens obtained during the course of the disease.

Conclusions

We have described a patient with M. fortuitum pulmonary infection who presented with migratory infiltrates. The pathological evidence and microbiological analysis suggested that M. fortuitum pulmonary infection was associated with lipoid pneumonia and chronic exposure to gastrointestinal fluid. Therefore, physicians should carefully monitor patients with M. fortuitum detected from lower respiratory tract specimens and consider antimicrobial therapy for M. fortuitum infection when the patient does not respond to adequate antibiotic therapy against common pneumonia pathogens.

Macrophage-Bacteria Interactions-A Lipid-Centric Relationship

Macrophages are professional phagocytes at the front line of immune defenses against foreign bodies and microbial pathogens. Various bacteria, which are responsible for deadly diseases including tuberculosis and salmonellosis, are capable of hijacking this important immune cell type and thrive intracellularly, either in the cytoplasm or in specialized vacuoles. Tight regulation of cellular metabolism is critical in shaping the macrophage polarization states and immune functions. Lipids, besides being the bulk component of biological membranes, serve as energy sources as well as signaling molecules during infection and inflammation. With the advent of systems-scale analyses of genes, transcripts, proteins, and metabolites, in combination with classical biology, it is increasingly evident that macrophages undergo extensive lipid remodeling during activation and infection. Each bacterium species has evolved its own tactics to manipulate host metabolism toward its own advantage. Furthermore, modulation of host lipid metabolism affects disease susceptibility and outcome of infections, highlighting the critical roles of lipids in infectious diseases. Here, we will review the emerging roles of lipids in the complex host-pathogen relationship and discuss recent methodologies employed to probe these versatile metabolites during the infection process. An improved understanding of the lipid-centric nature of infections can lead to the identification of the Achilles' heel of the pathogens and host-directed targets for therapeutic interventions. Currently, lipid-moderating drugs are clinically available for a range of non-communicable diseases, which we anticipate can potentially be tapped into for various infections.

Mycobacterial cord factor enhances migration of neutrophil-like HL-60 cells by prolonging AKT phosphorylation

Trehalose 6,6'-dimycolate (TDM), or cord factor, is a crucial stimulus of immune responses during Mycobacterium tuberculosis infection. Although TDM has immuno-stimulatory properties, including adjuvant activity and the ability to induce granuloma formation, the mechanisms underlying these remain unknown. We hypothesized that TDM stimulates transendothelial migration of neutrophils, which are the first immune cells to infiltrate the tissue upon infection. In this study, it was shown that TDM enhances N-formylmethionyl-leucyl-phenylalanine (fMLP)-induced chemotaxis and transendothelial movement by prolonging AKT phosphorylation in human neutrophils. TDM induced expression of macrophage-inducible C-type lectin, a receptor for TDM, and induced secretion of pro-inflammatory cytokines and chemokines in differentiated HL-60 cells. In 2- and 3-D neutrophil migration assays, TDM-stimulated neutrophils showed increased fMLP-induced chemotaxis and transendothelial migration. Interestingly, following fMLP stimulation of TDM-activated neutrophils, AKT, a crucial kinase for neutrophil polarization and chemotaxis, showed prolonged phosphorylation at serine 473. Taken together, these data suggest that TDM modulates transendothelial migration of neutrophils upon mycobacterial infection through prolonged AKT phosphorylation. AKT may therefore be a promising therapeutic target for enhancing immune responses to mycobacterial infection.

Microbial Offense vs Host Defense: Who Controls the TB Granuloma?

The granuloma is the hallmark of tuberculosis and simultaneously signifies acquisition of an infection and induction of a host immune response. But who benefits more from the development of the granuloma, the host or the pathogen? Is microbe or man dictating disease course and progression? Mycobacterial diseases affect humans and animals alike, and the concepts presented in this review reflect host-pathogen interactions that influence not only mycobacterial granulomas in humans and animals but also other infectious granulomatous diseases that are encountered in veterinary medicine. Current dogma supports that an organized granuloma is a mark of an adequate and “restrictive” host immune response. However, the formation of a granuloma also provides a niche for the maturation, growth, and persistence of numerous infectious agents, and these pathogens devote some portion of their genetic machinery to ensuring these structures’ form. An understanding of pathogens’ contributions to granuloma formation can aid the development of host-directed therapies and other antimicrobial and antiparasitic therapies that can tip this balance in favor of a restrictive host response and elimination—not just containment—of the infectious organism. This review discusses animal models that have aided our understanding of pathogens’ contribution to the host response and how mycobacterial virulence genes direct host pathology in ways that may aid disease transmission and/or persistence in the form of latent infection.

Pathology of Tuberculosis: How the Pathology of Human Tuberculosis Informs and Directs Animal Models

Tuberculosis (TB) is a chronic inflammatory disease caused by the pathogenic bacterium Mycobacterium tuberculosis. A wide variety of host- and pathogen-associated variables influence the clinical manifestation of TB in different individuals within the human population. As a consequence, the characteristic granulomatous lesions that develop within the lung are heterogeneous in size and cellular composition. Due to the lack of appropriate tissues from human TB patients, a variety of animal models are used as surrogates to study the basic pathogenesis and to test experimental vaccines and new drug therapies. Few animal models mimic the clinical course and pathological response of M. tuberculosis seen in the naturally occurring disease in people. In particular, post-primary TB, which accounts for the majority of cases of active TB and is responsible for transmission between individuals via aerosol exposers, cannot be reproduced in animals and therefore cannot be adequately modeled experimentally. This article describes a new paradigm that explains the pathogenesis of post-primary TB in humans. This new evidence was derived from histological examination of tissues from patients with different stages of M. tuberculosis infection and that had not been treated with antimicrobial drugs. Gaining a better understanding of this unique stage of TB disease will lead to more effective treatment, diagnostic, and prevention strategies.

Adjuvants Based on Synthetic Mycobacterial Cord Factor Analogues: Biophysical Properties of Neat Glycolipids and Nanoself-Assemblies with DDA

Synthetic mycobacterial cord factor analogues, e.g., trehalose 6,6'-dibehenate (TDB), are highly promising adjuvants due to their strong immunopotentiating capabilities, but their biophysical properties have remained poorly characterized. Here, we report the synthesis of an array of synthetic TDB analogues varying in acyl chain length, degree of acylation, and headgroup display, which was subjected to biophysical characterization of neat nondispersed self-assembled nanostructures in excess buffer and as aqueous dispersions with cationic dimethyldioctadecylammonium (DDA) bromide. The array comprised trehalose mono- (TMX) and diester (TDX) analogues with symmetrically shortened acyl chains [denoted by X: arachidate (A), stearate (S), palmitate (P), myristate (Myr), and laurate (L)] and an analogue with a short hydrophilic polyethylene glycol (PEG) linker inserted between the trehalose headgroup of TDS and the acyl chains (PEG-TDS). All dispersions were liposomes, but in contrast to the colloidally stable and highly cationic TDX-containing liposomes, the zeta-potential was significantly reduced for DDA/TMX and DDA/PEG-TDS liposomes, suggesting a charge-shielding effect, which compromises the colloidal stability. An increased d-spacing was observed for the lamellar phase of neat TDB analogues in excess buffer (TDS < TMS < PEG-TDS), confirming that the charge shielding is caused by an extended molecular configuration of the more flexible headgroup. Differential scanning calorimetry showed highly cooperative phase transitions for all tested dispersions albeit the monoesters destabilized the lipid bilayers. Langmuir experiments demonstrated that incorporation of TDXs and PEG-TDS stabilized DDA monolayers due to improved hydrogen bonding and reduced intermolecular repulsions. In conclusion, data suggest that the DDA/TDS dispersions exhibit favorable physicochemical properties rendering these DDA/TDS liposomes an attractive vaccine adjuvant, and they emphasize that not only the receptor binding and immune activation but also the biophysical properties of immunopotentiator formulations should be collectively considered when designing adjuvants with optimal safety, efficacy, and storage stability.

Hypervirulent Mycobacterium tuberculosis strain triggers necrotic lung pathology associated with enhanced recruitment of neutrophils in resistant C57BL/6 mice

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (Mtb) that in most cases induces irreversible necrosis of lung tissue as a result of excessive inflammatory reactions. The murine model of TB in resistant C57BL/6 mice infected with reference Mtb strains is widely used in TB studies; however, these mice do not show a necrotic pathology, which restricts their use in studies of irreversible tissue damage. Recently, we demonstrated that necrotic lung lesions could be induced in the C57BL/6 mice by highly virulent Mtb strains belonging to the modern Beijing sublineage. However, the pathogenic mechanisms leading to necrosis in this model were not elucidated. In this study, we investigated the dynamics of lung lesions in mice infected with highly virulent Beijing Mtb strain M299, compared with those infected with laboratory Mtb strain H37Rv. The data demonstrate that necrotic lung lesions in mice infected by the strain M299 were associated with enhanced recruitment of myeloid cells, especially neutrophils, and increased levels of proinflammatory cytokines, consistent with exacerbated inflammation. High levels of IFN-γ production contributed to the control of bacterial growth. Further progression to chronic disease was associated with a reduction in the levels of inflammatory mediators in the lungs, the accumulation of foamy macrophages and partial healing of the necrotic tissue by fibrosis. At a late stage of disease, degradation of foamy cells resulted in the liberation of accumulated lipids and persisting bacilli and further activation of inflammation, which promoted lung consolidation. Overall, our studies show that C57BL/6 mice infected with highly virulent Mtb strain may serve as a TB model reproducing an exacerbated inflammatory response in a resistant host to hypervirulent mycobacteria, leading to irreversible necrotic lung lesions.

A mathematical model predicting host mitochondrial pyruvate transporter activity to be a critical regulator of Mycobacterium tuberculosis pathogenicity

Modulation of host metabolic machinery by Mycobacterium tuberculosis is a well established phenomenon. In our earlier study (Mehrotra et al., 2014), we observed a marked increase in acetyl-CoA levels in cells bearing virulent M. tuberculosis infections compared to host cells harbouring avirulent infections. The difference was observed inspite of similar levels of total host cellular pyruvate in both infection types. The present study aimed in capturing the cause for such a phenomenon that defines the pathogenicity of M. tuberculosis. Through mathematical model, we dissected the relative importance of virulence mediated effect on Pyruvate dehydrogenase (PDH) activity, rate of acetyl-CoA consumption and mitochondrial pyruvate transporter (MPC) activity in causing the observed outcomes. Simulation results exhibit MPC to be the key regulatory junction perturbed by virulent strains of M. tuberculosis leading to alteration of mitochondrial metabolic flux and regulation of acetyl-CoA formation. As an experimental validation, drug mediated inhibition of MPC activity was sufficient to reduce virulent bacillary loads, pointing towards a possible mechanistic target for drug discovery.

Pathogenic mycobacteria achieve cellular persistence by inhibiting the Niemann-Pick Type C disease cellular pathway

BACKGROUND

Tuberculosis remains a major global health concern. The ability to prevent phagosome-lysosome fusion is a key mechanism by which intracellular mycobacteria, including Mycobacterium tuberculosis, achieve long-term persistence within host cells. The mechanisms underpinning this key intracellular pro-survival strategy remain incompletely understood. Host macrophages infected with persistent mycobacteria share phenotypic similarities with cells taken from patients suffering from Niemann-Pick Disease Type C (NPC), a rare lysosomal storage disease in which endocytic trafficking defects and lipid accumulation within the lysosome lead to cell dysfunction and cell death. We investigated whether these shared phenotypes reflected an underlying mechanistic connection between mycobacterial intracellular persistence and the host cell pathway dysfunctional in NPC.

METHODS

The induction of NPC phenotypes in macrophages from wild-type mice or obtained from healthy human donors was assessed via infection with mycobacteria and subsequent measurement of lipid levels and intracellular calcium homeostasis. The effect of NPC therapeutics on intracellular mycobacterial load was also assessed.

RESULTS

Macrophages infected with persistent intracellular mycobacteria phenocopied NPC cells, exhibiting accumulation of multiple lipid types, reduced lysosomal Ca²⁺ levels, and defects in intracellular trafficking. These NPC phenotypes could also be induced using only lipids/glycomycolates from the mycobacterial cell wall. These data suggest that persistent intracellular mycobacteria inhibit the NPC pathway, likely via inhibition of the NPC1 protein, and subsequently induce altered acidic store Ca²⁺ homeostasis. Reduced lysosomal calcium levels may provide a mechanistic explanation for the reduced levels of phagosome-lysosome fusion in mycobacterial infection. Treatments capable of correcting defects in NPC mutant cells via modulation of host cell calcium were of benefit in promoting clearance of mycobacteria from infected host cells.

CONCLUSION 

These findings provide a novel mechanistic explanation for mycobacterial intracellular persistence, and suggest that targeting interactions between the mycobacteria and host cell pathways may provide a novel avenue for development of anti-TB therapies.

Pathogenic mycobacteria achieve cellular persistence by inhibiting the Niemann-Pick Type C disease cellular pathway

BACKGROUND

Tuberculosis remains a major global health concern. The ability to prevent phagosome-lysosome fusion is a key mechanism by which intracellular mycobacteria, including Mycobacterium tuberculosis, achieve long-term persistence within host cells. The mechanisms underpinning this key intracellular pro-survival strategy remain incompletely understood. Host macrophages infected with persistent mycobacteria share phenotypic similarities with cells taken from patients suffering from Niemann-Pick Disease Type C (NPC), a rare lysosomal storage disease in which endocytic trafficking defects and lipid accumulation within the lysosome lead to cell dysfunction and cell death. We investigated whether these shared phenotypes reflected an underlying mechanistic connection between mycobacterial intracellular persistence and the host cell pathway dysfunctional in NPC.

METHODS

The induction of NPC phenotypes in macrophages from wild-type mice or obtained from healthy human donors was assessed via infection with mycobacteria and subsequent measurement of lipid levels and intracellular calcium homeostasis. The effect of NPC therapeutics on intracellular mycobacterial load was also assessed.

RESULTS

Macrophages infected with persistent intracellular mycobacteria phenocopied NPC cells, exhibiting accumulation of multiple lipid types, reduced lysosomal Ca2+ levels, and defects in intracellular trafficking. These NPC phenotypes could also be induced using only lipids/glycomycolates from the mycobacterial cell wall. These data suggest that persistent intracellular mycobacteria inhibit the NPC pathway, likely via inhibition of the NPC1 protein, and subsequently induce altered acidic store Ca2+ homeostasis. Reduced lysosomal calcium levels may provide a mechanistic explanation for the reduced levels of phagosome-lysosome fusion in mycobacterial infection. Treatments capable of correcting defects in NPC mutant cells via modulation of host cell calcium were of benefit in promoting clearance of mycobacteria from infected host cells.

CONCLUSION

These findings provide a novel mechanistic explanation for mycobacterial intracellular persistence, and suggest that targeting interactions between the mycobacteria and host cell pathways may provide a novel avenue for development of anti-TB therapies.

Association of pellicle growth morphological characteristics and clinical presentation of Mycobacterium tuberculosis isolates

Trehalose 6,6'dimycolate (TDM) is a glycolipid found in nearly pure form on the surface of virulent Mycobacterium tuberculosis (MTB). This manuscript investigated the production of TDM, growth rate and colony morphology of multiple strains of MTB, each of which had been isolated from both pulmonary (sputum) and extrapulmonary sites of multiple patients. Since sputum contains MTB primarily from cavities and extrapulmonary biopsies are typically granulomas, this provided an opportunity to compare the behavior of single strains of MTB that had been isolated from cavities and granulomas. The results demonstrated that MTB isolated from pulmonary sites produced more TDM (3.23 ± 1.75 μg TDM/mg MTB), grew more rapidly as thin spreading pellicles, demonstrated early cording, and climbed culture well walls. In contrast, extrapulmonary isolates produced less TDM (1.42 ± 0.58 μg TDM/mg MTB) (p < 0.001) and grew as discrete patches with little tendency to spread or climb. Both Beijing pulmonary isolates and the non-Beijing pulmonary isolates produced significantly more TDM (1.64 ± 0.46 μg TDM/mg MTB) and grew faster than the Beijing and non-Beijing extrapulmonary isolates (1.14 ± 0.63 μg TDM/mg MTB) (p < 0.001 and p < 0.005 respectively). These results indicate that MTB from pulmonary sites (cavities) grows faster and produces more TDM than strains isolated from extrapulmonary sites (granulomas). This report suggests a critical role for TDM in cavitary TB.