Foamy macrophages from tuberculous patients' granulomas constitute a nutrient-rich reservoir for M. tuberculosis persistence

Identification of a 1-acyl-glycerol-3-phosphate acyltransferase from Mycobacterium tuberculosis, a key enzyme involved in triacylglycerol biosynthesis

Latent tuberculosis, caused by dormant Mycobacterium tuberculosis (Mtb), poses a threat to global health through the incubation of undiagnosed infections within the community. Dormant Mtb, which is phenotypically tolerant to antibiotics, accumulates triacylglycerol (TAG) utilizing fatty acids obtained from macrophage lipid droplets. TAG is vital to mycobacteria, serving as a cell envelope component and energy reservoir during latency. TAG synthesis occurs by sequential acylation of glycerol-3-phosphate, wherein the second acylation step is catalyzed by acylglycerol-3-phosphate acyltransferase (AGPAT), resulting in the production of phosphatidic acid (PA), a precursor for the synthesis of TAG and various phospholipids. Here, we have characterized a putative acyltransferase of Mtb encoded by Rv3816c. We found that Rv3816c has all four characteristic motifs of AGPAT, exists as a membrane-bound enzyme, and functions as 1-acylglycerol-3-phosphate acyltransferase. The enzyme could transfer the acyl group to acylglycerol-3-phosphate (LPA) from monounsaturated fatty acyl-coenzyme A of chain length 16 or 18 to produce PA. Complementation of Escherichia coli PlsC mutant in vivo by Rv3816c confirmed that it functions as AGPAT. Its active site mutants, H43A and D48A, were incapable of transferring the acyl group to LPA in vitro and were not able to rescue the growth defect of E. coli PlsC mutant in vivo. Identifying Rv3816c as AGPAT and comparing its properties with other AGPAT homologs is not only a step toward understanding the TAG biosynthesis in mycobacteria but has the potential to explore it as a drug target.

The role of cholesterol and its oxidation products in tuberculosis pathogenesis

Mycobacterium tuberculosis causes tuberculosis (TB), one of the world's most deadly infections. Lipids play an important role in M. tuberculosis pathogenesis. M. tuberculosis grows intracellularly within lipid-laden macrophages and extracellularly within the cholesterol-rich caseum of necrotic granulomas and pulmonary cavities. Evolved from soil saprophytes that are able to metabolize cholesterol from organic matter in the environment, M. tuberculosis inherited an extensive and highly conserved machinery to metabolize cholesterol. M. tuberculosis uses this machinery to degrade host cholesterol; the products of cholesterol degradation are incorporated into central carbon metabolism and used to generate cell envelope lipids, which play important roles in virulence. The host also modifies cholesterol by enzymatically oxidizing it to a variety of derivatives, collectively called oxysterols, which modulate cholesterol homeostasis and the immune response. Recently, we found that M. tuberculosis converts host cholesterol to an oxidized metabolite, cholestenone, that accumulates in the lungs of individuals with TB. M. tuberculosis encodes cholesterol-modifying enzymes, including a hydroxysteroid dehydrogenase, a putative cholesterol oxidase, and numerous cytochrome P450 monooxygenases. Here, we review what is known about cholesterol and its oxidation products in the pathogenesis of TB. We consider the possibility that the biological function of cholesterol metabolism by M. tuberculosis extends beyond a nutritional role.

TOLLIP inhibits lipid accumulation and the integrated stress response in alveolar macrophages to control Mycobacterium tuberculosis infection

A polymorphism causing deficiencies in Toll-interacting protein (TOLLIP), an inhibitory adaptor protein affecting endosomal trafficking, is associated with increased tuberculosis (TB) risk. It is, however, unclear how TOLLIP affects TB pathogenesis. Here we show that TB severity is increased in Tollip-/- mice, characterized by macrophage- and T cell-driven inflammation, foam cell formation and lipid accumulation. Tollip-/- alveolar macrophages (AM) specifically accumulated lipid and underwent necrosis. Transcriptional and protein analyses of Mycobacterium tuberculosis (Mtb)-infected, Tollip-/- AM revealed increased EIF2 signalling and downstream upregulation of the integrated stress response (ISR). These phenotypes were linked, as incubation of the Mtb lipid mycolic acid with Mtb-infected Tollip-/- AM activated the ISR and increased Mtb replication. Correspondingly, the ISR inhibitor, ISRIB, reduced Mtb numbers in AM and improved Mtb control, overcoming the inflammatory phenotype. In conclusion, targeting the ISR offers a promising target for host-directed anti-TB therapy towards improved Mtb control and reduced immunopathology.

Fluvastatin Converts Human Macrophages into Foam Cells with Increased Inflammatory Response to Inactivated Mycobacterium tuberculosis H37Ra

Cholesterol biosynthesis inhibitors (statins) protect hypercholesterolemic patients against developing active tuberculosis, suggesting that these drugs could help the host to control the pathogen at the initial stages of the disease. This work studies the effect of fluvastatin on the early response of healthy peripheral blood mononuclear cells (PBMCs) to inactivated Mycobacterium tuberculosis (Mtb) H37Ra. We found that in fluvastatin-treated PBMCs, most monocytes/macrophages became foamy cells that overproduced NLRP3 inflammasome components in the absence of immune stimulation, evidencing important cholesterol metabolism/immunity connections. When both fluvastatin-treated and untreated PBMCs were exposed to Mtb H37Ra, a small subset of macrophages captured large amounts of bacilli and died, concentrating the bacteria in necrotic areas. In fluvastatin-untreated cultures, most of the remaining macrophages became epithelioid cells that isolated these areas of cell death in granulomatous structures that barely produced IFNγ. By contrast, in fluvastatin-treated cultures, foamy macrophages surrounded the accumulated bacteria, degraded them, markedly activated caspase-1 and elicited a potent IFNγ/cytotoxic response. In rabbits immunized with the same bacteria, fluvastatin increased the tuberculin test response. We conclude that statins may enhance macrophage efficacy to control Mtb, with the help of adaptive immunity, offering a promising tool in the design of alternative therapies to fight tuberculosis.

Pathogenic mycobacterium upregulates cholesterol 25-hydroxylase to promote granuloma development via foam cell formation

Pathogenic mycobacteria orchestrate the complex cell populations known as granuloma that is the hallmark of tuberculosis. Foam cells, a lipid-rich cell-type, are considered critical for granuloma formation; however, the causative factor in foam cell formation remains unclear. Atherosclerosis is a chronic inflammatory disease characterized by the abundant accumulation of lipid-laden-macrophage-derived foam cells during which cholesterol 25-hydroxylase (CH25H) is crucial in foam cell formation. Here, we show that M. marinum (Mm), a relative of M. tuberculosis, induces foam cell formation, leading to granuloma development following CH25H upregulation. Moreover, the Mm-driven increase in CH25H expression is associated with the presence of phthiocerol dimycocerosate, a determinant for Mm virulence and integrity. CH25H-null mice showed decreased foam cell formation and attenuated pathology. Atorvastatin, a recommended first-line lipid-lowering drug, promoted the elimination of M. marinum and concomitantly reduced CH25H production. These results define a previously unknown role for CH25H in controlling macrophage-derived foam cell formation and Tuberculosis pathology.

Epigenetic programming of host lipid metabolism associates with resistance to TST/IGRA conversion after exposure to Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) exposure leads to a range of outcomes including clearance, latent TB infection (LTBI), and pulmonary tuberculosis (TB). Some heavily exposed individuals resist tuberculin skin test (TST) and interferon gamma release assay (IGRA) conversion (RSTR), which suggests that they employ IFNγ-independent mechanisms of Mtb control. Here, we compare monocyte epigenetic profiles of RSTR and LTBI from a Ugandan household contact cohort. Chromatin accessibility did not differ between uninfected RSTR and LTBI monocytes. In contrast, methylation significantly differed at 174 CpG sites and across 63 genomic regions. Consistent with previous transcriptional findings in this cohort, differential methylation was enriched in lipid and cholesterol associated pathways including in the genes APOC3, KCNQ1, and PLA2G3. In addition, methylation was enriched in Hippo signaling, which is associated with cholesterol homeostasis and includes CIT and SHANK2. Lipid export and Hippo signaling pathways were also associated with gene expression in response to Mtb in RSTR as well as IFN stimulation in monocyte-derived macrophages (MDMs) from an independent healthy donor cohort. Moreover, serum-derived HDL from RSTR had elevated ABCA1-mediated cholesterol efflux capacity (CEC) compared to LTBI. Our findings suggest that resistance to TST/IGRA conversion is linked to regulation of lipid accumulation in monocytes, which could facilitate early Mtb clearance among RSTR subjects through IFNγ-independent mechanisms.

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.

Legionella pneumophila usurps host cell lipids for vacuole expansion and bacterial growth

Vacuolar pathogens reside in membrane-bound compartments within host cells. Maintaining the integrity of this compartment is paramount to bacterial survival and replication as it protects against certain host surveillance mechanisms that function to eradicate invading pathogens. Preserving this compartment during bacterial replication requires expansion of the vacuole membrane to accommodate the increasing number of bacteria, and yet, how this is accomplished remains largely unknown. Here, we show that the vacuolar pathogen Legionella pneumophila exploits multiple sources of host cell fatty acids, including inducing host cell fatty acid scavenging pathways, in order to promote expansion of the replication vacuole and bacteria growth. Conversely, when exogenous lipids are limited, the decrease in host lipid availability restricts expansion of the replication vacuole membrane, resulting in a higher density of bacteria within the vacuole. Modifying the architecture of the vacuole prioritizes bacterial growth by allowing the greatest number of bacteria to remain protected by the vacuole membrane despite limited resources for its expansion. However, this trade-off is not without risk, as it can lead to vacuole destabilization, which is detrimental to the pathogen. However, when host lipid resources become extremely scarce, for example by inhibiting host lipid scavenging, de novo biosynthetic pathways, and/or diverting host fatty acids to storage compartments, bacterial replication becomes severely impaired, indicating that host cell fatty acid availability also directly regulates L. pneumophila growth. Collectively, these data demonstrate dual roles for host cell fatty acids in replication vacuole expansion and bacterial proliferation, revealing the central functions for these molecules and their metabolic pathways in L. pneumophila pathogenesis.

Roles of Lipolytic enzymes in Mycobacterium tuberculosis pathogenesis

Mycobacterium tuberculosis (Mtb) is a bacterial pathogen that can endure for long periods in an infected patient, without causing disease. There are a number of virulence factors that increase its ability to invade the host. One of these factors is lipolytic enzymes, which play an important role in the pathogenic mechanism of Mtb. Bacterial lipolytic enzymes hydrolyze lipids in host cells, thereby releasing free fatty acids that are used as energy sources and building blocks for the synthesis of cell envelopes, in addition to regulating host immune responses. This review summarizes the relevant recent studies that used in vitro and in vivo models of infection, with particular emphasis on the virulence profile of lipolytic enzymes in Mtb. A better understanding of these enzymes will aid the development of new treatment strategies for TB. The recent work done that explored mycobacterial lipolytic enzymes and their involvement in virulence and pathogenicity was highlighted in this study. Lipolytic enzymes are expected to control Mtb and other intracellular pathogenic bacteria by targeting lipid metabolism. They are also potential candidates for the development of novel therapeutic agents.

Dual-specificity phosphatase 5-mediated fatty acid oxidation promotes Mycobacterium bovis BCG -induced inflammatory responses

Mycobacterium tuberculosis (Mtb) reprograms FAs metabolism of macrophages during infection and affects inflammatory reaction eventually, however, the mechanism remains poorly understood. Here we show that Mycobacterium bovis (BCG) induces DUSP5 expression through TLR2-MAPKs signaling pathway and promotes fatty acid oxidation (FAO). Silencing DUSP5 by adeno-associated virus vector (AAV) ameliorates lung injury and DUSP5 knockdown reduces the expression of IL-1β, IL-6 and inactivated NF-κB signaling in BCG-infected macrophages. Of note, DUSP5 specific siRNA increases the content of free fatty acids (FFAs) and triglyceride (TG), but represses the expression of FAO associated enzymes such as CPT1A and PPARα, suggesting DUSP5 mediated FAO during BCG infection. Moreover, Inhibiting FAO by pharmacological manner suppresses IL-1β, IL-6, TNF-α expression and relieves lung damage. Taken together, our data indicates DUSP5 mediates FAO reprogramming and promotes inflammatory response to BCG infection.

The Dual Roles of Activating Transcription Factor 3 (ATF3) in Inflammation, Apoptosis, Ferroptosis, and Pathogen Infection Responses

Transcription factors are pivotal regulators in the cellular life process. Activating transcription factor 3 (ATF3), a member of the ATF/CREB (cAMP response element-binding protein) family, plays a crucial role as cells respond to various stresses and damage. As a transcription factor, ATF3 significantly influences signal transduction regulation, orchestrating a variety of signaling pathways, including apoptosis, ferroptosis, and cellular differentiation. In addition, ATF3 serves as an essential link between inflammation, oxidative stress, and immune responses. This review summarizes the recent advances in research on ATF3 activation and its role in regulating inflammatory responses, cell apoptosis, and ferroptosis while exploring the dual functions of ATF3 in these processes. Additionally, this article discusses the role of ATF3 in diseases related to pathogenic microbial infections. Our review may be helpful to better understand the role of ATF3 in cellular responses and disease progression, thus promoting advancements in clinical treatments for inflammation and oxidative stress-related diseases.

Polyphosphate kinase-1 regulates bacterial and host metabolic pathways involved in pathogenesis of Mycobacterium tuberculosis

Inorganic polyphosphate (polyP) is primarily synthesized by Polyphosphate Kinase-1 (PPK-1) and regulates numerous cellular processes, including energy metabolism, stress adaptation, drug tolerance, and microbial pathogenesis. Here, we report that polyP interacts with acyl CoA carboxylases, enzymes involved in lipid biosynthesis in Mycobacterium tuberculosis. We show that deletion of ppk-1 in M. tuberculosis results in transcriptional and metabolic reprogramming. In comparison to the parental strain, the Δppk-1 mutant strain had reduced levels of virulence-associated lipids such as PDIMs and TDM. We also observed that polyP deficiency in M. tuberculosis is associated with enhanced phagosome-lysosome fusion in infected macrophages and attenuated growth in mice. Host RNA-seq analysis revealed decreased levels of transcripts encoding for proteins involved in either type I interferon signaling or formation of foamy macrophages in the lungs of Δppk-1 mutant-infected mice relative to parental strain-infected animals. Using target-based screening and molecular docking, we have identified raloxifene hydrochloride as a broad-spectrum PPK-1 inhibitor. We show that raloxifene hydrochloride significantly enhanced the activity of isoniazid, bedaquiline, and pretomanid against M. tuberculosis in macrophages. Additionally, raloxifene inhibited the growth of M. tuberculosis in mice. This is an in-depth study that provides mechanistic insights into the regulation of mycobacterial pathogenesis by polyP deficiency.

Mycobacterium tuberculosis response to cholesterol is integrated with environmental pH and potassium levels via a lipid metabolism regulator

Successful colonization of the host requires Mycobacterium tuberculosis (Mtb) to sense and respond coordinately to disparate environmental cues during infection and adapt its physiology. However, how Mtb response to environmental cues and the availability of key carbon sources may be integrated is poorly understood. Here, by exploiting a reporter-based genetic screen, we have unexpectedly found that overexpression of transcription factors involved in Mtb lipid metabolism altered the dampening effect of low environmental potassium concentrations ([K+]) on the pH response of Mtb. Cholesterol is a major carbon source for Mtb during infection, and transcriptional analyses revealed that Mtb response to acidic pH was augmented in the presence of cholesterol and vice versa. Strikingly, deletion of the putative lipid regulator mce3R had little effect on Mtb transcriptional response to acidic pH or cholesterol individually, but resulted specifically in loss of cholesterol response augmentation in the simultaneous presence of acidic pH. Similarly, while mce3R deletion had little effect on Mtb response to low environmental [K+] alone, augmentation of the low [K+] response by the simultaneous presence of cholesterol was lost in the mutant. Finally, a mce3R deletion mutant was attenuated for growth in foamy macrophages and for colonization in a murine infection model that recapitulates caseous necrotic lesions and the presence of foamy macrophages. These findings reveal the critical coordination between Mtb response to environmental cues and cholesterol, a vital carbon source, and establishes Mce3R as a transcription factor that crucially serves to integrate these signals.

Deciphering drug discovery and microbial pathogenesis research in tuberculosis during the two decades of postgenomic era using entity mining approach

We examined literature on Mycobacterium tuberculosis (Mtb) subsequent to its genome release, spanning years 1999-2020. We employed scientometric mapping, entity mining, visualization techniques, and PubMed and PubTator databases. Most popular keywords, most active research groups, and growth in quantity of publications were determined. By gathering annotations from the PubTator, we determined direction of research in the areas of drug hypersensitivity, drug resistance (AMR), and drug-related side effects. Additionally, we examined the patterns in research on Mtb metabolism and various forms of tuberculosis, including skin, brain, pulmonary, extrapulmonary, and latent tuberculosis. We discovered that 2011 had the highest annual growth rate of publications, at 19.94%. The USA leads the world in publications with 18,038, followed by China with 14,441, and India with 12,158 publications. Studies on isoniazid and rifampicin resistance showed an enormous increase. Non-tuberculous mycobacteria also been the subject of more research in effort to better understand Mtb physiology and as model organisms. Researchers also looked at co-infections like leprosy, hepatitis, plasmodium, HIV, and other opportunistic infections. Host perspectives like immune response, hypoxia, and reactive oxygen species, as well as comorbidities like arthritis, cancer, diabetes, and kidney disease etc. were also looked at. Symptomatic aspects like fever, coughing, and weight loss were also investigated. Vitamin D has gained popularity as a supplement during illness recovery, however, the interest of researchers declined off late. We delineated dominant researchers, journals, institutions, and leading nations globally, which is crucial for aligning ongoing and evolving landscape of TB research efforts. Recognising the dominant patterns offers important information about the areas of focus for current research, allowing biomedical scientists, clinicians, and organizations to strategically coordinate their efforts with the changing priorities in the field of tuberculosis research.

The novel bacterial effector protein CbEPF1 mediates ER-LD membrane contacts to regulate host lipid droplet metabolism

Effective intracellular communication between cellular organelles is pivotal for maintaining cellular homeostasis. Tether proteins, which are responsible for establishing membrane contact sites between cell organelles, enable direct communication between organelles and ultimately influence organelle function and host cell homeostasis. While recent research has identified tether proteins in several bacterial pathogens, their functions have predominantly been associated with mediating inter-organelle communication specifically between the bacteria containing vacuole (BCV) and the host endoplasmic reticulum (ER). However, this study reveals a novel bacterial effector protein, CbEPF1, which acts as a molecular tether beyond the confines of the BCV and facilitates interactions between host cell organelles. Coxiella burnetii, an obligate intracellular bacterial pathogen, encodes the FFAT motif-containing protein CbEPF1 which localizes to host lipid droplets (LDs). CbEPF1 establishes inter-organelle contact sites between host LDs and the ER through its interactions with VAP family proteins. Intriguingly, CbEPF1 modulates growth of host LDs in a FFAT motif-dependent manner. These findings highlight the potential for bacterial effector proteins to impact host cellular homeostasis by manipulating inter-organelle communication beyond conventional BCVs.

Organelles are miscommunicating: Membrane contact sites getting hijacked by pathogens

Membrane Contact Sites (MCS) are areas of close apposition of organelles that serve as hotspots for crosstalk and direct transport of lipids, proteins and metabolites. Contact sites play an important role in Ca2+ signalling, phospholipid synthesis, and micro autophagy. Initially, altered regulation of vesicular trafficking was regarded as the key mechanism for intracellular pathogen survival. However, emerging studies indicate that pathogens hijack MCS elements - a novel strategy for survival and replication in an intracellular environment. Several pathogens exploit MCS to establish direct contact between organelles and replication inclusion bodies, which are essential for their survival within the cell. By establishing this direct control, pathogens gain access to cytosolic compounds necessary for replication, maintenance, escaping endocytic maturation and circumventing lysosome fusion. MCS components such as VAP A/B, OSBP, and STIM1 are targeted by pathogens through their effectors and secretion systems. In this review, we delve into the mechanisms which operate in the evasion of the host immune system when intracellular pathogens hostage MCS. We explore targeting MCS components as a novel therapeutic approach, modifying molecular pathways and signalling to address the disease's mechanisms and offer more effective, tailored treatments for affected individuals.

From immunology to artificial intelligence: revolutionizing latent tuberculosis infection diagnosis with machine learning

Latent tuberculosis infection (LTBI) has become a major source of active tuberculosis (ATB). Although the tuberculin skin test and interferon-gamma release assay can be used to diagnose LTBI, these methods can only differentiate infected individuals from healthy ones but cannot discriminate between LTBI and ATB. Thus, the diagnosis of LTBI faces many challenges, such as the lack of effective biomarkers from Mycobacterium tuberculosis (MTB) for distinguishing LTBI, the low diagnostic efficacy of biomarkers derived from the human host, and the absence of a gold standard to differentiate between LTBI and ATB. Sputum culture, as the gold standard for diagnosing tuberculosis, is time-consuming and cannot distinguish between ATB and LTBI. In this article, we review the pathogenesis of MTB and the immune mechanisms of the host in LTBI, including the innate and adaptive immune responses, multiple immune evasion mechanisms of MTB, and epigenetic regulation. Based on this knowledge, we summarize the current status and challenges in diagnosing LTBI and present the application of machine learning (ML) in LTBI diagnosis, as well as the advantages and limitations of ML in this context. Finally, we discuss the future development directions of ML applied to LTBI diagnosis.

Mycobacterium tuberculosis suppresses host DNA repair to boost its intracellular survival

Mycobacterium tuberculosis (Mtb) triggers distinct changes in macrophages, resulting in the formation of lipid droplets that serve as a nutrient source. We discover that Mtb promotes lipid droplets by inhibiting DNA repair responses, resulting in the activation of the type-I IFN pathway and scavenger receptor-A1 (SR-A1)-mediated lipid droplet formation. Bacterial urease C (UreC, Rv1850) inhibits host DNA repair by interacting with RuvB-like protein 2 (RUVBL2) and impeding the formation of the RUVBL1-RUVBL2-RAD51 DNA repair complex. The suppression of this repair pathway increases the abundance of micronuclei that trigger the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway and subsequent interferon-β (IFN-β) production. UreC-mediated activation of the IFN-β pathway upregulates the expression of SR-A1 to form lipid droplets that facilitate Mtb replication. UreC inhibition via a urease inhibitor impaired Mtb growth within macrophages and in vivo. Thus, our findings identify mechanisms by which Mtb triggers a cascade of cellular events that establish a nutrient-rich replicative niche.

Tuberculosis diagnostic and treatment delays among patients in Uganda

Background

Mycobacterium tuberculosis, a bacterium that relies on its human host to achieve airborne transmission and existence, is the primary cause of tuberculosis (TB), a disease that is vital to public health.

Aim

To update the society on tuberculosis diagnostic and treatment delays among patients in Uganda.

Materials and Methods

The review paper utilized different search engines, such as Pubmed Central, Scopus, Web of Science, Google Scholar, and so forth, to conduct this review paper.

Results

Delays in diagnosis could cause diseases to spread throughout the community, progress more quickly, and increase mortality. With many populations experiencing TB diagnostic delay and less than a third of the population experiencing TB treatment delay, the rates of tuberculosis diagnosis and treatment delays are high.

Conclusion

The delay in diagnosing and treating tuberculosis in men is positively correlated with knowledge of the disease's symptoms and the regular use of a handkerchief or both hands to cover the mouth and nose while coughing or sneezing.

Use of single-cell technology to improve our understanding of the role of TLR2 in macrophage-Mycobacterium tuberculosis interaction

The interaction between Mycobacterium tuberculosis, the agent of tuberculosis (TB), and its host cell, the macrophage, is multifaceted, dynamic, and involves multiple molecular partners. A better understanding of this interaction could help researchers manipulate the immune system in order to design host-targeted immunotherapies and/or develop a novel vaccine protecting better adults against TB. Jani and coworkers studied the role of the macrophage receptor TLR2 in the response to M. tuberculosis using single-cell technologies (C. Jani, S. L. Solomon, J. M. Peters, and S. C. Pringle, et al., mSystems, https://doi.org/10.1128/msystems.00052-23, 2023). This work addresses the multiple challenges associated with such studies and shows how informative single-cell analysis can be for the study of heterogeneous and complex host-pathogen interactions.

Substrate DNA Promoting Binding of Mycobacterium tuberculosis MtrA by Facilitating Dimerization and Interpretation of Affinity by Minor Groove Width

In order to deepen the understanding of the role and regulation mechanisms of prokaryotic global transcription regulators in complex processes, including virulence, the associations between the affinity and binding sequences of Mycobacterium tuberculosis MtrA have been explored extensively. Analysis of MtrA 294 diversified 26 bp binding sequences revealed that the sequence similarity of fragments was not simply associated with affinity. The unique variation patterns of GC content and periodical and sequential fluctuation of affinity contribution curves were observed along the sequence in this study. Furthermore, docking analysis demonstrated that the structure of the dimer MtrA-DNA (high affinity) was generally consistent with other OmpR family members, while Arg 219 and Gly 220 of the wing domain interacted with the minor groove. The results of the binding box replacement experiment proved that box 2 was essential for binding, which implied the differential roles of the two boxes in the binding process. Furthermore, the results of the substitution of the nucleotide at the 20th and/or 21st positions indicated that the affinity was negatively associated with the value of minor groove width precisely at the 21st position. The dimerization of the unphosphorylated MtrA facilitated by a low-affinity DNA fragment was observed for the first time. However, the proportion of the dimer was associated with the affinity of substrate DNA, which further suggested that the affinity was actually one characteristic of the stability of dimers. Based on the finding of 17 inter-molecule hydrogen bonds identified in the interface of the MtrA dimer, including 8 symmetric complementary ones in the conserved α4-β5-α5 face, we propose that hydrogen bonds should be considered just as important as salt bridges and the hydrophobic patch in the dimerization. Our comprehensive study on a large number of binding fragments with quantitative affinity values provided new insight into the molecular mechanism of dimerization, binding specificity and affinity determination of MtrA and clues for solving the puzzle of how global transcription factors regulate a large quantity of target genes.

Mycobacterium tuberculosis: Pathogenesis and therapeutic targets

Tuberculosis (TB) remains a significant public health concern in the 21st century, especially due to drug resistance, coinfection with diseases like immunodeficiency syndrome (AIDS) and coronavirus disease 2019, and the lengthy and costly treatment protocols. In this review, we summarize the pathogenesis of TB infection, therapeutic targets, and corresponding modulators, including first-line medications, current clinical trial drugs and molecules in preclinical assessment. Understanding the mechanisms of Mycobacterium tuberculosis (Mtb) infection and important biological targets can lead to innovative treatments. While most antitubercular agents target pathogen-related processes, host-directed therapy (HDT) modalities addressing immune defense, survival mechanisms, and immunopathology also hold promise. Mtb's adaptation to the human host involves manipulating host cellular mechanisms, and HDT aims to disrupt this manipulation to enhance treatment effectiveness. Our review provides valuable insights for future anti-TB drug development efforts.

Exposure of Mycobacterium tuberculosis to human alveolar lining fluid shows temporal and strain-specific adaptation to the lung environment

Upon infection, Mycobacterium tuberculosis ( M.tb ) reaches the alveolar space and comes in close contact with human alveolar lining fluid (ALF) for an uncertain period of time prior to its encounter with alveolar cells. We showed that homeostatic ALF hydrolytic enzymes modify the M.tb cell envelope, driving M.tb -host cell interactions. Still, the contribution of ALF during M.tb infection is poorly understood. Here, we exposed 4 M.tb strains with different levels of virulence, transmissibility, and drug resistance (DR) to physiological concentrations of human ALF for 15-min and 12-h, and performed RNA sequencing. Gene expression analysis showed a temporal and strain-specific adaptation to human ALF. Differential expression (DE) of ALF-exposed vs. unexposed M.tb revealed a total of 397 DE genes associated with lipid metabolism, cell envelope and processes, intermediary metabolism and respiration, and regulatory proteins, among others. Most DE genes were detected at 12-h post-ALF exposure, with DR- M.tb strain W-7642 having the highest number of DE genes. Interestingly, genes from the KstR2 regulon, which controls the degradation of cholesterol C and D rings, were significantly upregulated in all strains post-ALF exposure. These results indicate that M.tb -ALF contact drives initial metabolic and physiologic changes in M.tb , with potential implications in infection outcome.

IMPORTANCE

Tuberculosis, caused by airborne pathogen Mycobacterium tuberculosis ( M.tb ), is one of the leading causes of mortality worldwide. Upon infection, M.tb reaches the alveoli and gets in contact with human alveolar lining fluid (ALF), where ALF hydrolases modify the M.tb cell envelope driving subsequent M.tb -host cell interactions. Still, the contributions of ALF during infection are poorly understood. We exposed 4 M.tb strains to ALF for 15-min and 12-h and performed RNA sequencing, demonstrating a temporal and strain-specific adaptation of M.tb to ALF. Interestingly, genes associated with cholesterol degradation were highly upregulated in all strains. This study shows for the first time that ALF drives global metabolic changes in M.tb during the initial stages of the infection, with potential implications in disease outcome. Biologically relevant networks and common and strain-specific bacterial determinants derived from this study could be further investigated as potential therapeutic candidates.

Revolutionizing control strategies against Mycobacterium tuberculosis infection through selected targeting of lipid metabolism

Lipid species play a critical role in the growth and virulence expression of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB). During Mtb infection, foamy macrophages accumulate lipids in granulomas, providing metabolic adaptation and survival strategies for Mtb against multiple stresses. Host-derived lipid species, including triacylglycerol and cholesterol, can also contribute to the development of drug-tolerant Mtb, leading to reduced efficacy of antibiotics targeting the bacterial cell wall or transcription. Transcriptional and metabolic analyses indicate that lipid metabolism-associated factors of Mtb are highly regulated by antibiotics and ultimately affect treatment outcomes. Despite the well-known association between major antibiotics and lipid metabolites in TB treatment, a comprehensive understanding of how altered lipid metabolites in both host and Mtb influence treatment outcomes in a drug-specific manner is necessary to overcome drug tolerance. The current review explores the controversies and correlations between lipids and drug efficacy in various Mtb infection models and proposes novel approaches to enhance the efficacy of anti-TB drugs. Moreover, the review provides insights into the efficacious control of Mtb infection by elucidating the impact of lipids on drug efficacy. This review aims to improve the effectiveness of current anti-TB drugs and facilitate the development of innovative therapeutic strategies against Mtb infection by making reverse use of Mtb-favoring lipid species.

Rifampicin drug resistance and host immunity in tuberculosis: more than meets the eye

Tuberculosis (TB) is the leading cause of death due to an infectious agent, with more than 1.5 million deaths attributed to TB annually worldwide. The global dissemination of drug resistance across Mycobacterium tuberculosis (Mtb) strains, causative of TB, resulted in an estimated 450 000 cases of drug-resistant (DR) TB in 2021. Dysregulated immune responses have been observed in patients with multidrug resistant (MDR) TB, but the effects of drug resistance acquisition and impact on host immunity remain obscure. In this review, we compile studies that span aspects of altered host-pathogen interactions and highlight research that explores how drug resistance and immunity might intersect. Understanding the immune processes differentially induced during DR TB would aid the development of rational therapeutics and vaccines for patients with MDR TB.

Mycobacterium tuberculosis response to cholesterol is integrated with environmental pH and potassium levels via a lipid utilization regulator

How bacterial response to environmental cues and nutritional sources may be integrated in enabling host colonization is poorly understood. Exploiting a reporter-based screen, we discovered that overexpression of Mycobacterium tuberculosis (Mtb) lipid utilization regulators altered Mtb acidic pH response dampening by low environmental potassium (K+). Transcriptional analyses unveiled amplification of Mtb response to acidic pH in the presence of cholesterol, a major carbon source for Mtb during infection, and vice versa. Strikingly, deletion of the putative lipid regulator mce3R resulted in loss of augmentation of (i) cholesterol response at acidic pH, and (ii) low [K+] response by cholesterol, with minimal effect on Mtb response to each signal individually. Finally, the ∆mce3R mutant was attenuated for colonization in a murine model that recapitulates lesions with lipid-rich foamy macrophages. These findings reveal critical coordination between bacterial response to environmental and nutritional cues, and establish Mce3R as a crucial integrator of this process.

Defensive-lipid droplets: Cellular organelles designed for antimicrobial immunity

Microbes have developed many strategies to subvert host organisms, which, in turn, evolved several innate immune responses. As major lipid storage organelles of eukaryotes, lipid droplets (LDs) are an attractive source of nutrients for invaders. Intracellular viruses, bacteria, and protozoan parasites induce and physically interact with LDs, and the current view is that they "hijack" LDs to draw on substrates for host colonization. This dogma has been challenged by the recent demonstration that LDs are endowed with a protein-mediated antibiotic activity, which is upregulated in response to danger signals and sepsis. Dependence on host nutrients could be a generic "Achilles' heel" of intracellular pathogens and LDs a suitable chokepoint harnessed by innate immunity to organize a front-line defense. Here, we will provide a brief overview of the state of the conflict and discuss potential mechanisms driving the formation of the 'defensive-LDs' functioning as hubs of innate immunity.

Tapping lipid droplets: A rich fat diet of intracellular bacterial pathogens

Lipid droplets (LDs) are dynamic and versatile organelles present in most eukaryotic cells. LDs consist of a hydrophobic core of neutral lipids, a phospholipid monolayer coat, and a variety of associated proteins. LDs are formed at the endoplasmic reticulum and have diverse roles in lipid storage, energy metabolism, membrane trafficking, and cellular signaling. In addition to their physiological cellular functions, LDs have been implicated in the pathogenesis of several diseases, including metabolic disorders, cancer, and infections. A number of intracellular bacterial pathogens modulate and/or interact with LDs during host cell infection. Members of the genera Mycobacterium, Legionella, Coxiella, Chlamydia, and Salmonella exploit LDs as a source of intracellular nutrients and membrane components to establish their distinct intracellular replicative niches. In this review, we focus on the biogenesis, interactions, and functions of LDs, as well as on their role in lipid metabolism of intracellular bacterial pathogens.

Lipid droplets: a cellular organelle vital in cancer cells

Lipid droplets (LDs) are cellular organelles comprising a core of neutral lipids (glycerides, sterols) encased within a single phospholipid membrane, responsible for storing surplus lipids and furnishing cellular energy. LDs engage in lipid synthesis, catabolism, and transport processes by interacting with other organelles (e.g., endoplasmic reticulum, mitochondria), and they play critical roles in regulating cellular stress and immunity. Recent research has uncovered that an elevated number of LDs is a hallmark of cancer cells, attributable to their enhanced lipid uptake and synthesis capacity, with lipids stored as LDs. Depletion of LDs in cancer cells induces apoptosis, prompting the emergence of small molecule antitumor drugs targeting LDs or key factors (e.g., FASN, SCD1) within the lipid synthesis pathway. Advancements in LD isolation and artificial synthesis have demonstrated their potential applicability in antitumor research. LDs extracted from murine adipose tissue and incubated with lipophilic antitumor drugs yield drug-coated LDs, which promote apoptosis in cancer cells. Furthermore, LDs have been employed as biological lenses to augment the resolution of subcellular structures (microfilaments, microtubules), facilitating the observation of intricate structures within thicker cells, including cancer cells. This review delineates the functional and metabolic mechanisms of LDs in cancer cells and encapsulates recent progress in LD-centered antitumor research, offering novel insights for tumor diagnosis and treatment.

Mammalian lipid droplets: structural, pathological, immunological and anti-toxicological roles

Mammalian lipid droplets (LDs) are specialized cytosolic organelles consisting of a neutral lipid core surrounded by a membrane made up of a phospholipid monolayer and a specific population of proteins that varies according to the location and function of each LD. Over the past decade, there have been significant advances in the understanding of LD biogenesis and functions. LDs are now recognized as dynamic organelles that participate in many aspects of cellular homeostasis plus other vital functions. LD biogenesis is a complex, highly-regulated process with assembly occurring on the endoplasmic reticulum although aspects of the underpinning molecular mechanisms remain elusive. For example, it is unclear how many enzymes participate in the biosynthesis of the neutral lipid components of LDs and how this process is coordinated in response to different metabolic cues to promote or suppress LD formation and turnover. In addition to enzymes involved in the biosynthesis of neutral lipids, various scaffolding proteins play roles in coordinating LD formation. Despite their lack of ultrastructural diversity, LDs in different mammalian cell types are involved in a wide range of biological functions. These include roles in membrane homeostasis, regulation of hypoxia, neoplastic inflammatory responses, cellular oxidative status, lipid peroxidation, and protection against potentially toxic intracellular fatty acids and lipophilic xenobiotics. Herein, the roles of mammalian LDs and their associated proteins are reviewed with a particular focus on their roles in pathological, immunological and anti-toxicological processes.

Ion mobility mass spectrometry for the study of mycobacterial mycolic acids

Lipids are highly structurally diverse molecules involved in a wide variety of biological processes. The involvement of lipids is even more pronounced in mycobacteria, including the human pathogen Mycobacterium tuberculosis, which produces a highly complex and diverse set of lipids in the cell envelope. These lipids include mycolic acids, which are among the longest fatty acids in nature and can contain up to 90 carbon atoms. Mycolic acids are ubiquitously found in mycobacteria and are alpha branched and beta hydroxylated lipids. Discrete modifications, such as alpha, alpha', epoxy, methoxy, keto, and carboxy, characterize mycolic acids at the species level. Here, we used high precision ion mobility-mass spectrometry to build a database including 206 mass-resolved collision cross sections (CCSs) of mycolic acids originating from the strict human pathogen M. tuberculosis, the opportunistic strains M. abscessus, M. marinum and M. avium, and the nonpathogenic strain M. smegmatis. Primary differences between the mycolic acid profiles could be observed between mycobacterial species. Acyl tail length and modifications were the primary structural descriptors determining CCS magnitude. As a resource for researchers, this work provides a detailed catalogue of the mass-resolved collision cross sections for mycolic acids along with a workflow to generate and analyse the dataset generated.

Identification of host regulators of Mycobacterium tuberculosis phenotypes uncovers a role for the MMGT1-GPR156 lipid droplet axis in persistence

The ability of Mycobacterium tuberculosis (Mtb) to establish latency affects disease and response to treatment. The host factors that influence the establishment of latency remain elusive. We engineered a multi-fluorescent Mtb strain that reports survival, active replication, and stressed non-replication states and determined the host transcriptome of the infected macrophages in these states. Additionally, we conducted a genome-wide CRISPR screen to identify host factors that modulated the phenotypic state of Mtb. We validated hits in a phenotype-specific manner and prioritized membrane magnesium transporter 1 (MMGT1) for a detailed mechanistic investigation. Mtb infection of MMGT1-deficient macrophages promoted a switch to persistence, upregulated lipid metabolism genes, and accumulated lipid droplets during infection. Targeting triacylglycerol synthesis reduced both droplet formation and Mtb persistence. The orphan G protein-coupled receptor GPR156 is a key inducer of droplet accumulation in ΔMMGT1 cells. Our work uncovers the role of MMGT1-GPR156-lipid droplets in the induction of Mtb persistence.

Modulation of innate immunity in airway epithelium for host-directed therapy

Innate immunity of the mucosal surfaces provides the first-line defense from invading pathogens and pollutants conferring protection from the external environment. Innate immune system of the airway epithelium consists of several components including the mucus layer, mucociliary clearance of beating cilia, production of host defense peptides, epithelial barrier integrity provided by tight and adherens junctions, pathogen recognition receptors, receptors for chemokines and cytokines, production of reactive oxygen species, and autophagy. Therefore, multiple components interplay with each other for efficient protection from pathogens that still can subvert host innate immune defenses. Hence, the modulation of innate immune responses with different inducers to boost host endogenous front-line defenses in the lung epithelium to fend off pathogens and to enhance epithelial innate immune responses in the immunocompromised individuals is of interest for host-directed therapy. Herein, we reviewed possibilities of modulation innate immune responses in the airway epithelium for host-directed therapy presenting an alternative approach to standard antibiotics.

M. tuberculosis relies on trace oxygen to maintain energy homeostasis and survive in hypoxic environments

The bioenergetic mechanisms by which Mycobacterium tuberculosis survives hypoxia are poorly understood. Current models assume that the bacterium shifts to an alternate electron acceptor or fermentation to maintain membrane potential and ATP synthesis. Counterintuitively, we find here that oxygen itself is the principal terminal electron acceptor during hypoxic dormancy. M. tuberculosis can metabolize oxygen efficiently at least two orders of magnitude below the concentration predicted to occur in hypoxic lung granulomas. Despite a difference in apparent affinity for oxygen, both the cytochrome bcc:aa₃ and cytochrome bd oxidase respiratory branches are required for hypoxic respiration. Simultaneous inhibition of both oxidases blocks oxygen consumption, reduces ATP levels, and kills M. tuberculosis under hypoxia. The capacity of mycobacteria to scavenge trace levels of oxygen, coupled with the absence of complex regulatory mechanisms to achieve hierarchal control of the terminal oxidases, may be a key determinant of long-term M. tuberculosis survival in hypoxic lung granulomas.

CD11b+Gr-1low cells that accumulate in M.leprae-induced granulomas of the footpad skin of nude mice have the characteristics of monocytic-myeloid-derived suppressor cells

CD11b⁺Gr-1^low cells that are increased in the lungs of a Mycobacterium (M) tuberculosis-infection mouse model have the characteristics of monocytic (M)-myeloid-derived suppressor cells (MDSCs) and harbor M.tuberculosis. Interestingly, a high number of M-MDSCs have also been observed in skin lesions of patients with lepromatous leprosy. We hypothesized that CD11b⁺Gr-1^low cells might be involved in the pathogenesis of leprosy, as they are in tuberculosis. In the current study, we investigated the issue of whether CD11b⁺Gr-1^low cells accumulate in Mycobacterium (M) leprae-induced granulomas of the footpad skin of nude mice. Our results show that CD11b⁺Gr-1^low cells began to accumulate in the 7-month-old M.leprae-induced granulomas and were replaced by other leukocytes, including CD11b⁺Gr-1^high over time during M.leprae infections. CD11b ⁺ Gr-1^low cells expressed the surface markers of M-MDSC, Ly6C^high and Ly6G^low. In addition, CD11b⁺Gr-1^low cells have the nuclei of a mononuclear cell type and expressed higher levels of arginase 1 (Arg1) and inducible NO synthetase (iNOS). Furthermore, they showed a higher infection rate by M.leprae. Taken together, our results indicate that the inoculation with M.leprae induced an accumulation of CD11b ⁺ Gr-1^low at a relatively early stage, 7-month-old M.leprae-induced granulomas, and that CD11b⁺Gr-1^low have the characteristics of M-MDSC and may act as a reservoir for M.leprae.

A terpene nucleoside from M. tuberculosis induces lysosomal lipid storage in foamy macrophages

Induction of lipid-laden foamy macrophages is a cellular hallmark of tuberculosis (TB) disease, which involves the transformation of infected phagolysosomes from a site of killing into a nutrient-rich replicative niche. Here, we show that a terpenyl nucleoside shed from Mycobacterium tuberculosis, 1-tuberculosinyladenosine (1-TbAd), caused lysosomal maturation arrest and autophagy blockade, leading to lipid storage in M1 macrophages. Pure 1-TbAd, or infection with terpenyl nucleoside-producing M. tuberculosis, caused intralysosomal and peribacillary lipid storage patterns that matched both the molecules and subcellular locations known in foamy macrophages. Lipidomics showed that 1-TbAd induced storage of triacylglycerides and cholesterylesters and that 1-TbAd increased M. tuberculosis growth under conditions of restricted lipid access in macrophages. Furthermore, lipidomics identified 1-TbAd-induced lipid substrates that define Gaucher's disease, Wolman's disease, and other inborn lysosomal storage diseases. These data identify genetic and molecular causes of M. tuberculosis-induced lysosomal failure, leading to successful testing of an agonist of TRPML1 calcium channels that reverses lipid storage in cells. These data establish the host-directed cellular functions of an orphan effector molecule that promotes survival in macrophages, providing both an upstream cause and detailed picture of lysosome failure in foamy macrophages.

The fat is in the lysosome: how Mycobacterium tuberculosis tricks macrophages into storing lipids

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), infects primarily macrophages, causing them to differentiate into lipid-laden foamy macrophages that are a primary source of tissue destruction in patients with TB. In this issue of the JCI, Bedard et al. demonstrate that 1-tuberculosinyladenosine, a virulence factor produced by M. tuberculosis, caused lysosomal dysfunction associated with lipid storage in the phagolysosome of macrophages in a manner that mimicked lysosomal storage diseases. This work sheds light on how M. tuberculosis manipulates host lipid metabolism for its survival and opens avenues toward host-directed therapy against TB.

Tuberculosis: Pathogenesis, Current Treatment Regimens and New Drug Targets

Mycobacterium tuberculosis (M. tb), the causative agent of TB, is a recalcitrant pathogen that is rife around the world, latently infecting approximately a quarter of the worldwide population. The asymptomatic status of the dormant bacteria escalates to the transmissible, active form when the host's immune system becomes debilitated. The current front-line treatment regimen for drug-sensitive (DS) M. tb strains is a 6-month protocol involving four different drugs that requires stringent adherence to avoid relapse and resistance. Poverty, difficulty to access proper treatment, and lack of patient compliance contributed to the emergence of more sinister drug-resistant (DR) strains, which demand a longer duration of treatment with more toxic and more expensive drugs compared to the first-line regimen. Only three new drugs, bedaquiline (BDQ) and the two nitroimidazole derivatives delamanid (DLM) and pretomanid (PMD) were approved in the last decade for treatment of TB-the first anti-TB drugs with novel mode of actions to be introduced to the market in more than 50 years-reflecting the attrition rates in the development and approval of new anti-TB drugs. Herein, we will discuss the M. tb pathogenesis, current treatment protocols and challenges to the TB control efforts. This review also aims to highlight several small molecules that have recently been identified as promising preclinical and clinical anti-TB drug candidates that inhibit new protein targets in M. tb.

Adaptation to the intracellular environment of primary human macrophages influences drug susceptibility of Mycobacterium tuberculosis

As a facultative intracellular pathogen, M. tuberculosis (Mtb) is highly adapted to evading antibacterial mechanisms in phagocytic cells. Both the macrophage and pathogen experience transcriptional and metabolic changes from the onset of phagocytosis. To account for this interaction in the assessment of intracellular drug susceptibility, we allowed a 3-day preadaptation phase post-macrophage infection prior to drug treatment. We found that intracellular Mtb in human monocyte-derived macrophages (MDM) presents dramatic alterations in susceptibility to isoniazid, sutezolid, rifampicin and rifapentine when compared to axenic culture. Infected MDM gradually accumulate lipid bodies, adopting a characteristic appearance reminiscent of foamy macrophages in granulomas. Furthermore, TB granulomas in vivo develop hypoxic cores with decreasing oxygen tension gradients across their radii. Accordingly, we evaluated the effects of hypoxia on preadapted intracellular Mtb in our MDM model. We observed that hypoxia induced greater lipid body formation and no additional shifts in drug tolerance, suggesting that the adaptation of intracellular Mtb to baseline host cell conditions under normoxia dominates changes to intracellular drug susceptibility. Using unbound plasma concentrations in patients as surrogates for free drug concentrations in lung interstitial fluid, we estimate that intramacrophage Mtb in granulomas are exposed to bacteriostatic concentrations of most study drugs.

Membrane damage and repair: a thin line between life and death

Bilayered membranes separate cells from their surroundings and form boundaries between intracellular organelles and the cytosol. Gated transport of solutes across membranes enables cells to establish vital ion gradients and a sophisticated metabolic network. However, an advanced compartmentalization of biochemical reactions makes cells also particularly vulnerable to membrane damage inflicted by pathogens, chemicals, inflammatory responses or mechanical stress. To avoid potentially lethal consequences of membrane injuries, cells continuously monitor the structural integrity of their membranes and readily activate appropriate pathways to plug, patch, engulf or shed the damaged membrane area. Here, we review recent insights into the cellular mechanisms that underly an effective maintenance of membrane integrity. We discuss how cells respond to membrane lesions caused by bacterial toxins and endogenous pore-forming proteins, with a primary focus on the intimate crosstalk between membrane proteins and lipids during wound formation, detection and elimination. We also discuss how a delicate balance between membrane damage and repair determines cell fate upon bacterial infection or activation of pro-inflammatory cell death pathways.

Apoptosis Inhibitor of Macrophages Contributes to the Chronicity of Mycobacterium avium Infection by Promoting Foamy Macrophage Formation

In Mycobacterium avium infections, macrophages play a critical role in the host defense response. Apoptosis inhibitor of macrophage (AIM), also known as CD5L, may represent a novel supportive therapy against various diseases, including metabolic syndrome and infectious diseases. The mechanisms of AIM include modulating lipid metabolism in macrophages and other host cells. We investigated the role of AIM in M. avium infections in vitro and in vivo. In a mouse model of M. avium pneumonia, foamy macrophages were induced 6 wk after infection. The bacteria localized in these macrophages. Flow cytometric analysis also confirmed that the percentage of CD11chighMHCclassIIhigh interstitial and alveolar macrophages, a cell surface marker defined as foamy macrophages, increased significantly after infection. AIM in alveolar lavage fluid and serum gradually increased after infection. Administration of recombinant AIM significantly increased the number of bacteria in the lungs of mice, accompanied by the induction of inflammatory cytokine and iNOS expression. In mouse bone marrow-derived macrophages, the mRNA expression of AIM after M. avium infection and the amount of AIM in the supernatant increased prior to the increase in intracellular bacteria. Infected cells treated with anti-AIM Abs had fewer bacteria and a higher percentage of apoptosis-positive cells than infected cells treated with isotype control Abs. Finally, AIM in the sera of patients with M. avium-pulmonary disease was measured and was significantly higher than in healthy volunteers. This suggests that AIM production is enhanced in M. avium-infected macrophages, increasing macrophage resistance to apoptosis and providing a possible site for bacterial growth.

Application Prospects of FTIR Spectroscopy and CLSM to Monitor the Drugs Interaction with Bacteria Cells Localized in Macrophages for Diagnosis and Treatment Control of Respiratory Diseases

Visualization of the interaction of drugs with biological cells creates new approaches to improving the bioavailability, selectivity, and effectiveness of drugs. The use of CLSM and FTIR spectroscopy to study the interactions of antibacterial drugs with latent bacterial cells localized in macrophages create prospects to solve the problems of multidrug resistance (MDR) and severe cases. Here, the mechanism of rifampicin penetration into E. coli bacterial cells was studied by tracking the changes in the characteristic peaks of cell wall components and intracellular proteins. However, the effectiveness of the drug is determined not only by penetration, but also by efflux of the drugs molecules from the bacterial cells. Here, the efflux effect was studied and visualized using FTIR spectroscopy, as well as CLSM imaging. We have shown that because of efflux inhibition, eugenol acting as an adjuvant for rifampicin showed a significant (more than three times) increase in the antibiotic penetration and the maintenance of its intracellular concentration in E. coli (up to 72 h in a concentration of more than 2 μg/mL). In addition, optical methods have been applied to study the systems containing bacteria localized inside of macrophages (model of the latent form), where the availability of bacteria for antibiotics is reduced. Polyethylenimine grafted with cyclodextrin carrying trimannoside vector molecules was developed as a drug delivery system for macrophages. Such ligands were absorbed by CD206+ macrophages by 60-70% versus 10-15% for ligands with a non-specific galactose label. Owing to presence of ligands with trimannoside vectors, the increase in antibiotic concentration inside macrophages, and thus, its accumulation into dormant bacteria, is observed. In the future, the developed FTIR+CLSM techniques would be applicable for the diagnosis of bacterial infections and the adjustment of therapy strategies.

Mycobacterium tuberculosis induces delayed lipid droplet accumulation in dendritic cells depending on bacterial viability and virulence

Tuberculosis remains a global health threat with high morbidity. Dendritic cells (DCs) participate in the acute and chronic inflammatory responses to Mycobacterium tuberculosis (Mtb) by directing the adaptive immune response and are present in lung granulomas. In macrophages, the interaction of lipid droplets (LDs) with mycobacteria-containing phagosomes is central to host-pathogen interactions. However, the data available for DCs are still a matter of debate. Here, we reported that bone marrow-derived DCs (BMDCs) were susceptible to Mtb infection and replication at similar rate to macrophages. Unlike macrophages, the analysis of gene expression showed that Mtb infection induced a delayed increase in lipid droplet-related genes and proinflammatory response. Hence, LD accumulation has been observed by high-content imaging in late periods. Infection of BMDCs with killed H37Rv demonstrated that LD accumulation depends on Mtb viability. Moreover, infection with the attenuated strains H37Ra and Mycobacterium bovis-BCG induced only an early transient increase in LDs, whereas virulent Mtb also induced delayed LD accumulation. In addition, infection with the BCG strain with the reintroduced virulence RD1 locus induced higher LD accumulation and bacterial replication when compared to parental BCG. Collectively, our data suggest that delayed LD accumulation in DCs is dependent on mycobacterial viability and virulence.

Atorvastatin Potentially Reduces Mycobacterial Severity through Its Action on Lipoarabinomannan and Drug Permeability in Granulomas

The majority of preclinical research has shown that Mycobacterium tuberculosis can modify host lipids in various ways. To boost its intramacrophage survival, M. tuberculosis causes host lipids to build up, resulting in the development of lipid-laden foam cells. M. tuberculosis binds to and enters the macrophage via the cell membrane cholesterol. Aggregation of cholesterol in the cell wall of M. tuberculosis and an increase in vascularity at the granuloma site reduce the permeability of rifampicin and isoniazid concentrations. However, very few studies have assessed the effect of statins on drug penetration. Here, we used atorvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, to observe its effect on the bacterial burden by increasing the drug concentration at the infection site. We looked into how atorvastatin could be used in conjunction with first-line drugs to promote drug permeation. In this study, we detected an accumulation of drugs at the peripheral sites of the lungs and impaired drug distribution to the diseased sites. The efficacy of antituberculosis drugs, with atorvastatin as an adjunct, on the viability of M. tuberculosis cells was demonstrated. A nontoxic statin dosage established phenotypic and normal granuloma vasculature and showed an additive effect with rifampicin and isoniazid. Our data show that statins help to reduce the tuberculosis bacterial burden. Our findings reveal that the bacterial load is connected with impaired drug permeability resulting from lipid accumulation in the bacterial cell wall. Statin therapy combined with antituberculosis medications have the potential to improve treatment in tuberculosis patients. IMPORTANCE Mycobacterium tuberculosis binds to and enters the macrophage via the cell membrane cholesterol. M. tuberculosis limits phagosomal maturation and activation without engaging in phagocytosis. Aggregation of cholesterol in the cell wall of M. tuberculosis and an increase in the vascularity at the granuloma site reduce the permeability of rifampicin and isoniazid concentrations. However, very few studies have assessed the effect of statins on drug penetration, which can be increased through a reduction in cholesterol and vascularity. Herein, we used atorvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, to observe its effect on bacterial burden through increasing the drug concentration at the infection site. Our main research goal is to diminish mycobacterial dissemination and attenuate bacterial growth by increasing drug permeability.

Nur77 influences immunometabolism to regulate the release of proinflammatory cytokines and the formation of lipid bodies during Mycobacterium tuberculosis infection of macrophages

Infection of macrophages with Mycobacterium tuberculosis induces innate immune responses designed to clear the invading bacterium. However, bacteria often survive within the intracellular environment by exploiting these responses triggered by macrophages. Here, the role of the orphan nuclear receptor Nur77 (Nr4a1) in regulating the response of macrophages infected with M. tuberculosis (Mtb) has been delineated. Nur77 is induced early during infection, regulates metabolism by binding directly at the promoter of the TCA cycle enzyme, isocitrate dehydrogenase 2 (IDH2), to act as its repressor, and shifts the balance from a proinflammatory to an anti-inflammatory phenotype. Depletion of Nur77 increased transcription of IDH2 and, consequently, the levels of intracellular succinate, leading to enhanced levels of the proinflammatory cytokine IL-1β. Further, Nur77 inhibited the production of antibacterial nitric oxide and IL-1β in a succinate dehydrogenase (SDH)-dependent manner, suggesting that its induction favors bacterial survival by suppressing bactericidal responses. Indeed, depletion of Nur77 inhibited the intracellular survival of Mtb. On the other hand, depletion of Nur77 enhanced lipid body formation, suggesting that the fall in Nur77 levels as infection progresses likely favors foamy macrophage formation and long-term survival of Mtb in the host milieu.

Tools to develop antibiotic combinations that target drug tolerance in Mycobacterium tuberculosis

Combination therapy is necessary to treat tuberculosis to decrease the rate of disease relapse and prevent the acquisition of drug resistance, and shorter regimens are urgently needed. The adaptation of Mycobacterium tuberculosis to various lesion microenvironments in infection induces various states of slow replication and non-replication and subsequent antibiotic tolerance. This non-heritable tolerance to treatment necessitates lengthy combination therapy. Therefore, it is critical to develop combination therapies that specifically target the different types of drug-tolerant cells in infection. As new tools to study drug combinations earlier in the drug development pipeline are being actively developed, we must consider how to best model the drug-tolerant cells to use these tools to design the best antibiotic combinations that target those cells and shorten tuberculosis therapy. In this review, we discuss the factors underlying types of drug tolerance, how combination therapy targets these populations of bacteria, and how drug tolerance is currently modeled for the development of tuberculosis multidrug therapy. We highlight areas for future studies to develop new tools that better model drug tolerance in tuberculosis infection specifically for combination therapy testing to bring the best drug regimens forward to the clinic.

Inflammation-mediated tissue damage in pulmonary tuberculosis and host-directed therapeutic strategies

Treatment of tuberculosis (TB) involves the administration of anti-mycobacterial drugs for several months. The emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb, the causative agent) together with increased disease severity in people with co-morbidities such as diabetes mellitus and HIV have hampered efforts to reduce case fatality. In severe disease, TB pathology is largely attributable to over-exuberant host immune responses targeted at controlling bacterial replication. Non-resolving inflammation driven by host pro-inflammatory mediators in response to high bacterial load leads to pulmonary pathology including cavitation and fibrosis. The need to improve clinical outcomes and reduce treatment times has led to a two-pronged approach involving the development of novel antimicrobials as well as host-directed therapies (HDT) that favourably modulate immune responses to Mtb. HDT strategies incorporate aspects of immune modulation aimed at downregulating non-productive inflammatory responses and augmenting antimicrobial effector mechanisms to minimise pulmonary pathology and accelerate symptom resolution. HDT in combination with existing antimycobacterial agents offers a potentially promising strategy to improve the long-term outcome for TB patients. In this review, we describe components of the host immune response that contribute to inflammation and tissue damage in pulmonary TB, including cytokines, matrix metalloproteinases, lipid mediators, and neutrophil extracellular traps. We then proceed to review HDT directed at these pathways.

Lipid droplets as multifunctional organelles related to the mechanism of evasion during mycobacterial infection

Tuberculosis (TB) is an infectious disease caused by the bacteria of the Mycobaterium tuberculosis (Mtb) complex. The modulation of the lipid metabolism has been implicated in the immune response regulation, including the formation of lipid droplets (LD)s, LD-phagosome association and eicosanoid synthesis. Mtb, M. bovis BCG and other pathogenic mycobacteria, as well as wall components, such as LAM, can induce LDs formation in a mechanism involving surface receptors, for instance TLRs, CD36, CD14, CD11b/CD18 and others. In addition, the activation of the lipid-activated nuclear receptor PPARγ is involved in the mechanisms of LD biogenesis, as well as in the modulation of the synthesis of lipid mediators. In infected cells, LDs are sites of compartmentalized prostaglandin E₂ synthesis involved in macrophage deactivation, bacterial replication and regulation of the host cytokine profile. LDs also have a function in vesicle traffic during infection. Rab7 and RILP, but not Rab5, are located on LDs of infected macrophages, suggesting that LDs and phagosomes could exchange essential proteins for phagosomal maturation, interfering in mycobacterial survival. The pharmacological inhibition of LDs biogenesis affects the bacterial replication and the synthesis of lipid mediators and cytokines, suggesting that LDs may be new targets for antimicrobial therapies. However, it is still controversial if the accumulation of LDs favors the mycobacterial survival acting as an escape mechanism, or promotes the host resistance to infection. Thus, in this mini-review we discuss recent advances in understanding the important role of LDs in the course of infections and the implications for the pathophysiology of mycobacteriosis.

Current Understanding on the Role of Lipids in Macrophages and Associated Diseases

Lipid metabolism is the major intracellular mechanism driving a variety of cellular functions such as energy storage, hormone regulation and cell division. Lipids, being a primary component of the cell membrane, play a pivotal role in the survival of macrophages. Lipids are crucial for a variety of macrophage functions including phagocytosis, energy balance and ageing. However, functions of lipids in macrophages vary based on the site the macrophages are residing at. Lipid-loaded macrophages have recently been emerging as a hallmark for several diseases. This review discusses the significance of lipids in adipose tissue macrophages, tumor-associated macrophages, microglia and peritoneal macrophages. Accumulation of macrophages with impaired lipid metabolism is often characteristically observed in several metabolic disorders. Stress signals differentially regulate lipid metabolism. While conditions such as hypoxia result in accumulation of lipids in macrophages, stress signals such as nutrient deprivation initiate lipolysis and clearance of lipids. Understanding the biology of lipid accumulation in macrophages requires the development of potentially active modulators of lipid metabolism.

Mannosylated Systems for Targeted Delivery of Antibacterial Drugs to Activated Macrophages

Macrophages are a promising target for drug delivery to influence macrophage-associated processes in the body, namely due to the presence of resistant microorganisms in macrophages. In this work, a series of mannosylated carriers based on mannan, polyethylenimine (PEI) and cyclodextrin (CD) was synthesized. The molecular architecture was studied using FTIR and ¹H NMR spectroscopy. The particle size, from small 10-50 nm to large 500 nm, depending on the type of carrier, is potentially applicable for the creation of various medicinal forms: intravenous, oral and inhalation. Non-specific capture by cells with a simultaneous increase in selectivity to CD206+ macrophages was achieved. ConA was used as a model mannose receptor, binding galactosylated (CD206 non-specific) carriers with constants of the order of 10⁴ M^-1 and mannosylated conjugates of 10⁶-10⁷ M^-1. The results of such primary "ConA-screening" of ligands are in a good agreement in terms of the comparative effectiveness of the interaction of ligands with the CD206+ macrophages: non-specific (up to 10%) absorption of highly charged and small particles; weakly specific uptake of galactosylated polymers (up to 50%); and high affine capture (more than 70-80%) of the ligands with grafted trimannoside was demonstrated using the cytometry method. Double and multi-complexes of antibacterials (moxifloxacin with its adjuvants from the class of terpenoids) were proposed as enhanced forms against resistant pathogens. In vivo pharmacokinetic experiments have shown that polymeric carriers significantly improve the efficiency of the antibiotic: the half-life of moxifloxacin is increased by 2-3 times in conjugate-loaded forms, bio-distribution to the lungs in the first hours after administration of the drug is noticeably greater, and, after 4 h of observation, free moxifloxacin was practically removed from the lungs of rats. Although, in polymer systems, its content is significant-1.2 µg/g. Moreover, the importance of the covalent crosslinking carrier with mannose label was demonstrated. Thus, this paper describes experimental, scientifically based methods of targeted drug delivery to macrophages to create enhanced medicinal forms.