PPARdelta: A key modulator in the pathogenesis of diabetes mellitus and Mycobacterium tuberculosis co-morbidity

The interplay between lipid metabolism and immune response in macrophages plays a pivotal role in various infectious diseases, notably tuberculosis (TB). Herein, we illuminate the modulatory effect of heat-killed Mycobacterium tuberculosis (HKMT) on macrophage lipid metabolism and its implications on the inflammatory cascade. Our findings demonstrate that HKMT potently activates the lipid scavenger receptor, CD36, instigating lipid accumulation. While CD36 inhibition mitigated lipid increase, it unexpectedly exacerbated the inflammatory response. Intriguingly, this paradoxical effect was linked to an upregulation of PPARδ. Functional analyses employing PPARδ modulation revealed its central role in regulating both lipid dynamics and inflammation, suggesting it as a potential therapeutic target. Moreover, primary monocytic cells from diabetic individuals, a demographic at amplified risk of TB, exhibited heightened PPARδ expression and inflammation, further underscoring its pathological relevance. Targeting PPARδ in these cells effectively dampened the inflammatory response, offering a promising therapeutic avenue against TB.

Heterogeneity of foam cell biogenesis across diseases

Foam cells are dysfunctional, lipid-laden macrophages associated with chronic inflammation of diverse origin. The long-standing paradigm that foam cells are cholesterol-laden derives from atherosclerosis research. We previously showed that, in tuberculosis, foam cells surprisingly accumulate triglycerides. Here, we utilized bacterial (Mycobacterium tuberculosis), fungal (Cryptococcus neoformans), and human papillary renal cell carcinoma (pRCC) models. We applied mass spectrometry-based imaging to assess the spatial distribution of storage lipids relative to foam-cell-rich areas in lesional tissues, and characterized lipid-laden macrophages generated under corresponding in vitro conditions. The in vivo data were consistent with in vitro findings showing that cryptococcus-infected macrophages accumulated triglycerides, while macrophages exposed to pRCC-conditioned-medium accumulated both triglycerides and cholesterol. Moreover, cryptococcus- and mycobacterium-infected macrophages accumulated triglycerides by different mechanisms. Collectively, our data indicate that the mechanisms of foam cell formation are disease-microenvironment-specific. Since foam cells are potential therapeutic targets, recognizing that their formation is disease-specific opens new biomedical research directions.

In situ lipidomics of Staphylococcus aureus osteomyelitis using imaging mass spectrometry

Osteomyelitis occurs when Staphylococcus aureus invades the bone microenvironment, resulting in a bone marrow abscess with a spatially defined architecture of cells and biomolecules. Imaging mass spectrometry and microscopy are invaluable tools that can be employed to interrogate the lipidome of S. aureus-infected murine femurs to reveal metabolic and signaling consequences of infection. Here, nearly 250 lipids were spatially mapped to healthy and infection-associated morphological features throughout the femur, establishing composition profiles for tissue types. Ether lipids and arachidonoyl lipids were significantly altered between cells and tissue structures in abscesses, suggesting their roles in abscess formation and inflammatory signaling. Sterols, triglycerides, bis(monoacylglycero)phosphates, and gangliosides possessed ring-like distributions throughout the abscess, indicating dysregulated lipid metabolism in a subpopulation of leukocytes that cannot be discerned with traditional microscopy. These data provide chemical insight into the signaling function and metabolism of cells in the fibrotic border of abscesses, likely characteristic of lipid-laden macrophages.

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.

Peroxisome Proliferator-Activated Receptor-Targeted Therapies: Challenges upon Infectious Diseases

Peroxisome proliferator-activated receptors (PPARs) α, β, and γ are nuclear receptors that orchestrate the transcriptional regulation of genes involved in a variety of biological responses, such as energy metabolism and homeostasis, regulation of inflammation, cellular development, and differentiation. The many roles played by the PPAR signaling pathways indicate that PPARs may be useful targets for various human diseases, including metabolic and inflammatory conditions and tumors. Accumulating evidence suggests that each PPAR plays prominent but different roles in viral, bacterial, and parasitic infectious disease development. In this review, we discuss recent PPAR research works that are focused on how PPARs control various infections and immune responses. In addition, we describe the current and potential therapeutic uses of PPAR agonists/antagonists in the context of infectious diseases. A more comprehensive understanding of the roles played by PPARs in terms of host-pathogen interactions will yield potential adjunctive personalized therapies employing PPAR-modulating agents.

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.

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.

Targeting hypoxia-induced tumor stemness by activating pathogen-induced stem cell niche defense

Tumor hypoxia and oxidative stress reprograms cancer stem cells (CSCs) to a highly aggressive and inflammatory phenotypic state of tumor stemness. Previously, we characterized tumor stemness phenotype in the ATP Binding Cassette Subfamily G Member 2 (ABCG2)–positive migratory side population (SPm) fraction of CSCs exposed to extreme hypoxia followed by reoxygenation. Here, we report that post-hypoxia/reoxygenation SPm+/ABCG2+ CSCs exerts defense against pathogen invasion that involves bystander apoptosis of non-infected CSCs. In an in vitro assay of cancer cell infection by Bacillus Calmette Guerin (BCG) or mutant Mycobacterium tuberculosis (Mtb) strain 18b (Mtb-m18b), the pathogens preferentially replicated intracellular to SPm+/ABCG2+ CSCs of seven cell lines of diverse cancer types including SCC-25 oral squamous cancer cell line. The conditioned media (CM) of infected CSCs exhibited direct anti-microbial activity against Mtb and BCG, suggesting niche defense against pathogen. Importantly, the CM of infected CSCs exhibited marked in vitro bystander apoptosis toward non-infected CSCs. Moreover, the CM-treated xenograft bearing mice showed 10- to 15-fold reduction (p < 0.001; n = 7) in the number of CSCs residing in the hypoxic niches. Our in vitro studies indicated that BCG-infected SPm+/ABCG2+ equivalent EPCAM+/ABCG2+ CSCs of SCC-25 cells underwent pyroptosis and released a high mobility group box protein 1 (HMGB1)/p53 death signal into the tumor microenvironment (TME). The death signal can induce a Toll-like receptor 2/4–mediated bystander apoptosis in non-infected CSCs by activating p53/MDM2 oscillation and subsequent activation of capase-3–dependent intrinsic apoptosis. Notably, SPm+/ABCG2+ but not SP cells undergoing bystander apoptosis amplified the death signal by further release of HMGB1/p53 complex into the TME. These results suggest that post-hypoxia SPm+/ABCG2+ CSCs serve a functional role as a tumor stemness defense (TSD) phenotype to protect TME against bacterial invasion. Importantly, the CM of TSD phenotype undergoing bystander apoptosis may have therapeutic uses against CSCs residing in the hypoxic niche.

Shrimp Lipid Droplet Protein Perilipin Involves in the Pathogenesis of AHPND-Causing Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), caused by a unique strain of Vibrio parahaemolyticus (Vp (AHPND)), has become the world’s most severe debilitating disease in cultured shrimp. Thus far, the pathogenesis of AHPND remains largely unknow. Herein, in Litopenaeus vannamei, we found that a Vp (AHPND) infection significantly increased the expression of lipid droplets (LDs) protein LvPerilipin, as well as promoted the formation of LDs. In addition, the knockdown of LvPerilipin increased the shrimp survival rate in response to the Vp (AHPND) infection, and inhibited the proliferation of Vp (AHPND). Furthermore, we demonstrated that LvPerilipin depletion could increase the production of reactive oxygen species (ROS), which may be responsible for the decreased Vp (AHPND) proliferation. Taken together, our current data for the first time reveal that the shrimp lipid droplets protein Perilipin is involved in the pathogenesis of Vp (AHPND) via promoting LDs accumulation and decreasing ROS production.

Comprehensive lipid and lipid-related gene investigations of host immune responses to characterize metabolism-centric biomarkers for pulmonary tuberculosis

Despite remarkable success in the prevention and treatment of tuberculosis (TB), it remains one of the most devastating infectious diseases worldwide. Management of TB requires an efficient and timely diagnostic strategy. In this study, we comprehensively characterized the plasma lipidome of TB patients, then selected candidate lipid and lipid-related gene biomarkers using a data-driven, knowledge-based framework. Among 93 lipids that were identified as potential biomarker candidates, ether-linked phosphatidylcholine (PC O–) and phosphatidylcholine (PC) were generally upregulated, while free fatty acids and triglycerides with longer fatty acyl chains were downregulated in the TB group. Lipid-related gene enrichment analysis revealed significantly altered metabolic pathways (e.g., ether lipid, linolenic acid, and cholesterol) and immune response signaling pathways. Based on these potential biomarkers, TB patients could be differentiated from controls in the internal validation (random forest model, area under the curve [AUC] 0.936, 95% confidence interval [CI] 0.865–0.992). PC(O-40:4), PC(O-42:5), PC(36:0), and PC(34:4) were robust biomarkers able to distinguish TB patients from individuals with latent infection and healthy controls, as shown in the external validation. Small changes in expression were identified for 162 significant lipid-related genes in the comparison of TB patients vs. controls; in the random forest model, their utilities were demonstrated by AUCs that ranged from 0.829 to 0.956 in three cohorts. In conclusion, this study introduced a potential framework that can be used to identify and validate metabolism-centric biomarkers.

Progressive Host-Directed Strategies to Potentiate BCG Vaccination Against Tuberculosis

The pursuit to improve the TB control program comprising one approved vaccine, M. bovis Bacille Calmette-Guerin (BCG) has directed researchers to explore progressive approaches to halt the eternal TB pandemic. Mycobacterium tuberculosis (M.tb) was first identified as the causative agent of TB in 1882 by Dr. Robert Koch. However, TB has plagued living beings since ancient times and continues to endure as an eternal scourge ravaging even with existing chemoprophylaxis and preventive therapy. We have scientifically come a long way since then, but despite accessibility to the standard antimycobacterial antibiotics and prophylactic vaccine, almost one-fourth of humankind is infected latently with M.tb. Existing therapeutics fail to control TB, due to the upsurge of drug-resistant strains and increasing incidents of co-infections in immune-compromised individuals. Unresponsiveness to established antibiotics leaves patients with no therapeutic possibilities. Hence the search for an efficacious TB immunization strategy is a global health priority. Researchers are paving the course for efficient vaccination strategies with the radically advanced operation of core principles of protective immune responses against M.tb. In this review; we have reassessed the progression of the TB vaccination program comprising BCG immunization in children and potential stratagems to reinforce BCG-induced protection in adults.

Bacille Calmette-Guérin vaccine reprograms human neonatal lipid metabolism in vivo and in vitro

Vaccines have generally been developed with limited insight into their molecular impact. While systems vaccinology enables characterization of mechanisms of action, these tools have yet to be applied to infants, who are at high risk of infection and receive the most vaccines. Bacille Calmette-Guérin (BCG) protects infants against disseminated tuberculosis (TB) and TB-unrelated infections via incompletely understood mechanisms. We employ mass-spectrometry-based metabolomics of blood plasma to profile BCG-induced infant responses in Guinea-Bissau in vivo and the US in vitro. BCG-induced lysophosphatidylcholines (LPCs) correlate with both TLR-agonist- and purified protein derivative (PPD, mycobacterial antigen)-induced blood cytokine production in vitro, raising the possibility that LPCs contribute to BCG immunogenicity. Analysis of an independent newborn cohort from The Gambia demonstrates shared vaccine-induced metabolites, such as phospholipids and sphingolipids. BCG-induced changes to the plasma lipidome and LPCs may contribute to its immunogenicity and inform the development of early life vaccines.

PPARγ Ameliorates Mycobacterium tuberculosis H37Ra-Induced Foamy Macrophage Formation via the ABCG1-Dependent Cholesterol Efflux Pathway in THP-1 Macrophages

Foamy macrophages are present during the course of Mycobacterium tuberculosis (Mtb) infection and seems to be nutrient-rich reservoir and secure reservoir for the bacilli, which leads to bacterial persistence and infection transmission. Peroxisome proliferator activated receptor γ (PPARγ) is a key transcription factor for cholesterol metabolism in macrophages and its role in regulating atherosclerosis related foamy macrophages (FMs) formation has been well-studied. However, knowledge about the mechanism of PPARγ regulating Mtb infection induced FM formation remains very limited. In this study, we investigate the functional role of PPARγ in Mtb H37Ra infection-induced foamy macrophages formation. H37Ra infection induced a time-dependent decreased expression of PPARγ that paralleled the augmented lipid body formation in THP1-derived macrophages. PPARγ antagonist GW9662 significantly potentiate H37Ra induced lipid body formation and inhibit ABCG1 expression, overexpression of ABCG1 by transduced macrophages with lentivirus significantly reversed the promotion effect of GW9662 on FM formation. Moreover, Treatment with a TLR2 neutralizing antibody ameliorated the activation of ABCG1 by Mtb H37Ra without significantly effecting the suppression of PPARγ, suggesting a greater role for TLR2 to regulate ABCG1 compared to PPARγ. Overall, this study showed that PPARγ is involved in ameliorating FM formation by regulating ABCG1 expression, these observations expose a novel role of PPARγ in the Mtb infection induced FM formation.

The Pathogenic Role of Foam Cells in Atherogenesis: Do They Represent Novel Therapeutic Targets?

BACKGROUND

Foam cells, mainly derived from monocytes-macrophages, contain lipid droplets essentially composed of cholesterol in their cytoplasm. They infiltrate the intima of arteries, contributing to the formation of atherosclerotic plaques.

PATHOGENESIS

Foam cells damage the arterial cell wall via the release of proinflammatory cytokines, free radicals, and matrix metalloproteinases, enhancing the plaque size up to its rupture.

THERAPY

A correct dietary regimen seems to be the most appropriate therapeutic approach to minimize obesity, which is associated with the formation of foam cells. At the same time, different types of antioxidants have been evaluated to arrest the formation of foam cells, even if the results are still contradictory. In any case, a combination of antioxidants seems to be more efficient in the prevention of atherosclerosis.

Lipid larceny: channelizing host lipids for establishing successful pathogenesis by bacteria

Lipids are complex organic compounds made up of carbon, oxygen, and hydrogen. These play a diverse and intricate role in cellular processes like membrane trafficking, protein sorting, signal transduction, and bacterial infections. Both Gram-positive bacteria (Staphylococcus sp., Listeria monocytogenes, etc.) and Gram-negative bacteria (Chlamydia sp., Salmonella sp., E. coli, etc.) can hijack the various host-lipids and utilize them structurally as well as functionally to mount a successful infection. The pathogens can deploy with various arsenals to exploit host membrane lipids and lipid-associated receptors as an attachment for toxins' landing or facilitate their entry into the host cellular niche. Bacterial species like Mycobacterium sp. can also modulate the host lipid metabolism to fetch its carbon source from the host. The sequential conversion of host membrane lipids into arachidonic acid and prostaglandin E2 due to increased activity of cPLA-2 and COX-2 upon bacterial infection creates immunosuppressive conditions and facilitates the intracellular growth and proliferation of bacteria. However, lipids' more debatable role is that they can also be a blessing in disguise. Certain host-lipids, especially sphingolipids, have been shown to play a crucial antibacterial role and help the host in combating the infections. This review shed light on the detailed role of host lipids in bacterial infections and the current understanding of the lipid in therapeutics. We have also discussed potential prospects and the need of the hour to help us cope in this race against deadly pathogens and their rapidly evolving stealthy virulence strategies.

Lipid droplet accumulation occurs early following Salmonella infection and contributes to intracellular bacterial survival and replication

Salmonellosis is a public health problem caused by Salmonella sp., a highly adapted facultative intracellular pathogen. After internalization, Salmonella sp. Manipulates several host processes, mainly through the activation of the type III secretion system (T3SS), including modification of host lipid metabolism and lipid droplet (LD) accumulation. LDs are dynamic and complex lipid-rich organelles involved in several cellular processes. The present study investigated the mechanism involved in LD biogenesis in Salmonella-infected macrophages and its role in bacterial pathogenicity. Here, we reported that S. Typhimurium induced a rapid time-dependent increase of LD formation in macrophages. The LD biogenesis was demonstrated to depend on Salmonella's viability and SPI1-related T3SS activity, with the participation of Toll-Like Receptor (TLR) signaling. We also observed that LD accumulation occurs through TLR2-dependent signaling and is counter-regulated by TLR4. Last, the pharmacologic modulation of LD formation by inhibiting diacylglycerol O-acyltransferase 1 (DGAT1) and cytosolic phospholipase A2 (cPLA2) significantly reduced the intracellular bacterial proliferation and impaired the prostaglandin E2 (PGE₂ ) synthesis. Collectively, our data suggest the role of LDs on S. typhimurium intracellular survival and replication in macrophages. This data set provides new perspectives for future investigations about LDs in host-pathogen interaction.

Impact of the Extracellular Vesicles Derived From Trypanosoma cruzi: A Paradox in Host Response and Lipid Metabolism Modulation

Chagas disease is a major public health problem, especially in the South and Central America region. Its incidence is related to poverty and presents a high rate of morbidity and mortality. The pathogenesis of Chagas disease is complex and involves many interactive pathways between the hosts and the Trypanosoma cruzi. Several factors have been implicated in parasite-host interactions, including molecules secreted by infected cells, lipid mediators and most recent, extracellular vesicles (EVs). The EVs of T. cruzi (EVsT) were reported for the first time in the epimastigote forms about 42 years ago. The EVsT are involved in paracrine communication during the infection and can have an important role in the inflammatory modulation and parasite escape mechanism. However, the mechanisms by which EVs employ their pathological effects are not yet understood. The EVsT seem to participate in the activation of macrophages via TLR2 triggering the production of cytokines and a range of other molecules, thus modulating the host immune response which promotes the parasite survival. Moreover, new insights have demonstrated that EVsT induce lipid body formation and PGE₂ synthesis in macrophages. This phenomenon is followed by the inhibition of the synthesis of pro-inflammatory cytokines and antigen presentation, causing decreased parasitic molecules and allowing intracellular parasite survival. Therefore, this mini review aims to discuss the role of the EVs from T. cruzi as well as its involvement in the mechanisms that regulate the host immune response in the lipid metabolism and its significance for the Chagas disease pathophysiology.

Host factors subverted by Mycobacterium tuberculosis: Potential targets for host directed therapy

INTRODUCTION

Despite new approaches in the diagnosis and treatment of tuberculosis (TB), it continues to be a major health burden. Several immunotherapies that potentiate the immune response have come up as adjuncts to drug therapies against drug resistant TB strains; however, there needs to be an urgent appraisal of host specific drug targets for improving their clinical management and to curtail disease progression. Presently, various host directed therapies (HDTs) exist (repurposed drugs, nutraceuticals, monoclonal antibodies and immunomodulatory agents), but these mostly address molecules that combat disease progression.

AREAS COVERED

The current review discusses major Mycobacterium tuberculosis (M. tuberculosis) survival paradigms inside the host and presents a plethora of host targets subverted by M. tuberculosis which can be further explored for future HDTs. The host factors unique to M. tuberculosis infection (in humans) have also been identified through an in-silico interaction mapping.

EXPERT OPINION

HDTs could become the next-generation adjunct therapies in order to counter antimicrobial resistance and virulence, as well as to reduce the duration of existing TB treatments. However, current scientific efforts are largely directed toward combatants rather than host molecules co-opted by M. tuberculosis for its survival. This might drive the immune system to a hyper-inflammatory condition; therefore, we emphasize that host factors subverted by M. tuberculosis, and their subsequent neutralization, must be considered for development of better HDTs.

Lipid droplets diversity and functions in inflammation and immune response

INTRODUCTION

Lipid droplets (LDs) are dynamic and evolutionary conserved lipid-enriched organelles composed of a core of neutral lipids surrounded by a monolayer of phospholipids associated with a diverse array of proteins that are cell- and stimulus-regulated. Far beyond being simply a deposit of neutral lipids, accumulating evidence demonstrate that LDs act as spatial and temporal local for lipid and protein compartmentalization and signaling organization.

AREAS COVERED

This review focuses on the progress in our understanding of LD protein diversity and LD functions in the context of cell signaling and immune responses, highlighting the relationship between LD composition with the multiple roles of this organelle in immunometabolism, inflammation and host-response to infection.

EXPERT OPINION

LDs are essential platforms for various cellular processes, including metabolic regulation, cell signaling, and immune responses. The functions of LD in infection and inflammatory disease are associated with the dynamic and complexity of their proteome. Our contemporary view place LDs as critical regulators of different inflammatory and infectious diseases and key markers of leukocyte activation.

Mesenchymal Stem Cells and Tuberculosis: Clinical Challenges and Opportunities

Tuberculosis (TB) is one of the communicable diseases caused by Mycobacterium tuberculosis (Mtb) infection, affecting nearly one-third of the world's population. However, because the pathogenesis of TB is still not fully understood and the development of anti-TB drug is slow, TB remains a global public health problem. In recent years, with the gradual discovery and confirmation of the immunomodulatory properties of mesenchymal stem cells (MSCs), more and more studies, including our team's research, have shown that MSCs seem to be closely related to the growth status of Mtb and the occurrence and development of TB, which is expected to bring new hope for the clinical treatment of TB. This article reviews the relationship between MSCs and the occurrence and development of TB and the potential application of MSCs in the treatment of TB.

Vitamin D receptor activated by vitamin D administration alleviates Mycobacterium tuberculosis-induced bone destruction by inhibiting NFκB-mediated aberrant osteoclastogenesis

Clinically, bone destruction caused by Mycobacterium tuberculosis was serious especially in patients with vitamin D (VD) deficiency. However, the role of VD in M. tuberculosis-induced bone destruction remains clear. In this context, we investigate the role of VD and vitamin D receptor (VDR) in the M. tuberculosis-induced bone destruction. First, we infected RAW264.7 and bone marrow-derived macrophages (BMMs) with Mycobacterium bovis Bacillus Calmette-Guérin (M. bovis BCG) in vitro. Then, we activated VDR through VD administration. TRAP and FAK staining, bone resorption assays, immunofluorescence staining, qPCR, and western blot were carried out. In vivo, the M. tuberculosis-induced osteolytic model on the murine skull was established and the μCT and histological analyses were performed. We found that VDR and TRAP were upregulated in bone tuberculosis tissue and proved that M. tuberculosis infection promoted osteoclastogenesis in RAW264.7 and BMMs. VD could inhibit osteoclasts differentiation, fusion, and bone resorption dose-dependently. However, when VDR was knocked down, the inhibitory effect of VD on osteoclasts disappeared. In mechanism, activation of VDR inhibits the phosphorylation of IκB α, thereby inhibiting NFκB signaling pathway and alleviating osteoclastogenesis. Furthermore, in the skull osteolysis model, VD administration reduced osteolysis, but not in VDR^-/- mice. Our study, for the first time, demonstrates that activation of VDR by VD administration inhibits M. tuberculosis-induced bone destruction. Our results reveal that VD and VDR are potential therapeutic targets for M. tuberculosis-induced bone destruction, and are of great clinical significance for the development of new therapeutic strategies.

Essential Roles of PPARs in Lipid Metabolism during Mycobacterial Infection

The mycobacterial cell wall is composed of large amounts of lipids with varying moieties. Some mycobacteria species hijack host cells and promote lipid droplet accumulation to build the cellular environment essential for their intracellular survival. Thus, lipids are thought to be important for mycobacteria survival as well as for the invasion, parasitization, and proliferation within host cells. However, their physiological roles have not been fully elucidated. Recent studies have revealed that mycobacteria modulate the peroxisome proliferator-activated receptor (PPAR) signaling and utilize host-derived triacylglycerol (TAG) and cholesterol as both nutrient sources and evasion from the host immune system. In this review, we discuss recent findings that describe the activation of PPARs by mycobacterial infections and their role in determining the fate of bacilli by inducing lipid metabolism, anti-inflammatory function, and autophagy.

The role of lipid droplets in microbial pathogenesis

The nonpolar lipids present in cells are mainly triacylglycerols and steryl esters. When cells are provided with an abundance of nutrients, these storage lipids accumulate. As large quantities of nonpolar lipids cannot be integrated into membranes, they are isolated from the cytosolic environment in lipid droplets. As specialized, inducible cytoplasmic organelles, lipid droplets have functions beyond the regulation of lipid metabolism, in cell signalling and activation, membrane trafficking and control of inflammatory mediator synthesis and secretion. Pathogens, including fungi, viruses, parasites, or intracellular bacteria can induce and may benefit from lipid droplets in infected cells. Here we review biogenesis of lipid droplets as well as the role of lipid droplets in the pathogenesis of selected viruses, bacteria, protists and yeasts.

Accessing BCG in infected macrophages by antibody-mediated drug delivery system and tracking by surface-enhanced Raman scattering spectroscopy

Gold nanoparticles (AuNP) modified with antibody and rifampicin (RP) were tested against Mycobacterium bovis Bacillus Calmette-Guérin (BCG), which previously generated in vitro infection of macrophages from mice. Such a drug delivery system works as nanocarrier for RP and presented lower toxicity for macrophages cells than each separated component. Surface-enhanced Raman scattering (SERS) spectroscopy and fluorescence microscopy were used as analytical tools for the characterization of the internalization of gold nanocarriers into macrophage cells. The effective antibiotic action of RP, when combined with gold nanocarrier, was confirmed by dead-live assay of BCG bacteria lysed from macrophages after incubation. Such results indicate the delivery of RP to BCG bacteria, which were infecting macrophages, occurred with remarkable efficiency. It was rationalized based on the strategy used for the adsorption of antibody molecules on gold surface.

Not too fat to fight: The emerging role of macrophage fatty acid metabolism in immunity to Mycobacterium tuberculosis

While the existence of a special relationship between Mycobacterium tuberculosis (Mtb) and host lipids has long been known, it remains a challenging enigma. It was clearly established that Mtb requires host fatty acids (FAs) and cholesterol to produce energy, build its distinctive lipid-rich cell wall, and produce lipid virulence factors. It was also observed that in infected hosts, Mtb constantly resides in a FA-rich environment that the pathogen contributes to generate by inducing a lipid-laden "foamy" phenotype in host macrophages. These observations and the proximity between lipid droplets and phagosomes containing bacteria within infected macrophages gave rise to the hypothesis that Mtb reprograms host cell lipid metabolism to ensure a continuous supply of essential nutrients and its long-term persistence in vivo. However, recent studies question this principle by indicating that in Mtb-infected macrophages, lipid droplet formation prevents bacterial acquisition of host FAs while supporting the production of FA-derived protective lipid mediators. Further, in vivo investigations reveal discrete macrophage phenotypes linking the FA metabolisms of host cell and intracellular pathogen. Notably, FA storage within lipid droplets characterizes both macrophages controlling Mtb infection and dormant intracellular Mtb. In this review, we integrate findings from immunological and microbiological studies illustrating the new concept that cytoplasmic accumulation of FAs is a metabolic adaptation of macrophages to Mtb infection, which potentiates their antimycobacterial responses and forces the intracellular pathogen to shift into fat-saving, survival mode.

Novel therapeutic evaluation biomarkers of lipid metabolism targets in uncomplicated pulmonary tuberculosis patients

Currently, the management of pulmonary tuberculosis (TB) lacks potent medications and accurate efficacy evaluation biomarkers. In view of the fact that the host lipids are the important energy source of Mycobacterium tuberculosis (Mtb), UPLC-MS/MS based on lipid metabolism was used to monitor the plasma lipid spectrum of TB patients from the initial diagnosis to cured. The analysis showed that TB patients presented aberrant metabolism of phospholipids, glycerides, and sphingolipids. Upon the treatment, the abnormal expression of Cer (d18:1/24:0), CerP (d18:1/20:3), LPE (0:0/22:0), LPA (0:0/16:0), and LPA (0:0/18:0) in TB patients were gradually normalized, indicating that the intervention of lipid metabolism could block energy metabolism and inhibit the cell wall synthesis of Mtb. Furthermore, the increase in ceramide (Cer) levels could promote autophagosome-lysosome fusion. LPA (0:0/16:0) and LPA (0:0/18:0) had a great potential in the early diagnosis (both sensitivity and specificity were 100%) and efficacy evaluation (both sensitivity and specificity were 100%) of TB, indicating that the above lipid metabolites could be used as potential biomarkers for TB.

Macrophage metabolic reprogramming during chronic lung disease

Airway macrophages (AMs) play key roles in the maintenance of lung immune tolerance. Tissue tailored, highly specialised and strategically positioned, AMs are critical sentinels of lung homoeostasis. In the last decade, there has been a revolution in our understanding of how metabolism underlies key macrophage functions. While these initial observations were made during steady state or using in vitro polarised macrophages, recent studies have indicated that during many chronic lung diseases (CLDs), AMs adapt their metabolic profile to fit their local niche. By generating reactive oxygen species (ROS) for pathogen defence, utilising aerobic glycolysis to rapidly generate cytokines, and employing mitochondrial respiration to fuel inflammatory responses, AMs utilise metabolic reprogramming for host defence, although these changes may also support chronic pathology. This review focuses on how metabolic alterations underlie AM phenotype and function during CLDs. Particular emphasis is given to how our new understanding of AM metabolic plasticity may be exploited to develop AM-focused therapies.

Repression of PPARγ reduces the ABCG2-mediated efflux activity of M2 macrophages

Even though subclasses of macrophage have distinct roles during progression of infectious diseases, it remains poorly understood whether there is a subset-specific difference in drug responses. Here, we report that ABCG2 was expressed specifically in M2-like macrophages and that it controlled their efflux activities. Abcg2 expression is markedly induced during polarization of PMA-primed macrophages toward an M2 type. IL-4 and IL-13 induced Pparg expression through STAT6 and PPARγ in turn acted on the Abcg2 promoter for its transcription activation. Once polarized to M2-like macrophages, these cells had sustained PPARγ transcription activation of Abcg2 gene. Accordingly, interruption of this machinery by T0070907, an inverse agonist of PPARγ, was shown to be effective in Abcg2 downregulation and its efflux activity in M2-like macrophages. Taken together, our results implicate that ABCG2 of M2 macrophages may function as an important pump that plays a potential role in drug efflux and that T0070907 may be used to increase the efficacy of M2 macrophage-targeting drugs such as antibiotics.

Targeting immunometabolism in host defence against Mycobacterium tuberculosis

In the face of ineffective vaccines, increasing antibiotic resistance and the decline in new antibacterial drugs in the pipeline, tuberculosis (TB) still remains pandemic. Exposure to Mycobacterium tuberculosis (Mtb), which causes TB, results in either direct elimination of the pathogen, most likely by the innate immune system, or infection and containment that requires both innate and adaptive immunity to form the granuloma. Host defence strategies against infectious diseases are comprised of both host resistance, which is the ability of the host to prevent invasion or to eliminate the pathogen, and disease tolerance, which is defined by limiting the collateral tissue damage. In this review, we aim to examine the metabolic demands of the immune cells involved in both host resistance and disease tolerance, chiefly the macrophage and T-lymphocyte. We will further discuss how baseline metabolic heterogeneity and inflammation-driven metabolic reprogramming during infection are linked to their key immune functions containing mycobacterial growth and instructing protective immunity. Targeting key players in immune cellular metabolism may provide a novel opportunity for treatments at different stages of TB disease.

The function of peroxisome proliferator-activated receptors PPAR-γ and PPAR-δ in Mycobacterium leprae-induced foam cell formation in host macrophages

Leprosy is a chronic infectious disease caused by Mycobacterium leprae (M. leprae). In lepromatous leprosy (LL), skin macrophages, harboring extensive bacterial multiplication, gain a distinctive foamy appearance due to increased intracellular lipid load. To determine the mechanism by which M. leprae modifies the lipid homeostasis in host cells, an in vitro M. leprae infection system, using human macrophage precursor THP-1 cells and M. leprae prepared from the footpads of nude mice, was employed. RNA extracted from skin smear samples of patients was used to investigate host gene expressions before and after multidrug therapy (MDT). We found that a cluster of peroxisome proliferator-activated receptor (PPAR) target genes associated with adipocyte differentiation were strongly induced in M. leprae-infected THP-1 cells, with increased intracellular lipid accumulation. PPAR-δ and PPAR-γ expressions were induced by M. leprae infection in a bacterial load-dependent manner, and their proteins underwent nuclear translocalization after infection, indicating activation of PPAR signaling in host cells. Either PPAR-δ or PPAR-γ antagonist abolished the effect of M. leprae to modify host gene expressions and inhibited intracellular lipid accumulation in host cells. M. leprae-specific gene expressions were detected in the skin smear samples both before and after MDT, whereas PPAR target gene expressions were dramatically diminished after MDT. These results suggest that M. leprae infection activates host PPAR signaling to induce an array of adipocyte differentiation-associated genes, leading to accumulation of intracellular lipids to accommodate M. leprae parasitization. Certain PPAR target genes in skin lesions may serve as biomarkers for monitoring treatment efficacy.

Leprosy is a chronic infectious disease caused by Mycobacterium leprae (M. leprae). Lipid-enriched intracellular environment is important for the parasitization of M. leprae. During anti-leprosy treatment, chemotherapy-killed bacilli can remain in host tissues for a long time, making it difficult to determine the treatment efficacy by Zeihl-Nelson’s staining-based bacterial index (BI) test. In this study, we found that host peroxisome proliferator-activated receptor (PPAR) signaling is responsible for modification of intracellular lipid homeostasis to accommodate M. leprae parasitization in host macrophages. In skin smear samples of patients, M. leprae-derived gene expressions were detected both before and after anti-leprosy treatment, whereas human PPAR target gene expressions were dramatically diminished after the treatment. These results further our understanding of M. leprae intracellular parasitization, and suggest that PPAR signaling may be a novel therapeutic target for treating M. leprae infection and monitoring the expressions of certain PPAR target genes in skin lesions may be helpful to evaluate the treatment efficacy and recurrent infection.

An Army Marches on Its Stomach: Metabolic Intermediates as Antimicrobial Mediators in Mycobacterium tuberculosis Infection

The cells of the immune system are reliant on their metabolic state to launch effective responses to combat mycobacterial infections. The bioenergetic profile of the cell determines the molecular fuels and metabolites available to the host, as well as to the bacterial invader. How cells utilize the nutrients in their microenvironment—including glucose, lipids and amino acids—to sustain their functions and produce antimicrobial metabolites, and how mycobacteria exploit this to evade the immune system is of great interest. Changes in flux through metabolic pathways alters the intermediate metabolites present. These intermediates are beginning to be recognized as key modulators of immune signaling as well as direct antimicrobial effectors, and their impact on tuberculosis infection is becoming apparent. A better understanding of how metabolism impacts immunity to Mycobacterium tuberculosis and how it is regulated and thus can be manipulated will open the potential for novel therapeutic interventions and vaccination strategies.

Host-Directed Therapy in Tuberculosis: Targeting Host Metabolism

Mycobacterium tuberculosis (Mtb) has complex and intricate interactions with host immune cells. Mtb can survive, persist, and grow within macrophages and thereby circumvent detection by the innate immune system. Recently, the field of immunometabolism, which focuses on the link between metabolism and immune function, has provided us with an improved understanding of the role of metabolism in modulating immune function. For example, host immune cells can switch from oxidative phosphorylation to glycolysis in response to infection, a phenomenon known as the Warburg effect. In this state, immune cells are capable of amplifying production of both antimicrobial pro-inflammatory mediators that are critical for the elimination of bacteria. Also, cells undergoing the Warburg effect upregulate production of nitric oxide augment the synthesis of bioactive lipids. In this review, we describe our current understanding of the Warburg effect and discuss its role in promoting host immune responses to Mtb. In most settings, immune cells utilize the Warburg effect to promote inflammation and thereby eliminate invading bacteria; interestingly, Mtb exploits this effect to promote its own survival. A better understanding of the dynamics of metabolism within immune cells together with the specific features that contribute to the pathogenesis of tuberculosis (TB) may suggest potential host-directed therapeutic targets for promoting clearance of Mtb and limiting its survival in vivo.

Inhibition of Fatty Acid Oxidation Promotes Macrophage Control of Mycobacterium tuberculosis

Macrophage activation involves metabolic reprogramming to support antimicrobial cellular functions. How these metabolic shifts influence the outcome of infection by intracellular pathogens remains incompletely understood. Mycobacterium tuberculosis (Mtb) modulates host metabolic pathways and utilizes host nutrients, including cholesterol and fatty acids, to survive within macrophages. We found that intracellular growth of Mtb depends on host fatty acid catabolism: when host fatty acid β-oxidation (FAO) was blocked chemically with trimetazidine, a compound in clinical use, or genetically by deletion of the mitochondrial fatty acid transporter carnitine palmitoyltransferase 2 (CPT2), Mtb failed to grow in macrophages, and its growth was attenuated in mice. Mechanistic studies support a model in which inhibition of FAO generates mitochondrial reactive oxygen species, which enhance macrophage NADPH oxidase and xenophagy activity to better control Mtb infection. Thus, FAO inhibition promotes key antimicrobial functions of macrophages and overcomes immune evasion mechanisms of Mtb.IMPORTANCEMycobacterium tuberculosis (Mtb) is the leading infectious disease killer worldwide. We discovered that intracellular Mtb fails to grow in macrophages in which fatty acid β-oxidation (FAO) is blocked. Macrophages treated with FAO inhibitors rapidly generate a burst of mitochondria-derived reactive oxygen species, which promotes NADPH oxidase recruitment and autophagy to limit the growth of Mtb. Furthermore, we demonstrate the ability of trimetazidine to reduce pathogen burden in mice infected with Mtb. These studies will add to the knowledge of how host metabolism modulates Mtb infection outcomes.

Target the Host, Kill the Bug; Targeting Host Respiratory Immunosuppressive Responses as a Novel Strategy to Improve Bacterial Clearance During Lung Infection

The lung is under constant pressure to protect the body from invading bacteria. An effective inflammatory immune response must be tightly orchestrated to ensure complete clearance of any invading bacteria, while simultaneously ensuring that inflammation is kept under strict control to preserve lung viability. Chronic bacterial lung infections are seen as a major threat to human life with the treatment of these infections becoming more arduous as the prevalence of antibiotic resistance becomes increasingly commonplace. In order to survive within the lung bacteria target the host immune system to prevent eradication. Many bacteria directly target inflammatory cells and cytokines to impair inflammatory responses. However, bacteria also have the capacity to take advantage of and strongly promote anti-inflammatory immune responses in the host lung to inhibit local pro-inflammatory responses that are critical to bacterial elimination. Host cells such as T regulatory cells and myeloid-derived suppressor cells are often enhanced in number and activity during chronic pulmonary infection. By increasing suppressive cell populations and cytokines, bacteria promote a permissive environment suitable for their prolonged survival. This review will explore the anti-inflammatory aspects of the lung immune system that are targeted by bacteria and how bacterial-induced immunosuppression could be inhibited through the use of host-directed therapies to improve treatment options for chronic lung infections.

Rab7 controls lipid droplet-phagosome association during mycobacterial infection

Lipid droplets (LDs) are organelles that have multiple roles in inflammatory and infectious diseases. LD act as essential platforms for immunometabolic regulation, including as sites for lipid storage and metabolism, inflammatory lipid mediator production, and signaling pathway compartmentalization. Accumulating evidence indicates that intracellular pathogens may exploit host LDs as source of nutrients and as part of their strategy to promote immune evasion. Notably, numerous studies have demonstrated the interaction between LDs and pathogen-containing phagosomes. However, the mechanism involved in this phenomenon remains elusive. Here, we observed LDs and PLIN2 surrounding M. bovis BCG-containing phagosomes, which included observations of a bacillus cell surrounded by lipid content inside a phagosome and LAM from mycobacteria co-localizing with LDs; these results were suggestive of exchange of contents between these compartments. By using beads coated with M.tb lipids, we demonstrated that LD-phagosome associations are regulated through the mycobacterial cell wall components LAM and PIM. In addition, we demonstrated that Rab7 and RILP, but not Rab5, localizes to LDs of infected macrophages and observed the presence of Rab7 at the site of interaction with an infected phagosome. Moreover, treatment of macrophages with the Rab7 inhibitor CID1067700 significantly inhibited the association between LDs and LAM-coated beads. Altogether, our data demonstrate that LD-phagosome interactions are controlled by mycobacterial cell wall components and Rab7, which enables the exchange of contents between LDs and phagosomes and may represent a fundamental aspect of bacterial pathogenesis and immune evasion.

Foam Cells: One Size Doesn't Fit All

Chronic inflammation in many infectious and metabolic diseases, and some cancers, is accompanied by the presence of foam cells. These cells form when the intracellular lipid content of macrophages exceeds their capacity to maintain lipid homeostasis. Concurrently, critical macrophage immune functions are diminished. Current paradigms of foam cell formation derive from studies of atherosclerosis. However, recent studies indicate that the mechanisms of foam cell biogenesis during tuberculosis differ from those operating during atherogenesis. Here, we review how foam cell formation and function vary with disease context. Since foam cells are therapeutic targets in atherosclerosis, further research on the disease-specific mechanisms of foam cell biogenesis and function is needed to explore the therapeutic consequences of targeting these cells in other diseases.

Immunometabolism of Phagocytes During Mycobacterium tuberculosis Infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) remains as a leading killer among infectious diseases worldwide. The nature of the host immune response dictates whether the initial Mtb infection is cleared or progresses toward active disease, and is ultimately determined by intricate host-pathogen interactions that are yet to be fully understood. The early immune response to infection is mediated by innate immune cells, including macrophages and neutrophils that can phagocytose Mtb and mount an antimicrobial response. However, Mtb can exploit these innate immune cells for its survival and dissemination. Recently, it has become clear that the immune response and metabolic remodeling are interconnected, which is highlighted by the rapid evolution of the interdisciplinary field of immunometabolism. It has been proposed that the net outcome to Mtb infection—clearance or chronic disease—is likely a result of combined immunologic and metabolic activities of the immune cells. Indeed, host cells activated by Mtb infection have strikingly different metabolic requirements than naïve/non-infected cells. Macrophages activated by Mtb-derived molecules or upon phagocytosis acquire a phenotype similar to M1 with elevated production of pro-inflammatory molecules and rely on glycolysis and pentose phosphate pathway to meet their bioenergetic and metabolic requirements. In these macrophages, oxidative phosphorylation and fatty acid oxidation are dampened. However, the non-infected/naive, M2-type macrophages are anti-inflammatory and derive their energy from oxidative phosphorylation and fatty acid oxidation. Similar metabolic adaptations also occur in other phagocytes, including dendritic cells, neutrophils upon Mtb infection. This metabolic reprogramming of innate immune cells during Mtb infection can differentially regulate their effector functions, such as the production of cytokines and chemokines, and antimicrobial response, all of which can ultimately determine the outcome of Mtb-host interactions within the granulomas. In this review, we describe key immune cells bolstering host innate response and discuss the metabolic reprogramming in these phagocytes during Mtb infection. We focused on the major phagocytes, including macrophages, dendritic cells and neutrophils and the key regulators involved in metabolic reprogramming, such as hypoxia-inducible factor-1, mammalian target of rapamycin, the cellular myelocytomatosis, peroxisome proliferator-activator receptors, sirtuins, arginases, inducible nitric acid synthase and sphingolipids.

Metabolic Programming of Macrophages: Implications in the Pathogenesis of Granulomatous Disease

Metabolic reprogramming is rapidly gaining appreciation in the etiology of immune cell dysfunction in a variety of diseases. Tuberculosis, schistosomiasis, and sarcoidosis represent an important class of diseases characterized by the formation of granulomas, where macrophages are causatively implicated in disease pathogenesis. Recent studies support the incidence of macrophage metabolic reprogramming in granulomas of both infectious and non-infectious origin. These publications identify the mechanistic target of rapamycin (mTOR), as well as the major regulators of lipid metabolism and cellular energy balance, peroxisome proliferator receptor gamma (PPAR-γ) and adenosine monophosphate-activated protein kinase (AMPK), respectively, as key players in the pathological progression of granulomas. In this review, we present a comprehensive breakdown of emerging research on the link between macrophage cell metabolism and granulomas of different etiology, and how parallels can be drawn between different forms of granulomatous disease. In particular, we discuss the role of PPAR-γ signaling and lipid metabolism, which are currently the best-represented metabolic pathways in this context, and we highlight dysregulated lipid metabolism as a common denominator in granulomatous disease progression. This review therefore aims to highlight metabolic mechanisms of granuloma immune cell fate and open up research questions for the identification of potential therapeutic targets in the future.

The interplay between depression and tuberculosis

Depression is a major mental health condition and is expected be the most debilitating and widespread health disorder by 2030. Tuberculosis (TB) is also a leading cause of morbidity and mortality worldwide and interestingly, is a common comorbidity of depression. As such, much attention has been paid to the association between these 2 pathologies. Based on clinical reports, the association between TB and depression seems to be bidirectional, with a substantial overlap in symptoms between the 2 conditions. TB infection or reactivation may precipitate depression, likely as a consequence of the host's inflammatory response and/or dysregulation of the hypothalamic-pituitary-adrenal axis. Nevertheless, few studies have considered whether patients with depression are at a higher risk for TB. In this review, we discuss the hypotheses on the association between depression and TB, highlighting the immuno-inflammatory response and lipid metabolism as potential mechanisms. Improving our understanding of the interplay between these 2 disorders should help guide TB clinical care and prevention both in patients with comorbid depression and in the general population.

Fat, fight, and beyond: The multiple roles of lipid droplets in infections and inflammation

Increased accumulation of cytoplasmic lipid droplets (LDs) in host nonadipose cells is commonly observed in response to numerous infectious diseases, including bacterial, parasite, and fungal infections. LDs are lipid-enriched, dynamic organelles composed of a core of neutral lipids surrounded by a monolayer of phospholipids associated with a diverse array of proteins that are cell and stimulus regulated. Far beyond being simply a deposit of neutral lipids, LDs have come to be seen as an essential platform for various cellular processes, including metabolic regulation, cell signaling, and the immune response. LD participation in the immune response occurs as sites for compartmentalization of several immunometabolic signaling pathways, production of inflammatory lipid mediators, and regulation of antigen presentation. Infection-driven LD biogenesis is a complexly regulated process that involves innate immune receptors, transcriptional and posttranscriptional regulation, increased lipid uptake, and new lipid synthesis. Accumulating evidence demonstrates that intracellular pathogens are able to exploit LDs as an energy source, a replication site, and/or a mechanism of immune response evasion. Nevertheless, LDs can also act in favor of the host as part of the immune and inflammatory response to pathogens. Here, we review recent findings that explored the new roles of LDs in the context of host-pathogen interactions.

Mycobacterium tuberculosis and macrophage nuclear receptors: What we do and don't know

Nuclear receptors (NRs) are ligand-activated transcription factors that are expressed in a wide variety of cells and play a major role in lipid signaling. NRs are key regulators of immune and metabolic functions in macrophages and are linked to macrophage responses to microbial pathogens. Pathogens are also known to induce the expression of specific NRs to promote their own survival. In this review, we focus on the NRs recently shown to influence macrophage responses to Mycobacterium tuberculosis (M.tb), a significant cause of morbidity and mortality worldwide. We provide an overview of NR-controlled transcriptional activity and regulation of macrophage activation. We also discuss in detail the contribution of specific NRs to macrophage responses to M.tb, including influence on macrophage phenotype, cell signaling, and cellular metabolism. We pay particular attention to PPARγ since it is required for differentiation of alveolar macrophages, an important niche for M.tb, and its role during M.tb infection is becoming increasingly appreciated. Research into NRs and M.tb is still in its early stages, therefore continuing to advance our understanding of the complex interactions between M.tb and macrophage NRs may reveal the potential of NRs as pharmacological targets for the treatment of tuberculosis.

Advances in Cardiovascular Disease Lipid Research Can Provide Novel Insights Into Mycobacterial Pathogenesis

Cardiovascular disease (CVD) is the leading cause of death in industrialized nations and an emerging health problem in the developing world. Systemic inflammatory processes associated with alterations in lipid metabolism are a major contributing factor that mediates the development of CVDs, especially atherosclerosis. Therefore, the pathways promoting alterations in lipid metabolism and the interplay between varying cellular types, signaling agents, and effector molecules have been well-studied. Mycobacterial species are the causative agents of various infectious diseases in both humans and animals. Modulation of host lipid metabolism by mycobacteria plays a prominent role in its survival strategy within the host as well as in disease pathogenesis. However, there are still several knowledge gaps in the mechanistic understanding of how mycobacteria can alter host lipid metabolism. Considering the in-depth research available in the area of cardiovascular research, this review presents an overview of the parallel areas of research in host lipid-mediated immunological changes that might be extrapolated and explored to understand the underlying basis of mycobacterial pathogenesis.

Lipid Droplets: A Significant but Understudied Contributor of Host⁻Bacterial Interactions

Lipid droplets (LDs) are cytosolic lipid storage organelles that are important for cellular lipid metabolism, energy homeostasis, cell signaling, and inflammation. Several bacterial, viral and protozoal pathogens exploit host LDs to promote infection, thus emphasizing the importance of LDs at the host–pathogen interface. In this review, we discuss the thus far reported relation between host LDs and bacterial pathogens including obligate and facultative intracellular bacteria, and extracellular bacteria. Although there is less evidence for a LD–extracellular bacterial interaction compared to interactions with intracellular bacteria, in this review, we attempt to compare the bacterial mechanisms that target LDs, the host signaling pathways involved and the utilization of LDs by these bacteria. Many intracellular bacteria employ unique mechanisms to target host LDs and potentially obtain nutrients and lipids for vacuolar biogenesis and/or immune evasion. However, extracellular bacteria utilize LDs to either promote host tissue damage or induce host death. We also identify several areas that require further investigation. Along with identifying LD interactions with bacteria besides the ones reported, the precise mechanisms of LD targeting and how LDs benefit pathogens should be explored for the bacteria discussed in the review. Elucidating LD–bacterial interactions promises critical insight into a novel host–pathogen interaction.

Host-directed therapies to combat tuberculosis and associated non-communicable diseases

Mycobacterium tuberculosis (Mtb) has coevolved with a human host to evade and exploit the immune system in multiple ways. Mtb is an enormously successful human pathogen that can remain undetected in hosts for decades without causing clinical disease. While tuberculosis (TB) represents a perfect prototype of host-pathogen interaction, it remains a major challenge to develop new therapies to combat mycobacterial infections. Additionally, recent studies emphasize on comorbidity of TB with different non-communicable diseases (NCDs), highlighting the impact of demographic and lifestyle changes on the global burden of TB. In the recent past, host-directed therapies have emerged as a novel and promising approach to treating TB. Drugs modulating host responses are likely to avoid the development of bacterial resistance which is a major public health concern for TB treatment. Interestingly, many of these drugs also form treatment strategies for non-communicable diseases. In general, technological advances along with novel host-directed therapies may open an exciting and promising research area, which can eventually deliver effective TB treatment as well as curtail the emergent synergy with NCDs.

Macrophage nuclear receptors: Emerging key players in infectious diseases

Nuclear receptors (NRs) are ligand-activated transcription factors that are expressed in a variety of cells, including macrophages. For decades, NRs have been therapeutic targets because their activity can be pharmacologically modulated by specific ligands and small molecule inhibitors. NRs regulate a variety of processes, including those intersecting metabolic and immune functions, and have been studied in regard to various autoimmune diseases. However, the complex roles of NRs in host response to infection are only recently being investigated. The NRs peroxisome proliferator-activated receptor γ (PPARγ) and liver X receptors (LXRs) have been most studied in the context of infectious diseases; however, recent work has also linked xenobiotic pregnane X receptors (PXRs), vitamin D receptor (VDR), REV-ERBα, the nuclear receptor 4A (NR4A) family, farnesoid X receptors (FXRs), and estrogen-related receptors (ERRs) to macrophage responses to pathogens. Pharmacological inhibition or antagonism of certain NRs can greatly influence overall disease outcome, and NRs that are protective against some diseases can lead to susceptibility to others. Targeting NRs as a novel host-directed treatment approach to infectious diseases appears to be a viable option, considering that these transcription factors play a pivotal role in macrophage lipid metabolism, cholesterol efflux, inflammatory responses, apoptosis, and production of antimicrobial byproducts. In the current review, we discuss recent findings concerning the role of NRs in infectious diseases with an emphasis on PPARγ and LXR, the two most studied. We also highlight newer work on the activity of emerging NRs during infection.

Crosstalk between Peroxisome Proliferator-Activated Receptors and Toll-Like Receptors: A Systematic Review

As one of the four major families of pattern recognition receptors (PRRs), toll like receptors (TLRs) are crucial and important components of the innate immune system. Peroxisome proliferatoractivated receptors (PPARs) with three isoforms are transcription factors classified as a subfamily of nuclear receptor proteins, and are of significant regulatory activity in cellular differentiation, development, metabolism, and tumorigenesis. It is well established that PPARs agonists display anti-inflammatory effects through inhibition of the nuclear factor-kappa B (NF-κB) pathway, a key regulator of immune and inflammatory responses, in a sense that TLRs signaling pathways are mainly toward activation of NF-κB. Through a systematic review of previous studies, we aimed to address and clarify the reciprocal interaction between TLRs and PPARs in hope to find alternative therapeutic approaches for inflammatory diseases. Among the available scientific database, 31 articles were selected for this review. A comprehensive review of this database confirms the presence of a cross-talk between PPARs and TLRs, indicating that not only PPARs stimulation may affect the expression level of TLRs via several mechanisms leading to modulating TLRs activities, but also TLRs have the potential to moderate the expression of PPARs. We, therefore, conclude that, as a key regulator of the innate immune system, the interaction between PPARs and TLRs is a potential therapeutic target in disease treatment.

Lipid metabolism and intracellular bacterial virulence: key to next-generation therapeutics

Lipid metabolism is thought to play a key role in the pathogenicity of several intracellular bacteria. Bacterial lipolytic enzymes hydrolyze lipids from the host cell to release free fatty acids which are used as an energy source and building blocks for the synthesis of cell envelope and also to modulate host immune responses. In this review, we discussed the role of lipid metabolism and lipolytic enzymes in the life cycle and virulence of Mycobacterium tuberculosis and other intracellular bacteria. The lipolytic enzymes appear to be potential candidates for developing novel therapeutics by targeting lipid metabolism for controlling M. tuberculosis and other intracellular pathogenic bacteria.

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Storage lipid studies in tuberculosis reveal that foam cell biogenesis is disease-specific

Foam cells are lipid-laden macrophages that contribute to the inflammation and tissue damage associated with many chronic inflammatory disorders. Although foam cell biogenesis has been extensively studied in atherosclerosis, how these cells form during a chronic infectious disease such as tuberculosis is unknown. Here we report that, unlike the cholesterol-laden cells of atherosclerosis, foam cells in tuberculous lung lesions accumulate triglycerides. Consequently, the biogenesis of foam cells varies with the underlying disease. In vitro mechanistic studies showed that triglyceride accumulation in human macrophages infected with Mycobacterium tuberculosis is mediated by TNF receptor signaling through downstream activation of the caspase cascade and the mammalian target of rapamycin complex 1 (mTORC1). These features are distinct from the known biogenesis of atherogenic foam cells and establish a new paradigm for non-atherogenic foam cell formation. Moreover, they reveal novel targets for disease-specific pharmacological interventions against maladaptive macrophage responses.

PGL I expression in live bacteria allows activation of a CD206/PPARγ cross-talk that may contribute to successful Mycobacterium leprae colonization of peripheral nerves

Mycobacterium leprae, an obligate intracellular bacillus, infects Schwann cells (SCs), leading to peripheral nerve damage, the most severe leprosy symptom. In the present study, we revisited the involvement of phenolic glycolipid I (PGL I), an abundant, private, surface M. leprae molecule, in M. leprae-SC interaction by using a recombinant strain of M. bovis BCG engineered to express this glycolipid. We demonstrate that PGL I is essential for bacterial adhesion and SC internalization. We also show that live mycobacterium-producing PGL I induces the expression of the endocytic mannose receptor (MR/CD206) in infected cells in a peroxisome proliferator-activated receptor gamma (PPARγ)-dependent manner. Of note, blocking mannose recognition decreased bacterial entry and survival, pointing to a role for this alternative recognition pathway in bacterial pathogenesis in the nerve. Moreover, an active crosstalk between CD206 and the nuclear receptor PPARγ was detected that led to the induction of lipid droplets (LDs) formation and prostaglandin E2 (PGE2), previously described as fundamental players in bacterial pathogenesis. Finally, this pathway was shown to induce IL-8 secretion. Altogether, our study provides evidence that the entry of live M. leprae through PGL I recognition modulates the SC phenotype, favoring intracellular bacterial persistence with the concomitant secretion of inflammatory mediators that may ultimately be involved in neuroinflammation.

Nerve damage is the most severe symptom of leprosy, an ancient disease that continues to be a major health problem in several countries. Nerve damage is due to the ability of Mycobacterium leprae, the etiologic agent, to invade SCs, the glial cells of the peripheral nervous system. Understanding the molecular basis of M. leprae–SC interaction is essential for the creation of new tools aiming to treat and, above all, prevent leprosy neuropathy. This study demonstrates the critical role of PGL I, an M. leprae-abundant specific cell wall lipid, in establishing infection. PGL I is not only a prerequisite in initiating bacterial adhesion to and subsequent invasion of SCs, but also for changing the repertoire of cell surface proteins to allow for the entrance of bacteria via alternative pathways. These new invasive pathways induced by PGL I involve recognition of other bacterial cell surface glycolipids that, in turn, evoke functional changes in the infected cell, including the accumulation of host cell-derived lipids, which favor bacterial survival. These pathways also promote the secretion of inflammatory mediators that may contribute to nerve damage. In an era of translational-oriented research, exploring these receptors in depth could lead to the development of attractive strategies to ensure the targeted intracellular delivery of therapeutics aiming to prevent neuropathy.