Pathogen destruction versus intracellular survival: the role of lipids as phagosomal fate determinants

An antibiotic-free platform for eliminating persistent Helicobacter pylori infection without disrupting gut microbiota

Helicobacter pylori (H. pylori) infection remains the leading cause of gastric adenocarcinoma, and its eradication primarily relies on the prolonged and intensive use of two antibiotics. However, antibiotic resistance has become a compelling health issue, leading to H. pylori eradication treatment failure worldwide. Additionally, the powerlessness of antibiotics against biofilms, as well as intracellular H. pylori and the long-term damage of antibiotics to the intestinal microbiota, have also created an urgent demand for antibiotic-free approaches. Herein, we describe an antibiotic-free, multifunctional copper-organic framework (HKUST-1) platform encased in a lipid layer comprising phosphatidic acid (PA), rhamnolipid (RHL), and cholesterol (CHOL), enveloped in chitosan (CS), and loaded in an ascorbyl palmitate (AP) hydrogel: AP@CS@Lip@HKUST-1. This platform targets inflammatory sites where H. pylori aggregates through electrostatic attraction. Then, hydrolysis by matrix metalloproteinases (MMPs) releases CS-encased nanoparticles, disrupting bacterial urease activity and membrane integrity. Additionally, RHL disperses biofilms, while PA promotes lysosomal acidification and activates host autophagy, enabling clearance of intracellular H. pylori. Furthermore, AP@CS@Lip@HKUST-1 alleviates inflammation and enhances mucosal repair through delayed Cu2+ release while preserving the intestinal microbiota. Collectively, this platform presents an advanced therapeutic strategy for eradicating persistent H. pylori infection without inducing drug resistance.

Acidification of the phagosome orchestrates the motor forces directing its transport

Phagosomes are dynamic organelles formed by macrophages to capture and destroy microbial pathogens. Phagosome transport from the cell periphery to the perinuclear region, is essential for fusion with lysosomes and the elimination of pathogens. Molecular motors, kinesin and dynein, generate opposing forces, transporting the phagosome away from and towards the lysosome, respectively. Luminal acidification plays a crucial role in determining the net directional movement of the phagosome. The mechanics of this regulation are not known. In this study, we used the sodium proton exchanger NHE9 to selectively modulate phagosomal acidification in macrophages. We then investigated its impact on the mechanical properties of kinesin and dynein motors through optical trapping experiments. We observed a negative correlation between the tenacity of dynein motors and pH under high resistive forces. Reduced luminal acidification impaired generation of dynein cooperative forces, which are crucial for transporting the phagosome to the lysosome. Conversely, the kinesin-powered motility of phagosomes is enabled by a decrease in phagosomal acidification. Given the various methods pathogens employ to limit phagosomal acidification, our findings are highly significant in the context of host-pathogen interactions.

Macrophage Function Modulated by tPA Signaling in Mouse Experimental Kidney Disease Models

Macrophage infiltration and accumulation is a hallmark of chronic kidney disease. Tissue plasminogen activator (tPA) is a serine protease regulating the homeostasis of blood coagulation, fibrinolysis, and matrix degradation, and has been shown to act as a cytokine to trigger various receptor-mediated intracellular signal pathways, modulating macrophage function in response to kidney injury. In this review, we discuss the current understanding of tPA-modulated macrophage function and underlying signaling mechanisms during kidney fibrosis and inflammation.

Liposome-based nanoparticles impact on regulatory and effector phenotypes of macrophages and T cells in multiple Sclerosis patients

Current available treatments of Multiple Sclerosis (MS) reduce neuroinflammation acting on different targets on the immune system, but potentially lead to severe side effects and have a limited efficacy in slowing the progression of the disease. Here, we evaluated in vitro the immunomodulatory potential of a new class of nanoparticles - liposomes, constituted by a double-layer of phosphatidylserine (PSCho/PS), and double-faced, with an outer layer of phosphatidylserine and an inner layer of phosphatidic acid (PSCho/PA), either alone or in the presence of the myelin basic protein (MBP) peptide (residues 85-99) (PSCho/PS-MBP and PSCho/PA-MBP). Results showed that PSCho/PS are equally and efficiently internalized by pro- and anti-inflammatory macrophages (M1 and M2 respectively), while PSCho/PA were internalized better by M2 than M1. PSCho/PS liposomes were able to inhibit the secretion of innate pro-inflammatory cytokine IL-1β. PSCho/PS liposomes expanded Tregs, reducing Th1 and Th17 cells, while PSCho/PA liposomes were unable to dampen pro-inflammatory T cells and to promote immune-regulatory phenotype (Treg). The ability of PSCho/PS liposomes to up-regulate Treg cells was more pronounced in MS patients with high basal expression of M2 markers. PSCho/PS liposomes were more effective in decreasing Th1 (but not Th17) cells in MS patients with a disease duration >3 months. On the other hand, down-modulation of Th17 cells was evident in MS patients with active, Gadolinium enhancing lesions at MRI and in MS patients with a high basal expression of M1-associated markers in the monocytes. The same findings were observed for the modulation of MBP-driven Th1/Th17/Treg responses. These observations suggest that early MS associate to a hard-wired pro-Th1 phenotype of M1 that is lost later during disease course. On the other hand, acute inflammatory events reflect a temporary decrease of M2 phenotype that however is amenable to restauration upon treatment with PSCho/PS liposomes. Thus, together these data indicate that monocytes/macrophages may play an important regulatory function during MS course and suggest a role for PSCho/PS and PSCho/PS-MBP as new therapeutic tools to dampen the pro-inflammatory immune responses and to promote its regulatory branch.

Mycobacterium abscessus: It's Complex

Mycobacterium abscessus (M. abscessus) is an opportunistic pathogen usually colonizing abnormal lung airways and is often seen in patients with cystic fibrosis. Currently, there is no vaccine available for M. abscessus in clinical development. The treatment of M. abscessus-related pulmonary diseases is peculiar due to intrinsic resistance to several commonly used antibiotics. The development of either prophylactic or therapeutic interventions for M. abscessus pulmonary infections is hindered by the absence of an adequate experimental animal model. In this review, we outline the critical elements related to M. abscessus virulence mechanisms, host-pathogen interactions, and treatment challenges associated with M. abscessus pulmonary infections. The challenges of effectively combating this pathogen include developing appropriate preclinical animal models of infection, developing proper diagnostics, and designing novel strategies for treating drug-resistant M. abscessus.

The sodium proton exchanger NHE9 regulates phagosome maturation and bactericidal activity in macrophages

Acidification of phagosomes is essential for the bactericidal activity of macrophages. Targeting machinery that regulates pH within the phagosomes is a prominent strategy employed by various pathogens that have emerged as major threats to public health. Nascent phagosomes acquire the machinery for pH regulation through a graded maturation process involving fusion with endolysosomes. In addition, meticulous coordination between proton pumping and leakage mechanisms is crucial for maintaining optimal pH within the phagosome. However, relative to mechanisms involved in acidifying the phagosome lumen, little is known about proton leakage pathways in this organelle. Sodium proton transporter NHE9 is a known proton leakage pathway located on the endosomes. As phagosomes acquire proteins through fusions with endosomes during maturation, NHE9 seemed a promising candidate for regulating proton fluxes on the phagosome. Here, using genetic and biophysical approaches, we show NHE9 is an important proton leakage pathway associated with the maturing phagosome. NHE9 is highly expressed in immune cells, specifically macrophages; however, NHE9 expression is strongly downregulated upon bacterial infection. We show that compensatory ectopic NHE9 expression hinders the directed motion of phagosomes along microtubules and promotes early detachment from the microtubule tracks. As a result, these phagosomes have shorter run lengths and are not successful in reaching the lysosome. In accordance with this observation, we demonstrate that NHE9 expression levels negatively correlate with bacterial survival. Together, our findings show that NHE9 regulates lumenal pH to affect phagosome maturation, and consequently, microbicidal activity in macrophages.

Specific labelling of phagosome-derived vesicles in macrophages with a membrane dye delivered with microfabricated microparticles

Phagocytosis performed by a macrophage involves complex membrane trafficking and reorganization among various membranous cellular structures including phagosomes and vesicles derived from the phagosomes known as phagosome-derived vesicles. The present work reports on development of a technique that allows to specifically label the phagosome-derived vesicles in macrophages with a membrane dye. The technique is based on the use of microfabricated microparticles that are made of a thermosensitive nonbiodegradable polymer poly(N-isopropylacrylamide) (PNIPAM) or its derivative and contain a membrane dye 1,1'-dialkyl-3,3,3',3'-tetramethylindodicarbocyanine (DiI). The microparticles can be phagocytosed by RAW264.7 macrophages into their phagosomes, resulting in formation of intracellular DiI-positive vesicles derived from the phagosomes. The DiI-positive vesicles are motile and acidic; can be stained by fluorescently labelled dextran added in the culture medium; and can accumulate around new phagosomes, indicating that they possess properties of lysosomes. This technique is also applicable to another membrane dye 3,3'-dioctadecyloxacarbocyanine (DiO) and holds great potential to be useful for advancing our understanding of phagocytosis. STATEMENT OF SIGNIFICANCE: Phagocytosis performed by macrophages is a cellular process of great importance to various applications of biomaterials such as drug delivery and medical implantation. This work reports on a technique for characterizing phagocytosis based on the use of poly(N-isopropylacrylamide), which is a major biomaterial with numerous applications. This technique is the first of its kind and has generated an original finding about phagocytosis. In addition to drug delivery and medical implantation, phagocytosis plays critical roles in diseases, injuries and vaccination. This work could thus attract immediate and widespread interests in the field of biomaterials science and engineering.

Liposomes as Adjuvants and Vaccine Delivery Systems

The review considers liposomes as systems of substantial interest as adjuvant carriers in vaccinology due to their versatility and maximal biocompatibility. Research and development on the use of liposomes and lipid nanoparticles to create subunit vaccines for the prevention and treatment of infectious diseases has been going on for several decades. In recent years, the area has seen serious progress due to the improvement of the technology of industrial production of various high-grade lipids suitable for parenteral administration and the emergence of new technologies and equipment for the production of liposomal preparations. When developing vaccines, it is necessary to take into account how the body’s immune system (innate and adaptive immunity) functions. The review briefly describes some of the fundamental mechanisms underlying the mobilization of immunity when encountering an antigen, as well as the influence of liposome carriers on the processes of internalization of antigens by immunocompetent cells and ways of immune response induction. The results of the studies on the interactions of liposomes with antigen-presenting cells in function of the liposome size, charge, and phase state of the bilayer, which depends on the lipid composition, are often contradictory and should be verified in each specific case. The introduction of immunostimulant components into the composition of liposomal vaccine complexes—ligands of the pathogen-associated molecular pattern receptors—permits modulation of the strength and type of the immune response. The review briefly discusses liposome-based vaccines approved for use in the clinic for the treatment and prevention of infectious diseases, including mRNA-loaded lipid nanoparticles. Examples of liposomal vaccines that undergo various stages of clinical trials are presented.

Combined Host- and Pathogen-Directed Therapy for the Control of Mycobacterium abscessus Infection

Mycobacterium abscessus is the etiological agent of severe pulmonary infections in vulnerable patients, such as those with cystic fibrosis (CF), where it represents a relevant cause of morbidity and mortality. Treatment of pulmonary infections caused by M. abscessus remains extremely difficult, as this species is resistant to most classes of antibiotics, including macrolides, aminoglycosides, rifamycins, tetracyclines, and β-lactams. Here, we show that apoptotic body like liposomes loaded with phosphatidylinositol 5-phosphate (ABL/PI5P) enhance the antimycobacterial response, both in macrophages from healthy donors exposed to pharmacological inhibition of cystic fibrosis transmembrane conductance regulator (CFTR) and in macrophages from CF patients, by enhancing phagosome acidification and reactive oxygen species (ROS) production. The treatment with liposomes of wild-type as well as CF mice, intratracheally infected with M. abscessus, resulted in about a 2-log reduction of pulmonary mycobacterial burden and a significant reduction of macrophages and neutrophils in bronchoalveolar lavage fluid (BALF). Finally, the combination treatment with ABL/PI5P and amikacin, to specifically target intracellular and extracellular bacilli, resulted in a further significant reduction of both pulmonary mycobacterial burden and inflammatory response in comparison with the single treatments. These results offer the conceptual basis for a novel therapeutic regimen based on antibiotic and bioactive liposomes, used as a combined host- and pathogen-directed therapeutic strategy, aimed at the control of M. abscessus infection, and of related immunopathogenic responses, for which therapeutic options are still limited. IMPORTANCE Mycobacterium abscessus is an opportunistic pathogen intrinsically resistant to many antibiotics, frequently linked to chronic pulmonary infections, and representing a relevant cause of morbidity and mortality, especially in immunocompromised patients, such as those affected by cystic fibrosis. M. abscessus-caused pulmonary infection treatment is extremely difficult due to its high toxicity and long-lasting regimen with life-impairing side effects and the scarce availability of new antibiotics approved for human use. In this context, there is an urgent need for the development of an alternative therapeutic strategy that aims at improving the current management of patients affected by chronic M. abscessus infections. Our data support the therapeutic value of a combined host- and pathogen-directed therapy as a promising approach, as an alternative to single treatments, to simultaneously target intracellular and extracellular pathogens and improve the clinical management of patients infected with multidrug-resistant pathogens such as M. abscessus.

Mapping Sphingolipid Metabolism Pathways during Phagosomal Maturation

Phagocytosis is an important physiological process, which, in higher organisms, is a means of fighting infections and clearing cellular debris. During phagocytosis, detrimental foreign particles (e.g. pathogens and apoptotic cells) are engulfed by phagocytes (e.g. macrophages), enclosed in membrane-bound vesicles called phagosomes, and transported to the lysosome for eventual detoxification. During this well-choreographed process, the nascent phagosome (also called early phagosome, EP) undergoes a series of spatiotemporally regulated changes in its protein and lipid composition and matures into a late phagosome (LP), which subsequently fuses with the lysosomal membrane to form the phagolysosome. While several elegant proteomic studies have identified the role of unique proteins during phagosomal maturation, the corresponding lipidomic studies are sparse. Recently, we reported a comparative lipidomic analysis between EPs and LPs and showed that ceramides are enriched on the LPs. Further, we found that this ceramide accumulation on LPs was orchestrated by ceramide synthase 2, inhibition of which hampers phagosomal maturation. Following up on this study, here, using biochemical assays, we first show that the increased ceramidase activity on EPs also significantly contributes to the accumulation of ceramides on LPs. Next, leveraging lipidomics, we show that de novo ceramide synthesis does not significantly contribute to the ceramide accumulation on LPs, while concomitant to increased ceramides, glucosylceramides are substantially elevated on LPs. We validate this interesting finding using biochemical assays and show that LPs indeed have heightened glucosylceramide synthase activity. Taken together, our studies provide interesting insights and possible new roles of sphingolipid metabolism during phagosomal maturation.

The loss of enzymatic activity of the PHARC-associated lipase ABHD12 results in increased phagocytosis that causes neuroinflammation

Phagocytosis is an important evolutionary conserved process, essential for clearing pathogens and cellular debris in higher organisms, including humans. This well‐orchestrated innate immunological response is intricately regulated by numerous cellular factors, important amongst which are the immunomodulatory lysophosphatidylserines (lyso‐PSs) and the pro‐apoptotic oxidized phosphatidylserines (PSs) signalling lipids. Interestingly, in mammals, both these signalling lipids are physiologically regulated by the lipase ABHD12, mutations of which cause the human neurological disorder PHARC. Despite the biomedical significance of this lipase, detailed mechanistic studies and the specific contribution of ABHD12 to innate processes like phagocytosis remain poorly understood. Here, by immunohistochemical and immunofluorescence approaches, using the murine model of PHARC, we show, that upon an inflammatory stimulus, activated microglial cells in the cerebellum of mice deficient in ABHD12 have an amoeboid morphology, increased soma size and display heightened phagocytosis activity. We also report that upon an inflammatory stimulus, cerebellar levels of ABHD12 increase to possibly metabolize the heightened oxidized PS levels, temper phagocytosis and, in turn, control neuroinflammation during oxidative stress. Next, to complement these findings, with the use of biochemical approaches in cultured microglial cells, we show that the pharmacological inhibition and/or genetic deletion of ABHD12 results in increased phagocytic uptake in a fluorescent bead uptake assay. Together, our studies provide compelling evidence that ABHD12 plays an important role in regulating phagocytosis in cerebellar microglial cells and provides a possible explanation, as to why human PHARC subjects display neuroinflammation and atrophy in the cerebellum.

Dectin-2 mediates phagocytosis of Lactobacillus paracasei KW3110 and IL-10 production by macrophages

Lactic acid bacteria (LAB) are most generally used as probiotics and some strains of LAB are known to have anti-inflammatory effects. A specific strain of lactic acid bacteria, Lactobacillus paracasei KW3110 (KW3110), activates macrophages to produce interleukin-10 (IL-10), an anti-inflammatory cytokine; however, the biological mechanism remains unclear. In this study, we showed that the amount of incorporated KW3110 into a macrophage cell line, RAW 264.7, was higher than other genetically related strains using fluorescence microscopy. RNA-seq analysis indicated that treatment of macrophages with KW3110 induced Dectin-2 gene expression, which is a pattern recognition receptor, recognizing α-mannose. In addition, antibody treatment and knock down of Dectin-2, or factors downstream in the signaling pathway, decreased the amount of incorporated KW3110 and IL-10 production. Substantial lectin array analysis also revealed that KW3110 had higher binding affinities to lectins, which recognize the carbohydrate chains comprised of α-mannose, than two other LAB. In conclusion, KW3110 is readily incorporated into macrophages, leading to IL-10 production. Dectin-2 mediated the phagocytosis of KW3110 into macrophages and this may be involved with the characteristic carbohydrate chains of KW3110.

Human Corneal Epithelial Cells Internalize Aspergillus flavus Spores by Actin-Mediated Endocytosis

Human corneal epithelial (HCE) cells play a significant role in the innate immune response by secreting cytokines and antimicrobial peptides when they encounter fungal pathogens. But the detailed mechanism of attachment and engulfment of the fungal conidia by HCE cells is not well understood. Here, we show the phagocytosis of Aspergillus flavus conidia by RCB2280 cells and primary HCE cultures using confocal microscopy and proteomic analysis of conidium-containing phagosomes. Phalloidin staining showed actin polymerization, leading to an actin ring around engulfed conidia. Cytochalasin D inhibited the actin-mediated endocytosis of the conidia. Immunolabeling of the early endosomal markers CD71 and early endosomal antigen (EEA1) and the late endosomal markers lysosome-associated membrane protein 1 (LAMP1), Rab7, and cathepsin G showed that endosomal proteins were recruited to the site of conidia and showed maturation of the conidium-containing phagosomes. Lysotracker red DND 99 labeling showed the acidification of the phagosomes containing conidia. Phagosome-specific proteome analysis confirmed the recruitment of various phagosomal and endosomal proteins to the conidium-containing phagosomes. These results show that the ocular surface epithelium contributes actively to antifungal defense by the phagocytosis of invading fungal conidia.

Helicobacter pylori inhibits autophagic flux and promotes its intracellular survival and colonization by down-regulating SIRT1

Helicobacter pylori (H. pylori) is the strong risk factor for a series of gastric pathological changes. Persistent colonization of H. pylori leading to chronic infection is responsible for gastritis and malignancy. Autophagy is an evolutionary conserved process which can protect cells and organisms from bacterial infection. Here, we demonstrated that H. pylori infection induced autophagosome formation but inhibited autophagic flux. SIRT1, a class III histone deacetylase, was down-regulated at both mRNA and protein levels by H. pylori infection in gastric cells. Further investigation showed that the transcriptional factor RUNX3 accounted for down-regulation of SIRT1 in H. pylori-infected gastric cells. SIRT1 promoted autophagic flux in gastric cells and activation of SIRT1 restored the autophagic flux inhibited by H. pylori infection. Furthermore, SIRT1 exerted inhibitory effects on intracellular survival and colonization of H. pylori. And activation of autophagic flux in SIRT1-inhibited gastric cells could significantly reduce intracellular load of H. pylori. Moreover, the relationship between H. pylori infection and SIRT1 expression was identified in clinical specimen. Our findings define the importance of SIRT1 in compromised autophagy induced by H. pylori infection and bacterial intracellular colonization. These results provide evidence that SIRT1 can serve as a therapeutic target to eradicate H. pylori infection.

Label-free screening of biochemical changes in macrophage-like cells following MoS2 exposure using Raman micro-spectroscopy

The emergence of large scale production techniques for 2D particulate materials has dramatically increased their applications potential. Understanding the interactions of biological cells with such particulate material is therefore of paramount importance, both for toxicological assessment and potential biomedical applications. Conventional in-vitro cytological assays commonly record only a single colorimetric end-point, and do not provide an in-depth analysis of how such materials are uptaken and processed within cells. To demonstrate its potential as an alternative, label free approach, confocal Raman micro-spectroscopy has been used to profile the cellular response of macrophage-like immune cells as a result of exposure to a sub-lethal dose of particulate MoS₂, as an example novel 2D material. Particles were seen to be uptaken and trafficked in sub-cellular vesicles, and this sensitive technique allows differences in the biochemical composition of the vesicles to be assessed and monitored as a function of time. Untreated macrophage-like cells contain lipidic vesicles which are found to be relatively rich in the membrane lipid sphingomyelin, key to the process of cell membrane regeneration. After exposure to MoS₂, the particulate material is seen to be invaginated in similar vesicles, the most prominent of which now, however, have spectroscopic signatures which are dominated by those of phosphatidyl family lipids, consistent with the phagocytotic pathway. The lipidic content of cells is seen to increase at all time-points (4, 24 and 72 h). although vesicles composed of sphingomyelin become more prominent again following a prolonged incubation of 72 h to a sub-lethal dose of MoS₂, as the immune cell has processed the particulate material and initiates recovery to a normal/untreated state. This study reveals Raman micro-spectroscopy is an effective method for monitoring cellular responses and evolution of organelle compositions in response to MoS₂ exposure. The additional benefit of using this technique is that cells can be monitored as a function of time, while it can also be used for screening other micro/nano materials for toxicology and/or establishing cell responses.

Development of an In Vitro Membrane Model to Study the Function of EsxAB Heterodimer and Establish the Role of EsxB in Membrane Permeabilizing Activity of Mycobacterium tuberculosis

EsxA and EsxB are secreted as a heterodimer and have been shown to play critical roles in phagosome rupture and translocation of Mycobacterium tuberculosis into the cytosol. Recent in vitro studies have suggested that the EsxAB heterodimer is dissociated upon acidification, which might allow EsxA insertion into lipid membranes. While the membrane permeabilizing activity (MPA) of EsxA has been well characterized in liposomes composed of di-oleoyl-phosphatidylcholine (DOPC), the MPA of EsxAB heterodimer has not been detected through in vitro assays due to its negligible activity with DOPC liposomes. In this study, we established a new in vitro membrane assay to test the MPA activity of N-terminal acetylated EsxA (N-EsxA). We established that a dose-dependent increase in anionic charged lipids enhances the MPA of N-EsxA. The MPA of both N-EsxA and EsxAB were significantly increased with this new liposome system and made it possible to characterize the MPA of EsxAB in more physiologically-relevant conditions. We tested, for the first time, the effect of temperature on the MPA of N-EsxA and EsxAB in this new system. Interestingly, the MPA of N-EsxA was lower at 37 °C than at RT, and on the contrary, the MPA of EsxAB was higher at 37 °C than at RT. Surprisingly, after incubation at 37 °C, the MPA of N-EsxA continuously decreased over time, while MPA of EsxAB remained stable, suggesting EsxB plays a key role in stabilizing N-EsxA to preserve its MPA at 37 °C. In summary, this study established a new in vitro model system that characterizes the MPA of EsxAB and the role of EsxB at physiological-relevant conditions.

An Interplay Between Autophagy and Immunometabolism for Host Defense Against Mycobacterial Infection

Autophagy, an intracellular catabolic pathway featuring lysosomal degradation, is a central component of the host immune defense against various infections including Mycobacterium tuberculosis (Mtb), the pathogen that causes tuberculosis. Mtb can evade the autophagic defense and drive immunometabolic remodeling of host phagocytes. Co-regulation of the autophagic and metabolic pathways may play a pivotal role in shaping the innate immune defense and inflammation during Mtb infection. Two principal metabolic sensors, AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) kinase, function together to control the autophagy and immunometabolism that coordinate the anti-mycobacterial immune defense. Here, we discuss our current understanding of the interplay between autophagy and immunometabolism in terms of combating intracellular Mtb, and how AMPK-mTOR signaling regulates antibacterial autophagy in terms of Mtb infection. We describe several autophagy-targeting agents that promote host antimicrobial defenses by regulating the AMPK-mTOR axis. A better understanding of the crosstalk between immunometabolism and autophagy, both of which are involved in host defense, is crucial for the development of innovative targeted therapies for tuberculosis.

Deletion of the lon gene augments expression of Salmonella Pathogenicity Island (SPI)-1 and metal ion uptake genes leading to the accumulation of bactericidal hydroxyl radicals and host pro-inflammatory cytokine-mediated rapid intracellular clearance

In the present study, we characterized the involvement of Lon protease in bacterial virulence and intracellular survival in Salmonella under abiotic stress conditions resembling the conditions of a natural infection. Wild type (JOL401) and the lon mutant (JOL909) Salmonella Typhimurium were exposed to low temperature, pH, osmotic, and oxidative stress conditions and changes in gene expression profiles related to virulence and metal ion uptake were investigated. Expression of candidate genes invF and hilC of Salmonella Pathogenicity Island (SPI)-1 and sifA and sseJ of SPI-2 revealed that Lon protease controls SPI-1 genes and not SPI-2 genes under all stress conditions tested. The lon mutant exhibited increased accumulation of hydroxyl (OH·) ions that lead to cell damage due to oxidative stress. This oxidative damage can also be linked to an unregulated influx of iron due to the upregulation of ion channel genes such as fepA in the lon mutant. The deletion of lon from the Salmonella genome causes oxidative damage and increased expression of virulence genes. It also prompts the secretion of host pro-inflammatory cytokines leading to early clearance of the bacteria from host cells. We conclude that poor bacterial recovery from mice infected with the lon mutant is a result of disrupted bacterial intracellular equilibrium and rapid activation of cytokine expression leading to bacterial lysis.

Liposomes Loaded With Phosphatidylinositol 5-Phosphate Improve the Antimicrobial Response to Pseudomonas aeruginosa in Impaired Macrophages From Cystic Fibrosis Patients and Limit Airway Inflammatory Response

Despite intensive antimicrobial and anti-inflammatory therapies, cystic fibrosis (CF) patients are subjected to chronic infections due to opportunistic pathogens, including multidrug resistant (MDR) Pseudomonas aeruginosa. Macrophages from CF patients show many evidences of reduced phagocytosis in terms of internalization capability, phagosome maturation, and intracellular bacterial killing. In this study, we investigated if apoptotic body-like liposomes (ABLs) loaded with phosphatidylinositol 5-phosphate (PI5P), known to regulate actin dynamics and vesicular trafficking, could restore phagocytic machinery while limiting inflammatory response in in vitro and in vivo models of MDR P. aeruginosa infection. Our results show that the in vitro treatment with ABL carrying PI5P (ABL/PI5P) enhances bacterial uptake, ROS production, phagosome acidification, and intracellular bacterial killing in human monocyte-derived macrophages (MDMs) with pharmacologically inhibited cystic fibrosis transmembrane conductance regulator channel (CFTR), and improve uptake and intracellular killing of MDR P. aeruginosa in CF macrophages with impaired bactericidal activity. Moreover, ABL/PI5P stimulation of CFTR-inhibited MDM infected with MDR P. aeruginosa significantly reduces NF-κB activation and the production of TNF-α, IL-1β, and IL-6, while increasing IL-10 and TGF-β levels. The therapeutic efficacy of ABL/PI5P given by pulmonary administration was evaluated in a murine model of chronic infection with MDR P. aeruginosa. The treatment with ABL/PI5P significantly reduces pulmonary neutrophil infiltrate and the levels of KC and MCP-2 cytokines in the lungs, without affecting pulmonary bacterial load. Altogether, these results show that the ABL/PI5P treatment may represent a promising host-directed therapeutic approach to improve the impaired phagocytosis and to limit the potentially tissue-damaging inflammatory response in CF.

Is targeting dysregulation in apoptosis splice variants in Mycobacterium tuberculosis (MTB) host interactions and splicing factors resulting in immune evasion by MTB strategies a possibility?

Mycobacterium tuberculosis (Mtb), is one of the foremost organisms causing mortality in humans, and has been for most of human history. When faced with an infection the human immune system is ordinarily very competent in killing both extracellular and intracellular bacilli. However, Mtb is able to evade the host immune system and is even able to establish a persistent infectious reservoir by "hiding" in the immune cells of the host. While the mechanisms by which the bacteria accomplishes this are not fully understood, it is known that the bacterium can subvert cellular processes in cells such as macrophages that prevent the lysis of the bacteria or the cell undergoing apoptosis. They are also able to interfere with immune cell signalling. One of the greatest effects that Mtb has is too alter the transcriptome of the macrophage. An easy way for the bacterium to accomplish this is to alter the alternative splicing patterns of the host. This can lead to a large change in the population of different protein isoforms, some of which have very different functions when compared to the original protein. At the same time the long history of Mtb infecting humans have led to specific immune reactions that occur in the host immune system in order to fight the infection. Many of these specific reactions involve new isoforms of host defence proteins. In this way the human host can use alternate splicing to create new isoforms of immune- related proteins that are more effective in defending against Mtb.

Cellular evasion strategies of Helicobacter pylori in regulating its intracellular fate

Helicobacter pylori colonizes human stomach mucosa and its infection causes gastrointestinal diseases with variable severity. Bacterial infection stimulates autophagy, which is a part of innate immunity used to eliminate intracellular pathogens. Several intracellular bacteria have evolved multipronged strategies to circumvent this conserved system and thereby enhance their chance of intracellular survival. Nonetheless, studies on H. pylori have produced inconsistent results, showing either elevated or reduced clearance efficiency of intracellular bacteria through autophagy. In this review, we summarize recent studies on the mechanisms involved in autophagy induced by H. pylori and the fate of intracellular bacteria.

Lipid markers and related transcripts during excitotoxic neurodegeneration in kainate-treated mice

Lipid homeostasis is dysregulated in some neurodegenerative diseases and after brain injuries due to excess glutamate or lack of oxygen. However the kinetics and cell specificity of dysregulation in different groups of lipids during excitotoxic neuronal death are not clear. Here we examined the changes during excitotoxic neuronal death induced by injecting kainic acid (KA) into the CA1 region of mouse hippocampus. We compared neuronal loss and glial cell proliferation with changes in lipid-related transcripts and markers for different lipid groups, over 12 days after KA-treatment. As neurons showed initial signs of damage, transcripts and proteins linked to fatty acid oxidation were up-regulated. Cholesterol biosynthesis induced by transcripts controlled by the transcription factor Srebp2 seems to be responsible for a transient increase in neuronal free cholesterol at 1 to 2 days. In microglia, but not in neurons, Perilipin-2 associated lipid droplets were induced and properties of Nile red emissions suggest lipid contents change over time. After microglial expression of phagocytotic markers at 2 days, some neutral lipid deposits co-localized with lysosome markers of microglia and were detected within putative phagocytotic cups. These data delineate distinct lipid signals in neurons and glial cells during excitotoxic processes from initial neuronal damage to engagement of the lysosome-phagosome system.

Lipidomics Suggests a New Role for Ceramide Synthase in Phagocytosis

Phagocytosis is an evolutionarily conserved biological process where pathogens or cellular debris are cleared by engulfing them in a membrane-enclosed cellular compartment called the phagosome. The formation, maturation, and subsequent degradation of a phagosome is an important immune response essential for protection against many pathogens. Yet, the global lipid profile of phagosomes remains unknown, especially as a function of their maturation in immune cells. Here, we show using mass spectrometry based quantitative lipidomics that the ceramide class of lipids, especially very long chain ceramides, are enriched on maturing phagosomes with a concomitant decrease in the biosynthetic precursors of ceramides. We thus posit a new function for the enzyme ceramide synthase during phagocytosis in mammalian macrophages. Biochemical assays, cellular lipid feeding experiments, and pharmacological blockade of ceramide synthase together show that this enzyme indeed controls the flux of ceramides on maturing phagosomes. We also find similar results in the primitive eukaryote Dictyostelium discoideum, suggesting that ceramide enrichment may be evolutionarily conserved and likely an indispensible step in phagosome maturation.

Coin Tossing Explains the Activity of Opposing Microtubule Motors on Phagosomes

How the opposing activity of kinesin and dynein motors generates polarized distribution of organelles inside cells is poorly understood and hotly debated [1, 2]. Possible explanations include stochastic mechanical competition [3, 4], coordinated regulation by motor-associated proteins [5-7], mechanical activation of motors [8], and lipid-induced organization [9]. Here, we address this question by using phagocytosed latex beads to generate early phagosomes (EPs) that move bidirectionally along microtubules (MTs) in an in vitro assay [9]. Dynein/kinesin activity on individual EPs is recorded as real-time force generation of the motors against an optical trap. Activity of one class of motors frequently coincides with, or is rapidly followed by opposite motors. This leads to frequent and rapid reversals of EPs in the trap. Remarkably, the choice between dynein and kinesin can be explained by the tossing of a coin. Opposing motors therefore appear to function stochastically and independently of each other, as also confirmed by observing no effect on kinesin function when dynein is inhibited on the EPs. A simple binomial probability calculation based on the geometry of EP-microtubule contact explains the observed activity of dynein and kinesin on phagosomes. This understanding of intracellular transport in terms of a hypothetical coin, if it holds true for other cargoes, provides a conceptual framework to explain the polarized localization of organelles inside cells.

The Multirole of Liposomes in Therapy and Prevention of Infectious Diseases

Liposomes are closed bilayer structures spontaneously formed by hydrated phospholipids that are widely used as efficient delivery systems for drugs or antigens, due to their capability to encapsulate bioactive hydrophilic, amphipathic, and lipophilic molecules into inner water phase or within lipid leaflets. The efficacy of liposomes as drug or antigen carriers has been improved in the last years to ameliorate pharmacokinetics and capacity to release their cargo in selected target organs or cells. Moreover, different formulations and variations in liposome composition have been often proposed to include immunostimulatory molecules, ligands for specific receptors, or stimuli responsive compounds. Intriguingly, independent research has unveiled the capacity of several phospholipids to play critical roles as intracellular messengers in modulating both innate and adaptive immune responses through various mechanisms, including (i) activation of different antimicrobial enzymatic pathways, (ii) driving the fusion–fission events between endosomes with direct consequences to phagosome maturation and/or to antigen presentation pathway, and (iii) modulation of the inflammatory response. These features can be exploited by including selected bioactive phospholipids in the bilayer scaffold of liposomes. This would represent an important step forward since drug or antigen carrying liposomes could be engineered to simultaneously activate different signal transduction pathways and target specific cells or tissues to induce antigen-specific T and/or B cell response. This lipid-based host-directed strategy can provide a focused antimicrobial innate and adaptive immune response against specific pathogens and offer a novel prophylactic or therapeutic option against chronic, recurrent, or drug-resistant infections.

Sphingolipid Metabolism of a Sea Anemone Is Altered by the Presence of Dinoflagellate Symbionts

In host-microbe interactions, signaling lipids function in interpartner communication during both the establishment and maintenance of associations. Previous evidence suggests that sphingolipids play a role in the mutualistic cnidarian-Symbiodinium symbiosis. Exogenously applied sphingolipids have been shown to alter this partnership, though endogenous host regulation of sphingolipids by the sphingosine rheostat under different symbiotic conditions has not been characterized. The rheostat regulates levels of pro-survival sphingosine-1-phosphate (S1P) and pro-apoptotic sphingosine (Sph) through catalytic activities of sphingosine kinase (SPHK) and S1P phosphatase (SGPP). The role of the rheostat in recognition and establishment of cnidarian-Symbiodinium symbiosis was investigated in the sea anemone Aiptasia pallida by measuring gene expression, protein levels, and sphingolipid metabolites in symbiotic, aposymbiotic, and newly recolonized anemones. Comparison of two host populations showed that symbiotic animals from one population had lower SGPP gene expression and Sph lipid concentrations compared to aposymbiotic animals, while the other population had higher S1P concentrations than their aposymbiotic counterparts. In both populations, the host rheostat trended toward host cell survival in the presence of symbionts. Furthermore, upregulation of both rheostat enzymes on the first day of host recolonization by symbionts suggests a role for the rheostat in host-symbiont recognition during symbiosis onset. Collectively, these data suggest a regulatory role of sphingolipid signaling in cnidarian-Symbiodinium symbiosis and symbiont uptake.

Unraveling the role of membrane microdomains during microbial infections

Infectious diseases pose major socioeconomic and health-related threats to millions of people across the globe. Strategies to combat infectious diseases derive from our understanding of the complex interactions between the host and specific bacterial, viral, and fungal pathogens. Lipid rafts are membrane microdomains that play important role in life cycle of microbes. Interaction of microbial pathogens with host membrane rafts influences not only their initial colonization but also their spread and the induction of inflammation. Therefore, intervention strategies aimed at modulating the assembly of membrane rafts and/or regulating raft-directed signaling pathways are attractive approaches for the. management of infectious diseases. The current review discusses the latest advances in terms of techniques used to study the role of membrane microdomains in various pathological conditions and provides updated information regarding the role of membrane rafts during bacterial, viral and fungal infections.

Liposomes loaded with bioactive lipids enhance antibacterial innate immunity irrespective of drug resistance

Phagocytosis is a key mechanism of innate immunity, and promotion of phagosome maturation may represent a therapeutic target to enhance antibacterial host response. Phagosome maturation is favored by the timely and coordinated intervention of lipids and may be altered in infections. Here we used apoptotic body-like liposomes (ABL) to selectively deliver bioactive lipids to innate cells, and then tested their function in models of pathogen-inhibited and host-impaired phagosome maturation. Stimulation of macrophages with ABLs carrying phosphatidic acid (PA), phosphatidylinositol 3-phosphate (PI3P) or PI5P increased intracellular killing of BCG, by inducing phagosome acidification and ROS generation. Moreover, ABLs carrying PA or PI5P enhanced ROS-mediated intracellular killing of Pseudomonas aeruginosa, in macrophages expressing a pharmacologically-inhibited or a naturally-mutated cystic fibrosis transmembrane conductance regulator. Finally, we show that bronchoalveolar lavage cells from patients with drug-resistant pulmonary infections increased significantly their capacity to kill in vivo acquired bacterial pathogens when ex vivo stimulated with PA- or PI5P-loaded ABLs. Altogether, these results provide the proof of concept of the efficacy of bioactive lipids delivered by ABL to enhance phagosome maturation dependent antimicrobial response, as an additional host-directed strategy aimed at the control of chronic, recurrent or drug-resistant infections.

Toxoplasma gondii GRA7-Targeted ASC and PLD1 Promote Antibacterial Host Defense via PKCα

Tuberculosis is a global health problem and at least one-third of the world’s population is infected with Mycobacterium tuberculosis (MTB). MTB is a successful pathogen that enhances its own intracellular survival by inhibiting inflammation and arresting phago-lysosomal fusion. We previously demonstrated that Toxoplasma gondii (T. gondii) dense granule antigen (GRA) 7 interacts with TNF receptor-associated factor 6 via Myeloid differentiation primary response gene 88, enabling innate immune responses in macrophages. To extend these studies, we found that GRA7 interacts with host proteins involved in antimicrobial host defense mechanisms as a therapeutic strategy for tuberculosis. Here, we show that protein kinase C (PKC)α-mediated phosphorylation of T. gondii GRA7-I (Ser52) regulates the interaction of GRA7 with PYD domain of apoptosis-associated speck-like protein containing a carboxy-terminal CARD, which is capable of oligomerization and inflammasome activation can lead to antimicrobial defense against MTB. Furthermore, GRA7-III interacted with the PX domain of phospholipase D1, facilitating its enzyme activity, phago-lysosomal maturation, and subsequent antimicrobial activity in a GRA7-III (Ser135) phosphorylation-dependent manner via PKCα. Taken together, these results underscore a previously unrecognized role of GRA7 in modulating antimicrobial host defense mechanism during mycobacterial infection.

We previously demonstrated that Toxoplasma gondii (T. gondii) dense granule antigen (GRA) 7 interacts with TRAF6 via MyD88, enabling innate immune responses in macrophages and effective protection against T. gondii infection in vivo. However, its exact role and how it regulates host innate immune responses have not been fully explained. Herein, we show that PKCα-mediated phosphorylation of GRA7 is essential for the interaction between GRA7 and ASC or PLD1, which can promote antimicrobial defense against Mycobacterium tuberculosis (MTB). Notably, PKCα specifically phosphorylated Ser52 and Ser135 of GRA7 in vitro and in vivo, indicating that GRA7 is a substrate of PKCα. The N-terminal of GRA7 (GRA7-I) was sufficient for interaction with the PYD domain of ASC, which is capable of ASC oligomerization and inflammasome activation. Furthermore, GRA7-III interacted with the PX domain of PLD1, facilitating its enzyme activity, phago-lysosomal maturation, and subsequent antimicrobial activity in a GRA7 phosphorylation-dependent manner. Interestingly, phosphomimetic mutation in GRA7 overcame the need for PKCα. Collectively, these results provide novel insight into how GRA7 can promote ASC and PLD1 activation in a PKCα-dependent manner as an antimicrobial host defense mechanism.

The sphingosine rheostat is involved in the cnidarian heat stress response but not necessarily in bleaching

Sphingolipids play important roles in mitigating cellular heat and oxidative stress by altering membrane fluidity, receptor clustering and gene expression. Accumulation of signaling sphingolipids that comprise the sphingosine rheostat, pro-apoptotic sphingosine (Sph) and pro-survival sphingosine-1-phosphate (S1P), is key to determining cell fate. Reef-building corals and other symbiotic cnidarians living in shallow tropical waters can experience elevated seawater temperature and high UV irradiance, two stressors that are increasing in frequency and severity with climate change. In symbiotic cnidarians, these stressors disrupt the photosynthetic machinery of the endosymbiont and ultimately result in the collapse of the partnership (dysbiosis), known as cnidarian bleaching. In a previous study, exogenously applied sphingolipids altered heat-induced bleaching in the symbiotic anemone Aiptasia pallida, but endogenous regulation of these lipids is unknown. Here, we characterized the role of the rheostat in the cnidarian heat stress response (HSR) and in dysbiosis. Gene expression of rheostat enzymes sphingosine kinase (AP-SPHK) and S1P phosphatase (AP-SGPP), and concentrations of sphingolipids were quantified from anemones incubated at elevated temperatures. We observed a biphasic HSR in A. pallida. At early exposure, rheostat gene expression and lipid levels were suppressed while gene expression of a heat stress biomarker increased and 40% of symbionts were lost. After longer incubations at the highest temperature, AP-SGPP and then Sph levels both increased. These results indicate that the sphingosine rheostat in A. pallida does not participate in initiation of dysbiosis, but instead functions in the chronic response to prolonged heat stress that promotes host survival.

Lipid - Motor Interactions: Soap Opera or Symphony?

Intracellular transport of organelles can be driven by multiple motor proteins that bind to the lipid membrane of the organelle and work as a team. We review present knowledge on how lipids orchestrate the recruitment of motors to a membrane. Looking beyond recruitment, we also discuss how heterogeneity and local mechanical properties of the membrane may influence function of motor-teams. These issues gain importance because phagocytosed pathogens use lipid-centric strategies to manipulate motors and survive in host cells.

Cytopiloyne, a polyacetylenic glucoside from Bidens pilosa, acts as a novel anticandidal agent via regulation of macrophages

ETHNOPHARMACOLOGICAL RELEVANCE

Bidens pilosa, a tropical and sub-tropical herbal plant, is used as an ethnomedicine for bacterial infection or immune modulation in Asia, America and Africa. It has been demonstrated that cytopiloyne (CP), a bioactive polyacetylenic glucoside purified from B. pilosa, increases the percentage of macrophages in the spleen but the specific effects on macrophages remain unclear.

AIM OF THE STUDY

The aim of this study was to evaluate the effects of CP on macrophage activity and host defense in BALB/c mice with Candida parapsilosis infection and investigate the likely mechanisms.

MATERIALS AND METHODS

RAW264.7 cells, a mouse macrophage cell line, were used to assess the effects of CP on macrophage activity by phagocytosis assay, colony forming assay and acridine orange/crystal violet stain. To evaluate the activity of CP against C. parapsilosis, BALB/c mouse infection models were treated with/without CP and histopathological examination was performed. The role of macrophages in the infection model was clarified by treatment with carrageenan, a selective macrophage-toxic agent. RAW264.7 macrophage activities influenced by CP were further investigated by lysosome staining, phagosomal acidification assay, lysosome enzyme activity and PKC inhibitor GF109203X.

RESULTS

The results showed that CP in vitro enhances the ability of RAW264.7 macrophages to engulf and clear C. parapsilosis. In the mouse model, CP treatment improved the survival rate of Candida-infected mice and lowered the severity of microscopic lesions in livers and spleens via a macrophage-dependent mechanism. Furthermore, with CP treatment, the fusion and acidification of phagolysosomes were accelerated and the lysosome enzyme activity of RAW264.7 macrophages was elevated. PKC inhibitor GF109203X reversed the increase in phagocytic activity by CP demonstrating that the PKC pathway is involved in the macrophage-mediated phagocytosis of C. parapsilosis.

CONCLUSIONS

Our data suggested that CP, as an immunomodulator, enhances macrophage activity against C. parapsilosis infections.

Deciphering the roles of phosphoinositide lipids in phagolysosome biogenesis

Professional phagocytes engulf microbial invaders into plasma membrane-derived phagosomes. These mature into microbicidal phagolysosomes, leading to killing of the ingested microbe. Phagosome maturation involves sequential fusion of the phagosome with early endosomes, late endosomes, and the main degradative compartments in cells, lysosomes. Some bacterial pathogens manipulate the phosphoinositide (PIP) composition of phagosome membranes and are not delivered to phagolysosomes, pointing at a role of PIPs in phagosome maturation. This hypothesis is supported by comprehensive microscopic studies. Recently, cell-free reconstitution of fusion between phagosomes and endo(lyso)somes identified phosphatidylinositol 4-phosphate [PI(4)P] and phosphatidylinositol 3-phosphate [PI(3)P] as key regulators of phagolysosome biogenesis. Here, we describe the emerging roles of PIPs in phagosome maturation and we present tools to study PIP involvement in phagosome trafficking using intact cells or purified compartments.

Tissue-type plasminogen activator (tPA) promotes M1 macrophage survival through p90 ribosomal S6 kinase (RSK) and p38 mitogen-activated protein kinase (MAPK) pathway

Macrophage accumulation is one of the hallmarks of progressive kidney disease. Resting macrophages have a finite lifespan, but become resistant to apoptosis in response to pathogenic cues, whereas the underlying mechanism remains unknown. Tissue-type plasminogen activator (tPA), a protease up-regulated in the kidneys with chronic injury, has been shown to promote macrophage accumulation and renal inflammation. We hypothesized that tPA may be the endogenous factor that promotes macrophage survival and extends their lifespan that leads to their accumulation in the injured kidneys. We examined the role of tPA in macrophage survival, and found that tPA protected macrophages from both staurosporine and H2O2-induced apoptosis. tPA promoted the survival of both resting and lipopolysaccharide- or interferon-γ-induced M1 macrophages, but failed to do so in the interleukin 4 (IL4)-induced M2 macrophages. In the kidneys with unilateral ureteral obstruction, there were significantly more apoptotic M1 macrophages in tPA-deficient mice than their wild-type counterparts, and obstruction-induced M1 macrophages accumulation and M1 chemokine expression were markedly reduced in these knock-out mice. The cytoprotective effect of tPA required its receptor, LDL receptor-related protein-1 (LRP-1). tPA induced the phosphorylation of Erk1/2, p90 ribosomal S6 kinase (RSK), and p38 in a temporal order. The tPA-mediated macrophage survival was eliminated by PD98059, BI-D1870, or sc68376, the specific inhibitors for Erk1/2, p90RSK, or p38, respectively. Thus, it is clear that tPA promoted M1 macrophage survival through its receptor LRP-1-mediated novel signaling cascade involving Erk1/2, p90RSK, and p38, which leads to the accumulation of these cells in the injured kidneys.

An improved flow cytometry assay to monitor phagosome acidification

Phago-lysosome formation is important for cell-autonomous immunity to intracellular pathogens, antigen presentation and metabolism. A hallmark feature of phago-lysosomal compartments is that they undergo progressive luminal acidification controlled by the activation of vacuolar V-ATPase. Acidification is required for many enzymatic processes taking place in phago-lysosomes, like proteolysis, and supports the microbicidal activity of macrophages. Here we present a new quantitative methodology to assess phagosome acidification by flow cytometry based on the use of bi-fluorescent particles. This method relies on the use of UV polystyrene beads labelled with the acid sensor pHrodo-succinimidyl ester (pHrodo(TM) SE red) and enables us to dissociate particle association with phagocytes from their engulfment in acidified compartments. This methodology is well suited to monitor the acidification of phagosomes formed in vivo after fluorescent bead administration.

Surviving the macrophage: tools and tricks employed by Mycobacterium tuberculosis

Mycobacterium tuberculosis has evolved to withstand one of the most inhospitable cells within the human body, namely the macrophage, a cell that is normally geared toward the destruction of any invading microbe. How M. tuberculosis achieves this is still incompletely understood; however, a number of mechanisms are now known that provide advantages to M. tuberculosis for its survival and proliferation inside the macrophage. While some of these mechanisms are mediated by factors released by M. tuberculosis, others rely on host components that are being hijacked to benefit survival of M. tuberculosis within the macrophage as well to avoid the generation of an effective immune response. Here, we describe several of these mechanisms, also pointing out the potential usage of this knowledge toward the development of novel strategies to treat tuberculosis. Furthermore, we attempt to put the 'macrophage niche' into context with other intracellular pathogens and discuss some of the generalities as well as specializations that M. tuberculosis employs to survive.

The M1 protein of Streptococcus pyogenes triggers an innate uptake mechanism into polarized human endothelial cells

Serotype M1 Streptococcus pyogenes is a major human pathogen associated with severe invasive diseases causing high morbidity and mortality. In a substantial number of cases, invasive disease develops in previously healthy individuals with no obvious port of entry. This has led to the hypothesis that the source of streptococci in these cases is a transient bacteraemia. This study focuses on the analysis of interaction of tissue-invasive serotype M1 S. pyogenes with human endothelial cells (EC) of the vascular system. We identify the M1 surface protein of S. pyogenes as the EC invasin which is recognised by polarized human blood EC, thereby triggering rapid, phagocytosis-like uptake of streptococci into polarized EC layers. Upon internalization, the M1 S. pyogenes serotype is incorporated into phagosomes which traffic via the endosomal/lysosomal pathway. However, some of the streptococci successfully evade this innate killing process and hereby mediate their escape into the cytoplasm of the host cell. The results of this study demonstrate that blood EC possess an efficient uptake mechanism for serotype M1 S. pyogenes. Despite efficient phagocytosis, streptococcal survival within EC constitutes one potential mechanism which favours intracellular persistence and thus facilitates continuous infection and dissemination from the primary side of infection into deep tissue.

Live-cell imaging of phosphoinositide dynamics and membrane architecture during Legionella infection

The causative agent of Legionnaires’ disease, Legionella pneumophila, replicates in amoebae and macrophages in a distinct membrane-bound compartment, the Legionella-containing vacuole (LCV). LCV formation is governed by the bacterial Icm/Dot type IV secretion system that translocates ~300 different “effector” proteins into host cells. Some of the translocated effectors anchor to the LCV membrane via phosphoinositide (PI) lipids. Here, we use the soil amoeba Dictyostelium discoideum, producing fluorescent PI probes, to analyze the LCV PI dynamics by live-cell imaging. Upon uptake of wild-type or Icm/Dot-deficient L. pneumophila, PtdIns(3,4,5)P₃ transiently accumulated for an average of 40 s on early phagosomes, which acquired PtdIns(3)P within 1 min after uptake. Whereas phagosomes containing ΔicmT mutant bacteria remained decorated with PtdIns(3)P, more than 80% of wild-type LCVs gradually lost this PI within 2 h. The process was accompanied by a major rearrangement of PtdIns(3)P-positive membranes condensing to the cell center. PtdIns(4)P transiently localized to early phagosomes harboring wild-type or ΔicmT L. pneumophila and was cleared within minutes after uptake. During the following 2 h, PtdIns(4)P steadily accumulated only on wild-type LCVs, which maintained a discrete PtdIns(4)P identity spatially separated from calnexin-positive endoplasmic reticulum (ER) for at least 8 h. The separation of PtdIns(4)P-positive and ER membranes was even more pronounced for LCVs harboring ΔsidC-sdcA mutant bacteria defective for ER recruitment, without affecting initial bacterial replication in the pathogen vacuole. These findings elucidate the temporal and spatial dynamics of PI lipids implicated in LCV formation and provide insight into host cell membrane and effector protein interactions.

The environmental bacterium Legionella pneumophila is the causative agent of Legionnaires’ pneumonia. The bacteria form in free-living amoebae and mammalian immune cells a replication-permissive compartment, the Legionella-containing vacuole (LCV). To subvert host cell processes, the bacteria secrete the amazing number of ~300 different proteins into host cells. Some of these proteins bind phosphoinositide (PI) lipids to decorate the LCV. PI lipids are crucial factors involved in host cell membrane dynamics and LCV formation. Using Dictyostelium amoebae producing one or two distinct fluorescent probes, we elucidated the dynamic LCV PI pattern in high temporal and spatial resolution. Notably, the endocytic PI lipid PtdIns(3)P was slowly cleared from LCVs, thus incapacitating the host cell’s digestive machinery, while PtdIns(4)P gradually accumulated on the LCV, enabling critical interactions with host organelles. The LCV PI pattern underlies the spatiotemporal configuration of bacterial effector proteins and therefore represents a crucial aspect of LCV formation.

Maintenance of vacuole integrity by bacterial pathogens

Many intracellular bacterial pathogens reside within a membrane-bound compartment. The biogenesis of these vacuolar compartments is complex, involving subversion of host cell secretory pathways by bacterial proteins. In recent years it has become clear that disruption of vacuole biogenesis may result in membrane rupture and escape of bacteria into the host cell cytosol. Correct modulation of the host cell cytoskeleton, signalling molecules such as small GTPases and the lipids of the vacuole membrane have all been shown to be critical in the maintenance of vacuole integrity. Increasing evidence suggests that vacuole rupture may result from aberrant mechanical forces exerted on the vacuole, possibly due to a defect in vacuole expansion.

The scavenger protein apoptosis inhibitor of macrophages (AIM) potentiates the antimicrobial response against Mycobacterium tuberculosis by enhancing autophagy

Apoptosis inhibitor of macrophages (AIM), a scavenger protein secreted by tissue macrophages, is transcriptionally regulated by the nuclear receptor Liver X Receptor (LXR) and Retinoid X Receptor (RXR) heterodimer. Given that LXR exerts a protective immune response against M. tuberculosis, here we analyzed whether AIM is involved in this response. In an experimental murine model of tuberculosis, AIM serum levels peaked dramatically early after infection with M. tuberculosis, providing an in vivo biological link to the disease. We therefore studied the participation of AIM in macrophage response to M. tuberculosis in vitro. For this purpose, we used the H37Rv strain to infect THP-1 macrophages transfected to stably express AIM, thereby increasing infected macrophage survival. Furthermore, the expression of this protein enlarged foam cell formation by enhancing intracellular lipid content. Phagocytosis assays with FITC-labeled M. tuberculosis bacilli indicated that this protein was not involved in bacterial uptake; however, AIM expression decreased the number of intracellular cfus by up to 70% in bacterial killing assays, suggesting that AIM enhances macrophage mycobactericidal activity. Accordingly, M. tuberculosis-infected AIM-expressing cells upregulated the production of reactive oxygen species. Moreover, real-time PCR analysis showed increased mRNA levels of the antimicrobial peptides cathelicidin and defensin 4B. These increases were concomitant with greater cellular concentrations of the autophagy-related molecules Beclin 1 and LC3II, as well as enhanced acidification of mycobacterial phagosomes and LC3 co-localization. In summary, our data support the notion that AIM contributes to key macrophage responses to M. tuberculosis.

The recruitment of p47(phox) and Rac2G12V at the phagosome is transient and phosphatidylserine dependent

BACKGROUND INFORMATION

During phagocytosis, neutrophils internalise pathogens in a phagosome and produce reactive oxygen species (ROS) by the NADPH oxidase to kill the pathogen. The cytosolic NADPH oxidase subunits p40(phox), p47(phox), p67(phox) and Rac2 translocate to the phagosomal membrane to participate in enzyme activation. The kinetics of this recruitment and the underlying signalling pathways are only partially understood. Anionic phospholipids, phosphatidylserine (PS) and phosphoinositides (PPI) provide an important attachment site for numerous proteins, including several oxidase subunits.

RESULTS

We investigated the kinetics of p47(phox) and Rac2 phagosomal membrane recruitment. Both subunits are known to interact with anionic phospholipids; we therefore addressed the role of PS in this recruitment. Phagosomal accumulation of p47(phox) and Rac2 tagged with fluorescent proteins was analysed by videomicroscopy. We used the C2 domain of lactadherin (lactC2) that interacts strongly and specifically with PS to monitor intracellular PS localisation and to decrease PS accessibility. During phagocytosis of opsonised zymosan, p47(phox) and constitutively active Rac2G12V briefly translocated to the phagosomal membrane, whereas ROS production continued for a longer period. However, in the presence of lactC2, Rac2G12V recruitment was inhibited and the kinetics of p47(phox) recruitment and detachment were delayed. A reduced phagosomal ROS production was also observed during the first 7 min following the phagosome closure.

CONCLUSIONS

These results suggest that p47(phox) and Rac2 accumulate only transiently at the phagosome at the onset of NADPH activity and detach from the phagosome before the end of ROS production. Furthermore, lactC2, by masking PS, interfered with the phagosomal recruitment of p47(phox) and Rac2 and disturbed NADPH oxidase activity. Thus, PS appears as a modulator of NADPH oxidase activation.

Cytotoxic aspects of gadolinium oxide nanostructures for up-conversion and NIR bioimaging

Bioimaging is an important diagnostic tool in the investigation and visualization of biological phenomena in cells and in medicine. In this context, up-converting Gd(2)O(3):Er(3+),Yb(3+) nanostructures (nanoparticles, nanorods) have been synthesized by precipitation methods and hydrothermal synthesis. Independent of size and morphology, Gd(2)O(3):Er(3+),Yb(3+) powders show up-conversion (550 nm, 670 nm) and near-infrared emission (1.5 μm) upon 980 nm excitation, which makes these structures interesting for application as biomarkers. With regard to their potential application in bioimaging, cytotoxicity is an important aspect and is strongly affected by the physico-chemical properties of the investigated nanostructures. Therefore, the cytotoxic effect of bare and poly(ethylene glycol)-b-poly(acrylic acid) block co-polymer-modified nanostructures on non-phagocytic and phagocytic cells (B-cell hybridoma cells and macrophages) was investigated. The observed cytotoxic behavior in the case of macrophages incubated with bare nanostructures was assigned to the poor chemical durability of gadolinium oxide, but could be overcome by surface modification.

LC3-Associated Phagocytosis (LAP): Connections with Host Autophagy

Autophagy is an intracellular degradative process with a number of roles, one of which can be the protection of eukaryotic cells from invading microbes. Microtubule-associated protein light-chain 3 (LC3) is a key autophagy-related protein that is recruited to the double-membrane autophagosome responsible for sequestering material intended for delivery to lysosomes. GFP-LC3 is widely used as a marker of autophagosome formation as denoted by the formation of green puncta when viewed by fluorescence microscopy. Recently, it has been demonstrated that LC3 can be recruited to other membranes including single-membrane phagosomes, in a process termed LC3-associated phagocytosis (LAP). Thus, the observation of green puncta in cells can no longer, by itself, be taken as evidence of autophagy. This review will clarify those features of LAP which serve to distinguish it from autophagy and that make connections with host autophagic responses in terms of infection by microbial pathogens. More specifically, it will refer to concurrent studies of the mechanism by which LAP is triggered in comparison to autophagy.

Bacillus anthracis factors for phagosomal escape

The mechanism of phagosome escape by intracellular pathogens is an important step in the infectious cycle. During the establishment of anthrax, Bacillus anthracis undergoes a transient intracellular phase in which spores are engulfed by local phagocytes. Spores germinate inside phagosomes and grow to vegetative bacilli, which emerge from their resident intracellular compartments, replicate and eventually exit from the plasma membrane. During germination, B. anthracis secretes multiple factors that can help its resistance to the phagocytes. Here the possible role of B. anthracis toxins, phospholipases, antioxidant enzymes and capsules in the phagosomal escape and survival, is analyzed and compared with that of factors of other microbial pathogens involved in the same type of process.

Lipid body-phagosome interaction in macrophages during infectious diseases: host defense or pathogen survival strategy?

Phagocytosis of invading microorganisms by specialized cells such as macrophages and neutrophils is a key component of the innate immune response. These cells capture and engulf pathogens and subsequently destroy them in intracellular vacuoles—the phagosomes. Pathogen phagocytosis and progression and maturation of pathogen-containing phagosomes, a crucial event to acquire microbicidal features, occurs in parallel with accentuated formation of lipid-rich organelles, termed lipid bodies (LBs), or lipid droplets. Experimental and clinical infections with different pathogens such as bacteria, parasites, and viruses induce LB accumulation in cells from the immune system. Within these cells, LBs synthesize and store inflammatory mediators and are considered structural markers of inflammation. In addition to LB accumulation, interaction of these organelles with pathogen-containing phagosomes has increasingly been recognized in response to infections and may have implications in the outcome or survival of the microorganism within host cells. In this review, we summarize our current knowledge on the LB-phagosome interaction within cells from the immune system, with emphasis on macrophages, and discuss the functional meaning of this event during infectious diseases.

Immunotherapy for TB

Mycobacterium tuberculosis was one of the first human pathogens to be identified as the cause of a specific disease – TB. TB was also one of the first specific diseases for which immunotherapy was attempted. In more than a century since, multiple different immunotherapies have been attempted, alongside vaccination and antibiotic treatment, with varying degrees of success. Despite this, TB remains a major worldwide health problem that causes nearly 2 million deaths annually and has infected an estimated 2 billion people. A major reason for this is that M. tuberculosis is an ancient human pathogen that has evolved complex strategies for persistence in the human host. It has thus been long understood that, to effectively control TB, we will need to address the ability of the pathogen to establish a persistent, latent infection in most infected individuals. This review discusses what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge has been used to design immunotherapeutic strategies.

Janus-faced liposomes enhance antimicrobial innate immune response in Mycobacterium tuberculosis infection

We have generated unique asymmetric liposomes with phosphatidylserine (PS) distributed at the outer membrane surface to resemble apoptotic bodies and phosphatidic acid (PA) at the inner layer as a strategy to enhance innate antimycobacterial activity in phagocytes while limiting the inflammatory response. Results show that these apoptotic body-like liposomes carrying PA (ABL/PA) (i) are more efficiently internalized by human macrophages than by nonprofessional phagocytes, (ii) induce cytosolic Ca(2+) influx, (iii) promote Ca(2+)-dependent maturation of phagolysosomes containing Mycobacterium tuberculosis (MTB), (iv) induce Ca(2+)-dependent reactive oxygen species (ROS) production, (v) inhibit intracellular mycobacterial growth in differentiated THP-1 cells as well as in type-1 and -2 human macrophages, and (vi) down-regulate tumor necrosis factor (TNF)-α, interleukin (IL)-12, IL-1β, IL-18, and IL-23 and up-regulate transforming growth factor (TGF)-β without altering IL-10, IL-27, and IL-6 mRNA expression. Also, ABL/PA promoted intracellular killing of M. tuberculosis in bronchoalveolar lavage cells from patients with active pulmonary tuberculosis. Furthermore, the treatment of MTB-infected mice with ABL/PA, in combination or not with isoniazid (INH), dramatically reduced lung and, to a lesser extent, liver and spleen mycobacterial loads, with a concomitant 10-fold reduction of serum TNF-α, IL-1β, and IFN-γ compared with that in untreated mice. Altogether, these results suggest that apoptotic body-like liposomes may be used as a Janus-faced immunotherapeutic platform to deliver polar secondary lipid messengers, such as PA, into phagocytes to improve and recover phagolysosome biogenesis and pathogen killing while limiting the inflammatory response.