Mycobacterial surface moieties are released from infected macrophages by a constitutive exocytic event

Bacterial Membrane Vesicles: Physiological Roles, Infection Immunology, and Applications

Bacterial or fungal membrane vesicles, traditionally considered as microbial metabolic wastes, are secreted mainly from the outer membrane or cell membrane of microorganisms. However, recent studies have shown that these vesicles play essential roles in direct or indirect communications among microorganisms and between microorganisms and hosts. This review aims to provide an updated understanding of the physiological functions and emerging applications of bacterial membrane vesicles, with a focus on their biogenesis, mechanisms of adsorption and invasion into host cells, immune stimulatory effects, and roles in the much-concerned problem of bacterial resistance. Additionally, the potential applications of these vesicles as biomarkers, vaccine candidates, and drug delivery platforms are discussed.

Cell wall and immune modulation by Rv1800 (PPE28) helps M. smegmatis to evade intracellular killing

Rv1800 is predicted as PPE family protein found in pathogenic mycobacteria only. Under acidic stress, the rv1800 gene was expressed in M. tuberculosis H37Ra. In-silico study showed lipase/esterase activity in C-terminus PE-PPE domain having pentapeptide motif with catalytic Ser-Asp-His residue. Full-length Rv1800 and C-terminus PE-PPE domain proteins showed esterase activity with pNP-C4 at the optimum temperature of 40 °C and pH 8.0. However, the N-terminus PPE domain showed no esterase activity, but involved in thermostability of Rv1800 full-length protein. M. smegmatis expressing rv1800 (MS_Rv1800) showed altered colony morphology and a significant resistance to numerous environmental stresses, antibiotics and higher lipid content. In extracellular and membrane fraction, Rv1800 protein was detected, while C terminus PE-PPE was present in cytoplasm, suggesting the role of N-terminus PPE domain in transportation of protein. MS_Rv1800 infected macrophage showed higher intracellular survival and low production of ROS, NO and expression levels of iNOS and pro-inflammatory cytokines, while induced expression of the anti-inflammatory cytokines. The Rv1800, PPE and PE-PPE showed antibody-mediated immunity in MDR-TB and PTB patients. Overall, these results confirmed the esterase activity in the C-terminus and function of N-terminus in thermostabilization and transportation; predicting the role of Rv1800 in immune/lipid modulation to support intracellular mycobacterium survival.

Rv3539 (PPE63) of Mycobacterium Tuberculosis Promotes Survival of Mycobacterium Smegmatis in Human Macrophages Cell Line via Cell Wall Modulation of Bacteria and Altering Host's Immune Response

The modulation of host's immune response plays an important role in the intracellular survival of Mycobacterium tuberculosis. The intracellular pathogen counteracts environmental stresses with help of the expression of several genes. The M. tuberculosis genome encodes several immune-modulatory proteins including PE (proline-glutamic acid)/PPE (proline-proline-glutamic acid) superfamily proteins. It is unclear how the unique PE/PPE proteins superfamily contributes to survival under different stress and pathophysiology conditions. Previously, we showed that PPE63 (Rv3539) has C-terminal esterase extension and was localized as a membrane attached and in extracellular compartment. Therefore, the probability of these proteins interacting with the host to modulate the host immune response cannot be ruled out. The physiological role of PPE63 was characterized by expressing the PPE63 in the M. smegmatis, a non-pathogenic strain intrinsically deficient of PPE63. The recombinant M. smegmatis expressing PPE63 altered the colony morphology, lipid composition, and integrity of the cell wall. It provided resistance to multiple hostile environmental stress conditions and several antibiotics. MS_Rv3539 demonstrated higher infection and intracellular survival in comparison to the MS_Vec in the PMA-differentiated THP-1 cells. The decreased intracellular level of ROS, NO, and expression of iNOS was observed in THP-1 cells upon infection with MS_Rv3539 in comparison to MS_Vec. Further, the decrease in expression of pro-inflammatory cytokines like IL-6, TNF-α, and IL-1β and enhanced anti-inflammatory cytokines like IL-10, pointed toward its role in immune modulation. Overall this study suggested the role of Rv3539 in enhanced intracellular survival of M. smegmatis via cell wall modulation and altered immune response of host.

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.

Macrophages disseminate pathogen associated molecular patterns through the direct extracellular release of the soluble content of their phagolysosomes

Recognition of pathogen-or-damage-associated molecular patterns is critical to inflammation. However, most pathogen-or-damage-associated molecular patterns exist within intact microbes/cells and are typically part of non-diffusible, stable macromolecules that are not optimally immunostimulatory or available for immune detection. Partial digestion of microbes/cells following phagocytosis potentially generates new diffusible pathogen-or-damage-associated molecular patterns, however, our current understanding of phagosomal biology would have these molecules sequestered and destroyed within phagolysosomes. Here, we show the controlled release of partially-digested, soluble material from phagolysosomes of macrophages through transient, iterative fusion-fission events between mature phagolysosomes and the plasma membrane, a process we term eructophagy. Eructophagy is most active in proinflammatory macrophages and further induced by toll like receptor engagement. Eructophagy is mediated by genes encoding proteins required for autophagy and can activate vicinal cells by release of phagolysosomally-processed, partially-digested pathogen associated molecular patterns. We propose that eructophagy allows macrophages to amplify local inflammation through the processing and dissemination of pathogen-or-damage-associated molecular patterns.

The detection of conserved motifs by pattern recognition receptors is a crucial component of the innate detection of pathogens and danger signals via conserved pattern recognition receptors. Here the authors define a pathway that transfers partially digested material from the phagolysosomal pathway of macrophages to release at the plasma membrane which is associated with enhanced inflammatory potential, by a process they introduce as eructophagy.

Emerging Roles of Extracellular Vesicles in Pneumococcal Infections: Immunomodulators to Potential Novel Vaccine Candidates

The Gram-positive bacterial pathogen, Streptococcus pneumoniae is a major global health threat that kills over one million people worldwide. The pneumococcus commonly colonizes the nasopharynx asymptomatically as a commensal, but is also capable of causing a wide range of life-threatening diseases such as pneumonia, meningitis and septicemia upon migration into the lower respiratory tract and spread to internal organs. Emergence of antibiotic resistant strains and non-vaccine serotypes has led to the classification of pneumococcal bacteria as a priority pathogen by the World Health Organization that needs urgent research into bacterial pathogenesis and development of novel vaccine strategies. Extracellular vesicles are spherical membrane bound structures that are released by both pathogen and host cells, and influence bacterial pathogenesis as well as the immune response. Recent studies have found that while bacterial vesicles shuttle virulence factors and toxins into host cells and regulate inflammatory responses, vesicles released from the infected host cells contain both bacterial and host proteins that are antigenic and immunomodulatory. Bacterial membrane vesicles have great potential to be developed as cell-free vaccine candidates in the future due to their immunogenicity and biostability. Host-derived vesicles isolated from patient biofluids such as blood and bronchoalveolar lavage could be used to identify potential diagnostic biomarkers as well as engineered to deliver desired payloads to specific target cells for immunotherapy. In this review, we summarize the recent developments on the role of bacterial and host vesicles in pneumococcal infections and future prospects in developing novel therapeutics and diagnostics for control of invasive pneumococcal diseases.

Correlation of over-expression of rv1900c with enhanced survival of M. smegmatis under stress conditions: Modulation of cell surface properties

Mycobacterium tuberculosis has distinct cell wall composition that helps in intracellular survival of bacteria. Rv1900c, a two domain protein, has been grouped in lip gene family. The expression of rv1900c was upregulated under acidic, nutritive and iron stress conditions in M. tuberculosis H37Ra. To investigate the biological effect of Rv1900c in mycobacterium physiology, rv1900c gene was cloned in M. smegmatis, a surrogate host. Its counterpart MSMEG_4477 in M. smegmatis demonstrated 38% protein similarity with Rv1900c. MSMEG_4477 gene was knocked out in M. smegmatis by homologous recombination. rv1900c and MSMEG_4477 genes, cloned in pVV16, were expressed in the M. smegmatis knockout strain (M. smegmatis ΔMSMEG_4477). Gene knockout significantly altered colony morphology and growth kinetics of M. smegmatis. M. smegmatis ΔMSMEG_1900 (pVV16::rv1900c) colonies were less wrinkled and had smooth surface as compared to M. smegmatis ΔMSMEG_4477. The changes were reverted back to normal upon expression of MSMEG_4477 in knockout strain M. smegmatis ΔMSMEG_4477 (pVV16::MSMEG_4477). The expression of rv1900c enhanced the biofilm formation and survival of bacteria under various in vitro stresses like acidic, nutritive stress, including lysozyme, SDS and multiple antibiotics treatment in comparison to control. On the other hand the expression of rv1900c decreased the cell wall permeability. The resistance provided by M. smegmatis ΔMSMEG_4477 (pVV16::MSMEG_4477) was comparable to M. smegmatis having vector alone (MS_vec). The lipid content of M. smegmatis ΔMSMEG_1900 (pVV16::rv1900c) was observed to be different from M. smegmatis ΔMSMEG_4477 (pVV16::MSMEG_4477). M. smegmatis ΔMSMEG_1900 (pVV16::rv1900c) was more tolerant to stress conditions in comparison to M. smegmatis ΔMSMEG_4477 (pVV16::MSMEG_4477). Expression of rv1900c enhanced the intracellular survival of mycobacteria. Therefore, the present study suggested an association of Rv1900c to the stress tolerance by cell wall modification that might have resulted in enhanced intracellular survival of the mycobacteria.

Helicobacter pylori Outer Membrane Vesicles and Extracellular Vesicles from Helicobacter pylori-Infected Cells in Gastric Disease Development

Extracellular vesicles (EVs) are cell-derived vesicles important in intercellular communication that play an essential role in host-pathogen interactions, spreading pathogen-derived as well as host-derived molecules during infection. Pathogens can induce changes in the composition of EVs derived from the infected cells and use them to manipulate their microenvironment and, for instance, modulate innate and adaptive inflammatory immune responses, both in a stimulatory or suppressive manner. Gastric cancer is one of the leading causes of cancer-related deaths worldwide and infection with Helicobacter pylori (H. pylori) is considered the main risk factor for developing this disease, which is characterized by a strong inflammatory component. EVs released by host cells infected with H. pylori contribute significantly to inflammation, and in doing so promote the development of disease. Additionally, H. pylori liberates vesicles, called outer membrane vesicles (H. pylori-OMVs), which contribute to atrophia and cell transformation in the gastric epithelium. In this review, the participation of both EVs from cells infected with H. pylori and H. pylori-OMVs associated with the development of gastric cancer will be discussed. By deciphering which functions of these external vesicles during H. pylori infection benefit the host or the pathogen, novel treatment strategies may become available to prevent disease.

The emerging role of exosomal miRNAs as a diagnostic and therapeutic biomarker in Mycobacterium tuberculosis infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), has been the world's driving fatal bacterial contagious disease globally. It continues a public health emergency, and around one-third of the global community has been affected by latent TB infection (LTBI). This is mostly due to the difficulty in diagnosing and treating patients with TB and LTBI. Exosomes are nanovesicles (40-100 nm) released from different cell types, containing proteins, lipids, mRNA, and miRNA, and they allow the transfer of one's cargo to other cells. The functional and diagnostic potential of exosomal miRNAs has been demonstrated in bacterial infections, including TB. Besides, it has been recognized that cells infected by intracellular pathogens such as Mtb can be secreting an exosome, which is implicated in the infection's fate. Exosomes, therefore, open a unique viewpoint on the investigative process of TB pathogenicity. This study explores the possible function of exosomal miRNAs as a diagnostic biomarker. Moreover, we include the latest data on the pathogenic and therapeutic role of exosomal miRNAs in TB.

Fatty acid oxidation of alternatively activated macrophages prevents foam cell formation, but Mycobacterium tuberculosis counteracts this process via HIF-1α activation

The ability of Mycobacterium tuberculosis (Mtb) to persist inside host cells relies on metabolic adaptation, like the accumulation of lipid bodies (LBs) in the so-called foamy macrophages (FM), which are favorable to Mtb. The activation state of macrophages is tightly associated to different metabolic pathways, such as lipid metabolism, but whether differentiation towards FM differs between the macrophage activation profiles remains unclear. Here, we aimed to elucidate whether distinct macrophage activation states exposed to a tuberculosis-associated microenvironment or directly infected with Mtb can form FM. We showed that the triggering of signal transducer and activator of transcription 6 (STAT6) in interleukin (IL)-4-activated human macrophages (M(IL-4)) prevents FM formation induced by pleural effusion from patients with tuberculosis. In these cells, LBs are disrupted by lipolysis, and the released fatty acids enter the β-oxidation (FAO) pathway fueling the generation of ATP in mitochondria. Accordingly, murine alveolar macrophages, which exhibit a predominant FAO metabolism, are less prone to become FM than bone marrow derived-macrophages. Interestingly, direct infection of M(IL-4) macrophages with Mtb results in the establishment of aerobic glycolytic pathway and FM formation, which could be prevented by FAO activation or inhibition of the hypoxia-inducible factor 1-alpha (HIF-1α)-induced glycolytic pathway. In conclusion, our results demonstrate that Mtb has a remarkable capacity to induce FM formation through the rewiring of metabolic pathways in human macrophages, including the STAT6-driven alternatively activated program. This study provides key insights into macrophage metabolism and pathogen subversion strategies.

Tuberculosis is a deadly disease caused by Mycobacterium tuberculosis. Although progress has been made in tuberculosis control, there are unexplored aspects of how the immune system deals with the pathogen that need to be addressed. M. tuberculosis primarily lives in macrophages, immune cells which can destroy mycobacteria. In spite of the multiple microbicidal properties, the bacterium still manipulates the metabolism of macrophages, reflected in the accumulation lipid droplets and consequent differentiation into foamy macrophages. These lipid-laden macrophages constitute a favorable niche for the bacteria to persist hidden from our immune defense. Macrophages are classified into different programs depending on how they are activated by environmental signals, ranging from classically (microbicidal/inflammatory) to alternatively (tissue remodeling/repair/growth) activated spectrum extremes. We found that, unlike other programs, alternatively activated macrophages are reluctant to accumulate lipid droplets due to the signal transducer and activator of transcription 6 (STAT6), which promotes the degradation of those lipids. Notwithstanding, M. tuberculosis counteracts lipolysis by rewiring the metabolism of alternatively activated macrophages towards the accumulation of lipid droplets via the activation of the hypoxia-inducible factor 1-alpha (HIF-1α). Therefore, this study contributes to the better understanding of how bacillus shifts the metabolism of macrophages, which can be crucial to therapeutic purposes.

Trafficking of Mycobacterium tuberculosis Envelope Components and Release Within Extracellular Vesicles: Host-Pathogen Interactions Beyond the Wall

Components of Mycobacterium tuberculosis (Mtb) envelope such as lipoproteins, lypoglycans, lipids, and glycolipids act as Pathogen Associated Molecular Patterns and/or antigens, hence contributing in different ways to the bacillus recognition, phagocytosis, and to immune responses modulation. However, Mtb envelope components are not only encountered at the bacillus-host direct contact but can act remotely from the bacillus envelope. Indeed, they are also released from the bacillus envelope and are detected in different compartments such as the infected cells endosomal compartments or in extracellular vesicles produced by the bacillus itself or by infected cells. Characterizing their trafficking is of main importance for our understanding of their role in host-pathogen interactions and consequently for their potential use as vaccine components. This review aims at providing an overview of the current knowledge of the nature of Mtb envelope components shuttled within extracellular vesicles, the interaction of these vesicles with host immune cells and the remaining black holes.

Mycobacterium tuberculosis (Mtb) lipid mediated lysosomal rewiring in infected macrophages modulates intracellular Mtb trafficking and survival

Intracellular pathogens commonly manipulate the host lysosomal system for their survival. However, whether this pathogen-induced alteration affects the organization and functioning of the lysosomal system itself is not known. Here, using in vitro and in vivo infections and quantitative image analysis, we show that the lysosomal content and activity are globally elevated in Mycobacterium tuberculosis (Mtb)-infected macrophages. We observed that this enhanced lysosomal state is sustained over time and defines an adaptive homeostasis in the infected macrophage. Lysosomal alterations are caused by mycobacterial surface components, notably the cell wall-associated lipid sulfolipid-1 (SL-1), which functions through the mTOR complex 1 (mTORC1)-transcription factor EB (TFEB) axis in the host cells. An Mtb mutant lacking SL-1, MtbΔpks2, shows attenuated lysosomal rewiring compared with the WT Mtb in both in vitro and in vivo infections. Exposing macrophages to purified SL-1 enhanced the trafficking of phagocytic cargo to lysosomes. Correspondingly, MtbΔpks2 exhibited a further reduction in lysosomal delivery compared with the WT. Reduced trafficking of this mutant Mtb strain to lysosomes correlated with enhanced intracellular bacterial survival. Our results reveal that global alteration of the host lysosomal system is a defining feature of Mtb-infected macrophages and suggest that this altered lysosomal state protects host cell integrity and contributes to the containment of the pathogen.

Identification of Mycobacterium tuberculosis Peptides in Serum Extracellular Vesicles from Persons with Latent Tuberculosis Infection

Identification of biomarkers for latent Mycobacterium tuberculosis infection and risk of progression to tuberculosis (TB) disease are needed to better identify individuals to target for preventive therapy, predict disease risk, and potentially predict preventive therapy efficacy. Our group developed multiple reaction monitoring mass spectrometry (MRM-MS) assays that detected M. tuberculosis peptides in serum extracellular vesicles from TB patients. We subsequently optimized this MRM-MS assay to selectively identify 40 M. tuberculosis peptides from 19 proteins that most commonly copurify with serum vesicles of patients with TB. Here, we used this technology to evaluate if M. tuberculosis peptides can also be detected in individuals with latent TB infection (LTBI). Serum extracellular vesicles from 74 individuals presumed to have latent M. tuberculosis infection (LTBI) based on close contact with a household member with TB or a recent tuberculin skin test (TST) conversion were included in this study. Twenty-nine samples from individuals with no evidence of TB infection by TST and no known exposure to TB were used as controls to establish a threshold to account for nonspecific/background signal. We identified at least one of the 40 M. tuberculosis peptides in 70 (95%) individuals with LTBI. A single peptide from the glutamine synthetase (GlnA1) enzyme was identified in 61/74 (82%) individuals with LTBI, suggesting peptides from M. tuberculosis proteins involved in nitrogen metabolism might be candidates for pathogen-specific biomarkers for detection of LTBI. The detection of M. tuberculosis peptides in serum extracellular vesicles from persons with LTBI represents a potential advance in the diagnosis of LTBI.

Mycobacterium tuberculosis PE31 (Rv3477) Attenuates Host Cell Apoptosis and Promotes Recombinant M. smegmatis Intracellular Survival via Up-regulating GTPase Guanylate Binding Protein-1

The Mycobacterium (M.) tuberculosis comprising proline–glutamic acid (PE) subfamily proteins associate with virulence, pathogenesis, and host-immune modulations. While the functions of most of this family members are not yet explored. Here, we explore the functions of “PE only” subfamily member PE31 (Rv3477) in virulence and host-pathogen interactions. We have expressed the M. tuberculosis PE31 in non-pathogenic Mycobacterium smegmatis strain (Ms_PE31) and demonstrated that PE31 significantly altered the cell facet features including colony morphology and biofilm formation. PE31 expressing M. smegmatis showed more resistant to the low pH, diamide, H₂O₂ and surface stress. Moreover, Ms_PE31 showed higher intracellular survival in macrophage THP-1 cells. Ms_PE31 significantly down-regulated the production of IL-12p40 and IL-6, while up-regulates the production of IL-10 in macrophages. Ms_PE31 also induced the expression of guanylate-binding protein-1 (GBP-1) in macrophages. Further analysis demonstrates that Ms_PE31 inhibits the caspase-3 activation and reduces the macrophages apoptosis. Besides, the NF-κB signaling pathway involves the interplay between Ms_PE31 and macrophages. Collectively, our finding identified that PE31 act as a functionally relevant virulence factor of M. tuberculosis.

Extracellular Vesicles in Mycobacterial Infections: Their Potential as Molecule Transfer Vectors

Extracellular vesicles are membrane-bound structures released by living cells and present in body fluids. Their composition includes proteins, lipids, carbohydrates, and nucleic acids and are involved in transfers between cells. Extracellular vesicles can deliver molecules to cells and tissues even if distant. As a consequence, they have a role in information transmission and in the modulation of the biological function of recipient cells. Among other things, they are involved in antigen presentation and the induction of secretion events by immune cells. Thus, extracellular vesicles participate in the regulation of immune responses during infections. We will discuss their potential as effectors and disease biomarkers concerning only mycobacterial infections.

Exosomes in tuberculosis: Still terra incognita?

Today, diagnosis, vaccination, and treatment of tuberculosis (TB) remain major clinical challenges. Therefore, an introduction of new diagnostic measures and biomarkers is necessary to improve infection control. The ideal biomarker for TB infection can be defined as a host or pathogen-derived biomolecule, which is potent for identifying infection and determining its clinical stage. Exosomes, defined as cell-derived nanovesicles released into biological fluids, are involved in cell-cell communication and immune modulation. These vesicles have emerged as a new platform for improving the clinical diagnosis and prognosis of different infectious diseases and cancers. The role of these nanovehicles, as alternative biomarkers for the improvement of TB diagnosis and treatment, has been demonstrated in a significant body of literature. In this review, we summarized recent progress in the clinical application of exosome-based biomarkers in TB infection.

Antibody to Poly-N-acetyl glucosamine provides protection against intracellular pathogens: Mechanism of action and validation in horse foals challenged with Rhodococcus equi

Immune correlates of protection against intracellular bacterial pathogens are largely thought to be cell-mediated, although a reasonable amount of data supports a role for antibody-mediated protection. To define a role for antibody-mediated immunity against an intracellular pathogen, Rhodococcus equi, that causes granulomatous pneumonia in horse foals, we devised and tested an experimental system relying solely on antibody-mediated protection against this host-specific etiologic agent. Immunity was induced by vaccinating pregnant mares 6 and 3 weeks prior to predicted parturition with a conjugate vaccine targeting the highly conserved microbial surface polysaccharide, poly-N-acetyl glucosamine (PNAG). We ascertained antibody was transferred to foals via colostrum, the only means for foals to acquire maternal antibody. Horses lack transplacental antibody transfer. Next, a randomized, controlled, blinded challenge was conducted by inoculating at ~4 weeks of age ~10⁶ cfu of R. equi via intrabronchial challenge. Eleven of 12 (91%) foals born to immune mares did not develop clinical R. equi pneumonia, whereas 6 of 7 (86%) foals born to unvaccinated controls developed pneumonia (P = 0.0017). In a confirmatory passive immunization study, infusion of PNAG-hyperimmune plasma protected 100% of 5 foals against R. equi pneumonia whereas all 4 recipients of normal horse plasma developed clinical disease (P = 0.0079). Antibodies to PNAG mediated killing of extracellular and intracellular R. equi and other intracellular pathogens. Killing of intracellular organisms depended on antibody recognition of surface expression of PNAG on infected cells, along with complement deposition and PMN-assisted lysis of infected macrophages. Peripheral blood mononuclear cells from immune and protected foals released higher levels of interferon-γ in response to PNAG compared to controls, indicating vaccination also induced an antibody-dependent cellular release of this critical immune cytokine. Overall, antibody-mediated opsonic killing and interferon-γ release in response to PNAG may protect against diseases caused by intracellular bacterial pathogens.

Development of effective vaccines for diseases such as tuberculosis, brucellosis and others caused by intracellular pathogens has proved challenging, as data exist supporting both antibody and cellular immune effectors as mediators of protection. To address this problem against an important, and representative, equine intracellular pathogen, we chose to test a vaccine candidate for the ability to protect horse foals challenged at 4 weeks of age with Rhodococcus equi. To make these foals immune, their pregnant mares were immunized with a vaccine targeting the conserved surface antigen found on many microbes, termed PNAG. Antibody in the pregnant mares was transferred to their foals and, after the foals were challenged, 91% of those born to vaccinated mares were protected against R. equi pneumonia. Meanwhile, 86% of the non-vaccinated controls developed pneumonia. We also showed antibody to PNAG could kill various bacteria that produce this antigen when residing inside of human macrophage cells, a new mechanism for antibody-mediated immunity to intracellular bacteria. These results support the development of PNAG as a vaccine for intracellular bacterial pathogens.

The PGRS Domain of Mycobacterium tuberculosis PE_PGRS Protein Rv0297 Is Involved in Endoplasmic Reticulum Stress-Mediated Apoptosis through Toll-Like Receptor 4

The genome of Mycobacterium tuberculosis, the causal organism of tuberculosis (TB), encodes a unique protein family known as the PE/PPE/PGRS family, present exclusively in the genus Mycobacterium and nowhere else in the living kingdom, with largely unexplored functions. We describe the functional significance of the PGRS domain of Rv0297, a member of this family. In silico analyses revealed the presence of intrinsically disordered stretches and putative endoplasmic reticulum (ER) localization signals in the PGRS domain of Rv0297 (Rv0297PGRS). The PGRS domain aids in ER localization, which was shown by infecting macrophage cells with M. tuberculosis and by overexpressing the protein by transfection in macrophage cells followed by activation of the unfolded protein response, as evident from increased expression of GRP78/GRP94 and CHOP/ATF4, leading to disruption of intracellular Ca²⁺ homeostasis and increased nitric oxide (NO) and reactive oxygen species (ROS) production. The consequent activation of the effector caspase-8 resulted in apoptosis of macrophages, which was Toll-like receptor 4 (TLR4) dependent. Administration of recombinant Rv0297PGRS (rRv0297PGRS) also exhibited similar effects. These results implicate a hitherto-unknown role of the PGRS domain of the PE_PGRS protein family in ER stress-mediated cell death through TLR4. Since this protein is already known to be present at later stages of infection in human granulomas it points to the possibility of it being employed by M. tuberculosis for its dissemination via an apoptotic mechanism.

Apoptosis is generally thought to be a defense mechanism in protecting the host against Mycobacterium tuberculosis in early stages of infection. However, apoptosis during later stages in lung granulomas may favor the bacterium in disseminating the disease. ER stress has been found to induce apoptosis in TB granulomas, in zones where apoptotic macrophages accumulate in mice and humans. In this study, we report ER stress-mediated apoptosis of host cells by the Rv0297-encoded PE_PGRS5 protein of M. tuberculosis exceptionally present in the pathogenic Mycobacterium genus. The PGRS domain of Rv0297 aids the protein in localizing to the ER and induces the unfolded protein response followed by apoptosis of macrophages. The effect of the Rv0297PGRS domain was found to be TLR4 dependent. This study presents novel insights on the strategies employed by M. tuberculosis to disseminate the disease.

Role of Extracellular Vesicles in Viral and Bacterial Infections: Pathogenesis, Diagnostics, and Therapeutics

Extracellular vesicles (EVs), or exosomes, are nanovesicles of endocytic origin that carry host and pathogen-derived protein, nucleic acid, and lipid cargos. They are secreted by most cell types and play important roles in normal cell-to-cell communications but can also spread pathogen- and host-derived molecules during infections to alter immune responses and pathophysiological processes. New research is beginning to decipher how EVs influence viral and bacterial pathogenesis. In this review, we will describe how EVs influence viral and bacterial pathogenesis by spreading pathogen-derived factors and how they can promote and inhibit the immune response to these pathogens. We will also discuss the emerging potential of EVs as diagnostic and therapeutic tools.

Immunomodulatory role for membrane vesicles released by THP-1 macrophages and respiratory pathogens during macrophage infection

Background

During infection, inflammation is partially driven by the release of mediators which facilitate intercellular communication. Amongst these mediators are small membrane vesicles (MVs) that can be released by both host cells and Gram-negative and -positive bacteria. Bacterial membrane vesicles are known to exert immuno-modulatory and -stimulatory actions. Moreover, it has been proposed that host cell-derived vesicles, released during infection, also have immunostimulatory properties. In this study, we assessed the release and activity of host cell-derived and bacterial MVs during the first hours following infection of THP-1 macrophages with the common respiratory pathogens non-typeable Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae, and Pseudomonas aeruginosa.

Results

Using a combination of flow cytometry, tunable resistive pulse sensing (TRPS)-based analysis and electron microscopy, we demonstrated that the release of MVs occurs by both host cells and bacteria during infection. MVs released during infection and bacterial culture were found to induce a strong pro-inflammatory response by naive THP-1 macrophages. Yet, these MVs were also found to induce tolerance of host cells to secondary immunogenic stimuli and to enhance bacterial adherence and the number of intracellular bacteria.

Conclusions

Bacterial MVs may play a dual role during infection, as they can both trigger and dampen immune responses thereby contributing to immune defence and bacterial survival.

Mycobacterium tuberculosis reactivates latent HIV-1 in T cells in vitro

Following proviral integration into the host cell genome and establishment of a latent state, the human immunodeficiency virus type 1 (HIV-1) can reenter a productive life cycle in response to various stimuli. HIV-1 reactivation occurs when transcription factors, such as nuclear factor-κB (NF-κB), nuclear factor of activated T cells (NFAT), and activator protein -1 (AP-1), bind cognate sites within the long terminal repeat (LTR) region of the HIV-1 provirus to promote transcription. Interestingly, pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) can reactivate latent HIV-1 through activation of the transcription factor NF-κB. Some PRRs are expressed on central memory CD4⁺ T cells (T_CM), which in HIV-1 patients constitute the main reservoir of latent HIV-1. Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), interacts with PRRs through membrane components. However, the ability of Mtb to reactivate latent HIV-1 has not been extensively studied. Here we show that phosphatidylinositol mannoside 6 (PIM6), a component of the Mtb membrane, in addition to whole bacteria in co-culture, can reactivate HIV-1 in a primary T_CM cell model of latency. Using a JLAT model of HIV-1 latency, we found this interaction to be mediated through Toll-like receptor-2 (TLR-2). Thus, we describe a mechanism by which Mtb can exacerbate HIV-1 infection. We hypothesize that chronic Mtb infection can drive HIV-1 reactivation. The phenomenon described here could explain, in part, the poor prognosis that characterizes HIV-1/Mtb co-infection.

A Functional Role for Antibodies in Tuberculosis

While a third of the world carries the burden of tuberculosis, disease control has been hindered by a lack of tools, including a rapid, point-of-care diagnostic and a protective vaccine. In many infectious diseases, antibodies (Abs) are powerful biomarkers and important immune mediators. However, in Mycobacterium tuberculosis (Mtb) infection, a discriminatory or protective role for humoral immunity remains unclear. Using an unbiased antibody profiling approach, we show that individuals with latent tuberculosis infection (Ltb) and active tuberculosis disease (Atb) have distinct Mtb-specific humoral responses, such that Ltb infection is associated with unique Ab Fc functional profiles, selective binding to FcγRIII, and distinct Ab glycosylation patterns. Moreover, compared to Abs from Atb, Abs from Ltb drove enhanced phagolysosomal maturation, inflammasome activation, and, most importantly, macrophage killing of intracellular Mtb. Combined, these data point to a potential role for Fc-mediated Ab effector functions, tuned via differential glycosylation, in Mtb control.

Transcriptomic Approaches in the Zebrafish Model for Tuberculosis-Insights Into Host- and Pathogen-specific Determinants of the Innate Immune Response

Mycobacterium marinum infection in zebrafish has become a well-established model of tuberculosis. Both embryonic and adult zebrafish infection studies have contributed to our knowledge of the development and function of tuberculous granulomas, which are typical of mycobacterial pathogenesis. In this review we discuss how transcriptome profiling studies have helped to characterize this infection process. We illustrate this using new RNA sequencing (RNA-Seq) data that reveals three main phases in the host response to M. marinum during the early stages of granuloma development in zebrafish embryos and larvae. The early phase shows induction of complement and transcription factors, followed by a relatively minor induction of pro-inflammatory cytokines within hours following phagocytosis of M. marinum. A minimal response is observed in the mid-phase, between 6 hours and 1day post infection, when the tissue dissemination of M. marinum begins. During subsequent larval development the granulomas expand and a late-phase response is apparent, which is characterized by progressively increasing induction of complement, transcription factors, pro-inflammatory cytokines, matrix metalloproteinases, and other defense and inflammation-related gene groups. This late-phase response shares common components with the strong and acute host transcriptome response that has previously been reported for Salmonella typhimurium infection in zebrafish embryos. In contrast, the early/mid-phase response to M. marinum infection, characterized by suppressed pro-inflammatory signaling, is strikingly different from the acute response to S. typhimurium infection. Furthermore, M. marinum infection shows a collective and strongly fluctuating regulation of lipoproteins, while S. typhimurium infection has pronounced effects on amino acid metabolism and glycolysis.

Extracellular vesicles and infectious diseases: new complexity to an old story

Exosomes and other extracellular microvesicles (ExMVs) have important functions in intercellular communication and regulation. During the course of infection, these vesicles can convey pathogen molecules that serve as antigens or agonists of innate immune receptors to induce host defense and immunity, or that serve as regulators of host defense and mediators of immune evasion. These molecules may include proteins, nucleic acids, lipids, and carbohydrates. Pathogen molecules may be disseminated by incorporation into vesicles that are created and shed by host cells, or they may be incorporated into vesicles shed from microbial cells. Involvement of ExMVs in the induction of immunity and host defense is widespread among many pathogens, whereas their involvement in immune evasion mechanisms is prominent among pathogens that establish chronic infection and is found in some that cause acute infection. Because of their immunogenicity and enrichment of pathogen molecules, exosomes may also have potential in vaccine preparations and as diagnostic markers. Additionally, the ability of exosomes to deliver molecules to recipient cells raises the possibility of their use for drug/therapy delivery. Thus, ExMVs play a major role in the pathogenesis of infection and provide exciting potential for the development of novel diagnostic and therapeutic approaches.

Mycobacterium tuberculosis: Manipulator of Protective Immunity

Mycobacterium tuberculosis (MTB) is one of the most successful pathogens in human history and remains a global health challenge. MTB has evolved a plethora of strategies to evade the immune response sufficiently to survive within the macrophage in a bacterial-immunological equilibrium, yet causes sufficient immunopathology to facilitate its transmission. This review highlights MTB as the driver of disease pathogenesis and presents evidence of the mechanisms by which MTB manipulates the protective immune response into a pathological productive infection.

A bug's life in the granuloma

The granuloma is the defining feature of the host response to infection with Mycobacterium tuberculosis (Mtb). Despite knowing of its existence for centuries, much remains unclear regarding the host and bacterial factors that contribute to granuloma formation, heterogeneity of presentation, and the forces at play within. Mtb is highly adapted to life within the granuloma and employs many unique strategies to both create a niche within the host as well as survive the stresses imposed upon it. Adding to the complexity of the granuloma is the vast range of pathology observed, often within the same individual. Here, we explore some of the many ways in which Mtb crafts the immune response to its liking and builds a variety of granuloma features that contribute to its survival. We also consider the multitude of ways that Mtb is adapted to life in the granuloma and how variability in the deployment of these strategies may result in different fates for both the bacterium and the host. It is through better understanding of these complex interactions that we may begin to strategize novel approaches for tuberculosis treatments.

Extracellular vesicles from infected cells: potential for direct pathogenesis

Infections that result in natural or manmade spread of lethal biological agents are a concern and require national and focused preparedness. In this manuscript, as part of an early diagnostics and pathogen treatment strategy, we have focused on extracellular vesicles (EVs) that arise following infections. Although the field of biodefense does not currently have a rich resource in EVs literature, none the less, similar pathogens belonging to the more classical emerging and non-emerging diseases have been studied in their EV/exosomal contents and function. These exosomes are formed in late endosomes and released from the cell membrane in almost every cell type in vivo. These vesicles contain proteins, RNA, and lipids from the cells they originate from and function in development, signal transduction, cell survival, and transfer of infectious material. The current review focuses on how different forms of infection exploit the exosomal pathway and how exosomes can be exploited artificially to treat infection and disease and potentially also be used as a source of vaccine. Virally-infected cells can secrete viral as well as cellular proteins and RNA in exosomes, allowing viruses to cause latent infection and spread of miRNA to nearby cells prior to a subsequent infection. In addition to virally-infected host cells, bacteria, protozoa, and fungi can all release small vesicles that contain pathogen-associated molecular patterns, regulating the neighboring uninfected cells. Examples of exosomes from both virally and bacterially infected cells point toward a re-programming network of pathways in the recipient cells. Finally, many of these exosomes contain cytokines and miRNAs that in turn can effect gene expression in the recipient cells through the classical toll-like receptor and NFκB pathway. Therefore, although exosomes do not replicate as an independent entity, they however facilitate movement of infectious material through tissues and may be the cause of many pathologies seen in infected hosts.

Ubiquitination as a Mechanism To Transport Soluble Mycobacterial and Eukaryotic Proteins to Exosomes

Exosomes are extracellular vesicles of endocytic origin that function in intercellular communication. Our previous studies indicate that exosomes released from Mycobacterium tuberculosis-infected macrophages contain soluble mycobacterial proteins. However, it was unclear how these secreted proteins were targeted to exosomes. In this study, we determined that exosome production by the murine macrophage cell line RAW264.7 requires the endosomal sorting complexes required for transport and that trafficking of mycobacterial proteins from phagocytosed bacilli to exosomes was dependent on protein ubiquitination. Moreover, soluble mycobacterial proteins, when added exogenously to RAW264.7 or human HEK293 cells, were endocytosed, ubiquitinated, and released via exosomes. This suggested that endocytosed proteins could be recycled from cells through exosomes. This hypothesis was supported using the tumor-associated protein He4, which, when endocytosed by RAW264.7 or HEK293 cells, was transported to exosomes in a ubiquitin-dependent manner. Our data suggest that ubiquitination is a modification sufficient for trafficking soluble proteins within the phagocytic/endocytic network to exosomes.

Bacterial Membrane Vesicles Mediate the Release of Mycobacterium tuberculosis Lipoglycans and Lipoproteins from Infected Macrophages

Mycobacterium tuberculosis is an intracellular pathogen that infects lung macrophages and releases microbial factors that regulate host defense. M. tuberculosis lipoproteins and lipoglycans block phagosome maturation, inhibit class II MHC Ag presentation, and modulate TLR2-dependent cytokine production, but the mechanisms for their release during infection are poorly defined. Furthermore, these molecules are thought to be incorporated into host membranes and released from infected macrophages within exosomes, 40-150-nm extracellular vesicles that derive from multivesicular endosomes. However, our studies revealed that extracellular vesicles released from infected macrophages include two distinct, largely nonoverlapping populations: one containing host cell markers of exosomes (CD9, CD63) and the other containing M. tuberculosis molecules (lipoglycans, lipoproteins). These vesicle populations are similar in size but have distinct densities, as determined by separation on sucrose gradients. Release of lipoglycans and lipoproteins from infected macrophages was dependent on bacterial viability, implicating active bacterial mechanisms in their secretion. Consistent with recent reports of extracellular vesicle production by bacteria (including M. tuberculosis), we propose that bacterial membrane vesicles are secreted by M. tuberculosis within infected macrophages and subsequently are released into the extracellular environment. Furthermore, extracellular vesicles released from M. tuberculosis-infected cells activate TLR2 and induce cytokine responses by uninfected macrophages. We demonstrate that these activities derive from the bacterial membrane vesicles rather than exosomes. Our findings suggest that bacterial membrane vesicles are the primary means by which M. tuberculosis exports lipoglycans and lipoproteins to impair effector functions of infected macrophages and circulate bacterial components beyond the site of infection to regulate immune responses by uninfected cells.

Effective expansion of forkhead box P3⁺ regulatory T cells via early secreted antigenic target 6 and antigen 85 complex B from Mycobacterium tuberculosis

The expansion of CD4+ CD25+ forkhead box (FOX)P3+ regulatory T (Treg) cells has been observed in patients with Mycobacterium (M.) tuberculosis; however, the mechanism of expansion remains to be elucidated. The aim of the present study was to examine the role of the early secreted antigenic target 6(ESAT‑6) and antigen 85 complex B (Ag85B) from M. tuberculosis on Treg cell expansion. To investigate the sensitivity of peripheral blood cultures to the M. tuberculosis ESAT‑6 and Ag85B antigens, the proportion of circulating CD4+ CD25+ FOXP3+ Treg cells was determined using flow cytometry and the levels of FOXP3 mRNA were determined using reverse transcription quantitative polymerase chain reaction. The mRNA levels of FOXP3 and the proportion of circulating CD4+ CD25+ FOXP3+ Treg cells were increased in multiplicitous drug‑resistant tuberculosis patients compared with those in healthy controls and patients with latent tuberculosis (TB) infection (LTBI) (P<0.001). The mycobacterial antigens ESAT‑6 and Ag85B increased the expansion of the CD4+ CD25+ FOXP3+ Treg cells and the mRNA levels of FOXP3 in healthy controls and LTBI patients compared with the effect of Bacillus Calmette‑Guerin (P<0.05). Additionally, the mRNA levels of FOXP3 were elevated in the LTBI patients following stimulations with the mycobacterial antigens (P=0.012). Therefore, the M. tuberculosis antigens ESAT‑6 and Ag85B induced CD4+ CD25+ FOXP3+ Treg‑cell expansion, particularly in patients with LTBI. These findings indicated that CD4+ CD25+ FOXP3+ Treg cells may have a primary role in the failure of the host immune system to eradicate M. tuberculosis.

Exosomes and other extracellular vesicles in host-pathogen interactions

An effective immune response requires the engagement of host receptors by pathogen-derived molecules and the stimulation of an appropriate cellular response. Therefore, a crucial factor in our ability to control an infection is the accessibility of our immune cells to the foreign material. Exosomes-which are extracellular vesicles that function in intercellular communication-may play a key role in the dissemination of pathogen- as well as host-derived molecules during infection. In this review, we highlight the composition and function of exosomes and other extracellular vesicles produced during viral, parasitic, fungal and bacterial infections and describe how these vesicles could function to either promote or inhibit host immunity.

Crosstalk between Mycobacterium tuberculosis and the host cell

The successful establishment and maintenance of a bacterial infection depend on the pathogen's ability to subvert the host cell's defense response and successfully survive, proliferate, or persist within the infected cell. To circumvent host defense systems, bacterial pathogens produce a variety of virulence factors that potentiate bacterial adherence and invasion and usurp host cell signaling cascades that regulate intracellular microbial survival and trafficking. Mycobacterium tuberculosis, probably one of the most successful pathogens on earth, has coexisted with humanity for centuries, and this intimate and persistent connection between these two organisms suggests that the pathogen has evolved extensive mechanisms to evade the human immune system at multiple levels. While some of these mechanisms are mediated by factors released by M. tuberculosis, others rely on host components that are hijacked to prevent the generation of an effective immune response thus benefiting the survival of M. tuberculosis within the host cell. Here, we describe several of these mechanisms, with an emphasis on the cyclic nucleotide signaling and subversion of host responses that occur at the intracellular level when tubercle bacilli encounter macrophages, a cell that becomes a safe-house for M. tuberculosis although it is specialized to kill most microbes.

Mycobacterium tuberculosis keto-mycolic acid and macrophage nuclear receptor TR4 modulate foamy biogenesis in granulomas: a case of a heterologous and noncanonical ligand-receptor pair

The cell wall of Mycobacterium tuberculosis is configured of bioactive lipid classes that are essential for virulence and potentially involved in the formation of foamy macrophages (FMs) and granulomas. Our recent work established crosstalk between M. tuberculosis cell wall lipids and the host lipid-sensing nuclear receptor TR4. In this study, we have characterized, identified, and adopted a heterologous ligand keto-mycolic acid from among M. tuberculosis lipid repertoire for the host orphan NR TR4. Crosstalk between cell wall lipids and TR4 was analyzed by transactivation and promoter reporter assays. Mycolic acid (MA) was found to transactivate TR4 significantly compared with other cell wall lipids. Among the MA, the oxygenated form, keto-MA, was responsible for transactivation, and the identity was validated by TR4 binding assays followed by TLC and nuclear magnetic resonance. Isothermal titration calorimetry revealed that keto-MA binding to TR4 is energetically favorable. This keto-MA-TR4 axis seems to be essential to this oxygenated MA induction of FMs and granuloma formation as evaluated by in vitro and in vivo model of granuloma formation. TR4 binding with keto-MA features a unique association of host nuclear receptor with a bacterial lipid and adds to the presently known ligand repertoire beyond dietary lipids. Pharmacologic modulation of this heterologous axis may hold promise as an adjunct therapy to frontline tuberculosis drugs.

Antigens for CD4 and CD8 T cells in tuberculosis

Tuberculosis (TB), caused by infection with Mycobacterium tuberculosis (MTB), represents an important cause of morbidity and mortality worldwide for which an improved vaccine and immunodiagnostics are urgently needed. CD4(+) and CD8(+) T cells play an important role in host defense to TB. Definition of the antigens recognized by these T cells is critical for improved understanding of the immunobiology of TB and for development of vaccines and diagnostics. Herein, the antigens and epitopes recognized by classically HLA class I- and II-restricted CD4(+) and CD8(+) T cells in humans infected with MTB are reviewed. Immunodominant antigens and epitopes have been defined using approaches targeting particular TB proteins or classes of proteins and by genome-wide discovery approaches. Antigens and epitopes recognized by classically restricted CD4(+) and CD8(+) T cells show extensive breadth and diversity in MTB-infected humans.

Host-pathogen interactions during Mycobacterium tuberculosis infections

The intimate and persistent connection between Mycobacterium tuberculosis and its human host suggests that the pathogen has evolved extensive mechanisms to evade eradication by the immune system. In particular, the organism has adapted to replicate within phagocytic cells, especially macrophages, which are specialized to kill microbes. Over the past decade of M. tuberculosis research, the means to manipulate both the organism and the host has ushered in an exciting time that has uncovered some of the mechanisms of the innate macrophage-pathogen interactions that lie at the heart of M. tuberculosis pathogenesis, though many interactions likely still await discovery. In this chapter, we will delve into some of these advances, with an emphasis on the interactions that occur on the cellular level when M. tuberculosis cells encounter macrophages. In particular, we focus on two major aspects of M. tuberculosis biology regarding the proximal physical interface between the bacterium and host, namely the interactions with the phagosomal membrane as well as the distinctive mycobacterial cell wall. Importantly, some of the emerging paradigms in M. tuberculosis pathogenesis and host response represent common themes in bacterial pathogenesis, such as the role of host cell membrane perforation in intracellular survival and host response. However, the array of unique bacterial lipid mediators and their interaction with host cells highlights the unique biology of this persistent pathogen.

The carrying pigeons of the cell: exosomes and their role in infectious diseases caused by human pathogens

Exosomes have recently been classified as the newest family members of 'bioactive vesicles' that function to promote intercellular communication. Long ignored and thought to be only a mechanism by which cellular waste is removed, exosomes have garnered a huge amount of interest in recent years as their critical functions in maintaining homeostasis through intercellular communication and also in different types of diseases have been demonstrated. Many groundbreaking studies of exosome functions have been performed in the cancer field and the infectious disease areas of study, revealing the importance and also the fascinating complexity of exosomal packaging, targeting, and functions. Selective packaging of exosomes in response to the type of infection, exosomal modulation of the immune response and host signaling pathways, exosomal regulation of pathogen spread, and effects of exosomes on the degree of pathogenesis have all been well documented. In this review, we provide a synthesis of the current understanding of the role of exosomes during infections caused by human pathogens and discuss the implications of these findings for a better understanding of pathogenic mechanisms and future therapeutic and diagnostic applications.

Analysis of Toll-like receptors, iNOS and cytokine profiles in patients with pulmonary tuberculosis during anti-tuberculosis treatment

Toll-like receptors (TLRs) play an important role in mycobacterial infection, although little is known about the roles of these receptors, cytokines and nitric oxide during anti-tuberculosis treatment. Our objective was to evaluate the mRNA and cell surface expression of TLR2 and TLR4; inducible nitric oxide synthase (iNOS) expression; and cytokine Th1, Th2 and Th17 profiles in pulmonary tuberculosis patients at different time points of anti-tuberculosis treatment. Peripheral blood mononuclear cells (PBMCs) were obtained from PPD⁺ healthy controls and from patients receiving anti-tuberculosis treatment. Gene expression quantification was performed by qPCR, cell surface expression was assessed using flow cytometry, and cytokine quantification was conducted using the CBA technique. The treated patients presented higher gene expression and higher numbers of receptors on the cell surface of lymphocytes and monocytes than did control individuals. IL-12 and IFN-γ levels increased after the start of treatment, whereas TNF-α levels were reduced. TGF-β presented the highest levels during treatment. IL-10 and IL-17 expression and production tended to increase during treatment. iNOS gene expression was reduced throughout treatment in patients. Our results suggest that anti-tuberculosis treatment modulates the immune response, inducing an increase in the expression of TLRs and pro- and anti-inflammatory cytokines to combat bacteria and reduce the inflammatory process.

Mycobacteria and the greasy macrophage: getting fat and frustrated

In the current issue of Infection and Immunity, Caire-Brändli and coworkers (Infect. Immun. 82:476-490, 2014, doi:10.1128/IAI.01196-13) describe a novel cell system for studying mycobacterial interactions with foamy macrophages and provide a magnificent series of electron microscopy-based observations providing major insight into the microbiology and cell biology of these interactions.

Mycobacterial phosphatidylinositol mannoside 6 (PIM6) up-regulates TCR-triggered HIV-1 replication in CD4+ T cells

Tuberculosis (TB) is the leading cause of mortality among those infected with human immunodeficiency virus (HIV-1) worldwide. HIV-1 load and heterogeneity are increased both locally and systemically in active TB. Mycobacterium tuberculosis (MTB) infection supports HIV-1 replication through dysregulation of host cytokines, chemokines, and their receptors. However the possibility that mycobacterial molecules released from MTB infected macrophages directly interact with CD4(+) T cells triggering HIV-1 replication has not been fully explored. We studied the direct effect of different MTB molecules on HIV-1 replication (R5-tropic strain Bal) in anti-CD3- stimulated CD4(+) T cells from healthy donors in an antigen presenting cell (APC)-free system. PIM6, a major glycolipid of the mycobacterial cell wall, induced significant increases in the percent of HIV-1 infected T cells and the viral production in culture supernatants. In spite of structural relatedness, none of the other three major MTB cell wall glycolipids had significant impact on HIV-1 replication in T cells. Increased levels of IFN-γ in culture supernatants from cells treated with PIM6 indicate that HIV-1 replication is likely dependent on enhanced T cell activation. In HEK293 cells transfected with TLR2, PIM6 was the strongest TLR2 agonist among the cell wall associated glycolipids tested. PIM6 increased the percentage of HIV infected cells and viral particles in the supernatant in a T-cell-based reporter cell line (JLTRg-R5) transfected with TLR1 and TLR2 but not in the cells transfected with the empty vector (which lack TLR2 expression) confirming that PIM6-induced HIV-1 replication depends at least partially on TLR2 signaling.

TAP mediates import of Mycobacterium tuberculosis-derived peptides into phagosomes and facilitates loading onto HLA-I

Processing and presentation of antigen on MHC-I class I molecules serves to present peptides derived from cytosolic proteins to CD8⁺ T cells. Infection with bacteria that remain in phagosomal compartments, such as Mycobacterium tuberculosis (Mtb), provides a challenge to this immune recognition as bacterial proteins are segregated from the cytosol. Previously we identified the Mtb phagosome itself as an organelle capable of loading MHC Class I molecules with Mtb antigens. Here, we find that the TAP transporter, responsible for importing peptides into the ER for loading in Class I molecules, is both present and functional in Mtb phagosomes. Furthermore, we describe a novel peptide reagent, representing the N-terminal domain of the bovine herpes virus UL49.5 protein, which is capable of specifically inhibiting the lumenal face of TAP. Together, these results provide insight into the mechanism by which peptides from intra-phagosomal pathogens are loaded onto Class I molecules.

Intracellular Mycobacterium tuberculosis exploits host-derived fatty acids to limit metabolic stress

Recent data indicate that the nutrients available to Mycobacterium tuberculosis (Mtb) inside its host cell are restricted in their diversity. Fatty acids and cholesterol appear more favored; however, their degradation can result in certain metabolic stresses. Their breakdown can generate propionyl-CoA, which gives rise to potentially toxic intermediates. Detoxification of propionyl-CoA relies on the activity of the methylcitrate cycle, the methylmalonyl pathway, or incorporation of the propionyl-CoA into methyl-branched lipids in the cell wall. The current work explores carbon flux through these pathways, focusing primarily on those pathways responsible for the incorporation of propionyl-CoA into virulence-associated cell wall lipids. Exploiting both genetic and biochemical rescue, we demonstrate that these metabolic pressures are experienced by Mtb inside its host macrophage and that the bacterium accesses host fatty acid stores. The metabolism of these host lipids expands the acetyl-CoA pool and alleviates the pressure from propionyl-CoA. These data have major implications for our appreciation of central metabolism of Mtb during the course of infection.

The evolutionary pressures that have molded Mycobacterium tuberculosis into an infectious adjuvant

Mycobacterium tuberculosis (Mtb) is highly immunogenic and appears to have evolved to preserve its antigenicity. The retention of antigenicity is important to the maintenance of a robust immune response that contributes greatly to the late-stage tissue damage required for transmission and completion of the pathogen's life cycle. Bacterial persistence is achieved through the remodeling of the tissue at site of infection and maintaining the lymphocytes at a distance from the infected macrophages in the granuloma core. The tissue metabolism within the granuloma leads to lipid sequestration that supports bacterial growth. However, growth on host lipids places metabolic stresses on Mtb, which has evolved to incorporate potentially harmful metabolic intermediates into the very cell wall lipids that induce the remodeling of the host tissue response.

Protective and survival efficacies of Rv0160c protein in murine model of Mycobacterium tuberculosis

The proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) multi-gene families code for approximately 10% of the Mycobacterium tuberculosis (Mtb) genome. These proteins are thought to be virulence factors that participate in impounding the host immune responses. While some members have been studied, the functions of most PE/PPE proteins are yet to be explored. The studies presented here have specifically characterized the roles of one of the PE proteins of Mtb, Rv0160c (PE4), in mycobacterial persistence and in prophylactic efficacy. We have expressed Rv0160c in a non-pathogenic fast-growing Mycobacterium smegmatis strain and demonstrated that the protein improves the survival of mycobacteria in macrophages and in mice. The protein has also shown its effect under physiological stress of bacteria, as evidenced by elevated expression in acidic and in hypoxic conditions. In mice, the level of Rv0160c was noticeably high during the chronic stage of tuberculosis. The seroreactivity of the protein against different categories of tuberculosis patients revealed a strong B-cell humoral response in freshly infected pulmonary tuberculosis patients. In mice, it exhibited increased IL-2, TNF, and IL-6 production. The antigenic properties of the protein directed towards the protective efficacy against the Mtb challenge. All together, our findings have identified Rv0160c as an in vivo expressed immunodominant antigen which plays a crucial role in the pathogenesis of mycobacterial disease and could prove to be a good preventive antigen for tuberculosis.

Exosomes isolated from mycobacteria-infected mice or cultured macrophages can recruit and activate immune cells in vitro and in vivo

More than 2 billion people are infected with Mycobacterium. tuberculosis; however, only 5-10% of those infected will develop active disease. Recent data suggest that containment is controlled locally at the level of the granuloma and that granuloma architecture may differ even within a single infected individual. Formation of a granuloma likely requires exposure to mycobacterial components released from infected macrophages, but the mechanism of their release is still unclear. We hypothesize that exosomes, which are small membrane vesicles containing mycobacterial components released from infected macrophages, could promote cellular recruitment during granuloma formation. In support of this hypothesis, we found that C57BL/6 mouse-derived bone marrow macrophages treated with exosomes released from M. tuberculosis-infected RAW264.7 cells secrete significant levels of chemokines and can induce migration of CFSE-labeled macrophages and splenocytes. Exosomes isolated from the serum of M. bovis bacillus Calmette-Guérin-infected mice could also stimulate macrophage production of chemokines and cytokines ex vivo, but the level and type differed during the course of a 60-d infection. Of interest, the exosome concentration in serum correlated strongly with mouse bacterial load, suggesting some role in immune regulation. Finally, hollow fiber-based experiments indicated that macrophages treated with exosomes released from M. tuberculosis-infected cells could promote macrophage recruitment in vivo. Exosomes injected intranasally could also recruit CD11b(+) cells into the lung. Overall, our study suggests that exosomes may play an important role in recruiting and regulating host cells during an M. tuberculosis infection.

Nonprotein structures from mycobacteria: emerging actors for tuberculosis control

Immune response to Mycobacterium tuberculosis, the causal agent of tuberculosis, is critical for protection. For many decades, consistent to classical biochemistry, most studies regarding immunity to the tubercle bacilli focused mainly on protein structures. But the atypical, highly impermeable and waxy coat of mycobacteria captured the interest of structural biologists very early, allowing the description of amazing molecules, such as previously unknown carbohydrates or fatty acids of astonishing lengths. From their discovery, cell wall components were identified as important structural pillars, but also as molecular motifs able to alter the human immune response. Recently, as new developments have emerged, classical conceptions of mycobacterial immune modulators have been giving place to unexpected discoveries that, at the turn of the last century, completely changed our perception of immunity vis-à-vis fat compounds. In this paper, current knowledge about chemical and ultrastructural features of mycobacterial cell-wall is overviewed, with an emphasis on the relationships between cell-wall nonpeptide molecules and immune response. Remarks regarding the potential of these molecules for the development of new tools against tuberculosis are finally discussed.

Exosomes released from Mycoplasma infected tumor cells activate inhibitory B cells

Mycoplasmas cause numerous human diseases and are common opportunistic pathogens in cancer patients and immunocompromised individuals. Mycoplasma infection elicits various host immune responses. Here we demonstrate that mycoplasma-infected tumor cells release exosomes (myco+ exosomes) that specifically activate splenic B cells and induce splenocytes cytokine production. Induction of cytokines, including the proinflammatory IFN-γ and the anti-inflammatory IL-10, was largely dependent on the presence of B cells. B cells were the major IL-10 producers. In splenocytes from B cell deficient μMT mice, induction of IFN-γ+ T cells by myco+ exosomes was greatly increased compared with wild type splenocytes. In addition, anti-CD3-stimulated T cell proliferation was greatly inhibited in the presence of myco+ exosome-treated B cells. Also, anti-CD3-stimulated T cell signaling was impaired by myco+ exosome treatment. Proteomic analysis identified mycoplasma proteins in exosomes that potentially contribute to the effects. Our results demonstrate that mycoplasma-infected tumor cells release exosomes carrying mycoplasma components that preferentially activate B cells, which in turn, are able to inhibit T cell activity. These results suggest that mycoplasmas infecting tumor cells can exploit the exosome pathway to disseminate their own components and modulate the activity of immune cells, in particular, activate B cells with inhibitory activity.

Escape from the Phagosome: The Explanation for MHC-I Processing of Mycobacterial Antigens?

Mycobacterium tuberculosis (Mtb) is thought to live in an altered phagosomal environment. In this setting, the mechanisms by which mycobacterial antigens access the major histocompatibility class I (MHC-I) processing machinery remain incompletely understood. There is evidence that Mtb antigens can be processed in both endocytic and cytosolic environments, with different mechanisms being proposed for how Mtb antigens can access the cytosol. Recently, electron microscopy was used to demonstrate that Mtb has the potential to escape the phagosome and reside in the cytosol. This was postulated as the primary mechanism by which Mtb antigens enter the MHC-I processing and presentation pathway. In this commentary, we will review data on the escape of Mtb from the cytosol and whether this escape is required for antigen presentation to CD8⁺ T cells.