Apoptosis of human monocytes/macrophages in Mycobacterium tuberculosis infection

In-depth systems biological evaluation of bovine alveolar macrophages suggests novel insights into molecular mechanisms underlying Mycobacterium bovis infection

OBJECTIVE

Bovine tuberculosis (bTB) is a chronic respiratory infectious disease of domestic livestock caused by intracellular Mycobacterium bovis infection, which causes ~$3 billion in annual losses to global agriculture. Providing novel tools for bTB managements requires a comprehensive understanding of the molecular regulatory mechanisms underlying the M. bovis infection. Nevertheless, a combination of different bioinformatics and systems biology methods was used in this study in order to clearly understand the molecular regulatory mechanisms of bTB, especially the immunomodulatory mechanisms of M. bovis infection.

METHODS

RNA-seq data were retrieved and processed from 78 (39 non-infected control vs. 39 M. bovis-infected samples) bovine alveolar macrophages (bAMs). Next, weighted gene co-expression network analysis (WGCNA) was performed to identify the co-expression modules in non-infected control bAMs as reference set. The WGCNA module preservation approach was then used to identify non-preserved modules between non-infected controls and M. bovis-infected samples (test set). Additionally, functional enrichment analysis was used to investigate the biological behavior of the non-preserved modules and to identify bTB-specific non-preserved modules. Co-expressed hub genes were identified based on module membership (MM) criteria of WGCNA in the non-preserved modules and then integrated with protein-protein interaction (PPI) networks to identify co-expressed hub genes/transcription factors (TFs) with the highest maximal clique centrality (MCC) score (hub-central genes).

RESULTS

As result, WGCNA analysis led to the identification of 21 modules in the non-infected control bAMs (reference set), among which the topological properties of 14 modules were altered in the M. bovis-infected bAMs (test set). Interestingly, 7 of the 14 non-preserved modules were directly related to the molecular mechanisms underlying the host immune response, immunosuppressive mechanisms of M. bovis, and bTB development. Moreover, among the co-expressed hub genes and TFs of the bTB-specific non-preserved modules, 260 genes/TFs had double centrality in both co-expression and PPI networks and played a crucial role in bAMs-M. bovis interactions. Some of these hub-central genes/TFs, including PSMC4, SRC, BCL2L1, VPS11, MDM2, IRF1, CDKN1A, NLRP3, TLR2, MMP9, ZAP70, LCK, TNF, CCL4, MMP1, CTLA4, ITK, IL6, IL1A, IL1B, CCL20, CD3E, NFKB1, EDN1, STAT1, TIMP1, PTGS2, TNFAIP3, BIRC3, MAPK8, VEGFA, VPS18, ICAM1, TBK1, CTSS, IL10, ACAA1, VPS33B, and HIF1A, had potential targets for inducing immunomodulatory mechanisms by M. bovis to evade the host defense response.

CONCLUSION

The present study provides an in-depth insight into the molecular regulatory mechanisms behind M. bovis infection through biological investigation of the candidate non-preserved modules directly related to bTB development. Furthermore, several hub-central genes/TFs were identified that were significant in determining the fate of M. bovis infection and could be promising targets for developing novel anti-bTB therapies and diagnosis strategies.

Utility of plasma cell-free DNA detection using homobifunctional imidoesters using a microfluidic system for diagnosing active tuberculosis

BACKGROUND

It is difficult to diagnose tuberculosis (TB), particularly sputum-scarce pulmonary TB and extrapulmonary TB, using conventional diagnostic tests. Since these cases require additional invasive procedures to obtain appropriate specimens, new non-invasive diagnostic tests are needed. Plasma cell-free DNA (cfDNA) detection has gained interest as a novel diagnostic test for TB as it is convenient and less invasive. Therefore, we investigated the performance of enriched cfDNA for diagnosing pulmonary TB and extrapulmonary TB.

METHODS

All patients suspected to have TB, who consented to the use of blood for detecting cfDNA, were prospectively enrolled from January 2019 to June 2020. We categorised the patients as confirmed, probable, possible TB, and not-TB. We compared the performance of cfDNA with those of conventional diagnostic tests.

RESULTS

Among the 96 patients enrolled, 40 (41.7%) had TB, including 34 with confirmed TB and six probable TB, and 41 (42.7%) did not have TB. Acid-fast bacilli microscopy, Xpert MTB/RIF, and mycobacterial culture results were positive in 12 (31.6%), 22 (61.1%), and 25 (65.8%) patients, respectively. The sensitivity and specificity of cfDNA were 80.0% and 78.1%, respectively. While the sensitivity and specificity of cfDNA were similar to those of interferon-gamma releasing assay (IGRA) (sensitivity 80.6% and specificity 71.4%), the combined sensitivity and specificity of the two assays were 94.4% and 64.3%, respectively, which can be used to rule out TB.

CONCLUSIONS

Plasma cfDNA assay seems to be a useful adjunct to the current tests for diagnosing TB, especially when used in combination with IGRA for ruling out TB.AbbreviationsTBtuberculosiscfDNAcell-free DNAPCRpolymerase chain reactionAFBacid-fast bacilliIGRAinterferon-gamma releasing assayCTcomputed tomographyHIVhuman immunodeficiency virus.

Mycobacterium tuberculosis Rv1515c antigen enhances survival of M. smegmatis within macrophages by disrupting the host defence

Mycobacterium tuberculosis (Mtb) infection is the major cause of tuberculosis. Mtb regions of difference (RD) genes are vital for survival of the pathogen within hosts and for the attenuation of the bacillus Calmette-Guérin vaccine. However, the function of most RD proteins largely remains unexplored. In the present study, we focused on Rv1515c, an RD6 member from M. tuberculosis, and characterised it as a cell surface-associated protein that functions in disrupting the cytokine profile and promoting endoplasmic reticulum stress-mediated apoptosis. Rv1515c expression in M. smegmatis, a nonpathogenic species, resulted in enhanced resistance of the bacterium to various in vitro stressors (such as low pH, sodium dodecyl sulfate, oxidative pressure, and nitrogen intermediate) and its cellular survival within macrophages. Our study is the first to identify the role of Rv1515c in the physiology and pathogenesis of mycobacterium.

Mycobacterium tuberculosis ESX-1-secreted substrate protein EspC promotes mycobacterial survival through endoplasmic reticulum stress-mediated apoptosis

EsxA, secreted by the ESAT-6 secretion system 1 (ESX-1) secretion system, is considered the major Mycobacterium tuberculosis (Mtb) virulence determinant. However, the roles of the individual ESX-1 substrates, such as EspC, remain unclear due to their interdependency for secretion with EsxA. Here, we validated that EspC triggered ER stress-mediated apoptosis in macrophages. The EspC-mediated ER stress was involved in pro-inflammatory cytokines generation, intracellular Ca₂₊ release, and reactive oxygen species accumulation. Mitochondrial transmembrane potential dissipation and mitochondrial outer membrane permeabilization occurred in EspC-treated macrophages, causing apoptosis. Furthermore, ER stress-mediated apoptosis was effectively induced in EspC-overexpressing Mycobacterium smegmatis-infected macrophages and mice. EspC overexpression caused a significant increase in bacterial survival in the macrophages, spleens, and lungs, and accelerated mouse death was observed. Moreover, the increased viability of bacteria in the macrophages was significantly reduced by pretreatment with the apoptosis inhibitor. Overall, our results revealed that EspC is an essential ESX-1 protein for Mtb–host interactions and EspC-induced ER stress-mediated apoptosis may be employed by Mtb to establish and spread infection. Given the critical roles of the ESX systems in Mtb pathogenesis and immunity, our findings offer new perspectives on the complex host-pathogen interactions and mechanisms underlying ESX-1-mediated pathogenesis.

The role of Mycobacterium tuberculosis complex species on apoptosis and necroptosis state of macrophages derived from active pulmonary tuberculosis patients

Objective

The role of Mycobacterium tuberculosis complex (MTBC) species in tuberculosis (TB) infection in human is still questioned. The aim of this study was to determine whether M. tuberculosis and M. bovis is associated with apoptosis and necroptosis by measuring the expression of specific signaling pathways components (Fas-associated protein with death domain (FADD) and receptor interacting protein 3 (RIP3)), and the level of apoptosis.

Results

We recruited 30 patients with pulmonary TB; 24 patients were infected with M. tuberculosis Beijing strain and six patients with M. bovis BCG strain. M. tuberculosis-infected patients were more likely to have severe lung damage compared to those infected with M. bovis (odds ratio [OR] 7.60; 95% confidence interval [CI] 1.07–54.09). M. tuberculosis infection was associated with lower expression of FADD and lower apoptosis level of macrophages compared to M. bovis. No significant different of RIP3 between MTBC species groups. In conclusion, M. tuberculosis Beijing strain was associated with severe pulmonary damage, inhibited FADD expression and reduced apoptosis level of macrophages derived from pulmonary TB patients. This suggests that the M. tuberculosis Beijing strain is potentially to be used as determinant of disease progressivity and tissue damage in TB cases.

Transcriptional Profiling of Human Peripheral Blood Mononuclear Cells Identifies Diagnostic Biomarkers That Distinguish Active and Latent Tuberculosis

Mycobacterium tuberculosis (M. tuberculosis) infection in humans can cause active disease or latent infection. However, the factors contributing to the maintenance of latent infection vs. disease progression are poorly understood. In this study, we used a genome-wide RNA sequencing (RNA-seq) approach to identify host factors associated with M. tuberculosis infection status and a novel gene signature that can distinguish active disease from latent infection. By RNA-seq, we characterized transcriptional differences in purified protein derivative (PPD)-stimulated peripheral blood mononuclear cells (PBMCs) among three groups: patients with active tuberculosis (ATB), individuals with latent TB infection (LTBI), and TB-uninfected controls (CON). A total of 401 differentially expressed genes enabled grouping of individuals into three clusters. A validation study by quantitative real-time PCR (qRT-PCR) confirmed the differential expression of TNFRSF10C, IFNG, PGM5, EBF3, and A2ML1 between the ATB and LTBI groups. Additional clinical validation was performed to evaluate the diagnostic performance of these five biomarkers using 130 subjects. The 3-gene signature set of TNFRSF10C, EBF3, and A2ML1 enabled correct classification of 91.5% of individuals, with a high sensitivity of 86.2% and specificity of 94.9%. Diagnostic performance of the 3-gene signature set was validated using a clinical cohort of 147 subjects with suspected ATB. The sensitivity and specificity of the 3-gene set for ATB were 82.4 and 92.4%, respectively. In conclusion, we detected distinct gene expression patterns in PBMCs stimulated by PPD depending on the status of M. tuberculosis infection. Furthermore, we identified a 3-gene signature set that could distinguish ATB from LTBI, which may facilitate rapid diagnosis and treatment for more effective disease control.

Cell death at the cross roads of host-pathogen interaction in Mycobacterium tuberculosis infection

Tuberculosis (TB) continues to be the leading cause of death by any single infectious agent, accounting for around 1.7 million annual deaths globally, despite several interventions and support programs by national and international agencies. With the development of drug resistance in Mycobacterium tuberculosis (M. tb), there has been a paradigm shift in TB research towards host-directed therapy. The potential targets include the interactions between host and bacterial proteins that are crucial for pathogenesis. Hence, collective efforts are being made to understand the molecular details of host-pathogen interaction for possible translation into host-directed therapy. The present review focuses on 'host cell death modalities' of host-pathogen interaction, which play a crucial role in determining the outcome of TB disease progression. Several cell death modalities that occur in response to mycobacterial infection have been identified in human macrophages either as host defences for bacterial clearance or as pathogen strategies for multiplication and dissemination. These cell death modalities include apoptosis, necrosis, pyroptosis, necroptosis, pyronecrosis, NETosis, and autophagy. These processes are highly overlapping with several mycobacterial proteins participating in more than one cell death pathway. Until now, reviews in M. tb and host cell death have discussed either focusing on host evasion strategies, apoptosis, autophagy, and necrosis or describing all these forms with limited discussions of their role in host-pathogen interactions. Here, we present a comprehensive review of various mycobacterial factors modulating host cell death pathways and the cross-talk between them. Besides this, we have discussed the networking of host cell death pathways including the interference of host miRNA during M. tb infection with their respective targets. Through this review, we present the host targets that overlap across several cell death modalities and the technical limitations of methodology in cell death research. Given the compelling need to discover alternative drug target(s), this review identifies these overlapping cell death factors as potential targets for host-directed therapy.

Transglutaminase type 2 plays a key role in the pathogenesis of Mycobacterium tuberculosis infection

BACKGROUND

Mycobacterium tuberculosis (MTB), the aetiological agent of tuberculosis (TB), is capable of interfering with the phagosome maturation pathway, by inhibiting phagosome-lysosome fusion and the autophagic process to ensure survival and replication in macrophages. Thus, it has been proposed that the modulation of autophagy may represent a therapeutic approach to reduce MTB viability by enhancing its clearance.

OBJECTIVE

The aim of this study was to investigate whether transglutaminase type 2 (TG2) is involved in the pathogenesis of MTB.

RESULTS

We have shown that either genetic or pharmacological inhibition of TG2 leads to a marked reduction in MTB replicative capacity. Infection of TG2 knockout mice demonstrated that TG2 is required for MTB intracellular survival in macrophages and host tissues. The same inhibitory effect can be reproduced in vitro using Z-DON, a specific inhibitor of the transamidating activity of TG2. Massive cell death observed in macrophages that properly express TG2 is hampered by the absence of the enzyme and can be largely reduced by the treatment of wild-type macrophages with the TG2 inhibitor. Our data suggest that reduced MTB replication in cells lacking TG2 is due to the impairment of LC3/autophagy homeostasis. Finally, we have shown that treatment of MTB-infected murine and human primary macrophages with cystamine, a TG2 inhibitor already tested in clinical studies, causes a reduction in intracellular colony-forming units in human macrophages similar to that achieved by the anti-TB drug capreomycin.

CONCLUSION

These results suggest that inhibition of TG2 activity is a potential novel approach for the treatment of TB.

PPD-induced monocyte mitochondrial damage is associated with a protective effect to develop tuberculosis in BCG vaccinated individuals: A cohort study

Introduction

The mechanisms of mononuclear phagocyte death have been associated with the permissiveness and resistance to mycobacterial replication, but it remains unknown whether or not they help predict the risk of developing TB.

Objective

To describe the factors associated with the induction of monocyte mitochondrial and membrane damage in response to PPD as well as determine if this type of damage might predict the susceptibility of developing active tuberculosis in a cohort of household contacts (HHCs) from Medellin, Colombia from 2005 to 2008.

Methods

The prospective cohort study contains 2060 HHCs patients with pulmonary tuberculosis who were meticulously followed for two years. A survey of the socio-demographic, clinical, epidemiological factors and blood samples were collected. Mononuclear cell cultures were stimulated with or without PPD and the type of monocyte death was determined by the flow of cytometry, an indicator was also used for its analysis. Logistic regression was adjusted by the Generalized Estimations Equations and the survival was estimated with the Kaplan-Meier and Cox regression. Confidence intervals were used for estimating the association.

Results

1,859 out of 2,060 blood samples of the HHCs patients analyzed showed monocyte death. In response to PPD, 83.4% underwent mitochondrial damage while 50.9% had membrane damage. The membrane damage in response to PPD was higher in children under 4 years (OR: 1.57; (95% CI: 1.1 to 2.4) and the HHCs who slept regularly in the same household has an index case of (OR: 1.54; 95% CI: 1.0 to 2.3). After adjustment by age, comorbidities, nutritional status, proximity to index case and overcrowding, the risk of developing active TB among BCG vaccinated HHCs individuals with induction of mitochondrial damage was HR = 0.19 (95% CI: 0.1 to 0.5).

Conclusions

The induction of monocytes mitochondrial damage by PPD stimulation correlates with protection of TB disease development in BCG-vaccinated HHCs. This represents a potential tool to predict susceptibility of developing active disease in this population.

Recombinant Lipoprotein Rv1016c Derived from Mycobacterium tuberculosis Is a TLR-2 Ligand that Induces Macrophages Apoptosis and Inhibits MHC II Antigen Processing

TLR2-dependent cellular signaling in Mycobacterium tuberculosis-infected macrophages causes apoptosis and inhibits class II major histocompatibility complex (MHC-II) molecules antigen processing, leading to evasion of surveillance. Mycobacterium tuberculosis (MTB) lipoproteins are an important class of Toll-like receptor (TLR) ligand, and identified as specific components that mediate these effects. In this study, we identified and characterized MTB lipoprotein Rv1016c (lpqT) as a cell wall associated-protein that was exposed on the cell surface and enhanced the survival of recombinants M. smegmatis_Rv1016c under stress conditions. We found that Rv1016c lipoprotein was a novel TLR2 ligand and able to induce macrophage apoptosis in a both dose- and time-dependent manner. Additionally, apoptosis induced by Rv1016c was reserved in THP-1 cells blocked with anti-TLR-2 Abs or in TLR2^−/− mouse macrophages, indicating that Rv1016c-induced apoptosis is dependent on TLR2. Moreover, we demonstrated that Rv1016c lipoprotein inhibited IFN-γ-induced MHC-II expression and processing of soluble antigens in a TLR2 dependent manner. Class II transactivator (CIITA) regulates MHC II expression. In this context, Rv1016c lipoprotein diminished IFN-γ-induced expression of CIITA IV through TLR2 and MAPK Signaling. TLR2-dependent apoptosis and inhibition of MHC-II Ag processing induced by Rv1016c during mycobacteria infection may promote the release of residual bacilli from apoptotic cells and decrease recognition by CD4⁺ T cells. These mechanisms may allow intracellular MTB to evade immune surveillance and maintain chronic infection.

Low Dose BCG Infection as a Model for Macrophage Activation Maintaining Cell Viability

Mycobacterium bovis BCG, the current vaccine against tuberculosis, is ingested by macrophages promoting the development of effector functions including cell death and microbicidal mechanisms. Despite accumulating reports on M. tuberculosis, mechanisms of BCG/macrophage interaction remain relatively undefined. In vivo, few bacilli are sufficient to establish a mycobacterial infection; however, in vitro studies systematically use high mycobacterium doses. In this study, we analyze macrophage/BCG interactions and microenvironment upon infection with low BCG doses and propose an in vitro model to study cell activation without affecting viability. We show that RAW macrophages infected with BCG at MOI 1 activated higher and sustained levels of proinflammatory cytokines and transcription factors while MOI 0.1 was more efficient for early stimulation of IL-1β, MCP-1, and KC. Both BCG infection doses induced iNOS and NO in a dose-dependent manner and maintained nuclear and mitochondrial structures. Microenvironment generated by MOI 1 induced macrophage proliferation but not MOI 0.1 infection. In conclusion, BCG infection at low dose is an efficient in vitro model to study macrophage/BCG interactions that maintains macrophage viability and mitochondrial structures. This represents a novel model that can be applied to BCG research fields including mycobacterial infections, cancer immunotherapy, and prevention of autoimmunity and allergies.

Synovial fluid from patients with rheumatoid arthritis modulates monocyte cell-surface phenotype

Objectives

To investigate the ability of synovial fluid from patients with rheumatoid arthritis (RA) or osteoarthritis (OA) to modulate cell-surface phenotype, function and viability of monocytes.

Methods

Monocytes from healthy donors were incubated with synovial fluid from patients with RA or OA. These were then cultured with autologous healthy CD4+ T-cells. Immunoglobulin-like transcript 4 (ILT4) and CD86 were evaluated on stimulated monocytes and CD4+ T-cells via fluorescence activated cell sorting.

Results

Monocytes incubated with synovial fluid from patients with RA (SF-RA; n = 12) had significantly lower ILT4 and higher CD86 levels than those incubated with synovial fluid from patients with OA (SF-OA; n = 12) or medium alone. In patients with RA, there was a significant negative correlation between ILT4 and disease activity score (DAS; r = −0.699), and a positive correlation between CD86 and DAS (r = 0.626). T-cells costimulated with monocytes cultured with SF-RA produced significantly more interferon-γ and tumour necrosis factor-α than those costimulated with monocytes cultured with SF-OA or controls.

Conclusions

Soluble mediators in SF-RA could contribute to modulating inflammation and local effectiveness of the immune response.

Mce4A protein of Mycobacterium tuberculosis induces pro inflammatory cytokine response leading to macrophage apoptosis in a TNF-α dependent manner

Mycobacterium tuberculosis subverts the host immune response through numerous immune-evasion strategies. Apoptosis has been identified as one such mechanism and has been well studied in M. tuberculosis infection. Here, we demonstrate that the Mce4A protein of mce4 operon is involved in the induction of host cell apoptosis. Earlier we have shown that the Mce4A was required for the invasion and survival of M. tuberculosis. In this report we present evidence to establish a role for Mce4A in the modulation of THP-1 cell survival. Recombinant Mce4A was expressed and purified from Escherichia coli as inclusion bodies and then refolded. Viability of THP-1 cells decreased in a dose-dependent manner when treated with Mce4A. The secretion of pro-inflammatory cytokines like tumor necrosis factor (TNF-α) or interferon gamma (IFN-γ), and enhanced nitric oxide release was observed when the THP-1 cells, were treated with Mce4A protein. The Mce4A induced apoptosis of the THP-1 cells was TNF-α dependent since blocking with anti TNF-α antibody abrogated this phenomenon. Collectively, these data suggest that Mce4A can induce the THP-1 cells to undergo apoptosis which primarily follows a TNF- α dependent pathway.

Indolyl Aryl Sulphones as HIV-1 Non-Nucleoside Reverse Transcriptase Inhibitors: Synthesis, Biological Evaluation and Binding Mode Studies of New Derivatives at Indole-2-carboxamide

New non-nucleoside reverse transcriptase inhibitors (NNRTIs) that are active against the commonly occurring mutations of HIV are urgently needed for the treatment of AIDS. We synthesized new NNRTIs of the indolyl aryl sulphone (IAS) family, which are endowed with high antiviral potency against HIV-1 wt (wild-type), and the Y181C and K103N-Y181C drug resistant mutant strains. Several new compounds were highly active in lymphocytes infected with primary isolates carrying the K103N-V108I-M184V and L100I-V108I mutations. The design of new IASs was based on three-dimensional quantitative structure-activity relationship (3D QSAR) studies and docking simulations. A cross-docking study was also undertaken to gain some insights in to the binding mode of the newly synthesized IASs in the wt and mutated isoforms of reverse transcriptase.

Discriminative expression of whole blood genes in HIV patients with latent and active TB in Ethiopia

BACKGROUND

Transcriptomic host biomarkers could assist in developing effective diagnostics, vaccines and therapeutics for tuberculosis (TB). However, different biomarkers may be discriminatory in different populations depending on the host and bacillary genetics and HIV infection, and need to be addressed.

METHODS

The expression levels of 45 genes that are known to be involved in or affected by TB pathogenesis were analyzed using dual color Reverse Transcriptase Multiplex Ligation-dependent Probe Amplification (dcRT-MLPA) assay in whole blood of 106 HIV positive individuals including active TB patients (TB(+)HIV(+), n = 29), and non TB patients that are tuberculin skin test positive (TST+) (TST(+)HIV(+), n = 26), or TST negative (TST(-)HIV(+), n = 51).

RESULTS

Between the two clinical groups (TB(+)HIV(+) vs. TST(-)HIV(+)) 8 genes were differently expressed (CCL19, CD14, CD8A, FPR1, IL7R, CCL22, TNFRSF1A, and FCGR1A); between TB(+)HIV(+) vs. TST(+)HIV(+), 6 genes (CD14, IL7R, TIMP2, CCL22, TNFRSF1A, and FCGR1A) were differently expressed. Since no difference in gene expression was revealed between TST(+)HIV(+) vs. TST(-)HIV(+), we clustered both the TST(+)HIV(+) and TST(-)HIV(+) individuals as one group (TST(+/-)HIV(+)) and compared gene expression with TB(+)HIV(+) patients. Thus, the results revealed that the levels of five genes (CD8A, TIMP2, CCL22, FCGR1A and TNFRSF1A) were the most accurate single gene markers for differentiation between TB(+)HIV(+) and TST(+/-)HIV(+), with AUCs of 0.71, 0.71, 0.79, 0.83 and 0.73, respectively. However, the combination of two genes (CCL22 + FCGR1A) and FCGR1A alone were the most accurate marker for differentiation between the two groups (TB(+)HIV(+) and TST(+/-)HIV(+)) with AUC of 0.85 and 0.83, respectively.

CONCLUSIONS

We showed that five genes (CD8A, TIMP2, CCL22, FCGR1A and TNFRSF1A), specifically FCGR1A and CCL22 have the potential to discriminate active TB from non-active TB in HIV patients in Ethiopia and could be used to improve diagnostic tools for active TB in HIV patients, and to understand the pathogenesis of TB/HIV coinfection.

T regulatory cells: Achilles' heel of Mycobacterium tuberculosis infection?

T regulatory cells (Treg) constitute a specialized subset of T cells that play a pivotal role in preventing the occurrence of autoimmune diseases by suppressing deleterious activities of immune cells. Contrarily, they can have adverse effect on immune response against infectious diseases where Treg weaken the host immunity leading to enhanced microbial load and thereby increase in severity of the disease. Here, we have attempted to review plethora of information documenting prevalence of Treg in tuberculosis (TB) and their involvement in progression and immunopathogenesis of the disease. Further, we have laid emphasis on the possible use of Treg as a biomarker for determining the TB treatment efficacy. Also, we have discussed the probable contribution of Treg in dampening the efficacy of BCG, the anti-TB vaccine. Finally, we have speculated some of the possible strategies which might be explored by exploiting Treg for enhancing the efficacy of TB management.

Mycobacterium tuberculosis PE25/PPE41 protein complex induces necrosis in macrophages: Role in virulence and disease reactivation?

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The Mycobacterium secreted protein PE25/PPE41 drives TNF-α secretion.

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PE25/PPE41 protein induces necrotic cell death, but not apoptosis, in macrophages.

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Necotic cell death induced by PE25/PPE41 is independent of TNF-α/NFκB/AP-1 pathways.

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PE25/PPE41 possibly acts as virulence factor, by an ‘immune quorum sensing’ mechanism.

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Necrotic cell death may help in mycobacterial dissemination and re-activation.

Necrotic cell death during TB infection is an important prerequisite for bacterial dissemination and virulence. The underlying mechanisms and the bacterial factors involved therein are not well understood. The Mycobacterium tuberculosis (M. tuberculosis) co-operonic PE25/PPE41 protein complex, similar to ESAT-6/CFP-10, belonging to the PE/PPE and ESAT-6 families of genes has co-expanded and co-evolved in the genomes of pathogenic mycobacteria. We report a novel role of this highly immunogenic PE25/PPE41 protein complex in inducing necrosis, but not apoptosis, in macrophages. We propose that these protein complexes of M. tuberculosis, secreted by similar/unique transport system (Type VII), have an important role in M. tuberculosis virulence and disease reactivation.

The Mycobacterium tuberculosis recombinant LprN protein of mce4 operon induces Th-1 type response deleterious to protection in mice

Lipoproteins are known to be effective immunogens and affect both innate and adaptive immunity. The lprN gene of Mycobacterium tuberculosis has been predicted to encode for a putative lipoprotein in silico. Here, we studied its function as an immunogen by in vivo studies in mice. The recombinant LprN protein, expressed and purified in Escherichia coli, triggered a cell-mediated immune response in BALB/c mice. This was observed by significantly higher T-cell proliferation and increased production of TNF-α and IFN-γ cytokines. However, pre-exposure to LprN protein failed to provide protection in mice after challenge with a virulent strain of M. tuberculosis. Histological examination showed an increase in tissue destruction in experimental animals, indicating an immunogenic potential for LprN protein that enhanced the virulence of bacilli.

Downregulation of immunoglobulin-like transcript-4 (ILT4) in patients with psoriatic arthritis

OBJECTIVE

The immunoglobulin-like transcript-4 (ILT4) is an inhibitory receptor that modulates the activity of innate immune agents. We determined the expression of ILT4 and analysed the relationship with the expression of costimulatory proteins and tumor necrosis factor-α (TNF-α) production in monocytes from patients with psoriatic arthritis (PsA) starting anti-TNF treatment.

METHODS

Peripheral blood monocytes from 15 healthy controls and from 16 patients with PsA were activated in vitro by CD40 ligand (CD40L) and analyzed for ILT4, CD40, CD80 and CD86 expression, and spontaneous lipopolysaccharide (LPS)-induced TNF-α production by flow cytometry, before and after treatment with adalimumab.

RESULTS

The percentage of ILT4-negative monocytes was greater in PsA patients compared to controls and negatively correlated with DAS44. Normal monocytes treated with sera of PsA patients showed a reduced expression of ILT4 compared with monocytes exposed to sera from controls. CD40, CD80 and CD86 expression was higher in patients compared to controls. Both spontaneous and LPS-induced TNF-α production was restricted to ILT4-negative monocytes and was greater in PsA patients compared to controls. Finally, twelve weeks-treatment with adalimumab resulted in a significant increase of ILT4 expression and a decrease of costimulatory molecules expression in PsA patients, compared to pre-therapy levels.

CONCLUSIONS

These data support the possibility that changes in the immunophenotype of monocytes play a role in the pathogenesis of PSA. Thus, modulation of the expression of ILT4 may represent an enticing new therapeutic target.

Upregulation of the inhibitory receptor ILT4 in monocytes from septic patients

Sepsis-induced immune dysfunction is a complex phenomenon that involves both innate and adaptive responses. Upregulation of the inhibitor receptor named immunoglobulin like transcript 4 (ILT4) is crucial to the tolerogenic function of monocytes. Here, ILT4 expression, endotoxin-induced IL-12 and IL-10 production and CD86 expression were investigated in circulating monocytes from 16 patients with severe sepsis and 16 age and sex matched controls. We found that monocytes from patients with severe sepsis express significantly higher levels of ILT4 than monocytes from controls. Upregulation of ILT4 expression appeared to be induced by soluble factors present in the serum of septic patients and directly correlated with the degree of organ dysfunction. ILT4(+) monocytes from septic patients also displayed an alteration in the cytokine response to endotoxin stimulation characterized by reduced IL-12 production and increased IL-10 production, and a reduced expression of the costimulatory molecule CD86. In conclusion, the increased ILT4 expression and IL-10 production and the decreased CD86 expression and IL-12 production indicate that during sepsis monocytes undergo substantial modulation of the surface and cytokine phenotype. These phenotypic changes may interfere with the antigen presenting cell activity of monocytes, which may contribute to the impairment of adaptive immune responses that takes place during sepsis.

Nitric oxide not apoptosis mediates differential killing of Mycobacterium bovis in bovine macrophages

To identify the resistance phenotype against Mycobacterium bovis in cattle, we used a bactericidal assay that has been considered a marker of this trait. Three of 24 cows (12.5%) were phenotyped as resistant and 21 as susceptible. Resistance of bovine macrophages (MΦ) to BCG challenge was evaluated for its association with SLC11A1 GT microsatellite polymorphisms within 3'UTR region. Twenty-three cows (95.8%) had a GT13 genotype, reported as resistant, consequently the SLC11A1 polymorphism was not in agreement with our bactericidal assay results. MΦ of cows with resistant or susceptible phenotype were challenged in vitro with virulent M. bovis field strain or BCG, and nitric oxide production, bacterial killing and apoptosis induction were measured in resting and LPS-primed states. M. bovis field strain induced more apoptosis than BCG, although the difference was not significant. Resistant MΦ controlled better the replication of M. bovis (P<0.01), produced more nitric oxide (P<0.05) and were slightly more prone to undergo apoptosis than susceptible cells. LPS pretreatment of MΦ enhanced all the functional parameters analyzed. Inhibition of nitric oxide production with n (G)-monomethyl-L-arginine monoacetate enhanced replication of M. bovis but did not modify apoptosis rates in both resistant and susceptible MΦ. We conclude that nitric oxide production not apoptosis is a major determinant of macrophage resistance to M. bovis infection in cattle and that the influence of SLC11A1 gene 3'UTR polymorphism is not associated with this event.

Human granuloma in vitro model, for TB dormancy and resuscitation

Tuberculosis (TB) is responsible for death of nearly two million people in the world annually. Upon infection, Mycobacterium tuberculosis (Mtb) causes formation of granuloma where the pathogen goes into dormant state and can live for decades before resuscitation to develop active disease when the immune system of the host is weakened and/or suppressed. In an attempt to better understand host-pathogen interactions, several groups have been developing in vitro models of human tuberculosis granuloma. However, to date, an in vitro granuloma model in which Mtb goes into dormancy and can subsequently resuscitate under conditions that mimic weakening of the immune system has not been reported. We describe the development of a biomimetic in vitro model of human tuberculosis granuloma using human primary leukocytes, in which the Mtb exhibited characteristics of dormant mycobacteria as demonstrated by (1) loss of acid-fastness, (2) accumulation of lipid bodies (3) development of rifampicin-tolerance and (4) gene expression changes. Further, when these micro granulomas were treated with immunosuppressant anti-tumor necrosis factor-alpha monoclonal antibodies (anti-TNFα mAbs), resuscitation of Mtb was observed as has been found in humans. In this human in vitro granuloma model triacylglycerol synthase 1deletion mutant (Δtgs1) with impaired ability to accumulate triacylglycerides (TG), but not the complemented mutant, could not go into dormancy. Deletion mutant of lipY, with compromised ability to mobilize the stored TG, but not the complemented mutant, was unable to come out of dormancy upon treatment with anti-TNFα mAbs. In conclusion, we have developed an in vitro human tuberculosis granuloma model that largely exhibits functional features of dormancy and resuscitation observed in human tuberculosis.

Relatively low level of antigen-specific monocytes detected in blood from untreated tuberculosis patients using CD4+ T-cell receptor tetramers

The in vivo kinetics of antigen-presenting cells (APCs) in patients with advanced and convalescent tuberculosis (TB) is not well characterized. In order to target Mycobacterium tuberculosis (MTB) peptides- and HLA-DR-holding monocytes and macrophages, 2 MTB peptide-specific CD4⁺ T-cell receptor (TCR) tetramers eu and hu were successfully constructed. Peripheral blood (PBL) samples from inpatients with advanced pulmonary TB (PTB) were analyzed using flow cytometry, and the percentages of tetramer-bound CD14⁺ monocytes ranged from 0.26–1.44% and 0.21–0.95%, respectively; significantly higher than those measured in PBL samples obtained from non-TB patients, healthy donors, and umbilical cords. These tetramers were also able to specifically detect macrophages in situ via immunofluorescent staining. The results of the continuous time-point tracking of the tetramer-positive rates in PBL samples from active PTB outpatients undergoing treatment show that the median percentages were at first low before treatment, increased to their highest levels during the first month, and then began to decrease during the second month until finally reaching and maintaining a relatively low level after 3–6 months. These results suggest that there is a relatively low level of MTB-specific monocytes in advanced and untreated patients. Further experiments show that MTB induces apoptosis in CD14⁺ cells, and the percentage of apoptotic monocytes dramatically decreases after treatment. Therefore, the relatively low level of MTB-specific monocytes is probably related to the apoptosis or necrosis of APCs due to live bacteria and their growth. The bactericidal effects of anti-TB drugs, as well as other unknown factors, would induce a peak value during the first month of treatment, and a relatively low level would be subsequently reached and maintained until all of the involved factors reached equilibrium. These tetramers have diagnostic potential and can provide valuable insights into the mechanisms of antigen presentation and its relationship with TB infection and latent TB infection.

Mycobacterium tuberculosis (MTB) is one of the most dangerous pathogens in the world. It is estimated that one-third of the world population contracts the bacteria during their lives. Approximately 5–10% of infected individuals will eventually develop an active form of the disease. Cellular immunity plays an important role in immunity against tuberculosis (TB); however, the host's defense mechanisms are not completely understood. Here, we developed a novel tool: MTB antigen-specific tetrameric CD4⁺ T-cell receptor (TCR) complexes that can detect MTB peptide-specific antigen presenting cells (APCs) in blood and local tissues. We found that a relatively low level of antigen-specific monocytes (i.e., APCs) was detected in peripheral blood (PBL) samples from untreated TB patients, and then increased to their peak levels during the first month after treatment, which probably had something to do with the decrease in APC apoptosis. Our research provides a new method for tracking dynamic changes in APCs that are associated with TB infection and latent TB infection, and an additional tool for the studies of TB immunity and its pathogenesis.

The balance of apoptotic and necrotic cell death in Mycobacterium tuberculosis infected macrophages is not dependent on bacterial virulence

Background

An important mechanism of Mycobacterium tuberculosis pathogenesis is the ability to control cell death pathways in infected macrophages: apoptotic cell death is bactericidal, whereas necrotic cell death may facilitate bacterial dissemination and transmission.

Methods

We examine M.tuberculosis control of spontaneous and chemically induced macrophage cell death using automated confocal fluorescence microscopy, image analysis, flow cytometry, plate-reader based vitality assays, and M.tuberculosis strains including H37Rv, and isogenic virulent and avirulent strains of the Beijing lineage isolate GC1237.

Results

We show that bacterial virulence influences the dynamics of caspase activation and the total level of cytotoxicity. We show that the powerful ability of M.tuberculosis to inhibit exogenously stimulated apoptosis is abrogated by loss of virulence. However, loss of virulence did not influence the balance of macrophage apoptosis and necrosis – both virulent and avirulent isogenic strains of GC1237 induced predominantly necrotic cell death compared to H37Rv which induced a higher relative level of apoptosis.

Conclusions

This reveals that macrophage necrosis and apoptosis are independently regulated during M. tuberculosis infection of macrophages. Virulence affects the level of host cell death and ability to inhibit apoptosis but other strain-specific characteristics influence the ultimate mode of host cell death and alter the balance of apoptosis and necrosis.

The recombinant tuberculosis vaccine rBCG ΔureC::hly(+) induces apoptotic vesicles for improved priming of CD4(+) and CD8(+) T cells

BACKGROUND

The recombinant BCG ΔureC::hly(+) (rBCG) vaccine candidate is more efficient than parental BCG (pBCG) against tuberculosis (TB) in preclinical models. Evidence exists for superior CD4 and CD8 T cell stimulation. Although the responsible immune mechanisms are incompletely understood, crosspriming of CD8 T cells has been proposed as a major mechanism underlying better protection of rBCG over pBCG. The present study investigates the role of apoptotic vesicles from pBCG- and rBCG-infected macrophages in crosspriming.

METHODS

Apoptotic vesicles were isolated from pBCG- and rBCG-infected mouse macrophages. The priming potential of the isolated vesicles was evaluated in terms of dendritic cell activation and specific T cell stimulation.

RESULTS

Apoptotic vesicles from both pBCG- and rBCG-infected macrophages activated dendritic cells but to a different degree. Overall, rBCG-infected apoptotic vesicles induced more profound CD4 and CD8 T cell responses as compared to pBCG.

CONCLUSIONS

These data support the notion that the improved vaccine efficacy of rBCG rests on enhanced crosspriming as a consequence of stronger apoptosis.

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.

Targeting of Mycobacterium tuberculosis heparin-binding hemagglutinin to mitochondria in macrophages

Mycobacterium tuberculosis heparin-binding hemagglutinin (HBHA), a virulence factor involved in extrapulmonary dissemination and a strong diagnostic antigen against tuberculosis, is both surface-associated and secreted. The role of HBHA in macrophages during M. tuberculosis infection, however, is less well known. Here, we show that recombinant HBHA produced by Mycobacterium smegmatis effectively induces apoptosis in murine macrophages. DNA fragmentation, nuclear condensation, caspase activation, and poly (ADP-ribose) polymerase cleavage were observed in apoptotic macrophages treated with HBHA. Enhanced reactive oxygen species (ROS) production and Bax activation were essential for HBHA-induced apoptosis, as evidenced by a restoration of the viability of macrophages pretreated with N-acetylcysteine, a potent ROS scavenger, or transfected with Bax siRNA. HBHA is targeted to the mitochondrial compartment of HBHA-treated and M. tuberculosis-infected macrophages. Dissipation of the mitochondrial transmembrane potential (ΔΨ_m) and depletion of cytochrome c also occurred in both macrophages and isolated mitochondria treated with HBHA. Disruption of HBHA gene led to the restoration of ΔΨ_m impairment in infected macrophages, resulting in reduced apoptosis. Taken together, our data suggest that HBHA may act as a strong pathogenic factor to cause apoptosis of professional phagocytes infected with M. tuberculosis.

Cell death is a common outcome during infection with a number of pathogenic microorganisms. Therefore, defining the factors responsible for killing of host cells is important to uncovering mechanisms of pathogenesis. World-wide, two billon people are latently infected with Mycobacterium tuberculosis, which is still killing 2–3 million people each year. Heparin-binding hemagglutinin (HBHA) protein of M. tuberculosis is known to interact specifically with non-phagocytic cells and to be involved in dissemination from lungs to other tissues. Nevertheless, the role of HBHA in phagocytic cells such as macrophages, which are the first cells of the immune system to encounter inhaled pathogens, has been unknown. In the present study, we suggest HBHA as a critical bacterial protein for macrophage cell death. After M. tuberculosis infection or HBHA treatment of macrophages, HBHA targeted to mitochondria and then caused mitochondrial damage and oxidative stress, which eventually lead to apoptosis. A mutant of M. tuberculosis lacking HBHA induced less apoptosis with moderated mitochondrial damage. These experiments provide a candidate virulence factor which may be a novel target for tuberculosis treatment.

Ipr1 gene mediates RAW 264.7 macrophage cell line resistance to Mycobacterium bovis

Tuberculosis caused by Mycobacterium bovis (M. bovis) seriously affects efficiency of animal production with impacts on public health as well. Effective programmes of prevention and eradication of M. bovis infection therefore are urgently needed. Intracellular pathogen resistance gene 1 (Ipr1) is well known to mediate innate immunity to Mycobacterium tuberculosis (MTB), but there are no reports as to whether Ipr1 can enhance the phagocytic ability of macrophage against M. bovis. In this investigation, RAW 264.7 macrophage was transduced with lentiviral vector carrying Ipr1 (named Lenti-Ipr1); transgenic cells were identified by RT-PCR and western blotting. Transgenic positive cells (R-Ipr1) were then infected with an M. bovis virulent strain, with non-transduced cells used as control. When cell proliferation, viability and apoptosis of the two groups were investigated, it was found that infected RAW 264.7 died by necrosis whereas R-Ipr1 underwent apoptosis. Furthermore, the numbers of intracellular bacteria in R-Ipr1 were lower than those in control cells (P < 0.05). To identify the role of Ipr1, we measured the genes of Casp3, Mcl-1 and NOS2A which associated with macrophage activation and apoptosis by real-time quantitative PCR. The results demonstrated that Ipr1 gene expression can enhance anti-M. bovis infection of macrophage. This establishes a basis for the future production of Ipr1-transgenic cattle to strengthen the tuberculosis resistance.

Inhaled therapies for tuberculosis and the relevance of activation of lung macrophages by particulate drug-delivery systems

Pathogenic strains of Mycobacterium tuberculosis (Mtb) induce ‘alternative activation’ of lung macrophages that they colonize, in order to create conditions that promote the establishment and progression of infection. There is some evidence to indicate that such macrophages may be rescued from alternative activation by inhalable microparticles containing a variety of drugs. This review summarizes the experience of various groups of researchers, relating to observations of induction of a number of classical macrophage activation pathways. Restoration of a ‘respiratory burst’ and upregulation of reactive oxygen species and nitrogen intermediates through the phagocyte oxidase and nitric oxide synthetase enzyme systems; induction of proinflammatory macrophage cytokines; and finally induction of apoptosis rather than necrosis of the infected macrophage are discussed. It is suggested that there is scope to co-opt host responses in the management of tuberculosis, through the route of pulmonary drug delivery.

Innate immune responses to M. tuberculosis infection

A prerequisite for successful establishment of Mycobacterium tuberculosis in the host is its ability to survive after internalization in alveolar macrophages that they encounter after inhalation. The innate immune response protects some individuals to the extent that they remain uninfected. In others, the innate immune system is not sufficient and an adaptive immune response is generated. This is usually protective, but not sterilizing, and individuals remain latently infected. In susceptible individuals, M. tuberculosis successfully escapes immune surveillance. The interplay between the host innate immune response and the bacterial mechanisms in play to offset this response, is of considerable importance in dictating the course of the disease. In order to gain an understanding of this interplay it is of importance to analyze how M. tuberculosis interacts with innate immune receptors and makes its entry into macrophages, how it subverts the bactericidal effects of macrophages, and dampens processes required for protective immunity, including cytokine and chemokine induction. This review will focus on some of the Indian efforts in these areas, concentrating mainly on the interaction of M. tuberculosis with macrophages and dendritic cells (DCs). The role of the PE/PPE family of proteins in regulating the immune response, will not be discussed in this chapter. The genome-wide approaches of analyzing host-M. tuberculosis interactions will also be discussed elsewhere.

HIV-1/mycobacterium tuberculosis coinfection immunology: how does HIV-1 exacerbate tuberculosis?

Human immunodeficiency virus type 1 (HIV) and Mycobacterium tuberculosis have become intertwined over the past few decades in a "syndemic" that exacerbates the morbidity and mortality associated with each pathogen alone. The severity of the coinfection has been extensively examined in clinical studies. The extrapolation of peripheral evidence from clinical studies has increased our basic understanding of how HIV increases susceptibility to TB. These studies have resulted in multiple hypotheses of how HIV exacerbates TB pathology through the manipulation of granulomas. Granulomas can be located in many tissues, most prominently the lungs and associated lymph nodes, and are made up of multiple immune cells that can actively contain M. tuberculosis. Granuloma-based research involving both animal models and clinical studies is needed to confirm these hypotheses, which will further our understanding of this coinfection and may lead to better treatment options. This review examines the data that support each hypothesis of how HIV manipulates TB pathology while emphasizing a need for more tissue-based experiments.

Modulation of cell death by M. tuberculosis as a strategy for pathogen survival

It has been clearly demonstrated that in vitro, virulent M. tuberculosis can favor necrosis over apoptosis in infected macrophages, and this has been suggested as a mechanism for evading the host immune response. We recently reported that an effect consistent with this hypothesis could be observed in cells from the blood of TB patients, and in this paper, we review what is known about evasion strategies employed by M. tuberculosis and in particular consider the possible interaction of the apoptosis-inhibiting effects of M. tuberculosis infection with another factor (IL-4) whose expression is thought to play a role in the failure to control M. tuberculosis infection. It has been noted that IL-4 may exacerbate TNF-α-induced pathology, though the mechanism remains unexplained. Since pathology in TB typically involves inflammatory aggregates around infected cells, where TNF-α plays an important role, we predicted that IL-4 would inhibit the ability of cells to remove M. tuberculosis by apoptosis of infected cells, through the extrinsic pathway, which is activated by TNF-α. Infection of human monocytic cells with mycobacteria in vitro, in the presence of IL-4, appears to promote necrosis over apoptosis in infected cells—a finding consistent with its suggested role as a factor in pathology during M. tuberculosis infection.

Expression of apoptosis-related genes in an Ethiopian cohort study correlates with tuberculosis clinical status

Mycobacterium tuberculosis remains one of the world's deadliest pathogens in part because of its ability to persist in the face of an active immune response. It has been suggested that apoptosis of infected macrophages is one way in which the host deals with intracellular pathogens and that M. tuberculosis can inhibit this process. To assess the relevance of this process for human disease, we compared the expression of multiple genes involved in the activation of the extrinsic ("death receptor initiated") pathway of apoptosis in 29 tuberculosis patients, 70 tuberculosis contacts and 27 community controls from Ethiopia. We found that there is a strong upregulation of genes for factors that promote apoptosis in PBMC from individuals with active disease, including TNF-alpha and its receptors, Fas and FasL and pro-Caspase 8. The anti-apoptotic factor FLIP, however, was also upregulated. A possible explanation for this dichotomy was given by fractionation of PBMC using CD14, which suggests that macrophage/monocytes may regulate several key molecules differently from non-monocytic cells (especially TNF-alpha and its receptors, a finding confirmed by protein ELISA) potentially reducing the sensitivity to apoptotic death of monocyte/macrophages--the primary host cell for M. tuberculosis. This may represent an important survival strategy for the pathogen.

Impaired activation of Stat1 and c-Jun as a possible defect in macrophages of patients with active tuberculosis

Studies of patients with active tuberculosis (TB) and infected healthy individuals have shown that interferon (IFN)-gamma is present in sites of Mycobacterium tuberculosis infection in comparable levels. This suggests that there is a deficiency in the macrophage response to IFN-gamma in TB patients. We used recombinant human IFN-gamma to stimulate adherent monocyte-derived macrophages from three groups of people: patients with active tuberculosis (TBP), their healthy household contacts (HHC) and healthy uninfected controls from the community (CC). We then evaluated the ability of the macrophages to inhibit the growth of M. tuberculosis H37Rv as well as their cytokine profile at early in infection (48 h). After IFN-gamma treatment, macrophages of healthy individuals (HHC and CC) controlled M. tuberculosis growth and produced mainly nitric oxide (NO) and interleukin (IL)-12p70, whereas TBP macrophages did not kill M. tuberculosis. Additionally, TBP macrophages produced low levels of NO and IL-12p70 and high levels of tumour necrosis factor (TNF)-alpha and IL-10. Transforming growth factor (TGF)-beta levels were similar among all three groups. M. tuberculosis infection had little effect on the cytokine response after IFN-gamma stimulus, but infection alone induced more IL-10 and TGF-beta in TBP macrophages. There were no differences in Stat1 nuclear translocation and DNA binding between the groups. However, the phosphorylated Stat1 and c-Jun (AP-1) in nuclear protein extracts was diminished in TBP macrophages compared to macrophages of healthy individuals. These results indicate an impairment of Stat1-dependent and Stat1-independent IFN-gamma signalling in macrophages of people with active tuberculosis, suggesting a different molecular regulation that could impact macrophage functionality and disease outcome.

Interaction of Mycobacterium tuberculosis with the host: consequences for vaccine development

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), remains a major worldwide health problem that causes more than 2 million deaths annually. In addition, an estimated 2 billion people are latently infected with M. tuberculosis. The bacterium is one of the oldest human pathogens and has evolved complex strategies for survival. Therefore, to be successful in the high endemic regions, any future TB vaccine strategy will have to be tailored in accordance with the resulting complexity of the TB infection and anti-mycobacterial immune response. In this review, we will discuss what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge is used in new and more advanced vaccine strategies.

Neutrophil apoptosis and the resolution of infection

Polymorphonuclear leukocytes (PMNs) are the most abundant white cell in humans and an essential component of the innate immune system. PMNs are typically the first type of leukocyte recruited to sites of infection or areas of inflammation. Ingestion of microorganisms triggers production of reactive oxygen species and fusion of cytoplasmic granules with forming phagosomes, leading to effective killing of ingested microbes. Phagocytosis of bacteria typically accelerates neutrophil apoptosis, which ultimately promotes the resolution of infection. However, some bacterial pathogens alter PMN apoptosis to survive and thereby cause disease. Herein, we review PMN apoptosis and the ability of microorganisms to alter this important process.

Mycobacterium tuberculosis 38-kDa lipoprotein is apoptogenic for human monocyte-derived macrophages

Mycobacterium tuberculosis is the main aetiologic agent of tuberculosis, a disease of great concern in less-developed regions. Apoptosis is a conspicuous event in macrophages infected in vitro with mycobacteria, a phenomenon also observed in vivo in granulomas of patients with tuberculosis. To determine its significance, it is important to define the mycobacterial moieties involved and how they cause apoptosis. Here we show that the 38-kDa lipoprotein induces macrophage caspase-dependent apoptosis involving TNF-alpha and FasL and, interestingly, with the upregulation of cell-death receptors TNFR1, TNFR2 and Fas. A role for the Toll-like receptor 2 was also demonstrated. In conclusion, the ability to induce apoptosis of host cells is another property of the 38-kDa lipoprotein, a molecule that has focused attention for being an immunodominant antigen that participates in phosphate transport.

Macrophage apoptosis in tuberculosis

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that infects alveolar macrophages following aerosol transmission. Lung macrophages provide a critical intracellular niche that is required for Mtb to establish infection in the human host. This parasitic relationship is made possible by the capacity of Mtb to block phagosome maturation following entry into the host macrophage, creating an environment that supports bacillary replication. Apoptosis is increasingly understood to play a role in host defense against intracellular pathogens including viruses, fungi, protozoa and bacteria. In the last 15 years an understanding of the role that macrophage apoptosis plays in TB has begun to emerge. Here we review the history and current state of the art of this topic and we offer a model of the macrophage-pathogen interaction that takes into the account the complexities of programmed cell death and the relationship between various death signaling pathways and host defense in TB.

1Alpha,25-dihydroxyvitamin D3 inhibits CD40L-induced pro-inflammatory and immunomodulatory activity in human monocytes

CD40 ligand (CD40L) stimulation induces proinflammatory and immunomodulatory activity in monocytes. Here, we report on the effects of the steroid hormone 1alpha,25-dihydroxyvitamin D3 (1,25D3) on human blood monocytes that have been stimulated with the CD40L ligand. Co-treatment of CD40L-stimulated monocytes with 1,25D3 resulted in reduced production and secretion of tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta, as well as in reduced expression of the surface co-stimulatory molecules CD80 and CD86. In addition, costimulation of CD4+ T lymphocytes by monocytes co-treated with CD40L and 1,25D3 resulted in reduced cell proliferation and diminished interferon (IFN)-gamma but enhanced IL-10 production by CD4+ T cells. Finally, 1,25D3 interfered with the ability of CD40L to rescue monocytes from apoptosis induced by serum withdrawal. These findings suggest that 1,25D3 may regulate the interaction of monocytes with T cells or other cell types that express CD40L, thus influencing the outcome of the immune or inflammatory response.

Living on the edge: inhibition of host cell apoptosis by Mycobacterium tuberculosis

Tuberculosis is a human disease of global importance caused by infection with Mycobacterium tuberculosis. Thus, an estimated one-third of the world's population is latently infected; there are 2-3 million annual deaths and an increasing amount of multidrug-resistant and extensive drug-resistant tuberculosis cases. M. tuberculosis is a highly adapted human pathogen that has evolved to employ multiple strategies in its attempt to avoid an efficient host immune response. The induction of host cell death is an ancient immune defense strategy that is conserved throughout the animal and plant kingdoms. Here we review the current status of the research involving interaction of mycobacteria with host cells, regarding the induction of host cell death by apoptosis. We conclude that virulent strains of M. tuberculosis employ several strategies to avoid the induction of macrophage cell death, and success in this process is clearly important for bacterial virulence. The molecular mechanisms of host cell apoptosis inhibition are little understood, but the recent identification of anti-apoptosis genes in the genome of M. tuberculosis has provided the tools necessary to investigate the details of this host-pathogen interaction. The results of these future studies may prove useful for the development of new drug targets and/or vaccine candidates.

Mycobacteria and innate cells: critical encounter for immunogenicity

Protective immunity against mycobacterial infections such as Mycobacterium tuberculosis is mediated by interactions between specific T cells and activated macrophages. To date,many aspects of mycobacterial immunity have shown that innate cells are the key elements that substantially influence the subsequent adaptive host response. During the early phases of infection,phagocytic cells and innate lymphocyte subsets play a pivotal role. Here we summarize the findings of recent investigations on macrophages,dendritic cells and gammadelta T lymphocytes in the response to mycobacteria.

Human splenic macrophages as a model for in vitro infection with Mycobacterium tuberculosis

Macrophages play an important role during Mycobacterium tuberculosis (MTB) infection. In humans most of the studies on MTB-macrophage interactions have been performed using circulating monocytes and monocyte-derived macrophages. However, little research has been performed on this interaction using tissue macrophages. Herein, we used human splenic macrophages to characterize particular responses to MTB infection. Based on morphological, biochemical, and immunological markers, splenic adherent cells exhibit characteristics of tissue macrophages. They were able to efficiently phagocytose both live and heat-killed (h-k) MTB H37Rv. Upon infection with live, but not h-k MTB, an increase in secreted TNF-alpha was elicited. Splenic macrophages produced high basal levels of IL-10; however, infection with live or h-k MTB resulted in decrease IL-10 secretion. Both IL-12p40 and IL-12p70 basal levels were also decreased upon infection with live or h-k MTB; however, while the reduction for IL-12p40 levels was observed at earlier time points (4h) for both live and h-k MTB, infection with live MTB, but not h-k MTB, resulted in a time-dependent secretion of IL-12p40 at 24 and 48h after infection. IL-12p70 levels were completely reduced upon infection by either live or h-k MTB. These results support that human splenic macrophages may represent a potential useful model to study MTB-macrophage interactions in vitro.

Lipopolysaccharide desensitizes monocytes-macrophages to CD40 ligand stimulation

Polymicrobial sepsis induces the suppression of macrophage function as determined by a reduction of pro-inflammatory cytokine production upon re-exposure to lipopolysaccharide (LPS) in vitro. Here, we examined whether macrophages were refractory to only LPS or if they were unable to respond to other stimuli such as CD40 ligand (CD40L). Monocytic cells exposed in vitro to LPS showed a dose-dependent reduction of their ability to produce interleukin-12 and tumour necrosis factor-alpha upon subsequent CD40L stimulation, as compared to cells stimulated with CD40L alone. Similarly, LPS interfered with the up-regulation of CD40, CD80 and CD86 induced by CD40L in monocytic cells. The effect of LPS on the response of monocytes to CD40L was similar whether these cells were directly exposed to LPS or cocultured with LPS-pretreated cells, indicating that soluble factors released by LPS stimulation could mediate tolerance to CD40L. We also show that the functional alterations induced by LPS in monocytes can be reversed by indomethacin, thus suggesting a role for inducible cyclooxygenase in mediating the LPS-induced hyporesponsive state of monocytes to CD40L. In conclusion, we propose that in vitro CD40L tolerance may be an appropriate model of monocyte alteration observed during septic immunosuppression and may help in the development of novel therapeutic strategies.

M. tuberculosis and M. leprae Translocate from the Phagolysosome to the Cytosol in Myeloid Cells

M. tuberculosis and M. leprae are considered to be prototypical intracellular pathogens that have evolved strategies to enable growth in the intracellular phagosomes. In contrast, we show that lysosomes rapidly fuse with the virulent M. tuberculosis- and M. leprae-containing phagosomes of human monocyte-derived dendritic cells and macrophages. After 2 days, M. tuberculosis progressively translocates from phagolysosomes into the cytosol in nonapoptotic cells. Cytosolic entry is also observed for M. leprae but not for vaccine strains such as M. bovis BCG or in heat-killed mycobacteria and is dependent upon secretion of the mycobacterial gene products CFP-10 and ESAT-6. The cytosolic bacterial localization and replication are pathogenic features of virulent mycobacteria, causing significant cell death within a week. This may also reveal a mechanism for MHC-based antigen presentation that is lacking in current vaccine strains.