Mycobacterium tuberculosis-induced activation accelerates apoptosis in peripheral blood neutrophils from patients with active tuberculosis

Variants of human DECTIN-1 rs16910526 are linked to differential reactive oxygen species production and susceptibility to tuberculosis

BACKGROUND

Dectin-1 is a transmembrane receptor that plays a pivotal role in recognising fungi and Mycobacterium tuberculosis (Mtb). A specific variant, DECTIN-1 rs16910526, results in a truncated receptor that disrupts membrane expression and ligand binding and is clinically associated with recurrent cutaneous mycoses. Previous research has clarified the role of Dectin-1 in boosting immune defenses against mycobacteria by enhancing reactive oxygen species (ROS) production in neutrophils (PMNs). Here, we investigated the association between the rs16910526 variant and Dectin-1 expression in PMNs, as well as intracellular ROS production in response to Mtb. Furthermore, we explored the potential link between the rs16910526 gene variant and TB outcomes in Argentina.

METHODS

DNA was extracted from blood samples obtained from a cohort of 178 TB patients and healthy subjects (HS) in Argentina. PCR amplification and sequencing were performed to identify the rs16910526 variant. Flow cytometry was utilised to assess Dectin-1 expression on the PMN plasma membrane and to measure intracellular ROS levels, as indicated by the oxidation of DHR123 in response to the Mtb antigen.

RESULTS

PMNs carrying the rs16910526 variant exhibited diminished Dectin-1 expression and ROS production in response to Mtb (p < 0.0001). In a case‒control study, the rs16910526 variant had an allelic frequency of 0.112 in TB patients and 0.051 in HS. Notably, 10 out of 88 HS and 18 out of 62 TB patients harboured the variant (odds ratio [OR]: 2.55 [95% CI 1.1-5.9, p = 0.03]), indicating a potential association with TB disease. Furthermore, TB patients with the rs16910526 variant exhibited a delayed sputum smear conversion time (p < 0.004) and 100% positivity for acid-fast bacilli smears (p < 0.00001).

CONCLUSION

Our study identified a significant association between the SNP variant rs16910526 in the DECTIN-1 gene and Dectin-1 expression in the PMN, leading to altered ROS production. The higher frequency of this variant in TB patients compared to HS suggests a possible link with susceptibility to TB disease in Argentina.

Detailed phenotyping reveals diverse and highly skewed neutrophil subsets in both the blood and airways during active tuberculosis infection

Introduction

Neutrophils play a complex and important role in the immunopathology of TB. Data suggest they are protective during early infection but become a main driver of immunopathology if infection progresses to active disease. Neutrophils are now recognized to exist in functionally diverse states, but little work has been done on how neutrophil states or subsets are skewed in TB disease.

Methods

To address this, we carried out comprehensive phenotyping by flow cytometry of neutrophils in the blood and airways of individuals with active pulmonary TB with and without HIV co-infection recruited in Durban, South Africa.

Results

Active TB was associated with a profound skewing of neutrophils in the blood toward phenotypes associated with activation and apoptosis, reduced phagocytosis, reverse transmigration, and immune regulation. This skewing was also apparently in airway neutrophils, particularly the regulatory subsets expressing PDL-1 and LOX-1. HIV co-infection did not impact neutrophil subsets in the blood but was associated with a phenotypic change in the airways and a reduction in key neutrophil functional proteins cathelicidin and arginase 1.

Discussion

Active TB is associated with profound skewing of blood and airway neutrophils and suggests multiple mechanisms by which neutrophils may exacerbate the immunopathology of TB. These data indicate potential avenues for reducing neutrophil-mediated lung pathology at the point of diagnosis.

Association between interleukin-8 -251A/T polymorphism and the risk of tuberculosis: A meta-analysis

Objective

The relationship between interleukin-8 (IL8) −251A/T polymorphism and tuberculosis (TB) risk remains controversial. Therefore, the present meta-analysis was performed by retrieving relevant studies from the available literature.

Methods

We comprehensively searched three databases to identify eligible literature on the relationship of IL8 −251A/T polymorphism with TB risk, calculated pooled odds ratios (OR) with 95% confidence intervals (CI), and subsequent evaluated the heterogeneity and publication bias.

Results

We found that IL8 −251A/T polymorphism increased TB risk (AA vs. TT: OR = 2.86, 95%CI: 1.46–5.60; AT vs. TT: OR = 1.64, 95%CI: 1.15–2.34; dominant model: OR = 1.88, 95%CI: 1.24–2.86; recessive model: OR = 1.77, 95%CI: 1.17–2.69). Subgroup analyses based on race revealed that the IL8 −251A/T polymorphism might be associated with the risk of TB in African but not Asian individuals.

Conclusion

The IL8 −251A/T polymorphism might be related to the risk of TB. Nevertheless, large-scale studies should be performed to confirm the role of IL8 −251A/T polymorphism on TB risk.

Anaphylatoxins Enhance Recruitment of Nonclassical Monocytes via Chemokines Produced by Pleural Mesothelial Cells in Tuberculous Pleural Effusion

In the present study, we sought to elucidate the mechanisms by which monocytes migrate into the pleural space in the presence of anaphylatoxins in tuberculous pleural effusion (TPE). Monocytes in both pleural effusion and blood were counted, and their phenotypic characteristics were analyzed. Activation of the complement system was detected in TPE. The effects of Mpt64 and anaphylatoxins on the production of chemokines in pleural mesothelial cells (PMCs) were measured. The chemoattractant activity of chemokines produced by PMCs for monocytes was observed. Levels of CD14⁺CD16⁺ monocytes were significantly higher in TPE than in blood. Three pathways of the complement system were activated in TPE. C3a-C3aR1, C5a-C5aR1, CCL2-CCR2, CCL7-CCR2, and CX3CL1-CX3CR1 were coexpressed in PMCs and monocytes isolated from TPE. Moreover, we initially found that Mpt64 stimulated the expression of C3a and C5a in PMCs. C3a and C5a not only induced CCL2, CCL7, and CX3CL1 expression in PMCs but also stimulated production of IL-1β, IL-17, and IL-27 in monocytes. C3a and C5a stimulated PMCs to secrete CCL2, CCL7, and CX3CL1, which recruited CD14⁺CD16⁺ monocytes to the pleural cavity. As a result, the infiltration of CD14⁺CD16⁺ monocytes engaged in the pathogenesis of TPE by excessive production of inflammatory cytokines.

N-chlorination mediates protective and immunomodulatory effects of oxidized human plasma proteins

Hypochlorous acid (HOCl), a powerful antimicrobial oxidant, is produced by neutrophils to fight infections. Here, we show that N-chlorination, induced by HOCl concentrations encountered at sites of inflammation, converts blood plasma proteins into chaperone-like holdases that protect other proteins from aggregation. This chaperone-like conversion was reversible by antioxidants and was abrogated by prior methylation of basic amino acids. Furthermore, reversible N-chlorination of basic amino acid side chains is the major factor that converts plasma proteins into efficient activators of immune cells. Finally, HOCl-modified serum albumin was found to act as a pro-survival molecule that protects neutrophils from cell death induced by highly immunogenic foreign antigens. We propose that activation and enhanced persistence of neutrophils mediated by HOCl-modified plasma proteins, resulting in the increased and prolonged generation of ROS, including HOCl, constitutes a potentially detrimental positive feedback loop that can only be attenuated through the reversible nature of the modification involved.

The Immune Interaction between HIV-1 Infection and Mycobacterium tuberculosis

The modulation of tuberculosis (TB)-induced immunopathology caused by human immunodeficiency virus (HIV)-1 coinfection remains incompletely understood but underlies the change seen in the natural history, presentation, and prognosis of TB in such patients. The deleterious combination of these two pathogens has been dubbed a "deadly syndemic," with each favoring the replication of the other and thereby contributing to accelerated disease morbidity and mortality. HIV-1 is the best-recognized risk factor for the development of active TB and accounts for 13% of cases globally. The advent of combination antiretroviral therapy (ART) has considerably mitigated this risk. Rapid roll-out of ART globally and the recent recommendation by the World Health Organization (WHO) to initiate ART for everyone living with HIV at any CD4 cell count should lead to further reductions in HIV-1-associated TB incidence because susceptibility to TB is inversely proportional to CD4 count. However, it is important to note that even after successful ART, patients with HIV-1 are still at increased risk for TB. Indeed, in settings of high TB incidence, the occurrence of TB often remains the first presentation of, and thereby the entry into, HIV care. As advantageous as ART-induced immune recovery is, it may also give rise to immunopathology, especially in the lower-CD4-count strata in the form of the immune reconstitution inflammatory syndrome. TB-immune reconstitution inflammatory syndrome will continue to impact the HIV-TB syndemic.

Low-Density Granulocytes Are Elevated in Mycobacterial Infection and Associated with the Severity of Tuberculosis

Tuberculosis remains a global health problem caused by infection with Mycobacterium tuberculosis. Numerous studies have established a close correlation between the development of tuberculosis and the roles of neutrophils. Recently, a distinct population of CD15+ granulocytes was found to be present in the peripheral blood mononuclear cell (PBMC) fraction in humans. This population of granulocytes, termed low-density granulocytes (LDGs), was reported to be elevated and associated with disease activity or severity in a number of different conditions including SLE, asthma and HIV infection. However, both the frequency and clinical significance of LDGs associated with tuberculosis are unclear. Here we determined LDG levels and made comparisons between subjects with active pulmonary tuberculosis (PTB) and healthy controls, between PTB patients with mild-to-moderate disease and patients with advanced disease, and among PTB patients following anti-tuberculous therapy of varying durations. The direct correlation between M. tuberculosis infection and LDG levels was confirmed by in vitro infection of whole peripheral blood and isolated granulocytes with mycobacteria. Our results demonstrated that PBMCs in PTB patients contained significantly elevated percentages of LDGs compared with control subjects. LDGs in tuberculosis expressed higher levels of activation markers compared to normal-density granulocytes (NDGs). M. tuberculosis induced the generation of LDGs in both whole blood and isolated NDGs from control subjects, which suggests that LDGs associated with M. tuberculosis infection are likely to originate from in situ activation. Furthermore, our results revealed that the frequency of LDGs is associated with the severity of tuberculosis.

Lung Mucosa Lining Fluid Modification of Mycobacterium tuberculosis to Reprogram Human Neutrophil Killing Mechanisms

We have shown that human alveolar lining fluid (ALF) contains homeostatic hydrolases capable of altering the Mycobacterium tuberculosis cell wall and subsequently its interaction with human macrophages. Neutrophils are also an integral part of the host immune response to M. tuberculosis infection. Here we show that the human lung mucosa influences M. tuberculosis interaction with neutrophils, enhancing the intracellular killing of ALF-exposed M. tuberculosis and up-regulating the expression of tumor necrosis factor and interleukin 8. In contrast, ALF-exposed M. tuberculosis does not induce neutrophil apoptosis or necrosis, degranulation, or release of extracellular traps, and it decreases the oxidative response. These results suggest an important role for the human alveolar mucosa: increasing the innate capacity of the neutrophil to recognize and kill M. tuberculosis by favoring the use of intracellular mechanisms, while at the same time limiting neutrophil extracellular inflammatory responses to minimize their associated tissue damage.

Neutrophils from pulmonary tuberculosis patients show augmented levels of chemokines MIP-1α, IL-8 and MCP-1 which further increase upon in vitro infection with mycobacterial strains

Neutrophils being innate cells initiate the immune defence against mycobacteria by sending signals to other immune cells. Chemokines being the vital link in signaling processes, it is of interest to study their secretion by neutrophils as a response to tuberculosis infection. The levels of various chemokines (MIP-1α, MCP-1, IL-8 and IP-10) and chemokine receptors (CXCR1, CXCR2 and CCR1) in neutrophils from healthy individuals and pulmonary tuberculosis patients were studied following infection with Mycobacterium tuberculosis strains (clinical--S7 and S10 and laboratory--H37Rv). The release of MIP-1α, IL-8 and MCP-1 is found to be greatly increased in patient neutrophils. Mycobacterial strains differentially influenced neutrophils affecting the release of chemokines to different extent. H37Rv significantly increased the release of MIP-1α and IL-8 in both normals and tuberculosis patients, while S10 up regulated only the release of MIP-1α in patients. Thus, during tuberculosis, neutrophils undergo functional alteration to combat infection. While H37Rv is greatly recognized by neutrophils and triggers the release of chemokines, clinical strains by some means try to suppress immune activation of neutrophils in their favor.

Outbreaks of Mycobacterium tuberculosis MDR strains differentially induce neutrophil respiratory burst involving lipid rafts, p38 MAPK and Syk

Background

Neutrophils (PMN) are the first cells to infiltrate the lung after infection, and they play a significant protective role in the elimination of pathogen, by releasing preformed oxidants and proteolytic enzymes from granules and generating ROS, thus limiting inflammation by succumbing to apoptosis. In a previous study, we found marked differences in ROS-induced apoptosis between two Mycobacterium tuberculosis (Mtb) strains, M and Ra, representative of widespread Mtb families in South America, i.e. Haarlem and Latin-American Mediterranean (LAM), being strain M able to generate further drug resistance and to disseminate aggressively.

Methods

In this study we evaluate the nature of bacteria-PMN interaction by assessing ROS production, apoptosis, lipid raft coalescence, and phagocytosis induced by Mtb strains.

Results

Dectin-1 and TLR2 participate in Mtb-induced ROS generation and apoptosis in PMN involving p38 MAPK and Syk activation with the participation of a TLR2-dependent coalescence of lipid rafts. Further, ROS production occurs during the phagocytosis of non-opsonized bacteria and involves α-glucans on the capsule. In contrast, strain M lacks the ability to induce ROS because of: 1) a reduced phagocytosis and 2) a failure in coalescence of lipid raft.

Conclusions

The differences in wall composition could explain the success of some strains which stay unnoticed by the host through inhibition of apoptosis and ROS but making possible its replication inside PMN as a potential evasion mechanism. Innate immune responses elicited by Mtb strain-to-strain variations need to be considered in TB vaccine development.

Investigation of the effect of Mycobacterium bovis infection on bovine neutrophils functions

Bovine tuberculosis is a disease in cattle caused by infection with Mycobacterium bovis. The disease has posed significant economic losses and remains a public health hazard worldwide. Interactions between M. bovis and bovine macrophages have been extensively characterized in various studies, while similar analyses in neutrophils, which are one of the other types of white blood cells in mammals, were often overlooked. Neutrophils provide defense against all microbes and can present a diverse collection of antimicrobial molecules, which play an important role in the control of tuberculosis progression. Much of the available data about the involvement of neutrophils in the killing M. bovis is controversial. In this study, we assessed the effect of in vitro infection with M. bovis on some parameters of neutrophils functions including phenotypic changes, apoptosis rate and inflammatory cytokines production. Our results demonstrated that phagocytosis of M. bovis activated and enhanced bovine neutrophils functions as well as initialed their defense mechanism, but failed to eliminate the mycobacteria. Moreover, autophagy might get involved in the defense infection process functioning as a protective mechanism, and inducible-autophagy by lipopolysaccharides stimulation and starvation treatment could efficiently reverse the inability of neutrophils for killing M. bovis, suggesting a potential target for anti-mycobacterial drug-therapy.

Clinical isolates of Mycobacterium tuberculosis differ in their ability to induce respiratory burst and apoptosis in neutrophils as a possible mechanism of immune escape

Tuberculosis pathogenesis was earlier thought to be mainly related to the host but now it appears to be clear that bacterial factors are also involved. Genetic variability of Mycobacterium tuberculosis (Mtb) could be slight but it may lead to sharp phenotypic differences. We have previously reported that nonopsonized Mtb H37Rv induce apoptosis of polymorphonuclear neutrophils (PMNs) by a mechanism that involves the p38 pathway. Here we evaluated the capability to induce PMN apoptosis of two prevalent Mtb lineages in Argentina, the Latin America and Mediterranean (LAM), and Haarlem, using the H37Rv as a reference strain. Results showed that LAM strains strongly induced apoptosis of PMN which correlated with the induction of reactive oxygen species (ROS) production and p38 activation. Interestingly, the highly prosperous multidrug-resistant M strain, belonging to the Haarlem lineage, lacked the ability to activate and to induce PMN apoptosis as a consequence of (1) a weak ROS production and (2) the contribution of antiapoptotic mechanisms mediated at least by ERK. Although with less skill, M is able to enter the PMN so that phenotypic differences could lead PMN to be a reservoir allowing some pathogens to prevail and persist over other strains in the community.

Mycobacterium tuberculosis-induced neutrophil ectosomes decrease macrophage activation

BACKGROUND

The existence of ectosome-like microvesicles released by neutrophils was proposed a few decades ago. Other studies revealed that the innate immune response during mycobacterial infection is accompanied by an intense migration of neutrophils to the site of infection, which may be important during the acute phase of tuberculosis. We found that the ectosomes derived from infected neutrophils are biologically active and can influence the survival of Mycobacterium tuberculosis within macrophages.

METHODS

Mycobacteria were cultured on supplemented Middlebrook-7H9 broth. All strains were grown to the exponential phase and quantitated by serial dilution. Human neutrophils and macrophages were infected with mycobacteria. Ectosomes from neutrophils were isolated post-infection and characterized by transmission electron microscopy and flow cytometry. To determine whether these microvesicles influenced mycobactericidal activity, mycobacteria-infected macrophages were treated with isolated ectosomes.

RESULTS

Ectosomes were released from neutrophils infected with mycobacteria. These ectosomes were derived from neutrophil plasma membrane and a small proportion stained with PKH26. These microvesicles, when incubated with infected macrophages, influenced antimycobacterial activity.

CONCLUSIONS

This is the first study to demonstrate that ectosomes that are shed from infected neutrophils influence mycobactericidal activity in macrophages in vitro, suggesting that these microvesicles have biological significance. Nevertheless, major gaps in our knowledge of microvesicle biology remain.

A highly efficient Ziehl-Neelsen stain: identifying de novo intracellular Mycobacterium tuberculosis and improving detection of extracellular M. tuberculosis in cerebrospinal fluid

Tuberculous meningitis leads to a devastating outcome, and early diagnosis and rapid chemotherapy are vital to reduce morbidity and mortality. Since Mycobacterium tuberculosis is a kind of cytozoic pathogen and its numbers are very few in cerebrospinal fluid, detecting M. tuberculosis in cerebrospinal fluid from tuberculous meningitis patients is still a challenge for clinicians. Ziehl-Neelsen stain, the current feasible microbiological method for the diagnosis of tuberculosis, often needs a large amount of cerebrospinal fluid specimen but shows a low detection rate of M. tuberculosis. Here, we developed a modified Ziehl-Neelsen stain, involving cytospin slides with Triton processing, in which only 0.5 ml of cerebrospinal fluid specimens was required. This method not only improved the detection rate of extracellular M. tuberculosis significantly but also identified intracellular M. tuberculosis in the neutrophils, monocytes, and lymphocytes clearly. Thus, our modified method is more effective and sensitive than the conventional Ziehl-Neelsen stain, providing clinicians a convenient yet powerful tool for rapidly diagnosing tuberculous meningitis.

Risk factors for immune reconstitution inflammatory syndrome under combination antiretroviral therapy can be aetiology-specific

In order to discriminate general from aetiology-specific risk factors for immune reconstitution inflammatory syndrome (IRIS), we followed up, during six months, 99 patients with advanced HIV infection commencing antiretroviral therapy (ART) without active opportunistic infections or evident inflammation. IRIS predictors were determined by univariate analysis using clinical data from 76 ART-responding patients either completing follow-up or developing IRIS, and by multivariate analysis of inflammation, disease progression and nutrition status variables. We identified 23 primary IRIS events (30.3%). Univariate predictors for all IRIS events were higher platelet counts and lower CD4/CD8 ratio, whereas subclinical inflammation was the multivariate predictor. Platelets, alkaline phosphatase levels and %CD8 T-cells in univariate analysis also predicted mycobacteria-associated IRIS independently, remaining elevated during follow-up. Herpesvirus IRIS was predicted by platelets and inflammation. Indicators of advanced HIV disease and subclinical inflammation jointly predict IRIS, and some are specific of the underlying microbial aetiology, possibly explaining previous reports.

Dendritic cell activation by sensing Mycobacterium tuberculosis-induced apoptotic neutrophils via DC-SIGN

Mycobacterium tuberculosis (Mtb) manipulates cells of the innate immune system to provide the bacteria with a sustainable intracellular niche. Mtb spread through aerosol carrying them deep into the lungs, where they are internalized by phagocytic cells, such as neutrophils (PMNs), dendritic cells (DCs), and macrophages. PMNs undergo accelerated apoptosis after interaction with the bacterium, and apoptotic cells are sequestered by neighboring phagocytes. Removal of aged apoptotic cells because of natural tissue turnover is described as an immunologically silent process facilitating resolution of inflammation and inhibition of DC maturation. Silencing of immune cells could be favorable for intracellular bacteria. The aim of this study was to clarify the interaction between Mtb-induced apoptotic PMNs and DCs, and evaluate whether this interaction follows the proposed anti-inflammatory pathway. In contrast to aged apoptotic cells, Mtb-induced apoptotic PMNs induced functional DC maturation. We found that the cell fraction from Mtb-induced apoptotic PMNs contained almost all stimulatory capacity, suggesting that cell-cell interaction is crucial for DC activation. Inhibitory studies showed that this cell contact-dependent activation required binding of the PMN Mac-1 (CD11b/CD18) to the DC via DC-SIGN and endocytic activity involving the alpha(v)beta(5) but did not involve the scavenger receptor CD36. Taken together, this study demonstrates that the DCs can distinguish between normal and infected apoptotic PMNs via cellular crosstalk, where the DCs can sense the presence of danger on the Mtb-infected PMNs and modulate their response accordingly.

B cells delay neutrophil migration toward the site of stimulus: tardiness critical for effective bacillus Calmette-Guérin vaccination against tuberculosis infection in mice

Mutations in the btk gene encoding Bruton's tyrosine kinase cause X-linked immune deficiency, with impaired B lymphocyte function as the major phenotype. Earlier, we demonstrated that CBA/N-xid mice, unlike the wild-type CBA mice, were not protected by bacillus Calmette-Guérin (BCG) vaccination against tuberculosis infection. Because IFN-gamma-producing T cells and activated macrophages are key elements of antituberculosis protection, it remained unclear how the mutation predominantly affecting B cell functions interferes with responses along the T cell-macrophage axis. In this study, we show that B cell deficiency leads to an abnormally rapid neutrophil migration toward the site of external stimulus. Using adoptive cell transfers and B cell genetic knockout, we demonstrate a previously unappreciated capacity of B cells to downregulate neutrophil motility. In our system, an advanced capture of BCG by neutrophils instead of macrophages leads to a significant decrease in numbers of IFN-gamma-producing T cells and impairs BCG performance in X-linked immune-deficient mice. The defect is readily compensated for by the in vivo neutrophil depletion.

Neutrophils are the predominant infected phagocytic cells in the airways of patients with active pulmonary TB

BACKGROUND

The exact role of neutrophils in the pathogenesis of TB is poorly understood. Recent evidence suggests that neutrophils are not simply scavenging phagocytes in Mycobacterium tuberculosis (Mtb) infection.

METHODS

Three different types of clinical specimens from patients with active pulmonary TB who underwent lung surgery were examined: sputum, BAL fluid, and cavity contents. Differential cell separation and quantification were performed for intracellular and extracellular bacteria, and bacterial length was measured using microscopy.

RESULTS

Neutrophils were more abundant than macrophages in sputum (86.6% +/- 2.2% vs 8.4% +/- 1.3%) and in BAL fluid (78.8% +/- 5.8% vs 11.8% +/- 4.1%). Inside the cavity, lymphocytes (41.3% +/- 11.2%) were the most abundant cell type, followed by neutrophils (38.8% +/- 9.4%) and macrophages (19.5% +/- 7.5%). More intracellular bacilli were found in neutrophils than macrophages in sputum (67.6% +/- 5.6% vs 25.2% +/- 6.5%), in BAL fluid (65.1% +/- 14.4% vs 28.3% +/- 11.6%), and in cavities (61.8% +/- 13.3% vs 23.9% +/- 9.3%). The lengths of Mtb were shortest in cavities (1.9+/- 0.1 microm), followed by in sputum (2.9 +/- 0.1 microm) and in BAL fluid (3.6 +/- 0.2 microm).

CONCLUSIONS

Our results show that neutrophils are the predominant cell types infected with Mtb in patients with TB and that these intracellular bacteria appear to replicate rapidly. These results are consistent with a role for neutrophils in providing a permissive site for a final burst of active replication of the bacilli prior to transmission.

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.

Neutrophil extracellular traps are induced by Mycobacterium tuberculosis

Due to the intracellular nature of mycobacterial infections, little attention has been paid to the possible extracellular role that neutrophils might play in tuberculosis. The recent discovery of neutrophil extracellular traps (NETs), composed of DNA and antimicrobial proteins,(1) introduces a new perspective to our understanding of the mechanism used by the innate immune system to contain and kill microorganisms. In this study, we tested in vitro whether Mycobacterium tuberculosis, an intracellular pathogen, can induce NETs formation and if this newly discovered mechanism is involved in a control response during mycobacterial infection. We found that two different genotypes of M. tuberculosis exerted, in vitro, a cytotoxic effect and induced subcellular changes on infected neutrophils, leading to NETs formation in a time dependent manner. NETs trapped mycobacteria but were unable to kill them. NETs formation induced by M. tuberculosis could help understand the early stages of mycobacterial pathogenesis.

Neutrophils recruited to the site of Mycobacterium bovis BCG infection undergo apoptosis and modulate lipid body biogenesis and prostaglandin E production by macrophages

Neutrophil influx to sites of mycobacterial infections is one of the first events of tuberculosis pathogenesis. However, the role of early neutrophil recruitment in mycobacterial infection is not completely understood. We investigated the rate of neutrophil apoptosis and the role of macrophage uptake of apoptotic neutrophils in a pleural tuberculosis model induced by BCG. Recruited neutrophils were shown to phagocyte BCG and a large number of neutrophils undergo apoptosis within 24 h. Notably, the great majority of apoptotic neutrophils were infected by BCG. Increased lipid body (lipid droplets) formation, accompanied by prostaglandin E(2) (PGE(2)) and TGF-beta1 synthesis, occurred in parallel to macrophage uptake of apoptotic cells. Lipid body and PGE(2) formation was observed after macrophage exposure to apoptotic, but not necrotic or live neutrophils. Blockage of BCG-induced lipid body formation significantly inhibited PGE(2) synthesis. Pre-treatment with the pan-caspase inhibitor zVAD inhibited BCG-induced neutrophil apoptosis and lipid body formation, indicating a role for apoptotic neutrophils in macrophage lipid body biogenesis in infected mice. In conclusion, BCG infection induced activation and apoptosis of infected neutrophils at the inflammatory site. The uptake of apoptotic neutrophils by macrophages leads to TGF-beta1 generation and PGE(2)-derived lipid body formation, and may have modulator roles in mycobacterial pathogenesis.

Innate immunity in tuberculosis: myths and truth

Tuberculosis is the most important bacterial infection world wide. The causative agent, Mycobacterium tuberculosis survives and proliferates within macrophages. Immune mediators such as interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha) activate macrophages and promote bacterial killing. IFN-gamma is predominantly secreted by innate cells (mainly natural killer (NK) cells) and by T cells upon instruction by interleukin 12 (IL-12) and IL-18. These cytokines are primarily produced by dendritic cells and macrophages in response to Toll-like receptor (TLR) signalling interaction with tubercle bacilli. These signals also induce pro-inflammatory cytokines (including IL-1beta and TNF-alpha), chemokines and defensins. The inflammatory environment further recruits innate effector cells such as macrophages, polymorphonuclear neutrophils (PMN) and NK cells to the infectious foci. This eventually leads to the downstream establishment of acquired T cell immunity which appears to be protective in more than 90% of infected individuals. Robust innate immune activation is considered an essential prerequisite for protective immunity and vaccine efficacy. However, data published so far provide a muddled view of the functional importance of innate immunity in tuberculosis. Here we critically discuss certain aspects of innate immunity, namely PMN, TLRs and NK cells, as characterised in tuberculosis to date, and their contribution to protection and pathology.

Rapid and potent induction of cell death and loss of NK cell cytotoxicity against oral tumors by F(ab')2 fragment of anti-CD16 antibody

Freshly isolated untreated NK cells undergo rapid apoptosis and lose their cytotoxic function upon the addition of F(ab')2 fragment of anti-CD16 antibodies. Loss of NK cell cytotoxic function after treatment with F(ab')2 fragment of anti-CD16 antibody can be seen against K562 and UCLA-2 oral tumor cells when either added immediately in the co-cultures of NK cells with the tumor cells or after pre-treatment of NK cells with the antibody before their addition to the tumor cells. Addition of Interleukin-2 (IL-2) in combination with anti-CD16 antibody to NK cells delayed the induction of DNA fragmentation in NK cells, and even though decreased cytotoxicity could still be observed against K562 and UCLA-2 oral tumors when compared to IL-2 alone treated NK cells, the cytotoxicity levels remained relatively higher and approached those obtained by untreated NK cells in the absence of antibody treatment. No increases in IFN-gamma, Granzymes A and B, Perforin and TRAIL genes could be seen in NK cells treated with anti-CD16 antibody. Neither secretion of IFN-gamma nor increased expression of CD69 activation antigen could be observed after the treatment of NK cells with anti-CD16 antibody. Furthermore, IL-2 mediated increase in CD69 surface antigens was down-modulated by anti-CD16 antibody. Finally, the addition of anti-CD16 antibody to co-cultures of NK cells with tumor target cells was not inhibitory for the secretion of VEGF by oral tumor cells, unlike those co-cultured with untreated or IL-2 treated NK cells. Thus, binding and triggering of CD16 receptor on NK cells may enhance oral tumor survival and growth by decreased ability of NK cells to suppress VEGF secretion or induce tumor cell death during the interaction of NK cells with oral tumor cells.

Mycobacterium tuberculosis-induced apoptotic neutrophils trigger a pro-inflammatory response in macrophages through release of heat shock protein 72, acting in synergy with the bacteria

Mycobacterium tuberculosis (Mtb) survive inside macrophages by manipulating microbicidal functions such as phago-lysosome fusion, production of reactive oxygen species and nitric oxide, and by rendering macrophages non-responsive to IFN-gamma. Mtb-infected lung tissue does however not only contain macrophages, but also significant numbers of infiltrating polymorphonuclear neutrophils (PMN). These are able to phagocytose and kill ingested Mtb, but are short-lived cells that constantly need to be removed from tissues to avoid tissue damage. Phagocytosis of aged or UV-induced apoptotic PMN by macrophages induce an anti-inflammatory response in macrophages. However, in the present study, we show that engulfment of Mtb-induced apoptotic PMN by macrophages initiates secretion of TNF-alpha from the macrophages, reflecting a pro-inflammatory response. Moreover, Mtb-induced apoptotic PMN up-regulate heat shock proteins 60 and 72 (Hsp60, Hsp72) intracellularly and also release Hsp72 extracellularly. We found that both recombinant Hsp72 and released Hsp72 enhanced the pro-inflammatory response to both Mtb-induced apoptotic PMN and Mtb. This stimulatory effect of the supernatant was abrogated by depleting the Hsp72 with immunoprecipitation. These findings indicate that released Hsp72 from Mtb-infected PMN can trigger macrophage activation during the early stage of Mtb infections, thereby creating a link between innate and adaptive immunity.

Spontaneous or Mycobacterium tuberculosis-induced apoptotic neutrophils exert opposite effects on the dendritic cell-mediated immune response

Polymorphonuclear neutrophils (PMN) modulate the adaptive immune response through interactions with immature dendritic cells (iDC) while spontaneous apoptotic neutrophils PMNapo (PMNapo) may have an inhibitory effect on DC functions. We investigate the effect exerted by PMNapo in DC maturation and the role of Mycobacterium tuberculosis (Mtb)-induced PMNapo in the cross-presentation of mycobacterial antigens. We demonstrate that Mtb triggers the maturation of iDC while it is impaired by the presence of PMNapo, which abrogate Mtb-induced expression of costimulatory and HLA class II molecules, reducing IL-12 and IFN-gamma release by DC and partially inhibiting Mtb-driven lymphocyte proliferation. This inhibitory effect is not observed in already Mtb-matured DC, and it involves a direct interaction between DC and PMNapo, as supernatants from PMNapo cultures do not reveal this effect. Although PMNapo do not alter Mtb/DC-SIGN interaction, they affect the intracellular signals leading to DC maturation without requiring their entry into DC. Phagocytosis of Mtb-induced PMNapo by iDC leads to lymphoproliferation, which is significantly reduced by blocking CD36 and not DC-SIGN on iDC. Therefore, cross-presentation of Mtb antigens is taking place. Our findings suggest that the inflammatory milieu is subjected to a fine balance between non-infected and Mtb-induced PMNapo: non-infected PMNapo limiting inflammation and Mtb-induced PMNapo generating a specific immune activity.

Functional characteristics of neutrophils and mononuclear cells from tuberculosis patients stimulated in vitro with heat killed M. tuberculosis

BACKGROUND

The major protective immune response against intracellular bacteria, such as Mycobacterium tuberculosis, is a cell-mediated immunity involving neutrophils (PMNs) and peripheral mononuclear cells (MCs), contributing to the clearance of this microorganism and the resolution of the infection. This study was addressed to evaluate PMNs and MCs for their bactericidal function.

METHODS

The sample comprised 14 tuberculosis (TB) inpatients (HIV-), and 10 healthy controls (HCo). Peripheral PMNs and MCs were separated by Ficoll-Hypaque and cultured in RPMI with or without heat-killed Mycobacterium tuberculosis (HK Mtb). Respiratory burst (RB), CD11b, IL-8 and TNFalpha receptor expression were assessed by flow cytometry in cells undergoing stimulation or not. Presence of IL-8 and TNFalpha in cell culture supernatants was determined by ELISA.

RESULTS

TB patients had a lower RB response than HCo for both cell types (MCs, p <0.05, PMNs, p <0.01) regardless of HK Mtb stimulation. Compared to HCo, PMNs and MCs from TB patients presented a reduced CD11b expression, with the two subject groups showing a decrease in this marker expression following HK. Mtb was added to both cell cultures. Whereas fewer IL-8 and TNFalpha receptors were found when studying MCs and PMNs from TB patients, antigen stimulation significantly raised the expression for both cytokine receptors. Culture supernatants from MCs and PMNs of TB patients contained increased amounts of IL-8 and TNFalpha.

CONCLUSIONS

The present findings may provide some explanation as to the different roles played by PMNs and MCs in TB immunopathology.

Immunological consequences of three doses of heat-killed Mycobacterium vaccae in the immunotherapy of tuberculosis

We report the first study of triple-dose immunotherapy with heat-killed Mycobacterium vaccae (SRL 172) combined with short-course, directly observed chemotherapy in newly diagnosed pulmonary tuberculosis patients. The study was carried out in Rosario, Argentina, where single-dose immunotherapy with M. vaccae has previously been shown effective. Twenty-two HIV seronegative patients, sputum-positive for tubercle bacilli, entered a randomised and partly blinded trial. Twelve patients received injections of SRL 172 and 10 patients received placebo on days 1, 30 and 60 of chemotherapy. All patients were followed up clinically, by sputum bacteriology, chest radiography and haematology. Patients receiving SRL 172 showed faster and more complete clinical improvement, accelerated disappearance of bacilli from sputum, better radiological clearance and a more rapid fall in ESR, than did those receiving placebo. Follow-up continued for a year after therapy and no patient failed treatment or relapsed. Special investigations included longitudinal assessments of respiratory bursts and expression of CD11b on separated polymorphonuclear and mononuclear leukocytes. Tumour necrosis factor alpha (TNF-alpha) was measured in the supernates of cultured cells and both TNF-alpha and interleukin-4 (IL-4) were measured in serum samples. Immunotherapy recipients showed a significantly faster return towards normal values in all the immunological parameters, than did placebo recipients. The results are consistent with a regulatory activity on cellular immunity, reducing the influence of Th2 and enhancing Th1 to the benefit of the patients. This could allow a reduced period of chemotherapy without loss of efficacy and help to prevent the development of multi-drug resistance.

Increased rate of apoptosis and diminished phagocytic ability of human neutrophils infected with Afa/Dr diffusely adhering Escherichia coli strains

The proinflammatory effect of Afa/Dr diffusely adhering Escherichia coli (Afa/Dr DAEC) strains have been recently demonstrated in vitro by showing that polymorphonuclear leukocyte (PMN) transepithelial migration is induced after bacterial colonization of apical intestinal monolayers. The effect of Afa/Dr DAEC-PMN interaction on PMN behavior has been not investigated. Because of the putative virulence mechanism of PMN apoptosis during infectious diseases and taking into account the high level of expression of the decay-accelerating factor (DAF, or CD55), the receptor of Afa/Dr DAEC on PMNs, we sought to determine whether infection of PMNs by Afa/Dr DAEC strains could promote cell apoptosis. We looked at the behavior of PMNs incubated with Afa/Dr DAEC strains once they had transmigrated across polarized monolayers of intestinal (T84) cells. Infection of PMNs by Afa/Dr DAEC strains induced PMN apoptosis characterized by morphological nuclear changes, DNA fragmentation, caspase activation, and a high level of annexin V expression. However, transmigrated and nontransmigrated PMNs incubated with Afa/Dr DAEC strains showed similar elevated global caspase activities. PMN apoptosis depended on their agglutination, induced by Afa/Dr DAEC, and was still observed after preincubation of PMNs with anti-CD55 and/or anti-CD66 antibodies. Low levels of phagocytosis of Afa/Dr DAEC strains were observed both in nontransmigrated and in transmigrated PMNs compared to that observed with the control E. coli DH5alpha strain. Taken together, these data strongly suggest that interaction of Afa/Dr DAEC with PMNs may increase the bacterial virulence both by inducing PMN apoptosis through an agglutination process and by diminishing their phagocytic capacity.

Mycobacterium tuberculosis triggers apoptosis in peripheral neutrophils involving toll-like receptor 2 and p38 mitogen protein kinase in tuberculosis patients

Polymorphonuclear neutrophils (PMN) exposed to Mycobacterium tuberculosis display bactericidal responses and produce inflammatory proteins. This PMN-mediated inflammatory response is regulated by an activation of the apoptotic program, which collaborates to avoid tissue injury. In vitro, circulating PMN from patients with tuberculosis (TB) show an increased spontaneous apoptosis, and M. tuberculosis-induced activation accelerates the PMN apoptosis. In this study, we evaluated the mechanisms involved in spontaneous and M. tuberculosis-induced apoptosis. We demonstrate that apoptosis of PMN is not induced by lipoarabinomannan or by a whole-cell lysate of M. tuberculosis and that neither tumor necrosis factor alpha nor CD11b, CD14, and Fcgamma receptors are involved. Apoptosis of PMN from patients with active TB (TB-PMN) is induced by the interaction with the whole M. tuberculosis via Toll-like receptor 2 (TLR2), and, in contrast to spontaneous apoptosis, it involves the p38 mitogen-activated protein kinase (MAPK) pathway. These results correlate with a high expression of phosphorylated p38 (p-p38) in circulating TB-PMN and with the ability of M. tuberculosis to induce in vitro the expression of p-p38 in PMN. Therefore, when the bacterial burden is low, TB-PMN could be detecting nonopsonized M. tuberculosis via TLR2, leading to the activation of the p38 MAPK pathway, which in turn would induce PMN activation and apoptosis. This mechanism needs further confirmation at the site of infection.