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

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.

The host-pathogen-environment triad: Lessons learned through the study of the multidrug-resistant Mycobacterium tuberculosis M strain

Multidrug-resistant tuberculosis is one of the major obstacles that face the tuberculosis eradication efforts. Drug-resistant Mycobacterium tuberculosis clones were initially disregarded as a public health threat, because they were assumed to have paid a high fitness cost in exchange of resistance acquisition. However, some genotypes manage to overcome the impact of drug-resistance conferring mutations, retain transmissibility and cause large outbreaks. In Argentina, the HIV-AIDS epidemics fuelled the expansion of the so-called M strain in the early 1990s, which is responsible for the largest recorded multidrug-resistant tuberculosis cluster of Latin America. The aim of this work is to review the knowledge gathered after nearly three decades of multidisciplinary research on epidemiological, microbiological and immunological aspects of this highly successful strain. Collectively, our results indicate that the successful transmission of the M strain could be ascribed to its unaltered virulence, low Th1/Th17 response, a low fitness cost imposed by the resistance conferring mutations and a high resistance to host-related stress. In the early 2000s, the incident cases due to the M strain steadily declined and stabilized in the latest years. Improvements in the management, diagnosis and treatment of multidrug-resistant tuberculosis along with societal factors such as the low domestic and international mobility of the patients affected by this strain probably contributed to the outbreak containment. This stresses the importance of sustaining the public health interventions to avoid the resurgence of this conspicuous multidrug-resistant strain.

IRAK1 and IRAK4 signaling proteins are dispensable in the response of human neutrophils to Mycobacterium tuberculosis infection

The involvement of neutrophils in the host response to Mycobacterium tuberculosis (Mtb) infection is not as well recognized as the involvement of macrophages and dendritic cells. Thus, this study gives more insight on the impact of the virulent Mtb H37Rv strain on proapoptotic and proinflammatory functions of human neutrophils in vitro. We found that neutrophils are not able to kill Mtb during the infection process, probably due to the lack of reactive oxygen species and nitric oxide production in response to bacteria. However, infected neutrophils effectively released cytokines, chemoattractant interleukin (IL) 8 and proinflammatory IL-1β. Moreover, Mtb enhanced the early apoptosis of neutrophils at 2 h postinfection. Additionally, this proapoptotic and proinflammatory response of neutrophils to Mtb infection occurred in an IRAK1- and IRAK4-independent manner. We also found that Mtb did not affect the surface expression of Toll-like receptor (TLR) 2 and slightly enhanced the surface expression of TLR4, but did not influence mRNA levels of both TLRs during the infection process. In conclusion, we show that the inhibition of signaling proteins activated by MyD88-dependent pathway did not participate in the biological activity of neutrophils against Mtb.

The multicopper oxidase of Mycobacterium tuberculosis (MmcO) exhibits ferroxidase activity and scavenges reactive oxygen species in activated THP-1 cells

The MmcO protein of Mycobacterium tuberculosis is a membrane-associated multicopper oxidase. Its natural substrate(s) and its role in pathogenesis are not well characterized. A recent report proposes that MmcO contributes to copper resistance in M. tuberculosis during infection. We have expressed and reconstituted the active enzyme from inclusion bodies in E. coli. MmcO exhibits maximal activity against the experimental substrate 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) or ABTS, at pH 4. The enzyme also exhibits ferroxidase activity at pH 4. Most notable was the finding that MmcO is able to scavenge the reactive oxygen species (ROS) generated by the xanthine/xanthine oxidase enzyme system. This ROS scavenging activity of MmcO was also evident against ROS generated by THP-1 cells. We propose that MmcO protects M. tuberculosis during infection against ROS attack in addition to providing copper resistance to the pathogen.

Mycobacterial Evolution Intersects With Host Tolerance

Over the past 200 years, tuberculosis (TB) has caused more deaths than any other infectious disease, likely infecting more people than it has at any other time in human history. Mycobacterium tuberculosis (Mtb), the etiologic agent of TB, is an obligate human pathogen that has evolved through the millennia to become an archetypal human-adapted pathogen. This review focuses on the evolutionary framework by which Mtb emerged as a specialized human pathogen and applies this perspective to the emergence of specific lineages that drive global TB burden. We consider how evolutionary pressures, including transmission dynamics, host tolerance, and human population patterns, may have shaped the evolution of diverse mycobacterial genomes.

The Immune Escape Mechanisms of Mycobacterium Tuberculosis

Epidemiological data from the Center of Disease Control (CDC) and the World Health Organization (WHO) statistics in 2017 show that 10.0 million people around the world became sick with tuberculosis. Mycobacterium tuberculosis (MTB) is an intracellular parasite that mainly attacks macrophages and inhibits their apoptosis. It can become a long-term infection in humans, causing a series of pathological changes and clinical manifestations. In this review, we summarize innate immunity including the inhibition of antioxidants, the maturation and acidification of phagolysosomes and especially the apoptosis and autophagy of macrophages. Besides, we also elaborate on the adaptive immune response and the formation of granulomas. A thorough understanding of these escape mechanisms is of major importance for the prevention, diagnosis and treatment of tuberculosis.

THE GROWTH RATE PHENOTYPIC PROPERTY OF MYCOBACTERIUM TUBERCULOSIS CLINICAL STRAINS: DEPENDENCE ON TUBERCULOSIS LOCALIZATION, TREATMENT, DRUG SUSCEPTIBILITY

The phenotypic properties of the M. tuberculosis strains obtained from patients with pulmonary or extra-pulmonary tuberculosis are  determined by a complex set of factors: the genetic characteristics  of the pathogen, its ability to adapt in vivo and in vitro, the influence of the host’s immune system and chemotherapy. The growth rate as  the phenotypic property is the most accessible for the study of the  host-pathogen relationships at the level of host/strain population  interactions. The aim of the study is to assess in vitro of the growth  rate of M. tuberculosis strains isolated from patients with pulmonary  and extra-pulmonary tuberculosis: untreated and treated (with  surgical and non-surgical treatment) and also sensitive and resistant isolates in comparison with the reference strain H37Rv. To estimate  the growth rate of 116 clinical isolates we have used the modified  method originally developed by von Groll and co-authors: to get the  bacteria growth curve the fluorescence intensity of growing strains  (with indicator resazurin) has been measured daily for 8 days in 96- well plate. The growth rate is determined as the slope of the growth  curve. The mean values of the growth rate have been calculated in  the following groups of patients: 1 — untreated patients with  pulmonary tuberculosis (PT), respiratory material; 2 — non-surgical  treated PT patients, respiratory material; 3 — surgical treated PT  patients (mainly with chronic and hyperchronic process), respiratory  material; 4 — patients like in 3rd group, surgical material; 5 — bone  and joint tuberculosis (BJT), surgical material. In addition, groups of  sensitive and resistant strains have been examined, but there are no  significant differences in growth rates. It has been obtained that  the growth rate of strains isolated from the PT patients is higher than in BJT patients: it can be explained less favorable  conditions for the pathogen vegetation in the BJT. In the case of a  closed tuberculous lesion where the pathogen transmission to  another host is impossible, then the selection of strains with the  property to survive in the tissues of the osteoarticular system is  impossible too, therefor it should be observed only an adaptation of  the pathogen strain population to the individual host. The growth  rate of isolates from untreated PT patients is higher than that of the  treated ones. Comparison of the growth parameters of only MDR  strains 1–5 groups to eliminate the influence of the  sensitivity/resistance has resulted in the same conclusions. We  suggest that the decrease in the growth rate of strains from the  treated PT patients is in not only result of the treatment, but also is  conditioned by adaptation of the pathogen to its external  environment, which is the internal environment of the  macroorganism. To confirm this assumption, the bacterial load of  1,083 diagnostic specimens grouped in a similar manner has been  estimated, taking into account only MDR/XDR strains. In the group  of treated patients the frequency of high bacterial load (CFU ≥ 100)  reached 52.5–63.8% that shows the conserved fitness of bacteria in  such patients. The mean values of the growth rate of the strain  H37Rv non-adapted to the macroorganism (due to numerous  passages on artificial media) are higher than in all groups of clinical  strains. Thus, heterogeneity of phenotypic properties of M.  tuberculosis clinical strains on the basis of growth rate has been  obtained. The growth rate of M. tuberculosis clinical strains is  depended on the tuberculosis localization (PT, BJT) and on the joint  effect of patient treatment and pathogen adaptation to the host. 

Mycobacterium tuberculosis Multidrug-Resistant Strain M Induces Low IL-8 and Inhibits TNF-α Secretion by Bronchial Epithelial Cells Altering Neutrophil Effector Functions

M strain, the most prevalent multidrug-resistant strain of Mycobacterium tuberculosis (Mtb) in Argentina, has mounted mechanisms to evade innate immune response. The role of human bronchial epithelium in Mtb infection remains unknown as well as its crosstalk with neutrophils (PMN). In this work, we evaluate whether M and H37Rv strains invade and replicate within bronchial epithelial cell line Calu-6 and how conditioned media (CM) derived from infected cells alter PMN responses. We demonstrated that M infects and survives within Calu-6 without promoting death. CM from M-infected Calu-6 (M-CM) did not attract PMN in correlation with its low IL-8 content compared to H37Rv-CM. Also, PMN activation and ROS production in response to irradiated H37Rv were impaired after treatment with M-CM due to the lack of TNF-α. Interestingly, M-CM increased H37Rv replication in PMN which would allow the spreading of mycobacteria upon PMN death and sustain IL-8 release. Thus, our results indicate that even at low invasion/replication rate within Calu-6, M induces the secretion of factors altering the crosstalk between these nonphagocytic cells and PMN, representing an evasion mechanism developed by M strain to persist in the host. These data provide new insights on the role of bronchial epithelium upon M infection.

Transmissible Mycobacterium tuberculosis Strains Share Genetic Markers and Immune Phenotypes

RATIONALE

Successful transmission of tuberculosis depends on the interplay of human behavior, host immune responses, and Mycobacterium tuberculosis virulence factors. Previous studies have been focused on identifying host risk factors associated with increased transmission, but the contribution of specific genetic variations in mycobacterial strains themselves are still unknown.

OBJECTIVES

To identify mycobacterial genetic markers associated with increased transmissibility and to examine whether these markers lead to altered in vitro immune responses.

METHODS

Using a comprehensive tuberculosis registry (n = 10,389) and strain collection in the Netherlands, we identified a set of 100 M. tuberculosis strains either least or most likely to be transmitted after controlling for host factors. We subjected these strains to whole-genome sequencing and evolutionary convergence analysis, and we repeated this analysis in an independent validation cohort. We then performed immunological experiments to measure in vitro cytokine production and neutrophil responses to a subset of the original strains with or without the identified mutations associated with increased transmissibility.

MEASUREMENTS AND MAIN RESULTS

We identified the loci espE, PE-PGRS56, Rv0197, Rv2813-2814c, and Rv2815-2816c as targets of convergent evolution among transmissible strains. We validated four of these regions in an independent set of strains, and we demonstrated that mutations in these targets affected in vitro monocyte and T-cell cytokine production, neutrophil reactive oxygen species release, and apoptosis.

CONCLUSIONS

In this study, we identified genetic markers in convergent evolution of M. tuberculosis toward enhanced transmissibility in vivo that are associated with altered immune responses in vitro.

Immunological consequences of strain variation within the Mycobacterium tuberculosis complex

In 2015, there were an estimated 10.4 million new cases of tuberculosis (TB) globally, making it one of the leading causes of death due to an infectious disease. TB is caused by members of the Mycobacterium tuberculosis complex (MTBC), with human disease resulting from infection by M. tuberculosis sensu stricto and M. africanum. Recent progress in genotyping techniques, in particular the increasing availability of whole genome sequence data, has revealed previously under appreciated levels of genetic diversity within the MTBC. Several studies have shown that this genetic diversity may translate into differences in TB transmission, clinical manifestations of disease, and host immune responses. This suggests the existence of MTBC genotype‐dependent host–pathogen interactions which may influence the outcome of infection and progression of disease. In this review, we highlight the studies demonstrating differences in innate and adaptive immunological outcomes consequent on MTBC genetic diversity, and discuss how these differences in immune response might influence the development of TB vaccines, diagnostics and new therapies.

The Defect in Autophagy Induction by Clinical Isolates of Mycobacterium Tuberculosis Is Correlated with Poor Tuberculosis Outcomes

Background

Tuberculosis (TB) represents a major global health problem. The prognosis of clinically active tuberculosis depends on the complex interactions between Mycobacterium tuberculosis (Mtb) and its host. In recent years, autophagy receives particular attention for its role in host defense against intracellular pathogens, including Mtb. In present study, we aim to investigate the relationship of autophagy induction by clinical isolates of Mtb with the clinical outcomes in patients with TB.

Methodology/Principal Findings

We collected 185 clinical isolates of Mtb, and determined the effect of these Mtb isolates on autophagy induction in macrophages. It was found that most of clinical isolates of Mtb were able to induce autophagosome formation in macrophages, however, the autophagy-inducing ability varied significantly among different isolates. Of importance, our results revealed that patients infected by Mtb with poor autophagy-inducing ability displayed more severe radiographic extent of disease (p<0.001), and were more likely to have unfavorable treatment outcomes (p<0.001). No significant association was observed between the extent of Mtb-induced autophagy with some socio-demographic characteristics (such as gender, age and tobacco consumption), and some laboratory tests (such as hemoglobin, leukocyte count and erythrocyte sedimentation rate). Furthermore, results from logistic regression analysis demonstrated that the defect in autophagy induction by clinical isolates of Mtb was an independent risk factor for far-advanced radiographic disease (aOR 4.710 [1.93–11.50]) and unfavorable treatment outcomes (aOR 8.309 [2.22–28.97]) in TB.

Conclusion/Significance

These data indicated that the defect in autophagy induction by Mtb isolates increased the risk of poor clinical outcomes in TB patients, and detection of clinical isolates-induced autophagosome formation might help evaluate the TB outcomes.

Nontuberculous mycobacterial osteomyelitis

Osteomyelitis caused by nontuberculous mycobacteria (NTM) can have severe consequences and a poor prognosis. Physicians therefore need to be alert to this condition, especially in immunocompromised patients. Although the pathogenesis of NTM osteomyelitis is still unclear, studies in immunodeficient individuals have revealed close relationships between NTM osteomyelitis and defects associated with the interleukin-12-interferon-γ-tumor necrosis factor-α axis, as well as human immunodeficiency virus infection, various immunosuppressive conditions, and diabetes mellitus. Culture and species identification from tissue biopsies or surgical debridement tissue play crucial roles in diagnosing NTM osteomyelitis. Suitable imaging examinations are also important. Adequate surgical debridement and the choice of appropriate, combined antibiotics for long-term anti-mycobacterial chemotherapy, based on in vitro drug susceptibility tests, are the main therapies for these bone infections. Bacillus Calmette-Guerin vaccination might have limited prophylactic value. The use of multiple drugs and long duration of treatment mean that the therapeutic process needs to be monitored closely to detect potential side effects. Adequate duration of anti-mycobacterial chemotherapy together with regular monitoring with blood and imaging tests are key factors determining the recovery outcome in patients with NTM osteomyelitis.

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.

Characterization of Proinflammatory Responses and Innate Signaling Activation in Macrophages Infected with Mycobacterium scrofulaceum

Mycobacterium scrofulaceum is an environmental and slow-growing atypical mycobacterium. Emerging evidence suggests that M. scrofulaceum infection is associated with cervical lymphadenitis in children and pulmonary or systemic infections in immunocompromised adults. However, the nature of host innate immune responses to M. scrofulaceum remains unclear. In this study, we examined the innate immune responses in murine bone marrow-derived macrophages (BMDMs) infected with different M. scrofulaceum strains including ATCC type strains and two clinically isolated strains (rough and smooth types). All three strains resulted in the production of proinflammatory cytokines in BMDMs mediated through toll-like receptor-2 and the adaptor MyD88. Activation of MAPKs (extracellular signal-regulated kinase 1/2, and p38, and c-Jun N-terminal kinase) and nuclear receptor (NF)-κB together with intracellular reactive oxygen species generation were required for the expression of proinflammatory cytokines in BMDMs. In addition, the rough morphotypes of M. scrofulaceum clinical strains induced higher levels of proinflammatory cytokines, MAPK and NF-κB activation, and ROS production than other strains. When mice were infected with different M. scrofulaceum strains, those infected with the rough strain showed the greatest hepatosplenomegaly, granulomatous lesions, and immune cell infiltration in the lungs. Notably, the bacterial load was higher in mice infected with rough colonies than in mice infected with ATCC or smooth strains. Collectively, these data indicate that rough M. scrofulaceum induces higher inflammatory responses and virulence than ATCC or smooth strains.

Cell death and autophagy in tuberculosis

Mycobacterium tuberculosis has succeeded in infecting one-third of the human race though inhibition or evasion of innate and adaptive immunity. The pathogen is a facultative intracellular parasite that uses the niche provided by mononuclear phagocytes for its advantage. Complex interactions determine whether the bacillus will or will not be delivered to acidified lysosomes, whether the host phagocyte will survive infection or die, and whether the timing and mode of cell death works to the advantage of the host or the pathogen. Here we discuss cell death and autophagy in TB. These fundamental processes of cell biology feature in all aspects of TB pathogenesis and may be exploited to the treatment or prevention of TB disease.

Differential expression of immunogenic proteins on virulent Mycobacterium tuberculosis clinical isolates

Molecular epidemiology has revealed that Mycobacterium tuberculosis (Mtb), formerly regarded as highly conserved species, displays a considerable degree of genetic variability that can influence the outcome of the disease as well as the innate and adaptive immune response. Recent studies have demonstrated that Mtb families found worldwide today differ in pathology, transmissibility, virulence, and development of immune response. By proteomic approaches seven proteins that were differentially expressed between a local clinical isolate from Latin-American-Mediterranean (LAM) and from Haarlem (H) lineages were identified. In order to analyze the immunogenic ability, recombinant Rv2241, Rv0009, Rv0407, and Rv2624c proteins were produced for testing specific antibody responses. We found that these proteins induced humoral immune responses in patients with drug-sensitive and drug-resistant tuberculosis with substantial cross-reactivity among the four proteins. Moreover, such reactivity was also correlated with anti-Mtb-cell surface IgM, but not with anti-ManLAM, anti-PPD, or anti-Mtb-surface IgG antibodies. Therefore, the present results describe new Mtb antigens with potential application as biomarkers of TB.

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.

A polysaccharide virulence factor of a human fungal pathogen induces neutrophil apoptosis via NK cells

Aspergillus fumigatus is an opportunistic human fungal pathogen that sheds galactosaminogalactan (GG) into the environment. Polymorphonuclear neutrophils (PMNs) and NK cells are both part of the first line of defense against pathogens. We recently reported that GG induces PMN apoptosis. In this study, we show that PMN apoptosis occurs via a new NK cell-dependent mechanism. Reactive oxygen species, induced by the presence of GG, play an indispensable role in this apoptotic effect by increasing MHC class I chain-related molecule A expression at the PMN surface. This increased expression enables interaction between MHC class I chain-related molecule A and NKG2D, leading to NK cell activation, which in turn generates a Fas-dependent apoptosis-promoting signal in PMNs. Taken together, our results demonstrate that the crosstalk between PMNs and NK cells is essential to GG-induced PMN apoptosis. NK cells might thus play a role in the induction of PMN apoptosis in situations such as unexplained neutropenia or autoimmune diseases.