Epithelial cells infected with Chlamydophila pneumoniae (Chlamydia pneumoniae) are resistant to apoptosis

Chlamydia Infection Remodels Host Cell Mitochondria to Alter Energy Metabolism and Subvert Apoptosis

Chlamydia infection represents an important cause for concern for public health worldwide. Chlamydial infection of the genital tract in females is mostly asymptomatic at the early stage, often manifesting as mucopurulent cervicitis, urethritis, and salpingitis at the later stage; it has been associated with female infertility, spontaneous abortion, ectopic pregnancy, and cervical cancer. As an obligate intracellular bacterium, Chlamydia depends heavily on host cells for nutrient acquisition, energy production, and cell propagation. The current review discusses various strategies utilized by Chlamydia in manipulating the cell metabolism to benefit bacterial propagation and survival through close interaction with the host cell mitochondrial and apoptotic pathway molecules.

Keeping the host alive - lessons from obligate intracellular bacterial pathogens

Mammals have evolved sophisticated host cell death signaling pathways as an important immune mechanism to recognize and eliminate cell intruders before they establish their replicative niche. However, intracellular bacterial pathogens that have co-evolved with their host have developed a multitude of tactics to counteract this defense strategy to facilitate their survival and replication. This requires manipulation of pro-death and pro-survival host signaling pathways during infection. Obligate intracellular bacterial pathogens are organisms that absolutely require an eukaryotic host to survive and replicate, and therefore they have developed virulence factors to prevent diverse forms of host cell death and conserve their replicative niche. This review encapsulates our current understanding of these host-pathogen interactions by exploring the most relevant findings of Anaplasma spp., Chlamydia spp., Rickettsia spp. and Coxiella burnetii modulating host cell death pathways. A detailed comprehension of the molecular mechanisms through which these obligate intracellular pathogens manipulate regulated host cell death will not only increase the current understanding of these difficult-to-study pathogens but also provide insights into new tools to study regulated cell death and the development of new therapeutic approaches to control infection.

Host cell death during infection with Chlamydia: a double-edged sword

The phylum Chlamydiae constitutes a group of obligate intracellular bacteria that infect a remarkably diverse range of host species. Some representatives are significant pathogens of clinical or veterinary importance. For instance, Chlamydia trachomatis is the leading infectious cause of blindness and the most common bacterial agent of sexually transmitted diseases. Chlamydiae are exceptionally dependent on their eukaryotic host cells as a consequence of their developmental biology. At the same time, host cell death is an integral part of the chlamydial infection cycle. It is therefore not surprising that the bacteria have evolved exquisite and versatile strategies to modulate host cell survival and death programs to their advantage. The recent introduction of tools for genetic modification of Chlamydia spp., in combination with our increasing awareness of the complexity of regulated cell death in eukaryotic cells, and in particular of its connections to cell-intrinsic immunity, has revived the interest in this virulence trait. However, recent advances also challenged long-standing assumptions and highlighted major knowledge gaps. This review summarizes current knowledge in the field and discusses possible directions for future research, which could lead us to a deeper understanding of Chlamydia's virulence strategies and may even inspire novel therapeutic approaches.

Exploring the "Multiple-Hit Hypothesis" of Neurodegenerative Disease: Bacterial Infection Comes Up to Bat

Despite major strides in personalized genomics, it remains poorly understood why neurodegenerative diseases occur in only a fraction of individuals with a genetic predisposition and conversely, why individuals with no genetic risk of a disorder develop one. Chronic diseases like Alzheimer's, Parkinson's, and Multiple sclerosis are speculated to result from a combination of genetic and environmental factors, a concept commonly referred to as the “multiple hit hypothesis.” A number of bacterial infections have been linked to increased risk of neurodegeneration, and in some cases, clearance of bacterial pathogens has been correlated with amelioration of central nervous system (CNS) deficits. Additionally, mutations in several genes known to contribute to CNS disorders like Parkinson's Disease have repeatedly been implicated in susceptibility to intracellular bacterial infection. Recent data has begun to demonstrate roles for these genes (PARK2, PINK1, and LRRK2) in modulating innate immune outcomes, suggesting that immune dysregulation may play an even more important role in neurodegeneration than previously appreciated. This review will broadly explore the connections between bacterial infection, immune dysregulation, and CNS disorders. Understanding this interplay and how bacterial pathogenesis contributes to the “multiple-hit hypothesis” of neurodegeneration will be crucial to develop therapeutics to effectively treat both neurodegeneration and infection.

Genetic Screen in Chlamydia muridarum Reveals Role for an Interferon-Induced Host Cell Death Program in Antimicrobial Inclusion Rupture

Multiple obligatory intracellular bacteria in the genus Chlamydia are important pathogens. In humans, strains of C. trachomatis cause trachoma, chlamydia, and lymphogranuloma venereum. These diseases are all associated with extended courses of infection and reinfection that likely reflect the ability of chlamydiae to evade various aspects of host immune responses. Interferon-stimulated genes, driven in part by the cytokine interferon gamma, restrict the host range of various Chlamydia species, but how these pathogens evade interferon-stimulated genes in their definitive host is poorly understood. Various Chlamydia species can inhibit death of their host cells and may have evolved this strategy to evade prodeath signals elicited by host immune responses. We present evidence that chlamydia-induced programmed cell death resistance evolved to counter interferon- and immune-mediated killing of Chlamydia-infected cells.

Interferon-regulated immune defenses protect mammals from pathogenically diverse obligate intracellular bacterial pathogens of the genus Chlamydia. Interferon gamma (IFN-γ) is especially important in controlling the virulence of Chlamydia species and thus impacts the modeling of human chlamydial infection and disease in mice. How IFN-γ contributes to cell-autonomous defenses against Chlamydia species and how these pathogens evade IFN-γ-mediated immunity in their natural hosts are not well understood. We conducted a genetic screen which identified 31 IFN-γ-sensitive (Igs) mutants of the mouse model pathogen Chlamydia muridarum. Genetic suppressor analysis and lateral gene transfer were used to map the phenotype of one of these mutants, Igs4, to a missense mutation in a putative chlamydial inclusion membrane protein, TC0574. We observed the lytic destruction of Igs4-occupied inclusions and accompanying host cell death in response to IFN-γ priming or various proapoptotic stimuli. However, Igs4 was insensitive to IFN-γ-regulated cell-autonomous defenses previously implicated in anti-Chlamydia trachomatis host defense in mice. Igs4 inclusion integrity was restored by caspase inhibitors, indicating that the IFN-γ-mediated destruction of Igs4 inclusions is dependent upon the function of caspases or related prodeath cysteine proteases. We further demonstrated that the Igs4 mutant is immune restricted in an IFN-γ-dependent manner in a mouse infection model, thereby implicating IFN-γ-mediated inclusion destruction and host cell death as potent in vivo host defense mechanisms to which wild-type C. muridarum is resistant. Overall, our results suggest that C. muridarum evolved resistance mechanisms to counter IFN-γ-elicited programmed cell death and the associated destruction of intravacuolar pathogens.

Low-Dose Aspirin May Prevent Trophoblast Dysfunction in Women With Chlamydia Pneumoniae Infection

OBJECTIVE

Previously, we demonstrated that live Chlamydia pneumoniae (Cp) impaired extravillous trophoblast (EVT) viability and invasion and that Cp DNA was detected in placentas from cases with preeclampsia. We sought to elucidate whether (1) inactive forms of Cp also affect EVT function; (2) potential therapeutic interventions protect against the effects of Cp; and (3) anti-Cp antibodies are associated with preeclampsia.

METHODS

Human first-trimester EVTs were infected with ultraviolet light-inactivated Cp. Subgroups of EVTs were pretreated with low-dose acetyl-salicylic acid (ASA), dexamethasone, heparin, and indomethacin. We conducted functional assays after infection with inactivated Cp and measured interleukin 8 (IL8), C-reactive protein (CRP), heat shock protein 60 (HSP60), and tumor necrosis factor-α (TNFα) in culture media. We measured anti-Cp IgG serum levels from women who developed preeclampsia (N = 105) and controls (N = 121).

RESULTS

Inactivated Cp reduced EVT invasion when compared to noninfected cells (P < .00001) without adversely affecting cell viability. Increased levels of IL8, CRP, HSP60, and TNFα were detected in EVTs infected with inactivated Cp compared to noninfected cells (P < .0001). Only pretreatment with low-dose ASA prevented reduced EVT invasion and decreased release of inflammatory mediators (P < .01). Elevated anti-Cp IgG antibodies were more prevalent in serum from cases with preeclampsia compared to controls (67/105 vs 53/121; adjusted P = .013); elevated IgG correlated significantly with elevated serum CRP and elevated soluble fms-like tyrosine kinase-1-placental growth factor ratio.

CONCLUSION

Inactivated Cp induces decreased EVT invasion and a proinflammatory response; these effects were abrogated by pretreatment with low-dose ASA. Our results suggest an association between Cp infection, trophoblast dysfunction, and preeclampsia.

Absence of Specific Chlamydia trachomatis Inclusion Membrane Proteins Triggers Premature Inclusion Membrane Lysis and Host Cell Death

Chlamydia trachomatis is a human pathogen associated with significant morbidity worldwide. As obligate intracellular parasites, chlamydiae must survive within eukaryotic cells for sufficient time to complete their developmental cycle. To promote host cell survival, chlamydiae express poorly understood anti-apoptotic factors. Using recently developed genetic tools, we show that three inclusion membrane proteins (Incs) out of eleven examined are required for inclusion membrane stability and avoidance of host cell death pathways. In the absence of specific Incs, premature inclusion lysis results in recognition by autophagolysosomes, activation of intrinsic apoptosis, and premature termination of the chlamydial developmental cycle. Inhibition of autophagy or knockdown of STING prevented host cell death and activation of intrinsic apoptosis. Significantly, these findings emphasize the importance of Incs in the establishment of a replicative compartment that sequesters the pathogen from host surveillance systems.

Chlamydia muridarum Infection of Macrophages Stimulates IL-1β Secretion and Cell Death via Activation of Caspase-1 in an RIP3-Independent Manner

Chlamydiae are Gram-negative bacteria, which replicate exclusively in the infected host cells. Infection of the host cells by Chlamydiae stimulates the innate immune system leading to an inflammatory response, which is manifested not only by secretion of proinflammatory cytokines such as IL-1β from monocytes, macrophages, and dendritic cells, but also possibly by cell death mediated by Caspase-1 pyroptosis. RIP3 is a molecular switch that determines the development of necrosis or inflammation. However, the involvement of RIP3 in inflammasome activation by Chlamydia muridarum infection has not been clarified. Here, we assessed the role of RIP3 in synergy with Caspase-1 in the induction of IL-1β production in BMDM after either LPS/ATP or Chlamydia muridarum stimulation. The possibility of pyroptosis and necroptosis interplays and the role of RIP3 in IL-1β production during Chlamydia muridarum infection in BMDM was investigated as well. The data indicated that RIP3 is involved in NLRP3 inflammasome activation in LPS/ATP-stimulated BMDMs but not in Chlamydia muridarum infection. Pyroptosis occurred in BMDM after LPS/ATP stimulation or Chlamydia muridarum infection. Moreover, the results also illuminated the important role of the Caspase-1-mediated pyroptosis process which does not involve RIP3. Taken together, these observations may help shed new light on details in inflammatory signaling pathways activated by Chlamydia muridarum infection.

Prediction of Chlamydia pneumoniae protein localization in host mitochondria and cytoplasm and possible involvements in lung cancer etiology: a computational approach

Collecting evidence suggests that the intercellular infection of Chlamydia pneumoniae in lungs contributes to the etiology of lung cancer. Many proteins of Chlamydia pneumoniae outmanoeuvre the various system of the host. The infection may regulate various factors, which can influence the growth of lung cancer in affected persons. In this in-silico study, we predict potential targeting of Chlamydia pneumoniae proteins in mitochondrial and cytoplasmic comportments of host cell and their possible involvement in growth and development of lung cancer. Various cellular activities are controlled in mitochondria and cytoplasm, where the localization of Chlamydia pneumoniae proteins may alter the normal functioning of host cells. The rationale of this study is to find out and explain the connection between Chlamydia pneumoniae infection and lung cancer. A sum of 183 and 513 proteins were predicted to target in mitochondria and cytoplasm of host cell out of total 1112 proteins of Chlamydia pneumoniae. In particular, many targeted proteins may interfere with normal growth behaviour of host cells, thereby altering the decision of program cell death. Present article provides a potential connection of Chlamydia pneumoniae protein targeting and proposed that various targeted proteins may play crucial role in lung cancer etiology through diverse mechanisms.

TNF-α inhibits the growth of Legionella pneumophila in airway epithelial cells by inducing apoptosis

BACKGROUND

TNF-α plays an important role in the pathogenesis of Legionella pneumophila (Lp)-induced pneumonia. Patients undergoing anti-TNF-α therapy are at an increased risk of Lp infection. Lp infects both phagocytic and non-phagocytic cells such as airway epithelial cells; however, the role of TNF-α in airway epithelial cells is unknown.

METHODS

Human airway epithelial cell line NCI-H292 was infected with Lp NUL1 strain. After infection, both intracellular growth of Lp and cell death were evaluated after treating the cells with or without TNF-α. Apoptosis was examined by performing activated caspase-3/7 staining and by using a pan-caspase inhibitor.

RESULTS

Lp infected and replicated in NCI-H292 cells in a time-dependent manner, and TNF-α treatment of Lp-infected NCI-H292 cells inhibited Lp replication. Inhibitory effects of TNF-α on Lp replication were suppressed after treatment with a TNF-α-neutralizing antibody. Lp infection increased extracellular lactate dehydrogenase levels and decreased the number of living cells. Increased number of Lp-infected NCI-H292 cells showed caspase-3/7 activation, indicating they underwent apoptosis. TNF-α treatment inhibited Lp replication by increasing the apoptosis of NCI-H292 cells.

CONCLUSIONS

Thus, our results suggested that airway epithelial cells were involved in the pathogenesis of Lp infection and that TNF-α played a protective role by inhibiting the intracellular replication of Lp and by increasing the apoptosis of Lp-infected airway epithelial cells. However, Lp infection should be investigated further in patients undergoing anti-TNF-α therapy who develop pneumonia.

An epistatic effect of apaf-1 and caspase-9 on chlamydial infection

Chlamydia is an obligate intracellular bacterial pathogen that replicates solely within a membrane-bound vacuole termed an inclusion. Chlamydia seems to perturb multiple cellular processes of the host, such as, rearrangement of the membrane trafficking system for its intracellular multiplication, and inhibition of host cell apoptosis for persistent infection. In an attempt to clarify host factor involvement in apoptosis regulation, we found that inhibition of Caspase-9 restricted, while Apaf-1 promoted, Chlamydia pneumoniae infection in HEp-2, HeLa, and mouse epithelial fibroblast (MEF) cells. These opposition contributions to the chlamydial infection were confirmed using caspase-9^−/− and apaf-1^−/− MEFs. Similar phenomena also appeared in the case of infection with Chlamydia trachomatis. Interestingly, caspase-9 in apaf-1^−/− MEFs was activated by chlamydial infection but during the infection caspase-3 was not activated. That is, caspase-9 was activated without support for multiplication and activation by Apaf-1, and the activated caspase-9 may be physically disconnected from the caspase cascade. This may be partially explained by the observation of caspase-9 accumulation within chlamydial inclusions. The sequestration of caspase-9 by chlamydia seems to result in apoptosis repression, which is crucial for the chlamydial development cycle. Because Apaf-1 shares domains with intracellular innate immune receptor NOD1, it may play a key role in the strategy to regulate chlamydial infection.

A Simple High Efficiency Intra-Islet Transduction Protocol Using Lentiviral Vectors

Successful normalization of blood glucose in patients transplanted with pancreatic islets isolated from cadaveric donors established the proof-of-concept that Type 1 Diabetes Mellitus is a curable disease. Nonetheless, major caveats to the widespread use of this cell therapy approach have been the shortage of islets combined with the low viability and functional rates subsequent to transplantation. Gene therapy targeted to enhance survival and performance prior to transplantation could offer a feasible approach to circumvent these issues and sustain a durable functional β-cell mass in vivo. However, efficient and safe delivery of nucleic acids to intact islet remains a challenging task. Here we describe a simple and easy-to-use lentiviral transduction protocol that allows the transduction of approximately 80 % of mouse and human islet cells while preserving islet architecture, metabolic function and glucose-dependent stimulation of insulin secretion. Our protocol will facilitate to fully determine the potential of gene expression modulation of therapeutically promising targets in entire pancreatic islets for xenotransplantation purposes.

Chlamydia pneumoniae and typical bacteria occurrence in adenoid in children qualified for adenoidectomy

OBJECTIVE

The ability to occur in a persistent form and to inhibit apoptosis in host cells are the features of Chlamydia pneumoniae which may be associated with adenoid hypertrophy. This study aimed to (1) demonstrate the presence of C. pneumoniae DNA in the adenoid in children qualified for adenoidectomy, (2) evaluate the correlation between the presence of C. pneumoniae in the adenoid and the child's age, the size of adenoid, the incidence of recurrent respiratory tract infections, (3) demonstrate the co-occurrence of C. pneumoniae and typical bacteria in the adenoid tissue.

METHODS

A group of 200 children aged from 2 to 16 years (mean age 6.4) undergoing adenoidectomy from February 2010 to May 2011 were enrolled to the study. Adenoid tissue removed during planned adenoidectomy was analyzed for the presence of C. pneumoniae by real-time PCR and for the presence of typical bacteria by bacteriological culture. Statistical analysis was conducted using SPSS PASW Statistics 18 software.

RESULTS

C. pneumoniae was detected in the adenoid in 5.5% of the 200 children. Positive results were obtained most frequently (24.1%, 7/29) in the eldest group (10-16 years). In PCR positive children the degree of choanal narrowing by an adenoid tended to be larger than in PCR negative children. The occurrence of C. pneumoniae in the adenoid was not associated with a presence of a specific typical bacterium.

CONCLUSIONS

The presence of C. pneumoniae in an adenoid was most frequently detected in the children of 10-16 years qualified for adenoidectomy.

Statistical genetic analysis of serological measures of common, chronic infections in Alaska Native participants in the GOCADAN study

This paper describes genetic investigations of seroreactivity to five common infectious pathogens in the Genetics of Coronary Artery Disease in Alaska Natives (GOCADAN) study. Antibody titers and seroprevalence were available for 495 to 782 (depending on the phenotype) family members at two time points, approximately 15 years apart, for Chlamydophila pneumoniae, Helicobacter pylori, cytomegalovirus (CMV), herpes simplex virus 1 (HSV-1), and herpes simplex virus 2 (HSV-2). Seroprevalence rates indicate that infections with most of these pathogens are common (≥20% for all of them, >80% for H. pylori, CMV, and HSV-1). Seropositive individuals typically remain seropositive over time, with seroreversion rates of <1% to 10% over ∼15 years. Antibody titers were significantly heritable for most pathogens, with the highest estimate being 0.61 for C. pneumoniae. Significant genome-wide linkage evidence was obtained for C. pneumoniae on chromosome 15 (logarithm of odds, LOD score of 3.13). These results demonstrate that individual host genetic differences influence antibody measures of common infections in this population, and further investigation may elucidate the underlying immunological processes and genes involved.

Chlamydial intracellular survival strategies

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen and the causative agent of blinding trachoma. Although Chlamydia is protected from humoral immune responses by residing within remodeled intracellular vacuoles, it still must contend with multilayered intracellular innate immune defenses deployed by its host while scavenging for nutrients. Here we provide an overview of Chlamydia biology and highlight recent findings detailing how this vacuole-bound pathogen manipulates host-cellular functions to invade host cells and maintain a replicative niche.

Review: apoptotic mechanisms in bacterial infections of the central nervous system

In this article we review the apoptotic mechanisms most frequently encountered in bacterial infections of the central nervous system (CNS). We focus specifically on apoptosis of neural cells (neurons and glia), and provide first an overview of the phenomenon of apoptosis itself and its extrinsic and intrinsic pathways. We then describe apoptosis in the context of infectious diseases and inflammation caused by bacteria, and review its role in the pathogenesis of the most relevant bacterial infections of the CNS.

Infectious agents and neurodegeneration

A growing body of epidemiologic and experimental data point to chronic bacterial and viral infections as possible risk factors for neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. Infections of the central nervous system, especially those characterized by a chronic progressive course, may produce multiple damage in infected and neighbouring cells. The activation of inflammatory processes and host immune responses cause chronic damage resulting in alterations of neuronal function and viability, but different pathogens can also directly trigger neurotoxic pathways. Indeed, viral and microbial agents have been reported to produce molecular hallmarks of neurodegeneration, such as the production and deposit of misfolded protein aggregates, oxidative stress, deficient autophagic processes, synaptopathies and neuronal death. These effects may act in synergy with other recognized risk factors, such as aging, concomitant metabolic diseases and the host’s specific genetic signature. This review will focus on the contribution given to neurodegeneration by herpes simplex type-1, human immunodeficiency and influenza viruses, and by Chlamydia pneumoniae.

CPAF: a Chlamydial protease in search of an authentic substrate

Bacteria in the genus Chlamydia are major human pathogens that cause an intracellular infection. A chlamydial protease, CPAF, has been proposed as an important virulence factor that cleaves or degrades at least 16 host proteins, thereby altering multiple cellular processes. We examined 11 published CPAF substrates and found that there was no detectable proteolysis when CPAF activity was inhibited during cell processing. We show that the reported proteolysis of these putative CPAF substrates was due to enzymatic activity in cell lysates rather than in intact cells. Nevertheless, Chlamydia-infected cells displayed Chlamydia-host interactions, such as Golgi reorganization, apoptosis resistance, and host cytoskeletal remodeling, that have been attributed to CPAF-dependent proteolysis of host proteins. Our findings suggest that other mechanisms may be responsible for these Chlamydia-host interactions, and raise concerns about all published CPAF substrates and the proposed roles of CPAF in chlamydial pathogenesis.

Chlamydia are bacteria that invade eukaryotic host cells and live within a membrane-bound compartment called the chlamydial inclusion. Growth and survival of these important human and animal pathogens depends on extensive interactions with the host cell, which allow chlamydiae to acquire critical nutrients and to avoid host anti-microbial defenses. Chlamydiae are proposed to cause many of these host-pathogen interactions through the cleavage or degradation of host proteins by the chlamydial protease CPAF, which is secreted into the host cytoplasm. Here, we raise questions about the proposed roles of this virulence factor during infection, as well as its published substrates. We found that there was no detectable cleavage or degradation of 11 previously reported CPAF substrates in Chlamydia-infected cells and that CPAF-mediated proteolysis of these host proteins occurs during cell harvest and lysis. However, we still observed host-pathogen interactions previously attributed to CPAF proteolysis of these proteins, suggesting that Chlamydia is likely to cause these effects on the host cell through other mechanisms. Our findings call for a re-evaluation of all published CPAF substrates as well as the proposed roles of this protease in chlamydial pathogenesis.

Lack of effective anti-apoptotic activities restricts growth of Parachlamydiaceae in insect cells

The fundamental role of programmed cell death in host defense is highlighted by the multitude of anti-apoptotic strategies evolved by various microbes, including the well-known obligate intracellular bacterial pathogens Chlamydia trachomatis and Chlamydia (Chlamydophila) pneumoniae. As inhibition of apoptosis is assumed to be essential for a successful infection of humans by these chlamydiae, we analyzed the anti-apoptotic capacity of close relatives that occur as symbionts of amoebae and might represent emerging pathogens. While Simkania negevensis was able to efficiently replicate within insect cells, which served as model for metazoan-derived host cells, the Parachlamydiaceae (Parachlamydia acanthamoebae and Protochlamydia amoebophila) displayed limited intracellular growth, yet these bacteria induced typical features of apoptotic cell death, including formation of apoptotic bodies, nuclear condensation, internucleosomal DNA fragmentation, and effector caspase activity. Induction of apoptosis was dependent on bacterial activity, but not bacterial de novo protein synthesis, and was detectable already at very early stages of infection. Experimental inhibition of host cell death greatly enhanced parachlamydial replication, suggesting that lack of potent anti-apoptotic activities in Parachlamydiaceae may represent an important factor compromising their ability to successfully infect non-protozoan hosts. These findings highlight the importance of the evolution of anti-apoptotic traits for the success of chlamydiae as pathogens of humans and animals.

HIF-1α is involved in mediating apoptosis resistance to Chlamydia trachomatis-infected cells

Chlamydiae are obligate intracellular Gram-negative bacteria that cause widespread diseases in humans. Due to the intimate association between bacterium and host, Chlamydia evolved various strategies to protect their host cell against death-inducing stimuli, allowing the bacterium to complete its development cycle. An RNA interference (RNAi)-based screen was used to identify host cell factors required for apoptosis resistance of human epithelial cells infected with Chlamydia trachomatis serovar L2. Among the 32 validated hits, the anti-apoptotic Bcl-2 family member Mcl-1 was identified as a target. Protein network analyses implicated the transcription factor hypoxia-induced factor 1 alpha (HIF-1α) to be central to the regulation of many of the identified targets. Further mechanistic investigations showed that HIF-1α was stabilized within the host cell cytoplasm during early infection time points, followed by its translocation to the nucleus and eventual transcriptional activation of Mcl-1. siRNA-mediated depletion of HIF-1α led to a drastic decrease in Mcl-1, rendering the cell sensitive to apoptosis induction. Taken together, our findings identify HIF-1α as responsible for upregulation of Mcl-1 and the maintenance of apoptosis resistance during Chlamydia infection.

Evolutionary conservation of infection-induced cell death inhibition among Chlamydiales

Control of host cell death is of paramount importance for the survival and replication of obligate intracellular bacteria. Among these, human pathogenic Chlamydia induces the inhibition of apoptosis in a variety of different host cells by directly interfering with cell death signaling. However, the evolutionary conservation of cell death regulation has not been investigated in the order Chlamydiales, which also includes Chlamydia-like organisms with a broader host spectrum. Here, we investigated the apoptotic response of human cells infected with the Chlamydia-like organism Simkania negevensis (Sn). Simkania infected cells exhibited strong resistance to apoptosis induced by intrinsic stress or by the activation of cell death receptors. Apoptotic signaling was blocked upstream of mitochondria since Bax translocation, Bax and Bak oligomerisation and cytochrome c release were absent in these cells. Infected cells turned on pro-survival pathways like cellular Inhibitor of Apoptosis Protein 2 (cIAP-2) and the Akt/PI3K pathway. Blocking any of these inhibitory pathways sensitized infected host cell towards apoptosis induction, demonstrating their role in infection-induced apoptosis resistance. Our data support the hypothesis of evolutionary conserved signaling pathways to apoptosis resistance as common denominators in the order Chlamydiales.

Chlamydia pneumoniae inhibits activated human T lymphocyte proliferation by the induction of apoptotic and pyroptotic pathways

Chlamydia pneumoniae is an omnipresent obligate intracellular bacterial pathogen that infects numerous host species. C. pneumoniae infections of humans are a common cause of community acquired pneumonia but have also been linked to chronic diseases such as atherosclerosis, Alzheimer's disease, and asthma. Persistent infection and immune avoidance are believed to play important roles in the pathophysiology of C. pneumoniae disease. We found that C. pneumoniae organisms inhibited activated but not nonactivated human T cell proliferation. Inhibition of proliferation was pathogen specific, heat sensitive, and multiplicity of infection dependent and required chlamydial entry but not de novo protein synthesis. Activated CD4(+) and CD8(+) T cells were equally sensitive to C. pneumoniae antiproliferative effectors. The C. pneumoniae antiproliferative effect was linked to T cell death associated with caspase 1, 8, 9, and IL-1β production, indicating that both apoptotic and pyroptotic cellular death pathways were activated after pathogen-T cell interactions. Collectively, these findings are consistent with the conclusion that C. pneumoniae could induce a local T cell immunosuppression and inflammatory response revealing a possible host-pathogen scenario that would support both persistence and inflammation.

The struggle within: microbial influences on colorectal cancer

Recently, an unprecedented effort has been directed at understanding the interplay between chronic inflammation and development of cancer, with the case of inflammatory bowel disease (IBD)-associated colorectal cancer at the forefront of this research endeavor. The last decade has been particularly fertile, with the discovery of numerous innovative paradigms linking various inflammatory, proliferative, and innate and adaptive immune signaling pathways to the development of colorectal cancer. Because of the preponderant role of the intestinal microbiota in the initiation and progression of IBD, recent efforts have been directed at understanding the relationship between bacteria and colorectal cancer. The microbiota and its collective genome, the microbiome, form a diverse and complex ecological community that profoundly impacts intestinal homeostasis and disease states. This review will discuss the differential influence of the microbiota on the development of IBD-associated colorectal cancer and highlight the role of innate immune sensor-dependent as well as -independent mechanisms in this pathology.

Interactions between bacterial pathogens and mitochondrial cell death pathways

The modulation of host cell death pathways by bacteria has been recognized as a major pathogenicity mechanism. Among other strategies, bacterial pathogens can hijack the cell death machinery of host cells by influencing the signalling pathways that converge on the mitochondria. In particular, many bacterial proteins have evolved to interact in a highly specific manner with host mitochondria, thereby modulating the decision between cell life and death. In this Review, we explore the intimate interactions between bacterial pathogens and mitochondrial cell death pathways.

Deficiency of XIAP leads to sensitization for Chlamydophila pneumoniae pulmonary infection and dysregulation of innate immune response in mice

Obligate intracellular Chlamydophila pneumoniae induce apoptosis resistance in host cells to escape eradication by immune effector cells. Apoptosis resistance depends on the increased expression and stabilization of cellular inhibitor of apoptosis proteins (cIAPs) and X-linked inhibitor of apoptosis protein (XIAP). Here we investigated the role of XIAP in experimental pulmonary infection of mice with C. pneumoniae. XIAP knock out (KO) mice were sensitized for C. pneumoniae infection compared with wild type mice. XIAP was involved in lipopolysaccharide (LPS)-induced production of nitric oxide (NO) and endotoxin shock. Hyper-secretion of tumor necrosis factor-alpha and lower NO in LPS-treated KO mouse macrophages revealed its regulatory role in inflammatory responses. Unexpectedly, activating stimuli like LPS, tumor necrosis factor-alpha, or interferon-gamma very efficiently induced apoptotic cell death in KO macrophages but not in wild type macrophages. Cell survival transcription factor nuclear factor kappaB (NF-kappaB) p65 levels were reduced in lungs and pulmonary macrophages of infected KO mice. Furthermore, a reduced CD8 T cell population and their increased sensitivity for concanavalin A and chlamydial HSP60 stimulation revealed a defect in CD8 T cells in XIAP KO mice. These data demonstrated a role of XIAP for the integrity of both innate and cellular immune responses during C. pneumoniae infection.

Chlamydial infection: the link with ocular adnexal lymphomas

Chlamydiae are obligate intracellular bacteria that grow in eukaryotic cells and cause a wide spectrum of diseases. They can establish persistent infections, are mitogenic in vitro, promote polyclonal cell proliferation in vivo and induce resistance to apoptosis in infected cells-properties that might contribute to tumorigenesis. In fact, Chlamydophila psittaci (Cp) has been linked to the development and maintenance of ocular adnexal marginal zone B-cell lymphoma (OAMZL). In this indolent malignancy, Cp is transported by monocytes and macrophages and causes both local and systemic infection. Cp elementary bodies are viable and infectious in the conjunctiva and peripheral blood of patients with OAMZL. Bacterial eradication with antibiotic therapy is often followed by lymphoma regression. Despite recent advances in the understanding of this bacterium-lymphoma association, several questions remain unanswered. For instance, prevalence variations among different geographical areas and related diagnostic and therapeutic implications remain a major investigational issue. We will focus on clinical and therapeutic implications of chlamydial infections in patients with lymphomas and summarize the current knowledge on the association between Cp infection and OAMZL. Available data on the epidemiology, biology and pathogenesis of this association are analyzed and new investigative and clinical approaches are discussed.

Killing me softly: chlamydial use of proteolysis for evading host defenses

Chlamydial infections in humans cause severe health problems, including blinding trachoma and sexually transmitted diseases. Although the involved pathogenic mechanisms remain unclear, the ability to replicate and maintain long-term residence in the infected cells seems to significantly contribute to chlamydial pathogenicity. These obligate intracellular parasites maintain a delicate balance between exploiting and protecting their host: they occupy intracellular space and acquire nutrients from the infected cells, but at the same time they have to maintain the integrity of the host cells for the completion of their intracellular growth. For this purpose, chlamydiae hijack certain signaling pathways that prevent the host cells from undergoing apoptosis induced by intracellular stress and protect the infected cells from recognition and attack by host defenses. Interestingly, one of the strategies that chlamydiae use for these purposes is the induction of limited proteolysis of host proteins, which is the main focus of this article.

Mycobacterium leprae induces insulin-like growth factor and promotes survival of Schwann cells upon serum withdrawal

Peripheral nerve lesions are considered the most relevant symptoms of leprosy, a chronic infectious disease caused by Mycobacterium leprae. The strategies employed by M. leprae to infect and multiply inside Schwann cells (SCs), however, remain poorly understood. In this study, it is shown that treatment of SCs with M. leprae significantly decreased cell death induced by serum deprivation. Not displayed by Mycobacterium smegmatis or Mycobacterium bovis BCG, the M. leprae survival effect was both dose dependent and specific. The conditioned medium (CM) of M. leprae-treated cultures was seen to mimic the protective effect of the bacteria, suggesting that soluble factors secreted by SCs in response to M. leprae were involved in cell survival. Indeed, by quantitative RT-PCR and dot blot/ELISA, it was demonstrated that M. leprae induced the expression and secretion of the SC survival factor insulin-like growth factor-I. Finally, the involvement of this hormone in M. leprae-induced SC survival was confirmed in experiments with neutralizing antibodies. Taken together, the results of this study delineate an important strategy for the successful colonization of M. leprae in the nerve based on the survival maintenance of the host cell through induction of IGF-I production.

Molecular pathogenesis of chronic Chlamydia pneumoniae infection: a brief overview

Owing to its unique host cell-dependent development cycle, Chlamydia pneumoniae occupies an intracellular niche that enables the bacterium to survive and to multiply, secluded from both the extracellular and the cytoplasmic environments. Within its separate chlamydial inclusion, it is able to genetically switch between a replicative and a persisting non-replicative state, linking the pathogen to acute as well as chronic diseases. Although its role in acute respiratory infection has been established, a potential link between chronic vascular infection with C. pneumoniae and the development of atherosclerosis remains enigmatic, in particular because chronic chlamydial infection cannot be eradicated by antibiotics. C. pneumoniae has developed numerous mechanisms to establish an adequate growth milieu involving the type III secretion-mediated release of chlamydial effector proteins that interact with cellular structures and reprogram host cell regulatory pathways. This brief overview of these pathomechanisms focuses on chronic vascular infection.

cIAP-1 controls innate immunity to C. pneumoniae pulmonary infection

The resistance of epithelial cells infected with Chlamydophila pneumoniae for apoptosis has been attributed to the induced expression and increased stability of anti-apoptotic proteins called inhibitor of apoptosis proteins (IAPs). The significance of cellular inhibitor of apoptosis protein-1 (cIAP-1) in C. pneumoniae pulmonary infection and innate immune response was investigated in cIAP-1 knockout (KO) mice using a novel non-invasive intra-tracheal infection method. In contrast to wildtype, cIAP-1 knockout mice failed to clear the infection from their lungs. Wildtype mice responded to infection with a strong inflammatory response in the lung. In contrast, the recruitment of macrophages was reduced in cIAP-1 KO mice compared to wildtype mice. The concentration of Interferon gamma (IFN-γ) was increased whereas that of Tumor Necrosis Factor (TNF-α) was reduced in the lungs of infected cIAP-1 KO mice compared to infected wildtype mice. Ex vivo experiments on mouse peritoneal macrophages and splenocytes revealed that cIAP-1 is required for innate immune responses of these cells. Our findings thus suggest a new immunoregulatory role of cIAP-1 in the course of bacterial infection.

Lack of an association betweenChlamydia psittaciand ocular adnexal lymphoma

The objective of this study was to assess whether there is PCR evidence for C. psittaci DNA in ocular adnexal lymphoma specimens collected in an academic institution in the U.S. This was a retrospective, single-center study of patients from 1994 - 2004. We used 28 ocular adnexal lymphoma biopsy specimens from adult patients, 16 control lymphoma specimens from patients with systemic lymphomas not involving the ocular adnexa, and five control benign adnexal tissue samples. The presence of C. psittaci DNA was investigated by polymerase chain reaction (PCR) in each group. Two different assays were utilized: (1) conventional PCR/gel based assay targeting a 111-bp fragment of the 16S gene and (2) a real-time PCR assay amplifying a 148-bp portion of the 16S gene with detection via a specific fluorescent probe. Amplification was carried out to 60 cycles. Positive controls consisted of isolated DNA from C. psittaci strains VS1, CP3, and FP. A human DNA internal control was used to assess sample DNA quality and amplification success. Mean outcome measure was the presence of C. psittaci DNA. Using both assays, all patient samples in all categories yielded negative results. Both assays detected C. psittaci DNA from isolated strains. Internationally, Chlamydia psittaci has been associated with ocular adnexal lymphomas with great variability. Similar to several other recent studies in the USA, our study could not confirm the presence of C. psittaci in ocular adnexal lymphomas. Differences in the prevalence of C. psittaci infection in various geographic regions or technical differences in the application of the assays may underlie the variability in the association between C. psittaci and ocular adnexal lymphoma.

Neisseria gonorrhoeae infection protects human endocervical epithelial cells from apoptosis via expression of host antiapoptotic proteins

Several microbial pathogens can modulate the host apoptotic response to infection, which may contribute to immune evasion. Various studies have reported that infection with the sexually transmitted disease pathogen Neisseria gonorrhoeae can either inhibit or induce apoptosis. N. gonorrhoeae infection initiates at the mucosal epithelium, and in women, cells from the ectocervix and endocervix are among the first host cells encountered by this pathogen. In this study, we defined the antiapoptotic effect of N. gonorrhoeae infection in human endocervical epithelial cells (End/E6E7 cells). We first established that N. gonorrhoeae strain FA1090B failed to induce cell death in End/E6E7 cells. Subsequently, we demonstrated that stimulation with N. gonorrhoeae protected these cells from staurosporine (STS)-induced apoptosis. Importantly, only End/E6E7 cells incubated with live bacteria and in direct association with N. gonorrhoeae were protected from STS-induced apoptosis, while heat-killed and antibiotic-killed bacteria failed to induce protection. Stimulation of End/E6E7 cells with live N. gonorrhoeae induced NF-kappaB activation and resulted in increased gene expression of the NF-kappaB-regulated antiapoptotic genes bfl-1, cIAP-2, and c-FLIP. Furthermore, cIAP-2 protein levels also increased in End/E6E7 cells incubated with gonococci. Collectively, our results indicate that the antiapoptotic effect of N. gonorrhoeae in human endocervical epithelial cells results from live infection via expression of host antiapoptotic proteins. Securing an intracellular niche through the inhibition of apoptosis may be an important mechanism utilized by N. gonorrhoeae for microbial survival and immune evasion in cervical epithelial cells.

Trophoblast infection with Chlamydia pneumoniae and adverse pregnancy outcomes associated with placental dysfunction

OBJECTIVE

We sought to determine whether Chlamydia pneumoniae impairs invasive trophoblast function and is associated with preeclampsia.

STUDY DESIGN

We conducted cell viability and invasion assays using primary extravillous trophoblast cells isolated from first-trimester placentas. We performed a case-control study to identify C pneumoniae in trophoblast cells dissected by laser capture microscopy from placentas in women with severe preeclampsia and control subjects who delivered at term.

RESULTS

Trophoblast cell viability and invasion through extracellular matrices were decreased after infection with C pneumoniae (both P < .05). C pneumoniae DNA was detected in trophoblast cells in 15/48 cases but only 3/30 controls (odds ratio, 4.1; P = .02). Positive and negative controls yielded expected results.

CONCLUSION

C pneumoniae infection can reduce trophoblast invasion into the uterine wall and is associated with preeclampsia. Further investigation of the mechanisms by which C pneumoniae induces trophoblast dysfunction, and the identification of therapies to prevent adverse outcomes attributed to trophoblast dysfunction, are warranted.

Apoptosis resistance in Chlamydia-infected cells: a fate worse than death?

Chlamydia has long been studied as an intracellular pathogen causing widespread diseases. In the last three decades, the field of apoptosis has rapidly emerged, and as a consequence, research on infectious diseases in general and on Chlamydia-host interaction in particular shifted to apoptosis modulation. Ten years ago, the first paper describing the drastic inhibition of apoptosis in Chlamydia-infected cells was published. In a reversal of roles, here was a pathogen that was strongly protecting cells in an organism against destruction by the organism's immune system. Since then, numerous studies have described apoptosis inhibition by Chlamydia and the mechanisms involved, but still there is a lack of general consensus on the subject. With a section of studies even reporting the induction of cell death by Chlamydia and not its inhibition, the field became even more diverse and complicated. In this review, an attempt is made to discuss the recent findings on apoptosis modulation by chlamydial species.

Premature apoptosis of Chlamydia-infected cells disrupts chlamydial development

The obligate intracellular development of Chlamydia suggests that the bacteria should be vulnerable to premature host cell apoptosis, but because Chlamydia-infected cells are apoptosis resistant, this has never been able to be tested. We have devised a system to circumvent the apoptotic block imposed by chlamydial infection. When the proapoptotic protein Bim(S) was experimentally induced, epithelial cells underwent apoptosis that was not blocked by chlamydial infection. Apoptosis during the developmental cycle prevented the generation of infectious bacteria and caused transcriptional changes of bacterial genes and loss of intracellular ATP. Intriguingly, although apoptosis resulted in destruction of host cell structures and of the Chlamydia inclusion, and prevented generation of elementary bodies, Bim(S) induction in the presence of a caspase inhibitor allowed differentiation into morphologically normal but noninfectious elementary bodies. These data show that chlamydial infection renders host cells apoptosis resistant at a premitochondrial step and demonstrate the consequences of premature apoptosis for development of the bacteria.

Chlamydia causes fragmentation of the Golgi compartment to ensure reproduction

The obligate intracellular bacterium Chlamydia trachomatis survives and replicates within a membrane-bound vacuole, termed the inclusion, which intercepts host exocytic pathways to obtain nutrients. Like many other intracellular pathogens, C. trachomatis has a marked requirement for host cell lipids, such as sphingolipids and cholesterol, produced in the endoplasmic reticulum and the Golgi apparatus. However, the mechanisms by which intracellular pathogens acquire host cell lipids are not well understood. In particular, no host cell protein responsible for transporting Golgi-derived lipids to the chlamydial inclusions has yet been identified. Here we show that Chlamydia infection in human epithelial cells induces Golgi fragmentation to generate Golgi ministacks surrounding the bacterial inclusion. Ministack formation is triggered by the proteolytic cleavage of the Golgi matrix protein golgin-84. Inhibition of golgin-84 truncation prevents Golgi fragmentation, causing a block in lipid acquisition and maturation of C. trachomatis. Golgi fragmentation by means of RNA-interference-mediated knockdown of distinct Golgi matrix proteins before infection enhances bacterial maturation. Our data functionally connect bacteria-induced golgin-84 cleavage, Golgi ministack formation, lipid acquisition and intracellular pathogen growth. We show that C. trachomatis subverts the structure and function of an entire host cell organelle for its own advantage.

Mcl-1 is a key regulator of apoptosis resistance in Chlamydia trachomatis-infected cells

Chlamydia are obligate intracellular bacteria that cause variety of human diseases. Host cells infected with Chlamydia are protected against many different apoptotic stimuli. The induction of apoptosis resistance is thought to be an important immune escape mechanism allowing Chlamydia to replicate inside the host cell. Infection with C. trachomatis activates the Raf/MEK/ERK pathway and the PI3K/AKT pathway. Here we show that inhibition of these two pathways by chemical inhibitors sensitized C. trachomatis infected cells to granzyme B-mediated cell death. Infection leads to the Raf/MEK/ERK-mediated up-regulation and PI3K-dependent stabilization of the anti-apoptotic Bcl-2 family member Mcl-1. Consistently, interfering with Mcl-1 up-regulation sensitized infected cells for apoptosis induced via the TNF receptor, DNA damage, granzyme B and stress. Our data suggest that Mcl-1 up-regulation is primarily required to maintain apoptosis resistance in C. trachomatis-infected cells.

The host adherens junction molecule nectin-1 is downregulated in Chlamydia trachomatis-infected genital epithelial cells

Nectin-1, a member of the immunoglobulin superfamily, is a Ca(2+)-independent cell adhesion protein implicated in the organization of E-cadherin-based adherens junctions (AJs) and claudin-based tight junctions (TJs) in epithelial cells. Nectin-1 also regulates cell-cell adhesion and cell polarization in a Cdc42- and Rac-dependent manner. Western blot analyses demonstrated that accumulation of host nectin-1 is decreased by 85 % at 48 hours post-infection (h.p.i.) in Chlamydia trachomatis serovar E-infected HeLa cells. Time-course experiments demonstrated that this decrease was sustained to 60 h.p.i. Nectin-1 downregulation in C. trachomatis-infected cells was prevented by both chloramphenicol exposure and prior inactivation of the chlamydiae with UV light, demonstrating that active C. trachomatis replication was required. Penicillin G-exposure studies demonstrated that nectin-1 accumulation was also altered during persistent infection. Finally, RT-PCR analyses indicated that chlamydial infection did not alter accumulation of any nectin-1 transcripts, demonstrating that nectin-1 accumulation is reduced at a post-transcriptional level. Intesrestingly, N-cadherin-dependent cell-cell junctions can be disrupted by C. trachomatis infection, as reported by Prozialeck et al. (2002). Because interaction of nectin molecules on adjacent cells is essential for AJ formation, these data suggest that C. trachomatis may disrupt AJs, at least in part, by diminishing nectin-1 accumulation. Notably, release of chlamydiae-infected epithelial cells has been observed both in vitro from polarized monolayers and in vivo from tissues, suggesting that chlamydia-modulated downregulation of adhesion molecules and the subsequent disruption of host cell adherence may be involved in chlamydial dissemination or pathogenesis.

Inhibition of apoptosis in neuronal cells infected with Chlamydophila (Chlamydia) pneumoniae

Background

Chlamydophila (Chlamydia) pneumoniae is an intracellular bacterium that has been identified within cells in areas of neuropathology found in Alzheimer disease (AD), including endothelia, glia, and neurons. Depending on the cell type of the host, infection by C. pneumoniae has been shown to influence apoptotic pathways in both pro- and anti-apoptotic fashions. We have hypothesized that persistent chlamydial infection of neurons may be an important mediator of the characteristic neuropathology observed in AD brains. Chronic and/or persistent infection of neuronal cells with C. pneumoniae in the AD brain may affect apoptosis in cells containing chlamydial inclusions.

Results

SK-N-MC neuroblastoma cells were infected with the respiratory strain of C. pneumoniae, AR39 at an MOI of 1. Following infection, the cells were either untreated or treated with staurosporine and then examined for apoptosis by labeling for nuclear fragmentation, caspase activity, and membrane inversion as indicated by annexin V staining. C. pneumoniae infection was maintained through 10 days post-infection. At 3 and 10 days post-infection, the infected cell cultures appeared to inhibit or were resistant to the apoptotic process when induced by staurosporine. This inhibition was demonstrated quantitatively by nuclear profile counts and caspase 3/7 activity measurements.

Conclusion

These data suggest that C. pneumoniae can sustain a chronic infection in neuronal cells by interfering with apoptosis, which may contribute to chronic inflammation in the AD brain.

Reduced display of tumor necrosis factor receptor I at the host cell surface supports infection with Chlamydia trachomatis

The obligate intracellular human pathogenic bacterium Chlamydia trachomatis has evolved multiple mechanisms to circumvent the host immune system. Infected cells exhibit a profound resistance to the induction of apoptosis and down-regulate the expression of major histocompatibility complex class I and class II molecules to evade the cytotoxic effect of effector immune cells. Here we demonstrate the down-regulation of tumor necrosis factor receptor 1 (TNFR1) on the surface of infected cells. Interestingly, other members of the TNFR family such as TNFR2 and CD95 (Fas/Apo-1) were not modulated during infection, suggesting a selective mechanism underlying surface reduction of TNFR1. The observed effect was not due to reduced expression since the overall amount of TNFR1 protein was increased in infected cells. TNFR1 accumulated at the chlamydial inclusion and was shed by the infected cell into the culture supernatant. Receptor shedding depended on the infection-induced activation of the MEK-ERK pathway and the metalloproteinase TACE (TNFalpha converting enzyme). Our results point to a new function of TNFR1 modulation by C. trachomatis in controlling inflammatory signals during infection.

In vitro models of acute and long-term continuous infection of human respiratory epithelial cells with Chlamydophila pneumoniae have opposing effects on host cell apoptosis

Persistent infection with the obligate intracellular pathogen Chlamydophila pneumoniae has been implicated in the pathogenesis of many chronic diseases, but its mechanism remains unclear. Many pathogens have been found to modulate cellular apoptosis in order to survive and multiply. Chlamydial species were shown to both induce and inhibit host cell apoptosis depending on the experimental conditions. We utilized in vitro models of acute and long-term continuous (LTC) infection with the same cell line (HEp-2) and chlamydial isolate (TW-183) used in both models. Host cell apoptosis in infected and uninfected cells was assessed by fluorescence microscopy and flow cytometry. While acute infection induced apoptosis 72 h post-infection, LTC-infected cells had low rates of apoptosis and showed resistance to different exogenous inducers of apoptosis (sorbitol, serum withdrawal, hydrogen peroxide) when compared to uninfected cells. Chronicity of infection appears to be a critical factor in the modulation of host cell apoptosis by C. pneumoniae. Induction of apoptosis may help to propagate the infection, while inhibition of apoptosis could help protect the organism in chronic infection.

Cytotoxic necrotizing factor 1 prevents apoptosis via the Akt/IkappaB kinase pathway: role of nuclear factor-kappaB and Bcl-2

Cytotoxic necrotizing factor 1 (CNF1) is a protein toxin produced by some pathogenic strains of Escherichia coli that specifically activates Rho, Rac, and Cdc42 GTPases. We previously reported that this toxin prevents the ultraviolet-B-induced apoptosis in epithelial cells, with a mechanism that remained to be defined. In this work, we show that the proteasomal degradation of the Rho GTPase is necessary to achieve cell death protection, because inhibition of Rho degradation abolishes the prosurvival activity of CNF1. We hypothesize that Rho inactivation allows the activity of Rac to become dominant. This in turn leads to stimulation of the phosphoinositide 3-kinase/Akt/IkappaB kinase/nuclear factor-kappaB prosurvival pathway and to a remarkable modification in the architecture of the mitochondrial network, mainly consisting in the appearance of elongated and interconnected mitochondria. Importantly, we found that Bcl-2 silencing reduces the ability of CNF1 to protect cells against apoptosis and that it also prevents the CNF1-induced mitochondrial changes. It is worth noting that the ability of a bacterial toxin to induce such a remodeling of the mitochondrial network is herein reported for the first time. The possible pathophysiological relevance of this finding is discussed.

The apicomplexan pathogen Neospora caninum inhibits host cell apoptosis in the absence of discernible NF-kappa B activation

Neospora caninum, a causative agent of bovine abortions, is an apicomplexan parasite that is closely related to the human pathogen Toxoplasma gondii. Since a number of intracellular parasites, including T. gondii, have been shown to modulate host cell apoptosis, the present study was conducted to establish whether N. caninum is similarly capable of subverting apoptotic pathways in its host cells. Our results indicated that death receptor-mediated apoptosis is repressed during N. caninum infection, and the data further showed that the executioner caspase, caspase 3, does not become activated in the infected cells. Surprisingly, nuclear translocation of the NF-kappaB subunit p65 was not detected in N. caninum-infected cells, although this host transcription factor has been shown to upregulate prosurvival genes in cells infected with T. gondii. Consistent with these findings, the distinct accumulation of phosphorylated IkappaB that is seen at the parasitophorous vacuole membrane (PVM) of T. gondii was not apparent on the N. caninum PVM. Although a putative IkappaB kinase activity was detected in N. caninum extracts, thereby implying that this parasite is capable of modulating NF-kappaB translocation into the host cell nucleus, the data collectively suggest that a profound and sustained activation of the NF-kappaB pathway is not central to the ability of N. caninum to prevent apoptosis of their host cells.

Host-Cell Survival and Death During Chlamydia Infection

Different Chlamydia trachomatis strains are responsible for prevalent bacterial sexually-transmitted disease and represent the leading cause of preventable blindness worldwide. Factors that predispose individuals to disease and mechanisms by which chlamydiae cause inflammation and tissue damage remain unclear. Results from recent studies indicate that prolonged survival and subsequent death of infected cells and their effect on immune effector cells during chlamydial infection may be important in determining the outcome. Survival of infected cells is favored at early times of infection through inhibition of the mitochondrial pathway of apoptosis. Death at later times displays features of both apoptosis and necrosis, but pro-apoptotic caspases are not involved. Most studies on chlamydial modulation of host-cell death until now have been performed in cell lines. The consequences for pathogenesis and the immune response will require animal models of chlamydial infection, preferably mice with targeted deletions of genes that play a role in cell survival and death.

NF-kappaB and inhibitor of apoptosis proteins are required for apoptosis resistance of epithelial cells persistently infected with Chlamydophila pneumoniae

Infection with Chlamydophila pneumoniae (Cpn) renders host cells resistant to apoptosis induced by a variety of stimuli. While modulation of apoptosis has been extensively studied in cells acutely infected with Cpn, very little is known on how persistent chlamydial infection influences host cell survival. Here we show that epithelial cells persistently infected with Cpn resist apoptosis induced with TNFalpha or staurosporine. Cpn induced the activation of nuclear factor kappa B (NF-kappaB) and inhibition of NF-kappaB with a chemical inhibitor or by silencing expression of the p65 subunit sensitized infected cells for apoptosis induction by staurosporine or TNFalpha. Persistent infection resulted in the upregulation of the NF-kappaB regulated inhibitor of apoptosis protein 2 (cIAP-2) but not inhibitor of apoptosis protein 1 (cIAP-1). Interestingly, silencing of either cIAP-1 or cIAP-2 sensitized infected cells, suggesting that IAPs play an important role in the apoptosis resistance of persistently infected cells.

Effect of PEX, a noncatalytic metalloproteinase fragment with integrin-binding activity, on experimental Chlamydophila pneumoniae infection

Chlamydophila pneumoniae is a pathogen that is involved in acute and chronic respiratory infections and that is associated with asthma and coronary artery diseases. In this study, we evaluated the effects of PEX, a noncatalytic metalloproteinase fragment with integrin-binding activity, against experimental infections caused by C. pneumoniae. Moreover, we investigated the relationships between C. pneumoniae and alpha(v)beta(3) integrin functions in order to explain the possible mechanism of action of PEX both in vitro and in vivo. For the in vitro experiments, HeLa cells were infected with C. pneumoniae and treated with either PEX or azithromycin. The results obtained with PEX were not significantly different (P > 0.05) from those achieved with azithromycin. Similar results were also obtained in a lung infection model. Male C57BL/J6 mice inoculated intranasally with 10(6) inclusion-forming units of C. pneumoniae were treated with either PEX or azithromycin plus rifampin. Infected mice treated with PEX showed a marked decrease in C. pneumoniae counts versus those for the controls; this finding did not differ significantly (P > 0.05) from the results observed for the antibiotic-treated group. Integrin alpha(v)beta(3) plays an important role in C. pneumoniae infection. Blockage of integrin activation led to a significant inhibition of C. pneumoniae infection in HeLa cells. Moreover, CHO(DHFR) alpha(v)beta(3)-expressing cells were significantly (P < 0.001) more susceptible to C. pneumoniae infection than CHO(DHFR) cells. These results offer new perspectives on the treatment of C. pneumoniae infection and indicate that alpha(v)beta(3) could be a promising target for new agents developed for activity against this pathogen.

Ocular immune privilege is circumvented by CD4+ T cells, leading to the rejection of intraocular tumors in an IFN-{gamma}-dependent manner

Although intraocular tumors reside in an immune-privileged site, they can circumvent immune privilege and undergo rejection, which typically follows one of two pathways. One pathway involves CD4(+) T cells, delayed-type hypersensitivity (DTH), and the culmination in ischemic necrosis of the tumor and phthisis (atrophy) of the eye. The second pathway is DTH-independent and does not inflict collateral injury to ocular tissues, and the eye is preserved. In this study, we used a well-characterized tumor, Ad5E1, to analyze the role of IFN-gamma in the nonphthisical form of intraocular tumor rejection. The results showed that IFN-gamma induced tumor cell apoptosis, inhibited tumor cell proliferation, and promoted rejection by inhibiting angiogenesis. Microarray analysis revealed that IFN-gamma induced up-regulation of five antiangiogenic genes and down-regulation of four proangiogenic genes in Ad5E1 tumor cells. Although IFN-gamma knockout (KO) mice have progressively growing intraocular tumors, IFN-gamma was not needed for the elimination of extraocular tumors, as all IFN-gamma KO mice rejected s.c. tumor inocula. This represents a heretofore unrecognized role for IFN-gamma in circumventing ocular immune privilege and eliminating intraocular tumors. The findings also reveal that some IFN-gamma-independent tumor rejection processes are excluded from the eye and may represent a new facet of ocular immune privilege.

IAP-IAP complexes required for apoptosis resistance of C. trachomatis-infected cells

Host cells infected with obligate intracellular bacteria Chlamydia trachomatis are profoundly resistant to diverse apoptotic stimuli. The molecular mechanisms underlying the block in apoptotic signaling of infected cells is not well understood. Here we investigated the molecular mechanism by which apoptosis induced via the tumor necrosis factor (TNF) receptor is prevented in infected epithelial cells. Infection with C. trachomatis leads to the up-regulation of cellular inhibitor of apoptosis (cIAP)-2, and interfering with cIAP-2 up-regulation sensitized infected cells for TNF-induced apoptosis. Interestingly, besides cIAP-2, cIAP-1 and X-linked IAP, although not differentially regulated by infection, are required to maintain apoptosis resistance in infected cells. We detected that IAPs are constitutively organized in heteromeric complexes and small interfering RNA-mediated silencing of one of these IAPs affects the stability of another IAP. In particular, the stability of cIAP-2 is modulated by the presence of X-linked IAP and their interaction is stabilized in infected cells. Our observations suggest that IAPs are functional and stable as heteromers, a thus far undiscovered mechanism of IAP regulation and its role in modulation of apoptosis.