Caspase Exploitation by Legionella pneumophila

Non-Canonical Inflammasome Pathway: The Role of Cell Death and Inflammation in Ehrlichiosis

Activating inflammatory caspases and releasing pro-inflammatory mediators are two essential functions of inflammasomes which are triggered in response to pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). The canonical inflammasome pathway involves the activation of inflammasome and its downstream pathway via the adaptor ASC protein, which causes caspase 1 activation and, eventually, the cleavage of pro-IL-1b and pro-IL-18. The non-canonical inflammasome pathway is induced upon detecting cytosolic lipopolysaccharide (LPS) by NLRP3 inflammasome in Gram-negative bacteria. The activation of NLRP3 triggers the cleavage of murine caspase 11 (human caspase 4 or caspase 5), which results in the formation of pores (via gasdermin) to cause pyroptosis. Ehrlichia is an obligately intracellular bacterium which is responsible for causing human monocytic ehrlichiosis (HME), a potentially lethal disease similar to toxic shock syndrome and septic shock syndrome. Several studies have indicated that canonical and non-canonical inflammasome activation is a crucial pathogenic mechanism that induces dysregulated inflammation and host cellular death in the pathophysiology of HME. Mechanistically, the activation of canonical and non-canonical inflammasome pathways affected by virulent Ehrlichia infection is due to a block in autophagy. This review aims to explore the significance of non-canonical inflammasomes in ehrlichiosis, and how the pathways involving caspases (with the exception of caspase 1) contribute to the pathophysiology of severe and fatal ehrlichiosis. Improving our understanding of the non-canonical inflammatory pathway that cause cell death and inflammation in ehrlichiosis will help the advancement of innovative therapeutic, preventative, and diagnostic approaches to the treatment of ehrlichiosis.

Insights into inflammasome regulation: cellular, molecular, and pathogenic control of inflammasome activation

Maintenance of immune homeostasis is an intricate process wherein inflammasomes play a pivotal role by contributing to innate and adaptive immune responses. Inflammasomes are ensembles of adaptor proteins that can trigger a signal following innate sensing of pathogens or non-pathogens eventuating in the inductions of IL-1β and IL-18. These inflammatory cytokines substantially influence the antigen-presenting cell's costimulatory functions and T helper cell differentiation, contributing to adaptive immunity. As acute and chronic disease conditions may accompany parallel tissue damage, we analyze the critical role of extracellular factors such as cytokines, amyloids, cholesterol crystals, etc., intracellular metabolites, and signaling molecules regulating inflammasome activation/inhibition. We develop an operative framework for inflammasome function and regulation by host cell factors and pathogens. While inflammasomes influence the innate and adaptive immune components' interplay modulating the anti-pathogen adaptive immune response, pathogens may target inflammasome inhibition as a survival strategy. As trapped between health and diseases, inflammasomes serve as promising therapeutic targets and their modus operandi serves as a scientific rationale for devising better therapeutic strategies.

Legionella pneumophila-Virulence Factors and the Possibility of Infection in Dental Practice

Legionella pneumophila is defined as a bacterium that can cause severe pneumonia. It is found in the natural environment and in water, and is often found in water tanks. It can be an integral part of biofilms in nature, and the protozoa in which it can live provide it with food and protect it from harmful influences; therefore, it has the ability to move into a sustainable but uncultured state (VBNC). L. pneumophila has been shown to cause infections in dental practices. The most common transmission route is aerosol generated in dental office water systems, which can negatively affect patients and healthcare professionals. The most common way of becoming infected with L. pneumophila in a dental office is through water from dental instruments, and the dental unit. In addition to these bacteria, patients and the dental team may be exposed to other harmful bacteria and viruses. Therefore, it is vital that the dental team regularly maintains and decontaminates the dental unit, and sterilizes all accessories that come with it. In addition, regular water control in dental offices is necessary.

Risk assessment and quantitative measurement along with monitoring of Legionella in hospital water sources

Legionella spp. as a causative agent of Legionnaires' disease (LD) and an opportunistic pathogen creates a public health problem. Isolation and quantification of this bacteria from clinic water sources are essential for hazard appraisal and sickness avoidance. This study aimed at risk assessment and quantitative measurement along with Legionella monitoring in educational hospital water sources in Tehran, Iran. A cross-sectional study was carried out in 1 year. The conventional culture method was used in this study to isolate Legionella from water samples. The polymerase chain reaction (PCR) technique was used to confirm the identity of the isolates and ensure that they were all Legionella. Quantitative PCR (qPCR) was used to determine the count of bacteria, and HeLa cell culture was used to determine the invasion of isolates. A total of 100 water samples were collected and inoculated on GVPC (glycine, vancomycin, polymyxin, and cycloheximide) agar; 12 (12%) and 42 (42%) cases were culture and PCR positive, respectively. Percentage of Legionella presence in PCR-positive samples by the qPCR method in <10³ GU/L, in about 10³ and lower than 10⁴ GU/L, and in 10⁴ GU/L was 40.47 (17 cases), 4.76% (two cases), and 54.76% (23 cases), respectively. Invasion analysis revealed that five and four isolates had invaded HeLa cells more than twice and equally, respectively, and the others had a lower invasion than the reference strain. The findings revealed that the spread of LD in hospitals was linked to the water system. Given the importance of nosocomial infections in the medical community, establishing a hospital water monitoring system is the most effective way to control these infections, particularly Legionella infections.

Early life microbial exposures and allergy risks: opportunities for prevention

Allergies, including asthma, food allergy and atopic dermatitis, are increasing in prevalence, particularly in westernized countries. Although a detailed mechanistic explanation for this increase is lacking, recent evidence indicates that, in addition to genetic predisposition, lifestyle changes owing to modernization have an important role. Such changes include increased rates of birth by caesarean delivery, increased early use of antibiotics, a westernized diet and the associated development of obesity, and changes in indoor and outdoor lifestyle and activity patterns. Most of these factors directly and indirectly impact the formation of a diverse microbiota, which includes bacterial, viral and fungal components; the microbiota has a leading role in shaping (early) immune responses. This default programme is markedly disturbed under the influence of environmental and lifestyle risk factors. Here, we review the most important allergy risk factors associated with changes in our exposure to the microbial world and the application of this knowledge to allergy prevention strategies.

The Challenge of the Sponge Suberites domuncula (Olivi, 1792) in the Presence of a Symbiotic Bacterium and a Pathogen Bacterium

Sponges, which are in close contact with numerous bacteria in prey/predator, symbiotic and pathogenic relationships, must provide an appropriate response in such situations. This starts with a discriminating recognition of the partner either by a physical contact or through secreted molecules or both. We investigated the expression of the Toll-like receptor, Caspase 3/7, Tumor Necrosis Factor receptor-associated factor 6, Bcl-2 homology protein-2 and macrophage expressed genes of axenic sponge cells in the presence of a symbiotic bacterium (Endozoicomonas sp. Hex311), a pathogen bacterium (Pseudoalteromonas sp. 1A1), their exoproducts and lipopolysaccharides. The vast majority of answers are in line with what could be observed with the symbiotic bacterium. The pathogenic bacterium seems to profit from the eukaryotic cell: suppression of the production of the antibacterial compound, inhibition of the apoptosis caspase-dependent pathway, deregulation of bacterial recognition. This work contributes new scientific knowledge in the field of immunology and apoptosis in early branching metazoan harboring within its tissue and cells a large number of symbiotic bacteria.

Inhibition of inflammasome activation by a clinical strain of Klebsiella pneumoniae impairs efferocytosis and leads to bacterial dissemination

Klebsiella pneumoniae is a Gram-negative bacterium responsible for severe cases of nosocomial pneumonia. During the infectious process, both neutrophils and monocytes migrate to the site of infection, where they carry out their effector functions and can be affected by different patterns of cell death. Our data show that clinical strains of K. pneumoniae have dissimilar mechanisms for surviving within macrophages; these mechanisms include modulation of microbicidal mediators and cell death. The A28006 strain induced high IL-1β production and pyroptotic cell death in macrophages; by contrast, the A54970 strain induced high IL-10 production and low IL-1β production by macrophages. Pyroptotic cell death induced by the A28006 strain leads to a significant increase in bacterial sensitivity to hydrogen peroxide, and efferocytosis of the pyroptotic cells results in efficient bacterial clearance both in vitro and in vivo. In addition, the A54970 strain was able to inhibit inflammasome activation and pyroptotic cell death by inducing IL-10 production. Here, for the first time, we present a K. pneumoniae strain able to inhibit inflammasome activation, leading to bacterial survival and dissemination in the host. The understanding of possible escape mechanisms is essential in the search for alternative treatments against multidrug-resistant bacteria.

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts by Legionella pneumophila

Legionella pneumophila is a ubiquitous environmental bacterium that has evolved to infect and proliferate within amoebae and other protists. It is thought that accidental inhalation of contaminated water particles by humans is what has enabled this pathogen to proliferate within alveolar macrophages and cause pneumonia. However, the highly evolved macrophages are equipped with more sophisticated innate defence mechanisms than are protists, such as the evolution of phagotrophic feeding into phagocytosis with more evolved innate defence processes. Not surprisingly, the majority of proteins involved in phagosome biogenesis (~80%) have origins in the phagotrophy stage of evolution. There are a plethora of highly evolved cellular and innate metazoan processes, not represented in protist biology, that are modulated by L. pneumophila, including TLR2 signalling, NF-κB, apoptotic and inflammatory processes, histone modification, caspases, and the NLRC-Naip5 inflammasomes. Importantly, L. pneumophila infects haemocytes of the invertebrate Galleria mellonella, kill G. mellonella larvae, and proliferate in and kill Drosophila adult flies and Caenorhabditis elegans. Although coevolution with protist hosts has provided a substantial blueprint for L. pneumophila to infect macrophages, we discuss the further evolutionary aspects of coevolution of L. pneumophila and its adaptation to modulate various highly evolved innate metazoan processes prior to becoming a human pathogen.

Geographic variation in the aetiology, epidemiology and microbiology of bronchiectasis

Bronchiectasis is a disease associated with chronic progressive and irreversible dilatation of the bronchi and is characterised by chronic infection and associated inflammation. The prevalence of bronchiectasis is age-related and there is some geographical variation in incidence, prevalence and clinical features. Most bronchiectasis is reported to be idiopathic however post-infectious aetiologies dominate across Asia especially secondary to tuberculosis. Most focus to date has been on the study of airway bacteria, both as colonisers and causes of exacerbations. Modern molecular technologies including next generation sequencing (NGS) have become invaluable tools to identify microorganisms directly from sputum and which are difficult to culture using traditional agar based methods. These have provided important insight into our understanding of emerging pathogens in the airways of people with bronchiectasis and the geographical differences that occur. The contribution of the lung microbiome, its ethnic variation, and subsequent roles in disease progression and response to therapy across geographic regions warrant further investigation. This review summarises the known geographical differences in the aetiology, epidemiology and microbiology of bronchiectasis. Further, we highlight the opportunities offered by emerging molecular technologies such as -omics to further dissect out important ethnic differences in the prognosis and management of bronchiectasis.

The Toolbox for Uncovering the Functions of Legionella Dot/Icm Type IVb Secretion System Effectors: Current State and Future Directions

The defective in organelle trafficking/intracellular multiplication (Dot/Icm) Type IVb secretion system (T4SS) is the essential virulence factor for the intracellular life style and pathogenicity of Legionella species. Screens demonstrated that an individual L. pneumophila strain can use the Dot/Icm T4SS to translocate an unprecedented number of more than 300 proteins into host cells, where these, so called Icm/Dot-translocated substrates (IDTS) or effectors, manipulate host cell functions to the benefit of the bacteria. Bioinformatic analysis of the pan-genus genome predicts at least 608 orthologous groups of putative effectors. Deciphering the function of these effectors is key to understanding Legionella pathogenesis; however, the analysis is challenging. Substantial functional redundancy renders classical, phenotypic screening of single gene deletion mutants mostly ineffective. Here, I review experimental approaches that were successfully used to identify, validate and functionally characterize T4SS effectors and highlight new methods, which promise to facilitate unlocking the secrets of Legionella's extraordinary weapons arsenal.

Interpretation and Relevance of Advanced Technique Results

Advanced techniques in the field of diagnostic microbiology have made amazing progress over the past 25 years due largely to a technological revolution in the molecular aspects of microbiology [1, 2]. In particular, rapid molecular methods for nucleic acid amplification and characterization combined with automation in the clinical microbiology laboratory as well as user-friendly software and robust laboratory informatics systems have significantly broadened the diagnostic capabilities of modern clinical microbiology laboratories. Molecular methods such as nucleic acid amplification tests (NAATs) rapidly are being developed and introduced in the clinical laboratory setting [3, 4]. Indeed, every section of the clinical microbiology laboratory, including bacteriology, mycology, mycobacteriology, parasitology, and virology, has benefited from these advanced techniques. Because of the rapid development and adaptation of these molecular techniques, the interpretation and relevance of the results produced by such molecular methods continues to lag behind. The purpose of this chapter is to review, update, and discuss the interpretation and relevance of results produced by these advanced molecular techniques.

The Influence of Programmed Cell Death in Myeloid Cells on Host Resilience to Infection with Legionella pneumophila or Streptococcus pyogenes

Pathogen clearance and host resilience/tolerance to infection are both important factors in surviving an infection. Cells of the myeloid lineage play important roles in both of these processes. Neutrophils, monocytes, macrophages, and dendritic cells all have important roles in initiation of the immune response and clearance of bacterial pathogens. If these cells are not properly regulated they can result in excessive inflammation and immunopathology leading to decreased host resilience. Programmed cell death (PCD) is one possible mechanism that myeloid cells may use to prevent excessive inflammation. Myeloid cell subsets play roles in tissue repair, immune response resolution, and maintenance of homeostasis, so excessive PCD may also influence host resilience in this way. In addition, myeloid cell death is one mechanism used to control pathogen replication and dissemination. Many of these functions for PCD have been well defined in vitro, but the role in vivo is less well understood. We created a mouse that constitutively expresses the pro-survival B-cell lymphoma (bcl)-2 protein in myeloid cells (CD68(bcl2tg), thus decreasing PCD specifically in myeloid cells. Using this mouse model we explored the impact that decreased cell death of these cells has on infection with two different bacterial pathogens, Legionella pneumophila and Streptococcus pyogenes. Both of these pathogens target multiple cell death pathways in myeloid cells, and the expression of bcl2 resulted in decreased PCD after infection. We examined both pathogen clearance and host resilience and found that myeloid cell death was crucial for host resilience. Surprisingly, the decreased myeloid PCD had minimal impact on pathogen clearance. These data indicate that the most important role of PCD during infection with these bacteria is to minimize inflammation and increase host resilience, not to aid in the clearance or prevent the spread of the pathogen.