The infectious hypoxia: occurrence and causes during Shigella infection

Shigella Detection and Molecular Serotyping With a Customized TaqMan Array Card in the Enterics for Global Health (EFGH): Shigella Surveillance Study

Background

Quantitative polymerase chain reaction (qPCR) targeting ipaH has been proven to be highly efficient in detecting Shigella in clinical samples compared to culture-based methods, which underestimate Shigella burden by 2- to 3-fold. qPCR assays have also been developed for Shigella speciation and serotyping, which is critical for both vaccine development and evaluation.

Methods

The Enterics for Global Health (EFGH) Shigella surveillance study will utilize a customized real-time PCR-based TaqMan Array Card (TAC) interrogating 82 targets, for the detection and differentiation of Shigella spp, Shigella sonnei, Shigella flexneri serotypes, other diarrhea-associated enteropathogens, and antimicrobial resistance (AMR) genes. Total nucleic acid will be extracted from rectal swabs or stool samples, and assayed on TAC. Quantitative analysis will be performed to determine the likely attribution of Shigella and other particular etiologies of diarrhea using the quantification cycle cutoffs derived from previous studies. The qPCR results will be compared to conventional culture, serotyping, and phenotypic susceptibility approaches in EFGH.

Conclusions

TAC enables simultaneous detection of diarrheal etiologies, the principal pathogen subtypes, and AMR genes. The high sensitivity of the assay enables more accurate estimation of Shigella-attributed disease burden, which is critical to informing policy and in the design of future clinical trials.

The importance of biofilm contamination control for dental unit waterlines: a multicenter assessment of the microbiota diversity of biofilm in dental unit waterlines

Background

The biofilm formation in Dental Unit Waterlines (DUWLs) could become an important cause of infection during dental care, which could put immunocompromised individuals at risk of cross-infection. The aim of this study was to characterize the microbial communities of biofilms among DUWLs using high-throughput sequencing technology.

Methods

Twenty-nine biofilm samples were obtained from 24 dental chair units at 5 hospitals and 2 dental clinics. The genomic DNA of the samples was extracted, then 16S rDNA and ITS2 gene were amplified and sequenced. Alpha-diversity and Beta-diversity were calculated with QIIME2 and the Kruskal - Wallis H-test was adopted for statistical analysis.

Results

Microbial communities with a high diversity of bacteria (377 genera) and fungi (83 genera) were detected in the biofilm samples. The dominant phylum of bacteria was Proteobacteria (93.27%) and that of fungi was Basidiomycota (68.15%). Potential human pathogens were detected including 7 genera of bacteria (Pseudomonas, Stenotrophomonas, Hafnia-Obesumbacterium, Burkholderia-Caballeronia-Paraburkholderia, Ralstonia, Enterobacter, Klebsiella) and 6 genera of fungi (Malassezia, Candida, Alternaria, Cryptococcus, Rhodotorula, Rhinocladiella).

Conclusions

This multicenter assessment revealed the infectious risk during dental care. It emphasized the importance of biofilm control due to biofilm accumulation and multiple kinds of opportunistic pathogens in DUWLs.

Bacterial conversion of a host weapon into a nutritional signal

Bacteria engulfed by phagocytic cells must resist oxidation damage and adapt to cellular hypoxia, but the mechanisms involved in this process are not completely elucidated. Recent work by Kim et al. in the Journal of Biological Chemistry investigated how the intracellular pathogen Salmonella enterica activates gene expression required to counteract oxidative damage. The authors show that this bacterium utilizes host oxidative molecules to activate regulatory proteins that enhance the production of effector molecules, counteracting the host weapon NADPH oxidase and inducing a protective response.

Hypoxia Effects on Trypanosoma cruzi Epimastigotes Proliferation, Differentiation, and Energy Metabolism

Trypanosoma cruzi, the causative agent of Chagas disease, faces changes in redox status and nutritional availability during its life cycle. However, the influence of oxygen fluctuation upon the biology of T. cruzi is unclear. The present work investigated the response of T. cruzi epimastigotes to hypoxia. The parasites showed an adaptation to the hypoxic condition, presenting an increase in proliferation and a reduction in metacyclogenesis. Additionally, parasites cultured in hypoxia produced more reactive oxygen species (ROS) compared to parasites cultured in normoxia. The analyses of the mitochondrial physiology demonstrated that hypoxic condition induced a decrease in both oxidative phosphorylation and mitochondrial membrane potential (ΔΨm) in epimastigotes. In spite of that, ATP levels of parasites cultivated in hypoxia increased. The hypoxic condition also increased the expression of the hexokinase and NADH fumarate reductase genes and reduced NAD(P)H, suggesting that this increase in ATP levels of hypoxia-challenged parasites was a consequence of increased glycolysis and fermentation pathways. Taken together, our results suggest that decreased oxygen levels trigger a shift in the bioenergetic metabolism of T. cruzi epimastigotes, favoring ROS production and fermentation to sustain ATP production, allowing the parasite to survive and proliferate in the insect vector.

Do Primocolonizing Bacteria Enable Bacteroides thetaiotaomicron Intestinal Colonization Independently of the Capacity To Consume Oxygen?

Aerobic bacteria are frequent primocolonizers of the human naive intestine. Their generally accepted role is to eliminate oxygen, which would allow colonization by anaerobes that subsequently dominate bacterial gut populations.

Aerobic bacteria are frequent primocolonizers of the human naive intestine. Their generally accepted role is to eliminate oxygen, which would allow colonization by anaerobes that subsequently dominate bacterial gut populations. In this hypothesis-based study, we revisited this dogma experimentally in a germfree mouse model as a mimic of the germfree newborn. We varied conditions leading to the establishment of the dominant intestinal anaerobe Bacteroides thetaiotaomicron. Two variables were introduced: Bacteroides inoculum size and preestablishment by bacteria capable or not of consuming oxygen. High Bacteroides inoculum size enabled its primocolonization. At low inocula, we show that bacterial preestablishment was decisive for subsequent Bacteroides colonization. However, even non-oxygen-respiring bacteria, a hemAEscherichia coli mutant and the intestinal obligate anaerobe Clostridium scindens, facilitated Bacteroides establishment. These findings, which are supported by recent reports, revise the long-held assumption that oxygen scavenging is the main role for aerobic primocolonizing bacteria. Instead, we suggest that better survival of aerobic bacteria ex vivo during vectorization between hosts could be a reason for their frequent primocolonization.

The selective advantage of facultative anaerobes relies on their unique ability to cope with changing oxygen levels during infection

Bacteria, including those that are pathogenic, have been generally classified according to their ability to survive and grow in the presence or absence of oxygen: aerobic and anaerobic bacteria, respectively. Strict aerobes require oxygen to grow (e.g., Neisseria), and strict anaerobes grow exclusively without, and do not survive oxygen exposure (e.g., Clostridia); aerotolerant bacteria (e.g., Lactobacilli) are insensitive to oxygen exposure. Facultative anaerobes (e.g., E. coli) have the unique ability to grow in the presence or in the absence of oxygen and are thus well-adapted to these changing conditions, which may constitute an underestimated selective advantage for infection. In the WHO antibiotic-resistant 'priority pathogens' list, facultative anaerobes are overrepresented (8 among 12 listed pathogens), consistent with clinical studies performed in populations particularly susceptible to infectious diseases. Bacteria aerobic respiratory chain plays a central role in oxygen consumption, leading to the formation of hypoxic infectious sites (infectious hypoxia). Facultative anaerobes have developed a wide diversity of aerotolerance and anaerotolerance strategies in vivo. However, at a single cell level, the modulation of the intracellular oxygen level in host infected cells remains elusive and will be discussed in this review. In conclusion, the ability of facultative bacteria to evolve in the presence or the absence of oxygen is essential for their virulence strategy and constitute a selective advantage. TAKE AWAY: Most life-threatening pathogenic bacteria are facultative anaerobes. Only facultative anaerobes are aerotolerant, anaerotolerant and capable of consuming O₂ . Facultative anaerobes induce and are well adapted to cellular hypoxia.

Computational Identification of Master Regulators Influencing Trypanotolerance in Cattle

African Animal Trypanosomiasis (AAT) is transmitted by the tsetse fly which carries pathogenic trypanosomes in its saliva, thus causing debilitating infection to livestock health. As the disease advances, a multistage progression process is observed based on the progressive clinical signs displayed in the host’s body. Investigation of genes expressed with regular monotonic patterns (known as Monotonically Expressed Genes (MEGs)) and of their master regulators can provide important clue for the understanding of the molecular mechanisms underlying the AAT disease. For this purpose, we analysed MEGs for three tissues (liver, spleen and lymph node) of two cattle breeds, namely trypanosusceptible Boran and trypanotolerant N’Dama. Our analysis revealed cattle breed-specific master regulators which are highly related to distinguish the genetic programs in both cattle breeds. Especially the master regulators MYC and DBP found in this study, seem to influence the immune responses strongly, thereby susceptibility and trypanotolerance of Boran and N’Dama respectively. Furthermore, our pathway analysis also bolsters the crucial roles of these master regulators. Taken together, our findings provide novel insights into breed-specific master regulators which orchestrate the regulatory cascades influencing the level of trypanotolerance in cattle breeds and thus could be promising drug targets for future therapeutic interventions.

Mechanisms controlling bacterial infection in myeloid cells under hypoxic conditions

Various factors of the tissue microenvironment such as the oxygen concentration influence the host-pathogen interaction. During the past decade, hypoxia-driven signaling via hypoxia-inducible factors (HIF) has emerged as an important factor that affects both the pathogen and the host. In this chapter, we will review the current knowledge of this complex interplay, with a particular emphasis given to the impact of hypoxia and HIF on the inflammatory and antimicrobial activity of myeloid cells, the bacterial responses to hypoxia and the containment of bacterial infections under oxygen-limited conditions. We will also summarize how low oxygen concentrations influence the metabolism of neutrophils, macrophages and dendritic cells. Finally, we will discuss the consequences of hypoxia and HIFα activation for the invading pathogen, with a focus on Pseudomonas aeruginosa, Mycobacterium tuberculosis, Coxiella burnetii, Salmonella enterica and Staphylococcus aureus. This includes a description of the mechanisms and microbial factors, which the pathogens use to sense and react to hypoxic conditions.

Overview of the rules of the microbial engagement in the gut microbiome: a step towards microbiome therapeutics

Human gut microbiome is a diversified, resilient, immuno-stabilized, metabolically active and physiologically essential component of the human body. Scientific explorations have been made to seek in-depth information about human gut microbiome establishment, microbiome functioning, microbiome succession, factors influencing microbial community dynamics and the role of gut microbiome in health and diseases. Extensive investigations have proposed the microbiome therapeutics as a futuristic medicine for various physiological and metabolic disorders. A comprehensive outlook of microbial colonization, host-microbe interactions, microbial adaptation, commensal selection and immuno-survivability is still required to catalogue the essential genetic and physiological features for the commensal engagement. Evolution of a structured human gut microbiome relies on the microbial flexibility towards genetic, immunological and physiological adaptation in the human gut. Key features for commensalism could be utilized in developing tailor-made microbiome-based therapy to overcome various physiological and metabolic disorders. This review describes the key genetics and physiological traits required for host-microbe interaction and successful commensalism to institute a human gut microbiome.

Comparing the performance of conventional PCR, RTQ-PCR, and droplet digital PCR assays in detection of Shigella

The incidence of foodborne infections caused by Shigella spp. is still very high in every year, which poses a great potential threat to public health. Conventional quantification methods based on culture techniques, biochemical, and serological identification are time-consuming and labor-intensive. To develop a more rapid and efficient detection method of Shigella spp., we compared the sensitivity and specificity of three different polymerase chain reaction (PCR) methods, including conventional PCR, quantitative real-time PCR (RTQ-PCR), and droplet digital PCR (ddPCR). Our results indicated that ddPCR method exhibited higher sensitivity, and the limit of detection was 10^-5 ng/μl for genomic DNA templates, 10^-1 cfu/ml for Shigella bacteria culture. In addition, we found that ddPCR was a time-saving method, which required a shorter pre-culturing time. Collectively, ddPCR assay was a reliable method for rapid and effective detection of Shigella spp.

Neutrophil Metabolic Shift during their Lifecycle: Impact on their Survival and Activation

Polymorphonuclear neutrophils (PMNs) are innate immune cells, which represent 50% to 70% of the total circulating leukocytes. How PMNs adapt to various microenvironments encountered during their life cycle, from the bone marrow, to the blood plasma fraction, and to inflamed or infected tissues remains largely unexplored. Metabolic shifts have been reported in other immune cells such as macrophages or lymphocytes, in response to local changes in their microenvironment, and in association with a modulation of their pro-inflammatory or anti-inflammatory functions. The potential contribution of metabolic shifts in the modulation of neutrophil activation or survival is anticipated even though it is not yet fully described. If neutrophils are considered to be mainly glycolytic, the relative importance of alternative metabolic pathways, such as the pentose phosphate pathway, glutaminolysis, or the mitochondrial oxidative metabolism, has not been fully considered during activation. This statement may be explained by the lack of knowledge regarding the local availability of key metabolites such as glucose, glutamine, and substrates, such as oxygen from the bone marrow to inflamed tissues. As highlighted in this review, the link between specific metabolic pathways and neutrophil activation has been outlined in many reports. However, the impact of neutrophil activation on metabolic shifts' induction has not yet been explored. Beyond its importance in neutrophil survival capacity in response to available metabolites, metabolic shifts may also contribute to neutrophil population heterogeneity reported in cancer (tumor-associated neutrophil) or auto-immune diseases (Low/High Density Neutrophils). This represents an active field of research. In conclusion, the characterization of neutrophil metabolic shifts is an emerging field that may provide important knowledge on neutrophil physiology and activation modulation. The related question of microenvironmental changes occurring during inflammation, to which neutrophils will respond to, will have to be addressed to fully appreciate the importance of neutrophil metabolic shifts in inflammatory diseases.

Prolonged exposure to hypoxia induces an autophagy-like cell survival program in human neutrophils

Neutrophils contribute to low oxygen availability at inflammatory sites through the generation of reactive oxidants. They are also functionally affected by hypoxia, which delays neutrophil apoptosis. However, the eventual fate of neutrophils in hypoxic conditions is unknown and this is important for their effective clearance and the resolution of inflammation. We have monitored the survival and function of normal human neutrophils exposed to hypoxia over a 48 h period. Apoptosis was delayed, and the cells remained intact even at 48 h. However, hypoxia promoted significant changes in neutrophil morphology with the appearance of many new cytoplasmic vesicles, often containing cell material, within 5 hours of exposure to low O₂ . This coincided with an increase in LC3B-II expression, indicative of autophagosome formation and an autophagy-like process. In hypoxic conditions, neutrophils preferentially lost myeloperoxidase, a marker of azurophil granules. Short-term (2 h) hypoxic exposure resulted in sustained potential to generate superoxide when O₂ was restored, but the capacity for oxidant production was lost with longer periods of hypoxia. Phagocytic ability was unchanged by hypoxia, and bacterial killing by neutrophils in both normoxic and hypoxic conditions was substantially diminished after 24 hours. However, pre-exposure to hypoxia resulted in an enhanced ability to kill bacteria by oxidant-independent mechanisms. Our data provide the first evidence for hypoxia as a driver of neutrophil autophagy that can influence the function and ultimate fate of these cells, including their eventual clearance and the resolution of inflammation.

Shigella-mediated oxygen depletion is essential for intestinal mucosa colonization

Pathogenic enterobacteria face various oxygen (O₂) levels during intestinal colonization from the O₂-deprived lumen to oxygenated tissues. Using Shigella flexneri as a model, we had previously demonstrated that epithelium invasion is promoted by O₂ in a Type III secretion system (T3SS)-dependent manner¹. However, subsequent pathogen adaptation to tissue oxygenation modulation remained unknown. Assessing single-cell distribution, together with tissue oxygenation, we demonstrate here that the colonic mucosa O₂ is actively depleted by Shigella flexneri aerobic respiration, not host neutrophils, during infection, leading to the formation of hypoxic foci of infection. This process is promoted by T3SS inactivation in infected tissues, favoring colonizers over explorers. We identify the molecular mechanisms supporting infectious hypoxia induction, and we demonstrate here how enteropathogens optimize their colonization capacity in relation to their ability to manipulate tissue oxygenation during infection.

Preferential phosphatidylinositol 5-phosphate binding contributes to a destabilization of the VHS domain structure of Tom1

Tom1 transports endosomal ubiquitinated proteins that are targeted for degradation in the lysosomal pathway. Infection of eukaryotic cells by Shigella flexneri boosts oxygen consumption and promotes the synthesis of phosphatidylinositol-5-phosphate (PtdIns5P), which triggers Tom1 translocation to signaling endosomes. Removing Tom1 from its cargo trafficking function hinders protein degradation in the host and, simultaneously, enables bacterial survival. Tom1 preferentially binds PtdIns5P via its VHS domain, but the effects of a reducing environment as well as PtdIns5P on the domain structure and function are unknown. Thermal denaturation studies demonstrate that, under reducing conditions, the monomeric Tom1 VHS domain switches from a three-state to a two-state transition behavior. PtdIns5P reduced thermostability, interhelical contacts, and conformational compaction of Tom1 VHS, suggesting that the phosphoinositide destabilizes the protein domain. Destabilization of Tom1 VHS structure was also observed with other phospholipids. Isothermal calorimetry data analysis indicates that, unlike ubiquitin, Tom1 VHS endothermically binds to PtdIns5P through two noncooperative binding sites, with its acyl chains playing a relevant role in the interaction. Altogether, these findings provide mechanistic insights about the recognition of PtdIns5P by the VHS domain that may explain how Tom1, when in a different VHS domain conformational state, interacts with downstream effectors under S. flexneri infection.

Immunologic balance of regulatory T cell/T helper 17 responses in gastrointestinal infectious diseases: Role of miRNAs

Gastrointestinal Infectious diseases (GIDs) are the second cause of death worldwide. T helper17 cells (Th17) play an important role in GIDs through production of IL-17A, IL-17F, and IL-22 cytokines. Because of their increased activities in GID, Th17 and its inflammatory cytokines can inhibit the progression and eliminate the infection. Actually, although Th17 have the best performance in the acute phase, regulatory T cells (Treg cells) are enhanced in the chronic phase and infection progress through its suppressive function. In addition, Treg cells prevent undesirable inflammatory damages developed by immune system components. On the other hand, miRNAs have important roles in the regulation of immune responses to eliminate bacterial infections and protect host organisms from harmful effects. Actually, miRNAs can reinforce innate and adaptive immunity to remove infections. Of note, miRNAs can develop a regulatory network with the immune system. Additionally, miRNAs can also serve in favor of bacteria to reduce immune responses. Therefore, balance of immune responses in Treg and Th17 cells can influence outcome of many infectious diseases. In conclusion, there is an imbalance in the Treg/Th17 ratio in GIDs; importantly, sets of miRNAs, particularly miR155 and miR146, were determined to be involved clearly in GIDs.

Stress-induced host membrane remodeling protects from infection by non-motile bacterial pathogens

While mucosal inflammation is a major source of stress during enteropathogen infection, it remains to be fully elucidated how the host benefits from this environment to clear the pathogen. Here, we show that host stress induced by different stimuli mimicking inflammatory conditions strongly reduces the binding of Shigella flexneri to epithelial cells. Mechanistically, stress activates acid sphingomyelinase leading to host membrane remodeling. Consequently, knockdown or pharmacological inhibition of the acid sphingomyelinase blunts the stress-dependent inhibition of Shigella binding to host cells. Interestingly, stress caused by intracellular Shigella replication also results in remodeling of the host cell membrane, in vitro and in vivo, which precludes re-infection by this and other non-motile pathogens. In contrast, Salmonella Typhimurium overcomes the shortage of permissive entry sites by gathering effectively at the remaining platforms through its flagellar motility. Overall, our findings reveal host membrane remodeling as a novel stress-responsive cell-autonomous defense mechanism that protects epithelial cells from infection by non-motile bacterial pathogens.

Enterobacteria and host resistance to infection

Enterobacteriaceae are a large family of Gram-negative, non-spore-forming bacteria. Although many species exist as part of the natural flora of animals including humans, some members are associated with both intestinal and extraintestinal diseases. In this review, we focus on members of this family that have important roles in human disease: Salmonella, Escherichia, Shigella, and Yersinia, providing a brief overview of the disease caused by these bacteria, highlighting the contribution of animal models to our understanding of their pathogenesis and of host genetic determinants involved in susceptibility or resistance to infection.

Fluorescence imaging host pathogen interactions: fifteen years benefit of hindsight…

We consider in review current state-of-the-art fluorescence microscopy for investigating the host-pathogen interface. Our perspective is honed from years with literally thousands of microbiologists using the variety of imaging technologies available within our dedicated BSL2/BSL3 optical imaging research service facilities at the Institut Pasteur Paris founded from scratch in 2001. During fifteen years learning from the success and failures of introducing different fluorescence imaging technologies, methods, and technical development strategies we provide here a synopsis review of our experience to date and a synthesis of how we see the future in perspective for fluorescence imaging at the host-pathogen interface.

Hypoxia, Hypoxia-Inducible Factor-1α, and Innate Antileishmanial Immune Responses

Low oxygen environments and accumulation of hypoxia-inducible factors (HIFs) are features of infected and inflamed tissues. Here, we summarize our current knowledge on oxygen levels found in Leishmania-infected tissues and discuss which mechanisms potentially contribute to local tissue oxygenation in leishmanial lesions. Moreover, we review the role of hypoxia and HIF-1 on innate antileishmanial immune responses.

MUB40 Binds to Lactoferrin and Stands as a Specific Neutrophil Marker

Neutrophils represent the most abundant immune cells recruited to inflamed tissues. A lack of dedicated tools has hampered their detection and study. We show that a synthesized peptide, MUB₄₀, binds to lactoferrin, the most abundant protein stored in neutrophil-specific and tertiary granules. Lactoferrin is specifically produced by neutrophils among other leukocytes, making MUB₄₀ a specific neutrophil marker. Naive mammalian neutrophils (human, guinea pig, mouse, rabbit) were labeled by fluorescent MUB₄₀ conjugates (-Cy5, Dylight405). A peptidase-resistant retro-inverso MUB₄₀ (RI-MUB₄₀) was synthesized and its lactoferrin-binding property validated. Neutrophil lactoferrin secretion during in vitro Shigella infection was assessed with RI-MUB₄₀-Cy5 using live cell microscopy. Systemically administered RI-MUB₄₀-Cy5 accumulated at sites of inflammation in a mouse arthritis inflammation model in vivo and showed usefulness as a potential tool for inflammation detection using non-invasive imaging. Improving neutrophil detection with the universal and specific MUB₄₀ marker will aid the study of broad ranges of inflammatory diseases.

Visualizing endocytic recycling and trafficking in live neurons by subdiffractional tracking of internalized molecules

Our understanding of endocytic pathway dynamics is restricted by the diffraction limit of light microscopy. Although super-resolution techniques can overcome this issue, highly crowded cellular environments, such as nerve terminals, can also dramatically limit the tracking of multiple endocytic vesicles such as synaptic vesicles (SVs), which in turn restricts the analytical dissection of their discrete diffusional and transport states. We recently introduced a pulse-chase technique for subdiffractional tracking of internalized molecules (sdTIM) that allows the visualization of fluorescently tagged molecules trapped in individual signaling endosomes and SVs in presynapses or axons with 30- to 50-nm localization precision. We originally developed this approach for tracking single molecules of botulinum neurotoxin type A, which undergoes activity-dependent internalization and retrograde transport in autophagosomes. This method was then adapted to localize the signaling endosomes containing cholera toxin subunit-B that undergo retrograde transport in axons and to track SVs in the crowded environment of hippocampal presynapses. We describe (i) the construction of a custom-made microfluidic device that enables control over neuronal orientation; (ii) the 3D printing of a perfusion system for sdTIM experiments performed on glass-bottom dishes; (iii) the dissection, culturing and transfection of hippocampal neurons in microfluidic devices; and (iv) guidance on how to perform the pulse-chase experiments and data analysis. In addition, we describe the use of single-molecule-tracking analytical tools to reveal the average and the heterogeneous single-molecule mobility behaviors. We also discuss alternative reagents and equipment that can, in principle, be used for sdTIM experiments and describe how to adapt sdTIM to image nanocluster formation and/or tubulation in early endosomes during sorting events. The procedures described in this protocol take ∼1 week.

HIF-1α is a key regulator in potentiating suppressor activity and limiting the microbicidal capacity of MDSC-like cells during visceral leishmaniasis

Leishmania donovani is known to induce myelopoiesis and to dramatically increase extramedullary myelopoiesis. This results in splenomegaly, which is then accompanied by disruption of the splenic microarchitecture, a chronic inflammatory environment, and immunosuppression. Chronically inflamed tissues are typically hypoxic. The role of hypoxia on myeloid cell functions during visceral leishmaniasis has not yet been studied. Here we show that L. donovani promotes the output from the bone marrow of monocytes with a regulatory phenotype that function as safe targets for the parasite. We also demonstrate that splenic myeloid cells acquire MDSC-like function in a HIF-1α-dependent manner. HIF-1α is also involved in driving the polarization towards M2-like macrophages and rendering intermediate stage monocytes more susceptible to L. donovani infection. Our results suggest that HIF-1α is a major player in the establishment of chronic Leishmania infection and is crucial for enhancing immunosuppressive functions and lowering leishmanicidal capacity of myeloid cells.

The protozoan parasite Leishmania donovani causes chronic infection in the spleen, which is accompanied by a chronic inflammatory environment, an enlargement of the organ, and immunosuppression. The environment of chronically inflamed tissues is characterized by low oxygen levels and tissue disruption, which induce the expression of the transcription factor HIF-1α in all cells. The kinetics of monocyte production and differentiation in the bone marrow and the spleen, and the role of hypoxia in myeloid cell functions during visceral leishmaniasis have not yet been studied. Here we show that L. donovani promotes the output from the bone marrow of monocytes with a regulatory phenotype that function as safe targets for the parasite. We also demonstrate that HIF-1α potentiates inhibitory functions of myeloid cells and is involved in driving the polarization towards M2-like macrophages and rendering them more susceptible to L. donovani infection. Our results suggest that HIF-1α is a major player in the establishment of chronic Leishmania infection and is crucial for enhancing immunosuppressive functions and lowering leishmanicidal capacity of myeloid cells.

Harnessing Neutrophil Survival Mechanisms during Chronic Infection by Pseudomonas aeruginosa: Novel Therapeutic Targets to Dampen Inflammation in Cystic Fibrosis

More than two decades after cloning the cystic fibrosis transmembrane regulator (CFTR) gene, the defective gene in cystic fibrosis (CF), we still do not understand how dysfunction of this ion channel causes lung disease and the tremendous neutrophil burden which persists within the airways; nor why chronic colonization by Pseudomonas aeruginosa develops in CF patients who are thought to be immunocompetent. It appears that the microenvironment within the lung of CF patients provides favorable conditions for both P. aeruginosa colonization and neutrophil survival. In this context, the ability of bacteria to induce hypoxia, which in turn affects neutrophil survival is an additional level of complexity that needs to be accounted for when controlling neutrophil fate in CF. Recent studies have underscored the importance of neutrophils in innate immunity and their functions appear to extend far beyond their well-described role in antibacterial defense. Perhaps a disturbance in neutrophil reprogramming during the course of an infection severely modulates the inflammatory response in CF. Furthermore there is an emerging concept that the CFTR itself may be an immune modulator and stimulating CFTR function in CF patients could promote neutrophil and macrophages antimicrobial function. Fostering the resolution of inflammation by favoring neutrophil apoptosis could preserve their microbicidal activities but decrease their proinflammatory potential. In this context, triggering neutrophil apoptosis with roscovitine may be a potential therapeutic option and this is currently being evaluated in CF patients. In the present review we discuss how neutrophils functions are disturbed in CF and how this may relate to chronic infection with P. aeuginosa and we propose novel research directions aimed at modulating neutrophil survival, dampening lung inflammation and ultimately leading to an amelioration of the lung disease.