Neutrophil and Alveolar Macrophage-Mediated Innate Immune Control of Legionella pneumophila Lung Infection via TNF and ROS

Innate immune responses and monocyte-derived phagocyte recruitment in protective immunity to pathogenic bacteria: insights from Legionella pneumophila

Legionella species are Gram-negative intracellular bacteria that evolved in soil and freshwater environments, where they infect and replicate within various unicellular protozoa. The primary virulence factor of Legionella is the expression of a type IV secretion system (T4SS), which contributes to the translocation of effector proteins that subvert biological processes of the host cells. Because of its evolution in unicellular organisms, T4SS effector proteins are not adapted to subvert specific mammalian signaling pathways and immunity. Consequently, Legionella pneumophila has emerged as an interesting infection model for investigating immune responses against pathogenic bacteria in multicellular organisms. This review highlights recent advances in our understanding of mammalian innate immunity derived from studies involving L. pneumophila. This includes recent insights into inflammasome-mediated mechanisms restricting bacterial replication in macrophages, mechanisms inducing cell death in response to infection, induction of effector-triggered immunity, activation of specific pulmonary cell types in mammalian lungs, and the protective role of recruiting monocyte-derived cells to infected lungs.

Bacterial Surface Appendages Modulate the Antimicrobial Activity Induced by Nanoflake Surfaces on Titanium

Bioinspired nanotopography is a promising approach to generate antimicrobial surfaces to combat implant-associated infection. Despite efforts to develop bactericidal 1D structures, the antibacterial capacity of 2D structures and their mechanism of action remains uncertain. Here, hydrothermal synthesis is utilized to generate two 2D nanoflake surfaces on titanium (Ti) substrates and investigate the physiological effects of nanoflakes on bacteria. The nanoflakes impair the attachment and growth of Escherichia coli and trigger the accumulation of intracellular reactive oxygen species (ROS), potentially contributing to the killing of adherent bacteria. E. coli surface appendages type-1 fimbriae and flagella are not implicated in the nanoflake-mediated modulation of bacterial attachment but do influence the bactericidal effects of nanoflakes. An E. coli ΔfimA mutant lacking type-1 fimbriae is more susceptible to the bactericidal effects of nanoflakes than the parent strain, while E. coli cells lacking flagella (ΔfliC) are more resistant. The results suggest that type-1 fimbriae confer a cushioning effect that protects bacteria upon initial contact with the nanoflake surface, while flagella-mediated motility can lead to elevated membrane abrasion. This finding offers a better understanding of the antibacterial properties of nanoflake structures that can be applied to the design of antimicrobial surfaces for future medical applications.

Transcriptomic insights into vibrio-induced mortality in the clam Meretrix petechialis under high temperature

In this study, we investigate the mortality of the clam Meretrix petechialis facing a vibrio challenge under different temperatures and the underlying molecular mechanisms. Our experiment distinctly revealed that clam mortality was predominantly observed under high temperature, highlighting the critical impact of thermal stress on clam susceptibility to infection. Using RNA-seq, we further compared the global transcriptional response to vibrio in clam gills between high and low temperatures. Compared to other groups, the differentially expressed genes in vibrio-challenged group at high temperature associated with immunity, oxidative stress, and membrane transport. Key results show a weakened immune response in clams at high temperature, especially in the TNF signaling pathway, and a decrease in membrane transport efficiency, notably in SLC proteins. Additionally, high temperature enhanced pro-inflammatory related unsaturated fatty acid metabolism, leading to increased oxidative damage. This was further evidenced by our biochemical assays, which showed significantly higher levels of lipid peroxidation and protein carbonylation in clams at high temperature, indicating heightened oxidative damage. RT-PCR validation of selected DEGs corroborated the RNA-seq findings. Our findings contribute to the understanding of more frequent shellfish mortality in summer, emphasizing the role of temperature in pathogen response, elucidating the molecular mechanisms underlying the synergistic effect of pathogen and high temperature stresses. The key genes identified provide potential targets for resistance-assisted breeding. This research has significant implications for bivalve aquaculture and their physiology, particularly in light of global climate changes affecting marine ecosystems.

The involvement of oxidative stress and the TLR4/NF-κB/NLRP3 pathway in acute lung injury induced by high-altitude hypoxia

OBJECTIVE

This study investigated the effect of oxidative stress and the TLR4/NF-κB/NLRP3 pathway on the pathogenesis of acute lung injury (ALI) induced by high-altitude hypoxia.

METHODS

Rats were placed in an animal hyperbaric oxygen chamber to establish a rat model of ALI induced by high-altitude hypoxia after treatment with N-acetylcysteine (NAC; a reactive oxygen species [ROS] inhibitor) or/and MCC950 (an NLPR3 inflammasome inhibitor). After modeling, the wet-to-dry weight ratio (W/D) of rat lung tissues was calculated. In lung tissues, ROS levels were detected with immunofluorescence, the enzyme activity was tested with the kit, and the expression of TLR4/NF-κB/NLRP3 pathway-related genes and proteins was measured with western blotting and qRT-PCR. The levels of inflammatory factors in the serum were quantified with ELISA.

RESULTS

After modeling, rats showed significantly increased W/D, ROS levels, and Malondialdehyde (MDA) concentrations and markedly diminished Superoxide dismutase (SOD) and Glutathione (GSH) concentrations in lung tissues (all P < 0.01), accompanied by substantially enhanced serum levels of TNF-α, IL-6, and IL-1β, significantly reduced serum levels of IL-10, and remarkably augmented TLR4, NLRP3, p-NF-κB p65, NF-κB p65 mRNA, and Caspase-1 expression in lung tissues (all P < 0.01). Furthermore, treatment with NAC or MCC950 alone or in combination prominently lowered the W/D of lung tissues (P < 0.01), serum levels of TNF-α (P < 0.05), IL-6 (P < 0.05), and IL-1β (P < 0.01), and NF-κB p65 expression and phosphorylation (P < 0.05, P < 0.01) while significantly increasing SOD and GSH concentrations (P < 0.05, P < 0.01) and serum levels of IL-10 (P < 0.01) in modeled rats. Meanwhile, treatment of NAC alone or combined with MCC950 significantly reduced MDA concentration and ROS levels (P < 0.05, P < 0.01) in modeled rats, and treatment of MCC950 alone or combined with NAC considerably declined TLR4, NLRP3, and Caspase-1 expression in modeled rats (P < 0.05, P < 0.01).

CONCLUSION

Inhibition of oxidative stress and the TLR4/NF-κB/NLRP3 pathway can ameliorate ALI in rats exposed to high-altitude hypoxia.

RIPK1 inhibitor ameliorates pulmonary injury by modulating the function of neutrophils and vascular endothelial cells

Acute lung injury (ALI) is an acute and progressive hypoxic respiratory failure that could progress to acute respiratory distress syndrome (ARDS) with a high mortality rate, thus immediate medical attention and supportive care are necessary. The pathophysiology of ALI is characterized by the disruption of the alveolar-capillary barrier and activation of neutrophils, leading to lung tissue damage. The receptor-interacting protein kinase 1 (RIPK1) has emerged as a promising target for the treatment of multiple inflammatory diseases, but the role of RIPK1 in the ALI remains poorly understood. In this study, we aimed to figure out the pathological role of RIPK1 in ALI, especially in the pulmonary immune microenvironment involving neutrophils and endothelial cells. In vivo experiments showed that RIPK1 inhibitor protected against lipopolysaccharide (LPS)-induced lung injury in mouse models, with reduced neutrophils and monocytes infiltration in the lungs. Further studies demonstrated that, besides the inhibitory action on necroptosis, RIPK1 inhibitor directly suppressed reactive oxygen species (ROS) generation and inflammatory cytokines secretion from neutrophils. Furthermore, RIPK1 inhibition maintains the barrier function in TNF-α-primed vascular endothelial cells and prevents their activation induced by the supernatant from LPS-stimulated neutrophils. Mechanistically, the aforementioned effects of RIPK1 inhibitor are associated with the NF-κB signaling pathway, which is partially independent of necroptosis inhibition. These results provide new evidence that RIPK1 inhibitor directly regulates the function of neutrophils and endothelial cells, as well as interferes with the interactions between these two cell types, therefore contributing to a better understanding of RIPK1 in ALI and providing a potential avenue for future therapeutic interventions.

Inflammatory Patterns Associated with Legionella in HIV and Pneumonia Coinfections

Legionella infections have a propensity for occurring in HIV-infected individuals, with immunosuppressed individuals tending to present with more severe disease. However, understanding regarding the Legionella host response in immune compromised individuals is lacking. This study investigated the inflammatory profiles associated with Legionella infection in patients hospitalized with HIV and pneumonia in Medellín, Colombia from February 2007 to April 2014, and correlated these profiles with clinical outcomes. Sample aliquots from the Colombian cohort were shipped to Canada where Legionella infections and systemic cytokine profiles were determined using real-time PCR and bead-based technology, respectively. To determine the effect of Legionella coinfection on clinical outcome, a patient database was consulted, comparing laboratory results and outcomes between Legionella-positive and -negative individuals. Principal component analysis revealed higher plasma concentrations of eotaxin, IP-10 and MCP-1 (p = 0.0046) during Legionella infection. Individuals with this immune profile also had higher rates of intensive care unit admissions (adjusted relative risk 1.047 [95% confidence interval 1.027-1.066]). Results demonstrate that systemic markers of monocyte/macrophage activation and differentiation (eotaxin, MCP-1, and IP-10) are associated with Legionella infection and worse patient outcomes. Further investigations are warranted to determine how this cytokine profile may play a role in Legionella pneumonia pathogenesis or immunity.

Peptide TK-HR from the Skin of Chinese Folk Medicine Frog Hoplobatrachus Rugulosus Accelerates Wound Healing via the Activation of the Neurokinin-1 Receptor

Wound healing is a complex process and remains a considerable challenge in clinical trials due to the lack of ideal therapeutic drugs. Here, a new peptide TK-HR identified from the skin of the frog Hoplobatrachus rugulosus was tested for its ability to heal cutaneous wounds in mice. Topical application of TK-HR at doses of 50-200 μg/mL significantly accelerated wound closure without causing any adverse effects in the animals. In vitro and in vivo investigations proved the regulatory role of the peptide on neutrophils, macrophages, keratinocytes, and vein endothelial cells involved in the inflammatory, proliferative, and remodeling phases of wound healing. Notably, TK-HR activated the MAPK and TGF-β-Smad signaling pathways by acting on NK1R in RAW264.7 cells and mice. The current work has identified that TK-HR is a potent wound healing regulator that can be applied for the treatment of wounds, including diabetic foot ulcers and infected wounds, in the future.

Acyloxyacyl hydrolase promotes pulmonary defense by preventing alveolar macrophage tolerance

Although alveolar macrophages (AMs) play important roles in preventing and eliminating pulmonary infections, little is known about their regulation in healthy animals. Since exposure to LPS often renders cells hyporesponsive to subsequent LPS exposures ("tolerant"), we tested the hypothesis that LPS produced in the intestine reaches the lungs and stimulates AMs, rendering them tolerant. We found that resting AMs were more likely to be tolerant in mice lacking acyloxyacyl hydrolase (AOAH), the host lipase that degrades and inactivates LPS; isolated Aoah-/- AMs were less responsive to LPS stimulation and less phagocytic than were Aoah+/+ AMs. Upon innate stimulation in the airways, Aoah-/- mice had reduced epithelium- and macrophage-derived chemokine/cytokine production. Aoah-/- mice also developed greater and more prolonged loss of body weight and higher bacterial burdens after pulmonary challenge with Pseudomonas aeruginosa than did wildtype mice. We also found that bloodborne or intrarectally-administered LPS desensitized ("tolerized") AMs while antimicrobial drug treatment that reduced intestinal commensal Gram-negative bacterial abundance largely restored the innate responsiveness of Aoah-/- AMs. Confirming the role of LPS stimulation, the absence of TLR4 prevented Aoah-/- AM tolerance. We conclude that commensal LPSs may stimulate and desensitize (tolerize) alveolar macrophages in a TLR4-dependent manner and compromise pulmonary immunity. By inactivating LPS in the intestine, AOAH promotes antibacterial host defenses in the lung.

Phenylboronic Ester-Bridged Chitosan/Myricetin Nanomicelle for Penetrating the Endothelial Barrier and Regulating Macrophage Polarization and Inflammation against Ischemic Diseases

The brain and liver are more susceptible to ischemia and reperfusion (IR) injury (IRI), which triggers the reactive oxygen species (ROS) burst and inflammatory cascade and results in severe neuronal damage or hepatic injury. Moreover, the damaged endothelial barrier contributes to proinflammatory activity and limits the delivery of therapeutic agents such as some macromolecules and nanomedicine despite the integrity being disrupted after IRI. Herein, we constructed a phenylboronic-decorated chitosan-based nanoplatform to deliver myricetin, a multifunctional polyphenol molecule for the treatment of cerebral and hepatic ischemia. The chitosan-based nanostructures are widely studied cationic carriers for endothelium penetration such as the blood-brain barrier (BBB) and sinusoidal endothelial barrier (SEB). The phenylboronic ester was chosen as the ROS-responsive bridging segment for conjugation and selective release of myricetin molecules, which meanwhile scavenged the overexpressed ROS in the inflammatory environment. The released myricetin molecules fulfill a variety of roles including antioxidation through multiple phenolic hydroxyl groups, inhibition of the inflammatory cascade by regulation of the macrophage polarization from M1 to M2, and endothelial injury repairment. Taken together, our present study provides valuable insight into the development of efficient antioxidant and anti-inflammatory platforms for potential application against ischemic disease.

TNF licenses macrophages to undergo rapid caspase-1, -11, and -8-mediated cell death that restricts Legionella pneumophila infection

The inflammatory cytokine tumor necrosis factor (TNF) is necessary for host defense against many intracellular pathogens, including Legionella pneumophila. Legionella causes the severe pneumonia Legionnaires' disease and predominantly affects individuals with a suppressed immune system, including those receiving therapeutic TNF blockade to treat autoinflammatory disorders. TNF induces pro-inflammatory gene expression, cellular proliferation, and survival signals in certain contexts, but can also trigger programmed cell death in others. It remains unclear, however, which of the pleiotropic functions of TNF mediate control of intracellular bacterial pathogens like Legionella. In this study, we demonstrate that TNF signaling licenses macrophages to die rapidly in response to Legionella infection. We find that TNF-licensed cells undergo rapid gasdermin-dependent, pyroptotic death downstream of inflammasome activation. We also find that TNF signaling upregulates components of the inflammasome response, and that the caspase-11-mediated non-canonical inflammasome is the first inflammasome to be activated, with caspase-1 and caspase-8 mediating delayed pyroptotic death. We find that all three caspases are collectively required for optimal TNF-mediated restriction of bacterial replication in macrophages. Furthermore, caspase-8 is required for control of pulmonary Legionella infection. These findings reveal a TNF-dependent mechanism in macrophages for activating rapid cell death that is collectively mediated by caspases-1, -8, and -11 and subsequent restriction of Legionella infection.

Effector-mediated subversion of proteasome activator (PA)28αβ enhances host defense against Legionella pneumophila under inflammatory and oxidative stress conditions

Legionella pneumophila is a natural pathogen of amoebae that causes Legionnaires' Disease in immunocompromised individuals via replication within macrophages. L. pneumophila virulence and intracellular replication hinges on hundreds of Dot/Icm-translocated effector proteins, which are essential for biogenesis of the replication-permissive Legionella-containing vacuole (LCV). However, effector activity can also enhance mammalian host defense via effector-triggered immunity. The L. pneumophila effector LegC4 is important for virulence in amoebae but enhances host defense against L. pneumophila in the mouse lung and, uniquely, within macrophages activated with either tumor necrosis factor (TNF) or interferon (IFN)-γ. The mechanism by which LegC4 potentiates cytokine-mediated host defense in macrophages is unknown. Here, we found that LegC4 enhances cytokine-mediated phagolysosomal fusion with Legionella-containing vacuole (LCV) and binds host proteasome activator (PA)28α, which forms a heterooligomer with PA28β to facilitate ubiquitin-independent proteasomal degradation of oxidant-damaged (carbonylated) proteins. We found that oxidative stress was sustained in the presence of LegC4 and that the LegC4 restriction phenotype was relieved in PA28αβ-deficient macrophages and in the lungs of mice in vivo. Our data also show that oxidative stress is sufficient for LegC4-mediated restriction in macrophages producing PA28αβ. PA28αβ has been traditionally associated with antigen presentation; however, our data support a novel mechanism whereby effector-mediated subversion of PA28αβ enhances cell-autonomous host defense against L. pneumophila under inflammatory and oxidative stress conditions. This work provides a solid foundation to evaluate induced proteasome regulators as mediators of innate immunity.

Cigarette smoke depletes alveolar macrophages and delays clearance of Legionella pneumophila

Legionella pneumophila is the main etiological agent of Legionnaires' disease, a severe bacterial pneumonia. L. pneumophila is initially engulfed by alveolar macrophages (AMs) and subvert normal cellular functions to establish a replicative vacuole. Cigarette smokers are particularly susceptible to developing Legionnaires' disease and other pulmonary infections; however, little is known about the cellular mechanisms underlying this susceptibility. To investigate this, we used a mouse model of acute cigarette smoke exposure to examine the immune response to cigarette smoke and subsequent L. pneumophila infection. Contrary to previous reports, we show that cigarette smoke exposure alone causes a significant depletion of AMs using enzymatic digestion to extract cells, or via imaging intact lung lobes by light-sheet microscopy. Furthermore, treatment of mice deficient in specific types of cell death with smoke suggests that NLRP3-driven pyroptosis is a contributor to smoke-induced death of AMs. After infection, smoke-exposed mice displayed increased pulmonary L. pneumophila loads and developed more severe disease compared with air-exposed controls. We tested if depletion of AMs was related to this phenotype by directly depleting them with clodronate liposomes and found that this also resulted in increased L. pneumophila loads. In summary, our results showed that cigarette smoke depleted AMs from the lung and that this likely contributed to more severe Legionnaires' disease. Furthermore, the role of AMs in L. pneumophila infection is more nuanced than simply providing a replicative niche, and our studies suggest they play a major role in bacterial clearance.

Innate immune responses in pneumonia

The lungs are an immunologically unique environment; they are exposed to innumerable pathogens and particulate matter daily. Appropriate clearance of pathogens and response to pollutants is required to prevent overwhelming infection, while preventing tissue damage and maintaining efficient gas exchange. Broadly, the innate immune system is the collection of immediate, intrinsic immune responses to pathogen or tissue injury. In this review, we will examine the innate immune responses of the lung, with a particular focus on their role in pneumonia. We will discuss the anatomic barriers and antimicrobial proteins of the lung, pathogen and injury recognition, and the role of leukocytes (macrophages, neutrophils, and innate lymphocytes) and lung stromal cells in innate immunity. Throughout the review, we will focus on new findings in innate immunity as well as features that are unique to the lung.

CLEC12A Binds to Legionella pneumophila but Has No Impact on the Host's Antibacterial Response

Legionella pneumophila is an intracellular pathogen that can cause severe pneumonia after the inhalation of contaminated aerosols and replication in alveolar macrophages. Several pattern recognition receptors (PRRs) have been identified that contribute to the recognition of L. pneumophila by the innate immune system. However, the function of the C-type lectin receptors (CLRs), which are mainly expressed by macrophages and other myeloid cells, remains largely unexplored. Here, we used a library of CLR-Fc fusion proteins to search for CLRs that can bind the bacterium and identified the specific binding of CLEC12A to L. pneumophila. Subsequent infection experiments in human and murine macrophages, however, did not provide evidence for a substantial role of CLEC12A in controlling innate immune responses to the bacterium. Consistently, antibacterial and inflammatory responses to Legionella lung infection were not significantly influenced by CLEC12A deficiency. Collectively, CLEC12A is able to bind to L. pneumophila-derived ligands but does not appear to play a major role in the innate defense against L. pneumophila.

Inflammasome activation and CCR2-mediated monocyte-derived dendritic cell recruitment restrict Legionella pneumophila infection

Flagellin-induced NAIP/NLRC4 inflammasome activation and pyroptosis are critical events restricting Legionella pneumophila infection. However, the cellular and molecular dynamics of the in vivo responses against this bacterium are still unclear. We have found temporal coordination of two independent innate immunity pathways in controlling Legionella infection, the inflammasome activation and the CCR2-mediated Mo-DC recruitment. Inflammasome activation was an important player at the early stage of infection by lowering the numbers of bacteria for an efficient bacterial clearance conferred by the Mo-DC at the late stage of the infection. Mo-DC emergence highly depended on CCR2-signaling and dispensed inflammasome activation and pyroptosis. Also, Mo-DC compartment did not rely on the inflammasome machinery to deliver proper immune responses and was the most abundant cytokine-producing among the monocyte-derived cells in the infected lung. Importantly, when the CCR2- and NLRC4-dependent axes of response were simultaneously ablated, we observed an aggravated bacterial burden in the lung of infected mice. Taken together, we showed that inflammasome activation and CCR2-mediated immune response interplay in distinct pathways to restrict pulmonary bacterial infection. These findings extend our understanding of the in vivo integration and cooperation of different innate immunity arms in controlling infectious agents.

Apoptosis in megakaryocytes: Safeguard and threat for thrombopoiesis

Platelets, generated from precursor megakaryocytes (MKs), are central mediators of hemostasis and thrombosis. The process of thrombopoiesis is extremely complex, regulated by multiple factors, and related to many cellular events including apoptosis. However, the role of apoptosis in thrombopoiesis has been controversial for many years. Some researchers believe that apoptosis is an ally of thrombopoiesis and platelets production is apoptosis-dependent, while others have suggested that apoptosis is dispensable for thrombopoiesis, and is even inhibited during this process. In this review, we will focus on this conflict, discuss the relationship between megakaryocytopoiesis, thrombopoiesis and apoptosis. In addition, we also consider why such a vast number of studies draw opposite conclusions of the role of apoptosis in thrombopoiesis, and try to figure out the truth behind the mystery. This review provides more comprehensive insights into the relationship between megakaryocytopoiesis, thrombopoiesis, and apoptosis and finds some clues for the possible pathological mechanisms of platelet disorders caused by abnormal apoptosis.

The protective effect of natural medicines against excessive inflammation and oxidative stress in acute lung injury by regulating the Nrf2 signaling pathway

Acute lung injury (ALI) is a common critical disease of the respiratory system that progresses into acute respiratory distress syndrome (ARDS), with high mortality, mainly related to pulmonary oxidative stress imbalance and severe inflammation. However, there are no clear and effective treatment strategies at present. Nuclear factor erythroid 2-related factor 2(Nrf2) is a transcription factor that interacts with multiple signaling pathways and regulates the activity of multiple oxidases (NOX, NOS, XO, CYP) related to inflammation and apoptosis, and exhibits antioxidant and anti-inflammatory roles in ALI. Recently, several studies have reported that the active ingredients of natural medicines show protective effects on ALI via the Nrf2 signaling pathway. In addition, they are cheap, naturally available, and possess minimal toxicity, thereby having good clinical research and application value. Herein, we summarized various studies on the protective effects of natural pharmaceutical components such as polyphenols, flavonoids, terpenoids, alkaloids, and polysaccharides on ALI through the Nrf2 signaling pathway and demonstrated existing gaps as well as future perspectives.

Rutile-TiO2/PtO2 Glass Coatings Disinfects Aquatic Legionella pneumophila via Morphology Change and Endotoxin Degradation under LED Irradiation

Legionella pneumophila (L. pneumophila) is the causative agent of Legionnaires’ disease and Pontiac fever, collectively known as legionellosis. L. pneumophila infection occurs through inhalation of contaminated aerosols from water systems in workplaces and institutions. The development of disinfectants that can eliminate L. pneumophila in such water systems without evacuating people is needed to prevent the spread of L. pneumophila. Photocatalysts are attractive disinfectants that do not harm human health. In particular, the TiO2 photocatalyst kills L. pneumophila under various conditions, but its mode of action is unknown. Here, we confirmed the high performance of TiO2 photocatalyst containing PtO2 via the degradation of methylene blue (half-value period: 19.2 min) and bactericidal activity against Escherichia coli (half-value period: 15.1 min) in water. Using transmission electron microscopy, we demonstrate that the disinfection of L. pneumophila (half-value period: 6.7 min) by TiO2 photocatalyst in water is accompanied by remarkable cellular membrane and internal damage to L. pneumophila. Assays with limulus amebocyte lysate and silver staining showed the release of endotoxin from L. pneumophila due to membrane damage and photocatalytic degradation of this endotoxin. This is the first study to demonstrate the disinfection mechanisms of TiO2 photocatalyst, namely, via morphological changes and membrane damage of L. pneumophila. Our results suggest that TiO2 photocatalyst might be effective in controlling the spread of L. pneumophila.

Development of a Novel Peptide Nucleic Acid Probe for the Detection of Legionella spp. in Water Samples

Legionella are opportunistic intracellular pathogens that are found throughout the environment. The Legionella contamination of water systems represents a serious social problem that can lead to severe diseases, which can manifest as both Pontiac fever and Legionnaires' disease (LD) infections. Fluorescence in situ hybridization using nucleic acid mimic probes (NAM-FISH) is a powerful and versatile technique for bacterial detection. By optimizing a peptide nucleic acid (PNA) sequence based on fluorescently selective binding to specific bacterial rRNA sequences, we established a new PNA-FISH method that has been successfully designed for the specific detection of the genus Legionella. The LEG22 PNA probe has shown great theoretical performance, presenting 99.9% specificity and 96.9% sensitivity. We also demonstrated that the PNA-FISH approach presents a good signal-to-noise ratio when applied in artificially contaminated water samples directly on filtration membranes or after cells elution. For water samples with higher turbidity (from cooling tower water systems), there is still the need for further method optimization in order to detect cellular contents and to overcome interferents' autofluorescence, which hinders probe signal visualization. Nevertheless, this work shows that the PNA-FISH approach could be a promising alternative for the rapid (3-4 h) and accurate detection of Legionella.

Glutathione Synthesis Regulated by CtrA Protects Ehrlichia chaffeensis From Host Cell Oxidative Stress

Ehrlichia chaffeensis, a small Gram-negative obligatory intracellular bacterium, infects human monocytes or macrophages, and causes human monocytic ehrlichiosis, one of the most prevalent, life-threatening emerging zoonoses. Reactive oxygen species are produced by the host immune cells in response to bacterial infections. The mechanisms exploited by E. chaffeensis to resist oxidative stress have not been comprehensively demonstrated. Here, we found that E. chaffeensis encodes two functional enzymes, GshA and GshB, to synthesize glutathione that confers E. chaffeensis the oxidative stress resistance, and that the expression of gshA and gshB is upregulated by CtrA, a global transcriptional regulator, upon oxidative stress. We found that in E. chaffeensis, the expression of gshA and gshB was upregulated upon oxidative stress using quantitative RT-PCR. Ehrlichia chaffeensis GshA or GshB restored the ability of Pseudomonas aeruginosa GshA or GshB mutant to cope with oxidative stress, respectively. Recombinant E. chaffeensis CtrA directly bound to the promoters of gshA and gshB, determined with electrophoretic mobility shift assay, and activated the expression of gshA and gshB determined with reporter assay. Peptide nucleic acid transfection of E. chaffeensis, which reduced the CtrA protein level, inhibited the oxidative stress-induced upregulation of gshA and gshB. Our findings provide insights into the function and regulation of the two enzymes critical for E. chaffeensis resistance to oxidative stress and may deepen our understanding of E. chaffeensis pathogenesis and adaptation in hosts.

Characterization, expression, and function analysis of AKR1A1 gene from yellow catfish (Tachysurus fulvidraco)

Aldehyde reductase (AKR1A1) is a carbonyl detoxification protein in toxic aldehyde removal. In the present study, the full-length cDNA of yellow catfish AKR1A1 (TfAKR1A1) was cloned. As expected, yellow catfish AKR1A1 showed similarities with that of other species. Subsequently, prokaryotic expression vector was constructed and recombinant TfAKR1A1 (rTfAKR1A1) was successfully induced and purified. rTfAKR1A1 exhibited reductive activity to many aldehydes and ketones. To determine whether TfAKR1A1 could confer stress tolerance in vitro, the viability of control and TfAKR1A1 expression E. coli under abiotic stress was compared by spot assay. Results showed that the recombinant strain had better stress resistance under cadmium, hydrogen peroxide, and DL-glyceraldehyde stress. Then, effects of an intraperitoneal injection of rTfAKR1A1 protein on cadmium-induced oxidative stress were evaluated. Results displayed that TfAKR1A1 and Nrf2 expression levels were significantly decreased, CAT and SOD expression levels were significantly increased, BCL-2 and IL-10 expression levels were significantly increased, and caspase3a, NF-κB, and IL-1β expression levels were significantly decreased in protein-injection group. Furthermore, oxidative stress indexes in livers under different protein injection doses were examined by ELISA. Results showed that CAT, SOD, and GSH-Px activities were upregulated, ROS and T-AOC contents were also improved, while MDA content was significantly decreased both in lower and middle dose injection groups. Finally, liver pathological section analysis was performed. Results displayed that liver injury degree in protein-injected groups was lower than that of PBS group under cadmium stress. These results suggested that TfAKR1A1 played important roles in response to cadmium stress in yellow catfish.

Dihydrolipoamide Acetyltransferase AceF Influences the Type III Secretion System and Resistance to Oxidative Stresses through RsmY/Z in Pseudomonas aeruginosa

Carbon metabolism plays an important role in bacterial physiology and pathogenesis. The type III secretion system (T3SS) of Pseudomonas aeruginosa is a virulence factor that contributes to acute infections. It has been demonstrated that bacterial metabolism affects the T3SS. Meanwhile, expression of T3SS genes is negatively regulated by the small RNAs RsmY and RsmZ. In this study, we studied the relationship between the dihydrolipoamide acetyltransferase gene aceF and the T3SS. Our results reveal an upregulation of RsmY and RsmZ in the aceF mutant, which represses the expression of the T3SS genes. Meanwhile, the aceF mutant is more tolerant to hydrogen peroxide. We demonstrate that the expression levels of the catalase KatB and the alkyl hydroperoxide reductase AhpB are increased in the aceF mutant. The simultaneous deletion of rsmY and rsmZ in the aceF mutant restored the expression levels of katB and ahpB, as well as bacterial susceptibility to hydrogen peroxide. Thus, we identify a novel role of AceF in the virulence and oxidative response of P. aeruginosa.

TNF-α response in macrophages depends on clinical Legionella pneumophila isolates genotypes

Legionnaires' Disease (LD) is a severe pneumonia mainly caused in Europe by Legionella pneumophila serogroup 1 (Lp1). Sequence-based typing methods reveal that some sequence types (ST) are overrepresented in clinical samples such as ST1 and ST47, suggesting that some strains are more fit for infection than others. In the present study, a collection of 108 Lp1 clinical isolates were used to evaluate the strain-dependent immune responses from human macrophages. Clinical Lp1 isolates induced differential TNFα secretion from macrophages. ST1 isolates induced a significantly higher TNF-α secretion than non-ST1, whereas ST47 isolates induced a significantly lower TNF-α secretion than non-ST47 isolates. ST1 isolates induced a significantly higher cell death than ST47 isolates evaluated by lactate dehydrogenase activity (cytotoxicity) and caspase-3 activity (apoptosis). Treatment of macrophages with anti-TNF-α antibodies significantly reduced the cell death in macrophages infected with ST1 or ST47 strains. The TNF-α secretion was neither explained by a differential bacterial replication nor by the number or type (bystander or infected) of TNF-α producing cells following infection but by a differential response from macrophages. The Paris ST1 reference strain elicited a significantly higher TNF-α gene transcription and a higher induction of NF-κB signaling pathway than the Lorraine ST47 reference strain.Clinical Lp1 isolates induce a diverse immune response and cell death, which could be related to the genotype. The two predominant sequence-types ST1 and ST47 trigger opposite inflammatory response that could be related to the host susceptibility.

Population analysis of Legionella pneumophila reveals a basis for resistance to complement-mediated killing

Legionella pneumophila is the most common cause of the severe respiratory infection known as Legionnaires' disease. However, the microorganism is typically a symbiont of free-living amoeba, and our understanding of the bacterial factors that determine human pathogenicity is limited. Here we carried out a population genomic study of 902 L. pneumophila isolates from human clinical and environmental samples to examine their genetic diversity, global distribution and the basis for human pathogenicity. We find that the capacity for human disease is representative of the breadth of species diversity although some clones are more commonly associated with clinical infections. We identified a single gene (lag-1) to be most strongly associated with clinical isolates. lag-1, which encodes an O-acetyltransferase for lipopolysaccharide modification, has been distributed horizontally across all major phylogenetic clades of L. pneumophila by frequent recent recombination events. The gene confers resistance to complement-mediated killing in human serum by inhibiting deposition of classical pathway molecules on the bacterial surface. Furthermore, acquisition of lag-1 inhibits complement-dependent phagocytosis by human neutrophils, and promoted survival in a mouse model of pulmonary legionellosis. Thus, our results reveal L. pneumophila genetic traits linked to disease and provide a molecular basis for resistance to complement-mediated killing.

Cytokines Induced by Edwardsiella tarda: Profile and Role in Antibacterial Immunity

Edwardsiella tarda is a Gram-negative bacterial pathogen with a broad range of hosts, including fish and mammals. In the present study, we used an advanced antibody array technology to identify the expression pattern of cytokines induced by E. tarda in a mouse infection model. In total, 31 and 24 differentially expressed cytokines (DECs) were identified in the plasma at 6 h and 24 h post-infection (hpi), respectively. The DECs were markedly enriched in the Gene Ontology (GO) terms associated with cell migration and response to chemokine and in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with immunity, diseases, and infection. Ten key DECs, including IL6 and TNF-α, were found to form extensive protein-protein interaction networks. IL6 was demonstrated to inhibit E. tarda infection and be required for E. tarda-induced inflammatory response. TNF-α also exerted an inhibitory effect on E. tarda infection, and knockdown of fish (Japanese flounder) TNF-α promoted E. tarda invasion in host cells. Together, the results of this study revealed a comprehensive profile of cytokines induced by E. tarda, thus adding new insights into the role of cytokine-associated immunity against bacterial infection and also providing the potential plasma biomarkers of E. tarda infection for future studies.

Cytomegalovirus subverts macrophage identity

Cytomegaloviruses (CMVs) have co-evolved with their mammalian hosts for millions of years, leading to remarkable host specificity and high infection prevalence. Macrophages, which already populate barrier tissues in the embryo, are the predominant immune cells at potential CMV entry sites. Here we show that, upon CMV infection, macrophages undergo a morphological, immunophenotypic, and metabolic transformation process with features of stemness, altered migration, enhanced invasiveness, and provision of the cell cycle machinery for viral proliferation. This complex process depends on Wnt signaling and the transcription factor ZEB1. In pulmonary infection, mouse CMV primarily targets and reprograms alveolar macrophages, which alters lung physiology and facilitates primary CMV and secondary bacterial infection by attenuating the inflammatory response. Thus, CMV profoundly perturbs macrophage identity beyond established limits of plasticity and rewires specific differentiation processes, allowing viral spread and impairing innate tissue immunity.

Updating the NLRC4 Inflammasome: from Bacterial Infections to Autoimmunity and Cancer

Inflammasomes comprise a family of cytosolic multi-protein complexes that modulate the activation of cysteine-aspartate-specific protease 1 (caspase-1) and promote the maturation and secretion of interleukin (IL)-1β and IL-18, leading to an inflammatory response. Different types of inflammasomes are defined by their sensor protein which recognizes pathogenic ligands and then directs inflammasome assembly. Although the specific molecular mechanisms underlying the activation of most inflammasomes are still unclear, NLRC4 inflammasomes have emerged as multifaceted agents of the innate immune response, playing important roles in immune defense against a variety of pathogens. Other studies have also expanded the scope of NLRC4 inflammasomes to include a range of inherited human autoimmune diseases as well as proposed roles in cancer. In this review article, we provide an updated overview of NLRC4 inflammasomes, describing their composition, activation mechanisms and roles in both microbial infections and other disease conditions.

Dot/Icm-Dependent Restriction of Legionella pneumophila within Neutrophils

The Dot/Icm type IV secretion system (T4SS) of Legionella pneumophila is essential for lysosomal evasion and permissiveness of macrophages for intracellular proliferation of the pathogen. In contrast, we show that polymorphonuclear cells (PMNs) respond to a functional Dot/Icm system through rapid restriction of L. pneumophila. Specifically, we show that the L. pneumophila T4SS-injected amylase (LamA) effector catalyzes rapid glycogen degradation in the PMNs cytosol, leading to cytosolic hyperglucose. Neutrophils respond through immunometabolic reprogramming that includes upregulated aerobic glycolysis. The PMNs become activated with spatial generation of intracellular reactive oxygen species within the Legionella-containing phagosome (LCP) and fusion of specific and azurophilic granules to the LCP, leading to rapid restriction of L. pneumophila. We conclude that in contrast to macrophages, PMNs respond to a functional Dot/Icm system, and specifically to the effect of the injected amylase effector, through rapid engagement of major microbicidal processes and rapid restriction of the pathogen. IMPORTANCE Legionella pneumophila is commonly found in aquatic environments and resides within a wide variety of amoebal hosts. Upon aerosol transmission to humans, L. pneumophila invades and replicates with alveolar macrophages, causing pneumonia designated Legionnaires' disease. In addition to alveolar macrophages, neutrophils infiltrate into the lungs of infected patients. Unlike alveolar macrophages, neutrophils restrict and kill L. pneumophila, but the mechanisms were previously unclear. Here, we show that the pathogen secretes an amylase (LamA) enzyme that rapidly breakdowns glycogen stores within neutrophils, and this triggers increased glycolysis. Subsequently, the two major killing mechanisms of neutrophils, granule fusion and production of reactive oxygen species, are activated, resulting in rapid killing of L. pneumophila.

Lymphotoxin β Receptor: a Crucial Role in Innate and Adaptive Immune Responses against Toxoplasma gondii

The lymphotoxin β receptor (LTβR) plays an essential role in the initiation of immune responses to intracellular pathogens. In mice, the LTβR is crucial for surviving acute toxoplasmosis; however, until now, a functional analysis was largely incomplete. Here, we demonstrate that the LTβR is a key regulator required for the intricate balance of adaptive immune responses. Toxoplasma gondii-infected LTβR-deficient (LTβR^−/−) mice show globally altered interferon-γ (IFN-γ) regulation, reduced IFN-γ-controlled host effector molecule expression, impaired T cell functionality, and an absent anti-parasite-specific IgG response, resulting in a severe loss of immune control of the parasites. Reconstitution of LTβR^−/− mice with toxoplasma immune serum significantly prolongs survival following T. gondii infection. Notably, analysis of RNA-seq data clearly indicates a specific effect of T. gondii infection on the B cell response and isotype switching. This study uncovers the decisive role of the LTβR in cytokine regulation and adaptive immune responses to control T. gondii.

Tissue-resident macrophages: guardians of organ homeostasis

Tissue-resident macrophages (M_TR) have recently emerged as a key rheostat capable of regulating the balance between organ health and disease. In most organs, ontogenetically and functionally distinct macrophage subsets fulfill a plethora of functions specific to their tissue environment. In this review, we summarize recent findings regarding the ontogeny and functions of macrophage populations in different mammalian tissues, describing how these cells regulate tissue homeostasis and how they can contribute to inflammation. Furthermore, we highlight new developments concerning certain general principles of tissue macrophage biology, including the importance of metabolism for understanding macrophage activation states and the influence of intrinsic and extrinsic factors on macrophage metabolic control. We also shed light on certain open questions in the field and how answering these might pave the way for tissue-specific therapeutic approaches.

Pathogenicity and Virulence of Legionella: Intracellular replication and host response

Bacteria of the genus Legionella are natural pathogens of amoebae that can cause a severe pneumonia in humans called Legionnaires’ Disease. Human disease results from inhalation of Legionella-contaminated aerosols and subsequent bacterial replication within alveolar macrophages. Legionella pathogenicity in humans has resulted from extensive co-evolution with diverse genera of amoebae. To replicate intracellularly, Legionella generates a replication-permissive compartment called the Legionella-containing vacuole (LCV) through the concerted action of hundreds of Dot/Icm-translocated effector proteins. In this review, we present a collective overview of Legionella pathogenicity including infection mechanisms, secretion systems, and translocated effector function. We also discuss innate and adaptive immune responses to L. pneumophila, the implications of Legionella genome diversity and future avenues for the field.

Anti-inflammatory, pro-proliferative and antimicrobial potential of the compounds isolated from Daemonorops draco (Willd.) Blume

ETHNO-PHARMACOLOGICAL RELEVANCE

Daemonorops draco (D. draco) commonly known as "Dragon's blood" is one of the most used plants by Momok, Anak Dalam and Talang Mamak tribes from Indonesia as a remedy for wound healing.

AIM OF THE STUDY

This study aimed to identify the extract, fractions and compounds responsible for the anti-inflammatory and pro-proliferative activities of the D. draco resin. Additionally, the antimicrobial activity against two bacteria and one yeast species was analysed.

MATERIALS AND METHODS

Bio-guided isolation of compounds with anti-inflammatory, pro-proliferative and antimicrobial activities from the D. draco resin was carried out by measuring: the inhibition of NF-κB and activation of Nrf2 in THP-1, HaCaT, NIH-3T3 cells; cell proliferation in NIH-3T3 and HaCaT cells; and the antimicrobial effect on E. coli, S. aureus and C. albicans.

RESULTS

Guided isolation by bioassay gave rise to the isolation and characterisation by nuclear magnetic resonance and mass spectrometry of three compounds: 1 (Bexarotene), 2 (Taspine) and 3 (2-hydroxy-1-naphthaldehyde isonicotinoyl hydrazone). All compounds showed NF-κB inhibitory activity with IC₅₀ values of 0.10-0.13, 0.22-0.24 and 3.75-4.78 μM, respectively, while the positive control, Celastrol, had an IC₅₀ of 7.96 μM. Likewise, all compounds showed an activating effect of Nrf2 with EC₅₀ values of 5.34-5.43, 163.20-169.20 and 300.82-315.56 nM, respectively, while the positive control, CDDO-Me, had an EC₅₀ of 0.11 nM. In addition, concerning the pro-proliferative activity, compound 1 (IC₅₀ = 8.62-8.71 nM) showed a capacity of 100%, compound 2 (IC₅₀ = 166-171 nM) showed a capacity of 75%, and compound 3 (IC₅₀ = 469-486 nM) showed a capacity of 65%, while FSB 10% (positive control) had a pro-proliferative activity of 100% in the NIH3T3 cell lines (fibroblasts) and HaCaT (keratinocytes). Finally, all the compounds showed antimicrobial activity with MIC values of 0.12-0.16, 0.31-0.39 and 3.96-3.99 μM, respectively, in S. aureus, E. coli and C. albicans strains, while the positive control, Ofloxacin, had a MIC of 27.65 μM.

CONCLUSION

This study managed to isolate, for the first time, three compounds (Bexarotene, Taspine and 2-hydroxy-1-naphthaldehyde isonicotinoyl hydrazone) from the resin of D. draco, with anti-inflammatory, and pro-proliferative as well as antimicrobial activities.

The Role of Innate Immunity in Pulmonary Infections

Innate immunity forms a protective line of defense in the early stages of pulmonary infection. The primary cellular players of the innate immunity against respiratory infections are alveolar macrophages (AMs), dendritic cells (DCs), neutrophils, natural killer (NK) cells, and innate lymphoid cells (ILCs). They recognize conserved structures of microorganisms through membrane-bound and intracellular receptors to initiate appropriate responses. In this review, we focus on the prominent roles of innate immune cells and summarize transmembrane and cytosolic pattern recognition receptor (PRR) signaling recognition mechanisms during pulmonary microbial infections. Understanding the mechanisms of PRR signal recognition during pulmonary pathogen infections will help us to understand pulmonary immunopathology and lay a foundation for the development of effective therapies to treat and/or prevent pulmonary infections.

Hypoxia-Inducible Factor-1: A Potential Target to Treat Acute Lung Injury

Acute lung injury (ALI) is an acute hypoxic respiratory insufficiency caused by various intra- and extrapulmonary injury factors. Presently, excessive inflammation in the lung and the apoptosis of alveolar epithelial cells are considered to be the key factors in the pathogenesis of ALI. Hypoxia-inducible factor-1 (HIF-1) is an oxygen-dependent conversion activator that is closely related to the activity of reactive oxygen species (ROS). HIF-1 has been shown to play an important role in ALI and can be used as a potential therapeutic target for ALI. This manuscript will introduce the progress of HIF-1 in ALI and explore the feasibility of applying inhibitors of HIF-1 to ALI, which brings hope for the treatment of ALI.

Investigating the role of L. pnuemophila LPS derivatives in formation of specific cell-mediated immune responses against the pathogen

Legionella pneumophila is a Gram-negative intracellular bacterium and causes legionnaire's disease an -atypical pneumonia in humans. Lipopolysaccharide (LPS) is the main antigen of Gram-negative bacteria but is less studied because of its carbohydrate nature. Here, we immunized mice with detoxified LPS and O-antigen polysaccharide in combination with bovine serum albumin (BSA) and explored the immunological responses of mice to the bacterial infection. LPS of L. pneumophila was extracted by hot phenol-water method. Purified LPS was detoxified by sodium hydroxide alkaline procedure. O-polysaccharide antigen (OPS) obtained by acetic acid treatment of LPS. BALB/c mice were immunized mainly with non-covalent combination of detoxified LPS (dLPS) or OPS with BSA separately. Pure polysaccharide antigens did not elicit significant serum IgG against LPS. Combination of the dLPS and OPS with BSA resulted in risen IgG and its subclasses (IgG1 and IgG2a) against lipopolysaccharide. Mice were challenged intravenously with sublethal dose of L. pneumpphila. Then, splenocytes were cultured and cytokine responses of splenocytes to pathogenic Legionella was studied by ELISA. Mice immunized with combination of the dLPS or OPS and BSA showed significant elevation of cytokine responses to pathogenic L. pneumophila. Our results suggest that combination of the polysaccharide antigen derived from Legionella LPS may confer raised cell-mediated responses against the pathogen when combined with a protein antigen which is capable of eliciting cell-mediated responses. Although not covalently bond, Legionella polysaccharides combined with BSA effectively elicited Th-1 type cytokines and humoral responses against L. pneumophila in BALB/c mice.

Effects of Endotoxin Tolerance Induced by Porphyromonas gingivalis Lipopolysaccharide on Inflammatory Responses in Neutrophils

Periodontitis is a dental plaque-induced chronic inflammatory disease. Long-term exposure of the host to periodontal pathogens leads to a hyporesponsive state to the following stimulations, which is described as endotoxin tolerance. Neutrophils are the most abundant innate immune cells in the body. To clarify the roles of endotoxin tolerance in periodontitis, inflammatory responses in Porphyromonas gingivalis (P. gingivalis) lipopolysaccharide (LPS)-tolerized neutrophils were explored in this study. Here, apoptosis and respiratory burst in neutrophils upon single or repeated P. gingivalis LPS stimulations were explored by flow cytometry. Cytokine production (TNF-α, IL-8, and IL-10) in tolerized neutrophils or neutrophils co-cultured with peripheral blood mononuclear cells was determined by ELISA. Phagocytosis of P. gingivalis by tolerized neutrophils was also assayed by flow cytometry. In addition, quality and quantitation of neutrophil extracellular trap (NET) formation were detected using immunofluorescence microscope and microplate reader, respectively. The protein expressions of extracellular signal-regulated kinase1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK) were examined to identify possible mechanisms for the abovementioned changes. Tolerance induced by P. gingivalis LPS significantly suppressed apoptosis, reactive oxygen species (ROS) generation, and phagocytosis in neutrophils (p < 0.05). In both neutrophils alone and co-culture system, repeated P. gingivalis LPS stimulations significantly decreased TNF-α production, but increased IL-10 secretion (p < 0.05). Moreover, in tolerized neutrophils, NET formations were strengthened and there were more released extracellular DNA (p < 0.05). In P. gingivalis LPS-tolerized neutrophils, phosphorylation of ERK1/2 was suppressed compared with that in non-tolerized cells. Taken together, immune responses in neutrophils were reprogrammed by P. gingivalis LPS-induced tolerance, which might be related with the development of inflammation in periodontal tissues. Moreover, ERK1/2 might play important roles in endotoxin tolerance triggered by P. gingivalis LPS.

Target the Host, Kill the Bug; Targeting Host Respiratory Immunosuppressive Responses as a Novel Strategy to Improve Bacterial Clearance During Lung Infection

The lung is under constant pressure to protect the body from invading bacteria. An effective inflammatory immune response must be tightly orchestrated to ensure complete clearance of any invading bacteria, while simultaneously ensuring that inflammation is kept under strict control to preserve lung viability. Chronic bacterial lung infections are seen as a major threat to human life with the treatment of these infections becoming more arduous as the prevalence of antibiotic resistance becomes increasingly commonplace. In order to survive within the lung bacteria target the host immune system to prevent eradication. Many bacteria directly target inflammatory cells and cytokines to impair inflammatory responses. However, bacteria also have the capacity to take advantage of and strongly promote anti-inflammatory immune responses in the host lung to inhibit local pro-inflammatory responses that are critical to bacterial elimination. Host cells such as T regulatory cells and myeloid-derived suppressor cells are often enhanced in number and activity during chronic pulmonary infection. By increasing suppressive cell populations and cytokines, bacteria promote a permissive environment suitable for their prolonged survival. This review will explore the anti-inflammatory aspects of the lung immune system that are targeted by bacteria and how bacterial-induced immunosuppression could be inhibited through the use of host-directed therapies to improve treatment options for chronic lung infections.

IFNγ receptor down-regulation facilitates Legionella survival in alveolar macrophages

Legionella pneumophila is an opportunistic human pathogen and causative agent of the acute pneumonia known as Legionnaire's disease. Upon inhalation, the bacteria replicate in alveolar macrophages (AM), within an intracellular vacuole termed the Legionella-containing vacuole. We recently found that, in vivo, IFNγ was required for optimal clearance of intracellular L. pneumophila by monocyte-derived cells (MC), but the cytokine did not appear to influence clearance by AM. Here, we report that during L. pneumophila lung infection, expression of the IFNγ receptor subunit 1 (IFNGR1) is down-regulated in AM and neutrophils, but not MC, offering a possible explanation for why AM are unable to effectively restrict L. pneumophila replication in vivo. To test this, we used mice that constitutively express IFNGR1 in AM and found that prevention of IFNGR1 down-regulation enhanced the ability of AM to restrict L. pneumophila intracellular replication. IFNGR1 down-regulation was independent of the type IV Dot/Icm secretion system of L. pneumophila indicating that bacterial effector proteins were not involved. In contrast to previous work, we found that signaling via type I IFN receptors was not required for IFNGR1 down-regulation in macrophages but rather that MyD88- or Trif- mediated NF-κB activation was required. This work has uncovered an alternative signaling pathway responsible for IFNGR1 down-regulation in macrophages during bacterial infection.

TOLLIP deficiency is associated with increased resistance to Legionella pneumophila pneumonia

Legionella pneumophila (Lp) is a flagellated, intracellular bacterium that can cause Legionnaires' disease (LD). Lp activates multiple innate immune receptors, and TOLLIP dampens MyD88-dependent signaling and may influence susceptibility to LD. We evaluated the effect of TOLLIP on innate immunity, pneumonia severity, and LD susceptibility in mouse lungs and human populations. To accomplish this, we evaluated the effect of TOLLIP on lung-specific Lp control and immune response and associated a common functional TOLLIP variant with Lp-induced innate immune responses and LD susceptibility in humans. After aerosol Lp infection, Tollip^-/- mice demonstrated significantly fewer bacterial colony-forming unit and increased cytokine responses from BAL fluid. Tollip^-/- macrophages also suppressed intracellular Lp replication in a flagellin-independent manner. The presence of a previously characterized, functionally active SNP associated with decreased TOLLIP mRNA transcript in monocytes was associated with increased TNF and IL-6 secretion after Lp stimulation of PBMC ex vivo. This genotype was separately associated with decreased LD susceptibility (309 controls, 88 cases, p = 0.008, OR 0.36, 95% CI 0.16-0.76) in a candidate gene association study. These results suggest that TOLLIP decreases lung-specific TLR responses to increase LD susceptibility in human populations. Better understanding of TOLLIP may lead to novel immunomodulatory therapies.

Viewing Legionella pneumophila Pathogenesis through an Immunological Lens

Legionella pneumophila is the causative agent of the severe pneumonia Legionnaires' disease. L. pneumophila is ubiquitously found in freshwater environments, where it replicates within free-living protozoa. Aerosolization of contaminated water supplies allows the bacteria to be inhaled into the human lung, where L. pneumophila can be phagocytosed by alveolar macrophages and replicate intracellularly. The Dot/Icm type IV secretion system (T4SS) is one of the key virulence factors required for intracellular bacterial replication and subsequent disease. The Dot/Icm apparatus translocates more than 300 effector proteins into the host cell cytosol. These effectors interfere with a variety of cellular processes, thus enabling the bacterium to evade phagosome-lysosome fusion and establish an endoplasmic reticulum-derived Legionella-containing vacuole, which facilitates bacterial replication. In turn, the immune system has evolved numerous strategies to recognize intracellular bacteria such as L. pneumophila, leading to potent inflammatory responses that aid in eliminating infection. This review aims to provide an overview of L. pneumophila pathogenesis in the context of the host immune response.

Potentiation of Cytokine-Mediated Restriction of Legionella Intracellular Replication by a Dot/Icm-Translocated Effector

Legionella pneumophila is ubiquitous in freshwater environments, where it replicates within unicellular protozoa. However, L. pneumophila is also an accidental human pathogen that can cause Legionnaires' disease in immunocompromised individuals by uncontrolled replication within alveolar macrophages. To replicate within eukaryotic phagocytes, L. pneumophila utilizes a Dot/Icm type IV secretion system to translocate a large arsenal of over 300 effector proteins directly into host cells. In mammals, translocated effectors contribute to innate immune restriction of L. pneumophila We found previously that the effector LegC4 is important for L. pneumophila replication within a natural host protist but is deleterious to replication in a mouse model of Legionnaires' disease. In the present study, we used cultured mouse primary macrophages to investigate how LegC4 attenuates L. pneumophila replication. We found that LegC4 enhanced restriction of L. pneumophila replication within macrophages activated with tumor necrosis factor (TNF) or interferon gamma (IFN-γ). In addition, expression of legC4 was sufficient to restrict Legionella longbeachae replication within TNF- or IFN-γ-activated macrophages. Thus, this study demonstrates that LegC4 contributes to L. pneumophila clearance from healthy hosts by potentiating cytokine-mediated host defense mechanisms.IMPORTANCE Legionella spp. are natural pathogens of protozoa and accidental pathogens of humans. Innate immunity in healthy individuals effectively controls Legionella infection due in part to rapid and robust production of proinflammatory cytokines resulting from detection of Dot/Icm-translocated substrates, including effectors. Here, we demonstrate that the effector LegC4 enhances proinflammatory host restriction of Legionella by macrophages. These data suggest that LegC4 may augment proinflammatory signaling or antimicrobial activity of macrophages, a function that has not previously been observed for another bacterial effector. Further insight into LegC4 function will likely reveal novel mechanisms to enhance immunity against pathogens.

Two sequential layers of antibody-mediated control of Legionella pneumophila infection

Protective immunity against intracellular pathogens, including bacteria, usually relies on cellular immunity. However, antibodies are also implicated in mediating protection against intracellular bacteria. In case of airway infection with Legionella pneumophila (Lpn), the causative agent of Legionnaires' disease, pre-existing Lpn-specific antibodies were shown to afford protection within two days of infection. Here we dissected the early kinetics of Ab-mediated protection against airway Lpn infection and observed two kinetically and mechanistically distinct phases of protection by passively administered antibodies. Within the first hour of infection, Lpn-opsonizing antibodies provided almost 10-fold protection in an antibody Fc-dependent, but FcR-independent manner. Later on, by two days post infection, Lpn-specific Ab-mediated protection strictly involved FcγR, Syk kinase activity in alveolar macrophages and induction of reactive oxygen species (ROS). The findings presented here contribute to the understanding of the mechanisms of Ab-mediated control of Lpn infection in actively or passively immunized individuals.

Interleukin 1 alpha (IL-1α) restricts Brucella abortus 544 survival through promoting lysosomal-mediated killing and NO production in macrophages

The interleukin-1 (IL-1) family of cytokines, particularly IL-1α and IL-1β, are potent regulators of innate immunity that play key roles in host defense against infection, hence we evaluated the role of these cytokines in the control of brucellosis within RAW 264.7 cells. Marked expression and secretion of IL-1α and IL-1β were observed during Brucella infection in macrophages. Blocking of IL-1α and IL-1β reduced induction of IL-10, IL-1β and TNF, and IL-6 and TNF, respectively. However, interference of IL-1α and not IL-1β signaling notably augmented susceptibility of macrophages to Brucella infection which indicates that IL-1α is required for a downstream signaling cascade of innate immunity for efficient clearance of Brucella. This protection requires binding to interleukin-1 receptor (IL-1R) mediated by myeloid differentiation factor 88 (MyD88) signaling and associated with increased lysosomal-mediated killing and nitric oxide (NO) production. Expression of pro-inflammatory cytokines was observed to be mediated via NF-κB-p50, HIF-1α and CEBPA, but negatively controlled by CEBPB while transcription of some important phagolysosomal genes was regulated via CEBPA and c-Jun which indicates the important role of these transcription factors in the control of Brucella infection in macrophages via IL-1α signaling pathway.

Differential expression of cytokines in PBMC of Bos indicus and Bos taurus × Bos indicus cattle due to Brucella abortus S19 antigen

Brucellosis is the most dreadful disease among bovines, although breed differences have been observed in prevalence of disease, worldwide. In present study, antibody response and relative expression of proinflammatory cytokines was compared in Bos indicus (zebu) and Bos taurus × Bos indicus (crossbred) cattle vaccinated by live attenuated Brucella abortus S19 antigen. Six female calves (4-6 months age) of both groups were vaccinated with B.abortus S19 strain. Blood samples were collected before vaccination (0d) and 7^th (7d), 14^th (14d) and 28^th (28d) days after vaccination. Indirect ELISA showed high (p < .05) anti-Brucella antibody level after vaccination; with no significant difference between the groups. During Real-time expression, IFNγ, TNFα, IL6 and IL10 genes initially showed down regulation followed by upregulation in both the groups; however, the trend was much prominent in crossbreds. The expressions of IFNγ, TNFα and IL6, proinflammatory molecules important for initial containment of the Brucella were significantly (p < .01) higher in crossbred. The study showed that the Sahiwal cattle were less responsive to B.abortus S19 antigen than crossbreds, indicating its lower sensitivity to the Brucella, comparatively. In contrary, higher expression of the proinflammatory molecules in crossbreds could be important for containment of the organism during initial stage of infection.

The Mouse as a Model for Pulmonary Legionella Infection

Infection of C57BL/6 mice with wild-type Legionella pneumophila typically results in very mild disease. However, in mice where the cytosolic recognition of flagellin is impaired by mutation, L. pneumophila infection results in more severe lung inflammation that is reminiscent of Legionnaires' disease. This can be replicated in wild-type mice by using aflagellated mutants of L. pneumophila. These models greatly facilitate the investigation of L. pneumophila virulence factors and the complex pulmonary immune system that is triggered by infection. Here we describe methods for infecting C57BL/6 mice with aflagellated L. pneumophila, the quantification of bacterial load in the lungs and isolation and analysis of invading immune cells. These assays enable the identification of phagocyte subsets and can determine whether phagocytic cells act as a replicative niche for L. pneumophila replication.

Histopathological Changes Caused by Inflammation and Oxidative Stress in Diet-Induced-Obese Mouse following Experimental Lung Injury

Obesity has been identified as a risk factor for adverse outcomes of various diseases. However, information regarding the difference between the response of obese and normal subjects to pulmonary inflammation is limited. Mice were fed with the control or high-fat diet to establish the lean and diet-induced obese (DIO) mice. Escherichia coli was intranasally instilled to reproduce non-fatal acute pneumonia model. After infection, serum samples and lung tissues were obtained at 0, 12, 24, and 72 h. DIO mice exhibited increased serum triglyceride (TG) and total cholesterol (TC) contents as well as pulmonary resistin, IL-6, and leptin levels compared with lean mice. E. coli infection caused an acute suppurative inflammation in the lung with increased lung index and serum TG and TC contents; elevated pulmonary tumor necrosis factor-α, interleukin (IL)-1β, IL-6, IL-8, and leptin levels; and oxidative stress in mice. Interestingly, almost all the above-mentioned parameters peaked at 12 h after infection in the lean-E. coli group but after 12 h in the DIO-E. coli group. These results indicated that the DIO mice presented a delayed inflammatory response and oxidative stress in non-fatal acute pneumonia induced by E. coli infection.

NLRP3 Inflammasome and Caspase-1/11 Pathway Orchestrate Different Outcomes in the Host Protection Against Trypanosoma cruzi Acute Infection

Infection with protozoan parasite Trypanosoma cruzi results in activation of nucleotide-binding domain and leucine-rich repeat containing receptors (NLRs). NLR activation leads to inflammasome formation, the activation of caspase-1, and the subsequent cleavage of IL-1β and IL-18. Considering that inflammasome activation and IL-1β induction by macrophages are key players for an appropriate T cell response, we investigated the relevance of NLR pyrin domain-containing 3 (NLRP3) and caspase-1/11 to elucidate their roles in the induction of different T cell phenotypes and the relationship with parasite load and hepatic inflammation during T. cruzi-Tulahuen strain acute infection. We demonstrated that infected nlrp3-/- and C57BL/6 wild type (WT) mice exhibited similar parasitemia and survival, although the parasite load was higher in the livers of nlrp3-/- mice than in those of WT mice. Increased levels of transaminases and pro-inflammatory cytokines were found in the plasma of WT and nlrp3-/- mice indicating that NLRP3 is dispensable to control the parasitemia but it is required for a better clearance of parasites in the liver. Importantly, we have found that NLRP3 and caspase-1/11-deficient mice differentially modulate T helper (Th1, Th2, and Th17) and cytotoxic T lymphocyte phenotypes. Strikingly, caspase-1/11-/- mice showed the most dramatic reduction in the number of IFN-γ- and IL-17-producing CD4+ and CD8+ T cells associated with higher parasitemia and lower survival. Additionally, caspase-1/11-/- mice demonstrated significantly reduced liver inflammation with the lowest alanine aminotransferase (ALT) levels but the highest hepatic parasitic load. These results unequivocally demonstrate that caspase-1/11 pathway plays an important role in the induction of liver adaptive immunity against this parasite infection as well as in hepatic inflammation.

Activation of the porcine alveolar macrophages via toll-like receptor 4/NF-κB mediated pathway provides a mechanism of resistin leading to inflammation

Resistin, a previously discovered cysteine-rich adipokine known to regulate glucose metabolism, has been emerged as a mediator in inflammation and immunity. Its level was supposed to be related to the expression of indicators, such as interleukin-1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α) in inflammation. Toll-like receptor 4 (TLR4) was reported to be a receptor for resistin in cells, like leukocytes and peripheral blood mononuclear cells (PBMC). However, the pro-inflammatory role of resistin and its intracellular mechanisms in alveolar macrophages have not been thoroughly validated. Here we found that the pro-inflammatory cytokine expression in porcine alveolar macrophages (PAMs) was positively correlated with resistin. Our results also showed that resistin induced the expression of TLR4, intracellular molecules myeloid differentiation primary response protein 88 (MyD88), TRIF-related adaptor molecule (TRAM) and nuclear factor κB (NF-κB) in PAMs. In contrast, inhibition of TLR4, MyD88, TRAM and NF-κB abrogated the pro-inflammatory effect of resistin on PAMs. Additionally, the associations among TLR4, MyD88/TRAM and NF-κB were investigated by introducing TLR4-siRNA, MyD88-siRNA and TRAM-siRNA respectively into PAMs prior to the treatment with resistin. Taken together, our findings demonstrated that resistin promoted the production of pro-inflammatory cytokine in PAMs via TLR4/NF-κB-mediated pathway (TLR4/MyD88/TRAM/NF-κB).

Interleukin 1α (IL-1α) Promotes Pathogenic Immature Myeloid Cells and IL-1β Favors Protective Mature Myeloid Cells During Acute Lung Infection

Bacterial pneumonia is a common risk factor for acute lung injury and sepsis-mediated death, but the mechanisms underlying the overt inflammation and accompanying pathology are unclear. Infiltration of immature myeloid cells and necrotizing inflammation mediate severe pathology and death during pulmonary infection with Francisella tularensis. However, eliciting mature myeloid cells provides protection. Yet, the host factors responsible for this pathologic immature myeloid cell response are unknown. Here, we report that while the influx of both mature and immature myeloid cells is strictly MyD88 dependent, the interleukin 1 (IL-1) receptor mediates an important dual function via its ligands IL-1α and IL-1β. Although IL-1β favors the appearance of bacteria-clearing mature myeloid cells, IL-1α contributes to lung infiltration by ineffective and pathologic immature myeloid cells. Finally, IL-1α and IL-1β are not the sole factors involved, but myeloid cell responses during acute pneumonia were largely unaffected by lung levels of interleukin 10, interleukin 17, CXCL1, granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor.

PutA Is Required for Virulence and Regulated by PruR in Pseudomonas aeruginosa

Pseudomonas aeruginosa, a Gram-negative opportunistic pathogenic bacterium, causes acute and chronic infections. Upon entering the host, P. aeruginosa alters global gene expression to adapt to host environment and avoid clearance by the host immune system. Proline utilization A (PutA) is a bifunctional enzyme, which converts proline to glutamate. Here we report that PutA was required for the virulence of P. aeruginosa in a murine acute pneumonia model. A putA mutant was more susceptible to oxidative stress compared to the wild type strain. An AraC/XylS family protein, PruR, directly bound to the upstream of −35 box in the putA promoter and activated putA expression. High concentration of proline in bacteria up-regulated pruR expression, which led to the activation of putA expression. As a feedback regulation, glutamate produced by PutA released PruR from the putA promoter and turned off the putA expression. PruR affected bacterial virulence through the regulation of the putA expression. Altogether, these data are the first to reveal that PutA plays an important role in the pathogenesis of P. aeruginosa, as well as to describe the genetic regulation of PutA in P. aeruginosa.