E-prostanoid 3 receptor deletion improves pulmonary host defense and protects mice from death in severe Streptococcus pneumoniae infection

MicroRNA-155 Modulates Macrophages' Response to Non-Tuberculous Mycobacteria through COX-2/PGE2 Signaling

Non-tuberculous mycobacteria (NTM) have been recognized as a causative agent of various human diseases, including severe infections in immunocompromised patients, such as people living with HIV. The most common species identified is the Mycobacterium avium-intracellulare complex (MAI/MAC), accounting for a majority of infections. Despite abundant information detailing the clinical significance of NTM, little is known about host-pathogen interactions in NTM infection. MicroRNAs (miRs) serve as important post-transcriptional regulators of gene expression. Using a microarray profile, we found that the expression of miR-155 and cyclo-oxygenase 2 (COX-2) is significantly increased in bone-marrow-derived macrophages from mice and human monocyte-derived macrophages from healthy volunteers that are infected with NTM. Antagomir against miR-155 effectively suppressed expression of COX-2 and reduced Prostaglandin E2(PGE2) secretion, suggesting that COX-2/PGE2 expression is dependent on miR-155. Mechanistically, we found that inhibition of NF-κB activity significantly reduced miR-155/COX-2 expression in infected macrophages. Most importantly, blockade of COX-2, E-prostanoid receptors (EP2 and EP4) enhanced killing of MAI in macrophages. These findings provide novel mechanistic insights into the role of miR-155/COX-2/PGE2 signalling and suggest that induction of these pathways enhances survival of mycobacteria in macrophages. Defining host-pathogen interactions can lead to novel immunomodulatory therapies for NTM infections which are difficult to treat.

E-cigarettes and respiratory health: the latest evidence

The E-cigarette market continues to expand at an alarming rate with thousands of flavours available for purchase and continuously evolving devices. Now that it is a multi-billion dollar industry and one without stringent regulation, there is rising concern over the safety of vaping products. Since June 2019, over 2800 cases of E-cigarette-associated acute lung toxicity have been reported in the USA, over 60 of which resulted in death. Many argue that E-cigarettes offer a safer alternative to smoking, but we are evidently far from fully understanding the potential hazards that they pose to respiratory health. Although the risk of an outbreak in the UK has been considered low due to tighter E-cigarette regulations, we cannot fully eliminate the possibility of similar events occurring in the future. With evidence frequently emerging of the harmful effects of E-cigarettes to pulmonary health, there is an urgent need to define the long-term implications of vaping. Studies show that E-cigarette exposure can disrupt pulmonary homeostasis, with reports of gas exchange disturbance, reduced lung function, increased airway inflammation and oxidative stress, downregulation of immunity, and increased risk of respiratory infection. In this review, the latest research on the effect of E-cigarette use on respiratory health will be presented.

Novel Function of Cyclooxygenase-2: Suppressing Mycobacteria by Promoting Autophagy via the Protein Kinase B/Mammalian Target of Rapamycin Pathway

In Mycobacterium tuberculosis-infected macrophages, cyclooxygenase-2 (COX-2) expression considerably increases to defend the body against mycobacteria by regulating adaptive immunity and restoring the mitochondrial inner membrane. Moreover, in cancer cells, COX-2 enhances the autophagy machinery, an important bactericidal mechanism. However, the association between M. tuberculosis-induced COX-2 and autophagy-mediated antimycobacterial response has not been explored. Here, COX-2 expression silencing reduced the autophagy and bactericidal activity against intracellular M. tuberculosis, while COX-2 overexpression reversed the above effects. In addition, enhancement of bactericidal activity was suppressed by inhibiting autophagy in COX-2-overexpressing cells, indicating that COX-2 accelerated mycobacterial elimination by promoting autophagy. Furthermore, the regulatory effects of COX-2 on autophagy were mediated by its catalytic products, which functioned through inhibiting the protein kinase B/mammalian target of rapamycin pathway. Thus, COX-2 contributes to host defense against mycobacterial infection by promoting autophagy, establishing the basis for development of novel therapeutic agents against tuberculosis by targeting COX-2.

Decidual stromal cell-derived PGE2 regulates macrophage responses to microbial threat

PROBLEM

Bacterial chorioamnionitis causes adverse pregnancy outcomes, yet host-microbial interactions are not well characterized within gestational membranes. The decidua, the outermost region of the membranes, is a potential point of entry for bacteria ascending from the vagina to cause chorioamnionitis. We sought to determine whether paracrine communication between decidual stromal cells and macrophages shaped immune responses to microbial sensing.

METHOD OF STUDY

Decidual cell-macrophage interactions were modeled in vitro utilizing decidualized, telomerase-immortalized human endometrial stromal cells (dTHESCs) and phorbol ester-differentiated THP-1 macrophage-like cells. The production of inflammatory mediators in response to LPS was monitored by ELISA for both cell types, while phagocytosis of bacterial pathogens (Escherichia coli and Group B Streptococcus (GBS)) was measured in THP-1 cells or primary human placental macrophages. Diclofenac, a non-selective cyclooxygenase inhibitor, and prostaglandin E₂ (PGE₂ ) were utilized to interrogate prostaglandins as decidual cell-derived paracrine immunomodulators. A mouse model of ascending chorioamnionitis caused by GBS was utilized to assess the colocalization of bacteria and macrophages in vivo and assess PGE₂ production.

RESULTS

In response to LPS, dTHESC and THP-1 coculture demonstrated enhancement of most inflammatory mediators, but a potent suppression of macrophage TNF-α generation was observed. This appeared to reflect a paracrine-mediated effect of decidual cell-derived PGE₂ . In mice with GBS chorioamnionitis, macrophages accumulated at sites of bacterial invasion with increased PGE₂ in amniotic fluid, suggesting such paracrine effects might hold relevance in vivo.

CONCLUSION

These data suggest key roles for decidual stromal cells in modulating tissue responses to microbial threat through release of PGE₂ .

Inflammatory Resolution Triggers a Prolonged Phase of Immune Suppression through COX-1/mPGES-1-Derived Prostaglandin E2

Acute inflammation is characterized by granulocyte infiltration followed by efferocytosing mononuclear phagocytes, which pave the way for inflammatory resolution. Until now, it was believed that resolution then leads back to homeostasis, the physiological state tissues experience before inflammation occurred. However, we discovered that resolution triggered a prolonged phase of immune suppression mediated by prostanoids. Specifically, once inflammation was switched off, natural killer cells, secreting interferon γ (IFNγ), infiltrated the post-inflamed site. IFNγ upregulated microsomal prostaglandin E synthase-1 (mPGES-1) alongside cyclo-oxygenase (COX-1) within macrophage populations, resulting in sustained prostaglandin (PG)E₂ biosynthesis. Whereas PGE₂ suppressed local innate immunity to bacterial infection, it also inhibited lymphocyte function and generated myeloid-derived suppressor cells, the net effect of which was impaired uptake/presentation of exogenous antigens. Therefore, we have defined a sequence of post-resolution events that dampens the propensity to develop autoimmune responses to endogenous antigens at the cost of local tissue infection.

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Inflammatory resolution triggers T/NK cell infiltration, which synthesizes IFNγ

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Through IP-10, IFNγ indirectly triggers monocyte-derived macrophage infiltration

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Macrophages are directly acted upon by IFNγ to make abundant PGE₂

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PGE₂ exerts a phase of post-inflammation immune suppression and tolerance

Identification of key genes in human airway epithelial cells in response to respiratory pathogens using microarray analysis

Background

Airway epithelium is the primary target for pathogens. It functions not only as a mechanical barrier, but also as an important sentinel of the innate immune system. However, the interactions and processes between host airway epithelium and pathogens are not fully understood.

Results

In this study, we identified responses of the human airway epithelium cells to respiratory pathogen infection. We retrieved three mRNA expression microarray datasets from the Gene Expression Omnibus database, and identified 116 differentially expressed genes common to all three datasets. Gene functional annotations were performed using Gene Ontology and pathway analyses. Using protein-protein interaction network analysis and text mining, we identified a subset of genes functioned as a group and associated with infection, inflammation, tissue adhesion, and receptor internalization in infected epithelial cells. These genes were further identified in BESE-2B cells in response to Talaromyces marneffei by Real-Time quantitative PCR (qRT-PCR). In addition, we performed an in silico prediction of microRNA-target interactions and examined our findings.

Conclusions

Using bioinformatics analysis, we identified several genes that may serve as biomarkers for the diagnosis or the surveillance of early respiratory tract infection, and identified additional genes and miRNAs that warrant further fundamental experimental research.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1187-7) contains supplementary material, which is available to authorized users.

Prostaglandin E2 as a Regulator of Immunity to Pathogens

The body is exposed to foreign pathogens every day, but remarkably, most pathogens are effectively cleared by the innate immune system without the need to invoke the adaptive immune response. Key cellular components of the innate immune system include macrophages and neutrophils and the recruitment and function of these cells are tightly regulated by chemokines and cytokines in the tissue space. Innate immune responses are also known to regulate development of adaptive immune responses often via the secretion of various cytokines. In addition to these protein regulators, numerous lipid mediators can also influence innate and adaptive immune functions. In this review, we cover one particular lipid regulator, prostaglandin E2 (PGE2) and describe its synthesis and signaling and what is known about the ability of this lipid to regulate immunity and host defense against viral, fungal and bacterial pathogens.

Ablation of the leptin receptor in myeloid cells impairs pulmonary clearance of Streptococcus pneumoniae and alveolar macrophage bactericidal function

Leptin is a pleiotropic hormone produced by white adipose tissue that regulates appetite and many physiological functions, including the immune response to infection. Genetic leptin deficiency in humans and mice impairs host defenses against respiratory tract infections. Since leptin deficiency is associated with obesity and other metabolic abnormalities, we generated mice that lack the leptin receptor (LepRb) in cells of the myeloid linage (LysM-LepRb-KO) to evaluate its impact in lean metabolically normal mice in a murine model of pneumococcal pneumonia. We observed higher lung and spleen bacterial burdens in LysM-LepRb-KO mice following an intratracheal challenge with Streptococcus pneumoniae. Although numbers of leukocytes recovered from bronchoalveolar lavage fluid did not differ between groups, we did observe higher levels of pulmonary IL-13 and TNFα in LysM-LepRb-KO mice 48 h post infection. Phagocytosis and killing of ingested S. pneumoniae were also impaired in alveolar macrophages (AMs) from LysM-LepRb-KO mice in vitro and were associated with reduced LTB₄ and enhanced PGE₂ synthesis in vitro. Pretreatment of AMs with LTB₄ and the cyclooxygenase inhibitor, indomethacin, restored phagocytosis but not bacterial killing in vitro. These results confirm our previous observations in leptin-deficient ( ob/ob) and fasted mice and demonstrate that decreased leptin action, as opposed to metabolic irregularities associated with obesity or starvation, is responsible for the defective host defense against pneumococcal pneumonia. They also provide novel targets for therapeutic intervention in humans with bacterial pneumonia.

Susceptibility to Aspergillus Infections in Rats with Chronic Obstructive Pulmonary Disease via Deficiency Function of Alveolar Macrophages and Impaired Activation of TLR2

Clinical evidence indicates that patients with severe chronic obstructive pulmonary disease (COPD) are more susceptible to Aspergillus. However, the exact mechanisms underlying this effect are not known. In this study, we used cigarette smoke exposure to generate COPD rat model. colony-forming units (CFU) count assessment and phagocytosis were applied to evaluate the defense function of COPD rats against Aspergillus challenge. ELISA, western blotting, and GST-Rac1 pull-down assays were conducted to determine the expressions of cytokines and TLR2-associated signaling pathway. Our data showed that Aspergillus burdens increased, phagocytosis of Aspergillus as well as the expressions of inflammatory cytokines from alveolar macrophages (AMs) were impaired in COPD rats compared with normal rats. Though TLR2 signaling-related proteins were induced in response to the stimulation of Aspergillus or Pam3csk4 (TLR2 agonist), the activation of TLR2-associated signaling pathway was apparently interfered in rats with COPD, compared to that in normal rats. Taken together, our study demonstrated that COPD caused the deficiency of AMs function and impaired the activation of TLR2/PI3K/Rac 1 signaling pathway, leading to invasion of Aspergillus infection, which also provides a future basis for the infection control in COPD patients.

Human lung fibroblasts produce proresolving peroxisome proliferator-activated receptor-γ ligands in a cyclooxygenase-2-dependent manner

Human lung fibroblasts (HLFs) act as innate immune sentinel cells that amplify the inflammatory response to injurious stimuli. Here, we use targeted lipidomics to explore the hypothesis that HLFs also play an active role in the resolution of inflammation. We detected cyclooxygenase-2 (COX-2)-dependent production of both proinflammatory and proresolving prostaglandins (PGs) in conditioned culture medium from HLFs treated with a proinflammatory stimulus, IL-1β. Among the proresolving PGs in the HLF lipidome were several known ligands for peroxisome proliferator-activated receptor-γ (PPARγ), a transcription factor whose activation in the lung yields potent anti-inflammatory, antifibrotic, and proresolving effects. Next, we used a cell-based luciferase reporter to confirm the ability of HLF supernatants to activate PPARγ, demonstrating, for the first time, that primary HLFs activated with proinflammatory IL-1β or cigarette smoke extract produce functional PPARγ ligands; this phenomenon is temporally regulated, COX-2- and lipocalin-type PGD synthase-dependent, and enhanced by arachidonic acid supplementation. Finally, we used luciferase reporter assays to show that several of the PGs in the lipidome of activated HLFs independently activate PPARγ and/or inhibit NFκB. These results indicate that HLFs, as immune sentinels, regulate both proinflammatory and proresolving responses to injurious stimuli. This novel endogenous resolution pathway represents a new therapeutic target for globally important inflammatory diseases such as chronic obstructive pulmonary disease.

Protective Effect of Galectin-1 during Histoplasma capsulatum Infection Is Associated with Prostaglandin E2 and Nitric Oxide Modulation

Histoplasma capsulatum is a dimorphic fungus that develops a yeast-like morphology in host's tissue, responsible for the pulmonary disease histoplasmosis. The recent increase in the incidence of histoplasmosis in immunocompromised patients highlights the need of understanding immunological controls of fungal infections. Here, we describe our discovery of the role of endogenous galectin-1 (Gal-1) in the immune pathophysiology of experimental histoplasmosis. All infected wild-type (WT) mice survived while only 1/3 of Lgals1^−/− mice genetically deficient in Gal-1 survived 30 days after infection. Although infected Lgals1^−/− mice had increased proinflammatory cytokines, nitric oxide (NO), and elevations in neutrophil pulmonary infiltration, they presented higher fungal load in lungs and spleen. Infected lung and infected macrophages from Lgals1^−/− mice exhibited elevated levels of prostaglandin E₂ (PGE₂, a prostanoid regulator of macrophage activation) and prostaglandin E synthase 2 (Ptgs2) mRNA. Gal-1 did not bind to cell surface of yeast phase of H. capsulatum, in vitro, suggesting that Gal-1 contributed to phagocytes response to infection rather than directly killing the yeast. The data provides the first demonstration of endogenous Gal-1 in the protective immune response against H. capsulatum associated with NO and PGE₂ as an important lipid mediator in the pathogenesis of histoplasmosis.

Increased lethality and defective pulmonary clearance of Streptococcus pneumoniae in microsomal prostaglandin E synthase-1-knockout mice

The production of prostaglandin E2 (PGE2) increases dramatically during pneumococcal pneumonia, and this lipid mediator impairs alveolar macrophage (AM)-mediated innate immune responses. Microsomal prostaglandin E synthase-1 (mPGES-1) is a key enzyme involved in the synthesis of PGE2, and its expression is enhanced during bacterial infections. Genetic deletion of mPGES-1 in mice results in diminished PGE2 production and elevated levels of other prostaglandins after infection. Since PGE2 plays an important immunoregulatory role during bacterial pneumonia we assessed the impact of mPGES-1 deletion in the host defense against pneumococcal pneumonia in vivo and in AMs in vitro. Wild-type (WT) and mPGES-1 knockout (KO) mice were challenged with Streptococcus pneumoniae via the intratracheal route. Compared with WT animals, we observed reduced survival and increased lung and spleen bacterial burdens in mPGES-1 KO mice 24 and 48 h after S. pneumoniae infection. While we found modest differences between WT and mPGES-1 KO mice in pulmonary cytokines, AMs from mPGES-1 KO mice exhibited defective killing of ingested bacteria in vitro that was associated with diminished inducible nitric oxide synthase expression and reduced nitric oxide (NO) synthesis. Treatment of AMs from mPGES-1 KO mice with an NO donor restored bacterial killing in vitro. These results suggest that mPGES-1 plays a critical role in bacterial pneumonia and that genetic ablation of this enzyme results in diminished pulmonary host defense in vivo and in vitro. These results suggest that specific inhibition of PGE2 synthesis by targeting mPGES-1 may weaken host defense against bacterial infections.

Prostaglandin E2 Regulation of Macrophage Innate Immunity

Globally, maternal and fetal health is greatly impacted by extraplacental inflammation. Group B Streptococcus (GBS), a leading cause of chorioamnionitis, is thought to take advantage of the uterine environment during pregnancy in order to cause inflammation and infection. In this study, we demonstrate the metabolic changes of murine macrophages caused by GBS exposure. GBS alone prompted a delayed increase in lactate production, highlighting its ability to redirect macrophage metabolism from aerobic to anaerobic respiration. This production of lactate is thought to aid in the development and propagation of GBS throughout the surrounding tissue. Additionally, this study shows that PGE2 priming was able to exacerbate lactate production, shown by the rapid and substantial lactate increases seen upon GBS exposure. These data provide a novel model to study the role of GBS exposure to macrophages with and without PGE2 priming.

Inflammatory Cyclooxygenase Activity and PGE2 Signaling in Models of Alzheimer's Disease

The inflammatory response is a fundamental driving force in the pathogenesis of Alzheimer’s disease (AD). In the setting of accumulating immunogenic Aß peptide assemblies, microglia, the innate immune cells of the brain, generate a non-resolving immune response and fail to adequately clear accumulating Aß peptides, accelerating neuronal and synaptic injury. Pathological, biomarker, and imaging studies point to a prominent role of the innate immune response in AD development, and the molecular components of this response are beginning to be unraveled. The inflammatory cyclooxygenase-PGE₂ pathway is implicated in pre-clinical development of AD, both in epidemiology of normal aging populations and in transgenic mouse models of Familial AD. The cyclooxygenase-PGE₂ pathway modulates the inflammatory response to accumulating Aß peptides through actions of specific E-prostanoid G-protein coupled receptors.

Adipose Tissue-Derived Mesenchymal Stem Cells Attenuate Pulmonary Infection Caused by Pseudomonas aeruginosa via Inhibiting Overproduction of Prostaglandin E2

RATIONALE

New strategies for treating Pseudomonas aeruginosa pulmonary infection are urgently needed. Adipose tissue-derived mesenchymal stem cells (ASCs) may have a potential therapeutic role in P. aeruginosa-induced pulmonary infection.

METHODS

The therapeutic and mechanistic effects of ASCs on P. aeruginosa pulmonary infection were evaluated in a murine model of P. aeruginosa pneumonia.

RESULTS

ASCs exhibited protective effects against P. aeruginosa pulmonary infection, evidenced by reduced bacterial burdens, inhibition of alveolar neutrophil accumulation, decreased levels of myeloperoxidase, macrophage inflammatory protein-2 and total proteins in broncho-alveolar lavage fluid (BALF), and attenuated severity of lung injury. ASCs had no effects on BALF and serum levels of keratinocyte growth factor or Ang-1. ASCs had no effects on the levels of insulin growth factor 1 (IGF-1) in BALF, but increased IGF-1 levels in serum. ASCs inhibited the overproduction of prostaglandin E2 (PGE2 ) by decreasing the expression of cyclooxygenase-2 (COX2) and enhancing the expression of 15-PGDH. In addition, the addition of exogenous PGE2 with ASCs abolished many of the protective effects of ASCs, and administrating PGE2 alone exacerbated lung infection. By inhibiting production of PGE2 , ASCs improved phagocytosis and the bactericidal properties of macrophages. Furthermore suppressing PGE2 signaling by COX2 inhibition or EP2 inhibition exhibited protective effects against pulmonary infection as well.

CONCLUSIONS

In a murine model of P. aeruginosa pneumonia, ASCs exhibited protective effects by inhibiting production of PGE2 , which subsequently improved phagocytosis and the bactericidal properties of macrophages. ASCs may provide a new strategy for managing pulmonary infection caused by P. aeruginosa.

Aspirin enhances opsonophagocytosis and is associated to a lower risk for Klebsiella pneumoniae invasive syndrome

Background

Klebsiella pneumoniae (KP) expressing hypermucoviscosity phenotype (HV-KP) has abundant capsular polysaccharide (CPS) and is capable of causing invasive syndrome. Sodium salicylate (SAL) reduces the production of CPS. The study was aimed to investigate the relationship between aspirin usage and KP-mediated invasive syndrome and the effect of SAL on HV-KP.

Methods

Patients with community-acquired KP bacteraemia were prospectively enrolled. KP-M1, a serotype-K1 HV-KP clinical isolate, was used in the following experiments: CPS production, HV-KP phenotype, and the effect of SAL on neutrophils phagocytosis. The effect of oral aspirin intake on the leukocyte bactericidal activity was evaluated.

Results

Patients infected by HV-KP and diabetic patients with poor glycemic control were at an increased risk for invasive syndrome (p < 0.01); those who had recent use of aspirin (p = 0.02) were at a lower risk. CPS production was significantly reduced in the presence of SAL. The HV-KP phenotype and resistance to neutrophil phagocytosis were both significantly reduced in the KP-M1 after incubation with SAL (p < 0.01). Aspirin treatment significantly enhanced the killing of KP-M1 by leukocytes (p < 0.01).

Conclusion

Treatment with SAL significantly reduces CPS production in HV-KP, thereby contributing to leukocyte phagocytosis and bactericidal activity against this pathogen.

Polarization of the innate immune response by prostaglandin E2: a puzzle of receptors and signals

Eicosanoids tailor the innate immune response by supporting local inflammation and exhibiting immunomodulatory properties. Prostaglandin (PG) E2 is the most abundant eicosanoid in the inflammatory milieu due to the robust production elicited by pathogen-associated molecular patterns on cells of the innate immune system. The different functions and cell distribution of E prostanoid receptors explain the difficulty encountered thus far to delineate the actual role of PGE2 in the immune response. The biosynthesis of eicosanoids includes as the first step the Ca(2+)- and kinase-dependent activation of the cytosolic phospholipase A2, which releases arachidonic acid from membrane phospholipids, and later events depending on the transcriptional regulation of the enzymes of the cyclooxygenase routes, where PGE2 is the most relevant product. Acting in an autocrine/paracrine manner in macrophages, PGE2 induces a regulatory phenotype including the expression of interleukin (IL)-10, sphingosine kinase 1, and the tumor necrosis factor family molecule LIGHT. PGE2 also stabilizes the suppressive function of myeloid-derived suppressor cells, inhibits the release of IL-12 p70 by macrophages and dendritic cells, and may enhance the production of IL-23. PGE2 is a central component of the inflammasome-dependent induction of the eicosanoid storm that leads to massive loss of intravascular fluid, increases the mortality rate associated with coinfection by Candida ssp. and bacteria, and inhibits fungal phagocytosis. These effects have important consequences for the outcome of infections and the polarization of the immune response into the T helper cell types 2 and 17 and can be a clue to develop pharmacological tools to address infectious, autoimmune, and autoinflammatory diseases.

Pathways mediating resolution of inflammation: when enough is too much

Patients with critical illness, and in particular sepsis, are now recognized to undergo unifying, pathogenic disturbances of immune function. Whilst scientific and therapeutic focus has traditionally been on understanding and modulating the initial pro-inflammatory limb, recent years have witnessed a refocusing on the development and importance of immunosuppressive 'anti-inflammatory' pathways. Several mechanisms are known to drive this phenomenon; however, no overriding conceptual framework justifies them. In this article we review the contribution of pro-resolution pathways to this phenotype, describing the observed immune alterations in terms of either a failure of resolution of inflammation or the persistence of pro-resolution processes causing inappropriate 'injurious resolution'-a novel hypothesis. The dysregulation of key processes in critical illness, including apoptosis of infiltrating neutrophils and their efferocytosis by macrophages, are discussed, along with the emerging role of specialized cell subtypes Gr1(+) CD11b(+) myeloid-derived suppressor cells and CD4(+) CD25(+) FoxP3(+) T-regulatory cells.

Induction of cyclooxygenase-2 signaling by Stomatococcus mucilaginosus highlights the pathogenic potential of an oral commensal

Stomatococcus mucilaginosus is an oral commensal that has been occasionally reported to cause severe infections in immunocompromised patients. There is no information about the pathogenic role of S. mucilaginosus in airway infections. In a cohort of 182 subjects with bronchiectasis, we found that 9% were colonized with S. mucilaginosus in their lower airways by culture growth from bronchoalveolar lavage. To address the pathogenic potential of S.mucilaginosus, we developed a murine model of S. mucilaginosus lung infection. Intratracheal injection of S. mucilaginosus in C57BL/6 mice resulted in a neutrophilic influx with production of proinflammatory cytokines, chemokines, and lipid mediators, mainly PGE₂ with induction of cyclooxygenase-2 (COX-2) in the lungs. Presence of TLR2 was necessary for induction of COX-2 and production of PGE₂ by S. mucilaginosus. TLR2-deficient mice showed an enhanced clearance of S. mucilaginosus compared with wild-type mice. Administration of PGE₂ to TLR2(-/-) mice resulted in impaired clearance of S. mucilaginosus, suggesting a key role for COX-2-induced PGE₂ production in immune response to S. mucilaginosus. Mechanistically, induction of COX-2 in macrophages was dependent on the p38-ERK/MAPK signaling pathway. Furthermore, mice treated with S. mucilaginosus and Pseudomonas aeruginosa showed an increased mortality compared with mice treated with PA103 or S. mucilaginosus alone. Inhibition of COX-2 significantly improved survival in mice infected with PA103 and S. mucilaginosus. These data provide novel insights into the bacteriology and personalized microbiome in patients with bronchiectasis and suggest a pathogenic role for S. mucilaginosus in patients with bronchiectasis.

PGE(2) suppression of innate immunity during mucosal bacterial infection

Prostaglandin E2 (PGE2) is an important lipid mediator in inflammatory and immune responses during acute and chronic infections. Upon stimulation by various proinflammatory stimuli such as lipopolysaccharide (LPS), interleukin (IL)-1β, and tumor necrosis factor (TNF)-α, PGE2 synthesis is upregulated by the expression of cyclooxygenases. Biologically active PGE2 is then able to signal through four primary receptors to elicit a response. PGE2 is a critical molecule that regulates the activation, maturation, migration, and cytokine secretion of several immune cells, particularly those involved in innate immunity such as macrophages, neutrophils, natural killer cells, and dendritic cells. Both Gram-negative and Gram-positive bacteria can induce PGE2 synthesis to regulate immune responses during bacterial pathogenesis. This review will focus on PGE2 in innate immunity and how bacterial pathogens influence PGE2 production during enteric and pulmonary infections. The conserved ability of many bacterial pathogens to promote PGE2 responses during infection suggests a common signaling mechanism to deter protective pro-inflammatory immune responses. Inhibition of PGE2 production and signaling during infection may represent a therapeutic alternative to treat bacterial infections. Further study of the immunosuppressive effects of PGE2 on innate immunity will lead to a better understanding of potential therapeutic targets within the PGE2 pathway.

Intrauterine group A streptococcal infections are exacerbated by prostaglandin E2

Streptococcus pyogenes (Group A Streptococcus; GAS) is a major cause of severe postpartum sepsis, a re-emerging cause of maternal morbidity and mortality worldwide. Immunological alterations occur during pregnancy to promote maternofetal tolerance, which may increase the risk for puerperal infection. PGE2 is an immunomodulatory lipid that regulates maternofetal tolerance, parturition, and innate immunity. The extent to which PGE2 regulates host immune responses to GAS infections in the context of endometritis is unknown. To address this, both an in vivo mouse intrauterine (i.u.) GAS infection model and an in vitro human macrophage-GAS interaction model were used. In C57BL/6 mice, i.u. GAS inoculation resulted in local and systemic inflammatory responses and triggered extensive changes in the expression of eicosanoid pathway genes. The i.u. administration of PGE2 increased the mortality of infected mice, suppressed local IL-6 and IL-17A levels, enhanced neutrophilic inflammation, reduced uterine macrophage populations, and increased bacterial dissemination. A role for endogenous PGE2 in the modulation of antistreptococcal host defense was suggested, because mice lacking the genes encoding the microsomal PGE2 synthase-1 or the EP2 receptor were protected from death, as were mice treated with the EP4 receptor antagonist, GW627368X. PGE2 also regulated GAS-macrophage interactions. In GAS-infected human THP-1 (macrophage-like) cells, PGE2 inhibited the production of MCP-1 and TNF-α while augmenting IL-10 expression. PGE2 also impaired the phagocytic ability of human placental macrophages, THP-1 cells, and mouse peritoneal macrophages in vitro. Exploring the targeted disruption of PGE2 synthesis and signaling to optimize existing antimicrobial therapies against GAS may be warranted.

Prospective diagnostic analysis of copy number variants using SNP microarrays in individuals with autism spectrum disorders

Copy number variants (CNVs) have repeatedly been found to cause or predispose to autism spectrum disorders (ASDs). For diagnostic purposes, we screened 194 individuals with ASDs for CNVs using Illumina SNP arrays. In several probands, we also analyzed candidate genes located in inherited deletions to unmask autosomal recessive variants. Three CNVs, a de novo triplication of chromosome 15q11-q12 of paternal origin, a deletion on chromosome 9p24 and a de novo 3q29 deletion, were identified as the cause of the disorder in one individual each. An autosomal recessive cause was considered possible in two patients: a homozygous 1p31.1 deletion encompassing PTGER3 and a deletion of the entire DOCK10 gene associated with a rare hemizygous missense variant. We also identified multiple private or recurrent CNVs, the majority of which were inherited from asymptomatic parents. Although highly penetrant CNVs or variants inherited in an autosomal recessive manner were detected in rare cases, our results mainly support the hypothesis that most CNVs contribute to ASDs in association with other CNVs or point variants located elsewhere in the genome. Identification of these genetic interactions in individuals with ASDs constitutes a formidable challenge.

Leukotriene B4 enhances innate immune defense against the puerperal sepsis agent Streptococcus pyogenes

Puerperal sepsis is a leading cause of maternal mortality worldwide. Streptococcus pyogenes [group A Streptococcus; (GAS)] is a major etiologic agent of severe postpartum sepsis, yet little is known regarding the pathogenesis of these infections. Tissue macrophages provide innate defense against GAS, and their actions are highly regulated. The intracellular second messenger cAMP can negatively regulate macrophage actions against GAS. Because leukotriene (LT) B(4) has been shown to suppress intracellular cAMP in macrophages, we hypothesized that it could enhance innate defenses against GAS. We assessed the capacity of LTB(4) to modulate antistreptococcal actions of human macrophages, including placental and decidual macrophages and used a novel intrauterine infection model of GAS in mice lacking the 5-lipoxygenase enzyme to determine the role of endogenous LTs in host defense against this pathogen. Animals lacking 5-lipoxygenase were significantly more vulnerable to intrauterine GAS infection than were wild-type mice and showed enhanced dissemination of bacteria out of the uterus and a more robust inflammatory response than did wild-type mice. In addition, LTB(4) reduced intracellular cAMP levels via the BLT1 receptor and was a potent stimulant of macrophage phagocytosis and NADPH oxidase-dependent intracellular killing of GAS. Importantly, interference was observed between the macrophage immunomodulatory actions of LTB(4) and the cAMP-inducing lipid PGE(2), suggesting that interplay between pro- and anti-inflammatory compounds may be important in vivo. This work underscores the potential for pharmacological targeting of lipid mediator signaling cascades in the treatment of invasive GAS infections.

Dying to live: how the death modality of the infected macrophage affects immunity to tuberculosis

Virulent Mycobacterium tuberculosis (Mtb) inhibits apoptosis and triggers necrosis of host macrophages to evade innate delay in the initiation of adaptive immunity. Necrosis is a mechanism used by bacteria to exit macrophage, evade the host defenses, and disseminate while apoptosis is associated with diminished pathogen viability. We have recently demonstrated that eicosanoids regulate cell death program of either human or murine macrophages infected with Mtb. We have defined prostaglandin E2 (PGE2) as a pro-apoptotic host lipid mediator which protects against necrosis. In contrast, lipoxin A4 (LXA4) is a pro-necrotic lipid mediator which suppresses PGE2 synthesis, resulting in mitochondrial damage and inhibition of plasma membrane repair mechanisms; this ultimately leads to the induction of necrosis. Thus, the balance between PGE2 and LXA4 determines whether Mtb-infected macrophages undergo apoptosis or necrosis and this balance determines the outcome of infection.

The resolution of inflammation

In 2012, Nature Reviews Immunology organized a conference that brought together scientists and clinicians from both academia and industry to discuss one of the most pressing questions in medicine--how do we turn off rampant, undesirable inflammation? There is a growing appreciation that, similarly to the initiation of inflammation, the resolution of inflammation is an intricate and active process. Can we therefore harness the mediators involved in resolution responses to treat patients with chronic inflammatory or autoimmune diseases? Here, we ask five of the speakers from the conference to share their thoughts on this emerging field.

Cholera toxin induces a shift from inactive to active cyclooxygenase 2 in alveolar macrophages activated by Mycobacterium bovis BCG

Intranasal vaccination stimulates formation of cyclooxygenases (COX) and release of prostaglandin E(2) (PGE(2)) by lung cells, including alveolar macrophages. PGE(2) plays complex pro- or anti-inflammatory roles in facilitating mucosal immune responses, but the relative contributions of COX-1 and COX-2 remain unclear. Previously, we found that Mycobacterium bovis BCG, a human tuberculosis vaccine, stimulated increased release of PGE(2) by macrophages activated in vitro; in contrast, intranasal BCG activated no PGE(2) release in the lungs, because COX-1 and COX-2 in alveolar macrophages were subcellularly dissociated from the nuclear envelope (NE) and catalytically inactive. This study tested the hypothesis that intranasal administration of BCG with cholera toxin (CT), a mucosal vaccine component, would shift the inactive, NE-dissociated COX-1/COX-2 to active, NE-associated forms. The results showed increased PGE(2) release in the lungs and NE-associated COX-2 in the majority of COX-2(+) macrophages. These COX-2(+) macrophages were the primary source of PGE(2) release in the lungs, since there was only slight enhancement of NE-associated COX-1 and there was no change in COX-1/COX-2 levels in alveolar epithelial cells following treatment with CT and/or BCG. To further understand the effect of CT, we investigated the timing of BCG versus CT administration for in vivo and in vitro macrophage activations. When CT followed BCG treatment, macrophages in vitro had elevated COX-2-mediated PGE(2) release, but macrophages in vivo exhibited less activation of NE-associated COX-2. Our results indicate that inclusion of CT in the intranasal BCG vaccination enhances COX-2-mediated PGE(2) release by alveolar macrophages and further suggest that the effect of CT in vivo is mediated by other lung cells.

Prostaglandin E2 and the suppression of phagocyte innate immune responses in different organs

The local and systemic production of prostaglandin E₂ (PGE₂) and its actions in phagocytes lead to immunosuppressive conditions. PGE₂ is produced at high levels during inflammation, and its suppressive effects are caused by the ligation of the E prostanoid receptors EP₂ and EP₄, which results in the production of cyclic AMP. However, PGE₂ also exhibits immunostimulatory properties due to binding to EP₃, which results in decreased cAMP levels. The various guanine nucleotide-binding proteins (G proteins) that are coupled to the different EP receptors account for the pleiotropic roles of PGE₂ in different disease states. Here, we discuss the production of PGE₂ and the actions of this prostanoid in phagocytes from different tissues, the relative contribution of PGE₂ to the modulation of innate immune responses, and the novel therapeutic opportunities that can be used to control inflammatory responses.

Cyclooxygenase inhibition in sepsis: is there life after death?

Prostaglandins are important mediators and modulators of the inflammatory response to infection. The prostaglandins participate in the pathogenesis of hemodynamic collapse, organ failure, and overwhelming inflammation that characterize severe sepsis and shock. In light of this, cyclooxygenase (COX) inhibiting pharmacological agents have been extensively studied for their capacity to ameliorate the aberrant physiological and immune responses during severe sepsis. Animal models of sepsis, using the systemic administration of pathogen-associated molecular patterns (PAMPs) or live pathogens, have been used to examine the effectiveness of COX inhibition as a treatment for severe sepsis. These studies have largely shown beneficial effects on mortality. However, human studies have failed to show clinical utility of COX inhibitor treatment in severely septic patients. Why this approach “worked” in animals but not in humans might reflect differences in the controlled nature of animal investigations compared to human studies. This paper contrasts the impact of COX inhibitors on mortality in animal models of sepsis and human studies of sepsis and examines potential reasons for differences between these two settings.

E-prostanoid 2 receptor signaling suppresses lung innate immunity against Streptococcus pneumoniae

Pneumonia is a major global health problem. Prostaglandin (PG) E(2) is an immunomodulatory lipid with anti-inflammatory, immunosuppressive, and pro-resolving actions. Data suggest that the E-prostanoid (EP) 2 receptor mediates immunomodulatory effects of PGE(2), but the extent to which this occurs in Streptococcus pneumoniae infection is unknown. Intratracheal lung infection of C57BL/6 mice possessing (EP2(+/+)) or lacking (EP2(-/-)) the EP2 receptor was performed, as were in vitro studies of alveolar macrophage (AM) host defense functions. Bacterial clearance and survival were significantly improved in vivo in EP2(-/-) mice and it correlated with greater neutrophilic inflammation and higher lung IL-12 levels. Upon ex vivo challenge with pneumococcus, EP2(-/-)cells expressed greater amounts of TNF-α and MIP-2 than did EP2(+/+) AMs, and had improved phagocytosis, intracellular killing, and reactive oxygen intermediate generation. These data suggest that PGE(2)-EP2 signaling may provide a novel pharmacological target for treating pneumococcal pneumonia in combination with antimicrobials.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Distinct protein kinase A anchoring proteins direct prostaglandin E2 modulation of Toll-like receptor signaling in alveolar macrophages

Toll-like receptors (TLRs) direct a proinflammatory program in macrophages. One mediator whose generation is induced by TLR ligation is prostaglandin E(2) (PGE(2)), which is well known to increase intracellular cAMP upon G protein-coupled receptor ligation. How PGE(2)/cAMP shapes the nascent TLR response and the mechanisms by which it acts remain poorly understood. Here we explored PGE(2)/cAMP regulation of NO production in primary rat alveolar macrophages stimulated with the TLR4 ligand LPS. Endogenous PGE(2) synthesis accounted for nearly half of the increment in NO production in response to LPS. The enhancing effect of PGE(2) on LPS-stimulated NO was mediated via cAMP, generated mainly upon ligation of the E prostanoid 2 receptor and acting via protein kinase A (PKA) rather than via the exchange protein activated by cAMP. Isoenzyme-selective cAMP agonists and peptide disruptors of protein kinase A anchoring proteins (AKAPs) implicated PKA regulatory subunit type I (RI) interacting with an AKAP in this process. Gene knockdown of potential RI-interacting AKAPs expressed in alveolar macrophages revealed that AKAP10 was required for PGE(2) potentiation of LPS-induced NO synthesis. AKAP10 also mediated PGE(2) potentiation of the expression of cytokines IL-10 and IL-6, whereas PGE(2) suppression of TNF-α was mediated by AKAP8-anchored PKA-RII. Our data illustrate the pleiotropic manner in which G protein-coupled receptor-derived cAMP signaling can influence TLR responses in primary macrophages and suggest that AKAP10 may coordinate increases in gene expression.

Disruption of leptin receptor-STAT3 signaling enhances leukotriene production and pulmonary host defense against pneumococcal pneumonia

The adipocyte-derived hormone leptin regulates energy homeostasis and the innate immune response. We previously reported that leptin plays a protective role in bacterial pneumonia, but the mechanisms by which leptin regulates host defense remain poorly understood. Leptin binding to its receptor, LepRb, activates multiple intracellular signaling pathways, including ERK1/2, STAT5, and STAT3. In this study, we compared the responses of wild-type and s/s mice, which possess a mutant LepRb that prevents leptin-induced STAT3 activation, to determine the role of this signaling pathway in pneumococcal pneumonia. Compared with wild-type animals, s/s mice exhibited greater survival and enhanced pulmonary bacterial clearance after an intratracheal challenge with Streptococcus pneumoniae. We also observed enhanced phagocytosis and killing of S. pneumoniae in vitro in alveolar macrophages (AMs) obtained from s/s mice. Notably, the improved host defense and AM antibacterial effector functions in s/s mice were associated with increased cysteinyl-leukotriene production in vivo and in AMs in vitro. Augmentation of phagocytosis in AMs from s/s mice could be blocked using a pharmacologic cysteinyl-leukotriene receptor antagonist. Phosphorylation of ERK1/2 and cytosolic phospholipase A(2) α, known to enhance the release of arachidonic acid for subsequent conversion to leukotrienes, was also increased in AMs from s/s mice stimulated with S. pneumoniae in vitro. These data indicate that ablation of LepRb-mediated STAT3 signaling and the associated augmentation of ERK1/2, cytosolic phospholipase A(2) α, and cysteinyl-leukotriene synthesis confers resistance to s/s mice during pneumococcal pneumonia. These data provide novel insights into the intracellular signaling events by which leptin contributes to host defense against bacterial pneumonia.

Evasion of innate immunity by Mycobacterium tuberculosis: is death an exit strategy?

Virulent Mycobacterium tuberculosis inhibits apoptosis and triggers necrosis of host macrophages to evade innate immunity and delay the initiation of adaptive immunity. By contrast, attenuated M. tuberculosis induces macrophage apoptosis, an innate defence mechanism that reduces bacterial viability. In this Opinion article, we describe how virulent M. tuberculosis blocks production of the eicosanoid lipid mediator prostaglandin E(2) (PGE(2)). PGE(2) production by infected macrophages prevents mitochondrial damage and initiates plasma membrane repair, two processes that are crucial for preventing necrosis and inducing apoptosis. Thus, M. tuberculosis-mediated modulation of eicosanoid production determines the death modality of the infected macrophage, which in turn has a substantial impact on the outcome of infection.

Inducible cyclooxygenase released prostaglandin E2 modulates the severity of infection caused by Streptococcus pyogenes

Streptococcus pyogenes is a significant human pathogen that can cause life-threatening invasive infections. Understanding the mechanism of disease is crucial to the development of more effective therapies. In this report, we explored the role of PGE(2), an arachidonic acid metabolite, and its rate-limiting enzyme cyclooxygenase 2 (COX-2) in the pathogenesis of severe S. pyogenes infections. We found that the COX-2 expression levels in tissue biopsies from S. pyogenes-infected patients, as well as in tissue of experimentally infected mice, strongly correlated with the severity of infection. This harmful effect was attributed to PGE(2)-mediated suppression of the bactericidial activity of macrophages through interaction with the G2-coupled E prostanoid receptor. The suppressive effect of PGE(2) was associated with enhanced intracellular cAMP production and was mimicked by the cAMP-elevating agent, forskolin. Activation of protein kinase A (PKA) was the downstream effector mechanisms of cAMP because treatment with PKI(14-22), a highly specific inhibitor of PKA, prevented the PGE(2)-mediated inhibition of S. pyogenes killing in macrophages. The inhibitory effect exerted by PKA in the generation of antimicrobial oxygen radical species seems to be the ultimate effector mechanism responsible for the PGE(2)-mediated downregulation of the macrophage bactericidal activity. Importantly, either genetic ablation of COX-2, pharmacological inhibition of COX-2 or treatment with the G2-coupled E prostanoid antagonist, AH6809, significantly improved the disease outcome in S. pyogenes infected mice. Therefore, the results of this study open up new perspectives on potential molecular pathways that are prone to pharmacological manipulation during severe streptococcal infections.

Priming innate immune responses to infection by cyclooxygenase inhibition kills antibiotic-susceptible and -resistant bacteria

Inhibition of cyclooxygenase (COX)-derived prostaglandins (PGs) by nonsteroidal anti-inflammatory drugs (NSAIDs) mediates leukocyte killing of bacteria. However, the relative contribution of COX1 versus COX2 to this process, as well as the mechanisms controlling it in mouse and humans, are unknown. Indeed, the potential of NSAIDs to facilitate leukocyte killing of drug-resistant bacteria warrants investigation. Therefore, we carried out a series of experiments in mice and humans, finding that COX1 is the predominant isoform active in PG synthesis during infection and that its prophylactic or therapeutic inhibition primes leukocytes to kill bacteria by increasing phagocytic uptake and reactive oxygen intermediate-mediated killing in a cyclic adenosine monophosphate (cAMP)-dependent manner. Moreover, NSAIDs enhance bacterial killing in humans, exerting an additive effect when used in combination with antibiotics. Finally, NSAIDs, through the inhibition of COX prime the innate immune system to mediate bacterial clearance of penicillin-resistant Streptococcus pneumoniae serotype 19A, a well-recognized vaccine escape serotype of particular concern given its increasing prevalence and multi-antibiotic resistance. Therefore, these data underline the importance of lipid mediators in host responses to infection and the potential of inhibitors of PG signaling pathways as adjunctive therapies, particularly in the con-text of antibiotic resistance.

Intrapulmonary administration of leukotriene B4 enhances pulmonary host defense against pneumococcal pneumonia

Leukotriene B(4) (LTB(4)) is a potent lipid mediator of inflammation formed by the 5-lipoxygenase (5-LO)-catalyzed oxidation of arachidonic acid. We have previously shown that (i) LTB(4) is generated during infection, (ii) its biosynthesis is essential for optimal antimicrobial host defense, (iii) LT deficiency is associated with clinical states of immunocompromise, and (iv) exogenous LTB(4) augments antimicrobial functions in phagocytes. Here, we sought to determine whether the administration of LTB(4) has therapeutic potential in a mouse model of pneumonia. Wild-type and 5-LO knockout mice were challenged with Streptococcus pneumoniae via the intranasal route, and bacterial burdens, leukocyte counts, and cytokine levels were determined. LTB(4) was administered via the intraperitoneal, intravenous, and intranasal routes prior to pneumococcal infection and by aerosol 24 h following infection. Leukocytes recovered from mice given S. pneumoniae and treated with aerosolized LTB(4) were evaluated for expression levels of the p47phox subunit of NADPH oxidase. Intrapulmonary but not systemic pretreatment with LTB(4) significantly reduced the lung S. pneumoniae burden in wild-type mice. Aerosolized LTB(4) was effective at improving lung bacterial clearance when administered postinoculation in animals with established infection and exhibited greater potency in 5-LO knockout animals, which also exhibited greater baseline susceptibility. Augmented bacterial clearance in response to LTB(4) was associated with enhanced monocyte recruitment and leukocyte expression of p47phox. The results of the current study in an animal model serve as a proof of concept for the potential utility of treatment with aerosolized LTB(4) as an immunostimulatory strategy in patients with bacterial pneumonia.

Cigarette smoke exposure impairs pulmonary bacterial clearance and alveolar macrophage complement-mediated phagocytosis of Streptococcus pneumoniae

Cigarette smoke exposure increases the risk of pulmonary and invasive infections caused by Streptococcus pneumoniae, the most commonly isolated organism from patients with community-acquired pneumonia. Despite this association, the mechanisms by which cigarette smoke exposure diminishes host defense against S. pneumoniae infections are poorly understood. In this study, we compared the responses of BALB/c mice following an intratracheal challenge with S. pneumoniae after 5 weeks of exposure to room air or cigarette smoke in a whole-body exposure chamber in vivo and the effects of cigarette smoke on alveolar macrophage phagocytosis of S. pneumoniae in vitro. Bacterial burdens in cigarette smoke-exposed mice were increased at 24 and 48 h postinfection, and this was accompanied by a more pronounced clinical appearance of illness, hypothermia, and increased lung homogenate cytokines interleukin-1beta (IL-1beta), IL-6, IL-10, and tumor necrosis factor alpha (TNF-alpha). We also found greater numbers of neutrophils in bronchoalveolar lavage fluid recovered from cigarette smoke-exposed mice following a challenge with heat-killed S. pneumoniae. Interestingly, overnight culture of alveolar macrophages with 1% cigarette smoke extract, a level that did not affect alveolar macrophage viability, reduced complement-mediated phagocytosis of S. pneumoniae, while the ingestion of unopsonized bacteria or IgG-coated microspheres was not affected. This murine model provides robust additional support to the hypothesis that cigarette smoke exposure increases the risk of pneumococcal pneumonia and defines a novel cellular mechanism to help explain this immunosuppressive effect.