Paradoxical role of alveolar macrophage-derived granulocyte-macrophage colony-stimulating factor in pulmonary host defense post-bone marrow transplantation

Effects of Hematopoietic Cell Transplantation on the Pulmonary Immune Response to Infection

Pulmonary infections are common in hematopoietic cell transplant (HCT) patients of all ages and are associated with high levels of morbidity and mortality. Bacterial, viral, fungal, and parasitic pathogens are all represented as causes of infection. The lung mounts a complex immune response to infection and this response is significantly affected by the pre-HCT conditioning regimen, graft characteristics, and ongoing immunomodulatory therapy. We review the published literature, including animal models as well as human data, to describe what is known about the pulmonary immune response to infection in HCT recipients. Studies have focused on the pulmonary immune response to Aspergillus fumigatus, gram-positive and gram-negative bacteria, and viruses, and show a range of defects associated with both the innate and adaptive immune responses after HCT. There are still many open areas for research, to delineate novel therapeutic targets for pulmonary infections as well as to explore linkages to non-infectious inflammatory lung conditions.

Lipidomic methodologies for biomarkers of chronic inflammation in nutritional research: ω-3 and ω-6 lipid mediators

The evolutionary history of hominins has been characterized by significant dietary changes, which include the introduction of meat eating, cooking, and the changes associated with plant and animal domestication. The Western pattern diet has been linked with the onset of chronic inflammation, and serious health problems including obesity, metabolic syndrome, and cardiovascular diseases. Diets enriched with ω-3 marine PUFAs have revealed additional improvements in health status associated to a reduction of proinflammatory ω-3 and ω-6 lipid mediators. Lipid mediators are produced from enzymatic and non-enzymatic oxidation of PUFAs. Interest in better understanding the occurrence of these metabolites has increased exponentially as a result of the growing evidence of their role on inflammatory processes, control of the immune system, cell signaling, onset of metabolic diseases, or even cancer. The scope of this review has been to highlight the recent findings on: a) the formation of lipid mediators and their role in different inflammatory and metabolic conditions, b) the direct use of lipid mediators as antiinflammatory drugs or the potential of new drugs as a new therapeutic option for the synthesis of antiinflammatory or resolving lipid mediators and c) the impact of nutritional interventions to modulate lipid mediators synthesis towards antiinflammatory conditions. In a second part, we have summarized methodological approaches (Lipidomics) for the accurate analysis of lipid mediators. Although several techniques have been used, most authors preferred the combination of SPE with LC-MS. Advantages and disadvantages of each method are herein addressed, as well as the main LC-MS difficulties and challenges for the establishment of new biomarkers and standardization of experimental designs, and finally to deepen the study of mechanisms involved on the inflammatory response.

Consequences of Hypoxia for the Pulmonary Alveolar Epithelial Cell Innate Immune Response

Pulmonary innate immune responses involve a highly regulated multicellular network to defend the enormous surface area of the lung. Disruption of these responses renders the host susceptible to pneumonia. Alveolar epithelial cells (AEC) are a critical source of innate immune molecules such as GM-CSF, which determine the functional maturation of alveolar macrophages. In many pulmonary diseases, heterogeneous ventilation leads to regional hypoxia in the lung. The effect of hypoxia on AEC innate immune function is unknown. We now report that exposure of primary murine AEC to hypoxia (1% oxygen) for 24 h results in significant suppression of key innate immune molecules, including GM-CSF, CCL2, and IL-6. This exposure did not cause toxicity but did induce stabilization of hypoxia-inducible factor 1α protein (HIF-1α) and shift to glycolytic metabolism. Focusing on GM-CSF, we found that hypoxia greatly decreased the rate of GM-CSF transcription. Hypoxia both decreased NF-κB signaling in AEC and induced chromosomal changes, resulting in decreased accessibility in the GM-CSF proximal promoter of target sequences for NF-κB binding. In mice exposed to hypoxia in vivo (12% oxygen for 2 d), lung GM-CSF protein expression was reduced. In vivo phagocytosis of fluorescent beads by alveolar macrophages was also suppressed, but this effect was reversed by treatment with GM-CSF. These studies suggest that in critically ill patients, local hypoxia may contribute to the susceptibility of poorly ventilated lung units to infection through complementary effects on several pathways, reducing AEC expression of GM-CSF and other key innate immune molecules.

Lipid Mediators in Inflammation

Lipids are potent signaling molecules that regulate a multitude of cellular responses, including cell growth and death and inflammation/infection, via receptor-mediated pathways. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. This diversity arises from their synthesis, which occurs via discrete enzymatic pathways and because they elicit responses via different receptors. This review will collate the bioactive lipid research to date and summarize the major pathways involved in their biosynthesis and role in inflammation. Specifically, lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins, and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins, and maresins) will be discussed herein.

Effect of irradiation/bone marrow transplantation on alveolar epithelial type II cells is aggravated in surfactant protein D deficient mice

Irradiation followed by bone marrow transplantation (BM-Tx) is a frequent therapeutic intervention causing pathology to the lung. Although alveolar epithelial type II (AE2) cells are essential for lung function and are damaged by irradiation, the long-term consequences of irradiation and BM-Tx are not well characterized. In addition, it is unknown whether surfactant protein D (SP-D) influences the response of AE2 cells to the injurious events. Therefore, wildtype (WT) and SP-D^-/- mice were subjected to a myeloablative whole body irradiation dose of 8 Gy and subsequent BM-Tx and compared with age- and sex-matched untreated controls. AE2 cell changes were investigated quantitatively by design-based stereology. Compared with WT, untreated SP-D^-/- mice showed a higher number of larger sized AE2 cells and a greater amount of surfactant-storing lamellar bodies. Irradiation and BM-Tx induced hyperplasia and hypertrophy in WT and SP-D^-/- mice as well as the formation of giant lamellar bodies. The experimentally induced alterations were more severe in the SP-D^-/- than in the WT mice, particularly with respect to the surfactant-storing lamellar bodies which were sometimes extremely enlarged in SP-D^-/- mice. In conclusion, irradiation and BM-Tx have profound long-term effects on AE2 cells and their lamellar bodies. These data may explain some of the clinical pulmonary consequences of this procedure. The data should also be taken into account when BM-Tx is used as an experimental procedure to investigate the impact of bone marrow-derived cells for the phenotype of a specific genotype in the mouse.

Inhibition of Neutrophil Extracellular Trap Formation after Stem Cell Transplant by Prostaglandin E2

RATIONALE

Autologous and allogeneic hematopoietic stem cell transplant (HSCT) patients are susceptible to pulmonary infections, including bacterial pathogens, even after hematopoietic reconstitution. We previously reported that murine bone marrow transplant (BMT) neutrophils overexpress cyclooxygenase-2, overproduce prostaglandin E2 (PGE2), and exhibit defective intracellular bacterial killing. Neutrophil extracellular traps (NETs) are DNA structures that capture and kill extracellular bacteria and other pathogens.

OBJECTIVES

To determine whether NETosis was defective after transplant and if so, whether this was regulated by PGE2 signaling.

METHODS

Neutrophils isolated from mice and humans (both control and HSCT subjects) were analyzed for NETosis in response to various stimuli in the presence or absence of PGE2 signaling modifiers.

MEASUREMENTS AND MAIN RESULTS

NETs were visualized by immunofluorescence or quantified by Sytox Green fluorescence. Treatment of BMT or HSCT neutrophils with phorbol 12-myristate 13-acetate or rapamycin resulted in reduced NET formation relative to control cells. NET formation after BMT was rescued both in vitro and in vivo with cyclooxygenase inhibitors. Additionally, the EP2 receptor antagonist (PF-04418948) or the EP4 antagonist (AE3-208) restored NET formation in neutrophils isolated from BMT mice or HSCT patients. Exogenous PGE2 treatment limited NETosis of neutrophils collected from normal human volunteers and naive mice in an exchange protein activated by cAMP- and protein kinase A-dependent manner.

CONCLUSIONS

Our results suggest blockade of the PGE2-EP2 or EP4 signaling pathway restores NETosis after transplantation. Furthermore, these data provide the first description of a physiologic inhibitor of NETosis.

Transforming growth factor-β induces microRNA-29b to promote murine alveolar macrophage dysfunction after bone marrow transplantation

Hematopoietic stem cell transplantation (HSCT) is complicated by pulmonary infections that manifest posttransplantation. Despite engraftment, susceptibility to infections persists long after reconstitution. Previous work using a murine bone marrow transplant (BMT) model implicated increased cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2) in promoting impaired alveolar macrophage (AM) responses. However, mechanisms driving COX-2 overexpression remained elusive. Previously, transforming growth factor-β (TGF-β) signaling after BMT was shown to promote hypomethylation of the COX-2 gene. Here, we provide mechanistic insight into how this occurs and show that TGF-β induces microRNA (miR)-29b while decreasing DNA methyltransferases (DNMT)1, DNMT3a, and DNMT3b in AMs after BMT. De novo DNMT3a and DNMT3b were decreased upon transient transfection of miR-29b, resulting in decreased methylation of the COX-2 promoter and induction of COX-2. As a consequence, miR-29b-driven upregulation of COX-2 promoted AM dysfunction, and transfection of BMT AMs with a miR-29b inhibitor rescued the bacterial-killing defect. MiR-29b-mediated defects in BMT AMs were dependent on increased levels of PGE2, as miR-29b-transfected AMs treated with a novel E prostanoid receptor 2 antagonist abrogated the impaired bacterial killing. We also demonstrate that patients that have undergone HSCT exhibit increased miR-29b; thus these studies highlight miR-29b in driving defective AM responses and identify this miRNA as a potential therapeutic target.

Defective pulmonary innate immune responses post-stem cell transplantation; review and results from one model system

Infectious pulmonary complications limit the success of hematopoietic stem cell transplantation (HSCT) as a therapy for malignant and non-malignant disorders. Susceptibility to pathogens in both autologous and allogeneic HSCT recipients persists despite successful immune reconstitution. As studying the causal effects of these immune defects in the human population can be limiting, a bone marrow transplant (BMT) mouse model can be used to understand the defect in mounting a productive innate immune response post-transplantation. When syngeneic BMT is performed, this system allows the study of BMT-induced alterations in innate immune cell function that are independent of the confounding effects of immunosuppressive therapy and graft-versus-host disease. Studies from several laboratories, including our own show that pulmonary susceptibility to bacterial infections post-BMT are largely due to alterations in the lung alveolar macrophages. Changes in these cells post-BMT include cytokine and eicosanoid dysregulations, scavenger receptor alterations, changes in micro RNA profiles, and alterations in intracellular signaling molecules that limit bacterial phagocytosis and killing. The changes that occur highlight mechanisms that promote susceptibility to infections commonly afflicting HSCT recipients and provide insight into therapeutic targets that may improve patient outcomes post-HSCT.

Prostaglandin E2-induced changes in alveolar macrophage scavenger receptor profiles differentially alter phagocytosis of Pseudomonas aeruginosa and Staphylococcus aureus post-bone marrow transplant

The effectiveness of hematopoietic stem cell transplantation as a therapy for malignant and nonmalignant conditions is complicated by pulmonary infections. Using our syngeneic bone marrow transplant (BMT) mouse model, BMT mice with a reconstituted hematopoietic system displayed increased susceptibility to Pseudomonas aeruginosa and Staphylococcus aureus. BMT alveolar macrophages (AMs) exhibited a defect in P. aeruginosa phagocytosis, whereas S. aureus uptake was surprisingly enhanced. We hypothesized that the difference in phagocytosis was due to an altered scavenger receptor (SR) profile. Interestingly, MARCO expression was decreased, whereas SR-AI/II was increased. To understand how these dysregulated SR profiles might affect macrophage function, CHO cells were transfected with SR-AI/II, and phagocytosis assays revealed that SR-AI/II was important for S. aureus uptake but not for P. aeruginosa. Conversely, AMs treated in vitro with soluble MARCO exhibited similar defects in P. aeruginosa internalization as did BMT AMs. The 3'-untranslated region of SR-AI contains a putative target region for microRNA-155 (miR-155), and miR-155 expression is decreased post-BMT. Anti-miR-155-transfected AMs exhibited an increase in SR-AI/II expression and S. aureus phagocytosis. Elevated PGE2 has been implicated in driving an impaired innate immune response post-BMT. In vitro treatment of AMs with PGE2 increased SR-AI/II and decreased MARCO and miR-155. Despite a difference in phagocytic ability, BMT AMs harbor a killing defect to both P. aeruginosa and S. aureus. Thus, our data suggest that PGE2-driven alterations in SR and miR-155 expression account for the differential phagocytosis of P. aeruginosa and S. aureus, but impaired killing ultimately confers increased susceptibility to pulmonary infection.

Collaboration between macrophages and vaccine-induced CD4+ T cells confers protection against lethal Pseudomonas aeruginosa pneumonia during neutropenia

The usefulness of vaccine-based strategies to prevent lethal bacterial infection in a host with neutropenia is not well-defined. Here, we show in a neutropenic mouse model that immunity induced by mucosal vaccination with a live-attenuated Pseudomonas aeruginosa vaccine is protective against lethal P. aeruginosa pneumonia caused by both vaccine-homologous and vaccine-heterologous strains, whereas passive immunization confers only vaccine-homologous protection. Cells in the macrophage lineage served as crucial innate cellular effectors in the neutropenic host after active immunization. Vaccine efficacy was CD4(+) T-cell dependent and associated with accumulation of macrophage-lineage cells in the alveolar space after infection, as well as with enhanced P. aeruginosa clearance from the lung. Adaptive CD4(+) T cells produced granulocyte-macrophage colony-stimulating factor (GM-CSF) on restimulation in vitro, and local GM-CSF was critical for vaccine efficacy. Thus, collaboration between the innate and adaptive effectors induced by mucosal vaccination can overcome neutropenia and confer protection against lethal bacterial infection in the profoundly neutropenic host.

COX-2 expression is upregulated by DNA hypomethylation after hematopoietic stem cell transplantation

Hematopoietic stem cell transplant therapy is limited by pulmonary infections. Mice with fully reconstituted hematopoietic compartments, including alveolar macrophages (AMs), after bone marrow transplantation (BMT) have impaired host defense against Gram-negative Pseudomonas aeruginosa. Impaired innate immunity is related to increased production of PGE(2) by AMs. Cyclooxygenase (COX)-2 is the rate-limiting enzyme for synthesis of PGE(2) from arachidonic acid, and COX-2 expression is elevated in AMs post-BMT. We hypothesized that epigenetic mechanisms may be responsible for upregulation of COX-2 in AMs. Using bisulfite sequencing, we observed the 5'-untranslated region and exon 1 of the COX-2 gene is hypomethylated in the AMs of BMT mice compared with control. COX-2 expression was increased in primary AMs and in the AM cell line (MHS) after treatment with 5-aza-2'-deoxycytidine (a methyltransferase inhibitor). Methylation by SssI methyltransferase of a 698-bp region of the COX-2 promoter including the beginning of exon 1 driving a luciferase reporter silenced luciferase expression. Because TGF-β1 is elevated in lungs post-BMT, we tested whether TGF-β1 could promote expression of COX-2 in a hypermethylated COX-2 vector, and observed TGF-β1-induced modest expression of COX-2, suggesting an ability to demethylate the promoter. Finally, BMTs performed with marrow from mice expressing a dominant-negative form of the TGF-βRII on CD11c-expressing cells (which includes AMs) demonstrated improved host defense and AM function. Our findings suggest impaired innate immunity and PGE(2) elevation post-BMT are due to hypomethylation of the COX-2 gene, which is at least partly regulated by TGF-β1.

TLR4-dependent GM-CSF protects against lung injury in Gram-negative bacterial pneumonia

Toll-like receptors (TLRs) are required for protective host defense against bacterial pathogens. However, the role of TLRs in regulating lung injury during Gram-negative bacterial pneumonia has not been thoroughly investigated. In this study, experiments were performed to evaluate the role of TLR4 in pulmonary responses against Klebsiella pneumoniae (Kp). Compared with wild-type (WT) (Balb/c) mice, mice with defective TLR4 signaling (TLR4(lps-d) mice) had substantially higher lung bacterial colony-forming units after intratracheal challenge with Kp, which was associated with considerably greater lung permeability and lung cell death. Reduced expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA and protein was noted in lungs and bronchoalveolar lavage fluid of TLR4 mutant mice postintratracheal Kp compared with WT mice, and primary alveolar epithelial cells (AEC) harvested from TLR4(lps-d) mice produced significantly less GM-CSF in vitro in response to heat-killed Kp compared with WT AEC. TLR4(lps-d) AEC underwent significantly more apoptosis in response to heat-killed Kp in vitro, and treatment with GM-CSF protected these cells from apoptosis in response to Kp. Finally, intratracheal administration of GM-CSF in TLR4(lps-d) mice significantly decreased albumin leak, lung cell apoptosis, and bacteremia in Kp-infected mice. Based on these observations, we conclude that TLR4 plays a protective role on lung epithelium during Gram-negative bacterial pneumonia, an effect that is partially mediated by GM-CSF.

Impaired pulmonary immunity post-bone marrow transplant

Infectious complications are a serious cause of morbidity and mortality following hematopoietic stem cell transplantation (HSCT), and the lung is a particular target organ post-transplant. Our laboratory has used a murine bone marrow transplant model to study alterations in immunity that occur as a result of transplantation. Our studies focus on immune responses that occur following immune cell reconstitution in the absence of immunosuppressive drug therapy or graft-versus-host disease. We have found that impaired clearance of both bacterial and viral pulmonary infections is related to specific alterations in immune cell function and cytokine production. Our data offer insight into mechanisms that contribute to opportunistic infections in HSCT recipients.

PTEN limits alveolar macrophage function against Pseudomonas aeruginosa after bone marrow transplantation

Hematopoietic stem cell transplant patients are susceptible to infection despite cellular reconstitution. In a murine model of syngeneic bone marrow transplantation (BMT), we previously reported that BMT mice have impaired host defense against Pseudomonas aeruginosa pneumonia due to overproduction of (PG)E(2) in lung. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is an effector in the PGE(2) signaling pathway that negatively regulates alveolar macrophage (AM) phagocytosis and bacterial killing. Therefore, examined whether overproduction of PGE(2) after BMT inhibits AM host defense by up-regulating PTEN phosphatase activity. We found that PTEN activity is elevated in BMT AMs in response to increased PGE(2) signaling and that pharmacological inhibition of PTEN activity in BMT AMs fully restores phagocytosis of serum-opsonized P. aeruginosa but only partially restores phagocytosis of nonopsonized P. aeruginosa. In wild-type mice transplanted with myeloid-specific conditional PTEN knockout (PTEN CKO) bone marrow, bacterial clearance is improved after challenge with P. aeruginosa pneumonia. Furthermore, PTEN CKO BMT AMs display improved TNF-α production and enhanced phagocytosis and killing of serum-opsonized P. aeruginosa despite overproduction of PGE(2). However, AM phagocytosis of nonopsonized P. aeruginosa is only partially restored in the absence of PTEN after BMT. This may be related to elevated AM expression of IL-1 receptor-associated kinase (IRAK)-M, a molecule previously identified in the PGE(2) signaling pathway to inhibit AM phagocytosis of nonopsonized bacteria. These data suggest that PGE(2) signaling up-regulates IRAK-M independently of PTEN and that these molecules differentially inhibit opsonized and nonopsonized phagocytosis of P. aeruginosa.

Old and new generation lipid mediators in acute inflammation and resolution

Originally regarded as just membrane constituents and energy storing molecules, lipids are now recognised as potent signalling molecules that regulate a multitude of cellular responses via receptor-mediated pathways, including cell growth and death, and inflammation/infection. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. The diversity of their actions arises because such metabolites are synthesised via discrete enzymatic pathways and because they elicit their response via different receptors. This review will collate the bioactive lipid research to date and summarise the findings in terms of the major pathways involved in their biosynthesis and their role in inflammation and its resolution. It will include lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins and maresins).

A role for IL-1 receptor-associated kinase-M in prostaglandin E2-induced immunosuppression post-bone marrow transplantation

Following immune reconstitution, hematopoietic stem cell transplant patients often display reduced immune function and are especially susceptible to lung infections. In a mouse model of syngeneic bone marrow transplantation (BMT), we previously reported that PGE(2) is overproduced in lungs of BMT mice, significantly impairing host defense against Pseudomonas aeruginosa. This impairment in host defense post-BMT is also marked by diminished alveolar macrophage (AM) phagocytosis, bacterial killing, and production of TNF-alpha and cysteinyl leukotrienes. However, a mechanism by which overproduction of PGE(2) suppresses pulmonary host defense post-BMT is unknown. As IL-1R-associated kinase (IRAK)-M is a known inhibitor of MyD88-dependent IL-1R/TLR signaling and macrophage function, we sought to determine whether IRAK-M is involved in PGE(2)-induced immunosuppression post-BMT. We found that IRAK-M expression is elevated 3.5-fold in BMT AMs relative to control AMs, and this is related to AM overproduction of PGE(2). Furthermore, genetic ablation of IRAK-M in the bone marrow of BMT mice restores host defense against P. aeruginosa. Despite AM overproduction of PGE(2) and elevated E prostanoid 2 receptor expression, AM phagocytosis, killing, and production of cysteinyl leukotrienes and TNF-alpha are restored in the absence of IRAK-M post-BMT. Also, treatment with PGE(2) does not inhibit AM phagocytosis in the absence of IRAK-M. These data suggest that the absence of IRAK-M in the hematopoietic compartment post-BMT enhances pulmonary host defense and mitigates AM sensitivity to the inhibitory effects of PGE(2). Therefore, strategies to limit IRAK-M elevation post-BMT may be efficacious in reducing patient susceptibility to infection.

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.