A phosphatidylserine species inhibits a range of TLR- but not IL-1beta-induced inflammatory responses by disruption of membrane microdomains

Compartmentalized regulation of lipid signaling in oxidative stress and inflammation: Plasmalogens, oxidized lipids and ferroptosis as new paradigms of bioactive lipid research

Perturbations in lipid homeostasis combined with conditions favoring oxidative stress constitute a hallmark of the inflammatory response. In this review we focus on the most recent results concerning lipid signaling in various oxidative stress-mediated responses and inflammation. These include phagocytosis and ferroptosis. The best characterized event, common to these responses, is the synthesis of oxygenated metabolites of arachidonic acid and other polyunsaturated fatty acids. Major developments in this area have highlighted the importance of compartmentalization of the enzymes and lipid substrates in shaping the appropriate response. In parallel, other relevant lipid metabolic pathways are also activated and, until recently, there has been a general lack of knowledge on the enzyme regulation and molecular mechanisms operating in these pathways. Specifically, data accumulated in recent years on the regulation and biological significance of plasmalogens and oxidized phospholipids have expanded our knowledge on the involvement of lipid metabolism in the progression of disease and the return to homeostasis. These recent major developments have helped to establish the concept of membrane phospholipids as cellular repositories for the compartmentalized production of bioactive lipids involved in cellular regulation. Importantly, an enzyme classically described as being involved in regulating the homeostatic turnover of phospholipids, namely the group VIA Ca²⁺-independent phospholipase A₂ (iPLA₂β), has taken center stage in oxidative stress and inflammation research owing to its key involvement in regulating metabolic and ferroptotic signals arising from membrane phospholipids. Understanding the role of iPLA₂β in ferroptosis and metabolism not only broadens our knowledge of disease but also opens possible new horizons for this enzyme as a target for therapeutic intervention.

In Vitro Effects of 5-Lipoxygenase Pathway Inhibition on Rhinovirus-Associated Bronchial Epithelial Inflammation

Introduction

The leukotriene pathway may be implicated in the induction of virus-induced inflammation. Respiratory epithelial cells may express low levels of 5-lipoxygenase (5-LO) and release leukotrienes (LTs) C4, D4, and E4, upon exposure to viruses or other stimuli. Enhanced expression of 5-LO pathway proteins after rhinovirus (RV) infection has previously been described. We hypothesized that anti-leukotriene treatment of epithelial cells, with or without exposure to RV-infected peripheral blood mononuclear cells (PBMCs)-conditioned media, may inhibit RV-induced up-regulation of inflammatory cytokines.

Methods

PBMCs from a healthy donor were exposed to RV1B and supernatants were harvested at 48 h post infection. BEAS-2B cells were infected with RV, with or without conditioning with the PBMC supernatant. Treatment with anti-LT agents was performed either on both PBMCs and BEAS-2B or at the bronchial epithelial level only, with varying concentrations of montelukast (CysLT receptor antagonist) or MK-886 [FLAP(5-lipoxygenase-activating-protein) inhibitor]. Evaluation of the inflammatory cytokines IL-8, RANTES, IL-11, IL-6, and IP-10 was performed using ELISA.

Results

Our results show that anti-LT treatment of RV-infected bronchial epithelial cells suppresses epithelial RV-mediated cytokine production, independent of conditioning.

Conclusions

This observation may represent an indirect mode of action of the anti-leukotrienes in virus-induced asthma.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41030-021-00152-x.

Inhibition of dengue virus infection by 1-stearoyl-2-arachidonoyl-phosphatidylinositol in vitro

Dengue fever is an acute febrile infectious disease caused by dengue virus (DENV). Despite the significant public health concerns posed by DENV, there are currently no effective anti-DENV therapeutic agents. To develop such drugs, a better understanding of the detailed mechanisms of DENV infection is needed. Both lipid metabolism and lipid synthesis are activated in DENV-infected cells, so we used lipid screening to identify potential antiviral lipid molecules. We identified 1-stearoyl-2-arachidonoyl-phosphatidylinositol (SAPI), which is the most abundant endogenous phosphatidylinositol (PI) molecular species, as an anti-DENV lipid molecule. SAPI suppressed the cytopathic effects induced by DENV2 infection as well as the replication of all DENV serotypes without inhibiting the entry of DENV2 into host cells. However, no other PI molecular species or PI metabolites, including lysophosphatidylinositols and phosphoinositides, displayed anti-DENV2 activity. Furthermore, SAPI suppressed the production of DENV2 infection-induced cytokines and chemokines, including C-C motif chemokine ligand (CCL)5, CCL20, C-X-C chemokine ligand 8, IL-6, and IFN-β. SAPI also suppressed the TNF-α production induced by LPS stimulation in macrophage cells differentiated from THP-1 cells. Our results demonstrated that SAPI is an endogenous inhibitor of DENV and modulated inflammatory responses in DENV2-infected cells, at least in part via TLR 4.-Sanaki, T., Wakabayashi, M., Yoshioka, T., Yoshida, R., Shishido, T., Hall, W. W., Sawa, H., Sato, A. Inhibition of dengue virus infection by 1-stearoyl-2-arachidonoyl-phosphatidylinositol in vitro.

Airway Epithelial Cells Generate Pro-inflammatory Tenascin-C and Small Extracellular Vesicles in Response to TLR3 Stimuli and Rhinovirus Infection

Viral infections are a common cause of asthma exacerbations, with human rhinoviruses (RV) the most common trigger. RV signals through a number of different receptors, including toll-like receptor (TLR)3. Tenascin-C (TN-C) is an immunomodulatory extracellular matrix protein present in high quantities in the airway of people with asthma, and expression is also upregulated in nasal lavage fluid in response to RV infection. Respiratory viral infection has been demonstrated to induce the release of small extracellular vesicles (sEV) such as exosomes, whilst exosomal cargo can also be modified in the bronchoalveolar lavage fluid of people with asthma. These sEVs may potentiate airway inflammation and regulate the immune response to infection. This study characterizes the relationship between RV infection of bronchial epithelial cells and the release of TN-C, and the release of sEVs following stimulation with the TLR3 agonist and synthetic viral mimic, poly(I:C), as well as the function of the released protein/vesicles. The BEAS-2B airway epithelial cell line and primary human bronchial epithelial cells (PBECs) from asthmatic and non-asthmatic donors were infected with RV or treated with poly(I:C). TN-C expression, release and localization to sEVs was quantified. TN-C expression was also assessed following intra-nasal challenge of C57BL/6 mice with poly(I:C). BEAS-2B cells and macrophages were subsequently challenged with TN-C, or with sEVs generated from BEAS-2B cells pre-treated with siRNA targeted to TN-C or control. The results revealed that poly(I:C) stimulation induced TN-C release in vivo, whilst both poly(I:C) stimulation and RV infection promoted release in vitro, with elevated TN-C release from PBECs obtained from people with asthma. Poly(I:C) also induced the release of TN-C-rich sEVs from BEAS-2B cells. TN-C, and sEVs from poly(I:C) challenged cells, induced cytokine synthesis in macrophages and BEAS-2B cells, whilst sEVs from control cells did not. Moreover, sEVs with ~75% reduced TN-C content did not alter the capacity of sEVs to induce inflammation. This study identifies two novel components of the inflammatory pathway that regulates the immune response following RV infection and TLR3 stimulation, highlighting TN-C release and pro-inflammatory sEVs in the airway as relevant to the biology of virally induced exacerbations of asthma.

TRAF3-interacting JNK-activating modulator promotes inflammation by stimulating translocation of Toll-like receptor 4 to lipid rafts

Toll-like receptors (TLRs) are key players of the innate immune system and contribute to inflammation and pathogen clearance. Although TLRs have been extensively studied, it remains unclear how exactly bacterial lipopolysaccharide (LPS)-induced conformational changes of the extracellular domain of the TLRs trigger the dimerization of their intracellular domain across the plasma membrane and thereby stimulate downstream signaling. Here, using LPS-stimulated THP-1-derived macrophages and murine macrophages along with immunoblotting and immunofluorescence and quantitative analyses, we report that in response to inflammatory stimuli, the coiled-coil protein TRAF3-interacting JNK-activating modulator (T3JAM) associates with TLR4, promotes its translocation to lipid rafts, and thereby enhances macrophage-mediated inflammation. T3JAM overexpression increased and T3JAM depletion decreased TLR4 signaling through both the MyD88-dependent pathway and TLR4 endocytosis. Importantly, deletion or mutation of T3JAM to disrupt its coiled-coil-mediated homoassociation abrogated TLR4 recruitment to lipid rafts. Consistently, T3JAM depletion in mice dampened TLR4 signaling and alleviated LPS-induced inflammatory damage. Collectively, our findings reveal an additional molecular mechanism by which TLR4 activity is regulated and suggest that T3JAM may function as a molecular clamp to "tighten up" TLR4 and facilitate its translocation to lipid rafts.

DUSP10 Negatively Regulates the Inflammatory Response to Rhinovirus through Interleukin-1β Signaling

Rhinoviruses are one of the causes of the common cold. In patients with asthma or chronic obstructive pulmonary disease, viral infections, including those with rhinovirus, are the commonest cause of exacerbations. Novel therapeutics to limit viral inflammation are clearly required. The work presented here identifies DUSP10 as an important protein involved in limiting the inflammatory response in the airway without affecting immune control of the virus.

Rhinoviral infection is a common trigger of the excessive inflammation observed during exacerbations of asthma and chronic obstructive pulmonary disease. Rhinovirus (RV) recognition by pattern recognition receptors activates the mitogen-activated protein kinase (MAPK) pathways, which are common inducers of inflammatory gene production. A family of dual-specificity phosphatases (DUSPs) can regulate MAPK function, but their roles in rhinoviral infection are not known. We hypothesized that DUSPs would negatively regulate the inflammatory response to RV infection. Our results revealed that the p38 and c-Jun N-terminal kinase (JNK) MAPKs play key roles in the inflammatory response of epithelial cells to RV infection. Three DUSPs previously shown to have roles in innate immunity (DUSPs 1, 4, and 10) were expressed in primary bronchial epithelial cells, and one of them, DUSP10, was downregulated by RV infection. Small interfering RNA-mediated knockdown of DUSP10 identified a role for the protein in negatively regulating inflammatory cytokine production in response to interleukin-1β (IL-1β), alone and in combination with RV, without any effect on RV replication. This study identifies DUSP10 as an important regulator of airway inflammation in respiratory viral infection.

IMPORTANCE Rhinoviruses are one of the causes of the common cold. In patients with asthma or chronic obstructive pulmonary disease, viral infections, including those with rhinovirus, are the commonest cause of exacerbations. Novel therapeutics to limit viral inflammation are clearly required. The work presented here identifies DUSP10 as an important protein involved in limiting the inflammatory response in the airway without affecting immune control of the virus.

Manufacture and characterization of anti-inflammatory liposomes from jumbo flying squid (Dosidicus gigas) skin phospholipid extraction

The anti-inflammation properties of marine phospholipids enriched with n-3 fatty acids contribute to anti-inflammatory and inflammation-resolving mediators. Functional squid-skin (SQ) liposomes were manufactured from squid-skin phospholipids, and their anti-inflammatory effects were investigated. SQ liposomes included phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylcholine (PC), and lysophosphatidylcholine (Lyso-PC), and had an approximate diameter of 100 mm. When RAW264.7 cells were treated with the SQ liposome, no (p > 0.05) cytotoxicity was observed below a concentration of 7.5 mg mL-1. An SQ-liposome pretreatment of lipopolysaccharide (LPS)-induced RAW 264.7 cells showed decreased (p < 0.05) prostaglandin E2 (PGE2), nitric oxide (NO), interleukin-1beta (IL-1β), IL-6, and tumor necrosis factor-alpha (TNF-α). The engulfment of SQ liposomes by the RAW264.7 cells resulted in lower (p < 0.05) LPS-induced intracellular levels of reactive oxygen species. Furthermore, an SQ-liposome administration ameliorated (p < 0.05) carrageenan-induced paw edema in mice. SQ liposomes may act via apoptotic mimicry to elicit the resolution of inflammation and prevent chronic inflammation-related diseases.

Lipid raft localization of TLR2 and its co-receptors is independent of membrane lipid composition

BACKGROUND

Toll like receptors (TLRs) are an important and evolutionary conserved class of pattern recognition receptors associated with innate immunity. The recognition of Gram-positive cell wall constituents strongly depends on TLR2. In order to be functional, TLR2 predominantly forms a heterodimer with TLR1 or TLR6 within specialized membrane microdomains, the lipid rafts. The membrane lipid composition and the physicochemical properties of lipid rafts are subject to modification by exogenous fatty acids. Previous investigations of our group provide evidence that macrophage enrichment with polyunsaturated fatty acids (PUFA) induces a reordering of lipid rafts and non-rafts based on the incorporation of supplemented PUFA as well as their elongation and desaturation products.

METHODS

In the present study we investigated potential constraining effects of membrane microdomain reorganization on the clustering of TLR2 with its co-receptors TLR1 and TLR6 within lipid rafts. To this end, RAW264.7 macrophages were supplemented with either docosahexaenoic acid (DHA) or arachidonic acid (AA) and analyzed for receptor expression and microdomain localization in context of TLR stimulation.

RESULTS AND CONCLUSIONS

Our analyses showed that receptor levels and microdomain localization were unchanged by PUFA supplementation. The TLR2 pathway, in contrast to the TLR4 signaling cascade, is not affected by exogenous PUFA at the membrane level.

The θ-defensin retrocyclin 101 inhibits TLR4- and TLR2-dependent signaling and protects mice against influenza infection

A member of the θ‐defensin family protects mice during infection with influenza, suggesting a new strategy for viral therapy in humans.

Despite widespread use of annual influenza vaccines, seasonal influenza‐associated deaths number in the thousands each year, in part because of exacerbating bacterial superinfections. Therefore, discovering additional therapeutic options would be a valuable aid to public health. Recently, TLR4 inhibition has emerged as a possible mechanism for protection against influenza‐associated lethality and acute lung injury. Based on recent data showing that rhesus macaque θ‐defensins could inhibit TLR4‐dependent gene expression, we tested the hypothesis that a novel θ‐defensin, retrocyclin (RC)‐101, could disrupt TLR4‐dependent signaling and protect against viral infection. In this study, RC‐101, a variant of the humanized θ‐defensin RC‐1, blocked TLR4‐mediated gene expression in mouse and human macrophages in response to LPS, targeting both MyD88‐ and TRIF‐dependent pathways. In a cell‐free assay, RC‐101 neutralized the biologic activity of LPS at doses ranging from 0.5 to 50 EU/ml, consistent with data showing that RC‐101 binds biotinylated LPS. The action of RC‐101 was not limited to the TLR4 pathway because RC‐101 treatment of macrophages also inhibited gene expression in response to a TLR2 agonist, Pam3CSK4, but failed to bind that biotinylated agonist. Mouse macrophages infected in vitro with mouse‐adapted A/PR/8/34 influenza A virus (PR8) also produced lower levels of proinflammatory cytokine gene products in a TLR4‐independent fashion when treated with RC‐101. Finally, RC‐101 decreased both the lethality and clinical severity associated with PR8 infection in mice. Cumulatively, our data demonstrate that RC‐101 exhibits therapeutic potential for the mitigation of influenza‐related morbidity and mortality, potentially acting through TLR‐dependent and TLR‐independent mechanisms.

Functional crosstalk between membrane lipids and TLR biology

Toll-like receptors (TLRs) are important transmembrane proteins of the innate immune system that detect invading pathogens and subsequently orchestrate an immune response. The ensuing inflammatory processes are connected to lipid metabolism at multiple levels. Here, we describe different aspects of how membrane lipids can shape the response of TLRs. Recent reports have uncovered the role of individual lipid species on membrane protein function and mouse models have contributed to the understanding of how changes in lipid metabolism alter TLR signaling, endocytosis, and cytokine secretion. Finally, we discuss the importance of systematic approaches to identify the function of individual lipid species or the composition of membrane lipids in TLR-related processes.

LPS- or Pseudomonas aeruginosa-mediated activation of the macrophage TLR4 signaling cascade depends on membrane lipid composition

It is well known that PUFA impede the LPS-mediated activation of the transcription factor NFkappaB. However, the underlying mode of action has not been clarified yet. To address this issue in a comprehensive approach, we used the monocyte/macrophage cell line RAW264.7 to investigate the consequences of a PUFA supplementation on the TLR4 pathway with a focus on (i) the gene expression of TLR4 itself as well as of its downstream mediators, (ii) the membrane microdomain localization of TLR4 and CD14, (iii) the stimulation-induced interaction of TLR4 and CD14. Our data indicate that the impairment of the TLR4-mediated cell activation by PUFA supplementation is not due to changes in gene expression of mediator proteins of the signaling cascade. Rather, our data provide evidence that the PUFA enrichment of macrophages affects the TLR4 pathway at the membrane level. PUFA incorporation into membrane lipids induces a reordering of membrane microdomains thereby affecting cellular signal transduction. It is important to note that this remodeling of macrophage rafts has no adverse effect on cell viability. Hence, microdomain disruption via macrophage PUFA supplementation has a potential as non-toxic strategy to attenuate inflammatory signaling.

Human rhinovirus-induced inflammatory responses are inhibited by phosphatidylserine containing liposomes

Human rhinovirus (HRV) infections are major contributors to the healthcare burden associated with acute exacerbations of chronic airway disease, such as chronic obstructive pulmonary disease and asthma. Cellular responses to HRV are mediated through pattern recognition receptors that may in part signal from membrane microdomains. We previously found Toll-like receptor signaling is reduced, by targeting membrane microdomains with a specific liposomal phosphatidylserine species, 1-stearoyl-2-arachidonoyl-sn-glycero-3-phospho-L-serine (SAPS). Here we explored the ability of this approach to target a clinically important pathogen. We determined the biochemical and biophysical properties and stability of SAPS liposomes and studied their ability to modulate rhinovirus-induced inflammation, measured by cytokine production, and rhinovirus replication in both immortalized and normal primary bronchial epithelial cells. SAPS liposomes rapidly partitioned throughout the plasma membrane and internal cellular membranes of epithelial cells. Uptake of liposomes did not cause cell death, but was associated with markedly reduced inflammatory responses to rhinovirus, at the expense of only modest non-significant increases in viral replication, and without impairment of interferon receptor signaling. Thus using liposomes of phosphatidylserine to target membrane microdomains is a feasible mechanism for modulating rhinovirus-induced signaling, and potentially a prototypic new therapy for viral-mediated inflammation.

A Conserved Circular Network of Coregulated Lipids Modulates Innate Immune Responses

Lipid composition affects the biophysical properties of membranes that provide a platform for receptor-mediated cellular signaling. To study the regulatory role of membrane lipid composition, we combined genetic perturbations of sphingolipid metabolism with the quantification of diverse steps in Toll-like receptor (TLR) signaling and mass spectrometry-based lipidomics. Membrane lipid composition was broadly affected by these perturbations, revealing a circular network of coregulated sphingolipids and glycerophospholipids. This evolutionarily conserved network architecture simultaneously reflected membrane lipid metabolism, subcellular localization, and adaptation mechanisms. Integration of the diverse TLR-induced inflammatory phenotypes with changes in lipid abundance assigned distinct functional roles to individual lipid species organized across the network. This functional annotation accurately predicted the inflammatory response of cells derived from patients suffering from lipid storage disorders, based solely on their altered membrane lipid composition. The analytical strategy described here empowers the understanding of higher-level organization of membrane lipid function in diverse biological systems.

Rhinoviral infection and asthma: the detection and management of rhinoviruses by airway epithelial cells

Human rhinoviruses (HRV) have been linked to the development of childhood asthma and recurrent acute asthma exacerbations throughout life, and contribute considerably to the healthcare and economic burden of this disease. However, the ability of HRV infections to trigger exacerbations, and the link between allergic status and HRV responsiveness, remains incompletely understood. Whilst the receptors on human airway cells that detect and are utilized by most HRV group A and B, but not C serotypes are known, how endosomal pattern recognition receptors (PRRs) detect HRV replication products that are generated within the cytoplasm remains somewhat of an enigma. In this article, we explore a role for autophagy, a cellular homeostatic process that allows the cell to encapsulate its own cytosolic constituents, as the crucial mechanism controlling this process and regulating the innate immune response of airway epithelial cells to viral infection. We will also briefly describe some of the recent insights into the immune responses of the airway to HRV, focusing on neutrophilic inflammation that is a potentially unwanted feature of the acute response to viral infection, and the roles of IL-1 and Pellinos in the regulation of responses to HRV.

Regulation of innate immune responses by transmembrane interactions: lessons from the TLR family

The mammalian innate immune response is responsible for the early stages of defense against invading pathogens. One of the major receptor families facilitating innate immune activation is the Toll-like receptor (TLR) family. These receptors are type 1 membrane proteins spanning the membrane with a single transmembrane domain (TMD). All TLRs form homo- and hetero-dimers within membranes and new data suggest that the single transmembrane domain of some of these receptors is involved in their dimerization and function. Newly identified TLR dimers are continuously reported but only little is known about the importance of the TMDs for their dimer assembly and signaling regulation. Uncontrolled or untimely activation of TLRs is related to a large number of pathologies ranging from cystic fibrosis to sepsis and cancer. In this review we will focus on the contribution of the TMDs of innate immune receptors - specifically TLR2-to their regulation and function. In addition, we will address the current issues remaining to be solved regarding the mechanistic insights of this regulation. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Roles of neutrophils in the regulation of the extent of human inflammation through delivery of IL-1 and clearance of chemokines

This study examined the establishment of neutrophilic inflammation in humans. We tested the hypotheses that neutrophil recruitment was associated with local CXCL8 production and that neutrophils themselves might contribute to the regulation of the size of the inflammatory response. Humans were challenged i.d. with endotoxin. Biopsies of these sites were examined for cytokine production and leukocyte recruitment by qPCR and IHC. Additional in vitro models of inflammation examined the ability of neutrophils to produce and sequester cytokines relevant to neutrophilic inflammation. i.d. challenge with 15 ng of a TLR4-selective endotoxin caused a local inflammatory response, in which 1% of the total biopsy area stained positive for neutrophils at 6 h, correlating with 100-fold up-regulation in local CXCL8 mRNA generation. Neutrophils themselves were the major source of the early cytokine IL-1β. In vitro, neutrophils mediated CXCL8 but not IL-1β clearance (>90% clearance of ≤2 nM CXCL8 over 24 h). CXCL8 clearance was at least partially receptor-dependent and modified by inflammatory context, preserved in models of viral infection but reduced in models of bacterial infection. In conclusion, in a human inflammatory model, neutrophils are rapidly recruited and may regulate the size and outcome of the inflammatory response through the uptake and release of cytokines and chemokines in patterns dependent on the underlying inflammatory stimulus.

Granulocytes in helminth infection -- who is calling the shots?

Helminths are parasitic organisms that can be broadly described as “worms” due to their elongated body plan, but which otherwise differ in shape, development, migratory routes and the predilection site of the adults and larvae. They are divided into three major groups: trematodes (flukes), which are leaf-shaped, hermaphroditic (except for blood flukes) flatworms with oral and ventral suckers; cestodes (tapeworms), which are segmented, hermaphroditic flatworms that inhabit the intestinal lumen; and nematodes (roundworms), which are dioecious, cylindrical parasites that inhabit intestinal and peripheral tissue sites. Helminths exhibit a sublime co-evolution with the host´s immune system that has enabled them to successfully colonize almost all multicellular species present in every geographical environment, including over two billion humans. In the face of this challenge, the host immune system has evolved to strike a delicate balance between attempts to neutralize the infectious assault versus limitation of damage to host tissues. Among the most important cell types during helminthic invasion are granulocytes: eosinophils, neutrophils and basophils. Depending on the specific context, these leukocytes may have pivotal roles in host protection, immunopathology, or facilitation of helminth establishment. This review provides an overview of the function of granulocytes in helminthic infections.

Pellino-1 selectively regulates epithelial cell responses to rhinovirus

Pellino-1 has recently been identified as a regulator of interleukin-1 (IL-1) signaling, but its roles in regulation of responses of human cells to human pathogens are unknown. We investigated the potential roles of Pellino-1 in the airways. We show for the first time that Pellino-1 regulates responses to a human pathogen, rhinovirus minor group serotype 1B (RV-1B). Knockdown of Pellino-1 by small interfering RNA (siRNA) was associated with impaired production of innate immune cytokines such as CXCL8 from human primary bronchial epithelial cells in response to RV-1B, without impairment in production of antiviral interferons (IFN), and without loss of control of viral replication. Pellino-1 actions were likely to be independent of interleukin-1 receptor-associated kinase-1 (IRAK-1) regulation, since Pellino-1 knockdown in primary epithelial cells did not alter responses to IL-1 but did inhibit responses to poly(I·C), a Toll-like receptor 3 (TLR3) activator that does not signal via IRAK-1 to engender a response. These data indicate that Pellino-1 represents a novel target that regulates responses of human airways to human viral pathogens, independently of IRAK signaling. Neutralization of Pellino-1 may therefore provide opportunities to inhibit potentially harmful neutrophilic inflammation of the airways induced by respiratory viruses, without loss of control of the underlying viral infection.

Interleukin-1β regulates CXCL8 release and influences disease outcome in response to Streptococcus pneumoniae, defining intercellular cooperation between pulmonary epithelial cells and macrophages

The success of Streptococcus pneumoniae (the pneumococcus) as a pulmonary pathogen is related to its restriction of innate immune responses by respiratory epithelial cells. The mechanisms used to overcome this restriction are incompletely elucidated. Pulmonary chemokine expression involves complex cellular and molecular networks, involving the pulmonary epithelium, but the specific cellular interactions and the cytokines that control them are incompletely defined. We show that serotype 2 or 4 pneumococci induce only modest levels of CXCL8 expression from epithelial cell lines, even in the absence of a polysaccharide capsule. In contrast, coculture of A549 cells with the macrophage-like THP-1 cell line, differentiated with vitamin D, or monocyte-derived macrophages enhanced CXCL8 release. Supernatants from the THP-1 cell line prime A549 cells to release CXCL8 at levels similar to cocultures. Interleukin-1Ra (IL-1Ra) inhibits CXCL8 release from cocultures and reduces the activity of macrophage-conditioned media, but inhibition of tumor necrosis factor alpha (TNF-α) had only a minimal effect on CXCL8 release. Release of IL-1β but not TNF-α was upregulated in cocultures. IL-1 type 1 receptor knockout C57BL/6 and BALB/c mice confirmed the importance of IL-1 signaling in CXC chemokine expression and neutrophil recruitment in vivo. In fulminant disease, increased IL-1 signaling resulted in increased neutrophils in the airway and more invasive disease. These results demonstrate that IL-1 is an important component of the cellular network involving macrophages and epithelial cells, which facilitates CXC chemokine expression and aids neutrophil recruitment during pneumococcal pneumonia. They also highlight a potential clinical role for anti-IL-1 treatment to limit excessive neutrophilic inflammation in the lung.

Role of interleukin-1 and MyD88-dependent signaling in rhinovirus infection

Rhinoviral infection is an important trigger of acute inflammatory exacerbations in patients with underlying airway disease. We have previously established that interleukin-1β (IL-1β) is central in the communication between epithelial cells and monocytes during the initiation of inflammation. In this study we explored the roles of IL-1β and its signaling pathways in the responses of airway cells to rhinovirus-1B (RV-1B) and further determined how responses to RV-1B were modified in a model of bacterial coinfection. Our results revealed that IL-1β dramatically potentiated RV-1B-induced proinflammatory responses, and while monocytes did not directly amplify responses to RV-1B alone, they played an important role in the responses observed with our coinfection model. MyD88 is the essential signaling adapter for IL-1β and most Toll-like receptors. To examine the role of MyD88 in more detail, we created stable MyD88 knockdown epithelial cells using short hairpin RNA (shRNA) targeted to MyD88. We determined that IL-1β/MyD88 plays a role in regulating RV-1B replication and the inflammatory response to viral infection of airway cells. These results identify central roles for IL-1β and its signaling pathways in the production of CXCL8, a potent neutrophil chemoattractant, in viral infection. Thus, IL-1β is a viable target for controlling the neutrophilia that is often found in inflammatory airway disease and is exacerbated by viral infection of the airways.

The microtubule regulator stathmin is an endogenous protein agonist for TLR3

TLR3 recognizes dsRNAs and is considered of key importance to antiviral host-defense responses. TLR3 also triggers neuroprotective responses in astrocytes and controls the growth of axons and neuronal progenitor cells, suggesting additional roles for TLR3-mediated signaling in the CNS. This prompted us to search for alternative, CNS-borne protein agonists for TLR3. A genome-scale functional screening of a transcript library from brain tumors revealed that the microtubule regulator stathmin is an activator of TLR3-dependent signaling in astrocytes, inducing the same set of neuroprotective factors as the known TLR3 agonist polyinosinic:polycytidylic acid. This activity of stathmin crucially depends on a long, negatively charged alpha helix in the protein. Colocalization of stathmin with TLR3 on astrocytes, microglia, and neurons in multiple sclerosis-affected human brain indicates that as an endogenous TLR3 agonist, stathmin may fulfill previously unsuspected regulatory roles during inflammation and repair in the adult CNS.

Diesel exhaust particles override natural injury-limiting pathways in the lung

Induction of effective inflammation in the lung in response to environmental and microbial stimuli is dependent on cooperative signaling between leukocytes and lung tissue cells. We explored how these inflammatory networks are modulated by diesel exhaust particles (DEP) using cocultures of human monocytes with epithelial cells. Cocultures, or monoculture controls, were treated with DEP in the presence or absence of LPS or flagellin. Production of cytokines was explored by Western blotting and ELISA; cell signaling was analyzed by Western blotting. Here, we show that responses of epithelial cells to DEP are amplified by the presence of monocytes. DEP amplified the responses of cellular cocultures to very low doses of TLR agonists. In addition, in the presence of DEP, the responses induced by LPS or flagellin were less amenable to antagonism by the physiological IL-1 antagonist, IL-1ra. This was paralleled by the uncoupling of IL-1 production and release from monocytes, potentially attributable to an ability of DEP to sequester or degrade extracellular ATP. These data describe a model of inflammation where DEP amplifies responses to low concentrations of microbial agonists and alters the nature of the inflammatory milieu induced by TLR agonists.

A central role for monocytes in Toll-like receptor-mediated activation of the vasculature

There is increasing evidence that activation of inflammatory responses in a variety of tissues is mediated co-operatively by the actions of more than one cell type. In particular, the monocyte has been implicated as a potentially important cell in the initiation of inflammatory responses to Toll-like receptor (TLR)-activating signals. To determine the potential for monocyte-regulated activation of tissue cells to underpin inflammatory responses in the vasculature, we established cocultures of primary human endothelial cells and monocytes and dissected the inflammatory responses of these systems following activation with TLR agonists. We observed that effective activation of inflammatory responses required bidirectional signalling between the monocyte and the tissue cell. Activation of cocultures was dependent on interleukin-1 (IL-1). Although monocyte-mediated IL-1beta production was crucial to the activation of cocultures, TLR specificity to these responses was also provided by the endothelial cells, which served to regulate the signalling of the monocytes. TLR4-induced IL-1beta production by monocytes was increased by TLR4-dependent endothelial activation in coculture, and was associated with increased monocyte CD14 expression. Activation of this inflammatory network also supported the potential for downstream monocyte-dependent T helper type 17 activation. These data define co-operative networks regulating inflammatory responses to TLR agonists, identify points amenable to targeting for the amelioration of vascular inflammation, and offer the potential to modify atherosclerotic plaque instability after a severe infection.