Type I alveolar epithelial cells mount innate immune responses during pneumococcal pneumonia

Altered circular RNA expressions in extracellular vesicles from bronchoalveolar lavage fluids in mice after bacterial infections

Circular RNAs (circRNAs) are a class of transcripts that often are generated by back-splicing that covalently connects the 3'end of the exon to the 5'end. CircRNAs are more resistant to nuclease and more stable than their linear counterparts. One of the well-recognized roles of circRNAs is the miRNA sponging effects that potentially lead to the regulation of downstream proteins. Despite that circRNAs have been reported to be involved in a wide range of human diseases, including cancers, cardiovascular, and neurological diseases, they have not been studied in inflammatory lung responses. Here, we analyzed the circRNA profiles detected in extracellular vesicles (EVs) obtained from the broncho-alveolar lavage fluids (BALF) in response to LPS or acid instillation in mice. Next, we validated two specific circRNAs in the BALF-EVs and BALF cells in response to endotoxin by RT-qPCR, using specific primers targeting the circular form of RNAs rather than the linear host RNAs. The expression of these selected circRNAs in the BALF inflammatory cells, alveolar macrophages (AMs), neutrophils, and lung tissue were analyzed. We further predicted the potential miRNAs that interact with these circRNAs. Our study is the first report to show that circRNAs are detectable in BALF EVs obtained from mice. The EV-cargo circRNAs are significantly altered by the noxious stimuli. The circRNAs identified using microarrays may be validated by RT-qPCR using primers specific to the circular but not the linear form. Future studies to investigate circRNA expression and function including miRNA sponging in lung inflammation potentially uncover novel strategies to develop diagnostic/therapeutic targets.

Lung repair and regeneration: Advanced models and insights into human disease

The respiratory system acts as both the primary site of gas exchange and an important sensor and barrier to the external environment. The increase in incidences of respiratory disease over the past decades has highlighted the importance of developing improved therapeutic approaches. This review will summarize recent research on the cellular complexity of the mammalian respiratory system with a focus on gas exchange and immunological defense functions of the lung. Different models of repair and regeneration will be discussed to help interpret human and animal data and spur the investigation of models and assays for future drug development.

Insights into the defensive roles of lncRNAs during Mycoplasma pneumoniae infection

Mycoplasma pneumoniae causes respiratory tract infections, affecting both children and adults, with varying degrees of severity ranging from mild to life-threatening. In recent years, a new class of regulatory RNAs called long non-coding RNAs (lncRNAs) has been discovered to play crucial roles in regulating gene expression in the host. Research on lncRNAs has greatly expanded our understanding of cellular functions involving RNAs, and it has significantly increased the range of functions of lncRNAs. In lung cancer, transcripts associated with lncRNAs have been identified as regulators of airway and lung inflammation in a process involving protein complexes. An excessive immune response and antibacterial immunity are closely linked to the pathogenesis of M. pneumoniae. The relationship between lncRNAs and M. pneumoniae infection largely involves lncRNAs that participate in antibacterial immunity. This comprehensive review aimed to examine the dysregulation of lncRNAs during M. pneumoniae infection, highlighting the latest advancements in our understanding of the biological functions and molecular mechanisms of lncRNAs in the context of M. pneumoniae infection and indicating avenues for investigating lncRNAs-related therapeutic targets.

Advances and Applications of Lung Organoids in the Research on Acute Respiratory Distress Syndrome (ARDS)

Acute Respiratory Distress Syndrome (ARDS) is a sudden onset of lung injury characterized by bilateral pulmonary edema, diffuse inflammation, hypoxemia, and a low P/F ratio. Epithelial injury and endothelial injury are notable in the development of ARDS, which is more severe under mechanical stress. This review explains the role of alveolar epithelial cells and endothelial cells under physiological and pathological conditions during the progression of ARDS. Mechanical injury not only causes ARDS but is also a side effect of ventilator-supporting treatment, which is difficult to model both in vitro and in vivo. The development of lung organoids has seen rapid progress in recent years, with numerous promising achievements made. Multiple types of cells and construction strategies are emerging in the lung organoid culture system. Additionally, the lung-on-a-chip system presents a new idea for simulating lung diseases. This review summarizes the basic features and critical problems in the research on ARDS, as well as the progress in lung organoids, particularly in the rapidly developing microfluidic system-based organoids. Overall, this review provides valuable insights into the three major factors that promote the progression of ARDS and how advances in lung organoid technology can be used to further understand ARDS.

Santos R, Brito CB, Ferrero MC, Ortiz Wilczyñski JM, Silva EAC, C. Oliveira S, Baldi PC. Role of the cGAS/STING pathway in the control of Brucella abortus infection acquired through the respiratory route

Despite the importance of the respiratory route for Brucella transmission, the lung immune response to this pathogen is scarcely characterized. We investigated the role of the cGAS/STING pathway of microbial DNA recognition in the control of respiratory Brucella infection. After in vitro B. abortus infection, CFU numbers were significantly higher in alveolar macrophages (AM) and lung explants from STING KO mice than in samples from wild type (WT) mice, but no difference was observed for cGAS KO samples. CFU were also increased in WT AM and lung epithelial cells preincubated with the STING inhibitor H151. Several proinflammatory cytokines (TNF-α, IL-1β, IL-6, IP-10/CXCL10) were diminished in Brucella-infected lung explants and/or AM from STING KO mice and cGAS KO mice. These cytokines were also reduced in infected AM and lung epithelial cells pretreated with H151. After intratracheal infection with B. abortus, STING KO mice exhibited increased CFU in lungs, spleen and liver, a reduced expression of IFN-β mRNA in lungs and spleen, and reduced levels of proinflammatory cytokines and chemokines in bronchoalveolar lavage fluid (BALF) and lung homogenates. Increased lung CFU and reduced BALF cytokines were also observed in cGAS KO mice. In summary, the cGAS/STING pathway induces the production of proinflammatory cytokines after respiratory Brucella infection, which may contribute to the STING-dependent control of airborne brucellosis.

Interaction between alveolar macrophages and epithelial cells during Mycoplasma pneumoniae infection

Mycoplasma pneumoniae, as one of the most common pathogens, usually causes upper respiratory tract infections and pneumonia in humans and animals. It accounts for 10% to 40% of community-acquired pneumonia in children. The alveolar epithelial cells (AECs) are the first barrier against pathogen infections, triggering innate immune responses by recruiting and activating immune cells when pathogens invade into the lung. Alveolar macrophages (AMs) are the most plentiful innate immune cells in the lung, and are the first to initiate immune responses with pathogens invasion. The cross-talk between the alveolar epithelium and macrophages is necessary to maintain physiological homeostasis and to eradicate invaded pathogen by regulating immune responses during Mycoplasma pneumoniae infections. This review summarizes the communications between alveolar macrophages and epithelial cells during Mycoplasma pneumoniae infections, including cytokines-medicated communications, signal transduction by extracellular vesicles, surfactant associated proteins-medicated signal transmission and establishment of intercellular gap junction channels.

Pneumonia-Induced Inflammation, Resolution and Cardiovascular Disease: Causes, Consequences and Clinical Opportunities

Pneumonia is inflammation in the lungs, which is usually caused by an infection. The symptoms of pneumonia can vary from mild to life-threatening, where severe illness is often observed in vulnerable populations like children, older adults, and those with preexisting health conditions. Vaccines have greatly reduced the burden of some of the most common causes of pneumonia, and the use of antimicrobials has greatly improved the survival to this infection. However, pneumonia survivors do not return to their preinfection health trajectories but instead experience an accelerated health decline with an increased risk of cardiovascular disease. The mechanisms of this association are not well understood, but a persistent dysregulated inflammatory response post-pneumonia appears to play a central role. It is proposed that the inflammatory response during pneumonia is left unregulated and exacerbates atherosclerotic vascular disease, which ultimately leads to adverse cardiac events such as myocardial infarction. For this reason, there is a need to better understand the inflammatory cross talk between the lungs and the heart during and after pneumonia to develop therapeutics that focus on preventing pneumonia-associated cardiovascular events. This review will provide an overview of the known mechanisms of inflammation triggered during pneumonia and their relevance to the increased cardiovascular risk that follows this infection. We will also discuss opportunities for new clinical approaches leveraging strategies to promote inflammatory resolution pathways as a novel therapeutic target to reduce the risk of cardiac events post-pneumonia.

Innate immune responses in pneumonia

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

The underappreciated role of resident epithelial cell populations in metastatic progression: contributions of the lung alveolar epithelium

Metastatic cancer is difficult to treat and is responsible for the majority of cancer-related deaths. After cancer cells initiate metastasis and successfully seed a distant site, resident cells in the tissue play a key role in determining how metastatic progression develops. The lung is the second most frequent site of metastatic spread, and the primary site of metastasis within the lung is alveoli. The most abundant cell type in the alveolar niche is the epithelium. This review will examine the potential contributions of the alveolar epithelium to metastatic progression. It will also provide insight into other ways in which alveolar epithelial cells, acting as immune sentinels within the lung, may influence metastatic progression through their various interactions with cells in the surrounding microenvironment.

New Insights into the Alveolar Epithelium as a Driver of Acute Respiratory Distress Syndrome

The alveolar epithelium serves as a barrier between the body and the external environment. To maintain efficient gas exchange, the alveolar epithelium has evolved to withstand and rapidly respond to an assortment of inhaled, injury-inducing stimuli. However, alveolar damage can lead to loss of alveolar fluid barrier function and exuberant, non-resolving inflammation that manifests clinically as acute respiratory distress syndrome (ARDS). This review discusses recent discoveries related to mechanisms of alveolar homeostasis, injury, repair, and regeneration, with a contemporary emphasis on virus-induced lung injury. In addition, we address new insights into how the alveolar epithelium coordinates injury-induced lung inflammation and review maladaptive lung responses to alveolar damage that drive ARDS and pathologic lung remodeling.

Minilungs from Human Embryonic Stem Cells to Study the Interaction of Streptococcus pneumoniae with the Respiratory Tract

The new generation of organoids derived from human pluripotent stem cells holds a promising strategy for modeling host-bacteria interaction studies. Organoids recapitulate the composition, diversity of cell types, and, to some extent, the functional features of the native organ. We generated lung bud organoids derived from human embryonic stem cells to study the interaction of Streptococcus pneumoniae (pneumococcus) with the alveolar epithelium. Invasive pneumococcal disease is an important health problem that may occur as a result of the spread of pneumococcus from the lower respiratory tract to sterile sites. We show here an efficient experimental approach to model the main events of the pneumococcal infection that occur in the human lung, exploring bacterial adherence to the epithelium and internalization and triggering an innate response that includes the interaction with surfactant and the expression of representative cytokines and chemokines. Thus, this model, based on human minilungs, can be used to study pneumococcal virulence factors and the pathogenesis of different serotypes, and it will allow therapeutic interventions in a reliable human context. IMPORTANCE Streptococcus pneumoniae is responsible for high morbidity and mortalities rates worldwide, affecting mainly children and adults older than 65 years. Pneumococcus is also the most common etiologic agent of bacterial pneumonia and nonepidemic meningitis, and it is a frequent cause of bacterial sepsis. Although the introduction of pneumococcal vaccines has decreased the burden of pneumococcal disease, the rise of antibiotic-resistant strains and nonvaccine types by serotype replacement is worrisome. To study the biology of pneumococcus and to establish a reliable human model for pneumococcal pathogenesis, we generated human minilungs from embryonic stem cells. The results show that these organoids can be used to model some events occurring during the interaction of pneumococcus with the lung, such as adherence, internalization, and the initial alveolar innate response. This model also represents a great alternative for studying virulence factors involved in pneumonia, drug screening, and other therapeutic interventions.

Coronaviral Infection and Interferon Response: The Virus-Host Arms Race and COVID-19

The recent pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in unprecedented morbidity and mortality worldwide. The host cells use a number of pattern recognition receptors (PRRs) for early detection of coronavirus infection, and timely interferon secretion is highly effective against SARS-CoV-2 infection. However, the virus has developed many strategies to delay interferon secretion and disarm cellular defense by intervening in interferon-associated signaling pathways on multiple levels. As a result, some COVID-19 patients suffered dramatic susceptibility to SARS-CoV-2 infection, while another part of the population showed only mild or no symptoms. One hypothesis suggests that functional differences in innate immune integrity could be the key to such variability. This review tries to decipher possible interactions between SARS-CoV-2 proteins and human antiviral interferon sensors. We found that SARS-CoV-2 actively interacts with PRR sensors and antiviral pathways by avoiding interferon suppression, which could result in severe COVID-19 pathogenesis. Finally, we summarize data on available antiviral pharmaceutical options that have shown potential to reduce COVID-19 morbidity and mortality in recent clinical trials.

Teleost swim bladder, an ancient air-filled organ that elicits mucosal immune responses

The air-filled organs (AOs) of vertebrates (lungs and swim bladders) have evolved unique functions (air-breathing or buoyancy control in water) to adapt to different environments. Thus far, immune responses to microbes in AOs have been described exclusively in the lungs of tetrapods. Similar to lungs, swim bladders (SBs) represent a mucosal surface, a feature that leads us to hypothesize a role for SB in immunity. In this study, we demonstrate that secretory IgT (sIgT) is the key SB immunoglobulin (Ig) responding to the viral challenge, and the only Ig involved in viral neutralization in that organ. In support of these findings, we found that the viral load of the SB from fish devoid of sIgT was much higher than that of control fish. Interestingly, similar to the lungs in mammals, the SB represents the mucosal surface in fish with the lowest content of microbiota. Moreover, sIgT is the main Ig class found coating their surface, suggesting a key role of this Ig in the homeostasis of the SB microbiota. In addition to the well-established role of SB in buoyancy control, our findings reveal a previously unrecognized function of teleost SB in adaptive mucosal immune responses upon pathogenic challenge, as well as a previously unidentified role of sIgT in antiviral defense. Overall, our findings indicate that despite the phylogenetic distance and physiological roles of teleost SB and mammalian lungs, they both have evolved analogous mucosal immune responses against microbes which likely originated independently through a process of convergent evolution.

Epithelial LIF signaling limits apoptosis and lung injury during bacterial pneumonia

During bacterial pneumonia, alveolar epithelial cells are critical for maintaining gas exchange and providing antimicrobial as well as pro-immune properties. We previously demonstrated that leukemia inhibitory factor (LIF), an IL-6 family cytokine, is produced by type II alveolar epithelial cells (ATII) and is critical for tissue protection during bacterial pneumonia. However, the target cells and mechanisms of LIF-mediated protection remain unknown. Here, we demonstrate that antibody-induced LIF blockade remodels the lung epithelial transcriptome in association with increased apoptosis. Based on these data, we performed pneumonia studies using a novel mouse model in which LIFR (the unique receptor for LIF) is absent in lung epithelium. Although LIFR is expressed on the surface of epithelial cells, its absence only minimally contributed to tissue protection during pneumonia. Single-cell RNA-sequencing (scRNAseq) was conducted to identify adult murine lung cell types most prominently expressing Lifr, revealing endothelial cells, mesenchymal cells, and ATIIs as major sources of Lifr. Sequencing data indicated that ATII cells were significantly impacted by pneumonia, with additional differences observed in response to LIF neutralization, including but not limited to gene programs related to cell death, injury, and inflammation. Overall, our data suggest that LIF signaling on epithelial cells alters responses in this cell type during pneumonia. However, our results also suggest separate and perhaps more prominent roles of LIFR in other cell types, such as endothelial cells or mesenchymal cells, which provide grounds for future investigation.

Regulation of MHC Class I Expression in Lung Epithelial Cells during Inflammation

Lung infections are a perennial leading cause of death worldwide. The lung epithelium comprises three main cell types: alveolar type I (AT1), alveolar type II (AT2), and bronchiolar cells. Constitutively, these three cell types express extremely low amounts of surface MHC class I (MHC I) molecules, that is, <1% of levels found on medullary thymic epithelial cells (ECs). We report that inhalation of the TLR4 ligand LPS upregulates cell surface MHC I by ∼25-fold on the three subtypes of mouse lung ECs. This upregulation is dependent on Nlrc5, Stat1, and Stat2 and caused by a concerted production of the three IFN families. It is nevertheless hampered, particularly in AT1 cells, by the limited expression of genes instrumental in the peptide loading of MHC I molecules. Genes involved in production and response to cytokines and chemokines were selectively induced in AT1 cells. However, discrete gene subsets were selectively downregulated in AT2 or bronchiolar cells following LPS inhalation. Genes downregulated in AT2 cells were linked to cell differentiation and cell proliferation, and those repressed in bronchiolar cells were primarily involved in cilium function. Our study shows a delicate balance between the expression of transcripts maintaining lung epithelium integrity and transcripts involved in Ag presentation in primary lung ECs.

GM-CSF: Orchestrating the Pulmonary Response to Infection

This review summarizes the structure and function of the alveolar unit, comprised of alveolar macrophage and epithelial cell types that work in tandem to respond to infection. Granulocyte-macrophage colony-stimulating factor (GM-CSF) helps to maintain the alveolar epithelium and pulmonary immune system under physiological conditions and plays a critical role in restoring homeostasis under pathologic conditions, including infection. Given the emergence of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and global spread of coronavirus disease 2019 (COVID-19), with subsequent acute respiratory distress syndrome, understanding basic lung physiology in infectious diseases is especially warranted. This review summarizes clinical and preclinical data for GM-CSF in respiratory infections, and the rationale for sargramostim (yeast-derived recombinant human [rhu] GM-CSF) as adjunctive treatment for COVID-19 and other pulmonary infectious diseases.

Adult stem cell-derived complete lung organoid models emulate lung disease in COVID-19

Background

SARS-CoV-2, the virus responsible for COVID-19, causes widespread damage in the lungs in the setting of an overzealous immune response whose origin remains unclear.

Methods

We present a scalable, propagable, personalized, cost-effective adult stem cell-derived human lung organoid model that is complete with both proximal and distal airway epithelia. Monolayers derived from adult lung organoids (ALOs), primary airway cells, or hiPSC-derived alveolar type II (AT2) pneumocytes were infected with SARS-CoV-2 to create in vitro lung models of COVID-19.

Results

Infected ALO monolayers best recapitulated the transcriptomic signatures in diverse cohorts of COVID-19 patient-derived respiratory samples. The airway (proximal) cells were critical for sustained viral infection, whereas distal alveolar differentiation (AT2→AT1) was critical for mounting the overzealous host immune response in fatal disease; ALO monolayers with well-mixed proximodistal airway components recapitulated both.

Conclusions

Findings validate a human lung model of COVID-19, which can be immediately utilized to investigate COVID-19 pathogenesis and vet new therapies and vaccines.

Funding

This work was supported by the National Institutes for Health (NIH) grants 1R01DK107585-01A1, 3R01DK107585-05S1 (to SD); R01-AI141630, CA100768 and CA160911 (to PG) and R01-AI 155696 (to PG, DS and SD); R00-CA151673 and R01-GM138385 (to DS), R01- HL32225 (to PT), UCOP-R00RG2642 (to SD and PG), UCOP-R01RG3780 (to P.G. and D.S) and a pilot award from the Sanford Stem Cell Clinical Center at UC San Diego Health (P.G, S.D, D.S). GDK was supported through The American Association of Immunologists Intersect Fellowship Program for Computational Scientists and Immunologists. L.C.A's salary was supported in part by the VA San Diego Healthcare System. This manuscript includes data generated at the UC San Diego Institute of Genomic Medicine (IGC) using an Illumina NovaSeq 6000 that was purchased with funding from a National Institutes of Health SIG grant (#S10 OD026929).

Adult Stem Cell-derived Complete Lung Organoid Models Emulate Lung Disease in COVID-19

SARS-CoV-2, the virus responsible for COVID-19, causes widespread damage in the lungs in the setting of an overzealous immune response whose origin remains unclear. We present a scalable, propagable, personalized, cost-effective adult stem cell-derived human lung organoid model that is complete with both proximal and distal airway epithelia. Monolayers derived from adult lung organoids (ALOs), primary airway cells, or hiPSC-derived alveolar type-II (AT2) pneumocytes were infected with SARS-CoV-2 to create in vitro lung models of COVID-19. Infected ALO-monolayers best recapitulated the transcriptomic signatures in diverse cohorts of COVID-19 patient-derived respiratory samples. The airway (proximal) cells were critical for sustained viral infection whereas distal alveolar differentiation (AT2→AT1) was critical for mounting the overzealous host immune response in fatal disease; ALO monolayers with well-mixed proximodistal airway components recapitulated both. Findings validate a human lung model of COVID-19 which can be immediately utilized to investigate COVID-19 pathogenesis, and vet new therapies and vaccines.

Microbial metabolism of L-tyrosine protects against allergic airway inflammation

The constituents of the gut microbiome are determined by the local habitat, which itself is shaped by immunological pressures, such as mucosal IgA. Using a mouse model of restricted antibody repertoire, we identified a role for antibody-microbe interactions in shaping a community of bacteria with an enhanced capacity to metabolize L-tyrosine. This model led to increased concentrations of p-cresol sulfate (PCS), which protected the host against allergic airway inflammation. PCS selectively reduced CCL20 production by airway epithelial cells due to an uncoupling of epidermal growth factor receptor (EGFR) and Toll-like receptor 4 (TLR4) signaling. Together, these data reveal a gut microbe-derived metabolite pathway that acts distally on the airway epithelium to reduce allergic airway responses, such as those underpinning asthma.

Regulatory mechanisms of neutrophil migration from the circulation to the airspace

The neutrophil, a short-lived effector leukocyte of the innate immune system best known for its proteases and other degradative cargo, has unique, reciprocal physiological interactions with the lung. During health, large numbers of ‘marginated’ neutrophils reside within the pulmonary vasculature, where they patrol the endothelial surface for pathogens and complete their life cycle. Upon respiratory infection, rapid and sustained recruitment of neutrophils through the endothelial barrier, across the extravascular pulmonary interstitium, and again through the respiratory epithelium into the airspace lumen, is required for pathogen killing. Overexuberant neutrophil trafficking to the lung, however, causes bystander tissue injury and underlies several acute and chronic lung diseases. Due in part to the unique architecture of the lung’s capillary network, the neutrophil follows a microanatomic passage into the distal airspace unlike that observed in other end-organs that it infiltrates. Several of the regulatory mechanisms underlying the stepwise recruitment of circulating neutrophils to the infected lung have been defined over the past few decades; however, fundamental questions remain. In this article, we provide an updated review and perspective on emerging roles for the neutrophil in lung biology, on the molecular mechanisms that control the trafficking of neutrophils to the lung, and on past and ongoing efforts to design therapeutics to intervene upon pulmonary neutrophilia in lung disease.

A biomathematical model of immune response and barrier function in mice with pneumococcal lung infection

Pneumonia is one of the leading causes of death worldwide. The course of the disease is often highly dynamic with unforeseen critical deterioration within hours in a relevant proportion of patients. Besides antibiotic treatment, novel adjunctive therapies are under development. Their additive value needs to be explored in preclinical and clinical studies and corresponding therapy schedules require optimization prior to introduction into clinical practice. Biomathematical modeling of the underlying disease and therapy processes might be a useful aid to support these processes. We here propose a biomathematical model of murine immune response during infection with Streptococcus pneumoniae aiming at predicting the outcome of different treatment schedules. The model consists of a number of non-linear ordinary differential equations describing the dynamics and interactions of the pulmonal pneumococcal population and relevant cells of the innate immune response, namely alveolar- and inflammatory macrophages and neutrophils. The cytokines IL-6 and IL-10 and the chemokines CCL2, CXCL1 and CXCL5 are considered as major mediators of the immune response. We also model the invasion of peripheral blood monocytes, their differentiation into macrophages and bacterial penetration through the epithelial barrier causing blood stream infections. We impose therapy effects on this system by modelling antibiotic therapy and treatment with the novel C5a-inactivator NOX-D19. All equations are derived by translating known biological mechanisms into equations and assuming appropriate response kinetics. Unknown model parameters were determined by fitting the predictions of the model to time series data derived from mice experiments with close-meshed time series of state parameters. Parameter fittings resulted in a good agreement of model and data for the experimental scenarios. The model can be used to predict the performance of alternative schedules of combined antibiotic and NOX-D19 treatment. We conclude that we established a comprehensive biomathematical model of pneumococcal lung infection, immune response and barrier function in mice allowing simulations of new treatment schedules. We aim to validate the model on the basis of further experimental data. We also plan the inclusion of further novel therapy principles and the translation of the model to the human situation in the near future.

Impact of Regulatory T Cells on Type 2 Alveolar Epithelial Cell Transcriptomes during Resolution of Acute Lung Injury and Contributions of IFN-γ

By enhancing tissue repair and modulating immune responses, Foxp3⁺ regulatory T cells (Tregs) play essential roles in resolution from lung injury. The current study investigated the effects that Tregs exert directly or indirectly on the transcriptional profiles of type 2 alveolar epithelial (AT2) cells during resolution in an experimental model of acute lung injury. Purified AT2 cells were isolated from uninjured mice or mice recovering from LPS-induced lung injury, either in the presence of Tregs or in Treg-depleted mice, and transcriptome profiling identified differentially expressed genes. Depletion of Tregs resulted in altered expression of 49 genes within AT2 cells during resolution, suggesting that Tregs present in this microenvironment influence AT2-cell function. Biological processes from Gene Ontology enriched in the absence of Tregs included those describing responses to IFN. Neutralizing IFN-γ in Treg-depleted mice reversed the effect of Treg depletion on inflammatory macrophages and B cells by preventing the increase in inflammatory macrophages and the decrease in B cells. Our results provide insight into the effects of Tregs on AT2 cells. Tregs directly or indirectly impact many AT2-cell functions, including IFN type I and II-mediated signaling pathways. Inhibition of IFN-γ expression and/or function may be one mechanism through which Tregs accelerate resolution after acute lung injury.

Epithelial membrane protein 2 governs transepithelial migration of neutrophils into the airspace

Whether respiratory epithelial cells regulate the final transit of extravasated neutrophils into the inflamed airspace or are a passive barrier is poorly understood. Alveolar epithelial type 1 (AT1) cells, best known for solute transport and gas exchange, have few established immune roles. Epithelial membrane protein 2 (EMP2), a tetraspan protein that promotes recruitment of integrins to lipid rafts, is highly expressed in AT1 cells but has no known function in lung biology. Here, we show that Emp2-/- mice exhibit reduced neutrophil influx into the airspace after a wide range of inhaled exposures. During bacterial pneumonia, Emp2-/- mice had attenuated neutrophilic lung injury and improved survival. Bone marrow chimeras, intravital neutrophil labeling, and in vitro assays suggested that defective transepithelial migration of neutrophils into the alveolar lumen occurs in Emp2-/- lungs. Emp2-/- AT1 cells had dysregulated surface display of multiple adhesion molecules, associated with reduced raft abundance. Epithelial raft abundance was dependent upon putative cholesterol-binding motifs in EMP2, whereas EMP2 supported adhesion molecule display and neutrophil transmigration through suppression of caveolins. Taken together, we propose that EMP2-dependent membrane organization ensures proper display on AT1 cells of a suite of proteins required to instruct paracellular neutrophil traffic into the alveolus.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Alcohol-induced mitochondrial DNA damage promotes injurious crosstalk between alveolar epithelial cells and alveolar macrophages

Excessive alcohol users have a higher risk for developing respiratory infections compared to individuals who do not chronically misuse alcohol, due to impaired host immune defense. In the lung, alveolar epithelial cells play a critical role in host immune defense against invading pathogens in the lower respiratory tract due to their capacity to maintain barrier integrity, and alveolar macrophages play a key role in pulmonary innate immunity by phagocytizing and clearing infiltrating microbes. Chronic alcohol misuse induces mitochondrial damage that results in release of mitochondrial DNA (mtDNA) in exosomes. We hypothesized that alcohol-induced cellular damage leads to release of exosomes containing damaged mtDNA, which can mediate injurious crosstalk between lung epithelial cells and macrophages. The mouse alveolar epithelial cell line, MLE-12, and the mouse alveolar macrophage cell line, MH-S, were transfected with a damaged mtDNA overexpression plasmid or exposed to ethanol in vitro. Overexpression of damaged mtDNA impaired MLE-12 barrier function and MH-S phagocytic capacity. Ethanol induced damage of mtDNA in MLE-12 and MH-S cells, and promoted release of exosomes enriched with damaged mtDNA from these cells. Exosomes from ethanol-exposed MH-S cells caused mtDNA damage and barrier dysfunction in MLE-12 cells, and exosomes from ethanol-exposed MLE-12 cells caused mtDNA damage and phagocytic dysfunction in MH-S cells. Collectively, these data show that ethanol-induced mtDNA damage in MLE-12 and MH-S cells stimulates release of damaged mtDNA-enriched exosomes and contributes to injurious crosstalk between the alveolar epithelium and macrophages, potentially leading to impaired host immune defense against respiratory infections.

Roles of interleukin-11 during acute bacterial pneumonia

Interleukin-11 (IL-11) is an interleukin-6 (IL-6) family cytokine shown to play a protective role in acute inflammatory settings including systemic infection. In this study we addressed the role of IL-11 in acute bacterial pneumonia using a mouse model of E. coli pneumonia. Compared with other related cytokines, IL-11 protein was maintained at high levels in the lung at baseline, with only mild alterations in whole lung and BALF levels during acute infection. The primary source of IL-11 in the lung was the epithelium, but steady state production was not dependent on the inflammatory transcription factor nuclear factor kappa B in cells of either myeloid or epithelial lineage. Blockade of IL-11 with neutralizing antibodies resulted in a mild but significant decrease in neutrophil recruitment and increase in pulmonary edema during pneumonia, without detectable alterations in bacterial clearance. Exogenous IL-11 administration, however, had no effect at baseline or during infection. Overall, we conclude that maintenance of lung IL-11 concentrations may influence acute pulmonary inflammation during infection, albeit modestly.

A study of the endocytosis mechanism and transendothelial activity of lung-targeted GALA-modified liposomes

The GALA peptide (WEAALAEALAEALAEHLAEALAEALEALAA) was originally designed to induce the destabilization of endosomal membranes based on its ability to undergo a pH-dependent conformational change from a random coil to an α-helix. We recently found that liposomes modified with GALA peptide (GALA-LPs) extensively accumulate in lung endothelial cells (ECs) after intravenous injection. However, the uptake mechanism of GALA-LPs and their ability to reach alveolar epithelium was unclear. We report herein that GALA-LPs are internalized into ECs via a clathrin-mediated pathway. Surprisingly, GALA-LPs had the ability to pass lung ECs and reach other cells through transcytosis. GALA-LPs were taken up by >70% of lung ECs, while they also accumulated in ~30% of type I alveolar epithelium (ATI). GALA-modified gold nanoparticles were detected in ECs, in the basement membrane and in other cells such as ATI, type II alveolar epithelium (ATII) and alveolar macrophages. Consistent with this result, a significant gene knockdown was achieved in lung epithelium cells using GALA-LPs encapsulating anti-podoplanin siRNA. This indicates that GALA-LPs can be used as a carrier for delivering macromolecules to parenchymal as well as to endothelial cells in the lung. Although caveolae are commonly linked to the transendothelial transport of proteins and antibodies, our data indicate that clathrin-mediated endocytosis might also participate in the transcytosis process.

Transcriptional analysis of Foxp3+ Tregs and functions of two identified molecules during resolution of ALI

Recovery from acute lung injury (ALI) is an active process. Foxp3+ Tregs contribute to recovery from ALI through modulating immune responses and enhancing alveolar epithelial proliferation and tissue repair. The current study investigates Treg transcriptional profiles during resolution of ALI in mice. Tregs from either lung or splenic tissue were isolated from uninjured mice or mice recovering from ALI and then examined for differential gene expression between these conditions. In mice with ALI, Tregs isolated from the lungs had hundreds of differentially expressed transcripts compared with those from the spleen, indicating that organ specificity and microenvironment are critical in Treg function. These regulated transcripts suggest which intracellular signaling pathways modulate Treg behavior. Interestingly, several transcripts having no prior recognized function in Tregs were differentially expressed by lung Tregs during resolution. Further investigation into 2 identified transcripts, Mmp12 and Sik1, revealed that Treg-specific expression of each plays a role in Treg-promoted ALI resolution. This study provides potentially novel information describing the signals that may expand resident Tregs, recruit or retain them to the lung during ALI, and modulate their function. The results provide insight into both tissue- and immune microenvironment-specific transcriptional differences through which Tregs direct their effects.

Alveolar-Capillary Membrane-Related Pulmonary Cells as a Target in Endotoxin-Induced Acute Lung Injury

The main function of the lungs is oxygen transport from the atmosphere into the blood circulation, while it is necessary to keep the pulmonary tissue relatively free of pathogens. This is a difficult task because the respiratory system is constantly exposed to harmful substances entering the lungs by inhalation or via the blood stream. Individual types of lung cells are equipped with the mechanisms that maintain pulmonary homeostasis. Because of the clinical significance of acute respiratory distress syndrome (ARDS) the article refers to the physiological role of alveolar epithelial cells type I and II, endothelial cells, alveolar macrophages, and fibroblasts. However, all these cells can be damaged by lipopolysaccharide (LPS) which can reach the airspaces as the major component of the outer membrane of Gram-negative bacteria, and lead to local and systemic inflammation and toxicity. We also highlight a negative effect of LPS on lung cells related to alveolar-capillary barrier and their response to LPS exposure. Additionally, we describe the molecular mechanism of LPS signal transduction pathway in lung cells.

Integrative Physiology of Pneumonia

Pneumonia is a type of acute lower respiratory infection that is common and severe. The outcome of lower respiratory infection is determined by the degrees to which immunity is protective and inflammation is damaging. Intercellular and interorgan signaling networks coordinate these actions to fight infection and protect the tissue. Cells residing in the lung initiate and steer these responses, with additional immunity effectors recruited from the bloodstream. Responses of extrapulmonary tissues, including the liver, bone marrow, and others, are essential to resistance and resilience. Responses in the lung and extrapulmonary organs can also be counterproductive and drive acute and chronic comorbidities after respiratory infection. This review discusses cell-specific and organ-specific roles in the integrated physiological response to acute lung infection, and the mechanisms by which intercellular and interorgan signaling contribute to host defense and healthy respiratory physiology or to acute lung injury, chronic pulmonary disease, and adverse extrapulmonary sequelae. Pneumonia should no longer be perceived as simply an acute infection of the lung. Pneumonia susceptibility reflects ongoing and poorly understood chronic conditions, and pneumonia results in diverse and often persistent deleterious consequences for multiple physiological systems.

Identification of miRNA-rich vesicles in bronchoalveolar lavage fluid: Insights into the function and heterogeneity of extracellular vesicles

Despite emerging interest in the role of extracellular vesicle (EV)-containing microRNAs (EV-miRNAs), the existence of functional EV-miRNAs under patho-physiological conditions has been viewed with skepticism. Due to the heterogenicity of EVs, several barriers related to EV-miRNA research are to be explored before the in vivo function of EV-miRNAs can be thoroughly delineated. For example, it has been reported that far less than one copy of a given miRNA can be detected per exosome. In this study, we demonstrated that miRNA-rich-EVs exist and can be consistently isolated using differential centrifugation & density-gradient fractionation from bronchoalveolar lavage fluid (BALF) in vivo. The absolute number of this 'miRNA-rich'-EV population is only about 7.05 × 10⁹ per mouse (6% of total EVs). However, the RNA amount detected in this population of EVs represents approximately 39% of the total EV RNAs in the BALF. In contrast, the remaining populations of BALF EVs (76% of total EVs) contain extremely low concentrations of RNAs and miRNAs. The miRNA-rich-EVs in BALF are likely derived from alveolar epithelial type-I cells (ATIs). Notably, caveolin-1, a lipid raft protein, is exclusively detected in the miRNA-rich-EVs, suggesting the lipid raft protein as a biomarker of EV-miRNA enrichment. We further demonstrated that miRNAs contained in the ATI-EVs are actively delivered into alveolar macrophages, subsequently promoting inflammasome activation, neutrophil recruitment, and M1-macrophage polarization in response to P. aeruginosa pneumonia in vitro and in vivo. Collectively, we are the first to identify and characterize the miRNA-rich-EVs in BALF. These miRNA-rich EVs endorse pro-inflammatory responses in bacterial lung infection. Our study provides a novel insight into the development of biomarkers, therapeutic strategies and underlying mechanisms for lung pathology.

Exosome-Mediated Small RNA Delivery: A Novel Therapeutic Approach for Inflammatory Lung Responses

Exosomes (EXOs) are a type of extracellular nanovesicles released from living cells. Accumulating evidence suggests that EXOs are involved in the pathogenesis of human diseases, including lung conditions. In recent years, the potential of EXO-mediated drug delivery has gained increasing interest. In this report, we investigated whether inhaled EXOs serve as an efficient and practical delivery vehicle to activate or inhibit alveolar macrophages (AMs), subsequently modulating pulmonary immune responses. We first identified the recipient cells of the inhaled EXOs, which were labeled with PKH26. We found that only lung macrophages efficiently take up intratracheally instilled EXOs in vivo. Using modified calcium chloride-mediated transformation, we manipulated small RNA molecules in serum-derived EXOs, including siRNAs, microRNA (miRNA) mimics, and miRNA inhibitors. Via intratracheal instillation, we successfully delivered siRNA and miRNA mimics or inhibitors into lung macrophages using the serum-derived EXOs as vehicles. Furthermore, EXO siRNA or miRNA molecules are functional in modulating LPS-induced lung inflammation in vivo. Beneficially, serum-derived EXOs themselves do not trigger lung immune responses, adding more favorable features to serve as drug delivery agents. Collectively, we developed a novel protocol using serum-derived EXOs to deliver designated small RNA molecules into lung macrophages in vivo, potentially shedding light on future gene therapy of human lung diseases.

Functional Evidence of Pulmonary Extracellular Vesicles in Infectious and Noninfectious Lung Inflammation

Acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) is a highly complex process that can be triggered by both noninfectious (sterile) and infectious stimuli. Inflammatory lung responses are one of the key features in the pathogenesis of this devastating syndrome. How ALI/ARDS-associated inflammation develops remains incompletely understood, particularly after exposure to sterile stimuli. Emerging evidence suggests that extracellular vesicles (EVs) regulate intercellular communication and inflammatory responses in various diseases. In this study, we characterized the generation and function of pulmonary EVs in the setting of ALI/ARDS, induced by sterile stimuli (oxidative stress or acid aspiration) and infection (LPS/Gram-negative bacteria) in mice. EVs detected in bronchoalveolar lavage fluid (BALF) were markedly increased after exposure of animals to both types of stimuli. After sterile stimuli, alveolar type-І epithelial cells were the main source of the BALF EVs. In contrast, infectious stimuli-induced BALF EVs were mainly derived from alveolar macrophages (AMs). Functionally, BALF EVs generated in both the noninfectious and infectious ALI models promoted the recruitment of macrophages in in vivo mouse models. Furthermore, BALF EVs differentially regulated AM production of cytokines and inflammatory mediators, as well as TLR expression in AMs in vivo. Regardless of their origin, BALF EVs contributed significantly to the development of lung inflammation in both the sterile and infectious ALI. Collectively, our results provide novel insights into the mechanisms by which EVs regulate the development of lung inflammation in response to diverse stimuli, potentially providing novel therapeutic and diagnostic targets for ALI/ARDS.

Extracellular Vesicle: An Emerging Mediator of Intercellular Crosstalk in Lung Inflammation and Injury

Inflammatory lung responses are one of the characterized features in the pathogenesis of many lung diseases, including acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD). Alveolar macrophages (AMs) and alveolar epithelial cells are the first line of host defense and innate immunity. Due to their central roles in both the initiation and resolution of inflammatory lung responses, AMs constantly communicate with other lung cells, including the alveolar epithelial cells. In the past, emerging evidence suggests that extracellular vesicles play an essential role in cell–cell crosstalk. In this review, we will discuss the recent findings on the intercellular communications between lung epithelial cells and alveolar macrophages, via EV-mediated signal transfer.

Long noncoding RNA HAGLROS regulates cell apoptosis and autophagy in lipopolysaccharides-induced WI-38 cells via modulating miR-100/NF-κB axis

Pneumonia is a lower respiratory disease caused by pathogens or other factors. This study aimed to explore the roles and mechanism of long noncoding RNA HAGLROS in lipopolysaccharides (LPS)-induced inflammatory injury in pneumonia. The HAGLROS expression in serum of patients with acute stage pneumonia was detected. To induce pulmonary injury, WI-38 human lung fibroblasts were stimulated with lipopolysaccharides (LPS). The HAGLROS expressions in LPS-treated WI-38 cells and the effects of HAGLROS knockdown on the viability, apoptosis, and autophagy of LPS-induced cells were detected. Moreover, the regulatory relationship between HAGLROS and miR-100 was explored as well as the functional targets of miR-100 were identified. Furthermore, the regulatory relationship between miR-100 and PI3K/AKT/NF-κB pathway was elucidated. LncRNA HAGLROS was higher expressed in serum of patients with acute stage pneumonia compared with that in serum of healthy control. LPS caused WI-38 cell injury and increased HAGLROS levels. Downregulation of HAGLROS alleviated LPS-induced cell injury via increasing cell viability, and inhibiting apoptosis and autophagy. Moreover, there was a negative correlation between HAGLROS and miR-100, and the effects of HAGLROS downregulation on LPS-induced apoptosis and autophagy in WI-38 cells were by regulation of miR-100. Furthermore, NFΚB3 was verified as a functional target of miR-100 and effects of miR-100 inhibition on LPS-induced WI-38 cell injury were alleviated by knockdown of NFΚB3. Besides, Knockdown of HAGLROS inhibited the activation of PI3K/AKT/NF-κB pathway. Our findings reveal that downregulation of HAGLROS may alleviate LPS-induced inflammatory injury in WI-38 cells via modulating miR-100/NF-κB axis. HAGLROS/miR-100/NF-κB axis may provide a new strategy for treating acute stage of pneumonia.

Regionally compartmentalized resident memory T cells mediate naturally acquired protection against pneumococcal pneumonia

As children age, they become less susceptible to the diverse microbes causing pneumonia. These microbes are pathobionts that infect the respiratory tract multiple times during childhood, generating immunological memory. To elucidate mechanisms of such naturally acquired immune protection against pneumonia, we modeled a relevant immunological history in mice by infecting their airways with mismatched serotypes of Streptococcus pneumoniae (pneumococcus). Previous pneumococcal infections provided protection against a heterotypic, highly virulent pneumococcus, as evidenced by reduced bacterial burdens and long-term sterilizing immunity. This protection was diminished by depletion of CD4⁺ cells prior to the final infection. The resolution of previous pneumococcal infections seeded the lungs with CD4⁺ resident memory T (T_RM) cells, which responded to heterotypic pneumococcus stimulation by producing multiple effector cytokines, particularly interleukin (IL)-17A. Following lobar pneumonias, IL-17-producing CD4⁺ T_RM cells were confined to the previously infected lobe, rather than dispersed throughout the lower respiratory tract. Importantly, pneumonia protection also was confined to that immunologically experienced lobe. Thus regionally localized memory cells provide superior local tissue protection to that mediated by systemic or central memory immune defenses. We conclude that respiratory bacterial infections elicit CD4⁺ T_RM cells that fill a local niche to optimize heterotypic protection of the affected tissue, preventing pneumonia.

Isolation and Purification of Epithelial and Endothelial Cells from Mouse Lung

It is well established that responses to inhaled environmental agents are controlled by the coordinated actions by multiple immune cell types, including macrophages, dendritic cells, and lymphocytes. Recent evidence indicates that some structural cells can also contribute to the initiation and propagation of immune responses. For example, airway epithelial cells can promote eosinophilic inflammation in response to allergen inhalation. Much remains to be learned, however, regarding how each of these cell types interact with the others, and how these interactions shape immune responses to inhaled agents. Such studies have been hampered by the lack of reliable methods to isolate multiple and distinct populations of cells from the same tissue sample. Consequently, investigators have had to choose between using different protocols to isolate different populations of cells from different animals and accept that for some populations, cell yields can be very low. To overcome these difficulties, we have developed a convenient and practical method to isolate and purify subpopulations of epithelial and endothelial cells from mouse lung. Here, we describe these methods in detail.

microRNA‑3941 targets IGF2 to control LPS‑induced acute pneumonia in A549 cells

The present study aimed to investigate the potential roles and regulatory mechanism of microRNA (miR)-3941 in lipopolysaccharides (LPS)‑induced acute pneumonia. The expression of miR‑3941 in child patients with acute pneumonia was detected and A549 cells were treated with LPS to establish the cellular model of acute pneumonia. The effects of miR‑3941 in LPS‑induced cell injury were investigated by assessing cell viability, apoptosis and inflammation. In addition, the regulatory relationship between miR‑3941 and insulin‑like growth factor 2 (IGF2) was explored, as well as the association between miR‑3941 and the phosphatidylinositol‑4,5‑bisphosphate 3‑kinase/protein kinase B (PI3K/AKT) pathway. miR‑3941 was significantly down‑regulated in patients with acute pneumonia (P<0.01). In the cell model of acute pneumonia, LPS treatment significantly induced cell injury via inhibiting cell viability (P<0.05 or P<0.01), inducing cell apoptosis (P<0.01) and enhancing the production of cytokines [interleukin (IL)‑6, IL‑8 and tumor necrosis factor‑α; P<0.01 or P<0.001]. LPS treatment also resulted in a significantly decreased expression of miR‑3941 in A549 cells (P<0.01) and the overexpression of miR‑3941 significantly alleviated LPS‑induced cell injury (P<0.05). In addition, IGF2 was confirmed as a direct target gene of miR‑3941. Knockdown of IGF2 significantly alleviated LPS‑induced cell injury (P<0.05, P<0.01 or P<0.001), which was significantly reversed by suppression of miR‑3941 (P<0.05, P<0.01 or P<0.001). Furthermore, inhibition of miR‑3941 was demonstrated to activate the PI3K/AKT pathway, which was inhibited following knockdown of IGF2. The present study indicates that miR‑3941 is downregulated in child patients with acute pneumonia and that downregulation of miR‑3941 may promote LPS‑induced cell injury in A549 cells via targeting IGF2 to regulate the activation of the PI3K/AKT pathway. Therefore, miR‑3941 may be a potential therapeutic target for the treatment of acute pneumonia in child patients.

Embryonic Lethality and Host Immunity of RelA-Deficient Mice Are Mediated by Both Apoptosis and Necroptosis

In mammalian cells, signaling pathways triggered by TNF can be switched from NF-κB activation to apoptosis and/or necroptosis. The in vivo mechanisms underlying the mutual regulation of these three signaling pathways are poorly understood. In this article, we report that the embryonic lethality of RelA-deficient mice is partially prevented by the deletion of Rip3 or Mlkl, but it is fully rescued by the combined ablation of Fadd and Rip3 or Mlkl or by blocking RIP1 kinase activity (RIP1^K45A). RelA^-/-Fadd^-/-Rip3^-/- triple-knockout (TKO) and RelA^-/-Rip1^K45A/K45A mice displayed bacterial pneumonia leading to death ∼2 wk after birth. Moreover, RelA^-/-Rip1^K45A/K45A mice, but not TKO mice, developed severe inflammation associated with inflammatory skin lesion. Antibiotic treatment improved bacterial pneumonia, extended the lifespan of TKO and RelA^-/-Rip1^K45A/K45A mice, and alleviated skin inflammation in RelA^-/-Rip1^K45A/K45A mice. These results show the mechanisms underlying the in vivo mutual regulation between NF-κB activation and the cell death pathway and provide new insights into this interplay in embryonic development and host immune homeostasis.

Impaired expression of CXCL5 and matrix metalloproteinases in the lungs of mice with high susceptibility to Streptococcus pneumoniae infection

Introduction

Streptococcus pneumoniae colonizes the nasopharynx of healthy individuals establishing a commensal relationship with the host. In some conditions, bacteria invade the lower respiratory tract and innate immune responses are crucial to avoid diseases such as pneumonia, sepsis, or meningitis.

Methods

Here, we compared the susceptibility to pneumococcal respiratory infection of two outbred mouse lines, AIRmin and AIRmax, selected for low or high acute inflammatory responses, respectively.

Results

AIRmin mice showed increased susceptibility to infection with different pneumococcal serotypes, when compared to AIRmax. Significant higher numbers of alveolar macrophages expressing the CD206 mannose receptor were observed in AIRmin mice when compared to AIRmax mice. Despite this difference, secretion of several cytokines and chemokines in the respiratory tract of AIRmin and AIRmax mice, after infection, was similar. The only exception was CXCL5, which was highly induced after pneumococcal infection in AIRmax mice but not in AIRmin mice. Reduced expression of the matrix metalloproteinases (MMP) 2, 3, 8, and 9, as well as reduced activities of MMPs were also observed in the lungs of AIRmin mice, after infection. Such impaired responses may have contributed to the low influx of neutrophils observed in the airways of these mice. Finally, high percentages of macrophages and neutrophils in apoptosis or necrosis, at the site of infection, were also observed in AIRmin mice, suggesting that leukocyte functionality is also compromised.

Conclusions

Our results indicate that CXCL5 and MMPs contribute to the resistance to pneumococcal infection in mice.

Neutrophils from Patients with Primary Ciliary Dyskinesia Display Reduced Chemotaxis to CXCR2 Ligands

Primary ciliary dyskinesia (PCD), cystic fibrosis (CF), and chronic obstructive airway disease are characterized by neutrophilic inflammation in the lungs. In CF and chronic obstructive airway disease, improper functioning of neutrophils has been demonstrated. We hypothesized that the pulmonary damage in PCD might be aggravated by abnormal functioning neutrophils either as a primary consequence of the PCD mutation or secondary to chronic inflammation. We analyzed chemotactic responses and chemoattractant receptor expression profiles of peripheral blood neutrophils from 36 patients with PCD, 21 healthy children and 19 healthy adults. We stimulated peripheral blood monocytes from patients and healthy controls and measured CXCL8 and IL-1β production with ELISA. PCD neutrophils displayed reduced migration toward CXCR2 ligands (CXCL5 and CXCL8) in the shape change, microchamber and microslide chemotaxis assays, whereas leukotriene B4 and complement component 5a chemotactic responses were not significantly different. The reduced response to CXCL8 was observed in all subgroups of patients with PCD (displaying either normal ultrastructure, dynein abnormalities or central pair deficiencies) and correlated with lung function. CXCR2 was downregulated in about 65% of the PCD patients, suggestive for additional mechanisms causing CXCR2 impairment. After treatment with the TLR ligands lipopolysaccharide and peptidoglycan, PCD monocytes produced more CXCL8 and IL-1β compared to controls. Moreover, PCD monocytes also responded stronger to IL-1β stimulation in terms of CXCL8 production. In conclusion, we revealed a potential link between CXCR2 and its ligand CXCL8 and the pathogenesis of PCD.

Expression of Piwi protein MIWI2 defines a distinct population of multiciliated cells

P-element-induced wimpy testes (Piwi) proteins are known for suppressing retrotransposon activation in the mammalian germline. However, whether Piwi protein or Piwi-dependent functions occur in the mammalian soma is unclear. Contrary to germline-restricted expression, we observed that Piwi-like Miwi2 mRNA is indeed expressed in epithelial cells of the lung in adult mice and that it is induced during pneumonia. Further investigation revealed that MIWI2 protein localized to the cytoplasm of a discrete population of multiciliated airway epithelial cells. Isolation and next-generation sequencing of MIWI2-positive multiciliated cells revealed that they are phenotypically distinct from neighboring MIWI2-negative multiciliated cells. Mice lacking MIWI2 exhibited an altered balance of airway epithelial cells, demonstrating fewer multiciliated cells and an increase in club cells. During pneumococcal pneumonia, Miwi2-deficient mice exhibited increased expression of inflammatory mediators and increased immune cell recruitment, leading to enhanced bacterial clearance. Taken together, our data delineate MIWI2-dependent functions outside of the germline and demonstrate the presence of distinct subsets of airway multiciliated cells that can be discriminated by MIWI2 expression. By demonstrating roles for MIWI2 in airway cell identity and pulmonary innate immunity, these studies elucidate unanticipated physiological functions for Piwi proteins in somatic tissues.

Myeloid-epithelial cross talk coordinates synthesis of the tissue-protective cytokine leukemia inhibitory factor during pneumonia

In bacterial pneumonia, lung damage resulting from epithelial cell injury is a major contributor to the severity of disease and, in some cases, can lead to long-term sequelae, especially in the setting of severe lung injury or acute respiratory distress syndrome. Leukemia inhibitory factor (LIF), a member of the IL-6 cytokine family, is a critical determinant of lung tissue protection during pneumonia, but the cellular sources of LIF and the signaling pathways leading to its production in the infected lung are not known. Here, we demonstrate that lung epithelium, specifically alveolar type II cells, is the predominant site of LIF transcript induction in pneumonic mouse lungs. Epithelial cell cultures were induced to express LIF by bacteria and by sterile bronchoalveolar lavage fluid from pneumonic mice. Reciprocal bone marrow chimera studies demonstrated that LIF deficiency in the nonhematopoietic compartment, but not LIF deficiency in hematopoietic cells, eliminated LIF induction during pneumonia. Although NF-κB RelA (p65) is essential for the expression of many cytokines during pneumonia, its targeted mutation in the lung epithelium was inconsequential for pneumonia-driven LIF induction. However, maximal expression of this epithelial-derived cytokine was dependent on NF-κB RelA in myeloid cells. Overall, our data suggest a signaling axis whereby activation of NF-κB RelA in myeloid cells promotes epithelial LIF induction during lung infections, representing a means through which these two cell types collaborate to improve tissue resilience during pneumonia.

Foxp3+ Regulatory T Cell Expression of Keratinocyte Growth Factor Enhances Lung Epithelial Proliferation

Repair of the lung epithelium after injury is a critical component for resolution; however, the processes necessary to drive epithelial resolution are not clearly defined. Published data demonstrate that Foxp3⁺ regulatory T cells (Tregs) enhance alveolar epithelial proliferation after injury, and Tregs in vitro directly promote type II alveolar epithelial cell (AT2) proliferation, in part by a contact-independent mechanism. Therefore, we sought to determine the contribution of Treg-specific expression of a growth factor that is known to be important in lung repair, keratinocyte growth factor (kgf). The data demonstrate that Tregs express kgf and that Treg-specific expression of kgf regulates alveolar epithelial proliferation during the resolution phase of acute lung injury and in a model of regenerative alveologenesis in vivo. In vitro experiments demonstrate that AT2 cells cocultured with Tregs lacking kgf have decreased rates of proliferation compared with AT2 cells cocultured with wild-type Tregs. Moreover, Tregs isolated from lung tissue and grown in culture express higher levels of two growth factors that are important for lung repair (kgf and amphiregulin) compared with Tregs isolated from splenic tissue. Lastly, Tregs isolated from human lung tissue can be stimulated ex vivo to induce kgf expression. This study reveals mechanisms by which Tregs direct tissue-reparative effects during resolution after acute lung injury, further supporting the emerging role of Tregs in tissue repair.

Lung Epithelial Cell-Derived Microvesicles Regulate Macrophage Migration via MicroRNA-17/221-Induced Integrin β1 Recycling

Robust lung inflammation is one of the prominent features in the pathogenesis of acute lung injury (ALI). Macrophage migration and recruitment are often seen at the early stage of lung inflammatory responses to noxious stimuli. Using an acid inhalation-induced lung injury model, we explored the mechanisms by which acid exposure initiates macrophage recruitment and migration during development of ALI. The lung epithelium comprises a large surface area and functions as a first-line defense against noxious insults. We found that acid exposure induced a remarkable microvesicle (MV) release from lung epithelium as detected in bronchoalveolar lavage fluid. Significantly elevated RNA, rather than protein, was found in these epithelium-derived MVs after acid and included several highly elevated microRNAs, including microRNA (miR)-17 and miR-221. Acid-induced epithelial MV release promoted macrophage migration in vitro and recruitment into the lung in vivo and required, in part, MV shuttling of miR-17 and/or miR-221. Mechanistically, acid-induced epithelial MV miR-17/221 promoted β₁ integrin recycling and presentation back onto the surface of macrophages, in part via a Rab11-mediated pathway. Integrin β₁ is known to play an essential role in regulating macrophage migration. Taken together, acid-induced ALI results in epithelial MV shuttling of miR-17/221 that in turn modulates macrophage β₁ integrin recycling, promoting macrophage recruitment and ultimately contributing to lung inflammation.

Epithelial Cell-Derived Secreted and Transmembrane 1a Signals to Activated Neutrophils during Pneumococcal Pneumonia

Airway epithelial cell responses are critical to the outcome of lung infection. In this study, we aimed to identify unique contributions of epithelial cells during lung infection. To differentiate genes induced selectively in epithelial cells during pneumonia, we compared genome-wide expression profiles from three sorted cell populations: epithelial cells from uninfected mouse lungs, epithelial cells from mouse lungs with pneumococcal pneumonia, and nonepithelial cells from those same infected lungs. Of 1,166 transcripts that were more abundant in epithelial cells from infected lungs compared with nonepithelial cells from the same lungs or from epithelial cells of uninfected lungs, 32 genes were identified as highly expressed secreted products. Especially strong signals included two related secreted and transmembrane (Sectm) 1 genes, Sectm1a and Sectm1b. Refinement of sorting strategies suggested that both Sectm1 products were induced predominantly in conducting airway epithelial cells. Sectm1 was induced during the early stages of pneumococcal pneumonia, and mutation of NF-κB RelA in epithelial cells did not diminish its expression. Instead, type I IFN signaling was necessary and sufficient for Sectm1 induction in lung epithelial cells, mediated by signal transducer and activator of transcription 1. For target cells, Sectm1a bound to myeloid cells preferentially, in particular Ly6G(bright)CD11b(bright) neutrophils in the infected lung. In contrast, Sectm1a did not bind to neutrophils from uninfected lungs. Sectm1a increased expression of the neutrophil-attracting chemokine CXCL2 by neutrophils from the infected lung. We propose that Sectm1a is an epithelial product that sustains a positive feedback loop amplifying neutrophilic inflammation during pneumococcal pneumonia.

Mapping differential cellular protein response of mouse alveolar epithelial cells to multi-walled carbon nanotubes as a function of atomic layer deposition coating

Carbon nanotubes (CNTs), a prototypical engineered nanomaterial, have been increasingly manufactured for a variety of novel applications over the past two decades. However, since CNTs possess fiber-like shape and cause pulmonary fibrosis in rodents, there is concern that mass production of CNTs will lead to occupational exposure and associated pulmonary diseases. The aim of this study was to use contemporary proteomics to investigate the mechanisms of cellular response in E10 mouse alveolar epithelial cells in vitro after exposure to multi-walled CNTs (MWCNTs) that were functionalized by atomic layer deposition (ALD). ALD is a method used to generate highly uniform and conformal nanoscale thin-film coatings of metals to enhance novel conductive properties of CNTs. We hypothesized that specific types of metal oxide coatings applied to the surface of MWCNTs by ALD would determine distinct proteomic profiles in mouse alveolar epithelial cells in vitro that could be used to predict oxidative stress and pulmonary inflammation. Uncoated (U)-MWCNTs were functionalized by ALD with zinc oxide (ZnO) to yield Z-MWCNTs or aluminum oxide (Al₂O₃) to yield A-MWCNTs. Significant differential protein expression was found in the following critical pathways: mTOR/eIF4/p70S6K signaling and Nrf-2 mediated oxidative stress response increased following exposure to Z-MWCNTs, interleukin-1 signaling increased following U-MWCNT exposure, and inhibition of angiogenesis by thrombospondin-1, oxidative phosphorylation, and mitochondrial dysfunction increased following A-MWCNT exposure. This study demonstrates that specific types of metal oxide thin film coatings applied by ALD produce distinct cellular and biochemical responses related to lung inflammation and fibrosis compared to uncoated MWCNT exposure in vitro.

Macrophage-derived apoptotic bodies promote the proliferation of the recipient cells via shuttling microRNA-221/222

Bacterial pneumonia is a common and serious clinical entity. Alveolar epithelial cells and alveolar macrophages are the first line of defense in the innate immunity against bacterial pathogens. Epithelial cells are known to release chemokines/cytokines that recruit and activate phagocytic cells. However, the signals sent from alveolar macrophages back to the lung epithelial cells remain largely unexplored. We found that LPS, a well-recognized stimulator derived from gram-negative (G^-) bacteria, rapidly and robustly induces the secretion of macrophage-derived extracellular vesicles (EVs). The main type of EVs found in the early stages after LPS stimulation are apoptotic bodies (ABs) and not microvesicles (MVs) or exosomes (Exos). Furthermore, LPS markedly up-regulate the levels of a repertoire of microRNAs (miRNAs) in the macrophage-derived ABs, including miR-221 and miR-222. Functionally, the LPS-induced, macrophage-derived ABs promote the proliferation of malignant and/or normal lung epithelial cells. We next directly transfected miR-221 and/or miR-222 inhibitors into the LPS-induced ABs. Deletion of miR-221/222 in ABs significantly reduces the AB-mediated proliferation of lung epithelial cells. Mechanistically, AB-shuttling miR-221/222 promote cell growth by modulating cyclin-dependent kinase inhibitor 1B (CDKN1B) pathways. Collectively, LPS-induced, macrophage-derived ABs promote the proliferation of their recipient epithelial cells, partially via AB-shuttling miRNAs.

Upregulation of interleukin-33 and thymic stromal lymphopoietin levels in the lungs of idiopathic pulmonary fibrosis

BACKGROUND

Innate T helper type 2 (Th2) immune responses mediated by interleukin (IL)-33, thymic stromal lymphopoietin (TSLP), and IL-25 have been shown to play an important role in pulmonary fibrosis of animal models; however, their clinical implications remain poorly understood.

METHODS

TSLP, IL-25, and IL-33 concentrations were measured in bronchoalveolar lavage fluids obtained from normal controls (NCs; n = 40) and from patients with idiopathic pulmonary fibrosis (IPF; n = 100), non-specific interstitial pneumonia (NSIP; n = 22), hypersensitivity pneumonitis (HP; n = 20), and sarcoidosis (n = 19).

RESULTS

The TSLP and IL-33 levels were significantly higher in patients with IPF relative to the NCs (p = 0.01 and p = 0.0001, respectively), NSIP (p = 4.95E - 7 and p = 0.0002, respectively), HP (p = 0.00003 and p = 0.000005, respectively), and sarcoidosis groups (p = 0.003 and p = 0.0001, respectively). However, the IL-25 levels were not significantly different between NC and IPF group (p = 0.432). Receiver operating characteristic curves of the TSLP and IL-33 levels revealed clear differences between the IPF and NC groups (AUC = 0.655 and 0.706, respectively), as well as between the IPF and the other lung disease groups (AUC = 0.786 and 0.781, respectively). Cut-off values of 3.52 pg/μg TSLP and 3.77 pg/μg IL-33 were shown to differentiate between the IPF and NC groups with 99.2 and 94.3% accuracy. Cut-off values of 4.66 pg/μg TSLP and 2.52 pg/μg IL-33 possessed 99.4 and 93.2% accuracy for differentiating among the IPF and other interstitial lung disease groups.

CONCLUSIONS

Innate immune responses may be associated with the development of IPF. Furthermore, the IL-33 and TSLP levels in BAL fluids may be useful for differentiating IPF from other chronic interstitial lung diseases.