New fronts emerge in the influenza cytokine storm

Influenza virus is a significant pathogen in humans and animals with the ability to cause extensive morbidity and mortality. Exuberant immune responses induced following infection have been described as a "cytokine storm," associated with excessive levels of proinflammatory cytokines and widespread tissue damage. Recent studies have painted a more complex picture of cytokine networks and their contributions to clinical outcomes. While many cytokines clearly inflict immunopathology, others have non-pathological delimited roles in sending alarm signals, facilitating viral clearance, and promoting tissue repair, such as the IL-33-amphiregulin axis, which plays a key role in resolving some types of lung damage. Recent literature suggests that type 2 cytokines, traditionally thought of as not involved in anti-influenza immunity, may play an important regulatory role. Here, we discuss the diverse roles played by cytokines after influenza infection and highlight new, serene features of the cytokine storm, while highlighting the specific functions of relevant cytokines that perform unique immune functions and may have applications for influenza therapy.

IL-33/ST2 pathway drives regulatory T cell dependent suppression of liver damage upon cytomegalovirus infection

Regulatory T (Treg) cells dampen an exaggerated immune response to viral infections in order to avoid immunopathology. Cytomegaloviruses (CMVs) are herpesviruses usually causing asymptomatic infection in immunocompetent hosts and induce strong cellular immunity which provides protection against CMV disease. It remains unclear how these persistent viruses manage to avoid induction of immunopathology not only during the acute infection but also during life-long persistence and virus reactivation. This may be due to numerous viral immunoevasion strategies used to specifically modulate immune responses but also induction of Treg cells by CMV infection. Here we demonstrate that liver Treg cells are strongly induced in mice infected with murine CMV (MCMV). The depletion of Treg cells results in severe hepatitis and liver damage without alterations in the virus load. Moreover, liver Treg cells show a high expression of ST2, a cellular receptor for tissue alarmin IL-33, which is strongly upregulated in the liver of infected mice. We demonstrated that IL-33 signaling is crucial for Treg cell accumulation after MCMV infection and ST2-deficient mice show a more pronounced liver pathology and higher mortality compared to infected control mice. These results illustrate the importance of IL-33 in the suppressive function of liver Treg cells during CMV infection.

Treg cells are crucial for immune homeostasis and for dampening immune response to several diseased conditions, including viral infections. Murine cytomegalovirus (MCMV) is a herpesvirus with pathogenic potential, so that early immune mechanisms are essential in controlling virus and protecting from virus-induced pathology. Studies on Foxp3⁺ Treg cells have revealed their inhibitory role on the early T cell response to MCMV infection and have suggested Treg cells as a target of MCMV’s immunoevasion mechanisms. Here we demonstrate that the number and activation status of liver Treg cells is strongly induced upon MCMV infection in order to protect the host from severe liver damage. They constitutively express high amounts of IL-33 receptor ST2 and their accumulation depends on IL-33, which is released as a tissue alarmin after the cell damage. For the first time, we show an immunoregulatory role of IL-33-dependent Treg cells in the liver of MCMV infected mice and their suppression of MCMV-induced immunopathology.

Transcriptomic profiling of a chicken lung epithelial cell line (CLEC213) reveals a mitochondrial respiratory chain activity boost during influenza virus infection

Avian Influenza virus (AIV) is a major concern for the global poultry industry. Since 2012, several countries have reported AIV outbreaks among domestic poultry. These outbreaks had tremendous impact on poultry production and socio-economic repercussion on farmers. In addition, the constant emergence of highly pathogenic AIV also poses a significant risk to human health. In this study, we used a chicken lung epithelial cell line (CLEC213) to gain a better understanding of the molecular consequences of low pathogenic AIV infection in their natural host. Using a transcriptome profiling approach based on microarrays, we identified a cluster of mitochondrial genes highly induced during the infection. Interestingly, most of the regulated genes are encoded by the mitochondrial genome and are involved in the oxidative phosphorylation metabolic pathway. The biological consequences of this transcriptomic induction result in a 2.5- to 4-fold increase of the ATP concentration within the infected cells. PB1-F2, a viral protein that targets the mitochondria was not found associated to the boost of activity of the respiratory chain. We next explored the possibility that ATP may act as a host-derived danger signal (through production of extracellular ATP) or as a boost to increase AIV replication. We observed that, despite the activation of the P2X7 purinergic receptor pathway, a 1mM ATP addition in the cell culture medium had no effect on the virus replication in our epithelial cell model. Finally, we found that oligomycin, a drug that inhibits the oxidative phosphorylation process, drastically reduced the AIV replication in CLEC213 cells, without apparent cellular toxicity. Collectively, our results suggest that AIV is able to boost the metabolic capacities of its avian host in order to provide the important energy needs required to produce progeny virus.

Endothelial IL-33 Expression Is Augmented by Adenoviral Activation of the DNA Damage Machinery

IL-33, required for viral clearance by cytotoxic T cells, is generally expressed in vascular endothelial cells in healthy human tissues. We discovered that endothelial IL-33 expression was stimulated as a response to adenoviral transduction. This response was dependent on MRE11, a sensor of DNA damage that can also be activated by adenoviral DNA, and on IRF1, a transcriptional regulator of cellular responses to viral invasion and DNA damage. Accordingly, we observed that endothelial cells responded to adenoviral DNA by phosphorylation of ATM and CHK2 and that depletion or inhibition of MRE11, but not depletion of ATM, abrogated IL-33 stimulation. In conclusion, we show that adenoviral transduction stimulates IL-33 expression in endothelial cells in a manner that is dependent on the DNA-binding protein MRE11 and the antiviral factor IRF1 but not on downstream DNA damage response signaling.

Dynamics of influenza-induced lung-resident memory T cells underlie waning heterosubtypic immunity

Lung-resident memory CD8 T cells (T_RM) induced by influenza A virus (IAV) that are pivotal for providing subtype-transcending protection against IAV infection (heterosubtypic immunity) are not maintained long term, causing gradual loss of protection. The short-lived nature of lung T_RM contrasts sharply with long-term maintenance of T_RM induced by localized infections in the skin and in other tissues. We show that the decline in lung T_RM is determined by an imbalance between apoptosis and lung recruitment and conversion to T_RM of circulating memory cells. We show that circulating effector memory cells (T_EM) rather than central memory cells (T_CM) are the precursors for conversion to lung T_RM Time-dependent changes in expression of genes critical for lymphocyte trafficking and T_RM differentiation, in concert with enrichment of T_CM, diminish the capacity of circulating memory CD8 T cells to form T_RM with time, explaining why IAV-induced T_RM are not stably maintained. Systemic booster immunization, through increasing the number of circulating T_EM, increases lung T_RM, providing a potential new avenue to enhance IAV vaccines.

Inflammation elevated IL-33 originating from the lung mediates inflammation in acute lung injury

Excessive inflammatory reactions occur with acute respiratory distress syndrome (ARDS), however, the underlying mechanisms of ARDS remain incompletely understood. Here we investigated whether interleukin (IL)-33 was elevated in ARDS patients. Serum samples were obtained from 14 ARDS patients and 24 control healthy volunteers. ELISA was used to measure the concentrations of IL-33. Besides, we established pulmonary ARDS and extrapulmonary ARDS models in mice, and serum and lung tissue samples were collected for analyses. The results showed that serum IL-33 concentrations were significantly higher in pulmonary ARDS patients compared to controls. Also, the levels of IFN-γ and IL-2 were positively correlated with IL-33 levels. We also showed that there were increased IL-33 levels in both the serum and lungs in the pulmonary ARDS model. This was not the case, however, in the extrapulmonary ARDS model. Pulmonary inflammation and injury in the pulmonary ARDS model was reduced with IL-33 neutralizing antibody treatment.

Regulation of IL-33 by Oncostatin M in Mouse Lung Epithelial Cells

IL-33 modulates both innate and adaptive immune responses at tissue sites including lung and may play critical roles in inflammatory lung disease. Although IL-33 expression can be altered upon NF-Kappa B activation, here we examine regulation by Oncostatin M, a gp130 cytokine family member, in mouse lung tissue. Responses were assessed in BALB/c mouse lung at day 7 of transient overexpression using endotracheally administered adenovirus encoding OSM (AdOSM) or empty vector (AdDel70). Whole lung extracts showed induction of IL-33 mRNA (>20-fold) and protein (10-fold increase in immunoblots) by AdOSM relative to AdDel70. Immunohistochemistry for IL-33 indicated a marked induction of nuclear staining in alveolar epithelial cells in vivo. Oncostatin M stimulated IL-33 mRNA and IL-33 full length protein in C10 mouse type 2 alveolar epithelial cells in culture in time-dependent and dose-dependent fashion, whereas IL-6, LIF, IL-31, IL-4, or IL-13 did not, and TGFβ repressed IL-33. IL-33 induction was associated with activation of STAT3, and pharmacological inhibition of STAT3 ameliorated IL-33 levels. These results indicate Oncostatin M as a potent inducer of IL-33 in mouse lung epithelial cells and suggest that an OSM/IL-33 axis may participate in innate immunity and inflammatory conditions in lung.

Tumor necrosis factor-α regulates interleukin-33 expression through extracellular signal-regulated kinase, p38, and nuclear factor-κB pathways in airway epithelial cells

BACKGROUND

Interleukin (IL)-33 plays an important role in controlling immune responses in barrier tissues, and is a potent mediator of inflammatory diseases such as asthma, rheumatoid disease, and chronic rhinosinusitis. The aims of the present study were 2-fold: (1) to determine the stimulatory effect of tumor necrosis factor-α (TNF-α) on IL-33 production in nasal epithelial and A549 cells; and (2) to identify downstream pathways that activate IL-33 production.

METHODS

Primary nasal epithelial cells (PNECs) from 5 normal patients were isolated and cultured. To identify which cytokines stimulate IL-33 production, we performed reverse-transcription polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), and immunofluorescence staining. Three mitogen-activated protein kinases (MAPKs) (p38, extracellular signal-regulated kinase [ERK], and c-Jun N-terminal kinase [JNK]) and nuclear factor κB (NF-κB) were evaluated as downstream signaling molecules by RT-PCR, ELISA, Western blot analysis, and luciferase reporter assay.

RESULTS

The IL-33 messenger RNA (mRNA) and protein levels were increased significantly by TNF-α in PNECs and A549 cells. TNF-α stimulated the expression of IL-33 in a dose- and time-dependent manner in A549 cells. PNECs and A549 cells were treated with TNF-α in the presence of specific inhibitors of p38, ERK, JNK, and NF-κB. In both cell types, inhibitors of ERK, p38, and NF-κB reversed TNF-α-induced IL-33 production. In the luciferase reporter assay, NF-κB activity was inhibited not only by an NF-κB inhibitor, but also by ERK and p38 inhibitors.

CONCLUSION

TNF-α stimulated IL-33 expression through ERK, p38, and NFκB pathways in PNECs and A549 cells.

The Influenza A Virus Genotype Determines the Antiviral Function of NF-κB

The role of NF-κB in influenza A virus (IAV) infection does not reveal a coherent picture, as pro- and also antiviral functions of this transcription factor have been described. To address this issue, we used clustered regularly interspaced short palindromic repeat with Cas9 (CRISPR-Cas9)-mediated genome engineering to generate murine MLE-15 cells lacking two essential components of the NF-κB pathway. Cells devoid of either the central NF-κB essential modulator (NEMO) scaffold protein and thus defective in IκB kinase (IKK) activation or cells not expressing the NF-κB DNA-binding and transactivation subunit p65 were tested for propagation of the SC35 virus, which has an avian host range, and its mouse-adapted variant, SC35M. While NF-κB was not relevant for replication of SC35M, the absence of NF-κB activity increased replication of the nonadapted SC35 virus. This antiviral effect of NF-κB was most prominent upon infection of cells with low virus titers as they usually occur during the initiation phase of IAV infection. The defect in NF-κB signaling resulted in diminished IAV-triggered phosphorylation of interferon regulatory factor 3 (IRF3) and expression of the antiviral beta interferon (IFN-β) gene. To identify the viral proteins responsible for NF-κB dependency, reassortant viruses were generated by reverse genetics. SC35 viruses containing the SC35M segment encoding neuraminidase (NA) were completely inert to the inhibitory effect of NF-κB, emphasizing the importance of the viral genotype for susceptibility to the antiviral functions of NF-κB.

IMPORTANCE

This study addresses two different issues. First, we investigated the role of the host cell transcription factor NF-κB in IAV replication by genetic manipulation of IAVs by reverse genetics combined with targeted genome engineering of host cells using CRISPR-Cas9. The analysis of these two highly defined genetic systems indicated that the IAV genotype can influence whether NF-κB displays an antiviral function and thus might in part explain incoherent results from the literature. Second, we found that perturbation of NF-κB function greatly improved the growth of a nonadapted IAV, suggesting that NF-κB may contribute to the maintenance of the host species barrier.

HMGB1 regulates IL-33 expression in acute respiratory distress syndrome

The development and progression of acute respiratory distress syndrome (ARDS) has been shown to be regulated by cytokines. IL-33 and HMGB1 are conventionally considered as nuclear proteins and have a proinflammatory role. Studies have confirmed that HMGB1 has a significant role in ARDS, but few studies have provided direct evidence to confirm that IL33 is involved in ARDS. The purpose of our study was to determine whether IL-33 is elevated in ARDS and the relationship between IL-33 and HMGB1 in ARDS. We established a mouse model of LPS-induced lung inflammation/injury. Serum, bronchoalveolar lavage fluid (BALF) and lung tissues were obtained to determine the related indicators. IL-33 levels in both the serum, BALF and lungs were significantly increased at 24h after LPS administration compared to the control group. We also found that HMGB1 and other Th1 cytokine/chemokine levels in serum and BALF were also significantly elevated, but the Th2 cytokine levels in serum and BALF didn't increase. To further study the relationship between IL-33 and HMGB1, mice were pretreated with glycyrrhizin (an inhibitor of HMGB1) prior to LPS administration. We found that the expression of IL-33 and HMGB1 were markedly lower than those in the LPS group and the lung injury was ameliorated. The levels of other Th1 cytokines and chemokines in serum and BALF were also significantly decreased. The results showed that IL-33 is likely a major factor in ARDS, and the release of HMGB1 may be correlated with up-regulation of IL-33 expression.

IL33 in rheumatoid arthritis: potential contribution to pathogenesis

A better understanding of the inflammatory mechanisms of rheumatoid arthritis and the development of biological therapy revolutionized its treatment, enabling an interference in the synovitis - structural damage - functional disability cycle. Interleukin 33 was recently described as a new member of the interleukin-1 family, whose common feature is its pro-inflammatory activity. Its involvement in the pathogenesis of a variety of diseases, including autoimmune diseases, raises the interest in the possible relationship with rheumatoid arthritis. Its action has been evaluated in experimental models of arthritis as well as in serum, synovial fluid and membrane of patients with rheumatoid arthritis. It has been shown that the administration of interleukin-33 exacerbates collagen-induced arthritis in experimental models, and a positive correlation between cytokine concentrations in serum and synovial fluid of patients with rheumatoid arthritis and disease activity was found. This review discusses evidence for the role of interleukin-33 with a focus on rheumatoid arthritis.

Interleukin-33, friend and foe in type-2 immune responses

IL-33 is the most recent addition to the IL-1 cytokine family, identified in 2005 as the ligand of T1/ST2 and inducer of type-2 immune responses. IL-33 has been implicated in a wide range of disease settings, in anti-inflammatory responses and homeostasis, and thus signalling must be strictly regulated. Altered gene expression, post-translational modification, decoy receptor, and receptor signalling are all modulatory mechanisms used to control the IL-33 pathway. Understanding both the genetic and post-translational factors influencing IL-33 activity will be critical for provision of safe effective treatment of type-2 disorders.

Insights into Group 2 Innate Lymphoid Cells in Human Airway Disease

Recent discoveries have led to the identification of a novel group of immune cells, the innate lymphoid cells (ILCs). The members of this group are divided into three subpopulations: ILC1s, ILC2s, and ILC3s. ILC2s produce Th2 cytokines, IL-4, IL-5, and IL-13, upon activation by epithelial cell-derived cytokines, lipid mediators (cysteinyl leukotrienes and prostaglandin D2), and TNF family member TL1A and promote structural and immune cell responses in the airways after antigen exposure. In addition, ILC2 function is also influenced by inducible T cell costimulator (ICOS)/ICOS-ligand (ICOS-L) interactions via direct contact between immune cells. The most common airway antigens are allergens and viruses which are highly linked to the induction of airway diseases with underlying type 2 inflammation including asthma and allergic rhinitis. Based on recent findings linking ILC2s and airway Th2 responses, there is intensive investigation into the role of ILC2s in human disease with the hope of a better understanding of the pathophysiology and the discovery of novel potential therapeutic targets. This review summarizes the recent advances made in elucidating ILC2 involvement in human Th2 airway disease.

Innate lymphoid cells in asthma: Will they take your breath away?

Asthma is a complex and heterogeneous disease that is characterized by airway hyper-reactivity (AHR) and airway inflammation. Although asthma was long thought to be driven by allergen-reactive TH 2 cells, it has recently become clear that the pathogenesis of asthma is more complicated and associated with multiple pathways and cell types. A very exciting recent development was the discovery of innate lymphoid cells (ILCs) as key players in the pathogenesis of asthma. ILCs do not express antigen receptors but react promptly to "danger signals" from inflamed tissue and produce an array of cytokines that direct the ensuing immune response. The roles of ILCs may differ in distinct asthma phenotypes. ILC2s may be critical for initiation of adaptive immune responses in inhaled allergen-driven AHR, but may also function independently of adaptive immunity, mediating influenza-induced AHR. ILC2s also contribute to resolution of lung inflammation through their production of amphiregulin. Obesity-induced asthma is associated with expansion of IL-17A-producing ILC3s in the lungs. Furthermore, ILCs may also contribute to steroid-resistant asthma. Although the precise roles of ILCs in different types of asthma are still under investigation, it is clear that inhibition of ILC function represents a potential target that could provide novel treatments for asthma.

Targeting the IL-33/IL-13 Axis for Respiratory Viral Infections

Lung diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are highly prevalent worldwide. One of the major factors that limits the efficacy of current medication in these patients are viral infections, leading to exacerbations of symptoms and decreased quality of life. Current pharmacological strategies targeting virus-induced lung disease are problematic due to antiviral resistance and the requirement for strain-specific vaccination. Thus, new therapeutic strategies are urgently required. In this Opinion article, we provide state-of-the-art evidence from humans and preclinical animal models implicating the interleukin (IL)-33/IL-13 axis in virus-induced lung disease. Thus, targeting the IL-33/IL-13 axis may be a feasible way to overcome the limitations of current therapy used to treat virus-induced exacerbations of lung disease.

The role of IL-33 and mast cells in allergy and inflammation

Interleukin-33 (IL-33) is a member of the interleukin-1 (IL-1) cytokine family. It is preferentially and constitutively expressed in different structural cells such as epithelial cells, endothelial cells, and smooth muscle cells. During necrosis of these cells (after tissue injury or cell damage), the IL-33 that is released may be recognized by different types of immune cells, such as eosinophils, basophils and, especially, mast cells. IL-33 needs the specific receptor ST2 (membrane-bound receptor) and Interleukin-1 receptor accessory protein heterodimer for its binding, which instigates the production of different types of cytokines and chemokines that have crucial roles in the exacerbation of allergic diseases and inflammation. IL-33 and mast cells have been influentially associated to the pathophysiology of allergic diseases and inflammation. IL-33 is a crucial regulator of mast cell functions and might be an attractive therapeutic target for the treatment of allergic and inflammatory diseases. In this review, we summarize the current knowledge regarding the roles of IL-33 and mast cells in the pathogenesis of allergies and inflammation.

Investigation of Neuropathogenesis in HIV-1 Clade B and C Infection Associated with IL-33 and ST2 Regulation

In present research work, for the first time, we demonstrate that neuropathogenesis in HIV-1 clade B and C infection is associated with IL-33 and ST2 dysregulation, that is, implication toward neuropathogenesis. It is known that neuropathogenesis of HIV infected individuals is clade dependent. Proinflammatory cytokines and related receptors play a significant role in the complex regulatory mechanisms of neuropathogenesis in HIV-1 infection. Among them, IL-33 is an inflammatory cytokine expressed in the central nervous system (CNS) and activates microglia cells and may affect neuroimmune inflammatory processes involved in HIV neuropathogenesis. Beside this, IL-33 receptor (ST2) plays a role in neuroinflammatory processes through the modulation of the biological action of IL-33. quantitative real time PCR (qRT-PCR), ELISA, Western blot (WB), and flow cytometry experiments were performed to elucidate the role of IL-33/ST2 in HIV neuropathogenesis in CNS cells. Apoptosis and mechanisms of IL-33 in neuronal cells were studied using caspase-3 assay and RT-PCR. Results of the studies suggest that the infection in CNS cells with HIV-1 clade B resulted in higher levels of IL-33/ST2L expression compared to HIV-1 clade C infection. Furthermore, higher concentrations of IL-33 were associated with a decrease in myocyte enhancer factor 2C (MEF2C) expression, a transcription factor that regulates synaptic function, and an increase in apoptosis, NOD2, and SLC11A1 in clade B infection. This led to neuroinflammation which dysregulates synaptic function and apoptosis. These parameters are common in neuroAIDS provoked by HIV infection.

IL-33 Receptor (ST2) Signalling is Important for Regulation of Th2-Mediated Airway Inflammation in a Murine Model of Acute Respiratory Syncytial Virus Infection

T1/ST2, an orphan receptor with homology with the interleukin (IL)-1 receptor family, is the ligand-binding component of the receptor for the cytokine IL-33, a newly identified cytokine known to amplify the Th2 cell-dominant immune responses. The function of IL-33/ST2 signalling during respiratory syncytial virus (RSV) infection is not fully known. In this study, following intranasal infection with RSV, BALB/c mice showed a marked increase in the production of IL-33, with an elevated expression of ST2 mRNA as well as a massive infiltration of CD45(+) ST2(+) cells in the lungs, suggesting that during the early phase of RSV infection, IL-33 target cells which express ST2 on cell surface, may play a critical role for the development of RSV-induced airway inflammation. Indeed, blocking ST2 signalling using anti-ST2 monoclonal antibody diminished not only RSV-induced eosinophil recruitment, but also the amounts of Th2-associated cytokines, particularly IL-13, and Th17-type cytokine IL-17A in the lungs of infected mice. However, anti-ST2 antibody treatment did not affect the production of Th1-type cytokine IFN-γ as well as pulmonary viral growth and clearance. These results indicate that IL-33/ST2 signalling is involved in RSV-induced, Th2-associated airway inflammation but not protective immunity.

Rhinovirus and asthma: a storied history of incompatibility

The human rhinovirus (HRV) is commonly associated with loss of asthma symptom control requiring escalation of care and emergency room visits in many patients. While the association is clear, the mechanisms behind HRV-induced asthma exacerbations remain uncertain. Immune dysregulation via aberrant immune responses, both deficient and exaggerated, have been proposed as mechanisms for HRV-induced exacerbations of asthma. Epithelium-derived innate immune cytokines that bias Th2 responses, including interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin (TSLP), have also been implicated as a means to bridge allergic conditions with asthma exacerbations. In this review, we discuss the literature supporting these positions. We also discuss new and emerging biotherapeutics that may target virus-induced exacerbations of asthma.

Membrane translocation of IL-33 receptor in ventilator induced lung injury

Ventilator-induced lung injury is associated with inflammatory mechanism and causes high mortality. The objective of this study was to discover the role of IL-33 and its ST2 receptor in acute lung injury induced by mechanical ventilator (ventilator-induced lung injury; VILI). Male Wistar rats were intubated after tracheostomy and received ventilation at 10 cm H2O of inspiratory pressure (PC10) by a G5 ventilator for 4 hours. The hemodynamic and respiratory parameters were collected and analyzed. The morphological changes of lung injury were also assessed by histological H&E stain. The dynamic changes of lung injury markers such as TNF-α and IL-1β were measured in serum, bronchoalveolar lavage fluid (BALF), and lung tissue homogenization by ELISA assay. During VILI, the IL-33 profile change was detected in BALF, peripheral serum, and lung tissue by ELISA analysis. The Il-33 and ST2 expression were analyzed by immunohistochemistry staining and western blot analysis. The consequence of VILI by H&E stain showed inducing lung congestion and increasing the expression of pro-inflammatory cytokines such as TNF-α and IL-1β in the lung tissue homogenization, serum, and BALF, respectively. In addition, rats with VILI also exhibited high expression of IL-33 in lung tissues. Interestingly, the data showed that ST2L (membrane form) was highly accumulated in the membrane fraction of lung tissue in the PC10 group, but the ST2L in cytosol was dramatically decreased in the PC10 group. Conversely, the sST2 (soluble form) was slightly decreased both in the membrane and cytosol fractions in the PC10 group compared to the control group. In conclusion, these results demonstrated that ST2L translocation from the cytosol to the cell membranes of lung tissue and the down-expression of sST2 in both fractions can function as new biomarkers of VILI. Moreover, IL-33/ST2 signaling activated by mechanically responsive lung injury may potentially serve as a new therapy target.

Crucial and diverse role of the interleukin-33/ST2 axis in infectious diseases

Interleukin-33 (IL-33) has now emerged as a cytokine with diverse and pleiotropic functions in various infectious and inflammatory diseases. IL-33 is expressed by epithelial cells, endothelial cells, fibroblasts, and hepatocytes. The target cells of IL-33 are Th2 cells, basophils, dendritic cells, mast cells, macrophages, NKT cells, and nuocytes, newly discovered natural helper cells/innate lymphoid cells bearing the ST2 receptor. IL-33 has dual functions, both as a traditional cytokine and as a nuclear factor that regulates gene transcription. IL-33 functions as an "alarmin" released following cell death, as a biomarker, and as a vaccine adjuvant, with proinflammatory and protective effects during various infections. The exacerbated or protective role of the IL-33/ST2 axis during different infections is dependent upon the organ involved, type of infectious agent, whether the infection is acute or chronic, the invasiveness of the infectious agent, the host immune compartment, and cellular and cytokine microenvironments. In this review, we focus on recent advances in the understanding of the role of the IL-33/ST2 axis in various viral, bacterial, fungal, helminth, and protozoal infectious diseases gained from animal models and studies in human patients. The functional role of IL-33 and ST2 during experimentally induced infections has been summarized by accumulating the data for IL-33- and ST2-deficient mice or for mice exogenously administered IL-33. In summary, exploring the crucial and diverse roles of the IL-33/ST2 axis during infections may help in the development of therapeutic interventions for a wide range of infectious diseases.

IL-33-dependent type 2 inflammation during rhinovirus-induced asthma exacerbations in vivo

RATIONALE

Rhinoviruses are the major cause of asthma exacerbations; however, its underlying mechanisms are poorly understood. We hypothesized that the epithelial cell-derived cytokine IL-33 plays a central role in exacerbation pathogenesis through augmentation of type 2 inflammation.

OBJECTIVES

To assess whether rhinovirus induces a type 2 inflammatory response in asthma in vivo and to define a role for IL-33 in this pathway.

METHODS

We used a human experimental model of rhinovirus infection and novel airway sampling techniques to measure IL-4, IL-5, IL-13, and IL-33 levels in the asthmatic and healthy airways during a rhinovirus infection. Additionally, we cultured human T cells and type 2 innate lymphoid cells (ILC2s) with the supernatants of rhinovirus-infected bronchial epithelial cells (BECs) to assess type 2 cytokine production in the presence or absence of IL-33 receptor blockade.

MEASUREMENTS AND MAIN RESULTS

IL-4, IL-5, IL-13, and IL-33 are all induced by rhinovirus in the asthmatic airway in vivo and relate to exacerbation severity. Further, induction of IL-33 correlates with viral load and IL-5 and IL-13 levels. Rhinovirus infection of human primary BECs induced IL-33, and culture of human T cells and ILC2s with supernatants of rhinovirus-infected BECs strongly induced type 2 cytokines. This induction was entirely dependent on IL-33.

CONCLUSIONS

IL-33 and type 2 cytokines are induced during a rhinovirus-induced asthma exacerbation in vivo. Virus-induced IL-33 and IL-33-responsive T cells and ILC2s are key mechanistic links between viral infection and exacerbation of asthma. IL-33 inhibition is a novel therapeutic approach for asthma exacerbations.

Respiratory viral infection, epithelial cytokines, and innate lymphoid cells in asthma exacerbations

Exacerbations of asthma are most commonly triggered by viral infections, which amplify allergic inflammation. Cytokines released by virus-infected AECs may be important in driving this response. This review focuses on accumulating evidence in support of a role for epithelial cytokines, including IL-33, IL-25, and TSLP, as well as their targets, type 2 innate lymphoid cells (ILC2s), in the pathogenesis of virus-induced asthma exacerbations. Production and release of these cytokines lead to recruitment and activation of ILC2s, which secrete mediators, including IL-5 and IL-13, which augment allergic inflammation. However, little information is currently available about the induction of these responses by the respiratory viruses that are strongly associated with exacerbations of asthma, such as rhinoviruses. Further human studies, as well as improved animal experimental models, are needed to investigate appropriately the pathogenetic mechanisms in virus-induced exacerbations of asthma, including the role of ILCs.

Importance of Both Innate Immunity and Acquired Immunity for Rapid Expulsion of S. venezuelensis

In the first part of this review, we described the relevant roles of endogenous IL-33 for accumulation of ILC2 and eosinophils even in the lungs of Rag2^−/− mice. Type II alveolar epithelial (ATII) cells express IL-33 in their nucleus and infection with Strongyloides venezuelensis induces IL-33 production by increasing the number of ATII cells possibly by the action of chitin. IL-33 from ATII cells induces ILC2 proliferation and at the same time activates them to produce IL-5 and IL-13, which in combination induce lung eosinophilic inflammation, aiding to expel infected worms in the lungs. In the second part, we showed that, although AID^−/− mice normally develop Th2 cells and intestinal mastocytosis after infection with S. venezuelensis, they need adoptive transfers of immune sera from S. venezuelensis infected mice to obtain the capacity to promptly expel S. venezuelensis. Thus, intestinal nematode infection induces various Th2 immune responses (e.g., Th2 cell, ILC2, goblet cell hyperplasia, intestinal mastocytosis, smooth muscle cell contraction, local and systemic eosinophilia, and high serum level of IgE and IgG1). However, all of them are not necessary for rapid expulsion of intestinal nematodes. Instead, some combinations of Th2 immune responses are essentially required.

The alarmin concept applied to human renal transplantation: evidence for a differential implication of HMGB1 and IL-33

The endogenous molecules high mobility group box 1 (HMGB1) and interleukin-33 (IL-33) have been identified as alarmins, capable of mediating danger signals during tissue damage. Here, we address their possible role as innate-immune mediators in ischemia-reperfusion injury (IRI) following human kidney transplantation. We analysed serum and urinary HMGB1 and IL-33 levels, all determined by enzyme-linked immunosorbent assay, in a cohort of 26 deceased renal transplant recipients. Urinary HMGB1 and IL-33 levels were significantly increased as soon as 30 min after reperfusion, as compared to those before treatment. Moreover, both serum and urinary IL-33 (but not HMGB1) increase was positively correlated with cold ischemia time, from 30 min to 3 days post-transplantation. In vitro, human umbilical vein endothelial cells subjected to hypoxia conditions released both HMGB-1 and IL-33, while only the latter was further increased upon subsequent re-oxygenation. Finally, we postulate that leukocytes from renal recipient patients are targeted by both HMGB1 and IL-33, as suggested by increased transcription of their respective receptors (TLR2/4 and ST2L) shortly after transplantation. Consistent with this view, we found that iNKT cells, an innate-like T cell subset involved in IRI and targeted by IL-33 but not by HMGB1 was activated 1 hour post-transplantation. Altogether, these results are in keeping with a potential role of IL-33 as an innate-immune mediator during kidney IRI in humans.

A direct and nonredundant role for thymic stromal lymphopoietin on antiviral CD8 T cell responses in the respiratory mucosa

Mucosally produced thymic stromal lymphopoietin (TSLP) regulates Th2 responses by signaling to dendritic cells and CD4 T cells. Activated CD8 T cells express the TSLP receptor (TSLPR), yet a direct role for TSLP in CD8 T cell immunity in the mucosa has not been described. Because TSLP shares signaling components with IL-7, a cytokine important for the development and survival of memory CD8 T cells in systemic infection models, we hypothesized that TSLP spatially and nonredundantly supports the development of these cells in the respiratory tract. In this study, we demonstrate that influenza infection induces the early expression of TSLP by lung epithelial cells with multiple consequences. The global loss of TSLP responsiveness in TSLPR(-/-) mice enhanced morbidity and delayed viral clearance. Using a competitive adoptive transfer system, we demonstrate that selective loss of TSLPR signaling on antiviral CD8 T cells decreases their accumulation specifically in the respiratory tract as early as day 8 after infection, primarily due to a proliferation deficiency. Importantly, the subsequent persistence of memory cells derived from this pool was also qualitatively and quantitatively affected. In this regard, the local support of antiviral CD8 T cells by TSLP is well suited to the mucosa, where responses must be tempered to prevent excessive inflammation. Taken together, these data suggest that TSLP uniquely participates in local immunity in the respiratory tract and modulation of TSLP levels may promote long-term CD8 T cell immunity in the mucosa when other prosurvival signals are limiting.

The amazing innate immune response to influenza A virus infection

Influenza A viruses (IAVs) remain a major health threat and a prime example of the significance of innate immunity. Our understanding of innate immunity to IAV has grown dramatically, yielding new concepts that change the way we view innate immunity as a whole. Examples include the role of p53, autophagy, microRNA, innate lymphocytes, endothelial cells and gut commensal bacteria in pulmonary innate immunity. Although the innate response is largely beneficial, it also contributes to major complications of IAV, including lung injury, bacterial super-infection and exacerbation of reactive airways disease. Research is beginning to dissect out which components of the innate response are helpful or harmful. IAV uses its limited genetic complement to maximum effect. Several viral proteins are dedicated to combating innate responses, while other viral structural or replication proteins multitask as host immune modulators. Many host innate immune proteins also multitask, having roles in cell cycle, signaling or normal lung biology. We summarize the plethora of new findings and attempt to integrate them into the larger picture of how humans have adapted to the threat posed by this remarkable virus. We explore how our expanded knowledge suggests ways to modulate helpful and harmful inflammatory responses, and develop novel treatments.

Differences in respiratory syncytial virus and influenza infection in a house-dust-mite-induced asthma mouse model: consequences for steroid sensitivity

A significant number of clinical asthma exacerbations are triggered by viral infection. We aimed to characterize the effect of virus infection in an HDM (house dust mite) mouse model of asthma and assess the effect of oral corticosteroids. HDM alone significantly increased eosinophils, lymphocytes, neutrophils, macrophages and a number of cytokines in BAL (bronchoalveolar lavage), all of which were sensitive to treatment with prednisolone (with the exception of neutrophils). Virus infection also induced cell infiltration and cytokines. RSV (respiratory syncytial virus) infection in HDM-treated animals further increased all cell types in BAL (except eosinophils, which declined), but induced no further increase in HDM-elicited cytokines. However, while HDM-elicited TNF-α (tumour necrosis factor-α), IFN-γ (interferon-γ), IL (interleukin)-2, IL-5 and IL-10 were sensitive to prednisolone treatment, concomitant infection with RSV blocked the sensitivity towards steroid. In contrast, influenza infection in HDM- challenged animals resulted in increased BAL lymphocytes, neutrophils, IFN-γ, IL-1β, IL-4, IL-5, IL-10 and IL-12, but all were attenuated by prednisolone treatment. HDM also increased eNO (exhaled NO), which was further increased by concomitant virus infection. This increase was only partially attenuated by prednisolone. RSV infection alone increased BAL mucin. However, BAL mucin was increased in HDM animals with virus infection. Chronic HDM challenge in mice elicits a broad inflammatory response that shares many characteristics with clinical asthma. Concomitant influenza or RSV infection elicits differing inflammatory profiles that differ in their sensitivity towards steroids. This model may be suitable for the assessment of novel pharmacological interventions for asthmatic exacerbation.

Group 2 innate lymphoid cell production of IL-5 is regulated by NKT cells during influenza virus infection

Respiratory virus infections, such as influenza, typically induce a robust type I (pro-inflammatory cytokine) immune response, however, the production of type 2 cytokines has been observed. Type 2 cytokine production during respiratory virus infection is linked to asthma exacerbation; however, type 2 cytokines may also be tissue protective. Interleukin (IL)-5 is a prototypical type 2 cytokine that is essential for eosinophil maturation and egress out of the bone marrow. However, little is known about the cellular source and underlying cellular and molecular basis for the regulation of IL-5 production during respiratory virus infection. Using a mouse model of influenza virus infection, we found a robust transient release of IL-5 into infected airways along with a significant and progressive accumulation of eosinophils into the lungs, particularly during the recovery phase of infection, i.e. following virus clearance. The cellular source of the IL-5 was group 2 innate lymphoid cells (ILC2) infiltrating the infected lungs. Interestingly, the progressive accumulation of eosinophils following virus clearance is reflected in the rapid expansion of c-kit⁺ IL-5 producing ILC2. We further demonstrate that the enhanced capacity for IL-5 production by ILC2 during recovery is concomitant with the enhanced expression of the IL-33 receptor subunit, ST2, by ILC2. Lastly, we show that NKT cells, as well as alveolar macrophages (AM), are endogenous sources of IL-33 that enhance IL-5 production from ILC2. Collectively, these results reveal that c-kit⁺ ILC2 interaction with IL-33 producing NKT and AM leads to abundant production of IL-5 by ILC2 and accounts for the accumulation of eosinophils observed during the recovery phase of influenza infection.

IL-5 is a cytokine that is typically associated with parasitic infections and allergic reactions. The primary role of IL-5 is thought to be for the development and maturation of an innate immune cell type, the eosinophil, which is also a culprit in allergic diseases such as asthma. During respiratory virus infection, such as influenza infection, IL-5 and eosinophils are not thought to play a major role in host defense. Here we show that IL-5 is produced in response to influenza infection and results in the progressive accumulation of eosinophils in the lung. We show that a newly discovered cell type, the group 2 innate lymphoid cell (ILC2), is responsible for IL-5 production during influenza infection and that the capacity of ILC2 to make IL-5 is greatly increased following virus clearance, i.e. during the recovery phase. The production of IL-5 by ILC2 is in part regulated by NKT cells and IL-33 produced by this cell type during the recovery phase of influenza infection.

Pathogenic mouse hepatitis virus or poly(I:C) induce IL-33 in hepatocytes in murine models of hepatitis

The IL-33/ST2 axis is known to be involved in liver pathologies. Although, the IL-33 levels increased in sera of viral hepatitis patients in human, the cellular sources of IL-33 in viral hepatitis remained obscure. Therefore, we aimed to investigate the expression of IL-33 in murine fulminant hepatitis induced by a Toll like receptor (TLR3) viral mimetic, poly(I:C) or by pathogenic mouse hepatitis virus (L2-MHV3). The administration of poly(I:C) plus D-galactosamine (D-GalN) in mice led to acute liver injury associated with the induction of IL-33 expression in liver sinusoidal endothelial cells (LSEC) and vascular endothelial cells (VEC), while the administration of poly(I:C) alone led to hepatocyte specific IL-33 expression in addition to vascular IL-33 expression. The hepatocyte-specific IL-33 expression was down-regulated in NK-depleted poly(I:C) treated mice suggesting a partial regulation of IL-33 by NK cells. The CD1d KO (NKT deficient) mice showed hepatoprotection against poly(I:C)-induced hepatitis in association with increased number of IL-33 expressing hepatocytes in CD1d KO mice than WT controls. These results suggest that hepatocyte-specific IL-33 expression in poly(I:C) induced liver injury was partially dependent of NK cells and with limited role of NKT cells. In parallel, the L2-MHV3 infection in mice induced fulminant hepatitis associated with up-regulated IL-33 expression as well as pro-inflammatory cytokine microenvironment in liver. The LSEC and VEC expressed inducible expression of IL-33 following L2-MHV3 infection but the hepatocyte-specific IL-33 expression was only evident between 24 to 32h of post infection. In conclusion, the alarmin cytokine IL-33 was over-expressed during fulminant hepatitis in mice with LSEC, VEC and hepatocytes as potential sources of IL-33.

Type 2 innate lymphocytes in allergic airway inflammation

CD4(+) T helper-2 (Th2) cells, which produce a unique profile of IL-4, IL-5 and IL-13 pro-inflammatory cytokines, are thought to be central in the orchestration and amplification of allergic asthma. However, a novel non-T/non-B lymphoid cell population, named type 2 innate lymphocytes (ILC2s), that produces high amounts of IL-5 and IL-13 was recently discovered. Unlike Th2 cells, these ILC2s are not antigen-restricted and are activated by epithelial cell-derived cytokines IL-25 and IL-33. In this review, we will focus on recent studies, mainly involving allergen-based mouse models, that have provided evidence for a significant contribution of ILC2 to allergic airway information.

T cell-mediated host immune defenses in the lung

Evidence has increasingly shown that the lungs are a major site of immune regulation. A robust and highly regulated immune response in the lung protects the host from pathogen infection, whereas an inefficient or deleterious response can lead to various pulmonary diseases. Many cell types, such as epithelial cells, dendritic cells, macrophages, neutrophils, eosinophils, and B and T lymphocytes, contribute to lung immunity. This review focuses on the recent advances in understanding how T lymphocytes mediate pulmonary host defenses against bacterial, viral, and fungal pathogens.

Virus/allergen interactions in asthma

Understanding the underlying mechanisms that cause and exacerbate allergic asthmatic disease is of great clinical interest. Clinical studies have revealed that allergies and viral respiratory illnesses are strongly linked to the inception and exacerbation of asthma, and suggest the possibility that there are interactive inflammatory mechanisms. Recent work has revealed a number of mechanisms of virus and allergen cross-talk that may play a role in the pathophysiology of allergic asthma, including (1) deficiency in virus-induced interferon responses, (2) defective epithelial barrier function, (3) increased release of epithelium-derived cytokines (e.g., thymic stromal lymphopoietin (TSLP), interleukin (IL)-25, IL-33), (4) dysregulation of lymphocytes [e.g., innate lymphoid cells (ILCs), regulatory T cells (Tregs)], and (5) altered activation of purinergic receptors. One or more of these processes may provide targets for new therapeutics to treat allergic asthma and prevent disease exacerbation.

IL-1 β and IL-6 upregulation in children with H1N1 influenza virus infection

The role of cytokines in relation to clinical manifestations, disease severity, and outcome of children with H1N1 virus infection remains thus far unclear. The aim of this study was to evaluate interleukin IL-1β and IL-6 plasma expressions and their association with clinical findings, disease severity, and outcome of children with H1N1 infection. We prospectively evaluated 15 children with H1N1 virus infection and 15 controls with lower respiratory tract infections (LRTI). Interleukin plasma levels were measured using immunoenzymatic assays. Significantly higher levels of IL-1β and IL-6 were detected in all patients with H1N1 virus infection compared to controls. It is noteworthy to mention that in H1N1 patients with more severe clinical manifestations of disease IL-1β and IL-6 expressions were significantly upregulated compared to H1N1 patients with mild clinical manifestations. In particular, IL-6 was significantly correlated with specific clinical findings, such as severity of respiratory compromise and fever. No correlation was found between interleukin expression and final outcome. In conclusion, H1N1 virus infection induces an early and significant upregulation of both interleukins IL1β and IL-6 plasma expressions. The upregulation of these cytokines is likely to play a proinflammatory role in H1N1 virus infection and may contribute to airway inflammation and bronchial hyperreactivity in these patients.

IL-22 is essential for lung epithelial repair following influenza infection

Influenza infection is widespread in the United States and the world. Despite low mortality rates due to infection, morbidity is common and little is known about the molecular events involved in recovery. Influenza infection results in persistent distal lung remodeling, and the mechanism(s) involved are poorly understood. Recently IL-22 has been found to mediate epithelial repair. We propose that IL-22 is critical for recovery of normal lung function and architecture after influenza infection. Wild-type and IL-22(-/-) mice were infected with influenza A PR8/34 H1N1 and were followed up for up to 21 days post infection. IL-22 receptor was localized to the airway epithelium in naive mice but was expressed at the sites of parenchymal lung remodeling induced by influenza infection. IL-22(-/-) mice displayed exacerbated lung injury compared with wild-type mice, which correlated with decreased lung function 21 days post infection. Epithelial metaplasia was observed in wild-type mice but was not evident in IL-22(-/-) animals that were characterized with an increased fibrotic phenotype. Gene expression analysis revealed aberrant expression of epithelial genes involved in repair processes, among changes in several other biological processes. These data indicate that IL-22 is required for normal lung repair after influenza infection. IL-22 represents a novel pathway involved in interstitial lung disease.

The evolutionary role of the IL-33/ST2 system in host immune defence

Interleukin (IL)-33 is a recently identified pleiotropic cytokine, which can orchestrate complex innate and adaptive immune responses in immunity and disease. It has been characterized as a cytokine of the IL-1 family and affects a wide range of immune cells by signalling through its receptor ST2L. Accumulating evidence suggests a crucial role of IL-33/ST2 in inducing and modifying host immune responses against a variety of pathogens including parasites, bacteria, viruses and fungi as well as sterile insults of both endogenous and exogenous source. In this review, we endeavour to give a comprehensive overview of the current knowledge about the role of IL-33 and its receptor ST2 in host defence against infections.

Blockade of IL-33 ameliorates Con A-induced hepatic injury by reducing NKT cell activation and IFN-γ production in mice

IL-33, a recently described member of the IL-1 family, has been identified as a cytokine endowed with pro-Th2 type functions. To date, there are only limited data on its role in physiological and pathological hepatic immune responses. In this study, we examined the role of IL-33 in immune-mediated liver injury by exploring the model of concanavalin A (Con A)-induced hepatitis. We observed that the level of IL-33 expression in the liver was dramatically increased at 12 h after Con A injection. Meanwhile, ST2L, the receptor of IL-33, was significantly up-regulated in lymphocytes including T and natural killer T (NKT) cells, especially in NKT cells. Moreover, administration of recombinant IL-33 exacerbated Con A-induced hepatitis, while pretreatment of IL-33-blocking antibody or psST2-Fc plasmids showed a protective effect probably by inhibiting the activation of late stage of T cells and NKT cells and also decreasing the production of the cytokine IFN-γ. Furthermore, depletion of NKT cells abolished the protective effect of IL-33-blocking antibody, and IL-33 failed to exacerbate Con A-induced hepatitis in IFN-γ(-/-) mice. These data suggested the critical roles of NKT cells and IFN-γ in the involvement of IL-33 in Con A-induced hepatitis. Blockade of IL-33 may represent a novel therapeutic strategy through IL-33/ST2L signal to prevent immune-mediated liver injury.

IL-33 and HMGB1 alarmins: sensors of cellular death and their involvement in liver pathology

'Alarmins' are a group of proteins or molecules that are released from cells during cellular demise to alert the host immune system. Two of them, Interleukin-33 (IL-33) and high-mobility group box-1 (HMGB1), share many similarities of cellular localization, functions and involvement in various inflammatory pathologies including hepatitis. The expressions of IL-33 and HMGB1, and their receptors ST2 and receptor for advanced glycation end products (RAGE), are substantially up-regulated during acute and chronic hepatitis. Recent data evidence a possible protective role of IL-33/ST2 axis during liver injury. A contrast in expression of IL-33 and HMGB1 alarmins were associated with type of hepatocellular death mediated by immune cells or hepato-toxic agents. The massive release of active form of IL-33 from hepatocytes may affect the recruitment and activation of its ST2-positive target immune cells in the liver to confer its alarmin functions. This review highlights the emerging roles of alarmin proteins in various liver pathologies, by focusing on classical HMGB1 and a newly discovered alarmin, the IL-33.

Recent insights into pulmonary repair following virus-induced inflammation of the respiratory tract

A hallmark of infection by respiratory viruses is productive infection of and the subsequent destruction of the airway epithelium. These viruses can also target other stromal cell types as well as in certain instances, CD45(+) hematopoietic cells either resident in the lungs or part of the inflammatory response to infection. The mechanisms by which the virus produces injury to these cell types include direct infection with cytopathic effects as a consequence of replication. Host mediated damage is also a culprit in pulmonary injury as both innate and adaptive immune cells produce soluble and cell-associated pro-inflammatory mediators. Recently, it has become increasingly clear that in addition to control of excess inflammation and virus elimination, the resolution of infection requires an active repair process, which is necessary to regain normal respiratory function and restore the lungs to homeostasis. The repair response must re-establish the epithelial barrier and regenerate the microarchitecture of the lung. Emerging areas of research have highlighted the importance of innate immune cells, particularly the newly described innate lymphoid cells, as well as alternatively activated macrophages and pulmonary stem cells in the repair process. The mechanisms by which respiratory viruses may impede or alter the repair response will be important areas of research for identifying therapeutic targets aimed at limiting virus and host mediated injury and expediting recovery.

Full-length IL-33 promotes inflammation but not Th2 response in vivo in an ST2-independent fashion

Expression of IL-33 is elevated in patients with pulmonary diseases, and full-length (not proteolytically processed) IL-33 is the predominant form in the lungs in health and disease. To determine whether activation of IL-33 is needed for functional effects, activities of full-length mouse and mature mouse (mm) forms of IL-33 were compared in vivo. Replication-deficient adenoviral constructs were used for gene delivery. Both isoforms caused pulmonary infiltration of lymphocytes and neutrophils, whereas mm IL-33 also caused pulmonary eosinophilia and goblet cell hyperplasia and increased expression of IL-4, IL-5, IL-13, IL-17, MCP-1, and KC. The different effects were not associated with differential release from IL-33-producing cells or by differences in subcellular distributions of IL-33 isoforms. Germline deficiency of the cell surface receptor chain ST2 abrogated the mm IL-33-induced Th2-associated effects (pulmonary eosinophilia, goblet cell hyperplasia, and increased IL-4 and IL-5), yet the lymphocytic infiltration induced by full-length mouse IL-33 or mm IL-33 was not fully abrogated by the absence of ST2. The similar effects of IL-33 isoforms were associated with comparable regulation of gene expression, notably matrix metalloproteinases 3, 10, and 13. Thus, full-length IL-33 is functionally active in vivo in an ST2-independent fashion, and its effects are partially different from those of mature IL-33. The different effects of these isoforms, particularly the pro-Th2 effects of mature IL-33, are due to differential utilization of the IL-33R chain ST2, whereas their similar effects result from regulation of gene expression.

Role of innate lymphocytes in infection and inflammation

Cooperation between the innate and adaptive immune responses is critical for enabling protective immunity against various invading microbes. Distinct types of effector T cells have different functions in adaptive immune responses. Th1 cells play important roles in the control of intracellular bacteria by producing IFN-γ to activate macrophages and in anti-viral immunity by producing IFN-γ and activating cytotoxic T lymphocytes. Th2 cell-derived cytokines are important in activating mast cells, eosinophils, and goblet cells in anti-helminth immunity. Th17 cells are pivotal for the inflammatory response mediated by neutrophils, which resists extracellular bacterial infection. In all cases, it is critical that the innate immune responses limit the growth and expansion of invading microbes until antigen-specific adaptive immune responses are established. Recent studies have identified multiple subsets in innate lymphocytes corresponding to previously defined Th subsets. Classical natural killer cells, RORγ⁺ lymphoid tissue inducer-related cells, and Th2-type innate lymphocytes play distinct roles in innate immune responses by producing Th1, Th17, and Th2 cytokines, respectively. Cooperation between innate lymphocytes and antigen-specific T and B cells are likely important in protective immunity against distinct types of microbes. The most recently identified subset is the RORγ-independent Lin⁻Thy-1⁺IL-7R⁺GATA3⁺ innate lymphocyte subset such as natural helper (NH) cell, which is Id2- and IL-7-dependent. This population produces Th2 cytokines, most notably IL-5 and IL-13, and plays a major role in innate immune responses during anti-helminth immunity. In addition, these cells are likely involved in the pathophysiology of some types of allergic diseases. We summarize here current knowledge regarding various innate lymphocyte subsets. In particular, we focus on the Th2-type innate lymphocyte subset.

The alarmin interleukin-33 drives protective antiviral CD8⁺ T cell responses

Pathogen-associated molecular patterns decisively influence antiviral immune responses, whereas the contribution of endogenous signals of tissue damage, also known as damage-associated molecular patterns or alarmins, remains ill defined. We show that interleukin-33 (IL-33), an alarmin released from necrotic cells, is necessary for potent CD8(+) T cell (CTL) responses to replicating, prototypic RNA and DNA viruses in mice. IL-33 signaled through its receptor on activated CTLs, enhanced clonal expansion in a CTL-intrinsic fashion, determined plurifunctional effector cell differentiation, and was necessary for virus control. Moreover, recombinant IL-33 augmented vaccine-induced CTL responses. Radio-resistant cells of the splenic T cell zone produced IL-33, and efficient CTL responses required IL-33 from radio-resistant cells but not from hematopoietic cells. Thus, alarmin release by radio-resistant cells orchestrates protective antiviral CTL responses.

Innate lymphoid cells promote lung-tissue homeostasis after infection with influenza virus

Innate lymphoid cells (ILCs), a heterogeneous cell population, are critical in orchestrating immunity and inflammation in the intestine, but whether ILCs influence immune responses or tissue homeostasis at other mucosal sites remains poorly characterized. Here we identify a population of lung-resident ILCs in mice and humans that expressed the alloantigen Thy-1 (CD90), interleukin 2 (IL-2) receptor a-chain (CD25), IL-7 receptor a-chain (CD127) and the IL-33 receptor subunit T1-ST2. Notably, mouse ILCs accumulated in the lung after infection with influenza virus, and depletion of ILCs resulted in loss of airway epithelial integrity, diminished lung function and impaired airway remodeling. These defects were restored by administration of the lung ILC product amphiregulin. Collectively, our results demonstrate a critical role for lung ILCs in restoring airway epithelial integrity and tissue homeostasis after infection with influenza virus.