Pulmonary Innate Lymphoid Cell Responses during Rhinovirus-induced Asthma Exacerbations In Vivo: A Clinical Trial

Tissue-specific features of innate lymphoid cells in antiviral defense

Innate lymphocytes (ILCs) rapidly respond to and protect against invading pathogens and cancer. ILCs include natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer (LTi) cells and include type I, type II, and type III immune cells. While NK cells have been well recognized for their role in antiviral immunity, other ILC subtypes are emerging as players in antiviral defense. Each ILC subset has specialized functions that uniquely impact the antiviral immunity and health of the host depending on the tissue microenvironment. This review focuses on the specialized functions of each ILC subtype and their roles in antiviral immune responses across tissues. Several viruses within infection-prone tissues will be highlighted to provide an overview of the extent of the ILC immunity within tissues and emphasize common versus virus-specific responses.

Therapeutic relevance of eosinophilic inflammation and airway viral interactions in severe asthma

The role of eosinophils in airway inflammation and asthma pathogenesis is well established, with raised eosinophil counts in blood and sputum associated with increased disease severity and risk of asthma exacerbation. Conversely, there is also preliminary evidence suggesting antiviral properties of eosinophils in the airways. These dual roles for eosinophils are particularly pertinent as respiratory virus infections contribute to asthma exacerbations. Biologic therapies targeting key molecules implicated in eosinophil-associated pathologies have been approved in patients with severe asthma and, therefore, the effects of depleting eosinophils in a clinical setting are of considerable interest. This review discusses the pathological and antiviral roles of eosinophils in asthma and exacerbations. We also highlight the significant reduction in asthma exacerbations seen with biologic therapies, even at the height of the respiratory virus season. Furthermore, we discuss the implications of these findings in relation to the role of eosinophils in inflammation and antiviral responses to respiratory virus infection in asthma.

Progesterone amplifies allergic inflammation and airway pathology in association with higher lung ILC2 responses

Perimenstrual worsening of asthma occurs in up to 40% of women with asthma, leading to increased acute exacerbations requiring clinical care. The role of sex hormones during these times remains unclear. In the current study, we used a translational approach to determine whether progesterone exacerbates allergic inflammation in the traditional chicken egg ovalbumin (OVA) model in BALB/c mice. Simultaneously, we used peripheral blood mononuclear cells (PBMC) from healthy human donors to assess the effects of progesterone on circulating group 2 innate lymphoid cells (ILC2). Briefly, lungs of ovariectomized (OVX) or sham-operated female (F-Sham) controls were implanted with a progesterone (P4, 25 mg) (OVX-P4) or placebo pellet (OVX-Placebo), followed by sensitization and challenge with ovalbumin (OVA). Progesterone increased total inflammatory histologic scores, increased hyper-responsiveness to methacholine (MCh), increased select chemokines in the bronchoalveolar lavage (BAL) and serum, and increased ILC2 and neutrophil numbers, along the airways compared with F-Sham-OVA and OVX-Placebo-OVA animals. Lung ILC2 were sorted from F-Sham-OVA, OVX-Placebo-OVA and OVX-P4-OVA treated animals and stimulated with IL-33. OVX-P4-OVA lung ILC2 were more responsive to interleukin 33 (IL-33) compared with F-Sham-OVA treated, producing more IL-13 and chemokines following IL-33 stimulation. We confirmed the expression of the progesterone receptor (PR) on human ILC2, and showed that P4 + IL-33 stimulation also increased IL-13 and chemokine production from human ILC2. We establish that murine ILC2 are capable of responding to P4 and thereby contribute to allergic inflammation in the lung. We confirmed that human ILC2 are also hyper-responsive to P4 and IL-33 and likely contribute to airway exacerbations following allergen exposures in asthmatic women with increased symptoms around the time of menstruation.NEW & NOTEWORTHY There is a strong association between female biological sex and severe asthma. We investigated the allergic immune response, lung pathology, and airway mechanics in the well-described chicken egg ovalbumin (OVA) model with steady levels of progesterone delivered throughout the treatment period. We found that progesterone enhances the activation of mouse group 2 innate lymphoid cells (ILC2). Human ILC2 are also hyper-responsive to progesterone and interleukin 33 (IL-33), and likely contribute to airway exacerbations following allergen exposures in women with asthma.

Innate lymphoid cells are activated in HFRS, and their function can be modulated by hantavirus-induced type I interferons

Hantaviruses cause the acute zoonotic diseases hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Infected patients show strong systemic inflammation and immune cell activation. NK cells are highly activated in HFRS, suggesting that also other innate lymphoid cells (ILCs) might be responding to infection. Here, we characterized peripheral ILC responses, and measured plasma levels of soluble factors and plasma viral load, in 17 Puumala virus (PUUV)-infected HFRS patients. This revealed an increased frequency of ILC2 in patients, in particular the ILC2 lineage-committed c-Kitlo ILC2 subset. Patients' ILCs showed an activated profile with increased proliferation and displayed altered expression of several homing markers. How ILCs are activated during viral infection is largely unknown. When analyzing PUUV-mediated activation of ILCs in vitro we observed that this was dependent on type I interferons, suggesting a role for type I interferons-produced in response to virus infection-in the activation of ILCs. Further, stimulation of naïve ILC2s with IFN-β affected ILC2 cytokine responses in vitro, causing decreased IL-5 and IL-13, and increased IL-10, CXCL10, and GM-CSF secretion. These results show that ILCs are activated in HFRS patients and suggest that the classical antiviral type I IFNs are involved in shaping ILC functions.

Update on asthma biology

This is an exciting time to be conducting asthma research. The recent development of targeted asthma biologics has validated the power of basic research to discover new molecules amenable to therapeutic intervention. Advances in high-throughput sequencing are providing a wealth of "omics" data about genetic and epigenetic underpinnings of asthma, as well as about new cellular interacting networks and potential endotypes in asthma. Airway epithelial cells have emerged not only as key sensors of the outside environment but also as central drivers of dysregulated mucosal immune responses in asthma. Emerging data suggest that the airway epithelium in asthma remembers prior encounters with environmental exposures, resulting in potentially long-lasting changes in structure and metabolism that render asthmatic individuals susceptible to subsequent exposures. Here we summarize recent insights into asthma biology, focusing on studies using human cells or tissue that were published in the past 2 years. The studies are organized thematically into 6 content areas to draw connections and spur future research (on genetics and epigenetics, prenatal and early-life origins, microbiome, immune and inflammatory pathways, asthma endotypes and biomarkers, and lung structural alterations). We highlight recent studies of airway epithelial dysfunction and response to viral infections and conclude with a framework for considering how bidirectional interactions between alterations in airway structure and mucosal immunity can lead to sustained lung dysfunction in asthma.

The SARS-CoV-2 pandemic and asthma: What we have learned and what is still unknown

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has brought new insights into the immunologic intricacies of asthma. In this review, we discuss the epidemiology of asthma in patients infected with SARS-CoV-2 and the risk of severe infection. Type 2 inflammation had an overall protective effect against SARS-CoV-2 infection by various mechanisms summarized in this review. Asthma, intranasal, and inhaled corticosteroids decreased the angiotensin-converting enzyme 2 receptor, an important receptor for SARS-CoV-2 entry into host cells. We summarize the nuances of the treatment of type 2 inflammation despite its underlying protective effects. Research to date has shown that patients on various allergen immunotherapies and biologics do benefit from being vaccinated.

Asthma in the era of COVID-19

Since its global invasion in 2019, COVID-19 has affected several aspects of patients' lives and posed a significant impact on the health care system. Several patient populations were identified to be at high risk of contracting SARS-CoV-2 infection and/or developing severe COVID-19-related sequelae. Conversely, anyone who has contracted SARS-CoV-2 is at risk to experience symptoms and signs consistent with post-COVID manifestations. Patients with asthma were initially thought to be at increased risk and severity for SARS-CoV-2 infection. However, accumulating evidence demonstrates that asthma endotypes/phenotypes and comorbidities influence the risk stratification in this population. Furthermore, initial concerns about the potentially increased risk of poor outcomes with asthma treatments such as inhaled corticosteroids and biologics have not been substantiated. In this review, we provide an update on COVID-19 and asthma, including risk of susceptibility, clinical manifestations and course in this population as well as discuss recommendations for management.

Interferons as negative regulators of ILC2s in allergic lung inflammation and respiratory viral infections

Group 2 innate lymphoid cells (ILC2s), characterized by a lack of antigen receptors, have been regarded as an important component of type 2 pulmonary immunity. Analogous to Th2 cells, ILC2s are capable of releasing type 2 cytokines and amphiregulin, thus playing an essential role in a variety of diseases, such as allergic diseases and virus-induced respiratory diseases. Interferons (IFNs), an important family of cytokines with potent antiviral effects, can be triggered by microbial products, microbial exposure, and pathogen infections. Interestingly, the past few years have witnessed encouraging progress in revealing the important role of IFNs and IFN-producing cells in modulating ILC2 responses in allergic lung inflammation and respiratory viral infections. This review underscores recent progress in understanding the role of IFNs and IFN-producing cells in shaping ILC2 responses and discusses disease phenotypes, mechanisms, and therapeutic targets in the context of allergic lung inflammation and infections with viruses, including influenza virus, rhinovirus (RV), respiratory syncytial virus (RSV), and severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2).

The Functional Role of Group 2 Innate Lymphoid Cells in Asthma

Asthma is a heterogeneous disease characterized by chronic airway inflammation. Group 2 innate lymphoid cells (ILC2) play an important role in the pathogenesis of asthma. ILC2s lack antigen-specific receptors and respond to epithelial-derived cytokines, leading to the induction of airway eosinophilic inflammation in an antigen-independent manner. Additionally, ILC2s might be involved in the mechanism of steroid resistance. Numerous studies in both mice and humans have shown that ILC2s induce airway inflammation through inflammatory signals, including cytokines and other mediators derived from immune or non-immune cells. ILC2s and T helper type 2 (Th2) cells collaborate through direct and indirect interactions to organize type 2 immune responses. Interestingly, the frequencies or numbers of ILC2 are increased in the blood and bronchoalveolar lavage fluid of asthma patients, and the numbers of ILC2s in the blood and sputum of severe asthmatics are significantly larger than those of mild asthmatics. These findings may contribute to the regulation of the immune response in asthma. This review article highlights our current understanding of the functional role of ILC2s in asthma.

Altered cell function and increased replication of rhinoviruses and EV-D68 in airway epithelia of asthma patients

Introduction

Rhinovirus (RV) infections constitute one of the main triggers of asthma exacerbations and an important burden in pediatric yard. However, the mechanisms underlying this association remain poorly understood.

Methods

In the present study, we compared infections of in vitro reconstituted airway epithelia originating from asthmatic versus healthy donors with representative strains of RV-A major group and minor groups, RV-C, RV-B, and the respiratory enterovirus EV-D68.

Results

We found that viral replication was higher in tissues derived from asthmatic donors for all tested viruses. Viral receptor expression was comparable in non-infected tissues from both groups. After infection, ICAM1 and LDLR were upregulated, while CDHR3 was downregulated. Overall, these variations were related to viral replication levels. The presence of the CDHR3 asthma susceptibility allele (rs6967330) was not associated with increased RV-C replication. Regarding the tissue response, a significantly higher interferon (IFN) induction was demonstrated in infected tissues derived from asthmatic donors, which excludes a defect in IFN-response. Unbiased transcriptomic comparison of asthmatic versus control tissues revealed significant modifications, such as alterations of cilia structure and motility, in both infected and non-infected tissues. These observations were supported by a reduced mucociliary clearance and increased mucus secretion in non-infected tissues from asthmatic donors.

Discussion

Altogether, we demonstrated an increased permissiveness and susceptibility to RV and respiratory EV infections in HAE derived from asthmatic patients, which was associated with a global alteration in epithelial cell functions. These results unveil the mechanisms underlying the pathogenesis of asthma exacerbation and suggest interesting therapeutic targets.

Asthma Management in Adults

Management of asthma in adults has advanced in the past 10 years. Central to these advances has been further clarification of type (T) 2 mechanisms of airway inflammation and utilization of T2 biomarkers, that is, eosinophils and fractional exhaled nitric oxide. In addition, epithelial cells are emerging as significant contributors to inflammation through generation of alarmins to initiate local injury as well as downstream pathways. Five new biologics, mepolizumab, benralizumab, reslizumab, dupilumab, and tezepelumab, were approved to join omalizumab and revolutionize severe asthma treatment. These biologics significantly prevent exacerbations to spare systemic corticosteroids use and their side effects. Guidelines attest to the effectiveness of inhaled corticosteroids/long-acting β-agonists (formoterol) for both maintenance and rescue therapy. Focused updates to the Expert Panel Report addressed limited but specific questions relevant to asthma control. Future guidelines should include phenotype/endotype-directed therapeutics to gain more precision-directed treatment.

Asthma exacerbations: the Achilles heel of asthma care

Asthma exacerbations significantly impact millions of patients worldwide to pose large disease burdens on affected patients, families, and health-care systems. Although numerous environmental factors cause asthma exacerbations, viral respiratory infections are the principal triggers. Advances in the pathophysiology of asthma have elucidated dysregulated protective immune responses and upregulated inflammation that create susceptibility and risks for exacerbation. Biologics for the treatment of severe asthma reduce rates of exacerbations and identify specific pathways of inflammation that contribute to altered pathophysiology, novel therapeutic targets, and informative biomarkers. Major steps to prevent exacerbations include the identification of molecular pathways whose blockage will prevent asthma attacks safely, predictably, and effectively.

The Therapeutic Potential for Targeting Group 2 Innate Lymphoid Cells in Asthma

T helper type 2 cells (Th2 cells) and group 2 innate lymphoid cells (ILC2s) play an important role in the pathophysiology of asthma, including airway eosinophilic inflammation. ILC2s are activated by epithelial-derived cytokines [interleukin-25 (IL-25), IL-33, and thymic stromal lymphopoietin (TSLP)] from airway epithelial cells, leading to the release of high amounts of type 2 cytokines, such as IL-5 and IL-13. ILC2s induce airway inflammation in an antigen-independent manner, and ILC2s are considered to be involved in the pathogenesis of asthma exacerbation. Furthermore, ILC2 activation might also confer steroid resistance. Many recent studies in humans and mice are increasingly demonstrating that the function of ILC2s is regulated not just by epithelial-derived cytokines but by a variety of cytokines and mediators derived from innate immune cells. Furthermore, the biologics targeting these cytokines and/or their receptors have been shown to reduce asthma exacerbations and improve lung function and quality of life in asthmatics. This article reviews the current treatment landscape for type 2 airway inflammation in asthma and discusses the therapeutic potential for targeting ILC2s.

Advancing Lung Immunology Research: An Official American Thoracic Society Workshop Report

The mammalian airways and lungs are exposed to a myriad of inhaled particulate matter, allergens, and pathogens. The immune system plays an essential role in protecting the host from respiratory pathogens, but a dysregulated immune response during respiratory infection can impair pathogen clearance and lead to immunopathology. Furthermore, inappropriate immunity to inhaled antigens can lead to pulmonary diseases. A complex network of epithelial, neural, stromal, and immune cells has evolved to sense and respond to inhaled antigens, including the decision to promote tolerance versus a rapid, robust, and targeted immune response. Although there has been great progress in understanding the mechanisms governing immunity to respiratory pathogens and aeroantigens, we are only beginning to develop an integrated understanding of the cellular networks governing tissue immunity within the lungs and how it changes after inflammation and over the human life course. An integrated model of airway and lung immunity will be necessary to improve mucosal vaccine design as well as prevent and treat acute and chronic inflammatory pulmonary diseases. Given the importance of immunology in pulmonary research, the American Thoracic Society convened a working group to highlight central areas of investigation to advance the science of lung immunology and improve human health.

Innate Immune Responses by Respiratory Viruses, Including Rhinovirus, During Asthma Exacerbation

Viral infection, especially with rhinovirus (RV), is a major cause of asthma exacerbation. The production of anti-viral cytokines such as interferon (IFN)-β and IFN-α from epithelial cells or dendritic cells is lower in patients with asthma or those with high IgE, which can contribute to viral-induced exacerbated disease in these patients. As for virus-related factors, RV species C (RV-C) induces more exacerbated disease than other RVs, including RV-B. Neutrophils activated by viral infection can induce eosinophilic airway inflammation through different mechanisms. Furthermore, virus-induced or virus-related proteins can directly activate eosinophils. For example, CXCL10, which is upregulated during viral infection, activates eosinophils in vitro. The role of innate immune responses, especially type-2 innate lymphoid cells (ILC2) and epithelial cell-related cytokines including IL-33, IL-25, and thymic stromal lymphopoietin (TSLP), in the development of viral-induced airway inflammation has recently been established. For example, RV infection induces the expression of IL-33 or IL-25, or increases the ratio of ILC2 in the asthmatic airway, which is correlated with the severity of exacerbation. A mouse model has further demonstrated that virus-induced mucous metaplasia and ILC2 expansion are suppressed by antagonizing or deleting IL-33, IL-25, or TSLP. For treatment, IFNs including IFN-β suppress not only viral replication but also ILC2 activation in vitro. Agonists of toll-like receptor (TLR) 3 or 7 can induce IFNs, which can then suppress viral replication and ILC2 activation. Therefore, if delivered in the airway, IFNs or TLR agonists could become innovative treatments for virus-induced asthma exacerbation.

Type I conventional dendritic cells relate to disease severity in virus-induced asthma exacerbations

RATIONALE

Rhinoviruses are the major precipitant of asthma exacerbations and individuals with asthma experience more severe/prolonged rhinovirus infections. Concurrent viral infection and allergen exposure synergistically increase exacerbation risk. Although dendritic cells orchestrate immune responses to both virus and allergen, little is known about their role in viral asthma exacerbations.

OBJECTIVES

To characterize dendritic cell populations present in the lower airways, and to assess whether their numbers are altered in asthma compared to healthy subjects prior to infection and during rhinovirus-16 infection.

METHODS

Moderately-severe atopic asthmatic patients and healthy controls were experimentally infected with rhinovirus-16. Bronchoalveolar lavage was collected at baseline, day 3 and day 8 post infection and dendritic cells isolated using fluorescence activated cell sorting.

MEASUREMENTS AND MAIN RESULTS

Numbers of type I conventional dendritic cells, which cross prime CD8⁺ T helper cells and produce innate interferons, were significantly reduced in the lower airways of asthma patients compared to healthy controls at baseline. This reduction was associated serum IgE at baseline and with reduced numbers of CD8⁺ T helper cells and with increased viral replication, airway eosinophils and reduced lung function during infection. IgE receptor expression on lower airway plasmacytoid dendritic cells was significantly increased in asthma, consistent with a reduced capacity to produce innate interferons.

CONCLUSIONS

Reduced numbers of anti-viral type I conventional dendritic cells in asthma are associated with adverse outcomes during rhinovirus infection. This, with increased FcεR1α expression on lower airway plasmacytoid DCs could mediate the more permissive respiratory viral infection observed in asthma patients.

Heterogeneity of type 2 innate lymphoid cells

More than a decade ago, type 2 innate lymphoid cells (ILC2s) were discovered to be members of a family of innate immune cells consisting of five subsets that form a first line of defence against infections before the recruitment of adaptive immune cells. Initially, ILC2s were implicated in the early immune response to parasitic infections, but it is now clear that ILC2s are highly diverse and have crucial roles in the regulation of tissue homeostasis and repair. ILC2s can also regulate the functions of other type 2 immune cells, including T helper 2 cells, type 2 macrophages and eosinophils. Dysregulation of ILC2s contributes to type 2-mediated pathology in a wide variety of diseases, potentially making ILC2s attractive targets for therapeutic interventions. In this Review, we focus on the spectrum of ILC2 phenotypes that have been described across different tissues and disease states with an emphasis on human ILC2s. We discuss recent insights in ILC2 biology and suggest how this knowledge might be used for novel disease treatments and improved human health.

Type 2 innate lymphoid cells (ILC2s) have diverse phenotypes across different tissues and disease states. Recent insights into ILC2 biology raise new possibilities for the improved treatment of cancer and of metabolic, infectious and chronic inflammatory diseases.

Rhinovirus Infections and Their Roles in Asthma: Etiology and Exacerbations

Rhinovirus infections can cause wheezing illnesses in all age groups. In preschool children, rhinovirus infections frequently initiate acute wheezing illnesses. Children who wheeze with rhinoviruses are at increased risk to go on to develop asthma. Once asthma is established, rhinovirus infections are potent triggers for acute airway obstruction and exacerbations in children and adults. Paradoxically, for most individuals, rhinovirus infections commonly cause cold symptoms with little or no involvement of the lower airways. This paradox has led investigators to identify specific risk factors and mechanisms for rhinovirus wheezing, and this review will outline progress in 3 main areas. First, the 3 species of rhinoviruses have different patterns of infection and virulence. Second, personal factors such as lung function and immunity influence lower respiratory outcomes of rhinovirus infection. The mucosal immune response is critical, and the quality of the interferon response and allergic inflammation interacts to determine the risk for rhinovirus wheezing. Finally, rhinovirus infections can promote pathogen-dominated airway microbiota that increase the risk for wheezing. Although specific antivirals for rhinovirus are still not available, identifying risk factors for wheezing illnesses has provided several other potential targets and strategies for reducing the risk of rhinovirus-induced wheezing and exacerbations of asthma.

Role of type-2 innate lymphoid cells (ILC2s) in type-2 asthma

PURPOSE OF REVIEW

The purpose of this review is to provide a synthesis of recent discoveries about type-2 innate lymphoid cells, especially, as they relate to the pathogenesis of asthma.

RECENT FINDINGS

We focused on features and characteristics of type-2 innate lymphoid cells (ILC2s) that distinguish them from other type-2 cells, especially Th2 cells. We collected and reviewed data related to human asthma and airway ILC2s. We examined the concept of ILC2 memory and trained immunity. We also analyzed steroid resistance of ILC2s, which is relevant for steroid-resistant asthma.

SUMMARY

The implications of the findings include an understanding of ILC2 inflammation, and pathways and molecules that can be targeted by biologics and other therapeutic agents for management severe and steroid-resistant asthma.