Human Bronchial Epithelial Cell-derived Factors from Severe Asthmatic Subjects Stimulate Eosinophil Differentiation

Immunometabolism in the pathogenesis of asthma

Bronchial asthma is a heterogeneous disease characterised by chronic airway inflammation. A variety of immune cells such as eosinophils, mast cells, T lymphocytes, neutrophils and airway epithelial cells are involved in the airway inflammation and airway hyperresponsiveness in asthma pathogenesis, resulting in extensive and variable reversible expiratory airflow limitation. However, the precise molecular mechanisms underlying the allergic immune responses, particularly immunometabolism, remains unclear. Studies have detected enhanced oxidative stress, and abnormal metabolic progresses of glycolysis, fatty acid and amino acid in various immune cells, inducing dysregulation of innate and adaptive immune responses in asthma pathogenesis. Immunometabolism mechanisms contain multiple signalling pathways, providing novel therapy targets for asthma. This review summarises the current knowledge on immunometabolism reprogramming in asthma pathogenesis, as well as potential therapy strategies.

Successful Treatment of Eosinophilic Chronic Rhinosinusitis and Secretory Otitis Media in Refractory Asthma With Thymic Stromal Lymphopoietin (TSLP) Receptor Monoclonal Antibody

Eosinophilic chronic rhinosinusitis (ECRS) is a type 2 inflammatory disease that frequently co-occurs with bronchial asthma. The current treatment options for ECRS include endoscopic sinus surgery and oral corticosteroid therapy (OCS). However, recurrence after surgery is common, and OCS therapy may cause side effects. We present the case of a 74-year-old woman with severe asthma, ECRS, and secretory otitis media with possible eosinophilic otitis media, who experienced significant improvement in both conditions after treatment with tezepelumab, an anti-thymic stromal lymphopoietin (TSLP) antibody. Tezepelumab treatment led to a reduction in blood and tissue eosinophil counts. It improved the nasal polyp and computed tomography scores, tympanic and hearing test results, and asthma symptoms without using OCSs. Our findings suggest that tezepelumab may be a promising option for those patients with asthma, ECRS, and secretory otitis media who do not respond well to conventional treatment because upstream of the type 2 inflammation pathway is suppressed. Further to this case report, future studies are required to confirm the long-term efficacy and safety of tezepelumab in treating ECRS and secretory otitis media due to type 2 inflammation.

Sounding the alarmins-The role of alarmin cytokines in asthma

The alarmin cytokines thymic stromal lymphopoietin (TSLP), interleukin (IL)-33, and IL-25 are epithelial cell-derived mediators that contribute to the pathobiology and pathophysiology of asthma. Released from airway epithelial cells exposed to environmental triggers, the alarmins drive airway inflammation through the release of predominantly T2 cytokines from multiple effector cells. The upstream positioning of the alarmins is an attractive pharmacological target to block multiple T2 pathways important in asthma. Blocking the function of TSLP inhibits allergen-induced responses including bronchoconstriction, airway hyperresponsiveness, and inflammation, and subsequent clinical trials of an anti-TSLP monoclonal antibody, tezepelumab, in asthma patients demonstrated improvements in lung function, airway responsiveness, inflammation, and importantly, a reduction in the rate of exacerbations. Notably, these improvements were observed in patients with T2-high and with T2-low asthma. Clinical trials blocking IL-33 and its receptor ST2 have also shown improvements in lung function and exacerbation rates; however, the impact of blocking the IL-33/ST2 axis in T2-high versus T2-low asthma is unclear. To date, there is no evidence that IL-25 blockade is beneficial in asthma. Despite the considerable overlap in the cellular functions of IL-25, IL-33, and TSLP, they appear to have distinct roles in the immunopathology of asthma.

CBX4 Regulates Long-Form Thymic Stromal Lymphopoietin-Mediated Airway Inflammation Through SUMOylation in HDM-Induced Asthma

Thymic stromal lymphopoietin (TSLP) presents in two distinct isoforms: short-form (sfTSLP) and long-form (lfTSLP). lfTSLP promotes inflammation while sfTSLP inhibits inflammation in allergic asthma. However, little is known about the regulation of lfTSLP and sfTSLP during allergic attack in asthma airway epithelium. Here, we report that SUMOylation was enhanced in HDM-induced allergic asthma airway epithelium. Inhibition of SUMOylation significantly alleviated airway Th2 inflammation and lfTSLP expression. Mechanistically, CBX4, a SUMOylation E3 ligase, enhanced lfTSLP mRNA translation, but not sfTSLP, through the RNA binding protein, MEX-3B. MEX-3B promoted lfTSLP translation by binding the lfTSLP mRNA through its KH domains. Furthermore, CBX4 regulated MEX-3B transcription in human bronchial epithelial cell (HBE) through enhancing SUMOylation levels of the transcription factor, TFII-I. In conclusion, we demonstrate an important mechanism whereby CBX4 promotes MEX-3B transcription through enhancing TFII-I SUMOylation, and MEX-3B enhances the expression of lfTSLP through binding to the lfTSLP mRNA and promoting its translation. Our findings uncover a novel target of CBX4 for therapeutic agents to lfTSLP-mediated asthma.

Targeting the Epithelium-Derived Innate Cytokines: From Bench to Bedside

When epithelial cells are exposed to potentially threatening external stimuli such as allergens, bacteria, viruses, and helminths, they instantly produce "alarmin" cytokines, namely, IL-33, IL-25, and TSLP. These alarmins alert the immune system about these threats, thereby mobilizing host immune defense mechanisms. Specifically, the alarmins strongly stimulate type-2 immune cells, including eosinophils, mast cells, dendritic cells, type-2 helper T cells, and type-2 innate lymphoid cells. Given that the alarm-raising role of IL-33, IL-25, and TSLP was first detected in allergic and infectious diseases, most studies on alarmins focus on their role in these diseases. However, recent studies suggest that alarmins also have a broad range of effector functions in other pathological conditions, including psoriasis, multiple sclerosis, and cancer. Therefore, this review provides an update on the epithelium-derived cytokines in both allergic and non-allergic diseases. We also review the progress of clinical trials on biological agents that target the alarmins and discuss the therapeutic potential of these agents in non-allergic diseases.

Monoclonal Antibodies Targeting Alarmins: A New Perspective for Biological Therapies of Severe Asthma

Alarmins are innate cytokines, including thymic stromal lymphopoietin (TSLP), interleukin-33 (IL-33), and interleukin-25 (IL-25), which are mainly produced by airway epithelium and exert a prominent role in asthma pathobiology. In particular, several environmental factors such as allergens, cigarette smoking, airborne pollutants, and infectious agents trigger the release of alarmins, which in turn act as upstream activators of pro-inflammatory pathways underlying type 2 (T2-high) asthma. Indeed, alarmins directly activate group 2 innate lymphoid cells (ILC2), eosinophils, basophils, and mast cells and also stimulate dendritic cells to drive the commitment of naïve T helper (Th) cells towards the Th2 immunophenotype. Therefore, TSLP, IL-33, and IL-25 represent suitable targets for add-on therapies of severe asthma. Within this context, the fully human anti-TSLP monoclonal antibody tezepelumab has been evaluated in very promising randomized clinical trials. Tezepelumab and other anti-alarmins are thus likely to become, in the near future, valuable therapeutic options for the biological treatment of uncontrolled severe asthma.

Tezepelumab: A Potential New Biological Therapy for Severe Refractory Asthma

Thymic stromal lymphopoietin (TSLP) is an innate cytokine, belonging to the group of alarmins, which plays a key pathogenic role in asthma by acting as an upstream activator of cellular and molecular pathways leading to type 2 (T2-high) airway inflammation. Released from airway epithelial cells upon tissue damage induced by several noxious agents including allergens, viruses, bacteria, and airborne pollutants, TSLP activates dendritic cells and group 2 innate lymphoid cells involved in the pathobiology of T2-high asthma. Tezepelumab is a fully human monoclonal antibody that binds to TSLP, thereby preventing its interaction with the TSLP receptor complex. Preliminary results of randomized clinical trials suggest that tezepelumab is characterized by a good safety and efficacy profile in patients with severe, uncontrolled asthma.

Eosinophil Lineage-Committed Progenitors as a Therapeutic Target for Asthma

Eosinophilic asthma is the most prevalent phenotype of asthma. Although most asthmatics are adequately controlled by corticosteroid therapy, a subset (5-10%) remain uncontrolled with significant therapy-related side effects. This indicates the need for a consideration of alternative treatment strategies that target airway eosinophilia with corticosteroid-sparing benefits. A growing body of evidence shows that a balance between systemic differentiation and local tissue eosinophilopoietic processes driven by traffic and lung homing of bone marrow-derived hemopoietic progenitor cells (HPCs) are important components for the development of airway eosinophilia in asthma. Interleukin (IL)-5 is considered a critical and selective driver of terminal differentiation of eosinophils. Studies targeting IL-5 or IL-5R show that although mature and immature eosinophils are decreased within the airways, there is incomplete ablation, particularly within the bronchial tissue. Eotaxin is a chemoattractant for mature eosinophils and eosinophil-lineage committed progenitor cells (EoP), yet anti-CCR3 studies did not yield meaningful clinical outcomes. Recent studies highlight the role of epithelial cell-derived alarmin cytokines, IL-33 and TSLP, (Thymic stromal lymphopoietin) in progenitor cell traffic and local differentiative processes. This review provides an overview of the role of EoP in asthma and discusses findings from clinical trials with various therapeutic targets. We will show that targeting single mediators downstream of the inflammatory cascade may not fully attenuate tissue eosinophilia due to the multiplicity of factors that can promote tissue eosinophilia. Blocking lung homing and local eosinophilopoiesis through mediators upstream of this cascade may yield greater improvement in clinical outcomes.

TSLP-Driven Chromatin Remodeling and Trained Systemic Immunity after Neonatal Respiratory Viral Infection

Our studies have previously shown a role for persistent TSLP production in the lungs of mice after early-life respiratory syncytial virus (RSV) infection that leads to an altered immune phenotype, including accumulation of "inflammatory" dendritic cells (DC). This study investigates the role of TSLP driving systemic trained immunity in DC in early-life RSV-infected mice. Bone marrow-derived DCs (BMDC) from early-life RSV-infected mice at 4 wk postinfection showed enhanced expression of costimulatory molecules and cytokines, including Tslp, that regulate immune cell function. The adoptive transfer of BMDC grown from early-life RSV-infected mice was sufficient to exacerbate allergic disease development. The addition of recombinant TSLP during differentiation of BMDC from naive mice induced a similar altered phenotype as BMDC grown from early-life RSV-infected mice, suggesting a role for TSLP in the phenotypic changes. To assess the role of TSLP in these changes, global transcriptomic characterization of TSLPR^-/- BMDC infected with RSV was performed and showed a higher upregulation of type 1 IFN genes and concomitant downregulation of inflammatory genes. Assay for transposase-accessible chromatin using sequencing analysis demonstrated that TSLPR^-/- BMDC had a parallel gain in physical chromatin accessibility near type 1 genes and loss in accessibility near genes related to RSV pathology, with IFN regulatory factor 4 (IRF4) and STAT3 predicted as top transcription factors binding within differentially accessible regions in wild-type. Importantly, these studies show that in the absence of TSLP signaling, BMDC are able to mount an appropriate type 1 IFN-associated antiviral response to RSV. In summary, RSV-induced TSLP alters chromatin structure in DC to drive trained innate immunity and activates pathogenic gene programs in mice.

Anti-alarmin approaches entering clinical trials

PURPOSE OF REVIEW

The alarmins, thymic stromal lymphopoietin (TSLP), interleukin (IL)-25 and IL-33, are upstream regulators of T2 (type 2) inflammation and found to be expressed at high levels in airway epithelium of patients with T2 asthma. This review will summarize how alarmins regulate the inflamed asthmatic airways through previously described and newly identified mechanisms.

RECENT FINDINGS

Alarmins drive allergic and nonallergic asthma through activation of innate lymphoid cell 2 (ILC2), which are a rich source of cytokines such as IL-5 and IL-13, with resulting effects on eosinophilopoeisis and remodelling, respectively. Findings from bronchial allergen challenges have illustrated widespread expression of alarmins and their receptors across many effector cells in airways, and recent studies have emphasized alarmin regulation of CD4 T lymphocytes, eosinophils and basophils, and their progenitors. Furthermore, a link between alarmins and lipid mediators is being uncovered.

SUMMARY

Alarmins can drive well defined inflammatory pathways through activation of dendritic cells and polarizing T cells to produce type 2 cytokines, as well as they can directly activate many other effector cells that play a central role in allergic and nonallergic asthma. Clinical trials support a central role for TSLP in driving airway inflammation and asthma exacerbations, while ongoing trials blocking IL-33 and IL-25 will help to define their respective role in asthma.

TSLP Inhibitors for Asthma: Current Status and Future Prospects

Thymic stromal lymphopoietin (TSLP) is an allarmin cytokine whose importance in human asthma has been repeatedly documented. Accordingly, targeting of TSLP and TSLP-mediated signalling is considered as an attractive therapeutic strategy to asthma. Tezepelumab, which is the first-in-class anti-TSLP monoclonal antibodies (mAb), is a fully human IgG2λ mAb that binds human TSLP, prevents interaction with its receptor and, consequently, inhibits multiple downstream inflammatory pathways. Because of the excellent results of Phase II trials, the Food and Drug Administration granted tezepelumab as a 'breakthrough' biological drug for the treatment of severe asthma. Several studies with this mAb are ongoing. CSJ117 is an Ab fragment that binds to TSLP and is delivered by inhalation but there is no published information on this biologic agent. Since new information suggests that targeting TSLP may be more likely to improve day-to-day asthma symptoms, in contrast to targeting mediators of the adaptive immune system, approaches that primarily act to ameliorate asthma exacerbations, novel approaches capable of blocking TSLP (for example, fully human single-chain fragment variables against TSLP, bifunctional drugs such as the one that combines an anti-IL-13 mAb with an anti-TSLP mAb, a fusion protein consisting of the ectodomains of TSLPR and IL-7Ra that extend into the extracellular space, also known as a TSLP-trap, fragments capable of disrupting the TSLP:TSLPR complex) are under preclinical investigation. However, some critical aspects remain to be clarified before being able to define this approach as the one that will probably better help patients suffering from severe asthma because of its holistic effects.

The Role of the TL1A/DR3 Axis in the Activation of Group 2 Innate Lymphoid Cells in Subjects with Eosinophilic Asthma

Rationale: Group 2 innate lymphoid cells (ILC2s) are critical for type 2 inflammation. In murine models of asthma, some ILC2s remain activated in the absence of epithelial cell-derived cytokine signaling, implicating alternate stimulatory pathways. DR3 (death receptor 3), a member of the tumor necrosis factor receptor superfamily, is expressed on ILC2s. Genome-wide association studies report an association between DR3 ligand, TL1A (tumor necrosis factor-like protein 1A), and chronic inflammatory conditions.Objectives: We investigated the TL1A/DR3 axis in airway ILC2 biology in eosinophilic asthma.Methods: Stable subjects with mild asthma were subject to allergen inhalation challenge, and DR3 expression on sputum cells was assessed. We investigated cytokine regulation of DR3 expression on ILC2s and steroid sensitivity. Airway TL1A was assessed in sputum from subjects with mild asthma and subjects with prednisone-dependent severe eosinophilic asthma.Measurements and Main Results: There was a significant increase in sputum DR3⁺ ILC2s 24 hours after allergen challenge, and DR3 expression on ILC2s was upregulated by IL-2, IL-33, or TSLP in vitro. Stimulation with TL1A significantly increased IL-5 expression by ILC2s and was attenuated by dexamethasone, an effect that was negated in the presence of TSLP. Airway TL1A levels were increased 24 hours after allergen challenge in subjects with mild asthma but were significantly greater in those with severe eosinophilic asthma. The highest levels were detected in subjects with severe asthma with airway autoimmune responses. C1q⁺ immune complexes from the sputa of subjects with severe asthma with high autoantibody levels stimulated TL1A production by monocytes.Conclusions: The TL1A/DR3 axis is a costimulator of ILC2s in asthma, particularly in the airways of patients with a predisposition to autoimmune responses.

Thymic stromal lymphopoietin: its role and potential as a therapeutic target in asthma

INTRODUCTION

Thymic stromal lymphopoietin (TSLP), an epithelial cytokine (alarmin), is a central regulator of the immune response to inhaled environmental insults such as allergens, viruses and pollutants, initiating a cascade of downstream inflammation. There is compelling evidence that TSLP plays a major role in the pathology of asthma, and therapies that aim to block its activity are in development.

AREAS COVERED

We review studies conducted in humans and human cells, largely published in PubMed January 2010-October 2019, that investigated the innate and adaptive immune mechanisms of TSLP in asthma relevant to type 2-driven (eosinophilic/allergic) inflammation and non-type 2-driven (non-eosinophilic/non-allergic) inflammation, and the role of TSLP as a mediator between immune cells and structural cells in the airway. Clinical data from studies evaluating TSLP blockade are also discussed.

EXPERT OPINION

The position of TSLP at the top of the inflammatory cascade makes it a promising therapeutic target in asthma. Systemic anti-TSLP monoclonal antibody therapy with tezepelumab has yielded positive results in clinical trials to date, reducing exacerbations and biomarkers of inflammation in patients across the spectrum of inflammatory endotypes. Inhaled anti-TSLP is an alternative route currently under evaluation. The long-term safety and efficacy of TSLP blockade need to be evaluated.

Mechanical forces induce an asthma gene signature in healthy airway epithelial cells

Bronchospasm compresses the bronchial epithelium, and this compressive stress has been implicated in asthma pathogenesis. However, the molecular mechanisms by which this compressive stress alters pathways relevant to disease are not well understood. Using air-liquid interface cultures of primary human bronchial epithelial cells derived from non-asthmatic donors and asthmatic donors, we applied a compressive stress and then used a network approach to map resulting changes in the molecular interactome. In cells from non-asthmatic donors, compression by itself was sufficient to induce inflammatory, late repair, and fibrotic pathways. Remarkably, this molecular profile of non-asthmatic cells after compression recapitulated the profile of asthmatic cells before compression. Together, these results show that even in the absence of any inflammatory stimulus, mechanical compression alone is sufficient to induce an asthma-like molecular signature.

The role of type 2 innate lymphoid cells in eosinophilic asthma

Eosinophilic asthma has conventionally been proposed to be a T helper 2 driven disease but emerging evidence supports a central role of type 2 innate lymphoid cells (ILC2s). These are non-T, non-B cells that lack antigen specificity and produce more IL-5 and IL-13 than CD4⁺ T lymphocytes, on a cell per cell basis, in vitro. Although it is clear that ILC2s and CD4⁺ T cells work in concert with each other to drive type 2 immune responses, kinetic studies in allergic asthma suggest that ILC2s may act locally within the airways to "initiate" eosinophilic responses, whereas CD4⁺ T cells act locally and systemically to "perpetuate" eosinophilic inflammatory responses. Importantly, ILC2s are increased within the airways of severe asthmatics, with the greatest number of IL-5⁺ IL-13⁺ ILC2s being detected in sputum from severe asthmatics with uncontrolled eosinophilia despite high-dose steroid therapy. Although the precise relationship between ILC2s and steroid sensitivity in asthma remains unclear, controlling the activation of ILC2s within the airways may provide an effective therapeutic target for eosinophilic inflammation in airways diseases.

Sex-associated TSLP-induced immune alterations following early-life RSV infection leads to enhanced allergic disease

Many studies have linked severe RSV infection during early-life with an enhanced likelihood of developing childhood asthma, showing a greater susceptibility in boys. Our studies show that early-life RSV infection leads to differential long-term effects based upon the sex of the neonate; leaving male mice prone to exacerbation upon secondary allergen exposure while overall protecting female mice. During initial viral infection, we observed better viral control in the female mice with correlative expression of interferon-β that was not observed in male mice. Additionally, we observed persistent immune alterations in male mice at 4 weeks post infection. These alterations include Th2 and Th17-skewing, innate cytokine expression (Tslp and Il33), and infiltration of innate immune cells (DC and ILC2). Upon exposure to allergen, beginning at 4 weeks following early-life RSV-infection, male mice show severe allergic exacerbation while female mice appear to be protected. Due to persistent expression of TSLP following early-life RSV infection in male mice, genetically modified TSLPR-/- mice were evaluated and demonstrated an abrogation of allergen exacerbation in male mice. These data indicate that TSLP is involved in the altered immune environment following neonatal RSV-infection that leads to more severe responses in males during allergy exposure, later in life. Thus, TSLP may be a clinically relevant therapeutic target early in life.

Airway Eosinophilopoietic and Autoimmune Mechanisms of Eosinophilia in Severe Asthma

Eosinophils are critical in asthma biology, contributing to symptoms, airflow obstruction, airway hyperresponsiveness, and remodeling. In severe asthma, in addition to local maturation in bone marrow, in situ eosinophilopoiesis plays a key role in the persistence of airway eosinophilia. Local milieu of structural, epithelial and inflammatory cells contribute by generating eosinophilopoietic cytokines in response to epithelial-derived alarmins. Another mechanism of persistent airway eosinophilia is glucocorticosteroid insensitivity, which is linked to recurrent airway infections and presence of local autoantibodies. Novel molecules are being developed to target specific immune pathways as potential steroid-sparing strategies.

Eosinophil Development, Disease Involvement, and Therapeutic Suppression

Human eosinophils have characteristic morphologic features, including a bilobed nucleus and cytoplasmic granules filled with cytotoxic and immunoregulatory proteins that are packaged in a specific manner. Eosinophil production in the bone marrow is exquisitely regulated by timely expression of a repertoire of transcription factors that work together via collaborative and hierarchical interactions to direct eosinophil development. In addition, proper granule formation, which occurs in a spatially organized manner, is an intrinsic checkpoint that must be passed for proper eosinophil production to occur. In eosinophil-associated disorders, eosinophils and their progenitors can be recruited in large numbers into tissues where they can induce proinflammatory organ damage in response to local signals. Eosinophils are terminally differentiated and do not proliferate once they leave the bone marrow. The cytokine IL-5 specifically enhances eosinophil production and, along with other mediators, promotes eosinophil activation. Indeed, eosinophil depletion with anti-IL-5 or anti-IL-5Rα is now proven to be clinically beneficial for several eosinophilic disorders, most notably severe asthma, and several therapeutics targeting eosinophil viability and production are now in development. Significant progress has been made in our understanding of eosinophil development and the consequences of tissue eosinophilia. Future research efforts focused on basic eosinophil immunobiology and translational efforts to assist in the diagnosis, treatment selection, and resolution of eosinophil-associated disorders will likely be informative and clinically helpful.