The Intriguing Role of Interleukin 13 in the Pathophysiology of Asthma

1Bronchial asthma is a chronic inflammatory disorder of the airways characterized by reversible airflow obstruction, bronchial hyperreactivity (BHR), mucus overproduction, angiogenesis, and airway remodeling. Asthma is a common disorder resulting in substantial morbidity, healthcare expenditure, and death. Worldwide, up to 300 million people are affected by asthma, making it one of the most common chronic diseases (World Health Organization. Global surveillance, prevention, and control of chronic respiratory diseases. http://www.who.int/gard/publications/GARD/2007) and approximately 250,000 people die from asthma each year. In the majority of patients, asthma can be controlled by combinations of inhaled glucocorticoids (ICS), short- or long-acting ß2-adrenergic agonists (LABA), long-acting muscarinic antagonists, and leukotriene receptor antagonists, according to the Global Initiative for Asthma (GINA) guidelines (GINA http://www.ginasthma.org/pdf). However, in approximately 15% of patients, the disease is refractory to conventional treatments and results in hospital admissions due to severe exarcerbations.

2As with many chronic inflammatory diseases, clinicians now realize that the traditional classification of asthma has been an oversimplification. Different asthma phenotypes, each with distinct pathophysiology, are now being defined as asthma endotypes. The heterogeneity of different forms of asthma reflects the involvement of different immune cell populations and inflammatory mediators. Asthma can be classified according to two major endotypes: “T2-high” asthma is characterized by increased levels of type 2 inflammation mainly mediated by mast cells, eosinophils, basophils, T-helper 2 cells, group 2 innate lymphoid cells (ILC2s), and immunoglobulin E (IgE)-producing B cells. Patients with T2-high asthma have eosinophilia and other signs of type 2 inflammation including high levels of IL-4 and IL-13. Increased blood and sputum levels of eosinophils, serum IgE, and the fraction of exhaled nitric oxide (FeNO) have been associated with the mechanisms of T2-high asthma. Another feature is the high expression of the prostaglandin D2 receptor chemoattractant receptor-homologous molecule (CRTH2) on Th2 lymphocytes. “T2-low” asthma is less well characterized and may include several different endotypes. It has been suggested that Th1 and Th17 pathways and neurogenic inflammation may be involved.

3Levels of IgE, blood eosinophils and FeNO can be useful to guide the selection of monoclonal antibodies (mAbs) in the treatment of different forms of severe uncontrolled asthma. For example, several studies have demonstrated the safety and efficacy of mAbs blocking IL-5 (i.e., mepolizumab and reslizumab) for treatment of patients with severe eosinophilic asthma. Benralizumab, a mAb against IL-5Rα, expressed on the surface of human eosinophils, is particularly effective in patients with severe asthma with high blood eosinophils. Because IgE and the high affinity receptor for IgE (FcεRI) play a central role in atopic asthma, a mAb anti-IgE (omalizumab) is indicated for the treatment of patients (aged ≥ 12 years) with moderate-to-severe uncontrolled allergic asthma. Serum concentrations of IgE are used to guide this treatment.

4IL-4 and IL-13 were among the first identified cytokines orchestrating Th2 inflammation. IL-4 and IL-13 are potent mediators of type 2 inflammation with both overlapping and distinct functions. Pascolizumab, a mAb selectively blocking IL-4, failed to produce positive effects. Two mAbs blocking IL-13 (i.e., anrukinzumab, lebrikizumab) have shown marginal effects in the treatment of asthmatic patients. Another mAb anti-IL-13 (i.e., tralokinumab, AstraZeneca) failed to reduce asthma exacerbation rate in severe uncontrolled asthmatics. Considering the individual relevance of IL-4 and IL-13 in the pathogenesis of asthma, these results are surprising and intriguing. A simplistic explanation is that individual blockade of IL-4 or IL-13 is insufficient to inhibit the complex orchestration of allergic inflammation and clinical consequences in severe asthma. This hypothesis is indirectly supported by the efficacy of dupilumab, which inhibits both IL-4 and IL-13 signaling mediated by IL-4Rα, in patients with severe uncontrolled asthma.

5In this review we analyze the biological and immunological effects of IL-13 in the context of experimental models of asthma and in asthmatic patients. Despite promising findings in several experimental models of allergic inflammation, the results of multicenter studies evaluating the efficacy of anti-IL-13 mAbs in patients with asthma were surprisingly negative. Possible explanations of these discrepancies are discussed.

6IL-13 is a pleiotropic cytokine originally cloned from activated human T-lymphocytes. The human IL13 gene is located on chromosome 5q31-33 in the cluster of genes encoding IL-4, IL-3, IL-5, IL-9, and granulocyte-macrophage colony-stimulating factor (GM-CSF). The gene encoding IL-13 is upstream of the IL4 gene, leading to the speculation that these genes arose as a duplication event during evolution. However, IL-13 has only 25% homology with IL-4 thus explaining why these cytokines share some, but not all functional properties. IL-13 can be produced by stimulated Th2 cells, B lymphocytes, CD8+ cells, type 2 ILCs, alveolar macrophages, human mast cells, and basophils.

7Figure 1 schematically illustrates the complex receptor system which mediates the signaling of IL-4 and IL-13. The IL-4Rα subunit is a component of both the type I and type II receptors. Type I receptors are composed of the IL-4Rα subunit complexed with common γ chain (γc); this receptor binds to IL-4 and is expressed on cells of hematopoietic stem cell origin. The type II receptor complex consists of IL-4Rα partnering with IL-13Rα1 and is found on many non-hematopoietic cells, such as bronchial epithelial cells, smooth muscle cells, fibroblasts, and keratinocytes. IL-4 signals through both the type I and type II receptor complexes whereas IL-13 signals only through the type II complex, because IL-13 binds to IL-13Rα1, whereas IL-4 primarily binds to IL-4Rα. In addition, the two cytokines have different functions and signaling. IL-4Rα, γc, and IL-13Rα1 all contain proline rich regions that can bind the Janus kinases JAK1, JAK2, JAK3, and TYK2. In hematopoietic cells that express γc and the associated JAK3, IL-4 binding to type I receptor results in the activation of JAK1, JAK2, and JAK3. IL-4 and IL-13 binding to type II receptor activate JAK1, JAK2, and TYK2. Activation of JAKs results in phosphorylation of cytoplasmic tyrosines leading to the recruitment of STAT6 to the receptor, followed by its phosphorylation and activation. The activation of STAT6 is the primary signaling event in the response to IL-4 or IL-13. In certain experimental conditions STAT1 and STAT3 can also be activated by both IL-4 and IL-13. The cytoplasmic domain of human IL-13Rα1 contains two tyrosine residues, which might serve as docking sites for STAT3. Phosphorylated STAT6 and STAT3 monomers dimerize and then translocate to the nucleus, bind to specific DNA elements to regulate transcription.

8LaPorte and collaborators have examined in detail the molecular and structural basis of the IL-4/IL-13 receptor system. They demonstrated that IL-4 first binds to IL-4Rα to form a binary complex which then binds to the γc to form the functional ternary complex (type I receptor). IL-4 and IL-13 can also bind with high affinity to IL-4Rα and IL-13Rα1, respectively (type II receptor). The authors also compared the kinetics and potency of IL-4 and IL-13 signaling. IL-4 induced tyrosine phosphorylation of STAT6 more rapidly and more potently than IL-13. The latter observation was supported by previous experiments on cultured airway smooth muscle cells. IL-4 is a central mediator for Th2 cell polarization, initiation of IgE synthesis, and recruitment of eosinophils. Although IL-13 has some redundancy in these effects, this cytokine has additional roles in mediating goblet cell hyperplasia, airway smooth muscle contractility, collagen deposition, and fibrosis.

9There is a distinct IL-13Rα2 subunit, to which only IL-13 binds. Initially this receptor was considered a decoy receptor involved in removing IL-13 by internalization. Although the IL-13Rα2 lacks canonical JAK-STAT signaling activity, this hypothesis has come into question because several studies have shown that, under certain circumstances, IL-13Rα2 can mediate IL-13 signaling. In human airways, it was found that IL-13Rα2 is involved in IL-13 signaling through the transcription factor activator protein-1 (AP-1) to induce the activation of TGF-β. Recently, it has been demonstrated that IL-13 induced phosphorylation of ERK1/2 and the downstream activation of AP-1-related genes in human nasal epithelial cells. The authors proposed that engagement of IL-13Rα2 by IL-13 activates mitogen-activated protein ERK1/2 pathway and downstream AP-1-related gene C-JUN.

10Figure 2 shows schematically that IL-13 is produced by several immune cells and has many diverse functions on a wide variety of cell types relevant to the pathogenesis of allergic disorders. IL-13 can be produced by activated ILC2, Th2 cells, mast cells, macrophages, basophils, eosinophils, and B cells. In human B cells IL-13 has similar effects as IL-4, including promoting B cell proliferation and inducing class switching to IgE and IgG4 in combination with CD40/CD40L and inducing expression of the low-affinity IgE receptor CD23. In macrophages IL-13 favors the M2 polarization. IL-13 promotes survival, activation, and recruitment of eosinophils. In addition, IL-13 stimulates eosinophil trafficking from the peripheral blood to the site of inflammation by inducing the production of IL-5 and eosinophil chemokines such as eotaxins IL-13 promotes FcεRI expression and proliferation of human mast.

11IL-13 also has important effects on non-hematopoietic cells, including endothelial cells, smooth muscle cells, fibroblasts, epithelial cells, and sensory neurons. IL-13 is a potent inducer of vascular cell adhesion molecule-1 (VCAM-1) on endothelial cells, an important aspect in the recruitment of eosinophils. In addition, IL-13 increases the expression of ß1 integrin and VCAM-1 on human lung fibroblasts and increases muscular contraction in response to acetylcholine. Furthermore, IL-13 enhances proliferation and cholinergic-induced contraction of smooth muscle cells and induces collagen synthesis in human fibroblasts contributing to airways remodeling. In epithelial cells, IL-13 is a potent inducer of eotaxin. Moreover, IL-13 induces mucus overproduction and goblet cell metaplasia. IL-13 induces vascular endothelial growth factors (VEGFs) which are pro-angiogenic factors relevant in bronchial asthma. Recently, it has been demonstrated that IL-13 and IL-4 directly activate mouse and human sensory neurons which express IL-13Rα1 and IL-4Rα.

12Seminal studies in animal models of allergic asthma demonstrated that selective neutralization of IL-13 reduced airway hypersensitivity (AHR), bronchoalveolar lavage (BAL) eosinophils, and mucus overproduction. Furthermore, IL-13 delivery to the airways caused all of these effects. Overexpression of IL-13 in the lung of mice caused mucus hypersecretion, subepithelial fibrosis, eotaxin production, and eosinophilic infiltration. Interestingly, mice with targeted deletion of IL-13 failed to develop allergen-induced AHR, despite the presence of eosinophilic pulmonary infiltration. Mice lacking STAT6 were protected from pulmonary effects of IL-13. Importantly, reconstitution of STAT6 in epithelial cells only was sufficient for IL-13-induced AHR and mucus production in the absence of inflammation and fibrosis. Administration of anti-IL-13 in a mouse model of chronic asthma inhibited eosinophil recruitment in the airways, goblet cell hyperplasia, and subepithelial fibrosis, but only marginally inhibited AHR. In another study, blockade of IL-13 with sIL-13Rα2-human IgG fusion protein inhibited AHR associated with brief allergen exposure, but did not modify AHR associated with chronic airway remodeling.

13An IL-13 vaccine prepared by inserting a murine IL-13 peptide into a viral carrier protein, induced sustained and intense anti-IL-13 IgG antibodies, associated with inhibition of ovalbumin (OVA)-induced acute airway allergic responses. In a more recent study, the same experimental approach suppressed BAL IL-13 concentration and eosinophils, subepithelial collagen deposition, and mucus hyperproduction. Interestingly, while IL-13 vaccine inhibited AHR development, it did not revert AHR. The latter findings suggest that IL-13 may be crucial in the development, but not in the maintenance of airway hyperesponsiveness.

14Il13ra2 gene silencing or blockade of IL-13Rα2 signaling led to marked downregulation of TGF-β1 production and collagen deposition in a model of lung fibrosis. In IL-13Rα2-deficient mice, AHR and airway inflammation (i.e., mucus production and BAL eosinophils) were attenuated compared to wild type mice following house dust challenge. More recently, it was reported that, in IL-4Rα-deficient mice, IL-13, but not IL-4, was required for development of OVA-mediated AHR and goblet cell hyperplasia. Munitz and collaborators studying Il13ra1 -/- mice found that IL-13Rα1 is critical for baseline IgE production, AHR, mucus production, and eotaxin production. By contrast, Th2 and IgE responses to antigen were IL-13Rα1-independent.

15In a novel mouse model of non-allergic asthma overexpression of the activator protein-1 (AP-1) subunit Fra2, caused airway inflammation with IL-13 overexpression, BAL eosinophilia, mucus hyperproduction, AHR, and peribronchial collagen deposition. Administration of anti-IL-13 antibody markedly decreased STAT6 phosphorylation in the lung, BAL eosinophilia, and goblet cell hyperplasia. However, peribronchial collagen deposition and bronchial smooth muscle width were not affected by anti-IL-13 administration. The interesting results obtained in this experimental model may be more reflective of severe asthma which exhibit poor response to mAb anti-IL-13.

16Genetic deletion of the IL-33 receptor in a mouse model of experimental asthma increased TSLP production, which stimulated the emergence of IL-13+ ILC2s and lung mast cells leading to airway hyperresponsiveness.

17Collectively, the results derived from different experimental models indicate that IL-13 plays an important role in the development of several aspects of asthma. However, it is becoming evident that IL-13 neutralization is not sufficient to reverse certain aspects of airway inflammation once they are established.

18Increased concentration of IL-13 have been found in the blood, sputum, bronchial mucosa, and BAL fluid of asthmatic patients compared to healthy individuals. Increased IL-13 expression in asthma was confirmed by IL-13 mRNA overexpression and by ex vivo stimulation of sputum T cells. Following allergen challenge, IL-13 is increased in BAL. Ex vivo BAL T cells express IL-13 mRNA; expression is inversely related to forced expiratory volume in 1 s (FEV1). Several studies have shown that IL-13 is expressed in bronchial biopsies in patients with asthma. IL-13+ ILC2 were increased in the circulation of asthmatic patients with levels correlating with asthma severity. High production of IL-13 by cord blood CD4+ T cells is a predictor of development of atopic disorders.

19Polymorphisms have been identified both in the IL-13 promoter (IL-13-1112 T) and the coding region (IL-13+Arg 130Gen, IL-13+2044G > A) in asthmatic patients. One of the polymorphisms (Arg 130 Gln) (A/G) identified occur in the region critical for receptor-ligand interactions. Multiple polymorphism in IL-4R gene have also been identified and associated with asthma. Recently, three meta-analyses of IL-13 polymorphisms in adults and children suggested that the IL-13+1923 C/T polymorphism is associated with increased risk of asthma.

20Several anti-IL-13 mAbs (anrukinzumab, lebrikizumab, tralokinumab) have provided an opportunity to investigate the role of this cytokine in the pathophysiology of severe asthma, as well as assessing treatment response. A phase 1 study evaluated the pharmacokinetics, safety, and tolerability of tralokinumab in asthmatic patients receiving three different i.v. doses: 1 mg/kg, 5 mg/kg, or 10 mg/kg. Despite the small sample size, pharmacokinetics were linear over the dose range studied. The half-life was found to be 2–3 weeks and tralokinumab exhibited an acceptable safety profile.

21A phase 2a, randomized, double-blind, placebo-controlled, parallel-group, multicenter study investigated the effects of different dose regimens of tralokinumab in 194 adults with moderate-to-severe asthma inadequately controlled with standard therapy. Three dose regimens were evaluated: 47 patients received s.c. tralokinumab 150 mg, 51 patients received tralokinumab 300 mg, 48 subjects received tralokinumab 600 mg, and 48 patients received placebo. The primary endpoint was the change from baseline in mean Asthma Control Questionnaire score (ACQ-6) at week 13. Secondary endpoints were change in FEV1, pre-bronchodilator lung function, patient-reported outcomes (PROs), rescue β2-agonist use, and safety outcomes. Mean ACQ-6 score improved in all treatment groups, from baseline to week 13. These changes in ACQ-6 persisted through week 24 and were greater in active patients with higher IL-13 sputum concentrations compared with subjects with lower IL-13 sputum concentrations or subjects receiving placebo. Improvement in FEV1 were higher in patients with peripheral eosinophil counts ≥ 300 cells/ml. Pulmonary function improvements were higher in tralokinumab patients with higher sputum IL-13 (≥ 10 pg/ml-1) compared to tralokinumab patients with lower sputum IL-13 (≤10 pg/ml-1) and patients receiving placebo. However, there was no significant difference in asthma exacerbation rate. The authors defined asthma exacerbations as either a progressive increase of asthma symptoms (cough, wheeze, chest tightness, and/or shortness of breath) or a reduction of >20% in peak expiratory flow or FEV1 from baseline that did not resolve after the initiation of rescue medications and resulted in an administration of systemic glucocorticoids.

22The phase 2b clinical trial run by Brightling and colleagues evaluated safety and tolerability profile and the reduction in exacerbation rate and FEV1 improvement. The authors randomized 452 severe asthmatic patients, all with two-to-six asthma exacerbations in the previous year, to receive tralokinumab (300 mg s.c. either every 2 weeks or every 2 weeks for 3 months and then every 4 weeks) or placebo as add-on therapy for 1 year. At the end of the study, they reported no changes in the annual exacerbation rate at week 52 (primary endpoint) in patients treated with tralokinumab, either every 2 or 4 weeks, versus placebo or in time to first exacerbation. The authors defined asthma exacerbation as an increase in asthma symptoms resulting in use or increase in dose of systemic glucocorticoids for three or more consecutive days. Similarly, secondary endpoints such as improvements in prebronchodilator FEV1, ACQ-6, and AQLQ(S) were not significant in patients treated with tralokinumab compared with placebo. They found a significant improvement in FEV1 in patients treated with tralokinumab every 2 weeks. In this study measurement of serum dipeptidyl peptidase-4 (DPP-4) and periostin concentrations were included as a predictive candidate biomarkers before the study was unmasked. Subgroup analyses of patients receiving tralokinumab every 2 weeks with airway reversibility at baseline, but not receiving oral glucocorticoids, showed some clinical improvements in the subgroup who had raised serum DPP-4 and periostin. These preliminary results suggested that certain subpopulations of patients with severe asthma might respond to tralokinumab treatment.

23A phase 2 multicenter, double-blind, randomized, placebo-controlled trial evaluated the effects of tralokinumab on eosinophilic airway inflammation in uncontrolled moderate-to-severe asthma (MESOS). In this study, participants aged 18–75 years were randomly assigned to receive tralokinumab (300 mg s.c. every 2 weeks) or placebo. The primary outcome measure was change from baseline to week 12 in bronchial biopsy eosinophil count. Secondary outcome measures included change in blood and sputum eosinophil counts. Exploratory outcomes included FeNO and blood IgE concentrations. Tralokinumab did not affect bronchial, peripheral blood, or eosinophil counts compared to placebo at week 12. FeNO concentrations and total blood IgE were significantly reduced. The authors concluded that IL-13 is not crucial for eosinophilic airway inflammation control in patients with moderate-to-severe asthma.

24Two large phase 3 clinical trials, STRATOS 1 and STRATOS 2, explored the use of periostin and DPP-4 as biomarkers of IL-13-driven inflammatory patterns in patients aged 12–75 years with severe uncontrolled asthma treated with tralokinumab (300 mg s.c. every 2 weeks for 52 weeks) or placebo. The primary endpoint was the annualized asthma exacerbation rate reduction at week 52 in the all-comers population for STRATOS 1 and in the biomarker-positive population for STRATOS 2. The results of both trials confirmed that tralokinumab did not improve the annual asthma exacerbation rate in the all-comers population with severe asthma. In contrast to the preliminary results of the phase 2 trial, periostin and DPP-4 were not shown to predict response to tralokinumab. In both trials, tralokinumab-treated participants had a small increase in blood eosinophil counts from baseline, whereas placebo-treated participants did not. A recent meta-analysis of six randomized clinical trials suggested that tralokinumab was well tolerated and modestly improved FEV1 but did not reduce asthma exacerbations in severe uncontrolled asthma.

25Given the need to reduce oral glucocorticoid administration in patients with severe asthma, treatments that may allow tapering of glucocorticoids without loss of disease control are needed. A 40-week, randomized, double-blind trial (TROPOS) evaluated the oral glucocorticoid-sparing potential of tralokinumab in patients with severe, uncontrolled asthma requiring maintenance glucocorticoid treatment plus ICS/LABA. One hundred forty patients were randomized to tralokinumab (300 mg s.c. every 2 weeks) or placebo. The primary endpoint was percentage change from baseline in average glucocorticoid dose at week 40, while maintaining asthma control. Secondary endpoints included patients with a prescribed maintenance glucocorticoid dose of ≤ 5 mg, those with greater than 50% reduction in prescribed maintenance glucocorticoids dose and annual asthma exacerbation rate. An asthma exacerbation was defined as worsening of asthma that required a temporary increase in systemic glucocorticoids for ⩾3 days or that resulted in an emergency-room or urgent-care visit that led to a temporary increase in systemic glucocorticoids for ⩾3 days to treat symptoms or an inpatient hospitalization due to asthma. There were no significant between-group differences for primary and secondary endpoints. Reporting of adverse events and serious adverse events were similar for the tralokinumab and placebo groups.

26A randomized, double-blind, placebo controlled study examined the effects of another anti-IL-13 mAb, lebrikizumab (250 mg s.c. once monthly for 6 months), on change in prebronchodilator FEV1 from baseline to week 12 in 219 adults with uncontrolled asthma. Lebrikizumab treatment was associated with greater improvement in percent change in FEV1 in patients with high pretreatment levels of serum periostin compared to patients with low periostin levels. In two replicate studies (LUTE and VERSE) in patients with moderate-to-severe uncontrolled asthma, lebrikizumab (37.5, 125 or 250 mg s.c. every 4 weeks) reduced asthma exacerbation rate by 60% compared to placebo in periostin-high patients and by 5% in periostin-low patients. In these studies the authors defined asthma exacerbation as new or increased asthma symptoms that led to treatment with systemic glucocorticoids or to hospitalization. Two replicate, phase 3 trials (LAVOLTA1 and LAVOLTA2) explored the use of periostin and eosinophilia (≥ 300 cells/µl) as biomarkers of IL-13-driven inflammatory patterns in patients with severe uncontrolled asthma treated with lebrikizumab (37.5 mg or 125 mg s.c. once every 4 weeks for 52 weeks or placebo). The primary endpoint was the reduction in the rate of asthma exacerbations over 52 weeks in biomarker-high patients (periostin ≥ 50 ng/ml or blood eosinophils ≥ 300 cells/µl. In contrast to the preliminary results of phase 2 trials, lebrikizumab did not consistently show significant reduction in asthma exacerbations in biomarker-high patients.

27Chronic rhinosinusitis with nasal polyps (CRSwNP) is a common and relevant comorbidity for severe asthma. The prevalence of CRSwNP is greater in asthmatics compared to the general population and it increases with the severity of asthma, with the highest prevalence rates in non-atopic, late-onset, severe asthmatics. More than 60% of patients with CRSwNP have asthma. Moreover, CRWwNP is one of the significant determinants of poor asthma control and a hallmark of refractory eosinophilic asthma. The clinical evidence of a strong relationship between CRSwNP and severe asthma raises the possibility of a shared pathogenesis. In particular both diseases seem to be dependent on eosinophilic inflammation mediated by epithelial cytokines such as thymic stromal lymphopoietin (TSLP), IL-25, and IL-33 secreted as a consequence of epithelial damage. The latter activation is mediated by ILC2 which produce several type 2 cytokines such as IL-13 and IL-5. Recently, it has been reported that there were significant increases in IL-13, IL-13Rα1, and IL-13Rα2 mRNA and protein concentrations in nasal polyp epithelium. Moreover, IL-13 treatment resulted in mucus overproduction and impairment of ciliary function of human nasal epithelial cells. Therefore, an anti-IL-13 strategy, such as tralokinumab, may be helpful in the management of patients with nasal polyps. Unfortunately, there are no clinical trials of treatment of CRSwNP with anti-IL13 mAbs. Interestingly, patients treated with dupilumab, a mAb against IL-4Rα, the common receptor for both IL-4 and IL-13 had a significant reduction in polyp size and improvement in symptoms and nasal and olfactory function.

28Three phase 2 clinical trials and three phase 3 clinical trials have shown that tralokinumab did not lower the annual exacerbation rate and did not improve ACQ-6 scores compared to placebo in severe uncontrolled asthmatic patients. These negative findings parallel the results of two replicate, phase 3 trials with lebrikizumab. These negative results are surprising given the wide spectrum of pro-inflammatory and pro-fibrogenic activities of IL-13 in experimental models of asthma and in asthmatic patients.

29There are several possible explanations of these negative findings. First, it is likely that IL-13 is not the main cytokine involved in the complex network of severe asthma pathogenesis, meaning blocking it alone is ineffective. Second, as shown in several trials, the biomarkers (e.g., periostin, DPP-4, peripheral eosinophil count) used to identify responders to anti-IL-13 therapy are not optimal. Third, in both experimental models and clinical studies blocking IL-13 appears to have no effect on reducing tissue or blood eosinophilia, the pathophysiological feature most closely linked to asthma exacerbations. Fourth, perhaps the route of administration (i.e., s.c.) and the size of the anti-IL-13 mAb particles are not ideal. For instance, it has been reported the preliminary efficacy of a nebulized inhaled anti-IL-13 mAb antigen-binding fragment in macaque model of asthma. Finally, we cannot exclude the possibility that some of these negative findings could be due to the inclusion in clinical trials of patients with Th2-low asthma.

30Interestingly, the initial attempts to develop cytokine therapies for asthma focusing on antagonizing IL-4 were also unsuccessful, as was an attempt to block the combined receptor. By contrast, dupilumab, which binds to IL-4Rα and consequently blocks both IL-4 and IL-13 signaling, decreases asthma exacerbations and improves respiratory symptoms in patients with persistent asthma. These observations suggest that only the effective simultaneous blockade of signaling from two main cytokines (i.e., IL-4 and IL-13) is effective in the treatment of severe asthma. Using allergic preclinical models, it has been demonstrated that the combined blockade of the IL-13 and IL-33 pathways leads to a greater inhibition of type 2 inflammation over inhibition of either pathway alone. Similarly, co-blockade of IL-13 and IL-25 attenuated AHR, eosinophil infiltration in the lung, and mucus hyperproduction in a mouse model of OVA-induced asthma. Recently, a novel dual antagonist anti-TSLP/IL-13 bispecific antibody has been described. It will be interesting to see whether combinatorial blockade of multiple cytokines, including IL-13, may yield additional efficacy over single-axis therapies alone.

31It is intriguing that IL-13 blockade modulates several aspects of different experimental models of allergic asthma. Moreover, recent results demonstrate that anti-IL-13 antibody improves bronchial hyperresponsiveness and mucus production in a mouse model of non-allergic asthma. These positive experimental results contrast with negative results in the treatment of asthmatic patients with different anti-IL-13 mAbs (tralokinumab and lebrikizumab). These findings highlight that the results from murine studies do not always predict clinical effectiveness.

32In conclusion, despite several efforts, attempts to demonstrate a benefit of anti-IL-13 in patients with severe asthma remain unproven. While this may be because the right predictive biomarkers or patient phenotypes have not yet been identified, it is perhaps more likely that there is enough redundancy in the pathophysiology of severe asthma to persist without IL-13. This is not to say, at this stage, that another clinical indication might not be found for IL-13-blocking drugs in future.

33All authors contributed to reviewing the current literature and writing of the manuscript and approved the final version of the paper. Conceptualization: GM, GS, GV. Original draft preparation: GM, GS, GV. Final editing: GM, FG, VP, AP, EH, SL, GS, GV.

34This work was supported in part by grants from CISI-Lab Project (University of Naples Federico II) and TIMING Project (Regione Campania).

35EH is advisory board member for AstraZeneca, Sanofi- Genzyme, Novartis, GSK, Circassia, Nestlè, and Purina.

36The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.