Understanding the key issues in the treatment of uncontrolled persistent asthma with type 2 inflammation

Inflammatory response

The interplay between innate and adaptive cells and mediators in type 2 inflammation underpins asthma pathophysiology (figure 2) [3, 16, 24]. Type 2 immune responses in the airway are mediated mainly by eosinophils, mast cells, basophils, Th2 cells, ILC2s and IgE-producing B-cells. Upstream events in the airway epithelium involving secretion of master regulators or alarmins such as IL-33, IL-25 or thymic stromal lymphopoietin (TSLP) regulate maturation of CD4⁺ T-cells to induce a Th2 adaptive immune response. Th2 cells migrate to the airway epithelium, where, together with ILC2 cells, they generate a variety of type 2 cytokines (IL-4, IL-5 and IL-13) that aid in eosinophil recruitment and B-cell class switching [13, 24]. These cytokines play key roles in the disease pathophysiology and drive a variety of downstream events, including activation of airway epithelial cells, chemoattraction of effector cells, regulation of airway smooth muscle and remodelling of the epithelial matrix. These responses, in turn, predispose towards hyperresponsiveness, airflow obstruction and exacerbations. In addition to the adaptive response to external stimuli, IL-25, IL-33 and TSLP can directly activate ILC2s to secrete IL-5, IL-13 and IL-9, the latter contributing to the further activation and survival of ILC2s [25, 26]. In vitro studies have shown that IL-4 and IL-13 can induce B-cell switching and IgE production, leading to the sensitisation of mast cells and basophils and consequent release of pro-inflammatory mediators such as type 2 cytokines (IL-4, IL-5 and IL-13), histamine, leukotrienes and prostaglandin D₂ upon allergen exposure [16]. The overlapping functions of IL-4 and IL-13 may be due to receptor and cytokine expression patterns, because both cytokines signal through two potentially heterodimeric receptors with a shared receptor moiety, the IL-4Rα chain [27].

The central clinical role of IL-4 and IL-13 in type 2 inflammation is recognised in the current GINA guidelines, which recommend blockade of IL-4Rα, the shared receptor component for IL-4 and IL-13, in the treatment of patients with severe type 2 asthma (as well as atopic dermatitis (AD) or chronic rhinosinusitis with nasal polyps (CRSwNP). By contrast, anti-IgE and anti-IL-5/anti-IL-5R antibodies are used as targeted therapies against components of type 2 inflammation and they are restricted to use in patients with severe allergic asthma and eosinophilic asthma, respectively (figure 1) [2, 21].

Epithelial barrier function and epithelium-derived cytokines

Epithelial barrier dysfunction in response to chronic airway injury is a pathophysiological feature shared across type 2 inflammatory airway diseases. Disruption of the epithelial barrier allows environmental agents (e.g. airway viruses, allergens, bacteria and fungi) to penetrate the epithelium and activate innate and adaptive immune responses, resulting in histological changes and functional abnormalities in the airway mucosal epithelium. The release of epithelium-derived cytokines such as TSLP, IL-25 and IL-33 from bronchial and nasal epithelial cells contributes to the overall pathophysiology of asthma, and the damage to barrier function increases mucosal permeability to foreign substances [28]. Epithelium-derived cytokines recruit dendritic cells and promote their maturation; they in turn activate T-cells via antigen presentation and co-stimulation [29]. They also act as chemoattractants for eosinophils and neutrophils, and secretion of TSLP, IL-25 and IL-33 can further stimulate the production of key type 2 cytokines (i.e. IL-4, IL-5 and IL-13) in inflammatory cells of the innate (ILC2s) and adaptive (Th2 cells) arms of the immune system [3, 30]. These cytokines contribute to epithelial barrier disruption, increasing epithelial permeability [31, 32]. Additionally, release of IL-13 by ILC2s stimulates epithelial cells to increase IL-33 production, resulting in a positive feed-forward loop [33]. Exposure of human bronchial epithelial cells to IL-4 and IL-13 in vitro substantially impairs the airway epithelial cell junctional complex structure and function in a Janus kinase-dependent manner. This suggests that Th2-cytokine-dependent barrier disruption may underlie the observed defects in barrier function seen in allergic asthma [31]. Disruption of bronchial epithelial tight junction barrier proteins by IL-13 in in vivo studies highlights another essential mechanism of asthma pathogenesis [32].

Similar results have also been observed with other pro-inflammatory cytokines (including tumor necrosis factor-α and interferon-γ) that disrupt the tight junction of human airway epithelium and promote inflammatory cytokine release [34]. Epithelial barrier fragility is also strongly associated with the release of epithelial cytokines, TSLP, IL-25 and IL-33, due to the activation of airway epithelial cells, dendritic cells and ILC2s [28].

Mucus secretion

The role of mucus plugging of the airways in the pathophysiology of asthma has long been recognised [35]. The principal cause of death from asthma is asphyxiation from profound airflow obstruction, which likely includes intraluminal airway obstruction by widespread mucus plugs [36]. Excess mucus production contributes to airflow obstruction [37], and the subsequent formation of mucus plugs is associated with decreased lung function. This pathobiology is commonly observed in patients with severe airflow obstruction.

Mucus metaplasia (i.e. increased surface epithelial mucin production) and an increased number of bronchial microvessels are components of airway remodelling in asthma and are a predisposition to mucus dysfunction [38]. These changes occur in patients with airway inflammation and are characterised by infiltration of the airway wall and luminal mucus with CD4⁺ T-cells, eosinophils and ILC2s that secrete Th2 cytokines [39].

In a mouse model, IL-13 signalling affects goblet cell count following an allergic stimulus and plays a significant role in the generation and persistence of airway inflammation, remodelling and dysfunction [40]. IL-13 also increases production of the glycoprotein mucin 5AC (MUC5AC), which is associated with a more adherent type of mucus, over MUC5B. Changes in mucus composition and organisation are likely to be major contributors to airway obstruction, morbidity and mortality in asthma [36].

Smooth muscle function and airway remodelling

Airway remodelling is a collective term that represents the multiple pathological changes that occur in the airway epithelium and submucosa and lead to permanent structural changes of the airway in patients with asthma [3]. IL-4 and IL-13 cause changes in the airway mucosa that may predispose patients with asthma to exacerbations by narrowing the baseline airway calibre and altering structural elements that then exaggerate responses to inhaled triggers (figure 3) [3, 16, 41]. IL-5 has also been implicated in epithelial desquamation and airway remodelling [42]. Ongoing inflammation and airway remodelling in asthma also contribute to a self-perpetuating cycle of exacerbations in the exacerbation-prone phenotype [43].

Airway smooth muscle cells express receptors for cytokines derived from CD4⁺ T-cells. These cytokines, particularly IL-13, can act directly on airway smooth muscle cells, leading to changes in contractile and relaxant responses, proliferation and the ability of smooth muscle cells to generate chemokines such as eotaxin and thymus and activation-regulated chemokine (TARC), thus potentiating mitogenic effects [44, 45].

IL-13 facilitates calcium influx (a mechanism that mediates airway smooth muscle contraction) in response to contraction to histamine in human airway smooth muscle cells [44]. IL-13 signalling also affects histological responses and is a key mediator of the tissue fibrosis caused by type 2 inflammation [46, 47]. The fibrogenic effects of IL-13 can be mediated by transforming growth factor-β (TGF-β), and IL-13 selectively stimulates TGF-β production in Clara cell 10-kDa (CC10)–IL-13 transgenic mice. TGF-β antagonists (soluble TGFβR-Fc) decrease IL-13-induced lung collagen production in mice [46]. There is also evidence that IL-13 induces sub-epithelial fibrosis through direct effects on fibroblast proliferation and collagen production, both dependent on [48] and independent of [49] TGF-β. IL-13 also increases fibroblast invasiveness in human airway fibroblasts in patients with asthma [47, 50].

Eosinophils are thought to also play an important role in the airway remodelling process. Epithelial-derived cytokines, including chemokine ligands CXCL8, CXCL5 and CCL11, promote the infiltration of eosinophils into the airway tissue [29]. Eosinophils secrete numerous pro-fibrotic molecules, such as eosinophil cationic protein and TGF-β1, which are key drivers of airway remodelling and pro-inflammatory cytokines (e.g. IL-5) perpetuating the tissue inflammation. In vitro studies have also demonstrated that eosinophils interact with both mast cells and epithelial cells, inducing epithelial secretion of inflammatory cytokines and stimulating mast cells to produce fibrogenic factors as well as tumor necrosis factor-α, which further stimulates TGF-β1 release from eosinophils [51, 52].

Chronic rhinosinusitis with nasal polyps

Epithelial barrier dysfunction and mucosal deficits contribute to Staphylococcus aureus colonisation of nasal polyps, which then contribute to the development of chronic inflammation in CRSwNP. S. aureus enterotoxins skew the cytokine response towards a type 2 phenotype with increased IL-4, IL-5 and IL-13 expression, which initiates and perpetuates the inflammatory response. These factors combine to drive the influx of a variety of immune cells, including eosinophils, mast cells, ILC2s and lymphocytes, all of which participate in chronic inflammatory response within the nasal polyps [16, 38, 65].

Allergic rhinitis

The inflammatory response in the nasal mucosa in AR includes an early IgE-mediated and a late-phase response that includes recruitment of eosinophils and basophils, and release of type 2 cytokines including IL-4, IL-5 and IL-13 [66]. Chemoattractants, including eotaxin, IL-5 and RANTES (regulated on activation, normal T-cell expressed and secreted), promote the characteristic infiltration by eosinophils, basophils, Th2 lymphocytes and mast cells seen in chronic AR. As our understanding of the basic pathophysiological features of AR evolves, the development of new diagnostic and treatment options may allow more effective modulation of key pathways in the immune system to alter atopic disease processes and associated morbidity [16, 67–69].

Atopic dermatitis

AD is a common skin disease characterised by a disturbed skin barrier, susceptibility to cutaneous infections and intractable pruritus [16]. AD provokes Th2-mediated immune responses to numerous environmental antigens, and evidence for type 2 inflammation is found in both acute and chronic skin lesions [70]. IL-4, IL-5 and IL-13 are implicated in the pathogenesis of AD, with increased levels leading to amplified signalling of type 2 cytokines and chemokines, activation of subsequent pro-inflammatory signalling pathways and effects on epidermal barrier function [16, 62]. Patients with AD also have increased levels of TARC, a chemokine that promotes migration of Th2 cells [71].

Therapeutic implications

Type 2 cytokines have integral roles in the pathogenesis of asthma, and a number of targeted biologics have been evaluated as potential treatments, including those that target IL-4, IL-5 and IL-13 pathways. Currently available monoclonal antibodies include omalizumab (anti-IgE); mepolizumab, reslizumab and benralizumab (anti-IL-5 pathway); and dupilumab (anti-IL-4/IL-13) (table 3). The major benefit of these new biologic agents has been in achieving disease control, as evidenced by significant reductions in exacerbations observed in clinical studies [18]. The relative efficacy of these agents correlates with target selection and the molecular characteristics of the inhibitor. Anti-IgE and anti-IL-5 agents that target downstream mediators of the type 2 pathway are effective in treating asthma and, although not currently approved, CRSwNP, but not AD. Dual blockade of IL-4 and IL-13 has demonstrated significant efficacy across multiple allergic diseases (asthma, AD, CRSwNP, EoE), confirming the commonality of the IL-4/IL-13 pathway in these atopic diseases [16, 94].

In patients with severe asthma, underlying type 2 airway inflammation may be identified, according to GINA, on the basis of elevated type 2 inflammatory biomarkers (blood and/or sputum eosinophils and/or F_eNO) and/or asthma that is clinically allergen driven and/or requires maintenance oral corticosteroids (figure 1) [2, 21, 78].

Biomarkers that reflect the characteristics of the underlying inflammatory process and the presence of type 2 inflammation are currently used to stratify patients in clinical trials for a number of these agents. Accurate classification of a type 2-high asthma phenotype requires a combination of clinical parameters such as allergic phenotype or responsiveness to corticosteroids, as well as type 2 biomarkers, alone or in combination [13].

Given the complexity of inflammatory pathways in asthma, there is likely to be considerable overlap in these biomarkers in each of the asthma phenotypes; consequently, there may be more than one dominant biomarker. Panels of type 2 biomarkers may more accurately reflect risks in asthma, and ultimately help physicians to select the most appropriate treatment. Two separate studies have shown that elevated levels of both F_eNO and blood eosinophils are associated with greater bronchial hyperresponsiveness and increased exacerbations [95, 96].