Mucosal Immunity

Abstract In addition to their critical role in allergic disorders, mast cells (MCs) are well recognized for their protective effector functions during bacteria and parasite infections. This review describes recent advancements of our understanding of the complex role of MCs in fungal infections. Specifically, we outline key features of the contribution of MCs to infections with six fungal pathogens, namely Sporothrix, Paracoccidioides, Aspergillus, Malassezia, Candida and Dermatophytes. Evidence from studies of these pathogens suggests that MCs can function as positive regulators that detect and contain fungi at the site of infection. However, it appears that the inflammation induced by MCs following fungal infections may not always and only be beneficial to the host. MC responses during fungal infections may primarily benefit the pathogen by facilitating its spreading and contributing to a greater severity of fungal infections. This review also highlights key drivers of MCs activation and effector mechanisms that have been identified for the multidimensional function of MCs in fungal diseases and in allergic diseases combined with fungal infection. 1 INTRODUCTION Fungi are a very diverse group of eukaryotic organisms that separate from plants and animals in their own kingdom. Today, fungal infections represent a serious public health problem around the world with more than 300 million people suffering from serious fungal infections and an estimated 1.6 million deaths annually.1 Fungi can cause a broad range of mycoses in humans: (a) Superficial fungal infections of the skin, nails and hair are commonly due to dermatophytes, for example Trichophyton rubrum or Trichophyton mentagrophytes, or yeasts, for example Candida or Malassezia furfur.2-5 (b) Subcutaneous fungal infections include sporotrichosis, chromoblastomycosis, mycetoma, phaeohyphomycosis, hyalohyphomycosis and lacaziosis.6, 7 (c) Paracoccidioidomycosis, coccidioidomycosis, histoplasmosis, mucormycosis and cryptococcosis are common systemic fungal infections.8 Subcutaneous and systemic mycoses occur mainly in immunosuppressed patients.7 Systemic fungal infections are linked to significant morbidity and mortality.8 In addition, fungal spores, for example, Aspergillus spores, are important allergenic agents and can cause persistent allergic inflammation of the airways.9 The immune mechanisms of defense against fungal infections are numerous, ranging from protective mechanisms that were present early in evolution (innate immunity) to sophisticated adaptive mechanisms that are induced specifically during infection and disease (adaptive immunity).10-12 Traditionally, mast cells (MCs) are seen as key effector cells in the elicitation of allergic signs and symptoms.13-15 However, there is an increasing number of studies that indicate that MCs are also essential players in innate immune responses.16-18 MCs are strategically located in tissues that are borders to the external environment such as the lung, the gut and the skin. Therefore, MCs are perfectly positioned to orchestrate protective immune responses against invading pathogens including fungi. Most studies of anti‐fungal immune responses are focused on the effects of epithelial cells, neutrophils, macrophages/monocytes and dendritic cells (DCs).19-23 As of now, only a few publications and reviews have addressed the role of MCs in the immune response to fungi infections. In the innate immune system, MCs act as sentinels for invading pathogens, as initiators of inflammatory responses and as regulatory cells in the coordination of raising and limiting innate immune responses.18, 24, 25 Recent findings suggest that, during innate immune responses against invading pathogens, MCs function as positive or negative immunoregulatory cells depending on the situation. In this review, we focus on the recent advancements of our understanding of the complex role of MCs in fungal infections—as positive regulators that detect and contain fungi at the site of infection and as negative regulators that promote a wider dissemination and a greater severity of fungal infections. Specifically, we highlight key features of the contribution of MCs to infections with six fungal pathogens, namely Sporothrix, Paracoccidioides, Aspergillus, Malassezia, Candida and Dermatophytes. 2 WHAT IS THE ROLE AND RELEVANCE OF MCs IN SPOROTHRIX SCHENCKII INFECTION? Sporothrix schenckii (S. schenckii) is an opportunistic dimorphic fungus with a global distribution.26, 27 S. schenckii is the cause of sporotrichosis, which usually affects the skin, less commonly the lungs, joints, bones, and brain of humans and other mammals.28 Veterinarians, animal technicians and caretakers, and owners of cats with sporotrichosis are regarded to be at increased risk for the acquisition of the disease.27 Sporotrichosis is transferred via the inoculation of S. schenckii (conidia or hyphae) from contaminated soil, plants or organic matter into sites of tissue injury such as wounds (traumatic inoculation).27 During interactions of S. schenckii with the host, the conidia‐phase of S. schenckii will transform into the yeast‐phase of S. schenckii in response to the host's temperature. Yeast is the dominant form of S. schenckii in sporotrichosis.29 In vitro studies showed that IgG‐seroreactive proteins from Sporothrix yeast cell extracts such as Gp 70 may be important for the pathogenesis and invasion of S. schenckii in human.30 In 2012, Romo‐Lozano et al. first reported that S. schenckii conidia can activate mouse peritoneal MCs to release early response cytokines such as TNF‐α and IL‐6 without degranulation.31, 32 By using an experimental murine model of systemic S. schenckii infection, which used intraperitoneal inoculation with S. schenckii conidia, they demonstrated that mice that had been depleted of peritoneal MCs presented with a decreased fungal load in organs and reduced severity of clinical manifestations. Their results suggest that mediators released by MCs may be implicated in promoting a wider dissemination of S. schenckii and a greater severity of the infection, indicating that MCs might act as negative immunoregulators in S. schenckii infection. However, this study used a functional MCs depletion model where MCs were depleted by injection of compound 48/80 into the peritoneal cavity. Moreover, their experimental model investigated systemic S. schenckii infection following intraperitoneal injection, whereas subcutaneous infection is the more common clinical manifestation of sporotrichosis. In summary, the in vivo evidence for a relevant role of MCs in S. schenckii infection is still limited. Based on the limited evidence available from mouse studies so far, MCs appear to promote S. schenkii, rather than protect from them. How MCs are involved in human sporotrichosis still needs to be clarified. 3 DO MCs PROMOTE PARACOCCIDIOIDES BRASILIENSIS INFECTION? The fungus Paracoccidioides brasiliensis (Pb) causes paracoccidioidomycosis (PCM), an autochthonous systemic mycosis found primarily in Central and South America.33 PCM is characterized by mucous membrane ulcerations of the mouth and nose with spreading through the lymphatic system. The route of infection is assumed to be inhalation. The lungs, lymph nodes and mucous membrane of the mouth are the most frequently infected tissues.34 That MCs interact with Pb has been shown by Valim et al., who demonstrated that co‐culturing of Pb yeast cells with MC lines rat basophilic leukaemia (RBL)‐2H3 MCs induced morphological changes of MCs and their degranulation.35 Furthermore, endogenous PbPga1, which is an immunoregulatory protein on the surface of Pb yeast cells, is able to activate rodent MCs through the NF‐κB pathway, resulting in the release of the inflammatory interleukin IL‐6. IL‐6 is involved in the induction of inflammatory responses that can lead to tissue destruction. Recently, a study of skin biopsies from patients with paracoccidioidomycosis suggested that MCs contribute to skin responses characterized by loose granuloma formation. This type of cutaneous response, which does not effectively contain or clear the infection, features markedly increased numbers of MCs and a Th2 cytokine profile.36 Interestingly, most MCs in these lesions were positive for IL‐10 suggesting that MCs are an important source of this immunosuppressive cytokine. Moreover, Batista et al. observed a downregulation of the inducible nitric oxide synthase (iNOS) in the tissues of oral paracoccidioidomycosis (OP). This results in reduced production of fungicidal nitric oxide (NO) in multinucleated giant cells and most mononuclear cells.37 MCs have been shown to be one of the major sources of NO in humans with fungus infections.38 These findings suggest that a low expression of iNOS in OP may represent a possible MC‐mediated mechanism that permits local fungal multiplication and maintenance of active oral lesions. Taken together, MCs appear to contribute to the morbidity of PCM, by several mechanisms. However, most of the above findings are from ex vivo or in vitro experiments, and proof that MCs play a harmful role in Pb infection needs to come from future in vivo studies. 4 MCs CONTRIBUTE TO THE MORBIDITY OF ASPERGILLUS FUMIGATUS INFECTION Aspergillus fumigatus (A. fumigatus) is a ubiquitous, spore‐forming fungus that has been associated with multiple pulmonary disorders. Inhaled conidia of A. fumigatus adhere to pulmonary epithelial cells, causing opportunistic infection followed by inflammatory responses to A. fumigatus antigens.39 Most studies on the role of MCs in chronic lung disease due to A. fumigatus focus on IgE‐mediated allergic bronchopulmonary aspergillosis (ABPA).40-42 ABPA is associated with early allergic and late‐phase inflammatory responses of the lungs to A. fumigatus antigens that occur in a minority (less than 1%) of patients with asthma and, more often (7%‐10%), in patients with cystic fibrosis.43 ABPA is characterized by elevated total and A. fumigatus‐specific immunoglobulin E (IgE) levels accompanied by eosinophilia, pulmonary infiltrates, bronchiectasis and fibrosis leading to asthmatic attacks and wheezing in affected patients. A. fumigatus‐specific IgE has been shown to activate MCs by cross‐linking of the high‐affinity receptor for IgE(FcεRI) leading to the immediate release of preformed mediators from intracellular granules and the de novo synthesis of lipid mediators and cytokines, all of which contribute to the subsequent development of inflammatory responses and obstruction of the airways.44-46 Proteases released from MCs can promote the release of growth factors from airway epithelial cells driving strong and irreversible remodelling reactions in airways of patients with ABPA, resulting in bronchiectasis lesions and fibrosis.46 Treatment with omalizumab, a humanized mAb that binds to IgE, has been shown to be an effective therapy for patients with severe allergic asthma and ABPA as shown by attenuated asthma symptoms and improved pulmonary function parameters in patients with ABPA.47 Recently, clinical trials demonstrated that omalizumab treatment not only provided a clinically important reduction in serum IgE, exacerbation rates and steroid requirement but also showed attenuated asthma symptoms and improved pulmonary function parameters in patients with ABPA.47 Interestingly, however, the majority of patients with airway colonization by A. fumigatus do not develop ABPA, yet they still exhibit a progressive decline in lung function.48 IgE‐independent MC degranulation in response to A. fumigates may be involved in this response. Urb and co‐workers found that direct contact with mature A. fumigatus hyphae can induce the degranulation of bone marrow‐derived mast cells (BMCMCs) in the absence of IgE, whereas no significant degranulation is induced by either conidia or immature hyphae.49 Although the exact mechanism of this is still unknown, the fungal proteins StuA and MedA, which control the composition of the cell surface molecules of mature A. fumigatus hyphae, were shown to induce BMCMC degranulation in the absence of IgE.49 Therefore, the authors speculated that A. fumigatus may directly worsen pulmonary disorders, by inducing degranulation in MCs that are in contact to hyphae, which may aggravate the inflammation and obstruction of airway rather than eliminate the fungus. Recent data shows that similar to ABPA in man, MCs may be implicated in recurrent airway obstruction (RAO), a type I hypersensitivity response to A. fumigatus in horses.49, 50 As in ABPA, specific IgE‐mediated cross‐linking of FcεRI results in MC degranulation and the release of histamine, serotonin, leukotrienes and eicosanoids such as thromboxane and 15‐hydroxyeicosatetraenoic acid. In addition, in experimental eosinophilic esophagitis induced by A. fumigatus, Niranjan et al. reported that MCs numbers are markedly increased and play a significant role in promoting oesophageal functional abnormalities, contributing to muscle cell hyperplasia and hypertrophy.51 Finally, a recent report suggests that MCs also contribute to A. fumigatus‐induced inflammation in acute invasive fungal rhinosinusitis (AIFR), an aggressive fungal infection with high morbidity and mortality rates in immunocompromised patients.52 Yan et al. created a rat model of AIFR based on A. fumigatus infection and investigated the role of MCs.53 Compared to control rats, the total number of MCs was unchanged, but MC degranulation was only found in the infected nasal cavities of AIFR rats. In summary, these findings suggest that MC responses to A. fumigatus appear to help the pathogen rather than the host. Further studies will be required to determine the underlying mechanisms of the contribution of MCs to A. fumigatus infection by valuable mice models.54 5 EXACERBATION OF MALASSEZIA‐INDUCED SKIN INFLAMMATION BY MCs IN PATIENTS WITH ATOPIC DERMATITIS Malassezia comprises a family of opportunistic pathogenic fungi, which includes 19 species found on the skin of humans and animals. These Malassezia species have been associated with dermatological diseases such as seborrhoeic dermatitis, pityriasis versicolor, atopic dermatitis (AD) and folliculitis.55 AD is one of the most common chronic inflammatory diseases of the skin.56 Malassezia sympodialis (M. sympodialis) is frequently isolated from AD patients and healthy individuals.57 Recently, it was demonstrated that mouse and human MCs respond to extracts from M. sympodialis.58-60 For example, M. sympodialis can enhance IgE‐mediated MC degranulation by signalling through the TLR2/MyD88 pathway. Interestingly, Ribbing et al. further observed that MCs derived from peripheral blood CD34+ progenitor cells of Malassezia‐infected patients contain more preformed granule mediators and exhibit enhanced IL‐6 secretion in response to M. sympodialis, as compared to MCs from healthy controls.60 In addition, Malassezia also can elicit specific IgE and T‐cell reactivity in AD patients.61, 62 Hiragun and colleagues identified the fungal protein, MGL_1304, derived from Malassezia globosa (M. globosa) in purified sweat, as an allergen in AD patients.63-65 They confirmed its ability to induce IgE‐dependent degranulation in human basophils and RBL cells expressing the human FcεRI receptor. Their results suggest that MGL_1304 is a major allergen in human sweat and could cause type I allergy in patients with AD. These findings indicate that M. sympodialis can activate MCs and thereby exacerbate inflammatory responses in AD patients. 6 DO MCs CONTRIBUTE TO THE PATHOGENIC EFFECTS OF CANDIDA ALBICANS? Candida albicans (C. albicans) is an opportunistic fungal pathogen, and they are normally grown as a harmless commensal on the skin and mucosal surfaces of healthy individuals.66 However, it can also cause superficial and systemic infections, especially in immunosuppression condition. C. albicans initially interacts with epithelial cells, resulting in fungal recognition and the formation of hyphae.67 The ability to undergo the morphological transition from yeast to hyphal growth is critical for its pathogenesis. Epithelial activation leads to the production of inflammatory mediators that recruit innate immune cells, which together work with epithelial cells to clear the fungal infection. The main immune cell in the defense against these candida infections is assigned to professional phagocytes, neutrophils and macrophages. In addition to the above cells, MCs also have been identified as the main cell types involved in the defense against this pathogen.68, 69 Yamaguchi et al. have shown that ovalbumin‐sensitized mice colonized with C. albicans show higher specific IgG and IgE titres than control mice, and colonized tissues promoted infiltration and degranulation of MCs.70 Their results suggest that gastrointestinal C. albicans colonization may promote sensitization against food antigens, at least partly due to MC‐mediated hyperpermeability in the gastrointestinal mucosa of mice. Furthermore, Takenaka and colleagues reported that increased levels of Candida‐specific IgE were linked to MC hyperplasia in lymph nodes in Kimura's disease.71 In addition, Segal and co‐workers observed that the injection of C. albicans together with Complete Freund's Adjuvant into the foot‐pads of guinea‐pigs provoked a state of hypersensitivity, characterized by direct degranulation of mouse peritoneal MCs when exposed in vitro to the antigen.72 Noverr and co‐workers reported that overgrowth of the yeast C. albicans following antibiotic treatment can promote the development of allergic airway disease characterized by increased levels of MCs.73 Taken together, these findings suggest that C. albicans may contribute to atopic diseases, at least in part by effects on MCs. Interestingly, MCs may also protect from C. albicans infection. Recent evidence shows that rat peritoneal MCs can kill C. albicans in the extracellular environment, very likely through secreted granular mediators.74 That C. albicans can degranulate rat peritoneal MCs to release mediators has been shown as early as in 1974 when Nosál et al. reported that glycoprotein from C. albicans can induce histamine release in isolated rat MCs, which was confirmed by later in vivo studies showing C. albicans‐induced MCs degranulation in the gut of mice.75 More recently, Nieto‐Patlán et al. have shown that both yeasts and hyphae of C. albicans can induce BMCMCs degranulation, production of TNF‐α, IL‐6, IL‐10, CCL3, CCL4, and ROS as well as NO involving TLR2 and dectin‐1.76 Moreover, MC seems to be able to phagocytose C. albicans. According to Lopes JP et al., the protective response of primary human MCs in C. albicans infections has three stages: Initially, during the first 3 hours, MCs reduce C. albicans viability by degranulation. During the intermediate stage of the response, 3‐12 hours after infection, MC cytokine secretion and the recruitment of neutrophils by releasing chemoattractants such as CXCL1/CXCL2 help to contain the infections by phagocytosis, the release of proinflammatory mediators and by recruiting other leucocytes. Later responses, that is after 12 hours, are characterized by the release of anti‐inflammatory cytokines and the recruitment of effector cells of adaptive immunity.77 Taking together, these results suggest that MCs can help the infected host by killing C. albicans and by containing the infection by recruiting neutrophils and macrophages to the sites of infection. As for the role of MCs in C. albicans infection, there are still many questions to be answered, and further evidence from in vivo studies is needed to arrive at robust conclusions.78 7 WHAT IS THE RELEVANCE OF MCs IN DERMATOPHYTE INFECTIONS? Dermatophytes, a group of keratinophilic fungi with approximately 40 species, are grouped into three genera: Trichophyton, Microsporum and Epidermophyton.79 Dermatophytes can invade human skin, hair, and nails and cause dermatophytosis (tinea).80 The role and relevance of MCs in dermatophyte infections are ill characterized. MCs have been observed to accumulate at sites of dermatophyte infections. For example, in Microsporum canis‐infected cats, the histopathological features of lesional skin include MC infiltration of the superficial dermis.81 Moskalewski et al. reported that the number of skin MCs is higher in mice infected with dermatophytes.82 In a Trichophyton quinckeanum dermatophytosis mouse model,83 MCs also accumulated in the dermis at the site of infection. This may or may not be relevant for the course of dermatophyte infections, and we will need further studies to clarify this. 8 WHAT CAN SPOROTHRIX, PARACOCCIDIOIDES, ASPERGILLUS, MALASSEZIA, CANDIDA AND DERMATOPHYTES TEACH US ABOUT THE EFFECTS OF MCs IN FUNGAL INFECTIONS? It appears that the inflammation induced by MCs following fungal infections may not always and only be beneficial to the host, but may primarily benefit the fungal pathogen by facilitating spreading (Figure 1 and Table 1). Additional support for this comes from studies with the fungal cell wall product zymosan.84-86 Zymosan‐induced peritonitis was found to be suppressed in MC‐deficient mice (WBB6F1‐W/Wv), and this was restored by adoptively transferred BMCMCs.87 MCs also appear to be involved in the morphine‐mediated inhibition of zymosan‐induced peritonitis in mice.88, 89 Gelatinase B/MMP‐9 as an inflammatory marker enzyme in mouse zymosan peritonitis was constitutively present in unstimulated MCs,90 and zymosan‐activated peritoneal MCs reportedly contain and produce more MMP‐9 after zymosan administration. Fungal species (family) Main colonized tissues Study host Study type IgE/non–IgE‐mediated response Reference Sporothrix schenckii (Ophiostomataceae) Skin, lungs, joints, bones, brain Mouse In vivo Non–IgE‐mediated response 31, 32 Paracoccidioides brasiliensis (Ajellomycetaceae) Lymph nodes, mucous membrane of the mouth Mouse In vivo Non–IgE‐mediated response 35 Human In vitro Non–IgE‐mediated response 36, 37 Aspergillus fumigatus (Trichocomaceae) Lung Mouse In vivo IgE‐mediated response 40 Human In vivo IgE‐mediated response 41, 44, 47 Mouse In vivo Non–IgE‐mediated response 45 Mouse In vitro Non–IgE‐mediated response 49 Horse In vivo IgE‐mediated response 50 Mouse In vivo, In vitro Non–IgE‐mediated response 51 Malassezia (Malasseziaceae) Skin Human In vitro IgE‐mediated response 59-61 Candida albicans (Saccharomycetaceae) Skin, mucosal surfaces Mouse In vivo IgE‐mediated response 69, 72 Guinea‐pig In vitro IgE‐mediated response 71 Rat In vitro Non–IgE‐mediated response 73 Mouse In vivo Non–IgE‐mediated response 74 Mouse In vitro Non–IgE‐mediated response 75 Human In vitro Non–IgE‐mediated response 76 Dermatophyte Skin Mouse In vivo Unclear 87, 88 While the published findings from studies on MCs in fungal infections point to a role of MCs that is different from that in bacteria and parasite infection, they do not provide the underlying mechanisms. In Sporotrichosis, for example, MCs promote a wider dissemination of S. schenckii and a greater severity of the infection in mice, but it remains unclear how MCs can mediate these negative effects. Also, most of the investigations performed so far, as described above, have been conducted with mouse models, and it has to be stressed that care should be taken in the translation of findings from the murine to the human system. MCs in mice and humans share many characteristic features, but also differ significantly, for example, in the expression of certain mediators such as proteases.91 In addition, the characterization of the role of MCs in fungal infections, as of now, has not made use of novel models and insights recently introduced to MC research, a limitation that will need to be addressed by future studies. Further evidence from ongoing and forthcoming research will need to provide a better picture of the mechanisms and the relevance of MC responses in fungal infections in humans. Finally, more focus has to come to the clinical relevance of characterizing the role of MCs in fungal infections. Several MC inhibitors are currently under development for the treatment of MC‐driven diseases. These inhibitors should be explored for their effects on fungal diseases. They may be especially helpful (and instructive) in the treatment of patients with allergic diseases and fungal infections. On the other hand, MC inhibitors may carry a risk of exacerbating some fungal infections, candidiasis for example, as MCs can reportedly contribute significantly to the clearance of C. albicans. In conclusion, further studies are needed to characterize the complex function of MCs in fungal diseases and, especially, in allergic diseases exacerbated by fungal infections. In the long run, this will lead to better strategies for preventing and treating fungal infections. ACKNOWLEDGEMENTS This work was supported by grants from A Joint Sino‐German Research project (no. GZ901) and Jiangsu Provincial Medical Youth Talent (no. QNRC2016736). CONFLICT OF INTEREST The authors confirm that there are no conflicts of interest. REFERENCES

Abstract United airways disease (UAD) is the concept that the upper and lower airways, which are anatomically and immunologically related, form a single organ. According to this concept, upper and lower airway diseases are frequently comorbid because they reflect manifestations of a single underlying disease at different sites of the respiratory tract. Allergic asthma‐allergic rhinitis is the archetypal UAD, but emerging data indicate that UAD is a heterogeneous condition and consists of multiple phenotypes (observable clinical characteristics) and endotypes (pathobiologic mechanisms). The UAD paradigm also extends to myriad sinonasal diseases (eg, chronic rhinosinusitis with or without nasal polyps) and lower airway diseases (eg, bronchiectasis, chronic obstructive pulmonary disease). Here, we review currently known phenoendotypes of UAD and propose a “treatable traits” approach for the classification and management of UAD, wherein pathophysiological mechanisms and factors contributing to disease are identified and targeted for treatment. Treatable traits in UAD can be analyzed according to a framework comprising airway inflammation (eosinophilic, neutrophilic), impaired airway mucosal defense (impaired mucociliary clearance, antibody deficiency), and exogenous cofactors (allergic sensitizers, tobacco smoke, microbes). Appreciation of treatable traits is necessary in advancing the effort to deliver precise treatments and achieve better outcomes in patients with UAD. 1 INTRODUCTION The respiratory tract is divided into the upper and lower airways at the level of the vocal cords. Despite this artificial distinction, they are anatomically contiguous and immunologically related. United airways disease (UAD) is the concept that upper and lower airways form a single organ, with upper and lower airway diseases co‐occurring frequently because they reflect different manifestations of a single underlying disease process.1 Possible mechanisms linking the upper and lower airways include aspiration of nasal inflammatory cells and mediators into the tracheobronchial tree, hematogenous propagation of inflammatory mediators from one part of the airway to another, stimulation of bone marrow progenitor cells, or the nasobronchial reflex.2 The archetypal UAD is allergic asthma‐rhinitis, but emerging data indicate that UAD is complex and heterogenous, and may present with other clinical phenotypes of sinonasal disease (eg, chronic rhinosinusitis with or without nasal polyps) and lower airway disease (eg, bronchiectasis, chronic obstructive pulmonary disease [COPD]). Even within a given clinical phenotype, there is further variability based on the underlying pathobiologic mechanisms, known as endotypes. Current guideline‐based management approaches (summarized in Table 1) stratify patients according to symptoms, disease control, future risk, and occasionally broad clinical phenotypes, but do not sufficiently consider complexity and variability in other important dimensions such as airway inflammation, airway mucosal defense, and exogenous cofactors. Failure to recognize this complexity or treat both upper and lower compartments of the airways inevitably leads to suboptimal outcomes for patients. Site Disease Approach to stratification of patients and selection of therapies Patient stratification groups Recommended treatments according to patient stratification Other possible treatments Upper airway Allergic rhinitis Depends on symptoms, time course, severity, control, patient preferences, availability of treatment, and cost. One approach is a step‐up/step‐down approach based on assessment of control Step 1: mild symptoms Intranasal or oral non‐sedating H1‐antihistamines Leukotriene antagonists Allergen and irritant avoidance Allergen immunotherapy Nasal filters or saline Step 2: moderate‐to‐severe symptoms or persistent AR Intranasal corticosteroids Step 3: uncontrolled at step 2 (current or historical) Combination intranasal corticosteroids and intranasal H1‐antihistamines Step 4: uncontrolled at step 3 Short course oral steroids Chronic rhinosinusitis Based on clinical phenotype CRSsNP Intranasal corticosteroids Antibiotics (second line) Consider short course of oral steroids Saline irrigation Sinus surgery for medically refractory disease CRSwNP Intranasal corticosteroids Short course of oral steroids Antibiotic if suspicion of infection Consider leukotriene receptor for acetylsalicylic acid sensitivity Lower airway Asthma Assessment of symptoms, exacerbations, and lung function followed by stepwise escalation/de‐escalation of therapy Step 1 Consider low‐dose ICS Leukotriene receptor antagonists Theophylline Allergen avoidance Allergen immunotherapy for patients with allergic rhinitis and sensitized to house dust mites, frequent exacerbations, and FEV1 > 70% Vaccinations Weight reduction for obese asthma patients Step 2 Low‐dose ICS Step 3 Low‐dose ICS + LABA Step 4 Medium‐/high‐dose ICS + LABA Step 5 Low‐dose oral corticosteroids Add‐on treatments: tiotropium, anti‐IgE, anti‐IL5, bronchial thermoplasty Patients may benefit from phenotyping and endotyping at Step 5 Chronic obstructive pulmonary disease Assessment of symptoms (CAT, mMRC), exacerbations, airflow obstruction (FEV1) Group A (CAT < 10, mMRC < 2, infrequent exacerbations) Bronchodilator Smoking cessation (including pharmacologic treatment) Education, self‐management, and pulmonary rehabilitation Pneumococcal and influenza vaccinations Long‐term oxygen therapy in the presence of severe resting hypoxemia Long‐term noninvasive ventilation for patients with severe chronic hypercapnia and a history of hospitalization for acute respiratory failure Bronchoscopic or surgical lung volume reduction surgery for selected patients Group B (CAT ≥ 10, mMRC ≥ 2, infrequent exacerbations) LABA or LAMA Increase to LAMA + LABA if persistent symptoms Group C (CAT < 10, mMRC < 2, frequent exacerbations) LAMA Increase to LAMA + LABA or change to LABA + ICS if further exacerbations Group D (CAT ≥ 10, mMRC ≥ 2, frequent exacerbations) LAMA + LABA Increase to LAMA + LABA + ICS if persistent symptoms/further exacerbations Consider macrolides Consider roflumilast if FEV1 < 50% predicted and patient has chronic bronchitis Bronchiectasis Assessment of microbial infection, exacerbations and airway symptoms Pseudomonas aeruginosa infection with frequent exacerbations Long‐term inhaled antibiotic treatment Escalate to combined oral and inhaled antibiotic treatment if inadequate response Physiotherapy, pulmonary rehabilitation, and/or airway clearance Pneumococcal and influenza vaccinations Long‐acting bronchodilators for patients with significant breathlessness Non‐Pseudomonas aeruginosa infection with frequent exacerbations Long‐term macrolide treatment Escalate to combined oral and inhaled antibiotic treatment if inadequate response Multiple sequential airway treatments Short‐ or long‐acting bronchodilators → Mucolytic/physiotherapy adjuncts → airway clearance → inhaled antibiotics AR, allergic rhinitis; CAT, COPD Assessment Tool; CRSsNP, chronic rhinosinusitis without nasal polyps; CRSwNP, chronic rhinosinusitis with nasal polyps; FEV1, forced expiratory volume in one second; ICS, inhaled corticosteroids; ICS + LABA, combination inhaled corticosteroids and long‐acting beta agonist; LABA, long‐acting beta agonist; LAMA, long‐acting muscarinic antagonist; mMRC, modified Medical Research Council dyspnea score. The treatable traits approach has been described for lower airway diseases,3 and in this review, we extend its application to combined upper and lower airway disease. We summarize the current evidence for known clinicopathobiologic subtypes (“phenoendotypes”) of UAD. We have grouped the clinical manifestations of UAD according to the “treatable traits” of airway inflammation, airway mucosal defense dysfunction, and exogenous cofactors (Figure 1 and Table 2). These traits are not mutually exclusive and multiple traits may contribute to disease in an individual at any one point in time. Treatable traits may also vary over time. It is with the intention of drawing the readers’ attention to traits that are common to both airway compartments that we have artificially classified the upper and lower airway conditions as presented in this review. We believe that targeting common disease processes, when used in combination with current patient stratification approaches, is likely to lead to clinical improvements at both sites of the respiratory tract. Trait Endotype/pathophysiology Method of assessment/biomarker Clinical phenotype Targeted treatment Upper airway Lower airway Inflammation Eosinophilic, high type 2/TH2‐cytokines IL‐4, IL‐5, IL‐13 Blood periostin, high FeNO, sputum eosinophilia, blood eosinophilia CRSwNP Eosinophilic asthma Corticosteroids (systemic, inhaled, intranasal), anti‐IL‐5, IL‐4, IL‐13, anti‐TSLP, ?CRTh2 antagonist Allergic, systemic CD4+ TH2 cells Blood total IgE, blood specific IgEs, skin prick tests Allergic rhinitis Allergic asthma Allergen avoidance, corticosteroids, antileukotrienes, anti‐IgE, immunotherapy Allergic, local Local nasal and bronchial mucosa specific IgE production Blood total IgE not raised, negative blood specific IgEs, negative skin prick tests, local IgE and specific IgE production, nasal allergen provocation test Local allergic rhinitis Intrinsic asthma ?Omalizumab, ?immunotherapy Non‐allergic eosinophilic IL‐25, IL‐33, TSLP, group 2 innate lymphoid cells Blood periostin, high FeNO, sputum eosinophilia, blood eosinophilia, non‐reactive skin prick tests, absent specific IgE in blood Non‐allergic rhinitis with eosinophilia syndrome (NARES) Adult onset eosinophilic asthma Anti‐TSLP, anti‐IL‐4, IL‐5, IL‐13, ?CRTh2 antagonist Neutrophilic TH17 cells, IL‐17, CXCL1, CXCL8 IL‐8, sputum neutrophilia CRSsNP COPD, neutrophilic asthma Smoking cessation, macrolides Impaired airway mucosal defense Cystic fibrosis Autosomal recessive disorder in CFTR gene leading to improper salt balance and viscous airway surface liquid Genetic testing, sweat chloride test CRSwNP (acid mucin) Bronchiectasis CFTR modulators Primary ciliary dyskinesia Autosomal recessive mutations causing dynein and radial spoke defects Low nasal nitric oxide, ciliary motion and ultrastructure CRSsNP Bronchiectasis No validated PCD‐specific therapies Primary antibody deficiency Deficient immunoglobulins or functional antibody response Quantitative serum immunoglobulin levels, functional antibody responses CRS Bronchiectasis Immunoglobulins, vaccinations Exogenous cofactors Occupational exposures High molecular weight sensitizers: IgE‐mediated type 2 inflammation Low molecular weight sensitizers: haptens Irritant‐induced: no immunological reaction Total IgE, skin prick tests Peak flow monitoring Specific bronchoprovocation challenge Occupational rhinitis, reactive upper airways dysfunction syndrome Occupational asthma, reactive airways dysfunction syndrome Exposure avoidance, respiratory protection devices, ?anti‐IgE Aspirin/non‐steroidal anti‐inflammatories Constitutive overexpression of 5‐lipoxygenase and leukotriene C4 synthase Aspirin/NSAID challenge, urinary leukotriene E4 CRSwNP Severe asthma Avoidance, desensitization, antileukotrienes Staphylococcus aureus enterotoxin Superantigens elicit Th1, Th17 and Th2 cytokines, including IL‐4, IL‐5, IL‐13 Staph aureus enterotoxin IgE's CRSwNP Severe asthma ?anti‐IgE Aspergillus/fungal hypersensitivity TH2 cells, IL‐4, IL‐5, IL‐13, IgE, eosinophilia Aspergillus skin test positivity, Aspergillus specific IgE, IgE > 1000 IU/mL, Blood eosinophil >500/μL, precipitating antibodies to Aspergillus Allergic fungal rhinosinusitis Allergic bronchopulmonary aspergillosis (asthma, bronchiectasis) Steroids, antifungal, ?anti‐IgE Upper respiratory tract infection Variable depending on microbe and underlying disease Respiratory cultures and nucleic acid tests Exacerbation of rhinitis/rhinosinusitis Exacerbation of asthma, COPD or bronchiectasis Antivirals, antibiotics, vaccinations Typical upper and lower airway clinical phenotypes associated with underlying treatable traits are listed. Taxonomy based on clinical phenotypes has significant limitations: each phenotype may represent comprise treatable traits, for example, CRSwNP and asthma may be neutrophilic or eosinophilic inflammation depending on the population studied. Hence, a more precise approach is needed to identify contributory treatable trait(s) in each case, which can aid in selection of targeted treatments. Question mark (?) indicates targeted therapies under investigation. Treatable traits and targeted therapies may be used to complement the current approaches for patient stratification and treatment selection—including non‐targeted treatments—shown in Table 1. 2 AIRWAY INFLAMMATION Airway inflammation is important in the pathogenesis of many upper and lower airway diseases. It is a dynamic trait which varies over time and is modulated by factors such as treatment, infection, environmental exposures, and natural history of disease.4 Clinicians commonly assume that asthma and chronic rhinosinusitis with nasal polyps (CRSwNP) are stereotypically eosinophilic in nature, and that chronic rhinosinusitis without nasal polyps (CRSsNP) or COPD is stereotypically neutrophilic. As discussed below, while these generalizations may hold true for many cases there is substantial variability in the underlying pathophysiological mechanisms for each clinical phenotype or disease. Identification and quantification of airway inflammation can help to direct appropriate treatment. Current methods to assess upper and lower airway inflammation are summarized in Table 3. Method Comments Strengths Limitations Nasal lavage Luminal proteins, cytokines and cells Simple and safe Correlates with, but less invasive than induced sputum Variable fluid recovery and dilution with lavages No sampling of the epithelium Nasal brush or scraping Samples the nasal epithelium Simple and minimally invasive Does not sample deeper layers of the mucosa or the nasal lumen Nasal biopsy Identifies tissue inflammatory cellular subtypes of rhinitis or rhinosinusitis Samples the total nasal mucosa Technically difficult No information about nasal lumen Blood periostin Marker of Th2, eosinophilic airway inflammation Predicts severity of chronic rhinosinusitis Predicts chronic rhinosinusitis with nasal polyps phenotype Noninvasive Not widely available Conflicting evidence on whether periostin is associated with sputum eosinophilia Sputum induction Neutrophil count ≥65% or >500 × 104/mL identifies neutrophilic airway inflammation Eosinophil count ≥3% identifies eosinophilic airway inflammation Noninvasive Well‐validated Not widely available Logistically, technically and practically difficult High failure rate Risk of inducing bronchoconstriction Peripheral blood eosinophils Eosinophil count ≥2% identifies eosinophilic airway inflammation Noninvasive, minimal risk Widely available, cheap Predicts with important clinical outcomes such as exacerbation risk and response to treatment Wide diurnal variation of readings (40%) within the same subject May not be applicable when only local mucosal inflammation occurs Fractional exhaled nitric oxide (FeNO) FeNO < 25 ppb indicates low likelihood of airway eosinophilia FeNO > 47 ppb suggests eosinophilic airway inflammation and corticosteroid responsiveness Noninvasive Quick, easy, and safe to perform Readings may be affected by smoking, respiratory tract infections Sputum eosinophilia and FeNO are not always concordant Urinary leukotriene E4 (LTE4) Marker of cysteinyl leukotriene activity Higher levels observed in aspirin‐exacerbated respiratory disease Collection of urine is simple and of no risk Special equipment and training are required to measure LTE4 2.1 Allergic asthma‐rhinitis The link between allergic asthma and allergic rhinitis (AR) is well established. Up to 40% of AR patients have asthma and almost all patients with asthma have AR.5 Many patients with AR also exhibit nonspecific bronchial hyperresponsiveness.6 IgE‐mediated hypersensitivity to aeroallergens results in type 2/eosinophilic inflammation in the respiratory tract, manifesting as rhinitis in the upper airways and asthma in the lower airways. Upon allergen exposure, sensitization occurs whereby specific IgE (sIgE) are generated, circulate in the blood, and attach to the surface of mast cells and basophils, including those that reside in the airway mucosa. Subsequent exposure to allergen triggers mast cells degranulation, release of chemical mediators, and synthesis of leukotrienes and cytokines, resulting in an immediate allergic airway reaction. The late phase response involves Th2 lymphocyte recruitment with production of inflammatory cytokines such as IL‐4, IL‐5, and IL‐13, as well as activation of eosinophils. The central role of sIgE in allergic asthma‐rhinitis is highlighted by a population‐based birth cohort study showing that presence of peripheral blood sIgE directed against common allergens is associated with increased odds of asthma and rhinitis in childhood.7 In adults with AR, experimental nasal provocation with house dust mite allergens induces not only nasal eosinophilia but also bronchial eosinophilic inflammation, bronchoconstriction, and bronchial hyperresponsiveness,8 suggesting inflammatory crosstalk between upper and lower airways. The allergic asthma‐rhinitis phenoendotype usually begins in childhood and can be identified by measuring peripheral blood total IgE which is elevated, plus demonstration of atopy by the presence of raised sIgE levels in blood or skin prick test positivity. Omalizumab, an anti‐IgE monoclonal antibody, is efficacious for both severe allergic asthma9 and uncontrolled AR.10 Therapeutic response to omalizumab is predicted by higher baseline periostin levels and reduced free serum IgE levels during period of treatment.11 In some cases, specific causative allergens can be identified and targeted with allergen immunotherapy. In both AR12 and allergic asthma,13 immunotherapy improves symptoms and reduces the need for pharmacotherapy in the short term, albeit with limited evidence for longer‐term efficacy. Treatment of AR patients with grass pollen sublingual immunotherapy tablets was associated with slower AR progression, less frequent asthma onset, and slower asthma progression.14 Candidate biomarkers that can identify and/or predict response to allergen immunotherapy include the sIgE:total IgE ratio, specific IgG4 and IgE‐FAB.15 2.2 Local allergic rhinitis‐non‐allergic asthma: local mucosal IgE production A subset of asthma and rhinitis patients demonstrate type 2/eosinophilic allergic airway responses mediated by sIgE which is produced and sequestered in the airway mucosa, with skin prick test non‐reactivity and no detectable circulating sIgE in peripheral blood. In the upper airway, this phenoendotype is known as local allergic rhinitis (LAR), a distinct entity characterized by local production of sIgE in the nasal mucosa without evidence of systemic atopy. LAR does not appear to represent a forme fruste of AR,16 and is associated with increased risk of asthma compared to healthy individuals without LAR.17 The lower airway analog of LAR may correspond to a subtype of non‐allergic asthma which is currently not well defined, but whose existence is supported by studies (Supporting Information: References S1‐S4). In the aforementioned entity, intramucosal IgE synthesis is observed in the lower airways, and functional sIgE can be found in the sputum despite negative skin prick tests and blood sIgE levels. Bronchial inhalational challenge with allergen induces a clinical lower airway allergic response characterized by sputum eosinophilia and bronchial hyperresponsiveness. It is not known why individuals with allergic asthma‐rhinitis demonstrate positive skin prick tests and elevated sIgE in blood, whereas non‐allergic asthmatics and patients with LAR do not although local mucosal IgE synthesis occurs in all those conditions. One possibility is that local mucosal IgE synthesis may not reach sufficient quantities to spill over into peripheral blood. Alternatively, IgE may be sequestered within the bronchial mucosa. Regardless, efficacy of omalizumab has been shown in non‐atopic asthma with evidence of local mucosal IgE production. Pillai et al. (Supporting Information: References S5) randomized 18 such subjects and obtained bronchial biopsies from subjects before and after treatment with 12‐14 weeks of omalizumab. The patients subsequently underwent supervised dose reduction of their regular asthma therapy. Compared to placebo, omalizumab therapy significantly reduced total bronchial mucosal IgE+ cells. Lung function declined in the placebo‐treated group but improved in the omalizumab‐treated group. This small but important study serves as proof‐of‐concept that anti‐IgE can target disease associated with local mucosal IgE production. 2.3 Non‐allergic rhinitis with eosinophilia syndrome‐non‐allergic eosinophilic asthma Type 2 (eosinophilic) inflammation can occur in UAD without evidence of IgE‐mediated hypersensitivity. In the upper airways, this endotype has been termed non‐allergic rhinitis with eosinophilia syndrome (NARES) and is characterized by persistent rhinitis and marked eosinophilia of the nasal mucosa in the absence of local or systemic IgE production, suggesting allergen‐independent induction of eosinophilic airway inflammation.18 Almost half of patients with NARES who do not report asthmatic symptoms nevertheless demonstrate bronchial hyperresponsiveness and sputum eosinophilia, supporting a link between NARES and asthma.19 The corresponding asthma phenoendotype is non‐allergic eosinophilic asthma, in which eosinophilic inflammation and type 2 cytokine production are not driven by allergen‐activated Th2 lymphocytes, but is instead postulated to be initiated by type 2 innate lymphoid cells (ILC2).20 ILC2s are a recently discovered population of lymphocytes capable of producing large amounts of type 2 cytokines (IL‐5 and IL‐13) and orchestrate eosinophilic inflammation in the absence of adaptive immunity, including Th2 cells, B cells, and IgE antibody.21 ILC2s are activated by epithelium‐derived cytokines IL‐25, IL‐33, and thymic stromal lymphopoietins (TSLP), which are released by epithelial cells in response to epithelial damage caused by a wide range of stimuli including microbiota and pollutants. ILC2s are increased in peripheral blood in eosinophilic asthmatics compared to non‐eosinophilic asthmatics and their numbers in sputum correlate with eosinophil counts.22 Increased ILC2s has also been observed in eosinophilic nasal polyps,23 but evidence for the involvement of ILC2 cells in NARES and non‐allergic eosinophilic asthma is lacking and should be the focus of future studies. Potential targeted treatments for this pathway include monoclonal antibodies targeting any of the type 2 cytokines, discussed in the section below. TSLP, targeted by the monoclonal antibody tezepelumab, was shown in a recent dose‐ranging proof‐of‐concept study to reduce exacerbations in patients with uncontrolled asthma, including in those with low IgE (<100 IU/mL).24 Antagonists of CRTH2, the receptor for prostaglandin D2 on ILC2 cells, are currently undergoing evaluation to assess clinical efficacy (Supporting Information: References S6, S7). 2.4 Asthma‐chronic rhinosinusitis Chronic rhinosinusitis (CRS) is broadly defined as an inflammatory disorder of the paranasal sinuses and mucosa of the nasal passages that lasts 12 weeks or longer. The condition is heterogeneous, with 2 broad clinical phenotypes, with nasal polyps (CRSwNP) and without nasal polyps (CRSsNP), as well as diverse endotypes.25 Epidemiologic studies have demonstrated higher prevalence of CRS in asthmatics compared to non‐asthmatics across different populations.26, 27 However, the inflammatory endotype of CRS varies according to the population being studied. In Caucasian subjects, CRSwNP is predominantly eosinophilic and CRSsNP is predominantly neutrophilic, whereas in Chinese subjects CRSwNP is mainly neutrophilic, with fewer Chinese subjects demonstrating eosinophilic nasal polyps.28 Various studies have elucidated the detailed phenoendotypes of either CRS or asthma separately, but there is a paucity of data about phenoendotypes of combined asthma‐CRS. Despite this, phenoendotypes of asthma and CRS are remarkably analogous to each other. Based on sputum inflammatory cells, asthma can be eosinophilic, neutrophilic, mixed eosinophilic and neutrophilic, or paucigranulocytic.29 Based on bronchoepithelial cell gene expression profiles, asthma inflammatory signatures can be partitioned into “Th2 high” (associated with elevated IL‐4, IL‐5, IL‐13, sputum, and blood eosinophilia) or “Th2 low”.30 CRS endotypes can be similarly classified into “IL‐5 high” and “IL‐5 low” clusters.31 Subjects with IL‐5 high CRS have high eosinophilic cationic protein and IgE from nasal specimens, and present clinically with CRSwNP and asthma. Low IL‐5 clusters clinically exhibit the CRSsNP phenotype. “IL‐5 high” CRS appears to be the upper airway analog of “Th2‐high” asthma. The inflammatory pattern of the upper and lower airways in patients with nasal polyps and comorbid asthma was evaluated in a recent study using nasal polyp biopsies and exhaled nitric oxide.32 Approximately, two‐thirds of patients were eosinophilic and one‐third was non‐eosinophilic, with concordance demonstrated between the upper and lower airways in individual subjects. More studies are needed to clarify concordance between upper and lower airway phenoendotypes across the spectrum of inflammatory endotypes in asthma and CRS. Systemic, inhaled, or intranasal corticosteroids are cost‐effective treatments for eosinophilic upper and lower airway inflammation. In cases that are refractory to corticosteroids or in patients who are unable to tolerate high doses of corticosteroids, several biologics targeting eosinophilic inflammation can be considered, such as anti‐IL‐13 (tralokinumab), anti‐IL‐5 (mepolizumab and reslizumab), antibodies against alpha chain of IL‐5 receptor (benralizumab), and alpha subunit of the IL‐4 receptor (dupilumab). Mepolizumab is efficacious in both eosinophilic nasal polyposis and severe eosinophilic asthma,33, 34 reducing polyp burden and need for surgery in the former and reducing exacerbations in the latter. Similarly, dupilumab was demonstrated in small phase 2 trials to reduce polyp burden in patients with chronic sinusitis and nasal polyposis,35 and was found to decrease exacerbations and improve forced expiratory volume in one second (FEV1) in patients with severe eosinophilic asthma.36 Patients with asthma and nasal polyps show greater improvement in Asthma Control Questionnaire score with reslizumab compared to asthmatics without nasal polyps.37 2.5 Chronic obstructive pulmonary disease‐chronic rhinosinusitis Chronic obstructive pulmonary disease is a disease characterized by chronic respiratory symptoms and persistent airflow limitation caused by significant exposure to noxious particles or gases, most commonly tobacco smoke. Although COPD has traditionally been thought to be a purely lower airway disease, there is increasing evidence to suggest frequent concomitant involvement of the upper airways. Multiple epidemiologic studies have shown a high prevalence of nasal symptoms among COPD patients. An early population‐based epidemiologic study conducted in the Swedish general population in 1992 found that 40.1% of individuals with self‐reported chronic bronchitis or emphysema had symptoms of rhinosinusitis, compared with 32.7% in the total study sample.38 In a follow‐up study in 2000, those who reported thick, yellow, nasal discharge and nasal blockage in 1992 had a higher incidence of COPD diagnosis over an 8‐year follow‐up period.39 Another cross‐sectional population‐based Canadian study also found an association between CRS and COPD, both self‐reported.40 Self‐report may lead to misclassification errors of COPD and sinonasal disease but are an inherent limitation of large‐scale epidemiologic studies. Medical record diagnoses, which are considered more reliable than self‐reported medical history, were used in one study from Taiwan which extracted International Classification of Diseases diagnostic codes from a national insurance database and found an increased risk of CRSsNP among COPD patients.41 Recent studies have utilized more stringent and objective criteria for classifying COPD and rhinosinusitis. Bergqvist et al.42 used spirometry for identifying COPD, defined as FEV1/forced vital capacity (FVC) ratio <0.7, and found that the 5‐year incidence of nasal symptoms without having a cold was higher in subjects who demonstrated obstruction on spirometry compared to subjects without baseline spirometric evidence of airflow limitation. Hansen et al43 evaluated the prevalence of paranasal sinus opacification at magnetic resonance imaging in COPD and asthma patients compared to a control group. COPD was defined using similar spirometric criteria as the Bergqvist et al study, whereas asthma was self‐reported. COPD and asthma patients were, respectively, found to be 6 and 2 times more likely than healthy controls to demonstrate paranasal sinus opacification. These findings support the link between upper and lower airway diseases in both asthma and COPD, but we cannot conclude from this study that the association with sinonasal disease is stronger for COPD than for asthma because misclassification bias may have occurred among asthma patients, who were diagnosed based on self‐report instead of objective evidence of variable airflow limitation. Similarly, uncontrolled COPD cohort studies have also found a high prevalence of up to 75% of sinonasal disease, when using spirometry for COPD diagnosis and CT paranasal sinuses for diagnosis of CRS.44, 45 An important limitation of all the aforementioned studies is the use of prebronchodilator, instead of postbronchodilator, FEV1/FVC ratio for identifying cases of COPD. Such a definition could result in other obstructive lung diseases (such as asthma or bronchiectasis) being misclassified as COPD. Only one small study thus far has evaluated COPD using postbronchodilator spirometry and found higher nasal symptoms scored on a visual analogue scale compared to healthy age‐matched controls.46 Although prevalence of sinonasal disease does not increase with declining FEV1,44, 45 the presence of sinonasal disease has profound effects on other important COPD‐related outcomes such as symptom morbidity and exacerbations, which are known to be relatively independent of FEV1. Upper airway symptoms worsen over the natural history of COPD,47 and are associated with lower airway symptoms44 and impaired quality of life.48 Sinonasal symptoms also feature prominently during COPD exacerbations, and are associated with frequent exacerbations47 as well as treatment failure of acute exacerbation of COPD.49 Tobacco smoke forms the common link between COPD and CRS,50 likely by inducing neutrophilic inflammation in the nasal and bronchial mucosa. Compared with non‐smoking healthy controls, smokers without COPD present with nasal inflammation characterized by increased neutrophils and CD8 T lymphocytes51 although this has not been consistently demonstrated.52 Nasal and bronchial IL‐8 levels, neutrophil counts, and CD8 Th1 cells are even higher in smoking COPD patients than in smoking “healthy” controls matched for pack‐year smoking history.51, 53 Nasal and sputum IL‐8 concentrations correlate in COPD patients,53 indicating concordance of neutrophilic inflammation between upper and lower airways in keeping with the UAD hypothesis. Release of neutrophil‐derived proteases such as elastase and matrix metalloproteinase‐9 causes alveolar wall destruction and mucous hypersecretion characteristic of COPD, and may contribute to tissue destruction and remodeling of the sinonasal mucosa seen in CRS. Neutrophilic inflammation results in airflow limitation in the lower airways in COPD, which is mirrored in the upper airways by reduced nasal patency54 and higher nasal resistance.55 Severity of lower airway obstruction, as measured by FEV1, correlates with severity of CRS symptoms.45 Smoking cessation alters the progressive clinical course of COPD by reducing lung function decline and mortality, hence remaining a cornerstone of COPD treatment in addition to other medical therapies. Additionally, in CRS, smoking cessation improves symptoms, quality of life and decreases CRS‐related antibiotic use.56Neutrophilic airway inflammation per se may potentially be targeted for treatment using macrolides, which exert a repertory of effects that suppress neutrophilic inflammation, including downregulation of IL‐8, elastase and matrix metalloproteinase‐9, and reduction of neutrophil chemotaxis and survival.57 Macrolides have proven efficacy in decreasing the frequency of COPD exacerbations,58 and in a small double‐blind randomized controlled trial was associated with improvement in symptoms, endoscopic scores, and nasal lavage IL‐8 levels specifically in Th1, neutrophilic CRS patients.59 Further studies are needed to confirm the efficacy of macrolides. In addition, long‐term macrolides should only be used in carefully selected patients after optimization of other standard medical therapies because of potential cardiac and otological adverse effects, as well as risk of inducing antibiotic resistance. 3 IMPAIRED AIRWAY MUCOSAL DEFENCE Major components of the airway mucosal defense mechanism include the mucociliary escalator, immunoglobulins, antimicrobial peptides, the epithelial barrier, and cells of the innate and adaptive immune system. Impaired mucosal airway defense has a causal role in several phenoendotypes of UAD. The predominant lower airway phenotype in UAD due to impaired mucosal defense is bronchiectasis, which is characterized by abnormal and irreversible dilatation of the lower airways. Frequent coexistence of bronchiectasis and sinonasal disease has long been recognized.60 Patients with bronchiectasis have a high prevalence of CRS when assessed using European Position Paper on Rhinosinusitis and Nasal Polyps criteria, that is, presence of symptoms and positive findings on either nasoendoscopy or CT of paranasal sinuses.61, 62 Conversely, patients with CRS more frequently demonstrate CT evidence of dilated airways compared to individuals without CRS.63 3.1 Cystic fibrosis: bronchiectasis‐chronic rhinosinusitis due to airway surface liquid viscosity Airway surface liquid (ASL) viscosity is regulated in part by the cystic fibrosis transmembrane regulator (CFTR) protein, which mediates ion transport at the airway epithelial surface. Pathological processes which impair CFTR activity can result in abnormal ion flux at the epithelial surface, increased sodium reabsorption, periciliary liquid depletion, and increased mucous viscosity, thereby impairing mucociliary clearance.64 Loss of CFTR‐mediated bicarbonate secretion in the airway epithelium also results in acidification of the ASL, which impairs the action of antimicrobial peptides in the ASL, rendering the airway more susceptible to colonization and infection by microbes.64 Cystic fibrosis (CF) is due to heterogeneous autosomal recessive mutations in the CFTR gene and affects mucociliary clearance at the sinoepithelial lining, lower airways, gastrointestinal tract, and exocrine organs. Sinonasal disease is ubiquitous in CF and characterized by neutrophilic polyps with acid mucin glands, mucoceles, or pseudomucoceles.65 Lower airway disease is characterized by intense neutrophilic inflammation resulting in bronchiectasis, which occurs in 70%‐75% of children by 5 years of age and represents the major cause of morbidity and death in this condition. Treatment of CF has been transformed by the discovery of novel CFTR modulators aimed at addressing the specific defects causing reduced CFTR activity. The most common gene mutation in CF is F508del which causes a processing defect that severely reduces CFTR levels at the epithelial surface and also disrupts channel opening. The latter gating defect is also seen in other mutations, such as the G551D mutation. Lumacaftor and tezacaftor are both CFTR correctors, which improve trafficking of CFTR to the epithelial cell surface. Ivacaftor is a CFTR potentiator and acts by increasing the probability of the channel remaining open. Randomized controlled trials have proven that treatment with these compounds is associated with improved lung function in CF,66-69 and improvement of CF‐associated CRS following treatment with CFTR modulators has been reported anecdotally.70, 71 There is emerging evidence that CFTR dysfunction is a trait in other UADs and may be a target for treatment. CF carrier mutations are implicated in CRS patients who do not have CF.72 Constituents in tobacco smoke such as cadmium, arsenic, and acrolein can cause CFTR dysfunction,73 either by reducing CFTR expression or by inducing gating defects. CFTR decrements are also seen in COPD, including reduced nasal and lower airway potential difference74 and elevated sweat chloride,75 and the degree of CFTR impairment correlates with severity of symptoms and airflow limitation.74, 75 Ivacaftor reverses the acute reduction of CFTR activity after exposure to cigarette smoke in human bronchoepithelial cells,76 and has been demonstrated to ameliorate chronic bronchitis symptoms and improve CFTR activity in a preliminary study.77 Roflumilast, a phosphodiesterase‐4 inhibitor, can stimulate CFTR by increasing cyclic adenosine monophosphate and reverses smoke‐induced inhibition of CFTR activity.78 Roflumilast is approved for reducing exacerbations in severe COPD associated with chronic bronchitis. 3.2 Primary ciliary dyskinesia (PCD): bronchiectasis‐chronic rhinosinusitis due to ciliary dysfunction Chronic rhinosinusitis and bronchiectasis are universal in adults with PCD. The underlying pathophysiology is absent or impaired ciliary motility due to heterogeneous recessive mutations affecting the biogenesis of cilia lining the upper and lower airways.79 In turn, this leads to impaired mucociliary clearance manifesting in the neonatal period as difficulty feeding due to nasal blockage/rhinitis, neonatal respiratory distress syndrome and later in life as recurrent sinopulmonary infections, and bronchiectasis. Nasal nitric oxide is an excellent screening test for patients with a history suggestive of PCD.80 Other tests to assess for ciliary dyskinesia include analysis of ciliary ultrastructure by electron microscopy, assessment of ciliary beat frequency and pattern by high resolution, high‐speed video microscopy, and immunofluorescent staining of ciliary proteins. Ciliary dysfunction occurs in other airway diseases but is usually acquired, for example, due to smoking or infection. In CRS, blunted ciliary response to environmental stimuli has been demonstrated, and this is reversed once the tissue is removed from infected/inflamed environment.81 In COPD, cigarette smoke is associated with ciliary abnormalities in both bronchial82 and nasal mucosae.83 Sputum from atopic and non‐atopic asthmatics have ciliary inhibitory effect on human bronchoepithelial cell cultures, which was reversible upon removal of the sputum.84 Bronchial biopsies from asthma patients demonstrate reduced ciliary number which recovers after treatment with inhaled corticosteroids.85 There are currently no validated effective therapies for primary ciliary dyskinesia and this should be a focus for future research. In acquired cases, removal of the causative factor may reverse ciliary impairment. 3.3 Primary antibody deficiency The most common immunodeficiencies associated with CRS and bronchiectasis are primary antibody deficiencies,86 of which common variable immunodeficiency (CVID), characterized by reduced serum IgG concentrations, is the most common in adults. One‐third and two‐thirds of patients with CVID have bronchiectasis and recurrent sinusitis, respectively.87 Specific antibody deficiency, which is associated with normal immunoglobulin and IgG subclass levels but impaired antibody response to encapsulated respiratory pathogens such as Streptococcus pneumoniae and Hemophilus influenza, is also associated with a high prevalence of CRS and bronchiectasis, accounting for as many as 50% of idiopathic bronchiectasis.88 Patients with primary antibody deficiency most likely require lifelong immunoglobulin replacement, vaccinations, and prophylactic antibiotics. There is also some evidence that antibody deficiency may be a treatable underlying contributor in other airway diseases. For example, low circulating IgG levels are associated with higher rate of exacerbations and shorter time to next exacerbation in two COPD cohorts.89 Treatment with immunoglobulin replacement is associated with significant reduction in exacerbations and rescue antibiotic use in a small case series of 14 COPD patients (Supporting Information: References S8), warranting future prospective, randomized clinical trials to assess the efficacy of IVIG in preventing exacerbations in selected COPD patients with antibody deficiency. A potential contributing role of antibody deficiency in asthma has also been suggested by a small case‐control study which found that asthma patients are more likely to have a diagnosis of CVID or selective IgA deficiency than non‐asthmatic individuals, potentially accounting for the increased risk of bacterial infections in some individuals with asthma.90 A preliminary proof‐of‐concept study found that IVIG significantly reduces the frequency of asthma exacerbations and improved asthma control in adults with IgG subclass deficiency.91 4 EXOGENOUS COFACTORS The respiratory tract is continuously in contact with air from the environment which contains particulates such as microbes, pollutants, and other noxious agents that can cause or aggravate UAD. Aeroallergens such as house dust mites, pollen, and animal dander are the archetypal examples of how specific exposures can cause and worsen allergic asthma‐rhinitis. There are several other well‐delineated phenoendotypes of UAD in which specific inciting or aggravating cofactors are implicated. 4.1 Occupational asthma‐rhinitis More than 90% of individuals with occupational asthma (OA) also have occupational rhinitis (OR).92 Different occupational exposures can cause OA‐OR by different mechanisms. High molecular weight sensitizers, such as proteins and glycopeptides, often cause symptoms after a latency period via IgE‐mediated hypersensitivity.93 In contrast, the mechanism by which low molecular weight agents induces OA‐OR is poorly understood. Single or multiple exposures to high concentrations of irritants can lead to nonspecific inflammation in both upper and lower airways, clinically recognized as reactive upper airways dysfunction syndrome and reactive airways dysfunction syndrome, respectively. In addition to usual care for asthma and rhinitis, the management of OA‐OR is centered around exposure avoidance and use of appropriate respiratory protection equipment. The efficacy of omalizumab for sensitizing agents with IgE‐dependent reactions requires future studies. 4.2 Aspirin‐exacerbated respiratory disease The clinical phenotype of aspirin‐exacerbated respiratory disease (AERD) consists of a triad of asthma, CRSwNP, and exacerbation of upper and lower respiratory tract symptoms within 30 minutes to 3 hours after exposure to aspirin and other cyclooxygenase‐1‐inhibiting drugs. Nasal polyps in AERD can be distinguished from CRSwNP in individuals without AERD by the presence of enhanced eosinophilia, GM‐CSF, and decreased tissue plasminogen activator.94 The endotype of AERD is not fully understood but current available data suggest an underlying dysregulation of arachidonic acid metabolism, with constitutive overproduction of proinflammatory cysteinyl leukotrienes and decreased levels of anti‐inflammatory mediators such as prostaglandin E2.95 This dysregulation is aggravated by cyclooxygenase inhibition following exposure to aspirin and other non‐steroidal anti‐inflammatory drugs in susceptible individuals. The gold standard for diagnosing AERD is aspirin challenge, but challenge testing carries the risk of severe bronchospasm. Urinary leukotriene E4, the end‐product of arachidonic acid metabolism, is raised in AERD and may have a role in diagnosing AERD.96 Treatment options include avoidance of aspirin and NSAIDs, desensitization, and leukotriene receptor antagonists. Omalizumab and mepolizumab may be helpful in AERD as suggested by small trials involving subsets of patients with AERD (Supporting Information: References S9, S10). Whether the observed clinical efficacy of these biologics can be attributed to target AERD‐specific endotype above and beyond their effect on type‐2 eosinophilic inflammation is unclear. 4.3 Asthma‐CRS: hypersensitivity to Staphylococcus aureus enterotoxins Staphylococcus aureus is consistently found in nasal tissues of CRS patients, and evidence supports a role for S. aureus in the pathogenesis of CRS‐asthma. S. aureus colonizes nasal polyps and secretes proteins, including enterotoxins and serine protease‐like proteins in the nasal mucosa, which have the ability to elicit a polyclonal IgE response resulting in an intense type 2, IL‐5 high, eosinophilic inflammation which is likely responsible for chronic severe inflammation of the airways. Presence of sIgE to S. aureus enterotoxins in nasal polyp tissue is strongly associated with asthma in patients with CRSwNP,31, 97 providing an etiological link between asthma‐CRS. In a small placebo‐controlled, proof‐of‐concept study, omalizumab was found to ameliorate nasal polyps and airway symptoms in a small cohort of patients with asthma‐CRSwNP who demonstrated sIgE toward S. aureus enterotoxins.98 4.4 Allergic bronchopulmonary aspergillosis‐allergic aspergillus rhinosinusitis: fungal hypersensitivity IgE‐mediated hypersensitivity to Aspergillus hyphae is the underlying endotype for allergic bronchopulmonary aspergillosis (ABPA) and allergic aspergillus rhinosinusitis (AARS). Upper airway disease is characterized by pansinusitis, passage of nasal plugs, and recurrent nasal polyposis. Lower airway disease is characterized by mucus inspissation within the bronchi and severe asthma, central bronchiectasis, mucous plugging, and fleeting pulmonary infiltrates. In a case series, 22/95 patients of ABPA had also concomitant AARS. All these patients had positive skin tests, positive precipitins, and raised total and sIgE (Supporting Information: References S11). This combination of ABPA and AAS has been termed “sinobronchial allergic mycosis” to recognize their shared etiology and association. The optimal treatment of ABPA and AAS is not known but includes the use of steroids, antifungals, or omalizumab. 5 CONCLUSION The evidence for UAD is strongest for asthma and AR/CRS. However, close associations between upper airway diseases and other lower respiratory tract disorders can also be appreciated. UAD is complex and heterogeneous but a “treatable traits” paradigm, based on assessment of airway inflammation, mucosal defense dysfunction, and exogenous cofactors can help to identify pathophysiological mechanisms involved in the development of disease and potential targets for intervention in individuals presenting with combined upper and lower airway disease. A given individual with disease can demonstrate multiple traits which may vary over time. We have summarized the various treatable traits in UAD and their proven or notional targeted treatments in Table 2. Appreciation of these treatable traits is necessary toward advancing the precision medicine approach in UAD. CONFLICT OF INTEREST The authors declare that they have no conflicts of interest. Supporting Information REFERENCES Notes : AR, allergic rhinitis; CAT, COPD Assessment Tool; CRSsNP, chronic rhinosinusitis without nasal polyps; CRSwNP, chronic rhinosinusitis with nasal polyps; FEV1, forced expiratory volume in one second; ICS, inhaled corticosteroids; ICS + LABA, combination inhaled corticosteroids and long‐acting beta agonist; LABA, long‐acting beta agonist; LAMA, long‐acting muscarinic antagonist; mMRC, modified Medical Research Council dyspnea score. Typical upper and lower airway clinical phenotypes associated with underlying treatable traits are listed. Taxonomy based on clinical phenotypes has significant limitations: each phenotype may represent comprise treatable traits, for example, CRSwNP and asthma may be neutrophilic or eosinophilic inflammation depending on the population studied. Hence, a more precise approach is needed to identify contributory treatable trait(s) in each case, which can aid in selection of targeted treatments. Question mark (?) indicates targeted therapies under investigation. Treatable traits and targeted therapies may be used to complement the current approaches for patient stratification and treatment selection—including non‐targeted treatments—shown in Table 1. Citing Literature Number of times cited: 2 Ana Margarida Pereira, Cristina Jácome, Rute Almeida and João Almeida Fonseca, How the Smartphone Is Changing Allergy Diagnostics, Current Allergy and Asthma Reports, 10.1007/s11882-018-0824-4, 18, 12, (2018). Crossref Umut Can Kucuksezer, Cevdet Ozdemir, Mubeccel Akdis and Cezmi A. Akdis, Precision/Personalized Medicine in Allergic Diseases and Asthma, Archivum Immunologiae et Therapiae Experimentalis, 10.1007/s00005-018-0526-6, (2018). Crossref

Via Krishan Maggon

Immunity. 2015 Sep 21. pii: S1074-7613(15)00352-0. doi: 10.1016/j.immuni.2015.08.020. [Epub ahead of print]

Pathogenesis of Nasal polyp has been primarily investigated on the basis of sinonasal mucosal immunity. Several inflammatory responses have been reported in nasal polyp pathogenesis to date. In this study the researchers have summarised new research on pathways for developing chronic nasal polyp rhinosinusitis (CRSwNP), which focuses on Th2 inflammation. The th2 inflammation may be driven by Staphylococcus aureus by the production of enterotoxins and a serine protease-like protein. Furthermore, S. aureus affected the activity of the mucosal obstruction directly and increased synthesis of Th2 cytokines by the rapid activation of endogenous epithelial cytokines. The synthesis of innate lymphoid epithelial-derived cytokines, including TSLP, IL-25, and IL-33, promotes th2 reactions. Mast cell produces IL-5, IL-13 and periostin and has a function to play in the pathogenesis of nasal polyps by orchestrating infiltration of eosinophils. The formation of extracellular eosinophil traps and Charcot-Leyden crystals is closely linked with the seriousness of disease and development of viscous mucus plugs. The role of S. aureus in innate and adaptive immunity is increasingly evident and contributes to the inflammation of Th2 in CRSwNP. Innate immunity, like epithelial cytokines, is critical in the production of CRSwNP, inducing different pathways, and must be further examined as Th2-controlled biomarkers. Neutrophilic inflammation has recently begun to be investigated but still remains uncertain. For latest news and updates By signing up, you will receive emails about Physician's Weekly products and you agree to our Terms & Conditions and Privacy Policy. Reference: https://journals.lww.com/co-allergy/Abstract/2020/02000/Th2_inflammatory_responses_in_the_development_of.2.aspx

A major aspect of the regulatory function of mast cells appears to be their role as
intermediaries between the nervous and immune systems. Mast cells are activated by
neurotransmitters allowing neural control of innate and adaptive immunity.