3.2. Asthma
As with COPD, asthma is a broad clinical entity that manifests with symptoms of wheezing, shortness of breath, cough, and chest tightness. Asthma is characterized by reversible airflow obstruction, in contrast with COPD where the airflow obstruction is (mostly) irreversible
[97][52]. Behind this clinical definition, asthma is a heterogeneous disease that relates to diverse pathophysiological mechanisms. Two main immune phenotypes are described, namely type 2 (T2-) high asthma, which includes several subsets according to the age of onset, allergic background, and comorbidities such as aspirin sensitivity and/or nasal polyposis; T2-low asthma, which includes obesity-related asthma
[97,98,99,100][52][53][54][55]. Both early-onset allergic (or extrinsic) asthma and some forms of late-onset non-allergic asthma are characterized by a T2-mediated eosinophilic inflammation, whose underlying upstream mechanisms include epithelial activation by viruses, allergens and/or air pollution. The subepithelial penetration of these irritants is further enhanced by epithelial barrier dysfunction that is due to the decreased tightness of apical junctional complexes, as well as zones of epithelial shedding
[77][45]. These airborne stimuli induce epithelial cells necrosis leading to the release of alarmins such as TSLP, IL-33, IL-25 and GM-CSF, the latter attracting and/or activating mast cells, basophils and eosinophils
[51][32]. In T2-low asthma, macrophages recruitment is also driven by the epithelial release of CCL2 and CCL3
[101][56], while in obesity-related asthma, leptin activates airway epithelial cells and induces the production of cytokines such as IL-8, further contributing to neutrophil recruitment
[101][56]. In turn, these inflammatory cells mediate airway remodelling, including goblet cells metaplasia, epithelial-to-mesenchymal transition and reticular basement membrane thickening, along with bronchial hyperresponsiveness and smooth muscle hypertrophy
[51[32][53],
98], as well as IgE class switch recombination in B cells
[51][32]. In addition the increased levels of allergen specific IgE, higher IgA levels have been observed in the airway mucosa of allergic asthma patients
[102,103,104][57][58][59].
Although the exact role(s) of S-IgA in the development of allergic disorders remains controversial, several lines of evidence suggest that the S-IgA system could play a protective role in asthma and allergic diseases
[2]. Thus, higher levels of S-IgA in breast milk were associated to a lower risk of atopic dermatitis up to the age of two
[108][60], and subjects with selective IgA deficiency display higher risks of developing allergic diseases
[109][61]. In addition, populations with higher serum IgA levels display lower rates of house dust mite sensitization and severe airway hyperresponsiveness
[110][62]. Conversely, a positive correlation exists between serum IgA specifically directed against cow’s milk β-lactoglobulin and wheat gliadin at the age of one, and IgE sensitization at the age of 6
[111][63].
3.3. Cystic Fibrosis
In contrast with asthma and COPD
[128][64], data concerning IgA in cystic fibrosis (CF) are scarcer. CF is a multisystemic disease mainly affecting the respiratory, digestive and reproductive tracts
[129][65]. Chronic pulmonary infections, notably due to
Pa, play a crucial role in the prognosis of the disease and constitute a major immune challenge for the airway epithelium
[130][66]. Recently,
we undertoo
k one study assessing the pIgR/IgA system in a multimodal project, showing increased epithelial IgA
+ B cells, IgA production and pIgR expression in the airway tissues, sputum, and serum from CF patients
[72,73][40][41]. In addition, a mice model mimicking chronic lung infection by using
Pa-coated microbeads instilled in the mice airways showed that infection could upregulate pIgR expression and IgA production in the lungs of F508del mice, partly in an IL-17-dependant manner
[72][40]. This was observed in vivo although in vitro, CF-derived airway epithelial cells displayed reduced pIgR expression, that appeared to be related to the activation unfolded protein response. In parallel, pIgR transcytosis is enhanced in CF, as evidenced by increased SC concentration in CF sputum, although SC seems to be dysfunctional in CF, notably preventing its neutralization capacity of IL-8/CXCL-8 due to an altered glycosylation pattern
[131][67]. Increased IgA concentration has been found in the serum from CF patients, as compared with controls
[132][68], in particular in chronically
Pa infected patients
[133][69] or with high clinical severity according to Shwachman Kulczycki scores
[134][70]. Similarly, increased IgA concentrations were found in BALF from CF patients as compared with controls
[135][71].
3.4. Bronchiolitis Obliterans Syndrome
Chronic lung allograft dysfunction (CLAD) represents a major hurdle in lung transplantation (LT), that burdens long-term survival
[137][72]. Depending on pulmonary function tests and computed tomography, CLAD is currently separated into obstructive CLAD (first described as bronchiolitis obliterans syndrome (BOS), restrictive CLAD (also known as restrictive allograft syndrome (RAS), mixed CLAD (combining BOS and RAS features) and undefined CLAD
[138][73]. BOS is the most frequent form of CLAD and is histologically characterized by epithelial injury, bronchocentric mononuclear inflammation, and fibrosis of small airways
[144][74].
As repetitive and/or chronic infections have widely been associated with CLAD risk
[145[75][76][77],
146,147], it can be questioned whether impaired airway mucosal immunity (and thus reduced microbial eviction) could favor CLAD/BOS. However, the literature in the field is scarce, with no study so far assessing the airway expression of pIgR after LT. One previous study assessed BALF S-IgA levels in LT recipients, showing reduced S-IgA levels during so-called “rejection episodes”
[148][78].
Of note is that, CLAD phenotypes had not been described at the time this study was published. More recently, Vandermeulen and colleagues showed increased total BALF IgA levels in LT recipients with RAS and BOS as compared with control LT recipients
[149][79]. Finally, low serum IgA levels prior to LT represent a risk factor for developing post-LT infections
[150[80][81],
151], while lower post-LT IgA serum levels have been observed in patients with BOS as compared with control LT recipients
[151][81].