Asthma is a global health problem. Prevalence of this disease has risen in recent decades, and it now thought to affect up to 358 million individuals worldwide. The problem is particularly significant in industrialized countries, which have witnessed a great increase in asthma rates in the last 50 years.1,2 Asthma currently affects between 1% and 16% of the world population,1 while prevalence in Spain ranges from 1.5% to 16.7% of the adult population and around 10% of children.3 Asthma patients have a poorer quality of life, more work and school absenteeism, and a greater comorbidity burden. As this a chronic disease, socio-economic costs are high.4

Bronchial asthma is defined as an inflammatory disease of the airways that causes bronchial hyperresponsiveness or airflow obstruction, and which is manifested by symptoms such as cough, wheezing or dyspnea.1,3 Asthma has traditionally been considered a disease associated with atopy or allergy with onset in childhood that can persist or resolve in adulthood.5 However, it is now thought of as a heterogeneous and multifactorial disease with several phenotypes, each with its own natural history and distinct response to treatment.6,7 In addition to allergic or extrinsic asthma and non-allergic or intrinsic asthma, other phenotypes have been defined in the last 20 years on the basis of clinical or physiological characteristics (severity, age at onset, grade of obstruction, resistance to treatment), triggering factors (exercise, allergens, occupation, aspirin-induced asthma), or type of inflammation (eosinophilic, neutrophilic or paucicellular).8 These definitions focus on partial features of the disease, and while they may be helpful, they do not fully explain the complexities of asthma. Indeed, issues such as the causes of increasing asthma rates, genetic susceptibility, or the interaction between environmental factors and the immune system, both in the genesis of asthma or as a trigger of exacerbations, have yet to be resolved.

Two strategies have been developed in the past 10 years to improve understanding of this condition. The first is to classify asthma according to phenotypes, using cluster analysis.9–11 Cluster analysis is based on multivariate mathematical algorithms that quantify similarities between individuals of the same population, in order to define the mechanisms of the disease and, thus, optimize treatment. The second strategy consists of the creation of different patient cohorts, in order to study the natural history of the disease. To date, 3 European cohorts,12–14 and 1 American cohort15 have been developed. The purpose of these cohorts is to optimize treatment according to the particular features of each patient. However, cohorts published to date focus on the study of severe or refractory asthma. To the best of our knowledge, no cohort studies have yet focused on the analysis of the overall asthma population. Moreover, none of these cohorts includes both lung function and inflammatory studies, nor has progressive bronchial imaging been collected.

This aim of this study, the MEGA project (Mechanism underlying the genesis and evolution of asthma), is to create a cohort that would give access to clinical, physiological, molecular, and genetic data in patients with varying degrees of asthma severity. The data collected will help establish the different physiopathological pathways that cause or impact on the heterogeneous expression of this disease, and will show what percentage of patients might progress to bronchiectasis or fixed bronchial obstruction, and identify factors that might promote or influence this progress. This is a multicenter study that will be performed in 8 hospitals, under the auspices of the Spanish Respiratory Disease Biomedical Research Network (CIBERES). Specific tests, including imaging, lung function, inflammation, and bronchial hyperresponsiveness, will be performed periodically to determine the relevant events that characterize the asthma population, the long-term parameters that can determine changes in severity, and the treatments that influence disease progression. The study, which is the first of its kind, will also seek to identify the causes of exacerbations and how they affect the course of the disease. In short, the aim is to improve our understanding of the natural history of the disease, without focusing exclusively on severe asthma (Fig. 1).

Fig. 1.

Study design.

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This article describes the cohort selection criteria, baseline characteristics and progress-related parameters that will be collected, as well as the methods used to obtain, process, and store the various biological samples in the CIBERES pulmonary biobank.

The main aim of creating this cohort is to contribute to the understanding of the heterogeneity of asthma. Accordingly, we will perform immunological studies to clarify the mechanisms involved in the development of differing clinical manifestations among patients, epigenetic studies to identify the factors leading to the appearance of asthma, studies to identify risk factors for exacerbations, and studies to identify patients who are susceptible to developing bronchiectasis and irreversible progressive airway obstruction.

A large proportion of current research in asthma is aimed at identifying the immunological pathways that determine disease, including disease phenotypes and endotypes and certain biomarkers that may help us design precision medicine.38 It is generally agreed that 3 possible immune responses can be distinguished in the development of asthma, namely type 2 immune response, non-type 2 immune response, and mixed type Th2/Th17 immune response,39 but the intimate mechanisms that generate these responses are still largely unknown. Type 2 response is the best known, and is generally observed in asthma patients with eosinophilic inflammation.40 The distinctive feature of this response is a Th2 pathway which leads to the final production of specific IgE, the basic effector cells of which appear to be Th2 lymphocytes and plasma cells. The main cytokines involved are IL-4, IL-5 and IL-13.41 The type 2 response also occurs in non-atopic patients, apparently driven by innate T lymphocyte (ILC2), with IL-5 as the main effector cytokine.42 Before differentiation, both pathways appear to have common bronchial epithelium-derived cytokines, such as IL-33, IL-31, IL-25 and thymic stromal lymphopoietin, and these molecules are currently under investigation as possible therapeutic targets43–45 (Fig. 3). It is unclear why this second pathway, with its low Th2 profile but marked eosinophilic inflammation, is associated with more severe asthma and a poorer response to corticosteroids.

Fig. 3.

Main pathways involved in type 2 immune response in bronchial asthma © 2018 Xavier Muñoz Gall.

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Non-type 2 immune response, also known as non-eosinophilic asthma,10,46 appears to include patients with Th17-dependent neutrophilic inflammation,47,48 patients with neutrophilic inflammation dependent on dysregulation of the innate immune response associated with IL-1b or CXCR2,49 and patients with neurogenic inflammation basically associated with the RTPA1 receptors.50,51 While these response types are the focus of several studies, the putative role of tumor necrosis factor-alpha, IL-6, IL-8, IL-37, IL-22 and IL-9 is under discussion.39

Although patients with a mixed Th17/type 2 response have been described, the relationship between the 2 response pathways is highly complex and little understood. IL-17 produced by T cells or by the Th17 cells themselves in response to possible epithelial damage was seen to increase the production of IL-4 and IL-13 by ILC2 and Th2 cells in an atopic dermatitis animal model.52 At the same time, IL-4 and IL-13 seem to be capable of amplifying the Th17 response by regulating CD209a expression in dendritic cells in murine schistosomiasis models.53 However, in murine asthma models, the neutralization of IL-4 or IL-13 results in an increase in Th17 cells and neutrophilic inflammation, while the neutralization of IL-13 and IL-17 prevents both eosinophilic and neutrophilic inflammation and the appearance of bronchial hyperresponsiveness.54 A better understanding of these pathways and their possible interactions obtained from the possible results of this cohort, may translate to a better selection of therapeutic targets and the implementation of combined treatments, such as double Th2/Th17 blockade.

Asthma is clearly a complex disease in which both genetic and environmental factors play a decisive role. However, neither genetic55 nor environmental56 factors alone can explain the variability observed in these patients. Epigenetic studies may help determine genetic-environmental interactions and may shed light on the hereditary component of asthma.57,58 Epigenetic mechanisms are known to regulate the expression of cytokines that play an important role in the differentiation of T lymphocytes and certain transcription factors,59,60 which in turn may be influenced by different environmental exposures.61 Indeed, exposure to diesel particulate matter has been associated with DNA methylation in peripheral blood and in the epithelial cells of the airways, and with altered T reg cell function when associated with exposure to polycyclic aromatic hydrocarbons.62,63 Although exposure to certain allergens has also been associated with methylation changes in the DNA of respiratory epithelial cells,62 very few studies have evaluated the influence of multiple exposures on epigenetic changes.

The interaction between changes in the microbiome and epigenetic changes is another interesting area. Epidemiological studies have described a lower incidence of asthma in individuals with less exposure to antibiotics in early life,64 and in babies delivered vaginally rather than by Cesarean section.65 In fact, there is a strong relationship between exposure to microorganisms and susceptibility to asthma in early life. Several authors believe that these early exposures may affect the composition of the microbiome of the individual and this, in turn, may generate epigenetic changes that will lead to a greater susceptibility to developing asthma.58 Although we have no plans to develop microbiome studies in this cohort, we will take into account these multiple factors that may determine epigenetic changes and, as such, may be responsible for the heterogeneity of asthma.

Little is understood about the factors that determine exacerbations in asthmatic patients. Various factors have been implicated, including respiratory infections, especially of viral origin,66–68 exposure to allergens,69 smoking,70 lack of adherence to treatment,71 rhinosinusitis,72,73 obesity, intolerance of non-steroidal anti-inflammatory drugs,74 and psychosocial factors.75 However, allergic sensitization is known to vary widely, depending on environmental and climatic conditions.76 Moreover, while viral infection seems to be a major trigger for exacerbations,66–68 the interaction between these infections and allergic sensitization associated with a specific bronchial inflammatory response has not been studied in depth, nor are studies available that associate certain factors of exacerbation with different asthma phenotypes.

While a relationship between the degree of control of asthma and the risk of exacerbations has been established,77 evidence that they are not always associated78 is supported by the fact that inflammation and bronchial hyperresponsiveness persist in patients with controlled asthma.79 It is difficult to achieve disease control in patients with severe asthma, and treatment is essentially aimed at avoiding exacerbations.3 Several markers have been proposed in an attempt to establish the risk of exacerbation, such as FEV1 or the number of exacerbations in the previous year.80 A clinical index for predicting exacerbations has recently been developed81; however, it was created from retrospective analyses and has not been fully validated, nor does it include measurements of inflammatory factors, such as FeNO or eosinophils in induced sputum, that are essential data in this disease.

Finally, some asthma patients are known to develop bronchiectasis,82 while others develop fixed airway obstruction practically indistinguishable from that experienced by patients with chronic obstructive pulmonary disease.83 Both factors predict a worse prognosis and a greater use of healthcare resources.84 These events have conventionally been associated with patients with more severe asthma or poorer disease control and a greater number of exacerbations85,86; however, the factors that determine which patients with similar clinical characteristics will go on to develop these complications remain unclear. Nor do we know if this poor prognosis can occur in less severe forms of the disease.

In conclusion, this large Spanish, prospective, multidisciplinary cohort has been created to clarify the molecular mechanisms that influence the different clinical presentations of the disease and the factors associated with certain mechanisms that may affect exacerbations or the appearance of bronchial changes in asthma patients, in the form of bronchiectasis or fixed progressive airway obstruction. Another aim is to identify potentially preventable epigenetic changes and to try to discover biomarkers to help determine the most effective treatment for each patient.

This study is funded by FIS PI15/0190 (Instituto de Salud Carlos III), the European Regional Development Fund (ERDF) and Sanofi. MJC receives funding from the Miguel Servet research program of the Instituto de Salud Carlos III (CP12/03101).

All authors of the manuscript have significantly contributed to the research, preparation, review, and final version of the manuscript.

The authors state that they have no conflict of interests.