How the microbiome affects asthma: new insights from a Spanish study

The World Asthma Foundation (WAF). WAF is a nonprofit organization dedicated to improving the lives of people with asthma through education, research, and advocacy. In this blog post, I want to share with you some exciting findings from a recent study on the microbiome and asthma, published by Spanish researchers in the journal Nutrients.

The microbiome is the collection of microorganisms that live in and on our bodies, such as bacteria, fungi, viruses, and parasites. The microbiome plays an important role in our health and immunity, and can also influence our susceptibility and response to various diseases, including asthma.

Asthma is a chronic inflammatory disease of the airways that affects millions of people worldwide. Asthma can be triggered by different factors, such as allergens, infections, pollution, stress, and diet. Asthma can also have different phenotypes (characteristics), such as allergic or non-allergic, eosinophilic or non-eosinophilic, mild or severe.

What is the microbiome and how does it affect asthma?

The study by Valverde-Molina and García-Marcos reviews the current evidence and challenges on the relationship between the microbiome and asthma, specifically how microbial dysbiosis (an imbalance of the microbial communities in the body) can influence the origins, phenotypes, persistence, and severity of asthma.

How different factors can influence the microbiome and asthma

The study explores how different factors, such as diet, environment, genetics, and infections, can affect the microbiome and asthma, and how modulating the microbiome could be a potential strategy for preventing or treating asthma. The study also reviews the different methods and techniques used to study the microbiome and its interactions with the immune system and the respiratory system.

The gut-lung axis: a key connection between the microbiome and asthma

One of the key points of the study is the importance of the gut-lung axis in the origin and persistence of asthma. The gut-lung axis is the concept that describes how the gut and lung microbiomes communicate with each other through various pathways, such as metabolites, cytokines, antibodies, and immune cells. The gut-lung axis can modulate inflammation and allergic responses in both organs.

The study shows that the process of microbial colonization in the first three years of life is fundamental for health, with the first hundred days of life being critical. Different factors are associated with early microbial dysbiosis, such as caesarean delivery, artificial lactation and antibiotic therapy, among others.

How microbial dysbiosis can lead to different asthma phenotypes and severity

Longitudinal cohort studies on gut and airway microbiome in children have found an association between microbial dysbiosis and asthma at later ages of life. A low ?-diversity (the number of different species) and relative abundance of certain commensal gut bacterial genera in the first year of life are associated with the development of asthma. Gut microbial dysbiosis, with a lower abundance of Phylum Firmicutes (a group of bacteria that includes lactobacilli), could be related with increased risk of asthma.

Upper airway microbial dysbiosis, especially early colonization by Moraxella spp. (a type of bacteria that can cause respiratory infections), is associated with recurrent viral infections and the development of asthma. Moreover, the bacteria in the respiratory system produce metabolites (substances produced by metabolism) that may modify the inception of asthma and its progression.

The role of the lung microbiome in asthma development has yet to be fully elucidated. Nevertheless, the most consistent finding in studies on lung microbiome is
the increased bacterial load (the number of bacteria) and the predominance of proteobacteria (a group of bacteria that includes Haemophilus spp. and Moraxella catarrhalis), especially in severe asthma.

Candida albicans: a fungal culprit in asthma development and exacerbation

The study also mentions Candida albicans (a type of fungus that can cause infections) as one of the fungal genera that can affect the gut and lung microbiome and asthma. Candida albicans can trigger inflammation and autoimmune responses in the body. Candida albicans can also induce a Th17 response (a type of immune response) in the gut and lungs. Candida albicans can also increase lung bacterial load and exacerbate airway inflammation.

This study is very relevant to our own research and findings on Candida’s role in inflammation and autoimmune response: implications for severe asthma. We published an article on this topic on our website on October 13th 2021 which features findings from Mayo Clinic researchers who examined how intestinal fungal microbiota affects lung resident memory CD4+ T cells (a type of immune cell) in patients with asthma.

You can find our article here: https://worldasthmafoundation.org/candidas-role-in-inflammation-and-autoimmune-response-implications-for-severe-asthma.htm

How modulating the microbiome could be a promising strategy for asthma prevention and treatment

We think that these studies complement each other well and provide valuable insights into this important and emerging topic. We believe that understanding the microbiome and its impact on asthma is crucial for developing new and effective strategies for prevention, diagnosis, and treatment of this chronic disease.

Gut Health and Asthma

The gut and lungs are anatomically distinct, but potential anatomic communications and complex pathways involving their respective microbiota have reinforced the existence of a gut–lung axis (GLA). Compared to the better-studied gut microbiota, the lung microbiota, only considered in recent years, represents a more discreet part of the whole microbiota associated to human hosts. Gut health is not the only area to think about.

While the majority of studies focused on the bacterial component of the microbiota in healthy and pathological conditions, recent works highlighted the contribution of fungal and viral kingdoms at both digestive and respiratory levels. Moreover, growing evidence indicates the key role of inter-kingdom crosstalks in maintaining host homeostasis and in disease evolution.

In fact, the recently emerged GLA concept involves host–microbe as well as microbe–microbe interactions, based both on localized and long-reaching effects. GLA can shape immune responses and interfere with the course of respiratory diseases. In this review, we aim to analyze how the lung and gut microbiota influence each other and may impact on respiratory diseases.

Due to the limited knowledge on the human virobiota, we focused on gut and lung bacteriobiota and mycobiota, with a specific attention on inter-kingdom microbial crosstalk. These are able to shape local or long-reached host responses within the GLA.

Introduction

Recent advances in microbiota explorations have led to an improved knowledge of the communities of commensal microorganisms within the human body. Human skin and mucosal surfaces are associated with rich and complex ecosystems (microbiota) composed of bacteria (bacteriobiota), fungi (mycobiota), viruses (virobiota), phages, archaea, protists, and helminths (Cho and Blaser, 2012).

The role of the gut bacteriobiota in local health homeostasis and diseases is being increasingly investigated, but its long-distance impacts still need to be clarified (Chiu et al., 2017). Among the relevant inter-organ connections, the gut–lung axis (GLA) remains less studied than the gut–brain axis.

So far, microbiota studies mainly focused on the bacterial component, neglecting other microbial kingdoms. However, the understanding of mycobiota involvement in human health and inter-organ connections should not be overlooked (Nguyen et al., 2015; Enaud et al., 2018).

Viruses are also known to be key players in numerous respiratory diseases and to interact with the human immune system, but technical issues still limit the amount of data regarding virobiota (Mitchell and Glanville, 2018). Therefore, we will focus on bacterial and fungal components of the microbiota and their close interactions that are able to shape local or long-reached host responses within the GLA.

While GLA mycobiota also influences chronic gut diseases such as IBD, we will not address this key role in the present review: we aimed at analyzing how lung and gut bacteriobiota and mycobiota influence each other, how they interact with the human immune system, and their role in respiratory diseases.

Gut Health

Microbial Interactions Within the Gut–Lung Axis

The gut microbiota has been the most extensively investigated in gut health. The majority of genes (99%) amplified in human stools are from bacteria, which are as numerous as human cells and comprise 150 distinct bacterial species, belonging mainly to Firmicutes and Bacteroidetes phyla. Proteobacteria, Actinobacteria, Cyanobacteria, and Fusobacteria are also represented in healthy people (Sekirov et al., 2010; Human Microbiome Project Consortium, 2012).

More recently, fungi have been recognized as an integral part of our commensal flora, and their role in health and diseases is increasingly considered (Huffnagle and Noverr, 2013; Huseyin et al., 2017). Fungi are about 100 times larger than bacteria, so even if fungal sequences are 100 to 1,000 times less frequent than bacterial sequences, fungi must not be neglected in the gastrointestinal ecosystem.

Mycobiota Diversity

In contrast with the bacteriobiota, the diversity of the gut mycobiota in healthy subjects is limited to few genera, with a high prevalence of Saccharomyces cerevisiae, Malassezia restricta, and Candida albicans (Nash et al., 2017).

Note from the WAF editorial board. We wish to acknowledge and thank Raphaël Enaud, Renaud Preve, Eleonora Ciarlo, Fabien Beaufils, Gregoire Wieërs, Benoit Guery and Laurence Delhaes for their support of Asthma education and research. For more information about Asthma or Gut Health, visit the World Asthma Foundation.

Although often dichotomized due to technical and analysis sequencing issues, critical interactions exist between bacteriobiota and mycobiota (Peleg et al., 2010). The most appropriate approach to decipher the role of gut microbiota is therefore considering the gut as an ecosystem in which inter-kingdom interactions occur and have major implications as suggested by the significant correlations between the gut bacteriobiota and mycobiota profiles among healthy subjects (Hoffmann et al., 2013).

Yeasts

Yeasts, e.g., Saccharomyces boulardii and C. albicans, or fungus wall components, e.g., ?-glucans, are able to inhibit the growth of some intestinal pathogens (Zhou et al., 2013; Markey et al., 2018). S. boulardii also produces proteases or phosphatases that inactivate the toxins produced by intestinal bacteria such as Clostridium difficile and Escherichia coli (Castagliuolo et al., 1999; Buts et al., 2006).

In addition, at physiological state and during gut microbiota disturbances (e.g., after a course of antibiotics), fungal species may take over the bacterial functions of immune modulation, preventing mucosal tissue damages (Jiang et al., 2017). Vice versa, bacteria can also modulate fungi: fatty acids locally produced by bacteria impact on the phenotype of C. albicans (Noverr and Huffnagle, 2004; Tso et al., 2018).

Microbiota

Beside the widely studied gut microbiota, microbiotas of other sites, including the lungs, are essential for host homeostasis and disease. The lung microbiota is now recognized as a cornerstone in the physiopathology of numerous respiratory diseases (Soret et al., 2019; Vandenborght et al., 2019).

Inter-Kingdom Crosstalk Within the Lung Microbiota

The lung microbiota represents a significantly lower biomass than the gut microbiota: about 10 to 100 bacteria per 1,000 human cells (Sze et al., 2012). Its composition depends on the microbial colonization from the oropharynx and upper respiratory tract through salivary micro-inhalations, on the host elimination abilities (especially coughing and mucociliary clearance), on interactions with the host immune system, and on local conditions for microbial proliferation, such as pH or oxygen concentration (Gleeson et al., 1997; Wilson and Hamilos, 2014).

The predominant bacterial phyla in lungs are the same as in gut, mainly Firmicutes and Bacteroidetes followed by Proteobacteria and Actinobacteria (Charlson et al., 2011). In healthy subjects, the main identified fungi are usually environmental: Ascomycota (Aspergillus, Cladosporium, Eremothecium, and Vanderwaltozyma) and Microsporidia (Systenostrema) (Nguyen et al., 2015; Vandenborght et al., 2019).

In contrast to the intestinal or oral microbiota, data highlighting the interactions between bacteria and fungi in the human respiratory tract are more scattered (Delhaes et al., 2012; Soret et al., 2019). However, data from both in vitro and in vivo studies suggest relevant inter-kingdom crosstalk (Delhaes et al., 2012; Xu and Dongari-Bagtzoglou, 2015; Lof et al., 2017; Soret et al., 2019).

Several Pathways

This dialogue may involve several pathways as physical interaction, quorum-sensing molecules, production of antimicrobial agents, immune response modulation, and nutrient exchange (Peleg et al., 2010). Synergistic interactions have been documented between Candida and Streptococcus, such as stimulation of Streptococcus growth by Candida, increasing biofilm formation, or enhancement of the Candida pathogenicity by Streptococcus (Diaz et al., 2012; Xu et al., 2014).

In vitro studies exhibited an increased growth of Aspergillus fumigatus in presence of Pseudomonas aeruginosa, due to the mold’s ability in to assimilate P. aeruginosa-derived volatile sulfur compounds (Briard et al., 2019; Scott et al., 2019). However, the lung microbiota modulation is not limited to local inter-kingdom crosstalk and also depends on inter-compartment crosstalk between the gut and lungs.
Microbial Inter-compartment Crosstalk

From birth throughout the entire life span, a close correlation between the composition of the gut and lung microbiota exists, suggesting a host-wide network (Grier et al., 2018). For instance, modification of newborns’ diet influences the composition of their lung microbiota, and fecal transplantation in rats induces changes in the lung microbiota (Madan et al., 2012; Liu et al., 2017).

Gut-Lung Interaction

The host’s health condition can impact this gut–lung interaction too. In cystic fibrosis (CF) newborns, gut colonizations with Roseburia, Dorea, Coprococcus, Blautia, or Escherichia presaged their respiratory appearance, and their gut and lung abundances are highly correlated over time (Madan et al., 2012). Similarly, the lung microbiota is enriched with gut bacteria, such as Bacteroides spp., after sepsis (Dickson et al., 2016).

Conversely, lung microbiota may affect the gut microbiota composition. In a pre-clinical model, influenza infection triggers an increased proportion of Enterobacteriaceae and decreased abundances of Lactobacilli and Lactococci in the gut (Looft and Allen, 2012). Consistently, lipopolysaccharide (LPS) instillation in the lungs of mice is associated with gut microbiota disturbances (Sze et al., 2014).

Although gastroesophageal content inhalations and sputum swallowing partially explain this inter-organ connection, GLA also involves indirect communications such as host immune modulation.

Gut–Lung Axis Interactions With Human Immune System

Gut microbiota effects on the local immune system have been extensively reviewed (Elson and Alexander, 2015). Briefly, the gut microbiota closely interacts with the mucosal immune system using both pro-inflammatory and regulatory signals (Skelly et al., 2019). It also influences neutrophil responses, modulating their ability to extravasate from blood (Karmarkar and Rock, 2013).

Receptor Signaling

Toll-like receptor (TLR) signaling is essential for microbiota-driven myelopoiesis and exerts a neonatal selection shaping the gut microbiota with long-term consequences (Balmer et al., 2014; Fulde et al., 2018). Moreover, the gut microbiota communicates with and influences immune cells expressing TLR or GPR41/43 by means of microbial associated molecular patterns (MAMPs) or short-chain fatty acids (SCFAs) (Le Poul et al., 2003).

Data focused on the gut mycobiota’s impact on the immune system are sparser. Commensal fungi seem to reinforce bacterial protective benefits on both local and systemic immunity, with a specific role for mannans, a highly conserved fungal wall component. Moreover, fungi are able to produce SCFAs (Baltierra-Trejo et al., 2015; Xiros et al., 2019). Therefore, gut mycobiota perturbations could be as deleterious as bacteriobiota ones (Wheeler et al., 2016; Jiang et al., 2017).

Lung Microbiota and Local Immunity

A crucial role of lung microbiota in the maturation and homeostasis of lung immunity has emerged over the last few years (Dickson et al., 2018). Colonization of the respiratory tract provides essential signals for maturing local immune cells with long-term consequences (Gollwitzer et al., 2014).

Pre-clinical studies confirm the causality between airway microbial colonization and the regulation and maturation of the airways’ immune cells. Germ-free mice exhibit increased local Th2-associated cytokine and IgE production, promoting allergic airway inflammation (Herbst et al., 2011).

Consistently, lung exposure to commensal bacteria reduces Th2-associated cytokine production after an allergen challenge and induces regulatory cells early in life (Russell et al., 2012; Gollwitzer et al., 2014). The establishment of resident memory B cells in lungs also requires encountering lung microbiota local antigens, especially regarding immunity against viruses such as influenza (Allie et al., 2019).

Interactions between lung microbiota and immunity are also a two-way process; a major inflammation in the lungs can morbidly transform the lung microbiota composition (Molyneaux et al., 2013).

Gut Health, Long-Reaching Immune Modulation Within Gut–Lung Axis

Beyond the local immune regulation by the site-specific microbiota, the long-reaching immune impact of gut microbiota is now being recognized, especially on the pulmonary immune system (Chiu et al., 2017).

The mesenteric lymphatic system is an essential pathway between the lungs and the intestine, through which intact bacteria, their fragments, or metabolites (e.g., SCFAs) may translocate across the intestinal barrier, reach the systemic circulation, and modulate the lung immune response (Trompette et al., 2014; Bingula et al., 2017; McAleer and Kolls, 2018).

SCFAs, mainly produced by the bacterial dietary fibers’ fermentation especially in case of a high-fiber diet (HFD), act in the lungs as signaling molecules on resident antigen-presenting cells to attenuate the inflammatory and allergic responses (Anand and Mande, 2018; Cait et al., 2018).

SCFA receptor–deficient mice show increased inflammatory responses in experimental models of asthma (Trompette et al., 2014). Fungi, including A. fumigatus, can also produce SCFAs or create a biofilm enhancing the bacterial production of SCFAs, but on the other hand, bacterial SCFAs can dampen fungal growth (Hynes et al., 2008; Baltierra-Trejo et al., 2015; Xiros et al., 2019). The impact of fungal production of SCFAs on the host has not been assessed so far.

Other Elements

Other important players of this long-reaching immune effect are gut segmented filamentous bacteria (SFBs), a commensal bacteria colonizing the ileum of most animals, including humans, and involved in the modulation of the immune system’s development (Yin et al., 2013). SFBs regulate CD4+ T-cell polarization into the Th17 pathway, which is implicated in the response to pulmonary fungal infections and lung autoimmune manifestations (McAleer et al., 2016; Bradley et al., 2017).

Recently, innate lymphoid cells, involved in tissue repair, have been shown to be recruited from the gut to the lungs in response to inflammatory signals upon IL-25 (Huang et al., 2018). Finally, intestinal TLR activation, required for the NF-?B–dependent pathways of innate immunity and inflammation, is associated with an increased influenza-related lung response in mice (Ichinohe et al., 2011).

Mechanisms

Other mechanisms may be involved in modulating the long-reaching immune response related to gut microbiota, exemplified by the increased number of mononuclear leukocytes and an increased phagocytic and lytic activity after treatment with Bifidobacterium lactis HN019 probiotics (Gill et al., 2001). Diet, especially fiber intake, which increases the systemic level of SCFAs, or probiotics influence the pulmonary immune response and thus impact the progression of respiratory disorders (King et al., 2007; Varraso et al., 2015; Anand and Mande, 2018).

The GLA immune dialogue remains a two-way process. For instance, Salmonella nasal inoculation promotes a Salmonella-specific gut immunization which depends on lung dendritic cells (Ruane et al., 2013). Respiratory influenza infection also modulates the composition of the gut microbiota as stated above. These intestinal microbial disruptions seem to be unrelated to an intestinal tropism of influenza virus but mediated by Th17 cells (Wang et al., 2014).

In summary, GLA results from complex interactions between the different microbial components of both the gut and lung microbiotas combined with local and long-reaching immune effects. All these interactions strongly suggest a major role for the GLA in respiratory diseases, as recently documented in a mice model (Skalski et al., 2018).
Gut–Lung Axis in Respiratory Diseases

Acute Infectious Diseases

Regarding influenza infection and the impact of gut and lung microbiota, our knowledge is still fragmentary; human data are not yet available. However, antibiotic treatment causes significantly reduced immune responses against influenza virus in mice (Ichinohe et al., 2011). Conversely, influenza-infected HFD-fed mice exhibit increased survival rates compared to infected controls thanks to an enhanced generation of Ly6c-patrolling monocytes. These monocytes increase the numbers of macrophages that have a limited capacity to produce CXCL1 locally, reducing neutrophil recruitment to the airways and thus tissue damage. In parallel, diet-derived SCFAs boost CD8+ T-cell effector function in HFD-fed mice (Trompette et al., 2018).

Both lung and gut microbiota are essential against bacterial pneumonia. The lung microbiota is able to protect against respiratory infections with Streptococcus pneumoniae and Klebsiella pneumoniae by priming the pulmonary production of granulocyte-macrophage colony-stimulating factor (GM-CSF) via IL-17 and Nod2 stimulation (Brown et al., 2017).

Gut Health and Lung Bacterial Infections

The gut microbiota also plays a crucial role in response to lung bacterial infections. Studies on germ-free mice showed an increased morbidity and mortality during K. pneumoniae, S. pneumoniae, or P. aeruginosa acute lung infection (Fagundes et al., 2012; Fox et al., 2012; Brown et al., 2017). The use of broad-spectrum antibiotic treatments, to disrupt mouse gut microbiota, results in worse outcome in lung infection mouse models (Schuijt et al., 2016; Robak et al., 2018).

Mechanistically, alveolar macrophages from mice deprived of gut microbiota through antibiotic treatment are less responsive to stimulation and show reduced phagocytic capacity (Schuijt et al., 2016). Interestingly, priming of antibiotic-treated animals with TLR agonists restores resistance to pulmonary infections (Fagundes et al., 2012). SFBs appear to be an important gut microbiota component for lung defense against bacterial infection thanks to their capacity to induce the production of the Th17 cytokine, IL-22, and to increase neutrophil counts in the lungs during Staphylococcus aureus pneumonia (Gauguet et al., 2015).

Modulating chronic infectious diseases will similarly depend on gut and lung microbiotas. For instance, Mycobacterium tuberculosis infection severity is correlated with gut microbiota (Namasivayam et al., 2018).

Chronic Respiratory Diseases

Multiple studies have addressed the impact of gut and lung microbiota on chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD), asthma, and CF (Table 1).

Table 1. Gut–lung axis in human chronic respiratory diseases. Gut Health.

Decreased lung microbiota diversity and Proteobacteria expansion are associated with both COPD severity and exacerbations (Garcia-Nuñez et al., 2014; Wang et al., 2016, 2018; Mayhew et al., 2018). The fact that patients with genetic mannose binding lectin deficiency exhibit a more diverse pulmonary microbiota and a lower risk of exacerbation suggests not only association but also causality (Dicker et al., 2018).

Besides the lung flora, the gut microbiota is involved in exacerbations, as suggested by the increased gastrointestinal permeability in patients admitted for COPD exacerbations (Sprooten et al., 2018). Whatever the permeability’s origin (hypoxemia or pro-inflammatory status), the level of circulating gut microbiota–dependent trimethylamine-N-oxide has been associated with mortality in COPD patients (Ottiger et al., 2018). This association being explained by comorbidities and age, its impact per se is not guaranteed. Further studies are warranted to investigate the role of GLA in COPD and to assess causality.

Early Life Perturbation

Early-life perturbations in fungal and bacterial gut colonization, such as low gut microbial diversity, e.g., after neonatal antibiotic use, are critical to induce childhood asthma development (Abrahamsson et al., 2014; Metsälä et al., 2015; Arrieta et al., 2018).

This microbial disruption is associated with modifications of fecal SCFA levels (Arrieta et al., 2018). Causality has been assessed in murine models. Inoculation of the bacteria absent in the microbiota of asthmatic patients decreases airways inflammation (Arrieta et al., 2015).

Fungi

Furthermore, Bacteroides fragilis seems to play a major role in immune homeostasis, balancing the host systemic Th1/Th2 ratio and therefore conferring protection against allergen-induced airway disorders (Mazmanian et al., 2005; Panzer and Lynch, 2015; Arrieta et al., 2018). Nevertheless, it is still not fully deciphered, as some studies conversely found that an early colonization with Bacteroides, including B. fragilis, could be an early indicator of asthma later in life (Vael et al., 2008).

Regarding fungi, gut fungal overgrowth (after antibiotic administration or a gut colonization protocol with Candida or Wallemia mellicola) increases the occurrence of asthma via IL-13 without any fungal expansion in the lungs (Noverr et al., 2005; Wheeler et al., 2016; Skalski et al., 2018). The prostaglandin E2 produced in the gut by Candida can reach the lungs and promotes lung M2 macrophage polarization and allergic airway inflammation (Kim et al., 2014).

Mouse & Human Gut Health

In mice, a gut overrepresentation of W. mellicola associated with several intestinal microbiome disturbances appears to have long-reaching effects on the pulmonary immune response and severity of asthma, by involving the Th2 pathways, especially IL-13 and to a lesser degree IL-17, goblet cell differentiation, fibroblasts activation, and IgE production by B cells (Skalski et al., 2018).

These results indicate that the GLA, mainly through the gut microbiota, is likely to play a major role in asthma.

Cystic Fibrosis and Gut Health

In CF patients, gut and lung microbiota are distinct from those of healthy subjects, and disease progression is associated with microbiota alterations. (Madan et al., 2012; Stokell et al., 2015; Nielsen et al., 2016). Moreover, the bacterial abundances at both sites are highly correlated and have similar trends over time (Madan et al., 2012). This is especially true regarding Streptococcus, which is found in higher proportion in CF stools, gastric contents, and sputa. (Al-Momani et al., 2016; Nielsen et al., 2016).

Moreover, CF patients with a documented intestinal inflammation exhibit a higher Streptococcus abundance in the gut (Enaud et al., 2019). That suggests the GLA’s involvement in intestinal inflammation. Of note, gut but not lung microbiota alteration is associated with early-life exacerbations. Some gut microbiota perturbations, such as a decrease of Parabacteroides, are predictive of airway colonization with P. aeruginosa (Hoen et al., 2015).

Furthermore, oral administration of probiotics to CF patients leads to a decreased number of exacerbations (Anderson et al., 2016). While the mycobiota has been recently studied in CF (Nguyen et al., 2015; Soret et al., 2019), no data on the role of the fungal component of the GLA are currently available in CF. This deserves to be more widely studied.

Improving Health in the Gut

The role of inter-compartment and inter-kingdom interactions within the GLA in those pulmonary diseases now has to be further confirmed and causality assessed. Diet, probiotics, or more specific modulations could be, in the near future, novel essential tools in therapeutic management of these respiratory diseases.

Conclusion

The gut–lung axis or GLA has emerged as a specific axis with intensive dialogues between the gut and lungs, involving each compartment in a two-way manner, with both microbial and immune interactions (Figure 1). Each kingdom and compartment plays a crucial role in this dialogue, and consequently in host health and diseases. The roles of fungal and viral kingdoms within the GLA still remain to be further investigated. Their manipulation, as for the bacterial component, could pave the way for new approaches in the management of several respiratory diseases such as acute infections, COPD, asthma, and/or CF.

WAF: Gut health is an important area of research for the foundation.

Gut Health and asthma, an interview with Rodney Dietert, PhD.

See also Dr. Dietert’s interview about the Gut and Lung connection.

Genetic Basis of Asthma

Asthma is the most common chronic childhood disease in developed nations and its prevalence has increased in the world over the last 25 years. It is a complex disease with both genetic and environmental risk factors. Asthma is caused by multiple interacting genes, some having a protective effect and others contributing to the disease pathogenesis, with each gene having its own tendency to be influenced by the environment. This article reviews the current state of the genetics of asthma in six categories, viz. epidemiology, management, aetiology, family and twin studies, segregation and linkage studies, and candidate genes and single nucleotide polymorphisms (SNPs).

Asthma is one of the most serious allergic diseases and the most common chronic childhood disease in developed nations1. It has been characterized by increased responsiveness of the tracheobronchial tree to a multiplicity of stimuli2–4, increased infiltration of various inflammatory cells especially eosinophils into the airway, epithelial damage, airway smooth-muscle hypertrophy5, constriction, variable airway obstruction usually associated with inflammation in the conducting airways of the lungs6 and mucous hypersecretion in the bronchiolar walls of the lung7. Asthma is critically dependent on a series of cell adhesion molecule-mediated interactions between vascular endothelium and leukocytes7, leading to symptoms8 and elevation in total serum IgE9. It is manifested physiologically by widespread narrowing of the air passages and clinically by paroxysms of dyspnoea, cough, wheezing and tightness, provoked by one or more triggers such as physical exertion and airway irritants (cold, dry air, smoke, etc.)4,10. It is an episodic disease, with acute exacerbations interspersed with symptom-free periods. Typically, most attacks are short-lived, lasting minutes to hours, and clinically the patient seems to recover completely after an attack. However, there can be a phase in which the patients experience some degree of airway obstruction daily. This phase can be mild, with or without superimposed severe episodes, or can be much more serious, with severe obstruction persisting for days or weeks; the latter condition is known as “acute severe asthma”. In unusual circumstances, acute episodes can cause death4. Asthma exacerbations are characteristically worse at night and can progress to severe airflow obstruction, shortness of breath, and respiratory distress and insufficiency. Rarely, severe sequel such as hypoxic seizures, respiratory failure, and death can occur.

Here we review the latest information on the genetic basis of asthma which is one of the most intriguing diseases affecting people of all ages, gender, race and ethnicities. Familial and segregation studies have an important role in asthma aetiology and several candidate genes on all the human chromosomes play their roles in initiation and/or inhibition of different pathways of asthma disease.

Note from the WAF editorial board: We wish to acknowledge and thank Mahdi Bijanzadeh, Padukudru A. Mahesh,* and Nallur B. Ramachandra
at the Indian Journal of Medical Research for their dedication to Asthma education and research.

Conclusion and future prospects

Asthma is one of the most serious and intriguing allergic diseases. Asthma aggregates within families and is a complex multifactorial disease with the involvement of environment and genetic components. Our preliminary pedigree analysis revealed that autosomal recessive pattern of inheritance was prominent in asthma; parental consanguinity100 and serum intracellular cell adhesion molecule-1 (ICAM-1)101 was significantly associated with asthma, whereas the ABO blood system102, IL-4 and ADAM33 specific gene variants81, and serum E-selectin101 were not associated with asthma. More than 100 loci have been reported to be associated with asthma and there are also indications that mutation in a major gene can cause asthma. Due to an increasing number of current studies being done in genetics of asthma, there is an increasing list of inducer and inhibitor candidate genes for asthma. There are more than 100 candidate genes in every chromosome which are identified to have an association with asthma and the strength of association of these SNPs with asthma varies in different parts of the world. More studies are needed to determine the exact function of these genes, gene-gene interactions and the gene-environment interactions which are undoubtedly complex and remain elusive for the time being even with whole genome-wide association studies.

Further studies on asthma with the genomics data and tools, to map, identify the specific gene/s, and phenotype specific SNPs will help to unravel the pathways involved in asthma aetiology and employ pharmacogenomics to design better drugs for an individualized treatment plan. Thus with a fruitful interaction among researchers involved in pathophysiology, epidemiology, clinical research and genetics of asthma, this century holds promise for a better understanding of the pathology, diagnosis, prevention, treatment and management of asthma.