World Asthma Day Summary

On the day after World Asthma Day, May 3, 2022, we scanned the globe to find a statement that best sums up the current state of affairs regarding Asthma.

Kudos to tbe U.S National Institute of Environmental Health NIH Statement on World Asthma Day 2022: Toward Improved Asthma Care

Good enough of summary that we want to publish this in its entirety.

Asthma is a serious lung disease; causes chest tightness, wheezing, and coughing; can often be controlled with proper treatment.

Today (May 3, 2022) on World Asthma Day, the National Institutes of Health reaffirms its commitment to biomedical research aimed at preventing the onset of asthma, understanding its underlying causes, and improving the treatment of it. This chronic airway disease, which is characterized by periodic worsening of inflammation that can make it hard to breathe, affects more than 25 million people in the United States, including more than 5 million children. Left untreated, it can be life-threatening.

While scientists have made substantial progress in understanding asthma diagnosis, management, and treatment, therapies to permanently improve breathing for those who suffer from asthma remain elusive. Researchers around the globe are working steadily toward this goal while they seek to better understand and find new ways to manage the disease. They also are continuing research on the underlying causes of disparities in the incidence, care, and prevention of the disease. On the heels of recently updated management and treatment guidelines, researchers anticipate a brighter future for people living with asthma.

Three NIH institutes primarily support and conduct studies on asthma — the National Heart, Lung, and Blood Institute (NHLBI); the National Institute of Allergy and Infectious Diseases (NIAID); and the National Institute of Environmental Health Sciences (NIEHS). Other NIH Institutes and Centers also support and conduct asthma research. NIH scientists and grantees made important advances in understanding, treating, and managing asthma in 2021, which are briefly highlighted as follows:

Asthma and COVID-19

An NHLBI-funded study showed that during the pandemic, asthma attacks, also known as asthma exacerbations, significantly decreased in a large group of children and adolescents, compared to the year before the pandemic. The study also found that telehealth visits among these patients increased dramatically during this time. The study included nearly 4,000 participants aged 5-17 years with a prior diagnosis of asthma. Researchers believe a better understanding of the factors that contributed to these improved outcomes could lead to better asthma control in all children and adolescents, as researchers noted no racial or ethnic differences in health outcomes in this population.

A NIAID-funded study found that asthma does not increase the risk of becoming infected with SARS-CoV-2, the virus that causes COVID-19. This finding came from a six-month household survey of more than 4,000 children and adults conducted between May 2020 and February 2021.

Asthma Disparities

Researchers have known for decades that social determinants of health – conditions like housing, neighborhood, education, income, and healthcare access – can affect the quality of life and asthma-related health outcomes of people living with the disease. NIH scientists are now reporting new advances in understanding the relationship between social determinants of health and asthma.

Black and Hispanic children who live in low-income urban environments in the United States are at particularly high risk for asthma attacks. These children tend to be underrepresented in large trials of new biologic therapies for asthma.

In a recent NIAID-supported clinical trial, the monoclonal antibody mepolizumab decreased asthma attacks by 27% in Black and Hispanic children and adolescents who have a form of severe asthma, are prone to asthma attacks, and live in low-income urban neighborhoods.

In one study, NHLBI-funded investigators demonstrated the importance of housing interventions in improving the health of children with asthma. Poor quality housing is associated with a high level of asthma triggers – including mold, cockroach, mouse, and dust mite allergens – that can pose a health threat to children with asthma. The study showed the feasibility of using targeted interventions – including better pest management, improved ventilation, and moisture reduction – to achieve healthy housing. It showed that such interventions can result in reduced symptoms and hospitalizations due to asthma.

Environmental Exposures and Asthma

Researchers have known for years that asthma can be triggered by substances in the indoor and outdoor environment. New research shows that exposure to some asthma triggers might even occur before birth.

In an NIH-supported study that included grant support from NIEHS and the NHLBI, researchers reported that prenatal exposure to tiny air pollution particles significantly increased the risk for developing asthma in children. The study, which analyzed data from two different study cohorts, focused on a group of mothers and their children, mostly Black or Hispanic, in the Boston area who lived near major roadways with heavy traffic. It found that more than 18% of the children who were exposed to high levels of these so-called ultrafine particles in the womb developed asthma in their preschool years, compared to 7% of children overall in the United States.

An NIEHS clinical study will assess how environmental factors affect disease progression in non-smoking adults who have moderate or severe asthma. The study will focus on the microbiological and genetic factors associated with atopic asthma, also known as allergic asthma, which is triggered by pollen, dust mites, and other allergens. A better understanding of this data might lead to improved treatments for people with this type of asthma, researchers say.

Climate Change and Asthma

Studies have shown that climate change can increase air pollutants such as ground-level ozone, fine particulates, wildfire smoke, and dust, and that these pollutants can exacerbate asthma. Climate change can also affect the production, distribution, and severity of airborne allergens.

NIEHS, NHLBI, and other NIH institutes and centers are leading the NIH Climate Change and Health Initiative. This is a cross-cutting NIH effort to reduce health threats such as asthma that can develop or worsen because of climate change. The initiative will look at these threats across the lifespan and find ways to build health resilience in individuals, communities, and nations around the world. A strategic framework for the Initiative will help guide NIH investments in this area.

An NIEHS-funded study provides examples of how extreme weather events can affect asthma outcomes. For example, as heat waves and droughts become more frequent and prolonged, the risk of large wildfires will likely increase, resulting in poor air quality that makes it more difficult to control asthma. Other climate-change events can lead to longer and more intense pollen seasons, while mold and dampness in homes may cause asthma to develop or worsen preexisting cases.

About the National Institute of Allergy and Infectious Diseases (NIAID): NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID website.

About the National Heart, Lung, and Blood Institute (NHLBI): NHLBI is the global leader in conducting and supporting research in heart, lung, and blood diseases and sleep disorders that advances scientific knowledge, improves public health, and saves lives. For more information, visit For additional information about NHLBI’s asthma resources, visit

About the National Institute of Environmental Health Sciences (NIEHS): NIEHS supports research to understand the effects of the environment on human health and is part of the National Institutes of Health. For more information on NIEHS or environmental health topics, visit

About the National Institutes of Health (NIH): NIH, the nation’s medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit

The World Asthma Foundation Announces Speakers for Microbiome First Summit

On this World Asthma Day, May 3, 2002, The Microbiome First – Pathway to Sustainable Healthcare Summit organization committee invites healthcare professionals, non-communicable disease community leaders, and stakeholders to participate in the inaugural Microbiome First Summit, a virtual event taking place online at this May, 17-19, 2022. FREE to participants.

For detailed information and to register, visit:

The event, Microbiome First – Pathway to Sustainable Healthcare Summit, kicks off the inaugural event underwritten and moderated by the
World Asthma Foundation (WAF), which is pleased to announce the
following speakers:

Event Keynote
Cornell University Professor Emeritus
Ithaca, NY, USA
Author of The Human Superorganism.
Keynote: “Big Picture View of Our Tiny Microbes”

Researcher Sessions
Associate Professor, departments of Physiology, Pharmacology, and Pediatrics, University of Calgary
Calgary AB, CANADA
Session: “The early-life mycobiome in immune and metabolic development”

Assistant Professor, Microbiome Program, Center for Individualized Medicine, Mayo Clinic.
Rochester, MN, USA
Session: “A predictive index for health status using species-level gut microbiome profiling”

Assistant Professor, Molecular Biology and Biochemistry School of Biological Sciences
Associate Director, UCI Microbiome Initiative
Irvine, CA, USA
Session: “High-Fiber, Whole-Food Dietary Intervention Alters the Human Gut Microbiome but Not Fecal Short-Chain Fatty Acids”

Cognitive neuroscientist; Expert in brain imaging, autism, body cognition
Associate Professor in the USC Chan Division of Occupational Science and Occupational Therapy
Los Angeles, CA, USA
Session: “Brain-Gut-Microbiome System: Pathways and Implications for Autism Spectrum Disorder”

Chief of Rheumatology and Clinical Immunology at University Hospital of Münster
Associate Professor Adjunct of Immunobiology at Yale School of Medicine.
Session: “Dietary Resistant Starch Effects on Gut Pathobiont Translocation and Systemic Autoimmunity”

Senior research scientist and Associate Professor in the Department of Microbiology and Immunology at the Stanford University School of Medicine.
Palo Alto, CA, USA
Session: “Gut-microbiota-targeted diets modulate human immune status”

Associate Professor
Principal Research Fellow
The University of Queensland Diamantina Institute
Faculty of Medicine
The University of Queensland
Translational Research Institute
Woolloongabba, QLD, AUSTRALIA
Session: “Metabolite-based Dietary Supplementation in Human Type 1 Diabetes is associated with Microbiota and Immune modulation”

Scientist, Wyss Institute of Harvard University and Institute of Medical Engineering and Science at Massachusetts Institute of Technology
Cambridge, MA, USA
Session: “Protecting the Gut Microbiota from Antibiotics with Engineered Live Biotherapeutics”

Gastroenterologist, Neuroscientist, Distinguished Research Professor
Department of Medicine, UCLA David Geffen School of Medicine
Executive Director, G. Oppenheimer Center for Neurobiology of Stress and Resilience at UCLA
Founding Director, UCLA Brain Gut Microbiome Center.
Los Angeles, CA, USA
Session: “The Gut–Brain Axis and the Microbiome: Mechanisms and Clinical Implications”

Principal Investigator, Chassaing Lab
Associate professor, French National Institute of Health and Medical Research.
Session: “Ubiquitous food additive and microbiota and intestinal environment”

Associate Professor
College of Medicine, University of Ulsan
Department of Pulmonary and Critical Care, Asan Medical Center
Seoul, KOREA
Session: “The Therapeutic Application of Gut-Lung Axis in Chronic Respiratory Disease”

Clinical and translational researcher
Research Fellow, The University of Western Australia
Honorary Research Associate, Telethon Kids Institute.
Session:House Dust Mite Shedding in Human Milk: a Neglected Cause of Allergy Susceptibility?”

Research Fellow, The University of Western Australia, Australia
Honorary Research Associate, Telethon Kids Institute.
Session: “Gut Microbiota by Breastfeeding: The Gateway to Allergy Prevention”

Rachel Carson Professor of Ecology and Evolutionary Biology, Yale University
Microbiology faculty member, Yale School of Medicine.
New Haven, CT, USA
Session: “New Yale Center to Advance Phage Research, Understanding, Treatments, Training, Education”

Scientist, Wyss Institute of Harvard University and Institute of Medical Engineering and Science of Massachusetts Institute of Technology MIT
Boston, MA, USA
Session: “Protecting the Gut Microbiota from Antibiotics with Engineered Live Biotherapeutics”

Emeritus Professor, Allergy/Immunology and Environmental Health University of Texas San Antonio, TX, USA
Session: “Toxicant-Induced Lost of Tolerance for Chemicals, Foods and Drugs: a Global Phenomenon”

Media Supporter Content
Microbiome Courses
London, England UK
Session “Educating Parents About ‘Seeding And Feeding’ A Baby’s Microbiome”

Summit Details:

The goal of the Microbiome First – Sustainable Healthcare Summit is to
improve quality of life at reduced cost by addressing the microbiome
first, as recent research shows that all of these non-communicable diseases have a relationship to the microbiome.

For additional information visit or on Twitter at @MicrobiomeFirst

Monoclonal Antibodies Anaphylaxis Risk For Severe Asthma Patients

Monoclonal antibodies

Most monoclonal antibodies increase the risk of anaphylaxis in severe asthma patients, according to a report published in Clinical and Translational Allergy. The study found 4 out of 5 common mAbs were associated with an increased risk of anaphylaxis. Each of the therapies had a different risk profile. One monoclonal antibody – dupilumab – exhibited a very low risk. The patients in this study were mainly young and middle-aged adults.

What You Need to Know

Monoclonal antibodies (mAbs) are a type of medication designed to work on one specific target, in contrast to conventional medications that often affect more than one site within the body. They differ from polyclonal treatments by binding specifically to a single place (an epitope) on their target protein. Monoclonal antibodies are found naturally, produced by cloned B cells called hybridomas.

For those with severe asthma, there are Clinical Safety Issues. Any severe asthmatic who experiences an anaphylactic reaction due to food or insect allergy must be made aware that the use of mAbs may cause an adverse reaction for them.

Severe asthma patients receiving monoclonal antibodies need close monitoring due to increased anaphylaxis risk, the new study found.

The report, published in Clinical and Translational Allergy, found 4 out of 5 common mAbs were associated with a heightened anaphylaxis risk, though the risk varied from therapy to therapy.

Key Takeaways

  • Monoclonal antibodies work with the immune system.
  • mAbs target cancer cells, viruses, bacteria, and other pathogens.
  • They may be beneficial in autoimmune diseases in place of other immunosuppressive agents.
  • mAbs generally prevent allergic reactions, but for some people, may cause a severe adverse reaction.
  • Regular doses of antihistamines and/or epinephrine may be used with the monoclonal antibodies.

Common monoclonal antibodies include:

– omalizumab (trade names Xolair and Omeclamox and used for allergies to asthma and insect bites)

– mepolizumab (trade name Nucala, a newer monoclonal antibody. It is used for Severe Persistent Asthma in those who have moderate to severe allergic asthma and do not respond well to conventional treatments.)

What Are Monoclonal Antibodies?

Monoclonal antibodies are secreted proteins that neutralize a pathogen or an undesirable substance. They derive from monocytes and can bind to target the protein which is responsible for activating B-cells during immune response. They function as a tool to modify the progression of disease by slowing down the symptoms of autoimmune reactions.

Monoclonal Antibodies (MAbs) function to eliminate pathogens or unwanted toxins. They may have diagnostic as well as treatment potentials in autoimmune disease-related disorders.

The World Asthma Foundation thanks the six expert researchers for their insight into how normally beneficial Monoclonal Antibodies may cause Anaphylactic reactions in severe asthmatics.

Li L, Wang Z, Cui L, Xu Y, Guan K, and Zhao B. “Anaphylactic risk related to omalizumab, benralizumab, reslizumab, mepolizumab, and dupilumab.” Published online June 3, 2021. doi:10.1002/clt2.12038

Monoclonal antibodies
Monoclonal antibodies, PR image.

Asthma and Bacteria: Nose to the Toes

Staphylococcus aureus enterotoxins (intestinal toxins) have a demonstrated effect on airway disease including Asthma in early life according to multiple studies. These bacteria are in the gut and on the skin.

To further the WAF misson to improve our understanding of what drives Severe Asthma, the World Asthma Foundation reached out to Rodney Dietert, PhD, for his thoughts on the topic of Asthma and Staphylococcus aureus.

Rodney Dietert, PhD is a Cornell University Professor Emeritus, Health Scientist Head of Translational Science + Education for SEED, and the Author of the Human Super-Organism How the Microbiome is Revolutionizing the Pursuit of a Healthy Life.

This is the third in the series of interviews on the topic of Asthma and Staphylococcus aureus with Rodney Dietert, Phd.

Today We Learn About

* Staphylococcus aureus beyond the nose including the skin and the gut

Video Interview

Bacteria – Staph A and Asthma

World Asthma Foundation: Dr. Dietert, can we talk about Staphylococcus aureus and Asthma beyond the nose? 

Rodney Dietert, PhD: There are skin and gut microbiome effects on the Staph A asthma connection as well. It’s not just the nose but the nose is a good starting point.

Staph A, diet consumption, the bacteria that are in place, particularly in the nose, but also to some extent in the gut and even the skin, can determine what’s going to happen later in the risk for conditions like asthma. I think the thing to realize is that that bacteria and early on, that’s when you’re still recruiting cells. Lung maturation is one of the late-maturing systems. The lung and brain are late compared to a lot of other physiological systems. You don’t really fully mature the lung until something like 18 or 20.

Those effects on recruiting and getting balance in your immune cells in the lung are really important. When you’ve got a bacterium there that is producing allergens, it is stimulating a population we didn’t use to know about, called T helper 9. These cells produce a cytokine called Interleukin-9. The important thing to know is that these cells interact exquisitely with mast cells. They actually have T helper 9 cells.

Immune cells have histamine receptors so they’re co-stimulating between these cells and mast cells. Imagine (the outcomes) when an infant is skewed toward producing that kind of immune cells in these tissues, like the lung, and them having that kind of interactions with mast cells.

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

Staph A bacteria - Dr. Dietert.

For full story and video follow the link below

Asthma and Bacteria: Nose to the Toes

Asthma and Bacteria Link says Study

To further the WAF mission to improve our understanding of what drives Severe Asthma, the World Asthma Foundation reached out to Rodney Dietert, PhD for his thoughts on the topic of Asthma and the bacteria Staphylococcus aureus.

Staphylococcus aureus (S. aureus) is a Gram positive (thick wall) bacterium that is believed to be carried by about one third of the general population and is responsible for common and serious diseases. A growing amount of medical literature suggest that Staphylococcus aureus enterotoxins (intestinal toxins)could affect airway disease including Asthma. 

Rodney Dietert, PhD is a Cornell University Professor Emeritus, Health Scientist Head of Translational Science + Education for SEED and the Author of the Human Super-Organism How the Microbiome is Revolutionizing the Pursuit of a Healthy Life

This is the first interview of three on the topic of Staphylococcus aureus with Rodney Dietert, Phd. We learn about:

* Connection between Asthma and bacteria Staph A
* Skin, nose and gut Microbiome
* Asthma and the immune system

World Asthma Foundation: Dr. Dietert, can you connect Staphylococcus Aureus or Staph A and Chronic Disease including Asthma for us?

Connection between Asthma and Bacteria Staph A: Video

Rodney Dietert PhD. connects Staph A and Asthma (and other chronic diseases)

Rodney Dietert, PhD: Yes, well, it’s very interesting because there’s a lot of research starting to come out on infectious agents and chronic diseases. We used to think that never the two shall meet but in fact, they do in many cases, either by inducing chronic diseases or by exacerbating those conditions.

Staph A, that is gram-positive bacterium, it is serious in terms of potential infections. We have it most often on the skin and in the nose. It can be either carried there or it can be a transient exposure. It also contributes to food poisoning. It’s one of the agents that, if contaminated, food can produce serious GI problems. It has a number of products that it makes including what are called enterotoxins or exotoxins. These can punch holes in cells. They can damage epithelial layers in the gut or in the airways.

They also are very interesting, in the case of Staph A or Staphylococcus aureus because some of these toxins can serve as allergens. They are actually sensitizing agents and that is the unknown, until recently, discovery, that when you’re looking for allergens that may contribute to asthma, you’d better include integral elements of that particular bacterium because that may be your allergen, that you may or may not have tested for.

World Asthma Foundation: Can you explain the interaction between the Microbiome, Staph A and Asthma?

Rodney Dietert, PhD:  Well, as with any potential opportunistic pathogen, the status of our microbiome in the body sites that carry it, and that would be where we’re exposed to the environment, so the airways, respiratory system, the skin, the gut, the urogenital tract, the status of that microbiome is incredibly important in terms of whether those pathogens can gain a foothold and then produce an infection. That is absolutely the case with Staph A.

Breaking the skin may give it (an opportunity to infect) , or surgery (as well). You have Staph A, and particularly drug-resistant Staph A, (as) a potential risk with surgeries but, (also) I mentioned food poisoning, breaking the skin. Also, dysbiosis, we call it, or a problem with the microbiome in the nose really can result in (Staph A problems), not just chronic sinusitis or reoccurring infections, but asthma. (The bacterium) can be inducing the condition, (and/or) it can be exacerbating already existing asthma.

Rodney Dietert, PhD: (Staph A) has been identified as one of the major culprits that is in the nose and where it can gain a foothold, (it) can produce some real problems.

World Asthma Foundation: What’s the distinction between infection and colonization in the context of Staph A and Asthma?

Rodney Dietert, PhD: Well, colonization is really where it’s able to attach to the proximity of the epithelium, or maybe directly to the surface. It can then produce its toxins and damage the epithelium and also have a nutrient source and spread. The thing to keep in mind is your friendly bacteria, your microbiota, that are mutualistic bacteria, or commensals they’re also called, the ones that we take as probiotics. Those actually have something like double-digit processes they can use to block pathogens like Staph A. Their being in place and metabolizing, in this case in the nose, is really important.

There is a recent really beautiful study that was published looking at early life and looking at colonization by bacteria in the nose and the prognosis for those children to develop asthma or not and some of the parameters related to that. That is where you can really see that starting to think about Staph A and asthma is critical immediately at birth and in the early few months. That’s where some of these distinctions are made and where, unfortunately, you can set up the immune system for inflammation in the lung.

World Asthma Foundation: You have a background in Immmunotoxicology. Can you define this role?

Rodney Dietert, PhD: Immunotoxicity is basically any environmental or external directed alteration to the immune system, in a negative way, damage to the immune system. That damage can take all kinds of different forms. Now, I have to say in my earlier years as a professor in the era of AIDS, HIV and AIDS, everybody thought, well, it’s all immunosuppression. My mantra has been, I contend that there are very few things that produce (only) pure immunosuppression. Something goes down and usually something else goes up (within the immune system) quite frankly. That part of what goes up is (often) allergy, autoimmunity, and inflammatory disease.

We used to measure, in the earlier days of immunology, we’ll measure things and say, “Wow, the antibody levels are reduced,” or something like that. We weren’t measuring more complex indicators for auto-immunity allergic diseases like asthma and psoriasis, inflammatory conditions. Had we been doing that (measuring the more complex indicators of immune-inflicted chronic diseases), we would see that some things we thought that were either immunosuppressive or not (and deemed safe), were not actually where the excitement (most significant risk) was for damage to the immune system.

The damage (connected to improper immune enhancement/balance) can (lead to) self-inflicted disease produced largely by immune cells (that are) out of control and misregulated. When you’re talking about asthma, that’s where you are.  (It) is (that) you have cells and mediators in the lung doing things out of balance that they shouldn’t be doing.

World Asthma Foundation: You advocate and have written extensively about Sustainable Healthcare. In fact, you write about the cost of chronic disease. Can you summarize the findings of the World Economics Council and Harvard study predicting that Chronic Disease will consume 48% of Global GDP by 2030?

Rodney Dietert, PhD:  That’s worldwide net worth, and we can’t afford it. If you’re thinking about healthcare being sustainable and being available for people in the future, for our children, for our grandchildren, then we’ve got to do things differently. We should do a better job of preventing chronic diseases like asthma and we certainly should do a better job of managing these (chronic diseases) with the life course in mind.

When a child presents with asthma, the pediatrician quite frankly, in my opinion, should be asking, “What can I do for that child today? What can I do to prevent comorbid diseases 10, 20, 30, 40 years from now?” I think the second part of that we have not yet fully embraced and dealt with.

World Asthma Foundation: What would you like researchers to know about the relationship between Staph A and chronic disease?

Rodney Dietert, PhD: I think they need to realize again that the starting point, birth and the first few months, is the time to do something. (In early life) it is easier (to make the most significant changes) and (those actions taken during early development are) likely to be more permanent (as the infant ages). (Also, there is an opportunity to insulate that child from even some problems, maybe diet or otherwise, later in life that (otherwise) could be a risk factor.

That’s the time to do something. Where you get bigger bang for the buck, is early (in life). Researchers simply need to know to look for infectious agents that are involved with conditions like asthma and to start to realize that their management needs to start and stop with the microbes that are protecting the individual, the friendly microbes or microbiota, and to ensure that that is in balance.

If you’re hoping to nudge the immune system in a more useful way, you’re hoping to control inflammation, it’s my contention that if you don’t correct the microbiome, you’re going to be back in the same boat, on the leakyboat, (the) sinking boat, shortly, with your treatments.

There’s sort of a fingerprint of (the) respiratory microbiome, and particularly in the nose, that reflects asthma existing. There’s the chicken or egg question, which comes first, and what’s a result of (what)? Nevertheless, if you don’t actually address that (the respiratory microbiome which affects both risk of Staph A infection and lung immune status) in any meaningful way, then you’ve got to know that the tendency is there that (it is) going to snap back at some point. It’s going to bite you (with an elevated risk of and/or exacerbation of asthma).

WAF will continue to investigate the link between asthma and bacteria.

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

Rodney Dietert PhD on the topic of Asthma and the bacteria Staphylococcus aureus.

For more information about Dr. Dietert, visit here.

Gut and Lung Connection to Asthma – Rodney Dietert, PhD

In this fifth in a series of interviews with Rodney Dietert PhD, he talks about communication between the gut and lung. Dr. Dietert is Cornell University Professor Emeritus, Health Scientist Head of Translational Science + Education for SEED and the Author of the Human Super-Organism How the Microbiome is Revolutionizing the Pursuit of a Healthy Life we learn about:

* The Gut and lung communication and its relationship to Asthma

World Asthma FoundationDefeating Asthma Series uncovers New Hope for Asthma Managementant

Asthmatics: Our understanding of Asthma and the way we treat it may soon be radically different from what currently exists, due to new research on the human microbiome and how the microbiome affects asthma.


World Asthma Foundation: Research into the Microbiome and its relationship to health has improved significantly in the last few years. For example, we now know about the relationship between the gut and health. We’ve also learned about communication between the gut and the lung and the impact on Asthma. Dr. Dietert, so there’s some crosstalk, right?

Video interview: Asthma Connection to Gut and Lung Cross Talk – Rodney Dietert, PhD

Dr. Dietert: Tremendous crosstalk, absolutely tremendous. You’re correct that if you’re looking at endpoints, something like risk of asthma or management of asthma, then you really, at a minimum, are going to focus both on the respiratory system microbiome and the gut microbiome. That’s not necessarily the exclusion of others but those two are really important. Just like the gut microbiome can affect the brain, it can affect behavior, mood. You don’t need lots of hardcore meds as an antidepressant when you’ve got the solution sitting right in your gut in terms of the microbiome.

With the respiratory system, you’ve got both the local microbes being extremely important but you have crosstalk, you have chemical interactions that are originating in the gut that are affecting the respiratory system as well.

World Asthma Foundation: Dr. Dietert, we certainly thank you for your time, all that you do for the microbiome and the community. Good afternoon, and thanks again.

Dr. Dietert: Well, and thank you for all you do with the World Asthma foundation, Bill. Pleasure.

To learn more about Dr. Dietert, go here.

Gut and Lung crosstalk interview with Rodney Dietert.

Asthma and the Microbiome – Martin J Blaser MD Interview

Defeating Asthma Series uncovers New Hope for Asthma Management

In this second interview with Martin J Blaser MD, Director of the Center for Advanced Biotechnology and Medicine at Rutgers Biomedical and Health Sciences and the Henry Rutgers Chair of the Human Microbiome and Professor of Medicine and Microbiology at the Rutgers Robert Wood Johnson Medical School in New Jersey and the Author of the “Missing Microbes – How the Overuse of Antibiotics is Fueling Our Modern Plagues.” we learn:

  • About the connection between Asthma and the Microbiome
  • About research and studies that predict Asthma in childhood
  • About bacteria not just in the stomach but in the colon
  • About C-sections and the likelihood to develop asthma
  • About the Mayo Clinic study on Asthma and antibiotics useage

Our understanding of Asthma and the way we treat it may soon be radically different from what currently exists, due to new research on the human microbiome and how the microbiome affects asthma.

World Asthma Foundation: Dr. Blaser, can you help us connect Asthma and the Microbiome?

[videopress kcn8lw8M]

Dr. Blaser: I’ve gotten very involved in studying the human microbiome in general, not just in the stomach, but in the colon. We and others are working on the relationship of the bacteria (microbiome) in the colon and asthma.

Again, there’s a paper that’s published. A young doctor from Denmark, Dr. Jakob Stokholm, came to work in my lab. This happened after Missing Microbes was published, so it’s not in the book. He’s part of a study in Copenhagen called the COPSAC study, the Copenhagen Open Study of Asthma in Children. They have cohorts of moms whose kids are going to have high risk of asthma, either because they have asthma or they already have a child who has asthma.

In 2010, if I remember correctly, they enrolled 750 moms with this high risk. They obtained fecal samples from the moms. They also got samples from the kids at one week, one month, and one year of life. Then they followed these kids until they were about six. The question was, is there anything that might predict who was going to get asthma at the age of six? We did a lot of work studying the microbiome in their fecal specimens, and what we found is consistent with what other people found: that the microbiome matures over time between one week and one month, and one year. It shows a pattern of maturation, but in some kids, their microbiome doesn’t mature in the normal way.

Then we made a very important observation. In those kids whose microbiome didn’t mature normally when you compare them to kids who did have normal maturation, the odds ratio, the chances that they were going to get asthma when they were six was 3, (300%) meaning a rate three times normal. Then we divided those kids by whether their mother previously had asthma or not. If their mother didn’t have asthma, the maturation pattern did not make a difference, but if their mother did have asthma, the odds ratio was 13.

We’re getting in the range of the association between smoking and lung cancer. That’s how strong that is. That was published about two years ago in Nature Communications. We have a new paper that now is in press. It is about cesarean sections. It’s known that kids born by C-section have a higher risk of developing asthma. The question is why?

From this study, again with the children in the Copenhagen study, we confirmed that kids born by C-section are more likely to develop asthma than those who didn’t. In those kids who had C-section, on average, their microbiome early was abnormal compared to those who were born vaginally. But by a year, in many of them, their microbiome had matured normally, but if it didn’t mature normally, those kids had a very high rate of getting asthma. Again, a high risk. That’s going to be published within a month or two because it’s been accepted already.

Now, what I will tell you is that with Dr. Müeller and with a graduate student in my lab, Tim Borbet, we’ve been doing a lot of mouse-asthma studies where we can experimentally give a mouse asthma or allergy. We already can show that if we perturb the microbiome early in life with antibiotics, they’re going to get more allergy and more asthma. That’s interesting because a paper was just published from British Columbia, showing that they had a really good program to diminish antibiotic use across the whole province. They showed that with diminishing antibiotic use, asthma rates are going down, so it’s all connected.

Furthermore, I’m part of another study that’s also in press. It’s going to be published probably in a month or two with scientists at the Mayo Clinic. I visited there a few years ago. The Mayo clinic is located in Olmsted County, Minnesota. It’s a pretty isolated place. In general, people don’t come, people don’t go, they stay there. It’s a very good stable population to study. I suggested to my colleagues there, why don’t you look at the effects of antibiotics in early life for certain marker diseases, including asthma and food allergy, and atopic dermatitis and allergic rhinitis. All these diseases go together. The group there is very active and outstanding, and they studied about 14,000 kids who were born in Olmsted County, and they were followed up to the time that they were 15 or 14. They had a lot of information from their health records because most of their medical care there is through the Mayo Clinic.

The bottom line is that if they received antibiotics in the first two years of life, their odds ratio of getting asthma was 2. They were twice as likely as kids who did not receive early-life antibiotics. Lots of things are pointing to the importance of the early life microbiome and the importance of when its being perturbed by antibiotics, that there’s increased risk. The relationship with moms, that’s this kind of transgenerational thing that each generation is stepping down.

World Asthma Foundation: A lot of these antibiotics are not only prescribed, but they’re ubiquitous in our diet and our food supply right?.

Dr. Blaser: Yes. Well, I’m very interested in that as well, although the prescribed antibiotic is more important because it’s higher dose. In mice, when we give low doses of antibiotics, it perturbs the immune system but not so much. When we give them the same kind of doses that kids get to treat their ear infections or their throat infections, it really perturbs their immune system and puts it on a different path. That’s also published.

Catch the video interview by clicking here .

Asthma and Indoor Air Pollution:

Key insights for Asthmatics:

  • Makes Asthma Worse
  • Significant Association with Exacerbations
  • Among this panel of relatively moderate to severe asthmatics, the respiratory irritants produced by several domestic combustion sources were associated with increased morbidity.
  • Although there is abundant clinical evidence of asthmatic responses to indoor aeroallergens, the symptomatic impacts of other common indoor air pollutants from gas stoves, fireplaces, and environmental tobacco smoke have been less well characterized. These combustion sources produce a complex mixture of pollutants, many of which are respiratory irritants.
  • Results of an analysis of associations between indoor pollution and several outcomes of respiratory morbidity in a population of adult asthmatics residing in the U.S. Denver, Colorado, metropolitan area. A panel of 164 asthmatics recorded in a daily diary the occurrence of several respiratory symptoms, nocturnal asthma, medication use, and restrictions in activity, as well as the use of gas stoves, wood stoves, or fireplaces, and exposure to environmental tobacco smoke.
  • Multiple logistic regression analysis suggests that the indoor sources of combustion have a statistically significant association with exacerbations of asthma. For example, after correcting for repeated measures and autocorrelation, the reported use of a gas stove was associated with moderate or worse shortness of breath (OR, 1.60; 95% CI, 1.11-2.32), moderate or worse cough (OR, 1.71; 95% CI, 0.97-3.01), nocturnal asthma (OR, 1.01; 95% CI, 0.91-1.13), and restrictions in activity (OR, 1.47; 95% CI, 1.0-2.16
  • The WAF Editorial Board wishes to thank and acknowledge B D Ostro 1 , M J Lipsett, J K Mann, M B Wiener, J Selner
    California Environmental Protection Agency, Berkeley for their contribution to Asthma education and research.

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.


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, 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).


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).


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).


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.


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.

Why Cell Biology of Asthma Matters

Cell types responsible for the major pathology in asthma:
1. Epithelial cells – initiate airway inflammation mucus, and
2. Smooth muscle cells – contract excessively to cause airway narrowing.

The clinical manifestations of asthma are caused by obstruction of the conducting airways of the lung. Two airway cell types are critical for asthma pathogenesis: epithelial cells and smooth muscle cells. Airway epithelial cells, which are the first line of defense against inhaled pathogens and particles, initiate airway inflammation and produce mucus, an important contributor to airway obstruction. The other main cause of airway obstruction is contraction of airway smooth muscle. Complementary experimental approaches involving cultured cells, animal models, and human clinical studies have provided many insights into diverse mechanisms that contribute to airway epithelial and smooth muscle cell pathology in this complex disease. Continued attention to the study of the cell biology of asthma will be crucial for generating new ideas for asthma prevention and treatment based on normalizing epithelial and smooth muscle function.

Note from the WAF editorial board: We wish to acknowledge and thank David J. Erle and Dean Sheppard, Lung Biology Center and Department of Medicine, University of California, San Francisco for their support for Asthma research and education.

Asthma is a common disease that affects up to 8% of children in the United States (Moorman et al., 2007) and is a major cause of morbidity worldwide. The principal clinical manifestations of asthma are repeated episodes of shortness of breath and wheezing that are at least partially reversible, recurrent cough, and excess airway mucus production. Because asthma involves an integrated response in the conducting airways of the lung to known or unknown triggers, it is a multicellular disease, involving abnormal responses of many different cell types in the lung (Locksley, 2010). Here we focus on the two cell types that are ultimately responsible for the major symptomatic pathology in asthma—epithelial cells that initiate airway inflammation in asthma and are the source of excess airway mucus, and smooth muscle cells that contract excessively to cause symptomatic airway narrowing. The current thinking about cell–cell communications that drive asthma (Fig. 1) is that known and unknown inhaled stimuli (i.e., proteases and other constituents of inhaled allergens, respiratory viruses, and air pollutants) stimulate airway epithelial cells to secrete the cytokines TSLP, interleukin (IL)-25, and IL-33, which act on subepithelial dendritic cells, mast cells, and innate lymphoid cells (iLCs) to recruit both innate and adaptive hematopoietic cells and initiate the release of T helper 2 (Th2) cytokines (principally IL-5 and IL-13; Locksley, 2010; Scanlon and McKenzie, 2012; Bando et al., 2013; Barlow et al., 2013; Nussbaum et al., 2013). Environmental stimuli also activate afferent nerves in the airway epithelium that can themselves release biologically active peptide mediators and also trigger reflex release of acetylcholine from efferent fibers in the vagus nerve. This initial response is amplified by the recruitment and differentiation of subsets of T cells that sustain secretion of these cytokines and in some cases secrete another cytokine, IL-17, at specific strategic sites in the airway wall. The released cytokines act on epithelial cells and smooth muscle cells and drive the pathological responses of these cells that contribute to symptomatic disease. The cell biology underlying the responses of the relevant hematopoietic lineages is not specific to asthma and has been discussed elsewhere (Locksley, 2010; Scanlon and McKenzie, 2012). We focus our discussion on the contributions of epithelial cells and airway smooth muscle cells.
An external file that holds a picture, illustration, etc. Object name is JCB_201401050_Fig1.jpg
Figure 1.

Cell–cell communication in the airway wall in asthma. Environmental triggers concurrently act on airway afferent nerves (which both release their own peptide mediators and stimulate reflex release of the bronchoconstrictor acetylcholine) and airway epithelial cells to initiate responses in multiple cell types that contribute to the mucous metaplasia and airway smooth muscle contraction that characterize asthma. Epithelial cells release TSLP and IL-33, which act on airway dendritic cells, and IL-25, which together with IL-33 acts on mast cells, basophils, and innate type 2 lymphocytes (iLC2). These secreted products stimulate dendritic cell maturation that facilitates the generation of effector T cells and triggers the release of both direct bronchoconstrictors and Th2 cytokines from innate immune cells, which feed back on both the epithelium and airway smooth muscle and further facilitate amplification of airway inflammation through subsequent adaptive T cell responses.

Cell biology of airway epithelium

The airway is covered with a continuous sheet of epithelial cells (Crystal et al., 2008; Ganesan et al., 2013). Two major airway cell types, ciliated and secretory cells, establish and maintain the mucociliary apparatus, which is critical for preserving airway patency and defending against inhaled pathogens and allergens. The apparatus consists of a mucus gel layer and an underlying periciliary layer. Ciliated cells each project ?300 motile cilia into the periciliary layer that are critical for propelling the mucus layer up the airway. In addition, cilia are coated with membrane-spanning mucins and tethered mucopolysaccharides that exclude mucus from the periciliary space and promote formation of a distinct mucus layer (Button et al., 2012). Secretory cells produce a different class of mucins, the polymeric gel-forming mucins. The two major airway gel-forming mucins are MUC5AC and MUC5B. Some secretory cells, known as mucous or goblet cells, produce mucins and store them within easily visualized collections of mucin granules, whereas other cells produce and secrete mucins (especially MUC5B) but lack prominent granules. Gel-forming mucins are secreted into the airway lumen and are responsible for the characteristic viscoelastic properties of the mucus gel layer.
Airway epithelial injury and remodeling in asthma

A variety of structural changes in the epithelium and other portions of the airway, termed “airway remodeling,” is frequently seen in individuals with asthma (Elias et al., 1999). These changes include airway wall thickening, epithelial hypertrophy and mucous metaplasia, subepithelial fibrosis, myofibroblast hyperplasia, and smooth muscle cell hyperplasia and hypertrophy. Airway remodeling is thought to represent a response to ongoing tissue injury caused by infectious agents, allergens, or inhaled particulates and by the host responses to these stimuli. Signs of frank epithelial injury, including loss of epithelial integrity, disruption of tight junctions, impairment of barrier function, and cell death, have been identified in some studies and may correlate with asthma severity (Laitinen et al., 1985; Jeffery et al., 1989; Barbato et al., 2006; Holgate, 2007). However, in many individuals asthma symptoms and features of airway remodeling, including mucous metaplasia and subepithelial fibrosis, are seen in the absence of signs of active airway infection or overt tissue injury (Ordoñez et al., 2000), suggesting that other processes account for the persistence of asthma in these individuals. Substantial evidence suggests that the persistence of asthma is driven by ongoing host immune responses that generate mediators driving airway remodeling and airway dysfunction. The epithelium is both a site of production of these mediators and a source of cells that respond to mediators produced by immune cells and other cells within the airway. How airway epithelial cells recognize and respond to viruses, allergens, and other stimuli has been comprehensively reviewed elsewhere (Lambrecht and Hammad, 2012). Here we will focus on the contribution of the epithelium to production of and responses to Th2 cytokines.
Airway epithelial contributions to Th2 responses.

Th2 cytokines, especially IL-13, play critical roles in asthma. Multiple cytokines, including TSLP, GM-CSF, IL-1, IL-25, and IL-33, are produced by the epithelium and promote production of Th2 cytokines by immune cells (Cates et al., 2004; Hammad et al., 2009; Locksley, 2010; Nagarkar et al., 2012). Genome-wide association studies implicate multiple Th2-related genes, including IL13, IL33, and TSLP, in asthma (Moffatt et al., 2010; Torgerson et al., 2011). IL-13 is produced by innate lymphoid cells (Neill et al., 2010; Price et al., 2010; Saenz et al., 2010; Hasnain et al., 2011) and Th2 cells (Grünig et al., 1998; Wills-Karp et al., 1998) during allergic inflammation and by macrophages in a mouse model of virus-induced airway disease (Kim et al., 2008). IL-13 induces characteristic changes in airway epithelial mRNA (Kuperman et al., 2005b; Woodruff et al., 2007; Zhen et al., 2007) and miRNA (Solberg et al., 2012) expression patterns in airway epithelial cells. The IL-13 transcriptional “signature” can be used to identify individuals with “Th2 high” and “Th2 low” asthma (Woodruff et al., 2009). The IL-13–induced protein periostin is secreted basally from airway epithelial cells and can be used as a biomarker for Th2 high asthma (Jia et al., 2012; Parulekar et al., 2014). Roughly half of individuals with asthma are Th2 high, and these individuals have better responses to treatment with inhaled corticosteroids (Woodruff et al., 2009) or anti–IL-13 antibody (Corren et al., 2011). The key drivers of Th2 low asthma remain poorly understood, although Th17 family cytokines may be important (Newcomb and Peebles, 2013).

Mucous metaplasia.

Although mucus is critical for host defense, pathological mucus production is an important contributor to asthma morbidity and mortality. In fatal asthma, airways are often plugged with tenacious mucus plugs that obstruct movement of gas (Kuyper et al., 2003). This catastrophic phenomenon likely reflects increased mucin production and secretion as well as changes in mucin cross-linking, mucus gel hydration, and mucus clearance. Abnormalities in mucus are not limited to severe asthma exacerbations because an increase in intracellular mucin stores (mucous metaplasia) is seen even in individuals with stable, mild to moderate asthma (Ordoñez et al., 2001). In mouse allergic airway disease models of asthma, mucous metaplasia results from increased production and storage of mucins (especially MUC5AC) in preexisting secretory cells, including club cells (Evans et al., 2004), rather than transdifferentiation of ciliated cells (Pardo-Saganta et al., 2013). However, in virus-driven models of asthma mucous cells might arise from transdifferentiation of ciliated cells (Tyner et al., 2006). A variety of stimuli and signaling pathways have been shown to regulate mucin production and secretion in airway epithelial cells.
IL-13 stimulates mucin production in Th2 high asthma.

Direct effects of IL-13 on airway epithelial cells induce mucous metaplasia in human airway epithelial cells in culture (Laoukili et al., 2001; Zhen et al., 2007) and in mouse airway epithelial cells in vivo (Kuperman et al., 2002). IL-13 is necessary for mucous metaplasia in many mouse asthma models (Grünig et al., 1998; Wills-Karp et al., 1998; Tyner et al., 2006). Individuals with Th2 high asthma have elevated levels of bronchial epithelial cell MUC5AC mRNA compared with healthy controls or individuals with Th2 low asthma (Woodruff et al., 2009). Recent transgenic mouse studies demonstrate roles for MUC5AC in clearance of enteric nematode infections (Hasnain et al., 2011) and protection against influenza infection (Ehre et al., 2012). Increased MUC5AC expression is therefore part of an integrated immune response that contributes to host defense against pathogens or inhaled particulates. A less well-recognized feature of Th2-high asthma is the substantial decrease in expression of MUC5B (Woodruff et al., 2009). The recent discovery that MUC5B is required for normal mucociliary clearance and defense against airway infection (Roy et al., 2014) suggests further attention should be directed to the possibility that a reduction in MUC5B may be an important contributor to airway dysfunction in asthma.

IL-13 is recognized by cell surface receptors expressed on almost all cell types, including airway epithelial cells (Fig. 2). The airway epithelial cell IL-13 receptor that is critical for mucous metaplasia is a heterodimer composed of IL-13R?1 and IL-4R?. Removal of this receptor in airway epithelial secretory cells (driven by the CCSP promoter) prevented mucous metaplasia in an allergic asthma model (Kuperman et al., 2005a). IL-13 binding leads to activation of Jak kinases associated with the receptor cytoplasmic domain and subsequent phosphorylation of signal transducer and activator of transcription 6 (STAT6). STAT6 activation is required for IL-13–induced mucous metaplasia (Kuperman et al., 2002).
An external file that holds a picture, illustration, etc. Object name is JCB_201401050_Fig2.jpg
Figure 2.

Mechanisms of IL-13–induced mucous metaplasia. IL-13 binds to its receptor on the surface of mucous cell progenitors (e.g., club cells) leading to phosphorylation of STAT6 and translocation of STAT6 heterodimers to the nucleus, where they bind to promoters of STAT6-responsive genes. STAT6-dependent processes that contribute to mucous metaplasia include a CLCA1-dependent pathway, a Serpin-dependent pathway, and a 15-lipoxygenase-1–dependent pathway. The transcription factor SPDEF is a master regulator of mucous cell differentiation. It inhibits FOXA2, which represses mucous cell differentiation, and activates transcription of other genes that are expressed in mucous cells.

The series of events that link STAT6 activation to mucous metaplasia are only partly understood. STAT6 does not appear to directly regulate MUC5AC transcription (Young et al., 2007) and the critical direct targets of STAT6 have not been determined. One pathway that depends upon STAT6 activation involves the protein calcium-activated chloride channel 1 (CLCA1). CLCA1 is among the most highly induced genes in airway epithelial cells from individuals with asthma (Hoshino et al., 2002; Toda et al., 2002). Despite its name, CLCA1 does not appear to function as an ion channel but instead undergoes extracellular secretion and cleavage. Extracellular CLCA1 can induce MUC5AC expression via activation of the MAP kinase MAPK13 (p38?-MAPK; Alevy et al., 2012), although the presumed CLCA1 receptor and the relevant MAPK13 targets have not yet been identified. A second pathway involves the protease inhibitor Serpin3a, the mouse orthologue of human SERPINB3 and SERPINB4. These serpins are induced by IL-13 in a STAT6-dependent fashion (Ray et al., 2005). After allergen challenge, Serpin3a?/? mice had less mucous metaplasia than wild-type mice (Sivaprasad et al., 2011), despite an intact inflammatory response. These results suggest that serpins inhibit proteases that normally degrade one or more proteins required for mucous metaplasia, although the relevant proteases and their protein substrates are not yet known. Another IL-13–induced pathway involves the enzyme 15-lipoxygenase-1 (15-LO-1; Zhao et al., 2009). 15-LO-1 converts arachidonic acid to 15-hydroxyeicosatetraenoic acid, which was shown to enhance MUC5AC expression in human airway epithelial cells.

IL-13– and STAT6-mediated mucous metaplasia depends upon changes in the activity of a network of transcription factors. Allergen-induced IL-13–mediated STAT6 activation leads to increased expression of the SAM-pointed domain–containing Ets-like factor (SPDEF; Park et al., 2007; Chen et al., 2009). The induction of SPDEF depends at least in part on FOXM1, a member of the Forkhead box (FOX) family of transcription factors (Ren et al., 2013). The SPDEF program is also important for mucous metaplasia triggered by other stimuli, including rhinoviruses (Korfhagen et al., 2012). Although SPDEF does not appear to directly regulate mucin gene transcription, SPDEF initiates a transcriptional program that is necessary and sufficient to induce mucous metaplasia. One of the effects of SPDEF is inhibition of the expression of another FOX family gene, FOXA2. In mice, deletion of Foxa2 in mucous cell precursors is sufficient to induce mucous metaplasia, and overexpression of FOXA2 inhibits allergen-induced mucous metaplasia (Zhen et al., 2007; G. Chen et al., 2010). The relationship between IL-13 and FOXA2 is complex. IL-13 inhibits expression of FOXA2, which contributes to mucous metaplasia. However, deletion of Foxa2 in airway epithelial cells during fetal development resulted in Th2 inflammation and production of IL-13 in the airway (G. Chen et al., 2010). The direct targets that are responsible for these effects of FOXA2 are not yet known.
The EGFR pathway induces mucin gene expression and mucous metaplasia.

Epidermal growth factor receptor (EGFR) binds multiple ligands including EGF, TGF-?, heparin-binding EGF, amphiregulin, ?-cellulin, and epiregulin. Ligand binding activates the EGFR kinase domain, initiating signaling cascades that are central to many fundamental biological processes, including cell proliferation, differentiation, survival, and migration. EGFR ligands induce expression of MUC5AC in human airway epithelial cell lines and a tyrosine kinase inhibitor that inhibits EGFR kinase prevents mucous metaplasia induced either by an EGFR ligand or by allergen challenge (Takeyama et al., 1999). Subsequent studies showed that bronchial epithelial EGFR levels are increased in asthma and correlate with disease severity (Takeyama et al., 2001a), and that epithelial EGFR signaling contributes to mucous metaplasia in a chronic asthma model (Le Cras et al., 2011).

Various stimuli, including bacterial products (Kohri et al., 2002; Lemjabbar and Basbaum, 2002;Koff et al., 2008), viruses (Tyner et al., 2006; Zhu et al., 2009; Barbier et al., 2012), cigarette smoke (Takeyama et al., 2001b; Basbaum et al., 2002), and inflammatory cell products (Burgel et al., 2001) can activate the EGFR pathway in airway epithelial cells. Some stimuli have been shown to initiate the EGFR signaling cascade by activating the PKC isoforms PKC ? and PKC ?, leading to recruitment of the NADPH oxidase subunits p47phox and p67phox to membrane-associated dual oxidase-1 and the generation of reactive oxygen species (ROS) at the cell surface (Shao and Nadel, 2005). ROS in turn activate latent TGF-?–converting enzyme resulting in cleavage of surface EGFR pro-ligands (Shao et al., 2003). EGFR ligand binding leads to activation of the Ras–Raf–MEK1/2–ERK1/2 pathway and MUC5AC transcriptional induction, which depends upon the Sp1 transcription factor and Sp1-binding sites within the MUC5AC promoter (Takeyama et al., 2000; Perrais et al., 2002). The IL-13 and EGFR pathways make critical but distinct contributions to gene regulation in airway epithelial cells (Zhen et al., 2007). Both pathways inhibit expression of FOXA2, suggesting that this transcription factor may represent a final common pathway for IL-13– and EGFR-induced mucous metaplasia.

Notch signaling regulates mucous cell differentiation.

Notch signaling is also important for mucous metaplasia (Tsao et al., 2011). Notch is a transmembrane receptor that binds to cell-surface ligands in the Delta-like and Jagged families. Ligand binding activates ?-secretase–mediated proteolytic cleavage and liberates the Notch intracellular domain, which enters the nucleus, associates with transcription factors, and drives expression of downstream Notch genes. Genetic manipulation of Notch signaling in mice has different effects depending on the developmental stage. In explanted embryonic lungs, addition of Notch ligand or expression of a constitutively active form of Notch increased MUC5AC-containing mucous cells, whereas a ?-secretase inhibitor reduced mucous cells (Guseh et al., 2009). Notch-induced mucous metaplasia did not require STAT6 activation, suggesting that the Notch and STAT6 pathways may operate in parallel. In contrast, in postnatal mouse lung, disruptions of Notch signaling induced mucous metaplasia (Tsao et al., 2011), a process that principally depends on the Notch ligand Jagged1 (Zhang et al., 2013). The Notch target Hes1 appears to be critical for inhibition of mucous metaplasia and MUC5AC transcription, although inactivation of Hes1 was not sufficient to induce mucous metaplasia (Ou-Yang et al., 2013). The observation that a ?-secretase inhibitor reduced IL-13–induced mucous metaplasia in cultured human airway epithelial cells (Guseh et al., 2009) suggests that further attention to the role of epithelial Notch signaling in asthma is warranted.

The secretory pathway in mucous cells

Mucin monomers are large (?5,000 amino acid residue) proteins that require extensive processing in the ER and Golgi. Each mucin monomer contains ?200 cysteine residues that can potentially participate in intra- and intermolecular disulfide bonds. The ER of mucous cells contains specialized molecules that are not widely expressed in other cell types and are required for efficient processing of mucins. One of these is anterior gradient 2 (AGR2) homologue, a member of the protein disulfide isomerase family. An active site cysteine residue in AGR2 forms mixed disulfide bonds with mucins in the ER and mice deficient in AGR2 have profound defects in intestinal mucin production (Park et al., 2009). In a mouse model of allergic asthma, AGR2-deficient mice had reduced mucus production compared with allergen-challenged wild-type mice (Schroeder et al., 2012). The reduction in mucus production was associated with activation of the unfolded protein response, a characteristic response to ER stress (Walter and Ron, 2011). AGR2 may therefore either have a direct role in mucin folding or another function necessary for maintaining normal function of the mucous cell ER. Another molecule found in the mucous cell ER is inositol-requiring enzyme 1? (IRE1?), a transmembrane ER stress sensor. IRE1? is found in mucus-producing cells in the intestine and the airways, but not in other cells. IRE1? regulates AGR2 transcription, and mice deficient in IRE1? had reduced AGR2 expression and impaired airway mucin production in an allergic asthma model (Martino et al., 2013). AGR2 and IRE1? have apparently evolved to meet the unusual demands posed by the need to produce large amounts of mucins.

ORMDL3, a member of the Orm family of transmembrane ER proteins, has also been implicated in asthma. Genetic polymorphisms at loci close to ORMDL3 were strongly associated with asthma in multiple genome-wide association studies (Moffatt et al., 2007; Galanter et al., 2008). Allergen challenge induced ORMDL3 expression in airway epithelial cells in a STAT6-dependent fashion, although ORMDL3 does not appear to be a direct target of STAT6 (Miller et al., 2012). Studies involving overexpression or knockdown of ORDML3 in HEK293 cells indicate that ORMDL3 is involved in regulating ER stress responses and ER-mediated calcium signaling (Cantero-Recasens et al., 2010). In addition, Orm proteins form complexes with serine palmitoyl-CoA transferase (SPT), the first and rate-limiting enzyme in sphingolipid production, and may thereby help coordinate lipid metabolism in the secretory pathway (Breslow et al., 2010). Genetic and pharmacologic reductions in SPT activity induced airway hyperresponsiveness in the absence of inflammation or mucous metaplasia (Worgall et al., 2013). Further studies are required to determine whether ORMDL3’s role in modulating sphingolipid production, ER stress, calcium signaling, or other ER functions in airway epithelial cells or other cells is important in asthma.

Mucins travel from the ER to the Golgi and then are packaged into large granules for secretion. In the Golgi, mucins are extensively O-glycosylated and undergo further multimerization before being released from the cell by regulated exocytosis. Throughout the airways of normal mice and in distal (smaller) airways of humans, basal secretion accounts for most mucin release, and mucin-producing cells retain too little mucin to detect using histological stains. However, mucous cells found in larger airways of humans and allergen-challenged mice contain readily detectable accumulations of mucin-containing granules that can be released by various stimuli, including the P2Y2 receptor ligands ATP and UTP and proteases that cleave protease-activated receptors. Mice lacking the exocytic priming protein Munc13-2 accumulate mucin in secretory cells that normally have minimal intracellular mucin (club cells) but can secrete mucin in response to stimulation (Zhu et al., 2008). In contrast, allergen-challenged mice lacking the low affinity calcium sensor synaptotagmin-2 have a severe defect in acute agonist-stimulated airway mucin secretion, but have preserved basal secretion and do not accumulate mucins in club cells (Tuvim et al., 2009). Agonist-stimulated secretion also depends upon the IL-13–inducible calcium-activated chloride channel TMEM16A, which is increased in mucous cells from individuals with asthma (Huang et al., 2012). Because increased production of MUC5AC via transgenic overexpression was not in itself sufficient to cause airway obstruction (Ehre et al., 2012), it seems likely that qualitative defects in mucin processing, secretion, or hydration that affect the physicochemical properties of mucus contribute to airway obstruction in asthma. Epithelial transport of water and ions, including H+ and bicarbonate, is important in maintaining the normal properties of mucus (E. Chen et al., 2010; Paisley et al., 2010; Garland et al., 2013). Rapid secretion of stored mucin, which is not fully hydrated, may result in the formation of concentrated, rubbery mucus that cannot be cleared normally by cilia or by coughing (Fahy and Dickey, 2010). Hence, IL-13 (Danahay et al., 2002; Nakagami et al., 2008) and other asthma mediators that affect airway epithelial cell water and ion transport could contribute to airway obstruction by altering the physicochemical properties of mucus.
Ciliated cell structure and function in asthma

In comparison with the extensive asthma literature regarding mucous cells, relatively few reports have focused on ciliated cells. One study of epithelial cell strips obtained by endobronchial brushing found decreased ciliary beat frequency and increases in abnormal ciliary beating patterns and ciliary ultrastructural defects in individuals with asthma compared with healthy controls (Thomas et al., 2010). These abnormalities were more pronounced in severe asthma. Ciliary abnormalities were accompanied by increases in the numbers of dead cells and evidence of loss of epithelial structural integrity, which suggests that ciliary dysfunction may be a consequence of a generalized epithelial injury. In any case, these results suggest that ciliary dysfunction might be an important contributor to impaired mucociliary clearance in asthma.
Cell biology of airway smooth muscle in asthma

The excessive airway narrowing that can lead to severe shortness of breath, respiratory failure, and death from asthma is largely due to contraction of the bands of smooth muscle present in the walls of large- and medium-sized conducting airways in the lung. In the large central airways of humans, these bands of muscle are present in the posterior portion of the airways and attach to the anterior airway cartilage rings, but in more peripheral airways smooth muscle is present circumferentially around the airways. In both locations, contraction of smooth muscle, which can be physiologically induced by release of acetylcholine from efferent parasympathetic nerves or by release of histamine and cysteinyl leukotrienes from mast cells and basophils, causes airway narrowing, with the most extensive narrowing in medium-sized airways. In healthy mammals, including humans, physiological responses to release of acetylcholine from efferent nerves or release of histamine and leukotrienes from mast cells and basophils causes only mild and generally asymptomatic airway narrowing. Normal mammals are also generally resistant to marked airway narrowing in response to pharmacologic administration of high concentrations of these contractile agonists directly into the airways. However, people with asthma have a marked increase in sensitivity to all of these agonists that can readily be demonstrated by dramatic increases in airway resistance and associated drops in maximal expiratory airflow rates during forced expiratory maneuvers (Boushey et al., 1980). Recent comparisons between responses to inhaled allergens in allergic asthmatic subjects and other subjects with similarly severe cutaneous immune responses to allergens makes it clear that all allergic humans release largely similar amounts of bronchoconstrictors into the airways (i.e., histamine and leukotrienes), but only asthmatics develop exaggerated airway narrowing in response to these mediators (Becky Kelly et al., 2003).
Mechanisms regulating generation of force by airway smooth muscle actin–myosin coupling

Force generation by airway smooth muscle is mediated by interactions between actin and myosin that depend on phosphorylation of the myosin light chain by the serine–threonine kinase, myosin light chain kinase (Fig. 3). This process is negatively regulated by myosin phosphatase. Increases in intracellular calcium concentration in smooth muscle cells induce contraction by two parallel pathways. When bound to calcium, the serine–threonine kinase calmodulin directly phosphorylates, and thereby activates, myosin light chain kinase. Increased calcium also increases GTP loading of the GTPase, RhoA, which increases the activity of its downstream effector kinases Rho-associated coiled-coil–containing protein kinases 1 and 2 (ROCK 1 and 2). ROCKs directly phosphorylate myosin light chain phosphatase, an effect that inactivates the phosphatase, further enhancing myosin phosphorylation. RhoA can also be activated independently of increases in intracellular calcium.

Core signaling pathways responsible for airway smooth muscle contraction. Airway smooth muscle contractile force is generated by cyclic cross-bridging of actin and smooth muscle myosin, which depends on myosin phosphorylation. Myosin phosphorylation is regulated by cyclic increases in cytosolic calcium (Ca2+) that activate calmodulin (CaM) to phosphorylate myosin light chain kinase (MLCK), which directly phosphorylates myosin. In parallel, the small GTPase, RhoA, is activated by both calcium-dependent and -independent pathways. Rho directly activates Rho-associated coiled-coil protein kinase (ROCK) which, in turn, phosphorylates and thereby inactivates myosin light chain phosphatase (MLCP), which normally dephosphorylates myosin. The most important physiological pathway for increasing cytosolic calcium in airway smooth muscle involves activation of G?q by G protein–coupled receptors that respond to extracellular contractile agonists, such as methacholine (Mch), serotonin (5-HT), and histamine. G?q activates phospholipase C ? (PLC?), which generates IP3 to bind to IP3 receptors on the sarcoplasmic reticulum and release sequestered Ca2+.

There are multiple upstream paths to increased i[Ca] in airway smooth muscle. Acetylcholine, released from post-ganglionic parasympathetic efferent nerves that innervate the muscle, activates G protein–coupled M2 muscarinic receptors, which are coupled to G?q. GTP-loaded G?q activates its downstream effector, PLC?, which phosphorylates PIP2 to generate IP3. IP3, in turn, binds to IP3 receptors on the sarcoplasmic reticulum to trigger translocation of calcium into the cytosol. Other contractile agonists, including histamine, bradykinin, and serotonin (5-HT; the specific agonists and receptors vary across mammalian species) bind to different G protein–coupled receptors to trigger the same pathway. Agonist-induced airway smooth muscle contraction is usually associated with cyclic oscillations in i[Ca], thought to be induced by local changes in cytosolic calcium triggering reuptake of calcium by the sarcoplasmic reticulum, and the magnitude of contractile force induced is most closely associated with the frequency of these calcium oscillations rather than their amplitude (Bergner and Sanderson, 2002).

Increases in cytosolic calcium concentration can also be induced by an influx of calcium from the extracellular space, generally due to the opening of voltage-gated calcium channels in the plasma membrane. These channels can be opened experimentally by increasing the extracellular concentration of potassium ions, which also induces airway smooth muscle contraction. Increased extracellular potassium concentrations also increase release of acetylcholine from post-ganglionic efferent nerves, so proper interpretation of the effects of KCl requires simultaneous addition of a muscarinic antagonist such as atropine.
Regulation of airway smooth muscle force generation by integrin-containing adhesion complexes

For smooth muscle cell contraction to be translated into the force required for airway narrowing, the contracting smooth muscle cell must be firmly tethered to the underlying ECM. Linkage to the ECM is accomplished through the organization of multi-protein complexes nucleated by integrins. The short cytoplasmic domains of integrins can organize surprisingly large multi-protein machines that modulate multiple signaling pathways and link integrins (and thus their ECM ligands) to the actin–myosin cytoskeleton (Yamada and Geiger, 1997; Zaidel-Bar et al., 2007). Many of the contractile agonists that stimulate myosin phosphorylation and actin–myosin interaction simultaneously enhance the formation of integrin signaling complexes, induce actin polymerization at sites of adhesion, and strengthen coupling between the actin–myosin cytoskeleton and the ECM (Mehta and Gunst, 1999; Tang et al., 1999, 2003; Gunst and Fredberg, 2003; Gunst et al., 2003; Opazo Saez et al., 2004). These events appear to also be quite important for generation of maximal contractile force because interventions that inhibit the formation or activity of adhesion complexes can inhibit the strength of contraction without affecting myosin phosphorylation (Mehta and Gunst, 1999; Tang et al., 2003; Opazo Saez et al., 2004).
Lessons from abnormal behavior of airway smooth muscle in animal models
Mice lacking ?9?1 integrin in airway smooth muscle.

Although there are large differences between the organization of airways in mice and humans, in vivo abnormalities in airway narrowing seen in mouse models do provide some insight into pathways that potentially contribute to abnormal airway smooth muscle contraction in asthma. For the purposes of this review, we will cite three illustrative examples. The integrin ?9?1 is highly expressed in airway smooth muscle (Palmer et al., 1993). Conditional knockout of the integrin ?9 subunit (uniquely found in the ?9?1 integrin) results in a spontaneous increase in in vivo airway responsiveness (as measured by increases in pulmonary resistance in response to intravenous acetylcholine), and to increased contractile responses to cholinergic agonists of both airways in lung slices and tracheal rings studied in an organ bath (Chen et al., 2012). Interestingly, although tracheal rings from these mice also have increased contractile responses to other G protein–coupled receptor agonists (e.g., serotonin), they have normal contractile responses to depolarization with KCl. These findings suggest that loss of ?9?1 increases airway responsiveness at some step upstream of calcium release from the sarcoplasmic reticulum (Fig. 4 A). In this case, increased airway responsiveness appears to be due to loss of co-localization of the polyamine-catabolizing enzyme spermidine/spermine N1-acetyltransferase (SSAT), which binds directly to the ?9 cytoplasmic domain (Chen et al., 2004), and the lipid kinase, PIP5K1?, which binds directly to talin, an integrin ?1 subunit binding partner. Spermine and spermidine are critical cofactors for PIP5K1?, so its juxtaposition with SSAT effectively reduces enzymatic activity. PIP5K1? converts PI4P to PIP2 and is responsible for most of the PIP2 produced in airway smooth muscle cells (Chen et al., 1998). PIP2 is the substrate for IP3 generation by PLC?, so when ?9?1 is present and ligated, contractile agonists that activate receptors coupled to G?q induce less IP3 generation (Chen et al., 2012) and thus less Ca2+ release through IP3 receptors in the sarcoplasmic reticulum. The importance of this pathway was confirmed by the observations that the frequency of Ca2+ oscillations induced by cholinergic agonists was reduced in lung slices from mice lacking ?9?1, and that all of the abnormalities in smooth muscle from these animals could be rescued by addition of a cell-permeable form of PIP2 (Chen et al., 2012).

Pathways that negatively regulate airway smooth muscle contraction. (A) The integrin ?9?1 negatively regulates airway smooth muscle contraction by colocalizing the polyamine-catabolizing enzyme, spermine spermidine acetyltransferase (SSAT), which directly binds to the ?9 subunit with the lipid kinase, PIP5K1?, the major source of PIP2 in airway smooth muscle, which binds to talin, a direct interactor with the ?1 subunit. PIP5K1? depends on spermine and spermidine for maximal activity, so the local breakdown of spermine and spermidine reduces PIP5K1? activity, thereby decreasing PIP2 concentrations and the amount of IP3 that is generated by activation of contractile G protein–coupled receptors (such as those activated by acetylcholine or serotonin [5-HT]). (B) The secreted scaffold protein, milk fat globule-EGF factor 8 (MFGE8), inhibits the smooth muscle hypercontractility induced by IL-13, IL-17, and tumor necrosis factor ? (TNF) by inhibiting the induction and activation of the small GTPase, RhoA. Active RhoA contributes to smooth muscle contraction by directly activating Rho-associated coiled-coil protein kinase (ROCK) which, in turn, phosphorylates and thereby inactivates myosin light chain phosphatase (MLCP), which normally dephosphorylates myosin.
Effects of T cell cytokines on airway smooth muscle contractility.

Several studies conducted over the past 15 years have suggested that cytokines released from T cells can contribute to airway hyperresponsiveness in allergic asthma (Locksley, 2010). The Th2 cytokine IL-13 has been most extensively studied, and can induce both mucous metaplasia and airway hyperresponsiveness when administered directly into the airways of mice (Grünig et al., 1998; Wills-Karp et al., 1998). In vitro, incubation of tracheal rings or lung slices increases narrowing of airways in lung slices and increases force generation by mouse tracheal rings, at least in part by inducing a dramatic increase in expression of the small GTPase, RhoA (Chiba et al., 2009), which is a critical effector of airway smooth muscle contraction (Fig. 4 B). Chronic allergen challenge or direct administration of IL-13 into the airways of mice also increased RhoA expression, in association with induction of airway hyperresponsiveness. A recent study suggested that IL-17 can also increase airway smooth muscle contractility and airway narrowing by induction of RhoA in airway smooth muscle cells (Kudo et al., 2012). In that study, mice lacking the ?v?8 integrin specifically on antigen-presenting dendritic cells were protected from allergen-induced airway hyperresponsiveness. These mice had the same degree of general airway inflammation and mucous metaplasia in response to allergen as wild-type control mice, but had a very specific defect in the generation of antigen-specific Th17 cells, an important source of IL-17 in lungs (Kudo et al., 2012). In vitro, IL-17 was shown to directly increase the contractility of mouse tracheal rings and to increase the levels of RhoA protein and its downstream effector, ROCK2, and to increase phosphorylation of the direct ROCK target, myosin phosphatase. Phosphorylation of myosin phosphatase inhibits its function, and IL-17 was also shown to consequently increase phosphorylation of myosin light chain kinase. Importantly, all of these biochemical effects were dramatically induced in vivo in airway smooth muscle of control mice in response to allergen sensitization and challenge, but all were markedly reduced in mice lacking ?v?8 on dendritic cells. Furthermore, tracheal rings removed from these knockout mice after allergen challenge had decreased in vitro contractility compared with rings from allergen challenged control mice, but this difference in contractility was eliminated by exogenous addition of IL-17. These findings strongly suggest that both IL-13 and IL-17 can contribute to airway hyperresponsiveness by directly inducing RhoA expression in airway smooth muscle (Fig. 4 B). Tumor necrosis factor ?, also implicated in asthma pathogenesis, has been shown to increase airway smooth muscle contractility by a similar mechanism (Goto et al., 2009).
Enhanced cytokine-mediated airway smooth muscle contraction in MFGE8-deficient mice.

Milk fat globule EGF factor 8 (MFGE8) is a secreted protein composed of two EGF repeats and two discoidin domains. MFGE8 was originally described to facilitate uptake of apoptotic cells by phagocytes (Hanayama et al., 2004). Mice lacking MFGE8 have normal baseline lung morphology and function, but have exaggerated airway responsiveness after allergen sensitization and challenge (Kudo et al., 2013). However, this abnormality did not appear to be related to any effects on reuptake of apoptotic cells. Immunostaining demonstrated that secreted MFGE8 was concentrated adjacent to airway smooth muscle. Tracheal rings removed from MFGE8 knockout mice had normal contractile responses at baseline, but had markedly enhanced contractile responses after overnight incubation with IL-13, and this increase in contractility could be rescued by addition of recombinant MFGE8 to the muscle bath. Importantly, rescue required the presence of at least one of the discoidin domains and of the integrin-binding RGD motif of the second EGF repeat. In mouse tracheal rings and cultured airway smooth muscle, loss of MFGE8 greatly enhanced the IL-13–induced increase in RhoA protein. These findings suggest that ligation of one or more RGD-binding integrins on airway smooth muscle by extracellular MFGE8 normally serves as a brake on cytokine-mediated RhoA induction and thereby limits maximal cytokine-induced airway hyperresponsiveness (Fig. 4 B). The specific integrin(s) involved in this response, the molecular mechanisms linking integrin ligation to inhibition of RhoA, and the role and binding partner(s) of the MFGE8 discoidin domains that are required for RhoA inhibition all remain to be determined.


Rapid progress has been made toward identifying epithelial and smooth muscle cell molecules and pathways that can produce many of the abnormalities found in individuals with asthma. Because these discoveries were made in diverse experimental systems, we still face major challenges in understanding how these molecules and pathways interact in vivo and in identifying the pathways that are most relevant in people with asthma. Asthma is a heterogeneous disease, and recent progress toward identifying subtypes with distinct pathophysiologic mechanisms promises to focus attention on certain pathways in epithelial and smooth muscle cells (Lötvall et al., 2011). It will be especially important to understand mechanisms underlying severe asthma. Approximately 5–10% of individuals with asthma have severe disease, with symptoms that persist despite standard therapy with bronchodilators and inhaled corticosteroids (Brightling et al., 2012). These individuals have high rates of asthma exacerbations leading to hospitalization and are at relatively high risk for fatal asthma attacks. Continued attention to the study of the cell biology of asthma will be crucial for generating new ideas for asthma prevention and treatment based on normalizing epithelial and smooth muscle function.