Mucus in the gut may help relieve bowel inflammation

Friday September 27 2013

"'Bottled mucus' may help gut disease," reports BBC News, which explains that mucus has "a role in calming the immune system". The idea of swigging down bottles of mucus is hard to stomach and, thankfully, is not what the researchers suggest.

Our digestive systems are subject to a delicate balancing act. The immune system has to protect against harmful bacteria while, at the same time, leaving alone the so-called "friendly" bacteria that help us to digest our food and not reacting to harmless substances in what we eat.

If the immune system mistakenly attacks the friendly bacteria, it can lead to unnecessary inflammation of the tissue lining. One theory is that this type of immune response may contribute to inflammatory bowel conditions such as Crohn’s disease and ulcerative colitis.

A layer of mucus helps stop this immune response by forming a physical barrier between the gut contents and the gut lining. Researchers in the current study wanted to see if the mucus also acts in other ways to stop gut inflammation. Through a series of experiments using mice and human cells in the lab, they found that mucus also suppresses the immune system's response to substances that would otherwise lead to inflammation.

The long-term goal is that these findings could help us to understand the role of mucus in gut better, and possibly use this knowledge to develop new ways to prevent and treat gut infections, food allergies and inflammatory bowel disease.

Where did the story come from?

The study was carried out by researchers from the Icahn School of Medicine at Mount Sinai, New York and other research centres in the US and Spain. Sources of funding were not reported.

The study was published in the peer-reviewed journal Science.

The stomach-turning headline on BBC News suggesting that drinking “bottled mucus” may help gut disease is premature. We just don’t know whether there will be any future treatments based on the findings, or how they might work. However, the body of the story gives good coverage of the findings and is fair and balanced.

What kind of research was this?

This was laboratory and animal research looking at what role mucus plays in the gut. The tissue lining of the gut is exposed to the bacteria that live in our digestive tract and help us digest our food, as well as the food itself, which may contain bacteria and other organisms. The gut lining needs to be able to tolerate these normal exposures without becoming inflamed.

Cells in the surface of the gut produce a layer of mucus that lines the digestive tract and acts as a physical barrier. However, the researchers who carried out this study thought that mucus might also play a more active role in preventing the immune system from reacting inappropriately to these substances.

This type of study allows scientists to do experiments they could not do in humans. As the basic biology of humans and other animals is similar, the findings give us an indication of how the human biology is likely to work. Once researchers have these clues, they can design ways of testing whether the findings hold true in humans – for example, by looking at human tissue samples in the lab, or samples of human gut contents.

What did the research involve?

The researchers focused on a protein called mucin 2, which is a key component of mucus. It binds to long carbohydrate chains, which hold onto water molecules and make mucus viscous (thick and sticky). They looked at the role of mucin 2 in the small intestine – the first part of the gut, which joins the stomach to the large intestine (or colon).

The researchers carried out a range of experiments in the laboratory and in mice. This included analysing:

  • the structure of the mucus layer in the small intestine of mice
  • how the presence of mucin 2 affects the reaction of human immune system cells that are present in the gut to bacterial proteins
  • how the small intestines of mice that are genetically engineered to lack the mucin 2 protein react to bacteria
  • a range of other experiments to look at exactly how mucin 2 has the effects that were found in the other experiments

What were the basic results?

The researchers found that the mucus layer lining the small intestine was porous and allowed bacteria to get through to the surface of the tissue lining the intestine. This contrasted with the mucus layer in the large intestine, which formed a dense barrier protecting the surface of the tissue.

Mucus is a viscous substance secreted by special membranes in the body called mucous membranes. It lines delicate internal surfaces of the body that come into contact with the exterior – such as the inside of the nose, lungs and gastrointestinal tract.

It stops these surfaces from drying out. It can also trap substances such as dirt and bacteria before they can get further into the body, and contains antibodies and other proteins to help fight invaders.

The researchers found that bacteria in the gut were coated with mucin 2, and ingested (“eaten”) by special immune system cells in the small intestine. When mucin 2 was present, these immune system cells produced less of the chemicals that cause inflammation than they did if they had ingested these bacteria by themselves.

The presence of mucin 2 also caused these immune system cells to produce chemicals to suppress inflammation. Mice that lacked mucin 2 had more bacteria stuck to the lining of their small intestine. The cells in the intestine were producing more of the inflammatory chemicals, and less of the anti-inflammatory chemicals.

How did the researchers interpret the results?

The researchers conclude that the mucus in the gut does not just act as a physical barrier to prevent inflammation of the gut lining. It also promotes signals to “dampen down” the immune response to foreign substances in the gut, helping prevent inflammatory responses to “good” gut bacteria and food.

Conclusion

The current study has discovered more about the important role of mucus in the gut. The findings come from mice and human cells in the laboratory, and there may be some differences in the human intestine. However, the basic biology of humans and other animals is very similar, so these findings give a good starting point for further study in human tissue.

Although these results can help scientists to learn more about the role of mucus in the gut, there is still much to learn, for example, how the effects of mucus are overridden in cases of harmful gut infections.

The researchers hope that these findings might help to develop better vaccines and treatments for gut infections, food allergies and inflammatory bowel disease (such as Crohn’s disease and ulcerative colitis). These are likely to be long-term goals, and not guaranteed to result from such findings, but the more researchers understand about our biology the better their chances of developing better therapies for these conditions.

Analysis by Bazian
Edited by NHS Choices