Gut bacteria may offer protection against respiratory illness

Date:

  • A new study in mice suggests the presence of certain gut bacteria transforms alveolar macrophages into respiratory virus neutralizers.
  • The results show that the macrophages disabled respiratory illnesses like COVID-19, RSV, and the flu.
  • The mechanisms behind this discovery are unclear, and the researchers are planning human trials to determine whether the findings can be replicated.

A new mouse study has found that certain segmented filamentous bacteria (SFB), when naturally present or introduced into the intestinal microbiome, cause the lungs’ alveolar macrophages to disable viral infections and protect against illness.

The results show that mice were more capable of resisting serious respiratory illnesses like respiratory syncytial virus (RSV) and SARS-CoV-2, the virus that causes COVID-19, as well as influenza viruses.

Alveoli are tiny air sacs in your lungs — an estimated 480 million of them — that take in oxygen and convert it to carbon dioxide.

In the presence of the segmented filamentous bacteria, macrophages resident in the alveoli somehow destroyed the pathogens directly without triggering the immune system’s larger inflammatory response.

During the study, mice were exposed both to RSV and SARS-CoV-2. In mice without segmented filamentous bacteria, alveolar macrophages became quickly depleted in the presence of the pathogens.

However, in mice with segmented filamentous bacteria, alveolar macrophages changed in two ways. First, they became resistant to the pathogens. Second, they became pathogen-neutralizers, disabling them.

Despite the promising implications, subsequent research in humans is needed to ascertain the value of these findings. If confirmed in humans, fighting serious respiratory infections could become as simple as supplementing one’s diet with the requisite gut flora.

The study is published in Cell Host & Microbe.

Dr. Jimmy Johannes, an internist, pulmonologist, and critical care medicine specialist at MemorialCare Medical Group in Long Beach, CA, not involved in the study, explained to Medical News Today:

“Macrophages are a key component of the immune system that exists in the tissues of the body. Certainly, they’re important in the lungs because the lungs are an important interface between the outside world and in your body.

[Macrophages] essentially kind of vacuum up any small debris that might exist on the alveoli, the lung sacs. They also help trigger the body’s immune response when an infection is present. [They] are one of the first responders for any new infection or pathogens that might come into contact with their lungs.”

The study’s co-senior author Andrew T. Gewirtz, Regents’ Professor and Distinguished University Professor at the Institute for Biomedical Sciences at Georgia State University, described the change in the lung macrophages when SFB was in mice’s intestines.

”Basically, [the macrophages] switched from a ‘smoke alarm’ phenotype where they made a lot of noise to attract other immune cells to a ‘sprinkler phenotype,’ wherein they directly quelled the fire,” he told MNT.

The researchers also observed additional indicators that the macrophages entirely drove the protection against respiratory viral illness.

“Our research had long focused on gut microbiota, so when the COVID pandemic began in March 2020, we started to investigate if microbiota composition might influence respiratory viral infection (RVI). This led us to discover that mice whose microbiota contained SFB were highly resistant to RVI.

We had no idea why that would be, so we investigated many possibilities, leading us to the surprising finding that lung macrophages completely changed phenotype when SFB was in the intestine.”

— Andrew T. Gewirtz, co-senior study author

Dr. Gewirtz said that it is his teams’ belief that other gut bacteria may be capable of playing a similar role, and that they are looking for them.

The paper’s authors suspect SFB may activate the complement system within the body’s greater immune system.

The complement system is a first-line immune defense that cleans out damaged cells, destroys pathogens, and helps the body heal.

“Complement is one of the oldest and longest appreciated components of the immune system, but nonetheless not fully understood,” said Dr. Gewirtz.

“One key function of complement is marking viruses for phagocytosis. Our findings indicate that, in addition, complement proteins directly inactivate some viruses, including influenza, irrespective of phagocytosis,” he added.

Research using mice does not necessarily translate to humans, but if the study’s findings could be replicated in humans, the implications could be significant.

“We are now planning human studies. We believe this will allow us to identify people whose gut microbes make them prone or resistant to RVI, and may lead to approaches to reduce the risk of severe RVI to those whom are prone to it,” Dr. Gewirtz said.

However, Dr. Johannes said that any discussion of clinical applications should wait for now.

“I think it’s too early to make any recommendations off this data. It would be interesting to see if they find something similar in humans and whether adding certain bacteria to the gut flora can change the immune response in humans to certain [viruses],” Dr. Johannes said.

The human microbiome is still a relatively new area of knowledge, and you may wonder about the safety of introducing non-native bacteria to an individual’s intestinal tract.

Dr. Johannes said he was not especially concerned, saying it “wouldn’t be the worst thing in the world to try to introduce certain gut flora that otherwise we think is benign.”

He cited how antibiotics manipulate the gut microbiome as an example and noted that humans “eat all sorts of foods that may change our microbiome.”

Read the full article here

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