The trillions of microbes in the gastrointestinal tract, known as the gut microbiome, have a major impact on human health and disease. These microbes help us digest food and absorb nutrients; they can also affect immune cells and signaling in the gut; and they generate untold numbers of molecules that can have a physiological effect on the human body. But scientists still have a lot more to learn about the exact mechanisms and processes that enable gut microbes to influence human health. A new study has now revealed more about how a protozoan that lives in the human gut microbiome can affect the health and immunity of the lungs. The findings have been reported in Cell.
"Peaceful gut microbes living inside our intestine are essential players in controlling our immune system. Growing evidence implicates these commensal microorganisms in conditions that affect other organs such as the lungs, brain, skin, or joints," noted corresponding study author Arthur Mortha, an associate professor at the University of Toronto, among other appointments. "Our aim was to understand how commensal protozoan species in the gut impact the outcome of diseases and our overall health."
Although researchers have been able to learn a lot about the bacteria in the gut microbiome, and some stuff about the viruses and archaea that live there too, this work focused on a lesser-known member of the gut microbiome: protozoa. While these single-celled organisms are similar in some ways to bacteria, their cells are more complex. Some protozoa are considered parasites, while others are symbiotic, and have a mutually beneficial relationship with their host. This work used a mouse model, however, and a protozoan called Tritrichomonas musculis that naturally lives in the mouse gut.
This model demonstrated that the presence of T. mu protozoa was associated with significantly higher levels of certain types of immune cells in the lungs. Some of those lung immune cells were found to have originated in the gut, and migrated to the lungs. Once there, these cells controlled the immune environment where they ended up, and altered outcomes of respiratory infections and illnesses.
The production, release, and movement of these immune cells seems to indicate that T. mu is working as a conductor in the gut that orchestrates immunity in other areas of the body, Mortha noted.
The effects of T. mu were mixed; the immune changes in the lungs that were triggered by T. mu appear to protect against respiratory infections while worsening the airway inflammation seen allergic asthma.
"This protozoan has a very strong impact on the immune system in the intestinal tract," Mortha added. "It exacerbates colorectal cancer development and inflammatory bowel disease, but it also gives the host the ability to withstand very severe infections."
In additional work conducted with sputum samples from severe asthma patients, the researchers found that these patients appear to carry higher levels of protozoa that are associated with people. So the results from the mouse model may be relevant to humans, although more research will be needed to confirm that.
Mortha is hopeful that this data will lead to better diagnostic and treatment options for patients with respiratory or inflammatory disorders.
Sources: University of Toronto, Cell
