The ensemble of organisms in our gut — the countless trillions of bacteria, profoundly affect our physical and mental well-being. The intestine, or the gastrointestinal tract, is the harborage of these organisms and they are responsible for us feeling either healthy (by fighting pathogens) or down in the dumps (by triggering autoimmune diseases).

Their major function, so far, was thought to be protecting the body from attacking pathogens, but in a new study published in Immunity, scientists have proposed that the gut immune system, apart from keeping us safe from invading organisms, also plays an important role in maintaining the rich and diverse population of gut microflora. The researchers propose that faults in the immune regulation affect the delicate balance of the bacterial community in the gut, which in turn affects the immune system.

The researchers, led by Sidonia Fagarasan from the RIKEN Center for Integrative Medical Science in Japan, worked on mice to show that the regulation by immune T-cells of immunoglobulin A (IgA), an antibody that plays a key role in the gut immunity, is critical for the maintenance of rich bacterial communities in mammal guts. The mice used for the research had several immune deficiencies. The researchers attempted to restore the deficiencies by reconstituting the missing components in some of the mice. They then monitored the micorflora in the mice’s guts with or without the reconstitutions and evaluated the flow of information between the immune system and bacteria. They discovered that the precise control of IgA production by regulatory T-cells promotes and balances the bacterial communities in the gut.

To check how the different bacterial communities interact with the host, the scientists looked at germ-free mice — mice born under sterile conditions and maintained such. They also worked with young pups that had been transplanted with different bacterial communities by either injecting or by painting the fur with fecal bacteria extracts from normal or immune-deficient mice.

They found that the immune system recognizes different sub-groups of bacteria and responds differently to them. Rich and balanced bacterial communities seem to be perceived as "self" and induce a quick maturation of the immune system and gut responses (induction of regulatory T-cells and IgA), while a poor and unbalanced bacterial community is apparently perceived as "non-self" and induces responses aimed at eliminating it (T-cells with inflammatory properties and IgG or IgE responses).

"This study should have an impact on the way we understand immune-related disorders associated with bacteria dysbiosis in the gut," Fagarasan said in a press release. "In order to re-establish a healthy state we need to interfere not only with the bacteria, by providing probiotics or through fecal transplantation, but also with the immune system, by correcting the faults caused either by inherited deficiencies or by aging."

"It was surprising," she continued, "to see how the reconstitution of T cell-deficient mice with a special regulatory T-cell type leads to dramatic changes in gut bacterial communities. It was spectacular to see how the immune system perceives and reacts to different bacteria communities. It gives us hopes that with a better knowledge of the symbiotic relationships between the immune system and bacteria in the gut, we could intervene and induce modifications aiming to re-establish balance and restore health." 

This research is another milestone in the study of how gut microflora affects our lives and the way our bodies function. Several studies have linked healthy gut flora to healthy bodies. And a healthy gut flora can be established with the right food, according to CNN, which states that natural fermented foods rich in probiotics and prebiotics are excellent sources to replenish the lost good gut bacteria.

Source: Fagarasan S, Kawamoto S, Maruya M et al. Foxp3+ T Cells Regulate Immunoglobulin A Selection and Facilitate Diversification of Bacterial Species Responsible for Immune Homeostasis. Immunity. 2014.