How your immune system promotes friendly gut bacteria

Scientists in Japan have uncovered a molecular mechanism through which antibodies influence gut bacteria to preserve health.

They found that immunoglobulin A (IgA) antibodies released by the gut can alter how bacteria express their genes.

This encourages the microorganisms to form communities that work together to defend against disease and safeguard the health of their "host."

Antibodies are involved in the immune response and have long been recognized as fighters of harmful agents. More recently, it has also emerged that they play an important role in regulating good bacteria in the gut.

But until the new study — now published in the Journal of Experimental Medicine — it was not clear how they did this.

Senior study author Dr. Keiichiro Suzuki, of the RIKEN Center for Integrative Medical Sciences in Japan, says that they already "knew that [IgA] contributed in some way to gut health."

But they were excited, he adds, to find that the "new mechanism" that they uncovered "actually promotes symbiosis among the bacteria that inhabit the mucus membrane of the gut."

Gut microbiota and IgA

Our guts contain "complex and dynamic" communities of bacteria and other microorganisms that play an important role in health and disease.

Collectively known as the gut microbiota, these tiny creatures have evolved in partnership with us over millennia to mutual benefit.

In their millions, they strengthen the hundreds of square feet of our guts, shape their lining, regulate metabolism, collect energy, defend against pathogens, and help develop our immune systems.

In previous work, the team had shown that IgA helps control the mix and location of bacteria in the gut, and that its stabilizing influence seems to comes from an ability to "coat" bacteria.

They found that a common species of human gut bacteria called Bacteroides thetaiotaomicron "was particularly susceptible to coating by IgA."

IgA alters gene expression

In the new study, the scientists probed the molecular underpinning of this activity. They discovered that IgA alters gene expression in B. theta.

Dr. Suzuki and team called these proteins "mucus-associated functional factors (MAFFs)," and they discovered that they seemed to be doing two things to promote friendly gut bacteria.

First, the MAFFs seemed to help B. theta grow in the mucus-secreting lining of the gut. And, second, they stimulated B. theta to make molecules that encouraged the growth of Clostridiales and other friendly bacteria.

The researchers confirmed this beneficial influence of MAFFs in mice. They injected the mice with B. theta that did not produce an abundance of MAFFs. The mice's gut bacteria changed and the animals become prone to colitis, or inflamed gut.

The team hopes that the findings will eventually lead to new treatments for inflammatory bowel disease.

"The MAFF system is also present in humans so it is an interesting target of research, but there is still much to be investigated."

Dr. Keiichiro Suzuki