Our immune system is made up of multiple smart defenses built into our bodies – but if those defenses go haywire, it can lead to diseases such as multiple sclerosis (MS). Scientists may have found a new way to control potentially dangerous autoimmune responses.

A team from Harvard Medical School at Brigham and Women's Hospital in the US developed a special probiotic that was able to regulate the activity of dendritic cells. These cells play a crucial role in managing immune cell responses.

When the probiotics were placed into the guts of mice with induced conditions similar to MS, autoimmunity was suppressed in key regions in the brain. If the treatment works in humans, it could help with some of the most damaging diseases that currently exist.

As well as its long-lasting, self-sustaining application, what's also particularly promising about the treatment is that it's more precise than existing options, and doesn't seem to come with much in the way of side effects.

"Engineered probiotics could revolutionize the way we treat chronic diseases," says neuroscientist Francisco Quintana, from Brigham and Women's.

"If we can use living microbes to produce medicine from within the body, they can keep producing the active compound as needed, which is essential when we consider lifelong diseases that require constant treatment."

To get the probiotic working, the team had to delve deeper into the function of dendritic cells in autoimmunity, which isn't yet fully understood. They found a new biochemical pathway in these cells that could 'put the brakes on' the immune system.

These brakes don't function properly in people with autoimmune diseases, which leaves the body vulnerable. The 'good bacteria' engineered by the researchers was designed to produce lactate, which in turn activated the immune system brake.

None of the engineered probiotic was observed in the bloodstreams of the mice, suggesting the gut and the brain were signaling to each other directly – and dendritic cells are found in both places in the body.

Although the study involved mouse models with a similar condition to MS, other immune mediated diseases could be targeted in the same way. Autoimmune diseases affect some 5-8 percent of the population in the US, but limited treatment options are available – not least because drugs can struggle to cross the blood-brain barrier.

Further down the line, researchers hope that different types of probiotics could be engineered to produce different compounds and get different results. That's going to take a lot more research, but this is a promising start.

"The ability to use living cells as a source of medicine in the body has tremendous potential to make more personalized and precise therapies," says Quintana.

"If these microbes living in the gut are powerful enough to influence inflammation in the brain, we're confident we'll be able to harness their power elsewhere as well."

The research has been published in Nature.