“It’s been suspected for years and years, both in humans and in the animal model, that the development of autoimmune diseases like arthritis is dependent on the gut microbiota,” says immunologist Diane Mathis of Harvard Medical School in Boston. Now, she says, those suspicions are beginning to be confirmed in humans. “It’s a very striking finding.”
Rheumatoid arthritis is a mysterious disease. It can strike at any age, typically beginning in young and middle-aged adults and causing painfully stiff, swollen joints in the hands and feet. It can also destroy bone and cartilage and damage organs like the lungs and kidneys. Scientists aren’t sure what causes rheumatoid arthritis, but they do know that it’s an autoimmune disorder, meaning that the body’s immune system is attacking its own tissues. And that’s where gut bacteria come in.
Gut bacteria have an intricate relationship with our immune system. We need to be able to tolerate helpful microbes while still recognizing and fighting invaders. Immunologist Dan Littman of New York University knew that gut microbes are important to the development of a particular type of immune cell his team studies, known as a Th17 cell. Mice that are reared in sterile conditions produce very few of these cells, and his group had previously found that mice bought from one supplier had far more Th17 cells than those that came from a different supplier. The difference turned out to be due to the rodents’ gut microbes.
When Littman presented that result at a conference several years ago, Mathis, who was in the audience, told him that she had seen a change in her lab animals when they were moved to a lab in a different town. Instead of spontaneously developing a mouse version of arthritis, they remained healthy. Littman and Mathis collaborated to find out why and tracked down the difference to a particular type of bacterium that, when present in the intestines, trains the immune system to produce Th17 cells, which in turn release molecules that cause inflammation and bone damage in arthritis.