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.So, mice that have the wrong bacteria produce lots of dangerous immune system cells. Eventually this led to studies of humans:
Littman wondered if rheumatoid arthritis in humans might also be due to specific gut microbes. His team tested fecal samples (which reflect the population of gut bacteria) from 114 residents of the New York City area. Some subjects were healthy; others had been living with rheumatoid arthritis for years; still others had psoriatic arthritis, a different autoimmune disease whose causes are also unknown; and some had been recently diagnosed with rheumatoid arthritis. Members of this latter group were especially important because, although they had rheumatoid arthritis, they hadn't yet been treated for it. In this group, a bacterium named Prevotella copri was present in 75% of patients' intestines. . . . P. copri only appeared in 37% of patients living with either rheumatoid or psoriatic arthritis and 21% of healthy controls.Which proves nothing, of course, but is certainly suggestive. For one thing it fits with one of the weird facts about RA, which is that some people (but only some) get relief by radically changing their diets.