From Scientific American:
Gut microbes deserve a lot of credit: They not only help digest our food, produce some nutrients, detoxify harmful substances, and protect us from pathogens—they are also important for the development of the immune system. Disturbances in the gut microbiota have been linked to allergies as well as disorders of the digestive and immune systems. Although intestinal organisms' impact on the digestive system's functioning is generally accepted, how they influence pathologies elsewhere in the body has remained a mystery. New research has begun to address this enigma. Diane Mathis, professor of pathology at Harvard Medical School, and her colleagues have found that one species of naturally occurring gut bacteria can set off arthritis in mice, in part by manipulating cells of the immune system. Their study appears in the June 25 issue of the journal Immunity.
Rheumatoid arthritis (RA) is a chronic, incurable disease characterized primarily by painful joint inflammation. Although its precise cause is unknown, RA is an autoimmune disease in which the immune system mistakes the body's own substances and cells for foreign invaders and releases friendly fire on them. To study how gut microbes affect the development of RA, the researchers made use of a specific strain of mice that naturally develop severe inflammatory arthritis. They raised the mice under germ-free conditions and found that the animals developed RA significantly more slowly than the controls that were naturally colonized with diverse, nonpathogenic microorganisms. What's more, the colonized mice produced a much greater level of an immunoregulatory protein known as IL-17. This molecule is produced by immune cells and promotes inflammation. Blocking IL-17 function in the mice prevented disease progression, demonstrating the important role of IL-17 in arthritis.