A fascinating new study on mice suggests that bacteria living in the gut may trigger the immune response that can result in rheumatoid arthritis.

Lead researcher Diane Mathis, a professor of pathology at Harvard Medical School, says her area of research is quite new and it's possible the findings may not translate to humans. But the research could lead to a new way of looking at autoimmune diseases.

For the study, Mathis and her team raised mice that were genetically prone to developing arthritis, in a germ-free environment. The mice had much lower levels of arthritis-causing auto-antibodies than mice raised in a non-germ-free facility.

At three weeks of age, some of the germ-free mice were transferred to a non-germ-free facility and the researchers fed the mice segmented filamentous bacteria. The bacteria species chosen were those commonly found in intestines.

The animals quickly and aggressively started making antibodies and developed full-blown rheumatoid arthritis within four days.

"In the absence of all bacteria, these mice didn't develop arthritis, but the introduction of a single bacterium was enough to jump-start the immune process that leads to development of the disease," says Mathis, an HMS professor of pathology.

Co-author Hsin-Jung Wu emphasized that the bacteria did not cause the mice to "catch" arthritis.

"It's more that they have the genetic susceptibility, and this bacterium creates an environment that allows this genetic susceptibility to play out," says Wu, a postdoctoral researcher at Harvard Medical School.

"It's an interaction between genetics and the environment."

The bacteria caused the mice to make more of a type of white blood cell called T Helper 17 cells. The immune system reacted to these cells as threatening antibodies which in turn triggered rheumatoid arthritis.

The findings appear in the June 25 issue of the journal Immunity.

Mathis says her team plans on tackling the molecular explanation of how these bacteria promote the development of this particular subset of T cells. They also want to explore connections between intestinal bacteria and other autoimmune diseases, in particular type-1 diabetes.

Rheumatoid arthritis, or RA, is a systemic condition marked by inflammation of connective tissues, which leads to the destruction of cartilage, often on many joints at a time. It differs from the more common osteoarthritis which is often due to "wear and tear" on specific joints.

While osteoarthritis often develops in older age, RA can affect people of all ages from toddlers to seniors.