Rheumatoid arthritis (RA) is a chronic autoimmune condition that causes painful, swollen joints, most often in the hands and wrists. Among chronic diseases in the U.S. RA affects more than 54 million people, and this number is expected to rise to 78 million in the next 20 years. Nearly half of those who suffer from RA become disabled and unable to work, causing them to struggle with medical bills, and even with support from insurance. 

Autoimmune conditions develop when the body’s immune system starts to attack itself. Until recently, the cause of this common health condition was unknown. But researchers have discovered that the origins of the antibodies that wage war on patients’ joints may lie in their guts, specifically in the microbiome, a community of organisms that live in the intestines. Because microbes live at the same sites as the antibodies that lead to RA, scientists hypothesized that those same bacteria could trigger these antibodies.

Researchers found that one previously unknown species of bacteria was present in the intestines of close to 20 percent of people who were either diagnosed with rheumatoid arthritis or produced the antibodies that cause the condition. An October 2022 study published in the journal Arthritis & Rheumatology confirms that an abnormal immune response to a fairly common species of gut bacteria could influence the development of arthritis. One specific bacterial strain, Prevotella copri, has been the focus of considerable research in the past few years.

A research team from University College London suggests that bacterial imbalances in the gut microbiome may play a major role in the development of RA. The preclinical study showed that damage to the gut lining directly correlates with joint inflammation arthritis severity and joint inflammation.

“We wanted to know what was happening in the gut and whether changes to the intestinal lining – which usually acts as a barrier to protect the body from bacteria – are a feature of the disease and contribute to its development,” co-lead author Claudia Mauri said, according to the website Newatlas.com.

The study showed that mice bred to have a genetic predisposition to gut permeability tended to develop signs of severe arthritis. Human patients suffering from RA had higher blood levels of lipopolysaccharide (LPS) and other protein molecules that are known markers of intestinal damage. Researchers have long reported consistent links between gut microbiome abnormalities and RA, and the study focused on the links between arthritis severity and bacteria-induced weakening of the gut wall.

An October 2022 study published in the journal Nature found that specific genes can impact our immune systems, either preventing or making us more susceptible to specific diseases such as RA. Researchers examined ancient DNA samples from the bones of more than 200 people in Europe who died from the bubonic plague in the 14th century and identified four genes that—depending on the variant— either protected or increased susceptibility to the bacteria that causes the disease. Some of these same genetic variants are associated with some autoimmune diseases, including RA.

Luis Barreiro, a senior author of the research, called our genome “a reflection of our whole evolutionary history,” according to Fortune.com, and pointed out the profound impact certain germs can have on our immune systems.

Barreiro and his team concluded that genes that may have helped our ancestors survive the plague may make us susceptible to certain diseases today. Researchers’ next goal is to uncover how common these specific gut bacteria are in the general population and test whether they may lead to the development of RA and other autoimmune diseases and determine the best way to both prevent and treat these conditions.