A newly discovered strain of bacteria could play a role in the development of rheumatoid arthritis, according to findings recently published in Science Translational Medicine.
Mice colonized with a strain of Subdoligranulum bacteria in their gut – a strain previously unidentified but now named Subdoligranulum didolesgii – developed joint swelling and inflammation as well as antibodies and T-cell responses similar to what is seen in RA, researchers reported.
Dr. Kristine Kuhn
“This was the first time that anyone has observed arthritis developing in a mouse that was not otherwise immunologically stimulated with an adjuvant of some kind, or genetically manipulated,” said Kristine Kuhn, MD, PhD, associate professor of rheumatology at the University of Colorado at Denver, Aurora, who led a team of researchers that also included investigators from Stanford (Calif.) University and Benaroya Research Institute in Seattle.
The findings offer the latest evidence – and perhaps the most compelling evidence – for the mucosal origins hypothesis, the idea that rheumatoid arthritis can start with an immune response somewhere in the mucosa because of environmental interactions, and then becomes systemic, resulting in symptoms in the joints. Anti-citrullinated protein antibodies (ACPA), hallmarks of RA, have been found at mucosal surfaces in the periodontium and the lungs, and there have been reports of them in the intestine and cervicovaginal mucosa as well.
The latest findings that implicate the new bacterium build on previous findings in which people at risk of RA, but without symptoms yet, had an expansion of B cells producing immunoglobulin A (IgA), an antibody found in the mucosa. A closer look at these B cells, using variable region sequencing, found that they arose from a family that includes both IgA and IgG members. Because IgG antibodies are systemic, this suggested a kind of evolution from an IgA-based, mucosal immune response to one that is systemic and could target the joints.
Researchers mixed monoclonal antibodies from these B cells with a pool of bacteria from the stool of a broad population of people, and then pulled out the bacteria bound by these antibodies, and sequenced them. They found that the antibodies had bound almost exclusively to Ruminococcaceae and Lachnospiraceae.
They then cultured the stool of an individual at risk of developing RA and ended up with five isolates within Ruminococcaceae – “all of which belonged to the Subdoligranulum genus,” Dr. Kuhn said. When they sequenced these, they found that they had a new strain, which was named by Meagan Chriswell, an MD-PhD candidate and member of the Cherokee Nation of Oklahoma, who chose a term based on the Cherokee word for rheumatism.
Researchers at Benaroya then mixed this strain with T cells of people with RA, and those of controls, and only the T cells of those with RA were stimulated by the bacterium, they found.
“When intestines of germ-free mice were colonized with the strain, we found that they were getting arthritis,” Dr. Kuhn said. Photos of the joints show a striking contrast between the swollen joints of the mice given Subdoligranulum didolesgii and those injected with Prevotella copri, another strain suspected of having a link to RA, as well as with another Subdoligranulum strain and a sterile media. Dr. Kuhn noted that the P. copri strain did not come from an RA-affected individual.
“We thought that our results closed the loop nicely to show that these immune responses truly were toward the Subdoligranulum, and also stimulating arthritis,” she said.
The researchers then assessed the prevalence of the strain in people at risk for RA or with RA, and in controls. They found it in 17% of those with or at risk for RA but didn’t see it at all in the healthy control population.
Dr. Kuhn and her research team, she said, are now looking at the prevalence of the strain in a larger population and doing more investigating into the link with RA.
“Does it really associate with the development of immune responses and the development of rheumatoid arthritis?” she said.