High levels of certain bacteria in the mouth have been associated with hallmarks of immune activity that can affect the joints in a small study
22 February 2023
Oral bacteria can enter the blood and trigger hallmarks of immune activity involved in rheumatoid arthritis, bolstering the idea that gum disease may contribute to the painful joint condition.
The study is among the first to show that the same antibodies that target the joints in rheumatoid arthritis also attack bacteria that live on the gums, and strengthens the advice that people with the condition should maintain good dental hygiene, says Dana Orange at Rockefeller University in New York.
Rheumatoid arthritis is an autoimmune condition, which means the immune system attacks tissues within the body. Past research has found that people with the condition are more likely to have gum disease, when the gums are inflamed and sometimes bleed, but it is unclear why.
In the latest study, Orange and her colleagues monitored five people with rheumatoid arthritis, two of whom also had gum disease, by asking them to provide weekly blood samples for up to four years. These samples were tested for bacterial genetic material, as well as several markers of immune system activity.
Unlike the three people without gum disease, those with the condition had frequent traces of genes from mouth bacteria, such as Streptococcus species, in their blood, usually seen every few weeks.
This is probably caused by episodes of bleeding gums, says Orange. “As the blood is coming out, the bacteria are going in.”
When the mouth bacteria peaked, there were also hallmarks of activity from a type of immune cell called monocytes, which are known to be involved in the immune response against the joints in arthritis.
The researchers also sometimes found traces of bacteria that normally live on the skin in the blood samples, but these weren’t accompanied by immune activity. This suggests the skin bacteria contaminated the sample while it was being drawn from a participant, says Orange. “We know [the immune system] doesn’t ignore bacteria in the blood – they are usually cleared very quickly.”
A second part of the study analysed one-off blood samples from 73 people, about half of whom had rheumatoid arthritis. One blood test for rheumatoid arthritis looks for antibodies that target a group of proteins with a chemical modification called citrullination. This alteration can happen to any protein, but certain citrullinated proteins are found at high levels in the joints of people with rheumatoid arthritis.
In the study, antibodies against citrullinated proteins from those who had rheumatoid arthritis also worked against citrullinated proteins made by the mouth bacteria that enter the blood. This suggests that mouth bacteria reach the blood, triggering immune cells to make antibodies against citrullinated proteins, which cause a flare-up of the immune attack on the joints, says Orange.
While the study was small, Paul Emery at the University of Leeds, UK, says sampling the participants’ blood every week adds to the validity of its results. “It’s the first one that shows a correlation between [markers of] rheumatoid arthritis flares and bacteria in the blood.”
Immune activity caused by gum disease has been linked with a growing number of diverse medical conditions, from heart attacks to Alzheimer’s disease, although different mechanisms and bacteria may be involved. In Alzheimer’s disease, for instance, mouth bacteria called Porphyromonas gingivalis have been implicated in the condition.
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