The idea that gum disease can be a driver of more detrimental systemic disorders has been kicked around the immunological community for a number of years; however, the molecular evidence linking the ideas together has been slow to accumulate. Yet now, investigators at Johns Hopkins University report they have new evidence that a bacterium, known to cause chronic inflammatory gum infections, also triggers the inflammatory autoimmune response characteristic of chronic, joint-destroying rheumatoid arthritis (RA). The researchers believe that these new findings have important implications for the prevention and treatment of RA.
“This is like putting together the last few pieces of a complicated jigsaw puzzle that has been worked on for many years,” remarked senior study investigator Felipe Andrade, M.D., Ph.D., associate professor of medicine at the Johns Hopkins University School of Medicine.
The findings from this study, which were published recently in Science Translational Medicine in an article entitled “Aggregatibacter actinomycetemcomitans–Induced Hypercitrullination Links Periodontal Infection to Autoimmunity in Rheumatoid Arthritis,” describes a common denominator the Hopkins team identified in periodontal disease and many people with RA—the bacteria A. actinomycetemcomitans. Infection with A. actinomycetemcomitans appears to induce the production of citrullinated proteins, which are suspected of activating the immune system and driving the cascade of events leading to RA.
“This research may be the closest we've come to uncovering the root cause of RA,” noted lead study investigator Maximilian Konig, M.D., a former Johns Hopkins University School of Medicine fellow now at Massachusetts General Hospital.
The clinical association between periodontal disease and RA was first noticed in the early 1900s, and over time, researchers have suspected that both diseases may be triggered by a common factor. In recent years, studies have focused on a bacterium known as Porphyromonas gingivalis, found in patients with gum disease. However, while major efforts are currently ongoing to demonstrate that this bacterium causes RA by inducing citrullinated proteins, all attempts by this research team have failed to corroborate such a link. Dr. Andrade and his colleagues have persisted on finding alternative bacterial drivers because of intriguing links between periodontal disease and RA.
“We used mass spectrometry to define the microbial composition and antigenic repertoire of gingival crevicular fluid in patients with periodontal disease and healthy controls,” the authors wrote. “Periodontitis was characterized by the presence of citrullinated autoantigens that are primary immune targets in RA. The citrullinome in periodontitis mirrored patterns of hypercitrullination observed in the rheumatoid joint, implicating this mucosal site in RA pathogenesis. Proteomic signatures of several microbial species were detected in hypercitrullinated periodontitis samples. Among these, Aggregatibacter actinomycetemcomitans (Aa), but not other candidate pathogens, induced hypercitrullination in host neutrophils.”
Citrullination happens naturally in everyone as a way to regulate the function of proteins. However, in people with RA, this process becomes overactive, resulting in the abnormal accumulation of citrullinated proteins. This drives the production of antibodies against these proteins that create inflammation and attack a person's own tissues, the hallmark of RA.
The Hopkins team found that A. actinomycetemcomitans was the only pathogen able to induce hypercitrullination in neutrophils, which are highly enriched with the peptidylarginine deiminase (PAD) enzymes required for citrullination. Neutrophils are the most abundant inflammatory cells found in the joints and the gums of patients with RA and periodontal disease, say the researchers. These cells have been studied for many years as the major source of hypercitrullination in RA.
Moreover, the investigators found that A. actinomycetemcomitans initiates hypercitrullination through the bacterial secretion of a toxin—leukotoxin A (LtxA)—as a self-defense strategy to kill host immune cells. The toxin creates holes on the surface of neutrophils, allowing a flux of high amounts of calcium into the cell where concentrations are normally kept low. Since the PAD enzymes are activated by calcium, the increased exposure overactivates these enzymes, generating hypercitrullination.
Gathering 196 samples from a large study of patients with RA, the researchers found that almost half of the patients—92 out of 196—had evidence of infection by A. actinomycetemcomitans. These data were similar to patients with periodontal disease, with approximately 60% positivity, but quite different in healthy controls, where 11% of people were positive for A. actinomycetemcomitans. More strikingly, exposure to A. actinomycetemcomitans was a major determinant in the production of antibodies to citrullinated proteins in patients with genetic susceptibility to RA.
However, the scientists cautioned against overinterpretation of the results, as more than 50% of the study participants who had RA had no evidence of infection with A. actinomycetemcomitans, which, they say, may indicate that other bacteria in the gut, lung, or elsewhere could be using a similar mechanism to induce hypercitrullination.