Research headed by scientists at the National Institute of Dental and Craniofacial Research (NIDCR) has shown how blocking the function of the blood clotting protein, fibrin, prevents bone loss from periodontal (gum) disease in mice. Drawing on animal and human data, the study—headed by NIDCR investigators Niki Moutsopoulos, DDS, PhD, and Thomas Bugge, PhD, found that build-up of fibrin triggers an overactive immune response that damages the gums and underlying bone. The results suggest that suppressing abnormal fibrin activity could hold promise for preventing or treating periodontal disease, as well as other inflammatory disorders—including arthritis and multiple sclerosis—that are marked by fibrin buildup.
”Severe periodontal disease can lead to tooth loss and remains a barrier to productivity and quality of life for far too many Americans, especially those lacking adequate access to dental care,” said NIDCR director Rena D’Souza, DDS, PhD. “By providing the most comprehensive picture yet of the underlying mechanisms of periodontal disease, this study brings us closer to more effective methods for prevention and treatment.”
Moutsopoulos, Bugge, and colleagues reported their findings in Science, in a paper titled, “Fibrin is a critical regulator of neutrophil effector function at the oral mucosal barrier.”
Periodontal disease is a bacterial infection of the tissues supporting the teeth. The condition affects nearly half of people in the United States who are over the age 30, and 70% of those who are 65 years and older. In its early stages, periodontal disease causes redness and swelling (inflammation) of the gums. In advanced stages, called periodontitis, the underlying bone becomes damaged, leading to tooth loss. While scientists have known that periodontitis is driven in part by an exaggerated immune cell response, until now, it was unclear what triggered the response, and how it caused tissue and bone damage.
At sites of injury or inflammation, fibrin normally plays a protective role, helping to form blood clots and activating immune cells to fight infection. But too much fibrin has been linked with health problems, including a rare form of periodontitis due to a condition called plasminogen (PLG) deficiency. In affected people, mutations in the PLG gene lead to fibrin buildup and disease at various body sites, including the mouth. “Homozygous or compound heterozygous mutations in the human PLG gene typically lead to severe mucosal disease, suggesting a critical role for this gene in mucosal immunity,” the authors wrote. “Such patients present with deposition of fibrin at various mucosal sites leading to oral disease (ligneous periodontitis); ocular disease (conjunctivitis); and lung, vaginal, and gastrointestinal disease.”
In the oral mucosa, local deposition of fibrin has been hypothesized to lead to severe soft tissue and bone destruction around teeth, and potentially loss of all teeth during adolescence. To explore the connection between abnormal fibrin buildup and periodontitis, the scientists studied PLG deficiency in mice and analyzed human genetic data. “To understand the mechanistic links between mucosal fibrin deposition and immunopathology, we employed an array of genetically engineered mouse models, complemented by histological and genetic studies in humans,” they wrote. “We chose to study the mechanisms involved in PLG deficiency–mediated immunopathology at the site of the oral mucosa because periodontitis is among the most common manifestations in humans with PLG deficiency.”
Their studies showed that, like humans with the condition, PLG-deficient mice developed periodontitis, including periodontal bone loss and elevated levels of fibrin in the gums. “Consistent with findings in plasminogen-deficient humans, mice lacking plasminogen displayed extravascular fibrin accumulation, gingival inflammation, and periodontal bone loss,” the team noted. The animals’ gums were also crowded with neutrophil immune cells, which are also found at high levels in common forms of periodontitis.
Neutrophils typically defend the oral cavity from harmful microbes. But an excessive neutrophil response is thought to cause tissue damage. To find out if fibrin was driving this overactive response, the researchers impaired its ability to interact with protein receptors on neutrophils. The weakened binding between fibrin and neutrophils completely prevented periodontal bone loss in PLG-deficient mice. Strikingly, it also reduced bone loss in normal mice with a common, age-related form of periodontitis, suggesting that similar mechanisms were at play in both forms of the disease. “We identify fibrin as a critical immune regulator of oral mucosal barrier homeostasis, mediated by the local engagement and activation of neutrophils,” the team commented.
“This study suggests that fibrin can cause neutrophil immunity to shift from protective to damaging in certain circumstances,” said Moutsopoulos, who credited postdoctoral fellow and study first author Lakmali Silva, PhD, for her research that led to the findings. “This fibrin-neutrophil engagement may be a driver of periodontitis.”
A genetic analysis of over 1,000 people seems to support the animal findings. Even in the absence of PLG deficiency, variations in the PLG gene were linked to an increased risk of severe periodontitis, consistent with the idea that similar processes contribute to rare and common forms of the disease. Taken together, the study results suggest that excessive build-up of fibrin in the gums—whether due to changes in genes, such as PLG, chronic inflammation from a bacterial infection, or some combination of the two—triggers an elevated and ultimately harmful neutrophil response that causes periodontal disease.
“ … this work demonstrates an essential role for plasmin-mediated fibrinolysis in oral mucosal homeostasis and disease and suggests that targeting fibrin-neutrophil engagement may be beneficial for the prevention or treatment of the prevalent human disease periodontitis,” the investigators concluded. “Our findings pave the way for the investigation of fibrin-neutrophil engagement as a target for the prevention or treatment of the prevalent human disease periodontitis,” they stated.
The results are also in line with findings from other research teams, which have found that elevated fibrin may contribute to inflammatory and autoimmune diseases such as arthritis and multiple sclerosis, and that interfering with fibrin activity could help treat these conditions. “Our data support the idea that targeting the fibrin-neutrophil interaction could be a promising treatment avenue to explore in both rare and common forms of periodontitis,” added Silva.