Mucosal barrier sites are continuously exposed to diverse environmental stimuli, and the fine-tuning of mucosal tissue–specific immune responses is key to maintaining tissue homeostasis. However, specific mechanisms of mucosal barrier immunity remain to be elucidated. In humans, congenital deficiency in plasminogen—the proenzyme form of the abundant plasma protease plasmin—leads to severe mucosal inflammatory disease with oral (ligneous periodontitis), ocular (ligneous conjunctivitis), lung, vaginal, and gastrointestinal tract involvement, which implicates plasmin in mucosal barrier homeostasis.

Fibrin is the principal physiological target of plasmin, and abnormal inflammatory responses associated with plasminogen deficiency in humans and in animal models have been linked to extravascular fibrin deposition. In the oral cavity, excess extravascular fibrin is hypothesized to underlie severe oral mucosal (gingival) inflammation, supporting bone (periodontal) destruction, and adolescent loss of dentition associated with plasminogen deficiency in humans. Mucosal inflammation surrounding dentition and destruction of underlying bone are also hallmarks of common forms of periodontitis. In this study, we sought to determine the mechanistic link between fibrin deposition and oral mucosal immunopathology.

We employed an array of genetically engineered mouse models and pharmacological inhibitors, complemented by histological and genetic studies in humans, to probe the association between fibrin deposition and local immunopathology. Consistent with findings in plasminogen-deficient humans, mice lacking plasminogen displayed extravascular fibrin accumulation, gingival inflammation, and periodontal bone loss. Oral immunopathology in plasminogen deficiency was prevented by genetic or pharmacological elimination of the fibrin precursor, fibrinogen, demonstrating the critical role of fibrin in plasminogen deficiency–associated mucosal immunopathology.

Notably, oral immunopathology was significantly reduced in plasminogen-deficient germ-free mice compared with specific pathogen–free mice. This suggests that commensal microbial communities represent a trigger for oral mucosal fibrin deposition and associated immunopathology.

Oral lesions in plasminogen-deficient mice were sites of neutrophil accumulation proximal to fibrin deposits. Notably, plasminogen-deficient mice expressing a mutant fibrin lacking the binding site for the myeloid integrin α_Mβ₂ did not develop periodontal disease, despite featuring unabated gingival fibrin deposition and neutrophil accumulation. Fibrin engagement by α_Mβ₂ promoted neutrophil effector functions, including the production of reactive oxygen species and neutrophil extracellular trap (NET) formation (NETosis). Accordingly, genetic or pharmacological ablation of NETosis reduced periodontal bone destruction in plasminogen-deficient mice. Thus, neutrophil activation, through fibrin-neutrophil engagement, triggered oral mucosal immunopathology. Abrogation of fibrin-neutrophil engagement also reduced age-associated periodontal immunopathology under normal fibrinolytic conditions, which was suggestive of a broader role for this mechanism in non-Mendelian forms of periodontitis. Consistent with this notion, variation in the human PLG gene, which encodes plasminogen, was associated with increased prevalence of common forms of severe periodontal disease in a cohort of Americans of European ancestry.

We identify fibrin as a critical immune regulator of oral mucosal barrier homeostasis, mediated by the local engagement …