Citrullination of fibrinogen induced by Porphyromonas gingivalis serve as a key pathway in breaking immune tolerance in rheumatoid arthritis

OBJECTIVE: This study investigates how Porphyromonas gingivalis (P. gingivalis) breaks immune tolerance to citrullinated antigens in rheumatoid arthritis (RA) and characterizes the immunogenicity of citrullinated fibrinogen generated by its secreted peptidylarginine deiminase (PPAD). METHODS: Subgingival microbiota from ACPA-positive/negative RA patients and healthy controls were profiled by 16S rRNA sequencing and qPCR. Collagen-induced arthritis (CIA) mice were orally colonized with wild-type P. gingivalis W83 or a PPAD-knockout strain, with arthritis severity, histopathology, and immune responses assessed. An in vitro citrullination system was used to produce modified fibrinogen, whose binding to ACPA, macrophage activation, and in vivo immunogenicity were evaluated. RESULTS: P. gingivalis was enriched in ACPA-positive RA patients, correlating with anti-CCP titers and disease activity. Wild-type, but not PPAD-knockout, P. gingivalis exacerbated CIA, boosting anti-CCP levels and splenic B cells. Fibrinogen was the primary substrate upregulated for citrullination in gingiva. The resulting citrullinated fibrinogen formed immune complexes with ACPA that drove M1 macrophage polarization via the TLR4/MyD88/NF-κB pathway. In vivo, this modified protein induced anti-CCP antibodies and aggravated arthritis; proteolytically cleaved fragments demonstrated even greater immunogenicity. CONCLUSION: By secreting PPAD to citrullinate oral fibrinogen, P. gingivalis initiates systemic autoimmunity and promotes ACPA-positive RA. This work strengthens the mucosal origin hypothesis and provides new targets for early intervention.