Independent Roles of Host and Bacterial PAD Enzymes in Periodontitis

Periodontitis is a chronic inflammatory disease that frequently continues to progress despite therapy, indicating that pathogenic mechanisms remain insufficiently understood. Citrullination, a posttranslational modification implicated in autoimmune diseases, has emerged as a potential contributor to chronic periodontal inflammation. In periodontitis, citrullination can be mediated by at least 3 distinct enzymes: host-derived peptidylarginine deiminase 2 (PAD2) and peptidylarginine deiminase 4 (PAD4) and bacteria-derived Porphyromonas gingivalis peptidylarginine deiminase (PPAD) secreted by the keystone pathogen P gingivalis. However, their relative stage- and progression-grade-dependent contributions to human periodontitis remain unclear. In this cross-sectional clinical study, 70 subjects were classified as possessing a healthy periodontium or showing symptoms of gingivitis or different periodontitis stages (stages I-IV). Periodontitis was additionally stratified by its progression grade (grades A-C). Gingival crevicular fluid (GCF) and periodontal tissues were collected. PAD2, PAD4, and PPAD levels in GCF were quantified by enzyme-linked immunosorbent assay, and periodontal tissues were examined by histology and immunofluorescence for citrullinated proteins, PAD2, PAD4, and PPAD. Citrullinated proteins were markedly increased in periodontal tissues from periodontitis patients. PAD2 and PAD4 expression levels were locally elevated within tissues. Opposite to PAD2 levels, PAD4 levels in GCF peaked in gingivitis and tended to decline with periodontitis stage, showing no association with progression grade. PAD2 expression increased with progression grade only when heavy smokers were excluded from the analysis. PPAD expression was increased in gingivitis, further increasing toward stage II periodontitis, after which PPAD expression leveled off. Increasing PPAD levels were strongly associated with the progression grade of periodontitis. Host PAD2 and PAD4 levels in GCF were not correlated and neither were host PADs and bacterial PPAD levels. These findings demonstrate that different host-derived PADs and bacteria-derived PPAD display distinct and independent expression dynamics across disease stages, with PPAD uniquely linked to progressive periodontal destruction. This differential regulation identifies citrullination as a stage-specific mechanism in periodontitis and highlights bacterial PPAD as a potential driver of disease progression.