An association between periodontal disease and RA has been studied on DR1 Tg mice, in which oral exposure to Porphyromonas gingivalis led to increased percentage of Th17 cells in blood and lymph nodes, systemic pro-inflammatory cytokine response, loss of bone density, and generation of anti-citrullinated protein antibodies (185). This evidence concerns the gene DR1 and rheumatoid arthritis.