HLA-DRB1 and rheumatoid arthritis: Of these, the rather recent epidemiological data support a role for periodontal bacteria Porphyromonas gingivalis in RA etiology wherein statistically significant interactions between elevated levels of anti-citrullinated protein antibodies (ACPAs) against its virulence factor, smoking and HLA-DRB1 shared epitope alleles were observed in ACPA-positive RA (13).