PADI4 and periodontitis: Considering the close association between periodontitis and RA, the role of gut microbiota in RA pathogenesis and the changes of gut microbiota induced by periodontopathic bacteria, we aimed to clarify whether the association of periodontal disease with RA is due to effects on gut microbiota and gut immune systems exerted by ingested P. gingivalis. To clarify the specific role of P. gingivalis, P. intermedia, another periodontopathic bacterium24 that does not express PAD activity, was used as a control.