Rheumatoid arthritis (RA) is an autoimmune synovitis that affects 0.5% of the world population. RA is characterized by intermittent flares of clinical arthritis that is thought to be mediated in part by anti- citrullinated protein autoimmune responses. The best established environmental risk factors for developing RA include cigarette smoking and periodontal disease, suggesting oral mucosa is a critical site for disease initiation. Nevertheless, the mechanisms by which these environmental exposures lead to RA development and progression remain poorly understood. We have established a clinical and technical protocol for repeated home finger stick blood collection in RA patients to allow for longitudinal RNA sequencing (RNAseq). Using this novel approach, we recently discovered bacteria characteristic of human oral mucosa in the blood of anti-CCP+ RA patients, followed by activation of a signature B cell immune response 3 weeks later, and then clinical flare of disease activity 2 weeks after that. We also investigated B cell responses to these pathogens. We demonstrated elevations in IgA blood plasmablasts both in pre-clinical RA as well as in established RA, with the continual re-activation of a distinct set of IgA/IgG plasmablast clonal families in established RA suggesting a persistent mucosally-driven germinal center reaction. We demonstrate that the recombinant Mabs encoded by the persistently reactivated IgA/G plamablast clonal families encode antibodies that react with both human citrullinated antigens and citrullinated isoforms of oral bacteria identified in the blood of patients antecedent to flares. We anticipate that RA plasmablast Mabs with distinct specificities, either alone or in immune complexes, mediate activation of distinct cellular responses that promote synovitis and tissue destruction in RA. This R01 proposal will test the hypothesis that mucosal breaks trigger plasmablast responses that encode anti-bacterial antibodies that cross-react with host citrullinated antigens. We further hypothesize that mucosal bacteria-induced ACPA activate cellular responses, including macrophage TNF production, NETosis and osteoclast activation, which promote synovitis and joint tissue destruction in RA. Aim 1 will identify the antibody repertoires responsive to pre-flare bacteremia in two independent cohorts of RA patients. Aim 2 will characterize RA plasmablast IgA/G Mabs and sera for reactivity to citrullinated isoforms of bacterial species derived from subgingival collections. Aim 3 will characterize periodontitis tissue for evidence of RA-related autoimmunity. Aim 4 will determine the mechanisms by which cross reactive Mabs, either alone or in immune complexes, mediate arthritis. Success of this proposal would demonstrate that citrullinated periodontal bacteria and mucosal breaks play a key role in mediating RA flare, findings that could lead to development of new diagnostic and therapeutic approaches.