Elucidating the Mechanisms that link the Shared Epitope, Periodontal Disease and Arthritis

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Periodontal disease (PD) and Rheumatoid Arthritis (RA) have many features in common. PD is a chronic inflammatory disease of the periodontium, the soft and hard tissues supporting the teeth, and is characterized by the destruction of the alveolar bone as a result of the chronic bacterial challenge and a compromised immune response. RA is a chronic inflammatory disease of the diarthrodial joints which results in reduced mobility. Both diseases exhibit a strong genetic component, the “shared epitope” (SE) HLA-DRβ1 as well as a lifestyle component such as tobacco use, a lifestyle choice that is prevalent among veterans. We have previously demonstrated that when the shared epitope is added as a transgene to a mouse that is known not to be susceptible to collagen-induced arthritis (CIA), this mouse (B10.M) becomes susceptible to CIA. C57BL/6 (B6) mice have been described as not being susceptible to bone loss or oral colonization with the putative PD pathogen, the gram negative anaerobe Porphyromonas gingivalis in studies of PD. We have engineered a B6 mouse to express a chimeric mouse/human SE as a transgene in the absence of its murine class II (B6.DR1 mouse), and we have begun studies to determine if expression of this HLA-DRβ1 transgene can provide the B6 mouse with the appropriate set of susceptibility genes necessary for colonization with P. gingivalis and for all the pathways that lead to bone destruction. The overarching goal of this program of study is to directly address the mechanistic basis behind the association between periodontitis and the development of arthritis. We hypothesize that periodontitis in the context of permissive tissue types (such as those bearing the SE) will provide the necessary pPAD enzymes to promote host protein citrulline modifications that drive T cell development and subsequent ACPA production. We further hypothesize that ACPAs form immune complexes that enhance vascular permeability and allow binding of any number of joint specific antibodies to the cartilage where innate mechanisms such as complement and FcR binding to propagate arthritis. This study is significant because it has very important implications for veterans bearing the shared epitope. Should our hypothesis prove correct, it will suggest that aggressive preventative measures including a more pro-active periodontal screening with an enhanced frequency of visits may be necessary to prevent arthritis in veterans bearing any allelic variation of the shared epitope. This might also include arthritis that develops following acute injury to the joint. Our preliminary data demonstrate 1) that we can establish an Pg oral infection model in the humanized B6.DR1 mice and that the bacteria are present for a prolong period in the oral cavity, and can be detected in the blood stream, 2) that infection of the oral cavity induces an immune response that can be detected by the presence of ACPA, the generation of Th17 cells, and the induction of several proinflammatory cytokines, 3) that the ACPA response is dependent on the expression of the HLA-DR1 molecule, 4) that Pg infection leads to significant bone loss both in periodontal bones as well as peri- articular bones, and 5) Pg infection induces the development of autoimmune arthritis in mice that have an established autoimmune response but otherwise lack overt signs of disease. Collectively, these data provide critical evidence in support of our primary hypothesis, and the specific aims we propose to use to test this hypothesis.