Immunization with Porphyromonas gingivalis enolase induces autoimmunity to mammalian [alpha]-enolase and arthritis in DR4-IE-transgenic mice

Objective To examine the hypothesis that the subset of rheumatoid arthritis (RA) characterized by antibodies to citrullinated [alpha]-enolase is mediated by Porphyromonas gingivalis enolase in the context of DR4 alleles. Methods Recombinant human [alpha]-enolase and P gingivalis enolase, either citrullinated or uncitrullinated, were used to immunize DR4-IE-transgenic mice and control mice (class II major histocompatibility complex-deficient [class II MHC-/-] and C57BL/6 wild-type mice). Arthritis was quantified by measurement of ankle swelling in the hind paws and histologic examination. Serum IgG reactivity with [alpha]-enolase and citrullinated [alpha]-enolase was assayed by Western blotting and enzyme-linked immunosorbent assay (ELISA). Antibodies to peptide 1 of citrullinated [alpha]-enolase (CEP-1) and its arginine-bearing control peptide, REP-1, were also assessed by ELISA. Results Significant hind-ankle swelling (≥0.3 mm) occurred in DR4-IE-transgenic mice immunized with citrullinated human [alpha]-enolase (9 of 12 mice), uncitrullinated human [alpha]-enolase (9 of 12 mice), citrullinated P gingivalis enolase (6 of 6 mice), and uncitrullinated P gingivalis enolase (6 of 6 mice). Swelling peaked on day 24. None of the control groups developed arthritis. The arthritic joints showed synovial hyperplasia and erosions, but there was a paucity of leukocyte infiltration. Antibodies to human [alpha]-enolase, both citrullinated and unmodified, and to CEP-1 and REP-1 were detectable in all immunized mice except the class II MHC-/- control mice. Conclusion This is the first animal model that links an immune response to P gingivalis enolase to an important subset of RA, defined by antibodies to citrullinated [alpha]-enolase in the context of DR4. The fact that arthritis and anti-CEP-1 antibodies were induced independent of citrullination of the immunizing antigen suggests that the unmodified form of [alpha]-enolase may be important in initiating the corresponding subset of human RA. [PUBLICATION ABSTRACT]