Rheumatoid arthritis (RA) is an autoimmune disease of complex aetiology and pathogenesis. In recent years it has become evident that apoptotically modified histones exert a central role in the induction of autoimmunity, for example in systemic lupus erythematosus (SLE) and RA. Because nitric oxide (NO)-related species have been found in inflamed joints of arthritis patients, we investigated whether nitrotyrosine is present in sera of RA patients, and whether peroxynitrite-modified H2A histone is likely to be involved in the induction and progression of RA.
Commercially available H2A histone was modified in vitro by peroxynitrite. The RA patients were divided into three groups on the basis of CRP, nitrite, total protein and IgG level. Sera of RA patients with high-titre rheumatoid factor (RF) were analysed for autoantibodies against native DNA and native and peroxynitrite-modified H2A histone. Binding characteristics and specificity of the autoantibodies were analysed by direct binding, inhibition enzyme-linked immunosorbent assay (ELISA), and band shift assay.
Sera from control subjects contained almost negligible amounts of 3-nitrotyrosine (3-NT); lower levels were found in group 1 RA patients in comparison to group 2 and group 3 patients, where the level of nitrotyrosine was progressively higher. Enzyme immunoassay data showed preferential binding of RA autoantibodies to peroxynitrite-modified H2A histone in comparison with native H2A histone or native DNA. The results suggest that peroxynitrite modification of self-antigen(s) can generate neoepitopes capable of inducing RA characteristic autoantibodies.
The preferential binding of peroxynitrite-modified histones by autoantibodies derived from RA sera indicates a role for oxidatively modified and nitrated proteins in the initiation/progression of RA.