Peroxynitrite-induced modification of H2A histone presents epitopes which are strongly bound by human anti-DNA autoantibodies: Role of peroxynitrite-modified-H2A in SLE induction and progression

Peroxynitrite is a potent oxidant and nitrating agent and has in vivo existence. It is a powerful proinflammatory substance and may increase vascular permeability in inflamed tissues. Systemic lupus erythematosus (SLE) is an autoimmune inflammatory disease of unknown etiology. Since its discovery, numerous self- and non-self, nuclear, and cytoplasmic antigens have been suggested as stimuli for SLE initiation, but the exact trigger is yet to be identified. In this study, an attempt has been made to investigate the binding characteristics of SLE anti-DNA autoantibodies to native DNA and native and peroxynitrite-modified H2A histone to explore the possible role of modified protein antigen(s) in SLE initiation and progression. The nuclear protein (H2A histone) was modified by peroxynitrite synthesized in our laboratory. The peroxynitrite-modified H2A revealed generation of nitrotyrosine, dityrosine, and carbonyls when subjected to investigation by physicochemical methods. Binding characteristics and specificity of SLE anti-DNA antibodies were analyzed by direct binding and inhibition enzyme-linked immunosorbent assay. The data show preferential binding of SLE autoantibodies to peroxynitrite-modified H2A histone in comparison with native H2A histone or native DNA. A band shift assay further substantiated the enhanced recognition of peroxynitirite-modified H2A histone by anti-DNA autoantibodies. The results suggest that peroxynitrite modification of self-antigen(s) can generate neoepitopes capable of inducing SLE characteristic autoantibodies. The preferential binding of peroxynitrite-modified H2A histone by SLE anti-DNA antibodies points out the likely role of oxidatively modified and nitrated H2A histone in the initiation/progression of SLE. Moreover, oxidatively modified and nitrated nuclear protein antigen, rather than nucleic acid antigens, appear to be more suitable as a trigger for SLE.