Nitration of H2B histone elicits an immune response in experimental animals

Histone H2B is an autoantigen that appears in circulation due to altered apoptosis/or insufficient clearance and is likely to be involved in the induction and progression of autoimmune diseases since modified-H2B is immunogenic. Our studies demonstrate that tyrosines of H2B histone spontaneously converts to free and nitrotyrosine bound protein in vivo. Commercially available H2B histone was modified with peroxynitrite in vitro. Modified H2B was found to be more immunogenic than native form in experimental animals. Furthermore, the sera of rabbits were analyzed for the native and modified forms of the H2B histone. The binding specificity of autoantibodies was characterized by competitive enzyme-linked immunosorbent assay (ELISA) and band shift assay. The free 3-nitrotyrosine in systemic lupus erythematosus sera was quantified by high-performance liquid chromatography. Peroxynitrite-modified H2B induced high titre antibodies as compared to native form which were directly proportional to the nitrotyrosine content. Furthermore, the induced antibodies showed specificity towards the immunogen and cross-reacted with tyrosine-nitrated proteins. ELISA showed preferential binding of induced anti-peroxynitrite modified H2B antibodies to modified H2B as compared to native H2B. The present study shows that peroxynitrite modification of self-antigen(s) generates neoepitopes capable of inducing modified-H2B autoantibodies in experimental animals.