Autoantibody reactivity with histone 5
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AbstractThe mechanisms responsible for the induction of the autoimmune state in juvenile rheumatoid arthritis (JRA), systemic lupus erythematosus (SLE) and drug-induced lupus (DIL) are poorly understood. One of the features of autoimmunity is the presence of antibodies that react with a variety of self-antigens including nuclear DNA and histone. In keeping with the concept that the autoimmune state is driven by autoantigens, it was hypothesized that autoantibodies from these three patient populations would not bind to the non-mammalian histone H5. The specificity of JRA sera for histone subclasses was examined by immunoblotting. Antibodies to Hl alone were found in 4 of 21 pauciarticular-onset JRA sera, 4 of 19 polyarticular-onset JRA sera, and 2 of 11 systemic-onset JRA sera. Antibodies to H5 alone were found in 1 of 21 pauciarticular JRA sera, 1 of 19 polyarticular JRA sera, and 3 of 11 systemic JRA sera. Antibodies to both Hl and H5 were found in 4 of 21 pauciarticular JRA sera, 4 of 19 polyarticular sera, and 1 of 11 systemic JRA sera. Only two sera reacted with the core histones. No reactivity to histones was observed in 30 sera from age-matched children with non-rheumatic disease. The presence of antibodies to H5 in some JRA patients, in conjunction with sequence similarity analysis, suggests that the immune response in these patients is directed to determinants that are not shared by sequences of mammalian proteins. In an analysis of other patient sera, 79/98 patients with SLE, 3/3 patients with hydralazine-induced lupus (HIL), and 4/4 patients with procainamide-induced lupus (PIL) bound to H5. Proteolytic digestion of H5 and subsequent immunoblotting of the resulting peptides with 9 SLE and 7 DIL sera, revealed binding to an antigenic determinant in the carboxyl terminus. Furthermore, while some H5 antibodies also bind to HI, adsorption and immunoblotting studies identified a population of antibodies that bind specifically to H5. As an approach to determining the linear epitopes on H5, a new solid-phase multi-pin peptide synthesis protocol was used. None of 8 SLE, 3 HIL or 2 PIL sera showed statistically significant reactivity with hexamers in the carboxy-terminus. Of interest, the peptides with the most reactivity were in the globular domain of H5. These observations suggest that the majority of H5 carboxy-terminal determinants observed in immunoblotting experiments are conformational epitopes. Furthermore, since no binding was found to the globular domain by immunoblotting, the linear epitopes in this domain may be unexposed in the native protein. These findings provide new insight into the epitopes on a non-mammalian protein (H5) and give additional information about the origin of autoantibodies in SLE, PIL and HIL.
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