Monoclonal anti-histone H1 autoantibodies from MRL lpr/lpr mice

Hybridomas producing anti-histone monoclonal antibodies were generated from a 2-month old MRL lpr/lpr mouse. Five IgM antibodies showed binding to histone H3 and three of these IgM antibodies also bound histone H1. Two IgG2a antibodies (MRA3 and MRA12) were specific for histone H1 and their binding was further characterized. Both reacted strongly with mouse and calf thymus histones H1, but showed limited (if any) binding to H1 from non-mammalian species such as duck, trout, or sea urchin sperm. Histone H1 is composed of three domains: N (N-terminal), G (globular) and C (C-terminal). The binding of MRA3 and MRA12 antibodies to the N, G and C domains of the histone H1 molecule was also investigated, using purified H1 fragments. Significant binding was observed only with the GC fragment but not with isolated NG, G or C fragments. Moreover, the integrity of most, if not all, of the G domain (residues 33-122) was necessary for antibody binding, since cleavage of the H1 molecule either at residue 72 or 106 abolished the binding to MRA3 and MRA12. Taken together, these results could indicate that MRA3 and MRA12 antibodies recognize a conformational determinant of the H1 molecule.