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The histone H2A-H2B dimer is a component of nucleosomes in chromatin and a frequent target of autoantibodies in spontaneous and drug-induced lupus. We obtained a panel of several lgG mAbs reacting with H2A-H2B or DNA from MRL mice which develop a spontaneous lupus-like syndrome. Several of these antibodies do not react with individual histones, but bind strongly to the H2A-H2B dimer and some bind even more strongly to the H2A-H2B-DNA complex. Moreover, these antibodies not only bind to H2A-H2B dimers in the absence of DNA, but also exhibit significant binding to DNA in the absence of histones, indicating an overlap between the anti-histone and anti-DNA specificities. The analysis of the variable region gene sequences of these antibodies shows a recurrent usage of similar VH genes, suggesting a dominant role for the heavy chain in determining binding specificity. The heavy chain third complementarity determining regions of these antibodies are also remarkable for their frequency of D-D fusions and of D segments read in unusual reading frames and for many arginine residues that may contribute to DNA binding. In addition, several antibodies obtained from an individual mouse are clonally related and some differ through somatic mutations, indicating that autoreactive clones are positively selected by nuclear antigens.

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Objective. To characterize the binding properties and variable-region sequences of LG4-1, a monoclonal antibody from an autoimmune MRL/Mp-lpr/lpr mouse that reacts specifically with nucleosome core particles and represents a new antinuclear antibody specificity.Methods. The reactivity of the antibody against various nuclear substrates was determined using an enzymatic immunoassay, and the variable-region genes were sequenced from messenger RNA, using the dideoxy chain termination method.Results.LG4-1 was found to react with nucleosome core particles but not with individual histones and DNA, or with various histone-histone and histoneDNA complexes. It was demonstrated that this antibody is encoded by a combination of variable-region genes and gene segments that have undergone few somatic mutations.Conclusion. The nucleosome core particle ex- Nucleosomes, the fundamental repeating units of the chromatin fiber, are formed of complexes of histones and DNA (1,2). The nucleosome core particle is composed of a central tetramer of 2 molecules each of histones H3 and H4 flanked by 2 dimers of histones H2A and H2B and surrounded by 2 superhelical turns of approximately 146 basepairs of DNA (2). The full nucleosome contains a molecule of histone H1 located outside of each core particle at the point where DNA enters and exits the nucleosome (1). The core histones (H2A, H2B, H3, and H4) have a heterogeneous distribution of amino acids, with basic residues concentrated in the amino-terminal third of the molecule while the rest of the molecule resembles a globular protein (H2A and H3 also have short, basic carboxyterminal domains). The histones in the core particle interact strongly through their globular domains, and only their unstructured basic regions are accessible to trypsin digestion (3). Thus, protease-digested nucleosomes composed of core histone-limit-peptides (corresponding to the globular domains) and DNA maintain an overall structure similar to that of native nucleosomes (3).Nucleosomes and their subcomponents can also be readily reconstituted in vitro, yielding complexes with structural properties similar to those of their native counterparts (4). For instance, H2A and H2B or H3 and H4 can spontaneously reassociate under physiologic conditions to form H2A-H2B dimers and H3,-H4, tetramers, respectively (5-7). The complete histone octamer will reassociate if the repulsive electrostatic interactions between cationic