Binding Properties of Human Anti‐DNA Antibodies to Cloned Human DNA Fragments

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Abstract
The DNA–anti-DNA antibody immune complexes were isolated from plasma of systemic lupus crythematosus (SLE) patients and DNA fragments separated from immune complexes were subjected to molecular cloning. The resulting recombinant DNA clones showed a molecular size of 37–79 base pairs, a high guanine and cytosine content, high frequencies of CpG dinucleotides, and palindromic sequences, and also clusters of G+C- and A+T-rich segments. These clones hybridized randomly to total human DNA. The reactivity of dsDNA antibodies, both monoclonal and polyclonal, from SLE was examined with a cloned SLE antigen DNA. A competitive inhibition assay showed that human monoclonal antibodies had at least one magnitude higher affinity to the cloned DNA than to the native DNA fragments. In order to characterize the factors that were recognized by antibodies, human G+C-rich and also A+T-rich 100 bp DNA fragments were cloned, and their base sequences determined. The antibody showed a higher affinity to the G+C-rich DNA fragment (71% G+C) than to the A+T-rich DNA fragment (46% G+C). When cytosines in CpG doublets in G+C-rich fragments were methylated (mCpG), the reactivity increased up to 100-fold. The native anti-DNA antibodies from SLE patients also showed preferential binding to G+C-rich fragments. These observations suggested that human anti-dsDNA antibodies may recognize some unique structures around the G+C regions or G+C clusters of DNA.