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Somatically mutated IgG anti‐DNA antibody clonally related to germ‐line encoded IgM anti‐DNA antibody

Among 15 anti‐DNA antibody‐producing hybridomas derived from a single NZB × NZW F1 mouse, an IgM and an IgG were shown to use the same VH gene of the Q52 family. Using a combination of two primers both consisting of a mixture of oligonucleotides (one complementary to the 5′ end of VH segment and one to the 3′ end of VH segment of Q52 family) we determined the sequences of several members of germ‐line VH genes in the Q52 family derived from NZB and NZW strains. Comparison of the sequences with those of cloned VH cDNA obtained from the hybridomas revealed that the VH sequence of the IgM anti‐DNA antibody was identical to that of a cloned NZW germ‐line VH gene, except for the priming sites. In contrast, the VH sequence of the IgG counterpart contained somatically mutated nucleotides. Because the IgG anti‐DNA antibody showed a higher DNA binding activity than did the IgM antibody, we conclude that these changes in nucleotide sequences were induced and selected through an antigen‐driven mechanism as is the case in a normal immune response. It is tempting to speculate that the germ‐line encoded, low‐affinity IgM autoantibody undergoes somatic mutations and isotype switching, resulting in generation of pathogenic, high‐affinity autoantibodies in autoimmune diseases.