A cross-sectional study of anti-DNA antibodies in the serum and IgG and IgM fraction of healthy individuals, patients with systemic lupus erythematosus and their relatives.

Sera from healthy individuals, relatives of lupus patients, and lupus patients with active disease and in remission were screened for IgG and IgM antibodies to ssDNA and dsDNA. The serum samples were also separated into IgG and IgM fractions which were screened for DNA reactivity at dilutions equivalent to those used for testing un-fractionated serum. The results show that whilst, as expected, antibodies of IgG or IgM isotype to both ss and dsDNA could be detected in high concentration in the serum of lupus patients with active disease, anti-ssDNA antibodies of the IgG or IgM isotype are also detectable in healthy individuals, relatives and spouses of lupus patients. Furthermore when IgG and IgM serum fractions were separated and screened for DNA reactivity, the IgM fraction in the healthy individuals frequently showed an increase in the level of binding to ssDNA compared to the serum or separated IgM fraction of SLE relatives. In contrast no increase in DNA reactivity of the IgG fraction was detected in the separated IgG fraction from healthy individuals. These observations infer that IgM autoantibodies are present in healthy individuals at levels higher than is reported normally, and at levels comparable with those found in relatives of SLE patients in which autoreactive antibodies are frequently reported, and their ability to bind ssDNA in whole serum is inhibited by IgG antibodies present within whole serum.