Detection of DNA anti-DNA immune complexes in systemic lupus erythematosus sera

Since DNA x anti-DNA immune complexes (DNA x IC) tend to be less susceptible to DNase digestion in the antibody excess commonly seen in the sera of patients with active SLE (Bruneau 1977), the detection of DNA x IC using DNAase digestion reported by Harbeck (1973) (DNase) becomes less sensitive. By using new procedures of DNase digestion of SLE sera pre-incubated with an excess amount of DNA (DNA+DNase), a markedly higher binding value of ¹⁴C x double stranded (ds) DNA was demonstrated compared to that of the DNase digestion method. In this condition of antigen excess, DNA x IC appears to be digested by DNase more easily. This phenomenon is thought to reflect the amount of DNA x IC in the circulating blood. The positive incidences of DNA x IC detection in 50 SLE sera using the DNase method and DNA+DNase method were 14% and 44%, respectively. Cases that were considered to have DNA x IC by the DNase method were also confirmed to have DNA x IC by the advanced DNA+DNase method. Serum fractionated by ultracentrifugation revealed that the DNA x IC were present mainly in the IgG area. Based on the finding that the DNA x IC detected by the DNA+DNase method correlated well with the anti-dsDNA antibody titers (r = 0.42, p < 0.01), DNA x IC seem to be present in the circulating blood in close relationship to the anti-dsDNA antibody. The DNA+DNase method is considered suitable for detecting DNA x IC in the sera of patients with SLE.