Antibodies to denatured DNA detected by millipore filter assay in patients with SLE and other rheumatic diseases

Since denatured DNA (d-DNA) is trapped to membrane filter by itself, the millipore filter assay, frequently used to detect antibodies to native DNA (n-DNA), has been thought to be unsuitable to disclose the antibodies to d-DNA. Application of sonication to the heat-denatured ¹⁴C-DNA obtained from E. Coli, however, prevented the direct attachment of d-DNA antigen to the filter membrane, and made it possible to measure the d-DNA binding activities of sera by millipore filter method. The optimal sonication time was 15 second at 2.5A using KUBOTA INSONATOR Model 200M. To avoid the nonspecific bindings between the ¹⁴C-d-DNA and sera, 0.1 molar of borate buffer (pH 8.3) was used throughout the study. All of the sera with active SLE and 64% of patients with inactive SLE revealed high d-DNA binding activities. Some of sera with inactive SLE, however, had high-grade d-DNA binding activities without any to n-DNA. In the group of other rheumatic diseases, several sera of dermatomyositis and Sjogren's syndrome had relatively high binding activities monospecific to d-DNA. In a group of sera with SLE, the correlations between the severity of lupus nephritis and the difference of reactivity to n-DNA and d-DNA were studied. In sera of patients with active lupus nephritis, high-grade binding activities to n-DNA and relatively lowgrade d-DNA bindings were detected. In contrast, the sera of patients with inactive lupus nephritis had low n-DNA binding activities and relatively high d-DNA bindings. The difference of reactivity to DNA in these two groups were statistically significant (P<0.02). These data suggest that d-DNA anti-d-DNA antibody system have less pathogenetic roles in developing active lupus nephritis.