[What are IgG antibodies to single-stranded DNA measured by enzyme immunoassay--re-evaluation of their clinical significance].

Enzyme immunoassay (EIA) to detect IgG antibodies to single-stranded DNA (ssDNA EIA) and EIA to detect anti double-stranded DNA antibodies (dsDNA EIA) have been developed and widely used in Japan. However, antigenecity of ssDNA and independence of anti ssDNA antibodies, and their clinical significance are still obscure. IgG anti dsDNA and ssDNA antibodies were simultaneously measured by dsDNA EIA and ssDNA EIA in sera of two hundred and eighty five patients from systemic lupus erythematosus (SLE) including other connective tissue diseases. Both EIAs were specific for SLE, although low levels of anti ssDNA antibodies were also detected in several other connective tissue diseases. Significant correlation was observed between two EIAs. All sera were divided into three group : 1 ) both EIAs were negative, 2 ) only ssDNA EIA was positive and 3 ) both EIAs were positive and in most sera ssDNA EIA levels were higher than dsDNA EIA levels. No sera were found which reacted only with dsDNA EIA or which reacted more strongly with dsDNA EIA than with ssDNA EIA. Both EIA titers were inhibited by dsDNA or ssDNA antigens by competitive inhipition study, but inhibition rate by ssDNA was higher than that by dsDNA. Previous reports by others that ssDNA EIA could be utilized to detect early stage of active SLE were not supported in this study. All these results suggested that most of what is called anti ssDNA antibedies detected by ssDNA EIA are not antibodies to purine and pyrimidine bases, but they might be anti dsDNA antibodies with low avidity which will react phosphate-ribose backbones or internal duplex structures in the ssDNA antigen. Only use of dsDNA-EIA is, thus, recommended for clinical purpose.